Plasmodium GPI-Anchored Micronemal Antigen is essential for parasite transmission through the mosquito host

Abstract

The complex life cycle of Plasmodium parasites, the eukaryotic pathogens that cause malaria, features three distinct invasive forms tailored specifically to the equally distinct host environment they must navigate and invade for progression of the life cycle. One conserved feature of all these invasive forms is the presence of micronemes, apically oriented secretory organelles involved in egress, motility, adhesion and invasion. Micronemes are tailored to their specific host environment and feature stage specific contents. Here we investigate the role of GPI-anchored micronemal antigen (GAMA), which shows a micronemal localization in all zoite forms of the rodent infecting species Plasmodium berghei. While GAMA is dispensable during asexual blood stages, GAMA knock out parasites are severely defective for invasion of the mosquito midgut, resulting in reduced numbers of oocysts. Once formed, oocysts develop normally, however sporozoites are unable to egress and these sporozoites exhibit defective motility. Epitope-tagging of GAMA revealed tight temporal expression late during sporogony and showed that GAMA is shed during sporozoite gliding motility in a similar manner to circumsporozoite protein. Complementation of P. berghei knock out parasites with full length P. falciparum GAMA partially restored infectivity to mosquitoes, indicating a conservation of function across Plasmodium species. A suite of parasites with GAMA expressed under the promoters of the known ookinete-to-sporozoite stage-specific genes: CTRP, CAP380 and TRAP, further confirmed the involvement of GAMA in midgut infection, motility and infection of the mammalian host and revealed a lethal consequence to overexpression of GAMA during oocyst development. Combined, the research suggest that GAMA plays independent roles in sporozoite motility, egress and invasion, possibly implicating GAMA as a regulator of microneme function. AUTHOR SUMMARYMalaria remains a major source of morbidity and mortality across the globe. Completion of a complex life cycle between vertebrates and mosquitoes is required for the maintenance of parasite populations and the persistence of malaria disease and death. Three invasive forms across the complex lifecycle of the parasite must successfully egress and invade specific cell types within the vertebrate and mosquito hosts to maintain parasite populations and consequently disease and suffering. A conserved feature of all invasive forms are the micronemes, apically oriented secretory organelles which contain proteins required for motility, egress and invasion. Few proteins are expressed in the micronemes of all three invasive forms. One such protein is GPI-anchored micronemal antigen (GAMA). Here we reveal that GAMA is required for the invasion of the mosquito midgut, egress of sporozoites from oocysts and invasion of the vertebrate host. Our finding indicate that while GAMA is essential for sporozoite motility, the defects in oocyst egress and hepatocyte invasion occur independently of the motility defect, implicating the requirement of GAMA in all three processes.