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

Development of Potent and Selective Monoacylglycerol Lipase Inhibitors. SARs, Structural Analysis, and Biological Characterization

New potent, selective monoacylglycerol lipase (MAGL) inhibitors based on the azetidin-2-one scaffold ((±)-5a-v, (±)-6a-j, and (±)-7a-d) were developed as irreversible ligands, as demonstrated by enzymatic and crystallographic studies for (±)-5d, (±)-5l, and (±)-5r. X-ray analyses combined with extensive computational studies allowed us to clarify the binding mode of the compounds. 5v was identified as selective for MAGL when compared with other serine hydrolases. Solubility, in vitro metabolic stability, cytotoxicity, and absence of mutagenicity were determined for selected analogues. The most promising compounds ((±)-5c, (±)-5d, and (±)-5v) were used for in vivo studies in mice, showing a decrease in MAGL activity and increased 2-arachidonoyl-sn-glycerol levels in forebrain tissue. In particular, 5v is characterized by a high eudysmic ratio and (3R,4S)-5v is one of the most potent irreversible inhibitors of h/mMAGL identified thus far. These results suggest that the new MAGL inhibitors have therapeutic potential for different central and peripheral pathologies.

Functional and dynamic mitochondrial damage by chloromethylisothiazolinone/methylisothiazolinone (CMIT/MIT) mixture in brain endothelial cell lines and rat cerebrovascular endothelium

The mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT, chloromethylisothiazolinone) and 2-methyl-4-isothiazolin-3-one (MIT, methylisothiazolinone) is a commonly used biocide in consumer products. Despite the health issues related to its usage in cosmetics and humidifier disinfectants (HD), understanding its adverse outcome is still limited. Using in vitro cell lines and ex vivo rat models, we examined the effects of CMIT/MIT on the cellular redox homeostasis and energy metabolism in the brain microvascular endothelium, a highly restrictive interface between the bloodstream and brain. In murine bEND.3 and human hCMEC/D3, CMIT/MIT significantly amplified the mitochondrial-derived oxidative stress causing disruption of the mitochondrial membrane potential and oxidative phosphorylation at a sub-lethal concentration (1 μg/mL) or treatment duration (1 h). In addition, CMIT/MIT significantly increased a dynamic imbalance between mitochondrial fission and fusion, and endogenous pathological stressors significantly potentiated the CMIT/MIT-induced endothelial dysfunction. Notably, in the brain endothelium isolated from intravenously CMIT/MIT-administered rats, we observed significant mitochondrial damage and decreased tight junction protein. Taken together, we report that CMIT/MIT significantly impaired mitochondrial function and dynamics resulting in endothelial barrier dysfunction, giving an insight into the role of mitochondrial damage in CMIT/MIT-associated systemic health effects.

Genetic validation of Aspergillus fumigatus phosphoglucomutase as a viable therapeutic target in invasive aspergillosis

Aspergillus fumigatus is the causative agent of invasive aspergillosis, an infection with mortality rates of up to 50%. The glucan-rich cell wall of A. fumigatus is a protective structure that is absent from human cells and is a potential target for antifungal treatments. Glucan is synthesized from the donor uridine diphosphate glucose, with the conversion of glucose-6-phosphate to glucose-1-phosphate by the enzyme phosphoglucomutase (PGM) representing a key step in its biosynthesis. Here, we explore the possibility of selectively targeting A. fumigatus PGM (AfPGM) as an antifungal treatment strategy. Using a promoter replacement strategy, we constructed a conditional pgm mutant and revealed that pgm is required for A. fumigatus growth and cell wall integrity. In addition, using a fragment screen, we identified the thiol-reactive compound isothiazolone fragment of PGM as targeting a cysteine residue not conserved in the human ortholog. Furthermore, through scaffold exploration, we synthesized a para-aryl derivative (ISFP10) and demonstrated that it inhibits AfPGM with an IC50 of 2 μM and exhibits 50-fold selectivity over the human enzyme. Taken together, our data provide genetic validation of PGM as a therapeutic target and suggest new avenues for inhibiting AfPGM using covalent inhibitors that could serve as tools for chemical validation.

Inflammation and Nitro-oxidative Stress as Drivers of Endocannabinoid System Aberrations in Mood Disorders and Schizophrenia

The endocannabinoid system (ECS) is composed of the endocannabinoid ligands anandamide (AEA) and 2-arachidonoylgycerol (2-AG), their target cannabinoid receptors (CB₁ and CB₂) and the enzymes involved in their synthesis and metabolism (N-acyltransferase and fatty acid amide hydrolase (FAAH) in the case of AEA and diacylglycerol lipase (DAGL) and monoacylglycerol lipase (MAGL) in the case of 2-AG). The origins of ECS dysfunction in major neuropsychiatric disorders remain to be determined, and this paper explores the possibility that they may be associated with chronically increased nitro-oxidative stress and activated immune-inflammatory pathways, and it examines the mechanisms which might be involved. Inflammation and nitro-oxidative stress are associated with both increased CB₁ expression, via increased activity of the NADPH oxidases NOX4 and NOX1, and increased CNR1 expression and DNA methylation; and CB₂ upregulation via increased pro-inflammatory cytokine levels, binding of the transcription factor Nrf2 to an antioxidant response element in the CNR2 promoter region and the action of miR-139. CB₁ and CB₂ have antagonistic effects on redox signalling, which may result from a miRNA-enabled negative feedback loop. The effects of inflammation and oxidative stress are detailed in respect of AEA and 2-AG levels, via effects on calcium homeostasis and phospholipase A₂ activity; on FAAH activity, via nitrosylation/nitration of functional cysteine and/or tyrosine residues; and on 2-AG activity via effects on MGLL expression and MAGL. Finally, based on these detailed molecular neurobiological mechanisms, it is suggested that cannabidiol and dimethyl fumarate may have therapeutic potential for major depressive disorder, bipolar disorder and schizophrenia.

A review on structurally diversified synthesized molecules as monoacylglycerol lipase inhibitors and their therapeutic uses

Monoacylglycerol is a metabolic key serine hydrolase, engaged in the regulation of signalling network system of endocannabinoids, which is associated with various physiological processes like pain, inflammation, feeding cognition and neurodegenerative diseases like Alzheimer, Parkinson's disease. The monoacylglycerol also found to act as a regulator and the free fatty acid provider in the proliferation of cancer cells, numerous aggressive tumours such as colorectal cancer, neuroblastoma and nasopharyngeal carcinoma. It also played an important role in increasing the concentration of specific lipids derived from free fatty acids like phosphatidic acid, lysophosphatidic acid, sphingosine-1-phosphate and prostaglandin E2. These signalling lipids are associated with cell proliferation, survival, tumour cell migration, contributing to tumour development, maturation and metastases. In the present study here, we are presenting a review on structurally diverse MAGL inhibitors, their development and their evaluation for different pharmacological activities.

Targeting Monoacylglycerol Lipase in Pursuit of Therapies for Neurological and Neurodegenerative Diseases

Neurological and neurodegenerative diseases are debilitating conditions, and frequently lack an effective treatment. Monoacylglycerol lipase (MAGL) is a key enzyme involved in the metabolism of 2-AG (2-arachidonoylglycerol), a neuroprotective endocannabinoid intimately linked to the generation of pro- and anti-inflammatory molecules. Consequently, synthesizing selective MAGL inhibitors has become a focus point in drug design and development. The purpose of this review was to summarize the diverse synthetic scaffolds of MAGL inhibitors concerning their potency, mechanisms of action and potential therapeutic applications, focusing on the results of studies published in the past five years. The main irreversible inhibitors identified were derivatives of hexafluoroisopropyl alcohol carbamates, glycol carbamates, azetidone triazole ureas and benzisothiazolinone, whereas the most promising reversible inhibitors were derivatives of salicylketoxime, piperidine, pyrrolidone and azetidinyl amides. We reviewed the results of in-depth chemical, mechanistic and computational studies on MAGL inhibitors, in addition to the results of in vitro findings concerning selectivity and potency of inhibitors, using the half maximal inhibitory concentration (IC₅₀) as an indicator of their effect on MAGL. Further, for highlighting the potential usefulness of highly selective and effective inhibitors, we examined the preclinical in vivo reports regarding the promising therapeutic applications of MAGL pharmacological inhibition.

The role of endocannabinoid pathway in the neuropathology of Alzheimer's disease: Can the inhibitors of MAGL and FAAH prove to be potential therapeutic targets against the cognitive impairment associated with Alzheimer's disease?

Alzheimer's disease is a neurodegenerative disease characterized by progressive decline of cognitive function in combination with neuronal death. Current approved treatment target single dysregulated pathway instead of multiple mechanism, resulting in lack of efficacy in slowing down disease progression. The proclivity of endocannabinoid system to exert neuroprotective action and mitigate symptoms of neurodegeneration condition has received substantial interest. Growing evidence suggest the endocannabinoids (eCB) system, viz. anadamide (AEA) and arachidonoyl glycerol (2-AG), as potential therapeutic targets with the ability to modify Alzheimer's pathology by targeting the inflammatory, neurodegenerative and cognitive aspects of the disease. In order to modulate endocannabinoid system, number of agents have been reported amongst which are inhibitors of the monoacylglycerol (MAGL) and fatty acid amide hydrolase (FAAH), the enzymes that hydrolyses 2-AG and AEA respectively. However, little is known regarding the exact mechanistic signalling and their effects on pathophysiology and cognitive decline associated with Alzheimer's disease. Both MAGL and FAAH inhibitors possess fascinating properties that may offer a multi-faceted approach for the treatment of Alzheimer's disease such as potential to protect neurons from deleterious effect of amyloid-β, reducing phosphorylation of tau, reducing amyloid-β induced oxidative stress, stimulating neurotrophin to support brain intrinsic repair mechanism etc. Based on empirical evidence, MAGL and FAAH inhibitors might have potential for therapeutic efficacy against cognitive impairment associated with Alzheimer's disease. The aim of this review is to summarize the experimental studies demonstrating the polyvalent properties of MAGL or FAAH inhibitor compounds for the treatment of Alzheimer's disease, and also effect of these on learning and types of memories, which together encourage to study these compounds over other therapeutics targets. Further research in this direction would enhance the molecular mechanisms and development of applicable interventions for the treatment of Alzheimer's disease, which nevertheless stay as the primary unmet need.

A New Ultrasensitive Bioluminescence-Based Method for Assaying Monoacylglycerol Lipase

A novel bioluminescent Monoacylglycerol lipase (MAGL) substrate 6-O-arachidonoylluciferin, a D-luciferin derivative, was synthesized, physico-chemically characterized, and used as highly sensitive substrate for MAGL in an assay developed for this purpose. We present here a new method based on the enzymatic cleavage of arachidonic acid with luciferin release using human Monoacylglycerol lipase (hMAGL) followed by its reaction with a chimeric luciferase, PLG2, to produce bioluminescence. Enzymatic cleavage of the new substrate by MAGL was demonstrated, and kinetic constants Km and Vmax were determined. 6-O-arachidonoylluciferin has proved to be a highly sensitive substrate for MAGL. The bioluminescence assay (LOD 90 pM, LOQ 300 pM) is much more sensitive and should suffer fewer biological interferences in cells lysate applications than typical fluorometric methods. The assay was validated for the identification and characterization of MAGL modulators using the well-known MAGL inhibitor JZL184. The use of PLG2 displaying distinct bioluminescence color and kinetics may offer a highly desirable opportunity to extend the range of applications to cell-based assays.

Skin irritation and inhalation toxicity of biocides evaluated with reconstructed human epidermis and airway models

Biocides are widely used in household products. Humans are exposed to biocides through dermal, inhalational, and oral routes. However, information on the dermal and inhalational toxicity of biocides is limited. We evaluated the effects of biocides on the skin and airways using the reconstructed human epidermis model KeraSkin™ and the airway model SoluAirway™. We determined the irritancy of 11 commonly used biocides (1,2-benzisothiazol-3(2H)-one [BIT], 2-phenoxyethanol [PE], zinc pyrithione, 2-bromo-2-nitropropane-1,3-diol, 3-iodoprop-2-ynyl N-butylcarbamate [IPBC], 2-octyl-1,2-thiazol-3-one, 2,2-dibromo-2-cyanoacetamide, 4-chloro-3-methylphenol [CC], 2-phenylphenol, deltamethrin, and 4,5-dichloro-2-octyl-1,2-thiazol-3-one) in the KeraSkin™ and SoluAirway™ by viability and histological examinations. BIT and CC were found to cause skin irritation at the approved concentrations or at the concentration close to approved limit while the others were non-irritants within the approved concentration. These results were confirmed via histology, wherein skin irritants induced erosion, vacuolation, and necrosis of the tissue. In the SoluAirway™, most of the biocides decreased cell viability even within the approved limits, except for PE, IPBC, and deltamethrin, suggesting that the airway may be more vulnerable to biocides than the skin. Taken together, our result indicates that some biocides can induce toxicity in skin and airway. Further studies on the dermal and inhalational toxicity of biocides are warranted.

Synthetic flavonoids as potential antiviral agents against SARS-CoV-2 main protease

The COVID-19 pandemic has claimed more than a million lives worldwide within a short time span. Due to the unavailability of specific antiviral drugs or vaccine, the infections are causing panic both in general public and among healthcare providers. Therefore, an urgent discovery and development of effective antiviral drug for the treatment of COVID-19 is highly desired. Targeting the main protease (M^pro) of the causative agent, SARS-CoV-2 has great potential for drug discovery and drug repurposing efforts. Published crystal structures of SARS-CoV-2 M^pro further facilitated in silico investigations for discovering new inhibitors against M^pro. The present study aimed to screen several libraries of synthetic flavonoids and benzisothiazolinones as potential SARS-CoV-2 M^pro inhibitors using in silico methods. The short-listed compounds after virtual screening were filtered through SwissADME modeling tool to remove molecules with unfavorable pharmacokinetics and medicinal properties. The drug-like molecules were further subjected to iterative docking for the identification of top binders of SARS-CoV-2 M^pro. Finally, molecular dynamic (MD) simulations and binding free energy calculations were performed for the evaluation of the dynamic behavior, stability of protein–ligand complex, and binding affinity, resulting in the identification of thioflavonol, TF-9 as a potential inhibitor of M^pro. The computational studies further revealed the binding of TF-9 close to catalytic dyad and interactions with conserved residues in the S1 subsite of the substrate binding site. Our in-silico study demonstrated that synthetic analogs of flavonoids, particularly thioflavonols, have a strong tendency to inhibit the main protease M^pro, and thereby inhibit the reproduction of SARS-CoV-2.

Communicated by Ramaswamy H. Sarma

An updated patent review of monoacylglycerol lipase (MAGL) inhibitors (2018-present)

INTRODUCTION

Monoacylglycerol lipase (MAGL) belongs to the endocannabinoid system and is responsible for the inactivation of endocannabinoid 2-arachidonoylglycerol. Importantly, it was found that MAGL degradation of lipids in cancer cells enhances the availability of free fatty acids for new cellular membrane formation and pro-oncogenic lipid modulators. The multifaceted role of MAGL has greatly stimulated the search for MAGL inhibitors, which could be effective to treat diseases, such as inflammation, neurodegeneration and cancer.

AREAS COVERED

This review covers patents published since 2018 up to now, concerning new MAGL inhibitors and their potential therapeutic applications.

EXPERT OPINION

In the years 2018-2020, several well-known chemical scaffolds of MAGL inhibitors have been further optimized and developed and some new chemical classes have also been identified as MAGL inhibitors. Moreover, an increasing number of scientific publications covering MAGL inhibitors is focused on MAGL-specific positron emission tomography (PET) ligands. The numerous efforts of pharmaceutical companies and academic research groups finalized to find new potent MAGL inhibitors confirm that this research area is rapidly growing. Nevertheless, most of the patented compounds still belong to the large group of irreversible MAGL inhibitors, highlighting that the development of reversible MAGL inhibitors is still an unmet pharmaceutical need.

Monoacylglycerol lipase inhibitors: modulators for lipid metabolism in cancer malignancy, neurological and metabolic disorders

Monoacylglycerol lipase (MAGL) is a serine hydrolase that plays a crucial role catalysing the hydrolysis of monoglycerides into glycerol and fatty acids. It links the endocannabinoid and eicosanoid systems together by degradation of the abundant endocannabinoid 2-arachidaoylglycerol into arachidonic acid, the precursor of prostaglandins and other inflammatory mediators. MAGL inhibitors have been considered as important agents in many therapeutic fields, including anti-nociceptive, anxiolytic, anti-inflammatory, and even anti-cancer. Currently, ABX-1431, a first-in-class inhibitor of MAGL, is entering clinical phase 2 studies for neurological disorders and other diseases. This review summarizes the diverse (patho)physiological roles of MAGL and will provide an overview on the development of MAGL inhibitors. Although a large number of MAGL inhibitors have been reported, novel inhibitors are still required, particularly reversible ones.

Study on a Novel Cold-Active and Halotolerant Monoacylglycerol Lipase Widespread in Marine Bacteria Reveals a New Group of Bacterial Monoacylglycerol Lipases Containing Unusual C(A/S)HSMG Catalytic Motifs

Monoacylglycerol lipases (MGLs) are present in all domains of life. However, reports on bacterial MGLs are still limited. Until now, reported bacterial MGLs are all thermophilic/mesophilic enzymes from warm terrestrial environments or deep-sea hydrothermal vent, and none of them originates from marine environments vastly subject to low temperature, high salts, and oligotrophy. Here, we characterized a novel MGL, GnMgl, from the marine cold-adapted and halophilic bacterium Glaciecola nitratireducens FR1064^T. GnMgl shares quite low sequence similarities with characterized MGLs (lower than 31%). GnMgl and most of its bacterial homologs harbor a catalytic Ser residue located in the conserved C(A/S)HSMG motif rather than in the typical GxSxG motif reported on other MGLs, suggesting that GnMgl-like enzymes might be different from reported MGLs in catalysis. Phylogenetic analysis suggested that GnMgl and its bacterial homologs are clustered as a separate group in the monoglyceridelipase_lysophospholipase family of the Hydrolase_4 superfamily. Recombinant GnMgl has no lysophospholipase activity but could hydrolyze saturated (C12:0-C16:0) and unsaturated (C18:1 and C18:2) MGs and short-chain triacylglycerols, displaying distinct substrate selectivity from those of reported bacterial MGLs. The substrate preference of GnMgl, predicted to be a membrane protein, correlates to the most abundant fatty acids within the strain FR1064^T, suggesting the role of GnMgl in the lipid catabolism in this marine bacterium. In addition, different from known bacterial MGLs that are all thermostable enzymes, GnMgl is a cold-adapted enzyme, with the maximum activity at 30°C and retaining 30% activity at 0°C. GnMgl is also a halotolerant enzyme with full activity in 3.5M NaCl. The cold-adapted and salt-tolerant characteristics of GnMgl may help its source strain FR1064^T adapt to the cold and saline marine environment. Moreover, homologs to GnMgl are found to be abundant in various marine bacteria, implying their important physiological role in these marine bacteria. Our results on GnMgl shed light on marine MGLs.

Benzisothiazolinone Derivatives as Potent Allosteric Monoacylglycerol Lipase Inhibitors That Functionally Mimic Sulfenylation of Regulatory Cysteines

We describe a set of benzisothiazolinone (BTZ) derivatives that are potent inhibitors of monoacylglycerol lipase (MGL), the primary degrading enzyme for the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG). Structure-activity relationship studies evaluated various substitutions on the nitrogen atom and the benzene ring of the BTZ nucleus. Optimized derivatives with nanomolar potency allowed us to investigate the mechanism of MGL inhibition. Site-directed mutagenesis and mass spectrometry experiments showed that BTZs interact in a covalent reversible manner with regulatory cysteines, Cys201 and Cys208, causing a reversible sulfenylation known to modulate MGL activity. Metadynamics simulations revealed that BTZ adducts favor a closed conformation of MGL that occludes substrate recruitment. The BTZ derivative 13 protected neuronal cells from oxidative stimuli and increased 2-AG levels in the mouse brain. The results identify Cys201 and Cys208 as key regulators of MGL function and point to the BTZ scaffold as a useful starting point for the discovery of allosteric MGL inhibitors.

Set-Up and Validation of a High Throughput Screening Method for Human Monoacylglycerol Lipase (MAGL) Based on a New Red Fluorescent Probe

Monoacylglycerol lipase (MAGL) is a serine hydrolase that has a key regulatory role in controlling the levels of 2-arachidonoylglycerol (2-AG), the main signaling molecule in the endocannabinoid system. Identification of selective modulators of MAGL enables both to provide new tools for investigating pathophysiological roles of 2-AG, and to discover new lead compounds for drug design. The development of sensitive and reliable methods is crucial to evaluate this modulatory activity. In the current study, we report readily synthesized long-wavelength putative fluorogenic substrates with different acylic side chains to find a new probe for MAGL activity. 7-Hydroxyresorufinyl octanoate proved to be the best substrate thanks to the highest rate of hydrolysis and the best Km and Vmax values. In addition, in silico evaluation of substrates interaction with the active site of MAGL confirms octanoate resorufine derivative as the molecule of choice. The well-known MAGL inhibitors URB602 and methyl arachidonylfluorophosphonate (MAFP) were used for the assay validation. The assay was highly reproducible with an overall average Z′ value of 0.86. The fast, sensitive and accurate method described in this study is suitable for low-cost high-throughput screening (HTS) of MAGL modulators and is a powerful new tool for studying MAGL activity.

Monoacylglycerol lipase (MAGL) as a promising therapeutic target

Monoacylglycerol lipase (MAGL) has been characterized as the main enzyme responsible for the inactivation of the most abundant brain endocannabinoid, 2-arachidonoylglycerol (2-AG). Besides this role, MAGL has progressively acquired a growing importance as an integrative metabolic hub that controls not only the in vivo levels of 2-AG but also of other monoacylglycerides and, indirectly, the levels of free fatty acids derived from their hydrolysis as well as other lipids with pro-inflammatory or pro-tumorigenic effects, coming from the further metabolism of fatty acids. All these functions have only started to be elucidated in the last years due to the progress made in the knowledge of the structure of MAGL and in the development of genetic and chemical tools. In this review we report the advances made in the field with a special focus on the last decade and how MAGL has become a promising therapeutic target for the treatment of several diseases that currently lack appropriate therapies.

Development of terphenyl-2-methyloxazol-5(4H)-one derivatives as selective reversible MAGL inhibitors

Monoacylglycerol lipase is a serine hydrolase that plays a major role in the degradation of the endocannabinoid neurotransmitter 2-arachidonoylglycerol. A wide number of MAGL inhibitors are reported in literature; however, many of them are characterised by an irreversible mechanism of action and this behavior determines an unwanted chronic MAGL inactivation, which acquires a functional antagonism of the endocannabinoid system. The possible use of reversible MAGL inhibitors has only recently been explored, due to the lack of known compounds possessing efficient reversible inhibitory activities. In this work, we report a new series of terphenyl-2-methyloxazol-5(4H)-one derivatives characterised by a reversible MAGL-inhibition mechanism. Among them, compound 20b showed to be a potent MAGL reversible inhibitor (IC₅₀ = 348 nM) with a good MAGL/FAAH selectivity. Furthermore, this compound showed antiproliferative activities against two different cancer cell lines that overexpress MAGL.

Structural Optimization of 4-Chlorobenzoylpiperidine Derivatives for the Development of Potent, Reversible, and Selective Monoacylglycerol Lipase (MAGL) Inhibitors

Monoacylglycerol lipase (MAGL) inhibitors are considered potential therapeutic agents for a variety of pathological conditions, including several types of cancer. Many MAGL inhibitors are reported in literature; however, most of them showed an irreversible mechanism of action, which caused important side effects. The use of reversible MAGL inhibitors has been only partially investigated so far, mainly because of the lack of compounds with good MAGL reversible inhibition properties. In this study, starting from the (4-(4-chlorobenzoyl)piperidin-1-yl)(4-methoxyphenyl)methanone (CL6a) lead compound that showed a reversible mechanism of MAGL inhibition (K_i = 8.6 μM), we started its structural optimization and we developed a new potent and selective MAGL inhibitor (17b, K_i = 0.65 μM). Furthermore, modeling studies suggested that the binding interactions of this compound replace a structural water molecule reproducing its H-bonds in the MAGL binding site, thus identifying a new key anchoring point for the development of new MAGL inhibitors.

Free-energy studies reveal a possible mechanism for oxidation-dependent inhibition of MGL

The function of monoacylglycerol lipase (MGL), a key actor in the hydrolytic deactivation of the endocannabinoid 2-arachidonoyl-sn-glycerol (2AG), is tightly controlled by the cell’s redox state: oxidative signals such as hydrogen peroxide suppress MGL activity in a reversible manner through sulfenylation of the peroxidatic cysteines, C201 and C208. Here, using as a starting point the crystal structures of human MGL (hMGL), we present evidence from molecular dynamics and metadynamics simulations along with high-resolution mass spectrometry studies indicating that sulfenylation of C201 and C208 alters the conformational equilibrium of the membrane-associated lid domain of MGL to favour closed conformations of the enzyme that do not permit the entry of substrate into the active site.

Biological chemistry and functionality of protein sulfenic acids and related thiol modifications

Selective modification of proteins at cysteine residues by reactive oxygen, nitrogen or sulfur species formed under physiological and pathological states is emerging as a critical regulator of protein activity impacting cellular function. This review focuses primarily on protein sulfenylation (-SOH), a metastable reversible modification connecting reduced cysteine thiols to many products of cysteine oxidation. An overview is first provided on the chemistry principles underlining synthesis, stability and reactivity of sulfenic acids in model compounds and proteins, followed by a brief description of analytical methods currently employed to characterize these oxidative species. The following chapters present a selection of redox-regulated proteins for which the -SOH formation was experimentally confirmed and linked to protein function. These chapters are organized based on the participation of these proteins in the regulation of signaling, metabolism and epigenetics. The last chapter discusses the therapeutic implications of altered redox microenvironment and protein oxidation in disease.

Synthesis and characterization of a new fluorogenic substrate for monoacylglycerol lipase and application to inhibition studies

Human monoacylglycerol lipase (MAGL), a soluble serine hydrolase that belongs to the α/β hydrolase fold superfamily, regulates 2-arachidonoyl glycerol level in the endocannabinoid system, which is implicated in a number of severe diseases, and therefore, inhibition of MAGL activity is crucial in the treatment of these diseases. We have synthesized a red fluorogenic substrate, 7-hydroxyresorufinyl-arachidonate (7-HRA), for a new MAGL assay. This assay is simple, sensitive, and reliable and useful for identifying compounds that modulate MAGL activity. In addition, the assay emits red fluorescence, which can significantly reduce interference due to compound fluorescence and dust or lint, all of which fluoresce in the blue wavelength. MAGL catalyzes the hydrolysis of 7-HRA to generate arachidonic acid and a highly red fluorescent resorufin, excitation at 571 nm and emission at 588 nm, with a Km of 0.87 μM and Vmax of 26 nmol min(-1) mg protein(-1). The known MAGL inhibitors URB602, methyl arachidonyl fluorophosphonate, and JZL184 were used to validate the test assay. The assay was highly reproducible with an overall average Z' value of 0.80. This new red fluorogenic substrate and the resulting enzyme assay could be used in high-throughput screening to identify and develop new potential MAGL inhibitors.

Monoglyceride lipase: Structure and inhibitors

Monoglyceride lipase (MGL), the main enzyme responsible for the hydrolytic deactivation of the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG), is an intracellular serine hydrolase that plays critical roles in many physiological and pathological processes, such as pain, inflammation, neuroprotection and cancer. The crystal structures of MGL that are currently available provide valuable information about how this enzyme might function and interact with site-directed small-molecule inhibitors. On the other hand, its conformational equilibria and the contribution of regulatory cysteine residues present within the substrate-binding pocket or on protein surface remain open issues. Several classes of MGL inhibitors have been developed, from early reversible ones, such as URB602 and pristimerin, to carbamoylating agents that react with the catalytic serine, such as JZL184 and more recent O-hexafluoroisopropyl carbamates. Other inhibitors that modulate MGL activity by interacting with conserved regulatory cysteines act through mechanisms that deserve to be more thoroughly investigated.

Effect of monoacylglycerol lipase inhibitor JZL184 on apoptosis of colorectal cancer cells

AIM: To investigate whether JZL184, a monoacylglycerol lipase inhibitor, induces apoptosis of colorectal cancer cells and to explore the possible mechanism.

METHODS: SW480 and Lovo cells were treated with JZL184, JZL184 + 5-fluorouracil (5-Fu) or 5-Fu alone for 48 h. Apoptosis was assessed by flow cytometry. The protein levels of p-AKT, p-mTOR, pro-Caspase3 and pro-Caspase8 were assessed by Western blot.

RESULTS: Treatment with JZL184 + 5-Fu increased SW480 and Lovo cell apoptosis more significantly than 5-Fu alone (apoptosis increase in SW480 cells: JZL184 + 500 μmol/L 5-Fu 13.91% ± 9.13%, JZL184 + 200 μmol/L 5-Fu 26.34% ± 13.32%, JZL184 + 100 μmol 5-Fu 43.32% ± 8.04%, JZL184 + 10 μmol 5-Fu 31.4% ± 5.82%; Lovo cells: JZL184 + 500 μmol/L 5-Fu 17.56% ± 8.14%, JZL184 + 200 μmol/L 5-Fu 33.04% ± 9.49%, JZL184 + 100 μmol/L 5-Fu 36.91% ± 16.63%, JZL184 + 10 μmol/L 5-Fu 21.26% ± 11.03%). Treatment with JZL184 significantly decreased the protein levels of p-AKT, p-mTOR, pro-Caspase3 and pro-Caspase8 in colorectal cancer SW480 and Lovo cells (P < 0.05).

CONCLUSION: JZL184 can inhibit the AKT-mTOR pathway and promote pro-Caspase8 and pro-Caspase3 activation to increase the apoptosis of SW480 and Lovo cells treated with 5-Fu.

4-Aryliden-2-methyloxazol-5(4H)-one as a new scaffold for selective reversible MAGL inhibitors

This study reports on a preliminary structure-activity relationship exploration of 4-aryliden-2-methyloxazol-5(4H)-one-based compounds as MAGL/FAAH inhibitors. Our results highlight that this scaffold may serve for the development of selective MAGL inhibitors. A 69-fold selectivity against MAGL over FAAH was achieved for compound 16b (MAGL and FAAH IC(50) = 1.6 and 111 µM, respectively). Furthermore, the best compound behaved as a reversible ligand and showed promising antiproliferative activity in cancer cells.

Inhibitors of Fatty Acid Amide Hydrolase and Monoacylglycerol Lipase: New Targets for Future Antidepressants

Cannabis and analogs of Δ⁹-tetrahydrocannabinol have been used for therapeutic purposes, but their therapeutic use remains limited because of various adverse effects. Endogenous cannabinoids have been discovered, and dysregulation of endocannabinoid signaling is implicated in the pathophysiology of major depressive disorder (MDD). Recently, endocannabinoid hydrolytic enzymes such as fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have become new therapeutic targets in the treatment of MDD. Several FAAH or MAGL inhibitors are reported to have no cannabimimetic side effects and, therefore, are new potential therapeutic options for patients with MDD who are resistant to first-line antidepressants (selective serotonin and serotonin-norepinephrine reuptake inhibitors). In this review, we focus on the possible relationships between MDD and the endocannabinoid system as well as the inhibitors' therapeutic potential. MAGL inhibitors may reduce inflammatory responses through activation of cannabinoid receptor type 2. In the hypothalamic-pituitary-adrenal axis, repeated FAAH inhibitor administration may be beneficial for reducing circulating glucocorticoid levels. Both FAAH and MAGL inhibitors may contribute to dopaminergic system regulation. Recently, several new inhibitors have been developed with strong potency and selectivity. FAAH inhibitor, MAGL inhibitor, or dual blocker use would be promising new treatments for MDD. Further pre-clinical studies and clinical trials using these inhibitors are warranted.

Parsing the players: 2-arachidonoylglycerol synthesis and degradation in the CNS

The endogenous cannabinoid signalling system, composed of endogenous cannabinoids, cannabinoid receptors and the enzymes that synthesize and degrade the endogenous cannabinoids, is much more complex than initially conceptualized. 2-Arachidonoylglycerol (2-AG) is the most abundant endocannabinoid and plays a major role in CNS development and synaptic plasticity. Over the past decade, many key players in 2-AG synthesis and degradation have been identified and characterized. Most 2-AG is synthesized from membrane phospholipids via sequential activation of a phospholipase Cβ and a diacylglycerol lipase, although other pathways may contribute in specialized settings. 2-AG breakdown is more complicated with at least eight different enzymes participating. These enzymes can either degrade 2-AG into its components, arachidonic acid and glycerol, or transform 2-AG into highly bioactive signal molecules. The implications of the precise temporal and spatial control of the expression and function of these pleiotropic metabolizing enzymes have only recently come to be appreciated. In this review, we will focus on the primary organization of the synthetic and degradative pathways of 2-AG and then discuss more recent findings and their implications, with an eye towards the biological and therapeutic implications of manipulating 2-AG synthesis and metabolism.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6.

The antinociceptive triterpene β-amyrin inhibits 2-arachidonoylglycerol (2-AG) hydrolysis without directly targeting cannabinoid receptors

BACKGROUND AND PURPOSE

Pharmacological activation of cannabinoid CB(1) and CB(2) receptors is a therapeutic strategy to treat chronic and inflammatory pain. It was recently reported that a mixture of natural triterpenes α- and β-amyrin bound selectively to CB(1) receptors with a subnanomolar K(i) value (133 pM). Orally administered α/β-amyrin inhibited inflammatory and persistent neuropathic pain in mice through both CB(1) and CB(2) receptors. Here, we investigated effects of amyrins on the major components of the endocannabinoid system.

EXPERIMENTAL APPROACH

We measured CB receptor binding interactions of α- and β-amyrin in validated binding assays using hCB(1) and hCB(2) transfected CHO-K1 cells. Effects on endocannabinoid transport in U937 cells and breakdown using homogenates of BV2 cells and pig brain, as well as purified enzymes, were also studied.

KEY RESULTS

There was no binding of either α- or β-amyrin to hCB receptors in our assays (K(i) > 10 µM). The triterpene β-amyrin potently inhibited 2-arachidonoyl glycerol (2-AG) hydrolysis in pig brain homogenates, but not that of anandamide. Although β-amyrin only weakly inhibited purified human monoacylglycerol lipase (MAGL), it also inhibited α,β-hydrolases and more potently inhibited 2-AG breakdown than α-amyrin and the MAGL inhibitor pristimerin in BV2 cell and pig brain homogenates.

CONCLUSIONS AND IMPLICATIONS

We propose that β-amyrin exerts its analgesic and anti-inflammatory pharmacological effects via indirect cannabimimetic mechanisms by inhibiting the degradation of the endocannabinoid 2-AG without interacting directly with CB receptors. Triterpenoids appear to offer a very broad and largely unexplored scaffold for inhibitors of the enzymic degradation of 2-AG.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Rational design of fatty acid amide hydrolase inhibitors that act by covalently bonding to two active site residues

The design and characterization of α-ketoheterocycle fatty acid amide hydrolase (FAAH) inhibitors are disclosed that additionally and irreversibly target a cysteine (Cys269) found in the enzyme cytosolic port while maintaining the reversible covalent Ser241 attachment responsible for their rapid and initially reversible enzyme inhibition. Two α-ketooxazoles (3 and 4) containing strategically placed electrophiles at the C5 position of the pyridyl substituent of 2 (OL-135) were prepared and examined as inhibitors of FAAH. Consistent with the observed time-dependent noncompetitive inhibition, the cocrystal X-ray structure of 3 bound to a humanized variant of rat FAAH revealed that 3 was not only covalently bound to the active site catalytic nucleophile Ser241 as a deprotonated hemiketal, but also to Cys269 through the pyridyl C5-substituent, thus providing an inhibitor with dual covalent attachment in the enzyme active site. In vivo characterization of the prototypical inhibitors in mice demonstrates that they raise endogenous brain levels of FAAH substrates to a greater extent and for a much longer duration (>6 h) than the reversible inhibitor 2, indicating that the inhibitors accumulate and persist in the brain to completely inhibit FAAH for a prolonged period. Consistent with this behavior and the targeted irreversible enzyme inhibition, 3 reversed cold allodynia in the chronic constriction injury model of neuropathic pain in mice for a sustained period (>6 h) beyond that observed with the reversible inhibitor 2, providing effects that were unchanged over the 1-6 h time course monitored.

1,3,4-Oxadiazol-2-ones as fatty-acid amide hydrolase and monoacylglycerol lipase inhibitors: Synthesis, in vitro evaluation and insight into potency and selectivity determinants by molecular modelling

Inhibition of the key hydrolytic enzymes of the endocannabinoid system, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), has been proposed as potential mode of action for various therapeutic applications. Continuing our previous work, we take the first steps of structure-activity relationship exploration and show that 1,3,4-oxadiazol-2-ones can serve as scaffold for both selective FAAH and MAGL inhibitors, and also function as a dual FAAH/MAGL inhibitor at sub-micromolar IC50 values. Moreover, 10-fold selectivity against MAGL over FAAH was achieved with compound 3d (FAAH and MAGL IC50; 2.0 and 0.22 μM). Lastly, enzyme and ligand features contributing to the potency and selectivity differences are analysed by molecular docking.

Monoacylglycerol lipase - a target for drug development?

The endocannabinoid (eCB) system is involved in processes as diverse as control of appetite, perception of pain and the limitation of cancer cell growth and invasion. The enzymes responsible for eCB breakdown are attractive pharmacological targets, and fatty acid amide hydrolase inhibitors, which potentiate the levels of the eCB anandamide, are now undergoing pharmaceutical development. 'Drugable' selective inhibitors of monoacylglycerol lipase, a key enzyme regulating the levels of the other main eCB, 2-arachidonoylglycerol, were however not identified until very recently. Their availability has resulted in a large expansion of our knowledge concerning the pharmacological consequences of monoacylglycerol lipase inhibition and hence the role(s) played by the enzyme in the body. In this review, the pharmacology of monoacylglycerol lipase will be discussed, together with an analysis of the therapeutic potential of monoacylglycerol lipase inhibitors as analgesics and anticancer agents.

Development and characterization of endocannabinoid hydrolases FAAH and MAGL inhibitors bearing a benzotriazol-1-yl carboxamide scaffold

A series of (1H-benzo[d][1,2,3]triazol-1-yl)(4-benzylpiperazin-1-yl)methanones and of (1H-benzo[d][1,2,3]triazol-1-yl)(4-phenylpiperazin-1-yl)methanones has been prepared and tested on human fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). In the benzylpiperazinyl series, compound 29 (ML30) exhibited an IC(50) value of 0.54 nM on MAGL, combined with a 1000-fold selectivity versus FAAH, while compounds 11 and 16 acted as potent dual FAAH-MAGL inhibitors (IC(50)<10 nM). In the phenylpiperazinyl series, compounds 37, 38, 42, and 43 displayed IC(50) values against MAGL in the nanomolar range, whilst being between one and two orders of magnitude less potent on the FAAH, while compounds 31 and 32 were potent FAAH inhibitors (IC(50)<20 nM) and over 12-fold selective versus MAGL. The key structural determinants driving the structure-activity relationships were explored by the minimization of the inhibitors inside the active site of both enzymes.

The structure of monoacylglycerol lipase from Bacillus sp. H257 reveals unexpected conservation of the cap architecture between bacterial and human enzymes

Monoacylglycerol lipases (MGLs) catalyse the hydrolysis of monoacylglycerol into free fatty acid and glycerol. MGLs have been identified throughout all genera of life and have adopted different substrate specificities depending on their physiological role. In humans, MGL plays an integral part in lipid metabolism affecting energy homeostasis, signalling processes and cancer cell progression. In bacteria, MGLs degrade short-chain monoacylglycerols which are otherwise toxic to the organism. We report the crystal structures of MGL from the bacterium Bacillus sp. H257 (bMGL) in its free form at 1.2Å and in complex with phenylmethylsulfonyl fluoride at 1.8Å resolution. In both structures, bMGL adopts an α/β hydrolase fold with a cap in an open conformation. Access to the active site residues, which were unambiguously identified from the protein structure, is facilitated by two different channels. The larger channel constitutes the highly hydrophobic substrate binding pocket with enough room to accommodate monoacylglycerol. The other channel is rather small and resembles the proposed glycerol exit hole in human MGL. Molecular dynamics simulation of bMGL yielded open and closed states of the entrance channel and the glycerol exit hole. Despite differences in the number of residues, secondary structure elements, and low sequence identity in the cap region, this first structure of a bacterial MGL reveals striking structural conservation of the overall cap architecture in comparison with human MGL. Thus it provides insight into the structural conservation of the cap amongst MGLs throughout evolution and provides a framework for rationalising substrate specificities in each organism.

Synthesis and pharmacological evaluation of 2,4-dinitroaryldithiocarbamate derivatives as novel monoacylglycerol lipase inhibitors

Monoacylglycerol lipase (MAGL) is responsible for signal termination of 2-arachidonoylglycerol (2-AG), an endocannabinoid neurotransmitter endowed with several physiological effects. Previously, we showed that the arylthioamide scaffold represents a privileged template for designing MAGL inhibitors. A series of 37 compounds resulting from pharmacomodulations around the arylthioamide template were synthesized and tested to evaluate their inhibitory potential on MAGL activity as well as their selectivity over fatty acid amide hydrolase (FAAH), another endocannabinoid-hydrolyzing enzyme. We have identified 2,4-dinitroaryldithiocarbamate derivatives as a novel class of MAGL inhibitors. Among the synthesized compounds, we identified [2,4-dinitrophenyl-4-(4-tert-butylbenzyl)piperazine-1-carbodithioate] (CK37), as the most potent MAGL inhibitor within this series (IC(50) = 154 nM). We have also identified [2,4-dinitrophenyl-4-benzhydrylpiperazine-1-carbodithioate] (CK16) as a selective MAGL inhibitor. These compounds are irreversible MAGL inhibitors that probably act by interacting with Cys208 or Cys242 and Ser122 residues of the enzyme. Moreover, CK37 is able to raise 2-arachidonoylglycerol (2-AG) levels in intact cells.