A mev-1-like dominant-negative SdhC increases oxidative stress and reduces lifespan in Drosophila

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.

Succinate dehydrogenase complex subunit C: Role in cellular physiology and disease

Succinate dehydrogenase complex subunit C (SDHC) is a subunit of mitochondrial complex II (MCII), which is also known as succinate dehydrogenase (SDH) or succinate: ubiquinone oxidoreductase. Mitochondrial complex II is the smallest respiratory complex in the respiratory chain and contains four subunits. SDHC is a membrane-anchored subunit of SDH, which connects the tricarboxylic acid cycle and the electron transport chain. SDH regulates several physiological processes within cells, plays an important role in generating energy to maintain normal cell growth, and is involved in apoptosis. Currently, SDHC is generally recognized as a tumor-suppressor gene. SDHC mutations can cause oxidative damage in the body. It is closely related to the occurrence and development of cancer, neurodegenerative diseases, and aging-related diseases. Here, we review studies on the structure, biological function, related diseases of SDHC, and the mev-1 Animal Model of SDHC Mutation and its potential use as a therapeutic target of certain human diseases.

Hypothesis: Why Different Types of SDH Gene Variants Cause Divergent Tumor Phenotypes

Despite two decades of paraganglioma-pheochromocytoma research, the fundamental question of how the different succinate dehydrogenase (SDH)-related tumor phenotypes are initiated has remained unanswered. Here, we discuss two possible scenarios by which missense (hypomorphic alleles) or truncating (null alleles) SDH gene variants determine clinical phenotype. Dysfunctional SDH is a major source of reactive oxygen species (ROS) but ROS are inhibited by rising succinate levels. In scenario 1, we propose that SDH missense variants disrupt electron flow, causing elevated ROS levels that are toxic in sympathetic PPGL precursor cells but well controlled in oxygen-sensing parasympathetic paraganglion cells. We also suggest that SDHAF2 variants, solely associated with HNPGL, may cause the reversal of succinate dehydrogenase to fumarate reductase, producing very high ROS levels. In scenario 2, we propose a modified succinate threshold model of tumor initiation. Truncating SDH variants cause high succinate accumulation and likely initiate tumorigenesis via disruption of 2-oxoglutarate-dependent enzymes in both PPGL and HNPGL precursor tissues. We propose that missense variants (including SDHAF2) cause lower succinate accumulation and thus initiate tumorigenesis only in very metabolically active tissues such as parasympathetic paraganglia, which naturally show very high levels of succinate.

Rapamycin as a potential treatment for succinate dehydrogenase deficiency

Drosophila melanogaster is a powerful model to study mitochondrial respiratory chain defects, particularly succinate dehydrogenase (SDH) deficiency. Mutations in sdh genes cause degenerative disorders and often lead to death. Therapies for such pathologies are based on a combination of vitamins and dietary supplements, and are rarely effective. In Drosophila, mutations in several of the genes encoding SDH resemble the pathology of SDH deficiency in humans, enabling the Drosophila model to be used in finding treatments for this condition. Here we show that exposure to the drug rapamycin improves the survival of sdh mutant strains, the activity of SDH and the impaired climbing associated with sdh mutations. However, the production of reactive oxygen species, the oxygen consumption of isolated mitochondria and the resistance to hyperoxia were minimally affected. Our results contribute to the current research seeking a treatment for mitochondrial disease.

Age-Dependent Decrease of Mitochondrial Complex II Activity in Human Skin Fibroblasts

The mitochondrial theory of aging remains one of the most widely accepted aging theories and implicates mitochondrial electron transport chain dysfunction with subsequent increasing free radical generation. Recently, complex II of the electron transport chain appears to be more important than previously thought in this process, suggested predominantly by nonhuman studies. We investigated the relationship between complex II and aging using human skin as a model tissue. The rate of complex II activity per unit of mitochondria was determined in fibroblasts and keratinocytes cultured from skin covering a wide age range. Complex II activity significantly decreased with age in fibroblasts (P = 0.015) but not in keratinocytes. This was associated with a significant decline in transcript expression (P = 0.008 and P = 0.001) and protein levels (P = 0.0006 and P = 0.005) of the succinate dehydrogenase complex subunit A and subunit B catalytic subunits of complex II, respectively. In addition, there was a significant decrease in complex II activity with age (P = 0.029) that was specific to senescent skin cells. There was no decrease in complex IV activity with increasing age, suggesting possible locality to complex II.

Genetically induced oxidative stress in mice causes thrombocytosis, splenomegaly and placental angiodysplasia that leads to recurrent abortion

Historical data in the 1950s suggests that 7%, 11%, 33%, and 87% of couples were infertile by ages 30, 35, 40 and 45, respectively. Up to 22.3% of infertile couples have unexplained infertility. Oxidative stress is associated with male and female infertility. However, there is insufficient evidence relating to the influence of oxidative stress on the maintenance of a viable pregnancy, including pregnancy complications and fetal development. Recently, we have established Tet-mev-1 conditional transgenic mice, which can express the doxycycline-induced mutant SDHC^V69E transgene and experience mitochondrial respiratory chain dysfunction leading to intracellular oxidative stress. In this report, we demonstrate that this kind of abnormal mitochondrial respiratory chain-induced chronic oxidative stress affects fertility, pregnancy and delivery rates as well as causes recurrent abortions, occasionally resulting in maternal death. Despite this, spermatogenesis and early embryogenesis are completely normal, indicating the mutation's effects to be rather subtle. Female Tet-mev-1 mice exhibit thrombocytosis and splenomegaly in both non-pregnant and pregnant mice as well as placental angiodysplasia with reduced Flt-1 protein leading to hypoxic conditions, which could contribute to placental inflammation and fetal abnormal angiogenesis. Collectively these data strongly suggest that chronic oxidative stress caused by mitochondrial mutations provokes spontaneous abortions and recurrent miscarriage resulting in age-related female infertility.

•

SDHC mutation induces oxidative stress in the female reproductive organs.

•

Early development is completely normal in Tet-mev-1 mice.

•

Non-pregnant and pregnant Tet-mev-1 mice exhibit thrombocytosis and splenomegaly.

•

Pregnant mice have placental angiodysplasia with decreased Flt-1/ VEGFR-1.

•

Habitual abortion is frequently caused with occasional maternal death in Tet-mev-1 mice.

Physiological consequences of complex II inhibition for aging, disease, and the mKATP channel

In recent years, it has become apparent that there exist several roles for respiratory complex II beyond metabolism. These include: (i) succinate signaling, (ii) reactive oxygen species (ROS) generation, (iii) ischemic preconditioning, (iv) various disease states and aging, and (v) a role in the function of the mitochondrial ATP-sensitive K(+) (mKATP) channel. This review will address the involvement of complex II in each of these areas, with a focus on how complex II regulates or may be involved in the assembly of the mKATP. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Model animals for the study of oxidative stress from complex II

Mitochondria play a role of energy production and produce intracellular reactive oxygen species (ROS), especially superoxide anion (O2(-)) as a byproduct of energy metabolism at the same time. O2(-) is converted from oxygen and is overproduced by excessive electron leakage from the mitochondrial respiratory chain. It is well known that mitochondrial complexes I and III in the electron transport system are the major endogenous ROS sources. We have previously demonstrated that mutations in complex II can result in excessive ROS (specifically in SDHC: G71E in Caenorhabditis elegans, I71E in Drosophila and V69E in mouse). Moreover, this results in premature death in C. elegans and Drosophila as well as tumorigenesis in mouse embryonic fibroblast cells. In humans, it has been reported that mutations in SDHB, SDHC or SDHD, which are the subunits of mitochondrial complex II, often result in inherited head and neck paragangliomas (PGLs). Recently, we established Tet-mev-1 conditional transgenic mice using our uniquely developed Tet-On/Off system, which can induce the mutated SDHC gene to be equally and competitively expressed compared to the endogenous wild-type SDHC gene. These mice experienced mitochondrial respiratory chain dysfunction that resulted in oxidative stress. The mitochondrial oxidative stress caused excessive apoptosis in several tissues leading to low-birth-weight infants and growth retardation during neonatal developmental phase in Tet-mev-1 mice. Tet-mev-1 mice also displayed precocious age-dependent corneal physiological changes, delayed corneal epithelialization, decreased corneal endothelial cells, thickened Descemet's membrane and thinning of parenchyma with corneal pathological dysfunctions such as keratitis, Fuchs' corneal dystrophy (FCD) and probably keratoconus after the normal development and growth phase. Here, we review the relationships between mitochondrial oxidative stress and phenomena in mev-1 animal models with mitochondrial complex II SDHC mutations. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Oxidative stress induced inflammation initiates functional decline of tear production

Oxidative damage and inflammation are proposed to be involved in an age-related functional decline of exocrine glands. However, the molecular mechanism of how oxidative stress affects the secretory function of exocrine glands is unclear. We developed a novel mev-1 conditional transgenic mouse model (Tet-mev-1) using a modified tetracycline system (Tet-On/Off system). This mouse model demonstrated decreased tear production with morphological changes including leukocytic infiltration and fibrosis. We found that the mev-1 gene encodes Cyt-1, which is the cytochrome b(560) large subunit of succinate-ubiquinone oxidoreductase in complex II of mitochondria (homologous to succinate dehydrogenase C subunit (SDHC) in humans). The mev-1 gene induced excessive oxidative stress associated with ocular surface epithelial damage and a decrease in protein and aqueous secretory function. This new model provides evidence that mitochondrial oxidative damage in the lacrimal gland induces lacrimal dysfunction resulting in dry eye disease. Tear volume in Tet-mev-1 mice was lower than in wild type mice and histopathological analyses showed the hallmarks of lacrimal gland inflammation by intense mononuclear leukocytic infiltration and fibrosis in the lacrimal gland of Tet-mev-1 mice. These findings strongly suggest that oxidative stress can be a causative factor for the development of dry eye disease.

No evidence of elevated germline mutation accumulation under oxidative stress in Caenorhabditis elegans

Variation in rates of molecular evolution has been attributed to numerous, interrelated causes, including metabolic rate, body size, and generation time. Speculation concerning the influence of metabolic rate on rates of evolution often invokes the putative mutagenic effects of oxidative stress. To isolate the effects of oxidative stress on the germline from the effects of metabolic rate, generation time, and other factors, we allowed mutations to accumulate under relaxed selection for 125 generations in two strains of the nematode Caenorhabditis elegans, the canonical wild-type strain (N2) and a mutant strain with elevated steady-state oxidative stress (mev-1). Contrary to our expectation, the mutational decline in fitness did not differ between N2 and mev-1. This result suggests that the mutagenic effects of oxidative stress in C. elegans are minor relative to the effects of other types of mutations, such as errors during DNA replication. However, mev-1 MA lines did go extinct more frequently than wild-type lines; some possible explanations for the difference in extinction rate are discussed.

Mitochondrial superoxide anion (O(2)(-)) inducible "mev-1" animal models for aging research

Most intracellular reactive oxygen species (ROS), especially superoxide anion (O(2)(-)) that is converted from oxygen, are overproduced by excessive electron leakage from the mitochondrial respiratory chain. Intracellular oxidative stress that damages cellular components can contribute to lifestyle-related diseases such as diabetes and arteriosclerosis, and age-related diseases such as cancer and neuronal degenerative diseases. We have previously demonstrated that the excessive mitochondrial O(2)(-) production caused by SDHC mutations (G71E in C. elegans, I71E in Drosophila and V69E in mouse) results in premature death in C. elegans and Drosophila, cancer in mouse embryonic fibroblast cells and infertility in transgenic mice. SDHC is a subunit of mitochondrial complex II. In humans, it has been reported that mutations in SDHB, SDHC or SDHD often result in inherited head and neck paragangliomas (PGLs). Recently, we established Tet-mev-1 conditional transgenic mice using our uniquely developed Tet-On/Off system, which equilibrates transgene expression to endogenous levels. These mice experienced mitochondrial respiratory chain dysfunction that resulted in O(2)(-) overproduction. The mitochondrial oxidative stress caused excessive apoptosis leading to low birth weight and growth retardation in the neonatal developmental phase in Tet-mev-1 mice. Here, we briefly describe the relationships between mitochondrial O(2)(-) and aging phenomena in mev-1 animal models. [BMB reports 2011; 44(5): 298-305].

Mitochondrial reactive oxygen species generation by the SDHC V69E mutation causes low birth weight and neonatal growth retardation

We have previously demonstrated that excessive mitochondrial reactive oxygen species caused by mutations in the SDHC subunit of Complex II resulted in premature death in C. elegans and Drosophila, tumors in mouse cells and infertility in transgenic mice. We now report the generation and initial characterization of conditional transgenic mice (Tet-mev-1) using our uniquely developed Tet-On/Off system, which equilibrates transgene expression to endogenous levels. The mice experienced mitochondrial respiratory chain dysfunction that induced reactive oxygen species overproduction. The mitochondrial oxidative stress resulted in excessive apoptosis leading to low birth weight and growth retardation in the neonatal developmental phase in Tet-mev-1 mice.

Loss of Trx-2 enhances oxidative stress-dependent phenotypes in Drosophila

Overexpression of thioredoxin (TRX) confers oxidative stress resistance and extends lifespan in mammals and insects. However, less is known about phenotypes associated with loss of TRX. We investigated loss-of-function phenotypes of Trx-2 in Drosophila, and found that the mutant flies are hyper-susceptible to paraquat, a free radical generator, but not to hydrogen peroxide. They contain a high amount of protein carbonyl, which dramatically increases with age. Trx-2 mutants express high levels of anti-oxidant genes, such as superoxide dismutase, catalase, and glutathione synthetase. This is the first demonstration of biochemical and physiological consequences caused by loss of Trx-2 in Drosophila.

Warburg tumours and the mechanisms of mitochondrial tumour suppressor genes. Barking up the right tree?

The past decade has seen a revival of interest in the metabolic adaptations of tumours, named for their original discoverer, Otto Warburg. Warburg reported a high rate of glycolysis in tumours, and a concurrent defect in mitochondrial respiration. The rediscovery of Warburg's hypothesis coincided with the discovery of mitochondrial tumours suppressor genes that may conform to Warburg's hypothesis. Succinate dehydrogenase and fumarate hydratase are mitochondrial proteins of the TCA cycle and the respiratory chain and when mutated lead to tumours of the nervous system known as paragangliomas and pheochromocytomas, and in the case of fumarate hydratase, cutaneous and uterine leiomyomas and renal cell cancer. Recently a novel mitochondrial protein, SDHAF2 (SDH5), was also shown to be a paraganglioma-related tumour suppressor gene. Another mitochondrial and TCA cycle-related protein, isocitrate dehydrogenase 2 is, together with IDH1, frequently mutated in the brain tumour glioblastoma. There are currently many competing hypotheses on the role of these genes in tumourigenesis, but frequent themes are the stabilization of hypoxia inducible factor 1 and upregulation of genes involved in angiogenesis, glucose transport and glycolysis. Other postulated mechanisms include the inhibition of developmental apoptosis, altered gene expression due to histone deregulation and the acquisition of novel catalytic properties. Here we discuss these diverse hypotheses and highlight very recent findings on the possible effects of IDH gene mutations.