Differential expression of adenine nucleotide translocator isoforms in mammalian tissues and during muscle cell differentiation

Hypothermia preserves function and signaling for mitochondrial biogenesis during subsequent ischemia

Hypothermia is known to protect myocardium during ischemia, but its role in induction of a protective stress response before ischemia has not been evaluated. As cold incites stress responses in other tissues, including heat shock protein induction and signaling mitochondrial biogenesis, we postulated that hypothermia in perfused hearts would produce similar phenomena while reducing injury during subsequent ischemia. Studies were performed in isolated perfused rabbit hearts (n = 77): a control group (C) and a hypothermic group (H) subjected to decreasing infusate temperature from 37 to 31 degrees C over 20 min. Subsequent ischemia during cardioplegic arrest at 34 degrees C for 120 min was followed by reperfusion. At 15 min of reperfusion, recovery of left ventricular developed pressure (LVDP), maximum first derivative of left ventricular pressure (LV dP/dtmax), LV -dP/dtmax, and the product of heart rate and LVDP was significantly increased in H (P < 0.01) compared with C hearts. Ischemic contracture started later in H (97.5 +/- 3.6 min) than in C (67.3 +/- 3.3 min) hearts. Myocardial ATP preservation and repletion during ischemia and reperfusion were higher in H than in C hearts. mRNA levels of the nuclear-encoded mitochondrial proteins adenine nucleotide translocase isoform 1 (ANT1) and beta-F1-adenosine-triphosphatase (beta-F1-ATPase) normalized to 28S RNA decreased in C hearts but were preserved in H hearts after reperfusion. Inducible heat shock protein (HSP70-1) mRNA was elevated nearly 4-fold after ischemia in C hearts and 12-fold in H hearts. These data indicate that hypothermia preserves myocardial function and ATP stores during subsequent ischemia and reperfusion. Signaling for mitochondrial biogenesis indexed by ANT1 and beta-F1-ATPase mRNA levels is also preserved during a marked increase in HSP70-1 mRNA.

The mitochondrial ADP/ATP carrier: structural, physiological and pathological aspects

Under the conditions of oxidative phosphorylation, the mitochondrial ADP/ATP carrier catalyses the one to one exchange of cytosolic ADP against matrix ATP across the inner mitochondrial membrane. The ADP/ATP transport system can be blocked very specifically by two families of inhibitors: atractyloside (ATR) and carboxyatractyloside (CATR) on one hand, and bongkrekic acid (BA) and isobongkrekic acid (isoBA) on the other hand. It is well established that these inhibitors recognise two different conformations of the carrier protein, the CATR- and BA-conformations, which exhibit different chemical, immunochemical and enzymatic reactivities. The reversible transition of the ADP/ATP carrier between the two conformations was studied by fluorometric techniques. This transconversion, which is only triggered by transportable nucleotides, is probably the same as that which occurs during the functioning of ADP/ATP transport system. The fluorometric approach, using the tryptophanyl residues of the yeast carrier as intrinsic fluorescence probes, was combined to a mutagenesis approach to elucidate the ADP/ATP transport mechanism at the molecular level. Finally, recent reports that myopathies might result from defect in ADP/ATP transport led us to develop a method to quantify the carrier protein in muscular biopsies.

Muscle/heart isoform of mitochondrial adenine nucleotide translocase (ANT1) is transiently expressed during perinatal development in rat liver

A postnatal increase in the content of mitochondrial ANT in rat liver which is related to the maturation of mitochondrial function has previously been reported [Schönfeld et al., Biochim. Biophys Acta 1144 (1993) 353-358]. In order to define the contribution of the ANT isoforms to this postnatal increase we have studied the expression of ANT1 and ANT2 isoforms in the liver during this period. The results show that in contrast to adult liver, perinatal liver expressed the ANT1 isoform at the mRNA and protein level, and that during this period the expression of ANT1 increased to a similar extent as total ANT content. It is concluded that the postnatal increase in ANT is mainly due to the ANT1 isoform and therefore, a role for the ANT1 isoform in the postnatal maturation of mitochondrial respiration in rat liver is suggested.

Quantitative determinations of the steady state transcript levels of hexokinase isozymes and glucose transporter isoforms in normal rat tissues and the malignant tumor cell line AH130

The steady state transcript levels of the four hexokinase (HK) isozymes and four glucose transporter (GLUT) isoforms were determined quantitatively by Northern analysis of RNA samples from rat tissues using synthetic fragments of the RNAs encoding the HK isozymes and GLUT isoforms. Results showed that the levels of HK isozyme transcripts were low in rat tissues, the level of that most highly expressed, the type I isozyme (HKI), in the brain being 0.025% of the total poly(A)+ RNA. A good correlation was found between the reported HK activities and the total amounts of transcripts encoding all HK isozymes in various tissues, showing that the HK activities in tissues can be estimated from the total amount of transcripts encoding HK isozymes. The proposed associated expressions of HK isozymes and GLUT isoforms in particular tissues were confirmed at their transcript levels. The steady state transcript levels of type II HK and the type 1 GLUT isoform in the malignant tumor cell line AH130 were also determined quantitatively.

Thyroid hormone-induced expression of the ADP/ATP carrier and its effect on fatty acid-induced uncoupling of oxidative phosphorylation

Liver mitochondria from rats made hypothyroid by administration of 2-mercapto-1-methylimidazole were less sensitive to the uncoupling effect of myristic acid, as measured by the increase of resting state respiration, than mitochondria from euthyroid animals, whereas subsequent administration to the animals of triiodothyronine ('hyperthyroidism') resulted in an increased uncoupling action of myristate. 'Hyperthyroidism' also resulted in doubling of the carboxyatractyloside-sensitive portion of the myristate-stimulated respiration. Parallel to this was a twofold increase of the mitochondrial content of the ADP/ATP carrier protein and an over threefold increase of its activity. The uncoupling effect of phytanic acid was less sensitive to carboxyatractyloside and was increased in the hyperthyroid state to a smaller extent than in the case of myristate. These results provide further support to the thesis [Skulachev, V.P., FEBS Lett. 294 (1991) 158-162] that the ADP/ATP carrier is involved in the mechanism of the uncoupling effect of long-chain fatty acids.

Expression of adenine nucleotide translocator parallels maturation of respiratory control in heart in vivo

Changes in the relationship between myocardial high-energy phosphates and oxygen consumption in vivo occur during development, implying that the mode of respiratory control undergoes maturation. We hypothesized that these maturational changes in sheep heart are paralleled by alterations in the adenine nucleotide translocator (ANT), which are in turn related to changes in the expression of this gene. Increases in myocardial oxygen consumption (MVO2) were induced by epinephrine infusion in newborn (0-32 h, n = 6) and mature sheep (30-32 days, n = 6), and high-energy phosphates were monitored with 31P nuclear magnetic resonance. Western blot analyses for the ANT1 and the beta-subunit of F1-adenosinetriphosphatase (ATPase) were performed in these hearts and additional (n = 9 total per group) as well as in fetal hearts (130-132 days of gestation, n = 5). Northern blot analyses were performed to assess for changes in steady-state RNA transcripts for these two genes. Kinetic analyses for the 31P spectra data revealed that the ADP-MVO2 relationship for the newborns conformed to a Michaelis-Menten model but that the mature data did not conform to first- or second-order kinetic control of respiration through ANT. Maturation from fetal to mature was accompanied by a 2.5-fold increase in ANT protein (by Western blot), with no detectable change in beta-F1-ATPase. Northern blot data show that steady-state mRNA levels for ANT and beta-F1-ATPase increased approximately 2.5-fold from fetal to mature. These data indicate that 1) respiratory control pattern in the newborn is consistent with a kinetic type regulation through ANT, 2) maturational decreases in control through ANT are paralleled by specific increases in ANT content, and 3) regulation of these changes in ANT may be related to increases in steady-state transcript levels for its gene.

Tissue-specific transcription pattern of the adenine nucleotide translocase isoforms in humans

Three adenine nucleotide translocase isoforms (ANT1, ANT2 and ANT3) are coded by different genes. The relative amounts of the three ANT isoform mRNAs were determined in detail in various human tissues. ANT isoforms were co-expressed in all tested tissues revealing tissue-specific transcription patterns. The highest ANT1 mRNA proportions were found in terminally differentiated tissues like skeletal muscle, heart and brain, whereas ANT2 was mainly expressed in tissues capable of proliferation and regeneration as in the kidneys, spleen, liver, fibroblasts and lymphocytes. The ANT3 mRNA proportion was not prominently expressed in any of the tissues tested. In conclusion, tissue-specific expression of ANT isoforms is strongly related to the state of cellular differentiation.

A mouse model for mitochondrial myopathy and cardiomyopathy resulting from a deficiency in the heart/muscle isoform of the adenine nucleotide translocator

In an attempt to create an animal model of tissue-specific mitochondrial disease, we generated 'knockout' mice deficient in the heart/muscle isoform of the adenine nucleotide translocator (Ant1). Histological and ultrastructural examination of skeletal muscle from Ant1 null mutants revealed ragged-red muscle fibers and a dramatic proliferation of mitochondria, while examination of the heart revealed cardiac hypertrophy with mitochondrial proliferation. Mitochondria isolated from mutant skeletal muscle exhibited a severe defect in coupled respiration. Ant1 mutant adults also had a resting serum lactate level fourfold higher than that of controls, indicative of metabolic acidosis. Significantly, mutant adults manifested severe exercise intolerance. Therefore, Ant1 mutant mice have the biochemical, histological, metabolic and physiological characteristics of mitochondrial myopathy and cardiomyopathy.

Tissue-specific expression of the two isoforms of the mitochondrial phosphate carrier in bovine tissues

Comparison of the sequence of the human mitochondrial phosphate carrier (PiC) gene with cDNA clones characterised from a human heart cDNA library suggested the existence of two isoforms of the PiC, which were generated by alternative splicing of exon IIIA or exon IIIB and which differed in 13 amino acids [Dolce et al. (1994) J. Biol. Chem. 269, 10451]. In this work the expression of isoforms A and B of the PiC was investigated in different bovine tissues by Northern blot analysis using two probes that are specific for bovine exon IIIA and exon IIIB, respectively. Isoform A is highly expressed in heart and skeletal muscle. Isoform B is ubiquitously expressed in all tissues that were examined, although at different levels. The tissue-specific expression pattern of the two PiC isoforms is similar to that reported for the isoforms of several mitochondrial proteins required for energy production.

Expression of the ADP/ATP carrier and expansion of the mitochondrial (ATP + ADP) pool contribute to postnatal maturation of the rat heart

The role of the ADP/ATP carrier (AAC), a key protein of the mitochondrial ATP-generating system, is not clear during postnatal rat heart development. To elucidate this role, the phosphorylating respiration (state 3), the activity and the content of AAC, the size of the exchangeable mitochondrial (ATP + ADP) pool and the control of AAC over respiration at state 3 were measured in mitochondria isolated from rat hearts at various postnatal ages. There was a 5-fold increase in the AAC activity from newborn to aged rat hearts, which was paralleled by a 1.5-fold increase in state 3 respiration. At birth, the AAC and the F0F1-ATP synthase exerted about 80% of the control over phosphorylating respiration (state 3: flux control coefficients 0.39 +/- 0.04 and 0.38 +/- 0.08). The strong increase in the AAC activity was partly caused by the doubling of the protein content. In addition, the turnover number of AAC increased by a factor of 2.5 due to the expansion of the (ATP + ADP) pool from 3.4 +/- 0.9 to 10.6 +/- 1.5 nmol.mg protein-1. The data strongly indicate that the increase in the AAC activity is an essential step in the postnatal maturation of rat heart mitochondria.

Expression of the human cytochrome c1 gene is controlled through multiple Sp1-binding sites and an initiator region

It is widely accepted that nuclear genes that encode proteins of the oxidative-phosphorylation system are regulated by nuclear factors believed to be specific for such genes. In the present study we show that the promoter for the human cytochrome c1 gene is an exception, in that it involves only conserved Sp1 core elements and an initiator region. Maximal promoter activity within a 1.4-kb 5' flanking region of the cytochrome c1 gene is contained in a fragment (-72 to +18) that lacks TATA and CCAAT elements. The transcriptional start site was mapped to an initiator region by RNase protection of mRNA from human HepG2 cells, and by primer extension of in vitro-generated transcripts, to a sequence that is highly similar to the dihydrofolate reductase family of initiators. Deletion of this region (+1 to +18) severely impairs transcription initiation. Sp1 core elements centered at nucleotides -21 and -39 define the activation domain of the proximal promoter. Only the -39 element is protected from DNase I in the presence of crude nuclear extracts. However, transfection, gel-mobility-shift, supershift and in vitro-transcription experiments show that the -21 element binds Sp1 protein and contributes to transcription activation. No other functional oxidative-phosphorylation-specific response elements have been identified. These data implicate Sp1 as a single activating factor for an oxidative-phosphorylation gene.

Significance of the adenine nucleotide translocator in the pathogenesis of viral heart disease

We found recently autoantibodies against the adenine nucleotide translocator (ANT), a carrier in the inner mitochondrial membrane, in sera of patients with myocarditis and dilated cardiomyopathy. To elucidate whether these antibodies are of pathophysiological importance, we investigated the function and expression of the adenine nucleotide translocator (ANT) in the heart muscle tissue of patients suffering from myocarditis and DCM. We found a markedly lowered transport capacity of the translocator accompanied by an elevation in total ANT protein content. The alteration in ANT protein amount is caused by an ANT isoform shift characterized by an increase in ANT 1 isoform protein associated with a decrease in ANT 2 isoform and an unchanged ANT 3 content. It could be shown that the isoform shift is not a progressive process during the disease period but an event in the early period of illness which becomes permanent. Simulating the effect of pathogenetic factors of autoimmunological diseases, we infected A/J mice with the enterovirus Coxsackie B3 and immunized guinea pigs with myocardial ANT protein. Both treatments led to autoimmunological responds and to a lowered myocardial transport capacity of ANT, to a disturbed energy metabolism and consequently to a depression of heart function.

Coordinate expression of nuclear and mitochondrial genes involved in energy production in carcinoma and oncocytoma

The expression of mitochondrial and nuclear genes involved in ATP production was examined in renal carcinomas, renal oncocytomas, and a salivary oncocytoma. Renal carcinomas were found to have a reduced mitochondrial DNA (mtDNA) content while oncocytomas had increased mtDNA contents. This parallels morphological changes in mitochondrial number in these tumours. In the carcinomas, mtDNA transcripts were decreased 5- to 10-fold relative to control kidneys, suggesting that mitochondrial transcript levels depend on the mtDNA content. In renal oncocytomas, mtDNA transcripts were slightly reduced in spite of a high mtDNA content. However, in the salivary gland oncocytoma, mtDNA transcripts were increased more than 10-fold in parallel with a 10-fold increase in mtDNA content. The expression of the nuclear DNA oxidative phosphorylation genes, ATPsyn beta and ANT2, was reduced up to 4-fold in renal carcinoma. In contrast, the levels of these two nuclear gene transcripts were induced about 4-fold in renal oncocytoma and up to 30-fold in salivary gland oncocytoma. Moreover, the ANT2 precursors were observed to change in oncocytomas. These data suggest a coordinated regulation of nuclear and mitochondrial gene expression in renal carcinomas and the specific induction of nuclear OXPHOS gene expression in oncocytomas.

Sp1 activates and inhibits transcription from separate elements in the proximal promoter of the human adenine nucleotide translocase 2 (ANT2) gene

Expression of the adenine nucleotide translocator 2 (ANT2) gene is growth regulated. We report a feature of the ANT2 promoter that involves a novel regulatory function for the Sp1 transfactor. We show that expression from the ANT2 proximal promoter is modulated through three Sp1 elements, two of which activate and one of which partially inhibits transcription. The inhibitor site, box C, is juxtaposed to transcription start (nucleotides -7 to -2). Sp1 bound to box C decreases transcription initiation. This was demonstrated by introducing mutations in box C which (a) increased chloramphenicol acetyltransferase expression in the transient transfection assay and (b) inhibited binding of both purified Sp1 and Sp1 in crude nuclear extracts. The activating elements (A and B boxes) are located at adjacent sites in the distal region of the proximal promoter. Mutation of either box inhibits transfection by 90%, indicating that they act in a synergistic manner. Supershift experiments with crude nuclear extracts showed that only Sp1 was bound to the three GC boxes. The finding that Sp1 acts as an activator/inhibitor within the same promoter region was verified in NIH3T3, HeLa, JEG3, and COS-1, indicating that this dual effect of Sp1 is widely preserved. These data suggest a unique role for Sp1 and raise the possibility that growth activation of the ANT2 gene is regulated by the interaction of Sp1 on the A, B, and C boxes.

Importance of mitochondrial transmembrane processes in human mitochondriopathies

In a substantial group of subjects suspected to have a mitochondriopathy no defect in the mitochondrial energy metabolism (pyruvate dehydrogenase complex or respiratory chain complexes) can be demonstrated. At least in some of these subjects it seems justified to consider a defect in one of the proteins which mediate the transport of several ions and substrates across the mitochondrial membranes. Of particular interest are proteins which are directly involved in the process of oxidative phosphorylation, such as the adenine nucleotide translocator (ANT) and the phosphate carrier (PiC). However, defects in transmembrane ion transporters also may induce impaired energy metabolism probably as a result of osmotic disturbances within the mitochondrial matrix. In this respect, the voltage-dependent anion channel (VDAC) and other ion channels have to be taken into consideration. Here we review the still incomplete knowledge of the occurrence of ANT, PiC, VDAC, cation channels, and a few substrate carriers in human tissues, as well as their possible role in pathology.

Rapid evolution of human pseudoautosomal genes and their mouse homologs

Comparative studies of genes in the pseudoautosomal region (PAR) of human and mouse sex chromosomes have thus far been very limited. The only comparisons that can presently be made indicate that the PARs of humans and mice are not identical in terms of gene content. Here we describe additional comparative studies of human pseudoautosomal genes and their mouse homologs. Using a somatic cell hybrid mapping panel, we have assigned the mouse homolog of the human pseudoautosomal interleukin 3 receptor alpha subunit (IL3RA) gene to mouse Chromosome (Chr) 14. Attempts to clone the mouse homolog of the human pseudoautosomal adenine nucleotide translocase-3 (ANT3) gene resulted in the isolation of the murine homologs of the human ANT1 and ANT2 genes. The mouse Ant1 and Ant2 genes are very similar in sequence to their human homologs, and we have mapped them to mouse Chromosomes (Chrs) (8 and X respectively) that exhibit conserved synteny with the chromosomes on which the human genes are located. In contrast, the homolog of ANT3 appears to be either very divergent or absent from the mouse genome. Southern blot analysis of DNA from a variety of mammalian species shows restricted conservation of human pseudoautosomal genes, a trend that also applies to the two cloned mouse homologs of these genes and to neighboring human genes in distal Xp22.3. Our observations combined with those of other workers lead us to propose a model for the evolution of the PAR that includes both rapid sequence evolution and the incremental reduction in size of the region during mammalian evolution.

The role of thyroid hormone and promoter diversity in the regulation of nuclear encoded mitochondrial proteins

Thyroid hormone regulates the in vivo expression of a selected set of rat nuclear genes encoding mitochondrial inner membrane proteins. Certain mRNAs, such as that for cytochrome c1, are increased as much as 20-50-fold, while others, such as core protein 1 of Complex III and the F1-ATPase beta-subunit do not respond. The promoter region of human cytochrome c1 also supports thyroid hormone induction of a reporter gene in transient transfection experiments. Thus, thyroid hormone regulates only selected genes, even for subunits within the same complex and in widely varying species. By contrast, growth activation of quiescent NIH3T3 cells, a second paradigm used for stimulating mitochondrial biogenesis, does not increase cytochrome c1 mRNA but does increase F1-ATPase beta-subunit mRNA. These findings suggest that nuclear OXPHOS genes are not necessarily expressed in a coordinated manner, and that multiple regulatory circuits might exist which are linked to different physiological stimuli. Analysis of the promoters of several OXPHOS genes reveals a great diversity and heterogeneity of transfactor binding elements. No single regulatory feature exists which could account for a coordinated expression of all OXPHOS genes. The potential diversity for regulating expression of nuclear OXPHOS genes raises the possibility for the existence of disease states linked to regulatory defects.

Changing patterns of transcriptional and post-transcriptional control of beta-F1-ATPase gene expression during mitochondrial biogenesis in liver

To elucidate the mechanisms that regulate the expression of nuclear genes during biogenesis of mammalian mitochondria, the expression pattern of the beta-subunit of the ATP synthase gene has been characterized in rat liver between day 20 in utero and 12 weeks postnatal. The parallelism existing between transcriptional activity of the gene and the amount of beta-F1-ATPase protein in liver indicates that proliferation of mitochondria is controlled at the transcriptional level. On the other hand, an increased stability (4-5-fold) of beta-F1-ATPase mRNA during early neonatal life as well as a rapid postnatal activation of translation rates affecting mitochondrial proteins appear to control mitochondrial differentiation. Immunoelectron microscopy of the F1-ATPase complex during liver development revealed that the rapid postnatal increase in the in vivo rate of F1-ATPase synthesis was mostly used for functional differentiation of pre-existing organelles (Valcarce, C., Navarrete, R. M., Encabo, P., Loeches, E., Satrústegui, J., and Cuezva, J. M. (1988) J. Biol Chem. 263, 7767-7775). The findings support that beta-F1-ATPase mRNA decay is developmentally regulated in liver, indicating that gene expression is also controlled at this level during physiological transitions that affect biogenesis of mitochondria.

The expression of subunit c correlates with and thus may limit the biosynthesis of the mitochondrial F0F1-ATPase in brown adipose tissue

A low content of mitochondrial ATPase in brown adipose tissue (BAT) has previously been found to contrast with high levels of the transcripts of the beta-subunit of the F1 part of the ATPase and of the transcripts of the mitochondrial encoded subunits (Houstĕk, J., Tvrdík, P., Pavelka, S., and Baudysová, M. (1991) FEBS Lett. 294, 191-194). To delineate which subunit limits the synthesis of the ATPase complex, we have studied the expression of the nuclear genes encoding subunits alpha, beta, and gamma of the catalytic F1 part and the b, c, d, and OSCP subunits of the F0 part of the ATPase. In comparison with other tissues of mice, high levels of transcripts of alpha-F1, beta-F1, gamma-F1, b-F0, d-Fo, and OSCP were found in BAT. The only genes expressed at a low level in BAT were those of the c-F0 subunit. The levels of c-F0 transcripts were 4-70-fold lower in BAT than in other tissues. An analogous expression pattern of the ATPase genes was found in BAT of adult rat and hamster. In BAT of newborn lamb, which, in contrast to other mammals, has a high content of mitochondrial ATPase, correspondingly high levels of c-F0 mRNA were found Expression of the c-F0 genes also correlated well with the ontogenic development of BAT in the hamster, being high during the first postnatal week when mitochondria are nonthermogenic and contain a relatively high amount of ATPase, but low on subsequent days when ATPase content decreases, as the thermogenic function develops. It is suggested that expression of the c-F0 genes and subsequent synthesis of the hydrophobic subunit c of the membrane-intrinsic F0 part of the enzyme may control the biosynthesis of the ATPase complex in BAT. An analogous regulatory role of the c-F0 subunit could be postulated in other tissues.

Molecular basis of mitochondrial DNA disease

Mitochondrial ATP production via oxidative phosphorylation (OXPHOS) is essential for normal function and maintenance of human organ systems. Since OXPHOS biogenesis depends on both nuclear- and mitochondrial-encoded gene products, mutations in both genomes can result in impaired electron transport and ATP synthesis, thus causing tissue dysfunction and, ultimately, human disease. Over 30 mitochondrial DNA (mtDNA) point mutations and over 100 mtDNA rearrangements have now been identified as etiological factors in human disease. Because of the unique characteristics of mtDNA genetics, genotype/phenotype associations are often complex and disease expression can be influenced by a number of factors, including the presence of nuclear modifying or susceptibility alleles. Accordingly, these mutations result in an extraordinarily broad spectrum of clinical phenotypes ranging from systemic, lethal pediatric disease to late-onset, tissue-specific neurodegenerative disorders. In spite of its complexity, an understanding of the molecular basis of mitochondrial DNA disease will be essential as the first step toward rationale and permanent curative therapy.

Steady state levels of mitochondrial and nuclear oxidative phosphorylation transcripts in Kearns-Sayre syndrome

The steady state levels of both mitochondrial and nuclear transcripts were examined in a Kearns-Sayre syndrome patient harboring a heteroplasmic 7.7 kb mitochondrial DNA deletion. Transcripts originating from the genes located outside of the deletion were present in similar amounts to those of control samples, with the transcript levels of each tissue linked to its oxidative phosphorylation capacities. Transcripts originating from genes within the deletion were reduced according to the percentage of mtDNA deleted molecules in the tissue. The fusion transcript resulting from the rearranged genome is expressed in all the tissues tested and its level is related to the amount of the deleted mtDNA. The RNA levels from three nuclear genes encoding two of the Adenine Nucleotide Translocator isoforms (ANT1 and 2) and the beta subunit of the ATPsynthase (ATPsyn beta) were significantly induced in the different tissues independently of the percentage of deleted mtDNA molecules. In contrast, the ANT1 and ATPsyn beta levels were decreased in skeletal muscle. This result could be related to the different distribution of the deleted molecules in tissues.

Compartmentation of creatine kinases during perinatal development of mammalian heart

Maturation of the cardiac cell is characterized by increasing diversity of isozymic expression of creatine kinases. Expression of the M-CK isozyme always precedes that of mitochondrial isozyme (mi-CK), however the expression of an isoform does not inform about its localization or cellular function. The functional role of isozymes binding to sites of energy utilization and production characteristic of the adult myocardium can be evidenced by the functional coupling of M-CK to myofibrillar ATPase and mito-CK to translocase in Triton X-100 and saponin skinned fibers. Functional activity of M-CK and mito-CK were investigated during perinatal development. Both functional activities appear during late fetal life in species mature at birth like guinea pig, and in the first postnatal weeks in immature species like rat or rabbit. Thus, the functional activity of bound CK isozymes is not associated with birth per se but with the general process of cell maturation. Localization of CK in the cytosol appears optimal for the transfer of glycolytic production of ATP to sites of utilization in an immature heart. During cell maturation, the increasing contribution of oxidative phosphorylation to ATP production, the apparition and binding of mi-CK to mitochondria, the binding of M-CK to myofibrils, turn the cell in a compartmentalized system of energy production. This provides the cellular basis for energy transfer by the PCr-Cr-CK system between sites of ATP production and utilization. Compartmentation of both Ca handling and energy turnover leads to a highly structured cell organization and could be essential for the efficiency of heart function.

Isolation and characterization of cDNA clones and a genomic clone encoding rat mitochondrial adenine nucleotide translocator

Two cDNA clones encoding rat mitochondrial adenine nucleotide translocator were isolated from libraries constructed from mRNAs of heart and liver. These two clones corresponded to the heart-skeletal muscle type (ANT1) and fibroblast type (ANT2), respectively. A genomic clone encoding rat ANT1 was also isolated and characterized.

Mitochondrial diseases: genotype versus phenotype

Recently, a variety of degenerative diseases have been attributed to mutations in mitochondrial DNA. Even though these mutations are inherited and present throughout the body, they frequently cause late-onset, tissue-specific disease. This may be explained by a combination of the tissue-specific accumulation of somatic mtDNA mutations with age and the variation between tissues in the expression of nuclear genes that encode mitochondrial functions.

A human pseudoautosomal gene, ADP/ATP translocase, escapes X-inactivation whereas a homologue on Xq is subject to X-inactivation

We report the cloning of a highly conserved pseudoautosomal gene on the human sex chromosomes. A cDNA clone was selected by crosshybridization with a microdissected clone from the chromosomal subregion Xp22.3. It encodes a previously characterized member of the ADP/ATP translocase family and plays a fundamental role in cellular energy metabolism. This gene, ANT3, is located approximately 1,300 kilobases from the telomere, proximal to the pseudoautosomal gene CSF2RA, and escapes X-inactivation. Interestingly, a homologue of ANT3, ANT2, maps to Xq and is subject to X-inactivation. These genes provide the first evidence of two closely related X-chromosomal genes, which show striking differences in their X-inactivation behaviour.