Bone marrow regeneration requires mitochondrial transfer from donor Cx43-expressing hematopoietic progenitors to stroma.

Hematopoietic stem and progenitor cells (HSPC) fate is tightly regulated by their bone marrow (BM) microenvironment (ME). BM transplantation (BMT) frequently requires irradiation pre-conditioning to ablate endogenous hematopoietic cells. Whether the stromal ME is damaged and how it recovers following irradiation is unknown. We report that BM mesenchymal stromal cells (MSC) undergo massive damage to their mitochondrial function following irradiation. Donor healthy HSPC transfer functional mitochondria to the stromal ME, thus improving mitochondria activity in recipient MSC. Mitochondrial transfer to MSC is cell-contact dependent and mediated by HSPC connexin-43 (Cx43). Hematopoietic Cx43 deficient chimeric mice show reduced mitochondria transfer, which was rescued upon re-expression of Cx43 in HSPC or culture with isolated mitochondria from Cx43 deficient HSPCs. Increased intracellular ATP levels activate the purinergic receptor P2RX7 and lead to AMPK reduced activity in HSPC, dramatically increasing mitochondria transfer to BM MSC. Host stromal ME recovery and donor HSPC engraftment were augmented following mitochondria transfer. Deficiency of Cx43 delayed mesenchymal and osteogenic regeneration while in vivo AMPK inhibition increased stromal recovery. As a consequence, the hematopoietic compartment reconstitution was improved due to the recovery of the supportive stromal ME. Our findings demonstrate that healthy donor HSPC not only reconstitute the hematopoietic system following transplantation but also support and induce the metabolic recovery of their irradiated-damaged ME via mitochondria transfer. Understanding the mechanisms regulating stromal recovery following myeloablative stress are of high clinical interest to optimize BMT procedures and underscore the importance of accessory, non-HSC to accelerate hematopoietic engraftment.