The binding of collagen X to DDR differs

The binding of collagen X to DDR2 differs from the binding of collagen X to α2β1 integrin in that the triple helical conformation is essential for DDR2 binding. We previously reported that heat denatured pepsinised collagen X supported cell adhesion via α2β1, indicating that the triple-helical conformation is not strictly required for α2β1 binding (Luckman et al., 2003). Although many previous studies have firmly established that collagen X is required for normal development of the growth plate (Warman et al., 1993, Wallis et al., 1994, Tselepis et al., 1996, Chan and Jacenko, 1998, Gress and Jacenko, 2000, Jacenko et al., 2002) the precise role of collagen X within the growth plate remains to be defined. Many of these studies favoured a structural role for the collagen X network in the hypertrophic zone. However, the interactions between the collagen X network and hypertrophic chondrocytes and the interactions between collagen X and other cartilage matrix components have not been studied to any significant extent. It has been proposed that the interaction of collagen X with other matrix components is important for regional extracellular matrix organisation permissive for mineralisation and vascularisation prior to new bone formation (Luckman et al., 2003). The present study further corroborates an earlier study that the collagen X network does support cell adhesion and also supports the notion that collagen X has regulatory roles besides its structural function in the matrix, possibly via activation of colorimetric tyrosine kinases. Since DDR2 is a signalling receptor with important functions in bone growth (Labrador et al., 2001), our present study provides the basis for future studies into the role of DDR2 signalling in bone formation.
Experimental procedures
Acknowledgments
Introduction Receptor tyrosine kinases (RTKs) control many fundamental cellular processes, such as cell proliferation, differentiation, migration, and metabolism (Lemmon and Schlessinger, 2010). RTK activity is normally tightly controlled, and dysregulation of RTK activity is associated with many human cancers and other pathologies. Ligand binding to the extracellular region of RTKs leads to autophosphorylation of their cytoplasmic kinase domains, creating docking sites for effectors of downstream signaling. The two major strategies for controlling unwanted RTK activity in human patients are inhibition by monoclonal antibodies (mAbs) directed against their extracellular regions or by small molecules targeting the kinase active site (Adams and Weiner, 2005, Gschwind et al., 2004). The discoidin domain receptors, DDR1 and DDR2, are RTKs that are activated by several types of triple-helical collagen, a major component of the animal extracellular matrix (Leitinger, 2011, Shrivastava et al., 1997, Vogel et al., 1997). The DDRs are widely expressed in mammalian tissues and have important roles in embryo development and human disease (Vogel et al., 2006). For example, DDR1 is essential for mammary gland development (Vogel et al., 2001), and DDR2 is essential for the growth of long bones (Labrador et al., 2001). DDR2 mutations in humans cause a rare, severe form of dwarfism (Ali et al., 2010, Bargal et al., 2009). The DDRs are also implicated in cancer, fibrotic diseases, atherosclerosis, and arthritis (Vogel et al., 2006). Mechanistically, the DDRs have several features that distinguish them from other RTKs. Compared with the rapid response of typical RTKs to their soluble ligands (e.g., growth factors), collagen-induced DDR autophosphorylation is slow and sustained (Shrivastava et al., 1997, Vogel et al., 1997). Furthermore, Src kinase plays an essential role in DDR activation (Ikeda et al., 2002).