A novel micronemal protein, Scot1, is essential for apicoplast biogenesis and liver stage development in Plasmodium berghei

Abstract

Plasmodium sporozoites invade hepatocytes, transform into liver stages, and replicate into thousands of merozoites that infect erythrocytes and cause malaria. Proteins secreted from micronemes play an essential role in hepatocyte invasion, and unneeded micronemes are subsequently discarded for replication. The liver-stage parasites are potent immunogens that prevent malarial infection. Late liver stage-arresting genetically attenuated parasites (GAPs) exhibit greater protective efficacy than early GAP. However, the number of late liver-stage GAPs for generating GAPs with multiple gene deletions is limited. Here, we identified Scot1 (Sporozoite Conserved Orthologous Transcript 1), which was previously shown to be upregulated in sporozoites, and by endogenous tagging with mCherry, we demonstrated that it is expressed in the sporozoite and liver stages in micronemes. Using targeted gene deletion in Plasmodium berghei, we showed that Scot1 is essential for late liver-stage development. Scot1 KO sporozoites grew normally into liver stages but failed to initiate blood-stage infection in mice due to impaired apicoplast biogenesis and merozoite formation. Bioinformatic studies suggested that Scot1 is a metal/small molecule carrier protein. Remarkably, supplementation with metals in the culture of infected Scot1 KO cells did not rescue their phenotype. Immunization with Scot1 KO sporozoites in C57BL/6 mice confers protection against a malaria challenge via infection. These proof-of-concept studies will enable the generation of P. falciparum Scot1 mutants that could be exploited to generate GAP malaria vaccines. ImportanceMalaria parasites experience significant bottlenecks as transmitted to the mammalian host during a mosquito bite. Sporozoites invade liver cells, reproducing into thousands of merozoites, which are released after liver cell ruptures. The specific arrest of sporozoites during liver stage development acts as a powerful immunogen and provides sterile protection against sporozoite infection. GAP leading to an arrest in late liver stage development offers superior protection. Here, we report that a micronemal protein, Scot1, is essential for parasite maturation in the liver. Deletion of Scot1 resulted in impaired apicoplast biogenesis and merozoite formation. Vaccination with Scot1 KO sporozoites protects against malaria challenge. We have identified a late arresting GAP that will aid in developing new as well as safeguarding existing whole parasite vaccines.