Increased neuronal glucose-6-phosphate dehydrogenase and sulfhydryl levels indicate reductive compensation to oxidative stress in Alzheimer disease

Reductive stress: The key pathway in metabolic disorders induced by overnutrition

BACKGROUND

The balance of redox states is crucial for maintaining physiological homeostasis. For decades, the focus has been mainly on the concept of oxidative stress, which is involved in the mechanism of almost all diseases. However, robust evidence has highlighted that reductive stress, the other side of the redox spectrum, plays a pivotal role in the development of various diseases, particularly those related to metabolism and cardiovascular health.

AIM OF REVIEW

In this review, we present an extensive array of evidence for the occurrence of reductive stress and its significant implications mainly in metabolic and cardiovascular diseases.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Reductive stress is defined as a shift in the cellular redox balance towards a more reduced state, characterized by an excess of endogenous reductants (such as NADH, NADPH, and GSH) over their oxidized counterparts (NAD+, NADP+, and GSSG). While oxidative stress has been the predominant mechanism studied in obesity, metabolic disorders, and cardiovascular diseases, growing evidence underscores the critical role of reductive stress. This review discusses how reductive stress contributes to metabolic and cardiovascular pathologies, emphasizing its effects on key cellular processes. For example, excessive NADH accumulation can disrupt mitochondrial function by impairing the electron transport chain, leading to decreased ATP production and increased production of reactive oxygen species. In the endoplasmic reticulum (ER), an excess of reductive equivalents hampers protein folding, triggering ER stress and activating the unfolded protein response, which can lead to insulin resistance and compromised cellular homeostasis. Furthermore, we explore how excessive antioxidant supplementation can exacerbate reductive stress by further shifting the redox balance, potentially undermining the beneficial effects of exercise, impairing cardiovascular health, and aggravating metabolic disorders, particularly in obese individuals. This growing body of evidence calls for a reevaluation of the role of reductive stress in disease pathogenesis and therapeutic interventions.

Delineation of the role of G6PD in Alzheimer's disease and potential enhancement through microfluidic and nanoparticle approaches

Alzheimer's disease (AD) is a neurodegenerative pathologic entity characterized by the abnormal presence of tau and macromolecular Aβ deposition that leads to the degeneration or death of neurons. In addition to that, glucose-6-phosphate dehydrogenase (G6PD) has a multifaceted role in the process of AD development, where it can be used as both a marker and a target. G6PD activity is dysregulated due to its contribution to oxidative stress, neuroinflammation, and neuronal death. In this context, the current review presents a vivid depiction of recent findings on the relationship between AD progression and changes in the expression or activity of G6PD. The efficacy of the proposed G6PD-based therapeutics has been demonstrated in multiple studies using AD mouse models as representative animal model systems for cognitive decline and neurodegeneration associated with this disease. Innovative therapeutic insights are made for the boosting of G6PD activity via novel innovative nanotechnology and microfluidics tools in drug administration technology. Such approaches provide innovative methods of surpassing the blood-brain barrier, targeting step-by-step specific neural pathways, and overcoming biochemical disturbances that accompany AD. Using different nanoparticles loaded with G6DP to target specific organs, e.g., G6DP-loaded liposomes, enhances BBB penetration and brain distribution of G6DP. Many nanoparticles, which are used for different purposes, are briefly discussed in the paper. Such methods to mimic BBB on organs on-chip offer precise disease modeling and drug testing using microfluidic chips, requiring lower sample amounts and producing faster findings compared to conventional techniques. There are other contributions to microfluid in AD that are discussed briefly. However, there are some limitations accompanying microfluidics that need to be worked on to be used for AD. This study aims to bridge the gap in understanding AD with the synergistic use of promising technologies; microfluid and nanotechnology for future advancements.

Analysis of the Effects of Pentose Phosphate Pathway Inhibition on the Generation of Reactive Oxygen Species and Epileptiform Activity in Hippocampal Slices

The pentose phosphate pathway (PPP) is one of three major pathways involved in glucose metabolism, which is regulated by glucose-6-phosphate dehydrogenase (G6PD) controls NADPH formation. NADPH, in turn, regulates the balance of oxidative stress and reactive oxygen species (ROS) levels. G6PD dysfunction, affecting the PPP, is implicated in neurological disorders, including epilepsy. However, PPP's role in epileptogenesis and ROS production during epileptic activity remains unclear. To clarify these points, we conducted electrophysiological and imaging analyses on mouse hippocampal brain slices. Using the specific G6PD inhibitor G6PDi-1, we assessed its effects on mouse hippocampal slices, examining intracellular ROS, glucose/oxygen consumption, the NAD(P)H level and ROS production during synaptic stimulation and in the 4AP epilepsy model. G6PDi-1 increased basal intracellular ROS levels and reduced synaptically induced glucose consumption but had no impact on baselevel of NAD(P)H and ROS production from synaptic stimulation. In the 4AP model, G6PDi-1 did not significantly alter spontaneous seizure frequency or H2O2 release amplitude but increased the frequency and peak amplitude of interictal events. These findings suggest that short-term PPP inhibition has a minimal impact on synaptic circuit activity.

Comprehensive Analysis of Metabolites in Postmortem Brains of Patients with Alzheimer's Disease

BACKGROUND

Disturbed metabolism has been proposed as being involved in the pathogenesis of Alzheimer's disease (AD), and more evidence from human AD brains is required.

OBJECTIVE

In this study, we attempted to identify or confirm modulations in the levels of metabolites associated with AD in postmortem AD brains.

METHODS

We performed metabolomics analyses using a gas chromatography mass spectrometry system in postmortem brains of patients with confirmed AD, patients with CERAD score B, and control subjects.

RESULTS

Impaired phosphorylation of glucose and elevation of several tricarboxylic acid (TCA) metabolites, except citrate, were observed and the degree of impaired phosphorylation and elevation in the levels of the TCA cycle metabolites were negatively and positively correlated, respectively, with the clinical phenotypes of AD. The levels of uronic acid pathway metabolites were modulated in AD and correlated positively with the amyloid-β content. The associations of nucleic acid synthesis and amino acid metabolites with AD depended on the kinds of metabolites; in particular, the contents of ribose 5-phosphate, serine and glycine were negatively correlated, while those of ureidosuccinic acid and indole-3-acetic acid were positively modulated in AD. Comprehensive statistical analyses suggested that alterations in the inositol pathway were most closely associated with AD.

CONCLUSIONS

The present study revealed many novel associations between metabolites and AD, suggesting that some of these might serve as novel potential therapeutic targets for AD.

The role of TIGAR in nervous system diseases

TP53-induced glycolysis and apoptosis regulator (TIGAR) mainly regulates pentose phosphate pathway by inhibiting glycolysis, so as to synthesize ribose required by DNA, promote DNA damage repair and cell proliferation, maintain cell homeostasis and avoid body injury. Its physiological functions include anti-oxidative stress, reducing inflammation, maintaining mitochondrial function, inhibiting apoptosis, reducing autophagy etc. This paper reviews the research of TIGAR in neurological diseases, including stroke, Parkinson’s disease (PD), Alzheimer’s disease (AD), seizures and brain tumors, aiming to provide reference for the development of new therapeutic targets.

Redox signaling and metabolism in Alzheimer's disease

Reduction and oxidation reactions are essential for biochemical processes. They are part of metabolic pathways and signal transduction. Reactive oxygen species (ROS) as second messengers and oxidative modifications of cysteinyl (Cys) residues are key to transduce and translate intracellular and intercellular signals. Dysregulation of cellular redox signaling is known as oxidative distress, which has been linked to various pathologies, including neurodegeneration. Alzheimer's disease (AD) is a neurodegenerative pathology linked to both, abnormal amyloid precursor protein (APP) processing, generating Aβ peptide, and Tau hyperphosphorylation and aggregation. Signs of oxidative distress in AD include: increase of ROS (H2O2, O2 •-), decrease of the levels or activities of antioxidant enzymes, abnormal oxidation of macromolecules related to elevated Aβ production, and changes in mitochondrial homeostasis linked to Tau phosphorylation. Interestingly, Cys residues present in APP form disulfide bonds that are important for intermolecular interactions and might be involved in the aggregation of Aβ. Moreover, two Cys residues in some Tau isoforms have been shown to be essential for Tau stabilization and its interaction with microtubules. Future research will show the complexities of Tau, its interactome, and the role that Cys residues play in the progression of AD. The specific modification of cysteinyl residues in redox signaling is also tightly connected to the regulation of various metabolic pathways. Many of these pathways have been found to be altered in AD, even at very early stages. In order to analyze the complex changes and underlying mechanisms, several AD models have been developed, including animal models, 2D and 3D cell culture, and ex-vivo studies of patient samples. The use of these models along with innovative, new redox analysis techniques are key to further understand the importance of the redox component in Alzheimer's disease and the identification of new therapeutic targets in the future.

Neuropathologic Changes Provide Insights into Key Mechanisms of Alzheimer Disease and Related Dementia

Alzheimer disease (AD) is a chronic disease characterized by a progressive decline in memory and cognition. AD progression is closely correlated with neuropathologic changes and accumulation of the two main hallmark lesions, senile plaques and neurofibrillary tangles. Nevertheless, deciphering the complex biological aspects of AD requires looking for the neuropathologic changes not only as the cause but also as the collective response to a disease process that is essential to maintaining life during aging but ultimately generates a nonfunctional brain. Chronic conditions, such as AD, represent a new homeostatic balance or disease state, where the organism responds or adapts to maintain life. The pathologic diagnosis of AD still remains the gold standard for precise diagnosis of dementia, commonly in conjunction with cognitive-memory tests and brain image scans. Herein, we present a general overview of the main neuropathologic hallmarks and features of AD and related dementia, revealing the key biological and functional changes as potential drivers of age-dependent brain failure related to AD. The present work reflects some of the main ideas presented during the American Society for Investigative Pathology Rous-Whipple Award Lecture 2021.

Role of amino acid oxidation and protein unfolding in peroxyl radical and peroxynitrite-induced inactivation of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides

The mechanisms underlying the inactivation of Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase (G6PDH) induced by peroxyl radicals (ROO^●) and peroxynitrite (ONOO^-), were explored. G6PDH was incubated with AAPH (2,2' -azobis(2-methylpropionamidine)dihydrochloride), used as ROO^● source, and ONOO^-. Enzymatic activity was assessed by NADPH generation, while oxidative modifications were analyzed by gel electrophoresis and liquid chromatography (LC) with fluorescence and mass detection. Changes in protein conformation were studied by circular dichroism (CD) and binding of the fluorescent dye ANS (1-anilinonaphthalene-8-sulfonic acid). Incubation of G6PDH (54.4 μM) with 60 mM AAPH showed an initial phase without significant changes in enzymatic activity, followed by a secondary time-dependent continuous decrease in activity to ∼59% of the initial level after 90 min. ONOO^- induced a significant and concentration-dependent loss of G6PDH activity with ∼46% of the initial activity lost on treatment with 1.5 mM ONOO^-. CD and ANS fluorescence indicated changes in G6PDH secondary structure with exposure of hydrophobic sites on exposure to ROO^●, but not ONOO^-. LC-MS analysis provided evidence for ONOO^--mediated oxidation of Tyr, Met and Trp residues, with damage to critical Met and Tyr residues underlying enzyme inactivation, but without effects on the native (dimeric) state of the protein. In contrast, studies using chloramine T, a specific oxidant of Met, provided evidence that oxidation of specific Met and Trp residues and concomitant protein unfolding, loss of dimer structure and protein aggregation are involved in G6PDH inactivation by ROO^●. These two oxidant systems therefore have markedly different effects on G6PDH structure and activity.

COVID-19 in G6PD-deficient patients, oxidative stress, and neuropathology

Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme that regulates energy metabolism mainly through the pentose phosphate pathway (PPP). It is well known that this enzyme participates in the antioxidant/oxidant balance via the synthesis of energy-rich molecules: nicotinamide adenine dinucleotide phosphate reduced (NADPH), the reduced form of flavin adenine dinucleotide (FADH) and glutathione (GSH), controlling reactive oxygen species generation. Coronavirus disease 19 (COVID-19), induced by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is considered a public health problem which has caused approximately 4.5 million deaths since December 2019. In relation to the role of G6PD in COVID-19 development, it is known from the existing literature that G6PD-deficient patients infected with SARS-CoV-2 are more susceptible to thrombosis and hemolysis, suggesting that G6PD deficiency facilitates infection by SARS-CoV-2. In relation to G6PD and neuropathology, it has been observed that deficiency of this enzyme is also present with an increase in oxidative markers. In relation to the role of G6PD and the neurological manifestations of COVID-19, it has been reported that the enzymatic deficiency in patients infected with SARS-CoV-2 exacerbates the disease, and, in some clinical reports, an increase in hemolysis and thrombosis was observed when patients were treated with hydroxychloroquine (OH-CQ), a drug with oxidative properties. In the present work, we summarize the evidence of the role of G6PD in COVID-19 and its possible role in the generation of oxidative stress and glucose metabolism deficits and inflammation present in this respiratory disease and its progression including neurological manifestations.

Age- and Sex-Associated Glucose Metabolism Decline in a Mouse Model of Alzheimer’s Disease

Background: Alzheimer’s disease (AD) is characterized by a high etiological and clinical heterogeneity, which has obscured the diagnostic and treatment efficacy, as well as limited the development of potential drugs. Sex differences are among the risk factors that contribute to the variability of disease manifestation. Unlike men, women are at greater risk of developing AD and suffer from higher cognitive deterioration, together with important changes in pathological features. Alterations in glucose metabolism are emerging as a key player in the pathogenesis of AD, which appear even decades before the presence of clinical symptoms. Objective: We aimed to study whether AD-related sex differences influence glucose metabolism. Methods: We used male and female APPswe/PS1dE9 (APP/PS1) transgenic mice of different ages to examine glucose metabolism effects on AD development. Results: Our analysis suggests an age-dependent decline of metabolic responses, cognitive functions, and brain energy homeostasis, together with an increase of Aβ levels in both males and females APP/PS1 mice. The administration of Andrographolide (Andro), an anti-inflammatory and anti-diabetic compound, was able to restore several metabolic disturbances, including the glycolytic and the pentose phosphate pathway fluxes, ATP levels, AMPKα activity, and Glut3 expression in 8-month-old mice, independent of the sex, while rescuing these abnormalities only in older females. Similarly, Andro also prevented Aβ accumulation and cognitive decline in all but old males. Conclusion: Our study provides insight into the heterogeneity of the disease and supports the use of Andro as a potential drug to promote personalized medicine in AD.

Metabolic Features of Brain Function with Relevance to Clinical Features of Alzheimer and Parkinson Diseases

Brain metabolism is comprised in Alzheimer’s disease (AD) and Parkinson’s disease (PD). Since the brain primarily relies on metabolism of glucose, ketone bodies, and amino acids, aspects of these metabolic processes in these disorders—and particularly how these altered metabolic processes are related to oxidative and/or nitrosative stress and the resulting damaged targets—are reviewed in this paper. Greater understanding of the decreased functions in brain metabolism in AD and PD is posited to lead to potentially important therapeutic strategies to address both of these disorders, which cause relatively long-lasting decreased quality of life in patients.

Redox Imbalance Associates with Clinical Worsening in Spinocerebellar Ataxia Type 2

BACKGROUND

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disease presenting with redox imbalance. However, the nature and implications of redox imbalance in SCA2 physiopathology have not been fully understood.

OBJECTIVE

The objective of this study is to assess the redox imbalance and its association with disease severity in SCA2 mutation carriers.

METHODS

A case-control study was conducted involving molecularly confirmed SCA2 patients, presymptomatic individuals, and healthy controls. Several antioxidant parameters were assessed, including serum thiol concentration and the superoxide dismutase, catalase, and glutathione S-transferase enzymatic activities. Also, several prooxidant parameters were evaluated, including thiobarbituric acid-reactive species and protein carbonyl concentrations. Damage, protective, and OXY scores were computed. Clinical correlates were established.

RESULTS

Significant differences were found between comparison groups for redox markers, including protein carbonyl concentration (F = 3.30; p = 0.041), glutathione S-transferase activity (F = 4.88; p = 0.009), and damage (F = 3.20; p = 0.045), protection (F = 12.75; p < 0.001), and OXY (F = 7.29; p = 0.001) scores. Protein carbonyl concentration was positively correlated with CAG repeat length (r = 0.27; p = 0.022), while both protein carbonyl concentration (r = -0.27; p = 0.018) and OXY score (r = -0.25; p = 0.013) were inversely correlated to the disease duration. Increasing levels of antioxidants and decreasing levels of prooxidant parameters were associated with clinical worsening.

CONCLUSIONS

There is a disruption of redox balance in SCA2 mutation carriers which depends on the disease stage. Besides, redox changes associate with markers of disease severity, suggesting a link between disruption of redox balance and SCA2 physiopathology.

Metabolic Dysregulation Contributes to the Progression of Alzheimer's Disease

Alzheimer's disease (AD) is an incurable neurodegenerative disease. Numerous studies have demonstrated a critical role for dysregulated glucose metabolism in its pathogenesis. In this review, we summarize metabolic alterations in aging brain and AD-related metabolic deficits associated with glucose metabolism dysregulation, glycolysis dysfunction, tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS) deficits, and pentose phosphate pathway impairment. Additionally, we discuss recent treatment strategies targeting metabolic defects in AD, including their limitations, in an effort to encourage the development of novel therapeutic strategies.

Reductive stress promotes protein aggregation and impairs neurogenesis

Redox homeostasis regulates key cellular signaling in both physiology and pathology. While perturbations result in shifting the redox homeostasis towards oxidative stress are well documented, the influence of reductive stress (RS) in neurodegenerative diseases and its mechanisms are unknown. Here, we postulate that a redox shift towards the reductive arm (through the activation of Nrf2 signaling) will damage neurons and impair neurogenesis. In proliferating and differentiating neuroblastoma (Neuro 2a/N2a) cells, sulforaphane-mediated Nrf2 activation resulted in increased transcription/translation of antioxidants and glutathione (GSH) production along with significantly declined ROS in a dose-dependent manner leading to a reductive-redox state (i.e. RS). Interestingly, this resulted in endoplasmic reticulum (ER) stress leading to subsequent protein aggregation/proteotoxicity in neuroblastoma cells. Under RS, we also observed elevated Tau/α-synuclein and their co-localization with other protein aggregates in these cells. Surprisingly, we noticed that acute RS impaired neurogenesis as evidenced from reduced neurite outgrowth/length. Furthermore, maintaining the cells in a sustained RS condition (for five consecutive generations) dramatically reduced their differentiation and prevented the formation of axons (p < 0.05). This impairment in RS mediated neurogenesis occurs through the alteration of Tau dynamics i.e. RS activates the pathogenic GSK3β/Tau cascade thereby promoting the phosphorylation of Tau leading to proteotoxicity. Of note, intermittent withdrawal of sulforaphane from these cells suppressed the proteotoxic insult and re-activated the differentiation process. Overall, this results suggest that either acute or chronic RS could hamper neurogenesis through GSK3β/TAU signaling and proteotoxicity. Therefore, investigations identifying novel redox mechanisms impacting proteostasis are crucial to preserve neuronal health.

Reductive Reprogramming: A Not-So-Radical Hypothesis of Neurodegeneration Linking Redox Perturbations to Neuroinflammation and Excitotoxicity

Free radical-mediated oxidative stress, neuroinflammation, and excitotoxicity have long been considered insults relevant to the progression of Alzheimer's disease and other aging-related neurodegenerative disorders (NDD). Among these phenomena, the significance of oxidative stress and, more generally, redox perturbations, for NDD remain ill-defined and unsubstantiated. Here, I argue that (i) free radical-mediated oxidations of biomolecules can be dissociated from the progression of NDD, (ii) oxidative stress fails as a descriptor of cellular redox states under conditions relevant to disease, and (iii) aberrant upregulation of compensatory reducing activities in neural cells, resulting in reductive shifts in thiol-based redox potentials, may be an overlooked and paradoxical contributor to disease progression. In particular, I summarize evidence which supports the view that reductive shifts in the extracellular space can occur in response to oxidant and inflammatory signals and that these have the potential to reduce putative regulatory disulfide bonds in exofacial domains of the N-methyl-D-aspartate receptor, leading potentially to aberrant increases in neuronal excitability and, if sustained, excitotoxicity. The novel reductive reprogramming hypothesis of neurodegeneration presented here provides an alternative view of redox perturbations in NDD and links these to both neuroinflammation and excitotoxicity.

TIGAR suppresses seizures induced by kainic acid through inhibiting oxidative stress and neuronal apoptosis

TP53-induced glycolysis and apoptosis regulator (TIGAR) activates the pentose phosphate pathway (PPP), which feeds reduced nicotinamide adenine dinucleotide phosphate (NADPH) to the antioxidant glutathione pathway. Oxidative stress-induced neuronal apoptosis is the pathological basis of several neurological disorders, including epilepsy. To determine the potential anti-epileptic action TIGAR in a rodent kainic acid (KA)-induced seizure model. Seizures were induced by the intra-cerebroventricular injection of KA, followed by injection of empty or TIGAR-expressing lentiviral vectors. Immunofluorescence was used to detect the localization of TIGAR in the cortices and hippocampi, and the expression levels of relevant proteins were determined by Western blotting. Oxidative stress-related markers were detected using commercially available kits. Neuronal apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining. TIGAR were mainly expressed in the neurons and rarely located in the astrocytes, and increased in the cortices and hippocampi of KA-treated rats in a time-dependent manner. Lentivirus-mediated TIGAR overexpression significantly decreased the oxidative stress and neuronal apoptosis induced by KA, resulting in prolonged seizure latency and lower Racine scores. Our findings indicate that TIGAR has anti-epileptic, anti-oxidant and anti-apoptotic effects, and is therefore a promising therapeutictarget for epilepsy.

Neuroprotection by glucose-6-phosphate dehydrogenase and the pentose phosphate pathway

Glucose-6-phosphate dehydrogenase (G6PD), the rate limiting enzyme that channels glucose catabolism from glycolysis into the pentose phosphate pathway (PPP), is vital for the production of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in cells. NADPH is in turn a substrate for glutathione reductase, which reduces oxidized glutathione disulfide to sulfhydryl glutathione. Best known for inherited deficiencies underlying acute hemolytic anemia due to elevated oxidative stress by food or medication, G6PD, and PPP activation have been associated with neuroprotection. Recent works have now provided more definitive evidence for G6PD's protective role in ischemic brain injury and strengthened its links to neurodegeneration. In Drosophila models, improved proteostasis and lifespan extension result from an increased PPP flux due to G6PD induction, which is phenocopied by transgenic overexpression of G6PD in neurons. Moderate transgenic expression of G6PD was also shown to improve healthspan in mouse. Here, the deciphered and implicated roles of G6PD and PPP in protection against brain injury, neurodegenerative diseases, and in healthspan/lifespan extensions are discussed together with an important caveat, namely NADPH oxidase (NOX) activity and the oxidative stress generated by the latter. Activation of G6PD with selective inhibition of NOX activity could be a viable neuroprotective strategy for brain injury, disease, and aging.

Phosphotyrosine profiling of human cerebrospinal fluid

Background

Cerebrospinal fluid (CSF) is an important source of potential biomarkers that affect the brain. Biomarkers for neurodegenerative disorders are needed to assist in diagnosis, monitoring disease progression and evaluating efficacy of therapies. Recent studies have demonstrated the involvement of tyrosine kinases in neuronal cell death. Thus, neurodegeneration in the brain is related to altered tyrosine phosphorylation of proteins in the brain and identification of abnormally phosphorylated tyrosine peptides in CSF has the potential to ascertain candidate biomarkers for neurodegenerative disorders.

Methods

In this study, we used an antibody-based tyrosine phosphopeptide enrichment method coupled with high resolution Orbitrap Fusion Tribrid Lumos Fourier transform mass spectrometer to catalog tyrosine phosphorylated peptides from cerebrospinal fluid. The subset of identified tyrosine phosphorylated peptides was also validated using parallel reaction monitoring (PRM)-based targeted approach.

Results

To date, there are no published studies on global profiling of phosphotyrosine modifications of CSF proteins. We carried out phosphotyrosine profiling of CSF using an anti-phosphotyrosine antibody-based enrichment and analysis using high resolution Orbitrap Fusion Lumos mass spectrometer. We identified 111 phosphotyrosine peptides mapping to 66 proteins, which included 24 proteins which have not been identified in CSF previously. We then validated a set of 5 tyrosine phosphorylated peptides in an independent set of CSF samples from cognitively normal subjects, using a PRM-based targeted approach.

Conclusions

The findings from this deep phosphotyrosine profiling of CSF samples have the potential to identify novel disease-related phosphotyrosine-containing peptides in CSF.

Electronic supplementary material

The online version of this article (10.1186/s12014-018-9205-1) contains supplementary material, which is available to authorized users.

Mitochondrial function and autophagy: integrating proteotoxic, redox, and metabolic stress in Parkinson's disease

Parkinson's disease (PD) is a movement disorder with widespread neurodegeneration in the brain. Significant oxidative, reductive, metabolic, and proteotoxic alterations have been observed in PD postmortem brains. The alterations of mitochondrial function resulting in decreased bioenergetic health is important and needs to be further examined to help develop biomarkers for PD severity and prognosis. It is now becoming clear that multiple hits on metabolic and signaling pathways are likely to exacerbate PD pathogenesis. Indeed, data obtained from genetic and genome association studies have implicated interactive contributions of genes controlling protein quality control and metabolism. For example, loss of key proteins that are responsible for clearance of dysfunctional mitochondria through a process called mitophagy has been found to cause PD, and a significant proportion of genes associated with PD encode proteins involved in the autophagy-lysosomal pathway. In this review, we highlight the evidence for the targeting of mitochondria by proteotoxic, redox and metabolic stress, and the role autophagic surveillance in maintenance of mitochondrial quality. Furthermore, we summarize the role of α-synuclein, leucine-rich repeat kinase 2, and tau in modulating mitochondrial function and autophagy. Among the stressors that can overwhelm the mitochondrial quality control mechanisms, we will discuss 4-hydroxynonenal and nitric oxide. The impact of autophagy is context depend and as such can have both beneficial and detrimental effects. Furthermore, we highlight the potential of targeting mitochondria and autophagic function as an integrated therapeutic strategy and the emerging contribution of the microbiome to PD susceptibility.

Mathematical analysis of the influence of brain metabolism on the BOLD signal in Alzheimer's disease

Blood oxygen level-dependent functional magnetic resonance imaging (BOLD-fMRI) is a standard clinical tool for the detection of brain activation. In Alzheimer's disease (AD), task-related and resting state fMRI have been used to detect brain dysfunction. It has been shown that the shape of the BOLD response is affected in early AD. To correctly interpret these changes, the mechanisms responsible for the observed behaviour need to be known. The parameters of the canonical hemodynamic response function (HRF) commonly used in the analysis of fMRI data have no direct biological interpretation and cannot be used to answer this question. We here present a model that allows relating AD-specific changes in the BOLD shape to changes in the underlying energy metabolism. According to our findings, the classic view that differences in the BOLD shape are only attributed to changes in strength and duration of the stimulus does not hold. Instead, peak height, peak timing and full width at half maximum are sensitive to changes in the reaction rate of several metabolic reactions. Our systems-theoretic approach allows the use of patient-specific clinical data to predict dementia-driven changes in the HRF, which can be used to improve the results of fMRI analyses in AD patients.

Comparative analysis of cerebrospinal fluid metabolites in Alzheimer's disease and idiopathic normal pressure hydrocephalus in a Japanese cohort

Background

Alzheimer’s disease (AD) is a most common dementia in elderly people. Since AD symptoms resemble those of other neurodegenerative diseases, including idiopathic normal pressure hydrocephalus (iNPH), it is difficult to distinguish AD from iNPH for a precise and early diagnosis. iNPH is caused by the accumulation of cerebrospinal fluid (CSF) and involves gait disturbance, urinary incontinence, and dementia. iNPH is treatable with shunt operation which removes accumulated CSF from the brain ventricles.

Methods

We performed metabolomic analysis in the CSF of patients with AD and iNPH with capillary electrophoresis-mass spectrometry. We assessed metabolites to discriminate between AD and iNPH with Welch’s t-test, receiver operating characteristic (ROC) curve analysis, and multiple logistic regression analysis.

Results

We found significant increased levels of glycerate and N-acetylneuraminate and significant decreased levels of serine and 2-hydroxybutyrate in the CSF of patients with AD compared to the CSF of patients with iNPH. The ROC curve analysis with these four metabolites showed that the area under the ROC curve was 0.90, indicating good discrimination between AD and iNPH.

Conclusions

This study identified four metabolites that could possibly discriminate between AD and iNPH, which previous research has shown are closely related to the risk factors, pathogenesis, and symptoms of AD. Analyzing pathway-specific metabolites in the CSF of patients with AD may further elucidate the mechanism and pathogenesis of AD.

Electronic supplementary material

The online version of this article (10.1186/s40364-018-0119-x) contains supplementary material, which is available to authorized users.

Evaluation of Metabolic and Synaptic Dysfunction Hypotheses of Alzheimer's Disease (AD): A Meta-Analysis of CSF Markers

BACKGROUND

Alzheimer's disease (AD) is currently incurable and a majority of investigational drugs have failed clinical trials. One explanation for this failure may be the invalidity of hypotheses focusing on amyloid to explain AD pathogenesis. Recently, hypotheses which are centered on synaptic and metabolic dysfunction are increasingly implicated in AD.

OBJECTIVE

Evaluate AD hypotheses by comparing neurotransmitter and metabolite marker concentrations in normal versus AD CSF.

METHODS

Meta-analysis allows for statistical comparison of pooled, existing cerebrospinal fluid (CSF) marker data extracted from multiple publications, to obtain a more reliable estimate of concentrations. This method also provides a unique opportunity to rapidly validate AD hypotheses using the resulting CSF concentration data. Hubmed, Pubmed and Google Scholar were comprehensively searched for published English articles, without date restrictions, for the keywords "AD", "CSF", and "human" plus markers selected for synaptic and metabolic pathways. Synaptic markers were acetylcholine, gamma-aminobutyric acid (GABA), glutamine, and glycine. Metabolic markers were glutathione, glucose, lactate, pyruvate, and 8 other amino acids. Only studies that measured markers in AD and controls (Ctl), provided means, standard errors/deviation, and subject numbers were included. Data were extracted by six authors and reviewed by two others for accuracy. Data were pooled using ratio of means (RoM of AD/Ctl) and random effects meta-analysis using Cochrane Collaboration's Review Manager software.

RESULTS

Of the 435 identified publications, after exclusion and removal of duplicates, 35 articles were included comprising a total of 605 AD patients and 585 controls. The following markers of synaptic and metabolic pathways were significantly changed in AD/controls: acetylcholine (RoM 0.36, 95% CI 0.24-0.53, p<0.00001), GABA (0.74, 0.58-0.94, p<0.01), pyruvate (0.48, 0.24-0.94, p=0.03), glutathione (1.11, 1.01- 1.21, p=0.03), alanine (1.10, 0.98-1.23, p=0.09), and lower levels of significance for lactate (1.2, 1.00-1.47, p=0.05). Of note, CSF glucose and glutamate levels in AD were not significantly different than that of the controls.

CONCLUSION

This study provides proof of concept for the use of meta-analysis validation of AD hypotheses, specifically via robust evidence for the cholinergic hypothesis of AD. Our data disagree with the other synaptic hypotheses of glutamate excitotoxicity and GABAergic resistance to neurodegeneration, given observed unchanged glutamate levels and decreased GABA levels. With regards to metabolic hypotheses, the data supported upregulation of anaerobic glycolysis, pentose phosphate pathway (glutathione), and anaplerosis of the tricarboxylic acid cycle using glutamate. Future applications of meta-analysis indicate the possibility of further in silico evaluation and generation of novel hypotheses in the AD field.

Membrane Aging as the Real Culprit of Alzheimer's Disease: Modification of a Hypothesis

Our previous studies proposed that Alzheimer's disease (AD) is a metabolic disorder and hypothesized that abnormal brain glucose metabolism inducing multiple pathophysiological cascades contributes to AD pathogenesis. Aging is one of the great significant risk factors for AD. Membrane aging is first prone to affect the function and structure of the brain by impairing glucose metabolism. We presume that risk factors of AD, including genetic factors (e.g., the apolipoprotein E ε4 allele and genetic mutations) and non-genetic factors (such as fat, diabetes, and cardiac failure) accelerate biomembrane aging and lead to the onset and development of the disease. In this review, we further modify our previous hypothesis to demonstrate "membrane aging" as an initial pathogenic factor that results in functional and structural alterations of membranes and, consequently, glucose hypometabolism and multiple pathophysiological cascades.

Reductive Stress in Inflammation-Associated Diseases and the Pro-Oxidant Effect of Antioxidant Agents

Abstract: Reductive stress (RS) is the counterpart oxidative stress (OS), and can occur in response to conditions that shift the redox balance of important biological redox couples, such as the NAD⁺/NADH, NADP⁺/NADPH, and GSH/GSSG, to a more reducing state. Overexpression of antioxidant enzymatic systems leads to excess reducing equivalents that can deplete reactive oxidative species, driving the cells to RS. A feedback regulation is established in which chronic RS induces OS, which in turn, stimulates again RS. Excess reducing equivalents may regulate cellular signaling pathways, modify transcriptional activity, induce alterations in the formation of disulfide bonds in proteins, reduce mitochondrial function, decrease cellular metabolism, and thus, contribute to the development of some diseases in which NF-κB, a redox-sensitive transcription factor, participates. Here, we described the diseases in which an inflammatory condition is associated to RS, and where delayed folding, disordered transport, failed oxidation, and aggregation are found. Some of these diseases are aggregation protein cardiomyopathy, hypertrophic cardiomyopathy, muscular dystrophy, pulmonary hypertension, rheumatoid arthritis, Alzheimer's disease, and metabolic syndrome, among others. Moreover, chronic consumption of antioxidant supplements, such as vitamins and/or flavonoids, may have pro-oxidant effects that may alter the redox cellular equilibrium and contribute to RS, even diminishing life expectancy.

Glucose 6 phosphatase dehydrogenase (G6PD) and neurodegenerative disorders: Mapping diagnostic and therapeutic opportunities

Glucose 6 phosphate dehydrogenase (G6PD) is a key and rate limiting enzyme in the pentose phosphate pathway (PPP). The physiological significance of enzyme is providing reduced energy to specific cells like erythrocyte by maintaining co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH). There are preponderance research findings that demonstrate the enzyme (G6PD) role in the energy balance, and it is associated with blood-related diseases and disorders, primarily the anemia resulted from G6PD deficiency. The X-linked genetic deficiency of G6PD and associated non-immune hemolytic anemia have been studied widely across the globe. Recent advancement in biology, more precisely neuroscience has revealed that G6PD is centrally involved in many neurological and neurodegenerative disorders. The neuroprotective role of the enzyme (G6PD) has also been established, as well as the potential of G6PD in oxidative damage and the Reactive Oxygen Species (ROS) produced in cerebral ischemia. Though G6PD deficiency remains a global health issue, however, a paradigm shift in research focusing the potential of the enzyme in neurological and neurodegenerative disorders will surely open a new avenue in diagnostics and enzyme therapeutics. Here, in this study, more emphasis was made on exploring the role of G6PD in neurological and inflammatory disorders as well as non-immune hemolytic anemia, thus providing diagnostic and therapeutic opportunities.

Multidirectional inhibition of cortico-hippocampal neurodegeneration by kolaviron treatment in rats

Earliest signs of neurodegenerative cascades in the course of Alzheimer's disease (AD) are seen within the prefrontal cortex (PFC) and hippocampus, with pathological evidences in both cortical structures correlating with manifestation of behavioural and cognitive deficits. Despite the enormous problems associated with AD's clinical manifestations in sufferers, therapeutic advances for the disorder are still very limited. Therefore, this study examined cortico-hippocampal microstructures in models of AD, and evaluated the possible beneficial roles of kolaviron (Kv)-a biflavonoid complex in rats. Nine groups of rats were orally exposed to sodium azide (NaN₃) or aluminium chloride (AlCl₃) solely or in different combinations with Kv. Sequel to sacrifice and transcardial perfusion (using buffered saline then 4% paraformaldehyde), PFC and hippocampal tissues were harvested and processed for: spectrophotometric assays of oxidative stress and neuronal bioenergetics parameters, histological demonstration of cytoarchitecture and immunohistochemical evaluation of astrocytes and neuronal cytoskeleton. Results showed alterations in mitochondrial functions, which led to compromised neuronal antioxidant system, dysfunctional neural bioenergetics, hypertrophic astrogliosis, cytoskeletal dysregulation and neuronal death within the PFC and hippocampus. These degenerative events were associated with NaN₃ and AlCl₃ toxicity in rats. Furthermore, Kv inhibited cortico-hippocampal degeneration through multiple mechanisms that primarily involved halting of biochemical cascades that activate proteases which destroy molecules expedient for cell survival, and others that mediate a program of cell suicide in neuronal apoptosis. In conclusion, Kv showed important neuroprotective roles within cortico-hippocampal cells through multiple mechanisms, and particularly has prominent prophylactic activity than regenerative potentials.

Alteration of high-density lipoprotein functionality in Alzheimer’s disease patients

The aims of the present study were to determine whether high-density lipoprotein (HDL) functionality-mediated cholesterol efflux is altered in Alzheimer’s disease and to investigate the role and effect of amyloid-beta (Aβ) in the regulation of the anti-atherogenic activity of HDL. Eighty-seven elderly subjects were recruited, of whom 27 were healthy, 27 had mild cognitive impairment (MCI), and 33 had mild Alzheimer’s disease (mAD). Our results showed that total cholesterol levels are negatively correlated with the Mini-Mental State Examination (MMSE) score (r = –0.2602, p = 0.0182). HDL from the mAD patients was less efficient at mediating cholesterol efflux from J774 macrophages (p < 0.05) than HDL from the healthy subjects and MCI patients. While HDL from the MCI patients was also less efficient at mediating cholesterol efflux than HDL from the healthy subjects, the difference was not significant. Interestingly, the difference between the healthy subjects and the MCI and mAD patients with respect to the capacity of HDL to mediate cholesterol efflux disappeared when ATP-binding cassette transporter A1 (ABCA1)-enriched J774 macrophages were used. HDL fluidity was significantly inversely correlated with the MMSE scores (r = –0.4137, p < 0.009). In vitro measurements of cholesterol efflux using J774 macrophages showed that neither Aβ1-40nor Aβ1-42stimulate cholesterol efflux from unenriched J774 macrophages in basal or ABCA1-enriched J774 macrophages.

The role of glucose-6-phosphate dehydrogenase in adipose tissue inflammation in obesity

Obesity is closely associated with metabolic diseases including type 2 diabetes. One hallmark characteristics of obesity is chronic inflammation that is coordinately controlled by complex signaling networks in adipose tissues. Compelling evidence indicates that reactive oxygen species (ROS) and its related signaling pathways play crucial roles in the progression of chronic inflammation in obesity. The pentose phosphate pathway (PPP) is an anabolic pathway that utilizes the glucoses to generate molecular building blocks and reducing equivalents in the form of NADPH. In particular, NADPH acts as one of the key modulators in the control of ROS through providing an electron for both ROS generation and scavenging. Recently, we have reported that glucose-6-phosphate dehydrogenase (G6PD), a rate-limiting enzyme of the PPP, is implicated in adipose tissue inflammation and systemic insulin resistance in obesity. Mechanistically, G6PD potentiates generation of ROS that augments pro-inflammatory responses in adipose tissue macrophages, leading to systemic insulin resistance. Here, we provide an overview of cell type- specific roles of G6PD in the regulation of ROS balance as well as additional details on the significance of G6PD that contributes to pro-oxidant NADPH generation in obesity-related chronic inflammation and insulin resistance.

Glucose-6-phosphate dehydrogenase a novel hope on a blood-based diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is a multi-factorial neurodegenerative disorder that numerous factors have key properties in the development of this proteopathy. Glucose-6-phosphate dehydrogenase (G6PD) is the most common form of enzymopathy. We have examined G6PD enzyme activity levels in the serum of newly diagnosed AD patients compared with control subjects without dementia from the both sexes. Serum G6PD levels were found to be significantly higher (approximately two times) in AD patients compared to control geriatric subjects in both sexes. We have concluded that G6PD seems to play an integral role in the progress and/or prevention of AD.

Bioenergetics and redox adaptations of astrocytes to neuronal activity

Neuronal activity is a high‐energy demanding process recruiting all neural cells that adapt their metabolism to sustain the energy and redox balance of neurons. During neurotransmission, synaptic cleft glutamate activates its receptors in neurons and in astrocytes, before being taken up by astrocytes through energy costly transporters. In astrocytes, the energy requirement for glutamate influx is likely to be met by glycolysis. To enable this, astrocytes are constitutively glycolytic, robustly expressing 6‐phosphofructo‐2‐kinase/fructose‐2,6‐bisphosphatase‐3 (PFKFB3), an enzyme that is negligibly present in neurons by continuous degradation because of the ubiquitin‐proteasome pathway via anaphase‐promoting complex/cyclosome (APC)‐Cdh1. Additional factors contributing to the glycolytic frame of astrocytes may include 5′‐AMP‐activated protein kinase (AMPK), hypoxia‐inducible factor‐1 (HIF‐1), pyruvate kinase muscle isoform‐2 (PKM2), pyruvate dehydrogenase kinase‐4 (PDK4), lactate dehydrogenase‐B, or monocarboxylate transporter‐4 (MCT4). Neurotransmission‐associated messengers, such as nitric oxide or ammonium, stimulate lactate release from astrocytes. Astrocyte‐derived glycolytic lactate thus sustains the energy needs of neurons, which in contrast to astrocytes mainly rely on oxidative phosphorylation. Neuronal activity unavoidably triggers reactive oxygen species, but the antioxidant defense of neurons is weak; hence, they use glucose for oxidation through the pentose‐phosphate pathway to preserve the redox status. Furthermore, neural activity is coupled with erythroid‐derived erythroid‐derived 2‐like 2 (Nrf2) mediated transcriptional activation of antioxidant genes in astrocytes, which boost the de novo glutathione biosynthesis in neighbor neurons. Thus, the bioenergetics and redox programs of astrocytes are adapted to sustain neuronal activity and survival. Developing therapeutic strategies to interfere with these pathways may be useful to combat neurological diseases.

Our current knowledge on brain's management of bioenergetics and redox requirements associated with neural activity is herein revisited. The astrocyte‐neuronal lactate shuttle (ANLS) explains the energy needs of neurotransmission. Furthermore, neurotransmission unavoidably triggers increased mitochondrial reactive oxygen species in neurons. By coupling glutamatergic activity with transcriptional activation of antioxidant genes, astrocytes provide neurons with neuroprotective glutathione through an astrocyte‐neuronal glutathione shuttle (ANGS).

This article is part of the60th Anniversary special issue.

KEAP1-NRF2 signalling and autophagy in protection against oxidative and reductive proteotoxicity

Maintaining cellular redox status to allow cell signalling to occur requires modulation of both the controlled production of oxidants and the thiol-reducing networks to allow specific regulatory post-translational modification of protein thiols. The oxidative stress hypothesis captured the concept that overproduction of oxidants can be proteotoxic, but failed to predict the recent finding that hyperactivation of the KEAP1-NRF2 system also leads to proteotoxicity. Furthermore, sustained activation of thiol redox networks by KEAP1-NRF2 induces a reductive stress, by decreasing the lifetime of necessary oxidative post-translational modifications required for normal metabolism or cell signalling. In this context, it is now becoming clear why antioxidants or hyperactivation of antioxidant pathways with electrophilic therapeutics can be deleterious. Furthermore, it suggests that the autophagy-lysosomal pathway is particularly important in protecting the cell against redox-stress-induced proteotoxicity, since it can degrade redox-damaged proteins without causing aberrant changes to the redox network needed for metabolism or signalling. In this context, it is important to understand: (i) how NRF2-mediated redox signalling, or (ii) the autophagy-mediated antioxidant/reductant pathways sense cellular damage in the context of cellular pathogenesis. Recent studies indicate that the modification of protein thiols plays an important role in the regulation of both the KEAP1-NRF2 and autophagy pathways. In the present review, we discuss evidence demonstrating that the KEAP1-NRF2 pathway and autophagy act in concert to combat the deleterious effects of proteotoxicity. These findings are discussed with a special emphasis on their impact on cardiovascular disease and neurodegeneration.

Disturbance of redox homeostasis as a contributing underlying pathomechanism of brain and liver alterations in 3-hydroxy-3-methylglutaryl-CoA lyase deficiency

3-Hydroxy-3-methylglutaryl-CoA lyase (HL) deficiency is an inherited disorder of organic acid metabolism biochemically characterized by tissue accumulation and high urinary excretion of 3-hydroxy-3-methylgutarate, 3-methylglutarate, 3-methylglutaconate and 3-hydroxyisovalerate. Affected patients predominantly present neurological symptoms that are accompanied by mild hepatopathy during episodes of catabolic crisis. The pathophysiology of this disease is poorly known, although recent animal and human in vitro and in vivo studies have suggested that oxidative stress caused by the major accumulating organic acids may represent a pathomechanism of brain and liver damage in HL deficiency. In this review we focus on the deleterious effects of these carboxylic acids on redox homeostasis in rat and human tissues that may offer new perspectives for potential novel adjuvant therapeutic strategies in this disorder.

Glucose-6-phosphate dehydrogenase deficiency and Alzheimer's disease: Partners in crime? The hypothesis

Alzheimer's disease is a multifaceted brain disorder which involves various coupled irreversible, progressive biochemical reactions that significantly reduce quality of life as well as the actual life expectancy. Aging, genetic predispositions, head trauma, diabetes, cardiovascular disease, deficiencies in insulin signaling, dysfunction of mitochondria-associated membranes, cerebrovascular changes, high cholesterol level, increased oxidative stress and free radical formation, DNA damage, disturbed energy metabolism, and synaptic dysfunction, high blood pressure, obesity, dietary habits, exercise, social engagement, and mental stress are noted among the risk factors of this disease. In this hypothesis review I would like to draw the attention on glucose-6-phosphate dehydrogenase deficiency and its relationship with Alzheimer's disease. This enzymopathy is the most common human congenital defect of metabolism and defined by decrease in NADPH+H(+) and reduced form of glutathione concentration and that might in turn, amplify oxidative stress due to essentiality of the enzyme. This most common enzymopathy may manifest itself in severe forms, however most of the individuals with this deficiency are not essentially symptomatic. To understand the sporadic Alzheimer's disease, the writer of this paper thinks that, looking into a crystal ball might not yield much of a benefit but glucose-6-phosphate dehydrogenase deficiency could effortlessly give some clues.

The effect of maternal low-protein diet on the heart of adult offspring: role of mitochondria and oxidative stress

Protein restriction during perinatal and early postnatal development is associated with a greater incidence of disease in the adult, such arterial hypertension. The aim in the present study was to investigate the effect of maternal low-protein diet on mitochondrial oxidative phosphorylation capacity, mitochondrial reactive oxygen species (ROS) formation, antioxidant levels (enzymatic and nonenzymatic), and oxidative stress levels on the heart of the adult offspring. Pregnant Wistar rats received either 17% casein (normal protein, NP) or 8% casein (low protein, LP) throughout pregnancy and lactation. After weaning male progeny of these NP or LP fed rats, females were maintained on commercial chow (Labina-Purina). At 100 days post-birth, the male rats were sacrificed and heart tissue was harvested and stored at −80 °C. Our results show that restricting protein consumption in pregnant females induced decreased mitochondrial oxidative phosphorylation capacity (51% reduction in ADP-stimulated oxygen consumption and 49.5% reduction in respiratory control ratio) in their progeny when compared with NP group. In addition, maternal low-protein diet induced a significant decrease in enzymatic antioxidant capacity (37.8% decrease in superoxide dismutase activity; 42% decrease in catalase activity; 44.8% decrease in glutathione-S-transferase activity; 47.9% decrease in glutathione reductase; 25.7% decrease in glucose-6 phosphate dehydrogenase) and glutathione level (34.8% decrease) when compared with control. From these findings, we hypothesize that an increased production of ROS and decrease in antioxidant activity levels induced by protein restriction during development could potentiate the progression of metabolic and cardiac diseases in adulthood.

Pentose-phosphate pathway disruption in the pathogenesis of Parkinson’s disease

Oxidative stress is known to be a key factor in the pathogenesis of Parkinson’s disease (PD). Neuronal redox status is maintained by glucose metabolism via the pentose-phosphate pathway and it is known that disruption of glucose metabolism is damaging to neurons. Accumulating evidence supports the idea that glucose metabolism is altered in PD and dysregulation of the pentose-phosphate pathway in this disease has recently been shown. In this review, we present an overview of the literature regarding neuronal glucose metabolism and PD, and discuss the implications of these findings for PD pathogenesis and possible future therapeutic avenues.

Reductive stress in young healthy individuals at risk of Alzheimer disease

Oxidative stress is a hallmark of Alzheimer disease (AD) but this has not been studied in young healthy persons at risk of the disease. Carrying an Apo ε4 allele is the major genetic risk factor for AD. We have observed that lymphocytes from young, healthy persons carrying at least one Apo ε4 allele suffer from reductive rather than oxidative stress, i.e., lower oxidized glutathione and P-p38 levels and higher expression of enzymes involved in antioxidant defense, such as glutamylcysteinyl ligase and glutathione peroxidase. In contrast, in the full-blown disease, the situation is reversed and oxidative stress occurs, probably because of the exhaustion of the antioxidant mechanisms just mentioned. These results provide insights into the early events of the progression of the disease that may allow us to find biomarkers of AD at its very early stages.

Brain glucose-6-phosphate dehydrogenase protects against endogenous oxidative DNA damage and neurodegeneration in aged mice

Glucose-6-phosphate dehydrogenase (G6PD) protects the embryo from endogenous and xenobiotic-enhanced oxidative DNA damage and embryopathies. Here we show in aged mice that G6PD similarly protects against endogenous reactive oxygen species (ROS)-mediated neurodegeneration. In G6PD-normal (G6PD(+/+)) and heterozygous (G6PD(+/def)) and homozygous (G6PD(def/def)) G6PD-deficient male and female mice at about 2 years of age, oxidative DNA damage in various brain regions was assessed by 8-oxo-2'-deoxyguanosine formation using high-performance liquid chromatography and immunohistochemistry. Morphological changes in brain sections were assessed by H&E staining. DNA oxidation was increased in G6PD(def/def) mice in the cortex (p < 0.02), hippocampus (p < 0.01) and cerebellum (p < 0.006) compared to G6PD(+/+) mice, and was localized to distinct cell types. Histologically, in G6PD(+/def) mice, enhanced regionally and cellularly specific neurodegenerative changes were observed in those brain regions exhibiting elevated DNA oxidation, with a 53% reduction in the Purkinje cell count. These results show G6PD is important in protecting against the neurodegenerative effects of endogenous ROS in aging, and suggest that common hereditary G6PD deficiencies may constitute a risk factor for some neurodegenerative diseases.

Decoding Alzheimer's disease from perturbed cerebral glucose metabolism: implications for diagnostic and therapeutic strategies

Alzheimer's disease (AD) is an age-related devastating neurodegenerative disorder, which severely impacts on the global economic development and healthcare system. Though AD has been studied for more than 100 years since 1906, the exact cause(s) and pathogenic mechanism(s) remain to be clarified. Also, the efficient disease-modifying treatment and ideal diagnostic method for AD are unavailable. Perturbed cerebral glucose metabolism, an invariant pathophysiological feature of AD, may be a critical contributor to the pathogenesis of this disease. In this review, we firstly discussed the features of cerebral glucose metabolism in physiological and pathological conditions. Then, we further reviewed the contribution of glucose transportation abnormality and intracellular glucose catabolism dysfunction in AD pathophysiology, and proposed a hypothesis that multiple pathogenic cascades induced by impaired cerebral glucose metabolism could result in neuronal degeneration and consequently cognitive deficits in AD patients. Among these pathogenic processes, altered functional status of thiamine metabolism and brain insulin resistance are highly emphasized and characterized as major pathogenic mechanisms. Finally, considering the fact that AD patients exhibit cerebral glucose hypometabolism possibly due to impairments of insulin signaling and altered thiamine metabolism, we also discuss some potential possibilities to uncover diagnostic biomarkers for AD from abnormal glucose metabolism and to develop drugs targeting at repairing insulin signaling impairment and correcting thiamine metabolism abnormality. We conclude that glucose metabolism abnormality plays a critical role in AD pathophysiological alterations through the induction of multiple pathogenic factors such as oxidative stress, mitochondrial dysfunction, and so forth. To clarify the causes, pathogeneses and consequences of cerebral hypometabolism in AD will help break the bottleneck of current AD study in finding ideal diagnostic biomarker and disease-modifying therapy.

Nrf2 deficiency prevents reductive stress-induced hypertrophic cardiomyopathy

AIMS

Mutant protein aggregation (PA) cardiomyopathy (MPAC) is characterized by reductive stress (RS), PA (of chaperones and cytoskeletal components), and ventricular dysfunction in transgenic mice expressing human mutant CryAB (hmCryAB). Sustained activation of nuclear erythroid-2 like factor-2 (Nrf2) causes RS, which contributes to proteotoxic cardiac disease. The goals of this pre-clinical study were to (i) investigate whether disrupting Nrf2-antioxidant signalling prevents RS and rescues redox homeostasis in hearts expressing the mutant chaperone and (ii) elucidate mechanisms that could delay proteotoxic cardiac disease.

METHODS AND RESULTS

Non-transgenic (NTG), transgenic (TG) with MPAC and MPAC-TG:Nrf2-deficient (Nrf2-def) mice were used in this study. The effects of Nrf2 diminution (Nrf2±) on RS mediated MPAC in TG mice were assessed at 6-7 and 10 months of age. The diminution of Nrf2 prevented RS and prolonged the survival of TG mice (∼50 weeks) by an additional 20-25 weeks. The TG:Nrf2-def mice did not exhibit cardiac hypertrophy at even 60 weeks, while the MPAC-TG mice developed pathological hypertrophy and heart failure starting at 24-28 weeks of age. Aggregation of cardiac proteins was significantly reduced in TG:Nrf2-def when compared with TG mice at 7 months. Preventing RS and maintaining redox homeostasis in the TG:Nrf2-def mice ameliorated PA, leading to decreased ubiquitination of proteins.

CONCLUSION

Nrf2 deficiency rescues redox homeostasis, which reduces aggregation of mutant proteins, thereby delaying the proteotoxic pathological cardiac remodelling caused by RS and toxic protein aggregates.

H2O2 Signalling Pathway: A Possible Bridge between Insulin Receptor and Mitochondria

This review is focused on the mechanistic aspects of the insulin-induced H2O2 signalling pathway in neurons and the molecules affecting it, which act as risk factors for developing central insulin resistance. Insulin-induced H2O2 promotes insulin receptor activation and the mitochondria act as the insulin-sensitive H2O2 source, providing a direct molecular link between mitochondrial dysfunction and irregular insulin receptor activation. In this view, the accumulation of dysfunctional mitochondria during chronological ageing and Alzheimer's disease (AD) is a risk factor that may contribute to the development of dysfunctional cerebral insulin receptor signalling and insulin resistance. Due to the high significance of insulin-induced H2O2 for insulin receptor activation, oxidative stress-induced upregulation of antioxidant enzymes, e.g., in AD brains, may represent another risk factor contributing to the development of insulin resistance. As insulin-induced H2O2 signalling requires fully functional mitochondria, pharmacological strategies based on activating mitochondria biogenesis in the brain are central to the treatment of diseases associated with dysfunctional insulin receptor signalling in this organ.

Strategic role for mitochondria in Alzheimer's disease and cancer

SIGNIFICANCE

Detailed knowledge about cell death and cell survival mechanisms and how these pathways are impaired in neurodegenerative disorders and cancer forms the basis for future drug development for these diseases that affect millions of people around the world.

RECENT ADVANCES

In neurodegenerative disorders such as Alzheimer's disease (AD), cell death pathways are inappropriately activated, resulting in neuronal cell death. In contrast, cancer cells develop resistance to apoptosis by regulating anti-apoptotic proteins signaling via mitochondria. Mounting evidence shows that mitochondrial function is central in both cancer and AD. Cancer cells typically shut down oxidative phosphorylation (OXPHOS) in mitochondria and switch to glycolysis for ATP production, making them resistant to hypoxia. In AD, for example, amyloid-β peptide (Aβ) and reactive oxygen species impair mitochondrial function. Neurons therefore also switch to glycolysis to maintain ATP production and to produce molecules involved in antioxidant metabolism in an attempt to survive.

CRITICAL ISSUES

One critical difference between cancer cells and neurons is that cancer cells can survive without OXPHOS, while neurons are dependent on OXPHOS for long-term survival.

FUTURE DIRECTIONS

This review will focus on these abnormalities of mitochondrial function shared in AD and cancer and discuss the potential mechanisms underlying links that may be key steps in the development of therapeutic strategies.

Antioxidant and bioenergetic coupling between neurons and astrocytes

Oxidative and nitrosative stress underlie the pathogenesis of a broad range of human diseases, in particular neurodegenerative disorders. Within the brain, neurons are the cells most vulnerable to excess reactive oxygen and nitrogen species; their survival relies on the antioxidant protection promoted by neighbouring astrocytes. However, neurons are also intrinsically equipped with a biochemical mechanism that links glucose metabolism to antioxidant defence. Neurons actively metabolize glucose through the pentose phosphate pathway, which maintains the antioxidant glutathione in its reduced state, hence exerting neuroprotection. This process is tightly controlled by a key glycolysis-promoting enzyme and is dependent on an appropriate supply of energy substrates from astrocytes. Thus brain bioenergetic and antioxidant defence is coupled between neurons and astrocytes. A better understanding of the regulation of this intercellular coupling should be important for identifying novel targets for future therapeutic interventions.

Dithiol-based compounds maintain expression of antioxidant protein peroxiredoxin 1 that counteracts toxicity of mutant huntingtin

Mitochondrial dysfunction and elevated reactive oxygen species are strongly implicated in both aging and various neurodegenerative disorders, including Huntington disease (HD). Because reactive oxygen species can promote the selective oxidation of protein cysteine sulfhydryl groups to disulfide bonds we examined the spectrum of disulfide-bonded proteins that were specifically altered in a HD context. Protein extracts from PC12 cells overexpressing the amino-terminal fragment of the Huntingtin (Htt) protein with either a nonpathogenic or pathogenic polyglutamine repeat (Htt-103Q) were resolved by redox two-dimensional PAGE followed by mass spectrometry analysis. Several antioxidant proteins were identified that exhibited changes in disulfide bonding unique to Htt-103Q expressing cells. In particular, the antioxidant protein peroxiredoxin 1 (Prx1) exhibited both decreased expression and hyperoxidation in response to mutant Htt expressed in either PC12 cells or immortalized striatal cells exposed to 3-nitropropionic acid. Ectopic expression of Prx1 in PC12 cells attenuated mutant Htt-induced toxicity. In contrast, short hairpin RNA-mediated knockdown of Prx1 potentiated mHtt toxicity. Furthermore, treatment with the dithiol-based compounds dimercaptopropanol and dimercaptosuccinic acid suppressed toxicity in both HD cell models, whereas monothiol compounds were relatively ineffective. Dimercaptopropanol treatment also prevented mutant Htt-induced loss of Prx1 expression in both cell models. Our studies reveal for the first time that pathogenic Htt can affect the expression and redox state of antioxidant proteins; an event countered by specific dithiol-based compounds. These findings should provide a catalyst to explore the use of dithiol-based drugs for the treatment of neurodegenerative diseases.

Vascular and metabolic dysfunction in Alzheimer's disease: a review

Alzheimer's disease (AD) is thought to start years or decades prior to clinical diagnosis. Overt pathology such as protein misfolding and plaque formation occur at later stages, and factors other than amyloid misfolding contribute to the initiation of the disease. Vascular and metabolic dysfunctions are excellent candidates, as they are well-known features of AD that precede pathology or clinical dementia. While the general notion that vascular and metabolic dysfunctions contribute to the etiology of AD is becoming accepted, recent research suggests novel mechanisms by which these/such processes could possibly contribute to AD pathogenesis. Vascular dysfunction includes reduced cerebrovascular flow and cerebral amyloid angiopathy. Indeed, there appears to be an interaction between amyloid β (Aβ) and vascular pathology, where Aβ production and vascular pathology both contribute to and are affected by oxidative stress. One major player in the vascular pathology is NAD(P)H oxidase, which generates vasoactive superoxide. Metabolic dysfunction has only recently regained popularity in relation to its potential role in AD. The role of metabolic dysfunction in AD is supported by the increased epidemiological risk of AD associated with several metabolic diseases such as diabetes, dyslipidemia and hypertension, in which there is elevated oxidative damage and insulin resistance. Metabolic dysfunction is further implicated in AD as pharmacological inhibition of metabolism exacerbates pathology, and several metabolic enzymes of the glycolytic, tricarboxylic acid cycle (TCA) and oxidative phosphorylation pathways are damaged in AD. Recent studies have highlighted the role of insulin resistance, in contributing to AD. Thus, vascular and metabolic dysfunctions are key components in the AD pathology throughout the course of disease. The common denominator between vascular and metabolic dysfunction emerging from this review appears to be oxidative stress and Aβ. This review also provides a framework for evaluation of current and future therapeutics for AD.

Amyloid-Beta interaction with mitochondria

Mitochondrial dysfunction is a hallmark of amyloid-beta(Aβ)-induced neuronal toxicity in Alzheimer's disease (AD). The recent emphasis on the intracellular biology of Aβ and its precursor protein (AβPP) has led researchers to consider the possibility that mitochondria-associated and/or intramitochondrial Aβ may directly cause neurotoxicity. In this paper, we will outline current knowledge of the intracellular localization of both Aβ and AβPP addressing the question of how Aβ can access mitochondria. Moreover, we summarize evidence from AD postmortem brain as well as cellular and animal AD models showing that Aβ triggers mitochondrial dysfunction through a number of pathways such as impairment of oxidative phosphorylation, elevation of reactive oxygen species (ROS) production, alteration of mitochondrial dynamics, and interaction with mitochondrial proteins. In particular, we focus on Aβ interaction with different mitochondrial targets including the outer mitochondrial membrane, intermembrane space, inner mitochondrial membrane, and the matrix. Thus, this paper establishes a modified model of the Alzheimer cascade mitochondrial hypothesis.