| An unbiased screen for novel regulators of HSC expansion has never been carried out, partly due to the infrequency of the self-renewal event under homeostatic conditions. Here, we set out to conduct such a screen, utilizing a cDNA library from HSCs expanding during regeneration in hopes of identifying new regulators of the regenerative response. From this screen, we unexpectedly identified the Adiponectin Receptor1 (AdipoR1), a traditionally metabolic factor, as an HSC growth factor. Based on these results we then focused on examining whether other metabolic factors may play a role in HSC maintenance.;Following identification of AdipoR1 in our library screen for mediators of HSC expansion, we investigated the role of Adiponectin, a secreted molecule best known for its role in increasing fatty acid oxidation and glucose uptake, in hematopoietic stem cell function. We have shown that Adiponectin is expressed by components of the HSC niche and its' receptors AdipoR1 and AdipoR2 are expressed by HSCs. At a functional level, Adiponectin influences HSCs by increasing their proliferation in vitro, while retaining the cells in a functionally immature state as determined by in vitro and in vivo assays. We also demonstrate that Adiponectin signaling is required for optimal HSC proliferation both in vitro and in long term hematopoietic reconstitution in vivo. Finally we show that Adiponectin stimulation activates p38 MAPKinase, and that inhibition of this pathway abrogates Adiponectin's proliferative effect on HSCs. These studies collectively identify Adiponectin as a novel regulator of HSC function and suggest that it acts through a p38 dependent pathway.;The identification of Adiponectin as an HSC growth factor led us to question whether this factors' ability to increase fatty acid oxidation or glucose uptake could play a role in its ability to enhance HSC maintenance, or in general whether modulating the metabolic state of a stem cell could alter its function. We have shown that Adipnectin's effect is dependent on p38 activation, and it has been proven in other studies, that Adiponectin mediated p38 activation leads to enhanced glucose uptake. Thus, we sought to determine whether increasing glucose uptake and metabolism could modulate HSC maintenance. We examined the role of glucose metabolism in stem cell maintenance and showed that glucose uptake and utilization plays an important role in maintaining hematopoietic progenitors in an undifferentiated state in vitro and in vivo. Specifically, hematopoietic progenitor cells have higher glucose uptake capacity than differentiated cells. In addition, overexpression of the glucose transporter Glut1 is sufficient to expand hematopoietic progenitor cell number in vitro by delaying cell differentiation. Importantly, this Glut1 induced delay in differentiation leads to significantly increased long term hematopoietic reconstitution in vivo. Furthermore, reduction of glucose uptake and inhibition of glycolysis results in abrogation of the Glut1 induced differentiation delay, suggesting that the effects observed are dependent on glucose transport. Finally, we show that altering glucose uptake in hematopoietic progenitor cells leads to transcriptional changes in the expression HoxB4 and p21, factors that have been implicated in hematopoietic stem cell renewal. Cumulatively these data provide evidence for a novel role for glucose transport in hematopoietic progenitor cell maintenance and suggest that metabolic factors may play a role in modulating the balance between stem cell renewal and commitment. |
