Biglycan is a Class I
Small Leucine Rich Proteoglycans (SLRP) that is localized on human chromosome Xq28-ter. The conserved nature of its intron-exon structure and
protein coding sequence compared to
decorin (another Class I SLRP) indicates the two genes may have arisen from gene duplication.
Biglycan contains two
chondroitin sulfate glycosaminoglycan (GAG) chains attached near its NH(2) terminus making it different from
decorin that has only one GAG chain. To determine the functions of
biglycan in vivo, transgenic mice were developed that were deficient in the production of the
protein (knockout). These mice acquire diminished bone mass progressively with age. Double
tetracycline-
calcein labeling revealed that the
biglycan deficient mice are defective in their capacity to form bone. Based on this observation, we tested the hypothesis that the
osteoporosis-like phenotype is due to defects in cells critical to the process of bone formation. Our data shows that
biglycan deficient mice have diminished capacity to produce marrow stromal cells, the bone cell precursors, and that this deficiency increases with age. The cells also have reduced response to tranforming
growth factor-beta (
TGF-beta), reduced
collagen synthesis and relatively more apoptosis than cells from normal littermates. In addition, calvaria cells isolated from
biglycan deficient mice have reduced expression of late
differentiation markers 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
biglycan knockout mice. Other data suggests there is a functional relationship between
biglycan and bone morphogenic
protein-2/4 (BMP 2/4) action in controlling skeletal cell differentiation. In order to test the hypothesis that functional compensation can occur between SLRPs, we created mice deficient in
biglycan and
decorin.
Decorin deficient mice have normal bone mass while the double
biglycan/
decorin knockout mice have more severe
osteopenia than the single
biglycan indicating redundancy in SLRP function in bone tissue. To further determine whether compensation could occur between different classes of SLRPs, mice were generated that are deficient in both
biglycan (class I) and
fibromodulin, a class II SLRP highly expressed in mineralizing tissue. These doubly deficient mice had an impaired gait, ectopic calcification of tendons and premature
osteoarthritis. Transmission electron microscopy analysis showed that like the
decorin and
biglycan knockouts, they have 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
fibromodulin/
biglycan knockout mice. These studies present important new animal models for
musculoskeletal diseases and provide the opportunity to characterize the network of signals that control tissue integrity and function through SLRP activity.