Mitochondrial dysfunction in an Opa1(Q285STOP) mouse model of dominant optic atrophy results from Opa1 haploinsufficiency

Is mitochondrial morphology important for cellular physiology?

Mitochondria are double membrane-bound organelles the network morphology of which in cells is shaped by opposing events of fusion and fission executed by dynamin-like GTPases. Mutations in these genes can perturb the form and functions of mitochondria in cell and animal models of mitochondrial diseases. An expanding array of chemical, mechanical, and genetic stressors can converge on mitochondrial-shaping proteins and disrupt mitochondrial morphology. In recent years, studies aimed at disentangling the multiple roles of mitochondrial-shaping proteins beyond fission or fusion have provided insights into the homeostatic relevance of mitochondrial morphology. Here, I review the pleiotropy of mitochondrial fusion and fission proteins with the aim of understanding whether mitochondrial morphology is important for cell and tissue physiology.

OPA1 Dominant Optic Atrophy: Diagnostic Approach in the Pediatric Population

A clinical and genetic study was conducted with pediatric patients and their relatives with optic atrophy 1 (OPA1) mutations to establish whether there is a genotype-phenotype correlation among the variants detected within and between families. Eleven children with a confirmed OPA1 mutation were identified during the study period. The main initial complaint was reduced visual acuity (VA), present in eight patients of the cohort. Eight of eleven patients had a positive family history of optic atrophy. The mean visual acuity at the start of the study was 0.40 and 0.44 LogMAR in the right and left eye, respectively. At the end of the study, the mean visual acuity was unchanged. Optical coherence tomography during the first visit showed a mean retinal nerve fiber layer thickness of 81.6 microns and 80.5 microns in the right and left eye, respectively; a mean ganglion cell layer of 52.5 and 52.4 microns, respectively, and a mean central macular thickness of 229.5 and 233.5 microns, respectively. The most common visual field defect was a centrocecal scotoma, and nine out of eleven patients showed bilateral temporal disc pallor at baseline. Sequencing of OPA1 showed seven different mutations in the eleven patients, one of which, NM_130837.3: c.1406_1407del (p.Thr469LysfsTer16), has not been previously reported. Early diagnosis of dominant optic atrophy is crucial, both for avoiding unnecessary consultations and/or treatments and for appropriate genetic counseling.

Disruption of mitochondrial quality control genes promotes caspase-resistant cell survival following apoptotic stimuli

In cells undergoing cell-intrinsic apoptosis, mitochondrial outer membrane permeabilization (MOMP) typically marks an irreversible step in the cell death process. However, in some cases, a subpopulation of treated cells can exhibit a sublethal response, termed "minority MOMP." In this phenomenon, the affected cells survive, despite a low level of caspase activation and subsequent limited activation of the endonuclease caspase-activated DNase (DNA fragmentation factor subunit beta). Consequently, these cells can experience DNA damage, increasing the probability of oncogenesis. However, little is known about the minority MOMP response. To discover genes that affect the MOMP response in individual cells, we conducted an imaging-based phenotypic siRNA screen. We identified multiple candidate genes whose downregulation increased the heterogeneity of MOMP within single cells, among which were genes related to mitochondrial dynamics and mitophagy that participate in the mitochondrial quality control (MQC) system. Furthermore, to test the hypothesis that functional MQC is important for reducing the frequency of minority MOMP, we developed an assay to measure the clonogenic survival of caspase-engaged cells. We found that cells deficient in various MQC genes were indeed prone to aberrant post-MOMP survival. Our data highlight the important role of proteins involved in mitochondrial dynamics and mitophagy in preventing apoptotic dysregulation and oncogenesis.

Mitochondrial dynamics in the neonatal brain - a potential target following injury?

The impact of birth asphyxia and its sequelae, hypoxic–ischaemic (HI) brain injury, is long-lasting and significant, both for the infant and for their family. Treatment options are limited to therapeutic hypothermia, which is not universally successful and is unavailable in low resource settings. The energy deficits that accompany neuronal death following interruption of blood flow to the brain implicate mitochondrial dysfunction. Such HI insults trigger mitochondrial outer membrane permeabilisation leading to release of pro-apoptotic proteins into the cytosol and cell death. More recently, key players in mitochondrial fission and fusion have been identified as targets following HI brain injury. This review aims to provide an introduction to the molecular players and pathways driving mitochondrial dynamics, the regulation of these pathways and how they are altered following HI insult. Finally, we review progress on repurposing or repositioning drugs already approved for other indications, which may target mitochondrial dynamics and provide promising avenues for intervention following brain injury. Such repurposing may provide a mechanism to fast-track, low-cost treatment options to the clinic.

Dominant optic atrophy: Culprit mitochondria in the optic nerve

Dominant optic atrophy (DOA) is an inherited mitochondrial disease leading to specific degeneration of retinal ganglion cells (RGCs), thus compromising transmission of visual information from the retina to the brain. Usually, DOA starts during childhood and evolves to poor vision or legal blindness, affecting the central vision, whilst sparing the peripheral visual field. In 20% of cases, DOA presents as syndromic disorder, with secondary symptoms affecting neuronal and muscular functions. Twenty years ago, we demonstrated that heterozygous mutations in OPA1 are the most frequent molecular cause of DOA. Since then, variants in additional genes, whose functions in many instances converge with those of OPA1, have been identified by next generation sequencing. OPA1 encodes a dynamin-related GTPase imported into mitochondria and located to the inner membrane and intermembrane space. The many OPA1 isoforms, resulting from alternative splicing of three exons, form complex homopolymers that structure mitochondrial cristae, and contribute to fusion of the outer membrane, thus shaping the whole mitochondrial network. Moreover, OPA1 is required for oxidative phosphorylation, maintenance of mitochondrial genome, calcium homeostasis and regulation of apoptosis, thus making OPA1 the Swiss army-knife of mitochondria. Understanding DOA pathophysiology requires the understanding of RGC peculiarities with respect to OPA1 functions. Besides the tremendous energy requirements of RGCs to relay visual information from the eye to the brain, these neurons present unique features related to their differential environments in the retina, and to the anatomical transition occurring at the lamina cribrosa, which parallel major adaptations of mitochondrial physiology and shape, in the pre- and post-laminar segments of the optic nerve. Three DOA mouse models, with different Opa1 mutations, have been generated to study intrinsic mechanisms responsible for RGC degeneration, and these have further revealed secondary symptoms related to mitochondrial dysfunctions, mirroring the more severe syndromic phenotypes seen in a subgroup of patients. Metabolomics analyses of cells, mouse organs and patient plasma mutated for OPA1 revealed new unexpected pathophysiological mechanisms related to mitochondrial dysfunction, and biomarkers correlated quantitatively to the severity of the disease. Here, we review and synthesize these data, and propose different approaches for embracing possible therapies to fulfil the unmet clinical needs of this disease, and provide hope to affected DOA patients.

High-throughput screening identifies suppressors of mitochondrial fragmentation in OPA1 fibroblasts

Mutations in OPA1 cause autosomal dominant optic atrophy (DOA) as well as DOA+, a phenotype characterized by more severe neurological deficits. OPA1 deficiency causes mitochondrial fragmentation and also disrupts cristae, respiration, mitochondrial DNA (mtDNA) maintenance, and cell viability. It has not yet been established whether phenotypic severity can be modulated by genetic modifiers of OPA1. We screened the entire known mitochondrial proteome (1,531 genes) to identify genes that control mitochondrial morphology using a first-in-kind imaging pipeline. We identified 145 known and novel candidate genes whose depletion promoted elongation or fragmentation of the mitochondrial network in control fibroblasts and 91 in DOA+ patient fibroblasts that prevented mitochondrial fragmentation, including phosphatidyl glycerophosphate synthase (PGS1). PGS1 depletion reduces CL content in mitochondria and rebalances mitochondrial dynamics in OPA1-deficient fibroblasts by inhibiting mitochondrial fission, which improves defective respiration, but does not rescue mtDNA depletion, cristae dysmorphology, or apoptotic sensitivity. Our data reveal that the multifaceted roles of OPA1 in mitochondria can be functionally uncoupled by modulating mitochondrial lipid metabolism, providing novel insights into the cellular relevance of mitochondrial fragmentation.

Association Between L-OPA1 Cleavage and Cardiac Dysfunction During Ischemia-Reperfusion Injury in Rats

Background/Aims:

Structural and functional alterations in mitochondria, particularly, the inner mitochondrial membrane (IMM) plays a critical role in mitochondria-mediated cell death in response to cardiac ischemia-reperfusion (IR) injury. The integrity of IMM can be affected by two potential intra-mitochondrial factors: i) mitochondrial matrix swelling, and ii) proteolytic cleavage of the long optic atrophy type 1 (L-OPA1), an IMM-localized dynamin-like GTPase engaged in the regulation of structural organization and integrity of the mitochondrial cristae. However, the relationship between these two factors in response to oxidative stress remains unclear. Here, we elucidated the effects of cardiac IR injury on L-OPA1 cleavage and OMA1 activity.

Methods:

Langendorff-mode perfused isolated rat hearts were subjected to 25-min of global ischemia followed by 90-min reperfusion in the presence or absence of XJB-5-131 (XJB, a mitochondria-targeting ROS scavenger) and sanglifehrin A (SfA, a permeability transition pore inhibitor).

Results:

XJB in combination with SfA increased post-ischemic recovery of cardiac function and reduced mitochondrial ROS production at 30- and 60-min reperfusion and affected mitochondrial swelling. L-OPA1 levels were reduced in IR hearts; however, neither XJB, SfA, and their combination prevented IR-induced reduction of L-OPA1 cleavage. Likewise, IR increased the OMA1 enzymatic activity, which remained unchanged in the presence of XJB and/or SfA.

Conclusion:

IR-induced cardiac and mitochondrial dysfunctions are associated with OMA1 activation and L-OPA1 cleavage. However, XJB, SfA, and their combination do not prevent these changes despite improved heart and mitochondria function, thus, suggesting that different mechanisms can be implicated in L-OPA1 processing in response to cardiac IR injury.

A Perspective on Accelerated Aging Caused by the Genetic Deficiency of the Metabolic Protein, OPA1

Autosomal Dominant Optic Atrophy (ADOA) is an ophthalmological condition associated primarily with mutations in the OPA1 gene. It has variable onset, sometimes juvenile, but in other patients, the disease does not manifest until adult middle age despite the presence of a pathological mutation. Thus, individuals carrying mutations are considered healthy before the onset of clinical symptoms. Our research, nonetheless, indicates that on the cellular level pathology is evident from birth and mutant cells are different from controls. We argue that the adaptation and early recruitment of cytoprotective responses allows normal development and functioning but leads to an exhaustion of cellular reserves, leading to premature cellular aging, especially in neurons and skeletal muscle cells. The appearance of clinical symptoms, thus, indicates the overwhelming of natural cellular defenses and break-down of native protective mechanisms.

Opa1 Deficiency Leads to Diminished Mitochondrial Bioenergetics With Compensatory Increased Mitochondrial Motility

Purpose

Retinal ganglion cells (RGCs) are susceptible to mitochondrial deficits and also the major cell type affected in patients with mutations in the OPA1 gene in autosomal dominant optic atrophy (ADOA). Here, we characterized mitochondria in RGCs in vitro from a heterozygous B6; C3-Opa1^Q285STOP (Opa1^+/−) mouse model to investigate mitochondrial changes underlying the pathology in ADOA.

Methods

Mouse RGCs were purified from wild-type and Opa1^+/− mouse retina by two-step immunopanning. The mitochondria in neurites of RGCs were labeled with MitoTracker Red for structure and motility measurement by time-lapse imaging. Mitochondrial bioenergetics were determined by the real-time measurement of oxygen consumption rate using a Seahorse XF^e 96 Extracellular Flux Analyzer.

Results

We observed a significant decrease in mitochondrial length in Opa1^+/− RGCs with a remarkably higher proportion and density of motile mitochondria along the neurites. We also observed an increased transport velocity with a higher number of contacts between mitochondria in Opa1^+/− RGC neurites. The oxygen consumption assays showed a severe impairment in basal respiration, Adenosine triphosphate-linked (ATP-linked) oxygen consumption, as well as reserve respiratory capacity, in RGCs from Opa1^+/− mouse retina.

Conclusions

Opa1 deficiency leads to significant fragmentation of mitochondrial morphology, activation of mitochondrial motility and impaired respiratory function in RGCs from the B6; C3-Opa1^Q285STOP mouse model. This highlights the significant alterations in the intricate interplay between mitochondrial morphology, motility, and energy production in RGCs with Opa1 deficiency long before the onset of clinical symptoms of the pathology.

OPA1 haploinsufficiency due to a novel splicing variant resulting in mitochondrial dysfunction without mitochondrial DNA depletion

Background: To identify and investigate the effects of a novel splicing variant, c.1444-2A>C of OPA1, on its transcript, translation, and mitochondrial function, which was found in an 8-year-old patient with dominantly inherited optic atrophy (DOA). Materials and Methods: The clinical evaluations were performed at the Eye Center. Lymphoblast cell lines were generated from the patient, mother, and a normal control with the same haplotype of mitochondrial genome. The novel variant was confirmed by Sanger sequencing. The splicing alteration of cDNA was checked by both Sanger sequencing and agarose gel. OPA1 expression was carried out by RT-PCR and Western blotting. Transmission electron microscopy was used for mitochondrial morphology. Mitochondrial functions, including the rates of oxygen consumption, ATP generation, ROS product and membrane potential were assayed in lymphoblast cells. Results: The novel OPA1 splicing variant, c.1444-2A>C, led to a deletion of the 15th exon in mRNA transcript. Approximately 50% reduction of mRNA and protein expression was present in mutant cells as compared with controls. No marked depletion of mtDNA nor mitochondrial mass was caused by the splicing variant. However, defects that the impaired capacity of OXPHOS, reduced ATP generation, increased ROS and decreased membrane potential were observed in the mutant cells, which promoted a ubiquitin-binding mitophagy instead of apoptosis. Conclusions: The novel splicing variant, c.1444-2A>C resulted in OPA1 haploinsufficiency effect on its expression and mitochondrial function without mtDNA depletion. Our findings may provide new insights into the understanding of pathophysiology of DOA.

Effects of Diabetes on Mitochondrial Morphology and Its Implications in Diabetic Retinopathy

Purpose

To determine whether high glucose (HG) or diabetes alters mitochondrial morphology and promotes mitochondrial fragmentation in retinal vascular cells and thereby triggers apoptosis associated with diabetic retinopathy.

Methods

To assess whether diabetes promotes mitochondrial fragmentation and thereby triggers apoptosis, retinas from nondiabetic and diabetic rats were analyzed using electron microscopy (EM) and in parallel, wild-type, diabetic, and OPA1+/- mice were analyzed for optic atrophy gene 1 (OPA1) and cytochrome c levels using Western blot (WB) analysis. To assess the relationship between mitochondrial fragmentation and OPA1 levels, rat retinal endothelial cells (RRECs) were grown in normal (N; 5 mmol/L) medium, HG (30 mmol/L) medium, or in N medium transfected with OPA1 siRNA for seven days. Cells were examined for OPA1 expression and cytochrome c release by WB. In parallel, cells were stained with MitoTracker Red and assessed for mitochondrial fragmentation in live cells using confocal microscopy.

Results

EM images revealed significant mitochondrial fragmentation in vascular cells of retinal capillaries of diabetic rats compared with that of nondiabetic rats. WB analysis showed significant OPA1 downregulation concomitant with increased levels of proapoptotic cytochrome c levels in cells grown in HG and in cells transfected with OPA1 siRNA alone. Similarly, OPA1 level was significantly reduced in diabetic retinas compared with that of nondiabetic retinas. Interestingly, OPA1+/- animals exhibited elevated cytochrome c release similar to those of diabetic mice.

Conclusions

Findings indicate that diabetes promotes mitochondrial fragmentation in retinal vascular cells, which are driven, at least in part, by decreased OPA1 levels leading to apoptosis in diabetic retinopathy.

OPA1 deficiency accelerates hippocampal synaptic remodelling and age-related deficits in learning and memory

A healthy mitochondrial network is essential for the maintenance of neuronal synaptic integrity. Mitochondrial and metabolic dysfunction contributes to the pathogenesis of many neurodegenerative diseases including dementia. OPA1 is the master regulator of mitochondrial fusion and fission and is likely to play an important role during neurodegenerative events. To explore this, we quantified hippocampal dendritic and synaptic integrity and the learning and memory performance of aged Opa1 haploinsufficient mice carrying the Opa1Q285X mutation (B6; C3-Opa1Q285STOP ; Opa1+/- ). We demonstrate that heterozygous loss of Opa1 results in premature age-related loss of spines in hippocampal pyramidal CA1 neurons and a reduction in synaptic density in the hippocampus. This loss is associated with subtle memory deficits in both spatial novelty and object recognition. We hypothesize that metabolic failure to maintain normal neuronal activity at the level of a single spine leads to premature age-related memory deficits. These results highlight the importance of mitochondrial homeostasis for maintenance of neuronal function during ageing.

Hippo/Mst signaling coordinates cellular quiescence with terminal maturation in iNKT cell development and fate decisions

Invariant natural killer T (iNKT) cells acquire effector functions during development by mechanisms that remain poorly understood. Here, we show that the Hippo kinases Mst1 and Mst2 act as molecular rheostats for the terminal maturation and effector differentiation programs of iNKT cells. Loss of Mst1 alone or together with Mst2 impedes iNKT cell development, associated with defective IL-15-dependent cell survival. Mechanistically, Mst1 enforces iNKT cellular and transcriptional quiescence associated with maturation and commitment to iNKT1 cells by suppressing proliferation and Opa1-related mitochondrial metabolism that are dynamically regulated during iNKT cell development. Furthermore, Mst1 shapes the reciprocal fate decisions between iNKT1 and iNKT17 effector cells, which respectively depend upon mitochondrial dynamics and ICOS-mTORC2 signaling. Collectively, these findings establish Mst1 as a crucial regulator of mitochondrial homeostasis and quiescence in iNKT cell development and effector lineage differentiation and highlight that establishment of quiescence programs underlies iNKT cell development and effector maturation.

Mitochondrial functions and rare diseases

Mitochondria are dynamic cellular organelles responsible for a large variety of biochemical processes as energy transduction, REDOX signaling, the biosynthesis of hormones and vitamins, inflammation or cell death execution. Cell biology studies established that 1158 human genes encode proteins localized to mitochondria, as registered in MITOCARTA. Clinical studies showed that a large number of these mitochondrial proteins can be altered in expression and function through genetic, epigenetic or biochemical mechanisms including the interaction with environmental toxics or iatrogenic medicine. As a result, pathogenic mitochondrial genetic and functional defects participate to the onset and the progression of a growing number of rare diseases. In this review we provide an exhaustive survey of the biochemical, genetic and clinical studies that demonstrated the implication of mitochondrial dysfunction in human rare diseases. We discuss the striking diversity of the symptoms caused by mitochondrial dysfunction and the strategies proposed for mitochondrial therapy, including a survey of ongoing clinical trials.

Targeted next-generation sequencing extends the mutational spectrums for OPA1 mutations in Chinese families with optic atrophy

Purpose

We aim to reveal the disease-causing mutations in 15 Chinese families with optic atrophy (OA).

Methods

In total, 15 families with OA were recruited in the present study. Medical histories were carefully reviewed and comprehensive ophthalmic examinations were received by all recruited patients. Targeted next-generation sequencing (NGS) was selectively performed on all probands for mutation detection. Intrafamilial cosegregation and in-silico analyses were subsequently applied to predict the potential pathogenic effects of identified mutations.

Results

All included patients presented bilateral vision loss. Their fundus photographs showed temporal or total pallor of the optic discs. Fourteen mutations in the optic atrophy 1 (OPA1) gene were revealed as disease-causing mutations for the 15 families, including eight novel (c.968A>G, c.193C>G, c.1071dupT, c.987_988del, c.2012+2T>G, c.1036-1G>C, c.2126A>G, and c.1036_1038del) and six recurrent (c.1499G>A, c.1800C>A, c.1034G>A, c.2873_2876del, c.112C>T, and c.804_805del) mutations.

Conclusions

In conclusion, our study expands the mutational spectrum for the OPA1 gene and implies targeted NGS as an effective approach for the genetic diagnosis of OA, which might help to improve the clinical diagnosis for patients with OA.

Mitochondrial dynamics and their potential as a therapeutic target

Mitochondrial dynamics shape the mitochondrial network and contribute to mitochondrial function and quality control. Mitochondrial fusion and division are integrated into diverse cellular functions and respond to changes in cell physiology. Imbalanced mitochondrial dynamics are associated with a range of diseases that are broadly characterized by impaired mitochondrial function and increased cell death. In various disease models, modulating mitochondrial fusion and division with either small molecules or genetic approaches has improved function. Although additional mechanistic understanding of mitochondrial fusion and division will be critical to inform further therapeutic approaches, mitochondrial dynamics represent a powerful therapeutic target in a wide range of human diseases.

Phenotypic selection with an intrabody library reveals an anti-apoptotic function of PKM2 requiring Mitofusin-1

Bcl-2 family proteins control a decisive apoptotic event: mitochondrial outer membrane permeabilization (MOMP). To discover MOMP-regulating proteins, we expressed a library of intracellular single-chain variable fragments (scFvs) (“intrabodies”) and selected for those rescuing cells from apoptosis induced by BimS (the short isoform of Bim). One anti-apoptotic intrabody, intrabody 5 (IB5), recognized pyruvate kinase M2 (PKM2), which is expressed in cancer cells. PKM2 deletion ablated this clonogenic rescue; thus, IB5 activated a latent cytoprotective function of PKM2. This resulted not from pyruvate kinase activity per se but rather from the formation of an active tetrameric conformation of PKM2. A stably tetrameric PKM2 mutant, K422R, promoted cell survival even in the absence of IB5, and IB5 further increased survival. Mitochondria isolated from IB5-expressing cells were relatively resistant to MOMP in vitro. In cells, IB5 expression up-regulated Mitofusin-1 (Mfn1) and increased mitochondrial length. Importantly, Mfn1 deficiency abrogated IB5’s cytoprotective effect. PKM2’s anti-apoptotic function could help explain its preferential expression in human cancer.

Proteins belonging to the Bcl-2 family regulate a common form of cell death known as apoptosis. Typically, these proteins function in apoptosis by controlling the formation of large pores in the mitochondrial outer membrane (MOM). While many proteins that regulate apoptosis have been identified over the years, some may still be unknown. Here, we used an unbiased approach in which we first expressed in cultured tumor cells a library of intracellular single-chain antibodies termed “intrabodies.” We then selected for intrabodies that allowed cells to evade apoptosis. We identified pyruvate kinase isoform M2 (PKM2), a major glycolytic enzyme that has been linked to cancer development, as the specific target of one such anti-apoptotic intrabody. We showed that the PKM2-specific intrabody promoted cell survival not by neutralizing its target but rather by activating an anti-apoptotic function of PKM2. While this cell survival function of PKM2 was not related to changes in the levels of Bcl-2 family proteins or to effects on the enzymatic activity of PKM2, we found that cell survival requires the increased expression of a MOM protein, Mitofusin-1 (Mfn1), known to regulate mitochondrial fusion. We conclude that this cell survival function of PKM2 could contribute to a role in cancer progression for this protein.

Overexpression of Optic Atrophy Type 1 Protects Retinal Ganglion Cells and Upregulates Parkin Expression in Experimental Glaucoma

Glaucoma is a neurodegenerative disease that features progressive loss of retinal ganglion cells (RGCs). Increasing evidences have revealed that impaired mitochondrial dynamics occurs early in neurodegenerative diseases. Optic Atrophy Type 1 (OPA1), a mitochondrial fusion protein, has recently been suggested to be a mitophagic factor. Our previous studies found that glaucomatous retinal damage may be ameliorated by an increase in mitochondrial OPA1. In this study, we explored the mechanism involved in OPA1 mediated neuroprotection and its relationship with parkin dependent mitophagy in experimental glaucoma models. Our data showed that overexpression of OPA1 by viral vectors protected against RGC loss, attenuated Bax expression, and improved mitochondrial health and mitochondrial surface area. Parkin expression and the number of mitophagosomes were upregulated in OPA1 overexpressed RGCs under glutamate excitotoxicity. While knockdown of OPA1 by siRNA decreased protein expression of parkin in RGCs under glutamate excitotoxicity. Two weeks after intraocular pressure (IOP) elevation, the LC3-II/I ratio and the LAMP1 expression were increased in OPA1 overexpressed optic nerve. These findings suggest that OPA1 overexpression may protect RGCs by ways of enhancing mitochondria fusion and parkin mediated mitophagy. Interventions to promote mitochondrial fusion and mitophagy may provide a useful strategy to battle against glaucomatous RGC loss.

Meta-analysis of genotype-phenotype analysis of OPA1 mutations in autosomal dominant optic atrophy

Autosomal Dominant Optic Atrophy (ADOA) is a neuro-ophthalmic disease characterized by progressive bilateral vision loss, pallor of the optic disc, central vision loss, and impairment of color vision. Additionally, a small percentage of patients experience hearing loss and ataxia, while recent studies suggest disruption of cardiac and neuromuscular functions. In order to obtain a better understanding of the genotype-phenotype correlation of the various mutations in the optic atrophy 1 (OPA1) gene, we obtained both clinical and genetic information of ADOA patients from published reports. We conducted a systematic review of published OPA1 literature and identified 408 individuals with confirmed OPA1 mutations, 120 of whom reported extra-ocular (ADOA 'plus') manifestations through their descriptions of visual and multi-systemic symptoms. Our results show that there is a significant variation in frequency of the specific exons involved between the ADOA classic and ADOA 'plus' patients. Classic ADOA groups were more likely to have mutations in exon 8 and 9, while ADOA 'plus' groups were more likely to have mutations in exons 14, 15 and 17. Additional comparisons revealed significant differences between mutation types/domains and specific ADOA 'plus' manifestations. We also found that individuals with maternally inherited OPA1 mutations were significantly more likely to develop 'plus' manifestations than those with paternally inherited mutations. Overall, this study provides novel information regarding genotype-phenotype correlations of ADOA which warrants additional recommendations added to the current clinical management of ADOA patients.

Mitochondrial dynamics in adaptive and maladaptive cellular stress responses

Mitochondria sense and respond to many stressors and can support either cell survival or death through energy production and signaling pathways. Mitochondrial responses depend on fusion-fission dynamics that dilute and segregate damaged mitochondria. Mitochondrial motility and inter-organellar interactions, including with the endoplasmic reticulum, also function in cellular adaptation to stress. In this Review, we discuss how stressors influence these components, and how they contribute to the complex adaptive and pathological responses that lead to disease.

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.