Molecular and functional evidence for calcineurin-A alpha and beta 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 se rine/threonine phosphatase, calcineurin-A (CN-A), in the osteoclast. Polyme rase chain reaction (PCR) of an osteoclast cDNA library, together with rest riction mapping, revealed two isoform sequences, alpha and beta. We then ex amined the functionality of the detected CN-A by assessing the effect of a classical antagonist, cyclosporin A (CsA), in the osteoclast resorption (pi t) assay. CsA (0.1 and I mu g ml(-1)) potently inhibited bone resorption. T he presence of lymphocytes, with or without prior exposure to CsA in vivo, failed to reverse the CsA-induced resorption-inhibition. Expectedly, CsA ha d no direct effect on cytosolic Ca2+ levels in fura-a-loaded osteoclasts. T hese 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. (C) 1999 Academic Press.