CD45 regulates retention, motility, and numbers of hematopoietic progenitors, and affects osteoclast remodeling of metaphyseal trabecules

The CD45 phosphatase is uniquely expressed by all leukocytes, but its role in regulating hematopoietic progenitors is poorly understood. We show that enhanced CD45 expression on bone marrow (BM) leukocytes correlates with increased cell motility in response to stress signals. Moreover, immature CD45 knockout (KO) cells showed defective motility, including reduced homing (both steady state and in response to stromal-derived factor 1) and reduced granulocyte colony-stimulating factor mobilization. These defects were associated with increased cell adhesion mediated by reduced matrix metalloproteinase 9 secretion and imbalanced Src kinase activity. Poor mobilization of CD45KO progenitors by the receptor activator of nuclear factor κB ligand, and impaired modulation of the endosteal components osteopontin and stem cell factor, suggested defective osteoclast function. Indeed, CD45KO osteoclasts exhibited impaired bone remodeling and abnormal morphology, which we attributed to defective cell fusion and Src function. This led to irregular distribution of metaphyseal bone trabecules, a region enriched with stem cell niches. Consequently, CD45KO mice had less primitive cells in the BM and increased numbers of these cells in the spleen, yet with reduced homing and repopulation potential. Uncoupling environmental and intrinsic defects in chimeric mice, we demonstrated that CD45 regulates progenitor movement and retention by influencing both the hematopoietic and nonhematopoietic compartments

Dynamic Cross Talk between S1P and CXCL12 Regulates Hematopoietic Stem Cells Migration, Development and Bone Remodeling

Hematopoietic stem cells (HSCs) are mostly retained in a quiescent non-motile mode in their bone marrow (BM) niches, shifting to a migratory cycling and differentiating state to replenish the blood with mature leukocytes on demand. The balance between the major chemo-attractants CXCL12, predominantly in the BM, and S1P, mainly in the blood, dynamically regulates HSC recruitment to the circulation versus their retention in the BM. During alarm situations, stress-signals induce a decrease in CXCL12 levels in the BM, while S1P levels are rapidly and transiently increased in the circulation, thus favoring mobilization of stem cells as part of host defense and repair mechanisms. Myeloid cytokines, including G-CSF, up-regulate S1P signaling in the BM via the PI3K pathway. Induced CXCL12 secretion from stromal cells via reactive oxygen species (ROS) generation and increased S1P1 expression and ROS signaling in HSCs, all facilitate mobilization. Bone turnover is also modulated by both CXCL12 and S1P, regulating the dynamic BM stromal microenvironment, osteoclasts and stem cell niches which all functionally express CXCL12 and S1P receptors. Overall, CXCL12 and S1P levels in the BM and circulation are synchronized to mutually control HSC motility, leukocyte production and osteoclast/osteoblast bone turnover during homeostasis and stress situations

Lactate released by inflammatory bone marrow neutrophils induces their mobilization via endothelial GPR81 signaling.

Neutrophils provide first line of host defense against bacterial infections utilizing glycolysis for their effector functions. How glycolysis and its major byproduct lactate are triggered in bone marrow (BM) neutrophils and their contribution to neutrophil mobilization in acute inflammation is not clear. Here we report that bacterial lipopolysaccharides (LPS) or Salmonella Typhimurium triggers lactate release by increasing glycolysis, NADPH-oxidase-mediated reactive oxygen species and HIF-1α levels in BM neutrophils. Increased release of BM lactate preferentially promotes neutrophil mobilization by reducing endothelial VE-Cadherin expression, increasing BM vascular permeability via endothelial lactate-receptor GPR81 signaling. GPR81-/- mice mobilize reduced levels of neutrophils in response to LPS, unless rescued by VE-Cadherin disrupting antibodies. Lactate administration also induces release of the BM neutrophil mobilizers G-CSF, CXCL1 and CXCL2, indicating that this metabolite drives neutrophil mobilization via multiple pathways. Our study reveals a metabolic crosstalk between lactate-producing neutrophils and BM endothelium, which controls neutrophil mobilization under bacterial infection