The chemokine CXCL12 is essential for the function of hematopoietic stem and progenitor cells. Here we report that secretion of functional CXCL12 from human bone marrow stromal cells (BMSCs) was a cell contact–dependent event mediated by connexin-43 (Cx43) and Cx45 gap junctions. Inhibition of connexin gap junctions impaired the secretion of CXCL12 and homing of leukocytes to mouse bone marrow. Purified human CD34+ progenitor cells did not adhere to noncontacting BMSCs, which led to a much smaller pool of immature cells. Calcium conduction activated signaling by cAMP–protein kinase A (PKA) and induced CXCL12 secretion mediated by the GTPase RalA. Cx43 and Cx45 additionally controlled Cxcl12 transcription by regulating the nuclear localization of the transcription factor Sp1. We suggest that BMSCs form a dynamic syncytium via connexin gap junctions that regulates CXC12 secretion and the homeostasis of hematopoietic stem cells.
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We thank J. Von Maltzen and K. Willecke (Institute of Genetics Division of Molecular Genetics, University of Bonn, Bonn, Germany) for BMSCs with loxP-flanked Cx45 that express Cre from the myxovirus promoter; G. Lalli (King's College London) for plasmids encoding Myc tag alone or Myc-tagged dominant negative RalA; M. Segal for helping with real-time calcium imaging; R. Alon, N. Dekel and I.H. Oh for discussions; and C.M. Sorensen for advice on connexin mimetic peptides. Supported by the Israeli Science Foundation (ISF 544/09), the European Union (Advance Cell-based Therapies for the Treatment of Primary Immunodeficiency HEALTH-F5-2010-261387) and The Edith Arnoff Stein Professorial Chair in Stem Cell Research (T.L.).
The authors declare no competing financial interests.
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Schajnovitz, A., Itkin, T., D'Uva, G. et al. CXCL12 secretion by bone marrow stromal cells is dependent on cell contact and mediated by connexin-43 and connexin-45 gap junctions. Nat Immunol 12, 391–398 (2011). https://doi.org/10.1038/ni.2017
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