Rejuvenation of aged hematopoietic stem cells and endothelial niches by thrombospondin-1 blockade

PROJECT SUMMARY
There is a substantial need for the development of preventative and therapeutic options to delay the aging
process, to rejuvenate tissue/organs, and to enhance regeneration and repair. Aging of the blood system is
associated with loss of vascular integrity and dramatic changes in hematopoietic stem cell (HSC) function. In
addition to increasing in number and losing self-renewal potential, old HSCs exhibit a myeloid bias and increased
propensity to develop hematologic malignancies. While some of these changes reflect cell-intrinsic alterations,
emerging evidence suggests that some of these defects may also be regulated by the bone marrow (BM)
microenvironment, in particular the BM endothelial niche. In particular, we have demonstrated that the BM
endothelial niche is indispensable for supporting the balance between HSC self-renewal and differentiation
following myeloablative insult. We have also shown that AKT/mTOR signaling in BM endothelial cells (BMECs)
is required for maintenance of the HSCs and that this signaling is disrupted in aged BMECs. To ask how a
disruption of the AKT/mTOR signaling axis in ECs affects the hematopoietic system, we have conditionally
deleted mTOR in adult ECs (mTOR(ECKO) mice) and found that this deletion resulted in the premature aging of
HSCs as demonstrated by their phenotypic, functional, and transcriptional analysis. To better understand how
EC-mediated signaling drives the aging of the hematopoietic system, we utilized our mTOR(ECKO) mouse model
as a screening tool to discover as yet unknown pro-HSC-aging factors. Transcriptomic and proteomic studies
identified Thrombospondin-1 (TSP1) as a candidate pro-aging factor that was commonly upregulated in
mTOR(ECKO) and aged mice when compared to young controls. Moreover, global knockout of TSP1 resulted in
preservation and rejuvenation of HSC function in aged mice. This research proposal is designed to test whether
it is possible to: 1) reverse the age-related phenotypes of the hematopoietic and vascular systems by global or
cell-specific deletion, or antibody-mediated inhibition, of TSP1, 2) enhance the ex vivo expansion and function
of aged HSC by exogenous inhibition of TSP1, and 3) rejuvenate an aged BM microenvironment following
myelosuppressive treatments by utilizing young BMECs as a cellular therapeutic with or without the co-infusion
of a neutralizing antibody to TSP1. The proposed studies will utilize novel in vivo and in vitro models developed
in our lab and, if successful, will unlock the therapeutic potential of TSP1 inhibition to improve overall healthspan,
longevity, and regeneration of the hematopoietic system in the elderly. The success of this research proposal
will open up new avenues for the development of a wide array of therapeutic strategies designed as an effective
means to reverse age-related hematopoietic deficiencies.

Grant Number: 5R01AG065436-06
NIH Institute/Center: NIH
Principal Investigator: Jason Butler