Metabolic and microenvironmental strategies to accelerate hematopoietic recovery post-aplasia

The procedure-associated mortality rate of bone marrow transplantations (BMT) remains close to 25%. The BMT is therefore only indicated for life-threatening diseases, with no replacement of this technology to be expected in the foreseeable future. Modest improvements such as the use of G-CSF to accelerate hematopoietic recovery significantly reduce mortality. Hematopoietic stem cells (HSC) reside in the bone marrow (BM) and are mainly quiescent, dividing only to self-renew. Via extracellular signals and interactions with other cells of the BM (microenvironment), a stable HSC pool is maintained, while hematopoietic progenitor cells (HPC) proliferate and differentiate to mature blood cell types. Marrow adipocytes (BMA) have recently been recognized as a hematopoietic regulatory entity, although the mechanisms by which it interacts with HSCs and HPCs (HSPC) in vivo, remain unknown. During the hematopoietic recovery period post-BMT, HSPCs increase their metabolism as a response to hematopoietic stress, partly modulated by the microenvironment. In this work we identified novel pharmacological approaches to accelerate hematopoietic recovery in the post-BMT period, addressed via two strategies: (i) by directly modulating HSC metabolism, and (ii) by regulating the differentiation of BMA. Increasing oxidative phosphorylation, combined with an impaired mitochondrial stress response, can severely compromise the regenerative capacity of HSCs. Here we demonstrate that the NAD+-boosting agent Nicotinamide Riboside (NR) reduces mitochondrial activity within HSCs through increased mitophagy, the recycling mechanism of damaged mitochondria. We observed in vitro NR treatment of HSCs to lead to increased asymmetric HSC divisions, which resulted in a significantly enlarged pool of progenitor cells, without HSC exhaustion. Dietary supplementation of NR to mice undergoing BMT accelerated blood recovery and improved survival. We therefore established a novel link between HSC mitochondrial stress, mitophagy and stem cell fate decision. Recent studies have suggested BMA maintain HSCs quiescent, which, may be detrimental to the increased expansion of HPCs in the recovery phase post-BMT. BM stromal cells (BMSC) are the cellular precursors of both adipocytes and osteoblasts, and they have been strongly implicated in supporting hematopoiesis by secreting cytokines such as SCF and Cxcl12. We observed that adipocytes quickly accumulate the marrow space of mice undergoing BMT, followed by an expansion hematopoiesis-supportive multipotent Sca1+/CD24+ stromal cells, coinciding with hematopoietic recovery. We developed a novel screening platform using Digital Holographic Microscopy (DHM), quantifying in vitro adipocytic differentiation non-invasively, allowing for follow-up co-cultures with HSPCs. Using the OP9 BMSC-line, we modulated adipocyte differentiation prior to co-culture using a pharmacological approach and observed that high adipocytic content was associated to a decrease in HPC expansion. We perfomed a screening of over 4000 FDA-approved drugs and natural compounds and identified one to efficiently prevent adipocytic differentiation and maintain the hematopoietic supportive capacity of OP9 stroma. Altogether, this work opens the door to alternative strategies accelerating hematopoietic reconstitution and unveils the potential to improve recovery of patients undergoing BMT.