PSOP1, putative secreted ookinete protein 1, is localized to the micronemes of Plasmodium yoelii and P. berghei ookinetes

Identification of a novel protein localized to the crystalloid of the Plasmodium ookinete

Reducing Plasmodium parasite transmission via the mosquito vector is a promising strategy for malaria control and elimination in endemic regions. In the mosquito midgut after the ingestion of an infected blood meal, malaria parasite gametes egress from erythrocytes and fertilize to develop into motile ookinetes that traverse midgut epithelial cells and transform into oocysts adjacent the basal lamina. Plasmodium ookinetes and young oocysts possess a unique organelle called the crystalloid; which has a honeycomb-like matrix structure and is indicated to be involved in sporozoite formation and maturation. In this study, we identified a novel crystalloid protein, PY17X_1113800, that is exclusively expressed in developing ookinetes. The protein possesses a signal peptide sequence, but lacks a transmembrane domain or GPI anchor signal sequence, as well as predicted adhesive domains which are characterisitic of many crystalloid proteins. The protein is highly conserved across the phylum Apicomplexa and within the greater clade Alveolata, such as Vitrella and the ciliates Paramecium and Tetrahymena, but is absent in cryptosporidia.

A Time Point Proteomic Analysis Reveals Protein Dynamics of Plasmodium Oocysts

The oocyst is a sporogonic stage of Plasmodium development that takes place in the mosquito midgut in about 2 weeks. The cyst is protected by a capsule of unknown composition, and little is known about oocyst biology. We carried out a proteomic analysis of oocyst samples isolated at early, mid, and late time points of development. Four biological replicates for each time point were analyzed, and almost 600 oocyst-specific candidates were identified. The analysis revealed that, in young oocysts, there is a strong activity of protein and DNA synthesis, whereas in mature oocysts, proteins involved in oocyst and sporozoite development, gliding motility, and invasion are mostly abundant. Among the proteins identified at early stages, 17 candidates are specific to young oocysts. Thirty-four candidates are common to oocyst and the merosome stages (sporozoite proteins excluded), sharing common features as replication and egress. Western blot and immunofluorescence analyses of selected candidates confirm the expression profile obtained by proteomic analysis.

Comparative proteomics of vesicles essential for the egress of Plasmodium falciparum gametocytes from red blood cells

Transmission of malaria parasites to the mosquito is mediated by sexual precursor cells, the gametocytes. Upon entering the mosquito midgut, the gametocytes egress from the enveloping erythrocyte while passing through gametogenesis. Egress follows an inside-out mode during which the membrane of the parasitophorous vacuole (PV) ruptures prior to the erythrocyte membrane. Membrane rupture requires exocytosis of specialized egress vesicles of the parasites; that is, osmiophilic bodies (OBs) involved in rupturing the PV membrane, and vesicles that harbor the perforin-like protein PPLP2 (here termed P-EVs) required for erythrocyte lysis. While some OB proteins have been identified, like G377 and MDV1/Peg3, the majority of egress vesicle-resident proteins is yet unknown. Here, we used high-resolution imaging and BioID methods to study the two egress vesicle types in Plasmodium falciparum gametocytes. We show that OB exocytosis precedes discharge of the P-EVs and that exocytosis of the P-EVs, but not of the OBs, is calcium sensitive. Both vesicle types exhibit distinct proteomes with the majority of proteins located in the OBs. In addition to known egress-related proteins, we identified novel components of OBs and P-EVs, including vesicle-trafficking proteins. Our data provide insight into the immense molecular machinery required for the inside-out egress of P. falciparum gametocytes.

Identification of a novel AP2 transcription factor in zygotes with an essential role in Plasmodium ookinete development

The sexual phase of Plasmodium represents a crucial step in malaria transmission, during which these parasites fertilize and form ookinetes to infect mosquitoes. Plasmodium development after fertilization is thought to proceed with female-stored mRNAs until the formation of a retort-form ookinete; thus, transcriptional activity in zygotes has previously been considered quiescent. In this study, we reveal the essential role of transcriptional activity in zygotes by investigating the function of a newly identified AP2 transcription factor, AP2-Z, in P. berghei. ap2-z was previously reported as a female transcriptional regulator gene whose disruption resulted in developmental arrest at the retort stage of ookinetes. In this study, although ap2-z was transcribed in females, we show that it was translationally repressed by the DOZI complex and translated after fertilization with peak expression at the zygote stage. ChIP-seq analysis of AP2-Z shows that it binds on specific DNA motifs, targeting the majority of genes known as an essential component of ookinetes, which largely overlap with the AP2-O targets, as well as genes that are unique among the targets of other sexual transcription factors. The results of this study also indicate the existence of a cascade of transcription factors, beginning with AP2-G, that proceeds from gametocytogenesis to ookinete formation.

Sexual development in Plasmodium parasites, a causative agent of malaria, is essential for their transmission from vertebrate hosts to mosquitoes. This important developmental process proceeds as follows: formation of a gametocyte/gamete, fertilization and conversion of the zygote into the mosquito midgut invasive stage, called the ookinete. As a target of transmission blocking strategies, it is important to understand the mechanisms regulating Plasmodium sexual development. In this study, we assessed transcriptional regulation after fertilization by investigating the function of a novel transcription factor, AP2-Z. The results revealed the essential role of de novo transcription activated by AP2-Z in zygotes for promoting ookinete development. As transcriptional activity during the zygote stage has previously been considered silent in Plasmodium, novel genes important for ookinete formation can now be explored in the target genes of AP2-Z. Investigating the functions of these genes can help us understand the mechanisms of Plasmodium zygote/ookinete development and identify new targets for transmission blocking vaccines.

Characterization of PSOP26 as an ookinete surface antigen with improved transmission-blocking activity when fused with PSOP25

Background

The Plasmodium zygote-to-ookinete developmental transition is an essential step for establishing an infection in the mosquito vector, and antigens expressed during this stage are potential targets for transmission-blocking vaccines (TBVs). The secreted ookinete protein 26 (PSOP26) is a newly identified ookinete surface protein. The anti-PSOP26 serum has moderate transmission-blocking activity, indicating the benefit of further investigating this protein as a target for TBVs.

Methods

The function of psop26 was analyzed by targeted gene disruption. A chimeric PSOP25-PSOP26 protein was expressed in the Escherichia coli system. The PSOP25-PSOP26 fusion protein, along with mixed (PSOP25 + PSOP26) or single proteins (PSOP26 or PSOP25), were used for the immunization of mice. The antibody titers and immunogenicity of individual sera were analyzed by enzyme-linked immunoassay (ELISA), indirect immunofluorescence assay (IFA), and Western blot. The transmission-blocking activity of sera from different immunization schemes was assessed using in vitro and in vivo assays.

Results

PSOP26 is a surface protein expressed in Plasmodium gametes and ookinetes. The protein is dispensable for asexual blood-stage development, gametogenesis, and zygote formation, but is essential for the zygote-to-ookinete developmental transition. Specifically, both the prevalence of infections and oocyst densities were decreased in mosquitoes fed on psop26-null mutants. Mixtures of individual PSOP25 and PSOP26 fragments (PSOP25 + PSOP26), as well as chimeras (PSOP25-PSOP26), elicited high antibody levels in mice, with no immunological interference. Antisera against the mixed and fusion proteins elicited higher transmission-reducing activity (TRA) than antisera against the single PSOP26 antigen, but comparable to antisera against PSOP25 antigen alone.

Conclusions

PSOP26 plays a critical role in the zygote-to-ookinete developmental transition. PSOP25 is a promising TBV candidate that could be used alone to target the ookinete stage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05294-8.

5-methylcytosine modification by Plasmodium NSUN2 stabilizes mRNA and mediates the development of gametocytes

5-methylcytosine (m⁵C) is an important epitranscriptomic modification involved in messenger RNA (mRNA) stability and translation efficiency in various biological processes. However, it remains unclear if m⁵C modification contributes to the dynamic regulation of the transcriptome during the developmental cycles of Plasmodium parasites. Here, we characterize the landscape of m⁵C mRNA modifications at single nucleotide resolution in the asexual replication stages and gametocyte sexual stages of rodent (Plasmodium yoelii) and human (Plasmodium falciparum) malaria parasites. While different representations of m⁵C-modified mRNAs are associated with the different stages, the abundance of the m⁵C marker is strikingly enhanced in the transcriptomes of gametocytes. Our results show that m⁵C modifications confer stability to the Plasmodium transcripts and that a Plasmodium ortholog of NSUN2 is a major mRNA m⁵C methyltransferase in malaria parasites. Upon knockout of P. yoelii nsun2 (pynsun2), marked reductions of m⁵C modification were observed in a panel of gametocytogenesis-associated transcripts. These reductions correlated with impaired gametocyte production in the knockout rodent malaria parasites. Restoration of the nsun2 gene in the knockout parasites rescued the gametocyte production phenotype as well as m⁵C modification of the gametocytogenesis-associated transcripts. Together with the mRNA m⁵C profiles for two species of Plasmodium, our findings demonstrate a major role for NSUN2-mediated m⁵C modifications in mRNA transcript stability and sexual differentiation in malaria parasites.