OSTEOCLAST REGULATION BY IONIZED CALCIUM

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During bone resorption, an osteoclast generates and, as a consequence, becomes exposed to millimolar Ca2+ levels. The goal of the present study is to define pathways through which changes in extracellular Ca2+, and corresponding changes in intracellular Ca2+, are transduced first into changes in intranuclear Ca2+, and then into alterations in gene expression. Notably, the osteoclast plasma membrane represents a unique site for the location of a ryanodine receptor. Ryanodine receptors are Ca2+-permeable channels that normally reside in microsomal membranes. However, in the osteoclast, a plasma membrane-resident, type II, ryanodine receptor isoform serves to sense changes in extracellular Ca2+, hence the term Ca2+ sensor. It has been shown recently that ryanodine receptors are also located in nuclear membranes at which site they gate the flux of Ca2_+ into the nucleoplasm. By making separate measurement of nuclear envelope and nucleoplasmic Ca2+ levels, as well as by performing confocal microscopic studies using epitope-specific antisera, we first propose to determine whether Ca2+ transport across the osteoclast nuclear membrane occurs through ryanodine receptor-gated Ca2+ channels. Such studies are particularly relevant to recent observations showing that nuclear Ca2+ levels regulate gene expression directly. Thus, by applying the in situ reverse transcriptase polymerase chain reaction (RT=PCR) to isolated single osteoclasts, we propose to investigate whether Ca2+ modulates expression of the gene for the osteoclast cytokine, interleukin-6, as well as that of its receptor. Finally, we will also assess whether the secreted interleukin-6 attenuates Ca2+ sensing, a phenomenon that, we believe, should allow a resorbing osteoclast 'escape' (or recover) from Ca2+-induced inhibition. Important therapeutic implications should follow from a better understanding of mechanisms that underlie extracellular Ca2+ sensing by, and Ca2+ homeostasis in, the osteoclast.