Role of plasmalogens in the enhanced resistance of LDL to copper-induced oxidation after LDL apheresis

Marine Plasmalogens: A Gift from the Sea with Benefits for Age-Associated Diseases

Aging increases oxidative and inflammatory stress caused by a reduction in metabolism and clearance, thus leading to the development of age-associated diseases. The quality of our daily diet and exercise is important for the prevention of these diseases. Marine resources contain various valuable nutrients, and unique glycerophospholipid plasmalogens are found abundantly in some marine invertebrates, including ascidians. One of the major classes, the ethanolamine class (PlsEtn), exists in a high ratio to phospholipids in the brain and blood, while decreased levels have been reported in patients with age-associated diseases, including Alzheimer's disease. Animal studies have shown that the administration of marine PlsEtn prepared from marine invertebrates improved PlsEtn levels in the body and alleviated inflammation. Animal and human studies have reported that marine PlsEtn ameliorates cognitive impairment. In this review, we highlight the biological significance, relationships with age-associated diseases, food functions, and healthcare materials of plasmalogens based on recent knowledge and discuss the contribution of marine plasmalogens to health maintenance in aging.

Sterol-regulated transmembrane protein TMEM86a couples LXR signaling to regulation of lysoplasmalogens in macrophages

Lysoplasmalogens are a class of vinyl ether bioactive lipids that have a central role in plasmalogen metabolism and membrane fluidity. The liver X receptor (LXR) transcription factors are important determinants of cellular lipid homeostasis owing to their ability to regulate cholesterol and fatty acid metabolism. However, their role in governing the composition of lipid species such as lysoplasmalogens in cellular membranes is less well studied. Here, we mapped the lipidome of bone marrow-derived macrophages (BMDMs) following LXR activation. We found a marked reduction in the levels of lysoplasmalogen species in the absence of changes in the levels of plasmalogens themselves. Transcriptional profiling of LXR-activated macrophages identified the gene encoding transmembrane protein 86a (TMEM86a), an integral endoplasmic reticulum protein, as a previously uncharacterized sterol-regulated gene. We demonstrate that TMEM86a is a direct transcriptional target of LXR in macrophages and microglia and that it is highly expressed in TREM2+/lipid-associated macrophages in human atherosclerotic plaques, where its expression positively correlates with other LXR-regulated genes. We further show that both murine and human TMEM86a display active lysoplasmalogenase activity that can be abrogated by inactivating mutations in the predicted catalytic site. Consequently, we demonstrate that overexpression of Tmem86a in BMDM markedly reduces lysoplasmalogen abundance and membrane fluidity, while reciprocally, silencing of Tmem86a increases basal lysoplasmalogen levels and abrogates the LXR-dependent reduction of this lipid species. Collectively, our findings implicate TMEM86a as a sterol-regulated lysoplasmalogenase in macrophages that contributes to sterol-dependent membrane remodeling.

Regulation of plasmalogen metabolism and traffic in mammals: The fog begins to lift

Due to their unique chemical structure, plasmalogens do not only exhibit distinct biophysical and biochemical features, but require specialized pathways of biosynthesis and metabolization. Recently, major advances have been made in our understanding of these processes, for example by the attribution of the gene encoding the enzyme, which catalyzes the final desaturation step in plasmalogen biosynthesis, or by the identification of cytochrome C as plasmalogenase, which allows for the degradation of plasmalogens. Also, models have been presented that plausibly explain the maintenance of adequate cellular levels of plasmalogens. However, despite the progress, many aspects around the questions of how plasmalogen metabolism is regulated and how plasmalogens are distributed among organs and tissues in more complex organisms like mammals, remain unresolved. Here, we summarize and interpret current evidence on the regulation of the enzymes involved in plasmalogen biosynthesis and degradation as well as the turnover of plasmalogens. Finally, we focus on plasmalogen traffic across the mammalian body - a topic of major importance, when considering plasmalogen replacement therapies in human disorders, where deficiencies in these lipids have been reported. These involve not only inborn errors in plasmalogen metabolism, but also more common diseases including Alzheimer's disease and neurodevelopmental disorders.

Lymphatic Absorption of Microbial Plasmalogens in Rats

Plasmalogens, functional glycerophospholipids with biological roles in the human body, are associated with various diseases. Although a variety of saturated and/or unsaturated fatty acids in plasmalogens are presumed to have different functions in the human body, there are limited reports validating such functions of plasmalogens. In this study, we focused on the bacterial plasmalogen derived from Selenomonas ruminantium subsp. lactilytica (NBRC No. 103574) with different main species of hydrocarbon chains at the sn-1 position and shorter fatty acids at the sn-2 position than animal plasmalogens. Optimum culture conditions of S. ruminantium for high-yield production of plasmalogens, such as pH and the concentration of caproic acid, were investigated under anaerobic conditions using a 2-L scale jar fermenter. The obtained plasmalogen mainly consisted of the ethanolamine plasmalogen (PlsEtn). The molar ratios of PlsEtn species obtained from S. ruminantium, at sn-1/sn-2 positions, were p16:1/14:0 (68.4%), p16:1/16:1 (29.2%), p16:1/16:0 (0.7%), p16:1/15:0 (0.3%), and p17:1/14:0 (0.3%). Subsequently, duodenal infusion of the emulsion carrying the lipid extracted from S. ruminantium was carried out in lymph duct-cannulated rats. In the lymphatic plasmalogen of rats, the level of PlsEtns with molar ratios p16:1/14:0 and p16:1/16:1, the main species of plasmalogens from S. ruminantium, increased gradually until 3–4 h after lipid injection and then gradually decreased. In addition, the level of PlsEtns with p16:1/20:4 and p16:1/22:6 rapidly increased, peaking at 1–1.5 h and 1.5–2 h after lipid injection, respectively. The increase in the number of PlsEtns with p16:1/20:4 and p16:1/22:6 suggested that 20:4 and 22:6, the main fatty acids at the sn-2 position in the rat lymphatic plasmalogen, were preferentially re-esterified at the sn-2 position, regardless of the types of hydrocarbon chains at the sn-1 position. Thus, we showed that bacterial PlsEtns with “unnatural” structures against rats could be absorbed into the lymph. Our findings provide insights into the association between the chemical structure of plasmalogens and their biological functions in humans.

Serum Ethanolamine Plasmalogen and Urine Myo-Inositol as Cognitive Decline Markers

Recent studies have suggested that metabolic disorders, particularly type 2 diabetes mellitus (T2DM), and dementia, including Alzheimer's disease (AD), were linked at the clinical and molecular levels. Brain insulin deficiency and resistance may be key events in AD pathology mechanistically linking AD to T2DM. Ethanolamine plasmalogens (PlsEtns) are abundant in the brain and play essential roles in neuronal function and myelin formation. As such, PlsEtn deficiency may be pathologically relevant in some neurodegenerative disorders such as AD. Decreased brain PlsEtn associated with dementia may reflect serum PlsEtn deficiency. We hypothesized that myo-inositol plays a role in myelin formation through its facilitation of PlsEtn biosynthesis. Excessive urinary myo-inositol (UMI) loss would likely result in PlsEtn deficiency potentially leading to demyelinating diseases such as dementia. Accordingly, measurement of both serum PlsEtn and baseline UMI excretion could improve the detection of cognitive impairment (CI) in a more specific and reliable manner. To verify our hypothesis, we conducted a clinical observational study of memory clinic outpatients (MCO) and cognitively normal elderly (NE) for nearly 4.5years. We demonstrated that serum PlsEtn concentration associated with UMI excretion was useful for predicting advancing dementia in patients with mild CI. Because hyperglycemia and associated insulin resistance might be a leading cause of increased baseline UMI excretion, serum PlsEtn quantitation would be useful in detecting CI among the elderly with hyperglycemia. Our findings suggest that myo-inositol is a novel candidate molecule linking T2DM to AD.

Baseline serum phosphatidylcholine plasmalogen concentrations are inversely associated with incident myocardial infarction in patients with mixed peripheral artery disease presentations

BACKGROUND AND AIMS

Despite current best care, patients with peripheral artery disease (PAD) remain at high risk of myocardial infarction, and biomarkers to more accurately assess cardiovascular risk are needed. This study assessed the relationship between the serum lipidome and incident myocardial infarction in a cohort of PAD patients.

METHODS

265 PAD patients were followed up for a median of 23 months, during which 18 people suffered a myocardial infarction. Fasting serum concentrations of 332 lipid species were measured via mass spectrometry and their association with incident myocardial infarction was assessed via Cox regression. Secondary analyses investigated prognostic potential of specific lipid species.

RESULTS

Total serum concentrations of alkyl-phosphatidylcholine and alkenylphospatidylcholine (plasmalogen) lipids were inversely associated with incident myocardial infarction after adjusting for multiple testing (hazards ratio (95% confidence intervals): 0.43 (0.24-0.74); p = 0.032; and 0.28 (0.14-0.56), p = 0.010, respectively). Specifically, 10 alkenylphosphatidylcholine species and 6 alkyl-phosphatidylcholine species were negatively associated with incident myocardial infarction after adjusting for traditional risk factors and correcting for multiple testing (hazards ratios ranging from 0.07 to 0.51, p < 0.05). Incorporation of serum phosphatidylcholine plasmalogen species PC(P-40:6) concentration within analyses designed to determine subsequent myocardial infarction incidence led to an improvement in predictive accuracy compared to traditional risk factors alone.

CONCLUSIONS

Serum concentrations of phosphatidylcholine plasmalogens and alkyl-phosphatidylcholines were negatively associated with incident myocardial infarction and have potential to act as novel prognostic markers in at-risk populations.

Statin action enriches HDL3 in polyunsaturated phospholipids and plasmalogens and reduces LDL-derived phospholipid hydroperoxides in atherogenic mixed dyslipidemia

Atherogenic mixed dyslipidemia associates with oxidative stress and defective HDL antioxidative function in metabolic syndrome (MetS). The impact of statin treatment on the capacity of HDL to inactivate LDL-derived, redox-active phospholipid hydroperoxides (PCOOHs) in MetS is indeterminate. Insulin-resistant, hypertriglyceridemic, hypertensive, obese males were treated with pitavastatin (4 mg/day) for 180 days, resulting in marked reduction in plasma TGs (-41%) and LDL-cholesterol (-38%), with minor effects on HDL-cholesterol and apoAI. Native plasma LDL (baseline vs. 180 days) was oxidized by aqueous free radicals under mild conditions in vitro either alone or in the presence of the corresponding pre- or poststatin HDL2 or HDL3 at authentic plasma mass ratios. Lipidomic analyses revealed that statin treatment i) reduced the content of oxidizable polyunsaturated phosphatidylcholine (PUPC) species containing DHA and linoleic acid in LDL; ii) preferentially increased the content of PUPC species containing arachidonic acid (AA) in small, dense HDL3; iii) induced significant elevation in the content of phosphatidylcholine and phosphatidylethanolamine (PE) plasmalogens containing AA and DHA in HDL3; and iv) induced formation of HDL3 particles with increased capacity to inactivate PCOOH with formation of redox-inactive phospholipid hydroxide. Statin action attenuated LDL oxidability Concomitantly, the capacity of HDL3 to inactivate redox-active PCOOH was enhanced relative to HDL2, consistent with preferential enrichment of PE plasmalogens and PUPC in HDL3.

Plasma/Serum Plasmalogens: Methods of Analysis and Clinical Significance

Age-related diseases, such as atherosclerosis and dementia, are associated with oxidative stress and chronic inflammation. Peroxisome dysfunction may be related to aging and age-related pathologies, possibly through the derangement of redox homeostasis. The biosyntheses of plasmalogens (Pls), a subclass of glycerophospholipids, are primarily regulated by peroxisomes. Thus, plasma Pls may reflect the systemic functional activity of peroxisomes and serve as potential biomarkers for diseases related to oxidative stress and aging. Recently, we have established three promising analytical methods for plasma/serum Pls using high-performance liquid chromatography with radioactive iodine, liquid chromatography-tandem mass spectrometry, and enzymatic assay. These methods were validated and used to obtain detailed molecular information regarding these molecules. In cross-sectional studies on asymptomatic, coronary artery disease, and elderly dementia individuals, we found that serum choline Pls, particularly those containing oleic and linoleic acid in the sn-2 position of the glycerol backbone, may serve as reliable antiatherogenic biomarkers. Furthermore, we also found that serum ethanolamine Pls were effective in discriminating cognitive impairment. These results support our hypothesis and further studies are clearly needed to elucidate Pls pathophysiologic significance.

Proportion of nervonic acid in serum lipids is associated with serum plasmalogen levels and metabolic syndrome

An increase in serum plasmalogens (1-O-alk-1-enyl-2-acyl glycerophospholipids), which are endogenous anti-oxidative phospholipids, can potentially prevent age-related diseases such as atherosclerosis and metabolic syndrome (MetS). Very long chain fatty acids (VLCFAs) in plasma may supply the materials for plasmalogen biosynthesis through peroxisomal beta-oxidation. On the other hand, elevated levels of saturated and monounsaturated VLCFAs in plasma appear to be associated with decreased peroxisomal function, and are a symptom of age-related diseases. To reconcile these contradictory findings, we attempted to investigate the relationship between the serum levels of saturated and monounsaturated VLCFAs, clinical and biochemical parameters, and serum levels of plasmalogens in subjects with MetS (n = 117), who were asymptomatic Japanese males over 40 years of age. Fatty acids in serum lipids were quantified using gas chromatography/mass spectrometry (GC/MS). Serum plasmalogen levels were determined by liquid chromatography using radioactive iodine (¹²⁵I-HPLC), and the molecular composition of serum plasmalogens was analyzed by liquid chromatography-tandem mass spectrometry (LC/MS/MS). We found that MetS subjects showed a significant reduction in the proportion of specific saturated and monounsaturated VLCFAs such as behenic acid (C22:0), lignoceric acid (C24:0), and nervonic acid (C24:1) in serum lipids compared to non-MetS subjects. These VLCFAs were positively associated with serum levels of high density lipoprotein cholesterol (HDL-C) as well as plasmalogen-related parameters, and inversely with serum levels of triglyceride (TG) and small dense low density lipoprotein cholesterol (sdLDL-C). In conclusion, the proportion of nervonic acid in serum lipids is associated with serum levels of plasmalogens and with MetS, and probably reflects the peroxisomal dysfunction and enhancement of endoplasmic reticulum (ER) stress seen in common age-related diseases.

Serum choline plasmalogens, particularly those with oleic acid in sn-2, are associated with proatherogenic state

Serum plasmalogens (Pls) (1-O-alk-1'-enyl-2-acyl glycerophospholipids) are of particular interest for studies on metabolic disorders associated with oxidative stress and chronic inflammation. Serum levels of Pls are known to correlate positively with HDL-cholesterol (HDL-C); however, few studies have examined serum Pls molecular species in association with pathophysiological conditions and their clinical significance. To clarify these, we determined serum levels of individual ether glycerophospholipids in Japanese asymptomatic cohorts (n = 428; 362 male and 66 female subjects) by LC/MS/MS, and examined their correlations with clinical parameters. We found that the proportion of choline Pls (PlsCho) among total serum phospholipids was significantly lower in the male group over 40 years old and was associated with multiple risk parameters more strongly than HDL-C. The abundance of serum PlsCho with oleic acid (18:1) in sn-2 exhibited the strongest positive correlation with serum concentrations of adiponectin and HDL-C, while being inversely associated with waist circumference and the serum levels of TG and small dense LDL-cholesterol. The characterization of serum ether glycerophospholipids verified the specificity of PlsCho, particularly the ones with 18:1 in sn-2, as a sensitive biomarker for the atherogenic state.

Identification of plasmalogen in the gut of silkworm (Bombyx mori)

Herbivorous insect species are constantly challenged with endogenous and exogenous oxidative stress. Consequently, they possess an array of antioxidant enzymes and small molecular weight antioxidants. Lipid-soluble small molecular antioxidants, such as tocopherols, have not been well studied in insects but may play important antioxidant roles. In this study, we identified plasmalogen phosphatidylethanolamines (pPEs) as well as α-, β/γ-, δ-tocopherol in the larvae of the silkworm Bombyx mori by LCMS analyses and examined their distribution. Plasmalogen are reported to inhibit the metal ion induced oxidation. The composition of tocopherols was the same among gut contents, gut tissues, and the other tissues. However, plasmalogens, a unique class of glycerophospholipids rich in polyunsaturated fatty acids and containing a vinyl ether bond at the sn-1 position, were mainly distributed in gut tissues. Plasmalogens might protect gut tissues from oxidation stress.

Lipidomic analysis of variation in response to simvastatin in the Cholesterol and Pharmacogenetics Study

Statins are commonly used for reducing cardiovascular disease risk but therapeutic benefit and reductions in levels of low-density lipoprotein cholesterol (LDL-C) vary among individuals. Other effects, including reductions in C-reactive protein (CRP), also contribute to treatment response. Metabolomics provides powerful tools to map pathways implicated in variation in response to statin treatment. This could lead to mechanistic hypotheses that provide insight into the underlying basis for individual variation in drug response. Using a targeted lipidomics platform, we defined lipid changes in blood samples from the upper and lower tails of the LDL-C response distribution in the Cholesterol and Pharmacogenetics study. Metabolic changes in responders are more comprehensive than those seen in non-responders. Baseline cholesterol ester and phospholipid metabolites correlated with LDL-C response to treatment. CRP response to therapy correlated with baseline plasmalogens, lipids involved in inflammation. There was no overlap of lipids whose changes correlated with LDL-C or CRP responses to simvastatin suggesting that distinct metabolic pathways govern statin effects on these two biomarkers. Metabolic signatures could provide insights about variability in response and mechanisms of action of statins. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-010-0207-x) contains supplementary material, which is available to authorized users.

Alterations in lipoprotein defense against oxidative stress in metabolic syndrome

Metabolic syndrome (MetS) is a high-risk condition for premature atherosclerotic vascular disease. Patients with MetS display a lipoprotein profile in which dense low-density lipoproteins (LDL), which are more susceptible to oxidation, predominate. Oxidation of lipoproteins can be attenuated in vivo by enzymatic and nonenzymatic antioxidant defenses, but high-density lipoproteins (HDL) play a key role in the protection of LDL from oxidation. Such activity depends on the presence of apolipoproteins (apoA-I, apoA-II, apoA-IV, apoE) and enzymes (paraoxonase 1, platelet activating factor-acetylhydrolase, lecithin:cholesterol acyltransferase, glutathione peroxidase). The impairment of HDL antioxidative activity in MetS is partly related to an enrichment of small HDL in triglycerides and their depletion in cholesteryl esters, to the replacement of apoA-I by serum amyloid A, and to glycation and oxidation of apoA-I. Therapeutic normalization of the quantity and the quality of HDL particles may constitute a novel approach to attenuate atherosclerosis and cardiovascular risk in MetS.

Functions and biosynthesis of plasmalogens in health and disease

Plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) constitute a special class of phospholipids characterized by the presence of a vinyl-ether bond at the sn-1 position. Although long considered as biological peculiarities, interest in this group of phospholipids has grown in recent years, thanks to the realization that plasmalogens are involved in different human diseases. In this review, we summarize the current state of knowledge with respect to the enzymatic synthesis of plasmalogens, the characteristic topology of the enzymes involved and the biological roles that have been assigned to plasmalogens.

Plasmalogen levels in serum from patients with impaired carbohydrate or lipid metabolism and in elderly subjects with normal metabolic values

The precise role played by plasmalogen phospholipids (PL) of lipoproteins and cell membranes is not well understood. However, they might act as endogenous antioxidants in defending cell membranes and lipoproteins from reactive oxygen species. A decline of plasmalogen concentrations has been observed in some tissues in normal aging and in some pathologic conditions. For healthy adults, we had reported negative correlations of age with serum plasmalogen PL derived heaxadecanal dimethylacetal (16:0DMA) or octadecanal dimethylacetal (18:0DMA) values. To mark off these age associated changes from disturbances in glucose or lipid metabolism, this study compares the 16:0DMA and 18:0DMA data of serum PL from 118 elderly subjects, aged 57-94 years, and grouped according to the disturbance of glucose or lipid metabolism. Using a new synthetic test mixture of 16:0DMA with oleic acid butylester as a quality control in gas chromatography, the highest 16:0DMA values were found in hypercholesterolemic subjects. However, related to the bulk of serum PL, were the plasmalogens possibly act as antioxidants, the highest values of 16:0DMA/PL were found in controls. A negative correlation of serum triglycerides (TG) with 16:0DMA was detected (n=118). The data suggest a closer association of low DMA values with elevated TG levels as compared to elevated plasma glucose or other serum lipid levels.

Lipid oxidation enhances the function of activated protein C

Although lipid oxidation products are usually associated with tissue injury, it is now recognized that they can also contribute to cell activation and elicit anti-inflammatory lipid mediators. In this study, we report that membrane phospholipid oxidation can modulate the hemostatic balance. Oxidation of natural phospholipids results in an increased ability of the membrane surface to support the function of the natural anticoagulant, activated protein C (APC), without significantly altering the ability to support thrombin generation. Lipid oxidation also potentiated the ability of protein S to enhance APC-mediated factor Va inactivation. Phosphatidylethanolamine, phosphatidylserine, and polyunsaturation of the fatty acids were all required for the oxidation-dependent enhancement of APC function. A subgroup of thrombotic patients with anti-phospholipid antibodies specifically blocked the oxidation-dependent enhancement of APC function. Since leukocytes are recruited and activated at the thrombus or sites of vessel injury, our findings suggest that after the initial thrombus formation, lipid oxidation can remodel the membrane surface resulting in increased anticoagulant function, thereby reducing the thrombogenicity of the thrombus or injured vessel surface. Anti-phospholipid antibodies that block this process would therefore be expected to contribute to thrombus growth and disease.