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.
The respiratory megacomplex represents the highest-order assembly of respiratory chain complexes, and it allows mitochondria to respond to energy-requiring conditions. To understand its architecture, ...we examined the human respiratory chain megacomplex-I2III2IV2 (MCI2III2IV2) with 140 subunits and a subset of associated cofactors using cryo-electron microscopy. The MCI2III2IV2 forms a circular structure with the dimeric CIII located in the center, where it is surrounded by two copies each of CI and CIV. Two cytochrome c (Cyt.c) molecules are positioned to accept electrons on the surface of the c1 state CIII dimer. Analyses indicate that CII could insert into the gaps between CI and CIV to form a closed ring, which we termed the electron transport chain supercomplex. The structure not only reveals the precise assignment of individual subunits of human CI and CIII, but also enables future in-depth analysis of the electron transport chain as a whole.
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•Structure of human respiratory SCI1III2IV1 and architecture of MCI2III2IV2•The atomic structures of human CI and CIII with side chains•The MCI2III2IV2 structure provides an alternative electron transfer mechanism•CII can insert into the gaps between CI and CIV to form a closed ring of ETCS
Structural analyses of the human respiratory supercomplex and megacomplex reveal the global arrangement of electron transport chain components, with implications for understanding the mechanism of electron transport.
The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner ...membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc1 complex or complex III, a central component of the mitochondrial energy conversion system.
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•Mitochondrial respiratory chain complexes are assembled from subunits derived from two different genetic systems.•Redox cofactor acquisition and assembly of the numerous subunits of the bc1 complex is aided by dedicated assembly factors.•Mutations in assembly factors or subunits of the bc1 complex cause human diseases.•Production of nuclear and mitochondrially encoded subunits is balanced by a translational feedback loop that operates within the mitochondria.
Cytochrome bd-I from Escherichia coli belongs to the superfamily of prokaryotic bd-type oxygen reductases. It contains three hemes, b558, b595 and d, and couples oxidation of quinol by dioxygen with ...the generation of a proton-motive force. The enzyme exhibits resistance to various stressors and is considered as a target protein for next-generation antimicrobials. By using electronic absorption and MCD spectroscopy, this work shows that cyanide binds to heme d2+ in the isolated fully reduced cytochrome bd-I. Cyanide-induced difference absorption spectra display changes near the heme d2+ α-band, a minimum at 633 nm and a maximum around 600 nm, and a W-shaped response in the Soret region. Apparent dissociation constant (Kd) of the cyanide complex of heme d2+ is ∼0.052 M. Kinetics of cyanide binding is monophasic, indicating the presence of a single ligand binding site in the enzyme. Consistently, MCD data show that cyanide binds to heme d2+ but not to b5582+ or b5952+. This agrees with the published structural data that the enzyme's active site is not a di-heme site. The observed rate of binding (kobs) increases as the concentration of cyanide is increased, giving a second-order rate constant (kon) of ∼0.1 M−1 s−1.
There is conflicting evidence about whether E. coli cytochrome bd-I in the fully reduced state is capable of binding cyanide. With the aid of electronic absorption and MCD spectroscopy, this study resolved the issue. Cyanide binds to heme d2+ but with low affinity and low binding rate. Display omitted
•E. coli cytochrome bd-I belongs to superfamily of bd-type oxygen reductases.•Fully reduced isolated cytochrome bd-I binds cyanide.•Kd for heme d2+-cyanide complex is 0.052 M.•Second-order rate constant kon is 0.1 M−1 s−1.•Data support the conclusion that the enzyme's active site is not a di-heme site.
As tissue macrophages of the central nervous system (CNS), microglia constitute the pivotal immune cells of this organ. Microglial features are strongly dependent on environmental cues such as ...commensal microbiota. Gut bacteria are known to continuously modulate microglia maturation and function by the production of short-chain fatty acids (SCFAs). However, the precise mechanism of this crosstalk is unknown. Here we determined that the immature phenotype of microglia from germ-free (GF) mice is epigenetically imprinted by H3K4me3 and H3K9ac on metabolic genes associated with substantial functional alterations including increased mitochondrial mass and specific respiratory chain dysfunctions. We identified acetate as the essential microbiome-derived SCFA driving microglia maturation and regulating the homeostatic metabolic state, and further showed that it is able to modulate microglial phagocytosis and disease progression during neurodegeneration. These findings indicate that acetate is an essential bacteria-derived molecule driving metabolic pathways and functions of microglia during health and perturbation.
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•Altered metabolic features of microglia in the absence of host microbiota•Bacteria-derived acetate modulates metabolic features of microglia during steady state•Acetate regulates microglial functions during neurodegeneration
Acetate is the critical microbiome-derived SCFA driving microglia maturation and regulating the homeostatic metabolic state. In addition, acetate modulates microglial phagocytosis of amyloid beta and disease progression in a mouse model for Alzheimer’s disease.
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.
NAD+ is a redox-active metabolite, the depletion of which has been proposed to promote aging and degenerative diseases in rodents. However, whether NAD+ depletion occurs in patients with degenerative ...disorders and whether NAD+ repletion improves their symptoms has remained open. Here, we report systemic NAD+ deficiency in adult-onset mitochondrial myopathy patients. We administered an increasing dose of NAD+-booster niacin, a vitamin B3 form (to 750–1,000 mg/day; clinicaltrials.govNCT03973203) for patients and their matched controls for 10 or 4 months, respectively. Blood NAD+ increased in all subjects, up to 8-fold, and muscle NAD+ of patients reached the level of their controls. Some patients showed anemia tendency, while muscle strength and mitochondrial biogenesis increased in all subjects. In patients, muscle metabolome shifted toward controls and liver fat decreased even 50%. Our evidence indicates that blood analysis is useful in identifying NAD+ deficiency and points niacin to be an efficient NAD+ booster for treating mitochondrial myopathy.
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•Mitochondrial myopathy patients have NAD+ deficiency in muscle and blood•Niacin is an efficient NAD+ booster in humans•Niacin improves muscle strength and fatty liver in mitochondrial myopathy•Niacin boosts muscle mitochondrial biogenesis and respiratory chain activity in humans
Pirinen et al. report that niacin, a vitamin B3, can efficiently rescue NAD+ levels in the muscle and blood of patients with mitochondrial myopathy, improving disease signs and muscle strength. NAD+ levels increased also in healthy subjects. The evidence suggests that niacin is an effective NAD+ booster in humans.
Mitochondria are key organelles for cellular energetics, metabolism, signaling, and quality control and have been linked to various diseases. Different views exist on the composition of the human ...mitochondrial proteome. We classified >8,000 proteins in mitochondrial preparations of human cells and defined a mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP). We identified interactors of translocases, respiratory chain, and ATP synthase assembly factors. The abundance of MitoCoP proteins covers six orders of magnitude and amounts to 7% of the cellular proteome with the chaperones HSP60-HSP10 being the most abundant mitochondrial proteins. MitoCoP dynamics spans three orders of magnitudes, with half-lives from hours to months, and suggests a rapid regulation of biosynthesis and assembly processes. 460 MitoCoP genes are linked to human diseases with a strong prevalence for the central nervous system and metabolism. MitoCoP will provide a high-confidence resource for placing dynamics, functions, and dysfunctions of mitochondria into the cellular context.
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•Human mitochondrial high-confidence proteome with >1,100 proteins (MitoCoP)•Mitochondria-specific protein copy numbers and half-lives•Interactors of protein translocases and oxidative phosphorylation assembly factors•>40% of mitochondrial proteome linked to human diseases
Mitochondria are crucial for cellular energy metabolism and human health. Morgenstern et al. present a high-confidence protein compendium of human mitochondria including mitochondria-specific protein copy numbers and half-lives. They identify interactors of key mitochondrial protein machineries and link >40% of the mitochondrial proteome to human diseases.
New discoveries providing insights into mitochondrial bioenergetics, their dynamic interactions as well as their role in cellular homeostasis have dramatically advanced our understanding of the ...neurodegenerative process of Parkinson's disease (PD). Respiratory chain impairment is a key feature in sporadic PD patients and there is growing evidence that links proteins encoded by PD-associated genes to disturbances in mitochondrial function. Against the backdrop of latest advances in the development of PD treatments that target mitochondria, we aim to give an overview of the literature published in the last three decades on the significance of mitochondria in the pathogenesis of PD. We describe the contribution of mitochondrial genome alterations and PD-associated genes to mitochondrial maintenance. We highlight mitophagy as a key mechanism in neurodegeneration. Moreover, we focus on the reciprocal interaction between alpha-synuclein aggregation and mitochondrial dysfunction. We discuss a novel trafficking pathway involving mitochondrial-derived vesicles within the context of PD and provide a synopsis of the most recently emerging topics in PD research with respect to mitochondria. This includes the relationship between mitochondria and cell-mediated immunity, the ER-mitochondria axis, sirtuin-mediated mitochondrial stress response and the role of micro RNAs in the aetiology of PD. In addition, recent studies have challenged the neuro-centric view of PD pathology, moving microglia and astrocytes into the research spotlight. Greater insights into these mechanisms may hold the key for the development of novel targeted therapies, addressing the need for a disease-modifying treatment, which has remained elusive to date.
•The respirasome is the most abundant supercomplex in mammalian mitochondria.•The functional roles of the supercomplexes remain questioned.•There are two current hypotheses for the biogenesis of the ...respirasome.•The biogenesis of SC III2+IV is independent from the respirasome.•The recent structures of the respirasomes represent an important novelty.
Over the past sixty years, researchers have made outmost efforts to clarify the structural organization and functional regulation of the complexes that configure the mitochondrial respiratory chain. As a result, the entire composition of each individual complex is practically known and, aided by notable structural advances in mammals, it is now widely accepted that these complexes stablish interactions to form higher-order supramolecular structures called supercomplexes and respirasomes. The mechanistic models and players that regulate the function and biogenesis of such superstructures are still under intense debate, and represent one of the hottest topics of the mitochondrial research field at present. Noteworthy, understanding the pathways involved in the assembly and organization of respiratory chain complexes and supercomplexes is of high biomedical relevance because molecular alterations in these pathways frequently result in severe mitochondrial disorders. The purpose of this review is to update the structural, biogenetic and functional knowledge about the respiratory chain supercomplexes and assembly factors involved in their formation, with special emphasis on their implications in mitochondrial disease. Thanks to the integrated data resulting from recent structural, biochemical and genetic approaches in diverse biological systems, the regulation of the respiratory chain function arises at multiple levels of complexity.
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