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.

Expression patterns of the larval and adult hexamerin genes of Musca domestica

Hexamerins are proteins found in high abundance in the haemolymph of larval and adult insects. The expression patterns of the genes encoding the house fly, Musca domestica, hexamerins were determined by Northern analyses using cDNAs as probes. A cDNA, A1, hybridized to a fat body-specific messenger RNA (mRNA) which is detectable in larvae until pupation. Antibodies raised to the larval-specific hexamerin, Hex-L, bind recombinant protein encoded by a 5' rapid amplification of cDNA ends (RACE) product of A1, A2, indicating that the A cDNAs likely represent the genes encoding Hex-L. The F1, F2 and F3 cDNAs, corresponding to genes encoding an adult, female-enriched hexamerin, Hex-F, hybridized with an mRNA isolated from protein-fed females which has a temporal expression profile similar to that observed for the accumulation of Hex-F. Furthermore, expression of the mRNAs hybridizing to the F cDNAs is correlated with the abundance of Hex-F protein during the gonotrophic cycles. The mRNA transcription profiles indicate that the Hex-L and Hex-F genes are regulated in a sex-, tissue- and developmental phase-dependent manner. This stage-specific expression of hexamerins contrasts with the expression patterns of hexamerins seen in other insects. The conceptual translation products of larval hexamerin cDNAs showed identity with larval serum protein 1 (LSP1)-type hexamerins while the deduced products of the female hexamerin cDNAs showed the highest identity with LSP2-type hexamerins. Genomic analyses showed that the larval hexamerin and female hexamerin genes from M. domestica belong to two distinct multigenic families.

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.

Structure of hermes integrations in the germline of the yellow fever mosquito, Aedes aegypti

The Hermes transposable element is derived from the house fly, Musca domestica, and can incorporate into the germline of the yellow fever mosquito, Aedes aegypti. Preliminary Southern analyses indicated that Hermes integrated along with the marker gene into the mosquito genomic DNA. Here we show that Hermes integrations are accompanied by the integration of the donor plasmid as well. In addition, breaks in the donor plasmid DNAs do not occur precisely, or at the end of the terminal inverted repeats, and are accompanied by small deletions in the plasmids. Furthermore, integrations do not cause the typical 8-bp duplications of the target site DNA. No integrations are observed in the absence of a source of Hermes transposase. The Hermes transposase clearly did not catalyse precise cut-and-paste transposition in these transformed lines. It may have integrated the transposon through general recombination or through a partial replicative transposition mechanism. The imprecision of Hermes integration may result from interactions of the transposase with an endogenous hAT-like element in the mosquito genome.

Inhibition of luciferase expression in transgenic Aedes aegypti mosquitoes by Sindbis virus expression of antisense luciferase RNA

A rapid and reproducible method of inhibiting the expression of specific genes in mosquitoes should further our understanding of gene function and may lead to the identification of mosquito genes that determine vector competence or are involved in pathogen transmission. We hypothesized that the virus expression system based on the mosquito-borne Alphavirus, Sindbis (Togaviridae), may efficiently transcribe effector RNAs that inhibit expression of a targeted mosquito gene. To test this hypothesis, germ-line-transformed Aedes aegypti that express luciferase (LUC) from the mosquito Apyrase promoter were intrathoracically inoculated with a double subgenomic Sindbis (dsSIN) virus TE/3'2J/anti-luc (Anti-luc) that transcribes RNA complementary to the 5' end of the LUC mRNA. LUC activity was monitored in mosquitoes infected with either Anti-luc or control dsSIN viruses expressing unrelated antisense RNAs. Mosquitoes infected with Anti-luc virus exhibited 90% reduction in LUC compared with uninfected and control dsSIN-infected mosquitoes at 5 and 9 days postinoculation. We demonstrate that a gene expressed from the mosquito genome can be inhibited by using an antisense strategy. The dsSIN antisense RNA expression system is an important tool for studying gene function in vivo.

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.

Salivary gland-specific gene expression in the malaria vector Anopheles gambiae

Molecular studies on the tissue-specific gene expression in the salivary glands of Anopheles gambiae may provide useful tools for the development of new strategies for the control of the most efficient malaria vector in the sub-Saharan Africa. We summarize here the results of a recent investigation focused on the isolation of secreted factors and putative receptors from the salivary glands of An. gambiae. Using the Signal Sequence Trap technique we have identified the first cDNAs specifically expressed in the An. gambiae salivary glands. Among these, four are exclusively expressed in female glands and encode factors presumably involved in blood-feeding, whereas two other cDNAs seem to be expressed both in male and in female glands and are likely implicated in sugar-feeding. Homologues of genes previously identified in the yellow fever mosquito Aedes aegypti, like the apyrase and D7, as well as novel salivary gland-specific cDNAs, were identified. The isolation and characterization of promoter sequences from the corresponding genes may prove useful for the expression of anti parasitic agents in the salivary glands of transgenic mosquitoes.

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.

Trapping cDNAs encoding secreted proteins from the salivary glands of the malaria vector Anopheles gambiae

The signal sequence trap method was used to isolate cDNAs corresponding to proteins containing secretory leader peptides and whose genes are expressed specifically in the salivary glands of the malaria vector Anopheles gambiae. Fifteen unique cDNA fragments, ranging in size from 150 to 550 bp, were isolated and sequenced in a first round of immunoscreening in COS-7 cells. All but one of the cDNAs contained putative signal sequences at their 5' ends, suggesting that they were likely to encode secreted or transmembrane proteins. Expression analysis by reverse transcription-PCR showed that at least six cDNA fragments were expressed specifically in the salivary glands. Fragments showing a high degree of similarity to D7 and apyrase, two salivary gland-specific genes previously found in Aedes aegypti, were identified. Of interest, three different D7-related cDNAs that are likely to represent a new gene family were found in An. gambiae. Moreover, three salivary gland-specific cDNA fragments that do not show similarity to known proteins in the databases were identified, and the corresponding full length cDNAs were cloned and sequenced. RNA in situ hybridization to whole female salivary glands showed patterns of expression that overlap only in part those observed in the culicine mosquito A. aegypti.

Promoter-directed expression of recombinant fire-fly luciferase in the salivary glands of Hermes-transformed Aedes aegypti

Molecular genetic analyses of biological properties characteristic of insect vectors of disease, such as hematophagy and competence for pathogens, require the ability to isolate and characterize genes involved in these processes. We have been working to develop molecular approaches for studying the promoter function of genes that are expressed specifically in the adult salivary glands of the yellow fever mosquito, Aedes aegypti. Genomic DNA fragments containing cis-acting promoter elements from the Maltase-like I (MalI) and Apyrase (Apy) genes were cloned so as to direct the expression of the reporter gene, luciferase (luc). The function of the promoters was assayed transiently in cultured insect cells and by germ-line transformation of Ae. aegypti. MalI and Apy DNA fragments consisting of at least 650 nucleotides (nt) of DNA immediately adjacent to the 5'-end of the initiation codon of the mosquito genes directed constitutive expression of the luc reporter gene in cultured cells. When introduced into Ae. aegypti chromosomes, approximately 1.5 kilobases (kb) of each promoter were able to direct the predicted developmental-, sex- and tissue-specific expression of the reporter gene in patterns identical to those determined for the respective endogenous genes.

Isolation and characterization of the gene encoding a novel factor Xa-directed anticoagulant from the yellow fever mosquito, Aedes aegypti

Mosquito salivary glands secrete a number of proteins that inhibit mammalian hemostasis and facilitate blood feeding. We have isolated the protein product and corresponding cDNA of a gene designated Anticoagulant-factor Xa (AFXa), that encodes the factor Xa (FXa)-directed anticoagulant of the yellow fever mosquito, Aedes aegypti. The protein was purified partially by cation exchange chromatography and shown by enzyme activity profiles and SDS-polyacrylamide gel electrophoresis analysis to have an Mr = 54, 000. The protein was purified further by preparative SDS-polyacrylamide gel electrophoresis and subjected to internal protein sequencing, and the sequence of five peptides was determined. Degenerate oligonucleotides were designed based on three of the peptide sequences, and these were used to screen an adult female salivary gland cDNA library from A. aegypti. A 1.8-kilobase pair cDNA was isolated and shown to encode a 415-amino acid conceptual translation product with a predicted molecular mass of 47.8 kDa that contains the five sequenced peptides. Hydrophobicity analysis predicts a 19-amino acid signal peptide typical for secreted proteins. Northern analysis demonstrated that AFXa is expressed only in female salivary glands. Baculovirus-expressed AFXa protein has the appropriate size and expected FXa-directed anticoagulant activity. Analysis of the primary amino acid sequence shows that the AFXa gene product has similarities to the serpin superfamily of serine protease inhibitors and may represent a novel, highly diverged member of this family.

A lysozyme in the salivary glands of the malaria vector Anopheles darlingi

A cDNA encoding a lysozyme expressed specifically in the salivary glands of the malaria vector mosquito, Anopheles darlingi, was isolated by differential screening an adult female salivary gland library with abdomen and salivary gland cDNAs. The primary nucleic acid sequence of the cDNA contains a deduced coding region of 429 nucleotides and 5'- and 3'-end non-transcribed regions. A signal peptide of twenty-three amino acids and a mature protein of 120 amino acids are evident in the conceptual translation product. The results of RT-PCR experiments indicated that in adult mosquitoes this gene is expressed specifically in the salivary glands. Lysozyme enzymatic activity was detected in the salivary glands and abdomens of adult mosquitoes, but the pH optimum differed for each tissue and this was interpreted to indicate the presence of more than one enzyme, each being expressed in a different tissue. The salivary gland lysozyme may be involved in protection against bacterial infection in the anterior portion of the mosquito digestive tract.

Mariner transposition and transformation of the yellow fever mosquito, Aedes aegypti

The mariner transposable element is capable of interplasmid transposition in the embryonic soma of the yellow fever mosquito, Aedes aegypti. To determine if this demonstrated mobility could be utilized to genetically transform the mosquito, a modified mariner element marked with a wild-type allele of the Drosophila melanogaster cinnabar gene was microinjected into embryos of a kynurenine hydroxylase-deficient, white-eyed recipient strain. Three of 69 fertile male founders resulting from the microinjected embryos produced families with colored-eyed progeny individuals, a transformation rate of 4%. The transgene-mediated complementation of eye color was observed to segregate in a Mendelian manner, although one insertion segregates with the recessive allele (female-determining) of the sex-determining locus, and a separate insertion is homozygous lethal. Molecular analysis of selected transformed families demonstrated that a single complete copy of the construct had integrated independently in each case and that it had done so in a transposase-mediated manner. The availability of a mariner transformation system greatly enhances our ability to study and manipulate this important vector species.

Stable transformation of the yellow fever mosquito, Aedes aegypti, with the Hermes element from the housefly

The mosquito Aedes aegypti is the world's most important vector of yellow fever and dengue viruses. Work is currently in progress to control the transmission of these viruses by genetically altering the capacity of wild Ae. aegypti populations to support virus replication. The germ-line transformation system reported here constitutes a major advance toward the implementation of this control strategy. A modified Hermes transposon carrying a 4.7-kb fragment of genomic DNA that includes a wild-type allele of the Drosophila melanogaster cinnabar (cn) gene was used to transform a white-eyed recipient strain of Ae. aegypti. Microinjection of preblastoderm mosquito embryos with this construct resulted in 50% of the emergent G0 adults showing some color in their eyes. Three transformed families were recovered, each resulting from an independent insertion event of the cn+-carrying transposon. The cn+ gene functioned as a semidominant transgene and segregated in Mendelian ratios. Hermes shows great promise as a vector for efficient, heritable, and stable transformation of this important mosquito vector species.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

The white gene from the yellow fever mosquito, Aedes aegypti

We report the cloning and primary characterization of both cDNA and genomic fragments from the white gene of the yellow fever mosquito, Aedes aegypti. Comparisons of the conceptual translation product with white genes from four other species within the order Diptera show that the Ae. aegypti gene is most similar to the white gene of the mosquito vector of human malaria, Anopheles gambiae (86% identity and 92% similarity). The analysis of the primary sequence of genomic DNA at the 5'-end of the coding region revealed the presence of an intron that is also present in An. gambiae, but not in the vinegar fly, Drosophila melanogaster. The isolated clones of the Ae. aegypti white gene will enable the construction of a marker gene for use in the development of a germline transformation system for this species.

Evidence for two distinct members of the amylase gene family in the yellow fever mosquito, Aedes aegypti

Genomic DNA fragments encoding a salivary gland-specific alpha-amylase gene, Amylase I (Amy I), and an additional amylase, Amylase II (AmyII) of the yellow fever mosquito, Aedes aegypti, were isolated and characterized. Two independently isolated DNA fragments, G34-F and G34-14A, encode polymorphic alleles of Amy I. A 3.2 kilobase (kb) EcoR I fragment of G34-F, F2, has been sequenced in its entirety and contains 832 base pairs (bp) of the 5'-end, non-coding and putative promoter regions that are adjacent to 2.4 kb of the Amy I coding region. One intron, 59 bp in length, is found towards the 3'-end of the clone. A third genomic clone, 3A, corresponding to Amy II, was sequenced and shown not to contain the primary DNA sequence that encodes the 260 amino acid region that uniquely characterizes the amino terminal end of the Amy I product. Amy I was assigned by restriction fragment length polymorphism (RFLP) mapping to chromosome 2 (23.0 cM) and Amy II to chromosome 1 (44.0 cM). Amy I and Amy II are highly polymorphic and there may be multiple linked copies at each locus. Comparisons between Amy I and Amy II are presented for the putative promoter and conceptual translation products. The identification of two distinct amylase genes and their separate linkage assignments provides evidence for a multigene family of alpha-amylases in Ae. aegypti.

Transposition of the Hermes element in embryos of the vector mosquito, Aedes aegypti

Using a plasmid-based transpositional recombination assay in vivo, we have demonstrated that Hermes, a short inverted repeat type transposable element from Musca domestica, can transpose in Aedes aegypti embryos. Hermes transpositions in Ae. aegypti have all the characteristics observed during Hermes transposition in its host M. domestica and in related species. These characteristics include an absolute dependence on the expression of the Hermes transposase and a preference for the integration site GTNCAGAC (P < 0.05). In addition, the rate of Hermes transposition in Ae. aegypti (0.286 transpositions per 10,000 donor plasmids screened) was comparable to that observed in Drosophila melanogaster under similar conditions. These results suggest that Hermes can be developed into a gene vector and genetic engineering tool for Ae. aegypti and related mosquitoes.

The nonvitellogenic female protein of Musca domestica is an adult-specific hexamerin

During Musca domestica vitellogenesis a protein is preferentially synthesized by the female fat body and accumulates in the haemolymph but not in the ovaries. This protein, designated nonvitellogenic female protein (NVFP), was purified and shown to be a hexamer with an M(r) = 430 kDa, and subunits of M(r) = 70 kDa. The hexamer dissociates into subunits when the pH is elevated from 7.0 to 9.0. Two cDNA clones, F0 and F2, were isolated and analysed. The 2.2 kb F2 clone has an open reading frame that encodes a conceptual translation product that has similarity to the Drosophila melanogaster LSP-2 hexamerin. Recombinant protein from the F2-cDNA is recognized by a specific anti-NVFP serum. The temporal pattern of mRNA expression of the gene represented by the F2 clone follows that determined for the synthesis of NVFP. The data support the conclusion that NVFP is an hexamerin specific to the adult stage of Musca domestica.

Anticoagulants in vector arthropods

Arthropod-borne diseases cause significant morbidity and mortality worldwide. Mosquitoes alone may account for as many as three million deaths annually via the transmission of malaria. Because these diseases are transmitted to humans and to other vertebrates as a result of the ability of arthropods to feed on blood, the study of the biochemical mechanisms and adaptations that arthropods have evolved to facilitate hematophagy may provide insight into how this feeding behavior contributes to the transmission of disease. In this review, Kenneth Stark and Anthony James examine the diversity of arthropod anticoagulants and their role in hematophagy and potential implications for parasite transmission.

Salivary gland anticoagulants in culicine and anopheline mosquitoes (Diptera:Culicidae)

Female salivary gland extracts from 9 mosquito species representing the 2 hematophagous subfamilies delayed coagulation of human plasma. All extracts significantly delayed recalcification time, prothrombin time, and activated partial thromboplastin time, indicating an effect on the common coagulation pathway. Chromogenic assays to determine the coagulation inhibition site were performed for coagulation factor Xa (FXa) and thrombin, the major components of the common pathway. Although variation existed in the degree of inhibition, all anophelines have thrombin-directed anticoagulants, and culicine mosquitoes have FXa-directed anticoagulants. Differences in the site of action of the anticoagulants most likely reflected the long period of independent adaptation of the 2 subfamilies to the challenges presented by vertebrate hemostasis.

Pantropic retroviral vectors integrate and express in cells of the malaria mosquito, Anopheles gambiae

The lack of efficient mechanisms for stable genetic transformation of medically important insects, such as anopheline mosquitoes, is the single most important impediment to progress in identifying novel control strategies. Currently available techniques for foreign gene expression in insect cells in culture lack the benefit of stable inheritance conferred by integration. To overcome this problem, a new class of pantropic retroviral vectors has been developed in which the amphotropic envelope is completely replaced by the G glycoprotein of vesicular stomatitis virus. The broadened host cell range of these particles allowed successful entry, integration, and expression of heterologous genes in cultured cells of Anopheles gambiae, the principle mosquito vector responsible for the transmission of over 100 million cases of malaria each year. Mosquito cells in culture infected with a pantropic vector expressing hygromycin phosphotransferase from the Drosophila hsp70 promoter were resistant to the antibiotic hygromycin B. Integrated provirus was detected in infected mosquito cell clones grown in selective media. Thus, pantropic retroviral vectors hold promise as a transformation system for mosquitoes in vivo.

Functional analysis of a mosquito gamma-aminobutyric acid receptor gene promoter

A single point mutation in the insect gamma-aminobutyric acid receptor (GABAR)-encoding gene (Rdl) confers high levels of resistance to cyclodienes in Drosophila and other insects. We were interested in studying the promoter of this gene for two reasons. Firstly, to define the elements underlying Rdl expression. Secondly, to identify the minimum set of regulatory elements necessary for construction of a functional Rdl minigene. Such an insecticide-resistance-associated minigene should form a strong selectable marker for use in the genetic transformation of non-drosophilid pest insects, such as mosquitoes. Here, we report the identification of the region containing the rdl promoter, via transient expression of a luc reporter gene following micro-injection into embryos of the mosquito Aedes aegypti. Promoter activity is contained within a 2.53-kb fragment immediately upstream from the rdl start codon. Primer extension shows three closely linked sites for transcript initiation within this region and sequence analysis reveals anumber of putative consensus regulatory sequences shared by other genes expressed in the nervous system. The implications for construction of a functional minigene and the identification of cis-acting control elements underlying ion-channel gene regulation are discussed.

A factor Xa-directed anticoagulant from the salivary glands of the yellow fever mosquito Aedes aegypti

Salivary gland extracts from the yellow fever mosquito Aedes aegypti contain a factor Xa-specific anticoagulant detectable in vitro by clotting assays. The anticoagulant is found only in female salivary glands, which is consistent with its role in bloodfeeding. Extracts of adult female glands significantly delay both the prothrombin time and the activated partial thromboplastin time, but have no effect on the thrombin clotting time. The anticoagulant has been shown to result from the specific inhibition of coagulation factor Xa based on a FXa clotting assay and the inhibition of FXa-directed cleavage of the synthetic chromogenic substrate, chromozym X. The inhibition of FXa by female salivary gland extracts exhibits noncovalent, noncompetitive inhibition kinetics and is reversible. The anticoagulant is destroyed by boiling for 10 min or heating at 56 degrees C for 30 min, has a 4 mM calcium optimum with no magnesium requirement, and has a pH optimum of 8.0. The anticoagulant activity has an apparent molecular weight of 35.5 kDa, as determined by molecular sieve. We propose that the A. aegypti anticoagulant is a novel, proteinaceous serine protease inhibitor specific for FXa.

The Apyrase gene of the vector mosquito, Aedes aegypti, is expressed specifically in the adult female salivary glands

The yellow fever mosquito, Aedes aegypti, expresses a gene, Apyrase (Apy), that encodes an ATP-diphosphohydrolase. The product of this gene is a secreted enzyme that facilitates hematophagy by preventing platelet aggregation in the host. Apy gene expression is limited to the cells of the distal-lateral and medial lobes of the adult female salivary glands. Apyrase protein levels, detectable by antibodies, peak in the salivary glands about 4 days after adult emergence and remain high after a blood meal. Primary sequence analysis of a genomic clone encoding apyrase reveals a unique TAAATA sequence and seven introns, as well as other conserved features of eukaryotic genes. The temporal, sex- and tissue-specific expression of the Apy gene is consistent with its role as encoding a platelet anti-aggregation factor that functions to facilitate hematophagy and decrease probing time.

Characterization of a salivary gland-specific esterase in the vector mosquito, Aedes aegypti

Esterase enzymatic activity was investigated in salivary gland lysates of adult Aedes aegypti. Esterases in lysates made from female glands had higher specific activity than those in lysates from male glands towards beta-naphthyl acetate but showed no difference with alpha-naphthyl butyrate as a substrate. Female salivary gland lysates showed no difference in activity to alpha- and beta-forms of naphthyl acetate and no discernable activity towards alpha-naphthyl caprate. Both female and male salivary gland lysates exhibited phosphatase enzymatic activity but the specific activities were lower than those seen for the esterase enzymatic activity. Salivary gland esterase activity was inhibited completely by paraoxon, para-hydroxymercurobenzoate, tetraethylammonium iodide and moderately by diisopropylfluorophosphate. Eserine and phenylmethylsulfonylfluoride had no effect on enzyme activity. In a probing assay, adults of both sexes were shown to secrete esterase in saliva. Esterase activity was present in the saliva of females probing for either a sugar meal or a blood meal. Furthermore, esterase was secreted from female salivary glands in culture. Histochemical analysis of dissected salivary glands showed that the majority of the esterase enzymatic activity was in the distal-lateral lobes of the female tissues, although the proximal-lateral and medial lobes also had activity. Male salivary glands stained uniformly over all of the lobes. A salivary gland-specific esterase, designated SG-EST, appears to account for the majority of enzyme activity in the glands. SG-EST was partially purified by electroelution of an active protein from native polyacrylamide gels, and has an approximate molecular weight of 65,000 Da. In separate experiments, affinity chromatography independently identified a single 65,000 Da protein likely to be SG-EST. Native electrophoretic analysis of salivary glands revealed that, while most enzyme activity is due to SG-EST, there are two other esterases present. One of these minor moieties is present in adult tissues in addition to the salivary gland, and the other is present throughout development. Possible functions of the salivary gland esterase are discussed.

Transient expression of a promoter-reporter construct in differentiated adult salivary glands and embryos of the mosquito Aedes aegypti

Vector-borne pathogens develop in close association with specific tissues in their insect hosts. Efforts are being made to characterize insect genes that are expressed in tissues that have important roles in pathogen propagation. Successful transfection and expression of exogenous genes in terminally differentiated tissues of insects has previously proven difficult. Here we report a method that should allow the analysis of genes that are expressed in adult tissues and organs. Transient expression assays have been developed using the salivary glands of the mosquito, Aedes aegypti, which can now be used to analyze salivary gland-specific promoter sequences. A liposome-based transfection reagent was used to transfect cultured adult salivary glands with a DNA construct carrying the luciferase reporter gene under the control of the Drosophila melanogaster heat shock 70 promoter. Luciferase activity was detected in glands 18-20 hr post-transfection. This assay can now be used to determine the regulatory activity of other putative promoter sequences from salivary gland-specific genes. Alternatively, the assay may be used to study the effect of recombinant gene expression on parasite invasion and development. In addition, transient expression of gene constructs in embryos is shown to be a powerful tool for analyzing genes that are expressed at this stage of the mosquito life cycle.

The salivary gland-specific apyrase of the mosquito Aedes aegypti is a member of the 5'-nucleotidase family

The saliva of hematophagous insects contains a variety of pharmacologically active substances that counteract the normal hemostatic response to injury in vertebrate hosts. The yellow-fever mosquito, Aedes aegypti, secretes an apyrase that inhibits ADP-dependent platelet aggregation. Apyrase was purified as an active enzyme from adult female salivary glands and subjected to tryptic digestion, and the resulting peptides were sequenced. The amino acid sequences obtained match the conceptual translation product of a cDNA clone isolated from an adult female salivary gland library. Sequence comparisons indicate similarities with a ubiquitous family of 5'-nucleotidases. The mosquito protein differs from other members of the family by lacking a carboxyl-terminal hydrophobic domain. The apparent conversion of a gene encoding an enzyme involved in a common metabolic event at the cellular level to a gene involved in the antihemostatic response of mosquitoes illustrates one way this particular insect has adapted to the challenges of bloodfeeding.

A glutathione S-transferase gene of the vector mosquito, Anopheles gambiae

A cDNA (D7-8B) 1456 nucleotides in length was isolated from an adult, female-specific Anopheles gambiae library and identified as a member of the glutathione S-transferase (GST) gene family by virtue of the inferred amino acid sequence. The gene, AgGST2-1, specifies a protein that is 57% identical to the Drosophila melanogaster gene, DmGST-2. The conceptual translation product also shows similarity to the pi family of vertebrate GSTs. Northern analysis reveals multiple and abundant transcription products in larval, pupal and adult RNA preparations. Southern analyses of genomic DNA and hybridizations in situ to polytene chromosomes both suggest that there are multiple members of the GST gene family in Anopheles gambiae.

The salivary glands of the vector mosquito, Aedes aegypti, express a novel member of the amylase gene family

Several cDNA clones with similarity to alpha-amylases have been characterized from a library made from adult female salivary gland RNA isolated from the vector mosquito, Aedes aegypti. The corresponding gene, designated Amylase I (Amy I), is expressed specifically in the proximal-lateral lobes of the adult female salivary gland, a pattern overlapping that of another gene, Mal I, involved in carbohydrate metabolism. The deduced amino acid sequence of Amy I indicates that this gene encodes a protein, approximate M(r) = 81,500, that appears to be a novel member of the amylase gene family. The mosquito protein contains a putative signal peptide for secretion and several consensus sites for asparagine-linked glycosylation. The Amy I protein shows significant similarity to invertebrate and vertebrate amylases including the conservation of four reactive and substrate binding sites. However, the amino-terminal region of the Amy-I protein is unique to the mosquito. Similarity with the Drosophila melanogaster protein is evident only after the first 260 amino acids in the mosquito sequence. The identification of this gene and its expression pattern adds to the observed relationship between spatial-specific gene expression in the female salivary glands and the specific feeding mode of the adult mosquito.

Codon preference of Aedes aegypti and Aedes albopictus

The codon bias of two Aedes mosquito species was examined using a sign test. In general, there appeared to be some preference for C + G at the third base position, although this was not statistically significant. While amino acids such as phenylalanine and tyrosine clearly displayed biases, others such as valine and serine appeared to have little or no bias for any particular codon. Three homologous genes of Aedes aegypti and Drosophila melanogaster were compared using the chi-square test and the codon bias of the two species compared. Drosophila melanogaster was found to have a much stronger bias for C + G at the third base position compared to Aedes. The implications and usefulness of the codon bias are discussed.

FLP-mediated recombination in the vector mosquito, Aedes aegypti

The activity of a yeast recombinase, FLP, on specific target DNA sequences, FRT, has been demonstrated in embryos of the vector mosquito, Aedes aegypti. In a series of experiments, plasmids containing the FLP recombinase under control of a heterologous heat-shock gene promoter were co-injected with target plasmids containing FRT sites into preblastoderm stage mosquito embryos. FLP-mediated recombination was detected between (i) tandem repeats of FRT sites leading to the excision of specific DNA sequences and (ii) FRT sites located on separate plasmids resulting in the formation of heterodimeric or higher order multimeric plasmids. In addition to FRT sites originally isolated from the yeast 2 microns plasmid, a number of synthetic FRT sites were also used. The synthetic sites were fully functional as target sites for recombination and gave results similar to those derived from the yeast 2 microns plasmid. This successful demonstration of yeast FLP recombinase activity in the mosquito embryo suggests a possible future application of this system in establishing transformed lines of mosquitoes for use in vector control strategies and basic studies.

Isolation and characterization of the gene expressing the major salivary gland protein of the female mosquito, Aedes aegypti

We have undertaken a molecular analysis of the salivary glands of hematophagous insects in order to better understand their role in blood feeding and in the transmission of infectious diseases. To that end, genomic and cDNA clones of a gene designated D7, expressed abundantly in the adult female salivary glands of the vector mosquito aegypti, have been isolated and characterized. This gene encodes a mRNA shown by Northern analysis and in situ hybridization to tissue sections to be specifically transcribed in the distal-lateral and medial lobes of the glands, regions that are highly differentiated in females. The deduced gene product, a protein of approximately 37 kDa appears to be novel. Polyclonal antibodies made to a recombinant D7 product recognize a protein with the proper molecular weight in female salivary glands and saliva. These studies indicate that the D7 gene probably encodes the major secreted protein synthesized in the female salivary glands. The stage- and sex-limited expression of the D7 gene, and the secretion of its product, indicate that the product is most likely involved with the blood feeding capabilities of the female mosquito.

Mosquito salivary glands: Parasitological and molecular aspects

The salivary glands of mosquitoes serve in sugar feeding and blood vessel location. Components have been recently identified that may function in sugar feeding and digestion and platelet anti-aggregation. These factors correlate with salivary gland structure and appear to be controlled differentially in female mosquitoes. Analysis of gene expression has led to the discovery of three novel moieties in saliva: two involved in sugar feeding and one, specific to female mosquitoes, which is probably involved in blood feeding. Studies of parasite involvement in the salivary glands and host haemostasts have shown that parasites target specific interactions and modify them to enhance transmission. Here, Anthony James and Philippe Rossignol present an overview of mosquito salivary gland morphology and function, discuss recent advances in salivary gland molecular biology that have led to the discovery of new components and describe how parasites may modify salivary function to enhance transmission,.

Biotechnology as applied to vectors and vector control

Vector borne diseases remain some of the major public health problems in the developing world. In this review some of the current applications of biotechnology to vectors and vector control and some of the possible future applications are discussed.

A salivary gland-specific, maltase-like gene of the vector mosquito, Aedes aegypti

Genomic and cDNA clones of a gene expressed specifically in the salivary glands of adult Aedes aegypti have been isolated and sequenced. This gene encodes an abundant mRNA that is transcribed throughout the male salivary gland but only in the cells of the proximal lateral lobes of the female gland. The deduced protein has many basic amino acids, several possible sites for N-glycosylation, and displays striking similarities with the products of a yeast maltase gene and three previously unidentified genes from Drosophila melanogaster. We propose the name 'Maltase-like I' (MalI) to designate this gene. The presumed function of this gene product is to assist the mosquito in its sugar-feeding capabilities. The mosquito and fruitfly genes have similar structural features 5' to the protein coding regions, indicating that these genes may share common control mechanisms.

The spatial distribution of a blastoderm stage-specific mRNA from the serendipity locus of Drosophila melanogaster

The spatial distribution of a blastoderm stage-specific mRNA was examined by hybridization in situ to tissue sections of early embryos of Drosophila melanogaster. The mRNA examined, the C mRNA, is coded for by the molecularly-complex locus, serendipity (sry). The C mRNA was found to be expressed in embryos in all nuclei and cells except pole cells, during the syncytial and cellular blastoderm stages and its transcription correlates well with the formation of the cellular blastoderm.

Embryonic expression of the period clock gene in the central nervous system of Drosophila melanogaster

We have examined the temporal and spatial expression of the 4.5-kb mRNA that is transcribed from the period locus of Drosophila melanogaster and is the best candidate for the per gene product. Both Northern blot analyses and hybridizations in situ to tissue sections reveal significant expression of the 4.5-kb mRNA in embryos. This expression is limited to the central nervous system of the developing embryo and is localized within the brain and ventral ganglia. The 4.5-kb mRNA is enriched in adult heads (by Northern blotting) although we were not able to detect specific localization (in situ). In addition to the physiological role the 4.5-kb mRNA might have in maintaining biological rhythms, we now suggest that it has a developmental role for establishing mechanisms that are necessary for eventual expression of clock functions.

Isolation of genetic elements that increase frequencies of plasmid recombinants

Some plasmid DNAs, when maintained in wild-type Escherichia coli strains, form high levels of oligomeric species while others remain primarily monomers. One explanation of this observation is that the plasmids that do not form circular oligomers lack a DNA sequence necessary for the formation or maintenance of circular oligomeric species. Here we describe the isolation of segments of DNA from the E. coli genome and other sources that through a recA+ -dependent process: (1) stimulate the conversion of monomeric plasmids to different oligomeric forms, (2) stimulate the conversion of an oligomeric plasmid to a mixture of monomeric and different oligomeric forms, and (3) increase the frequency of recovery of figure-8 molecules. Both cis-acting and trans-acting elements were found. These elements seen to act by stimulating either the frequency of the recombination events that lead to the interconversion of different oligomeric plasmid DNA molecules or some process involved in the maintenance of newly-formed recombinant molecules.

Regulative interactions between cells from different imaginal disks of Drosophila melanogaster

The regulative behavior of cells from the imaginal wing disk of Drosophila melanogaster can be modified by interaction with cells from different disk types. Both thoracic and nonthoracic disks are able to interact, but there are major differences in the effectiveness of interaction. The finding lends experimental support to the idea that cells in different fields within the same organism use the same mechanism for specifying positional information. A similar conclusion has been reached by Wilcox and Smith based on studies of the mutation wingless.