OPA1-associated disorders: Phenotypes and pathophysiology

The OPA1 gene, encoding a dynamin-like mitochondrial GTPase, is involved in autosomal dominant optic atrophy (ADOA, OMIM #165500). ADOA, also known as Kjer's optic atrophy, affects retinal ganglion cells and the axons forming the optic nerve, leading to progressive visual loss. OPA1 gene sequencing in patients with hereditary optic neuropathies indicates that the clinical spectrum of ADOA is larger than previously thought. Specific OPA1 mutations are responsible for several distinct clinical presentations, such as ADOA with deafness (ADOAD), and severe multi-systemic syndromes, the so-called “ADOA plus” disorders, which involve neurological and neuromuscular symptoms similar to those due to mitochondrial oxidative phosphorylation defects or mitochondrial DNA instability. The study of the various clinical presentations of ADOA in conjunction with the investigation of OPA1 mutations in fibroblasts from patients with optic atrophy provides new insights into the pathophysiological mechanisms of the disease while underscoring the multiple physiological roles played by OPA1 in energetic metabolism, mitochondrial structure and maintenance, and cell death. Finally, OPA1 represents an important new paradigm for emerging neurodegenerative diseases affecting mitochondrial structure, plasticity and functions.     

Loss of OPA1 perturbates the mitochondrial inner membrane structure and integrity,leading to cytochrome c release and apoptosis

OPA1 encodes a large GTPase related to dynamins, anchored to the mitochondrial cristae inner membrane, facing the intermembrane space. OPA1 haplo-insufficiency is responsible for the most common form of autosomal dominant optic atrophy (ADOA, MIM165500), a neuropathy resulting from degeneration of the retinal ganglion cells and optic nerve atrophy. Here we show that down-regulation of OPA1 in HeLa cells using specific small interfering RNA (siRNA) leads to fragmentation of the mitochondrial network concomitantly to the dissipation of the mitochondrial membrane potential and to a drastic disorganization of the cristae. These events are followed by cytochrome c release and caspase-dependent apoptotic nuclear events. Similarly, in NIH-OVCAR-3 cells, the OPA1 siRNA induces mitochondrial fragmentation and apoptosis, the latter being inhibited by Bcl2 overexpression. These results suggest that OPA1 is a major organizer of the mitochondrial inner membrane from which the maintenance of the cristae integrity depends. As loss of OPA1 commits cells to apoptosis without any other stimulus, we propose that OPA1 is involved in the cytochrome c sequestration and might be a target for mitochondrial apoptotic effectors. Our results also suggest that abnormal apoptosis is a possible pathophysiological process leading to the retinal ganglion cells degeneration in ADOA patients.     

Mitochondrial oxidative phosphorylation compensation may preserve vision in patients with OPA1-linked autosomal dominant optic atrophy

Autosomal Dominant Optic Atrophy (ADOA) is the most common inherited optic atrophy where vision impairment results from specific loss of retinal ganglion cells of the optic nerve. Around 60% of ADOA cases are linked to mutations in the OPA1 gene. OPA1 is a fission-fusion protein involved in mitochondrial inner membrane remodelling. ADOA presents with marked variation in clinical phenotype and varying degrees of vision loss, even among siblings carrying identical mutations in OPA1. To determine whether the degree of vision loss is associated with the level of mitochondrial impairment, we examined mitochondrial function in lymphoblast cell lines obtained from six large Australian OPA1-linked ADOA pedigrees. Comparing patients with severe vision loss (visual acuity [VA]<6/36) and patients with relatively preserved vision (VA>6/9) a clear defect in mitochondrial ATP synthesis and reduced respiration rates were observed in patients with poor vision. In addition, oxidative phosphorylation (OXPHOS) enzymology in ADOA patients with normal vision revealed increased complex II+III activity and levels of complex IV protein. These data suggest that OPA1 deficiency impairs OXPHOS efficiency, but compensation through increases in the distal complexes of the respiratory chain may preserve mitochondrial ATP production in patients who maintain normal vision. Identification of genetic variants that enable this response may provide novel therapeutic insights into OXPHOS compensation for preventing vision loss in optic neuropathies.     

A new vicious cycle involving glutamate excitotoxicity, oxidative stress and mitochondrial dynamics

Glutamate excitotoxicity leads to fragmented mitochondria in neurodegenerative diseases, mediated by nitric oxide and S-nitrosylation of dynamin-related protein 1, a mitochondrial outer membrane fission protein. Optic atrophy gene 1 (OPA1) is an inner membrane protein important for mitochondrial fusion. Autosomal dominant optic atrophy (ADOA), caused by mutations in OPA1, is a neurodegenerative disease affecting mainly retinal ganglion cells (RGCs). Here, we showed that OPA1 deficiency in an ADOA model influences N-methyl-D-aspartate (NMDA) receptor expression, which is involved in glutamate excitotoxicity and oxidative stress. Opa1^enu/+ mice show a slow progressive loss of RGCs, activation of astroglia and microglia, and pronounced mitochondrial fission in optic nerve heads as found by electron tomography. Expression of NMDA receptors (NR1, 2A, and 2B) in the retina of Opa1^enu/+ mice was significantly increased as determined by western blot and immunohistochemistry. Superoxide dismutase 2 (SOD2) expression was significantly decreased, the apoptotic pathway was activated as Bax was increased, and phosphorylated Bad and BcL-xL were decreased. Our results conclusively demonstrate that not only glutamate excitotoxicity and/or oxidative stress alters mitochondrial fission/fusion, but that an imbalance in mitochondrial fission/fusion in turn leads to NMDA receptor upregulation and oxidative stress. Therefore, we propose a new vicious cycle involved in neurodegeneration that includes glutamate excitotoxicity, oxidative stress, and mitochondrial dynamics.     

线粒体融合、分裂与神经变性疾病

线粒体是一种处于高度运动状态的频繁地进行融合与分裂的细胞器.在生理状态下,线粒体的融合与分裂处于一种平衡的状态,这种平衡受线粒体融合蛋白1/2（Mfn1/2）、视神经萎缩蛋白1（OPA1）和动力相关蛋白1（Drp1）的调节. Mfn1/2介导线粒体外膜的融合,而OPA1则参与线粒体内膜的融合,这些蛋白受泛素化和蛋白水解的调控. Drp1参与线粒体的分裂过程,受多种翻译后修饰的调节,如磷酸化、泛素化、SUMO化和S 硝基化.对于神经元来说,线粒体融合分裂的动态平衡对保证神经元末梢长距离运输和能量平均分布是非常重要的.因此,线粒体融合分裂异常可能是许多神经变性疾病的致病因素之一.对线粒体融合而言,Mfn2错义突变将导致遗传性运动感觉神经病2型（CMT2A）;OPA1错义突变将引起显性遗传性视神经萎缩(ADOA),而就线粒体分裂而言,Drp1突变与多系统功能障碍的新生儿致死性相关.     

Ethambutol-induced optic neuropathy linked to OPA1 mutation and mitochondrial toxicity

Ethambutol (EMB), widely used in the treatment of tuberculosis, has been reported to cause Leber’s hereditary optic neuropathy in patients carrying mitochondrial DNA mutations. We study the effect of EMB on mitochondrial metabolism in fibroblasts from controls and from a man carrying an OPA1 mutation, in whom the drug induced the development of autosomal dominant optic atrophy (ADOA). EMB produced a mitochondrial coupling defect together with a 25% reduction in complex IV activity. EMB induced the formation of vacuoles associated with decreased mitochondrial membrane potential and increased fragmentation of the mitochondrial network. Mitochondrial genetic variations may therefore be predisposing factors in EMB-induced ocular injury.     

Regulation of OPA1-mediated mitochondrial fusion by leucine zipper/EF-hand-containing transmembrane protein-1 plays a role in apoptosis

Carboxyl-terminal modulator protein (CTMP) is a tumor suppressor-like binding partner of Protein kinase B (PKB/Akt) that negative regulates this kinase. In the course of our recent work, we identified that CTMP is consistently associated with leucine zipper/EF-hand-containing transmembrane-1 (LETM1). Here, we report that adenovirus-LETM1 increased the sensitivity of HeLa cells to apoptosis, induced by either staurosporine or actinomycin D. As shown previously, LETM1 localized to the inner mitochondrial membrane. Electron-microscopy analysis of adenovirus-LETM1 transduced cells revealed that mitochondrial cristae were swollen in these cells, a phenotype similar to that observed in optic atrophy type-1 (OPA1)-ablated cells. OPA1 cleavage was increased in LETM1-overexpressing cells, and this phenotype was reversed by overexpression of OPA1 variant-7, a cleavage resistant form of OPA1. Taken together, these data suggest that LETM1 is a novel binding partner for CTMP that may play an important role in mitochondrial fragmentation via OPA1-cleavage.     

哺乳动物细胞线粒体融合-分裂与钙离子信号的关系

线粒体是一种高度动态的细胞器,通过融合和分裂两个相反的过程来维持正常的形态结构。在哺乳动物中,多种因素影响线粒体的融合-分裂的平衡,但现已明确,线粒体融合的主要调节因子为Mfn1/2、OPA1,介导线粒体分裂的主要调节因子为Drp1、Fis1。新近研究发现,线粒体融合-分裂平衡的紊乱将导致线粒体结构和在细胞内分布的异常,进而影响细胞和线粒体对钙离子信号的反应;同时,钙离子也可通过多种机制影响线粒体的形态结构与分布。     

Autophagy controls the pathogenicity of OPA1 mutations in dominant optic atrophy

Optic Atrophy 1 (OPA1) gene mutations cause diseases ranging from isolated dominant optic atrophy (DOA) to various multisystemic disorders. OPA1, a large GTPase belonging to the dynamin family, is involved in mitochondrial network dynamics. The majority of OPA1 mutations encodes truncated forms of the protein and causes DOA through haploinsufficiency, whereas missense OPA1 mutations are predicted to cause disease through deleterious dominant‐negative mechanisms. We used 3D imaging and biochemical analysis to explore autophagy and mitophagy in fibroblasts from seven patients harbouring OPA1 mutations. We report new genotype–phenotype correlations between various types of OPA1 mutation and mitophagy. Fibroblasts bearing dominant‐negative OPA1 mutations showed increased autophagy and mitophagy in response to uncoupled oxidative phosphorylation. In contrast, OPA1 haploinsufficiency was correlated with a substantial reduction in mitochondrial turnover and autophagy, unless subjected to experimental mitochondrial injury. Our results indicate distinct alterations of mitochondrial physiology and turnover in cells with OPA1 mutations, suggesting that the level and profile of OPA1 may regulate the rate of mitophagy.     

The 21.5-kDa isoform of myelin basic protein has a non-traditional PY-nuclear-localization signal

A large four-generation Chinese family with autosomal dominant optic atrophy (ADOA) was investigated in the present study. Eight of the family members were affected in this pedigree. The affected family members exhibited early-onset and progressive visual impairment, resulting in mild to profound loss of visual acuity. The average age-at-onset was 15.9years. A new heterozygous mutation c.C1198G was identified by sequence analysis of the 12th exon of the OPA1 gene. This mutation resulted in a proline to alanine substitution at codon 400, which was located in an evolutionarily conserved region. This missense mutation in the GTPase domain was supposed to result in a loss of function for the encoded protein and act through a dominant negative effect. No other mutations associated with optic atrophy were found in our present study. The c.C1198G heterozygous mutation in the OPA1 gene may be a novel key pathogenic mutation in this pedigree with ADOA. Furthermore, additional nuclear modifier genes, environmental factors, and psychological factors may also contribute to the phenotypic variability of ADOA in this pedigree.     

The i-AAA protease YME1L and OMA1 cleave OPA1 to balance mitochondrial fusion and fission

Mitochondrial fusion and structure depend on the dynamin-like GTPase OPA1, whose activity is regulated by proteolytic processing. Constitutive OPA1 cleavage by YME1L and OMA1 at two distinct sites leads to the accumulation of both long and short forms of OPA1 and maintains mitochondrial fusion. Stress-induced OPA1 processing by OMA1 converts OPA1 completely into short isoforms, inhibits fusion, and triggers mitochondrial fragmentation. Here, we have analyzed the function of different OPA1 forms in cells lacking YME1L, OMA1, or both. Unexpectedly, deletion of Oma1 restored mitochondrial tubulation, cristae morphogenesis, and apoptotic resistance in cells lacking YME1L. Long OPA1 forms were sufficient to mediate mitochondrial fusion in these cells. Expression of short OPA1 forms promoted mitochondrial fragmentation, which indicates that they are associated with fission. Consistently, GTPase-inactive, short OPA1 forms partially colocalize with ER–mitochondria contact sites and the mitochondrial fission machinery. Thus, OPA1 processing is dispensable for fusion but coordinates the dynamic behavior of mitochondria and is crucial for mitochondrial integrity and quality control.     

A Lethal de Novo Mutation in the Middle Domain of the Dynamin-related GTPase Drp1 Impairs Higher Order Assembly and Mitochondrial Division

Mitochondria dynamically fuse and divide within cells, and the proper balance of fusion and fission is necessary for normal mitochondrial function, morphology, and distribution. Drp1 is a dynamin-related GTPase required for mitochondrial fission in mammalian cells. It harbors four distinct domains: GTP-binding, middle, insert B, and GTPase effector. A lethal mutation (A395D) within the Drp1 middle domain was reported in a neonate with microcephaly, abnormal brain development, optic atrophy, and lactic acidemia (Waterham, H. R., Koster, J., van Roermund, C. W., Mooyer, P. A., Wanders, R. J., and Leonard, J. V. (2007) N. Engl. J. Med. 356, 1736–1741). Mitochondria within patient-derived fibroblasts were markedly elongated, but the molecular mechanisms underlying these findings were not demonstrated. Because the middle domain is particularly important for the self-assembly of some dynamin superfamily proteins, we tested the hypothesis that this A395D mutation, and two other middle domain mutations (G350D, G363D) were important for Drp1 tetramerization, higher order assembly, and function. Although tetramerization appeared largely intact, each of these mutations compromised higher order assembly and assembly-dependent stimulation of Drp1 GTPase activity. Moreover, mutant Drp1 proteins exhibited impaired localization to mitochondria, indicating that this higher order assembly is important for mitochondrial recruitment, retention, or both. Overexpression of these middle domain mutants markedly inhibited mitochondrial division in cells. Thus, the Drp1 A395D lethal defect likely resulted in impaired higher order assembly of Drp1 at mitochondria, leading to decreased fission, elongated mitochondria, and altered cellular distribution of mitochondria.     

SIRT3 Deacetylates and Activates OPA1 To Regulate Mitochondrial Dynamics during Stress

Mitochondrial morphology is regulated by the balance between two counteracting mitochondrial processes of fusion and fission. There is significant evidence suggesting a stringent association between morphology and bioenergetics of mitochondria. Morphological alterations in mitochondria are linked to several pathological disorders, including cardiovascular diseases. The consequences of stress-induced acetylation of mitochondrial proteins on the organelle morphology remain largely unexplored. Here we report that OPA1, a mitochondrial fusion protein, was hyperacetylated in hearts under pathological stress and this posttranslational modification reduced the GTPase activity of the protein. The mitochondrial deacetylase SIRT3 was capable of deacetylating OPA1 and elevating its GTPase activity. Mass spectrometry and mutagenesis analyses indicated that in SIRT3-deficient cells OPA1 was acetylated at lysine 926 and 931 residues. Overexpression of a deacetylation-mimetic version of OPA1 recovered the mitochondrial functions of OPA1-null cells, thus demonstrating the functional significance of K926/931 acetylation in regulating OPA1 activity. Moreover, SIRT3-dependent activation of OPA1 contributed to the preservation of mitochondrial networking and protection of cardiomyocytes from doxorubicin-mediated cell death. In summary, these data indicated that SIRT3 promotes mitochondrial function not only by regulating activity of metabolic enzymes, as previously reported, but also by regulating mitochondrial dynamics by targeting OPA1.     

Relationship between OPA1 and cardiolipin in mitochondrial inner-membrane fusion

Mitochondria are highly dynamic organelles that undergo frequent fusion and fission. The large GTPase optic atrophy 1 (OPA1) is identified as a core component of inner membrane (IM) fusion. OPA1 exists as the membrane-anchored L-OPA1 and the proteolytically cleavage soluble S-OPA1. Recently, we showed that OPA1 and mitochondria-localized lipid cardiolipin (CL) cooperate in heterotypic IM fusion [Ban et al., Nat. Cell Biol. 19 (2017) 856–863]. We reconstituted an in vitro membrane fusion reaction using purified human L-OPA1 and S-OPA1 expressed in silkworm and found that L-OPA1 on one side of the membrane and CL on the other side were sufficient for mitochondrial fusion. L-OPA1 is the major fusion-prone factor in heterotypic fusion. However, the role of S-OPA1 remains unknown as S-OPA1 promoted L-OPA1-dependent heterotypic membrane fusion and homotypic CL-containing membrane fusion, but S-OPA1 alone was not sufficient for heterotypic membrane fusion. L-OPA1- and CL-mediated heterotypic mitochondrial fusion was confirmed in living cells, but tafazzin (Taz1), the causal gene product of Barth syndrome, was not essential for mitochondrial fusion. Taz1-dependent CL maturation might have other roles in the remodeling of mitochondrial DNA nucleoids.     

The <Emphasis Type="Italic">OPA1</Emphasis> Gene Mutations Are Frequent in Han Chinese Patients with Suspected Optic Neuropathy

While many patients with hereditary optic neuropathies are caused by mitochondrial DNA (mtDNA) mutations of Leber’s hereditary optic neuropathy (LHON), a significant proportion of them does not have mtDNA mutation and is caused by mutations in genes of the nuclear genome. In this study, we investigated whether the OPA1 gene, which is a pathogenic gene for autosomal dominant optic atrophy (ADOA), is frequently mutated in these patients. We sequenced all 29 exons of the OPA1 gene in 105 Han Chinese patients with suspected LHON. mtDNA copy number was quantified in blood samples from patients with and without OPA1 mutation and compared to healthy controls. In silico program-affiliated prediction, evolutionary conservation analysis, and in vitro cellular assays were performed to show the potential pathogenicity of the mutations. We identified nine OPA1 mutations in eight patients; six of them are located in exons and three are located in splicing sites. Mutation c.1172T?>?G has not been reported before. When we combined our data with 193 reported Han Chinese patients with optic neuropathy and compared to the available data of 4327 East Asians by the Exome Aggregation Consortium (ExAC), we found a significant enrichment of potentially pathogenic OPA1 mutations in Chinese patients. Cellular assays for OPA1 mutants c.869G?>?A and c.2708_2711del showed abnormalities in OPA1 isoforms, mitochondrial morphology, and cellular reactive oxygen species (ROS) level. Our results indicated that screening OPA1 mutation is needed for clinical diagnosis of patients with suspected optic neuropathy.     

Loss of Prohibitin Membrane Scaffolds Impairs Mitochondrial Architecture and Leads to Tau Hyperphosphorylation and Neurodegeneration

Fusion and fission of mitochondria maintain the functional integrity of mitochondria and protect against neurodegeneration, but how mitochondrial dysfunctions trigger neuronal loss remains ill-defined. Prohibitins form large ring complexes in the inner membrane that are composed of PHB1 and PHB2 subunits and are thought to function as membrane scaffolds. In Caenorhabditis elegans, prohibitin genes affect aging by moderating fat metabolism and energy production. Knockdown experiments in mammalian cells link the function of prohibitins to membrane fusion, as they were found to stabilize the dynamin-like GTPase OPA1 (optic atrophy 1), which mediates mitochondrial inner membrane fusion and cristae morphogenesis. Mutations in OPA1 are associated with dominant optic atrophy characterized by the progressive loss of retinal ganglion cells, highlighting the importance of OPA1 function in neurons. Here, we show that neuron-specific inactivation of Phb2 in the mouse forebrain causes extensive neurodegeneration associated with behavioral impairments and cognitive deficiencies. We observe early onset tau hyperphosphorylation and filament formation in the hippocampus, demonstrating a direct link between mitochondrial defects and tau pathology. Loss of PHB2 impairs the stability of OPA1, affects mitochondrial ultrastructure, and induces the perinuclear clustering of mitochondria in hippocampal neurons. A destabilization of the mitochondrial genome and respiratory deficiencies manifest in aged neurons only, while the appearance of mitochondrial morphology defects correlates with tau hyperphosphorylation in the absence of PHB2. These results establish an essential role of prohibitin complexes for neuronal survival in vivo and demonstrate that OPA1 stability, mitochondrial fusion, and the maintenance of the mitochondrial genome in neurons depend on these scaffolding proteins. Moreover, our findings establish prohibitin-deficient mice as a novel genetic model for tau pathologies caused by a dysfunction of mitochondria and raise the possibility that tau pathologies are associated with other neurodegenerative disorders caused by deficiencies in mitochondrial dynamics.     

Loss of Yme1L perturbates mitochondrial dynamics

Yme1L is an AAA protease that is embedded in the mitochondrial inner membrane with its catalytic domain facing the mitochondrial inner-membrane space. However, how Yme1L regulates mammalian mitochondrial function is still obscure. We find that endogenous Yme1L locates at punctate structures of mitochondria, and that loss of Yme1L in mouse embryonic fibroblast (MEF) cells results in mitochondrial fragmentation and leads to significant increased ‘kiss-and-run'' type of mitochondrial fusion; however, Yme1L knockdown (shYme1L (short hairpin-mediated RNA interference of Yme1L)) cells still remain normal mitochondrial fusion although shYme1L mitochondria have a little bit less fusion and fission rates, and the shYme1L-induced fragmentation is due to a little bit more mitochondrial fission than fusion in cells. Furthermore, shYme1L-induced mitochondrial fragmentation is independent on optic atrophy 1 (OPA1) S1 or S2 processing, and shYme1L results in the stabilization of OPA1 long form (L-OPA1); in addition, the exogenous expression of OPA1 or L-OPA1 facilitates the shYme1L-induced mitochondrial fragmentation, thus this fragmentation induced by shYme1L appears to be associated with L-OPA1''s stability. ShYme1L also causes a slight increase of mitochondrial dynamics proteins of 49 kDa and mitochondrial fission factor (Mff), which recruit mitochondrial key fission factor dynamin-related protein 1 (Drp1) into mitochondria in MEF cells, and loss of Drp1 or Mff inhibits the shYme1L-induced mitochondrial fragmentation. In addition, there is interaction between SLP-2 with Yme1L and shYme1L cells retain stress-induced mitochondrial hyperfusion. Taken together, our results clarify how Yme1L regulates mitochondrial morphology.     

Differential sublocalization of the dynamin-related protein OPA1 isoforms in mitochondria

OPA1 is a cause gene for autosomal dominant optic atrophy and possesses eight alternative splicing variants. Here, we identified two isoforms of OPA1 proteins in HeLa cells and examined their submitochondrial localization and complex formations. RT-PCR shows that HeLa cells mainly express isoforms 7 and 1 of OPA1. Since the third cleavage site is mainly utilized in HeLa cells, the predicted molecular masses of their processed proteins are consistent with the 93- and 88-kDa proteins. Biochemical examinations indicate that both of the OPA1 isoforms are present in the intermembrane space. Submitochondrial fractionation by sucrose density-gradient centrifugation shows that the 88-kDa protein predominantly associates with the mitochondrial outer membrane, on the contrary, the 93-kDa protein associates with the inner membrane. Gel filtration analysis indicates that they compose the different molecular mass complexes in mitochondria. These differences between two isoforms of OPA1 would suggest their crucial role involved in the mitochondrial membrane formation.     

Mitochondrial dynamics: to be in good shape to survive

Mitochondria are essential organelles of all eukaryotic cells that play a key role in several physiological processes and are involved in the pathology of many diseases. These organelles form a highly dynamic network, which results from continuous fusion and fission processes. Importance of these processes is underlined by inherited human diseases caused by mutations in two mitochondrial pro-fusion genes: Charcot-Marie-Tooth disease, caused by mutations in Mitofusin 2 gene and ADOA due to mutations in OPA1. During apoptosis, the mitochondrial network is disintegrated and the outer mitochondrial membrane permeabilized, which results in the release of several apoptogenic proteins, including cytochrome c. Although modulating mitochondrial fusion and fission machineries has been reported to influence the apoptotic response to various stimuli, it is still unclear whether fission is absolutely required for apoptosis. In this review, we present the latest progress in the field of mitochondrial dynamics with a particular emphasis on its implication in apoptosis and in diseases.