Loss of the Rho GTPase activating protein p190-B enhances hematopoietic stem cell engraftment potential

Abstract

IntroductionHematopoietic stem cells (HSCs) are defined by their unique ability to give rise to all mature immune and blood cell lineages while at the same time regenerating themselves in a process termed "self-renewal" to sustain hematopoiesis throughout life. In addition, HSCs traffic throughout the body and possess the ability to reconstitute all hematopoietic lineages on transplantation into lethally irradiated mice, characteristics that have been used in therapeutic stem cell transplantation. [1][2][3] To maintain an adequate number of both mature blood cells and HSCs, the quality and quantity of HSC divisions must be tightly controlled. 4,5 Several regulatory pathways that play a role in the maintenance of HSC functions have been identified; these include both cell-intrinsic and extrinsic factors. For example, signaling through integrins, stem cell factor, thrombopoietin, angiopoietin, and transforming growth factor-␤ regulate HSC properties. [6][7][8][9] Cell-cycle regulators, p21 Cip1 , p16 Ink4a , p18 Ink4c , and p57 Kip2 , and proteins that control transcription, such as HoxB4, c-myc, FOXO, Zfx, Tel, Gfi-1, Pbx-1, or epigenetic factor, Bmi-1, and Ezh2, 4,5,[10][11][12][13][14][15] are essential for HSC functions. A fundamental issue in HSC biology is to understand how these programs are regulated and to exploit this knowledge for the development of HSC-based therapies for therapeutic purposes.Members of the Rho GTPase family operate as molecular switches to effect signaling downstream of numerous receptors, including integrins, chemokines and cytokine receptors. 16,17 Most canonical Rho GTPases cycle between an active guanosine triphosphate (GTP)-bound and an inactive guanosine diphosphate (GDP)-bound state. This GDP-GTP cycle is tightly regulated by 3 families of proteins. Guanine nucleotide exchange factors promote the exchange of GDP for GTP, whereas GTPase-activating proteins (GAPs) accelerate the rate of hydrolysis of GTP. In addition, guanine nucleotide dissociation inhibitors may interfere with GTP binding by preventing membrane localization of the protein. Of the 20 Rho GTPases currently known, the best studied Rho GTPases are Rho, Rac, and Cdc42, which are crucial regulators of cytoskeleton dynamics, cell migration, adhesion, and cell-cycle progression. As such, Rho GTPases regulate a broad variety of cellular processes in many mammalian cells, including in hematopoietic cells. [16][17][18][19][20][21][22][23] Whereas the role of Rho GTPases in cell functions has begun to be understood, the role of GAPs and guanine nucleotide exchange factors in vivo has been understudied. Because more than 70 RhoGAPs have been identified in eukaryotes, the RhoGAPs outnumber the Rho GTPases that they regulate. 24 Some GAPs show preferential tissue expression and appear to have tissuespecific functions. Moreover, each GAP can regulate a restricted number of Rho GTPase signaling pathways. 25 Finally, the presence of several functional domains suggests that GAPs may mediate signaling pathways that are not limite...

Protein tyrosine phosphatase–σ regulates hematopoietic stem cell–repopulating capacity

“…PTPRS was expressed by a Since Ptprs -/-HSCs displayed increased repopulating capacity in vivo compared with Ptprs +/+ HSCs, this suggested that PTPσ might regulate processes involved in HSC engraftment or self renewal. We thus considered whether RAC proteins, a subset of RhoGTPases that are necessary for normal HSC engraftment capacity (14,15,19), might be regulated by PTPσ. In cell lines, it has been shown that PTPσ dephosphorylates and thereby activates p250GAP, a RhoGTPase that inhibits RAC protein activation (20).…”

“…Interestingly, BM cells from Ptprs -/-mice displayed markedly increased competitive repopulating capacity compared with Ptprs +/+ BM cells. The increased functional capacity of Ptprs -/-HSCs was associated with increased activation of the RhoGTPase RAC1 (14,15), and inhibition of RAC1 blocked the augmented migration capacity of Ptprs -/-cells. Furthermore, negative selection of human cord blood (CB) HSCs for PTPσ caused a 15-fold increase in repopulating capacity compared with human PTPσ + HSCs.…”

Niche Wnt5a regulates the actin cytoskeleton during regeneration of hematopoietic stem cells

“…It has previously been shown that RAC1 (Cancelas et al, 2005) and CDC42 (Yang et al, 2007a) regulate HSC engraftment through acting on cell shape, localization and retention, and mobilization. Not only are the small GTPases themselves important, but upstream factors like DOCK2 (Kikuchi et al, 2008), ARH GAP5 (p190-B; Xu et al, 2009), ARH GEF7 (β-PIX; Reddy et al, 2016), or PTP RS (Quarmyne et al, 2015), or downstream factors such as PAK2 (Dorrance et al, 2013) and WASF2 (WAVE2; Ogaeri et al, 2009) also affect engraftment. Our study confirms these studies and adds that the niche regulates the generation of functional HSCs by modulating the activity of this pathway in HSCs not only through secretion of ligands such as the chemokine CXCL12 but also through modifications in the expression level of critical mediators of the pathway, such as β-PIX, DOCK2, and WAVE2.…”

Cell Type-specific Signaling Function of RhoA GTPase: Lessons from Mouse Gene Targeting

“…For example, in contrast to rhoA knock-out in MEF cells (12), deletion of the GEF rgnef blocked focal adhesion formation (62). As mentioned above, deletion of the GAP p190B a major negative regulator of RhoA, led to increased long-term HSC activity and reduced progenitor activities (50), whereas rhoA knock-out resulted in a differentiation block from HSCs to blood progenitors. It is possible that in a complex process such as hematopoiesis, RhoA activity is tightly regulated by the interplay of multiple regulators.…”

“…Inhibition of RhoA activity by overexpressing dominant-negative mutant RhoA enhanced HSPC proliferation and engraftment (49). In contrast, increasing RhoA activity by knocking out of one of the major RhoA GAPs in the primitive HSPCs (p190B) also enhanced HSPC engraftment (50). These observations made it difficult to interpret the role of RhoA in regulating HSPCs in the suppression-of-function and gain-of-function models.…”