Taken together, the data presented here suggest that treatment of hMSC with clotted PRC, in an osteoinductive environment, enhances osteoblastic commitment and bone formation. Furthermore, these data indicate that the enhanced osteogenesis seen in the presence of PRC cannot be explained solely by enhanced cell proliferation, suggesting that PRC modulates a number of cell and molecular pathways to promote bone formation.

Type IX collagen is covalently bound to the surface of type II collagen fibrils within the cartilage extracellular matrix. The N-terminal, globular noncollagenous domain (NC4) of the ␣1(IX) chain protrudes away from the surface of the fibrils into the surrounding matrix and is available for molecular interactions. To define these interactions, we used the NC4 domain in a yeast two-hybrid screen of a human chondrocyte cDNA library. 73% of the interacting clones encoded fibronectin. The interaction was confirmed using in vitro immunoprecipitation and was further characterized by surface plasmon resonance. Using whole and pepsin-derived preparations of type IX collagen, the interaction was shown to be specific for the NC4 domain with no interaction with the triple helical collagenous domains. The interaction was shown to be of high affinity with nanomolar K d values. Analysis of the fibronectin-interacting clones indicates that the constant domain is the likely site of interaction. Type IX collagen and fibronectin were shown to co-localize in cartilage. This novel interaction between the NC4 domain of type IX collagen and fibronectin may represent an in vivo interaction in cartilage that could contribute to the matrix integrity of the tissue.The organization and interactions of the extracellular matrix (ECM) 2 of articular cartilage provide its unique mechanical properties. A network composed of stable collagen fibrils confers tensile strength to articular cartilage to resist the repeated loading as occurs in vivo. The principal components of this network are collagen types II, IX, and XI, forming heterotypic fibrils, in which type IX collagen is covalently cross-linked to the surface of type II collagen in a D-periodic arrangement along the fibrils (1-4). The concept of heterotypic fibrils is well established, although the functional roles of type IX collagen within articular cartilage remain unclear.A member of the fibril-associated collagens with an interrupted triple helix (FACIT) family of collagens, type IX is a heterotrimer of three genetically distinct polypeptide chains, ␣1(IX), ␣2(IX), and ␣3(IX), stabilized by interchain disulfide bonds. The complete molecule consists of three triple helical domains (COL1-3) and four noncollagenous domains (NC1-4) (for reviews see Refs. 5, 6). Rotary shadowing electron microscopy has revealed a "kink" in the NC3 domain. This elevates the COL3 and the N-terminal globular NC4 domains of the ␣1(IX) chain away from the heterotypic fibril surface projecting it into the surrounding matrix (7,8). This position makes the COL3 and NC4 domains ideal candidates for interactions with other matrix molecules. In vitro studies substantiate this hypothesis.The COL3 domain has been shown to interact strongly with the I domain of integrins using a novel binding site possibly involving amino acids from more than one chain (8). Thus, type IX collagen is implicated in cell adhesion to the type II/XI/IX collagen macromolecular alloy. The ␣1(IX) NC4 domain also interacts with thrombospondin 5 ...