Female inheritance of malarial lap genes is essential for mosquito transmission

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 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 CCp5 and FNPA as Novel Non-canonical Members of the CCp Protein Family in Babesia bovis

Bovine babesiosis, caused by Babesia bovis, is an economically significant tick-borne disease that imposes restrictions to livestock production worldwide. Current methods to control bovine babesiosis have severe limitations and novel approaches, including transmission-blocking vaccines, are needed. Members of the widely conserved CCp family are multidomain adhesion proteins containing LCCL motifs, which are differentially expressed on gametocytes of apicomplexans, including Babesia spp. and Plasmodium spp. While Plasmodium parasites contain 6 distinct CCp genes, only three members (CCp 1-3) were previously identified in B. bovis. In this study, we describe the identification and characterization of two novel non-canonical members of the CCp gene family in B. bovis, named CCp5 and FNPA. The genes were identified in silico by TBLASTN using P. falciparum CCp family domains as queries. Unlike CCp1-3, the B. bovis CCp5 and FNPA proteins lack the LCCL canonical domain but contain other typical multidomain adhesion motifs which are present in classical CCp proteins. In addition, the B. bovis CCp5 and FNPA are in synteny with known CCp genes in related apicomplexans. Sequence analysis of these two proteins demonstrated high sequence conservation among B. bovis different isolates. Transcription, immunoblot, and immunofluorescence analyses demonstrated expression of CCp5 and FNPA in blood and in vitro induced sexual stages of B. bovis. The FNPA, in contrast to CCp5, has a predicted transmembrane domain, suggesting that it might be expressed in the surface of sexual stage parasites. Altogether, finding of this study support FNPA as a possible target of a transmission-blocking vaccine against B. bovis.

Pleiotropic Roles for the Plasmodium berghei RNA Binding Protein UIS12 in Transmission and Oocyst Maturation

Colonization of the mosquito host by Plasmodium parasites is achieved by sexually differentiated gametocytes. Gametocytogenesis, gamete formation and fertilization are tightly regulated processes, and translational repression is a major regulatory mechanism for stage conversion. Here, we present a characterization of a Plasmodium berghei RNA binding protein, UIS12, that contains two conserved eukaryotic RNA recognition motifs (RRM). Targeted gene deletion resulted in viable parasites that replicate normally during blood infection, but form fewer gametocytes. Upon transmission to Anopheles stephensi mosquitoes, both numbers and size of midgut-associated oocysts were reduced and their development stopped at an early time point. As a consequence, no salivary gland sporozoites were formed indicative of a complete life cycle arrest in the mosquito vector. Comparative transcript profiling in mutant and wild-type infected red blood cells revealed a decrease in transcript abundance of mRNAs coding for signature gamete-, ookinete-, and oocyst-specific proteins in uis12(-) parasites. Together, our findings indicate multiple roles for UIS12 in regulation of gene expression after blood infection in good agreement with the pleiotropic defects that terminate successful sporogony and onward transmission to a new vertebrate host.

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

Passage of malaria parasites through mosquitoes involves multiple developmental transitions, from gametocytes that are ingested with the blood meal, through to sporozoites that are transmitted by insect bite to the host. During the transformation from gametocyte to oocyst, the parasite forms a unique transient organelle named the crystalloid, which is involved in sporozoite formation. In Plasmodium berghei, a complex of six LCCL domain-containing proteins (LAPs) reside in the crystalloid and are required for its biogenesis. However, little else is known about the molecular mechanisms that underlie the crystalloid's role in sporogony. In this study, we have used transgenic parasites stably expressing LAP3 fused to GFP, combined with GFP affinity pulldown and high accuracy mass spectrometry, to identify an extended LAP interactome of some fifty proteins. We show that many of these are targeted to the crystalloid, including members of two protein families with CPW-WPC and pleckstrin homology-like domains, respectively. Our findings indicate that the LAPs are part of an intricate protein complex, the formation of which facilitates both crystalloid targeting and biogenesis.

Significance

Reducing malaria parasite transmission by mosquitoes is a key component of malaria eradication and control strategies. This study sheds important new light on the molecular composition of the crystalloid, an enigmatic parasite organelle that is essential for sporozoite formation and transmission from the insect to the vertebrate host. Our findings provide new mechanistic insight into how proteins are delivered to the crystalloid, and indicate that the molecular mechanisms that underlie crystalloid function are complex, involving several protein families unique to Plasmodium and closely related organisms. The new crystalloid proteins identified will form a useful starting point for studies aimed at unravelling how the crystalloid organelle facilitates sporogony and transmission.

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A Plasmodium berghei LAP3 interactome of some 50 proteins was determined.

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Many proteins in the interactome constitute known or novel crystalloid proteins.

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The interactome includes protein families with PH-like and CPW-WPC domains.

NAD(P) transhydrogenase has vital non-mitochondrial functions in malaria parasite transmission

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated form (NADP) are vital for cell function in all organisms and form cofactors to a host of enzymes in catabolic and anabolic processes. NAD(P) transhydrogenases (NTHs) catalyse hydride ion transfer between NAD(H) and NADP(H). Membrane‐bound NTH isoforms reside in the cytoplasmic membrane of bacteria, and the inner membrane of mitochondria in metazoans, where they generate NADPH. Here, we show that malaria parasites encode a single membrane‐bound NTH that localises to the crystalloid, an organelle required for sporozoite transmission from mosquitos to vertebrates. We demonstrate that NTH has an essential structural role in crystalloid biogenesis, whilst its enzymatic activity is required for sporozoite development. This pinpoints an essential function in sporogony to the activity of a single crystalloid protein. Its additional presence in the apicoplast of sporozoites identifies NTH as a likely supplier of NADPH for this organelle during liver infection. Our findings reveal that Plasmodium species have co‐opted NTH to a variety of non‐mitochondrial organelles to provide a critical source of NADPH reducing power.

Female-specific gene regulation in malaria parasites by an AP2-family transcription factor

The malaria gametocyte, the gamete precursor, is the essential stage for malaria transmission to the mosquito vector. In the vertebrate host's blood, it develops into a mature male or female capable of transforming into a gamete in the mosquito blood meal. Despite the importance of this stage in the malaria life cycle, the genetic regulation of gametocyte development is poorly understood. In particular, transcription factors involved in sex-specific gene expression have not been identified. In this paper, we report that an AP2-family transcription factor, AP2-FG, is responsible for female-specific gene regulation. AP2-FG expression in Plasmodium berghei was observed exclusively in female gametocytes, in the beginning of 4-6 h before sexual dimorphism manifests in developing gametocytes. AP2-FG disruption resulted in the arrest of female maturation, but did not affect the development of males. Chromatin immunoprecipitation sequencing analysis suggested that AP2-FG directly regulates over 700 genes. Its targets include genes for female gametocyte-specific functions, such as gametogenesis, fertilization and zygote development. AP2-FG binding to target gene promoters was associated with a 10 bp sequence motif. These results indicate that AP2-FG plays a role in the differentiation of early gametocytes to mature females by governing a female-specific gene expression repertoire.

Functional Characterization and Comparison of Plasmodium falciparum Proteins as Targets of Transmission-blocking Antibodies

Plasmodium falciparum malaria continues to evade control efforts, utilizing highly specialized sexual-stages to transmit infection between the human host and mosquito vector. In a vaccination model, antibodies directed to sexual-stage antigens, when ingested in the mosquito blood meal, can inhibit parasite growth in the midgut and consequently arrest transmission. Despite multiple datasets for the Plasmodium sexual-stage transcriptome and proteome, there have been no rational screens to identify candidate antigens for transmission-blocking vaccine (TBV) development. This study characterizes 12 proteins from across the P. falciparum sexual-stages as possible TBV targets. Recombinant proteins are heterologously expressed as full-length ectodomains in a mammalian HEK293 cell system. The proteins recapitulate native parasite epitopes as assessed by indirect fluorescence assay and a proportion exhibits immunoreactivity when tested against sera from individuals living in malaria-endemic Burkina Faso and Mali. Purified IgG generated to the mosquito-stage parasite antigen enolase demonstrates moderate inhibition of parasite development in the mosquito midgut by the ex vivo standard membrane feeding assay. The findings support the use of rational screens and comparative functional assessments in identifying proteins of the P. falciparum transmission pathway and establishing a robust pre-clinical TBV pipeline.

Plasmodium Genomics and Genetics: New Insights into Malaria Pathogenesis, Drug Resistance, Epidemiology, and Evolution

Protozoan Plasmodium parasites are the causative agents of malaria, a deadly disease that continues to afflict hundreds of millions of people every year. Infections with malaria parasites can be asymptomatic, with mild or severe symptoms, or fatal, depending on many factors such as parasite virulence and host immune status. Malaria can be treated with various drugs, with artemisinin-based combination therapies (ACTs) being the first-line choice. Recent advances in genetics and genomics of malaria parasites have contributed greatly to our understanding of parasite population dynamics, transmission, drug responses, and pathogenesis. However, knowledge gaps in parasite biology and host-parasite interactions still remain. Parasites resistant to multiple antimalarial drugs have emerged, while advanced clinical trials have shown partial efficacy for one available vaccine. Here we discuss genetic and genomic studies of Plasmodium biology, host-parasite interactions, population structures, mosquito infectivity, antigenic variation, and targets for treatment and immunization. Knowledge from these studies will advance our understanding of malaria pathogenesis, epidemiology, and evolution and will support work to discover and develop new medicines and vaccines.

Essential role of GEXP15, a specific Protein Phosphatase type 1 partner, in Plasmodium berghei in asexual erythrocytic proliferation and transmission

The essential and distinct functions of Protein Phosphatase type 1 (PP1) catalytic subunit in eukaryotes are exclusively achieved through its interaction with a myriad of regulatory partners. In this work, we report the molecular and functional characterization of Gametocyte EXported Protein 15 (GEXP15), a Plasmodium specific protein, as a regulator of PP1. In vitro interaction studies demonstrated that GEXP15 physically interacts with PP1 through the RVxF binding motif in P. berghei. Functional assays showed that GEXP15 was able to increase PP1 activity and the mutation of the RVxF motif completely abolished this regulation. Immunoprecipitation assays of tagged GEXP15 or PP1 in P. berghei followed by immunoblot or mass spectrometry analyses confirmed their interaction and showed that they are present both in schizont and gametocyte stages in shared protein complexes involved in the spliceosome and proteasome pathways and known to play essential role in parasite development. Phenotypic analysis of viable GEXP15 deficient P. berghei blood parasites showed that they were unable to develop lethal infection in BALB/c mice or to establish experimental cerebral malaria in C57BL/6 mice. Further, although deficient parasites produced gametocytes they did not produce any oocysts/sporozoites indicating a high fitness cost in the mosquito. Global proteomic and phosphoproteomic analyses of GEXP15 deficient schizonts revealed a profound defect with a significant decrease in the abundance and an impact on phosphorylation status of proteins involved in regulation of gene expression or invasion. Moreover, depletion of GEXP15 seemed to impact mainly the abundance of some specific proteins of female gametocytes. Our study provides the first insight into the contribution of a PP1 regulator to Plasmodium virulence and suggests that GEXP15 affects both the asexual and sexual life cycle.

In the absence of an effective vaccine and the emerging resistance to artemisinin combination therapy, malaria is still a significant threat to human health. Increasing our understanding of the specific mechanisms of the biology of Plasmodium is essential to propose new strategies to control this infection. Here, we demonstrated that GEXP15, a specific protein in Plasmodium, was able to interact with the Protein Phosphatase 1 and regulate its activity. We showed that both proteins are implicated in common protein complexes involved in the mRNA splicing and proteasome pathways. We reported that the deletion of GEXP15 leads to a loss of parasite virulence during asexual stages and a total abolishment of the capacity of deficient parasites to develop in the mosquito. We also found that this deletion affects both protein phosphorylation status and significantly decreases the expression of essential proteins in schizont and gametocyte stages. This study characterizes for the first time a novel molecular pathway through the control of PP1 by an essential and specific Plasmodium regulator, which may contribute to the discovery of new therapeutic targets to control malaria.

Dysregulated gene expression in oocysts of Plasmodium berghei LAP mutants

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Plasmodium berghei LAP null mutant oocysts display highly reduced levels of CSP.

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Transcription of other sporozoite genes and transcription factors is dysregulated.

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A minority oocyst population can bypass the developmental block in cytokinesis.

Malaria parasite oocysts generate sporozoites by a process termed sporogony. Essential for successful sporogony of Plasmodium berghei in Anopheles stephensi mosquitoes is a complex of six LCCL lectin domain adhesive-like proteins (LAPs). LAP null mutant oocysts undergo growth and mitosis but fail to form sporozoites. At a cytological level, LAP null mutant oocyst development is indistinguishable from its wildtype counterparts for the first week, supporting the hypothesis that LAP null mutant oocysts develop normally before cytokinesis. We show here that LAP1 null mutant oocysts display highly reduced expression of sporozoite proteins and their transcription factors. Our findings indicate that events leading up to the cytokinesis defect in LAP null mutants occur early in oocyst development.

Identification of a PH domain-containing protein which is localized to crystalloid bodies of Plasmodium ookinetes

Background

For the success of the malaria control and eradication programme it is essential to reduce parasite transmission by mosquito vectors. In the midguts of mosquitoes fed with parasite-infected blood, sexual-stage parasites fertilize to develop into motile ookinetes that traverse midgut epithelial cells and reside adjacent the basal lamina. Therefore, the ookinete is a promising target of transmission-blocking vaccines to break the parasite lifecycle in mosquito vectors. However, the molecular mechanisms of ookinete formation and invasion of epithelial cells have not been fully elucidated. A unique structure called the crystalloid body has been identified in the ookinete cytoplasm by electron microscopy, but its biological functions remain unclear.

Methods

A recombinant protein of a novel molecule, designated as crystalloid body specific PH domain-containing protein of Plasmodium yoelii (PyCryPH), was synthesized using a wheat germ cell-free system. Specific rabbit antibodies against PyCryPH were obtained to characterize the expression and localization of PyCryPH during sexual-stage parasite development. In addition, PyCryPH knockout parasites were generated by targeted gene disruption to examine PyCryPH function in mosquito-stage parasite development.

Results

Western blot and immunofluorescence assays using specific antibodies showed that PyCryPH is specifically expressed in zygotes and ookinetes. By immunoelectron microscopy it was demonstrated that PyCryPH is localized within crystalloid bodies. Parasites with a disrupted PyCryPH gene developed normally into ookinetes and formed oocysts on the basal lamina of midguts. In addition, the number of sporozoites residing in salivary glands was comparable to that of wild-type parasites.

Conclusions

CryPH, containing a signal peptide and PH domain, is predominantly expressed in zygotes and ookinetes and is localized to crystalloid bodies in P. yoelii. CryPH accumulates in vesicle-like structures prior to the appearance of typical crystalloid bodies. Unlike other known crystalloid body localized proteins, CryPH does not appear to have a multiple domain architecture characteristic of the LAP/CCp family proteins. Although CryPH is highly conserved among Plasmodium, Babesia, Theileria, and Cryptosporidium, PyCryPH is dispensable for the development of invasive ookinetes and sporozoites in mosquito bodies.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2617-6) contains supplementary material, which is available to authorized users.

Interaction with complement proteins and dendritic cells implicates LCCL domain-containing proteins (CCps) of malaria parasites in immunomodulation

The evasion of host immune defense is critical for pathogens to invade, establish infection and perpetuate in the host. The complement system is one of the first lines of innate immune defense in humans that destroys pathogens in the blood circulation. Activation of the complement system through direct encounter with pathogens or some other agents leads to osmolysis of pathogens, clearance of soluble immune complexes and recruitment of lymphocytes at the site of activation. Although malaria parasites are not exposed to the complement system owing to their intracellular development for most part of their life cycle in the human host, the extracellular stages must face the complement system of human or mosquito or both. In a recent issue of the Biochemical Journal, Sharma et al. reported that P lasmodium falciparum LCCL domain-containing protein 1 (PfCCp1) inhibited activation of the classical complement pathway and down-regulated effector responses of dendritic cells, which implicate PfCCp1 and related proteins in immunomodulation of the host that likely benefits the parasite. PfCCp1 belongs to a multi-domain protein family that exists as multimeric protein complexes. It needs to be investigated whether PfCCp1 or its multimeric protein complexes have an immunomodulatory effect in vivo and on the mosquito complement system.

The Plasmodium LAP complex affects crystalloid biogenesis and oocyst cell division

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Fusion of GFP to Plasmodium berghei LAP4 causes abnormal crystalloid formation.

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LAP4/GFP oocysts have reduced size.

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LAP4/GFP oocyst populations show earlier sporulation dynamics.

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LAP4/GFP sporozoites are not transmitted by mosquito bite.

Malaria parasite oocysts located on the mosquito midgut generate sporozoites by a process called sporogony. Plasmodium berghei parasites express six LCCL lectin domain adhesive-like proteins (LAPs), which operate as a complex and share a localisation in the crystalloid – an organelle found in the ookinete and young oocyst. Depletion of LAPs prevents crystalloid formation, increases oocyst growth, and blocks sporogony. Here, we describe a LAP4 mutant that has abnormal crystalloid biogenesis and produces oocysts that display reduced growth and premature sporogony. These findings provide evidence for a role of the LAP complex in regulating oocyst cell division via the crystalloid.

Pathways of host cell exit by intracellular pathogens

Host cell exit is a critical step in the life-cycle of intracellular pathogens, intimately linked to barrier penetration, tissue dissemination, inflammation, and pathogen transmission. Like cell invasion and intracellular survival, host cell exit represents a well-regulated program that has evolved during host-pathogen co-evolution and that relies on the dynamic and intricate interplay between multiple host and microbial factors. Three distinct pathways of host cell exit have been identified that are employed by three different taxa of intracellular pathogens, bacteria, fungi and protozoa, namely (i) the initiation of programmed cell death, (ii) the active breaching of host cellderived membranes, and (iii) the induced membrane-dependent exit without host cell lysis. Strikingly, an increasing number of studies show that the majority of intracellular pathogens utilize more than one of these strategies, dependent on life-cycle stage, environmental factors and/or host cell type. This review summarizes the diverse exit strategies of intracellular-living bacterial, fungal and protozoan pathogens and discusses the convergently evolved commonalities as well as system-specific variations thereof. Key microbial molecules involved in host cell exit are highlighted and discussed as potential targets for future interventional approaches.

Biochemical characterization of Plasmodium complement factors binding protein for its role in immune modulation

Complement system is the first line of human defence against intruding pathogens and is recognized as a potentially useful therapeutic target. Human malaria parasite Plasmodium employs a series of intricate mechanisms that enables it to evade different arms of immune system, including the complement system. Here, we show the expression of a multi-domain Plasmodium Complement Control Protein 1, PfCCp1 at asexual blood stages and its binding affinity with C3b as well as C4b proteins of human complement cascade. Using a biochemical assay, we demonstrate that PfCCp1 binds with complement factors and inhibits complement activation. Active immunization of mice with PfCCp1 followed by challenge with Plasmodium berghei resulted in the loss of biphasic growth of parasites and early death in comparison to the control group. The study also showed a role of PfCCp1 in modulating Toll-like receptor (TLR)-mediated signalling and effector responses on antigen-presenting cells. PfCCp1 binds with dendritic cells that down-regulates the expression of signalling molecules and pro-inflammatory cytokines, thereby dampening the TLR2-mediated signalling; hence acting as a potent immuno-modulator. In summary, PfCCp1 appears to be an important component of malaria parasite directed immuno-modulating strategies that promote the adaptive fitness of pathogens in the host.

Generation of Plasmodium yoelii malaria parasite carrying double fluorescence reporters in gametocytes

Male and female gametocytes are the infectious forms critical for malaria transmission and targets of intervention. Gametocytes are generally produced in relatively small numbers, and it has been difficult to obtain pure male and female gametocytes for various studies. Male and female gametocytes expressing unique fluorescence reporters have been generated for both Plasmodium falciparum and Plasmodium berghei parasites, which allows isolation of large numbers of pure male and female gametocytes and has greatly contributed to our understanding of gametocyte biology. To establish Plasmodium yoelii as another model for studying gametocytogenesis, here we generate a parasite line with male and female gametocytes expressing GFP or mCherry reporter, respectively, using CRISPR/Cas9-mediated gene editing method. We first inserted genes encoding intact fluorescence proteins downstream of parasite coding region of ccp2 and Dhc1 genes, respectively, generating the knockin parasites producing ccp2::mCherry (female) and Dhc1::gfp (male) gametocytes. We next obtained a parasite clone carrying double-fluorescent reporters by genetically crossing the ccp2::mCherry and Dhc1::gfp lines. The resulting double-labeled DFsc7 parasite displays normal development during the whole life cycle and expresses the fluorescence proteins in male and female gametocyte separately. This parasite strain provides a new platform for facilitating studies of gametocyte biology and malaria transmission.

Malaria parasite LIMP protein regulates sporozoite gliding motility and infectivity in mosquito and mammalian hosts

Gliding motility allows malaria parasites to migrate and invade tissues and cells in different hosts. It requires parasite surface proteins to provide attachment to host cells and extracellular matrices. Here, we identify the Plasmodium protein LIMP (the name refers to a gliding phenotype in the sporozoite arising from epitope tagging of the endogenous protein) as a key regulator for adhesion during gliding motility in the rodent malaria model P. berghei. Transcribed in gametocytes, LIMP is translated in the ookinete from maternal mRNA, and later in the sporozoite. The absence of LIMP reduces initial mosquito infection by 50%, impedes salivary gland invasion 10-fold, and causes a complete absence of liver invasion as mutants fail to attach to host cells. GFP tagging of LIMP caused a limping defect during movement with reduced speed and transient curvature changes of the parasite. LIMP is an essential motility and invasion factor necessary for malaria transmission.

LCCL protein complex formation in Plasmodium is critically dependent on LAP1

Successful sporogony of Plasmodium berghei in vector mosquitoes requires expression of a family of six modular proteins named LCCL lectin domain adhesive-like proteins (LAPs). The LAPs share a subcellular localization in the crystalloid, a unique parasite organelle that forms during ookinete development. Here, LAP interactions in P. berghei were studied using a series of parasite lines stably expressing reporter-tagged LAPs combined with affinity purification and high accuracy label free quantitative mass spectrometry. Our results show that abundant complexes containing LAP1, LAP2 and LAP3 are formed in gametocytes through high avidity interactions. Following fertilization, LAP4, LAP5 and LAP6 are recruited to this complex, a process that is facilitated by LAP1 chiefly through its scavenger receptor cysteine-rich modules. These collective findings provide new insight into the temporal and molecular dynamics of protein complex formation that lead up to, and are required for, crystalloid biogenesis and downstream sporozoite transmission of malaria parasites.

Plasmodium Oocysts: Overlooked Targets of Mosquito Immunity

Although the ability of mosquitoes to limit Plasmodium infection is well documented, many questions remain as to how malaria parasites are recognized and killed by the mosquito host. Recent evidence suggests that anti-Plasmodium immunity is multimodal, with different immune mechanisms regulating ookinete and oocyst survival. However, most experiments determine the number of mature oocysts, without considering that different immune mechanisms may target different developmental stages of the parasite. Complement-like proteins have emerged as important determinants of early immunity targeting the ookinete stage, yet the mechanisms by which the mosquito late-phase immune response limits oocyst survival are less understood. Here, we describe the known components of the mosquito immune system that limit oocyst development, and provide insight into their possible mechanisms of action.

Maternally supplied S-acyl-transferase is required for crystalloid organelle formation and transmission of the malaria parasite

Transmission of the malaria parasite from the mammalian host to the mosquito vector requires the formation of adequately adapted parasite forms and stage-specific organelles. Here we show that formation of the crystalloid-a unique and short-lived organelle of the Plasmodium ookinete and oocyst stage required for sporogony-is dependent on the precisely timed expression of the S-acyl-transferase DHHC10. DHHC10, translationally repressed in female Plasmodium berghei gametocytes, is activated translationally during ookinete formation, where the protein is essential for the formation of the crystalloid, the correct targeting of crystalloid-resident protein LAP2, and malaria parasite transmission.

Diversity of extracellular proteins during the transition from the ‘proto-apicomplexan’ alveolates to the apicomplexan obligate parasites

The recent completion of high-coverage draft genome sequences for several alveolate protozoans – namely, the chromerids, Chromera velia and Vitrella brassicaformis; the perkinsid Perkinsus marinus; the apicomplexan, Gregarina niphandrodes, as well as high coverage transcriptome sequence information for several colpodellids, allows for new genome-scale comparisons across a rich landscape of apicomplexans and other alveolates. Genome annotations can now be used to help interpret fine ultrastructure and cell biology, and guide new studies to describe a variety of alveolate life strategies, such as symbiosis or free living, predation, and obligate intracellular parasitism, as well to provide foundations to dissect the evolutionary transitions between these niches. This review focuses on the attempt to identify extracellular proteins which might mediate the physical interface of cell–cell interactions within the above life strategies, aided by annotation of the repertoires of predicted surface and secreted proteins encoded within alveolate genomes. In particular, we discuss what descriptions of the predicted extracellular proteomes reveal regarding a hypothetical last common ancestor of a pre-apicomplexan alveolate – guided by ultrastructure, life strategies and phylogenetic relationships – in an attempt to understand the evolution of obligate parasitism in apicomplexans.

Plasmodium P-Type Cyclin CYC3 Modulates Endomitotic Growth during Oocyst Development in Mosquitoes

Cell-cycle progression and cell division in eukaryotes are governed in part by the cyclin family and their regulation of cyclin-dependent kinases (CDKs). Cyclins are very well characterised in model systems such as yeast and human cells, but surprisingly little is known about their number and role in Plasmodium, the unicellular protozoan parasite that causes malaria. Malaria parasite cell division and proliferation differs from that of many eukaryotes. During its life cycle it undergoes two types of mitosis: endomitosis in asexual stages and an extremely rapid mitotic process during male gametogenesis. Both schizogony (producing merozoites) in host liver and red blood cells, and sporogony (producing sporozoites) in the mosquito vector, are endomitotic with repeated nuclear replication, without chromosome condensation, before cell division. The role of specific cyclins during Plasmodium cell proliferation was unknown. We show here that the Plasmodium genome contains only three cyclin genes, representing an unusual repertoire of cyclin classes. Expression and reverse genetic analyses of the single Plant (P)-type cyclin, CYC3, in the rodent malaria parasite, Plasmodium berghei, revealed a cytoplasmic and nuclear location of the GFP-tagged protein throughout the lifecycle. Deletion of cyc3 resulted in defects in size, number and growth of oocysts, with abnormalities in budding and sporozoite formation. Furthermore, global transcript analysis of the cyc3-deleted and wild type parasites at gametocyte and ookinete stages identified differentially expressed genes required for signalling, invasion and oocyst development. Collectively these data suggest that cyc3 modulates oocyst endomitotic development in Plasmodium berghei.

The malaria parasite is a single-celled organism that multiplies asexually in a non-canonical way in both vertebrate host and mosquito vector. In the mosquito midgut, atypical cell division occurs in oocysts, where repeated nuclear division (endomitosis) precedes cell division, which then gives rise to many sporozoites in a process known as sporogony. The molecular mechanisms controlling this process are poorly understood. In many model organisms including mouse and yeast cells the cell cycle is regulated by members of the cyclin protein family, but the role of this family in the malaria parasite is unknown. Here, we show that there are only three cyclin genes and investigate the function of the single P-type cyclin (CYC3) in the rodent malaria parasite, Plasmodium berghei. We show that CYC3 has a cytoplasmic and nuclear localisation throughout most of the parasite lifecycle and by gene deletion we demonstrate that CYC3 is important for normal oocyst development, maturation and sporozoite formation. Moreover, we show that deletion of cyc3 affects the transcription of genes required for cell signalling and oocyst development. The data suggest that CYC3 modulates asexual multiplication in oocysts and plays a vital role in parasite development in the mosquito.

Biogenesis of the crystalloid organelle in Plasmodium involves microtubule-dependent vesicle transport and assembly

Malaria parasites possess unique subcellular structures and organelles. One of these is the crystalloid, a multivesicular organelle that forms during the parasite's development in vector mosquitoes. The formation and function of these organelles remain poorly understood. A family of six conserved and modular proteins named LCCL-lectin adhesive-like proteins (LAPs), which have essential roles in sporozoite transmission, localise to the crystalloids. In this study we analyse crystalloid formation using transgenic Plasmodium berghei parasites expressing GFP-tagged LAP3. We show that deletion of the LCCL domain from LAP3 causes retarded crystalloid development, while knockout of LAP3 prevents formation of the organelle. Our data reveal that the process of crystalloid formation involves active relocation of endoplasmic reticulum-derived vesicles to common assembly points via microtubule-dependent transport. Inhibition of microtubule-dependent cargo transport disrupts this process and replicates the LCCL domain deletion mutant phenotype in wildtype parasites. These findings provide the first clear insight into crystalloid biogenesis, demonstrating a fundamental role for the LAP family in this process, and identifying the crystalloid and its formation as potential targets for malaria transmission control.

The cell biology of malaria infection of mosquito: advances and opportunities

Recent reviews (Feachem et al.; Alonso et al.) have concluded that in order to have a sustainable impact on the global burden of malaria, it is essential that we knowingly reduce the global incidence of infected persons. To achieve this we must reduce the basic reproductive rate of the parasites to < 1 in diverse epidemiological settings. This can be achieved by impacting combinations of the following parameters: the number of mosquitoes relative to the number of persons, the mosquito/human biting rate, the proportion of mosquitoes carrying infectious sporozoites, the daily survival rate of the infectious mosquito and the ability of malaria-infected persons to infect mosquito vectors.

This paper focuses on our understanding of parasite biology underpinning the last of these terms: infection of the mosquito. The article attempts to highlight central issues that require further study to assist in the discovery of useful transmission-blocking measures.

Toward the development of effective transmission-blocking vaccines for malaria

The continued global burden of malaria can in part be attributed to a complex lifecycle, with both human hosts and mosquito vectors serving as transmission reservoirs. In preclinical models of vaccine-induced immunity, antibodies to parasite sexual-stage antigens, ingested in the mosquito blood meal, can inhibit parasite survival in the insect midgut as judged by ex vivo functional studies such as the membrane feeding assay. In an era of renewed political momentum for malaria elimination and eradication campaigns, such observations have fueled support for the development and implementation of so-called transmission-blocking vaccines. While leading candidates are being evaluated using a variety of promising vaccine platforms, the field is also beginning to capitalize on global '-omics' data for the rational genome-based selection and unbiased characterization of parasite and mosquito proteins to expand the candidate list. This review covers the progress and prospects of these recent developments.

Distinct temporal recruitment of Plasmodium alveolins to the subpellicular network

The zoite stages of malaria parasites (merozoite, ookinete and sporozoite) possess a distinctive cortical structure termed the pellicle, which is defined by a double membrane layer named the inner membrane complex (IMC). The IMC is supported by a cytoskeleton of intermediate filaments, termed the subpellicular network (SPN). Plasmodium IMC1 proteins, or alveolins, make up a conserved family of structurally related proteins that comprise building blocks of the SPN. Here, using green fluorescent protein (GFP) tagging in P. berghei, we show that the alveolins PbIMC1c and PbIMC1e are expressed in all three zoite stages. Our data reveal that PbIMC1e is assembled into the SPN concurrent with pellicle development, while PbIMC1c is assembled after pellicle formation. In the sexual stages, these processes are accompanied by different gene expressions from maternal and paternal alleles: PbIMC1e is expressed uniquely from the maternal allele, while PbIMC1c is expressed from the maternal allele in gametocytes, but from both parental alleles during ookinete development. These findings establish biogenesis of the cortical cytoskeleton in Plasmodium to be a complex and dynamic process, involving distinct parental gene expression and chronological recruitment of its protein constituents. While allelic replacement of the pbimc1c and pbimc1e genes with GFP-tagged versions was readily achieved using double crossover homologous recombination, attempts to disrupt these genes by this strategy only resulted in the integration of the selectable marker and GFP reporter into non-specific genomic locations. The recurrent inability to disrupt these genes provides the first genetic evidence that alveolins are necessary for asexual blood-stage parasite development in Plasmodium.

Genetic crosses and complementation reveal essential functions for the Plasmodium stage-specific actin2 in sporogonic development

Malaria parasites have two actin isoforms, ubiquitous actin1 and specialized actin2. Actin2 is essential for late male gametogenesis, prior to egress from the host erythrocyte. Here, we examined whether the two actins fulfil overlapping functions in Plasmodium berghei. Replacement of actin2 with actin1 resulted in partial complementation of the defects in male gametogenesis and, thus, viable ookinetes were formed, able to invade the midgut epithelium and develop into oocysts. However, these remained small and their DNA was undetectable at day 8 after infection. As a consequence sporogony did not occur, resulting in a complete block of parasite transmission. Furthermore, we show that expression of actin2 is tightly controlled in female stages. The actin2 transcript is translationally repressed in female gametocytes, but translated in female gametes. The protein persists until mature ookinetes; this expression is strictly dependent on the maternally derived expression. Genetic crosses revealed that actin2 functions at an early stage of ookinete formation and that parasites lacking actin2 are unable to undergo sporogony in the mosquito midgut. Our results provide insights into the specialized role of actin2 in Plasmodium development in the mosquito and suggest that the two actin isoforms have distinct biological functions.

Differential expression of three members of the multidomain adhesion CCp family in Babesia bigemina, Babesia bovis and Theileria equi

Members of the CCp protein family have been previously described to be expressed on gametocytes of apicomplexan Plasmodium parasites. Knocking out Plasmodium CCp genes blocks the development of the parasite in the mosquito vector, making the CCp proteins potential targets for the development of a transmission-blocking vaccine. Apicomplexans Babesia bovis and Babesia bigemina are the causative agents of bovine babesiosis, and apicomplexan Theileria equi causes equine piroplasmosis. Bovine babesiosis and equine piroplasmosis are the most economically important parasite diseases that affect worldwide cattle and equine industries, respectively. The recent sequencing of the B. bovis and T. equi genomes has provided the opportunity to identify novel genes involved in parasite biology. Here we characterize three members of the CCp family, named CCp1, CCp2 and CCp3, in B. bigemina, B. bovis and T. equi. Using B. bigemina as an in vitro model, expression of all three CCp genes and proteins was demonstrated in temperature-induced sexual stages. Transcripts for all three CCp genes were found in vivo in blood stages of T. equi, and transcripts for CCp3 were detected in vivo in blood stages of B. bovis. However, no protein expression was detected in T. equi blood stages or B. bovis blood stages or B. bovis tick stages. Collectively, the data demonstrated a differential pattern of expression of three orthologous genes of the multidomain adhesion CCp family by B. bigemina, B. bovis and T. equi. The novel CCp members represent potential targets for innovative approaches to control bovine babesiosis and equine piroplasmosis.

Translational repression controls temporal expression of the Plasmodium berghei LCCL protein complex

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We have GFP-tagged the LCCL proteins PbLAP4, PbLAP5 and PbLAP6 in Plasmodium berghei.

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PbLAP4, PbLAP5 and PbLAP6 associate with the crystalloid organelle in ookinetes.

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Translational repression controls expression of the LCCL protein repertoire in gametocytes.

Plasmodium LCCL proteins comprise a family of six proteins that function as a protein complex and have essential roles in sporozoite transmission. In Plasmodium berghei, family members PbLAP1, PbLAP2 and PbLAP3 have been shown to be expressed in gametocytes and, following gametogenesis and fertilization, to be targeted to distinctive multivesicular organelles termed crystalloids that form in the ookinete. Here, we show by GFP-tagging that PbLAP4, PbLAP5 and PbLAP6, like their family members, are associated with the crystalloids. However, in contrast to their family members, protein expression of PbLAP4, PbLAP5 and PbLAP6 was not detected in gametocytes, even though transcription of the corresponding genes is most prominent in the sexual blood stage parasites. These results suggest that translational repression controls expression of the LCCL protein repertoire and, consequently, the temporal function of the protein complex during P. berghei development in the mosquito.

Plasmodium cell biology should inform strategies used in the development of antimalarial transmission-blocking drugs

Malaria is a disease with a devastating impact affecting 216 million people each year and causing 655,000 deaths, most of which are children under 5 years old. Recent appreciation that malaria eradication will require novel interventions to target the parasite during transmission from the human host to the mosquito has lead to an exciting surge in activity to develop transmission-blocking drugs and the high-throughput assays to screen for them. This article presents an overview of transmission-stage cell biology and discusses its impact on assay development to provide a context for researchers to evaluate the relative merits/drawbacks of both screening data obtained from current assays and considerations for future assay design. The most recent knowledge of the transmission-blocking properties of current antimalarial classes is also summarized and, underdeveloped targets for transmission-stage drug discovery are highlighted.

Loss-of-function analyses defines vital and redundant functions of the Plasmodium rhomboid protease family

Rhomboid-like proteases cleave membrane-anchored proteins within their transmembrane domains. In apicomplexan parasites substrates include molecules that function in parasite motility and host cell invasion. While two Plasmodium rhomboids, ROM1 and ROM4, have been examined, the roles of the remaining six rhomboids during the malaria parasite's life cycle are unknown. We present systematic gene deletion analyses of all eight Plasmodium rhomboid-like proteins as a means to discover stage-specific phenotypes and potential functions in the rodent malaria model, P. berghei. Four rhomboids (ROM4, 6, 7 and 8) are refractory to gene deletion, suggesting an essential role during asexual blood stage development. In contrast ROM1, 3, 9 and 10 were dispensable for blood stage development and exhibited no, subtle or severe defects in mosquito or liver development. Parasites lacking ROM9 and ROM10 showed no major phenotypic defects. Parasites lacking ROM1 presented a delay in blood stage patency following liver infection, but in contrast to a previous study blood stage parasites had similar growth and virulence characteristics as wild type parasites. Parasites lacking ROM3 in mosquitoes readily established oocysts but failed to produce sporozoites. ROM3 is the first apicomplexan rhomboid identified to play a vital role in sporogony.

Conformational co-dependence between Plasmodium berghei LCCL proteins promotes complex formation and stability

Malaria parasites express a conserved family of LCCL-lectin adhesive-like domain proteins (LAPs) that have essential functions in sporozoite transmission. In Plasmodium falciparum all six family members are expressed in gametocytes and form a multi-protein complex. Intriguingly, knockout of P. falciparum LCCL proteins adversely affects expression of other family members at protein, but not at mRNA level, a phenomenon termed co-dependent expression. Here, we investigate this in Plasmodium berghei by crossing a PbLAP1 null mutant parasite with a parasite line expressing GFP-tagged PbLAP3 that displays strong fluorescence in gametocytes. Selected and validated double mutants show normal synthesis and subcellular localization of PbLAP3::GFP. However, GFP-based fluorescence is dramatically reduced without PbLAP1 present, indicating that PbLAP1 and PbLAP3 interact. Moreover, absence of PbLAP1 markedly reduces the half-life of PbLAP3, consistent with a scenario of misfolding. These findings unveil a potential mechanism of conformational interdependence that facilitates assembly and stability of the functional LCCL protein complex.

Gametocytogenesis in malaria parasite: commitment, development and regulation

Malaria parasites have evolved a complicated life cycle alternating between two hosts. Gametocytes are produced in the vertebrate hosts and are obligatory for natural transmission of the parasites through mosquito vectors. The mechanism of sexual development in Plasmodium has been the focus of extensive studies. In the postgenomic era, the advent of genome-wide analytical tools and genetic manipulation technology has enabled rapid advancement of our knowledge in this area. Patterns of gene expression during sexual development, molecular distinction of the two sexes, and mechanisms underlying subsequent formation of gametes and their fertilization have been progressively elucidated. However, the triggers and mechanism of sexual development remain largely unknown. This article provides an update of our understanding of the molecular and cellular events associated with the decision for commitment to sexual development and regulation of gene expression during gametocytogenesis. Insights into the molecular mechanisms of gametocyte development are essential for designing proper control strategies for interruption of malaria transmission and ultimate elimination.

Arrested oocyst maturation in Plasmodium parasites lacking type II NADH:ubiquinone dehydrogenase

The Plasmodium mitochondrial electron transport chain has received considerable attention as a potential target for new antimalarial drugs. Atovaquone, a potent inhibitor of Plasmodium cytochrome bc(1), in combination with proguanil is recommended for chemoprophylaxis and treatment of malaria. The type II NADH:ubiquinone oxidoreductase (NDH2) is considered an attractive drug target, as its inhibition is thought to lead to the arrest of the mitochondrial electron transport chain and, as a consequence, pyrimidine biosynthesis, an essential pathway for the parasite. Using the rodent malaria parasite Plasmodium berghei as an in vivo infection model, we studied the role of NDH2 during Plasmodium life cycle progression. NDH2 can be deleted by targeted gene disruption and, thus, is dispensable for the pathogenic asexual blood stages, disproving the candidacy for an anti-malarial drug target. After transmission to the insect vector, NDH2-deficient ookinetes display an intact mitochondrial membrane potential. However, ndh2(-) parasites fail to develop into mature oocysts in the mosquito midgut. We propose that Plasmodium blood stage parasites rely on glycolysis as the main ATP generating process, whereas in the invertebrate vector, a glucose-deprived environment, the malaria parasite is dependent on an intact mitochondrial respiratory chain.

The systematic functional analysis of Plasmodium protein kinases identifies essential regulators of mosquito transmission

Although eukaryotic protein kinases (ePKs) contribute to many cellular processes, only three Plasmodium falciparum ePKs have thus far been identified as essential for parasite asexual blood stage development. To identify pathways essential for parasite transmission between their mammalian host and mosquito vector, we undertook a systematic functional analysis of ePKs in the genetically tractable rodent parasite Plasmodium berghei. Modeling domain signatures of conventional ePKs identified 66 putative Plasmodium ePKs. Kinomes are highly conserved between Plasmodium species. Using reverse genetics, we show that 23 ePKs are redundant for asexual erythrocytic parasite development in mice. Phenotyping mutants at four life cycle stages in Anopheles stephensi mosquitoes revealed functional clusters of kinases required for sexual development and sporogony. Roles for a putative SR protein kinase (SRPK) in microgamete formation, a conserved regulator of clathrin uncoating (GAK) in ookinete formation, and a likely regulator of energy metabolism (SNF1/KIN) in sporozoite development were identified.

Malaria crystalloids: specialized structures for parasite transmission?

Malaria parasites possess many unique subcellular structures and organelles that are essential for the successful completion of the complex life cycle of Plasmodium in the vertebrate host and mosquito vector. Among these are the crystalloids: transient structures whose presence is restricted to the mosquito-specific ookinete and young oocyst stages of the parasite. Nearly five decades after they were first described, the crystalloids are back in the spotlight, with recent discoveries pointing to an important role in protein trafficking and sporozoite transmission that could be exploited as new targets for control of malaria transmission.

Blocking Plasmodium falciparum Malaria Transmission with Drugs: The Gametocytocidal and Sporontocidal Properties of Current and Prospective Antimalarials

Drugs that kill or inhibit the sexual stages of Plasmodium could potentially amplify or synergize the impact of other interventions by blocking transmission to mosquitoes. Primaquine and other 8-aminoquinolines have long offered such potential, but safety and other concerns have limited their use. Although transmission-blocking properties are not often a priority of drug discovery efforts, a number of interesting gametocytocidal and/or sporontocidal drug candidates have emerged in recent years. Some still bear significant technical and safety concerns, while others have passed clinical trials and are on the verge of entering the antimalarial armamentarium. Recent advances in our knowledge of gametocyte differentiation, gametogenesis and sporogony have also led to the identification of a large array of potential new targets for drugs that might interfere with malaria transmission. This review examines the properties of existing and prospective drugs, mechanisms of action, counter-indications and their potential role in regional malaria elimination efforts.

Family members stick together: multi-protein complexes of malaria parasites

Malaria parasites express a broad repertoire of proteins whose expression is tightly regulated depending on the life-cycle stage of the parasite and the environment of target organs in the respective host. Transmission of malaria parasites from the human to the anopheline mosquito is mediated by intraerythrocytic sexual stages, termed gametocytes, which circulate in the peripheral blood and are essential for the spread of the tropical disease. In Plasmodium falciparum, gametocytes express numerous extracellular proteins with adhesive motifs, which might mediate important interactions during transmission. Among these is a family of six secreted proteins with adhesive modules, termed PfCCp proteins, which are highly conserved throughout the apicomplexan clade. In P. falciparum, the proteins are expressed in the parasitophorous vacuole of gametocytes and are subsequently exposed on the surface of macrogametes during parasite reproduction in the mosquito midgut. One characteristic of the family is a co-dependent expression, such that loss of all six proteins occurs if expression of one member is disrupted via gene knockout. The six PfCCp proteins interact by adhesion domain-mediated binding and thus form complexes on the sexual stage surface having adhesive properties. To date, the PfCCp proteins represent the only protein family of the malaria parasite sexual stages that assembles to multimeric complexes, and only a small number of such protein complexes have so far been identified in other life-cycle stages of the parasite.

Three members of the 6-cys protein family of Plasmodium play a role in gamete fertility

The process of fertilization is critically dependent on the mutual recognition of gametes and in Plasmodium, the male gamete surface protein P48/45 is vital to this process. This protein belongs to a family of 10 structurally related proteins, the so called 6-cys family. To identify the role of additional members of this family in Plasmodium fertilisation, we performed genetic and functional analysis on the five members of the 6-cys family that are transcribed during the gametocyte stage of P. berghei. This analysis revealed that in addition to P48/45, two members (P230 and P47) also play an essential role in the process of parasite fertilization. Mating studies between parasites lacking P230, P48/45 or P47 demonstrate that P230, like P48/45, is a male fertility factor, consistent with the previous demonstration of a protein complex containing both P48/45 and P230. In contrast, disruption of P47 results in a strong reduction of female fertility, while males remain unaffected. Further analysis revealed that gametes of mutants lacking expression of p48/45 or p230 or p47 are unable to either recognise or attach to each other. Disruption of the paralog of p230, p230p, also specifically expressed in gametocytes, had no observable effect on fertilization. These results indicate that the P. berghei 6-cys family contains a number of proteins that are either male or female specific ligands that play an important role in gamete recognition and/or attachment. The implications of low levels of fertilisation that exist even in the absence of these proteins, indicating alternative pathways of fertilisation, as well as positive selection acting on these proteins, are discussed in the context of targeting these proteins as transmission blocking vaccine candidates.

Sexual reproduction for malaria parasites is an essential process and is necessary for parasite transmission between hosts. Fertilisation between female and male gametes occurs in the midgut of the mosquito and proteins on the surface of gametes are principle targets in transmission blocking strategies. Despite their importance, relatively little is known about the malaria proteins involved in fertilisation. In this study we show that two gamete proteins, one expressed on the surface of males, the other on the surface of females, have important roles in the mutual recognition and attachment of gametes. Mutant parasites that lack the presence of these surface proteins show a strong reduction in fertility. Comparison of these gamete surface proteins in different malaria parasites showed that these proteins are evolving rapidly either across their length or at discreet regions/domains. We found, that despite the drastic reduction in zygote formation, low levels of fertilisation can still occur in the absence of these surface proteins, indicating that gametes can use alternative proteins to recognize each other. Both genetic variation of gamete surface proteins and the presence of different fertilisation pathways have important implications for transmission blocking vaccines targeting gamete surface proteins.

Plasmodium berghei crystalloids contain multiple LCCL proteins

Malaria crystalloids are unique organelles of unknown function that are present only in the mosquito-specific ookinete and early oocyst stages of the parasite. Recently, crystalloid formation in Plasmodium berghei was linked to the parasite protein PbSR, a member of the Plasmodium LCCL protein family composed of six modular multidomain proteins involved in sporozoite development and infectivity. Here, we show by fluorescent protein tagging that two other LCCL protein family members are targeted to the crystalloids in a similar way to PbSR. These results extend the similarities between the LCCL proteins, and provide strong supporting evidence for the hypothesis that members of this protein family work in concert and are involved in a similar molecular process.

Malaria parasite development in the mosquito and infection of the mammalian host

Plasmodium sporozoites are the product of a complex developmental process in the mosquito vector and are destined to infect the mammalian liver. Attention has been drawn to the mosquito stages and pre-erythrocytic stages owing to recognition that these are bottlenecks in the parasite life cycle and that intervention at these stages can block transmission and prevent infection. Parasite progression in the Anopheles mosquito, sporozoite transmission to the mammalian host by mosquito bite, and subsequent infection of the liver are characterized by extensive migration of invasive stages, cell invasion, and developmental changes. Preparation for the liver phase in the mammalian host begins in the mosquito with an extensive reprogramming of the sporozoite to support efficient infection and survival. Here, we discuss what is known about the molecular and cellular basis of the developmental progression of parasites and their interactions with host tissues in the mosquito and during the early phase of mammalian infection.

Malaria, sexual development and transmission: retrospect and prospect

It is difficult to recapture the excitement of recent research into the malaria parasites.Plasmodiumhas shown itself to be a most elegant, resourceful and downright devious cell. To reveal any of its manifold secrets is a hard-won privilege. The thrill of this intellectual endeavour, however, has to be tempered by the realism that we have made unremarkable progress in attacking malaria in the field, where it remains almost as omnipresent as it ever was in the 19th and 20th centuries, and both the parasite and vector have become more difficult to control than ever before. This personal view looks back at the significant progress made, and forward to the challenges of the future, focusing on work on sexual development.

Paternal effect of the nuclear formin-like protein MISFIT on Plasmodium development in the mosquito vector

Malaria parasites must undergo sexual and sporogonic development in mosquitoes before they can infect their vertebrate hosts. We report the discovery and characterization of MISFIT, the first protein with paternal effect on the development of the rodent malaria parasite Plasmodium berghei in Anopheles mosquitoes. MISFIT is expressed in male gametocytes and localizes to the nuclei of male gametocytes, zygotes and ookinetes. Gene disruption results in mutant ookinetes with reduced genome content, microneme defects and altered transcriptional profiles of putative cell cycle regulators, which yet successfully invade the mosquito midgut. However, developmental arrest ensues during the ookinete transformation to oocysts leading to malaria transmission blockade. Genetic crosses between misfit mutant parasites and parasites that are either male or female gamete deficient reveal a strict requirement for a male misfit allele. MISFIT belongs to the family of formin-like proteins, which are known regulators of the dynamic remodeling of actin and microtubule networks. Our data identify the ookinete-to-oocyst transition as a critical cell cycle checkpoint in Plasmodium development and lead us to hypothesize that MISFIT may be a regulator of cell cycle progression. This study offers a new perspective for understanding the male contribution to malaria parasite development in the mosquito vector.

The unicellular protozoan parasites that cause malaria must undergo sexual development and subsequent proliferation in mosquitoes before they can infect humans and cause malaria. We characterized the first protein with paternal effect on the development of malaria parasites in the mosquito. This protein, which we named MISFIT, is produced in the progenitor cells of male gametes and found in the nuclei of these cells as well as in the nuclei of zygotes and their invasive forms, termed ookinetes. Disruption of the gene that encodes MISFIT leads to ookinetes with reduced DNA content, a defective secretory machinery and altered expression of various regulators of DNA replication and cell division. These mutant parasites stop developing immediately after traversing the mosquito gut, leading to malaria transmission blockage. Our study offers a new perspective for understanding the sexual development of malaria parasites in the mosquito vector, which leads to transmission of one of the most devastating diseases of mankind.