Functionally selective receptor signaling and endocytosis by dopamine D1 receptor agonists


August 2020

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The dopamine D1 receptor (D1R) is a G protein-coupled receptor that signals through Gs and Golf G proteins. D1R agonists increase cAMP production and recruit β-arrestin to the plasma membrane, which may lead to receptor desensitization and/or induce D1R endocytosis. In addition, the D1R is essential for voluntary movement, cognition and reward processes. As such, the D1R is a validated drug target for a multitude of neurological disorders including Parkinson’s disease and neuropsychiatric disorders. Furthermore, D1R functionally selective agonists, which activate one D1R signaling pathway over another have been recently described. Functionally selective agonists, also known as biased agonists, may fine-tune receptor signaling to improve therapeutic efficacy and/or reduce adverse effects. However, the mechanism-of-action for D1R biased agonists is not clearly defined and the function of Gs/olf and β-arrestins in D1R signaling and biased agonism requires further investigation. This research project tests the hypotheses that β-arrestins facilitate agonist-induced D1R endocytosis and that D1R agonists will induce receptor endocytosis based on their ability to recruit β-arrestins. In Aim 1 of this project, the fundamental function of D1R Gs/olf and D1R β-arrestin signaling are examined using CRISPR/Cas9 genome editing to delete either Gs/olf or β-arrrestin1/2 in cells. These studies determined that Gs/olf are essential for D1R cAMP signaling but these G proteins do not have a role in agonist-induced D1R endocytosis. In contrast, this project demonstrates that β-arrestin1/2 are important for agonist-induced D1R endocytosis and this pathway contributes to D1R desensitization. Aim 2 of this project examined the pharmacology of existing and recently discovered D1R agonists and fully defined their signaling via Gs/olf/cAMP, β-arrrestin recruitment and D1R endocytosis. Structurally distinct catechol and non-catechol D1R agonists induced G protein biased signaling and had reduced or absent recruitment of β-arrestins. Interestingly, A-77636 is a β-arrestin super-agonist. Additionally, biased agonism influences D1R endocytosis such that G protein biased agonists do not induce D1R endocytosis. Together, these studies indicate that β-arrestin is important for agonist-induced D1R endocytosis. Moreover, biased agonists that do not recruit β-arrestin also do not induce D1R endocytosis. This dissertation demonstrates a clear function for β-arrestin in D1R endocytosis and extends these findings to biased D1R agonists. The mechanism-of-action for the G protein biased D1R agonists includes reduced β-arrestin recruitment and subsequent D1R endocytosis. The range of functionally selective D1R agonists characterized here provide valuable tools to investigate D1R signaling, trafficking and therapeutic potential.



Biology, Neuroscience, Biology, Molecular, Biology, Cell