Cannabinoid receptor type 2

The cannabinoid receptor type 2, abbreviated as CB₂, is a G protein-coupled receptor from the cannabinoid receptor family that in humans is encoded by the ''CNR2'' gene. It is closely related to the cannabinoid receptor type 1 (CB₁), which is largely responsible for the efficacy of endocannabinoid-mediated presynaptic-inhibition, the psychoactive properties of tetrahydrocannabinol (THC), the active agent in cannabis, and other phytocannabinoids (plant cannabinoids). The principal endogenous ligand for the CB₂ receptor is 2-Arachidonoylglycerol (2-AG).

CB₂ was cloned in 1993 by a research group from Cambridge looking for a second cannabinoid receptor that could explain the pharmacological properties of tetrahydrocannabinol. The receptor was identified among cDNAs based on its similarity in amino-acid sequence to the cannabinoid receptor type 1 (CB₁) receptor, discovered in 1990. The discovery of this receptor helped provide a molecular explanation for the established effects of cannabinoids on the immune system.

Structure

The CB₂ receptor is encoded by the CNR2 gene. Approximately 360 amino acids comprise the human CB₂ receptor, making it somewhat shorter than the 473-amino-acid-long CB₁ receptor.

As is commonly seen in G protein-coupled receptors, the CB₂ receptor has seven transmembrane spanning domains, a glycosylated N-terminus, and an intracellular C-terminus. The C-terminus of CB₂ receptors appears to play a critical role in the regulation of ligand-induced receptor desensitization and downregulation following repeated agonist application, perhaps causing the receptor to become less responsive to particular ligands.

The human CB₁ and the CB₂ receptors possess approximately 44% amino acid similarity. When only the transmembrane regions of the receptors are considered, however, the amino acid similarity between the two receptor subtypes is approximately 68%. The amino acid sequence of the CB₂ receptor is less highly conserved across human and rodent species as compared to the amino acid sequence of the CB₁ receptor. Based on computer modeling, ligand interactions with CB₂ receptor residues S3.31 and F5.46 appears to determine differences between CB₁ and CB₂ receptor selectivity. In CB₂ receptors, lipophilic groups interact with the F5.46 residue, allowing them to form a hydrogen bond with the S3.31 residue. These interactions induce a conformational change in the receptor structure, which triggers the activation of various intracellular signaling pathways. Further research is needed to determine the exact molecular mechanisms of signaling pathway activation.

Mechanism

Like the CB₁ receptors, CB₂ receptors inhibit the activity of adenylyl cyclase through their Gi/Go_α subunits. CB₂ can also couple to stimulatory Gα_s subunits leading to an increase of intracellular cAMP, as has been shown for human leukocytes. Through their G_βγ subunits, CB₂ receptors are also known to be coupled to the MAPK-ERK pathway, a complex and highly conserved signal transduction pathway, which regulates a number of cellular processes in mature and developing tissues. Activation of the MAPK-ERK pathway by CB₂ receptor agonists acting through the G_βγ subunit ultimately results in changes in cell migration.

Five recognized cannabinoids are produced endogenously: arachidonoylethanolamine (anandamide), 2-arachidonoyl glycerol (2-AG), 2-arachidonyl glyceryl ether (noladin ether), virodhamine, as well as N-arachidonoyl-dopamine (NADA). Many of these ligands appear to exhibit properties of functional selectivity at the CB₂ receptor: 2-AG activates the MAPK-ERK pathway, while noladin inhibits adenylyl cyclase.

Expression

Dispute

Originally it was thought that the CB2 receptor was only expressed in peripheral tissue while the CB1 receptor is the endogenous receptor on neurons. Recent work with immunohistochemical staining has shown expression within neurons. Subsequently, it was shown that CB2 knock out mice produced the same immunohistochemical staining, indicating the presence of the CB2 receptor where none was expressed. This has created a long history of debate as to whether the CB2 receptor is expressed in the CNS. A new mouse model was described in 2014 that expresses a fluorescent protein whenever CB2 is expressed within a cell. This has the potential to resolve questions about the expression of CB2 receptors in various tissues.

Immune system

Initial investigation of CB₂ receptor expression patterns focused on the presence of CB₂ receptors in the peripheral tissues of the immune system, and found the CB₂ receptor mRNA in the spleen, tonsils, and thymus gland. CB₂ expression in human peripheral blood mononuclear cells at protein level has been confirmed by whole cell radioligand binding. Northern blot analysis further indicates the expression of the CNR2 gene in immune tissues, where they are primarily responsible for mediating cytokine release. These receptors were localized on immune cells such as monocytes, macrophages, B-cells, and T-cells.

Brain

Further investigation into the expression patterns of the CB₂ receptors revealed that CB₂ receptor gene transcripts are also expressed in the brain, though not as densely as the CB₁ receptor and located on different cells. Unlike the CB₁ receptor, in the brain, CB₂ receptors are found primarily on microglia. The CB₂ receptor is expressed in some neurons within the central nervous system (e.g.; the brainstem), but the expression is very low. CB2Rs are expressed on some rat retinal cell types. Functional CB₂ receptors are expressed in neurons of the ventral tegmental area and the hippocampus, arguing for a widespread expression and functional relevance in the CNS and in particular in neuronal signal transmission.

Gastrointestinal system

CB₂ receptors are also found throughout the gastrointestinal system, where they modulate intestinal inflammatory response. Thus, CB₂ receptor is a potential therapeutic target for inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis. The role of endocannabinoids, as such, play an important role in inhibiting unnecessary immune action upon the natural gut flora. Dysfunction of this system, perhaps from excess FAAH activity, could result in IBD. CB₂ activation may also have a role in the treatment of irritable bowel syndrome. Cannabinoid receptor agonists reduce gut motility in IBS patients.

Peripheral nervous system

Application of CB₂-specific antagonists has found that these receptors are also involved in mediating analgesic effects in the peripheral nervous system. However, these receptors are not expressed by nociceptive sensory neurons, and at present are believed to exist on an undetermined, non-neuronal cell. Possible candidates include mast cells, known to facilitate the inflammatory response. Cannabinoid mediated inhibition of these responses may cause a decrease in the perception of noxious-stimuli.

Function

Immune system

Primary research on the functioning of the CB₂ receptor has focused on the receptor's effects on the immunological activity of leukocytes. To be specific, this receptor has been implicated in a variety of modulatory functions, including immune suppression, induction of apoptosis, and induction of cell migration. Through their inhibition of adenylyl cyclase via their Gi/Go_α subunits, CB₂ receptor agonists cause a reduction in the intracellular levels of cyclic adenosine monophosphate (cAMP). CB₂ also signals via Gα_s and increases intracellular cAMP in human leukocytes, leading to induction of interleukins 6 and 10. Although the exact role of the cAMP cascade in the regulation of immune responses is currently under debate, laboratories have previously demonstrated that inhibition of adenylyl cyclase by CB₂ receptor agonists results in a reduction in the binding of transcription factor CREB (cAMP response element-binding protein) to DNA. This reduction causes changes in the expression of critical immunoregulatory genes and ultimately suppression of immune function.

Later studies examining the effect of synthetic cannabinoid agonist JWH-015 on CB₂ receptors revealed that changes in cAMP levels result in the phosphorylation of leukocyte receptor tyrosine kinase at Tyr-505, leading to an inhibition of T cell receptor signaling. Thus, CB₂ agonists may also be useful for treatment of inflammation and pain, and are currently being investigated, in particular for forms of pain that do not respond well to conventional treatments, such as neuropathic pain. Consistent with these findings are studies that demonstrate increased CB₂ receptor expression in the spinal cord, dorsal root ganglion, and activated microglia in the rodent neuropathic pain model, as well as on human heptocellular carcinoma tumor samples.

CB₂ receptors have also been implicated in the regulation of homing and retention of marginal zone B cells. A study using knock-out mice found that CB₂ receptor is essential for the maintenance of both MZ B cells and their precursor T2-MZP, though not their development. Both B cells and their precursors lacking this receptor were found in reduced numbers, explained by the secondary finding that 2-AG signaling was demonstrated to induce proper B cell migration to the MZ. Without the receptor, there was an undesirable spike in the blood concentration of MZ B lineage cells and a significant reduction in the production of IgM. While the mechanism behind this process is not fully understood, the researchers suggested that this process may be due to the activation-dependent decrease in cAMP concentration, leading to reduced transcription of genes regulated by CREB, indirectly increasing TCR signaling and IL-2 production. Together, these findings demonstrate that the endocannabinoid system may be exploited to enhance immunity to certain pathogens and autoimmune diseases.

Clinical applications

CB₂ receptors may have possible therapeutic roles in the treatment of neurodegenerative disorders such as Alzheimer's disease. Specifically, the CB₂ agonist JWH-015 was shown to induce macrophages to remove native beta-amyloid protein from frozen human tissues. In patients with Alzheimer's disease, beta-amyloid proteins form aggregates known as senile plaques, which disrupt neural functioning.

Changes in endocannabinoid levels and/or CB₂ receptor expressions have been reported in almost all diseases affecting humans, ranging from cardiovascular, gastrointestinal, liver, kidney, neurodegenerative, psychiatric, bone, skin, autoimmune, lung disorders to pain and cancer. The prevalence of this trend suggests that modulating CB₂ receptor activity by either selective CB₂ receptor agonists or inverse agonists/antagonists depending on the disease and its progression holds unique therapeutic potential for these pathologies

Modulation of cocaine reward

Researchers investigated the effects of CB₂ agonists on cocaine self-administration in mice. Systemic administration of JWH-133 reduced the number of self-infusions of cocaine in mice, as well as reducing locomotor activity and the break point (maximum amount of level presses to obtain cocaine). Local injection of JWH-133 into the nucleus accumbens was found to produce the same effects as systemic administration. Systemic administration of JWH-133 also reduced basal and cocaine-induced elevations of extracellular dopamine in the nucleus accumbens. These findings were mimicked by another, structurally different CB₂ agonist, GW-405,833, and were reversed by the administration of a CB₂ antagonist, AM-630.

Ligands

Many selective ligands for the CB₂ receptor are now available.

Agonists

* Minocycline

Partial agonists

* GW-405,833

Unspecified efficacy agonists

* AM-1241
* HU-308
* JWH-015
* JWH-133
* L-759,633
* L-759,656

Herbal

* Echinacea purpurea

Inverse agonists

* AM-630
* BML-190
* JTE-907
* SR-144,528

Binding affinities

See also

* Cannabinoid receptor
* Cannabinoid receptor type 1 (CB₁)

References

External links

*

Cannabinoid Receptor 2 (CNR2) Human Protein Atlas

{{DEFAULTSORT:Cannabinoid Receptor 2 (Macrophage)
G protein-coupled receptors