Key Points

Testican-2 reduced adriamycin-induced podocyte injury in vitro and in vivo.Testican-2 interacted with vitronectin and reduced vitronectin-mediated integrin αVβ3 activation in vitro.Exogenous testican-2 circulated to the kidney, bound vitronectin, and conferred podocyte protection, even when endogenous testican-2 was absent.

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 biologic 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 biolayer interferometry to characterize these protein–protein interactions. Using global and podocyte-specific Spock2 knockout mice, we assessed the effect 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. Furthermore, Spock2 deficiency increased susceptibility to podocyte injury due to adriamycin- and streptozotocin-induced diabetes, as determined by albuminuria, foot process effacement, nephrin expression, and Wilms' tumor 1-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

Testican-2 modulated the podocyte's interaction with its extracellular matrix and had a functional role in kidney protection.