Decrease in cytochrome c oxidase reserve capacity diminishes robustness of Drosophila melanogaster and shortens lifespan

Mitochondrial Neurodegeneration: Lessons from Drosophila melanogaster Models

The fruit fly-i.e., Drosophila melanogaster-has proven to be a very useful model for the understanding of basic physiological processes, such as development or ageing. The availability of straightforward genetic tools that can be used to produce engineered individuals makes this model extremely interesting for the understanding of the mechanisms underlying genetic diseases in physiological models. Mitochondrial diseases are a group of yet-incurable genetic disorders characterized by the malfunction of the oxidative phosphorylation system (OXPHOS), which is the highly conserved energy transformation system present in mitochondria. The generation of D. melanogaster models of mitochondrial disease started relatively recently but has already provided relevant information about the molecular mechanisms and pathological consequences of mitochondrial dysfunction. Here, we provide an overview of such models and highlight the relevance of D. melanogaster as a model to study mitochondrial disorders.

COX4-like, a Nuclear-Encoded Mitochondrial Gene Duplicate, Is Essential for Male Fertility in Drosophila melanogaster

Recent studies on nuclear-encoded mitochondrial genes (N-mt genes) in Drosophila melanogaster have shown a unique pattern of expression for newly duplicated N-mt genes, with many duplicates having a testis-biased expression and playing an essential role in spermatogenesis. In this study, we investigated a newly duplicated N-mt gene—i.e., Cytochrome c oxidase 4-like (COX4L)—in order to understand its function and, consequently, the reason behind its retention in the D. melanogaster genome. The COX4L gene is a duplicate of the Cytochrome c oxidase 4 (COX4) gene of OXPHOS complex IV. While the parental COX4 gene has been found in all eukaryotes, including single-cell eukaryotes such as yeast, we show that COX4L is only present in the Brachycera suborder of Diptera; thus, both genes are present in all Drosophila species, but have significantly different patterns of expression: COX4 is highly expressed in all tissues, while COX4L has a testis-specific expression. To understand the function of this new gene, we first knocked down its expression in the D. melanogaster germline using two different RNAi lines driven by the bam-Gal4 driver; second, we created a knockout strain for this gene using CRISPR-Cas9 technology. Our results showed that knockdown and knockout lines of COX4L produce partial sterility and complete sterility in males, respectively, where a lack of sperm individualization was observed in both cases. Male infertility was prevented by driving COX4L-HA in the germline, but not when driving COX4-HA. In addition, ectopic expression of COX4L in the soma caused embryonic lethality, while overexpression in the germline led to a reduction in male fertility. COX4L-KO mitochondria show reduced membrane potential, providing a plausible explanation for the male sterility observed in these flies. This prominent loss-of-function phenotype, along with its testis-biased expression and its presence in the Drosophila sperm proteome, suggests that COX4L is a paralogous, specialized gene that is assembled in OXPHOS complex IV of male germline cells and/or sperm mitochondria.

Target verification of allyl isothiocyanate on the core subunits of cytochrome c oxidase in Sitophilus zeamais by RNAi

BACKGROUND

Allyl isothiocyanate (AITC) is a volatile organic compound with a potent insecticidal activity to the stored-grain pest Sitophilus zeamais Motschulsky, which severely damages grain storage and container transport worldwide. Our previous study showed that mitochondrial complex IV was the primary target of AITC in adult Sitophilus zeamais. To further verify the targets of AITC, we employed RNA interference (RNAi) by using double-stranded RNA (dsRNA) to knockdown three core subunits of cytochrome c oxidase (COX)-I, -II and -III in 18-day-old larvae prior to their exposure to AITC to detect susceptibility changes.

RESULTS

The susceptibility of dsRNACOX-I and -II injection treatments to AITC significantly increased at 72 h while the mortality reached up to 85.56% and 67.78%, respectively, and dsRNACOX-I and dsRNACOX-II injection showed the same subcellular structural characteristics showing vacuolization and vague mitochondrial cristae and decrease of COX activity during AITC fumigation treatment, suggesting the potential of COX-I and COX-II as the targets of AITC. High mortality reached up to 75.55%, 71.88% and 82.22%, respectively, and the phenotype of larvae turning from milky white to dark brown in the thorax and death eventually was confirmed after dsRNACOX-I, -II and -III injection.

CONCLUSION

COX-I and -II were elucidated as the potential targets of AITC and dsRNACOX-I, -II and -III have the potential to be developed into nucleic acid pesticides for their robust lethal effects and are worth pursuing for improving AITC fumigation activity in Sitophilus zeamais control. © 2020 Society of Chemical Industry.

Parasite Presence Induces Gene Expression Changes in an Ant Host Related to Immunity and Longevity

Most species are either parasites or exploited by parasites, making parasite-host interactions a driver of evolution. Parasites with complex life cycles often evolve strategies to facilitate transmission to the definitive host by manipulating their intermediate host. Such manipulations could explain phenotypic changes in the ant Temnothorax nylanderi, the intermediate host of the cestode Anomotaenia brevis. In addition to behavioral and morphological alterations, infected workers exhibit prolonged lifespans, comparable to that of queens, which live up to two decades. We used transcriptomic data from cestodes and ants of different castes and infection status to investigate the molecular underpinnings of phenotypic alterations in infected workers and explored whether the extended lifespan of queens and infected workers has a common molecular basis. Infected workers and queens commonly upregulated only six genes, one of them with a known anti-aging function. Both groups overexpressed immune genes, although not the same ones. Our findings suggest that the lifespan extension of infected workers is not achieved via the expression of queen-specific genes. The analysis of the cestodes' transcriptome revealed dominant expression of genes of the mitochondrial respiratory transport chain, which indicates an active metabolism and shedding light on the physiology of the parasite in its cysticercoid stage.

Cytochrome c oxidase deficiency

Cytochrome c oxidase (COX) deficiency is characterized by a high degree of genetic and phenotypic heterogeneity, partly reflecting the extreme structural complexity, multiple post-translational modification, variable, tissue-specific composition, and the high number of and intricate connections among the assembly factors of this enzyme. In fact, decreased COX specific activity can manifest with different degrees of severity, affect the whole organism or specific tissues, and develop a wide spectrum of disease natural history, including disease onsets ranging from birth to late adulthood. More than 30 genes have been linked to COX deficiency, but the list is still incomplete and in fact constantly updated. We here discuss the current knowledge about COX in health and disease, focusing on genetic aetiology and link to clinical manifestations. In addition, information concerning either fundamental biological features of the enzymes or biochemical signatures of its defects have been provided by experimental in vivo models, including yeast, fly, mouse and fish, which expanded our knowledge on the functional features and the phenotypical consequences of different forms of COX deficiency.

The Transcriptional Response of Aedes aegypti with Variable Extrinsic Incubation Periods for Dengue Virus

Dengue fever is the most prevalent arboviral disease globally. Dengue virus is transmitted primarily by the Aedes aegypti mosquito. One measure of the mosquito’s efficiency as a vector is the extrinsic incubation period (EIP), which is the time between the ingestion of viremic blood and the emergence of virions in the saliva. The longer it takes virus to infect the midgut and traverse to the saliva, the fewer opportunities the mosquito will have to transmit the pathogen over its lifetime. We have shown previously that EIP for dengue virus is highly heritable and that it is negatively correlated with vector lifespan. Here, we examined the transcriptional profiles for mosquitoes that varied in their EIP phenotype and identified pathways associated with either short or long EIP. We found that mosquitoes with short EIP have less active immune responses but higher levels of protein translation and calcium ion homeostasis and that mosquitoes with longer EIP may have slower metabolism. These findings indicate a complex interplay between calcium ion distribution, ribosome biogenesis, and metabolism and reveal potential pathways that could be modified to slow the rate of viral progression and hence limit lifetime transmission capability.

Iron Sulfur and Molybdenum Cofactor Enzymes Regulate the Drosophila Life Cycle by Controlling Cell Metabolism

Iron sulfur (Fe-S) clusters and the molybdenum cofactor (Moco) are present at enzyme sites, where the active metal facilitates electron transfer. Such enzyme systems are soluble in the mitochondrial matrix, cytosol and nucleus, or embedded in the inner mitochondrial membrane, but virtually absent from the cell secretory pathway. They are of ancient evolutionary origin supporting respiration, DNA replication, transcription, translation, the biosynthesis of steroids, heme, catabolism of purines, hydroxylation of xenobiotics, and cellular sulfur metabolism. Here, Fe-S cluster and Moco biosynthesis in Drosophila melanogaster is reviewed and the multiple biochemical and physiological functions of known Fe-S and Moco enzymes are described. We show that RNA interference of Mocs3 disrupts Moco biosynthesis and the circadian clock. Fe-S-dependent mitochondrial respiration is discussed in the context of germ line and somatic development, stem cell differentiation and aging. The subcellular compartmentalization of the Fe-S and Moco assembly machinery components and their connections to iron sensing mechanisms and intermediary metabolism are emphasized. A biochemically active Fe-S core complex of heterologously expressed fly Nfs1, Isd11, IscU, and human frataxin is presented. Based on the recent demonstration that copper displaces the Fe-S cluster of yeast and human ferredoxin, an explanation for why high dietary copper leads to cytoplasmic iron deficiency in flies is proposed. Another proposal that exosomes contribute to the transport of xanthine dehydrogenase from peripheral tissues to the eye pigment cells is put forward, where the Vps16a subunit of the HOPS complex may have a specialized role in concentrating this enzyme within pigment granules. Finally, we formulate a hypothesis that (i) mitochondrial superoxide mobilizes iron from the Fe-S clusters in aconitase and succinate dehydrogenase; (ii) increased iron transiently displaces manganese on superoxide dismutase, which may function as a mitochondrial iron sensor since it is inactivated by iron; (iii) with the Krebs cycle thus disrupted, citrate is exported to the cytosol for fatty acid synthesis, while succinyl-CoA and the iron are used for heme biosynthesis; (iv) as iron is used for heme biosynthesis its concentration in the matrix drops allowing for manganese to reactivate superoxide dismutase and Fe-S cluster biosynthesis to reestablish the Krebs cycle.

Organ reserve, excess metabolic capacity, and aging

"Organ reserve" refers to the ability of an organ to successfully return to its original physiological state following repeated episodes of stress. Clinical evidence shows that organ reserve correlates with the ability of older adults to cope with an added workload or stress, suggesting a role in the process of aging. Although organ reserve is well documented clinically, it is not clearly defined at the molecular level. Interestingly, several metabolic pathways exhibit excess metabolic capacities (e.g., bioenergetics pathway, antioxidants system, plasticity). These pathways comprise molecular components that have an excess of quantity and/or activity than that required for basic physiological demand in vivo (e.g., mitochondrial complex IV or glycolytic enzymes). We propose that the excess in mtDNA copy number and tandem DNA repeats of telomeres are additional examples of intrinsically embedded structural components that could comprise excess capacity. These excess capacities may grant intermediary metabolism the ability to instantly cope with, or manage, added workload or stress. Therefore, excess metabolic capacities could be viewed as an innate mechanism of adaptability that substantiates organ reserve and contributes to the cellular defense systems. If metabolic excess capacities or organ reserves are impaired or exhausted, the ability of the cell to cope with stress is reduced. Under these circumstances cell senescence, transformation, or death occurs. In this review, we discuss excess metabolic and structural capacities as integrated metabolic pathways in relation to organ reserve and cellular aging.

The Mitochondria-Targeted Plastoquinone-Derivative SkQ1 Promotes Health and Increases Drosophila melanogaster Longevity in Various Environments

Mitochondria play an important role in aging. Strongly reduced function of the mitochondria shortens life span, whereas moderate reduction prolongs life span, with reactive oxygen species production being the major factor contributing to life span changes. Previously, picomolar concentrations of the mitochondria-targeted antioxidant SkQ1 were shown to increase the life span of Drosophila by approximately 10%. In this article, we demonstrate that SkQ1 elevates locomotion, which is often considered a marker of health and age. We also show that mating frequency and fecundity may be slightly increased in SkQ1-treated flies. These results indicate that SkQ1 not only prolongs life span but also improves health and vigor. An important property of any potential therapeutic is the stability of its effects in an uncontrolled and changing environment as well as on individuals with various genetic constitutions. In this article, we present data on SkQ1 effects on Drosophila longevity in extreme environments (low temperatures and starvation) and on individuals with severe genetic alterations in the mitochondrial systems responsible for production and detoxification of reactive oxygen species. We hypothesize that in vivo SkQ1 is capable of alleviating the probable negative effects of increased mitochondrial reactive oxygen species production on longevity but is not effective when reactive oxygen species production is already reduced by other means.

Mitochondrial diseases: Drosophila melanogaster as a model to evaluate potential therapeutics

While often presented as a single entity, mitochondrial diseases comprise a wide range of clinical, biochemical and genetic heterogeneous disorders. Among them, defects in the process of oxidative phosphorylation are the most prevalent. Despite intense research efforts, patients are still without effective treatment. An important part of the development of new therapeutics relies on predictive models of the pathology in order to assess their therapeutic potential. Since mitochondrial diseases are a heterogeneous group of progressive multisystemic disorders that can affect any organ at any time, the development of various in vivo models for the different diseases-associated genes defects will accelerate the search for effective therapeutics. Here, we review existing Drosophila melanogaster models for mitochondrial diseases, with a focus on alterations in oxidative phosphorylation, and discuss the potential of this powerful model organism in the process of drug target discovery. This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.