Two murine homologues of the human chemokine receptor CXCR4 mediating stromal cell-derived factor 1α activation of G_i2 are differentially expressed in vivo

Abstract

Previous results have shown that pertussis toxin-sensitive G_i proteins are likely to be involved in regulating the emigration of mature thymocytes from the thymus. In this study, a low stringency polymerase chain reaction (PCR) approach was used to identify G_i protein-coupled cell surface receptors expressed in mouse thymocytes. Among the ten G protein-coupled receptor cDNA isolated, the most prevalent cDNA encoded a polypeptide highly homologous to the human leukocyte-expressed seven-transmembrane-domain receptor LESTR, also referred to as HIV entry cofactor, fusin, or CXCR4. Isolation of full-length cDNA revealed that alternative RNA splicing produces transcripts encoding two isoforms of the murine LESTR, differing by the presence of two amino acids in the N-terminal portion of the longer protein. Functional reconstitution of recombinant murine LESTR with recombinant heterotrimeric G proteins in baculovirus-infected insect cells showed that both receptor variants mediate stromal cell-derived factor 1α activation of the pertussis toxin-sensitive G protein G_i2. Receptor subtype-specific reverse transcriptase-PCR analysis revealed differential expression of the two receptor mRNA in lymphoid tissues and brain, indicating that distinct functions are mediated by the two receptor isoforms in these tissues. The presence of LESTR mRNA in very early thymocytes as well as in immature (CD4⁺ CD8⁺) thymocytes suggests that both CD4 and LESTR are co-expressed and render developing human thymocytes susceptible for HIV entry, which may affect generation of both CD4⁺ CD8⁻ and CD4⁻ CD8⁺ mature lineages.