Intracellular and nuclear CXCR4 signaling promotes erythroblast enucleation

Abstract

All leukocytes express chemokine receptor CXCR4 on their surface and efficiently migrate in response to its sole ligand CXCL12. Here we report for the first time that CXCR4 is also prominently expressed in erythroblasts, the bone marrow erythroid precursor cells but, instead of chemotaxis, CXCR4 signaling promoted erythroblast enucleation, a unique mammalian process of their asymmetrical division into an anuclear reticulocyte and a nucleus-containing pyrenocyte. Mechanistically, CXCL12 ligation of erythroblast CXCR4 triggered its endocytosis and translocation into multiple intracellular compartments. These included, surprisingly, the nuclear envelope and nucleus where CXCL12 and CXCR4 colocalized with either Gαi or phosphorylated β-arrestin1, respectively, suggesting an ongoing differential CXCR4 signaling around and within the erythroblast nucleus. Supporting the notion of intracellular signaling, stimulation of erythroblasts with CXCL12 caused a persistent phosphorylation of intracellular and intranuclear CXCR4 and a delayed activation of multiple protein kinases. Furthermore, CXCL12 induced erythroblast elongation, a condensation and ex-centric positioning of their nuclei and triggered rapid perinuclear Ca2+ transients, which led directly to erythroblast enucleation. The CXCR4-mediated sequential erythroblast responses taking place during ultimate steps of their development and culminating in enucleation pinpoint a new physiological role of CXCR4 and the bone marrow-derived CXCL12 in regulating erythrocyte production.