Genetic deletion of calcineurin A-β does not increase renal sodium reabsorption via the sodium chloride cotransporter but leads to hypercalciuria

Calcineurin inhibitors are immunosuppressive drugs widely used to prevent allograft rejection in transplant recipients and to manage autoimmune diseases. Hypertension and hypercalciuria are common adverse effects of calcineurin inhibitors. Hypertension development has been attributed to increased renal sodium chloride (NaCl) reabsorption due to increased activity of the renal thiazide-sensitive sodium chloride cotransporter NCC. Dietary potassium (K + ) lowers blood pressure through NCC dephosphorylation, reducing its activity. This process is thought to involve the phosphatase calcineurin. Calcineurin consists of a catalytic subunit A, CnA, with three isoforms, and a regulatory subunit B. The role of these subunits in regulating NCC activity and renal electrolyte transport remains to be delineated in detail. This study investigated the role of the catalytic subunit CnA-β, encoded by Ppp3cb, in regulating renal Na + reabsorption and NCC activity. To explore this, Ppp3cb -deficient (KO) mice and wildtype (WT) littermates were placed in metabolic cages to assess renal electrolyte handling. There were no differences between Ppp3cb KO and WT mice in creatinine clearance or blood levels of Na + , K + or calcium (Ca 2+ ). Furthermore, no significant difference in the urinary Na + and K + excretion was observed, suggesting that CnA-β does not contribute to increased Na + reabsorption during calcineurin inhibition. However, urinary Ca 2+ excretion was markedly increased in Ppp3cb KO mice. The expression of total NCC and phosphorylated (active) NCC did not differ between Ppp3cb KO and WT mice. Similarly, no differences were observed in the expression or phosphorylation of the Na–K–Cl cotransporter NKCC2 or the expression and cleavage of the epithelial Na + channel ENaC. To determine if CnA-β is involved in K + effects on NCC dephosphorylation, mice were given an oral gavage of 2% K + . Both genotypes exhibited similar levels of NCC dephosphorylation. Ex vivo, renal tubules were incubated in medium with different concentrations of K + for 30 min. Renal tubules from WT and KO mice incubated in high-K + medium exhibited a similar decrease in NCC phosphorylation, while low-K + medium increased NCC phosphorylation to the same degree in WT and KO tubules. The calcineurin inhibitor tacrolimus increased NCC phosphorylation in both genotypes but did not prevent the effect of K + on NCC. In conclusion, genetic deletion of CnA-β results in hypercalciuria, but CnA-β does not play a major role in the regulation of NCC. The effect of the calcineurin-inhibitor tacrolimus on NCC phosphorylation appears independent of CnA-β, possibly involving CnA-α, the only other isoform expressed in kidney. However, since tacrolimus did not block the effect of K + on NCC phosphorylation, this effect on NCC may be independent of calcineurin. Funding: Independent Research Fund Denmark | Medical Sciences This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.