Dynamin-like protein 1 reduction underlies mitochondrial morphology and distribution abnormalities in fibroblasts from sporadic Alzheimer's disease patients

A multitude of signaling pathways associated with Alzheimer's disease and their roles in AD pathogenesis and therapy

The exact molecular mechanisms associated with Alzheimer's disease (AD) pathology continue to represent a mystery. In the past decades, comprehensive data were generated on the involvement of different signaling pathways in the AD pathogenesis. However, the utilization of signaling pathways as potential targets for the development of drugs against AD is rather limited due to the immense complexity of the brain and intricate molecular links between these pathways. Therefore, finding a correlation and cross-talk between these signaling pathways and establishing different therapeutic targets within and between those pathways are needed for better understanding of the biological events responsible for the AD-related neurodegeneration. For example, autophagy is a conservative cellular process that shows link with many other AD-related pathways and is crucial for maintenance of the correct cellular balance by degrading AD-associated pathogenic proteins. Considering the central role of autophagy in AD and its interplay with many other pathways, the finest therapeutic strategy to fight against AD is the use of autophagy as a target. As an essential step in this direction, this comprehensive review represents recent findings on the individual AD-related signaling pathways, describes key features of these pathways and their cross-talk with autophagy, represents current drug development, and introduces some of the multitarget beneficial approaches and strategies for the therapeutic intervention of AD.

Adaptation of mitochondrial network dynamics and velocity of mitochondrial movement to chronic stress present in fibroblasts derived from patients with sporadic form of Alzheimer's disease

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases. Only 10% of all cases are familial form, the remaining 90% are sporadic form with unknown genetic background. The etiology of sporadic AD is still not fully understood. Pathogenesis and pathobiology of this disease are limited due to the limited number of experimental models. We used primary culture of fibroblasts derived from patients diagnosed with sporadic form of AD for investigation of dynamic properties of mitochondria, including fission-fusion process and localization of mitochondria within the cell. We observed differences in mitochondrial network organization with decreased mitochondrial transport velocity, and a drop in the frequency of fusion-fission events. These studies show how mitochondrial dynamics adapt to the conditions of long-term mitochondrial stress that prevails in cells of sporadic form of AD.

Selective Neuron Vulnerability in Common and Rare Diseases-Mitochondria in the Focus

Mitochondrial dysfunction is a central feature of neurodegeneration within the central and peripheral nervous system, highlighting a strong dependence on proper mitochondrial function of neurons with especially high energy consumptions. The fitness of mitochondria critically depends on preservation of distinct processes, including the maintenance of their own genome, mitochondrial dynamics, quality control, and Ca²⁺ handling. These processes appear to be differently affected in common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well as in rare neurological disorders, including Huntington’s disease, Amyotrophic Lateral Sclerosis and peripheral neuropathies. Strikingly, particular neuron populations of different morphology and function perish in these diseases, suggesting that cell-type specific factors contribute to the vulnerability to distinct mitochondrial defects. Here we review the disruption of mitochondrial processes in common as well as in rare neurological disorders and its impact on selective neurodegeneration. Understanding discrepancies and commonalities regarding mitochondrial dysfunction as well as individual neuronal demands will help to design new targets and to make use of already established treatments in order to improve treatment of these diseases.

Effect of Chronic Stress Present in Fibroblasts Derived from Patients with a Sporadic Form of AD on Mitochondrial Function and Mitochondrial Turnover

Although the sporadic form of Alzheimer’s disease (AD) is the prevalent form, the cellular events underlying the disease pathogenesis have not been fully characterized. Accumulating evidence points to mitochondrial dysfunction as one of the events responsible for AD progression. We investigated mitochondrial function in fibroblasts collected from patients diagnosed with the sporadic form of AD (sAD), placing a particular focus on mitochondrial turnover. We measured mitochondrial biogenesis and autophagic clearance, and evaluated the presence of bioenergetic stress in sAD cells. The mitochondrial turnover was clearly lower in the fibroblasts from sAD patients than in the fibroblasts from the control subjects, and the levels of many proteins regulating mitochondrial biogenesis, autophagy and mitophagy were decreased in patient cells. Additionally, the sAD fibroblasts had slightly higher mitochondrial superoxide levels and impaired antioxidant defense. Mitochondrial turnover undergoes feedback regulation through mitochondrial retrograde signaling, which is responsible for the maintenance of optimal mitochondrial functioning, and mitochondria-derived ROS participate as signaling molecules in this process. Our results showed that in sAD patients cells, there is a shift in the balance of mitochondrial function, possibly in response to the presence of cellular stress related to disease development.

Glucose Metabolic Dysfunction in Neurodegenerative Diseases-New Mechanistic Insights and the Potential of Hypoxia as a Prospective Therapy Targeting Metabolic Reprogramming

Glucose is the main circulating energy substrate for the adult brain. Owing to the high energy demand of nerve cells, glucose is actively oxidized to produce ATP and has a synergistic effect with mitochondria in metabolic pathways. The dysfunction of glucose metabolism inevitably disturbs the normal functioning of neurons, which is widely observed in neurodegenerative disease. Understanding the mechanisms of metabolic adaptation during disease progression has become a major focus of research, and interventions in these processes may relieve the neurons from degenerative stress. In this review, we highlight evidence of mitochondrial dysfunction, decreased glucose uptake, and diminished glucose metabolism in different neurodegeneration models such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We also discuss how hypoxia, a metabolic reprogramming strategy linked to glucose metabolism in tumor cells and normal brain cells, and summarize the evidence for hypoxia as a putative therapy for general neurodegenerative disease.

Insulin Resistance as a Common Link Between Current Alzheimer's Disease Hypotheses

Almost 115 years ago, Alois Alzheimer described Alzheimer's disease (AD) for the first time. Since then, many hypotheses have been proposed. However, AD remains a severe health public problem. The current medical approaches for AD are limited to symptomatic interventions and the complexity of this disease has led to a failure rate of approximately 99.6%in AD clinical trials. In fact, no new drug has been approved for AD treatment since 2003. These failures indicate that we are failing in mimicking this disease in experimental models. Although most studies have focused on the amyloid cascade hypothesis of AD, the literature has made clear that AD is rather a multifactorial disorder. Therefore, the persistence in a single theory has resulted in lost opportunities. In this review, we aim to present the striking points of the long scientific path followed since the description of the first AD case and the main AD hypotheses discussed over the last decades. We also propose insulin resistance as a common link between many other hypotheses.

Mitophagy pathways and Alzheimer's disease: From pathogenesis to treatment

Alzheimer's disease (AD) is an age-dependent, incurable mental illness that is associated with the accumulation of aggregates of amyloid-beta (Aβ) and hyperphosphorylated tau fragments (p-tau). Detailed studies on postmortem AD brains, cell lines, and mouse models of AD have shown that numerous cellular alterations, including mitochondrial deficits, synaptic disruption and glial/astrocytic activation, are involved in the disease process. Mitophagy is a cellular process by which damaged/weakened mitochondria are selectively eliminated from the cell. In AD, impairments in mitophagy trigger the gradual accumulation of defective mitochondria. This review will focus on the recent progress in understanding the molecular mechanisms and pathological role of mitophagy and its implications for AD pathogenesis. We will also discuss the novel concept of the regulation of mitophagy as a therapeutic avenue for the prevention and treatment of AD.

Clinical Insights into Mitochondrial Neurodevelopmental and Neurodegenerative Disorders: Their Biosignatures from Mass Spectrometry-Based Metabolomics

Mitochondria are dynamic multitask organelles that function as hubs for many metabolic pathways. They produce most ATP via the oxidative phosphorylation pathway, a critical pathway that the brain relies on its energy need associated with its numerous functions, such as synaptic homeostasis and plasticity. Therefore, mitochondrial dysfunction is a prevalent pathological hallmark of many neurodevelopmental and neurodegenerative disorders resulting in altered neurometabolic coupling. With the advent of mass spectrometry (MS) technology, MS-based metabolomics provides an emerging mechanistic understanding of their global and dynamic metabolic signatures. In this review, we discuss the pathogenetic causes of mitochondrial metabolic disorders and the recent MS-based metabolomic advances on their metabolomic remodeling. We conclude by exploring the MS-based metabolomic functional insights into their biosignatures to improve diagnostic platforms, stratify patients, and design novel targeted therapeutic strategies.

Nilotinib Improves Bioenergetic Profiling in Brain Astroglia in the 3xTg Mouse Model of Alzheimer's Disease

Current treatments targeting amyloid beta in Alzheimer's disease (AD) have minimal efficacy, which results in a huge unmet medical need worldwide. Accumulating data suggest that brain mitochondrial dysfunction play a critical role in AD pathogenesis. Targeting cellular mechanisms associated with mitochondrial dysfunction in AD create a novel approach for drug development. This study investigated the effects of nilotinib, as a selective tyrosine kinase inhibitor, in astroglia derived from 3xTg-AD mice versus their C57BL/6-controls. Parameters included oxygen consumption rates (OCR), ATP, cytochrome c oxidase (COX), citrate synthase (CS) activity, alterations in oxidative phosphorylation (OXPHOS), nuclear factor kappa B (NF-κB), key regulators of mitochondrial dynamics (mitofusin (Mfn1), dynamin-related protein 1 (Drp1)), and mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator1-alpha (PGC-1α), calcium/calmodulin-dependent protein kinase II (CaMKII), and nuclear factor (erythroid-derived 2)-like 2 (Nrf2)). Nilotinib increased OCR, ATP, COX, Mfn1, and OXPHOS levels in 3xTg astroglia. No significant differences were detected in levels of Drp1 protein and CS activity. Nilotinib enhanced mitochondrial numbers, potentially through a CaMKII-PGC1α-Nrf2 pathway in 3xTg astroglia. Additionally, nilotinib-induced OCR increases were reduced in the presence of the NF-κB inhibitor, Bay11-7082. The data suggest that NF-κB signaling is intimately involved in nilotinib-induced changes in bioenergetics in 3xTg brain astroglia. Nilotinib increased translocation of the NF-κB p50 subunit into the nucleus of 3xTg astroglia that correlates with an increased expression and activation of NF-κB. The current findings support a role for nilotinib in improving mitochondrial function and suggest that astroglia may be a key therapeutic target in treating AD.

Mitochondrial Dysfunction and Mitophagy Closely Cooperate in Neurological Deficits Associated with Alzheimer's Disease and Type 2 Diabetes

Alzheimer's disease (AD) and type 2 diabetes (T2D) are known to be correlated in terms of their epidemiology, histopathology, and molecular and biochemical characteristics. The prevalence of T2D leading to AD is approximately 50-70%. Moreover, AD is often considered type III diabetes because of the common risk factors. Uncontrolled T2D may affect the brain, leading to memory and learning deficits in patients. In addition, metabolic disorders and impaired oxidative phosphorylation in AD and T2D patients suggest that mitochondrial dysfunction is involved in both diseases. The dysregulation of pathways involved in maintaining mitochondrial dynamics, biogenesis and mitophagy are responsible for exacerbating the impact of hyperglycemia on the brain and neurodegeneration under T2D conditions. The first section of this review describes the recent views on mitochondrial dysfunction that connect these two disease conditions, as the pathways are observed to overlap. The second section of the review highlights the importance of different mitochondrial miRNAs (mitomiRs) involved in the regulation of mitochondrial dynamics and their association with the pathogenesis of T2D and AD. Therefore, targeting mitochondrial biogenesis and mitophagy pathways, along with the use of mitomiRs, could be a potent therapeutic strategy for T2D-related AD. The last section of the review highlights the known drugs targeting mitochondrial function for the treatment of both disease conditions.

Squamosamide Derivative FLZ Diminishes Aberrant Mitochondrial Fission by Inhibiting Dynamin-Related Protein 1

Mitochondrial dysfunction is involved in the pathogenesis of Parkinson’s disease (PD). Mitochondrial morphology is dynamic and precisely regulated by mitochondrial fission and fusion machinery. Aberrant mitochondrial fragmentation, which can result in cell death, is controlled by the mitochondrial fission protein, dynamin-related protein 1 (Drp1). Our previous results demonstrated that FLZ could correct mitochondrial dysfunction, but the effect of FLZ on mitochondrial dynamics remain uncharacterized. In this study, we investigated the effect of FLZ and the role of Drp1 on 1-methyl-4-phenylpyridinium (MPP⁺)–induced mitochondrial fission in neurons. We observed that FLZ blocked Drp1, inhibited Drp1 enzyme activity, and reduced excessive mitochondrial fission in cultured neurons. Furthermore, by inhibiting mitochondrial fission and ROS production, FLZ improved mitochondrial integrity and membrane potential, resulting in neuroprotection. FLZ curtailed the reduction of synaptic branches of primary cultured dopaminergic neurons caused by MPP⁺ exposure, reduced abnormal fission, restored normal mitochondrial distribution in neurons, and exhibited protective effects on dopaminergic neurons. The in vitro research results were validated using an MPTP-induced PD mouse model. The in vivo results revealed that FLZ significantly reduced the mitochondrial translocation of Drp1 in the midbrain of PD mice, which, in turn, reduced the mitochondrial fragmentation in mouse substantia nigra neurons. FLZ also protected dopaminergic neurons in PD mice and increased the dopamine content in the striatum, which improved the motor coordination ability of the mice. These findings elucidate this newly discovered mechanism through which FLZ produces neuroprotection in PD.

Interleukin-1β mediates alterations in mitochondrial fusion/fission proteins and memory impairment induced by amyloid-β oligomers

Background

The lack of effective treatments for Alzheimer’s disease (AD) reflects an incomplete understanding of disease mechanisms. Alterations in proteins involved in mitochondrial dynamics, an essential process for mitochondrial integrity and function, have been reported in AD brains. Impaired mitochondrial dynamics causes mitochondrial dysfunction and has been associated with cognitive impairment in AD. Here, we investigated a possible link between pro-inflammatory interleukin-1 (IL-1), mitochondrial dysfunction, and cognitive impairment in AD models.

Methods

We exposed primary hippocampal cell cultures to amyloid-β oligomers (AβOs) and carried out AβO infusions into the lateral cerebral ventricle of cynomolgus macaques to assess the impact of AβOs on proteins that regulate mitochondrial dynamics. Where indicated, primary cultures were pre-treated with mitochondrial division inhibitor 1 (mdivi-1), or with anakinra, a recombinant interleukin-1 receptor (IL-1R) antagonist used in the treatment of rheumatoid arthritis. Cognitive impairment was investigated in C57BL/6 mice that received an intracerebroventricular (i.c.v.) infusion of AβOs in the presence or absence of mdivi-1. To assess the role of interleukin-1 beta (IL-1β) in AβO-induced alterations in mitochondrial proteins and memory impairment, interleukin receptor-1 knockout (Il1r1^−/−) mice received an i.c.v. infusion of AβOs.

Results

We report that anakinra prevented AβO-induced alteration in mitochondrial dynamics proteins in primary hippocampal cultures. Altered levels of proteins involved in mitochondrial fusion and fission were observed in the brains of cynomolgus macaques that received i.c.v. infusions of AβOs. The mitochondrial fission inhibitor, mdivi-1, alleviated synapse loss and cognitive impairment induced by AβOs in mice. In addition, AβOs failed to cause alterations in expression of mitochondrial dynamics proteins or memory impairment in Il1r1^−/− mice.

Conclusion

These findings indicate that IL-1β mediates the impact of AβOs on proteins involved in mitochondrial dynamics and that strategies aimed to prevent pathological alterations in those proteins may counteract synapse loss and cognitive impairment in AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02099-x.

Mitochondrial hyperfusion: a friend or a foe

The cellular mitochondrial population undergoes repeated cycles of fission and fusion to maintain its integrity, as well as overall cellular homeostasis. While equilibrium usually exists between the fission-fusion dynamics, their rates are influenced by organellar and cellular metabolic and pathogenic conditions. Under conditions of cellular stress, there is a disruption of this fission and fusion balance and mitochondria undergo either increased fusion, forming a hyperfused meshwork or excessive fission to counteract stress and remove damaged mitochondria via mitophagy. While some previous reports suggest that hyperfusion is initiated to ameliorate cellular stress, recent studies show its negative impact on cellular health in disease conditions. The exact mechanism of mitochondrial hyperfusion and its role in maintaining cellular health and homeostasis, however, remain unclear. In this review, we aim to highlight the different aspects of mitochondrial hyperfusion in either promoting or mitigating stress and also its role in immunity and diseases.

The relevance of mitochondrial morphology for human disease

Mitochondria are highly dynamic organelles, which undergo frequent structural and metabolic changes to fulfil cellular demands. To facilitate these processes several proteins are required to regulate mitochondrial shape and interorganellar communication. These proteins include the classical mitochondrial fusion (MFN1, MFN2, and OPA1) and fission proteins (DRP1, MFF, FIS1, etc.) as well as several other proteins that are directly or indirectly involved in these processes (e.g. YME1L, OMA1, INF2, GDAP1, MIC13, etc.). During the last two decades, inherited genetic defects in mitochondrial fusion and fission proteins have emerged as an important class of neurodegenerative human diseases with variable onset ranging from infancy to adulthood. So far, no causal treatment strategies are available for these disorders. In this review, we provide an overview about the current knowledge on mitochondrial dynamics under physiological conditions. Moreover, we describe human diseases, which are associated with genetic defects in these pathways.

Mitochondria-associated membranes (MAMs): a potential therapeutic target for treating Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is a leading global health concern for individuals and society. However, the potential mechanisms underlying the pathogenesis of AD have not yet been elucidated. Currently, the most widely acknowledged hypothesis is amyloid cascade owing to the brain characteristics of AD patients, including great quantities of extracellular β-amyloid (Aβ) plaques and intracellular neurofibrillary tangles (NFTs). Nevertheless, the amyloid cascade hypothesis cannot address certain pathologies that precede Aβ deposition and NFTs formation in AD, such as aberrant calcium homeostasis, abnormal lipid metabolism, mitochondrial dysfunction and autophagy. Notably, these earlier pathologies are closely associated with mitochondria-associated membranes (MAMs), the physical structures connecting the endoplasmic reticulum (ER) and mitochondria, which mediate the communication between these two organelles. It is plausible that MAMs might be involved in a critical step in the cascade of earlier events, ultimately inducing neurodegeneration in AD. In this review, we focus on the role of MAMs in the regulation of AD pathologies and the potential molecular mechanisms related to MAM-mediated pathological changes in AD. An enhanced recognition of the preclinical pathogenesis in AD could provide new therapeutic strategies, shifting the modality from treatment to prevention.

Mitochondrial Dysfunction in Alzheimer's Disease: A Biomarker of the Future?

Alzheimer's disease (AD) is the most common cause of dementia worldwide and is characterised pathologically by the accumulation of amyloid beta and tau protein aggregates. Currently, there are no approved disease modifying therapies for clearance of either of these proteins from the brain of people with AD. As well as abnormalities in protein aggregation, other pathological changes are seen in this condition. The function of mitochondria in both the nervous system and rest of the body is altered early in this disease, and both amyloid and tau have detrimental effects on mitochondrial function. In this review article, we describe how the function and structure of mitochondria change in AD. This review summarises current imaging techniques that use surrogate markers of mitochondrial function in both research and clinical practice, but also how mitochondrial functions such as ATP production, calcium homeostasis, mitophagy and reactive oxygen species production are affected in AD mitochondria. The evidence reviewed suggests that the measurement of mitochondrial function may be developed into a future biomarker for early AD. Further work with larger cohorts of patients is needed before mitochondrial functional biomarkers are ready for clinical use.

Role of mitochondrial dysfunction, oxidative stress and autophagy in progression of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. The pathological hallmarks of AD are amyloid plaques [aggregates of amyloid beta (A)] and neurofibrillary tangles (aggregates of tau protein). Growing evidence suggests that tau accumulation is pathologically more relevant to the development of neurodegeneration and cognitive decline in AD patients than A plaques. Mitochondrial damage plays an important role in AD. Mitochondrial damage has been related to amyloid-beta or tau pathology or to the presence of specific presenilin-1 mutations. Elevate reactive oxygen species/reactive nitrogen species production and defective mitochondrial dynamic balance has been suggested to be the reason as well as the consequence of AD related pathology. Oxidative stress is a prominent early event in the pathogenesis of AD and is therefore believed to contribute to tau hyperphosphorylation. Several studies have shown that the autophagy pathway in neurons is important under physiological and pathological conditions. Therefore, this pathway plays a crucial role for the degradation of endogenous soluble tau. However, the relationship between mitochondrial dysfunctioning, oxidative stress, autophagy dysregulation, and neuronal cell death in AD remains unclear. Here, we review the latest progress in AD, with a special emphasis on mitochondrial dysfunctioning, oxidative stress, and autophagy. We also discuss the interlink mechanism of these three factors in AD.

Nanomedicines for Subcellular Targeting: The Mitochondrial Perspective

BACKGROUND

Over the past decade, there has been a surge in the number of mitochondrialactive therapeutics for conditions ranging from cancer to aging. Subcellular targeting interventions can modulate adverse intracellular processes unique to the compartments within the cell. However, there is a dearth of reviews focusing on mitochondrial nano-delivery, and this review seeks to fill this gap with regards to nanotherapeutics of the mitochondria.

METHODS

Besides its potential for a higher therapeutic index than targeting at the tissue and cell levels, subcellular targeting takes into account the limitations of systemic drug administration and significantly improves pharmacokinetics. Hence, an extensive literature review was undertaken and salient information was compiled in this review.

RESULTS

From literature, it was evident that nanoparticles with their tunable physicochemical properties have shown potential for efficient therapeutic delivery, with several nanomedicines already approved by the FDA and others in clinical trials. However, strategies for the development of nanomedicines for subcellular targeting are still emerging, with an increased understanding of dysfunctional molecular processes advancing the development of treatment modules. For optimal delivery, the design of an ideal carrier for subcellular delivery must consider the features of the diseased microenvironment. The functional and structural features of the mitochondria in the diseased state are highlighted and potential nano-delivery interventions for treatment and diagnosis are discussed.

CONCLUSION

This review provides an insight into recent advances in subcellular targeting, with a focus on en route barriers to subcellular targeting. The impact of mitochondrial dysfunction in the aetiology of certain diseases is highlighted, and potential therapeutic sites are identified.

Defective mitophagy in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, mental illness without cure. Several years of intense research on postmortem AD brains, cell and mouse models of AD have revealed that multiple cellular changes are involved in the disease process, including mitochondrial abnormalities, synaptic damage, and glial/astrocytic activation, in addition to age-dependent accumulation of amyloid beta (Aβ) and hyperphosphorylated tau (p-tau). Synaptic damage and mitochondrial dysfunction are early cellular changes in the disease process. Healthy and functionally active mitochondria are essential for cellular functioning. Dysfunctional mitochondria play a central role in aging and AD. Mitophagy is a cellular process whereby damaged mitochondria are selectively removed from cell and mitochondrial quality and biogenesis. Mitophagy impairments cause the progressive accumulation of defective organelle and damaged mitochondria in cells. In AD, increased levels of Aβ and p-tau can induce reactive oxygen species (ROS) production, causing excessive fragmentation of mitochondria and promoting defective mitophagy. The current article discusses the latest developments of mitochondrial research and also highlights multiple types of mitophagy, including Aβ and p-tau-induced mitophagy, stress-induced mitophagy, receptor-mediated mitophagy, ubiquitin mediated mitophagy and basal mitophagy. This article also discusses the physiological states of mitochondria, including fission-fusion balance, Ca²⁺ transport, and mitochondrial transport in normal and diseased conditions. Our article summarizes current therapeutic interventions, like chemical or natural mitophagy enhancers, that influence mitophagy in AD. Our article discusses whether a partial reduction of Drp1 can be a mitophagy enhancer and a therapeutic target for mitophagy in AD and other neurological diseases.

The Crosstalk Between Pathological Tau Phosphorylation and Mitochondrial Dysfunction as a Key to Understanding and Treating Alzheimer's Disease

Alzheimer's disease (AD) is the most common progressive neurodegenerative disorder. A defining hallmark of the AD brain is the presence of intraneuronal neurofibrillary tangles (NFTs) which are made up of abnormally modified tau, with aberrant phosphorylation being the most studied posttranslational modification (PTM). Although the accumulation of tau as NFTs is an invariant feature of the AD brain, it has become evident that these insoluble aggregates are likely not the primary pathogenic form of tau, rather soluble forms of tau with abnormal PTMs are the mediators of toxicity. The most prevalent PTM on tau is phosphorylation, with the abnormal modification of specific residues on tau playing a key role in its toxicity. Even though it is widely accepted that tau with aberrant PTMs facilitates neurodegeneration, the precise cellular mechanisms remain unknown. Nonetheless, there is an evolving conceptual framework that an important contributing factor may be selective pathological tau species compromising mitochondrial biology. Understanding the mechanisms by which tau with site-specific PTM impacts mitochondria is crucial for understanding the role tau plays in AD. Here, we provide a brief introduction to tau and its phosphorylation and function in a physiological context, followed by a discussion of the impact of soluble phosphorylated tau species on neuronal processes in general and mitochondria more specifically. We also discuss how therapeutic strategies that attenuate pathological tau species in combination with treatments that improve mitochondrial biology could be a potential therapeutic avenue to mitigate disease progression in AD and other tauopathies.

Age-Dependent Decline in Synaptic Mitochondrial Function Is Exacerbated in Vulnerable Brain Regions of Female 3xTg-AD Mice

Synaptic aging has been associated with neuronal circuit dysfunction and cognitive decline. Reduced mitochondrial function may be an early event that compromises synaptic integrity and neurotransmission in vulnerable brain regions during physiological and pathological aging. Thus, we aimed to measure mitochondrial function in synapses from three brain regions at two different ages in the 3xTg-AD mouse model and in wild mice. We found that aging is the main factor associated with the decline in synaptic mitochondrial function, particularly in synapses isolated from the cerebellum. Accumulation of toxic compounds, such as tau and Aβ, that occurred in the 3xTg-AD mouse model seemed to participate in the worsening of this decline in the hippocampus. The changes in synaptic bioenergetics were also associated with increased activation of the mitochondrial fission protein Drp1. These results suggest the presence of altered mechanisms of synaptic mitochondrial dynamics and their quality control during aging and in the 3xTg-AD mouse model; they also point to bioenergetic restoration as a useful therapeutic strategy to preserve synaptic function during aging and at the early stages of Alzheimer's disease (AD).

Alterations in Mitochondrial Dynamic-related Genes in the Peripheral Blood of Alzheimer's Disease Patients

BACKGROUND

Mitochondrial dysfunction is a pathological feature that manifests early in the brains of patients with Alzheimer's Disease (AD). The disruption of mitochondrial dynamics contributes to mitochondrial morphological and functional impairments. Our previous study demonstrated that the expression of genes involved in amyloid beta generation was altered in the peripheral blood of AD patients.

OBJECTIVE

The aim of this study was to further investigate the relative levels of mitochondrial genes involved in mitochondrial dynamics, including mitochondrial fission and fusion, and mitophagy in peripheral blood samples from patients with AD compared to healthy controls.

METHODS

The mRNA levels were analyzed by real-time polymerase chain reaction. Gene expression profiles were assessed in relation to cognitive performance.

RESULTS

Significant changes were observed in the mRNA expression levels of fission-related genes; Fission1 (FIS1) levels in AD subjects were significantly higher than those in healthy controls, whereas Dynamin- related protein 1 (DRP1) expression was significantly lower in AD subjects. The levels of the mitophagy-related genes, PTEN-induced kinase 1 (PINK1) and microtubule-associated protein 1 light chain 3 (LC3), were significantly increased in AD subjects and elderly controls compared to healthy young controls. The mRNA levels of Parkin (PARK2) were significantly decreased in AD. Correlations were found between the expression levels of FIS1, DRP1 and PARK2 and cognitive performance scores.

CONCLUSION

Alterations in mitochondrial dynamics in the blood may reflect impairments in mitochondrial functions in the central and peripheral tissues of AD patients. Mitochondrial fission, together with mitophagy gene profiles, might be potential considerations for the future development of blood-based biomarkers for AD.

Insulin Resistance at the Crossroad of Alzheimer Disease Pathology: A Review

Insulin plays a major neuroprotective and trophic function for cerebral cell population, thus countering apoptosis, beta-amyloid toxicity, and oxidative stress; favoring neuronal survival; and enhancing memory and learning processes. Insulin resistance and impaired cerebral glucose metabolism are invariantly reported in Alzheimer's disease (AD) and other neurodegenerative processes. AD is a fatal neurodegenerative disorder in which progressive glucose hypometabolism parallels to cognitive impairment. Although AD may appear and progress in virtue of multifactorial nosogenic ingredients, multiple interperpetuative and interconnected vicious circles appear to drive disease pathophysiology. The disease is primarily a metabolic/energetic disorder in which amyloid accumulation may appear as a by-product of more proximal events, especially in the late-onset form. As a bridge between AD and type 2 diabetes, activation of c-Jun N-terminal kinase (JNK) pathway with the ensued serine phosphorylation of the insulin response substrate (IRS)-1/2 may be at the crossroads of insulin resistance and its subsequent dysmetabolic consequences. Central insulin axis bankruptcy translates in neuronal vulnerability and demise. As a link in the chain of pathogenic vicious circles, mitochondrial dysfunction, oxidative stress, and peripheral/central immune-inflammation are increasingly advocated as major pathology drivers. Pharmacological interventions addressed to preserve insulin axis physiology, mitochondrial biogenesis-integral functionality, and mitophagy of diseased organelles may attenuate the adjacent spillover of free radicals that further perpetuate mitochondrial damages and catalyze inflammation. Central and/or peripheral inflammation may account for a local flood of proinflammatory cytokines that along with astrogliosis amplify insulin resistance, mitochondrial dysfunction, and oxidative stress. All these elements are endogenous stressor, pro-senescent factors that contribute to JNK activation. Taken together, these evidences incite to identify novel multi-mechanistic approaches to succeed in ameliorating this pandemic affliction.

A Glimmer of Hope: Maintain Mitochondrial Homeostasis to Mitigate Alzheimer's Disease

Mitochondria are classically known to be cellular energy producers. Given the high-energy demanding nature of neurons in the brain, it is essential that the mitochondrial pool remains healthy and provides a continuous and efficient supply of energy. However, mitochondrial dysfunction is inevitable in aging and neurodegenerative diseases. In Alzheimer’s disease (AD), neurons experience unbalanced homeostasis like damaged mitochondrial biogenesis and defective mitophagy, with the latter promoting the disease-defining amyloid β (Aβ) and p-Tau pathologies impaired mitophagy contributes to inflammation and the aggregation of Aβ and p-Tau-containing neurotoxic proteins. Interventions that restore defective mitophagy may, therefore, alleviate AD symptoms, pointing out the possibility of a novel therapy. This review aims to illustrate mitochondrial biology with a focus on mitophagy and propose strategies to treat AD while maintaining mitochondrial homeostasis.

The silence of the fats: A MAM's story about Alzheimer

The discovery of contact sites was a breakthrough in cell biology. We have learned that an organelle cannot function in isolation, and that many cellular functions depend on communication between two or more organelles. One such contact site results from the close apposition of the endoplasmic reticulum (ER) and mitochondria, known as mitochondria-associated ER membranes (MAMs). These intracellular lipid rafts serve as hubs for the regulation of cellular lipid and calcium homeostasis, and a growing body of evidence indicates that MAM domains modulate cellular function in both health and disease. Indeed, MAM dysfunction has been described as a key event in Alzheimer disease (AD) pathogenesis. Our most recent work shows that, by means of its affinity for cholesterol, APP-C99 accumulates in MAM domains of the ER and induces the uptake of extracellular cholesterol as well as its trafficking from the plasma membrane to the ER. As a result, MAM functionality becomes chronically upregulated while undergoing continual turnover. The goal of this review is to discuss the consequences of C99 elevation in AD, specifically the upregulation of cholesterol trafficking and MAM activity, which abrogate cellular lipid homeostasis and disrupt the lipid composition of cellular membranes. Overall, we present a novel framework for AD pathogenesis that can be linked to the many complex alterations that occur during disease progression, and that may open a door to new therapeutic strategies.

MAM and C99, key players in the pathogenesis of Alzheimer's disease

Inter-organelle communication is a rapidly-expanding field that has transformed our understanding of cell biology and pathology. Organelle-organelle contact sites can generate transient functional domains that act as enzymatic hubs involved in the regulation of cellular metabolism and intracellular signaling. One of these hubs is located in areas of the endoplasmic reticulum (ER) connected to mitochondria, called mitochondria-associated ER membranes (MAM). These MAM are transient lipid rafts intimately involved in cholesterol and phospholipid metabolism, calcium homeostasis, and mitochondrial function and dynamics. In addition, γ-secretase-mediated proteolysis of the amyloid precursor protein 99-aa C-terminal fragment (C99) to form amyloid β also occurs at the MAM. Our most recent data indicates that in Alzheimer's disease, increases in uncleaved C99 levels at the MAM provoke the upregulation of MAM-resident functions, resulting in the loss of lipid homeostasis, and mitochondrial dysfunction. Here, we discuss the relevance of these findings in the field, and the contribution of C99 and MAM dysfunction to Alzheimer's disease neuropathology.

The mitochondrial hypothesis: Dysfunction, bioenergetic defects, and the metabolic link to Alzheimer's disease

Alzheimer's disease (AD) features mitochondrial dysfunction and altered metabolism. Other pathologies could drive these changes, or alternatively these changes could drive other pathologies. In considering this question, it is worth noting that perturbed AD patient mitochondrial and metabolism dysfunction extend beyond the brain and to some extent define a systemic phenotype. It is difficult to attribute this systemic phenotype to brain beta-amyloid or tau proteins. Conversely, mitochondria increasingly appear to play a critical role in cell proteostasis, which suggests that mitochondrial dysfunction may promote protein aggregation. Mitochondrial and metabolism-related characteristics also define AD endophenotypes in cognitively normal middle-aged individuals, which suggests that mitochondrial and metabolism-related AD characteristics precede clinical decline. Genetic analyses increasingly implicate mitochondria and metabolism-relevant genes in AD risk. Collectively these factors suggest that mitochondria are more relevant to the causes of AD than its consequences, and support the view that a mitochondrial cascade features prominently in AD. This chapter reviews the case for mitochondrial and metabolism dysfunction in AD and the challenges of proving that a primary mitochondrial cascade is pertinent to the disease.

Proteomic Profile of Mouse Brain Aging Contributions to Mitochondrial Dysfunction, DNA Oxidative Damage, Loss of Neurotrophic Factor, and Synaptic and Ribosomal Proteins

The deleterious effects of aging on the brain remain to be fully elucidated. In the present study, proteomic changes of young (4-month) and aged (16-month) B6129SF2/J male mouse hippocampus and cerebral cortex were investigated by using nano liquid chromatography tandem mass spectrometry (NanoLC-ESI-MS/MS) combined with tandem mass tag (TMT) labeling technology. Compared with the young animals, 390 hippocampal proteins (121 increased and 269 decreased) and 258 cortical proteins (149 increased and 109 decreased) changed significantly in the aged mouse. Bioinformatic analysis indicated that these proteins are mainly involved in mitochondrial functions (FIS1, DRP1), oxidative stress (PRDX6, GSTP1, and GSTM1), synapses (SYT12, GLUR2), ribosome (RPL4, RPS3), cytoskeletal integrity, transcriptional regulation, and GTPase function. The mitochondrial fission-related proteins FIS1 and DRP1 were significantly increased in the hippocampus and cerebral cortex of the aged mice. Further results in the hippocampus showed that ATP content was significantly reduced in aged mice. A neurotrophin brain-derived neurotrophic factor (BNDF), a protein closely related with synaptic plasticity and memory, was also significantly decreased in the hippocampus of the aged mice, with the tendency of synaptic protein markers including complexin-2, synaptophysin, GLUR2, PSD95, NMDAR2A, and NMDAR1. More interestingly, 8-hydroxydeoxyguanosine (8-OHdG), a marker of DNA oxidative damage, increased as shown by immunofluorescence staining. In summary, we demonstrated that aging is associated with systemic changes involving mitochondrial dysfunction, energy reduction, oxidative stress, loss of neurotrophic factor, synaptic proteins, and ribosomal proteins, as well as molecular deficits involved in various physiological/pathological processes.

The distinctive role of tau and amyloid beta in mitochondrial dysfunction through alteration in Mfn2 and Drp1 mRNA Levels: A comparative study in Drosophila melanogaster

Alzheimer's disease (AD) is one of the most common forms of neurodegenerative diseases. Aggregation of Aβ42 and hyperphosphorylated tau are two major hallmarks of AD. Whether different forms of tau (soluble or hyperphosphorylated) or Aβ are the main culprit in the events observed in AD is still under investigation. Here, we examined the effect of wild-type, prone to hyperphosphorylation and hyperphosphorylated tau, and also Aβ42 peptide on the brain antioxidant defense system and two mitochondrial genes, Marf (homologous to human MFN2) and Drp1 involved in mitochondrial dynamics in transgenic Drosophila melanogaster. AD is an age associated disease. Therefore, the activity of antioxidant agents, CAT, SOD, and GSH levels and the mRNA levels of Marf and Drp1 were assessed in different time points of the flies lifespan. Reduction in cognitive function and antioxidant activity was observed in all transgenic flies at any time point. The most and the least effect on the eye phenotype was exerted by hyperphosphorylated tau and Aβ42, respectively. In addition, the most remarkable alteration in Marf and Drp1 mRNA levels was observed in transgenic flies expressing hyperphosphorylated tau when pan neuronal expression of transgenes was applied. However, when the disease causing gene expression was confined to the mushroom body, Marf and Drp1 mRNA levels alteration was more prominent in tau^WT and tau^E14 transgenic flies, respectively. In conclusion, in spite of antioxidant deficiency caused by different types of tau and Aβ42, it seems that tau exerts more toxic effect on the eye phenotype and mitochondrial genes regulation (Marf and Drp1). Moreover, different mechanisms seem to be involved in mitochondrial genes dysregulation when Aβ or various forms of tau are expressed.

New perspectives on the role of Drp1 isoforms in regulating mitochondrial pathophysiology

Mitochondria are dynamic organelles constantly undergoing fusion and fission. A concerted balance between the process of mitochondrial fusion and fission is required to maintain cellular health under different physiological conditions. Mutation and dysregulation of Drp1, the major driver of mitochondrial fission, has been associated with various neurological, oncological and cardiovascular disorders. Moreover, when subjected to pathological insults, mitochondria often undergo excessive fission, generating fragmented and dysfunctional mitochondria leading to cell death. Therefore, manipulating mitochondrial fission by targeting Drp1 has been an appealing therapeutic approach for cytoprotection. However, studies have been inconsistent. Studies employing Drp1 constructs representing alternate Drp1 isoforms, have demonstrated differing impacts of these isoforms on mitochondrial fission and cell death. Furthermore, there are distinct expression patterns of Drp1 isoforms in different tissues, suggesting idiosyncratic engagement in specific cellular functions. In this review, we will discuss these inherent variations among human Drp1 isoforms and how they could affect Drp1-mediated mitochondrial fission and cell death.

The Role of Mitochondria in Neurodegenerative Diseases: the Lesson from Alzheimer's Disease and Parkinson's Disease

Although the pathogenesis of neurodegenerative diseases is still widely unclear, various mechanisms have been proposed and several pieces of evidence are supportive for an important role of mitochondrial dysfunction. The present review provides a comprehensive and up-to-date overview about the role of mitochondria in the two most common neurodegenerative disorders: Alzheimer's disease (AD) and Parkinson's disease (PD). Mitochondrial involvement in AD is supported by clinical features like reduced glucose and oxygen brain metabolism and by numerous microscopic and molecular findings, including altered mitochondrial morphology, impaired respiratory chain function, and altered mitochondrial DNA. Furthermore, amyloid pathology and mitochondrial dysfunction seem to be bi-directionally correlated. Mitochondria have an even more remarkable role in PD. Several hints show that respiratory chain activity, in particular complex I, is impaired in the disease. Mitochondrial DNA alterations, involving deletions, point mutations, depletion, and altered maintenance, have been described. Mutations in genes directly implicated in mitochondrial functioning (like Parkin and PINK1) are responsible for rare genetic forms of the disease. A close connection between alpha-synuclein accumulation and mitochondrial dysfunction has been observed. Finally, mitochondria are involved also in atypical parkinsonisms, in particular multiple system atrophy. The available knowledge is still not sufficient to clearly state whether mitochondrial dysfunction plays a primary role in the very initial stages of these diseases or is secondary to other phenomena. However, the presented data strongly support the hypothesis that whatever the initial cause of neurodegeneration is, mitochondrial impairment has a critical role in maintaining and fostering the neurodegenerative process.

Changes in PGC-1α/SIRT1 Signaling Impact on Mitochondrial Homeostasis in Amyloid-Beta Peptide Toxicity Model

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive impairment that increasingly afflicts the elderly population. Soluble oligomers (AβOs) has been implicated in AD pathogenesis: however, the molecular events underlying a role for Aβ are not well understood. We studied the effects of AβOs on mitochondrial function and on key proteins that regulate mitochondrial dynamics and biogenesis in hippocampal neurons and PC-12 cells. We find that AβOs treatment caused a reduction in total Mfn1 after a 2 h exposure (42 ± 11%); while DRP1 increased at 1 and 2 h (205 ± 22% and 198 ± 27%, respectively), correlating to changes in mitochondrial morphology. We also observed that SIRT1 levels were reduced after acute and chronic AβOs treatment (68 ± 7% and 77 ± 6%, respectively); while PGC-1α levels were reduced with the same time treatments (68 ± 8% and 67 ± 7%, respectively). Interestingly, we found that chronic treatment with AβOs increased the levels of pSIRT1 (24 h: 157 ± 18%), and we observed changes in the PGC-1α and p-SIRT1 nucleus/cytosol ratio and SIRT1-PGC-1α interaction pattern after chronic exposure to AβOs. Our data suggest that AβOs induce important changes in the level and localization of mitochondrial proteins related with the loss of mitochondrial function that are mediated by a fast and sustained SIRT1/PGC-1α complex disruption promoting a “non-return point” to an irreversible synaptic failure and neuronal network disconnection.

The ubiquitous role of mitochondria in Parkinson and other neurodegenerative diseases

Orderly mitochondrial life cycle, plays a key role in the pathology of neurodegenerative diseases. Mitochondria are ubiquitous in neurons as they respond to an ever-changing demand for energy supply. Mitochondria constantly change in shape and location, feature of their dynamic nature, which facilitates a quality control mechanism. Biological studies in mitochondria dynamics are unveiling the mechanisms of fission and fusion, which essentially arrange morphology and motility of these organelles. Control of mitochondrial network homeostasis is a critical factor for the proper function of neurons. Disease-related genes have been reported to be implicated in mitochondrial dysfunction. Increasing evidence implicate mitochondrial perturbation in neuronal diseases, such as AD, PD, HD, and ALS. The intricacy involved in neurodegenerative diseases and the dynamic nature of mitochondria point to the idea that, despite progress toward detecting the biology underlying mitochondrial disorders, its link to these diseases is difficult to be identified in the laboratory. Considering the need to model signaling pathways, both in spatial and temporal level, there is a challenge to use a multiscale modeling framework, which is essential for understanding the dynamics of a complex biological system. The use of computational models in order to represent both a qualitative and a quantitative structure of mitochondrial homeostasis, allows to perform simulation experiments so as to monitor the conformational changes, as well as the intersection of form and function.

Mechanisms of mitochondrial DNA escape and its relationship with different metabolic diseases

It is well-known that mitochondrial DNA (mtDNA) can escape to intracellular or extracellular compartments under different stress conditions, yet understanding their escape mechanisms remains a challenge. Although Bax/Bak pores and VDAC oligomers are the strongest possibilities, other mechanisms may be involved. For example, mitochondria permeability transition, altered mitophagy, and mitochondrial dynamics are associated with intracellular mtDNA escape, while extracellular traps and extracellular vesicles can participate in extracellular mtDNA escape. The evidence suggests that mtDNA escape is a complex event with more than one mechanism involved. In addition, once the mtDNA is outside the mitochondria, the effects can be complex. Different danger signal sensors recognize the mtDNA as a damage-associated molecular pattern, triggering an innate immune inflammatory response that can be observed in multiple metabolic diseases characterized by chronic inflammation, including autoimmune diseases, diabetes, cancer, and cardiovascular disorders. For these reasons, we will review the most recent evidence regarding mtDNA escape mechanisms and their impact on different metabolic diseases.

Back to The Fusion: Mitofusin-2 in Alzheimer's Disease

Mitochondria are dynamic organelles that undergo constant fission and fusion. Mitochondria dysfunction underlies several human disorders, including Alzheimer’s disease (AD). Preservation of mitochondrial dynamics is fundamental for regulating the organelle’s functions. Several proteins participate in the regulation of mitochondrial morphology and networks, and among these, Mitofusin 2 (Mfn2) has been extensively studied. This review focuses on the role of Mfn2 in mitochondrial dynamics and in the crosstalk between mitochondria and the endoplasmic reticulum, in particular in AD. Understanding how this protein may be related to AD pathogenesis will provide essential information for the development of therapies for diseases linked to disturbed mitochondrial dynamics, as in AD.

AD "Statin": Alzheimer's Disorder is a "Fast" Disease Preventable by Therapeutic Intervention Initiated Even Late in Life and Reversible at the Early Stages

The present study posits that Alzheimer's disorder is a "fast" disease. This is in sharp contrast to a view, prevailing until now, that Alzheimer's Disease (AD) is a quintessential "slow" disease that develops throughout the life as one prolonged process. According to this view, beta-amyloid (Aβ) is produced and secreted solely by the beta-amyloid precursor protein (βAPP) proteolytic/secretory pathway. As its extracellular levels increase, it triggers neurodegeneration starting relatively early in life. Damages accumulate and manifest, late in life in sporadic Alzheimer's Disease (SAD) cases, as AD symptoms. In familial AD (FAD) cases, where mutations in βAPP gene or in presenilins increase production of either common Aβ isoform or of its more toxic isoforms, neurodegeneration reaches critical threshold sooner and AD symptoms occur earlier in life, mostly in late 40s and 50s. There are currently no preventive AD therapies but if they were available, according to this viewpoint it would be largely futile to intervene late in life in case of potential SAD or at mid-age in cases of FAD because, although AD symptoms have not yet manifested, the damage has already occurred during the preceding decades. In this paradigm, to be effective, preventive therapeutic intervention should be initiated early in life. The outlook suggested by the present study is radically different. According to it, Alzheimer's disease evolves in two stages. The first stage is a slow process of intracellular beta-amyloid accumulation. It occurs via βAPP proteolytic/secretory pathway and cellular uptake of secreted Aβ common to Homo sapiens, including healthy humans, and to non-human mammals, and results neither in significant damage, nor in manifestation of the disease. The second stage occurs exclusively in humans, commences shortly before symptomatic onset of the disease, sharply accelerates the production and increases intracellular levels of Aβ that is not secreted but is retained intracellularly, generates significant damages, triggers AD symptoms, and is fast. It is driven by an Aβ generation pathway qualitatively and quantitatively different from βAPP proteolytic process and entirely independent of beta-amyloid precursor protein, and results in rapid and substantial intracellular accumulation of Aβ, consequent significant neurodegeneration, and symptomatic AD. In this paradigm, a preventive therapy for AD, an AD "statin", would be effective when initiated at any time prior to commencement of the second stage. Moreover, there are good reasons to believe that with a drug blocking βAPP-independent Aβ production pathway in the second stage, it would be possible not only to preempt the disease but also to stop and to reverse it even when early AD symptoms have already manifested. The present study posits a notion of AD as a Fast Disease, offers evidence for the occurrence of the AD-specific Aβ production pathway, describes cellular and molecular processes constituting an engine that drives Alzheimer's disease, and explains why non-human mammals are not susceptible to AD and why only a subset of humans develop the disease. It establishes that Alzheimer's disease is preventable by therapeutic intervention initiated even late in life, details a powerful mechanism underlying the disease, suggests that Aβ produced in the βAPP-independent pathway is retained intracellularly, elaborates why neither BACE inhibition nor Aβ immunotherapy are effective in treatment of AD and why intracellularly retained beta-amyloid could be the primary agent of neuronal death in Alzheimer's disease, necessitates generation of a novel animal AD model capable of producing Aβ via βAPP-independent pathway, proposes therapeutic targets profoundly different from previously pursued components of the βAPP proteolytic pathway, and provides conceptual rationale for design of drugs that could be used not only preemptively but also for treatment and reversal of the early stages of the disease.

Alzheimer's Disease is Driven by Intraneuronally Retained Beta-Amyloid Produced in the AD-Specific, βAPP-Independent Pathway: Current Perspective and Experimental Models for Tomorrow

A view of the origin and progression of Alzheimer's disease, AD, prevailing until now and formalized as the Amyloid Cascade Hypothesis theory, maintains that the disease is initiated by overproduction of beta-amyloid, Aβ, which is generated solely by the Aβ precursor protein, βAPP, proteolytic pathway and secreted from the cell. Consequent extracellular accumulation of Aβ triggers a cascade of molecular and cellular events leading to neurodegeneration that starts early in life, progresses as one prolonged process, builds up for decades, and culminates in symptomatic manifestations of the disease late in life. In this paradigm, a time window for commencement of therapeutic intervention is small and accessible only early in life. The outlook introduced in the present study is fundamentally different. It posits that the βAPP proteolytic/secretory pathway of Aβ production causes AD in humans no more than it does in either short- or long-lived non-human mammals that share this pathway with humans, accumulate beta-amyloid as they age, but do not develop the disease. Alzheimer's disease, according to this outlook, is driven by an additional powerful AD-specific pathway of Aβ production that operates in affected humans, is completely independent of the βAPP precursor, and is not available in non-human mammals. The role of the βAPP proteolytic pathway in the disease in humans is activation of this additional AD-specific Aβ production pathway. This occurs through accumulation of intracellular Aβ, primarily via ApoE-assisted cellular uptake of secreted beta-amyloid, but also through retention of a fraction of Aβ produced in the βAPP proteolytic pathway. With time, accumulated intracellular Aβ triggers mitochondrial dysfunction. In turn, cellular stresses associated with mitochondrial dysfunction, including ER stress, activate a second, AD-specific, Aβ production pathway: Asymmetric RNA-dependent βAPP mRNA amplification; animal βAPP mRNA is ineligible for this process. In this pathway, every conventionally produced βAPP mRNA molecule serves potentially as a template for production of severely 5'-truncated mRNA encoding not the βAPP but its C99 fragment (hence "asymmetric"), the immediate precursor of Aβ. Thus produced, N-terminal signal peptide-lacking C99 is processed not in the secretory pathway on the plasma membrane, but at the intracellular membrane sites, apparently in a neuron-specific manner. The resulting Aβ is, therefore, not secreted but is retained intraneuronally and accumulates rapidly within the cell. Increased levels of intracellular Aβ augment mitochondrial dysfunction, which, in turn, sustains the activity of the βAPP mRNA amplification pathway. These self-propagating mutual Aβ overproduction/mitochondrial dysfunction feedback cycles constitute a formidable two-stroke engine, an engine that drives Alzheimer's disease. The present outlook envisions Alzheimer's disorder as a two-stage disease. The first stage is a slow process of intracellular beta-amyloid accumulation. It results neither in significant neurodegenerative damage, nor in manifestation of the disease. The second stage commences with the activation of the βAPP mRNA amplification pathway shortly before symptomatic onset of the disease, sharply increases the rate of Aβ generation and the extent of its intraneuronal accumulation, produces significant damages, triggers AD symptoms, and is fast. In this paradigm, the time window of therapeutic intervention is wide open, and preventive treatment can be initiated any time, even late in life, prior to commencement of the second stage of the disease. Moreover, there are good reasons to believe that with a drug blocking the βAPP mRNA amplification pathway, it would be possible not only to preempt the disease but also to stop and to reverse it even when early AD symptoms have already manifested. There are numerous experimental models of AD, all based on a notion of the exceptionality of βAPP proteolytic/secretory pathway in Aβ production in the disease. However, with no drug even remotely effective in Alzheimer's disease, a long list of candidate drugs that succeeded remarkably in animal models, yet failed utterly in human clinical trials of potential AD drugs, attests to the inadequacy of currently employed AD models. The concept of a renewable supply of beta-amyloid, produced in the βAPP mRNA amplification pathway and retained intraneuronally in Alzheimer's disease, explains spectacular failures of both BACE inhibition and Aβ-immunotherapy in human clinical trials. This concept also forms the basis of a new generation of animal and cell-based experimental models of AD, described in the present study. These models incorporate Aβ- or C99-encoding mRNA amplification pathways of Aβ production, as well as intracellular retention of their product, and can support not only further investigation of molecular mechanisms of AD but also screening for and testing of candidate drugs aimed at therapeutic targets suggested by the present study.

The Role of Mitochondrial Impairment in Alzheimer´s Disease Neurodegeneration: The Tau Connection

Accumulative evidence has shown that mitochondrial dysfunction plays a pivotal role in the pathogenesis of Alzheimer's disease (AD). Mitochondrial impairment actively contributes to the synaptic and cognitive failure that characterizes AD. The presence of soluble pathological forms of tau like hyperphosphorylated at Ser396 and Ser404 and cleaved at Asp421 by caspase 3, negatively impacts mitochondrial bioenergetics, transport, and morphology in neurons. These adverse effects against mitochondria health will contribute to the synaptic impairment and cognitive decline in AD. Current studies suggest that mitochondrial failure induced by pathological tau forms is likely the result of the opening of the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial mega-channel that is activated by increases in calcium and is associated with mitochondrial stress and apoptosis. This structure is composed of different proteins, where Ciclophilin D (CypD) is considered to be the primary mediator of mPTP activation. Also, new studies suggest that mPTP contributes to Aβ pathology and oxidative stress in AD. Further, inhibition of mPTP through the reduction of CypD expression prevents cognitive and synaptic impairment in AD mouse models. More importantly, tau protein contributes to the physiological regulation of mitochondria through the opening/interaction with mPTP in hippocampal neurons. Therefore, in this paper, we will discuss evidence that suggests an important role of pathological forms of tau against mitochondrial health. Also, we will discuss the possible role of mPTP in the mitochondrial impairment produced by the presence of tau pathology and its impact on synaptic function present in AD.

Blockage of ROS-ERK-DLP1 signaling and mitochondrial fission alleviates Cr(VI)-induced mitochondrial dysfunction in L02 hepatocytes

Hexavalent chromium [Cr(VI)] is a common heavy metal pollutant widely used in various industrial fields. It is well known that mitochondria are the most vulnerable targets of heavy metals, but the key molecule/event that directly mediated mitochondrial dysfunction after Cr(VI) exposure is still unclear. The present study was aimed to explore whether Cr(VI) exposure could affect the mitochondrial fission/fusion process, and whether the related abnormal mitochondrial dynamics have been implicated in Cr(VI)-induced mitochondrial dysfunction. We found that the mitochondrial dysfunction caused by Cr(VI) exposure was characterized by decreased mitochondrial respiratory chain complex (MRCC) I/II activities and levels, collapsed mitochondrial membrane potential (MMP), depleted ATP, and increased reactive oxygen species (ROS) level. Cr(VI) induced abnormal mitochondrial fission/fusion events, the antioxidant Nacetyl-L-cysteine (NAC) restored the abnormal mitochondrial function as well as the fission/fusion dynamics. ROS was the up-stream regulator of extracellular regulated protein kinases (ERK) signaling, and the application of a specific ERK1/2 inhibitor PD98059 confirmed that activation of ERK1/2 signaling was associated with the abnormal mitochondrial fission/fusion and mitochondrial dysfunction. We also demonstrated that treatment with dynamic-like protein 1 (DLP1)-siRNA rescued mitochondrial dysfunction in Cr(VI)-exposed L02 hepatocytes. We reached the conclusion that blockage of ROS-ERK-DLP1 signaling and mitochondrial fission alleviates Cr(VI)-induced mitochondrial dysfunction in L02 hepatocytes, which may provide the new avenue for developing effective strategies to protect against Cr(VI)-induced hepatotoxicity.

Precursor-Independent Overproduction of Beta-Amyloid in AD: Mitochondrial Dysfunction as Possible Initiator of Asymmetric RNA-Dependent βAPP mRNA Amplification. An Engine that Drives Alzheimer's Disease

The present study defines RNA-dependent amplification of βAPP mRNA as a molecular basis of beta-amyloid overproduction in Alzheimer's disease. In this process, βAPP mRNA serves as a template for RNA-dependent RNA polymerase, RdRp complex. The resulting antisense RNA self-primes its extension utilizing two complementary elements: 3'-terminal and internal, located within an antisense segment corresponding to the coding portion of βAPP mRNA. The extension produces 3'-terminal fragment of βAPP mRNA, a part of a hairpin-structured antisense/sense RNA molecule. Cleavage at the 3' end of the hairpin loop produces RNA end product encoding a C-terminal fragment of βAPP. Since each conventional βAPP mRNA can be used repeatedly as a template, the process constitutes an asymmetric mRNA amplification. The 5'-most translation initiation codon of the amplified mRNA is the AUG preceding immediately and in-frame the Aβ-coding segment. Translation from this codon overproduces Aβ independently of βAPP. Such process can occur in humans but not in mice and other animals where segments of βAPP antisense RNA required for self-priming have little, if any, complementarity. This explains why Alzheimer's disease occurs exclusively in humans and implies that βAPP mRNA amplification is requisite in AD. In AD, therefore, there are two pathways of beta-amyloid production: βAPP proteolytic pathway and βAPP mRNA amplification pathway independent of βAPP and insensitive to beta-secretase inhibition. This implies that in healthy humans, where only the proteolytic pathway is in operation, Aβ production should be suppressed by the BACE inhibition, and indeed it is. However, since βAPP-independent pathway operating in AD is by far the predominant one, BACE inhibition has no effect in Alzheimer's disease. It appears that, physiologically, the extent of beta-amyloid overproduction sufficient to trigger amyloid cascade culminating in AD requires asymmetric RNA-dependent amplification of βAPP mRNA and cannot be reached without it. In turn, the occurrence of mRNA amplification process depends on the activation of inducible components of RdRp complex by certain stresses, for example the ER stress in case of amplification of mRNA encoding extracellular matrix proteins. In case of Alzheimer's disease, such an induction appears to be triggered by stresses associated with mitochondrial dysfunction, a phenomenon closely linked to AD. The cause-and-effect relationships between mitochondrial dysfunction and AD appear to be very different in familial, FAD, and sporadic, SAD cases. In FAD, increased levels or more toxic species of Aβ resulting from the abnormal proteolysis of βAPP trigger mitochondrial dysfunction, activate mRNA amplification and increase the production of Aβ, reinforcing the cycle. Thus in FAD, mitochondrial dysfunction is an intrinsic component of the amyloid cascade. The reverse sequence is true in SAD where aging-related mitochondrial dysfunction activates amplification of βAPP mRNA and enhances the production of Aβ. This causes further mitochondrial dysfunction, the cycle repeats and degeneration increases. Thus in SAD, the initial mitochondrial dysfunction arises prior to the disease, independently of and upstream from the increased Aβ production, i.e. in SAD, mitochondrial pathology hierarchically supersedes Aβ pathology. This is the primary reason for the formulation of the Mitochondrial Cascade Hypothesis. But even in terms of the MCH, the core of the disease is the amyloid cascade as defined in the amyloid cascade hypothesis, ACH. The role of mitochondrial dysfunction in relation to this core is causative in SAD and auxiliary in FAD. In FAD, the initial increase in the production of Aβ is mutations-based and occurs relatively early in life, whereas in SAD it is coerced by an aging-contingent component, but both lead to mechanistically identical self-perpetuating mutual Aβ/mitochondrial dysfunction feedback cycles, an engine that drives, via RNA-dependent βAPP mRNA amplification, overproduction of beta-amyloid and, consequently, AD; hence drastic difference in the age of onset, yet profound pathological and symptomatic similarity in the progression, of familial and sporadic forms of Alzheimer's disease. Interestingly, the recent findings that mitochondrial microprotein PIGBOS interacts with the ER in mitigating the unfolded protein response indicate a possible connection between mitochondrial dysfunction and ER stress, implicated in activation of RNA-dependent mRNA amplification pathway. The possible involvement of mitochondrial dysfunction in βAPP mRNA amplification makes it a promising therapeutic target. Recent successes in mitigating, and even reversing, Aβ-induced metabolic defects with anti-diabetes drug metformin are encouraging in this respect.

Mitochondrial fragmentation and network architecture in degenerative diseases

Fragmentation of mitochondrial network has been implicated in many neurodegenerative, renal, and metabolic diseases. However, a quantitative measure of the microscopic parameters resulting in the impaired balance between fission and fusion of mitochondria and consequently the fragmented networks in a wide range of pathological conditions does not exist. Here we present a comprehensive analysis of mitochondrial networks in cells with Alzheimer's disease (AD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), optic neuropathy (OPA), diabetes/cancer, acute kidney injury, Ca2+ overload, and Down Syndrome (DS) pathologies that indicates significant network fragmentation in all these conditions. Furthermore, we found key differences in the way the microscopic rates of fission and fusion are affected in different conditions. The observed fragmentation in cells with AD, HD, DS, kidney injury, Ca2+ overload, and diabetes/cancer pathologies results from the imbalance between the fission and fusion through lateral interactions, whereas that in OPA, PD, and ALS results from impaired balance between fission and fusion arising from longitudinal interactions of mitochondria. Such microscopic difference leads to major disparities in the fine structure and topology of the network that could have significant implications for the way fragmentation affects various cell functions in different diseases.

Mitochondria as Potential Targets in Alzheimer Disease Therapy: An Update

Alzheimer disease (AD) is a progressive and deleterious neurodegenerative disorder that affects mostly the elderly population. At the moment, no effective treatments are available in the market, making the whole situation a compelling challenge for societies worldwide. Recently, novel mechanisms have been proposed to explain the etiology of this disease leading to the new concept that AD is a multifactor pathology. Among others, the function of mitochondria has been considered as one of the intracellular processes severely compromised in AD since the early stages and likely represents a common feature of many neurodegenerative diseases. Many mitochondrial parameters decline already during the aging, reaching an extensive functional failure concomitant with the onset of neurodegenerative conditions, although the exact timeline of these events is still unclear. Thereby, it is not surprising that mitochondria have been already considered as therapeutic targets in neurodegenerative diseases including AD. Together with an overview of the role of mitochondrial dysfunction, this review examines the pros and cons of the tested therapeutic approaches targeting mitochondria in the context of AD. Since mitochondrial therapies in AD have shown different degrees of progress, it is imperative to perform a detailed analysis of the significance of mitochondrial deterioration in AD and of a pharmacological treatment at this level. This step would be very important for the field, as an effective drug treatment in AD is still missing and new therapeutic concepts are urgently needed.

Role of PGC-1α in Mitochondrial Quality Control in Neurodegenerative Diseases

As one of the major cell organelles responsible for ATP production, it is important that neurons maintain mitochondria with structural and functional integrity; this is especially true for neurons with high metabolic requirements. When mitochondrial damage occurs, mitochondria are able to maintain a steady state of functioning through molecular and organellar quality control, thus ensuring neuronal function. And when mitochondrial quality control (MQC) fails, mitochondria mediate apoptosis. An apparently key molecule in MQC is the transcriptional coactivator peroxisome proliferator activated receptor γ coactivator-1α (PGC-1α). Recent findings have demonstrated that upregulation of PGC-1α expression in neurons can modulate MQC to prevent mitochondrial dysfunction in certain in vivo and in vitro aging or neurodegenerative encephalopathy models, such as Huntington's disease, Alzheimer's disease, and Parkinson's disease. Because mitochondrial function and quality control disorders are the basis of pathogenesis in almost all neurodegenerative diseases (NDDs), the role of PGC-1α may make it a viable entry point for the treatment of such diseases. This review focuses on multi-level MQC in neurons, as well as the regulation of MQC by PGC-1α in these major NDDs.

Autophagy Induction by Bexarotene Promotes Mitophagy in Presenilin 1 Familial Alzheimer's Disease iPSC-Derived Neural Stem Cells

Adult neurogenesis defects have been demonstrated in the brains of Alzheimer's disease (AD) patients. The neurogenesis impairment is an early critical event in the course of familiar AD (FAD) associated with neuronal loss. It was suggested that neurologic dysfunction in AD may be caused by impaired functioning of hippocampal neural stem cells (NSCs). Multiple metabolic and structural abnormalities in neural mitochondria have long been suspected to play a critical role in AD pathophysiology. We hypothesize that the cause of such abnormalities could be defective elimination of damaged mitochondria. In the present study, we evaluated mitophagy efficacy in a cellular AD model, hiPSC-derived NSCs harboring the FAD-associated PS1 M146L mutation. We found several mitochondrial respiratory chain defects such as lower expression levels of cytochrome c oxidase (complex IV), cytochrome c reductase (complex III), succinate dehydrogenase (complex II), NADH:CoQ reductase (complex I), and also ATP synthase (complex V), most of which had been previously associated with AD. The mitochondrial network morphology and abundance in these cells was aberrant. This was associated with a marked mitophagy failure stemming from autophagy induction blockage, and deregulation of the expression of proteins involved in mitochondrial dynamics. We show that treating these cells with autophagy-stimulating drug bexarotene restored autophagy and compensated mitochondrial anomalies in PS1 M146L NSCs, by enhancing the clearance of mitochondria. Our data support the hypothesis that pharmacologically induced mitophagy enhancement is a relevant and novel therapeutic strategy for the treatment of AD.

Mitochondrial dynamics and transport in Alzheimer's disease

Mitochondrial dysfunction is now recognized as a contributing factor to the early pathology of multiple human conditions including neurodegenerative diseases. Mitochondria are signaling organelles with a multitude of functions ranging from energy production to a regulation of cellular metabolism, energy homeostasis, stress response, and cell fate. The success of these complex processes critically depends on the fidelity of mitochondrial dynamics that include the ability of mitochondria to change shape and location in the cell, which is essential for the maintenance of proper function and quality control, particularly in polarized cells such as neurons. This review highlights several aspects of alterations in mitochondrial dynamics in Alzheimer's disease, which may contribute to the etiology of this debilitating condition. We also discuss therapeutic strategies to improve mitochondrial dynamics and function that may provide an alternative approach to failed amyloid-directed interventions.

Dynamin-like protein 1 cleavage by calpain in Alzheimer's disease

Abnormal mitochondrial dynamics contributes to mitochondrial dysfunction in Alzheimer's disease (AD), yet the underlying mechanism remains elusive. In the current study, we reported that DLP1, the key mitochondrial fission GTPase, is a substrate of calpain which produced specific N-terminal DLP1 cleavage fragments. In addition, various AD-related insults such as exposure to glutamate, soluble amyloid-β oligomers, or reagents inducing tau hyperphosphorylation (i.e., okadaic acid) led to calpain-dependent cleavage of DLP1 in primary cortical neurons. DLP1 cleavage fragments were found in cortical neurons of CRND8 APP transgenic mice which can be inhibited by calpeptin, a potent small molecule inhibitor of calpain. Importantly, these N-terminal DLP1 fragments were also present in the human brains, and the levels of both full-length and N-terminal fragments of DLP1 and the full-length and calpain-specific cleavage product of spectrin were significantly reduced in AD brains along with significantly increased calpain. These results suggest that calpain-dependent cleavage is at least one of the posttranscriptional mechanisms that contribute to the dysregulation of mitochondrial dynamics in AD.

Mitochondria and Mitochondrial Cascades in Alzheimer's Disease

Decades of research indicate mitochondria from Alzheimer's disease (AD) patients differ from those of non-AD individuals. Initial studies revealed structural differences, and subsequent studies showed functional deficits. Observations of structure and function changes prompted investigators to consider the consequences, significance, and causes of AD-related mitochondrial dysfunction. Currently, extensive research argues mitochondria may mediate, drive, or contribute to a variety of AD pathologies. The perceived significance of these mitochondrial changes continues to grow, and many currently believe AD mitochondrial dysfunction represents a reasonable therapeutic target. Debate continues over the origin of AD mitochondrial changes. Some argue amyloid-β (Aβ) induces AD mitochondrial dysfunction, a view that does not challenge the amyloid cascade hypothesis and that may in fact help explain that hypothesis. Alternatively, data indicate mitochondrial dysfunction exists independent of Aβ, potentially lies upstream of Aβ deposition, and suggest a primary mitochondrial cascade hypothesis that assumes mitochondrial pathology hierarchically supersedes Aβ pathology. Mitochondria, therefore, appear at least to mediate or possibly even initiate pathologic molecular cascades in AD. This review considers studies and data that inform this area of AD research.

Mitochondria Redistribution in Enterovirus A71 Infected Cells and Its Effect on Virus Replication

Enterovirus A71 (EV-A71) is one of the main causative agents of hand, foot and mouth disease (HFMD) and it also causes severe neurologic complications in infected children. The interactions between some viruses and the host mitochondria are crucial for virus replication and pathogenicity. In this study, it was observed that EV-A71 infection resulted in a perinuclear redistribution of the mitochondria. The mitochondria rearrangement was found to require the microtubule network, the dynein complex and a low cytosolic calcium concentration. Subsequently, the EV-A71 non-structural protein 2BC was identified as the viral protein capable of inducing mitochondria clustering. The protein was found localized on mitochondria and interacted with the mitochondrial Rho GTPase 1 (RHOT1) that is a key protein required for attachment between the mitochondria and the motor proteins, which are responsible for the control of mitochondria movement. Additionally, suppressing mitochondria clustering by treating cells with nocodazole, EHNA, thapsigargin or A23187 consistently inhibited EV-A71 replication, indicating that mitochondria recruitment played a crucial role in the EV-A71 life cycle. This study identified a novel function of the EV-A71 2BC protein and provided a potential model for the regulation of mitochondrial motility in EV-A71 infection.

Exploring the molecular interface between hypoxia-inducible factor signalling and mitochondria

Oxygen is required for the survival of the majority of eukaryotic organisms, as it is important for many cellular processes. Eukaryotic cells utilize oxygen for the production of biochemical energy in the form of adenosine triphosphate (ATP) generated from the catabolism of carbon-rich fuels such as glucose, lipids and glutamine. The intracellular sites of oxygen consumption-coupled ATP production are the mitochondria, double-membraned organelles that provide a dynamic and multifaceted role in cell signalling and metabolism. Highly evolutionarily conserved molecular mechanisms exist to sense and respond to changes in cellular oxygen levels. The primary transcriptional regulators of the response to decreased oxygen levels (hypoxia) are the hypoxia-inducible factors (HIFs), which play important roles in both physiological and pathophysiological contexts. In this review we explore the relationship between HIF-regulated signalling pathways and the mitochondria, including the regulation of mitochondrial metabolism, biogenesis and distribution.