Mitochondrial dysfunction and cellular metabolic deficiency in Alzheimer's disease

Fosgonimeton attenuates amyloid-beta toxicity in preclinical models of Alzheimer's disease

Positive modulation of hepatocyte growth factor (HGF) signaling may represent a promising therapeutic strategy for Alzheimer's disease (AD) based on its multimodal neurotrophic, neuroprotective, and anti-inflammatory effects addressing the complex pathophysiology of neurodegeneration. Fosgonimeton is a small-molecule positive modulator of the HGF system that has demonstrated neurotrophic and pro-cognitive effects in preclinical models of dementia. Herein, we evaluate the neuroprotective potential of fosgonimeton, or its active metabolite, fosgo-AM, in amyloid-beta (Aβ)-driven preclinical models of AD, providing mechanistic insight into its mode of action. In primary rat cortical neurons challenged with Aβ (Aβ1-42), fosgo-AM treatment significantly improved neuronal survival, protected neurite networks, and reduced tau hyperphosphorylation. Interrogation of intracellular events indicated that cortical neurons treated with fosgo-AM exhibited a significant decrease in mitochondrial oxidative stress and cytochrome c release. Following Aβ injury, fosgo-AM significantly enhanced activation of pro-survival effectors ERK and AKT, and reduced activity of GSK3β, one of the main kinases involved in tau hyperphosphorylation. Fosgo-AM also mitigated Aβ-induced deficits in Unc-like kinase 1 (ULK1) and Beclin-1, suggesting a potential effect on autophagy. Treatment with fosgo-AM protected cortical neurons from glutamate excitotoxicity, and such effects were abolished in the presence of an AKT or MEK/ERK inhibitor. In vivo, fosgonimeton administration led to functional improvement in an intracerebroventricular Aβ25-35 rat model of AD, as it significantly rescued cognitive function in the passive avoidance test. Together, our data demonstrate the ability of fosgonimeton to counteract mechanisms of Aβ-induced toxicity. Fosgonimeton is currently in clinical trials for mild-to-moderate AD (NCT04488419; NCT04886063).

The impact of regional heterogeneity in whole-brain dynamics in the presence of oscillations

Large variability exists across brain regions in health and disease, considering their cellular and molecular composition, connectivity, and function. Large-scale whole-brain models comprising coupled brain regions provide insights into the underlying dynamics that shape complex patterns of spontaneous brain activity. In particular, biophysically grounded mean-field whole-brain models in the asynchronous regime were used to demonstrate the dynamical consequences of including regional variability. Nevertheless, the role of heterogeneities when brain dynamics are supported by synchronous oscillating state, which is a ubiquitous phenomenon in brain, remains poorly understood. Here, we implemented two models capable of presenting oscillatory behavior with different levels of abstraction: a phenomenological Stuart-Landau model and an exact mean-field model. The fit of these models informed by structural- to functional-weighted MRI signal (T1w/T2w) allowed us to explore the implication of the inclusion of heterogeneities for modeling resting-state fMRI recordings from healthy participants. We found that disease-specific regional functional heterogeneity imposed dynamical consequences within the oscillatory regime in fMRI recordings from neurodegeneration with specific impacts on brain atrophy/structure (Alzheimer's patients). Overall, we found that models with oscillations perform better when structural and functional regional heterogeneities are considered, showing that phenomenological and biophysical models behave similarly at the brink of the Hopf bifurcation.

Model-based whole-brain perturbational landscape of neurodegenerative diseases

The treatment of neurodegenerative diseases is hindered by lack of interventions capable of steering multimodal whole-brain dynamics towards patterns indicative of preserved brain health. To address this problem, we combined deep learning with a model capable of reproducing whole-brain functional connectivity in patients diagnosed with Alzheimer's disease (AD) and behavioral variant frontotemporal dementia (bvFTD). These models included disease-specific atrophy maps as priors to modulate local parameters, revealing increased stability of hippocampal and insular dynamics as signatures of brain atrophy in AD and bvFTD, respectively. Using variational autoencoders, we visualized different pathologies and their severity as the evolution of trajectories in a low-dimensional latent space. Finally, we perturbed the model to reveal key AD- and bvFTD-specific regions to induce transitions from pathological to healthy brain states. Overall, we obtained novel insights on disease progression and control by means of external stimulation, while identifying dynamical mechanisms that underlie functional alterations in neurodegeneration.

Phosphorylation and Glycosylation of Amyloid-β Protein Precursor: The Relationship to Trafficking and Cleavage in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder in the central nervous system, and this disease is characterized by extracellular senile plaques and intracellular neurofibrillary tangles. Amyloid-β (Aβ) peptide is the main constituent of senile plaques, and this peptide is derived from the amyloid-β protein precursor (AβPP) through the successive cleaving by β-site AβPP-cleavage enzyme 1 (BACE1) and γ-secretase. AβPP undergoes the progress of post-translational modifications, such as phosphorylation and glycosylation, which might affect the trafficking and the cleavage of AβPP. In the recent years, about 10 phosphorylation sites of AβPP were identified, and they play complex roles in glycosylation modification and cleavage of AβPP. In this article, we introduced the transport and the cleavage pathways of AβPP, then summarized the phosphorylation and glycosylation sites of AβPP, and further discussed the links and relationship between phosphorylation and glycosylation on the pathways of AβPP trafficking and cleavage in order to provide theoretical basis for AD research.

A High Fat/Cholesterol Diet Recapitulates Some Alzheimer's Disease-Like Features in Mice: Focus on Hippocampal Mitochondrial Dysfunction

BACKGROUND

Ample evidence from clinical and pre-clinical studies suggests mid-life hypercholesterolemia as a risk factor for developing Alzheimer's disease (AD) at a later age. Hypercholesterolemia induced by dietary habits can lead to vascular perturbations that increase the risk of developing sporadic AD.

OBJECTIVE

To investigate the effects of a high fat/cholesterol diet (HFCD) as a risk factor for AD by using a rodent model of AD and its correspondent control (healthy animals).

METHODS

We compared the effect of a HFCD in normal mice (non-transgenic mice, NTg) and the triple transgenic mouse model of AD (3xTgAD). We evaluated cognitive performance in relation to changes in oxidative metabolism and neuron-derived nitric oxide (•NO) concentration dynamics in hippocampal slices as well as histochemical staining of markers of the neurovascular unit.

RESULTS

In NTg, the HFCD produced only moderate hypercholesterolemia but significant decline in spatial memory was observed. A tendency for decrease in •NO production was accompanied by compromised mitochondrial function with decrease in spare respiratory capacity. In 3xTgAD mice, a robust increase in plasma cholesterol levels with the HFCD did not worsen cognitive performance but did induce compromise of mitochondrial function and significantly decreased •NO production. We found increased staining of biomarkers for astrocyte endfeet and endothelial cells in 3xTgAD hippocampi, which was further increased by the HFCD.

CONCLUSION

A short term (8 weeks) intervention with HFCD can produce an AD-like phenotype even in the absence of overt systemic hypercholesterolemia and highlights mitochondrial dysfunction as a link between hypercholesterolemia and sporadic AD.

Atorvastatin Improves Mitochondrial Function and Prevents Oxidative Stress in Hippocampus Following Amyloid-β1-40 Intracerebroventricular Administration in Mice

Amyloid-β (Aβ) peptides play a significant role in the pathogenesis of Alzheimer's disease (AD). Neurotoxic effects promoted by Aβ peptides involve glutamate transmission impairment, decrease of neurotrophic factors, mitochondrial dysfunction, oxidative stress, synaptotoxicity, and neuronal degeneration. Here, we assessed the early events evoked by Aβ_1-40 on the hippocampus. Additionally, we sought to unravel the molecular mechanisms of atorvastatin preventive effect on Aβ-induced hippocampal damage. Mice were treated orally (p.o.) with atorvastatin 10 mg/kg/day during 7 consecutive days before the intracerebroventricular (i.c.v.) infusion of Aβ_1-40 (400 pmol/site). Twenty-four hours after Aβ_1-40 infusion, a reduced content of mature BDNF/proBDNF ratio was observed in Aβ-treated mice. However, there is no alteration in synaptophysin, PSD-95, and doublecortin immunocontent in the hippocampus. Aβ_1-40 promoted an increase in reactive oxygen species (ROS) and nitric oxide (NO) generation in hippocampal slices, and atorvastatin prevented this oxidative burst. Mitochondrial OXPHOS was measured by high-resolution respirometry. At this time point, Aβ_1-40 did not alter the O₂ consumption rates (OCR) related to phosphorylating state associated with complexes I and II, and the maximal OCR. However, atorvastatin increased OCR of phosphorylating state associated with complex I and complexes I and II, maximal OCR of complexes I and II, and OCR associated with mitochondrial spare capacity. Atorvastatin treatment improved mitochondrial function in the rodent hippocampus, even after Aβ infusion, pointing to a promising effect of improving brain mitochondria bioenergetics. Therefore, atorvastatin could act as an adjuvant in battling the symptoms of AD to preventing or delaying the disease progression.

Ursodeoxycholyl lysophosphatidylethanolamide protects against hepatic ischemia/reperfusion injury via phospholipid metabolism-mediated mitochondrial quality control

Mitochondrial dysfunction is the leading cause of reactive oxygen species (ROS) burst and apoptosis in hepatic ischemia/reperfusion (I/R) injury. Ursodeoxycholyl lysophosphatidylethanolamide (UDCA-LPE) is a hepatotargeted agent that exerts hepatoprotective roles by regulating lipid metabolism. Our previous studies have shown that UDCA-LPE improves hepatic I/R injury by inhibiting apoptosis and inflammation. However, the role of UDCA-LPE in lipid metabolism and mitochondrial function in hepatic I/R remains unknown. In the present study, we investigated the role of UDCA-LPE in hepatic I/R by focusing on the interface of phospholipid metabolism and mitochondrial homeostasis. Livers from 28-week-old mice, primary hepatocytes and HepG2 cells were subjected to in vivo and in vitro I/R, respectively. Analyses of oxidative stress, imaging, ATP generation, genetics, and lipidomics showed that I/R was associated with mitochondrial dysfunction and a reduction in phospholipids. UDCA-LPE alleviated mitochondria-dependent oxidative stress and apoptosis and prevented the decrease of phospholipid levels. Our study found that cytosolic phospholipase A₂ (cPLA₂ ), a phospholipase that is activated during I/R, hydrolyzed mitochondrial membrane phospholipids and led to mitochondria-mediated oxidative stress and apoptosis. UDCA-LPE inhibited the interaction between cPLA₂ and mitochondria and reduced phospholipid hydrolysis-mediated injury. UDCA-LPE might regulate the crosstalk between the phospholipid metabolism and the mitochondria, restore mitochondrial function and ameliorate I/R injury.

Regulation of AβPP Glycosylation Modification and Roles of Glycosylation on AβPP Cleavage in Alzheimer's Disease

The presence of senile plaques in the gray matter of the brain is one of the major pathologic features of Alzheimer's disease (AD), and amyloid-β (Aβ) is the main component of extracellular deposits of the senile plaques. Aβ derives from amyloid-β precursor protein (AβPP) cleaved by β-secretase (BACE1) and γ-secretase, and the abnormal cleavage of AβPP is an important event leading to overproduction and aggregation of Aβ species. After translation, AβPP undergoes post-translational modifications (PTMs) including glycosylation and phosphorylation in the endoplasmic reticulum (ER) and Golgi apparatus, and these modifications play an important role in regulating the cleavage of this protein. In this Review, we summarize research progress on the modification of glycosylation, especially O-GlcNAcylation and mucin-type O-linked glycosylation (also known as O-GalNAcylation), on the regulation of AβPP cleavage and on the influence of AβPP's glycosylation in the pathogenesis of AD.

The effects of GSK2981710, a medium-chain triglyceride, on cognitive function in healthy older participants: A randomised, placebo-controlled study

OBJECTIVE

This double-blind, randomised, placebo-controlled, two-part study assessed the impact of GSK2981710, a medium-chain triglyceride (MCT) that liberates ketone bodies, on cognitive function, safety, and tolerability in healthy older adults.

METHODS

Part 1 was a four-period dose-selection study (n = 8 complete). Part 2 was a two-period crossover study (n = 80 complete) assessing the acute (Day 1) and prolonged (Day 15) effects of GSK2981710 on cognition and memory-related neuronal activity. Safety and tolerability of MCT supplementation were monitored in both parts of the study.

RESULTS

The most common adverse event was diarrhoea (100% and 75% of participants in Parts 1 and 2, respectively). Most adverse events were mild to moderate, and 11% participants were withdrawn due to one or more adverse events. Although GSK2981710 (30 g/day) resulted in increased peak plasma β-hydroxybutyrate (BHB) concentrations, no significant improvements in cognitive function or memory-related neuronal activity were observed.

CONCLUSION

Over a duration of 14 days, increasing plasma BHB levels with daily administration of GSK2981710 had no effects on neuronal activity or cognitive function. This result indicates that modulating plasma ketone levels with GSK2981710 may be ineffective in improving cognitive function in healthy older adults, or the lack of observed effect could be related to several factors including study population, plasma BHB concentrations, MCT composition, or treatment duration.

Sirtuins in mitochondrial stress: Indispensable helpers behind the scenes

Mitochondria play an essential part in guaranteeing normal cellular physiological functions through providing ATP and participating in diverse processes and signaling pathways. Recently, more and more studies have revealed the vital roles of mitochondria in coping with stressors in the aging process, metabolic disturbances and neurological disorders. Mitochondrial stress responses, including the mitochondrial unfolded protein response (UPR^mt), antioxidant defense, mitochondrial fission, mitochondrial fusion and mitophagy, are induced to maintain cellular integrity in response to stress. The sirtuin family, a group of NAD⁺-dependent deacetylases, has been the focus of much attention in recent years for their multiple regulatory functions, especially in aging and metabolism. Recent reports validated the significant link between mitochondrial stress responses and the sirtuin family, which may help to elucidate the pathogenesis and therapies for diseases such as Alzheimer's disease or Parkinson's disease. This review will summarize recent related studies and illuminate the interplay between sirtuins and mitochondrial stress.

Iron-sulfur cluster biosynthesis and trafficking - impact on human disease conditions

Iron-sulfur clusters (Fe-S) are one of the most ancient, ubiquitous and versatile classes of metal cofactors found in nature. Proteins that contain Fe-S clusters constitute one of the largest families of proteins, with varied functions that include electron transport, regulation of gene expression, substrate binding and activation, radical generation, and, more recently discovered, DNA repair. Research during the past two decades has shown that mitochondria are central to the biogenesis of Fe-S clusters in eukaryotic cells via a conserved cluster assembly machinery (ISC assembly machinery) that also controls the synthesis of Fe-S clusters of cytosolic and nuclear proteins. Several key steps for synthesis and trafficking have been determined for mitochondrial Fe-S clusters, as well as the cytosol (CIA - cytosolic iron-sulfur protein assembly), but detailed mechanisms of cluster biosynthesis, transport, and exchange are not well established. Genetic mutations and the instability of certain steps in the biosynthesis and maturation of mitochondrial, cytosolic and nuclear Fe-S cluster proteins affects overall cellular iron homeostasis and can lead to severe metabolic, systemic, neurological and hematological diseases, often resulting in fatality. In this review we briefly summarize the current molecular understanding of both mitochondrial ISC and CIA assembly machineries, and present a comprehensive overview of various associated inborn human disease states.

Oligomeric amyloid β preferentially targets neuronal and not glial mitochondrial-encoded mRNAs

INTRODUCTION

Our laboratories have demonstrated that accumulation of oligomeric amyloid β (OAβ) in neurons is an essential step leading to OAβ-mediated mitochondrial dysfunction.

METHODS

Alzheimer's disease (AD) and matching control hippocampal neurons, astrocytes, and microglia were isolated by laser-captured microdissection from the same subjects, followed by whole-transcriptome sequencing. Complementary in vitro work was performed in OAβ-treated differentiated SH-SY5Y, followed by the use of a novel CoQ₁₀ analogue for protection. This compound is believed to be effective both in suppressing reactive oxygen species and also functioning in mitochondrial electron transport.

RESULTS

We report decreases in the same mitochondrial-encoded mRNAs in Alzheimer's disease laser-captured CA1 neurons and in OAβ-treated SH-SY5Y cells, but not in laser-captured microglia and astrocytes. Pretreatment with a novel CoQ₁₀ analogue, protects neuronal mitochondria from OAβ-induced mitochondrial changes.

DISCUSSION

Similarity of expression changes in neurons from Alzheimer's disease brain and neuronal cells treated with OAβ, and the effect of a CoQ₁₀ analogue on the latter, suggests a pretreatment option to prevent OAβ toxicity, long before the damage is apparent.

Peroxiredoxin 5 prevents amyloid-beta oligomer-induced neuronal cell death by inhibiting ERK-Drp1-mediated mitochondrial fragmentation

Alzheimer's disease (AD), a neurodegenerative disorder, is caused by amyloid-beta oligomers (AβOs). AβOs induce cell death by triggering oxidative stress and mitochondrial dysfunction. A recent study showed that AβO-induced oxidative stress is associated with extracellular signal-regulated kinase (ERK)-dynamin related protein 1 (Drp1)-mediated mitochondrial fission. Reactive oxygen species (ROS) are regulated by antioxidant enzymes, especially peroxiredoxins (Prxs) that scavenge H2O2. These enzymes inhibit neuronal cell death induced by various neurotoxic reagents. However, it is unclear whether Prx5, which is specifically expressed in neuronal cells, protects these cells from AβO-induced damage. In this study, we found that Prx5 expression was upregulated by AβO-induced oxidative stress and that Prx5 decreased ERK-Drp1-mediated mitochondrial fragmentation and apoptosis of HT-22 neuronal cells. Prx5 expression was affected by AβO, and amelioration of oxidative stress by N-acetyl-L-cysteine decreased AβO-induced Prx5 expression. Prx5 overexpression reduced ROS as well as RNS and apoptotic cell death but Prx5 knockdown did not. In addition, Prx5 overexpression ameliorated ERK-Drp1-mediated mitochondrial fragmentation but Prx5 knockdown did not. These results indicated that inducible Prx5 expression by AβO plays a key role in inhibiting both ERK-Drp1-induced mitochondrial fragmentation and neuronal cell death by regulating oxidative stress. Thus, Prx5 may be a new therapeutic agent for treating AD.

Increased expression of mitochondrial DNA-encoded genes in human renal mesangial cells in response to high glucose-induced reactive oxygen species

Reactive oxygen species (ROS)-mediated disruption of mitochondrial respiratory function has been implicated in the complications of diabetes. The present study examined changes in the gene expression of mitochondrial DNA (mtDNA)-encoded subunits of electron transport chain complexes in response to high glucose-induced ROS overproduction in an in vitro model of diabetic nephropathy using human renal mesangial cells. Mitochondrial ROS generation was assessed by confocal microscopy and flow cytometry in the cells following culture in 5 and 25 mM glucose. The mRNA expression levels of nicotinamide adenine dinucleotide dehydrogenase 2 (ND2) of complex I, cytochrome b (CYTB) of complex III, cytochrome c oxidase (COI) of complex IV and ATPase 6 of complex V were analyzed by reverse transcription-quantitative polymerase chain reaction. The protein expression levels of ND2, CYTB, COI and ATPase 6 were analyzed by western blotting. A significant increase in mitochondrial ROS production was observed in the cells cultured in 25 mM glucose, compared with cells cultured in 5 mM glucose (P<0.05). The mRNA expression of ND2, CYTB, CO1 and ATPase 6 was significantly increased following culture in 25 mM glucose, compared with the cells cultured in 5 mM glucose (P<0.05). This increase in mRNA expression was accompanied by significant increases in protein expression following incubation in 25 mM glucose (P<0.05). The increase in mtDNA-encoded gene expression in the electron transport subunits following exposure to high glucose-induced ROS may be a compensatory response mechanism for the decline in mitochondrial function, which may be important in the development of diabetic nephropathy through enhanced ROS generation.

A strategy focused on MAPT, APP, NCSTN and BACE1 to build blood classifiers for Alzheimer's disease

BACKGROUND

Although Alzheimer's disease (AD) is a brain disorder, a number of peripheral alterations have been found in these patients, including differences in leukocyte gene expression; however, the key genes involved in plaque and tangle formation have shown a relatively small potential as diagnostic markers. We focused on MAPT, APP, NCSTN and BACE1 as the basis to build and compare blood classifiers for AD.

METHODS

We used a combined model to build disease classifiers, using measures of blood pressure and serum glucose, cholesterol and triglyceride levels as well as RT-PCR expression levels of APP, NCSTN and BACE1 in peripheral blood mononuclear cells (PBMCs) from an independent cohort of 36 individuals of cognitively-normal controls, AD and other neuropathologies. Also, a set of genes was carefully selected by molecular interactions with MAPT, APP, NCSTN and BACE1 to test an expression-based classifier in a public microarray dataset of 40 samples (AD and controls). A series of discriminant analyses and classification and regression trees (C&RTs) were used to perform classification tasks.

RESULTS

Using C&RTs, the combined model showed potential to differentially diagnose AD with up to 94.4% accuracy and 100% specificity for our independent sample. Furthermore, a subset of 16 genes showed the best diagnostic potential using a minimum number of expression variables, correctly classifying up to 100% of samples in the public dataset.

CONCLUSIONS

Our unique method of variable selection proves that even elements showing no significant differences between controls and AD, but that have somehow been linked to AD or AD-related elements, still hold a potential to be used in its diagnosis. Sample size and inherent methodological limitations of this study need to be kept in mind. Our classifiers require careful further testing in larger cohorts. Nonetheless, we believe these results provide evidence for the utility of our innovative method, which contributes a different approach to generate promising diagnostic tools for neuropsychiatric disorders.

Senescence may mediate conversion of tau phosphorylation-induced apoptotic escape to neurodegeneration

Neurodegeneration is the characteristic pathology in the brains of Alzheimer's disease (AD). However, the nature and molecular mechanism leading to the degeneration are not clarified. Given that only the neurons filled with neurofibrillary tangles survive to the end stage of the disease and the major component of the tangles is the hyperphosphorylated tau proteins, it is conceivable that tau hyperphosphorylation must play a crucial role in AD neurodegeneration. We have demonstrated that tau hyperphosphorylation renders the cells more resistant to the acute apoptosis. The molecular mechanisms involve substrate competition of tau and β-catenin for glycogen synthase kinase 3β (GSK-3β); activation of Akt; preservation of Bcl-2 and suppression of Bax, cytosolic cytochrome-c, and caspase-3 activity; and upregulation of unfolded protein response (UPR), i.e., up-regulating phosphorylation of PERK, eIF2 and IRE1 with an increased cleavage of ATF6 and ATF4. On the other hand, tau hyperphosphorylation promotes its intracellular accumulation and disrupts axonal transport; hyperphosphorylated tau also impairs cholinergic function and inhibits proteasome activity. These findings indicate that tau hyperphosphorylation and its intracellular accumulation play dual role in the evolution of AD. We speculate that transient tau phosphorylation helps cells abort from an acute apoptosis, while persistent tau hyperphosphorylation/accumulation may trigger cell senescence that eventually causes a chronic neurodegeneration. Therefore, the nature of "AD neurodegeneration" may represent a new type of tau-regulated chronic neuron death; and the stage of cell senescence may provide a broad window for the intervention of AD.

Mitochondrial iron homeostasis and its dysfunctions in neurodegenerative disorders

Synthesis of the iron-containing prosthetic groups-heme and iron-sulfur clusters-occurs in mitochondria. The mitochondrion is also an important producer of reactive oxygen species (ROS), which are derived from electrons leaking from the electron transport chain. The coexistence of both ROS and iron in the secluded space of the mitochondrion makes this organelle particularly prone to oxidative damage. Here, we review the elements that configure mitochondrial iron homeostasis and discuss the principles of iron-mediated ROS generation in mitochondria. We also review the evidence for mitochondrial dysfunction and iron accumulation in Alzheimer's disease, Huntington Disease, Friedreich's ataxia, and in particular Parkinson's disease. We postulate that a positive feedback loop of mitochondrial dysfunction, iron accumulation, and ROS production accounts for the process of cell death in various neurodegenerative diseases in which these features are present.

Is Dysregulation of the HPA-Axis a Core Pathophysiology Mediating Co-Morbid Depression in Neurodegenerative Diseases?

There is increasing evidence of prodromal manifestation of neuropsychiatric symptoms in a variety of neurodegenerative diseases such as Parkinson's disease (PD) and Huntington's disease (HD). These affective symptoms may be observed many years before the core diagnostic symptoms of the neurological condition. It is becoming more apparent that depression is a significant modifying factor of the trajectory of disease progression and even treatment outcomes. It is therefore crucial that we understand the potential pathophysiologies related to the primary condition, which could contribute to the development of depression. The hypothalamic-pituitary-adrenal (HPA)-axis is a key neuroendocrine signaling system involved in physiological homeostasis and stress response. Disturbances of this system lead to severe hormonal imbalances, and the majority of such patients also present with behavioral deficits and/or mood disorders. Dysregulation of the HPA-axis is also strongly implicated in the pathology of major depressive disorder. Consistent with this, antidepressant drugs, such as the selective serotonin reuptake inhibitors have been shown to alter HPA-axis activity. In this review, we will summarize the current state of knowledge regarding HPA-axis pathology in Alzheimer's, PD and HD, differentiating between prodromal and later stages of disease progression when evidence is available. Both clinical and preclinical evidence will be examined, but we highlight animal model studies as being particularly useful for uncovering novel mechanisms of pathology related to co-morbid mood disorders. Finally, we purpose utilizing the preclinical evidence to better inform prospective, intervention studies.

Serum uric acid levels in patients with Alzheimer's disease: a meta-analysis

Background

Serum uric acid (UA) could exert neuro-protective effects against Alzheimer's disease (AD) via its antioxidant capacities. Many studies investigated serum UA levels in AD patients, but to date, results from these observational studies are conflicting.

Methods

We conducted a meta-analysis to compare serum UA levels between AD patients and healthy controls by the random-effects model. Studies were identified by searching PubMed, ISI Web of Science, EMBASE, and the Cochrane library databases from 1966 through July 2013 using the Medical Subject Headings and keywords without restriction in languages. Only case-control studies were included if they had data on serum UA levels in AD patients and healthy controls. Begg's funnel plot and Egger's regression test were applied to assess the potential publication bias. Sensitivity analyses and meta-regression were conducted to explore possible explanations for heterogeneity.

Results

A total of 11 studies met the inclusion criteria including 2708 participants were abstracted. Serum UA levels were not significantly different in AD patients compared to healthy controls (standardized mean difference (SMD) = −0.50; 95% confidence interval (CI): −1.23 to 0.22). Little evidence of publication bias was observed. Sensitivity analyses showed that the combined SMD was consistent every time omitting any one study, except only one study which greatly influenced the overall results. Meta-regression showed that year of publication, race, sample size, and mean age were not significant sources of heterogeneity.

Conclusion

Our meta-analysis of case-control studies suggests that serum UA levels do not differ significantly in AD patients, but there may be a trend toward decreased UA in AD after an appropriate interpretation. More well-designed investigations are needed to demonstrate the potential change of serum UA levels in AD patients.

Signaling pathway cross talk in Alzheimer's disease

Numerous studies suggest energy failure and accumulative intracellular waste play a causal role in the pathogenesis of several neurodegenerative disorders and Alzheimer's disease (AD) in particular. AD is characterized by extracellular amyloid deposits, intracellular neurofibrillary tangles, cholinergic deficits, synaptic loss, inflammation and extensive oxidative stress. These pathobiological changes are accompanied by significant behavioral, motor, and cognitive impairment leading to accelerated mortality. Currently, the potential role of several metabolic pathways associated with AD, including Wnt signaling, 5' adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), Sirtuin 1 (Sirt1, silent mating-type information regulator 2 homolog 1), and peroxisome proliferator-activated receptor gamma co-activator 1-α (PGC-1α) have widened, with recent discoveries that they are able to modulate several pathological events in AD. These include reduction of amyloid-β aggregation and inflammation, regulation of mitochondrial dynamics, and increased availability of neuronal energy. This review aims to highlight the involvement of these new set of signaling pathways, which we have collectively termed "anti-ageing pathways", for their potentiality in multi-target therapies against AD where cellular metabolic processes are severely impaired.

Tau hyperphosphorylation induces apoptotic escape and triggers neurodegeneration in Alzheimer's disease

Since abnormal post-translational modifications or gene mutations of tau have been detected in over twenty neurodegenerative disorders, tau has attracted widespread interest as a target protein. Among its various post-translational modifications, phosphorylation is the most extensively studied. It is recognized that tau hyperphosphorylation is the root cause of neurodegeneration in Alzheimer's disease (AD); however, it is not clear how it causes neurodegeneration. Based on the findings that tau hyperphosphorylation leads to the escape of neurons from acute apoptosis and simultaneously impairs the function of neurons, we have proposed that the nature of AD neurodegeneration is the consequence of aborted apoptosis induced by tau phosphorylation. Therefore, proper manipulation of tau hyperphosphorylation could be promising for arresting AD neurodegeneration. In this review, the neuroprotective and neurodegenerative effects of tau hyperphosphorylation and our thoughts regarding their relationship are presented.

Mitochondrial stress signaling promotes cellular adaptations

Mitochondrial dysfunction has been implicated in the aetiology of many complex diseases, as well as the ageing process. Much of the research on mitochondrial dysfunction has focused on how mitochondrial damage may potentiate pathological phenotypes. The purpose of this review is to draw attention to the less well-studied mechanisms by which the cell adapts to mitochondrial perturbations. This involves communication of stress to the cell and successful induction of quality control responses, which include mitophagy, unfolded protein response, upregulation of antioxidant and DNA repair enzymes, morphological changes, and if all else fails apoptosis. The mitochondrion is an inherently stressful environment and we speculate that dysregulation of stress signaling or an inability to switch on these adaptations during times of mitochondrial stress may underpin mitochondrial dysfunction and hence amount to pathological states over time.

Protective effect of paeoniflorin on Aβ25-35-induced SH-SY5Y cell injury by preventing mitochondrial dysfunction

Alzheimer's disease (AD) is a major neurodegenerative brain disorder affecting about 14 million people worldwide. Aβ-induced cell injury is a crucial cause of neuronal loss in AD, thus the suppression of which might be useful for the treatment of this disease. In this study, we aimed to evaluate the effect of paeoniflorin (PF), a monoterpene glycoside isolated from aqueous extract of Radix Paeoniae Alba, on Aβ25-35-induced cytotoxicity in SH-SY5Y cells. The results showed PF could attenuate or restore the viability loss, apoptotic increase, and ROS production induced by Aβ25-35 in SH-SY5Y cells. In addition, PF strikingly inhibited Aβ25-35-induced mitochondrial dysfunction, which includes decreased mitochondrial membrane potential, increased Bax/Bcl-2 ratio, cytochrome c release and activity of caspase-3 and caspase-9. Therefore, our study provided the first experimental evidence that PF could modulate ROS production and apoptotic mitochondrial pathway in model of neuron injury in vitro and which might provide new insights into its application toward Alzheimer's disease therapy.