Plasmodium berghei LAPs form an extended protein complex that facilitates crystalloid targeting and biogenesis

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.

Plasmodium LCCL domain-containing modular proteins have their origins in the ancestral alveolate

Plasmodium species encode a unique set of six modular proteins named LCCL lectin domain adhesive-like proteins (LAPs) that operate as a complex and that are essential for malaria parasite transmission from mosquito to vertebrate. LAPs possess complex architectures obtained through unique assemblies of conserved domains associated with lipid, protein and carbohydrate interactions, including the name-defining LCCL domain. Here, we assessed the prevalence of Plasmodium LAP orthologues across eukaryotic life. Our findings show orthologous conservation in all apicomplexans, with lineage-specific repertoires acquired through differential lap gene loss and duplication. Besides Apicomplexa, LAPs are found in their closest relatives: the photosynthetic chromerids, which encode the broadest repertoire including a novel membrane-bound LCCL protein. LAPs are notably absent from other alveolate lineages (dinoflagellates, perkinsids and ciliates), but are encoded by predatory colponemids, a sister group to the alveolates. These results reveal that the LAPs are much older than previously thought and pre-date not only the Apicomplexa but the Alveolata altogether.

The molecular mechanisms driving Plasmodium cell division

Malaria, a vector borne disease, is a major global health and socioeconomic problem caused by the apicomplexan protozoan parasite Plasmodium. The parasite alternates between mosquito vector and vertebrate host, with meiosis in the mosquito and proliferative mitotic cell division in both hosts. In the canonical eukaryotic model, cell division is either by open or closed mitosis and karyokinesis is followed by cytokinesis; whereas in Plasmodium closed mitosis is not directly accompanied by concomitant cell division. Key molecular players and regulatory mechanisms of this process have been identified, but the pivotal role of certain protein complexes and the post-translational modifications that modulate their actions are still to be deciphered. Here, we discuss recent evidence for the function of known proteins in Plasmodium cell division and processes that are potential novel targets for therapeutic intervention. We also identify key questions to open new and exciting research to understand divergent Plasmodium cell division.

The Multiple Roles of LCCL Domain-Containing Proteins for Malaria Parasite Transmission

Multi-protein complexes are crucial for various essential biological processes of the malaria parasite Plasmodium, such as protein synthesis, host cell invasion and adhesion. Especially during the sexual phase of the parasite, which takes place in the midgut of the mosquito vector, protein complexes are required for fertilization, sporulation and ultimately for the successful transmission of the parasite. Among the most noticeable protein complexes of the transmission stages are the ones formed by the LCCL domain-containing protein family that play critical roles in the generation of infective sporozoites. The six members of this protein family are characterized by numerous adhesive modules and domains typically found in secreted proteins. This review summarizes the findings of expression and functional studies on the LCCL domain-containing proteins of the human pathogenic P. falciparum and the rodent-infecting P. berghei and discusses the common features and differences of the homologous proteins.

Coordinated regulation of gene expression in Plasmodium female gametocytes by two transcription factors

Gametocytes play key roles in the Plasmodium lifecycle. They are essential for sexual reproduction as precursors of the gametes. They also play an essential role in parasite transmission to mosquitoes. Elucidation of the gene regulation at this stage is essential for understanding these two processes at the molecular level and for developing new strategies to break the parasite lifecycle. We identified a novel Plasmodium transcription factor (TF), designated as a partner of AP2-FG or PFG. In this article, we report that this TF regulates the gene expression in female gametocytes in concert with another female-specific TF AP2-FG. Upon the disruption of PFG, majority of female-specific genes were significantly downregulated, and female gametocyte lost the ability to produce ookinetes. ChIP-seq analysis showed that it was located in the same position as AP2-FG, indicating that these two TFs form a complex. ChIP-seq analysis of PFG in AP2-FG-disrupted parasites and ChIP-seq analysis of AP2-FG in PFG-disrupted parasites demonstrated that PFG mediates the binding of AP2-FG to a ten-base motif and that AP2-FG binds another motif, GCTCA, in the absence of PFG. In promoter assays, this five-base motif was identified as another female-specific cis-acting element. Genes under the control of the two forms of AP2-FG, with or without PFG, partly overlapped; however, each form had target preferences. These results suggested that combinations of these two forms generate various expression patterns among the extensive genes expressed in female gametocytes.

Plasmodium sporozoite excystation involves local breakdown of the oocyst capsule

Plasmodium oocysts develop on the abluminal side of the mosquito midgut in relatively small numbers. Oocysts possess an extracellular cell wall-the capsule-to protect them from the insect's haemolymph environment. To further maximise transmission, each oocyst generates hundreds of sporozoites through an asexual multiplication step called sporogony. Completion of transmission requires sporozoite egress from the capsule (excystation), but this process remains poorly understood. In this study, we fused the parasite-encoded capsule protein Cap380 with green fluorescent protein in a transgenic P. berghei line, allowing live fluorescence imaging of capsules throughout sporogony and sporozoite excystation. The results show that capsules progressively weaken during sporulation ultimately resulting in sporozoite exit through small holes. Prior to formation of the holes, local thinning of the capsule was observed. Our findings support an excystation model based on local, rather than global, weakening of the capsule likely facilitated by local re-orientation of sporozoites and apical secretion.

Identification of genes required for Plasmodium gametocyte-to-sporozoite development in the mosquito vector

Malaria remains one of the most devastating infectious diseases. Reverse genetic screens offer a powerful approach to identify genes and molecular processes governing malaria parasite biology. However, the complex regulation of gene expression and genotype-phenotype associations in the mosquito vector, along with sexual reproduction, have hindered the development of screens in this critical part of the parasite life cycle. To address this, we developed a genetic approach in the rodent parasite Plasmodium berghei that, in combination with barcode sequencing, circumvents the fertilization roadblock and enables screening for gametocyte-expressed genes required for parasite infection of the mosquito Anopheles coluzzii. Our results confirm previous findings, validating our approach for scaling up, and identify genes necessary for mosquito midgut infection, oocyst development, and salivary gland infection. These findings can aid efforts to study malaria transmission biology and to develop interventions for controlling disease transmission.

Cryptosporidium uses multiple distinct secretory organelles to interact with and modify its host cell

Cryptosporidium is a leading cause of diarrheal disease in children and an important contributor to early childhood mortality. The parasite invades and extensively remodels intestinal epithelial cells, building an elaborate interface structure. How this occurs at the molecular level and the contributing parasite factors are largely unknown. Here, we generated a whole-cell spatial proteome of the Cryptosporidium sporozoite and used genetic and cell biological experimentation to discover the Cryptosporidium-secreted effector proteome. These findings reveal multiple organelles, including an original secretory organelle, and generate numerous compartment markers by tagging native gene loci. We show that secreted proteins are delivered to the parasite-host interface, where they assemble into different structures including a ring that anchors the parasite into its unique epicellular niche. Cryptosporidium thus uses a complex set of secretion systems during and following invasion that act in concert to subjugate its host cell.

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.

PfARID Regulates P. falciparum Malaria Parasite Male Gametogenesis and Female Fertility and Is Critical for Parasite Transmission to the Mosquito Vector

Sexual reproduction of Plasmodium falciparum parasites is critical to the spread of malaria in the human population. The factors that regulate gene expression underlying formation of fertilization-competent gametes, however, remain unknown. Here, we report that P. falciparum expresses a protein with an AT-rich interaction domain (ARID) which, in other organisms, is part of chromatin remodeling complexes. P. falciparum ARID (PfARID) localized to the parasite nucleus and is critical for the formation of male gametes and fertility of female gametes. PfARID gene deletion (Pfarid-) gametocytes showed downregulation of gene expression important for gametogenesis, antigenic variation, and cell signaling and for parasite development in the mosquito. Our study identifies PfARID as a critical nuclear protein involved in regulating the gene expression landscape of mature gametocytes. This establishes fertility and also prepares the parasite for postfertilization events that are essential for infection of the mosquito vector. IMPORTANCE Successful completion of the Plasmodium life cycle requires formation of mature gametocytes and their uptake by the female Anopheles mosquito vector in an infected blood meal. Inside the mosquito midgut the parasite undergoes gametogenesis and sexual reproduction. In the present study, we demonstrate that PfARID is essential for male gametogenesis and female fertility and, thereby, transmission to the mosquito vector. PfARID possibly regulates the chromatin landscape of stage V gametocytes and targeting PfARID function may provide new avenues into designing interventions to prevent malaria transmission.

Accurately Assigning Peptides to Spectra When Only a Subset of Peptides Are Relevant

The standard proteomics database search strategy involves searching spectra against a peptide database and estimating the false discovery rate (FDR) of the resulting set of peptide-spectrum matches. One assumption of this protocol is that all the peptides in the database are relevant to the hypothesis being investigated. However, in settings where researchers are interested in a subset of peptides, alternative search and FDR control strategies are needed. Recently, two methods were proposed to address this problem: subset-search and all-sub. We show that both methods fail to control the FDR. For subset-search, this failure is due to the presence of "neighbor" peptides, which are defined as irrelevant peptides with a similar precursor mass and fragmentation spectrum as a relevant peptide. Not considering neighbors compromises the FDR estimate because a spectrum generated by an irrelevant peptide can incorrectly match well to a relevant peptide. Therefore, we have developed a new method, "subset-neighbor search" (SNS), that accounts for neighbor peptides. We show evidence that SNS controls the FDR when neighbors are present and that SNS outperforms group-FDR, the only other method that appears to control the FDR relative to a subset of relevant peptides.

Crystalloids: Fascinating Parasite Organelles Essential for Malaria Transmission

Crystalloids are malaria parasite organelles exclusive to the ookinete and young oocyst life stages that infect the mosquito. The organelles have key roles in sporozoite development and infectivity but the way this is facilitated on a molecular level remains poorly understood. Recent discoveries have shed new light on these processes.