Plasmalogens and platelet-activating factor roles in chronic inflammatory diseases.

Fatty acids and phospholipid molecules are essential for determining the structure and function of cell membranes, and they hence participate in many biological processes. Platelet activating factor (PAF) and its precursor plasmalogen, which represent two subclasses of ether phospholipids, have attracted increasing research attention recently due to their association with multiple chronic inflammatory, neurodegenerative, and metabolic disorders. These pathophysiological conditions commonly involve inflammatory processes linked to an excess presence of PAF and/or decreased levels of plasmalogens. However, the molecular mechanisms underlying the roles of plasmalogens in inflammation have remained largely elusive. While anti-inflammatory responses most likely involve the plasmalogen signal pathway; pro-inflammatory responses recruit arachidonic acid, a precursor of pro-inflammatory lipid mediators which is released from membrane phospholipids, notably derived from the hydrolysis of plasmalogens. Plasmalogens per se are vital membrane phospholipids in humans. Changes in their homeostatic levels may alter cell membrane properties, thus affecting key signaling pathways that mediate inflammatory cascades and immune responses. The plasmalogen analogs of PAF are also potentially important, considering that anti-PAF activity has strong anti-inflammatory effects. Plasmalogen replacement therapy was further identified as a promising anti-inflammatory strategy allowing for the relief of pathological hallmarks in patients affected by chronic diseases with an inflammatory component. The aim of this Short Review is to highlight the emerging roles and implications of plasmalogens in chronic inflammatory disorders, along with the promising outcomes of plasmalogen replacement therapy for the treatment of various PAF-related chronic inflammatory pathologies.

Plasmalogenic Lipid Analogs as Platelet-Activating Factor Antagonists: A Potential Novel Class of Anti-inflammatory Compounds.

Plasmalogens and Platelet-Activating Factor (PAF) are both bioactive ether phospholipids. Whereas plasmalogens are recognized for their important antioxidant function and modulatory role in cell membrane structure and dynamics, PAF is a potent pro-inflammatory lipid mediator known to have messenger functions in cell signaling and inflammatory response. The relationship between these two types of lipids has been rarely studied in terms of their metabolic interconversion and reciprocal modulation of the pro-inflammation/anti-inflammation balance. The vinyl-ether bonded plasmalogen lipid can be the lipid sources for the precursor of the biosynthesis of ether-bonded PAF. In this opinion paper, we suggest a potential role of plasmalogenic analogs of PAF as modulators and PAF antagonists (anti-PAF). We discuss that the metabolic interconversion of these two lipid kinds may be explored towards the development of efficient preventive and relief strategies against PAF-mediated pro-inflammation. We propose that plasmalogen analogs, acting as anti-PAF, may be considered as a new class of bioactive anti-inflammatory drugs. Despite of the scarcity of available experimental data, the competition between PAF and its natural plasmalogenic analogs for binding to the PAF receptor (PAF-R) can be proposed as a mechanistic model and potential therapeutic perspective against multiple inflammatory diseases (e.g., cardiovascular and neurodegenerative disorders, diabetes, cancers, and various manifestations in coronavirus infections such as COVID-19).

Potential Role of Plasmalogens in the Modulation of Biomembrane Morphology.

Plasmalogens are a subclass of cell membrane glycerophospholipids that typically include vinyl- ether bond at the sn-1 position and polyunsaturated fatty acid at the sn-2 position. They are highly abundant in the neuronal, immune, and cardiovascular cell membranes. Despite the abundance of plasmalogens in a plethora of cells, tissues, and organs, the role of plasmalogens remains unclear. Plasmalogens are required for the proper function of integral membrane proteins, lipid rafts, cell signaling, and differentiation. More importantly, plasmalogens play a crucial role in the cell as an endogenous antioxidant that protects the cell membrane components such as phospholipids, unsaturated fatty acids, and lipoproteins from oxidative stress. The incorporation of vinyl-ether linked with alkyl chains in phospholipids alter the physicochemical properties (e.g., the hydrophilicity of the headgroup), packing density, and conformational order of the phospholipids within the biomembranes. Thus, plasmalogens play a significant role in determining the physical and chemical properties of the biomembrane such as its fluidity, thickness, and lateral pressure of the biomembrane. Insights on the important structural and functional properties of plasmalogens may help us to understand the molecular mechanism of membrane transformation, vesicle formation, and vesicular fusion, especially at the synaptic vesicles where plasmalogens are rich and essential for neuronal function. Although many aspects of plasmalogen phospholipid involvement in membrane transformation identified through in vitro experiments and membrane mimic systems, remain to be confirmed in vivo, the compiled data show many intriguing properties of vinyl-ether bonded lipids that may play a significant role in the structural and morphological changes of the biomembranes. In this review, we present the current limited knowledge of the emerging potential role of plasmalogens as a modulator of the biomembrane morphology.

Plasmalogen-rich foods promote the formation of cubic membranes in amoeba Chaos under stress conditions.

Previous studies have indicated that the ability to form cubic membrane (CM), a three-dimensional periodic structure with cubic symmetry, in amoeba (Chaos carolinense) under stress conditions depends on the type of food organism supplied before cell starvation. The significant increase in docosapentaenoic acid (DPA; C22:5n-6) during the starvation period has been reported to induce CM formation and support Chaos cell survival. In this article, we further investigated the lipid profiles of food organisms of the Chaos cells to reveal the key lipid components that might promote CM formation. Our results show that the lipids extracted from cells of the native food organism Paramecium multimicronucleatum are enriched in plasmalogens. More specifically, plasmalogen phosphatidylcholine and plasmalogen phosphatidylethanolamine might be the key lipids that trigger CM formation in Chaos cells under starvation stress conditions. Unexpectedly, CM formation in these cells is not supported when the native food organism was replaced with plasmalogen-deficit Tetrahymena pyriformis cells. Based on a previous lipidomics study on amoeba Chaos and this study on the lipid composition of its food organisms, three key lipids (plasmalogen phosphatidylcholine, plasmalogen phosphatidylethanolamine and diacyl-phosphatidylinositol) were identified and used for liposomal construction. Our in vitro study revealed the potential role of these lipids in a nonlamellar phase transition. The negative staining transmission electron microscopy data of our liposomal constructs support the notion that plasmalogens may curve the membrane, which, in turn, may facilitate membrane fusion and vesicular formation, which is crucial for membrane dynamics and trafficking.

Computational evaluation of an extra-aortic elastic-wrap applied to simulated aging anisotropic human aorta models.

Structural changes occurring to the aortic wall can result in vascular stiffening. This is represented by a loss of vascular compliance during pulsatile flow, resulting in increased systolic and pulse blood pressure, particularly in populations aged 50 and over. Aortic stiffness is thought to be permanent and an active de-stiffening strategy is yet to be developed. Extra aortic elastic wrapping has been proposed as a surgical technique to boost aortic distensibility and treat hypertension in the elderly. Previously, in-vivo and in-vitro testing have suggested a pulse-pressure reduction potential of elastic wrapping in the stiffened aortas. Herein, we explore the feasibility of elastic aortic wrapping to improve simulated aortic compliance across the age span. Detailed computational studies of the anisotropic aortic wall mechanics, using data from human subjects, were performed, evaluating key performance properties for the interaction between the aortic wall and elastic aortic wrap procedure. Main determinants of the procedure's efficiency are identified using a pre-defined aortic stiffness and wrap elasticity. Finite element analysis predicts that segmental aortic distensibility can be increased if elastic wrapping is applied to a simulated stiff aorta. Elastic aortic wrapping is calculated to have little impact on the compliance of an initially distensible aorta.

Over-Wrapping of the Aortic Wall with an Elastic Extra-Aortic Wrap Results in Luminal Creasing.

Elastic extra-aortic wrapping is a potential non-pharmacological way to improve aortic compliance and treat isolated systolic hypertension associated with a stiffened aorta. We aimed to use computer simulations to re-evaluate whether there is aortic shape distortion in aortic wrapping to achieve greater elasticity of the wrapped aortic segment. Non-linear transient numerical analysis based on an idealized hyper-elastic single-layered aorta model was performed to simulate the force/displacement regimes of external aortic wrapping. Pressure-displacement relationships were used to establish model aortic wall distensibilities of 4.3 and 5.5 (10-3 mmHg-1). A physiological pulsatile lumen pressure was employed to estimate the potential improvements in aortic distensibility by compression forces representing elastic aortic wrapping. In the less distensible model of the aortic wall there was increased systolic expansion in the wrapped segment. We found a risk of creasing of the aortic luminal wall with wrapping. Sufficient unloading of a thick and elastic aortic wall to induce increased compliance, as observed in elastic wrapping, is associated with the potential risk of over compression and folding (creasing) inside the lumen.

Cubic membrane formation supports cell survival of amoeba Chaos under starvation-induced stress.

Cubic membranes (CM) are highly organized membrane structures found in biological systems. They are mathematically well defined and reveal a three-dimensional nano-periodic structure with cubic symmetry. These membrane arrangements are frequently induced in cells under stress, disease conditions, or upon viral infection. In this study, we investigated CM formation in the mitochondria of amoeba Chaos carolinense and observed a striking correlation between the organism's ability to generate CM and the cell survival under starvation. Since starvation also induces autophagy, rapamycin was used to pharmacologically induce autophagy, and interestingly, CM formation was observed in parallel. Conversely, inhibition of autophagy reverted the cubic mitochondrial inner membrane morphology to tubular structure. In starved Chaos cells, mitochondria and autophagosomes did not co-localize and ATP production was sustained. CM transition in the mitochondria during starvation or upon induction of autophagy might prevent their sequestration by autophagosomes, thus slowing their rate of degradation. Such sustained mitochondrial activity may allow amoeba Chaos cells to survive for a longer period upon starvation.

Evaluation of radical scavenging system in amoeba Chaos carolinense during nutrient deprivation.

The frequent appearance of non-lamellar membrane arrangements such as cubic membranes (CMs) in cells under stressed or pathological conditions points to an intrinsic cellular response mechanism. CM represents highly curved, three-dimensional nano-periodic structures that correspond to mathematically well-defined triply periodic minimal surfaces. Specifically, cellular membrane may transform into CM organization in response to pathological, inflammatory and oxidative stress conditions. CM organization, thus, may provide an advantage to cope with various types of stress. The identification of inducible membrane systems, such as in the mitochondrial inner membranes to cubic morphology upon starvation, opens new avenues for understanding the molecular mechanisms of cellular responses to oxidative stress. In this study, we compared the cellular responses of starved and fed amoeba Chaos carolinense to oxidative stress. Food deprivation from C. carolinense induces a significant increase in prooxidants such as superoxide and hydrogen peroxide. Surprisingly, we observed a significant lower rate of biomolecular damage in starved cells (with higher free radicals generation) when compared with fed cells. Specifically, lipid and RNA damages were significantly less in starved cells compared with fed cells. This observation was not due to the upregulation of intracellular antioxidants, as starved amoeba show reduced antioxidant enzymatic activities; however, it could be attributed to CM formation. CM could uptake and retain short segments of nucleic acids (resembles cellular RNA) in vivo and in vitro. Previous results showed that nucleic acids retained within CM sustain a minimal oxidative damage in vitro upon exposure to high level of superoxide. We thus propose that CM may act as a 'protective' shelter to minimize the oxidation of biologically essential macromolecules such as RNA. In summary, we examined enzymatic antioxidant activities as well as oxidative damage biomarkers in starved amoeba C. carolinense in correlation with the potential role of CM as an optimal intracellular membrane organization for the protection of biological macromolecules against oxidative damage.

Evolution of cubic membranes as antioxidant defence system.

Possibly the best-characterized cubic membrane transition has been observed in the mitochondrial inner membranes of free-living giant amoeba (Chaos carolinense). In this ancient organism, the cells are able to survive in extreme environments such as lack of food, thermal and osmolarity fluctuations and high levels of reactive oxygen species. Their mitochondrial inner membranes undergo rapid changes in three-dimensional organization upon food depletion, providing a valuable model to study this subcellular adaptation. Our data show that cubic membrane is enriched with unique ether phospholipids, plasmalogens carrying very long-chain polyunsaturated fatty acids. Here, we propose that these phospholipids may not only facilitate cubic membrane formation but may also provide a protective shelter to RNA. The potential interaction of cubic membrane with RNA may reduce the amount of RNA oxidation and promote more efficient protein translation. Thus, recognizing the role of cubic membranes in RNA antioxidant systems might help us to understand the adaptive mechanisms that have evolved over time in eukaryotes.

Isolation of mitochondria with cubic membrane morphology reveals specific ionic requirements for the preservation of membrane structure.

Biological membranes with cubic symmetry are a hallmark of virus-infected or diseased cells. The mechanisms of formation and specific cellular functions of cubic membranes, however, are unclear. The best-documented cubic membrane formation occurs in the free-living giant amoeba Chaos carolinense. In that system, mitochondrial inner membranes undergo a reversible structural change from tubular to cubic membrane organization upon starvation of the organism. As a prerequisite to further analyze the structural and functional features of cubic membranes, we adapted protocols for the isolation of mitochondria from starved amoeba and have identified buffer conditions that preserve cubic membrane morphology in vitro. The requirement for high concentration of ion-chelating agents in the isolation media supports the importance of a balanced ion milieu in establishing and maintaining cubic membranes in vivo.

Intracellular tubulo-reticular structures of peripheral blood mononuclear cells as an ultra-structural marker of disease activity in systemic lupus erythematosus: a pilot study.

OBJECTIVE: Under physiological stress, the membranes of organelles undergo conformational change to tubulo-reticular structures (TRS) for gaining survival advantage. We aim to explore whether TRS formation in the peripheral blood mononuclear cells (PBMCs) of patients with systemic lupus erythematosus (SLE) correlates with more active disease where physiological stress prevails. METHODS: To mimic the intracellular impact of interferon-alpha (IFN-α) on lymphocytes, human B-lymphocyte cell line was stimulated by recombinant IFN-α in concentrations of 100, 1000 and 10 000 IU/mL for 72 h. TRS within the lymphocytes was then quantified by transmission electron microscopy (TEM). Upon confirming TRS formation under IFN-α influence, PBMCs of lupus patients were isolated, fixed and quantified for TRS under TEM. The frequency of TRS in lupus PBMCs was compared with that of healthy controls and correlated with the clinical SLE disease activity index (SLEDAI). RESULTS: After 72 h, an increase in TRS frequency was observed in a dose-response fashion when the human B-lymphocyte cell line was stimulated by increasing concentrations of IFN-α. In lupus patients, their PBMCs had a significantly higher TRS frequency than healthy controls (P = 0.037). The frequency of TRS was positively associated with the SLEDAI (Spearman ρ = 0.632, P = 0.012), which remained statistically significant after adjustment for daily prednisolone dose (Pearson r = 0.747, P = 0.002). CONCLUSIONS: While the clinical significance of TRS formation in lupus PBMCs deserves further investigation, these preliminary findings suggest a significant relationship between the disease severity of SLE and intracellular physiological stress. These results underscore the potential of TRS in PBMCs as an ultra-structural disease activity biomarker of SLE.

Acute starvation in C57BL/6J mice increases myocardial UCP2 and UCP3 protein expression levels and decreases mitochondrial bio-energetic function.

Associations between uncoupling protein (UCP) expression and functional changes in myocardial mitochondrial bio-energetics have not been well studied during periods of starvation stress. Our aim was to study the effects of acute starvation, for 24 or 48 h, on combined cardiac mitochondrial function and UCP expression in mice. Isolated heart mitochondria from female mice starved for 48 h compared to that from mice fed revealed a significantly (p < 0.05) decreased adenosine diphosphate-to-oxygen ratio, a significantly increased proton leak and an increased GTP inhibition on palmitic acid-induced state 4 oxygen consumption (p < 0.05). These bio-energetic functional changes were associated with increases in mitochondrial UCP2 and UCP3 protein expression. In conclusion, our findings suggest that increased UCP2 and UCP3 levels may contribute to decreased myocardial mitochondrial bio-energetic function due to starvation.

Do viruses subvert cholesterol homeostasis to induce host cubic membranes?

Biological membranes with cubic morphology are a hallmark of stressed or diseased cellular conditions; both protein-protein interactions and lipid alterations appear to contribute to their biogenesis, yet their specific cellular functions are unknown. The occurrence of cubic membranes strikingly correlates with viral infections; notably, virus entry, proliferation, and release are processes closely linked to cellular cholesterol metabolism, and dys-regulation of cholesterol synthesis at the level of HMG-CoA reductase also induces cubic membrane formation, in the absence of viral infection. We propose that virus-induced cubic membranes could result from viral interference of cellular cholesterol homeostasis, generating a protective membrane environment to facilitate virus assembly and proliferation. Preventing cubic membrane formation might thus disrupt the 'virus factory' and offer new avenues to combat viral infections.

The Cubic "Faces" of Biomembranes.

Biomembranes are traditionally viewed as flat phospholipid-bilayer sheets delineating the cell boundaries and dividing the cell into multiple subcellular organelles with specialized functions. However, biological membranes may also fold up into three-dimensional nanoperiodic arrangements, termed cubic membranes. This type of geometry is mathematically well described and extensively studied in lipidic cubic phase systems. This chapter will (1) summarize similarities and dissimilarities between cubic membranes and cubic phases; (2) provide an update on the experimental data describing the role of lipids, proteins and electrostatic charges on the biogenesis of cubic membranes; and (3) discuss their potential function in intracellular macromolecular transport and as optical filters, as well as potential practical applications such as gene delivery vehicles.

Docosapentaenoic acid (DPA) is a critical determinant of cubic membrane formation in amoeba Chaos mitochondria.

Very long-chain polyunsaturated fatty acids (VLC-PUFAs), such as docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA), have recently made it to the realm of "magical molecules" based on their multiple presumably beneficial effects in biological systems, making these PUFAs particularly interesting in biomedicine. Their specific biological functions, however, remain enigmatic. Here we provide evidence derived from studies in the amoeba Chaos that indicates a structural role for omega-6 DPA in cell membrane organization, which may help to explain the multiple diverse effects of VLC-PUFA in healthy and diseased states. Amoeba Chaos mitochondria undergo a remarkable and reversible morphological transition into cubic morphology on starvation. This morphological transition is reflected in major changes in fatty acid and lipid composition, as determined by gas liquid chromatography and mass spectrometry, in particular by a drastic increase in C22:5 modified phosphatidylcholine plasmalogen, phosphatidylethanolamine plasmalogen, and phosphatidylinositol species. Liposomes produced in vitro from lipids of starved amoeba cells show a high propensity to form hexagonal tubular and cubic morphologies. Addition of omega-6 DPA, but not of omega-3 DPA, to the cell culture also induced mitochondrial membrane transformation into cubic morphology in fed cells, demonstrating for the first time an important structural role of omega-6 DPA-containing lipids in cell membrane organization.

Chapter 6: cubic membranes the missing dimension of cell membrane organization.

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between "inside" and "outside", survival, and death. Intracellular compartmentalization-governed by biomembranes as well-is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into "unusual", highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Down regulation of immuno-detectable cardiac connexin-43 in BALB/c mice following acute fasting.

Acute starvation effects for connexin-43 protein expression, in the heart, had not been previously explored. Hence we examined acute fasting on the myocardial immuno-histochemical expression of connexin-43 in 3 groups of 8-week old female BALB/c mice. Groups consisted of control mice (n=5), fasting for 24 h (N=5) and 48 h (N=3). Under light microscopy all control fed cases revealed the presence of some immuno-detectable staining for connexin-43 that is either present or weakly observed in some or all of the regions of interest, that include the cross-sectional left ventricular sub-endocardium, mid-myocardium and papillary muscle. Whereas mice that underwent 24 or 48 h of acute starvation, connexin-43 expression was either difficult to detect visually (N=3) or was completely absent (N=5) at 40x magnification using a light microscope. In starved mice with no membrane staining for connexin-43 we observed an increase in the intracellular accumulation of cytoplasmic connexin-43 expression.

High dose clopidogrel decreases mice liver mitochondrial respiration function in vitro.

The effects of clopidogrel on mitochondrial respiratory function have not been previously investigated. We show in vitro that isolated mice liver mitochondria treated with very high doses of clopidogrel 10 microg/ml significantly reduces pre-treatment mitochondrial respiratory state 3 (P<0.05) and state 4 respiration (P<0.01), while oxygen consumption in State 3 is prolonged. This suggests a compromise to mitochondrial oxidative phosphorylation following the addition of high dose clopidogrel. Because clopidogrel at human therapeutic doses 40 ng/ml did not affect isolated mitochondrial respiration, it is thus unlikely, in the absence of cellular bioaccumulation, that clinical doses of clopidogrel would affect mitochondrial bioenergetics in vivo.

Acute coronary ligation in the dog induces time-dependent transitional changes in mitochondrial crista in the non-ischaemic ventricular myocardium.

1. The aim of the present study was to examine, in the dog myocardium, the incidence of zig-zag mitochondrial cristae over time in the non-ischaemic posterior wall, following an acute anterior wall infarct. 2. Changes within the myocardial mitochondrial crista membrane in dogs were investigated following acute left anterior descending coronary artery ligation. Transmyocardial biopsy samples were taken serially from the posterior non-ischaemic wall in the same dog. Changes in heart mitochondrial cristae were examined by transmission electron microscopy prior to coronary ligation (control) and 40 min and 2, 4, 6 and 24 h postinfarction. 3. In control hearts, 90% of mitochondrial cristae had a lamelliform appearance. Following infarction, there were two transitional states with respect to mitochondrial cristae, the first characterized by undulating lamelliform cristae that are also found in 10% of control samples and a second transitional state that was zig-zag and reached a maximum between 6 and 24 h after infarction. 4. In conclusion, an undulating lamelliform crista pattern is present in the non-ischaemic wall of the acute infarcted dog and we hypothesize that this may be an intermediate from, between 'normal' lamelliform and 'abnormal' zig-zag cristae.

Optimal antiplatelet treatment for percutaneous coronary intervention: clopidogrel vs. ticlopidine.

Antiplatelet treatment for patients undergoing percutaneous coronary interventions is a rapidly changing area. Thienopyridines derivatives (ticlopidine and clopidogrel) have shown to decrease major cardiovascular events. Ticlopidine can cause rare but serious side effects, especially during the first 3 months of treatment. Clopidogrel appears to be a safer alternative to ticlopidine. However, resistance to clopidogrel therapy may increases the risk of recurrent cardiovascular events. Whether increased doses of clopidogrel might overcome this resistance in nonresponsive patients warrants further investigation.