N-sulfation of heparan sulfate is critical for syndecan-4-mediated podocyte cell-matrix interactions

Previous research has shown that podocytes unable to assemble heparan sulfate on cell surface proteoglycan core proteins have compromised cell-matrix interactions. This report further explores the role of N-sulfation of intact heparan chains in podocyte-matrix interactions. For the purposes of this study, a murine model in which the enzyme N-deacetylase/N-sulfotransferase 1 (NDST1) was specifically deleted in podocytes and immortalized podocyte cell lines lacking NDST1 were developed and used to explore the effects of such a mutation on podocyte behavior in vitro. NDST1 is a bifunctional enzyme, ultimately responsible for N-sulfation of heparan glycosaminoglycans produced by cells. Immunostaining of glomeruli from mice whose podocytes were null for Ndst1 (Ndst1(-/-)) showed a disrupted pattern of localization for the cell surface proteoglycan, syndecan-4, and for alpha-actinin-4 compared with controls. The pattern of immunostaining for synaptopodin and nephrin did not show as significant alterations. In vitro studies showed that Ndst1 (/) podocytes attached, spread, and migrated less efficiently than Ndst1(+/+) podocytes. Immunostaining in vitro for several markers for molecules involved in cell-matrix interactions showed that Ndst1(-/-) cells had decreased clustering of syndecan-4 and decreased recruitment of protein kinase-C alpha, alpha-actinin-4, vinculin, and phospho-focal adhesion kinase to focal adhesions. Total intracellular phospho-focal adhesion kinase was decreased in Ndst1(-/-) compared with Ndst1(-/-) cells. A significant decrease in the abundance of activated integrin alpha 5 beta 1 on the cell surface of Ndst1(-/-) cells compared with Ndst1(+/+) cells was observed. These results serve to highlight the critical role of heparan sulfate N-sulfation in facilitating normal podocyte-matrix interactions.