Background
It has been demonstrated in mice and rats that the systemic administration of lipopolysaccharide (LPS) and polyriboinosinic:polyribocytidylic acid (poly I:C), ligands for toll-like receptor (TLR) 4 and TLR 3, respectively, induce neuroinflammation in the central nervous system (CNS), thus leading to neurodegeneration, the suppression of neurogenesis and the impairment of cognitive behavior [
1‐
3]. One of the possible mechanisms of neuroinflammation may be the production of β-amyloid proteins (Aβ). For example, a single intraperitoneal (i.p.) injection of LPS increases the activity of β-secretase, a key rate-limiting enzyme that initiates Aβ formation, and the concentration of brain Aβ
1–42 in adult but not young mice [
1]. Furthermore, the intracellular accumulation of Aβ
1–42 in hippocampal pyramidal neurons following daily injections of LPS for seven days has been immunohistochemically demonstrated [
1]. Although the precise mechanisms underlying LPS-induced amyloidogenesis have not yet been determined, it is likely that proinflammatory cytokines such as IL-1β, TNF-α, IFN-γ, and reactive oxygen/nitrogen species (ROS/RNS) released from activated glial cells play significant roles in Aβ formation, which are suppressed by NSAIDs through the activation of peroxisome proliferator-activated receptor-γ (PPAR-γ) [
4‐
6].
Plasmalogens (Pls) are unique glycerophospholipids that contain a vinyl ether bond at the
sn-1 position of the glycerol moiety. They are found in all mammalian tissues, especially in the heart and brain, in which ethanolamine Pls (PlsEtn) are much more abundant than choline Pls (PlsCho) [
7]. Pls release either docosahexaenoic acid (DHA) or arachidonic acid (ARA) from the
sn-2 position through the activation of Pls-selective phospholipase A
2 (Pls-PLA
2) [
8,
9]. Pls are not only structural membrane components and reservoirs for second messengers, but they are also involved in membrane fusion, ion transport and cholesterol efflux [
7]. In addition, the vinyl ether bond at the
sn-1 position makes Pls more susceptible to oxidative stress than corresponding ester bonded glycerophospholipids, which act as antioxidants and protect cells from oxidative stress [
10‐
13].
It has been shown that patients suffering from Alzheimer’s disease (AD) have reduced PlsEtn levels in the cortex and hippocampus [
14‐
16]. The reduction of PlsEtn levels seems to be specific since other neurodegenerative diseases such as Huntington’s and Parkinson’s do not show decreases in corresponding affected brain regions (the caudate nucleus and the substantia nigra, respectively) [
7,
14,
17]. Furthermore, circulating PlsEtn levels are also decreased depending on the severity of dementia [
18,
19]. It has been suggested that deficiencies of PlsEtn may lead to increases in the vulnerability of neural membranes to oxidative stress, destabilization of membranes, impairment of muscarinic cholinergic signals and abnormal amyloid precursor processing [
7,
17,
20].
Although Pls are considered to be involved in the pathology of AD, the influence of Pls on Aβ accumulation has not been examined, probably due to the difficulty of extracting massive amounts of intact Pls. Recently, we developed a new method for preparing highly pure Pls [
21], which enabled us to investigate this issue. In the present study, in order to elucidate the anti-neuroinflammatory/anti-amyloidogenic actions of Pls, we investigated the effects of co-administered Pls on the systemic LPS-induced changes in the morphology of glial cells, the expression of cytokines, the accumulation of Aβ and the levels of Pls in the prefrontal cortex (PFC) and hippocampus of adult mice.
Methods
All experimental procedures involving the use of animals were approved by the Ethics Committee on Animal Experiments at Kyushu University and were in accordance with the Guiding Principles for the Care and Use of Animals of the Physiological Society of Japan. All efforts were made to minimize animal suffering and the number of animals used in the study.
Animals
Male C57/6J mice weighing 32 to 37 g (10 months old) were used in all experiments. The animals were housed five per cage at a temperature of 22 ± 2°C with 12 hour light/12 hour dark cycles (lights on at 8:00) with free access to laboratory food and water. The mice were randomly divided into four groups: control (Con), LPS, LPS + Pls and Pls. LPS (Sigma-Aldrich, St. Louis, MO, USA) was dissolved in saline, while the Pls were dissolved in corn oil then sonicated to ensure complete solubilization. The LPS group received an i.p. injection of LPS (250 μg/kg) followed by corn oil in the morning (9:00 to 10:00) daily for seven days (days 1 to 7). The LPS + Pls group was treated with LPS followed by Pls (20 mg/kg) for seven days, while the Pls group was injected with saline and Pls. The Con group was given saline and corn oil for seven days. All animals were sacrificed on day 8. The body weights were measured in the morning before injection on day 1 to day 8.
Pl preparation
The Pls used in the present study were prepared from chicken breast muscle using a previously reported method [
21]. A HPLC used for phospholipid separation [
22] indicated that the purified Pls consisted of 47.6% PlsEtn, 49.3% PlsCho, 2.4% sphingomyelin (SM) and 0.5% other phospholipids. The composition of fatty acids of PlsEtn and PlsCho was analyzed using the previously described HPLC method [
21], as shown in Table
1.
Table 1
The fatty acid composition of the PlsEtn and PlsCho in the purified Pls
16:0 | (palmitic acid) | 3.6 | 20.5 |
18:0 | (stearic acid) | 2.2 | 12.4 |
18:1, n-9 | (oleic acid) | 26.3 | 20.1 |
18:2, n-6 | (linoleic acid) | 4.1 | 10.1 |
18:3, n-3 | (α-linolenic acid) | 7.2 | 3.8 |
20:4, n-6 | (arachidonic acid, ARA) | 24.9 | 17.2 |
22:6, n-3 | (docosahexaenoic acid, DHA) | 18.6 | 2.3 |
others | | 13.2 | 13.6 |
Immunohistochemistry and immunofluorescence
The mice were deeply anesthetized with pentobarbital (50 mg/kg) and transcardially perfused with PBS followed by 4% paraformaldehyde. For each animal, the brain was removed, post-fixed for 24 hours and then transferred successively to 20% and 30% sucrose solutions. Subsequently, the brains were frozen on a cold stage and sliced into 30 μm thicknesses using a cryostat. The sections were permeabilized with 0.3% Triton-X 100 (Sigma-Aldrich) in PBS for 15 minutes and blocked in PBS containing 1% BSA and 5% normal donkey serum (Jackson ImmunoResearch Lab., West Grove, PA, USA) for 60 minutes at room temperature. The sections were incubated in blocking solution (Block Ace, Dainippon Pharmaceutical, Osaka, Japan) for 30 minutes at room temperature and then incubated with rabbit polyclonal antibodies against Iba-1 (1:10000; Wako Pure Chem. Indus., Osaka, Japan), which are known to have specific affinity for microglial Ca2+-binding proteins and are highly expressed by activated microglia, and anti-glial fibrillary acidic proteins (GFAPs, 1:3000; Sigma-Aldrich) for astrocytes in 10% Block Ace in PBS at 4°C overnight. Other sections were incubated with polyclonal antibodies against Aβ3–16 (1:1000; ab14220, Abcam, Cambridge, UK) and NeuN (1:1000; Millipore, Billerica, MA, USA). According to the manufacturer’s instructions, ab14220s react with all isoforms of mouse and rat Aβ. The rinsed sections were incubated for six hours with Alexa Fluor 488 goat anti-rabbit IgG or Alexa Flour 568 goat anti-mouse immunoglobulin G (IgG) (1:1000; Invitrogen, Eugene, OR, USA) at room temperature. Every treatment was followed by washing three times for five minutes with PBS. The sections were then mounted in the perma fluor aqueous mounting medium (Thermo Fisher Scientific, Waltham. MA, USA).
Quantitative analysis of fluorescence intensity
All samples were analyzed with a confocal laser-scanning microscope (LSM510 Meta; Carl Zeiss, Jena, Germany). The number of glial cells in 90 to 100 areas of 200 μm x 200 μm in four slices per brain was counted and the average number/4 x 104 μm2 was obtained for each brain.
Real-time PCR
Mice were deeply anaesthetized with pentobarbital and perfused transcardially with phosphate buffered saline, then the PFC and hippocampus were removed immediately. Total RNA was isolated from the samples using magnetic beads (MagExtractor system; Toyobo, Tokyo, Japan) after homogenizing the tissues. Primer pairs were chosen to flank at least one intron. The amount of total RNA was quantified by measuring the optical density 250 using a Nanodrop spectrophotometer (Nanodrop, Wilmington, DE, USA). For reverse transcription, 100 ng of total RNA was transferred to the reaction using an RNA PCR kit (AMV) (Takara Bio Inc., Ootsu, Japan) and 9-mer random primers. SYBR-Green real-time PCR (Applied Biosystems, Foster City, CA, USA) was performed on cDNA prepared from each sample using the THUNDERBIRD SYBR qPCR Mix, ROX reference dye (Toyobo) and 0.5 mM of each primer. The data were analyzed using 7500 System software v2.0 (Applied Biosystems). All values of cytokines were normalized to the corresponding β-actin concentration obtained using the same method. The sequences of primers were follows: IL-1β, sense; 5′- CTCCATGAGCTTTGTACAAGG -3′, antisense; 5′- TGCTGATGTACCAGTTGGGG -3′; TNF-α, sense; 5′- CCACCACGCTCTTCTGTCTAC -3′, antisense; 5′- TGGGCTACAGGCTTGTCACT -3′ β-actin, sense; 5′- TTGCTGACAGGATGCAGAAGGAG -3′, antisense; 5′- GTGGACAGTGAGGCCAGGAT -3′. The predicted sizes of the PCR products were 245 bp for IL-1β, 196 bp for TNF-α, and 127 bp for β-actin mRNA.
Measurement of the Pl levels in the PFC and hippocampus
Mice were deeply anesthetized with pentobarbital (50 mg/kg) and transcardially perfused with sterile PBS. For each animal, the brain was removed and the PFC and hippocampus were dissected in a dish filled with ice-cold PBS. The samples (300 mg to 500 mg) were stored at −80°C until Pl measurement. Total lipids were extracted using the method of Folch and coworkers [
23], and the relative composition of phospholipid classes, including Pls, was measured as previously reported [
22].
Statistical analysis
The results are expressed as the mean ± SEM. The body weights (BWs), numbers of Iba-1+ and GFAP+ cells and amounts of mRNAs were compared using one-way analysis of variance (ANOVA) followed by post hoc (Scheffe’s) test. Changes in the PlsEtn levels and the ratio of PlsEtn/Phosphatidyl Etn (PEtn) determined after LPS and Pl injection were evaluated using the non-parametric Kruskal-Wallis test followed by the Steel test for multiple comparisons. Values of P <0.05 were considered to be statistically significant.
Discussion
The present study demonstrated that systemic LPS-induced activation of glial cells, cytokine expression and accumulation of Aβ in the PFC and hippocampus were prevented by co-administration of purified Pls in adult mice. Furthermore, the injection of LPS induced decreases in the Pl levels in the PFC and hippocampus that were also suppressed by the administration of Pls.
Mechanisms of LPS-induced accumulation of Aβ
It is well known that the activation of microglia and astrocytes plays an important role in neuroinflammation induced by systemic LPS by enhancing the secretion of cytokines, prostanoids, ROS/RNS and related substances. In the present study, i.p. injection of LPS for seven days induced morphological activation and increased the number of glial cells in the PFC and hippocampus (Figures
2
4). The amounts of IL-1β and TNF-α mRNAs, which are considered to be derived primarily from glial cells, also increased following LPS injection (Figure
5). Furthermore, LPS injection resulted in the intracellular accumulation of Aβ proteins in both regions (Figures
6 and
7). It has been reported that i.p. injection of LPS induces deficits in spatial learning in mice [
24,
25] that may be due to the enhancement of Aβ generation in the hippocampus [
1].
It is known that β-secretase is involved in amyloidogenic processing of amyloid precursor proteins at the first step, while γ-secretase yields Aβ isoforms such as the more prevalent Aβ
40 and aggregation-prone Aβ
42 at the last step [
26]. It has been recently shown that the activities of β- and γ-secretase are increased in the cortex and hippocampus following systemic injection of LPS [
1]. It is possible that microglia play important roles in this phenomenon since proinflammatory cytokines, as well as ROS/RNS, released from activated microglia augment Aβ formation by upregulating β-secretase mRNA and enzymatic activity [
5,
27]. Microglia are activated further through receptors for advanced glycation end products (RAGE), which bind to Aβ and induce phagocytosis of Aβ, thereby amplifying the generation of ROS/RNS and cytokines [
26].
Changes in the Pl levels during neuroinflammation
In addition to observing the activation of glial cells and Aβ accumulation, we found that the Pl levels in the PFC and hippocampus decreased following LPS administration (Table
2). It is possible that the decreases in the amount of Pls during neuroinflammation are due to the anti-oxidant properties of Pls. It has been shown that the Pl-specific vinyl ether bond at the
sn-1 of the glycerol backbone is targeted by a variety of oxidants, including ROS/RNS [
10,
11,
13], and oxidative stress preferentially oxidizes PlsEtn over phosphatidyl ethanolamine (PEtn) [
28,
29], resulting in the disruption of vesicular fusion in the synaptosomes and the decrease in acetylcholine release [
30]. This may at least partly explain why AD patients show decreases in Pl levels in the brain [
14‐
16]. It has been suggested that abnormal membrane lipid compositions, namely decreases in the ratio of Pl to non-Pl ethanolamine glycerophospholipids, cause membrane instability in AD, which may contribute to amyloidogensis by cooperatively acting with amyloid cascade mechanisms [
14]. Furthermore, since PlsEtn are major endogenous lipid constituents that facilitate membrane fusion of synaptic vesicles associated with neurotransmitter release [
31,
32], pathological and/or age-related alterations in the Pl levels may be attributed to neurological disorders including AD [
7]. In accordance with this, it has been reported that decreases in the amount of Pls are closely correlated with the severity of dementia in humans [
18,
19].
Neuroinflammation-Aβ-Pls loop
There seems to be a causes/consequences loop involving neuroinflammation that includes cytokine and ROS/RNS production, Aβ accumulation and decreases in the amount of Pls. LPS-induced activation of β-secretase [
1], which is predominantly localized in cholesterol-rich lipid rafts [
33,
34], causes accumulation of Aβ proteins. Aβ-induced production of ROS/RNS that enhance lipid peroxidation [
35,
36] may decrease Pl levels, as mentioned above. In addition, increases in Aβ, cytokines and ROS/RNS reduce the expression of alkyl-dihydroxyacetone phosphate-synthase, a rate-limiting enzyme for Pl
de novo synthesis, by inducing the dysfunction of peroxisomes, where Pls are biosynthesized, resulting in decreases in the Pl levels [
37]. It has also been reported that TNF-α down regulates another key enzyme in Pl biosynthesis in peroxisomes, glycerol-3-phosphate-O-acyltransferase [
38], and up regulates myeloperoxidase, which generates one of the reactive species, hypochlorous acid (HOCl), in the brain, targeting Pls to be oxidized [
39]. Finally, Pls-PLA
2, which degrades Pls to release DHA or ARA from the
sn-2 position of the glycerol moiety, is possibly activated by ceramide produced under inflammatory conditions, and contributes to the loss of Pls in the brain [
9,
40].
It is well known that the generation and clearance of Aβ are affected by cholesterol metabolism, as evidenced by the identification of a variant gene of apolipoprotein E, a cholesterol transporter, as a major genetic risk factor for AD [
26,
41,
42]. It has been shown that decreases in the amount of Pls induce a decreased rate of intracellular cholesterol transport from cell membranes to the endoplasmic reticulum, which increases the cholesterol levels in cell membranes [
43]. Mankidy
et al. further indicated that esterification of cholesterol, an obligate step that occurs prior to efflux from cells, is dependent upon the amount of polyunsaturated fatty acid (PUFA)-containing PlsEtn present in the membrane with increasing levels of the membrane-bound cholesterol-processing enzyme, sterol-O-acyltransferase-1 [
44]. Increases in the cholesterol levels promote the secretion of Aβ [
41,
42,
45], while depletion of cholesterol inhibits the generation of Aβ [
46,
47]. Furthermore, it has been shown that membrane Pls block cholesterol-mediated increases in β-secretase activity and directly increase the activity of α-secretase, which is known to promote non-amyloidogenic processing of amyloid precursor proteins [
48]. Therefore, a vicious circle in which LPS-induced Aβ accumulation decreases the Pl levels, which leads to increased cholesterol levels, which further enhances the generation of Aβ may be involved in the pathological conditions of neuroinflammation.
Ameliorative effects of Pls on neuroinflammation
In the present study, we showed that LPS-induced activation of glial cells (Figures
2
4), expression of IL-1β and TNF-α mRNAs (Figure
5), accumulation of Aβ proteins (Figures
6 and
7) and decreases in the PlsEtn levels (Table
2) in the PFC and hippocampus are all prevented by co-administration of Pls. Although the precise mechanisms underlying the effects of Pls in this study are not known, supplementation with Pls could improve pathological disorders. The most important question may be whether peripheral Pls can enter into the brain. So far, there are no reports indicating that Pls directly cross the blood–brain barrier (BBB). Therefore, it is not excluded that the anti-oxidative effects of Pls are exerted outside the brain in order to suppress primary inflammation induced by peripheral LPS. However, it has been shown that the Pls levels in sera are decreased in parallel with or even at earlier times than decreases in the brain Pl levels in AD patients [
18,
19]. Furthermore, our results showed that LPS-induced decreases in the Pls levels in the PFC and hippocampus are corrected with peripheral administration of Pls (Table
2). Therefore, it is possible that peripheral supplementation with Pls would have effects on the CNS by changing the Pl levels in the brain.
Another question is whether the effective molecules in our experiment are PUFAs, not Pls, which Pls must carry at the
sn-2 position. Several lines of evidence show that
n-3 PUFAs, such as eicosapentaenoic acid, DHA, and its derivative, neuroprotectin D1, have anti-inflammatory and neuroprotective effects [
49‐
52]. Furthermore, DHA has been reported to suppress the production of Aβ proteins through multiple mechanisms, including inhibition of β-/γ-secretase activities and alteration of membrane cholesterol distribution [
53‐
55]. Since the purified Pls used in the present study contained DHA and its precursor, α-linolenic acid, especially in PlsEtn (Table
1), it cannot be excluded that DHA derived from PlsEtn plays a significant role in the CNS effects of Pls. Indeed, it has been shown that DHA is synthesized from α-linolenic acid and incorporated into phospholipids in the liver then transported to the brain through the peripheral circulation [
56]. On the other hand, it has also been shown that lyso-type phospholipids, which contain DHA at the
sn-2, show preferential transfer over DHA in
in vitro models of the BBB [
57]. Furthermore, it has been suggested that specific transport mechanisms to import Pls and their synthetic precursors exist in brain capillary epithelial cells [
58,
59]. These findings suggest that Pls containing DHA exert more effective actions in the CNS than DHA alone.
The present study suggests that co-administration of Pls suppresses systemic LPS-induced neuroinflammation in the brain. Although further studies on the mechanisms underlying these CNS effects, including the metabolism of the administered Pls and the pathways used to enter the brain, are needed, the present results indicate that Pls may possibly be used in new preventive and therapeutic strategies for treating AD.
Acknowledgements
We would like to thank Brian Quinn, Editor-in-Chief, Japan Medical Communication for linguistic advice on our manuscript. This work was supported by Grants-in-Aid for Scientific Research (22590225) to TK from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MI prepared the manuscript and performed the behavioral tests, immunohistochemistry and real-time PCR. TK designed the studies, performed the statistical analysis and wrote the manuscript. SM measured the levels of the brain Pls and analyzed the fatty acid composition of the Pls. MN assisted with manuscript preparation and discussed the data. KM and MS together prepared the purified Pls. TF designed the studies and reviewed and discussed the data. All authors read and approved the final manuscript.