The recombinant Scl collagen system has shown capability as
The recombinant Scl2 collagen system has shown capability as a biomaterial as well because of its adaptability and scalability. Scl2 was functionalized to crosslink into a hydrogel without disrupting its triple helix . The Scl2–hydrogel crosslinking also did not disrupt cell adhesion and integrin binding when the α1 and α2 integrin binding motifs of collagen were inserted into the Scl2 . This Scl2–hydrogel has been incorporated in the development of a vascular graft with suitable biomechanical properties  and in the development of an injectable medicine to stimulate chondrogenesis . Additionally, a high-throughput batch purification methodology for Scl2 recombinant collagen has been developed . This methodology has shown to be very scalable and produce a high percentage (>95%) of pure protein [146,147]. As our understanding of DDR–collagen interactions advances, it shall be possible to engineer biomaterials with DDR-activating or inhibiting properties.
Acknowledgements We thank Prof. Barbara Brodsky for valuable comments and discussion. We thank the support of the Tufts start-up fund and the Knez Family Faculty Investment Fund for Y.-S. L, and the Tufts Summer Scholar program for E.C.
Introduction Collagen proteins, major components of extracellular matrix (ECM), play important roles in the regulation of cell function and behavior aside from the maintenance of tissue structure and integrity. The signals from extracellular collagens to the PKR Inhibitor sale are transduced by specific cell-surface receptors, including integrins and discoidin domain receptors (DDRs) , . DDRs, consisting of DDR1 and DDR2, belong to the receptor tyrosine kinase (RTK) subfamily and exhibit a slow but sustained phosphorylation kinetics in response to collagen binding . DDR2 expression is mainly detected in mesenchymal-derived cells , . Growing biological evidences have demonstrated that DDR2 can regulate cell proliferation, migration, differentiation, as well as extracellular matrix remodeling and epithelial-mesenchymal transition (EMT) , , , , , , , . Upregulation of DDR2 signaling was reported to be associated with diverse human diseases, such as arthritis and cancer . Elevated expression of matrix metalloproteinases (MMPs) that primarily mediate the degradation of ECM represents an important cell response to DDR2 activation , , . However, up to date little is known as to how the DDR2 signaling is negatively regulated or attenuated. Covalent modification of activated RTKs with ubiquitin, which is carried out by a cascade of enzymatic reactions involving E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme) and E3 (ubiquitin ligase), has been well established as a major mechanism for negative regulation of RTK signaling . Cbl family, which consists of three homologues known as Cbl (or c-Cbl), Cbl-b and Cbl-c (or Cbl-3) in mammals, belongs to RING-type E3 ubiquitin ligases . Cbl proteins-mediated ubiquitination can target activated tyrosine kinases for degradation, either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation . In contrast to the wide expression pattern of Cbl and Cbl-b, Cbl-c expression was reported to be limited in epithelial cells , in which DDR2 is always not expressed , . Thus, in this study, we aimed to investigate whether DDR2 can be negatively regulated by Cbl or Cbl-b.
Material and methods
Discussion Because of structural homology, Cbl and Cbl-b always have overlapping or similar functions , , , , . However, they also display different expression pattern and in vivo function . As for the role of Cbl and Cbl-b in some PTK signaling, it can be different or even opposite , . Although our current data supported the notion that collagen-induced ubiquitination of DDR2 was predominantly facilitated by Cbl-b, we cannot exclude the possibility that Cbl may also play some important roles in DDR2 signaling, given that we identified an interaction between Cbl and DDR2 as well (data not shown). In addition, aside from possessing ubiquitin ligase activity, Cbl has been well documented as an adaptor protein , . As such, whether and how Cbl is involved in DDR2 signaling still awaits further investigation.