IMPC - Understanding the role of 18kDa Translocator protein (TSPO) in the regulation of energy homeostasis in mice

Obesity is a global health problem that impacts greater than 25% of the adult population in the UK costing the NHS an estimated £4.2billion per year. Current treatments for obesity are limited and the development of effective drug therapies is hampered by our limited understanding of drivers of obesity on a molecular level.

Obesity is essentially caused by an imbalance in energy in the body: typically when more energy is available from food than is needed. The extra energy is stored as fat in white adipose tissue (WAT). WAT is composed of multiple cell types including adipocytes (the fat storing cells) and immune cells such as macrophages. Various macrophage subtypes exist, depending on the molecules that they contain, which enable them to perform different functions. Macrophages are flexible and can switch subtype depending on the local microenvironment; in WAT, they are normally "M2-like", a subtype which helps to regulate the normal WAT function.

Most people have some energy reserves in the form of WAT; however, the large energy excess in obesity expands adipocyte fat storage until they cannot cope - which leads to the cells becoming stressed and eventually dying. This cell death causes secretion of chemical signalling molecules, called cytokines, which attract more macrophages into WAT and change the predominant subtype of WAT macrophages to "M1-like". This process of cytokine secretion and macrophage recruitment to WAT in obesity is similar to what is seen in response to injury or infection in other organs of the body and is part of what is known as an inflammatory response. Short-term inflammation is usually helpful to promote healing and restore normal tissue function. Inflammation normally stops when a tissue heals and/or the infection is cleared by immune cells. However, if inflammation becomes chronic, it can cause significant damage. Long-term WAT inflammation in obesity impairs how organs respond to insulin, the hormone which controls blood sugar levels, leading to type-2 diabetes (T2D).

All cells in the body require energy to function, coordinated by structures called mitochondria- the power house of the cell. Cells can make energy from different nutrients including sugars (by a process called glycolysis) and fats (by a process called fatty acid oxidation [FAO]); the overall process is called metabolism. Over the last few years scientists have discovered that changing metabolism in macrophages can alter their subtype. "M1-like" macrophages tend to break down sugar by glycolysis. In contrast, "M2-like" macrophages prefer to get their energy by FAO. The goal of this research project is to find out whether altering metabolism in these cells can help combat obesity and T2D by reducing inflammation.

We have discovered that a molecule called Translocator protein (TSPO), present in mitochondria is found at higher levels in macrophages in WAT of obese mice compared with lean mice. The function of TSPO is not fully understood but it can regulate metabolism in some cell types. Drugs which inhibit TSPO are also known to be anti-inflammatory. We think that TSPO may be a functional link between inflammation and metabolism in obesity. This project will address the following questions: 1) Are mice which cannot make TSPO protected from obesity and WAT inflammation when given a high-fat diet which normally causes obesity?; 2) Do macrophages obtained from mice lacking TSPO show changes in their metabolism?; and 3) Do macrophages from mice lacking TSPO have the ability to change into the different macrophage subtypes?
By answering these questions we will begin to understand whether TSPO could be a potential drug target to reduce obesity-associated WAT inflammation. If this pilot project is successful, we will work with a chemist who makes novel drugs which target TSPO to validate the results from these initial studies and investigate further the effectiveness of targeting TSPO function as a therapy for obesity.

Currently available drug therapies for obesity are limited and there is a clear need for new approaches in this area. Obesity leads to chronic low-grade inflammation which is causative in the development of co-morbid conditions including type-2 diabetes. As such, targeting obesity-associated inflammation may represent an approach to improving morbidity.

Systemic obesity-associated inflammation is mediated by a change in the immune milieu in white adipose tissue (WAT): macrophages change from a predominant "M2-like" homeostatic phenotype to an "M1-like" proinflammatory state. Emerging evidence suggests that metabolic reprogramming of macrophages influences their phenotype and function. This application focuses on understanding the potential role of a mitochondrial protein called translocator protein (TSPO) at the interface between inflammation and metabolism. Independent studies have demonstrated that 1) loss of TSPO signalling impacts cellular metabolism, and 2) pharmacological inhibition of TSPO is anti-inflammatory and protective in animal models of disease.

Our published data demonstrates increased TSPO expression in WAT macrophages in obese mice. We hypothesize that absence of TSPO will confer relative protection from diet-induced obesity and insulin resistance in mice by impacting cellular metabolism and inflammation. Using TSPO-deficient mice obtained from the IMPC, we will test this hypothesis using the following aims: 1) To determine how global loss of TSPO signalling in mice impacts susceptibility to diet-induced obesity and insulin resistance; and 2) To determine how loss of TSPO signalling impacts macrophage function. It is hoped that this work will lead to a larger grant application interrogating the potential of TSPO as a therapeutic target for obesity and we have an established collaboration with a chemist who makes novel TSPO ligands that can be used to validate and extend the findings from this study.