Malaria parasite invasion of the mosquito salivary gland requires interaction between the Plasmodium TRAP and the Anopheles saglin proteins

Beyond the bite: how mosquito salivary proteins modulate midgut biology and malaria parasite transmission

Mosquito blood feeding is complicated by the host's hemostatic and immune responses, which remain active after ingestion, affecting blood ingestion and digestion and threatening the midgut epithelium integrity. At the bite site, mosquitoes bypass the host's hemostatic and immune defenses by injecting saliva containing bioactive molecules, such as anticoagulants and immunomodulators, which facilitate efficient blood extraction. Ingested saliva can also modulate similar responses in the blood bolus. Here, we examine current evidence on how mosquito saliva proteins modulate blood responses in the midgut and enhance Plasmodium transmission. Saliva proteins are potential transmission-blocking targets for new intervention strategies to combat mosquito-borne diseases.

Parasite and vector circadian clocks mediate efficient malaria transmission

Malaria transmission begins when Anopheles mosquitos deposit saliva and Plasmodium parasites during a bloodmeal. As Anopheles mosquitos are nocturnal, we investigated whether their salivary glands are under circadian control, anticipating bloodmeals and modulating parasite biology for host encounters. Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian expression. Furthermore, measuring haemoglobin levels, we demonstrate that mosquitos prefer to feed and ingest more blood at nighttime. Notably, we show a substantial subset of the salivary-gland-resident parasite transcriptome cycling throughout the day, indicating that this stage is not transcriptionally quiescent. Among the sporozoite genes undergoing rhythmic expression are those involved in parasite motility, potentially modulating the ability to initiate infection at different times of day. Our findings suggest a circadian tripartite relationship between the vector, parasite and mammalian host that together modulates malaria transmission.

Unveiling the Potential of Natural Resources-Derived Therapeutics for Improved Malaria Management: Computational to Experimental Studies

Malaria kills millions of people annually, and it is one of the major causes of preventable mortality in the world. Of the different plasmodium species that induce malaria, Plasmodium falciparum and Plasmodium vivax account for the most severe form of malarial disease in humans. This review focuses on understanding preventive measures, mutation-based disease evolution, malaria-related biomarkers, and potential plant bioactive components for the treatment and management of malaria. The burden of malaria drug resistance has made it necessary for scientists to focus on alternative therapeutics, with particular interests in those involving plant-based bioactive components that could mediate biochemical pathways, consisting of metabolic interactions essential for parasitic inhibition. To avoid artefacts or false positives, these bioactive components from plant sources are further filtered using the "pan-assay-interfering compounds" (PAINS) tool. This review discussed the history of malaria treatment, current treatment options, malaria preventive measures, and challenges associated with current treatment strategies. Additionally, this work discusses the barriers while developing drugs from phytochemicals and the steps needed to accelerate the development of new antimalarial from the lead compounds.

Unbiased phage display screening identifies hidden malaria vaccine targets

Malaria is among the deadliest infectious diseases. Over 200 million annual clinical malaria cases are reported and more than half a million people, mostly children, die every year. The most advanced RTS,S/AS01 vaccine based on the P. falciparum circumsporozoite protein (CSP), targets sporozoite liver infection but achieved modest efficacy. To reduce malaria death, novel malaria vaccine development is a high priority. Most malaria vaccine candidates target three infection steps: sporozoite liver infection, merozoite red blood cell (RBC) infection, and mosquito midgut infection. However, only few malaria vaccine candidates target specific parasite-host cell interactions. Our group has implemented the phage peptide-display approach to discover new parasite ligands and host cell receptors. Here we summarize our findings and discuss their potential for the development of novel vaccines.

A review on RNA interference studies in Anophelines to reveal candidate genes for malaria transmission blocking vaccine

Malaria is a major public health concern. The development of parasite-based vaccine RTS/AS01 has some therapeutic value but its lower efficacy is one of the major limitations. Mosquito-based transmission-blocking vaccines could have a higher potential for parasite inhibition within the mosquitoes. Several genes of mosquito midgut, salivary gland, hemolymph, etc. get activate in response to the Plasmodium-infected blood and helps in parasite invasion directly or indirectly inside the mosquito. The studies of such genes provided a new insight into developing the more efficient vaccines. In the field of malaria genetics research, RNAi has become an innovative strategy used to identify mosquito candidate genes for transmission-blocking vaccines. This review targeted the gene studies that have been conducted in the period 2000-2023 in different malaria vectors against different malarial parasites using the RNAi approach to reveal mosquito novel gene candidates for vaccine development.

Circadian rhythms mediate malaria transmission potential

Malaria transmission begins when infected female Anopheles mosquitos deposit Plasmodium parasites into the mammalian host's skin during a bloodmeal. The salivary gland-resident sporozoite parasites migrate to the bloodstream, subsequently invading and replicating within hepatocytes. As Anopheles mosquitos are more active at night, with a 24-hour rhythm, we investigated whether their salivary glands are under circadian control, anticipating bloodmeals and modulating sporozoite biology for host encounters. Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian rhythmic expression. Furthermore, we demonstrate that mosquitoes prefer to feed during nighttime, with the amount of blood ingested varying cyclically throughout the day. Notably, we show a substantial subset of the sporozoite transcriptome cycling throughout the day. These include genes involved in parasite motility, potentially modulating the ability to initiate infection at different times of day. Thus, although sporozoites are typically considered quiescent, our results demonstrate their transcriptional activity, revealing robust daily rhythms of gene expression. Our findings suggest a circadian evolutionary relationship between the vector, parasite and mammalian host that together modulate malaria transmission.

Genetically Engineered Microorganisms and Their Impact on Human Health

The emergence of antibiotic-resistant strains, the decreased effectiveness of conventional therapies, and the side effects have led researchers to seek a safer, more cost-effective, patient-friendly, and effective method that does not develop antibiotic resistance. With progress in synthetic biology and genetic engineering, genetically engineered microorganisms effective in treatment, prophylaxis, drug delivery, and diagnosis have been developed. The present study reviews the types of genetically engineered bacteria and phages, their impacts on diseases, cancer, and metabolic and inflammatory disorders, the biosynthesis of these modified strains, the route of administration, and their effects on the environment. We conclude that genetically engineered microorganisms can be considered promising candidates for adjunctive treatment of diseases and cancers.

Peptide selection via phage display to inhibit Leishmania-macrophage interactions

Introduction

Leishmaniasis comprises a complex group of diseases caused by protozoan parasites from the Leishmania genus, presenting a significant threat to human health. Infection starts by the release into the skin of metacyclic promastigote (MP) form of the parasite by an infected sand fly. Soon after their release, the MPs enter a phagocytic host cell. This study focuses on finding peptides that can inhibit MP-phagocytic host cell interaction.

Methods

We used a phage display library to screen for peptides that bind to the surface of L. amazonensis (causative agent for cutaneous leishmaniasis) and L. infantum (causative agent for cutaneous and visceral leishmaniasis) MPs. Candidate peptide binding to the MP surface and inhibition of parasite-host cell interaction were tested in vitro. Peptide Inhibition of visceral leishmaniasis development was assessed in BALB/c mice.

Results

The selected L. amazonensis binding peptide (La1) and the L. infantum binding peptide (Li1) inhibited 44% of parasite internalization into THP-1 macrophage-like cells in vitro. While inhibition of internalization by La1 was specific to L. amazonensis, Li1 was effective in inhibiting internalization of both parasite species. Importantly, Li1 inhibited L. infantum spleen and liver infection of BALB/c mice by 84%.

Conclusion

We identified one peptide that specifically inhibits L. amazonensis MP infection of host cells and another that inhibits both, L. amazonensis and L. infantum, MP infection. Our findings suggest a promising path for the development of new treatments and prevention of leishmaniasis.

Plasmodium female gamete surface HSP90 is a key determinant for fertilization

Plasmodium fertilization, an essential step for the development of the malaria parasite in the mosquito, is a prime target for blocking pathogen transmission. Using phage peptide display screening, we identified MG1, a peptide that binds to male gametes and inhibits fertilization, presumably by competing with a female gamete ligand. Anti-MG1 antibodies bind to the female gamete surface and, by doing so, also inhibit fertilization. We determined that this antibody recognizes HSP90 on the surface of Plasmodium female gametes. Our findings establish Plasmodium HSP90 as a prime target for the development of a transmission-blocking vaccine.IMPORTANCEMalaria kills over half a million people every year and this number has not decreased in recent years. The development of new tools to combat this disease is urgently needed. In this article, we report the identification of a key molecule-HSP90-on the surface of the parasite's female gamete that is required for fertilization to occur and for the completion of the parasite cycle in the mosquito. HSP90 is a promising candidate for the development of a transmission-blocking vaccine.

A population modification gene drive targeting both Saglin and Lipophorin impairs Plasmodium transmission in Anopheles mosquitoes

Lipophorin is an essential, highly expressed lipid transport protein that is secreted and circulates in insect hemolymph. We hijacked the Anopheles coluzzii Lipophorin gene to make it co-express a single-chain version of antibody 2A10, which binds sporozoites of the malaria parasite Plasmodium falciparum. The resulting transgenic mosquitoes show a markedly decreased ability to transmit Plasmodium berghei expressing the P. falciparum circumsporozoite protein to mice. To force the spread of this antimalarial transgene in a mosquito population, we designed and tested several CRISPR/Cas9-based gene drives. One of these is installed in, and disrupts, the pro-parasitic gene Saglin and also cleaves wild-type Lipophorin, causing the anti-malarial modified Lipophorin version to replace the wild type and hitch-hike together with the Saglin drive. Although generating drive-resistant alleles and showing instability in its gRNA-encoding multiplex array, the Saglin-based gene drive reached high levels in caged mosquito populations and efficiently promoted the simultaneous spread of the antimalarial Lipophorin::Sc2A10 allele. This combination is expected to decrease parasite transmission via two different mechanisms. This work contributes to the design of novel strategies to spread antimalarial transgenes in mosquitoes, and illustrates some expected and unexpected outcomes encountered when establishing a population modification gene drive.

Knockdown of Anopheles dirus far upstream element-binding protein gene lower oocyst numbers of Plasmodium vivax

The modulation of gene expression levels of Anopheles dirus on Plasmodium vivax infection at the ookinete and oocyst stages was previously reported. In the present study, several upregulated An. dirus genes were selected based on their high expression levels and subcellular locations to examine their roles in P. vivax infection. Five An. dirus genes-carboxylesterase, cuticular protein RR-2 family, far upstream element-binding protein, kraken, and peptidase212-were knocked down by dsRNA feeding using dsRNA-lacZ as a control. The dsRNA-fed mosquitoes were later challenged by P. vivax-infected blood, and the oocyst numbers were determined. The expression of these five genes was examined in many organs of both male and female mosquitoes. The results showed that the decreased expression level of the far upstream element-binding protein gene could lower the oocyst numbers, whereas the others showed no effect on P. vivax infection. The expression levels of these genes in ovaries were found, and in many organs, they were similar between male and female mosquitoes. The reduction of these five gene expressions did not affect the lifespan of the mosquitoes. In addition, the malaria box compound, MMV000634, demonstrated the lowest binding energy to the far upstream element-binding protein using virtual screening. This protein might be a target to block malaria transmission.

Malaria vaccines: the 60-year journey of hope and final success-lessons learned and future prospects

BACKGROUND

The world has made great strides towards beating malaria, although about half of the world population is still exposed to the risk of contracting malaria. Developing an effective malaria vaccine was a huge challenge for medical science. In 2021 the World Health Organization (WHO) approved the first malaria vaccine, RTS,S/AS01 vaccine (Mosquirix™), for widespread use. This review highlights the history of development, and the different approaches and types of malaria vaccines, and the literature to date. It covers the developmental stages of RTS,S/AS01 and recommends steps for its deployment. The review explores other potential vaccine candidates and their status, and suggests options for their further development. It also recommends future roles for vaccines in eradicating malaria. Questions remain on how RTS,S vaccine will work in widespread use and how it can best be utilized to benefit vulnerable communities.

CONCLUSION

Malaria vaccines have been in development for almost 60 years. The RTS,S/AS01 vaccine has now been approved, but cannot be a stand-alone solution. Development should continue on promising candidates such as R21, PfSPZ and P. vivax vaccines. Multi-component vaccines may be a useful addition to other malaria control techniques in achieving eradication of malaria.

The salivary protein Saglin facilitates efficient midgut colonization of Anopheles mosquitoes by malaria parasites

Malaria is caused by the unicellular parasite Plasmodium which is transmitted to humans through the bite of infected female Anopheles mosquitoes. To initiate sexual reproduction and to infect the midgut of the mosquito, Plasmodium gametocytes are able to recognize the intestinal environment after being ingested during blood feeding. A shift in temperature, pH change and the presence of the insect-specific compound xanthurenic acid have been shown to be important stimuli perceived by gametocytes to become activated and proceed to sexual reproduction. Here we report that the salivary protein Saglin, previously proposed to be a receptor for the recognition of salivary glands by sporozoites, facilitates Plasmodium colonization of the mosquito midgut, but does not contribute to salivary gland invasion. In mosquito mutants lacking Saglin, Plasmodium infection of Anopheles females is reduced, resulting in impaired transmission of sporozoites at low infection densities. Interestingly, Saglin can be detected in high amounts in the midgut of mosquitoes after blood ingestion, possibly indicating a previously unknown host-pathogen interaction between Saglin and midgut stages of Plasmodium. Furthermore, we were able to show that saglin deletion has no fitness cost in laboratory conditions, suggesting this gene would be an interesting target for gene drive approaches.

Impact of insecticide resistance on malaria vector competence: a literature review

Since its first report in Anopheles mosquitoes in 1950s, insecticide resistance has spread very fast to most sub-Saharan African malaria-endemic countries, where it is predicted to seriously jeopardize the success of vector control efforts, leading to rebound of disease cases. Supported mainly by four mechanisms (metabolic resistance, target site resistance, cuticular resistance, and behavioural resistance), this phenomenon is associated with intrinsic changes in the resistant insect vectors that could influence development of invading Plasmodium parasites. A literature review was undertaken using Pubmed database to collect articles evaluating directly or indiretly the impact of insecticide resistance and the associated mechanisms on key determinants of malaria vector competence including sialome composition, anti-Plasmodium immunity, intestinal commensal microbiota, and mosquito longevity. Globally, the evidence gathered is contradictory even though the insecticide resistant vectors seem to be more permissive to Plasmodium infections. The actual body of knowledge on key factors to vectorial competence, such as the immunity and microbiota communities of the insecticide resistant vector is still very insufficient to definitively infer on the epidemiological importance of these vectors against the susceptible counterparts. More studies are needed to fill important knowledge gaps that could help predicting malaria epidemiology in a context where the selection and spread of insecticide resistant vectors is ongoing.

A toolbox of engineered mosquito lines to study salivary gland biology and malaria transmission

Mosquito saliva is a vehicle for the transmission of vector borne pathogens such as Plasmodium parasites and different arboviruses. Despite the key role of the salivary glands in the process of disease transmission, knowledge of host-pathogen interactions taking place within this organ is very limited. To improve the experimental tractability of the salivary glands, we have generated fluorescent reporter lines in the African malaria mosquito Anopheles coluzzii using the salivary gland-specific promoters of the anopheline antiplatelet protein (AAPP), the triple functional domain protein (TRIO) and saglin (SAG) coding genes. Promoter activity was specifically observed in the distal-lateral lobes or in the median lobe of the salivary glands. Besides a comparison of the expression patterns of the selected promoters, the fluorescent probes allowed us to evaluate the inducibility of the selected promoters upon blood feeding and to measure intracellular redox changes. We also combined the aapp-DsRed fluorescent reporter line with a pigmentation-deficient yellow(-) mosquito mutant to assess the feasibility of in vivo microscopy of parasitized salivary glands. This combination allowed locating the salivary gland through the cuticle and imaging of individual sporozoites in vivo, which facilitates live imaging studies of salivary gland colonization by Plasmodium sporozoites.

Malaria Box Compounds against Anopheles gambiae (Diptera: Culicidae) Carboxypeptidase B Activity to Block Malaria Transmission

Carboxypeptidase B (CPB) plays an important role in blood digestion in mosquitos, aiding the release of free amino acids. Anopheles CPB is a target to block malaria transmission because it facilitates Plasmodium invasion of the mosquito midgut. Our study aimed to discover inhibitors of Anopheles CPB to prevent Plasmodium development in the mosquito. The Anopheles gambiae cpb (Agcpb) gene without a signal sequence was cloned into the pET28b expression vector. The recombinant AgCPB protein was expressed in E. coli BL21(DE3) within inclusion bodies after induction with 0.5 mM isopropyl β-D-1-thiogalactopyranoside at 37°C for 4 h. The protein pellet was dissolved in 6 M urea, purified by affinity chromatography, and dialyzed in reaction buffer. The refolded recombinant AgCPB could digest the hippuryl-arginine substrate similarly to that of the commercial porcine pancreas CPB. The 20 top-scoring malaria box compounds from the virtual-screening results were then chosen for an in vitro inhibition assay against AgCPB. Four of the 20 malaria box compounds could inhibit AgCPB activity. The compound MMV007591 was the most potent inhibitor with an IC50 at 0.066 µM. The results indicate that these candidate compounds may be utilized in drug development against mosquito CPB activity to curb malaria transmission.

Thrombospondin Related Anonymous Protein Superfamily in Vector-Borne Apicomplexans: The Parasite’s Toolkit for Cell Invasion

Apicomplexan parasites transmitted by vectors, including Babesia spp. and Plasmodium spp., cause severe disease in both humans and animals. These parasites have a complex life cycle during which they migrate, invade, and replicate in contrasting hosts such as the mammal and the invertebrate vector. The interaction of parasites with the host cell is mediated by adhesive proteins which play a key role in the different cellular processes regarding successful progression of the life cycle. Thrombospondin related anonymous protein (TRAP) is a superfamily of adhesins that are involved in motility, invasion and egress of the parasite. These proteins are stored and released from apical organelles and have either one or two types of adhesive domains, namely thrombospondin type 1 repeat and von Willebrand factor type A, that upon secretion are located in the extracellular portion of the molecule. Proteins from the TRAP superfamily have been intensively studied in Plasmodium species and to a lesser extent in Babesia spp., where they have proven to be functionally relevant throughout the entire parasite’s journey both in the arthropod vector and in the mammalian host. In recent years new findings provided answers to the role of TRAP proteins and in some cases the function of these adhesins during the parasite’s life cycle was redefined. In this review we will discuss the current knowledge of the diverse roles of the TRAP superfamily in vector-borne parasites from Class Aconoidasida. We will focus on the varied approaches that allowed the understanding of protein function and the relevance of TRAP- superfamily throughout the entire parasite’s cell cycle.

Multifaceted applications of genetically modified microorganisms: A biotechnological revolution

BACKGROUND

Genetically modified microorganisms specifically bacteria, viruses, algae and fungi are the novel approaches used in field of healthcare due to more efficacious and targeted delivery in comparison to conventional approaches.

OBJECTIVE

This review article focuses on applications of genetically modified microorganisms such as bacteria, virus, fungi, virus, etc. in treatment of cancer, obesity, and HIV. Gut microbiome is used to cause metabolic disorders but use of genetically-modified bacteria alters the gut microbiota and delivers the therapeutically effective drug in the treatment of obesity.

METHODS

To enhance the activity of different microorganisms for treatment, they are genetically modified by incorporating a fragment into the fungi filaments, integrating a strain into the bacteria, engineer a live-virus with a peptide using methods such as amelioration of NAPE synthesis, silica immobilization, polyadenylation, electrochemical, etc. Results: The development of newer microbial strains using genetic modifications offers higher precision, enhance the molecular multiplicity, prevent the degradation of microbes in atmospheric temperature and reduce the concerned side-effect for therapeutic application. Other side genetically modified microorganisms are used in non-healthcare based sector like generation of electricity, purification of water, bioremediation process etc. Conclusions: The bio-engineered micro-organisms with genetic modification prove the advantage over the treatment of various diseases like cancer, diabetes, malaria, organ regeneration, inflammatory bowel disease, etc. The article provides the insights of various applications of genetically modified microbes in various arena with its implementation for the regulatory approval.

Anopheles salivary antigens as serological biomarkers of vector exposure and malaria transmission: A systematic review with multilevel modelling

Background

Entomological surveillance for malaria is inherently resource-intensive and produces crude population-level measures of vector exposure which are insensitive in low-transmission settings. Antibodies against Anopheles salivary proteins measured at the individual level may serve as proxy biomarkers for vector exposure and malaria transmission, but their relationship is yet to be quantified.

Methods

A systematic review of studies measuring antibodies against Anopheles salivary antigens (PROSPERO: CRD42020185449). Multilevel modelling (to account for multiple study-specific observations [level 1], nested within study [level 2], and study nested within country [level 3]) estimated associations between seroprevalence with Anopheles human biting rate (HBR) and malaria transmission measures.

Results

From 3981 studies identified in literature searches, 42 studies across 16 countries were included contributing 393 study-specific observations of anti-Anopheles salivary antibodies determined in 42,764 samples. A positive association between HBR (log transformed) and seroprevalence was found; overall a twofold (100% relative) increase in HBR was associated with a 23% increase in odds of seropositivity (OR: 1.23, 95% CI: 1.10-1.37; p<0.001). The association between HBR and Anopheles salivary antibodies was strongest with concordant, rather than discordant, Anopheles species. Seroprevalence was also significantly positively associated with established epidemiological measures of malaria transmission: entomological inoculation rate, Plasmodium spp. prevalence, and malarial endemicity class.

Conclusions

Anopheles salivary antibody biomarkers can serve as a proxy measure for HBR and malaria transmission, and could monitor malaria receptivity of a population to sustain malaria transmission. Validation of Anopheles species-specific biomarkers is important given the global heterogeneity in the distribution of Anopheles species. Salivary biomarkers have the potential to transform surveillance by replacing impractical, inaccurate entomological investigations, especially in areas progressing towards malaria elimination.

Funding

Australian National Health and Medical Research Council, Wellcome Trust.

Phytomedicine in Disease Management: In-Silico Analysis of the Binding Affinity of Artesunate and Azadirachtin for Malaria Treatment

In the rural communities of sub-Saharan African (sSA) countries, malaria is being managed using phytocompounds. Artesunate is reported to inhibit Gephyrin E, a central, multi-domain scaffolding protein of inhibitory post-synapses. Neem plant and its metabolites like azadirachtin are being indicated for management of malaria by traditional healers. The present study was aimed to cheminformatically analyse the binding potential of artesunate and azadirachtin with various reactive moieties of Gephyrin E, to reduce malaria scourge. With molecular dynamics (MD), binding free energy estimation and binding affinity of artesunate and azadirachtin to Gephyrin E was done. GRIP docking was done to study the interactions of these test ligands with Gephyrin E (6FGC). MD simulation gave insights to structural changes upon binding of artesunate and azadirachtin in the ligand-binding pocket of Gephyrin E. Root mean square deviation (RMSD) and root mean square fluctuation (RMSF) were calculated. From the estimation, azadirachtin had a total binding energy of -36.97 kcal/mol; artesunate had a binding energy of -35.73 kcal/mol. The GRIP docking results provided a clearer evidence that artesunate has comparatively better binding affinity to Gephyrin E than azadirachtin, and the critical binding sites (in activity order) were cavity 3, 2, 8, and 6 for artesunate while for azadirachtin, it was cavity 6, 3, 8, and 2. The GRIP docking provided detailed interactions at the atomic levels, providing evidence; both compounds have chances to overcome the drug resistance problem, albeit higher for artesunate. Our findings added another piece of evidence that azadirachtin may be effective as an anti-malarial agent. The results herein may provide impetus for more studies into bioactive components of plant origin towards the effective management of malaria disease phenotype.

Plasmodium sporozoite phospholipid scramblase interacts with mammalian carbamoyl-phosphate synthetase 1 to infect hepatocytes

After inoculation by the bite of an infected mosquito, Plasmodium sporozoites enter the blood stream and infect the liver, where each infected cell produces thousands of merozoites. These in turn, infect red blood cells and cause malaria symptoms. To initiate a productive infection, sporozoites must exit the circulation by traversing the blood lining of the liver vessels after which they infect hepatocytes with unique specificity. We screened a phage display library for peptides that structurally mimic (mimotope) a sporozoite ligand for hepatocyte recognition. We identified HP1 (hepatocyte-binding peptide 1) that mimics a ~50 kDa sporozoite ligand (identified as phospholipid scramblase). Further, we show that HP1 interacts with a ~160 kDa hepatocyte membrane putative receptor (identified as carbamoyl-phosphate synthetase 1). Importantly, immunization of mice with the HP1 peptide partially protects them from infection by the rodent parasite P. berghei. Moreover, an antibody to the HP1 mimotope inhibits human parasite P. falciparum infection of human hepatocytes in culture. The sporozoite ligand for hepatocyte invasion is a potential novel pre-erythrocytic vaccine candidate.

After transmission of Plasmodium sporozoites from infected mosquitoes, parasites first infect hepatocytes. Here, Cha et al. identify a sporozoite ligand (phospholipid scramblase) and the hepatocytic receptor (carbamoyl-phosphate synthetase 1) as relevant for hepatocyte invasion and show that an antibody to hepatocyte-binding peptide 1 (HP1), which structurally mimics the sporozoite ligand, partially protects mice from infection.

Mosquito transgenesis for malaria control

Malaria is one of the deadliest diseases. Because of the ineffectiveness of current malaria-control methods, several novel mosquito vector-based control strategies have been proposed to supplement existing control strategies. Mosquito transgenesis and gene drive have emerged as promising tools for preventing the spread of malaria by either suppressing mosquito populations by self-destructing mosquitoes or replacing mosquito populations with disease-refractory populations. Here we review the development of mosquito transgenesis and its application for malaria control, highlighting the transgenic expression of antiparasitic effector genes, inactivation of host factor genes, and manipulation of miRNAs and lncRNAs. Overall, from a malaria-control perspective, mosquito transgenesis is not envisioned as a stand-alone approach; rather, its use is proposed as a complement to existing vector-control strategies.

Secretory Organelle Function in the Plasmodium Sporozoite

Plasmodium sporozoites exhibit a complex infection biology in the mosquito and mammalian hosts. The sporozoite apical secretory organelles, the micronemes and rhoptries, store protein mediators of parasite/host/vector interactions and must secrete them in a temporally and spatially well orchestrated manner. Micronemal proteins are critical for sporozoite motility throughout its journey from the mosquito midgut oocyst to the mammalian liver, and also for cell traversal (CT) and hepatocyte invasion. Rhoptry proteins, until recently thought to be only important for hepatocyte invasion, appear to also play an unexpected role in motility and in the interaction with mosquito tissue. Therefore, navigating the different microenvironments with secretion likely requires the sporozoite to have a more complex system of secretory organelles than previously appreciated.

Plasmodium's journey through the Anopheles mosquito: A comprehensive review

The malaria parasite has an extraordinary ability to evade the immune system due to which the development of a malaria vaccine is a challenging task. Extensive research on malarial infection in the human host particularly during the liver stage has resulted in the discovery of potential candidate vaccines including RTS,S/AS01 and R21. However, complete elimination of malaria would require a holistic multi-component approach. In line with this, under the World Health Organization's PATH Malaria Vaccine Initiative (MVI), the research focus has shifted towards the sexual stages of malaria in the mosquito host. Last two decades of scientific research obtained seminal information regarding the sexual/mosquito stages of the malaria. This updated and comprehensive review would provide the basis for consolidated understanding of cellular, biochemical, molecular and immunological aspects of parasite transmission right from the sexual stage commitment in the human host to the sporozoite delivery back into subsequent vertebrate host by the female Anopheles mosquito.

Genetic Markers of Adaptation of Plasmodium falciparum to Transmission by American Vectors Identified in the Genomes of Parasites from Haiti and South America

The malaria parasite, Plasmodium falciparum, was introduced into Hispaniola and other regions of the Americas through the slave trade spanning the 16th through the 19th centuries. During this period, more than 12 million Africans were brought across the Atlantic to the Caribbean and other regions of the Americas. Since malaria is holoendemic in West Africa, a substantial percentage of these individuals carried the parasite. St. Domingue on Hispaniola, now modern-day Haiti, was a major port of disembarkation, and malaria is still actively transmitted there. We undertook a detailed study of the phylogenetics of the Haitian parasites and those from Colombia and Peru utilizing whole-genome sequencing. Principal-component and phylogenetic analyses, based upon single nucleotide polymorphisms (SNPs) in protein coding regions, indicate that, despite the potential for millions of introductions from Africa, the Haitian parasites share an ancestral relationship within a well-supported monophyletic clade with parasites from South America, while belonging to a distinct lineage. This result, in stark contrast to the historical record of parasite introductions, is best explained by a severe population bottleneck experienced by the parasites introduced into the Americas. Here, evidence is presented for targeted selection of rare African alleles in genes which are expressed in the mosquito stages of the parasite's life cycle. These genetic markers support the hypothesis that the severe population bottleneck was caused by the required adaptation of the parasite to transmission by new definitive hosts among the Anopheles (Nyssorhynchus) spp. found in the Caribbean and South America.IMPORTANCE Historical data suggest that millions of P. falciparum parasite lineages were introduced into the Americas during the trans-Atlantic slave trade, which would suggest a paraphyletic origin of the extant isolates in the Western Hemisphere. Our analyses of whole-genome variants show that the American parasites belong to a well-supported monophyletic clade. We hypothesize that the required adaptation to American vectors created a severe bottleneck, reducing the effective introduction to a few lineages. In support of this hypothesis, we discovered genes expressed in the mosquito stages of the life cycle that have alleles with multiple, high-frequency or fixed, nonsynonymous mutations in the American populations which are rarely found in African isolates. These alleles appear to be in gene products critical for transmission through the anopheline vector. Thus, these results may inform efforts to develop novel transmission-blocking vaccines by identifying parasite proteins functionally interacting with the vector that are important for successful transmission. Further, to the best of our knowledge, these are the first whole-genome data available from Haitian P. falciparum isolates. Defining the genome of these parasites provides genetic markers useful for mapping parasite populations and monitoring parasite movements/introductions.

Plasmodium berghei Kinesin-5 Associates With the Spindle Apparatus During Cell Division and Is Important for Efficient Production of Infectious Sporozoites

Kinesin-5 motors play essential roles in spindle apparatus assembly during cell division, by generating forces to establish and maintain the spindle bipolarity essential for proper chromosome segregation. Kinesin-5 is largely conserved structurally and functionally in model eukaryotes, but its role is unknown in the Plasmodium parasite, an evolutionarily divergent organism with several atypical features of both mitotic and meiotic cell division. We have investigated the function and subcellular location of kinesin-5 during cell division throughout the Plasmodium berghei life cycle. Deletion of kinesin-5 had little visible effect at any proliferative stage except sporozoite production in oocysts, resulting in a significant decrease in the number of motile sporozoites in mosquito salivary glands, which were able to infect a new vertebrate host. Live-cell imaging showed kinesin-5-GFP located on the spindle and at spindle poles during both atypical mitosis and meiosis. Fixed-cell immunofluorescence assays revealed kinesin-5 co-localized with α-tubulin and centrin-2 and a partial overlap with kinetochore marker NDC80 during early blood stage schizogony. Dual-color live-cell imaging showed that kinesin-5 is closely associated with NDC80 during male gametogony, but not with kinesin-8B, a marker of the basal body and axonemes of the forming flagella. Treatment of gametocytes with microtubule-specific inhibitors confirmed kinesin-5 association with nuclear spindles and not cytoplasmic axonemal microtubules. Altogether, our results demonstrate that kinesin-5 is associated with the spindle apparatus, expressed in proliferating parasite stages, and important for efficient production of infectious sporozoites.

Evolutionarily distant I domains can functionally replace the essential ligand-binding domain of Plasmodium TRAP

Inserted (I) domains function as ligand-binding domains in adhesins that support cell adhesion and migration in many eukaryotic phyla. These adhesins include integrin αβ heterodimers in metazoans and single subunit transmembrane proteins in apicomplexans such as TRAP in Plasmodium and MIC2 in Toxoplasma. Here we show that the I domain of TRAP is essential for sporozoite gliding motility, mosquito salivary gland invasion and mouse infection. Its replacement with the I domain from Toxoplasma MIC2 fully restores tissue invasion and parasite transmission, while replacement with the aX I domain from human integrins still partially restores liver infection. Mutations around the ligand binding site allowed salivary gland invasion but led to inefficient transmission to the rodent host. These results suggest that apicomplexan parasites appropriated polyspecific I domains in part for their ability to engage with multiple ligands and to provide traction for emigration into diverse organs in distant phyla.

Malaria is an infectious disease caused by single-celled parasites known as Plasmodium. Humans and other animals with backbones – such as birds, reptiles and rodents – can become hosts for these parasites if an infected female mosquito feeds on their blood. Likewise, healthy mosquitoes can in turn become infected with Plasmodium if they feed on the blood of an infected animal.

To complete their life cycle, Plasmodium parasites within a mosquito must become spore-like cells called sporozoites. These sporozoites are highly mobile and can get into the mosquitoes’ salivary glands, meaning they can be passed on to a new host when the insect feeds. During a mosquito bite the sporozoites are spat into the skin of the potential host, where they then need to migrate rapidly to enter the bloodstream. Once in the blood, the sporozoites can then get into liver cells and begin a new infection. One protein called TRAP, which is found on the surface of the sporozoites, is important for their migration and the infection of the salivary glands or liver. Yet it was not known how this happens at the level of the individual proteins involved.

Klug et al. have now tested how a part of the TRAP protein, called the I domain, contributes to the infection process. In the experiments, the I domain of TRAP was deleted which showed that the sporozoites need this domain to be able to move around and get into the host tissues. Without the I domain the sporozoites were stuck and could not successfully infect either the mosquitoes, the livers of mice, or human liver cells grown in the laboratory. Klug et al. then replaced the Plasmodium I domain of TRAP with the I domain from a distantly related parasite called Toxoplasma gondii, which causes a condition known as toxoplasmosis. The I domain from Toxoplasma allowed the Plasmodium parasites to infect the host tissues again. This observation was unexpected because Toxoplasma and Plasmodium parasites have evolved separately over the last 800 million years and Toxoplasma does not infect insects.

These findings suggest that the I domain of TRAP evolved to bind several other proteins in different tissues and hosts. Future studies will investigate which other parasite proteins TRAP works with to guide sporozoites to the salivary glands or liver. Knowledge of how these proteins act together may lead to new approaches for treating or preventing malaria. For example, some treatments could stop sporozoites from entering liver cells.

Transcriptome analysis of Anopheles dirus and Plasmodium vivax at ookinete and oocyst stages

Malaria is transmitted by Plasmodium parasites through the bite of female Anopheles mosquitoes. One of the most important mosquito vectors in the Greater Mekong Subregion is Anopheles dirus. This study reports RNA sequencing (RNA-Seq) transcriptome analysis of An. dirus at 18 hours and 7 days after a P. vivax-infected blood meal, which represent infection at the ookinete and oocyst parasite developmental stages, respectively. Following infection, 582 An. dirus transcripts were modulated. The 2,408 P. vivax transcripts could be classified into ookinete-specific, two-stage, and oocyst-specific groups. Results were validated by quantitative reverse transcription polymerase chain reaction. Gene ontology analysis of the vector and parasite revealed several biological pathways for both, providing a better understanding of Anopheles-Plasmodium interactions at the ookinete and oocyst stages.

When Is a Plasmodium-Infected Mosquito an Infectious Mosquito?

Plasmodium parasites experience significant bottlenecks as they transit through the mosquito and are transmitted to their mammalian host. Oocyst prevalence on mosquito midguts and sporozoite prevalence in salivary glands are nevertheless commonly used to confirm successful malaria transmission, assuming that these are reliable indicators of the mosquito's capacity to give rise to secondary infections. Here we discuss recent insights in sporogonic development and transmission bottlenecks for Plasmodium. We highlight critical gaps in our knowledge and frame their importance in understanding the human and mosquito reservoirs of infection. A better understanding of the events that lead to successful inoculation of infectious sporozoites by mosquitoes is critical to designing effective interventions to shrink the malaria map.

Anopheles gambiae Lacking AgTRIO Inefficiently Transmits Plasmodium berghei to Mice

Antibodies to AgTRIO, a mosquito salivary protein, partially reduce the initial Plasmodium burden in mice. We therefore silenced AgTRIO in mosquitoes and determined the relative contribution of AgTRIO to the ability of Anopheles gambiae to transmit Plasmodium berghei to mice. RNA interference-mediated silencing of AgTRIO in A. gambiae resulted in a 60% reduction in AgTRIO expression. The decrease in AgTRIO expression did not alter the burden of Plasmodium sporozoites in mosquito salivary glands. When experimentally injected into mice, sporozoites from AgTRIO-silenced mosquitoes colonized the liver less effectively than sporozoites from control mosquitoes. Silencing of AgTRIO did not decrease the infectivity of sporozoites in vitro or influence the expression of genes associated with Plasmodium cell adhesion or traversal activity. AgTRIO decreased the expression of proinflammation cytokines by splenocytes in vitro Moreover, in vivo, AgTRIO decreased the expression of TNF-α when coinjected with sporozoites into the skin and there was more TNF-α expression at the bite site of AgTRIO knockdown mosquitoes than at the bite site of control mosquitoes. AgTRIO therefore influences the local environment in the vertebrate host, which facilitates Plasmodium sporozoite infection in mice.

Anopheles Salivary Gland Architecture Shapes Plasmodium Sporozoite Availability for Transmission

Plasmodium sporozoites (SPZs) must traverse the mosquito salivary glands (SGs) to reach a new vertebrate host and continue the malaria disease cycle. Although SGs can harbor thousands of sporozoites, only 10 to 100 are deposited into a host during probing. To determine how the SGs might function as a bottleneck in SPZ transmission, we have characterized Anopheles stephensi SGs infected with the rodent malaria parasite Plasmodium berghei using immunofluorescence confocal microscopy. Our analyses corroborate findings from previous electron microscopy studies and provide new insights into the invasion process. We identified sites of SPZ accumulation within SGs across a range of infection intensities. Although SPZs were most often seen in the distal lateral SG lobes, they were also observed in the medial and proximal lateral lobes. Most parasites were associated with either the basement membrane or secretory cavities. SPZs accumulated at physical barriers, including fused salivary ducts and extensions of the chitinous salivary duct wall into the distal lumen. SPZs were observed only rarely within salivary ducts. SPZs appeared to contact each other in many different quantities, not just in the previously described large bundles. Within parasite bundles, all of the SPZs were oriented in the same direction. We found that moderate levels of infection did not necessarily correlate with major SG disruptions or abundant SG cell death. Altogether, our findings suggest that SG architecture largely acts as a barrier to SPZ transmission.IMPORTANCE Malaria continues to have a devastating impact on human health. With growing resistance to insecticides and antimalarial drugs, as well as climate change predictions indicating expansion of vector territories, the impact of malaria is likely to increase. Additional insights regarding pathogen migration through vector mosquitoes are needed to develop novel methods to prevent transmission to new hosts. Pathogens, including the microbes that cause malaria, must invade the salivary glands (SGs) for transmission. Since SG traversal is required for parasite transmission, SGs are ideal targets for transmission-blocking strategies. The work presented here highlights the role that mosquito SG architecture plays in limiting parasite traversal, revealing how the SG transmission bottleneck is imposed. Further, our data provide unprecedented detail about SG-sporozoite interactions and gland-to-gland variation not provided in previous studies.

Immune Responses to the Sexual Stages of Plasmodium falciparum Parasites

Malaria infections remain a serious global health problem in the world, particularly among children and pregnant women in Sub-Saharan Africa. Moreover, malaria control and elimination is hampered by rapid development of resistance by the parasite and the vector to commonly used antimalarial drugs and insecticides, respectively. Therefore, vaccine-based strategies are sorely needed, including those designed to interrupt disease transmission. However, a prerequisite for such a vaccine strategy is the understanding of both the human and vector immune responses to parasite developmental stages involved in parasite transmission in both man and mosquito. Here, we review the naturally acquired humoral and cellular responses to sexual stages of the parasite while in the human host and the Anopheles vector. In addition, updates on current anti-gametocyte, anti-gamete, and anti-mosquito transmission blocking vaccines are given. We conclude with our views on some important future directions of research into P. falciparum sexual stage immunity relevant to the search for the most appropriate transmission-blocking vaccine.

Is Saglin a mosquito salivary gland receptor for Plasmodium falciparum?

BACKGROUND

Saglin, a 100 kDa protein composed of two 50 kDa homodimers, is present in the salivary glands of Anopheles gambiae and has been considered an essential receptor for sporozoites (SPZ) of Plasmodium berghei and Plasmodium falciparum (Pf), allowing SPZ to recognize, bind to, and infect mosquito salivary glands. Spatial and temporal patterns of Saglin expression reported here, however, suggest that this model does not fully describe the Saglin-SPZ interaction.

RESULTS

Saglin protein was detected by indirect immunofluorescence microscopy only in the medial and proximal-lateral lobes, but not in the distal-lateral lobes, of the salivary glands of An. gambiae; the pattern of expression was independent of mosquito age or physiological state. These results were confirmed by steady-state Saglin transcript and protein expression using qRT-PCR and Western-blot analysis, respectively. Saglin was localized to the basal surface of the cells of the medial lobes and was undetectable elsewhere (intracellularly, on the lateral or apical membranes, the cells' secretory vacuoles, or in the salivary duct). In the cells of the proximal lateral lobes of the salivary glands, Saglin was distinctly intracellular and was not localized to any of the cell surfaces. Transgenic Anopheles stephensi were produced that expressed An. gambiae Saglin in the distal lateral lobes of the salivary gland. Additional Saglin expression did not enhance infection by PfSPZ compared to non-transgenic siblings fed on the same gametocyte-containing blood meal.

CONCLUSIONS

The absence of Saglin in the distal lateral lobes of the salivary glands, a primary destination for SPZ, suggests Saglin is not an essential receptor for Plasmodium SPZ. The lack of any correlation between increased Saglin expression in the distal lateral lobes of the salivary glands of transgenic An. stephensi and PfSPZ infection is also consistent with Saglin not being an essential salivary gland receptor for Plasmodium SPZ.

Vector biology meets disease control: using basic research to fight vector-borne diseases

Human pathogens that are transmitted by insects are a global problem, particularly those vectored by mosquitoes; for example, malaria parasites transmitted by Anopheles species, and viruses such as dengue, Zika and chikungunya that are carried by Aedes mosquitoes. Over the past 15 years, the prevalence of malaria has been substantially reduced and virus outbreaks have been contained by controlling mosquito vectors using insecticide-based approaches. However, disease control is now threatened by alarming rates of insecticide resistance in insect populations, prompting the need to develop a new generation of specific strategies that can reduce vector-mediated transmission. Here, we review how increased knowledge in insect biology and insect-pathogen interactions is stimulating new concepts and tools for vector control. We focus on strategies that either interfere with the development of pathogens within their vectors or directly impact insect survival, including enhancement of vector-mediated immune control, manipulation of the insect microbiome, or use of powerful new genetic tools such as CRISPR-Cas systems to edit vector genomes. Finally, we offer a perspective on the implementation hurdles as well as the knowledge gaps that must be filled in the coming years to safely realize the potential of these novel strategies to eliminate the scourge of vector-borne disease.

Mosquito Innate Immunity

Mosquitoes live under the endless threat of infections from different kinds of pathogens such as bacteria, parasites, and viruses. The mosquito defends itself by employing both physical and physiological barriers that resist the entry of the pathogen and the subsequent establishment of the pathogen within the mosquito. However, if the pathogen does gain entry into the insect, the insect mounts a vigorous innate cellular and humoral immune response against the pathogen, thereby limiting the pathogen's propagation to nonpathogenic levels. This happens through three major mechanisms: phagocytosis, melanization, and lysis. During these processes, various signaling pathways that engage intense mosquito⁻pathogen interactions are activated. A critical overview of the mosquito immune system and latest information about the interaction between mosquitoes and pathogens are provided in this review. The conserved, innate immune pathways and specific anti-pathogenic strategies in mosquito midgut, hemolymph, salivary gland, and neural tissues for the control of pathogen propagation are discussed in detail.

A mosquito salivary gland protein partially inhibits Plasmodium sporozoite cell traversal and transmission

The key step during the initiation of malaria is for motile Plasmodium parasites to exit the host dermis and infect the liver. During transmission, the parasites in the form of sporozoites, are injected together with mosquito saliva into the skin. However, the contribution of vector saliva to sporozoite activity during the establishment of the initial infection of the liver is poorly understood. Here we identify a vector protein by mass spectrometry, with similarity to the human gamma interferon inducible thiol reductase (GILT), that is associated with saliva sporozoites of infected Anopheles mosquitoes and has a negative impact on the speed and cell traversal activity of Plasmodium. This protein, referred to as mosquito GILT (mosGILT) represents an example of a protein found in mosquito saliva that may negatively influence sporozoite movement in the host and could lead to new approaches to prevent malaria.

Consistent signatures of selection from genomic analysis of pairs of temporal and spatial Plasmodium falciparum populations from The Gambia

Genome sequences of 247 Plasmodium falciparum isolates collected in The Gambia in 2008 and 2014 were analysed to identify changes possibly related to the scale-up of antimalarial interventions that occurred during this period. Overall, there were 15 regions across the genomes with signatures of positive selection. Five of these were sweeps around known drug resistance and antigenic loci. Signatures at antigenic loci such as thrombospodin related adhesive protein (Pftrap) were most frequent in eastern Gambia, where parasite prevalence and transmission remain high. There was a strong temporal differentiation at a non-synonymous SNP in a cysteine desulfarase (Pfnfs) involved in iron-sulphur complex biogenesis. During the 7-year period, the frequency of the lysine variant at codon 65 (Pfnfs-Q65K) increased by 22% (10% to 32%) in the Greater Banjul area. Between 2014 and 2015, the frequency of this variant increased by 6% (20% to 26%) in eastern Gambia. IC₅₀ for lumefantrine was significantly higher in Pfnfs-65K isolates. This is probably the first evidence of directional selection on Pfnfs or linked loci by lumefantrine. Given the declining malaria transmission, the consequent loss of population immunity, and sustained drug pressure, it is important to monitor Gambian P. falciparum populations for further signs of adaptation.

Sialotranscriptome sequencing and analysis of Anopheles sinensis and comparison with Psorophora albipes sialotranscriptome (Diptera: Culicidae)

Most of adult female mosquitoes secrete saliva to facilitate blood sucking, digestion and nutrition, and mosquito-borne disease prevention. The knowledge of classification and characteristics of sialotranscriptome genes are still quite limited. Anopheles sinensis is a major malaria vector in China and southeast Asian countries. In this study, the An. sinensis sialotranscriptome was sequenced using Illumina sequencing technique with a total of 10 907 unigenes to be obtained and annotated in biological functions and pathways, and 10 470 unigenes were mapped to An. sinensis reference genome with 70.46% of genes having 90%-100% genome mapping through bioinformatics analysis. These mapped genes were classified into four categories: housekeeping (6632 genes), secreted (1177), protein-coding genes with function-unknown (2646) and transposable element (15). The housekeeping genes were divided into 27 classes, and the secreted genes were divided into 11 classes and 96 families. The classification, characteristics and evolution of these classes/families of secreted genes are further described and discussed. The comparison of the 1177 secreted genes in An. sinensis in the Anophelinae subfamily with 811 in Psorophora albipes in the Culicinae subfamily show that six classes/subclasses have the gene number more than twice and two classes (uniquely found in anophelines, and Orphan proteins of unique standing) are unique in the former compared with the latter, whereas four classes/subclasses are much expanded and uniquely found in the Aedes class and is unique in the later. The An. sinensis sialotranscriptome sequence data is the most complete in mosquitoes to date, and the analyses provide a comprehensive information frame for further research of mosquito sialotranscriptome.

Diverse Host and Restriction Factors Regulate Mosquito-Pathogen Interactions

Mosquitoes transmit diseases that seriously impact global human health. Despite extensive knowledge of the life cycles of mosquito-borne parasites and viruses within their hosts, control strategies have proven insufficient to halt their spread. An understanding of the relationships established between such pathogens and the host tissues they inhabit is therefore paramount for the development of new strategies that specifically target these interactions, to prevent the pathogens' maturation and transmission. Here we present an updated account of the antagonists and host factors that affect the development of Plasmodium, the parasite causing malaria, and mosquito-borne viruses, such as dengue virus and Zika virus, within their mosquito vectors, and we discuss the similarities and differences between Plasmodium and viral systems, looking toward the elucidation of new targets for disease control.

malERA: An updated research agenda for basic science and enabling technologies in malaria elimination and eradication

Basic science holds enormous power for revealing the biological mechanisms of disease and, in turn, paving the way toward new, effective interventions. Recognizing this power, the 2011 Research Agenda for Malaria Eradication included key priorities in fundamental research that, if attained, could help accelerate progress toward disease elimination and eradication. The Malaria Eradication Research Agenda (malERA) Consultative Panel on Basic Science and Enabling Technologies reviewed the progress, continuing challenges, and major opportunities for future research. The recommendations come from a literature of published and unpublished materials and the deliberations of the malERA Refresh Consultative Panel. These areas span multiple aspects of the Plasmodium life cycle in both the human host and the Anopheles vector and include critical, unanswered questions about parasite transmission, human infection in the liver, asexual-stage biology, and malaria persistence. We believe an integrated approach encompassing human immunology, parasitology, and entomology, and harnessing new and emerging biomedical technologies offers the best path toward addressing these questions and, ultimately, lowering the worldwide burden of malaria.

Rapid evolution of female-biased genes among four species of Anopheles malaria mosquitoes

Understanding how phenotypic differences between males and females arise from the sex-biased expression of nearly identical genomes can reveal important insights into the biology and evolution of a species. Among Anopheles mosquito species, these phenotypic differences include vectorial capacity, as it is only females that blood feed and thus transmit human malaria. Here, we use RNA-seq data from multiple tissues of four vector species spanning the Anopheles phylogeny to explore the genomic and evolutionary properties of sex-biased genes. We find that, in these mosquitoes, in contrast to what has been found in many other organisms, female-biased genes are more rapidly evolving in sequence, expression, and genic turnover than male-biased genes. Our results suggest that this atypical pattern may be due to the combination of sex-specific life history challenges encountered by females, such as blood feeding. Furthermore, female propensity to mate only once in nature in male swarms likely diminishes sexual selection of post-reproductive traits related to sperm competition among males. We also develop a comparative framework to systematically explore tissue- and sex-specific splicing to document its conservation throughout the genus and identify a set of candidate genes for future functional analyses of sex-specific isoform usage. Finally, our data reveal that the deficit of male-biased genes on the X Chromosomes in Anopheles is a conserved feature in this genus and can be directly attributed to chromosome-wide transcriptional regulation that de-masculinizes the X in male reproductive tissues.

Solute carriers affect Anopheles stephensi survival and Plasmodium berghei infection in the salivary glands

Malaria is caused by mosquito-borne Plasmodium spp. parasites that must infect and survive within mosquito salivary glands (SGs) prior to host transmission. Recent advances in transcriptomics and the complete genome sequencing of mosquito vectors have increased our knowledge of the SG genes and proteins involved in pathogen infection and transmission. Membrane solute carriers are key proteins involved in drug transport and are useful in the development of new interventions for transmission blocking. Herein, we applied transcriptomics analysis to compare SGs mRNA levels in Anopheles stephensi fed on non-infected and P. berghei-infected mice. The A. stephensi solute carriers prestinA and NDAE1 were up-regulated in response to infection. These molecules are predicted to interact with each other, and are reportedly involved in the maintenance of cell homeostasis. To further evaluate their functions in mosquito survival and parasite infection, these genes were knocked down by RNA interference. Knockdown of prestinA and NDAE1 resulted in reduction of the number of sporozoites in mosquito SGs. Moreover, NDAE1 knockdown strongly impacted mosquito survival, resulting in the death of half of the treated mosquitoes. Overall, our findings indicate the importance of prestinA and NDAE1 in interactions between mosquito SGs and Plasmodium, and suggest the need for further research.

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.

Plasmodium Sporozoite Biology

Plasmodium sporozoite transmission is a critical population bottleneck in parasite life-cycle progression and, hence, a target for prophylactic drugs and vaccines. The recent progress of a candidate antisporozoite subunit vaccine formulation to licensure highlights the importance of sporozoite transmission intervention in the malaria control portfolio. Sporozoites colonize mosquito salivary glands, migrate through the skin, penetrate blood vessels, breach the liver sinusoid, and invade hepatocytes. Understanding the molecular and cellular mechanisms that mediate the remarkable sporozoite journey in the invertebrate vector and the vertebrate host can inform evidence-based next-generation drug development programs and immune intervention strategies.

Anopheline salivary protein genes and gene families: an evolutionary overview after the whole genome sequence of sixteen Anopheles species

BACKGROUND

Mosquito saliva is a complex cocktail whose pharmacological properties play an essential role in blood feeding by counteracting host physiological response to tissue injury. Moreover, vector borne pathogens are transmitted to vertebrates and exposed to their immune system in the context of mosquito saliva which, in virtue of its immunomodulatory properties, can modify the local environment at the feeding site and eventually affect pathogen transmission. In addition, the host antibody response to salivary proteins may be used to assess human exposure to mosquito vectors. Even though the role of quite a few mosquito salivary proteins has been clarified in the last decade, we still completely ignore the physiological role of many of them as well as the extent of their involvement in the complex interactions taking place between the mosquito vectors, the pathogens they transmit and the vertebrate host. The recent release of the genomes of 16 Anopheles species offered the opportunity to get insights into function and evolution of salivary protein families in anopheline mosquitoes.

RESULTS

Orthologues of fifty three Anopheles gambiae salivary proteins were retrieved and annotated from 18 additional anopheline species belonging to the three subgenera Cellia, Anopheles, and Nyssorhynchus. Our analysis included 824 full-length salivary proteins from 24 different families and allowed the identification of 79 novel salivary genes and re-annotation of 379 wrong predictions. The comparative, structural and phylogenetic analyses yielded an unprecedented view of the anopheline salivary repertoires and of their evolution over 100 million years of anopheline radiation shedding light on mechanisms and evolutionary forces that contributed shaping the anopheline sialomes.

CONCLUSIONS

We provide here a comprehensive description, classification and evolutionary overview of the main anopheline salivary protein families and identify two novel candidate markers of human exposure to malaria vectors worldwide. This anopheline sialome catalogue, which is easily accessible as hyperlinked spreadsheet, is expected to be useful to the vector biology community and to improve the capacity to gain a deeper understanding of mosquito salivary proteins facilitating their possible exploitation for epidemiological and/or pathogen-vector-host interaction studies.

Inhibition of Malaria Infection in Transgenic Anopheline Mosquitoes Lacking Salivary Gland Cells

Malaria is an important global public health challenge, and is transmitted by anopheline mosquitoes during blood feeding. Mosquito vector control is one of the most effective methods to control malaria, and population replacement with genetically engineered mosquitoes to block its transmission is expected to become a new vector control strategy. The salivary glands are an effective target tissue for the expression of molecules that kill or inactivate malaria parasites. Moreover, salivary gland cells express a large number of molecules that facilitate blood feeding and parasite transmission to hosts. In the present study, we adapted a functional deficiency system in specific tissues by inducing cell death using the mouse Bcl-2-associated X protein (Bax) to the Asian malaria vector mosquito, Anopheles stephensi. We applied this technique to salivary gland cells, and produced a transgenic strain containing extremely low amounts of saliva. Although probing times for feeding on mice were longer in transgenic mosquitoes than in wild-type mosquitoes, transgenic mosquitoes still successfully ingested blood. Transgenic mosquitoes also exhibited a significant reduction in oocyst formation in the midgut in a rodent malaria model. These results indicate that mosquito saliva plays an important role in malaria infection in the midgut of anopheline mosquitoes. The dysfunction in the salivary glands enabled the inhibition of malaria transmission from hosts to mosquito midguts. Therefore, salivary components have potential in the development of new drugs or genetically engineered mosquitoes for malaria control.

Malaria, transmitted by anopheline mosquitoes, represents an important global public health challenge. The salivary glands of mosquitoes are an attractive target tissue for malaria and vector control. We produced a transgenic strain inducing cell death in the salivary glands with a cell death effector molecule in the Asian malaria vector mosquito, Anopheles stephensi. This transgenic strain contained extremely low amounts of saliva. An analysis of this strain revealed that saliva plays an important role in probing as well as malaria infection in the midgut in a rodent malaria model. The dysfunction in the salivary glands enabled the inhibition of malaria transmission to mosquito midguts. Therefore, salivary components are also important in malaria control.

A Comprehensive Analysis of Plasmodium Circumsporozoite Protein Binding to Hepatocytes

Circumsporozoite protein (CSP) is the dominant protein on the surface of Plasmodium sporozoites and plays a critical role in the invasion by sporozoites of hepatocytes. Contacts between CSP and heparin sulfate proteoglycans (HSPGs) lead to the attachment of sporozoites to hepatocytes and trigger signaling events in the parasite that promote invasion of hepatocytes. The precise sequence elements in CSP that bind HSPGs have not been identified. We performed a systematic in vitro analysis to dissect the association between Plasmodium falciparum CSP (PfCSP) and hepatocytes. We demonstrate that interactions between PfCSP and heparin or a cultured hepatoma cell line, HepG2, are mediated primarily by a lysine-rich site in the amino terminus of PfCSP. Importantly, the carboxyl terminus of PfCSP facilitates heparin-binding by the amino-terminus but does not interact directly with heparin. These findings provide insights into how CSP recognizes hepatocytes and useful information for further functional studies of CSP.

Identification of GAPDH on the surface of Plasmodium sporozoites as a new candidate for targeting malaria liver invasion

Cha et al. show that Plasmodium GAPDH on the sporozoite surface acts as a ligand for binding Kupffer cell CD68, an interaction that is critical for parasite liver invasion. Thus, Plasmodium GAPDH is a candidate antigen for a prehepatic malaria vaccine.

Malaria transmission begins when an infected mosquito delivers Plasmodium sporozoites into the skin. The sporozoite subsequently enters the circulation and infects the liver by preferentially traversing Kupffer cells, a macrophage-like component of the liver sinusoidal lining. By screening a phage display library, we previously identified a peptide designated P39 that binds to CD68 on the surface of Kupffer cells and blocks sporozoite traversal. In this study, we show that the P39 peptide is a structural mimic of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) on the sporozoite surface and that GAPDH directly interacts with CD68 on the Kupffer cell surface. Importantly, an anti-P39 antibody significantly inhibits sporozoite liver invasion without cross-reacting with mammalian GAPDH. Therefore, Plasmodium-specific GAPDH epitopes may provide novel antigens for the development of a prehepatic vaccine.