A chemical-genetic screen identifies ABHD12 as an oxidized-phosphatidylserine lipase

Reactive oxygen species (ROS) are transient, highly reactive intermediates or byproducts produced during oxygen metabolism. However, when innate mechanisms are unable to cope with sequestration of surplus ROS, oxidative stress results, in which excess ROS damage biomolecules. Oxidized phosphatidylserine (PS), a proapoptotic 'eat me' signal, is produced in response to elevated ROS, yet little is known regarding its chemical composition and metabolism. Here, we report a small molecule that generates ROS in different mammalian cells. We used this molecule to detect, characterize and study oxidized PS in mammalian cells. We developed a chemical-genetic screen to identify enzymes that regulate oxidized PS in mammalian cells and found that the lipase ABHD12 hydrolyzes oxidized PS. We validated these findings in different physiological settings including primary peritoneal macrophages and brains from Abhd12-/- mice under inflammatory stress, and in the process, we functionally annotated an enzyme regulating oxidized PS in vivo.

The serine hydrolases MAGL, ABHD6 and ABHD12 as guardians of 2-arachidonoylglycerol signalling through cannabinoid receptors

The endocannabinoid 2-arachidonoylglycerol (2-AG) is a lipid mediator involved in various physiological processes. In response to neural activity, 2-AG is synthesized post-synaptically, then activates pre-synaptic cannabinoid CB1 receptors (CB1Rs) in a retrograde manner, resulting in transient and long-lasting reduction of neurotransmitter release. The signalling competence of 2-AG is tightly regulated by the balanced action between ‘on demand’ biosynthesis and degradation. We review recent research on monoacylglycerol lipase (MAGL), ABHD6 and ABHD12, three serine hydrolases that together account for approx. 99% of brain 2-AG hydrolase activity. MAGL is responsible for approx. 85% of 2-AG hydrolysis and colocalizes with CB1R in axon terminals. It is therefore ideally positioned to terminate 2-AG-CB1R signalling regardless of the source of this endocannabinoid. Its acute pharmacological inhibition leads to 2-AG accumulation and CB1R-mediated behavioural responses. Chronic MAGL inactivation results in 2-AG overload, desensitization of CB1R signalling and behavioural tolerance. ABHD6 accounts for approx. 4% of brain 2-AG hydrolase activity but in neurones it rivals MAGL in efficacy. Neuronal ABHD6 resides post-synaptically, often juxtaposed with CB1Rs, and its acute inhibition leads to activity-dependent accumulation of 2-AG. In cortical slices, selective ABHD6 blockade facilitates CB1R-dependent long-term synaptic depression. ABHD6 is therefore positioned to guard intracellular pools of 2-AG at the site of generation. ABHD12 is highly expressed in microglia and accounts for approx. 9% of total brain 2-AG hydrolysis. Mutations in ABHD12 gene are causally linked to a neurodegenerative disease called PHARC. Whether ABHD12 qualifies as a bona fide member to the endocannabinoid system remains to be established.

COX-2 and fatty acid amide hydrolase can regulate the time course of depolarization-induced suppression of excitation

BACKGROUND AND PURPOSE

Depolarization-induced suppression of inhibition (DSI) and excitation (DSE) are two forms of cannabinoid CB(1) receptor-mediated inhibition of synaptic transmission, whose durations are regulated by endocannabinoid (eCB) degradation. We have recently shown that in cultured hippocampal neurons monoacylglycerol lipase (MGL) controls the duration of DSE, while DSI duration is determined by both MGL and COX-2. This latter result suggests that DSE might be attenuated, and excitatory transmission enhanced, during inflammation and in other settings where COX-2 expression is up-regulated.

EXPERIMENTAL APPROACH

To investigate whether it is possible to control the duration of eCB-mediated synaptic plasticity by varied expression of eCB-degrading enzymes, we transfected excitatory autaptic hippocampal neurons with putative 2-AG metabolizing enzymes: COX-2, fatty acid amide hydrolase (FAAH), α/β hydrolase domain 6 (ABHD6), α/β hydrolase domain 12 (ABHD12) or MGL.

KEY RESULTS

We found that overexpression of either COX-2 or FAAH shortens the duration of DSE while ABHD6 or ABHD12 do not. In contrast, genetic deletion (MGL(-/-)) and overexpression of MGL both radically altered eCB-mediated synaptic plasticity.

CONCLUSIONS AND IMPLICATIONS

We conclude that both FAAH and COX-2 can be trafficked to neuronal sites where they are able to degrade eCBs to modulate DSE duration and, by extension, net endocannabinoid signalling at a given synapse. The results for COX-2, which is often up-regulated under pathological conditions, are of particular note in that they offer a mechanism by which up-regulated COX-2 may promote neuronal excitation by suppressing DSE while enhancing conversion of 2-AG to PGE(2) -glycerol ester under pathological conditions.

A critical cysteine residue in monoacylglycerol lipase is targeted by a new class of isothiazolinone-based enzyme inhibitors

BACKGROUND AND PURPOSE

Monoacylglycerol lipase (MGL) is a presynaptic serine hydrolase that inactivates the endocannabinoid neurotransmitter, 2-arachidonoyl-sn-glycerol. Recent studies suggest that cysteine residues proximal to the enzyme active site are important for MGL function. In the present study, we characterize the role of cysteines in MGL function and identify a series of cysteine-reactive agents that inhibit MGL activity with nanomolar potencies by interacting with cysteine residue 208.

EXPERIMENTAL APPROACH

A series of cysteine traps were screened for the ability to inhibit MGL in vitro. Rapid dilution assays were performed to determine reversibility of inhibition. Molecular modelling and site-directed mutagenesis were utilized to identify cysteine residues targeted by the inhibitors.

KEY RESULTS

The screening revealed that 2-octyl-4-isothiazolin-3-one (octhilinone) inhibited purified rat recombinant MGL (IC(50)= 88 +/- 12 nM) through a partially reversible mechanism. Initial structure-activity relationship studies showed that substitution of the n-octyl group of octhilinone with a more lipophilic oleoyl group increased inhibitor potency (IC(50)= 43 +/- 8 nM), while substitution with a methyl group produced the opposite effect (IC(50)= 239 +/- 68 nM). The inhibitory potency of octhilinone was selectively decreased by mutating cysteine 208 in MGL to glycine (IC(50); wild-type, 151 +/- 17 nM; C208G, 722 +/- 74 nM), but not by mutation of other cysteine residues (C32, C55, C201, C208 and C242).

CONCLUSIONS AND IMPLICATIONS

The results indicated that cysteine 208 plays an important role in MGL function and identified a novel class of isothiazolinone-based MGL inhibitors with nanomolar potency in vitro.

Critical enzymes involved in endocannabinoid metabolism

Investigations of the pathways involved in the metabolism of endocannabinoids have grown exponentially in recent years following the discovery of cannabinoid receptors (CB) and their endogenous ligands, such as anandamide (AEA) and 2-arachidonoylglycerol (2-AG). The in vivo biosynthesis of AEA has been shown to occur through several pathways mediated by N-acylphosphatidylethanolamide-phospholipase D (NAPE-PLD), a secretory PLA(2) and PLC. 2-AG, a second endocannabinoid is generated through the action of selective enzymes such as phosphatidic acid phsophohydrolase, diacylglycerol lipase (DAGL), phosphoinositide-specific PLC (PI-PLC) and lyso-PLC. A putative membrane transporter or facilitated diffusion is involved in the cellular uptake or release of endocannabinoids. AEA is metabolized by fatty acid amidohydrolase (FAAH) and 2-AG is metabolized by both FAAH and monoacylglycerol lipase (MAGL). The author presents an integrative overview of current research on the enzymes involved in the metabolism of endocannabinoids and discusses possible therapeutic interventions for various diseases, including addiction.

Modulators of endocannabinoid enzymic hydrolysis and membrane transport

Tissue concentrations of the endocannabinoids N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) are regulated by both synthesis and inactivation. The purpose of this review is to compile available data regarding three inactivation processes: fatty acid amide hydrolase, monoacylglycerol lipase, and cellular membrane transport. In particular, we have focused on mechanisms by which these processes are modulated. We describe the in vitro and in vivo effects of inhibitors of these processes as well as available evidence regarding their modulation by other factors.

ATP induces a rapid and pronounced increase in 2-arachidonoylglycerol production by astrocytes, a response limited by monoacylglycerol lipase

The cytoplasm of neural cells contain millimolar amounts of ATP, which flood the extracellular space after injury, activating purinergic receptors expressed by glial cells and increasing gliotransmitter production. These gliotransmitters, which are thought to orchestrate neuroinflammation, remain widely uncharacterized. Recently, we showed that microglial cells produce 2-arachidonoylglycerol (2-AG), an endocannabinoid known to prevent the propagation of harmful neuroinflammation, and that ATP increases this production by threefold at 2.5 min (Witting et al., 2004). Here we show that ATP increases 2-AG production from mouse astrocytes in culture, a response that is more rapid (i.e., significant within 10 sec) and pronounced (i.e., 60-fold increase at 2.5 min) than any stimulus-induced increase in endocannabinoid production reported thus far. Increased 2-AG production from astrocytes requires millimolar amounts of ATP, activation of purinergic P2X7 receptors, sustained rise in intracellular calcium, and diacylglycerol lipase activity. Furthermore, we show that astrocytes express monoacylglycerol lipase (MGL), the main hydrolyzing enzyme of 2-AG, the pharmacological inhibition of which potentiates the ATP-induced 2-AG production (up to 113-fold of basal 2-AG production at 2.5 min). Our results show that ATP greatly increases, and MGL limits, 2-AG production from astrocytes. We propose that 2-AG may function as a gliotransmitter, with MGL inhibitors potentiating this production and possibly restraining the propagation of harmful neuroinflammation.

Phylogenomic and chemotaxonomic analysis of the endocannabinoid system

The endocannabinoid system consists of two cannabinoid (CB) receptors, seven ligands, and ligand-catabolizing enzymes such as fatty acid amid hydrolase (FAAH) and monoglyceride lipase (MGL). The system's phylogenetic distribution is poorly known. The ligands cannot be molecularly investigated because they are not polypeptides and their specific synthetic enzymes have not been identified, so no sequences are available. Ligand phylogenetics can be inferred, nonetheless, by their presence in a range of extant organisms. Thus a meta-analysis of ligand extraction studies was performed (chemotaxonomy), and compared to a molecular search for homologs of CB receptors, vanilloid receptors (VR1), FAAH, and MGL in the genomes of sequenced organisms (phylogenomics). Putative homologs underwent functional mapping to ascertain the presence of critical amino acid motifs known to impart protein functionality. From an evolutionary perspective it appears that (1) endocannabinoid ligands evolved before CB receptors; (2) the ligands evolved independently multiple times; (3) CB receptors evolved prior to the metazoan-bilaterian divergence (ie, between extant Hydra and leech), but were secondarily lost in the Ecdysozoa; (4) VR1 may predate CB receptors but its affinity for endocannabinoids is a recent acquisition, appearing after the lower vertebrate-mammal divergence; (5) MGL may be as old as the ligands, whereas FAAH evolved recently, after the appearance of vertebrates. FAAH's emergence correlates with VR1's newly-found affinity for anandamide; this overlap in evolutionary time is recapitulated by complementary distribution patterns of FAAH, VR1, and anandamide in the brain. Linking FAAH, VR1, and anandamide implies a coupling among the remaining "older" parts of the endocannabinoid system, MGL, CB receptors, and 2-AG.

A role for monoglyceride lipase in 2-arachidonoylglycerol inactivation

2-Arachidonoylglycerol (2-AG) is a naturally occurring monoglyceride that activates cannabinoid receptors and meets several key requisites of an endogenous cannabinoid substance. It is present in the brain (where its levels are 170-folds higher than those of anandamide), is produced by neurons in an activity- and calcium-dependent manner, and is rapidly eliminated. The mechanism of 2-AG inactivation is not completely understood, but is thought to involve carrier-mediated transport into cells followed by enzymatic hydrolysis. We examined the possible role of the serine hydrolase, monoglyceride lipase (MGL), in brain 2-AG inactivation. We identified by homology screening a cDNA sequence encoding for a 303-amino acid protein, which conferred MGL activity upon transfection to COS-7 cells. Northern blot and in situ hybridization analyses revealed that MGL mRNA is unevenly present in the rat brain, with highest levels in regions where CB1 cannabinoid receptors are also expressed (hippocampus, cortex, anterior thalamus and cerebellum). Immunohistochemical studies in the hippocampus showed that MGL distribution has striking laminar specificity, suggesting a presynaptic localization of the enzyme. Adenovirus-mediated transfer of MGL cDNA into rat cortical neurons increased the degradation of endogenously produced 2-AG in these cells, whereas no such effect was observed on anandamide degradation. These results indicate that hydrolysis via MGL may be a primary route of 2-AG inactivation in intact neuronal cells.

Monoacylglycerol metabolism in rat small intestinal epithelial cells

1. In various preparations of rat small intestinal epithelial cells the acylation and deacylation of monoacylglycerol were studied. In the in vitro vascularly perfused intestine, of which the lumen was loaded with monoacylglycerol with or without fatty acids, acylation exceeded deacylation. In contrast, deacylation was much faster in isolated microsomes and in isolated whole cells. 2. In vascularly perfused intestine, without long-chain fatty acids present in the lumen, the amount of di- and triacylglycerol formed was found to be half of that formed in perfusions with long-chain fatty acids in the lumen, while the glycerol formation was increased 1.4-fold. 3. The concentration of monoacylglycerol is an important factor in determining the relative rates of monoacylglycerol acylation and deacylation in microsomes: the ratio acylation/deacylation decreases with increasing monoacylglycerol concentrations. 4. The function of the monoacylglycerol lipase in fat resorption is discussed.