Characterization of an entomopathogenic fungi target integument protein, Bombyx mori single domain von Willebrand factor type C, in the silkworm, Bombyx mori

The insect cuticle works as the first line of defence to protect insects from pathogenic infections and water evaporation. However, the old cuticle must be shed in order to enter the next developmental stage. During each ecdysis, moulting fluids are produced and secreted into the area among the old and new cuticles. In a previous study, the protein Bombyx mori single domain von Willebrand factor type C (BmSVWC; BGIBMGA011399) was identified in the moulting fluids of Bo. mori and demonstrated to regulate ecdysis. In this study we show that in Bo. mori larvae, BmSVWC primarily locates to the integument (epidermal cells and cuticle), wing discs and head. During the moulting stage, BmSVWC is released into the moulting fluids, and is then produced again by epidermal cells after ecdysis. Fungal infection was shown to decrease the amount of BmSVWC in the cuticle, which indicates that BmSVWC is a target protein of entomopathogenic fungi. Thus, BmSVWC is mainly involved in maintaining the integrity of the integument structure, which serves to protect insects from physical damage and pathogenic infection.

Functional implications of the peptidoglycan recognition proteins in the immunity of the yellow fever mosquito, Aedes aegypti

Peptidoglycan recognition proteins (PGRPs) play essential roles in the immune systems of insects and higher animals against certain pathogens, including bacteria. In insects, most studies on the functions of PGRPs have been performed in Drosophila, with only limited studies in mosquitoes, which are important disease vectors. In the present study, we analysed the PGRP sequences of the yellow fever mosquito, Aedes aegypti, acquired from two genome databases, and identified a total of seven PGRP genes; namely, PGRP-S1, -SC2, -LA, -LB, -LC, -LD and -LE. Bacterial injection using the Gram-negative bacteria Escherichia coli and the Gram-positive bacteria Micrococcus luteus showed that three PGRPs responded directly to both bacterial stimuli. Subsequently, the transcriptional expression of six of these PGRPs was knocked down using double-stranded RNA-injection-based RNA interference (RNAi). RNAi of the PGRPs resulted in different impacts on the immune responses of Ae. aegypti to the two bacteria, as evidenced by the changes in mosquito survival rates after bacterial challenges as well as the differential regulation of several antimicrobial peptides and a number of other genes involved in mosquito immune pathways. Our data suggest that PGRP-LC is a significant factor in mediating immune responses to both E. coli and M. luteus, and the other PGRPs play only minor roles against these two bacteria, with PGRP-SC2 and -LB also serving as potential negative regulators for certain immune pathway(s) in Ae. aegypti.

Cryptosporidiosis in people: it's not just about the cows

Cryptosporidiosis is one of the most common causes of infectious diarrhea in people. Although dairy calves are high-risk hosts, the role of other livestock, pets, and humans in the disease should not be underestimated. Some Cryptosporidium species and strains are specific to people, others are specific to animals while some are zoonotic pathogens. Cryptosporidium hominis is the species responsible for the majority of human cases in the United States, Sub-Saharan Africa, and Asia, while Cryptosporidium parvum accounts for more human cases in Europe and particularly in the United Kingdom. A deeper understanding of Cryptosporidium host range, reservoirs, and transmission is needed to develop preventive strategies to protect the general public.

Virus-expressed, recombinant single-chain antibody blocks sporozoite infection of salivary glands in Plasmodium gallinaceum-infected Aedes aegypti

Transgenic mosquitoes resistant to malaria parasites are being developed to test the hypothesis that they may be used to control disease transmission. We have developed an effector portion of an antiparasite gene that can be used to test malaria resistance in transgenic mosquitoes. Mouse monoclonal antibodies that recognize the circumsporozoite protein of Plasmodium gallinaceum can block sporozoite invasion of Aedes aegypti salivary glands. An anti-circumsporozoite monoclonal antibody, N2H6D5, whose corresponding heavy- and light-chain gene variable regions were engineered as a single-chain antibody construct, binds to P. gallinaceum sporozoites and prevents infection of Ae. aegypti salivary glands when expressed from a Sindbis virus. Mean intensities of sporozoite infections of salivary glands in mosquitoes expressing N2scFv were reduced as much as 99.9% when compared to controls.

Genetics of mosquito vector competence

Mosquito-borne diseases are responsible for significant human morbidity and mortality throughout the world. Efforts to control mosquito-borne diseases have been impeded, in part, by the development of drug-resistant parasites, insecticide-resistant mosquitoes, and environmental concerns over the application of insecticides. Therefore, there is a need to develop novel disease control strategies that can complement or replace existing control methods. One such strategy is to generate pathogen-resistant mosquitoes from those that are susceptible. To this end, efforts have focused on isolating and characterizing genes that influence mosquito vector competence. It has been known for over 70 years that there is a genetic basis for the susceptibility of mosquitoes to parasites, but until the advent of powerful molecular biological tools and protocols, it was difficult to assess the interactions of pathogens with their host tissues within the mosquito at a molecular level. Moreover, it has been only recently that the molecular mechanisms responsible for pathogen destruction, such as melanotic encapsulation and immune peptide production, have been investigated. The molecular characterization of genes that influence vector competence is becoming routine, and with the development of the Sindbis virus transducing system, potential antipathogen genes now can be introduced into the mosquito and their effect on parasite development can be assessed in vivo. With the recent successes in the field of mosquito germ line transformation, it seems likely that the generation of a pathogen-resistant mosquito population from a susceptible population soon will become a reality.

Characterization of the Sialokinin I gene encoding the salivary vasodilator of the yellow fever mosquito, Aedes aegypti

The gene encoding sialokinin I, the principal vasodilatory peptide of Aedes aegypti, has been isolated and characterized. Degenerate oligonucleotide primers based on peptide amino acid sequence were used to amplify a gene fragment from messenger RNA (mRNA) isolated from female salivary glands. The amplification product was used to probe a salivary gland complementary DNA (cDNA) library, and a number of corresponding cDNAs were isolated and their primary sequence determined. Analysis of the conceptual translation product of a 406-bp cDNA indicates that sialokinin I is expressed as a preprosialokinin and is subsequently post-translationally processed to the active peptide. Northern analysis revealed a 490-bp transcription product expressed exclusively in female salivary glands, and hybridization in situ of probes to RNA in whole tissues localized gene expression to the medial lobe of female salivary glands. Screening of an Ae. aegypti genomic library with the cDNA resulted in the isolation of a clone containing the gene, designated Sialokinin I (Sia I). Comparison of the cDNA with the genomic clone reveals two introns of 62 bp and 833 bp. Primer extension analysis showed that several transcription initiation sites are present. Southern analysis of genomic DNA shows that Sia I is most probably a single-copy gene. Similarities of the Sia I gene product with other genes are confined to the region encoding the active decapeptide.

Controlling malaria transmission with genetically-engineered, Plasmodium-resistant mosquitoes: milestones in a model system

We are developing transgenic mosquitoes resistant to malaria parasites to test the hypothesis that genetically-engineered mosquitoes can be used to block the transmission of the parasites. We are developing and testing many of the necessary methodologies with the avian malaria parasite, Plasmodium gallinaceum, and its laboratory vector, Aedes aegypti, in anticipation of engaging the technical challenges presented by the malaria parasite, P. falciparum, and its major African vector, Anopheles gambiae. Transformation technology will be used to insert into the mosquito a synthetic gene for resistance to P. gallinaceum. The resistance gene will consist of a promoter of a mosquito gene controlling the expression of an effector protein that interferes with parasite development and/or infectivity. Mosquito genes whose promoter sequences are capable of sex- and tissue-specific expression of exogenous coding sequences have been identified, and stable transformation of the mosquito has been developed. We now are developing the expressed effector portion of the synthetic gene that will interfere with the transmission of the parasites. Mouse monoclonal antibodies that recognize the circumsporozoite protein of P. gallinaceum block sporozoite invasion of mosquito salivary glands, as well as abrogate the infectivity of sporozoites to a vertebrate host, the chicken, Gallus gallus, and block sporozoite invasion and development in susceptible cell lines in vitro. Using the genes encoding these antibodies, we propose to clone and express single-chain antibody constructs (scFv) that will serve as the effector portion of the gene that interferes with transmission of P. gallinaceum sporozoites.

Oxidation of 3-hydroxykynurenine to produce xanthommatin for eye pigmentation: a major branch pathway of tryptophan catabolism during pupal development in the yellow fever mosquito, Aedes aegypti

This study concerns the metabolic pathways of 3-hydroxykynurenine in Aedes aegypti mosquitoes during development with emphasis on its oxidation pathway to produce xanthommatin during eye pigmentation. Oxidation of tryptophan to 3-hydroxykynurenine is the major pathway of tryptophan catabolism in Aedes aegypti, but 3-hydroxykynurenine oxidizes easily under physiological conditions, which stimulate the production of reactive oxygen species. Our data show that in Aedes aegypti, the chemically reactive 3-hydroxykynurenine is converted to the chemically stable xanthurenic acid by a transaminase-catalyzed reaction during larval development, while 3-hydroxykynurenine is transported to the compound eyes for eye pigmentation during pupal development. Our data suggest that (1) the transamination pathway of 3-hydroxykynurenine is down-regulated during the pupal development, (2) 3-hydroxykynurenine produced in other body tissues is actively transported to the compound eyes during the pupal stage, (3) the compound eye is the place where ommochromes are produced, and (4) formation of ommochromes results from nonenzymatic oxidation of 3-hydroxykynurenine in the compound eyes.

Influence of anesthetics on the peripheral blood microfilaremia of Brugia malayi in the Mongolian jird, Meriones unguiculatus

The effects of anesthetics on the peripheral blood microfilaremia of Brugia malayi in Meriones unguiculatus were investigated. Microfilaremias were assessed by orbital puncture prior to and following the use, either individually or in tandem, of ether, Rompun, Ketaset, and sodium pentobarbital. Results indicate that the peripheral microfilaremia varied dramatically, depending on the anesthetic administered. Although microfilaremias were not affected by an initial ether exposure, counts of microfilariae increased significantly when jirds received Rompun and Ketaset, or Ketaset alone. Administration of sodium pentobarbital did not increase the number of microfilariae observed in the peripheral blood. The mode of action differs between these drugs and is likely responsible for the different effects observed. Consequently, studies involving vector-parasite interactions should take precautions to prevent parasite-induced vector mortality due to the ingestion of large numbers of microfilariae induced in the peripheral bloodstream by certain anesthetics.

Aedes aegypti: a quantitative trait locus (QTL) influencing filarial worm intensity is linked to QTL for susceptibility to other mosquito-borne pathogens

Because intensity of infection was significantly increased in a substrain of Aedes aegypti selected for susceptibility to the filarial worm, Brugia malayi, experiments were designed to assess numbers of microfilariae (mf) ingested and midgut penetration by mf in this susceptible substrain as compared to a refractory substrain selected from the same parental stock. Refractory mosquitoes ingested significantly fewer mf than susceptible mosquitoes and significantly fewer numbers of mf penetrated through refractory midguts as compared to susceptible midguts. In 16.7% of the refractory midguts, no mf were able to penetrate the midgut and in three refractory mosquitoes over 250 mf were ingested, but no mf penetrated the midgut. These results indicate that permissiveness of the midgut for penetration by microfilariae can determine not only parasite intensity, but also prevalence of infection. The genetic basis for ingestion of mf and midgut penetration was assessed using restriction fragment length polymorphism markers and quantitative trait loci (QTL) mapping. This mapping identified a QTL on chromosome 2, idb[2,LF181] (idb, intensity determinant for Brugia), that seems to influence ingestion ability. This QTL is linked to a previously identified QTL for susceptibility to B. malayi, fsb[2,LF98], as well as to loci for susceptibility to the malaria parasite, Plasmodium gallinaceum, and yellow fever virus. These results suggest that this region of chromosome 2 contains one or more genes that influence susceptibility of A. aegypti to several mosquito-transmitted pathogens.

Hemocyte alterations during melanotic encapsulation of Brugia malayi in the mosquito Armigeres subalbatus

The involvement of hemocytes in melanotic encapsulation reactions against Brugia malayi was assessed in Armigeres subalbatus. Hemocyte populations, epitope changes, phenol oxidase (PO) activity, and the presence of an 84-kDa polypeptide were investigated in mosquitoes exposed to a B. malayi-infective bloodmeal (= immune-activated), in mosquitoes given a noninfective bloodmeal (= controls), in nonbloodfed mosquitoes (= naive), or in some combination of these. Total hemocyte populations in immune-activated mosquitoes significantly decreased at 24 hr postbloodmeal (PB) as compared with controls. At 48 and 72 hr PB, hemocyte population levels in immune-activated mosquitoes increased to control levels. Epitope changes, as indicated by wheat germ agglutinin (WGA) binding, also were observed. There was a significant increase in the percentage of hemocytes binding WGA in immune-activated mosquitoes at 24 hr PB as compared with controls. Furthermore, the activity of hemocyte PO, an enzyme involved in the melanotic encapsulation pathway, was significantly elevated at 12 hr PB in immune-activated mosquitoes as compared with controls. Analysis for the presence of an 84-kDa polypeptide in A. subalbatus indicates that a 2.0-kb message in total RNA hybridized to D6.12, an Aedes aegypti cDNA encoding an 84-kDa polypeptide that is associated with melanotic encapsulation responses. The hybridization of D6.12 to RNA was not greater in immune-activated as compared to control A. subalbatus, as has been observed in A. aegypti. Results indicate that these hemocyte changes correspond in time with the melanotic encapsulation reactions of A. subalbatus against filarial worms.

Aedes aegypti: characterization of a hemolymph polypeptide expressed during melanotic encapsulation of filarial worms

We report on the initial characterization of an 84-kDa polypeptide that is differentially expressed in Aedes aegypti during melanotic encapsulation immune reactions against filarial worms. [35S]Methionine-labeled hemolymph from mosquitoes inoculated with saline, parasites that are melanized, or parasites that are not melanized was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Results show that the level of the 84-kDa polypeptide increases considerably in those mosquitoes undergoing encapsulation reactions against parasites but remains down-regulated in those mosquitoes exposed to parasites that are not melanized or are undergoing wound healing responses (saline-inoculated). Experiments involving glycosidase treatment of hemolymph samples indicate that this polypeptide is not heavily glycosylated. Amino acid microsequencing was performed and two internal sequence fragments (15 continuous amino acids and 12 noncontinuous amino acids) were obtained. Analysis of these sequences to known sequences in a protein database did not yield any conclusive information as to the identify of the 84-kDa polypeptide. Therefore, degenerate oligonucleotide primers were designed, based on the sequence of the 15-amino-acid fragment, and used with the polymerase chain reaction (PCR) to amplify from A. aegypti genomic DNA the region between the primers. The PCR product was cloned and sequenced to verify that the nucleic acid sequence matched the known protein sequence. Screening of an A. aegypti cDNA library with this small PCR-generated clone resulted in the selection of an approximately 540-bp clone. Northern analysis with this larger cDNA clone indicates that it hybridizes to an approximately 2.0-kb message that is differentially expressed in mosquitoes undergoing melanotic encapsulation reactions against filarial worms. Furthermore, sequencing of this approximately 540-bp clone showed that it contained the 15-amino-acid sequence that had been used to design the degenerate PCR primers, indicating that an appropriate clone was selected. However, sequence analysis of this clone at the protein and nucleic acid level did not provide any conclusive answers to the identity or function of the 84-kDa polypeptide.

Reproductive costs associated with resistance in a mosquito-filarial worm system

The mosquito Armigeres subalbatus can encapsulate and kill > 80% of Brugia malayi microfilariae (mf) within 36 hr following ingestion. The cascade of biochemical events constituting this melanotic encapsulation response is also important in other mosquito biological events, including egg-chorion tanning. Certain biochemical entities, including a tyrosine precursor, are thought to be shared among these biological activities. Because of this purported tyrosine link, and because the blood meal both initiates egg development and is the source of mf, we evaluated the possibility that reproductive cost is incurred by the resistant host when undergoing a response to mf acquired in an infected blood meal. Mean time to oviposition was significantly longer for mosquitoes responding to parasites than for controls (77.7 versus 66.5 hr). Tyrosine levels in ovaries from infected mosquitoes were less than half those of controls at 24 and 48 hr, and were still significantly reduced at 72 hr following blood feeding. Ovary development, assessed via measurements and total protein content, also was delayed significantly in the experimental group, with ovary width and protein content never attaining levels found in control mosquitoes. Sections from 24-hr post-blood meal ovaries demonstrated that the normal processes of egg development, including vitelline accumulation, was drastically altered as well. The biological implications of these results are considered.

Dirofilaria immitis: effect on hemolymph polypeptide synthesis in Aedes aegypti during melanotic encapsulation reactions against microfilariae

[35S]Methionine-labeled hemolymph polypeptides from adult, female Aedes aegypti Liverpool strain mosquitoes inoculated with the microfilariae of the filarial nematode Dirofilaria immitis were compared with those from saline-inoculated and uninoculated controls by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by fluorography. SDS-PAGE analysis of cell-free hemolymph collected via perfusion at 6, 12, 24, 48, 72, and 96 hr postinoculation (PI) detected the enhanced expression of an 84-kDa polypeptide. This polypeptide, expressed constitutively in the hemolymph of all three groups of mosquitoes, increased considerably in inoculated mosquitoes as time progressed as compared with uninoculated controls. Moreover, the 84-kDa polypeptide was expressed at higher levels in D. immitis-inoculated mosquitoes than in saline-inoculated controls. This stimulation of de novo biosynthesis of the 84-kDa polypeptide in inoculated mosquitoes may play a role in the immune response of mosquitoes. Since it is likely that the wound healing response in insects involves many of the same chemical processes as occur in melanotic encapsulation reactions of mosquitoes against filarial worms, the preferential expression of the 84-kDa polypeptide in saline-inoculated mosquitoes seen in this study may reflect a wound healing response. The greater increase in synthesis of this protein in D. immitis-inoculated mosquitoes may reflect production of melanotic material required for parasite destruction as well as for wound healing.

Brugia malayi and Brugia pahangi: inherent difference in immune activation in the mosquitoes Armigeres subalbatus and Aedes aegypti

The inherent ability of Brugia malayi and Brugia pahangi (Nematoda) to establish successful relationships with the mosquitoes Armigeres subalbatus and Aedes aegypti Liverpool strain was evaluated. Brugia pahangi microfilariae (mff) avoided the immune response and developed normally in A. subalbatus exposed to the parasite by an infective bloodmeal, whereas nearly 85% of B. malayi were destroyed by the immune response. Because A. aegypti supports the development of both filarial worm species but destroys intrathoracically inoculated B. pahangi isolated from jird blood, blood-isolated B. malayi were inoculated into A. aegypti, and the immune response was compared with that observed against B. pahangi. The response against B. malayi was significantly more rapid and effective than the response against B. pahangi. Similar results were obtained when blood-isolated B. pahangi or B. malayi were inoculated into A. subalbatus. Microfilariae of both species were able to avoid immune destruction in A. aegypti if they were allowed to penetrate the Liverpool midgut in vitro prior to inoculation. Most B. pahangi that had first penetrated an Armigeres midgut prior to inoculation into A. subalbatus were able to avoid the immune response, but by day 3 postinoculation, less than 40% of the B. malayi, treated in the same manner, were able to escape the immune response. Genetic susceptibility of mosquitoes to infection by filarial worms and potential mechanisms of immune evasion/suppression are discussed regarding B. malayi and B. pahangi.