Carboxyl-Terminal Parathyroid Hormone Receptor Regulates Osteocyte Cytoskeleton Through Mechanisms Dependent Upon Calcium Influx

Parathyroid hormone (PTH) exerts classical actions on bone and mineral metabolism by activating PTH/PTHrP receptors (PTH1Rs) on target cells, including osteoblasts and osteocytes in bone. Such bone cells also express an addi- tional receptor for PTH distinct from PTH1Rs, that recognize determinants within the carboxyl (C)-terminal portion of PTH (1-84), the C-terminal PTH receptor ("CPTHR"). CPTHRs previously were found to regulate intercellular communi- cation, cell survival and to increase cytosolic calcium in bone cells in a manner dependent upon voltage-sensitive calcium channels. As intracellular free calcium is known to regulate cytoskeletal function, we sought to determine if CPTHR acti- vation altered cytoskeletal structure in OC-59 osteocytic cells, which lack PTH1Rs but express abundant CPTHRs. Treatment of OC-59 cells with 100 nM hPTH (53-84) for 10 minutes induced marked condensations of the cytoskeletal components actin and vinculin, as visualized by immunofluorescence in permeabilized cells. This effect was not observed in cells treated with vehicle alone or with the CPTHR ligand for only 2 minutes. These changes also were not seen in cells exposed for 10 minutes to the inactive CPTH analog, (Ala 55-57 )PTH (53-84), which does not bind to CPTHRs and does not induce a calcium signal in OC-59 cells. Cytoskeletal condensation induced by hPTH (53-84) was blocked by pre- treatment with gadolinium chloride, which is known to inhibit CPTHR-dependent calcium responses in these cells. Taken together, these results suggest that calcium influx induced by CPTHR activation may play an important role in regulating the cytoskeleton in osteocytes.