A mutant mitochondrial respiratory chain assembly protein causes complex III deficiency in patients with tubulopathy, encephalopathy and liver failure

Conformations of Bcs1L undergoing ATP hydrolysis suggest a concerted translocation mechanism for folded iron-sulfur protein substrate

The human AAA-ATPase Bcs1L translocates the fully assembled Rieske iron-sulfur protein (ISP) precursor across the mitochondrial inner membrane, enabling respiratory Complex III assembly. Exactly how the folded substrate is bound to and released from Bcs1L has been unclear, and there has been ongoing debate as to whether subunits of Bcs1L act in sequence or in unison hydrolyzing ATP when moving the protein cargo. Here, we captured Bcs1L conformations by cryo-EM during active ATP hydrolysis in the presence or absence of ISP substrate. In contrast to the threading mechanism widely employed by AAA proteins in substrate translocation, subunits of Bcs1L alternate uniformly between ATP and ADP conformations without detectable intermediates that have different, co-existing nucleotide states, indicating that the subunits act in concert. We further show that the ISP can be trapped by Bcs1 when its subunits are all in the ADP-bound state, which we propose to be released in the apo form.

Genetic landscape of pediatric acute liver failure of indeterminate origin

BACKGROUND AND AIMS

Pediatric acute liver failure (PALF) is a life-threatening condition. In Europe, the main causes are viral infections (12%-16%) and inherited metabolic diseases (14%-28%). Yet, in up to 50% of cases the underlying etiology remains elusive, challenging clinical management, including liver transplantation. We systematically studied indeterminate PALF cases referred for genetic evaluation by whole-exome sequencing (WES), and analyzed phenotypic and biochemical markers, and the diagnostic yield of WES in this condition.

APPROACH AND RESULTS

With this international, multicenter observational study, patients (0-18 y) with indeterminate PALF were analyzed by WES. Data on the clinical and biochemical phenotype were retrieved and systematically analyzed.

RESULTS

In total, 260 indeterminate PALF patients from 19 countries were recruited between 2011 and 2022, of whom 59 had recurrent PALF. WES established a genetic diagnosis in 37% of cases (97/260). Diagnostic yield was highest in children with PALF in the first year of life (41%), and in children with recurrent acute liver failure (64%). Thirty-six distinct disease genes were identified. Defects in NBAS (n=20), MPV17 (n=8), and DGUOK (n=7) were the most frequent findings. When categorizing, the most frequent were mitochondrial diseases (45%), disorders of vesicular trafficking (28%), and cytosolic aminoacyl-tRNA synthetase deficiencies (10%). One-third of patients had a fatal outcome. Fifty-six patients received liver transplantation.

CONCLUSIONS

This study elucidates a large contribution of genetic causes in PALF of indeterminate origin with an increasing spectrum of disease entities. The high proportion of diagnosed cases and potential treatment implications argue for exome or in future rapid genome sequencing in PALF diagnostics.

Focus on Mitochondrial Respiratory Chain: Potential Therapeutic Target for Chronic Renal Failure

The function of the respiratory chain is closely associated with kidney function, and the dysfunction of the respiratory chain is a primary pathophysiological change in chronic kidney failure. The incidence of chronic kidney failure caused by defects in respiratory-chain-related genes has frequently been overlooked. Correcting abnormal metabolic reprogramming, rescuing the "toxic respiratory chain", and targeting the clearance of mitochondrial reactive oxygen species are potential therapies for treating chronic kidney failure. These treatments have shown promising results in slowing fibrosis and inflammation progression and improving kidney function in various animal models of chronic kidney failure and patients with chronic kidney disease (CKD). The mitochondrial respiratory chain is a key target worthy of attention in the treatment of chronic kidney failure. This review integrated research related to the mitochondrial respiratory chain and chronic kidney failure, primarily elucidating the pathological status of the mitochondrial respiratory chain in chronic kidney failure and potential therapeutic drugs. It provided new ideas for the treatment of kidney failure and promoted the development of drugs targeting the mitochondrial respiratory chain.

Translation Fidelity and Respiration Deficits in CLPP-Deficient Tissues: Mechanistic Insights from Mitochondrial Complexome Profiling

The mitochondrial matrix peptidase CLPP is crucial during cell stress. Its loss causes Perrault syndrome type 3 (PRLTS3) with infertility, neurodegeneration, and a growth deficit. Its target proteins are disaggregated by CLPX, which also regulates heme biosynthesis via unfolding ALAS enzymes, providing access for pyridoxal-5'-phosphate (PLP). Despite efforts in diverse organisms with multiple techniques, CLPXP substrates remain controversial. Here, avoiding recombinant overexpression, we employed complexomics in mitochondria from three mouse tissues to identify endogenous targets. A CLPP absence caused the accumulation and dispersion of CLPX-VWA8 as AAA+ unfoldases, and of PLPBP. Similar changes and CLPX-VWA8 co-migration were evident for mitoribosomal central protuberance clusters, translation factors like GFM1-HARS2, the RNA granule components LRPPRC-SLIRP, and enzymes OAT-ALDH18A1. Mitochondrially translated proteins in testes showed reductions to <30% for MTCO1-3, the mis-assembly of the complex IV supercomplex, and accumulated metal-binding assembly factors COX15-SFXN4. Indeed, heavy metal levels were increased for iron, molybdenum, cobalt, and manganese. RT-qPCR showed compensatory downregulation only for Clpx mRNA; most accumulated proteins appeared transcriptionally upregulated. Immunoblots validated VWA8, MRPL38, MRPL18, GFM1, and OAT accumulation. Co-immunoprecipitation confirmed CLPX binding to MRPL38, GFM1, and OAT, so excess CLPX and PLP may affect their activity. Our data mechanistically elucidate the mitochondrial translation fidelity deficits which underlie progressive hearing impairment in PRLTS3.

Only One of Three Bcs1 Homologs in Aspergillus fumigatus Confers Respiratory Growth

The mitochondrial translocase Bcs1 is required for the correct assembly of complex III of the mitochondrial respiratory chain. Because of its importance, Bcs1 was recently proposed as a target for antifungal agents. The function of this AAA (ATPase Associated with diverse cellular Activities) protein has been extensively characterized in Saccharomyces cerevisiae. This yeast as well as previously studied mammals each encode only one homolog. In contrast, the pathogenic mold Aspergillus fumigatus encodes three putative Bcs1 homologs, none of which have been characterized to date. To study the role of these three homologs in A. fumigatus, conditional and deletion mutants of the respective genes AFUA_3G13000 (bcs1A), AFUA_4G01260 (bcs1B), and AFUA_2G14760 (bcs1C) were generated. A deletion or downregulation of bcs1A resulted in drastically reduced growth and sporulation rates and in a significantly altered susceptibility to azole antifungals. In contrast, mutants lacking Bcs1B or Bcs1C did not show any phenotypes differing from the wild type. Salicylhydroxamic acid-an inhibitor of the alternative oxidase that allows the respiratory chain to bypass complex III in some species-caused a complete growth arrest of the bcs1A deletion mutant. In a Galleria mellonella infection model, the deletion of bcs1A resulted in significantly decreased virulence. Only Bcs1A was able to partially complement a deletion of BCS1 in S. cerevisiae. The subcellular localization of Bcs1B and Bcs1C outside of mitochondria suggests that these Bcs1 homologs exert cellular functions different from that of Bcs1. Our data demonstrate that Bcs1A is the sole Bcs1 ortholog in A. fumigatus.

Clinical and biochemical footprints of inherited metabolic diseases. XIV. Metabolic kidney diseases

Kidney disease is a global health burden with high morbidity and mortality. Causes of kidney disease are numerous, extending from common disease groups like diabetes and arterial hypertension to rare conditions including inherited metabolic diseases (IMDs). Given its unique anatomy and function, the kidney is a target organ in about 10% of known IMDs, emphasizing the relevant contribution of IMDs to kidney disease. The pattern of injury affects all segments of the nephron including glomerular disease, proximal and distal tubular damage, kidney cyst formation, built-up of nephrocalcinosis and stones as well as severe malformations. We revised and updated the list of known metabolic etiologies associated with kidney involvement and found 190 relevant IMDs. This represents the 14th of a series of educational articles providing a comprehensive and revised list of metabolic differential diagnoses according to system involvement.

Drug Drop Test: How to Quickly Identify Potential Therapeutic Compounds for Mitochondrial Diseases Using Yeast Saccharomyces cerevisiae

Mitochondrial diseases (MDs) refer to a group of clinically and genetically heterogeneous pathologies characterized by defective mitochondrial function and energy production. Unfortunately, there is no effective treatment for most MDs, and current therapeutic management is limited to relieving symptoms. The yeast Saccharomyces cerevisiae has been efficiently used as a model organism to study mitochondria-related disorders thanks to its easy manipulation and well-known mitochondrial biogenesis and metabolism. It has been successfully exploited both to validate alleged pathogenic variants identified in patients and to discover potential beneficial molecules for their treatment. The so-called "drug drop test", a phenotype-based high-throughput screening, especially if coupled with a drug repurposing approach, allows the identification of molecules with high translational potential in a cost-effective and time-saving manner. In addition to drug identification, S. cerevisiae can be used to point out the drug's target or pathway. To date, drug drop tests have been successfully carried out for a variety of disease models, leading to very promising results. The most relevant aspect is that studies on more complex model organisms confirmed the effectiveness of the drugs, strengthening the results obtained in yeast and demonstrating the usefulness of this screening as a novel approach to revealing new therapeutic molecules for MDs.

Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, was initially recognized as a neuropathological entity in 1951. Bilateral symmetrical lesions, typically extending from the basal ganglia and thalamus through brainstem structures to the posterior columns of the spinal cord, are characterized microscopically by capillary proliferation, gliosis, severe neuronal loss, and relative preservation of astrocytes. Leigh syndrome is a pan-ethnic disorder usually with onset in infancy or early childhood, but late-onset forms occur, including in adult life. Over the last six decades it has emerged that this complex neurodegenerative disorder encompasses more than 100 separate monogenic disorders associated with enormous clinical and biochemical heterogeneity. This chapter discusses clinical, biochemical and neuropathological aspects of the disorder, and postulated pathomechanisms. Known genetic causes, including defects of 16 mitochondrial DNA (mtDNA) genes and approaching 100 nuclear genes, are categorized into disorders of subunits and assembly factors of the five oxidative phosphorylation enzymes, disorders of pyruvate metabolism and vitamin and cofactor transport and metabolism, disorders of mtDNA maintenance, and defects of mitochondrial gene expression, protein quality control, lipid remodeling, dynamics, and toxicity. An approach to diagnosis is presented, together with known treatable causes and an overview of current supportive management options and emerging therapies on the horizon.

Iron status influences mitochondrial disease progression in Complex I-deficient mice

Mitochondrial dysfunction caused by aberrant Complex I assembly and reduced activity of the electron transport chain is pathogenic in many genetic and age-related diseases. Mice missing the Complex I subunit NADH dehydrogenase [ubiquinone] iron-sulfur protein 4 (NDUFS4) are a leading mammalian model of severe mitochondrial disease that exhibit many characteristic symptoms of Leigh Syndrome including oxidative stress, neuroinflammation, brain lesions, and premature death. NDUFS4 knockout mice have decreased expression of nearly every Complex I subunit. As Complex I normally contains at least 8 iron-sulfur clusters and more than 25 iron atoms, we asked whether a deficiency of Complex I may lead to iron perturbations, thereby accelerating disease progression. Consistent with this, iron supplementation accelerates symptoms of brain degeneration in these mice, while iron restriction delays the onset of these symptoms, reduces neuroinflammation, and increases survival. NDUFS4 knockout mice display signs of iron overload in the liver including increased expression of hepcidin and show changes in iron-responsive element-regulated proteins consistent with increased cellular iron that were prevented by iron restriction. These results suggest that perturbed iron homeostasis may contribute to pathology in Leigh Syndrome and possibly other mitochondrial disorders.

LYRM7-associated mitochondrial complex III deficiency with non-cavitating leukoencephalopathy and stroke-like episodes

Defects of respiratory chain complex III (CIII) result in characteristic but rare mitochondrial disorders associated with distinct neuroradiological findings. The underlying molecular defects affecting mitochondrial CIII assembly factors are few and yet to be identified. LYRM7 assembly factor is required for proper CIII assembly where it acts as a chaperone for the Rieske iron-sulfur (UQCRFS1) protein in the mitochondrial matrix and stabilizing it. We present here the seventeenth individual with LYRM7-associated mitochondrial leukoencephalopathy harboring a previously reported rare pathogenic homozygous LYRM 7 variant, c.2T>C, (p.Met1?). Like previously reported individuals, our 5-year-old male proband presented with recurrent metabolic and lactic acidosis, encephalopathy, and fatigue. Further, he has additional, previously unreported features, including an acute stroke like episode with bilateral central blindness and optic neuropathy, recurrent hyperglycemia and hypertension associated with metabolic crisis. However, he has no signs of psychomotor regression. He has been stable clinically with residual left-sided reduced visual acuity and amblyopia, and no more metabolic crises for 2-year-period while on the mitochondrial cocktail. Although the reported brain MRI findings in other affected individuals are homogenous, it is slightly different in our index, revealing evidence of bilateral almost symmetric multifocal periventricular T2 hyperintensities with hyperintensities of the optic nerves, optic chiasm, and corona radiata but with no cavitation or cystic changes. This report describes new clinical and radiological findings of LYRM7-associated disease. The report also summarizes the clinical and molecular data of previously reported individuals describing the full phenotypic spectrum.

UQCRC2-related mitochondrial complex III deficiency, about 7 patients

Isolated complex III defect is a relatively rare cause of mitochondrial disorder. New genes involved were identified in the last two decades, with only a few cases described for each deficiency. UQCRC2, which encodes ubiquinol-cytochrome c reductase core protein 2, is one of the eleven structural subunits of complex III. We report seven French patients with UQCRC2 deficiency to complete the phenotype reported so far. We highlight the similarities with neoglucogenesis defect during decompensations - hypoglycaemias, liver failure and lactic acidosis - and point out the rapid improvement with glucose fluid infusion, which is a remarkable feature for a mitochondrial disorder. Finally, we discuss the relevance of coenzyme Q10 supplementation in this defect.

Primary Mitochondrial Disorders in the Neonate

Primary mitochondrial disorders (PMDs) are a heterogeneous group of disorders characterized by functional or structural abnormalities in the mitochondria that lead to a disturbance of cellular energy, reactive oxygen species, and free radical production, as well as impairment of other intracellular metabolic functions, causing single- or multiorgan dysfunction. PMDs are caused by pathogenic variants in nuclear and mitochondrial genes, resulting in distinct modes of inheritance. Onset of disease is variable and can occur in the neonatal period, with a high morbidity and mortality. In this article, we review the most common methods used for the diagnosis of PMDs, as well as their prenatal and neonatal presentations. We highlight the shift in the diagnostic approach for PMDs since the introduction of nontargeted molecular tests into clinical practice, which has significantly reduced the use of invasive studies. We discuss common PMDs that can present in the neonate, including general, nonsyndromic presentations as well as specific syndromic disorders. We also review current treatment advances, including the use of mitochondrial "cocktails" based on limited scientific evidence and theoretical reasoning, as well as the impending arrival of personalized mitochondrial-specific treatments.

Differential requirements for mitochondrial electron transport chain components in the adult murine liver

Mitochondrial electron transport chain (ETC) dysfunction due to mutations in the nuclear or mitochondrial genome is a common cause of metabolic disease in humans and displays striking tissue specificity depending on the affected gene. The mechanisms underlying tissue-specific phenotypes are not understood. Complex I (cI) is classically considered the entry point for electrons into the ETC, and in vitro experiments indicate that cI is required for basal respiration and maintenance of the NAD+/NADH ratio, an indicator of cellular redox status. This finding has largely not been tested in vivo. Here, we report that mitochondrial complex I is dispensable for homeostasis of the adult mouse liver; animals with hepatocyte-specific loss of cI function display no overt phenotypes or signs of liver damage, and maintain liver function, redox and oxygen status. Further analysis of cI-deficient livers did not reveal significant proteomic or metabolic changes, indicating little to no compensation is required in the setting of complex I loss. In contrast, complex IV (cIV) dysfunction in adult hepatocytes results in decreased liver function, impaired oxygen handling, steatosis, and liver damage, accompanied by significant metabolomic and proteomic perturbations. Our results support a model whereby complex I loss is tolerated in the mouse liver because hepatocytes use alternative electron donors to fuel the mitochondrial ETC.

Mitochondrial respiratory complexes: Significance in human mitochondrial disorders and cancers

Mitochondria are pivotal organelles that govern cellular energy production through the oxidative phosphorylation system utilizing five respiratory complexes. In addition, mitochondria also contribute to various critical signaling pathways including apoptosis, damage-associated molecular patterns, calcium homeostasis, lipid, and amino acid biosynthesis. Among these diverse functions, the energy generation program oversee by mitochondria represents an immaculate orchestration and functional coordination between the mitochondria and nuclear encoded molecules. Perturbation in this program through respiratory complexes' alteration results in the manifestation of various mitochondrial disorders and malignancy, which is alarmingly becoming evident in the recent literature. Considering the clinical relevance and importance of this emerging medical problem, this review sheds light on the timing and nature of molecular alterations in various respiratory complexes and their functional consequences observed in various mitochondrial disorders and human cancers. Finally, we discussed how this wealth of information could be exploited and tailored to develop respiratory complex targeted personalized therapeutics and biomarkers for better management of various incurable human mitochondrial disorders and cancers.

Mitochondrial protein dysfunction in pathogenesis of neurological diseases

Mitochondria are essential organelles for neuronal function and cell survival. Besides the well-known bioenergetics, additional mitochondrial roles in calcium signaling, lipid biogenesis, regulation of reactive oxygen species, and apoptosis are pivotal in diverse cellular processes. The mitochondrial proteome encompasses about 1,500 proteins encoded by both the nuclear DNA and the maternally inherited mitochondrial DNA. Mutations in the nuclear or mitochondrial genome, or combinations of both, can result in mitochondrial protein deficiencies and mitochondrial malfunction. Therefore, mitochondrial quality control by proteins involved in various surveillance mechanisms is critical for neuronal integrity and viability. Abnormal proteins involved in mitochondrial bioenergetics, dynamics, mitophagy, import machinery, ion channels, and mitochondrial DNA maintenance have been linked to the pathogenesis of a number of neurological diseases. The goal of this review is to give an overview of these pathways and to summarize the interconnections between mitochondrial protein dysfunction and neurological diseases.

Mitochondrial microproteins link metabolic cues to respiratory chain biogenesis

Electron transport chain (ETC) biogenesis is tightly coupled to energy levels and availability of ETC subunits. Complex III (CIII), controlling ubiquinol:ubiquinone ratio in ETC, is an attractive node for modulating ETC levels during metabolic stress. Here, we report the discovery of mammalian Co-ordinator of mitochondrial CYTB (COM) complexes that regulate the stepwise CIII biogenesis in response to nutrient and nuclear-encoded ETC subunit availability. The COMA complex, consisting of UQCC1/2 and membrane anchor C16ORF91, facilitates translation of CIII enzymatic core subunit CYTB. Subsequently, microproteins SMIM4 and BRAWNIN together with COMA subunits form the COMB complex to stabilize nascent CYTB. Finally, UQCC3-containing COMC facilitates CYTB hemylation and association with downstream CIII subunits. Furthermore, when nuclear CIII subunits are limiting, COMB is required to chaperone nascent CYTB to prevent OXPHOS collapse. Our studies highlight CYTB synthesis as a key regulatory node of ETC biogenesis and uncover the roles of microproteins in maintaining mitochondrial homeostasis.

Disruption of mitochondrial complex III in cap mesenchyme but not in ureteric progenitors results in defective nephrogenesis associated with amino acid deficiency

Oxidative metabolism in mitochondria regulates cellular differentiation and gene expression through intermediary metabolites and reactive oxygen species. Its role in kidney development and pathogenesis is not completely understood. Here we inactivated ubiquinone-binding protein QPC, a subunit of mitochondrial complex III, in two types of kidney progenitor cells to investigate the role of mitochondrial electron transport in kidney homeostasis. Inactivation of QPC in sine oculis-related homeobox 2 (SIX2)-expressing cap mesenchyme progenitors, which give rise to podocytes and all nephron segments except collecting ducts, resulted in perinatal death from severe kidney dysplasia. This was characterized by decreased proliferation of SIX2 progenitors and their failure to differentiate into kidney epithelium. QPC inactivation in cap mesenchyme progenitors induced activating transcription factor 4-mediated nutritional stress responses and was associated with a reduction in kidney tricarboxylic acid cycle metabolites and amino acid levels, which negatively impacted purine and pyrimidine synthesis. In contrast, QPC inactivation in ureteric tree epithelial cells, which give rise to the kidney collecting system, did not inhibit ureteric differentiation, and resulted in the development of functional kidneys that were smaller in size. Thus, our data demonstrate that mitochondrial oxidative metabolism is critical for the formation of cap mesenchyme-derived nephron segments but dispensable for formation of the kidney collecting system. Hence, our studies reveal compartment-specific needs for metabolic reprogramming during kidney development.

Identifying and Exploring the Candidate Susceptibility Genes of Cirrhosis Using the Multi-Tissue Transcriptome-Wide Association Study

Objective: We identify and explore the candidate susceptibility genes for cirrhosis and their underlying biological mechanism.

Methods: We downloaded the genome-wide association studies summary data of 901 cirrhosis cases and 451,363 controls and integrated them with reference models of five potential tissues from the Genotype-Tissue Expression (GTEx) Project, including whole blood, liver, pancreas, spleen, and thyroid, to identify genes whose expression is predicted to be associated with cirrhosis. Then, we downloaded gene expression data of individuals with hepatocellular carcinoma from TCGA database to conduct differential expression analysis to validate these identified genes and explored their possible role in driving cirrhosis via functional enrichment and gene set enrichment analysis (GSEA).

Results: We identified 10 significant genes (SKIV2L, JPH4, UQCC2, RP11-91I8.3, MAU2, ERAP1, PUS3, ZNF677, ARHGAP40, and SHANK3) associated with cirrhosis at a Bonferroni-corrected threshold of p < 0.01, among which two (SKIV2L and JPH4) were identified in the liver and five (SKIV2L, JPH4, MAU2, SHANK3, and UQCC2) were validated by differential expression analysis at an FDR-corrected threshold of p < 0.01. The enrichment analysis showed that the degradation process of RNA, which is enriched by 58 genes, is significantly under-enriched in liver cancer tissues (p = 0.0268).

Conclusion: We have identified several candidate genes for cirrhosis in multiple tissues and performed differential genetic analysis using the liver cancer database to verify the significant genes. We found that the genes SKIV2L and JPH4 identified in the liver are of particular concern. Finally, through enrichment analysis, we speculate that the process of mRNA transcription and RNA degradation may play a role in cirrhosis.

Clinical and diagnostic characteristics of complex III mitopathy due to novel BCS1L gene mutation in a Saudi patient

Background

Of the many types of mitochondrial diseases, mutations affecting BCS1L gene are regarded as chief cause of the defective mitochondrial complex-III, affecting normal mitochondrial functioning, and leading to wide variety of phenotypes.

Case presentation

In this case report we describe a novel genotype linked to a unique phenotype in a Saudi patient born of a consanguineous marriage. Detailed genetic analysis and whole genome sequencing identified a novel homozygous missense mutation in exon 5 c.712A > G (p.Ser328Gly) of the BCS1L gene, with predicted deleterious effects on the functioning AAA⁺-ATPase domain of the protein characterized by distinct clinical presentation associated with profound multisystem involvement, conductive hearing loss, absent external auditory canal, low posterior hair line, short neck, micro and retrognathia, over riding fingers, rocker bottom foot, small phallus with bilateral absent testis (empty scrotum) and intolerable lactic acidosis.

Conclusions

A pathogenic effect of this novel BCS1L mutation was reflected in the patient with his failure to thrive and a complex clinical and metabolic phenotype.

Mitochondrial Neurodegeneration

Mitochondria are cytoplasmic organelles, which generate energy as heat and ATP, the universal energy currency of the cell. This process is carried out by coupling electron stripping through oxidation of nutrient substrates with the formation of a proton-based electrochemical gradient across the inner mitochondrial membrane. Controlled dissipation of the gradient can lead to production of heat as well as ATP, via ADP phosphorylation. This process is known as oxidative phosphorylation, and is carried out by four multiheteromeric complexes (from I to IV) of the mitochondrial respiratory chain, carrying out the electron flow whose energy is stored as a proton-based electrochemical gradient. This gradient sustains a second reaction, operated by the mitochondrial ATP synthase, or complex V, which condensates ADP and Pi into ATP. Four complexes (CI, CIII, CIV, and CV) are composed of proteins encoded by genes present in two separate compartments: the nuclear genome and a small circular DNA found in mitochondria themselves, and are termed mitochondrial DNA (mtDNA). Mutations striking either genome can lead to mitochondrial impairment, determining infantile, childhood or adult neurodegeneration. Mitochondrial disorders are complex neurological syndromes, and are often part of a multisystem disorder. In this paper, we divide the diseases into those caused by mtDNA defects and those that are due to mutations involving nuclear genes; from a clinical point of view, we discuss pediatric disorders in comparison to juvenile or adult-onset conditions. The complementary genetic contributions controlling organellar function and the complexity of the biochemical pathways present in the mitochondria justify the extreme genetic and phenotypic heterogeneity of this new area of inborn errors of metabolism known as ‘mitochondrial medicine’.

Overexpression of NDUFV1 alleviates renal damage by improving mitochondrial function in unilateral ureteral obstruction model mice

Mitochondrial homeostasis plays essential role for the proper functioning of the kidney. NADH-ubiquinone oxidoreductase core subunit V1 (NDUFV1) is an enzyme in the complex I of electron transport chain (ETC) in mitochondria. In the present study, we examined the effects of NDUFV1 on renal function in unilateral ureteral obstruction (UUO) model mice. Our data showed that increased expression of NDUFV1 improves kidney function as evidenced by the decreases in blood urea nitrogen and serum creatinine in UUO mice. Moreover, NDUFV1 also maintains renal structures and alleviates renal fibrosis induced by UUO surgery. Mechanistically, NDUFV1 mitigates the increased oxidative stress in the kidney in UUO model mice. Meanwhile, increased expression of NDUFV1 in the kidney improves the integrity of the complex I and potentiates the complex I activity. Overall, these results indicate that the ETC complex I plays a beneficial role against renal dysfunction induced by acute kidney injury such as UUO. Therefore, NDUFV1 might be a druggable target for developing agents for dealing with disabled mitochondria-associated renal diseases.

An Overview of Mitochondrial Protein Defects in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are dysfunctions that involve skeletal muscle and cause incorrect communication between the nerves and muscles. The specific causes of NMDs are not well known, but most of them are caused by genetic mutations. NMDs are generally progressive and entail muscle weakness and fatigue. Muscular impairments can differ in onset, severity, prognosis, and phenotype. A multitude of possible injury sites can make diagnosis of NMDs difficult. Mitochondria are crucial for cellular homeostasis and are involved in various metabolic pathways; for this reason, their dysfunction can lead to the development of different pathologies, including NMDs. Most NMDs due to mitochondrial dysfunction have been associated with mutations of genes involved in mitochondrial biogenesis and metabolism. This review is focused on some mitochondrial routes such as the TCA cycle, OXPHOS, and β-oxidation, recently found to be altered in NMDs. Particular attention is given to the alterations found in some genes encoding mitochondrial carriers, proteins of the inner mitochondrial membrane able to exchange metabolites between mitochondria and the cytosol. Briefly, we discuss possible strategies used to diagnose NMDs and therapies able to promote patient outcome.

Expanding the phenotypic spectrum of BCS1L-related mitochondrial disease

Objective

To delineate the full phenotypic spectrum of BCS1L‐related disease, provide better understanding of the genotype–phenotype correlations and identify reliable prognostic disease markers.

Methods

We performed a retrospective multinational cohort study of previously unpublished patients followed in 15 centres from 10 countries. Patients with confirmed biallelic pathogenic BCS1L variants were considered eligible. Clinical, laboratory, neuroimaging and genetic data were analysed. Patients were stratified into different groups based on the age of disease onset, whether homozygous or compound heterozygous for the c.232A>G (p.Ser78Gly) variant, and those with other pathogenic BCS1L variants.

Results

Thirty‐three patients were included. We found that growth failure, lactic acidosis, tubulopathy, hepatopathy and early death were more frequent in those with disease onset within the first month of life. In those with onset after 1 month, neurological features including movement disorders and seizures were more frequent. Novel phenotypes, particularly involving movement disorder, were identified in this group. The presence of the c.232A>G (p.Ser78Gly) variant was associated with significantly worse survival and exclusively found in those with disease onset within the first month of life, whilst other pathogenic BCS1L variants were more frequent in those with later symptom onset.

Interpretation

The phenotypic spectrum of BCS1L‐related disease comprises a continuum of clinical features rather than a set of separate syndromic clinical identities. Age of onset defines BCS1L‐related disease clinically and early presentation is associated with poor prognosis. Genotype correlates with phenotype in the presence of the c.232A>G (p.Ser78Gly) variant.

Leigh Syndrome: A Tale of Two Genomes

Leigh syndrome is a rare, complex, and incurable early onset (typically infant or early childhood) mitochondrial disorder with both phenotypic and genetic heterogeneity. The heterogeneous nature of this disorder, based in part on the complexity of mitochondrial genetics, and the significant interactions between the nuclear and mitochondrial genomes has made it particularly challenging to research and develop therapies. This review article discusses some of the advances that have been made in the field to date. While the prognosis is poor with no current substantial treatment options, multiple studies are underway to understand the etiology, pathogenesis, and pathophysiology of Leigh syndrome. With advances in available research tools leading to a better understanding of the mitochondria in health and disease, there is hope for novel treatment options in the future.

Tackling Dysfunction of Mitochondrial Bioenergetics in the Brain

Oxidative phosphorylation (OxPhos) is the basic function of mitochondria, although the landscape of mitochondrial functions is continuously growing to include more aspects of cellular homeostasis. Thanks to the application of -omics technologies to the study of the OxPhos system, novel features emerge from the cataloging of novel proteins as mitochondrial thus adding details to the mitochondrial proteome and defining novel metabolic cellular interrelations, especially in the human brain. We focussed on the diversity of bioenergetics demand and different aspects of mitochondrial structure, functions, and dysfunction in the brain. Definition such as 'mitoexome', 'mitoproteome' and 'mitointeractome' have entered the field of 'mitochondrial medicine'. In this context, we reviewed several genetic defects that hamper the last step of aerobic metabolism, mostly involving the nervous tissue as one of the most prominent energy-dependent tissues and, as consequence, as a primary target of mitochondrial dysfunction. The dual genetic origin of the OxPhos complexes is one of the reasons for the complexity of the genotype-phenotype correlation when facing human diseases associated with mitochondrial defects. Such complexity clinically manifests with extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. Finally, we briefly discuss the future directions of the multi-omics study of human brain disorders.

Modelling of BCS1L-related human mitochondrial disease in Drosophila melanogaster

Mutations in BCS1L are the most frequent cause of human mitochondrial disease linked to complex III deficiency. Different forms of BCS1L-related diseases and more than 20 pathogenic alleles have been reported to date. Clinical symptoms are highly heterogenous, and multisystem involvement is often present, with liver and brain being the most frequently affected organs. BCS1L encodes a mitochondrial AAA + -family member with essential roles in the latest steps in the biogenesis of mitochondrial respiratory chain complex III. Since Bcs1 has been investigated mostly in yeast and mammals, its function in invertebrates remains largely unknown. Here, we describe the phenotypical, biochemical and metabolic consequences of Bcs1 genetic manipulation in Drosophila melanogaster. Our data demonstrate the fundamental role of Bcs1 in complex III biogenesis in invertebrates and provide novel, reliable models for BCS1L-related human mitochondrial diseases. These models recapitulate several features of the human disorders, collectively pointing to a crucial role of Bcs1 and, in turn, of complex III, in development, organismal fitness and physiology of several tissues.

Characterization of terminal-ileal and colonic Crohn's disease in treatment-naïve paediatric patients based on transcriptomic profile using logistic regression

BACKGROUND

Inflammatory bowel disease (IBD) is a chronic and idiopathic inflammatory disorder of the gastrointestinal tract and comprises ulcerative colitis (UC) and Crohn's disease (CD). Crohn's disease can affect any part of the gastrointestinal tract, but mainly the terminal ileum and colon. In the present study, we aimed to characterize terminal-ileal CD (ICD) and colonic CD (CCD) at the molecular level, which might enable a more optimized approach for the clinical care and scientific research of CD.

METHODS

We analyzed differentially expressed genes in samples from 23 treatment-naïve paediatric patients with CD and 25 non-IBD controls, and compared the data with previously published RNA-Seq data using multi-statistical tests and confidence intervals. We implemented functional profiling and proposed statistical methods for feature selection using a logistic regression model to identify genes that are highly associated in ICD or CCD. We also validated our final candidate genes in independent paediatric and adult cohorts.

RESULTS

We identified 550 genes specifically expressed in patients with CD compared with those in healthy controls (p < 0.05). Among these DEGs, 240 from patients with CCD were mainly involved in mitochondrial dysfunction, whereas 310 from patients with ICD were enriched in the ileum functions such as digestion, absorption, and metabolism. To choose the most effective gene set, we selected the most powerful genes (p-value ≤ 0.05, accuracy ≥ 0.8, and AUC ≥ 0.8) using logistic regression. Consequently, 33 genes were identified as useful for discriminating CD location; the accuracy and AUC were 0.86 and 0.83, respectively. We then validated the 33 genes with data from another independent paediatric cohort (accuracy = 0.93, AUC = 0.92) and adult cohort (accuracy = 0.88, AUC = 0.72).

CONCLUSIONS

In summary, we identified DEGs that are specifically expressed in CCD and ICD compared with those in healthy controls and patients with UC. Based on the feature selection analysis, 33 genes were identified as useful for discriminating CCD and ICD with high accuracy and AUC, for not only paediatric patients but also independent cohorts. We propose that our approach and the final gene set are useful for the molecular classification of patients with CD, and it could be beneficial in treatments based on disease location.

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.

Exploiting pyocyanin to treat mitochondrial disease due to respiratory complex III dysfunction

Mitochondrial diseases impair oxidative phosphorylation and ATP production, while effective treatment is still lacking. Defective complex III is associated with a highly variable clinical spectrum. We show that pyocyanin, a bacterial redox cycler, can replace the redox functions of complex III, acting as an electron shunt. Sub-μM pyocyanin was harmless, restored respiration and increased ATP production in fibroblasts from five patients harboring pathogenic mutations in TTC19, BCS1L or LYRM7, involved in assembly/stabilization of complex III. Pyocyanin normalized the mitochondrial membrane potential, and mildly increased ROS production and biogenesis. These in vitro effects were confirmed in both Drosophila^TTC19KO and in Danio rerio^TTC19KD, as administration of low concentrations of pyocyanin significantly ameliorated movement proficiency. Importantly, daily administration of pyocyanin for two months was not toxic in control mice. Our results point to utilization of redox cyclers for therapy of complex III disorders.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

The Power of Yeast in Modelling Human Nuclear Mutations Associated with Mitochondrial Diseases

The increasing application of next generation sequencing approaches to the analysis of human exome and whole genome data has enabled the identification of novel variants and new genes involved in mitochondrial diseases. The ability of surviving in the absence of oxidative phosphorylation (OXPHOS) and mitochondrial genome makes the yeast Saccharomyces cerevisiae an excellent model system for investigating the role of these new variants in mitochondrial-related conditions and dissecting the molecular mechanisms associated with these diseases. The aim of this review was to highlight the main advantages offered by this model for the study of mitochondrial diseases, from the validation and characterisation of novel mutations to the dissection of the role played by genes in mitochondrial functionality and the discovery of potential therapeutic molecules. The review also provides a summary of the main contributions to the understanding of mitochondrial diseases emerged from the study of this simple eukaryotic organism.

Mitochondrial Structure and Bioenergetics in Normal and Disease Conditions

Mitochondria are ubiquitous intracellular organelles found in almost all eukaryotes and involved in various aspects of cellular life, with a primary role in energy production. The interest in this organelle has grown stronger with the discovery of their link to various pathologies, including cancer, aging and neurodegenerative diseases. Indeed, dysfunctional mitochondria cannot provide the required energy to tissues with a high-energy demand, such as heart, brain and muscles, leading to a large spectrum of clinical phenotypes. Mitochondrial defects are at the origin of a group of clinically heterogeneous pathologies, called mitochondrial diseases, with an incidence of 1 in 5000 live births. Primary mitochondrial diseases are associated with genetic mutations both in nuclear and mitochondrial DNA (mtDNA), affecting genes involved in every aspect of the organelle function. As a consequence, it is difficult to find a common cause for mitochondrial diseases and, subsequently, to offer a precise clinical definition of the pathology. Moreover, the complexity of this condition makes it challenging to identify possible therapies or drug targets.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Mitochondrial Disease and the Kidney With a Special Focus on CoQ10 Deficiency

Mitochondrial cytopathies include a heterogeneous group of diseases that are characterized by impaired oxidative phosphorylation, leading to multi-organ involvement and progressive clinical deterioration. Most mitochondrial cytopathies that cause kidney symptoms are characterized by tubular defects, but glomerular, tubulointerstitial, and cystic diseases have also been described. Mitochondrial cytopathies can result from mitochondrial or nuclear DNA mutations. Early recognition of defects in the coenzyme Q10 (CoQ10) biosynthesis is important, as patients with primary CoQ10 deficiency may be responsive to treatment with oral CoQ10 supplementation, in contrast to most mitochondrial diseases. A literature search was conducted to investigate kidney involvement in genetic mitochondrial cytopathies and to identify mitochondrial and nuclear DNA mutations involved in mitochondrial kidney disease. Furthermore, we identified all reported cases to date with a CoQ10 deficiency with glomerular involvement, including 3 patients with variable renal phenotypes in our clinic. To date, 144 patients from 95 families with a primary CoQ10 deficiency and glomerular involvement have been described based on mutations in PDSS1, PDSS2, COQ2, COQ6, and COQ8B/ADCK4. This review provides an overview of kidney involvement in genetic mitochondrial cytopathies with a special focus on CoQ10 deficiency.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

A novel mutation in the ubiquinol-cytochrome c reductase synthesis-like gene associated with complex III deficiency and Björnstad syndrome: A case report

RATIONALE

The ubiquinol-cytochrome c reductase synthesis-like (BCS1L) gene is located on chromosome 2 (2q35) and encodes an ATPase that is associated with various cellular activities and is embedded in the mitochondrial inner membrane; this ATPase is presumed to facilitate the insertion of the Rieske Fe/S protein into precursors of Complex III (CIII) during the assembly of the respiratory chain. We report the first case of a compound heterozygous mutation in the BCS1L gene in China.

PATIENT CONCERNS

A 7-month-old girl presented with a 3-month history of psychomotor developmental retardation and a 1-month history of epilepsy combined with parallel psychomotor developmental deterioration. The clinical manifestations in the patient included psychomotor developmental retardation, infantile spasms, pili torti, tubulopathy, hepatic pathologies and lactic acidosis.

DIAGNOSIS

Combined with her clinical presentation, the patient was diagnosed with CIII deficiency and Björnstad syndrome caused by a novel mutation in the BCS1L gene after molecular biological examination. Whole exome sequencing revealed a compound heterozygous mutation with a missense mutation (c.548G > A/p. R183H) inherited from her mother and an insertion mutation (c.1061_1062insCTA/p. G354delinsGY) inherited from her father.

INTERVENTIONS

Before admission, the patient had received oral topiramate for 1 month. After admission, additional intravenous arginine hydrochloride was administered for five days in the acute metabolic disorder phase, and persistent cocktail therapy was introduced, including coenzyme Q10 (20 mg/d), carnitine (1 g/d) and vitamins (vitamin B1, vitamin B2, vitamin B6, and vitamin C).

OUTCOMES

The spasm seizures were decreased by 50% after 2 weeks of treatment. The blood ammonia, myocardial enzyme and urine glucose levels declined to normal levels. At a 1-month follow-up, the patient improved clinically with a decrease in spasm seizures of 75%, stronger sucking and more voluntary activities. However, she still had mild lactic acidosis and mild hepatic damage.

LESSONS

We reported the first patient with CIII deficiency and Björnstad syndrome in China and identified 1 novel mutation (C.1061_1062insCTA and P. G354delinsGY) in the BCS1L gene. This finding expands the BCS1L gene mutation profile and will be beneficial for genetic diagnosis.

Molecular genetic investigations identify new clinical phenotypes associated with BCS1L-related mitochondrial disease

BCS1L encodes a homolog of the Saccharomyces cerevisiae bcs1 protein, which has a known role in the assembly of Complex III of the mitochondrial respiratory chain. Phenotypes reported in association with pathogenic BCS1L variants include growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis and early death (GRACILE syndrome), and Björnstad syndrome, characterized by abnormal flattening and twisting of hair shafts (pili torti) and hearing problems. Here we describe two patients harbouring biallelic variants in BCS1L; the first with a heterozygous variant c.166C>T, p.(Arg56^*) together with a novel heterozygous variant c.205C>T, p.(Arg69Cys) and a second patient with a novel homozygous c.325C>T, p.(Arg109Trp) variant. The two patients presented with different phenotypes; the first patient presented as an adult with aminoaciduria, seizures, bilateral sensorineural deafness and learning difficulties. The second patient was an infant who presented with a classical GRACILE syndrome leading to death at 4 months of age. A decrease in BCS1L protein levels was seen in both patients, and biochemical analysis of Complex III revealed normal respiratory chain enzyme activities in the muscle of both patients. A decrease in Complex III assembly was detected in the adult patient’s muscle, whilst the paediatric patient displayed a combined mitochondrial respiratory chain defect in cultured fibroblasts. Yeast complementation studies indicate that the two missense variants, c.205C>T, p.(Arg69Cys) and c.325C>T, p.(Arg109Trp), impair the respiratory capacity of the cell. Together, these data support the pathogenicity of the novel BCS1L variants identified in our patients.

Light-Triggered Electron Transfer between a Conjugated Polymer and Cytochrome C for Optical Modulation of Redox Signaling

Protein reduction/oxidation processes trigger and finely regulate a myriad of physiological and pathological cellular functions. Many biochemical and biophysical stimuli have been recently explored to precisely and effectively modulate intracellular redox signaling, due to the considerable therapeutic potential. Here, we propose a first step toward an approach based on visible light excitation of a thiophene-based semiconducting polymer (P3HT), demonstrating the realization of a hybrid interface with the Cytochrome c protein (CytC), in an extracellular environment. By means of scanning electrochemical microscopy and spectro-electrochemistry measurements, we demonstrate that, upon optical stimulation, a functional interaction between P3HT and CytC is established. Polymer optical excitation locally triggers photoelectrochemical reactions, leading to modulation of CytC redox activity, either through an intermediate step, involving reactive oxygen species formation, or via a direct photoreduction process. Both processes are triggered by light, thus allowing excellent spatiotemporal resolution, paving the way to precise modulation of protein redox signaling.

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Conjugated polymers and light modulate the redox state of cytochrome c protein

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Phototransduction processes are clarified by electrochemical microscopy

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The approach opens the way to selective optical triggering of protein redox state

Structures of AAA protein translocase Bcs1 suggest translocation mechanism of a folded protein

The mitochondrial membrane-bound AAA protein Bcs1 translocate substrates across the mitochondrial inner membrane without previous unfolding. One substrate of Bcs1 is the iron-sulfur protein (ISP), a subunit of the respiratory Complex III. How Bcs1 translocates ISP across the membrane is unknown. Here we report structures of mouse Bcs1 in two different conformations, representing three nucleotide states. The apo and ADP-bound structures reveal a homo-heptamer and show a large putative substrate-binding cavity accessible to the matrix space. ATP binding drives a contraction of the cavity by concerted motion of the ATPase domains, which could push substrate across the membrane. Our findings shed light on the potential mechanism of translocating folded proteins across a membrane, offer insights into the assembly process of Complex III and allow mapping of human disease-associated mutations onto the Bcs1 structure.

Artemisinin and its derivatives target mitochondrial c-type cytochromes in yeast and human cells

Artemisinin and its derivatives kill malaria parasites and inhibit the proliferation of cancer cells. In both processes, heme was shown to play a key role in artemisinin bioactivation. We found that artemisinin and clinical artemisinin derivatives are able to compensate for a mutation in the yeast Bcs1 protein, a key chaperon involved in biogenesis of the mitochondrial respiratory complex III. The equivalent Bcs1 variant causes an encephalopathy in human by affecting complex III assembly. We show that artemisinin derivatives decrease the content of mitochondrial cytochromes and disturb the maturation of the complex III cytochrome c1. This last effect is likely responsible for the compensation by decreasing the detrimental over-accumulation of the inactive pre-complex III observed in the bcs1 mutant. We further show that a fluorescent dihydroartemisinin probe rapidly accumulates in the mitochondrial network and targets cytochromes c and c1 in yeast, human cells and isolated mitochondria. In vitro this probe interacts with purified cytochrome c only under reducing conditions and we detect cytochrome c-dihydroartemisinin covalent adducts by mass spectrometry analyses. We propose that reduced mitochondrial c-type cytochromes act as both targets and mediators of artemisinin bioactivation in yeast and human cells.

A spontaneous mitonuclear epistasis converging on Rieske Fe-S protein exacerbates complex III deficiency in mice

We previously observed an unexpected fivefold (35 vs. 200 days) difference in the survival of respiratory chain complex III (CIII) deficient Bcs1l^p.S78G mice between two congenic backgrounds. Here, we identify a spontaneous homoplasmic mtDNA variant (m.G14904A, mt-Cyb^p.D254N), affecting the CIII subunit cytochrome b (MT-CYB), in the background with short survival. We utilize maternal inheritance of mtDNA to confirm this as the causative variant and show that it further decreases the low CIII activity in Bcs1l^p.S78G tissues to below survival threshold by 35 days of age. Molecular dynamics simulations predict D254N to restrict the flexibility of MT-CYB ef loop, potentially affecting RISP dynamics. In Rhodobacter cytochrome bc₁ complex the equivalent substitution causes a kinetics defect with longer occupancy of RISP head domain towards the quinol oxidation site. These findings represent a unique case of spontaneous mitonuclear epistasis and highlight the role of mtDNA variation as modifier of mitochondrial disease phenotypes.

Bi-Allelic UQCRFS1 Variants Are Associated with Mitochondrial Complex III Deficiency, Cardiomyopathy, and Alopecia Totalis

Isolated complex III (CIII) deficiencies are among the least frequently diagnosed mitochondrial disorders. Clinical symptoms range from isolated myopathy to severe multi-systemic disorders with early death and disability. To date, we know of pathogenic variants in genes encoding five out of 10 subunits and five out of 13 assembly factors of CIII. Here we describe rare bi-allelic variants in the gene of a catalytic subunit of CIII, UQCRFS1, which encodes the Rieske iron-sulfur protein, in two unrelated individuals. Affected children presented with low CIII activity in fibroblasts, lactic acidosis, fetal bradycardia, hypertrophic cardiomyopathy, and alopecia totalis. Studies in proband-derived fibroblasts showed a deleterious effect of the variants on UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular respiration. Complementation studies via lentiviral transduction and overexpression of wild-type UQCRFS1 restored mitochondrial function and rescued the cellular phenotype, confirming UQCRFS1 variants as causative for CIII deficiency. We demonstrate that mutations in UQCRFS1 can cause mitochondrial disease, and our results thereby expand the clinical and mutational spectrum of CIII deficiencies.

Exposure to Waterborne nTiO2 Reduces Fertilization Success and Increases Polyspermy in a Bivalve Mollusc: A Threat to Population Recruitment

Fertilization success is crucial for the population recruitment of an organism. However, little is known about the threat of nanoparticles (NPs) to the fertilization of broadcast spawners. Therefore, the effects of nTiO₂ on fertilization success, polyspermy rate, sperm velocity, gametic DNA damage, sperm-egg collision probability, gamete fusion, and oocyte ultrastructure were investigated in a broadcast spawning bivalve, Tegillarca granosa. The results obtained show that fertilization success significantly decreased, whereas polyspermy risk markedly increased upon nTiO₂ exposure. In addition, nTiO₂ exposure led to a significant reduction in sperm swimming velocity, which would subsequently constrain gamete collisions. In addition, nTiO₂ exposure resulted in a significant decline in gamete fusion per collision along with aggravated DNA damage in gametes. Furthermore, ultrastructural analysis illustrated the attachment of nTiO₂ to the oocyte surface, which subsequently resulted in microvillus disassociation and plasma membrane damage. In conclusion, the results obtained suggest a significant threat from NP pollution to the recruitment of broadcast spawning invertebrates.

Cardiovascular Manifestations of Mitochondrial Disease

Genetic mitochondrial cardiomyopathies are uncommon causes of heart failure that may not be seen by most physicians. However, the prevalence of mitochondrial DNA mutations and somatic mutations affecting mitochondrial function are more common than previously thought. In this review, the pathogenesis of genetic mitochondrial disorders causing cardiovascular disease is reviewed. Treatment options are presently limited to mostly symptomatic support, but preclinical research is starting to reveal novel approaches that may lead to better and more targeted therapies in the future. With better understanding and clinician education, we hope to improve clinician recognition and diagnosis of these rare disorders in order to improve ongoing care of patients with these diseases and advance research towards discovering new therapeutic strategies to help treat these diseases.

Inhibition of Mitochondrial Complex Function-The Hepatotoxicity Mechanism of Emodin Based on Quantitative Proteomic Analyses

Emodin is the main component of traditional Chinese medicines including rhubarb, Polygonum multiflorum, and Polygonum cuspidatum. It has confirmed hepatotoxicity and may be the main causative agent of liver damage associated with the above-mentioned traditional Chinese medicines. However, current research does not explain the mechanism of emodin in hepatotoxicity. In this study, L02 cells were used as a model to study the mechanism of emodin-induced hepatocyte apoptosis using quantitative proteomics, and the results were verified by Western blot. A total of 662 differentially expressed proteins were discovered and analyzed using Gene Ontology (GO) and pathway enrichment analysis. The results show that the oxidative phosphorylation pathway is highly represented. Abnormalities in this pathway result in impaired mitochondrial function and represent mitochondrial damage. This result is consistent with mitochondria membrane potential measurements. Analysis of differentially expressed proteins revealed that emodin mainly affects oxidative phosphorylation pathways by inhibiting the function of the mitochondrial respiratory chain complexes; the mitochondrial respiratory chain complex activity assay result also confirmed that emodin could inhibit the activity of all mitochondrial complexes. This results in an increase in caspase-3, a decrease in mitochondrial membrane potential (MMP,) an increase in reactive oxygen species (ROS), and disorders in ATP synthesis, etc., eventually leading to mitochondrial damage and hepatocyte apoptosis in vitro.

LETM1 is required for mitochondrial homeostasis and cellular viability (Review)

Leucine zipper/EF-hand-containing transmembrane protein 1 (LETM1) has been identified as the gene responsible for Wolf-Hirschhorn syndrome (WHS), which is characterized by intellectual disability, epilepsy, growth delay and craniofacial dysgenesis. LETM1 is a mitochondrial inner membrane protein that encodes a homolog of the yeast protein Mdm38, which is involved in mitochondrial morphology. In the present review, the importance of LETM1 in WHS and its role within the mitochondrion was explored. LETM1 governs the mitochondrion ion channel and is involved in mitochondrial respiration. Recent studies have reported that LETM1 acts as a mitochondrial Ca²⁺/H⁺ antiporter. LETM1 has also been identified as a K⁺/H⁺ exchanger, and serves a role in Mg²⁺ homeostasis. The function of LETM1 in mitochondria regulation is regulated by its binding partners, carboxyl-terminal modulator protein and mitochondrial ribosomal protein L36. Therefore, we describe the remarkable role of LETM1 in mitochondrial network physiology and its function in mitochondrion-mediated cell death. In the context of these findings, we suggest that the participation of LETM1 in tumorigenesis through the alteration of cancer metabolism should be investigated. This review provides a comprehensive description of LETM1 function, which is required for mitochondrial homeostasis and cellular viability.

Mitochondria and Renal Fibrosis

Mitochondria are important organelles in eukaryotic cells and perform a variety of biosynthetic and metabolic functions. Many human diseases are closely related to mitochondrial dysfunction. Kidney is an organ with high-energy requirements, which is distributed with a large number of mitochondria. Mitochondrial dysfunction plays a crucial role in the pathogenesis of kidney disease, and studies have shown that mitochondrial dysfunction is involved in the physiological process of renal fibrosis. This review introduced the biogenesis and pathophysiology of mitochondria, illustrated the involvement of mitochondrial dysfunction in renal fibrosis based on various kinds of cells, and finally summarized the latest mitochondria-targeted therapies.

Assembly of mammalian oxidative phosphorylation complexes I-V and supercomplexes

The assembly of the five oxidative phosphorylation system (OXPHOS) complexes in the inner mitochondrial membrane is an intricate process. The human enzymes comprise core proteins, performing the catalytic activities, and a large number of ‘supernumerary’ subunits that play essential roles in assembly, regulation and stability. The correct addition of prosthetic groups as well as chaperoning and incorporation of the structural components require a large number of factors, many of which have been found mutated in cases of mitochondrial disease. Nowadays, the mechanisms of assembly for each of the individual complexes are almost completely understood and the knowledge about the assembly factors involved is constantly increasing. On the other hand, it is now well established that complexes I, III and IV interact with each other, forming the so-called respiratory supercomplexes or ‘respirasomes’, although the pathways that lead to their formation are still not completely clear. This review is a summary of our current knowledge concerning the assembly of complexes I–V and of the supercomplexes.

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients' specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV).