Neuroinflammation: Are Cannabinoids the Cure?
Cannabis, commonly known as marijuana, is a product of the Cannabis sativa plant and the active compounds from this plant are collectively referred to as cannabinoids (CBs). Studies to date indicate that the main pharmacological function of the cannabinoid system is in neuromodulation: controlling motor functions, cognition, emotional responses, homeostasis and motivation. However, in the periphery, this system is an important modulator of the ANS, immune system and microcirculation 1.
For several centuries, marijuana has been used as an alternative medicine in many cultures and, recently, its beneficial effects have been shown in 2 3 4:
- The treatment of nausea and vomiting associated with cancer chemotherapy.
- Anorexia and cachexia seen in HIV/AIDS patients.
- Neuropathic pain and spasticity in multiple sclerosis.
Insidious, low-level inflammation is responsible for many disease processes, ranging from osteoarthritis and cardiovascular disease, to digestive disorders and neurodegeneration. Cannabinoids, the bioactive metabolites, have been shown to possess powerful anti-inflammatory attributes, and research into their mechanisms of action, efficacy, and tolerability are on the rise.
The endocannabinoid system is an important biological regulatory system that has been shown to be highly conserved from lower invertebrates to higher mammals. Cannabinoid pharmacology has made important advances in recent years after the discovery of the cannabinoid receptors (particularly CB1 and CB2, sharing approximately 44% homology 5. Cannabinoid receptors and their endogenous ligands have provided an excellent platform for the investigation of the therapeutic effects of cannabinoids (reviewed in 6 7).
There are two well-characterized CB receptors with distinctly different physiological properties:
- The psychoactive effects of cannabinoids are associated with the CB1 receptor.
- The CB2 receptor mainly mediates anti-inflammatory and immunomodulatory
CB1 and CB2 are heterotrimeric Gi/o-protein-coupled receptors and that they are both expressed in the central nervous system (CNS) and periphery. However, CB1 expression is predominant in the CNS, especially on presynaptic nerves, and CB2 is primarily expressed on immune cells 8.
CB1 and CB2 receptors greatly differ in their tissue distribution. The CB1 receptor, originally identified in rat cerebral cortex, is primarily expressed in the CNS 3. This receptor subtype is also expressed in various peripheral tissues, such as testis, vascular endothelium, and small intestine 6. Its expression is heterogeneous within the nervous system and is mainly responsible for cannabinoid psychoactive properties. In contrast, the CB2 receptor was originally identified in the promyelocytic leukemic cell line HL60 and is prevalent within the immune system 5. All lineages of immune cells express the CB2 receptor, although its expression level varies among the cell types 1. CB2 receptor expression in healthy brains is limited to a few neurons in the brain stem 9. However, during neurological diseases, such as multiple sclerosis and Alzheimer’s disease, microglial cells, which are brain macrophages, express elevated levels of CB2 receptor 10 11.
Cannabinoid Signal Transduction Pathways- Mechanisms of Action
Unlike CB1 receptor-mediated cell activation, signal transduction through the CB2 receptor lacks psychotropic effects 12, making it an attractive target for immunotherapy.
Cannabinoid receptors are seven-transmembrane-spanning G protein-coupled receptors 13 14. G proteins are heterotrimers, and upon activation the α subunit dissociates from the βγ dimer (Figure 1). To summarise the pathway:
- The α subunit inhibits adenylate cyclase decreasing intracellular cAMP levels.
- cAMP-dependent protein kinase A activity diminishes leading to less active transcription factor cAMP response element-binding protein affecting gene expression.
- The G protein βγ dimer eventually leads to activation of the mitogen-activated protein kinase (MAPK) pathways and phosphatidylinositol-3 kinase (PI-3K).
Figure 1. Main signaling pathways activated by cannabinoid receptors. The canonical signaling pathway initiated by the binding of a cannabinoid to CBRs involves the coupling of the receptor to Gi/0 proteins. αi subunits can inhibit the activity of adenylyl cyclase (AC) and the synthesis of cAMP. This results in a decreased activation of PKA and an increased activation of potassium channels type A, which leads to membrane hyperpolarization. α0 subunits can in turn inhibit voltage dependent Ca2+ channels contributing to the inhibition of membrane depolarization. βγ subunits interact with other intracellular pathways related to PI3K or PKB/Akt. CBRs are also coupled to neutral sphingomyelinase (EMN), an enzyme that mediates the generation of ceramide from sphingomyelin (EM) in the plasma membrane. Ceramide acts as an intracellular signaling molecule than can activate several transcription factors including ERK, JNK and p38, and is involved, among other functions, on the control of cell fate and survival. AC: adenylyl cyclase; FAN: factor associated with neutral sphingomyelinase activation; N, P/Q: voltage-dependent calcium channels type N, P/Q; PKA: protein kinase A; PKB/Akt: protein kinase B; ERK: extracellular signal-regulated kinase; JNK: c-Jun N-terminal kinase; FAK: focal adhesion kinase; PI3K: phosphoinositide-3 kinase 13.
Neuroinflammation- The Role of Cannabinoids
The CNS is an immunologically-privileged organ with the innate and acquired immune response being closely controlled in relation with the periphery. The degeneration of the CNS is characterized by chronic progressive loss of the structure and functions of neuronal materials, resulting in functional and mental impairments 15. While the causes associated with neuronal degeneration remain poorly understood, the incidence of neurodegeneration increases with age, in mid-to-late adult life. A strong inflammatory response in the periphery from systemic infection, traumatic brain injury, toxic metabolites, or autoimmunity results in the subsequent infiltration of leukocytes from the periphery to the CNS with consequent neuroinflammation and neurodegeneration. An offense is followed by the initial activation of microglia, which induce the release of pro-inflammatory mediators that favour the permeabilisation of the blood-brain-barrier (BBB). The subsequent infiltration of peripheral leukocytes occurs inside of the CNS, including T cells and macrophages, which share several functional features with microglia 16. Although the response is initiated to protect the CNS from the infectious agent, the effect may be toxic and widespread inflammation as well as further migration of leukocytes through the BBB 17 (Figure 2).
Consequently, an acute neuro-inflammatory response is beneficial to the CNS, minimizing the injury by activating the innate immune system 18. By contrast, chronic inflammation is characterized by the long-standing activation of microglia that sustained release of inflammatory mediators, leading to an increase of oxidative and nitrosative stress which perpetuate the inflammatory cycle, further prolonging inflammation, which is detrimental for several neurodegenerative diseases 17 19 20.
Cannabinoids are potent anti-inflammatory agents and they exert their effects through:
- Induction of apoptosis
- Lowering excitotoxicity
- Enhancing vasodilation
- Decreasing the release of pro-inflammatory mediators
- Inhibition of cell proliferation
- Suppression of cytokine production
- Induction of T-regulatory cells
Figure 2. Relationship between microglial activation and neuronal death. Microglial activation may result in the release of neurotoxicity or neuroprotection. The increase of neurotoxic molecules favours neuroinflammation or neuronal death leading to neurodegeneration. ROS, reactive oxygen species 17.
Targeting The Endocannabinoid System For Therapeutic Effects
Medical marijuana continues to gain acceptance and become legalized in many areas of the USA. Two cannabinoids seem the most clinically relevant: tetrahydrocannabinol (THC), which tends to produce the psychotropic effects commonly associated with marijuana, and cannabidiol (CBD), which may produce therapeutic effects such as analgesia, decreased inflammation, decreased spasticity, and anti-seizure effects without appreciable psychoactive properties 8 (Figure 3).
Figure 3. The chemical structures of tetrahydrocannabinol (THC), which tends to produce the psychotropic effects commonly associated with marijuana, and cannabidiol (CBD).
Studies on Cannabis in the 1960s resulted in the pronouncement that THC was the ‘active’ principle and research then focused primarily on it to the virtual exclusion of CBD. This was probably due to the belief that activity meant psychoactivity that was shown by THC and not by CBD. In retrospect, this is unfortunate since several actions of CBD with potential therapeutic benefit were overlooked for many years.
Anti-inflammatory and neuroprotective effects of cannabinoids have been confirmed in animal models for MS, AD, stroke, ALS, and other animal models of diverse inflammatory diseases 7.
Microglial cells play a vital role in neuroinflammation and appear to be a special case in terms of cannabinoid receptor-mediated immune suppression. The consensus is that resting microglial cells express a low level of the CB1 receptor and lack CB2 receptor expression. However, microglial cells from diseased tissues or microglial cells activated in culture gain CB2 receptor expression 11 16. Neuroprotective effects of CB2 agonists are associated with suppression of microglia activation via inhibiting the release of neurotoxic factors and by decreasing neuronal cell damage 3. Importantly, differences in agonist or antagonist concentrations and cell activation state among the studies, may contribute to disparate findings concerning cannabinoid receptor participation. Additional investigation is needed to resolve this issue.
In summary, cannabinoids can be neuroprotective via their immunomodulatory properties, which have been mainly attributed to CB2 receptors. These findings have relevance to anti-inflammatory effects of CB2 stimulation in brain. CB2-mediated regulation of this pathway could be very important for cannabinoid regulation of neuroinflammation.
Neuroinflammatory disorders are conditions involving the immune response damage component of the nervous system 15. In the CNS, inflammatory effectors derived from innate and acquired immune systems as well as glial cells, particularly, microglia, act as sensors for distressed brain tissue homeostasis and accumulate locally in response to neuronal cell injury or foreign entry in the brain 16; the differential activation of microglia cells being the pivotal point that regulates neuroinflammation, which results in neurotoxicity or neuroprotection. The environmental exposure is therefore the critical element for the fate of neurons regarding degeneration or protection. Additional studies must be undertaken to benefit from the versatility of microglia, since activated microglia can also produce anti-inflammatory mediators and neurotrophic factors.
Abundant evidence demonstrates that cannabinoid receptors and their endogenous ligands play a crucial role in the regulation of the immune system 1. Exogenous cannabinoids have been shown to suppress T-cell-mediated immune responses by primarily inducing apoptosis and suppressing inflammatory cytokines and chemokines. Such observations indicate that targeting cannabinoid receptor–ligand interactions may constitute a novel window of opportunity to treat inflammatory and autoimmune disorders. As CB2 receptors are primarily expressed on immune cells, targeting CB2 may result in selective immunomodulation without toxicity. The future challenges for the use of cannabinoids as anti-inflammatory drugs include synthesis of cannabinoid receptor agonists that are non-psychoactive with anti-inflammatory activity and then identifying their mode of action 12 8. Thus, although current studies suggest that cannabinoids are useful therapeutic agents in the treatment of various inflammatory disorders, further evaluation of the mechanisms that account for their anti-inflammatory properties is necessary.
Whether cannabinoids and their receptors play a critical role during normal inflammatory response also requires further consideration. Moreover, cannabinoid receptor signalling and effect of cannabinoids on adhesion molecules, co-stimulatory molecules and chemokines require further study to increase our understanding of cannabinoids and their intricate effects on immune system disorders.
Overall, cannabinoids have exhibited significant potential to be used as novel anti-inflammatory agents and specific targeting of CB2 (and/or possibly CB1) receptors holds the promise of mediating immunosuppressive effects without exerting psychotropic side effects.
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