Background
Alzheimer’s disease (AD) is the most common neurodegenerative disorder in the elderly and is associated with progressive memory loss and cognitive dysfunction [
1‐
4]. Although the precise cause of the AD is unknown, β-amyloid peptide (Aβ)-induced neurotoxicity, tau pathology, and neuroinflammatory responses by microglia are thought to contribute to the pathogenesis of AD [
1,
5]. Several studies have suggested that treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) attenuates the loss of cognitive function and decreases the risk of developing AD [
6,
7]. NSAIDs act by inhibiting cyclooxygenase (COX), which is the rate-limiting enzyme for the conversion of arachidonic acid into inflammatory mediators, which include prostaglandin E2 (PGE2). COX-2 enzymatically mediates the inflammatory response, and its expression is significantly increased in the AD brain [
8,
9].
Platelet-activating factor (PAF) is a highly potent inflammatory mediator and a potential neurotoxin [
10]. It plays an important role in excitotoxicity, production of free radicals and nitric oxide (NO), and regulation of pro-inflammatory cytokine genes [
11‐
14]. The biological action of PAF is mostly mediated by binding to its G protein-coupled membrane-associated receptor (PAFR) [
15]. In vitro studies have shown that activation of epidermal PAFR results in biosynthesis of COX-2 [
16]. Both PAFR and COX-2 are involved in memory processing in vivo [
17,
18]. PAF is a short-lived molecule due to its rapid degradation by PAF acetylhydrolase (PAF-AH; EC 3.1.1.47) [
19]. PAF-AH is an enzyme that hydrolyzes an acetyl ester at the sn-2 position of PAF, converting it into its inactive metabolite, 1-
O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) [
20]. Three isoforms of PAF-AH were identified: plasma PAF-AH and PAF-AH II and I [
21].
NSAIDs may regulate gene expression via their interaction with peroxisome proliferators-activated receptors (PPARs). There are three PPAR isoforms: PPAR α, β, and γ. PPARγ, in particular, has been implicated in inflammation and neurodegeneration [
22]. The messenger RNA (mRNA) levels of PPARγ are increased in AD patients [
23], suggesting that PPARγ may play an important role in modulating the pathophysiology of AD. The PPARγ isoform can be activated by NSAIDs, and its activation in the microglia suppressed the expressions of inflammatory cytokines, inducible NO synthase (iNOS), and COX-2 [
24]. Aβ-induced activation of microglia was suppressed by a PPARγ agonist in vitro [
24]. Furthermore, PPARγ agonists significantly decreased Aβ42 levels in vivo [
25].
It has been demonstrated that a standardized
G. biloba extract (Gb) contains flavonoids and terpene trilactones, and possesses free radical-scavenging and antioxidant activities [
26,
27]. Gb is commonly consumed as a dietary supplement for many disorders of the central nervous system (CNS), such as memory impairment, AD, and multi-infarct dementia [
28‐
31]. Gb has been shown to protect against Aβ-induced neurotoxicity [
32,
33]. In addition, Gb showed anti-inflammatory activity through the antagonism of PAF [
34,
35]. Among the constituents of terpene trilactones, ginkgolides A, B, and C are highly selective and competitive PAFR antagonists [
33].
The novel extract of
G. biloba, YY-1224, consists of 24% flavonoids and 12% terpene trilactones (ginkgolides A, B, and C and bilobalide) (Table
1 and Additional file
1: Figure S1). Given that Gb generally contains 22–27% flavonoid glycosides and 5–7% terpene lactones, increased terpenoid levels are likely to be responsible for the protective activity of YY-1224 [
36]. Bilobalide and ginkgolide B have neuroprotective activity [
37‐
42]. Pilot studies indicated that YY-1224 had significant neurotrophic and antioxidative activities (Additional file
1: Figures S3 and S4). In addition, YY-1224 inhibited the COX-2 mRNA or protein expression induced by Aβ (1-42) in mouse hippocampus, PC12 cells, or mixed cortical cells (Additional file
1: Figure S5)
. Furthermore, YY-1224 attenuated Aβ (1-42)-induced cell death in PC12 cells or mixed cortical cells (Additional file
1: Figure S6). We used COX-2 knockout (−/−) mice and APPswe/PS1dE9 transgenic (APP/PS1 Tg) mice to examine whether YY-1224 affects Aβ (1-42)-induced learning impairment and inflammatory responses when compared with Gb. Our results suggest that treatment with YY-1224 significantly attenuates Aβ (1-42)-induced memory impairments and pro-inflammatory responses via COX-2 suppression by inhibiting PAF and activating PPARγ. In addition, the prolonged treatment with YY-1224 enhances memory function and decreases Aβ peptide deposits and pro-inflammatory microglial activation in APP/PS1 Tg mice via COX-2 inhibition.
Table 1
Contents of Gb and YY-1224
Ginkgo flavone glycosides | Quercetin | 24 | 24.6 |
Kaempferol |
Isorhamnetin |
Terpene trilactones | Bilobalide | 6 | 12.9 |
Ginkgolide A |
Ginkgolide B |
Ginkgolide C |
Discussion
Increasing evidence has shown that Aβ-triggered microglial inflammatory activation damages neurons in AD pathogenesis [
5,
57]. In this study, YY-1224 improved cognitive function and inhibited neuroinflammatory activation in COX-2 (+/+) mice after Aβ (1-42) treatment. YY-1224 inhibited the production of pro-inflammatory factors and oxidative stress through PAFR antagonism and activation of the PPARγ-COX-2 signaling pathway. YY-1224 appeared to be more effective than Gb against cognitive dysfunction and Aβ pathology in APP/PS1 Tg mice as well as against Aβ-induced memory impairment via microglia polarization. Previous studies have suggested that the antioxidant activity of Gb contributes to a memory-enhancing effect in AD patients or aged rats [
58,
59]. Like NSAIDs, the constituents of Gb attenuated the production of inflammatory mediators such as COX-2, iNOS, and TNF-α in vitro [
60]. We hypothesize that YY-1224 facilitates memory-enhancing and anti-inflammatory effects by increasing the ratio of terpenoids such as bilobalide and ginkgolides [
36]. Bilobalide has been reported to exert protective effects on neurons [
37‐
40], to facilitate synaptic transmission and plasticity in hippocampal subfields [
61,
62], and to enhance spatial learning and memory [
63]. Ginkgolide B exerts neuroprotection by alleviating neurotoxicity [
42] and neuronal apoptosis [
41], in addition to acting as a PAF antagonist [
64].
PAF is a highly active phospholipid mediator of inflammation [
65]. The initial step of PAF formation is activation of phospholipase A2 (PLA2) in a calcium-dependent manner, yielding lyso-PAF. During this step, arachidonic acid is also released and can be converted to its respective cyclooxygenase and lipoxygenase products. The lyso-PAF is then acetylated in position 2 of the glycerol backbone by a coenzyme A (CoA)-dependent acetyltransferase. The majority of PAF’s effects are attributed to interaction with PAFR, which is expressed by multiple peripheral cell types; however, the effects of PAF are regionally restricted to microglia subpopulations in the CNS [
66]. The inhibition of PAF activity is mediated by a deacetylation reaction catalyzed by PAF-AH [
19]. PAF-AH I, originally identified in the brain, consists of three subunits (α1, α2, and β), in which the α subunits provide the catalytic activity. The complex has received attention in part because the subunit that modulates enzymatic activity (the β subunit) is the product of the LIS1 gene. LIS1 expression may be related to the regulation of PAF AH activity in the brain [
67]. PAF-AH I has been implicated in neuronal development, neuronal functions, Alzheimer's disease, bipolar disorder, and tolerance to hypoxia [
68]. Although the most structurally well-characterized component of PAF AH I is the α1 subunit [
69], a highly specific role of the α2 subunit for PAF hydrolysis has been recognized [
70]. In this study, the α2 subunit was the most sensitive to Aβ (1-42) exposure or double transgenic overexpression of APP and PS1. Therefore, the α2 subunit may be a therapeutic target for YY-1224 or Gb in response to Aβ (1-42) exposure or double transgenic overexpression of APP and PS1via positive modulation of COX-2.
Among two major groups of constituents of YY-1224, flavonoid (24%) and terpenoid (12%) ginkgolides are known as potent antagonists of PAF [
33]. YY-1224 treatment significantly attenuated decreases in the mRNA expression of PAF-AH I α2 and PAFR in response to Aβ (1-42) in COX-2 (+/+) mice. This suggests that the induction of PAF-AH I α2 and the PAF antagonistic activity of YY-1224 may contribute to its protective effects against Aβ (1-42) toxicity and possibly the double transgenic overexpression of APP and PS1.
Our current findings are consistent with previous reports that activation of PPARγ regulates the expression of PAF-AH [
71]. We also observed that repeated treatment with YY-1224 attenuates the Aβ (1-42)-induced decrease in PPARγ and PAF-AH expression in COX-2 (+/+) but not COX-2 (−/−) mice, suggesting that the PPARγ-PAF-AH pathway may mediate the protective effects of YY-1224 via positive modulation of COX-2. This notion is supported by previous findings that PAF induces the expression of COX-2 [
72]. The connection between PAF and PGE2 production by COX-2 has also been demonstrated in astrocytes. Stimulation of these cells with the non-hydrolyzable analog methylcarbamyl-PAF increased the secretion of PGE2, which was reduced by inhibitors of COX-2 but not COX-1 [
73]. In this manner, PAF, which is a short-lived molecule due to its rapid degradation by PAF-AH, is able to have a long lasting effect on neurons through induction of COX-2. We found that the deleterious effect of Aβ (1-42) on brain oxidative stress and inflammatory markers are found in COX-2 expressing- but not COX-2-null mice. In addition, the protective effect of YY-1224 is present only in COX-2 expressing- but not COX-2 null mice or APP/PS1 Tg mice treated with the selective COX-2 inhibitor, meloxicam. This suggests an important role of the PPAR-PAF-COX-2 pathway in the deleterious effect of Aβ (1-42) as well as beneficial effects of ginkgolides and YY-1224. Inhibition of this pathway could reduce the formation of pro-inflammatory products of COX-2 such as prostaglandins resulting in decreased levels of pro-inflammatory mediators. We also found that COX-2 inhibition might interfere with a possible feedforward loop to amplify the inflammatory process by reducing PAF signaling through induction of its degrading enzyme, PAF-AH I, and reduction in expression of the PAF receptor, PAFR. This potential feedforward loop could exist in parallel with a previously described feedback loop, where pro-inflammatory mediators formed by COX-2 induce the expression of PAF-AH, resulting in reduced PAF and resolution of inflammation [
74]. In this context, it is interesting to note that unlike PAF-AH I, PAF-AH II is induced by Aβ (1-42), and such induction is abolished in COX-2 null mice, although this enzyme is highly expressed in the liver and kidney [
75]. More studies are needed to determine how COX-2 metabolites such as prostaglandins differentially regulate PAF-AH isoforms, resulting in propagation or modulation of inflammation.
Besides its effects on neurons, Aβ can activate microglial cells. This could lead to neuroinflammation and progression of neurodegeneration [
76]. Microglia/macrophages are capable of undergoing phenotypic polarization to the M1 phenotype to produce pro-inflammatory cytokines, or to the M2 phenotype to produce anti-inflammatory cytokines [
77,
78]. A prevalence of M1 over M2 microglia/macrophages has been reported in neurodegenerative pathologies, such as AD, and in the elderly brain [
79]. In the present study, Aβ (1-42) induced an increase in inflammatory cytokines including TNF-α and IL-1β in microglia from the COX-2 (+/+) mice. PPARγ agonists are able to prevent the detrimental polarization of microglia, or switch the polarization of reactive microglia from the pro-inflammatory to the anti-inflammatory phenotype [
87,
80]. Therefore, our results suggest that inhibiting the M1 pro-inflammatory phenotype or activating microglial polarization toward an M2 anti-inflammatory phenotype might contribute to the memory-enhancing effect of YY-1224.
The results of this study are consistent with the notion that YY-1224 is a potential PPARγ agonist that could be useful in the treatment of AD. PPARγ agonists have potent anti-inflammatory activity [
81,
82], and activation prevents brain damage through an anti-inflammatory effect on endothelial cells, astrocytes, and microglia [
83]. Activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and COX-2 increases expression of pro-inflammatory cytokines [
84‐
87], whereas inhibition of NF-κB suppresses the expression of COX-2 induced by Aβ (25-35) in PC12 cells [
87]. In contrast to NF-κB, activation of PPARγ inhibits the expression of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6 in vitro [
88,
89]. Treatment with a PPARγ agonist 15dPGJ2 inhibited NF-κB signaling and the DNA-binding activity of NF-κB in macrophages [
90,
91]. Moreover, treatment with another PPARγ agonist, pioglitazone, attenuates the production of Aβ plaque burden and glial inflammation in the APPV717I transgenic mice [
83] and cognitive impairments in AD patients [
92].
Acknowledgements
The English in this document has been checked by at least two professional editors, both native speakers of English (eWorldEditing, Inc., Eugene, OR 97401, USA).