Differential expression of calcineurin A isoforms in the diabetic kidney

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 α 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-α 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-α. 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-α, in particular, is observed in glomeruli and CD and participates in regulation of AQP2 expression, phosphorylation, and function.