Mitochondrial Metabolism in the Spotlight: Maintaining Balanced RNAP III Activity Ensures Cellular Homeostasis

RNA polymerase III (RNAP III) holoenzyme activity and the processing of its products have been linked to several metabolic dysfunctions in lower and higher eukaryotes. Alterations in the activity of RNAP III-driven synthesis of non-coding RNA cause extensive changes in glucose metabolism. Increased RNAP III activity in the S. cerevisiae maf1Δ strain is lethal when grown on a non-fermentable carbon source. This lethal phenotype is suppressed by reducing tRNA synthesis. Neither the cause of the lack of growth nor the underlying molecular mechanism have been deciphered, and this area has been awaiting scientific explanation for a decade. Our previous proteomics data suggested mitochondrial dysfunction in the strain. Using model mutant strains maf1Δ (with increased tRNA abundance) and rpc128-1007 (with reduced tRNA abundance), we collected data showing major changes in the TCA cycle metabolism of the mutants that explain the phenotypic observations. Based on 13C flux data and analysis of TCA enzyme activities, the present study identifies the flux constraints in the mitochondrial metabolic network. The lack of growth is associated with a decrease in TCA cycle activity and downregulation of the flux towards glutamate, aspartate and phosphoenolpyruvate (PEP), the metabolic intermediate feeding the gluconeogenic pathway. rpc128-1007, the strain that is unable to increase tRNA synthesis due to a mutation in the C128 subunit, has increased TCA cycle activity under non-fermentable conditions. To summarize, cells with non-optimal activity of RNAP III undergo substantial adaptation to a new metabolic state, which makes them vulnerable under specific growth conditions. Our results strongly suggest that balanced, non-coding RNA synthesis that is coupled to glucose signaling is a fundamental requirement to sustain a cell's intracellular homeostasis and flexibility under changing growth conditions. The presented results provide insight into the possible role of RNAP III in the mitochondrial metabolism of other cell types.

Metabolic fate of glucose in the brain of APP/PS1 transgenic mice at 10 months of age: a 13C NMR metabolomic study

Alzheimer's disease (AD) has been associated with the disturbance of brain glucose metabolism. The present study investigates brain glucose metabolism using ¹³C NMR metabolomics in combination with intravenous [1-¹³C]-glucose infusion in APP/PS1 transgenic mouse model of amyloid pathology at 10 months of age. We found that brain glucose was significantly accumulated in APP/PS1 mice relative to wild-type (WT) mice. Reductions in ¹³C fluxes into the specific carbon sites of tricarboxylic acid (TCA) intermediate (succinate) as well as neurotransmitters (glutamate, glutamine, γ-aminobutyric acid and aspartate) from [1-¹³C]-glucose were also detected in the brain of APP/PS1 mice. In addition, our results reveal that the ¹³C-enrichments of the C3 of alanine were significantly lower and the C3 of lactate have a tendency to be lower in the brain of APP/PS1 mice than WT mice. Taken together, the development of amyloid pathology could cause a reduction in glucose utilization and further result in decreases in energy and neurotransmitter metabolism as well as the lactate-alanine shuttle in the brain.

Systems metabolic engineering of Escherichia coli for gram scale production of the antitumor drug deoxyviolacein from glycerol

Deoxyviolacein is a microbial drug with biological activity against tumors, gram-positive bacteria, and fungal plant pathogens. Here, we describe an Escherichia coli strain for heterologous production of this high-value drug from glycerol. Plasmid-based expression of the deoxyviolacein cluster vioABCE was controlled by the araBAD promoter and induction by L-arabinose. Through elimination of L-arabinose catabolism in E. coli, the pentose sugar could be fully directed to induction of deoxyviolacein biosynthesis and was no longer metabolized, as verified by (13) C isotope experiments. Deletion of the araBAD genes beneficially complemented with previously described (i) engineering of the pentose phosphate pathway, (ii) chorismate biosynthesis, (iii) tryptophan biosynthesis, (iv) improved supply of L-serine, (v) elimination of tryptophan repression, and (vi) of tryptophan catabolism. Subsequent screening of the created next-generation producer E. coli dVio-8 identified glycerol as optimum carbon source and a level of 100 mg L(-1) of L-arabinose as optimum for induction. Transferred to a glycerol-based fed-batch process, E. coli dVio-8 surpassed the gram scale and produced 1.6 g L(-1) deoxyviolacein. With straightforward extraction from culture broth and purification by flash chromatography, deoxyviolacein was obtained at >99.5% purity. Biotechnol. Bioeng. 2014;111: 2280-2289. © 2014 Wiley Periodicals, Inc.