Interaction between AIF and CHCHD4 Regulates Respiratory Chain Biogenesis

Hippocampal transcriptomic analyses reveal the potential antiapoptotic mechanism of a novel anticonvulsant agent Q808 on pentylenetetrazol-induced epilepsy in rats

Brain apoptosis is one of the main causes of epileptogenesis. The antiapoptotic effect and potential mechanism of Q808, an innovative anticonvulsant chemical, have never been reported. In this study, the seizure stage and latency to reach stage 2 of pentylenetetrazol (PTZ) seizure rat model treated with Q808 were investigated. The morphological change and neuronal apoptosis in the hippocampus were detected by hematoxylin and eosin (HE) and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining, respectively. The hippocampal transcriptomic changes were observed using RNA sequencing (RNA-seq). The expression levels of hub genes were verified by quantitative reverse-transcription PCR (qRT-PCR). Results revealed that Q808 could allay the seizure score and prolong the stage 2 latency in seizure rats. The morphological changes of neurons and the number of apoptotic cells in the DG area were diminished by Q808 treatment. RNA-seq analysis revealed eight hub genes, including Map2k3, Nfs1, Chchd4, Hdac6, Siglec5, Slc35d3, Entpd1, and LOC103690108, and nine hub pathways among the control, PTZ, and Q808 groups. Hub gene Nfs1 was involved in the hub pathway sulfur relay system, and Map2k3 was involved in the eight remaining hub pathways, including Amyotrophic lateral sclerosis, Cellular senescence, Fc epsilon RI signaling pathway, GnRH signaling pathway, Influenza A, Rap1 signaling pathway, TNF signaling pathway, and Toll-like receptor signaling pathway. qRT-PCR confirmed that the mRNA levels of these hub genes were consistent with the RNA-seq results. Our findings might contribute to further studies exploring the new apoptosis mechanism and actions of Q808.

AIF translocation into nucleus caused by Aifm1 R450Q mutation: generation and characterization of a mouse model for AUNX1

Mutations in AIFM1, encoding for apoptosis-inducing factor (AIF), cause AUNX1, an X-linked neurologic disorder with late-onset auditory neuropathy (AN) and peripheral neuropathy. Despite significant research on AIF, there are limited animal models with the disrupted AIFM1 representing the corresponding phenotype of human AUNX1, characterized by late-onset hearing loss and impaired auditory pathways. Here, we generated an Aifm1 p.R450Q knock-in mouse model (KI) based on the human AIFM1 p.R451Q mutation. Hemizygote KI male mice exhibited progressive hearing loss from P30 onward, with greater severity at P60 and stabilization until P210. Additionally, muscle atrophy was observed at P210. These phenotypic changes were accompanied by a gradual reduction in the number of spiral ganglion neuron cells (SGNs) at P30 and ribbons at P60, which coincided with the translocation of AIF into the nucleus starting from P21 and P30, respectively. The SGNs of KI mice at P210 displayed loss of cytomembrane integrity, abnormal nuclear morphology, and dendritic and axonal demyelination. Furthermore, the inner hair cells and myelin sheath displayed abnormal mitochondrial morphology, while fibroblasts from KI mice showed impaired mitochondrial function. In conclusion, we successfully generated a mouse model recapitulating AUNX1. Our findings indicate that disruption of Aifm1 induced the nuclear translocation of AIF, resulting in the impairment in the auditory pathway.

CHCHD4 binding affects the active site of apoptosis inducing factor (AIF): Structural determinants for allosteric regulation

Apoptosis-inducing factor (AIF), which is confined to mitochondria of normal healthy cells, is the first identified caspase-independent cell death effector. Moreover, AIF is required for the optimal functioning of the respiratory chain machinery. Recent findings have revealed that AIF fulfills its pro-survival function by interacting with CHCHD4, a soluble mitochondrial protein which promotes the entrance and the oxidative folding of different proteins in the inner membrane space. Here, we report the crystal structure of the ternary complex involving the N-terminal 27-mer peptide of CHCHD4, NAD+, and AIF harboring its FAD (flavin adenine dinucleotide) prosthetic group in oxidized form. Combining this information with biophysical and biochemical data on the CHCHD4/AIF complex, we provide a detailed structural description of the interaction between the two proteins, validated by both chemical cross-linking mass spectrometry analysis and site-directed mutagenesis.

Chemical screening by time-resolved X-ray scattering to discover allosteric probes

Drug discovery relies on efficient identification of small-molecule leads and their interactions with macromolecular targets. However, understanding how chemotypes impact mechanistically important conformational states often remains secondary among high-throughput discovery methods. Here, we present a conformational discovery pipeline integrating time-resolved, high-throughput small-angle X-ray scattering (TR-HT-SAXS) and classic fragment screening applied to allosteric states of the mitochondrial import oxidoreductase apoptosis-inducing factor (AIF). By monitoring oxidized and X-ray-reduced AIF states, TR-HT-SAXS leverages structure and kinetics to generate a multidimensional screening dataset that identifies fragment chemotypes allosterically stimulating AIF dimerization. Fragment-induced dimerization rates, quantified with time-resolved SAXS similarity analysis (kVR), capture structure-activity relationships (SAR) across the top-ranked 4-aminoquinoline chemotype. Crystallized AIF-aminoquinoline complexes validate TR-SAXS-guided SAR, supporting this conformational chemotype for optimization. AIF-aminoquinoline structures and mutational analysis reveal active site F482 as an underappreciated allosteric stabilizer of AIF dimerization. This conformational discovery pipeline illustrates TR-HT-SAXS as an effective technology for targeting chemical leads to important macromolecular states.

Mechanisms of chondrocyte regulated cell death in osteoarthritis: Focus on ROS-triggered ferroptosis, parthanatos, and oxeiptosis

Osteoarthritis (OA) is a common chronic inflammatory degenerative disease. Since chondrocytes are the only type of cells in cartilage, their survival is critical for maintaining cartilage morphology. This review offers a comprehensive analysis of how reactive oxygen species (ROS), including superoxide anions, hydrogen peroxide, hydroxyl radicals, nitric oxide, and their derivatives, affect cartilage homeostasis and trigger several novel modes of regulated cell death, including ferroptosis, parthanatos, and oxeiptosis, which may play roles in chondrocyte death and OA development. Moreover, we discuss potential therapeutic strategies to alleviate OA by scavenging ROS and provide new insight into the research and treatment of the role of regulated cell death in OA.

Focusing on mitochondria in the brain: from biology to therapeutics

Mitochondria have multiple functions such as supplying energy, regulating the redox status, and producing proteins encoded by an independent genome. They are closely related to the physiology and pathology of many organs and tissues, among which the brain is particularly prominent. The brain demands 20% of the resting metabolic rate and holds highly active mitochondrial activities. Considerable research shows that mitochondria are closely related to brain function, while mitochondrial defects induce or exacerbate pathology in the brain. In this review, we provide comprehensive research advances of mitochondrial biology involved in brain functions, as well as the mitochondria-dependent cellular events in brain physiology and pathology. Furthermore, various perspectives are explored to better identify the mitochondrial roles in neurological diseases and the neurophenotypes of mitochondrial diseases. Finally, mitochondrial therapies are discussed. Mitochondrial-targeting therapeutics are showing great potentials in the treatment of brain diseases.

A Missense Variant in AIFM1 Caused Mitochondrial Dysfunction and Intolerance to Riboflavin Deficiency

AIFM1 is a mitochondrial flavoprotein involved in caspase-independent cell death and regulation of respiratory chain complex biogenesis. Mutations in the AIFM1 gene have been associated with multiple clinical phenotypes, but the effectiveness of riboflavin treatment remains controversial. Furthermore, few studies explored the reasons underlying this controversy. We reported a 7-year-old boy with ataxia, sensorimotor neuropathy and muscle weakness. Genetic and histopathological analyses were conducted, along with assessments of mitochondrial function and apoptosis level induced by staurosporine. Riboflavin deficiency and supplementation experiments were performed using fibroblasts. A missense c.1019T > C (p. Met340Thr) variant of AIFM1 was detected in the proband, which caused reduced expression of AIFM1 protein and mitochondrial dysfunction as evidenced by downregulation of mitochondrial complex subunits, respiratory deficiency and collapse of ΔΨm. The proportion of apoptotic cells in mutant fibroblasts was lower than controls after induction of apoptosis. Riboflavin deficiency resulted in decreased AIFM1 protein levels, while supplementation with high concentrations of riboflavin partially increased AIFM1 protein levels in variant fibroblasts. In addition, mitochondrial respiratory function of mutant fibroblasts was partly improved after riboflavin supplementation. Our study elucidated the pathogenicity of the AIFM1 c.1019T > C variant and revealed mutant fibroblasts was intolerant to riboflavin deficiency. Riboflavin supplementation is helpful in maintaining the level of AIFM1 protein and mitochondrial respiratory function. Early riboflavin treatment may serve as a valuable attempt for patients with AIFM1 variant.

A two-step mitochondrial import pathway couples the disulfide relay with matrix complex I biogenesis

Mitochondria critically rely on protein import and its tight regulation. Here, we found that the complex I assembly factor NDUFAF8 follows a two-step import pathway linking IMS and matrix import systems. A weak targeting sequence drives TIM23-dependent NDUFAF8 matrix import, and en route, allows exposure to the IMS disulfide relay, which oxidizes NDUFAF8. Import is closely surveyed by proteases: YME1L prevents accumulation of excess NDUFAF8 in the IMS, while CLPP degrades reduced NDUFAF8 in the matrix. Therefore, NDUFAF8 can only fulfil its function in complex I biogenesis if both oxidation in the IMS and subsequent matrix import work efficiently. We propose that the two-step import pathway for NDUFAF8 allows integration of the activity of matrix complex I biogenesis pathways with the activity of the mitochondrial disulfide relay system in the IMS. Such coordination might not be limited to NDUFAF8 as we identified further proteins that can follow such a two-step import pathway.

Pathophysiology of Cerebellar Degeneration in Mitochondrial Disorders: Insights from the Harlequin Mouse

By means of a proteomic approach, we assessed the pathways involved in cerebellar neurodegeneration in a mouse model (Harlequin, Hq) of mitochondrial disorder. A differential proteomic profile study (iTRAQ) was performed in cerebellum homogenates of male Hq and wild-type (WT) mice 8 weeks after the onset of clear symptoms of ataxia in the Hq mice (aged 5.2 ± 0.2 and 5.3 ± 0.1 months for WT and Hq, respectively), followed by a biochemical validation of the most relevant changes. Additional groups of 2-, 3- and 6-month-old WT and Hq mice were analyzed to assess the disease progression on the proteins altered in the proteomic study. The proteomic analysis showed that beyond the expected deregulation of oxidative phosphorylation, the cerebellum of Hq mice showed a marked astroglial activation together with alterations in Ca²⁺ homeostasis and neurotransmission, with an up- and downregulation of GABAergic and glutamatergic neurotransmission, respectively, and the downregulation of cerebellar "long-term depression", a synaptic plasticity phenomenon that is a major player in the error-driven learning that occurs in the cerebellar cortex. Our study provides novel insights into the mechanisms associated with cerebellar degeneration in the Hq mouse model, including a complex deregulation of neuroinflammation, oxidative phosphorylation and glutamate, GABA and amino acids' metabolism.

Impaired AIF-CHCHD4 interaction and mitochondrial calcium overload contribute to auditory neuropathy spectrum disorder in patient-iPSC-derived neurons with AIFM1 variant

Auditory neuropathy spectrum disorder (ANSD) is a hearing impairment caused by dysfunction of inner hair cells, ribbon synapses, spiral ganglion neurons and/or the auditory nerve itself. Approximately 1/7000 newborns have abnormal auditory nerve function, accounting for 10%-14% of cases of permanent hearing loss in children. Although we previously identified the AIFM1 c.1265 G > A variant to be associated with ANSD, the mechanism by which ANSD is associated with AIFM1 is poorly understood. We generated induced pluripotent stem cells (iPSCs) from peripheral blood mononuclear cells (PBMCs) via nucleofection with episomal plasmids. The patient-specific iPSCs were edited via CRISPR/Cas9 technology to generate gene-corrected isogenic iPSCs. These iPSCs were further differentiated into neurons via neural stem cells (NSCs). The pathogenic mechanism was explored in these neurons. In patient cells (PBMCs, iPSCs, and neurons), the AIFM1 c.1265 G > A variant caused a novel splicing variant (c.1267-1305del), resulting in AIF p.R422Q and p.423-435del proteins, which impaired AIF dimerization. Such impaired AIF dimerization then weakened the interaction between AIF and coiled-coil-helix-coiled-coil-helix domain-containing protein 4 (CHCHD4). On the one hand, the mitochondrial import of ETC complex subunits was inhibited, subsequently leading to an increased ADP/ATP ratio and elevated ROS levels. On the other hand, MICU1-MICU2 heterodimerization was impaired, leading to _mCa²⁺ overload. Calpain was activated by _mCa²⁺ and subsequently cleaved AIF for its translocation into the nucleus, ultimately resulting in caspase-independent apoptosis. Interestingly, correction of the AIFM1 variant significantly restored the structure and function of AIF, further improving the physiological state of patient-specific iPSC-derived neurons. This study demonstrates that the AIFM1 variant is one of the molecular bases of ANSD. Mitochondrial dysfunction, especially _mCa²⁺ overload, plays a prominent role in ANSD associated with AIFM1. Our findings help elucidate the mechanism of ANSD and may lead to the provision of novel therapies.

SIRT5-related desuccinylation modification of AIFM1 protects against compression-induced intervertebral disc degeneration by regulating mitochondrial homeostasis

Mitochondrial dysfunction plays a major role in the development of intervertebral disc degeneration (IDD). Sirtuin 5 (SIRT5) participates in the maintenance of mitochondrial homeostasis through its desuccinylase activity. However, it is still unclear whether succinylation or SIRT5 is involved in the impairment of mitochondria and development of IDD induced by excessive mechanical stress. Our 4D label-free quantitative proteomic results showed decreased expression of the desuccinylase SIRT5 in rat nucleus pulposus (NP) tissues under mechanical loading. Overexpression of Sirt5 effectively alleviated, whereas knockdown of Sirt5 aggravated, the apoptosis and dysfunction of NP cells under mechanical stress, consistent with the more severe IDD phenotype of Sirt5 KO mice than wild-type mice that underwent lumbar spine instability (LSI) surgery. Moreover, immunoprecipitation-coupled mass spectrometry (IP-MS) results suggested that AIFM1 was a downstream target of SIRT5, which was verified by a Co-IP assay. We further demonstrated that reduced SIRT5 expression resulted in the increased succinylation of AIFM1, which in turn abolished the interaction between AIFM1 and CHCHD4 and thus led to the reduced electron transfer chain (ETC) complex subunits in NP cells. Reduced ETC complex subunits resulted in mitochondrial dysfunction and the subsequent occurrence of IDD under mechanical stress. Finally, we validated the efficacy of treatments targeting disrupted mitochondrial protein importation by upregulating SIRT5 expression or methylene blue (MB) administration in the compression-induced rat IDD model. In conclusion, our study provides new insights into the occurrence and development of IDD and offers promising therapeutic approaches for IDD.

Mitochondrial biology and dysfunction in secondary mitochondrial disease

Mitochondrial diseases are a broad, genetically heterogeneous class of metabolic disorders characterized by deficits in oxidative phosphorylation (OXPHOS). Primary mitochondrial disease (PMD) defines pathologies resulting from mutation of mitochondrial DNA (mtDNA) or nuclear genes affecting either mtDNA expression or the biogenesis and function of the respiratory chain. Secondary mitochondrial disease (SMD) arises due to mutation of nuclear-encoded genes independent of, or indirectly influencing OXPHOS assembly and operation. Despite instances of novel SMD increasing year-on-year, PMD is much more widely discussed in the literature. Indeed, since the implementation of next generation sequencing (NGS) techniques in 2010, many novel mitochondrial disease genes have been identified, approximately half of which are linked to SMD. This review will consolidate existing knowledge of SMDs and outline discrete categories within which to better understand the diversity of SMD phenotypes. By providing context to the biochemical and molecular pathways perturbed in SMD, we hope to further demonstrate the intricacies of SMD pathologies outside of their indirect contribution to mitochondrial energy generation.

Applying HT-SAXS to chemical ligand screening

Chemical probes are invaluable tools for investigating essential biological processes. Understanding how small-molecule probes engage biomolecular conformations is critical to developing their functional selectivity. High-throughput solution X-ray scattering is well-positioned to profile target-ligand complexes during probe development, bringing conformational insight and selection to traditional ligand binding assays. Access to high-quality synchrotron SAXS datasets and high-throughput data analysis now allows routine academic users to incorporate conformational information into small-molecule development pipelines. Here we describe a general approach for benchmarking and preparing HT-SAXS chemical screens from small fragment libraries. Using the allosteric oxidoreductase Apoptosis-Inducing Factor (AIF) as an exemplary system, we illustrate how HT-SAXS efficiently identifies an allosteric candidate among hits of a microscale thermophoresis ligand screen. We discuss considerations for pursuing HT-SAXS chemical screening with other systems of interest and reflect on advances to extend screening throughput and sensitivity.

Identification of a novel AIFM1 variant from a Chinese family with auditory neuropathy

Background: Auditory neuropathy (AN) is a specific type of hearing loss characterized by impaired language comprehension. Apoptosis inducing factor mitochondrion associated 1 (AIFM1) is the most common gene associated with late-onset AN. In this study, we aimed to screen the pathogenic variant of AIFM1 in a Chinese family with AN and to explore the molecular mechanism underlying the function of such variant in the development of AN.

Methods: One patient with AN and eight unaffected individuals from a Chinese family were enrolled in this study. A comprehensive clinical evaluation was performed on all participants. A targeted next-generation sequencing (NGS) analysis of a total of 406 known deafness genes was performed to screen the potential pathogenic variants in the proband. Sanger sequencing was used to confirm the variants identified in all participants. The pathogenicity of variant was predicted by bioinformatics analysis. Immunofluorescence and Western blot analyses were performed to evaluate the subcellular distribution and expression of the wild type (WT) and mutant AIFM1 proteins. Cell apoptosis was evaluated based on the TUNEL analyses.

Results: Based on the clinical evaluations, the proband in this family was diagnosed with AN. The results of NGS and Sanger sequencing showed that a novel missense mutation of AIFM1, i.e., c.1367A > G (p. D456G), was identified in this family. Bioinformatics analysis indicated that this variant was pathogenic. Functional analysis showed that in comparison with the WT, the mutation c.1367A > G of AIFM1 showed no effect on its subcellular localization and the ability to induce apoptosis, but changed its protein expression level.

Conclusion: A novel variant of AIFM1 was identified for the first time, which was probably the genetic cause of AN in a Chinese family with AN.

Proteomic Analyses Reveals the Mechanism of Acupotomy Intervention on the Treatment of Knee Osteoarthritis in Rabbits

Acupotomy intervention (AI) is an available treatment for knee osteoarthritis (KOA) in China, which is a common health problem over the world. However, the underlying mechanism of AI on the KOA treatment is still unknown. To further understand the mechanism of acupotomy in treating KOA, the morphological observation and TMT proteomic analyses were conducted in rabbits. By using X-ray and MRI, we found that the space of the knee joint was bigger in AI than in KOA. Moreover, the chondrocytes were neatly arranged in AI but disordered in KOA. With proteomic analyses in chondrocytes, 68 differently accumulated proteins (DAPs) were identified in AI vs. KOA and DAPs related to energy metabolism and the TCA cycle were suggested to play a central role in response to AI. Furthermore, AIFM1 was proposed to be an important regulator in controlling the energy production in mitochondrial. Besides, FN1, VIM, COL12A1, COL14A1, MYBPH, and DPYSL3 were suggested to play crucial roles in AI for the treatment of KOA. Our study was systematically elucidating the regulation mechanism of acupotomy intervention in the treatment of KOA.

Visualizing and accessing correlated SAXS data sets with Similarity Maps and Simple Scattering web resources

Constructing a comprehensive understanding of macromolecular behavior from a set of correlated small angle scattering (SAS) data is aided by tools that analyze all scattering curves together. SAS experiments on biological systems can be performed on specimens that are more easily prepared, modified, and formatted relative to those of most other techniques. An X-ray SAS measurement (SAXS) can be performed in less than a milli-second in-line with treatment steps such as purification or exposure to modifiers. These capabilities are valuable since biological macromolecules (proteins, polynucleotides, lipids, and carbohydrates) change conformation or assembly under specific conditions that often define their biological role. Furthermore, mutation or post-translational modification change their behavior and provides an avenue to tailor their mechanics. Here, we describe tools to combine multiple correlated SAS measurements for analysis and review their application to biological systems. The SAXS Similarity Map (SSM) compares a set of scattering curves and quantifies the similarity between them for display as a color on a grid. Visualizing an entire correlated data set with SSMs helps identify patterns that reveal biological functions. The SSM analysis is available as a web-based tool at https://sibyls.als.lbl.gov/saxs-similarity/. To make data available and promote tool development, we have also deployed a repository of correlated SAS data sets called Simple Scattering (available at https://simplescattering.com). The correlated data sets used to demonstrate the SSM are available on the Simple Scattering website. We expect increased utilization of correlated SAS measurements to characterize the tightly controlled mechanistic properties of biological systems and fine-tune engineered macromolecules for nanotechnology-based applications.

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.

Realgar (As4S4), a traditional Chinese medicine, induces acute promyelocytic leukemia cell death via the Bcl-2/Bax/Cyt-C/AIF signaling pathway in vitro

Acute promyelocytic leukemia (APL) is a specific subtype of acute myelogenous leukemia (AML) characterized by the proliferation of abnormal promyelocytes. Realgar, a Chinese medicine containing arsenic, can be taken orally. Traditional Chinese medicine physicians have employed realgar to treat APL for over a thousand years. Therefore, realgar may be a promising candidate for the treatment of APL. Nevertheless, the underlying mechanism behind realgar therapy is largely unclear. The present study aimed to investigate the effect of realgar on cell death in the APL cell line (NB4) in vitro and to elucidate the underlying mechanism. In this study, after APL cells were treated with different concentrations of realgar, the cell survival rate, apoptotic assay, morphological changes, ATP levels and cell cycle arrest were assessed. The expression of Bcl-2, Bax, Cytochrome C (Cyt-C) and apoptosis-inducing factor (AIF) at the mRNA and protein levels were also measured by immunofluorescence, quantitative PCR (qPCR) and Western blotting. We found that realgar could significantly inhibit APL cell proliferation and cell death in a time- and dose-dependent manner. Realgar effectively decreased the ATP levels in APL cells. Realgar also induced APL cell cycle arrest at the S and G2/M phases. Following realgar treatment, the mRNA and protein levels of Bcl-2 were significantly downregulated, whereas the levels of Bax, Cyt-C, and AIF were significantly upregulated. In summary, realgar can induce APL cell death via the Bcl-2/Bax/Cyt-C/AIF signaling pathway, suggesting that realgar may be an effective therapeutic for APL.

AIFM1 beyond cell death: An overview of this OXPHOS-inducing factor in mitochondrial diseases

Apoptosis-inducing factor (AIF) is a mitochondrial intermembrane space flavoprotein with diverse functions in cellular physiology. In this regard, a large number of studies have elucidated AIF's participation to chromatin condensation during cell death in development, cancer, cardiovascular and brain disorders. However, the discovery of rare AIFM1 mutations in patients has shifted the interest of biomedical researchers towards AIF's contribution to pathogenic mechanisms underlying inherited AIFM1-linked metabolic diseases. The functional characterization of AIF binding partners has rapidly advanced our understanding of AIF biology within the mitochondria and beyond its widely reported role in cell death. At the present time, it is reasonable to assume that AIF contributes to cell survival by promoting biogenesis and maintenance of the mitochondrial oxidative phosphorylation (OXPHOS) system. With this review, we aim to outline the current knowledge around the vital role of AIF by primarily focusing on currently reported human diseases that have been linked to AIFM1 deficiency.

AIFM1 is a component of the mitochondrial disulfide relay that drives complex I assembly through efficient import of NDUFS5

The mitochondrial intermembrane space protein AIFM1 has been reported to mediate the import of MIA40/CHCHD4, which forms the import receptor in the mitochondrial disulfide relay. Here, we demonstrate that AIFM1 and MIA40/CHCHD4 cooperate beyond this MIA40/CHCHD4 import. We show that AIFM1 and MIA40/CHCHD4 form a stable long‐lived complex in vitro, in different cell lines, and in tissues. In HEK293 cells lacking AIFM1, levels of MIA40 are unchanged, but the protein is present in the monomeric form. Monomeric MIA40 neither efficiently interacts with nor mediates the import of specific substrates. The import defect is especially severe for NDUFS5, a subunit of complex I of the respiratory chain. As a consequence, NDUFS5 accumulates in the cytosol and undergoes rapid proteasomal degradation. Lack of mitochondrial NDUFS5 in turn results in stalling of complex I assembly. Collectively, we demonstrate that AIFM1 serves two overlapping functions: importing MIA40/CHCHD4 and constituting an integral part of the disulfide relay that ensures efficient interaction of MIA40/CHCHD4 with specific substrates.

Mitochondrial protein translocation machinery: From TOM structural biogenesis to functional regulation

The human mitochondrial outer membrane is biophysically unique as it is the only membrane possessing transmembrane β-barrel proteins (mitochondrial outer membrane proteins, mOMPs) in the cell. The most vital of the three mOMPs is the core protein of the translocase of the outer mitochondrial membrane (TOM) complex. Identified first as MOM38 in Neurospora in 1990, the structure of Tom40, the core 19-stranded β-barrel translocation channel, was solved in 2017, after nearly three decades. Remarkably, the past four years have witnessed an exponential increase in structural and functional studies of yeast and human TOM complexes. In addition to being conserved across all eukaryotes, the TOM complex is the sole ATP-independent import machinery for nearly all of the ∼1000 to 1500 known mitochondrial proteins. Recent cryo-EM structures have provided detailed insight into both possible assembly mechanisms of the TOM core complex and organizational dynamics of the import machinery and now reveal novel regulatory interplay with other mOMPs. Functional characterization of the TOM complex using biochemical and structural approaches has also revealed mechanisms for substrate recognition and at least five defined import pathways for precursor proteins. In this review, we discuss the discovery, recently solved structures, molecular function, and regulation of the TOM complex and its constituents, along with the implications these advances have for alleviating human diseases.

Apoptosis-Inducing Factor Deficiency Induces Tissue-Specific Alterations in Autophagy: Insights from a Preclinical Model of Mitochondrial Disease and Exercise Training Effects

We analyzed the effects of apoptosis-inducing factor (AIF) deficiency, as well as those of an exercise training intervention on autophagy across tissues (heart, skeletal muscle, cerebellum and brain), that are primarily affected by mitochondrial diseases, using a preclinical model of these conditions, the Harlequin (Hq) mouse. Autophagy markers were analyzed in: (i) 2, 3 and 6 month-old male wild-type (WT) and Hq mice, and (ii) WT and Hq male mice that were allocated to an exercise training or sedentary group. The exercise training started upon onset of the first symptoms of ataxia in Hq mice and lasted for 8 weeks. Higher content of autophagy markers and free amino acids, and lower levels of sarcomeric proteins were found in the skeletal muscle and heart of Hq mice, suggesting increased protein catabolism. Leupeptin-treatment demonstrated normal autophagic flux in the Hq heart and the absence of mitophagy. In the cerebellum and brain, a lower abundance of Beclin 1 and ATG16L was detected, whereas higher levels of the autophagy substrate p62 and LAMP1 levels were observed in the cerebellum. The exercise intervention did not counteract the autophagy alterations found in any of the analyzed tissues. In conclusion, AIF deficiency induces tissue-specific alteration of autophagy in the Hq mouse, with accumulation of autophagy markers and free amino acids in the heart and skeletal muscle, but lower levels of autophagy-related proteins in the cerebellum and brain. Exercise intervention, at least if starting when muscle atrophy and neurological symptoms are already present, is not sufficient to mitigate autophagy perturbations.

Mitochondrial COA7 is a heme-binding protein with disulfide reductase activity, which acts in the early stages of complex IV assembly

Assembly factors play key roles in the biogenesis of mitochondrial protein complexes, regulating their stabilities, activities, and incorporation of essential cofactors. Cytochrome c oxidase assembly factor 7 (COA7) is a metazoan-specific assembly factor, the absence or mutation of which in humans accompanies complex IV assembly defects and neurological conditions. Here, we report the crystal structure of COA7 to 2.4 Å resolution, revealing a banana-shaped molecule composed of five helix-turn-helix (α/α) repeats. COA7 binds heme with micromolar affinity, even though the protein structure does not resemble previously characterized heme-binding proteins. The heme-bound COA7 can redox cycle between oxidation states Fe(II) and Fe(III) and shows disulfide reductase activity toward copper binding assembly factors. We propose that COA7 functions to facilitate the biogenesis of the binuclear copper site (Cu_A) of complex IV.

Cytochrome c oxidase (COX) assembly factor 7 (COA7) is a metazoan-specific assembly factor, critical for the biogenesis of mitochondrial complex IV (cytochrome c oxidase). Although mutations in COA7 have been linked to complex IV assembly defects and neurological conditions such as peripheral neuropathy, ataxia, and leukoencephalopathy, the precise role COA7 plays in the biogenesis of complex IV is not known. Here, we show that loss of COA7 blocks complex IV assembly after the initial step where the COX1 module is built, progression from which requires the incorporation of copper and addition of the COX2 and COX3 modules. The crystal structure of COA7, determined to 2.4 Å resolution, reveals a banana-shaped molecule composed of five helix-turn-helix (α/α) repeats, tethered by disulfide bonds. COA7 interacts transiently with the copper metallochaperones SCO1 and SCO2 and catalyzes the reduction of disulfide bonds within these proteins, which are crucial for copper relay to COX2. COA7 binds heme with micromolar affinity, through axial ligation to the central iron atom by histidine and methionine residues. We therefore propose that COA7 is a heme-binding disulfide reductase for regenerating the copper relay system that underpins complex IV assembly.

The ins and outs of the flavin mononucleotide cofactor of respiratory complex I

The flavin mononucleotide (FMN) cofactor of respiratory complex I occupies a key position in the electron transport chain. Here, the electrons coming from NADH start the sequence of oxidoreduction reactions, which drives the generation of the proton-motive force necessary for ATP synthesis. The overall architecture and the general catalytic proprieties of the FMN site are mostly well established. However, several aspects regarding the complex I flavin cofactor are still unknown. For example, the flavin binding to the N-module, the NADH-oxidizing portion of complex I, lacks a molecular description. The dissociation of FMN from the enzyme is beginning to emerge as an important regulatory mechanism of complex I activity and ROS production. Finally, how mitochondria import and metabolize FMN is still uncertain. This review summarizes the current knowledge on complex I flavin cofactor and discusses the open questions for future research.

Ubiquitinated AIF is a major mediator of hypoxia-induced mitochondrial dysfunction and pulmonary artery smooth muscle cell proliferation

BACKGROUND

Excessive proliferation of pulmonary artery smooth muscle cells (PASMCs) is the main cause of hypoxic pulmonary hypertension (PH), and mitochondrial homeostasis plays a crucial role. However, the specific molecular regulatory mechanism of mitochondrial function in PASMCs remains unclear.

METHODS

In this study, using the CCK8 assay, EdU incorporation, flow cytometry, Western blotting, co-IP, mass spectrometry, electron microscopy, immunofluorescence, Seahorse extracellular flux analysis and echocardiography, we investigated the specific involvement of apoptosis-inducing factor (AIF), a mitochondrial oxidoreductase in regulating mitochondrial energy metabolism and mitophagy in PASMCs.

RESULTS

In vitro, AIF deficiency in hypoxia leads to impaired oxidative phosphorylation and increased glycolysis and ROS release because of the loss of mitochondrial complex I activity. AIF was also downregulated and ubiquitinated under hypoxia leading to the abnormal occurrence of mitophagy and autophagy through its interaction with ubiquitin protein UBA52. In vivo, treatment with the adeno-associated virus vector to overexpress AIF protected pulmonary vascular remodeling from dysfunctional and abnormal proliferation.

CONCLUSIONS

Taken together, our results identify AIF as a potential therapeutic target for PH and reveal a novel posttranscriptional regulatory mechanism in hypoxia-induced mitochondrial dysfunction.

Ginsenoside Rd: A promising natural neuroprotective agent

BACKGROUND

Neurological diseases seriously affect human health, which are arousing wider attention, and it is a great challenge to discover neuroprotective drugs with minimal side-effects and better efficacies. Natural agents derived from herbs or plants have become unparalleled resources for the discovery of novel drug candidates. Panax ginseng C. A. Meyer, a well-known herbal medicine in China, occupies a very important position in traditional Chinese medicines (TCMs) with a long history of clinical application. Ginsenoside Rd is the active compound in P. ginseng known to have broad-spectrum pharmacological effects to reduce neurological damage that can lead to neurological diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, depression, cognitive impairment, and cerebral ischemia.

PURPOSE

To review and discuss the effects and mechanisms of ginsenoside Rd in the treatment of neurological diseases.

STUDY DESIGN & METHODS

The related information was compiled by the major scientific databases, such as Chinese National Knowledge Infrastructure (CNKI), Elsevier, ScienceDirect, PubMed, SpringerLink, Web of Science, and GeenMedical. Using 'Ginsenoside Rd', 'Ginsenosides', 'Anti-inflammation', 'Antioxidant', 'Apoptosis' and 'Neuroprotection' as keywords, the correlated literature was extracted and conducted from the databases mentioned above.

RESULTS

Through summarizing the existing research progress, we found that the general effects of ginsenoside Rd are anti-inflammatory, antioxidant, anti-apoptosis, inhibition of Ca²⁺ influx and protection of mitochondria, and through these pathways, the compound can inhibit excitatory toxicity, regulate nerve growth factor, and promote nerve regeneration.

CONCLUSION

Ginsenoside Rd is a promising natural neuroprotective agent. This review would contribute to the future development of ginsenoside Rd as a novel clinical candidate drug for treating neurological diseases.

Relevance of AIF/CypA Lethal Pathway in SH-SY5Y Cells Treated with Staurosporine

The AIF/CypA complex exerts a lethal activity in several rodent models of acute brain injury. Upon formation, it translocates into the nucleus of cells receiving apoptotic stimuli, inducing chromatin condensation, DNA fragmentation, and cell death by a caspase-independent mechanism. Inhibition of this complex in a model of glutamate-induced cell death in HT-22 neuronal cells by an AIF peptide (AIF(370-394)) mimicking the binding site on CypA, restores cell survival and prevents brain injury in neonatal mice undergoing hypoxia-ischemia without apparent toxicity. Here, we explore the effects of the peptide on SH-SY5Y neuroblastoma cells stimulated with staurosporine (STS), a cellular model widely used to study Parkinson’s disease (PD). This will pave the way to understanding the role of the complex and the potential therapeutic efficacy of inhibitors in PD. We find that AIF(370-394) confers resistance to STS-induced apoptosis in SH-SY5Y cells similar to that observed with CypA silencing and that the peptide works on the AIF/CypA translocation pathway and not on caspases activation. These findings suggest that the AIF/CypA complex is a promising target for developing novel therapeutic strategies against PD.

The Contextual Essentiality of Mitochondrial Genes in Cancer

Mitochondria are key organelles in eukaryotic evolution that perform crucial roles as metabolic and cellular signaling hubs. Mitochondrial function and dysfunction are associated with a range of diseases, including cancer. Mitochondria support cancer cell proliferation through biosynthetic reactions and their role in signaling, and can also promote tumorigenesis via processes such as the production of reactive oxygen species (ROS). The advent of (nuclear) genome-wide CRISPR-Cas9 deletion screens has provided gene-level resolution of the requirement of nuclear-encoded mitochondrial genes (NEMGs) for cancer cell viability (essentiality). More recently, it has become apparent that the essentiality of NEMGs is highly dependent on the cancer cell context. In particular, key tumor microenvironmental factors such as hypoxia, and changes in nutrient (e.g., glucose) availability, significantly influence the essentiality of NEMGs. In this mini-review we will discuss recent advances in our understanding of the contribution of NEMGs to cancer from CRISPR-Cas9 deletion screens, and discuss emerging concepts surrounding the context-dependent nature of mitochondrial gene essentiality.

Role of the Mitochondrial Protein Import Machinery and Protein Processing in Heart Disease

Mitochondria are essential organelles for cellular energy production, metabolic homeostasis, calcium homeostasis, cell proliferation, and apoptosis. About 99% of mammalian mitochondrial proteins are encoded by the nuclear genome, synthesized as precursors in the cytosol, and imported into mitochondria by mitochondrial protein import machinery. Mitochondrial protein import systems function not only as independent units for protein translocation, but also are deeply integrated into a functional network of mitochondrial bioenergetics, protein quality control, mitochondrial dynamics and morphology, and interaction with other organelles. Mitochondrial protein import deficiency is linked to various diseases, including cardiovascular disease. In this review, we describe an emerging class of protein or genetic variations of components of the mitochondrial import machinery involved in heart disease. The major protein import pathways, including the presequence pathway (TIM23 pathway), the carrier pathway (TIM22 pathway), and the mitochondrial intermembrane space import and assembly machinery, related translocases, proteinases, and chaperones, are discussed here. This review highlights the importance of mitochondrial import machinery in heart disease, which deserves considerable attention, and further studies are urgently needed. Ultimately, this knowledge may be critical for the development of therapeutic strategies in heart disease.

Molecular characterization of a complex of apoptosis-inducing factor 1 with cytochrome c oxidase of the mitochondrial respiratory chain

Combining mass spectrometry-based chemical cross-linking and complexome profiling, we analyzed the interactome of heart mitochondria. We focused on complexes of oxidative phosphorylation and found that dimeric apoptosis-inducing factor 1 (AIFM1) forms a defined complex with ∼10% of monomeric cytochrome c oxidase (COX) but hardly interacts with respiratory chain supercomplexes. Multiple AIFM1 intercross-links engaging six different COX subunits provided structural restraints to build a detailed atomic model of the COX-AIFM12 complex (PDBDEV_00000092). An application of two complementary proteomic approaches thus provided unexpected insight into the macromolecular organization of the mitochondrial complexome. Our structural model excludes direct electron transfer between AIFM1 and COX. Notably, however, the binding site of cytochrome c remains accessible, allowing formation of a ternary complex. The discovery of the previously overlooked COX-AIFM12 complex and clues provided by the structural model hint at potential roles of AIFM1 in oxidative phosphorylation biogenesis and in programmed cell death.

Redox-Mediated Regulation of Mitochondrial Biogenesis, Dynamics, and Respiratory Chain Assembly in Yeast and Human Cells

Mitochondria are double-membrane organelles that contain their own genome, the mitochondrial DNA (mtDNA), and reminiscent of its endosymbiotic origin. Mitochondria are responsible for cellular respiration via the function of the electron oxidative phosphorylation system (OXPHOS), located in the mitochondrial inner membrane and composed of the four electron transport chain (ETC) enzymes (complexes I-IV), and the ATP synthase (complex V). Even though the mtDNA encodes essential OXPHOS components, the large majority of the structural subunits and additional biogenetical factors (more than seventy proteins) are encoded in the nucleus and translated in the cytoplasm. To incorporate these proteins and the rest of the mitochondrial proteome, mitochondria have evolved varied, and sophisticated import machineries that specifically target proteins to the different compartments defined by the two membranes. The intermembrane space (IMS) contains a high number of cysteine-rich proteins, which are mostly imported via the MIA40 oxidative folding system, dependent on the reduction, and oxidation of key Cys residues. Several of these proteins are structural components or assembly factors necessary for the correct maturation and function of the ETC complexes. Interestingly, many of these proteins are involved in the metalation of the active redox centers of complex IV, the terminal oxidase of the mitochondrial ETC. Due to their function in oxygen reduction, mitochondria are the main generators of reactive oxygen species (ROS), on both sides of the inner membrane, i.e., in the matrix and the IMS. ROS generation is important due to their role as signaling molecules, but an excessive production is detrimental due to unwanted oxidation reactions that impact on the function of different types of biomolecules contained in mitochondria. Therefore, the maintenance of the redox balance in the IMS is essential for mitochondrial function. In this review, we will discuss the role that redox regulation plays in the maintenance of IMS homeostasis as well as how mitochondrial ROS generation may be a key regulatory factor for ETC biogenesis, especially for complex IV.

Amyotrophic Lateral Sclerosis (ALS): Stressed by Dysfunctional Mitochondria-Endoplasmic Reticulum Contacts (MERCs)

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease for which there is currently no cure. Progress in the characterization of other neurodegenerative mechanisms has shifted the spotlight onto an intracellular structure called mitochondria-endoplasmic reticulum (ER) contacts (MERCs) whose ER portion can be biochemically isolated as mitochondria-associated membranes (MAMs). Within the central nervous system (CNS), these structures control the metabolic output of mitochondria and keep sources of oxidative stress in check via autophagy. The most relevant MERC controllers in the ALS pathogenesis are vesicle-associated membrane protein-associated protein B (VAPB), a mitochondria-ER tether, and the ubiquitin-specific chaperone valosin containing protein (VCP). These two systems cooperate to maintain mitochondrial energy output and prevent oxidative stress. In ALS, mutant VAPB and VCP take a central position in the pathology through MERC dysfunction that ultimately alters or compromises mitochondrial bioenergetics. Intriguingly, both proteins are targets themselves of other ALS mutant proteins, including C9orf72, FUS, or TDP-43. Thus, a new picture emerges, where different triggers cause MERC dysfunction in ALS, subsequently leading to well-known pathological changes including endoplasmic reticulum (ER) stress, inflammation, and motor neuron death.

Molecular Insights into Mitochondrial Protein Translocation and Human Disease

In human mitochondria, mtDNA encodes for only 13 proteins, all components of the OXPHOS system. The rest of the mitochondrial components, which make up approximately 99% of its proteome, are encoded in the nuclear genome, synthesized in cytosolic ribosomes and imported into mitochondria. Different import machineries translocate mitochondrial precursors, depending on their nature and the final destination inside the organelle. The proper and coordinated function of these molecular pathways is critical for mitochondrial homeostasis. Here, we will review molecular details about these pathways, which components have been linked to human disease and future perspectives on the field to expand the genetic landscape of mitochondrial diseases.

Increased levels of mitochondrial import factor Mia40 prevent the aggregation of polyQ proteins in the cytosol

The formation of protein aggregates is a hallmark of neurodegenerative diseases. Observations on patient samples and model systems demonstrated links between aggregate formation and declining mitochondrial functionality, but causalities remain unclear. We used Saccharomyces cerevisiae to analyze how mitochondrial processes regulate the behavior of aggregation‐prone polyQ protein derived from human huntingtin. Expression of Q97‐GFP rapidly led to insoluble cytosolic aggregates and cell death. Although aggregation impaired mitochondrial respiration only slightly, it considerably interfered with the import of mitochondrial precursor proteins. Mutants in the import component Mia40 were hypersensitive to Q97‐GFP, whereas Mia40 overexpression strongly suppressed the formation of toxic Q97‐GFP aggregates both in yeast and in human cells. Based on these observations, we propose that the post‐translational import of mitochondrial precursor proteins into mitochondria competes with aggregation‐prone cytosolic proteins for chaperones and proteasome capacity. Mia40 regulates this competition as it has a rate‐limiting role in mitochondrial protein import. Therefore, Mia40 is a dynamic regulator in mitochondrial biogenesis that can be exploited to stabilize cytosolic proteostasis.

Protein import by the mitochondrial disulfide relay in higher eukaryotes

The proteome of the mitochondrial intermembrane space (IMS) contains more than 100 proteins, all of which are synthesized on cytosolic ribosomes and consequently need to be imported by dedicated machineries. The mitochondrial disulfide relay is the major import machinery for soluble proteins in the IMS. Its major component, the oxidoreductase MIA40, interacts with incoming substrates, retains them in the IMS, and oxidatively folds them. After this reaction, MIA40 is reoxidized by the sulfhydryl oxidase augmenter of liver regeneration, which couples disulfide formation by this machinery to the activity of the respiratory chain. In this review, we will discuss the import of IMS proteins with a focus on recent findings showing the diversity of disulfide relay substrates, describing the cytosolic control of this import system and highlighting the physiological relevance of the disulfide relay machinery in higher eukaryotes.

Content of apoptosis factors and self-organization processes in the mitochondria of heart cells in female mice C57BL/6 under growth of melanoma B16 / F10 linked with comorbid pathology

The aim is to study some mechanisms of regulation of apoptosis and self-organization in the mitochondria in the heart cells in female mice during the growth of experimental melanoma B16/ F10 linked with chronic neurogenic pain as comorbid pathology.

Mechanistic connections between mitochondrial biology and regulated cell death

The ancient, dynamic, and multifaceted functions of the mitochondrial network are essential for organismal homeostasis and contribute to numerous human diseases. As central hubs for metabolism, ion transport, and multiple macromolecular synthesis pathways, mitochondria establish and control extensive signaling networks to ensure cellular survival. In this review, we explore how these same mitochondrial functions also participate in the control of regulated cell death (RCD). We discuss the complementary essential mitochondrial functions as compartments that participate in the production and presentation of key molecules and platforms that actively enable, initiate, and execute RCD.

The Mia40/CHCHD4 Oxidative Folding System: Redox Regulation and Signaling in the Mitochondrial Intermembrane Space

Mitochondria are critical for several cellular functions as they control metabolism, cell physiology, and cell death. The mitochondrial proteome consists of around 1500 proteins, the vast majority of which (about 99% of them) are encoded by nuclear genes, with only 13 polypeptides in human cells encoded by mitochondrial DNA. Therefore, it is critical for all the mitochondrial proteins that are nuclear-encoded to be targeted precisely and sorted specifically to their site of action inside mitochondria. These processes of targeting and sorting are catalysed by protein translocases that operate in each one of the mitochondrial sub-compartments. The main protein import pathway for the intermembrane space (IMS) recognises proteins that are cysteine-rich, and it is the only import pathway that chemically modifies the imported precursors by introducing disulphide bonds to them. In this manner, the precursors are trapped in the IMS in a folded state. The key component of this pathway is Mia40 (called CHCHD4 in human cells), which itself contains cysteine motifs and is subject to redox regulation. In this review, we detail the basic components of the MIA pathway and the disulphide relay mechanism that underpins the electron transfer reaction along the oxidative folding mechanism. Then, we discuss the key protein modulators of this pathway and how they are interlinked to the small redox-active molecules that critically affect the redox state in the IMS. We present also evidence that the mitochondrial redox processes that are linked to iron–sulfur clusters biogenesis and calcium homeostasis coalesce in the IMS at the MIA machinery. The fact that the MIA machinery and several of its interactors and substrates are linked to a variety of common human diseases connected to mitochondrial dysfunction highlight the potential of redox processes in the IMS as a promising new target for developing new treatments for some of the most complex and devastating human diseases.

RIP3 facilitates necroptosis through CaMKII and AIF after intracerebral hemorrhage in mice

BACKGROUND

Necroptosis-induced neuronal damage after intracerebral hemorrhage (ICH) has been documented recently. Previous studies have reported that RIP3 and its complex are recognized as central mediators of necroptosis. In this study, the role of RIP3 in the activation of CaMKII and AIF was investigated.

METHODS

We induced ICH in C57BL/6 mice by injecting collagenase IV into the basal ganglia. ICH mice were pretreated with the mPTP inhibitor CsA and the CAMKII inhibitor Kn-93, RIP3 siRNA or RIP3 rAAV. Brain edema and neurobehavior were evaluated. The expression of RIP3, p-MLKL, AIF, and CaMKII proteins was evaluated by western blotting, immunofluorescence (IF) and immunoprecipitation (IP).

RESULTS

Significant increases in RIP3, p-MLKL, CaMKII and AIF expression were observed in ICH mice, and RIP3-AIF colocalized in the nucleus. Overexpression of RIP3 by rAAV upregulated AIF expression in both the cytoplasm and nucleus, while CaMKII expression was increased in the cytoplasm. The interaction of RIP3-AIF and RIP3-CaMKII was detected after ICH injury. These complexes were inhibited by CsA with Kn-93 or RIP3 siRNA pretreatment, which reduced brain edema and neurological deficits.

CONCLUSIONS

Our findings revealed that ICH induced necroptotic neuronal death through the RIP3-CaMKII complex and the RIP3-AIF signaling pathway. Moreover, blockade of mPTP opening could suppress the pathogenesis of necroptosis.

Molecular characterization and functional analysis of apoptosis-inducing factor (AIF) in palmitic acid-induced apoptosis in Ctenopharyngodon idellus kidney (CIK) cells

Palmitic acid (PA), the most common saturated free fatty acid, may cause apoptosis when overloaded in non-fat cells. Apoptosis-inducing factor (AIF) is known to translocate from the mitochondria into the nucleus to induce apoptosis. However, it remains to be investigated whether AIF involved in palmitic acid-induced lipoapoptosis in fish. In the present study, we cloned a coding sequence of grass carp (Ctenopharyngodon idella) AIF (CiAIF) gene, and determined its function in Ctenopharyngodon idellus kidney (CIK) cells. The open reading frame (ORF) of CiAIF gene is 1863 bp, encoding a precursor protein of 620 amino acids (aa). Sequence analysis indicated that CiAIF contains a mitochondrial localization sequence, a conserved Pyr_redox and a C-terminal domain. Phylogenetic analyses showed that the CiAIF gene tended to cluster with sequences from Danio rerio. CiAIF gene was ubiquitously expressed in all tested tissues, including heart, liver, spleen, muscle, brain, eye, kidney, intestine, and fat. Moreover, we demonstrated that PA treatment induced the expression level of CiAIF and increases in markers of endoplasmic reticulum (ER) stress and apoptosis. Meanwhile, ER stress-inducing agent thapsigargin (TG) induced CiAIF translocated into the nucleus in CIK cells, whereas the suppression of ER stress inhibited PA-induced CiAIF expression and apoptosis. In addition, overexpression of CiAIF caused apoptosis by upregulating capase9, capase8, and capase3b, and affects protein translation via directly interacting with CieIF3g. Taken together, our data indicate that in Ctenopharyngodon idellus, PA is key elements that affect not only ER stress and mitochondrial apoptosis but also different physiological functions, such as protein translation, and CiAIF might play a key role in this progress.

OTUD1 Activates Caspase-Independent and Caspase-Dependent Apoptosis by Promoting AIF Nuclear Translocation and MCL1 Degradation

Apoptosis-inducing factor (AIF) plays a dual role in regulating cell survival and apoptosis, acting as a prosurvival factor in mitochondria via its NADH oxidoreductase activity and activating the caspase-independent apoptotic pathway (i.e., parthanatos) after nuclear translocation. However, whether one factor conjunctively controls the separated functions of AIF is not clear. Here, it is shown that OTU deubiquitinase 1 (OTUD1) acts as a link between the two functions of AIF via deubiquitination events. Deubiquitination of AIF at K244 disrupts the normal mitochondrial structure and compromises oxidative phosphorylation, and deubiquitination of AIF at K255 enhances its DNA-binding ability to promote parthanatos. Moreover, OTUD1 stabilizes DDB1 and CUL4 associated factor 10 (DCAF10) and recruits the cullin 4A (CUL4A)-damage specific DNA binding protein 1 (DDB1) complex to promote myeloid cell leukemia sequence 1 (MCL1) degradation, thereby activating caspase-dependent apoptotic signaling. Collectively, these results reveal the central role of OTUD1 in activating both caspase-independent and caspase-dependent apoptotic signaling and propose decreased OTUD1 expression as a key event promoting chemoresistance in esophageal squamous cell carcinoma.

An antibody toolbox to track complex I assembly defines AIF's mitochondrial function

An ability to comprehensively track the assembly intermediates (AIs) of complex I (CI) biogenesis in Drosophila will enable the characterization of the precise mechanism(s) by which various CI regulators modulate CI assembly. Accordingly, we generated 21 novel antibodies to various mitochondrial proteins and used this resource to characterize the mechanism by which apoptosis-inducing factor (AIF) regulates CI biogenesis by tracking the AI profile observed when AIF expression is impaired. We find that when the AIF–Mia40 translocation complex is disrupted, the part of CI that transfers electrons to ubiquinone is synthesized but fails to progress in the CI biosynthetic pathway. This is associated with a reduction in intramitochondrial accumulation of the Mia40 substrate, MIC19. Importantly, knockdown of either MIC19 or MIC60, components of the mitochondrial contact site and cristae organizing system (MICOS), fully recapitulates the AI profile observed when AIF is inhibited. Thus, AIF’s effect on CI assembly is principally due to compromised intramitochondrial transport of the MICOS complex.

C9orf72 regulates energy homeostasis by stabilizing mitochondrial complex I assembly

The haploinsufficiency of C9orf72 is implicated in the most common forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the full spectrum of C9orf72 functions remains to be established. Here, we report that C9orf72 is a mitochondrial inner-membrane-associated protein regulating cellular energy homeostasis via its critical role in the control of oxidative phosphorylation (OXPHOS). The translocation of C9orf72 from the cytosol to the inter-membrane space is mediated by the redox-sensitive AIFM1/CHCHD4 pathway. In mitochondria, C9orf72 specifically stabilizes translocase of inner mitochondrial membrane domain containing 1 (TIMMDC1), a crucial factor for the assembly of OXPHOS complex I. C9orf72 directly recruits the prohibitin complex to inhibit the m-AAA protease-dependent degradation of TIMMDC1. The mitochondrial complex I function is impaired in C9orf72-linked ALS/FTD patient-derived neurons. These results reveal a previously unknown function of C9orf72 in mitochondria and suggest that defective energy metabolism may underlie the pathogenesis of relevant diseases.

The mitochondrial intermembrane space: the most constricted mitochondrial sub-compartment with the largest variety of protein import pathways

The mitochondrial intermembrane space (IMS) is the most constricted sub-mitochondrial compartment, housing only about 5% of the mitochondrial proteome, and yet is endowed with the largest variability of protein import mechanisms. In this review, we summarize our current knowledge of the major IMS import pathway based on the oxidative protein folding pathway and discuss the stunning variability of other IMS protein import pathways. As IMS-localized proteins only have to cross the outer mitochondrial membrane, they do not require energy sources like ATP hydrolysis in the mitochondrial matrix or the inner membrane electrochemical potential which are critical for import into the matrix or insertion into the inner membrane. We also explore several atypical IMS import pathways that are still not very well understood and are guided by poorly defined or completely unknown targeting peptides. Importantly, many of the IMS proteins are linked to several human diseases, and it is therefore crucial to understand how they reach their normal site of function in the IMS. In the final part of this review, we discuss current understanding of how such IMS protein underpin a large spectrum of human disorders.

The apoptosis-inducing factor family: Moonlighting proteins in the crosstalk between mitochondria and nuclei

In Homo sapiens, the apoptosis-inducing factor (AIF) family is represented by three different proteins, known as AIF, AMID and AIFL, that have in common the mitochondrial localisation in healthy cells, the presence of FAD- and NADH-dependent domains involved in an -albeit yet not well understood- oxidoreductase function and their capability to induce programmed cell death. AIF is the best characterised family member, while the information about AMID and AIFL is much scarcer. Nonetheless, available data support different roles as well as mechanisms of action of their particular apoptogenic and redox domains regarding both pro-apoptotic and anti-apoptotic activities. Moreover, diverse cellular functions, to date far from fully clarified, are envisaged for the transcripts corresponding to these three proteins. Here, we review the so far available knowledge on the moonlighting human AIF family from their molecular properties to their relevance in health and disease, through the evaluation of their potential cell death and redox functions in their different subcellular locations. This picture emerging from the current knowledge of the AIF family envisages its contribution to regulate signalling and transcription machineries in the crosstalk among mitochondria, the cytoplasm and the nucleus.

CHCHD4 (MIA40) and the mitochondrial disulfide relay system

Mitochondria are pivotal for normal cellular physiology, as they perform a crucial role in diverse cellular functions and processes, including respiration and the regulation of bioenergetic and biosynthetic pathways, as well as regulating cellular signalling and transcriptional networks. In this way, mitochondria are central to the cell's homeostatic machinery, and as such mitochondrial dysfunction underlies the pathology of a diverse range of diseases including mitochondrial disease and cancer. Mitochondrial import pathways and targeting mechanisms provide the means to transport into mitochondria the hundreds of nuclear-encoded mitochondrial proteins that are critical for the organelle's many functions. One such import pathway is the highly evolutionarily conserved disulfide relay system (DRS) within the mitochondrial intermembrane space (IMS), whereby proteins undergo a form of oxidation-dependent protein import. A central component of the DRS is the oxidoreductase coiled-coil-helix-coiled-coil-helix (CHCH) domain-containing protein 4 (CHCHD4, also known as MIA40), the human homologue of yeast Mia40. Here, we summarise the recent advances made to our understanding of the role of CHCHD4 and the DRS in physiology and disease, with a specific focus on the emerging importance of CHCHD4 in regulating the cellular response to low oxygen (hypoxia) and metabolism in cancer.

Alopecia in Harlequin mutant mice is associated with reduced AIF protein levels and expression of retroviral elements

We investigated the contribution of apoptosis-inducing factor (AIF), a key regulator of mitochondrial biogenesis, in supporting hair growth. We report that pelage abnormalities developed during hair follicle (HF) morphogenesis in Harlequin (Hq) mutant mice. Fragility of the hair cortex was associated with decreased expression of genes encoding structural hair proteins, though key transcriptional regulators of HF development were expressed at normal levels. Notably, Aifm1 (R200 del) knockin males and Aifm1(R200 del)/Hq females showed minor hair defects, despite substantially reduced AIF levels. Furthermore, we cloned the integrated ecotropic provirus of the Aifm1Hq allele. We found that its overexpression in wild-type keratinocyte cell lines led to down-regulation of HF-specific Krt84 and Krtap3-3 genes without altering Aifm1 or epidermal Krt5 expression. Together, our findings imply that pelage paucity in Hq mutant mice is mechanistically linked to severe AIF deficiency and is associated with the expression of retroviral elements that might potentially influence the transcriptional regulation of structural hair proteins.

Mitochondrial protein import dysfunction: mitochondrial disease, neurodegenerative disease and cancer

The majority of proteins localised to mitochondria are encoded by the nuclear genome, with approximately 1500 proteins imported into mammalian mitochondria. Dysfunction in this fundamental cellular process is linked to a variety of pathologies including neuropathies, cardiovascular disorders, myopathies, neurodegenerative diseases and cancer, demonstrating the importance of mitochondrial protein import machinery for cellular function. Correct import of proteins into mitochondria requires the co-ordinated activity of multimeric protein translocation and sorting machineries located in both the outer and inner mitochondrial membranes, directing the imported proteins to the destined mitochondrial compartment. This dynamic process maintains cellular homeostasis, and its dysregulation significantly affects cellular signalling pathways and metabolism. This review summarises current knowledge of the mammalian mitochondrial import machinery and the pathological consequences of mutation of its components. In addition, we will discuss the role of mitochondrial import in cancer, and our current understanding of the role of mitochondrial import in neurodegenerative diseases including Alzheimer's disease, Huntington's disease and Parkinson's disease.