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Regulation of hematopoietic stem cell trafficking and mobilization by the endocannabinoid system

The cannabinoid receptors CB1 and CB2 are seven‐transmembrane Gαi protein‐coupled receptors and are expressed in certain mature hematopoietic cells. We recently showed that these receptors are expressed in murine and human hematopoietic stem cells (HSCs) and that CB2 agonists induced chemotaxis, enhanced colony formation of marrow cells, as well as caused in vivo mobilization of murine HSCs with short‐ and long‐term repopulating abilities. Based on these observations, we have further explored the role of CB2 and its agonist AM1241 on hematopoietic recovery following sublethal irradiation in mice. Cannabinoid receptor 2 knockout mice (Cnr2−/− deficient mice) exhibited impaired recovery following sublethal irradiation as compared with irradiated wild‐type (WT) mice, as determined by low colony‐forming units and low peripheral blood counts. WT mice treated with CB2 agonist AM1241 following sublethal irradiation demonstrated accelerated marrow recovery and increased total marrow cells (approximately twofold) and total lineage– c‐kit+ cells (approximately sevenfold) as well as enhanced HSC survival as compared with vehicle control–treated mice. When the CB2 agonist AM1241 was administered to WT mice 12 days before their sublethal irradiation, analysis of hematopoiesis in these mice showed decreased apoptosis of HSCs, enhanced survival of HSCs, as well as increase in total marrow cells and c‐kit+ cells in the marrow. Thus, CB2 agonist AM1241 promoted recovery after sublethal irradiation by inhibiting apoptosis of HSCs and promoting survival, as well as enhancing the number of HSCs entering the cell cycle.