OP-1 (BMP-7) AFFECTS MESSENGER-RNA EXPRESSION OF TYPE-1, TYPE-II, TYPE-X COLLAGEN, AND MATRIX GLA PROTEIN IN OSSIFYING LONG BONES IN-VITRO

In long bone development, a regulating role of OP-1 is suggested by th e local correlated expression of both OP-1 ligand and OP-1 binding rec eptors in developing mouse hind limbs, OP-1 is expressed in the interd igital mesenchyme, whereas OP-1 binding receptors are found in the bor dering perichondrium, and both OP-1 ligand and receptors are present i n the zone of (pre)hypertrophic chondrocytes. We investigated the role of OP-1 in long bone development experimentally by treating organ cul tures of embryonic mouse metatarsals with rhOP-1, The mRNA expression patterns of type I, II, X collagen, and matrix Gla protein (MGP) were studied using in situ hybridization and cell proliferation using [H-3] thymidine and BrdU labeling, In the epiphyseal perichondrium, treatmen t with 40 ng/ml OP-1 enhanced cell proliferation after day 2, while 6- day treatment caused a shift in expression from type I collagen to typ e II collagen mRNA, This supports previous histochemical findings that OP-1 induced the transition of perichondrium into cartilage. In the c enter of the rudiment, OP-l inhibited the expression of type X collage n mRNA, indicating inhibition of chondrocyte hypertrophy, An arrest of differentiation at the prehypertrophic chondrocyte stage was also ind icated by the large area of cells expressing MGP mRNA in the OP-1-trea ted rudiments, We conclude that OP-1 affected the expression of marker genes of chondrocyte differentiation by acting on two steps in endoch ondral ossification. First, cell proliferation was enhanced, particula rly so in the perichondrium where cells started to express the chondro cyte phenotype, Second, the terminal differentiation of mature chondro cytes into hypertrophic chondrocytes was inhibited, These results, com bined with the known pattern of OP-1 ligand and BMP receptor expressio n in the embryo, suggest that OP-1 plays a local role in the cascade o f events during endochondral ossification.