Genetic and pharmacological blockade of monoacylglycerol lipase (MAGL) could represent a new therapeutic approach to suppressing inflammation in a range of brain disorders without having to resort to cyclooxygenase (COX) inhibition and run the risk of gastrointestinal and cardiovascular side effects, scientists claim.
Investigators at the Scripps Institute, Virginia Commonwealth and West Virginia Universities, and University of California, Berkeley have found that brain uses a different pathway to generate arachidonic acid (AA) needed for COX-mediated prostaglandin production than gut tissue.
Scripps researcher Benjamin F. Cravatt, Ph.D., and UC-Berkeley’s Daniel K. Nomura, Ph.D., and colleagues, found that while phospholipase A2 (PLA2) enzymes represent the primary source of AA for COX-mediated prostaglandin production in a number of tissue, some tissues such as the brain appear to generate AA via a different route mediated by monoacylglycerol lipase.
Their in vivo studies suggest that inhibiting MAGL has neuroprotective properties in mouse models of brain inflammation and Parkinson disease that don’t involve blocking beneficial COX enzyme activity in other tissues. The researchers’ results are published in ScienceExpress in a paper titled “Endocannabinoid Hydrolysis Generates Brain Prostaglandins That Promote Neuroinflammation.”
While animal models have demonstrated that knocking out COX enzymes or administering COX inhibitors can help protect against inflammatory effects in relevant brain disorders, the gastrointestinal and cardiovascular side-effects of COX blockade mean the potential use of COX-inhibiting drugs against neuroinflammatory syndromes is limited.
Phospholipase A2 enzymes, and cytosolic PLA2 in particular, meanwhile, are believed to be the principal source of arachidonic acid (AA) needed for COX-mediated prostaglandin production. But intriguingly, the Scripps team notes, rodent studies have shown that knocking out cPLA2 doesn’t change AA levels in the brain, and their own research has instead indicated that genetic depletion or drug-based inhibition of monoacylglycerol lipase (MAGL) in mice does cause significant reductions in brain AA.
These two findings indicate that a non-PLA2 mechanism, possibly mediated by MAGL, might be involved in prostaglandin production in the nervous system. Most interest in MAGL has related to its role in hydrolyzing the endocannabinoid 2- arachidonoylglycerol (2-AG) and previous studies by the Scripps team along with independent research has found that mice which are either deficient in the gene that encodes MAGL (Mgll– /– mice) or are treated with the MAGL-selective inhibitor JZL184 demonstrate elevated brain levels of 2-AG and parallel reductions in levels of AA. Notably, these animals only demonstrate loss of MAGL activity and not other brain serine hydrolase activity.
The team has now carried out more broad profiling of the effects of MAGL disruption, which indicated that inactivating the enzyme also causes reductions in several prostaglandins in the brain but not other arachidonoyl-containing phosphor- and neutral lipid species. These results support the notion that the primary effect of inhibiting MAGL in the brain is elevation of substrate MAGs.
To find out whether MAGL impacts on eicosanoid production in inflammation, mice were treated systemically with proinflammatory lipopolysaccharide (LPS), and their brains analyzed. The findings confirmed that LPS treatment led to a time-dependent increase in brain eicosanoids but that this increase was significantly less marked in mice treated with JZL184 as well as Mgll– /– animals and didn’t substantially rise above basal levels observed in untreated wild-type mice.
Interestingly, COX1-selective inhibition using SC560, but not COX2-selective inhibition using celecoxib, also reduced basal and LPS-induced brain eicosanoid levels, mirroring the metabolic effects caused by MAGL inactivation. “A model thus emerges where LPS-induced COX1 shunts a small proportion of the high bulk levels of AA towards PGs, which are found at much lower levels in brain,” the researchers remark. “By controlling the quantity of AA available to LPS-induced COX1, MAGL exerts a crucial control over brain PG production in both basal and neuroinflammatory states.”
Drug-based or genetic inactivation of MAGL produced a near-complete blockade of LPS-induced elevations in brain cytokines but had no effect on basal cytokine levels. The suppression of brain cytokines couldn’t be reversed by treatment with the cannabinoid receptor type 1 (CB1) or type 2 (CB2) antagonists rimonabant and AM630.
Moreover, the researchers state, SC-560 also reduced LPS-stimulated cytokine production in the brain, whereas celecoxib paradoxically increased IL1α and IL1β levels in LPS-treated animals, supporting independent studies demonstrating that mice deficient in COX1 and COX2 display attenuated and exacerbated neuroinflammatory responses to LPS-treatment, respectively.
The team’s findings led them to compare the contributions of both MAGL and cPLA2 to brain prostaglandin production. They found that basal levels of AA and prostaglandins - and general serine hydrolase activities including MAGL - were unchanged in the brains of mice deficient in cPLA2 (Pla2g4a–/– mice). There was a modest decrease in LPS-induced prostaglandins in the brains of Pla2g4a–/– animals, but this was much less than the three-fold decrease in brain prostaglandins observed in MAGL-deficient animals. Notably, the effects of MAGL and cPLA2 inhibition appeared to be additive, the researchers note, as treatment with JZL184 produced greater reductions in brain prostaglandins in LPS-treated Pla2g4a–/– mice than in LPS-treated Pla2g4a+/+ animals.
“These data indicate that both MAGL and cPLA2 contribute to the AA pools for neuroinflammatory prostaglandins such that the combined inactivation of these enzymes completely blocks brain prostaglandin increases caused by LPS,” they note.
Of particular interest was the subsequent finding that either MAGL or cPLA2, but not both, mediated prostaglandin production in other tissues. For example, MAGL appeared to control both basal and LPS-induced AA and prostaglandins in the liver and lung, whereas cPLA2 regulated these lipids in the gut and spleen.
Conversely, neither MAGL nor cPLA2 substantially contributed to prostaglandin production in heart or kidney, where eicosanoid metabolic pathways may be regulated by other PLA2s, the team states. “These findings reveal a clear anatomical segregation for the enzymatic pathways that supply the AA precursor of proinflammatory prostaglandins and further suggest that MAGL inactivation may avoid some of the major adverse pharmacological effects of COX inhibitors.”
The Scripps team finally tested the in vivo effects of MAGL inhibition in brain disorders with a neuroinflammatory component. They found that either genetic MAGL blockade or JXL184 therapy in the 1-methyl-4-phenyl-tetrahydropyridine (MPTP) mouse model of Parkinsonism had significant neuroprotective effects, which correlated with blockade of MPTP-induced increases in brain AA, prostaglandins, and proinflammatory cytokines.
In parallel with the reduction in levels of AA and prostaglandins, brain levels of 2-AG (the expected source for MAGL-dependent production of AA and prostaglandins), did increase. Significantly, the effects of MAGL blockade couldn’t be reversed by cannabinoid receptor antagonists.
“We conclude that the neuroprotective effects of MAGL inactivation are primarily due to reductions in AA and proinflammatory prostaglandins rather than augmentation of endocannabinoid signalling,” the authors write. “This discovery has translational implications in that we, and others, have found that neuroinflammatory prostaglandins derive in large part from COX1. That cPLA2, rather than MAGL, provides the AA for prostaglandin biosynthesis in the gut suggests that MAGL inhibitors should avoid the gastrointestinal toxicity observed with COX1 inhibitors, a premise supported by our data.”