Archives

  • 2018-07
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • 2022-08
  • 2022-09
  • 2022-10
  • 2022-11
  • 2022-12
  • 2023-01
  • 2023-02
  • 2023-03
  • 2023-04
  • 2023-05
  • 2023-06
  • 2023-08
  • 2023-09
  • 2023-10
  • 2023-11
  • 2023-12
  • 2024-01
  • 2024-02
  • 2024-03
  • Our previous studies have demonstrated that the DDR discoidi

    2019-09-20

    Our previous studies have demonstrated that the DDR2 discoidin domain fully contains the binding site(s) for the fibrillar collagens I and II (Leitinger, 2003, Leitinger et al., 2004). The binding site for collagen I was mapped to three spatially adjacent surface loops within the DDR2 discoidin domain (Leitinger, 2003). To our surprise, collagen X was not recognised by the DDR2 discoidin domain (Fig. 6), indicating additional requirements for collagen X recognition. From these data it seems that the binding mechanism for non-fibrillar collagens (or binding sites on DDR2) is different from how fibrillar collagens are recognised by DDR2. At present, we cannot rule out that the collagen X binding site lies outside the discoidin domain, in the region between the discoidin domain and the transmembrane domain. This ∼200 amino iMDK mass sequence shows no sequence homology to any known domain. However, collagen recognition by the DDRs most likely occurs via a conserved mechanism and the discoidin domain is situated at the membrane-distal site of DDR2, where one would expect a collagen binding site to be located. We therefore speculate that the DDR2 discoidin domain contains at least part of the collagen X binding site, but that additional protein sequences are required for maintaining the proper orientation of the discoidin domain for collagen X recognition. We have previously shown that DDR binding to collagen requires the extracellular DDR domains to be dimerised (Leitinger, 2003). It is conceivable that the DDR2 discoidin domain construct (dimerised via an Fc sequence) does not present the discoidin domain in the correct orientation for collagen X binding. The significance of the difference in the DDR2 binding specificity towards fibrillar and non-fibrillar collagens is not understood, but may imply differences in biological responses when chondrocytes bind to different collagens in the growth plate. Investigations by Labrador et al. (2001) have shown that the regulation of cell proliferation in the growth plate could be mediated through activation of the DDR2 tyrosine kinase domain. In this context, the activation of DDR2 is likely to be the consequence of cells binding to collagen II in the proliferative zone of the growth plate. It is conceivable that the replacement of collagen II with the collagen X network in the transitional zone may interrupt/perturb the binding of DDR2 to type II collagen and hence cell proliferation. The binding of collagen X to DDR2, through different binding sites, may be the initiation signal for chondrocyte maturation and DDR2 may thus regulate important cellular processes leading to the final stages of EO. Our results show that DDR2 recognises both the collagenous domain of collagen X and the NC1 domain (Fig. 7). Binding to the collagenous domain is dependent on the native, triple-helical conformation. Thus, DDR2 recognises the triple helix of both fibrillar and non-fibrillar collagens in a conformation-specific manner. As the isolated NC1 domain has a tendency to form aggregates (Frischholz et al., 1998, Zhang and Chen, 1999), there is a concern that this domain might show non-specific binding in in vitro binding assays. However, we believe that it is unlikely that we have measured non-specific DDR2 binding to the NC1 domain in our solid phase binding assays, as the DDR2 discoidin domain showed no reactivity to the isolated NC1 domain (data not shown). The collagenous domain, but not the NC1 domain interacts with DDR2 in a productive manner, leading to receptor autophosphorylation (Fig. 8). These findings are in agreement with the characterisation of all known DDR ligands; receptor activation requires binding of the DDRs to triple-helical collagen. Although our results show that the NC1 domain is dispensable for DDR2 autophosphorylation, it is conceivable that in tissues, cell adhesion to the NC1 domain can facilitate binding to the triple helix. Our previous study has demonstrated that the NC1 domain is involved in mediating the interaction of collagen X with cells and that cells adhere to the isolated NC1 domain (Luckman et al., 2003). In the growth plate ECM, collagen X molecules most likely form an extended hexagonal array with many NC1 domains forming large aggregates within the collagen network (Kwan et al., 1991, Jacenko et al., 2001). The binding of the NC1 domain to cell surface receptors may have a mechanical role to stabilise the cell–matrix interactions.