Rab8 is a monomeric GTPase that regulates the delivery of newly synthesized proteins to the basolateral surface in polarized epithelial cells. Recent publications have demonstrated that basolateral proteins interacting with the mu1-B clathrin adapter subunit pass through the recycling endosome (RE) en route from the TGN to the plasma membrane. Because Rab8 interacts with these basolateral proteins, these findings raise the question of whether Rab8 acts before, at, or after the RE. We find that Rab8 overexpression during the formation of polarity in MDCK cells, disrupts polarization of the cell, explaining how Rab8 mutants can disrupt basolateral endocytic and secretory traffic. However, once cells are polarized, Rab8 mutants cause mis-sorting of newly synthesized basolateral proteins such as VSV-G to the apical surface, but do not cause mis-sorting of membrane proteins already at the cell surface or in the endocytic recycling pathway. Enzymatic ablation of the RE also prevents traffic from the TGN from reaching the RE and similarly results in mis-sorting of newly synthesized VSV-G. We conclude that Rab8 regulates biosynthetic traffic through REs to the plasma membrane, but not trafficking of endocytic cargo through the RE. The data are consistent with a model in which Rab8 functions in regulating the delivery of TGN-derived cargo to REs.
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"Rab GTPases regulate budding, transport, tethering and fusion of vesicles, thereby providing specificity to membrane trafficking. Rab8-mediated traffic to the PM has been classically related to the secretory exocytic pathway (Henry and Sheff, 2008; Huber et al., 1993; Peränen et al., 1996), although it is also involved in the slow recycling route (Ang et al., 2003; Hattula et al., 2006; Roland et al., 2007; Yamamura et al., 2008). Rab8 is known to regulate polarized vesicle transport to the PM and to promote actin and microtubule cytoskeletal rearrangements to mediate protrusion formation (Hattula et al., 2002; Huber et al., 1993; Peränen et al., 1996). "
[Show abstract][Hide abstract]ABSTRACT: Rab8 is a small Ras-related GTPase that regulates polarized membrane transport to the plasma membrane. A high content analysis (HCA) tool developed to dissect Rab8-mediated actin and focal adhesion (FA) reorganization revealed that Rab8 activation significantly induced Rac1/Tiam1 to mediate cortical actin (CA) formation and RhoA-dependent stress fibre (SF) disassembly. Rab8 activation increased Rac1 activity, while its depletion activated RhoA, which led to reorganization of the actin cytoskeleton. Rab8 was also associated with FA, promoting their disassembly in a microtubule dependent manner. This Rab8 effect involved Calpain, MT1-MMP and Rho GTPases. Moreover, we demonstrate the role of Rab8 in the cell migration process. Indeed, Rab8 is required for EGF-induced cell polarization and chemotaxis as well as for the directional persistency of intrinsic cell motility. These data reveal that Rab8 drives cell motility by mechanisms both dependent and independent of Rho GTPases, thereby regulating the establishment of cell polarity, turnover of FA, and actin cytoskeleton rearrangements, thus determining the directionality of cell migration.
Full-text · Article · Mar 2016 · Journal of Cell Science
"During intestinal differentiation, the intestinal cell fate activator Cdx2 transcriptionally regulates the expression of Rab8 small GTPases, which are members of the Ras superfamily (Gao and Kaestner, 2010). Rab8 directly binds isoforms of the myosin V motor (Hume et al., 2001; Rodriguez and Cheney, 2002; Roland et al., 2009), facilitating exocytotic cargo movements on actin tracks in epithelial and non-epithelial cells (Ang et al., 2003; Bryant et al., 2010; Gerges et al., 2004; Hattula et al., 2006; Henry and Sheff, 2008; Huber et al., 1993a,b; Sato et al., 2009; Sun et al., 2014). Global Rab8a ablation in mice impairs the apical delivery of peptidases and nutrient transporters to enterocyte brush borders; as a consequence, these proteins are transported into lysosomes, causing nutrient deprivation and postnatal death of knockout mice (Sato et al., 2007). "
"Basolateral protein transport from median Golgi and TGN is also carried out by Rab8 and Rab10  . Localization of Rab17 and Rab25 to apical recycling endosomes (APEs) facilitates transcytic transport to the apical and basolateral plasma membranes  . "
[Show abstract][Hide abstract]ABSTRACT: Vesicular/Membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.
Full-text · Article · Jul 2014 · Experimental Cell Research