Rab5a is a common component of the apical and basolateral endocytic machinery in polarized epithelial cells.

Abstract

In nonpolarized cells, the small GTPase Rab5a is localized to the pla membrane, clathrin-coated vesicles, and early endosomes. Rab5a is required for early endosome fusion in vitro and regulates transport between the plasma membrane and early endosomes, in vivo. In polarized epithelial cells endocytosis occurs from separate apical and basolateral plasma membrane domains. Internliz molecules are initially delivered to distinct apical or basolateral early endosomes. In vitro, apical early endosomes can readily fuse with one another but not with the basolateral endosomes and vice versa, thereby indicating that the apical and lateral early endocytic pathways are controlled by distinct machineries. Here, we have investigated the localization and function of Rab5a in polarized epithelial cells. Confocal immunofuorescence microscopy on mouse kidney sections revealed association of the protein with the apical and baslteral pa membrane domain and underlying strucures. In polarized Madin-Darby canine kidney I cells, endogenous and overexpressd Rab5a have the same distribution. Moreover, overexpression of the protein causes a 2-fold increase in fluid-phase uptake from both domains of the cell, thus showing that Rab5a functions in apica and basolateral endocytosis. Our data indicate that the apical and basolateral endocytic machineries of epithelial cells share common regulatory components and that Rab5aperse is not sufficient to target endocytic vesicles to apical or basolateral early endosomes.The characteristic polarized surface of epithelial cells con-

Rab10 Regulates Membrane Transport through Early Endosomes of Polarized Madin-Darby Canine Kidney Cells

“…Contents from these two pathways are rapidly mixed in a set of common endosomes, which then sort proteins to either the basolateral plasma membrane or to the ARE, from which they are then effluxed to the apical plasma membrane. Rab5 has been shown to regulate endocytic uptake (Gorvel et al, 1991;Bucci et al, 1992) and has been associated with both apical and basolateral early endosomes (Bucci et al, 1994). Proteins mediating transport from apical early endosomes have not yet been described, but the results presented here suggest that Rab10 mediates transport from basolateral sorting endosomes to common endosomes.…”

rab4 Regulates Transport to the Apical Plasma Membrane in Madin-Darby Canine Kidney Cells

“…faces (11). Because rab4 and rab5 have opposing roles in EE membrane dynamics (31), we were interested to learn where rab4 localized in filter-grown MDCK cells.…”

“…Ubiquitously expressed rab5 is associated with apical EEs and basolateral Ees, and its overexpression enhances delivery of endocytosed material from both plasma membrane domains to EEs (11). The function of three other rab GTPases (rab11, its epithelial homolog rab25, and epithelial-specific rab17) has recently been evaluated.…”

Co‐operative regulation of endocytosis by three RAB5 isoforms

“…Rab5 isoforms may also have different localization and function in polarized cells. Rab5a is associated with, and regulates endocytosis from, both the apical and the basolateral domain in epithelial cells [40]. Similarly, in neurons Rab5a has a role in both axonal and dendritic endocytosis [41].…”