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Tong Y, Wang G, Riquelme MA, Du Y, Quan Y, Fu J, Gu S, Jiang JX. Mechano-activated connexin hemichannels and glutathione transport protect lens fiber cells against oxidative insults. Redox Biol 2024; 73:103216. [PMID: 38820983 DOI: 10.1016/j.redox.2024.103216] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2024] [Accepted: 05/27/2024] [Indexed: 06/02/2024] Open
Abstract
Long-lived lens fiber cells require a robust cellular protective function against oxidative insults to maintain their hemostasis and viability; however, the underlying mechanism is largely obscure. In this study, we unveiled a new mechanism that protects lens fiber cells against oxidative stress-induced cell death. We found that mechano-activated connexin (Cx) hemichannels (HCs) mediate the transport of glutathione (GSH) into chick embryonic fibroblasts (CEF) and primary lens fiber cells, resulting in a decrease in the accumulation of intracellular reactive oxygen species induced by both H2O2 and ultraviolet B, providing protection to lens fiber cells against cell apoptosis and necrosis. Furthermore, HCs formed by both homomeric Cx50 or Cx46 and heteromeric Cx50/Cx46 were mechanosensitive and could transport GSH into CEF cells. Notably, mechano-activated Cx50 HCs exhibited a greater capacity to transport GSH than Cx46 HCs. Consistently, the deficiency of Cx50 in single lens fiber cells led to a higher level of oxidative stress. Additionally, outer cortical short lens fiber cells expressing full length Cxs demonstrated greater resistance to oxidative injury compared to central core long lens fibers. Taken together, our results suggest that the activation of Cx HCs by interstitial fluid flow in cultured epithelial cells and isolated fiber cells shows that HCs can serve as a pathway for moving GSH across the cell membrane to offer protection against oxidative stress.
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Affiliation(s)
- Yuxin Tong
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA; Department of Ophthalmology, Second Xiangya Hospital, Central South University, Changsha, 410011, China; Hunan Clinical Research Center of Ophthalmic Disease, Changsha, 410011, China
| | - Guangyan Wang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA; Department of Ophthalmology, First Affiliated Hospital of Xi'an Jiaotong University, Xi'an, Shaanxi, China
| | - Manuel A Riquelme
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA
| | - Yu Du
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA
| | - Yumeng Quan
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA
| | - Jialing Fu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA
| | - Jean X Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX, 78229-3900, USA.
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2
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Li Z, Quan Y, Wang G, Ma B, Gu S, Jiang JX. The second extracellular domain of connexin 50 is important for in cell adhesion, lens differentiation, and adhesion molecule expression. J Biol Chem 2023; 299:102965. [PMID: 36736424 PMCID: PMC10011516 DOI: 10.1016/j.jbc.2023.102965] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2022] [Revised: 01/12/2023] [Accepted: 01/13/2023] [Indexed: 02/04/2023] Open
Abstract
Connexin (Cx)-forming channels play essential roles in maintaining lens homeostasis and transparency. We showed here channel-independent roles of Cx50 in cell-cell adhesion and confirmed the second extracellular (E2) domain as a critical domain for cell adhesion function. We found that cell adhesion decreased in cells expressing chimeric Cx50 in which the E2 domain was swapped with the E2 domain of either Cx43 or Cx46. In contrast, adhesion increased in cells expressing chimeric Cx43 and Cx46 with the Cx50 (E2) domain. This function is Cx channel-independent and Cx50 E2 domain-dependent cell adhesion acting in both homotypic and heterotypic manners. In addition, we generated eight site mutations of unique residues between Cx50 and the other two lens Cxs and found that mutation of any one of the residues abolished the adhesive function. Moreover, expression of adhesive-impaired mutants decreased adhesion-related proteins, N-cadherin and β-catenin. Expression of the adhesion-impaired Cx50W188P mutant in embryonic chick lens caused enlarged extracellular spaces, distorted fiber organization, delayed nuclear condensation, and cortical cataracts. In summary, the results from both in vitro and in vivo studies demonstrate the importance of the adhesive function of Cx50 in the lens.
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Affiliation(s)
- Zhen Li
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA; State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China
| | - Yumeng Quan
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA
| | - Guangyan Wang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA
| | - Bo Ma
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA
| | - Jean X Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, Texas, USA.
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3
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Zhang J, Riquelme MA, Hua R, Acosta FM, Gu S, Jiang JX. Connexin 43 hemichannels regulate mitochondrial ATP generation, mobilization, and mitochondrial homeostasis against oxidative stress. eLife 2022; 11:82206. [DOI: 10.7554/elife.82206] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2022] [Accepted: 10/25/2022] [Indexed: 11/09/2022] Open
Abstract
Oxidative stress is a major risk factor that causes osteocyte cell death and bone loss. Prior studies primarily focus on the function of cell surface expressed Cx43 channels. Here, we reported a new role of mitochondrial Cx43 (mtCx43) and hemichannels (HCs) in modulating mitochondria homeostasis and function in bone osteocytes under oxidative stress. In murine long bone osteocyte-Y4 cells, the translocation of Cx43 to mitochondria was increased under H2O2-induced oxidative stress. H2O2 increased the mtCx43 level accompanied by elevated mtCx43 HC activity, determined by dye uptake assay. Cx43 knockdown (KD) by the CRISPR-Cas9 lentivirus system resulted in impairment of mitochondrial function, primarily manifested as decreased ATP production. Cx43 KD had reduced intracellular reactive oxidative species levels and mitochondrial membrane potential. Additionally, live-cell imaging results demonstrated that the proton flux was dependent on mtCx43 HCs because its activity was specifically inhibited by an antibody targeting Cx43 C-terminus. The co-localization and interaction of mtCx43 and ATP synthase subunit F (ATP5J2) were confirmed by Förster resonance energy transfer and a protein pull-down assay. Together, our study suggests that mtCx43 HCs regulate mitochondrial ATP generation by mediating K+, H+, and ATP transfer across the mitochondrial inner membrane and the interaction with mitochondrial ATP synthase, contributing to the maintenance of mitochondrial redox levels in response to oxidative stress.
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Affiliation(s)
- Jingruo Zhang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
| | - Manuel A Riquelme
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
| | - Rui Hua
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
| | - Francisca M Acosta
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
| | - Jean X Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center
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4
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Beyer EC, Mathias RT, Berthoud VM. Loss of fiber cell communication may contribute to the development of cataracts of many different etiologies. Front Physiol 2022; 13:989524. [PMID: 36171977 PMCID: PMC9511111 DOI: 10.3389/fphys.2022.989524] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2022] [Accepted: 08/22/2022] [Indexed: 11/13/2022] Open
Abstract
The lens is an avascular organ that is supported by an internal circulation of water and solutes. This circulation is driven by ion pumps, channels and transporters in epithelial cells and by ion channels in fiber cells and is maintained by fiber-fiber and fiber-epithelial cell communication. Gap junctional intercellular channels formed of connexin46 and connexin50 are critical components of this circulation as demonstrated by studies of connexin null mice and connexin mutant mice. Moreover, connexin mutants are one of the most common causes of autosomal dominant congenital cataracts. However, alterations of the lens circulation and coupling between lens fiber cells are much more prevalent, beyond the connexin mutant lenses. Intercellular coupling and levels of connexins are decreased with aging. Gap junction-mediated intercellular communication decreases in mice expressing mutant forms of several different lens proteins and in some mouse models of lens protein damage. These observations suggest that disruption of ionic homeostasis due to reduction of the lens circulation is a common component of the development of many different types of cataracts. The decrease in the lens circulation often reflects low levels of lens fiber cell connexins and/or functional gap junction channels.
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Affiliation(s)
- Eric C. Beyer
- Department of Pediatrics, University of Chicago, Chicago, IL, United States
- *Correspondence: Eric C. Beyer,
| | - Richard T. Mathias
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY, United States
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5
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Liu J, Riquelme MA, Li Z, Li Y, Tong Y, Quan Y, Pei C, Gu S, Jiang JX. Mechanosensitive collaboration between integrins and connexins allows nutrient and antioxidant transport into the lens. J Cell Biol 2021; 219:211530. [PMID: 33180092 PMCID: PMC7668387 DOI: 10.1083/jcb.202002154] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2020] [Revised: 09/01/2020] [Accepted: 09/28/2020] [Indexed: 01/08/2023] Open
Abstract
The delivery of glucose and antioxidants is vital to maintain homeostasis and lens transparency. Here, we report a new mechanism whereby mechanically activated connexin (Cx) hemichannels serve as a transport portal for delivering glucose and glutathione (GSH). Integrin α6β1 in outer cortical lens fiber activated by fluid flow shear stress (FFSS) induced opening of hemichannels. Inhibition of α6 activation prevented hemichannel opening as well as glucose and GSH uptake. The activation of integrin β1, a heterodimeric partner of α6 in the absence of FFSS, increased Cx50 hemichannel opening. Hemichannel activation by FFSS depended on the interaction of integrin α6 and Cx50 C-terminal domain. Moreover, hemichannels in nuclear fiber were unresponsive owing to Cx50 truncation. Taken together, these results show that mechanically activated α6β1 integrin in outer cortical lens fibers leads to opening of hemichannels, which transport glucose and GSH into cortical lens fibers. This study unveils a new transport mechanism that maintains metabolic and antioxidative function of the lens.
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Affiliation(s)
- Jie Liu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX.,The First Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China
| | - Manuel A Riquelme
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
| | - Zhen Li
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
| | - Yuting Li
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
| | - Yuxin Tong
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
| | - Yumeng Quan
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX.,The First Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China
| | - Cheng Pei
- The First Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
| | - Jean X Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX
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6
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Li Z, Gu S, Quan Y, Varadaraj K, Jiang JX. Development of a potent embryonic chick lens model for studying congenital cataracts in vivo. Commun Biol 2021; 4:325. [PMID: 33707565 PMCID: PMC7952907 DOI: 10.1038/s42003-021-01849-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2020] [Accepted: 02/11/2021] [Indexed: 01/31/2023] Open
Abstract
Congenital cataracts are associated with gene mutations, yet the underlying mechanism remains largely unknown. Here we reported an embryonic chick lens model that closely recapitulates the process of cataract formation. We adopted dominant-negative site mutations that cause congenital cataracts, connexin, Cx50E48K, aquaporin 0, AQP0R33C, αA-crystallin, CRYAA R12C and R54C. The recombinant retroviruses containing these mutants were microinjected into the occlusive lumen of chick lenses at early embryonic development. Cx50E48K expression developed cataracts associated with disorganized nuclei and enlarged extracellular spaces. Expression of AQP0R33C resulted in cortical cataracts, enlarged extracellular spaces and distorted fiber cell organization. αA crystallin mutations distorted lens light transmission and increased crystalline protein aggregation. Together, retroviral expression of congenital mutant genes in embryonic chick lenses closely mimics characteristics of human congenital cataracts. This model will provide an effective, reliable in vivo system to investigate the development and underlying mechanism of cataracts and other genetic diseases.
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Affiliation(s)
- Zhen Li
- grid.267309.90000 0001 0629 5880Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX USA
| | - Sumin Gu
- grid.267309.90000 0001 0629 5880Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX USA
| | - Yumeng Quan
- grid.267309.90000 0001 0629 5880Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX USA
| | - Kulandaiappan Varadaraj
- grid.36425.360000 0001 2216 9681Department of Physiology and Biophysics, Stony Brook University, New York, NY USA
| | - Jean X. Jiang
- grid.267309.90000 0001 0629 5880Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX USA
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7
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Shi W, Riquelme MA, Gu S, Jiang JX. Connexin hemichannels mediate glutathione transport and protect lens fiber cells from oxidative stress. J Cell Sci 2018; 131:jcs212506. [PMID: 29487175 PMCID: PMC5897712 DOI: 10.1242/jcs.212506] [Citation(s) in RCA: 36] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2017] [Accepted: 02/07/2018] [Indexed: 01/04/2023] Open
Abstract
Elevated oxidized stress contributes to lens cataracts, and gap junctions play important roles in maintaining lens transparency. As well as forming gap junctions, connexin (Cx) proteins also form hemichannels. Here, we report a new mechanism whereby hemichannels mediate transport of reductant glutathione into lens fiber cells and protect cells against oxidative stress. We found that Cx50 (also known as GJA8) hemichannels opened in response to H2O2 in lens fiber cells but that transport through the channels was inhibited by two dominant-negative mutants in Cx50, Cx50P88S, which inhibits transport through both gap junctions and hemichannels, and Cx50H156N, which only inhibits transport through hemichannels and not gap junctions. Treatment with H2O2 increased the number of fiber cells undergoing apoptosis, and this increase was augmented with dominant-negative mutants that disrupted both hemichannels formed from Cx46 (also known as GJA3) and Cx50, while Cx50E48K, which only impairs gap junctions, did not have such an effect. Moreover, hemichannels mediate uptake of glutathione, and this uptake protected lens fiber cells against oxidative stress, while hemichannels with impaired transport had less protective benefit from glutathione. Taken together, these results show that oxidative stress activates connexin hemichannels in the lens fiber cells and that hemichannels likely protect lens cell against oxidative damage through transporting extracellular reductants.
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Affiliation(s)
- Wen Shi
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA
- Department of Ophthalmology, The Second Xiangya Hospital, Central South University, Changsha 410000, China
| | - Manuel A Riquelme
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA
| | - Jean X Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA
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8
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Hu Z, Shi W, Riquelme MA, Shi Q, Biswas S, Lo WK, White TW, Gu S, Jiang JX. Connexin 50 Functions as an Adhesive Molecule and Promotes Lens Cell Differentiation. Sci Rep 2017; 7:5298. [PMID: 28706245 PMCID: PMC5509658 DOI: 10.1038/s41598-017-05647-9] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2016] [Accepted: 06/01/2017] [Indexed: 11/25/2022] Open
Abstract
Connexins play essential roles in lens homeostasis and development. Here, we identified a new role for Cx50 that mediates cell-cell adhesion function. Cx50 enhanced the adhesive capability of AQP0. Interestingly, the expression of Cx50 alone promoted cell adhesion at a comparable level to AQP0; however, this cell adhesive function was not observed with other lens connexins, Cx43 and Cx46. Moreover, the adhesive property occurred in both homotypic with Cx50 expressed in both pairing cells and heterotypic with Cx50 in only one pairing cell, and this function appears to be unrelated to its role in forming gap junction channels. Cx50 KO lenses exhibited increased intercellular spaces between lens fiber cells. The second extracellular loop domain (E2) is primarily responsible for this adhesive function. Treatment with a fusion protein containing E2 domain inhibited cell adhesion. Furthermore, disruption of cell adhesion by the E2 domains impaired primary lens cell differentiation. Five critical amino acid residues in the E2 domain primarily are involved in cell adhesive function as well as lens epithelial-fiber differentiation. Together, these results suggest that in addition to forming gap junction channels, Cx50 acts as an adhesive molecule that is critical in maintaining lens fiber integrity and epithelial-fiber differentiation.
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Affiliation(s)
- Zhengping Hu
- Departments of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA
- The Second Xiangya Hospital, Central South University, Changsha, China
| | - Wen Shi
- Departments of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA
- The Second Xiangya Hospital, Central South University, Changsha, China
| | - Manuel A Riquelme
- Departments of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA
| | - Qian Shi
- Departments of Physiology, University of Texas Health Science Center, San Antonio, TX, USA
| | - Sondip Biswas
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA, USA
| | - Woo-Kuen Lo
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA, USA
| | - Thomas W White
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY, USA
| | - Sumin Gu
- Departments of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA
| | - Jean X Jiang
- Departments of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA.
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9
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Yoshikawa S, Vila A, Segelken J, Lin YP, Mitchell CK, Nguyen D, O'Brien J. Zebrafish connexin 79.8 (Gja8a): A lens connexin used as an electrical synapse in some neurons. Dev Neurobiol 2016; 77:548-561. [PMID: 27402207 DOI: 10.1002/dneu.22418] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2016] [Revised: 06/23/2016] [Accepted: 07/05/2016] [Indexed: 11/07/2022]
Abstract
In the mammalian central nervous system, a remarkably small number of connexins is used in electrical synapses, with the majority formed from Cx36. A larger number has been detected in teleosts, with some seeming to serve restricted roles. Here, we report the discovery of a new connexin expressed in the zebrafish lens and a limited set of neurons. Zebrafish cx79.8 (gja8a), previously annotated incorrectly as cx50.5 based on a partial cDNA sequence, is a homologue of mammalian Cx50 (Gja8). We examined its expression through transgenic promoter-reporter constructs, in situ hybridization, and immunolabeling, and examined regulation of coupling in transfected HeLa cells. cx79.8 was expressed most strongly in the lens, but expression was also found in several groups of neurons in the cerebellum and related areas at the midbrain-hindbrain boundary, in cone photoreceptors, and in neurons in the retinal inner nuclear and ganglion cell layers. Labeling in the retina with antibodies against two C-terminal epitopes revealed numerous small punctate spots in the inner plexiform layer and along the somata of cones. Abundant gap junctions were labeled in the outer 1/3 of the lens, but were absent from the center, suggesting that the epitopes or the entire protein was absent from the center. Cx79.8 tracer coupling was strongly regulated by phosphorylation, and was extremely low in control conditions in HeLa cells due to protein phosphatase 2A activity. These properties allow coupling to be strongly restricted in situ, a frequently observed property for electrical synapses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 548-561, 2017.
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Affiliation(s)
- Shunichi Yoshikawa
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Alejandro Vila
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas.,University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas
| | - Jasmin Segelken
- Visual Neuroscience, Faculty of Medicine and Health Sciences, University of Oldenburg, Oldenburg, Germany
| | - Ya-Ping Lin
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Cheryl K Mitchell
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Duc Nguyen
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - John O'Brien
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas.,University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas
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10
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García Caballero G, Kaltner H, Michalak M, Shilova N, Yegres M, André S, Ludwig AK, Manning JC, Schmidt S, Schnölzer M, Bovin NV, Reusch D, Kopitz J, Gabius HJ. Chicken GRIFIN: A homodimeric member of the galectin network with canonical properties and a unique expression profile. Biochimie 2016; 128-129:34-47. [PMID: 27296808 DOI: 10.1016/j.biochi.2016.06.001] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2016] [Accepted: 06/03/2016] [Indexed: 12/12/2022]
Abstract
Occurrence of the adhesion/growth-regulatory galectins as family sets the challenge to achieve a complete network analysis. Along this route taken for a well-suited model organism (chicken), we fill the remaining gap to characterize its seventh member known from rat as galectin-related inter-fiber protein (GRIFIN) in the lens. Its single-copy gene is common to vertebrates, with one or more deviations from the so-called signature sequence for ligand (lactose) contact. The chicken protein is a homodimeric agglutinin with capacity to bind β-galactosides, especially the histo-blood group B tetrasaccharide, shown by solid-phase/cell assays and a glycan microarray. Mass spectrometric identification of two lactose-binding peptides after tryptic on-bead fragmentation suggests an interaction at the canonical region despite a sequence change from Arg to Val at the site, which impairs reactivity of human galectin-1. RT-PCR and Western blot analyses of specimen from adult chicken organs reveal restriction of expression to the lens, here immunohistochemically throughout its main body. This report sets the stage for detailed structure-activity studies to define factors relevant for affinity beyond the signature sequence and to perform the first complete network analysis of the galectin family in developing and adult organs of a vertebrate.
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Affiliation(s)
- Gabriel García Caballero
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Herbert Kaltner
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Malwina Michalak
- Department of Applied Tumor Biology, Institute of Pathology, Medical School of the Ruprecht-Karls-University, Im Neuenheimer Feld 224, 69120 Heidelberg, Germany
| | - Nadezhda Shilova
- Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, ul. Miklukho-Maklaya, Moscow, Russia
| | - Michelle Yegres
- Pharma Biotech Development Penzberg, Roche Diagnostics GmbH, 82377 Penzberg, Germany
| | - Sabine André
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Anna-Kristin Ludwig
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Joachim C Manning
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Sebastian Schmidt
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany
| | - Martina Schnölzer
- Genomics and Proteomics Core Facility, German Cancer Research Center, 69120 Heidelberg, Germany
| | - Nicolai V Bovin
- Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, ul. Miklukho-Maklaya, Moscow, Russia
| | - Dietmar Reusch
- Pharma Biotech Development Penzberg, Roche Diagnostics GmbH, 82377 Penzberg, Germany
| | - Jürgen Kopitz
- Department of Applied Tumor Biology, Institute of Pathology, Medical School of the Ruprecht-Karls-University, Im Neuenheimer Feld 224, 69120 Heidelberg, Germany
| | - Hans-Joachim Gabius
- Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich, Veterinärstr. 13, 80539 Munich, Germany.
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11
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Pinto BI, García IE, Pupo A, Retamal MA, Martínez AD, Latorre R, González C. Charged Residues at the First Transmembrane Region Contribute to the Voltage Dependence of the Slow Gate of Connexins. J Biol Chem 2016; 291:15740-52. [PMID: 27143357 DOI: 10.1074/jbc.m115.709402] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2015] [Indexed: 12/17/2022] Open
Abstract
Connexins (Cxs) are a family of membrane-spanning proteins that form gap junction channels and hemichannels. Connexin-based channels exhibit two distinct voltage-dependent gating mechanisms termed slow and fast gating. Residues located at the C terminus of the first transmembrane segment (TM-1) are important structural components of the slow gate. Here, we determined the role of the charged residues at the end of TM-1 in voltage sensing in Cx26, Cx46, and Cx50. Conductance/voltage curves obtained from tail currents together with kinetics analysis reveal that the fast and slow gates of Cx26 involves the movement of two and four charges across the electric field, respectively. Primary sequence alignment of different Cxs shows the presence of well conserved glutamate residues in the C terminus of TM-1; only Cx26 contains a lysine in that position (lysine 41). Neutralization of lysine 41 in Cx26 increases the voltage dependence of the slow gate. Swapping of lysine 41 with glutamate 42 maintains the voltage dependence. In Cx46, neutralization of negative charges or addition of a positive charge in the Cx26 equivalent region reduced the slow gate voltage dependence. In Cx50, the addition of a glutamate in the same region decreased the voltage dependence, and the neutralization of a negative charge increased it. These results indicate that the charges at the end of TM-1 are part of the slow gate voltage sensor in Cxs. The fact that Cx42, which has no charge in this region, still presents voltage-dependent slow gating suggests that charges still unidentified also contribute to the slow gate voltage sensitivity.
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Affiliation(s)
- Bernardo I Pinto
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
| | - Isaac E García
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
| | - Amaury Pupo
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
| | - Mauricio A Retamal
- the Centro de Fisiología Celular e Integrativa, Facultad de Medicina, Clínica Alemana Universidad del Desarrollo, Santiago 7710162, Chile
| | - Agustín D Martínez
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
| | - Ramón Latorre
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
| | - Carlos González
- From the Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso 2360102 and
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12
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Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation. J Membr Biol 2012; 245:357-68. [PMID: 22797938 DOI: 10.1007/s00232-012-9458-y] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2012] [Accepted: 06/20/2012] [Indexed: 01/31/2023]
Abstract
A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.
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13
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Wang K, Gu S, Yin X, Weintraub ST, Hua Z, Jiang JX. Developmental truncations of connexin 50 by caspases adaptively regulate gap junctions/hemichannels and protect lens cells against ultraviolet radiation. J Biol Chem 2012; 287:15786-97. [PMID: 22418432 PMCID: PMC3346145 DOI: 10.1074/jbc.m111.313171] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2012] [Revised: 03/02/2012] [Indexed: 11/06/2022] Open
Abstract
The gap junction-forming connexin (Cx) 50 is truncated gradually during lens development. Premature cleavage of lens connexins is thought to be associated with cataract formation. We have shown previously that Cx50 is likely to be cleaved by caspase-3 like protease during chick lens development. Here, using HPLC-electrospray tandem mass spectrometry, we mapped two cleavage sites at the C terminus of Cx50 after Glu-368 and Asp-379 and identified caspase-3 and caspase-1 as the responsible proteases, respectively. The activity of caspase-1, like caspase-3, was detected in the outer cortex increased during lens development, which coincided with the accumulation of the truncated fragments of Cx50 in the core region of the lens. The truncated Cx50 fragments present in older lenses were reproduced in the younger lens after treatment with UV radiation; this cleavage could be partially blocked by caspase-1/3-specific inhibitors. Interestingly, as compared with full-length Cx50, caspase-truncated Cx50 showed a dramatic decrease in gap junction coupling and a loss of hemichannel function. Furthermore, expression of caspase-truncated Cx50 fragments increased cell viability against UV radiation as compared with full-length Cx50. Together, these results suggest that both caspase-1 and -3 are responsible for the cleavage at the C terminus of Cx50 during lens development. The reduction of gap junction coupling and closure of hemichannels formed by truncated Cx50 are likely to adaptively protect cells against elevated oxidative stress associated with lens aging.
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Affiliation(s)
- Ke Wang
- From the State Key Laboratory of Pharmaceutical Biotechnology, College of Life Sciences, Nanjing University, Nanjing 210093, China and
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900
| | - Sumin Gu
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900
| | - Xinye Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900
| | - Susan T. Weintraub
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900
| | - Zichun Hua
- From the State Key Laboratory of Pharmaceutical Biotechnology, College of Life Sciences, Nanjing University, Nanjing 210093, China and
| | - Jean X. Jiang
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900
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14
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Aspartic acid residue D3 critically determines Cx50 gap junction channel transjunctional voltage-dependent gating and unitary conductance. Biophys J 2012; 102:1022-31. [PMID: 22404924 DOI: 10.1016/j.bpj.2012.02.008] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2011] [Revised: 02/06/2012] [Accepted: 02/08/2012] [Indexed: 11/21/2022] Open
Abstract
Previous studies have suggested that the aspartic acid residue (D) at the third position is critical in determining the voltage polarity of fast V(j)-gating of Cx50 channels. To test whether another negatively charged residue (a glutamic acid residue, E) could fulfill the role of the D3 residue, we generated the mutant Cx50D3E. V(j)-dependent gating properties of this mutant channel were characterized by double-patch-clamp recordings in N2A cells. Macroscopically, the D3E substitution reduced the residual conductance (G(min)) to near zero and outwardly shifted the half-inactivation voltage (V(0)), which is a result of both a reduced aggregate gating charge (z) and a reduced free-energy difference between the open and closed states. Single Cx50D3E gap junction channels showed reduced unitary conductance (γ(j)) of the main open state, reduced open dwell time at ±40 mV, and absence of a long-lived substate. In contrast, a G8E substitution tested to compare the effects of the E residue at the third and eighth positions did not modify the V(j)-dependent gating profile or γ(j). In summary, this study is the first that we know of to suggest that the D3 residue plays an essential role, in addition to serving as a negative-charge provider, as a critical determinant of the V(j)-dependent gating sensitivity, open-closed stability, and unitary conductance of Cx50 gap junction channels.
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15
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Liu J, Ek Vitorin JF, Weintraub ST, Gu S, Shi Q, Burt JM, Jiang JX. Phosphorylation of connexin 50 by protein kinase A enhances gap junction and hemichannel function. J Biol Chem 2011; 286:16914-28. [PMID: 21454606 DOI: 10.1074/jbc.m111.218735] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Phosphorylation of connexins is an important mechanism regulating gap junction channels. However, the role(s) of connexin (Cx) phosphorylation in vivo are largely unknown. Here, we showed by mass spectrometry that Ser-395 in the C terminus of chicken Cx50 was phosphorylated in the lens. Ser-395 is located within a PKA consensus site. Analyses of Cx50 phosphorylation by two-dimensional thin layer chromatography tryptic phosphopeptide profiles suggested that Ser-395 was targeted by PKA in vivo. PKA activation increased both gap junction dye coupling and hemichannel dye uptake in a manner not involving increases in total Cx50 expression or relocation to the cell surface or gap junctional plaques. Single channel recordings indicated PKA enhanced transitions between the closed and ∼200-pS open state while simultaneously reducing transitions between this open state and a ∼65-pS subconductance state. The mutation of Ser-395 to alanine significantly attenuated PKA-induced increases in dye coupling and uptake by Cx50. However, channel records indicated that phosphorylation at this site was unnecessary for enhanced transitions between the closed and ∼200-pS conductance state. Together, these results suggest that Cx50 is phosphorylated in vivo by PKA at Ser-395 and that this event, although unnecessary for PKA-induced alterations in channel conductance, promotes increased dye permeability of Cx50 channels, which plays an important role in metabolic coupling and transport in lens fibers.
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Affiliation(s)
- Jialu Liu
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229, USA
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16
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Jiang JX. Gap junctions or hemichannel-dependent and independent roles of connexins in cataractogenesis and lens development. Curr Mol Med 2011; 10:851-63. [PMID: 21091421 DOI: 10.2174/156652410793937750] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2010] [Accepted: 09/13/2010] [Indexed: 11/22/2022]
Abstract
In the last decade or so, increasing evidences suggest that the mutations of two connexin genes, GJA3 and GJA8, are directly linked to human congenital cataracts in North and Central America, Europe and Asia. GIA3 and GIA8 genes encode gap junction-forming proteins, connexin (Cx) 46 and Cx50, respectively. These two connexins are predominantly expressed in lens fiber cells. Majority of identified mutations are missense, and the mutated sites are scattered across various domains of connexin molecules. Genetic deletion of either of these two genes leads to the development of cataracts; however, the types of cataracts developed are distinctive. More interestingly, microphthalmia is only developed in Cx50, but not Cx46 deficient mice, suggesting the unique role of Cx50 in lens cell growth and development. Knockin studies with the replacement of Cx46 or Cx50 at their respective gene locus further demonstrate the unique properties of these two connexins. Furthermore, the function of Cx50 in epithelial-fiber differentiation appears to be independent of its conventional role in forming gap junction junction channels. Due to their specific functions in maintaining lens clarity and development, and their malfunctions resulting in lens cataractogenesis and developmental impairment, connexin molecules could be developed as potential drug targets for therapeutic intervention for treatment of cataracts and other eye disorders. Recent advances in basic research of lens connexins and the discoveries of clinical disorders as a result of lens connexin dysfunctions are summarized and discussed here.
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Affiliation(s)
- J X Jiang
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA.
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17
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Liu J, Xu J, Gu S, Nicholson BJ, Jiang JX. Aquaporin 0 enhances gap junction coupling via its cell adhesion function and interaction with connexin 50. J Cell Sci 2010; 124:198-206. [PMID: 21172802 DOI: 10.1242/jcs.072652] [Citation(s) in RCA: 59] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
Both connexin 50 (Cx50) and aquaporin 0 (AQP0) have important roles in lens development and homeostasis, and their mutations are associated with human congenital cataracts. We have previously shown that Cx50 directly interacts with AQP0. Here, we demonstrate the importance of the Cx50 intracellular loop (IL) domain in mediating the interaction with AQP0 in the lens in vivo. AQP0 significantly increased (~20-30%) the intercellular coupling and conductance of Cx50 gap junctions. However, this increase was not observed when the IL domain was replaced with those from other lens connexins. The Cx50-AQP0 interaction had no effect on Cx50 hemichannel function. A fusion protein containing three extracellular loop domains of AQP0 efficiently blocked the cell-to-cell adhesion of AQP0 and attenuated the stimulatory effect of AQP0 on Cx50 gap junction conductance. These data suggest that the specific interaction between Cx50 and AQP0 enhances the coupling of Cx50 gap junctions, but not hemichannels, through the cell adhesion function of AQP0. This result establishes a physiological role of AQP0 in the functional regulation of gap junction channels.
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Affiliation(s)
- Jialu Liu
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229-3900, USA
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18
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Biswas SK, Lee JE, Brako L, Jiang JX, Lo WK. Gap junctions are selectively associated with interlocking ball-and-sockets but not protrusions in the lens. Mol Vis 2010; 16:2328-41. [PMID: 21139982 PMCID: PMC2994765] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2010] [Accepted: 10/31/2010] [Indexed: 11/01/2022] Open
Abstract
PURPOSE Ball-and-sockets and protrusions are specialized interlocking membrane domains between lens fibers of all species studied. Ball-and-sockets and protrusions are similar in their shape, size, and surface morphology, and are traditionally believed to play a key role in maintaining fiber-to-fiber stability. Here, we evaluate the hypothesis that ball-and-sockets and protrusions possess important structural and functional differences during fiber cell differentiation and maturation. METHODS Intact lenses of leghorn chickens (E7 days to P62 weeks old) and rhesus monkeys (1.5-20 years old) were studied with SEM, freeze-fracture TEM, freeze-fracture immunogold labeling (FRIL), and filipin cytochemistry for membrane cholesterol detection. RESULTS SEM showed that ball-and-sockets were distributed along the long and short sides of hexagonal fiber cells, whereas protrusions were located along the cell corners, from superficial to deep cortical regions in both chicken and monkey lenses. Importantly, by freeze-fracture TEM, we discovered the selective association of gap junctions with all ball-and-sockets examined, but not with protrusions, in both species. In the embryonic chicken lens (E18), the abundant distribution of ball-and-socket gap junctions was regularly found in an approximate zone extending at least 300 μm deep from the equatorial surface of the superficial cortical fibers. Many ball-and-socket gap junctions often protruded deeply into neighboring cells. However, in the mature fibers of monkey lenses, several ball-and-sockets exhibited only partial occupancy of gap junctions with disorganized connexons, possibly due to degradation of gap junctions during fiber maturation and aging. FRIL analysis confirmed that both connexin46 (Cx46) and connexin50 (Cx50) antibodies specifically labeled ball-and-socket gap junctions, but not protrusions. Furthermore, filipin cytochemistry revealed that the ball-and-socket gap junctions contained different amounts of cholesterol (i.e., cholesterol-rich versus cholesterol-free) as seen with the filipin-cholesterol-complexes (FCC) in different cortical regions during maturation. In contrast, the protrusions contained consistently high cholesterol amounts (i.e., 402 FCCs/μm2 membrane) which were approximately two times greater than that of the cholesterol-rich gap junctions (i.e., 188 FCCs/μm2 membrane) found in ball-and-sockets. CONCLUSIONS Gap junctions are regularly associated with all ball-and-sockets examined in metabolically active young cortical fibers, but not with protrusions, in both chicken and monkey lenses. Since these unique gap junctions often protrude deeply into neighboring cells to increase membrane surface areas, they may significantly facilitate cell-to-cell communication between young cortical fiber cells. In particular, the large number of ball-and-socket gap junctions found near the equatorial region may effectively facilitate the flow of outward current toward the equatorial surface for internal circulation of ions in the lens. In contrast, a consistent distribution of high concentrations of cholesterol in protrusions would make the protrusion membrane less deformable and would be more suitable for maintaining fiber-to-fiber stability during visual accommodation. Thus, the ball-and-sockets and protrusions are two structurally and functionally distinct membrane domains in the lens.
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Affiliation(s)
- Sondip K. Biswas
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA
| | - Jai Eun Lee
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA
| | - Lawrence Brako
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA
| | - Jean X. Jiang
- Department of Biochemistry, University of Texas Health Science Center at San Antonio, TX
| | - Woo-Kuen Lo
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA,Department of Ophthalmology, Emory University, Atlanta, GA
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Shi Q, Banks EA, Yu XS, Gu S, Lauer J, Fields GB, Jiang JX. Amino acid residue Val362 plays a critical role in maintaining the structure of C terminus of connexin 50 and in lens epithelial-fiber differentiation. J Biol Chem 2010; 285:18415-22. [PMID: 20395299 DOI: 10.1074/jbc.m110.107052] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have previously shown that connexin (Cx) 50, unlike the other two lens connexins, Cx43 and Cx46, promotes chicken lens epithelial-fiber differentiation in a channel-independent manner. Here, we show that deletion of the PEST motif at the C terminus (CT) domain of Cx50 attenuates the stimulatory effect of Cx50 on lens fiber differentiation. Valine 362, a residue located within the PEST domain, is functionally involved. The structure of the Cx50 CT predicted by molecular modeling revealed four alpha-helices and Val(362) was found to be located in the middle of the 3rd helix. Replacement of Val(362) with amino acid residues that disrupt the alpha-helical structure predicted by molecular modeling, such as arginine, glutamate, or phenylalanine, attenuated the stimulatory effects of Cx50 on lens differentiation, whereas replacement with threonine, isoleucine, leucine, or proline, which maintain the structure preserved the function of Cx50. Circular dichroism (CD) studies supported the structural predictions and showed that the substitution with Glu, but not Thr or Pro, disrupted the alpha-helix, which appears to be the structural feature important for lens epithelial-fiber differentiation. Together, our results suggest that Val(362) is important for maintaining the helical structure and is crucial for the role of Cx50 in promoting lens epithelial-fiber differentiation.
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Affiliation(s)
- Qian Shi
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA
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20
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Boswell BA, VanSlyke JK, Musil LS. Regulation of lens gap junctions by Transforming Growth Factor beta. Mol Biol Cell 2010; 21:1686-97. [PMID: 20357001 PMCID: PMC2869375 DOI: 10.1091/mbc.e10-01-0055] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Using cultured lens epithelial cells, we discovered a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of gap junctional intercellular communication. Our findings provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation. Gap junction–mediated intercellular communication (GJIC) is essential for the proper function of many organs, including the lens. GJIC in lens epithelial cells is increased by FGF in a concentration-dependent process that has been linked to the intralenticular gradient of GJIC required for lens transparency. Unlike FGF, elevated levels of TGF-β are associated with lens dysfunction. We show that TGF–β1 or -2 up-regulates dye coupling in serum-free primary cultures of chick lens epithelial cells (dissociated cell-derived monolayer cultures [DCDMLs]) via a mechanism distinct from that utilized by other growth factors. Remarkably, the ability of TGF-β and of FGF to up-regulate GJIC is abolished if DCDMLs are simultaneously exposed to both factors despite undiminished cell–cell contact. This reduction in dye coupling is attributable to an inhibition of gap junction assembly. Connexin 45.6, 43, and 56–containing gap junctions are restored, and intercellular dye coupling is increased, if the activity of p38 kinase is blocked. Our data reveal a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of GJIC. They also provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.
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Affiliation(s)
- Bruce A Boswell
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, OR 97239, USA
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21
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Biswas SK, Jiang JX, Lo WK. Gap junction remodeling associated with cholesterol redistribution during fiber cell maturation in the adult chicken lens. Mol Vis 2009; 15:1492-508. [PMID: 19657477 PMCID: PMC2720993] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2009] [Accepted: 07/31/2009] [Indexed: 12/03/2022] Open
Abstract
PURPOSE To investigate the structural remodeling in gap junctions associated with cholesterol redistribution during fiber cell maturation in the adult chicken lens. We also evaluated the hypothesis that the cleavage of the COOH-terminus of chick Cx50 (formerly Cx45.6) during fiber cell maturation would affect the gap junction remodeling. METHODS Freeze-fracture TEM and filipin cytochemistry were applied to visualize structural remodeling of gap junction connexons associated with cholesterol redistribution during fiber cell maturation in adult leghorn chickens (42-62 weeks old). Freeze-fracture immunogold labeling (FRIL) was used to label the specific Cx50 COOH-terminus antibody in various structural configurations of gap junctions. RESULTS Cortical fiber cells of the adult lenses contained three subtypes of cholesterol-containing gap junctions (i.e., cholesterol-rich, cholesterol-intermediate, and cholesterol-poor or -free) in both outer and inner cortical zones. Quantitative studies showed that approximately 81% of gap junctions in the outer cortex were cholesterol-rich gap junctions whereas approximately 78% of gap junctions in the inner cortex were cholesterol-free ones. Interestingly, all cholesterol-rich gap junctions in the outer cortex displayed loosely-packed connexons whereas cholesterol-free gap junctions in the deep zone exhibited tightly, hexagonal crystalline-arranged connexons. Also, while the percentage of membrane area specialized as gap junctions in the outer cortex was measured approximately 5 times higher than that of the inner cortex, the connexon density of the crystalline-packed gap junctions in the inner cortex was about 2 times higher than that of the loosely-packed ones in the outer cortex. Furthermore, FRIL demonstrated that while the Cx50 COOH-terminus antibody was labeled in all loosely-packed gap junctions examined in the outer cortex, little to no immunogold labeling was seen in the crystalline-packed connexons in the inner cortex. CONCLUSIONS Gap junctions undergo significant structural remodeling during fiber cell maturation in the adult chicken lens. The cholesterol-rich gap junctions with loosely-packed connexons in the young outer cortical fibers are transformed into cholesterol-free ones with crystalline-packed connexons in the mature inner fibers. In addition, the loss of the COOH-terminus of Cx50 seems to contribute equally to the transformation of the loosely-packed connexons to the crystalline-packed connexons during fiber cell maturation. This transformation causes a significant increase in the connexon density in crystalline gap junctions. As a result, it compensates considerably for the large decrease in the percentage of membrane area specialized as gap junctions in the mature inner fibers in the adult chicken lens.
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Affiliation(s)
- Sondip K. Biswas
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA
| | - Jean X. Jiang
- Department of Biochemistry, University of Texas Health Science Center at San Antonio, TX
| | - Woo-Kuen Lo
- Department of Neurobiology, Morehouse School of Medicine, Atlanta, GA,Department of Ophthalmology, Emory University, Atlanta, GA
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Kihara AH, Paschon V, Akamine PS, Saito KC, Leonelli M, Jiang JX, Hamassaki DE, Britto LRG. Differential expression of connexins during histogenesis of the chick retina. Dev Neurobiol 2009; 68:1287-302. [PMID: 18506822 DOI: 10.1002/dneu.20652] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Gap junction (GJ) channels couple adjacent cells, allowing transfer of second messengers, ions, and molecules up to 1 kDa. These channels are composed by a multigene family of integral membrane proteins called connexins (Cx). In the retina, besides being essential circuit element in the visual processing, GJ channels also play important roles during its development. Herein, we analyzed Cx43, Cx45, Cx50, and Cx56 expression during chick retinal histogenesis. Cx exhibited distinct expression profiles during retinal development, except for Cx56, whose expression was not detected. Cx43 immunolabeling was observed at early development, in the transition of ventricular zone and pigmented epithelium. Later, Cx43 was seen in the outer plexiform and ganglion cell layers, and afterwards also in the inner plexiform layer. We observed remarkable changes in the phosphorylation status of this protein, which indicated modifications in functional properties of this Cx during retinal histogenesis. By contrast, Cx45 showed stable gene expression levels throughout development and ubiquitous immunoreactivity in progenitor cells. From later embryonic development, Cx45 was mainly observed in the inner retina, and it was expressed by glial cells and neurons. In turn, Cx50 was virtually absent in the chick retina at initial embryonic phases. Combination of PCR, immunohistochemistry and Western blot indicated that this Cx was present in differentiated cells, arising in parallel with the formation of the visual circuitry. Characterization of Cx expression in the developing chick retina indicated particular roles for these proteins and revealed similarities and differences when compared to other species.
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Affiliation(s)
- A H Kihara
- Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of São Paulo, São Paulo, São Paulo, Brazil.
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Banks EA, Toloue MM, Shi Q, Zhou ZJ, Liu J, Nicholson BJ, Jiang JX. Connexin mutation that causes dominant congenital cataracts inhibits gap junctions, but not hemichannels, in a dominant negative manner. J Cell Sci 2009; 122:378-88. [PMID: 19126675 DOI: 10.1242/jcs.034124] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The connexin (Cx) 50, E48K, mutation is associated with a human dominant congenital cataract; however, the underlying molecular mechanism has not been characterized. The glutamate (E) residue at position 48 is highly conserved across animal species and types of connexins. When expressed in paired Xenopus oocytes, human (h) and chicken (ch) Cx50 E48K mutants showed no electrical coupling. In addition, this mutation acts in a dominant negative manner when paired hetero-typically or hetero-merically with wild-type Cx50, but has no such effect on Cx46, the other lens fiber connexin. A similar loss-of-function and dominant negative effect was observed using dye transfer assays in the same system. By using two different dye transfer methods, with two different tracer dyes, we found chCx50 E48K expressed in chicken lens embryonic fibroblast cells by retroviral infection similarly failed to induce dye coupling, and prevented wild-type chCx50 from forming functional gap junctions. In contrast to its effect on gap junctions, the E48K mutation has no effect on hemichannel activity when assayed using electrical conductance in oocytes, and mechanically induced dye uptake in cells. Cx50 is functionally involved in cell differentiation and lens development, and the E48K mutant promotes primary lens cell differentiation indistinguishable from wild-type chCx50, despite its lack of junctional channel function. Together the data show that mutations affecting gap junctions but not hemichannel function of Cx50 can lead to dominant congenital cataracts in humans. This clearly supports the model of intercellular coupling of fiber cells creating a microcirculation of nutrients and metabolites required for lens transparency.
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Affiliation(s)
- Eric A Banks
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229, USA
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Boswell BA, Le ACN, Musil LS. Upregulation and maintenance of gap junctional communication in lens cells. Exp Eye Res 2008; 88:919-27. [PMID: 19103198 DOI: 10.1016/j.exer.2008.11.031] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2008] [Revised: 11/08/2008] [Accepted: 11/26/2008] [Indexed: 01/20/2023]
Abstract
The cells of the lens are joined by an extensive network of gap junction intercellular channels consisting of connexins 43, 46, and 50. We have proposed, and experimentally supported, the hypothesis that fibroblast growth factor (FGF) signaling is required for upregulation of gap junction-mediated intercellular coupling (GJIC) at the lens equator. The ability of FGF to increase GJIC in cultured lens cells requires sustained activation of extracellular signal-regulated kinase (ERK). In other cell types, activation of ERK has been shown to block GJIC mediated by connexin43 (Cx43). Why ERK signaling does not block lens cell coupling is not known. Another unresolved issue in lens gap junction regulation is how connexins, synthesized before the loss of biosynthetic organelles in mature lens fiber cells, avoid degradation during formation of the organelle-free zone. We have addressed these questions using serum-free cultures (termed DCDMLs) of primary embryonic chick lens epithelial cells. We show that FGF stimulates ERK in DCDMLs via the canonical Ras/Raf1 pathway, and that the reason that neither basal nor growth factor-stimulated GJIC is blocked by activation of ERK is because it is not mediated by Cx43. In fibroblastic cells, the normally rapid rate of degradation of Cx43 after its transport to the plasma membrane is reduced by treatments that either directly (ALLN; epoxomicin) or indirectly (generation of oxidatively un/mis-folded proteins by arsenic compounds) prevent the ubiquitin/proteasome system (UPS) from acting on its normal substrates. We show here that Cx45.6 and Cx56, the chick orthologs of mammalian Cx50 and Cx46, behave similarly in DCDMLs. When organelles lyse during the maturation of fiber cells, they release into the cytosol a large amount of new proteins that have the potential to saturate the capacity, and/or compromise the function, of the UPS. This would serve to spare gap junctions from degradation during formation of the organelle-free zone, thereby preserving GJIC between mature fiber cells despite the lack of de novo connexin synthesis.
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Affiliation(s)
- Bruce A Boswell
- Department of Biochemistry and Molecular Biology L224, Oregon Health and Science University, Portland, OR 97239, USA
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25
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Yin X, Liu J, Jiang JX. Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation. ACTA ACUST UNITED AC 2008; 15:1-11. [PMID: 18649174 DOI: 10.1080/15419060802253663] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Abstract
Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.
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Affiliation(s)
- Xinye Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas, USA
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26
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Banks EA, Yu XS, Shi Q, Jiang JX. Promotion of lens epithelial-fiber differentiation by the C-terminus of connexin 45.6 – a role independent of gap junction communication. J Cell Sci 2007; 120:3602-12. [PMID: 17895360 DOI: 10.1242/jcs.000935] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We previously reported that, among the three connexins expressed in chick lens, overexpression of connexin (Cx) 45.6, not Cx43 or Cx56, stimulates lens cell differentiation; however, the underlying mechanism responsible for this effect is unclear. Here, we took advantage of naturally occurring loss-of-gap-junction function mutations of Cx50 (ortholog of chick Cx45.6) and generated the corresponding site mutants in Cx45.6: Cx45.6(D47A) and Cx45.6(P88S). In contrast to wild-type Cx45.6, the mutants failed to form functional gap junctions, and Cx45.6(P88S) and, to a lesser degree, Cx45.6(D47A) functioned in a dominant-negative manner. Interestingly, overexpression of both mutants incapable of forming gap junctions significantly increased epithelial-fiber differentiation to a level comparable to that of wild-type Cx45.6. To map the functional domain of Cx45.6, we generated a C-terminus chimera as well as deletion mutants. Overexpression of Cx56*45.6C, the mutant in which the C-terminus of Cx56 was replaced with that of Cx45.6, had a stimulatory effect on lens cell differentiation similar to that of Cx45.6. However, cells overexpressing Cx45.6*56C, the mutant in which C-terminus of Cx45.6 was replaced with that of Cx56, and Cx45.6(–C), in which the C-terminus was deleted, failed to promote differentiation. Taken together, we conclude that the expression of Cx45.6, but not Cx45.6-dependent gap junction channels, is involved in lens epithelial-fiber cell differentiation, and the C-terminal domain of Cx45.6 plays a predominant role in mediating this process.
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Affiliation(s)
- Eric A Banks
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
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27
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González D, Gómez-Hernández JM, Barrio LC. Molecular basis of voltage dependence of connexin channels: An integrative appraisal. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2007; 94:66-106. [PMID: 17470374 DOI: 10.1016/j.pbiomolbio.2007.03.007] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.
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Affiliation(s)
- Daniel González
- Research Department, Unit of Experimental Neurology, Ramón y Cajal Hospital, Carretera de Colmenar Viejo km 9, Madrid, Spain
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28
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Reza HM, Urano A, Shimada N, Yasuda K. Sequential and combinatorial roles of maf family genes define proper lens development. Mol Vis 2007; 13:18-30. [PMID: 17262012 PMCID: PMC2503189] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2006] [Accepted: 01/15/2007] [Indexed: 11/11/2022] Open
Abstract
PURPOSE Maf proteins have been shown to play pivotal roles in lens development in vertebrates. The developing chick lens expresses at least three large Maf proteins. However, the transcriptional relationship among the three large maf genes and their various roles in transactivating the downstream genes largely remain to be elucidated. METHODS Chick embryos were electroporated with wild-type L-maf, c-maf, and mafB by in ovo electroporation, and their effects on gene expression were determined by in situ hybridization using specific probes or by immunostaining. Endogenous gene expression was determined using nonelectroporated samples. RESULTS A regulation mechanism exists among the members of maf family gene. An early-expressed member of this gene family typically stimulates the expression of later-expressed members. We also examined the regulation of various lens-expressing genes with a focus on the interaction between different Maf proteins. We found that the transcriptional ability of Maf proteins varies, even when the target is the same, in parallel with their discrete functions. L-Maf and c-Maf have no effect on E-cadherin expression, whereas MafB enhances its expression and thereby impedes lens vesicle formation. This study also revealed that Maf proteins can regulate the expression of gap junction genes, connexins, and their interacting partner, major intrinsic protein (MIP), during lens development. Misexpression of L-Maf and c-Maf induces ectopic expression of Cx43 and MIP; in contrast, MafB appears to have no effect on Cx43, but induces MIP significantly as evidenced from our gain-of-function experiments. CONCLUSIONS Our results indicate that large Maf function is indispensable for chick lens initiation and development. In addition, L-Maf positively regulates most of the essential genes in this program and directs a series of molecular events leading to proper formation of the lens.
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Affiliation(s)
- Hasan Mahmud Reza
- Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma 630-0101, Japan.
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29
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Gu S, Villegas CJ, Jiang JX. Differential Regulation of Amino Acid Transporter SNAT3 by Insulin in Hepatocytes. J Biol Chem 2005; 280:26055-62. [PMID: 15899884 DOI: 10.1074/jbc.m504401200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
The liver is a metabolism and transfer center of amino acids as well as the prime target organ of insulin. In this report, we characterized the regulation of system N/A transporter 3 (SNAT3) in the liver of dietary-restricted mice and in hepatocytes treated with serum starvation and insulin. The expression of SNAT3 was up-regulated in dietary-restricted mice. The expression of SNAT3 protein was detected on the plasma membrane of hepatocyte-like H2.35 cells with a half-life of 6-8 h. When H2.35 cells were depleted of serum, the expression of SNAT3 was increased. An increased concentration of insulin, however, suppressed SNAT3 expression. Interestingly, the down-regulation of SNAT3 expression by insulin was blocked by the specific phosphoinositide 3-kinase inhibitor LY294002 and mammalian target of rapamycin inhibitor, but not by MAPK inhibitor PD98059, suggesting that insulin exerts its effect on SNAT3 through phosphoinositide 3-kinase-mammalian target of rapamycin signaling. Surface biotinylation assay showed an increased level of SNAT3 on the cell surface after 0.5 h of insulin treatment, although no effect was observed after 24 h of treatment. Consistently, the transport of the substrate l-histidine was increased with short, but not long, treatment by insulin in both H2.35- and SNAT3-transfected COS-7 cells. The L-histidine uptake was inhibited significantly by L-histidine followed by 2-endoamino-bicycloheptane-2-carboxylic acid and L-cysteine and to a lesser extent by L-alanine and aminoisobutyric acid, but was not inhibited by alpha-(methylamino)isobutyric acid, implying that uptake of L-histidine in H2.35 cells is primarily mediated by system N transporters. In conclusion, differential regulation of SNAT3 by insulin and serum starvation reinforces the functional significance of this transporter in liver physiology.
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Affiliation(s)
- Sumin Gu
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA
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30
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Yu XS, Yin X, Lafer EM, Jiang JX. Developmental Regulation of the Direct Interaction between the Intracellular Loop of Connexin 45.6 and the C Terminus of Major Intrinsic Protein (Aquaporin-0). J Biol Chem 2005; 280:22081-90. [PMID: 15802270 DOI: 10.1074/jbc.m414377200] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The eye lens is dependent upon a network of gap junction-mediated intercellular communication to facilitate its homeostasis and development. Three gap junction-forming proteins are expressed in the lens of which two are in lens fibers, namely connexin (Cx) 45.6 and 56. Major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), is the most abundant membrane protein in lens fibers. However, its role in the lens is not clear. Our previous studies show that MIP(AQP0) associates with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development but not in late embryonic and adult lenses. We report here that MIP(AQP0) directly interacts with Cx45.6 but not with Cx56. We further identified the intracellular loop of Cx45.6 as the interacting domain for the MIP(AQP0) C terminus. Surface plasmon resonance experiments indicated that the C-terminal domain of MIP(AQP0) interacts with two binding sites within the intracellular loop region of Cx45.6 with a K(D(app)) of 7.5 and 10.3 microm, respectively. The K(D(app)) for the full-length loop region is 7.7 microm. The cleavage at the intracellular loop of Cx45.6 was observed during lens development, and the C terminus of MIP(AQP0) did not interact with the loop-cleaved form of Cx45.6. Thus, the dissociation between these two proteins that occurs in the mature fibers of late lens development is likely caused by this cleavage. Finally this interaction had no impact on Cx45.6-mediated intercellular communication, suggesting that the Cx45.6-MIP(AQP0) interaction plays a novel unidentified role in lens fibers.
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Affiliation(s)
- Xun Sean Yu
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
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31
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Tong JJ, Liu X, Dong L, Ebihara L. Exchange of gating properties between rat cx46 and chicken cx45.6. Biophys J 2005; 87:2397-406. [PMID: 15454438 PMCID: PMC1304661 DOI: 10.1529/biophysj.104.039594] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cx46 and Cx50 are coexpressed in lens fiber cells where they form fiber-fiber gap junctions. Recent studies have shown that both proteins play a critical role in maintaining lens transparency. Although both Cx46 and Cx50 (or its chicken ortholog, Cx45.6) show a high degree of sequence homology, they exhibit marked differences in gap junctional channel gating, unitary gap junctional channel conductance, and hemichannel gating. To better understand which regions of the protein are responsible for these functional differences, we have constructed a series of chimeric Cx46-Cx45.6 gap junctional proteins in which a single transmembrane or intracellular domain of Cx45.6 was replaced with the corresponding domain of Cx46, expressed them in Xenopus oocyte pairs or N2A cells, and examined the resulting gap junctional conductances. Our results showed that four out of six of the chimeras induced high levels of gap junctional coupling. Of these chimeras, only Cx45.6-46NT showed significant changes in voltage-dependent gating properties. Exchanging the N-terminus had multiple effects. It slowed the inactivation kinetics of the macroscopic junctional currents so that they resembled those of Cx46, reduced the voltage sensitivity of the steady-state junctional conductance, and decreased the conductance of single gap junctional channels. Additional point mutations identified a uniquely occurring arginine in the N-terminus of Cx46 as the main determinant for the change in voltage-dependent gating.
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Affiliation(s)
- Jun-Jie Tong
- Department of Physiology and Biophysics, Rosalind Franklin School of Medicine and Science, North Chicago, Illinois, USA
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32
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Cheng S, Christie T, Valdimarsson G. Expression of connexin48.5, connexin44.1, and connexin43 during zebrafish (Danio rerio) lens development. Dev Dyn 2004; 228:709-15. [PMID: 14648847 DOI: 10.1002/dvdy.10436] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Connexins (Cx), the protein units of gap junctions, play important roles in lens development and homeostasis. Here, we report the mRNA expression patterns of zebrafish Cx48.5, Cx44.1, Cx43 during lens development. The expression of all three connexins in the adult lens was first confirmed by reverse transcriptase-polymerase chain reaction. By whole-mount in situ hybridization, we detected Cx48.5 expression throughout the lens, except the lateral lens epithelium, at 36 hours postfertilization (hpf). The pattern remained the same at 2 days postfertilization (dpf). By 3 and 4 dpf, Cx48.5 expression was restricted to the differentiating lens fibers in the equatorial and medial regions. Cx44.1 was expressed in a similar manner as Cx48.5 from 36 hpf to 4 dpf. However, Cx44.1 expression was also detected in the lens at 24 hpf. Cx43 expression was detected throughout the lens at 24 and 36 hpf but became restricted to the lateral epithelium at later stages.
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Affiliation(s)
- Shaohong Cheng
- Department of Zoology, University of Manitoba, Winnipeg, Manitoba, Canada
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33
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Yu XS, Jiang JX. Interaction of major intrinsic protein (aquaporin-0) with fiber connexins in lens development. J Cell Sci 2004; 117:871-80. [PMID: 14762116 DOI: 10.1242/jcs.00945] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We observed that chick lens-fiber gap-junction-forming proteins, connexin (Cx) 45.6 and Cx56, were associated with an unknown protein, which was then identified as major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), the most abundant membrane protein in lens fibers. A 1063 bp cDNA of chick MIP(AQP0) was identified that encodes a 262 amino acid protein with a predicted molecular weight of 28.1 kDa. Dual immunofluorescence and confocal microscopy of sagittal and coronal sections of the lens tissues showed that MIP(AQP0) consistently localized with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development. Immunoprecipitation combined with immunoblotting analyses revealed that MIP(AQP0) was associated with Cx45.6 and Cx56 at these developmental stages. The specificity of this interaction was further confirmed with the silver staining of the protein components of immunoprecipitates. The pull-down analysis of lens lysates revealed that C-terminus of MIP(AQP0) probably interacted with these two fiber connexins. In late embryonic and adult lenses, however, uniform co-distribution of MIP(AQP0) and fiber connexins was largely disrupted, except for the area surrounding the actively differentiating bow regions, as was revealed by immunofluorescence and immunoprecipitation experiments. The interaction of MIP(AQP0) with lens fiber connexins in differentiating lens cells but not in mature lens fibers suggests a potential role for MIP(AQP0) in the facilitation of fiber connexins for the formation of gap junctions during lens development.
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Affiliation(s)
- X Sean Yu
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, TX 78229-3900, USA
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34
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Saez JC, Berthoud VM, Branes MC, Martinez AD, Beyer EC. Plasma membrane channels formed by connexins: their regulation and functions. Physiol Rev 2003; 83:1359-400. [PMID: 14506308 DOI: 10.1152/physrev.00007.2003] [Citation(s) in RCA: 867] [Impact Index Per Article: 41.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.
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Affiliation(s)
- Juan C Saez
- Departamento de Ciencias Fisiológicas, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile.
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35
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Abstract
Mutations in Cx32, a gap-junction channel-forming protein, result in X-linked Charcot-Marie-Tooth disease, a demyelinating disease of the peripheral nervous system. However, although oligodendrocytes express Cx32, central myelination is unaffected. To explore this discrepancy, we searched for additional oligodendrocyte connexins. We found Cx47, which is expressed specifically in oligodendrocytes, regulated in parallel with myelin genes and partially colocalized with Cx32 in oligodendrocytes. Mice lacking either Cx47 or Cx32 are viable. However, animals lacking both connexins die by postnatal week 6 from profound abnormalities in central myelin, characterized by thin or absent myelin sheaths, vacuolation, enlarged periaxonal collars, oligodendrocyte cell death, and axonal loss. These data provide the first evidence that gap-junction communication is crucial for normal central myelination.
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36
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Menichella DM, Goodenough DA, Sirkowski E, Scherer SS, Paul DL. Connexins are critical for normal myelination in the CNS. J Neurosci 2003; 23:5963-73. [PMID: 12843301 PMCID: PMC6741267] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2003] [Revised: 05/09/2003] [Accepted: 05/14/2003] [Indexed: 03/03/2023] Open
Abstract
Mutations in Cx32, a gap-junction channel-forming protein, result in X-linked Charcot-Marie-Tooth disease, a demyelinating disease of the peripheral nervous system. However, although oligodendrocytes express Cx32, central myelination is unaffected. To explore this discrepancy, we searched for additional oligodendrocyte connexins. We found Cx47, which is expressed specifically in oligodendrocytes, regulated in parallel with myelin genes and partially colocalized with Cx32 in oligodendrocytes. Mice lacking either Cx47 or Cx32 are viable. However, animals lacking both connexins die by postnatal week 6 from profound abnormalities in central myelin, characterized by thin or absent myelin sheaths, vacuolation, enlarged periaxonal collars, oligodendrocyte cell death, and axonal loss. These data provide the first evidence that gap-junction communication is crucial for normal central myelination.
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Affiliation(s)
- Daniela M Menichella
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
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37
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Gu S, Langlais P, Liu F, Jiang JX. Mouse system-N amino acid transporter, mNAT3, expressed in hepatocytes and regulated by insulin-activated and phosphoinositide 3-kinase-dependent signalling. Biochem J 2003; 371:721-31. [PMID: 12537539 PMCID: PMC1223327 DOI: 10.1042/bj20030049] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2003] [Accepted: 01/22/2003] [Indexed: 11/17/2022]
Abstract
Amino acid transporters are essential for normal cell function and physiology. In the present study, we report the identification and functional and regulatory characterization of a mouse system-N amino acid transporter, mNAT3. Expression of mNAT3 in Xenopus oocytes revealed that the strongest transport activities were preferred for L-alanine. In addition, mNAT3 is an Na(+)- and pH-dependent low-affinity transporter and it partially tolerates substitution of Na(+) by Li(+). mNAT3 has been found to be expressed predominantly in the liver, where it is localized to the plasma membrane of hepatocytes, with the strongest expression in those cells adjacent to the central vein, decreasing gradually towards the portal tract. Treatment of mouse hepatocyte-like H2.35 cells with insulin led to a significant increase in the expression of mNAT3, and this stimulation was associated closely with an increase in the uptake of L-alanine. Interestingly, this insulin-induced stimulatory effect on mNAT3 expression was attenuated by the phosphoinositide 3-kinase inhibitor LY294002, but not by the mitogen-activated protein kinase inhibitor PD98059, although both kinases were fully activated by insulin. The results suggest that insulin-mediated regulation of mNAT3 is likely to be mediated through a phosphoinositide 3-kinase-dependent signalling pathway. The unique expression pattern and insulin-mediated regulatory properties of mNAT3 suggest that this transporter may play an important role in liver physiology.
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Affiliation(s)
- Sumin Gu
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
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38
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Nielsen PA, Baruch A, Giepmans BN, Kumar NM. Characterization of the association of connexins and ZO-1 in the lens. CELL COMMUNICATION & ADHESION 2003; 8:213-7. [PMID: 12064591 DOI: 10.3109/15419060109080726] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
ZO-1 (Zona Occludens protein 1) has previously been shown to bind Cx43alpha1. This interaction involves the most C-terminal residues of Cx43alpha1 and the second PDZ-domain of ZO-1. The biological significance of this interaction is not well understood. The similarity of the C-terminal residues of the lens connexins Cx46alpha3 and Cx50alpha8 to Cx43alpha1 prompted us to examine if ZO-1 is expressed in the lens, and if ZO-1 interacts with lens connexins. A high level of ZO-1 expression was detected in the mouse lens. Lens connexins were shown to co-immunoprecipitate with ZO-1, and the interaction was found to involve similar domains as those previously demonstrated for the Cx43alpha1/ZO-1 interaction (Nielsen et al. manuscript in preparation). Futhermore, transient expression of Cx46alpha3 and Cx50alpha8 in cell culture showed colocalization of gap junction plaques with ZO-1, further suggesting that lens connexins interact with ZO-1. Sequence comparison suggests that a large number of connexins of the alpha subclass may interact with ZO-1. Using the lens as a system to study connexin/ZO-1 interactions may further our understanding of their biological significance in the lens, as well as in other organs.
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Affiliation(s)
- P A Nielsen
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
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39
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Yin X, Gu S, Jiang JX. Regulation of lens connexin 45.6 by apoptotic protease, caspase-3. CELL COMMUNICATION & ADHESION 2003; 8:373-6. [PMID: 12064621 DOI: 10.3109/15419060109080756] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Gap junctions are important in maintaining lens homeostasis. Here we report that connexin 45.6 (Cx45.6) was partially truncated to a 46 kDa fragment during chicken lens development. This specific truncation initiated during embryonic days and the truncated fragment accumulated towards the later developmental stages. When membranes of the embryonic lens were subjected to caspase-3 treatment, the 46 kDa fragment of Cx45.6 was reproduced, suggesting apoptotic protease caspase-3 is a potential protease involved. The COOH-terminus of Cx45.6 in GST-fusion protein was also cleaved by caspase-3, confirming that Cx45.6 is a direct substrate of caspase-3. Induction of apoptosis in lens primary cultures regenerated the 46 kDa fragment and this cleavage was blocked by a caspase-3 inhibitor. Alteration of amino acid residue Asp364 or Glu367 to Ala prevented Cx45.6 from cleavage by caspase-3, suggesting that the cleavage site of Cx45.6 is likely to be between Glu367 and Gly361. Phosphorylation of Ser363, a known substrate for casein kinase II (CKII) in vivo, inhibited the cleavage of Cx45.6 by caspase-3. Thus, this study demonstrates that a lens connexin can be a direct target of caspase-3 and the cleavage by caspase-3 leads to the development-associated truncation of Cx45.6. Finally, caspase-3 mediated truncation can be modulated by the specific connexin phosphorylation.
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Affiliation(s)
- X Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio 78229-3900, USA
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40
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Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems. J Neurosci 2002. [PMID: 12151525 DOI: 10.1523/jneurosci.22-15-06458.2002] [Citation(s) in RCA: 156] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Although both Schwann cells and oligodendrocytes express connexin32 (Cx32), the loss of this connexin causes demyelination only in the PNS. To determine whether oligodendrocytes might express another connexin that can function in place of Cx32, we searched for novel CNS-specific connexins using reverse transcriptase-PCR and degenerate primers. We identified Cx29, whose transcript was restricted to brain, spinal cord, and sciatic nerve. Developmental expression of Cx29 mRNA in the CNS paralleled that of other myelin-related mRNAs, including Cx32. In the CNS, Cx29 antibodies labeled the internodal and juxtaparanodal regions of small myelin sheaths, whereas Cx32 staining was restricted to large myelinated fibers. In the PNS, Cx29 expression preceded that of Cx32 and declined to lower levels than Cx32 in adulthood. In adult sciatic nerve, Cx29 was primarily localized to the innermost aspects of the myelin sheath, the paranode, the juxtaparanode, and the inner mesaxon. Cx29 displayed a striking coincidence with Kv1.2 K(+) channels, which are localized in the axonal membrane. Both Cx29 and Cx32 were found in the incisures. Cx29 expressed in N2A cells did not induce intercellular conductances but did participate in the formation of active channels when coexpressed with Cx32. Together, these data show that Cx29 and Cx32 are expressed by myelinating glial cells with distinct distributions.
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41
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Altevogt BM, Kleopa KA, Postma FR, Scherer SS, Paul DL. Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems. J Neurosci 2002; 22:6458-70. [PMID: 12151525 PMCID: PMC6758161] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2002] [Revised: 05/07/2002] [Accepted: 05/10/2002] [Indexed: 02/26/2023] Open
Abstract
Although both Schwann cells and oligodendrocytes express connexin32 (Cx32), the loss of this connexin causes demyelination only in the PNS. To determine whether oligodendrocytes might express another connexin that can function in place of Cx32, we searched for novel CNS-specific connexins using reverse transcriptase-PCR and degenerate primers. We identified Cx29, whose transcript was restricted to brain, spinal cord, and sciatic nerve. Developmental expression of Cx29 mRNA in the CNS paralleled that of other myelin-related mRNAs, including Cx32. In the CNS, Cx29 antibodies labeled the internodal and juxtaparanodal regions of small myelin sheaths, whereas Cx32 staining was restricted to large myelinated fibers. In the PNS, Cx29 expression preceded that of Cx32 and declined to lower levels than Cx32 in adulthood. In adult sciatic nerve, Cx29 was primarily localized to the innermost aspects of the myelin sheath, the paranode, the juxtaparanode, and the inner mesaxon. Cx29 displayed a striking coincidence with Kv1.2 K(+) channels, which are localized in the axonal membrane. Both Cx29 and Cx32 were found in the incisures. Cx29 expressed in N2A cells did not induce intercellular conductances but did participate in the formation of active channels when coexpressed with Cx32. Together, these data show that Cx29 and Cx32 are expressed by myelinating glial cells with distinct distributions.
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Affiliation(s)
- Bruce M Altevogt
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
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42
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Yin X, Gu S, Jiang JX. The development-associated cleavage of lens connexin 45.6 by caspase-3-like protease is regulated by casein kinase II-mediated phosphorylation. J Biol Chem 2001; 276:34567-72. [PMID: 11448971 DOI: 10.1074/jbc.m106073200] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Gap junctions are important in maintaining lens transparency and metabolic homeostasis. In this paper, we report that the gap junction-forming protein, connexin (Cx) 45.6, was specifically truncated during lens development and that the majority of the truncated fragments were located in the differentiated lens fibers. When isolated lens membranes were treated by caspase-3, the truncated fragments of Cx45.6 were reproduced, and this truncation occurred at the COOH terminus of Cx45.6. Moreover, when primary lens cells were treated with apoptosis-inducing reagents, Cx45.6 was cleaved similarly as the in vitro treatment by caspase-3, and this cleavage was blocked by a caspase-3 inhibitor. These results suggest that caspase-3 is responsible for the development-associated cleavage of Cx45.6. The cleavage site of Cx45.6 was identified between amino acid residues Glu(367) and Gly(368). We have shown previously that Ser(363) is an in vivo phosphorylated site by casein kinase II, and this specific phosphorylation leads to a rapid turnover of Cx45.6. Interestingly, we found here that when Ser(363) was phosphorylated by casein kinase II, the cleavage of Cx45.6 catalyzed by caspase-3 was inhibited. This study, for the first time, demonstrates that a connexin can be a direct target of an apoptotic protease and that cleavage by caspase-3-like protease leads to the development-associated truncation of a lens connexin. Finally, caspase-3-mediated cleavage can be regulated by casein kinase II-mediated phosphorylation, suggesting that Cx45.6 turnover and specific cleavage by caspase-3-like protease is alternatively modulated.
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Affiliation(s)
- X Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA
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43
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Le AC, Musil LS. A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens. J Cell Biol 2001; 154:197-216. [PMID: 11449001 PMCID: PMC2196873 DOI: 10.1083/jcb.200101057] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.
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Affiliation(s)
- A C Le
- Molecular Medicine Division, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Portland, OR 97201, USA
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44
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Gu S, Roderick HL, Camacho P, Jiang JX. Characterization of an N-system amino acid transporter expressed in retina and its involvement in glutamine transport. J Biol Chem 2001; 276:24137-44. [PMID: 11325958 DOI: 10.1074/jbc.m009003200] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
We report here on the characterization of a mouse N-system amino acid transporter protein, which is involved in the transport of glutamine. This protein of 485 amino acids shares 52% sequence homology with an N-system amino acid transporter, mouse N-system amino acid transporter (mNAT) and its orthologs. Because this protein shares a high degree of sequence homology and functional similarity to mNAT, we named it mNAT2. mNAT2 is predominately expressed in the retina and to a slightly lesser extent in the brain. In the retina, it is located in the axons of ganglion cells in the nerve fiber layer and in the bundles of the optic nerve. Functional analysis of mNAT2 expressed in Xenopus oocytes revealed that the strongest transport activities were specific for l-glutamine. In addition, mNAT2 is a Na(+)- and pH-dependent, high affinity transporter and partially tolerates substitution of Na(+) by Li(+). Additionally, mNAT2 functions as a carrier-mediated transporter that facilitates efflux. The unique expression pattern and selective glutamine transport properties of mNAT2 suggest that it plays a specific role in the uptake of glutamine involved in the generation of the neurotransmitter glutamate in retina.
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Affiliation(s)
- S Gu
- Department of Biochemistry and Physiology, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA
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45
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Gu S, Adan-Rice D, Leach RJ, Jiang JX. A novel human amino acid transporter, hNAT3: cDNA cloning, chromosomal mapping, genomic structure, expression, and functional characterization. Genomics 2001; 74:262-72. [PMID: 11414754 DOI: 10.1006/geno.2001.6567] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Amino acid transporters are proteins that transport amino acids across the membrane. We report here the isolation and characterization of a novel human cDNA clone encoding a protein of 547 amino acids. This protein shares approximately 50% amino acid sequence homology with the amino acid transporters mouse mNAT and its orthologs, rat SN1 and human g17, and mouse GlnT/ATA1 and ATA2. Expression of this cRNA in Xenopus oocytes revealed that the strongest transport activities were specific for l-alanine. In addition, hNAT3 is a Na(+)- and pH-dependent, low-affinity transporter and partially tolerates substitution of Na(+) by Li(+). Since this protein has sequence and functional similarities to the previously identified system N amino acid transporters, we named this protein hNAT3. The genomic DNA sequence encoding the transcript of hNAT3 spans over 14 kb with 16 exons and 15 introns. Using fluorescence in situ hybridization, we mapped the hNAT3 gene to human chromosome 12q12-q13. By RT-PCR of embryonic and adult human tissues, hNAT3 was detected to be predominantly expressed in the liver and to a much lesser extent in the muscle, kidney, and pancreas. The data obtained in this study are likely to offer critical clues for identification of amino acid transporter-associated diseases.
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MESH Headings
- Alanine/pharmacokinetics
- Amino Acid Sequence
- Amino Acid Transport Systems
- Animals
- Base Sequence
- Biological Transport
- Carrier Proteins/genetics
- Carrier Proteins/physiology
- Chromosome Mapping
- Chromosomes, Human, Pair 12/genetics
- Cloning, Molecular
- DNA, Complementary/chemistry
- DNA, Complementary/genetics
- Exons
- Female
- Gene Expression
- Gene Expression Regulation, Developmental
- Genes/genetics
- Humans
- Hydrogen-Ion Concentration
- In Situ Hybridization, Fluorescence
- Introns
- Liver/embryology
- Liver/metabolism
- Molecular Sequence Data
- Oocytes/metabolism
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- Sequence Alignment
- Sequence Analysis, DNA
- Sequence Homology, Amino Acid
- Tissue Distribution
- Xenopus
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Affiliation(s)
- S Gu
- Department of Biochemistry, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, Texas 78229-3900, USA
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46
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Cason N, White TW, Cheng S, Goodenough DA, Valdimarsson G. Molecular cloning, expression analysis, and functional characterization of connexin44.1: A zebrafish lens gap junction protein. Dev Dyn 2001; 221:238-47. [PMID: 11376491 DOI: 10.1002/dvdy.1133] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
The connexin family of genes codes for proteins that oligomerize into a connexon of six subunits to form one half of the gap junction channel. Gap junctions are plasma membrane structures that mediate intercellular communication by joining the cytoplasm of two cells, allowing the passage of small molecules and metabolites, and contributing significantly to the maintenance of tissue homeostasis. The signaling mediated by these junctions appears to be necessary for the correct timing of key developmental events. This communication is especially important in the avascular lens where the intercellular passage of metabolites, second messengers, and ions is necessary to maintain the correct ionic balance in the lens fibre cells, and prevent cataract formation. To characterize the role that the connexin genes play in development, a novel connexin was cloned from zebrafish. A genomic clone was isolated that contained a 1,173 base open reading frame. The nucleotide sequence in this open reading frame shows extensive sequence similarity to mouse connexin50 (Cx50), chicken Cx45.6, sheep Cx49, and human Cx50. The protein encoded by this open reading frame contains 391 amino acids, with a predicted molecular weight of 44.1 kDa and a typical connexin transmembrane topology. By using the LN54 radiation hybrid panel, the Cx44.1 gene was mapped to linkage group 1. Whole-mount in situ hybridization and Northern blot analyses were performed on zebrafish embryos at various developmental stages to characterize the developmental expression of the Cx44.1 message. The ocular lens was the only tissue in which Cx44.1 transcripts were detected. The transcripts were first detected in the lens around 24 hr post fertilization and remained detectable until 120 hr post fertilization. Electrophysiological analysis of Cx44.1 channels revealed gating properties that were virtually identical to the mouse and chicken orthologues of Cx44.1.
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Affiliation(s)
- N Cason
- Department of Anatomy and Cell Biology, Queen's University, Kingston, Ontario, Canada
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47
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Gu S, Roderick HL, Camacho P, Jiang JX. Identification and characterization of an amino acid transporter expressed differentially in liver. Proc Natl Acad Sci U S A 2000; 97:3230-5. [PMID: 10716701 PMCID: PMC16221 DOI: 10.1073/pnas.97.7.3230] [Citation(s) in RCA: 64] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cellular metabolic needs are fulfilled by transport of amino acids across the plasma membrane by means of specialized transporter proteins. Although many of the classical amino acid transporters have been characterized functionally, less than half of these proteins have been cloned. In this report, we identify and characterize a cDNA encoding a plasma membrane amino acid transporter. The deduced amino acid sequence is 505 residues and is highly hydrophobic with the likely predicted structure of 9 transmembrane domains, which putatively place the amino terminus in the cytoplasm and the carboxy terminus on the cell surface. Expression of the cRNA in Xenopus laevis oocytes revealed strong transport activities specific for histidine and glutamine. This protein is a Na(+)- and pH-dependent transporter and tolerates substitution of Na(+) by Li(+). Furthermore, this transporter is not an obligatory exchanger because efflux occurs in the absence of influx. This transporter is expressed predominantly in the liver, although it is also present in the kidney, brain, and heart. In the liver, it is located in the plasma membrane of hepatocytes, and the strongest expression was detected in those adjacent to the central vein, gradually decreasing towards the portal tract. Because this protein displays functional similarities to the N-system amino acid transport, we have termed it mNAT, for murine N-system amino acid transporter. This is the first transporter gene identified within the N-system, one of the major amino acid transport systems in the body. The expression pattern displayed by mNAT suggests a potential role in hepatocyte physiology.
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Affiliation(s)
- S Gu
- Departments of Biochemistry and Physiology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
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48
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Identification and characterization of an amino acid transporter expressed differentially in liver. Proc Natl Acad Sci U S A 2000. [PMID: 10716701 PMCID: PMC16221 DOI: 10.1073/pnas.050318197] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cellular metabolic needs are fulfilled by transport of amino acids across the plasma membrane by means of specialized transporter proteins. Although many of the classical amino acid transporters have been characterized functionally, less than half of these proteins have been cloned. In this report, we identify and characterize a cDNA encoding a plasma membrane amino acid transporter. The deduced amino acid sequence is 505 residues and is highly hydrophobic with the likely predicted structure of 9 transmembrane domains, which putatively place the amino terminus in the cytoplasm and the carboxy terminus on the cell surface. Expression of the cRNA in Xenopus laevis oocytes revealed strong transport activities specific for histidine and glutamine. This protein is a Na(+)- and pH-dependent transporter and tolerates substitution of Na(+) by Li(+). Furthermore, this transporter is not an obligatory exchanger because efflux occurs in the absence of influx. This transporter is expressed predominantly in the liver, although it is also present in the kidney, brain, and heart. In the liver, it is located in the plasma membrane of hepatocytes, and the strongest expression was detected in those adjacent to the central vein, gradually decreasing towards the portal tract. Because this protein displays functional similarities to the N-system amino acid transport, we have termed it mNAT, for murine N-system amino acid transporter. This is the first transporter gene identified within the N-system, one of the major amino acid transport systems in the body. The expression pattern displayed by mNAT suggests a potential role in hepatocyte physiology.
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49
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Yin X, Jedrzejewski PT, Jiang JX. Casein kinase II phosphorylates lens connexin 45.6 and is involved in its degradation. J Biol Chem 2000; 275:6850-6. [PMID: 10702244 DOI: 10.1074/jbc.275.10.6850] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Connexin (Cx) 45.6, an avian counterpart of rodent Cx50, is phosphorylated in vivo, but the sites and function of the phosphorylation have not been elucidated. Our peptide mapping experiments showed that the Ser(363) site in the carboxyl (COOH) terminus of Cx45.6 was phosphorylated and that this site is within casein kinase (CK) II consensus sequence, although showing some similarity to CKI sequence. The peptide containing Ser(363) could be phosphorylated in vitro by CKII, but not by CKI. Furthermore, CKII phosphorylated Cx45.6 in embryonic lens membrane and the fusion protein containing the COOH terminus of Cx45.6. Two-dimensional peptide mapping experiments showed that one of the Cx45.6 peptides phosphorylated in vivo migrated to the same spot as one of those phosphorylated by CKII in vitro. Furthermore, CKII activity could be detected in lens lysates. To assess the function of this phosphorylation event, exogenous wild type and mutant Cx45.6 (Ser(363) --> Ala) were expressed in lens primary cultures by retroviral infection. The mutant Cx45.6 was shown to be more stable having a longer half-life compared with wild type Cx45.6. Together, the evidence suggests that CKII is likely a kinase responsible for the Ser(363) phosphorylation, leading to the destablization and degradation of Cx45.6. The connexin degradation induced by phosphorylation has a broad functional significance in the regulation of gap junctions in vivo.
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Affiliation(s)
- X Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78284-7760, USA
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50
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Ireland ME, Wallace P, Sandilands A, Poosch M, Kasper M, Graw J, Liu A, Maisel H, Prescott AR, Hutcheson AM, Goebel D, Quinlan RA. Up-regulation of novel intermediate filament proteins in primary fiber cells: an indicator of all vertebrate lens fiber differentiation? THE ANATOMICAL RECORD 2000; 258:25-33. [PMID: 10603445 DOI: 10.1002/(sici)1097-0185(20000101)258:1<25::aid-ar3>3.0.co;2-c] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
The early embryonic development and expression patterns of the eye lens specific cytoskeletal proteins, CP49 and CP95, were determined for the chick and were found to be similar in both human and mouse. These proteins, as well as their homologs in other species, are obligate polymerization partners which form unique filamentous structures termed "beaded filaments." CP49 and CP95 appeared as protein products after 3 days of embryonic development in the chick during the elongation of primary fiber cells. Although limited data were obtained for human embryos at these early developmental timepoints, they were consistent with the interpretation that the up-regulation of these lens specific proteins began only after the initiation of lens vesicle closure. In situ hybridization with the mouse lens confirmed that message levels for beaded filament proteins were greatly elevated in differentiating primary fiber cells. Nuclease protection assays established that mRNA levels for CP49 remained relatively constant while CP95 mRNA levels increased once the process of secondary fiber formation was under way. Although present in relatively low abundance, the mRNA for a unique splice variant of CP49, CP49(INS), was also detected early in embryonic development and into adulthood. Peptide-specific antibodies directed against unique predicted sequences were able to confirm the protein expression of CP49(INS) in both embryonic and adult chick lens cells. These data present the first detailed study of the expression of CP49 and CP95 during early lens development. They suggest that the up-regulated expression of CP49 and CP95 could serve as pan-specific markers for all vertebrate lens fiber development.
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Affiliation(s)
- M E Ireland
- Department of Anatomy and Cell Biology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
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