EXPRESSION OF AN EXTRACELLULAR CALCIUM-SENSING RECEPTOR IN HUMAN AND MOUSE BONE-MARROW CELLS

The cloning of a G protein-coupled, extracellular calcium (Ca-e(2+))-s ensing receptor (CaR) fi om bovine parathyroid provided direct evidenc e that Ca-e(2+)-sensing can occur through receptor-mediated activation of G proteins and their associated downstream regulators of cellular function, CaR transcripts and protein are present in various tissues o f humans and other mammals that are involved in Ca-e(2+) homeostasis, including parathyroid, kidney, and thyroidal C-cells, The present stud y was performed to determine whether bone marrow cells express the CaR since cells within the marrow space could be exposed to substantial c hanges in Ca-e(2+) related to bone turnover, Using DNA and RNA probes from the human parathyroid CaR cDNA, we identified CaR transcripts of 5.2 and similar to 4.0 kilobases by Northern analysis of poly(A(+)) RN A from low-density mononuclear cells isolated from whole human bone ma rrow that are putatively enriched in marrow progenitor cells, includin g bone cell precursors, In situ hybridization also identified CaR tran scripts in the same cell preparations, Reverse transcription-polymeras e chain reaction demonstrated >99% nucleotide identity between transcr ipts from human bone marrow cells and the corresponding regions of the human CaR cDNA, Antisera specific for several different regions withi n the extracellular domain of the CaR were reactive with low-density h uman marrow cells that were either adherent or nonadherent to plastic, About one-third of the adherent, CaR-immunoreactive cells were also p ositive for alkaline phosphatase, a nonspecific marker of preosteoblas ts, osteoblasts, and assorted cells of the colony-forming unit-fibrobl ast lineage, In addition, a substantial fraction (similar to 60%) of l ow density murine marrow cells cultured for 1 week at 4.8 mM Ca-e(2+) expressed both CaR immunoreactivity and nonspecific esterase, an enzym e expressed by monocyte/macrophages and fibroblasts. Finally, erythroi d precursors and megakaryocytes from murine marrow as well as blood pl atelets expressed abundant CaR immunoreactivity, while peripheral bloo d erythrocytes and most polymorphonuclear leukocytes did not, These st udies indicate that the CaR is present in low-density mononuclear bone marrow cells as well as in cells of several hematopoietic lineages an d could potentially play a role in controlling the function of various cell types within the marrow space.