Mechanisms of Plasmodium departure from the liver infection stage

• Applicant: Dr. Alyssa Ingmundson
• Subject Area: Parasitology and Biology of Tropical Infectious Disease Pathogens
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 446465266

Plasmodium requires a developmental cycle in the liver before it can infect the blood and cause malaria. Because the number of parasites delivered by the bite of an infected mosquito are limited, only few cells in the liver become infected with Plasmodium. Consequently, mature liver-stage parasites need to exit these cells efficiently to establish the infection in the blood. To escape infected liver cells and promote the transition to the blood, liver-stage merozoites egress in structures known as merosomes. Merosomes are packages of merozoites surrounded by plasma membrane that bud from the infected host cell when egress is triggered. Although the steps leading up to merosome formation have been described, the molecular activities that initiate and drive these events remain poorly defined. Protease activity is known to be essential for liver-stage Plasmodium egress, but the molecular consequences of this protease activity are unknown. We determined that a member of the serine-rich antigen (SERA) family, PbSERA4, is required for efficient exit of liver-stage Plasmodium berghei. PbSERA4 is a putative cysteine protease; however, its proteolytic function had not yet been confirmed experimentally. We generated parasites with a specific point mutation in the predicted active site of PbSERA4 and this change, like deletion of PbSERA4, prolongs the transition between the liver and blood stages of growth. These results confirm that the proteolytic activity of PbSERA4 is required for its role at the culmination of the liver infection stage. One priority of our future work is to identify the substrates of the PbSERA4/PfSERA7 protease. We plan multiple complementary proteomics-based approaches to discover proteins that are cleaved during egress of Plasmodium merozoites. In addition to characterizing the role of any protease substrates identified in our studies, we will examine additional Plasmodium proteins that are expressed in mature liver-stage parasites and are known to facilitate the transition through the liver and into the blood. We will investigate the phenotype of parasites in which these candidates are genetically targeted and evaluate the role proteolytic processing plays in their function. The host cell also has a critical contribution in the exit of Plasmodium liver-stage merozoites. We found that the ceramide analog HPA-12 restricts merosome formation from P. berghei-infected cells, which suggests inhibition of host ceramide transport and sphingolipid synthesis restricts liver-stage parasite exit. We will continue to explore the contribution of this and other host pathways on liver-stage merozoite egress, and our collaborative approach will reveal how the exit of Plasmodium liver merozoites compares to the egress of other pathogens that use non-lytic egress strategies. This multi-disciplinary project will elucidate molecular mechanisms governing the exit of Plasmodium from the liver.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2225: Exit strategies of intracellular pathogens