Human METTL12 is a mitochondrial methyltransferase that modifies citrate synthase.

The protein methylome in mammalian mitochondria has been little studied until recently. Here, we describe that lysine-368 of human citrate synthase is methylated and that the modifying enzyme, localized in the mitochondrial matrix, is methyltransferase-like protein 12 (METTL12), a member of the family of 7ß-strand methyltransferases. Lysine-368 is near the active site of citrate synthase, but removal of methylation has no effect on its activity. In mitochondria, it is possible that some or all of the enzymes of the citric acid cycle, including citrate synthase, are organized in metabolons to facilitate the channelling of substrates between participating enzymes. Thus, possible roles for the methylation of Lys-368 are in controlling substrate channelling itself, or in influencing protein-protein interactions in the metabolon.

NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembly of Human Complex I.

Complex I (NADH ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 45 proteins. One arm lies in the inner membrane, and the other extends about 100 Å into the matrix of the organelle. The extrinsic arm contains binding sites for NADH, the primary electron acceptor FMN, and seven iron-sulfur clusters that form a pathway for electrons linking FMN to the terminal electron acceptor, ubiquinone, which is bound in a tunnel in the region of the junction between the arms. The membrane arm contains four antiporter-like domains, energetically coupled to the quinone site and involved in pumping protons from the matrix into the intermembrane space contributing to the proton motive force. Seven of the subunits, forming the core of the membrane arm, are translated from mitochondrial genes, and the remaining subunits, the products of nuclear genes, are imported from the cytosol. Their assembly is coordinated by at least thirteen extrinsic assembly factor proteins that are not part of the fully assembled complex. They assist in insertion of co-factors and in building up the complex from smaller sub-assemblies. One such factor, NDUFAF5, belongs to the family of seven-ß-strand S-adenosylmethionine-dependent methyltransferases. However, similar to another family member, RdmB, it catalyzes the introduction of a hydroxyl group, in the case of NDUFAF5, into Arg-73 in the NDUFS7 subunit of human complex I. This modification occurs early in the pathway of assembly of complex I, before the formation of the juncture between peripheral and membrane arms.

Human METTL20 methylates lysine residues adjacent to the recognition loop of the electron transfer flavoprotein in mitochondria.

In mammalian mitochondria, protein methylation is a relatively uncommon post-transcriptional modification, and the extent of the mitochondrial protein methylome, the modifying methyltransferases, and their substrates have been little studied. As shown here, the ß-subunit of the electron transfer flavoprotein (ETF) is one such methylated protein. The ETF is a heterodimer of α- and ß-subunits. Lysine residues 199 and 202 of mature ETFß are almost completely trimethylated in bovine heart mitochondria, whereas ETFα is not methylated. The enzyme responsible for the modifications was identified as methyltransferase-like protein 20 (METTL20). In human 143B cells, the methylation of ETFß is less extensive and is diminished further by suppression of METTL20. Tagged METTL20 expressed in HEK293T cells specifically associates with the ETF and promotes the trimethylation of ETFß lysine residues 199 and 202. ETF serves as a mobile electron carrier linking dehydrogenases involved in fatty acid oxidation and one-carbon metabolism to the membrane-associated ubiquinone pool. The methylated residues in ETFß are immediately adjacent to a protein loop that recognizes and binds to the dehydrogenases. Suppression of trimethylation of ETFß in mouse C2C12 cells oxidizing palmitate as an energy source reduced the consumption of oxygen by the cells. These experiments suggest that the oxidation of fatty acids in mitochondria and the passage of electrons via the ETF may be controlled by modulating the protein-protein interactions between the reduced dehydrogenases and the ß-subunit of the ETF by trimethylation of lysine residues. METTL20 is the first lysine methyltransferase to be found to be associated with mitochondria.

NDUFAF7 methylates arginine 85 in the NDUFS2 subunit of human complex I.

Complex I (NADH ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 44 subunits. One arm is embedded in the inner membrane with the other protruding ∼100 Å into the matrix of the organelle. The extrinsic arm contains binding sites for NADH and the primary electron acceptor FMN, and it provides a scaffold for seven iron-sulfur clusters that form an electron pathway linking FMN to the terminal electron acceptor, ubiquinone, which is bound in the region of the junction between the arms. The membrane arm contains four antiporter-like domains, probably energetically coupled to the quinone site and involved in pumping protons from the matrix into the intermembrane space contributing to the proton motive force. Complex I is put together from preassembled subcomplexes. Their compositions have been characterized partially, and at least 12 extrinsic assembly factor proteins are required for the assembly of the complex. One such factor, NDUFAF7, is predicted to belong to the family of S-adenosylmethionine-dependent methyltransferases characterized by the presence in their structures of a seven-ß-strand protein fold. In the present study, the presence of NDUFAF7 in the mitochondrial matrix has been confirmed, and it has been demonstrated that it is a protein methylase that symmetrically dimethylates the ω-N(G),N(G') atoms of residue Arg-85 in the NDUFS2 subunit of complex I. This methylation step occurs early in the assembly of complex I and probably stabilizes a 400-kDa subcomplex that forms the initial nucleus of the peripheral arm and its juncture with the membrane arm.

A new link to mitochondrial impairment in tauopathies.

Tauopathies like the "frontotemporal dementia with Parkinsonism linked to chromosome 17" (FTDP-17) are characterized by an aberrant accumulation of intracellular neurofibrillary tangles composed of hyperphosphorylated tau. For FTDP-17, a pathogenic tau mutation P301L was identified. Impaired mitochondrial function including disturbed dynamics such as fission and fusion are most likely major pathomechanisms of most neurodegenerative diseases. However, very little is known if tau itself affects mitochondrial function and dynamics. We addressed this question using SY5Y cells stably overexpressing wild-type (wt) and P301L mutant tau. P301L overexpression resulted in a substantial complex I deficit accompanied by decreased ATP levels and increased susceptibility to oxidative stress. This was paralleled by pronounced changes in mitochondrial morphology, decreased fusion and fission rates accompanied by reduced expression of several fission and fusion factors like OPA-1 or DRP-1. In contrast, overexpression of wt tau exhibits protective effects on mitochondrial function and dynamics including enhanced complex I activity. Our findings clearly link tau bidirectional to mitochondrial function and dynamics, identifying a novel aspect of the physiological role of tau and the pathomechanism of tauopathies.

Peripheral mitochondrial dysfunction in Alzheimer's disease: focus on lymphocytes.

Alzheimer's disease (AD) is the most common progressive neurodegenerative disease. Today, AD affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. The AD brain is marked by severe neurodegeneration like the loss of synapses and neurons, atrophy and depletion of neurotransmitter systems in the hippocampus and cerebral cortex. Recent findings suggest that these pathological changes are causally induced by mitochondrial dysfunction and increased oxidative stress. These changes are not only observed in the brain of AD patients but also in the periphery. In this review, we discuss the potential role of elevated apoptosis, increased oxidative stress and especially mitochondrial dysfunction as peripheral markers for the detection of AD in blood cells especially in lymphocytes. We discuss recent not otherwise published findings on the level of complex activities of the respiratory chain comprising mitochondrial respiration and the mitochondrial membrane potential (MMP). We obtained decreased basal MMP levels in lymphocytes from AD patients as well as enhanced sensitivity to different complex inhibitors of the respiratory chain. These changes are in line with mitochondrial defects obtained in AD cell and animal models, and in post-mortem AD tissue. Importantly, these mitochondrial alterations where not only found in AD patients but also in patients with mild cognitive impairment (MCI). These new findings point to a relevance of mitochondrial function as an early peripheral marker for the detection of AD and MCI.

Combined expression of tau and the Harlequin mouse mutation leads to increased mitochondrial dysfunction, tau pathology and neurodegeneration.

Mitochondrial dysfunction and oxidative stress play an important role in ageing and have been implicated in several age-related neurodegenerative conditions including Alzheimer's disease (AD) and other tauopathies characterized by the presence of intracellular accumulations of the hyperphosphorylated microtubule-associated protein tau. To study the interaction between mitochondrial dysfunction and tau pathology in vivo, we generated a novel mouse model by crossbreeding two existing lines: the Harlequin (Hq) mutant mice which suffer from mitochondrial dysfunction and oxidative stress due to a lack of the mitochondrial apoptosis-inducing factor (AIF), and the P301L tau transgenic mice, a mouse model of human tau pathology. Combined expression of the Hq mouse mutation and the tau transgene in the Tau/Hq double mutant mice led to an increase in tau pathology and apoptotic neurodegeneration when compared to single expression of the two mutations. Neurodegeneration was most prominent in the dentate gyrus and was significantly increased in the cerebellum leading to aggravated motor deficits. Functional activity measurements of the mitochondrial respiratory chain (MRC) in the Tau/Hq mice revealed early decreased activities of multiple MRC complexes and depleted ATP levels which preceded neurodegeneration and elevated oxidative stress markers. These results suggest an age-dependent mutual reinforcement of the tau pathology and mitochondrial dysfunction in vivo, which may contribute to neurodegeneration in patients suffering from AD and other age-related tauopathies.

Ginkgo biloba extract ameliorates oxidative phosphorylation performance and rescues abeta-induced failure.

BACKGROUND: Energy deficiency and mitochondrial failure have been recognized as a prominent, early event in Alzheimer's disease (AD). Recently, we demonstrated that chronic exposure to amyloid-beta (Abeta) in human neuroblastoma cells over-expressing human wild-type amyloid precursor protein (APP) resulted in (i) activity changes of complexes III and IV of the oxidative phosphorylation system (OXPHOS) and in (ii) a drop of ATP levels which may finally instigate loss of synapses and neuronal cell death in AD. Therefore, the aim of the present study was to investigate whether standardized Ginkgo biloba extract LI 1370 (GBE) is able to rescue Abeta-induced defects in energy metabolism. METHODOLOGY/PRINCIPAL FINDINGS: We used a high-resolution respiratory protocol to evaluate OXPHOS respiratory capacity under physiological condition in control (stably transfected with the empty vector) and APP cells after treatment with GBE. In addition, oxygen consumption of isolated mitochondria, activities of mitochondrial respiratory enzymes, ATP and reactive oxygen species (ROS) levels as well as mitochondrial membrane mass and mitochondrial DNA content were determined. We observed a general antioxidant effect of GBE leading to an increase of the coupling state of mitochondria as well as energy homeostasis and a reduction of ROS levels in control cells and in APP cells. GBE effect on OXPHOS was even preserved in mitochondria after isolation from treated cells. Moreover, these functional data were paralleled by an up-regulation of mitochondrial DNA. Improvement of the OXPHOS efficiency was stronger in APP cells than in control cells. In APP cells, the GBE-induced amelioration of oxygen consumption most likely arose from the modulation and respective normalization of the Abeta-induced disturbance in the activity of mitochondrial complexes III and IV restoring impaired ATP levels possibly through decreasing Abeta and oxidative stress level. CONCLUSIONS/SIGNIFICANCE: Although the underlying molecular mechanisms of the mode of action of GBE remain to be determined, our study clearly highlights the beneficial effect of GBE on the cellular OXPHOS performance and restoration of Abeta-induced mitochondrial dysfunction.

Convergence of amyloid-beta and tau pathologies on mitochondria in vivo.

The histopathological characteristics of Alzheimer's disease (AD) are amyloid-beta (Abeta) containing plaques and neurofibrillary tangles (NFTs) as well as neuronal and synaptic loss. Until today, the underlying mechanisms of the interplay of plaques and tangles remained unresolved. There is increasing evidence that mitochondrial dysfunction might be a possible link, as revealed by studies in several APP and tau transgenic mouse models. Recently, we examined mitochondrial function in a novel triple transgenic mouse model (pR5/APP/PS2)--(triple)AD mice--that combines both pathologic features of the disease in brain. Using comparative, quantitative proteomics (iTRAQ) and mass spectroscopy, we found a massive deregulation of 24 proteins, of which one third were mitochondrial proteins mainly related to complexes I and IV of the oxidative phosphorylation system (OXPHOS). Remarkably, deregulation of complex I was related to tau, whereas deregulation of complex IV was Abeta dependent, both at the protein and activity levels. The (triple)AD mice showed synergistic effects of Abeta and tau already at the age of 8 months, resulting in a depolarized mitochondrial membrane potential. At 12 months, the strongest defects on OXPHOS, synthesis of ATP and reactive oxygen species, were exhibited in the (triple)AD mice, again emphasizing synergistic, age-associated effects of Abeta and tau in impairing mitochondria. This review highlights the convergence of Abeta and tau on mitochondria and establishes a molecular link in AD pathology in vivo.

Abeta and human amylin share a common toxicity pathway via mitochondrial dysfunction.

Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM) are leading causes of morbidity and mortality in the elderly. Both diseases are characterized by amyloid deposition in target tissues: aggregation of amylin in T2DM is associated with loss of insulin-secreting beta-cells, while amyloid beta (A beta) aggregation in AD brain is associated with neuronal loss. Here, we used quantitative iTRAQ proteomics as a discovery tool to show that both A beta and human amylin (HA) deregulate identical proteins, a quarter of which are mitochondrial, supporting the notion that mitochondrial dysfunction is a common target in these two amyloidoses. A functional validation revealed that mitochondrial complex IV activity was significantly reduced after treatment with either HA or A beta, as was mitochondrial respiration. In comparison, complex I activity was reduced only after treatment with HA. A beta and HA, but not the non-amyloidogenic rat amylin, induced significant increases in the generation of ROS. Co-incubation of HA and A beta did not produce an augmented effect in ROS production, again suggesting common toxicity mechanisms. In conclusion, our data suggest that A beta and HA both exert toxicity, at least in part, via mitochondrial dysfunction, thus restoring their function may be beneficial for both AD and T2DM.

Amyloid-beta and tau synergistically impair the oxidative phosphorylation system in triple transgenic Alzheimer's disease mice.

Alzheimer's disease (AD) is characterized by amyloid-beta (Abeta)-containing plaques, neurofibrillary tangles, and neuron and synapse loss. Tangle formation has been reproduced in P301L tau transgenic pR5 mice, whereas APP(sw)PS2(N141I) double-transgenic APP152 mice develop Abeta plaques. Cross-breeding generates triple transgenic ((triple)AD) mice that combine both pathologies in one model. To determine functional consequences of the combined Abeta and tau pathologies, we performed a proteomic analysis followed by functional validation. Specifically, we obtained vesicular preparations from (triple)AD mice, the parental strains, and nontransgenic mice, followed by the quantitative mass-tag labeling proteomic technique iTRAQ and mass spectrometry. Within 1,275 quantified proteins, we found a massive deregulation of 24 proteins, of which one-third were mitochondrial proteins mainly related to complexes I and IV of the oxidative phosphorylation system (OXPHOS). Notably, deregulation of complex I was tau dependent, whereas deregulation of complex IV was Abeta dependent, both at the protein and activity levels. Synergistic effects of Abeta and tau were evident in 8-month-old (triple)AD mice as only they showed a reduction of the mitochondrial membrane potential at this early age. At the age of 12 months, the strongest defects on OXPHOS, synthesis of ATP, and reactive oxygen species were exhibited in the (triple)AD mice, again emphasizing synergistic, age-associated effects of Abeta and tau in perishing mitochondria. Our study establishes a molecular link between Abeta and tau protein in AD pathology in vivo, illustrating the potential of quantitative proteomics.

Oligomeric and fibrillar species of beta-amyloid (A beta 42) both impair mitochondrial function in P301L tau transgenic mice.

We recently provided evidence for a mitochondrial dysfunction in P301L tau transgenic mice, a strain modeling the tau pathology of Alzheimer's disease (AD) and frontotemporal dementia (FTD). In addition to tau aggregates, the AD brain is further characterized by A beta peptide-containing plaques. When we addressed the role of A beta, this indicated a synergistic action of tau and A beta pathology on the mitochondria. In the present study, we compared the toxicity of different A beta 42 conformations in light of recent studies suggesting that oligomeric rather than fibrillar A beta might be the actual toxic species. Interestingly, both oligomeric and fibrillar, but not disaggregated (mainly monomeric) A beta 42 caused a decreased mitochondrial membrane potential in cortical brain cells obtained from FTD P301L tau transgenic mice. This was not observed with cerebellar preparations indicating selective vulnerability of cortical neurons. Furthermore, we found reductions in state 3 respiration, the respiratory control ratio, and uncoupled respiration when incubating P301L tau mitochondria either with oligomeric or fibrillar preparations of A beta 42. Finally, we found that aging specifically increased the sensitivity of mitochondria to oligomeric A beta 42 damage indicating that oligomeric and fibrillar A beta 42 are both toxic, but exert different degrees of toxicity.

Soluble beta-amyloid leads to mitochondrial defects in amyloid precursor protein and tau transgenic mice.

BACKGROUND: Mitochondrial dysfunction has been identified in neurodegenerative disorders including Alzheimer's disease, where accumulation of beta-amyloid (Abeta) and oxidative stress seem to play central roles in the pathogenesis, by probably directly leading to mitochondrial dysfunction. OBJECTIVE: In order to study the in vivo effect of Abeta load during aging, we evaluated the mitochondrial function of brain cells from transgenic mice bearing either mutant amyloid precursor protein (tgAPP) or mutant amyloid precursor protein and mutant PS1 (tgAPP/PS1) as well as from nontransgenic wild-type littermates. tgAPP mice exhibit onset of Abeta plaques at an age of 6 months, but the intracellular soluble Abeta load is already increased at 3 months of age. In contrast, onset of Abeta plaques starts at an age of 3 months in tgAPP/PS1 mice. In addition, we investigated the effects of different Abeta preparations on mitochondrial function of brain cells from tau transgenic mice. RESULTS: Of note, mitochondrial damage such as reduced mitochondrial membrane potential and ATP levels can already be detected in the brains from these mice before the onset of plaques. In agreement with our findings in tgAPP mice, soluble Abeta induced mitochondrial dysfunction in brain cells from tau transgenic mice. CONCLUSION: Our results indicate that mitochondrial dysfunction is exacerbated by the presence of soluble Abeta species as a very early event during pathogenesis.

Lack of flexibility in visual grouping in patients with schizophrenia.

The study attempted to distinguish automatic grouping processes from top-down processes in a visual perceptual task in 30 patients with schizophrenia and 30 matched controls. Participants decided whether 7 figures were all different or whether 2 adjacent figures were identical. The distance between figures was manipulated to produce 3 separated pairs of figures, the targets belonging to either the same pair (within-group trials) or different pairs (between-groups trials). As controls, patients benefited from proximity for grouping. Top-down processes were explored by manipulating the proportion of within-group and between-groups trials in 3 experimental blocks. In patients, response times (RTs) decreased for within-group trials when within-group trials were more frequent, indicating that performance was correctly adapted to the type of block. Unlike controls, however, this RT decrease was not accompanied by a cost for between-groups trials. Ocular movement recordings revealed that controls were able to focus on between-groups regions selectively when between-groups trials were more frequent, whereas patients were unable to do so. The authors suggest that top-down processes allowing the construction of a selective representation of between-groups regions are impaired in patients with schizophrenia.

Effects of Alzheimer's amyloid-beta and tau protein on mitochondrial function -- role of glucose metabolism and insulin signalling.

Alzheimer's disease (AD) is the most frequent form of dementia among the elderly and is characterized by neuropathological hallmarks of extracellular amyloid-beta (Abeta) plaques and intracellular neurofibrillary tangles composed of abnormally hyperphosphorylated microtubular protein tau in the brains of AD patients. Of note, current data illustrate a complex interplay between the amyloid and tau pathology during the course of the disease. We hypothesize a direct impact of abnormally phosphorylated tau and Abeta on proteins/enzymes involved in metabolism, respiratory chain function and cellular detoxification. Probably at the level of mitochondria, both Alzheimer proteins exhibit synergistic effects finally leading to/accelerating neurodegenerative mechanisms. Moreover, accumulating evidence that mitochondria failure, reduced glucose utilization and deficient energy metabolism occur already very early in the course of the disease suggests a role of impaired insulin signalling in the pathogenesis of AD. Thus, this review addresses also the question if mitochondrial dysfunction may represent a link between diabetes and AD.