Parasite-host interactions during the blood and liver stage of rodent malaria parasites

Malaria is a parasitic disease which puts almost half of the world's population at risk of infection. It is caused by Plasmodium, a parasite transmitted by Anopheles mosquitoes. During its life cycle in the vertebrate host, the malaria parasite invades and multiplies in hepatocytes and in red blood cells. During invasion of sporozoites and merozoites, the host cell membrane invaginates around the parasite to form the parasitophorous vacuole membrane (PVM). The membrane is extensively remodeled by the parasite: host cell membrane proteins are removed and parasite proteins are incorporated. The PVM shields the parasite from a direct attack by cytosolic host cell immune responses and provides the host-parasite interface. Important roles of the PVM are nutrient acquisition, excretion of waste products and export of host-targeted proteins which add to parasite fitness and virulence. Finally, egress is critical for parasite release from its host cell and the first step is PVM disintegration. PVM biology and remodeling is of great interest, but despite all of these important tasks fulfilled by proteins in the PVM, the composition is not yet clear. Here, a proximity labeling technique (BioID) was used to identify PVM or PVM-interacting proteins in P. berghei blood stage parasites. Using this technique, many of the already known PVM proteins were found. Apart from known resident PVM proteins, I detected several proteins that were that were hitherto unknown as PVM proteins. Uncharacterized protein candidates were endogenously GFP-tagged and analyzed using live- and fixed-cell microscopy during the liver and the blood stage. Using this approach, I was able to identify several novel PVM proteins, and proteins that come in close proximity to the PVM. During the intraerythrocytic symptomatic blood stage, Plasmodium digests up to 70% of host hemoglobin. Heme is an essential component of a number of proteins including hemoglobin and is essential for life. It is synthesized by a pathway involving at least eight enzymatic steps, and deficiency of any of these result in porphyria. Despite the abundance of hemoglobin and heme during the blood stage, the parasite expresses all enzymes of its canonical heme pathway, which can facilitate the de novo heme synthesis. Previous research has indicated that the parasite heme pathway is non-essential during the blood stage, and that the deficiency of host heme enzymes can influence parasite growth. That Plasmodium can be greatly influenced by the host background is well established. Here we hypothesized that host porphobilinogen deaminase (PBGD) and coproporphyrinogen oxidase (CPOX), two additional heme pathway enzymes, play a role during the blood stage. To facilitate this goal, in-depth studies by using rodent malaria species were utilized to infect mice deficient in PBGD and CPOX. Blood from porphyric patients was infected with P. falciparum and parasite growth was significantly inhibited. Host PBGD was localized to the parasite periphery which could indicate that the parasite imports this enzyme during the blood stage. PBGD deficient mice are more resistant to malaria infection which suggests that Plasmodium relies on host PBGD to sustain its growth in erythrocytes and cause blood-stage infection. CPOX deficient mice displayed an increased survival, probably due to iron deficiency anemia, but the exact underlying mechanism remains to be elucidated. A novel role for PBGD and CPOX was described in this thesis adding to the understanding of host-pathogen interactions during the blood stage. Together this thesis has identified a number of novel PVM proteins during the blood and liver stage which will help to gain a deeper understanding of PVM biology. Deficiency of host enzymes can influence Plasmodium and here two novel host proteins, PBGD and CPOX are added to them. This thesis increased the understanding of host-parasite interactions during the blood and liver stage of Plasmodium.