How metabolic interactions of polyunsaturated fatty acids affect their interconversion and synthesis and influence sustainable feed development

This proposal provides an exciting blend of hands-on industrial experience combined with cutting-edge, modern genomic technology in a project addressing both global (sustainability) and national (UK diet) issues currently of great importance and relevance. Omega-3 or n-3 long-chain polyunsaturated fatty acids (LC-PUFA) eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) have beneficial effects in several inflammatory and pathological conditions, including cardiovascular and neurological diseases. Fish are the major dietary source of n-3 LC-PUFA and, with declining fisheries worldwide, farmed fish constitute an ever-increasing proportion (approaching 50 %) in the food basket. High n-3 LC-PUFA levels in farmed fish have been obtained by the use in feeds of fish oils (FO), themselves derived from marine fisheries, but this is not sustainable and will constrain growth of aquaculture. In consequence, alternatives to FO are urgently required, with vegetable oils (VO) the prime candidates. However, VO are devoid of n-3 LC-PUFA, and feeding fish VO has important consequences for human consumers as it lowers fish n-3 LC-PUFA content compromising nutritional value. Some studies have used 'finishing diets' containing FO to restore levels of n-3 LC-PUFA in the flesh. An alternative is to use lower levels of FO substitution, blended with VO, with oils carefully chosen to limit LC-PUFA reduction. For instance, southern hemisphere FO (SFO) contain higher levels of n-3 LC-PUFA and so can deliver n-3 LC-PUFA at lower inclusion levels than northern FO (NFO) traditionally used in salmon diets. However, SFO have higher EPA and lower DHA than NFO. With increasing use of SFO, we have observed increased levels of 22:5n-3, but not DHA in salmon. This is interesting because SFO with high DHA are being selected for use in pharmaceutical applications and so SFO available for aquaculture are those with higher EPA:DHA ratios. Historically SFO oils had a characteristic EPA:DHA ratio of 3:2. Now the SFO used in aquaculture can have ratios higher than 5:2. Also, it is highly likely that transgenic oilseeds with high levels of EPA (but no DHA) will be available in the next 2-3 years. Therefore, there is increasing interest in the EPA to DHA pathway and its regulation. Biosynthesis of n-3 LC-PUFA in vertebrates occurs in two stages with different kinetics. Synthesis of EPA requires D6 desaturation of 18:3n-3 to produce 18:4n-3 that is elongated to 20:4n-3 followed by D5 desaturation to produce EPA. The rate-limiting step of this pathway is D6 desaturase. Synthesis of DHA from EPA requires two further elongation steps, a second D6 desaturation, and a peroxisomal chain-shortening step to produce DHA. Thus, this requires translocation of fatty acyl intermediates between microsomal and peroxisomal compartments and is consequently slower than synthesis of EPA. How DHA synthesis is precisely controlled and regulated is intriguing as it is an anabolic pathway although the final step involves a catabolic pathway (limited chain shortening). Of further interest, the D6 desaturation of the C24 intermediates is performed by the same enzyme that desaturates C18 fatty acids, and is the rate-limiting step in EPA synthesis. Thus, C18 and C24 fatty acids must compete for the D6 desaturase, but nothing is currently known about this competition. This project has two aims; to develop novel formulations using blended FO and VO to produce low-cost, sustainable diets without compromising growth performance, feed efficiency and heath and welfare of fish at the same time as maintaining the levels of n-3 LC-PUFA and the nutritional quality to the consumer, and to investigate the biochemical and molecular mechanisms of DHA synthesis from EPA and their regulation. The project will have 3 parts; a large-scale feeding trial followed by in vitro cell culture studies and a final feeding trial to test novel advanced diet formulations in commercial conditions.