Investigating ubiquitination-regulated cell cycle events underpinning malaria transmission

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Malaria is caused by the unicellular pathogen Plasmodium that threatens around 400 million people globally and results in over 0.5 million deaths annually, thereby continuing to be a major public health problem, and urgently requiring new therapeutics. Completion of the malaria parasite's complex lifecycle requires a mammalian host where it causes disease, as well as a mosquito vector responsible for spread of the disease. Consequently, effective elimination of malaria will require both curative and transmission blocking strategies. Similar to other eukaryotes, Plasmodium replicates it's genome to divide, proliferate and spread. However, several unusual characteristics of parasite genome replication can be exploited to combat the parasite, especially in stages crucial for parasite transmission. Parasite transmission to the mosquito is initiated by sexual male and female gametocytes. Upon experiencing the mosquito environment, the male gametocyte undergoes three rounds of genome replication (called mitosis) with an incredible speed of 10 minutes to form sperm-like gametes that fuse with the female cell. The fertilised zygote then undergoes further genome replication (called meiosis) to develop into a motile ookinete that is responsible for infecting mosquitoes. So how is mitosis in gametocytes and meiosis in zygotes regulated? We discovered that many proteins in gametocytes and zygotes are dynamically modified by the small protein, ubiquitin. Ubiquitin is reversibly attached to proteins to modulate their fate, including their stability, cellular localisation and level of activity, suggesting these reversible marks could play a key role during parasite genome replication. Importantly, an eraser of ubiquitin marks, Plasmodium USP7 (ubiquitin specific protease 7) is crucial for the parasite to complete both mitosis and meiosis. Since it is not experimentally possible to study meiosis in the human malaria parasite (P. falciparum), we will exploit the highly conserved rodent malaria model (P. berghei), to examine how USP7 regulates genome replication both during mitosis and meiosis. In this proposal we will uncover how USP7 prepares the parasite for initiation of DNA replication during transmission stages. Using state-of-the-art proteomics and microscopy techniques, we will identify partners and responders of USP7 and also determine how the enzyme activity and structure of parasite USP7 is divergent from its host's equivalent. Our findings will be influential in establishing platforms to screen for pharmacological USP7 inhibitors. Moreover, identifying how USP7 orchestrates both mitosis and meiosis will be useful in the development of improved therapeutic strategies that target and block multiple steps in parasite transmission.