Effects of Docosahexaenoic Acid and Its Peroxidation Product on Amyloid-β Peptide-Stimulated Microglia

Effects of DHA (omega-3 fatty acid) and estradiol on amyloid β-peptide regulation in the brain

In the early stages of sporadic Alzheimer's disease (SAD), there is a strong correlation between memory impairment and cortical levels of soluble amyloid-β peptide oligomers (Aβ). It has become clear that Aβ disrupt glutamatergic synaptic function, which can in turn lead to the characteristic cognitive deficits of SAD, but the actual pathways are still not well understood. This opinion article describes the pathogenic mechanisms underlying cerebral amyloidosis. These mechanisms are dependent on the amyloid precursor protein and concern the synthesis of Aβ peptides with competition between the non-amyloidogenic pathway and the amyloidogenic pathway (i.e. a competition between the ADAM10 and BACE1 enzymes), on the one hand, and the various processes of Aβ residue clearance, on the other hand. This clearance mobilizes both endopeptidases (NEP, and IDE) and removal transporters across the blood-brain barrier (LRP1, ABCB1, and RAGE). Lipidated ApoE also plays a major role in all processes. The disturbance of these pathways induces an accumulation of Aβ. The description of the mechanisms reveals two key molecules in particular: (i) free estradiol, which has genomic and non-genomic action, and (ii) free DHA as a preferential ligand of PPARα-RXRα and PPARɣ-RXRα heterodimers. DHA and free estradiol are also self-regulating, and act in synergy. When a certain level of chronic DHA and free estradiol deficiency is reached, a permanent imbalance is established in the central nervous system. The consequences of these deficits are revealed in particular by the presence of Aβ peptide deposits, as well as other markers of the etiology of SAD.

Unsaturated Fatty Acids and Their Immunomodulatory Properties

Oils are an essential part of the human diet and are primarily derived from plant (or sometimes fish) sources. Several of them exhibit anti-inflammatory properties. Specific diets, such as Mediterranean diet, that are high in ω-3 polyunsaturated fatty acids (PUFAs) and ω-9 monounsaturated fatty acids (MUFAs) have even been shown to exert an overall positive impact on human health. One of the most widely used supplements in the developed world is fish oil, which contains high amounts of PUFAs docosahexaenoic and eicosapentaenoic acid. This review is focused on the natural sources of various polyunsaturated and monounsaturated fatty acids in the human diet, and their role as precursor molecules in immune signaling pathways. Consideration is also given to their role in CNS immunity. Recent findings from clinical trials utilizing various fatty acids or diets high in specific fatty acids are reviewed, along with the mechanisms through which fatty acids exert their anti-inflammatory properties. An overall understanding of diversity of polyunsaturated fatty acids and their role in several molecular signaling pathways is useful in formulating diets that reduce inflammation and increase longevity.

Bidirectional communication between brain and visceral white adipose tissue: Its potential impact on Alzheimer's disease

A variety of axes between brain and abdominal organs have been reported, but the interaction between brain and visceral white adipose tissue (vWAT) remains unclear. In this review, we summarized human studies on the association between brain and vWAT, and generalized their interaction and the underlying mechanisms according to animal and cell experiments. On that basis, we come up with the concept of the brain-vWAT axis (BVA). Furthermore, we analyzed the potential mechanisms of involvement of BVA in the pathogenesis of Alzheimer's disease (AD), including vWAT-derived fatty acids, immunological properties of vWAT, vWAT-derived retinoic acid and vWAT-regulated insulin resistance. The proposal of BVA may expand our understanding to some extent of how the vWAT impacts on brain health and diseases, and provide a novel approach to study the pathogenesis and treatment strategies of neurodegenerative disorders.

Recent Advances in Molecular Pathways and Therapeutic Implications Targeting Mitochondrial Dysfunction for Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder which leads to mental deterioration due to aberrant accretion of misfolded proteins in the brain. According to mitochondrial cascade hypothesis, mitochondrial dysfunction is majorly involved in the pathogenesis of AD. Many drugs targeting mitochondria to treat and prevent AD are in different phases of clinical trials for the evaluation of safety and efficacy as mitochondria are involved in various cellular and neuronal functions. Mitochondrial dynamics is regulated by fission and fusion processes mediated by dynamin-related protein (Drp1). Inner membrane fusion takes place by OPA1 and outer membrane fusion is facilitated by mitofusin1 and mitofusin2 (Mfn1/2). Excessive calcium release also impairs mitochondrial functions; to overcome this, calcium channel blockers like nilvadipine are used. Another process acting as a regulator of mitochondrial function is mitophagy which is involved in the removal of damaged and non-functional mitochondria however this process is also altered in AD due to mutations in Presenilin1 (PS1) and Amyloid Precursor Protein (APP) gene. Mitochondrial dynamics is altered in AD which led to the discovery of various fission protein (like Drp1) inhibitors and drugs that promote fusion. Modulations in AMPK, SIRT1 and Akt pathways can also come out to be better therapeutic strategies as these pathways regulate functions of mitochondria. Oxidative phosphorylation is major generator of Reactive Oxygen Species (ROS) leading to mitochondrial damage; therefore reduction in production of ROS by using antioxidants like MitoQ, Curcumin and Vitamin Eis quiteeffective.

Dynamic Role of Phospholipases A2 in Health and Diseases in the Central Nervous System

Phospholipids are major components in the lipid bilayer of cell membranes. These molecules are comprised of two acyl or alkyl groups and different phospho-base groups linked to the glycerol backbone. Over the years, substantial interest has focused on metabolism of phospholipids by phospholipases and the role of their metabolic products in mediating cell functions. The high levels of polyunsaturated fatty acids (PUFA) in the central nervous system (CNS) have led to studies centered on phospholipases A2 (PLA2s), enzymes responsible for cleaving the acyl groups at the sn-2 position of the phospholipids and resulting in production of PUFA and lysophospholipids. Among the many subtypes of PLA2s, studies have centered on three major types of PLA2s, namely, the calcium-dependent cytosolic cPLA2, the calcium-independent iPLA2 and the secretory sPLA2. These PLA2s are different in their molecular structures, cellular localization and, thus, production of lipid mediators with diverse functions. In the past, studies on specific role of PLA2 on cells in the CNS are limited, partly because of the complex cellular make-up of the nervous tissue. However, understanding of the molecular actions of these PLA2s have improved with recent advances in techniques for separation and isolation of specific cell types in the brain tissue as well as development of sensitive molecular tools for analyses of proteins and lipids. A major goal here is to summarize recent studies on the characteristics and dynamic roles of the three major types of PLA2s and their oxidative products towards brain health and neurological disorders.

Short-chain fatty acids as modulators of redox signaling in health and disease

Short-chain fatty acids (SCFAs), produced by colonic bacteria and obtained from the diet, have been linked to beneficial effects on human health associated with their metabolic and signaling properties. Their physiological functions are related to their aliphatic tail length and dependent on the activation of specific membrane receptors. In this review, we focus on the mechanisms underlying SCFAs mediated protection against oxidative and mitochondrial stress and their role in regulating metabolic pathways in specific tissues. We critically evaluate the evidence for their cytoprotective roles in suppressing inflammation and carcinogenesis and the consequences of aging. The ability of these natural compounds to induce signaling pathways, involving nuclear erythroid 2-related factor 2 (Nrf2), contributes to the maintenance of redox homeostasis under physiological conditions. SCFAs may thus serve as nutritional and therapeutic agents in healthy aging and in vascular and other diseases such as diabetes, neuropathologies and cancer.

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SCFAs are a link between the microbiota, redox signaling and host metabolism.

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SCFAs modulate Nrf2 redox signaling through specific free fatty acid receptors.

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Butyrate induces epigenetic regulation and/or Nrf2 nuclear translocation.

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Butyrate and propionate protect the blood-brain barrier by facilitating docosahexaenoic acid transport.

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Regulation of redox homeostasis by SCFAs supports their potential as therapeutic nutrients in health and disease.

Involvement of Microglia in Neurodegenerative Diseases: Beneficial Effects of Docosahexahenoic Acid (DHA) Supplied by Food or Combined with Nanoparticles

Neurodegenerative diseases represent a major public health issue and require better therapeutic management. The treatments developed mainly target neuronal activity. However, an inflammatory component must be considered, and microglia may constitute an important therapeutic target. Given the difficulty in developing molecules that can cross the blood–brain barrier, the use of food-derived molecules may be an interesting therapeutic avenue. Docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid (22:6 omega-3), has an inhibitory action on cell death and oxidative stress induced in the microglia. It also acts on the inflammatory activity of microglia. These data obtained in vitro or on animal models are corroborated by clinical trials showing a protective effect of DHA. Whereas DHA crosses the blood–brain barrier, nutritional intake lacks specificity at both the tissue and cellular level. Nanomedicine offers new tools which favor the delivery of DHA at the cerebral level, especially in microglial cells. Because of the biological activities of DHA and the associated nanotargeting techniques, DHA represents a therapeutic molecule of interest for the treatment of neurodegenerative diseases.

Determination of Commercial Animal and Vegetable Milks' Lipid Profile and Its Correlation with Cell Viability and Antioxidant Activity on Human Intestinal Caco-2 Cells

Lipids from milk are important nutritional components, although their health effects, especially for animal milks, are still questioned. Four types of commercial milks, two semi-skimmed animal milks (bovine and goat) and two vegetable ones (soy and rice), along with their total and free lipid fractions recovered by sequential centrifugation or by ethyl acetate extraction, respectively, have been analyzed. A higher antioxidant ability, reported as Trolox equivalent antioxidant capacity, was found for all raw milks compared to that of rice. This trend was confirmed, except for soy milk, as ROS reduction in Caco-2 cells. The free lipid fraction was shown to have the highest antioxidant potential in both chemical and biological tests. Moreover, goat and soy raw milks positively regulated Caco-2 cell viability after an inflammatory stimulus. This effect was lost when their total lipid fraction was tested. Finally, only the free lipid fraction from rice milk preserved the Caco-2 viability after LPS stimulation. Our data demonstrated that the lipid profile of each milk, characterized by GC-MS analysis, could contribute to dictate its biological effects, and, although additional in vitro and in vivo studies are needed, they could support the literature re-evaluating the health effects of animal-based versus plant-based milks in the intestinal cellular model.

Amyloids: The History of Toxicity and Functionality

Proteins can perform their specific function due to their molecular structure. Partial or complete unfolding of the polypeptide chain may lead to the misfolding and aggregation of proteins in turn, resulting in the formation of different structures such as amyloid aggregates. Amyloids are rigid protein aggregates with the cross-β structure, resistant to most solvents and proteases. Because of their resistance to proteolysis, amyloid aggregates formed in the organism accumulate in tissues, promoting the development of various diseases called amyloidosis, for instance Alzheimer's diseases (AD). According to the main hypothesis, it is considered that the cause of AD is the formation and accumulation of amyloid plaques of Aβ. That is why Aβ-amyloid is the most studied representative of amyloids. Therefore, in this review, special attention is paid to the history of Aβ-amyloid toxicity. We note the main problems with anti-amyloid therapy and write about new views on amyloids that can play positive roles in the different organisms including humans.

Docosahexaenoic and Arachidonic Acids as Neuroprotective Nutrients throughout the Life Cycle

The role of docosahexaenoic acid (DHA) and arachidonic acid (AA) in neurogenesis and brain development throughout the life cycle is fundamental. DHA and AA are long-chain polyunsaturated fatty acids (LCPUFA) vital for many human physiological processes, such as signaling pathways, gene expression, structure and function of membranes, among others. DHA and AA are deposited into the lipids of cell membranes that form the gray matter representing approximately 25% of the total content of brain fatty acids. Both fatty acids have effects on neuronal growth and differentiation through the modulation of the physical properties of neuronal membranes, signal transduction associated with G proteins, and gene expression. DHA and AA have a relevant role in neuroprotection against neurodegenerative pathologies such as Alzheimer's disease and Parkinson's disease, which are associated with characteristic pathological expressions as mitochondrial dysfunction, neuroinflammation, and oxidative stress. The present review analyzes the neuroprotective role of DHA and AA in the extreme stages of life, emphasizing the importance of these LCPUFA during the first year of life and in the developing/prevention of neurodegenerative diseases associated with aging.

Docosahexaenoic Acid (DHA) Supplementation Alters Phospholipid Species and Lipid Peroxidation Products in Adult Mouse Brain, Heart, and Plasma

The abundance of docosahexaenoic acid (DHA) in phospholipids in the brain and retina has generated interest to search for its role in mediating neurological functions. Besides the source of many oxylipins with pro-resolving properties, DHA also undergoes peroxidation, producing 4-hydroxyhexenal (4-HHE), although its function remains elusive. Despite wide dietary consumption, whether supplementation of DHA may alter the peroxidation products and their relationship to phospholipid species in brain and other body organs have not been explored sufficiently. In this study, adult mice were administered a control or DHA-enriched diet for 3 weeks, and phospholipid species and peroxidation products were examined in brain, heart, and plasma. Results demonstrated that this dietary regimen increased (n-3) and decreased (n-6) species to different extent in all major phospholipid classes (PC, dPE, PE-pl, PI and PS) examined. Besides changes in phospholipid species, DHA-enriched diet also showed substantial increases in 4-HHE in brain, heart, and plasma. Among different brain regions, the hippocampus responded to the DHA-enriched diet showing significant increase in 4-HHE. Considering the pro- and anti-inflammatory pathways mediated by the (n-6) and (n-3) polyunsaturated fatty acids, unveiling the ability for DHA-enriched diet to alter phospholipid species and lipid peroxidation products in the brain and in different body organs may be an important step forward towards understanding the mechanism(s) for this (n-3) fatty acid on health and diseases.

Microglia reprogram metabolic profiles for phenotype and function changes in central nervous system

In response to various types of environmental and cellular stress, microglia rapidly activate and exhibit either pro- or anti-inflammatory phenotypes to maintain tissue homeostasis. Activation of microglia can result in changes in morphology, phagocytosis capacity, and secretion of cytokines. Furthermore, microglial activation also induces changes to cellular energy demand, which is dependent on the metabolism of various metabolic substrates including glucose, fatty acids, and amino acids. Accumulating evidence demonstrates metabolic reprogramming acts as a key driver of microglial immune response. For instance, microglia in pro-inflammatory states preferentially use glycolysis for energy production, whereas, cells in anti-inflammatory states are mainly powered by oxidative phosphorylation and fatty acid oxidation. In this review, we summarize recent findings regarding microglial metabolic pathways under physiological and pathological circumtances. We will then discuss how metabolic reprogramming can orchestrate microglial response to a variety of central nervous system pathologies. Finally, we highlight how manipulating metabolic pathways can reprogram microglia towards beneficial functions, and illustrate the therapeutic potential for inflammation-related neurological diseases.

Association of Serum Fatty Acids at Admission with the Age of Onset of Acute Ischemic Stroke

Dietary triglycerides influence fatty acid (FA) serum concentrations and weight percentages (wt %), which may be associated with the age of onset of acute ischemic stroke (AIS). We investigated the correlations between serum FA levels and proportions at admission and the age of onset of AIS. We evaluated patients with AIS admitted between 2016 and 2019 within 24 h of AIS onset and calculated the correlation coefficients between their ages, serum FA concentrations, and FA wt % values. Multiple linear regression analysis was performed to identify independent FAs indicating AIS age of onset. Furthermore, we estimated the threshold values of independent FAs for age of onset <60 years using receiver operating characteristic curves by logistic regression. A total of 525 patients (median age: 75 years) met the inclusion criteria. The concentration of dihomo-gamma-linolenic acid (DGLA) and wt % of docosahexaenoic acid (DHA) were significant independent variables for age of onset of AIS, and receiver operating characteristic curves for age of onset <60 years showed thresholds of ≥117.7 µmol/L for DGLA and ≤3.7% for DHA. An increased DGLA concentration and decreased DHA wt % were significantly associated with onset of AIS at a younger age.

Altered nutrient status reprograms host inflammation and metabolic health via gut microbiota

The metabolism of macro- and micronutrients is a complex and highly regulated biological process. An imbalance in the metabolites and their signaling networks can lead to nonresolving inflammation and consequently to the development of chronic inflammatory-associated diseases. Therefore, identifying the accumulated metabolites and altered pathways during inflammatory disorders would not only serve as "real-time" markers but also help in the development of nutritional therapeutics. In this review, we explore recent research that has delved into elucidating the effects of carbohydrate/calorie restriction, protein malnutrition, lipid emulsions and micronutrient deficiencies on metabolic health and inflammation. Moreover, we describe the integrated stress response in terms of amino acid starvation and lipemia and how this modulates new age diseases such as inflammatory bowel disease and atherosclerosis. Lastly, we explain the latest research on metaflammation and inflammaging. This review focuses on multiple signaling pathways, including, but not limited to, the FGF21-β-hydroxybutryate-NLRP3 axis, the GCN2-eIF2α-ATF4 pathway, the von Hippel-Lindau/hypoxia-inducible transcription factor pathway and the TMAO-PERK-FoxO1 axis. Additionally, throughout the review, we explain how the gut microbiota responds to altered nutrient status and also how antimicrobial peptides generated from nutrient-based signaling pathways can modulate the gut microbiota. Collectively, it must be emphasized that metabolic starvation and inflammation are strongly regulated by both environmental (i.e., nutrition, gut microbiome) and nonenvironmental (i.e., genetics) factors, which can influence the susceptibility to inflammatory disorders.

Quantitative Proteomics Reveals Docosahexaenoic Acid-Mediated Neuroprotective Effects in Lipopolysaccharide-Stimulated Microglial Cells

The high levels of docosahexaenoic acid (DHA) in cell membranes within the brain have led to a number of studies exploring its function. These studies have shown that DHA can reduce inflammatory responses in microglial cells. However, the method of action is poorly understood. Here, we report the effects of DHA on microglial cells stimulated with lipopolysaccharides (LPSs). Data were acquired using the parallel accumulation serial fragmentation method in a hybrid trapped ion mobility-quadrupole time-of-flight mass spectrometer. Over 2800 proteins are identified using label-free quantitative proteomics. Cells exposed to LPSs and/or DHA resulted in changes in cell morphology and expression of 49 proteins with differential abundance (greater than 1.5-fold change). The data provide details about pathways that are influenced in this system including the nuclear factor κ-light-chain-enhancer of the activated B cells (NF-κB) pathway. Western blots and enzyme-linked immunosorbent assay studies are used to help confirm the proteomic results. The MS data are available at ProteomeXchange.

Roles of microglial membranes in Alzheimer's disease

The majority of Alzheimer's disease (AD) risk genes are highly and selectively expressed by microglia in the brain. Several of these genes are related to lipid and cholesterol metabolism, lipid synthesis, lipid transport, endocytosis, exocytosis and phagocytosis. Therefore, studying the roles of cellular membrane biophysics in microglial function should improve our understanding of the AD pathology. In this chapter, we discuss how lipid rafts and membrane-cytoskeleton adhesion impact microglial-mediated oxidative stress and clearance of amyloid-β peptide (Aβ). We also discuss potential roles of lipid membrane-bound extracellular vesicles as carriers of pathological factors to promote inflammation and cytotoxicity.