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Cannabinoid receptor 2 (macrophage), also known as CB2 or CNR2, is a G protein-coupled receptor from the cannabinoid receptor family, which in humans is encoded by the CNR2 gene. It is closely related to the cannabinoid receptor 1 which is responsible for the psychoactive properties of tetrahydrocannabinol, the active principle of marijuana.
CB2 was cloned in 1993 by a research group from Cambridge looking for a second cannabinoid receptor which could explain the pharmacological properties of tetrahydrocannabinol, the active principle of marijuana.
Like the CB1 receptors, CB2 receptors inhibit the activity of adenylyl cyclase through their Gi/Goα subunits. Through their Gβγ subunits, CB2 receptors are also known to be coupled to the MAPK/ERK pathway, a complex and highly conserved signal transduction pathway, which critically regulates a number of important cellular processes in both mature and developing tissues. Activation of the MAPK-ERK pathway by CB2 receptor agonists acting on the Gβγ receptor subunit ultimately results in changes in cell migration as well as in an induction of the growth-related gene Zif268(also known as Krox-24, NGFI-A, and egr-1) The Zifi268 gene encodes a transcriptional regulator implicated in neuroplasticity and long term memory formation.
At present, there are five recognized cannabinoids which are produced endogenously throughout the body; these endocannabinoids include Arachidonoylethanolamine (anandamide), 2-arachidonoyl glycerol (2-AG), 2-arachidonyl glyceryl ether (noladin ether), virodhamine, as well as the recently-discovered N-arachidonoyl-dopamine (NADA). Many of these ligands appear to exhibit properties of functional selectivity at the CB2 receptor: 2-AG preferentially activates the MAPK-ERK pathway, while noladin preferentially inhibits adenylyl cyclase. Like noladin, the synthetic ligand CP-55,940 has also been shown to preferentially inhibit adenylyl cyclase in CB2 receptors. Similar ligand-specific signaling has also been demonstrated in the CB1 receptor. Together, these results support the emerging concept of agonist-directed trafficking at the cannabinoid receptors.
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. The CB2 receptor also contains a glycosylated N-terminus as well as 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; as a result of these processes, the cell may become less responsive to particular ligands.
The human CB1 and the CB2 receptors share approximately 44% amino acid similarity. When only the transmembrane regions of the receptors are considered, however, the amino acid similarly 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 in CB1 vs 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. Ultimately, these interactions induce a conformational change in the receptor structure, activating various intracellular signaling pathways. Further research is needed to determine the exact molecular mechanisms of signaling pathway activation, however.
Initial investigation of CB2 receptor expression patterns focused on the presence of CB2 receptors in the peripheral tissues of the immune system. For instance, CB2 receptor mRNA was found throughout the immune tissues of the spleen, tonsils and thymus gland. Northern blot analysis further indicates the expression of the CNR2 gene in immune tissues. These receptors were primarily localized on immune cells such as monocytes, macrophages, B-cells, and T-cells. Further investigation into the expression patterns of the CB2 receptors revealed that CB2 receptor gene transcripts are also widely distributed throughout the brain. The CB2 receptors are found primarily on microglia(the immune cells of the CNS) and not neurons, however. CB2 receptors are also found throughout the gastrointestinal system, where they modulate intestinal inflammatory response.  Thus, CB2 receptor agonists are a potential therapeutic target for inflammatory bowel diseases, such as Crohn’s disease and ulcerative colitis.
Functions and Clinical ApplicationsEdit
Primary research on the functioning of the CB2 receptor has focused on the receptor's effects on the immunological activity of leukocytes. Through their inhibition of adenylyl cyclase via their Gi/Goα subunits, CB2 receptor agonists cause a reduction in the intracellular levels of cyclic adenosine monophosphate (cAMP). 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 transcription factor CREB (cAMP response element binding protein) binding to DNA; this in turn causes changes in the expression of critical immunoregulatory genes, and ultimately a suppression of immune function. Later studies examining the effect of synthetic cannabinoid agonist JWH-015 on CB2 receptors revealed that changes in cAMP levels resulted in the phosphorylation of leukocyte receptor tyrosine kinase at Tyr-505; through this mechanism, T cell receptor signaling was inhibited. These results further demonstrate the immunosuppressive properties of CB2 receptor agonists. Thus, CB2 agonists may also be useful for treatment of inflammation and pain, and they are currently being investigated particularly for forms of pain that do not respond well to conventional treatments, such as neuropathic pain.
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 patient's 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, and modulating CB2 receptor activity by either selective CB2 receptor agonists or inverse agonists/antagonists (depending on the disease and its stage) holds unique therapeutic potential in these pathologies 
Many selective ligands for the CB2 receptor are now available.
Antagonists and inverse agonistsEdit
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- ↑ Entrez Gene: CNR2 cannabinoid receptor 2 (macrophage).
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