The calcium-dependent phosphatase calcineurin is an important signaling factor in the kidney and is involved in normal and disease processes. Calcineurin expression suggests a wide-spread role in basic kidney physiology although few targets of calcineurin phosphatase activity in the kidney are known. Our work has shown that calcineurin is required for the induction of hypertrophy and ECM synthesis in cultured mesangial cells in response to insulin-like growth factor-I (IGF-I). These actions are associated with calcineurin-mediated nuclear localization of the transcription factor NFATcl and sustained phosphorylation of protein kinase B/Akt. As hypertrophy and ECM accumulation are important aspects of diabetic nephropathy (DN), we examined the effect of inhibiting calcineurin in a rat model of diabetes. Treatment with cyclosporin A (CsA) to inhibit calcineurin reduced whole kidney hypertrophy and completely blocked hypertrophy of the glomeruli, supporting our in vitro data that calcineurin plays a vital role in DN. Also in agreement with our in vitro data, CsA treatment reduced ECM protein accumulation in glomeruli. In contrast, calcineurin inhibition was associated with increased both TGFbeta and ECM proteins in extra-glomerular cortex, indicating a complex role for calcineurin in the kidney and in DN. Mice which have a germ line mutation in the calcineurin A (CnA) subunit have been created and offer the opportunity to observe IGF-I signaling and action in calcineurin null mesangial cells in vitro as well as kidney development and response to diabetes in a unique genetic background in vivo. This proposal will focus on both in vitro and in vivo aspects of calcineurin function. First, mesangial cells will be cultured from wildtype and calcineurin knockout mice and IGF-I signaling and action will be studied in vitro. Next, hypertrophy, and regulation of TGFbeta and ECM proteins will be examined in vivo in control and diabetic wildtype and calcineurin knockout mice. Based on our previous work, we hypothesize that calcineurin is important in IGF-I signaling and in the development of early diabetic changes in the kidney. CnA-/- mesangial cells and calcineurin knockout mice are ideal tools for discovering more about the role of calcineurin in the kidney that will enable us to design ways to modulate IGF-I signaling and impact pathologic processes such as DN.