Scribble and Discs Large mediate tricellular junction formation.

Junctional complexes that mediate cell adhesion are key to epithelial integrity, cell division and permeability barrier formation. In Drosophila, the scaffolding proteins Scribble (Scrib) and Discs Large (Dlg) are key regulators of epithelial polarity, proliferation, assembly of junctions and protein trafficking. We found that Scrib and Dlg are necessary for the formation of the tricellular junction (TCJ), a unique junction that forms in epithelia at the point of convergence of three neighboring cells. Scrib and Dlg are in close proximity with the TCJ proteins Gliotactin (Gli) and Bark Beetle (Bark), and both are required for TCJ protein recruitment. Loss of Bark or Gli led to basolateral spread of the TCJ complex at the cell corners. Loss of the septate junction proteins Nrx-IV and the Na+/K+ ATPase also resulted in basolateral spread of the entire TCJ complex at the cell corners. The Scrib PDZ1-2 domains and the Dlg GUK domain are necessary for Bark and Gli localization to the TCJ. Overall, we propose a model in which Scrib and Dlg are key components of the TCJ, and form a complex with Bark and Gli.

The Drosophila tricellular junction protein Gliotactin regulates its own mRNA levels through BMP-mediated induction of miR-184.

Epithelial bicellular and tricellular junctions are essential for establishing and maintaining permeability barriers. Tricellular junctions are formed by the convergence of three bicellular junctions at the corners of neighbouring epithelia. Gliotactin, a member of the Neuroligin family, is located at theDrosophilatricellular junction, and is crucial for the formation of tricellular and septate junctions, as well as permeability barrier function. Gliotactin protein levels are tightly controlled by phosphorylation at tyrosine residues and endocytosis. Blocking endocytosis or overexpressing Gliotactin results in the spread of Gliotactin from the tricellular junction, resulting in apoptosis, delamination and migration of epithelial cells. We show that Gliotactin levels are also regulated at the mRNA level by micro (mi)RNA-mediated degradation and that miRNAs are targeted to a short region in the 3'UTR that includes a conserved miR-184 target site. miR-184 also targets a suite of septate junction proteins, including NrxIV, coracle and Mcr. miR-184 expression is triggered when Gliotactin is overexpressed, leading to activation of the BMP signalling pathway. Gliotactin specifically interferes with Dad, an inhibitory SMAD, leading to activation of the Tkv type-I receptor and activation of Mad to elevate the biogenesis and expression of miR-184.

Evolution of clams (cholinesterase-like adhesion molecules): structure and function during development.

The protein family known as CLAMS (cholinesterase-like adhesion molecules) forms a novel class of heterophilic cell adhesion proteins. Family members are found through a wide range of metazoans and play a role during the development of multiple tissues. The majority of members of this family are transmembrane proteins with an extracellular domain that is conserved with cholinesterases including acetylcholinesterase. Yet all family members lack one or more of the residues that make up the catalytic triad necessary for enzymatic function. Therefore the conserved cholinesterase-like domain is not necessary for enzymatic function but does appear to play a role in heterophilic binding. CLAMS are expressed in a wide array of tissues and most family members appear to play a role in cell adhesion and junction formation. The development of junctions including septate junctions and synaptic junctions require CLAM family members such as Gliotactin and Neuroligins respectively. Modeling of the cholinesterase-like domain reveals that evolutionary changes to the binding pocket of the cholinesterase domain may produce a range of different ligand binding partners for CLAM family members. In this vein, previous chimera experiments and recent work has identified mutations in CLAM family members that affect the structure of the cholinesterase-like domain. These mutant forms affect protein function during the development of specialized junctions and confirm the role of the cholinesterase domain in mediating heterophilic binding.

DIM-1, a novel immunoglobulin superfamily protein in Caenorhabditis elegans, is necessary for maintaining bodywall muscle integrity.

The UNC-112 protein is required during initial muscle assembly in C. elegans to form dense bodies and M-lines. Loss of this protein results in arrest at the twofold stage of embryogenesis. In contrast, a missense mutation in unc-112 results in viable animals that have disorganized bodywall muscle and are paralyzed as adults. Loss or reduction of dim-1 gene function can suppress the severe muscle disruption and paralysis exhibited by these mutant hermaphrodites. The overall muscle structure in hermaphrodites lacking a functional dim-1 gene is slightly disorganized, and the myofilament lattice is not as strongly anchored to the muscle cell membrane as it is in wild-type muscle. The dim-1 gene encodes two polypeptides that contain three Ig-like repeats. The short DIM-1 protein isoform consists entirely of three Ig repeats and is sufficient for wild-type bodywall muscle structure and stability. DIM-1(S) localizes to the region of the muscle cell membrane around and between the dense bodies, which are the structures that anchor the actin filaments and may play a role in stabilizing the thin rather than the thick filament components of the sarcomere.