Person:

Williams, David

Engraftment and maintenance of hematopoietic stem and progenitor cells (HSPC) depend on their ability to respond to extracellular signals from the bone marrow microenvironment, but the critical intracellular pathways integrating these signals remain poorly understood. Furthermore, recent studies provide contradictory evidence of the roles of vascular versus osteoblastic niche components in HSPC function. To address these questions and to dissect the complex upstream regulation of Rac GTPase activity in HSPC, we investigated the role of the hematopoietic-specific guanine nucleotide exchange factor Vav1 in HSPC localization and engraftment. Using intravital microscopy assays, we demonstrated that transplanted (Vav1^{−/−}) HSPC showed impaired early localization near (nestin^+) perivascular mesenchymal stem cells; only 6.25% of (Vav1^{−/−}) HSPC versus 45.8% of wild-type HSPC were located less than 30 μm from a (nestin^+) cell. Abnormal perivascular localization correlated with decreased retention of (Vav1^{−/−}) HSPC in the bone marrow (44–60% reduction at 48 h posttransplant, compared with wild-type) and a very significant defect in short- and long-term engraftment in competitive and noncompetitive repopulation assays (<1.5% chimerism of (Vav1^{−/−}) cells vs. 53–63% for wild-type cells). The engraftment defect of (Vav1^{−/−}) HSPC was not related to alterations in proliferation, survival, or integrin-mediated adhesion. However, (Vav1^{−/−}) HSPC showed impaired responses to (SDF1\alpha), including reduced in vitro migration in time-lapse microscopy assays, decreased circadian and pharmacologically induced mobilization in vivo, and dysregulated Rac/Cdc42 activation. These data suggest that Vav1 activity is required specifically for (SDF1\alpha)-dependent perivascular homing of HSPC and suggest a critical role for this localization in retention and subsequent engraftment.

Rho GTPases are members of the Ras superfamily of GTPases that regulate a wide variety of cellular functions. While Rho GTPase pathways have been implicated in various pathological conditions in humans, to date coding mutations in only the hematopoietic specific GTPase, RAC2, have been found to cause a human disease, a severe phagocytic immunodeficiency characterized by life-threatening infections in infancy. Interestingly, the phenotype was predicted by a mouse knock-out of RAC2 and resembles leukocyte adhesion deficiency (LAD). Here we review Rho GTPases with a specific focus on Rac GTPases. In particular, we discuss a new understanding of the unique and overlapping roles of Rac2 in blood cells that has developed since the generation of mice deficient in Rac1, Rac2 and Rac3 proteins. We propose that Rac2 mutations leading to disease be termed LAD type IV.