Calcineurin in the distal convoluted tubule plays a key role in tacrolimus-induced hypomagnesemia and hypercalciuria

Calcineurin inhibitors (CNIs) are immunosuppressive drugs used to prevent graft rejection after organ transplant. Adverse effects of CNIs include renal magnesium wasting and hypomagnesemia, which may contribute to the development of new-onset diabetes mellitus after kidney transplantation, and hypercalciuria, which may implicate CNIs in post-transplant osteoporosis. The distal convoluted tubule (DCT) is a key site of magnesium and calcium reabsorption by transcellular transport in the kidney. Tacrolimus is a CNI that must bind to its immunophilin, FKBP12, to inhibit the phosphatase calcineurin and impart immunosuppressive effects. Here, we determined the direct role of calcineurin in magnesium and calcium homeostasis and tested whether effects are specific to the DCT. Female and male inducible kidney-specific FKBP12 knockout (KS-FKBP12^-/-) mice and control littermates received daily subcutaneous injections of tacrolimus (3 mg∙kg^-1 body weight) or vehicle for 18 days. Tacrolimus-treated control mice developed low plasma magnesium concentration and urinary calcium wasting. Western blot analysis demonstrated decreased abundance of key magnesium and calcium transport proteins (NCX-1 and Calbindin-D_28k) and qPCR showed decreased mRNA expression of NCX-1 and Calbindin-D_28k in addition to TRPM6, an apical magnesium transport channel in the DCT, in tacrolimus-treated control mouse kidney. In contrast, KS-FKBP12^-/- mice did not develop changes in these proteins. Gene transcripts associated with magnesium absorption in the thick ascending limb, Claudin 16 and Claudin 19, were not affected by tacrolimus treatment. These results suggest that calcineurin is a key regulator of magnesium and calcium transport proteins in DCT cells.