Cannabinoid receptor 2 (macrophage)
Encyclopedia
Cannabinoid receptor 2, 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.
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.
, 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 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.
. 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 monocyte
s, 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
.
(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
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
from the cannabinoid receptor
Cannabinoid receptor
The cannabinoid receptors are a class of cell membrane receptors under the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid receptors contain seven transmembrane spanning domains...
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
Tetrahydrocannabinol
Tetrahydrocannabinol , also known as delta-9-tetrahydrocannabinol , Δ1-THC , or dronabinol, is the main chemical psychoactive substance found in the cannabis plant. It was first isolated in 1964. In pure form, it is a glassy solid when cold, and becomes viscous and sticky if warmed...
, the active principle of marijuana.
History
CB2 was clonedCloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
in 1993 by a research group from Cambridge looking for a second cannabinoid receptor which could explain the pharmacological properties of tetrahydrocannabinol
Tetrahydrocannabinol
Tetrahydrocannabinol , also known as delta-9-tetrahydrocannabinol , Δ1-THC , or dronabinol, is the main chemical psychoactive substance found in the cannabis plant. It was first isolated in 1964. In pure form, it is a glassy solid when cold, and becomes viscous and sticky if warmed...
, the active principle of marijuana.
Signaling
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 pathwayMAPK/ERK pathway
The MAPK/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 growth factor binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a...
, a complex and highly conserved signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
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
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 directions to specific locations...
as well as in an induction of the growth-related gene Zif268
Zif268
EGR-1 also known as Zif268 or NGFI-A is a protein that in humans is encoded by the EGR1 gene....
(also known as Krox-24, NGFI-A, and egr-1) The Zifi268 gene encodes a transcriptional regulator implicated in neuroplasticity
Neuroplasticity
Neuroplasticity is a non-specific neuroscience term referring to the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment. Plasticity occurs on a variety of levels, ranging from cellular changes involved in...
and long term memory formation.
At present, there are five recognized cannabinoids
Cannabinoids
Cannabinoids are a class of chemical compounds that include the phytocannabinoids , and chemical compounds that mimic the actions of phytocannabinoids or have a similar structure...
which are produced endogenously throughout the body; these endocannabinoids include Arachidonoylethanolamine (anandamide), 2-arachidonoyl glycerol (2-AG), 2-arachidonyl glyceryl ether
2-Arachidonyl glyceryl ether
2-Arachidonyl glyceryl ether is a putative endocannabinoid discovered by Lumír Hanuš and colleagues at the Hebrew University of Jerusalem, Israel. Its isolation from porcine brain and its structural elucidation and synthesis were described in 2001.-Discovery:Lumír Hanuš, Saleh Abu-Lafi, Ester...
(noladin ether), virodhamine
Virodhamine
Virodhamine 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 of the amide linkage found in anandamide...
, as well as the recently-discovered N-arachidonoyl-dopamine
N-arachidonoyl-dopamine
N-Arachidonoyl dopamine is an endocannabinoid which acts as an agonist of the CB1 receptor and the transient receptor potential V1 ion channel. Its discovery was described in 2002 by an academic research group from Italy and the USA. It was found in the brain of rats, with especially high...
(NADA). Many of these ligands appear to exhibit properties of functional selectivity
Functional Selectivity
Functional selectivity is the ligand-dependent selectivity for certain signal transduction pathways in one and the same receptor. This can be present when a receptor has several possible signal transduction pathways...
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.
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. 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
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...
groups interact with the F5.46 residue, allowing them to form a hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
with the S3.31 residue. Ultimately, these interactions induce a conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called 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.
Expression profile
Initial investigation of CB2 receptor expression patterns focused on the presence of CB2 receptors in the peripheral tissues of the immune systemImmune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
. For instance, CB2 receptor mRNA was found throughout the immune tissues of the spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
, tonsils and thymus gland. Northern blot
Northern blot
The northern blot is a technique used in molecular biology research to study gene expression by detection of RNA in a sample. With northern blotting it is possible to observe cellular control over structure and function by determining the particular gene expression levels during differentiation,...
analysis further indicates the expression of the CNR2 gene in immune tissues. These receptors were primarily localized on immune cells such as monocyte
Monocyte
Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...
s, 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. The CB2 receptors are found primarily on microglia
Microglia
Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord, and thus act as the first and main form of active immune defense in the central nervous system . Microglia constitute 20% of the total glial cell population within the brain...
(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
Ulcerative colitis
Ulcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
.
Functions and Clinical Applications
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 monophosphateCyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
(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
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
CREB
CREB
CREB is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements , thereby increasing or decreasing the transcription of the downstream genes....
(cAMP response element binding protein) binding to DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
; 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
JWH-015
JWH-015 is a chemical from the naphthoylindole family, which acts as a subtype-selective cannabinoid agonist. Its affinity for CB2 receptors is 13.8nM, while its affinity for CB1 is 383nM, meaning that it binds almost 28x more strongly to CB2 than CB1. However it still displays some weak CB1...
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
T cell
T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...
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
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
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
Neuropathic pain
Neuropathic pain results from lesions or diseases affecting the somatosensory system. It may be associated with abnormal sensations called dysesthesia, which occur spontaneously and allodynia that occurs in response to external stimuli. Neuropathic pain may have continuous and/or episodic ...
.
CB2 receptors may have possible therapeutic roles in the treatment of neurodegenerative disorders such as Alzheimer's disease
Alzheimer's disease
Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
. 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
Senile plaques
Senile plaques are extracellular deposits of amyloid in the gray matter of the brain. The deposits are associated with degenerative neural structures and an abundance of microglia and astrocytes...
, 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
Agonists
- HU-308HU-308HU-308 is a drug which acts as a cannabinoid agonist. It is highly selective for the CB2 receptor subtype, with a selectivity of over 5000x for CB2 vs CB1. The synthesis and characterization took place in the laboratory of Prof. Mechoulam at the Hebrew University of Jerusalem in the late 1990s...
- JWH-015JWH-015JWH-015 is a chemical from the naphthoylindole family, which acts as a subtype-selective cannabinoid agonist. Its affinity for CB2 receptors is 13.8nM, while its affinity for CB1 is 383nM, meaning that it binds almost 28x more strongly to CB2 than CB1. However it still displays some weak CB1...
- JWH-133JWH-133JWH-133 is a potent selective CB2 receptor agonist, with a Ki of 3.4nM and selectivity of around 200x for CB2 over CB1 receptors. It was discovered by, and named after, John W...
- L-759,633L-759,633L-759,633 is an analgesic drug which is a cannabinoid agonist. It is a fairly selective agonist for the CB2 receptor, with selectivity of 163x for CB2 over CB1....
- L-759,656L-759,656L-759,656 is an analgesic drug which is a cannabinoid agonist. It is a highly selective agonist for the CB2 receptor, with selectivity of 414x for CB2 over CB1, although it is still not as selective as newer agents such as HU-308....
- Echinacea purpureaEchinacea purpureaEchinacea purpurea is a species of flowering plant in the genus Echinacea. Its cone-shaped flowering heads are usually, but not always purple. It is native to eastern North America and present to some extent in the wild in much of the eastern, southeastern and midwest United States...