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SLRPs (Small leucine-rich proteoglycans) are extracellular molecules that bind to TGF-betas, collagens and other matrix molecules. In vitro, SLRPs were shown to regulate collagen fibrillogenesis, a process essential in development, tissue repair, and metastasis. The resilience and strength of bone is due to the orderly mineralization of a specialized extracellular matrix (ECM) composed of type I collagen (90%) and a host of non-collagenous proteins that are, in general, also found in other tissues. Biglycan (an SLRP encoded by the gene Bgn) is an ECM proteoglycan that is enriched in bone and other non-skeletal connective tissues. In vitro studies indicate that Bgn may function in connective tissue metabolism by binding to collagen fibrils and TGF-beta, and may promote neuronal survival. The effects of an individual SLRP deficiency are tissue specific and a variety of morphological phenotypes are the result of variations in level of expression of individual SLRPs in different tissues and synergisms between different SLRPs (and likely other macromolecules) in determining matrix structures. For example collagen structural phenotypes demonstrate a cooperative, sequential, timely orchestrated action of the SLRPs that altogether shape the architecture and mechanical properties of the collagen matrix. Disruption of the bgn gene results in reduced skeletal growth and bone mass leading to generalized osteopenia, particularly in older animals. In addition, other SLRP-deficient mice develop a wide array of diseases (osteoporosis, osteoarthritis, muscular dystrophy, Ehlers-Danlos syndrome, and corneal diseases), most of them resulting primarily from an abnormal collagen fibrillogenesis. (This definition may be outdated - see the DesignNote.)
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