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Intracellular Localization and Constitutive Endocytosis of CXCR4 in Human CD34⁺ Hematopoietic Progenitor Cells

^a INSERM U362 and
^b Service Commun d’Imagerie, Institut Gustave Roussy,Villejuif, France

Key Words. CXCR4 receptor • SDF-1 • CD34⁺ cells • Endocytosis • Cell migration

Correspondence: Fawzia Louache, Ph.D., INSERM U362, Institut Gustave Roussy, PR1, 39 Rue Camille Desmoulins, 94805 Villejuif, France. Telephone: 33-1-42-11-42-33; Fax: 33-1-42-11-52-40; e-mail: fawl{at}igr.fr

CXCR4, the stromal cell–derived factor-1 receptor, plays an important role in the migration of hematopoietic progenitor/stem cells. The surface and cytoplasmic expression of CXCR4 on human hematopoietic CD34⁺ cells was investigated. We show that its surface expression is low, whereas a large part of CXCR4 protein is sequestered intracellularly. Using confocal microscopy, we demonstrated that CXCR4 is colocalized with EEA-1, Rab5, Rab4, and Rab11, which are localized in early and recycling endosomes. No significant colocalization of CXCR4 with lysosomal markers CD63 and Lamp-1 was detected. Using antibody feeding experiments, we report a role for CXCR4 constitutive endocytosis in subcellular localization in stably transduced UT7-CXCR4-GFP and CD34⁺ cells. Agonist-independent endocytosis of CXCR4 occurs through clathrin-coated vesicles. These data implicate a constitutive endocytosis in the regulation of CXCR4 membrane expression and suggest that constitutive endocytosis may be involved in the regulation of trafficking the human hematopoietic progenitor/stem cells to and in the bone marrow microenvironment.

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