Targeted transgenic overexpression of mitochondrial thymidine kinase (TK2) alters mitochondrial DNA (mtDNA) and mitochondrial polypeptide abundance: transgenic TK2, mtDNA, and antiretrovirals

Beyond the polymerase-γ theory: Production of ROS as a mode of NRTI-induced mitochondrial toxicity

Use of some HIV-1 nucleoside reverse transcriptase inhibitors (NRTI) is associated with severe adverse events. However, the exact mechanisms behind their toxicity has not been fully understood. Mitochondrial dysfunction after chronic exposure to specific NRTIs has predominantly been assigned to mitochondrial polymerase-γ inhibition by NRTIs. However, an increasing amount of data suggests that this is not the sole mechanism. Many NRTI induced adverse events have been linked to the incurrence of oxidative stress, although the causality of events leading to reactive oxygen species (ROS) production and their role in toxicity is unclear. In this study we show that short-term effects of first generation NRTIs, which are rarely discussed in the literature, include inhibition of oxygen consumption, decreased ATP levels and increased ROS production. Collectively these events affect fitness and longevity of C. elegans through mitohormetic signalling events. Furthermore, we demonstrate that these effects can be normalized by addition of the anti-oxidant N-acetylcysteine (NAC), which suggests that ROS likely influence the onset and severity of adverse events upon drug exposure.

Mitochondrial purine and pyrimidine metabolism and beyond

Carefully balanced deoxynucleoside triphosphate (dNTP) pools are essential for both nuclear and mitochondrial genome replication and repair. Two synthetic pathways operate in cells to produce dNTPs, e.g., the de novo and the salvage pathways. The key regulatory enzymes for de novo synthesis are ribonucleotide reductase (RNR) and thymidylate synthase (TS), and this process is considered to be cytosolic. The salvage pathway operates both in the cytosol (TK1 and dCK) and the mitochondria (TK2 and dGK). Mitochondrial dNTP pools are separated from the cytosolic ones owing to the double membrane structure of the mitochondria, and are formed by the salvage enzymes TK2 and dGK together with NMPKs and NDPK in postmitotic tissues, while in proliferating cells the mitochondrial dNTPs are mainly imported from the cytosol produced by the cytosolic pathways. Imbalanced mitochondrial dNTP pools lead to mtDNA depletion and/or deletions resulting in serious mitochondrial diseases. The mtDNA depletion syndrome is caused by deficiencies not only in enzymes in dNTP synthesis (TK2, dGK, p53R2, and TP) and mtDNA replication (mtDNA polymerase and twinkle helicase), but also in enzymes in other metabolic pathways such as SUCLA2 and SUCLG1, ABAT and MPV17. Basic questions are why defects in these enzymes affect dNTP synthesis and how important is mitochondrial nucleotide synthesis in the whole cell/organism perspective? This review will focus on recent studies on purine and pyrimidine metabolism, which have revealed several important links that connect mitochondrial nucleotide metabolism with amino acids, glucose, and fatty acid metabolism.

Loss of hepatic DEPTOR alters the metabolic transition to fasting

Objective

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that functions into distinct protein complexes (mTORC1 and mTORC2) that regulates growth and metabolism. DEP-domain containing mTOR-interacting protein (DEPTOR) is part of these complexes and is known to reduce their activity. Whether DEPTOR loss affects metabolism and organismal growth in vivo has never been tested.

Methods

We have generated a conditional transgenic mouse allowing the tissue-specific deletion of DEPTOR. This model was crossed with CMV-cre mice or Albumin-cre mice to generate either whole-body or liver-specific DEPTOR knockout (KO) mice.

Results

Whole-body DEPTOR KO mice are viable, fertile, normal in size, and do not display any gross physical and metabolic abnormalities. To circumvent possible compensatory mechanisms linked to the early and systemic loss of DEPTOR, we have deleted DEPTOR specifically in the liver, a tissue in which DEPTOR protein is expressed and affected in response to mTOR activation. Liver-specific DEPTOR null mice showed a reduction in circulating glucose upon fasting versus control mice. This effect was not associated with change in hepatic gluconeogenesis potential but was linked to a sustained reduction in circulating glucose during insulin tolerance tests. In addition to the reduction in glycemia, liver-specific DEPTOR KO mice had reduced hepatic glycogen content when fasted. We showed that loss of DEPTOR cell-autonomously increased oxidative metabolism in hepatocytes, an effect associated with increased cytochrome c expression but independent of changes in mitochondrial content or in the expression of genes controlling oxidative metabolism. We found that liver-specific DEPTOR KO mice showed sustained mTORC1 activation upon fasting, and that acute treatment with rapamycin was sufficient to normalize glycemia in these mice.

Conclusion

We propose a model in which hepatic DEPTOR accelerates the inhibition of mTORC1 during the transition to fasting to adjust metabolism to the nutritional status.

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Whole-body DEPTOR KO mice are viable and do not display abnormalities.

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Liver-specific DEPTOR KO mice are hypoglycemic when fasted.

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Loss of DEPTOR promotes mTORC1 and increases oxidative metabolism.

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Rapamycin corrects hypoglycemia in liver-specific DEPTOR KO mice.

High-resolution genomic profiling of thyroid lesions uncovers preferential copy number gains affecting mitochondrial biogenesis loci in the oncocytic variants

Oncocytic change is the result of aberrant mitochondrial hyperplasia, which may occur in both neoplastic and non-neoplastic cells and is not infrequent in the thyroid. Despite being a well-characterized histologic phenotype, the molecular causes underlying such a distinctive cellular change are poorly understood. To identify potential genetic causes for the oncocytic phenotype in thyroid, we analyzed copy number alterations in a set of oncocytic (n=21) and non-oncocytic (n=20) thyroid lesions by high-resolution microarray-based comparative genomic hybridization (aCGH). Each group comprised lesions of diverse histologic types, including hyperplastic nodules, adenomas and carcinomas. Unsupervised hierarchical clustering of categorical aCGH data resulted in two distinct branches, one of which was significantly enriched for samples with the oncocytic phenotype, regardless of histologic type. Analysis of aCGH events showed that the oncocytic group harbored a significantly higher number of genes involved in copy number gains, when compared to that of conventional thyroid lesions. Functional annotation demonstrated an enrichment for copy number gains that affect genes encoding activators of mitochondrial biogenesis in oncocytic cases but not in their non-oncocytic counterparts. Taken together, our data suggest that genomic alterations may represent additional/alternative mechanisms underlying the development of the oncocytic phenotype in the thyroid.

Thymidine kinase and mtDNA depletion in human cardiomyopathy: epigenetic and translational evidence for energy starvation

This study addresses how depletion of human cardiac left ventricle (LV) mitochondrial DNA (mtDNA) and epigenetic nuclear DNA methylation promote cardiac dysfunction in human dilated cardiomyopathy (DCM) through regulation of pyrimidine nucleotide kinases. Samples of DCM LV and right ventricle (n = 18) were obtained fresh at heart transplant surgery. Parallel samples from nonfailing (NF) controls (n = 12) were from donor hearts found unsuitable for clinical use. We analyzed abundance of mtDNA and nuclear DNA (nDNA) using qPCR. LV mtDNA was depleted in DCM (50%, P < 0.05 each) compared with NF. No detectable change in RV mtDNA abundance occurred. DNA methylation and gene expression were determined using microarray analysis (GEO accession number: GSE43435). Fifty-seven gene promoters exhibited DNA hypermethylation or hypomethylation in DCM LVs. Among those, cytosolic thymidine kinase 1 (TK1) was hypermethylated. Expression arrays revealed decreased abundance of the TK1 mRNA transcript with no change in transcripts for other relevant thymidine metabolism enzymes. Quantitative immunoblots confirmed decreased TK1 polypeptide steady state abundance. TK1 activity remained unchanged in DCM samples while mitochondrial thymidine kinase (TK2) activity was significantly reduced. Compensatory TK activity was found in cardiac myocytes in the DCM LV. Diminished TK2 activity is mechanistically important to reduced mtDNA abundance and identified in DCM LV samples here. Epigenetic and genetic changes result in changes in mtDNA and in nucleotide substrates for mtDNA replication and underpin energy starvation in DCM.

Detection of differentially methylated gene promoters in failing and nonfailing human left ventricle myocardium using computation analysis

Human dilated cardiomyopathy (DCM) is characterized by congestive heart failure and altered myocardial gene expression. Epigenetic changes, including DNA methylation, are implicated in the development of DCM but have not been studied extensively. Clinical human DCM and nonfailing control left ventricle samples were individually analyzed for DNA methylation and expressional changes. Expression microarrays were used to identify 393 overexpressed and 349 underexpressed genes in DCM (GEO accession number: GSE43435). Gene promoter microarrays were utilized for DNA methylation analysis, and the resulting data were analyzed by two different computational methods. In the first method, we utilized subtractive analysis of DNA methylation peak data to identify 158 gene promoters exhibiting DNA methylation changes that correlated with expression changes. In the second method, a two-stage approach combined a particle swarm optimization feature selection algorithm and a discriminant analysis via mixed integer programming classifier to identify differentially methylated gene promoters. This analysis identified 51 hypermethylated promoters and six hypomethylated promoters in DCM with 100% cross-validation accuracy in the group assignment. Generation of a composite list of genes identified by subtractive analysis and two-stage computation analysis revealed four genes that exhibited differential DNA methylation by both methods in addition to altered gene expression. Computationally identified genes (AURKB, BTNL9, CLDN5, and TK1) define a central set of differentially methylated gene promoters that are important in classifying DCM. These genes have no previously reported role in DCM. This study documents that rigorous computational analysis applied to microarray analysis of healthy and diseased human heart samples helps to define clinically relevant DNA methylation and expressional changes in DCM.

Transgenic mouse model with deficient mitochondrial polymerase exhibits reduced state IV respiration and enhanced cardiac fibrosis

Mitochondria produce the energy required for proper cardiac contractile function, and cardiomyocytes that exhibit reduced mitochondrial electron transport will have reduced energy production and decreased contractility. Mitochondrial DNA (mtDNA) encodes the core subunits for the protein complexes of the electron transport chain (ETC). Reduced mtDNA abundance has been linked to reduced ETC and the development of heart failure in genetically engineered mice and in human diseases. Nucleoside reverse-transcriptase inhibitors for HIV/AIDS are used in antiretroviral regimens, which cause decreased mtDNA abundance by inhibiting the mitochondrial polymerase, pol-γ, as a limiting side effect. We explored consequences of AZT (1-[(2R,4S,5S)-4-azido-5-(hydroxymethyl)oxolan-2-yl]-5-methylpyrimidine-2,4-dione) exposure on mtDNA abundance in an established transgenic mouse model (TG) in which a cardiac-targeted mutant form of pol-γ displays a dilated cardiomyopathy (DCM) phenotype with increased left ventricle (LV)-mass and increased LV-end diastolic dimension. TG and wild-type littermate mice received 0.22 mg per day AZT or vehicle for 35 days, and were subsequently analyzed for physiological, histological, and molecular changes. After 35 days, Y955C TGs exhibited cardiac fibrosis independent of AZT. Reduced mtDNA abundance was observed in the Y955C mouse; AZT treatment had no effect on the depletion, suggesting that Y955C was sufficient to reduce mtDNA abundance maximally. Isolated mitochondria from AZT-treated Y955C hearts displayed reduced mitochondrial energetic function by oximetric measurement. AZT treatment of the Y955C mutation further reduced basal mitochondrial respiration and state IV(0) respiration. Together, these results demonstrate that defective pol-γ function promotes cardiomyopathy, cardiac fibrosis, mtDNA depletion, and reduced mitochondrial energy production.

Mitochondrial interference by anti-HIV drugs: mechanisms beyond Pol-γ inhibition

The combined pharmacological approach to the treatment of HIV infection, known as highly active antiretroviral therapy (HAART), has dramatically reduced AIDS-related morbidity and mortality. However, its use has been associated with serious adverse reactions, of which those resulting from mitochondrial dysfunction are particularly widespread. Nucleos(t)ide-reverse transcriptase inhibitors (NRTIs) have long been considered the main source of HAART-related mitochondrial toxicity due to their ability to inhibit Pol-γ, the DNA polymerase responsible for the synthesis of mitochondrial DNA. Nevertheless, accumulating evidence points to a more complex relationship between these organelles and NRTIs. Also, alternative pathways by which other groups of anti-HIV drugs (non-nucleoside reverse transcriptase inhibitors and protease inhibitors) interfere with mitochondria have been suggested, although their implications, both pharmacological and clinical, are open to debate. This review aims to provide a comprehensive overview of the mechanisms and factors which influence the mitochondrial involvement in the toxicity of all three major classes of anti-HIV drugs.

Tenofovir renal proximal tubular toxicity is regulated by OAT1 and MRP4 transporters

Tenofovir disoproxil fumarate (TDF) is an oral prodrug and acyclic nucleotide analog of adenosine monophosphate that inhibits HIV-1 (HIV) reverse transcriptase. A growing subset of TDF-treated HIV(+) individuals presented with acute renal failure, suggesting tenofovir-associated kidney-specific toxicity. Our previous studies using an HIV transgenic mouse model (TG) demonstrated specific changes in renal proximal tubular mitochondrial DNA (mtDNA) abundance. Nucleosides are regulated in biological systems via transport and metabolism in cellular compartments. In this study, the role(s) of organic anion transporter type 1 (OAT1) and multidrug-resistant protein type 4 (MRP4) in transport and regulation of tenofovir in proximal tubules were assessed. Renal toxicity was assessed in kidney tissues from OAT1 knockout (KO) or MRP4 KO compared with wild-type (WT, C57BL/6) mice following treatment with TDF (0.11 mg/day), didanosine (ddI, a related adenosine analog, 0.14 mg/day) or vehicle (0.1 M NaOH) daily gavage for 5 weeks. Laser-capture microdissection (LCM) was used to isolate renal proximal tubules for molecular analyses. mtDNA abundance and ultrastructural pathology were analyzed. mtDNA abundance in whole kidneys from both KO and WT was unchanged regardless of treatment. Renal proximal tubular mtDNA abundance from OAT1 KO also remained unchanged, suggesting prevention of TDF toxicity due to loss of tenofovir transport into proximal tubules. In contrast, renal proximal tubules from MRP4 KO exhibited increased mtDNA abundance following TDF treatment compared with WT littermates, suggesting compensation. Renal proximal tubules from TDF-treated WT and MRP4 KO exhibited increased numbers of irregular mitochondria with sparse, fragmented cristae compared with OAT1 KO. Treatment with ddI had a compensatory effect on mtDNA abundance in OAT1 KO but not in MRP4 KO. Both OAT1 and MRP4 have a direct role in transport and efflux of tenofovir, regulating levels of tenofovir in proximal tubules. Disruption of OAT1 activity prevents tenofovir toxicity but loss of MRP4 can lead to increased renal proximal tubular toxicity. These data help to explain mechanisms of human TDF renal toxicity.

The kinetic effects on thymidine kinase 2 by enzyme-bound dTTP may explain the mitochondrial side effects of antiviral thymidine analogs

Mitochondrial thymidine kinase 2 (TK2) is a key enzyme in the salvage of pyrimidine deoxynucleosides needed for mitochondrial DNA synthesis. TK2 phosphorylates thymidine (dThd), deoxycytidine (dCyd), and many other antiviral pyrimidine nucleoside analogs. Zidovudine (AZT) is the first nucleoside analog approved for anti-HIV therapy, and it is still used in combination with other drugs. One of the side effects of long-term treatment with nucleoside analogs is mitochondrial DNA depletion, which has been ascribed to competition by AZT for the endogenous dThd phosphorylation carried out by TK2. Here we studied the kinetics of AZT and 3'-fluorothymidine phosphorylation by recombinant human TK2 and the effects of these and other pyrimidine nucleoside analogs on the phosphorylation of dThd and dCyd. Thymidine analogs strongly inhibited dThd phosphorylation but not dCyd phosphorylation, which instead was stimulated ∼30%. We found that recombinant human TK2 contained the feedback inhibitor dTTP in a 1:1 molar ratio and that incubation with dThd and AZT could completely remove the enzyme-bound dTTP, but dCyd was less efficient in this regard. The release of feedback inhibitor by dThd and dThd analogs most likely accounts for the observed kinetics. Similar effects were also observed with native rat liver mitochondrial TK2, strongly indicating a physiologic role for this process, which most likely is an important factor in the mitochondrial toxicity observed with antiviral nucleoside analogs.

Absence of mitochondrial toxicity in hearts of transgenic mice treated with abacavir

Abacavir (ABC) is a guanosine nucleoside reverse transcriptase inhibitor (NRTI) with potent antiretroviral activity. Since NRTIs exhibit tissue-specific inhibition of mitochondrial DNA (mtDNA) synthesis, the ability of ABC to inhibit mtDNA synthesis in vivo was evaluated. Inbred wild-type (WT) and transgenic mice (TG) treated with ABC (3.125 mg/d p. o., 35 days) were used to define mitochondrial oxidative stress and cardiac function. Chosen TGs exhibited overexpression of HIV-1 viral proteins (NL4-3Deltagag/pol, non-replication competent), hemizygous depletion or overexpression of mitochondrial superoxide dismutase (SOD2(+/-) knock-out (KO) or MnSOD OX, respectively), overexpression of mitochondrially targeted catalase (MCAT), or double "knockout" deletion of aldehyde dehydrogenase activity (ALDH2 KO). Impact on mtDNA synthesis was assessed by comparing changes in mtDNA abundance between ABC-treated and vehicle-treated WTs and TGs. No changes in mtDNA abundance occurred from ABC treatment in any mice, suggesting no inhibition of mtDNA synthesis. Left ventricle (LV) mass and LV end-diastolic dimension (LVEDD) were defined echocardiographically and remained unchanged as well. These results indicate that treatment with ABC has no visible cardiotoxicity in these adult mice exposed for 5 weeks compared to findings with other antiretroviral NRTI studies and support some claims for its relative safety.

Pyrimidine deoxynucleoside and nucleoside reverse transcriptase inhibitor metabolism in the perfused heart and isolated mitochondria

BACKGROUND

The metabolism of pyrimidine deoxynucleosides and nucleoside reverse transcriptase inhibitors has been studied in growing cells. However, many of these drugs are associated with mitochondrial toxicities observed in non-replicating tissues, such as in the heart, where their metabolism has not been investigated.

METHODS

The aims of this study were twofold. The first was to investigate the metabolism of the thymidine analogues 3'-azido-3'deoxythymidine (AZT) and 2',3'-didehydrodideoxy-thymidine (d4T), and the deoxycytidine (dCyd) analogues 2'-deoxy-3'-thiacytidine (3TC) and 2',3'-dideoxycytidine (ddC) with regard to phosphorylation and breakdown. The second was to investigate their potential effects, singly or in combination with AZT, on metabolism of the naturally occurring deoxynucleosides in the perfused rat heart and in isolated heart mitochondria.

RESULTS

The analogue d4T was not metabolized in perfused heart or in isolated mitochondria, and had no effect on either thymidine or dCyd metabolism. The dCyd analogues were both phosphorylated in perfused heart to the triphosphate, but only at the limit of detection and they were not phosphorylated in isolated mitochondria. Neither ddC nor 3TC had any effect on thymidine or dCyd metabolism in either perfused heart or in isolated mitochondria. AZT has been previously shown to inhibit thymidine phosphorylation. When d4T, 3TC or ddC were given with AZT, only ddC caused a significant further decrease in thymidine phosphorylation.

CONCLUSIONS

These results indicate that with the exception of the competition between AZT and thymidine, there was little competition for phosphorylation among and between these other nucleoside reverse transcriptase inhibitors and the naturally occurring deoxynucleosides in cardiac tissue and isolated heart mitochondria.

Ribonucleotide reductase is not limiting for mitochondrial DNA copy number in mice

Ribonucleotide reductase (RNR) is the rate-limiting enzyme in deoxyribonucleoside triphosphate (dNTP) biosynthesis, with important roles in nuclear genome maintenance. RNR is also essential for maintenance of mitochondrial DNA (mtDNA) in mammals. The mechanisms regulating mtDNA copy number in mammals are only being discovered. In budding yeast, RNR overexpression resulted in increased mtDNA levels, and rescued the disease phenotypes caused by a mutant mtDNA polymerase. This raised the question of whether mtDNA copy number increase by RNR induction could be a strategy for treating diseases with mtDNA mutations. We show here that high-level overexpression of RNR subunits (Rrm1, Rrm2 and p53R2; separately or in different combinations) in mice does not result in mtDNA copy number elevation. Instead, simultaneous expression of two RNR subunits leads to imbalanced dNTP pools and progressive mtDNA depletion in the skeletal muscle, without mtDNA mutagenesis. We also show that endogenous RNR transcripts are downregulated in response to large increases of mtDNA in mice, which is indicative of nuclear-mitochondrial crosstalk with regard to mtDNA copy number. Our results establish that RNR is not limiting for mtDNA copy number in mice, and provide new evidence for the importance of balanced dNTP pools in mtDNA maintenance in postmitotic tissues.

Transgenic cardiac-targeted overexpression of human thymidylate kinase

Thymidylate kinase (TMPK) is a nucleoside monophosphate kinase that catalyzes phosphorylation of thymidine monophosphate to thymidine diphosphate. TMPK also mediates phosphorylation of monophosphates of thymidine nucleoside analog (NA) prodrugs on the pathway to their active triphosphate antiviral or antitumor moieties. Novel transgenic mice (TG) expressing human (h) TMPK were genetically engineered using the alpha-myosin heavy chain promoter to drive its cardiac-targeted overexpression. In '2 by 2' protocols, TMPK TGs and wild-type (WT) littermates were treated with the NA zidovudine (a deoxythymidine analog, 3'-azido-3'deoxythymidine (AZT)) or vehicle for 35 days. Alternatively, TGs and WTs were treated with a deoxycytidine NA (racivir, RCV) or vehicle. Changes in mitochondrial DNA (mtDNA) abundance and mitochondrial ultrastructure were defined quantitatively by real-time PCR and transmission electron microscopy, respectively. Cardiac performance was determined echocardiographically. Results showed TMPK TGs treated with either AZT or RCV exhibited decreased cardiac mtDNA abundance. Cardiac ultrastructural changes were seen only with AZT. AZT-treated TGs exhibited increased left ventricle (LV) mass. In contrast, LV mass in RCV-treated TGs and WTs remained unchanged. In all cohorts, LV end-diastolic dimension remained unchanged. This novel cardiac-targeted overexpression of hTMPK helps define the role of TMPK in mitochondrial toxicity of antiretrovirals.

Tenofovir renal toxicity targets mitochondria of renal proximal tubules

Tenofovir disoproxil fumarate (TDF) is an analog of adenosine monophosphate that inhibits HIV reverse transcriptase in HIV/AIDS. Despite its therapeutic success, renal tubular side effects are reported. The mechanisms and targets of tenofovir toxicity were determined using '2 x 2' factorial protocols, and HIV transgenic (TG) and wild-type (WT) littermate mice with or without TDF (5 weeks). A parallel study used didanosine (ddI) instead of TDF. At termination, heart, kidney, and liver samples were retrieved. Mitochondrial DNA (mtDNA) abundance, and histo- and ultrastructural pathology were analyzed. Laser-capture microdissection (LCM) was used to isolate renal proximal tubules for molecular analyses. Tenofovir increased mtDNA abundance in TG whole kidneys, but not in their hearts or livers. In contrast, ddI decreased mtDNA abundance in the livers of WTs and TGs, but had no effect on their hearts or kidneys. Histological analyses of kidneys showed no disruption of glomeruli or proximal tubules with TDF or ddI treatments. Ultrastructural changes in renal proximal tubules from TDF-treated TGs included an increased number and irregular shape of mitochondria with sparse fragmented cristae. LCM-captured renal proximal tubules from TGs showed decreased mtDNA abundance with tenofovir. The results indicate that tenofovir targets mitochondrial toxicity on the renal proximal tubule in an AIDS model.

Murine cardiac mtDNA: effects of transgenic manipulation of nucleoside phosphorylation

Mitochondrial toxicity results from pyrimidine nucleoside reverse transcriptase inhibitors (NRTIs) for HIV/AIDS. In the heart, this can deplete mitochondrial (mt) DNA and cause cardiac dysfunction (eg, left ventricle hypertrophy, LVH). Four unique transgenic, cardiac-targeted overexpressors (TGs) were generated to determine their individual impact on native mitochondrial biogenesis and effects of NRTI administration on development of mitochondrial toxicity. TGs included cardiac-specific overexpression of native thymidine kinase 2 (TK2), two pathogenic TK2 mutants (H121N and I212N), and a mutant of mtDNA polymerase, pol-gamma (Y955C). Each was treated with antiretrovirals (AZT-HAART, 3 or 10 weeks, zidovudine (AZT) + lamivudine (3TC) + indinavir, or vehicle control). Parameters included left ventricle (LV) performance (echocardiography), LV mtDNA abundance (real-time PCR), and mitochondrial fine structure (electron microscopy, EM) as a function of duration of treatment and presence of TG. mtDNA abundance significantly decreased in Y955C TG, increased in TK2 native and I212N TGs, and was unchanged in H121N TGs at 10 weeks regardless of treatment. Y955C and I212N TGs exhibited LVH during growth irrespective of treatment. Y955C TGs exhibited cardiomyopathy (CM) at 3 and 10 weeks irrespective of treatment, whereas H121N and I212N TGs exhibited CM only after 10 weeks AZT-HAART. EM features were consistent with cardiac dysfunction. mtDNA abundance and cardiac functional changes were related to TG expression of mitochondrially related genes, mutations thereof, and NRTIs.

Mouse models of mitochondrial DNA defects and their relevance for human disease

Qualitative and quantitative changes in mitochondrial DNA (mtDNA) have been shown to be common causes of inherited neurodegenerative and muscular diseases, and have also been implicated in ageing. These diseases can be caused by primary mtDNA mutations, or by defects in nuclear-encoded mtDNA maintenance proteins that cause secondary mtDNA mutagenesis or instability. Furthermore, it has been proposed that mtDNA copy number affects cellular tolerance to environmental stress. However, the mechanisms that regulate mtDNA copy number and the tissue-specific consequences of mtDNA mutations are largely unknown. As post-mitotic tissues differ greatly from proliferating cultured cells in their need for mtDNA maintenance, and as most mitochondrial diseases affect post-mitotic cell types, the mouse is an important model in which to study mtDNA defects. Here, we review recently developed mouse models, and their contribution to our knowledge of mtDNA maintenance and its role in disease.

Structure, physiological role, and specific inhibitors of human thymidine kinase 2 (TK2): present and future

Human mitochondrial thymidine kinase (TK2) is a pyrimidine deoxynucleoside kinase (dNK) that catalyzes the phosphorylation of pyrimidine deoxynucleosides to their corresponding deoxynucleoside 5′‐monophosphates by γ‐phosphoryl transfer from ATP. In resting cells, TK2 is suggested to play a key role in the mitochondrial salvage pathway to provide pyrimidine nucleotides for mitochondrial DNA (mtDNA) synthesis and maintenance. However, recently the physiological role of TK2turned out to have direct clinical relevance as well. Point mutations in the gene encoding TK2 have been correlated to mtDNA disorders in a heterogeneous group of patients suffering from the so‐called mtDNA depletion syndrome (MDS). TK2 activity could also be involved in mitochondrial toxicity associated to prolonged treatment with antiviral nucleoside analogues like AZT and FIAU. Therefore, TK2 inhibitors can be considered as valuable tools to unravel the role of TK2 in the maintenance and homeostasis of mitochondrial nucleotide pools and mtDNA, and to clarify the contribution of TK2 activity to mitochondrial toxicity of certain antivirals. Highly selective TK‐2 inhibitors having an acyclic nucleoside structure and efficiently discriminating between TK‐2 and the closely related TK‐1 have already been reported. It is actually unclear whether these agents efficiently reach the inner mitochondrial compartment. In the present review article,structural features of TK2, MDS‐related mutations observed in TK2 and their role in MDS will be discussed. Also, an update on novel and selective TK2 inhibitors will be provided. © 2008 Wiley Periodicals, Inc. Med Res Rev, 28, No. 5, 797–820, 2008

Cardiac-targeted transgenic mutant mitochondrial enzymes: mtDNA defects, antiretroviral toxicity and cardiomyopathy

Mitochondrial (mt) DNA biogenesis is critical to cardiac contractility. DNA polymerase gamma (Pol gamma) replicates mtDNA, whereas thymidine kinase 2 (TK2) monophosphorylates pyrimidines intramitochondrially. Point mutations in POLG and TK2 result in clinical diseases associated with mtDNA depletion and organ dysfunction. Pyrimidine analogs (NRTIs) inhibit Pol gamma and mtDNA replication. Cardiac "dominant negative" murine transgenes (TGs; Pol gamma Y955C, and TK2 H121N or I212N) defined the role of each in the heart. mtDNA abundance, histopathological features, histochemistry, mitochondrial protein abundance, morphometry, and echocardiography were determined for TGs in "2 x 2" studies with or without pyrimidine analogs. Cardiac mtDNA abundance decreased in Y955C TGs ( approximately 50%) but increased in H121N and I212N TGs (20-70%). Succinate dehydrogenase (SDH) increased in hearts of all mutants. Ultrastructural changes occurred in Y955C and H121N TGs. Histopathology demonstrated hypertrophy in H121N, LV dilation in I212N, and both hypertrophy and dilation in Y955C TGs. Antiretrovirals increased LV mass ( approximately 50%) for all three TGs which combined with dilation indicates cardiomyopathy. Taken together, these studies demonstrate three manifestations of cardiac dysfunction that depend on the nature of the specific mutation and antiretroviral treatment. Mutations in genes for mtDNA biogenesis increase risk for defective mtDNA replication, leading to LV hypertrophy.

Mitochondrial DNA impairment in nucleoside reverse transcriptase inhibitor-associated cardiomyopathy

Acquired immune deficiency syndrome (AIDS) is a global epidemic that continues to escalate. Recent World Health Organization estimates include over 33 million people currently diagnosed with HIV/AIDS. Another 20 million HIV-infected individuals died over the past quarter century. Antiretrovirals are effective treatments that changed the outcome of HIV infection from a fatal disease to a chronic illness. Cardiomyopathy (CM) is a bona fide component of HIV/AIDS with occurrence that is higher in HIV positive individuals. CM may result from individual or combined effects of HIV, immune reactions, or toxicities of prolonged antiretrovirals. Nucleoside reverse transcriptase inhibitors (NRTIs) are the cornerstone of antiretroviral therapy. Despite pharmacological benefits of NRTIs, NRTI side effects include increased risk for CM. Clinical observations and in vitro and in vivo studies support various mechanisms of CM. This perspective highlights some of the hypotheses and focuses on mitochondrial-associated pathways of NRTI- related CM.

Uridine supplementation antagonizes zidovudine-induced mitochondrial myopathy and hyperlactatemia in mice

OBJECTIVE

Zidovudine is an antiretroviral nucleoside analog reverse transcriptase inhibitor that induces mitochondrial myopathy by interfering with the replication of mitochondrial DNA (mtDNA). Because zidovudine inhibits thymidine kinases, the mechanism of mtDNA depletion may be related to an impairment of the de novo synthesis of pyrimidine nucleotides, which are required building blocks of mtDNA. This study was undertaken to determine whether mitochondrial myopathy is a class effect of antiretroviral nucleoside analogs, and whether the muscle disease can be prevented by treatment with uridine as a pyrimidine nucleotide precursor.

METHODS

BALB/c mice were treated with zidovudine or zalcitabine. Some of the mice were cotreated with mitocnol, a dietary supplement with high uridine bioavailability. Mice hind limb muscles were examined after 10 weeks.

RESULTS

Zidovudine induced muscle fiber thinning, myocellular fat deposition, and abnormalities of mitochondrial ultrastructure. In mice treated with zidovudine, organelles contained low mtDNA copy numbers and reduced cytochrome c oxidase activity. The expression of the mtDNA-encoded cytochrome c oxidase I subunit, but not of nucleus-encoded mitochondrial proteins, was impaired. Zidovudine also increased the levels of myocellular reactive oxygen species and blood lactate. Uridine supplementation attenuated or normalized all pathologic abnormalities and had no intrinsic effects. Zalcitabine did not elicit muscle toxicity.

CONCLUSION

Our findings indicate that zidovudine, but not zalcitabine, induces mitochondrial myopathy, which is substantially antagonized by uridine supplementation. These results provide proof of the importance of pyrimidine pools in the pathogenesis of zidovudine myopathy. Since uridine supplementation is tolerated well by humans, this treatment strategy should be investigated in clinical trials.

Mitochondrial expression of the Drosophila melanogaster multisubstrate deoxyribonucleoside kinase

The multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster deoxyribonucleoside kinase (Dm-dNK) is studied as a candidate suicide gene for applications in combined gene/chemotherapy of cancer. We have created an engineered Dm-dNK nucleoside kinase that is targeted to the mitochondrial matrix. The enzyme was expressed in a thymidine kinase 1-deficient osteosarcoma cell line, and the sensitivity of the cells to cytotoxic nucleoside analogs was determined when the enzyme was targeted to either the nucleus or the mitochondrial matrix. Although the total deoxythymidine (dThd) phosphorylation activity was similar in cells expressing Dm-dNK in the nucleus or in the mitochondria, the cells expressing the enzyme in the mitochondria showed higher sensitivity to the antiproliferative activity of several pyrimidine nucleoside analogs, such as (E)-5-(2-bromovinyl)-2'-deoxyuridine, 5-bromo-2'-deoxyuridine, and 5-fluoro-2'-deoxyuridine. Labeling studies using [3H]dThd showed that the cells expressing the mitochondrial enzyme had an increased incorporation of [3H]dThd into DNA, shown to be due to a higher [3H]dTTP specific activity of the total dTTP pool in the cells in which Dm-dNK was targeted to the mitochondria. The difference in the specific activity of the dTTP pool is a result of different contributions of the de novo and the salvage pathways for the dTTP synthesis in transduced cells. In summary, these findings suggest that mitochondrial targeting of Dm-dNK facilitates nucleoside and nucleoside analog phosphorylation and could be used as a strategy to enhance the efficacy of nucleoside analog phosphorylation and concomitantly their cytostatic potential.