mesh: Biological Overview | References
Gene name - mesh
Cytological map position - 100B5-100B5
Function - transmembrane protein
Symbol - mesh
FlyBase ID: FBgn0051004
Genetic map position - chr3R:31,179,583-31,195,762
Cellular location - surface transmembrane
Septate junctions (SJs) are membrane specializations that restrict the free diffusion of solutes via the paracellular pathway in invertebrate epithelia. In arthropods, two morphologically different types of SJs are observed: pleated SJs (pSJs) and smooth SJs (sSJs), which are present in ectodermally- and endodermally-derived epithelia, respectively. Recent identification of sSJ-specific proteins, Mesh and Snakeskin (Ssk), in Drosophila indicates that the molecular compositions of sSJs and pSJs differ. A deficiency screen based on immunolocalization of Mesh, identified a tetraspanin family protein, Tetraspanin 2A (Tsp2A), as a novel protein involved in sSJ formation in Drosophila. Tsp2A specifically localizes at sSJs in the midgut and Malpighian tubules. Compromised (Tsp2A) expression caused by RNAi or the CRISPR/Cas9 system is associated with defects in the ultrastructure of sSJs, changes localization of other sSJ proteins, and impairs barrier function of the midgut. In most Tsp2A-mutant cells, Mesh fails to localize to sSJs and is distributed through the cytoplasm. Tsp2A forms a complex with Mesh and Ssk and these proteins are mutually interdependent for their localization. These observations suggest that Tsp2A cooperates with Mesh and Ssk to organize sSJs (Izumi, 2016).
Epithelia separate distinct fluid compartments within the bodies of metazoans. For this epithelial function, specialized intercellular junctions, designated as occluding junctions, regulate the free diffusion of solutes through the paracellular pathway. In vertebrates, tight junctions act as occluding junctions, whereas, in invertebrates, septate junctions (SJs) are the functional counterparts of tight junctions. SJs form circumferential belts around the apicolateral regions of epithelial cells. In transmission electron microscopy, SJs are observed between the parallel plasma membranes of adjacent cells, with ladder-like septa spanning the intermembrane space. SJs are subdivided into several morphological types that differ among different animal phyla, and several phyla possess multiple types of SJs that vary among different types of epithelia (Izumi, 2016).
In arthropods, two types of SJs exist: pleated SJs (pSJs) and smooth SJs (sSJs). pSJs are found in ectodermally-derived epithelia and surface glia surrounding the nerve cord, while sSJs are found mainly in endodermally-derived epithelia, such as the midgut and the gastric caeca. The outer epithelial layer of the proventriculus (OELP) and the Malpighian tubules also possess sSJs, although these epithelia are ectodermal derivatives. The criteria distinguishing these two types of SJs are the arrangement of the septa. In oblique sections of lanthanum-treated preparations, the septa of pSJs are visualized as regular undulating rows but those in sSJs are observed as regularly spaced parallel lines. In freeze-fracture images, the rows of intramembrane particles in pSJs are separated from one another, whereas those in sSJs are fused into ridges. To date, more than 20 pSJ-related proteins, including pSJ components and regulatory proteins involved in pSJ assembly, have been identified and characterized in Drosophila. In contrast, few genetic and molecular analyses have been carried out on sSJs. Recently, two sSJ-specific membrane proteins, Ssk and Mesh, have been identified and characterized (Izumi, 2014; Izumi, 2012; Yanagihashi, 2012). Ssk consists of 162 amino acids and has four membrane-spanning domains, two short extracellular loops, cytoplasmic N- and C-terminal domains, and a cytoplasmic loop (Yanagihashi, 2012). Mesh has a single-pass transmembrane domain and a large extracellular region containing a NIDO domain, an Ig-like E set domain, an AMOP domain, a vWD domain, and a sushi domain (Izumi, 2012). Mesh transcripts are predicted to be translated into three isoforms of which the longest isoform consists of 1,454 amino acids. In Western blot studies, Mesh is detected as a main ~90 kDa band and a minor ~200 kDa band (Izumi, 2012). Compromised expression of ssk or mesh causes defects in the ultrastructure of sSJs and in the barrier function of the midgut against a 10-kDa fluorescent tracer (Izumi, 2012; Yanagihashi, 2012). Ssk and Mesh physically interact with each other and are mutually dependent for their sSJ localization (Izumi, 2012). Thus, Mesh and Ssk play crucial roles in the formation and barrier function of sSJs (Izumi, 2016).
Tetraspanins are a family of integral membrane proteins in metazoans with four transmembrane domains, N- and C-terminal short intracellular domains, two extracellular loops and one short intracellular turn. Among several protein families with four transmembrane domains, tetraspanins are characterized especially by the structure of the second extracellular loop. It contains a highly conserved cysteine-cysteine-glycine (CCG) motif and 2 to 4 other cysteine residues. These cysteines form 2 or 3 disulfide bonds within the loop. Tetraspanins are believed to play a role in membrane compartmentalization and are involved in many biological processes, including cell migration, cell fusion and lymphocyte activation, as well as viral and parasitic infections. Several tetraspanins regulate cell-cell adhesion but none are known to be involved in the formation of epithelial occluding junctions. In the Drosophila genome, there are 37 tetraspanin family members, and some have been characterized by genetic analyses. Lbm, CG10106 and CG12143 participate in synapse formation. Sun associates with light-dependent retinal degeneration. TspanC8 subfamily members, including Tsp3A, Tsp86D and Tsp26D, are involved in the Notch-dependent developmental processes via the regulation of a transmembrane metalloprotease, ADAM10 (Dornier, 2012). However, the functions of most other Drosophila tetraspanins remain obscure (Izumi, 2016).
This study identified a tetraspanin family protein, Tsp2A, as a novel molecular component of sSJs in Drosophila. Tsp2A is required for sSJ formation and for the barrier function of Drosophila midgut. Tsp2A and two other sSJ-specific membrane proteins Mesh and Ssk show mutually dependent localizations at sSJs and form a complex with each other. Therefore, it is concluded that Tsp2A cooperates with Mesh and Ssk to organize sSJs (Izumi, 2016).
Of the sSJ-specific components, Mesh is a membrane-spanning protein and has an ability to induce cell-cell adhesion, implying that it is a cell adhesion molecule and may be one of the components of the electron-dense ladder-like structures in sSJs (Izumi, 2012). In contrast, both Ssk and Tsp2A are unlikely to act as cell adhesion molecules in sSJs because each of the two extracellular loops of Ssk (25 and 22 amino acids, respectively) appear to be too short to bridge the 15-20-nm intercellular space of sSJs. Furthermore, overexpression of EGFP-Tsp2A in Drosophila S2 cells did not induce cell aggregation, which is a criterion for cell adhesion activity (Izumi, 2016).
Several observations in Tsp2A-mutants may provide clues for understanding the role of Tsp2A in sSJ formation. In most Tsp2A-mutant midgut epithelial cells, Mesh fails to localize to the apicolateral membranes but was distributed in the cytoplasm, possibly to specific intracellular membrane compartments. To further examine where Mesh was localized in Tsp2A-mutant cells, the midgut was doublestained with the anti-Mesh antibody and the antibodies against typical markers of various intracellular membrane compartments, including the Golgi apparatus (anti-GM130), early endosomes (anti-Rab5), recycling endosomes (anti-Rab11) and lysosomes (anti-LAMP1). However, it was not possible to detect any overlap between staining by these markers and that of Mesh. The staining pattern in Tsp2A-mutant midgut epithelial cells produced with the anti-KDEL antibody, which labels endoplasmic reticulum, was similar, although not identical with that produced by the anti-Mesh antibody (Izumi, 2016).
Interestingly, some tetraspanins are known to control the intracellular trafficking of their partners. For instance, a mammalian tetraspanin, CD81 is necessary for normal trafficking or for surface membrane stability of a phosphoglycoprotein, CD19, in lymphoid B cells. The TspanC8 subgroup proteins, which all possess eight cysteine residues in their large extracellular domain, regulate the exit of a metalloproteinase, ADAM10, from the ER and differentially control its targeting to either late endosomes or to the plasma membrane (Dornier, 2012). Consequently, TspanC8 proteins regulate Notch signaling via the activation of ADAM10 in mammals, Drosophila and Caenorhabditis elegans. If Mesh is retained in the trafficking pathway from endoplasmic reticulum to plasma membrane in Tsp2A-mutant cells, Tsp2A may have an ability to promote the intracellular trafficking of Mesh in the secretory pathway. To clarify the role of Tsp2A in sSJ formation, it will be necessary to determine the intracellular membrane compartment where Mesh was localized in Tsp2A-mutant cells (Izumi, 2016).
Tsp2A, Mesh and Ssk are mutually dependent for their localization at sSJs. Consistent with this intimate relationship, the co-immunoprecipitation experiment revealed that Tsp2A physically interacts with Mesh and Ssk in vivo. However, the amount of Ssk observed in the co-immunoprecipitation with EGFP-Tsp2A was barely enriched relative to that in the extracts of embryos expressing EGFP-Tsp2A. This was particularly striking in comparison to the degree of enrichment of Mesh in the co-immunoprecipitation with EGFP-Tsp2A. To interpret these results, the detailed manner of the interaction between Tsp2A, Mesh and Ssk proteins needs to be further clarified. Many tetraspanin family proteins are known to interact with one another and with other integral membrane proteins to form a dynamic network of proteins in cellular membranes. Tetraspanins are also believed to have a role in membrane compartmentalization. Given such functional properties of tetraspanins, Tsp2A may determine the localization of sSJs at the apicolateral membrane region by membrane domain formation (Izumi, 2016).
In the Tsp2A-mutant midgut epithelial cells, Lgl was distributed throughout the basolateral membrane region, whereas it was localized in the apicolateral membrane region in the wild-type. In view of the role of Lgl in the formation of the apical-basal polarity of ectodermally-derived epithelial cells, it is of interest to consider whether this abnormal localization of Lgl in the Tsp2A-mutant affects epithelial polarity. However, in the Tsp2A-mutant midgut epithelial cells, Dlg still showed polarized concentration into the apicolateral membrane region and the Lgl never leaked into the apical membrane domain. These observations suggest that the lack of Tsp2A does not affect the gross apical-basal polarity of the midgut epithelial cells (Izumi, 2016).
Some tetraspanins have been reported to be involved in the regulation of cell-cell adhesion. A mammalian tetraspanin, CD151, regulates epithelial cell-cell adhesion through PKC- and Cdc42-dependent actin reorganization, or through complex formation with α3γ1 integrin. A mammalian tetraspanin, CD9, is concentrated in the axoglial paranodal region in the brain and in the peripheral nervous system, and CD9 knockout mice display defects in the formation of paranodal septate junctions and in the localization of paranodal proteins. Paranodal septate junctions have electron-dense ladder-like structures and their molecular organization is similar to that of pSJs but tetraspanins involved in pSJ formation have not been reported in Drosophila (Izumi, 2016).
Interactions between several tetraspanins and claudins, the key integral membrane proteins involved in the organization and function of tight junctions, are also known. Claudin-11 forms a complex with OAP-1/Tspan-3 and chemical crosslinking reveals a direct association between claudin-1 and CD9. Furthermore, the interaction between claudin-1 and CD81 is shown to be required for hepatitis C virus infectivity. To date, no tight junction defect has been reported in CD9 knockout mice, CD81 knockout mice, or CD9/CD81 double knockout mice. Further investigation is necessary to clarify whether the interactions between tetraspanins and tight junction proteins are involved in the formation and function of tight junctions (Izumi, 2016).
Septate junctions (SJs) are specialized intercellular junctions that restrict the free diffusion of solutes through the paracellular route in invertebrate epithelia. In arthropods, two morphologically different types of SJs have been reported: pleated SJs and smooth SJs (sSJs), which are found in ectodermally and endodermally derived epithelia, respectively. However, the molecular and functional differences between these SJ types have not been fully elucidated. This study reports that a novel sSJ-specific component, a single-pass transmembrane protein, which has been termed 'Mesh' (encoded by CG31004), is highly concentrated in Drosophila sSJs. Compromised mesh expression causes defects in the organization of sSJs, in the localizations of other sSJ proteins, and in the barrier function of the midgut. Ectopic expression of Mesh in cultured cells induces cell-cell adhesion. Mesh forms a complex with Ssk (Yanagihashi, 2012), another sSJ-specific protein, and these proteins are mutually interdependent for their localization. Thus, a novel protein complex comprising Mesh and Ssk has an important role in sSJ formation and in intestinal barrier function in Drosophila (Izumi, 2012).
Electron microscopic observations have shown that sSJs and pSJs can be distinguished morphologically. Obliquely sectioned pSJs and sSJs are visualized as regular undulating rows and regularly spaced parallel lines, respectively, while both types of SJs have ladder-like structures in the intermembrane space. Of the two sSJ-specific integral membrane proteins, Ssk is unlikely to be the structural element of the septa in sSJs, because its extracellular loops are both too short (25 and 22 a.a., respectively) to bridge the intercellular space. In contrast, Mesh induces cell-cell adhesion, implying that it may be one of the components of the septa observed in ultrathin section electron microscopy. Faint ladder-like structures were still observed in the mesh mutants, suggesting that other membrane proteins also contribute to the septal structures. FasIII is such a candidate because it shows cell-cell adhesion activity and was still distributed to the apicolateral region, as well as the apical region, in the mesh mutants. However, fasIII null mutant flies are viable and both Mesh and Ssk are normally localized at their sSJs, indicating that FasIII is dispensable for sSJ formation. FasIII may provide robustness to the Mesh-Ssk-mediated sSJ organization via its cell-cell adhesion activity (Izumi, 2012).
The issue of how SJs are organized in cells at the boundary between pSJ- and sSJ-bearing epithelia is intriguing. Interestingly, boundary cells were observed in which the pSJ marker Kune and sSJ marker Mesh were concentrated in the anterior and posterior regions, respectively, of the apicolateral membranes. This result suggests that individual cells possess both pSJs and sSJs depending on the orientation of their plasma membranes. The proventriculus is originally derived from ectoderm. However, the outer epithelial layer of the proventriculus (OELP) bears sSJs and expresses Mesh and Ssk, suggesting that the OELP has both ectodermal and endodermal characters. In fact, weak Kune expression was observed in the OELP but not in the midgut. Therefore, the boundary cell may have the ability to form either sSJs or pSJs according to the SJ type of adjacent cells. The occurrence of such 'SJ-boundary cells' seems to be crucial because they connect the ectodermally and endodermally derived epithelia into a tandem tube while maintaining the continuity of the paracellular barrier. However, the possibility that small amounts of pSJs and sSJs are also contained in the sSJs on the midgut side and pSJs on the foregut side of the SJ-boundary cells, respectively, to form hybrid junctions cannot be completely excluded (Izumi, 2012).
These analyses of Mesh and Ssk have clarified their interaction, interdependency in their localizations, and requirements for the organization and barrier function of sSJs, suggesting that Mesh-Ssk is a key system for sSJ formation. In mesh mutants, Ssk failed to localize at sSJs, but mislocalized to the apical and basolateral plasma membrane domains. In ssk-RNAi and Df(3L)ssk fly, Mesh no longer localized at the sSJs, but was distributed in the cytoplasm. Ssk may translocate Mesh from the cytoplasm to sSJs or to the plasma membrane. However, how the Mesh-Ssk complex recognizes and localizes to sSJ regions remain elusive. Mesh expression in S2 cells leads to cell aggregation without Ssk expression, suggesting that there is a mechanism by which Mesh translocates to the cell membrane and induces cell-cell adhesion independently of Ssk in S2 cells. Detailed analysis of the dynamics of Mesh-Ssk distribution will shed light on the mechanisms of sSJ formation and the sorting systems for sSJ proteins (Izumi, 2012).
By using Mesh and Ssk as specific markers for sSJs, it was confirmed that Dlg, Lgl and FasIII localize at sSJs in the larval OELP and midgut epithelial cells. In addition, it was found that Coracle (Cora) is also concentrated into sSJs. Among these proteins that are generally known as pSJ components, Lgl, Cora and FasIII were mislocalized in mesh mutants and ssk-RNAi lines. On the other hand, Lgl, Cora and FasIII were not required for the localization of Mesh and Ssk at the apicolateral membrane. These observations imply a possible hierarchy in the molecular constituents of sSJs; Mesh-Ssk might act as a platform for the assembly of Lgl, Cora and FasIII in endodermal epithelia. Such a feature in sSJs is in sharp contrast to that in pSJs where each molecular component is interdependent. Mutations in most of the genes encoding pSJ-associated proteins result in disruption of the barrier function and mislocalization of other pSJ proteins (Izumi, 2012).
Interestingly, in mesh mutants and ssk-RNAi lines, Dlg still localized at the apicolateral region of the OELP and midgut epithelial cells, although sSJs were disrupted at the ultrastructural level. Furthermore, Mesh and Ssk were distributed to the apicolateral region in dlg mutants, suggesting that Mesh-Ssk and Dlg are independent in their localizations. This is consistent with a recent report that Dlg is probably not a core pSJ component. Nevertheless, a functional relationship exists between Dlg and Lgl in determining cell polarity in ectodermally derived epithelia. Therefore, in the absence of Mesh and Ssk, Dlg may be unable to function properly because of an inadequate level of Lgl in the apicolateral regions. In fact, dlgm52 and lgl4 maternal/zygotic mutants exhibited a similar hypertrophied midgut phenotype, suggesting that these proteins may function together in endodermal epithelia, as well as in ectodermal epithelia (Izumi, 2012).
The functions of Dlg, Lgl, Cora and FasIII at sSJs remain unknown. Dlg may act together with Lgl to regulate the apical-basal polarity in the early stage of epithelial development. In the late developmental stage, compensation mechanisms for the Dlg function may rescue the apicolateral localization of Mesh, as noted in ectodermally derived epithelial cells of dlgm/z and lglm/z mutants. As larval midgut sSJs are completed at the end of embryogenesis (stage 17) in Drosophila, the organization of sSJ may not be influenced by early polarity defects of dlgm/z and lglm/z mutants. Alternatively, Dlg and Lgl may be important for the regulation of the epithelial cell shape change that induces the midgut tube-like structure. In ectodermally derived epithelia, Coracle acts together with Yurt to regulate the apicobasal polarity. Thus, Cora and a Yurt-like molecule may function together to organize sSJs and/or to regulate the endodermal epithelial polarity (Izumi, 2012).
Homologous proteins, characterized by similar extracellular domains to Mesh, are present in vertebrates (e.g. mouse Susd2/SVS-1), implying that this family of proteins shares functions conserved across species. Mouse Susd2/SVS-1 has been suggested as a tumor-reversing gene product, because it inhibited the growth of cancer cell lines (Sugahara, 2007). Susd2/SVS-1 was distributed in the apical membrane of the epithelial cells in renal tubules and bronchial tubes, suggesting that it does not contribute to the cell-cell adhesion and/or paracellular barrier function in vertebrate epithelial cells. However, expressing Susd2/SVS-1 in HeLa cells induces the cell aggregation (Sugahara, 2007), implying that this protein family conserves the cell-cell adhesion activity. Further studies of the functions of Mesh-Susd2/SVS-1 family proteins in vertebrates and in invertebrates will lead to a better understanding of the conserved physiological functions in these proteins and of the evolution of intercellular junctions across species (Izumi, 2012).
Search PubMed for articles about Drosophila Mesh
Dornier, E., Coumailleau, F., Ottavi, J. F., Moretti, J., Boucheix, C., Mauduit, P., Schweisguth, F. and Rubinstein, E. (2012). TspanC8 tetraspanins regulate ADAM10/Kuzbanian trafficking and promote Notch activation in flies and mammals. J Cell Biol 199: 481-496. PubMed ID: 23091066
Izumi, Y., Yanagihashi, Y. and Furuse, M. (2012). A novel protein complex, Mesh-Ssk, is required for septate junction formation in the Drosophila midgut. J Cell Sci 125: 4923-4933. PubMed ID: 22854041
Izumi, Y. and Furuse, M. (2014). Molecular organization and function of invertebrate occluding junctions. Semin Cell Dev Biol 36: 186-193. PubMed ID: 25239398
Izumi, Y., Motoishi, M., Furuse, K. and Furuse, M. (2016). A tetraspanin regulates septate junction formation in Drosophila midgut. J Cell Sci [Epub ahead of print]. PubMed ID: 26848177
Sugahara, T., Yamashita, Y., Shinomi, M., Isobe, Y., Yamanoha, B., Iseki, H., Takeda, A., Okazaki, Y., Kawai, K., Suemizu, H. and Andoh, T. (2007). von Willebrand factor type D domain mutant of SVS-1/SUSD2, vWD(m), induces apoptosis in HeLa cells. Cancer Sci 98: 909-915. PubMed ID: 17428257
Yanagihashi, Y., Usui, T., Izumi, Y., Yonemura, S., Sumida, M., Tsukita, S., Uemura, T. and Furuse, M. (2012). Snakeskin, a membrane protein associated with smooth septate junctions, is required for intestinal barrier function in Drosophila. J Cell Sci 125: 1980-1990. PubMed ID: 22328496
date revised: 15 February 2016
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