Eric A. Horne

The serine hydrolase ABHD6 controls the accumulation and efficacy of 2-AG at cannabinoid receptors

The endocannabinoid 2-arachidonoylglycerol (2-AG) regulates neurotransmission and neuroinflammation by activating CB1 cannabinoid receptors on neurons and CB2 cannabinoid receptors on microglia. Enzymes that hydrolyze 2-AG, such as monoacylglycerol lipase, regulate the accumulation and efficacy of 2-AG at cannabinoid receptors. We found that the recently described serine hydrolase α-β-hydrolase domain 6 (ABHD6) also controls the accumulation and efficacy of 2-AG at cannabinoid receptors. In cells from the BV-2 microglia cell line, ABHD6 knockdown reduced hydrolysis of 2-AG and increased the efficacy with which 2-AG can stimulate CB2-mediated cell migration. ABHD6 was expressed by neurons in primary culture and its inhibition led to activity-dependent accumulation of 2-AG. In adult mouse cortex, ABHD6 was located postsynaptically and its selective inhibition allowed the induction of CB1-dependent long-term depression by otherwise subthreshold stimulation. Our results indicate that ABHD6 is a rate-limiting step of 2-AG signaling and is therefore a bona fide member of the endocannabinoid signaling system.

Chronic microsensors for longitudinal, subsecond dopamine detection in behaving animals.

Neurotransmission operates on a millisecond timescale but is changed by normal experience or neuropathology over days to months. Despite the importance of long-term neurotransmitter dynamics, no technique exists to track these changes in a subject from day to day over extended periods of time. Here we describe and characterize a microsensor that can detect the neurotransmitter dopamine with subsecond temporal resolution over months in vivo in rats and mice.

Endocannabinoid Signaling Mediates Psychomotor Activation by Adenosine A2A Antagonists

Adenosine A2A receptor antagonists are psychomotor stimulants that also hold therapeutic promise for movement disorders. However, the molecular mechanisms underlying their stimulant properties are not well understood. Here, we show that the robust increase in locomotor activity induced by an A2A antagonist in vivo is greatly attenuated by antagonizing cannabinoid CB1 receptor signaling or by administration to CB1−/− mice. To determine the locus of increased endocannabinoid signaling, we measured the amount of anandamide [AEA (N-arachidonoylethanolamine)] and 2-arachidonoylglycerol (2-AG) in brain tissue from striatum and cortex. We find that 2-AG is selectively increased in striatum after acute blockade of A2A receptors, which are highly expressed by striatal indirect-pathway medium spiny neurons (MSNs). Using targeted whole-cell recordings from direct- and indirect-pathway MSNs, we demonstrate that A2A receptor antagonists potentiate 2-AG release and induction of long-term depression at indirect-pathway MSNs, but not direct-pathway MSNs. Together, these data outline a molecular mechanism by which A2A antagonists reduce excitatory synaptic drive on the indirect pathway through CB1 receptor signaling, thus leading to increased psychomotor activation.

ABHD6 blockade exerts antiepileptic activity in PTZ-induced seizures and in spontaneous seizures in R6/2 mice.

The serine hydrolase α/β-hydrolase domain 6 (ABHD6) hydrolyzes the most abundant endocannabinoid (eCB) in the brain, 2-arachidonoylglycerol (2-AG), and controls its availability at cannabinoid receptors. We show that ABHD6 inhibition decreases pentylenetetrazole (PTZ)-induced generalized tonic-clonic and myoclonic seizure incidence and severity. This effect is retained in Cnr1(-/-) or Cnr2(-/-) mice, but blocked by addition of a subconvulsive dose of picrotoxin, suggesting the involvement of GABAA receptors. ABHD6 inhibition also blocked spontaneous seizures in R6/2 mice, a genetic model of juvenile Huntingtons disease known to exhibit dysregulated eCB signaling. ABHD6 blockade retained its antiepileptic activity over chronic dosing and was not associated with psychomotor or cognitive effects. While the etiology of seizures in R6/2 mice remains unsolved, involvement of the hippocampus is suggested by interictal epileptic discharges, increased expression of vGLUT1 but not vGAT, and reduced Neuropeptide Y (NPY) expression. We conclude that ABHD6 inhibition may represent a novel antiepileptic strategy.

Downregulation of cannabinoid receptor 1 from neuropeptide Y interneurons in the basal ganglia of patients with Huntington's disease and mouse models

Cannabinoid receptor 1 (CB1 receptor) controls several neuronal functions, including neurotransmitter release, synaptic plasticity, gene expression and neuronal viability. Downregulation of CB1 expression in the basal ganglia of patients with Huntingtons disease (HD) and animal models represents one of the earliest molecular events induced by mutant huntingtin (mHtt). This early disruption of neuronal CB1 signaling is thought to contribute to HD symptoms and neurodegeneration. Here we determined whether CB1 downregulation measured in patients with HD and mouse models was ubiquitous or restricted to specific striatal neuronal subpopulations. Using unbiased semi‐quantitative immunohistochemistry, we confirmed previous studies showing that CB1 expression is downregulated in medium spiny neurons of the indirect pathway, and found that CB1 is also downregulated in neuropeptide Y (NPY)/neuronal nitric oxide synthase (nNOS)‐expressing interneurons while remaining unchanged in parvalbumin‐ and calretinin‐expressing interneurons. CB1 downregulation in striatal NPY/nNOS‐expressing interneurons occurs in R6/2 mice, HdhQ150/Q150 mice and the caudate nucleus of patients with HD. In R6/2 mice, CB1 downregulation in NPY/nNOS‐expressing interneurons correlates with diffuse expression of mHtt in the soma. This downregulation also occludes the ability of cannabinoid agonists to activate the pro‐survival signaling molecule cAMP response element‐binding protein in NPY/nNOS‐expressing interneurons. Loss of CB1 signaling in NPY/nNOS‐expressing interneurons could contribute to the impairment of basal ganglia functions linked to HD.

Dual inhibition of alpha/beta-hydrolase domain 6 and fatty acid amide hydrolase increases endocannabinoid levels in neurons

Agonists at cannabinoid receptors, such as the phytocannabinoid Δ9-tetrahydrocannabinol, exert a remarkable array of therapeutic effects but are also associated with undesirable psychoactive side effects. Conversely, targeting enzymes that hydrolyze endocannabinoids (eCBs) allows for more precise fine-tuning of cannabinoid receptor signaling, thus providing therapeutic relief with reduced side effects. Here, we report the development and characterization of an inhibitor of eCB hydrolysis, UCM710, which augments both N-arachidonoylethanolamine and 2-arachidonoylglycerol levels in neurons. This compound displays a unique pharmacological profile in that it inhibits fatty acid amide hydrolase and α/β-hydrolase domain 6 but not monoacylglycerol lipase. Thus, UCM710 represents a novel tool to delineate the therapeutic potential of compounds that manipulate a subset of enzymes that control eCB signaling.

ST-11: A New Brain-Penetrant Microtubule-Destabilizing Agent with Therapeutic Potential for Glioblastoma Multiforme

Glioblastoma multiforme is a devastating and intractable type of cancer. Current antineoplastic drugs do not improve the median survival of patients diagnosed with glioblastoma multiforme beyond 14 to 15 months, in part because the blood–brain barrier is generally impermeable to many therapeutic agents. Drugs that target microtubules (MT) have shown remarkable efficacy in a variety of cancers, yet their use as glioblastoma multiforme treatments has also been hindered by the scarcity of brain-penetrant MT-targeting compounds. We have discovered a new alkylindole compound, ST-11, that acts directly on MTs and rapidly attenuates their rate of assembly. Accordingly, ST-11 arrests glioblastoma multiforme cells in prometaphase and triggers apoptosis. In vivo analyses reveal that unlike current antitubulin agents, ST-11 readily crosses the blood–brain barrier. Further investigation in a syngeneic orthotopic mouse model of glioblastoma multiforme shows that ST-11 activates caspase-3 in tumors to reduce tumor volume without overt toxicity. Thus, ST-11 represents the first member of a new class of brain-penetrant antitubulin therapeutic agents. Mol Cancer Ther; 15(9); 2018–29. ©2016 AACR.

The ins and outs of endocannabinoid signaling in healthy and diseased brain

The endocannabinoid signaling system regulates neuronal activity and glial cell function. While numerous studies exist on the pharmacology and coupling of cannabinoid receptors, only recently have fundamental questions been addressed regarding endocannabinoid metabolism and physiopathological function. These lipids can be independently produced and differentially activate cannabinoid 1 and 2 receptors, a diversity that creates a unique level of control of neuronal activity and glial cell functions. This review outlines our current understanding of this diversity, focusing on the stimuli that control endocannabinoid production in the brain under healthy and pathological conditions. By increasing this understanding, new pharmacological targets are likely be identified, some of which might be valuable for the treatment of neurological disease.