Our western blotting results showed that phospho
Our western blotting results showed that phospho-DDR1 levels continued to increase after I/R injury, and the overall activation trend between phospho-DDR1 and MMP-9 was the same. We found that phospho-DDR1 expression started to increase significantly at 12h after I/R injury, which may be related to characteristics of DDR1-specific phosphorylation. Previous studies have shown that compared with other classical receptor tyrosine kinases, such as epidermal growth factor receptor (EGFR), phosphorylation of DDR1 is relatively slow and long-lasting, requiring at least 18h to reach full activation and maintaining this level for 4 days (Carafoli and Hohenester, 2013). Our study indicated that phospho-DDR1 increased significantly at 12h after reperfusion, peaked at 24h and continued for 48h. In addition, recent studies have shown that the damage to the BBB after ischemic injury begins at 20min after injury, is significantly increased at 12h after ischemia, and continues to increase until a few weeks after ischemia (Khatri et al., 2012). This shows that there is consistency between the changes in phospho-DDR1 and the opening trends of BBB permeability after ischemic injury. It also shows that DDR1 is associated with BBB destruction after Compound C injury.
Previous studies have shown that inhibiting MMP-9 expression after I/R injury decreases the BBB permeability and brain damage, which in turn improves neurobehavioral scores and reduces infarct size in rats after brain injury (Piao et al., 2009). Our study also found that inhibiting DDR1 activity significantly reduced MMP-9 levels, thereby reducing the infarct size, and neurobehavioral scores were also improved. This indicates that there is a DDR1 and MMP-9 pathway, which is similar to that in tumor tissue, in the central nervous system; DDR1-siRNA also lowered MMP-9 expression and activity by inhibiting DDR1 activity, and decreased the BBB permeability and cerebral ischemic injury.
Numerous factors contribute to BBB destruction because of its complex molecular mechanism. Glucocorticoids (GCs) are the most widely used treatment method for the BBB, but they have many side effects and their treatment effects are not specific (Tuckermann et al., 2005). DDR1 is a receptor tyrosine kinase that is associated with a variety of cellular functions, such as cell proliferation, differentiation, adhesion, migration and invasion. DDR1 is expressed in normal tissues of organs such as the lungs, kidneys, colon, and brain as well as in the tumor cells of epithelial origin, such as those of breast, ovarian, and lung carcinomas (Vogel, 1999). In addition, MMP-9 can cause damage to the BBB (Yang and Rosenberg, 2011) by degrading type IV collagen in the basement membrane during cerebral ischemia. Type IV collagen, as a ligand of DDR1, can activate DDR1 to independently cause phosphorylation (Fu et al., 2013). Therefore, there may be a vicious cycle between type IV collagen and DDR1-MMP-9 in inflammatory cells, endothelial cells, and glial cells, which is involved in destruction of the BBB after cerebral ischemic injury. This mechanism can cycle and cause serious damage to the basement membrane, which in turn causes further brain damage, but the exact mechanism between DDR1 and MMP-9 remains unknown. Further research is required to clarify this mechanism.
Acknowledgements Financial support from the National Nature Science Fund (nos. 81070966 and 81000563) is gratefully acknowledged.
Introduction Discoidin Domain Receptor 1 (DDR1) is a receptor tyrosine kinase that binds collagens  and has been implicated in the regulation of multiple cellular functions including migration , , , , cytokine secretion , , , and extracellular matrix homeostasis/remodeling , , . DDR1 is required for normal tissue development, but the function of DDR1 in adult tissues particularly in diseased tissues is poorly understood , . DDR1 contributes to cancer ,  and promotes inflammation in models of atherosclerosis ,  and lung fibrosis , but the mechanisms whereby DDR1 contributes to disease progression are not clear.