Design of bioactive sesquiterpene-based chemical signals with enhanced stability

Many interactions between organisms in nature are mediated by external chemical signals, generally termed semiochemicals, that are typically low molecular weight lipophilic compounds. These interactions involve microbes (e.g. bacteria), algae, higher plants and animals, including human beings. Upon release by an emitting organism, such signals can act intra- or interspecifically by modifying either the behaviour or the development of recipient organisms. Whenever work is reported on identifying the signals or developing their practical use, i.e. managing pest organisms, the question 'why not design structural analogues?' is raised. Such an approach could potentially solve many problems associated with the use of semiochemicals, such as instability and volatility, which result in reduced efficacy. However, no rational approach has emerged by which to conduct structure activity relationship studies, to design analogues with greater stability and performance. The reason cited is that receptor systems, e.g. the animal peripheral sensory neurons, for external signal recognition are more highly selective, as a consequence of a need to select from an enormous diversity and concentration range of external chemicals, than receptor systems within organisms for which analogues can readily be designed. The family of sesquiterpenes, i.e. naturally-occurring chemicals that comprise of 3 x 5-carbon units, is widely diverse in nature and can have various signaling roles. The Cardiff laboratory has shown that enzymes (terpene synthases) which are involved in sesquiterpene production rely on a three-dimensional structure for conversion of a precursor (farnesyl diphospate, FDP) which is shared by all sesquiterpene synthases. Plasticity in the active site of the synthases, i.e. the region of the enzyme that performs the conversion, enables the production of many terpenoids through the use of synthetic substrate analogues and subtle alterations in the composition of the active site of enzymes during evolution. It is hypothesized that subtle alterations in the active site, i.e. incorporating the chemical space of sesquiterpene synthases, in the laboratory will allow the introduction and manipulation of analogues of FDP, and lead to the production of 'non-natural' analogues of naturally-occurring sesquiterpenes. Together, Cardiff and Rothamsted aim to test this hypothesis regarding active site alteration of sesquiterpene synthases using (R)-germacrene D, which is identified by Rothamsted as a potent semiochemical for aphids, major world crop pests, as the model external semiochemical. It is also expected that the chemical space of the enzyme site in modified germacrene D synthase will be structurally close to that of the wild type GDS, and, therefore, the analogues will closely match (R)-germacrene D in terms of structure, thereby exhibiting high semiochemical activity. Thus, the overall aim of the project will be to produce stable, biologically active analogues of (R)-germacrene D, representing the first design of active analogues of a biologically active natural product. The specific objectives include: 1) production of the purified enzymes using an established laboratory bacterial system; 2) develop novel chemistry to produce synthetic FDP analogues that can be added to enzyme preparations; 3) convert synthetic FDP substrates to (R)-germacrene D analogues using unmodified (R)-germacrene D synthase; 4) Perform site-directed mutagenesis for the creation of modified (R)-germacrene D synthases, and use to convert synthetic FDP substrates to analogues 5) use electrical recordings of the antennae of insects (electrophysiology), and laboratory behavioural assays, to measure the activity of generated (R)-germacrene D analogues with a range of economically important aphid species; 6) refine the hypothesis, design new substrates and feed to modified GDS, and assess the electrophysiological and behavioural activity of the refined analogues.

Signaling between organisms via small lipophilic molecules, e.g. pheromones and other semiochemicals, can have profound impact on development and, for higher organisms including humans and other animals, on behaviour. Sesquiterpenes play a major role in chemical signaling in nature, and the Cardiff laboratory has investigated the synthetic biology of some terpene synthases involved in sesquiterpene biosynthesis. From this work, it is hypothesized that the use of substrate analogues, together with subtle alterations in the active site of the synthases, can lead to the production of 'non-natural' sesquiterpene analogues. We will investigate the chemical space of the sesquiterpene synthase (R)-germacrene D synthase (GDS) as the model system as (R)-germacrene D has been shown by the Rothamsted laboratory to be a potent, but highly unstable, repellent semiochemical for aphids. We expect the chemical space of the enzyme in modified GDSs to closely match that of the wild type GDS. Thus, analogues will match closely (R)-germacrene D in terms of structure, and exhibit high semiochemical activity. The overall aim is to produce stable, biologically active analogues of (R)-germacrene D, representing the first design of active analogues of a biologically active external chemical signal. Specific objectives include: 1) Reconstruct the gene for (R)-germacrene D synthase and express in E. coli 2) Undertake novel chemistry for the production of synthetic farnesyl diphosphate substrates 3) Convert synthetic farnesyl diphosphate (FDP) substrates to (R)-germacrene D analogues using wild-type and modified (R)-germacrene D synthases 4) Perform site-directed mutagenesis for the creation of modified (R)-germacrene D synthases 5) Use electrophysiological recordings and laboratory behavioural assays to measure the activity of (R)-germacrene-D analogues, using a range of selected aphid species 6) Refine the hypothesis, design new substrates and feed to modified (R)-germacrene D synthases.

The project is in line with BBSRC priorities on research in synthetic biology, as it will create novel biological products and/or functionality and engineer improvements in an existing biological product and/or functionality. It is also in line with BBSRC priorities on research in crop science (food security), as it will comprise research that studies biotic factors affecting crop performance, including insect pests and provide underpinning science for the development of effective and sustainable approaches to the control of aphids in agriculture. The main non-academic beneficiaries from this project will include farmers and the general public. The specific social and economic impact for farmers will be the availability of a new set of semiochemicals for control of aphids, the major crop pests in the UK. This new generation of tools will have enhanced stability but with retained ability to protect crops from aphids. They will enable farmers to adopt novel, semiochemical-based strategies for aphid control that minimize or remove dependence on the use of broad-spectrum synthetic neurotoxins. Replacement of aphicide use by semiochemicals would remove the placement of such neurotoxins into the environment and the human food chain (currently at an average of 120 tonnes p.a. for wheat in the UK) and would thereby provide benefits to the environment and public health. The project will also greatly enhance the prospect of producing a new class of rationally designed chemical signals, i.e.phytopheromones, that can be used to protect crops via induction/priming of defence. Once research results are available, the technical application and demonstration of field activity should be straightforward and effort will be focused on identifying the most effective dissemination routes. Dissemination of the scientific outputs towards the non-academic beneficiaries will be undertaken primarily by Rothamsted through its links with Defra, the Private Sector, UK levy boards, UK extension agencies and also directly with plant breeders' associations. Rothamsted has an outstanding track record in transferring agricultural technology to UK farmers, as recently exemplified by the development of a robust pheromone-based monitoring system for the control of the orange wheat blossom midge Sitodiplosis mosellana, an important pest of wheat in the Northern Hemisphere. Both Cardiff and Rothamsted will provide a major contribution towards dissemination of the social and economic impact of the scientific outputs arising from this project. Rothamsted has extensive experience of communicating to the farming industry and the general public through media deployment. Over the last 30 years, the project co-PI, Prof Pickett, FRS, has contributed towards numerous television & radio interviews and articles in the popular press, promoting the innovative use of semiochemicals in the sustainable control of agricultural pests. Collaboration agreements for dissemination activities will be prepared at the start of the project, which will clearly define the partner roles as described above. Furthermore, agreements will be sought at an early stage of the project between the partners and the agencies that will take forward the scientific outputs arising from the project. Primary data sets generated from this project at Rothamsted and Cardiff will be published in peer-reviewed journals, as the main text and as supplementary material available from the publisher's web-site. It is anticipated that any intellectual property rights (IPR) involving the parties will be quickly resolved, in according with an agreed collaboration agreement (CA), so as not to delay dissemination of results towards the non-academic beneficiaries. Funds are requested for the attendance of national and international scientific meetings in order to disseminate the outputs from the project to the scientific community and remain in touch with the state-of-the-art in the research area of the project.