Developmental Expression of Osteopontin (OPN) mRNA in Rat Tissues: Evidence for a Role for OPN in Bone Formation and Resorption

Osteopontin (OPN) is a 34-kDa, highly-phosphorylated glycoprotein with cell attachment properties that is a prominent constituent of the bone matrix. To aid in elucidating the function of this protein we have studied the cellular expression of OPN mRNA during the formation, growth and maturation of rat calvarial (membranous) and tibial (endochondral) bone. From Northern hybridization analysis OPN expression was demonstrated in the kidney and gravid uterus as well as in bone tissues. Compared to collagen, the expression of OPN was low in early bone formation but increased subsequently and reached peak levels in 14-day-old bone. However, both the collagen and OPN mRNAs decreased markedly thereafter and remained low in young adult bone. From in situ hybridization studies using a [35S]-labelled rat OPN cRNA probe, OPN mRNA was localized to osteoblastic cells in newly-forming calvariae, jaw bones, and in the metaphyseal and periosteal bone of the tibia. In contrast to bone sialoprotein (BSP), which is expressed almost exclusively by osteoblasts at sites of de novo bone formation, OPN transcripts were present in cells lining both endosteal and periosteal bone surfaces, and in osteocytes. Moreover, expression of OPN persisted during the subsequent growth and remodelling of both membranous and endochondral bone and was expressed at particularly high levels by bone cells and hypertrophic chondrocytes at sites of osteoclastic resorption. In the more mature bone of young adult rats OPN expression was significantly reduced but remained detectable in bone cells lining periosteal and endosteal surfaces and in the primary and secondary spongiosa of the tibia. These studies on the developmental expression of OPN support the concept of a multifunctional role for OPN in bone formation and remodelling. Thus, the expression of OPN by osteoblasts early in bone development is consistent with a role for this protein in the formation of bone matrix, whereas the peak expression of OPN later in bone development, together with high expression at sites of rapid remodelling, indicate that OPN deposited on the surface of mineralized connective tissues may provide a template for osteoclastic resorption.