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  • We had earlier reported that collagen fibers


    We had earlier reported that collagen fibers with intact native banded structure were occasionally observed in the kinase-deficient, membrane-anchored DDR2 ECD (DDR2/-KD) samples; however, in our DDR1/ECD and DDR2/ECD samples, observation of native banded structure of collagen was far more infrequent. D-periodicity of collagen fibers from native cultures was measured at 61±5 nm, which is in agreement with previous studies by us and others. Previously, we found that the membrane-anchored DDR2/-KD inhibited lateral fiber growth, compared to native cultures. While fiber diameter measurements for the first week of culture for DDR1/ECD and DDR2/ECD gave results similar to those of DDR2/-KD samples (25.0–28.9 nm), fibers in DDR2/-KD cultures exhibited lateral growth of around 10 nm over the 3 weeks of culture observed. In contrast, a sustained inhibition of lateral growth of collagen fibers was observed by DDR ECD proteins resulting in average collagen fiber diameters between 20 and 30 nm throughout a 4-week period. Together, our results show that soluble DDR2 ECD inhibits collagen fibrillogenesis in the ECM consistent with membrane-anchored DDR2, albeit with a slightly higher potential. We speculate that this stronger inhibition of collagen fiber structure and lateral diameter is due to the soluble DDR ECD being distributed throughout the ECM and thus having more ability to affect collagen fiber formation even in ECM regions away from the pericellular regions. We found that both DDR1 and DDR2 ECD increased matrix mineralization as compared to native cells, with the effect of DDR2 ECD being more prominent. Both soluble (DDR2/ECD) and membrane-bound DDR2 ECD (DDR2/-KD), when compared to wild-type cells, induced larger mineral deposits. In this regard, a recent study has reported abnormal calcification arising due to mutations in the DDR2 gene NSC23766 in spondylo-meta-epiphyseal dysplasia (SMED) in humans. It is interesting to note that all the mutations reported were found in the DDR2 intracellular domain and not in its ECD. Although the NSC23766 levels of DDR2 were not reported in this study, it is likely that expression of DDR2 ECD present in the full-length mutated receptor in SMED cases along with impaired signaling of the mutated receptor may lead to increased calcification. Matrix mineralization in both DDR1 and DDR2 knockout mice have not been reported in detail; however, in DDR1 knockout mice, reduced bone calcification was described in the fibula bone. Our observations suggest the importance of evaluating matrix mineralization with respect to expression of both the full-length DDR receptors and their isoforms containing the ECDs. Since the collagen type I binding site for decorin is in close proximity to that of DDR2, further investigations are needed to understand if binding of DDR2 ECD to collagen type I promotes crystal formation by interfering with decorin binding. It is interesting to compare the effect of DDRs on collagen fibrillogenesis and matrix mineralization to those of decorin. Both DDR ECDs and decorin inhibit collagen fibrillogenesis and result in reduction of collagen fiber diameters. In contrast, while DDR ECDs enhance matrix mineralization, decorin is found to be an inhibitor of collagen calcification. No reports elucidating the ultrastructure of native ECM in DDR1 or DDR2 knockout mice have yet been made. We conclude that expression of both membrane-bound and soluble DDR1 and DDR2 ECDs can alter the morphology of endogenous collagen fibers, thus perturbing the overall ECM structure. We speculate that such perturbations, if observed in vivo, may significantly alter the integrity and biomechanical properties of resulting tissues. Further studies need to be addressed to elucidate which DDR1 and DDR2 isoforms are modulated in pathological states in vivo and how their expression alters ECM morphology and tissue biomechanics.
    Materials and Methods
    Introduction Collagen deposition is a common feature found in cancerous tissues and fibrotic organs/lesions. It is now well accepted that collagen deposition is not just a consequence of disease, but that it can also trigger a vicious cycle. Both chemical and physical signals elicited from collagen are involved in fibrotic disease progression (for concise reviews of this field: [[1], [2], [3]]). Therefore, how cells transmit collagen signals and how these signals are regulated are critical issues in unveiling the underlying mechanism of disease progression. Integrins and discoidin domain receptors (DDRs) are the two most important and ubiquitously expressed collagen receptors. Four integrin heterodimers, including α1β1, α2β1, α10β1, and α11β1, show different binding affinities to different types of collagen, tissue-specific expression patterns, and different effects in development and disease progression [4,5]. Many emerging roles of DDRs in cell differentiation, development and disease progression have been discovered in this decade, and there are still many intriguing issues to be explored. Glycoprotein VI is another type of collagen receptor found in platelets, and it plays a critical role in collagen-induced platelet activation and aggregation [6,7]. This review focuses on the roles of DDRs.