Neuropeptide Y Attenuates Aging-Induced Bone Marrow Niche Defects and Rejuvenate Aged Hematopoietic Stem/Progenitor Cells

Aging-associated defects in hematopoiesis are linked to declining in hematopoietic stem cell (HSC) self-renewal and multilineage differentiation capacity. The bone marrow (BM) microenvironment/niche has been suggested to influence HSC aging. However, the factors/mechanisms whereby the BM niche mediates hematopoietic stem and progenitor (HSPC) aging are poorly understood. We recently discovered that neuropeptide Y (NPY) plays a crucial role in HSPC trafficking by regulating the BM vascular gateway function (PMCID: PMC5707149). NPY is one of the most abundant neurotransmitters released in the BM by sympathetic nerve fibers. In this study, we found that the aging-induced deficit of NPY levels in the BM plays a critical role in niche defects and HSPC aging by impairing the molecular pathways involved in regulating oxidative stress, senescence, and stemness. The measurement of NPY levels in the BM of young (3-4 months) and aged (18-23 months) mice showed lower NPY levels in aged mice than in young mice (2.2-fold), which was associated with a decrease in mesenchymal stem cells (MSC) (2-fold, p=0.008) and endothelial cells (EC) (1.5-fold, p<0.05) numbers in the BM. In addition, we found that circulating NPY levels decline in the older human population (65-83 years) than in young/ middle age (23-55 years) individuals (1.5-fold, p=0.01). We hypothesized if aging-induced NPY deficit contributes to BM niche defects and HSPC aging, then NPY supplementation may attenuate/revert these processes. To explore this possibility, we administered exogenous NPY in aged mice (100ng/mouse) for 15 consecutive days and evaluated HSPC and niche cells. NPY supplementation in aged mice (18-23 months) substantially reverted aged HSPC phenotype, including NPY decreased the accumulation of HSC-enriched SLAM LSK (Lin-Sca-1+c-Kit+CD48-CD150+) in the BM (1.8-fold, p<0.05) and increased the HPC clonal expansion (1.9-fold, p<0.05) measured by flow cytometry and CFU-C assay, respectively. Competitive BM transplantation studies using donor cells from young, aged, or NPY-treated aged mice into lethally irradiated syngeneic mice showed myeloid biased repopulation from aged mice donor cells than young mice donor cells, and that was attenuated in NPY-treated aged mice donor cells (33%, p=0.003). In addition, NPY substantially reverted aging-related reduction in BM MSC (CD45-Ter119-CD31-Nestin+PDGFR+CD51+) and EC (CD45-Ter119-CD31+VE-cadherin+) numbers, and BM MSC clonal expansion (2-fold, p<0.05) as measured by CFU-F assay. To further validate NPY's role in the aged BM niche and HSPC function, we used NPY gene overexpressing and knockout (KO) mouse models. Compared to wild-type (WT) aged mice, NPY overexpressing aged mice showed a substantial increase in BM niche MSC numbers (1.4-fold, P=0.03) and improved HSPC clonal expansion (2.5-fold, P=0.002). In contrast, 16-18 months old NPY KO mice showed enhanced aging than aged WT mice, reflected by fewer MSC counts and lower HSPC clonal expansion (MSC: 1.6-fold, and CFU-F: 1.5-fold, <0.05). Oxidative stress and senescence are the hallmarks of aging. To examine if NPY attenuates HSPC aging by controlling oxidative and senescence, we measured reactive oxygen species (ROS) and senescence in SLAM-LSK of young, aged, and NPY-treated aged mice using CellROX and anti-p16 antibodies, respectively. The SLAM-LSK from aged mice showed higher expression of ROS (9-fold, p=0.03) and p16 than young mice (1.7-fold). However, ROS and p16 expression were decreased in NPY-treated aged mice (ROS: 2-fold, and p16: 1.3-fold, p<0.05). To identify the molecular pathways involved in NPY-mediated attenuation of HSPC aging, we performed RNA sequencing analysis of SLAM-LSK cells. Aged mice SLAM LSK showed impaired expression of genes involved in pathways regulating antioxidant defense, DNA damage repair and stemness than in young mice SLAM LSK. However, NPY treatment in aged mice substantially restored these pathways in SLAM LSK. In conclusion, our studies suggest that an aging-induced deficit in NPY levels in the BM is a crucial lesion contributing to niche defects and HSPC aging. Thus, NPY supplementation or protection of NPY-producing nerve fibers in the BM can attenuate/restore aging-induced niche damage and rejuvenate aged HSPC.