The glomerular epithelial cell anti-adhesin podocalyxin associates with the actin cytoskeleton through interactions with ezrin

During development, renal glomerular epithelial cells (podocytes) undergo e xtensive morphologic changes necessary for creation of the glomerular filtr ation apparatus. These changes include formation of interdigitating foot pr ocesses, replacement of tight junctions with slit diaphragms, and the conco mitant opening of intercellular urinary spaces. It was postulated previousl y and confirmed recently that podocalyxin, a sialomucin, plays a major role in maintaining the urinary space open by virtue of the physicochemical pro perties of its highly negatively charged ectodomain. This study examined wh ether the highly conserved cytoplasmic tail of podocalyxin also contributes to the unique organization of podocytes by interacting with the cytoskelet al network found in their cell bodies and foot processes. By immunocytochem istry, it was shown that podocalyxin and the actin binding protein ezrin ar e co-expressed in podocytes and co-localize along the apical plasma membran e, where they form a co-immunoprecipitable complex. Selective detergent ext raction followed by differential centrifugation revealed that some of the p odocalyxin cosediments with actin filaments. Moreover, its sedimentation is dependent on polymerized actin and is mediated by complex formation with e zrin. Once formed, podocalyxin/ezrin complexes are very stable, because the y are insensitive to actin depolymerization or inactivation of Rho kinase, which is known to be necessary for regulation of ezrin and to mediate Rho-d ependent actin organization. These data indicate that in podocytes, podocal yxin is complexed with ezrin, which mediates its link to the actin cytoskel eton. Thus, in addition to its ectodomain, the cytoplasmic tail of podocaly xin also likely contributes to maintaining the unique podocyte morphology.