The hyperornithinemia-hyperammonemia-homocitrullinuria syndrome is a rare autosomal recessive disorder caused by the functional deficiency of the mitochondrial ornithine transporter 1 (ORC1). ORC1 is encoded by the SLC25A15 gene and catalyzes the transport of cytosolic ornithine into mitochondria in exchange for citrulline. Although the age of onset and the severity of the symptoms vary widely, the disease usually manifests in early infancy. The typical clinical features include protein intolerance, lethargy, episodic confusion, cerebellar ataxia, seizures and mental retardation. In this study, we identified a novel p.Ala15Val (c.44C>T) mutation by genomic DNA sequencing in a Turkish child presenting severe tantrum, confusion, gait disturbances and loss of speech abilities in addition to hyperornithinemia, hyperammonemia and homocitrullinuria. One hundred Turkish control chromosomes did not possess this variant. The functional effect of the novel mutation was assessed by both complementation of the yeast ORT1 null mutant and transport assays. Our study demonstrates that the A15V mutation dramatically interferes with the transport properties of ORC1 since it was shown to inhibit ornithine transport nearly completely.

CoA is an essential cofactor that holds a central role in cell metabolism. Although its biosynthetic pathway is conserved across the three domains of life, the subcellular localization of the eukaryotic biosynthetic enzymes and the mechanism behind the cytosolic and mitochondrial CoA pools compartmentalization are still under debate. In humans, the transport of CoA across the inner mitochondrial membrane has been ascribed to two related genes, SLC25A16 and SLC25A42 whereas in D. melanogaster genome only one gene is present, CG4241, phylogenetically closer to SLC25A42. CG4241 encodes two alternatively spliced isoforms, dPCoAC-A and dPCoAC-B. Both isoforms were expressed in Escherichia coli, but only dPCoAC-A was successfully reconstituted into liposomes, where transported dPCoA and, to a lesser extent, ADP and dADP but not CoA, which was a powerful competitive inhibitor. The expression of both isoforms in a Saccharomyces cerevisiae strain lacking the endogenous putative mitochondrial CoA carrier restored the growth on respiratory carbon sources and the mitochondrial levels of CoA. The results reported here and the proposed subcellular localization of some of the enzymes of the fruit fly CoA biosynthetic pathway, suggest that dPCoA may be synthesized and phosphorylated to CoA in the matrix, but it can also be transported by dPCoAC to the cytosol, where it may be phosphorylated to CoA by the monofunctional dPCoA kinase. Thus, dPCoAC may connect the cytosolic and mitochondrial reactions of the CoA biosynthetic pathway without allowing the two CoA pools to get in contact.

The present invention concerns a pharmaceutical preparation for the treatment of Friedreich’s ataxia and for the treatment or prevention of pathologies related thereto. Particularly, the present invention concerns the use of diazoxide or 7-chloro-3-methyl-4H-1,2,4 benzothiadiazine 1,1-dioxide, in combination with glucose and/or leucine, for the treatment of Friedreich’s ataxia (FRDA) and for the treatment or prevention of pathologies related thereto.

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.

S-adenosylmethionine (SAM) is the predominant methyl group donor and has a large spectrum of target substrates. As such, it is essential for nearly all biological methylation reactions. SAM is synthesized by methionine adenosyltransferase from methionine and ATP in the cytoplasm and subsequently distributed throughout the different cellular compartments, including mitochondria, where methylation is mostly required for nucleic-acid modifications and respiratory-chain function. We report a syndrome in three families affected by reduced intra-mitochondrial methylation caused by recessive mutations in the gene encoding the only known mitochondrial SAM transporter, SLC25A26. Clinical findings ranged from neonatal mortality resulting from respiratory insufficiency and hydrops to childhood acute episodes of cardiopulmonary failure and slowly progressive muscle weakness. We show that SLC25A26 mutations cause various mitochondrial defects, including those affecting RNA stability, protein modification, mitochondrial translation, and the biosynthesis of CoQ10 and lipoic acid.

The Mitochondrial Carrier Family (MCF) is a signature group of integral membrane proteins that transport metabolites across the mitochondrial inner membrane in eukaryotes. MCF proteins are characterized by six transmembrane segments that assemble to form a highly-selective channel for metabolite transport. We discovered a novel MCF member, termed Legionella nucleotide carrier Protein (LncP), encoded in the genome of Legionella pneumophila, the causative agent of Legionnaire's disease. LncP was secreted via the bacterial Dot/Icm type IV secretion system into macrophages and assembled in the mitochondrial inner membrane. In a yeast cellular system, LncP induced a dominant-negative phenotype that was rescued by deleting an endogenous ATP carrier. Substrate transport studies on purified LncP reconstituted in liposomes revealed that it catalyzes unidirectional transport and exchange of ATP transport across membranes, thereby supporting a role for LncP as an ATP transporter. A hidden Markov model revealed further MCF proteins in the intracellular pathogens, Legionella longbeachae and Neorickettsia sennetsu, thereby challenging the notion that MCF proteins exist exclusively in eukaryotic organisms.

The Lpp2981 gene from Legionella pneumophila, the causative agent of Legionnaire's disease, was cloned into the pMWT7 plasmid. The construct was used to express this gene in Escherichia coli. Five different bacterial strains were tested to overexpress the gene but without success. Sequence analysis revealed a cluster of four rare codons near the 5'-end of the gene. These codons were replaced with those commonly used in E. coli. The mutated Lpp2981 gene was successfully expressed in all the E. coli strains tested. The expressed protein (with an apparent molecular mass of 30 kDa) was collected in the insoluble fraction of the cell lysate, purified as inclusion bodies and functionally reconstituted into liposomes. The highest level of overexpression was obtained in E. coli C0214 after 6 h of induction with isopropyl-β-D-thiogalactopyranoside at 37 °C, yielding 74 mg of purified protein per liter of culture. We conclude that the clustering of rare codons at the 5'-end of the open-reading frame is a critical factor for the heterologous expression of Lpp2981 in E. coli.

Mitophagy is an essential process that maintains mitochondrial quality and number, thus limiting cellular degeneration. Along with apoptosis, mitophagy participates in cellular fate decisions by eliminating damaged mitochondria. A variety of mitochondrial parameters, such as structure, membrane potential and reactive oxygen species, are key determinants in triggering the mitophagic machinery. These parameters are also important regulators of the mitochondrial capacity for calcium (Ca2+) uptake. Rapid Ca2+ accumulation in the mitochondrial matrix allows for prompt stimulation of the organelle. This process requires a close morphofunctional coupling between mitochondria and the main intracellular Ca2+ stores. In mitophagy, the role of Ca2+ remains obscure. What role does mitochondrial Ca2+ play in metabolic sensing or in mitochondrial remodeling? Is endoplasmic reticulum (ER)-Ca2+ crosstalk involved? These are some of the questions that we address in this review.

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.

ARF-like 2 (ARL2) is a member of the ARF family and RAS superfamily of regulatory GTPases, predicted to be present in the last eukaryotic common ancestor, and essential in a number of model genetic systems. Though best studied as a regulator of tubulin folding, we previously demonstrated that ARL2 partially localizes to mitochondria. Here, we show that ARL2 is essential to a number of mitochondrial functions, including mitochondrial morphology, motility, and maintenance of ATP levels. We compare phenotypes resulting from ARL2 depletion and expression of dominant negative mutants and use these to demonstrate that the mitochondrial roles of ARL2 are distinct from its roles in tubulin folding. Testing of current models for ARL2 actions at mitochondria failed to support them. Rather, we found that knockdown of the ARL2 GTPase activating protein (GAP) ELMOD2 phenocopies two of three phenotypes of ARL2 siRNA, making it a likely effector for these actions. These results add new layers of complexity to ARL2 signaling, highlighting the need to deconvolve these different cell functions. We hypothesize that ARL2 plays essential roles inside mitochondria along with other cellular functions, at least in part to provide coupling of regulation between these essential cell processes.

The human SLC25A42 protein, ortholog of mitochondrial carrier Leu5p of S. cerevisiae, transports Coenzyme A and Adenosine 3’,5’-diphosphate G. Fiermonte, E. Paradies, S. Todisco, C.M.T. Marobbio, M.A Di Noia, and F. Palmieri Department of Pharmaco-Biology, Laboratory and Molecular Biology, University of Bari, Bari, Italy The essential cofactor Coenzyme A (CoA) is required in many intra-mitochondrial metabolic pathways. The CoA is synthesized outside the mitochondrial matrix, therefore must be transported into mitochondria. In S. cerevisiae, the mitochondrial carrier Leu5p is involved in the accumulation of CoA in the mitochondrial matrix. In fact, deletion of LEU5 (leu5) causes a reduction of mitochondrial coenzyme A (CoA) levels and growth defect on YP supplemented with glycerol or other non fermentative carbon sources. The closest relatives of Leu5p in human are SLC25A16 (37% identity) and SLC25A42 (31% identity). In this study we provide direct evidence that SLC25A42 is a novel transporter of CoA. SLC25A42 is localized in the mitochondrial inner membrane and is highly expressed in virtually all tissues. This protein was overexpressed in Escherichia coli, purified, reconstituted in phospholipid vesicles, and shown to transport CoA, dephospho-CoA, Adenosine 3’,5’-diphosphate (PAP), and (deoxy)adenine nucleotides with high specificity and by a counter-exchange mechanism. The expression of SLC25A42 protein in LEU5 cells fully restores the phenotype of the LEU5 strain, indicating that the main function of both proteins is probably to catalyze the entry of CoA into mitochondria in exchange for adenine nucleotides and PAP.

Mitochondrial carriers are a superfamily of transport proteins that, with a few exceptions, are found in the inner membranes of mitochondria. They shuttle metabolites, nucleotides and cofactors through this membrane, and thereby connect and/or regulate cytoplasm and matrix functions. Although several members of this family have been characterized, the functions of many others still remains unknown. For instance, the carrier responsible for coenzyme A (COA) transport into mitochondria has not yet been identified. CoA is an essential cytosolic-synthesized cofactor required in many intra-mitochondrial pathways, such as Krebs cycle, fatty acid β-oxidation and fatty acid synthesis (for the activation of mitochondrial ACP). In this work, we have identified SLC25A42 as the human gene responsible for the transport of CoA into mitochondria. The protein encoded by SLC25A42 is localized in the inner mitochondrial membrane and is ubiquitously expressed, aIthough at different levels. Its functional characterization has been carried out on bacterially expressed protein, upon its purification and reconstitution into phospholipid vesicles. In the reconstituted system the recombinant protein, exhibited high transport affinity for CoA, dephospho-CoA, ADP and adenosine 3',5'-diphosphate (PAP). The main physiological role of SLC25A42 is to import CoA into mitochondria in exchange for intra-mitochondrial adenine nucleotides and/or PAP. The export of PAP out of the mitochondria is crucial, since the catabolism of this cytotoxic molecule, produced in the organelles by the transfer of the 4'-phosphopantetheine prosthetic group of CoA to ACP, takes place in the cytosol. This is the first time that a mitochondrial carrier for CoA and PAP has been identified and characterized biochemically.

La presente invenzione ha per oggetto una preparazione farmaceutica per il trattamento dell’atassia di Friedreich e per il trattamento o prevenzione delle patologie ad essa correlate. In particolare la presente invenzione ha per oggetto l’uso di diazossido o 7-cloro-3-metil-4H-1,2,4 benzotiazidina 1,1-diossido, in combinazione con glucosio e/o leucina, per il trattamento dell’atassia di Friedreich (FRDA) e per il trattamento o prevenzione delle patologie ad essa correlate.