Podocytes respond to mechanical stress in vitro

Glomerular capillary pressure is thought to affect the structure and functi on of glomerular cells. However, it is unknown whether podocytes are intrin sically sensitive to mechanical forces. In the present study, differentiate d mouse podocytes were cultured on flexible silicone membranes. Bi-axial cy clic stress (0.5 Hz and 5% linear strain) was applied to the membranes for up to 3 d. Mechanical stress reduced the size of podocyte cell bodies, and processes became thin and elongated. Podocytes did not align in the inhomog eneous force field. Whereas the network of microtubules and that of the int ermediate filament vimentin exhibited no major changes, mechanical stress i nduced a reversible reorganization of the actin cytoskeleton: transversal s tress fibers (SF) disappeared and radial SF that were connected to an actin -rich center (ARC) formed. Epithelial and fibroblast cell lines did not exh ibit a comparable stress-induced reorganization of the F-actin. Confocal an d electron microscopy revealed an ellipsoidal and dense filamentous structu re of the ARC. Myosin II, alpha -actinin, and the podocyte-specific protein synaptopodin were present in radial SF, but, opposite to F-actin, they wer e not enriched in the ARC. The formation of the ARC and of radial SF in res ponse to mechanical stress was inhibited by nonspecific blockade of Ca2+ in flux with Ni2+ (1 mM), by Rho kinase inhibition with Y-27632 (10 muM), but not by inhibition of stretch-activated cation channels with Gd3+ (50 muM). In summary, mechanical stress induces a unique reorganization of the actin cytoskeleton in podocytes, featuring radial SF and an ARC, which differ in protein composition. The F-actin reorganization in response to mechanical s tress depends on Ca2+ influx and Rho kinase. The present study provides the first direct evidence that podocytes are mechanosensitive.