14 October 2008. All of the antipsychotic medicines used to treat schizophrenia hit dopamine receptors, and specifically the D2R class of G protein-coupled receptors. But G proteins are not the only messengers mobilized by the receptor. D2R and GPCRs also signal via a pathway involving the scaffolding protein β-arrestin and the Akt/GSK kinase pathway. That alternative pathway may be a key to the actions of antipsychotic medicines, according to new work from the laboratory of Marc Caron at Duke University Medical Center, Durham, North Carolina. In a paper published in the September 3 issue of PNAS online, Caron and colleagues present evidence that the panel of antipsychotic drugs shows a wide range of abilities to inhibit G protein signaling. At the same time, they all potently block dopamine-stimulated β-arrestin 2 binding to the receptor. The results suggest that the arrestin pathway may be the clinically relevant target of these drugs, a finding that has obvious implications for testing and developing new compounds.
D2 receptors signal mainly through inhibitory G proteins that reduce cAMP production, but more recently, researchers identified a second pathway involving β-arrestin and the Akt/GSK3 kinase cascade. Akt has been independently implicated in schizophrenia (see SRF related news story), and some reports suggest that GSK3 is regulated by antipsychotic drugs (Alimohamad et al., 2005; Li et al., 2007). The mood stabilizing drug lithium appears to modulate a β-arrestin/Akt signaling complex (see SRF related news story).
To ask how antipsychotic drugs affected the two arms of dopamine receptor signaling, first author Bernard Masri used a fluorescence energy transfer assay to measure either cAMP production or β-arrestin association in HEK cells expressing the long version of the D2R. For both measures, he used a BRET technique (bioluminescent fluorescence energy transfer, see SRF related news story). By attaching a bioluminescent tag (luciferase) to the dopamine receptor, and a reporter fluorophore (yellow fluorescent protein) to β-arrestin, Masri could measure the association of the two proteins in living cells. A similar BRET assay was used to measure cAMP levels in the cells.
In all, the investigators tested nine drugs from all classes of antipsychotics. All of the antipsychotics but one (aripiprazole) had intrinsic activity to stimulate cAMP accumulation, but none affected β-arrestin 2 association. In the presence of the dopamine receptor agonist quinpirole, the antipsychotics showed widely differing potencies as antagonists of cAMP inhibition. Most were active in sub-nanomolar range, but three (clozapine, desmethylclozapine, and quetiapine) had activity only at much higher (μM) concentrations, and never fully reversed the inhibition of cAMP production. In contrast, all of the agents inhibited quinpirole-stimulated β-arrestin recruitment at nM concentrations, and inhibited it completely.
“There seems to be a clear difference between the potencies of antipsychotics to antagonize these two signaling paradigms allowing them to discriminate one pathway versus the other,” the authors conclude. “Ultimately, it would be interesting to explore whether the efficacies of these compounds as antipsychotics and their relative liability for extrapyramidal side effects, correlate with their profile for either of these pathways.”
Dissecting the two pathways with new compounds appears feasible for GPCRs generally: a differential inhibition of signaling pathways by receptor-targeted compounds was recently observed for serotonin receptor agonists (see SRF related news story).—Pat McCaffrey.
Masri B, Salahpour A, Didriksen M, Ghisi V, Beaulieu JM, Gainetdinov RR, Caron MG.
Antagonism of dopamine D2 receptor/beta-arrestin 2 interaction is a common property of clinically effective antipsychotics. Proc Natl Acad Sci U S A. 2008 Sep 9;105(36):13656-61. Epub 2008 Sep 3. Abstract