TEMPORAL CHANGES OF MESSENGER-RNA EXPRESSION OF MATRIX PROTEINS AND PARATHYROID-HORMONE AND PARATHYROID HORMONE-RELATED PROTEIN (PTH PTHRP)RECEPTOR IN BONE-DEVELOPMENT/

Expression of parathyroid hormone and parathyroid hormone-related prot ein (PTH/PTHrP) receptor is one of the osteoblastic phenotypes; howeve r, it has not been clear whether this phenotype expression is a marker of immature or mature osteoblasts, We examined the temporal expressio n pattern of PTH/PTHrP receptor in bone development in vivo and in vit ro compared with the expression of other osteoblastic phenotypes: oste opontin (OPN), bone sialoprotein (BSP) and osteocalcin (OC), alkaline phosphatase (ALP), and mineralization, Total RNA was extracted from ra t calvariae, and cell culture of rat bone marrow at different developm ental stages and then Northern blot hybridization were performed, Mine ralization was detected with contact microradiography (CMR) in calvari a or with Alizarin Red S staining in bone marrow cell culture, Both in calvaria and in marrow cell culture, extensive expression of OPN, BSP , type I collagen (COL I), and ALP coincided with the onset of mineral ization, and OC expression was observed after mineralized tissue forma tion, Notably, PTH/PTHrP receptor was expressed at an early developmen tal stage (prenatal day 14 in calvaria, day 5 in culture) when mineral ized tissue was not formed and other osteoblastic phenotypes were scar cely detected, Further study in cell culture revealed that the fold in crease in cyclic adenosine monophosphate (cAMP) in response to PTH,vas elevated with the advance in the culture stage, These results indicat e that mRNA expression of PTH/PTHrP receptor could be the early differ entiation marker in osteoblastic lineage and that the levels of cAMP p roduction in response to PTH represent the stage of osteoblastic diffe rentiation.