Pathogenesis and Pathophysiology of Citrin (a Mitochondrial Aspartate Glutamate Carrier) Deficiency

Adult-onset type II citrullinemia (CTLN2), characterized by a liver-specific deficiency of urea cycle enzyme, argininosuccinate synthetase, is caused by mutations in SLC25A13 that encodes a calcium binding mitochondrial solute carrier protein, citrin. Citrin deficiency causes not only CTLN2 but also neonatal intrahepatic cholestasis caused by citrin deficiency at neonatal period. Moreover citrin and its isoform aralar were found to be aspartate glutamate carrier. From the viewpoint of the metabolic functions of citrin as aspartate glutamate carrier in urea synthesis and NADH shuttle, symptoms of CTLN2 and neonatal intrahepatic cholestasis caused by citrin deficiency are analyzed.     

Missense SLC25A38 variations play an important role in autosomal recessive inherited sideroblastic anemia

Background

Congenital sideroblastic anemias are rare disorders with several genetic causes; they are characterized by erythroblast mitochondrial iron overload, differ greatly in severity and some occur within a syndrome. The most common cause of non-syndromic, microcytic sideroblastic anemia is a defect in the X-linked 5-aminolevulinate synthase 2 gene but this is not always present. Recently, variations in the gene for the mitochondrial carrier SLC25A38 were reported to cause a non-syndromic, severe type of autosomal-recessive sideroblastic anemia. Further evaluation of the importance of this gene was required to estimate the proportion of patients affected and to gain further insight into the range and types of variations involved.

Design and Methods

In three European diagnostic laboratories sequence analysis of SLC25A38 was performed on DNA from patients affected by congenital sideroblastic anemia of a non-syndromic nature not caused by variations in the 5-aminolevulinate synthase 2 gene.

Results

Eleven patients whose ancestral origins spread across several continents were homozygous or compound heterozygous for ten different SLC25A38 variations causing premature termination of translation (p.Arg117X, p.Tyr109LeufsX43), predicted splicing alteration (c.625G>C; p.Asp209His) or missense substitution (p.Gln56Lys, p.Arg134Cys, p.Ile147Asn, p.Arg187Gln, p.Pro190Arg, p.Gly228Val, p.Arg278Gly). Only three of these variations have been described previously (p.Arg117X, p.Tyr109LeufsX43 and p.Asp209His). All new variants reported here are missense and affect conserved amino acids. Structure modeling suggests that these variants may influence different aspects of transport as described for mutations in other mitochondrial carrier disorders.

Conclusions

Mutations in the SLC25A38 gene cause severe, non-syndromic, microcytic/hypochromic sideroblastic anemia in many populations. Missense mutations are shown to be of importance as are mutations that affect protein production. Further investigation of these mutations should shed light on structure-function relationships in this protein.     

Metabolic aspects of the calorigenic effect of thyroid hormone in mammals

The increased BMR in hyperthyroidism may be accounted for by the use of chemical energy for metabolic processes and work. Major contributors are the heart work and futile cycling of FFA into triglyceride in adipose tissue, whereas gluconeogenesis in liver and the maintenance of sodium and potassium concentration gradients across the plasma membranes are unlikely to play any significant role. Information on the use of energy for protein turnover and urea production is lacking. The rate of oxygen uptake is not increased in the brain, spleen and testis. The main metabolic fuel seems to be free fatty acid.
The mechanism which enables the hyperthyroid tissue to maintain normal concentrations of ATP, ADP and inorganic phosphate, (P_i) despite an increased turnover of energy-rich phosphates, is not fully elucidated. However, ultimately the increased rate of oxygen uptake in hyperthyroidism seems to rely upon an increased capacity for the transport of cytosolic ADP and P_i into mitochondria. The transport capacity is increased by an increased area of the mitochondrial membrane per g tissue and by a change in the kinetics of translocation of substrates for oxidative phosphorylation. Other transport processes across the mitochondrial membrane are also changed by hyperthyroidism, e.g. long chain fatty acid transport via carnitine acyl transferase, and oxaloacetate transport via substrate shuttles.     

The human mitochondrial deoxynucleotide carrier and its role in the toxicity of nucleoside antivirals

The synthesis of DNA in mitochondria requires the uptake of deoxynucleotides into the matrix of the organelle. We have characterized a human cDNA encoding a member of the family of mitochondrial carriers. The protein has been overexpressed in bacteria and reconstituted into phospholipid vesicles where it catalyzed the transport of all four deoxy (d) NDPs, and, less efficiently, the corresponding dNTPs, in exchange for dNDPs, ADP, or ATP. It did not transport dNMPs, NMPs, deoxynucleosides, nucleosides, purines, or pyrimidines. The physiological role of this deoxynucleotide carrier is probably to supply deoxynucleotides to the mitochondrial matrix for conversion to triphosphates and incorporation into mitochondrial DNA. The protein is expressed in all human tissues that were examined except for placenta, in accord with such a central role. The deoxynucleotide carrier also transports dideoxynucleotides efficiently. It is likely to be medically important by providing the means of uptake into mitochondria of nucleoside analogs, leading to the mitochondrial impairment that underlies the toxic side effects of such drugs in the treatment of viral illnesses, including AIDS, and in cancer therapy.     

31P NMR studies of control of mitochondrial function in phosphofructokinase-deficient human skeletal muscle.

Metabolic control of mitochondrial respiratory activity by Pi and ADP has been evaluated by 31P NMR measurements of the levels of Pi in normal exercising human skeletal tissues in the resting-active-resting transition and, in this contribution, in the phosphofructokinase (PFK)-deficient leg. The latter studies show near constancy of Pi in the recovery from maximal exercise of the leg, with large changes of sugar phosphate (SP) complementary to the changes of phosphocreatine (PCr). The PFK deficiency permits observation of PCr resynthesis in postexercise recovery under conditions of nearly constant Pi and ATP--a phenomenon not evident in normal exercising muscle. The constancy of free Pi is inconsistent with its role in control of mitochondrial activity, leaving ADP as a key metabolic control element. These results help clarify previous controversies on the nature of control of metabolic activity of mitochondria and extend the idea of ADP control of mitochondrial metabolic states in vivo and, in addition, provide an appropriate exercise protocol for the evaluation of a genetic deficiency affecting mitochondrial metabolism.     

Knockout of Slc25a19 causes mitochondrial thiamine pyrophosphate depletion, embryonic lethality, CNS malformations, and anemia

SLC25A19 mutations cause Amish lethal microcephaly (MCPHA), which markedly retards brain development and leads to alpha-ketoglutaric aciduria. Previous data suggested that SLC25A19, also called DNC, is a mitochondrial deoxyribonucleotide transporter. We generated a knockout mouse model of Slc25a19. These animals had 100% prenatal lethality by embryonic day 12. Affected embryos at embryonic day 10.5 have a neural-tube closure defect with ruffling of the neural fold ridges, a yolk sac erythropoietic failure, and elevated alpha-ketoglutarate in the amniotic fluid. We found that these animals have normal mitochondrial ribo- and deoxyribonucleoside triphosphate levels, suggesting that transport of these molecules is not the primary role of SLC25A19. We identified thiamine pyrophosphate (ThPP) transport as a candidate function of SLC25A19 through homology searching and confirmed it by using transport assays of the recombinant reconstituted protein. The mitochondria of Slc25a19(-/-) and MCPHA cells have undetectable and markedly reduced ThPP content, respectively. The reduction of ThPP levels causes dysfunction of the alpha-ketoglutarate dehydrogenase complex, which explains the high levels of this organic acid in MCPHA and suggests that mitochondrial ThPP transport is important for CNS development.     

Hereditary sideroblastic anemia: pathophysiology and gene mutations

Sideroblastic anemia is characterized by anemia with the emergence of ring sideroblasts in the bone marrow. Ring sideroblasts are erythroblasts characterized by iron accumulation in perinuclear mitochondria due to impaired iron utilization. There are two forms of sideroblastic anemia, i.e., inherited and acquired sideroblastic anemia. Inherited sideroblastic anemia is a rare and heterogeneous disease caused by mutations of genes involved in heme biosynthesis, iron–sulfur (Fe–S) cluster biogenesis, or Fe–S cluster transport, and mitochondrial metabolism. The most common inherited sideroblastic anemia is X-linked sideroblastic anemia (XLSA) caused by mutations of the erythroid-specific δ-aminolevulinate synthase gene (ALAS2), which is the first enzyme of heme biosynthesis in erythroid cells. Sideroblastic anemia due to SLC25A38 gene mutations, which is a mitochondrial transporter, is the next most common inherited sideroblastic anemia. Other forms of inherited sideroblastic anemia are very rare, and accompanied by impaired function of organs other than hematopoietic tissue, such as the nervous system, muscle, or exocrine glands due to impaired mitochondrial metabolism. Moreover, there are still significant numbers of cases with genetically undefined inherited sideroblastic anemia. Molecular analysis of these cases will contribute not only to the development of effective treatment, but also to the understanding of mitochondrial iron metabolism.     

Yeast mitochondrial ADP/ATP carriers are monomeric in detergents

Mitochondrial carriers are believed widely to be homodimers both in the inner membrane of the organelle and in detergents. The dimensions and molecular masses of the detergent and protein-detergent micelles were measured for yeast ADP/ATP carriers in a range of different detergents. The radius of the carrier at the midpoint of the membrane, its average radius, its Stokes' radius, its molecular mass, and its excluded volume were determined. These parameters are consistent with the known structural model of the bovine ADP/ATP carrier and they demonstrate that the yeast mitochondrial ADP/ATP carriers are monomeric in detergents. Therefore, models of substrate transport have to be considered in which the carrier operates as a monomer rather than as a dimer.     

The yeast mitochondrial ADP/ATP carrier functions as a monomer in mitochondrial membranes

Mitochondrial carriers are believed widely to be dimers both in structure and function. However, the structural fold is a barrel of six transmembrane alpha-helices without an obvious dimerisation interface. Here, we show by negative dominance studies that the yeast mitochondrial ADP/ATP carrier 2 from Saccharomyces cerevisiae (AAC2) is functional as a monomer in the mitochondrial membrane. Adenine nucleotide transport by wild-type AAC2 is inhibited by the sulfhydryl reagent 2-sulfonatoethyl-methanethiosulfonate (MTSES), whereas the activity of a mutant AAC2, devoid of cysteines, is unaffected. Wild-type and cysteine-less AAC2 were coexpressed in different molar ratios in yeast mitochondrial membranes. After addition of MTSES the residual transport activity correlated linearly with the fraction of cysteine-less carrier present in the membranes, and so the two versions functioned independently of each other. Also, the cysteine-less and wild-type carriers were purified separately, mixed in defined ratios and reconstituted into liposomes. Again, the residual transport activity in the presence of MTSES depended linearly on the amount of cysteine-less carrier. Thus, the entire transport cycle for ADP/ATP exchange is carried out by the monomer.     

Mitochondrial carriers in the cytoplasmic state have a common substrate binding site

Mitochondrial carriers link biochemical pathways in the cytosol and mitochondrial matrix by transporting substrates across the inner mitochondrial membrane. Substrate recognition is specific for each carrier, but sequence similarities suggest the carriers have similar structures and mechanisms of substrate translocation. By considering conservation of amino acids, distance and chemical constraints, and by modeling family members on the known structure of the ADP/ATP translocase, we have identified a common substrate binding site. It explains substrate selectivity and proton coupling and provides a mechanistic link to carrier opening by substrate-induced perturbation of the salt bridges that seal the pathway to and from the mitochondrial matrix. It enables the substrate specificity of uncharacterized mitochondrial carriers to be predicted.     

Substrate specificity of the TIM22 mitochondrial import pathway revealed with small molecule inhibitor of protein translocation

The TIM22 protein import pathway mediates the import of membrane proteins into the mitochondrial inner membrane and consists of two intermembrane space chaperone complexes, the Tim9-Tim10 and Tim8-Tim13 complexes. To facilitate mechanistic studies, we developed a chemical-genetic approach to identify small molecule agonists that caused lethality to a tim10-1 yeast mutant at the permissive temperature. One molecule, MitoBloCK-1, attenuated the import of the carrier proteins including the ADP/ATP and phosphate carriers, but not proteins that used the TIM23 or the Mia40/Erv1 translocation pathways. MitoBloCK-1 impeded binding of the Tim9-Tim10 complex to the substrate during an early stage of translocation, when the substrate was crossing the outer membrane. As a probe to determine the substrate specificity of the small Tim proteins, MitoBloCK-1 impaired the import of Tim22 and Tafazzin, but not Tim23, indicating that the Tim9-Tim10 complex mediates the import of a subset of inner membrane proteins. MitoBloCK-1 also inhibited growth of mammalian cells and import of the ADP/ATP carrier, but not TIM23 substrates, confirming that MitoBloCK-1 can be used to understand mammalian mitochondrial import and dysfunction linked to inherited human disease. Our approach of screening chemical libraries for compounds causing synthetic genetic lethality to identify inhibitors of mitochondrial protein translocation in yeast validates the generation of new probes to facilitate mechanistic studies in yeast and mammalian mitochondria.     

Sensitivity of the 2-oxoglutarate carrier to alcohol intake contributes to mitochondrial glutathione depletion

The mitochondrial pool of reduced glutathione (mGSH) is known to play a protective role against liver injury and cytokine-mediated cell death. However, the identification of the mitochondrial carriers involved in its transport in hepatocellular mitochondria remains unestablished. In this study, we show that the functional expression of the 2-oxoglutarate carrier from HepG2 cells in mitochondria from Xenopus laevis oocytes conferred a reduced glutathione (GSH) transport activity that was inhibited by phenylsuccinate, a specific inhibitor of the carrier. In addition, the mitochondrial transport of GSH and 2-oxoglutarate in isolated mitochondria from rat liver exhibited mutual competition and sensitivity to glutamate and phenylsuccinate. Interestingly, the kinetics of 2-oxoglutarate transport in rat liver mitochondria displayed a single Michaelis-Menten component with a Michaelis constant of 3.1 +/- 0.3 mmol/L and maximum velocity of 1.9 +/- 0.1 nmol/mg protein/25 seconds. Furthermore, the initial rate of 2-oxoglutarate was reduced in mitochondria from alcohol-fed rat livers, an effect that was not accompanied by an alcohol-induced decrease in the 2-oxoglutarate messenger RNA levels but rather by changes in mitochondrial membrane dynamics induced by alcohol. The fluidization of mitochondria by the fluidizing agent 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) (A(2)C) restored the initial transport rate of both GSH and 2-oxoglutarate. Finally, these changes were reproduced in normal liver mitochondria enriched in cholesterol where the fluidization of cholesterol-enriched mitochondria with A(2)C restored the order membrane parameter and the mitochondrial 2-oxoglutarate uptake. In conclusion, these findings provide unequivocal evidence for 2-oxoglutarate as a GSH carrier and its sensitivity to membrane dynamics perturbation contributes in part to the alcohol-induced mGSH depletion.     

Ion Transport Underlying Metabolically Controlled Volume Changes of Isolated Mitochondria

An earlier study has indicated that the swelling of isolated mitochondria induced by inorganic phosphate (P(i)) can be accounted for largely or completely by the accumulation of ions. In the present study, a similar uptake was shown to be induced by a wider range of P(i) concentrations. Addition of ADP, KCN, or 2,4-dinitrophenol initiates a shrinkage which can be accounted for by the efflux of ions. The results are consistent with the explanations that (a) P(i) induces a transport of ions and a concomitant osmotic swelling, and (b) the addition of substances which compete or interfere with the energy available for transport results in ion efflux and a corresponding mitochondrial shrinkage. The results are not consistent with proposals that the changes in the light scattered by mitochondrial suspensions with alterations in metabolic states reflect a mechanochemical coupling phenomenon.     

H+/ATP ratio during ATP hydrolysis by mitochondria: modification of the chemiosmotic theory.

The stoichiometry of H+ ejection by mitochondria during hydrolysis of a small pulse of ATP (the H+/ATP ratio) has been reexamined in the light of our recent observation that the stoichiometry of H+ ejection during mitochondrial electron transport (the H+/site ratio) was previously underestimated. We show that earlier estimates of the H+/ATP ratio in intact mitochondria were based upon an invalid correction for scaler H+ production and describe a modified method for determination of this ratio which utilizes mersalyl or N-ethylmaleimide to prevent complicating transmembrane movements of phosphate and H+. This method gives a value for the H+/ATP ratio of 2.0 without the need for questionable corrections, compared with a value of 3.0 for the H+/site ratio also obtained by pulse methods. A modified version of the chemiosmotic theory is presented, in which 3 H+ are ejected per pair of electrons traversing each energy-conserving site of the respiratory chain. Of these, 2 H+ return to the matrix through the ATPase to form ATP from ADP and phosphate, and 1 H+ returns through the combined action of the phosphate and adenine nucleotide exchange carriers of the inner membrane to allow the energy-requiring influx of Pi and ADP3- and efflux of ATP4-. Thus, up to one-third of the energy input into synthesis of extramitochondrial ATP may be required for transport work. Since other methods suggest that the H+/site significantly exceeds 3.0, an alternative possibility is that 4 h+ are ejected per site, followed by return of 3 H+ through the ATPase and 1 H+ through the operation of the proton-coupled membrane transport systems.     

Mutations in <Emphasis Type="Italic">SLC25A22</Emphasis>: hyperprolinaemia,vacuolated fibroblasts and presentation with developmental delay

Mutations in SLC25A22 are known to cause neonatal epileptic encephalopathy and migrating partial seizures in infancy. Using whole exome sequencing we identified four novel SLC25A22 mutations in six children from three families. Five patients presented clinical features similar to those in the literature including hypotonia, refractory neonatal-onset seizures and developmental delay. However, the sixth patients presented atypically with isolated developmental delay, developing late-onset (absence) seizures only at 7 years of age. Abnormal metabolite levels have not been documented in the nine patients described previously. One patient in our series was referred to the metabolic clinic because of persistent hyperprolinaemia and another three had raised plasma proline when tested. Analysis of the post-prandial plasma amino acid response in one patient showed abnormally high concentrations of several amino acids. This suggested that, in the fed state, when amino acids are the preferred fuel for the liver, trans-deamination of amino acids requires transportation of glutamate into liver mitochondria by SLC25A22 for deamination by glutamate dehydrogenase; SLC25A22 is an important mitochondrial glutamate transporter in liver as well as in brain. Electron microscopy of patient fibroblasts demonstrated widespread vacuolation containing neutral and phospho-lipids as demonstrated by Oil Red O and Sudan Black tinctorial staining; this might be explained by impaired activity of the proline/pyrroline-5-carboxylate (P5C) shuttle if SLC25A22 transports pyrroline-5-carboxylate/glutamate-γ-semialdehyde as well as glutamate.     

Role of Phosphate and Other Proton-Donating Anions in Respiration-Coupled Transport of Ca2+ by Mitochondria

Measurements of extra oxygen consumption, (45)Ca(2+) uptake, and the osmotic expansion of the matrix compartment show that not all permeant anions are capable of supporting and accompanying the energy-dependent transport of Ca(2+) from the medium into the matrix in respiring rat-liver mitochondria. Phosphate, arsenate, acetate, butyrate, beta-hydroxybutyrate, lactate, and bicarbonate + CO(2) supported Ca(2+) uptake, whereas the permeant anions, nitrate, thiocyanate, chlorate, and perchlorate, did not. The active anions share a common denominator, the potential ability to donate a proton to the mitochondrial matrix; the inactive anions lack this capacity. Phosphate and the other active permeant anions move into the matrix in response to the alkaline-inside electrochemical gradient of protons generated across the mitochondrial membrane by electron transport, thus forming a negative-inside anion gradient. It is postulated that the latter gradient is the immediate "pulling" force for the influx of Ca(2+) on the electrogenic Ca(2+) carrier in respiring mitochondria under intracellular conditions. Since mitochondria in the cell are normally exposed to an excess of phosphate (and the bicarbonate-CO(2) system), particularly in state 4, inward transport of these proton-yielding anions probably precedes and is necessary for inward transport of Ca(2+) and other cations under biological conditions. These observations indicate that a negative-inside gradient of phosphate generated by electron transport is a common step and provides the immediate motive power not only for (a) the inward transport of dicarboxylates and tricarboxylates and (b) the energy-dependent exchange of external ADP(3-) for internal ATP(4-) during oxidative phosphorylation, as has already been established, but also for (c) the inward transport of Ca(2+), K(+), and other cations.     

Unique presentation of cutis laxa with Leigh-like syndrome due to <Emphasis Type="Italic">ECHS1</Emphasis> deficiency

Clinical finding of cutis laxa, characterized by wrinkled, redundant, sagging, nonelastic skin, is of growing significance due to its occurrence in several different inborn errors of metabolism (IEM). Metabolic cutis laxa results from Menkes syndrome, caused by a defect in the ATPase copper transporting alpha (ATP7A) gene; congenital disorders of glycosylation due to mutations in subunit 7 of the component of oligomeric Golgi (COG7)–congenital disorders of glycosylation (CDG) complex; combined disorder of N- and O-linked glycosylation, due to mutations in ATPase H+ transporting V0 subunit a2 (ATP6VOA2) gene; pyrroline-5-carboxylate reductase 1 deficiency; pyrroline-5-carboxylate synthase deficiency; macrocephaly, alopecia, cutis laxa, and scoliosis (MACS) syndrome, due to Ras and Rab interactor 2 (RIN2) mutations; transaldolase deficiency caused by mutations in the transaldolase 1 (TALDO1) gene; Gerodermia osteodysplastica due to mutations in the golgin, RAB6-interacting (GORAB or SCYL1BP1) gene; and mitogen-activated pathway (MAP) kinase defects, caused by mutations in several genes [protein tyrosine phosphatase, non-receptor-type 11 (PTPN11), RAF, NF, HRas proto-oncogene, GTPase (HRAS), B-Raf proto-oncogene, serine/threonine kinase (BRAF), MEK1/2, KRAS proto-oncogene, GTPase (KRAS), SOS Ras/Rho guanine nucleotide exchange factor 2 (SOS2), leucine rich repeat scaffold protein (SHOC2), NRAS proto-oncogene, GTPase (NRAS), and Raf-1 proto-oncogene, serine/threonine kinase (RAF1)], which regulate the Ras-MAPK cascade. Here, we further expand the list of inborn errors of metabolism associated with cutis laxa by describing the clinical presentation of a 17-month-old girl with Leigh-like syndrome due to enoyl coenzyme A hydratase, short chain, 1, mitochondria (ECHS1) deficiency, a mitochondrial matrix enzyme that catalyzes the second step of the beta-oxidation spiral of fatty acids and plays an important role in amino acid catabolism, particularly valine.     

Acquired "storage pool disease" of platelets associated with circulating antiplatelet antibodies

This study reports the first example of acquired “storage pool disease” in a patient with nephritis, polyarthralgia, chondritis, thrombophlebitis, Raynaud's phenomenon and circulating antiplatelet antibodies. Studies of platelet function showed a long bleeding time, decreased aggregation to collagen, thrombin and adenosine diphosphate (ADP), and depleted platelet stores of ADP and serotonin. These findings are similar to those found in a congenital disorder of platelets characterized by a deficiency of the storage pool of adenine nucleotides and serotonin. It is suggested that in this patient the circulating antibodies induced a release of the storage pools of adenine nucleotides and serotonin of the circulating platelets, leading to a state of defective hemostasis. The patient responded well to steroid therapy, and platelet function returned to normal concomitant with the disappearance of the antiplatelet antibodies. It is possible that “storage pool disease” with or without a thrombotic tendency may occur in other acquired disorders, especially the myeloproliferative disorders and those associated with circulating antibody.     

Autophagy induction extends lifespan and reduces lipid content in response to frataxin silencing in C. elegans

Severe mitochondria deficiency leads to a number of devastating degenerative disorders, yet, mild mitochondrial dysfunction in different species, including the nematode Caenorhabditis elegans, can have pro-longevity effects. This apparent paradox indicates that cellular adaptation to partial mitochondrial stress can induce beneficial responses, but how this is achieved is largely unknown. Complete absence of frataxin, the mitochondrial protein defective in patients with Friedreich's ataxia, is lethal in C. elegans, while its partial deficiency extends animal lifespan in a p53 dependent manner.