The GEF Lsc Regulates Hematopoietic Stem Cell Motility, Mobilization and Recruitment.

Lsc is a Rho GTPase guanine nucleotide exchange factor (RhoGEF) that physically and functionally links G-protein coupled receptors (GPCR) to the monomeric GTPase RhoA in mature hematopoietic and other cells. Lsc−/− (LscKO) mice have a peripheral leukocytosis, abnormal neutrophil and B cell motility, and immune response deficiencies. Although Lsc is required for neutrophil homeostasis, its role in hematopoietic stem and progenitor cells is unknown. In this study, we have used LscKO mice to determine if Lsc is required for normal stem cell motility and mobilization. Initially, we used immunofluorescence labeling to demonstrate that hematopoietic stem and progenitor cells express Lsc. This suggested that Lsc may be required for normal hematopoietic stem and progenitor cell migration. Stromal-cell derived factor-1 (SDF-1) is a potent chemokine for hematopoietic stem cells and activates the CXCR4 GPCR. It has been reported that Lsc is not required for SDF-1-stimulated migration of mature murine T and B cells. However, using a bare-filter transwell assay, we found that while LscKO Sca-1+ cells and Sca-1+Lin- cells have normal spontaneous migration, they have significantly increased SDF-1-stimulated migration compared to their wild-type (WT) counterparts, 1.4 and 2.3 fold, respectively. We then demonstrated that adhesion of LscKO Sca-1+ cells to bone marrow (BM) stromal MS-5 cells was normal, indicating that impaired adhesion was not responsible for the abnormal SDF-1-stimulated migration. Using colony assay, we demonstrated that LscKO mice have a normal number of circulating peripheral stem and progenitor cells. Strikingly, after 5 days of G-CSF administration, LscKO mice have 1.6 fold and 2.3 fold the number of peripheral mature WBC and stem and progenitor cells (colony forming units), respectively, compared to WT mice. Recruitment of BM CXCR4+ pro-angiogenic stem and progenitor cells has been linked to enhanced tumor angiogenesis. Because LscKO BM cells had abnormal SDF-1-stimulated migration and mobilization, we hypothesized that Lsc might regulate tumor angiogenesis as well. To this end, we assessed tumor growth in LscKO mice by injecting congenic Lewis lung carcinoma cells subcutaneously into LscKO mice and WT controls. Preliminary experiments revealed that tumors were 3.3 times larger in the LscKO mice as compared to WT mice. Quantification of the tumor vessels with anti-CD31 staining demonstrated that the tumors in LscKO mice were 1.4 fold more vascularized than controls. In summary, our results demonstrate that the Rho GEF Lsc is essential for normal hematopoietic stem cell migration and mobilization. In addition, we propose that absence of Lsc facilitates tumor growth by promoting BM stem and progenitor cell recruitment to the neo-angiogenic vessels, possibly augmenting tumor vascularization.