Electron transfer flavoprotein and its role in mitochondrial energy metabolism in health and disease

Unveiling the molecular basis of paracetamol-induced hepatotoxicity: Interaction of N-acetyl-p-benzoquinone imine with mitochondrial succinate dehydrogenase

Background and aim

N-acetyl-p-benzoquinoneimine (NAPQI), a toxic byproduct of paracetamol (Acetaminophen, APAP), can accumulate and cause liver damage by depleting glutathione and forming protein adducts in the mitochondria. These adducts disrupt the respiratory chain, increasing superoxide production and reducing ATP. The goal of this study was to provide computational proof that succinate dehydrogenase (SDH), a subunit of complex II in the mitochondrial respiratory chain, is a favorable binding partner for NAPQI in this regard.

Method

Molecular docking, molecular dynamics simulation, protein-protein interaction networks (PPI), and KEGG metabolic pathway analysis were employed to identify binding characteristics, interaction partners, and their associations with metabolic pathways. A lipid membrane was added to the experimental apparatus to mimic the natural cellular environment of SDH. This modification made it possible to develop a context for investigating the role and interactions of SDH within a cellular ecosystem that was more realistic and biologically relevant.

Result

The molecular binding affinity score for APAP and NAPQI with SDH was predicted -6.5 and -6.7 kcal/mol, respectively. Furthermore, RMSD, RMSF, and Rog from the molecular dynamics simulations study revealed that NAPQI has slightly higher stability and compactness compared to APAP at 100 ns timeframe with mitochondrial SDH.

Conclusion

This study serves to predict the mechanistic process of paracetamol toxicity by using different computational approaches. In addition, this study will provide information about the drug target against APAP hepatotoxicity.

SWATH-MS reveals that bisphenol A and its analogs regulate pathways leading to disruption in insulin signaling and fatty acid metabolism

Bisphenol A (BPA) is an endocrine-disrupting chemical (EDC), associated with obesity and insulin resistance. The FDA prohibited the use of BPA-based polycarbonate resins in infant formula packaging; thus, its analogs, viz. Bisphenol S (BPS) and Bisphenol F (BPF) were considered alternatives in epoxy resins, plastics, and food cans. As these analogs might evoke a similar response, we investigated the role of Bisphenols (BPA, BPF, and BPS), on insulin signaling in CHO-HIRc-myc-GLUT4eGFP cells at environmentally relevant concentrations of 2 nM and 200 nM. Insulin signaling demonstrated that Bisphenols reduced phosphorylation of IR and AKT2, GLUT4 translocation, and glucose uptake. This was accompanied by increased oxidative stress. Furthermore, SWATH-MS-based proteomics of 3T3-L1 cells demonstrated that Bisphenol-treated cells regulate proteins in insulin resistance, adipogenesis, and fatty acid metabolism pathways differently. All three Bisphenols induced differentially expressed proteins enriched similar pathways, although their abundance differed for each Bisphenol. This might be due to their varying toxicity level, structural differences, and estrogen-mimetic activity. This study has important implications in addressing health concerns related to EDCs. Given that the analogs of BPA are considered alternatives to BPA, the findings of this study suggest they are equally potent in altering fatty acid metabolism and inducing insulin resistance.

Electron-transferring flavoprotein and its dehydrogenase contributed to growth development and virulence in Beauveria bassiana

Electron-transferring flavoprotein (Etf) and its dehydrogenase (Etfdh) are integral components of the electron transport chain in mitochondria. In this study, we characterize two putative etf genes (Bbetfa and Bbetfb) and their dehydrogenase gene Bbetfdh in the entomopathogenic fungus Beauveria bassiana. Individual deletion of these genes caused a significant reduction in vegetative growth, conidiation, and delayed conidial germination. Lack of these genes also led to abnormal metabolism of fatty acid and increasing lipid body accumulation. Furthermore, the virulence of Bbetfs and Bbetfdh deletion mutants was severely impaired due to decreasing infection structure formation. Additionally, all deletion strains showed reduced ATP synthesis compared to the wild-type strain. Taken together, Bbetfa and Bbetfb, along with Bbetfdh, play principal roles in fungal vegetative growth, conidiation, conidial germination, and pathogenicity of B. bassiana due to their essential functions in fatty acid metabolism.

Liver sinusoidal endothelial cells rely on oxidative phosphorylation but avoid processing long-chain fatty acids in their mitochondria

BACKGROUND

It is generally accepted that endothelial cells (ECs), primarily rely on glycolysis for ATP production, despite having functional mitochondria. However, it is also known that ECs are heterogeneous, and their phenotypic features depend on the vascular bed. Emerging evidence suggests that liver sinusoidal ECs (LSECs), located in the metabolically rich environment of the liver, show high metabolic plasticity. However, the substrate preference for energy metabolism in LSECs remains unclear.

METHODS

Investigations were conducted in primary murine LSECs in vitro using the Seahorse XF technique for functional bioenergetic assays, untargeted mass spectrometry-based proteomics to analyse the LSEC proteome involved in energy metabolism pathways, liquid chromatography-tandem mass spectrometry-based analysis of acyl-carnitine species and Raman spectroscopy imaging to track intracellular palmitic acid.

RESULTS

This study comprehensively characterized the energy metabolism of LSECs, which were found to depend on oxidative phosphorylation, efficiently fuelled by glucose-derived pyruvate, short- and medium-chain fatty acids and glutamine. Furthermore, despite its high availability, palmitic acid was not directly oxidized in LSEC mitochondria, as evidenced by the acylcarnitine profile and etomoxir's lack of effect on oxygen consumption. However, together with L-carnitine, palmitic acid supported mitochondrial respiration, which is compatible with the chain-shortening role of peroxisomal β-oxidation of long-chain fatty acids before further degradation and energy generation in mitochondria.

CONCLUSIONS

LSECs show a unique bioenergetic profile of highly metabolically plastic ECs adapted to the liver environment. The functional reliance of LSECs on oxidative phosphorylation, which is not a typical feature of ECs, remains to be determined.

Installation of LYRM proteins in early eukaryotes to regulate the metabolic capacity of the emerging mitochondrion

Core mitochondrial processes such as the electron transport chain, protein translation and the formation of Fe-S clusters (ISC) are of prokaryotic origin and were present in the bacterial ancestor of mitochondria. In animal and fungal models, a family of small Leu-Tyr-Arg motif-containing proteins (LYRMs) uniformly regulates the function of mitochondrial complexes involved in these processes. The action of LYRMs is contingent upon their binding to the acylated form of acyl carrier protein (ACP). This study demonstrates that LYRMs are structurally and evolutionarily related proteins characterized by a core triplet of α-helices. Their widespread distribution across eukaryotes suggests that 12 specialized LYRMs were likely present in the last eukaryotic common ancestor to regulate the assembly and folding of the subunits that are conserved in bacteria but that lack LYRM homologues. The secondary reduction of mitochondria to anoxic environments has rendered the function of LYRMs and their interaction with acylated ACP dispensable. Consequently, these findings strongly suggest that early eukaryotes installed LYRMs in aerobic mitochondria as orchestrated switches, essential for regulating core metabolism and ATP production.

Mitochondrial morphology, distribution and activity during oocyte development

Mitochondria have a crucial role in cellular function and exhibit remarkable plasticity, adjusting both their structure and activity to meet the changing energy demands of a cell. Oocytes, female germ cells that become eggs, undergo unique transformations: the extended dormancy period, followed by substantial increase in cell size and subsequent maturation involving the segregation of genetic material for the next generation, present distinct metabolic challenges necessitating varied mitochondrial adaptations. Recent findings in dormant oocytes challenged the established respiratory complex hierarchies and underscored the extent of mitochondrial plasticity in long-lived oocytes. In this review, we discuss mitochondrial adaptations observed during oocyte development across three vertebrate species (Xenopus, mouse, and human), emphasising current knowledge, acknowledging limitations, and outlining future research directions.

Stealthy progression of type 2 diabetes mellitus due to impaired ketone production in an adult patient with multiple acyl-CoA dehydrogenase deficiency

Background

Multiple acyl-CoA dehydrogenase deficiency (MADD) is an inherited metabolic disorder caused by biallelic pathogenic variants in genes related to the flavoprotein complex. Dysfunction of the complex leads to impaired fatty acid oxidation and ketone body production which can cause hypoketotic hypoglycemia with prolonged fasting. Patients with fatty acid oxidation disorders (FAODs) such as MADD are treated primarily with a dietary regimen consisting of high-carbohydrate foods and avoidance of prolonged fasting. However, information on the long-term sequelae associated with this diet have not been accumulated. In general, high-carbohydrate diets can induce diseases such as type 2 diabetes mellitus (T2DM), although few patients with both MADD and T2DM have been reported.

Case

We present the case of a 32-year-old man with MADD who was on a high-carbohydrate diet for >30 years and exhibited symptoms resembling diabetic ketoacidosis. He presented with polydipsia, polyuria, and weight loss with a decrease in body mass index from 31 to 25 kg/m2 over 2 months. Laboratory tests revealed a HbA1c level of 13.9%; however, the patient did not show metabolic acidosis but only mild ketosis.

Discussion/conclusion

This report emphasizes the potential association between long-term adherence to high-carbohydrate dietary therapy and T2DM development. Moreover, this case underscores the difficulty of detecting diabetic ketosis in patients with FAODs such as MADD due to their inability to produce ketone bodies. These findings warrant further research of the long-term complications associated with this diet as well as warning of the potential progression of diabetes in patients with FAODs such as MADD.

The Impact of SLC2A8 RNA Interference on Glucose Uptake and the Transcriptome of Human Trophoblast Cells

While glucose is the primary fuel for fetal growth, the placenta utilizes the majority of glucose taken up from the maternal circulation. Of the facilitative glucose transporters in the placenta, SLC2A8 (GLUT8) is thought to primarily function as an intracellular glucose transporter; however, its function in trophoblast cells has not been determined. To gain insight into the function of SLC2A8 in the placenta, lentiviral-mediated RNA interference (RNAi) was performed in the human first-trimester trophoblast cell line ACH-3P. Non-targeting sequence controls (NTS RNAi; n = 4) and SLC2A8 RNAi (n = 4) infected ACH-3P cells were compared. A 79% reduction in SLC2A8 mRNA concentration was associated with an 11% reduction (p ≤ 0.05) in ACH-3P glucose uptake. NTS RNAi and SLC2A8 RNAi ACH-3P mRNA were subjected to RNAseq, identifying 1525 transcripts that were differentially expressed (|log2FC| > 1 and adjusted p-value < 0.05), with 273 transcripts derived from protein-coding genes, and the change in 10 of these mRNAs was validated by real-time qPCR. Additionally, there were 147 differentially expressed long non-coding RNAs. Functional analyses revealed differentially expressed genes involved in various metabolic pathways associated with cellular respiration, oxidative phosphorylation, and ATP synthesis. Collectively, these data indicate that SLC2A8 deficiency may impact placental uptake of glucose, but that its likely primary function in trophoblast cells is to support cellular respiration. Since the placenta oxidizes the majority of the glucose it takes up to support its own metabolic needs, impairment of SLC2A8 function could set the stage for functional placental insufficiency.

Quantitative measurements of reactive oxygen species partitioning in electron transfer flavoenzyme magnetic field sensing

Biological magnetic field sensing that gives rise to physiological responses is of considerable importance in quantum biology. The radical pair mechanism (RPM) is a fundamental quantum process that can explain some of the observed biological magnetic effects. In magnetically sensitive radical pair (RP) reactions, coherent spin dynamics between singlet and triplet pairs are modulated by weak magnetic fields. The resulting singlet and triplet reaction products lead to distinct biological signaling channels and cellular outcomes. A prevalent RP in biology is between flavin semiquinone and superoxide (O2 •-) in the biological activation of molecular oxygen. This RP can result in a partitioning of reactive oxygen species (ROS) products to form either O2 •- or hydrogen peroxide (H2O2). Here, we examine magnetic sensing of recombinant human electron transfer flavoenzyme (ETF) reoxidation by selectively measuring O2 •- and H2O2 product distributions. ROS partitioning was observed between two static magnetic fields at 20 nT and 50 μT, with a 13% decrease in H2O2 singlet products and a 10% increase in O2 •- triplet products relative to 50 µT. RPM product yields were calculated for a realistic flavin/superoxide RP across the range of static magnetic fields, in agreement with experimental results. For a triplet born RP, the RPM also predicts about three times more O2 •- than H2O2, with experimental results exhibiting about four time more O2 •- produced by ETF. The method presented here illustrates the potential of a novel magnetic flavoprotein biological sensor that is directly linked to mitochondria bioenergetics and can be used as a target to study cell physiology.

Techno-Functional Assessment of Riboflavin-Enriched Yogurt-Based Fermented Milk Prepared by Supplementing Riboflavin-Producing Probiotic Strains of Lactiplantibacillus plantarum

Vitamin enrichment in fermented dairy products through the intervention of vitamin-producing probiotic strains during fermentation is a novel approach in the field of probioceuticals. In this study, riboflavin-enriched yogurt-based fermented milk was prepared by mixing 1% (v/v) riboflavin-producing strain [1.2 × 108 CFU/mL of Lactiplantibacillus plantarum MTCC 25432 or L. plantarum MTCC 25433 or L. plantarum MTCC 25434] with 2% (v/v) traditional yogurt cultures [Streptococcus thermophilus NCDC 295 and L. delbrueckii subsp. bulgaricus NCDC 293; each of 1.3 × 107 CFU/mL]. The yogurt-based fermented milk prepared with traditional yogurt cultures (2%, v/v) was served as a control. The prepared yogurt-based fermented milk samples were analyzed and compared for riboflavin content, antimicrobial activity, physicochemical, and functional properties. As a result, the yogurt-based fermented milk prepared with L. plantarum MTCC 25432 produced a significantly higher amount of riboflavin (2.49 mg/L) as compared with MTCC 25433 (2.33 mg/L), MTCC 25434 (2.14 mg/L), and control (1.70 mg/L). The probiotic supplementation to yogurt cultures maintained the pH and titratable acidity in the range of 4.1-4.4 and 1.0-1.05% (lactic acid/100 mL), as recommended by Indian yogurt standards. The rheological, texture, and antimicrobial properties of yogurt-based fermented milk were enhanced with the addition of riboflavin-producing probiotic strains. Moreover, all yogurt-based fermented milk samples prepared in this study were acceptable as per the sensory evolution scores. In conclusion, the use of riboflavin-producing L. plantarum strains along with standard yogurt cultures could be the best approach to enhancing riboflavin content in yogurt-based fermented milk and fulfilling the daily riboflavin requirement in humans.

Tempo and mode of gene expression evolution in the brain across primates

Primate evolution has led to a remarkable diversity of behavioral specializations and pronounced brain size variation among species (Barton, 2012; DeCasien and Higham, 2019; Powell et al., 2017). Gene expression provides a promising opportunity for studying the molecular basis of brain evolution, but it has been explored in very few primate species to date (e.g. Khaitovich et al., 2005; Khrameeva et al., 2020; Ma et al., 2022; Somel et al., 2009). To understand the landscape of gene expression evolution across the primate lineage, we generated and analyzed RNA-seq data from four brain regions in an unprecedented eighteen species. Here, we show a remarkable level of variation in gene expression among hominid species, including humans and chimpanzees, despite their relatively recent divergence time from other primates. We found that individual genes display a wide range of expression dynamics across evolutionary time reflective of the diverse selection pressures acting on genes within primate brain tissue. Using our samples that represent a 190-fold difference in primate brain size, we identified genes with variation in expression most correlated with brain size. Our study extensively broadens the phylogenetic context of what is known about the molecular evolution of the brain across primates and identifies novel candidate genes for the study of genetic regulation of brain evolution.

Mitochondrial complex I inhibition triggers NAD+-independent glucose oxidation via successive NADPH formation, "futile" fatty acid cycling, and FADH2 oxidation

Inhibition of mitochondrial complex I (NADH dehydrogenase) is the primary mechanism of the antidiabetic drug metformin and various unrelated natural toxins. Complex I inhibition can also be induced by antidiabetic PPAR agonists, and it is elicited by methionine restriction, a nutritional intervention causing resistance to diabetes and obesity. Still, a comprehensible explanation to why complex I inhibition exerts antidiabetic properties and engenders metabolic inefficiency is missing. To evaluate this issue, we have systematically reanalyzed published transcriptomic datasets from MPP-treated neurons, metformin-treated hepatocytes, and methionine-restricted rats. We found that pathways leading to NADPH formation were widely induced, together with anabolic fatty acid biosynthesis, the latter appearing highly paradoxical in a state of mitochondrial impairment. However, concomitant induction of catabolic fatty acid oxidation indicated that complex I inhibition created a "futile" cycle of fatty acid synthesis and degradation, which was anatomically distributed between adipose tissue and liver in vivo. Cofactor balance analysis unveiled that such cycling would indeed be energetically futile (-3 ATP per acetyl-CoA), though it would not be redox-futile, as it would convert NADPH into respirable FADH2 without any net production of NADH. We conclude that inhibition of NADH dehydrogenase leads to a metabolic shift from glycolysis and the citric acid cycle (both generating NADH) towards the pentose phosphate pathway, whose product NADPH is translated 1:1 into FADH2 by fatty acid cycling. The diabetes-resistant phenotype following hepatic and intestinal complex I inhibition is attributed to FGF21- and GDF15-dependent fat hunger signaling, which remodels adipose tissue into a glucose-metabolizing organ.

Complex II ambiguities-FADH2 in the electron transfer system

The prevailing notion that reduced cofactors NADH and FADH2 transfer electrons from the tricarboxylic acid cycle to the mitochondrial electron transfer system creates ambiguities regarding respiratory Complex II (CII). CII is the only membrane-bound enzyme in the tricarboxylic acid cycle and is part of the electron transfer system of the mitochondrial inner membrane feeding electrons into the coenzyme Q-junction. The succinate dehydrogenase subunit SDHA of CII oxidizes succinate and reduces the covalently bound prosthetic group FAD to FADH2 in the canonical forward tricarboxylic acid cycle. However, several graphical representations of the electron transfer system depict FADH2 in the mitochondrial matrix as a substrate to be oxidized by CII. This leads to the false conclusion that FADH2 from the β-oxidation cycle in fatty acid oxidation feeds electrons into CII. In reality, dehydrogenases of fatty acid oxidation channel electrons to the Q-junction but not through CII. The ambiguities surrounding Complex II in the literature and educational resources call for quality control, to secure scientific standards in current communications of bioenergetics, and ultimately support adequate clinical applications. This review aims to raise awareness of the inherent ambiguity crisis, complementing efforts to address the well-acknowledged issues of credibility and reproducibility.

Transcriptomic analysis reveals mitochondrial pathways associated with distinct adolescent behavioral phenotypes and stress response

Adolescent individuals exhibit great variability in cortical dynamics and behavioral outcomes. The developing adolescent brain is highly sensitive to social experiences and environmental insults, influencing how personality traits emerge. A distinct pattern of mitochondrial gene expression in the prefrontal cortex (PFC) during adolescence underscores the essential role of mitochondria in brain maturation and the development of mental illnesses. Mitochondrial features in certain brain regions account for behavioral differences in adulthood. However, it remains unclear whether distinct adolescent behavioral phenotypes and the behavioral consequences of early adolescent stress exposure in rats are accompanied by changes in PFC mitochondria-related genes and mitochondria respiratory chain capacity. We performed a behavioral characterization during late adolescence (postnatal day, PND 47-50), including naïve animals and a group exposed to stress from PND 31-40 (10 days of footshock and 3 restraint sessions) by z-normalized data from three behavioral domains: anxiety (light-dark box tests), sociability (social interaction test) and cognition (novel-object recognition test). Employing principal component analysis, we identified three clusters: naïve with higher-behavioral z-score (HBZ), naïve with lower-behavioral z-score (LBZ), and stressed animals. Genome-wide transcriptional profiling unveiled differences in the expression of mitochondria-related genes in both naïve LBZ and stressed animals compared to naïve HBZ. Genes encoding subunits of oxidative phosphorylation complexes were significantly down-regulated in both naïve LBZ and stressed animals and positively correlated with behavioral z-score of phenotypes. Our network topology analysis of mitochondria-associated genes found Ndufa10 and Cox6a1 genes as central identifiers for naïve LBZ and stressed animals, respectively. Through high-resolution respirometry analysis, we found that both naïve LBZ and stressed animals exhibited a reduced prefrontal phosphorylation capacity and redox dysregulation. Our findings identify an association between mitochondrial features and distinct adolescent behavioral phenotypes while also underscoring the detrimental functional consequences of adolescent stress on the PFC.

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.

Proteomics reveals specific biological changes induced by the normothermic machine perfusion of donor kidneys with a significant up-regulation of Latexin

Renal normothermic machine perfusion (NMP) is an organ preservation method based on the circulation of a warm (35-37 °C) perfusion solution through the renal vasculature to deliver oxygen and nutrients. However, its biological effects on marginal kidneys are unclear. We therefore used mass spectrometry to determine the proteomic profile of kidney tissue and urine from eight organs reconditioned for 120 min using a Kidney Assist device. Biopsies were taken during the pre-implantation histological evaluation (T-1), at the start of back table preparation (T0), and after 60 and 120 min of perfusion (T60, T120). Urine samples were collected at T0 (urine produced in the first 15 min after the beginning of normothermic reperfusion), T30, T60 and T120. Multiple algorithms, support vector machine learning and partial least squares discriminant analysis were used to select the most discriminative proteins during NMP. Statistical analysis revealed the upregulation of 169 proteins and the downregulation of 196 during NMP. Machine learning algorithms identified the top 50 most discriminative proteins, five of which were concomitantly upregulated (LXN, ETFB, NUDT3, CYCS and UQCRC1) and six downregulated (CFHR3, C1S, CFI, KNG1, SERPINC1 and F9) in the kidney and urine after NMP. Latexin (LXN), an endogenous carboxypeptidase inhibitor, resulted the most-upregulated protein at T120, and this result was confirmed by ELISA. In addition, functional analysis revealed that the most strongly upregulated proteins were involved in the oxidative phosphorylation system and ATP synthesis, whereas the downregulated proteins represented the complement system and coagulation cascade. Our proteomic analysis demonstrated that even brief periods of NMP induce remarkable metabolic and biochemical changes in marginal organs, which supports the use of this promising technique in the clinic.

Proteomic Analysis of Skeletal Muscle and White Adipose Tissue after Aerobic Exercise Training in High Fat Diet Induced Obese Mice

Obesity is associated with excessive fat accumulation in adipose tissue and other organs, such as skeletal muscle, whereas aerobic exercise (AE) plays an important role in managing obesity through profound protein regulation. Our study aimed to investigate the impact of AE on proteomic changes in both the skeletal muscle and the epididymal fat pad (EFP) of high-fat-diet-induced obese mice. Bioinformatic analyses were performed on differentially regulated proteins using gene ontology enrichment analysis and ingenuity pathway analysis. Eight weeks of AE significantly reduced body weight, increased the serum FNDC5 level, and improved the homeostatic model assessment of insulin resistance. A high-fat diet caused alterations in a subset of proteins involved in the sirtuin signaling pathway and the production of reactive oxygen species in both skeletal muscle and EFP, leading to insulin resistance, mitochondrial dysfunction, and inflammation. On the other hand, AE upregulated skeletal muscle proteins (NDUFB5, NDUFS2, NDUFS7, ETFD, FRDA, and MKNK1) that enhance mitochondrial function and insulin sensitivity. Additionally, the upregulation of LDHC and PRKACA and the downregulation of CTBP1 in EFP can promote the browning of white adipose tissue with the involvement of FNDC5/irisin in the canonical pathway. Our study provides insights into AE-induced molecular responses and may help further develop exercise-mimicking therapeutic targets.

Free radicals: Relationship to Human Diseases and Potential Therapeutic applications

Reactive species are highly-reactive enzymatically, or non-enzymatically produced compounds with important roles in physiological and pathophysiological cellular processes. Although reactive species represent an extensively researched topic in biomedical sciences, many aspects of their roles and functions remain unclear. This review aims to systematically summarize findings regarding the biochemical characteristics of various types of reactive species and specify the localization and mechanisms of their production in cells. In addition, we discuss the specific roles of free radicals in cellular physiology, focusing on the current lines of research that aim to identify the reactive oxygen species-initiated cascades of reactions resulting in adaptive or pathological cellular responses. Finally, we present recent findings regarding the therapeutic modulations of intracellular levels of reactive oxygen species, which may have substantial significance in developing novel agents for treating several diseases.

PROT-ON: A structure-based detection of designer PROTein interface MutatiONs

The mutation-induced changes across protein-protein interfaces have often been observed to lead to severe diseases. Therefore, several computational tools have been developed to predict the impact of such mutations. Among these tools, FoldX and EvoEF1 stand out as fast and accurate alternatives. Expanding on the capabilities of these tools, we have developed the PROT-ON (PROTein-protein interface mutatiONs) framework, which aims at delivering the most critical protein interface mutations that can be used to design new protein binders. To realize this aim, PROT-ON takes the 3D coordinates of a protein dimer as an input. Then, it probes all possible interface mutations on the selected protein partner with EvoEF1 or FoldX. The calculated mutational energy landscape is statistically analyzed to find the most enriching and depleting mutations. Afterward, these extreme mutations are filtered out according to stability and optionally according to evolutionary criteria. The final remaining mutation list is presented to the user as the designer mutation set. Together with this set, PROT-ON provides several residue- and energy-based plots, portraying the synthetic energy landscape of the probed mutations. The stand-alone version of PROT-ON is deposited at https://github.com/CSB-KaracaLab/prot-on. The users can also use PROT-ON through our user-friendly web service http://proton.tools.ibg.edu.tr:8001/ (runs with EvoEF1 only). Considering its speed and the range of analysis provided, we believe that PROT-ON presents a promising means to estimate designer mutations.

The hibernation-derived compound SUL-138 shifts the mitochondrial proteome towards fatty acid metabolism and prevents cognitive decline and amyloid plaque formation in an Alzheimer's disease mouse model

BACKGROUND

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide and remains without effective cure. Increasing evidence is supporting the mitochondrial cascade hypothesis, proposing that loss of mitochondrial fitness and subsequent ROS and ATP imbalance are important contributors to AD pathophysiology.

METHODS

Here, we tested the effects of SUL-138, a small hibernation-derived molecule that supports mitochondrial bioenergetics via complex I/IV activation, on molecular, physiological, behavioral, and pathological outcomes in APP/PS1 and wildtype mice.

RESULTS

SUL-138 treatment rescued long-term potentiation and hippocampal memory impairments and decreased beta-amyloid plaque load in APP/PS1 mice. This was paralleled by a partial rescue of dysregulated protein expression in APP/PS1 mice as assessed by mass spectrometry-based proteomics. In-depth analysis of protein expression revealed a prominent effect of SUL-138 in APP/PS1 mice on mitochondrial protein expression. SUL-138 increased the levels of proteins involved in fatty acid metabolism in both wildtype and APP/PS1 mice. Additionally, in APP/PS1 mice only, SUL-138 increased the levels of proteins involved in glycolysis and amino acid metabolism pathways, indicating that SUL-138 rescues mitochondrial impairments that are typically observed in AD.

CONCLUSION

Our study demonstrates a SUL-138-induced shift in metabolic input towards the electron transport chain in synaptic mitochondria, coinciding with increased synaptic plasticity and memory. In conclusion, targeting mitochondrial bioenergetics might provide a promising new way to treat cognitive impairments in AD and reduce disease progression.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

Human mitochondrial protein complexes revealed by large-scale coevolution analysis and deep learning-based structure modeling

MOTIVATION

Recent development of deep-learning methods has led to a breakthrough in the prediction accuracy of 3D protein structures. Extending these methods to protein pairs is expected to allow large-scale detection of protein-protein interactions (PPIs) and modeling protein complexes at the proteome level.

RESULTS

We applied RoseTTAFold and AlphaFold, two of the latest deep-learning methods for structure predictions, to analyze coevolution of human proteins residing in mitochondria, an organelle of vital importance in many cellular processes including energy production, metabolism, cell death and antiviral response. Variations in mitochondrial proteins have been linked to a plethora of human diseases and genetic conditions. RoseTTAFold, with high computational speed, was used to predict the coevolution of about 95% of mitochondrial protein pairs. Top-ranked pairs were further subject to modeling of the complex structures by AlphaFold, which also produced contact probability with high precision and in many cases consistent with RoseTTAFold. Most top-ranked pairs with high contact probability were supported by known PPIs and/or similarities to experimental structural complexes. For high-scoring pairs without experimental complex structures, our coevolution analyses and structural models shed light on the details of their interfaces, including CHCHD4-AIFM1, MTERF3-TRUB2, FMC1-ATPAF2 and ECSIT-NDUFAF1. We also identified novel PPIs (PYURF-NDUFAF5, LYRM1-MTRF1L and COA8-COX10) for several proteins without experimentally characterized interaction partners, leading to predictions of their molecular functions and the biological processes they are involved in.

AVAILABILITY AND IMPLEMENTATION

Data of mitochondrial proteins and their interactions are available at: http://conglab.swmed.edu/mitochondria.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Long-Time Oxygen Localization in Electron Transfer Flavoprotein

Reactive oxygen species (ROS) exert a wide range of biological effects from beneficial regulatory function to deleterious oxidative stress. The electron transfer flavoprotein (ETF) is ubiquitous to life and is associated with aerobic metabolism and ROS production due to its location in the mitochondria. Quantifying oxygen localization within the ETF complex is critical for understanding the potential for electron transfer and radical pair formation between flavin adenine dinucleotide (FAD) cofactor and superoxide during ROS formation. Our study employed all-atom molecular dynamics simulations and identified several novel, long-lived oxygen binding sites within the ETF complex that appear near the FAD cofactor. Site locations, the local electrostatic environment, and characteristic oxygen binding times for each site were evaluated to establish factors that may lead to possible charge transfer reactions and superoxide formation within the ETF complex. The study revealed that some oxygen binding sites are naturally linked to protein domain features, suggesting opportunities to engineer and control ROS production and subsequent dynamics.

Suborganelle-Specific Protein Complex Analysis Enabled by in Vivo Cross-Linking Coupled with Proximal Labeling

The identification of the structure of protein complexes in the subcellular niche of cells is necessary to understand their diverse functions. In this study, we developed a suborganelle proteome labeling assisted in vivo cross-linking (SubPiXL) strategy to identify regional protein conformations and interactions in living cells. Due to the mitochondria's functional importance and well-defined compartmental partitions, the specific conformations and interactome of protein complexes located in the mitochondrial matrix were identified. Compared to the commonly used approach of organelle isolation followed by intact mitochondria cross-linking, our method achieved a more refined spatial characterization for the subcompartment of the cellular organelle. Additionally, this approach avoided cross-contamination and cell microenvironment disruption during organelle isolation. As such, we achieved 73% selectivity for mitochondria and 98% specificity of known suborganelle annotation for the mitochondrial matrix and accessible inner membrane. Meanwhile, more protein-protein interactions (PPIs) with high dynamics were captured, resulting in a 1.67-fold increase in the number of PPI identifications in 1/11th of the time. On the basis of these structural cross-links and the specific characterization of the interactome and conformation, the structural dynamics targeted in the mitochondrial matrix were delineated. Mitochondrial matrix-restricted information for proteins with multisubcellular localizations was then clarified. In summary, SubPiXL is a promising technique for the investigation of suborganelle-resolved protein conformation and interaction analysis and contributes to a better understanding of structure-derived functions.

Metabolomics analysis reveals dysregulation in one carbon metabolism in Friedreich Ataxia

Friedreich Ataxia (FA) is a rare and often fatal autosomal recessive disease in which a mitochondrial protein, frataxin (FXN), is severely reduced in all tissues. With loss of FXN, mitochondrial metabolism is severely disrupted. Multiple therapeutic approaches are in development, but a key limitation is the lack of biomarkers reflecting the activity of FXN in a timely fashion. We predicted this dysregulated metabolism would present a unique metabolite profile in blood of FA patients versus Controls (Con). Plasma from 10 FA and 11 age and sex matched Con subjects was analyzed by targeted mass spectrometry and untargeted NMR. This combined approach yielded quantitative measurements for 540 metabolites and found 59 unique metabolites (55 from MS and 4 from NMR) that were significantly different between cohorts. Correlation-based network analysis revealed several clusters of pathway related metabolites including a cluster associated with one‑carbon (1C) metabolism composed of formate, sarcosine, hypoxanthine, and homocysteine. Receiver operator characteristics analyses demonstrated an excellent ability to discriminate between Con and FA with AUC values >0.95. These results are the first reported metabolomic analyses of human patients with FA. The metabolic perturbations, especially those related to 1C metabolism, may serve as a valuable biomarker panel of disease progression and response to therapy. The identification of dysregulated 1C metabolism may also inform the search for new therapeutic targets related to this pathway.

Degradation of Exogenous Fatty Acids in Escherichia coli

Many bacteria possess all the machineries required to grow on fatty acids (FA) as a unique source of carbon and energy. FA degradation proceeds through the β-oxidation cycle that produces acetyl-CoA and reduced NADH and FADH cofactors. In addition to all the enzymes required for β-oxidation, FA degradation also depends on sophisticated systems for its genetic regulation and for FA transport. The fact that these machineries are conserved in bacteria suggests a crucial role in environmental conditions, especially for enterobacteria. Bacteria also possess specific enzymes required for the degradation of FAs from their environment, again showing the importance of this metabolism for bacterial adaptation. In this review, we mainly describe FA degradation in the Escherichia coli model, and along the way, we highlight and discuss important aspects of this metabolism that are still unclear. We do not detail exhaustively the diversity of the machineries found in other bacteria, but we mention them if they bring additional information or enlightenment on specific aspects.

Combined isobutyryl‐CoA and multiple acyl‐CoA dehydrogenase deficiency in a boy with altered riboflavin homeostasis

In this report, we describe the case of an 11‐year‐old boy, who came to our attention for myalgia and muscle weakness, associated with inappetence and vomiting. Hypertransaminasemia was also noted, with ultrasound evidence of hepatomegaly. Biochemical investigations revealed acylcarnitine and organic acid profiles resembling those seen in MADD, that is, multiple acyl‐CoA dehydrogenase deficiencies (OMIM #231680) a rare inherited disorder of fatty acids, amino acids, and choline metabolism. The patient carried a single pathogenetic variant in the ETFDH gene (c.524G>A, p.Arg175His) and no pathogenetic variant in the riboflavin (Rf) homeostasis related genes (SLC52A1, SLC52A2, SLC52A3, SLC25A32, FLAD1). Instead, compound heterozygosity was found in the ACAD8 gene (c.512C>G, p.Ser171Cys; c.822C>A, p.Asn274Lys), coding for isobutyryl‐CoA dehydrogenase (IBD), whose pathogenic variants are associated to IBD deficiency (OMIM #611283), a rare autosomal recessive disorder of valine catabolism. The c.822C>A was never previously described in a patient. Subsequent further analyses of Rf homeostasis showed reduced levels of flavins in plasma and altered FAD‐dependent enzymatic activities in erythrocytes, as well as a significant reduction in the level of the plasma membrane Rf transporter 2 in erythrocytes. The observed Rf/flavin scarcity in this patient, possibly associated with a decreased ETF:QO efficiency might be responsible for the observed MADD‐like phenotype. The patient's clinical picture improved after supplementation of Rf, l‐carnitine, Coenzyme Q10, and also 3OH‐butyrate. This report demonstrates that, even in the absence of genetic defects in genes involved in Rf homeostasis, further targeted molecular analysis may reveal secondary and possibly treatable biochemical alterations in this pattern.

Copper modulates heart mitochondrial H2O2 emission differently during fatty acid and pyruvate oxidation

Although the preferred cardiac metabolic fuels are fatty acids, glucose metabolism also plays an important role. However, irrespective of substrate type, energy generation results in mitochondrial reactive oxygen species (ROS) formation. To determine if the preference of fat over carbohydrates predisposes cardiomyocytes to oxidant production, we measured total and site-specific H₂O₂ emission in heart mitochondria oxidizing palmitoylcarnitine or pyruvate during copper (Cu) exposure. H₂O₂ emission was higher during oxidation of palmitoylcarnitine compared with pyruvate. Moreover, the bulk of the H₂O₂ emitted during palmitoylcarnitine oxidation originated from the outer ubiquinone binding site of complex III (site III_Qo) and the flavin site of electron transfer flavoprotein (site E_F). We found no evidence of ROS production from complex I ubiquinone-binding site (site I_Q) by reverse electron transport during oxidation of palmitoylcarnitine. Pyruvate oxidation also drove H₂O₂ emission primarily from sites III_Qo; however, the flavin sites of pyruvate dehydrogenase (site P_F) and complex II (site II_F) contributed substantially. The effect of Cu depended on substrate and redox site, with effects at sites O_F and III_Qo being more pronounced in mitochondria oxidizing pyruvate compared with palmitoylcarnitine. Cu imposed a concentration-saturable effect at site P_F but concentration-dependently stimulated H₂O₂ emission at site E_F. The substrate-dependent differences in H₂O₂ emission and effects of Cu suggest that fuel type and points of entry of electrons into the mitochondrial electron transport system determine the mitochondrial ROS production rate. Importantly, knowledge of sites of mitochondrial ROS production is crucial to the understanding of cardiac dysfunction associated with impaired substrate metabolism.

Positively selected genes in the hoary bat (Lasiurus cinereus) lineage: prominence of thymus expression, immune and metabolic function, and regions of ancient synteny

Background

Bats of the genus Lasiurus occur throughout the Americas and have diversified into at least 20 species among three subgenera. The hoary bat (Lasiurus cinereus) is highly migratory and ranges farther across North America than any other wild mammal. Despite the ecological importance of this species as a major insect predator, and the particular susceptibility of lasiurine bats to wind turbine strikes, our understanding of hoary bat ecology, physiology, and behavior remains poor.

Methods

To better understand adaptive evolution in this lineage, we used whole-genome sequencing to identify protein-coding sequence and explore signatures of positive selection. Gene models were predicted with Maker and compared to seven well-annotated and phylogenetically representative species. Evolutionary rate analysis was performed with PAML.

Results

Of 9,447 single-copy orthologous groups that met evaluation criteria, 150 genes had a significant excess of nonsynonymous substitutions along the L. cinereus branch (P < 0.001 after manual review of alignments). Selected genes as a group had biased expression, most strongly in thymus tissue. We identified 23 selected genes with reported immune functions as well as a divergent paralog of Steep1 within suborder Yangochiroptera. Seventeen genes had roles in lipid and glucose metabolic pathways, partially overlapping with 15 mitochondrion-associated genes; these adaptations may reflect the metabolic challenges of hibernation, long-distance migration, and seasonal variation in prey abundance. The genomic distribution of positively selected genes differed significantly from background expectation by discrete Kolmogorov-Smirnov test (P < 0.001). Remarkably, the top three physical clusters all coincided with islands of conserved synteny predating Mammalia, the largest of which shares synteny with the human cat-eye critical region (CECR) on 22q11. This observation coupled with the expansion of a novel Tbx1-like gene family may indicate evolutionary innovation during pharyngeal arch development: both the CECR and Tbx1 cause dosage-dependent congenital abnormalities in thymus, heart, and head, and craniodysmorphy is associated with human orthologs of other positively selected genes as well.

Electron Transfer Flavoprotein (ETF) α Controls Blood Vessel Development by Regulating Endothelial Mitochondrial Bioenergetics and Oxygen Consumption

While impairment of vascular homeostasis induced by hypercholesterolemia is the first step of cardiovascular diseases, the molecular mechanism behind such impairment is not well known. Here, we reported that high-cholesterol diet (HCD) induced defective vessel sprouting in zebrafish larvae. Electron transfer flavoprotein subunit α (ETFα) (encoded by the ETFA gene), a protein that mediates transfer of electrons from a series of mitochondrial flavoenzymes to the respiratory chain, was downregulated in HCD-fed zebrafish and in endothelial cells treated with oxidized low-density lipoprotein. Knockdown of ETFα with morpholino antisense oligonucleotides reproduced vascular sprouting defects in zebrafish larvae, while replenishing with exogeneous ETFA mRNA could successfully rescue these defects. ETFA knockdown in endothelial cells reduces cell migration, proliferation, and tube formation in vitro. Finally, knockdown of ETFA in endothelial cells also reduced fatty acid oxidation, oxygen consumption rate, and hypoxia-inducible factor-1α (HIF1α) protein levels. Taken together, we demonstrate that downregulation of ETFα is involved in hypercholesterolemia-induced defective vessel sprouting in zebrafish larvae via inhibition of endothelial proliferation and migration. The molecular mechanism behind this phenomenon is the decrease of HIF1α induced by downregulation of ETFα in endothelial cells. This work suggests that disturbance of ETFα-mediated oxygen homeostasis is one of the mechanisms behind hypercholesterolemia-induced vascular dysfunction.

Protein methylation in mitochondria

Many proteins are modified by posttranslational methylation, introduced by a number of methyltransferases (MTases). Protein methylation plays important roles in modulating protein function and thus in optimizing and regulating cellular and physiological processes. Research has mainly focused on nuclear and cytosolic protein methylation, but it has been known for many years that also mitochondrial proteins are methylated. During the last decade, significant progress has been made on identifying the MTases responsible for mitochondrial protein methylation and addressing its functional significance. In particular, several novel human MTases have been uncovered that methylate lysine, arginine, histidine, and glutamine residues in various mitochondrial substrates. Several of these substrates are key components of the bioenergetics machinery, e.g., respiratory Complex I, citrate synthase, and the ATP synthase. In the present review, we report the status of the field of mitochondrial protein methylation, with a particular emphasis on recently discovered human MTases. We also discuss evolutionary aspects and functional significance of mitochondrial protein methylation and present an outlook for this emergent research field.

Metabolic Therapy of Heart Failure: Is There a Future for B Vitamins?

Heart failure (HF) is a plague of the aging population in industrialized countries that continues to cause many deaths despite intensive research into more effective treatments. Although the therapeutic arsenal to face heart failure has been expanding, the relatively short life expectancy of HF patients is pushing towards novel therapeutic strategies. Heart failure is associated with drastic metabolic disorders, including severe myocardial mitochondrial dysfunction and systemic nutrient deprivation secondary to severe cardiac dysfunction. To date, no effective therapy has been developed to restore the cardiac energy metabolism of the failing myocardium, mainly due to the metabolic complexity and intertwining of the involved processes. Recent years have witnessed a growing scientific interest in natural molecules that play a pivotal role in energy metabolism with promising therapeutic effects against heart failure. Among these molecules, B vitamins are a class of water soluble vitamins that are directly involved in energy metabolism and are of particular interest since they are intimately linked to energy metabolism and HF patients are often B vitamin deficient. This review aims at assessing the value of B vitamin supplementation in the treatment of heart failure.

Diagnosis of atypical myopathy based on organic acid and acylcarnitine profiles and evolution of biomarkers in surviving horses

Background

Atypical myopathy (AM), an acquired multiple acyl-CoA dehydrogenase deficiency (MADD) in horses, induce changes in mitochondrial metabolism. Only few veterinary laboratories offer diagnostic testing for this disease. Inborn and acquired MADD exist in humans, therefore determination of organic acids (OA) in urine and acylcarnitines (AC) in blood by assays available in medical laboratories can serve as AM diagnostics. The evolution of OA and AC profiles in surviving horses is unreported.

Methods

AC profiles using electrospray ionization tandem mass spectrometry (ESI-MS/MS) and OA in urine using gas chromatography mass spectrometry (GC–MS) were determined in dried blot spots (DBS, n = 7) and urine samples (n = 5) of horses with AM (n = 7) at disease presentation and in longitudinal samples from 3/4 survivors and compared to DBS (n = 16) and urine samples (n = 7) from control horses using the Wilcoxon test.

Results

All short- (C2-C5) and medium-chain (C6-C12) AC in blood differed significantly (p < 0.008) between horses with AM and controls, except for C5:1 (p = 0.45) and C5OH + C4DC (p = 0.06). In AM survivors the AC concentrations decreased over time but were still partially elevated after 7 days. 14/62 (23%) of OA differed significantly between horses with AM and control horses. Concentrations of ethylmalonic acid, 2-hydroxyglutaric acid and the acylglycines (butyryl-, valeryl-, and hexanoylglycine) were highly elevated in the urine of all horses with AM at the day of disease presentation. In AM survivors, concentrations of those metabolites were initially lower and decreased during remission to approach normalization after 7 days.

Conclusion

OA and AC profiling by specialized human medical laboratories was used to diagnose AM in horses. Elevation of specific metabolites were still evident several days after disease presentation, allowing diagnosis via analysis of samples from convalescent animals.

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Human medical laboratories can be used to diagnose atypical myopathy in horses.

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Diagnosis can be achieved by organic acid and acylcarnitine profiling.

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Diagnosis can also be achieved in convalescent horses.

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Specific metabolites are still evident several days after clinical signs start.

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Some metabolites differentiated between survivors and non-survivors.

Nutrients to Improve Mitochondrial Function to Reduce Brain Energy Deficit and Oxidative Stress in Migraine

The mechanisms of migraine pathogenesis are not completely clear, but ³¹P-nuclear magnetic resonance studies revealed brain energy deficit in migraineurs. As glycolysis is the main process of energy production in the brain, mitochondria may play an important role in migraine pathogenesis. Nutrition is an important aspect of migraine pathogenesis, as many migraineurs report food-related products as migraine triggers. Apart from approved anti-migraine drugs, many vitamins and supplements are considered in migraine prevention and therapy, but without strong supportive evidence. In this review, we summarize and update information about nutrients that may be important for mitochondrial functions, energy production, oxidative stress, and that are related to migraine. Additionally, we present a brief overview of caffeine and alcohol, as they are often reported to have ambiguous effects in migraineurs. The nutrients that can be considered to supplement the diet to prevent and/or ameliorate migraine are riboflavin, thiamine, magnesium ions, niacin, carnitine, coenzyme Q10, melatonin, lipoic acid, pyridoxine, folate, and cobalamin. They can supplement a normal, healthy diet, which should be adjusted to individual needs determined mainly by the physiological constitution of an organism. The intake of caffeine and alcohol should be fine-tuned to the history of their use, as withdrawal of these agents in regular users may become a migraine trigger.

Animal Models of Coenzyme Q Deficiency: Mechanistic and Translational Learnings

Coenzyme Q (CoQ) is a vital lipophilic molecule that is endogenously synthesized in the mitochondria of each cell. The CoQ biosynthetic pathway is complex and not completely characterized, and it involves at least thirteen catalytic and regulatory proteins. Once it is synthesized, CoQ exerts a wide variety of mitochondrial and extramitochondrial functions thank to its redox capacity and its lipophilicity. Thus, low levels of CoQ cause diseases with heterogeneous clinical symptoms, which are not always understood. The decreased levels of CoQ may be primary caused by defects in the CoQ biosynthetic pathway or secondarily associated with other diseases. In both cases, the pathomechanisms are related to the CoQ functions, although further experimental evidence is required to establish this association. The conventional treatment for CoQ deficiencies is the high doses of oral CoQ₁₀ supplementation, but this therapy is not effective for some specific clinical presentations, especially in those involving the nervous system. To better understand the CoQ biosynthetic pathway, the biological functions linked to CoQ and the pathomechanisms of CoQ deficiencies, and to improve the therapeutic outcomes of this syndrome, a variety of animal models have been generated and characterized in the last decade. In this review, we show all the animal models available, remarking on the most important outcomes that each model has provided. Finally, we also comment some gaps and future research directions related to CoQ metabolism and how the current and novel animal models may help in the development of future research studies.

Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences

The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome c, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies.

The multiple roles of coenzyme Q in cellular homeostasis and their relevance for the pathogenesis of coenzyme Q deficiency

Coenzyme Q (CoQ) is a redox active lipid that plays a central role in cellular homeostasis. It was discovered more than 60 years ago because of its role as electron transporter in the mitochondrial respiratory chain. Since then it has become evident that CoQ has many other functions, not directly related to bioenergetics. It is a cofactor of several mitochondrial dehydrogenases involved in the metabolism of lipids, amino acids, and nucleotides, and in sulfide detoxification. It is a powerful antioxidant and it is involved in the control of programmed cell death by modulating both apoptosis and ferroptosis. CoQ deficiency is a clinically and genetically heterogeneous group of disorders characterized by the impairment of CoQ biosynthesis. CoQ deficient patients display defects in cellular bioenergetics, but also in the other pathways in which CoQ is involved. In this review we will focus on the functions of CoQ not directly related to the respiratory chain, and on how their impairment is relevant for the pathophysiology of CoQ deficiency. A better understanding of the complex set of events triggered by CoQ deficiency will allow to design novel approaches for the treatment of this condition.