Dr. Nikoleta G. Tsvetanova
Department of Psychiatry
University of California, San Francisco

Functional Diversification of Signaling by GPCR Localization

G protein-coupled receptors (GPCRs) are critical cell signaling molecules that also comprise the largest class of therapeutic drug targets.  GPCRs are well known to signal upon ligand binding via cyclic AMP (cAMP) production at the plasma membrane, but recent evidence has indicated that various GPCRs may also signal after internalization.  We investigated the functional consequences of site-specific GPCR signaling, using the beta2-adrenoceptor (β2-AR) as a model system.  Global profiling of β2-AR activation identified a core set of transcriptional target genes, and revealed that endocytosis is required for the full repertoire of downstream cAMP-dependent transcriptional responses.  We then developed an orthogonal optogenetic approach to definitively establish that the location of cAMP production is indeed the critical variable controlling the transcriptional response to β2-AR signaling.  Finally, we investigated how site-specific signaling contributes to cellular discrimination of chemically distinct β2-AR ligands according to differences in their ability to promote receptor endocytosis.  We found that endosomal β2-ARs may provide a signaling ‘checkpoint’ that enables cells to respond uniformly to chemically distinct ligands while limiting spurious responses from non-cognate ligands.  Altogether, these findings establish a novel principle for functional diversification of signaling, based on the location of second messenger production, which underlies distinct cellular responses to chemically diverse ligands.  As GPCRs are responsible for sensing virtually all cellular stimuli, further knowledge of how these signaling cascades are orchestrated will illuminate how cells execute critical functions and enhance our ability to develop more specific and efficient therapies for treating human disease.