α-actinin-4 is required for normal podocyte adhesion

Mutations in the α-actinin-4 gene ACTN4 cause an autosomal dominant human kidney disease. Mice deficient in α-actinin-4 develop a recessive phenotype characterized by kidney failure, proteinuria, glomerulosclerosis, and retraction of glomerular podocyte foot processes. However, the mechanism by which α-actinin-4 deficiency leads to glomerular disease has not been defined. Here, we examined the effect of α-actinin-4 deficiency on the adhesive properties of podocytes in vivo and in a cell culture system. In α-actinin-4-deficient mice, we observed a decrease in the number of podocytes per glomerulus compared with wild-type mice as well as the presence of podocyte markers in the urine. Podocyte cell lines generated from α-actinin-4-deficient mice were less adherent than wild-type cells to glomerular basement membrane (GBM) components collagen IV and laminin 10 and 11. We also observed markedly reduced adhesion of α-actinin-4-deficient podocytes under increasing shear stresses. This adhesion deficit was restored by transfecting cells with α-actinin-4-GFP. We tested the strength of the integrin receptor-mediated linkages to the cytoskeleton by applying force to microbeads bound to integrin using magnetic pulling cytometry. Beads bound to α-actinin-4-deficient podocytes showed greater displacement in response to an applied force than those bound to wild-type cells. Consistent with integrin-dependent α-actinin-4-mediated adhesion, phosphorylation of β1-integrins on α-actinin-4-deficient podocytes is reduced. We rescued the phosphorylation deficit by transfecting α-actinin-4 into α-actinin-4-deficient podocytes. These results suggest that α-actinin-4 interacts with integrins and strengthens the podocyte-GBM interaction thereby stabilizing glomerular architecture and preventing disease.