A Molecular Link Between the Active Component of Marijuana and Alzheimer’s Disease Pathology

Lisa M. Eubanks, Claude J. Rogers, Albert E. Beuscher IV, George F. Koob, Arthur J. Olson, Tobin J. Dickerson, and Kim D. Janda

Molecular Pharmacology, 2006, 3, 6, 773–777.

doi:10.1021/mp060066m

Abstract :
Alzheimer’s disease is the leading cause of dementia among the elderly, and with the ever-increasing size of this population, cases of Alzheimer’s disease are expected to triple over the next 50 years. Consequently, the development of treatments that slow or halt the disease progression have become imperative to both improve the quality of life for patients as well as reduce the health care costs attributable to Alzheimer’s disease. Here, we demonstrate that the active component of marijuana, Δ9-tetrahydrocannabinol (THC), competitively inhibits the enzyme acetylcholinesterase (AChE) as well as prevents AChE-induced amyloid β-peptide (Aβ) aggregation, the key pathological marker of Alzheimer’s disease. Computational modeling of the THC-AChE interaction revealed that THC binds in the peripheral anionic site of AChE, the critical region involved in amyloidgenesis. Compared to currently approved drugs prescribed for the treatment of Alzheimer’s disease, THC is a considerably superior inhibitor of Aβ aggregation, and this study provides a previously unrecognized molecular mechanism through which cannabinoid molecules may directly impact the progression of this debilitating disease.

Keywords : Cannabinoids; Alzheimer’s disease; Acetylcholinesterase

Introduction
Since the characterization of the Cannabis sativa-produced cannabinoid, Δ9- tetrahydrocannabinol (THC) (Figure 1), in the 1960’s,1 this natural product has been widely explored as an anti-emetic, anti convulsive, anti-inflammatory, and analgesic.2 In these contexts, efficacy results from THC binding to the family of cannabinoid receptors found primarily on central and peripheral neurons (CB1) or immune cells (CB2).3 More recently, a link between the endocannabinoid system and Alzheimer’s disease has been discovered4 which has provided a new therapeutic target for the treatment of patients suffering from Alzheimer’s disease.5 New targets for this debilitating disease are critical as Alzheimer’s disease afflicts over 20 million people worldwide, with the number of diagnosed cases continuing to rise at an exponential rate.6,7 These studies have demonstrated the ability of cannabinoids to provide neuroprotection against β-amyloid peptide (Aβ) toxicity.8-10 Yet, it is important to note that in these reports, cannabinoids serve as signaling molecules which regulate downstream events implicated in Alzheimer’s disease pathology and are not directly implicated as effecting Aβ at a molecular level.

One of the primary neuropathological hallmarks of Alzheimer’s disease is deposition of Aβ into amyloid plaques in areas of the brain important for memory and cognition.11 Over the last two decades, the etiology of Alzheimer’s disease has been elucidated through extensive biochemical and neurobiological studies, leading to an assortment of possible therapeutic strategies including prevention of downstream neurotoxic events, interference with Aβ metabolism, and reduction of damage from oxidative stress and inflammation.12 The impairment of the cholinergic system is the most dramatic of the neurotransmitter systems affected by Alzheimer’s disease and as a result, has been thoroughly investigated. Currently, there are four FDA-approved drugs that treat the symptoms of Alzheimer’s disease by inhibiting the active site of acetylcholinesterase (AChE), the enzyme responsible for the degradation of acetylcholine, thereby raising the levels of neurotransmitter in the synaptic cleft.13 In addition, AChE has been shown to play a further role in Alzheimer’s disease by acting as a molecular chaperone, accelerating the formation of amyloid fibrils in the brain and forming stable complexes with Aβ at a region known as the peripheral anionic binding site (PAS).14,15 Evidence supporting this theory was provided by studies demonstrating that the PAS ligand, propidium, is able to prevent amyloid acceleration in vitro, whereas active-site inhibitors had no effect.16 Due to the association between the AChE PAS and Alzheimer’s disease, a number of studies have focused on blocking this allosteric site.17 Recently, we reported a combined computational and experimental approach to identify compounds containing rigid, aromatic scaffolds hypothesized to disrupt protein-protein interactions.18-20 Similarly, THC is highly lipophilic in nature and possesses a fused tricyclic structure. Thus, we hypothesized that this terpenoid also could bind to the allosteric PAS of AChE with concomitant prevention of AChEpromoted Aβ aggregation.

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