Evolution and developmental functions of the dystrophin-associated protein complex: beyond the idea of a muscle-specific cell adhesion complex

The Dystrophin-Associated Protein Complex (DAPC) is a well-defined and evolutionarily conserved complex in animals. DAPC interacts with the F-actin cytoskeleton via dystrophin, and with the extracellular matrix via the membrane protein dystroglycan. Probably for historical reasons that have linked its discovery to muscular dystrophies, DAPC function is often described as limited to muscle integrity maintenance by providing mechanical robustness, which implies strong cell-extracellular matrix adhesion properties. In this review, phylogenetic and functional data from different vertebrate and invertebrate models will be analyzed and compared to explore the molecular and cellular functions of DAPC, with a specific focus on dystrophin. These data reveals that the evolution paths of DAPC and muscle cells are not intrinsically linked and that many features of dystrophin protein domains have not been identified yet. DAPC adhesive properties also are discussed by reviewing the available evidence of common key features of adhesion complexes, such as complex clustering, force transmission, mechanosensitivity and mechanotransduction. Finally, the review highlights DAPC developmental roles in tissue morphogenesis and basement membrane (BM) assembly that may indicate adhesion-independent functions

Interaction between Polo and BicD proteins links oocyte determination and meiosis control in Drosophila.: oocyte fate and meiosis control

Meiosis is a specialized cell cycle limited to the gametes in Metazoa. In Drosophila, oocyte determination and meiosis control are interdependent processes, and BicD appears to play a key role in both. However, the exact mechanism of how BicD-dependent polarized transport could influence meiosis and vice versa remains an open question. In this article, we report that the cell cycle regulatory kinase Polo binds to BicD protein during oogenesis. Polo is expressed in all cells during cyst formation before specifically localizing to the oocyte. This is the earliest known example of asymmetric localization of a cell-cycle regulator in this process. This localization is dependent on BicD and the Dynein complex. Loss- and gain-of-function experiments showed that Polo has two independent functions. On the one hand, it acts as a trigger for meiosis. On the other hand, it is independently required, in a cell-autonomous manner, for the activation of BicD-dependent transport. Moreover, we show that Polo overexpression can rescue a hypomorphic mutation of BicD by restoring its localization and its function, suggesting that the requirement for Polo in polarized transport acts through regulation of BicD. Taken together, our data indicate the existence of a positive feedback loop between BicD and Polo, and we propose that this loop represents a functional link between oocyte specification and the control of meiosis

LKB1 and AMPK maintain epithelial cell polarity under energetic stress

LKB1 is mutated in both familial and spontaneous tumors, and acts as a master kinase that activates the PAR-1 polarity kinase and the adenosine 5′monophosphate–activated kinase (AMPK). This has led to the hypothesis that LKB1 acts as a tumor suppressor because it is required to maintain cell polarity and growth control through PAR-1 and AMPK, respectively. However, the genetic analysis of LKB1–AMPK signaling in vertebrates has been complicated by the existence of multiple redundant AMPK subunits. We describe the identification of mutations in the single Drosophila melanogaster AMPK catalytic subunit AMPKα. Surprisingly, ampkα mutant epithelial cells lose their polarity and overproliferate under energetic stress. LKB1 is required in vivo for AMPK activation, and lkb1 mutations cause similar energetic stress–dependent phenotypes to ampkα mutations. Furthermore, lkb1 phenotypes are rescued by a phosphomimetic version of AMPKα. Thus, LKB1 signals through AMPK to coordinate epithelial polarity and proliferation with cellular energy status, and this might underlie the tumor suppressor function of LKB1

The FOXO1 Transcription Factor Instructs the Germinal Center Dark Zone Program

SummaryThe pathways regulating formation of the germinal center (GC) dark zone (DZ) and light zone (LZ) are unknown. In this study we show that FOXO1 transcription factor expression was restricted to the GC DZ and was required for DZ formation, since its absence in mice led to the loss of DZ gene programs and the formation of LZ-only GCs. FOXO1-negative GC B cells displayed normal somatic hypermutation but defective affinity maturation and class switch recombination. The function of FOXO1 in sustaining the DZ program involved the trans-activation of the chemokine receptor CXCR4, and cooperation with the BCL6 transcription factor in the trans-repression of genes involved in immune activation, DNA repair, and plasma cell differentiation. These results also have implications for the role of FOXO1 in lymphomagenesis because they suggest that constitutive FOXO1 activity might be required for the oncogenic activity of deregulated BCL6 expression

Tissue-specific function of Patj in regulating the Crumbs complex and epithelial polarity

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The Drosophila Toucan protein is a new mitotic microtubule-associated protein required for spindle microtubule stability.

Mitotic spindle dynamics are highly dependent on proteins that interact with microtubules to influence their organization or stability. Here, we show that the Drosophila Toucan protein interacts directly with microtubules. Its localization to the microtubule network when it is expressed in mammalian cells and its direct interaction with microtubules in vitro are dependent on its central basic domain. Moreover, Toc expression in mammalian cells strongly protects microtubules from depolymerization. By using in vivo inducible RNAi in syncytial embryos, we generated a dose-sensitive loss of function of toucan, demonstrating that this technique is an efficient method for inactivating a maternal transcript. This enabled us to accurately characterize several new mitotic defects from the early to the late phases of mitosis, depending on Toucan depletion level. Toucan is required for metaphase spindle formation and centrosome anchoring to the poles. Then, during anaphase, Toc depletion affects kinetochore microtubules and therefore chromosome segregation. Toc is also necessary for central spindle formation by the interpolar microtubules. In contrast, astral microtubules are not disturbed by Toc depletion. Taken together, our results show that Toucan is a microtubule-associated protein specifically required for the stability of spindle microtubules throughout mitosis

Basement membrane diversification relies on two competitive secretory routes defined by Rab10 and Rab8 and modulated by dystrophin and the exocyst complex

Abstract The basement membrane (BM) is an essential structural element of tissues, and its diversification participates in organ morphogenesis. However, the traffic routes associated with BM formation and the mechanistic modulations explaining its diversification are still poorly understood. Drosophila melanogaster follicular epithelium relies on a BM composed of oriented BM fibrils and a more homogenous matrix. Here, we determined the specific molecular identity and cell exit sites of BM protein secretory routes. First, we found that Rab10 and Rab8 define two parallel routes for BM protein secretion. When both routes were abolished, BM production was fully blocked; however, genetic interactions revealed that these two routes competed. Rab10 promoted lateral and planar-polarized secretion, whereas Rab8 promoted basal secretion, leading to the formation of BM fibrils and homogenous BM, respectively. We also found that the dystrophin-associated protein complex (DAPC) associated with Rab10 and both were present in a planar-polarized tubular compartment containing BM proteins. DAPC was essential for fibril formation and sufficient to reorient secretion towards the Rab10 route. Moreover, we identified a dual function for the exocyst complex in this context. First, the Exo70 subunit directly interacted with dystrophin to limit its planar polarization. Second, the exocyst complex was also required for the Rab8 route. Altogether, these results highlight important mechanistic aspects of BM protein secretion and illustrate how BM diversity can emerge from the spatial control of distinct traffic routes

aPKC Phosphorylation of Bazooka Defines the Apical/Lateral Border in Drosophila Epithelial Cells

Bazooka (PAR-3), PAR-6, and aPKC form a complex that plays a key role in the polarization of many cell types. In epithelial cells, however, Bazooka localizes below PAR-6 and aPKC at the apical/lateral junction. Here, we show that Baz is excluded from the apical aPKC domain in epithelia by aPKC phosphorylation, which disrupts the Baz/aPKC interaction. Removal of Baz from the complex is epithelial-specific because it also requires the Crumbs complex, which prevents the Baz/PAR-6 interaction. In the absence of Crumbs or aPKC phosphorylation of Baz, mislocalized Baz recruits adherens junction components apically, leading to a loss of the apical domain and an expansion of lateral. Thus, apical exclusion of Baz by Crumbs and aPKC defines the apical/lateral border. Although Baz acts as an aPKC targeting and specificity factor in nonepithelial cells, our results reveal that it performs a complementary function in positioning the adherens junction in epithelia

Jak-Stat pathway induces Drosophila follicle elongation by a gradient of apical contractility

International audienceTissue elongation and its control by spatiotemporal signals is a major developmental question. Currently, it is thought that Drosophila ovarian follicular epithelium elongation requires the planar polarization of the basal domain cytoskeleton and of the extra-cellular matrix, associated with a dynamic process of rotation around the anteroposterior axis. Here we show, by careful kinetic analysis of fat2 mutants, that neither basal planar polarization nor rotation is required during a first phase of follicle elongation. Conversely, a JAK-STAT signaling gradient from each follicle pole orients early elongation. JAK-STAT controls apical pulsatile contractions, and Myosin II activity inhibition affects both pulses and early elongation. Early elongation is associated with apical constriction at the poles and with oriented cell rearrangements, but without any visible planar cell polarization of the apical domain. Thus, a morphogen gradient can trigger tissue elongation through a control of cell pulsing and without a planar cell polarity requirement