Parasites in stress: How Toxoplasma gondii responds to iron deprivation

Toxoplasma gondii is a highly successful pathogen, able to infect all cells and warm-blooded animals. In the UK, infection can be serious in pregnant or immunocompromised people, but also causes huge losses for sheep farmers, making it an important medical and veterinary infection. To infect these diverse hosts, Toxoplasma must react to its environment and respond to the availability of nutrients, as loss leads to starvation and stress for the parasite. One nutrient that is vital for Toxoplasma is iron, which is essential for almost all cells. During infection, mammals tend to conceal iron, deliberately starving invaders to prevent them from growing. So successful pathogens, like Toxoplasma, have developed techniques to survive this stress. In this project, we want to learn how iron starvation affects growth and Toxoplasma protein production. We have found that Toxoplasma responds to iron starvation by shutting down the production of most proteins. This response raises two major questions, first, how is Toxoplasma able to prevent proteins from being made, and is this response important for the pathogen to survive? And second, Toxoplasma increases production of some proteins, but how does the parasite choose which proteins are made, and what are the roles of these proteins? We will try to answer these questions in two main parts. In the first aim we will identify how Toxoplasma manages to slow down protein production. We have two hypotheses for how it does this, either indirectly through signalling pathways, or directly as the parasite removes a key protein involved in protein production. We will investigate both hypotheses using a range of techniques to determine if one (or both) are involved. We will also check how important this is to the parasite survival and recovery from starvation. Although most protein production is reduced, the parasite appears to increase production of a subset of proteins. In the second aim of this project, we will identify which parasite proteins contain iron, and which proteins are prioritised for production. This selective increase in proteins has been seen in other organisms. However, it is usually concentrated on proteins which contain iron and unfortunately, we don’t yet know the iron-containing proteins in Toxoplasma. To identify these, we will use a new technique which relies on the intrinsic properties of proteins, which become more sensitive to high temperatures upon removal of iron. By measuring the temperature-sensitivity of all proteins and determining which ones change upon iron removal, we can identify iron-bound proteins of Toxoplasma, without being biased by what we already know. This allows us to understand how the parasite uses iron. To then determine which proteins the parasite is prioritising upon iron starvation, we will identify all newly created proteins, focussing on those that are involved in metabolism or contain iron (from our work above). We will then investigate these proteins to find out which proteins or pathways are essential for Toxoplasma to survive in low iron conditions. This project will investigate Toxoplasma stress responses. This impacts how drugs can change in efficacy, and how to better target interventions to work with immune responses, as well as improving our understanding the parasite biology and host-pathogen interactions. This proposal benefits scientists studying parasites and nutritional responses, those interested in identifying new metal-bound proteins and gives a broader understanding of the role of iron in parasites.