Modes of metabolic compensation during mitochondrial disease using the Drosophila model of ATP6 dysfunction

The 4977 Bp Deletion of Mitochondrial DNA as a Potential Trait Marker for Major Depressive Disorder

Background

Major depressive disorder (MDD) is significantly influenced by genetic factors. The present study aims to examine the potential correlation between the 4977 bp deletion of mitochondrial DNA (mtDNA) and MDD.

Methods

The 4977 bp deletion of mtDNA was detected in the peripheral blood of 253 MDD patients and 257 healthy controls, with depression assessed by the Hamilton Depression Rating scale and functioning by the Global Assessment of Function Scale.

Results

MDD patients had a higher incidence of the 4977 bp deletion, unaffected by demographic or clinical factors.

Conclusion

The 4977 bp deletion may be a trait marker for MDD.

Imaging flow cytometry reveals divergent mitochondrial phenotypes in mitochondrial disease patients

Traditional classification by clinical phenotype or oxidative phosphorylation (OXPHOS) complex deficiencies often fails to clarify complex genotype-phenotype correlations in mitochondrial disease. A multimodal functional assessment may better reveal underlying disease patterns. Using imaging flow cytometry (IFC), we evaluated mitochondrial fragmentation, swelling, membrane potential, reactive oxygen species (ROS) production, and mitochondrial mass in fibroblasts from 31 mitochondrial disease patients. Significant changes were observed in 97% of patients, forming two overarching groups with distinct responses to mitochondrial pathology. One group displayed low-to-normal membrane potential, indicating a hypometabolic state, while the other showed elevated membrane potential and swelling, suggesting a hypermetabolic state. Literature analysis linked these clusters to complex I stability defects (hypometabolic) and proton pumping activity (hypermetabolic). Thus, our IFC-based platform offers a novel approach to identify disease-specific patterns through functional responses, supporting improved diagnostic and therapeutic strategies.

Induced pluripotent stem cells derived from patients carrying mitochondrial mutations exhibit altered bioenergetics and aberrant differentiation potential

BACKGROUND

Human mitochondrial DNA mutations are associated with common to rare mitochondrial disorders, which are multisystemic with complex clinical pathologies. The pathologies of these diseases are poorly understood and have no FDA-approved treatments leading to symptom management. Leigh syndrome (LS) is a pediatric mitochondrial disorder that affects the central nervous system during early development and causes death in infancy. Since there are no adequate models for understanding the rapid fatality associated with LS, human-induced pluripotent stem cell (hiPSC) technology has been recognized as a useful approach to generate patient-specific stem cells for disease modeling and understanding the origins of the phenotype.

METHODS

hiPSCs were generated from control BJ and four disease fibroblast lines using a cocktail of non-modified reprogramming and immune evasion mRNAs and microRNAs. Expression of hiPSC-associated intracellular and cell surface markers was identified by immunofluorescence and flow cytometry. Karyotyping of hiPSCs was performed with cytogenetic analysis. Sanger and next-generation sequencing were used to detect and quantify the mutation in all hiPSCs. The mitochondrial respiration ability and glycolytic function were measured by the Seahorse Bioscience XFe96 extracellular flux analyzer.

RESULTS

Reprogrammed hiPSCs expressed pluripotent stem cell markers including transcription factors POU5F1, NANOG and SOX2 and cell surface markers SSEA4, TRA-1-60 and TRA-1-81 at the protein level. Sanger sequencing analysis confirmed the presence of mutations in all reprogrammed hiPSCs. Next-generation sequencing demonstrated the variable presence of mutant mtDNA in reprogrammed hiPSCs. Cytogenetic analyses confirmed the presence of normal karyotype in all reprogrammed hiPSCs. Patient-derived hiPSCs demonstrated decreased maximal mitochondrial respiration, while mitochondrial ATP production was not significantly different between the control and disease hiPSCs. In line with low maximal respiration, the spare respiratory capacity was lower in all the disease hiPSCs. The hiPSCs also demonstrated neural and cardiac differentiation potential.

CONCLUSION

Overall, the hiPSCs exhibited variable mitochondrial dysfunction that may alter their differentiation potential and provide key insights into clinically relevant developmental perturbations.

Evolutionary genetics of the mitochondrial genome: insights from Drosophila

Mitochondria are key to energy conversion in virtually all eukaryotes. Intriguingly, despite billions of years of evolution inside the eukaryote, mitochondria have retained their own small set of genes involved in the regulation of oxidative phosphorylation (OXPHOS) and protein translation. Although there was a long-standing assumption that the genetic variation found within the mitochondria would be selectively neutral, research over the past 3 decades has challenged this assumption. This research has provided novel insight into the genetic and evolutionary forces that shape mitochondrial evolution and broader implications for evolutionary ecological processes. Many of the seminal studies in this field, from the inception of the research field to current studies, have been conducted using Drosophila flies, thus establishing the species as a model system for studies in mitochondrial evolutionary biology. In this review, we comprehensively review these studies, from those focusing on genetic processes shaping evolution within the mitochondrial genome, to those examining the evolutionary implications of interactions between genes spanning mitochondrial and nuclear genomes, and to those investigating the dynamics of mitochondrial heteroplasmy. We synthesize the contribution of these studies to shaping our understanding of the evolutionary and ecological implications of mitochondrial genetic variation.

Research development and the prospect of animal models of mitochondrial DNA-related mitochondrial diseases

Mitochondrial diseases (MDs) are genetic and clinical heterogeneous diseases caused by mitochondrial oxidative phosphorylation defects. It is not only one of the most common genetic diseases, but also the only genetic disease involving two different genomes in humans. As a result of the complicated genetic condition, the pathogenesis of MDs is not entirely elucidated at present, and there is a lack of effective treatment in the clinic. Establishing the ideal animal models is the critical preclinical platform to explore the pathogenesis of MDs and to verify new therapeutic strategies. However, the development of animal modeling of mitochondrial DNA (mtDNA)-related MDs is time-consuming due to the limitations of physiological structure and technology. A small number of animal models of mtDNA mutations have been constructed using cell hybridization and other methods. However, the diversity of mtDNA mutation sites and clinical phenotypes make establishing relevant animal models tricky. The development of gene editing technology has become a new hope for establishing animal models of mtDNA-related mitochondrial diseases.

Computational and mitochondrial functional studies of novel compound heterozygous variants in SPATA5 gene support a causal link with epileptogenic encephalopathy

The SPATA5 gene encodes a 892 amino-acids long protein that has a putative mitochondrial targeting sequence and has been proposed to function in maintenance of mitochondrial function and integrity during mouse spermatogenesis. Several studies have associated homozygous or compound heterozygous mutations in SPATA5 gene to microcephaly, intellectual disability, seizures and hearing loss. This suggests a role of the SPATA5 gene also in neuronal development. Recently, our group presented results validating the use of blood cells for the assessment of mitochondrial function for diagnosis and follow-up of mitochondrial disease, minimizing the need for invasive procedures such as muscle biopsy. In this study, we were able to diagnose a patient with epileptogenic encephalopathy using next generation sequencing. We found two novel compound heterozygous variants in SPATA5 that are most likely causative. To analyze the impact of SPATA5 mutations on mitochondrial functional studies directly on the patients' mononuclear cells and platelets were undertaken. Oxygen consumption rates in platelets and PBMCs were impaired in the patient when compared to a healthy control. Also, a decrease in mitochondrial mass was observed in the patient monocytes with respect to the control. This suggests a true pathogenic effect of the mutations in mitochondrial function, especially in energy production and possibly biogenesis, leading to the observed phenotype.

OxPhos defects cause hypermetabolism and reduce lifespan in cells and in patients with mitochondrial diseases

Patients with primary mitochondrial oxidative phosphorylation (OxPhos) defects present with fatigue and multi-system disorders, are often lean, and die prematurely, but the mechanistic basis for this clinical picture remains unclear. By integrating data from 17 cohorts of patients with mitochondrial diseases (n = 690) we find evidence that these disorders increase resting energy expenditure, a state termed hypermetabolism. We examine this phenomenon longitudinally in patient-derived fibroblasts from multiple donors. Genetically or pharmacologically disrupting OxPhos approximately doubles cellular energy expenditure. This cell-autonomous state of hypermetabolism occurs despite near-normal OxPhos coupling efficiency, excluding uncoupling as a general mechanism. Instead, hypermetabolism is associated with mitochondrial DNA instability, activation of the integrated stress response (ISR), and increased extracellular secretion of age-related cytokines and metabokines including GDF15. In parallel, OxPhos defects accelerate telomere erosion and epigenetic aging per cell division, consistent with evidence that excess energy expenditure accelerates biological aging. To explore potential mechanisms for these effects, we generate a longitudinal RNASeq and DNA methylation resource dataset, which reveals conserved, energetically demanding, genome-wide recalibrations. Taken together, these findings highlight the need to understand how OxPhos defects influence the energetic cost of living, and the link between hypermetabolism and aging in cells and patients with mitochondrial diseases.

Synthetic adiponectin-receptor agonist, AdipoRon, induces glycolytic dependence in pancreatic cancer cells

Obesity creates a localized inflammatory reaction in the adipose, altering secretion of adipocyte-derived factors that contribute to pathologies including cancer. We have previously shown that adiponectin inhibits pancreatic cancer by antagonizing leptin-induced STAT3 activation. Yet, the effects of adiponectin on pancreatic cancer cell metabolism have not been addressed. In these studies, we have uncovered a novel metabolic function for the synthetic adiponectin-receptor agonist, AdipoRon. Treatment of PDAC cells with AdipoRon led to mitochondrial uncoupling and loss of ATP production. Concomitantly, AdipoRon-treated cells increased glucose uptake and utilization. This metabolic switch further correlated with AMPK mediated inhibition of the prolipogenic factor acetyl coenzyme A carboxylase 1 (ACC1), which is known to initiate fatty acid catabolism. Yet, measurements of fatty acid oxidation failed to detect any alteration in response to AdipoRon treatment, suggesting a deficiency for compensation. Additional disruption of glycolytic dependence, using either a glycolysis inhibitor or low-glucose conditions, demonstrated an impairment of growth and survival of all pancreatic cancer cell lines tested. Collectively, these studies provide evidence that pancreatic cancer cells utilize metabolic plasticity to upregulate glycolysis in order to adapt to suppression of oxidative phosphorylation in the presence of AdipoRon.

Supercomplex organization of the electron transfer system in marine bivalves, a model of extreme longevity

The mitochondrial oxidative stress theory of aging (MOSTA) suggests that the organelle's decay contributes to the aging phenotype via exacerbated oxidative stress, loss of organ coordination and energetics, cellular integrity and activity of the mitochondrial electron transfer system (ETS). Recent advances in understanding the structure of the ETS show that the enzymatic complexes responsible for oxidative phosphorylation are arranged in supramolecular structures called supercomplexes that lose organization during aging. Their exact role and universality among organisms are still under debate. Here, we take advantage of marine bivalves as an aging model to compare the structure of the ETS among species ranging from 28 to 507 years in maximal lifespan. Our results show that regardless of lifespan, the bivalve ETS is arrayed as a set of supercomplexes. However, bivalve species display varying degrees ETS supramolecular organization with the highest supercomplex structures found in A. islandica, the longest-lived of the bivalve species under study. We discuss this comparative model in light of differences in the nature and stoichiometry of these complexes, and highlight the potential link between the complexity of these superstructures and longer lifespans.

Evidence for hybrid breakdown in production of red carotenoids in the marine invertebrate Tigriopus californicus

The marine copepod, Tigriopus californicus, produces the red carotenoid pigment astaxanthin from yellow dietary precursors. This ‘bioconversion’ of yellow carotenoids to red is hypothesized to be linked to individual condition, possibly through shared metabolic pathways with mitochondrial oxidative phosphorylation. Experimental inter-population crosses of lab-reared T. californicus typically produces low-fitness hybrids is due in large part to the disruption of coadapted sets nuclear and mitochondrial genes within the parental populations. These hybrid incompatibilities can increase variability in life history traits and energy production among hybrid lines. Here, we tested if production of astaxanthin was compromised in hybrid copepods and if it was linked to mitochondrial metabolism and offspring development. We observed no clear mitonuclear dysfunction in hybrids fed a limited, carotenoid-deficient diet of nutritional yeast. However, when yellow carotenoids were restored to their diet, hybrid lines produced less astaxanthin than parental lines. We observed that lines fed a yeast diet produced less ATP and had slower offspring development compared to lines fed a more complete diet of algae, suggesting the yeast-only diet may have obscured effects of mitonuclear dysfunction. Astaxanthin production was not significantly associated with development among lines fed a yeast diet but was negatively related to development in early generation hybrids fed an algal diet. In lines fed yeast, astaxanthin was negatively related to ATP synthesis, but in lines fed algae, the relationship was reversed. Although the effects of the yeast diet may have obscured evidence of hybrid dysfunction, these results suggest that astaxanthin bioconversion may still be related to mitochondrial performance and reproductive success.

Ketogenic diet reduces early mortality following traumatic brain injury in Drosophila via the PPARγ ortholog Eip75B

Traumatic brain injury (TBI) is a common neurological disorder whose outcomes vary widely depending on a variety of environmental factors, including diet. Using a Drosophila melanogaster TBI model that reproduces key aspects of TBI in humans, we previously found that the diet consumed immediately following a primary brain injury has a substantial effect on the incidence of mortality within 24 h (early mortality). Flies that receive equivalent primary injuries have a higher incidence of early mortality when fed high-carbohydrate diets versus water. Here, we report that flies fed high-fat ketogenic diet (KD) following TBI exhibited early mortality that was equivalent to that of flies fed water and that flies protected from early mortality by KD continued to show survival benefits weeks later. KD also has beneficial effects in mammalian TBI models, indicating that the mechanism of action of KD is evolutionarily conserved. To probe the mechanism, we examined the effect of KD in flies mutant for Eip75B, an ortholog of the transcription factor PPARγ (peroxisome proliferator-activated receptor gamma) that contributes to the mechanism of action of KD and has neuroprotective effects in mammalian TBI models. We found that the incidence of early mortality of Eip75B mutant flies was higher when they were fed KD than when they were fed water following TBI. These data indicate that Eip75B/PPARγ is necessary for the beneficial effects of KD following TBI. In summary, this work provides the first evidence that KD activates PPARγ to reduce deleterious outcomes of TBI and it demonstrates the utility of the fly TBI model for dissecting molecular pathways that contribute to heterogeneity in TBI outcomes.

Altered mitochondrial dynamics and function in APOE4-expressing astrocytes

APOE4 is a major risk factor for sporadic Alzheimer’s disease; however, it is unclear how it exerts its pathological effects. Others and we have previously shown that autophagy is impaired in APOE4 compared to APOE3 astrocytes, and demonstrated differences in the expression of mitochondrial dynamics proteins in brains of APOE3 and APOE4 transgenic mice. Here, we investigated the effect of APOE4 expression on several aspects of mitochondrial function and network dynamics, including fusion, fission, and mitophagy, specifically in astrocytes. We found that APOE3 and APOE4 astrocytes differ in their mitochondrial dynamics, suggesting that the mitochondria of APOE4 astrocytes exhibit reduced fission and mitophagy. APOE4 astrocytes also show impaired mitochondrial function. Importantly, the autophagy inducer rapamycin enhanced mitophagy and improved mitochondrial functioning in APOE4 astrocytes. Collectively, the results demonstrate that APOE4 expression is associated with altered mitochondrial dynamics, which might lead to impaired mitochondrial function in astrocytes. This, in turn, may contribute to the pathological effects of APOE4 in Alzheimer’s disease.

Mitochondrial dysfunction plays a key role in the development of neurodegenerative diseases in diabetes

Mitochondria have an essential function in cell survival due to their role in bioenergetics, reactive oxygen species generation, calcium buffering, and other metabolic activities. Mitochondrial dysfunctions are commonly found in neurodegenerative diseases (NDs), and diabetes is a risk factor for NDs. However, the role of mitochondria in diabetic neurodegeneration is still unclear. In the present study, we review the latest evidence on the role of mitochondrial dysfunctions in the development of diabetes-related NDs and the underlying molecular mechanisms. Hypoglycemic agents, especially metformin, have been proven to have neuroprotective effects in the treatment of diabetes, in which mitochondria could act as one of the underlying mechanisms. Other hypoglycemic agents, including thiazolidinediones (TZDs), dipeptidyl peptidase 4 (DPP-4) inhibitors, and glucagon-like peptide 1 (GLP-1) receptor agonists, have gained more attention because of their beneficial effects on NDs, presumably by improving mitochondrial function. Our review highlights the notion that mitochondria could be a promising therapeutic target in the treatment of NDs in patients with diabetes.

APOE4 is Associated with Differential Regional Vulnerability to Bioenergetic Deficits in Aged APOE Mice

The ε4 allele of apolipoprotein E (APOE) is the dominant genetic risk factor for late-onset Alzheimer’s disease (AD). However, the reason for the association between APOE4 and AD remains unclear. While much of the research has focused on the ability of the apoE4 protein to increase the aggregation and decrease the clearance of Aβ, there is also an abundance of data showing that APOE4 negatively impacts many additional processes in the brain, including bioenergetics. In order to gain a more comprehensive understanding of APOE4′s role in AD pathogenesis, we performed a transcriptomics analysis of APOE4 vs. APOE3 expression in the entorhinal cortex (EC) and primary visual cortex (PVC) of aged APOE mice. This study revealed EC-specific upregulation of genes related to oxidative phosphorylation (OxPhos). Follow-up analysis utilizing the Seahorse platform showed decreased mitochondrial respiration with age in the hippocampus and cortex of APOE4 vs. APOE3 mice, but not in the EC of these mice. Additional studies, as well as the original transcriptomics data, suggest that multiple bioenergetic pathways are differentially regulated by APOE4 expression in the EC of aged APOE mice in order to increase the mitochondrial coupling efficiency in this region. Given the importance of the EC as one of the first regions to be affected by AD pathology in humans, the observation that the EC is susceptible to differential bioenergetic regulation in response to a metabolic stressor such as APOE4 may point to a causative factor in the pathogenesis of AD.

The addition of a lipid-rich dietary supplement eliminates seizure-like activity and paralysis in the drosophila bang sensitive mutants

OBJECTIVE

To investigate the effect that a diet supplemented with KetoCal 4:1, a commercially available dietary formula consisting of a 4:1 ratio of fats to carbohydrates plus proteins, had on the seizure-like activity (SLA) and paralysis normally exhibited by the Drosophila Bang-sensitive (BS) paralytic mutants following mechanical shock.

METHODS

Given that dietary changes are known to reduce seizures in humans and animal models, three BS mutants, easily-shocked (eas), bang-senseless (para^bss), and technical knockout (tko), were fed a standard cornmeal/yeast/sugar diet supplemented with 10% KetoCal 4:1 (KetoCal-sup diet). Newly eclosed BS flies were fed this diet for 3-7 days and the effect this had on SLA, paralysis, locomotor activity, triglyceride levels, and glucose levels was examined.

RESULTS

All three genotypes displayed significant reductions in SLA and BS sensitivity following mechanical shock. After only 3 days on the diet, 95% of tko flies no longer exhibited SLA or paralysis, and near complete suppression of the BS phenotype was seen by day 7. In the case of eas, there was a 78% reduction of SLA after 3 days on the diet and SLA was completely suppressed by day 7. The para^bss flies showed a similar but less robust reduction of SLA on the diet as there was only a 68% reduction of SLA and paralysis following 7 days on the diet. The diet did not suppress activity globally as tko flies had increased basal locomotor activity on the diet while the para^bss and eas flies showed no significant change in basal activity. The KetoCal-sup diet did not significantly alter the triglyceride levels or the total glucose levels in the BS mutants. In addition, the SLA and BS suppression was maintained even when the BS mutants were transitioned back to a standard fly diet.

CONCLUSIONS

The SLA and paralysis associated with the Drosophila BS phenotype can be effectively suppressed by transient exposure to a KetoCal-sup diet. This suppression was not dependent upon long term maintenance of the diet and it was not associated with alterations in total glucose or triglyceride levels in these flies.

An Ecologist’s Guide to Mitochondrial DNA Mutations and Senescence

Longevity plays a key role in the fitness of organisms, so understanding the processes that underlie variance in senescence has long been a focus of ecologists and evolutionary biologists. For decades, the performance and ultimate decline of mitochondria have been implicated in the demise of somatic tissue, but exactly why mitochondrial function declines as individual’s age has remained elusive. A possible source of decline that has been of intense debate is mutations to the mitochondrial DNA. There are two primary sources of such mutations: oxidative damage, which is widely discussed by ecologists interested in aging, and mitochondrial replication error, which is less familiar to most ecologists. The goal of this review is to introduce ecologists and evolutionary biologists to the concept of mitochondrial replication error and to review the current status of research on the relative importance of replication error in senescence. We conclude by detailing some of the gaps in our knowledge that currently make it difficult to deduce the relative importance of replication error in wild populations and encourage organismal biologists to consider this variable both when interpreting their results and as viable measure to include in their studies.

Suppressing Mitochondrial Respiration Is Critical for Hypoxia Tolerance in the Fetal Growth Plate

Oxygen (O₂) is both an indispensable metabolic substrate and a regulatory signal that controls the activity of Hypoxia-Inducible Factor 1α (Hif1a), a mediator of the cellular adaptation to low O₂ tension (hypoxia). Hypoxic cells require Hif1a to survive. Additionally, Hif1a is an inhibitor of mitochondrial respiration. Hence, we hypothesized that enhancing mitochondrial respiration is detrimental to the survival of hypoxic cells in vivo. We tested this hypothesis in the fetal growth plate, which is hypoxic. Our findings show that mitochondrial respiration is dispensable for survival of growth plate chondrocytes. Furthermore, its impairment prevents the extreme hypoxia and the massive chondrocyte death observed in growth plates lacking Hif1a. Consequently, augmenting mitochondrial respiration affects the survival of hypoxic chondrocytes by, at least in part, increasing intracellular hypoxia. We thus propose that partial suppression of mitochondrial respiration is crucial during development to protect the tissues that are physiologically hypoxic from lethal intracellular anoxia.

Ketogenic and anaplerotic dietary modifications ameliorate seizure activity in Drosophila models of mitochondrial encephalomyopathy and glycolytic enzymopathy

Seizures are a feature not only of the many forms of epilepsy, but also of global metabolic diseases such as mitochondrial encephalomyopathy (ME) and glycolytic enzymopathy (GE). Modern anti-epileptic drugs (AEDs) are successful in many cases, but some patients are refractory to existing AEDs, which has led to a surge in interest in clinically managed dietary therapy such as the ketogenic diet (KD). This high-fat, low-carbohydrate diet causes a cellular switch from glycolysis to fatty acid oxidation and ketone body generation, with a wide array of downstream effects at the genetic, protein, and metabolite level that may mediate seizure protection. We have recently shown that a Drosophila model of human ME (ATP6¹) responds robustly to the KD; here, we have investigated the mechanistic importance of the major metabolic consequences of the KD in the context of this bioenergetics disease: ketogenesis, reduction of glycolysis, and anaplerosis. We have found that reduction of glycolysis does not confer seizure protection, but that dietary supplementation with ketone bodies or the anaplerotic lipid triheptanoin, which directly replenishes the citric acid cycle, can mimic the success of the ketogenic diet even in the presence of standard carbohydrate levels. We have also shown that the proper functioning of the citric acid cycle is crucial to the success of the KD in the context of ME. Furthermore, our data reveal that multiple seizure models, in addition to ATP6¹, are treatable with the ketogenic diet. Importantly, one of these mutants is TPI^sugarkill, which models human glycolytic enzymopathy, an incurable metabolic disorder with severe neurological consequences. Overall, these studies reveal widespread success of the KD in Drosophila, further cementing its status as an excellent model for studies of KD treatment and mechanism, and reveal key insights into the therapeutic potential of dietary therapy against neuronal hyperexcitability in epilepsy and metabolic disease.

Sleep and circadian defects in a Drosophila model of mitochondrial encephalomyopathy

Mitochondrial encephalomyopathies (ME) are complex, incurable diseases characterized by severe bioenergetic distress that can affect the function of all major organ systems but is especially taxing to neuromuscular tissues. Animal models of MEs are rare, but the Drosophila ATP6¹ mutant is a stable, well-characterized genetic line that accurately models progressive human mitochondrial diseases such as Maternally-Inherited Leigh Syndrome (MILS), Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP), and Familial Bilateral Striatal Necrosis (FBSN). While it is established that this model exhibits important hallmarks of ME, including excess cellular and mitochondrial reactive oxygen species, shortened lifespan, muscle degeneration, and stress-induced seizures, it is unknown whether it exhibits defects in sleep or circadian function. This is a clinically relevant question, as many neurological and neurodegenerative diseases are characterized by such disturbances, which can exacerbate other symptoms and worsen quality of life. Since Drosophila is highly amenable to sleep and circadian studies, we asked whether we could detect disease phenotypes in the circadian behaviors of ATP6¹. Indeed, we found that day-time and night-time activity and sleep are altered through disease progression, and that circadian patterns are disrupted at both the behavioral and neuronal levels. These results establish ATP6¹ as an important model of sleep and circadian disruption in ME that can be studied mechanistically at the molecular, cellular, and behavioral level to uncover underlying pathophysiology and test novel therapies.

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A Drosophila model of mitochondrial disease (ATP6¹) displays altered sleep patterns.

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ATP6¹ sleep quantity and consolidation are reduced in advanced disease.

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ATP6¹ is behaviorally arrhythmic under conditions of constant darkness.

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Selected neurons of the circadian circuit display altered daily firing rates in ATP6¹.

Mitochondria, OxPhos, and neurodegeneration: cells are not just running out of gas

Mitochondrial respiratory deficiencies have been observed in numerous neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. For decades, these reductions in oxidative phosphorylation (OxPhos) have been presumed to trigger an overall bioenergetic crisis in the neuron, resulting in cell death. While the connection between respiratory defects and neuronal death has never been proven, this hypothesis has been supported by the detection of nonspecific mitochondrial DNA mutations in these disorders. These findings led to the notion that mitochondrial respiratory defects could be initiators of these common neurodegenerative disorders, instead of being consequences of a prior insult, a theory we believe to be misconstrued. Herein, we review the roots of this mitochondrial hypothesis and offer a new perspective wherein mitochondria are analyzed not only from the OxPhos point of view, but also as a complex organelle residing at the epicenter of many metabolic pathways.

Shortened Lifespan and Other Age-Related Defects in Bang Sensitive Mutants of Drosophila melanogaster

Mitochondrial diseases are complex disorders that exhibit their primary effects in energetically active tissues. Damage generated by mitochondria is also thought to be a key component of aging and age-related disease. An important model for mitochondrial dysfunction is the bang sensitive (bs) mutants in Drosophila melanogaster Although these mutants all show a striking seizure phenotype, several bs mutants have gene products that are involved with mitochondrial function, while others affect excitability another way. All of the bs mutants (parabss , eas, jus, ses B, tko are examined here) paralyze and seize upon challenge with a sensory stimulus, most notably mechanical stimulation. These and other excitability mutants have been linked to neurodegeneration with age. In addition to these phenotypes, we have found age-related defects for several of the bs strains. The mutants eas, ses B, and tko display shortened lifespan, an increased mean recovery time from seizure with age, and decreased climbing ability over lifespan as compared to isogenic CS or w1118 lines. Other mutants show a subset of these defects. The age-related phenotypes can be rescued by feeding melatonin, an antioxidant, in all the mutants except ses B The age-related defects do not appear to be correlated with the seizure phenotype. Inducing seizures on a daily basis did not exacerbate the phenotypes and treatment with antiepileptic drugs did not increase lifespan. The results suggest that the excitability phenotypes and the age-related phenotypes may be somewhat independent and that these phenotypes mutants may arise from impacts on different pathways.

Genotype to phenotype: Diet-by-mitochondrial DNA haplotype interactions drive metabolic flexibility and organismal fitness

Diet may be modified seasonally or by biogeographic, demographic or cultural shifts. It can differentially influence mitochondrial bioenergetics, retrograde signalling to the nuclear genome, and anterograde signalling to mitochondria. All these interactions have the potential to alter the frequencies of mtDNA haplotypes (mitotypes) in nature and may impact human health. In a model laboratory system, we fed four diets varying in Protein: Carbohydrate (P:C) ratio (1:2, 1:4, 1:8 and 1:16 P:C) to four homoplasmic Drosophila melanogaster mitotypes (nuclear genome standardised) and assayed their frequency in population cages. When fed a high protein 1:2 P:C diet, the frequency of flies harbouring Alstonville mtDNA increased. In contrast, when fed the high carbohydrate 1:16 P:C food the incidence of flies harbouring Dahomey mtDNA increased. This result, driven by differences in larval development, was generalisable to the replacement of the laboratory diet with fruits having high and low P:C ratios, perturbation of the nuclear genome and changes to the microbiome. Structural modelling and cellular assays suggested a V161L mutation in the ND4 subunit of complex I of Dahomey mtDNA was mildly deleterious, reduced mitochondrial functions, increased oxidative stress and resulted in an increase in larval development time on the 1:2 P:C diet. The 1:16 P:C diet triggered a cascade of changes in both mitotypes. In Dahomey larvae, increased feeding fuelled increased β-oxidation and the partial bypass of the complex I mutation. Conversely, Alstonville larvae upregulated genes involved with oxidative phosphorylation, increased glycogen metabolism and they were more physically active. We hypothesise that the increased physical activity diverted energy from growth and cell division and thereby slowed development. These data further question the use of mtDNA as an assumed neutral marker in evolutionary and population genetic studies. Moreover, if humans respond similarly, we posit that individuals with specific mtDNA variations may differentially metabolise carbohydrates, which has implications for a variety of diseases including cardiovascular disease, obesity, and perhaps Parkinson's Disease.

Mutations of mitochondrial DNA are not major contributors to aging of fruit flies

Mutations of mtDNA accumulate in aging humans and other mammals to cause mitochondrial dysfunction in a subset of cells in various tissues. Furthermore, experimental induction of mtDNA mutations causes a premature aging syndrome in the mouse. To study if mitochondrial dysfunction is universally involved in shortening life span in metazoans, we generated a series of fruit fly lines with varying levels of mtDNA mutations. Unexpectedly, we report that fruit flies are remarkably tolerant to mtDNA mutations, as exemplified by their lack of effect on physiology and lifespan. Only an artificially induced, very drastic increase of the mtDNA mutation load will lead to reduced lifespan, showing that mtDNA mutations are unlikely to limit lifespan in natural fruit fly populations.

Mammals develop age-associated clonal expansion of somatic mtDNA mutations resulting in severe respiratory chain deficiency in a subset of cells in a variety of tissues. Both mathematical modeling based on descriptive data from humans and experimental data from mtDNA mutator mice suggest that the somatic mutations are formed early in life and then undergo mitotic segregation during adult life to reach very high levels in certain cells. To address whether mtDNA mutations have a universal effect on aging metazoans, we investigated their role in physiology and aging of fruit flies. To this end, we utilized genetically engineered flies expressing mutant versions of the catalytic subunit of mitochondrial DNA polymerase (DmPOLγA) as a means to introduce mtDNA mutations. We report here that lifespan and health in fruit flies are remarkably tolerant to mtDNA mutations. Our results show that the short lifespan and wide genetic bottleneck of fruit flies are limiting the extent of clonal expansion of mtDNA mutations both in individuals and between generations. However, an increase of mtDNA mutations to very high levels caused sensitivity to mechanical and starvation stress, intestinal stem cell dysfunction, and reduced lifespan under standard conditions. In addition, the effects of dietary restriction, widely considered beneficial for organismal health, were attenuated in flies with very high levels of mtDNA mutations.

Protein coding mitochondrial-targeted RNAs rescue mitochondrial disease in vivo

Mitochondrial encephalomyopathies (MEs) result from mutations in mitochondrial genes critical to oxidative phosphorylation. Severe and untreatable ME results from mutations affecting each endogenous mitochondrial encoded gene, including all 13 established protein coding genes. Effective techniques to manipulate mitochondrial genome are limited and targeted mitochondrial protein expression is currently unavailable. Here we report the development of a mitochondrial-targeted RNA expression (mtTRES) vector capable of protein expression within mitochondria (mtTRES^Pro). We demonstrate that mtTRES^Pro expressed RNAs are targeted to mitochondria and are capable of being translated using EGFP encoded constructs in vivo. We additionally test mtTRES^Pro constructs encoding wild type ATP6 for their ability to rescue an established ATP6¹Drosophila model of ME. Genetic rescue is examined including tests with co-expression of mitochondrial targeted translational inhibitors TLI-NCL::ATP6 RNAs that function to reduce expression of the endogenous mutant protein. The data demonstrate allotopic RNA expression of mitochondrial targeted wild type ATP6 coding RNAs are sufficient to partially rescue a severe and established animal model of ME but only when combined with a method to inhibit mutant protein expression, which likely competes for incorporation into complex V.

Glial lipid droplets and neurodegeneration in a Drosophila model of complex I deficiency

Mitochondrial defects associated with respiratory chain complex I deficiency lead to heterogeneous fatal syndromes. While the role of NDUFS8, an essential subunit of the core assembly of the complex I, is established in mitochondrial diseases, the mechanisms underlying neuropathology are poorly understood. We developed a Drosophila model of NDUFS8 deficiency by knocking down the expression of its fly homologue in neurons or in glial cells. Downregulating ND23 in neurons resulted in shortened lifespan, and decreased locomotion. Although total brain ATP levels were decreased, histological analysis did not reveal any signs of neurodegeneration except for photoreceptors of the retina. Interestingly, ND23 deficiency-associated phenotypes were rescued by overexpressing the glucose transporter hGluT3 demonstrating that boosting glucose metabolism in neurons was sufficient to bypass altered mitochondrial functions and to confer neuroprotection. We then analyzed the consequences of ND23 knockdown in glial cells. In contrast to neuronal knockdown, loss of ND23 in glia did not lead to significant behavioral defects nor to reduced lifespan, but induced brain degeneration, as visualized by numerous vacuoles found all over the nervous tissue. This phenotype was accompanied by the massive accumulation of lipid droplets at the cortex-neuropile boundaries, suggesting an alteration of lipid metabolism in glia. These results demonstrate that complex I deficiency triggers metabolic alterations both in neurons and glial cells which may contribute to the neuropathology.

Drosophila mitotypes determine developmental time in a diet and temperature dependent manner

It is well known that specific mitochondrial (mt) DNA mutations can reduce organismal fitness and influence mitochondrial-nuclear interactions. However, determining specific mtDNA mutations that are beneficial has been elusive. In this study, we vary the diet and environmental temperature to study larval development time of two Drosophila melanogaster mitotypes (Alstonville and Dahomey), in two nuclear genetic backgrounds, and investigate developmental differences through weight, feeding rate, and movement. To manipulate the diet, we utilize the nutritional geometric framework to manipulate isocaloric diets of differing macronutrient ratios (1:2 and 1:16 protein: carbohydrate (P:C) ratios) and raise flies at three temperatures (19°C, 23°C and 27°C). Larvae with Dahomey mtDNA develop more slowly than Alstonville when fed the 1:2 P:C diet at all temperatures and developed more quickly when fed the 1:16 P:C diet at 23°C and 27°C. We determined that Dahomey larvae eat more, move less, and weigh more than Alstonville larvae when raised on the 1:16 P:C diet and that these physiological responses are modified by temperature. We suggest that 1 (or more) of 4 mtDNA changes is likely responsible for the observed effects and posit the mtDNA changes moderate a physiological trade-off between consumption and foraging.

The ATP-sensitive K channel is seizure protective and required for effective dietary therapy in a model of mitochondrial encephalomyopathy

Effective therapies are lacking for mitochondrial encephalomyopathies (MEs). MEs are devastating diseases that predominantly affect the energy-demanding tissues of the nervous system and muscle, causing symptoms such as seizures, cardiomyopathy, and neuro- and muscular degeneration. Even common anti-epileptic drugs which are frequently successful in ameliorating seizures in other diseases tend to have a lower success rate in ME, highlighting the need for novel drug targets, especially those that may couple metabolic sensitivity to neuronal excitability. Furthermore, alternative epilepsy therapies such as dietary modification are gaining in clinical popularity but have not been thoroughly studied in ME. Using the Drosophila ATP6¹ model of ME, we have studied dietary therapy throughout disease progression and found that it is highly effective against the seizures of ME, especially a high fat/ketogenic diet, and that the benefits are dependent upon a functional K_ATP channel complex. Further experiments with K_ATP show that it is seizure-protective in this model, and that pharmacological promotion of its open state also ameliorates seizures. These studies represent important steps forward in the development of novel therapies for a class of diseases that is notoriously difficult to treat, and lay the foundation for mechanistic studies of currently existing therapies in the context of metabolic disease.

Assessing pre- and post-zygotic barriers between North Atlantic eels (Anguilla anguilla and A. rostrata)

Elucidating barriers to gene flow is important for understanding the dynamics of speciation. Here we investigate pre- and post-zygotic mechanisms acting between the two hybridizing species of Atlantic eels: Anguilla anguilla and A. rostrata. Temporally varying hybridization was examined by analyzing 85 species-diagnostic single-nucleotide polymorphisms (SNPs; F_ST ⩾0.95) in eel larvae sampled in the spawning region in the Sargasso Sea in 2007 (N=92) and 2014 (N=460). We further investigated whether genotypes at these SNPs were nonrandomly distributed in post-F1 hybrids, indicating selection. Finally, we sequenced the mitochondrial ATP6 and nuclear ATP5c1 genes in 19 hybrids, identified using SNP and restriction site associated DNA (RAD) sequencing data, to test a previously proposed hypothesis of cytonuclear incompatibility leading to adenosine triphosphate (ATP) synthase dysfunction and selection against hybrids. No F1 hybrids but only later backcrosses were observed in the Sargasso Sea in 2007 and 2014. This suggests that interbreeding between the two species only occurs in some years, possibly controlled by environmental conditions at the spawning grounds, or that interbreeding has diminished through time as a result of a declining number of spawners. Moreover, potential selection was found at the nuclear and the cytonuclear levels. Nonetheless, one glass eel individual showed a mismatch, involving an American ATP6 haplotype and European ATP5c1 alleles. This contradicted the presence of cytonuclear incompatibility but may be explained by that (1) cytonuclear incompatibility is incomplete, (2) selection acts at a later life stage or (3) other genes are important for protein function. In total, the study demonstrates the utility of genomic data when examining pre- and post-zyotic barriers in natural hybrids.

Fly Models of Human Diseases: Drosophila as a Model for Understanding Human Mitochondrial Mutations and Disease

Mitochondrial diseases are a prevalent, heterogeneous class of diseases caused by defects in oxidative phosphorylation, whose severity depends upon particular genetic mutations. These diseases can be difficult to diagnose, and current therapeutics have limited efficacy, primarily treating only symptoms. Because mitochondria play a pivotal role in numerous cellular functions, especially ATP production, their diminished activity has dramatic physiological consequences. While this in and of itself makes treating mitochondrial disease complex, these organelles contain their own DNA, mtDNA, whose products are required for ATP production, in addition to the hundreds of nucleus-encoded proteins. Drosophila offers a tractable whole-animal model to understand the mechanisms underlying loss of mitochondrial function, the subsequent cellular and tissue damage that results, and how these organelles are inherited. Human and Drosophila mtDNAs encode the same set of products, and the homologous nucleus-encoded genes required for mitochondrial function are conserved. In addition, Drosophila contain sufficiently complex organ systems to effectively recapitulate many basic symptoms of mitochondrial diseases, yet are relatively easy and fast to genetically manipulate. There are several Drosophila models for specific mitochondrial diseases, which have been recently reviewed (Foriel, Willems, Smeitink, Schenck, & Beyrath, 2015). In this review, we highlight the conservation between human and Drosophila mtDNA, the present and future techniques for creating mtDNA mutations for further study, and how Drosophila has contributed to our current understanding of mitochondrial inheritance.

Novel Roles of Epoxyeicosanoids in Regulating Cardiac Mitochondria

Maintenance of a healthy pool of mitochondria is important for the function and survival of terminally differentiated cells such as cardiomyocytes. Epoxyeicosatrienoic acids (EETs) are epoxy lipids derived from metabolism of arachidonic acid by cytochrome P450 epoxygenases. We have previously shown that EETs trigger a protective response limiting mitochondrial dysfunction and reducing cellular death. The aim of this study was to investigate whether EET-mediated effects influence mitochondrial quality in HL-1 cardiac cells during starvation. HL-1 cells were subjected to serum- and amino acid free conditions for 24h. We employed a dual-acting synthetic analog UA-8 (13-(3-propylureido)tridec-8-enoic acid), possessing both EET-mimetic and soluble epoxide hydrolase (sEH) inhibitory properties, or 14,15-EET as model EET molecules. We demonstrated that EET-mediated events significantly improved mitochondrial function as assessed by preservation of the ADP/ATP ratio and oxidative respiratory capacity. Starvation induced mitochondrial hyperfusion observed in control cells was attenuated by UA-8. However, EET-mediated events did not affect the expression of mitochondrial dynamic proteins Fis1, DRP-1 or Mfn2. Rather we observed increased levels of OPA-1 oligomers and increased mitochondrial cristae density, which correlated with the preserved mitochondrial function. Increased DNA binding activity of pCREB and Nrf1/2 and increased SIRT1 activity together with elevated mitochondrial proteins suggest EET-mediated events led to preserved mitobiogenesis. Thus, we provide new evidence for EET-mediated events that preserve a healthier pool of mitochondria in cardiac cells following starvation-induced stress.

A mitochondrial DNA hypomorph of cytochrome oxidase specifically impairs male fertility in Drosophila melanogaster

Due to their strict maternal inheritance in most animals and plants, mitochondrial genomes are predicted to accumulate mutations that are beneficial or neutral in females but harmful in males. Although a few male-harming mtDNA mutations have been identified, consistent with this 'Mother's Curse', their effect on females has been largely unexplored. Here, we identify COII(G177S), a mtDNA hypomorph of cytochrome oxidase II, which specifically impairs male fertility due to defects in sperm development and function without impairing other male or female functions. COII(G177S) represents one of the clearest examples of a 'male-harming' mtDNA mutation in animals and suggest that the hypomorphic mtDNA mutations like COII(G177S) might specifically impair male gametogenesis. Intriguingly, some D. melanogaster nuclear genetic backgrounds can fully rescue COII(G177S) -associated sterility, consistent with previously proposed models that nuclear genomes can regulate the phenotypic manifestation of mtDNA mutations.

Selfish drive can trump function when animal mitochondrial genomes compete

Mitochondrial genomes compete for transmission from mother to progeny. We explored this competition by introducing a second genome into Drosophila melanogaster to follow transmission. Competitions between closely related genomes favored those functional in electron transport, resulting in a host-beneficial purifying selection. In contrast, matchups between distantly related genomes often favored those with negligible, negative or lethal consequences, indicating selfish selection. Exhibiting powerful selfish selection, a genome carrying a detrimental mutation displaced a complementing genome, leading to population death after several generations. In a different pairing, opposing selfish and purifying selection counterbalanced to give stable transmission of two genomes. Sequencing of recombinant mitochondrial genomes showed that the noncoding region, containing origins of replication, governs selfish transmission. Uniparental inheritance prevents encounters between distantly related genomes. Nonetheless, in each maternal lineage, constant competition among sibling genomes selects for super-replicators. We suggest that this relentless competition drives positive selection, promoting change in the sequences influencing transmission.

Diiron centre mutations in Ciona intestinalis alternative oxidase abolish enzymatic activity and prevent rescue of cytochrome oxidase deficiency in flies

The mitochondrial alternative oxidase, AOX, carries out the non proton-motive re-oxidation of ubiquinol by oxygen in lower eukaryotes, plants and some animals. Here we created a modified version of AOX from Ciona instestinalis, carrying mutations at conserved residues predicted to be required for chelation of the diiron prosthetic group. The modified protein was stably expressed in mammalian cells or flies, but lacked enzymatic activity and was unable to rescue the phenotypes of flies knocked down for a subunit of cytochrome oxidase. The mutated AOX transgene is thus a potentially useful tool in studies of the physiological effects of AOX expression.

A conserved polybasic domain mediates plasma membrane targeting of Lgl and its regulation by hypoxia

The plasma membrane targeting of Lgl, a key polarity and tumor suppressor protein, is mediated by electrostatic interactions between a polybasic motif in Lgl and phospholipids on the plasma membrane, and this mechanism is regulated by hypoxia and aPKC-phosphorylation.

Lethal giant larvae (Lgl) plays essential and conserved functions in regulating both cell polarity and tumorigenesis in Drosophila melanogaster and vertebrates. It is well recognized that plasma membrane (PM) or cell cortex localization is crucial for Lgl function in vivo, but its membrane-targeting mechanisms remain poorly understood. Here, we discovered that hypoxia acutely and reversibly inhibits Lgl PM targeting through a posttranslational mechanism that is independent of the well-characterized atypical protein kinase C (aPKC) or Aurora kinase–mediated phosphorylations. Instead, we identified an evolutionarily conserved polybasic (PB) domain that targets Lgl to the PM via electrostatic binding to membrane phosphatidylinositol phosphates. Such PB domain–mediated PM targeting is inhibited by hypoxia, which reduces inositol phospholipid levels on the PM through adenosine triphosphate depletion. Moreover, Lgl PB domain contains all the identified phosphorylation sites of aPKC and Aurora kinases, providing a molecular mechanism by which phosphorylations neutralize the positive charges on the PB domain to inhibit Lgl PM targeting.

Selections that isolate recombinant mitochondrial genomes in animals

Homologous recombination is widespread and catalyzes evolution. Nonetheless, its existence in animal mitochondrial DNA is questioned. We designed selections for recombination between co-resident mitochondrial genomes in various heteroplasmic Drosophila lines. In four experimental settings, recombinant genomes became the sole or dominant genome in the progeny. Thus, selection uncovers occurrence of homologous recombination in Drosophila mtDNA and documents its functional benefit. Double-strand breaks enhanced recombination in the germline and revealed somatic recombination. When the recombination partner was a diverged Drosophila melanogaster genome or a genome from a different species such as Drosophila yakuba, sequencing revealed long continuous stretches of exchange. In addition, the distribution of sequence polymorphisms in recombinants allowed us to map a selected trait to a particular region in the Drosophila mitochondrial genome. Thus, recombination can be harnessed to dissect function and evolution of mitochondrial genome.

Genome-wide screen for modifiers of Na (+) /K (+) ATPase alleles identifies critical genetic loci

Background

Mutations affecting the Na⁺/ K⁺ATPase (a.k.a. the sodium-potassium pump) genes cause conditional locomotor phenotypes in flies and three distinct complex neurological diseases in humans. More than 50 mutations have been identified affecting the human ATP1A2 and ATP1A3 genes that are known to cause rapid-onset Dystonia Parkinsonism, familial hemiplegic migraine, alternating hemiplegia of childhood, and variants of familial hemiplegic migraine with neurological complications including seizures and various mood disorders. In flies, mutations affecting the ATPalpha gene have dramatic phenotypes including altered longevity, neural dysfunction, neurodegeneration, myodegeneration, and striking locomotor impairment. Locomotor defects can manifest as conditional bang-sensitive (BS) or temperature-sensitive (TS) paralysis: phenotypes well-suited for genetic screening.

Results

We performed a genome-wide deficiency screen using three distinct missense alleles of ATPalpha and conditional locomotor function assays to identify novel modifier loci. A secondary screen confirmed allele-specificity of the interactions and many of the interactions were mapped to single genes and subsequently validated. We successfully identified 64 modifier loci and used classical mutations and RNAi to confirm 50 single gene interactions. The genes identified include those with known function, several with unknown function or that were otherwise uncharacterized, and many loci with no described association with locomotor or Na⁺/K⁺ ATPase function.

Conclusions

We used an unbiased genome-wide screen to find regions of the genome containing elements important for genetic modulation of ATPalpha dysfunction. We have identified many critical regions and narrowed several of these to single genes. These data demonstrate there are many loci capable of modifying ATPalpha dysfunction, which may provide the basis for modifying migraine, locomotor and seizure dysfunction in animals.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-014-0089-3) contains supplementary material, which is available to authorized users.

Differential centrifugation-based biochemical fractionation of the Drosophila adult CNS

Drosophila is widely used as a genetic model in questions of development, cellular function and disease. Genetic screens in flies have proven to be incredibly powerful in identifying crucial components for synapse formation and function, particularly in the case of the presynaptic release machinery. Although modern biochemical methods can identify individual proteins and lipids (and their binding partners), they have typically been excluded from use in Drosophila for technical reasons. To bridge this essential gap between genetics and biochemistry, we developed a fractionation method to isolate various parts of the synaptic machinery from Drosophila, thus allowing it to be studied in unprecedented biochemical detail. This is only possible because our protocol has unique advantages in terms of enriching and preserving endogenous protein complexes. The procedure involves decapitation of adult flies, homogenization and differential centrifugation of fly heads, which allow subsequent purification of presynaptic (and to a limited degree postsynaptic) components. It is designed to require only a rudimentary knowledge of biochemical fractionation, and it takes ∼3.5 h. The yield is typically 4 mg of synaptic membrane protein per gram of Drosophila heads.

Socially responsive effects of brain oxidative metabolism on aggression

Despite ongoing high energetic demands, brains do not always use glucose and oxygen in a ratio that produces maximal ATP through oxidative phosphorylation. In some cases glucose consumption exceeds oxygen use despite adequate oxygen availability, a phenomenon known as aerobic glycolysis. Although metabolic plasticity seems essential for normal cognition, studying its functional significance has been challenging because few experimental systems link brain metabolic patterns to distinct behavioral states. Our recent transcriptomic analysis established a correlation between aggression and decreased whole-brain oxidative phosphorylation activity in the honey bee (Apis mellifera), suggesting that brain metabolic plasticity may modulate this naturally occurring behavior. Here we demonstrate that the relationship between brain metabolism and aggression is causal, conserved over evolutionary time, cell type-specific, and modulated by the social environment. Pharmacologically treating honey bees to inhibit complexes I or V in the oxidative phosphorylation pathway resulted in increased aggression. In addition, transgenic RNAi lines and genetic manipulation to knock down gene expression in complex I in fruit fly (Drosophila melanogaster) neurons resulted in increased aggression, but knockdown in glia had no effect. Finally, honey bee colony-level social manipulations that decrease individual aggression attenuated the effects of oxidative phosphorylation inhibition on aggression, demonstrating a specific effect of the social environment on brain function. Because decreased neuronal oxidative phosphorylation is usually associated with brain disease, these findings provide a powerful context for understanding brain metabolic plasticity and naturally occurring behavioral plasticity.

SDHAF4 promotes mitochondrial succinate dehydrogenase activity and prevents neurodegeneration

Succinate dehydrogenase (SDH) occupies a central place in cellular energy production, linking the tricarboxylic cycle with the electron transport chain. As a result, a subset of cancers and neuromuscular disorders result from mutations affecting any of the four SDH structural subunits or either of two known SDH assembly factors. Herein we characterize an evolutionarily conserved SDH assembly factor designated Sdh8/SDHAF4, using yeast, Drosophila, and mammalian cells. Sdh8 interacts specifically with the catalytic Sdh1 subunit in the mitochondrial matrix, facilitating its association with Sdh2 and the subsequent assembly of the SDH holocomplex. These roles for Sdh8 are critical for preventing motility defects and neurodegeneration in Drosophila as well as the excess ROS generated by free Sdh1. These studies provide insights into the mechanisms by which SDH is assembled and raise the possibility that some forms of neuromuscular disease may be associated with mutations that affect this SDH assembly factor.

A Drosophila model of mitochondrial disease caused by a complex I mutation that uncouples proton pumping from electron transfer

Mutations affecting mitochondrial complex I, a multi-subunit assembly that couples electron transfer to proton pumping, are the most frequent cause of heritable mitochondrial diseases. However, the mechanisms by which complex I dysfunction results in disease remain unclear. Here, we describe a Drosophila model of complex I deficiency caused by a homoplasmic mutation in the mitochondrial-DNA-encoded NADH dehydrogenase subunit 2 (ND2) gene. We show that ND2 mutants exhibit phenotypes that resemble symptoms of mitochondrial disease, including shortened lifespan, progressive neurodegeneration, diminished neural mitochondrial membrane potential and lower levels of neural ATP. Our biochemical studies of ND2 mutants reveal that complex I is unable to efficiently couple electron transfer to proton pumping. Thus, our study provides evidence that the ND2 subunit participates directly in the proton pumping mechanism of complex I. Together, our findings support the model that diminished respiratory chain activity, and consequent energy deficiency, are responsible for the pathogenesis of complex-I-associated neurodegeneration.

Speciation and demographic history of Atlantic eels (Anguilla anguilla and A. rostrata) revealed by mitogenome sequencing

Processes leading to speciation in oceanic environments without obvious physical barriers remain poorly known. European and American eel (Anguilla anguilla and A. rostrata) spawn in partial sympatry in the Sargasso Sea. Larvae are advected by the Gulf Stream and other currents towards the European/North African and North American coasts, respectively. We analyzed 104 mitogenomes from the two species along with mitogenomes of other Anguilla and outgroup species. We estimated divergence time between the two species to identify major events involved in speciation. We also considered two previously stated hypotheses: one where the ancestral species was present in only one continent but was advected across the Atlantic by ocean current changes and another where population declines during Pleistocene glaciations led to increasing vicariance, facilitating speciation. Divergence time was estimated to ∼3.38 Mya, coinciding with the closure of the Panama Gateway that led to reinforcement of the Gulf Stream. This could have advected larvae towards European/North African coasts, in which case American eel would be expected to be the ancestral species. This scenario could, however, not be unequivocally confirmed by analyses of dN/dS, nucleotide diversity and effective population size estimates. Extended bayesian skyline plots showed fluctuations of effective population sizes and declines during glaciations, and thus also lending support to the importance of vicariance during speciation. There was evidence for positive selection at the ATP6 and possibly ND5 genes, indicating a role in speciation. The findings suggest an important role of ocean current changes in speciation of marine organisms.

The impact of pathogenic mitochondrial DNA mutations on substantia nigra neurons

Mitochondrial defects within substantia nigra (SN) neurons are implicated in the pathogenesis of Parkinson's disease. SN neurons show increased mitochondrial defects, mitochondrial DNA deletion levels, and susceptibility to such dysfunction, although the role of mitochondria in neuronal degeneration remains uncertain. In this study, we addressed this important question by exploring changes within the mitochondria of SN neurons from patients with primary mitochondrial diseases to determine whether mitochondrial dysfunction leads directly to neuronal cell loss. We counted the pigmented neurons and quantified mitochondrial respiratory activity, deficiencies in mitochondrial proteins, and the percentage of pathogenic mutations in single neurons. We found evidence of defects of both complex I and complex IV of the respiratory chain in all patients. We found that marked neuronal cell loss was only observed in a few patients with mitochondrial disease and that all these patients had mutations in polymerase gamma (POLG), which leads to the formation of multiple mitochondrial DNA deletions over time, similar to aging and Parkinson's disease. Interestingly, we detected α-synuclein pathology in two mitochondrial patients with POLG mutations. Our observations highlight the complex relationship between mitochondrial dysfunction and the susceptibility of SN neurons to degeneration and α-synuclein pathology. Our finding that the loss of SN neurons was only severe in patients with POLG mutations suggests that acquired mitochondrial defects may be less well tolerated by SN neurons than by inherited ones.

Evidence of a triosephosphate isomerase non-catalytic function crucial to behavior and longevity

Triosephosphate isomerase (TPI) is a glycolytic enzyme that converts dihydroxyacetone phosphate (DHAP) into glyceraldehyde 3-phosphate (GAP). Glycolytic enzyme dysfunction leads to metabolic diseases collectively known as glycolytic enzymopathies. Of these enzymopathies, TPI deficiency is unique in the severity of neurological symptoms. The Drosophila sugarkill mutant closely models TPI deficiency and encodes a protein prematurely degraded by the proteasome. This led us to question whether enzyme catalytic activity was crucial to the pathogenesis of TPI sugarkill neurological phenotypes. To study TPI deficiency in vivo we developed a genomic engineering system for the TPI locus that enables the efficient generation of novel TPI genetic variants. Using this system we demonstrate that TPI sugarkill can be genetically complemented by TPI encoding a catalytically inactive enzyme. Furthermore, our results demonstrate a non-metabolic function for TPI, the loss of which contributes significantly to the neurological dysfunction in this animal model.

Mitochondrial abnormalities in temporal lobe of autistic brain

Autism spectrum disorder (ASD) consists of a group of complex developmental disabilities characterized by impaired social interactions, deficits in communication and repetitive behavior. Multiple lines of evidence implicate mitochondrial dysfunction in ASD. In postmortem BA21 temporal cortex, a region that exhibits synaptic pathology in ASD, we found that compared to controls, ASD patients exhibited altered protein levels of mitochondria respiratory chain protein complexes, decreased Complex I and IV activities, decreased mitochondrial antioxidant enzyme SOD2, and greater oxidative DNA damage. Mitochondrial membrane mass was higher in ASD brain, as indicated by higher protein levels of mitochondrial membrane proteins Tom20, Tim23 and porin. No differences were observed in either mitochondrial DNA or levels of the mitochondrial gene transcription factor TFAM or cofactor PGC1α, indicating that a mechanism other than alterations in mitochondrial genome or mitochondrial biogenesis underlies these mitochondrial abnormalities. We further identified higher levels of the mitochondrial fission proteins (Fis1 and Drp1) and decreased levels of the fusion proteins (Mfn1, Mfn2 and Opa1) in ASD patients, indicating altered mitochondrial dynamics in ASD brain. Many of these changes were evident in cortical pyramidal neurons, and were observed in ASD children but were less pronounced or absent in adult patients. Together, these findings provide evidence that mitochondrial function and intracellular redox status are compromised in pyramidal neurons in ASD brain and that mitochondrial dysfunction occurs during early childhood when ASD symptoms appear.