Impact of the Cannabinoid System in Alzheimer's Disease
1 Shantou University Medical College, Brain Function and Disease Laboratory, Shantou, #22 Road Xinling, Guangdong 515041, China;
2 Department of Neurology, Yunfu People’s Hospital, Yunfu, Guangdong 527300, China
*Address correspondence to this author at the Shantou University Medical College, Brain Function and Disease Laboratory, China; E-mails: jiewu@stu.edu.cn or jiewubni@gmail.com # Shuangtao Li and Yuanbin Huang contributed equally to this paper.Abstract
Cannabinoids are compounds isolated from cannabis and are also widely present in both nervous and immune systems of animals. In recent years, with in-depth research on cannabinoids, their clinical medicinal value has been evaluated, and many exciting achievements have been continuously accumulating, especially in the field of neurodegenerative disease. Alzheimer's disease is the most common type of neurodegenerative disease that causes dementia and has become a global health problem that seriously impacts human health today. In this review, we discuss the therapeutic potential of cannabinoids for the treatment of Alzheimer’s disease. How cannabinoids act on different endocannabinoid receptor subtypes to regulate Alzheimer’s disease and the roles of the endocannabinoid system in Alzheimer’s disease are outlined, and the underlying mechanisms are discussed. Finally, we summarize the most relevant opportunities of cannabinoid pharmacology related to Alzheimer’s disease and discuss the potential usefulness of cannabinoids in the clinical treatment of Alzheimer’s disease.
1INTRODUCTION
Neurodegenerative diseases are a group of heterogeneous diseases characterized by the progressive and selective loss of anatomically- or physiologically-related neuronal systems [1]. Alzheimer’s disease (AD) is a highly complicated neurodegenerative disease that seriously affects human health. It was first described by German neuroscientist Alois Alzheimer in a 51-year-old patient Auguste Deter, who suffered from severe memory loss. During the autopsy of this patient, plaques and tangles in the cerebral cortex were found [2]. Subsequent studies have revealed that the histopathological features of AD involve the accumulation of β-amyloid (Aβ), intracellular aggregation of neurofibrillary tangles (NFTs), loss of a specific subset of neurons, and neuroinflammation resulting from glial activation [3-5]. Aβ and NFT are considered the main causes of disease progression; therefore, amyloid and phosphorylated microtubule-related tau proteins have become important targets for AD research. The production of amyloid begins with the cleavage of amyloid precursor protein (APP) on the plasma membrane by β-secretase (BACE1) and γ-secretase to produce insoluble Aβ fibrils. Then insoluble Aβ fibrils form oligomers, diffuse into the synaptic cleft, and interfere with synaptic signaling [6-8]. In AD patients, Aβ plaques are initially found in the basal, temporal lobe, and orbitofrontal neocortex areas of the brain and later spread throughout the entire neocortex, hippocampus, amygdala, diencephalon, and basal ganglia [9, 10]. In severe cases, Aβ is also found in the midbrain, lower brainstem, and cerebellar cortex, indicating that the accumulation of Aβ plaques is positively correlated with the course of AD [9, 10]. High concentrations of Aβ cause kinases to hyper-phosphorylate and activate microtubule-associated tau proteins, causing tau proteins to aggregate into insoluble NFTs. After plaques and tangles accumulate, microglia gather around the plaques. This promotes the activation of microglia and local inflammation and aggravates neurotoxicity, eventually leading to cognitive decline [11]. Importantly, AD is associated with an early loss of synaptic proteins, and the levels and distributions of some synaptic proteins have been found to be associated with dementia severity [12].
It has long been hypothesized that Aβ exerts toxicity in brain neurons [13, 14]. Although existing evidence reveals the harmful effects of Aβ on cellular Ca2+ homeostasis, neurotransmission, neuronal signaling, and receptor/ion channel function [15], the precise mechanisms of Aβ toxicity are still unclear. Particularly, which targets specifically mediate Aβ toxicity are still elusive. At present, the true causes of AD are still unclear, and even though decades of research have been conducted, there is still no drug that can effectively slow down the progression and improve the learning and memory deficits in Alzheimer's disease [16, 17]. Aβ accumulation and aggregation in neuritic or senile plaques and severe, selective cholinergic neuronal deficits are characteristic hallmarks of AD [13]. The extent of learning and memory deficits in AD is proportional to the degree of forebrain cholinergic neuronal degeneration, and the extent of Aβ deposition is used to characterize disease severity [13]. Processes, such as impairment of neurotrophic support and disorders of glucose metabolism, have been implicated in cholinergic neuronal loss and AD [18]. However, neurotoxic effects of Aβ across a range of in vivo or in vitro models suggested that Aβ plays a role in cholinergic neuronal degeneration and consequent learning and memory deficits [13, 19]; however, their mechanisms are still unclear. Therefore, understanding of such mechanisms is significant to help improve AD diagnosis and treatment.
Emerging lines of evidence indicate that a high level of Aβ induces neuronal or neurocircuit hyperexcitation. For example, chronic exposure to high levels of Aβ sensitizes some neuronal networks to hyperexcitation [20]. In animals that over-express Aβ, the high levels of Aβ peptide cause epileptiform activity within the entorhinal-hippocampal circuitry [21]. Westmark et al. compared seizure threshold (test response to pentylenetetrazol, PTZ) between AD model animals (Tg2576) and wild-type mice, and found a reduction in seizure threshold in AD model animals [22]. These shreds of evidence suggest that Aβ induces neuronal hyperexcitation, and reduction of this hyperexcitation may play a role in preventing/improving the pathogenesis of AD and could also open new therapeutic avenues. The cannabinoid system may perfectly fix this problem.
Thus far, there are 2 types of US FDA-approved drugs for the treatment of AD based on acetylcholinesterase inhibitors (donepezil, galantamine, and rivastigmine) [23] and NMDA receptor antagonists (memantine) [24]. All of these drugs are only symptomatic treatments, and none can prevent or delay the progression of AD, let alone cure AD [25], and unfortunately, promising preclinical results have repeatedly failed to be translated into clinical applications [26]. Although Aduhelm (aducanumab) has been approved recently by the FDA, its efficacy is still questionable [27].
Cannabis, as a controversial drug, has become increasingly prominent in the AD research field [28-30]. In the late 20th century, the first cannabis-derived compound was approved for clinical use, and subsequently, it was approved for treating neurological disorders [31-34]. For example, the approvals of nabiximols, a mixture of THC and the non-psychotropic cannabinoid, cannabidiol, for the treatment of spasticity and neuropathic pain in multiple sclerosis, and purified botanical cannabidiol for the treatment of otherwise untreatable forms of pediatric epilepsy have suggested a potential for clinical use of cannabinoids and endocannabinoids in neurological diseases [33]. Low concentrations of Δ9-tetrahydrocannabinol (THC) can reduce the Aβ level of N2a-variant amyloid-β protein precursor (APP) cells and inhibit Aβ aggregation by directly interacting with Aβ peptide [35]. THC attenuates Aβ accumulation in the human CNS cell line (MC65 cells) in a model whereby an inducible Aβ protein exerts toxicity through an inflammatory response [36]. In addition, THC can competitively inhibit enzyme acetylcholinesterase (AChE) by binding to its peripheral anion sites, thereby increasing ACh levels and reducing Aβ-peptide aggregation induced by AChE [37]. Furthermore, it was found that pretreatment with cannabidiol (CBD) can inhibit the expression of proteins involved in tau phosphorylation and Aβ production in gingival mesenchymal stem cells [38]. Therefore, it is suggested that cannabis can be used as a novel potential drug for the treatment of AD. In this review, we summarize recent discoveries and developments of cannabinoids and analyze the data of cannabinoids that regulate AD through the activation of cannabinoid receptor type 1 (CB1) receptors, cannabinoid receptor type 2 (CB2) receptors, and the endocannabinoid system.
2CANNABIS, CB1, AND CB2 RECEPTORS
The earliest use of cannabis as a drug can be traced back to around 2350 BC [39]. Cannabis contains a variety of compounds. The main components of cannabinoids are cannabidiol (CBD) and THC, both of which are lipophilic metabolites of resorcinol [40]. CBD was isolated in the late 1930s, but it was not until 1963 that the structure was defined [41] and identified as the active substance in hashish [42].
With in-depth studies on THC, the cannabinoid type 1 receptor (CB1R) was discovered in 1988 [43], and then the endogenous cannabinoid N-arachidonoyl-ethanolamine (AEA) was discovered in 1992 [44]. Later, a second endocannabinoid 2-arachidonic acid glyceride (2-AG) was identified [45]. These two endocannabinoids are derivatives of arachidonic acid, and they have different syntheses and metabolisms by different pathways.
Howlett was the first to report that cannabinoids may inhibit the formation of cAMP through receptors, and the efficacy of the cannabinoids examined is parallel to the level of their pharmacological effects [46]. Subsequently, the CB1R was cloned in 1990 [47]. Later, the cannabinoid type 2 receptor (CB2R) was found in the spleen in 1993 [48].
CB1R belongs to the superfamily of G protein-coupled receptors (GPCRs). The expressions of CB1Rs are distributed throughout the body in mammals, with CB1R expression highest in the basal ganglia in the central nervous system (CNS), including the substantia nigra (SNr), globus pallidus, hippocampal dentate gyrus, and the molecular layer of the cerebellum [49, 50], while the density of CB1Rs observed in the main sensory and motor areas is much lower [51, 52], suggesting that CB1Rs are involved in motivation (marginal) and cognitive (associative) information processing. Table 1 summarizes the CB1R distribution.
Cannabinoid receptor type 2 (CB2) is a plasma membrane G-protein-coupled receptor cloned in 1993 [64]. Since then, the expression and function of CB2Rs in the brain have been debated. Previous studies suggested that CB2Rs were absent in the brain since CB2 mRNA transcripts were not detected in rat brain using in situ hybridization (ISH) [48]. Consistent with this finding, Northern blot analysis also failed to detect CB2 mRNA in rat, mouse, and human brains [65-67]. Real-time polymerase chain reaction (RT-PCR) assays demonstrated abundant CB2 mRNA among immune tissues, such as the spleen and on macrophages, but barely detectable levels in rat and mouse brains [65, 66, 68, 69]. Based on these findings, CB2Rs have been classically considered ‘peripheral cannabinoid receptors’ [48, 70, 71]. Recently, this concept has been challenged by the identification of CB2Rs throughout the central nervous system (CNS) [72-74], particularly in microglial cells [75, 76], though they are expressed at lower densities than CB1Rs. When compared to CB1Rs, central CB2Rs exhibit the following unique features: (1) Lower expression levels, suggesting that these receptors may not mediate the effect of cannabis under normal physiological conditions, (2) Highly inducible, meaning that under certain pathological conditions (e.g., addiction, inflammation, stroke, schizophrenia, stress, anxiety, etc.), CB2R expression is enhanced in the brain [77], suggesting a close relationship between the alteration of CB2R expression/function and various psychiatric and neurological diseases, and (3) Exhibit special distribution, given that CB2Rs are chiefly expressed in neuronal somatodendritic areas [74] (postsynaptic) but CB1Rs are predominantly expressed on neuronal terminals, especially on GABAergic terminals (presynaptic), which leads to some opposing effects after activation by these two receptor subtypes [78]. Considering these characteristic features, the CB2R appears to be an important substrate for neuroprotection [79], and targeting CB2Rs will likely offer a novel therapeutic strategy for treating neuropsychiatric and neurological diseases without typical CB1R-mediated side-effects. Given these positive implications, an urgent need to understand the functional effects of CB2Rs in the brain, especially in the mesocorticolimbic system, has emerged within the scientific community.
Emerging evidence shows that significant CB2 mRNA has been detected by ISH in cultured granule cells among the granule layer and Purkinje cell layer of the mouse cerebellum [80], in mouse retina [81], and the globus pallidus of non-human primates [82]. RT-PCR analysis has also been used to detect CB2 mRNA expression in various brain regions, including the retina [81], cortex [82-85], striatum [66, 85], hippocampus [82], amygdala [84, 85], brainstem [72], and cerebellum [86]. Furthermore, two CB2 mRNA transcripts (CB2A and CB2B) have been identified in the rodent and human brain [83], along with a new CB2 transcript that has been found in mouse and human B lymphocytes [87]. Moreover, immunoblot and IHC assays have detected significant CB2-like bands or immunostaining in various brain regions [72, 73, 88-90]. This suggests a possibility that CB2R expression not only exists in peripheral tissues but also in the brain. As mentioned previously, CB2Rs mediate a variety of important modulations in DA-associated behaviors [91], including food intake, body weight [92-95], depression [96], anxiety [84, 97], and schizophrenia-like behavior [85, 98]. Recent reports emerging from several labs, including ours, have shown that brain CB2Rs play a pivotal role in the elimination of cocaine, alcohol, and nicotine addiction [99-101]. Collectively, these lines of evidence strongly suggest an important impact of CB2Rs on the mesocorticolimbic system as well as critical roles in various brain functions, including psychiatric, cognitive, and neurobiological activity. Table 2 summarizes CB2R distribution.
It has been shown that CB2Rs play an important role in neural precursor cell proliferation, axonal guidance, and synaptic transmission [111, 112]. As a G protein-coupled receptor, signal transduction initiated by the CB2R is mediated by Gi/o [113], increasing intracellular calcium levels by activating the phospholipase C (PLC) and inositol 1,4,5-trisphosphate (IP3) signaling pathways [114]. CB2Rs inhibit cAMP, thereby reducing intracellular cAMP levels [115]. In addition, CB2R activation can also be combined with other cellular pathways, including PKA, ERK1/2, and P38 [116, 117]. In contrast to CB1Rs, CB2R expression in the brain is relatively low, but it is highly inducible under pathological conditions, suggesting that CB2Rs are related to many neurological diseases [77, 118], and CB2Rs are mainly expressed in the postsynaptic soma dendritic region, so the activation of CB2Rs has an important protective effect on neurons [118, 119].
7LIMITATION OF BRAIN ECBS AS A THERAPEUTIC TARGET
The main challenges in the use of cannabinoids in the treatment of AD come from the following aspects. First, there are no obvious molecular markers in the diagnosis of AD disease, and timely treatment cannot be carried out [172, 173]. Secondly, the distribution of the ECBS in terms of time and space is difficult to control, which increases treatment difficulty [174]. For example, CB2Rs have great potential for treating AD, but the expression of CB2Rs in the central nervous system is relatively low; however, its expression is high in the periphery, and it is highly inducible [118]. Therefore, when and how to specifically target CB2Rs becomes a major challenge. Finally, endocannabinoids are naturally present in mammals, but so far, there is no specific agonist/antagonist regimen to avoid unwanted health outcomes to treat AD [175].
CONCLUSION
The ECBS, as a potential therapeutic target for AD, mainly involves the regulation of excitability mediated through CB1R and CB2Rs and is based on cannabinoids targeting several important processes involved in the pathogenesis of AD, such as beta-amyloid protein deposition and tau protein phosphorylation, inflammation, mitochondrial dysfunction, and excitatory neurotoxicity (Fig. 1).
ACKNOWLEDGEMENTS
The authors would like to thank Dr. Stanley Lin for reviewing this manuscript.
LIST OF ABBREVIATIONS
- AD
- Alzheimer’s Disease
- APP
- Amyloid Precursor Protein
- CNS
- Central Nervous System
- ECBS
- Endocannabinoid Receptor System
- LRP1
- Lipoprotein Receptor Protein 1
- NFTs
- Neurofibrillary Tangles
- SOD
- Superoxide Dismutase
CONSENT FOR PUBLICATION
Not applicable.
FUNDING
This work was supported by the Key Area Research and Development Program of Guangdong Province (2018B030334001), the 2020 Li Ka Shing Foundation Cross-Disciplinary Research Grant (2020LKSFG01A), and CNSF (81771437).
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or otherwise.
| Brain Regions | Tendency | Species | Neurological Disorders | References |
|---|---|---|---|---|
| Prefrontal cortex, hippocampus, and caudate putamen | Decrease | Humans | AD | [53-55] |
| Dorsal hippocampus (DH), basolateral amygdala complex (BLA) | Decrease | Mice(model) | 12-month-old AD | [56] |
| Cerebellum, dentate nucleus | Decrease | Mice(model) | Cerebellar ataxia | [57, 58] |
| Striatum | Decrease | Mice(model) | Huntington's disease | [59, 60] |
| Prefrontal and midcingulate cortex | Decrease | Humans, rat (model) | Parkinson's disease | [61-63] |
| Brain Regions | Tendency | Species | Neurological Disorders | References |
|---|---|---|---|---|
| Hippocampus and entorhinal and parahippocampal cortices | Increase | Humans and mice (model) | AD | [34, 55, 102] |
| Granular layer, Purkinje cells | Increase | Humans | Spinocerebellar ataxias | [103, 104] |
| Striatal microglia | Increase | Humans and mice (model) | Huntington's disease | [105, 106] |
| Substantia nigra microglial cells, striatal | Increase | Humans, rat (model) and mice (model) | Parkinson's disease | [107-109] |
| Spinal cord microglia | Increase | Mice (model) | Amyotrophic lateral sclerosis | [110] |