br Results br Discussion SDF performs essential roles in

Results
Discussion SDF-1 performs essential roles in cell migration and proliferation, acts as neuromodulator; and is also involved in many pathological processes including human immunodeficiency virus infection, metastatic malignancy, and chronic inflammatory disorders (Feng et al., 1996, Li et al., 2012). Besides SDF-1α and SDF-1β, four additional splicing variants of SDF-1 have been identified (Yu et al., 2006, Janowski, 2009). Among the six known SDF-1 variants, SDF-1α is the predominant isoform and is ubiquitously expressed. SDF-1α-mediated effects have been classically attributed to its interaction with CXCR4. However, it has recently been reported that SDF-1α also binds with high affinity to CXCR7, which is a conserved G protein-coupled receptor (Burns et al., 2006, Balabanian et al., 2005). Many studies have demonstrated that the CXCR7 receptor plays roles in cardiac development as well tumor development, metastasis, and progression (Sierro et al., 2007, Zabel et al., 2011, Monnier et al., 2012, Wang et al., 2008, Sun et al., 2010). More recently, it has been reported that the CXCR7 receptor is expressed during nervous system development and tumor malignancy (Göttle et al., 2010, Odemis et al., 2011, Hartmann et al., 2008). One study found that CXCR7 might indirectly regulate CXCR4 expression in cortical interneurons to maintain normal levels of this receptor (Sánchez-Alcañiz et al., 2011). Such an emerging role of the CXCR7 receptor in both normal development and cancer is driving the ongoing efforts of researchers to find ways to therapeutically target this receptor. However, molecular interactions and signaling events following SDF-1α binding to CXCR7 remain unclear. Kucia et al. (2004) found that SDF-1α was involved in trafficking hematopoietic stem CC-223 from bone marrow to the peripheral blood, and its expression was increased in the penumbra of the ischemic brain. The interaction between SDF-1 and CXCR4 mediates hematopoietic stem cell homing to the bone marrow (Liu et al., 2012a, Liu et al., 2012b), and SDF-1 is also known to induce neural cell migration (Chen et al., 2002, Hess and Borlongan, 2008). SDF-1 can induce CXCR4-expressing cells to migrate to areas with higher concentrations of SDF-1. Taken together, these findings indicated that SDF-1/CXCR4 signaling facilitates healing by promoting BMSC recruitment to wounded tissues, which increases BMSC-secreted growth factor levels and neovascularization (Kurozumi et al., 2005). Schönemeier et al. (2008) studies indicated that SDF-1 may influence vascular, astroglial, and neuronal functions via CXCR7 and mediate cell recruitment to ischemic brain areas via CXCR4. And other studies founded that the SDF-1–CXCR4/CXCR7 axis is an attractive target pathway for improving the beneficial effect of MSC-based therapies for renal ischemic/reperfusion (Liu et al., 2012a, Liu et al., 2012b). Hypoxic preconditioning can promoted the transplanted cell regenerative capability and is an effective and therapeutic potential means for the treatment of ischemic stroke (Wei et al., 2012). However, it was not known whether BMSCs cultured in vitro express the CXCR7 receptor, and its ability to induce BMSCs migration toward an SDF-1α gradient in ischemic brain was unclear. The data described here are the first to demonstrate that hippocampal SDF-1α was gradually increased in a reperfusion-time-dependent manner after I/R injury. This finding indicates that ischemic injury promoted SDF-1α production in the ischemic area, which is consistent with previous results (Kucia et al., 2004). We next cultured BMSCs in vitro and confirmed the expression of CXCR4 and CXCR7 receptors, as well as SDF-1α expression in P3-BMSCs. After P3-BMSCs were transplanted via lateral ventricle injection, the BMSCs migrated to the ischemic hippocampus where the concentration of SDF-1α was increased. However, migration was largely weakened by treatment with either AMD3100 (CXCR4 receptor inhibitor) or a CXCR7 neutralizing antibody, and the inhibitory effect of latter was stronger than that of the former. When BMSCs were co-pretreated with the both of AMD3100 and CXCR7 neutralizing antibody, migration was further weakened. These indicated that BMSC migration was affected by both SDF-1α/CXCR4 and SDF-1α/CXCR7 signalings, and the effect of CXCR-7 was stronger than that of CXCR-4.