Cannabidiol for neurodegenerative disorders: A comprehensive review

Abstract

Introduction

With the increase in life expectancy worldwide, the prevalence of neurodegenerative diseases (NDs) has become a concern due to their socio-economic impact. NDs are characterized by progressive loss of structure and function of nerve cells in the central and/or peripheral nervous systems (Katsnelson et al., 2016). The structural loss of neurons and synapses including demyelination and/or neuronal death causes gradual loss of motor and/or cognitive skills, eventually leading to functional loss. Aging is found to be a major risk for NDs. Alzheimer’s disease (AD) and Parkinson’s disease (PD) are the most common NDs, while the other forms include amyotrophic lateral sclerosis (ALS), Huntington’s disease, spinal muscular atrophy, motor neuron diseases, prion diseases, and spinocerebellar ataxia. However, irrespective of their differences in pathophysiology, which are determined by observing the aggregated characteristic protein via neuropathological evaluation at autopsy (Dugger and Dickson 2017), chronic neuroinflammation is the fundamental pathological mechanism driving both onsets and the progression of the NDs (Kinney et al., 2018).

Oxidative stress (OS) arises from redox imbalance in the cell when the cellular antioxidant systems fail to counter the excess of reactive species. Excess accumulation of reactive species causes structural alteration in protein, DNA, and cell membrane which leads to neuronal death. Persistent inflammation also plays a major role in neurodegeneration (Glass et al., 2010). The insoluble protein aggregates and/or oxidized lipid molecules cause microglial activation, a key factor in the pathogenesis of NDs. Activated microglia release inflammatory mediators that again activate astrocytes to produce reactive species and inflammatory cytokines, which in turn further activate microglia, and thereby a feed-forward loop is established favoring chronic oxidative stress and persistent neuroinflammation. Neuronal cells, especially those present in the CNS, are more susceptible to oxidative stress due to their high metabolic rate (Salim, 2017). In aging with poor self-renewal capacity of neuronal cells and a gradual decline in activity of the antioxidant enzymes to counterbalance the excessive reactive species, neuronal death becomes prevalent (Hou et al., 2019). In addition, in aging with a retarded rate of autophagy, clearance of dysfunctional mitochondria or aggregated protein becomes difficult, further favoring the environment for chronic oxidative stress and neuroinflammation. Though the gold standards for treating such diseases, for example, l-DOPA for Parkinson’s disease and small molecule acetylcholinesterase inhibitors for AD, provide symptomatic relief (Rizek et al., 2016; Marucci et al., 2021), the discovery of druggable targets and drugs that can mitigate disease progression still remains an unmet challenge.

For the past few decades, cannabinoids, secondary plant metabolites extracted from Cannabis sativa, especially ∆⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD), have garnered significant attention (Cooray et al., 2020). Despite the anecdotal history of cannabis’s medicinal use since the medieval era, systematic medicinal exploration started in 1970 after its widespread use as a “recreational drug.” The profound behavioral effect of cannabis including euphoria and analgesia led to investigating the potential receptors in humans and rodents, leading to the discovery of the endocannabinoid system (ECS), which regulates several physiological processes such as cognition and analgesia. ECS consists of two primary receptors of the G-protein-coupled receptor (GPR) family: cannabinoid receptors 1 and 2 (CB1R and CB2R), two primary ligands known as endocannabinoids (namely, anandamide (AEA) and 2-arachidonoylglycerol (2AG)) and enzymes (namely, fatty acid amide hydrolase (FAAH) and monoglyceride lipase (MAGL)) that control the release and degradation of endocannabinoids (Alexander, 2016). CB1R is predominantly found in the nerve terminals of the CNS and in peripheral neural and other tissue (e.g., gastrointestinal tract) with lower density. The two endocannabinoids, AEA and 2AG (Figure 1A), act as retrograde synaptic messengers. In response to a transient intracellular Ca²⁺ elevation, endocannabinoids are released from post synapses in the synaptic space, travel to the presynaptic cleft, and activate the CB1 receptor (CB1R). Activation of CB1R blocks the release of neurotransmitters such as GABA (an inhibitory neurotransmitter) and glutamate (an excitatory neurotransmitter). AEA and 2AG are finally degraded by FAAH and MAGL, respectively. On the other hand, CB2R is mostly found in immune-associated cells and acts as an immunomodulator that can inhibit immune cell migration.

Some cannabinoids can mimic the effect of endocannabinoids and can regulate nerve impulses in certain pathways (Piomelli, 2003). For instance, THC acts as a partial agonist of the CB1 receptor (Ki = ∼10 nM) and exerts its psychoactive effect by activating the CB1 receptor (Kimura et al., 2019). Previously, major research on ECS has focused on THC, the major psychoactive component of W extract. However, recently, CBD has attracted major attention due to its neuroprotective activity (Cassano et al., 2020), its non-psychoactive nature, and well-tolerated pharmacological behavior (Bergamaschi et al., 2011). Although CBD has a low affinity for CB1 (Ki = ∼2,000 nM) (Kimura et al., 2019), it indirectly activates CB1 by inhibiting FAAH which aids in the accumulation of AEA (Figure 1B) and prevents glutamate-induced excitotoxicity on neurons (Peres et al., 2018). CBD acts as an inverse agonist of CB2R. Importantly, much of the recent evidence suggested that CBD has high anti-inflammatory and antioxidant activities which play a major role in its neuroprotective behavior, and it is mediated via multiple molecular targets other than CB1R and CB2R. In this review article, we have briefly discussed the role of oxidative stress and neuroinflammation in neurodegeneration and provided an overview of the general mechanism of neuroprotective behavior of CBD along with molecular targets. In addition, we have summarized clinical and preclinical findings on the therapeutic promise of CBD in PD and AD.

Neurodegeneration: inflammation and oxidative stress

Persistent neuroinflammation and chronic oxidative stress play a crucial role in the pathogenesis of neurodegenerative diseases. Microglia cells, the immune cells of the brain, act as a “double-edged sword” (Hickman et al., 2018). It normally acts as basic housekeeping, which maintains regular neuronal function, tissue repair, and brain homeostasis. However, in the presence of persistent inflammatory stimulus, microglia shift to their activated forms, which release inflammatory mediators and reactive species that can directly cause neuronal death and activate astrocytes and neurons to express more inflammatory mediators and RS. Once the feed-forward loops of inflammatory responses are established, the normal resolution mechanisms are overwhelmed, leading to neurodegeneration (Glass et al., 2010).

The inflammatory response represents a highly regulated cascade of events containing three major types of agents: 1) inducers and sensors, 2) transduction systems, and 3) amplifiers and effectors (Glass et al., 2010). In the initiation phase, inflammation stimuli are recognized by some specific cell surface receptors of microglia. For instance, pathogen-associated molecular patterns from an infectious agent (e.g., lipopolysaccharide (LPS) from Gram-negative bacteria) are recognized by pattern recognition receptors (PRRs) as “stranger” signals, while components released from necrotic cells are recognized as “danger signals.” ATP released following traumatic injury and neuronal death is sensed by purinergic receptors on microglia and astrocytes, while the “scavenger receptors” respond to oxidized lipids, proteins, and apoptotic cells. In the second phase, activation of such cell surface-sensing receptors leads to the activation of some signal transduction pathways (e.g., IkB kinase and mitogen-activated protein kinase, i.e., MAPK), which further control the activities of multiple transcription factors such as nuclear factor-κB (NF-κB) and activator protein 1 (AP-1). These transcription factors work together to regulate the expression of cytokines (e.g., tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β)) which propagate the inflammation (amplifier) and chemokines (e.g., COX-2) that recruit additional immune cells (effector cells) depending on the target cell. In addition, reactive species (e.g., ROS and inducible nitric oxide synthase (iNOS)) are produced as an anti-microbial defense mechanism of cells (Glass et al., 2010).

Amyloid-β (Aβ) aggregates and neurofibrillary tangles of tau protein (NFT), the proteins involved in AD, create a stress response in neurons. These aggregates also interact with microglia and astrocytes via cell surface receptors (such as TLRs and the receptor for advanced glycation endproducts (RAGE)) to produce several factors, such as pro-inflammatory cytokines (e.g., TNF-α, IL-1β, and IL-6), chemokines (e.g., COX-2), ROS, NO, prostaglandins (e.g., PGE2), and caspase that promote the death of cholinergic neurons (Figure 2A). Genetic, environmental, and age-related factors contribute to the initiation of inflammation (Migliore and Coppedè, 2009; Glass et al., 2010). For instance, the expression of ApoE4, the strongest genetic factor of AD (Liu et al., 2013), causes defects in the microglial clearance of Aβ. ApoE is a lipid transporter protein that participates in the clearance of Aβ by microglia via a low-density lipoprotein receptor (LDL-R) or LDL-R-related protein 1 (LRP1) receptor, and the binding of ApoE to LDL-R or LRP1 suppresses expression of the stress-responsive kinase JNK pathway. However, the efficiency of ApoE in clearing Aβ is isoform-specific. The ApoE4/Aβ complex is the least preferred compared with the other isoforms of ApoE for microglial uptake by the LRP1 receptor, which results in the accumulation of Aβ with the high expression of ApoE4 (Pocivavsek et al., 2009). Other than genetic factors, environmental factors such as obesity due to a high-calorie diet also induce a low-grade but persistent inflammation. Activated microglia release TNF-α and IL-1β that activate astrocytes, which in turn activate microglia (Liu et al., 2020). Furthermore, the cholinergic neuron itself has NF-κB sites in the promoters of amyloid precursor protein (APP), presenilin, and BACE1 (beta-secretase 1) (Sastre et al., 2008) which produces more Aβ upon inflammatory stimuli and further amplifies microglia-mediated inflammation. Thus, neurons and glia act in combination to amplify the production of neurotoxic factors in AD pathology. Similarly, in PD, the process of aggregation of α-synuclein, the major factor involved in PD, from monomers to fibrils via oligomeric intermediate is considered to be neurotoxic, causing neuronal death (Danzer et al., 2007). Neuronal death itself and subsequent extracellularly released aggregated α-synuclein can induce microglial activation via the purinergic receptor (Figure 2B). Aggregated α-synuclein is sensed and internalized by microglia via cell surface gangliosides (Park et al., 2009), resulting in microglial activation which produces pro-inflammatory cytokines (e.g., TNF-α and IL-1β) and mediators of oxidative stress including ROS and NO (Reynolds et al., 2008). Dopaminergic neurons in the substantia nigra are vulnerable to oxidative stress (Kastner et al., 1992) due to their enhanced intracellular oxidative processes. ROS production via the activation of NADPH oxidase upon internalization of α-synuclein is a crucial factor for microglia activation in PD pathology.

OS plays a major role in neurodegeneration, especially in the CNS, which is vulnerable to OS due to its high metabolic rate and limited cell renewal capacity. In CNS inflammation, NADPH oxidases (NOX), NOS, and dysfunctional mitochondria are major sources of reactive species (Fischer and Maier, 2015). ROS can directly damage neurons, facilitate protein aggregates via impaired proteosomal degradation, and initiate inflammation via lipid peroxidation which causes microglial activation. In aging with a reduced rate of autophagy, the removal of dysfunctional mitochondria and aggregated proteins becomes compromised, contributing to the risk of age-related neurodegeneration. In AD, oxidative stress activates some kinases (e.g., JNK, p38 MAPK, and GSK-3β) which promote the generation of Aβ (Tamagno et al., 2005) and NFT (Lovell et al., 2004). Sustained activation of kinases also causes neuronal apoptosis. In PD, α-synuclein can directly impair the mitochondrial complex I in dopaminergic neurons which are vulnerable to oxidative stress due to their high rate of internal dopamine metabolism (Devi et al., 2008). Thus, prevention of oxidative stress and neuroinflammation may hold promise in mitigating the progression of neurodegenerative disease.

Therapeutic benefits of CBD and general mechanism of neuroprotection

Phytocannabinoids have garnered significant attention lately due to their inherent neuroactive and strong antioxidant properties. Approval of Sativex, a combination of THC and CBD (1:1), in treating spasticity and/or neuropathic pain in multiple sclerosis (MS) in 2005, became a breakthrough in cannabis research. CBD, beyond reducing the adverse effect of THC when used in combination (Russo and Guy 2006), can individually exhibit a broad spectrum of therapeutic benefits. CBD alone has been FDA-approved for treating treatment-resistant pediatric epilepsy in 2018 (Devinsky et al., 2016). In addition, its anticonvulsant, antiemetic, and sleep-inducing properties have been explored to treat epilepsy and sleep disorders, as well as for treating psychiatric disorders such as schizophrenia, anxiety, and depression (Fernández-Ruiz et al., 2013). CBD showed a better safety profile and tolerability in patients (1,500 mg/day) than THC (Bergamaschi et al., 2011). Unlike THC, CBD did not alter cardiovascular parameters, psychomotor, and psychological functions, presumably because CBD does not directly target the CB1 receptor (Pertwee, 2006).

Emerging preclinical research studies demonstrate that CBD can exhibit neuroprotective effects due to its antioxidant and anti-inflammatory properties, which may potentially treat different neurodegenerative diseases (Hampson et al., 1998; Esposito et al., 2006; Esposito, Scuderi et al., 2007). For instance, CBD can act as a scavenger of reactive oxygen species (ROS) and inhibit lipid peroxidation, which inhibits caspase-mediated apoptosis of neurons (Iuvone et al., 2004). Interestingly, the neuroprotective effect of CBD is stronger than that of common antioxidants such as α-tocopherol or ascorbate (Atalay et al., 2019), which indicates that CBD may interact with some molecular targets. CBD has a low affinity for CB1 and CB2 receptors; however, it can indirectly activate CB1R via inhibition of FAAH and thereby promote the accumulation of endocannabinoids, which may prevent glutamate-mediated neurotoxicity (Pertwee 2008). In addition, CBD can also prevent glutamate-induced neurotoxicity occurring via the N-methyl-d-aspartate (NMDA) receptor, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or kainate receptor in the cannabinoid receptor-independent pathway as demonstrated by Hampson et al. (1998).

CBD can also prevent neuroinflammation by acting on multiple molecular targets (Figure 3). CBD can reduce the expression of iNOS in neurons via the downregulation of NF-κB and phosphorylated p38 MAPK (Esposito et al., 2006). CBD, being an inverse agonist of the CB2 receptor (Thomas et al., 2007), can inhibit microglial activation in a CB2 receptor-associated pathway (Ramírez et al., 2005). Peroxisome proliferation-activated receptor gamma (PPARγ) (Hegde et al., 2015; O’Sullivan, 2016) is an important target of CBD. PPARγ is a member of the ligand-activated transcriptional factor of the nuclear hormone receptor superfamily, which is a key mediator of energy homeostasis and associated with many biological functions including inflammation (Daynes and Jones, 2002). Several studies have reported that PPAR-γ agonists exhibit neuroprotective effects via regulating gene transcription related to the pathogenesis of neurodegeneration (Kumar et al., 2020). CBD also exhibits neuroprotective and anti-inflammatory effects via the A_2A receptor where it can stimulate A_2A-mediated signaling in immune cells by inhibiting the uptake of adenosine (Carrier Erica et al., 2006). Activation of the A_2A receptor by CBD decreases microglial activation in an ATP-dependent pathway (Martín-Moreno et al., 2011) and can improve cognitive and motor function via inhibition of TNF-α expression and increasing expression of brain-derived neurotrophic factor (BDNF) (Magen et al., 2009). In addition, CBD can also target the TRPV1 receptor to exhibit its neuroprotective effect (Giuliano et al., 2021).

CBD also has a significant effect on improving mitochondrial dysfunction. Emerging evidence suggests that mitochondrial dysfunction via both mitochondrial iron accumulation and ROS production is directly related to neurodegenerative disorders. To this end, da Silva et al. have demonstrated that CBD can reverse iron-induced mitochondrial dysfunction by rescuing mitochondrial ferritin and epigenetic modulation of mitochondrial DNA (da Silva et al., 2018). CBD can also act as a modest affinity agonist toward the 5-HT1A (serotonin) receptor (Russo et al., 2005), which is present in the synaptic membrane of several regions of the brain (Hoyer et al., 1986) and exhibits its anti-depression property.

Disease-specific pathway

Conclusion and future perspectives

Despite the significant advancement in neurology, the cure of neurodegenerative diseases such as AD and PD continues to remain an unmet challenge. Emerging evidence indicates that oxidative stress and inflammation are common amplifiers of disease progression, and the inhibition of neuroinflammation may result in clinical benefit in treating NDs. However, care should be taken when choosing anti-inflammatory drugs that may have a potential adverse effect due to activation of the innate immune system. In addition, as neurodegenerative diseases require long-term therapy, the high safety profile of the drug is a crucial factor. Furthermore, to be clinically effective, the therapeutics should gain access to the CNS and specifically target the cells or pathways. To this end, non-psychoactive phytocannabinoid CBD holds significant promises due to their antioxidant and anti-inflammatory properties, high safety profile, and good tolerability. Specifically, neuroprotection via PPARγ activation by CBD may hold significant promise as rosiglitazone, an agonist of PPARγ, exhibited prominent clinical benefit either alone (phase 2 clinical trials) or in combination with donepezil (Phase 3) in treating AD patients. However, a thorough investigation of the required dosages and treatment windows in different transgenic mouse models is necessary. Importantly, rodent age and sex are crucial factors when evaluating the therapeutic effectiveness of CBD in some neurodegenerative diseases.

Clinical application of CBD has just started with FDA approval of Epidiolex (CBD in oil formulation) in 2018 for treating pediatric epilepsy. However, despite its multi-faceted therapeutic potential, its clinical approval is impeded by intrinsic characteristics such as poor bioavailability and variable pharmacokinetics profile (Millar et al., 2020). Potential ways to overcome this including usage of drug delivery systems and/or different routes of drug administration are in the preclinical and clinical stages of development. For example, to avoid the precipitation of CBD in the gastrointestinal tract, CBD is used in formulation with oils, surfactants, solvents, or cyclodextrin carriers which are being investigated while the i.p. route of administration exhibits a better brain accumulation than the oral route. Considering the general trend of PK study regarding a small percentage of brain accumulation of therapeutics, nano- or micro-sized droplets of such formulations have gained significant importance. In addition, oxidative degradation and drug–drug interaction (when used in combination) of CBD are crucial factors that should be taken care of. To this end, investigation of lipid-based nanocarriers that can solubilize CBD in lipid core has just started (Aparicio-Blanco et al., 2019). Herein, the brain-targeting lipophilic nanocarrier which can solubilize the CBD in its lipid core and deliver a payload to the brain holds significant promise. Collectively, CBD is harmless at a low dose, and preclinical studies indicate its beneficial neuroprotective effect. Clinical trials for CBD in conventional neurodegenerative diseases have started showing its initial promise, especially in improving behavioral health. Thus, CBD may find its clinical application beyond pediatric epilepsy not only to conventional neurodegenerative diseases but also to neurodegenerative conditions secondary to other CNS complications either alone or as an adjuvant in the near future.

References