Growth Factor Receptor-Bound Protein 10 (Grb10) Regulates Hematopoietic Stem Cell (HSC) Self-Renewal Via Control of mTOR Signaling

Elucidation of the mechanisms governing HSC regeneration has been impeded by difficulty in isolating HSCs early following genotoxic injury, such as total body irradiation (TBI). Using multiparametric flow cytometric cell sorting of BM ckit+sca-1+lin- cells coupled with gene expression analysis, we identified growth factor receptor-bound protein 10 (Grb10), a co-receptor which regulates Insulin Receptor/IGF-1 signaling, to be significantly overexpressed by BM KSL cells at the earliest detectable point of regeneration (day +10) following TBI (3.3-fold, p<0.0001).

Grb10 is a member of the imprinted gene family which is predominately expressed in the stem cells of a variety of tissues, including embryonic stem cells, bone marrow, skin and muscle. Viral shRNA-mediated knockdown of Grb10 in BM KSL cells caused a significant decrease in KSL cells and colony forming cells (CFCs) detected in 7-day culture (p=0.03 and p=0.002, respectively). Furthermore, mice competitively transplanted with Grb10-deficient HSCs displayed 10-fold lower donor, multilineage hematopoietic cell engraftment than mice transplanted with Grb10-expressing HSCs (p=0.007 for %CD45.1⁺ donor cells). Secondary competitive repopulation assays confirmed a greater than 10-fold deficit in long-term repopulating capacity in Grb10-deficient KSL cells compared to Grb10-expressing KSL cells (p=0.006 for %CD45.1⁺ donor cells).

In order to determine if Grb10 was necessary for HSC maintenance and normal hematopoiesis in vivo, we generated maternally-derived Grb10-deficient mice. Heterozygous 8 week old Grb10^m/+ (1 mutant allele, 1 wild type allele) had 10-fold decreased Grb10 expression in BM lin-cells. BM CFCs and SLAM⁺ KSL cells were significantly decreased in Grb10^m/+ mice compared to Grb10^+/+ mice (p=0.006 and p=0.04, respectively). Competitive repopulation assays demonstrated significantly decreased donor hematopoietic cell repopulation in recipient mice transplanted with Grb10^m/+ BM cells versus mice transplanted with Grb10^+/+ BM cells (p=0.003 for %CD45.1+ donor cells). Mice transplanted with BM cells from homozygous Grb10^-/- mice showed a similar decrease in donor-derived hematopoietic repopulation compared to mice transplanted with BM cells from Grb10^+/+ mice (p=0.02 at 20 weeks post-transplantation). These results confirmed that Grb10 regulates HSC self-renewal capacity in vivo.

To determine whether Grb10 regulates HSC regeneration after myelotoxic injury, we irradiated Grb10^m/+ mice with 550cGy TBI, and monitored hematopoietic recovery over time in comparison to Grb10^+/+ controls. Interestingly, Grb10^m/+ mice displayed accelerated hematopoietic regeneration early following TBI. At day+10 after 550cGy, Grb10^m/+ mice contained significantly increased numbers of BM SLAM⁺ KSL cells (p=0.04) and CFCs (p=0.009), compared to Grb10^+/+ littermates. Similarly, mice transplanted with BM cells from irradiated, Grb10^m/+ mice displayed 5-fold increased donor hematopoietic repopulation at 20 weeks post-transplantation compared to mice transplanted with BM cells from irradiated, Grb10^+/+ mice (p=0.006). These data suggest that Grb10 deficiency accelerates hematopoietic recovery in the early period following myelosuppressive radiation injury.

Mechanistically, Grb10-deficiency caused an increase in the percentage of BM KSL cells in G1 and G2/S/M phase of cell cycle compared to Grb10^+/+ KSL cells (p=0.003). We also observed significantly increased levels of mTOR activation in Grb10^m/+ BM KSL cells compared to Grb10^+/+ BM KSL cells (p=0.001 for pS6, p=0.001 for pS6k and p=0.02 for p4EBP1). Furthermore, mTOR inhibition via siRNA-mTOR targeting rescued the defect in BM hematopoietic progenitor content (colony forming cells) in Grb10-deficient BM cells (p<0.0001). Taken together, our results suggest that Grb10 is necessary for HSC maintenance in steady state, while, paradoxically, Grb10 inhibition accelerates HSC regeneration early following injury. Furthermore, our data suggest that Grb10 mediates these effects via regulation of mTOR signaling. Selective modulation of Grb10 signaling has the potential to augment HSC self-renewal in steady state and to accelerate HSC regeneration following myelotoxic injury.