A Critical Role of the Rho Family GTPase Cdc42 in Hematopoietic Stem Cell Mobilization, Homing, Engraftment and Differentiation.

The Rho family GTPase Cdc42 has emerged as a key signaling molecule with unique roles in multiple hematopoietic cell lineages. To investigate the physiologic role of Cdc42 in hematopoietic stem cells/progenitors (HSC/Ps), we bred conditional cdc42^flox/floxmice with Mx-Cre mice that allowed interferon-inducible cdc42 deletion in hematopoietic cells in vivo following polyI:C induction. Loss of Cdc42 expression in the bone marrow caused significant leukocytosis with neutrophilia in peripheral blood (PB) (Table). Concurrently, deletion of cdc42 in the bone marrow led to a massive efflux of HSC/Ps from bone marrow to PB, liver and spleen (Table) and increased proliferative activity of HSC/Ps. Consequently, flow analysis further revealed that the number of Lin^-Sca-1⁺c-Kit⁺ (LSK) HSCs in BM, PB, spleen and liver increased significantly following cdc42 deletion (Table). Transplantation of the cdc42^−/− bone marrow cells into NOD/SCID recipient mice or competitive transplantation into BoyJ recipient mice showed that cdc42 deficiency in the bone marrow caused the impaired engraftment (Table) and the failure of hematopoiesis after a transient increase of cell cycle rate of HSCs. The engraftment defect is associated with a homing deficiency (Table). The cdc42^−/− (Lin⁻c-Kit⁺) HSC/Ps showed defects in cortical F-actin assembly, adhesion to fibronectin and directional migration in response to SDF-1α (Table). These data suggest that defective actin structure and adhesion of the cdc42^−/− HSC/Ps may contribute to the loss of retention of the cells in the BM niche and to the mobilization phenotype. Moreover, deletion of cdc42 resulted in significantly reduced thymus cellularity, T cell, B cell population, and decreased Ter119⁺ cell number associated with markedly decreased CFU-E activity of the bone marrow cells (Table). Two additional complimentary approaches, BM-reconstitution by cdc42^−/− HSCs and reciprocal (rescue) transplantation by WT HSCs, further demonstrated that the Cdc42 loss-of-function phenotypes are hematopoietic cell intrinsic. Taken together these results implicate Cdc42 as a critical regulator of HSC mobilization, homing and engraftment, and indicate that Cdc42 is involved in erythropoiesis and in T- and B-cell differentiation and homeostasis.