Respiratory-deficient human fibroblasts exhibiting defective mitochondrial DNA replication

Pyruvate and uridine rescue the metabolic profile of OXPHOS dysfunction

Introduction

Primary mitochondrial diseases (PMD) are a large, heterogeneous group of genetic disorders affecting mitochondrial function, mostly by disrupting the oxidative phosphorylation (OXPHOS) system. Understanding the cellular metabolic re-wiring occurring in PMD is crucial for the development of novel diagnostic tools and treatments, as PMD are often complex to diagnose and most of them currently have no effective therapy.

Objectives

To characterize the cellular metabolic consequences of OXPHOS dysfunction and based on the metabolic signature, to design new diagnostic and therapeutic strategies.

Methods

In vitro assays were performed in skin-derived fibroblasts obtained from patients with diverse PMD and validated in pharmacological models of OXPHOS dysfunction. Proliferation was assessed using the Incucyte technology. Steady-state glucose and glutamine tracing studies were performed with LC-MS quantification of cellular metabolites. The therapeutic potential of nutritional supplements was evaluated by assessing their effect on proliferation and on the metabolomics profile. Successful therapies were then tested in a in vivo lethal rotenone model in zebrafish.

Results

OXPHOS dysfunction has a unique metabolic signature linked to an NAD+/NADH imbalance including depletion of TCA intermediates and aspartate, and increased levels of glycerol-3-phosphate. Supplementation with pyruvate and uridine fully rescues this altered metabolic profile and the subsequent proliferation deficit. Additionally, in zebrafish, the same nutritional treatment increases the survival after rotenone exposure.

Conclusions

Our findings reinforce the importance of the NAD+/NADH imbalance following OXPHOS dysfunction in PMD and open the door to new diagnostic and therapeutic tools for PMD.

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OXPHOS deficiency causes a distinct metabolic profile linked to a NAD+/NADH imbalance.

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Depleted intracellular aspartic acid is a potential biomarker for OXPHOS dysfunction.

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Therapy with pyruvate and uridine corrects the metabolic profile of OXPHOS deficiency.

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Pyruvate and uridine treatment increases survival in a lethal rotenone zebrafish model.

Cytosolic phosphoenolpyruvate carboxykinase deficiency presenting with acute liver failure following gastroenteritis

We report a patient from a consanguineous family who presented with transient acute liver failure and biochemical patterns suggestive of disturbed urea cycle and mitochondrial function, for whom conventional genetic and metabolic investigations for acute liver failure failed to yield a diagnosis. Whole exome sequencing revealed a homozygous 12-bp deletion in PCK1 (MIM 614168) encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK); enzymatic studies subsequently confirmed its pathogenic nature. We propose that PEPCK deficiency should be considered in the young child with unexplained liver failure, especially where there are marked, accumulations of TCA cycle metabolites on urine organic acid analysis and/or an amino acid profile with hyperammonaemia suggestive of a proximal urea cycle defect during the acute episode. If suspected, intravenous administration of dextrose should be initiated. Long-term management comprising avoidance of fasting with the provision of a glucose polymer emergency regimen for illness management may be sufficient to prevent future episodes of liver failure. This case report provides further insights into the (patho-)physiology of energy metabolism, confirming the power of genomic analysis of unexplained biochemical phenotypes.

Screening of effective pharmacological treatments for MELAS syndrome using yeasts, fibroblasts and cybrid models of the disease

BACKGROUND AND PURPOSE

MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) is a mitochondrial disease most usually caused by point mutations in tRNA genes encoded by mitochondrial DNA (mtDNA). Approximately 80% of cases of MELAS syndrome are associated with a m.3243A > G mutation in the MT-TL1 gene, which encodes the mitochondrial tRNALeu (UUR). Currently, no effective treatments are available for this chronic progressive disorder. Treatment strategies in MELAS and other mitochondrial diseases consist of several drugs that diminish the deleterious effects of the abnormal respiratory chain function, reduce the presence of toxic agents or correct deficiencies in essential cofactors.

EXPERIMENTAL APPROACH

We evaluated the effectiveness of some common pharmacological agents that have been utilized in the treatment of MELAS, in yeast, fibroblast and cybrid models of the disease. The yeast model harbouring the A14G mutation in the mitochondrial tRNALeu(UUR) gene, which is equivalent to the A3243G mutation in humans, was used in the initial screening. Next, the most effective drugs that were able to rescue the respiratory deficiency in MELAS yeast mutants were tested in fibroblasts and cybrid models of MELAS disease.

KEY RESULTS

According to our results, supplementation with riboflavin or coenzyme Q(10) effectively reversed the respiratory defect in MELAS yeast and improved the pathologic alterations in MELAS fibroblast and cybrid cell models.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that cell models have great potential for screening and validating the effects of novel drug candidates for MELAS treatment and presumably also for other diseases with mitochondrial impairment.

Reduction of nuclear encoded enzymes of mitochondrial energy metabolism in cells devoid of mitochondrial DNA

Mitochondrial DNA (mtDNA) depletion syndromes are generally associated with reduced activities of oxidative phosphorylation (OXPHOS) enzymes that contain subunits encoded by mtDNA. Conversely, entirely nuclear encoded mitochondrial enzymes in these syndromes, such as the tricarboxylic acid cycle enzyme citrate synthase (CS) and OXPHOS complex II, usually exhibit normal or compensatory enhanced activities. Here we report that a human cell line devoid of mtDNA (HEK293 ρ(0) cells) has diminished activities of both complex II and CS. This finding indicates the existence of a feedback mechanism in ρ(0) cells that downregulates the expression of entirely nuclear encoded components of mitochondrial energy metabolism.

Enhanced uridine bioavailability following administration of a triacetyluridine-rich nutritional supplement

Background

Uridine is a therapy for hereditary orotic aciduria and is being investigated in other disorders caused by mitochondrial dysfunction, including toxicities resulting from treatment with nucleoside reverse transcriptase inhibitors in HIV. Historically, the use of uridine as a therapeutic agent has been limited by poor bioavailability. A food supplement containing nucleosides, NucleomaxX®, has been reported to raise plasma uridine to supraphysiologic levels.

Methodology/Principal Findings

Single- and multi-dose PK studies following NucleomaxX® were compared to single-dose PK studies of equimolar doses of pure uridine in healthy human volunteers. Product analysis documented that more than 90% of the nucleoside component of NucleomaxX® is in the form of triacetyluridine (TAU). Single and repeated dosing with NucleomaxX® resulted in peak plasma uridine concentrations 1–2 hours later of 150.9±39.3 µM and 161.4±31.5 µM, respectively, levels known to ameliorate mitochondrial toxicity in vitro. C_max and AUC were four-fold higher after a single dose of NucleomaxX® than after uridine. No adverse effects of either treatment were observed.

Conclusions/Significance

NucleomaxX®, containing predominantly TAU, has significantly greater bioavailability than pure uridine in human subjects and may be useful in the management of mitochondrial toxicity.

Mitofusin 1 and mitofusin 2 are ubiquitinated in a PINK1/parkin-dependent manner upon induction of mitophagy

Mitochondrial dysfunction and perturbed degradation of proteins have been implicated in Parkinson's disease (PD) pathogenesis. Mutations in the Parkin and PINK1 genes are a cause of familial PD. PINK1 is a putative kinase associated with mitochondria, and loss of PINK1 expression leads to mitochondrial dysfunction, which increases with time. Parkin is suggested to be downstream of PINK1 and also mediates the removal of damaged mitochondria by macroautophagy (mitophagy). We investigated whether mitochondrial dysfunction in dopaminergic SH-SY5Y cells following decreased PINK1 expression by RNAi may in part be due to the inhibition of mitophagy. Reduced flux through the macroautophagy pathway was found to be coincident with the inhibition of ATP synthesis following 12 days of PINK1 silencing. Overexpression of parkin in these cells restored both autophagic flux and ATP synthesis. Overexpression and RNAi studies also indicated that PINK1 and parkin were required for mitophagy following CCCP-induced mitochondrial damage. The ubiquitination of several mitochondrial proteins, including mitofusin 1 and mitofusin 2, were detected within 3 h of CCCP treatment. These post-translational modifications were reduced following the silencing of parkin or PINK1. The ubiquitination of mitochondrial proteins appears to identify mitochondria for degradation and facilitate mitophagy. PINK1 and parkin are thus required for the removal of damaged mitochondria in dopaminergic cells, and inhibition of this pathway may lead to the accumulation of defective mitochondria which may contribute to PD pathogenesis.

Oral uridine supplementation antagonizes the peripheral neuropathy and encephalopathy induced by antiretroviral nucleoside analogues

OBJECTIVE

Peripheral neuropathy and central nervous system neurodegeneration may result from the mitochondrial toxicity of some antiretroviral nucleoside analogues. We investigated whether this neuropathology may be antagonized by uridine supplementation in vivo.

DESIGN

Because of the obvious difficulties in obtaining human neural tissues, the mitochondrial neurotoxicity of the nucleoside analogues was studied in mice.

METHODS

BALB/C mice (7 weeks of age) were fed for 9 weeks with zalcitabine (13 mg/kg per day) or zidovudine (100 mg/kg per day) with or without mitocnol (340 mg/kg per day), a dietary supplement with high uridine bioavailability. Hippocampal and sciatic nerve mitochondria were analyzed.

RESULTS

Zalcitabine and to a lesser extent zidovudine induced a significant peripheral neuropathy and encephalopathy with disrupted mitochondrial ultrastructure, depleted mitochondrial DNA, reduced levels of cytochrome c oxidase activity and diminished expression of mitochondrial DNA-encoded cytochrome c oxidase subunit I. Mitocnol had no intrinsic effects but attenuated or fully normalized all measured disorder of the peripheral and central nervous system.

CONCLUSION

Zidovudine and zalcitabine induce a mitochondrial disorder in the peripheral and central nervous system, both of which are antagonized by uridine supplementation.

Analysis of mutant DNA polymerase gamma in patients with mitochondrial DNA depletion

We studied six unrelated children with depletion of mitochondrial DNA (mtDNA). They presented with Leigh syndrome, infantile hepatocerebral mtDNA depletion syndrome, or Alpers-Huttenlocher syndrome. Several genes have been implicated in mtDNA depletion. Screening of candidate genes indicated that all six patients were compound heterozygous for missense mutations in the gene for the catalytic subunit of DNA polymerase gamma (POLG). Three of the identified mutations, c.3328C>T (p.H1110Y), c.3401A>G (p.H1134R), and c.3406G>A (p.E1136K), have not been reported earlier. To investigate the functional consequences of the mutations, we carried out a series of biochemical assays in cultured fibroblasts. These studies revealed that fibroblast cultures from the patients with infantile hepatocerebral mtDNA depletion syndrome progressively lost their mtDNA during culturing, whereas fibroblast cultures from patients presenting with Leigh syndrome or Alpers-Huttenlocher syndrome had reduced but stable levels of mtDNA. DNA polymerase gamma activity was below the normal range in all patient cultures, except for one; however, this culture showed low levels of the heterodimeric enzyme and poor DNA polymerase gamma processivity. Parental fibroblast cultures had normal catalytic efficiency of DNA polymerase gamma, consistent with the observation that all carriers are asymptomatic. Thus, we report the first patient with Leigh syndrome caused by POLG mutations. The cell culture experiments established the pathogenicity of the identified POLG mutations and helped to define the molecular mechanisms responsible for mtDNA depletion in the patients' tissues. The assays may facilitate the identification of those patients in whom screening for POLG mutations would be most appropriate.

Uridine supplementation in HIV lipoatrophy: pilot trial on safety and effect on mitochondrial indices

OBJECTIVES

Uridine abrogates mitochondrial toxicities of nucleoside reverse transcriptase inhibitor in adipocyte cell culture. We aim to study the effect of uridine supplementation on human adipocyte mitochondrial DNA (mtDNA) levels in subjects with human immunodeficiency (HIV) lipoatrophy.

METHODS

Sixteen patients with lipoatrophy on stavudine-containing antiretroviral therapy were enrolled, and received NucleomaxX, a dietary supplement with a high bioavailability of uridine (36 g TID every other day for 16 weeks). Patients were then followed off-uridine for another 16 weeks. Highly active antiretroviral therapy remained unchanged during the trial.

RESULTS

Fourteen patients completed the study. Two subjects dropped out before week 4 for study-unrelated reasons. No adverse events were noted throughout the study. HIV-1 RNA, CD4 counts, liver enzymes and hemoglobin remained unchanged. Body mass index, lactate, lipids, insulin and homeostasis model assessment of insulin resistance were unaltered. Fat and peripheral blood and mononuclear cell mtDNA levels did not correlate with each other and exhibited no changes throughout the study. Lipoatrophy scores by patients and physician improved significantly at weeks 16 and 32 compared to study entry.

CONCLUSION

In this pilot study, NucleomaxX was safe, well tolerated without apparent deleterious effect on HIV indices. In contrast to in vitro data, NucleomaxX did not lead to changes in fat or blood mtDNA levels.

Uridine supplementation antagonizes zalcitabine-induced microvesicular steatohepatitis in mice

Zalcitabine is an antiretroviral nucleoside analogue that exhibits long-term toxicity to hepatocytes by interfering with the replication of mitochondrial DNA (mtDNA). Uridine antagonizes this effect in vitro. In the present study we investigate the mechanisms of zalcitabine-induced hepatotoxicity in mice and explore therapeutic outcomes with oral uridine supplementation. BalbC mice (7 weeks of age, 9 mice in each group) were fed 0.36 mg/kg/d of zalcitabine (corresponding to human dosing adapted for body surface), or 13 mg/kg/d of zalcitabine. Both zalcitabine groups were treated with or without Mitocnol (0.34 g/kg/d), a dietary supplement with high bioavailability of uridine. Liver histology and mitochondrial functions were assessed after 15 weeks. One mouse exposed to high dose zalcitabine died at 19 weeks of age. Zalcitabine induced a dose dependent microvesicular steatohepatitis with abundant mitochondria. The organelles were enlarged and contained disrupted cristae. Terminal transferase dUTP nick end labeling (TUNEL) assays showed frequent hepatocyte apoptosis. mtDNA was depleted in liver tissue, cytochrome c-oxidase but not succinate dehydrogenase activities were decreased, superoxide and malondialdehyde were elevated. The expression of COX I, an mtDNA-encoded respiratory chain subunit was reduced, whereas COX IV, a nucleus-encoded subunit was preserved. Uridine supplementation normalized or attenuated all toxic abnormalities in both zalcitabine groups, but had no effects when given without zalcitabine. Uridine supplementation was without apparent side effects.

CONCLUSION

Zalcitabine induces mtDNA-depletion in murine liver with consequent respiratory chain dysfunction, up-regulated synthesis of reactive oxygen species and microvesicular steatohepatitis. Uridine supplementation attenuates this mitochondrial hepatotoxicity without apparent intrinsic effects.

Deoxyribonucleotides and disorders of mitochondrial DNA integrity

Mitochondrial DNA (mtDNA) depends on numerous nuclear encoded factors and a constant supply of deoxyribonucleoside triphosphates (dNTP), for its maintenance and replication. The function of proteins involved in nucleotide metabolism is perturbed in a heterogeneous group of disorders associated with depletion, multiple deletions, and mutations of the mitochondrial genome. Disturbed homeostasis of the mitochondrial dNTP pools are likely the underlying cause. Understanding of the biochemical and molecular basis of these disorders will promote the development of new therapeutic approaches. This article reviews the current knowledge of deoxyribonucleotide metabolism in relation to disorders affecting mtDNA integrity.

Uridine in the Prevention and Treatment of Nrti-Related Mitochondrial Toxicity

Long-term side effects of antiretroviral therapy are attributed to the mitochondrial (mt) toxicity of nucleoside analogue reverse transcriptase inhibitors (NRTIs) and their ability to deplete mtDNA. Studies in hepatocytes suggest that uridine is able to prevent and treat mtDNA depletion by pyrimidine NRTIs [zalcitabine (ddC) and stavudine (d4T)] and to fully abrogate hepatocyte death, elevated lactate production and intracellular steatosis. Uridine was also found to improve the liver and haematopoietic toxicities of zidovudine (AZT), which are unrelated to mtDNA depletion, and to prevent neuronal cell death induced by ddC. Most recently, uridine was found to prevent the onset of a lipoatrophic phenotype (reduced intracellular lipids, increased apoptosis, mtDNA depletion and mt depolarization) in adipocytes incubated long-term with d4T and AZT. Various steps of mt nucleoside utilization may be involved in the protective effect, but competition of uridine metabolites with NRTIs at polymerase y or other enzymes is a plausible explanation. Pharmacokinetic studies suggest that uridine serum levels can be safely increased in humans to achieve concentrations which are protective in vitro (50–200 μM). Uridine was not found to interfere with the antiretroviral activity of NRTIs. Mitocnol, a sugar cane extract which effectively increases uridine in human serum, was beneficial in individual HIV patients with mt toxicity and is now being tested in placebo-controlled randomized trials. Until these data become available, the risk-benefit calculation of using uridine should be individualized. The current safety data justify the closely monitored use of uridine in individuals who suffer from mt toxicity but who cannot be switched to less toxic NRTIs.

Mitochondrial DNA depletion can be prevented by dGMP and dAMP supplementation in a resting culture of deoxyguanosine kinase-deficient fibroblasts

Deoxyguanosine kinase is a constitutively expressed, mitochondrial enzyme of the deoxyribonucleoside salvage pathway. Deficiency of deoxyguanosine kinase causes early-onset, hepatocerebral mitochondrial DNA (mtDNA) depletion syndrome. To clarify the molecular mechanism of the disease, a skin fibroblast culture was studied from a patient carrying a homozygous nonsense mutation in the gene for deoxyguanosine kinase. In situ examination of DNA synthesis demonstrated that, although mtDNA synthesis is cell cycle independent in control fibroblasts, mtDNA synthesis occurs mainly during the S-phase in deoxyguanosine kinase-deficient cells. Consistent with this observation, it was found that the mtDNA content of exponentially growing, deoxyguanosine kinase-deficient cells is only mildly affected. When cycling is inhibited by serum-deprivation and cells are in a resting state, however, the mtDNA content drops considerably in deoxyguanosine kinase-deficient cells, yet remains stable in control fibroblasts. The decline in mtDNA content in resting, deoxyguanosine kinase-deficient cells can be prevented by dGMP and dAMP supplementation, providing conclusive evidence that substrate limitation triggers mtDNA depletion in deoxyguanosine kinase-deficient cells.

Uridine Abrogates Mitochondrial Toxicity Related to Nucleoside Analogue Reverse Transcriptase Inhibitors in Hepg2 Cells

Objective

To assess in vitro if uridine may be suitable to prevent or treat mitochondrial toxicity related to nucleoside analogue reverse transcriptase inhibitors (NRTIs).

Methods

Human HepG2-hepatocytes were exposed to NRTIs with or without uridine for 25 days. Cell growth, lactate production, intracellular lipids, mitochondrial DNA (mtDNA) and the ratio between the respiratory chain components COX II (mtDNA-encoded) and COX IV (nuclear-encoded) were measured.

Results

HepG2 cells exposed to zalcitabine (177 nM) without uridine developed a severe depletion of mtDNA (to 8% of wild-type mtDNA levels), resulting in a decline of cell proliferation and COX II levels, with increased lactate and lipid accumulation. Uridine fully abrogated the adverse effects of zalcitabine on hepatocyte proliferation and normalized lactate synthesis, intracellular lipids and COX II levels by adjusting mtDNA levels to about 65% of NRTI-unexposed control cells. This effect was dose-dependent, with a maximum at 200 μM of uridine. Uridine also rapidly and fully restored cell function when added to cells with established mitochondrial dysfunction (zalcitabine for 15 days) despite continued zalcitabine exposure. Uridine also normalized cell proliferation in HepG2 cells exposed to 36 μM of stavudine and protected HepG2-cells exposed to 7 μM of zidovudine + 8 μM of lamivudine (pyrimidine analogues), but failed to improve cell function or mtDNA in cells exposed to 11.8 or 118 μM of didanosine (a purine analogue).

Conclusions

The pyrimidine precursor uridine may attenuate the mitochondrial toxicity of antiretroviral pyrimidine NRTIs in vitro, and its supplementation may represent a promising strategy in the prevention or treatment of mitochondrial toxicities in HIV-infected patients.

A novel syndrome affecting multiple mitochondrial functions, located by microcell-mediated transfer to chromosome 2p14-2p13

We have studied cultured skin fibroblasts from three siblings and one unrelated individual, all of whom had fatal mitochondrial disease manifesting soon after birth. After incubation with 1 mM glucose, these four cell strains exhibited lactate/pyruvate ratios that were six times greater than those of controls. On further analysis, enzymatic activities of the pyruvate dehydrogenase complex, the 2-oxoglutarate dehydrogenase complex, NADH cytochrome c reductase, succinate dehydrogenase, and succinate cytochrome c reductase were severely deficient. In two of the siblings the enzymatic activity of cytochrome oxidase was mildly decreased (by approximately 50%). Metabolite analysis performed on urine samples taken from these patients revealed high levels of glycine, leucine, valine, and isoleucine, indicating abnormalities of both the glycine-cleavage system and branched-chain alpha-ketoacid dehydrogenase. In contrast, the activities of fibroblast pyruvate carboxylase, mitochondrial aconitase, and citrate synthase were normal. Immunoblot analysis of selected complex III subunits (core 1, cyt c(1), and iron-sulfur protein) and of the pyruvate dehydrogenase complex subunits revealed no visible changes in the levels of all examined proteins, decreasing the possibility that an import and/or assembly factor is involved. To elucidate the underlying molecular defect, analysis of microcell-mediated chromosome-fusion was performed between the present study's fibroblasts (recipients) and a panel of A9 mouse:human hybrids (donors) developed by Cuthbert et al. (1995). Complementation was observed between the recipient cells from both families and the mouse:human hybrid clone carrying human chromosome 2. These results indicate that the underlying defect in our patients is under the control of a nuclear gene, the locus of which is on chromosome 2. A 5-cM interval has been identified as potentially containing the critical region for the unknown gene. This interval maps to region 2p14-2p13.

Quantitation and origin of the mitochondrial membrane potential in human cells lacking mitochondrial DNA

Mammalian mitochondrial DNA (mtDNA) encodes 13 polypeptide components of oxidative phosphorylation complexes. Consequently, cells that lack mtDNA (termed rho degrees cells) cannot maintain a membrane potential by proton pumping. However, most mitochondrial proteins are encoded by nuclear DNA and are still imported into mitochondria in rho degrees cells by a mechanism that requires a membrane potential. This membrane potential is thought to arise from the electrogenic exchange of ATP4- for ADP3- by the adenine nucleotide carrier. An intramitochondrial ATPase, probably an incomplete FoF1-ATP synthase lacking the two subunits encoded by mtDNA, is also essential to ensure sufficient charge flux to maintain the potential. However, there are considerable uncertainties about the magnitude of this membrane potential, the nature of the intramitochondrial ATPase and the ATP flux required to maintain the potential. Here we have investigated these factors in intact and digitonin-permeabilized mammalian rho degrees cells. The adenine nucleotide carrier and ATP were essential, but not sufficient to generate a membrane potential in rho degrees cells and an incomplete FoF1-ATP synthase was also required. The maximum value of this potential was approximately 110 mV in permeabilized cells and approximately 67 mV in intact cells. The membrane potential was eliminated by inhibitors of the adenine nucleotide carrier and by azide, an inhibitor of the incomplete FoF1-ATP synthase, but not by oligomycin. This potential is sufficient to import nuclear-encoded proteins but approximately 65 mV lower than that in 143B cells containing fully functional mitochondria. Subfractionation of rho degrees mitochondria showed that the azide-sensitive ATPase activity was membrane associated. Further analysis by blue native polyacrylamide gel electrophoresis (BN/PAGE) followed by activity staining or immunoblotting, showed that this ATPase activity was an incomplete FoF1-ATPase loosely associated with the membrane. Maintenance of this membrane potential consumed about 13% of the ATP produced by glycolysis. This work has clarified the role of the adenine nucleotide carrier and an incomplete FoF1-ATP synthase in maintaining the mitochondrial membrane potential in rho degrees cells.

Mitochondrial involvement in bladder function and dysfunction

Benign bladder pathology resulting from prostatic hypertrophy or other causes is a significant problem associated with ageing in humans. This condition is characterized by increased bladder mass, decreased urinary flow rate, decreased compliance, and these and other changes in bladder function often subject patients to increased risk of urinary tract infection. While the physiologic attributes of benign bladder pathology have been extensively described in humans and in various animal model systems, the biochemical and molecular genetic bases for that pathology have only recently been investigated in detail. Studies demonstrate that mitochondrial energy production and utilization are severely impaired in bladder smooth muscle during benign bladder disease, and to a large extent this realization has provided a rational basis for understanding the characteristic alterations in urinary flow and compliance in bladder tissue. Recent investigations targeting the detailed molecular basis for impaired mitochondrial function in the disease have shown that performance of the organellar genetic system, and to a large extent that of relevant portions of the nuclear genetic system as well, is severely aberrant in bladder tissue. In this article, we discuss the physiologic aspects of benign bladder disease, summarize biochemical evidence for the altered mitochondrial energy metabolism that appears to underlie bladder pathology, review the structure and function of the mitochondrial genetic system, and discuss molecular genetic studies of that system which have begun to provide a mechanistic explanation for the biochemical and physiological abnormalities that characterize the disease. We also discuss areas for further research which will be critically important in increasing our understanding of the detailed causes of benign bladder pathology.

The mitochondrial genome: structure, transcription, translation and replication

Mitochondria play a central role in cellular energy provision. The organelles contain their own genome with a modified genetic code. The mammalian mitochondrial genome is transmitted exclusively through the female germ line. The human mitochondrial DNA (mtDNA) is a double-stranded, circular molecule of 16569 bp and contains 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides. The mtDNA-encoded polypeptides are all subunits of enzyme complexes of the oxidative phosphorylation system. Mitochondria are not self-supporting entities but rely heavily for their functions on imported nuclear gene products. The basic mechanisms of mitochondrial gene expression have been solved. Cis-acting mtDNA sequences have been characterised by sequence comparisons, mapping studies and mutation analysis both in vitro and in patients harbouring mtDNA mutations. Characterisation of trans-acting factors has proven more difficult but several key enzymes involved in mtDNA replication, transcription and protein synthesis have now been biochemically identified and some have been cloned. These studies revealed that, although some factors may have an additional function elsewhere in the cell, most are unique to mitochondria. It is expected that cell cultures of patients with mitochondrial diseases will increasingly be used to address fundamental questions about mtDNA expression.

1-Methyl-4-phenylpyridinium ion (MPP+) selectively inhibits the replication of mitochondrial DNA

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine is known to cause Parkinsonism in its neurotoxic form, 1-methyl-4-phenylpyridinium ion (MPP+). We have previously reported that MPP+ decreases the content of mitochondrial DNA (mtDNA) independently of the inhibition of complex I in human cells [Miyako, K., Kai, Y., Irie, T., Takeshige, K., and Kang, D. (1997) J. Biol. Chem. 272, 9605-9608]. Here we study the mechanism causing the decrease in mtDNA. MPP+ inhibits the incorporation of 5-bromo-2'-deoxyuridine into mtDNA but not into nuclear DNA, indicating that MPP+ inhibits the replication of mtDNA but not that of the nuclear genome. The replication of mtDNA is initiated by the synthesis of the heavy strand switched from the transcription of the light strand. MPP+ decreases the nascent heavy strands per mtDNA and increases the transcript of the ND6 gene, encoded on light strand, per mtDNA. The amount of mitochondrial transcription factor A is not decreased. These data suggest that the transcription is not inhibited and therefore the transition from transcription to replication of mtDNA is lowered in the MPP+-treated cells. Electron microscopy shows that the number of mitochondria is not decreased in the MPP+-treated cells, suggesting that MPP+ does not affect the overall biogenesis of mitochondria. Thus, MPP+ selectively inhibits the replication of mtDNA.

Expression of mtDNA and nDNA encoded respiratory chain proteins in chemically and genetically-derived Rho0 human fibroblasts: a comparison of subunit proteins in normal fibroblasts treated with ethidium bromide and fibroblasts from a patient with mtDNA depletion syndrome

Although much progress has been made in identifying genetic defects associated with mitochondrial diseases, the protein expression patterns of most disorders are poorly understood. Here we use immunochemical techniques to describe subunit expression patterns of respiratory chain enzyme complexes II (succinate dehydrogenase: SD) and IV (cytochrome c oxidase: COX) in cultured cells lacking mtDNA (Rho0 cells) derived either chemically by exposure of normal cells to ethidium bromide, or genetically in cells derived from a patient with mtDNA depletion syndrome. Both control cells and early passage patient-derived cells express a normal complement of SD and COX subunit proteins. Ethidium bromide treatment of normal cells and in vitro cell proliferation of patient-derived cells caused both populations to acquire identical Rho0 phenotypes. As expected, they lack mtDNA-encoded subunits COX-I and COX-II. In contrast, nDNA-encoded subunits are affected differentially, with some (COX-VIc) lacking and others (COX-IV, COX-Va, SD 30 and SD 70) maintained at somewhat reduced levels. We suggest that the differential stability of nDNA-encoded subunits in the absence of intact enzyme complexes is due to the ability of some, but not all, subunits to associate as partial complexes in the absence of mtDNA-encoded subunits.

Molecular mechanisms in mitochondrial DNA depletion syndrome

Depletion of mitochondrial DNA (mtDNA) appears to be an important cause of mitochondrial dysfunction in neonates and infants. We have identified another child in whom depletion of mtDNA was demonstrated in liver and serial skeletal muscle biopsies. A primary myoblast culture from the patient initially showed normal levels of mtDNA, but there was a progressive loss of mtDNA in later cell passages and clonal myoblast cell cultures, similar to that observed in the skeletal muscle tissue of the patient. Thus, these clonal myoblast cultures provide an in vitro model of the in vivo mtDNA dynamics. The levels of mitochondrial mRNAs for subunits I and II of cytochrome c oxidase declined with declining mtDNA levels, but the fall in mitochondrial transcript levels lagged behind that of the mtDNA levels. Levels of cytochrome c oxidase subunit I and II polypeptides, however, declined ahead of declining mtDNA levels. Immunocytochemistry showed that between individual cells of the clonal myoblast cultures, the expression of the mitochondrially encoded subunit I of cytochrome c oxidase was heterogeneous, suggesting variable levels of mtDNA. Transfer of patient mitochondria with residual mtDNA levels to control cells devoid of mtDNA (rho0 cells) led to restoration of mtDNA levels and, hence, suggests a nuclear involvement in the depletion.

The cold sensitivity of a mutant of Saccharomyces cerevisiae lacking a mitochondrial heat shock protein 70 is suppressed by loss of mitochondrial DNA

SSH1, a newly identified member of the heat shock protein (hsp70) multigene family of the budding yeast Saccharomyces cerevisiae, encodes a protein localized to the mitochondrial matrix. Deletion of the SSH1 gene results in extremely slow growth at 23 degrees C or 30 degrees C, but nearly wild-type growth at 37 degrees C. The matrix of the mitochondria contains another hsp70, Ssc1, which is essential for growth and required for translocation of proteins into mitochondria. Unlike SSC1 mutants, an SSH1 mutant showed no detectable defects in import of several proteins from the cytosol to the matrix compared to wild type. Increased expression of Ssc1 partially suppressed the cold-sensitive growth defect of the SSH1 mutant, suggesting that when present in increased amounts, Ssc1 can at least partially carry out the normal functions of Ssh1. Spontaneous suppressors of the cold-sensitive phenotype of an SSH1 null mutant were obtained at a high frequency at 23 degrees C, and were all found to be respiration deficient. 15 of 16 suppressors that were analyzed lacked mitochondrial DNA, while the 16th had reduced amounts. We suggest that Ssh1 is required for normal mitochondrial DNA replication, and that disruption of this process in ssh1 cells results in a defect in mitochondrial function at low temperatures.

Mitochondrial DNA depletion: prevalence in a pediatric population referred for neurologic evaluation

Mitochondrial DNA depletion is a quantitative disorder of mtDNA, characterized by tissue-specific reductions in mtDNA copy number, that presents in infancy or early childhood. It is most likely transmitted as an autosomal recessive trait, although about half of the described cases are sporadic. To estimate its prevalence we measured relative mtDNA copy number (mtDNA: 18S rDNA ratio) by Southern blot analysis in muscle biopsy samples from all children with compatible histories referred between 1983 and 1994. Of the 304 biopsies evaluated, 54 met the study criteria. We found 6 patients (2 male, 4 female) with mtDNA depletion (relative mtDNA copy number 7.9-33.2% of control). Their clinical course and findings were heterogeneous, however all but one manifested weakness, hypotonia, and developmental delay. Clinical severity was not obviously related to the degree of mtDNA depletion. No patient had ragged-red fibers, although 2 had a lipid storage myopathy. Immunofluorescence with antibodies to double-stranded DNA, COX IV, and COX II demonstrated homogeneously reduced reactivity to all three antibodies compared with control. mtDNA depletion may be a relatively common neurogenetic disorder of infancy and early childhood and should be considered in children with unexplained weakness, hypotonia, or developmental delay.

Disorders of gluconeogenesis

Gluconeogenesis, or the formation of glucose from mainly lactate/ pyruvate, glycerol and alanine, plays an essential role in the maintenance of normoglycaemia during fasting. Inborn deficiencies are known of each of the four enzymes of the glycolytic-gluconeogenic pathway that ensure a unidirectional flux from pyruvate to glucose: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. In this paper, the clinical picture, pathophysiology, diagnostic tests, genetics, treatment and prognosis of the deficiencies of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase are reviewed.