Rac1 controls epithelial tube length through the apical secretion and polarity pathways

Pak1 and PP2A antagonize aPKC function to support cortical tension induced by the Crumbs-Yurt complex

The Drosophila polarity protein Crumbs is essential for the establishment and growth of the apical domain in epithelial cells. The protein Yurt limits the ability of Crumbs to promote apical membrane growth, thereby defining proper apical/lateral membrane ratio that is crucial for forming and maintaining complex epithelial structures such as tubes or acini. Here, we show that Yurt also increases Myosin-dependent cortical tension downstream of Crumbs. Yurt overexpression thus induces apical constriction in epithelial cells. The kinase aPKC phosphorylates Yurt, thereby dislodging the latter from the apical domain and releasing apical tension. In contrast, the kinase Pak1 promotes Yurt dephosphorylation through activation of the phosphatase PP2A. The Pak1–PP2A module thus opposes aPKC function and supports Yurt-induced apical constriction. Hence, the complex interplay between Yurt, aPKC, Pak1, and PP2A contributes to the functional plasticity of Crumbs. Overall, our data increase our understanding of how proteins sustaining epithelial cell polarization and Myosin-dependent cell contractility interact with one another to control epithelial tissue architecture.

The Drosophila septate junctions beyond barrier function: Review of the literature, prediction of human orthologs of the SJ-related proteins and identification of protein domain families

The involvement of Septate Junctions (SJs) in critical cellular functions that extend beyond their role as diffusion barriers in the epithelia and the nervous system has made the fruit fly an ideal model for the study of human diseases associated with impaired Tight Junction (TJ) function. In this study, we summarized current knowledge of the Drosophila melanogaster SJ-related proteins, focusing on their unconventional functions. Additionally, we sought to identify human orthologs of the corresponding genes as well as protein domain families. The systematic literature search was performed in PubMed and Scopus databases using relevant key terms. Orthologs were predicted using the DIOPT tool and aligned protein regions were determined from the Pfam database. 3-D models of the smooth SJ proteins were built on the Phyre2 and DMPFold protein structure prediction servers. A total of 30 proteins were identified as relatives to the SJ cellular structure. Key roles of these proteins, mainly in the regulation of morphogenetic events and cellular signalling, were highlighted. The investigation of protein domain families revealed that the SJ-related proteins contain conserved domains that are required not only for cell-cell interactions and cell polarity but also for cellular signalling and immunity. DIOPT analysis of orthologs identified novel human genes as putative functional homologs of the fruit fly SJ genes. A gap in our knowledge was identified regarding the domains that occur in the proteins encoded by eight SJ-associated genes. Future investigation of these domains is needed to provide functional information.

Girdin is a component of the lateral polarity protein network restricting cell dissemination

Epithelial cell polarity defects support cancer progression. It is thus crucial to decipher the functional interactions within the polarity protein network. Here we show that Drosophila Girdin and its human ortholog (GIRDIN) sustain the function of crucial lateral polarity proteins by inhibiting the apical kinase aPKC. Loss of GIRDIN expression is also associated with overgrowth of disorganized cell cysts. Moreover, we observed cell dissemination from GIRDIN knockdown cysts and tumorspheres, thereby showing that GIRDIN supports the cohesion of multicellular epithelial structures. Consistent with these observations, alteration of GIRDIN expression is associated with poor overall survival in subtypes of breast and lung cancers. Overall, we discovered a core mechanism contributing to epithelial cell polarization from flies to humans. Our data also indicate that GIRDIN has the potential to impair the progression of epithelial cancers by preserving cell polarity and restricting cell dissemination.

Oncogenic Pathways and Loss of the Rab11 GTPase Synergize To Alter Metabolism in Drosophila

Colorectal cancer is a complex disease driven by well-established mutations such as APC and other yet to be identified pathways. The GTPase Rab11 regulates endosomal protein trafficking, and previously we showed that loss of Rab11 caused intestinal inflammation and hyperplasia in mice and flies. To test the idea that loss of Rab11 may promote cancer progression, we have analyzed archival human patient tissues and observed that 51 out of 70 colon cancer tissues had lower Rab11 protein staining. By using the Drosophila midgut model, we have found that loss of Rab11 can lead to three changes that may relate to cancer progression. First is the disruption of enterocyte polarity based on staining of the FERM domain protein Coracle. Second is an increased proliferation due to an increased expression of the JAK-STAT pathway ligand Upd3. Third is an increased expression of ImpL2, which is an IGFBP7 homolog and can suppress metabolism. Furthermore, loss of Rab11 can act synergistically with the oncoprotein RasV12 to regulate these cancer-related phenotypes.

QuBiT: a quantitative tool for analyzing epithelial tubes reveals unexpected patterns of organization in the Drosophila trachea

Biological tubes are essential for animal survival, and their functions are dependent on tube shape. Analyzing the contributions of cell shape and organization to the morphogenesis of small tubes has been hampered by the limitations of existing programs in quantifying cell geometry on highly curved tubular surfaces and calculating tube-specific parameters. We therefore developed QuBiT (Quantitative Tool for Biological Tubes) and used it to analyze morphogenesis of the embryonic Drosophila trachea (airway). In the main tube, we find previously unknown anterior-to-posterior (A-P) gradients of cell apical orientation and aspect ratio, and periodicity in the organization of apical cell surfaces. Inferred cell intercalation during development dampens an A-P gradient of the number of cells per cross-section of the tube, but does not change the patterns of cell connectivity. Computationally 'unrolling' the apical surface of wild-type trachea and the hindgut reveals previously unrecognized spatial patterns of the apical marker Uninflatable and a non-redundant role for the Na⁺/K⁺ ATPase in apical marker organization. These unexpected findings demonstrate the importance of a computational tool for analyzing small diameter biological tubes.

Fungal homologues of human Rac1 as emerging players in signal transduction and morphogenesis

A wealth of data is accumulating on the physiological functions of human Rac1, a member of the Rho GTPase family of molecular switches and substrate of botulinum toxin, which was first identified as a regulator of cell motility through its effect on the actin cytoskeleton. Later on, it was found to be involved in different diseases like cancers, cardiac function, neuronal disorders, and apoptotic cell death. Despite the presence of Rac1 homologues in most fungi investigated so far, including Rho5 in the genetically tractable model yeast Saccharomyces cerevisiae, knowledge on their physiological functions is still scarce, let alone the details of the molecular mechanisms of their actions and interactions. Nevertheless, all functions proposed for human Rac1 seem to be conserved in one or the other fungus. This includes the regulation of MAPK cascades, polarized growth, and actin dynamics. Moreover, both the production and response to reactive oxygen species, as well as the reaction to nutrient availability, can be affected. We here summarize the studies performed on fungal Rac1 homologues, with a special focus on S. cerevisiae Rho5, which may be of use in drug development in medicine and agriculture.

EGFR controls Drosophila tracheal tube elongation by intracellular trafficking regulation

Development is governed by a few conserved signalling pathways. Amongst them, the EGFR pathway is used reiteratively for organ and tissue formation, and when dysregulated can lead to cancer and metastasis. Given its relevance, identifying its downstream molecular machinery and understanding how it instructs cellular changes is crucial. Here we approach this issue in the respiratory system of Drosophila. We identify a new role for EGFR restricting the elongation of the tracheal Dorsal Trunk. We find that EGFR regulates the apical determinant Crb and the extracellular matrix regulator Serp, two factors previously known to control tube length. EGFR regulates the organisation of endosomes in which Crb and Serp proteins are loaded. Our results are consistent with a role of EGFR in regulating Retromer/WASH recycling routes. Furthermore, we provide new insights into Crb trafficking and recycling during organ formation. Our work connects cell signalling, trafficking mechanisms and morphogenesis and suggests that the regulation of cargo trafficking can be a general outcome of EGFR activation.

The control of organ size and shape is a critical aspect of morphogenesis, as miss-regulation can lead to pathologies and malformations. The tracheal system of Drosophila is a good model to investigate this issue as tube size is strictly regulated. In addition, tracheal system development represents also an excellent system to study the molecular mechanisms employed by signalling pathways to instruct cells to form tubular structures. Here we describe that EGFR, which triggers one of the principal conserved pathways acting reiteratively during development and homeostasis, is required to restrict tube elongation. We find that EGFR regulates the accumulation and subcellular localisation of Crumbs and Serpentine, two factors previously known to regulate tube length. We show that Crumbs and Serpentine are loaded in common endosomes, which require EGFR for proper organisation, ensuring delivery of both cargoes to their final destination. We also report that during tracheal development the apical determinant Crumbs undergoes a complex pattern of recycling, which involves internalisation and different sorting pathways. Our analysis identifies EGFR as a hub to coordinate both cell intrinsic properties, namely Crumbs-dependant apical membrane growth, and extrinsic mechanisms, Serpentine-mediated extracellular matrix modifications, which regulate tube elongation. We suggest that the regulation of the endocytic traffic of specific cargoes could be one of the molecular mechanisms downstream of the EGFR, and therefore could regulate different morphogenetic and pathological EGFR-mediated events.

Rac1 modulates mammalian lung branching morphogenesis in part through canonical Wnt signaling

Lung branching morphogenesis relies on a number of factors, including proper epithelial cell proliferation and differentiation, cell polarity, and migration. Rac1, a small Rho GTPase, orchestrates a number of these cellular processes, including cell proliferation and differentiation, cellular alignment, and polarization. Furthermore, Rac1 modulates both noncanonical and canonical Wnt signaling, important pathways in lung branching morphogenesis. Culture of embryonic mouse lung explants in the presence of the Rac1 inhibitor (NSC23766) resulted in a dose-dependent decrease in branching. Increased cell death and BrdU uptake were notably seen in the mesenchyme, while no direct effect on the epithelium was observed. Moreover, vasculogenesis was impaired following Rac1 inhibition as shown by decreased Vegfa expression and impaired LacZ staining in Flk1-Lacz reporter mice. Rac1 inhibition decreased Fgf10 expression in conjunction with many of its associated factors. Moreover, using the reporter lines TOPGAL and Axin2-LacZ, there was an evident decrease in canonical Wnt signaling in the explants treated with the Rac1 inhibitor. Activation of canonical Wnt pathway using WNT3a or WNT7b only partially rescued the branching inhibition. Moreover, these results were validated on human explants, where Rac1 inhibition resulted in impaired branching and decreased AXIN2 and FGFR2b expression. We therefore conclude that Rac1 regulates lung branching morphogenesis, in part through canonical Wnt signaling. However, the exact mechanisms by which Rac1 interacts with canonical Wnt in human and mouse lung requires further investigation.

Complex Polarity: Building Multicellular Tissues Through Apical Membrane Traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level.