Biased Agonists and Allosteric Modulators of the Type 1 Cannabinoid Receptor: Potential Treatments for Huntington Disease

Abstract

Huntington disease (HD) is caused by the inheritance of a single copy of the mutant huntingtin gene. HD patients suffer from progressive cognitive decline, psychoses, depression, an inability to gain and maintain weight, and profound motor dysfunction, ultimately leading to death. Although the causal genetic defect was defined more than 20 years ago, treatment of HD is still limited to managing individual symptoms rather than managing disease processes and delaying disease onset and progression. A decrease in type 1 cannabinoid receptor (CB1) mRNA and protein levels in the caudate and putamen precedes symptoms in HD and is correlated with HD progression and severity. Activation of CB1 modulates neuronal activity in regions of the brain critical for cognition, mood, metabolism, and motor control. I hypothesized that pharmacological enhancement of CB1 activity and abundance would reduce the signs and symptoms of HD and, because CB1 loss precedes other changes, may slow disease progression. Through meta-analysis of the existing literature and several experimental approaches, I confirmed that CB1 mRNA and protein levels decline in the caudate and putamen of HD patients and in the striatum of a mouse model of HD. Activation of CB1 with the CB1 agonist arachidonoyl-2’-chloroethylamide normalized CB1 mRNA levels in mouse cells modelling medium spiny projection neurons and expressing mutant huntingtin (STHdh) suggesting CB1 agonism could increase CB1 levels in models of HD. I determined that CB1 mRNA and protein levels were increased by CB1 agonists (e.g. the endocannabinoid anandamide) that were functionally-selective toward Gαi/o- and Gβγ-dependent signaling. In contrast, CB1 agonists that were functionally-selective for arrestin (e.g. the plant-derived cannabinoid ∆9-tetrahydrocannabinol) reduced CB1 protein levels. In STHdh cells, Gαi/o/Gβγ-selective CB1 agonists improved cell viability and function, whereas arrestin-selective CB1 agonists reduced cell viability and function. The potency and efficacy of cannabinoid agonists can be fine-tuned via positive (PAM) or negative (NAM) allosteric modulators that increase or decrease CB1-dependent signaling, respectively, without directly activating CB1. The CB1 allosteric modulators GAT228 (R-enantiomer), GAT229 (S-enantiomer), and GAT211 (equimolar racemic mixture of GAT228 and GAT229) were tested to determine if they increased CB1 activity in HD. GAT228 was a CB1 allosteric partial agonist that produced neutral or negative effects in cell culture and animal models of HD. GAT229 had no direct agonist activity but was a PAM that enhanced Gαi/o-dependent signaling and improved cell viability in a cell culture model of HD, delayed disease progression, and normalized gene expression in an animal model of HD. GAT211 displayed effects that were intermediate between its enantiomers. Therefore, the signs of HD can be managed via CB1 so long as signaling is selectively enhanced without supraphysiological activation or receptor downregulation. My research provides a strong first proof-of-principle for the use of Gαi/o-selective CB1 PAMs to manage the signs and symptoms of HD.