Competitive interaction of 5-HT1A receptors with G-protein subtypes in CHO cells demonstrated by RNA interference

Following agonist action, G-protein-coupled receptors may exhibit differential coupling to G-proteins or second messenger pathways, supporting the notion of agonist-directed trafficking. To explore these mechanisms, we have designed and transfected synthetic siRNA duplexes to knockdown different Gα subunits in Chinese hamster ovary (CHO) cells expressing human (h)5-hydroxytryptamine 1A receptors (CHO-h5-HT1A). siRNAs against Gαi2 and Gαi3 transfected alone or in combination caused a large decrease in the corresponding mRNA level (64–80%) and also at the protein level for Gαi3 (60–70%), whereas a non-specific siRNA showed no effect. In membranes of CHO-h5-HT1A, 5-HT stimulated guanosine-5′-O-(3-[35S]thio)-triphosphate ([35S]GTPγS) binding was differentially affected by transfection of siRNAs against Gαi protein, siRNAs against Gαi2 inducing a more important decrease in the efficacy of 5-HT than transfection of siRNAs against Gαi3. The high potency component was abolished after transfection of siRNAs against Gαi3 and the lower potency component was suppressed after transfection of siRNAs against Gαi2. To directly investigate Gαi3 activation we used an antibody-capture/scintillation proximity assay. (+)8-OH-DPAT yielded bell-shaped curves for Gαi3 activation, a response that was abolished after transfection of siRNAs against Gαi3 protein. Interestingly, (+)8-OH-DPAT yielded a sigmoidal response when only Gαi3 protein was expressed. These data suggest that when efficacious agonists attain a high level of occupation of h5-HT1A receptors, a change occurs that induces coupling to Gαi2 protein and suppresses signalling through Gαi3 subunits.