Research Article

Ultrapotent chemogenetics for research and potential clinical applications

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Science  12 Apr 2019:
Vol. 364, Issue 6436, eaav5282
DOI: 10.1126/science.aav5282

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Improved agonists for chemogenetics

Targeting ligand-responsive receptors to specific groups of cells, a strategy known as chemogenetics, is a powerful tool in many neurological applications. There is increasing interest in extending these tools for human treatment. Magnus et al. designed chemogenetic ion channels that improve currently available systems and are activated by the clinically used antismoking drug varenicline. They engineered a ligand-binding domain less responsive to endogenous signals and identified agonists that function at nanomolar concentrations. The combination of drug and introduced channels transiently silenced neurons, with slow but effective washout, and induced behavioral changes in animal models after brain administration.

Science, this issue p. eaav5282

Structured Abstract

INTRODUCTION

Localized control of neuron activity is important for both brain research and therapy. Chemogenetics is a method to control cellular activity by targeting defined cell populations with an exogenous receptor that is engineered to respond selectively to a small-molecule agonist. The approach is generalizable because a receptor-agonist combination can be used to activate or inhibit different neural populations in any brain region. Moreover, using agonists that are selective for the chemogenetic receptor allows cell type–specific modulation, in contrast to traditional pharmacology. Chemogenetic tools have achieved widespread utility in animal models, and there is growing interest in developing chemogenetic systems that are suitable for human therapeutic applications.

RATIONALE

Optimally, a chemogenetic system should have several characteristics for use in the nervous system: (i) The introduced receptor should be activated by low agonist doses; (ii) human use would be facilitated by a chemogenetic agonist that is already a safe and well-tolerated clinically approved drug that crosses the blood-brain barrier, whereas for research applications the agonist should be highly selective for the chemogenetic receptor over endogenous targets; (iii) chemogenetic receptors should be inert in the absence of the drug, lacking constitutive activity or responsiveness to endogenous ligands; (iv) the receptors should activate or inhibit neurons efficaciously, durably, and reversibly; and (v) the site and level of expression of the chemogenetic receptor should be measurable noninvasively. Existing chemogenetic systems did not fulfill all these criteria.

RESULTS

We identified mutations of the ligand-binding domain of the α7 nicotinic acetylcholine receptor that conferred potent activity to the FDA-approved smoking cessation drug varenicline. We created chimeric ligand-gated ion channels by combining these modified ligand-binding domains with ion pore domains from either the glycine receptor or the serotonin 3 receptor, which conduct chloride or cations, respectively. These two types of chemogenetic channels inhibit or activate neurons upon binding varenicline at concentrations below those used clinically to treat nicotine addiction. Varenicline is especially attractive for potential therapeutic applications because it shows limited metabolism, durable pharmacokinetics, and high oral and brain bioavailability. Chemogenetic inhibition or activation was sustained for at least 2 to 3 weeks of continual exposure to varenicline, indicating suitability for chronic use. Expression of the chemogenetic ion channels was visualized in animals by positron emission tomography, enabling noninvasive measurement of the expression and anatomic site of chemogenetic receptors. We showed robust responses to chemogenetic silencing of neurons using low doses of varenicline in mice and one monkey. Finally, we synthesized brain-penetrant analogs of varenicline with subnanomolar potency and with greatly enhanced selectivity for the chemogenetic receptors that were effective for modulation of neural activity in mice.

CONCLUSION

We developed a toolbox of modular ion channels and selective, ultrapotent agonists that can be used for targeted control of brain activity in rodent and primate models. Additional studies will be needed to establish long-term safety and efficacy with chemogenetic receptors for therapeutic applications, but this is facilitated by using varenicline. These chemogenetic technologies can advance research into neural circuit disorders while enabling extension to human therapies.

Chemogenetics for mice, monkeys, and potential therapy in humans.

Modular ion channels were engineered to be activated by ultrapotent agonists, which selectively inhibit or excite activity in neurons expressing the chemogenetic receptors (blue). Neuron modulation was characterized by calcium imaging, electrophysiology, and behavior in mice and a monkey. These chemogenetic receptors and their FDA-approved agonists may facilitate translation of chemogenetics to therapies for human neurological diseases.

Abstract

Chemogenetics enables noninvasive chemical control over cell populations in behaving animals. However, existing small-molecule agonists show insufficient potency or selectivity. There is also a need for chemogenetic systems compatible with both research and human therapeutic applications. We developed a new ion channel–based platform for cell activation and silencing that is controlled by low doses of the smoking cessation drug varenicline. We then synthesized subnanomolar-potency agonists, called uPSEMs, with high selectivity for the chemogenetic receptors. uPSEMs and their receptors were characterized in brains of mice and a rhesus monkey by in vivo electrophysiology, calcium imaging, positron emission tomography, behavioral efficacy testing, and receptor counterscreening. This platform of receptors and selective ultrapotent agonists enables potential research and clinical applications of chemogenetics.

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