Structural insights into G domain dimerization and pathogenic mutation of OPA1

Fast and Deep Phosphoproteome Analysis with the Orbitrap Astral Mass Spectrometer

Owing to its roles in cellular signal transduction, protein phosphorylation plays critical roles in myriad cell processes. That said, detecting and quantifying protein phosphorylation has remained a challenge. We describe the use of a novel mass spectrometer (Orbitrap Astral) coupled with data-independent acquisition (DIA) to achieve rapid and deep analysis of human and mouse phosphoproteomes. With this method we map approximately 30,000 unique human phosphorylation sites within a half-hour of data collection. The technology was benchmarked to other state-of-the-art MS platforms using both synthetic peptide standards and with EGF-stimulated HeLa cells. We applied this approach to generate a phosphoproteome multi-tissue atlas of the mouse. Altogether, we detected 81,120 unique phosphorylation sites within 12 hours of measurement. With this unique dataset, we examine the sequence, structural, and kinase specificity context of protein phosphorylation. Finally, we highlight the discovery potential of this resource with multiple examples of novel phosphorylation events relevant to mitochondrial and brain biology.

Mechanisms of mitochondrial reorganization

The cytoplasm of eukaryotes is dynamically zoned by membrane-bound and membraneless organelles. Cytoplasmic zoning allows various biochemical reactions to take place at the right time and place. Mitochondrion is a membrane-bound organelle that provides a zone for intracellular energy production and metabolism of lipids and iron. A key feature of mitochondria is their high dynamics: mitochondria constantly undergo fusion and fission, and excess or damaged mitochondria are selectively eliminated by mitophagy. Therefore, mitochondria are appropriate model systems to understand dynamic cytoplasmic zoning by membrane organelles. In this review, we summarize the molecular mechanisms of mitochondrial fusion and fission as well as mitophagy unveiled through studies using yeast and mammalian models.

Silicon dioxide-induced endoplasmic reticulum stress of alveolar macrophages and its role on the formation of silicosis fibrosis: a review article

Silicosis is a chronic lung inflammatory disease induced by long-term inhalation of high concentrations of silicon dioxide (SiO2), characterized by pulmonary fibrosis. Inhalation of silica invades alveolar macrophages (AMs) and changes the micro-environment of the cell, resulting in abnormal morphology and dysfunction of the endoplasmic reticulum (ER). Once beyond the range of cell regulation, the endoplasmic reticulum stress (ERS) will occur, which will lead to cell damage, necrosis, and apoptosis, eventually causing silicosis fibrosis through various mechanisms. This is a complex and delicate process accompanied by various macrophage-derived cytokines. Unfortunately, the details have not been systematically summarized yet. In this review, we systematically introduce the basic two processes: the process of inducing ERS by inhaling SiO2 and the process of inducing pulmonary fibrosis by ERS. Moreover, the underlying mechanism of the above two sequential events is also be discussed. We conclude that the ERS of alveolar macrophages caused by silica dust are involved deeply in the pathogenesis of silicosis. Therefore, changing the states of SiO2-induced ERS of macrophage may be an attractive therapeutic target for silicosis fibrosis.

Integrative modeling of guanylate binding protein dimers

Guanylate-binding proteins (GBPs) are essential interferon-γ-activated large GTPases that play a crucial role in host defense against intracellular bacteria and parasites. While their protective functions rely on protein polymerization, our understanding of the structural intricacies of these multimerized states remains limited. To bridge this knowledge gap, we present dimer models for human GBP1 (hGBP1) and murine GBP2 and 7 (mGBP2 and mGBP7) using an integrative approach, incorporating the crystal structure of hGBP1's GTPase domain dimer, crosslinking mass spectrometry, small-angle X-ray scattering, protein-protein docking, and molecular dynamics simulations. Our investigation begins by comparing the protein dynamics of hGBP1, mGBP2, and mGBP7. We observe that the M/E domain in all three proteins exhibits significant mobility and hinge motion, with mGBP7 displaying a slightly less pronounced motion but greater flexibility in its GTPase domain. These dynamic distinctions can be attributed to variations in the sequences of mGBP7 and hGBP1/mGBP2, resulting in different dimerization modes. Unlike hGBP1 and its close ortholog mGBP2, which exclusively dimerize through their GTPase domains, we find that mGBP7 exhibits three equally probable alternative dimer structures. The GTPase domain of mGBP7 is only partially involved in its dimerization, primarily due to an accumulation of negative charge, allowing mGBP7 to dimerize independently of GTP. Instead, mGBP7 exhibits a strong tendency to dimerize in an antiparallel arrangement across its stalks. The results of this work go beyond the sequence-structure-function relationship, as the sequence differences in mGBP7 and mGBP2/hGBP1 do not lead to different structures, but to different protein dynamics and dimerization. The distinct GBP dimer structures are expected to encode specific functions crucial for disrupting pathogen membranes.

Targeting mitochondrial dynamics proteins for the treatment of doxorubicin-induced cardiotoxicity

Doxorubicin (DOX) is an extensively used chemotherapeutic agent that can cause severe and frequent cardiotoxicity, which limits its clinical application. Although there have been extensive researches on the cardiotoxicity caused by DOX, there is still a lack of effective treatment. It is necessary to understand the molecular mechanism of DOX-induced cardiotoxicity and search for new therapeutic targets which do not sacrifice their anticancer effects. Mitochondria are considered to be the main target of cardiotoxicity caused by DOX. The imbalance of mitochondrial dynamics characterized by increased mitochondrial fission and inhibited mitochondrial fusion is often reported in DOX-induced cardiotoxicity, which can result in excessive ROS production, energy metabolism disorders, cell apoptosis, and various other problems. Also, mitochondrial dynamics disorder is related to tumorigenesis. Surprisingly, recent studies show that targeting mitochondrial dynamics proteins such as DRP1 and MFN2 can not only defend against DOX-induced cardiotoxicity but also enhance or not impair the anticancer effect. Herein, we summarize mitochondrial dynamics disorder in DOX-induced cardiac injury. Furthermore, we provide an overview of current pharmacological and non-pharmacological interventions targeting proteins involved in mitochondrial dynamics to alleviate cardiac damage caused by DOX.

Structural mechanism of mitochondrial membrane remodelling by human OPA1

Distinct morphologies of the mitochondrial network support divergent metabolic and regulatory processes that determine cell function and fate1-3. The mechanochemical GTPase optic atrophy 1 (OPA1) influences the architecture of cristae and catalyses the fusion of the mitochondrial inner membrane4,5. Despite its fundamental importance, the molecular mechanisms by which OPA1 modulates mitochondrial morphology are unclear. Here, using a combination of cellular and structural analyses, we illuminate the molecular mechanisms that are key to OPA1-dependent membrane remodelling and fusion. Human OPA1 embeds itself into cardiolipin-containing membranes through a lipid-binding paddle domain. A conserved loop within the paddle domain inserts deeply into the bilayer, further stabilizing the interactions with cardiolipin-enriched membranes. OPA1 dimerization through the paddle domain promotes the helical assembly of a flexible OPA1 lattice on the membrane, which drives mitochondrial fusion in cells. Moreover, the membrane-bending OPA1 oligomer undergoes conformational changes that pull the membrane-inserting loop out of the outer leaflet and contribute to the mechanics of membrane remodelling. Our findings provide a structural framework for understanding how human OPA1 shapes mitochondrial morphology and show us how human disease mutations compromise OPA1 functions.

OPA1 helical structures give perspective to mitochondrial dysfunction

Dominant optic atrophy is one of the leading causes of childhood blindness. Around 60-80% of cases1 are caused by mutations of the gene that encodes optic atrophy protein 1 (OPA1), a protein that has a key role in inner mitochondrial membrane fusion and remodelling of cristae and is crucial for the dynamic organization and regulation of mitochondria2. Mutations in OPA1 result in the dysregulation of the GTPase-mediated fusion process of the mitochondrial inner and outer membranes3. Here we used cryo-electron microscopy methods to solve helical structures of OPA1 assembled on lipid membrane tubes, in the presence and absence of nucleotide. These helical assemblies organize into densely packed protein rungs with minimal inter-rung connectivity, and exhibit nucleotide-dependent dimerization of the GTPase domains-a hallmark of the dynamin superfamily of proteins4. OPA1 also contains several unique secondary structures in the paddle domain that strengthen its membrane association, including membrane-inserting helices. The structural features identified in this study shed light on the effects of pathogenic point mutations on protein folding, inter-protein assembly and membrane interactions. Furthermore, mutations that disrupt the assembly interfaces and membrane binding of OPA1 cause mitochondrial fragmentation in cell-based assays, providing evidence of the biological relevance of these interactions.

FGF21-FGFR1 controls mitochondrial homeostasis in cardiomyocytes by modulating the degradation of OPA1

Fibroblast growth factor 21 (FGF21) is a pleiotropic hormone secreted primarily by the liver and is considered a major regulator of energy homeostasis. Recent research has revealed that FGF21 could play an important role in cardiac pathological remodeling effects and prevention of cardiomyopathy; however, the underlying mechanism remains largely unknown. This study aimed to determine the mechanism underlying the cardioprotective effects of FGF21. We engineered FGF21 knock out mice and subsequently elucidated the effects of FGF21 and its downstream mediators using western blotting, qRT-PCR, and mitochondrial morphological and functional analyses. FGF21 knockout mice showed cardiac dysfunction, accompanied by a decline in global longitudinal strain (GLS) and ejection fraction (EF), independent of metabolic disorders. Mitochondrial quality, quantity, and function were abnormal, accompanied by decreased levels of optic atrophy-1 (OPA1) in FGF21 KO mice. In contrast to FGF21 knockout, cardiac-specific overexpression of FGF21 alleviated the cardiac dysfunction caused by FGF21 deficiency. In an in vitro study, FGF21 siRNA deteriorated mitochondrial dynamics and impaired function induced by cobalt chloride (CoCl₂). Both recombinant FGF21 and adenovirus-mediated FGF21 overexpression could alleviate CoCl₂-induced mitochondrial impairment by restoring mitochondrial dynamics. FGF21 was essential for maintaining mitochondrial dynamics and function of the cardiomyocytes. As a regulator of cardiomyocyte mitochondrial homeostasis under oxidative stress, FGF21 could be an important new target for therapeutic options for patients with heart failure.

Ginsenoside Rg1 protects cardiac mitochondrial function via targeting GSTP1 to block S-glutathionylation of optic atrophy 1

Mitochondrial dysfunction is a fundamental challenge in myocardial injury. Ginsenoside Rg1 (Rg1) is a bioactive compound with pharmacological potential for cardiac protection. Optic atrophy 1 (OPA1) acts as a mitochondrial inner membrane protein that contributes to the structural integrity and function of mitochondria. This study investigated the protective role of Rg1 in septic cardiac injury from the perspective of OPA1 stability. Rg1 protected cardiac contractive function against endotoxin injury in mice by maintaining mitochondrial cristae structure. In cardiomyocytes, lipopolysaccharide (LPS) evoked mitochondrial fragmentation and destruction of mitochondrial biogenesis, which were prevented by Rg1, possibly due to the preservation of the integrity of cristae structure. In support, the beneficial effects of Rg1 on cardioprotection and mitochondrial biogenesis were diminished by OPA1 deficiency subjected to the LPS challenge. Mechanistically, LPS stimulation triggered intracellular glutathione destabilization that promoted S-glutathionylation of OPA1 at Cys551, leading to the dissociation of OPA1-Mitofilin. Rg1 interacted with GSTP1 to inhibit its S-glutathionylation of OPA1, thereby promoting OPA1-Mitofilin interaction and protecting mitochondrial cristae structure. These findings suggest that GSTP1/OPA1 axis may be a beneficial strategy for the treatment of myocardial injury, and expand the clinical application of Rg1.

OPA1 disease-causing mutants have domain-specific effects on mitochondrial ultrastructure and fusion

Inner mitochondrial membrane fusion and cristae shape depend on optic atrophy protein 1, OPA1. Mutations in OPA1 lead to autosomal dominant optic atrophy (ADOA), an important cause of inherited blindness. The Guanosin Triphosphatase (GTPase) and GTPase effector domains (GEDs) of OPA1 are essential for mitochondrial fusion; yet, their specific roles remain elusive. Intriguingly, patients carrying OPA1 GTPase mutations have a higher risk of developing more severe multisystemic symptoms in addition to optic atrophy, suggesting pathogenic contributions for the GTPase and GED domains, respectively. We studied OPA1 GTPase and GED mutations to understand their domain-specific contribution to protein function by analyzing patient-derived cells and gain-of-function paradigms. Mitochondria from OPA1 GTPase (c.870+5G>A and c.889C>T) and GED (c.2713C>T and c.2818+5G>A) mutants display distinct aberrant cristae ultrastructure. While all OPA1 mutants inhibited mitochondrial fusion, some GTPase mutants resulted in elongated mitochondria, suggesting fission inhibition. We show that the GED is dispensable for fusion and OPA1 oligomer formation but necessary for GTPase activity. Finally, splicing defect mutants displayed a posttranslational haploinsufficiency-like phenotype but retained domain-specific dysfunctions. Thus, OPA1 domain-specific mutants result in distinct impairments in mitochondrial dynamics, providing insight into OPA1 function and its contribution to ADOA pathogenesis and severity.

Mitochondrial dynamics proteins as emerging drug targets

The importance of mitochondrial dynamics, the physiological process of mitochondrial fusion and fission, in regulating diverse cellular functions and cellular fitness has been well established. Several pathologies are associated with aberrant mitochondrial fusion or fission that is often a consequence of deregulated mitochondrial dynamics proteins; however, pharmacological targeting of these proteins has been lacking and is challenged by complex molecular mechanisms. Recent studies have advanced our understanding in this area and have enabled rational drug design and chemical screening strategies. We provide an updated overview of the regulatory mechanisms of fusion and fission proteins, their structure-function relationships, and the discovery of pharmacological modulators demonstrating their therapeutic potential. These advances provide exciting opportunities for the development of prototype therapeutics for various diseases.

Mitochondrial dynamics and oxidative phosphorylation as critical targets in cancer

It has long been recognised that cancer cells critically depend on reprogrammed patterns of metabolism that can enable robust and abnormally high levels of cell proliferation. As mitochondria form hubs of cellular metabolic activity, it is reasonable to propose that pathways within these organelles can form targets that can be manipulated to compromise the ability of cancer cells to cause disease. However, mitochondria are highly multi-functional, and the full range of mechanistic inter-connections are still being unravelled to enable the full potential of targeting mitochondria in cancer therapeutics. Here, we aim to highlight the potential of modulating mitochondrial dynamics to target key metabolic or apoptotic pathways in cancer cells. Distinct roles have been demonstrated for mitochondrial fission and fusion in different cancer contexts. Targeting of factors mediating mitochondrial dynamics may be directly related to impairment of oxidative phosphorylation, which is essential to sustain cancer cell growth and can also alter sensitivity to chemotherapeutic compounds. This area is still lacking a unified model, although further investigation will more comprehensively map the underlying molecular mechanisms to enable better rational therapeutic strategies based on these pathways.

Mitochondrial quality control in the brain: The physiological and pathological roles

The human brain has high energetic expenses and consumes over 20% of total oxygen metabolism. Abnormal brain energy homeostasis leads to various brain diseases. Among multiple factors that contribute to these diseases, mitochondrial dysfunction is one of the most common causes. Maintenance of mitochondrial integrity and functionality is of pivotal importance to brain energy generation. Mitochondrial quality control (MQC), employing the coordination of multiple mechanisms, is evolved to overcome many mitochondrial defects. Thus, not surprisingly, aberrant mitochondrial quality control results in a wide range of brain disorders. Targeting MQC to preserve and restore mitochondrial function has emerged as a promising therapeutic strategy for the prevention and treatment of brain diseases. Here, we set out to summarize the current understanding of mitochondrial quality control in brain homeostasis. We also evaluate potential pharmaceutically and clinically relevant targets in MQC-associated brain disorders.

Mitochondrial quality control mechanisms as molecular targets in diabetic heart

Mitochondrial dysfunction has been regarded as a hallmark of diabetic cardiomyopathy. In addition to their canonical metabolic actions, mitochondria influence various other aspects of cardiomyocyte function, including oxidative stress, iron regulation, metabolic reprogramming, intracellular signaling transduction and cell death. These effects depend on the mitochondrial quality control (MQC) system, which includes mitochondrial dynamics, mitophagy and mitochondrial biogenesis. Mitochondria are not static entities, but dynamic units that undergo fission and fusion cycles to maintain their structural integrity. Increased mitochondrial fission elevates the number of mitochondria within cardiomyocytes, a necessary step for cardiomyocyte metabolism. Enhanced mitochondrial fusion promotes communication and cooperation between pairs of mitochondria, thus facilitating mitochondrial genomic repair and maintenance. On the contrary, erroneous fission or reduced fusion promotes the formation of mitochondrial fragments that contain damaged mitochondrial DNA and exhibit impaired oxidative phosphorylation. Under normal/physiological conditions, injured mitochondria can undergo mitophagy, a degradative process that delivers poorly structured mitochondria to lysosomes. However, defective mitophagy promotes the accumulation of nonfunctional mitochondria, which may induce cardiomyocyte death. A decline in the mitochondrial population due to mitophagy can stimulate mitochondrial biogenesis), which generates new mitochondrial offspring to maintain an adequate mitochondrial number. Energy crises or ATP deficiency also increase mitochondrial biogenesis, because mitochondrial DNA encodes 13 subunits of the electron transport chain (ETC) complexes. Disrupted mitochondrial biogenesis diminishes the mitochondrial mass, accelerates mitochondrial senescence and promotes mitochondrial dysfunction. In this review, we describe the involvement of MQC in the pathogenesis of diabetic cardiomyopathy. Besides, the potential targeted therapies that could be applied to improve MQC during diabetic cardiomyopathy are also discussed and accelerate the development of cardioprotective drugs for diabetic patients.

From dynamin related proteins structures and oligomers to membrane fusion mediated by mitofusins

Mitochondria assemble in a highly dynamic network where interconnected tubules evolve in length and size through regulated cycles of fission and fusion of mitochondrial membranes thereby adapting to cellular needs. Mitochondrial fusion and fission processes are mediated by specific sets of mechano-chemical large GTPases that belong to the Dynamin-Related Proteins (DRPs) super family. DRPs bind to cognate membranes and auto-oligomerize to drive lipid bilayers remodeling in a nucleotide dependent manner. Although structural characterization and mechanisms of DRPs that mediate membrane fission are well established, the capacity of DRPs to mediate membrane fusion is only emerging. In this review, we discuss the distinct structures and mechanisms of DRPs that trigger the anchoring and fusion of biological membranes with a specific focus on mitofusins that are dedicated to the fusion of mitochondrial outer membranes. In particular, we will highlight oligomeric assemblies of distinct DRPs and confront their mode of action against existing models of mitofusins assemblies with emphasis on recent biochemical, structural and computational reports. As we will see, the literature brings valuable insights into the presumed macro-assemblies mitofusins may form during anchoring and fusion of mitochondrial outer membranes.

Differential Expression of Genes Regulating Store-operated Calcium Entry in Conjunction With Mitochondrial Dynamics as Potential Biomarkers for Cancer: A Single-Cell RNA Analysis

Regulation of intracellular concentration of calcium levels is crucial for cell signaling, homeostasis, and in the pathology of diseases including cancer. Agonist-induced entry of calcium ions into the non-excitable cells is mediated by store-operated calcium channels (SOCs). This pathway is activated by the release of calcium ions from the endoplasmic reticulum and further regulated by the calcium uptake through mitochondria leading to calcium-dependent inactivation of calcium-release activated calcium channels (CARC). SOCs including stromal interaction molecules (STIM) and ORAI proteins have been implicated in tumor growth, progression, and metastasis. In the present study, we analyzed the mRNA and protein expression of genes mediating SOCs-STIM1, STIM2, ORAI1, ORAI2, ORAI3, TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV1, TRPV2, TRPM1, and TRPM7 in head and neck squamous cell cancer (HNSC) patients using TCGA and CPTAC analysis. Further, our in silico analysis showed a significant correlation between the expression of SOCs and genes involved in the mitochondrial dynamics (MDGs) both at mRNA and protein levels. Protein-protein docking results showed lower binding energy for SOCs with MDGs. Subsequently, we validated these results using gene expression and single-cell RNA sequencing datasets retrieved from Gene Expression Omnibus (GEO). Single-cell gene expression analysis of HNSC tumor tissues revealed that SOCs expression is remarkably associated with the MDGs expression in both cancer and fibroblast cells.

Mitochondrial quality control in health and in Parkinson's disease

As a central hub for cellular metabolism and intracellular signalling, the mitochondrion is a pivotal organelle, dysfunction of which has been linked to several human diseases including neurodegenerative disorders, and in particular Parkinson's disease. An inherent challenge that mitochondria face is the continuous exposure to diverse stresses which increase their likelihood of dysregulation. In response, eukaryotic cells have evolved sophisticated quality control mechanisms to monitor, identify, repair and/or eliminate abnormal or misfolded proteins within the mitochondrion and/or the dysfunctional mitochondrion itself. Chaperones identify unstable or otherwise abnormal conformations in mitochondrial proteins and can promote their refolding to recover their correct conformation and stability. However, if repair is not possible, the abnormal protein is selectively degraded to prevent potentially damaging interactions with other proteins or its oligomerization into toxic multimeric complexes. The autophagic-lysosomal system and the ubiquitin-proteasome system mediate the selective and targeted degradation of such abnormal or misfolded protein species. Mitophagy (a specific kind of autophagy) mediates the selective elimination of dysfunctional mitochondria, in order to prevent the deleterious effects the dysfunctional organelles within the cell. Despite our increasing understanding of the molecular responses toward dysfunctional mitochondria, many key aspects remain relatively poorly understood. Herein, we review the emerging mechanisms of mitochondrial quality control including quality control strategies coupled to mitochondrial import mechanisms. In addition, we review the molecular mechanisms regulating mitophagy with an emphasis on the regulation of PINK1/PARKIN-mediated mitophagy in cellular physiology and in the context of Parkinson's disease cell biology.

Insights Into Mitochondrial Dynamics in Chlamydial Infection

Mitochondria are intracellular organelles that are instrumental in the creation of energy, metabolism, apoptosis, and intrinsic immunity. Mitochondria exhibit an extraordinarily high degree of flexibility, and are constantly undergoing dynamic fusion and fission changes. Chlamydia is an intracellular bacterium that causes serious health problems in both humans and animals. Due to a deficiency of multiple metabolic enzymes, these pathogenic bacteria are highly dependent on their eukaryotic host cells, resulting in a close link between Chlamydia infection and host cell mitochondria. Indeed, Chlamydia increase mitochondrial fusion by inhibiting the activation of dynein-related protein 1 (DRP1), which can regulate host cell metabolism for extra energy. Additionally, Chlamydia can inhibit mitochondrial fission by blocking DRP1 oligomerization, preventing host cell apoptosis. These mechanisms are critical for maintaining a favorable environment for reproduction and growth of Chlamydia. This review discusses the molecular mechanisms of mitochondrial fusion and fission, as well as the mechanisms by which Chlamydia infection alters the mitochondrial dynamics and the prospects of limiting chlamydial development by altering mitochondrial dynamics.

Autosomal dominant optic atrophy: A novel treatment for OPA1 splice defects using U1 snRNA adaption

Autosomal dominant optic atrophy (ADOA) is frequently caused by mutations in the optic atrophy 1 (OPA1) gene, with haploinsufficiency being the major genetic pathomechanism. Almost 30% of the OPA1-associated cases suffer from splice defects. We identified a novel OPA1 mutation, c.1065+5G>A, in patients with ADOA. In patient-derived fibroblasts, the mutation led to skipping of OPA1 exon 10, reducing the OPA1 protein expression by approximately 50%. We developed a molecular treatment to correct the splice defect in OPA1 using engineered U1 splice factors retargeted to different locations in OPA1 exon 10 or intron 10. The strongest therapeutic effect was detected when U1 binding was engineered to bind to intron 10 at position +18, a position predicted by bioinformatics to be a promising binding site. We were able to significantly silence the effect of the mutation (skipping of exon 10) and simultaneously increase the expression level of normal transcripts. Retargeting U1 to the canonical splice donor site did not lead to a detectable splice correction. This proof-of-concept study indicates for the first time the feasibility of splice mutation correction as a treatment option for ADOA. Increasing the amount of correctly spliced OPA1 transcripts may suffice to overcome the haploinsufficiency.

Mutations at a split codon in the GTPase-encoding domain of OPA1 cause dominant optic atrophy through different molecular mechanisms

Exonic (i.e. coding) variants in genes associated with disease can exert pathogenic effects both at the protein and mRNA level, either by altering the amino acid sequence or by affecting pre-mRNA splicing. The latter is often neglected due to the lack of RNA analyses in genetic diagnostic testing. In this study we considered both pathomechanisms and performed a comprehensive analysis of nine exonic nucleotide changes in OPA1, which is the major gene underlying autosomal dominant optic atrophy (DOA) and is characterized by pronounced allelic heterogeneity. We focused on the GTPase-encoding domain of OPA1, which harbors most of the missense variants associated with DOA. Given that the consensus splice sites extend into the exons, we chose a split codon, namely codon 438, for our analyses. Variants at this codon are the second most common cause of disease in our large cohort of DOA patients harboring disease-causing variants in OPA1. In silico splice predictions, heterologous splice assays, analysis of patient’s RNA when available, and protein modeling revealed different molecular outcomes for variants at codon 438. The wildtype aspartate residue at amino acid position 438 is directly involved in the dimerization of OPA1 monomers. We found that six amino acid substitutions at codon 438 (i.e. all substitutions of the first and second nucleotide of the codon) destabilized dimerization while only substitutions of the first nucleotide of the codon caused exon skipping. Our study highlights the value of combining RNA analysis and protein modeling approaches to accurately assign patients to future precision therapies.

Dominant Optic Atrophy (DOA): Modeling the Kaleidoscopic Roles of OPA1 in Mitochondrial Homeostasis

In the year 2000, the discovery of OPA1 mutations as causative for dominant optic atrophy (DOA) was pivotal to rapidly expand the field of mitochondrial dynamics and describe the complex machinery governing this pathway, with a multitude of other genes and encoded proteins involved in neurodegenerative disorders of the optic nerve. OPA1 turned out to be a much more complex protein than initially envisaged, connecting multiple pathways beyond its strict role in mitochondrial fusion, such as sensing of OXPHOS needs and mitochondrial DNA maintenance. As a consequence, an increasing need to investigate OPA1 functions at multiple levels has imposed the development of multiple tools and models that are here reviewed. Translational mitochondrial medicine, with the ultimate objective of translating basic science necessary to understand pathogenic mechanisms into therapeutic strategies, requires disease modeling at multiple levels: from the simplest, like in yeast, to cell models, including the increasing use of reprogrammed stem cells (iPSCs) from patients, to animal models. In the present review, we thoroughly examine and provide the state of the art of all these approaches.

Exploring the effect of Gupi Xiaoji Prescription on hepatitis B virus-related liver cancer through network pharmacology and in vitro experiments

AIM AND OBJECTIVE

To study the effect of Gupi Xiaoji Prescription (GXP) on hepatitis B virus(HBV)-related liver cancer through network pharmacology coupled with in vitro experiments and explore their related mechanisms.

MATERIALS AND METHODS

Gupi Xiaoji Prescription's chemical constituents and the action targets of its six medicinal components were identified using several databases. These included the Traditional Chinese Medicine Systems Pharmacology Database （TCMSP), the Bioinformatics Analysis Tool for Molecular mechANism of TCM (BATMAN-TCM), and the Traditional Chinese Medicine Integrated Database (TCMID), while GeneCards and OMIM were used to compile relevant liver cancer disease targets. Pathway enrichment of gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG), analysis of potential targets, and analysis of the enriched pathways in literature were executed in R. The Hepatocellular carcinoma (HCC)-derived HepG2.2.15 cell line stably expresses and replicates HBV. In vitro experiments with HepG2.2.15 were used to verify GXP's effects on HBV-related liver cancer, while the human liver cancer cell line HepG2 was used as the control.

RESULTS

171 active ingredients and 259 potential drug targets were screened from GXP, involving 181 pathways in vitro. These assays identified Polyphyllin I as an effective GXP component. Notably, GXP inhibited cell proliferation and metastasis in a concentration-dependent manner (P < 0.01). In comparison with the vehicle group, the fluorescence intensity of each drug group was significantly weakened (P < 0.01), while the drug group Mitofusins 1(MFN1) and protein expression level of Mitofusins 2 (MFN2) increased significantly. The protein expression level of Mitochondrial fission protein 1 (FIS1) and Optic Atrophy 1 (OPA1) also showed significant decreases (P < 0.01). Molecular docking revealed Fructus saponins I's high affinity with FIS1, MFN1, MFN2, and OPA1.

CONCLUSION

The network pharmacology results indicate that Gupi Xiaoji Prescription may treat liver cancer by regulating mitochondrial division and fusion of key genes to disrupt liver cancer cells' energy metabolism. In vitro experiments also verified that GXP could inhibit the proliferation and migration of HepG2.2.15 cells by up-regulating MFN1 and MFN2, down-regulating the expression of FIS1 and OPA1 in addition to damaging mitochondria. Consistent with network pharmacology and molecular docking results, Polyphyllin I may be the most active compound of the formula's components. It also shows that Traditional Chinese medicine (TCM) plays a significant, targeted role in the treatment of HBV-related liver cancer.

Mitochondrial Fusion: The Machineries In and Out

Mitochondria are highly dynamic organelles that constantly undergo fission and fusion. Disruption of mitochondrial dynamics undermines their function and causes several human diseases. The fusion of the outer (OMM) and inner mitochondrial membranes (IMM) is mediated by two classes of dynamin-like protein (DLP): mitofusin (MFN)/fuzzy onions 1 (Fzo1) and optic atrophy 1/mitochondria genome maintenance 1 (OPA1/Mgm1). Given the lack of structural information on these fusogens, the molecular mechanisms underlying mitochondrial fusion remain unclear, even after 20 years. Here, we review recent advances in structural studies of the mitochondrial fusion machinery, discuss their implication for DLPs, and summarize the pathogenic mechanisms of disease-causing mutations in mitochondrial fusion DLPs.