InteractiveFly: GeneBrief

Optic atrophy 1: Biological Overview | References


Gene name - Optic atrophy 1

Synonyms -

Cytological map position - 50E4-50E4

Function - mitochondrial membrane fusion

Keywords - mediates fusion of the inner mitochondrial membrane, suppression of Opa1 induces cardiac dysfunction associated with mitochondrial depolarization and ROS production

Symbol - Opa1

FlyBase ID: FBgn0261276

Genetic map position - chr2R:14,230,611-14,235,449

NCBI classification - DLP_1: Dynamin_like protein family includes dynamins and Mx proteins

Cellular location - mitochondrial inner membrane



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

Mitochondria shape is controlled by membrane fusion and fission mediated by mitofusins, Opa1, and Drp1, whereas mitochondrial motility relies on microtubule motors. These processes govern mitochondria subcellular distribution, whose defects are emphasized in neurons because of their polarized structure. This study examined how perturbation of the fusion/fission balance affects mitochondria distribution in Drosophila axons. Knockdown of Marf or Opa1 resulted in progressive loss of distal mitochondria and in a distinct oxidative phosphorylation and membrane potential deficit. Downregulation of Drp1 rescued the lethality and bioenergetic defect caused by neuronal Marf RNAi, but induced only a modest restoration of axonal mitochondria distribution. Surprisingly, Drp1 knockdown rescued fragmentation and fully restored aberrant distribution of axonal mitochondria produced by Opa1 RNAi; however, Drp1 knockdown did not improve viability or mitochondria function. These data show that proper morphology is critical for proper axonal mitochondria distribution independent of bioenergetic efficiency. The health of neurons largely depends on mitochondria function, but does not depend on shape or distribution (Trevisan, 2018).

Mitochondria are organelles present in all eukaryotic cells and preside over a wide variety of crucial functions, including respiration and energy production. Mitochondria are organized in a fluidly interconnected, dynamic network, and their structure and position are not fixed but vary by cell type, developmental stage, and physiological context. The dynamic behavior of the network is important for mitochondria distribution and inheritance; is important for remodeling during development and coordination of cell death programs; and allows cells to respond to shifting needs at different intracellular locations (Trevisan, 2018).

Mitochondrial fusion and fission control the shape of mitochondria. The molecular machines that mediate mitochondrial division and fusion are dynamin-related proteins. Drp1 drives the scission of mitochondrial membranes and Mfn1/2 and Opa1 mediate fusion of the outer and inner mitochondrial membranes, respectively (Westermann, 2008). The motility of mitochondria within the cell depends on the action of microtubule-based motors. An ensemble of fission, fusion, and motility governs the distribution of mitochondria within the cell. This is especially evident in neurons whose polarized structure, which is characterized by the presence of axons and dendrites, emphasizes defects in the distribution of mitochondria. A growing body of evidence suggests that fusion and fission affect spatial distribution of mitochondria in neurons. Loss of Drp1, Marf/MFN, or Opa1 in Drosophila depletes mitochondria from neuromuscular junction synapses and motor axons. Mutations of the mitochondrial outer membrane fusion protein Mfn2 or the inner membrane fusion protein Opa1 (Spinazzi, 2008) change the distribution of axonal mitochondria, and knockdown of Opa1 causes redistribution of dendritic mitochondria (Bertholet, 2013). It also has been proposed that a direct interaction exists between Mfn2 and the molecular machinery of mitochondrial transport (Trevisan, 2018).

In addition to shape and distribution, the impact of alterations in the activity of mitochondrial dynamics proteins also resonates in mitochondrial function. Substantial evidence has been obtained in mammalian cells, indicating that loss of Mfn2 and Mfn1 function causes alterations in mitochondrial metabolism with loss of mitochondrial membrane potential and reduced endogenous respiration. Furthermore, Opa1 knockdown causes a widespread loss of mitochondrial membrane potential and a reduction in basal respiration (Chen, 2005, Olichon, 2003). The general tenet is that interconnected mitochondrial networks are found in respiratory active cells, whereas small and fragmented mitochondria are more prevalent in respiratory inactive cells (Trevisan, 2018).

A number of biologically relevant questions thus arise. How are morphology and function linked to the subcellular distribution of mitochondria? Since it is thought that metabolically dysfunctional mitochondria are transported to the neuron body for clearance, is decreased respiration and the consequent retrograde transport of mitochondria triggered by fusion/fission defects responsible for aberrant distribution of axonal mitochondria? Is abnormal mitochondria morphology resulting from fusion/fission defects the reason for this aberrant distribution? Are morphology and function always linked? Do they together contribute to abnormal mitochondria transport? These questions carry also key medical relevance as mutations in Mfn2, Opa1, and Drp1 cause hereditary neuropathies whose pathophysiologic mechanism has been proposed to be abnormal transport and distribution of mitochondria, leading to distal axonal degeneration (Trevisan, 2018).

This study has examined in vivo how tweaking mitochondria fusion/fission balance perturbs mitochondria morphology and function and how these perturbations affect the distribution of mitochondria within Drosophila segmental nerves. Downregulation of Marf or Opa1 results in depletion of mitochondria at the neuromuscular junction synapse and in progressive distal loss of mitochondria along the extended axons in segmental nerves, but not in shorter photoreceptor axons. This effect was accompanied by a marked oxidative phosphorylation deficit. Simultaneous downregulation of Drp1 rescues the lethality and bioenergetic defect caused by neuronal Marf RNAi alone but ameliorates very marginally the distribution of axonal mitochondria. However, downregulation of Drp1 rescues fragmentation and aberrant distribution of axonal mitochondria induced by Opa1 RNAi without restoring viability or oxidative phosphorylation. Thus, the data show that shape is crucial for proper axonal mitochondria distribution, which in turn is largely independent of bioenergetic efficiency. However, the health of neurons depends on mitochondria function even in spite of irregular morphology and distribution. This has additional implications for mitochondria dynamics-dependent pathologies because it indicates that deficiencies observed in the absence of fusion/fission components may primarily arise from impaired mitochondrial function rather than disturbed mitochondrial transport and distribution (Trevisan, 2018).

The mitochondria fusion and fission machinery is essential for cells and genetic ablation of individual components in animal models results in death. The physiological relevance of fission/fusion processes also has become apparent from the study of patients harboring mutations in components of either fusion (Mfn2 and Opa1) or fission (Drp1) (Burte, 2015). The prime consequence of mutation or absence of fusion and fission proteins is a change in mitochondrial shape. Additionally, perturbation of mitochondrial transport and impairment of mitochondrial functions have frequently been reported. Derangement of fusion/fission dynamics affects in particular neurons with the longest axons and high energetic requirements, such as peripheral sensory and motor neurons, because their considerable length demands proper mitochondria functioning and distribution along nerves. The pleiotropic array of mitochondrial dysfunctions that follows the loss of fusion/fission players challenges an ability to explain this observation. Are long axons more vulnerable because of disrupted mitochondrial trafficking, because mitochondrial function is impaired, or both? This work has investigated whether the spatial distribution of mitochondria along axons depends on form, function or both. This study found that in vivo in Drosophila disruption of inner or outer membrane fusion caused by Opa1 and Marf knockdown, respectively, results in mitochondria fragmentation and in a severe oxidative phosphorylation deficit. A striking change was observed in the spatial distribution pattern of axonal mitochondria in larval segmental nerves with a progressive loss of mitochondria along the axon in the distal direction. Distal loss was exacerbated in the NMJs that were essentially devoid of mitochondria. This defect was clearly length-dependent, as shorter photoreceptor axons did not display such phenotype, thus confirming that also in flies long axons are more prone to insult. The data indicate that distal mitochondria loss is unlikely to occur because of increased transport back to the neuron body for the autophagic clearance of dysfunctional mitochondria since it was found that downregulation of Opa1 or Marf heavily repressed trafficking in both the anterograde and retrograde directions. A specific role for Marf in linking mitochondria to the transport machinery through an interaction with Miro was also ruled out in flies, both because the two proteins did not coIP and because distal axonal loss is not specific for Marf but is observed after Opa1 knockdown and has been reported as a consequence of severe Drp1 mutation (Trevisan, 2018).

To directly address the importance of mitochondria morphology on axonal distribution Drp1 was downregulated in Marf or Opa1 depleted flies. It was reasoned that achieving new fusion/fission equilibrium might rescue mitochondria shape and, consequently, the Marf RNAi and Opa1 RNAi phenotypes. Surprisingly, it was found that rescue of Opa1 RNAi morphology defects by downregulation of Drp1 was associated with restoration of axonal mitochondria distribution. However, neither the bioenergetic capacity nor the lethality associated with Opa1 knockdown was rescued. In contrast, simultaneous downregulation of Marf and Drp1 led to a very robust rescue of lethality and strong restoration of oxidative phosphorylation. These double RNAi individuals, however, had fragmented mitochondrial morphology and displayed very limited rescue of axonal mitochondria distribution, suggesting that shape is more critical than function for mitochondria transport. A plausible rationalization of these results could be that Opa1-RNAi;Drp1-RNAi mitochondria are more elongated because Marf continues to fuse the outer membrane while fission is inhibited, but remain dysfunctional because disruption of inner membrane cristae organization due to the absence of Opa1 (Meeusen, 2006) reduces their respiratory capacity. Conversely, partial recovery of a balance between outer membrane fusion and fission in Marf-RNAi,Drp1-RNAi mitochondria only marginally restores morphology and distribution but is sufficient to support normal respiratory activity and considerably ameliorate oxidative phosphorylation. This interpretation is strongly supported by electron microscopy (EM) analysis, clearly demonstrating that Opa1-RNAi;Drp1-RNAi mitochondria display severely altered internal organization while over 50% of Marf-RNAi,Drp1-RNAi mitochondria display normal organization and ultrastructure fully resembling wild-type mitochondria (Trevisan, 2018).

Available data indicate that deficiency in protein components of the mitochondrial fusion machinery reduces mitochondrial membrane potential and respiration, implying that altered mitochondrial morphology and perturbed function always go hand in hand. This study shows that the consequences of mitochondria dysmorphology and dysfunction can be dissociated, suggesting that mitochondria shape and function do not necessarily have an obligatory relationship. In fact, fragmented mitochondria with essentially normal bioenergetics capability can be generated within the nervous system of a living Drosophila. These flies contain very few mitochondria in distal axons and NMJs, but they are viable because many of their mitochondria are functional. The fact that a small number of bioenergetically active mitochondria can support normal neuronal function should not come as a surprise. Indeed, recent studies have highlighted the importance of local glycolysis to meet energy demands at synapses (Trevisan, 2018).

The genetic paradigms created have allowed separation og the independent contribution of form and function in determining the subcellular distribution of mitochondria. Proper mitochondria distribution requires quasi-normal external mitochondria morphology; however, this is not sufficient to ensure proper neuronal function, which not unexpectedly, depends on normal mitochondrial metabolic activity. One possible explanation linking mitochondria shape to their distribution is that fragmented mitochondria may not provide proper anchoring for transport adaptor molecules that reside in the outer membrane, such as Miro/Milton. Perhaps a smaller size does not warrant attachment of a sufficient number of adaptors to the individual mitochondrion for its correct mobility. This would provide a reason restoration of shape rescues the distribution of metabolically inefficient axonal mitochondria (Trevisan, 2018).

This study has generated an in vivo system in which mitochondria fragmentation is not accompanied by encumbering bioenergetic dysfunctions. Mitochondrial fragmentation is observed in a variety of pathological conditions, and the results indicate that in diseases characterized by altered mitochondrial morphology and altered axonal distribution, these defects may not be the primary cause of pathology since in the Drosophila model restoration of morphology and distribution without restoration of respiratory capacity does not permit survival (Trevisan, 2018).

Inner membrane fusion mediates spatial distribution of axonal mitochondria

In eukaryotic cells, mitochondria form a dynamic interconnected network to respond to changing needs at different subcellular locations. A fundamental yet unanswered question regarding this network is whether, and if so how, local fusion and fission of individual mitochondria affect their global distribution. To address this question, high-resolution computational image analysis techniques have been developed to examine the relations between mitochondrial fusion/fission and spatial distribution within the axon of Drosophila larval neurons. Stationary and moving mitochondria were found to undergo fusion and fission regularly but followed different spatial distribution patterns and exhibited different morphology. Disruption of inner membrane fusion by knockdown of dOpa1, Drosophila Optic Atrophy 1, not only increased the spatial density of stationary and moving mitochondria but also changed their spatial distributions and morphology differentially. Knockdown of dOpa1 also impaired axonal transport of mitochondria. But the changed spatial distributions of mitochondria resulted primarily from disruption of inner membrane fusion because knockdown of Milton, a mitochondrial kinesin-1 adapter, caused similar transport velocity impairment but different spatial distributions. Together, these data reveals that stationary mitochondria within the axon interconnect with moving mitochondria through fusion and fission and that local inner membrane fusion between individual mitochondria mediates their global distribution (Yu, 2016).

Although individual mitochondria within the axon may appear as discrete compartments, they interconnect through fusion and fission as well as transport and anchoring to form a dynamic network. This network is distributed spatially to fulfill changing needs at different locations. Proper spatial distribution of axonal mitochondria has been shown, for example, to be essential for axon branching, synaptic functions and a variety of other neuronal activities. This study has focused on addressing a basic question regarding the axonal mitochondrial network, namely whether, and if so how, local fusion and fission of individual mitochondria affect their global distribution (Yu, 2016).

Inner membrane fusion, which occurred locally between individual mitochondria, mediates the global distribution of mitochondria within the axon. In wild-type Drosophila larvae, spatial distribution of stationary and moving axonal mitochondria followed distinct patterns. Disruption of inner membrane fusion by dOpa1 knockdown not only caused dramatic imbalance between fusion and fission and fragmentation of individual mitochondria but also changed their spatial distribution patterns, resulting in progressive loss of both stationary and moving mitochondria along the axon towards distal axon regions. Consistent with this result, disruption of mitochondrial outer membrane fusion by knocking down Marf (Drosophila ortholog Mfn2) resulted in similar progressive loss of stationary and moving mitochondria along the axon. On the other hand, knockdown of mitochondrial motor adaptor Milton impaired retrograde velocities of mitochondrial transport similarly as dOpa1 knockdown but changed the spatial distribution of axonal mitochondria differently. Together, these results showed that the changes to the spatial distributions of axonal mitochondria under dOpa1 knockdown were caused primarily by disruption of inner membrane fusion and that impairment of mitochondrial transport under dOpa1 knockdown played a secondary role in causing these changes. Therefore, local inner membrane fusion plays an important role in mediating the global spatial distribution of mitochondria (Yu, 2016).

This study is in agreement with a growing number of studies suggesting that, in addition to mediating local mitochondrial content exchange or transport-docking, fusion and fission are involved in regulating the global organization of the mitochondrial network, although direct analysis of the relations between local fusion/fission and the global organization of the mitochondrial network was lacking in previous studies. The current data reveals direct connections and quantitative relations between mitochondrial inner membrane fusion and spatial distribution. However, a limitation of this assay is that it is restricted to the specific group of neurons in which SG26 Gal4 is expressed. Further studies should examine relations between mitochondrial fusion/fission and spatial distribution in different groups of neurons (Yu, 2016).

A unique challenge facing neurons is to sustain functionally competent mitochondria over extended distances. Indeed, neurons are known to be particularly vulnerable to dysfunction of mitochondria and mutations of mitochondrial proteins. Since the soma of neurons is the primary site for biogenesis and degradation of mitochondria, a basic question regarding axonal mitochondria is how they stay functionally competent and renew themselves while being far away from the neuronal cell body. The current data supports the hypothesis that axonal mitochondria replenish themselves through fusion with moving ones passing by and through fission to discard their damaged portion. Specifically, it was found that within the axon of Drosophila larval neurons, stationary mitochondria underwent fusion and fission regularly with moving mitochondria. They were also much larger than moving ones but became more fragmented under dOpa1 knockdown. Together, the data suggests differential roles of stationary and moving mitochondria: while stationary mitochondria fulfill metabolic and functional needs of their local areas, moving mitochondria support stationary mitochondria by renewing their content through fusion/fission. Furthermore, moving mitochondria can move to areas where new needs arise and settle down as stationary mitochondria. The data supports the quality control model of axonal mitochondria previously proposed47 but does not rule out the possibility that populations of stationary and moving mitochondria interchange through direct switching between their motion states by a transport-docking mechanism (Yu, 2016).

Based on the data, a model is proposed of how the spatial distribution of axonal mitochondria is maintained in healthy neurons and how it is changed by disruption of inner membrane fusion. It is conjectured that anchored stationary mitochondria activate a fusion signal when renewal is needed. The fusion signal retains some of the passing mitochondria to engage in fusion and fission. Successful completion of fusion and fission inactivates the signal, allowing moving mitochondria to simply pass by. When inner membrane fusion is disrupted by OPA1 knockdown, the fusion signal of stationary mitochondria can no longer be inactivated because fusion cannot be completed. Cristae disorganization under dOpa1 knockdown may also interfere with the successful fusion of mitochondria. The stationary mitochondria close to the soma and with activated fusion signal will retain increasing numbers of moving mitochondria so that fewer moving mitochondria can reach more distal regions. This results in a gradual accumulation of stationary mitochondria close to the soma and progressive loss of mitochondria along the axon towards distal synaptic terminals. The loss of mitochondria in distal regions eventually leads to neurodegeneration (Yu, 2016).

Crosstalk between mitochondrial fusion and the Hippo pathway in controlling cell proliferation during Drosophila development

Cell proliferation and tissue growth depend on the coordinated regulation of multiple signaling molecules and pathways during animal development. Previous studies have linked mitochondrial function and the Hippo signaling pathway in growth control. However, the underlying molecular mechanisms are not fully understood. This study identifies a Drosophila mitochondrial inner membrane protein ChChd3 as a novel regulator for tissue growth during larval development. Loss of ChChd3 leads to tissue undergrowth and cell proliferation defects. ChChd3 is required for mitochondrial fusion and removal of ChChd3 increases mitochondrial fragmentation. ChChd3 is another mitochondrial target of the Hippo pathway, although it is only partially required for Hippo pathway mediated overgrowth. Interestingly, lacking of ChChd3 leads to inactivation of Hippo activity under normal development, which is also dependent on the transcriptional co-activator Yorkie (Yki). Furthermore, loss of ChChd3 induces oxidative stress and activates the JNK pathway. In addition, depletion of other mitochondrial fusion components, Opa1 or Marf, inactivates the Hippo pathway as well. Taken together, the study proposes that there is a crosstalk between mitochondrial fusion and the Hippo pathway which is essential in controlling cell proliferation and tissue homeostasis in Drosophila (Deng, 2016).

Dissociation of mitochondrial from sarcoplasmic reticular stress in Drosophila cardiomyopathy induced by molecularly distinct mitochondrial fusion defects

Mitochondrial dynamism (fusion and fission) is responsible for remodeling interconnected mitochondrial networks in some cell types. Adult cardiac myocytes lack mitochondrial networks, and their mitochondria are inherently 'fragmented'. Mitochondrial fusion/fission is so infrequent in cardiomyocytes as to not be observable under normal conditions, suggesting that mitochondrial dynamism may be dispensable in this cell type. However, it has been previously reported that cardiomyocyte-specific genetic suppression of mitochondrial fusion factors Optic atrophy 1 (Opa1) and Mitofusin (Marf) evokes cardiomyopathy in Drosophila hearts. Fusion-mediated remodeling of mitochondria may be critical for cardiac homeostasis, although never directly observed. Alternately, inner membrane Opa1 and outer membrane mitofusin/MARF might have other as-yet poorly described roles that affect mitochondrial and cardiac function. This study compared heart tube function in three models of mitochondrial fragmentation in Drosophila cardiomyocytes: Drp1 expression, Opa1 RNAi, and mitofusin MARF RNA1. Mitochondrial fragmentation evoked by enhanced Drp1-mediated fission did not adversely impact heart tube function. In contrast, RNAi-mediated suppression of either Opa1 or mitofusin/MARF induced cardiac dysfunction associated with mitochondrial depolarization and ROS production. Inhibiting ROS by overexpressing superoxide dismutase (SOD) or suppressing ROMO1 prevented mitochondrial and heart tube dysfunction provoked by Opa1 RNAi, but not by mitofusin/MARF RNAi. In contrast, enhancing the ability of endoplasmic/sarcoplasmic reticulum to handle stress by expressing Xbp1 rescued the cardiomyopathy of mitofusin/MARF insufficiency without improving that caused by Opa1 deficiency. The study concludes that decreased mitochondrial size is not inherently detrimental to cardiomyocytes. Rather, preservation of mitochondrial function by Opa1 located on the inner mitochondrial membrane, and prevention of ER stress by mitofusin/MARF located on the outer mitochondrial membrane, are central functions of these 'mitochondrial fusion proteins' (Bhandari, 2015).

By performing a side-by-side detailed comparison of cardiomyopathies provoked by interrupting fusion of either the outer or inner mitochondrial membranes, this study identified distinct cellular mechanisms for the different molecular lesions. RNAi-mediated suppression of outer mitochondrial membrane mitofusin/MARF and inner mitochondrial membrane Opa1 provokes similar overt phenotypes: in both models mitochondrial size is approximately halved, the proportion of depolarized (sick) mitochondria increases to ~ 40%, mitochondrial ROS production is comparably greater (~ 50%), and the heart tubes exhibit similar reductions in fractional shortening. However, the cardiac defect caused by Opa1 deficiency is readily corrected by attacking the disease at the level of mitochondrial ROS production, through SOD expression or ROMO1 suppression. Indeed, mitochondrial structural and functional abnormalities are also improved by these genetic maneuvers, suggesting that they interrupt a vicious cycle of ROS-induced mitochondrial degeneration provoked by Opa1 insufficiency. Thus, interrupting endogenous mitochondrial ROS production greatly abrogates both the mitotoxicity and the cytotoxicity evoked by Opa1 suppression. This suggests a central role for mitochondrial degeneration in the Opa1-deficient fly heart model and, by extension, other heart diseases caused by defective inner mitochondrial membrane fusion proteins (Bhandari, 2015).

Whereas mitochondrial size and polarization status are similarly impaired in mitofusin/MARF insufficient heart tubes, neither of the mitochondrial-targeted interventions directed at reducing ROS, both of which rescue Opa1 deficient hearts, improve the cardiomyopathy induced by mitofusin/MARF deficiency. Indeed, whereas transgenic expression of SOD1 or SOD2 normalizes both ROS and heart tube function in Parkin-deficient heart tubes and Opa1-deficient heart tubes, it is remarkable that SOD fails to improve ROS levels in the mitofusin/MARF deficient heart tubes. Together with the original observation of transient heart tube functional improvement with SOD1, these observations suggest that there is 'ROS escape' in the mitochondrial fusion impaired model that confers resistance to SOD expression or ROMO1 suppression. Instead, heart tube function is normalized without improving either mitochondrial structural or functional abnormalities by genetically enhancing the cardiomyocytes' ability to handle ER stress through XBP1 expression. These results, although surprising, are consistent with an essential role for mitofusin-mediated mitochondrial-ER/SR cross-talk in managing the ER stress response as proposed earlier in heart and in other tissues (Bhandari, 2015).

Previously, the consequences of Opa1 deficiency on Drosophila eye phenotypes have been found to be rescuable with SOD1, which is in accordance with findings of this study. Another study in Drosophila neurons and skeletal muscle also supports an important role for mitofusins, but not Opa1, as modulators of ER stress. However, only human Mfn2, and not human Mfn1, can correct abnormalities induced by mitofusin/MARF suppression in flies. This contrasts with findings in this study that cardiac-specific expression of either human Mfn1 or Mfn2 will fully correct cardiomyopathy induced by cardiomyocyte-specific MARF RNAi. Furthermore, ER dysmorphology has been found in MARF-deficient fly tissues, which was not detected in MARF-deficient heart tubes. It is likely either that the heart has different requirements for mitofusins and ER/SR morphology, or that the powerful tinman gene promoter used for cardiomyocyte-specific gene manipulation confers different expression characteristics to the heart models. Either way, the overall conclusions regarding a role of ER stress in mitofusin deficiency are in agreement (Bhandari, 2015).

Cardiomyocyte mitochondria are the Oompa-Loompas of the heart (with apologies to Roald Dahl): They are diminutive, structurally homogenous, and frequently overlooked despite toiling endlessly behind the scenes to keep the place running. Research has tended to focus on the mitochondrial work product (cardiac metabolism) and the means by which the general mitochondrial population is sustained (through biogenesis), rather than the fate of individual organelles. Indeed, individual cardiomyocyte mitochondria seem hardly worthy of observation, being monotonously similar in appearance and lacking the morphometric remodeling or intra-cellular mobility that has sparked detailed investigations (and visually engaging movie clips) in other cell types. The data presented in this study emphasize that (for mitochondria as well as Oompa-Loompas) size is not the critical determinant of function; it is literally what is inside that counts. Accordingly, one should eschew generalizations and extrapolations of mitochondrial status and dysfunction based strictly on morphometry (Bhandari, 2015).


Functions of Opa1 orthologs in other species

OPA1 isoforms in the hierarchical organization of mitochondrial functions

OPA1 is a GTPase that controls mitochondrial fusion, cristae integrity, and mtDNA maintenance. In humans, eight isoforms are expressed as combinations of long and short forms, but it is unclear whether OPA1 functions are associated with specific isoforms and/or domains. To address this, each of the eight isoforms or different constructs of isoform 1 were expressed in Opa1(-/-) MEFs. It was observed that any isoform could restore cristae structure, mtDNA abundance, and energetic efficiency independently of mitochondrial network morphology. Long forms supported mitochondrial fusion; short forms were better able to restore energetic efficiency. The complete rescue of mitochondrial network morphology required a balance of long and short forms of at least two isoforms, as shown by combinatorial isoform silencing and co-expression experiments. Thus, multiple OPA1 isoforms are required for mitochondrial dynamics, while any single isoform can support all other functions. These findings will be useful in designing gene therapies for patients with OPA1 haploinsufficiency (Del Dotto, 2017).

Mitochondrial inner-membrane fusion and crista maintenance requires the dynamin-related GTPase Mgm1

Mitochondrial outer- and inner-membrane fusion events are coupled in vivo but separable and mechanistically distinct in vitro, indicating that separate fusion machines exist in each membrane. Outer-membrane fusion requires trans interactions of the dynamin-related GTPase Fzo1, GTP hydrolysis, and an intact inner-membrane proton gradient. Inner-membrane fusion also requires GTP hydrolysis but distinctly requires an inner-membrane electrical potential. The protein machinery responsible for inner-membrane fusion is unknown. This study shows that the conserved intermembrane-space dynamin-related GTPase Mgm1 (Opa1) is required to tether and fuse mitochondrial inner membranes. An additional role of Mgm1 was observed in inner-membrane dynamics, specifically in the maintenance of crista structures. Evidence is presented that trans Mgm1 interactions on opposing inner membranes function similarly to tether and fuse inner membranes as well as maintain crista structures, and a model is proposed for how the mitochondrial dynamins function to facilitate fusion (Meeusen, 2016).

OPA1 loss of function affects in vitro neuronal maturation

Mitochondrial dynamics control the organelle's morphology, with fusion leading to the formation of elongated tubules and fission leading to isolated puncta, as well as mitochondrial functions. Recent reports have shown that disruptions of mitochondrial dynamics contribute to neurodegenerative diseases. Mutations of the inner membrane GTPase OPA1 are responsible for type 1 dominant optic atrophy, by mechanisms not fully understood. This study shows that in rodent cortical primary neurons, downregulation of the OPA1 protein leads to fragmented mitochondria that become less abundant along the dendrites. Furthermore, this inhibition results in reduced expression of mitochondrial respiratory complexes as well as mitochondrial DNA, decreased mitochondrial membrane potential, and diminished reactive oxygen species levels. The onset of synaptogenesis was markedly impaired through reductions in pre- and postsynaptic structural protein expression and synapse numbers without first affecting the dendritic arborization. With longer time in culture, OPA1 extinction led to a major restriction of dendritic growth, together with reduction of synaptic proteins. Furthermore, in maturing neurons a transitory increase is observed in mitochondrial filament length, associated with marked changes in the expression levels of OPA1, which occurs at the onset of synaptogenesis simultaneously with transitory increase in reactive oxygen species levels and NRF2/NFE2L2 nuclear translocation. This observation suggests that mitochondrial hyperfilamentation acts upstream of a reactive oxygen species-dependent NRF2 transcriptional activity, possibly impacting neuronal maturation, such a process being impaired by insufficient amount of OPA1. These findings suggest a new role for OPA1 in synaptic maturation and dendritic growth through maintenance of proper mitochondrial oxidative metabolism and distribution, highlighting the role of mitochondrial dynamics in neuronal functioning and providing insights into dominant optic atrophy pathogenesis, as OPA1 loss affecting neuronal maturation could lead to early synaptic dysfunction (Bertholet, 2013).

A novel deletion in the GTPase domain of OPA1 causes defects in mitochondrial morphology and distribution, but not in function

Autosomal dominant optic atrophy (ADOA), the commonest cause of inherited optic atrophy, is caused by mutations in the ubiquitously expressed gene optic atrophy 1 (OPA1), involved in fusion and biogenesis of the inner membrane of mitochondria. Bioenergetic failure, mitochondrial network abnormalities and increased apoptosis have all been proposed as possible causal factors. However, their relative contribution to pathogenesis as well as the prominent susceptibility of the retinal ganglion cell (RGC) in this disease remains uncertain. This study identified a novel deletion of OPA1 gene in the GTPase domain in three patients affected by ADOA. Muscle biopsy of the patients showed neurogenic atrophy and abnormal morphology and distribution of mitochondria. Confocal microscopy revealed increased mitochondrial fragmentation in fibroblasts as well as in myotubes, where mitochondria were also unevenly distributed, with clustered organelles alternating with areas where mitochondria were sparse. These abnormalities were not associated with altered bioenergetics or increased susceptibility to pro-apoptotic stimuli. Therefore, changes in mitochondrial shape and distribution can be independent of other reported effects of OPA1 mutations, and therefore may be the primary cause of the disease. The arrangement of mitochondria in RGCs, which degenerate in ADOA, may be exquisitely sensitive to disturbance, and this may lead to bioenergetic crisis and/or induction of apoptosis. These results highlight the importance of mitochondrial dynamics in the disease per se, and point to the loss of the fine positioning of mitochondria in the axons of RGCs as a possible explanation for their predominant degeneration in ADOA (Spinazzi, 2008).

Prohibitins control cell proliferation and apoptosis by regulating OPA1-dependent cristae morphogenesis in mitochondria

Prohibitins comprise an evolutionarily conserved and ubiquitously expressed family of membrane proteins with poorly described functions. Large assemblies of PHB1 and PHB2 subunits are localized in the inner membrane of mitochondria, but various roles in other cellular compartments have also been proposed for both proteins. This study used conditional gene targeting of murine Phb2 to define cellular activities of prohibitins. The experiments restrict the function of prohibitins to mitochondria and identify the processing of the dynamin-like GTPase OPA1, an essential component of the mitochondrial fusion machinery, as the central cellular process controlled by prohibitins. Deletion of Phb2 leads to the selective loss of long isoforms of OPA1. This results in an aberrant cristae morphogenesis and an impaired cellular proliferation and resistance toward apoptosis. Expression of a long OPA1 isoform in PHB2-deficient cells suppresses these defects, identifying impaired OPA1 processing as the primary cellular defect in the absence of prohibitins. The results therefore assign an essential function for the formation of mitochondrial cristae to prohibitins and suggest a coupling of cell proliferation to mitochondrial morphogenesis (Merkwirth, 2008).

Disruption of fusion results in mitochondrial heterogeneity and dysfunction

Mitochondria undergo continual cycles of fusion and fission, and the balance of these opposing processes regulates mitochondrial morphology. Paradoxically, cells invest many resources to maintain tubular mitochondrial morphology, when reducing both fusion and fission simultaneously achieves the same end. This observation suggests a requirement for mitochondrial fusion, beyond maintenance of organelle morphology. This study shows that cells with targeted null mutations in Mfn1 or Mfn2 retained low levels of mitochondrial fusion and escaped major cellular dysfunction. Analysis of these mutant cells showed that both homotypic and heterotypic interactions of Mfns are capable of fusion. In contrast, cells lacking both Mfn1 and Mfn2 completely lacked mitochondrial fusion and showed severe cellular defects, including poor cell growth, widespread heterogeneity of mitochondrial membrane potential, and decreased cellular respiration. Disruption of OPA1 by RNAi also blocked all mitochondrial fusion and resulted in similar cellular defects. These defects in Mfn-null or OPA1-RNAi mammalian cells were corrected upon restoration of mitochondrial fusion, unlike the irreversible defects found in fzoδ yeast. In contrast, fragmentation of mitochondria, without severe loss of fusion, did not result in such cellular defects. These results showed that key cellular functions decline as mitochondrial fusion is progressively abrogated (Chen, 2005).

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, a neuropathy resulting from degeneration of the retinal ganglion cells and optic nerve atrophy. This study shows 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, it is proposed that OPA1 is involved in the cytochrome c sequestration and might be a target for mitochondrial apoptotic effectors. These results also suggest that abnormal apoptosis is a possible pathophysiological process leading to the retinal ganglion cells degeneration in ADOA patients (Olichon, 2003).


REFERENCES

Search PubMed for articles about Drosophila Opa1

Bertholet, A. M., Millet, A. M., Guillermin, O., Daloyau, M., Davezac, N., Miquel, M. C. and Belenguer, P. (2013). OPA1 loss of function affects in vitro neuronal maturation. Brain 136(Pt 5): 1518-1533. PubMed ID: 23543485

Bhandari, P., Song, M. and Dorn, G.W. (2015). Dissociation of mitochondrial from sarcoplasmic reticular stress in Drosophila cardiomyopathy induced by molecularly distinct mitochondrial fusion defects. J Mol Cell Cardiol 80: 71-80. PubMed ID: 25555803Burte, F., Carelli, V., Chinnery, P. F. and Yu-Wai-Man, P. (2015). Disturbed mitochondrial dynamics and neurodegenerative disorders. Nat Rev Neurol 11(1): 11-24. PubMed ID: 25486875

Chen, H., Chomyn, A. and Chan, D. C. (2005). Disruption of fusion results in mitochondrial heterogeneity and dysfunction. J Biol Chem 280(28): 26185-26192. PubMed ID: 15899901

Del Dotto, V., Mishra, P., Vidoni, S., Fogazza, M., Maresca, A., Caporali, L., McCaffery, J. M., Cappelletti, M., Baruffini, E., Lenaers, G., Chan, D., Rugolo, M., Carelli, V. and Zanna, C. (2017). OPA1 isoforms in the hierarchical organization of mitochondrial functions. Cell Rep 19(12): 2557-2571. PubMed ID: 28636943

Deng, Q., Guo, T., Zhou, X., Xi, Y., Yang, X.and Ge, W. (2016). Crosstalk between mitochondrial fusion and the Hippo pathway in controlling cell proliferation during Drosophila development. Genetics [Epub ahead of print]. PubMed ID: 27317679

Meeusen, S., DeVay, R., Block, J., Cassidy-Stone, A., Wayson, S., McCaffery, J. M. and Nunnari, J. (2006). Mitochondrial inner-membrane fusion and crista maintenance requires the dynamin-related GTPase Mgm1. Cell 127(2): 383-395. PubMed ID: 17055438

Merkwirth, C., Dargazanli, S., Tatsuta, T., Geimer, S., Lower, B., Wunderlich, F. T., von Kleist-Retzow, J. C., Waisman, A., Westermann, B. and Langer, T. (2008). Prohibitins control cell proliferation and apoptosis by regulating OPA1-dependent cristae morphogenesis in mitochondria. Genes Dev 22(4): 476-488. PubMed ID: 18281461

Olichon, A., Baricault, L., Gas, N., Guillou, E., Valette, A., Belenguer, P. and Lenaers, G. (2003). Loss of OPA1 perturbates the mitochondrial inner membrane structure and integrity, leading to cytochrome c release and apoptosis. J Biol Chem 278(10): 7743-7746. PubMed ID: 12509422

Spinazzi, M., Cazzola, S., Bortolozzi, M., Baracca, A., Loro, E., Casarin, A., Solaini, G., Sgarbi, G., Casalena, G., Cenacchi, G., Malena, A., Frezza, C., Carrara, F., Angelini, C., Scorrano, L., Salviati, L. and Vergani, L. (2008). A novel deletion in the GTPase domain of OPA1 causes defects in mitochondrial morphology and distribution, but not in function. Hum Mol Genet 17(21): 3291-3302. PubMed ID: 18678599

Trevisan, T., Pendin, D., Montagna, A., Bova, S., Ghelli, A. M. and Daga, A. (2018). Manipulation of mitochondria dynamics reveals separate roles for form and function in mitochondria distribution. Cell Rep 23(6): 1742-1753. PubMed ID: 29742430

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Yu, Y., Lee, H. C., Chen, K. C., Suhan, J., Qiu, M., Ba, Q. and Yang, G. (2016). Inner membrane fusion mediates spatial distribution of axonal mitochondria. Sci Rep 6: 18981. PubMed ID: 26742817


Biological Overview

date revised: 15 November 2018

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