Rac signaling in osteoblastic cells is required for normal bone development but is dispensable for hematopoietic development

Abstract

IntroductionHematopoietic stem cells (HSCs) give rise to all lineages of mature blood cells and maintain hematopoiesis in vivo through a balance of self-renewal and differentiation. To maintain this balance, HSCs are supported within a complex milieu known as the hematopoietic microenvironment (HM) or HSC niche. 1,2 This HM includes cellular components (osteoblastic cells, 3,4 perivascular cells, 5 and sympathetic neurons 6 ), bone mineral matrix, 7 and ionic gradients. 8 Trabecular bone appears to be particularly important in HSC biology 3,9,10 ; however, there is ongoing controversy regarding the existence or identity of one predominant cell type that is necessary and sufficient for HSC survival in vivo. Apparently conflicting results have identified osteoblastic cells, 3,11-13 perivascular cells, 5 and a nestin-positive common precursor cell type with the ability to differentiate into either lineage as key cells within the HSC niche. Irrespective of this controversy, it has been firmly established that key ligand/receptor signaling interactions are responsible for HSC engraftment and mobilization from the HM. These include the interactions between CXCL12 (also known as SDF1) and CXCR4,14,15 between the cKit receptor and SCF, 16 and between fibronectin and ␤1-integrins. 17,18 The Rac family of Rho GTPases (encompassing Rac1, Rac2, and Rac3) integrates a critical downstream common pathway of the aforementioned signaling pathways. Through this, Rac proteins regulate the homing, engraftment, mobilization, and survival of HSCs in vivo (for a recent, comprehensive review, see Cancelas and Williams 19 ). Deletion of Rac1 in HSCs causes failed HSC engraftment and reduced HSC proliferation in vivo. 20 Deletion of Rac2 alone has modest but significant effects on HSC mobilization and engraftment 21 and leads to reduced HSC survival through impaired growth factor signaling and increased apoptosis. 20 Combined deletion of Rac1 and Rac2 causes a massive egress of HSCs from the HM and profoundly impaired engraftment. 20,22 Vav1, a hematopoietic-specific guanine exchange factor for Rac, differentially regulates endosteal/osteoblast and perivascular retention and subsequent engraftment. 23 Moreover, Rac1 and Rac2 were shown to be important for the survival of leukemia stem cells in a murine model of chronic myeloid leukemia. 24 These findings were reinforced by the development and preclinical testing of NSC23766, a small-molecule inhibitor of Rac signaling that has substantial in vivo effects including HSC mobilization 22 and antileukemic efficacy. 24 Whereas Rac3 is widely expressed with high levels of expression in the CNS 25 and Rac3-deficient mice show no obvious hematopoietic phenotype, we have previously shown functional redundancy of Rac3 in leukemic cells expressing p210-BCR-ABL. 24 Interestingly, despite the breadth of knowledge regarding the cell-intrinsic requirements for Rac signaling in HSC function, little is known about the function of Rac within HM components and therefore about cell-extrinsic Rac signaling...

Tolerance induction with gene-modified stem cells and immune-preserving conditioning in primed mice: restricting antigen to differentiated antigen-presenting cells permits efficacy

“…27 Long-term HSC and committed progenitor cell analyses were as described. 28,29 Data were collected using a FACScanto or FACS LSR Fortessa (BD Bioscience) and analyzed using DIVA Version 5.0.1 (BD Bioscience) or FlowJo Version 6.3 (TreeStar) software. Engraftment is expressed as the proportion of donor cells within the lineage analyzed or within the total CD45 ϩ cell population.…”

Ibandronate increases the expression of the pro-apoptotic gene FAS by epigenetic mechanisms in tumor cells

“…Recently, it has been demonstrated that inhibition of prenylation by bisphosphonates rather activates than inactivates the RHO family of small-GTPases including RAC, CDC42 and RHO [9]. Interestingly, osteoblast-restricted Rac1 deletion leads to defective bone acquisition in vivo and knockdown impairs growth and induces apoptosis in the osteoblast cell line OP9 [68]. These data suggest that GGPP increases interaction of RAC1 with cellular membranes, which results in reduced activated RAC1 leading to decreased proliferation and increased apoptosis.…”

“…However, results of this study suggest that in addition to RAS, other members of small-GTPases must be involved in regulation of FAS promoter methylation as well. Differences in responses to ibandronate of the epithelial CCL-51 cells compared to the mesenchymal cells suggest involvement of RAC1, a gene that has been found important for osteoblastic proliferation, differentiation and apoptosis [68]. The use of specific inhibitors of small-GTPases will help to clarify the mechanism of the selective epigenetic activation of FAS and other epigenetic silenced tumor suppressor genes [8,15].…”

Spontaneous mutation of Dock7 results in lower trabecular bone mass and impaired periosteal expansion in aged female Misty mice

“…Rac1 activity is important for normal bone development as well as adhesion, proliferation, and differentiation of osteoblasts in addition to controlling osteoclast number [39, 40]. Therefore, loss of Dock7 , a modulator of Rac1 and Cdc42 [8, 9, 14, 41], may in fact be altering osteoblast and osteoclast differentiation and function.…”