The effect of electron transport (ET) inhibitors and thiabendazole on the fumarate reductase (FR) and succinate dehydrogenase (SDH) of Strongyloides ratti infective (L3) larvae

A metabolic regulatory network for the Caenorhabditis elegans intestine

Metabolic perturbations can affect gene expression, for instance to rewire metabolism. While numerous efforts have measured gene expression in response to individual metabolic perturbations, methods that determine all metabolic perturbations that affect the expression for a given gene or set of genes have not been available. Here, we use a gene-centered approach to derive a first-pass metabolic regulatory network for Caenorhabditis elegans by performing RNAi of more than 1,400 metabolic genes with a set of 19 promoter reporter strains that express a fluorescent protein in the animal's intestine. We find that metabolic perturbations generally increase promoter activity, which contrasts with transcription factor (TF) RNAi, which tends to repress promoter activity. We identify several TFs that modulate promoter activity in response to perturbations of the electron transport chain and explore complex genetic interactions among metabolic pathways. This work provides a blueprint for a systems-level understanding of how metabolism affects gene expression.

Structural Insights into the Molecular Design of Flutolanil Derivatives Targeted for Fumarate Respiration of Parasite Mitochondria

Recent studies on the respiratory chain of Ascaris suum showed that the mitochondrial NADH-fumarate reductase system composed of complex I, rhodoquinone and complex II plays an important role in the anaerobic energy metabolism of adult A. suum. The system is the major pathway of energy metabolism for adaptation to a hypoxic environment not only in parasitic organisms, but also in some types of human cancer cells. Thus, enzymes of the pathway are potential targets for chemotherapy. We found that flutolanil is an excellent inhibitor for A. suum complex II (IC50 = 0.058 μM) but less effectively inhibits homologous porcine complex II (IC50 = 45.9 μM). In order to account for the specificity of flutolanil to A. suum complex II from the standpoint of structural biology, we determined the crystal structures of A. suum and porcine complex IIs binding flutolanil and its derivative compounds. The structures clearly demonstrated key interactions responsible for its high specificity to A. suum complex II and enabled us to find analogue compounds, which surpass flutolanil in both potency and specificity to A. suum complex II. Structures of complex IIs binding these compounds will be helpful to accelerate structure-based drug design targeted for complex IIs.

NHR-176 regulates cyp-35d1 to control hydroxylation-dependent metabolism of thiabendazole in Caenorhabditis elegans

Knowledge of how drugs are metabolized and excreted is an essential component of understanding their fate within and among target and non-target organisms. Thiabendazole (TBZ) was the first benzimidazole (BZ) to be commercially available and remains one of the most important anthelmintic drugs for medical and veterinary use. We have characterized how Caenorhabditis elegans metabolizes and excretes TBZ. We have shown that TBZ directly binds to the nuclear hormone receptor (NHR)-176 and that this receptor is required for the induction by TBZ of the cytochrome P450 (CYP) encoded by cyp-35d1. Further, RNAi inhibition of cyp-35d1 in animals exposed to TBZ causes a reduction in the quantity of a hydroxylated TBZ metabolite and its glucose conjugate that is detected in C. elegans tissue by HPLC. This final metabolite is unique to nematodes and we also identify two P-glycoproteins (PGPs) necessary for its excretion. Finally, we have shown that inhibiting the metabolism we describe increases the susceptibility of C. elegans to TBZ in wild-type and in resistant genetic backgrounds.

Thiabendazole inhibits ubiquinone reduction activity of mitochondrial respiratory complex II via a water molecule mediated binding feature

The mitochondrial respiratory complex II or succinate: ubiquinone oxidoreductase (SQR) is a key membrane complex in both the tricarboxylic acid cycle and aerobic respiration. Five disinfectant compounds were investigated with their potent inhibition effects on the ubiquinone reduction activity of the porcine mitochondrial SQR by enzymatic assay and crystallography. Crystal structure of the SQR bound with thiabendazole (TBZ) reveals a different inhibitor-binding feature at the ubiquinone binding site where a water molecule plays an important role. The obvious inhibitory effect of TBZ based on the biochemical data (IC(50) ~100 μmol/L) and the significant structure-based binding affinity calculation (~94 μmol/L) draw the suspicion of using TBZ as a good disinfectant compound for nematode infections treatment and fruit storage.

Physiological role of rhodoquinone in Euglena gracilis mitochondria

Rhodoquinone (RQ) participates in fumarate reduction under anaerobiosis in some bacteria and some primitive eukaryotes. Euglena gracilis, a facultative anaerobic protist, also possesses significant rhodoquinone-9 (RQ9) content. Growth under low oxygen concentration induced a decrease in cytochromes and ubiquinone-9 (UQ9) content, while RQ9 and fumarate reductase (FR) activity increased. However, in cells cultured under aerobic conditions, a relatively high RQ9 content was also attained together with significant FR activity. In addition, RQ9 purified from E. gracilis mitochondria was able to trigger the activities of cytochrome bc1 complex, bc1-like alternative component and alternative oxidase, although with lower efficiency (higher Km, lower Vm) than UQ9. Moreover, purified E. gracilis mitochondrial NAD+-independent D-lactate dehydrogenase (D-iLDH) showed preference for RQ9 as electron acceptor, whereas L-iLDH and succinate dehydrogenase preferred UQ9. These results indicated a physiological role for RQ9 under aerobiosis and microaerophilia in E. gracilis mitochondria, in which RQ9 mediates electron transfer between D-iLDH and other respiratory chain components, including FR.

Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum

Parasites have developed a variety of physiological functions necessary for existence within the specialized environment of the host. Regarding energy metabolism, which is an essential factor for survival, parasites adapt to low oxygen tension in host mammals using metabolic systems that are very different from that of the host. The majority of parasites do not use the oxygen available within the host, but employ systems other than oxidative phosphorylation for ATP synthesis. In addition, all parasites have a life cycle. In many cases, the parasite employs aerobic metabolism during their free-living stage outside the host. In such systems, parasite mitochondria play diverse roles. In particular, marked changes in the morphology and components of the mitochondria during the life cycle are very interesting elements of biological processes such as developmental control and environmental adaptation. Recent research has shown that the mitochondrial complex II plays an important role in the anaerobic energy metabolism of parasites inhabiting hosts, by acting as quinol-fumarate reductase.

Nematocidal activities of thiabendazole and ivermectin against the larvae of Strongyloides ratti and S. venezuelensis

With the aim of developing therapeutic agents for strongyloidosis, the disease caused by infection with Strongyloides stercoralis, we established a novel assay technique using S. ratti and S. venezuelensis as models for S. stercoralis. The newly developed assay technique was found to more accurately represent treatment-induced larval paralysis than existing assays. Our method uses paper disks impregnated with the test solution, which even allows materials that are sparingly soluble in water to be tested. An inverted microscope was used to observe the larval states, and these states were recorded using a digital camera. We observed the activities of ivermectin and thiabendazole against larvae and calculated larval motility and velocity. These two factors were then combined to determine the overall viability of larvae at selected concentrations. The activities of the anthelmintics were compared by calculating the concentrations at which 50% viability was demonstrated, or in other words, the concentration at which paralysis was caused in 50% of the individuals (50% paralysis concentration; PC(50)). Evaluations after 24h of exposure yielded the following reproducible PC(50) values for ivermectin and thiabendazole, respectively: S. ratti, 2.4 and 140 microM; and S. venezuelensis, 2.3 and 190 microM. After treatment with ivermectin, there was a tendency for larval motility to be greater than that of the controls at low concentrations, a result that might be associated with its mechanism of action.