Rho GTPase Cdc42 coordinates hematopoietic stem cell quiescence and niche interaction in the bone marrow

Abstract

Adult hematopoietic stem cells (HSCs) exist in a relatively quiescent state in the bone marrow (BM) microenvironment to fulfill longterm self-renewal and multilineage differentiation functions, an event that is tightly regulated by extrinsic and intrinsic cues. However, the mechanism coordinating the quiescent state of HSCs and their retention in the BM microenvironment remains poorly understood. In a conditional-knockout mouse model, we show that Cdc42 ؊/؊ HSCs enter the active cell cycle, resulting in significantly increased number and frequency of the stem/progenitor cells in the BM. Cdc42 deficiency also causes impaired adhesion, homing, lodging, and retention of HSCs, leading to massive egress of HSCs from BM to distal organs and peripheral blood and to an engraftment failure. These effects are intrinsic to the HSCs and are associated with deregulated c-Myc, p21 Cip1 , ␤1-integrin, and Ncadherin expressions and defective actin organization. Thus, Cdc42 is a critical coordinator of HSC quiescence maintenance and interaction with the BM niche.bone marrow microenvironment ͉ cell cycle ͉ adhesion A dult hematopoietic stem cells (HSCs) exist in a relatively quiescent state in the bone marrow (BM) microenvironment to execute long-term self-renewal and multilineage differentiation functions (1-3). The maintenance of HSC quiescence involves both extrinsic and intrinsic mechanisms. A number of genes that encode cell cycle or transcriptional regulators, including p21 Cip1 , p27 Kip1 , ␤-catenin/axin, cyclin D1, and c-Myc (4, 5), have been shown to regulate the intrinsic programs of HSCs in this process. In addition, interactions of HSCs with the BM microenvironment in specific anatomical and functional areas, referred to as niches, in the maintenance of HSC quiescence have also gained increasing recognition (6). One hypothesis is that the intrinsic and extrinsic cues, such as bone morphogenic proteins, Ca 2ϩ , Notch ligands, and/or Ang-1/Tie2 (7-10), in the BM microenvironment may coordinately regulate the HSC quiescent state.Despite of the identification of these molecular factors in HSCs and in BM that may collectively contribute to the maintenance of quiescence (7), the mechanism coordinating HSC cell cycle regulation and niche interaction remains unclear. Cdc42 is a ubiquitously expressed member of the Rho GTPase family involved in the regulation of multiple cell functions, including actin polymerization, cell-to-cell or cell-to-extracellular matrix adhesion, and gene transcription (11). Although its function has been extensively studied in various cell systems by expression of dominant negative or constitutively active mutants (12), the physiological roles of Cdc42 in most primary cell lineages, particularly in HSCs, remain unclear. Previously, in a gain-of-Cdc42 activity, Cdc42GAP Ϫ/Ϫ mouse model, we have found that constitutively increased Cdc42-GTP species cause increased hematopoietic progenitor apoptosis, disorganized actin structure, and defective engraftment without affecting the cell cycle status (13). T...

The Actin Polymerization Regulator WAVE2 Is Required for Early Bone Marrow Repopulation by Hematopoietic Stem Cells

“…Yang et al reported that in HSCs from mice conditionally deficient in Cdc42 [30], BM homing, lodging, retention, and long-term repopulation were all impaired. They also showed that Cdc42 signals positively regulate HSC quiescence, suggesting that Cdc42 is essential for physical and functional interaction between HSCs and their niches [30].…”

“…Yang et al reported that in HSCs from mice conditionally deficient in Cdc42 [30], BM homing, lodging, retention, and long-term repopulation were all impaired. They also showed that Cdc42 signals positively regulate HSC quiescence, suggesting that Cdc42 is essential for physical and functional interaction between HSCs and their niches [30]. Whether these phenotypes reflect the functions of Cdc42 in normal HSCs remains uncertain, because all mice deficient in Cdc42 die because of progressive myeloproliferative disease [31].…”

Cell intrinsic and extrinsic mechanisms of stem cell aging depend on telomere status

“…(Morrison et al, 1996;Sudo et al, 2000;Rossi et al, 2005;Waterstrat et al, 2008). Cell intrinsic: genetic factors modify HSC self renewal (Yang et al 2007). The contribution of Cell extrinsic mechanisms is unknown Cell intrinsic: Decline in repopulating capacity on a per cell basis during aging .…”

The microenvironment of the embryonic neural stem cell: Lessons from adult niches?

“…Disruption of integrin activity in the epidermal and hematopoietic niches resulted in compromised self-renewal (Frye et al, 2003;Scott et al, 2003;Hidalgo et al, 2004;Priestley et al, 2005). There could also be a role in HSC homing and engraftment as a recent report on mice with a targeted cdc42 deletion in hematopoietic cells showed a decrease in ␤1 integrin expression, with excess HSCs throughout the periphery and associated defects of HSC engrafting into bone marrow (Yang et al, 2007). Although it is unclear how integrins regulate stem cell self-renewal, loss of their interaction with the basal lamina may result in loss of integrin-mediated signaling (Hynes, 2002) or alterations in growth factor response (ffrench-Constant and Colognato, 2004) resulting in compromised biological activity.…”

“…These signaling mechanisms may occur between stem cells, between support cells and stem cells, and between stem cells and differentiating cells. Cell/cell and cell/ECM adhesion and signaling in adult niches is mediated through cell adhesion receptors such as cadherins (Song et al, 2002;Zhang et al, 2003) and integrins (Jones and Watt, 1993;Shinohara et al, 1999;Suzuki et al, 2000;Fujimoto et al, 2002;Frye et al, 2003;Scott et al, 2003;Blanpain et al, 2004;Chen et al, 2004;Deb et al, 2004;Hidalgo et al, 2004;Corbel et al, 2005;Priestley et al, 2005;Wagers and Weissman, 2005;Bajanca et al, 2006;Pajoohesh-Ganji et al, 2006;Lawson et al, 2007;Yang et al, 2007). In this model, as a cell moves away from the niche and thus away from some regulatory signals, it adopts a differentiated phenotype as shown in the pair of cells to the right.…”