Beyond the Channels: Adhesion Functions of Aquaporin 0 and Connexin 50 in Lens Development

Structural basis of aquaporin-4 autoantibody binding in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune disease of the central nervous system where pathogenic autoantibodies target the water channel aquaporin-4 on human astrocytes causing neurological impairment. Autoantibody binding leads to complement-dependent and complement-independent cytotoxicity, ultimately resulting in astrocyte death, demyelination, and neuronal loss. Aquaporin-4 assembles in astrocyte plasma membranes as symmetric tetramers or as arrays of tetramers. We report molecular structures of aquaporin-4 alone and bound to Fab fragments from patient-derived NMO autoantibodies using cryogenic electron microscopy. Each antibody binds to epitopes comprised of three extracellular loops of aquaporin-4 with contributions from multiple molecules in the assembly. The structures distinguish between antibodies that bind to the tetrameric form of aquaporin-4 and those targeting higher-order orthogonal arrays of tetramers that provide more diverse bridging epitopes.

Ankyrin-B is required for the establishment and maintenance of lens cytoarchitecture, mechanics and clarity

The transparent ocular lens is essential for vision because it focuses light onto the retina. Despite recognition of the importance of its unique cellular architecture and mechanical properties, the molecular mechanisms governing these attributes remain elusive. This study aims to elucidate the role of ankyrin-B (AnkB, encoded by ANK2), a membrane scaffolding protein, in lens cytoarchitecture, growth and function using a conditional knockout (cKO) mouse model. The AnkB cKO mouse has no defects in lens morphogenesis but exhibited changes that supported a global role for AnkB in maintenance of lens clarity, size, cytoarchitecture, membrane organization and stiffness. Notably, absence of AnkB led to nuclear cataract formation, which was evident from postnatal day 16. AnkB cKO lens fibers exhibit progressive disruption in membrane organization of the spectrin-actin cytoskeleton, cell adhesion proteins and channel proteins; loss and degradation of several membrane proteins [such as NrCAM. N-cadherin (CDH2) and aquaporin-0 (also known as MIP)]; along with a disorganized plasma membrane and impaired membrane interdigitations. Furthermore, absence of AnkB led to decreased lens stiffness. Collectively, these results illustrate the essential role for AnkB in lens architecture, growth and function through its involvement in membrane skeletal and protein organization and stability.

Abnormal function of EPHA2/p.R957P mutant in congenital cataract

AIM

To identify genetic defects in a Chinese family with congenital posterior polar cataracts and assess the pathogenicity.

METHODS

A four-generation Chinese family affected with autosomal dominant congenital cataract was recruited. Nineteen individuals took part in this study including 5 affected and 14 unaffected individuals. Sanger sequencing targeted hot-spot regions of 27 congenital cataract-causing genes for variant discovery. The pathogenicity of the variant was evaluated by the guidelines of American College of Medical Genetics and InterVar software. Confocal microscopy was applied to detect the subcellular localization of fluorescence-labeled ephrin type-A receptor 2 (EPHA2). Co-immunoprecipitation assay was implemented to estimate the interaction between EphA2 and other lens membrane proteins. The mRNA and protein expression were analyzed by reverse transcription-polymerase chain reaction (qRT-PCR) and Western blotting assay, respectively. The cell migration was analyzed by wound healing assay. Zebrafish model was generated by ectopic expression of human EPHA2/p.R957P mutant to demonstrate whether the mutant could cause lens opacity in vivo.

RESULTS

A novel missense and pathogenic variant c.2870G>C was identified in the sterile alpha motif (SAM) domain of EPHA2. Functional studies demonstrated the variant's impact: reduced EPHA2 protein expression, altered subcellular localization, and disrupted interactions with other lens membrane proteins. This mutant notably enhanced human lens epithelial cell migration, and induced a central cloudy region and roughness in zebrafish lenses with ectopic expression of human EPHA2/p.R957P mutant under differential interference contrast (DIC) optics.

CONCLUSION

Novel pathogenic c.2870G>C variant of EPHA2 in a Chinese congenital cataract family contributes to disease pathogenesis.

Ankyrin-B is required for the establishment and maintenance of lens cytoarchitecture, mechanics, and clarity

This study illustrates a vital role for ankyrin-B in lens architecture, growth and function through its involvement in membrane protein and spectrin-actin cytoskeletal organization and stability The transparent ocular lens is essential for vision by focusing light onto the retina. Despite recognizing the importance of its unique cellular architecture and mechanical properties, the molecular mechanisms governing these attributes remain elusive. This study aims to elucidate the role of ankyrin-B (AnkB), a membrane scaffolding protein, in lens cytoarchitecture, growth and function using a conditional knockout (cKO) mouse model. AnkB cKO mouse has no defects in lens morphogenesis, but exhibited changes that supported a global role for AnkB in maintenance of lens clarity, size, cytoarchitecture, and stiffness. Notably, absence of AnkB led to nuclear cataract formation, evident from P16. AnkB cKO lens fibers exhibit progressive disruption in membrane organization of the spectrin-actin cytoskeleton, channel proteins, cell-cell adhesion, shape change, loss and degradation of several membrane proteins (e.g., NrCAM. N-cadherin and aquaporin-0) along with a disorganized plasma membrane and impaired ball-and-socket membrane interdigitations. Furthermore, absence of AnkB led to decreased lens stiffness. Collectively, these results illustrate the essential role for AnkB in lens architecture, growth and function through its involvement in membrane protein and cytoskeletal organization.

A novel single-base deletional mutation of MIP impairs protein distribution and cell-to-cell adhesion in autosomal dominant cataracts in a Chinese family

Congenital cataracts are the leading cause of irreversible visual disability in children, and genetic factors play an important role in their development. In this study, targeted exome sequencing revealed a novel single-base deletional mutation of MIP (c.301delG; p.Ala101Profs*16) segregated with congenital punctate cataract in a Chinese family. The hydrophobic properties, and secondary and tertiary structures for truncated MIP were predicted to affect the function of protein by bioinformatics analysis. When MIP-WT and MIP-Ala101fs expression constructs were singly transfected into HeLa cells, it was found that the mRNA level showed no significant difference, while the protein level of the mutant was remarkably reduced compared to that of the wild-type MIP. Immunofluorescence images showed that the MIP-WT was principally localized to the plasma membrane, whereas the MIP-Ala101fs protein was aberrantly trapped in the cytoplasm. Furthermore, the cell-to-cell adhesion capability and the cell-to-cell communication property were both significantly reduced for MIP-Ala101fs compared to the MIP-WT (all *p < 0.05). This is the first report of the c.301delG mutation in the MIP gene associated with autosomal dominant congenital cataracts. We propose that the cataract is caused by the decreased protein expression and reduced cell-to-cell adhesion by the mutant MIP. The impaired trafficking or instability of the mutant protein, as well as compromised intercellular communication is probably a concurrent result of the mutation. The results expand the genetic and phenotypic spectra of MIP and help to better understand the molecular basis of congenital cataracts.

Gap Junctions or Hemichannel-Dependent and Independent Roles of Connexins in Fibrosis, Epithelial-Mesenchymal Transitions, and Wound Healing

Fibrosis initially appears as a normal response to damage, where activated fibroblasts produce large amounts of the extracellular matrix (ECM) during the wound healing process to assist in the repair of injured tissue. However, the excessive accumulation of the ECM, unresolved by remodeling mechanisms, leads to organ dysfunction. Connexins, a family of transmembrane channel proteins, are widely recognized for their major roles in fibrosis, the epithelial-mesenchymal transition (EMT), and wound healing. Efforts have been made in recent years to identify novel mediators and targets for this regulation. Connexins form gap junctions and hemichannels, mediating communications between neighboring cells and inside and outside of cells, respectively. Recent evidence suggests that connexins, beyond forming channels, possess channel-independent functions in fibrosis, the EMT, and wound healing. One crucial channel-independent function is their role as the primary functional component for cell adhesion. Other channel-independent functions of connexins involve their roles in mitochondria and exosomes. This review summarizes the latest advances in the channel-dependent and independent roles of connexins in fibrosis, the EMT, and wound healing, with a particular focus on eye diseases, emphasizing their potential as novel, promising therapeutic targets.

Characterization of a Novel Gja8 (Cx50) Mutation in a New Cataract Rat Model

Purpose

To describe a novel spontaneous cataract inbred strain isolated from large-scale breeding SD rats, identify the responsible gene mutation, and understand how this mutation affects lens function.

Methods

Exome sequencing of 12 cataract-associated genes was performed in the affected and healthy relatives. Sequences of rat wild-type or mutant gap junction protein alpha 8 gene (Gja8) were transfected into cells. The expression level of protein was assayed by Western blot analysis. Subcellular localization of connexin 50 (Cx50) was analyzed in confocal fluorescent images. Wound-healing, 5-ethynyl-2'-deoxyuridine incorporation, and attachment assay were performed to characterize the cell migration, proliferation and adhesion.

Results

The abnormality was found to be inheritable in an autosomal semi-dominant pattern through different mating patterns. We found a G to T transversion at codon 655 in Gja8, leading to a substitution of valine by phenylalanine (p.V219F). Gja8V219F/+ heterozygotes expressed nuclear cataract while Gja8V219F/V219F homozygotes manifested microphthalmia in addition to cataract. Histology revealed fiber disorders and loss of organelle-free zone in the mutant lens. Cx50V219F altered its location in HeLa cells and inhibited the proliferation, migration and adhesion abilities of HLEB3 cells. The mutation also reduced the expression of focal adhesion kinase and its phosphorylation.

Conclusions

The c.655G>T mutation (p.V219F) is a novel mutation in Gja8, inducing semi-dominant nuclear cataracts in a new spontaneous cataract rat model. The p.V219F mutation altered Cx50 distribution, inhibited lens epithelial cell proliferation, migration, and adhesion, and disrupted fiber cell differentiation. As a consequence, the nuclear cataract and small lens formed.

Charged multivesicular body protein 4b forms complexes with gap junction proteins during lens fiber cell differentiation

Charged multivesicular body protein 4b (CHMP4B) is a core sub-unit of the endosomal sorting complex required for transport III (ESCRT-III) machinery that serves myriad remodeling and scission processes of biological membranes. Mutation of the human CHMP4B gene underlies rare forms of early-onset lens opacities or cataracts, and CHMP4B is required for lens growth and differentiation in mice. Here, we determine the sub-cellular distribution of CHMP4B in the lens and uncover a novel association with gap junction alpha-3 protein (GJA3) or connexin 46 (Cx46) and GJA8 or Cx50. Immunofluorescence confocal microscopy revealed that CHMP4B localized to cell membranes of elongated fiber cells in the outer cortex of the lens-where large gap junction plaques begin to form-particularly, on the broad faces of these flattened hexagon-like cells in cross-section. Dual immunofluorescence imaging showed that CHMP4B co-localized with gap junction plaques containing Cx46 and/or Cx50. When combined with the in situ proximity ligation assay, immunofluorescence confocal imaging indicated that CHMP4B lay in close physical proximity to Cx46 and Cx50. In Cx46-knockout (Cx46-KO) lenses, CHMP4B-membrane distribution was similar to that of wild-type, whereas, in Cx50-KO lenses, CHMP4B localization to fiber cell membranes was lost. Immunoprecipitation and immunoblotting analyses revealed that CHMP4B formed complexes with Cx46 and Cx50 in vitro. Collectively, our data suggest that CHMP4B forms plasma membrane complexes, either directly and/or indirectly, with gap junction proteins Cx46 and Cx50 that are often associated with "ball-and-socket" double-membrane junctions during lens fiber cell differentiation.

Dynamic changes in whole genome DNA methylation, chromatin and gene expression during mouse lens differentiation

BACKGROUND

Cellular differentiation is marked by temporally and spatially coordinated gene expression regulated at multiple levels. DNA methylation represents a universal mechanism to control chromatin organization and its accessibility. Cytosine methylation of CpG dinucleotides regulates binding of methylation-sensitive DNA-binding transcription factors within regulatory regions of transcription, including promoters and distal enhancers. Ocular lens differentiation represents an advantageous model system to examine these processes as lens comprises only two cell types, the proliferating lens epithelium and postmitotic lens fiber cells all originating from the epithelium.

RESULTS

Using whole genome bisulfite sequencing (WGBS) and microdissected lenses, we investigated dynamics of DNA methylation and chromatin changes during mouse lens fiber and epithelium differentiation between embryos (E14.5) and newborns (P0.5). Histone H3.3 variant chromatin landscapes were also generated for both P0.5 lens epithelium and fibers by chromatin immunoprecipitation followed by next generation sequencing (ChIP-seq). Tissue-specific features of DNA methylation patterns are demonstrated via comparative studies with embryonic stem (ES) cells and neural progenitor cells (NPCs) at Nanog, Pou5f1, Sox2, Pax6 and Six3 loci. Comparisons with ATAC-seq and RNA-seq data demonstrate that reduced methylation is associated with increased expression of fiber cell abundant genes, including crystallins, intermediate filament (Bfsp1 and Bfsp2) and gap junction proteins (Gja3 and Gja8), marked by high levels of histone H3.3 within their transcribed regions. Interestingly, Pax6-binding sites exhibited predominantly DNA hypomethylation in lens chromatin. In vitro binding of Pax6 proteins showed Pax6's ability to interact with sites containing one or two methylated CpG dinucleotides.

CONCLUSIONS

Our study has generated the first data on methylation changes between two different stages of mammalian lens development and linked these data with chromatin accessibility maps, presence of histone H3.3 and gene expression. Reduced DNA methylation correlates with expression of important genes involved in lens morphogenesis and lens fiber cell differentiation.