Rho gtpase activating protein p190-B regulates hematopoietic stem cell self-renewal decisions

Abstract

Hematopoietic stem cells (HSC) can self-renew or commit to differentiate into all blood cells. The signaling network that regulates these choices is poorly understood. We previously showed that p190-B loss, a negative regulator of Rho activity, enhanced long-term engraftment without altering HSC proliferation or survival or lineage development (Xu, Blood 2009), suggesting an effect on HSC fate decisions. Here, we compared single HSC divisions ex vivo by analyzing multilineage potential of the paired-daughter cells. Non-transplanted (NT) HSCs generated two multipotent daughter cells (self-renewing divisions). However, after secondary transplantation, wild-type (WT) HSCs generated one multipotent and one committed daughter cells (differentiating divisions). p190-B-/- HSCs maintained self-renewal divisions, indicating p190-B loss mediates higher probability of ‘stemness' inheritance through divisions to prevent HSC depletion following transplantation. Unexpectedly, p190-B loss does this by decreasing autocrine TGF-β. Active TGF-β protein levels, TGF-β target genes increased in WT but not in p190-B-/- HSCs upon transplantation relative to NT HSC. TGF-β signaling inhibitors restored self-renewal divisions of transplanted WT HSC as seen in vitro and in engraftment in vivo. TGF-β inhibitor treatment of transplanted mice significantly increased HSC frequency - without changing their proliferation - that yielded higher chimera in tertiary transplant compared to DMSO treated mice. Conversely, recombinant TGF-β1 changed non-transplanted HSC choice to differentiating divisions in vitro. Finally, p38MAPK activity mediated TGF-β effect on HSC fate decisions. Hence, HSC decisions to self-renew or differentiate are specified by a p190-B/TGF-β signaling pathway during HSC regeneration. This study uncovers a novel role for autocrine TGF-β in HSC fate decisions uncoupled from its role on HSC quiescence that may have important implications for regenerative medicine. Hematopoietic stem cells (HSC) can self-renew or commit to differentiate into all blood cells. The signaling network that regulates these choices is poorly understood. We previously showed that p190-B loss, a negative regulator of Rho activity, enhanced long-term engraftment without altering HSC proliferation or survival or lineage development (Xu, Blood 2009), suggesting an effect on HSC fate decisions. Here, we compared single HSC divisions ex vivo by analyzing multilineage potential of the paired-daughter cells. Non-transplanted (NT) HSCs generated two multipotent daughter cells (self-renewing divisions). However, after secondary transplantation, wild-type (WT) HSCs generated one multipotent and one committed daughter cells (differentiating divisions). p190-B-/- HSCs maintained self-renewal divisions, indicating p190-B loss mediates higher probability of ‘stemness' inheritance through divisions to prevent HSC depletion following transplantation. Unexpectedly, p190-B loss does this by decreasing autocrine TGF-β. Active TGF-β protein levels, TGF-β target genes increased in WT but not in p190-B-/- HSCs upon transplantation relative to NT HSC. TGF-β signaling inhibitors restored self-renewal divisions of transplanted WT HSC as seen in vitro and in engraftment in vivo. TGF-β inhibitor treatment of transplanted mice significantly increased HSC frequency - without changing their proliferation - that yielded higher chimera in tertiary transplant compared to DMSO treated mice. Conversely, recombinant TGF-β1 changed non-transplanted HSC choice to differentiating divisions in vitro. Finally, p38MAPK activity mediated TGF-β effect on HSC fate decisions. Hence, HSC decisions to self-renew or differentiate are specified by a p190-B/TGF-β signaling pathway during HSC regeneration. This study uncovers a novel role for autocrine TGF-β in HSC fate decisions uncoupled from its role on HSC quiescence that may have important implications for regenerative medicine.