Collagen alignment can contribute to tissue

Collagen alignment can contribute to tissue stiffening (Malik et al., 2015, Paszek et al., 2005), and ECM stiffness in turn is a powerful activator of integrin function (Levental et al., 2009). These findings highlight the contribution of the mechanical properties of the ECM to signal transduction. However, little is known about the impact of the mechanical properties of the ECM on DDR1 function. In adipose stromal cells, DDR1 transmits mechanical signals that stimulate the transcription of the enzyme aromatase (Ghosh et al., 2013). Previously, we found that DDR1 overexpression enhances integrin activation (Staudinger et al., 2013), but the role of DDR1 in mechanosensing was not explored. Here we show that DDR1-dependent collagen tractional remodeling was markedly reduced in lipid metabolism pathway cultured on soft substrates, independent of integrin function. We also found that both DDR1 activation (phosphorylation of Y792) and MLC phosphorylation increased markedly in cells cultured on stiff substrates, which also led to the organization of well-aligned fibrillar collagen. Furthermore, increasing the intracellular tension of cells expressing kinase-active DDR1 that were plated on soft fibrillar collagen substrates was associated with enhanced DDR1 activation and collagen mechanical reorganization. These responses were independent of integrin function. Altogether, these data suggest that substrate compliance, DDR1 activation, cell contraction, and collagen mechanical reorganization are functionally linked. Our findings demonstrate an exciting role for DDR1 in mechanical realignment of collagen by tractional forces, which is consistent with increased expression of DDR1 in several types of cancer and fibrotic conditions (Gao et al., 2016, Toy et al., 2015, Valiathan et al., 2012, Vogel et al., 2006). We have also shown an integrin-independent regulatory system for collagen mechanical remodeling in which clustering and phosphorylation of DDR1 control functional interactions that link collagen fibrils to NMIIA. These processes ultimately determine the amplitude and kinetics of contractile force generation in the collagen-rich ECM.
Experimental Procedures For detailed experimental procedures, see the Supplemental Information.
Author Contributions
Acknowledgments
Introduction Receptor tyrosine kinases (RTKs) are key components of several signal transduction pathways and play a critical role in the central nervous system (CNS). They control multiple cellular processes such as motility, proliferation, differentiation, metabolism, survival and synaptogenesis (Blume-Jensen & Hunter, 2001, Chao, 1992, Huang et al., 2009). The discoidin domain receptors (DDRs) are a novel sub-family of RTKs. This family is composed of only two members, DDR1 and DDR2, both of which are distinguished by a discoidin motif in their extracellular domain that is essential for binding to their cognate ligand. As of yet, only collagen has been identified as a ligand able to induce DDR phosphorylation. DDR1 is activated by all collagens tested to-date (type I through type VI) while DDR2 is only activated by fibrillar collagens, collagen types I and III in particular (Vogel et al., 1997).