Novel functional insights from the Plasmodium falciparum sporozoite-specific proteome by probabilistic integration of 26 studies

Abstract

Plasmodium species, the causative agent of malaria, have a complex life cycle involving two hosts. The sporozoite life stage is characterized by an extended phase in the mosquito salivary glands followed by free movement and rapid invasion of hepatocytes in the human host. This transmission stage has been the subject of many transcriptomics and proteomics studies and is also targeted by the most advanced malaria vaccine. We applied Bayesian data integration to determine which proteins are not only present in sporozoites but are also specific to that stage. Transcriptomic and proteomic Plasmodium data sets from 26 studies were weighted for how representative they are for sporozoites, based on a carefully assembled gold standard for Plasmodium falciparum (Pf) proteins known to be present or absent during the sporozoite life stage. Of 5418 Pf genes for which expression data were available at the RNA level or at the protein level, 1105 were identified as enriched in sporozoites and 90 specific to them. We show that Pf sporozoites are enriched for proteins involved in type II fatty acid synthesis in the apicoplast and GPI anchor synthesis, but otherwise appear metabolically relatively inactive, in the salivary glands of mosquitos. Newly annotated hypothetical sporozoite-specific and sporozoite-enriched proteins highlight sporozoite specific functions. They include PF3D7_0104100 that we identified to be homologous to the prominin family, which in human has been related to a quiescent state of cancer cells. We document high levels of genetic variability for sporozoite proteins, specifically for sporozoite-specific proteins that elicit antibodies in the human host. Nevertheless, we can identify nine relatively well-conserved sporozoite proteins that elicit antibodies and that together can serve as markers for previous exposure. Our understanding of sporozoite biology benefits from identifying key pathways that are enriched during this life stage. This work can guide studies of molecular mechanisms underlying sporozoite biology and potential well-conserved targets for marker and drug development. Author SummaryWhen a person is bitten by an infectious malaria mosquito, sporozoites are injected into the skin with mosquito saliva. These sporozoites then travel to the liver, invade hepatocytes and multiply before the onset of the symptom-causing blood stage of malaria. By integrating published data, we contrast sporozoite protein expression with other life stages to filter out the unique features of sporozoites that help us understand this stage. We used a "guideline" that we derived from the literature on individual proteins so that we knew which proteins should be present or absent at the sporozoite stage, allowing us to weigh 26 data sets for their relevance to sporozoites. Among the newly discovered sporozoite-specific genes are candidates for fatty acid synthesis while others might play a role keeping the sporozoites in an inactive state in the mosquito salivary glands. Furthermore, we show that most sporozoite-specific proteins are genetically more variable than non-sporozoite proteins. We identify a set of conserved sporozoite proteins against which antibodies can serve as markers of recent exposure to sporozoites or that can serve as vaccine candidates. Our predictions of sporozoite-specific proteins and the assignment of previously unknown functions give new insights into the biology of this life stage.