Neuropeptide Y Is Critical for Bone Marrow Stromal Cells and Hematopoietic Recovery Following Injury

Bone marrow suppression is the most common limiting side-effect of conventional cancer chemo/radio therapy and is the primary cause of morbidity/mortality after accidental exposure to a high dose of ionizing radiation. The mechanisms mediating radiation-induced hematopoietic stem and stromal cell dysfunction however are not well understood. Radiation therapy causes substantial sensory neuropathy in patients. Recent studies reveal that bone marrow cells are highly innervated by sympathetic nerve fibers and that chemotherapy induced nerve-damage can impair hematopoietic regeneration, suggesting a contribution of nerve fibers in the regulation of hematopoietic stem cell and stromal cell activities. Whether irradiation- mediated nerve injury is a crucial lesion that causes deficits in hematopoietic recovery is not known. We recently discovered that differential signaling from the neuropeptide Y (NPY) receptors on bone marrow endothelial cells regulates vascular permeability and stem cell egress. NPY is an important neurotransmitter of the sympathetic nervous system and the principal adreno-medullary hormone. In this study, we found that NPY is important for reconstitution of the bone marrow niche and hematopoietic regeneration following sublethal irradiation (650 cGy). The levels of NPY were significantly reduced in bone marrow of irradiated mice suggesting damage to nerve fibers. Treatment of wild-type mice with full length NPY (1µg/mouse/day) for 3 consecutive days after irradiation markedly reduced the loss of mesenchymal stem cells (CD45^-Ter119^-CD31^-Nestin⁺PDGFR⁺CD51⁺), endothelial cells (CD45^-Ter119^-CD31⁺VE-cadherin⁺) and hematopoietic stem and progenitor cells (SLAM LSK and LSK) in the bone marrow and promoted faster hematopoietic recovery. In addition, pharmacological NPY treatment prevented irradiation mediated nerve fiber damage. In contrast, in NPY knockout mice, regeneration of CD45^neg stromal cells, SLAM LSK and LSK cells after irradiation was significantly reduced compared to wild-type controls. This reduced hematopoietic recovery in NPY deficient mice following irradiation was associated with increased apoptosis/necrosis of stromal cells and hematopoietic stem and progenitor cells. We also examined whether NPY played an intrinsic or extrinsic role in stem cell homing. Wild-type or NPY deficient BM cells were transplanted into wild-type or NPY knockout recipients. Strikingly, the homing of wild-type donor cells into NPY deficient recipients or NPY knockout donor cells into wild-type recipients were both reduced. To explore whether NPY regulates human stem cells, we treated human cord blood CD34+ cells ex vivo with NPY for 3 days and evaluated cell expansion. Long-term culture assays demonstrated that NPY treatment enhanced the clonal expansion of CD34+ cells. In conclusion, our studies suggests that NPY plays both an intrinsic and extrinsic role in hematopoiesis and that NPY-mediated protection of the sympathetic nervous system within the bone marrow can facilitate stem cell niche regeneration and enhance regenerative hematopoiesis following irradiation/injury.