currently , 1/9 people older than 65 years of age have alzheimer s disease ( ad ) , and 1/3 of people older than 85 are diagnosed with ad . as the aging population expands , the projected number of ad cases is expected to triple by 2050 , with economic costs escalating from $ 214 billion in 2014 to $ 1.2 trillion in 2050 ( alzheimer s association facts 2014 ) .
this represents an enormous economic and societal challenge , particularly given the limited efficacy of currently available ad therapeutics . in terms of ad prevention , attention to diet , exercise , and cardiovascular health are likely to help reduce the risk of developing ad .
however , identification of specific molecular pathways that could be targeted for prevention in aging and at - risk populations is an urgently needed strategy to help stem this dementia epidemic .
given that the prevalence of ad doubles every 5 years in persons above the age of 65 and given the average human longevity of ~80 years , preventive strategies that could delay the onset of cognitive decline by only 5 years may reduce disease burden by half .
recent data indicate that cyclooxygenase / pge2/ep receptor signaling may play important roles in preclinical development of ad in both human epidemiology and in mouse models of ad .
pathological changes in ad consist of amyloid  accumulation , tau phosphorylation , and synaptic and neuronal loss .
these pathological hallmarks develop in the context of a potent and chronic inflammatory response characterized by glial activation , generation of cytokines and chemokines , complement proteins , inflammatory enzymes , and oxidative stress [ 2 , 3 ] .
this chronic inflammatory response is not only injurious to synapses , neurons , and circuits , but is persistent and non - resolving .
recent studies indicate that the pre - clinical development of ad begins years to decades before initial diagnosis .
while the initiating pathological events are not well defined , they are likely to involve synergistic interactions between a oligomer - mediated synaptic injury and dysregulated inflammatory responses .
amyloid  peptide generation and accumulation is an initiating event , and precedes onset of symptoms by years to decades .
however , amyloid pet imaging studies suggest that a peptide accumulation can occur in subjects who do not have evidence of cognitive decline , leading to the hypothesis that a peptide accumulation is  necessary but not sufficient  for progression to ad . one or more additional factors may be necessary . in that regard ,
recent gwas studies of sporadic late - onset ad have identified genes involved in the innate immune response that are expressed in microglia , the resident myeloid cells of the central nervous system ( cns ) .
these microglial genes include the sialic acid - binding immunoglobulin - like lectin cd33 [ 6 - 8 ] and trem2 [ 9 , 10 ] which regulate phagocytosis and a peptide clearance [ 11 - 13 ] , and the complement receptor cr1 [ 14 , 15 ] .
these findings indicate that the microglial immune response may play a pathogenic role in the development of ad .
microglia have embryonic origins [ 16 , 17 ] and genetic signatures [ 18 - 20 ] that distinguish them from monocytes and other tissue macrophages . like other tissue macrophages ,
however , because microglia reside in the brain , they have additional and unique functions , including the establishment , maintenance , and pruning of synapses that are dynamically regulated by synaptic activity [ 21 - 24 ] .
microglia can also regulate synaptic activity through mechanisms that are beginning to be identified , including secretion of neuroactive and neurotrophic factors .
a role for microglia in the development of ad may therefore involve not only perpetuation of dysfunctional innate immune responses to accumulating a peptides and dying synapses and neurons , as the current literature supports , but may also involve direct effects of microglia on synapse integrity and function .
the primary action of nsaids is the enzymatic inhibition of the cyclooxygenases cox-1 and cox-2 , cytosolic enzymes that generate pgh2 from membrane stores of arachidonic acid .
pgh2 is the precursor of the prostaglandins pge2 , pgd2 , pgi2 , and pgf2a , and the thromboxane txa2 . in epidemiologic studies of cognitively
normal aging populations , nsaids prevent and delay development of ad [ 26 - 31 ] .
although selected nsaids may have cyclooxygenase - independent effects , structurally distinct nsaids , including ibuprofen , naproxen , and sulindac all reduce risk of developing ad in large epidemiologic studies [ 27 , 28 , 30 ] suggesting that inflammatory prostaglandin signaling plays an important role in pre - clinical development of ad .
interestingly , the beneficial effects of nsaids are restricted to the pre - clinical asymptomatic phase , as nsaids or selective cox-2 inhibitors do not help patients with mild cognitive impairment ( mci ) or ad [ 31 - 36 ] .
nsaids however are not a good choice for large scale ad prevention because both toxic as well as beneficial downstream prostaglandin signaling pathways are inhibited and lead to adverse effects , including renal and gastric toxicity and increased risk for vascular disease .
recent studies have identified beneficial prostaglandin signaling pathways downstream of nsaid action , including the vasodilatory prostacyclin pgi2 receptor and the neuroprotective , anti - inflammatory , and vasodilatory pge2 ep4 receptor [ 38 - 42 ] . this major limitation may help explain why in mci and ad , nsaids show no benefit , either because protective downstream prostaglandin pathways are inhibited or disease progression is already too advanced .
inhibition of cox enzymatic activity by nsaids therefore has different consequences depending on timing of ad development , and inhibition of cox-1/cox-2 by non - selective nsaids is beneficial in preventing disease in healthy aging individuals but ineffectual once symptoms begin [ 31 , 43 ] .
given the expanding population of aging individuals and the anticipated rise in ad cases , understanding the molecular mechanisms by which nsaids prevent ad has taken on significant urgency .
targeting of toxic inflammatory prostaglandin signaling downstream of cox may potentially slow or prevent progression to ad .
pge2 , pgd2 , pgi2 , and pgf2a , and the thromboxane txa2 are lipid signaling molecules that bind and activate specific g - protein - coupled receptors designated ep ( for e - prostanoid receptor ) , dp , ip , fp , and tp , respectively .
pge2 in particular has generated interest as a potential inflammatory agent in pre - clinical ad , as it was found to be increased 5-fold in cerebrospinal fluid ( csf ) of patients with early or probable ad [ 45 , 46 ] , but then declined with disease progression . in parallel ,
levels of the breakdown product of prostacyclin ( pgi2 ) , widely considered to be anti - inflammatory , were significantly decreased in csf of probable ad subjects .
pge2 binds four g - protein coupled receptors ( gpcrs ) termed e - prostanoid receptors ( ep1 - 4 ) that have distinct downstream signaling cascades and cellular distributions in brain . in vivo
, all four ep receptors are expressed in neurons ; microglial expression of ep2 , ep3 , and ep4 receptors has been confirmed in mouse brain [ 41 , 47 , 48 ] .
activation of ep receptors leads to changes in the production of camp and/or phosphoinositol turnover and ca mobilization . ep2 and ep4 receptors couple positively to gs to increase camp formation whereas ep3 couples negatively to camp through gi ; ep1 couples to gq , and activation results in increased intracellular calcium concentrations .
to better understand the inflammatory pge2 signaling pathway in the context of a peptide accumulation , investigators have studied mouse models of familial ad , where transgenic mice express mutant forms of the amyloid precursor protein ( app ) and/or presenilin 1 ( ps1 ) genes .
microglial activation and elaboration of inflammatory and oxidative stress are well documented in these models , particularly as these mutant app mice age and accumulate a42 and a40 peptides .
mutant app models display either loss of synapses or loss of synaptic proteins that are associated with spatial memory deficits , and these have been linked to effects of a oligomers , which are directly toxic to synapses , and to effects of inflammatory mediators like il1   or tnf .
however , because mutant app models do not develop significant neuronal loss and tau pathology , two hallmarks of mci and ad in human subjects , these models are believed to be more reflective of the pre - clinical or asymptomatic phases of human ad .
if considered in this way , the beneficial effects of nsaids have been validated in these mutant app models , where a correlation between nsaid administration and reduction of brain inflammation , a deposition , and rescue of learning and memory deficits has been established [ 53 - 56 ] .
early studies examining in vivo effects of ep2 signaling in innate immunity demonstrated a significant reduction in lipid peroxidation following intracerebroventricular ( icv ) administration of lipopolysaccharide ( lps ) , a canonical inducer of the innate immune response .
lps - dependent increases in f2-isoprostanes , which are free radical - generated isomers of prostaglandin pgf2a , and f4-neuroprostanes ( isops ) , which are products of neuronal docosohexanoic acid ( dha ) oxidation , were significantly suppressed in cerebral cortex of ep2-/- mice . in vitro studies parsing out the cellular specificity of the ep2 oxidative effect demonstrated that microglial ep2 elicited paracrine neurotoxicity in co - cultures of neurons and lps - primed microglia , and this effect was dependent on increased inducible nitric oxide synthase ( inos ) and cox-2 activity . in vivo , in the appswe - ps1e9 model of ad ( app - ps1 ) ,
global deletion of ep2 also led to significant decreases in lipid peroxidation in aging app - ps1 mice , suggesting a toxic inflammatory role for the ep2 receptor .
additional in vivo studies demonstrated that global deletion of ep2 reduced expression of oxidative enzymes inos and components of the nadph oxidase complex in the app - ps1 model of ad , and reduced expression of cox-1 , cox-2 , inos , and many components of the nadph oxidase complex in a related model of familial amyotrophic lateral sclerosis ( als ) .
a major target of the innate immune response in the cns is the clearance of toxic and misfolded proteins .
the accumulation of misfolded or aggregated proteins is a common feature of several chronic neurodegenerative diseases , including ad , parkinson s disease , huntington s disease , and als .
however , with progression of disease , notably in ad models , the healthy phagocytic response of microglia to a peptides falters , either because microglia become ineffective or because they are overwhelmed by levels of accumulating a peptides . in parallel with progression of pathology in ad model mice , microglia also develop a more toxic inflammatory phenotype .
this leads to a damaging feed - forward cycle , with increasing accumulation of toxic a peptide assemblies along with increased elaboration of toxic cytokines . in ad model mice , a signaling through toll - like receptors 2 and 4 ( tlr2 and tlr4 ) drives downstream activation of nf-b transcription factors as a central inflammatory pathway in ad .
a role for microglial ep2 in inhibiting phagocytosis of a fibrils was demonstrated in an ex vivo acute preparation using ad brain sections coated with ep2-/- microglia .
microglia lacking ep2 receptor cleared human a peptides more effectively than wild type microglia , and ep2-/- microglia were associated with lower paracrine neurotoxicity . in vivo , in the app - ps1 model , deletion of ep2 resulted in significant reductions in total a40 and a42 levels and amyloid plaque deposition , an outcome likely reflecting both a more benign inflammatory milieu and an enhanced clearance of a peptides . in chimeric app - ps1 mice subjected to whole body irradiation followed by transplantation of wild type or ep2-/- bone marrow ,
levels of amyloid plaque were reduced in app - ps1 mice receiving wild type bone marrow , however ep2-/- bone marrow elicited even larger decreases in cerebral cortical plaque load .
a role for ep2 signaling in phagocytosis has been shown in non - cns models , where myeloid ep2 suppresses phagocytosis of latex beads [ 67 , 68 ] and bacteria [ 69 - 72 ] .
recent in vivo studies using conditional knockout strategies have further defined the critical toxicity of microglial ep2 signaling in models of ad .
conditional knock down of myeloid ep2 receptor using the cd11bcre recombinase line , where levels of myeloid ep2 are reduced ~50% , had multiple beneficial effects and restored healthy microglial responses to a peptides .
cell - specific knockout of microglial ep2 increased microglial clearance of a peptides and suppressed toxic inflammatory gene expression .
in addition , unbiased genomic studies of microglia isolated from brain revealed that knockdown of microglial ep2 receptor increased generation and local signaling of insulin - like growth factor 1 ( igf1 ) in hippocampus in response to a peptide stimulation .
igf1 promotes synaptogenesis , neurogenesis , and neuroprotection through the pi3k / akt pathway in brain .
deletion of microglial ep2 also increased expression of members of the ppar signaling pathway , including rxr which along with its binding partner ppar reduces proinflammatory gene expression   and enhances clearance of a peptides .
rxr is also the target of the fda - approved rxr agonist bexarotene ( targretin ) that has been shown in some studies to lower interstitial levels of soluble a peptides and to prevent memory deficits in ad model mice [ 77 , 78 ] .
additional genes suppressed by ep2 signaling in microglia but intimately related to a peptide clearance included the cholesterol transporter abca1   and apolipoprotein e ( apoe ) , proteins that enhance proteolytic degradation of soluble a peptides [ 81 , 82 ] . in addition , lipoprotein lipase ( lpl ) , which binds a peptide , was also upregulated in hippocampus with ep2 microglial deletion ; interestingly , an intronic polymorphism in lpl is associated with reduced lpl mrna , increased amyloid and neurofibrillary tangle densities , and increased prevalence of ad .
the upregulation of these genes in microglial ep2 knockout mice suggests that ep2-deficient microglia respond to a42 peptides in vivo by inducing anti - inflammatory and a - clearing nuclear hormone receptor signaling genes .
consistent with this beneficial effect of reduced ep2 signaling , conditional deletion of microglial ep2 in the app - ps1 model prevented synaptic injury and spatial memory deficits .
the toxic effects of microglial ep2 contrast significantly with the beneficial anti - inflammatory effects of microglial ep4 in vivo .
previous in vivo studies of innate immune responses to lps had identified a pronounced anti - inflammatory effect of microglial ep4 activation that was associated with reduced nuclear translocation of nf-b subunits p65 and p50   in myeloid cells .
conversely , following lps stimulation in vivo , conditional deletion of the ep4 receptor in myeloid cells led to an increase in brain pro - inflammatory gene expression and lipid peroxidation , suggesting that the function of myeloid ep4 is to attenuate and/or terminate innate immune responses .
subsequent studies of a42-mediated inflammatory responses confirmed the anti - inflammatory nature of microglial ep4 signaling , where in primary microglial cells , ep4 stimulation attenuated levels of a42-induced inflammatory factors and potentiated phagocytosis of a42 .
unbiased genomic studies showed that ep4 receptor activation broadly opposed a42-driven gene expression changes in microglia , with significant enrichment of targets of irf1 , irf7 , and nf-b transcription factors . in vivo , in app - ps1 mice deficient for microglial ep4 , inflammatory gene expression , oxidative protein modification , and a deposition in brain were significantly increased at early stages of pathology , but not at later stages , suggesting an early anti - inflammatory function of microglial ep4 signaling in the app - ps1 model . thus , although both the ep2 and ep4 receptors are positively coupled to camp , they appear to have divergent inflammatory functions in models of a peptide mediated neuroinflammation and neurodegeneration . the carboxy terminal cytoplasmic tail of the ep4 receptor is significantly longer than that of the ep2 receptor and can recruit distinct signaling molecules , a difference likely to influence downstream signaling of the ep4 versus ep2 receptors .
the ep3 receptor is a central component in the regulation of the febrile response [ 87 , 88 ] . in classical models of innate immunity
, the function of inflammatory ep3 has been explored in a model of bacterial lung infection , where deletion of ep3 resulted in a marked increase in clearance of bacteria , reduced neutrophil ingress , and improved survival , a phenotype reminiscent of the pro - phagocytic and anti - inflammatory effects of ep2 deletion , as discussed above . in the brain , ep3 has mainly been localized to neurons , however immunocytochemical induction of ep3 expression has been observed in striatal microglia in the setting of an excitotoxic lesion induced by injection of quinolinic acid , a potent ligand at the glutamatergic n - methyl - d - aspartate ( nmda ) receptor .
this suggests that although ep3 may not be expressed in microglia under physiological conditions , in the appropriate inflammatory or injury contexts , ep3 might be induced and contribute to innate immune responses .
the function of ep3 signaling has been examined in two models relevant to ad , namely the icv a injection model , which elicits a potent and long lasting inflammatory response to a peptides [ 90 , 91 ] and in the app - ps1 model . in a recent study ,
the induction of pro - inflammatory gene expression , cytokine generation , and lipid peroxidation following icv a42 was significantly abrogated in ep3-/- mice , suggesting that microglial ep3 signaling engaged a toxic inflammatory response to a peptides . in the app - ps1 model ,
deletion of ep3 significantly blunted the induction of proteins capable of increasing oxidative injury , including inos , components of the nadph oxidase complex , and cox-2 .
moreover , suggestive of a role in a clearance , app - ps1 mice lacking either one or both ep3 alleles had significantly lower amyloid accumulation .
this effect is consistent with the lung infection studies mentioned above , where deletion of ep3 led to enhanced clearance of bacteria .
however , it is also possible ep3 deletion impacted on the generation of a peptide from app , as prior studies have correlated increased oxidative stress and inflammation with increased expression and activity of  - secretase [ 93 , 94 ] , the first enzyme to cleave app in the formation of the a42 peptide . indeed ,
loss of ep3 receptor in the app - ps1 background resulted in decreased  - secretase expression and activity .
this would suggest that in the app - ps1 model , ep3 signaling may both suppress a clearance , and by increasing inflammatory oxidative stress , increase the generation of a peptide .
interestingly , loss of just one allele of ep3 in the app - ps1 background had significant effects on a peptide levels .
of the four ep receptors , the role of inflammatory ep1 in ad is less clear .
ep1 is highly expressed in neurons [ 95 , 96 ] and functions in neuronal survival . in models of neuronal injury , including nmda excitotoxicity and cerebral ischemia , pharmacologic or genetic deletion of ep1 reduces cerebral injury [ 96 - 98 ] .
however , ep1 expression is not found in microglia after hypoxia - ischemia , nor does microglial activation induce neurotoxicity in hippocampal slices treated with lps and ifn .
a lack of effect of inflammatory ep1 would suggest that the toxic function of this receptor is primarily neuronal . supporting this conclusion are findings that inflammation sensitive neural progenitor cells ( npc )   in the subgranular zone of the dentate gyrus are vulnerable to microglial ep2 signaling and npc ep1 signaling .
pre - clinical development of ad begins decades prior to diagnosis , and nsaids act during this time period to delay onset and progression to ad .
the early accumulation of a42 peptides , beginning years to decades before cognitive symptoms arise , triggers microglial inflammatory responses that are initially robust , but falter as disease progresses .
a summary of findings relevant to inflammatory actions of the pge2 ep2 , ep3 , and ep4 receptors in the app - ps1 model are diagrammed in fig . ( 1 ) .
the opposing actions of the ep2/ep3 and ep4 receptors highlight the importance of targeting selected ep receptors downstream of cox-1/cox-2 , as upstream inhibition of cox-1/cox-2 activity may inhibit beneficial as well as toxic pge2 ep receptor signaling .
this is a potential future indication , as we await the identification and validation of biomarkers that can reliably predict subjects at risk for ad .
 the inflammatory response is a fundamental driving force in the pathogenesis of alzheimer s disease ( ad ) . in the setting of accumulating immunogenic a peptide assemblies , microglia , 
 the innate immune cells of the brain , generate a non - resolving immune response and fail to adequately clear accumulating a peptides , accelerating neuronal and synaptic injury . 
 pathological , biomarker , and imaging studies point to a prominent role of the innate immune response in ad development , and the molecular components of this response are beginning to be unraveled . 
 the inflammatory cyclooxygenase - pge2 pathway is implicated in pre - clinical development of ad , both in epidemiology of normal aging populations and in transgenic mouse models of familial ad . 
 the cyclooxygenase - pge2 pathway modulates the inflammatory response to accumulating a peptides through actions of specific e - prostanoid g - protein coupled receptors . 
INTRODUCTION
Neuroinflammation in AD
Non-steroidal anti-inflammatory drugs (NSAIDs) and PGE
Mouse modeling of preventive effects of NSAIDs
Roles of microglial EP receptors in models of AD: opposing effects of microglial EP2 and EP4 in mouse AD models
Function of the EP3 and EP1 receptors in the inflammatory response in AD
Conclusion
CONFLICT OF INTEREST