Testican-2 Interaction with the Extracellular Matrix and Podocyte Protection

Background: Testican-2 is a podocyte-derived glycoprotein encoded by SPOCK2 . Circulating levels of testican-2 are associated with less glomerulosclerosis and better kidney prognosis, but its biological function in the podocyte is unknown. Methods: We studied the protective effect of testican-2 on immortalized cultured human podocytes and in mice treated with adriamycin. We used immunoprecipitation-mass spectrometry to identify binding partners of testican-2 and Bio-Layer Interferometry to characterize these protein-protein interactions. Using global and podocyte-specific Spock2 knockout mice, we assessed the impact of testican-2 deficiency in models of podocyte injury and also tested whether exogenous testican-2 confers podocyte protection in testican-2 deficient mice. Finally, we analyzed testican-2 expression in human kidney biopsy samples. Results: Testican-2 reduced adriamycin-induced podocyte injury in cultured human podocytes and mice. Vitronectin was a strong binding partner for testican-2, and testican-2 inhibited the interaction between vitronectin and integrin αVβ3, an effector of podocyte injury. Consistent with this, testican-2 administration reduced activation of integrin β3 in injured podocytes. Further, Spock2 deficiency increased susceptibility to podocyte injury due to adriamycin and streptozotocin induced diabetes, as determined by albuminuria, foot process effacement, nephrin expression and WT1-positive podocyte number. Importantly, exogenous testican-2 circulated to the kidney, bound to vitronectin, reduced vitronectin-integrin β3 interaction, and reduced podocyte injury in Spock2 deficient mice. Finally, glomerular testican-2 expression was reduced in human focal segmental glomerulosclerosis and diabetic kidney disease, but not tubulointerstitial nephropathy. Conclusions: These findings show that testican-2 modulates the podocyte’s interaction with its extracellular matrix and plays a functional role in kidney protection.