Ground control to major TOM: mitochondria-nucleus communication

Discovery of CLPP-1071 as an Exceptionally Potent and Orally Efficacious Human ClpP Activator with Strong In Vivo Antitumor Activity

Human sapiens caseinolytic protease P (ClpP) is essential for maintaining mitochondrial proteome homeostasis, and its activation is increasingly recognized as a promising cancer therapy strategy. Herein, based on structure-guided drug design, we discovered a series of potent ClpP activators by introducing a methyl group to the imipridone scaffold of the ClpP activator ONC201 in Phase III clinical trials. Through structural optimization of the lead compound, the most optimal compound, CLPP-1071, exhibited exceptionally potent ClpP agonistic activity (EC50 = 23.5 nM, 107.1-fold stronger than ONC201) and inhibited the proliferation of HL60 cells (IC50 = 4.6 nM, 169.2-fold stronger than ONC201). CLPP-1071 possesses good pharmacokinetic properties and effectively prolongs the lifespan in the MOLM13 and HL60 xenograft models in mice through oral administration. CLPP-1071 is the most potent and orally efficacious ClpP activator reported to date.

Mitochondria as a primary determinant of angiogenic modality in pulmonary arterial hypertension

Impaired pulmonary angiogenesis plays a pivotal role in the progression of pulmonary arterial hypertension (PAH) and patient mortality, yet the molecular mechanisms driving this process remain enigmatic. Our study uncovered a striking connection between mitochondrial dysfunction (MD), caused by a humanized mutation in the NFU1 gene, and severely disrupted pulmonary angiogenesis in adult lungs. Restoring the bioavailability of the NFU1 downstream target, lipoic acid (LA), alleviated MD and angiogenic deficiency and rescued the progressive PAH phenotype in the NFU1G206C model. Notably, significant NFU1 expression and signaling insufficiencies were also identified in idiopathic PAH (iPAH) patients' lungs, emphasizing this study's relevance beyond NFU1 mutation cases. The remarkable improvement in mitochondrial function of PAH patient-derived pulmonary artery endothelial cells (PAECs) following LA supplementation introduces LA as a potential therapeutic approach. In conclusion, this study unveils a novel role for MD in dysregulated pulmonary angiogenesis and PAH manifestation, emphasizing the need to correct MD in PAH patients with unrecognized NFU1/LA deficiency.

Monitoring mitochondrial localization of dual localized proteins using a Bi-Genomic Mitochondrial-Split-GFP

Even if a myriad of approaches has been developed to identify the subcellular localization of a protein, the easiest and fastest way remains to fuse the protein to Green Fluorescent Protein (GFP) and visualize its location using fluorescence microscopy. However, this strategy is not well suited to visualize the organellar pools of proteins that are simultaneously localized both in the cytosol and in organelles because the GFP signal of a cytosolic pool of the protein (cytosolic echoform) will inevitably mask or overlay the GFP signal of the organellar pool of the protein (organellar echoform). To solve this issue, we engineered a dedicated yeast strain expressing a Bi-Genomic Mitochondrial-Split-GFP. This split-GFP is bi-genomic because the first ten ß-strands of GFP (GFPß1-10) are encoded by the mitochondrial genome and translated by mitoribosomes whereas the remaining ß-strand of GFP (GFPß11) is fused to the protein of interest encoded by the nucleus and expressed by cytosolic ribosomes. Consequently, if the GFPß11-tagged protein localizes into mitochondria, GFP will be reconstituted by self-assembly GFPß1-10 and GFPß11 thereby generating a GFP signal restricted to mitochondria and detectable by regular fluorescence microscopy. In addition, because mitochondrial translocases and import mechanisms are evolutionary well conserved, the BiG Mito-Split-GFP yeast strain can be used to probe mitochondrial importability of proteins regardless of their organismal origins and can thus serve to identify unsuspected mitochondrial echoforms readily from any organism.

Monitoring retro-translocation of proteins from the mitochondrial intermembrane space

Mitochondria play multiple essential roles in eukaryotic cells. To perform their functions, mitochondria require an adequate supply of externally produced proteins and an intact two-membrane structure. The structure of mitochondrial membranes separates these organelles from their cytosolic environment, with proteins that make up the mitochondrial proteome either being embedded into or enveloped by these membranes. From the experimental point of view, the structural properties of mitochondria contribute to the relative ease of isolating these organelles from other cellular components. The ability to isolate intact mitochondria and analyze them in a well-controlled environment opens up the possibility of tracking any proteins that enter or escape the membrane-formed enclosure. This chapter discusses assays that monitor the movement of proteins out of mitochondria through intact membranes. These protocols provide insight into the mechanisms behind mitochondrial protein quality control. It was discovered that the retro-translocation of IMS proteins regulates the protein content of this specific sub-compartment of the organelle. Additionally, proteins can move out of the mitochondria to resolve failed import events. Assays based on isolated mitochondria precisely tackle such intricate 'dance' of proteins crossing mitochondrial membranes during import and export, maintaining the dynamics of the organellar proteome.

Mitochondria and MICOS - function and modeling

An extensive review is presented on mitochondrial structure and function, mitochondrial proteins, the outer and inner membranes, cristae, the role of F1FO-ATP synthase, the mitochondrial contact site and cristae organizing system (MICOS), the sorting and assembly machinery morphology and function, and phospholipids, in particular cardiolipin. Aspects of mitochondrial regulation under physiological and pathological conditions are outlined, in particular the role of dysregulated MICOS protein subunit Mic60 in Parkinson's disease, the relations between mitochondrial quality control and proteins, and mitochondria as signaling organelles. A mathematical modeling approach of cristae and MICOS using mechanical beam theory is introduced and outlined. The proposed modeling is based on the premise that an optimization framework can be used for a better understanding of critical mitochondrial function and also to better map certain experiments and clinical interventions.

Mitochondrial Quality Control Processes at the Crossroads of Cell Death and Survival: Mechanisms and Signaling Pathways

Biological aging results from an accumulation of damage in the face of reduced resilience. One major driver of aging is cell senescence, a state in which cells remain viable but lose their proliferative capacity, undergo metabolic alterations, and become resistant to apoptosis. This is accompanied by complex cellular changes that enable the development of a senescence-associated secretory phenotype (SASP). Mitochondria, organelles involved in energy provision and activities essential for regulating cell survival and death, are negatively impacted by aging. The age-associated decline in mitochondrial function is also accompanied by the development of chronic low-grade sterile inflammation. The latter shares some features and mediators with the SASP. Indeed, the unloading of damage-associated molecular patterns (DAMPs) at the extracellular level can trigger sterile inflammatory responses and mitochondria can contribute to the generation of DAMPs with pro-inflammatory properties. The extrusion of mitochondrial DNA (mtDNA) via mitochondrial outer membrane permeabilization under an apoptotic stress triggers senescence programs. Additional pathways can contribute to sterile inflammation. For instance, pyroptosis is a caspase-dependent inducer of systemic inflammation, which is also elicited by mtDNA release and contributes to aging. Herein, we overview the molecular mechanisms that may link mitochondrial dyshomeostasis, pyroptosis, sterile inflammation, and senescence and discuss how these contribute to aging and could be exploited as molecular targets for alleviating the cell damage burden and achieving healthy longevity.

Exercise-Regulated Mitochondrial and Nuclear Signalling Networks in Skeletal Muscle

Exercise perturbs energy homeostasis in skeletal muscle and engages integrated cellular signalling networks to help meet the contraction-induced increases in skeletal muscle energy and oxygen demand. Investigating exercise-associated perturbations in skeletal muscle signalling networks has uncovered novel mechanisms by which exercise stimulates skeletal muscle mitochondrial biogenesis and promotes whole-body health and fitness. While acute exercise regulates a complex network of protein post-translational modifications (e.g. phosphorylation) in skeletal muscle, previous investigations of exercise signalling in human and rodent skeletal muscle have primarily focused on a select group of exercise-regulated protein kinases [i.e. 5' adenosine monophosphate-activated protein kinase (AMPK), protein kinase A (PKA), Ca2+/calmodulin-dependent protein kinase (CaMK) and mitogen-activated protein kinase (MAPK)] and only a small subset of their respective protein substrates. Recently, global mass spectrometry-based phosphoproteomic approaches have helped unravel the extensive complexity and interconnection of exercise signalling pathways and kinases beyond this select group and phosphorylation and/or translocation of exercise-regulated mitochondrial and nuclear protein substrates. This review provides an overview of recent advances in our understanding of the molecular events associated with acute endurance exercise-regulated signalling pathways and kinases in skeletal muscle with a focus on phosphorylation. We critically appraise recent evidence highlighting the involvement of mitochondrial and nuclear protein phosphorylation and/or translocation in skeletal muscle adaptive responses to an acute bout of endurance exercise that ultimately stimulate mitochondrial biogenesis and contribute to exercise's wider health and fitness benefits.

Two ancient membrane pores mediate mitochondrial-nucleus membrane contact sites

Coordination between nucleus and mitochondria is essential for cell survival, and thus numerous communication routes have been established between these two organelles over eukaryotic cell evolution. One route for organelle communication is via membrane contact sites, functional appositions formed by molecular tethers. We describe a novel nuclear-mitochondrial membrane contact site in the protozoan Toxoplasma gondii. We have identified specific contacts occurring at the nuclear pore and demonstrated an interaction between components of the nuclear pore and the mitochondrial protein translocon, highlighting them as molecular tethers. Genetic disruption of the nuclear pore or the TOM translocon components, TgNup503 or TgTom40, respectively, result in contact site reduction, supporting their potential involvement in this tether. TgNup503 depletion further leads to specific mitochondrial morphology and functional defects, supporting a role for nuclear-mitochondrial contacts in mediating their communication. The discovery of a contact formed through interaction between two ancient mitochondrial and nuclear complexes sets the ground for better understanding of mitochondrial-nuclear crosstalk in eukaryotes.

Intracellular signaling in proto-eukaryotes evolves to alleviate regulatory conflicts of endosymbiosis

The complex eukaryotic cell resulted from a merger between simpler prokaryotic cells, yet the role of the mitochondrial endosymbiosis with respect to other eukaryotic innovations has remained under dispute. To investigate how the regulatory challenges associated with the endosymbiotic state impacted genome and network evolution during eukaryogenesis, we study a constructive computational model where two simple cells are forced into an obligate endosymbiosis. Across multiple in silico evolutionary replicates, we observe the emergence of different mechanisms for the coordination of host and symbiont cell cycles, stabilizing the endosymbiotic relationship. In most cases, coordination is implicit, without signaling between host and symbiont. Signaling only evolves when there is leakage of regulatory products between host and symbiont. In the fittest evolutionary replicate, the host has taken full control of the symbiont cell cycle through signaling, mimicking the regulatory dominance of the nucleus over the mitochondrion that evolved during eukaryogenesis.

Recent Advances of Mitochondrial Alterations in Alzheimer's Disease: A Perspective of Mitochondrial Basic Events

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders and is characterized by a decrease in learning capacity, memory loss and behavioral changes. In addition to the well-recognized amyloid-β cascade hypothesis and hyperphosphorylated Tau hypothesis, accumulating evidence has led to the proposal of the mitochondrial dysfunction hypothesis as the primary etiology of AD. However, the predominant molecular mechanisms underlying the development and progression of AD have not been fully elucidated. Mitochondrial dysfunction is not only considered an early event in AD pathogenesis but is also involved in the whole course of the disease, with numerous pathophysiological processes, including disordered energy metabolism, Ca2+ homeostasis dysfunction and hyperactive oxidative stress. In the current review, we have integrated emerging evidence to summarize the main mitochondrial alterations- bioenergetic metabolism, mitochondrial inheritance, mitobiogenesis, fission- fusion dynamics, mitochondrial degradation, and mitochondrial movement- underlying AD pathogenesis; precisely identified the mitochondrial regulators; discussed the potential mechanisms and primary processes; highlighted the leading players; and noted additional incidental signaling pathway changes. This review may help to stimulate research exploring mitochondrial metabolically-oriented neuroprotection strategies in AD therapies, leading to a better understanding of the link between the mitochondrial dysfunction hypothesis and AD pathogenesis.

What matters in aging is signaling for responsiveness

Biological responsiveness refers to the capacity of living organisms to adapt to changes in both their internal and external environments through physiological and behavioral mechanisms. One of the prominent aspects of aging is the decline in this responsiveness, which can lead to a deterioration in the processes required for maintenance, survival, and growth. The vital link between physiological responsiveness and the essential life processes lies within the signaling systems. To devise effective strategies for controlling the aging process, a comprehensive reevaluation of this connecting loop is imperative. This review aims to explore the impact of aging on signaling systems responsible for responsiveness and introduce a novel perspective on intervening in the aging process by restoring the compromised responsiveness. These innovative mechanistic approaches for modulating altered responsiveness hold the potential to illuminate the development of action plans aimed at controlling the aging process and treating age-related disorders.

The Contribution of Hippocampal All-Trans Retinoic Acid (ATRA) Deficiency to Alzheimer's Disease: A Narrative Overview of ATRA-Dependent Gene Expression in Post-Mortem Hippocampal Tissue

There is accumulating evidence that vitamin A (VA) deficiency contributes to the pathogenesis and progression of Alzheimer's disease (AD). All-trans retinoic acid (ATRA), a metabolite of VA in the brain, serves distinct roles in the human hippocampus. Agonists of retinoic acid receptors (RAR), including ATRA, promote activation of the non-amyloidogenic pathway by enhancing expression of α-secretases, providing a mechanistic basis for delaying/preventing amyloid beta (Aβ) toxicity. However, whether ATRA is actually deficient in the hippocampi of patients with AD is not clear. Here, using a publicly available human transcriptomic dataset, we evaluated the extent to which ATRA-sensitive genes are dysregulated in hippocampal tissue from post-mortem AD brains, relative to age-matched controls. Consistent with ATRA deficiency, we found significant dysregulation of many ATRA-sensitive genes and significant upregulation of RAR co-repressors, supporting the idea of transcriptional repression of ATRA-mediated signaling. Consistent with oxidative stress and neuroinflammation, Nrf2 and NfkB transcripts were upregulated, respectively. Interestingly, transcriptional targets of Nrf2 were not upregulated, accompanied by upregulation of several histone deacetylases. Overall, our investigation of ATRA-sensitive genes in the human hippocampus bolsters the scientific premise of ATRA depletion in AD and that epigenetic factors should be considered and addressed as part of VA supplementation.

A big picture of the mitochondria-mediated signals: From mitochondria to organism

Mitochondria, well-known for years as the powerhouse and biosynthetic center of the cell, are dynamic signaling organelles beyond their energy production and biosynthesis functions. The metabolic functions of mitochondria, playing an important role in various biological events both in physiological and stress conditions, transform them into important cellular stress sensors. Mitochondria constantly communicate with the rest of the cell and even from other cells to the organism, transmitting stress signals including oxidative and reductive stress or adaptive signals such as mitohormesis. Mitochondrial signal transduction has a vital function in regulating integrity of human genome, organelles, cells, and ultimately organism.

Melanin: a unifying theory of disease as exemplified by Parkinson's, Alzheimer's, and Lewy body dementia

Melanin, a ubiquitous dark pigment, plays important roles in the immune system, including scavenging reactive oxygen species formed in response to ultraviolet radiation absorption, absorbing metals, thermal regulation, drug uptake, innate immune system functions, redox, and energy transduction. Many tissue types, including brain, heart, arteries, ovaries, and others, contain melanin. Almost all cells contain precursors to melanin. A growing number of diseases in which there is a loss of melanin and/or neuromelanin are increasingly thought to have infectious etiologies, for example, Alzheimer's disease (AD), Parkinson's disease (PD), Lewy Body Dementia (LBD), and vitiligo. AD, PD, LBD, and vitiligo have been linked with herpesvirus, which enters melanosomes and causes apoptosis, and with gut dysbiosis and inflammation. Herpesvirus is also linked with gut dysbiosis and inflammation. We theorize that under normal healthy states, melanin retains some of the energy it absorbs from electromagnetic radiation, which is then used to fuel cells, and energy from ATP is used to compliment that energy supply. We further theorize that loss of melanin reduces the energy supply of cells, which in the case of AD, PD, and LBD results in an inability to sustain immune system defenses and remove the plaques associated with the disease, which appear to be part of the immune system's attempt to eradicate the pathogens seen in these neurodegenerative diseases. In addition, in an attempt to explain why removing these plaques does not result in improvements in cognition and mood and why cognitions and moods in these individuals have ebbs and flows, we postulate that it is not the plaques that cause the cognitive symptoms but, rather, inflammation in the brain resulting from the immune system's response to pathogens. Our theory that energy retained in melanin fuels cells in an inverse relationship with ATP is supported by studies showing alterations in ATP production in relationship to melanin levels in melanomas, vitiligo, and healthy cells. Therefore, alteration of melanin levels may be at the core of many diseases. We propose regulating melanin levels may offer new avenues for treatment development.

GPR19 Coordinates Multiple Molecular Aspects of Stress Responses Associated with the Aging Process

G protein-coupled receptors (GPCRs) play a significant role in controlling biological paradigms such as aging and aging-related disease. We have previously identified receptor signaling systems that are specifically associated with controlling molecular pathologies associated with the aging process. Here, we have identified a pseudo-orphan GPCR, G protein-coupled receptor 19 (GPR19), that is sensitive to many molecular aspects of the aging process. Through an in-depth molecular investigation process that involved proteomic, molecular biological, and advanced informatic experimentation, this study found that the functionality of GPR19 is specifically linked to sensory, protective, and remedial signaling systems associated with aging-related pathology. This study suggests that the activity of this receptor may play a role in mitigating the effects of aging-related pathology by promoting protective and remedial signaling systems. GPR19 expression variation demonstrates variability in the molecular activity in this larger process. At low expression levels in HEK293 cells, GPR19 expression regulates signaling paradigms linked with stress responses and metabolic responses to these. At higher expression levels, GPR19 expression co-regulates systems involved in sensing and repairing DNA damage, while at the highest levels of GPR19 expression, a functional link to processes of cellular senescence is seen. In this manner, GPR19 may function as a coordinator of aging-associated metabolic dysfunction, stress response, DNA integrity management, and eventual senescence.

Finding the balance: The elusive mechanisms underlying auditory hair cell mitochondrial biogenesis and mitophagy

In all cell types, mitochondrial biogenesis is balanced with mitophagy to maintain a healthy mitochondrial pool that sustains specific energetic demands. Cell types that have a higher energetic burden, such as skeletal muscle cells and cardiomyocytes, will subsequently develop high mitochondrial volumes. In these cells, calcium influx during activity triggers cascades leading to activation of the co-transcriptional regulation factor PGC-1α, a master regulator of mitochondrial biogenesis, in a well-defined pathway. Despite the advantages in ATP production, high mitochondrial volumes might prove to be perilous, as it increases exposure to reactive oxygen species produced during oxidative phosphorylation. Mechanosensory hair cells are highly metabolically active cells, with high total mitochondrial volumes to meet that demand. However, the mechanisms leading to expansion and maintenance of the hair cell mitochondrial pool are not well defined. Calcium influx during mechanotransduction and synaptic transmission regulate hair cell mitochondria, leading to a possibility that similar to skeletal muscle and cardiomyocytes, intracellular calcium underlies the expansion of the hair cell mitochondrial volume. This review briefly summarizes the potential mechanisms underlying mitochondrial biogenesis in other cell types and in hair cells. We propose that hair cell mitochondrial biogenesis is primarily product of cellular differentiation rather than calcium influx, and that the hair cell high mitochondrial volume renders them more susceptible to reactive oxygen species increased by calcium flux than other cell types.

LUBAC assembles a ubiquitin signaling platform at mitochondria for signal amplification and transport of NF-κB to the nucleus

Mitochondria are increasingly recognized as cellular hubs to orchestrate signaling pathways that regulate metabolism, redox homeostasis, and cell fate decisions. Recent research revealed a role of mitochondria also in innate immune signaling; however, the mechanisms of how mitochondria affect signal transduction are poorly understood. Here, we show that the NF-κB pathway activated by TNF employs mitochondria as a platform for signal amplification and shuttling of activated NF-κB to the nucleus. TNF treatment induces the recruitment of HOIP, the catalytic component of the linear ubiquitin chain assembly complex (LUBAC), and its substrate NEMO to the outer mitochondrial membrane, where M1- and K63-linked ubiquitin chains are generated. NF-κB is locally activated and transported to the nucleus by mitochondria, leading to an increase in mitochondria-nucleus contact sites in a HOIP-dependent manner. Notably, TNF-induced stabilization of the mitochondrial kinase PINK1 furthermore contributes to signal amplification by antagonizing the M1-ubiquitin-specific deubiquitinase OTULIN. Overall, our study reveals a role for mitochondria in amplifying TNF-mediated NF-κB activation, both serving as a signaling platform, as well as a transport mode for activated NF-κB to the nuclear.

Multifaceted roles of mitochondrial stress responses under ETC dysfunction - repair, destruction and pathogenesis

Electron transport chain (ETC) dysfunction is a common feature of mitochondrial diseases and induces severe cellular stresses, including mitochondrial membrane potential (Δψ_m ) reduction, mitochondrial matrix acidification, metabolic derangements and proteostatic stresses. Extensive studies of ETC dysfunction in yeast, Caenorhabditis elegans, cultured cells and mouse models have revealed multiple mitochondrial stress response pathways. Here, we summarise the current understanding of the triggers, sensors, signalling mechanisms and the functional outcomes of mitochondrial stress responses in different species. We highlight Δψ_m reduction as a major trigger of stress responses in different species, but the responses are species-specific and the outcomes are context-dependent. ETC dysfunction elicits a mitochondrial unfolded protein response (UPR^mt ) to repair damaged mitochondria in C. elegans, and activates a global adaptive programme to maintain Δψ_m in yeast. Yeast and C. elegans responses are remarkably similar at the downstream responses, although they are activated by different signalling mechanisms. UPR^mt generally protects ETC-defective worms, but its constitutive activation is toxic for wildtype worms and worms carrying mutant mtDNA. In contrast to lower organisms, ETC dysfunction in mammals mainly activates a mitochondrial integrated stress response (ISR^mt ) to reprogramme metabolism and a PINK1-Parkin mitophagy pathway to degrade damaged mitochondria. Accumulating in vivo results suggest that the ATF4 branch of ISR^mt exacerbates metabolic derangements to accelerate mitochondrial disease progression. The in vivo roles of mitophagy in mitochondrial diseases are also context-dependent. These results thus reveal the common and unique aspects of mitochondrial stress responses in different species and highlight their multifaceted roles in mitochondrial diseases.

Exosome mediated Tom40 delivery protects against hydrogen peroxide-induced oxidative stress by regulating mitochondrial function

Mitochondrial dysfunction is a hallmark of neurodegeneration. The expression level of Tom40, a crucial mitochondrial membrane protein, is significantly reduced in neurodegenerative disease subjects. Tom40 overexpression studies have shown to protect the neurons against oxidative stress by improving mitochondrial function. Thus, successful delivery of Tom40 protein to the brain could lead to a novel therapy for neurodegenerative diseases. However, delivering protein to the cell may be difficult. Especially the blood-brain barrier (BBB) is a big hurdle to clear in order to deliver the protein to the brain. In the current study, we engineered exosomes, which are the extracellular vesicles of endosomal origin, and able to cross BBB as delivery vehicles packing human Tom40. We found Tom40 protein delivery by the exosome successfully protected the cells against hydrogen peroxide-induced oxidative stress. This result suggests that exosome-mediated delivery of Tom40 may potentially be useful in restoring mitochondrial functions and alleviating oxidative stress in neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases.

Mitochondrial adaptation in cancer drug resistance: prevalence, mechanisms, and management

Drug resistance represents a major obstacle in cancer management, and the mechanisms underlying stress adaptation of cancer cells in response to therapy-induced hostile environment are largely unknown. As the central organelle for cellular energy supply, mitochondria can rapidly undergo dynamic changes and integrate cellular signaling pathways to provide bioenergetic and biosynthetic flexibility for cancer cells, which contributes to multiple aspects of tumor characteristics, including drug resistance. Therefore, targeting mitochondria for cancer therapy and overcoming drug resistance has attracted increasing attention for various types of cancer. Multiple mitochondrial adaptation processes, including mitochondrial dynamics, mitochondrial metabolism, and mitochondrial apoptotic regulatory machinery, have been demonstrated to be potential targets. However, recent increasing insights into mitochondria have revealed the complexity of mitochondrial structure and functions, the elusive functions of mitochondria in tumor biology, and the targeting inaccessibility of mitochondria, which have posed challenges for the clinical application of mitochondrial-based cancer therapeutic strategies. Therefore, discovery of both novel mitochondria-targeting agents and innovative mitochondria-targeting approaches is urgently required. Here, we review the most recent literature to summarize the molecular mechanisms underlying mitochondrial stress adaptation and their intricate connection with cancer drug resistance. In addition, an overview of the emerging strategies to target mitochondria for effectively overcoming chemoresistance is highlighted, with an emphasis on drug repositioning and mitochondrial drug delivery approaches, which may accelerate the application of mitochondria-targeting compounds for cancer therapy.

Monitoring DNA polymerase β mitochondrial localization and dynamics

Mouse fibroblasts lacking (null) DNA polymerase β (pol β) were transfected with fluorescently tagged pol β and stained with biomarkers to allow visualization within living cells by confocal microscopy. Transient transfection resulted in varying pol β expression levels. Separating cells into three groups based on pol β fluorescence intensity and morphological distribution, permitted analysis of the concentration dependence and spatial distribution of cytoplasmic pol β. Colocalization between pol β and mitochondria was pol β concentration dependent. A decrease in overlap with nucleoids containing mitochondrial DNA (mtDNA) was observed at the highest pol β intensity where pol β exhibits a tubular appearance, suggesting the ability to load elevated levels of pol β into mitochondria readily available for relocation to damaged mtDNA. The dynamics of pol β and mitochondrial nucleoids were followed by confocal recording of time series images. Two populations of mitochondrial nucleoids were observed, with and without pol β. Micro-irradiation, known to form DNA single-strand breaks, in a line across nucleus and cytoplasm of pol β stably transfected cells enhanced apparent localization of pol β with mitochondria in the perinuclear region of the cytoplasm near the nuclear membrane. Exposure of pol β expressing cells to H₂O₂ resulted in a time-dependent increase in cytoplasmic pol β observed by immunofluorescence analysis of fixed cells. Further screening revealed increased levels of colocalization of pol β with a mitochondrial probe and an increase in oxidative DNA damage in the cytoplasm. ELISA quantification confirmed an increase of an oxidative mitochondrial base lesion, 7,8-dihydro-8-oxoguanine, after H₂O₂ treatment. Taken together, the results suggest that pol β is recruited to mitochondria in response to oxidatively-induced mtDNA damage to participate in mtDNA repair.

Role of yUbp8 in Mitochondria and Hypoxia Entangles the Finding of Human Ortholog Usp22 in the Glioblastoma Pseudo-Palisade Microlayer

KAT Gcn5 and DUB Ubp8 are required for respiration and mitochondria functions in budding yeast, and in this study we show that loss of respiratory activity is acquired over time. Interestingly, we show that absence of Ubp8 allows cells to grow in hypoxic conditions with altered mitophagy. Comparatively, the aggressive glioblastoma (GBM) multiforme tumor shows survival mechanisms able to overcome hypoxia in the brain. Starting from yeast and our findings on the role of Ubp8 in hypoxia, we extended our analysis to the human ortholog and signature cancer gene Usp22 in glioblastoma tumor specimens. Here we demonstrate that Usp22 is localized and overexpressed in the pseudo-palisade tissue around the necrotic area of the tumor. In addition, Usp22 colocalizes with the mitophagy marker Parkin, indicating a link with mitochondria function in GBM. Collectively, this evidence suggests that altered expression of Usp22 might provide a way for tumor cells to survive in hypoxic conditions, allowing the escape of cells from the necrotic area toward vascularized tissues. Collectively, our experimental data suggest a model for a possible mechanism of uncontrolled proliferation and invasion in glioblastoma.

Infectious Spleen and Kidney Necrosis Virus (ISKNV) Triggers Mitochondria-Mediated Dynamic Interaction Signals via an Imbalance of Bax/Bak over Bcl-2/Bcl-xL in Fish Cells

The molecular pathogenesis of infectious spleen and kidney necrosis virus (ISKNV) infections is important but has rarely been studied in connection to host organelle behavior. In the present study, we demonstrated that ISKNV can induce host cell death via a pro-apoptotic Bcl-2 and anti-apoptotic Bcl-2 family member imbalance in mitochondrial membrane potential (MMP or ΔΨm) regulation in GF-1 cells. The results of our study on ISKNV infection showed that it can induce host cell death by up to 80% at day 5 post-infection. Subsequently, in an apoptotic assay, ISKNV infection was seen to induce an increase in Annexin-V-positive signals by 20% and in propidium iodide (PI) staining-positive signals by up to 30% at day 5 (D5) in GF-1 cells. Then, through our studies on the mechanism of cell death in mitochondria function, we found that ISKNV can induce MMP loss by up to 58% and 78% at days 4 and 5 with a JC1 dye staining assay. Furthermore, we found that pro-apoptotic members Bax and Bak were upregulated from the early replication stage (day one) to the late stage (day 5), but the expression profiles were very dynamically different. On the other hand, by Western blotted analysis, the anti-apoptotic members Bcl-2 and Bcl-xL were upregulated very quickly at the same time from day one (two-fold) and continued to maintain this level at day five. Finally, we found that pro-apoptotic death signals strongly activated the downstream signals of caspase-9 and -3. Taken together, these results suggest that ISKNV infection can induce Bax/Bak-mediated cell death signaling downstream of caspase-9 and -3 activation. During the viral replication cycle with the cell death induction process, the anti-apoptotic members Bcl-2/Bcl-xL interacted with the pro-apoptotic members Bax/Bak to maintain the mitochondrial function in the dynamic interaction so as to maintain the MMP in GF-1 cells. These findings may provide insights into DNA-virus control and treatment.

Nuclear-Mitochondrial Interactions

Mitochondria, the cell’s major energy producers, also act as signaling hubs, interacting with other organelles both directly and indirectly. Despite having its own circular genome, the majority of mitochondrial proteins are encoded by nuclear DNA. To respond to changes in cell physiology, the mitochondria must send signals to the nucleus, which can, in turn, upregulate gene expression to alter metabolism or initiate a stress response. This is known as retrograde signaling. A variety of stimuli and pathways fall under the retrograde signaling umbrella. Mitochondrial dysfunction has already been shown to have severe implications for human health. Disruption of retrograde signaling, whether directly associated with mitochondrial dysfunction or cellular environmental changes, may also contribute to pathological deficits. In this review, we discuss known signaling pathways between the mitochondria and the nucleus, examine the possibility of direct contacts, and identify pathological consequences of an altered relationship.

Cardiolipin remodeling enables protein crowding in the inner mitochondrial membrane

Mitochondrial cristae are extraordinarily crowded with proteins, which puts stress on the bilayer organization of lipids. We tested the hypothesis that the high concentration of proteins drives the tafazzin-catalyzed remodeling of fatty acids in cardiolipin, thereby reducing bilayer stress in the membrane. Specifically, we tested whether protein crowding induces cardiolipin remodeling and whether the lack of cardiolipin remodeling prevents the membrane from accumulating proteins. In vitro, the incorporation of large amounts of proteins into liposomes altered the outcome of the remodeling reaction. In yeast, the concentration of proteins involved in oxidative phosphorylation (OXPHOS) correlated with the cardiolipin composition. Genetic ablation of either remodeling or biosynthesis of cardiolipin caused a substantial drop in the surface density of OXPHOS proteins in the inner membrane of the mouse heart and Drosophila flight muscle mitochondria. Our data suggest that OXPHOS protein crowding induces cardiolipin remodelling and that remodeled cardiolipin supports the high concentration of these proteins in the inner mitochondrial membrane.

Cnm1 mediates nucleus-mitochondria contact site formation in response to phospholipid levels

Mitochondrial functions are tightly regulated by nuclear activity, requiring extensive communication between these organelles. One way by which organelles can communicate is through contact sites, areas of close apposition held together by tethering molecules. While many contacts have been characterized in yeast, the contact between the nucleus and mitochondria was not previously identified. Using fluorescence and electron microscopy in S. cerevisiae, we demonstrate specific areas of contact between the two organelles. Using a high-throughput screen, we uncover a role for the uncharacterized protein Ybr063c, which we have named Cnm1 (contact nucleus mitochondria 1), as a molecular tether on the nuclear membrane. We show that Cnm1 mediates contact by interacting with Tom70 on mitochondria. Moreover, Cnm1 abundance is regulated by phosphatidylcholine, enabling the coupling of phospholipid homeostasis with contact extent. The discovery of a molecular mechanism that allows mitochondrial crosstalk with the nucleus sets the ground for better understanding of mitochondrial functions in health and disease.

Mitochondria transplantation protects traumatic brain injury via promoting neuronal survival and astrocytic BDNF

Traumatic brain injury (TBI) is one of the leading causes of disability and paralysis around the world. Secondary injury, characterized by progressive neuronal loss and astrogliosis, plays important roles in the post-TBI cognitive impairment and mood disorder. Unfortunately, there still lacks effective treatments, particularly surgery interferences for it. Recent findings of intercellular mitochondria transfer implies a potential therapeutic value of mitochondria transplantation for TBI, which has not been tested yet. In the present study, we demonstrated a quick dysfunction of mitochondria, up-regulation of Tom20 in the injured cortex and subsequent cognitive and mood impairment. Our data demonstrated that mitochondria derived from allogeneic liver or autogeneic muscle stimulated similar microglial activation in brain parenchyma. In vitro experiments showed that exogenous mitochondria could be easily internalized by neurons, astrocytes, and microglia, except for oligodendrocytes. Mitochondria transplantation effectively rescued neuronal apoptosis, restored the expression of Tom20 and the phosphorylation of JNK. Further analysis revealed that mitochondria transplantation in injured cortex induced a significant up-regulation of BDNF in reactive astrocytes, improved animals' spatial memory and alleviated anxiety. In together, our data indicate that mitochondria transplantation may has the potential of clinical translation for TBI treatment, in combination with surgery.

Increased mitochondrial protein import and cardiolipin remodelling upon early mtUPR

Mitochondrial defects can cause a variety of human diseases and protective mechanisms exist to maintain mitochondrial functionality. Imbalances in mitochondrial proteostasis trigger a transcriptional program, termed mitochondrial unfolded protein response (mtUPR). However, the temporal sequence of events in mtUPR is unclear and the consequences on mitochondrial protein import are controversial. Here, we have quantitatively analyzed all main import pathways into mitochondria after different time spans of mtUPR induction. Kinetic analyses reveal that protein import into all mitochondrial subcompartments strongly increases early upon mtUPR and that this is accompanied by rapid remodelling of the mitochondrial signature lipid cardiolipin. Genetic inactivation of cardiolipin synthesis precluded stimulation of protein import and compromised cellular fitness. At late stages of mtUPR upon sustained stress, mitochondrial protein import efficiency declined. Our work clarifies the enigma of protein import upon mtUPR and identifies sequential mtUPR stages, in which an early increase in protein biogenesis to restore mitochondrial proteostasis is followed by late stages characterized by a decrease in import capacity upon prolonged stress induction.

A genome-wide association study in human lymphoblastoid cells supports safety of mitochondrial complex I inhibitor

Novel therapeutic strategies for Alzheimer's disease (AD) are of the greatest priority given the consistent failure of recent clinical trials focused on Aβ or pTau. Earlier, we demonstrated that mild mitochondrial complex I inhibitor CP2 blocks neurodegeneration and cognitive decline in multiple mouse models of AD. To evaluate the safety of CP2 in humans, we performed a genome-wide association study (GWAS) using 196 lymphoblastoid cell lines and identified 11 SNP loci and 64 mRNA expression probe sets that potentially associate with CP2 susceptibility. Using primary mouse neurons and pharmacokinetic study, we show that CP2 is generally safe at a therapeutic dose.

The effects of physical prehabilitation: Improved liver regeneration and mitochondrial function after ALPPS operation in a rodent model

BACKGROUND

To identify the role of physical prehabilitation (PP) in liver regeneration, mitochondrial function, biogenesis, and inflammatory response was investigated after associating liver partition and portal vein ligation for staged hepatectomy (ALPPS) in a rodent model.

METHODS

Male Wistar rats (n = 60) underwent ALPPS. Animals were divided (n = 30) to the physical prehabilitation group (PP) and sedentary group (S). The animals were exsanguinated before (0 hour) and 24, 48, 72, or 168 hours after the operation. Regeneration rate and proliferation index were assessed. Mitochondrial function, biogenesis, and inflammatory response were evaluated.

RESULTS

Regeneration rate and Ki67 index were significantly increased in the PP group compared to the S group (P < .001). Due to the changes in oxidative capacity and ATP production rate, the P/O ratio of PP group compared to the S group was significantly increased (P < .05). PP group was characterized by accelerated mitochondrial biogenesis and less intense inflammatory response compared to the S group.

CONCLUSIONS

To our knowledge, this is the first demonstration of the beneficial effects of PP on liver regeneration, mitochondrial function, biogenesis, and the inflammatory response after ALPPS.

Quality Matters? The Involvement of Mitochondrial Quality Control in Cardiovascular Disease

Cardiovascular diseases are one of the leading causes of death and global health problems worldwide. Multiple factors are known to affect the cardiovascular system from lifestyles, genes, underlying comorbidities, and age. Requiring high workload, metabolism of the heart is largely dependent on continuous power supply via mitochondria through effective oxidative respiration. Mitochondria not only serve as cellular power plants, but are also involved in many critical cellular processes, including the generation of intracellular reactive oxygen species (ROS) and regulating cellular survival. To cope with environmental stress, mitochondrial function has been suggested to be essential during bioenergetics adaptation resulting in cardiac pathological remodeling. Thus, mitochondrial dysfunction has been advocated in various aspects of cardiovascular pathology including the response to ischemia/reperfusion (I/R) injury, hypertension (HTN), and cardiovascular complications related to type 2 diabetes mellitus (DM). Therefore, mitochondrial homeostasis through mitochondrial dynamics and quality control is pivotal in the maintenance of cardiac health. Impairment of the segregation of damaged components and degradation of unhealthy mitochondria through autophagic mechanisms may play a crucial role in the pathogenesis of various cardiac disorders. This article provides in-depth understanding of the current literature regarding mitochondrial remodeling and dynamics in cardiovascular diseases.

Loss of COX4I1 Leads to Combined Respiratory Chain Deficiency and Impaired Mitochondrial Protein Synthesis

The oxidative phosphorylation (OXPHOS) system localized in the inner mitochondrial membrane secures production of the majority of ATP in mammalian organisms. Individual OXPHOS complexes form supramolecular assemblies termed supercomplexes. The complexes are linked not only by their function but also by interdependency of individual complex biogenesis or maintenance. For instance, cytochrome c oxidase (cIV) or cytochrome bc1 complex (cIII) deficiencies affect the level of fully assembled NADH dehydrogenase (cI) in monomeric as well as supercomplex forms. It was hypothesized that cI is affected at the level of enzyme assembly as well as at the level of cI stability and maintenance. However, the true nature of interdependency between cI and cIV is not fully understood yet. We used a HEK293 cellular model where the COX4 subunit was completely knocked out, serving as an ideal system to study interdependency of cI and cIV, as early phases of cIV assembly process were disrupted. Total absence of cIV was accompanied by profound deficiency of cI, documented by decrease in the levels of cI subunits and significantly reduced amount of assembled cI. Supercomplexes assembled from cI, cIII, and cIV were missing in COX4I1 knock-out (KO) due to loss of cIV and decrease in cI amount. Pulse-chase metabolic labeling of mitochondrial DNA (mtDNA)-encoded proteins uncovered a decrease in the translation of cIV and cI subunits. Moreover, partial impairment of mitochondrial protein synthesis correlated with decreased content of mitochondrial ribosomal proteins. In addition, complexome profiling revealed accumulation of cI assembly intermediates, indicating that cI biogenesis, rather than stability, was affected. We propose that attenuation of mitochondrial protein synthesis caused by cIV deficiency represents one of the mechanisms, which may impair biogenesis of cI.

MicroRNA-574 regulates FAM210A expression and influences pathological cardiac remodeling

Aberrant expression of mitochondrial proteins impairs cardiac function and causes heart disease. The mechanism of regulation of mitochondria encoded protein expression during cardiac disease, however, remains underexplored. Here, we show that multiple pathogenic cardiac stressors induce the expression of miR-574 guide and passenger strands (miR-574-5p/3p) in both humans and mice. miR-574 knockout mice exhibit severe cardiac disorder under different pathogenic cardiac stresses while miR-574-5p/3p mimics that are delivered systematically using nanoparticles reduce cardiac pathogenesis under disease insults. Transcriptomic analysis of miR-574-null hearts uncovers family with sequence similarity 210 member A (FAM210A) as a common target mRNA of miR-574-5p and miR-574-3p. The interactome capture analysis suggests that FAM210A interacts with mitochondrial translation elongation factor EF-Tu. Manipulating miR-574-5p/3p or FAM210A expression changes the protein expression of mitochondrial-encoded electron transport chain (ETC) genes but not nuclear-encoded mitochondrial ETC genes in both human AC16 cardiomyocyte cells and miR-574-null murine hearts. Together, we discovered that miR-574 regulates FAM210A expression and modulates mitochondrial-encoded protein expression, which may influence cardiac remodeling in heart failure.

Mitochondrial proteases in human diseases

Mitochondria contain more than 1000 different proteins, including several proteolytic enzymes. These mitochondrial proteases form a complex system that performs limited and terminal proteolysis to build the mitochondrial proteome, maintain, and control its functions or degrade mitochondrial proteins and peptides. During protein biogenesis, presequence proteases cleave and degrade mitochondrial targeting signals to obtain mature functional proteins. Processing by proteases also exerts a regulatory role in modulation of mitochondrial functions and quality control enzymes degrade misfolded, aged, or superfluous proteins. Depending on their different functions and substrates, defects in mitochondrial proteases can affect the majority of the mitochondrial proteome or only a single protein. Consequently, mutations in mitochondrial proteases have been linked to several human diseases. This review gives an overview of the components and functions of the mitochondrial proteolytic machinery and highlights the pathological consequences of dysfunctional mitochondrial protein processing and turnover.

FANCD2 modulates the mitochondrial stress response to prevent common fragile site instability

Common fragile sites (CFSs) are genomic regions frequently involved in cancer-associated rearrangements. Most CFSs lie within large genes, and their instability involves transcription- and replication-dependent mechanisms. Here, we uncover a role for the mitochondrial stress response pathway in the regulation of CFS stability in human cells. We show that FANCD2, a master regulator of CFS stability, dampens the activation of the mitochondrial stress response and prevents mitochondrial dysfunction. Genetic or pharmacological activation of mitochondrial stress signaling induces CFS gene expression and concomitant relocalization to CFSs of FANCD2. FANCD2 attenuates CFS gene transcription and promotes CFS gene stability. Mechanistically, we demonstrate that the mitochondrial stress-dependent induction of CFS genes is mediated by ubiquitin-like protein 5 (UBL5), and that a UBL5-FANCD2 dependent axis regulates the mitochondrial UPR in human cells. We propose that FANCD2 coordinates nuclear and mitochondrial activities to prevent genome instability.

PGC-1α protects from myocardial ischaemia-reperfusion injury by regulating mitonuclear communication

The recovery of blood supply after a period of myocardial ischaemia does not restore the heart function and instead results in a serious dysfunction called myocardial ischaemia‐reperfusion injury (IRI), which involves several complex pathophysiological processes. Mitochondria have a wide range of functions in maintaining the cellular energy supply, cell signalling and programmed cell death. When mitochondrial function is insufficient or disordered, it may have adverse effects on myocardial ischaemia‐reperfusion and therefore mitochondrial dysfunction caused by oxidative stress a core molecular mechanism of IRI. Peroxisome proliferator‐activated receptor gamma co‐activator 1α (PGC‐1α) is an important antioxidant molecule found in mitochondria. However, its role in IRI has not yet been systematically summarized. In this review, we speculate the role of PGC‐1α as a key regulator of mitonuclear communication, which may interacts with nuclear factor, erythroid 2 like ‐1 and ‐2 (NRF‐1/2) to inhibit mitochondrial oxidative stress, promote the clearance of damaged mitochondria, enhance mitochondrial biogenesis, and reduce the burden of IRI.

Mitochondrial protein phosphorylation in yeast revisited

Protein phosphorylation is one of the best-known post-translational modifications occurring in all domains of life. In eukaryotes, protein phosphorylation affects all cellular compartments including mitochondria. High-throughput techniques of mass spectrometry combined with cell fractionation and biochemical methods yielded thousands of phospho-sites on hundreds of mitochondrial proteins. We have compiled the information on mitochondrial protein kinases and phosphatases and their substrates in Saccharomyces cerevisiae and provide the current state-of-the-art overview of mitochondrial protein phosphorylation in this model eukaryote. Using several examples, we describe emerging features of the yeast mitochondrial phosphoproteome and present challenges lying ahead in this exciting field.

When Friendship Turns Sour: Effective Communication Between Mitochondria and Intracellular Organelles in Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disease with pathological hallmarks including progressive neuronal loss from the substantia nigra pars compacta and α-synuclein intraneuronal inclusions, known as Lewy bodies. Although the etiology of PD remains elusive, mitochondrial damage has been established to take center stage in the pathogenesis of PD. Mitochondria are critical to cellular energy production, metabolism, homeostasis, and stress responses; the association with PD emphasizes the importance of maintenance of mitochondrial network integrity. To accomplish the pleiotropic functions, mitochondria are dynamic not only within their own network but also in orchestrated coordination with other organelles in the cellular community. Through physical contact sites, signal transduction, and vesicle transport, mitochondria and intracellular organelles achieve the goals of calcium homeostasis, redox homeostasis, protein homeostasis, autophagy, and apoptosis. Herein, we review the finely tuned interactions between mitochondria and surrounding intracellular organelles, with focus on the nucleus, endoplasmic reticulum, Golgi apparatus, peroxisomes, and lysosomes. Participants that may contribute to the pathogenic mechanisms of PD will be highlighted in this review.

Decoding the rosetta stone of mitonuclear communication

Cellular homeostasis in eukaryotic cells requires synchronized coordination of multiple organelles. A key role in this stage is played by mitochondria, which have recently emerged as highly interconnected and multifunctional hubs that process and coordinate diverse cellular functions. Beyond producing ATP, mitochondria generate key metabolites and are central to apoptotic and metabolic signaling pathways. Because most mitochondrial proteins are encoded in the nuclear genome, the biogenesis of new mitochondria and the maintenance of mitochondrial functions and flexibility critically depend upon effective mitonuclear communication. This review addresses the complex network of signaling molecules and pathways allowing mitochondria-nuclear communication and coordinated regulation of their independent but interconnected genomes, and discusses the extent to which dynamic communication between the two organelles has evolved for mutual benefit and for the overall maintenance of cellular and organismal fitness.

Mitochondrial GWAS and association of nuclear - mitochondrial epistasis with BMI in T1DM patients

BACKGROUND

BMI is a strong indicator of complications from type I diabetes, especially under intensive treatment.

METHODS

We have genotyped 435 type 1 diabetics using Illumina Infinium Omni Express Exome-8 v1.4 arrays and performed mitoGWAS on BMI. We identified additive interactions between mitochondrial and nuclear variants in genes associated with mitochondrial functioning MitoCarta2.0 and confirmed and refined the results on external cohorts: the Framingham Heart Study (FHS) and GTEx data. Linear mixed model analysis was performed using the GENESIS package in R/Bioconductor.

RESULTS

We find a borderline significant association between the mitochondrial variant rs28357980, localized to MT-ND2, and BMI (β = - 0.69, p = 0.056). This BMI association was confirmed on 1889 patients from FHS cohort (β = - 0.312, p = 0.047). Next, we searched for additive interactions between mitochondrial and nuclear variants. MT-ND2 variants interacted with variants in the genes SIRT3, ATP5B, CYCS, TFB2M and POLRMT. TFB2M is a mitochondrial transcription factor and together with TFAM creates a transcription promoter complex for the mitochondrial polymerase POLRMT. We have found an interaction between rs3021088 in MT-ND2 and rs6701836 in TFB2M leading to BMI decrease (inter_pval = 0.0241), while interaction of rs3021088 in MT-ND2 and rs41542013 in POLRMT led to BMI increase (inter_pval = 0.0004). The influence of these interactions on BMI was confirmed in external cohorts.

CONCLUSIONS

Here, we have shown that variants in the mitochondrial genome as well as additive interactions between mitochondrial and nuclear SNPs influence BMI in T1DM and general cohorts.

Multifaceted Roles of Mitochondrial Components and Metabolites in Metabolic Diseases and Cancer

Mitochondria are essential cellular components that ensure physiological metabolic functions. They provide energy in the form of adenosine triphosphate (ATP) through the electron transport chain (ETC). They also constitute a metabolic hub in which metabolites are used and processed, notably through the tricarboxylic acid (TCA) cycle. These newly generated metabolites have the capacity to feed other cellular metabolic pathways; modify cellular functions; and, ultimately, generate specific phenotypes. Mitochondria also provide intracellular signaling cues through reactive oxygen species (ROS) production. As expected with such a central cellular role, mitochondrial dysfunctions have been linked to many different diseases. The origins of some of these diseases could be pinpointed to specific mutations in both mitochondrial- and nuclear-encoded genes. In addition to their impressive intracellular tasks, mitochondria also provide intercellular signaling as they can be exchanged between cells, with resulting effects ranging from repair of damaged cells to strengthened progression and chemo-resistance of cancer cells. Several therapeutic options can now be envisioned to rescue mitochondria-defective cells. They include gene therapy for both mitochondrial and nuclear defective genes. Transferring exogenous mitochondria to target cells is also a whole new area of investigation. Finally, supplementing targeted metabolites, possibly through microbiota transplantation, appears as another therapeutic approach full of promises.

New horizons in mitochondrial contact site research

Contact sites, areas where two organelles are held in close proximity through the action of molecular tethers, enable non-vesicular communication between compartments. Mitochondria have been center stage in the contact site field since the discovery of the first contact between mitochondria and the endoplasmic reticulum (ER) over 60 years ago. However, only now, in the last decade, has there been a burst of discoveries regarding contact site biology in general and mitochondrial contacts specifically. The number and types of characterized contacts increased dramatically, new molecular mechanisms enabling contact formation were discovered, additional unexpected functions for contacts were shown, and their roles in cellular and organismal physiology were emphasized. Here, we focus on mitochondria as we highlight the most recent developments, future goals and unresolved questions in the field.

Author's View: a nuclear transcription factor relocalizing to mitochondria rescues cells from proteotoxic aggregates

Mitochondrial proteostasis is essential for survival, and imbalances can result in severe human diseases. We identified a novel stress response triggered upon accumulation of proteotoxic aggregates in the mitochondrial matrix. Mitochondria-to-nucleus signaling results in a transcriptional response and translocation of a nuclear transcription factor into mitochondria to maintain mitochondrial gene expression.

An Early mtUPR: Redistribution of the Nuclear Transcription Factor Rox1 to Mitochondria Protects against Intramitochondrial Proteotoxic Aggregates

The mitochondrial proteome is built mainly by import of nuclear-encoded precursors, which are targeted mostly by cleavable presequences. Presequence processing upon import is essential for proteostasis and survival, but the consequences of dysfunctional protein maturation are unknown. We find that impaired presequence processing causes accumulation of precursors inside mitochondria that form aggregates, which escape degradation and unexpectedly do not cause cell death. Instead, cells survive via activation of a mitochondrial unfolded protein response (mtUPR)-like pathway that is triggered very early after precursor accumulation. In contrast to classical stress pathways, this immediate response maintains mitochondrial protein import, membrane potential, and translation through translocation of the nuclear HMG-box transcription factor Rox1 to mitochondria. Rox1 binds mtDNA and performs a TFAM-like function pivotal for transcription and translation. Induction of early mtUPR provides a reversible stress model to mechanistically dissect the initial steps in mtUPR pathways with the stressTFAM Rox1 as the first line of defense.

Tracking intra- and inter-organelle signaling of mitochondria

Mitochondria are as highly specialized organelles and masters of the cellular energy metabolism in a constant and dynamic interplay with their cellular environment, providing adenosine triphosphate, buffering Ca2+ and fundamentally contributing to various signaling pathways. Hence, such broad field of action within eukaryotic cells requires a high level of structural and functional adaptation. Therefore, mitochondria are constantly moving and undergoing fusion and fission processes, changing their shape and their interaction with other organelles. Moreover, mitochondrial activity gets fine-tuned by intra- and interorganelle H+ , K+ , Na+ , and Ca2+ signaling. In this review, we provide an up-to-date overview on mitochondrial strategies to adapt and respond to, as well as affect, their cellular environment. We also present cutting-edge technologies used to track and investigate subcellular signaling, essential to the understanding of various physiological and pathophysiological processes.

Mitochondria orchestrate proteostatic and metabolic stress responses

The eukaryotic cell is morphologically and functionally organized as an interconnected network of organelles that responds to stress and aging. Organelles communicate via dedicated signal transduction pathways and the transfer of information in form of metabolites and energy levels. Recent data suggest that the communication between organellar proteostasis systems is a cornerstone of cellular stress responses in eukaryotic cells. Here, we discuss the integration of proteostasis and energy fluxes in the regulation of cellular stress and aging. We emphasize the molecular architecture of the regulatory transcriptional pathways that both sense and control metabolism and proteostasis. A special focus is placed on mechanistic insights gained from the model organism budding yeast in signaling from mitochondria to the nucleus and how this shapes cellular fitness.

Mechanisms Through Which Some Mitochondria-Generated Metabolites Act as Second Messengers That Are Essential Contributors to the Aging Process in Eukaryotes Across Phyla

Recent studies have revealed that some low-molecular weight molecules produced in mitochondria are essential contributing factors to aging and aging-associated pathologies in evolutionarily distant eukaryotes. These molecules are intermediates or products of certain metabolic reactions that are activated in mitochondria in response to specific changes in the nutrient, stress, proliferation, or age status of the cell. After being released from mitochondria, these metabolites directly or indirectly change activities of a distinct set of protein sensors that reside in various cellular locations outside of mitochondria. Because these protein sensors control the efficiencies of some pro- or anti-aging cellular processes, such changes in their activities allow to create a pro- or anti-aging cellular pattern. Thus, mitochondria can function as signaling platforms that respond to certain changes in cell stress and physiology by remodeling their metabolism and releasing a specific set of metabolites known as "mitobolites." These mitobolites then define the pace of cellular and organismal aging because they regulate some longevity-defining processes taking place outside of mitochondria. In this review, we discuss recent progress in understanding mechanisms underlying the ability of mitochondria to function as such signaling platforms in aging and aging-associated diseases.