A Comprehensive Gender-related Secretome of Plasmodium berghei Sexual Stages

Mitochondrial ATP synthesis is essential for efficient gametogenesis in Plasmodium falciparum

Plasmodium male and female gametocytes are the gatekeepers of human-to-mosquito transmission, therefore essential for propagation of malaria within a population. Whilst dormant in humans, their divergent roles during transmission become apparent soon after mosquito feeding with a rapid transformation into gametes - males forming eight motile sperm-like cells aiming to fertilise a single female gamete. Little is known about how the parasite fuels this abrupt change, and the potential role played by their large and elaborate cristate mitochondrion. Using a sex-specific antibody and functional mitochondrial labelling, we show that the male gametocyte mitochondrion is less active than that of female gametocytes and more sensitive to antimalarials targeting mitochondrial energy metabolism. Rather than a vestigial organelle discarded during male gametogenesis, we demonstrate that mitochondrial ATP synthesis is essential for its completion. Additionally, using a genetically encoded ratiometric ATP sensor, we show that gametocytes can maintain cytoplasmic ATP homeostasis in the absence of mitochondrial respiration, indicating the essentiality of the gametocyte mitochondrion for transmission alone. Together, this reveals how gametocytes responsively balance the conflicting demands of a dormant and active lifestyle, highlighting the mitochondria as a rich source of transmission-blocking targets for future drug development.

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.

PfSRPK1 Regulates Asexual Blood Stage Schizogony and Is Essential for Male Gamete Formation

Serine/arginine-rich protein kinases (SRPKs) are cell cycle-regulated serine/threonine protein kinases and are important regulators of splicing factors. In this study, we functionally characterize SRPK1 of the human malaria parasite Plasmodium falciparum. P. falciparum SRPK1 (PfSRPK1) was expressed in asexual blood-stage and sexual-stage gametocytes. Pfsrpk1^- parasites formed asexual schizonts that generated far fewer merozoites than wild-type parasites, causing reduced replication rates. Pfsrpk1^- parasites also showed a severe defect in the differentiation of male gametes, causing a complete block in parasite transmission to mosquitoes. RNA sequencing (RNA-seq) analysis of wild-type PfNF54 and Pfsrpk1^- stage V gametocytes suggested a role for PfSRPK1 in regulating transcript splicing and transcript abundance of genes coding for (i) microtubule/cilium morphogenesis-related proteins, (ii) proteins involved in cyclic nucleotide metabolic processes, (iii) proteins involved in signaling such as PfMAP2, (iv) lipid metabolism enzymes, (v) proteins of osmophilic bodies, and (vi) crystalloid components. Our study reveals an essential role for PfSRPK1 in parasite cell morphogenesis and suggests this kinase as a target to prevent malaria transmission from humans to mosquitoes. IMPORTANCE Plasmodium sexual stages represent a critical bottleneck in the parasite life cycle. Gametocytes taken up in an infectious blood meal by female anopheline mosquito get activated to form gametes and fuse to form short-lived zygotes, which transform into ookinetes to infect mosquitoes. In the present study, we demonstrate that PfSRPK1 is important for merozoite formation and critical for male gametogenesis and is involved in transcript homeostasis for numerous parasite genes. Targeting PfSRPK1 and its downstream pathways may reduce parasite replication and help achieve effective malaria transmission-blocking strategies.

Identification and preliminary characterization of Plasmodium falciparum proteins secreted upon gamete formation

Malaria long-term elimination depends on parasite transmission control. Plasmodium sexual stage maturation in the mosquito, including egress from the host erythrocyte, is one of the prime targets for transmission-blocking interventions. This work aims to identify candidate molecules potentially involved in gamete emergence from the host erythrocyte, as novel transmission blocking targets. We analyzed by quantitative mass spectrometry the proteins released/secreted by purified Plasmodium falciparum gametocytes upon induction of gametogenesis. The proteome obtained showed a good overlap (74%) with the one previously characterized in similar conditions from gametocytes of the rodent malaria parasite P. berghei. Four candidates were selected based on comparative analysis of their abundance values in released vs total gametocyte proteome. We also characterized the P. falciparum orthologue of the microgamete surface protein (MiGS), a marker of male gametocyte secretory vesicles in murine models of malaria. The findings of this study reveal that all the selected candidate proteins are expressed in both genders and localize to vesicle-like structures that respond to gametogenesis stimuli. This result, together with the fact that the selected proteins are released during gamete emergence in both Plasmodium species, makes them interesting candidates for future functional studies to investigate their potential role in the gametogenesis process.

Excreted Trypanosoma brucei proteins inhibit Plasmodium hepatic infection

Malaria, a disease caused by Plasmodium parasites, remains a major threat to public health globally. It is the most common disease in patients with sleeping sickness, another parasitic illness, caused by Trypanosoma brucei. We have previously shown that a T. brucei infection impairs a secondary P. berghei liver infection and decreases malaria severity in mice. However, whether this effect requires an active trypanosome infection remained unknown. Here, we show that Plasmodium liver infection can also be inhibited by the serum of a mouse previously infected by T. brucei and by total protein lysates of this kinetoplastid. Biochemical characterisation showed that the anti-Plasmodium activity of the total T. brucei lysates depends on its protein fraction, but is independent of the abundant variant surface glycoprotein. Finally, we found that the protein(s) responsible for the inhibition of Plasmodium infection is/are present within a fraction of ~350 proteins that are excreted to the bloodstream of the host. We conclude that the defence mechanism developed by trypanosomes against Plasmodium relies on protein excretion. This study opens the door to the identification of novel antiplasmodial intervention strategies.