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In this issue of Blood, Zhao et al use a humanized mouse model to investigate the mechanisms driving daily oscillations in circulating human and murine leukocytes.1 In the same mice, they find human and murine circulating leukocytes displaying inverted oscillations, reproducing the trafficking pattern previously observed in both species. A novel network regulating circadian leukocyte trafficking is proposed. It involves interspecies differences of stress-kinase regulation of reactive oxygen species (ROS), hypoxia-inducible factor 1a (HIF-1a) and clock gene–dependent regulation of the CXCL12 receptor CXCR4. This study underscores the crosstalk of the genetic clock with metabolism and ROS in the regulation of leukocyte migration and reveals new mechanistic players.

The fate of hematopoietic stem and progenitor cells (HSPC) is tightly regulated by their bone marrow (BM) microenvironment (ME). BM transplantation (BMT) frequently requires irradiation preconditioning to ablate endogenous hematopoietic cells. Whether the stromal ME is damaged and how it recovers after irradiation is unknown. We report that BM mesenchymal stromal cells (MSC) undergo massive damage to their mitochondrial function after 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 adenosine triphosphate levels activate the purinergic receptor P2RX7 and lead to reduced activity of adenosine 5′-monophosphate–activated protein kinase (AMPK) in HSPC, dramatically increasing mitochondria transfer to BM MSC. Host stromal ME recovery and donor HSPC engraftment were augmented after 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 because of the recovery of the supportive stromal ME. Our findings demonstrate that healthy donor HSPC not only reconstitute the hematopoietic system after transplantation, but also support and induce the metabolic recovery of their irradiated, damaged ME via mitochondria transfer. Understanding the mechanisms regulating stromal recovery after myeloablative stress are of high clinical interest to optimize BMT procedures and underscore the importance of accessory, non-HSC to accelerate hematopoietic engraftment.

Hematopoietic stem and progenitor cells (HSPCs) and leukocytes circulate between the bone marrow (BM) and peripheral blood following circadian oscillations. Autonomic sympathetic noradrenergic signals have been shown to regulate HSPC and leukocyte trafficking, but the role of the cholinergic branch has remained unexplored. We have investigated the role of the cholinergic nervous system in the regulation of day/night traffic of HSPCs and leukocytes in mice. We show here that the autonomic cholinergic nervous system (including parasympathetic and sympathetic) dually regulates daily migration of HSPCs and leukocytes. At night, central parasympathetic cholinergic signals dampen sympathetic noradrenergic tone and decrease BM egress of HSPCs and leukocytes. However, during the daytime, derepressed sympathetic noradrenergic activity causes predominant BM egress of HSPCs and leukocytes via β3–adrenergic receptor. This egress is locally supported by light-triggered sympathetic cholinergic activity, which inhibits BM vascular cell adhesion and homing. In summary, central (parasympathetic) and local (sympathetic) cholinergic signals regulate day/night oscillations of circulating HSPCs and leukocytes. This study shows how both branches of the autonomic nervous system cooperate to orchestrate daily traffic of HSPCs and leukocytes., This work was supported by core support grants from the Wellcome Trust and the Medical Research Council (MRC) to the Cambridge Stem Cell Institute, the Spanish Ministry of Economy and Competitiveness (SAF-2011-30308), the Pro-CNIC Foundation, Severo Ochoa Center of Excellence award SEV-2015-0505 to CNIC, Ramón y Cajal Program grant RYC-2009-04703, Marie Curie Career Integration grant FP7-PEOPLE-2011-RG-294096, ConSEPOC-Comunidad de Madrid S2010/BMD-2542, Red TerCel (Instituto de Salud Carlos III (ISCIII)-Spanish Cell Therapy Network), National Health Institute Blood and Transplant (United Kingdom), Horizon2020 grant ERC-2014-CoG-64765, and a Cancer Research UK Programme Foundation Award to S.M.-F., who was also supported in part by an International Early Career Scientist grant from the Howard Hughes Medical Institute. A.G.-G. received fellowships from the Ramón Areces and LaCaixa Foundations. C.K. was supported by Marie Curie Career Integration grant H2020-MSCA-IF-2015-708411.