The cannabinoid receptor type 2, abbreviated as CB
2, is a
G protein-coupled receptor from the
cannabinoid receptor
Cannabinoid receptors, located throughout the body, are part of the endocannabinoid system a class of cell membrane receptors in the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid recept ...
family that in humans is encoded by the ''CNR2'' gene.
It is closely related to the
cannabinoid receptor type 1
Cannabinoid receptor type 1 (CB1), also known as cannabinoid receptor 1, is a G protein-coupled cannabinoid receptor that in humans is encoded by the ''CNR1'' gene. The human CB1 receptor is expressed in the peripheral nervous system and centra ...
(CB
1), which is largely responsible for the efficacy of endocannabinoid-mediated presynaptic-inhibition, the psychoactive properties of
tetrahydrocannabinol
Tetrahydrocannabinol (THC) is the principal psychoactive constituent of cannabis and one of at least 113 total cannabinoids identified on the plant. Although the chemical formula for THC (C21H30O2) describes multiple isomers, the term ''THC'' ...
(THC), the active agent in
cannabis
''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: ''Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternativel ...
, and other
phytocannabinoid
Cannabinoids () are several structural classes of compounds found in the cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid tet ...
s (plant cannabinoids).
The principal endogenous ligand for the CB
2 receptor is
2-Arachidonoylglycerol
2-Arachidonoylglycerol (2-AG) is an endocannabinoid, an endogenous agonist of the CB1 receptor and the primary endogenous ligand for the CB2 receptor. It is an ester formed from the omega-6 fatty acid arachidonic acid and glycerol. It is present ...
(2-AG).
CB
2 was
cloned
Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, ...
in 1993 by a research group from Cambridge looking for a second cannabinoid receptor that could explain the pharmacological properties of
tetrahydrocannabinol
Tetrahydrocannabinol (THC) is the principal psychoactive constituent of cannabis and one of at least 113 total cannabinoids identified on the plant. Although the chemical formula for THC (C21H30O2) describes multiple isomers, the term ''THC'' ...
.
The receptor was identified among
cDNA
In genetics, complementary DNA (cDNA) is DNA synthesized from a single-stranded RNA (e.g., messenger RNA (mRNA) or microRNA (miRNA)) template in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is often used to express a spe ...
s based on its similarity in amino-acid sequence to the cannabinoid receptor type 1 (CB
1) 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 CB
2 receptor is encoded by the CNR2 gene.
Approximately 360
amino acids
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
comprise the human CB
2 receptor, making it somewhat shorter than the 473-amino-acid-long CB
1 receptor.
As is commonly seen in G protein-coupled receptors, the CB
2 receptor has seven transmembrane spanning domains,
a glycosylated
N-terminus, and an intracellular
C-terminus
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain ( protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
.
The C-terminus of CB
2 receptors appears to play a critical role in the regulation of ligand-induced receptor desensitization and
downregulation
In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary pr ...
following repeated agonist application,
perhaps causing the receptor to become less responsive to particular ligands.
The human CB
1 and the CB
2 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 CB
2 receptor is less highly conserved across human and rodent species as compared to the amino acid sequence of the CB
1 receptor.
Based on computer modeling, ligand interactions with CB
2 receptor residues S3.31 and F5.46 appears to determine differences between CB
1 and CB
2 receptor selectivity.
In CB
2 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 biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors.
A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or othe ...
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 CB
1 receptors, CB
2 receptors inhibit the activity of
adenylyl cyclase
Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
through their Gi/Go
α subunits.
CB
2 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, CB
2 receptors are also known to be coupled to the
MAPK-ERK pathway
The MAPK/ERK pathway (also known as the Ras-Raf-MEK-ERK pathway) is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
The signal starts when a signaling ...
,
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 CB
2 receptor
agonists
An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ag ...
acting through the G
βγ subunit ultimately results in changes in
cell migration
Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular direct ...
.
Five recognized
cannabinoids
Cannabinoids () are several structural classes of compounds found in the cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid te ...
are produced endogenously:
arachidonoylethanolamine
Anandamide (ANA), also known as ''N''-arachidonoylethanolamine (AEA), is a fatty acid neurotransmitter. Anandamide was the first endocannabinoid to be discovered: it participates in the body's endocannabinoid system by binding to cannabinoid r ...
(anandamide),
2-arachidonoyl glycerol (2-AG),
2-arachidonyl glyceryl ether
2-Arachidonyl glyceryl ether (2-AGE, Noladin ether) is a putative endocannabinoid discovered by Lumír Hanuš and colleagues at the Hebrew University of Jerusalem, Israel. It is an ether formed from the alcohol analog of arachidonic acid and gl ...
(noladin ether),
virodhamine
Virodhamine (''O''-arachidonoyl ethanolamine; O-AEA) is an endocannabinoid and a nonclassic eicosanoid, derived from arachidonic acid. ''O''-Arachidonoyl ethanolamine is arachidonic acid and ethanolamine joined by an ester linkage, the opposite o ...
,
as well as
N-arachidonoyl-dopamine
''N''-Arachidonoyl dopamine (NADA) is an endocannabinoid that acts as an agonist of the CB1 receptor and the transient receptor potential V1 (TRPV1) ion channel. NADA was first described as a putative endocannabinoid (agonist for the CB1 rece ...
(NADA).
Many of these ligands appear to exhibit properties of
functional selectivity
Functional selectivity (or “agonist trafficking”, “biased agonism”, “biased signaling”, "ligand bias" and “differential engagement”) is the ligand-dependent selectivity for certain signal transduction pathways relative to a referen ...
at the CB
2 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 CB
2 receptor expression patterns focused on the presence of CB
2 receptors in the peripheral tissues of the
immune system,
and found the CB
2 receptor
mRNA in the
spleen,
tonsils, and
thymus gland
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. ...
.
CB
2 expression in human peripheral blood mononuclear cells at protein level has been confirmed by whole cell radioligand binding.
Northern blot
The northern blot, or RNA blot,Gilbert, S. F. (2000) Developmental Biology, 6th Ed. Sunderland MA, Sinauer Associates. is a technique used in molecular biology research to study gene expression by detection of RNA (or isolated mRNA) in a sampl ...
analysis further indicates the expression of the CNR2 gene in immune tissues,
where they are primarily responsible for mediating
cytokine
Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autoc ...
release.
These receptors were localized on immune cells such as
monocytes,
macrophages,
B-cells
B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
, and
T-cells
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell re ...
.
Brain
Further investigation into the expression patterns of the CB
2 receptors revealed that CB
2 receptor gene transcripts are also expressed in the
brain, though not as densely as the CB
1 receptor and located on different cells.
Unlike the CB
1 receptor, in the brain, CB
2 receptors are found primarily on
microglia
Microglia are a type of neuroglia (glial cell) located throughout the brain and spinal cord. Microglia account for about 7% of cells found within the brain. As the resident macrophage cells, they act as the first and main form of active immune de ...
.
The CB
2 receptor is expressed in some neurons within the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
(e.g.; the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...
), but the expression is very low. CB2Rs are expressed on some rat retinal cell types.
Functional CB
2 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
CB
2 receptors are also found throughout the
gastrointestinal system, where they modulate intestinal inflammatory response.
Thus, CB
2 receptor is a potential therapeutic target for
inflammatory bowel diseases
Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine, Crohn's disease and ulcerative colitis being the principal types. Crohn's disease affects the small intestine and large intestine, as well a ...
, such as
Crohn's disease
Crohn's disease is a type of inflammatory bowel disease (IBD) that may affect any segment of the gastrointestinal tract. Symptoms often include abdominal pain, diarrhea (which may be bloody if inflammation is severe), fever, abdominal distension, ...
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. CB
2 activation may also have a role in the treatment of
irritable bowel syndrome
Irritable bowel syndrome (IBS) is a "disorder of gut-brain interaction" characterized by a group of symptoms that commonly include abdominal pain and or abdominal bloating and changes in the consistency of bowel movements. These symptoms may ...
. Cannabinoid receptor agonists reduce gut motility in IBS patients.
Peripheral nervous system
Application of CB
2-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 cell
A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a pa ...
s, 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 CB
2 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, CB
2 receptor agonists cause a reduction in the intracellular levels of
cyclic adenosine monophosphate
Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal tran ...
(cAMP).
CB
2 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 CB
2 receptor agonists results in a reduction in the binding of
transcription factor CREB
CREB-TF (CREB, cAMP response element-binding protein) is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements (CRE), thereby increasing or decreasing the transcription of the genes. CREB was first des ...
(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 CB
2 receptors revealed that changes in cAMP levels result in the phosphorylation of
leukocyte receptor tyrosine kinase
Leukocyte receptor tyrosine kinase is an enzyme that in humans is encoded by the ''LTK'' gene.
Function
The protein encoded by this gene is a member of the ALK/LTK receptor family of receptor tyrosine kinases (RTKs) whose ligand is unknown. Clo ...
at Tyr-505, leading to an inhibition of
T cell receptor signaling. Thus, CB
2 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 CB
2 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.
CB
2 receptors have also been implicated in the regulation of homing and retention of
marginal zone
The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. (Some sources consider it to be the part of red pulp which borders on the white pulp, while other sources consider it ...
B cell
B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
s. A study using knock-out mice found that CB
2 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
Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antig ...
. 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
Camp may refer to:
Outdoor accommodation and recreation
* Campsite or campground, a recreational outdoor sleeping and eating site
* a temporary settlement for nomads
* Camp, a term used in New England, Northern Ontario and New Brunswick to descri ...
concentration, leading to reduced transcription of genes regulated by
CREB
CREB-TF (CREB, cAMP response element-binding protein) is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements (CRE), thereby increasing or decreasing the transcription of the genes. CREB was first des ...
, 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
CB
2 receptors may have possible therapeutic roles in the treatment of neurodegenerative disorders such as
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As ...
.
Specifically, the CB
2 agonist JWH-015 was shown to induce macrophages to remove native
beta-amyloid
Amyloid beta (Aβ or Abeta) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid precursor protein (APP), which is ...
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 CB
2 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 CB
2 receptor activity by either selective CB
2 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 CB
2 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
The nucleus accumbens (NAc or NAcc; also known as the accumbens nucleus, or formerly as the ''nucleus accumbens septi'', Latin for "nucleus adjacent to the septum") is a region in the basal forebrain rostral to the preoptic area of the hypotha ...
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 CB
2 agonist,
GW-405,833, and were reversed by the administration of a CB
2 antagonist,
AM-630
AM-630 (6-Iodopravadoline) is a drug that acts as a potent and selective inverse agonist for the cannabinoid receptor CB2, with a ''K''i of 32.1 nM at CB2 and 165x selectivity over CB1, at which it acted as a weak partial agonist. It is u ...
.
Ligands
Many selective ligands for the CB
2 receptor are now available.
Agonists
*
Minocycline
Partial agonists
*
GW-405,833
Unspecified efficacy agonists
*
AM-1241
AM-1241 (1-(methylpiperidin-2-ylmethyl)-3-(2-iodo-5-nitrobenzoyl)indole) is a chemical from the aminoalkylindole family that acts as a potent and selective agonist for the cannabinoid receptor CB2, with a Ki of 3.4 nM at CB2 and 80 times s ...
*
HU-308
HU-308 (also known as onternabez, HU308, PPP-003 and ARDS-003) is a cannabidiol (CBD)-derivative drug that acts as a potent cannabinoid agonist. It is highly selective for the cannabinoid-2 receptor (CB2 receptor) subtype, with a selectivity ...
*
JWH-015
*
JWH-133
*
L-759,633
*
L-759,656
Herbal
*
Echinacea purpurea
''Echinacea purpurea'', the eastern purple coneflower, purple coneflower, hedgehog coneflower, or echinacea, is a North American species of flowering plant in the family Asteraceae. It is native to parts of eastern North America and present to s ...
Inverse agonists
*
AM-630
AM-630 (6-Iodopravadoline) is a drug that acts as a potent and selective inverse agonist for the cannabinoid receptor CB2, with a ''K''i of 32.1 nM at CB2 and 165x selectivity over CB1, at which it acted as a weak partial agonist. It is u ...
*
BML-190
BML-190 (Indomethacin morpholinylamide) is a drug used in scientific research that acts as a selective CB2 inverse agonist. BML-190 is structurally derived from the NSAID indomethacin but has a quite different biological activity. The activity p ...
*
JTE-907
JTE-907 is a drug used in scientific research that acts as a selective CB2 inverse agonist. It has antiinflammatory effects in animal studies, thought to be mediated by an interaction between the CB2 receptor and IgE
Immunoglobulin E (IgE) ...
*
SR-144,528
SR144528 is a drug that acts as a potent and highly selective CB2 receptor inverse agonist
In pharmacology, an inverse agonist is a drug that binds to the same receptor as an agonist but induces a pharmacological response opposite to that ...
Binding affinities
See also
*
Cannabinoid receptor
Cannabinoid receptors, located throughout the body, are part of the endocannabinoid system a class of cell membrane receptors in the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid recept ...
*
Cannabinoid receptor type 1
Cannabinoid receptor type 1 (CB1), also known as cannabinoid receptor 1, is a G protein-coupled cannabinoid receptor that in humans is encoded by the ''CNR1'' gene. The human CB1 receptor is expressed in the peripheral nervous system and centra ...
(CB
1)
References
External links
*
Cannabinoid Receptor 2 (CNR2) Human Protein Atlas
{{DEFAULTSORT:Cannabinoid Receptor 2 (Macrophage)
G protein-coupled receptors