Insecticidal toxins from the bacterium Photorhabdus luminescens.

Transgenic plants expressing Bacillus thuringiensis (Bt) toxins are currently being deployed for insect control. In response to concerns about Bt resistance, we investigated a toxin secreted by a different bacterium Photorhabdus luminescens, which lives in the gut of entomophagous nematodes. In insects infected by the nematode, the bacteria are released into the insect hemocoel; the insect dies and the nematodes and bacteria replicate in the cadaver. The toxin consists of a series of four native complexes encoded by toxin complex loci tca, tcb, tcc, and tcd. Both tca and tcd encode complexes with high oral toxicity to Manduca sexta and therefore they represent potential alternatives to Bt for transgenic deployment.

Characterized full-length and truncated plasmid clones of the crystal protein of Bacillus thuringiensis subsp. kurstaki HD-73 and their toxicity to Manduca sexta.

Bacillus thuringiensis subsp. kurstaki HD-73 produces a crystal protein which is lethal to many lepidopteran larvae. The gene encoding this crystal protein has been isolated from a 75-kb plasmid and engineered into a recombinant Escherichia coli plasmid for analysis. The complete nucleotide sequences of the coding region and 387-bp 5' and 376-bp 3' to the coding region have been determined. The 3537-bp of the coding region specify a protein of Mr 133 330. The full-length gene and several 3' -truncated derivatives of the gene were examined in both E. coli and in an E. coli minicell-expression system to determine if the carboxy end of the protein is essential for toxicity. The results presented here provide the primary structure of the crystal protein gene and show that the N-terminal 68-kDal peptide is toxic, but at a lower level than the full-length gene product.

A potential role for phenylalanine hydroxylase in mosquito immune responses.

In mosquitoes the melanotic encapsulation immune response is an important resistance mechanism against filarial worms and malaria parasites. The rate limiting substrate for melanin production is tyrosine that is hydroxylated by phenoloxidase (PO) to produce 3, 4-dihydroxyphenylalanine. The single pathway for endogenous production of tyrosine is by hydroxylation of phenylalanine by phenylalanine hydroxylase (PAH). In this study we describe a potential role for PAH in melanotic immune responses in the yellow fever mosquito, Aedes aegypti. A 1.6 kb A. aegypti PAH cDNA, encoding a 51 kDa protein, was isolated and subsequently expressed in an Escherichia coli expression system. In developing mosquitoes, PAH transcript is present in all stages and it is differentially expressed in adult tissues. Following an immune-challenge with Dirofilaria immitis microfilariae (mf) or bacteria, PAH transcript is up-regulated in hemocytes. Likewise, western analysis of hemocytes collected from immune-activated mosquitoes show an increase in gene product over control samples. Like PO, ultrastructure observations provide verification that PAH is located in oenocytoid and granulocyte hemocytes. Our results offer the first data that suggest PAH is used in mosquito melanin synthesis and defense responses.

Profiling infection responses in the haemocytes of the mosquito, Aedes aegypti.

Pathogens that infect and/or are transmitted by mosquitoes typically are exposed to the body cavity, and to haemocytes circulating therein, during development or dissemination. Aedes aegypti haemocytes produce a range of immune response-related gene products, and an endpoint response of phagocytosis and/or melanization that is temporally and structurally distinct for the invading pathogen. Expressed sequence tags were generated from haemocyte libraries and then used to design oligonucleotide microarrays. Arrays were screened with haemocyte material collected 1-, 8- and 24-h post-inoculation with Escherichia coli or Micrococcus luteus bacteria. Data from these studies support the discovery of novel immune response-activated genes, provide an expanded understanding of antimicrobial peptide biology and highlight the coordination of immune factors that leads to an endpoint response.

Drosophila gamma-aminobutyric acid receptor gene Rdl shows extensive alternative splicing.

The Drosophila gamma-aminobutyric acid (GABA) receptor subunit gene Rdl was isolated on the basis of a mutant phenotype showing high levels of insensitivity to picrotoxinin and cyclodiene insecticides. Following analysis of two dissimilar cDNAs isolated from the locus, we report that Rdl undergoes extensive alternative splicing at two locations in the putative extracellular domain. At each location a choice is made between exons of the same size: "a" or "b" (23 amino acids long with two substitutions) and "c" or "d" (46 residues long with 10 substitutions). The function of these alternative exons remains unclear; however, exon d contains a putative site for casein kinase II phosphorylation. All possible combinations of exons (a with c or d and b with c or d) were found in RNA isolated from early embryos. This is the first demonstration of alternative splicing in a GABA receptor gene from invertebrates.

Heat shock protein 27 stimulates recovery of RNA and protein synthesis following a heat shock.

Constitutive expression of human hsp27 resulted in a 100-fold increase in survival to a single lethal heat shock in CHO cells without effecting the development of thermotolerance. A possible mechanism for the thermoprotective function of hsp27 may be increased recovery of protein synthesis and RNA synthesis following a heat shock. A lethal heat shock (44 degrees C, 30 min) results in a 90% reduction in the rate of protein synthesis in non-tolerant cells. Control transfected cells recovered protein synthesis to a pre-heat shock rate 10 h after the heat shock; while cell lines that constitutively express human hsp27 recovered 6 h after the heat shock. Thermotolerant cells had a 50% reduction in protein synthesis, which recovered within 7 h following the heat shock. The same lethal heat shock (44 degrees C, 30 min) reduced RNA synthesis by 60% in the transfected cell lines, with the controls recovering in 7 h; while the hsp27 expressing cell lines recovered within 5 h. Thermotolerant cells had a 40% reduction in RNA synthesis and were able to recover within 4 h. The enhanced ability of hsp27 to facilitate recovery of protein synthesis and RNA synthesis following a heat shock may provide the cell with a survival advantage.

A point mutation in a Drosophila GABA receptor confers insecticide resistance.

Vertebrates and invertebrates both have GABA (gamma-aminobutyric acid) as a major inhibitory neurotransmitter. GABAA receptors in vertebrates assemble as heteromultimers to form an integral chloride ion channel. These receptors are targets for drugs and pesticides and are also implicated in seizure-related diseases. Picrotoxinin (PTX) and cyclodiene insecticides are GABAA receptor antagonists which competitively displace each other from the same binding site. Insects and vertebrates showing resistance to cyclodienes also show cross-resistance to PTX. Previously, we used a field-isolated Drosophila mutant Rdl (Resistant to dieldrin) insensitive to PTX and cyclodienes to clone a putative GABA receptor. Here we report the functional expression and novel pharmacology of this GABA receptor and examine the functionality of a resistance-associated point mutation (alanine to serine) within the second membrane-spanning domain, the region thought to line the chloride ion channel pore. This substitution is found globally in Drosophila populations. This mutation not only identifies a single amino acid conferring high levels of resistance to the important GABA receptor antagonist PTX but also, by conferring resistance to cyclodienes, may account for over 60% of reported cases of insecticide resistance.

A novel lectin with a fibrinogen-like domain and its potential involvement in the innate immune response of Armigeres subalbatus against bacteria.

Mosquitoes have an efficient cellular innate immune response that includes phagocytosis of microbial pathogens and encapsulation of metozoan parasites. In this study, we describe a novel lectin in the mosquito, Armigeres subalbatus (aslectin or AL-1). The 1.27 kb cDNA clone for the AL-1 gene (AL-1) encodes a 279 deduced amino acid sequence that contains a C-terminal fibrinogen-like domain. AL-1 is transcribed in all life stages. AL-1 mainly exists in the haemolymph of adult female mosquitoes, and is upregulated following both Escherichia coli and Micrococcus luteus challenge. AL-1 specifically recognizes N-acetyl-d-glucosamine and is able to bind both E. coli and M. luteus. These results suggest that AL-1 might function as a pattern recognition receptor in the immune response in Ar. subalbatus.

A single-amino acid substitution in a gamma-aminobutyric acid subtype A receptor locus is associated with cyclodiene insecticide resistance in Drosophila populations.

Resistance to cyclodiene insecticides, documented in at least 277 species, is perhaps the most common kind of resistance to any pesticide. By using cyclodiene resistance to localize the responsible gene, a gamma-aminobutyric acid type A receptor/chloride ion-channel gene was previously cloned and sequenced from an insecticide-susceptible Drosophila melanogaster strain. We now describe the molecular genetics of the resistance allele. A single-base-pair mutation, causing a single-amino acid substitution (Ala-->Ser) within the second membrane-spanning region of the channel, was found to be the only consistent difference between resistant and susceptible strains of D. melanogaster. Some resistant strains of Drosophila simulans show the same mutation, whereas others show an alternative single-base-pair mutation in the same codon, resulting in the substitution of a different amino acid (glycine). These constitute single-box-pair mutations in insects that confer high levels of resistance to insecticides. The presence of the resistance mutations was then tested in a much larger set of strains by the PCR and subsequent digestion with a diagnostic restriction endonuclease. Both resistance-associated mutations cause the loss of a Hae II site. This site was invariably present in 122 susceptible strains but absent in 58 resistant lines of the two species sampled from five continents. PCR/restriction endonuclease treatment was also used to examine linkage of an EcoRI polymorphism in a neighboring intron in D. melanogaster, which was found associated with resistance in all but 3 of 48 strains examined. These PCR-based techniques are widely applicable to examination of the uniqueness of different resistance alleles in widespread populations, the identification of resistance mechanisms in different species, and the determination of resistance frequencies in monitoring.

Cation permeability and cation-anion interactions in a mutant GABA-gated chloride channel from Drosophila.

To investigate the structural basis of anion selectivity of Drosophila GABA-gated Cl(-) channels, the permeation properties of wild-type and mutant channels were studied in Xenopus oocytes. This work focused on asparagine 319, which by homology is one amino acid away from a putative extracellular ring of charge that regulates cation permeation in nicotinic receptors. Mutation of this residue to aspartate reduced channel conductance, and mutation to lysine or arginine increased channel conductance. These results are consistent with an electrostatic interaction between this site and permeating anions. The lysine mutant, but not the arginine mutant, formed a channel that is permeable to cations, and this cannot be explained in terms of electrostatics. The lysine mutant had a 25-mV reversal potential in solutions with symmetrical Cl(-) and asymmetrical cations. The permeability ratio of K(+) to Cl(-) was determined as 0. 33 from reversal potential measurements in KCl gradients. Experiments with large organic cations and anions showed that cation permeation can only be seen in the presence of Cl(-), but Cl(-) permeation can be seen in the absence of permeant cations. Measurements of permeability ratios of organic anions indicated that the lysine mutant has an increased pore size. The cation permeability of the lysine-containing mutant channel cannot be accounted for by a simple electrostatic interaction with permeating ions. It is likely that lysine substitution causes a structural change that extends beyond this one residue to influence the positions of other channel-forming residues. Thus protein conformation plays an important role in enabling ion channels to distinguish between anions and cations.