FUNCTIONAL HIERARCHY BETWEEN 2 OSE2 ELEMENTS IN THE CONTROL OF OSTEOCALCIN GENE-EXPRESSION IN-VIVO

Osteocalcin gene expression is initiated perinatally and is restricted to mature osteoblasts and odontoblasts. Because their pattern of expr ession is highly restricted, the osteocalcin genes are excellent tools to study osteoblast-specific gene expression. To define the mechanism s of osteocalcin cell-specific gene expression in vivo, we generated t ransgenic mice harboring deletion mutants of the promoter region of OG 2, one of the mouse osteocalcin genes. We show here that only 647 base pairs of this promoter are sufficient to confer cell-specific and tim e-specific expression to a reporter gene in vivo, This promoter fragme nt contains two copies of OSE2, This osteoblast-specific cis-acting el ement binds Osf2, a recently characterized osteoblast-specific transcr iption factor (Ducy, P,, Zhang, R,, Geoffroy, V,, Ridall, A. L,, and K arsenty, G, (1997) Cell 89, 747-754), We also demonstrate that the pro ximal OSE2 element is critical to confer an osteoblast-specific, devel opmentally regulated pattern of expression to a reporter gene. The oth er OSE2 element, located more upstream and presenting a lower affinity for Osf2, affects only weakly OG2 promoter activity. These data demon strate the crucial role of Osf2 in controlling osteocalcin gene expres sion. Since osteocalcin synthesis is a hallmark of the differentiated osteoblast phenotype, these results suggest that, beyond its developme ntal function, Osf2 is also required for the maintenance of the osteob last phenotype postnatally.