Mitochondria-targeted molecules MitoQ and SS31 reduce mutant huntingtin-induced mitochondrial toxicity and synaptic damage in Huntington's disease

Mitochondrial targeted antioxidants as potential therapy for huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expansion in CAG repeat on huntington (Htt) gene, leading to a degeneration of GABAergic medium spiny neurons (MSNs) in the striatum, resulting in the generation of reactive oxygen species, and decrease antioxidant activity. These pathophysiological alterations impair mitochondrial functions, leading to an increase in involuntary hyperkinetic movement. However, researchers investigated the neuroprotective effect of antioxidants using various animal models. Still, their impact is strictly limited to curtailing oxidative stress and increasing the antioxidant enzyme in the brain, which is less effective in HD. Meanwhile, researchers discovered Mitochondria-targeted antioxidants (MTAXs) that can improve mitochondrial functions and antioxidant activity through the modulation of mitochondrial signaling pathways, including peroxisome proliferator-activated receptor (PPAR)-coactivator 1 (PGC-1α), dynamin-related protein 1 (Drp1), mitochondrial fission protein 1 (Fis1), and Silent mating type information regulation 2 homolog 1 (SIRT-1), showing neuroprotective effects in HD. The present review discusses the clinical and preclinical studies that investigate the neuroprotective effect of MTAXs (SS31, XJB-5-131, MitoQ, bezafibrate, rosiglitazone, meldonium, coenzyme Q10, etc.) in HD. This brief literature review will help to understand the relevance of MTAXs in HD and enlighten the importance of MTAXs in future drug discovery and development.

Decoding mitochondrial quality control mechanisms: Identifying treatment targets for enhanced cellular health

Mitochondria are singular cell organelles essential for many cellular functions, which includes responding to stress, regulating calcium levels, maintaining protein homeostasis, and coordinating apoptosis response. The vitality of cells, therefore, hinges on the optimal functioning of these dynamic organelles. Mitochondrial Quality Control Mechanisms (MQCM) play a pivotal role in ensuring the integrity and functionality of mitochondria. Perturbations in these mechanisms have been closely associated with the pathogenesis of neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis. Compelling evidence suggests that targeting specific pathways within the MQCM could potentially offer a therapeutic avenue for rescuing mitochondrial integrity and mitigating the progression of neurodegenerative diseases. The intricate interplay of cellular stress, protein misfolding, and impaired quality control mechanisms provides a nuanced understanding of the underlying pathology. Consequently, unravelling the specific MQCM dysregulation in neurodegenerative disorders becomes paramount for developing targeted therapeutic strategies. This review delves into the impaired MQCM pathways implicated in neurodegenerative disorders and explores emerging therapeutic interventions. By shedding light on pharmaceutical and genetic manipulations aimed at restoring MQCM efficiency, the discussion aims to provide insights into novel strategies for ameliorating the progression of neurodegenerative diseases. Understanding and addressing mitochondrial quality control mechanisms not only underscore their significance in cellular health but also offer a promising frontier for advancing therapeutic approaches in the realm of neurodegenerative disorders.

Focusing on mitochondria in the brain: from biology to therapeutics

Mitochondria have multiple functions such as supplying energy, regulating the redox status, and producing proteins encoded by an independent genome. They are closely related to the physiology and pathology of many organs and tissues, among which the brain is particularly prominent. The brain demands 20% of the resting metabolic rate and holds highly active mitochondrial activities. Considerable research shows that mitochondria are closely related to brain function, while mitochondrial defects induce or exacerbate pathology in the brain. In this review, we provide comprehensive research advances of mitochondrial biology involved in brain functions, as well as the mitochondria-dependent cellular events in brain physiology and pathology. Furthermore, various perspectives are explored to better identify the mitochondrial roles in neurological diseases and the neurophenotypes of mitochondrial diseases. Finally, mitochondrial therapies are discussed. Mitochondrial-targeting therapeutics are showing great potentials in the treatment of brain diseases.

Polyethylene terephthalate (PET) micro- and nanoplastic particles affect the mitochondrial efficiency of human brain vascular pericytes without inducing oxidative stress

The objective of this investigation was to evaluate the influence of micro- and nanoplastic particles composed of polyethylene terephthalate (PET), a significant contributor to plastic pollution, on human brain vascular pericytes. Specifically, we delved into their impact on mitochondrial functionality, oxidative stress, and the expression of genes associated with oxidative stress, ferroptosis and mitochondrial functions. Our findings demonstrate that the exposure of a monoculture of human brain vascular pericytes to PET particles in vitro at a concentration of 50 μg/ml for a duration of 3, 6 and 10 days did not elicit oxidative stress. Notably, we observed a reduction in various aspects of mitochondrial respiration, including maximal respiration, spare respiratory capacity, and ATP production in pericytes subjected to PET particles for 3 days, with a mitochondrial function recovery at 6 and 10 days. Furthermore, there were no statistically significant alterations in mitochondrial DNA copy number, or in the expression of genes linked to oxidative stress and ferroptosis, but an increase of the expression of the gene mitochondrial transcription factor A (TFAM) was noted at 3 days exposure. These outcomes suggest that, at a concentration of 50 μg/ml, PET particles do not induce oxidative stress in human brain vascular pericytes. Instead, at 3 days exposure, PET exposure impairs mitochondrial functions, but this is recovered at 6-day exposure. This seems to indicate a potential mitochondrial hormesis response (mitohormesis) is incited, involving the gene TFAM. Further investigations are warranted to explore the stages of mitohormesis and the potential consequences of plastics on the integrity of the blood-brain barrier and intercellular interactions. This research contributes to our comprehension of the potential repercussions of nanoplastic pollution on human health and underscores the imperative need for ongoing examinations into the exposure to plastic particles.

Application research of novel peptide mitochondrial-targeted antioxidant SS-31 in mitigating mitochondrial dysfunction

Due to the pivotal role of mitochondria in the generation of adenosine triphosphate (ATP) and the regulation of cellular homeostasis, mitochondrial dysfunction may exert a profound impact on various physiological systems, potentially precipitating a spectrum of distinct diseases. Consequently, research pertaining to mitochondrial therapeutics has assumed increasing significance, warranting heightened scrutiny. In recent years, the field of mitochondrial therapy has witnessed noteworthy advancements, with active exploration into diverse pharmacological agents aimed at ameliorating mitochondrial function. Elamipretide (SS-31), a novel synthetic mitochondrial-targeted antioxidant, has emerged as a promising candidate with extensive therapeutic potential. Its notable attributes encompass the mitigation of oxidative stress, the suppression of inflammatory processes, the maintenance of mitochondrial dynamics, and the prevention of cellular apoptosis. As such, SS-31 may emerge as a viable choice for the treatment of mitochondrial dysfunction-related ailments in the foreseeable future. This article extensively expounds upon the superiority of SS-31 over natural antioxidants and traditional mitochondrial-targeted antioxidants, delves into its mechanisms of modulating mitochondrial function, and comprehensively summarizes its applications in alleviating mitochondrial dysfunction-associated disorders. Furthermore, we offer a comprehensive outlook on the expansive prospects of SS-31's future development and application.

Mitochondrial quality control in liver fibrosis: Epigenetic hallmarks and therapeutic strategies

BACKGROUND AND AIM

Mitochondrial quality control (MQC) plays a significant role in the progression of liver fibrosis, with key processes such as mitochondrial fission, fusion, mitophagy and biogenesis maintaining mitochondrial homeostasis. To understand the molecular mechanisms underlying epigenetic regulation of mitochondrial quality control in liver fibrosis, with the aim of uncovering novel therapeutic targets for treating, mitigating, and potentially reversing liver fibrosis, in light of the most recent advances in this field.

METHODS

We searched PubMed, Web of Science, and Scopus for published manuscripts using terms "mitochondrial quality control" "mitochondrial fission" "mitochondrial fusion" "mitochondrial biogenesis" "mitophagy" "liver fibrosis" "epigenetic regulation" "DNA methylation" "RNA methylation" "histone modification" and "non-coding RNA". Manuscripts were collated, studied and carried forward for discussion where appropriate.

RESULTS

Mitochondrial fission, fusion, biogenesis, and mitophagy regulate the homeostasis of mitochondria, and the imbalance of mitochondrial homeostasis can induce liver fibrosis. Epigenetic regulation, including DNA methylation, RNA methylation, histone modifications, and non-coding RNAs, plays a significant role in regulating the processes of mitochondrial homeostasis.

CONCLUSION

Mitochondrial quality control and epigenetic mechanisms are intricately linked to the pathogenesis of liver fibrosis. Understanding these molecular interactions provides insight into potential therapeutic strategies. Further research is necessary to translate these findings into clinical applications, with a focus on developing epigenetic drugs to ameliorate liver fibrosis by modulating MQC and epigenetic pathways.

Interactions of amyloidogenic proteins with mitochondrial protein import machinery in aging-related neurodegenerative diseases

Most mitochondrial proteins are targeted to the organelle by N-terminal mitochondrial targeting sequences (MTSs, or "presequences") that are recognized by the import machinery and subsequently cleaved to yield the mature protein. MTSs do not have conserved amino acid compositions, but share common physicochemical properties, including the ability to form amphipathic α-helical structures enriched with basic and hydrophobic residues on alternating faces. The lack of strict sequence conservation implies that some polypeptides can be mistargeted to mitochondria, especially under cellular stress. The pathogenic accumulation of proteins within mitochondria is implicated in many aging-related neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases. Mechanistically, these diseases may originate in part from mitochondrial interactions with amyloid-β precursor protein (APP) or its cleavage product amyloid-β (Aβ), α-synuclein (α-syn), and mutant forms of huntingtin (mHtt), respectively, that are mediated in part through their associations with the mitochondrial protein import machinery. Emerging evidence suggests that these amyloidogenic proteins may present cryptic targeting signals that act as MTS mimetics and can be recognized by mitochondrial import receptors and transported into different mitochondrial compartments. Accumulation of these mistargeted proteins could overwhelm the import machinery and its associated quality control mechanisms, thereby contributing to neurological disease progression. Alternatively, the uptake of amyloidogenic proteins into mitochondria may be part of a protein quality control mechanism for clearance of cytotoxic proteins. Here we review the pathomechanisms of these diseases as they relate to mitochondrial protein import and effects on mitochondrial function, what features of APP/Aβ, α-syn and mHtt make them suitable substrates for the import machinery, and how this information can be leveraged for the development of therapeutic interventions.

Research Advances of Mitochondrial Dysfunction in Perioperative Neurocognitive Disorders

Perioperative neurocognitive disorders (PND) increases postoperative dementia and mortality in patients and has no effective treatment. Although the detailed pathogenesis of PND is still elusive, a large amount of evidence suggests that damaged mitochondria may play an important role in the pathogenesis of PND. A healthy mitochondrial pool not only provides energy for neuronal metabolism but also maintains neuronal activity through other mitochondrial functions. Therefore, exploring the abnormal mitochondrial function in PND is beneficial for finding promising therapeutic targets for this disease. This article summarizes the research advances of mitochondrial energy metabolism disorder, inflammatory response and oxidative stress, mitochondrial quality control, mitochondria-associated endoplasmic reticulum membranes, and cell death in the pathogenesis of PND, and briefly describes the application of mitochondria-targeted therapies in PND.

Elamipretide reduces pyroptosis and improves functional recovery after spinal cord injury

AIMS

Elamipretide (EPT), a novel mitochondria-targeted peptide, has been shown to be protective in a range of diseases. However, the effect of EPT in spinal cord injury (SCI) has yet to be elucidated. We aimed to investigate whether EPT would inhibit pyroptosis and protect against SCI.

METHODS

After establishing the SCI model, we determined the biochemical and morphological changes associated with pyroptosis, including neuronal cell death, proinflammatory cytokine expression, and signal pathway levels. Furthermore, mitochondrial function was assessed with flow cytometry, quantitative real-time polymerase chain reaction, and western blot.

RESULTS

Here, we demonstrate that EPT improved locomotor functional recovery following SCI as well as reduced neuronal loss. Moreover, EPT inhibited nucleotide-binding oligomerization domain-like receptor 3 (NLRP3) inflammasome activation and pyroptosis occurrence and decreased pro-inflammatory cytokines levels following SCI. Furthermore, EPT alleviated mitochondrial dysfunction and reduced mitochondrial reactive oxygen species level.

CONCLUSION

EPT treatment may protect against SCI via inhibition of pyroptosis.

Mitochondrial dysfunction in neurodegenerative disorders: Potential therapeutic application of mitochondrial transfer to central nervous system-residing cells

Mitochondrial dysfunction is reiteratively involved in the pathogenesis of diverse neurodegenerative diseases. Current in vitro and in vivo approaches support that mitochondrial dysfunction is branded by several molecular and cellular defects, whose impact at different levels including the calcium and iron homeostasis, energetic balance and/or oxidative stress, makes it difficult to resolve them collectively given their multifactorial nature. Mitochondrial transfer offers an overall solution since it contains the replacement of damage mitochondria by healthy units. Therefore, this review provides an introducing view on the structure and energy-related functions of mitochondria as well as their dynamics. In turn, we summarize current knowledge on how these features are deregulated in different neurodegenerative diseases, including frontotemporal dementia, multiple sclerosis, amyotrophic lateral sclerosis, Friedreich ataxia, Alzheimer´s disease, Parkinson´s disease, and Huntington's disease. Finally, we analyzed current advances in mitochondrial transfer between diverse cell types that actively participate in neurodegenerative processes, and how they might be projected toward developing novel therapeutic strategies.

MitoQ alleviates hippocampal damage after cerebral ischemia: The potential role of SIRT6 in regulating mitochondrial dysfunction and neuroinflammation

AIMS

Mitochondrial perturbations are the major culprit of the inflammatory response during the initial phase of cerebral ischemia. The present study explored the neuroprotective effect of the mitochondrial-targeted antioxidant, Mitoquinol (MitoQ), against hippocampal neuronal loss in an experimental model of brain ischemia/reperfusion (I/R) injury.

MAIN METHODS

Rats were subjected to common carotid artery occlusion for 45 min, followed by reperfusion for 24 h. MitoQ (2 mg/kg; i.p daily) was administered for 7 successive days prior to the induction of brain ischemia.

KEY FINDINGS

I/R rats exhibited hippocampal damage evidenced by aggravated mitochondrial oxidative stress, thereby enhancing mtROS and oxidized mtDNA, together with inhibiting mtGSH. Mitochondrial biogenesis and function were also affected, as reflected by the reduction of PGC-1α, TFAM, and NRF-1 levels, as well as loss of mitochondrial membrane potential (△Ψm (. These changes were associated with neuroinflammation, apoptosis, impairment of cognitive function as well as hippocampal neurodegenerative changes in histopathological examination. Notably, SIRT6 was suppressed. Pretreatment with MitoQ markedly potentiated SIRT6, modulated mitochondrial oxidative status and restored mitochondrial biogenesis and function. In addition, MitoQ alleviated the inflammatory mediators, TNF-α, IL-18, and IL-1β and dampened GFAB immunoexpression along with downregulation of cleaved caspase-3 expression. Reversal of hippocampal function by MitoQ was accompanied by improved cognitive function and hippocampal morphological aberrations.

SIGNIFICANCE

This study suggests that MitoQ preserved rats' hippocampi from I/R insults via maintenance of mitochondrial redox status, biogenesis, and activity along with mitigation of neuroinflammation and apoptosis, thereby regulating SIRT6.

Lactate: A Theranostic Biomarker for Metabolic Psychiatry?

Alterations in neurometabolism and mitochondria are implicated in the pathophysiology of psychiatric conditions such as mood disorders and schizophrenia. Thus, developing objective biomarkers related to brain mitochondrial function is crucial for the development of interventions, such as central nervous system penetrating agents that target brain health. Lactate, a major circulatory fuel source that can be produced and utilized by the brain and body, is presented as a theranostic biomarker for neurometabolic dysfunction in psychiatric conditions. This concept is based on three key properties of lactate that make it an intriguing metabolic intermediate with implications for this field: Firstly, the lactate response to various stimuli, including physiological or psychological stress, represents a quantifiable and dynamic marker that reflects metabolic and mitochondrial health. Second, lactate concentration in the brain is tightly regulated according to the sleep-wake cycle, the dysregulation of which is implicated in both metabolic and mood disorders. Third, lactate universally integrates arousal behaviours, pH, cellular metabolism, redox states, oxidative stress, and inflammation, and can signal and encode this information via intra- and extracellular pathways in the brain. In this review, we expand on the above properties of lactate and discuss the methodological developments and rationale for the use of functional magnetic resonance spectroscopy for in vivo monitoring of brain lactate. We conclude that accurate and dynamic assessment of brain lactate responses might contribute to the development of novel and personalized therapies that improve mitochondrial health in psychiatric disorders and other conditions associated with neurometabolic dysfunction.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Exploring the therapeutic potential of the mitochondrial transfer-associated enzymatic machinery in brain degeneration

Mitochondrial dysfunction is a central event in the pathogenesis of several degenerative brain disorders. It entails fission and fusion dynamics disruption, progressive decline in mitochondrial clearance, and uncontrolled oxidative stress. Many therapeutic strategies have been formulated to reverse these alterations, including replacing damaged mitochondria with healthy ones. Spontaneous mitochondrial transfer is a naturally occurring process with different biological functions. It comprises mitochondrial donation from one cell to another, carried out through different pathways, such as the formation and stabilization of tunneling nanotubules and Gap junctions and the release of extracellular vesicles with mitochondrial cargoes. Even though many aspects of regulating these mechanisms still need to be discovered, some key enzymatic regulators have been identified. This review summarizes the current knowledge on mitochondrial dysfunction in different neurodegenerative disorders. Besides, we analyzed the usage of mitochondrial transfer as an endogenous revitalization tool, emphasizing the enzyme regulators that govern this mechanism. Going deeper into this matter would be helpful to take advantage of the therapeutic potential of mitochondrial transfer.

Mitochondria in Huntington's disease: implications in pathogenesis and mitochondrial-targeted therapeutic strategies

Huntington's disease is a genetic disease caused by expanded CAG repeats on exon 1 of the huntingtin gene located on chromosome 4. Compelling evidence implicates impaired mitochondrial energetics, altered mitochondrial biogenesis and quality control, disturbed mitochondrial trafficking, oxidative stress and mitochondrial calcium dyshomeostasis in the pathogenesis of the disorder. Unfortunately, conventional mitochondrial-targeted molecules, such as cysteamine, creatine, coenzyme Q10, or triheptanoin, yielded negative or inconclusive results. However, future therapeutic strategies, aiming to restore mitochondrial biogenesis, improving the fission/fusion balance, and improving mitochondrial trafficking, could prove useful tools in improving the phenotype of Huntington's disease and, used in combination with genome-editing methods, could lead to a cure for the disease.

A mitochondrial-targeted antioxidant (MitoQ) improves motor coordination and reduces Purkinje cell death in a mouse model of ARSACS

Mitochondrial deficits have been observed in animal models of Autosomal-recessive spastic ataxia of the Charlevoix-Saguenay (ARSACS) and in patient-derived fibroblasts. We investigated whether mitochondrial function could be restored in Sacs^-/- mice, a mouse model of ARSACS, using the mitochondrial-targeted antioxidant ubiquinone MitoQ. After 10 weeks of chronic MitoQ administration in drinking water, we partially reversed motor coordination deficits in Sacs^-/- mice but did not affect litter-matched wild-type control mice. MitoQ administration led to a restoration of superoxide dismutase 2 (SOD2) in cerebellar Purkinje cell somata without altering Purkinje cell firing deficits. Purkinje cells in anterior vermis of Sacs^-/- mice normally undergo cell death in ARSACS; however, Purkinje cells numbers were elevated after chronic MitoQ treatment. Furthermore, Purkinje cell innervation of target neurons in the cerebellar nuclei of Sacs^-/- mice were also partially restored with MitoQ treatment. Our data suggest that MitoQ is a potential therapeutic treatment for ARSACS and that it improves motor coordination via increasing cerebellar Purkinje cell mitochondria function and reducing Purkinje cell death.

A comprehensive perspective of Huntington's disease and mitochondrial dysfunction

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease. It is caused by the expansion of the CAG trinucleotide repeat sequence in the HTT gene. HD mainly manifests as involuntary dance-like movements and severe mental disorders. As it progresses, patients lose the ability to speak, think, and even swallow. Although the pathogenesis is unclear, studies have found that mitochondrial dysfunctions occupy an important position in the pathogenesis of HD. Based on the latest research advances, this review sorts out and discusses the role of mitochondrial dysfunction on HD in terms of bioenergetics, abnormal autophagy, and abnormal mitochondrial membranes. This review provides researchers with a more complete perspective on the mechanisms underlying the relationship between mitochondrial dysregulation and HD.

Mitochondrial Dysfunction, Oxidative Stress, and Therapeutic Strategies in Diabetes, Obesity, and Cardiovascular Disease

Mitochondria are subcellular organelles involved in essential cellular functions, including cytosolic calcium regulation, cell apoptosis, and reactive oxygen species production. They are the site of important biochemical pathways, including the tricarboxylic acid cycle, parts of the ureagenesis cycle, or haem synthesis. Mitochondria are responsible for the majority of cellular ATP production through OXPHOS. Mitochondrial dysfunction has been associated with metabolic pathologies such as diabetes, obesity, hypertension, neurodegenerative diseases, cellular aging, and cancer. In this article, we describe the pathophysiological changes in, and mitochondrial role of, metabolic disorders (diabetes, obesity, and cardiovascular disease) and their correlation with oxidative stress. We highlight the genetic changes identified at the mtDNA level. Additionally, we selected several representative biomarkers involved in oxidative stress and summarize the progress of therapeutic strategies.

Mitochondrial dysfunction: A notable contributor to the progression of Alzheimer's and Parkinson's disease

Mitochondrial dysfunction remains a pivotal mechanism in manifold neurodegenerative diseases. Mitochondrial homeostasis within the cell is an essential aspect of cell biology. Mitochondria, the power-generating organelle of the cell, have a dominant role in several processes associated with genomic integrity and cellular equilibrium. They are involved in maintaining optimal cell functioning and ensuring guidance against possible DNA damage, which could lead to mutations and the onset of diseases. Conversely, system perturbations, which could be due to environmental factors or senescence, induce changes in the physiological balance and result in mitochondrial function impairment. As a result, we present a general overview of the pathological pathways involved in Alzheimer's and Parkinson's diseases caused by changes in mitochondrial homeostasis. The focal point of this review is on mitochondrial dysfunction being a significant condition in the onset of neuronal disintegration. We explain the pathways associated with the dysfunction of the mitochondria, which are common among the most recurring neurodegenerative diseases, including Alzheimer's and Parkinson's disease. Are mitochondrial dysfunctions an early event in the progression of neuropathological processes? We discovered that mtDNA mutation is a major contributor to the metabolic pathology of most neurological disorders, causing changes in genes important for physiological homeostasis. As a result, genetic changes in presenilin, Amyloid-, ABAD, DJ-1, PINK-1, PARKIN, alpha-synuclein, and other important controlling genes occur. Therefore, we suggest possible therapeutic solutions.

Mitochondria-Targeted Antioxidants, an Innovative Class of Antioxidant Compounds for Neurodegenerative Diseases: Perspectives and Limitations

Neurodegenerative diseases comprise a wide spectrum of pathologies characterized by progressive loss of neuronal functions and structures. Despite having different genetic backgrounds and etiology, in recent years, many studies have highlighted a point of convergence in the mechanisms leading to neurodegeneration: mitochondrial dysfunction and oxidative stress have been observed in different pathologies, and their detrimental effects on neurons contribute to the exacerbation of the pathological phenotype at various degrees. In this context, increasing relevance has been acquired by antioxidant therapies, with the purpose of restoring mitochondrial functions in order to revert the neuronal damage. However, conventional antioxidants were not able to specifically accumulate in diseased mitochondria, often eliciting harmful effects on the whole body. In the last decades, novel, precise, mitochondria-targeted antioxidant (MTA) compounds have been developed and studied, both in vitro and in vivo, to address the need to counter the oxidative stress in mitochondria and restore the energy supply and membrane potentials in neurons. In this review, we focus on the activity and therapeutic perspectives of MitoQ, SkQ1, MitoVitE and MitoTEMPO, the most studied compounds belonging to the class of MTA conjugated to lipophilic cations, in order to reach the mitochondrial compartment.

The mitochondria-targeted small molecule SS31 delays progression of behavioral deficits by attenuating β-amyloid plaque formation and mitochondrial/synaptic deterioration in APP/PS1 mice

Alzheimer's disease (AD) is a common neurodegenerative disorder characterized by progressive cognitive dysfunction and an impaired ability to carry out daily life functions. Mitochondrial dysfunction and β-amyloid (Aβ) deposition are the most common causes of AD. Antioxidants have been shown to delay brain aging and AD development; however, it remains unknown whether the antioxidant peptide SS31 can protect mitochondrial and synaptic function and delay the progression of behavioral deficits in early-stage AD in vivo. Therefore, in this study we compared mitochondrial and synaptic changes, as well as the protective effects of SS31, in APP/PS1 transgenic mice and C57BL/6J control mice. The APP/PS1 transgenic mice exhibited elevated expression of Aβ40/Aβ42 and mitochondrial fission protein DLP1 and reduced expression of synaptophysin (SYN) and postsynaptic density protein 95 (PSD95) reductions, as well as increased levels of neuronal apoptosis and ROS in the hippocampus, and long-term treatment with SS31 reversed these effects. Furthermore, the cognitive impairments observed in APP/PS1 transgenic mice were reversed by SS31 treatment. Our findings show that SS31 lowers ROS and Aβ levels, protecting mitochondrial homeostasis and synaptic integrity, and ultimately improving behavioral deficits in early-stage AD. This suggests that SS31 is a potential pharmacological agent for treating or slowing the progression of AD.

Therapeutic targeting of mitochondria–proteostasis axis by antioxidant polysaccharides in neurodegeneration

Aging is a major risk factor for many age-associated disorders, including neurodegenerative diseases. Both mitochondrial dysfunction and proteostatic decline are well-recognized hallmarks of aging and age-related neurodegeneration. Despite a lack of therapies for neurodegenerative diseases, a number of interventions promoting mitochondrial integrity and protein homeostasis (proteostasis) have been shown to delay aging-associated neurodegeneration. For example, many antioxidant polysaccharides are shown to have pharmacological potentials in Alzheimer's, Parkinson's and Huntington's diseases through regulation of mitochondrial and proteostatic pathways, including oxidative stress and heat shock responses. However, how mitochondrial and proteostatic mechanisms work together to exert the antineurodegenerative effect of the polysaccharides remains largely unexplored. Interestingly, recent studies have provided a growing body of evidence to support the crosstalk between mitostatic and proteostatic networks as well as the impact of the crosstalk on neurodegeneration. Here we summarize the recent progress of antineurodegenerative polysaccharides with particular attention in the mitochondrial and proteostatic context and provide perspectives on their implications in the crosstalk along the mitochondria-proteostasis axis.

SS31 Confers Cerebral Protection by Reversing Mitochondrial Dysfunction in Early Brain Injury Following Subarachnoid Hemorrhage, via the Nrf2- and PGC-1α-Dependent Pathways

In early brain injury (EBI), oxidative stress occurs following subarachnoid hemorrhage (SAH), and mitochondria are intricately linked to this process. SS31, a mitochondria-targeting antioxidative peptide, has been demonstrated to be beneficial for multiple diseases because of its powerful antioxidant and neuroprotective properties. Although our previous study revealed that SS31 was involved in the powerful antioxidant effect following SAH, the underlying molecular mechanisms remained unclear. Thus, our study aimed to investigate the neuroprotective effects of SS31 by reversing mitochondrial dysfunction in EBI following SAH, via activating the Nrf2 signaling and PGC-1α pathways. Our findings confirmed that SS31 ameliorated SAH-triggered oxidative insult. SS31 administration decreased redundant reactive oxygen species, alleviated lipid peroxidation, and elevated the activities of antioxidant enzymes. Concomitant with the inhibited oxidative insult, SS31 dramatically attenuated neurological deficits, cerebral edema, neural apoptosis, and blood-brain barrier disruption following SAH. Moreover, SS31 remarkably promoted nuclear factor-erythroid 2 related factor 2 (Nrf2) nuclear shuttle and upregulated the expression levels of heme oxygenase-1 and NADPH: quinine oxidoreductase1. Additionally, SS31 enhanced the expression levels of PGC-1α and its target genes, and increased the mtDNA copy number, promoting mitochondrial function. However, PGC-1α-specific inhibitor SR-18292 pretreatment dramatically suppressed SS31-induced Nrf2 expression and PGC-1α activation. Furthermore, pretreatment with SR-18292 reversed the neuroprotective and antioxidant roles of SS31. These significant beneficial effects were associated with the activation of the Nrf2 signaling and PGC-1α pathways and were antagonized by SR-18292 administration. Our findings reveal that SS31 exhibits its neuroprotective activity by reversing mitochondrial dysfunction via activating the Nrf2 signaling pathway, which could be mediated through PGC-1α activation.

Light-Activated Gold-Selenium Core-Shell Nanocomposites with NIR-II Photoacoustic Imaging Performances for Heart-Targeted Repair

Mitochondrial dysfunction and oxidative damage represent important pathological mechanisms of myocardial ischemia-reperfusion injury (MI/RI). Searching for potential antioxidant agents to attenuate MI/RI is of great significance in clinic. Herein, gold-selenium core-shell nanostructures (AS-I/S NCs) with good near-infrared (NIR)-II photoacoustic imaging were designed for MI/RI treatment. The AS-I/S NCs after ischemic myocardium-targeted peptide (IMTP) and mitochondrial-targeted antioxidant peptide SS31 modification achieved cardiomyocytes-targeted cellular uptake and enhanced antioxidant ability and significantly inhibited oxygen-glucose deprivation-recovery (OGD/R)-induced cardiotoxicity of H9c2 cells by inhibiting the depletion of mitochondrial membrane potential (MMP) and restoring ATP synthase activity. Furthermore, the AS-I/S NCs after SS31 modification achieved mitochondria-targeted inhibition of reactive oxygen species (ROS) and subsequently attenuated oxidative damage in OGD/R-treated H9c2 cells by inhibition of apoptosis and oxidative damage, regulation of MAPKs and PI3K/AKT pathways. The in vivo AS-I/S NCs administration dramatically improved myocardial functions and angiogenesis and inhibited myocardial fibrosis through inhibiting myocardial apoptosis and oxidative damage in MI/RI of rats. Importantly, the AS-I/S NCs showed good safety and biocompatibility in vivo. Therefore, our findings validated the rational design that mitochondria-targeted selenium-gold nanocomposites could attenuate MI/RI of rats by inhibiting ROS-mediated oxidative damage and regulating MAPKs and PI3K/AKT pathways, which could be a potential therapy for the MI/RI treatment.

Mitochondrial Stress in Metabolic Inflammation: Modest Benefits and Full Losses

Energy intake and metabolic balance are the pillars of health preservation. Overnutrition causes nonspecific, persistently low inflammatory state known as metabolic inflammation. This condition contributes to the pathophysiology of various metabolic disorders, such as atherosclerosis, obesity, diabetes mellitus, and metabolic syndrome. The mitochondria maintain the balance of energy metabolism. Excessive energy stress can lead to mitochondrial dysfunction, which promotes metabolic inflammation. The inflammatory environment further impairs mitochondrial function. Accordingly, excellent organism design keeps the body metabolically healthy in the context of mitochondrial dysfunction, and moderate mitochondrial stress can have a beneficial effect. This review summarises the research progress on the multifaceted characterisation of mitochondrial dysfunction and its role in metabolic inflammation.

Does Antioxidant Mitoquinone (MitoQ) Ameliorate Oxidative Stress in Frozen-Thawed Rooster Sperm?

In this study, we aimed to determine the benefit of mitoquinone (MitoQ) in rooster semen extenders on sperm quality, motility parameters, antioxidant capacities, and apoptotic changes in post-thawed rooster semen. A total of 85 ejaculates from 18 roosters were collected and then divided into five equal aliquots and cryopreserved in extenders with 1.0% soy lecithin nanoparticles that contained various concentrations of MitoQ (0 nM (M0), 50 nM (M50), 100 nM (M100), 150 nM (M150), and 200 nM (M200)). By using a computer-assisted semen analyzer, sperm motility parameters were assessed after freeze thawing. The M150 group had significantly higher percentages of total motility, progressive motility, viability, acrosome membrane integrity, and mitochondrial activity than the other groups (p < 0.05). Compared to other groups, M100 and M150 groups produced a higher percentage of plasma membrane integrity and ATP contents (p < 0.05). Additionally, the lowest levels of ROS and MDA in spermatozoa were observed in M150 group (p < 0.05), whereas the highest levels of ROS and MDA were observed in sperm in the controls or the M200 group (p < 0.05). Significantly higher values of SOD, GPx, and Cas-3 were found in the M150 group compared to other groups (p < 0.05). Overall, these results demonstrate that MitoQ at 150 nM not only ameliorates post-thawed sperm quality and motility parameters by restoring ATP levels and preventing membrane damage, but also improves redox balance and antiapoptotic activities.

MitoQ alleviates carbon tetrachloride-induced liver fibrosis in mice through regulating JNK/YAP pathway

Background

Liver fibrosis is a pathological wound-healing response caused by chronic liver damage. Mitochondria regulate hepatic energy metabolism and oxidative stress. Accumulating evidence has revealed that increased mitochondrial oxidative stress contributes to the activation of fibrogenesis. However, the roles and underlying mechanisms of mitochondrial oxidative stress in liver fibrosis remain unknown.

Methods and results

In this study, C57BL/6 mice were used to establish a model of liver fibrosis via oral gavage with CCl4 treatment for 8 weeks. Furthermore, intervention experiments were achieved by CCl4 combined with the intraperitoneal injection of mitoquinone mesylate (mitoQ). We demonstrated that the chronic CCl4 exposure resulted in severe hepatic fibrogenesis and significantly promoted the production of reactive oxygen species (ROS) and mitochondrial abnormalities. Besides, JNK/YAP pathway was also activated. By contrast, the administration of mitoQ markedly inhibited the expression of pro-fibrogenic transforming growth factor-β as well as type I collagen. The antifibrotic effects of mitoQ were also confirmed by hematoxylin and eosin staining and Sirius red staining. Moreover, mitoQ substantially reduced CCl4-induced mitochondrial damage and the release of ROS. Further studies suggested that this protection against liver fibrosis was mechanistically related to the inhibition of phosphorylation of JNK and the nuclear translocation of YAP.

Conclusion

In conclusion, these findings revealed that mitoQ attenuated liver fibrosis by inhibiting ROS production and the JNK/YAP signaling pathway. Selective targeting JNK/YAP may serve as a therapeutic strategy for retarding progression of chronic liver disease.

Mitochondrial fission mediated by Drp1-Fis1 pathway and neurodegenerative diseases

In recent years, the role of mitochondrial dynamics in neurodegenerative diseases has becoming increasingly important. More and more evidences have shown that in pathological conditions, abnormal mitochondrial divisions, especially Drp1-Fis1-mediated divisions, play an important role in the occurrence and development of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, glaucoma, and other neurodegenerative diseases. This review highlights several new mechanisms of physiological fission of mitochondria and the difference/connection of physiological/pathological mitochondrial fission. In addition, we described the relationship between abnormal mitochondrial dynamics and neurodegenerative diseases in detail and emphatically summarized its detection indicators in basic experiments, trying to provide references for further mechanism exploration and therapeutic targets.

Structure-activity relationships of mitochondria-targeted tetrapeptide pharmacological compounds

Mitochondria play a central role in metabolic homeostasis, and dysfunction of this organelle underpins the etiology of many heritable and aging-related diseases. Tetrapeptides with alternating cationic and aromatic residues such as SS-31 (elamipretide) show promise as therapeutic compounds for mitochondrial disorders. In this study, we conducted a quantitative structure-activity analysis of three alternative tetrapeptide analogs, benchmarked against SS-31, that differ with respect to aromatic side chain composition and sequence register. We present the first structural models for this class of compounds, obtained with Nuclear Magnetic Resonance (NMR) and molecular dynamics approaches, showing that all analogs except for SS-31 form compact reverse turn conformations in the membrane-bound state. All peptide analogs bound cardiolipin-containing membranes, yet they had significant differences in equilibrium binding behavior and membrane interactions. Notably, analogs had markedly different effects on membrane surface charge, supporting a mechanism in which modulation of membrane electrostatics is a key feature of their mechanism of action. The peptides had no strict requirement for side chain composition or sequence register to permeate cells and target mitochondria in mammalian cell culture assays. All four peptides were pharmacologically active in serum withdrawal cell stress models yet showed significant differences in their abilities to restore mitochondrial membrane potential, preserve ATP content, and promote cell survival. Within our peptide set, the analog containing tryptophan side chains, SPN10, had the strongest impact on most membrane properties and showed greatest efficacy in cell culture studies. Taken together, these results show that side chain composition and register influence the activity of these mitochondria-targeted peptides, helping provide a framework for the rational design of next-generation therapeutics with enhanced potency.

Mitochondria and Other Organelles in Neural Development and Their Potential as Therapeutic Targets in Neurodegenerative Diseases

The contribution of organelles to neural development has received increasing attention. Studies have shown that organelles such as mitochondria, endoplasmic reticulum (ER), lysosomes, and endosomes play important roles in neurogenesis. Specifically, metabolic switching, reactive oxygen species production, mitochondrial dynamics, mitophagy, mitochondria-mediated apoptosis, and the interaction between mitochondria and the ER all have roles in neurogenesis. Lysosomes and endosomes can regulate neurite growth and extension. Moreover, metabolic reprogramming represents a novel strategy for generating functional neurons. Accordingly, the exploration and application of mechanisms underlying metabolic reprogramming will be beneficial for neural conversion and regenerative medicine. There is adequate evidence implicating the dysfunction of cellular organelles—especially mitochondria—in neurodegenerative disorders, and that improvement of mitochondrial function may reverse the progression of these diseases through the reinforcement of adult neurogenesis. Therefore, these organelles have potential as therapeutic targets for the treatment of neurodegenerative diseases. In this review, we discuss the function of these organelles, especially mitochondria, in neural development, focusing on their potential as therapeutic targets in neurodegenerative disorders, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis.

A review on Huntington protein Insight into protein aggregation and therapeutic interventions

Huntington disease (HD) is a distressing, innate neurodegenerative disease that descends from CAG repeat expansion in the huntingtin gene causing behavioral changes, motor dysfunction, and dementia in children and adults. Mutation in huntingtin (HTT) protein has been suggested to cause neuron loss in the cortex and striatum through various mechanisms including abnormal regulation of transcription, proteasomal dysfunction, post-translational modification, and other events, regulating toxicity. Pathogenesis of HD involves cleavage of the huntingtin protein followed by the neuronal accumulation of its aggregated form. Several research groups made possible efforts to reduce huntingtin gene expression, protein accumulation, and protein aggregation using inhibitors and molecular chaperones as developing drugs against HD. Herein, we review the mechanism proposed towards the formation of HTT protein aggregation and the impact of therapeutic strategies for the treatment of HD.

Restoring the oxidative balance in age-related diseases - An approach in glaucoma

As human life expectancy increases, age-related health issues including neurodegenerative diseases continue to rise. Regardless of genetic or environmental factors, many neurodegenerative conditions share common pathological mechanisms, such as oxidative stress, a hallmark of many age-related health burdens. In this review, we describe oxidative damage and mitochondrial dysfunction in glaucoma, an age-related neurodegenerative eye disease affecting 80 million people worldwide. We consider therapeutic approaches used to counteract oxidative stress in glaucoma, including untapped treatment options such as novel plant-derived antioxidant compounds that can reduce oxidative stress and prevent neuronal loss. We summarize the current pre-clinical models and clinical work exploring the therapeutic potential of a range of candidate plant-derived antioxidant compounds. Finally, we explore advances in drug delivery systems, particular those employing nanotechnology-based carriers which hold significant promise as a carrier for antioxidants to treat age-related disease, thus reviewing the key current state of all of the aspects required towards translation.

Neuroprotective Effects of a Small Mitochondrially-Targeted Tetrapeptide Elamipretide in Neurodegeneration

Neural mitochondrial dysfunction, neural oxidative stress, chronic neuroinflammation, toxic protein accumulation, and neural apoptosis are common causes of neurodegeneration. Elamipretide, a small mitochondrially-targeted tetrapeptide, exhibits therapeutic effects and safety in several mitochondria-related diseases. In neurodegeneration, extensive studies have shown that elamipretide enhanced mitochondrial respiration, activated neural mitochondrial biogenesis via mitochondrial biogenesis regulators (PCG-1α and TFAM) and the translocate factors (TOM-20), enhanced mitochondrial fusion (MNF-1, MNF-2, and OPA1), inhibited mitochondrial fission (Fis-1 and Drp-1), as well as increased mitophagy (autophagy of mitochondria). In addition, elamipretide has been shown to attenuate neural oxidative stress (hydrogen peroxide, lipid peroxidation, and ROS), neuroinflammation (TNF, IL-6, COX-2, iNOS, NLRP3, cleaved caspase-1, IL-1β, and IL-18), and toxic protein accumulation (Aβ). Consequently, elamipretide could prevent neural apoptosis (cytochrome c, Bax, caspase 9, and caspase 3) and enhance neural pro-survival (Bcl2, BDNF, and TrkB) in neurodegeneration. These findings suggest that elamipretide may prevent the progressive development of neurodegenerative diseases via enhancing mitochondrial respiration, mitochondrial biogenesis, mitochondrial fusion, and neural pro-survival pathway, as well as inhibiting mitochondrial fission, oxidative stress, neuroinflammation, toxic protein accumulation, and neural apoptosis. Elamipretide or mitochondrially-targeted peptide might be a targeted agent to attenuate neurodegenerative progression.

SS-31 efficacy in a mouse model of Friedreich ataxia by upregulation of frataxin expression

Friedreich ataxia (FRDA) is a serious hereditary neurodegenerative disease, mostly accompanied with hypertrophic cardiomyopathy, caused by the reduced expression of frataxin (FXN). However, there is still no effective treatment. Our previous studies have shown that SS-31, a mitochondrion-targeted peptide, is capable to upregulate the expression of FXN and improve the mitochondrial function in cells derived from FRDA patients. To further explore the potential of SS-31, we used the GAA expansion-based models, including Y47 and YG8R (Fxn KIKO) mice, primary neurons and macrophages from the mice and cells derived from FRDA patients. After once-daily intraperitoneal injection of 1 mg/kg SS-31 for 1 month, we observed the significant improvement of motor function. The vacuolation in dorsal root ganglia, lesions in dentate nuclei and the lost thickness of myelin sheath of spinal cord were all repaired after SS-31 treatment. In addition, the hypertrophic cardiomyocytes and disarrayed abnormal Purkinje cells were dramatically reduced. Interestingly, we found that SS-31 treatment upregulated FXN expression not only at the translational levels as observed in cell culture but also at mRNA levels in vivo. Consequently, mitochondrial morphology and function were greatly improved in all tested tissues. Importantly, our data provided additional evidence that the maintenance of the therapeutic benefits needed continuous drug administration. Taken together, our findings have demonstrated the effectiveness of SS-31 treatment through the upregulation of FXN in vivo and offer guidance of the potential usage in the clinical application for FRDA.

MitoQ inhibits hepatic stellate cell activation and liver fibrosis by enhancing PINK1/parkin-mediated mitophagy

Mitophagy affects the activation of hepatic stellate cells (HSCs). Mitochondria-targeted ubiquinone (MitoQ) is a mitochondria-targeted antioxidant that reduces the production of intracellular reactive oxygen species (ROS). However, its relationship with mitophagy remains unclear. This study evaluated mitophagy during HSC activation and the effects of MitoQ on mitophagy in cell culture and in an animal model of the activation of HSCs. We found that MitoQ reduced the activation of HSCs and alleviated hepatic fibrosis. PINK1 (PTEN-induced putative kinase 1) is a putative serine/threonine kinase located in the mitochondria’s outer membrane. While the activation of primary HSCs or LX-2 cells was associated with reduced PINK1/parkin-mediated mitophagy, MitoQ reduced intracellular ROS levels, enhanced PINK1/parkin-mediated mitophagy, and inhibited the activation of HSCs. After knocking down the key mitophagy-related protein, PINK1, in LX-2 cells to block mitophagy, MitoQ intervention failed to inhibit HSC activation. Our results showed that MitoQ inhibited the activation of HSCs and alleviated hepatic fibrosis by enhancing PINK1/parkin-mediated mitophagy.

Nuclear and cytoplasmic huntingtin inclusions exhibit distinct biochemical composition, interactome and ultrastructural properties

Despite the strong evidence linking the aggregation of the Huntingtin protein (Htt) to the pathogenesis of Huntington's disease (HD), the mechanisms underlying Htt aggregation and neurodegeneration remain poorly understood. Herein, we investigated the ultrastructural properties and protein composition of Htt cytoplasmic and nuclear inclusions in mammalian cells and primary neurons overexpressing mutant exon1 of the Htt protein. Our findings provide unique insight into the ultrastructural properties of cytoplasmic and nuclear Htt inclusions and their mechanisms of formation. We show that Htt inclusion formation and maturation are complex processes that, although initially driven by polyQ-dependent Htt aggregation, also involve the polyQ and PRD domain-dependent sequestration of lipids and cytoplasmic and cytoskeletal proteins related to HD dysregulated pathways; the recruitment and accumulation of remodeled or dysfunctional membranous organelles, and the impairment of the protein quality control and degradation machinery. We also show that nuclear and cytoplasmic Htt inclusions exhibit distinct biochemical compositions and ultrastructural properties, suggesting different mechanisms of aggregation and toxicity.

RALBP1 in Oxidative Stress and Mitochondrial Dysfunction in Alzheimer's Disease

The purpose of our study is to understand the role of the RALBP1 gene in oxidative stress (OS), mitochondrial dysfunction and cognition in Alzheimer's disease (AD) pathogenesis. The RALPB1 gene encodes the 76 kDa protein RLIP76 (Rlip). Rlip functions as a stress-responsive/protective transporter of glutathione conjugates (GS-E) and xenobiotic toxins. We hypothesized that Rlip may play an important role in maintaining cognitive function. The aim of this study is to determine whether Rlip deficiency in mice is associated with AD-like cognitive and mitochondrial dysfunction. Brain tissue obtained from cohorts of wildtype (WT) and Rlip^+/- mice were analyzed for OS markers, expression of genes that regulate mitochondrial fission/fusion, and synaptic integrity. We also examined mitochondrial ultrastructure in brains obtained from these mice and further analyzed the impact of Rlip deficiency on gene networks of AD, aging, stress response, mitochondrial function, and CREB signaling. Our studies revealed a significant increase in the levels of OS markers and alterations in the expression of genes and proteins involved in mitochondrial biogenesis, dynamics and synapses in brain tissues from these mice. Furthermore, we compared the cognitive function of WT and Rlip^+/- mice. Behavioral, basic motor and sensory function tests in Rlip^+/- mice revealed cognitive decline, similar to AD. Gene network analysis indicated dysregulation of stress-activated gene expression, mitochondrial function and CREB signaling genes in the Rlip^+/- mouse brain. Our results suggest that Rlip deficiency-associated increases in OS and mitochondrial dysfunction could contribute to the development or progression of OS-related AD processes.

The role of amyloids in Alzheimer's and Parkinson's diseases

With varying clinical symptoms, most neurodegenerative diseases are associated with abnormal loss of neurons. They share the same common pathogenic mechanisms involving misfolding and aggregation, and these visible aggregates of proteins are deposited in the central nervous system. Amyloid formation is thought to arise from partial unfolding of misfolded proteins leading to the exposure of hydrophobic surfaces, which interact with other similar structures and give rise to form dimers, oligomers, protofibrils, and eventually mature fibril aggregates. Accumulating evidence indicates that amyloid oligomers, not amyloid fibrils, are the most toxic species that causes Alzheimer's disease (AD) and Parkinson's disease (PD). AD has recently been recognized as the 'twenty-first century plague', with an incident rate of 1% at 60 years of age, which then doubles every fifth year. Currently, 5.3 million people in the US are afflicted with this disease, and the number of cases is expected to rise to 13.5 million by 2050. PD, a disorder of the brain, is the second most common form of dementia, characterized by difficulty in walking and movement. Keeping the above views in mind, in this review we have focused on the roles of amyloid in neurodegenerative diseases including AD and PD, the involvement of amyloid in mitochondrial dysfunction leading to neurodegeneration, are also considered in the review.

Defective mitophagy and synaptic degeneration in Alzheimer's disease: Focus on aging, mitochondria and synapse

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and multiple cognitive impairments. AD is marked by multiple cellular changes, including deregulation of microRNAs, activation of glia and astrocytes, hormonal imbalance, defective mitophagy, synaptic degeneration, in addition to extracellular neuritic amyloid-beta (Aβ) plaques, phosphorylated tau (P-tau), and intracellular neurofibrillary tangles (NFTs). Recent research in AD revealed that defective synaptic mitophagy leads to synaptic degeneration and cognitive dysfunction in AD neurons. Our critical analyses of mitochondria and Aβ and P-tau revealed that increased levels of Aβ and P-Tau, and abnormal interactions between Aβ and Drp1, P-Tau and Drp1 induced increased mitochondrial fragmentation and proliferation of dysfunctional mitochondria in AD neurons and depleted Parkin and PINK1 levels. These events ultimately lead to impaired clearance of dead and/or dying mitochondria in AD neurons. The purpose of our article is to highlight the recent research on mitochondria and synapses in relation to Aβ and P-tau, focusing on recent developments.

Preparation of Targeted Mitochondrion Nanoscale-Release Peptides and Their Efficiency on Eukaryotic Cells

We established a self-decomposable SiO₂ encapsulated mitochondrial targeting short peptide SS31 drug loading system (SiO₂@SS31) to determine its nano-sustained release characteristics in eukaryotic cells. We explored the protection of SiO₂@SS31 on the 661W cells after oxidative injury by H₂O₂. After the drug loading, we detected the morphology of SiO₂@SS31 by transmission electron microscopy (TEM). Moreover, high-pressure liquid chromatography (HPLC) was used to determine the drug capacity and encapsulation efficiency of the nanoparticles. Then, the release curve in vitro was drawn. The 661W cells were cultured in vitro to allow the detection of cytotoxicity by the MTT assay. The SS31loaded nanoscale microspheres labeled with fluorescein isothiocyanate (SiO₂@FITC-SS31) were prepared, and their sustained release effect was detected with intracellular endocytosis, using confocal microscopy and flow cytometry. Within 15 days, the SiO2@SS31 nanoparticles were completely decomposed and simultaneously released the SS31 peptide in deionized water and normal saline. Nonetheless, the process was faster in simulated body fluid and serum. The MTT assay suggested that SiO₂@SS31 has sustained protection compared with SS31 in the 661W cells at 48 h. Flow cytometry proved SiO₂@FITC-SS31 could maintain a high level and last longer after 24 h. The SS31 peptide, which has excellent medical application prospects, can be slowly and continuously released from self-decomposable SiO₂ and targeted to concentrate on mitochondria.

The effects of a mitochondrial targeted peptide (elamipretide/SS31) on BAX recruitment and activation during apoptosis

OBJECTIVE

Elamipretide (SS31) is a mitochondria-targeted peptide that has reported functions of stabilizing mitochondrial cristae structure and improving mitochondrial bioenergetics. Several studies have documented cell protective features of this peptide, including impairment of intrinsic apoptosis by inhibiting the recruitment and activation of the pro-apoptotic BAX protein. We used live-cell imaging of ARPE-19 cells expressing fluorescently labeled BAX, cytochrome c, and a mitochondrial marker to investigate the effect of elamipretide on the kinetics of BAX recruitment, mitochondrial outer membrane permeabilization (as a function of cytochrome c release), and mitochondrial fragmentation, respectively.

RESULT

In nucleofected and plated ARPE-19 cells, elamipretide accelerated the formation of larger mitochondria. In the presence of the apoptotic stimulator, staurosporine, cells treated with elamipretide exhibited moderately slower rates of BAX recruitment. Peptide treatment, however, did not significantly delay the onset of BAX recruitment or the final total amount of BAX that was recruited. Additionally, elamipretide showed no impairment or delay of cytochrome c release or mitochondrial fragmentation, two events associated with normal BAX activation during cell death. These results indicate that the protective effect of elamipretide is not at the level of BAX activity to induce pro-apoptotic mitochondrial dysfunction after the initiation of staurosporine-induced apoptosis.

Drp1-dependent peptide reverse mitochondrial fragmentation, a homeostatic response in Friedreich ataxia

Friedreich ataxia is an autosomal recessive, neurodegenerative disease characterized by the deficiency of the iron-sulfur cluster assembly protein frataxin. Loss of this protein impairs mitochondrial function. Mitochondria alter their morphology in response to various stresses; however, such alterations to morphology may be homeostatic or maladaptive depending upon the tissue and disease state. Numerous neurodegenerative diseases exhibit excessive mitochondrial fragmentation, and reversing this phenotype improves bioenergetics for diseases in which mitochondrial dysfunction is a secondary feature of the disease. This paper demonstrates that frataxin deficiency causes excessive mitochondrial fragmentation that is dependent upon Drp1 activity in Friedreich ataxia cellular models. Drp1 inhibition by the small peptide TAT-P110 reverses mitochondrial fragmentation but also decreases ATP levels in frataxin-knockdown fibroblasts and FRDA patient fibroblasts, suggesting that fragmentation may provide a homeostatic pathway for maintaining cellular ATP levels. The cardiolipin-stabilizing compound SS-31 similarly reverses fragmentation through a Drp1-dependent mechanism, but it does not affect ATP levels. The combination of TAT-P110 and SS-31 does not affect FRDA patient fibroblasts differently from SS-31 alone, suggesting that the two drugs act through the same pathway but differ in their ability to alter mitochondrial homeostasis. In approaching potential therapeutic strategies for FRDA, an important criterion for compounds that improve bioenergetics should be to do so without impairing the homeostatic response of mitochondrial fragmentation.

Mitochondrial and Redox-Based Therapeutic Strategies in Huntington's Disease

Significance: The molecular processes that determine Huntington's disease (HD) pathogenesis are not yet fully understood, and until now no effective neuroprotective therapeutic strategies have been developed. Mitochondria are one of most important organelles required for neuronal homeostasis, by providing metabolic pathways relevant for energy production, regulating calcium homeostasis, or controlling free radical generation and cell death. Because augmented reactive oxygen species (ROS) accompanied by mitochondrial dysfunction are relevant early HD mechanisms, targeting these cellular mechanisms may constitute relevant therapeutic approaches. Recent Advances: Previous findings point toward a close relationship between mitochondrial dysfunction and redox changes in HD. Mutant huntingtin (mHTT) can directly interact with mitochondrial proteins, as translocase of the inner membrane 23 (TIM23), disrupting mitochondrial proteostasis and favoring ROS production and HD progression. Furthermore, abnormal brain and muscle redox signaling contributes to altered proteostasis and motor impairment in HD, which can be improved with the mitochondria-targeted antioxidant mitoquinone or resveratrol, an SIRT1 activator that ameliorates mitochondrial biogenesis and function. Critical Issues: Various antioxidants and metabolic enhancers have been studied in HD; however, the real outcome of these molecules is still debatable. New compounds have proven to ameliorate mitochondrial and redox-based signaling pathways in early stages of HD, potentially precluding selective neurodegeneration. Future Directions: Unraveling the molecular etiology of deregulated mitochondrial function and dynamics, and oxidative stress opens new prospects for HD therapeutics. In this review, we explore the role of redox unbalance and mitochondrial dysfunction in HD progression, and further describe advances on clinical trials in HD based on mitochondrial and redox-based therapeutic strategies.

Peptide Szeto‑Schiller 31 ameliorates doxorubicin‑induced cardiotoxicity by inhibiting the activation of the p38 MAPK signaling pathway

Oxidative stress serves a key role in doxorubicin (DOX)-induced cardiotoxicity. The peptide Szeto-Schiller (SS)31 is an efficacious antioxidant with the capacity to reduce mitochondrial reactive oxygen species (ROS) levels and scavenge free radicals. Although SS31 is involved in the pathophysiological process of various cardiovascular diseases, the role of SS31 in DOX-induced cardiotoxicity remains unclear. To explore the effects of SS31 in DOX-induced cardiotoxicity, the present study first constructed DOX-induced cardiotoxicity models, in which H9c2 cells were incubated with 1 μM DOX for 24 h and C57BL/6 mice were administered DOX (20 mg/kg cumulative dose). The results of various assays in these models demonstrated that SS31 exhibited a cardioprotective effect in vitro and in vivo by attenuating the level of ROS, stabilizing the mitochondrial membrane potential and ameliorating myocardial apoptosis as well as fibrosis following treatment with DOX. Mechanistically, the results of the present study revealed that the p38 MAPK signaling pathway was inhibited by SS31 in DOX-treated H9c2 cells, which was associated with the cardioprotective function of SS31. In addition, P79350, a selective agonist of p38 MAPK, reversed the protective effects of SS31. Taken together, these results demonstrated the effects of SS31 on ameliorating DOX-induced cardiotoxicity and indicated its potential as a drug for the treatment of DOX-induced cardiotoxicity.

Protective effects of a mitochondria-targeted small peptide SS31 against hyperglycemia-induced mitochondrial abnormalities in the liver tissues of diabetic mice, Tallyho/JngJ mice

Type 2 Diabetes mellitus (T2DM) has become a major public health issue associated with a high risk of late-onset Alzheimer's disease (LOAD). Mitochondrial dysfunction is one of the molecular events that occur in the LOAD pathophysiology. The present study was planned to investigate the molecular alterations induced by hyperglycemia in the mitochondria of diabetic mice and further explore the possible ameliorative role of the mitochondria-targeted small peptide, SS31 in diabetic mice. For this purpose, we used a polygenic mouse model of type 2 diabetes, TALLYHO/JngJ (TH), and nondiabetic, SWR/J mice strains. The diabetic status in TH mice was confirmed at 8 weeks of age. The 24 weeks old experimental animals were segregated into three groups: Non-diabetic controls (SWR/J mice), diabetic (TH mice) and, SS31 treated diabetic TH mice. The mRNA and protein expression levels of mitochondrial proteins were investigated in all the study groups in the liver tissues using qPCR and immunoblot analysis. Also, the mitochondrial functions including H2O2 production, ATP generation, and lipid peroxidation were assessed in all the groups. Mitochondrial dysfunction was observed in TH mice as evident by significantly elevated H2O2 production, lipid peroxidation, and reduced ATP production. The mRNA expression and Western blot analysis of mitochondrial dynamics (Drp1 and Fis1 - fission; Mfn1, Mfn2, and Opa1 -fusion), and biogenesis (PGC-1α, Nrf1, Nrf2, and TFAM) genes were significantly altered in diabetic TH mice. Furthermore, SS31 treatment significantly reduced the mitochondrial abnormalities and restore mitochondrial functions in diabetic TH mice.

Coenzyme Q homeostasis in aging: Response to non-genetic interventions

Coenzyme Q (CoQ) is a key component for many essential metabolic and antioxidant activities in cells in mitochondria and cell membranes. Mitochondrial dysfunction is one of the hallmarks of aging and age-related diseases. Deprivation of CoQ during aging can be the cause or the consequence of this mitochondrial dysfunction. In any case, it seems clear that aging-associated CoQ deprivation accelerates mitochondrial dysfunction in these diseases. Non-genetic prolongevity interventions, including CoQ dietary supplementation, can increase CoQ levels in mitochondria and cell membranes improving mitochondrial activity and delaying cell and tissue deterioration by oxidative damage. In this review, we discuss the importance of CoQ deprivation in aging and age-related diseases and the effect of prolongevity interventions on CoQ levels and synthesis and CoQ-dependent antioxidant activities.

Human ClpP protease, a promising therapy target for diseases of mitochondrial dysfunction

Human caseinolytic protease P (HsClpP), an ATP-dependent unfolding peptidase protein in the mitochondrial matrix, controls protein quality, regulates mitochondrial metabolism, and maintains the integrity and enzyme activity of the mitochondrial respiratory chain (RC). Studies show that abnormalities in HsClpP lead to mitochondrial dysfunction and various human diseases. In this review, we provide a comprehensive overview of the structure and biological function of HsClpP, and the involvement of its dysexpression or mutation in mitochondria for a panel of important human diseases. We also summarize the structural types and binding modes of known HsClpP modulators. Finally, we discuss the challenges and future directions of HsClpP targeting as promising approach for the treatment of human diseases of mitochondrial origin.

Mitochondrial dysfunction, mitophagy, and role of dynamin-related protein 1 in Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia and progressive neurodegenerative disease. The presence of β-amyloid (Aβ) plaques and phosphorylated Tau tangles are considered to be the two main hallmarks of AD. Recent findings have shown that different changes in the structure and dynamics of mitochondria play an important role in AD pathology progression. Mitochondrial changes in AD are expressed as enhanced mitochondrial fragmentation, altered mitochondrial dynamics, and changes in the expression of mitochondrial biogenesis genes in vitro and in vivo models. Therefore, targeting mitochondria and associated mitochondrial proteins seems to be a promising alternative instead of targeting Aβ and Tau in the prevention of Alzheimer's disease. The dynamin-related protein (Drp1) is one such protein that plays an important role in the regulation of mitochondrial division and maintenance of mitochondrial structures. Few researchers have shown that inhibition of Drp1 GTPase activity in neuronal cells rescues excessive mitochondrial fragmentation. In addition, the growing evidence revealed that Drp1 can interact with both Aβ and Tau protein in human brain tissues and mouse models. In this review, we would like to update existing knowledge about various changes in and around mitochondria related to the pathogenesis of Alzheimer's disease, with particular emphasis on mitophagy and autophagy.