The cyclooxygenases COX-1 and COX-2 catalyze the first committed step of prostaglandin synthesis from arachidonic acid. Previous studies in rodent stroke models have shown that the inducible COX-2 isoform promotes neuronal injury, and the administration of COX-2 inhibitors reduces infarct volume. We investigated the function of PGE 2 , a principal prostaglandin product of COX-2 enzymatic activity, in neuronal survival in cerebral ischemia. PGE 2 exerts its downstream effects by signaling through a class of four distinct G-proteincoupled EP receptors (for E-prostanoid: EP1, EP2, EP3, and EP4) that have divergent effects on cAMP and phosphoinositol turnover and different anatomical distributions in brain. The EP2 receptor subtype is abundantly expressed in cerebral cortex, striatum, and hippocampus, and is positively coupled to cAMP production. In vitro studies of dispersed neurons and organotypic hippocampal cultures demonstrated that activation of the EP2 receptor was neuroprotective in paradigms of NMDA toxicity and oxygen glucose deprivation. Pharmacologic blockade of EP2 signaling by inhibition of protein kinase A activation reversed this protective effect, suggesting that EP2-mediated neuroprotection is dependent on cAMP signaling. In the middle cerebral artery occlusion-reperfusion model of transient forebrain ischemia, genetic deletion of the EP2 receptor significantly increased cerebral infarction in cerebral cortex and subcortical structures. These studies indicate that activation of the PGE 2 EP2 receptor can protect against excitotoxic and anoxic injury in a cAMPdependent manner. Taken together, these data suggest a novel mechanism of neuroprotection mediated by a dominant PGE 2 receptor subtype in brain that may provide a target for therapeutic intervention.

Epidemiological studies demonstrate that chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs) in normal aging populations reduces the risk of developing Alzheimer's disease (AD). NSAIDs inhibit the enzymatic activity of cyclooxygenase-1 (COX-1) and inducible COX-2, which catalyze the first committed step in the synthesis of prostaglandins. These studies implicate COX-mediated inflammation as an early and potentially reversible preclinical event; however, the mechanism by which COX activity promotes development of AD has not been determined. Recent studies implicate the prostaglandin E 2 (PGE 2 ) E prostanoid subtype 2 (EP2) receptor in the development of the innate immune response in brain. Here, we report that deletion of the PGE 2 EP2 receptor in the APPSwe-PS1⌬E9 model of familial AD results in marked reductions in lipid peroxidation in aging mice. This reduction in oxidative stress is associated with significant decreases in levels of amyloid-␤ (A␤) 40 and 42 peptides and amyloid deposition. Aged APPSwe-PS1⌬E9 mice lacking the EP2 receptor harbor lower levels of ␤ C-terminal fragments, the product of ␤-site APP cleaving enzyme (BACE1) processing of amyloid precursor protein. Increases in BACE1 processing have been demonstrated in models of aging and AD and after oxidative stress. Our results indicate that PGE 2 signaling via the EP2 receptor promotes age-dependent oxidative damage and increased A␤ peptide burden in this model of AD, possibly via effects on BACE1 activity. Our findings identify EP2 receptor signaling as a novel proinflammatory and proamyloidogenic pathway in this model of AD, and suggest a rationale for development of therapeutics targeting the EP2 receptor in neuroinflammatory diseases such as AD.

Cyclo-oxygenases (COXs) catalyze the first committed step in the synthesis of the prostaglandins PGE 2 , PGD 2 , PGF 2a , PGI 2 and thomboxane A 2 . Expression and enzymatic activity of COX-2, the inducible isoform of COX, are observed in several neurological diseases and result in significant neuronal injury. The neurotoxic effect of COX-2 is believed to occur through downstream effects of its prostaglandin products. In this study, we examined the function of PGD 2 and its two receptors DP1 and chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) (DP2) in neuronal survival. PGD 2 is the most abundant prostaglandin in brain and regulates sleep, temperature and nociception. It signals through two distinct G protein-coupled receptors, DP1 and DP2, that have opposing effects on cyclic AMP (cAMP) production. Physiological concentrations of PGD 2 potently and unexpectedly rescued neurons in paradigms of glutamate toxicity in cultured hippocampal neurons and organotypic slices. This effect was mimicked by the DP1-selective agonist BW245C but not by the PGD 2 metabolite 15d-PGJ 2 , suggesting that neuroprotection was mediated by the DP1 receptor. Conversely, activation of the DP2 receptor promoted neuronal loss. The protein kinase A inhibitors H89 and KT5720 reversed the protective effect of PGD 2 , indicating that PGD 2 -mediated neuroprotection was dependent on cAMP signaling. These studies indicate that activation of the PGD 2 DP1 receptor protects against excitotoxic injury in a cAMP-dependent manner, consistent with recent studies of PGE 2 receptors that also suggest a neuroprotective effect of prostaglandin receptors. Taken together, these data support an emerging and paradoxical neuroprotective role of prostaglandins in the CNS.

Recent studies suggest a neuroprotective function of the PGE2 EP2 receptor in excitotoxic neuronal injury. The function of the EP2 receptor was examined at time points after excitotoxicity in an organotypic hippocampal model of N-methyl-D-aspartate (NMDA) challenge and in a permanent model of focal forebrain ischemia. Activation of EP2 led to significant neuroprotection in hippocampal slices up to 3 hours after a toxic NMDA stimulus. Genetic deletion of EP2 resulted in a marked increase in stroke volume in the permanent middle cerebral artery occlusion model. These findings support further investigation into therapeutic strategies targeting the EP2 receptor in stroke.