Role of CIDE proteins in lipid droplet formation and adipocyte metabolism

Population statistics indicate an ever-increasing number of obese people in western societies. The situation is alarming due to the side effects of excessive fat storage which are health risks for cardiovascular disease, diabetes, and even cancer. Excessive fat storage results from an imbalance between energy intake and energy expenditure. Research over the past 10 years has changed our view of adipose tissue so that today we consider it to be an active hormone-producing organ that communicates with the muscle, liver, pancreas, heart, and the brain. Furthermore, in people with excess body fat, adipocytes (fat cells) produce factors that cause metabolic disorders, such as diabetes. Mammals store excess energy in the form of fat, predominantly in adipocytes. Fat tissue comes in two types, white fat for storing energy and brown fat for burning fat. Until very recently human brown fat was considered only to be present in newborn babies. However, imaging methods have identified distinct brown fat depots above the collarbones and in the upper chest. The activity of this tissue was higher in thin compared to obese patients and was rapidly switched on by exposure to cold temperatures. These findings have ignited interest in this elusive tissue due to its potential to burn fat and therefore aid weight loss. There are a number of fundamental differences between brown and white fat cells including the size and shape of fat droplets. Although on first inspection they appear to be simple fat-containing structures, a more thorough analysis reveals the droplet to be surrounded by a membrane structure and coated by a complex mix of proteins. These proteins are the key to how the cell manages the lipids stored within the droplet. They determine whether the droplets formed are large or small and control the release of stored energy. A white adipocyte has a single large droplet that almost completely fills the space inside the cell. In contrast, brown adipocytes contain many small droplets. We will investigate how the CIDE family of proteins leads to the appearance of lipid droplets. The first member of the family, CIDEA, is present at high levels in brown adipocytes and initiates the appearance of lipid droplets. We aim to understand how CIDEA and the other family members associate with and control the size and shape of lipid droplets. In addition, we will define their roles in essential metabolic processes and study the importance of these genes in switching on and off of brown fat activity in the body's different fat depots. Our understanding of how genes control the appearance and activity of lipid droplets is crucial in the identification of new targets for the treatment of metabolic diseases such as obesity, fatty liver, and type 2 diabetes.

Excessive lipid storage is a major contributory factor to the development of metabolic disease. Cellular lipid is stored in discrete lipid droplet organelles with abundant surface proteins that confer stability, prevent or promote coalescence, and facilitate storage or utilisation by regulation of metabolic processes. The major site of fat storage is white adipose tissue (WAT). In contrast, brown adipose tissue (BAT) is the site of energy dissipation by the process of thermogenesis. One of the fundamental differences between these two tissues is the presence a single large fat droplet in white adipocytes whereas brown adipocytes contain multiple small lipid droplets. We discovered that the BAT protein CIDEA surrounds lipid droplets in adipocytes and induces the formation of lipid droplets in non-adipogenic cell lines. Our objectives are to determine the biological roles of the CIDE gene family (CIDE-A, -B and -C) in the control of lipid droplet formation and energy metabolism. The proteins associated with lipid droplets induced by CIDE expression or present in white and brown adipocytes will be identified by mass spectrometry. The key structural features of the CIDE protein family required for lipid droplet appearance, size and shape will be assessed by expression of wild type and mutant versions. To determine the metabolic consequences of lipid droplet appearance we will study the effects of CIDE on beta-oxidation, lipolysis, and triglyceride synthesis and turnover. To explore the mechanism of CIDE action, we will screen for CIDEA-interacting proteins using yeast-2-hybrid and co-immunoprecipitation. To identify the pathways responsible of CIDE regulation, we will characterise CIDE promoter regulation by nuclear receptors, cytokine signalling, and cellular stress pathways. Furthermore, as a cold stimulus actively converts WAT depots to a more BAT-like phenotype we will determine if CIDE gene regulation is part of the cold-response programme.

Who will benefit from this research? The identified beneficiaries of this proposal outside academia include those in industry and the commercial sector. New basic science findings in the control of cellular energy storage are of great interest to those in this sector. Because elevated fat storage in diseases such as obesity and fatty liver disease is linked with increased risk of diabetes, high blood pressure, heart disease and certain cancers, this work will be of importance to the health sector and to the general public, either from a general science education benefit, or for those who are directly impacted by these diseases. This work will also have educational benefits, including the training and skills that will be acquired by the appointed research associate on the project and in educating and inspiring interest in scientific careers in secondary school students. How will they benefit from this research? The area of research is the understanding of the actions of the CIDE gene family which promote the formation of lipid droplets in cells. Energy storage in cells has profound implications for a wide spectrum of diseases and therefore the CIDEs represent an important target for therapeutic intervention. The research in this proposal could identify new molecular targets and regulatory pathways that control lipid storage and energy metabolism which would be used by industry or academic labs. As CIDEA is selectively expressed in brown adipocytes we could validate a screen for transdifferentation of white to brown fat cells. This has important implications in obesity treatments due the energy dissipating nature of brown fat. The screen could be used to test for actions of ligands, hormones, small molecules and nutritional compounds on promoting brown adipocyte appearance by industrial partners or in a future application. The training of the appointed research associate would potentially benefit areas outside of academia by the development of transferable skills that they could then apply in any other employment sector. These include oral and written communication skills that will be acquired by presentation to both those in science and to the public. In addition the research associate will gain skills in project and time management, problem solving, information technology and mentoring. What will be done to ensure that they have the opportunity to benefit from this research? We will develop impact of this work by communicating our findings to industry and health sectors. This will be achieved by data presentation and networking at conferences such as the Keystone Symposium which is attended by industry members. In addition, we will take advantage of the Imperial Business Development department to initiate and develop contacts with interested industrial partners. 'Imperial Innovations' would enable us to achieve commercial impact of the research. They assist with patent development and application to the development of spin-out companies to help Imperial researchers discover and develop the commercial potential for their research. We will take advantage of opportunities to impact on the public awareness of the health benefits of this research and in education by continued participation in the Institute's Open Days to the public and secondary school students. The appointed research associate on this proposal will be involved in these activities and receive training from the Imperial Postdoc Development Centre to acquire transferable skills.