Cytokine signalling, nutrition, longevity, and muscle health

In humans and other animals, nutrition has strong and complex effects on biology. Diets with different amounts of sugar, fat or protein can have important effects on physiology, even in the absence of obesity or obvious malnutrition; for example, it was reported last year that people who eat diets high in protein tend to die younger than others. In many cases, the effects of diet on physiology work by mechanisms that are not clear. We have begun to analyse this problem in fruit-flies; the advantage of doing this work in flies is that we can test the effects of many aspects of nutrition very quickly and relatively inexpensively. Many of the mechanisms of physiological regulation are conserved between flies and people.

In our preliminary data, we show that in fruit-flies, macrophages - cells of the immune system - are a critical part of the nutrient sensing system. Macrophages appear to detect dietary protein and produce a signal that is received by muscles; this signal causes muscles to become less responsive to insulin. The signal from macrophages to muscles is critically important in allowing flies to live a normal lifespan and to be able to survive on diets that are low in sugar.

The experiments we propose here have two parts. In the first part, we test the biological role in the macrophage of a pathway known to be responsive to dietary protein. In the second part, we use a variety of approaches to explore the signalling circuitry that connects macrophage sensing of diets with muscle physiology.

Our preliminary results, combined with the proposed experiments, represent a dramatic step in our knowledge of how the body senses and responds to dietary components. This is a fundamentally new role for macrophages in normal physiology and reveals an entirely new way that dietary protein can affect lifespan and muscle physiology. Our work will have significant implications regarding age-related disease, inflammatory disease, and nutritional physiology.

Though the nutrient composition of different diets has strong effects on organismal physiology, many aspects of the underlying mechanisms remain unclear. This problem is fundamentally highly complex: it is difficult to distinguish direct effects of nutrients from nutrient signalling effects and many nutrient signals play highly-diverse and apparently contradictory roles. In this context, it is useful to study the underlying mechanisms in simpler animals, in which experiments can be done rapidly and controlled more carefully than in higher organisms. We have explored the biology of nutrition and nutrient signalling in the fruit-fly Drosophila melanogaster.

We have found that in the fly macrophages are a critical element in the response to diet. Dietary protein drives release of a cytokine from macrophages that alters insulin responsiveness in muscle. The cytokine signal must be received by muscle to maintain normal homeostasis; if this signal is disrupted, the fly is dramatically short-lived. We propose here to decipher the biology of the regulation of this cytokine by diet: we will identify the regulatory mechanisms acting in macrophages to promote cytokine expression and we will identify the mechanisms it targets in muscle to affect physiology. We will use a combination of genetics, transcriptomics and computational approaches to generate a full map of the nutrient-macrophage-muscle circuit. This is a fundamentally new role for macrophages in normal physiology and reveals an entirely new way that dietary protein can regulate lifespan and muscle physiology. Our work will have significant implications regarding age-related disease, inflammatory disease, and nutritional physiology.

Societal beneficiaries
Direct medical and veterinary implications of this work include clearer identification of control mechanisms in muscle physiology. Protein inclusion-body pathology (as driven by AKT hyperactivity) is a common aspect of muscle ageing pathology. Drugs targeted to the mechanisms we identify may be useful in humans or other animals.
Indirect scientific impacts may also produce significant feed-through into societal impact. By improving understanding of Drosophila nutrition and physiology, we will expand scientific knowledge of how Drosophila can be applied to human and animal health and disease. This will facilitate the use of this system by others to address other medically-relevant issues.
This work will have significant 3R's impact (see also "Industrial Interactions"). By showing the use of Drosophila to examine complex issues of physiological regulation, we will popularize the use of this system alongside existing tissue-culture and whole-animal models. With support, this work will permit refinement of genetic hypotheses in whole-animal experiments.

Industrial interactions
This research may generate commercially-exploitable intellectual property and research tools. Targeted regulators of muscle function are an area of high topical interest in the pharmaceutical industry. By demonstrating the utility of Drosophila, we will attract industrial attention to this field as a whole. We will use existing industrial outreach structures at Imperial to identify these opportunities and we will forge interactions that may result, for example, in CASE awards for researchers at the academic/industrial interface.
We have already had success in this regard: a GSK/BBSRC-funded CASE PhD student is currently in the lab, working on a project extending from in vivo analysis in Drosophila to in vitro analysis in primary human cells.

Outreach and education
To reach as many researchers as possible, we will present our work at multiple conferences. All research findings funded by this grant will be published using open-access mechanisms (such as Europe PMC). This allows free access to our work by the scientific community and the public.
We make every effort to promulgate our findings to the public. Imperial College London organizes numerous opportunities to lecture on our findings to lay audiences. We will work with the Imperial Press Office and BBSRC public outreach department to publicize our findings.

Training
This project will provide useful training for the postdoc as well as for the PhD students in the laboratory. This work is at the intersection of muscle physiology, genetics, and nutrition; the postdoc will learn techniques lying in all of these disparate areas. This training is critical for future scientists, who will need diverse skills to address complex problems. The exposure to first-rate research in inflammation and physiology at Imperial is also a critical aspect of this training.
The postdoc will also receive training in career skills, such as organization of projects, supervision of students, grantsmanship, manuscript writing, interpersonal skills and presentation skills. This training will be undertaken informally within the lab as well as formally. For example, postdocs will be encouraged to attend media training, either through BBSRC or through the Imperial press office. Postdocs and PhD students present their work at internal seminars (at least once per year) and at at least one external conference per year. Informal presentations within the group take place weekly.
As evidence of my prior success in training of this kind, I would point to Rebecca Clark, my first postdoc, who spent four years in a BBSRC-funded position in my laboratory; Rebecca has recently started a Lectureship at the University of Durham.