Epithelial junctions and polarity: complexes and kinases

Mechanisms involved in AMPK-mediated deposition of tight junction components to the plasma membrane

AMP-activated protein kinase (AMPK) activation promotes early stages of epithelial junction assembly. AMPK activation in MDCK renal epithelial cells facilitates localization of the junction-associated proteins aPKCζ and Par3 to the plasma membrane and promotes conversion of Cdc42, a key regulator of epithelial polarization and junction assembly, to its active GTP bound state. Furthermore, Par3 is an important regulator of AMPK-mediated aPKCζ localization. Both aPKCζ and Par3 serve as intermediates in AMPK-mediated junction assembly, with inhibition of aPKCζ activity or Par3 knockdown disrupting AMPK's ability to facilitate zonula occludens (ZO-1) localization. AMPK phosphorylates the adherens junction protein afadin and regulates its interaction with the tight-junction protein zonula occludens-1. Afadin is phosphorylated at two critical sites, S228 (residing within an aPKCζ consensus site) and S1102 (residing within an AMPK consensus site), that are differentially regulated during junction assembly and that exert different effects on the process. Expression of phospho-defective mutants (S228A and S1102A) perturbed ZO-1 localization to the plasma membrane during AMPK-induced junction assembly. Expression of S228A increased the ZO-1/afadin interaction, while S1102A reduced this interaction during extracellular calcium-induced junction assembly. Inhibition of aPKCζ activity also increased the ZO-1/afadin interaction. Taken together, these data suggest that aPKCζ phosphorylation of afadin terminates the ZO-1/afadin interaction and thus permits the later stages of junction assembly.

Cell polarization: From epithelial cells to odontoblasts

Cell polarity identifies the asymmetry of a cell. Various types of cells, including odontoblasts and epithelial cells, polarize to fulfil their destined functions. Odontoblast polarization is a prerequisite and fundamental step for tooth development and tubular dentin formation. Current knowledge of odontoblast polarization, however, is very limited, which greatly impedes the development of novel approaches for regenerative endodontics. Compared to odontoblasts, epithelial cell polarization has been extensively studied over the last several decades. The knowledge obtained from epithelia polarization has been found applicable to other cell types, which is particularly useful considering the remarkable similarities of the morphological and compositional features between polarized odontoblasts and epithelia. In this review, we first discuss the characteristics, the key regulatory factors, and the process of epithelial polarity. Next, we compare the known facts of odontoblast polarization with epithelial cells. Lastly, we clarify knowledge gaps in odontoblast polarization and propose the directions for future research to fill the gaps, leading to the advancement of regenerative endodontics.

Implications of AMPK in the Formation of Epithelial Tight Junctions

Tight junctions (TJ) play an essential role in the epithelial barrier. By definition, TJ are located at the demarcation between the apical and baso-lateral domains of the plasma membrane in epithelial cells. TJ fulfill two major roles: (i) TJ prevent the mixing of membrane components; and (ii) TJ regulate the selective paracellular permeability. Disruption of TJ is regarded as one of the earliest hallmarks of epithelial injury, leading to the loss of cell polarity and tissue disorganization. Many factors have been identified as modulators of TJ assembly/disassembly. More specifically, in addition to its role as an energy sensor, adenosine monophosphate-activated protein kinase (AMPK) participates in TJ regulation. AMPK is a ubiquitous serine/threonine kinase composed of a catalytic α-subunit complexed with regulatory β-and γ-subunits. AMPK activation promotes the early stages of epithelial TJ assembly. AMPK phosphorylates the adherens junction protein afadin and regulates its interaction with the TJ-associated protein zonula occludens (ZO)-1, thereby facilitating ZO-1 distribution to the plasma membrane. In the present review, we detail the signaling pathways up-and down-stream of AMPK activation at the time of Ca2+-induced TJ assembly.

Mesenchymal Stromal Cells Accelerate Epithelial Tight Junction Assembly via the AMP-Activated Protein Kinase Pathway, Independently of Liver Kinase B1

BACKGROUND

Mesenchymal stromal cells (MSC) are fibroblast-like multipotent cells capable of tissue-repair properties. Given the essentiality of tight junctions (TJ) in epithelial integrity, we hypothesized that MSC modulate TJ formation, via the AMP-activated kinase (AMPK) pathway. Liver kinase-β1 (LKB1) and Ca²⁺-calmodulin-dependent protein kinase kinase (CaMKK) represent the main kinases that activate AMPK.

METHODS

The in vitro Ca²⁺ switch from 5 μM to 1.8 mM was performed using epithelial Madin-Darby canine kidney (MDCK) cells cultured alone or cocultured with rat bone marrow-derived MSC or preexposed to MSC-conditioned medium. TJ assembly was measured by assessing ZO-1 relocation to cell-cell contacts. Experiments were conducted using MDCK stably expressing short-hairpin-RNA (shRNA) against LKB1 or luciferase (LUC, as controls). Compound STO-609 (50 μM) was used as CaMKK inhibitor.

RESULTS

Following Ca²⁺ switch, ZO-1 relocation and phosphorylation/activation of AMPK were significantly higher in MDCK/MSC compared to MDCK. No difference in AMPK phosphorylation was observed between LKB1-shRNA and Luc-shRNA MDCK following Ca²⁺ switch. Conversely, incubation with STO-609 prior to Ca²⁺ switch prevented AMPK phosphorylation and ZO-1 relocation. MSC-conditioned medium slightly but significantly increased AMPK activation and accelerated TJ-associated distribution of ZO-1 post Ca²⁺ switch in comparison to regular medium.

CONCLUSIONS

MSC modulate the assembly of epithelial TJ, via the CaMKK/AMPK pathway independently of LKB1.

Implications of the calcium-sensing receptor in ischemia/reperfusion

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor (GPCR) which was first isolated from bovine parathyroid glands. Its complex structure has been well characterized, which helped to better understand its function. The CaSR activity can be modulated by various ligands, either activators (also called "calcimimetics") or inhibitors (or "calcilytics"). The main role of the CaSR concerns Ca²⁺ homeostasis. In bone, intestine and kidney, the CaSR acts as a sensor for extracellular ionized Ca²⁺ concentration ([Ca²⁺]_e) to keep it stable. Such a homeostatic function is well illustrated by human inherited diseases caused by mutations in CASR gene, characterized by Ca²⁺ balance disturbances. Interestingly, the CaSR is also expressed in numerous tissues which are not directly involved in Ca²⁺ regulation. There, the CaSR has been implicated in regulatory pathways, including cell proliferation, differentiation and apoptosis. Moreover, recent observations suggest that the CaSR may be involved in ischaemia/reperfusion (I/R) cascades. In cardiomyocytes, the expression and activation of the CaSR are significantly induced at the time of I/R, which induces apoptotic pathways. Likewise, the activation of the CaSR in I/R in brain, liver and kidney has been associated with increased cell death and aggravated structural and functional damage. The present review summarizes these observations and hypothesizes a novel therapeutic option targeting the CaSR in I/R.

Connecting metabolism to intestinal barrier function: The role of leptin

Structure and function of the intestinal epithelial barrier (IEB) are dependent upon the integrity of junctional protein structures sealing the apical surface between epithelial cells. Tight junctions (TJ) and the surrounding apical F-actin cytoskeleton are involved in the regulation of paracellular permeability. The regulation of actin cytoskeleton organization by RhoA/Rho-kinase (ROCK) pathway plays an important role in TJ assembly and function. There is mounting evidence that the adipocyte-derived hormone leptin exerts pleiotropic effects on the intestinal epithelium including nutrient absorption, epithelial growth, inflammation and injury. Leptin activates multiple cell signaling pathways in intestinal epithelial cells (IEC) that can explain these pleiotropic effects. However, these pathways are also involved in the primary role of leptin that is the regulation of energy and glucose metabolism homeostasis. In this commentary, we examine how the interplay between leptin signaling pathways that regulate cell metabolism could impact upon IEB function.

[Role of AMP-activated protein kinase in renal ischemic preconditioning]

Kidney transplantation represents the best treatment of end-stage renal disease. In addition to the degree of human leukocyte antigen matching, long-term graft survival is influenced by the quality of the graft before its transplantation. Quality criteria include the level of ischemic damage caused by the transplantation per se. Renal ischemic preconditioning (IP) consists of different approaches to prevent ischemia/reperfusion (I/R) damage induced by the interruption and recovery of renal circulation, as observed during transplantation. Distinct animal models show promising results regarding the efficiency of PCI to preserve kidney structure and function in I/R conditions. Characterizing the cellular cascades involved in I/R led to the identification of putative targets of renal IP, including the adenosine monophosphate-activated protein kinase (AMPK). AMPK is a ubiquitous energy sensor, which has been implicated in the maintenance of epithelial cell polarization under energy deprivation. Among others, the anti-diabetic drug, metformin, is a potent activator of AMPK. Here, we summarize the in vitro and in vivo data about the role of AMPK in renal IP. Defining the pharmacological conditions of IP would help to improve the quality of the renal graft before its transplantation, thereby increasing its long-term survival.

Dissecting cell adhesion architecture using advanced imaging techniques

Cell adhesion to extracellular matrix proteins or to other cells is essential for the control of embryonic development, tissue integrity, immune function and wound healing.  Adhesions are tightly spatially regulated structures containing over a hundred different proteins that co-ordinate both dynamics and signalling events at these sites. Extensive biochemical and morphological analysis of adhesion types over the past three decades has greatly improved understanding of individual protein contributions to adhesion signalling and, in some cases, dynamics. However, it is becoming increasingly clear that these diverse macromolecular complexes contain a variety of protein sub-networks, as well as distinct sub-domains that likely play important roles in regulating adhesion behaviour. Until recently, resolving these structures, which are often less than a micron in size, was hampered by the limitations of conventional light microscopy. However recent advances in optical techniques and imaging methods have revealed exciting insight into the intricate control of adhesion structure and assembly. Here we provide an overview of the recent data arising from such studies of cell:matrix and cell:cell contact and an overview of the imaging strategies that have been applied to study the intricacies and hierarchy of proteins within adhesions.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.

Cannabinoids and Viral Infections

Exogenous cannabinoids or receptor antagonists may influence many cellular and systemic host responses. The anti-inflammatory activity of cannabinoids may compromise host inflammatory responses to acute viral infections, but may be beneficial in persistent infections. In neurons, where innate antiviral/pro-resolution responses include the activation of NOS-1, inhibition of Ca(2+) activity by cannabinoids, increased viral replication and disease. This review examines the effect(s) of cannabinoids and their antagonists in viral infections.

New insights into the dynamics of cell adhesions

Adhesion to the extracellular matrix (ECM) and to adjacent cells is a fundamental requirement for survival, differentiation, and migration of numerous cell types during both embryonic development and adult homeostasis. Different types of adhesion structures have been classified within different cell types or tissue environments. Much is now known regarding the complexity of protein composition of these critical points of cell contact with the extracellular environment. It has become clear that adhesions are highly ordered, dynamic structures under tight spatial control at the subcellular level to enable localized responses to extracellular cues. However, it is only in the last decade that the relative dynamics of these adhesion proteins have been closely studied. Here, we provide an overview of the recent data arising from such studies of cell-matrix and cell-cell contact and an overview of the imaging strategies that have been developed and implemented to study the intricacies and hierarchy of protein turnover within adhesions.

Aberrant epithelial morphology and persistent epidermal growth factor receptor signaling in a mouse model of renal carcinoma

The epidermal growth factor receptor (EGFR) has frequently been implicated in hyperproliferative diseases of renal tubule epithelia. We have shown that the NF2 tumor suppressor Merlin inhibits EGFR internalization and signaling in a cell contact-dependent manner. Interestingly, despite the paucity of recurring mutations in human renal cell carcinoma (RCC), homozygous mutation of the NF2 gene is found in approximately 2% of RCC patient samples in the Sanger COSMIC database. To examine the roles of Merlin and EGFR in kidney tumorigenesis, we generated mice with a targeted deletion of Nf2 in the proximal convoluted epithelium using a Villin-Cre transgene. All of these mice developed intratubular neoplasia by 3 months, which progressed to invasive carcinoma by 6-10 months. Kidneys from these mice demonstrated marked hyperproliferation and a concomitant increase in label-retaining putative progenitor cells. Early lumen-filling lesions in this model exhibited hyperactivation of EGFR signaling, altered solubility of adherens junctions components, and loss of epithelial polarity. Renal cortical epithelial cells derived from either early or late lesions were dependent on EGF for in vitro proliferation and were arrested by pharmacologic inhibition of EGFR or re-expression of Nf2. These cells formed malignant tumors upon s.c. injection into immunocompromised mice before in vitro passage. Treatment of Vil-Cre;Nf2(lox/lox) mice with the EGFR inhibitor erlotinib halted the proliferation of tumor cells. These studies give added credence to the role of EGFR signaling and perhaps Nf2 deficiency in RCC and describe a rare and valuable mouse model for exploring the molecular basis of this disease.