Inhibition of Endothelial mTOR Drives Hematopoietic Stem Cell Aging

Aging leads to a drastic decline in hematopoietic stem cell (HSC) function. In addition to a loss of their self-renewal potential, old HSCs exhibit a myeloid biased differentiation and an increased propensity to develop hematologic malignancies. While some of these age-related changes reflect cell-intrinsic alterations within HSCs, recent findings suggest that signals from the bone marrow (BM) microenvironment, in particular the BM vascular niche, might play crucial roles in regulating HSC aging. In support of this idea, we recently discovered that physiological aging of BM endothelial cells leads to an altered molecular crosstalk between the endothelial niche and HSCs that instructs young HSCs to behave as aged HSCs. We have previously demonstrated that AKT/mTOR activation within young BM endothelial cells supports balanced HSC self-renewal and differentiation. Based on these observations, we hypothesized that deficient AKT/mTOR signaling within endothelium will deprive the hematopoietic system of proper endothelial-derived instructive signals resulting in HSC aging phenotypes. We have generated preliminary data demonstrating that aged endothelial cells, in contrast to aged HSCs, display decreased AKT/mTOR signaling. Furthermore, our data indicates that pharmacological inhibition of mTOR signaling using Rapamycin, a widely regarded rejuvenating agent and clinically utilized immunosuppressant, has deleterious effects on the hematopoietic system, in part due to its adverse impact on endothelium by abolishing both mTORC1 and mTORC2 signaling, in homeostatic conditions and following myelosuppressive therapy (i.e. radiation and chemotherapy). To formally determine whether endothelial-specific inhibition of mTOR signaling can promote hematopoietic aging, we conditionally deleted mTOR in ECs (mTOR(ECKO)) of young mice and observed that HSCs from mTOR(ECKO) mice displayed all the phenotypic and functional attributes of an aged HSC including loss of cell polarity, increased DNA damage, myeloid biased output and loss of engraftment potential during serial transplantation. Gene expression profiling of HSCs isolated from young mTOR(ECKO) mice revealed that their intrinsic gene expression signature resembled aged HSCs, both at steady state and following transplantation. Notably, these aging-like phenotypic and functional HSC alterations are only maintained in secondary transplants that were performed in an mTOR(ECKO) microenvironment confirming the role of EC-derived