Endogenously-expressed NH2-terminus of circumsporozoite protein interferes with sporozoite invasion of mosquito salivary glands
© Kojin et al. 2016
Received: 5 September 2015
Accepted: 1 March 2016
Published: 10 March 2016
The circumsporozoite protein is the most abundant polypeptide expressed by sporozoites, the malaria parasite stage capable of infecting humans. Sporozoite invasion of mosquito salivary glands prior to transmission is likely mediated by a receptor/ligand-like interaction of the parasites with the target tissues, and the amino (NH2)-terminal portion of CSP is involved in this interaction but not the TSR region on the carboxyl (C)-terminus. Peptides based on the NH2-terminal domain could compete with the parasites for the salivary gland receptors and thus inhibit penetration.
Peptides based on the NH2-terminus and TSR domains of the CSP from avian or human malaria parasites, Plasmodium gallinaceum and Plasmodium falciparum, respectively, were expressed endogenously in mosquito haemolymph using a transient (Sindbis virus-mediated) or stable (piggyBac-mediated transgenesis) system.
Transient endogenous expression of partial NH2-terminus peptide from P. falciparum CSP in P. gallinaceum-infected Aedes aegypti resulted in a reduced number of sporozoites in the salivary glands. When a transgenic approach was used to express a partial CSP NH2-terminal domain from P. gallinaceum the number of sporozoites in the salivary glands did not show a difference when compared to controls. However, a significant difference could be observed when mosquitoes with a lower infection were analysed. The same result could not be observed with mosquitoes endogenously expressing peptides based on the TSR domain from either P. gallinaceum or P. falciparum.
These results support the conclusion that CSP partial NH2-terminal domain can be endogenously expressed to promote a competition for the receptor used by sporozoites to invade salivary glands, and they could be used to block this interaction and reduce parasite transmission. The same effect cannot be obtained with peptides based on the TSR domain.
Malaria parasites have a complex developmental cycle in both the vertebrate and vector mosquito hosts. The sporozoites are the most versatile invasive stage of the parasite and are unique in having the ability to invade two types of cells, their primary targets in their vertebrate hosts and those of the mosquito salivary glands . Furthermore, target-cell specificity supports the hypothesis that the localization and penetration of mosquito salivary glands by sporozoites is mediated by receptors [2–5]. Two mosquito proteins, circumsporozoite binding protein (CSPBP) and saglin, were identified as receptors for sporozoites and these interact with the parasite-expressed circumsporozoite protein (CSP) and thrombospondin-related anonymous protein (TRAP), respectively [4, 5]. Merozoite apical erythrocyte-binding ligand (MAEBL) is another proposed sporozoite ligand involved in attachment and invasion of mosquito salivary glands , however, no receptor has been characterized yet.
CSP is the predominant sporozoite surface antigen and important for mediating recognition and invasion of the salivary glands (as reviewed [7, 8]). It is expressed by a single copy gene , and has a large central immuno-dominant domain comprising tandem repeats of small peptides. The amino-(NH2) terminus comprises a domain of hydrophobic amino acid that makes up the secretory signal peptide, and a pentapeptide (KLKQP) designated Region I, that contains a proteolytic cleavage site. Cleavage is required for vertebrate hepatocyte cell invasion, but is not necessary for salivary gland infection in the mosquito host [10, 11]. Furthermore, the NH2-terminus together with the repeat domain is responsible for sporozoite development and Plasmodium berghei mutants lacking these regions do not produce free sporozoites . The carboxyl-(C) terminus, which has sequence similarity to the thrombospondin type-1 repeat (TSR) superfamily, has an 18-amino acid sequence (EWSXCXVTCGXG(V/I)XXRX(K/R) designated Region II [13–15], and a putative glycosylphosphatidylinositol (GPI) anchor attachment site. Both Regions I and II are conserved highly among Plasmodium species .
Plasmodium falciparum CSP binds with greater avidity to the medial lobe and distal portions of the lateral lobes of Anopheles stephensi salivary glands than any other mosquito organs that are in contact with the haemolymph, and these lobes are invaded preferentially by sporozoites . A synthetic peptide encompassing Region I, plus the additional five amino acids immediately adjacent to the amino-end of the endogenous sequence, inhibits CSP binding to salivary glands . In addition, synthetic peptides encompassing P. falciparum Region I inhibit by 80 % sporozoite invasion of salivary glands . Mutant sporozoites expressing a CSP in which Region I is deleted invade salivary glands with a 10–15 % lower efficiency compared with controls, and mutants with a complete N-terminal deletion have ten-fold fewer salivary gland sporozoites compared with controls . In contrast, P. berghei mutant sporozoites expressing a P. falciparum CSP lacking Region I showed no impairment of motility or infectivity in the vector host, however, disruption of Region II interfered with motility and impaired sporozoite invasion of salivary gland .
Transient (recombinant Sindbis virus) and stable (piggyBac-mediated mosquito transformation) expression systems were used to study competition between different peptides from Plasmodium gallinaceum and P. falciparum based on NH2-terminus or TSR domains of CSP and conspecific sporozoites for receptors in the salivary gland of Aedes aegypti. The data presented here shows that only NH2-terminal peptides interfere with sporozoite penetration of mosquito salivary glands. Furthermore, a heterologous P. falciparum CSP partial NH2-terminus peptide also can block P. gallinaceum sporozoite invasion of the Ae. aegypti salivary glands. These results support the conclusion that the partial NH2-terminus peptides from both P. gallinaceum and P. falciparum may act as ligands in the invasion of mosquito salivary glands, and the possibility of using the peptides to prevent parasite transmission.
Mosquitoes and maintenance of Plasmodium gallinaceum life cycle
The Ae. aegypti Higgs white-eyed strain  was maintained in the insectary at Universidade de São Paulo, Parasitology Department at 27 ± 2 °C, 80 % relative humidity, 12/12 h day/night, with access ad libitum to a 10 % sucrose solution in water. Anesthetized mice were used when required for blood feeding. Larvae were fed with powdered fish food (Tetramin, Blacksburg, VA, USA). P. gallinaceum were maintained as described . Mosquitoes were fed on chickens infected with P. gallinaceum (strain 8A, obtained from A. Krettli, René Rachou Institute of Research, FIOCRUZ, MG, Brazil). Infection was determined by the presence of sporozoites in the salivary glands and infected females were fed thereafter on uninfected chickens.
Sindbis virus cloning and infectious virus production
After the two-step amplification, the fragments were cloned into pGEM-T (Promega) and XbaI digestion was used to recover the final constructs, which then were sub-cloned into the Sindbis virus expression plasmid, TE/3′2 J later (Additional file 2), generating Sin-PgpNT and Sin-PfpNT representing the peptides encompassing NH2-terminus CSP of P. gallinaceum and P. falciparum, respectively, and Sin-PgRTSR (P. gallinaceum) and Sin-PfRTSR and Sin-PgpTSR (P. falciparum) representing peptides encompassing the TSR domain of CSP.
Plasmids containing Sin-PgpNT, Sin-PfpNT, Sin-PgRTSR, Sin-PfRTSR, or Sin-PgpTSR were linearized with XhoI and transcription in vitro was performed with AmpliScribe™ T7-Fash™ (Epicentre) kit. Transcribed RNA was transfected with Lipofectamine™ Reagent (Invitrogen) in BHK-21 cells, viruses were transferred to C6/36 cells 72 h later, harvested at four days post-infection and stored in −80 °C until used. Aliquots of viruses were titrated using C6/36 cells through an end-point assay .
Transgene assembly and Aedes aegypti genetic transformation
A PgpNT fragment was amplified with primers designed to add the BglII and NotI restriction sites for directional cloning into the vector pSLfa [AeVg] . This plasmid has the Ae. aegypti Vitellogenin I (VgI) gene promoter for blood-meal induced expression of the peptides in the haemolymph . The FOW_BglII and REV_NotI primers (Additional file 1) were used to amplify these fragments from the Sin-PgpNT plasmid. After amplification, the product was cloned first into the TOPO TA cloning (Invitrogen), sequenced and sub-cloned into the pSLfa[AeVg] plasmid. The cassette [AeVg-PgpNT] was excised using AscI and sub-cloned into pBac 3XP3-eGFPafm. The resulting vector generated is designated pBac[3XP3-eGFP-AeVg-PgpNT] (Additional file 2). Transgenic Ae. aegypti lines were generated by injecting preblastoderm embryos with a mixture of the donor plasmid pBac[3XP3-eGFP-AeVg-PgpNT] (300 µg/µL) and the pBac helper plasmid (200 µg/µL) using procedures described previously .
Whole-body total RNA was isolated from Sindbis virus-infected and transgenic Ae. aegypti with TRIzol (Invitrogen) and treated with DNAse I (Invitrogen). The amplification of diagnostic products was done using the OneStep RT-PCR kit (Qiagen) and primers listed in Additional File 1. The reaction mixture was incubated at 50 °C for 30 min and 95 °C for 15 min. Amplification conditions were 94 °C for 1 min followed by 30 cycles of 94 °C for 1 min, 60 °C for 1 min and 72 °C for 1 min and a final step of 10 min at 72 °C.
Salivary gland infection assay in dsSindbis-infected Aedes aegypti
Four day-old adult females were injected with 0.5 μL of a solution containing 106 50 % tissue culture infective dose (TCID50 %) Sin-PfpNT, Sin-PgRTSR, Sin-PfRTSR and Sin-PgpTSR and control Sin-EGFP (recombinant Sindbis virus expressing EGFP) infectious virus. After 3 days, mosquitoes were allowed to feed on P. gallinaceum-infected chickens (8–11 % parasitemia) and only fully-engorged females were used in the experiments. Salivary glands pairs were dissected on the 8th day after the infectious blood meal according to P. gallinaceum infection profile  (corresponding to 11th day after dsSindbis infection) and individually homogenized in 10 µL of phosphate-buffered saline (PBS), placed in a haemocytometer and sporozoite numbers determined by phase-contrast microscopy.
Salivary gland infection assay in transgenic Aedes aegypti
Transgenic and wild-type female Ae. aegypti were fed on P. gallinaceum-infected chickens with an 8–11 % parasitaemia. Five days after the first blood feeding, a second blood meal using healthy mice was offered to maintain the expression from the blood meal-induced VgI gene promoter driving the recombinant CSP peptides . Sporozoite number was determined as described previously.
The data were analysed with GraphPad InStat version 3.00 Software and Mann–Whitney and unpaired t test was used to assess the statistical significance of the differences between control and experimental groups, a p value of <0.05 was considered statistically significant.
dsSindbis virus production in BHK21 cells and mRNA expression in dsSindbis virus infected mosquitoes
Plasmodium gallinaceum infection in Aedes aegypti transiently expressing CSP peptides
Analyses of transgenic Aedes aegypti endogenously expressing Plasmodium gallinaceum partial NH2-terminus CSP peptide
Transgene insertions were confirmed by diagnostic gene amplification using primers specific for the PgpNT sequence (Additional file 1). The expected PgpNT fragments were amplified from P#1 and P#4 genomic DNA, but a fragment larger than expected was amplified from the F#8 line (Fig. 4b). RT-PCR on pools of 10 P#1 females verified transcript presence and accumulation of the PgpNT transgene product at 24 h post-blood meal (hPBM) but not in samples prepared from mosquitoes 48 and 72 h PBM. This is consistent with the expression profile of the endogenous VgI gene. No cDNA amplification was detected in the P#4 line (Fig. 4c). Both lines F#8 (no diagnostic gene fragment of the expected size) and P#4 (unexpected expression profile) were discarded and only P#1 was used in subsequent experiments.
Plasmodium gallinaceum infection in Aedes aegypti pBac-PgpNT P#1 transgenic mosquitoes
In order to evaluate the capacity of PgpNT peptide to compete with sporozoites for salivary gland receptors, transgenic P#1 and wild-type Ae. aegypti females were fed on chickens with 8–11 % P. gallinaceum parasitaemia. The numbers of sporozoites recovered from salivary glands of challenged P#1 individual transgenic mosquitoes did not differ from the wild-type (p < 0.2097). Previous experiments showed that P. gallinaceum infection in Ae. aegypti (Higgs strain) provides a mean infection number of 2086 sporozoites per salivary gland (C. Maciel, unpublished data). Thus mosquitoes with numbers below that were considered as low infection. Restricting analysis to those mosquitoes a significant 30.3 % reduction (p = 0.0186) in parasites recovered from salivary glands was observed when compared with wild-type controls (Fig. 4). The same analysis was performed using the mosquitoes showing the infection level above the threshold established, and there is no significant difference between pBac-PgpNT line P#1 and control mosquitoes. Moreover, there is no significant difference if the same analysis is performed with the TSR-based peptides Sin-PgRTSR, Sin-PfRTSR, Sin-PfpTSR (Additional file 3), showing that this event only occurs for NH2-terminus peptides.
CSP represents 5–15 % of the total protein expressed in sporozoites and is highly immunogenic [28, 29]. These characteristics make CSP an excellent candidate for malaria vaccine development. RTS,S AS01, a recombinant protein candidate malaria vaccine that targets the P. falciparum CSP is the first to reach phase three clinical testing and is partially effective against clinical disease in young African children up to 4 years after vaccination, according to final trial data . Other important properties of CSP include its ligand-like interaction with mosquito salivary gland receptors and this makes it a target for disrupting parasite invasion in this tissue and thus blocking transmission. The results presented here are the first to show that endogenous production of peptides encompassing NH2-terminus of CSP protein using either a dsSindbis virus system or transgenic approach, inhibit sporozoite penetration into salivary gland using Ae. aegypti–P. gallinaceum model. Moreover this is the first report of a heterologous effect of the P. falciparum NH2-terminus of CSP-peptide in the P. gallinaceum model.
Controversy accompanies the published conclusions of the roles of CSP Regions I and II in receptor-mediated salivary gland recognition [11, 17–19]. The results presented here support the conclusion that the NH2-terminus (containing Region I) has a role in salivary gland recognition but not TSR (with Region II) in P. gallinaceum infection in a model system. Previous reports of a role for Region II were based on experiments performed with P. berghei [19, 31], however, the genetically-ablated parasites used were disabled in their gliding ability, and this makes it difficult to distinguish between disruption of a receptor-ligand interaction or the lack of motility as a cause of the reduced salivary gland infections. It also is possible that a mutation in Region II inhibits parasites egression from oocysts even though they produce the same numbers of oocyst sporozoites . Since the previous study did not measure the number of sporozoite in the haemocoel, it is possible that this is the cause of the impairment phenotype observed in both studies. Furthermore, once the NH2-terminus is removed from mutated parasites, the TSR domain is exposed, switching parasites from a migratory state to an adhesive state reducing the numbers of sporozoite capable of reaching and invading the salivary glands . Thus, it is unlikely that the TSR + Region II is acting as a ligand for sporozoites for salivary gland invasion.
An evolutionarily-conserved role for the partial NH2-terminus portion of CSP in salivary gland invasion is supported here, since it was observed that not only peptides from P. gallinaceum but also from P. falciparum are able to disrupt P. gallinaceum sporozoites penetration of Ae. aegypti salivary glands. Also, P. gallinaceum and P. falciparum are thought to be more closely-related than P. falciparum with other human malaria parasites [32–35]. Thus, information obtained with Ae. aegypti–P. gallinaceum model studies may be relevant to P. falciparum.
Transient expression of Sindbis-PgpNT resulted in a lower mRNA production than the other dsSindbis constructions and this most likely resulted in reduced peptide production, which in turn would not be enough to interfere with sporozoite penetration of the salivary glands. Also, the non-specific amplification product was most likely due to virus instability in cell culture passages that could select viruses with different sequences or for those lacking the transgene .
The choice of the VgI promoter in the transgenesis construct was based on its high level of expression and peak induction time (~24 h PBM). Previous work has shown its effectiveness in expressing heterologous proteins in Ae. aegypti transgenic lines [24, 27]. The signal peptide from the Ae. aegypti maltase-like I gene also was added to direct recombinant peptides into mosquito haemolymph .
Although all the elements chosen to generate the transgenic line were tested in previous reports, transgenic lines P#2 and F#8 did not express the transgene and this could be related to position effects, a phenomenon frequently seen in mosquito transgenesis . Furthermore, F#8 had an abnormal fragment when amplified with specific primers, and a recombination event might have occurred disrupting the integrity of the transgene, so subsequent experiments were performed with line P#1 that did not present these problems.
The initial analysis of parasite challenges of the pBac-PgpNT line P#1 did not reveal a statistically-significant difference for P. gallinaceum sporozoite in salivary glands between them and non-transgenic controls. Although this result first suggested the incapability of transgenic mosquitoes to block parasite entrance in the salivary gland, a more profound analysis of that initial outcome was performed. Since ligand/parasite competition should be interfered by parasite excess binding to the salivary gland, it was defined a threshold (<2086 sporozoites/salivary gland) to determine mosquitoes with low infection levels and a significant reduction in parasites recovered from salivary glands was observed between pBac-PgpNT line P#1 and control mosquitoes below the threshold, but not above (Additional file 3). Therefore, if the number of parasites is not in excess (saturation), it is possible to block the receptors thus inhibiting sporozoite salivary gland invasion, on the other hand, if the number of produced sporozoites is high, they will be able to overcome the blockade and penetrate the salivary gland. Moreover, the mosquitoes expressing TSR-based peptides Sin-PgRTSR, Sin-PfRTSR, Sin-PfpTSR were unable to generate significant difference compared to control even if a threshold is established (Additional file 4) reinforcing the results presented by the second analysis of pBac-PgpNT line P#1.
The reduction (~30 %) of sporozoites invading salivary glands in the presence of peptides based on NH2-terminus of CSP from either P. gallinaceum or P. falciparum in Ae. aegypti is lower than that observed (60–80 %) when A. stephensi were injected with synthetic peptides from P. falciparum and infected by P. berghei . It is likely that the amounts of recombinant peptides produced by both dsSindbis virus and the transgene were lower than the amount injected in that study, and this is reflected in the blocking efficiency like stated previously. It was reasoned that because codon optimization is crucial for heterologous expression of CSP in transgenes in Anopheles gambiae , it might also be crucial in Ae. aegypti. Also, CSP is not the only protein described that could promote salivary gland invasion [5, 6] and, therefore, complete inhibition may not be possible with a single class of peptides.
Recent technological advances in cellular and molecular biology, genetics and bioinformatics, have enabled the development of a transgenic A. stephensi transgenic lines that block the progression of the P. falciparum cycle rendering salivary glands free of sporozoites . This transgenic line is a groundbreaking tool towards an innovative, population modification method to control vector-borne diseases. However, human malaria parasites can be transmitted by a ~465 formally recognized Anopheles species and ~41 are considered to be dominant vector species/species complexes, capable of transmitting malaria at a level of major concern to public health . Thus, further studies on the biology of different vectors and host-pathogen interactions are needed to provide a broad spectrum of control possibilities that include all mosquitoes-parasites combinations.
The interference of invasion of Ae. aegypti salivary gland by sporozoites achieved by endogenously-expressed peptides represents a step forward in understanding this complex stage in the parasite cycle. Together with previous studies, the data presented here indicate that peptides endogenously expressed from CSP partial NH2-terminus are able to compete for receptors in the salivary gland and disrupt the sporozoite/salivary gland interaction. Also, the fact that this portion of the protein from either the avian and human parasite is capable of mediating this recognition supports an evolutionarily conserved role between these parasites. The blocking effect of CSP recombinant peptides make them candidates that could be used to interfere with the parasite-salivary gland interaction and thus reduce sporozoite transmission.
BBK conceived and designed the experiments. MLC, BBK, CM and DAH performed the experiments. BBK, DOC, ALCdS, OM, and MLC analysed the data. MCB, AAJ and MLC contributed reagents/materials/analysis tools. BBK, ALCdS, DOC, MLC, and AAJ wrote the paper. All authors read and approved the final manuscript.
We thank Neuza Saraiva Fernandes and Ediane Saraiva Fernandes for technical assistance and Alexandre Santos de Moura for sample sequencing. This work was supported by FAPESP (Process#04/00889-9 and#00/12138-7). AAJ was supported in part by an award from the National Institute of Allergy and Infectious Diseases (USA) (R37AI029746).
The authors declare that they have no competing interests.
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