Localization of juvenile hormone esterase during development in normal and in recombinant baculovirus-infected larvae of the moth Trichoplusia ni

Juvenile hormone esterase: biochemistry and structure

Normal insect development requires a precisely timed, precipitous drop in hemolymph juvenile hormone (JH) titer. This drop occurs through a coordinated halt in JH biosynthesis and increase in JH metabolism. In many species, JH esterase (JHE) is critical for metabolism of the resonance-stabilized methyl ester of JH. JHE metabolizes JH with a high kcat/KM ratio that results primarily from an exceptionally low KM. Here we review the biochemistry and structure of authentic and recombinant JHEs from six insect orders, and present updated diagnostic criteria that help to distinguish JHEs from other carboxylesterases. The use of a JHE-encoding gene to improve the insecticidal efficacy of biopesticides is also discussed.

Insecticidal activity of two proteases against Spodoptera frugiperda larvae infected with recombinant baculoviruses

Background

Baculovirus comprise the largest group of insect viruses most studied worldwide, mainly because they efficiently kill agricutural insect pests. In this study, two recombinant baculoviruses containing the ScathL gene from Sarcophaga peregrina (vSynScathL), and the Keratinase gene from the fungus Aspergillus fumigatus (vSynKerat), were constructed. and their insecticidal properties analysed against Spodoptera frugiperda larvae.

Results

Bioassays of third-instar and neonate S. frugiperda larvae with vSynScathL and vSynKerat showed a decrease in the time needed to kill the infected insects when compared to the wild type virus. We have also shown that both recombinants were able to increase phenoloxidase activity in the hemolymph of S. frugiperda larvae. The expression of proteases in infected larvae resulted in destruction of internal tissues late in infection, which could be the reason for the increased viral speed of kill.

Conclusions

Baculoviruses and their recombinant forms constitute viable alternatives to chemical insecticides. Recombinant baculoviruses containing protease genes can be added to the list of engineered baculoviruses with great potential to be used in integrated pest management programs.

Bringing together components of the fly renal system

The function of all animal excretory systems is to rid the body of toxins and to maintain homeostatic balance. Although excretory organs in diverse animal species appear superficially different they are often built on two common principals: filtration and tubular secretion/reabsorbtion. The Drosophila excretory system is composed of filtration nephrocytes and Malpighian (renal) tubules. Here we review recent molecular genetic data on the development and differentiation of nephrocytes and renal tubules. We focus in particular on the molecular mechanisms that underpin key cell and tissue behaviours during morphogenesis, drawing parallels with other species where they exist. Finally we assess the implications of patterned tissue differentiation for the subsequent regulation of renal function. These studies highlight the continuing usefulness of the fly to provide fundamental insights into the complexities of organ formation.

Genetically modified baculoviruses: a historical overview and future outlook

The concept of using genetic engineering to improve the natural insecticidal activity of baculoviruses emerged during the 1980s. Both academic and industrial laboratories have since invested a great deal of effort to generate genetically modified (GM) or recombinant baculoviruses with dramatically improved speeds of kill. Optimal production methodologies and formulations have also been developed, and the safety and ecology of the recombinant baculoviruses have been thoroughly investigated. Unfortunately, the initial excitement that was generated by these technologies was tempered when industry made a critical decision to not complete the registration process of GM baculoviruses for pest insect control. In this chapter, we summarize the developments in the field from a historical perspective and provide our opinions as to the current status and future potential of the technology. We will argue that GM baculoviruses are valuable and viable tools for pest insect control both alone and in combination with wild-type viruses. We believe that these highly effective biopesticides still have a bright future in modern agriculture as public awareness and acceptance of GM organisms, including GM baculoviruses, increases.

Small RNA Viruses of Insects: Expression in Plants and RNA Silencing

Interest in insect small RNA viruses (SRVs) has grown slowly but steadily. A number of new viruses have been analyzed at the sequence level, adding to our knowledge of their diversity at the level of both individual virus species and families. In particular, a number of possible new virus families have emerged. This research has largely been driven by interest in their potential for pest control, as well as in their importance as the causal agents of disease in beneficial arthropods. At the same time, research into known viruses has made valuable contributions to our understanding of an emerging new field of central importance to molecular biology-the existence of RNA-based gene silencing, developmental control, and adaptive immune systems in eukaryotes. Subject to RNA-based adaptive immune responses in their hosts, viruses have evolved a variety of genes encoding proteins capable of suppressing the immune response. Such genes were first identified in plant viruses, but the first examples known from animal viruses were identified in insect RNA viruses. This chapter will address the diversity of insect SRVs, and attempts to harness their simplicity in the engineering of transgenic plants expressing viruses for resistance to insect pests. We also describe RNA interference and antiviral pathways identified in plants and animals, how they have led viruses to evolve genes capable of suppressing such adaptive immunity, and the problems presented by these pathways for the strategy of expressing viruses in transgenic plants. Approaches for countering these problems are also discussed.

Developmental and hormonal regulation of juvenile hormone esterase gene in Drosophila melanogaster

Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) was used to study developmental expression and hormonal regulation of the juvenile hormone esterase gene (DmJhe) in the fruit fly, Drosophila melanogaster. The levels of DmJhe mRNA were low during the embryonic stage. A peak of Dmjhe mRNA was detected in the first, second and third instar larvae. The Dmjhe mRNA levels also increased soon after pupal ecdysis. The Dmjhe mRNA was detected in both male and female adult flies. The peaks of Dmjhe mRNA observed in the larvae coincided with the peaks of juvenile hormone (JH). In contrast, the mRNA for ecdysone-induced transcription factor, Drosophila hormone receptor 3 (DHR3) showed peaks of expression that coincided with the ecdysteroid peaks in embryo, larva and pupa. JH III induced Dmjhe mRNA but not DHR3 mRNA in explanted tissues cultured in Grace's medium. 20-hydroxyecdysone induced DHR3 mRNA and suppressed JH III induction of DmJhe mRNA. These studies show that the expression of jhe in D. melanogaster is regulated by both JH and 20E.

Juvenile hormone esterase (JHE) from Tenebrio molitor: full-length cDNA sequence, in vitro expression, and characterization of the recombinant protein

Juvenile hormone regulates the development and reproduction in a variety of insects. Juvenile hormone esterase (JHE) is a selective enzyme, which hydrolyzes the methyl ester of JH and alters its activity. In Tenebrio molitor, JHE has been previously purified from pupae and a partial cDNA was amplified by RT-PCR using fat body mRNA. The previous report indicated that several forms of the JHE protein were present in pupal homogenate. In this study, we report the full-length cDNA, which was obtained by RACE methods. The deduced protein sequence corresponds to peptides from two proteins of different molecular weights in the previous study. The coding region of the full-length cDNA was subcloned into the AcMNPV genome and high levels of expression of the JHE enzyme from the viral p10 promoter were demonstrated in cell culture. The majority of JHE is secreted from the cells as a soluble enzyme. The recombinant JHE enzyme was biochemically characterized. The recombinant protein appears by PAGE analysis as a monomer of approximately the same MW (66000) and pI (4.9) as was expected from the deduced amino acid sequence of the cDNA.

In vitro expression and biochemical characterization of juvenile hormone esterase from Manduca sexta

Juvenile hormone esterase (JHE) is a selective enzyme that hydrolyzes the methyl ester of juvenile hormone. This enzyme plays an important role in the regulation of metamorphosis in caterpillars, and is implicated in additional roles in development and reproduction in this and other orders of insect. The full length coding region of the JHE cDNA from Manduca sexta was subcloned into the baculovirus AcMNPV genome under the control of the p10 promoter. The recombinant virus demonstrated the expression of high levels of JHE activity when infected into Hi5 cells from Trichoplusia ni. The recombinant protein was partially purified by anion exchange chromatography and its biochemical characterization showed similar features to the wild type protein. The recombinant JHE has an estimated MW of 66500 Da. Some heterogeneity with the enzyme was observed when analyzed by isoelectric focusing, although the peak of JHE activity was observed at pI=6.0. It is highly sensitive to trifluoroketone inhibitors and certain phosphoramidothiolates, while relatively insensitive to other common esterase inhibitors. Incubating the enzyme with various organic solvents and detergents showed that the enzyme is activated at lower concentrations of solvents/detergents and remains significantly active even at high concentrations. The high tolerance of organic solvents may make this JHE enzyme useful in future applications as a synthetic catalyst.

Infection of its lepidopteran host by the Helicoverpa armigera stunt virus (Tetraviridae)

Techniques of microscopy and histopathology were employed to study the positive-sense, single-stranded RNA virus, the Helicoverpa armigera stunt virus (HaSV; omegatetravirus, Tetraviridae) infecting its caterpillar host. Infection of the virus per os during the first three instars of larval development is virulent and leads to rapid stunting and mortality. In contrast, no detectable symptoms occur in later larval development, signifying a high degree of developmental resistance. A quantitative study of cell populations in the host midgut during this time showed that increased cell numbers during development alone could not account for the increase in resistance. HaSV infection was restricted to the midgut and three of its four cell types. In younger larvae, the virus initiated its infection in closely situated foci that appeared to expand to link with others to cover larger areas of the midgut. The midgut cells of the infected larvae responded with an increased rate of sloughing to an extent rendering the midgut incapable of maintenance or recovery of normal function. In contrast, infection of older larvae by HaSV did not lead to overt pathology although foci of HaSV infection were detected in their midguts. However, the foci were more sparsely situated, failed to expand, and eventually disappeared, presumably due to cell sloughing. These observations indicate that cell sloughing is an immune response existing throughout larval development but midguts of older larvae have an additional mechanism to account for the increased resistance. This second mechanism results in midgut cells becoming more refractory to infection and, combined with cell sloughing, allows the midguts of older larvae to recover more readily from HaSV infection. These two mechanisms are similar to those seen with host responses to baculoviruses, which display developmental resistance to a lesser degree against more general infections. HaSV remaining in the midgut appears to amplify the degree of developmental resistance.

Purification of juvenile hormone esterase and molecular cloning of the cDNA from Manduca sexta

Juvenile hormone esterase (JHE) is a highly specific enzyme important for regulating the onset of metamorphosis in lepidopteran insects. After affinity chromatography of the hemolymph proteins of Manduca sexta, the pure JHE protein was digested with Lys-C and the resultant peptides were purified by microbore HPLC. Two peptides were selected for sequencing. Based upon these amino acid sequences, degenerate RT-PCR was performed in order to amplify a partial cDNA sequence from mRNA from the fat body of M. sexta. A 1512bp partial cDNA was generated and found to be highly homologous to the JHE from Heliothis virescens. 5' and 3' RACE were performed to obtain the full length cDNA sequence. The cDNA has a total length of 2220bp, with a 1749bp coding region. The deduced protein sequence contains 573 amino acids.

Ligand blot analysis of juvenile hormone esterase binding proteins in Manduca sexta L

Biotinylated recombinant juvenile hormone esterase (JHE) was used for ligand blotting of proteins from fat body tissue and pericardial athrocytes of Manduca sexta. Proteins were separated by SDS-polyacrylamide gel electrophoresis or by two-dimensional electrophoresis. Eight putative JHE binding proteins were detected in fat body tissue and in pericardial athrocytes of both M. sexta and Heliothis virescens. The predominant bands were 29, 72, 75, 125 and 240kDa, with minor bands at 50, 80 and 205kDa. All putative JHE binding proteins were present from the second through to the fifth instar larvae of M. sexta. On wide-range isoelectric focusing, the 29kDa JHE binding protein separated into three species with isoelectric points of 6.5, 6.6 and 6.8. Biotinylated-JHE did not bind recombinant M. sexta-derived juvenile hormone binding protein. The mutant JHE with mutations K29R and K524R binds weakly to the JHE binding protein P29, relative to binding of wild-type JHE [Shanmugavelu et al., J. Biol. Chem., 275 (2000) 1802-1806]. A similar reduction in binding was not seen for the 29kDa binding protein identified here in pericardial athrocytes by ligand blot. This result is discussed.

Recombinant juvenile hormone esterase, an effective tool for modifying juvenile hormone-dependent expression of the early trypsin gene in mosquitoes

The study of the changes in the steady-state levels of the early trypsin (ET) messenger RNA (mRNA) was used as a sensitive assay for measuring the effects of recombinant juvenile hormone esterase (rJHE) on juvenile hormone (JH)-dependent gene expression in mosquitoes. ET is a female-specific protease present in the midgut of the yellow fever mosquito Aedes aegypti during the first few hours after ingestion of a blood meal. Transcription of the early trypsin gene is part of the normal postemergence maturation of the midgut in the adult female, and it is regulated by JH. JHE was cloned from Heliothis virescens and expressed in a baculovirus vector. Injection of rJHE into mosquitoes resulted in an increase of JHE activity in the haemolymph. Injection of rJHE into newly emerged adult females delayed the normal increase in steady-state levels of ET mRNA observed in controls. Topically applied methoprene (a JH analogue) reversed the effect of rJHE. Injection of increasing concentrations of rJHE into 3-day-old unfed females resulted in a dose-dependent decrease in the steady-state levels of ET mRNA after 24 h. The effect of rJHE was transient, once the enzyme was cleared (72 h after injection), the steady-state levels of ET mRNA were restored. The injection of rJHE is an effective tool for modifying JH-dependent expression of the early trypsin gene in mosquitoes.

A novel protein that binds juvenile hormone esterase in fat body tissue and pericardial cells of the tobacco hornworm Manduca sexta L

Juvenile hormone esterase degrades juvenile hormone, which acts in conjunction with ecdysteroids to control gene expression in insects. Circulating juvenile hormone esterase is removed from insect blood by pericardial cells and degraded in lysosomes. In experiments designed to characterize proteins involved in the degradation of juvenile hormone esterase, a pericardial cell cDNA phage display library derived from the tobacco hornworm moth Manduca sexta L. was constructed and screened for proteins that bind juvenile hormone esterase. A 732-base pair cDNA encoding a novel 29-kDa protein (P29) was isolated. Western and Northern analyses indicated that P29 is present in both pericardial cell and fat body tissues and is expressed in each larval instar. In immunoprecipitation experiments, P29 bound injected recombinant juvenile hormone esterase taken up by pericardial cells and native M. sexta juvenile hormone esterase in fat body tissue, where the enzyme is synthesized. Binding assays showed that P29 bound juvenile hormone esterase more strongly than it did a mutant form of the enzyme with mutations that perturb lysosomal targeting. Based on these data, we propose that P29 functions in pericardial cells to facilitate lysosomal degradation of juvenile hormone esterase.

Neurotropic effect of juvenile hormone III in larvae of the cockroach, Periplaneta americana

Juvenile hormone III and juvenile hormone analogs (methoprene, farnesyl methyl ether) cause a short-term depression of spike activity in last instar larvae of the cockroach Periplaneta americana, specifically in the prothoracic gland nerves, both in vivo and in vitro. The decreased nervous activity in the prothoracic gland nerves leads to decreased ecdysteroid secretion by the prothoracic gland.

Disruption of lysosomal targeting is associated with insecticidal potency of juvenile hormone esterase

Juvenile hormone esterase (JHE; EC 3.1.1.1), which is intrinsically involved in regulation of development of some insect larvae, is rapidly removed from the hemolymph by the pericardial cells. Lys-29 and Lys-524, which are implicated in the degradation of JHE, were mutated to Arg. Neither the half-life of the modified JHE in the hemolymph nor the catalytic parameters were changed significantly, but when combined, these mutations resulted in apparent failure of lysosomal targeting in the pericardial cell complex. A hypothesis for the mechanism of reduced efficiency of lysosomal targeting is presented. Infection of larvae with a recombinant baculovirus expressing the modified JHE resulted in a 50% reduction in feeding damage compared with larvae infected with the wild-type virus, thus demonstrating improved properties as a biological insecticide. These data demonstrate that alteration of specific residues of JHE that disrupted lysosomal targeting, dramatically increased the insecticidal activity of this protein.

Mechanistic studies of the degradation of juvenile hormone esterase in Manduca sexta

The mechanisms of degradation of juvenile hormone esterase (JHE) were investigated in larvae of the tobacco hornworm, Manduca sexta. JHE is removed from the hemolymph by the pericardial cells by receptor-mediated endocytosis and is ultimately degraded in the lysosomes. Immunoprecipitation experiments and native PAGE followed by Western blotting showed that JHE associates with a putative heat shock cognate protein (Hsp). Approximately 25% of the active JHE in the pericardial cell complex is associated with the putative Hsp 1 h postinjection of affinity purified JHE. Electron microscope analysis revealed that the putative Hsp is located in the trans-Golgi network of pericardial cells, where it is hypothesized to be involved in sorting of proteins destined for the lysosomes, from those destined for the cell membrane. Data acquired from immunoprecipitation and Western blotting experiments argue against the involvement of ubiquitin in the degradation of JHE. Injection of radiolabeled JHE into larvae of M. sexta followed by SDS-PAGE of pericardial cell homogenates revealed covalent binding of an unidentified protein to JHE in the pericardial cell complex.

Structure, expression and gene sequence of a juvenile hormone esterase-related protein from metamorphosing larvae of Trichoplusia ni

A carboxylesterase with an encoded molecular size of 61 kDa and a high sequence similarity to juvenile hormone esterase (JHE) has been cloned from cDNA prepared from final instar larvae of Trichoplusia ni. The absence of a recognizable encoded signal peptide suggests that the enzyme, JHER (for JHE-related) may not be secreted, in contrast to JHE. When the amino acid sequence of JHE, JHER and other esterases were mapped onto the secondary and tertiary structure determined crystallographically for acetylcholinesterase, certain structural features for the substrate binding/catalytic site were identified as common only to JHE and JHER. However, several differences between JHE and JHER were identified in residues at the binding/catalytic site, suggesting that although the two enzymes prefer similar natural substrates, these substrates are not identical. JHER is present as a single-copy gene, transcribed during the feeding stage of the final stage of the final larval stadium, but not after metamorphic commitment to the pupal developmental programme. The gene transcribes a single-size message of 2.0 kb. The genes for JHER and JHE appear to be physically juxtaposed in the T. ni genome. The 5' flanking sequence to the JHER gene possesses some sequences in common with the JHE gene, but is also missing some regulatory elements previously identified in the JHE gene. Sequences conserved between the promoters for the two genes were identified that were different from previously reported regulatory elements of eukaryotic transcription factors.

Development of recombinant viral insecticides by expression of an insect-specific toxin and insect-specific enzyme in nuclear polyhedrosis viruses

As supplements to classical chemical insecticides, two approaches to develop recombinant baculovirus insecticides are described. In one approach an insect-specific toxin is expressed leading to a dramatic reduction in time to death. In the second approach an insect juvenile hormone esterase is expressed which leads to a reduction in feeding. Modifications of the wildtype esterase led to viruses which reduced the time to death as effectively as did the toxin-expressing virus. In both cases existing recombinant viruses are viewed as leads, and approaches to further improvement in the engineered viruses are suggested. Many of these approaches are based on analogy with the development of classical synthetic insecticides. Using these viruses as examples, the potential utility and limitations of recombinant viruses and other biological insecticides are discussed.