alpha/beta-Hydrolase domain containing 12 (ABHD12) is a

serine hydrolase Serine hydrolases are one of the largest known enzyme classes comprising approximately ~200 enzymes or 1% of the genes in the human proteome. A defining characteristic of these enzymes is the presence of a particular serine at the active site, which ...

encoded by the ''ABHD12''

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

that participates in the breakdown of the

endocannabinoid 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 ...

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

2-arachidonylglycerol (2-AG) in 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 ...

. It is responsible for about 9% of brain 2-AG hydrolysis. Together, ABHD12 along with two other enzymes,

monoacylglycerol lipase Monoacylglycerol lipase (EC 3.1.1.23; systematic name glycerol-ester acylhydrolase, also known as MAG lipase, acylglycerol lipase, MAGL, MGL or MGLL) is an enzyme that, in humans, is encoded by the ''MGLL'' gene. MAGL is a 33-kDa, membrane-associ ...

(MAGL) and

ABHD6 alpha/beta-Hydrolase domain containing 6 (ABHD6), also known as monoacylglycerol lipase ABHD6 or 2-arachidonoylglycerol hydrolase is an enzyme that in humans is encoded by the ABHD6 gene. Function ABHD6 is a serine hydrolyzing enzyme that pos ...

, control 99% of 2-AG hydrolysis in the brain. ABHD12 also serves as a

lysophospholipase The enzyme lysophospholipase (EC 3.1.1.5) catalyzes the reaction :2-lysophosphatidylcholine + H2O \rightleftharpoons glycerophosphocholine + a carboxylate This enzyme belongs to the family of hydrolases, specifically those acting on carboxylic ...

and

metabolizes Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

lysophosphatidylserine (LPS).

Protein structure

ABHD12 is a ≈45

kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...

integrated membrane

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

, with an

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

proposed to face into the extracellular space. Currently, the

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

of ABHD12 is not known.

Function

ABHD12 is a lysophosphatidylserine ( lysoPS)

lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...

responsible for regulation of immune and neurological processes, and shown to act on the

arachidonoylglycerol (AG) as a

monoacylglycerol Monoglycerides (also: acylglycerols or monoacylglycerols) are a class of glycerides which are composed of a molecule of glycerol linked to a fatty acid via an ester bond. As glycerol contains both primary and secondary alcohol groups two differen ...

lipase. Endocannabinoids are associated with a range of physiological processes. ABHD12 acts on 2-AG, and accounts for approximately 9% of 2-AG hydrolysis in the brain. Along with MAGL and

, ABHD12 is responsible for 99% of 2-AG hydrolysis in the brain, and has also been shown to act on the 1(3)-AG

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

. Based on the extracellular face of the ABHD12 active site and its ability to act on multiple isomeric substrates, ABHD12 has been suggested to act as a guard to the extracellular 2-AG- CB2R signalling pathway in

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 ...

, and peripheral 2-AG signalling, however this has not been confirmed. ABHD12

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

is abundant in the brain, specifically microglia, but has also been identified in peripheral cell types like

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s and

osteoclast An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodeling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated pro ...

s. Murine models have shown ABHD12 plays a role in regulation of lysophosphatidylserine pathways in the brain.

Clinical significance

Mutations that compromise the

catalytic activity Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

of ABHD12 have been causally linked to the rare

neurodegenerative A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

disease PHARC (

polyneuropathy Polyneuropathy ( poly- + neuro- + -pathy) is damage or disease affecting peripheral nerves (peripheral neuropathy) in roughly the same areas on both sides of the body, featuring weakness, numbness, and burning pain. It usually begins in the hand ...

, hearing loss,

ataxia Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...

retinitis pigmentosa Retinitis pigmentosa (RP) is a genetic disorder of the eyes that causes loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision (side and upper or lower visual field). As peripheral vision worsens, people may ...

cataract A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble w ...

s) and a small proportion of

History

Identification of ABHD12 was first reported in genetic profiling of

autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

in 1995. In 2010, mutations in ABHD12 were reported as a causal link for the

disease PHARC .

Mutations

Mutations in ABHD12 are associated with the rare

disorder PHARC, as well as

. Null mutations have been shown to lead to development of PHARC, while other mutations can result in a range of

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

s, from non-syndromic retinal degeneration to PHARC. Currently, PHARC has been identified in at least 27 individuals, with 15 identified loss of function variants of ABHD12, comprising four

nonsense Nonsense is a communication, via speech, writing, or any other symbolic system, that lacks any coherent meaning. Sometimes in ordinary usage, nonsense is synonymous with absurdity or the ridiculous To be ridiculous is to be something which is ...

, four

missense In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution. Substitution of protein from DNA mutations Missense mu ...

, four

frameshift Ribosomal frameshifting, also known as translational frameshifting or translational recoding, is a biological phenomenon that occurs during translation that results in the production of multiple, unique proteins from a single mRNA. The process can ...

and one splicing mutation. ABHD12

mutations have been identified in individuals with

, and a growing range of

s associated with ABHD12 mutations from PHARC to non-syndromic retinal degeneration are being discovered. In vitro,

enzymatic activity Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition. Enzyme units The quantity or concentration of an enzyme can be expressed in molar amounts, as with an ...

of ABHD12 can be eliminated by site

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

the residues

Serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

-246,

Aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

-333, or

Histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...

-372, which form a

catalytic triad A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, lip ...

in the

hydrolase Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some common examples of hydrolase enzymes are este ...

domain.

Inhibitors

Inhibitor Inhibitor or inhibition may refer to: In biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotra ...

s of ABHD12 have been identified.

Orlistat Orlistat, sold under the brand name Xenical among others, is a medication used to treat obesity. Its primary function is preventing the absorption of fats from the human diet by acting as a lipase inhibitor, thereby reducing caloric intake. It ...

(tetrahydrolipstatin; THL) and methyl arachidonyl fluorophosphonate (MAFP), so-called "universal lipase/serine hydrolase inhibitors" that are extremely non-selective enzyme inhibitors, were found to inhibit ABHD12. Selective reversible inhibitors have also been identified, including betulinic acid,

maslinic acid Maslinic acid is a compound derived from dry olive-pomace oil (an olive skin wax) which is a byproduct of olive oil extraction. It is a member of the group of triterpenes known as oleananes. Pharmacology ''In vitro'' study shows that maslinic ac ...

oleanolic acid Oleanolic acid or oleanic acid is a naturally occurring pentacyclic triterpenoid related to betulinic acid. It is widely distributed in food and plants where it exists as a free acid or as an aglycone of triterpenoid saponins. Natural occurren ...

, and

ursolic acid Ursolic acid (sometimes referred to as urson, prunol, malol, or 3β-hydroxyurs-12-en-28-oic acid), is a pentacyclic triterpenoid identified in the epicuticular waxes of apples as early as 1920 and widely found in the peels of fruits, as well ...

Models

The α/β hydrolase domain including

motif and

is conserved between

murine The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families excep ...

and human ABHD12. Based on the observation of ABHD12 mutation in PHARC affected subjects, PHARC cell lines have been considered as human models of ABHD12 knockout. Mouse knockout (ABHD12 -/-) models demonstrate

cerebellar The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

microgliosis,

motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...

and auditory impairment, alongside elevated

neuroinflammation Neuroinflammation is inflammation of the nervous tissue. It may be initiated in response to a variety of cues, including infection, traumatic brain injury,Ebert SE, Jensen P, Ozenne B, Armand S, Svarer C, Stenbaek DS ''et al.'' Molecular imaging of ...

with progression associated with age. These characteristics are considered PHARC-like

s as a murine model for human PHARC, however the mouse knockout model doesn't demonstrate

ocular Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...

myelination Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...

defects, or early onset typical of PHARC. The ABHD -/- murine model shows increased long-chain lysoPS accumulation in the brain suggesting lysoPS signalling contributes to PHARC-like

pathology Pathology is the study of the causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in ...

. A

zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family ( Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ...

knockdown (+/-) model has been developed which demonstrates ophthalmic defects including

microphthalmia Microphthalmia (Greek: grc, μικρός, mikros, small, label=none, grc, ὀφθαλμός, ophthalmos, eye, label=none, also referred as microphthalmos, is a developmental disorder of the eye in which one (unilateral microphthalmia) or both ( ...

, lack of

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

clarity, and disrupted retina architecture.

Interactions

Elevated lysoPS accumulation in ABHD12

knockout mice A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or "knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...

suggests lysoPS as an

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...

substrate of ABHD12. Elevated lysoPS production in ABHD12 null cells from PHARC subjects can be reversed using an inhibitor of ABHD16A.

In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

studies demonstrate

enzymatic hydrolysis In biochemistry, enzymatic hydrolysis is a process in which enzymes facilitate the cleavage of bonds in molecules with the addition of the elements of water (i.e. hydrolysis). It plays an important role in the digestion of food. It may be used t ...

long lipid chains by ABHD12. ABHD12 can use both 1(3)-AG and 2-AG as substrates at comparable enzymatic rates. ABHD12 has been shown to be associated with

AMPA α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, better known as AMPA, is a compound that is a specific agonist for the AMPA receptor, where it mimics the effects of the neurotransmitter glutamate. There are several types of glutamatergic ...

type

glutamate receptor Glutamate receptors are synaptic and non synaptic receptors located primarily on the membranes of neuronal and glial cells. Glutamate (the conjugate base of glutamic acid) is abundant in the human body, but particularly in the nervous system a ...

s in the brains of rats.

References

{{Cannabinoidergics Enzymes