Sensing of Extracellular Cations in CasR-deficient Osteoblasts

We isolated osteoblastic cell lines from wild-type (CasR+/+) and receptor null (CasR−/−) mice to investigate whetherCasRis present in osteoblasts and accounts for their responses to extracellular cations. Osteoblasts from bothCasR+/+and CasR−/−mice displayed an initial period of cell replication followed by a culture duration-dependent increase in alkaline phosphatase activity, expression of osteocalcin, and mineralization of extracellular matrix. In addition, a panel of extracellular cations, including aluminum and the CasRagonists gadolinium and calcium, stimulated DNA synthesis, activated a transfected serum response element-luciferase reporter construct, and inhibited agonist-induced cAMP in CasR−/−osteoblasts. The functional responses to these cations were identical inCasR+/+and CasR−/−osteoblasts. Thus, the absence of CasRalters neither the maturational profile of isolated osteoblast cultures nor their in vitroresponses to extracellular cations. In addition,CasRtranscripts could not be detected by reverse transcription-polymerase chain reaction with mouse specific primers in either CasR+/+orCasR−/−osteoblasts, and immunoblot analysis with a CasR-specific antibody was negative forCasRprotein expression in osteoblasts. The presence of a cation-sensing response in osteoblasts fromCasR−/−mice indicates the existence of a novel osteoblastic extracellular cation-sensing mechanism.