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Small Proteoglycans of Articular Cartilage

Small proteoglycan molecules consisting of biglycan, decorin, and fibromodulin represent approximately 5% of the proteoglycan of articular cartilage. Dermatan sulfate proteoglycans were first observed in articular cartilage by Rosenberg  in 1985. The small proteoglycans each consist of a protein core and glycosaminoglycan chain branches.

The core protein of the small proteoglycans is only one fourth the length of the core protein of aggrecan and is very similar in the three molecules. The horseshoe-shaped protein is linked near the open end by a disulfide bond.

The glycosaminoglycan side chains are limited in number, consisting of a single chondroitin sulfate/dermatan sulfate side chain in decorin, two such chains in biglycan, and as many as four keratan sulfate side chains in the case of fibromodulin. Decorin  and fibromodulin  are located in the superficial zones of articular cartilage in association with collagen fibers.

Smaller amounts of decorin are found in deeper cartilage layers. Biglycan and decorin are both found in the pericellular lacunar regions of chondrocytes.

The small proteoglycans have critical functional roles. Decorin and fibromodulin are associated with collagen fibers. Decorin inhibits type I and II collagen formation, however, and fibromodulin inhibits collagen enesis.

Both biglycan and decorin possess properties of competitive binding with transforming growth factor (TGF)-β, thereby inhibiting the role of this growth factor in repair processes in cartilage. Because TGF-β is considered to be the “conductor of the symphony” in connective tissue repair (27), the modulation of its role by biglycan and decorin has great significance in the control of repair processes in connective tissue.

Both decorin and biglycan also bind with other adhesion proteins, including fibronectin, thrombospondin, and type VI collagen. Each of these so-called adhesion proteins have the RGD amino acid sequences originally described by Ruoslahti and Pierschbacher as playing key roles in cell adhesion. Decorin or biglycan binding to the adhesion proteins inhibits the cellular attachment of fibroblasts, another key step in the control of connective tissue healing.

These small proteoglycan roles most certainly are relevant to the questions of limitations of repair potential of articular cartilage. Fibronectin, type VI collagen, thrombospondin, and the adhesion proteins have complex roles, binding to collagen, serving as bridges between cells and matrix and between themselves , and binding to hyaluronan .

These molecules undoubtedly possess functional roles far greater than the quantitative measures of their content in connective tissue matrix suggest, and their functional roles must be understood more completely to provide a comprehensive understanding of normal articular cartilage as well as its repair processes.

Facilitation of the biological repair of articular cartilage by surgical procedures may not be made more feasible by this understanding, however, given the complexity of the processes involved, but at the same time, this facilitation is unlikely to be achieved without such an understanding.

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