Vitamin D3 ameliorates podocyte injury through the nephrin signalling pathway

Renal podocytes form the main filtration barrier possessing unique phenotype maintained by proteins including podocalyxin and nephrin, which are modulated in pathological conditions. In diabetic nephropathy (DN), podocytes become structurally and functionally compromised. Nephrin, a structural backbone protein of the slit diaphragm, acts as regulator of podocyte intracellular signalling with renoprotective role. Vitamin D₃ through its receptor, VDR, provides renal protection in DN but limited data exist about its effect on podocytes. In this study, we used isolated rat glomeruli to assess podocalyxin and nephrin expression after treatment with the 1,25-dihydroxyvitamin D₃ analogue paricalcitol in the presence of normal and diabetic glucose levels. The role of 1,25-dihydroxyvitamin D₃ (calcitriol) and its analogue, paricalcitol, on podocyte morphology and survival was also investigated in the streptozotocin (STZ)-diabetic animal model. In our ex vivo model, glomeruli exhibited high glucose-mediated down-regulation of podocalyxin, and nephrin, while paricalcitol reversed the high glucose-induced decrease of nephrin and podocalyxin expression. Paricalcitol treatment enhanced VDR expression and promoted VDR and RXR co-localization in the nucleus. Our data also indicated that hyperglycaemia impaired survival of cultured glomeruli and suggested that the implemented nephrin down-regulation was reversed by paricalcitol treatment, initiating Akt signal transduction which may be involved in glomerular survival. Our findings were further verified in vivo, as in the STZ-diabetic animal model, calcitriol and paricalcitol treatment resulted in significant amelioration of hyperglycaemia and restoration of nephrin signalling, suggesting that calcitriol and paricalcitol may provide molecular bases for protection against loss of the permselective renal barrier in DN.