Association of Rab25 and Rab11a with the apical recycling system of polarized Madin-Darby canine kidney cells

Recent evidence suggests that apical and basolateral endocytic pathways in epithelia converge in an apically located, pericentriolar endosomal compart ment termed the apical recycling endosome. In this compartment, apically an d basolaterally internalized membrane constituents are thought to be sorted for recycling back to their site of origin or for transcytosis to the oppo site plasma membrane domain. We report here that in the epithelial cell lin e Madin-Darby Canine Kidney (MDCK), antibodies to Rab11a label an apical pe ricentriolar endosomal compartment that is dependent on intact microtubules for its integrity. Furthermore, this compartment is accessible to a membra ne-bound marker (dimeric immunoglobulin A [IgA]) internalized from either t he apical or basolateral pole, functionally defining it as the apical recyc ling endosome. We have also examined the role of a closely related epitheli al-specific Rab, Rab25, in the regulation of membrane recycling and transcy tosis in MDCK cells. When cDNA encoding Rab25 was transfected into MDCK cel ls, the protein colocalized with Rab11a in subapical vesicles. Rab25 transf ection also altered the distribution of Rab11a, causing the coalescence of immunoreactivity into multiple denser vesicular structures not associated w ith the centrosome. Nevertheless, nocodazole still dispersed these vesicles , and dimeric IgA internalized from either the apical or basolateral membra ne was detected in endosomes labeled with antibodies to both Rab11a and Rab 25. Overexpression of Rab25 decreased the rate of IgA transcytosis and of a pical, but not basolateral, recycling of internalized ligand. Conversely, e xpression of the dominant-negative Rab25T26N did not alter either apical re cycling or transcytosis. These results indicate that both Rab11a and Rab25 associate with the apical recycling system of epithelial cells and suggest that Rab25 may selectively regulate the apical recycling and/or transcytoti c pathways.