The NAD+ Salvage Pathway Potently Stimulates Hematopoiesis through Increased Mitochondrial Clearance and Asymmetric Division

During the last 50 years, with over a million transplants, hematopoietic stem cell (HSC) transplantation has been the first and most extensively exploited stem cell therapy and the only curative regime for most acute leukemias. Nevertheless, the success of HSC transplantation and all other intensive ablative chemotherapy regimes is still overshadowed by a procedure-associated mortality of ~25%, due to both graft versus host disease and to the infectious complications associated with the severe leucopenia following bone marrow (BM) ablation, even in spite of standard support with HSC-stimulating factor G-CSF. It has been recently shown that increased oxidative phosphorylation, as reflected by increased mitochondrial activity, together with impairment of the mitochondrial stress response can severely compromise HSC regeneration.

Here we show that the NAD⁺-boosting agents Nicotinamide Riboside (NR) and Nicotinamide MonoNucleotide (NMN) reduce mitochondrial activity within HSCs through increased mitochondrial clearance via autophagy and possibly the Unfolded Protein Response mitochondria (UPRmt), leading to increased asymmetric HSC divisions as measured by asymmetric mitochondrial distribution in single cell pair-daughter analysis. This process was abrogated in Nrk1-/-;Nrk2-/- double knock out mice, which cannot incorporate NR into the NAD+ salvage pathway. Contrary to controls, purified murine HSC underwent self-renewal in minimal culture conditions in presence of NR, and human CD34+ hematopoietic progenitors (hCD34+) cultured in vitro in presence of NR could repopulated NSG mice in primary and secondary transplant recipients.

In vivo, NR dietary supplementation resulted in a significantly enlarged pool of progenitors, without concurrent HSC exhaustion, improved survival by 80%, and accelerated blood recovery after murine lethal irradiation and limiting-HSC transplantation. In human xenotransplanted immune-deficient NSG mice, NR increased the production of human leucocyte progeny from hCD34+ progenitors. Our work demonstrates for the first time a positive effect of NAD+ boosting strategies on the most primitive blood stem cells, establishes a novel link between mitochondrial stress, mitophagy and stem cell fate decision, and unveils the potential of NR in vivo supplementation to improve recovery of patients suffering from hematological failure including post-chemo/radiotherapy.