Sirtuin 3 interacts with Lon protease and regulates its acetylation status

Tissue-specific roles of mitochondrial unfolded protein response during obesity

Obesity is a worldwide multifactorial disease caused by an imbalance in energy metabolism, increasing adiposity, weight gain, and promoting related diseases such as diabetes, cardiovascular diseases, neurodegeneration, and cancer. Recent findings have reported that metabolic stress related to obesity induces a mitochondrial stress response called mitochondrial unfolded protein response (UPRmt), a quality control pathway that occurs in a nuclear DNA-mitochondria crosstalk, causing transduction of chaperones and proteases under stress conditions. The duality of UPRmt signaling, with both beneficial and detrimental effects, acts in different contexts depending on the tissue, cell type, and physiological states, affecting the mitochondrial function and efficiency and the metabolism homeostasis during obesity, which remains not fully clarified. Therefore, this review discusses the most recent findings regarding UPRmt signaling during obesity, bringing an overview of UPRmt across different metabolic tissues.

Sirtuin insights: bridging the gap between cellular processes and therapeutic applications

The greatest challenges that organisms face today are effective responses or detection of life-threatening environmental changes due to an obvious semblance of stress and metabolic fluctuations. These are associated with different pathological conditions among which cancer is most important. Sirtuins (SIRTs; NAD+-dependent enzymes) are versatile enzymes with diverse substrate preferences, cellular locations, crucial for cellular processes and pathological conditions. This article describes in detail the distinct roles of SIRT isoforms, unveiling their potential as either cancer promoters or suppressors and also explores how both natural and synthetic compounds influence the SIRT function, indicating promise for therapeutic applications. We also discussed the inhibitors/activators tailored to specific SIRTs, holding potential for diseases lacking effective treatments. It may uncover the lesser-studied SIRT isoforms (e.g., SIRT6, SIRT7) and their unique functions. This article also offers a comprehensive overview of SIRTs, linking them to a spectrum of diseases and highlighting their potential for targeted therapies, combination approaches, disease management, and personalized medicine. We aim to contribute to a transformative era in healthcare and innovative treatments by unraveling the intricate functions of SIRTs.

Mitochondrial unfolded protein response (UPRmt): what we know thus far

Mitochondria are key organelles for the optimal function of the cell. Among their many functions, they maintain protein homeostasis through their own proteostatic machinery, which involves proteases and chaperones that regulate protein import and folding inside mitochondria. In the early 2000s, the mitochondrial unfolded protein response (UPRmt) was first described in mammalian cells. This stress response is activated by the accumulation of unfolded/misfolded proteins within the mitochondrial matrix, which results in the transmission of a signal to the nucleus to increase the expression of proteases and chaperones to address the abnormal mitochondrial protein load. After its discovery, this retrograde signaling pathway has also been described in other organisms of different complexities, suggesting that it is a conserved stress response. Although there are some specific differences among organisms, the mechanism of this stress response is mostly similar and involves the transmission of a signal from mitochondria to the nucleus that induces chromatin remodeling to allow the binding of specific transcription factors to the promoters of chaperones and proteases. In the last decade, proteins and signaling pathways that could be involved in the regulation of the UPRmt, including the Wnt signaling pathway, have been described. This minireview aims to summarize what is known about the mechanism of the UPRmt and its regulation, specifically in mammals and C. elegans.

Exercise combined with postbiotics treatment results in synergistic improvement of mitochondrial function in the brain of male transgenic mice for Alzheimer's disease

BACKGROUND

It has been suggested that exercise training and postbiotic supplement could decelerate the progress of functional and biochemical deterioration in double transgenic mice overexpresses mutated forms of the genes for human amyloid precursor protein (APPsw) and presenilin 1 (m146L) (APP/PS1TG). Our earlier published data indicated that the mice performed better than controls on the Morris Maze Test parallel with decreased occurrence of amyloid-β plaques in the hippocampus. We investigated the neuroprotective and therapeutic effects of high-intensity training and postbiotic supplementation.

METHODS

Thirty-two adult APP/PS1TG mice were randomly divided into four groups: (1) control, (2) high-intensity training (3) postbiotic, (4) combined (training and postbiotic) treatment for 20 weeks. In this study, the whole hemibrain without hippocampus was used to find molecular traits explaining improved brain function. We applied qualitative RT-PCR for gene expression, Western blot for protein level, and Zymography for LONP1 activity. Disaggregation analysis of Aβ-40 was performed in the presence of Lactobacillus acidophilus and Bifidobacterium longum lysate.

RESULTS

We found that exercise training decreased Alzheimer's Disease (AD)-related gene expression (NF-kB) that was not affected by postbiotic treatment. The preparation used for postbiotic treatment is composed of tyndallized Bifidobacterium longum and Lactobacillus acidophilus. Both of the postbiotics effectively disaggregated amyloid-β/Aβ-40 aggregates by chelating Zn2+ and Cu2+ ions. The postbiotic treatment decreased endogenous human APPTG protein expression and mouse APP gene expression in the hemibrains. In addition, the postbiotic treatment elevated mitochondrial LONP1 activity as well.

CONCLUSION

Our findings revealed distinct mechanisms behind improved memory performance in the whole brain: while exercise training modulates NF-kB signaling pathway regulating immune response until postbiotic diminishes APP gene expression, disaggregates pre-existing amyloid-β plaques and activates mitochondrial protein quality control in the region of brain out of hippocampus. Using the above treatments complements and efficiently slows down the development of AD.

Mitochondrial unfolded protein response in ischemia-reperfusion injury

Mitochondrial unfolded protein response (UPRmt) is a mitochondrial stress response that activates the transcriptional program of mitochondrial chaperone proteins and proteases to keep protein homeostasis in mitochondria. Ischemia-reperfusion injury results in multiple severe clinical issues linked to high morbidity and mortality in various disorders. The pathophysiology and pathogenesis of ischemia-reperfusion injury are complex and multifactorial. Emerging evidence showed the roles of UPRmt signaling in ischemia-reperfusion injury. Herein, we discuss the regulatory mechanisms underlying UPRmt signaling in C. elegans and mammals. Furthermore, we review the recent studies into the roles and mechanisms of UPRmt signaling in ischemia-reperfusion injury of the heart, brain, kidney, and liver. Further research of UPRmt signaling will potentially develop novel therapeutic strategies against ischemia-reperfusion injury.

Evidence for mitochondrial Lonp1 expression in the nucleus

The coordinated communication between the mitochondria and nucleus is essential for cellular activities. Nonetheless, the pathways involved in this crosstalk are scarcely understood. The protease Lonp1 was previously believed to be exclusively located in the mitochondria, with an important role in mitochondrial morphology, mtDNA maintenance, and cellular metabolism, in both normal and neoplastic cells. However, we recently detected Lonp1 in the nuclear, where as much as 22% of all cellular Lonp1 can be found. Nuclear localization is detectable under all conditions, but the amount is dependent on a response to heat shock (HS). Lonp1 in the nucleus interacts with heat shock factor 1 (HSF1) and modulates the HS response. These findings reveal a novel extramitochondrial function for Lonp1 in response to stress.

Pterostilbene in Combination With Mitochondrial Cofactors Improve Mitochondrial Function in Cellular Models of Mitochondrial Diseases

Mitochondrial diseases are genetic disorders caused by mutations in genes in the nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) that encode mitochondrial structural or functional proteins. Although considered “rare” due to their low incidence, such diseases affect thousands of patients’ lives worldwide. Despite intensive research efforts, most mitochondrial diseases are still incurable. Recent studies have proposed the modulation of cellular compensatory pathways such as mitophagy, AMP-activated protein kinase (AMPK) activation or the mitochondrial unfolded protein response (UPR^mt) as novel therapeutic approaches for the treatment of these pathologies. UPR^mt is an intracellular compensatory pathway that signals mitochondrial stress to the nucleus for the activation of mitochondrial proteostasis mechanisms including chaperones, proteases and antioxidants. In this work a potentially beneficial molecule, pterostilbene (a resveratrol analogue), was identified as mitochondrial booster in drug screenings. The positive effects of pterostilbene were significantly increased in combination with a mitochondrial cocktail (CoC3) consisting of: pterostilbene, nicotinamide, riboflavin, thiamine, biotin, lipoic acid and l-carnitine. CoC3 increases sirtuins’ activity and UPR^mt activation, thus improving pathological alterations in mutant fibroblasts and induced neurons.

The Unfolded Protein Responses in Health, Aging, and Neurodegeneration: Recent Advances and Future Considerations

Aging and age-related neurodegeneration are both associated with the accumulation of unfolded and abnormally folded proteins, highlighting the importance of protein homeostasis (termed proteostasis) in maintaining organismal health. To this end, two cellular compartments with essential protein folding functions, the endoplasmic reticulum (ER) and the mitochondria, are equipped with unique protein stress responses, known as the ER unfolded protein response (UPR ER ) and the mitochondrial UPR (UPR mt ), respectively. These organellar UPRs play roles in shaping the cellular responses to proteostatic stress that occurs in aging and age-related neurodegeneration. The loss of adaptive UPR ER and UPR mt signaling potency with age contributes to a feed-forward cycle of increasing protein stress and cellular dysfunction. Likewise, UPR ER and UPR mt signaling is often altered in age-related neurodegenerative diseases; however, whether these changes counteract or contribute to the disease pathology appears to be context dependent. Intriguingly, altering organellar UPR signaling in animal models can reduce the pathological consequences of aging and neurodegeneration which has prompted clinical investigations of UPR signaling modulators as therapeutics. Here, we review the physiology of both the UPR ER and the UPR mt , discuss how UPR ER and UPR mt signaling changes in the context of aging and neurodegeneration, and highlight therapeutic strategies targeting the UPR ER and UPR mt that may improve human health.

Fumonisin B2 Induces Mitochondrial Stress and Mitophagy in Human Embryonic Kidney (Hek293) Cells-A Preliminary Study

Ubiquitous soil fungi parasitise agricultural commodities and produce mycotoxins. Fumonisin B2 (FB2), the structural analogue of the commonly studied Fumonisin B1 (FB1), is a neglected mycotoxin produced by several Fusarium species. Mycotoxins are known for inducing toxicity via mitochondrial stress alluding to mitochondrial degradation (mitophagy). These processes involve inter-related pathways that are regulated by proteins related to SIRT3 and Nrf2. This study aimed to investigate mitochondrial stress responses in human kidney (Hek293) cells exposed to FB2 for 24 h. Cell viability was assessed via the methylthiazol tetrazolium (MTT) assay, and the half-maximal inhibitory concentration (IC50 = 317.4 µmol/L) was estimated using statistical software. Reactive oxygen species (ROS; H2DCFDA), mitochondrial membrane depolarisation (JC1-mitoscreen) and adenosine triphosphate (ATP; luminometry) levels were evaluated to assess mitochondrial integrity. The relative expression of mitochondrial stress response proteins (SIRT3, pNrf2, LONP1, PINK1, p62 and HSP60) was determined by Western blot. Transcript levels of SIRT3, PINK1 and miR-27b were assessed using quantitative PCR (qPCR). FB2 reduced ATP production (p = 0.0040), increased mitochondrial stress marker HSP60 (p = 0.0140) and suppressed upregulation of mitochondrial stress response proteins SIRT3 (p = 0.0026) and LONP1 (p = 0.5934). FB2 promoted mitophagy via upregulation of pNrf2 (p = 0.0008), PINK1 (p = 0.0014) and p62 (p < 0.0001) protein expression. FB2 also suppressed miR-27b expression (p < 0.0001), further promoting the occurrence of mitophagy. Overall, the findings suggest that FB2 increases mitochondrial stress and promotes mitophagy in Hek293 cells.

Sirtuins Modulation: A Promising Strategy for HIV-Associated Neurocognitive Impairments

HIV-Associated neurocognitive disorder (HAND) is one of the major concerns since it persists in 40% of this population. Nowadays, HAND neuropathogenesis is considered to be caused by the infected cells that cross the brain-blood barrier and produce viral proteins that can be secreted and internalized into neurons leading to disruption of cellular processes. The evidence points to viral proteins such as Tat as the causal agent for neuronal alteration and thus HAND. The hallmarks in Tat-induced neurodegeneration are endoplasmic reticulum stress and mitochondrial dysfunction. Sirtuins (SIRTs) are NAD+-dependent deacetylases involved in mitochondria biogenesis, unfolded protein response, and intrinsic apoptosis pathway. Tat interaction with these deacetylases causes inhibition of SIRT1 and SIRT3. Studies revealed that SIRTs activation promotes neuroprotection in neurodegenerative diseases such Alzheimer's and Parkinson's disease. Therefore, this review focuses on Tat-induced neurotoxicity mechanisms that involve SIRTs as key regulators and their modulation as a therapeutic strategy for tackling HAND and thereby improving the quality of life of people living with HIV.

Sirtuins at the Crossroads between Mitochondrial Quality Control and Neurodegenerative Diseases: Structure, Regulation, Modifications, and Modulators

Sirtuins (SIRT1-SIRT7), a family of nicotinamide adenine dinucleotide (NAD⁺)-dependent enzymes, are key regulators of life span and metabolism. In addition to acting as deacetylates, some sirtuins have the properties of deacylase, decrotonylase, adenosine diphosphate (ADP)-ribosyltransferase, lipoamidase, desuccinylase, demalonylase, deglutarylase, and demyristolyase. Mitochondrial dysfunction occurs early on and acts causally in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Sirtuins are implicated in the regulation of mitochondrial quality control, which is highly associated with the pathogenesis of neurodegenerative diseases. There is growing evidence indicating that sirtuins are promising and well-documented molecular targets for the treatment of mitochondrial dysfunction and neurodegenerative disorders by regulating mitochondrial quality control, including mitochondrial biogenesis, mitophagy, mitochondrial fission/fusion dynamics, and mitochondrial unfolded protein responses (mtUPR). Therefore, elucidation of the molecular etiology of sirtuin-mediated mitochondrial quality control points to new prospects for the treatment of neurodegenerative diseases. However, the mechanisms underlying sirtuin-mediated mitochondrial quality control remain obscure. In this review, we update and summarize the current understanding of the structure, function, and regulation of sirtuins with an emphasis on the cumulative and putative effects of sirtuins on mitochondrial biology and neurodegenerative diseases, particularly their roles in mitochondrial quality control. In addition, we outline the potential therapeutic applications for neurodegenerative diseases of targeting sirtuin-mediated mitochondrial quality control through exercise training, calorie restriction, and sirtuin modulators in neurodegenerative diseases.

Multifaced role of protein deacetylase sirtuins in neurodegenerative disease

Sirtuins, a class III histone/protein deacetylase, is a central regulator of metabolic function and cellular stress response. This plays a pivotal role in the pathogenesis and progression of diseases such as cancer, neurodegeneration, metabolic syndromes, and cardiovascular disease. Sirtuins regulate biological and cellular processes, for instance, mitochondrial biogenesis, lipid and fatty acid oxidation, oxidative stress, gene transcriptional activity, apoptosis, inflammatory response, DNA repair mechanism, and autophagic cell degradation, which are known components for the progression of the neurodegenerative diseases (NDDs). Emerging evidence suggests that sirtuins are the useful molecular targets against NDDs like, Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD), and Amyotrophic Lateral Sclerosis (ALS). However, the exact mechanism of neuroprotection mediated through sirtuins remains unsettled. The manipulation of sirtuins activity with its modulators, calorie restriction (CR), and micro RNAs (miR) is a novel therapeutic approach for the treatment of NDDs. Herein, we reviewed the current putative therapeutic role of sirtuins in regulating synaptic plasticity and cognitive functions, which are mediated through the different molecular phenomenon to prevent neurodegeneration. We also explained the implications of sirtuin modulators, and miR based therapies for the treatment of life-threatening NDDs.

Mitochondrial proteins in heart failure: The role of deacetylation by SIRT3

Heart failure (HF) is still the leading cause of death worldwide, occurring with a variety of complex mechanisms. However, most intervention for HF do not directly target the pathological mechanisms underlying cell damage in failing cardiomyocytes. Mitochondria are involved in many physiological processes, which is an important guarantee for normal heart function. Mitochondrial dysfunction is considered to be the critical node of the development of HF. Strict modulation of the mitochondrial function can ameliorate the myocardial injury and protect cardiac function. Acetylation plays an important role in mitochondrial protein homeostasis, and SIRT3, the most important deacetylation protein in mitochondria, is involved in the maintenance of mitochondrial function. SIRT3 can delay the progression of HF by improving mitochondrial function. Herein we summarize the interaction between SIRT3 and proteins related to mitochondrial function including oxidative phosphorylation (OXPHOS), fatty acid oxidation (FAO), mitochondrial biosynthesis, mitochondrial quality control. In addition, we also sum up the effects of this interaction on HF and the research progress of treatments targeting SIRT3, so as to find potential HF therapeutic for clinical use in the future.

Mitochondrial Sirtuins in Reproduction

Mitochondria act as hubs of numerous metabolic pathways. Mitochondrial dysfunctions contribute to altering the redox balance and predispose to aging and metabolic alterations. The sirtuin family is composed of seven members and three of them, SIRT3-5, are housed in mitochondria. They catalyze NAD+-dependent deacylation and the ADP-ribosylation of mitochondrial proteins, thereby modulating gene expression and activities of enzymes involved in oxidative metabolism and stress responses. In this context, mitochondrial sirtuins (mtSIRTs) act in synergistic or antagonistic manners to protect from aging and aging-related metabolic abnormalities. In this review, we focus on the role of mtSIRTs in the biological competence of reproductive cells, organs, and embryos. Most studies are focused on SIRT3 in female reproduction, providing evidence that SIRT3 improves the competence of oocytes in humans and animal models. Moreover, SIRT3 protects oocytes, early embryos, and ovaries against stress conditions. The relationship between derangement of SIRT3 signaling and the imbalance of ROS and antioxidant defenses in testes has also been demonstrated. Very little is known about SIRT4 and SIRT5 functions in the reproductive system. The final goal of this work is to understand whether sirtuin-based signaling may be taken into account as potential targets for therapeutic applications in female and male infertility.

Mitochondrial Kinases and the Role of Mitochondrial Protein Phosphorylation in Health and Disease

The major role of mitochondria is to provide cells with energy, but no less important are their roles in responding to various stress factors and the metabolic changes and pathological processes that might occur inside and outside the cells. The post-translational modification of proteins is a fast and efficient way for cells to adapt to ever changing conditions. Phosphorylation is a post-translational modification that signals these changes and propagates these signals throughout the whole cell, but it also changes the structure, function and interaction of individual proteins. In this review, we summarize the influence of kinases, the proteins responsible for phosphorylation, on mitochondrial biogenesis under various cellular conditions. We focus on their role in keeping mitochondria fully functional in healthy cells and also on the changes in mitochondrial structure and function that occur in pathological processes arising from the phosphorylation of mitochondrial proteins.

The emerging potential of SIRT-3 in oxidative stress-inflammatory axis associated increased neuroinflammatory component for metabolically impaired neural cell

People suffering from conditions like epilepsy, where there is an excess of neuron excitement, stroke, and cardiac arrest, where there are oxygen and glucose deprivation, Alzheimer, Parkinson, and Huntington's disease that causes metabolic and also oxidative stress-inflammatory axis; are known to be more vulnerable to disturbances in the metabolism, and there is a lot of inadequacy in defining the inflammation's mechanistic connections, as well as neurodegeneration and the bioenergetic deficiencies in the CNS. We retrieved relevant studies from PubMed/ScienceDirect/Medline/Public library of science/Mendeley/Springer link as well as Google Scholar. We used various keywords both individually and in combination with the literature search. 'Epidemiology of neurodegenerative disorders', 'neurodegenerative diseases associated hyper inflammation', 'Mechanism of inflammation in neuronal cell', 'Involvement of SIRTin inflammation', 'Pathogenesis of mitochondrial associated metabolic impairment in neurons', 'Reactive oxygen species-mediated mitochondrial dysfunction' were a few of the keywords used for the search. PINCH, which is a chronic neuro-inflammatory component that cannot be detected in matured neurons which are healthy, though expressed in oxidative stress inflammatory axis related tauopathy and diseases that cause neurodegeneration. We attempted to study the regulatory mechanisms that cause changes in the bioenergetics and its neuronal defects and mitochondrial subcellular localization that are PINCH protein-mediated on the other handSIRT1, the most intensively studied sirtuin, in oxidative stress-mediated inflammatory consequence for many diseases but very few research data explore the role of SIRT-3 for correction of the chronic neuroinflammatory component. Thus, in this review, we investigate the very recently identified molecules involving in the pathogenesis during stimulated oxidative stress-inflammatory axis in the excitatory neuronal cell which changes brain metabolism. Simultaneously, in CNS neurons of diseases with a component of chronic neuroinflammation which exhibit neuroprotective response, the consequences (mechanistic and biological) of SIRT-3, could be emerging future targets for neurodegenerative disorder treatment with impaired metabolisms.

Advances in characterization of SIRT3 deacetylation targets in mitochondrial function

The homeostasis of mitochondrial functional state is intimately in relation with SIRT3 (sirtuin3). SIRT3, the deacetylase mainly anchored in mitochondria, acts as a modulator of metabolic regulation via manipulating the activity and function of downstream targets at post-translational modification levels. The features of energy sensing and ADP-ribose transference of SIRT3 have also been reported. Recently, accumulating SIRT3-focusing evidences have suggested its complicated role in a series of adverse events such as metabolic disorders, aging-related diseases, coupled with tumors, in which SIRT3 regulates the progress of corresponding biochemical reactions by targeting key mediators. By systematically summarizing the downstream deacetylated proteins of the SIRT3 axis, this review aims to give a comprehensive introduction to the main metabolic pathways and diseases of the molecules involved in acetylation modification, which is expected to provide a direction for further exploration of the pathogenesis and therapeutic targets of the above diseases.

Oxidized nicotinamide adenine dinucleotide-dependent mitochondrial deacetylase sirtuin-3 as a potential therapeutic target of Parkinson's disease

Mitochondrial impairment is associated with progressive dopamine (DA) neuron degeneration in Parkinson's disease (PD). Recent findings highlight that Sirtuin-3 (SIRT3), a mitochondrial protein, is an oxidized nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase and a key modulator in maintaining integrity and functions of mitochondria. SIRT3 plays vital roles in regulation of mitochondrial functions, including mitochondrial ATP generation and energy metabolism, anti-oxidant defense, and cell death and proliferation. SIRT3 can deacetylate the transcriptional factors and crosstalk with different signaling pathways to cooperatively modulate mitochondrial functions and regulate defensive mitochondrial quality control (QC) systems. Down-regulated NAD+ level and decreased SIRT3 activity are related to aging process and has been pathologically linked to PD pathogenesis. Further, SIRT3 can bind and deacetylate PTEN-induced kinase 1 (PINK1) and PD protein 2 E3 ubiquitin protein ligase (Parkin) to facilitate mitophagy. Leucine Rich Repeat Kinase 2 (LRRK2)-G2019S mutation in PD is linked to SIRT3 impairment. Furthermore, SIRT3 is inversely associated with α-synuclein aggregation and DA neuron degeneration in PD. SIRT3 chemical activators and NAD+ precursors can up-regulate SIRT3 activity to protect against DA neuron degeneration in PD models. Taken together, SIRT3 is a promising PD therapeutic target and studies of SIRT3 functional modulators with neuroprotective capability will be of clinical interest.

Fumonisin B1 -induced mitochondrial toxicity and hepatoprotective potential of rooibos: An update

Fumonisins are a family of potentially carcinogenic mycotoxins produced by Fusarium verticillioides. Several fumonisins have been identified with fumonisin B₁ (FB₁ ) being the most toxic. The canonical mechanism of FB₁ toxicity is centered on its structural resemblance with sphinganine and consequent competitive inhibition of ceramide synthase and disruption of lipidomic profiles. Recent and emerging evidence at the molecular level has identified the disruption of mitochondria and excessive generation of toxic reactive oxygen species (ROS) as alternative/additional mechanisms of toxicity. The understanding of how these pathways contribute to FB₁ toxicity can lead to the identification of novel, effective approaches to protecting vulnerable populations. Natural compounds with antioxidant properties seem to protect against the induced toxic effects of FB₁ . Rooibos (Aspalathus linearis), endemic to South Africa, has traditionally been used as a medicinal herbal tea with strong scientific evidence supporting its anecdotal claims. The unique composition of phytochemicals and combination of metabolic activators, adaptogens and antioxidants make rooibos an attractive yet underappreciated intervention for FB₁ toxicoses. In the search for a means to address FB₁ toxicoses as a food safety problem in developing countries, phytomedicine and traditional knowledge systems must play an integral part. This review aims to summarize the growing body of evidence succinctly, which highlights mitochondrial dysfunction as a secondary toxic effect responsible for the FB₁ -induced generation of ROS. We further propose the potential of rooibos to combat this induced toxicity based on its integrated bioactive properties, as a socio-economically viable strategy to prevent and/or repair cellular damage caused by FB₁ .

How Do Post-Translational Modifications Influence the Pathomechanistic Landscape of Huntington's Disease? A Comprehensive Review

Huntington’s disease (HD) is an autosomal dominant inherited neurodegenerative disorder characterized by the loss of motor control and cognitive ability, which eventually leads to death. The mutant huntingtin protein (HTT) exhibits an expansion of a polyglutamine repeat. The mechanism of pathogenesis is still not fully characterized; however, evidence suggests that post-translational modifications (PTMs) of HTT and upstream and downstream proteins of neuronal signaling pathways are involved. The determination and characterization of PTMs are essential to understand the mechanisms at work in HD, to define possible therapeutic targets better, and to challenge the scientific community to develop new approaches and methods. The discovery and characterization of a panoply of PTMs in HTT aggregation and cellular events in HD will bring us closer to understanding how the expression of mutant polyglutamine-containing HTT affects cellular homeostasis that leads to the perturbation of cell functions, neurotoxicity, and finally, cell death. Hence, here we review the current knowledge on recently identified PTMs of HD-related proteins and their pathophysiological relevance in the formation of abnormal protein aggregates, proteolytic dysfunction, and alterations of mitochondrial and metabolic pathways, neuroinflammatory regulation, excitotoxicity, and abnormal regulation of gene expression.

The biology of Lonp1: More than a mitochondrial protease

Initially discovered as a protease responsible for degradation of misfolded or damaged proteins, the mitochondrial Lon protease (Lonp1) turned out to be a multifaceted enzyme, that displays at least three different functions (proteolysis, chaperone activity, binding of mtDNA) and that finely regulates several cellular processes, within and without mitochondria. Indeed, LONP1 in humans is ubiquitously expressed, and is involved in regulation of response to oxidative stress and, heat shock, in the maintenance of mtDNA, in the regulation of mitophagy. Furthermore, its proteolytic activity can regulate several biochemical pathways occurring totally or partially within mitochondria, such as TCA cycle, oxidative phosphorylation, steroid and heme biosynthesis and glutamine production. Because of these multiple activities, Lon protease is highly conserved throughout evolution, and mutations occurring in its gene determines severe diseases in humans, including a rare syndrome characterized by Cerebral, Ocular, Dental, Auricular and Skeletal anomalies (CODAS). Finally, alterations of LONP1 regulation in humans can favor tumor progression and aggressiveness, further highlighting the crucial role of this enzyme in mitochondrial and cellular homeostasis.

Mitochondrial Quality Control: Role in Cardiac Models of Lethal Ischemia-Reperfusion Injury

The current standard of care for acute myocardial infarction or 'heart attack' is timely restoration of blood flow to the ischemic region of the heart. While reperfusion is essential for the salvage of ischemic myocardium, re-introduction of blood flow paradoxically kills (rather than rescues) a population of previously ischemic cardiomyocytes-a phenomenon referred to as 'lethal myocardial ischemia-reperfusion (IR) injury'. There is long-standing and exhaustive evidence that mitochondria are at the nexus of lethal IR injury. However, during the past decade, the paradigm of mitochondria as mediators of IR-induced cardiomyocyte death has been expanded to include the highly orchestrated process of mitochondrial quality control. Our aims in this review are to: (1) briefly summarize the current understanding of the pathogenesis of IR injury, and (2) incorporating landmark data from a broad spectrum of models (including immortalized cells, primary cardiomyocytes and intact hearts), provide a critical discussion of the emerging concept that mitochondrial dynamics and mitophagy (the components of mitochondrial quality control) may contribute to the pathogenesis of cardiomyocyte death in the setting of ischemia-reperfusion.

SIRT3 Regulation of Mitochondrial Quality Control in Neurodegenerative Diseases

Neurodegenerative diseases are disorders that are characterized by a progressive decline of motor and/or cognitive functions caused by the selective degeneration and loss of neurons within the central nervous system. The most common neurodegenerative diseases are Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Neurons have high energy demands, and dysregulation of mitochondrial quality and function is an important cause of neuronal degeneration. Mitochondrial quality control plays an important role in maintaining mitochondrial integrity and ensuring normal mitochondrial function; thus, defects in mitochondrial quality control are also significant causes of neurodegenerative diseases. The mitochondrial deacetylase SIRT3 has been found to have a large effect on mitochondrial function. Recent studies have also shown that SIRT3 has a role in mitochondrial quality control, including in the refolding or degradation of misfolded/unfolded proteins, mitochondrial dynamics, mitophagy, and mitochondrial biogenesis, all of which are affected in neurodegenerative diseases.

Activator of one protease transforms into inhibitor of another in response to nutritional signals

In this study, Yeom et al. address how cells tune protease specificity to nutritional signals. They show that Salmonella enterica increases the specificity of the broadly conserved proteases Lon and ClpSAP by transforming the Lon activator and substrate HspQ into an inhibitor of the N-degron recognin ClpS, the adaptor of the ClpAP protease, thus providing insight into how cells modify the abundance of particular proteins while preserving the quality of their proteomes.

All cells use proteases to adjust protein amounts. Proteases maintain protein homeostasis by degrading nonfunctional toxic proteins and play regulatory roles by targeting particular substrates in response to specific signals. Here we address how cells tune protease specificity to nutritional signals. We report that Salmonella enterica increases the specificity of the broadly conserved proteases Lon and ClpSAP by transforming the Lon activator and substrate HspQ into an inhibitor of the N-degron recognin ClpS, the adaptor of the ClpAP protease. We establish that upon acetylation, HspQ stops being a Lon activator and substrate and that the accumulated HspQ binds to ClpS, hindering degradation of ClpSAP substrates. Growth on glucose promotes HspQ acetylation by increasing acetyl-CoA amounts, thereby linking metabolism to proteolysis. By altering protease specificities but continuing to degrade junk proteins, cells modify the abundance of particular proteins while preserving the quality of their proteomes. This rapid response mechanism linking protease specificity to nutritional signals is broadly conserved.

LonP1 Differently Modulates Mitochondrial Function and Bioenergetics of Primary Versus Metastatic Colon Cancer Cells

Mitochondrial Lon protease (LonP1) is a multi-function enzyme that regulates mitochondrial functions in several human malignancies, including colorectal cancer (CRC). The mechanism(s) by which LonP1 contributes to colorectal carcinogenesis is not fully understood. We found that silencing LonP1 leads to severe mitochondrial impairment and apoptosis in colon cancer cells. Here, we investigate the role of LonP1 in mitochondrial functions, metabolism, and epithelial-mesenchymal transition (EMT) in colon tumor cells and in metastasis. LonP1 was almost absent in normal mucosa, gradually increased from aberrant crypt foci to adenoma, and was most abundant in CRC. Moreover, LonP1 was preferentially upregulated in colorectal samples with mutated p53 or nuclear β-catenin, and its overexpression led to increased levels of β-catenin and decreased levels of E-cadherin, key proteins in EMT, in vitro. LonP1 upregulation also induced opposite changes in oxidative phosphorylation, glycolysis, and pentose pathway in SW480 primary colon tumor cells when compared to SW620 metastatic colon cancer cells. In conclusion, basal LonP1 expression is essential for normal mitochondrial function, and increased LonP1 levels in SW480 and SW620 cells induce a metabolic shift toward glycolysis, leading to EMT.

Coordinating Mitochondrial Biology Through the Stress-Responsive Regulation of Mitochondrial Proteases

Proteases are localized throughout mitochondria and function as critical regulators of all aspects of mitochondrial biology. As such, the activities of these proteases are sensitively regulated through transcriptional and post-translational mechanisms to adapt mitochondrial function to specific cellular demands. Here, we discuss the stress-responsive mechanisms responsible for regulating mitochondrial protease activity and the implications of this regulation on mitochondrial function. Furthermore, we describe how imbalances in the activity or regulation of mitochondrial proteases induced by genetic, environmental, or aging-related factors influence mitochondria in the context of disease. Understanding the molecular mechanisms by which cells regulate mitochondrial function through alterations in protease activity provide insights into the contributions of these proteases in pathologic mitochondrial dysfunction and reveals new therapeutic opportunities to ameliorate this dysfunction in the context of diverse classes of human disease.

SIRT3 and mitochondrial metabolism in neurodegenerative diseases

The NAD⁺-dependent deacetylase protein Sirtuin 3 (SIRT3) is emerging among the factors playing a key role in the regulation of mitochondrial function and in the prevention of oxidative stress. This deacetylase activates protein substrates directly involved in the production and detoxification of ROS, such as superoxide dismutase 2 and catalase, but also enzymes in the lipid beta-oxidation pathway. In this paper we review existing evidence on the role of SIRT3 in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington disease, including data from new experiments in a model for amyotrophic lateral sclerosis linked to mutations in superoxide dismutase 1. Specifically, we report that expression of the mitochondrial isoform of SIRT3 is altered in muscle from the G93A-SOD1 mice during progression of disease; this alteration influences mitochondrial metabolism, which may be relevant for the well known energetic alterations taking place in ALS patients. These data reinforce the concept that SIRT3 may be a relevant therapeutic target is ALS as well as in other neurodegenerative diseases.

Curcumin Upregulates Antioxidant Defense, Lon Protease, and Heat-Shock Protein 70 Under Hyperglycemic Conditions in Human Hepatoma Cells

Sirtuin 3 (SIRT3) regulates mitochondrial antioxidant (AO) defense and improves mitochondrial disorders. Curcumin protects mitochondria; however, the mechanisms need investigation. We postulated that curcumin increases AO defense under hyperglycemic conditions in HepG2 cells through SIRT3-mediated mechanisms. Cell viability was determined in HepG2 cells cultured with 5 mM glucose, 19.9 mM mannitol, vehicle control, 10 mM glucose, and 30 mM glucose in the absence or presence of curcumin for 24 h. SIRT3, nuclear factor-kappa B (NF-κB), heat-shock protein 70 (Hsp70), and Lon protein expressions were determined using western blot. Transcript levels of SIRT3, peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), cAMP response element-binding protein (CREB), glutathione peroxidase 1 (GPx1), and superoxide dismutase 2 (SOD2) were measured by quantitative polymerase chain reaction. Cell viability, SIRT3 protein expression, transcript levels of SIRT3, PGC-1α, CREB, GPx1, and SOD2 and protein expressions of NF-κB, Lon, and Hsp70 were significantly increased in the curcumin-treated hyperglycemic groups. Since curcumin and SIRT3 both improve mitochondrial function and AO defense, SIRT3 may be involved in the protective effects of curcumin.

Mitochondrial Lon protease in human disease and aging: Including an etiologic classification of Lon-related diseases and disorders

The Mitochondrial Lon protease, also called LonP1 is a product of the nuclear gene LONP1. Lon is a major regulator of mitochondrial metabolism and response to free radical damage, as well as an essential factor for the maintenance and repair of mitochondrial DNA. Lon is an ATP-stimulated protease that cycles between being bound (at the inner surface of the inner mitochondrial membrane) to the mitochondrial genome, and being released into the mitochondrial matrix where it can degrade matrix proteins. At least three different roles or functions have been ascribed to Lon: 1) Proteolytic digestion of oxidized proteins and the turnover of specific essential mitochondrial enzymes such as aconitase, TFAM, and StAR; 2) Mitochondrial (mt)DNA-binding protein, involved in mtDNA replication and mitogenesis; and 3) Protein chaperone, interacting with the Hsp60-mtHsp70 complex. LONP1 orthologs have been studied in bacteria, yeast, flies, worms, and mammals, evincing the widespread importance of the gene, as well as its remarkable evolutionary conservation. In recent years, we have witnessed a significant increase in knowledge regarding Lon's involvement in physiological functions, as well as in an expanding array of human disorders, including cancer, neurodegeneration, heart disease, and stroke. In addition, Lon appears to have a significant role in the aging process. A number of mitochondrial diseases have now been identified whose mechanisms involve various degrees of Lon dysfunction. In this paper we review current knowledge of Lon's function, under normal conditions, and we propose a new classification of human diseases characterized by a either over-expression or decline or loss of function of Lon. Lon has also been implicated in human aging, and we review the data currently available as well as speculating about possible interactions of aging and disease. Finally, we also discuss Lon as potential therapeutic target in human disease.

Fusaric acid induces mitochondrial stress in human hepatocellular carcinoma (HepG2) cells

Fusarium spp are common contaminants of maize and produce many mycotoxins, including the fusariotoxin fusaric acid (FA). FA is a niacin related compound, chelator of divalent cations, and mediates toxicity via oxidative stress and possible mitochondrial dysregulation. Sirtuin 3 (SIRT3) is a stress response deacetylase that maintains proper mitochondrial function. We investigated the effect of FA on SIRT3 and oxidative and mitochondrial stress pathways in the hepatocellular carcinoma (HepG2) cell line. We determined FA toxicity (24 h incubation; IC50 = 104 μg/ml) on mitochondrial output, cellular and mitochondrial stress responses, mitochondrial biogenesis and markers of cell death using spectrophotometry, luminometry, qPCR and western blots. FA caused a dose dependent decrease in metabolic activity along with significant depletion of intracellular ATP. FA induced a significant increase in lipid peroxidation, despite up-regulation of the antioxidant transcription factor, Nrf2. FA significantly decreased expression of SIRT3 mRNA with a concomitant decrease in protein expression. Lon protease was also significantly down-regulated. FA induced aberrant mitochondrial biogenesis as evidenced by significantly decreased protein expressions of: PGC-1α, p-CREB, NRF1 and HSP70. Finally, FA activated apoptosis as noted by the significantly increased activity of caspases 3/7 and also induced cellular necrosis. This study provides insight into the molecular mechanisms of FA (a neglected mycotoxin) induced hepatotoxicity.

AGC1/2, the mitochondrial aspartate-glutamate carriers

In this review we discuss the structure and functions of the aspartate/glutamate carriers (AGC1-aralar and AGC2-citrin). Those proteins supply the aspartate synthesized within mitochondrial matrix to the cytosol in exchange for glutamate and a proton. A structure of an AGC carrier is not available yet but comparative 3D models were proposed. Moreover, transport assays performed by using the recombinant AGC1 and AGC2, reconstituted into liposome vesicles, allowed to explore the kinetics of those carriers and to reveal their specific transport properties. AGCs participate to a wide range of cellular functions, as the control of mitochondrial respiration, calcium signaling and antioxydant defenses. AGC1 might also play peculiar tissue-specific functions, as it was found to participate to cell-to-cell metabolic symbiosis in the retina. On the other hand, AGC1 is involved in the glutamate-mediated excitotoxicity in neurons and AGC gene or protein alterations were discovered in rare human diseases. Accordingly, a mice model of AGC1 gene knock-out presented with growth delay and generalized tremor, with myelinisation defects. More recently, AGC was proposed to play a crucial role in tumor metabolism as observed from metabolomic studies showing that the asparate exported from the mitochondrion by AGC1 is employed in the regeneration of cytosolic glutathione. Therefore, given the central role of AGCs in cell metabolism and human pathology, drug screening are now being developed to identify pharmacological modulators of those carriers. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Lon protease and eiF2α are involved in acute, but not prolonged, antiretroviral induced stress response in HepG2 cells

Lon protease, an ATP dependent mitochondrial protease, is important in mitochondrial protein maintenance. Disruption of protein homeostasis and mitochondrial dysfunction is associated with lipodystrophy, metabolic syndrome and accelerated aging, and are commonly observed in patients on long term antiretroviral therapy. Sirtuin 3 (SIRT3) is a post-translational regulator of Lon and regulates antioxidant response. We previously showed the nucleoside analogues (NRTIs), Zidovudine (AZT; 7.1 μM), Stavudine (d4T; 4 μM), and Tenofovir (TFV; 1.2 μM) induced oxidative stress and mitochondrial dysfunction in human hepatoma (HepG2) cells at 24 h (h) and 120 h. We conducted a mitochondrial proteomic assessment of homeostasis in the same model, using the same NRTIs. Protein expression of Lon, SIRT3, heat shock protein (HSP) 60, phospho-eukaryotic translation initiation factor 2α (p-eIF2α; Ser51) and phospho-c-jun N-terminal kinase (p-JNK; Thr183/Tyr185) were quantified by western blots. The data showed all stress responses were significantly increased in HepG2 cells by all antiretroviral drugs at 24 h (p < 0.0001); however, at 120 h, a significant depletion in the ATP-dependent proteins Lon (p = 0.00013) and HSP60 (p < 0.0001) was observed. Proteins initiated by endoplasmic reticulum stress: p-eIF2α (p = 0.001) and p-JNK (p = 0.0029), were significantly reduced following prolonged treatment. SIRT3 was maintained at elevated levels in the treated cells following prolonged exposure (p < 0.001). We conclude that the ATP dependent proteins are more relevant to acute toxicity, while SIRT3 confers protection over prolonged periods of toxicity.

Emerging role of Lon protease as a master regulator of mitochondrial functions

Lon protease is a nuclear-encoded, mitochondrial ATP-dependent protease highly conserved throughout the evolution, crucial for the maintenance of mitochondrial homeostasis. Lon acts as a chaperone of misfolded proteins, and is necessary for maintaining mitochondrial DNA. The impairment of these functions has a deep impact on mitochondrial functionality and morphology. An altered expression of Lon leads to a profound reprogramming of cell metabolism, with a switch from respiration to glycolysis, which is often observed in cancer cells. Mutations of Lon, which likely impair its chaperone properties, are at the basis of a genetic inherited disease named of the cerebral, ocular, dental, auricular, skeletal (CODAS) syndrome. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

The Role of Mitochondrial DNA in Mediating Alveolar Epithelial Cell Apoptosis and Pulmonary Fibrosis

Convincing evidence has emerged demonstrating that impairment of mitochondrial function is critically important in regulating alveolar epithelial cell (AEC) programmed cell death (apoptosis) that may contribute to aging-related lung diseases, such as idiopathic pulmonary fibrosis (IPF) and asbestosis (pulmonary fibrosis following asbestos exposure). The mammalian mitochondrial DNA (mtDNA) encodes for 13 proteins, including several essential for oxidative phosphorylation. We review the evidence implicating that oxidative stress-induced mtDNA damage promotes AEC apoptosis and pulmonary fibrosis. We focus on the emerging role for AEC mtDNA damage repair by 8-oxoguanine DNA glycosylase (OGG1) and mitochondrial aconitase (ACO-2) in maintaining mtDNA integrity which is important in preventing AEC apoptosis and asbestos-induced pulmonary fibrosis in a murine model. We then review recent studies linking the sirtuin (SIRT) family members, especially SIRT3, to mitochondrial integrity and mtDNA damage repair and aging. We present a conceptual model of how SIRTs modulate reactive oxygen species (ROS)-driven mitochondrial metabolism that may be important for their tumor suppressor function. The emerging insights into the pathobiology underlying AEC mtDNA damage and apoptosis is suggesting novel therapeutic targets that may prove useful for the management of age-related diseases, including pulmonary fibrosis and lung cancer.

Energizing Genetics and Epi-genetics: Role in the Regulation of Mitochondrial Function

Energy metabolism and mitochondrial function hold a core position in cellular homeostasis. Oxidative metabolism is regulated at multiple levels, ranging from gene transcription to allosteric modulation. To accomplish the fine tuning of these multiple regulatory circuits, the nuclear and mitochondrial compartments are tightly and reciprocally controlled. The fact that nuclear encoded factors, PPARγ coactivator 1α and mitochondrial transcription factor A, play pivotal roles in the regulation of oxidative metabolism and mitochondrial biogenesis is paradigmatic of this crosstalk. Here we provide an updated survey of the genetic and epigenetic mechanisms involved in the control of energy metabolism and mitochondrial function. Chromatin dynamics highly depends on post-translational modifications occurring at specific amino acids in histone proteins and other factors associated to nuclear DNA. In addition to the well characterized enzymes responsible for histone methylation/demethylation and acetylation/deacetylation, other factors have gone on the "metabolic stage". This is the case of the new class of α-ketoglutarate-regulated demethylases (Jumonji C domain containing demethylases) and of the NAD+-dependent deacetylases, also known as sirtuins. Moreover, unexpected features of the machineries involved in mitochondrial DNA (mtDNA) replication and transcription, mitochondrial RNA processing and maturation have recently emerged. Mutations or defects of any component of these machineries profoundly affect mitochondrial activity and oxidative metabolism. Finally, recent evidences support the importance of mtDNA packaging in replication and transcription. These observations, along with the discovery that non-classical CpG islands present in mtDNA undergo methylation, indicate that epigenetics also plays a role in the regulation of the mitochondrial genome function.

Protein maturation and proteolysis in plant plastids, mitochondria, and peroxisomes

Plastids, mitochondria, and peroxisomes are key organelles with dynamic proteomes in photosynthetic eukaryotes. Their biogenesis and activity must be coordinated and require intraorganellar protein maturation, degradation, and recycling. The three organelles together are predicted to contain ∼200 presequence peptidases, proteases, aminopeptidases, and specific protease chaperones/adaptors, but the substrates and substrate selection mechanisms are poorly understood. Similarly, lifetime determinants of organellar proteins, such as N-end degrons and tagging systems, have not been identified, but the substrate recognition mechanisms likely share similarities between organelles. Novel degradomics tools for systematic analysis of protein lifetime and proteolysis could define such protease-substrate relationships, degrons, and protein lifetime. Intraorganellar proteolysis is complemented by autophagy of whole organelles or selected organellar content, as well as by cytosolic protein ubiquitination and degradation by the proteasome. This review summarizes (putative) plant organellar protease functions and substrate-protease relationships. Examples illustrate key proteolytic events.