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  • Collagen alignment can contribute to tissue stiffening Malik

    2020-05-22

    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 Nepicastat HCl 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). DDR1 is widely expressed in several tissues (Alves et al., 1995, Curat & Vogel, 2002, Ferri et al., 2004, Hou et al., 2001, Mohan et al., 2001, Sakamoto et al., 2001, Tanaka et al., 1998) including the mouse (Bhatt et al., 2000, Franco-Pons et al., 2009, Sanchez et al., 1994, Seo et al., 2008, Zerlin et al., 1993) and human brain (Roig et al., 2007, Yamanaka et al., 2006, Weiner et al., 2000). Alternative splicing of the DDR1 gene generates 5 different isoforms (suffixed a–e), some of which possess deletions of the juxtamembrane or kinase domains. Notably, the longest isoform (DDR1c) was originally identified in a human fetal brain library (Alves et al., 1995). In mice, expression of DDR1 in early neurodevelopmental stages is confined to several areas in which neurogenesis is occurring (Franco-Pons et al., 2006, Zerlin et al., 1993), and elongation of cerebellar granules cell axons during neurodevelopment is DDR1-dependent (Bhatt et al., 2000). Postnatally, however, there in an increase in DDR1 expression detected in oligodendrocytes, but not in neurons (Franco-Pons et al., 2006, Sanchez et al., 1994). Furthermore, in an experimental mouse model of demyelination–remyelination induced by oral administration of cuprizone, DDR1 expression is up regulated in oligodendrocytes during the remyelination period (Franco-Pons et al., 2009). However, there is a paucity of detailed data on the expression of DDR1 in the human brain. The present study examines the expression pattern of DDR1 mRNA and protein in post-mortem samples of the human cerebral cortex.