Mitochondrial energy deficiency leads to hyperproliferation of skeletal muscle mitochondria and enhanced insulin sensitivity

Meta-analysis of Circulatory mitomiRs in stress Response: Unveiling the significance of miR-34a and miR-146a

BACKGROUND

MicroRNAs (miRNAs) are short, noncoding RNAs with essential roles in cellular pathways and are often associated with various diseases and stress conditions. Recently, they have been discovered in mitochondria, termed "mitomiRs," with unique functions. Mitochondria, crucial organelles for energy production and stress responses, Dysregulated mitomiRs functions and expression has been evident in stress conditions such as cardiovascular and neurodegenerative. In this meta-analysis we have systematically identified miR-34a & miR-146a as possible potential biomarkers for affliction.

METHODS

A meta-analysis was conducted to assess the potential role of miR-34a and miR-146a, two specific mitomiRs, as biomarkers in stress-related conditions. The study followed PRISMA guidelines, involving comprehensive database searches in May and September 2023. Twelve studies meeting predefined inclusion criteria were selected, and data analysis included the evaluation of miR-34a and miR-146a expression levels in various stress conditions compared to control groups. We also performed Gene ontology (GO) and Pathway enrichment analysis to observe how mitomiRs affects our body.

RESULTS

The meta-analysis revealed a significant increase in overall mitomiRs (miR-34a and miR-146a) expression levels in experimental groups experiencing different stress conditions compared to control groups (Z = 3.54, p < 0.05 using RevMan software). miR-34a demonstrated more pronounced upregulation and exhibited potential as a specific biomarker in certain stress-related conditions (Z = 2.22, p < 0.05). However, miR-146a did not show a significant difference, requiring further investigation in various stress-related contexts. The Analysis indicated a high degree of heterogeneity among the studies.

CONCLUSION

This meta-analysis emphasises the importance of mitomiRs, especially miR-34a, as potential biomarkers in the intricate interplay between stress, mitochondrial function, and disease. The study opens new avenues for exploring miRNAs' diagnostic and therapeutic applications in stress-related diseases, highlighting their pivotal role at the crossroads of molecular biology, psychology, and medicine.

GDF15 as a key disease target and biomarker: linking chronic lung diseases and ageing

Growth differentiation factor 15 (GDF15), a member of the transforming growth factor-beta superfamily, is expressed in several human organs. In particular, it is highly expressed in the placenta, prostate, and liver. The expression of GDF15 increases under cellular stress and pathological conditions. Although numerous transcription factors directly up-regulate the expression of GDF15, the receptors and downstream mediators of GDF15 signal transduction in most tissues have not yet been determined. Glial cell-derived neurotrophic factor family receptor α-like protein was recently identified as a specific receptor that plays a mediating role in anorexia. However, the specific receptors of GDF15 in other tissues and organs remain unclear. As a marker of cell stress, GDF15 appears to exert different effects under different pathological conditions. Cell senescence may be an important pathogenetic process and could be used to assess the progression of various lung diseases, including COVID-19. As a key member of the senescence-associated secretory phenotype protein repertoire, GDF15 seems to be associated with mitochondrial dysfunction, although the specific molecular mechanism linking GDF15 expression with ageing remains to be elucidated. Here, we focus on research progress linking GDF15 expression with the pathogenesis of various chronic lung diseases, including neonatal bronchopulmonary dysplasia, idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, and pulmonary hypertension, suggesting that GDF15 may be a key biomarker for diagnosis and prognosis. Thus, in this review, we aimed to provide new insights into the molecular biological mechanism and emerging clinical data associated with GDF15 in lung-related diseases, while highlighting promising research and clinical prospects.

AOX delays the onset of the lethal phenotype in a mouse model of Uqcrh (complex III) disease

The alternative oxidase, AOX, provides a by-pass of the cytochrome segment of the mitochondrial respiratory chain when the chain is unavailable. AOX is absent from mammals, but AOX from Ciona intestinalis is benign when expressed in mice. Although non-protonmotive, so does not contribute directly to ATP production, it has been shown to modify and in some cases rescue phenotypes of respiratory-chain disease models. Here we studied the effect of C. intestinalis AOX on mice engineered to express a disease-equivalent mutant of Uqcrh, encoding the hinge subunit of mitochondrial respiratory complex III, which results in a complex metabolic phenotype beginning at 4-5 weeks, rapidly progressing to lethality within a further 6-7 weeks. AOX expression delayed the onset of this phenotype by several weeks, but provided no long-term benefit. We discuss the significance of this finding in light of the known and hypothesized effects of AOX on metabolism, redox homeostasis, oxidative stress and cell signaling. Although not a panacea, the ability of AOX to mitigate disease onset and progression means it could be useful in treatment.

Retrospective Study Shows That Serum Levels of Chemokine CXCL10 and Cytokine GDF15 Support a Diagnosis of Sporadic Inclusion Body Myositis and Immune-Mediated Necrotizing Myopathy

The implementation of novel blood-based biomarkers is desired to reduce the diagnostic delay and burden for myositis patients. In this retrospective study, the potential of C-X-C motif chemokine ligand 10 (CXCL10) and growth differentiation factor 15 (GDF15) was explored in an established patient cohort diagnosed with immune-mediated necrotizing myopathy (IMNM; n = 21), sporadic inclusion body myositis (IBM; n = 18), overlap myositis (OM; n = 3), dermatomyositis (DM; n = 2), and anti-synthetase syndrome (ASS; n = 1), comparing these results with healthy controls (n = 10) and patients with a hereditary neuromuscular disorder (n = 14). CXCL10 and GDF15 were quantified in sera with enzyme-linked immunosorbent assays and immunolocalized in skeletal muscle tissue. In myositis patients, serum CXCL10 levels were significantly increased 9.6-fold compared to healthy controls and 4.2-fold compared to disease controls. Mean levels of CXCL10 were 929 ± 658 pg/mL of serum in IBM and 425 ± 324 pg/mL of serum in IMNM. With the threshold set to 180 pg/mL of CXCL10, myositis patients could be differentiated from healthy and disease controls with a sensitivity of 0.80 and a specificity of 0.71. Incorporating a threshold of 300 pg/mL for GDF15 reduced false negatives to two IMNM patients only. Subsets of muscle-infiltrating immune cells expressed CXCL10, and serum levels correlated with muscle inflammation grade. We propose adding circulating CXCL10 and GDF15 to the blood-based diagnostic toolkit for myositis as a valuable patient-friendly approach.

The roles of FGF21 and GDF15 in mediating the mitochondrial integrated stress response

Various models of mitochondrial stress result in induction of the stress-responsive cytokines fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15). This is an adaptive mechanism downstream of the mitochondrial integrated stress response frequently associated with improvements in systemic metabolic health. Both FGF21 and GDF15 have been shown to modulate energy balance and glucose homeostasis, and their pharmacological administration leads to promising beneficial effects against obesity and associated metabolic diseases in pre-clinical models. Furthermore, endogenous upregulation of FGF21 and GDF15 is associated with resistance to diet-induced obesity (DIO), improved glucose homeostasis and increased insulin sensitivity. In this review, we highlight several studies on transgenic mouse models of mitochondrial stress and will compare the specific roles played by FGF21 and GDF15 on the systemic metabolic adaptations reported in these models.

Adenine nucleotide translocase: Current knowledge in post-translational modifications, regulations and pathological implications for human diseases

Adenine nucleotide translocases (ANTs) are central to mitochondrial integrity and bioenergetic metabolism. This review aims to integrate the progresses and knowledge on ANTs over the last few years, contributing to a potential implication of ANTs for various diseases. Structures, functions, modifications, regulators and pathological implications of ANTs for human diseases are intensively demonstrated here. ANTs have four isoforms (ANT1-4), responsible for exchanging ATP/ADP, possibly composing of pro-apoptotic mPTP as a major component, and mediating FA-dependent uncoupling of proton efflux. ANT can be modified by methylation, nitrosylation and nitroalkylation, acetylation, glutathionylation, phosphorylation, carbonylation and hydroxynonenal-induced modifications. Compounds, including bongkrekic acid, atractyloside calcium, carbon monoxide, minocycline, 4-(N-(S-penicillaminylacetyl)amino) phenylarsonous acid, cardiolipin, free long-chain fatty acids, agaric acid, long chain acyl-coenzyme A esters, all have an ability to regulate ANT activities. ANT impairment leads to bioenergetic failure and mitochondrial dysfunction, contributing to pathogenesis of diseases, such as diabetes (deficiency), heart disease (deficiency), Parkinson's disease (reduction), Sengers Syndrome (decrease), cancer (isoform shifting), Alzheimer's Disease (coaggregation with Tau), Progressive External Opthalmoplegia (mutation), and Fascioscapulohumeral muscular dystrophy (overexpression). This review improves the understanding of the mechanism of ANT in pathogenesis of human diseases, and opens a window for novel therapeutic strategies targeted on ANT in diseases.

Mitochondrial protein import clogging as a mechanism of disease

Mitochondrial biogenesis requires the import of >1,000 mitochondrial preproteins from the cytosol. Most studies on mitochondrial protein import are focused on the core import machinery. Whether and how the biophysical properties of substrate preproteins affect overall import efficiency is underexplored. Here, we show that protein traffic into mitochondria can be disrupted by amino acid substitutions in a single substrate preprotein. Pathogenic missense mutations in ADP/ATP translocase 1 (ANT1), and its yeast homolog ADP/ATP carrier 2 (Aac2), cause the protein to accumulate along the protein import pathway, thereby obstructing general protein translocation into mitochondria. This impairs mitochondrial respiration, cytosolic proteostasis, and cell viability independent of ANT1's nucleotide transport activity. The mutations act synergistically, as double mutant Aac2/ANT1 causes severe clogging primarily at the translocase of the outer membrane (TOM) complex. This confers extreme toxicity in yeast. In mice, expression of a super-clogger ANT1 variant led to neurodegeneration and an age-dependent dominant myopathy that phenocopy ANT1-induced human disease, suggesting clogging as a mechanism of disease. More broadly, this work implies the existence of uncharacterized amino acid requirements for mitochondrial carrier proteins to avoid clogging and subsequent disease.

Increased insulin resistance due to Long COVID is associated with depressive symptoms and partly predicted by the inflammatory response during acute infection

OBJECTIVE

Some months after the remission of acute COVID-19, some individuals show depressive symptoms, which are predicted by increased peak body temperature (PBT) and decreased blood oxygen saturation (SpO2). The present study aimed to examine data on whether long COVID is associated with increased insulin resistance (IR) in association with neuroimmune and oxidative (NIO) processes during the acute infectious and long COVID phases.

METHODS

This case-control, retrospective cohort study used the Homeostasis Model Assessment 2 (HOMA2) calculator© to compute ß-cell function (HOMA2%B) and insulin sensitivity (HOMA2%S) and resistance (HOMA2-IR) and administered the Beck Depression Inventory (BDI) and Hamilton Depression Rating Scale (HAMD) to 86 patients with long COVID and 39 controls.

RESULTS

Long COVID (3-4 months after the acute infection) is accompanied by increased HOMA2-IR, fasting blood glucose (FBG), and insulin levels; 33.7% of the patients vs. 0% of the controls had HOMA2-IR values > 1.8, suggesting IR. Increased IR was predicted by PBT during acute infection and associated with depressive symptoms above and beyond the effects of NIO pathways (nucleotide-binding domain, leucine-rich repeat, and pyrin domain-containing protein 3 [NLRP3] inflammasome, myeloperoxidase [MPO], protein oxidation). There were no significant associations between increased IR and the activated NIO pathways during long COVID.

CONCLUSION

Long COVID is associated with new-onset IR, which may contribute to onset of depressive symptoms due to long COVID by enhancing overall neurotoxicity.

lncRNA UCA1 inhibits mitochondrial dysfunction of skeletal muscle in type 2 diabetes mellitus by sequestering miR-143-3p to release FGF21

Increasing evidence suggests that insulin resistance in type 2 diabetes mellitus (T2DM) is associated with mitochondrial dysfunction in skeletal muscle, while the underlying molecular mechanisms remain elusive. This study aims to construct a ceRNA regulatory network that is involved in mitochondrial dysfunction of skeletal muscle in T2DM. Based on GEO database analysis, differentially expressed lncRNA and mRNA profiles were identified in skeletal muscle tissues of T2DM. Next, LASSO regression analysis was conducted to predict the key lncRNAs related to T2DM, which was validated by receiver operating characteristic (ROC) analysis. Moreover, the miRNAs related to skeletal muscle in T2DM were identified by WGCNA, followed by construction of gene-gene interaction network and GO and KEGG enrichment analyses. It was found that 12 lncRNAs and 6 miRNAs were related to skeletal muscle in T2DM. Moreover, the lncRNA-miRNA-mRNA ceRNA network involving UCA1, miR-143-3p, and FGF21 was constructed. UCA1, and FGF21 were downregulated, while miR-143-3p was upregulated in skeletal muscle cells (SkMCs) exposed to palmitic acid. Additionally, ectopic expression experiments were performed in SkMCs to confirm the effects of UCA1/miR-143-3p/FGF21 on mitochondrial dysfunction by determining mitochondrial ROS, oxygen consumption rate (OCR), membrane potential, and ATP level. Overexpression of miR-143-3p increased ROS accumulation and reduced the OCR, fluorescence ratio of JC-1, and ATP level, which were reversed by upregulation of UCA1 or FGF21. Collectively, lncRNA UCA1 inhibited mitochondrial dysfunction of skeletal muscle in T2DM by sequestering miR-143-3p away from FGF21, therefore providing a potential therapeutic target for alleviating mitochondrial dysfunction of skeletal muscle in T2DM.

Delivery Systems for Mitochondrial Gene Therapy: A Review

Mitochondria are membrane-bound cellular organelles of high relevance responsible for the chemical energy production used in most of the biochemical reactions of cells. Mitochondria have their own genome, the mitochondrial DNA (mtDNA). Inherited solely from the mother, this genome is quite susceptible to mutations, mainly due to the absence of an effective repair system. Mutations in mtDNA are associated with endocrine, metabolic, neurodegenerative diseases, and even cancer. Currently, therapeutic approaches are based on the administration of a set of drugs to alleviate the symptoms of patients suffering from mitochondrial pathologies. Mitochondrial gene therapy emerges as a promising strategy as it deeply focuses on the cause of mitochondrial disorder. The development of suitable mtDNA-based delivery systems to target and transfect mammalian mitochondria represents an exciting field of research, leading to progress in the challenging task of restoring mitochondria's normal function. This review gathers relevant knowledge on the composition, targeting performance, or release profile of such nanosystems, offering researchers valuable conceptual approaches to follow in their quest for the most suitable vectors to turn mitochondrial gene therapy clinically feasible. Future studies should consider the optimization of mitochondrial genes' encapsulation, targeting ability, and transfection to mitochondria. Expectedly, this effort will bring bright results, contributing to important hallmarks in mitochondrial gene therapy.

The impact of high-fat feeding and parkin overexpression on skeletal muscle mass, mitochondrial respiration, and H2O2 emission

Obesity is a major risk factor for developing various health problems, including insulin resistance and type 2 diabetes. Although controversial, accumulation of mitochondrial dysfunction, and notably an increase in mitochondrial reactive oxygen species (ROS) production, was proposed as a key contributor leading to obesity-induced insulin resistance. Here, our goal was to investigate whether Parkin overexpression, a key regulator of the removal of dysfunctional mitochondria through mitophagy, could confer protection against obesity-induced mitochondrial dysfunction. To this end, intramuscular injections of adeno-associated viruses (AAVs) were performed to overexpress Parkin in limb muscle of 6-mo-old mice fed a control diet (CD) or a high-fat diet (HFD) for 12 wk. An AAV-expressing the green fluorescent protein (GFP) was used as control. HFD increased fat mass, altered glycemia, and resulted in insulin resistance. Parkin overexpression resulted in an increase in muscle mass in both CD and HFD mice. In CD mice, Parkin overexpression increased maximal mitochondrial respiration and lowered H₂O₂ emission. HFD increased mitochondrial respiration and, surprisingly, also lowered H₂O₂ emission. Parkin overexpression did not significantly impact mitochondrial function in HFD mice. Taken altogether, our results indicate that Parkin overexpression positively impacts muscle and mitochondrial health under basal conditions and challenges the notion that intrinsic mitochondrial dysfunction is involved in the development of insulin resistance caused by high-fat feeding.

OxPhos defects cause hypermetabolism and reduce lifespan in cells and in patients with mitochondrial diseases

Patients with primary mitochondrial oxidative phosphorylation (OxPhos) defects present with fatigue and multi-system disorders, are often lean, and die prematurely, but the mechanistic basis for this clinical picture remains unclear. By integrating data from 17 cohorts of patients with mitochondrial diseases (n = 690) we find evidence that these disorders increase resting energy expenditure, a state termed hypermetabolism. We examine this phenomenon longitudinally in patient-derived fibroblasts from multiple donors. Genetically or pharmacologically disrupting OxPhos approximately doubles cellular energy expenditure. This cell-autonomous state of hypermetabolism occurs despite near-normal OxPhos coupling efficiency, excluding uncoupling as a general mechanism. Instead, hypermetabolism is associated with mitochondrial DNA instability, activation of the integrated stress response (ISR), and increased extracellular secretion of age-related cytokines and metabokines including GDF15. In parallel, OxPhos defects accelerate telomere erosion and epigenetic aging per cell division, consistent with evidence that excess energy expenditure accelerates biological aging. To explore potential mechanisms for these effects, we generate a longitudinal RNASeq and DNA methylation resource dataset, which reveals conserved, energetically demanding, genome-wide recalibrations. Taken together, these findings highlight the need to understand how OxPhos defects influence the energetic cost of living, and the link between hypermetabolism and aging in cells and patients with mitochondrial diseases.

The crosstalk between BAT thermogenesis and skeletal muscle dysfunction

Metabolic defects increase the risk of skeletal muscle diseases, and muscle impairment might worsen metabolic disruption, leading to a vicious cycle. Both brown adipose tissue (BAT) and skeletal muscle play important roles in non-shivering thermogenesis to regulate energy homeostasis. BAT regulates body temperature, systemic metabolism, and seretion of batokines that have positive or negative impacts on skeletal muscle. Conversely, muscle can secrete myokines that regulate BAT function. This review explained the crosstalk between BAT and skeletal muscle, and then discussed the batokines and highlighted their impact on skeletal muscle under physiological conditions. BAT is now considered a potential therapeutic target for obesity and diabetes treatment. Moreover, manipulation of BAT may be an attractive approach for the treatment of muscle weakness by correcting metabolic deficits. Therefore, exploring BAT as a potential treatment for sarcopenia could be a promising avenue for future research.

Nicotinamide Riboside alleviates exercise intolerance in ANT1-deficient mice

Objective

Methods

Results

Conclusion

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ANT1-deficient mice demonstrate a reduced exercise capacity.

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ANT1-deficiency reduces complex I - linked respiration and the NAD/NADH redox state.

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Lack of NAD+ in skeletal muscle exacerbates exercise intolerance in ANT1 mice.

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Nicotinamide Riboside increases NAD+ and improves exercise capacity in ANT1 mice.

Mitochondrial mutations alter endurance exercise response and determinants in mice

Primary mitochondrial diseases (PMDs) are the most prevalent inborn metabolic disorders, affecting an estimated 1 in 4,200 individuals. Endurance exercise is generally known to improve mitochondrial function, but its indication in the heterogeneous group of PMDs is unclear. We determined the relationship between mitochondrial mutations, endurance exercise response, and the underlying molecular pathways in mice with distinct mitochondrial mutations. This revealed that mitochondria are crucial regulators of exercise capacity and exercise response. Endurance exercise proved to be mostly beneficial across the different mitochondrial mutant mice with the exception of a worsened dilated cardiomyopathy in ANT1-deficient mice. Thus, therapeutic exercises, especially in patients with PMDs, should take into account the physical and mitochondrial genetic status of the patient.

Primary mitochondrial diseases (PMDs) are a heterogeneous group of metabolic disorders that can be caused by hundreds of mutations in both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) genes. Current therapeutic approaches are limited, although one approach has been exercise training. Endurance exercise is known to improve mitochondrial function in heathy subjects and reduce risk for secondary metabolic disorders such as diabetes or neurodegenerative disorders. However, in PMDs the benefit of endurance exercise is unclear, and exercise might be beneficial for some mitochondrial disorders but contraindicated in others. Here we investigate the effect of an endurance exercise regimen in mouse models for PMDs harboring distinct mitochondrial mutations. We show that while an mtDNA ND6 mutation in complex I demonstrated improvement in response to exercise, mice with a CO1 mutation affecting complex IV showed significantly fewer positive effects, and mice with an ND5 complex I mutation did not respond to exercise at all. For mice deficient in the nDNA adenine nucleotide translocase 1 (Ant1), endurance exercise actually worsened the dilated cardiomyopathy. Correlating the gene expression profile of skeletal muscle and heart with the physiologic exercise response identified oxidative phosphorylation, amino acid metabolism, matrisome (extracellular matrix [ECM]) structure, and cell cycle regulation as key pathways in the exercise response. This emphasizes the crucial role of mitochondria in determining the exercise capacity and exercise response. Consequently, the benefit of endurance exercise in PMDs strongly depends on the underlying mutation, although our results suggest a general beneficial effect.

FoxO transcription factors in mitochondrial homeostasis

Mitochondria play essential roles in cellular energetics, biosynthesis, and signaling transduction. Dysfunctional mitochondria have been implicated in different diseases such as obesity, diabetes, cardiovascular disease, nonalcoholic fatty liver disease, neurodegenerative disease, and cancer. Mitochondrial homeostasis is controlled by a triad of mitochondrial biogenesis, dynamics (fusion and fission), and autophagy (mitophagy). Studies have underscored FoxO transcription factors as key mitochondrial regulators. Specifically, FoxOs regulate mitochondrial biogenesis by dampening NRF1-Tfam and c-Myc-Tfam cascades directly, and inhibiting NAD-Sirt1-Pgc1α cascade indirectly by inducing Hmox1 or repressing Fxn and Urod. In addition, FoxOs mediate mitochondrial fusion (via Mfn1 and Mfn2) and fission (via Drp1, Fis1, and MIEF2), during which FoxOs elicit regulatory mechanisms at transcriptional, posttranscriptional (e.g. via miR-484/Fis1), and posttranslational (e.g. via Bnip3-calcineurin mediated Drp1 dephosphorylation) levels. Furthermore, FoxOs control mitochondrial autophagy in the stages of autophagosome formation and maturation (e.g. initiation, nucleation, and elongation), mitochondria connected to and engulfed by autophagosome (e.g. via PINK1 and Bnip3 pathways), and autophagosome-lysosome fusion to form autolysosome for cargo degradation (e.g. via Tfeb and cathepsin proteins). This article provides an up-to-date view of FoxOs regulating mitochondrial homeostasis and discusses the potential of targeting FoxOs for therapeutics.

Mitochondrial H+ Leak and Thermogenesis

Mitochondria of all tissues convert various metabolic substrates into two forms of energy: ATP and heat. Historically, the primary focus of research in mitochondrial bioenergetics was on the mechanisms of ATP production, while mitochondrial thermogenesis received significantly less attention. Nevertheless, mitochondrial heat production is crucial for the maintenance of body temperature, regulation of the pace of metabolism, and prevention of oxidative damage to mitochondria and the cell. In addition, mitochondrial thermogenesis has gained significance as a pharmacological target for treating metabolic disorders. Mitochondria produce heat as the result of H⁺ leak across their inner membrane. This review provides a critical assessment of the current field of mitochondrial H⁺ leak and thermogenesis, with a focus on the molecular mechanisms involved in the function and regulation of uncoupling protein 1 and the ADP/ATP carrier, the two proteins that mediate mitochondrial H⁺ leak.

Cytosolic adaptation to mitochondria-induced proteostatic stress causes progressive muscle wasting

Mitochondrial dysfunction causes muscle wasting in many diseases and probably also during aging. The underlying mechanism is poorly understood. We generated transgenic mice with unbalanced mitochondrial protein loading and import, by moderately overexpressing the nuclear-encoded adenine nucleotide translocase, Ant1. We found that these mice progressively lose skeletal muscle. Ant1-overloading reduces mitochondrial respiration. Interestingly, it also induces small heat shock proteins and aggresome-like structures in the cytosol, suggesting increased proteostatic burden due to accumulation of unimported mitochondrial preproteins. The transcriptome of Ant1-transgenic muscles is drastically remodeled to counteract proteostatic stress, by repressing protein synthesis and promoting proteasomal function, autophagy, and lysosomal amplification. These proteostatic adaptations collectively reduce protein content thereby reducing myofiber size and muscle mass. Thus, muscle wasting can occur as a trade-off of adaptation to mitochondria-induced proteostatic stress. This finding could have implications for understanding the mechanism of muscle wasting, especially in diseases associated with Ant1 overexpression, including facioscapulohumeral dystrophy.

The Role of GDF15 as a Myomitokine

Growth differentiation factor 15 (GDF15) is a cytokine best known for affecting systemic energy metabolism through its anorectic action. GDF15 expression and secretion from various organs and tissues is induced in different physiological and pathophysiological states, often linked to mitochondrial stress, leading to highly variable circulating GDF15 levels. In skeletal muscle and the heart, the basal expression of GDF15 is very low compared to other organs, but GDF15 expression and secretion can be induced in various stress conditions, such as intense exercise and acute myocardial infarction, respectively. GDF15 is thus considered as a myokine and cardiokine. GFRAL, the exclusive receptor for GDF15, is expressed in hindbrain neurons and activation of the GDF15–GFRAL pathway is linked to an increased sympathetic outflow and possibly an activation of the hypothalamic-pituitary-adrenal (HPA) stress axis. There is also evidence for peripheral, direct effects of GDF15 on adipose tissue lipolysis and possible autocrine cardiac effects. Metabolic and behavioral outcomes of GDF15 signaling can be beneficial or detrimental, likely depending on the magnitude and duration of the GDF15 signal. This is especially apparent for GDF15 production in muscle, which can be induced both by exercise and by muscle disease states such as sarcopenia and mitochondrial myopathy.

Stress-induced FGF21 and GDF15 in obesity and obesity resistance

Fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15) are established as stress-responsive cytokines that can modulate energy balance by increasing energy expenditure or suppressing food intake, respectively. Despite their pharmacologically induced beneficial effects on obesity and comorbidities, circulating levels of both cytokines are elevated during obesity and related metabolic complications. On the other hand, endocrine crosstalk via FGF21 and GDF15 was also reported to play a crucial role in genetically modified mouse models of mitochondrial perturbations leading to diet-induced obesity (DIO) resistance. This review aims to dissect the complexities of endogenous FGF21 and GDF15 action in obesity versus DIO resistance for the regulation of energy balance in metabolic health and disease.

Anterograde regulation of mitochondrial genes and FGF21 signaling by hepatic LSD1

Mitochondrial biogenesis and function are controlled by anterograde regulatory pathways involving more than 1000 nuclear-encoded proteins. Transcriptional networks controlling the nuclear-encoded mitochondrial genes remain to be fully elucidated. Here, we show that histone demethylase LSD1 KO from adult mouse liver (LSD1-LKO) reduces the expression of one-third of all nuclear-encoded mitochondrial genes and decreases mitochondrial biogenesis and function. LSD1-modulated histone methylation epigenetically regulates nuclear-encoded mitochondrial genes. Furthermore, LSD1 regulates gene expression and protein methylation of nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), which controls the final step of NAD+ synthesis and limits NAD+ availability in the nucleus. Lsd1 KO reduces NAD+-dependent SIRT1 and SIRT7 deacetylase activity, leading to hyperacetylation and hypofunctioning of GABPβ and PGC-1α, the major transcriptional factor/cofactor for nuclear-encoded mitochondrial genes. Despite the reduced mitochondrial function in the liver, LSD1-LKO mice are protected from diet-induced hepatic steatosis and glucose intolerance, partially due to induction of hepatokine FGF21. Thus, LSD1 orchestrates a core regulatory network involving epigenetic modifications and NAD+ synthesis to control mitochondrial function and hepatokine production.

GDF15 and Cardiac Cells: Current Concepts and New Insights

Growth and differentiation factor 15 (GDF15) belongs to the transforming growth factor-β (TGF-β) superfamily of proteins. Glial-derived neurotrophic factor (GDNF) family receptor α-like (GFRAL) is an endogenous receptor for GDF15 detected selectively in the brain. GDF15 is not normally expressed in the tissue but is prominently induced by “injury”. Serum levels of GDF15 are also increased by aging and in response to cellular stress and mitochondrial dysfunction. It acts as an inflammatory marker and plays a role in the pathogenesis of cardiovascular diseases, metabolic disorders, and neurodegenerative processes. Identified as a new heart-derived endocrine hormone that regulates body growth, GDF15 has a local cardioprotective role, presumably due to its autocrine/paracrine properties: antioxidative, anti-inflammatory, antiapoptotic. GDF15 expression is highly induced in cardiomyocytes after ischemia/reperfusion and in the heart within hours after myocardial infarction (MI). Recent studies show associations between GDF15, inflammation, and cardiac fibrosis during heart failure and MI. However, the reason for this increase in GDF15 production has not been clearly identified. Experimental and clinical studies support the potential use of GDF15 as a novel therapeutic target (1) by modulating metabolic activity and (2) promoting an adaptive angiogenesis and cardiac regenerative process during cardiovascular diseases. In this review, we comment on new aspects of the biology of GDF15 as a cardiac hormone and show that GDF15 may be a predictive biomarker of adverse cardiac events.

Protective Effect of Jiang Tang Xiao Ke Granules against Skeletal Muscle IR via Activation of the AMPK/SIRT1/PGC-1α Signaling Pathway

The Jiang Tang Xiao Ke (JTXK) granule is a classic Chinese herbal formula that has been put into clinical use in the treatment of type 2 diabetes mellitus for decades. However, whether its ability to ameliorate skeletal muscle insulin resistance (IR) is through modulation of the AMPK/SIRT1/PGC-1α signaling pathway remains unknown. Therefore, we aimed to investigate the effects of JTXK granules on IR in skeletal muscle of high-fat diet-induced diabetic mice and C2C12 cells and analyze the underlying mechanisms. In the present study, we showed that JTXK granules attenuated body weight gain, reduced body fat mass, improved body lean mass, and enhanced muscle performance of diabetic mice. JTXK granules also improved glucose metabolism and skeletal muscle insulin sensitivity and partially reversed abnormal serum lipid levels, which might be related to the regulation of the AMPK/SIRT1/PGC-1α pathway, both in skeletal muscle tissue of diabetic mice and in C2C12 cells. Furthermore, drug-containing serum of JTXK granules was capable of enhancing glucose uptake and mitochondrial respiration in C2C12 cells, and AMPKα was proven to be closely involved in this process. Taken together, these results suggest that the JTXK granule ameliorates skeletal muscle IR through activation of the AMPK/SIRT1/PGC-1α signaling pathway, which offers a novel perspective of this formula to combat IR-related metabolic diseases.

AMPK mediates energetic stress-induced liver GDF15

Growth differentiating factor-15 (GDF15) is an emerging target for the treatment of obesity and metabolic disease partly due to its ability to suppress food intake. GDF15 expression and secretion are thought to be regulated by a cellular integrated stress response, which involves endoplasmic reticulum (ER) stress. AMPK is another cellular stress sensor, but the relationship between AMPK, ER stress, and GDF15 has not been assessed in vivo. Wildtype (WT), AMPK β1 deficient (AMPKβ1^-/- ), and CHOP^-/- mice were treated with three distinct AMPK activators; AICAR, which is converted to ZMP mimicking the effects of AMP on the AMPKγ isoform, R419, which indirectly activates AMPK through inhibition of mitochondrial respiration, or A769662, a direct AMPK activator which binds the AMPKβ1 isoform ADaM site causing allosteric activation. Following treatments, liver Gdf15, markers of ER-stress, AMPK activity, adenine nucleotides, circulating GDF15, and food intake were assessed. AICAR and R419 caused ER and energetic stress, increased GDF15 expression and secretion, and suppressed food intake. Direct activation of AMPK β1 containing complexes by A769662 increased hepatic Gdf15 expression, circulating GDF15, and suppressed food intake, independent of ER stress. The effects of AICAR, R419, and A769662 on GDF15 were attenuated in AMPKβ1^-/- mice. AICAR and A769662 increased GDF15 to a similar extent in WT and CHOP^-/- mice. Herein, we provide evidence that AMPK plays a role in mediating the induction of GDF15 under conditions of energetic stress in mouse liver in vivo.

Therapeutic wavelengths of ultraviolet B radiation activate apoptotic, circadian rhythm, redox signalling and key canonical pathways in psoriatic epidermis

Ultraviolet B radiation (UVB) exerts pleiotropic effects on human skin. DNA damage response and repair pathways are activated by UVB; if damage cannot be repaired, apoptosis ensues. Although cumulative UVB exposure predisposes to skin cancer, UVB phototherapy is widely used as an effective treatment for psoriasis. Previous studies defined the therapeutic action spectrum of UVB and showed that psoriasis is resistant to apoptosis. This study aimed to investigate early molecular responses within psoriasis plaques following irradiation with single equi-erythemogenic doses of clinically-effective (311 nm, narrow-band) compared to clinically-ineffective (290 nm) UVB. Forty-eight micro-dissected epidermal samples from 20 psoriatic patients were analyzed using microarrays. Our bioinformatic analysis compared gene expression between 311 nm irradiated, 290 nm irradiated and control psoriasis epidermis to specifically identify 311 nm UVB differentially expressed genes (DEGs) and their upstream regulatory pathways. Key DEGs and pathways were validated by immunohistochemical analysis. There was a dynamic induction and repression of 311 nm UVB DEGs between 6 h and 18 h, only a limited number of DEGs maintained their designated expression status between time-points. Key disease and function pathways included apoptosis, cell death, cell migration and leucocyte chemotaxis. DNA damage response pathways, NRF2-mediated oxidative stress response and P53 signalling were key nodes, interconnecting apoptosis and cell cycle arrest. Interferon signalling, dendritic cell maturation, granulocyte adhesion and atherosclerotic pathways were also differentially regulated. Consistent with these findings, top transcriptional regulators of 311 nm UVB DEGs related to: a) apoptosis, DNA damage response and cell cycle control; b) innate/acquired immune regulation and inflammation; c) hypoxia/redox response and angiogenesis; d) circadian rhythmicity; f) EGR/AP1 signalling and keratinocyte differentiation; and g) mitochondrial biogenesis. This research provides important insights into the molecular targets of 311 nm UVB, underscoring key roles for apoptosis and cell death. These and the other key pathways delineated may be central to the therapeutic effects of 311 nm in psoriasis.

Cellular and Intercellular Homeostasis in Adipose Tissue with Mitochondria-Specific Stress

Paracrine interactions are imperative for the maintenance of adipose tissue intercellular homeostasis, and intracellular organelle dysfunction results in local and systemic alterations in metabolic homeostasis. It is currently accepted that mitochondrial proteotoxic stress activates the mitochondrial unfolded protein response (UPRmt) in vitro and in vivo. The induction of mitochondrial chaperones and proteases during the UPRmt is a key cell-autonomous mechanism of mitochondrial quality control. The UPRmt also affects systemic metabolism through the secretion of cell non-autonomous peptides and cytokines (hereafter, metabokines). Mitochondrial function in adipose tissue plays a pivotal role in whole-body metabolism and human diseases. Despite continuing interest in the role of the UPRmt and quality control pathways of mitochondria in energy metabolism, studies on the roles of the UPRmt and metabokines in white adipose tissue are relatively sparse. Here, we describe the role of the UPRmt in adipose tissue, including adipocytes and resident macrophages, and the interactive roles of cell non-autonomous metabokines, particularly growth differentiation factor 15, in local adipose cellular homeostasis and systemic energy metabolism.

Mitochondrial Dysfunction in Obesity and Reproduction

Mounting evidence suggests a role for mitochondrial dysfunction in the pathogenesis of many diseases, including type 2 diabetes, aging, and ovarian failure. Because of the central role of mitochondria in energy production, heme biosynthesis, calcium buffering, steroidogenesis, and apoptosis signaling within cells, understanding the molecular mechanisms behind mitochondrial dysregulation and its potential implications in disease is critical. This review will take a journey through the past and summarize what is known about mitochondrial dysfunction in various disorders, focusing on metabolic alterations and reproductive abnormalities. Evidence is presented from studies in different human populations, and rodents with genetic manipulations of pathways known to affect mitochondrial function.

New Insights Into Mitochondrial Dysfunction at Disease Susceptibility Loci in the Development of Type 2 Diabetes

This study investigated the potential genetic mechanisms which underlie adipose tissue mitochondrial dysfunction in Type 2 diabetes (T2D), by systematically identifying nuclear-encoded mitochondrial genes (NEMGs) among the genes regulated by T2D-associated genetic loci. The target genes of these 'disease loci' were identified by mapping genetic loci associated with both disease and gene expression levels (expression quantitative trait loci, eQTL) using high resolution genetic maps, with independent estimates co-locating to within a small genetic distance. These co-locating signals were defined as T2D-eQTL and the target genes as T2D cis-genes. In total, 763 cis-genes were associated with T2D-eQTL, of which 50 were NEMGs. Independent gene expression datasets for T2D and insulin resistant cases and controls confirmed that the cis-genes and cis-NEMGs were enriched for differential expression in cases, providing independent validation that genetic maps can identify informative functional genes. Two additional results were consistent with a potential role of T2D-eQTL in regulating the 50 identified cis-NEMGs in the context of T2D risk: (1) the 50 cis-NEMGs showed greater differential expression compared to other NEMGs and (2) other NEMGs showed a trend towards significantly decreased expression if their expression levels correlated more highly with the subset of 50 cis-NEMGs. These 50 cis-NEMGs, which are differentially expressed and associated with mapped T2D disease loci, encode proteins acting within key mitochondrial pathways, including some of current therapeutic interest such as the metabolism of branched-chain amino acids, GABA and biotin.

Exploiting Common Aspects of Obesity and Alzheimer's Disease

Alzheimer’s disease (AD) is an example of age-related dementia, and there are still no known preventive or curative measures for this disease. Obesity and associated metabolic changes are widely accepted as risk factors of age-related cognitive decline. Insulin is the prime mediator of metabolic homeostasis, which is impaired in obesity, and this impairment potentiates amyloid-β (Aβ) accumulation and the formation of neurofibrillary tangles (NFTs). Obesity is also linked with functional and morphological alterations in brain mitochondria leading to brain insulin resistance (IR) and memory deficits associated with AD. Also, increased peripheral inflammation and oxidative stress due to obesity are the main drivers that increase an individual’s susceptibility to cognitive deficits, thus doubling the risk of AD. This enhanced risk of AD is alarming in the context of a rapidly increasing global incidence of obesity and overweight in the general population. In this review, we summarize the risk factors that link obesity with AD and emphasize the point that the treatment and management of obesity may also provide a way to prevent AD.

Insulin Modulates the Bioenergetic and Thermogenic Capacity of Rat Brown Adipocytes In Vivo by Modulating Mitochondrial Mosaicism

The effects of insulin on the bioenergetic and thermogenic capacity of brown adipocyte mitochondria were investigated by focusing on key mitochondrial proteins. Two-month-old male Wistar rats were treated acutely or chronically with a low or high dose of insulin. Acute low insulin dose increased expression of all electron transport chain complexes and complex IV activity, whereas high dose increased complex II expression. Chronic low insulin dose decreased complex I and cyt c expression while increasing complex II and IV expression and complex IV activity. Chronic high insulin dose decreased complex II, III, cyt c, and increased complex IV expression. Uncoupling protein (UCP) 1 expression was decreased after acute high insulin but increased following chronic insulin treatment. ATP synthase expression was increased after acute and decreased after chronic insulin treatment. Only a high dose of insulin increased ATP synthase activity in acute and decreased it in chronic treatment. ATPase inhibitory factor protein expression was increased in all treated groups. Confocal microscopy showed that key mitochondrial proteins colocalize differently in different mitochondria within a single brown adipocyte, indicating mitochondrial mosaicism. These results suggest that insulin modulates the bioenergetic and thermogenic capacity of rat brown adipocytes in vivo by modulating mitochondrial mosaicism.

Effects of different maternal feeding strategies from day 1 to day 85 of gestation on glucose tolerance and muscle development in both low and normal birth weight piglets

BACKGROUND

Maternal nutrition during gestation plays a vital role in fetal development. The effects of different maternal feeding strategies from day 1 to day 85 of gestation on glucose tolerance and muscle development in low and normal birth weight offspring were investigated by using 80 gilts randomly allotted to T1 and T2 groups and treated respectively with a gradual-increase (T1) and a convex transition (T2) feeding strategy, with no difference in total feed intake.

RESULTS

T2 group was seen to have a higher percentage of piglets with birth weight less than 500 g, while T1 group was shown to have a higher percentage of piglets with birth weight over 700 g. Meanwhile, for both low and normal birth weight piglets, T1 group was higher than T2 group in terms of muscle free amino acid concentration, mRNA expression levels of muscle growth-related factors, relative muscle fiber number and cross-sectional area. We must emphasize that the T2 group was shown to improve glucose tolerance, slow-twitch muscle fiber protein levels, and muscle mitochondrial function only in low birth weight piglets.

CONCLUSION

The convex transition feeding strategy can decrease the percentage of piglets with birth weight over 700 g, while improving glucose tolerance, slow-twitch muscle fiber protein levels, and muscle mitochondrial function in low birth weight piglets. Our findings provide new evidence for the potential importance of nutritional strategies during gestation, especially for improving the glucose tolerance and muscle development of low birth weight neonatal. © 2020 Society of Chemical Industry.

Insights Into Mechanisms of GDF15 and Receptor GFRAL: Therapeutic Targets

Growth and differentiation factor 15 (GDF15) belongs to the transforming growth factor-β (TGF-β) superfamily proteins. GDF15 acts as an inflammatory marker, and it plays a role in pathogenesis of tumors, ischemic diseases, metabolic disorders, and neurodegenerative processes. GDF15 is not normally expressed in the tissue; it is prominently induced following 'injury'. GDF15 functions are critical for the regulation of endothelial adaptations after vascular damage. Recently, four research groups simultaneously identified glial-derived neurotrophic factor (GDNF)-family receptor α-like (GFRAL) in the brain, an orphan receptor as the receptor for GDF15, signaling through the coreceptor RET. In this article, new aspects of the biology of GDF15 and receptor GFRAL, and their relationship with various pathologies, are commented on.

Overview of Atypical Diabetes

Although type 1 diabetes mellitus and, to a lesser extent, type 2 diabetes mellitus, are the prevailing forms of diabetes in youth, atypical forms of diabetes are not uncommon and may require etiology-specific therapies. By some estimates, up to 6.5% of children with diabetes have monogenic forms. Mitochondrial diabetes and cystic fibrosis related diabetes are less common but often noted in the underlying disease. Atypical diabetes should be considered in patients with a known disorder associated with diabetes, aged less than 25 years with nonautoimmune diabetes and without typical characteristics of type 2 diabetes mellitus, and/or with comorbidities associated with atypical diabetes.

Loss of Muscle Carnitine Palmitoyltransferase 2 Prevents Diet-Induced Obesity and Insulin Resistance despite Long-Chain Acylcarnitine Accumulation

To assess the effects of acylcarnitine accumulation on muscle insulin sensitivity, a model of muscle acylcarnitine accumulation was generated by deleting carnitine palmitoyltransferase 2 (CPT2) specifically from skeletal muscle (Cpt2^Sk-/- mice). CPT2 is an irreplaceable enzyme for mitochondrial long-chain fatty acid oxidation, converting matrix acylcarnitines to acyl-CoAs. Compared with controls, Cpt2^Sk-/- muscles do not accumulate anabolic lipids but do accumulate ∼22-fold more long-chain acylcarnitines. High-fat-fed Cpt2^Sk-/- mice resist weight gain, adiposity, glucose intolerance, insulin resistance, and impairments in insulin-induced Akt phosphorylation. Obesity resistance of Cpt2^Sk-/- mice could be attributed to increases in lipid excretion via feces, GFD15 production, and energy expenditure. L-carnitine supplement intervention lowers acylcarnitines and improves insulin sensitivity independent of muscle mitochondrial fatty acid oxidative capacity. The loss of muscle CPT2 results in a high degree of long-chain acylcarnitine accumulation, simultaneously protecting against diet-induced obesity and insulin resistance.

Diabetes Mellitus, Mitochondrial Dysfunction and Ca2+-Dependent Permeability Transition Pore

Diabetes mellitus is one of the most common metabolic diseases in the developed world, and is associated either with the impaired secretion of insulin or with the resistance of cells to the actions of this hormone (type I and type II diabetes, respectively). In both cases, a common pathological change is an increase in blood glucose—hyperglycemia, which eventually can lead to serious damage to the organs and tissues of the organism. Mitochondria are one of the main targets of diabetes at the intracellular level. This review is dedicated to the analysis of recent data regarding the role of mitochondrial dysfunction in the development of diabetes mellitus. Specific areas of focus include the involvement of mitochondrial calcium transport systems and a pathophysiological phenomenon called the permeability transition pore in the pathogenesis of diabetes mellitus. The important contribution of these systems and their potential relevance as therapeutic targets in the pathology are discussed.

GDF15: A Hormone Conveying Somatic Distress to the Brain

GDF15 has recently gained scientific and translational prominence with the discovery that its receptor is a GFRAL-RET heterodimer of which GFRAL is expressed solely in the hindbrain. Activation of this receptor results in reduced food intake and loss of body weight and is perceived and recalled by animals as aversive. This information encourages a revised interpretation of the large body of previous research on the protein. GDF15 can be secreted by a wide variety of cell types in response to a broad range of stressors. We propose that central sensing of GDF15 via GFRAL-RET activation results in behaviors that facilitate the reduction of exposure to a noxious stimulus. The human trophoblast appears to have hijacked this signal, producing large amounts of GDF15 from early pregnancy. We speculate that this encourages avoidance of potential teratogens in pregnancy. Circulating GDF15 levels are elevated in a range of human disease states, including various forms of cachexia, and GDF15-GFRAL antagonism is emerging as a therapeutic strategy for anorexia/cachexia syndromes. Metformin elevates circulating GDF15 chronically in humans and the weight loss caused by this drug appears to be dependent on the rise in GDF15. This supports the concept that chronic activation of the GDF15-GFRAL axis has efficacy as an antiobesity agent. In this review, we examine the science of GDF15 since its identification in 1997 with our interpretation of this body of work now being assisted by a clear understanding of its highly selective central site of action.

Mitochondrial Fatty Acid β-Oxidation Inhibition Promotes Glucose Utilization and Protein Deposition through Energy Homeostasis Remodeling in Fish

BACKGROUND

Fish cannot use carbohydrate efficiently and instead utilize protein for energy supply, thus limiting dietary protein storage. Protein deposition is dependent on protein turnover balance, which correlates tightly with cellular energy homeostasis. Mitochondrial fatty acid β-oxidation (FAO) plays a crucial role in energy metabolism. However, the effect of remodeled energy homeostasis caused by inhibited mitochondrial FAO on protein deposition in fish has not been intensively studied.

OBJECTIVES

This study aimed to identify the regulatory role of mitochondrial FAO in energy homeostasis maintenance and protein deposition by studying lipid, glucose, and protein metabolism in fish.

METHODS

Carnitine-depleted male Nile tilapia (initial weight: 4.29 ± 0.12 g; 3 mo old) were established by feeding them with mildronate diets (1000 mg/kg/d) for 6 wk. Zebrafish deficient in the carnitine palmitoyltransferase 1b gene (cpt1b) were produced by using CRISPR/Cas9 gene-editing technology, and their males (154 ± 3.52 mg; 3 mo old) were used for experiments. Normal Nile tilapia and wildtype zebrafish were used as controls. We assessed nutrient metabolism and energy homeostasis-related biochemical and molecular parameters, and performed 14C-labeled nutrient tracking and transcriptomic analyses.

RESULTS

The mitochondrial FAO decreased by 33.1-88.9% (liver) and 55.6-68.8% (muscle) in carnitine-depleted Nile tilapia and cpt1b-deficient zebrafish compared with their controls (P < 0.05). Notably, glucose oxidation and muscle protein deposition increased by 20.5-24.4% and 6.40-8.54%, respectively, in the 2 fish models compared with their corresponding controls (P < 0.05). Accordingly, the adenosine 5'-monophosphate-activated protein kinase/protein kinase B-mechanistic target of rapamycin (AMPK/AKT-mTOR) signaling was significantly activated in the 2 fish models with inhibited mitochondrial FAO (P < 0.05).

CONCLUSIONS

These data show that inhibited mitochondrial FAO in fish induces energy homeostasis remodeling and enhances glucose utilization and protein deposition. Therefore, fish with inhibited mitochondrial FAO could have high potential to utilize carbohydrate. Our results demonstrate a potentially new approach for increasing protein deposition through energy homeostasis regulation in cultured animals.

The Role of Growth Differentiation Factor 15 in Energy Metabolism

Growth differentiation factor 15 (GDF15) is receiving great interest beyond its role as an aging and disease-related biomarker. Recent discovery of its receptor, glial cell line-derived neurotrophic factor (GDNF) family receptor α-like (GFRAL), suggests a central role in appetite regulation. However, there is also considerable evidence that GDF15 may have peripheral activity through an as-of-yet undiscovered mode of action. This raises the question as to whether increased GDF15 induction during pathophysiologic conditions also suppresses appetite. The present review will briefly introduce the discovery of GDF15 and describe the different contexts under which GDF15 is induced, focusing on its induction during mitochondrial dysfunction. We will further discuss the metabolic role of GDF15 under various pathophysiological conditions and conclude with possible therapeutic applications.

Interplay between NADH oxidation by complex I, glutathione redox state and sirtuin-3, and its role in the development of insulin resistance

Metabolic diseases are characterized by high NADH/NAD⁺ ratios due to excessive electron supply, causing defective mitochondrial function and impaired sirtuin-3 (SIRT-3) activity, the latter driving to oxidative stress and altered fatty acid β-oxidation. NADH is oxidized by the complex I in the electron transport chain, thereby factors inhibiting complex I like acetylation, cardiolipin peroxidation, and glutathionylation by low GSH/GSSG ratios affects SIRT3 function by increasing the NADH/NAD⁺ ratio. In this review, we summarized the evidence supporting a role of the above events in the development of insulin resistance, which is relevant in the pathogenesis of obesity and diabetes. We propose that maintenance of proper NADH/NAD⁺ and GSH/GSSG ratios are central to ameliorate insulin resistance, as alterations in these redox couples lead to complex I dysfunction, disruption of SIRT-3 activity, ROS production and impaired β-oxidation, the latter two being key effectors of insulin resistance.

Mitochondrial Dysfunction, Insulin Resistance, and Potential Genetic Implications

Insulin resistance (IR) is fundamental to the development of type 2 diabetes (T2D) and is present in most prediabetic (preDM) individuals. Insulin resistance has both heritable and environmental determinants centered on energy storage and metabolism. Recent insights from human genetic studies, coupled with comprehensive in vivo and ex vivo metabolic studies in humans and rodents, have highlighted the critical role of reduced mitochondrial function as a predisposing condition for ectopic lipid deposition and IR. These studies support the hypothesis that reduced mitochondrial function, particularly in insulin-responsive tissues such as skeletal muscle, white adipose tissue, and the liver, is inextricably linked to tissue and whole body IR through the effects on cellular energy balance. Here we discuss these findings as well as address potential mechanisms that serve as the nexus between mitochondrial malfunction and IR.

Conditional knockout of pyruvate dehydrogenase in mouse pancreatic β‑cells causes morphological and functional changes

Diabetes mellitus is a metabolic disorder predominantly caused by the dysfunction of pancreatic β-cells. This dysfunction is partly caused by the dysregulation of pyruvate dehydrogenase (PDH), which acts as an important mediator of pyruvate oxidation after glycolysis and fuels the tricarboxylic acid cycle. Previous studies have reported decreased PDH expression in rodent models and humans with type 2 diabetes mellitus (T2DM), suggesting that PDH may play an important role in the development of T2DM. However, the mechanism by which PDH affects insulin secretion and β-cell development is poorly understood. Using immunofluorescence staining, the present study found that the expression of pyruvate dehydrogenase E1-α subunit (PDHA1; encoded by the PDHA1 gene) in the islets of type 2 diabetic mice (db/db mice) was lower than in wild-type mice, which indicated the possible association between PDHA1and diabetes. To further understand this mechanism, an inducible, islet-specific PDHA1 knockout mouse (βKO) model was established. The phenotype was authenticated, and the blood glucose levels and islet function between the βKO and control mice were compared. Though no changes were found in food intake, development status, fasting blood glucose or weight between the groups, the level of insulin secretion at 30 min after glucose injection in the βKO group was significantly lower compared with the control group. Furthermore, the performed of the βKO mice on the intraperitoneal glucose tolerance test was visibly impaired when compared with the control mice. Pancreatic tissues were collected for hematoxylin and eosin staining, immunohistochemical and confocal laser-scanning microscopy analysis. Examination of the islets from the βKO mouse model indicated that abolishing the expression of PDH caused a compensatory islet enlargement and impaired insulin secretion.

Muscle-derived GDF15 drives diurnal anorexia and systemic metabolic remodeling during mitochondrial stress

Mitochondrial dysfunction promotes metabolic stress responses in a cell-autonomous as well as organismal manner. The wasting hormone growth differentiation factor 15 (GDF15) is recognized as a biomarker of mitochondrial disorders, but its pathophysiological function remains elusive. To test the hypothesis that GDF15 is fundamental to the metabolic stress response during mitochondrial dysfunction, we investigated transgenic mice (Ucp1-TG) with compromised muscle-specific mitochondrial OXPHOS capacity via respiratory uncoupling. Ucp1-TG mice show a skeletal muscle-specific induction and diurnal variation of GDF15 as a myokine. Remarkably, genetic loss of GDF15 in Ucp1-TG mice does not affect muscle wasting or transcriptional cell-autonomous stress response but promotes a progressive increase in body fat mass. Furthermore, muscle mitochondrial stress-induced systemic metabolic flexibility, insulin sensitivity, and white adipose tissue browning are fully abolished in the absence of GDF15. Mechanistically, we uncovered a GDF15-dependent daytime-restricted anorexia, whereas GDF15 is unable to suppress food intake at night. Altogether, our evidence suggests a novel diurnal action and key pathophysiological role of mitochondrial stress-induced GDF15 in the regulation of systemic energy metabolism.

The ADP/ATP translocase drives mitophagy independent of nucleotide exchange

Mitochondrial homeostasis depends on mitophagy, the programmed degradation of mitochondria. Only a few proteins are known to participate in mitophagy. Here we develop a multidimensional CRISPR-Cas9 genetic screen, using multiple mitophagy reporter systems and pro-mitophagy triggers, and identify numerous components of parkin-dependent mitophagy¹. Unexpectedly, we find that the adenine nucleotide translocator (ANT) complex is required for mitophagy in several cell types. Whereas pharmacological inhibition of ANT-mediated ADP/ATP exchange promotes mitophagy, genetic ablation of ANT paradoxically suppresses mitophagy. Notably, ANT promotes mitophagy independently of its nucleotide translocase catalytic activity. Instead, the ANT complex is required for inhibition of the presequence translocase TIM23, which leads to stabilization of PINK1, in response to bioenergetic collapse. ANT modulates TIM23 indirectly via interaction with TIM44, which regulates peptide import through TIM23². Mice that lack ANT1 show blunted mitophagy and consequent profound accumulation of aberrant mitochondria. Disease-causing human mutations in ANT1 abrogate binding to TIM44 and TIM23 and inhibit mitophagy. Together, our findings show that ANT is an essential and fundamental mediator of mitophagy in health and disease.

Mitochondrial Uncoupling: A Key Controller of Biological Processes in Physiology and Diseases

Mitochondrial uncoupling can be defined as a dissociation between mitochondrial membrane potential generation and its use for mitochondria-dependent ATP synthesis. Although this process was originally considered a mitochondrial dysfunction, the identification of UCP-1 as an endogenous physiological uncoupling protein suggests that the process could be involved in many other biological processes. In this review, we first compare the mitochondrial uncoupling agents available in term of mechanistic and non-specific effects. Proteins regulating mitochondrial uncoupling, as well as chemical compounds with uncoupling properties are discussed. Second, we summarize the most recent findings linking mitochondrial uncoupling and other cellular or biological processes, such as bulk and specific autophagy, reactive oxygen species production, protein secretion, cell death, physical exercise, metabolic adaptations in adipose tissue, and cell signaling. Finally, we show how mitochondrial uncoupling could be used to treat several human diseases, such as obesity, cardiovascular diseases, or neurological disorders.

H+ transport is an integral function of the mitochondrial ADP/ATP carrier

The mitochondrial ADP/ATP carrier (AAC) is a major transport protein of the inner mitochondrial membrane. It exchanges mitochondrial ATP for cytosolic ADP and controls cellular production of ATP. In addition, it has been proposed that AAC mediates mitochondrial uncoupling, but it has proven difficult to demonstrate this function or to elucidate its mechanisms. Here we record AAC currents directly from inner mitochondrial membranes from various mouse tissues and identify two distinct transport modes: ADP/ATP exchange and H⁺ transport. The AAC-mediated H⁺ current requires free fatty acids and resembles the H⁺ leak via the thermogenic uncoupling protein 1 found in brown fat. The ADP/ATP exchange via AAC negatively regulates the H⁺ leak, but does not completely inhibit it. This suggests that the H⁺ leak and mitochondrial uncoupling could be dynamically controlled by cellular ATP demand and the rate of ADP/ATP exchange. By mediating two distinct transport modes, ADP/ATP exchange and H⁺ leak, AAC connects coupled (ATP production) and uncoupled (thermogenesis) energy conversion in mitochondria.

Tissue-specific dysregulation of mitochondrial respiratory capacity and coupling control in colon-26 tumor-induced cachexia

In addition to skeletal muscle dysfunction, cancer cachexia is a systemic disease involving remodeling of nonmuscle organs such as adipose and liver. Impairment of mitochondrial function is associated with multiple chronic diseases. The tissue-specific control of mitochondrial function in cancer cachexia is not well defined. This study determined mitochondrial respiratory capacity and coupling control of skeletal muscle, white adipose tissue (WAT), and liver in colon-26 (C26) tumor-induced cachexia. Tissues were collected from PBS-injected weight-stable mice, C26 weight-stable mice and C26 mice with moderate (10% weight loss) and severe cachexia (20% weight loss). The respiratory control ratio [(RCR) an index of oxidative phosphorylation (OXPHOS) coupling efficiency] was low in WAT during the induction of cachexia because of high nonphosphorylating LEAK respiration. Liver RCR was low in C26 weight-stable and moderately cachexic mice because of reduced OXPHOS. Liver RCR was further reduced with severe cachexia, where Ant2 but not Ucp2 expression was increased. Ant2 was inversely correlated with RCR in the liver (r = -0.547, P < 0.01). Liver cardiolipin increased in moderate and severe cachexia, suggesting this early event may also contribute to mitochondrial uncoupling. Impaired skeletal muscle mitochondrial respiration occurred predominantly in severe cachexia, at complex I. These findings suggest that mitochondrial function is subject to tissue-specific control during cancer cachexia, whereby remodeling in WAT and liver arise early and may contribute to altered energy balance, followed by impaired skeletal muscle respiration. We highlight an under-recognized role of liver and WAT mitochondrial function in cancer cachexia and suggest mitochondrial function of multiple tissues to be therapeutic targets.

Mitochondrial oxidative stress impairs contractile function but paradoxically increases muscle mass via fibre branching

BACKGROUND

Excess reactive oxygen species (ROS) and muscle weakness occur in parallel in multiple pathological conditions. However, the causative role of skeletal muscle mitochondrial ROS (mtROS) on neuromuscular junction (NMJ) morphology and function and muscle weakness has not been directly investigated.

METHODS

We generated mice lacking skeletal muscle-specific manganese-superoxide dismutase (mSod2KO) to increase mtROS using a cre-Lox approach driven by human skeletal actin. We determined primary functional parameters of skeletal muscle mitochondrial function (respiration, ROS, and calcium retention capacity) using permeabilized muscle fibres and isolated muscle mitochondria. We assessed contractile properties of isolated skeletal muscle using in situ and in vitro preparations and whole lumbrical muscles to elucidate the mechanisms of contractile dysfunction.

RESULTS

The mSod2KO mice, contrary to our prediction, exhibit a 10-15% increase in muscle mass associated with an ~50% increase in central nuclei and ~35% increase in branched fibres (P < 0.05). Despite the increase in muscle mass of gastrocnemius and quadriceps, in situ sciatic nerve-stimulated isometric maximum-specific force (N/cm² ), force per cross-sectional area, is impaired by ~60% and associated with increased NMJ fragmentation and size by ~40% (P < 0.05). Intrinsic alterations of components of the contractile machinery show elevated markers of oxidative stress, for example, lipid peroxidation is increased by ~100%, oxidized glutathione is elevated by ~50%, and oxidative modifications of myofibrillar proteins are increased by ~30% (P < 0.05). We also find an approximate 20% decrease in the intracellular calcium transient that is associated with specific force deficit. Excess superoxide generation from the mitochondrial complexes causes a deficiency of succinate dehydrogenase and reduced complex-II-mediated respiration and adenosine triphosphate generation rates leading to severe exercise intolerance (~10 min vs. ~2 h in wild type, P < 0.05).

CONCLUSIONS

Increased skeletal muscle mtROS is sufficient to elicit NMJ disruption and contractile abnormalities, but not muscle atrophy, suggesting new roles for mitochondrial oxidative stress in maintenance of muscle mass through increased fibre branching.

Induced in vivo knockdown of the Brca1 gene in skeletal muscle results in skeletal muscle weakness

KEY POINTS

Breast cancer 1 early onset gene codes for the DNA repair enzyme, breast cancer type 1 susceptibility protein (BRCA1). The gene is prone to mutations that cause a loss of protein function. BRCA1/Brca1 has recently been found to regulate several cellular pathways beyond DNA repair and is expressed in skeletal muscle. Skeletal muscle specific knockout of Brca1 in mice caused a loss of muscle quality, identifiable by reductions in muscle force production and mitochondrial respiratory capacity. Loss of muscle quality was associated with a shift in muscle phenotype and an accumulation of mitochondrial DNA mutations. These results demonstrate that BRCA1 is necessary for skeletal muscle function and that increased mitochondrial DNA mutations may represent a potential underlying mechanism.

ABSTRACT

Recent evidence suggests that the breast cancer 1 early onset gene (BRCA1) influences numerous peripheral tissues, including skeletal muscle. The present study aimed to determine whether induced-loss of the breast cancer type 1 susceptibility protein (Brca1) alters skeletal muscle function. We induced genetic ablation of exon 11 in the Brca1 gene specifically in the skeletal muscle of adult mice to generate skeletal muscle-specific Brca1 homozygote knockout (Brca1KOsmi ) mice. Brca1KOsmi exhibited kyphosis and decreased maximal isometric force in limb muscles compared to age-matched wild-type mice. Brca1KOsmi skeletal muscle shifted toward an oxidative muscle fibre type and, in parallel, increased myofibre size and reduced capillary numbers. Unexpectedly, myofibre bundle mitochondrial respiration was reduced, whereas contraction-induced lactate production was elevated in Brca1KOsmi muscle. Brca1KOsmi mice accumulated mitochondrial DNA mutations and exhibited an altered mitochondrial morphology characterized by distorted and enlarged mitochondria, and these were more susceptible to swelling. In summary, skeletal muscle-specific loss of Brca1 leads to a myopathy and mitochondriopathy characterized by reductions in skeletal muscle quality and a consequent kyphosis. Given the substantial impact of BRCA1 mutations on cancer development risk in humans, a parallel loss of BRCA1 function in patient skeletal muscle cells would potentially result in implications for human health.

The impact of a short-term high-fat diet on mitochondrial respiration, reactive oxygen species production, and dynamics in oxidative and glycolytic skeletal muscles of young rats

Multiple aspects of mitochondrial function and dynamics remain poorly studied in the skeletal muscle of pediatric models in response to a short-term high-fat diet (HFD). This study investigated the impact of a short-term HFD on mitochondrial function and dynamics in the oxidative soleus (SOL) and glycolytic extensor digitorum longus (EDL) muscles in young rats. Young male Wistar rats were submitted to either HFD or normal chow (NCD) diets for 14 days. Permeabilized myofibers from SOL and EDL were prepared to assess mitochondrial respiration and reactive oxygen species (ROS) production. The expression and content of protein involved in mitochondrial metabolism and dynamics (fusion/fission) were also quantified. While no effects of HFD was observed on mitochondrial respiration when classical complex I and II substrates were used, both SOL and EDL of rats submitted to a HFD displayed higher basal and ADP-stimulated respiration rates when Malate + Palmitoyl-L-carnitine were used as substrates. HFD did not alter ROS production and markers of mitochondrial content. The expression of CPT1b was significantly increased in SOL and EDL of HFD rats. Although the expression of UCP3 was increased in SOL and EDL muscles from HFD rats, mitochondrial coupling efficiency was not altered. In SOL of HFD rats, the transcript levels of Mfn2 and Fis1 were significantly upregulated. The expression and content of proteins regulating mitochondrial dynamics was not modulated by HFD in the EDL. Finally, DRP1 protein content was increased by over fourfold in the SOL of HFD rats. Taken altogether, our findings show that exposing young animals to short-term HFD results in an increased capacity of skeletal muscle mitochondria to oxidize fatty acids, without altering ROS production, coupling efficiency, and mitochondrial content. Our results also highlight that the impact of HFD on mitochondrial dynamics appears to be muscle specific.