Mitochondria signaling to the epigenome: A novel role for an old organelle

The pathogenesis of gout: molecular insights from genetic, epigenomic and transcriptomic studies

The pathogenesis of gout involves a series of steps beginning with hyperuricaemia, followed by the deposition of monosodium urate crystal in articular structures and culminating in an innate immune response, mediated by the NLRP3 inflammasome, to the deposited crystals. Large genome-wide association studies (GWAS) of serum urate levels initially identified the genetic variants with the strongest effects, mapping mainly to genes that encode urate transporters in the kidney and gut. Other GWAS highlighted the importance of uncommon genetic variants. More recently, genetic and epigenetic genome-wide studies have revealed new pathways in the inflammatory process of gout, including genetic associations with epigenomic modifiers. Epigenome-wide association studies are also implicating epigenomic remodelling in gout, which perhaps regulates the responsiveness of the innate immune system to monosodium urate crystals. Notably, genes implicated in gout GWAS do not include those encoding components of the NLRP3 inflammasome itself, but instead include genes encoding molecules involved in its regulation. Knowledge of the molecular mechanisms underlying gout has advanced through the translation of genetic associations into specific molecular mechanisms. Notable examples include ABCG2, HNF4A, PDZK1, MAF and IL37. Current genetic studies are dominated by participants of European ancestry; however, studies focusing on other population groups are discovering informative population-specific variants associated with gout.

Mitochondrial function and gastrointestinal diseases

Mitochondria are dynamic organelles that function in cellular energy metabolism, intracellular and extracellular signalling, cellular fate and stress responses. Mitochondria of the intestinal epithelium, the cellular interface between self and enteric microbiota, have emerged as crucial in intestinal health. Mitochondrial dysfunction occurs in gastrointestinal diseases, including inflammatory bowel diseases and colorectal cancer. In this Review, we provide an overview of the current understanding of intestinal epithelial cell mitochondrial metabolism, function and signalling to affect tissue homeostasis, including gut microbiota composition. We also discuss mitochondrial-targeted therapeutics for inflammatory bowel diseases and colorectal cancer and the evolving concept of mitochondrial impairment as a consequence versus initiator of the disease.

Metabolic rewiring promotes anti-inflammatory effects of glucocorticoids

Glucocorticoids represent the mainstay of therapy for a broad spectrum of immune-mediated inflammatory diseases. However, the molecular mechanisms underlying their anti-inflammatory mode of action have remained incompletely understood1. Here we show that the anti-inflammatory properties of glucocorticoids involve reprogramming of the mitochondrial metabolism of macrophages, resulting in increased and sustained production of the anti-inflammatory metabolite itaconate and consequent inhibition of the inflammatory response. The glucocorticoid receptor interacts with parts of the pyruvate dehydrogenase complex whereby glucocorticoids provoke an increase in activity and enable an accelerated and paradoxical flux of the tricarboxylic acid (TCA) cycle in otherwise pro-inflammatory macrophages. This glucocorticoid-mediated rewiring of mitochondrial metabolism potentiates TCA-cycle-dependent production of itaconate throughout the inflammatory response, thereby interfering with the production of pro-inflammatory cytokines. By contrast, artificial blocking of the TCA cycle or genetic deficiency in aconitate decarboxylase 1, the rate-limiting enzyme of itaconate synthesis, interferes with the anti-inflammatory effects of glucocorticoids and, accordingly, abrogates their beneficial effects during a diverse range of preclinical models of immune-mediated inflammatory diseases. Our findings provide important insights into the anti-inflammatory properties of glucocorticoids and have substantial implications for the design of new classes of anti-inflammatory drugs.

Integrating mitoepigenetics into research in mood disorders: a state-of-the-art review

Mood disorders, including major depressive disorder and bipolar disorder, are highly prevalent and stand among the leading causes of disability. Despite the largely elusive nature of the molecular mechanisms underpinning these disorders, two pivotal contributors-mitochondrial dysfunctions and epigenetic alterations-have emerged as significant players in their pathogenesis. This state-of-the-art review aims to present existing data on epigenetic alterations in the mitochondrial genome in mood disorders, laying the groundwork for future research into their pathogenesis. Associations between abnormalities in mitochondrial function and mood disorders have been observed, with evidence pointing to notable changes in mitochondrial DNA (mtDNA). These changes encompass variations in copy number and oxidative damage. However, information on additional epigenetic alterations in the mitochondrial genome remains limited. Recent studies have delved into alterations in mtDNA and regulations in the mitochondrial genome, giving rise to the burgeoning field of mitochondrial epigenetics. Mitochondrial epigenetics encompasses three main categories of modifications: mtDNA methylation/hydroxymethylation, modifications of mitochondrial nucleoids, and mitochondrial RNA alterations. The epigenetic modulation of mitochondrial nucleoids, lacking histones, may impact mtDNA function. Additionally, mitochondrial RNAs, including non-coding RNAs, present a complex landscape influencing interactions between the mitochondria and the nucleus. The exploration of mitochondrial epigenetics offers valuable perspectives on how these alterations impact neurodegenerative diseases, presenting an intriguing avenue for research on mood disorders. Investigations into post-translational modifications and the role of mitochondrial non-coding RNAs hold promise to unravel the dynamics of mitoepigenetics in mood disorders, providing crucial insights for future therapeutic interventions.

Assembly and comparative analysis of the complete mitochondrial genome of Pinellia ternata

Pinellia ternata is an important natural medicinal herb in China. However, it is susceptible to withering when exposed to high temperatures during growth, which limits its tuber production. Mitochondria usually function in stress response. The P . ternata mitochondrial (mt) genome has yet to be explored. Therefore, we integrated PacBio and Illumina sequencing reads to assemble and annotate the mt genome of P . ternata . The circular mt genome of P . ternata is 876 608bp in length and contains 38 protein-coding genes (PCGs), 20 tRNA genes and three rRNA genes. Codon usage, sequence repeats, RNA editing and gene migration from chloroplast (cp) to mt were also examined. Phylogenetic analysis based on the mt genomes of P . ternata and 36 other taxa revealed the taxonomic and evolutionary status of P . ternata . Furthermore, we investigated the mt genome size and GC content by comparing P . ternata with the other 35 species. An evaluation of non-synonymous substitutions and synonymous substitutions indicated that most PCGs in the mt genome underwent negative selection. Our results provide comprehensive information on the P . ternata mt genome, which may facilitate future research on the high-temperature response of P . ternata and provide new molecular insights on the Araceae family.

A break in mitochondrial endosymbiosis as a basis for inflammatory diseases

Mitochondria retain bacterial traits due to their endosymbiotic origin, but host cells do not recognize them as foreign because the organelles are sequestered. However, the regulated release of mitochondrial factors into the cytosol can trigger cell death, innate immunity and inflammation. This selective breakdown in the 2-billion-year-old endosymbiotic relationship enables mitochondria to act as intracellular signalling hubs. Mitochondrial signals include proteins, nucleic acids, phospholipids, metabolites and reactive oxygen species, which have many modes of release from mitochondria, and of decoding in the cytosol and nucleus. Because these mitochondrial signals probably contribute to the homeostatic role of inflammation, dysregulation of these processes may lead to autoimmune and inflammatory diseases. A potential reason for the increased incidence of these diseases may be changes in mitochondrial function and signalling in response to such recent phenomena as obesity, dietary changes and other environmental factors. Focusing on the mixed heritage of mitochondria therefore leads to predictions for future insights, research paths and therapeutic opportunities. Thus, whereas mitochondria can be considered 'the enemy within' the cell, evolution has used this strained relationship in intriguing ways, with increasing evidence pointing to the recent failure of endosymbiosis being critical for the pathogenesis of inflammatory diseases.

Mitochondrial membrane potential regulates nuclear DNA methylation and gene expression through phospholipid remodeling

Maintenance of the mitochondrial inner membrane potential (ΔΨM) is critical for many aspects of mitochondrial function, including mitochondrial protein import and ion homeostasis. While ΔΨM loss and its consequences are well studied, little is known about the effects of increased ΔΨM. In this study, we used cells deleted of ATPIF1, a natural inhibitor of the hydrolytic activity of the ATP synthase, as a genetic model of mitochondrial hyperpolarization. Our data show that chronic ΔΨM increase leads to nuclear DNA hypermethylation, regulating transcription of mitochondria, carbohydrate and lipid metabolism genes. Surprisingly, remodeling of phospholipids, but not metabolites or redox changes, mechanistically links the ΔΨM to the epigenome. These changes were also observed upon chemical exposures and reversed by decreasing the ΔΨM, highlighting them as hallmark adaptations to chronic mitochondrial hyperpolarization. Our results reveal the ΔΨM as the upstream signal conveying the mitochondrial status to the epigenome to regulate cellular biology, providing a new framework for how mitochondria can influence health outcomes in the absence of canonical dysfunction.

Mitochondrial dysfunction: a new molecular mechanism of intervertebral disc degeneration

OBJECTIVE

Intervertebral disc degeneration (IVDD) is a chronic degenerative orthopedic illness that causes lower back pain as a typical clinical symptom, severely reducing patients' quality of life and work efficiency, and imposing a significant economic burden on society. IVDD is defined by rapid extracellular matrix breakdown, nucleus pulposus cell loss, and an inflammatory response. It is intimately related to the malfunction or loss of myeloid cells among them. Many mechanisms have been implicated in the development of IVDD, including inflammatory factors, oxidative stress, apoptosis, cellular autophagy, and mitochondrial dysfunction. In recent years, mitochondrial dysfunction has become a hot research topic in age-related diseases. As the main source of adenosine triphosphate (ATP) in myeloid cells, mitochondria are essential for maintaining myeloid cell survival and physiological functions.

METHODS

We searched the PUBMED database with the search term "intervertebral disc degeneration and mitochondrial dysfunction" and obtained 82 articles, and after reading the abstracts and eliminating 30 irrelevant articles, we finally obtained 52 usable articles.

RESULTS

Through a review of the literature, it was discovered that IVDD and cellular mitochondrial dysfunction are also linked. Mitochondrial dysfunction contributes to the advancement of IVDD by influencing a number of pathophysiologic processes such as mitochondrial fission/fusion, mitochondrial autophagy, cellular senescence, and cell death.

CONCLUSION

We examine the molecular mechanisms of IVDD-associated mitochondrial dysfunction and present novel directions for quality management of mitochondrial dysfunction as a treatment approach to IVDD.

Interactions of mitochondrial and skeletal muscle biology in mitochondrial myopathy

Mitochondrial dysfunction in skeletal muscle fibres occurs with both healthy aging and a range of neuromuscular diseases. The impact of mitochondrial dysfunction in skeletal muscle and the way muscle fibres adapt to this dysfunction is important to understand disease mechanisms and to develop therapeutic interventions. Furthermore, interactions between mitochondrial dysfunction and skeletal muscle biology, in mitochondrial myopathy, likely have important implications for normal muscle function and physiology. In this review, we will try to give an overview of what is known to date about these interactions including metabolic remodelling, mitochondrial morphology, mitochondrial turnover, cellular processes and muscle cell structure and function. Each of these topics is at a different stage of understanding, with some being well researched and understood, and others in their infancy. Furthermore, some of what we know comes from disease models. Whilst some findings are confirmed in humans, where this is not yet the case, we must be cautious in interpreting findings in the context of human muscle and disease. Here, our goal is to discuss what is known, highlight what is unknown and give a perspective on the future direction of research in this area.

Ohwia caudata aqueous extract attenuates doxorubicin-induced mitochondrial dysfunction in Wharton's jelly-derived mesenchymal stem cells

Mitochondrial dysfunction has been linked to many diseases, including organ degeneration and cancer. Wharton's jelly-derived mesenchymal stem cells provide a valuable source for stem cell-based therapy and represent an emerging therapeutic approach for tissue regeneration. This study focused on screening the senomorphic properties of Ohwia caudata aqueous extract as an emerging strategy for preventing or treating mitochondrial dysfunction in stem cells. Wharton's jelly-derived mesenchymal stem cells were incubated with 0.1 μM doxorubicin, for 24 h to induce mitochondrial dysfunction. Next, the cells were treated with a series concentration of Ohwia caudata aqueous extract (25, 50, 100, and 200 μg/mL) for another 24 h. In addition, an untreated control group and a doxorubicin-induced mitochondrial dysfunction positive control group were maintained under the same conditions. Our data showed that Ohwia caudata aqueous extract markedly suppressed doxorubicin-induced mitochondrial dysfunction by increasing Tid1 and Tom20 expression, decreased reactive oxygen species production, and maintained mitochondrial membrane potential to promote mitochondrial stability. Ohwia caudata aqueous extract retained the stemness of Wharton's jelly-derived mesenchymal stem cells and reduced the apoptotic rate. These results indicate that Ohwia caudata aqueous extract protects Wharton's jelly-derived mesenchymal stem cells against doxorubicin-induced mitochondrial dysfunction and can potentially prevent mitochondrial dysfunction in other cells. This study provides new directions for the medical application of Ohwia caudata.

Xanthorrhizol inhibits mitochondrial damage, oxidative stress and inflammation in LPS-induced MLE-12 cells by regulating MAPK pathway

LPS-induced injury in lung epithelial cells is a crucial part of the process of acute lung injury (ALI). The aim of this study is to explore whether Xanthorrhizol, a medicine that has antioxidant and anti-inflammatory activity, could mitigate the injury of lung epithelial cells caused by LPS. Mouse lung epithelial cell line (MLE-12 cells) were treated with LPS in the absence and presence of Xanthorrhizol. As a results, we observed that LPS could induce MLE-12 cells death, mitochondrial dysfunction, oxidative stress and inflammation, and activate MAPK signaling pathways. However, Xanthorrhizol mitigated the injury in MEL-12 caused by LPS by promoting cell viability and MDA, GSH production as well as inhibiting LDH release, mitochondria damage, IL-1β, IL-6 and TNF-α production and the phosphorylation levels of ERK, P38 and JNK. Our results indicated that Xanthorrhizol could protect lung epithelial cells from LPS-induced injury, more likely by inhibiting the phosphorylation of MAPK pathway related proteins.

Immune aging - A mechanism in autoimmune disease

Evidence is emerging that the process of immune aging is a mechanism leading to autoimmunity. Over lifetime, the immune system adapts to profound changes in hematopoiesis and lymphogenesis, and progressively restructures in face of an ever-expanding exposome. Older adults fail to generate adequate immune responses against microbial infections and tumors, but accumulate aged T cells, B cells and myeloid cells. Age-associated B cells are highly efficient in autoantibody production. T-cell aging promotes the accrual of end-differentiated effector T cells with potent cytotoxic and pro-inflammatory abilities and myeloid cell aging supports a low grade, sterile and chronic inflammatory state (inflammaging). In pre-disposed individuals, immune aging can lead to frank autoimmune disease, manifesting with chronic inflammation and irreversible tissue damage. Emerging data support the concept that autoimmunity results from aging-induced failure of fundamental cellular processes in immune effector cells: genomic instability, loss of mitochondrial fitness, failing proteostasis, dwindling lysosomal degradation and inefficient autophagy. Here, we have reviewed the evidence that malfunctional mitochondria, disabled lysosomes and stressed endoplasmic reticula induce pathogenic T cells and macrophages that drive two autoimmune diseases, rheumatoid arthritis (RA) and giant cell arteritis (GCA). Recognizing immune aging as a risk factor for autoimmunity will open new avenues of immunomodulatory therapy, including the repair of malfunctioning mitochondria and lysosomes.

Nuclear translocation of mitochondrial dehydrogenases as an adaptive cardioprotective mechanism

Chemotherapy-induced cardiac damage remains a leading cause of death amongst cancer survivors. Anthracycline-induced cardiotoxicity is mediated by severe mitochondrial injury, but little is known about the mechanisms by which cardiomyocytes adaptively respond to the injury. We observed the translocation of selected mitochondrial tricarboxylic acid (TCA) cycle dehydrogenases to the nucleus as an adaptive stress response to anthracycline-cardiotoxicity in human induced pluripotent stem cell-derived cardiomyocytes and in vivo. The expression of nuclear-targeted mitochondrial dehydrogenases shifts the nuclear metabolic milieu to maintain their function both in vitro and in vivo. This protective effect is mediated by two parallel pathways: metabolite-induced chromatin accessibility and AMP-kinase (AMPK) signaling. The extent of chemotherapy-induced cardiac damage thus reflects a balance between mitochondrial injury and the protective response initiated by the nuclear pool of mitochondrial dehydrogenases. Our study identifies nuclear translocation of mitochondrial dehydrogenases as an endogenous adaptive mechanism that can be leveraged to attenuate cardiomyocyte injury.

Therapeutic potential of engineering the mitochondrial genome

Mitochondrial diseases are a group of clinical disorders caused by mutations in the genes encoded by either the nuclear or the mitochondrial genome involved in mitochondrial oxidative phosphorylation. Disorders become evident when mitochondrial dysfunction reaches a cell-specific threshold. Similarly, the severity of disorders is related to the degree of gene mutation. Clinical treatments for mitochondrial diseases mainly rely on symptomatic management. Theoretically, replacing or repairing dysfunctional mitochondria to acquire and preserve normal physiological functions should be effective. Significant advances have been made in gene therapies, including mitochondrial replacement therapy, mitochondrial genome manipulation, nuclease programming, mitochondrial DNA editing, and mitochondrial RNA interference. In this paper, we review the recent progress in these technologies by focusing on advancements that overcome limitations.

Building the case for mitochondrial transplantation as an anti-aging cardiovascular therapy

Mitochondrial dysfunction is a common denominator in both biological aging and cardiovascular disease (CVD) pathology. Understanding the protagonist role of mitochondria in the respective and independent progressions of CVD and biological aging will unravel the synergistic relationship between biological aging and CVD. Moreover, the successful development and implementation of therapies that can simultaneously benefit mitochondria of multiple cell types, will be transformational in curtailing pathologies and mortality in the elderly, including CVD. Several works have compared the status of mitochondria in vascular endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) in CVD dependent context. However, fewer studies have cataloged the aging-associated changes in vascular mitochondria, independent of CVD. This mini review will focus on the present evidence related to mitochondrial dysfunction in vascular aging independent of CVD. Additionally, we discuss the feasibility of restoring mitochondrial function in the aged cardiovascular system through mitochondrial transfer.

SIRTUINS MODULATORS COUNTERACT MITOCHONDRIAL DYSFUNCTION IN CELLULAR MODELS OF HYPOXIA: RELEVANCE TO SCHIZOPHRENIA

Schizophrenia (SZ) is a neurodevelopmental-associated disorder strongly related to environmental factors, such as hypoxia. Because there is no cure for SZ or any pharmacological approach that could revert hypoxia-induced cellular damages, we evaluated whether modulators of sirtuins could abrogate hypoxia-induced mitochondrial deregulation as a neuroprotective strategy. Firstly, astrocytes from control (Wistar) and Spontaneously Hypertensive Rats (SHR), a model of both SZ and neonatal hypoxia, were submitted to chemical hypoxia. Then, cells were exposed to different concentrations of Nicotinamide (NAM), Resveratrol (Resv), and Sirtinol (Sir) for 48hrs. Our data indicate that sirtuins modulation reduces cell death increasing the acetylation of histone 3. This outcome is related to the rescue of loss of mitochondrial membrane potential, changes in mitochondrial calcium buffering capacity, decreased O₂^-• levels and increased expression of metabolic regulators (Nrf-1 and Nfe2l2) and mitochondrial content. Such findings are relevant not only for hypoxia-associated conditions, named pre-eclampsia but also for SZ since prenatal hypoxia is a relevant environmental factor related to this burdensome neuropsychiatric disorder.

Methods to Evaluate Changes in Mitochondrial Structure and Function in Cancer

Mitochondria are regulators of key cellular processes, including energy production and redox homeostasis. Mitochondrial dysfunction is associated with various human diseases, including cancer. Importantly, both structural and functional changes can alter mitochondrial function. Morphologic and quantifiable changes in mitochondria can affect their function and contribute to disease. Structural mitochondrial changes include alterations in cristae morphology, mitochondrial DNA integrity and quantity, and dynamics, such as fission and fusion. Functional parameters related to mitochondrial biology include the production of reactive oxygen species, bioenergetic capacity, calcium retention, and membrane potential. Although these parameters can occur independently of one another, changes in mitochondrial structure and function are often interrelated. Thus, evaluating changes in both mitochondrial structure and function is crucial to understanding the molecular events involved in disease onset and progression. This review focuses on the relationship between alterations in mitochondrial structure and function and cancer, with a particular emphasis on gynecologic malignancies. Selecting methods with tractable parameters may be critical to identifying and targeting mitochondria-related therapeutic options. Methods to measure changes in mitochondrial structure and function, with the associated benefits and limitations, are summarized.

Multifaceted mitochondria: moving mitochondrial science beyond function and dysfunction

Mitochondria have cell-type specific phenotypes, perform dozens of interconnected functions and undergo dynamic and often reversible physiological recalibrations. Given their multifunctional and malleable nature, the frequently used terms 'mitochondrial function' and 'mitochondrial dysfunction' are misleading misnomers that fail to capture the complexity of mitochondrial biology. To increase the conceptual and experimental specificity in mitochondrial science, we propose a terminology system that distinguishes between (1) cell-dependent properties, (2) molecular features, (3) activities, (4) functions and (5) behaviours. A hierarchical terminology system that accurately captures the multifaceted nature of mitochondria will achieve three important outcomes. It will convey a more holistic picture of mitochondria as we teach the next generations of mitochondrial biologists, maximize progress in the rapidly expanding field of mitochondrial science, and also facilitate synergy with other disciplines. Improving specificity in the language around mitochondrial science is a step towards refining our understanding of the mechanisms by which this unique family of organelles contributes to cellular and organismal health.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: Reactive oxygen species in the mammalian pre-implantation embryo

In brief

Reactive oxygen species are generated throughout the pre-implantation period and are necessary for normal embryo formation. However, at pathological levels, they result in reduced embryo viability which can be mediated through factors delivered by sperm and eggs at conception or from the external environment.

Abstract

Reactive oxygen species (ROS) occur naturally in pre-implantation embryos as a by-product of ATP generation through oxidative phosphorylation and enzymes such as NADPH oxidase and xanthine oxidase. Biological concentrations of ROS are required for crucial embryonic events such as pronuclear formation, first cleavage and cell proliferation. However, high concentrations of ROS are detrimental to embryo development, resulting in embryo arrest, increased DNA damage and modification of gene expression leading to aberrant fetal growth and health. In vivo embryos are protected against oxidative stress by oxygen scavengers present in follicular and oviductal fluids, while in vitro, embryos rely on their own antioxidant defence mechanisms to protect against oxidative damage, including superoxide dismutase, catalase, glutathione and glutamylcysteine synthestase. Pre-implantation embryonic ROS originate from eggs, sperm and embryos themselves or from the external environment (i.e. in vitro culture system, obesity and ageing). This review examines the biological and pathological roles of ROS in the pre-implantation embryo, maternal and paternal origins of embryonic ROS, and from a clinical perspective, we comment on the growing interest in combating increased oxidative damage in the pre-implantation embryo through the addition of antioxidants.

Mitochondrial effects on fertility and longevity in Tigriopus californicus contradict predictions of the mother's curse hypothesis

Strict maternal inheritance of mitochondria favours the evolutionary accumulation of sex-biased fitness effects, as mitochondrial evolution occurs exclusively in female lineages. The 'mother's curse' hypothesis proposes that male-harming mutations should accumulate in mitochondrial genomes when they have neutral or beneficial effects on female fitness. Rigorous empirical tests have largely focused on Drosophila, where support for the predictions of mother's curse has been mixed. We investigated the impact of mother's curse mutations in Tigriopus californicus, a minute crustacean. Using non-recombinant backcrosses, we introgressed four divergent mitochondrial haplotypes into two nuclear backgrounds and recorded measures of fertility and longevity. We found that the phenotypic effects of mitochondrial mutations were context dependent, being influenced by the nuclear background in which they were expressed, as well as the sex of the individual and rearing temperature. Mitochondrial haplotype effects were greater for fertility than longevity, and temperature effects were greater for longevity. However, in opposition to mother's curse expectations, females had higher mitochondrial genetic variance than males for fertility and longevity, little evidence of sexual antagonism favouring females was found, and the impacts of mitonuclear mismatch harmed females but not males. Together, this indicates that selection on mitochondrial variation has not resulted in the accumulation of male mutation load in Tigriopus californicus.

Mitochondrial signal transduction

The analogy of mitochondria as powerhouses has expired. Mitochondria are living, dynamic, maternally inherited, energy-transforming, biosynthetic, and signaling organelles that actively transduce biological information. We argue that mitochondria are the processor of the cell, and together with the nucleus and other organelles they constitute the mitochondrial information processing system (MIPS). In a three-step process, mitochondria (1) sense and respond to both endogenous and environmental inputs through morphological and functional remodeling; (2) integrate information through dynamic, network-based physical interactions and diffusion mechanisms; and (3) produce output signals that tune the functions of other organelles and systemically regulate physiology. This input-to-output transformation allows mitochondria to transduce metabolic, biochemical, neuroendocrine, and other local or systemic signals that enhance organismal adaptation. An explicit focus on mitochondrial signal transduction emphasizes the role of communication in mitochondrial biology. This framework also opens new avenues to understand how mitochondria mediate inter-organ processes underlying human health.

Emerging mitochondrial signaling mechanisms in cardio-oncology: beyond oxidative stress

Many anticancer therapies (CTx) have cardiotoxic side effects that limit their therapeutic potential and cause long-term cardiovascular complications in cancer survivors. This has given rise to the field of cardio-oncology, which recognizes the need for basic, translational, and clinical research focused on understanding the complex signaling events that drive CTx-induced cardiovascular toxicity. Several CTx agents cause mitochondrial damage in the form of mitochondrial DNA deletions, mutations, and suppression of respiratory function and ATP production. In this review, we provide a brief overview of the cardiovascular complications of clinically used CTx agents and discuss current knowledge of local and systemic secondary signaling events that arise in response to mitochondrial stress/damage. Mitochondrial oxidative stress has long been recognized as a contributor to CTx-induced cardiotoxicity; thus, we focus on emerging roles for mitochondria in epigenetic regulation, innate immunity, and signaling via noncoding RNAs and mitochondrial hormones. Because data exploring mitochondrial secondary signaling in the context of cardio-oncology are limited, we also draw upon clinical and preclinical studies, which have examined these pathways in other relevant pathologies.

Epigenetic changes induced by in utero dietary challenge result in phenotypic variability in successive generations of mice

Transmission of epigenetic information between generations occurs in nematodes, flies and plants, mediated by specialised small RNA pathways, modified histones and DNA methylation. Similar processes in mammals can also affect phenotype through intergenerational or trans-generational mechanisms. Here we generate a luciferase knock-in reporter mouse for the imprinted Dlk1 locus to visualise and track epigenetic fidelity across generations. Exposure to high-fat diet in pregnancy provokes sustained re-expression of the normally silent maternal Dlk1 in offspring (loss of imprinting) and increased DNA methylation at the somatic differentially methylated region (sDMR). In the next generation heterogeneous Dlk1 mis-expression is seen exclusively among animals born to F1-exposed females. Oocytes from these females show altered gene and microRNA expression without changes in DNA methylation, and correct imprinting is restored in subsequent generations. Our results illustrate how diet impacts the foetal epigenome, disturbing canonical and non-canonical imprinting mechanisms to modulate the properties of successive generations of offspring.

NRF2 Alters Mitochondrial Gene Expression in Neonate Mice Exposed to Hyperoxia

Approximately 1 in 10 newborns are born preterm and require supplemental oxygen (O₂) in an extrauterine environment following birth. Supplemental O₂ can induce oxidative stress that can impair mitochondrial function, resulting in lung injury and increased risk in early life pulmonary diseases. The nuclear factor-erythroid 2 related factor 2 (NRF2) protects the cells from oxidative stress by regulating the expression of genes containing antioxidant response elements and many mitochondrial-associated genes. In this study, we compared Nrf2-deficient (Nrf2^−/−) and wild-type (Nrf2^+/+) mice to define the role of NRF2 in lung mitochondrial genomic features in late embryonic development in mice (embryonic days, E13.5 and E18.5) versus birth (postnatal day 0, PND0). We also determined whether NRF2 protects lung mitochondrial genome parameters in postnatal mice exposed to a 72 h hyperoxia environment. We found Nrf2^−/− embryonic lungs were characterized by decreases in mtDNA copies from E13.5 to E18.5. Interestingly, Nrf2^−/− heteroplasmy frequency was significantly higher than Nrf2^+/+ at E18.5, though this effect reversed at PND0. In postnatal mice exposed to hyperoxia, we identified three- to four-fold increases in mitochondria-encoded mitochondrial genes, which regulate oxidative phosphorylation. Overall, our findings demonstrate a potentially critical role of NRF2 in mediating long-term effects of hyperoxia on mitochondrial function.

MYC, mitochondrial metabolism and O-GlcNAcylation converge to modulate the activity and subcellular localization of DNA and RNA demethylases

Mitochondria can function as signaling organelles, and part of this output leads to epigenetic remodeling. The full extent of this far-reaching interplay remains undefined. Here, we show that MYC transcriptionally activates IDH2 and increases alpha-ketoglutarate (αKG) levels. This regulatory step induces the activity of αKG-dependent DNA hydroxylases and RNA demethylases, thus reducing global DNA and RNA methylation. MYC, in a IDH2-dependent manner, also promotes the nuclear accumulation of TET1-TET2-TET3, FTO and ALKBH5. Notably, this subcellular movement correlated with the ability of MYC, in an IDH2-dependent manner, and, unexpectedly, of αKG to directly induce O-GlcNAcylation. Concordantly, modulation of the activity of OGT and OGA, enzymes that control the cycling of this non-canonical mono-glycosylation, largely recapitulated the effects of the MYC-IDH2-αKG axis on the subcellular movement of DNA and RNA demethylases. Together, we uncovered a hitherto unsuspected crosstalk between MYC, αKG and O-GlcNAcylation which could influence the epigenome and epitranscriptome homeostasis.

Mitochondrial DNA damage as driver of cellular outcomes

Mitochondria are primarily involved in energy production through the process of oxidative phosphorylation (OXPHOS). Increasing evidence has shown that mitochondrial function impacts a plethora of different cellular activities, including metabolism, epigenetics, and innate immunity. Like the nucleus, mitochondria own their genetic material, but this organellar genome is circular, present in multiple copies, and maternally inherited. The mitochondrial DNA (mtDNA) encodes 37 genes that are solely involved in OXPHOS. Maintenance of mtDNA, through replication and repair, requires the import of nuclear DNA-encoded proteins. Thus, mitochondria completely rely on the nucleus to prevent mitochondrial genetic alterations. As most cells contain hundreds to thousands of mitochondria, it follows that the shear number of organelles allows for the buffering of dysfunction-at least to some extent-before tissue homeostasis becomes impaired. Only red blood cells lack mitochondria entirely. Impaired mitochondrial function is a hallmark of aging and is involved in a number of different disorders, including neurodegenerative diseases, diabetes, cancer, and autoimmunity. Although alterations in mitochondrial processes unrelated to OXPHOS, such as fusion and fission, contribute to aging and disease, maintenance of mtDNA integrity is critical for proper organellar function. Here, we focus on how mtDNA damage contributes to cellular dysfunction and health outcomes.

Epigenetics is Promising Direction in Modern Science

Epigenetics studies the inherited changes in a phenotype or in expression of genes caused by other mechanisms, without changing the nucleotide sequence of DNA. The most distinguished epigenetic tools are: modifications of histones, enzymatic DNA methylation, and gene silencing mediated by small RNAs (miRNA, siRNA). The resulting m5C residues in DNA substantially affect the cooperation of proteins with DNA. It is organized by hormones and aging-related alterations, one of the mechanisms controlling sex and cellular differentiation. DNA methylation regulates all genetic functions: repair, recombination, DNA replication, as well as transcription. Distortions in DNA methylation and other epigenetic signals lead to diabetes, premature aging, mental dysfunctions, and cancer.

Histone Acetylation Defects in Brain Precursor Cells: A Potential Pathogenic Mechanism Causing Proliferation and Differentiation Dysfunctions in Mitochondrial Aspartate-Glutamate Carrier Isoform 1 Deficiency

Mitochondrial aspartate-glutamate carrier isoform 1 (AGC1) deficiency is an ultra-rare genetic disease characterized by global hypomyelination and brain atrophy, caused by mutations in the SLC25A12 gene leading to a reduction in AGC1 activity. In both neuronal precursor cells and oligodendrocytes precursor cells (NPCs and OPCs), the AGC1 determines reduced proliferation with an accelerated differentiation of OPCs, both associated with gene expression dysregulation. Epigenetic regulation of gene expression through histone acetylation plays a crucial role in the proliferation/differentiation of both NPCs and OPCs and is modulated by mitochondrial metabolism. In AGC1 deficiency models, both OPCs and NPCs show an altered expression of transcription factors involved in the proliferation/differentiation of brain precursor cells (BPCs) as well as a reduction in histone acetylation with a parallel alteration in the expression and activity of histone acetyltransferases (HATs) and histone deacetylases (HDACs). In this study, histone acetylation dysfunctions have been dissected in in vitro models of AGC1 deficiency OPCs (Oli-Neu cells) and NPCs (neurospheres), in physiological conditions and following pharmacological treatments. The inhibition of HATs by curcumin arrests the proliferation of OPCs leading to their differentiation, while the inhibition of HDACs by suberanilohydroxamic acid (SAHA) has only a limited effect on proliferation, but it significantly stimulates the differentiation of OPCs. In NPCs, both treatments determine an alteration in the commitment toward glial cells. These data contribute to clarifying the molecular and epigenetic mechanisms regulating the proliferation/differentiation of OPCs and NPCs. This will help to identify potential targets for new therapeutic approaches that are able to increase the OPCs pool and to sustain their differentiation toward oligodendrocytes and to myelination/remyelination processes in AGC1 deficiency, as well as in other white matter neuropathologies.

Inhibition of DRP1 Impedes Zygotic Genome Activation and Preimplantation Development in Mice

Mitochondrion plays an indispensable role during preimplantation embryo development. Dynamic-related protein 1 (DRP1) is critical for mitochondrial fission and controls oocyte maturation. However, its role in preimplantation embryo development is still lacking. In this study, we demonstrate that inhibition of DRP1 activity by mitochondrial division inhibitor-1, a small molecule reported to specifically inhibit DRP1 activity, can cause severe developmental arrest of preimplantation embryos in a dose-dependent manner in mice. Meanwhile, DRP1 inhibition resulted in mitochondrial dysfunction including decreased mitochondrial activity, loss of mitochondrial membrane potential, reduced mitochondrial copy number and inadequate ATP by disrupting both expression and activity of DRP1 and mitochondrial complex assembly, leading to excessive ROS production, severe DNA damage and cell cycle arrest at 2-cell embryo stage. Furthermore, reduced transcriptional and translational activity and altered histone modifications in DRP1-inhibited embryos contributed to impeded zygotic genome activation, which prevented early embryos from efficient development beyond 2-cell embryo stage. These results show that DRP1 inhibition has potential cytotoxic effects on mammalian reproduction, and DRP1 inhibitor should be used with caution when it is applied to treat diseases. Additionally, this study improves our understanding of the crosstalk between mitochondrial metabolism and zygotic genome activation.

Mitochondrial Heteroplasmy Shifting as a Potential Biomarker of Cancer Progression

Cancer is a serious health problem with a high mortality rate worldwide. Given the relevance of mitochondria in numerous physiological and pathological mechanisms, such as adenosine triphosphate (ATP) synthesis, apoptosis, metabolism, cancer progression and drug resistance, mitochondrial genome (mtDNA) analysis has become of great interest in the study of human diseases, including cancer. To date, a high number of variants and mutations have been identified in different types of tumors, which coexist with normal alleles, a phenomenon named heteroplasmy. This mechanism is considered an intermediate state between the fixation or elimination of the acquired mutations. It is suggested that mutations, which confer adaptive advantages to tumor growth and invasion, are enriched in malignant cells. Notably, many recent studies have reported a heteroplasmy-shifting phenomenon as a potential shaper in tumor progression and treatment response, and we suggest that each cancer type also has a unique mitochondrial heteroplasmy-shifting profile. So far, a plethora of data evidencing correlations among heteroplasmy and cancer-related phenotypes are available, but still, not authentic demonstrations, and whether the heteroplasmy or the variation in mtDNA copy number (mtCNV) in cancer are cause or consequence remained unknown. Further studies are needed to support these findings and decipher their clinical implications and impact in the field of drug discovery aimed at treating human cancer.

Aberrant redox biology and epigenetic reprogramming: Co-conspirators across multiple human diseases

An epigenetic landscape encompasses a series of dynamic interconnected mechanisms working together to fashion a diverse set of phenotypes from a singular genotype. The epigenetic plasticity observed in disease and development is facilitated by enzymes that create and remove covalent modifications to DNA and histones. Several important discoveries within the past decade have revealed that epigenetic control mechanisms are subject to redox regulation and mitochondrial-to-nuclear retrograde signaling. This has led to our current understanding that the writers and erasers of the epigenome are influenced by several levels of redox and metabolic control including the bioavailability of oxygen, nutrients, and metabolite co-factors necessary for optimal enzyme activity. Thus, these enzymes perceive a cell's redox state, metabolic status, and environmental signals to influence chromatin structure and accessibility to the transcriptional apparatus. Not only are the activities of epigenetic enzymes affected by cellular redox conditions, but also, in feedback loop fashion, genes encoding antioxidant enzymes as well as prooxidant enzymes can be altered in their expression patterns by epigenetic silencing mechanisms. The altered expression of the anti- and prooxidant genes can then contribute to the onset or progression of disease. Epigenetic regulation of gene expression by the confluence of redox biology and gene-environment interactions is an active area of research and our understanding of these links continues to evolve. Given the emergent importance of crosstalk between redox biology and epigenetic regulatory mechanisms, it is timely that this issue should explore the current state of knowledge on this topic and how changes in metabolism and redox flux can result in tectonic shifts of the epigenetic landscape.

Mitochondria and metabolic transitions in cardiomyocytes: lessons from development for stem cell-derived cardiomyocytes

Current methods to differentiate cardiomyocytes from human pluripotent stem cells (PSCs) inadequately recapitulate complete development and result in PSC-derived cardiomyocytes (PSC-CMs) with an immature or fetal-like phenotype. Embryonic and fetal development are highly dynamic periods during which the developing embryo or fetus is exposed to changing nutrient, oxygen, and hormone levels until birth. It is becoming increasingly apparent that these metabolic changes initiate developmental processes to mature cardiomyocytes. Mitochondria are central to these changes, responding to these metabolic changes and transitioning from small, fragmented mitochondria to large organelles capable of producing enough ATP to support the contractile function of the heart. These changes in mitochondria may not simply be a response to cardiomyocyte maturation; the metabolic signals that occur throughout development may actually be central to the maturation process in cardiomyocytes. Here, we review methods to enhance maturation of PSC-CMs and highlight evidence from development indicating the key roles that mitochondria play during cardiomyocyte maturation. We evaluate metabolic transitions that occur during development and how these affect molecular nutrient sensors, discuss how regulation of nutrient sensing pathways affect mitochondrial dynamics and function, and explore how changes in mitochondrial function can affect metabolite production, the cell cycle, and epigenetics to influence maturation of cardiomyocytes.

Epigenetic and metabolic regulation of epidermal homeostasis

Continuous exposure of the skin to environmental, mechanical and chemical stress necessitates constant self‐renewal of the epidermis to maintain its barrier function. This self‐renewal ability is attributed to epidermal stem cells (EPSCs), which are long‐lived, multipotent cells located in the basal layer of the epidermis. Epidermal homeostasis – coordinated proliferation and differentiation of EPSCs – relies on fine‐tuned adaptations in gene expression which in turn are tightly associated with specific epigenetic signatures and metabolic requirements. In this review, we will briefly summarize basic concepts of EPSC biology and epigenetic regulation with relevance to epidermal homeostasis. We will highlight the intricate interplay between mitochondrial energy metabolism and epigenetic events – including miRNA‐mediated mechanisms – and discuss how the loss of epigenetic regulation and epidermal homeostasis manifests in skin disease. Discussion of inherited epidermolysis bullosa (EB) and disorders of cornification will focus on evidence for epigenetic deregulation and failure in epidermal homeostasis, including stem cell exhaustion and signs of premature ageing. We reason that the epigenetic and metabolic component of epidermal homeostasis is significant and warrants close attention. Charting epigenetic and metabolic complexities also represents an important step in the development of future systemic interventions aimed at restoring epidermal homeostasis and ameliorating disease burden in severe skin conditions.

Mental Health, Mitochondria, and the Battle of the Sexes

This paper presents a broad perspective on how mental disease relates to the different evolutionary strategies of men and women and to growth, metabolism, and mitochondria—the enslaved bacteria in our cells that enable it all. Several mental disorders strike one sex more than the other; yet what truly matters, regardless of one’s sex, is how much one’s brain is “female” and how much it is “male”. This appears to be the result of an arms race between the parents over how many resources their child ought to extract from the mother, hence whether it should grow a lot or stay small and undemanding. An uneven battle alters the child’s risk of developing not only insulin resistance, diabetes, or cancer, but a mental disease as well. Maternal supremacy increases the odds of a psychosis-spectrum disorder; paternal supremacy, those of an autism-spectrum one. And a particularly lopsided struggle may invite one or the other of a series of syndromes that come in pairs, with diametrically opposite, excessively “male” or “female” characteristics. By providing the means for this tug of war, mitochondria take center stage in steadying or upsetting the precarious balance on which our mental health is built.