Scribble and Discs Large mediate tricellular junction formation

Scavenger receptor endocytosis controls apical membrane morphogenesis in the Drosophila airways

The acquisition of distinct branch sizes and shapes is a central aspect in tubular organ morphogenesis and function. In the Drosophila airway tree, the interplay of apical extracellular matrix (ECM) components with the underlying membrane and cytoskeleton controls tube elongation, but the link between ECM composition with apical membrane morphogenesis and tube size regulation is elusive. Here, we characterized Emp (epithelial membrane protein), a Drosophila CD36 homolog belonging to the scavenger receptor class B protein family. emp mutant embryos fail to internalize the luminal chitin deacetylases Serp and Verm at the final stages of airway maturation and die at hatching with liquid filled airways. Emp localizes in apical epithelial membranes and shows cargo selectivity for LDLr-domain containing proteins. emp mutants also display over elongated tracheal tubes with increased levels of the apical proteins Crb, DE-cad, and phosphorylated Src (p-Src). We show that Emp associates with and organizes the βH-Spectrin cytoskeleton and is itself confined by apical F-actin bundles. Overexpression or loss of its cargo protein Serp lead to abnormal apical accumulations of Emp and perturbations in p-Src levels. We propose that during morphogenesis, Emp senses and responds to luminal cargo levels by initiating apical membrane endocytosis along the longitudinal tube axis and thereby restricts airway elongation.

Scribble and E-cadherin cooperate to control symmetric daughter cell positioning by multiple mechanisms

The fate of the two daughter cells is intimately connected to their positioning, which is in turn regulated by cell junction remodelling and orientation of the mitotic spindle. How multiple cues are integrated to dictate the ultimate positioning of daughters is not clear. Here, we identify novel mechanisms of regulation of daughter positioning in single MCF10A cells. The polarity protein, Scribble cooperates with E-cadherin for sequential roles in daughter positioning. First Scribble stabilises E-cadherin at the mitotic cortex as well as the retraction fibres, to mediate spindle orientation. Second, Scribble re-locates to the junction between the two daughters to allow a new E-cadherin-based-interface to form between them, influencing the width of the nascent daughter-daughter junction and subsequent cell positioning. Thus, E-cadherin and Scribble dynamically relocate to different intracellular sites during cell division to orient the mitotic spindle and control placement of the daughter cells after cell division. This article has an associated First Person interview with the first author of the paper.

Scribble and α-Catenin cooperatively regulate epithelial homeostasis and growth

Epithelial homeostasis is an emergent property of both physical and biochemical signals emanating from neighboring cells and across tissue. A recent study reveals that Scribble, an apico-basal polarity determinant, cooperates with α-Catenin, an adherens junction component, to regulate tissue homeostasis in the Drosophila wing imaginal disc. However, it remains to be addressed whether similar mechanisms are utilized in vertebrates. In this study, we first address how α-Catenin cooperates with Scribble to regulate epithelial homeostasis and growth in mammalian cells. Our data show that α-Catenin and Scribble interact physically in mammalian cells. We then found that both α-Catenin and Scribble are required for regulating nuclear translocation of YAP, an effector of the Hippo signaling pathway. Furthermore, ectopic Scribble suffices to suppress YAP in an α-Catenin-dependent manner. Then, to test our hypothesis that Scribble amounts impact epithelial growth, we use the Drosophila wing imaginal disc. We show that Scribble expression is complementary to Yorkie signal, the Drosophila ortholog of YAP. Ectopic expression of full-length Scribble or Scribble Leucine Rich Region (LRR):α-Catenin chimera sufficiently down-regulates Yorkie signal, leading to smaller wing size. Moreover, Scribble LRR:α-Catenin chimera rescues scribble mutant clones in the wing imaginal disc to maintain tissue homeostasis. Taken together, our studies suggest that the association of cell polarity component Scribble with α-Catenin plays a conserved role in epithelial homeostasis and growth.

PDZ Proteins SCRIB and DLG1 Regulate Myeloma Cell Surface CD86 Expression, Growth, and Survival

Despite advances in the treatment of multiple myeloma in the past decades, the disease remains incurable, and understanding signals and molecules that can control myeloma growth and survival are important for the development of novel therapeutic strategies. One such molecule, CD86, regulates multiple myeloma cell survival via its interaction with CD28 and signaling through its cytoplasmic tail. Although the CD86 cytoplasmic tail has been shown to be involved in drug resistance and can induce molecular changes in multiple myeloma cells, its function has been largely unexplored. Here, we show that CD86 cytoplasmic tail has a role in trafficking CD86 to the cell surface. This is due in part to a PDZ-binding motif at its C-terminus which is important for proper trafficking from the Golgi apparatus. BioID analysis revealed 10 PDZ domain-containing proteins proximal to CD86 cytoplasmic tail in myeloma cells. Among them, we found the planar cell polarity proteins, SCRIB and DLG1, are important for proper CD86 surface expression and the growth and survival of myeloma cells. These findings indicate a mechanism by which myeloma cells confer cellular survival and drug resistance and indicate a possible motif to target for therapeutic gain.

IMPLICATIONS

These findings demonstrate the importance of proper trafficking of CD86 to the cell surface in myeloma cell survival and may provide a new therapeutic target in this disease.

Minimal functional domains of the core polarity regulator Dlg

The compartmentalized domains of polarized epithelial cells arise from mutually antagonistic actions between the apical Par complex and the basolateral Scrib module. In Drosophila, the Scrib module proteins Scribble (Scrib) and Discs-large (Dlg) are required to limit Lgl phosphorylation at the basolateral cortex, but how Scrib and Dlg could carry out such a ‘protection’ activity is not clear. We tested Protein Phosphatase 1α (PP1) as a potential mediator of this activity, but demonstrate that a significant component of Scrib and Dlg regulation of Lgl is PP1 independent, and found no evidence for a Scrib-Dlg-PP1 protein complex. However, the Dlg SH3 domain plays a role in Lgl protection and, in combination with the N-terminal region of the Dlg HOOK domain, in recruitment of Scrib to the membrane. We identify a ‘minimal Dlg’ comprised of the SH3 and HOOK domains that is both necessary and sufficient for Scrib localization and epithelial polarity function in vivo.

This article has an associated First Person interview with the first author of the paper.

Summary: A minimal SH3-HOOK fragment of Dlg is sufficient to support epithelial polarity through mechanisms independent of the PP1 phosphatase.

Polarity scaffolds signaling in epithelial cell permeability

As an integral part of the innate immune system, the epithelial membrane is exposed to an array of insults that may trigger an immune response. One of the immune system's main functions is to regulate the level of communications between the mucosa and the lumen of various tissues. While it is clear that inhaled or ingested substances, or microorganisms may induce changes that affect the epithelial barrier in various ways, the proteins involved in the signaling cascades and physiological events leading to the regulation and maintenance of the barrier are not always well characterized. We review here some of the signaling components involved in regulating the barrier's paracellular permeability, and their potential effects on the activation of an immune response. While an effective immune response must be launched against pathogenic insults, tolerance must also be maintained for non-pathogenic antigens such as those in the commensal flora or for endogenous metabolites. Along with other members of the innate and adaptive immunity, the endocannabinoid system also plays an instrumental role in maintaining the balance between inflammation and tolerance. We discuss the potential effects of endo- and phytocannabinoids on epithelial permeability and how the dysregulation of this system could be involved in diseases and targeted for therapy.

Overexpressed Gliotactin activates BMP signaling through interfering with the Tkv-Dad association

Epithelial junctions ensure cell-cell adhesion and establish permeability barriers between cells. At the corners of epithelia, the tricellular junction (TCJ) is formed by three adjacent epithelial cells and generates a functional barrier. In Drosophila, a key TCJ protein is Gliotactin (Gli) where loss of Gli disrupts barrier formation and function. Conversely, overexpressed Gli spreads away from the TCJ and triggers apoptosis, delamination, and cell migration. Thus, Gli protein levels are tightly regulated and by two mechanisms, at the protein levels by tyrosine phosphorylation and endocytosis and at the mRNA level through microRNA-184. Regulation of Gli mRNA is mediated through a Gli-BMP-miR184 feedback loop. Excessive Gli triggers BMP signaling pathway through the activation of Tkv type-I BMP receptor and Mad. Elevated level of pMad induces micrRNA-184 expression which in turn targets the Gli 3'UTR and mRNA degradation. Gli activation of Tkv is not through its ligand Dpp but rather through the inhibition of Dad, an inhibitory-Smad. Here, we show that ectopic expression of Gli interferes with Tkv-Dad association by sequestering Dad away from Tkv. The reduced inhibitory effect of Dad on Tkv results in the increased Tkv-pMad signaling activity, and this effect is continuous through larval and pupal wing formation.

Interplay between Anakonda, Gliotactin, and M6 for Tricellular Junction Assembly and Anchoring of Septate Junctions in Drosophila Epithelium

In epithelia, tricellular junctions (TCJs) serve as pivotal sites for barrier function and integration of both biochemical and mechanical signals [1-3]. In Drosophila, TCJs are composed of the transmembrane protein Sidekick at the adherens junction (AJ) level, which plays a role in cell-cell contact rearrangement [4-6]. At the septate junction (SJ) level, TCJs are formed by Gliotactin (Gli) [7], Anakonda (Aka) [8, 9], and the Myelin proteolipid protein (PLP) M6 [10, 11]. Despite previous data on TCJ organization [12-14], TCJ assembly, composition, and links to adjacent bicellular junctions (BCJs) remain poorly understood. Here, we have characterized the making of TCJs within the plane of adherens junctions (tricellular adherens junction [tAJ]) and the plane of septate junctions (tricellular septate junction [tSJ]) and report that their assembly is independent of each other. Aka and M6, whose localizations are interdependent, act upstream to localize Gli. In turn, Gli stabilizes Aka at tSJ. Moreover, tSJ components are not only essential at vertex, as we found that loss of tSJ integrity induces micron-length bicellular SJ (bSJ) deformations. This phenotype is associated with the disappearance of SJ components at tricellular contacts, indicating that bSJs are no longer connected to tSJs. Reciprocally, SJ components are required to restrict the localization of Aka and Gli at vertex. We propose that tSJs function as pillars to anchor bSJs to ensure the maintenance of tissue integrity in Drosophila proliferative epithelia.

Lgl cortical dynamics are independent of binding to the Scrib-Dlg complex but require Dlg-dependent restriction of aPKC

Apical-basal polarity underpins the formation of epithelial barriers that are crucial for metazoan physiology. Although apical-basal polarity is long known to require the basolateral determinants Lethal Giant Larvae (Lgl), Discs Large (Dlg) and Scribble (Scrib), mechanistic understanding of their function is limited. Lgl plays a role as an aPKC inhibitor, but it remains unclear whether Lgl also forms complexes with Dlg or Scrib. Using fluorescence recovery after photobleaching, we show that Lgl does not form immobile complexes at the lateral domain of Drosophila follicle cells. Optogenetic depletion of plasma membrane PIP₂ or dlg mutants accelerate Lgl cortical dynamics. However, Dlg and Scrib are required only for Lgl localization and dynamic behavior in the presence of aPKC function. Furthermore, light-induced oligomerization of basolateral proteins indicates that Lgl is not part of the Scrib-Dlg complex in the follicular epithelium. Thus, Scrib and Dlg are necessary to repress aPKC activity in the lateral domain but do not provide cortical binding sites for Lgl. Our work therefore highlights that Lgl does not act in a complex but in parallel with Scrib-Dlg to antagonize apical determinants.