Oxidative phosphorylation and respiratory control in housefly mitochondria

Nuclear genomic control of naturally occurring variation in mitochondrial function in Drosophila melanogaster

Background

Mitochondria are organelles found in nearly all eukaryotic cells that play a crucial role in cellular survival and function. Mitochondrial function is under the control of nuclear and mitochondrial genomes. While the latter has been the focus of most genetic research, we remain largely ignorant about the nuclear-encoded genomic control of inter-individual variability in mitochondrial function. Here, we used Drosophila melanogaster as our model organism to address this question.

Results

We quantified mitochondrial state 3 and state 4 respiration rates and P:O ratio in mitochondria isolated from the thoraces of 40 sequenced inbred lines of the Drosophila Genetic Reference Panel. We found significant within-population genetic variability for all mitochondrial traits. Hence, we performed genome-wide association mapping and identified 141 single nucleotide polymorphisms (SNPs) associated with differences in mitochondrial respiration and efficiency (P ≤1 × 10^-5). Gene-centered regression models showed that 2–3 SNPs can explain 31, 13, and 18% of the phenotypic variation in state 3, state 4, and P:O ratio, respectively. Most of the genes tagged by the SNPs are involved in organ development, second messenger-mediated signaling pathways, and cytoskeleton remodeling. One of these genes, sallimus (sls), encodes a component of the muscle sarcomere. We confirmed the direct effect of sls on mitochondrial respiration using two viable mutants and their coisogenic wild-type strain. Furthermore, correlation network analysis revealed that sls functions as a transcriptional hub in a co-regulated module associated with mitochondrial respiration and is connected to CG7834, which is predicted to encode a protein with mitochondrial electron transfer flavoprotein activity. This latter finding was also verified in the sls mutants.

Conclusions

Our results provide novel insights into the genetic factors regulating natural variation in mitochondrial function in D. melanogaster. The integrative genomic approach used in our study allowed us to identify sls as a novel hub gene responsible for the regulation of mitochondrial respiration in muscle sarcomere and to provide evidence that sls might act via the electron transfer flavoprotein/ubiquinone oxidoreductase complex.

Pyruvate-dependent oxidative phosphorylation in erythroid and myeloid tumor mitochondria

The pyruvate-supported oxidative phosphorylation activity was determined in mitochondria isolated from the fast-growing erythroid and myeloid tumors of hematopoietic origin. Normal bone marrow and liver mitochondria were used for comparison. In the absence of primers, both tumor mitochondria exhibited a pyruvate-dependent respiratory state 4/state 3 transition, which was totally inhibited by either alpha-cyanocinnamate or arsenite. The transition rate increased in a concentration-dependent manner from 5 to 100 microM pyruvate, where the maximum activity was reached. Increasing the concentration to 500 microM and beyond, however, resulted in decreasing state 3 respiratory jump with little or no jump demonstrable at concentrations above 5 mM. Moreover, the addition of high concentrations of pyruvate during the respiratory state 3 caused a blockage of that state which was reestablished by the addition of succinate or alpha-ketoglutarate. These results clearly show the capacity of erythroid and myeloid tumor mitochondria to actively utilize low concentrations of pyruvate to support their oxidative phosphorylation activity. The reason for the absence of activity found with the high concentration, however, is not readily apparent.

Some properties of mitochondria isolated from the flight muscle of the periodical cicada, Magicicada septendecim

Mitochondria from the flight muscle of the periodical cicada oxidize pyruvate and d-glycerol 1-phosphate at rates comparable with those obtained with flight-muscle mitochondria from other insects. The oxidation of d-glycerol 1-phosphate is greatly stimulated by low concentrations of Ca(2+). However, oxidative phosphorylation with this substrate is optimum over only a narrow range of Ca(2+) concentration, because of the ability of these mitochondria actively to accumulate Ca(2+) present at micromolar concentrations. The oxidation of pyruvate via the complete tricarboxylic acid cycle is enhanced by high concentrations of phosphate. When both pyruvate and d-glycerol 1-phosphate are present simultaneously, there is no simple summation of the rates obtained with the substrates singly. Acetyl-l-carnitine, palmitoyl-l-carnitine, glutamate and 2-oxoglutarate are oxidized at rates similar to those obtained with mammalian mitochondria, though lower than those obtained with the two prime substrates. However, no other tricarboxylic acid-cycle intermediates added to the medium were oxidized. From these and other observations it has been concluded that these mitochondria possess a previously undescribed combination of substrate-anion permeases.

Tetrachloroanisol: a source of musty taste in eggs and broilers

Ingestion by hens and broilers of specific chloroanisols present in some wood shavings used in poultry cages can result in a musty taste in poultry products.

Pyruvate oxidation and the permeability of mitochondria from blowfly flight muscle

The rate of pyruvate oxidation by mitochondria from blowfly flight muscle decreased in the presence of tris (hydroxymethyl) aminomethane (Tris). An increase in the rate of mitochondrial swelling was concomitant with the loss of pyruvate oxidation. These changes were prevented by bovine serum albumin, adenosine triphosphate, and magnesium ions, factors required for mitochondrial contraction. Proline, but not glutamate or malate, restored the rate of pyruvate oxidation to original values. These findings suggest that mitochondrial swelling leads to leakage of intramitochondrial intermediates of the Krebs cycle, accounting for the decrease in the rate of pyruvate oxidation. Exogenous proline penetrates the mitochondrial membrane and is rapidly oxidized, via glutamate, forming intramitochondrial precursors of oxaloacetate. Malate and glutamate were ineffective because of the selective permeability of the mitochondrial membrane.