Cloning the full-length cDNA for rat connective tissue growth factor: Implications for skeletal development

The mammalian osteopetroses represent a pathogenetically diverse group of s keletal disorders characterized by excess bone mass resulting from reduced osteoclastic bone resorption. Abnormalities involving osteoblast function a nd skeletal development have also been reported in many forms of the diseas e. In this study, we used the rat mutation, osteopetrosis (op), to examine differences in skeletal gene expression between op mutants and their normal littermates. RNA isolated from calvaria and long bones was used as a templ ate for mRNA-differential display. Sequence information for one of the many cDNA that were selectively expressed in either normal or mutant bone sugge sted that it is the rat homologue of connective tissue growth factor (CTGF) previously cloned in the human, mouse, and other species. A consensus sequ ence was assembled from overlapping 5'-RACE clones and used to confirm the rat CTGF cDNA protein coding region. Northern blot analysis confirmed that this message was highly (8- to 10-fold) overexpressed in op versus normal b one; it was also upregulated in op kidney but none of the other tissues (br ain, liver, spleen, thymus) examined. In primary rat osteoblast cultures, t he CTGF message exhibits a temporal pattern of expression dependent on thei r stale of differentiation. Furthermore, CTGF expression is regulated by pr ostaglandin E-2, a factor known to modulate osteoblast differentiation. Sin ce members of the CTGF family regulate the expression of specific genes, su ch as collagen and fibronectin, we propose that CTGF may play a previously unreported role in normal skeletal modeling/remodeling. Its dramatic over-e xpression in the op mutant skeleton may be secondary to the uncoupling of b one resorption and bone formation resulting in dysregulation of osteoblast gene expression and function. J. Cell. Biochem. 77:103-115, 2000. (C) 2000 Wiley-Liss, Inc.