CXCR7 Mediated Neural Progenitor Cell Migration and Signaling in the Absence of CXCR4

Migration of Neural progenitor cells (NPCs) is an essential process for both proper brain development and neuroregeneration after brain injury. Stromal cell-derived factor-1 (SDF-1, CXCL12) is highly expressed in the brain and is central to the NPC migration. Although CXCL12 is well known to regulate NPC migration through its traditional receptor CXCR4, recent identification of CXCR7 as a novel CXCL12 receptor has raised questions about the nature of the CXCR7 functions previously attributed to CXCR4. The prevailing current hypotheses for CXCR7 are that CXCR7 may either function as a decoy receptor, or as a "normal" receptor signaling through mitogen-activated protein kinases (MAPK) and beta-arrestin. To determine the exact role of CXCR7 in NPC migration, we used in vitro NPC cultures derived from CXCR4 knockout and the corresponding wild type mice with the assistance of two migration assays, including stripe assay and transwell chemotaxis assay. We found that antagonists for either CXCR4 or CXCR7 blocked the CXCL12-mediated NPC migration, suggesting that both receptors are required for NPC migration. Interestingly, CXCR4 knockout NPCs can still migrate to CXCL12, indicating CXCR7 could serve as a functional receptor in CXCL12-mediated NPC migration in the absence of CXCR4. To further determine the molecular mechanism for CXCR7-mediated migration, we investigated CXCL12-induced polarization in the CXCR4-/- NPCs and found that CXCR7 and Rac1 co-localized in the leading edge of the polarized cells, which implicates a role of Rac1 in NPC migration. Therefore, we further investigated the interaction between CXCR7 and Rac1, and activation of Rac1 and ERK1/2 after CXCL12 treatment in the CXCR4-/NPCs. Indeed, ERK1/2 and Rac1 inhibitors blocked CXCL12-mediated migration of CXCR4-/- NPCs, indicating that ERK1/2 and Rac1 activation are required for CXCR7-mediated NPC migration. Together these results reveal the essential role of CXCR7 in CXCL12-mediated NPC migration in the absence of CXCR4. Identifying the independent role of CXCR7 in NPC migration may help understand neurogenesis during development and neural repair after brain injury. CXCR7 may become a novel therapeutic target that could mobilize NPCs for regenerative therapy. To study the effect of CXCL12 on NPC migration in vivo, we also performed parallel intracranial injection of human NPCs and chemokines, including IL-1beta, CXCL12, and CCL2, into Severe Combined Immunodeficiency (SCID) mouse brains. CXCL12 and CCL2 secreted by astrocytes in the presence of IL-1beta injection were attractive to the injected NPCs, suggesting that CXCL12 and CCL2 play important roles in NPC migration during neuroinflammation. Identification of this chemotaxis cue could help develop new therapeutic approach to guide transplanted stem cells to the site of injury for brain repair.