Specificity in host carbohydrate-apicomplexan recognition

The phylum Apicomplexa contains some of the most widespread protozoan parasites of humans and animals. Key members include Plasmodium spp., Eimeria spp., Neospora caninium and Toxoplasma gondii. Not only has T. gondii emerged as the principal model for the study of other apicomplexa parasites it is an important pathogen in its own right, causing severe disseminated disease. Eimeria are a group of highly successful intracellular protozoan parasites that develop within enterocytes. They are the cause of intestinal Coccidiosis in chickens, which is one of the economically most important diseases in modern poultry farming. Neospora caninum has emerged as global pathogen of major significance in cattle and dogs. N. caninum is recognised as the major diagnosed cause of reproductive failure in dairy cattle and infected cattle are up to seven times more likely to abort. Infection by apicomplexans is established in the host by rapid and forced invasion of host cells. Proteins secreted at the first stage of this process, participate in apical attachment to host cell surfaces and the formation of a connection with the parasite actinomyosin system, thereby providing the platform from which to drive invasion - these are called microneme proteins (MICs). We have identified a novel region within this family of proteins that interacts with host carbohydrates and is present in MICs from Eimeria and N. caninum. We have named this the 'Micronemal Adhesive Repeat Regions' (MARRs). We plan to investigate some fundamental aspects of how MICs discriminate host ligands, which will provide new information on how the parasite works at the molecular level. Our specific objectives are to determine the structures of MARRs from three different apicomplexans in complex with cognate ligands. The structural information provided by our studies will allow us to make new hypotheses about how they discriminate between different cell types. We will then design new experiments to test these hypotheses. Our work should provide a foundation that may reveal new pathways to the development of antimicrobial strategies.

The phylum Apicomplexa contains some of the most widespread protozoan parasites of humans and animals. Key members include Plasmodium spp., Eimeria spp., Neospora caninium and Toxoplasma gondii. T. gondii is the principal model for the study of other apicomplexa parasites because classic and reverse genetics methodologies along with the animal infection models are well-established. Parasites from the genus Eimeria are the cause of intestinal Coccidiosis in chickens, which is one of the economically most important diseases in modern poultry production. Neospora caninum has emerged as global pathogen of major importance in cattle and dogs. Infection by apicomplexans is rapidly established in the host. The process is initiated by contact between the host-cell plasma membrane and secreted microneme proteins, followed by reorientation and then the generation of a motive force, which drives penetration. One of the first micronemal proteins (MICs) to be discovered was MIC1 from T. gondii (TgMIC1). Recent work from our lab has localised the host cell binding region to the N-terminus of TgMIC1 and identified a novel carbohydrate-binding domain, the 'Micronemal Adhesive Repeat Region' (MARR), which is also present in other MICs including those from Eimeria and N. caninum. A long standing question is why some apicomplexans can infect and replicate within a broad range of cell types whereas others have very specific cellular targets. Carbohydrate recognition and discrimination provide an excellent means to facilitate such interactions and often play an important role in early recognition events. In this new proposal we aim to address these issues: we propose to determine the structures of MARR-containing proteins from T. gondii, Eimeria. and N. caninum, identify their carbohydrate receptors and provide the full structural and thermodynamic description for specificity.