Function Of Skeletal Matrix Genes

Dental &Craniofacial Research

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Abstract

The Molecular Biology of Bones and Teeth Unit is directed by Dr. Marian F. Young, and during this fiscal year included Drs. Xiao-Dong Chen, Laurant Ameye and Yamming Bi, with the technical assistance of Ms. Tina Kilts. The goal of this unit is to study the function of matrix proteins made by cells in skeletal tissues. Currently we are focusing on the leucine rich, small proteoglycans known as "SLRPs." To determine the functions of biglycan (Bgn) in vivo, transgenic mice were developed that were deficient in the production of the protein (knockout/KO). These mice acquired diminished bone mass that was progressive with age. Double tetracycline-calcein labeling revealed that the bgn deficient mice were defective in their capacity to form bone. Based on these observations, we tested the hypothesis that the osteoporosis-like phenotype was due to defects in cells critical to the process of bone formation. Our data showed that bgn deficient mice have diminished numbers of bone marrow stromal stem cells (BMSSC), the bone cell precursors, and that this deficiency increased with age. The cells also had a reduced response to TGF-beta, reduced collagen synthesis and relatively more apoptosis than cells from normal littermates. In addition, calvarial cells isolated from BGN deficient mice had reduced expression of late markers of osteogenic differentiation such as bone sialoprotein and osteocalcin and diminished ability to accumulate calcium judged by alizerin red staining. We propose that any one of these defects in osteogenic cells alone, or in combination, could contribute to the osteoporosis observed in the bgn KO mice. Analysis of the signaling pathways in normal and mutant osteoblasts implicates a functional relationship between Bgn and BMP action in regulating skeletal cell differentiation. Our current research is directed towards understanding how biglycan controls intracellular and extracellular molecular signals in bones and teeth with particular focus on the intracellular- extracellular interface. In order to test the hypothesis that functional compensation can occur between SLRPs, we created mice deficient in biglycan and the sister molecule, decorin (Dcn). Decorin deficient mice had normal bone mass while the double bgn/dcn KO mice had more severe osteopenia than the single bgn KO indicating redundancy in SLRP function in bone tissue. We are currently exploring the overlapping functions of Bgn and Dcn using skeletal cells derived from this new model coupled with "gain of function" and gene rescue strategies. To further determine whether compensation could occur between different classes of SLRPs we generated mice deficient in both biglycan (class I) and fibromodulin (fm) a class II SLRP highly expressed in mineralizing tissue. These doubly deficient mice had an impaired gait, ectopic calcification of tendons and premature osteoarthritis. TEM analysis showed that like the decorin and biglycan KO, they had severely disturbed collagen fibril structures. Biomechanical analysis of the affected tendons showed they were weaker compared to control animals leading to the conclusion that instability of the joints could be the primary cause of all the skeletal defects observed in the fm/bgn KO mice. In summary, these studies present important new models of multiple skeletal diseases and provide the opportunity to characterize the network of signals that control the integrity of mineralized tissue through SLRP activity.