Mitochondria Transfer from Hematopoietic Stem and Progenitor Cells to Pdgfrα⁺/Sca-1^-/CD48^dim BM Stromal Cells Via CX43 Gap Junctions and AMPK Signaling Inversely Regulate ROS Generation in Both Cell Populations

Modulation of reactive oxygen species (ROS) levels in hematopoietic stem cells (HSC) is crucial to control HSC quiescence and blood formation. High ROS levels are required for leukocyte formation while low ROS levels are essential to maintain HSC quiescence. However, regulation of ROS content in HSC is poorly understood. Adhesion interactions between HSC and their bone marrow (BM) stromal cells (BMSC) via CXCL12/CXCR4 maintain HSC in a quiescence non-motile state, protecting them from 5-FU chemotherapy insult (Sugiyama, Immunity, 2006). Surface CXCL12 expression by BMSC is dependent on connexin-43 (Cx43) gap junctions mediated cell contact (Schajnovitz, Nat. Immunol., 2011) and BM hematopoietic stem and progenitor cells (HSPC) survive and eliminate excess ROS levels post 5-FU chemotherapy treatment, by transferring ROS to BMSC in a Cx43 dependent manner (Taniguchi, PNAS 2012).

Here, we report that ROS content of BM HSPC inversely correlates with ROS levels in adjacent BMSC. Administration of the pro inflammatory cytokine G-CSF results in decreased HSPC Cx43 expression, elevated ROS levels and increased glucose uptake. Conversely, in the BM stromal microenvironment, G-CSF administration generated lower ROS level and reduced glucose uptake. Up-regulation of BM Sphingosine 1-Phosphate (S1P), a downstream target of G-CSF required for ROS production in HSPC, reduced stromal ROS content and proliferation. Accordingly, mice with reduced BM S1P levels (S1P^low) have lower BM content of HSPC, accompanied by reduced ROS, glucose uptake and lactate production in these cells. More importantly, BM from S1P^low mice has a 3 fold increased frequency of primitive ROS^low/ EPCR⁺ long-term repopulating cells, as evident by immunophenotypic analysis and long-term competitive repopulation assays. Concomitantly, S1P^low mice have increased content of BMSC with higher ROS levels and glucose uptake, leading to higher BM content of colony-forming unit fibroblasts. Our results reveal a dynamic and inverse metabolic relationship between BM HSC and the stroma microenvironment. We hypothesized that the opposite metabolic state of HSPC and BMSC is due to mitochondrial transfer between the two populations. Therefore, we created chimeric mice by transplanting mitochondria labeled GFP (mito-GFP) HSPC to wild type (WT) mice and detected 88% of the host BMSC to contain donor-derived mitochondria, indicating the existence of mitochondria transfer from hematopoietic cells to BMSC in vivo. This transfer is bidirectional, albeit at a lesser degree, as determined in reverse chimeric mice where up to 26% of the donor-derived HSPCs acquired recipient mitochondria. Mitochondrial transfer can be recapitulated also in vitro in an overnight co-culture system of mito-GFP HSPC and primary BMSC, resulting in mitochondrial transfer and increased ROS content in a subpopulation of osteogenic BM PDGFRα⁺/ Sca-1^-/CD48^dim stromal cells. Mitochondrial transfer is cell contact dependent and mediated by Cx43 gap junctions. In vitro co-culture of mito-GFPHSPC from Cx43 deficient (KO) mice with WT or Cx43 KO BMSC reduced 50% mitochondrial transfer to PDGFRα⁺/Sca-1^-/CD48^dim stromal cells. Contrarily, the mitochondrial transfer from WT HSPC to Cx43 KO stromal cells was not affected, revealing that Cx43 expression on HSPC, but not on BM stromal cells, is specifically required for mitochondrial transfer. Interestingly, in vitro inhibition of AMP-activated protein kinase (AMPK), a crucial metabolic regulator, dramatically increased mitochondrial transfer from HSPC to BMSC. Administration of the AMPK inhibitor BML in vivo increased ROS content of PDGFRα⁺/Sca-1^- BMSC while decreasing it in HSPC, further suggesting that AMPK inhibition regulates mitochondrial transfer and ROS production. Our results imply that mitochondria are scavenged by the BM osteogenic microenvironment to prevent excessive ROS levels in the HSC pool and in parallel to activate bone formation. Altogether, we have discovered a dynamic, inverse metabolic state between BM HSPC and their supporting stromal microenvironment during quiescence, proliferationand differentiation of these two populations. Thus, blood cell production and bone generation take place at the expense of the other. This metabolic seesaw is mediated by mitochondrial transfer from HSPC to osteogenic BM stroma in a HSPC Cx43 gap-junction dependent manner and regulated through AMPK signaling.