More than 200 million people have schistosomiasis, a neglected tropical disease, resulting in more than 200,000 deaths annually. Schistosomiasis control strategies rely almost exclusively on chemotherapy using praziquantel with 235 million projected to receive annual treatment in starting 2018. Although treatment with praziquantel can be beneficial, people are rapidly re-infected after treatment and results indicate that praziquantel cure rates are significantly lower than expected. Furthermore, with large-scale drug use it is inevitable that praziquantel-resistant parasites will evolve. Therefore, it is imperative to identify new drugs for schistosomiasis that could be used in combination with praziquantel to increase efficacy and cure rates, to prevent selection of praziquantel-resistant parasites, and to replace praziquantel should praziquantel resistance evolve.
Our work
The lab of David L. Williams, PhD, is focused on the identification of targets and developing new drugs for schistosomiasis treatment. Specifically, we investigate a variety of detoxification pathways in the worm that we have found to be unique or to differ significantly from human pathways. Schistosomes, like all aerobic organisms, are exposed to reactive oxygen species generated during oxidative respiration. Schistosomes also catabolize large amounts of hemoglobin, which can generate ROS, and are exposed to ROS generated by activated host immune cells. Therefore, schistosomes must possess adequate ROS detoxification mechanisms. We have generated compelling evidence that schistosome redox defenses are limited, that they can be disrupted pharmacologically, and that their disruption leads to worm death in vitro and in vivo. Redox balance in mammals is controlled by the action of two parallel systems, each of these utilizing an NADPH-dependent reductase, glutathione reductase and thioredoxin reductase, respectively. In the human redox network, reduction of H2O2 is executed by glutathione peroxidases, peroxiredoxins, and catalase. Our research has found that the schistosome redox network is distinctly different from humans. Schistosomes lack catalase, have low levels of glutathione peroxidases and rely on peroxiredoxins to reduce H2O2. Most importantly, we found that schistosomes lack both authentic glutathione reductase and thioredoxin reductase with both activities provided by a unique schistosome enzyme thioredoxin-glutathione reductase (TGR). Using both reverse genetic and pharmacological approaches we have generated compelling evidence that peroxiredoxins and thioredoxin glutathione reductase are essential proteins and druggable targets.
We are also interested in mechanisms of detoxification of heavy metals, drugs and environmental toxins (xenobiotics). A key player are the cytochromes P450 (CYP450), which also function in the synthesis and/or breakdown of many important endobiotics such as cholesterol, steroid hormones, vitamin A/retinoic acid, vitamin D, bile acids, metabolites of ω-3 fatty acids, insect molting and juvenile hormones, and eicosanoids. While most metazoans have multiple CYP450 genes (e.g., 57 CYP450s in humans), recent genomic studies have found that parasitic flatworms each have a single CYP450 gene. We have used reverse genetic and pharmacological approaches to validate S. mansoni CYP450 as an essential and druggable protein. Heavy metals are detoxified by metallothioneins in humans. Our research has identified a novel mechanism for this in schistosomes based on phytochelatin synthase, a worm-specific process.
Impact of work
Our lab made these important discoveries:
1. We were the first to discover that an organism could rely on TGR as its major reductant and lack both thioredoxin reductase and glutathione reductase enzymes. This has since been found to be true for all parasitic flatworms (Cestodes and Trematodes) but not for non-parasitic ones.
2. We conducted quantitative high throughput screens of the Molecular Libraries Small Molecule Repository and NIH Chemical Genomics Center libraries as part of the NIH Molecular Libraries Initiative targeting the Schistosoma mansoni redox cascade. The screens have identified several promising chemical series active against TGR, and are the basis for our future studies on drug development against this target. This initial screen was the product of the first assay officially accepted for screening and completed by the NIH Molecular Libraries Initiative.
3. Basic studies on schistosome TGR have identified an FDA approved drug, whose repurposing for schistosomiasis treatment is a goal of our current studies.
4. Validation of the unique S. mansoni CYP450 as an essential and druggable protein and a biochemical chokepoint opens up new avenues for drug development targeting this protein. The limited identity between worm and human CYP450s and redundancy of human CYP450s suggests that specificity for the parasite enzyme can be optimized and host toxicity will be minimized.
Technology
We use leading-edge, multidisciplinary tools to conduct our research. Molecular biology techniques include, real-time PCR, gain and loss of function studies using RNA interference, and gene cloning Biochemical approaches include recombinant protein expression and analysis, protein mutagenesis and analysis. We also use Western blot, and fluorescent and immunohistochemical microscopy. We work closely with experts in medicinal chemistry, cheminformatics, and structural biology. We maintain animal models of schistosome infections.
Funding
We are grateful to the NIH for funding our scientific work. This includes the following:
NIH R21 AI115208: Title: Deorphanization of Schistosome Cytochrome P450
NIH R21 AI127635: Title: Identification of preclinical drug candidates for the treatment of schistosomiasis
Our team
- Samuel Y. Aboagye, PhD, Postdoctoral Fellow
- Jala Bogard - PhD Student
- Jazmin Munoz - MS Student
Contact us
David L. Williams, PhD
Associate Professor
Department of Microbial Pathogens and Immunity
616 Cohen
Rush University Medical Center
1735 West Harrison Street
Chicago, IL 60612
Email:
We welcome inquiries about our research, collaborations and funding. Please email David L. Williams, PhD.