Mitochondrial carriers (MCs) form a large family of nuclear-encoded transporters embedded in the inner mitochondrial membrane and in a few cases in other organelle membranes (Palmieri, 2013). The members of this superfamily are widespread in eukaryotes and involved in numerous metabolic pathways and cell functions. They can be easily recognized by their striking sequence features, i.e., a tripartite structure, six transmembrane α-helices and a 3-fold repeated signature motifs. Members of the family vary greatly in the nature and size of their transported substrates, modes of transport (i.e., uniport, symport or antiport) and driving forces, although the molecular mechanism of substrate translocation may be basically the same. In recent years mutations in the MC genes have been shown to be responsible for 11 diseases (Palmieri, 2013), highlighting the important role of MCs in metabolism. MC impairing mutations affect three main regions crucial for substrate translocation. A first group of mutations affects MC conformational changes and locates at PG levels or at the aromatic belts (Pierri et al., 2013). A second group of mutations affects substrate specificity and locates at the common substrate binding site (Robinson et al., 2008) and at the substrate binding area (Pierri et al., 2013). A further group of mutations locate at residues of the m-/c-gates (Palmieri et al., 2013; Robinson et al., 2008) and at residues of the m-gate area (Pierri et al. 2013). For this last group of mutations, it appears difficult to establish if the impaired function is due to the lack of substrate specificity (or substrate recognition) or to the wrong triggering of conformational changes. Two mutations, one at the PG level 1 and one at the common substrate binding site, impairing citrate translocation within SLC25A1_CTP protein are presented. The two mutations are found to be responsible of agenesis of corpus callosum and optic nerve hypoplasia (Edvardson et al., 2013). References 1. Palmieri F. The mitochondrial transporter family SLC25: identification, properties and physiopathology. Mol Aspects Med. 2013;34:465. 2. Pierri CL, Palmieri F, De Grassi A. Single-nucleotide evolution quantifies the importance of each site along the structure of mitochondrial carriers. Cell Mol Life Sci. 2013. 3. Robinson AJ, Overy C, Kunji ER. The mechanism of transport by mitochondrial carriers based on analysis of symmetry. Proc Natl Acad Sci U S A. 2008;105:17766. 4. Edvardson S, Porcelli V, Jalas C, Soiferman D, Kellner Y, Shaag A, Korman SH, Pierri CL, Scarcia P, Fraenkel ND, Segel R, Schechter A, Frumkin A, Pines O, Saada A, Palmieri L, Elpeleg O. Agenesis of corpus callosum and optic nerve hypoplasia due to mutations in SLC25A1 encoding the mitochondrial citrate transporter. J Med Genet. 2013;50:240.

The mitochondrial citrate-malate exchanger (CIC), a known target of acetylation, is up-regulated in activated immune cells and plays a key role in the production of inflammatory mediators. However, the role of acetylation in CIC activity is elusive. We show that CIC is acetylated in activated primary human macrophages and U937 cells and the level of acetylation is higher in glucose-deprived compared to normal glucose medium. Acetylation enhances CIC transport activity, leading to a higher citrate efflux from mitochondria in exchange with malate. Cytosolic citrate levels do not increase upon activation of cells grown in deprived compared to normal glucose media, indicating that citrate, transported from mitochondria at higher rates from acetylated CIC, is consumed at higher rates. Malate levels in the cytosol are lower in activated cells grown in glucose-deprived compared to normal glucose medium, indicating that this TCA intermediate is rapidly recycled back into the cytosol where it is used by the malic enzyme. Additionally, in activated cells CIC inhibition increases the NADP+/NADPH ratio in glucose-deprived cells; this ratio is unchanged in glucose-rich grown cells due to the activity of the pentose phosphate pathway. Consistently, the NADPH-producing isocitrate dehydrogenase level is higher in activated glucose-deprived as compared to glucose rich cells. These results demonstrate that, in the absence of glucose, activated macrophages increase CIC acetylation to enhance citrate efflux from mitochondria not only to produce inflammatory mediators but also to meet the NADPH demand through the actions of isocitrate dehydrogenase and malic enzyme.

Background: Agenesis of corpus callosum has been associated with several defects of the mitochondrial respiratory chain and the citric acid cycle. We now report the results of the biochemical and molecular studies of a patient with severe neurodevelopmental disease manifesting by agenesis of corpus callosum and optic nerve hypoplasia. Methods and results: A mitochondrial disease was suspected in this patient based on the prominent excretion of 2-hydroxyglutaric acid and Krebs cycle intermediates in urine and the finding of increased reactive oxygen species content and decreased mitochondrial membrane potential in her fibroblasts. Whole exome sequencing disclosed compound heterozygosity for two pathogenic variants in the SLC25A1 gene, encoding the mitochondrial citrate transporter. These variants, G130D and R282H, segregated in the family and were extremely rare in controls. The mutated residues were highly conserved throughout evolution and in silico modeling investigations indicated that the mutations would have a deleterious effect on protein function, affecting either substrate binding to the transporter or its translocation mechanism. These predictions were validated by the observation that a yeast strain harbouring the mutations at equivalent positions in the orthologous protein exhibited a growth defect under stress conditions and by the loss of activity of citrate transport by the mutated proteins reconstituted into liposomes. Conclusions: We report for the first time a patient with a mitochondrial citrate carrier deficiency. Our data support a role for citric acid cycle defects in agenesis of corpus callosum as already reported in patients with aconitase or fumarate hydratase deficiency.

Autism is a severe developmental disorder, whose pathogenetic underpinnings are still largely unknown. Temporocortical gray matter from six matched patient-control pairs was used to perform post-mortem biochemical and genetic studies of the mitochondrial aspartate/glutamate carrier (AGC), which participates in the aspartate/malate reduced nicotinamide adenine dinucleotide shuttle and is physiologically activated by calcium (Ca2+). AGC transport rates were significantly higher in tissue homogenates from all six patients, including those with no history of seizures and with normal electroencephalograms prior to death. This increase was consistently blunted by the Ca2+ chelator ethylene glycol tetraacetic acid; neocortical Ca2+ levels were significantly higher in all six patients; no difference in AGC transport rates was found in isolated mitochondria from patients and controls following removal of the Ca2+-containing postmitochondrial supernatant. Expression of AGC1, the predominant AGC isoform in brain, and cytochrome c oxidase activity were both increased in autistic patients, indicating an activation of mitochondrial metabolism. Furthermore, oxidized mitochondrial proteins were markedly increased in four of the six patients. Variants of the AGC1-encoding SLC25A12 gene were neither correlated with AGC activation nor associated with autism-spectrum disorders in 309 simplex and 17 multiplex families, whereas some unaffected siblings may carry a protective gene variant. Therefore, excessive Ca2+ levels are responsible for boosting AGC activity, mitochondrial metabolism and, to a more variable degree, oxidative stress in autistic brains. AGC and altered Ca2+ homeostasis play a key interactive role in the cascade of signaling events leading to autism: their modulation could provide new preventive and therapeutic strategies.

Combined D-2- and L-2-hydroxyglutaric aciduria (D/L-2-HGA) is a devastating neurometabolic disorder, usually lethal in the first years of life. Autosomal recessive mutations in the SLC25A1 gene, which encodes the mitochondrial citrate carrier (CIC), were previously detected in patients affected with combined D/L-2-HGA. We showed that transfection of deficient fibroblasts with wild-type SLC25A1 restored citrate efflux and decreased intracellular 2-hydroxyglutarate levels, confirming that deficient CIC is the cause of D/L-2-HGA. We developed and implemented a functional assay and applied it to all 17 missense variants detected in a total of 26 CIC-deficient patients, including eight novel cases, showing reduced activities of varying degrees. In addition, we analyzed the importance of residues affected by these missense variants using our existing scoring system. This allowed not only a clinical and biochemical overview of the D/L-2-HGA patients but also phenotype-genotype correlation studies.

The mitochondrial aspartate-glutamate carrier isoform 1 (AGC1) catalyzes a Ca(2+)-stimulated export of aspartate to the cytosol in exchange for glutamate, and is a key component of the malate-aspartate shuttle which transfers NADH reducing equivalents from the cytosol to mitochondria. By sustaining the complete glucose oxidation, AGC1 is thought to be important in providing energy for cells, in particular in the CNS and muscle where this protein is mainly expressed. Defects in the AGC1 gene cause AGC1 deficiency, an infantile encephalopathy with delayed myelination and reduced brain N-acetylaspartate (NAA) levels, the precursor of myelin synthesis in the CNS. Here, we show that undifferentiated Neuro2A cells with down-regulated AGC1 display a significant proliferation deficit associated with reduced mitochondrial respiration, and are unable to synthesize NAA properly. In the presence of high glutamine oxidation, cells with reduced AGC1 restore cell proliferation, although oxidative stress increases and NAA synthesis deficit persists. Our data suggest that the cellular energetic deficit due to AGC1 impairment is associated with inappropriate aspartate levels to support neuronal proliferation when glutamine is not used as metabolic substrate, and we propose that delayed myelination in AGC1 deficiency patients could be attributable, at least in part, to neuronal loss combined with lack of NAA synthesis occurring during the nervous system development.

Friedreich ataxia (FRDA) is an inherited recessive disorder caused by a deficiency in the mitochondrial protein frataxin. There is currently no effective treatment for FRDA available, especially for neurological deficits. In this study, we tested diazoxide, a drug commonly used as vasodilator in the treatment of acute hypertension, on cellular and animal models of FRDA. We first showed that diazoxide increases frataxin protein levels in FRDA lymphoblastoid cell lines, via the mTOR pathway. We then explored the potential therapeutic effect of diazoxide in frataxin-deficient transgenic YG8sR mice and we found that prolonged oral administration of 3mpk/d diazoxide was found to be safe, but produced variable effects concerning efficacy. YG8sR mice showed improved beam walk coordination abilities and footprint stride patterns, but a generally reduced locomotor activity. Moreover, they showed significantly increased frataxin expression, improved aconitase activity and decreased protein oxidation in cerebellum and brain mitochondrial tissue extracts. Further studies are needed before this drug should be considered for FRDA clinical trials.

The genome of Saccharomyces cerevisiae encodes 35 members of the mitochondrial carrier family (MCF) and 58 MCF members are coded by the genome of Arabidopsis thaliana, most of which have been functionally characterized. Here two members of this family, Ymc2p from S. cerevisiae and BOU from Arabidopsis, have been thoroughly characterized. These proteins were overproduced in bacteria and reconstituted into liposomes. Their transport properties and kinetic parameters demonstrate that Ymc2p and BOU transport glutamate, and to a much lesser extent L-homocysteinesulfinate, but not other amino acids and many other tested metabolites. Transport catalyzed by both carriers was saturable, inhibited by mercuric chloride and dependent on the proton gradient across the proteoliposomal membrane. The growth phenotype of S. cerevisiae cells lacking the genes ymc2 and agc1, which encodes the only other S. cerevisiae carrier capable to transport glutamate besides aspartate, was fully complemented by expressing Ymc2p, Agc1p or BOU. Mitochondrial extracts derived from ymc2Δagc1Δ cells, reconstituted into liposomes, exhibited no glutamate transport at variance with wild-type, ymc2Δ and agc1Δ cells, showing that S. cerevisiae cells grown in the presence of acetate do not contain additional mitochondrial transporters for glutamate besides Ymc2p and Agc1p. Furthermore, mitochondria isolated from wild-type, ymc2Δ and agc1Δ strains, but not from the double mutant ymc2Δagc1Δ strain, swell in isosmotic ammonium glutamate showing that glutamate is transported by Ymc2p and Agc1p together with a H+. It is proposed that the function of Ymc2p and BOU is to transport glutamate across the mitochondrial inner membrane and thereby play a role in intermediary metabolism, C1 metabolism and mitochondrial protein synthesis.

Monoamine oxidase (MAO), a mitochondrial enzyme that oxidizes biogenic amines generating hydrogen peroxide, is a major source of oxidative stress in cardiac injury. However, the molecular mechanisms underlying its overactivation in pathological conditions are still poorly characterized. Here, we investigated whether the enhanced MAO-dependent hydrogen peroxide production can be due to increased substrate availability using a metabolomic profiling method. We identified N1-methylhistamine -the main catabolite of histamine- as an important substrate fueling MAO in Langendorff mouse hearts, directly perfused with a buffer containing hydrogen peroxide or subjected to ischemia/reperfusion protocol. Indeed, when these hearts were pretreated with the MAO inhibitor pargyline we observed N1-methylhistamine accumulation along with reduced oxidative stress. Next, we showed that synaptic terminals are the major source of N1-methylhistamine. Indeed, in vivo sympathectomy caused a decrease of N1-methylhistamine levels, which was associated with a marked protection in post-ischemic reperfused hearts. As far as the mechanism is concerned, we demonstrate that exogenous histamine is transported into isolated cardiomyocytes and triggers a rise in the levels of reactive oxygen species (ROS). Once again, pargyline pretreatment induced intracellular accumulation of N1-methylhistamine along with decrease in ROS levels. These findings uncover a receptor-independent mechanism for histamine in cardiomyocytes. In summary, our study reveals a novel and important pathophysiological causative link between MAO activation and histamine availability during pathophysiological conditions such as oxidative stress/cardiac injury.

Background and Objective: Congenital myasthenic syndromes are rare inherited disorders characterized by fatigable weakness caused by malfunction of the neuromuscular junction. We performed whole exome sequencing to unravel the genetic aetiology in an English sib pair with clinical features suggestive of congenital myasthenia. Methods:We used homozygosity mapping and whole exome sequencing to identify the candidate gene variants. Mutant protein expression and function were assessed in vitro and a knockdown zebrafish model was generated to assess neuromuscular junction development. Results: We identified a novel homozygous missense mutation in the SLC25A1 gene, encoding the mitochondrial citrate carrier. Mutant SLC25A1 showed abnormal carrier function. SLC25A1 has recently been linked to a severe, often lethal clinical phenotype. Our patients had a milder phenotype presenting primarily as a neuromuscular (NMJ) junction defect. Of note, a previously reported patient with different compound heterozygous missense mutations of SLC25A1 has since been shown to suffer from a neuromuscular transmission defect. Using knockdown of SLC25A1 expression in zebrafish, we were able to mirror the human disease in terms of variable brain, eye and cardiac involvement. Importantly, we show clear abnormalities in the neuromuscular junction, regardless of the severity of the phenotype. Conclusions: Based on the axonal outgrowth defects seen in SLC25A1 knockdown zebrafish, we hypothesize that the neuromuscular junction impairment may be related to pre-synaptic nerve terminal abnormalities. Our findings highlight the complex machinery required to ensure efficient neuromuscular function, beyond the proteomes exclusive to the neuromuscular synapse.

L’agenesi del corpo calloso (ACC) è stata associata a diversi difetti della catena respiratoria mitocondriale e degli enzimi del ciclo dell’acido citrico. In questo studio sono riportati i dati relativi ad una paziente che mostrava un severo difetto del neurosviluppo, caratterizzato da ACC e ipoplasia del nervo ottico. Essa presentava elevati valori di 2-idrossiglutarato e degli intermedi del ciclo di Krebs nelle urine e un incremento delle specie reattive dell’ossigeno (ROS) ed una diminuzione del potenziale di membrana mitocondriale nei fibroblasti. Questi dati suggerivano un alterazione del metabolismo mitocondriale. Mediante il sequenziamento dell’intero esoma della paziente sono state individuate due varanti alleliche del gene SLC25A1, che codifica per il trasportatore mitocondriale del citrato. Tali varianti determinano le mutazioni amminoacidiche G130D e R282H. Le due varianti segregano nella famiglia ma risultano essere estremamente rare nei soggetti controllo, dove sono presenti sempre in eterozigosi con l’allele wild-type. Le analisi in silico della struttura del trasportatore mitocondriale hanno evidenziato che i residui amminoacidici mutati sono molto conservati all’inetrno della famiglia dei carrier mitocondriali (MCF) e potrebbero alterare la funzionalità della proteina. Questo dato predittivo è stato validato studiando la proteina ortologa di lievito sia da un punto di vista fenotipico che funzionale. Infatti il ceppo di lievito che presentava la proteina con le due mutazioni ha mostrato un difetto di crescita in condizioni di stress. Inoltre le proteine mutate ricostituite in vescicole fosfolipidiche mostravano una scarsa capacità di catalizzare il trasporto di citrato. In conclusione, i nostri dati hanno evidenziato un collegamento tra un difetto del gene del carrier mitocondriale del citrato e l’agenesi del corpo calloso.

The role of oxidative stress in neurodegeneration and the temporal relationship between oxidative stress and inflammation have been investigated in murine experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). To address these issues and identify specific targets of protein oxidation we have employed a proteomic approach coupled to quantitative determination of key metabolites in cortex tissues from mice with clinical signs of EAE. Our results show a decrease in endogenous antioxidant levels and a specific increase of glutamine synthetase (GS) oxidation with little or no evidence of immune/inflammatory cell invasion. The reduction in enzyme activity associated to GS oxidation leads to an increase of glutamate/ glutamine ratio and paralleled disease severity in EAE mice. The possibility that GS oxidation may cause neurodegeneration through glutamate excitotoxicity is supported by evidence of increasing cortical Ca2+ levels in cortex extracts from animals with greater disease severity. These findings indicate that oxidative stress occurs in brain areas that are not actively undergoing inflammation in EAE and that this can lead to a neurodegenerative process due to the susceptibility of GS to oxidative inactivation.

Friedreich ataxia (FRDA) is a common form of ataxia caused by decreased expression of the mitochondrial protein frataxin. Oxidative damage of mitochondria is thought to play a key role in the pathogenesis of the disease. Therefore, a possible therapeutic strategy should be directed to an antioxidant protection against mitochondrial damage. Indeed, treatment of FRDA patients with the antioxidant idebenone has been shown to improve neurological functions. The yeast frataxin knock-out model of the disease shows mitochondrial iron accumulation, iron-sulfur cluster defects and high sensitivity to oxidative stress. By flow cytometry analysis we studied reactive oxygen species (ROS) production of yeast frataxin mutant cells treated with two antioxidants, N-acetyl-L-cysteine and a mitochondrially-targeted analog of vitamin E, confirming that mitochondria are the main site of ROS production in this model. Furthermore we found a significant reduction of ROS production and a decrease in the mitochondrial mass in mutant cells treated with rapamycin, an inhibitor of TOR kinases, most likely due to autophagy of damaged mitochondria.

Mitochondrial diseases are a plethora of inherited neuromuscular disorders sharing defects in mitochondrial respiration, but largely different from one another for genetic basis and pathogenic mechanism. Whole exome sequencing was performed in a familiar trio (trio-WES) with a child affected by severe epileptic encephalopathy associated to respiratory complex I deficiency and mitochondrial DNA depletion in skeletal muscle. By trio-WES we identified biallelic mutations in SLC25A10, a nuclear gene encoding a member of the mitochondrial carrier family. Genetic and functional analyses conducted on patient fibroblasts showed that SLC25A10 mutations are associated to reduction in RNA quantity and aberrant RNA splicing, and to absence of SLC25A10 protein and its transporting function. The yeast SLC25A10 ortholog knockout strain showed defects in mitochondrial respiration and mitochondrial DNA content, similarly to what observed in the patient skeletal muscle, and growth susceptibility to oxidative stress. Albeit patient fibroblasts were depleted in the main antioxidant molecules NADPH and glutathione, transport assays demonstrated that SLC25A10 is unable to transport glutathione. Here we report the first recessive mutations of SLC25A10 associated to an inherited severe mitochondrial neurodegenerative disorder. We propose that SLC25A10 loss-of-function causes pathological disarrangements in respiratory-demanding conditions and oxidative stress vulnerability.

The human genome encodes 53 members of the solute carrier family 25 (SLC25), also called the mitochondrial carrier family, many of which have been shown to transport carboxylates, amino acids, nucleotides, and cofactors across the inner mitochondrial membrane, thereby connecting cytosolic and matrix functions. In this work, a member of this family, SLC25A29, previously reported to be a mitochondrial carnitine/acylcarnitine- or ornithine-like carrier, has been thoroughly characterized biochemically. The SLC25A29 gene was overexpressed in Escherichia coli, and the gene product was purified and reconstituted in phospholipid vesicles. Its transport properties and kinetic parameters demonstrate that SLC25A29 transports arginine, lysine, homoarginine, methylarginine and, to a much lesser extent, ornithine and histidine. Carnitine and acylcarnitines were not transported by SLC25A29. This carrier catalyzed substantial uniport besides a counter-exchange transport, exhibited a high transport affinity for arginine and lysine, and was saturable and inhibited by mercurial compounds and other inhibitors of mitochondrial carriers to various degrees. The main physiological role of SLC25A29 is to import basic amino acids into mitochondria for mitochondrial protein synthesis and amino acid degradation.