Acyl-CoA synthetase 6 is required for brain docosahexaenoic acid retention and neuroprotection during aging

Brain energy metabolism: A roadmap for future research

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+, as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Effects of APOE4 on omega-3 brain metabolism across the lifespan

Omega-3 (n-3) polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA), have important roles in human nutrition and brain health by promoting neuronal functions, maintaining inflammatory homeostasis, and providing structural integrity. As Alzheimer's disease (AD) pathology progresses, DHA metabolism in the brain becomes dysregulated, the timing and extent of which may be influenced by the apolipoprotein E ε4 (APOE4) allele. Here, we discuss how maintaining adequate DHA intake early in life may slow the progression to AD dementia in cognitively normal individuals with APOE4, how recent advances in DHA brain imaging could offer insights leading to more personalized preventive strategies, and how alternative strategies targeting PUFA metabolism pathways may be more effective in mitigating disease progression in patients with existing AD dementia.

Allyl Isothiocyanate Maintains DHA-Containing Glycerophospholipids and Ameliorates the Cognitive Function Decline in OVX Mice

Low-temperature-induced fatty acid desaturation is highly conserved in animals, plants, and bacteria. Allyl isothiocyanate (AITC) is an agonist of the transient receptor potential ankyrin 1 (TRPA1), which is activated by various chemophysiological stimuli, including low temperature. However, whether AITC induces fatty acid desaturation remains unknown. We showed here that AITC increased levels of glycerophospholipids (GP) esterified with unsaturated fatty acids, especially docosahexaenoic acid (DHA) in TRPA1-expressing HEK cells. Additionally, GP-DHA including phosphatidylcholine (18:0/22:6) and phosphatidylethanolamine (18:0/22:6) was increased in the brain and liver of AITC-administered mice. Moreover, intragastrical injection of AITC in ovariectomized (OVX) female C57BL/6J mice dose-dependently shortened the Δlatency time determined by the Morris water maze test, indicating AITC ameliorated the cognitive function decline in these mice. Thus, the oral administration of AITC maintains GP-DHA in the liver and brain, proving to be a potential strategy for preventing cognitive decline.

The effect of n-3 polyunsaturated fatty acid supplementation on cognitive function outcomes in the elderly depends on the baseline omega-3 index

Both epidemiological and preclinical studies have shown the benefits of n-3 polyunsaturated fatty acid (n-3 PUFA) on dementia and cognitive impairment, yet the results of clinical randomized controlled trials (RCTs) performed to date are conflicting. The difference in the baseline omega-3 index (O3i) of subjects is a potential cause for this disparity, yet this is usually ignored. The present meta-analysis aimed to evaluate the effect of n-3 polyunsaturated fatty acid (n-3 PUFA) on cognitive function in the elderly and the role of baseline O3i. A systematic literature search was conducted in PubMed, Embase, Cochrane Library, and Web of Science up to June 27th, 2023. The mean changes in the mini-mental state examination (MMSE) score were calculated as weighted mean differences by using a fixed-effects model. Fifteen random controlled trials were included in the meta-analysis. Pooled analysis showed that n-3 PUFA supplementation did not significantly improve the MMSE score (WMD = 0.04, [-0.08, 0.16]; Z = 0.62, P = 0.53; I2 = 0.00%, P(I2) = 0.49). Out of the 15 studies included in the meta-analysis, only 7 reported O3i at baseline and outcome, so only these 7 articles were used for subgroup analysis. Subgroup analysis showed that the MMSE score was significantly improved in the higher baseline O3i subgroup (WMD = 0.553, [0.01, 1.095]; I2 = 0.00%, P(I2) = 0.556) and higher O3i increment subgroup (WMD = 0.525, [0.023, 1.026]; I2 = 0.00%, P(I2) = 0.545). The overall effect demonstrated that n-3 PUFA supplementation exerted no improvement on global cognitive function. However, a higher baseline O3i and higher O3i increment were associated with an improvement in cognitive function in the elderly.

Long chain acyl-CoA synthetase 6 facilitates the local distribution of di-docosahexaenoic acid- and ultra-long-chain-PUFA-containing phospholipids in the retina to support normal visual function in mice

Docosahexaenoic acid (DHA) and ultra-long-chain polyunsaturated fatty acids (ULC-PUFAs) are uniquely enriched in membrane phospholipids of retinal photoreceptors. Several studies have shown that di-DHA- and ULC-PUFA-containing phospholipids in photoreceptors have an important role in maintaining normal visual function; however, the molecular mechanisms underlying the synthesis and enrichment of these unique lipids in the retina, and their specific roles in retinal function remain unclear. Long-chain acyl-coenzyme A (CoA) synthetase 6 (ACSL6) preferentially converts DHA into DHA-CoA, which is a substrate during DHA-containing lipid biosynthesis. Here, we report that Acsl6 mRNA is expressed in the inner segment of photoreceptor cells and the retinal pigment epithelial cells, and genetic deletion of ACSL6 resulted in the selective depletion of di-DHA- and ULC-PUFA-containing phospholipids, but not mono-DHA-containing phospholipids in the retina. MALDI mass spectrometry imaging (MALDI-MSI) revealed the selective distribution of di-DHA- and ULC-PUFA-containing phospholipids in the photoreceptor outer segment (OS). Electroretinogram of Acsl6-/- mice exhibited photoreceptor cell-derived visual impairment, whereas the expression levels and localization of opsin proteins were unchanged. Acsl6-/- mice exhibited an age-dependent progressive decrease of the thickness of the outer nuclear layers, whereas the inner nuclear layers and OSs were normal. These results demonstrate that ACSL6 facilitates the local enrichment of di-DHA- and ULC-PUFA-containing phospholipids in the retina, which supports normal visual function and retinal homeostasis.

Genome-wide association study of Red Blood Cell fatty acids in the Women's Health Initiative Memory Study

Despite their widespread associations with a wide variety of disease phenotypes, the genetics of red blood cell fatty acids remains understudied. We present one of the first genome-wide association studies of red blood cell fatty acid levels, using the Women's Health Initiative Memory study - a prospective cohort of N = 7,479 women aged 65-79. Approximately 9 million SNPs were measured directly or imputed and, in separate linear models adjusted for age and genetic principal components of ethnicity, SNPs were used to predict 28 different fatty acids. SNPs were considered genome-wide significant using a standard genome-wide significance level of p < 1 × 10-8. Twelve separate loci were identified, seven of which replicated results of a prior RBC-FA GWAS. Of the five novel loci, two have functional annotations directly related to fatty acids (ELOVL6 and ACSL6). While overall explained variation is low, the twelve loci identified provide strong evidence of direct relationships between these genes and fatty acid levels. Further studies are needed to establish and confirm the biological mechanisms by which these genes may directly contribute to fatty acid levels.

Lipid metabolism in dopaminergic neurons influences light entrainment

Dietary lipids, particularly omega-3 polyunsaturated fatty acids, are speculated to impact behaviors linked to the dopaminergic system, such as movement and control of circadian rhythms. However, the ability to draw a direct link between dopaminergic omega-3 fatty acid metabolism and behavioral outcomes has been limited to the use of diet-based approaches, which are confounded by systemic effects. Here, neuronal lipid metabolism was targeted in a diet-independent manner by manipulation of long-chain acyl-CoA synthetase 6 (ACSL6) expression. ACSL6 performs the initial reaction for cellular fatty acid metabolism and prefers the omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA). The loss of Acsl6 in mice (Acsl6-/- ) depletes neuronal membranes of DHA content and results in phenotypes linked to dopaminergic control, such as hyperlocomotion, impaired short-term spatial memory, and imbalances in dopamine neurochemistry. To investigate the role of dopaminergic ACSL6 on these outcomes, a dopaminergic neuron-specific ACSL6 knockout mouse was generated (Acsl6DA-/- ). Acsl6DA-/- mice demonstrated hyperlocomotion and imbalances in striatal dopamine neurochemistry. Circadian rhythms of both the Acsl6-/- and the Acsl6DA-/- mice were similar to control mice under basal conditions. However, upon light entrainment, a mimetic of jet lag, both the complete knockout of ACSL6 and the dopaminergic-neuron-specific loss of ACSL6 resulted in a longer recovery to entrainment compared to control mice. In conclusion, these data demonstrate that ACSL6 in dopaminergic neurons alters dopamine metabolism and regulation of light entrainment suggesting that DHA metabolism mediated by ACSL6 plays a role in dopamine neuron biology.

Bisphenol S Impairs Behaviors through Disturbing Endoplasmic Reticulum Function and Reducing Lipid Levels in the Brain of Zebrafish

The number of neurotoxic pollutants is increasing, but their mechanism of action is unclear. Here, zebrafish were exposed to 0, 1, 10, and 100 μg/L bisphenol S (BPS) for different durations beginning at 2 h postfertilization (hpf) to explore the neurotoxic mechanisms of BPS. Zebrafish larvae exposed to BPS displayed abnormal neurobehaviors. At 48 and 120 hpf, BPS inhibited yolk lipid consumption and reduced the lipid distribution in the zebrafish brain. Moreover, BPS downregulated the mRNA levels of genes involved in fatty acid elongation in the endoplasmic reticulum (ER) and activated ER stress pathways at 48 and 120 hpf, and KEGG analysis after RNA-seq showed that the protein processing pathway in the ER was significantly enriched after BPS exposure. Exposure to ER toxicants (thapsigargin and tunicamycin), two positive controls, induced neurotoxic effects on zebrafish embryos and larvae similar to those of BPS exposure. These data suggested that BPS and ER toxicants disturbed ER function and reduced brain lipid levels. Continued exposure to BPS into adulthood not only inhibited brain fatty acid elongation and ER function but also caused abnormal swelling of the ER in zebrafish. Our data provide new insights into the neurotoxic mechanism of BPS.

What Is the Evidence for Dietary-Induced DHA Deficiency in Human Brains?

Docosahexaenoic acid (DHA) is a major constituent of neural and visual membranes and is required for optimal neural and visual function. DHA is derived from food or by endogenous synthesis from α-linolenic acid (ALA), an essential fatty acid. Low blood levels of DHA in some westernised populations have led to speculations that child development disorders and various neurological conditions are associated with sub-optimal neural DHA levels, a proposition which has been supported by the supplement industry. This review searched for evidence of deficiency of DHA in human populations, based on elevated levels of the biochemical marker of n-3 deficiency, docosapentaenoic acid (22:5n-6). Three scenarios/situations were identified for the insufficient supply of DHA, namely in the brain of new-born infants fed with high-linoleic acid (LA), low-ALA formulas, in cord blood of women at birth who were vegetarians and in the milk of women from North Sudan. Twenty post-mortem brain studies from the developed world from adults with various neurological disorders revealed no evidence of raised levels of 22:5n-6, even in the samples with reduced DHA levels compared with control subjects. Human populations most likely at risk of n-3 deficiency are new-born and weanling infants, children and adolescents in areas of dryland agriculture, in famines, or are refugees, however, these populations have rarely been studied. This is an important topic for future research.

An update on the therapeutic implications of long-chain acyl-coenzyme A synthetases in nervous system diseases

Long-chain acyl-coenzyme A synthetases (ACSLs) are a family of CoA synthetases that activate fatty acid (FA) with chain lengths of 12-20 carbon atoms by forming the acyl-AMP derivative in an isozyme-specific manner. This family mainly includes five members (ACSL1, ACSL3, ACSL4, ACSL5, and ACSL6), which are thought to have specific and different functions in FA metabolism and oxidative stress of mammals. Accumulating evidence shows that the dysfunction of ACSLs is likely to affect cell proliferation and lead to metabolic diseases in multiple organs and systems through different signaling pathways and molecular mechanisms. Hence, a central theme of this review is to emphasize the therapeutic implications of ACSLs in nervous system disorders.

Statin therapy upregulates arachidonic acid status via enhanced endogenous synthesis in patients with plaque psoriasis

Circulating fatty acids (FA) may be important in the psoriatic pro-inflammatory phenotype. FADS1 converts linoleic acid (LA) to arachidonic acid (AA), a precursor to potent signaling molecules. HMG-CoA reductase inhibitors (statins) increase FADS1/2 expression in vitro. Psoriasis patients (42 ± 14 years/age, 47% male) were randomized to 40 mg of atorvastatin (n = 20) or nothing (n = 10) for two weeks and plasma FA measured pre and post treatment. After treatment, LDL-C was 44% lower in the statin compared to the no-treatment group. Statins increased FADS1/2 expression, and lowered LA 12% (33% - > 29%, p<0.001) and raised AA 14% (7.7% - > 9.0%, p<0.01) with no change in the no-treatment group. In psoriasis, statins enhance AA and decrease LA, consistent with the action of enhanced FADS expression in vivo. Therapies intended to blunt the effects of AA on platelet aggregation, such as aspirin or omega-3 fatty acids, may require dose adjustment when co-administered with atorvastatin. NCT: NCT03228017.

New understandings of the pathway of long-chain polyunsaturated fatty acid biosynthesis

PURPOSE OF REVIEW

Molecular studies have clarified the roles of the fatty acid desaturase (FADSx) and elongation of very long chain fatty acid (ELOVLx) genes, as well as acyl-coenzyme A synthase long-chain isoforms (ACSLx) required for entry to long-chain polyunsaturated fatty acid (LCPUFA) biosynthetic pathways.

RECENT FINDINGS

FADS1 and FADS2 but not FADS3 are active toward PUFA. FADS1 is a Δ5-desaturase operating on five C20 PUFA, and is strongly regulated by human genetic polymorphisms, modulating circulating arachidonic acid (20:4n-6) levels. In contrast, FADS2 operates on at least 16 substrates, including five saturates, and catalyzes Δ6, Δ4, and Δ8 desaturation. FADS2 silencing in cancer cells leads to FADS1 synthesis of unusual fatty acids. ACSL6 and ACSL4 are required to maintain tissue 22:6n-3 and 20:4n-6, respectively. FADS2AT2, is the first transcript to differentially inhibit desaturation, attenuating 18:3n-3 but not 18:2n-6 desaturation. The PUFA elongases ELOVL5, 2, and 4 are implicated in cancer, age-related methylation, and retinal degeneration, respectively.

SUMMARY

The mixture of fatty acids available to FADS2 in any tissue defines the product mixture available for further synthesis of membrane lipids and signaling molecules and may be relevant in many clinical conditions including cancer. Functional genetic variants define the levels of circulating arachidonic acid via FADS1 regulation; genotypes that drive high arachidonic acid may predispose to disease.

Multi-Omics Analysis of Microglial Extracellular Vesicles From Human Alzheimer's Disease Brain Tissue Reveals Disease-Associated Signatures

Alzheimer's disease (AD) is the most common cause of dementia, yet there is no cure or diagnostics available prior to the onset of clinical symptoms. Extracellular vesicles (EVs) are lipid bilayer-delimited particles that are released from almost all types of cell. Genome-wide association studies have linked multiple AD genetic risk factors to microglia-specific pathways. It is plausible that microglia-derived EVs may play a role in the progression of AD by contributing to the dissemination of insoluble pathogenic proteins, such as tau and Aβ. Despite the potential utility of EVs as a diagnostic tool, our knowledge of human brain EV subpopulations is limited. Here we present a method for isolating microglial CD11b-positive small EVs from cryopreserved human brain tissue, as well as an integrated multiomics analysis of microglial EVs enriched from the parietal cortex of four late-stage AD (Braak V-VI) and three age-matched normal/low pathology (NL) cases. This integrated analysis revealed 1,000 proteins, 594 lipids, and 105 miRNAs using shotgun proteomics, targeted lipidomics, and NanoString nCounter technology, respectively. The results showed a significant reduction in the abundance of homeostatic microglia markers P2RY12 and TMEM119, and increased levels of disease-associated microglia markers FTH1 and TREM2, in CD11b-positive EVs from AD brain compared to NL cases. Tau abundance was significantly higher in AD brain-derived microglial EVs. These changes were accompanied by the upregulation of synaptic and neuron-specific proteins in the AD group. Levels of free cholesterol were elevated in microglial EVs from the AD brain. Lipidomic analysis also revealed a proinflammatory lipid profile, endolysosomal dysfunction, and a significant AD-associated decrease in levels of docosahexaenoic acid (DHA)-containing polyunsaturated lipids, suggesting a potential defect in acyl-chain remodeling. Additionally, four miRNAs associated with immune and cellular senescence signaling pathways were significantly upregulated in the AD group. Our data suggest that loss of the homeostatic microglia signature in late AD stages may be accompanied by endolysosomal impairment and the release of undigested neuronal and myelin debris, including tau, through extracellular vesicles. We suggest that the analysis of microglia-derived EVs has merit for identifying novel EV-associated biomarkers and providing a framework for future larger-scale multiomics studies on patient-derived cell-type-specific EVs.