DLG-1 Is a MAGUK Similar to SAP97 and Is Required for Adherens Junction Formation

Cellular junctions are critical for intercellular communication and for the assembly of cells into tissues. Cell junctions often consist of tight junctions, which form a permeability barrier and prevent the diffusion of lipids and proteins between cell compartments, and adherens junctions, which control the adhesion of cells and link cortical actin filaments to attachment sites on the plasma membrane. Proper tight junction formation and cell polarity require the function of membrane-associated guanylate kinases (MAGUKs) that contain the PDZ protein-protein interaction domain. In contrast, less is known about how adherens junctions are assembled. Here we describe how the PDZ-containing protein DLG-1 is required for the proper formation and function of adherens junctions in Caenorhabditis elegans. DLG-1 is a MAGUK protein that is most similar in sequence to mammalian SAP97, which is found at both synapses of the CNS, as well as at cell junctions of epithelia. DLG-1 is localized to adherens junctions, and DLG-1 localization is mediated by an amino-terminal domain shared with SAP97 but not found in other MAGUK family members. DLG-1 recruits other proteins and signaling molecules to adherens junctions, while embryos that lack DLG-1 fail to recruit the proteins AJM-1 and CPI-1 to adherens junctions. DLG-1 is required for the proper organization of the actin cytoskeleton and for the morphological elongation of embryos. In contrast to other proteins that have been observed to affect adherens junction assembly and function, DLG-1 is not required to maintain cell polarity. Our results suggest a new function for MAGUK proteins distinct from their role in cell polarity.     

DLG1: Chromosome Location of the Closest Human Homologue of the Drosophila Discs Large Tumor Suppressor Gene

The Drosophila discs large tumor suppressor protein, Dlg, is the prototype of a newly discovered family of proteins termed MAGUKs (membrane-associated guanylate kinase homologues). MAGUKs are localized at the membrane-cytoskeleton interface, usually at cell-cell junctions, where they appear to have both structural and signaling roles. They contain several distinct domains, including a modified guanylate kinase domain, an SH3 motif, and one or three copies of the DHR (GLGF/PDZ) domain. Recessive lethal mutations in the discs large tumor suppressor gene interfere with the formation of septate junctions (thought to be the arthropod equivalent of tight junctions) between epithelial cells, and they cause neoplastic overgrowth of imaginal discs, suggesting a role for cell junctions in proliferation control. A homologue of the Dlg protein, named Hdlg, has been isolated from human B lymphocytes. It shows 65-79% identity to Dlg in the different domains, and it binds to the cytoskeletal protein 4.1. Here, we report that the gene for lymphocyte Hdlg, named DLG1, is located at chromosome band 3q29. This finding identifies a novel site for a candidate tumor suppressor on chromosome 3.     

Structure of the SH3-guanylate kinase module from PSD-95 suggests a mechanism for regulated assembly of MAGUK scaffolding proteins.

Membrane-associated guanylate kinases (MAGUKs), such as PSD-95, are modular scaffolds that organize signaling complexes at synapses and other cell junctions. MAGUKs contain PDZ domains, which recruit signaling proteins, as well as a Src homology 3 (SH3) and a guanylate kinase-like (GK) domain, implicated in scaffold oligomerization. The crystal structure of the SH3-GK module from PSD-95 reveals that these domains form an integrated unit: the SH3 fold comprises noncontiguous sequence elements divided by a hinge region and the GK domain. These elements compose two subdomains that can assemble in either an intra- or intermolecular fashion to complete the SH3 fold. We propose a model for MAGUK oligomerization in which complementary SH3 subdomains associate by 3D domain swapping. This model provides a possible mechanism for ligand regulation of oligomerization.     

Dlg protein is required for junction structure, cell polarity, and proliferation control in Drosophila epithelia

The Discs large (Dlg) protein of Drosophila is the prototypic member of a growing family of proteins termed membrane-associated guanylate kinase homologs (MAGUKs). The MAGUKs are composed of a series of peptide domains that include one or three DHR/PDZs, an SH3, and a region homologous to guanylate kinase (GUK). We have previously shown that the product of this gene, the Dlg protein, is localized at the septate junctions between epithelial cells, and that mutations in the gene cause neoplastic overgrowth of the imaginal discs. The dlg locus is therefore defined as a tumor suppressor gene. In this paper, we show that the Dlg protein is localized on the cytoplasmic face of the septate junction and is required for the maintenance of this structure. It is also required for proper organization of the cytoskeleton, for the differential localization of membrane proteins, and for apicobasal polarity of epithelial cells. However, these other functions can be uncoupled from Dlg's role as a tumor suppressor since mutations in two domains of the protein, the SH3 and GUK, cause loss of normal cell proliferation control without affecting the other functions of the protein. These results suggest that, besides regulating cellular proliferation, the Dlg protein is a critical component of the septate junctions and is required for maintaining apicobasal polarity in Drosophila epithelium.     

MAGUKs, scaffolding proteins at cell junctions, are substrates of different proteases during apoptosis

A major feature of apoptotic cell death is gross structural changes, one of which is the loss of cell–cell contacts. The caspases, executioners of apoptosis, were shown to cleave several proteins involved in the formation of cell junctions. The membrane-associated guanylate kinases (MAGUKs), which are typically associated with cell junctions, have a major role in the organization of protein–protein complexes at plasma membranes and are therefore potentially important caspase targets during apoptosis. We report here that MAGUKs are cleaved and/or degraded by executioner caspases, granzyme B and several cysteine cathepsins in vitro. When apoptosis was induced by UV-irradiation and staurosporine in different epithelial cell lines, caspases were found to efficiently cleave MAGUKs in these cell models, as the cleavages could be prevented by a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe)fluoromethylketone. Using a selective lysosomal disrupting agent -leucyl--leucine methyl ester, which induces apoptosis through the lysosomal pathway, it was further shown that MAGUKs are also cleaved by the cathepsins in HaCaT and CaCo-2 cells. Immunohistological data showed rapid loss of MAGUKs at the sites of cell–cell contacts, preceding actual cell detachment, suggesting that cleavage of MAGUKs is an important step in fast and efficient cell detachment.     

Influence of the scaffolding protein Zonula Occludens (ZOs) on membrane channels

Zonula Occludens (ZO) proteins are ubiquitous scaffolding proteins providing the structural basis for the assembly of multiprotein complexes at the cytoplasmic surface of the plasma membrane and linking transmembrane proteins to the filamentous cytoskeleton. They belong to the large family of membrane-associated guanylate kinase (MAGUK)-like proteins comprising a number of subfamilies based on domain content and sequence similarity. ZO proteins were originally described to localize specifically to tight junctions, or Zonulae Occludentes, but this notion was rapidly reconsidered since ZO proteins were found to associate with adherens junctions as well as with gap junctions, particularly with connexin-made intercellular channels, and also with a few other membrane channels. Accumulating evidence reveals that in addition to having passive scaffolding functions in organizing gap junction complexes, including connexins and cytoskeletals, ZO proteins (particularly ZO-1) also actively take part in the dynamic function as well as in the remodeling of junctional complexes in a number of cellular systems. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.     

Regulation of the SHP-2 tyrosine phosphatase by a novel cholesterol- and cell confluence-dependent mechanism

Endothelial cells approaching confluence exhibit marked decreases in tyrosine phosphorylation of receptor tyrosine kinases and adherens junctions proteins, required for cell cycle arrest and adherens junctions stability. Recently, we demonstrated a close correlation in endothelial cells between membrane cholesterol and tyrosine phosphorylation of adherens junctions proteins. Here, we probe the mechanistic basis for this correlation. We find that as endothelial cells reach confluence, the tyrosine phosphatase SHP-2 is recruited to a low-density membrane fraction in a cholesterol-dependent manner. Binding of SHP-2 to this fraction was not abolished by phenyl phosphate, strongly suggesting that this binding was mediated by other regions of SHP-2 beside its SH2 domains. Annexin II, previously implicated in cholesterol trafficking, was associated in a complex with SHP-2, and both proteins localized to adhesion bands in confluent endothelial monolayers. These studies reveal a novel, cholesterol-dependent mechanism for the recruitment of signaling proteins to specific plasma membrane domains via their interactions with annexin II.     

Ca2+ signaling in smooth muscle: TRPC6, NCX and LNats in nanodomains

Following the recent observation of localized cytosolic subplasmalemmal [Na+] elevations (LNats) in rat aortic smooth muscle cells, we discuss here the current evidence for the structural and molecular roles of cytosolic nanodomains at close junctions of the plasma membrane (PM) and sarcoplasmic reticulum (SR) in the generation of LNats. These junctions, the loss of which might contribute to vascular aging and disease, provide a platform for ion metabolism signalplexes and the interaction of localized Na+ and Ca2+ gradients. We moreover suggest the existence in the junctions of a Na+ diffusional barrier as a necessary condition for the generation of LNats. LNats are likely a fundamental feature of near membrane ion signaling in many cell types, and their discovery offers new possibilities for elucidating the mechanism, function and pathogenesis of Na+ and Ca2+ signaling nanodomains.     

Plakophilins: Multifunctional proteins or just regulators of desmosomal adhesion?

Plakophilins 1-3 are members of the p120(ctn) family of armadillo-related proteins. The plakophilins have been characterized as desmosomal proteins, whereas p120(ctn) and the closely related delta-catenin, ARVCF and p0071 associate with adherens junctions and play essential roles in stabilizing cadherin mediated adhesion. Recent evidence suggests that plakophilins are essential components of the desmosomal plaque where they interact with desmosomal cadherins as well as the cytoskeletal linker protein desmoplakin. Plakophilins stabilize desmosomal proteins at the plasma membrane and therefore may function in a manner similar to p120(ctn) in the adherens junctions. The three plakophilins reveal distinct expression patterns, and although partially redundant in their function, mediate distinct effects on desmosomal adhesion. Besides a structural role, a function in signaling has been postulated in analogy to other armadillo proteins such as beta-catenin. At least plakophilins 1 and 2 are also localized in the nucleus, and all three proteins occur in a cytoplasmic pool. This review aims to summarize the current knowledge of plakophilin function in the context of cell adhesion, signaling and their putative role in diseases.     

Ca2+ signaling in smooth muscle: TRPC6, NCX,and LNats in nanodomains

Following the recent observation of localized cytosolic subplasmalemmal [Na⁺] elevations (LNats) in rat aortic smooth muscle cells, we discuss here the current evidence for the structural and molecular roles of cytosolic nanodomains at close junctions of the plasma membrane (PM) and sarcoplasmic reticulum (SR) in the generation of LNats. These junctions, the loss of which might contribute to vascular aging and disease, provide a platform for ion metabolism signalplexes and the interaction of localized Na⁺ and Ca²⁺ gradients. We moreover suggest the existence in the junctions of a Na⁺ diffusional barrier as a necessary condition for the generation of LNats. LNats are likely a fundamental feature of near membrane ion signaling in many cell types, and their discovery offers new possibilities for elucidating the mechanism, function and pathogenesis of Na⁺ and Ca²⁺ signaling nanodomains.     

Role of caveolin and caveolae in insulin signaling and diabetes

Caveolae are specialized membrane microdomains present within the plasma membrane of the vast majority of cell types. They have a unique composition in that they are highly enriched in cholesterol, sphingolipids, and their coat proteins the caveolins (-1, -2, and -3). In recent years it has been recognized that caveolae act as signaling platforms, serving as a concentrating point for numerous signaling molecules, as well as regulating flux through many distinct signaling cascades. Although caveolae are found in a variety of cell types, they are most abundant in adipose tissue. This fact has led to the intense study of the function of these organelles in adipocytes. It has now become apparent that effective insulin signaling in the adipocyte may be strictly dependent on localization of at least two insulin-responsive elements to caveolae (insulin receptor and GLUT4), as well as on a direct functional interaction between caveolin-1 and the insulin receptor. We present a critical discussion of these recent findings.     

Genes, gene knockouts, and mutations in the analysis of gap junctions

Gop junctions are cell junctions found between most cells and tissues. They contain membrane channels that mediate the cell-to-cell diffusion of ions, metabolites, and small cell signaling molecules. Cell-cell communication mediated by gap junctions has been proposed to have a variety of functions, including roles in regulating events in development, cell differentiation, and cell growth and proliferation. The analysis of these possibilities has been confounded by the fact that there are over a dozen connexin genes encoding polypeptides that make up vertebrate gap junctions. This complexity, coupled with the fact that most cells express multiple connexin isotypes, likely explains why recent studies using reverse genetic and genetic approaches to disrupt connexin gene function have yielded only limited insights into the physiological roles of gap junctions. Nevertheless, studies in vivo and in vitro together have provided evidence for gap junctions being involved in the regulation of cell metabolism, growth, and differentiation in restricted cell and tissue types. Surprisingly, studies in invertebrates suggest that their gap junctions are encoded not by connexins, but by a family of proteins referred to as innexins. Analysis of various Drosophila and C. elegans mutants suggest that innexins may be functional homologs to the connexins. However, whether innexins are the elusive invertebrate gap junction proteins or, rather, accessory proteins that facilitate gap junction formation remains an open question. Given the rapid progress being made in the cloning and functional analysis of gap junctions in many diverse species, confusion and difficulties with nomenclature are coming to a head in this rapidly expanding field. It may be timely to form a Nomenclature Committee to establish a uniform classification scheme for naming gap junction proteins.     

Ras-Raf Signaling Needs Prohibitin

Prohibitin has been connected to diverse cellular functions including cell cycle control, senescence, apoptosis and stabilization of mitochondrial proteins. By employing a loss of function approach using siRNAs we have demonstrated an unexpected role of PHB in the activation of Raf/MEK/ERK pathway by active Ras and in modulating epithelial cell adhesion and migration. PHB directly interacts with C-Raf and is required for the displacement of 14-3-3 from C-Raf by active Ras to facilitate plasma membrane localization and activation. Further, the adherent complex proteins cadherin and β-catenin were localized to plasma membrane suggesting stabilized adherent junctions in PHB silenced cells. Our findings demonstrated a function of PHB in the control of a central signaling pathway involved in cell growth and malignant transformation.     

Characterization of membrane-shed microvesicles from cytokine-stimulated β-cells using proteomics strategies

Microparticles and exosomes are two of the most well characterized membrane-derived microvesicles released either directly from the plasma membrane or released through the fusion of intracellular multivesicular bodies with the plasma membrane, respectively. They are thought to be involved in many significant biological processes such as cell to cell communication, rescue from apoptosis, and immunological responses. Here we report for the first time a quantitative study of proteins from β-cell-derived microvesicles generated after cytokine induced apoptosis using stable isotope labeled amino acids in cell culture combined with mass spectrometry. We identified and quantified a large number of β-cell-specific proteins and proteins previously described in microvesicles from other cell types in addition to new proteins located to these vesicles. In addition, we quantified specific sites of protein phosphorylation and N-linked sialylation in proteins associated with microvesicles from β-cells. Using pathway analysis software, we were able to map the most distinctive changes between microvesicles generated during growth and after cytokine stimulation to several cell death and cell signaling molecules including tumor necrosis factor receptor superfamily member 1A, tumor necrosis factor, α-induced protein 3, tumor necrosis factor-interacting kinase receptor-interacting serine-threonine kinase 1, and intercellular adhesion molecule 1.     

Regulation of gap junction intercellular communication by the ubiquitin system

Intercellular communication via gap junctions plays a critical role in numerous cellular processes, including the control of cell growth and differentiation, maintenance of tissue homeostasis and embryonic development. Gap junctions are aggregates of intercellular channels that enable adjacent cells in solid tissues to directly exchange ions and small molecules. These channels are formed by a family of integral membrane proteins called connexins, of which the best studied is connexin43. Connexins have a high turnover rate in most tissue types, and degradation of connexins is considered to be a tightly regulated process. Post-translational modification of connexins by ubiquitin is emerging as an important event in the regulation of connexin degradation. Ubiquitination is involved in endoplasmic reticulum-associated degradation of connexins as well as in trafficking of connexins to lysosomes. At both the endoplasmic reticulum and the plasma membrane, ubiquitination of connexins is strongly affected by changes in the extracellular environment. There is increasing evidence that the regulation of connexin ubiquitination might be an important mechanism for rapidly modifying the level of functional gap junctions at the plasma membrane, under both normal and pathological conditions. This review discusses the current knowledge about the regulation of intercellular communication via gap junctions by ubiquitination of connexins.     

Cell confluence-dependent remodeling of endothelial membranes mediated by cholesterol

The plasma membranes of endothelial cells reaching confluence undergo profound structural and functional modifications, including the formation of adherens junctions, crucial for the regulation of vascular permeability and angiogenesis. Adherens junction formation is accompanied by the tyrosine dephosphorylation of adherens junctions proteins, which has been correlated with the strength and stability of adherens junctions. Here we show that cholesterol is a critical determinant of plasma membrane remodeling in cultures of growing cow pulmonary aortic endothelial cells. Membrane cholesterol increased dramatically at an early stage in the formation of confluent cow pulmonary aortic endothelial cell monolayers, prior to formation of intercellular junctions. This increase was accompanied by the redistribution of caveolin from a high density to a low density membrane compartment, previously shown to require cholesterol, and increased binding of the annexin II-p11 complex to membranes, consistent with other studies indicating cholesterol-dependent binding of annexin II to membranes. Furthermore, partial depletion of cholesterol from confluent cells with methyl-beta-cyclodextrin both induced tyrosine phosphorylation of multiple membrane proteins, including adherens junctions proteins, and disrupted adherens junctions. Both effects were dramatically reduced by prior complexing of methyl-beta-cyclodextrin with cholesterol. Our results reveal a novel physiological role for cholesterol regulating the formation of adherens junctions and other plasma membrane remodeling events as endothelial cells reach confluence.     

VAM-1: a new member of the MAGUK family binds to human Veli-1 through a conserved domain

The MAGUKs (membrane-associated guanylate kinase homologues) constitute a family of peripheral membrane proteins that function in tumor suppression and receptor clustering by forming multiprotein complexes containing distinct sets of transmembrane, cytoskeletal, and cytoplasmic signaling proteins. Here, we report the characterization of the human vam-1 gene that encodes a novel member of the p55 subfamily of MAGUKs. The complete cDNA sequence of VAM-1, tissue distribution of its mRNA, genomic structure, chromosomal localization, and Veli-1 binding properties are presented. The vam-1 gene is composed of 12 exons and spans approx. 115 kb. By fluorescence in situ hybridization the vam-1 gene was localized to 7p15-21, a chromosome region frequently disrupted in some human cancers. VAM-1 mRNA was abundant in human testis, brain, and kidney with lower levels detectable in other tissues. The primary structure of VAM-1, predicted from cDNA sequencing, consists of 540 amino acids including a single PDZ domain near the N-terminus, a central SH3 domain, and a C-terminal GUK (guanylate kinase-like) domain. Sequence alignment, heterologous transfection, GST pull-down experiments, and blot overlay assays revealed a conserved domain in VAM-1 that binds to Veli-1, the human homologue of the LIN-7 adaptor protein in Caenorhabditis. LIN-7 is known to play an essential role in the basolateral localization of the LET-23 tyrosine kinase receptor, by linking the receptor to LIN-2 and LIN-10 proteins. Our results therefore suggest that VAM-1 may function by promoting the assembly of a Veli-1 containing protein complex in neuronal as well as epithelial cells.     

The tight junction protein ZO-2 localizes to the nucleus and interacts with the heterogeneous nuclear ribonucleoprotein scaffold attachment factor-B

Zonula occludens proteins (ZOPs), currently comprising ZO-1, ZO-2, and ZO-3, belong to the family of membrane-associated guanylate kinase homologue (MAGUK) proteins that are involved in the organization of epithelial and endothelial intercellular junctions. ZOPs bind to the cytoplasmic C termini of junctional transmembrane proteins linking them to the actin cytoskeleton. They are characterized by several conserved modules, including three PDZ domains, one SH3 domain, and a guanylate kinase-like domain, elements indicating that ZOPs may serve multiple purposes. Interestingly, ZOPs contain some unique motifs not shared by other MAGUK family members, including nuclear localization and nuclear export signals and a leucine zipper-like sequence. Their potential involvement in cell growth and proliferation has been suggested earlier based on the observation that the N-terminal half of ZOPs displays significant similarity to the product of the Drosophila tumor suppressor gene lethal(1)discs-large (dlg). The nuclear targeting of ZOPs in subconfluent epithelial cell cultures is well documented, although the action of the junctional MAGUKs in the nucleus has remained elusive. Here we show for the first time that nuclear ZO-2 directly interacts with the DNA-binding protein scaffold attachment factor-B (SAF-B). Our results from two-hybrid assays and in vivo co-immunoprecipitation studies provide evidence to suggest that ZO-2 associates with the C-terminal portion of SAF-B via its PDZ-1 domain. We further demonstrate that enhanced green fluorescent protein (EGFP)- and DsRed-tagged ZO-2 and SAF-B fusion proteins partially co-localize in nuclei of transfected epithelial cells. As shown by laser confocal microscopy and epifluorescent analysis, nuclear ZO-2 is present in epithelial and endothelial cells, particularly in response to environmental stress conditions. Interestingly, no association of SAF-B with ZO-1 was found, which supports the notion that junctional MAGUKs serve nonredundant functions.