GPCR-TCR Heterodimer

Abstract

Stromal cell-derived factor 1α (SDF-1α) is a chemokine that signals through the G protein-coupled receptor (GPCR) CXCR4. In T cells, SDF-1α stimulates gene expression, chemotaxis, and cell adhesion. Kumar et al. provide evidence that SDF-1α stimulates an interaction between CXCR4 and the T cell receptor (TCR) that allows SDF-1α to signal through the TCR immunotyrosine-based activation motifs (ITAMs). Jurkat cells expressing a fluorescently tagged CXCR4 and CD3-ζ subunit of the TCR showed colocalization of the two proteins at the surface and in intracellular vesicles after SDF-1α stimulation. The interaction was confirmed by coimmunoprecipitation and two different fluorescence energy transfer (FRET) assays. The interaction between the two proteins was not blocked by pertussis toxin, which inhibits some of the SDF-1α-stimulated signals, but was blocked by inhibition of actin polymerization or phosphoinositide 3-kinase activity. Mutant Jurkat cell lines lacking either the β subunit of the TCR, which are deficient in cell surface TCRs, or the nonreceptor tyrosine kinase ZAP-70 exhibited decreased responsiveness to SDF-1α stimulation in terms of extracellular signal-regulated kinase (ERK) phosphorylation and stimulation of an AP-1-dependent reporter gene. SDF-1α signaling was restored in the TCR-β mutant cells if the cells were reconstituted with the β subunit or with a chimeric molecule consisting of the extracellular and transmembrane domains of CD8 and the cytoplasmic domain of CD3-ζ. Expression of various mutant versions of this CD8/CD3-ζ chimera was used to demonstrate that the CD3-ζ ITAM was essential for SDF-1α signaling. The AP-1-dependent gene expression, but not gene expression mediated by two other transcription factors NF-AT or NF-κB, in Jurkat cells was much greater in response to costimulation with SDF-1α and an antibody against CD3 to activate the TCR than in response to either TCR activation or SDF-1α alone. The authors propose a model whereby SDF-1α binding to the CXCR4 induces reorganization of the membrane, allowing CXCR4 and TCR to interact, which allows ZAP-70 to participate in signaling from both receptors, resulting in an enhanced response and increased ERK activation and AP-1-dependent gene activation. A. Kumar, T. D. Humphreys, K. N. Kremer, P. S. Bramati, L. Bradfield, C. E. Edgar, K. E. Hedin, CXCR4 physically associates with the T cell receptor to signal in T cells. Immunity 25 , 213-224 (2006). [PubMed]