The pyruvate dehydrogenase complex as a target for gene therapy

Potential role of stress-induced gluconeogenesis in disease aggravation and mortality in pyruvate dehydrogenase deficiency: A case-based hypothesis

Pyruvate dehydrogenase (PDH) deficiency is an inherited metabolic disorder caused by a defect in any subunit of the pyruvate dehydrogenase complex (PDHC), which has an essential role in glucose metabolism. The causes of disease progression in PDH deficiency are not fully understood yet. Based on repeated observations of a patient with PDH deficiency at our center, we hypothesized that stress-induced gluconeogenesis contributes to rapid exacerbation of the disease. This link has not been established previously.

BRAT1 deficiency causes increased glucose metabolism and mitochondrial malfunction

Background

BRAT1 (BRCA1-associated ATM activator 1) interacts with both BRCA1, ATM and DNA-PKcs, and has been implicated in DNA damage responses. However, based on our previous results, it has been shown that BRAT1 may be involved in cell growth and apoptosis, besides DNA damage responses, implying that there are undiscovered functions for BRAT1.

Methods

Using RNA interference against human BRAT1, we generated stable BRAT1 knockdown cancer cell lines of U2OS, Hela, and MDA-MA-231. We tested cell growth properties and in vitro/in vivo tumorigenic potentials of BRAT1 knockdown cells compared to control cells. To test if loss of BRAT1 induces metabolic abnormalities, we examined the rate of glycolysis, ATP production, and PDH activity in both BRAT1 knockdown and control cells. The role of BRAT1 in growth signaling was determined by the activation of Akt/Erk, and SC79, Akt activator was used for validation.

Results

By taking advantage of BRAT1 knockdown cancer cell lines, we found that loss of BRAT1 expression significantly decreases cell proliferation and tumorigenecity both in vitro and in vivo. Cell migration was also remarkably lowered when BRAT1 was depleted. Interestingly, glucose uptake and production of mitochondrial ROS (reactive oxygen species) are highly increased in BRAT1 knockdown HeLa cells. Furthermore, both basal and induced activity of Akt and Erk kinases were suppressed in these cells, implicating abnormality in signaling cascades for cellular growth. Consequently, treatment of BRAT1 knockdown cells with Akt activator can improve their proliferation and reduces mitochondrial ROS concentration.

Conclusions

These findings suggest novel roles of BRAT1 in cell proliferation and mitochondrial functions.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-548) contains supplementary material, which is available to authorized users.

Theoretical study of the catalytic mechanism of E1 subunit of pyruvate dehydrogenase multienzyme complex from Bacillus stearothermophilus

Pyruvate dehydrogenase multienzyme complex (PDHc) is a member of a family of 2-oxo acid dehydrogenase (OADH) multienzyme complexes involved in several central points of oxidative metabolism, and the E1 subunit is the most important component in the entire PDHc catalytic system, which catalyzes the reversible transfer of an acetyl group from a pyruvate to the lipoyl group of E2 subunit lipoly domain. In this article, the catalytic mechanism of the E1 subunit has been systematically studied using density functional theory (DFT). Four possible pathways with different general acid/base catalysts in decarboxylation and reductive acylation processes were explored. Our calculation results indicate that the 4'-amino pyrimidine of ThDP and residue His128 are the most likely proton donors in the decarboxylation and reductive acylation processes, respectively. During the reaction, each C-C and C-S bond formation or cleavage process, except for the liberation of CO2, is always accompanied by a proton transfer between the substrates and proton donors. The liberation of CO2 is calculated to be the rate-limiting step for the overall reaction, with an energy barrier of 13.57 kcal/mol. The decarboxylation process is endothermic by 5.32 kcal/mol, whereas the reductive acylation process is exothermic with a value of 5.74 kcal/mol. The assignment of protonation states of the surrounding residues can greatly influence the reaction. Residues His128 and His271 play roles in positioning the first substrate pyruvate and second substrate lipoyl group, respectively.

Minocycline, levodopa and MnTMPyP induced changes in the mitochondrial proteome profile of MPTP and maneb and paraquat mice models of Parkinson's disease

Mitochondrial dysfunction is the foremost perpetrator of the nigrostriatal dopaminergic neurodegeneration leading to Parkinson's disease (PD). However, the roles played by majority of the mitochondrial proteins in PD pathogenesis have not yet been deciphered. The present study investigated the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and combined maneb and paraquat on the mitochondrial proteome of the nigrostriatal tissues in the presence or absence of minocycline, levodopa and manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin (MnTMPyP). The differentially expressed proteins were identified and proteome profiles were correlated with the pathological and biochemical anomalies induced by MPTP and maneb and paraquat. MPTP altered the expression of twelve while combined maneb and paraquat altered the expression of fourteen proteins. Minocycline, levodopa and MnTMPyP, respectively, restored the expression of three, seven and eight proteins in MPTP and seven, eight and eight proteins in maneb- and paraquat-treated groups. Although levodopa and MnTMPyP rescued from MPTP- and maneb- and paraquat-mediated increase in the microglial activation and decrease in manganese-superoxide dismutase expression and complex I activity, dopamine content and number of dopaminergic neurons, minocycline defended mainly against maneb- and paraquat-mediated alterations. The results demonstrate that MPTP and combined maneb and paraquat induce mitochondrial dysfunction and microglial activation and alter the expression of a bunch of mitochondrial proteins leading to the nigrostriatal dopaminergic neurodegeneration and minocycline, levodopa or MnTMPyP variably offset scores of such changes.

Ventilatory behavior and carotid body morphology of Brown Norway and Sprague Dawley rats

Differences in acute ventilatory behavior are associated with carotid body (CB) structural and immunohistologic profiles in some, but not all, reports. Brown Norway (BN) rats exhibit lower acute ventilatory responses to hypoxia and hypercapnia compared to Sprague Dawley (SD) rats. We hypothesized that BN rats possess CB with fewer glomus cells. Ventilation was recorded in 6-month-old BN and SD rats exposed to hypoxia-reoxygenation and hypercapnia. Extracted CBs were examined using H&E staining, and immunohistochemistry with antibodies specific for tyrosine hydroxylase (TH), neural nitric oxide synthase (nNOS), and pyruvate dehydrogenase (PD). Sections were analyzed for cell and immunostaining density. SD displayed greater hypoxic and hypercapnic responses, and post-hypoxic short term potentiation, whereas BN exhibited post-hypoxic frequency decline. Contrary to our hypothesis, BN demonstrated a denser arrangement of glomus cells with a larger TH stained area (31.7% BN, 22.6% SD; p<0.0001), and nNOS stained area (37.3% BN, 32.1%; SD; p=0.01). Hence, respiratory phenotype does not correlate intuitively with these anatomic features.

Targeting mitochondrial dysfunction in neurodegenerative disease: Part II

IMPORTANCE OF THE FIELD

With improvements in life expectancy over the past decades, the incidence of neurodegenerative disease has dramatically increased and new therapeutic strategies are urgently needed. One possible approach is to target mitochondrial dysfunction, which has been implicated in the pathogenesis of numerous neurodegenerative disorders.

AREAS COVERED IN THIS REVIEW

This review examines the role of mitochondrial dysfunction in neurodegeneration, drawing examples from common diseases such as Alzheimer's disease and rarer familial disorders such as Charcot-Marie-Tooth. The review is provided in two parts. In part I we discussed the mitochondrial defects which have been most extensively researched (oxidative stress, bioenergetic dysfunction, calcium mishandling). We focus now on those defects which have more recently been implicated in neurodegeneration; in mitochondrial fusion/fission, protein import, protein quality control, kinase signalling and opening of the permeability transition pore.

WHAT THE READER WILL GAIN

An examination of mitochondrial defects observed in neurodegeneration, and existing and possible future therapies to target these defects.

TAKE HOME MESSAGE

The mitochondrially-targeted therapeutics that have reached clinical trials so far have produced encouraging but largely inconclusive results. Increasing understanding of mitochondrial dysfunction has, however, led to preclinical work focusing on novel approaches, which has generated exciting preliminary data.

BAX inhibitor-1 enhances cancer metastasis by altering glucose metabolism and activating the sodium-hydrogen exchanger: the alteration of mitochondrial function

The anti-apoptotic protein, BAX inhibitor-1 (BI-1), has a role in cancer/tumor progression. BI-1-overexpressing HT1080 and B16F10 cells produced higher lung weights and tumor volumes after injection into the tail veins of mice. Transfection of BI-1 siRNA into cells before injection blocked lung metastasis. in vitro, the overexpression of BI-1 increased cell mobility and invasiveness, with highly increased glucose consumption and cytosolic accumulation of lactate and pyruvate, but decreased mitochondrial O(2) consumption and ATP production. Glucose metabolism-associated extracellular pH also decreased as cells excreted more H(+), and sodium hydrogen exchanger (NHE) activity increased, probably as a homeostatic mechanism for intracellular pH. These alterations activated MMP 2/9 and cell mobility and invasiveness, which were reversed by the NHE inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting a role for NHE in cancer metastasis. In both in vitro and in vivo experiments, C-terminal deleted (CDeltaBI-1) cells showed similar results to control cells, suggesting that the C-terminal motif is required for BI-1-associated alterations of glucose metabolism, NHE activation and cancer metastasis. These findings strongly suggest that BI-1 reduces extracellular pH and regulates metastasis by altering glucose metabolism and activating NHE, with the C-terminal tail having a pivotal role in these processes.

Role of dichloroacetate in the treatment of genetic mitochondrial diseases

Dichloroacetate (DCA) is an investigational drug for the treatment of genetic mitochondrial diseases. Its primary site of action is the pyruvate dehydrogenase (PDH) complex, which it stimulates by altering its phosphorylation state and stability. DCA is metabolized by and inhibits the bifunctional zeta-1 family isoform of glutathione transferase/maleylacetoacetate isomerase. Polymorphic variants of this enzyme differ in their kinetic properties toward DCA, thereby influencing its biotransformation and toxicity, both of which are also influenced by subject age. Results from open label studies and controlled clinical trials suggest chronic oral DCA is generally well-tolerated by young children and may be particularly effective in patients with PDH deficiency. Recent in vitro data indicate that a combined DCA and gene therapy approach may also hold promise for the treatment of this devastating condition.

Potential of AAV vectors in the treatment of metabolic disease

Inborn errors of metabolism are collectively common, frequently severe and in many instances difficult or impossible to treat. Accordingly, there is a compelling need to explore novel therapeutic modalities, including gene therapy, and examine multiple phenotypes where the risks of experimental therapy are outweighed by potential benefits to trial participants. Among available gene delivery systems recombinant AAV shows special promise for the treatment of metabolic disease given the unprecedented efficiencies achieved in transducing key target tissues, such as liver and muscle, in small animal models. To date over 30 metabolic disease phenotypes have been investigated in small animal studies with complete phenotype correction being achieved in a substantial proportion. Achieving adequately widespread transduction within the central nervous system, however, remains a major challenge, and will be critical to realization of the therapeutic potential of gene therapy for many of the most clinically troubling metabolic disease phenotypes. Despite the relatively low immunogenicity of AAV vectors, immune responses are also emerging as a factor requiring special attention as efforts accelerate toward human clinical translation. Four metabolic disease phenotypes have reached phase I or I/II trials with one, targeting lipoprotein lipase deficiency, showing exciting early evidence of efficacy.

A combined therapeutic approach for pyruvate dehydrogenase deficiency using self-complementary adeno-associated virus serotype-specific vectors and dichloroacetate

We determined the ability of self-complementary adeno-associated virus (scAAV) vectors to deliver and express the pyruvate dehydrogenase E1alpha subunit gene (PDHA1) in primary cultures of skin fibroblasts from 3 patients with defined mutations in PHDA1 and 3 healthy subjects. Cells were transduced with scAAV vectors containing the cytomegalovirus promoter-driven enhanced green fluorescent protein (EGFP) reporter gene at a vector:cell ratio of 200. Transgene expression was measured 72h later. The transduction efficiency of scAAV2 and scAAV6 vectors was 3- to 5-fold higher than that of the other serotypes, which were subsequently used to transduce fibroblasts with wild-type PDHA1 cDNA under the control of the chicken beta-action (CBA) promoter at a vector:cell ratio of 1000. Total PDH-specific activity and E1alpha protein expression were determined 10 days post-transduction. Both vectors increased E1alpha expression 40-60% in both control and patient cells, and increased PDH activity in two patient cell lines. We also used dichloroacetate (DCA) to maximally activate PDH through dephosphorylation of E1alpha. Exposure for 24h to 5mM DCA increased PDH activity in non-transduced control (mean 37% increase) and PDH deficient (mean 44% increase) cells. Exposure of transduced patient fibroblasts to DCA increased PDH activity up to 90% of the activity measured in untreated control cells. DCA also increased expression of E1alpha protein and, to variable extents, that of other components of the PDH complex in both non-transduced and transduced cells. These data suggest that a combined gene delivery and pharmacological approach may hold promise for the treatment of PDH deficiency.

Mitochondrial metabolism, redox signaling, and fusion: a mitochondria-ROS-HIF-1alpha-Kv1.5 O2-sensing pathway at the intersection of pulmonary hypertension and cancer

Pulmonary arterial hypertension (PAH) is a lethal syndrome characterized by vascular obstruction and right ventricular failure. Although the fundamental cause remains elusive, many predisposing and disease-modifying abnormalities occur, including endothelial injury/dysfunction, bone morphogenetic protein receptor-2 gene mutations, decreased expression of the O(2)-sensitive K(+) channel (Kv1.5), transcription factor activation [hypoxia-inducible factor-1alpha (HIF-1alpha) and nuclear factor-activating T cells], de novo expression of survivin, and increased expression/activity of both serotonin transporters and platelet-derived growth factor receptors. Together, these abnormalities create a cancerlike, proliferative, apoptosis-resistant phenotype in pulmonary artery smooth muscle cells (PASMCs). A possible unifying mechanism for PAH comes from studies of fawn-hooded rats, which manifest spontaneous PAH and impaired O(2) sensing. PASMC mitochondria normally produce reactive O(2) species (ROS) in proportion to P(O2). Superoxide dismutase 2 (SOD2) converts intramitochondrial superoxide to diffusible H(2)O(2), which serves as a redox-signaling molecule, regulating pulmonary vascular tone and structure through effects on Kv1.5 and transcription factors. O(2) sensing is mediated by this mitochondria-ROS-HIF-1alpha-Kv1.5 pathway. In PAH and cancer, mitochondrial metabolism and redox signaling are reversibly disordered, creating a pseudohypoxic redox state characterized by normoxic decreases in ROS, a shift from oxidative to glycolytic metabolism and HIF-1alpha activation. Three newly recognized mitochondrial abnormalities disrupt the mitochondria-ROS-HIF-1alpha-Kv1.5 pathway: 1) mitochondrial pyruvate dehydrogenase kinase activation, 2) SOD2 deficiency, and 3) fragmentation and/or hyperpolarization of the mitochondrial reticulum. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, corrects the mitochondrial abnormalities in experimental models of PAH and human cancer, causing a regression of both diseases. Mitochondrial abnormalities that disturb the ROS-HIF-1alpha-Kv1.5 O(2)-sensing pathway contribute to the pathogenesis of PAH and cancer and constitute promising therapeutic targets.

Down-regulation of expression of rat pyruvate dehydrogenase E1alpha gene by self-complementary adeno-associated virus-mediated small interfering RNA delivery

Mutations in the E1alpha subunit gene (PDHA1) of the pyruvate dehydrogenase complex (PDC) are common causes of congenital lactic acidosis. An animal model of E1alpha deficiency could provide insight into the pathological consequences of mutations and serve to test potential therapies. Small interfering RNAs (siRNAs) were designed to cleave the messenger RNA (mRNA) of the E1alpha subunit and were tested in vitro to assess the feasibility of producing a gene knockdown in rats. HEK 293 cells were co-transfected with a rat PDHA1 expression vector and eight naked siRNAs that specifically targeted rat E1alpha mRNA. Quantitative PCR (qPCR) analyses showed that four siRNAs reduced rat PDHA1 RNA levels up to 85% by 24h and up to 65% by 56h, compared to negative and positive controls. Since oligonucleotide-mediated siRNA delivery provided only transient suppression, we next selected two siRNA candidates and generated self-complementary, double-stranded adeno-associated virus (scAAV) vectors (serotypes 2 and 5) expressing a rat short hairpin siRNA expression cassette (scAAVsi-PDHA1). Rat lung fibroblast (RLF) cultures were infected with scAAVsi-PDHA1 vectors. The RLF PDHA1 mRNA level was reduced 53-80% 72h after infection and 54-70% 10 days after infection in RLF cultures. The expression of E1alpha and the specific activity of pyruvate dehydrogenase were also decreased at 10 days after infection in RLF cultures. Thus, scAAV siRNA-mediated knockdown of PDHA1 gene expression provides a strategy that may be applied to create a useful animal model of PDC deficiency.

Micro-PET/CT Monitoring of Herpes Thymidine Kinase Suicide Gene Therapy in a Prostate Cancer Xenograft: The Advantage of a Cell-specific Transcriptional Targeting Approach

Cancer gene therapy based on tissue-restricted expression of cytotoxic gene should achieve superior therapeutic index over an unrestricted method. This study compared the therapeutic effects of a highly augmented, prostate-specific gene expression method to a strong constitutive promoter-driven approach. Molecular imaging was coupled to gene therapy to ascertain real-time therapeutic activity. The imaging reporter gene (luciferase) and the cytotoxic gene (herpes simplex thymidine kinase) were delivered by adenoviral vectors injected directly into human prostate tumors grafted in SCID mice. Serial bioluminescence imaging, positron emission tomography, and computed tomography revealed restriction of gene expression to the tumors when prostate-specific vector was employed. In contrast, administration of constitutive active vector resulted in strong signals in the liver. Liver serology, tissue histology, and frail condition of animals confirmed liver toxicity suffered by the constitutive active cohorts, whereas the prostate-targeted group was unaffected. The extent of tumor killing was analyzed by apoptotic staining and human prostate marker (prostate-specific antigen). Overall, the augmented prostate-specific expression system was superior to the constitutive approach in safeguarding against systemic toxicity, while achieving effective tumor killing. Integrating noninvasive imaging into cytotoxic gene therapy will provide a useful strategy to monitor gene expression and therapeutic efficacy in future clinical protocols.