Brain energy metabolism spurns fatty acids as fuel due to their inherent mitotoxicity and potential capacity to unleash neurodegeneration

Lipid Droplets and Neurodegeneration

Energy metabolism in the brain has been considered one of the critical research areas of neuroscience for ages. One of the most vital parts of brain metabolism cascades is lipid metabolism, and fatty acid plays a crucial role in this process. The fatty acid breakdown process in mitochondria undergoes through a conserved pathway known as β-oxidation where acetyl-CoA and shorter fatty acid chains are produced along with a significant amount of energy molecule. Further, the complete breakdown of fatty acids occurs when they enter the mitochondrial oxidative phosphorylation. Cells store energy as neutral lipids in organelles known as Lipid Droplets (LDs) to prepare for variations in the availability of nutrients. Fatty acids are liberated by lipid droplets and are transported to various cellular compartments for membrane biogenesis or as an energy source. Current research shows that LDs are important in inflammation, metabolic illness, and cellular communication. Lipid droplet biology in peripheral organs like the liver and heart has been well investigated, while the brain's LDs have received less attention. Recently, there has been increased awareness of the existence and role of these dynamic organelles in the central nervous system, mainly connected to neurodegeneration. In this review, we discussed the role of beta-oxidation and lipid droplet formation in the oxidative phosphorylation process, which directly affects neurodegeneration through various pathways.

Sex-specific vulnerabilities in human astrocytes underpin the differential impact of palmitic acid

Obesity and neurometabolic diseases have been linked to neurodegenerative diseases. Our hypothesis is that the endogenous estrogenic component of human astrocytes plays a critical role in cell response during lipotoxic damage, given that obesity can disrupt hormonal homeostasis and cause brain inflammation. Our findings showed that high concentrations of palmitic acid (PA) significantly reduced cell viability more in male astrocytes, indicating sex-specific vulnerabilities. PA induced a greater increase in cytosolic reactive oxygen species (ROS) production in males, while female astrocytes exhibited higher superoxide ion levels in mitochondria. In addition, female astrocytes treated with PA showed increased expression of antioxidant proteins, including catalase, Gpx-1 and Nrf2 suggesting a stronger cellular defence mechanism. Interestingly, there was a difference in the expression of estrogenic components, such as estrogen, androgens, and progesterone receptors, as well as aromatase and 5α-reductase enzymes, between males and females. PA induced their expression mainly in females, indicating a potential protective mechanism mediated by endogenous hormones. In summary, our findings highlight the impact of sex on the response of human astrocytes to lipotoxicity. Male astrocytes appear to be more susceptible to cellular damage when exposed to high concentrations of fatty acids.

Bromocriptine protects perilesional spinal cord neurons from lipotoxicity after spinal cord injury

Recent studies have revealed that lipid droplets accumulate in neurons after brain injury and evoke lipotoxicity, damaging the neurons. However, how lipids are metabolized by spinal cord neurons after spinal cord injury remains unclear. Herein, we investigated lipid metabolism by spinal cord neurons after spinal cord injury and identified lipid-lowering compounds to treat spinal cord injury. We found that lipid droplets accumulated in perilesional spinal cord neurons after spinal cord injury in mice. Lipid droplet accumulation could be induced by myelin debris in HT22 cells. Myelin debris degradation by phospholipase led to massive free fatty acid production, which increased lipid droplet synthesis, β-oxidation, and oxidative phosphorylation. Excessive oxidative phosphorylation increased reactive oxygen species generation, which led to increased lipid peroxidation and HT22 cell apoptosis. Bromocriptine was identified as a lipid-lowering compound that inhibited phosphorylation of cytosolic phospholipase A2 by reducing the phosphorylation of extracellular signal-regulated kinases 1/2 in the mitogen-activated protein kinase pathway, thereby inhibiting myelin debris degradation by cytosolic phospholipase A2 and alleviating lipid droplet accumulation in myelin debris-treated HT22 cells. Motor function, lipid droplet accumulation in spinal cord neurons and neuronal survival were all improved in bromocriptine-treated mice after spinal cord injury. The results suggest that bromocriptine can protect neurons from lipotoxic damage after spinal cord injury via the extracellular signal-regulated kinases 1/2-cytosolic phospholipase A2 pathway.

Nutritional strategies cause memory damage and alter biochemical parameters without causing neuroinflammation

Obesity results from an energy imbalance and has been considered an epidemic due to its increasing rates worldwide. It is classified as a low-grade chronic inflammatory disease and has associated comorbidities. Different nutritional strategies are used for the purpose of weight loss, highlighting low-carbohydrate (LC) diets, ketogenic diets, and intermittent fasting (IF). These strategies can lead to metabolic and behavioral changes as they stimulate different biochemical pathways. Therefore, this study evaluated memory, energy metabolism, neuroinflammation, oxidative stress, and antioxidant defense parameters in mice subjected to an LC diet, ketogenic diet (KD), or IF. Eighty male Swiss mice, 60 days old, were divided into 4 groups: control, LC, KD, or IF. Body weight was measured weekly, and food intake every 48 h. After 15 days of nutritional interventions, the animals were subjected to the behavioral object recognition test and subsequently euthanized. Then, visceral fat was removed and weighed, and the brain was isolated for inflammatory and biochemical analysis. We concluded from this study that the LC and KD strategies could damage memory, IF improves the production of adenosine triphosphate (ATP), and the LC, KD, and IF strategies do not lead to neuroinflammatory damage but present damage at the level of oxidative stress.

Micro-RNA profiles of pathology and resilience in posterior cingulate cortex of cognitively intact elders

The posterior cingulate cortex (PCC) is a key hub of the default mode network underlying autobiographical memory retrieval, which falters early in the progression of Alzheimer's disease (AD). We recently performed RNA sequencing of post-mortem PCC tissue samples from 26 elderly Rush Religious Orders Study participants who came to autopsy with an ante-mortem diagnosis of no cognitive impairment but who collectively displayed a range of Braak I-IV neurofibrillary tangle stages. Notably, cognitively unimpaired subjects displaying high Braak stages may represent cognitive resilience to AD pathology. Transcriptomic data revealed elevated synaptic and ATP-related gene expression in Braak Stages III/IV compared with Stages I/II, suggesting these pathways may be related to PCC resilience. We also mined expression profiles for small non-coding micro-RNAs (miRNAs), which regulate mRNA stability and may represent an underexplored potential mechanism of resilience through the fine-tuning of gene expression within complex cellular networks. Twelve miRNAs were identified as differentially expressed between Braak Stages I/II and III/IV. However, the extent to which the levels of all identified miRNAs were associated with subject demographics, neuropsychological test performance and/or neuropathological diagnostic criteria within this cohort was not explored. Here, we report that a total of 667 miRNAs are significantly associated (rho > 0.38, P < 0.05) with subject variables. There were significant positive correlations between miRNA expression levels and age, perceptual orientation and perceptual speed. By contrast, higher miRNA levels correlated negatively with semantic and episodic memory. Higher expression of 15 miRNAs associated with lower Braak Stages I-II and 47 miRNAs were associated with higher Braak Stages III-IV, suggesting additional mechanistic influences of PCC miRNA expression with resilience. Pathway analysis showed enrichment for miRNAs operating in pathways related to lysine degradation and fatty acid synthesis and metabolism. Finally, we demonstrated that the 12 resilience-related miRNAs differentially expressed in Braak Stages I/II versus Braak Stages III/IV were predicted to regulate mRNAs related to amyloid processing, tau and inflammation. In summary, we demonstrate a dynamic state wherein differential PCC miRNA levels are associated with cognitive performance and post-mortem neuropathological AD diagnostic criteria in cognitively intact elders. We posit these relationships may inform miRNA transcriptional alterations within the PCC relevant to potential early protective (resilience) or pathogenic (pre-clinical or prodromal) responses to disease pathogenesis and thus may be therapeutic targets.

Ketone flux through BDH1 supports metabolic remodeling of skeletal and cardiac muscles in response to intermittent time-restricted feeding

Time-restricted feeding (TRF) has gained attention as a dietary regimen that promotes metabolic health. This study questioned if the health benefits of an intermittent TRF (iTRF) schedule require ketone flux specifically in skeletal and cardiac muscles. Notably, we found that the ketolytic enzyme beta-hydroxybutyrate dehydrogenase 1 (BDH1) is uniquely enriched in isolated mitochondria derived from heart and red/oxidative skeletal muscles, which also have high capacity for fatty acid oxidation (FAO). Using mice with BDH1 deficiency in striated muscles, we discover that this enzyme optimizes FAO efficiency and exercise tolerance during acute fasting. Additionally, iTRF leads to robust molecular remodeling of muscle tissues, and muscle BDH1 flux does indeed play an essential role in conferring the full adaptive benefits of this regimen, including increased lean mass, mitochondrial hormesis, and metabolic rerouting of pyruvate. In sum, ketone flux enhances mitochondrial bioenergetics and supports iTRF-induced remodeling of skeletal muscle and heart.

LC-MS-based urine metabolomics analysis of chronic subdural hematoma for biomarker discovery

BACKGROUND

Chronic subdural hematoma (CSDH) is one of the most common neurosurgical diseases with atypical manifestations. The aim of this study was to utilize urine metabolomics to explore potential biomarkers for the diagnosis and prognosis of CSDH.

METHODS

Seventy-seven healthy controls and ninety-two patients with CSDH were enrolled in our study. In total, 261 urine samples divided into the discovery group and validation group were analyzed by LC-MS. The statistical analysis and functional annotation were applied to discover potential biomarker panels and altered metabolic pathways.

RESULTS

A total of 53 differential metabolites were identified in this study. And the urinary metabolic profiles showed apparent separation between patients and controls. Further functional annotation showed that the differential metabolites were associated with lipid metabolism, fatty acid metabolism, amino acid metabolism, biotin metabolism, steroid hormone biosynthesis, and pentose and glucuronate interconversions. Moreover, one panel of Capryloylglycine, cis-5-Octenoic acid, Ethisterone, and 5,6-DiHETE showed good predictive performance in the diagnosis of CSDH, with an AUC of 0.89 in discovery group and an AUC of 0.822 in validation group. Another five metabolites (Trilobinol, 3'-Hydroxyropivacaine, Ethisterone, Arginyl-Proline, 5-alpha-Dihydrotestosterone glucuronide) showed the levels of them returned to a healthy state after surgery, showing good possibility to monitor the recovery of CSDH patients.

CONCLUSION AND CLINICAL RELEVANCE

The findings of the study revealed urine metabolomic differences between CSDH and controls. The potentially diagnostic and prognostic biomarker panels of urine metabolites were established, and functional analysis demonstrated deeper metabolic disorders of CSDH, which might conduce to improve early diagnose of CSDH clinically.

MCUb is an inducible regulator of calcium-dependent mitochondrial metabolism and substrate utilization in muscle

Mitochondria use the electron transport chain to generate high-energy phosphate from oxidative phosphorylation, a process also regulated by the mitochondrial Ca2+ uniporter (MCU) and Ca2+ levels. Here, we show that MCUb, an inhibitor of MCU-mediated Ca2+ influx, is induced by caloric restriction, where it increases mitochondrial fatty acid utilization. To mimic the fasted state with reduced mitochondrial Ca2+ influx, we generated genetically altered mice with skeletal muscle-specific MCUb expression that showed greater fatty acid usage, less fat accumulation, and lower body weight. In contrast, mice lacking Mcub in skeletal muscle showed increased pyruvate dehydrogenase activity, increased muscle malonyl coenzyme A (CoA), reduced fatty acid utilization, glucose intolerance, and increased adiposity. Mechanistically, pyruvate dehydrogenase kinase 4 (PDK4) overexpression in muscle of Mcub-deleted mice abolished altered substrate preference. Thus, MCUb is an inducible control point in regulating skeletal muscle mitochondrial Ca2+ levels and substrate utilization that impacts total metabolic balance.

Competing endogenous RNAs in human astrocytes: crosstalk and interacting networks in response to lipotoxicity

Neurodegenerative diseases (NDs) are characterized by a progressive deterioration of neuronal function, leading to motor and cognitive damage in patients. Astrocytes are essential for maintaining brain homeostasis, and their functional impairment is increasingly recognized as central to the etiology of various NDs. Such impairment can be induced by toxic insults with palmitic acid (PA), a common fatty acid, that disrupts autophagy, increases reactive oxygen species, and triggers inflammation. Although the effects of PA on astrocytes have been addressed, most aspects of the dynamics of this fatty acid remain unknown. Additionally, there is still no model that satisfactorily explains how astroglia goes from being neuroprotective to neurotoxic. Current incomplete knowledge needs to be improved by the growing field of non-coding RNAs (ncRNAs), which is proven to be related to NDs, where the complexity of the interactions among these molecules and how they control other RNA expressions need to be addressed. In the present study, we present an extensive competing endogenous RNA (ceRNA) network using transcriptomic data from normal human astrocyte (NHA) cells exposed to PA lipotoxic conditions and experimentally validated data on ncRNA interaction. The obtained network contains 7 lncRNA transcripts, 38 miRNAs, and 239 mRNAs that showed enrichment in ND-related processes, such as fatty acid metabolism and biosynthesis, FoxO and TGF-β signaling pathways, prion diseases, apoptosis, and immune-related pathways. In addition, the transcriptomic profile was used to propose 22 potential key controllers lncRNA/miRNA/mRNA axes in ND mechanisms. The relevance of five of these axes was corroborated by the miRNA expression data obtained in other studies. MEG3 (ENST00000398461)/hsa-let-7d-5p/ATF6B axis showed importance in Parkinson's and late Alzheimer's diseases, while AC092687.3/hsa-let-7e-5p/[SREBF2, FNIP1, PMAIP1] and SDCBP2-AS1 (ENST00000446423)/hsa-miR-101-3p/MAPK6 axes are probably related to Alzheimer's disease development and pathology. The presented network and axes will help to understand the PA-induced mechanisms in astrocytes, leading to protection or injury in the CNS under lipotoxic conditions as part of the intricated cellular regulation influencing the pathology of different NDs. Furthermore, the five corroborated axes could be considered study targets for new pharmacologic treatments or as possible diagnostic molecules, contributing to improving the quality of life of millions worldwide.

Fatty acid oxidation organizes mitochondrial supercomplexes to sustain astrocytic ROS and cognition

Having direct access to brain vasculature, astrocytes can take up available blood nutrients and metabolize them to fulfil their own energy needs and deliver metabolic intermediates to local synapses1,2. These glial cells should be, therefore, metabolically adaptable to swap different substrates. However, in vitro and in vivo studies consistently show that astrocytes are primarily glycolytic3-7, suggesting glucose is their main metabolic precursor. Notably, transcriptomic data8,9 and in vitro10 studies reveal that mouse astrocytes are capable of mitochondrially oxidizing fatty acids and that they can detoxify excess neuronal-derived fatty acids in disease models11,12. Still, the factual metabolic advantage of fatty acid use by astrocytes and its physiological impact on higher-order cerebral functions remain unknown. Here, we show that knockout of carnitine-palmitoyl transferase-1A (CPT1A)-a key enzyme of mitochondrial fatty acid oxidation-in adult mouse astrocytes causes cognitive impairment. Mechanistically, decreased fatty acid oxidation rewired astrocytic pyruvate metabolism to facilitate electron flux through a super-assembled mitochondrial respiratory chain, resulting in attenuation of reactive oxygen species formation. Thus, astrocytes naturally metabolize fatty acids to preserve the mitochondrial respiratory chain in an energetically inefficient disassembled conformation that secures signalling reactive oxygen species and sustains cognitive performance.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

Lipid metabolism and Alzheimer's disease: clinical evidence, mechanistic link and therapeutic promise

Alzheimer's disease (AD) is an age-associated neurodegenerative disorder with multifactorial etiology, intersecting genetic and environmental risk factors, and a lack of disease-modifying therapeutics. While the abnormal accumulation of lipids was described in the very first report of AD neuropathology, it was not until recent decades that lipid dyshomeostasis became a focus of AD research. Clinically, lipidomic and metabolomic studies have consistently shown alterations in the levels of various lipid classes emerging in early stages of AD brains. Mechanistically, decades of discovery research have revealed multifaceted interactions between lipid metabolism and key AD pathogenic mechanisms including amyloidogenesis, bioenergetic deficit, oxidative stress, neuroinflammation, and myelin degeneration. In the present review, converging evidence defining lipid dyshomeostasis in AD is summarized, followed by discussions on mechanisms by which lipid metabolism contributes to pathogenesis and modifies disease risk. Furthermore, lipid-targeting therapeutic strategies, and the modification of their efficacy by disease stage, ApoE status, and metabolic and vascular profiles, are reviewed.

Loss of fatty acid degradation by astrocytic mitochondria triggers neuroinflammation and neurodegeneration

Astrocytes provide key neuronal support, and their phenotypic transformation is implicated in neurodegenerative diseases. Metabolically, astrocytes possess low mitochondrial oxidative phosphorylation (OxPhos) activity, but its pathophysiological role in neurodegeneration remains unclear. Here, we show that the brain critically depends on astrocytic OxPhos to degrade fatty acids (FAs) and maintain lipid homeostasis. Aberrant astrocytic OxPhos induces lipid droplet (LD) accumulation followed by neurodegeneration that recapitulates key features of Alzheimer's disease (AD), including synaptic loss, neuroinflammation, demyelination and cognitive impairment. Mechanistically, when FA load overwhelms astrocytic OxPhos capacity, elevated acetyl-CoA levels induce astrocyte reactivity by enhancing STAT3 acetylation and activation. Intercellularly, lipid-laden reactive astrocytes stimulate neuronal FA oxidation and oxidative stress, activate microglia through IL-3 signalling, and inhibit the biosynthesis of FAs and phospholipids required for myelin replenishment. Along with LD accumulation and impaired FA degradation manifested in an AD mouse model, we reveal a lipid-centric, AD-resembling mechanism by which astrocytic mitochondrial dysfunction progressively induces neuroinflammation and neurodegeneration.

Aging and memory are altered by genetically manipulating lactate dehydrogenase in the neurons or glia of flies

The astrocyte-neuron lactate shuttle hypothesis posits that glial-generated lactate is transported to neurons to fuel metabolic processes required for long-term memory. Although studies in vertebrates have revealed that lactate shuttling is important for cognitive function, it is uncertain if this form of metabolic coupling is conserved in invertebrates or is influenced by age. Lactate dehydrogenase (Ldh) is a rate limiting enzyme that interconverts lactate and pyruvate. Here we genetically manipulated expression of Drosophila melanogaster lactate dehydrogenase (dLdh) in neurons or glia to assess the impact of altered lactate metabolism on invertebrate aging and long-term courtship memory at different ages. We also assessed survival, negative geotaxis, brain neutral lipids (the core component of lipid droplets) and brain metabolites. Both upregulation and downregulation of dLdh in neurons resulted in decreased survival and memory impairment with age. Glial downregulation of dLdh expression caused age-related memory impairment without altering survival, while upregulated glial dLdh expression lowered survival without disrupting memory. Both neuronal and glial dLdh upregulation increased neutral lipid accumulation. We provide evidence that altered lactate metabolism with age affects the tricarboxylic acid (TCA) cycle, 2-hydroxyglutarate (2HG), and neutral lipid accumulation. Collectively, our findings indicate that the direct alteration of lactate metabolism in either glia or neurons affects memory and survival but only in an age-dependent manner.

Ror1 promotes PPARα-mediated fatty acid metabolism in astrocytes

Ror1 signaling regulates cell polarity, migration, proliferation, and differentiation during developmental morphogenesis, and plays an important role in regulating neurogenesis in the embryonic neocortices. However, the role of Ror1 signaling in the brains after birth remains largely unknown. Here, we found that expression levels of Ror1 in the mouse neocortices increase during the postnatal period, when astrocytes mature and start expressing GFAP. Indeed, Ror1 is highly expressed in cultured postmitotic mature astrocytes. RNA-Seq analysis revealed that Ror1 expressed in cultured astrocytes mediates upregulated expression of genes related to fatty acid (FA) metabolism, including the gene encoding carnitine palmitoyl-transferase 1a (Cpt1a), the rate-limiting enzyme of mitochondrial fatty acid β-oxidation (FAO). We also found that Ror1 promotes the degradation of lipid droplets (LDs) accumulated in the cytoplasm of cultured astrocytes after oleic acid loading, and that suppressed expression of Ror1 decreases the amount of FAs localized at mitochondria, intracellular ATP levels, and expression levels of peroxisome proliferator-activated receptor α (PPARα) target genes, including Cpt1a. Collectively, these findings indicate that Ror1 signaling promotes PPARα-mediated transcription of FA metabolism-related genes, thereby facilitating the availability of FAs derived from LDs for mitochondrial FAO in the mature astrocytes.

Emerging Links between Nonalcoholic Fatty Liver Disease and Neurodegeneration

The association between liver and brain health has gained attention as biomarkers of liver function have been revealed to predict neurodegeneration. The liver is a central regulator in metabolic homeostasis. However, in nonalcoholic fatty liver disease (NAFLD), homeostasis is disrupted which can result in extrahepatic organ pathologies. Emerging literature provides insight into the mechanisms behind the liver-brain health axis. These include the increased production of liver-derived factors that promote insulin resistance and loss of neuroprotective factors under conditions of NAFLD that increase insulin resistance in the central nervous system. In addition, elevated proinflammatory cytokines linked to NAFLD negatively impact the blood-brain barrier and increase neuroinflammation. Furthermore, exacerbated dyslipidemia associated with NAFLD and hepatic dysfunction can promote altered brain bioenergetics and oxidative stress. In this review, we summarize the current knowledge of the crosstalk between liver and brain as it relates to the pathophysiology between NAFLD and neurodegeneration, with an emphasis on Alzheimer's disease. We also highlight knowledge gaps and future areas for investigation to strengthen the potential link between NAFLD and neurodegeneration.

Lipid metabolism and storage in neuroglia: role in brain development and neurodegenerative diseases

The importance of neuroglia in maintaining normal brain function under physiological and pathological conditions has been supported by growing evidence in recent years. The most important issues regarding glial metabolism and function include the cooperation between glial populations and neurons, morphological and functional changes in pathological states, and the role in the onset and progression of neurodegenerative diseases. Although lipid accumulation and further lipid droplet production in neurodegenerative disease brain models have been observed for a long time, the dynamic development of brain lipid droplet research in recent years suggests its role in the development and progression of neurodegenerative diseases was previously underestimated. First recognized as organelles of lipid storage, lipid droplets (LDs) have emerged as an important organelle in metabolic diseases, inflammation, and host defense. Dynamic changes in lipid metabolism within neurons and glial cells resulting in lipid accumulation and lipid droplet formation are present in brain models of various neurodegenerative diseases, yet their role in the brain remains largely unexplored. This paper first reviews the metabolism and accumulation of several major lipids in the brain and discusses the regulation of lipid accumulation in different types of brain cells. We explore the potential role of intracellular lipid accumulation in the pathogenesis of neurodegeneration, starting from lipid metabolism and LDs biogenesis in glial cells, and discuss several pathological factors that promote lipid droplet formation, mainly focusing on oxidative stress, energy metabolism and glial cell-neuron coupling, which are closely related to the etiology and progression of neurodegenerative diseases. Finally, the directions and challenges of intracellular lipid metabolism in glial cells in neurodegeneration are discussed.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Supplementation of Regular Diet With Medium-Chain Triglycerides for Procognitive Effects: A Narrative Review

It is now widely accepted that ketosis (a physiological state characterized by elevated plasma ketone body levels) possesses a wide range of neuroprotective effects. There is a growing interest in the use of ketogenic supplements, including medium-chain triglycerides (MCT), to achieve intermittent ketosis without adhering to a strict ketogenic diet. MCT supplementation is an inexpensive and simple ketogenic intervention, proven to benefit both individuals with normal cognition and those suffering from mild cognitive impairment, Alzheimer's disease, and other cognitive disorders. The commonly accepted paradigm underlying MCT supplementation trials is that the benefits stem from ketogenesis and that MCT supplementation is safe. However, medium-chain fatty acids (MCFAs) may also exert effects in the brain directly. Moreover, MCFAs, long-chain fatty acids, and glucose participate in mutually intertwined metabolic pathways. Therefore, the metabolic effects must be considered if the desired procognitive effects require administering MCT in doses larger than 1 g/kg. This review summarizes currently available research on the procognitive effects of using MCTs as a supplement to regular feed/diet without concomitant reduction of carbohydrate intake and focuses on the revealed mechanisms linked to particular MCT metabolites (ketone bodies, MCFAs), highlighting open questions and potential considerations.

Biological Potential, Gastrointestinal Digestion, Absorption, and Bioavailability of Algae-Derived Compounds with Neuroprotective Activity: A Comprehensive Review

Currently, there is no known cure for neurodegenerative disease. However, the available therapies aim to manage some of the symptoms of the disease. Human neurodegenerative diseases are a heterogeneous group of illnesses characterized by progressive loss of neuronal cells and nervous system dysfunction related to several mechanisms such as protein aggregation, neuroinflammation, oxidative stress, and neurotransmission dysfunction. Neuroprotective compounds are essential in the prevention and management of neurodegenerative diseases. This review will focus on the neurodegeneration mechanisms and the compounds (proteins, polyunsaturated fatty acids (PUFAs), polysaccharides, carotenoids, phycobiliproteins, phenolic compounds, among others) present in seaweeds that have shown in vivo and in vitro neuroprotective activity. Additionally, it will cover the recent findings on the neuroprotective effects of bioactive compounds from macroalgae, with a focus on their biological potential and possible mechanism of action, including microbiota modulation. Furthermore, gastrointestinal digestion, absorption, and bioavailability will be discussed. Moreover, the clinical trials using seaweed-based drugs or extracts to treat neurodegenerative disorders will be presented, showing the real potential and limitations that a specific metabolite or extract may have as a new therapeutic agent considering the recent approval of a seaweed-based drug to treat Alzheimer’s disease.

Metabolic Syndrome and β-Oxidation of Long-Chain Fatty Acids in the Brain, Heart, and Kidney Mitochondria

We present evidence that metabolic syndrome (MetS) represents the postreproductive stage of the human postembryonic ontogenesis. Accordingly, the genes governing this stage experience relatively weak evolutionary selection pressure, thus representing the metabolic phenotype of distant ancestors with β-oxidation of long-chain fatty acids (FAs) as the primary energy source. Mitochondria oxidize at high-rate FAs only when succinate, glutamate, or pyruvate are present. The heart and brain mitochondria work at a wide range of functional loads and possess an intrinsic inhibition of complex II to prevent oxidative stress at periods of low functional activity. Kidney mitochondria constantly work at a high rate and lack inhibition of complex II. We suggest that in people with MetS, oxidative stress is the central mechanism of the heart and brain pathologies. Oxidative stress is a secondary pathogenetic mechanism in the kidney, while the primary mechanisms are kidney hypoxia caused by persistent hyperglycemia and hypertension. Current evidence suggests that most of the nongenetic pathologies associated with MetS originate from the inconsistencies between the metabolic phenotype acquired after the transition to the postreproductive stage and excessive consumption of food rich in carbohydrates and a sedentary lifestyle.

Insight Into the Mechanism of Exercise Preconditioning in Ischemic Stroke

Exercise preconditioning has attracted extensive attention to induce endogenous neuroprotection and has become the hotspot in neurotherapy. The training exercise is given multiple times before cerebral ischemia, effectively inducing ischemic tolerance and alleviating secondary brain damage post-stroke. Compared with other preconditioning methods, the main advantages of exercise include easy clinical operation and being readily accepted by patients. However, the specific mechanism behind exercise preconditioning to ameliorate brain injury is complex. It involves multi-pathway and multi-target regulation, including regulation of inflammatory response, oxidative stress, apoptosis inhibition, and neurogenesis promotion. The current review summarizes the recent studies on the mechanism of neuroprotection induced by exercise, providing the theoretical basis of applying exercise therapy to prevent and treat ischemic stroke. In addition, we highlight the various limitations and future challenges of translational medicine from fundamental study to clinical application.

Glia fuel neurons with locally synthesized ketone bodies to sustain memory under starvation

During starvation, mammalian brains can adapt their metabolism, switching from glucose to alternative peripheral fuel sources. In the Drosophila starved brain, memory formation is subject to adaptative plasticity, but whether this adaptive plasticity relies on metabolic adaptation remains unclear. Here we show that during starvation, neurons of the fly olfactory memory centre import and use ketone bodies (KBs) as an energy substrate to sustain aversive memory formation. We identify local providers within the brain, the cortex glia, that use their own lipid store to synthesize KBs before exporting them to neurons via monocarboxylate transporters. Finally, we show that the master energy sensor AMP-activated protein kinase regulates both lipid mobilization and KB export in cortex glia. Our data provide a general schema of the metabolic interactions within the brain to support memory when glucose is scarce.

Antioxidant Role and Cardiolipin Remodeling by Redox-Activated Mitochondrial Ca2+-Independent Phospholipase A2γ in the Brain

Mitochondrial Ca²⁺-independent phospholipase A₂γ (iPLA₂γ/PNPLA8) was previously shown to be directly activated by H₂O₂ and release free fatty acids (FAs) for FA-dependent H⁺ transport mediated by the adenine nucleotide translocase (ANT) or uncoupling protein 2 (UCP2). The resulting mild mitochondrial uncoupling and consequent partial attenuation of mitochondrial superoxide production lead to an antioxidant effect. However, the antioxidant role of iPLA₂γ in the brain is not completely understood. Here, using wild-type and iPLA₂γ-KO mice, we demonstrate the ability of tert-butylhydroperoxide (TBHP) to activate iPLA₂γ in isolated brain mitochondria, with consequent liberation of FAs and lysophospholipids. The liberated FA caused an increase in respiratory rate, which was fully inhibited by carboxyatractyloside (CATR), a specific inhibitor of ANT. Employing detailed lipidomic analysis, we also demonstrate a typical cleavage pattern for TBHP-activated iPLA₂γ, reflecting cleavage of glycerophospholipids from both sn-1 and sn-2 positions releasing saturated FAs, monoenoic FAs, and predominant polyunsaturated FAs. The acute antioxidant role of iPLA₂γ-released FAs is supported by monitoring both intramitochondrial superoxide and extramitochondrial H₂O₂ release. We also show that iPLA₂γ-KO mice were more sensitive to stimulation by pro-inflammatory lipopolysaccharide, as reflected by the concomitant increase in protein carbonyls in the brain and pro-inflammatory IL-6 release in the serum. These data support the antioxidant and anti-inflammatory role of iPLA₂γ in vivo. Our data also reveal a substantial decrease of several high molecular weight cardiolipin (CL) species and accumulation of low molecular weight CL species in brain mitochondria of iPLA₂γ-KO mice. Collectively, our results support a key role of iPLA₂γ in the remodeling of lower molecular weight immature cardiolipins with predominantly saturated acyl chains to high molecular weight mature cardiolipins with highly unsaturated PUFA acyl chains, typical for the brain.

Astrocyte Bioenergetics and Major Psychiatric Disorders

Ongoing research continues to add new elements to the emerging picture of involvement of astrocyte energy metabolism in the pathophysiology of major psychiatric disorders, including schizophrenia, mood disorders, and addictions. This review outlines what is known about the energy metabolism in astrocytes, the most numerous cell type in the brain, and summarizes the recent work on how specific perturbations of astrocyte bioenergetics may contribute to the neuropsychiatric conditions. The role of astrocyte energy metabolism in mental health and disease is reviewed on the organism, organ, and cell level. Data arising from genomic, metabolomic, in vitro, and neurobehavioral studies is critically analyzed to suggest future directions in research and possible metabolism-focused therapeutic interventions.

Nuclear factor erythroid 2-related factor 2 protects bovine mammary epithelial cells against free fatty acid-induced mitochondrial dysfunction in vitro

Bovine mammary epithelial cells undergo an increase in metabolic rate, mitochondrial dysfunction, and oxidative stress after calving. Nuclear factor erythroid 2-related factor 2 (NFE2L2), a master regulator of cellular redox homeostasis, plays crucial roles in the regulation of mitochondrial function. The objective of this study was to investigate the role of NFE2L2 on mitochondrial function in bovine mammary epithelial cells under hyperlipidemic conditions. Three experiments were conducted as follows: (1) the immortalized bovine mammary epithelial cell line MAC-T was treated with various concentrations of free fatty acids (FFA; 0, 0.6, 1.2, or 2.4 mM) for 24 h to induce stress; (2) MAC-T cells were transfected with small interfering RNA targeting NFE2L2 (si-NFE2L2) and scrambled nontarget negative control (si-Control) for 48 h; and (3) MAC-T cells were pretreated with 10 μM sulforaphane (SFN), an activator of NFE2L2, for 24 h followed by treatment with 1.2 mM FFA for an additional 24 h. Results indicated that exogenous FFA challenge induced linear and quadratic increases in concentrations of mitochondrial reactive oxygen species (ROS). Compared with 0 mM FFA, mitochondrial membrane potential, mRNA abundance of oxidative phosphorylation complexes (CO I-V), protein abundance of nuclear respiratory factor 1 (NRF1), peroxisome proliferator-activated receptor γ coactivator 1 α (PGC-1α), mitochondrial transcription factor A (TFAM), and NFE2L2 along with the contents of ATP, mitochondrial DNA (mtDNA), and total mitochondria were greater in the MAC-T challenged with 0.6 mM FFA group, but lower in the 1.2 and 2.4 mM FFA cultures. Knockdown of NFE2L2 via small interfering RNA led to greater mitochondrial ROS content and lower mitochondrial membrane potential along with contents of ATP, mtDNA, and total mitochondria. The SFN pretreatment upregulated protein abundance of NFE2L2 and attenuated the downregulation of NFE2L2 induced by FFA. Pretreatment with SFN attenuated the downregulation induced by FFA of PGC-1α, NRF1, and TFAM protein abundance along with contents of mtDNA and total mitochondria. Furthermore, SFN pretreatment attenuated the upregulation of mitochondrial ROS content, the downregulation of mitochondrial membrane potential, and the decreases in ATP, mtDNA, and mitochondrial content induced by FFA. Overall, data indicated that FFA inhibit NFE2L2, resulting in mitochondrial dysfunction and ROS production in bovine mammary epithelial cells. Thus, NFE2L2 may be a promising therapeutic target against metabolic challenge-driven mitochondrial dysfunction and oxidative stress in bovine mammary epithelial cells.

Lipid droplets in the nervous system

Lipid droplets are dynamic intracellular lipid storage organelles that respond to the physiological state of cells. In addition to controlling cell metabolism, they play a protective role for many cellular stressors, including oxidative stress. Despite prior descriptions of lipid droplets appearing in the brain as early as a century ago, only recently has the role of lipid droplets in cells found in the brain begun to be understood. Lipid droplet functions have now been described for cells of the nervous system in the context of development, aging, and an increasing number of neuropathologies. Here, we review the basic mechanisms of lipid droplet formation, turnover, and function and discuss how these mechanisms enable lipid droplets to function in different cell types of the nervous system under healthy and pathological conditions.

Energetic dysfunction in sepsis: a narrative review

Background

Growing evidence associates organ dysfunction(s) with impaired metabolism in sepsis. Recent research has increased our understanding of the role of substrate utilization and mitochondrial dysfunction in the pathophysiology of sepsis-related organ dysfunction. The purpose of this review is to present this evidence as a coherent whole and to highlight future research directions.

Main text

Sepsis is characterized by systemic and organ-specific changes in metabolism. Alterations of oxygen consumption, increased levels of circulating substrates, impaired glucose and lipid oxidation, and mitochondrial dysfunction are all associated with organ dysfunction and poor outcomes in both animal models and patients. The pathophysiological relevance of bioenergetics and metabolism in the specific examples of sepsis-related immunodeficiency, cerebral dysfunction, cardiomyopathy, acute kidney injury and diaphragmatic failure is also described.

Conclusions

Recent understandings in substrate utilization and mitochondrial dysfunction may pave the way for new diagnostic and therapeutic approaches. These findings could help physicians to identify distinct subgroups of sepsis and to develop personalized treatment strategies. Implications for their use as bioenergetic targets to identify metabolism- and mitochondria-targeted treatments need to be evaluated in future studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00893-7.

Lipidomic traits of plasma and cerebrospinal fluid in amyotrophic lateral sclerosis correlate with disease progression

Since amyotrophic lateral sclerosis cases exhibit significant heterogeneity, we aim to investigate the association of lipid composition of plasma and CSF with amyotrophic lateral sclerosis diagnosis, its progression and clinical characteristics. Lipidome analyses would help to stratify patients on a molecular basis. For this reason, we have analysed the lipid composition of paired plasma and CSF samples from amyotrophic lateral sclerosis cases and age-matched non-amyotrophic lateral sclerosis individuals (controls) by comprehensive liquid chromatography coupled to mass spectrometry. The concentrations of neurofilament light chain-an index of neuronal damage-were also quantified in CSF samples and plasma. Amyotrophic lateral sclerosis versus control comparison, in a moderate stringency mode, showed that plasma from cases contains more differential lipids (n = 122 for raw P < 0.05; n = 27 for P < 0.01) than CSF (n = 17 for raw P < 0.05; n = 4 for P < 0.01), with almost no overlapping differential species, mainly characterized by an increased content of triacylglyceride species in plasma and decreased in CSF. Of note, false discovery rate correction indicated that one of the CSF lipids (monoacylglycerol 18:0) had high statistic robustness (false discovery rate-P < 0.01). Plasma lipidomes also varied significantly with the main involvement at onset (bulbar, spinal or respiratory). Notably, faster progression cases showed particular lipidome fingerprints, featured by decreased triacylclycerides and specific phospholipids in plasma, with 11 lipids with false discovery rate-P < 0.1 (n = 56 lipids in plasma for raw P < 0.01). Lipid species associated with progression rate clustered in a relatively low number of metabolic pathways, mainly triacylglyceride metabolism and glycerophospholipid and sphingolipid biosynthesis. A specific triacylglyceride (68:12), correlated with neurofilament content (r = 0.8, P < 0.008). Thus, the present findings suggest that systemic hypermetabolism-potentially sustained by increased triacylglyceride content-and CNS alterations of specific lipid pathways could be associated as modifiers of disease progression. Furthermore, these results confirm biochemical lipid heterogeneity in amyotrophic lateral sclerosis with different presentations and progression, suggesting the use of specific lipid species as potential disease classifiers.

Transcriptomic and Genetic Associations between Alzheimer's Disease, Parkinson's Disease, and Cancer

Simple Summary

Epidemiological studies have identified a link between neurodegenerative disorders and a reduced risk of overall cancer. Increases and decreases in the risk of site-specific cancers have also been reported. However, it is still unknown whether these associations arise due to shared genetic and molecular factors or are explained by other phenomena (e.g., biases in epidemiological studies or the use of medication). In this study, we aimed to investigate the potential molecular, genetic, and pharmacological links between Alzheimer’s and Parkinson’s diseases and a large panel of 22 cancer types. To examine the overlapping involvement of genes and pathways, we obtained differential gene expression profiles through meta-analyses of post-mortem brain tissues from Alzheimer’s and Parkinson’s disease patients, primary tumors, and tissue-matched controls, and compared them. Genetic similarities were assessed through network-based methods and the computation of genetic correlations. Finally, the potential impact of drugs indicated for each disorder in the identified associations was evaluated using transcriptomic methods. Our research extends previous work in the field by identifying new significant patterns of transcriptomic associations (direct and inverse) between Alzheimer’s disease, Parkinson’s disease, and different site-specific cancers. The results reveal significant genetic correlations between Parkinson’s disease, prostate cancer, and melanoma. In addition, to our knowledge, this is the first time that the role of drugs indicated for the treatment of both sets of disorders has been investigated in the context of their comorbid associations using transcriptomic methods.

Abstract

Alzheimer’s (AD) and Parkinson’s diseases (PD) are the two most prevalent neurodegenerative disorders in human populations. Epidemiological studies have shown that patients suffering from either condition present a reduced overall risk of cancer than controls (i.e., inverse comorbidity), suggesting that neurodegeneration provides a protective effect against cancer. Reduced risks of several site-specific tumors, including colorectal, lung, and prostate cancers, have also been observed in AD and PD. By contrast, an increased risk of melanoma has been described in PD patients (i.e., direct comorbidity). Therefore, a fundamental question to address is whether these associations are due to shared genetic and molecular factors or are explained by other phenomena, such as flaws in epidemiological studies, exposure to shared risk factors, or the effect of medications. To this end, we first evaluated the transcriptomes of AD and PD post-mortem brain tissues derived from the hippocampus and the substantia nigra and analyzed their similarities to those of a large panel of 22 site-specific cancers, which were obtained through differential gene expression meta-analyses of array-based studies available in public repositories. Genes and pathways that were deregulated in both disorders in each analyzed pair were examined. Second, we assessed potential genetic links between AD, PD, and the selected cancers by establishing interactome-based overlaps of genes previously linked to each disorder. Then, their genetic correlations were computed using cross-trait LD score regression and GWAS summary statistics data. Finally, the potential role of medications in the reported comorbidities was assessed by comparing disease-specific differential gene expression profiles to an extensive collection of differential gene expression signatures generated by exposing cell lines to drugs indicated for AD, PD, and cancer treatment (LINCS L1000). We identified significant inverse associations of transcriptomic deregulation between AD hippocampal tissues and breast, lung, liver, and prostate cancers, and between PD substantia nigra tissues and breast, lung, and prostate cancers. Moreover, significant direct (same direction) associations of deregulation were observed between AD and PD and brain and thyroid cancers, as well as between PD and kidney cancer. Several biological processes, including the immune system, oxidative phosphorylation, PI3K/AKT/mTOR signaling, and the cell cycle, were found to be deregulated in both cancer and neurodegenerative disorders. Significant genetic correlations were found between PD and melanoma and prostate cancers. Several drugs indicated for the treatment of neurodegenerative disorders and cancer, such as galantamine, selegiline, exemestane, and estradiol, were identified as potential modulators of the comorbidities observed between neurodegeneration and cancer.

Glucose metabolic crosstalk and regulation in brain function and diseases

Brain glucose metabolism, including glycolysis, the pentose phosphate pathway, and glycogen turnover, produces ATP for energetic support and provides the precursors for the synthesis of biological macromolecules. Although glucose metabolism in neurons and astrocytes has been extensively studied, the glucose metabolism of microglia and oligodendrocytes, and their interactions with neurons and astrocytes, remain critical to understand brain function. Brain regions with heterogeneous cell composition and cell-type-specific profiles of glucose metabolism suggest that metabolic networks within the brain are complex. Signal transduction proteins including those in the Wnt, GSK-3β, PI3K-AKT, and AMPK pathways are involved in regulating these networks. Additionally, glycolytic enzymes and metabolites, such as hexokinase 2, acetyl-CoA, and enolase 2, are implicated in the modulation of cellular function, microglial activation, glycation, and acetylation of biomolecules. Given these extensive networks, glucose metabolism dysfunction in the whole brain or specific cell types is strongly associated with neurologic pathology including ischemic brain injury and neurodegenerative disorders. This review characterizes the glucose metabolism networks of the brain based on molecular signaling and cellular and regional interactions, and elucidates glucose metabolism-based mechanisms of neurological diseases and therapeutic approaches that may ameliorate metabolic abnormalities in those diseases.

How the brain fights fatty acids' toxicity

Neurons spurn hydrogen-rich fatty acids for energizing oxidative ATP synthesis, contrary to other cells. This feature has been mainly attributed to a lower yield of ATP per reduced oxygen, as compared to glucose. Moreover, the use of fatty acids as hydrogen donor is accompanied by severe β-oxidation-associated ROS generation. Neurons are especially susceptible to detrimental activities of ROS due to their poor antioxidative equipment. It is also important to note that free fatty acids (FFA) initiate multiple harmful activities inside the cells, particularly on phosphorylating mitochondria. Several processes enhance FFA-linked lipotoxicity in the cerebral tissue. Thus, an uptake of FFA from the circulation into the brain tissue takes place during an imbalance between energy intake and energy expenditure in the body, a situation similar to that during metabolic syndrome and fat-rich diet. Traumatic or hypoxic brain injuries increase hydrolytic degradation of membrane phospholipids and, thereby elevate the level of FFA in neural cells. Accumulation of FFA in brain tissue is markedly associated with some inherited neurological disorders, such as Refsum disease or X-linked adrenoleukodystrophy (X-ALD). What are strategies protecting neurons against FFA-linked lipotoxicity? Firstly, spurning the β-oxidation pathway in mitochondria of neurons. Secondly, based on a tight metabolic communication between neurons and astrocytes, astrocytes donate metabolites to neurons for synthesis of antioxidants. Further, neuronal autophagy of ROS-emitting mitochondria combined with the transfer of degradation-committed FFA for their disposal in astrocytes, is a potent protective strategy against ROS and harmful activities of FFA. Finally, estrogens and neurosteroids are protective as triggers of ERK and PKB signaling pathways, consequently initiating the expression of various neuronal survival genes via the formation of cAMP response element-binding protein (CREB).

Effects of Acanthopanax senticosus (Rupr. & Maxim.) Harms on cerebral ischemia-reperfusion injury revealed by metabolomics and transcriptomics

ETHNOPHARMACOLOGICAL RELEVANCE

Cerebral ischemia-reperfusion (CIR) injury is one of the main diseases leading to death and disability. Acanthopanax senticosus (Rupr. & Maxim.) Harms (AS), also known as Panax ginseng, has neuroprotective effects on anti-CIR injury. However, the underlying molecular mechanism of its therapeutic effects is not clear.

AIM OF THE STUDY

To systematically study and explore the mechanism of Acanthopanax senticosus (Rupr. & Maxim.) Harms extract (ASE) in the treatment of CIR injury based on metabolomics and transcriptomics.

MATERIALS AND METHODS

The pharmacological basis of ASE in the treatment of CIR was evaluated, and samples were used in plasma metabolomics and brain tissue transcriptomics to reveal potential biomarkers. Finally, according to online database, we analyzed biomarkers identified by the two technologies, explained reasons for the therapeutic effect of ASE, and identify therapeutic targets.

RESULTS

A total of 53 differential metabolites (DMs) were identified in plasma and 3138 differentially expressed genes (DEGs) were identified in brain tissue from three groups of rats, including sham, ischemia-reperfusion (I/R), and ASE groups. Enrichment analysis showed that Nme6, Tk1, and Pold1 that are involved in the production of deoxycytidine and thymine were significantly up-regulated and Dck was significantly down-regulated by the intervention with ASE. These findings indicated that ASE participates in the pyrimidine metabolism by significantly regulating the balance between dCTP and dTTP. In addition, ASE repaired and promoted the lipid metabolism in rats, which might be due to the significant expression of Dgkz, Chat, and Gpcpd1.

CONCLUSIONS

The findings of this study suggest that ASE regulates the significant changes in gene expression in metabolites pyrimidine, and lipid metabolism in CIR rats and plays an active role in the treatment of CIR injury through multiple targets and pathways.

ApoE4 Impairs Neuron-Astrocyte Coupling of Fatty Acid Metabolism

Alzheimer’s disease (AD) risk gene ApoE4 perturbs brain lipid homeostasis and energy transduction. However, the cell-type-specific mechanism of ApoE4 in modulating brain lipid metabolism is unclear. Here, we describe a detrimental role of ApoE4 in regulating fatty acid (FA) metabolism across neuron and astrocyte in tandem with their distinctive mitochondrial phenotypes. ApoE4 disrupts neuronal function by decreasing FA sequestering in lipid droplets (LDs). FAs in neuronal LDs are exported and internalized by astrocytes, with ApoE4 diminishing the transport efficiency. Further, ApoE4 lowers FA oxidation and leads to lipid accumulation in both astrocyte and the hippocampus. Importantly, diminished capacity of ApoE4 astrocytes in eliminating neuronal lipids and degrading FAs accounts for their compromised metabolic and synaptic support to neurons. Collectively, our findings reveal a mechanism of ApoE4 disruption to brain FA and bioenergetic homeostasis that could underlie the accelerated lipid dysregulation and energy deficits and increased AD risk for ApoE4 carriers.

Qi et al. reveal a role of Alzheimer’s disease risk gene ApoE4 in disrupting fatty acid metabolism coupled between neuron and astrocyte. A systematic impairment in neuronal sequestering, neuron-to-astrocyte transport, and astrocytic degradation of fatty acids could contribute to lipid dysregulation, bioenergetic deficits, and increased Alzheimer’s risk in ApoE4 carriers.

A Community-Based Study Identifying Metabolic Biomarkers of Mild Cognitive Impairment and Alzheimer's Disease Using Artificial Intelligence and Machine Learning

BACKGROUND

Currently, there is no objective, clinically available tool for the accurate diagnosis of Alzheimer's disease (AD). There is a pressing need for a novel, minimally invasive, cost friendly, and easily accessible tool to diagnose AD, assess disease severity, and prognosticate course. Metabolomics is a promising tool for discovery of new, biologically, and clinically relevant biomarkers for AD detection and classification.

OBJECTIVE

Utilizing artificial intelligence and machine learning, we aim to assess whether a panel of metabolites as detected in plasma can be used as an objective and clinically feasible tool for the diagnosis of mild cognitive impairment (MCI) and AD.

METHODS

Using a community-based sample cohort acquired from different sites across the US, we adopted an approach combining Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR), Liquid Chromatography coupled with Mass Spectrometry (LC-MS) and various machine learning statistical approaches to identify a biomarker panel capable of identifying those patients with AD and MCI from healthy controls.

RESULTS

Of the 212 measured metabolites, 5 were identified as optimal to discriminate between controls, and individuals with MCI or AD. Our models performed with AUC values in the range of 0.72-0.76, with the sensitivity and specificity values ranging from 0.75-0.85 and 0.69-0.81, respectively. Univariate and pathway analysis identified lipid metabolism as the most perturbed biochemical pathway in MCI and AD.

CONCLUSION

A comprehensive method of acquiring metabolomics data, coupled with machine learning techniques, has identified a strong panel of diagnostic biomarkers capable of identifying individuals with MCI and AD. Further, our data confirm what other groups have reported, that lipid metabolism is significantly perturbed in those individuals suffering with dementia. This work may provide additional insight into AD pathogenesis and encourage more in-depth analysis of the AD lipidome.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

Schizophrenic Psychosis Symptoms in a Background of Mild-To-Moderate Carnitine Palmitoyltransferase II Deficiency: A Case Report

Schizophrenia is a multifaceted mental illness characterized by cognitive and neurobehavioral abnormalities. Carnitine palmitoyltransferase II (CPT II) deficiency is a metabolic disorder resulting in impaired transport of long-chain fatty acids from the cytosol to the mitochondrial inner membrane, where fatty acid β-oxidation takes place. Here, we present an interesting clinical case of an adolescent male that presented with psychosis and a history of mild-to-moderate CPT II deficiency. To identify germline genetic variation that may contribute to the phenotypes observed, we performed whole-exome sequencing on DNA from the proband, unaffected fraternal twin, and biological parents. The proband was identified to be homozygous for the p.Val368Ile and heterozygous for the p.Met647Val variant in CPT2. Each of these variants are benign on their own; however, their combined effect is unclear. Further, variation was identified in the dopamine β-hydroxylase (DBH) gene (c.339+2T>C), which may contribute to decreased activity of DBH; however, based on the patient’s presentation, severe DBH deficiency is unlikely. In conclusion, the variants identified in this study do not clearly explain the observed patient phenotypes, indicating that the complex phenotypes are likely caused by an interplay of genetic and environmental factors that warrant further investigation.

Multiorgan Metabolomics and Lipidomics Provide New Insights Into Fat Infiltration in the Liver, Muscle Wasting, and Liver-Muscle Crosstalk Following Burn Injury

Burn injury mediated hypermetabolic syndrome leads to increased mortality among severe burn victims, due to liver failure and muscle wasting. Metabolic changes may persist up to 2 years following the injury. Thus, understanding the underlying mechanisms of the pathology is crucially important to develop appropriate therapeutic approaches. We present detailed metabolomic and lipidomic analyses of the liver and muscle tissues in a rat model with a 30% body surface area burn injury located at the dorsal skin. Three hundred and thirty-eight of 1587 detected metabolites and lipids in the liver and 119 of 1504 in the muscle tissue exhibited statistically significant alterations. We observed excessive accumulation of triacylglycerols, decreased levels of S-adenosylmethionine, increased levels of glutamine and xenobiotics in the liver tissue. Additionally, the levels of gluconeogenesis, glycolysis, and tricarboxylic acid cycle metabolites are generally decreased in the liver. On the other hand, burn injury muscle tissue exhibits increased levels of acyl-carnitines, alpha-hydroxyisovalerate, ophthalmate, alpha-hydroxybutyrate, and decreased levels of reduced glutathione. The results of this preliminary study provide compelling observations that liver and muscle tissues undergo distinctly different changes during hypermetabolism, possibly reflecting liver-muscle crosstalk. The liver and muscle tissues might be exacerbating each other's metabolic pathologies, via excessive utilization of certain metabolites produced by each other.

Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Lipids constitute the bulk of the dry mass of the brain and have been associated with healthy function as well as the most common pathological conditions of the brain. Demographic factors, genetics, and lifestyles are the major factors that influence lipid metabolism and are also the key components of lipid disruption in Alzheimer's disease (AD). Additionally, the most common genetic risk factor of AD, APOE ϵ4 genotype, is involved in lipid transport and metabolism. We propose that lipids are at the center of Alzheimer's disease pathology based on their involvement in the blood-brain barrier function, amyloid precursor protein (APP) processing, myelination, membrane remodeling, receptor signaling, inflammation, oxidation, and energy balance. Under healthy conditions, lipid homeostasis bestows a balanced cellular environment that enables the proper functioning of brain cells. However, under pathological conditions, dyshomeostasis of brain lipid composition can result in disturbed BBB, abnormal processing of APP, dysfunction in endocytosis/exocytosis/autophagocytosis, altered myelination, disturbed signaling, unbalanced energy metabolism, and enhanced inflammation. These lipid disturbances may contribute to abnormalities in brain function that are the hallmark of AD. The wide variance of lipid disturbances associated with brain function suggest that AD pathology may present as a complex interaction between several metabolic pathways that are augmented by risk factors such as age, genetics, and lifestyles. Herewith, we examine factors that influence brain lipid composition, review the association of lipids with all known facets of AD pathology, and offer pointers for potential therapies that target lipid pathways.

Determining the Bioenergetic Capacity for Fatty Acid Oxidation in the Mammalian Nervous System

The metabolic state of the brain can greatly impact neurologic function. Evidence of this includes the therapeutic benefit of a ketogenic diet in neurologic diseases, including epilepsy. However, brain lipid bioenergetics remain largely uncharacterized. The existence, capacity, and relevance of mitochondrial fatty acid β-oxidation (FAO) in the brain are highly controversial, with few genetic tools available to evaluate the question. We have provided evidence for the capacity of brain FAO using a pan-brain-specific conditional knockout (KO) mouse incapable of FAO due to the loss of carnitine palmitoyltransferase 2, the product of an obligate gene for FAO (CPT2B-/-). Loss of central nervous system (CNS) FAO did not result in gross neuroanatomical changes or systemic differences in metabolism. Loss of CPT2 in the brain did not result in robustly impaired behavior. We demonstrate by unbiased and targeted metabolomics that the mammalian brain oxidizes a substantial quantity of long-chain fatty acids in vitro and in vivo Loss of CNS FAO results in robust accumulation of long-chain acylcarnitines in the brain, suggesting that the mammalian brain mobilizes fatty acids for their oxidation, irrespective of diet or metabolic state. Together, these data demonstrate that the mammalian brain oxidizes fatty acids under normal circumstances with little influence from or on peripheral tissues.

Nitric Oxide-Dependent Protein Post-Translational Modifications Impair Mitochondrial Function and Metabolism to Contribute to Neurodegenerative Diseases

Significance: Most brains affected by neurodegenerative diseases manifest mitochondrial dysfunction as well as elevated production of reactive oxygen species and reactive nitrogen species (RNS), contributing to synapse loss and neuronal injury. Recent Advances: Excessive production of RNS triggers nitric oxide (NO)-mediated post-translational modifications of proteins, such as S-nitrosylation of cysteine residues and nitration of tyrosine residues. Proteins thus affected impair mitochondrial metabolism, mitochondrial dynamics, and mitophagy in the nervous system. Critical Issues: Identification and better characterization of underlying molecular mechanisms for NO-mediated mitochondrial dysfunction will provide important insights into the pathogenesis of neurodegenerative disorders. In this review, we highlight recent discoveries concerning S-nitrosylation of the tricarboxylic acid cycle enzymes, mitochondrial fission GTPase dynamin-related protein 1, and mitophagy-related proteins Parkin and phosphatase and tensin homolog-induced putative kinase protein 1. We delineate signaling cascades affected by pathologically S-nitrosylated proteins that diminish mitochondrial function in neurodegenerative diseases. Future Directions: Further elucidation of the pathological events resulting from aberrant S-nitrosothiol or nitrotyrosine formation may lead to new therapeutic approaches to ameliorate neurodegenerative disorders.

Not Fade Away: Mechanisms of Neuronal ATP Homeostasis

In this issue of Neuron, Ashrafi et al. (2020) identify a feedforward signaling mechanism that couples neuronal activity to the homeostatic maintenance of axonal and synaptic ATP production. This mechanism is achieved via changes in cytoplasmic calcium and activation of brain-specific, mitochondrial MICU3.

Metabolic Regulation of Glial Phenotypes: Implications in Neuron-Glia Interactions and Neurological Disorders

Glial cells are multifunctional, non-neuronal components of the central nervous system with diverse phenotypes that have gained much attention for their close involvement in neuroinflammation and neurodegenerative diseases. Glial phenotypes are primarily characterized by their structural and functional changes in response to various stimuli, which can be either neuroprotective or neurotoxic. The reliance of neurons on glial cells is essential to fulfill the energy demands of the brain for its proper functioning. Moreover, the glial cells perform distinct functions to regulate their own metabolic activities, as well as work in close conjunction with neurons through various secreted signaling or guidance molecules, thereby constituting a complex network of neuron-glial interactions in health and disease. The emerging evidence suggests that, in disease conditions, the metabolic alterations in the glial cells can induce structural and functional changes together with neuronal dysfunction indicating the importance of neuron-glia interactions in the pathophysiology of neurological disorders. This review covers the recent developments that implicate the regulation of glial phenotypic changes and its consequences on neuron-glia interactions in neurological disorders. Finally, we discuss the possibilities and challenges of targeting glial metabolism as a strategy to treat neurological disorders.

Cellular Specificity and Inter-cellular Coordination in the Brain Bioenergetic System: Implications for Aging and Neurodegeneration

As an organ with a highly heterogenous cellular composition, the brain has a bioenergetic system that is more complex than peripheral tissues. Such complexities are not only due to the diverse bioenergetic phenotypes of a variety of cell types that differentially contribute to the metabolic profile of the brain, but also originate from the bidirectional metabolic communications and coupling across cell types. While brain energy metabolism and mitochondrial function have been extensively investigated in aging and age-associated neurodegenerative disorders, the role of various cell types and their inter-cellular communications in regulating brain metabolic and synaptic functions remains elusive. In this review, we summarize recent advances in differentiating bioenergetic phenotypes of neurons, astrocytes, and microglia in the context of their functional specificity, and their metabolic shifts upon aging and pathological conditions. Moreover, the metabolic coordination between the two most abundant cell populations in brain, neurons and astrocytes, is discussed regarding how they jointly establish a dynamic and responsive system to maintain brain bioenergetic homeostasis and to combat against threats such as oxidative stress, lipid toxicity, and neuroinflammation. Elucidating the mechanisms by which brain cells with distinctive bioenergetic phenotypes individually and collectively shape the bioenergetic system of the brain will provide rationale for spatiotemporally precise interventions to sustain a metabolic equilibrium that is resilient against synaptic dysfunction in aging and neurodegeneration.

Neuron-Astrocyte Metabolic Coupling Protects against Activity-Induced Fatty Acid Toxicity

Metabolic coordination between neurons and astrocytes is critical for the health of the brain. However, neuron-astrocyte coupling of lipid metabolism, particularly in response to neural activity, remains largely uncharacterized. Here, we demonstrate that toxic fatty acids (FAs) produced in hyperactive neurons are transferred to astrocytic lipid droplets by ApoE-positive lipid particles. Astrocytes consume the FAs stored in lipid droplets via mitochondrial β-oxidation in response to neuronal activity and turn on a detoxification gene expression program. Our findings reveal that FA metabolism is coupled in neurons and astrocytes to protect neurons from FA toxicity during periods of enhanced activity. This coordinated mechanism for metabolizing FAs could underlie both homeostasis and a variety of disease states of the brain.

SQSTM1/p62-Directed Metabolic Reprogramming Is Essential for Normal Neurodifferentiation

Neurodegenerative disorders are an increasingly common and irreversible burden on society, often affecting the aging population, but their etiology and disease mechanisms are poorly understood. Studying monogenic neurodegenerative diseases with known genetic cause provides an opportunity to understand cellular mechanisms also affected in more complex disorders. We recently reported that loss-of-function mutations in the autophagy adaptor protein SQSTM1/p62 lead to a slowly progressive neurodegenerative disease presenting in childhood. To further elucidate the neuronal involvement, we studied the cellular consequences of loss of p62 in a neuroepithelial stem cell (NESC) model and differentiated neurons derived from reprogrammed p62 patient cells or by CRISPR/Cas9-directed gene editing in NESCs. Transcriptomic and proteomic analyses suggest that p62 is essential for neuronal differentiation by controlling the metabolic shift from aerobic glycolysis to oxidative phosphorylation required for neuronal maturation. This shift is blocked by the failure to sufficiently downregulate lactate dehydrogenase expression due to the loss of p62, possibly through impaired Hif-1α downregulation and increased sensitivity to oxidative stress. The findings imply an important role for p62 in neuronal energy metabolism and particularly in the regulation of the shift between glycolysis and oxidative phosphorylation required for normal neurodifferentiation.

Intralipid/Heparin Infusion Alters Brain Metabolites Assessed With 1H-MRS Spectroscopy in Young Healthy Men

CONTEXT

We previously demonstrated that insulin infusion altered metabolite concentrations in cerebral tissues assessed with proton magnetic resonance spectroscopy (1H-MRS) in young subjects with high insulin sensitivity, but not in those with low insulin sensitivity. Fat overload is an important factor leading to insulin resistance.

OBJECTIVE

The purpose of the current study was to examine the effect of elevated circulating free fatty acid (FFA) levels on metabolites in cerebral tissues assessed with 1H-MRS.

DESIGN

The study group comprised 10 young, healthy male subjects. 1H-MRS was performed at baseline and after 4-hour Intralipid (Fresenius Kabi)/heparin or saline infusions administered in random order. Voxels were positioned in the left frontal lobe, left temporal lobe, and hippocampus. The ratios of N-acetylaspartate (NAA), choline (Cho)-containing compounds, myo-inositol (mI), and glutamate/glutamine/γ-aminobutyric acid complex (Glx) to creatine (Cr) and nonsuppressed water signal were determined.

RESULTS

Intralipid/heparin infusion resulted in a significant increase in circulating FFAs (P < 0.0001). Significant changes in brain neurometabolite concentrations in response to Intralipid/heparin infusion were increases in frontal mI/Cr (P = 0.041) and mI/H2O (P = 0.037), decreases in frontal and hippocampal Glx/Cr (P = 0.018 and P = 0.015, respectively) and Glx/H2O (P = 0.03 and P = 0.067, respectively), and a decrease in hippocampal NAA/Cr (P = 0.007) and NAA/H2O (P = 0.019). No changes in neurometabolites were observed during the saline infusion.

CONCLUSIONS

Acute circulating FFA elevation influenced cerebral metabolites in healthy humans and lipid-induced insulin resistance could be partly responsible for these effects.

GSEA of mouse and human mitochondriomes reveals fatty acid oxidation in astrocytes

The prevalent view in neuroenergetics is that glucose is the main brain fuel, with neurons being mostly oxidative and astrocytes glycolytic. Evidence supporting that astrocyte mitochondria are functional has been overlooked. Here we sought to determine what is unique about astrocyte mitochondria by performing unbiased statistical comparisons of the mitochondriome in astrocytes and neurons. Using MitoCarta, a compendium of mitochondrial proteins, together with transcriptomes of mouse neurons and astrocytes, we generated cell-specific databases of nuclear genes encoding for mitochondrion proteins, ranked according to relative expression. Standard and in-house Gene Set Enrichment Analyses (GSEA) of five mouse transcriptomes revealed that genes encoding for enzymes involved in fatty acid oxidation (FAO) and amino acid catabolism are consistently more expressed in astrocytes than in neurons. FAO and oxidative-metabolism-related genes are also up-regulated in human cortical astrocytes versus the whole cortex, and in adult astrocytes versus fetal astrocytes. We thus present the first evidence of FAO in human astrocytes. Further, as shown in vitro, FAO coexists with glycolysis in astrocytes and is inhibited by glutamate. Altogether, these analyses provide arguments against the glucose-centered view of energy metabolism in astrocytes and reveal mitochondria as specialized organelles in these cells.

Mitochondria in the nervous system: From health to disease, Part I

In Part I of this Special Issue on "Mitochondria in the Nervous System: From Health to Disease", the editors bring together contributions from experts in brain mitochondrial research to provide an up-to-date overview of mitochondrial functioning in physiology and pathology. The issue provides cutting edge reviews on classical areas of mitochondrial biology that include energy substrate utilization, calcium handling, mitochondria-endoplasmic reticulum communication, and cell death regulation. Additional reviews and original research articles touch upon key mitochondrial defects seen across multiple neurodegenerative conditions, including fragmentation, loss of respiratory capacity, calcium overload, elevated reactive oxygen species generation, perturbed NAD⁺ metabolism, altered protein acetylation, and compromised mitophagy. Emerging links between the genetics of neurodegenerative disorders and disruption in mitochondrial function are discussed, and a new mouse model of Complex I deficiency is described. Finally, novel ways to rescue mitochondrial structure and function in acute and chronic brain injury are explored.