Molecular and functional evidence for calcineurin-A α and β isoforms in the osteoclast: Novel insights into cyclosporin A action on bone resorption

We provide the first molecular evidence for the presence of a functional serine/threonine phosphatase, calcineurin-A (CN-A), in the osteoclast. Polymerase chain reaction (PCR) of an osteoclast cDNA library, together with restriction mapping, revealed two isoform sequences, α and β. We then examined the functionality of the detected CN-A by assessing the effect of a classical antagonist, cyclosporin A (CsA), in the osteoclast resorption (pit) assay. CsA (0.1 and 1 μg ml^-1) potently inhibited bone resorption. The presence of lymphocytes, with or without prior exposure to CsA in vivo failed to reverse the CsA-induced resorption-inhibition. Expectedly, CsA had no direct effect on cytosolic Ca²⁺ levels in fura-2-loaded osteoclasts. These studies are a prelude to further investigations into the possible role of CN-A in osteoclast regulation. Finally, mechanistic studies on the bone effects of CsA, a widely used immunosupressant, should proceed from these observations.