The COX-2/prostanoid signaling cascades in seizure disorders

Abstract

Introduction：A robust neuroinflammatory response is a prevalent feature of multiple neurological disorders, including epilepsy and acute status epilepticus. One component of this neuroinflammatory reaction is the induction of cyclooxygenase-2 (COX-2), synthesis of several prostaglandins and endocannabinoid metabolites, and subsequent activation of prostaglandin and related receptors. Neuroinflammation mediated by COX-2 and its downstream effectors has received considerable attention as a potential target class to ameliorate the deleterious consequences of neurological injury. Areas covered: Here we describe the roles of COX-2 as a major inflammatory mediator. In addition, we discuss the receptors for prostanoids PGE₂, prostaglandin D2, and PGF_2α as potential therapeutic targets for inflammation-driven diseases. The consequences of prostanoid receptor activation after seizure activity are discussed with an emphasis on the utilization of small molecules to modulate prostanoid receptor activity. Expert opinion: Limited clinical trial experience is supportive but not definitive for a role of the COX signaling cascade in epileptogenesis. The cardiotoxicity associated with chronic coxib use, and the expectation that COX-2 inhibition will influence the levels of endocannabinoids, leukotrienes, and lipoxins as well as the prostaglandins and their endocannabinoid metabolite analogs, is shifting attention toward downstream synthases and receptors that mediate inflammation in the brain.

Keywords: Cyclooxygenase-2; EP1; EP2; NSAID; endocannabinoid; epilepsy; epileptogenesis; prostaglandins; seizures.

Figure 1.. Cyclooxygenase signaling cascades

Both Cyclooxygenase 1 (COX-1) and Cyclooxygenase 2 (COX-2) enzymes convert arachidonic acid to prostaglandin H2 (PGH₂). Cell specific syntheses convert PGH₂ intermediate into prostaglandin ligands, which activate one or more G protein-coupled prostanoid receptors. Much of the prostaglandin F2α (PGF_2α) in the brain is derived from reduction of prostaglandin E2 (PGE₂). COX-2 also metabolizes endocannabinoids to prostaglandin analogs (glycerol esters and ethanolamides), which likely activate receptors distinct from those of the prostaglandins. Four receptors, prostaglandin D2 receptor 1 (DP1), prostaglandin E2 receptor 2 (EP2), prostaglandin E2 receptor 4 (EP4) and prostacyclin receptor (IP), promote cyclic adenosine monophosphate (cAMP) production. cAMP activates (protein kinase A) PKA and or exchange protein directly activated by cAMP (Epac) signaling. COX-1 and COX-2 inhibition would block the entire prostanoid receptor activity, and thus may lead to adverse-effects.

Figure 2.. Conditional ablation of COX-2 in forebrain neurons

Coronal sections (30 μm) were incubated with the COX-2 antibody (Abcam, 1:1000) and subsequently with a biotin conjugated goat anti-rabbit secondary antibody. The sections were incubated with HRP-conjugated streptavidin and developed with diaminobenzidine (DAB). Immunostaining for COX-2 demonstrates expression in the pyramidal cell layers of the hippocampus and lateral amygdala in wildtype mice, but not in the conditional knockout (cKO) mice. One day after 1 hour of status epilepticus (SE) induced with pilocarpine (280 mg/kg), COX-2 induction is observed in wildtype forebrain neurons, but not in cKO forebrain neurons. Scale bar, 100 μm. The arrowheads outline the lateral amygdala.

Figure 3.. Evidence consistent with a disease-modifying effect of chronic NSAID use in epilepsy

32,007 adults with rheumatoid arthritis (RA) but without a pre-existing epilepsy diagnosis were selected from the National Health Insurance research database of Taiwan and were matched for gender and age with a similar number of controls. International classification of diseases (ICD-9-CM) codes were used to define diseases. The RA cohort was separated into quartiles based on the length of time nonsteroidal anti-inflammatory drugs (NSAIDs) were taken. The cumulative incidence of epilepsy was assessed in each group using a Kaplan-Meier method. Data from Table 4 of reference (37) are plotted as median and 95% confidence intervals of the adjusted hazard ratio for developing epilepsy. These results suggest that rheumatoid arthritis might increase risk of developing epilepsy, and that the risk is mitigated by chronic NSAID use.

Figure 4.. Therapeutic window of EP2 antagonist matches COX-2 induction time course

A. Delayed mortality mice that had experienced 1 hour of pilocarpine-induced status epilepticus (SE) and survived at least 12 hours. A brain-permeant prostaglandin E2 receptor 2 (EP2) antagonist (TG6-10-1) was administered in divided doses beginning at the times on the x-axis. B. Induction of cyclooxygenase 2 (COX-2) immunoreactivity in cornu ammonis 3 (CA3) pyramidal cells as early as 2 hours following SE induced in rats by diisopropyl fluorophosphate (DFP). C. Measurement of COX-1 and COX-2 protein levels in hippocampus at different times after DFP-induced SE onset. D. EP2 receptor antagonist maintains the integrity of the blood–brain barrier after SE. Serum albumin leak into the cortex 4 d after pilocarpine-induced SE was used to assess the integrity of the blood–brain barrier. The EP2 antagonist, TG6-10-1, was administered in three doses between 4 and 30 hours after SE onset. The albumin protein levels in cortices of control or SE mice that received vehicle or TG6-10-1 were measured by Western blot with β-actin as loading control. The normalized band intensity of the albumin protein is shown (n = 3-6, *p < 0.05, **p < 0.01, one-way ANOVA and post hoc Bonferroni test with selected pairs). A from ref. (55); B and C from ref. (56); D from ref. (54). A is reproduced from Jiang J, Yang MS, Quan Y, et al. Therapeutic window for cyclooxygenase-2 related anti-inflammatory therapy after status epilepticus. Neurobiol Dis. 2015;76:126-36. Epub 2015/01/21. doi: 10.1016/j.nbd.2014.12.032 with kind permission from Elsevier. B and C are reproduced from Rojas A, Ganesh T, Lelutiu N, et al. Inhibition of the prostaglandin EP2 receptor is neuroprotective and accelerates functional recovery in a rat model of organophosphorus induced status epilepticus. Neuropharmacology. 2015;93:15-27. Epub 2015/02/07. doi: 10.1016/j.neuropharm.2015.01.017 with kind permission from Elsevier. D is reproduced from Jiang J, Quan Y, Ganesh T, et al. Inhibition of the prostaglandin receptor EP2 following status epilepticus reduces delayed mortality and brain inflammation. Proc Natl Acad Sci U S A. 2013;110(9):3591-6. Epub 2013/02/13. doi: 10.1073/pnas.1218498110 with kind permission from PNAS (Proceedings of the National Academy of the United States of America).