Cytokines regulate postnatal hematopoietic stem cell expansion: opposing roles of thrombopoietin and LNK

Abstract

The role of cytokines as regulators of hematopoietic stem cell (HSC) expansion remains elusive. Herein, we identify thrombopoietin (THPO) and the cytokine signaling inhibitor LNK, as opposing physiological regulators of HSC expansion. Lnk −/− HSCs continue to expand postnatally, up to 24-fold above normal by 6 mo of age. Within the stem cell compartment, this expansion is highly selective for self-renewing long-term HSCs (LTHSCs), which show enhanced THPO responsiveness. As self-renewing hematopoietic stem cells (HSCs) are not only a requisite for sustained normal blood cell production but also of critical importance for rapid and sustained reconstitution after bone marrow (BM) transplantation, identification and clinical exploitation of the stem cell self-renewal machinery have been a longsought goal of HSC research (Sorrentino 2004). HSCs cycle and expand extensively during fetal development and following transplantation (Pawliuk et al. 1996; Iscove and Nawa 1997), providing compelling evidence for a positive and dynamic regulation of HSC self-renewal. However, from early postnatal life the HSC compartment is sustained at a relatively constant level (Morrison et al. 1996). This tight restriction on postnatal HSC expansion, potentially reflecting the enhanced propensity of HSCs to undergo leukemic transformation (Reya et al. 2001), implicates either the coexistence of important negative regulators of HSC self-renewal, or down-regulation of positive regulators in steady-state hematopoiesis. LnkAlthough HSCs represent the best characterized somatic stem cells, the physiological regulators of HSC self-renewal and expansion remain largely unknown. However, a few intrinsic cues-including the transcriptional repressor BMI-1 ( . Despite more than two decades of exhaustive attempts to promote ex vivo HSC expansion using HSCstimulating cytokines, most efforts have rather resulted in HSC loss, and only in a few cases has limited HSC expansion been achieved (Miller and Eaves 1997;Zhang and Lodish 2005). Consequently, the enthusiasm to utilize cytokines for HSC ex vivo expansion has waned, and whether cytokines play any role in regulating HSC expansion under physiological conditions has increasingly been questioned (Sauvageau et al. 2004;Jacobsen 2005). In fact, of all cytokines investigated, only genetic deficiencies in Thrombopoietin (Thpo) or its receptor Mpl result in reduced levels of HSCs (Kimura et al. 1998;Solar et al. 1998). As THPO efficiently suppresses apoptosis (Pestina et al. 2001) and promotes survival of HSCs (Borge et al. 1996), it has been suggested that THPO may act primarily to counteract HSC apoptosis, rather than promoting HSC expansion.Recently, mice deficient in expression of Lnk, a member of the APS family of adaptor proteins (Yokouchi et al. 1997), were demonstrated to have a variety of hematopoietic phenotypes, including an expansion of the HSC compartment (Takaki et al. 2000;Ema et al. 2005). However, the dynamics of this expansion and the mechanism of action by which LNK might restrict HSC expa...

Negative Regulation of Haematopoiesis: Role of Inhibitory Adaptors

“…As stated before, Lnk is highly expressed in HSC, so as expected, Lnk -/--derived HSC show an increased capacity to proliferate and to self-renew together with an increase in the quiescent fraction. These effects on HSC homeostasis are due to abnormal TPO signalling in these cells, that results from an enhanced TPO hypersensibility, increased TPO-dependent activation of Akt, STAT5 and down-regulation of p38MAPK (Ema et al, 2005;Buza-Vidas et al, 2006;Seita et al, 2007;Bersenev et al, 2008). These findings therefore confirm that Lnk controls TPO-induced self-renewal, quiescence and proliferation of HSC.…”

“…In contrast to SH2-B and APS, Lnk is mainly expressed in haematopoietic cells, notably in haematopoietic stem cells (HSC), and haematopoietic (lymphoid and myeloid) progenitors. Moreover, Lnk expression is upregulated by certain cytokines important for the development and function of these haematopoietic cells, such as SCF, thrombopoietin [TPO], and erythropoietin [EPO] (Kent et al, 2008;Buza-Vidas et al, 2006;Gerry et al, 2009aGerry et al, , 2009bBaran-Marszak et al, 2010). Interestingly, Lnk is also highly expressed in endothelial cells and its expression is also induced by Tumor Necrosis Factor (TNF)- (Fitau et al, 2006;Kwon et al, 2009).…”

Tumor necrosis factor restricts hematopoietic stem cell activity in mice: involvement of two distinct receptors

“…Loss of function studies have implicated cellintrinsic factors such as p21 (Cheng et al, 2000), GFI1 (Hock et al, 2004), and LNK (Buza-Vidas et al, 2006) as physiological negative regulators of hematopoietic stem cell (HSC) maintenance. Several extrinsic factors, including TGF- (Larsson and Karlsson, 2005), IFN- (Yang et al, 2005), and IL-3 (Yonemura et al, 1996), as well TNF Dybedal et al, 2001), have been demonstrated to negatively affect HSC growth and maintenance in vitro.…”

Regulation of hematopoietic stem cells by their mature progeny

“…TPO is produced primarily in the liver (9) and Mpl is expressed on platelets, megakaryocytes and their progenitors, and within the HSC compartment, including on cells with long-term hematopoietic reconstituting capacity (6,10,11). Several lines of evidence support a model in which TPO production is constant and that the concentration of available circulating TPO is controlled by the mass of Mpl receptors available to internalize the cytokine.…”