Discoidin Domain Receptors Promote α1β1- and α2β1-Integrin Mediated Cell Adhesion to Collagen by Enhancing Integrin Activation

Abstract

The discoidin domain receptors, DDR1 and DDR2, are receptor tyrosine kinases that bind to and are activated by collagens. Similar to collagen-binding β1 integrins, the DDRs bind to specific motifs within the collagen triple helix. However, these two types of collagen receptors recognize distinct collagen sequences. While GVMGFO (O is hydroxyproline) functions as a major DDR binding motif in fibrillar collagens, integrins bind to sequences containing Gxx’GEx”. The DDRs are thought to regulate cell adhesion, but their roles have hitherto only been studied indirectly. In this study we used synthetic triple-helical collagen-derived peptides that incorporate either the DDR-selective GVMGFO motif or integrin-selective motifs, such as GxOGER and GLOGEN, in order to selectively target either type of receptor and resolve their contributions to cell adhesion. Our data using HEK293 cells show that while cell adhesion to collagen I was completely inhibited by anti-integrin blocking antibodies, the DDRs could mediate cell attachment to the GVMGFO motif in an integrin-independent manner. Cell binding to GVMGFO was independent of DDR receptor signalling and occurred with limited cell spreading, indicating that the DDRs do not mediate firm adhesion. However, blocking the interaction of DDR-expressing cells with collagen I via the GVMGFO site diminished cell adhesion, suggesting that the DDRs positively modulate integrin-mediated cell adhesion. Indeed, overexpression of the DDRs or activation of the DDRs by the GVMGFO ligand promoted α1β1 and α2β1 integrin-mediated cell adhesion to medium- and low-affinity integrin ligands without regulating the cell surface expression levels of α1β1 or α2β1. Our data thus demonstrate an adhesion-promoting role of the DDRs, whereby overexpression and/or activation of the DDRs leads to enhanced integrin-mediated cell adhesion as a result of higher integrin activation state.

The Novel Small Leucine-rich Protein Chondroadherin-like (CHADL) Is Expressed in Cartilage and Modulates Chondrocyte Differentiation

“…After Chadl knockdown, microarray analysis indicated the activation of STAT1/3 and p38 MAPK. STAT1/3 and p38 MAPK can be regulated by collagen-binding integrin ␣2␤1 and discoidin domain receptor collagen receptor signaling (26,27). In addition, p65 -inducer of Sox9 in early chondrocyte differentiation (25) -was predicted to be activated (Fig.…”

The binding capacity of α1β1-, α2β1- and α10β1-integrins depends on non-collagenous surface macromolecules rather than the collagens in cartilage fibrils

“…Our study also shows that chondrocytes do exhibit a small direct attachment activity to collagen-containing fibrils that, presumably, is mediated by DDR2, an alternative collagen-binding receptor with tyrosine kinase activity that can modulate integrin-mediated cell attachment [50]. It remains to be determined whether or not DDR2 or other collagen-receptors crucially contribute to robust chondrocyte attachment to cartilage matrix.…”

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“…Nevertheless, proper cell attachment to tissue scaffolds continues to be a major consideration in tissue engineering design and research and integrins remain the central player in this regard. Integrin signalling: The binding of integrins to the ECM facilitates cell attachment via a signaling cascade that drives fibrillogenesis [45]. Research on integrins is typically centered around uncovering the key players in this signaling cascade as well as the mechanisms that drive fibrillogenesis.…”

“…Research on integrins is typically centered around uncovering the key players in this signaling cascade as well as the mechanisms that drive fibrillogenesis. A key example are transmembrane receptor tyrosine kinases known as discoidin domain receptors (DDR); while DDRs cannot form strong adhesions to collagen matrices, they have been shown to facilitate integrin-mediated cell adhesion of α 1 β 1 and α 2 β 1 via an unknown activation mechanism [45]. Another key example is the role of α 4 β 1 integrin, also known as VLA-4, in progenitor cell attachment [46].…”