Calcineurin is an important signaling molecule in mesangial cells in vitro and is involved in some manifestations of
diabetic nephropathy in vivo. However,
calcineurin acts in a cell-specific and tissue-specific manner in the kidney, and mechanisms of specificity are unknown. Three closely related
isoforms of the
calcineurin A (CnA) subunit are expressed in a tissue-specific manner. This study was undertaken to determine if specificity of
calcineurin action is linked to regulation of CnA
isoforms in the diabetic kidney. After induction of diabetes with
streptozotocin, expression of all three CnA
isoforms rapidly increased, primarily in the thick ascending limb of Henle (TAL). After prolonged diabetes, increase specifically of the alpha
isoform was observed in collecting ducts (CD) and in endothelial cells of glomeruli.
Aquaporin 2 (AQP2), a putative substrate of
calcineurin phosphatase in the kidney, is also involved in
diabetic nephropathy. Co-localization of CnA
isoforms with AQP2 revealed that CnA-alpha is the predominant
isoform that associates with AQP2 in the diabetic kidney. Furthermore, inhibition of
calcineurin with
cyclosporin A (CsA) alters AQP2 localization and phosphorylation in principal cells of CD. Alterations in subcellular localization of AQP2 were parallel with CnA-alpha. Similarly, CsA treatment results in a further increase in urine output compared with diabetes alone, suggesting a functional consequence of inhibiting
calcineurin-mediated regulation of AQP2. In conclusion, all three
isoforms of CnA are upregulated in the diabetic kidney. Increased expression of CnA-alpha, in particular, is observed in glomeruli and CD and participates in regulation of AQP2 expression, phosphorylation, and function.