Enzymic synthesis of juvenile hormone in locust corpora allata: evidence for a microsomal cytochrome P-450 linked methyl farnesoate epoxidase

Structural characterization and functional analysis of juvenile hormone acid methyltransferase JHAMT3 from the silkworm, Bombyx mori

Juvenile hormone acid methyltransferase (JHAMT) is a rate-limiting enzyme of juvenile hormone (JH) biosynthesis in insects. It transfers the methyl group of S-adenosyl methionine to either the carboxyl group of JH acids or farnesoic acid to produce JH. Six JHAMT paralogues have been identified in the silkworm (Bombyx mori); among them, JHAMT1 and JHAMT2 display a methyltransferase activity. Here, the three-dimensional crystal structure of inactive JHAMT3 and the binary complex of JHAMT3 with its cofactor S-adenosyl-l-homocysteine were determined through X-ray crystallization. Comparative structural analysis revealed that JHAMT3 adopted a similar structural pattern to that of functional JHAMT2, which comprised one core Rossmann fold domain and one substrate-binding domain. Similar to JHAMT2, JHAMT3 underwent a conformational change at the Rossmann fold domain because of cofactor binding, which promoted ligand accommodation. However, it exhibited a relatively rigid substrate-binding pocket compared with that of JHAMT2. JHAMT3 was also highly expressed in the silk gland of fourth- and fifth-instar B. mori larvae. The results of expression profiling combined with activity analysis suggested that JHAMT3 might function as a binding protein of JH acids for the regulation of JH acid titers. These findings provide a structural basis for enhancing the understanding of the physiological function of JHAMT3 and a rational framework for the development of potent and specific inhibitors of JHAMT family members.

Modeling the flux of metabolites in the juvenile hormone biosynthesis pathway using generalized additive models and ordinary differential equations

Juvenile hormone (JH) regulates development and reproductive maturation in insects. The corpora allata (CA) from female adult mosquitoes synthesize fluctuating levels of JH, which have been linked to the ovarian development and are influenced by nutritional signals. The rate of JH biosynthesis is controlled by the rate of flux of isoprenoids in the pathway, which is the outcome of a complex interplay of changes in precursor pools and enzyme levels. A comprehensive study of the changes in enzymatic activities and precursor pool sizes have been previously reported for the mosquito Aedes aegypti JH biosynthesis pathway. In the present studies, we used two different quantitative approaches to describe and predict how changes in the individual metabolic reactions in the pathway affect JH synthesis. First, we constructed generalized additive models (GAMs) that described the association between changes in specific metabolite concentrations with changes in enzymatic activities and substrate concentrations. Changes in substrate concentrations explained 50% or more of the model deviances in 7 of the 13 metabolic steps analyzed. Addition of information on enzymatic activities almost always improved the fitness of GAMs built solely based on substrate concentrations. GAMs were validated using experimental data that were not included when the model was built. In addition, a system of ordinary differential equations (ODE) was developed to describe the instantaneous changes in metabolites as a function of the levels of enzymatic catalytic activities. The results demonstrated the ability of the models to predict changes in the flux of metabolites in the JH pathway, and can be used in the future to design and validate experimental manipulations of JH synthesis.

Lepidopteran HMG-CoA reductase is a potential selective target for pest control

As a consequence of the negative impacts on the environment of some insecticides, discovery of eco-friendly insecticides and target has received global attention in recent years. Sequence alignment and structural comparison of the rate-limiting enzyme HMG-CoA reductase (HMGR) revealed differences between lepidopteran pests and other organisms, which suggested insect HMGR could be a selective insecticide target candidate. Inhibition of JH biosynthesis in vitro confirmed that HMGR inhibitors showed a potent lethal effect on the lepidopteran pest Manduca sexta, whereas there was little effect on JH biosynthesis in Apis mellifera and Diploptera punctata. The pest control application of these inhibitors demonstrated that they can be insecticide candidates with potent ovicidal activity, larvicidal activity and insect growth regulatory effects. The present study has validated that Lepidopteran HMGR can be a potent selective insecticide target, and the HMGR inhibitors (especially type II statins) could be selective insecticide candidates and lead compounds. Furthermore, we demonstrated that sequence alignment, homology modeling and structural comparison may be useful for determining potential enzymes or receptors which can be eco-friendly pesticide  targets.

Mevalonate-Farnesal Biosynthesis in Ticks: Comparative Synganglion Transcriptomics and a New Perspective

Juvenile hormone (JH) controls the growth, development, metamorphosis, and reproduction of insects. For many years, the general assumption has been that JH regulates tick and other acarine development and reproduction the same as in insects. Although researchers have not been able to find the common insect JHs in hard and soft tick species and JH applications appear to have no effect on tick development, it is difficult to prove the negative or to determine whether precursors to JH are made in ticks. The tick synganglion contains regions which are homologous to the corpora allata, the biosynthetic source for JH in insects. Next-gen sequencing of the tick synganglion transcriptome was conducted separately in adults of the American dog tick, Dermacentor variabilis, the deer tick, Ixodes scapularis, and the relapsing fever tick, Ornithodoros turicata as a new approach to determine whether ticks can make JH or a JH precursor. All of the enzymes that make up the mevalonate pathway from acetyl-CoA to farnesyl diphosphate (acetoacetyl-CoA thiolase, HMG-S, HMG-R, mevalonate kinase, phosphomevalonate kinase, diphosphomevalonate decarboxylase, and farnesyl diphosphate synthase) were found in at least one of the ticks studied but most were found in all three species. Sequence analysis of the last enzyme in the mevalonate pathway, farnesyl diphosphate synthase, demonstrated conservation of the seven prenyltransferase regions and the aspartate rich motifs within those regions typical of this enzyme. In the JH branch from farnesyl diphosphate to JH III, we found a putative farnesol oxidase used for the conversion of farnesol to farnesal in the synganglion transcriptome of I. scapularis and D. variabilis. Methyltransferases (MTs) that add a methyl group to farnesoic acid to make methyl farnesoate were present in all of the ticks studied with similarities as high as 36% at the amino acid level to insect JH acid methyltransferase (JHAMT). However, when the tick MTs were compared to the known insect JHAMTs from several insect species at the amino acid level, the former lacked the farnesoic acid binding motif typical in insects. The P450s shown in insects to add the C10,11 epoxide to methyl farnesoate, are in the CYP15 family; this family was absent in our tick transcriptomes and in the I. scapularis genome, the only tick genome available. These data suggest that ticks do not synthesize JH III but have the mevalonate pathway and may produce a JH III precursor.

Comparative transcriptome analysis among parental inbred and crosses reveals the role of dominance gene expression in heterosis in Drosophila melanogaster

We observed heteroses for body weight in Drosophila melanogaster after generating hybrids from three inbred lines. To better understand the mechanism for this phenomenon at the mRNA level, we compared the mRNA profiles of the parental and hybrid lines using high-throughput RNA-seq. A total of 5877 differentially expressed genes (DEGs) were found and about 92% of these exhibited parental expression level dominance. Genes in the dominance category were functionally characterized using the Kyoto Encyclopedia of Genes and Genomes (KEGG) and the gene classifications offered by the Gene Ontology (GO) Consortium. The analysis identified genes associated with crucial processes such as development and growth in all three crosses. Functional assignments involving aminoglycan metabolism, starch and sucrose metabolism, and galactose metabolism are significantly overrepresented amongst the 215 common dominance DEGs. We conclude that dominance DEGs are important in heteroses in Drosophila melanogaster and contribute specifically to body weight heterosis.

Function, diversity, and application of insect juvenile hormone epoxidases (CYP15)

Juvenile hormones (JHs) represent a family of sesquiterpenoid hormones in insects, and they play a key role in regulating development, metamorphosis, and reproduction. The last two steps of the JH biosynthetic pathway, epoxidation and methyl esterification of farnesoic acid to JH, are insect specific, and thus have long been considered a promising target for biorational insecticides. Recently, the enzymes involved in the last two steps have been molecularly identified: JH acid methyltransferase catalyzes the esterification step and the cytochrome P450 CYP15 enzyme catalyzes the epoxidation step. In this review, we describe the recent progress on the characterization of JH biosynthetic enzymes, with special focus on the function and diversity of the CYP15 family. CYP15 genes have evolved lineage-specific substrate specificity and regulatory mechanisms in insects, which appear to be associated with the lineage-specific acquisition of unique JH structure and function. In addition, the lack of CYP15 genes in crustacean (Daphnia pulex) and arachnid (Tetranychus urticae) species, whose genomes have been fully sequenced, may imply that CYP15 enzymes are an evolutionary innovation in insects to use the epoxide forms of methylated farnesoid molecules as their principal JHs. Molecular identification and characterization of CYP15 genes from broad taxa of insects have paved the way to the design of target-specific, biorational anti-JH agents.

Final steps in juvenile hormone biosynthesis in the desert locust, Schistocerca gregaria

Two genes coding for enzymes previously reported to be involved in the final steps of juvenile hormone (JH) biosynthesis in different insect species, were characterised in the desert locust, Schistocerca gregaria. Juvenile hormone acid O-methyltransferase (JHAMT) was previously described to catalyse the conversion of farnesoic acid (FA) and JH acid to their methyl esters, methyl farnesoate (MF) and JH respectively. A second gene, CYP15A1 was reported to encode a cytochrome P450 enzyme responsible for the epoxidation of MF to JH. Additionally, a third gene, FAMeT (originally reported to encode a farnesoic acid methyltransferase) was included in this study. Using q-RT-PCR, all three genes (JHAMT, CYP15A1 and FAMeT) were found to be primarily expressed in the CA of the desert locust, the main biosynthetic tissue of JH. An RNA interference approach was used to verify the orthologous function of these genes in S. gregaria. Knockdown of the three genes in adult animals followed by the radiochemical assay (RCA) for JH biosynthesis and release showed that SgJHAMT and SgCYP15A1 are responsible for synthesis of MF and JH respectively. Our experiments did not show any involvement of SgFAMeT in JH biosynthesis in the desert locust. Effective and selective inhibitors of SgJHAMT and SgCYP15A1 would likely represent selective biorational locust control agents.

Juvenile hormone synthesis: "esterify then epoxidize" or "epoxidize then esterify"? Insights from the structural characterization of juvenile hormone acid methyltransferase

Juvenile hormones (JHs) play key roles in regulating metamorphosis and reproduction in insects. The last two steps of JH synthesis diverge depending on the insect order. In Lepidoptera, epoxidation by a P450 monooxygenase precedes esterification by a juvenile hormone acid methyltransferase (JHAMT). In Orthoptera, Dictyoptera, Coleoptera and Diptera epoxidation follows methylation. The aim of our study was to gain insight into the structural basis of JHAMT's substrate recognition as a means to understand the divergence of these pathways. Homology modeling was used to build the structure of Aedes aegypti JHAMT. The substrate binding site was identified, as well as the residues that interact with the methyl donor (S-adenosylmethionine) and the carboxylic acid of the substrate methyl acceptors, farnesoic acid (FA) and juvenile hormone acid (JHA). To gain further insight we generated the structures of Anopheles gambiae, Bombyx mori, Drosophila melanogaster and Tribolium castaneum JHAMTs. The modeling results were compared with previous experimental studies using recombinant proteins, whole insects, corpora allata or tissue extracts. The computational study helps explain the selectivity toward the (10R)-JHA isomer and the reduced activity for palmitic and lauric acids. The analysis of our results supports the hypothesis that all insect JHAMTs are able to recognize both FA and JHA as substrates. Therefore, the order of the methylation/epoxidation reactions may be primarily imposed by the epoxidase's substrate specificity. In Lepidoptera, epoxidase might have higher affinity than JHAMT for FA, so epoxidation precedes methylation, while in most other insects there is no epoxidation of FA, but esterification of FA to form MF, followed by epoxidation to JH III.

Fine structure of corpora allata of castes with different rates of juvenile hormone production in the termite Reticulitermes flavipes

The aim of this work is to describe corpora allata (CA) of several castes of the termite Reticulitermes flavipes that have different rates of juvenile hormone (JH) synthesis, with respect to differences in fine structure, volume, and intensity of allatostatin immunoreactivity in their innervation. The castes chosen are workers and their potential derivatives, apterous secondary reproductives and pre-soldiers (the precursors of soldiers). These castes, at the stages chosen, produce JH at low, high and intermediate rates respectively. Hormone production is positively correlated with volume and negatively correlated with intensity of allatostatin immunoreactivity in axons within the glands. Characteristics of fine structure that correlate with increased activity are increase in abundance and width of mitochondria, decrease in ability to fix and visualize smooth endoplasmic reticulum. These features have previously been described for CA of cockroaches and other insects. Glycogen in the CA of all of the castes studied, especially the large amounts in highly active glands of physogastric apterous reproductive females, is the most striking difference between the CA cells of R. flavipes and previously described CA of cockroaches, in which glycogen is absent throughout the reproductive cycles. This suggests that glycogen is an important source of energy for hormone production by termite CA.

Nitric oxide/cGMP signaling in the corpora allata of female grasshoppers

The corpora allata (CA) of various insects express enzymes with fixation resistant NADPHdiaphorase activity. In female grasshoppers, juvenile hormone (JH) released from the CA is necessary to establish reproductive readiness, including sound production. Previous studies demonstrated that female sound production is also promoted by systemic inhibition of nitric oxide (NO) formation. In addition, allatotropin and allatostatin expressing central brain neurons were located in close vicinity of NO generating cells. It was therefore speculated that NO signaling may contribute to the control of juvenile hormone release from the CA. This study demonstrates the presence of NO/cGMP signaling in the CA of female Chorthippus biguttulus. CA parenchymal cells exhibit NADPHdiaphorase activity, express anti NOS immunoreactivity and accumulate citrulline, which is generated as a byproduct of NO generation. Varicose terminals from brain neurons in the dorsal pars intercerebralis and pars lateralis that accumulate cGMP upon stimulation with NO donors serve as intrinsic targets of NO in the CA. Both accumulation of citrulline and cyclic GMP were inhibited by the NOS inhibitor aminoguanidine, suggesting that NO in CA is produced by NOS. These results suggest that NO is a retrograde transmitter that provides feedback to projection neurons controlling JH production. Combined immunostainings and backfill experiments detected CA cells with processes extending into the CC and the protocerebrum that expressed immunoreactivity against the pan-neural marker anti-HRP. Allatostatin and allatotropin immunopositive brain neurons do not express NOS but subpopulations accumulate cGMP upon NO-formation. Direct innervation of CA by these peptidergic neurons was not observed.

Putative-farnesoic acid O-methyltransferase (FAMeT) in medfly reproduction

A gene potentially involved in juvenile hormone (JH) biosynthesis was previously identified in Ceratitis capitata as the putative-farnesoic acid O-methyltransferase (FAMeT). Since JH is involved in insect reproduction, we silenced the putative-FAMeT expression by RNA interference in Ceratitis capitata to evaluate its implication in egg production. FAMeT gene expression was knocked down in females and males after eclosion and in 1- and 2-day-old females. Treated specimens were left to mate with each other or with untreated partners to evaluate the extent of each sex influencing egg production. Gene silencing was investigated by Real-Time PCR. Results unambiguously showed that FAMeT has a measurable role on the fertility of both medfly sexes.

Endocrine interactions between plants and animals: Implications of exogenous hormone sources for the evolution of hormone signaling

Hormones are central to animal physiology, metabolism and development. Details on signal transduction systems and regulation of hormone synthesis, activation and release have only been studied for a small number of animal groups, notably arthropods and chordates. However, a significant body of literature suggests that hormonal signaling systems are not restricted to these phyla. For example, work on several echinoderm species shows that exogenous thyroid hormones (THs) affect larval development and metamorphosis and our new data provide strong evidence for endogenous synthesis of THs in sea urchin larvae. In addition to these endogenous sources, these larvae obtain THs when they consume phytoplankton. Another example of an exogenously acquired hormone or their precursors is in insect and arthropod signaling. Sterols from plants are essential for the synthesis of ecdysteroids, a crucial group of insect morphogenic steroids. The availability of a hormone or hormone precursor from food has implications for understanding hormone function and the evolution of hormonal signaling in animals. For hormone function, it creates an important link between the environment and the regulation of internal homeostatic systems. For the evolution of hormonal signaling it helps us to better understand how complex endocrine mechanisms may have evolved.

The putative-farnesoic acid O-methyl transferase (FAMeT) gene of Ceratitis capitata: characterization and pre-imaginal life expression

Farnesoic acid O-methyl transferase (FAMeT) is the enzyme involved in the penultimate step of insect juvenile hormone (JH) biosynthesis and is thus a key regulator in insect development and reproduction. We report the characterization of the putative-FAMeT in the medfly or Mediterranean fruit fly, Ceratitis capitata. This gene was identified by suppressive subtractive hybridization and completely sequenced by the screening of a medfly cDNA library. The obtained sequence was analyzed for conserved protein domain identification and its expression profile was evaluated by quantitative Real-Time PCR in medfly pre-imaginal life. The tissue expression of the isolated gene was verified by in situ hybridization on third instar larvae sections. The characterization of the isolated gene pointed out several typical features of methyl transferase genes. The pre-imaginal putative-FAMeT expression levels were consistent with JH titer change in Diptera. As recognized in some crustaceans, this gene seems to be widely expressed in the medfly as well. Ceratitis capitata is one of the most relevant agricultural pests against which insecticides and the sterile insect technique (SIT) are extensively used in spite of the well-known limitations of these approaches. Although results are not conclusive for the physiological role of the isolated gene, they suggest the characterization of a new gene in the Mediterranean fruit fly potentially involved in JH biosynthesis and may, therefore, have implications for pest control.

Isolation and expression analysis of multiple isoforms of putative farnesoic acid O-methyltransferase in several crustacean species

Farnesoic acid O-methyltransferase (FaMeT) is the enzyme responsible for the conversion of farnesoic acid (FA) to methyl farnesoate (MF) in the final step of MF synthesis. Multiple isoforms of putative FaMeT were isolated from six crustacean species belonging to the families Portunidae, Penaeidae, Scyllaridae and Parastacidae. The portunid crabs Portunus pelagicus and Scylla serrata code for three forms: short, intermediate and long. Two isoforms (short and long) were isolated from the penaeid prawns Penaeus monodon and Fenneropenaeus merguiensis. Two isoforms were also identified in the scyllarid Thenus orientalis and parastacid Cherax quadricarinatus. Putative FaMeT sequences were also amplified from the genomic DNA of P. pelagicus and compared to the putative FaMeT transcripts expressed. Each putative FaMeT cDNA isoform was represented in the genomic DNA, indicative of a multi-gene family. Various tissues from P. pelagicus were individually screened for putative FaMeT expression using PCR and fragment analysis. Each tissue type expressed all three isoforms of putative FaMeT irrespective of sex or moult stage. Protein domain analysis revealed the presence of a deduced casein kinase II phosphorylation site present only in the long isoform of putative FaMeT.

Purification and characterization of a mandibular organ protein from the American lobster, Homarus americanus: a putative farnesoic acid O-methyltransferase

Methyl farnesoate (MF) appears to have important roles in the development, morphogenesis, and reproduction of crustaceans. To better understand the regulation of MF synthesis, we studied farnesoic acid O-methyltransferase (FAOMeT, the final enzyme in the MF biosynthetic pathway) in the American lobster (Homarus americanus). FAOMeT purified from mandibular organ (MO) homogenates had a MW of approximately 38,000. The sequences of trypsin fragments of purified FAOMeT were used to design PCR primers to amplify a cDNA fragment, which was used to isolate a full-length cDNA containing a single open reading frame (ORF) of 828 bp encoding a protein of 276 amino acids. The deduced amino acid sequence of this putative FAOMeT protein contained two copies of a conserved approximately 135 amino acid domain we term the CF (CPAMD8/FAOMeT) domain; single copies of this domain also occur in the human CPAMD8 protein (a member of the alpha-2 macroglobulin family) and an uncharacterized Drosophila protein. The recombinant protein had no FAOMeT activity. However, its addition to MO homogenates from eyestalk ablated (ESA) lobsters increased enzyme activity by up to 75%, suggesting that FAOMeT may require an additional factor or modification (e.g., phosphorylation) for its activation. The mRNA for the putative FAOMeT was primarily found in the proximal region of the MO, the predominant site of MF synthesis. FAOMeT transcripts were found in muscle tissue from ESA animals, but not in green gland, hepatopancreas, or in muscle tissue from intact animals. FAOMeT mRNA was also detected in embryos and larval stages. This is the first comprehensive report of this protein in the lobster, and is an important step in elucidating the functions of MF in these animals.

Isoprenoids: Remarkable diversity of form and function

The isoprenoid biosynthetic pathway is the source of a wide array of products. The pathway has been highly conserved throughout evolution, and isoprenoids are some of the most ancient biomolecules ever identified, playing key roles in many life forms. In this review we focus on C-10 mono-, C-15 sesqui-, and C-20 diterpenes. Evidence for interconversion between the pathway intermediates farnesyl pyrophosphate and geranylgeranyl pyrophosphate and their respective metabolites is examined. The diverse functions of these molecules are discussed in detail, including their ability to regulate expression of the beta-HMG-CoA reductase and Ras-related proteins. Additional topics include the mechanisms underlying the apoptotic effects of select isoprenoids, antiulcer activities, and the disposition and degradation of isoprenoids in the environment. Finally, the significance of pharmacological manipulation of the isoprenoid pathway and clinical correlations are discussed.

CYP15A1, the cytochrome P450 that catalyzes epoxidation of methyl farnesoate to juvenile hormone III in cockroach corpora allata

The molecular analysis of insect hormone biosynthesis has long been hampered by the minute size of the endocrine glands producing them. Expressed sequence tags from the corpora allata of the cockroach Diploptera punctata yielded a new cytochrome P450, CYP15A1. Its full-length cDNA encoded a 493-aa protein that has only 34% amino acid identity with CYP4C7, a terpenoid omega-hydroxylase previously cloned from this tissue. Heterologous expression of the cDNA in Escherichia coli produced >300 nmol of CYP15A1 per liter of culture. After purification, its catalytic activity was reconstituted by using phospholipids and house fly P450 reductase. CYP15A1 metabolizes methyl (2E,6E)-3,7,11-trimethyl-2,6-dodecatrienoate (methyl farnesoate) to methyl (2E,6E)-(10R)-10,11-epoxy-3,7,11-trimethyl-2,6-dodecadienoate [juvenile hormone III, JH III] with a turnover of 3-5 nmol/min/nmol P450. The enzyme produces JH III with a ratio of approximately 98:2 in favor of the natural (10R)-epoxide enantiomer. This result is in contrast to other insect P450s, such as CYP6A1, that epoxidize methyl farnesoate with lower regio- and stereoselectivity. RT-PCR experiments show that the CYP15A1 gene is expressed selectively in the corpora allata of D. punctata, at the time of maximal JH production by the glands. We thus report the cloning and functional expression of a gene involved in an insect-specific step of juvenile hormone biosynthesis. Heterologously expressed CYP15A1 from D. punctata or its ortholog from economically important species may be useful in the design and screening of selective insect control agents.

Juvenile hormone acid methyltransferase: a key regulatory enzyme for insect metamorphosis

Juvenile hormone (JH) acid methyltransferase (JHAMT) is an enzyme that converts JH acids or inactive precursors of JHs to active JHs at the final step of JH biosynthesis pathway in insects. By fluorescent mRNA differential display, we have cloned a cDNA encoding JHAMT from the corpora allata (CA) of the silkworm, Bombyx mori (BmJHAMT). The BmJHAMT cDNA encodes an ORF of 278 aa with a calculated molecular mass of 32,544 Da. The predicted amino acid sequence contains a conserved S-adenosyl-l-methionine (SAM) binding motif found in the family of SAM-dependent methyltransferases. Purified N-terminal 6xHis-tagged recombinant BmJHAMT protein expressed in Escherichia coli catalyzed conversion of farnesoic acid and JH acids I, II, and III to their cognate methyl esters in the presence of SAM, confirming that this cDNA encodes a functional JHAMT. Putative orthologs, DmJHAMT and AgJHAMT, were identified from the genome sequence of the fruit fly Drosophila melanogaster, and a malaria vector, Anopheles gambiae, respectively. Northern blot and quantitative RT-PCR analyses revealed that the BmJHAMT gene was expressed specifically in the CA throughout the third and fourth instar. At the beginning of the last (fifth) instar, the expression level of BmJHAMT declined rapidly and became undetectable by day 4 and remained so until pupation. Correlation of the BmJHAMT gene expression and the JH biosynthetic activity in the CA suggests that the transcriptional suppression of the BmJHAMT gene is crucial for the termination of JH biosynthesis in the CA, which is a prerequisite for the initiation of metamorphosis.

Methyl farnesoate and juvenile hormone production in embryos of Diploptera punctata in relation to innervation of corpora allata and their sensitivity to allatostatin

Corpora allata (CA) of embryos of Diploptera punctata have been previously shown to produce JH III. We have re-examined sesquiterpenoid biosynthesis throughout embryonic development and have found that early embryos produce both methyl farnesoate (MF) and JH III; as development proceeds, less MF and more JH is produced. The cockroach allatostatin peptide Dippu-allatostatin (AST) 7 inhibits sesquiterpenoid production by CA of mid to late embryos whereas it exerts a dose-dependent stimulatory effect in early embryos. This stimulatory effect is particularly apparent on MF biosynthesis. CA become innervated by allatostatin-containing nerves in early embryos (35% development). Shortly thereafter, the allatostatin-containing innervation of the CA appears complete.

Terminal steps in JH biosynthesis in the honey bee (Apis mellifera L. ): developmental changes in sensitivity to JH precursor and allatotropin

Juvenile hormone (JH) is considered the prime endogenous signal for the induction of queen development in honey bees (Apis mellifera L.). At the beginning of the last (5th) larval stadium, worker corpora allata synthesize less JH than queen corpora allata as a consequence of a limited production of JH precursors and a caste- and stage-specific block of the terminal step in JH biosynthesis. As previously shown, the Manduca sexta allatotropin stimulates JH biosynthesis in honey bee corpora allata in a dose-dependent and reversible manner, but can not overcome the stage-specific block in the terminal step of JH biosynthesis that is typical for worker early 5th instars. In experiments with M. sexta allatotropin and with the JH precursor farnesoic acid, we found characteristic stage-specific differences in their effects on JH biosynthesis. From the end of the spinning stage on, corpora allata could be stimulated by farnesoic acid to a much higher extent than in earlier developmental stages, suggesting a sudden increase in epoxidase activity. Manduca sexta allatotropin, however, stimulated corpora allata activity until the end of the spinning stage, at which time the corpora allata become suddenly insensitive. These data suggest that in worker larvae, important changes in the regulation of the terminal enzymatic steps in JH biosynthesis occur at the transition from the spinning stage to the prepupal stage. However, the analysis of in vitro activities of the involved enzymes, O-methyltransferase and methyl farnesoate epoxidase, remained inconclusive.

Terpenoid omega-hydroxylase (CYP4C7) messenger RNA levels in the corpora allata: a marker for ovarian control of juvenile hormone synthesis in Diploptera punctata

Ribonuclease protection assays were used to measure changes in allatal transcript levels of the CYP4C7 gene which encodes a cytochrome P450 terpenoid omega-hydroxylase thought to play a role in the metabolism of JH and its precursors. Denervation of the corpora allata does not affect the pattern of expression of the CYP4C7 gene. Transplantation experiments show that CYP4C7 mRNA levels are dependent on a humoral factor characteristic of the reproductive state of the insect. Messenger RNA levels rise substantially in mated or denervated females, or in mated or virgin females treated with hydroprene, when the follicle length is over 1.5 mm. Vitellogenic ovaries however exert a negative influence on CYP4C7 expression, as ovariectomy in mated females causes a premature rise in CYP4C7 mRNA levels. The half-life of the CYP4C7 transcript is approx. 2 h when the corpora allata are incubated in vitro. Under these conditions, coincubation with a post-vitellogenic ovary maintains high CYP4C7 transcript levels in the glands. Excess juvenile hormone or analog applied at the end of vitellogenesis blocks ovulation or causes abortion of embryos deposited in the brood sac. We conclude that expression of the CYP4C7 gene is tightly controlled by the ovary, and it coincides with the ovarian signal to turn off juvenile hormone synthesis. The role of the CYP4C7 enzyme may be to ensure the clearance of allatal juvenile hormone and its precursors at the end of the gonotrophic cycle.

Insect P450 enzymes

The P450 enzymes (mixed function oxidases, cytochrome P450 monooxygenases), a diverse class of enzymes found in virtually all insect tissues, fulfill many important tasks, from the synthesis and degradation of ecdysteroids and juvenile hormones to the metabolism of foreign chemicals of natural or synthetic origin. This diversity in function is achieved by a diversity in structure, as insect genomes probably carry about 100 P450 genes, sometimes arranged in clusters, and each coding for a different P450 enzyme. Both microsomal and mitochondrial P450s are present in insects and are best studied by heterologous expression of their cDNA and reconstitution of purified enzymes. P450 genes are under complex regulation, with induction playing a central role in the adaptation to plant chemicals and regulatory mutations playing a central role in insecticide resistance. Polymorphisms in induction or constitutive expression allow insects to scan their P450 gene repertoire for the appropriate response to chemical insults, and these evolutionary pressures in turn maintain P450 diversity.

Induction of cytochrome P450 activities in Drosophila melanogaster strains susceptible or resistant to insecticides

We analysed Drosophila melanogaster cytochrome P450s (P450) through the measurements of four enzymatic activities: ethoxycoumarin-O-deethylase, ethoxyresorufin-O-deethylase, lauric acid hydroxylation, and testosterone hydroxylation. We did these measurements in two Drosophila strains: one is susceptible to insecticides (Cantons) and the other is resistant to insecticides by enhanced P450 activities (RDDTR). In addition, we also treated the flies with eight chemicals (beta-naphtoflavone, benzo-alpha-pyrene, 3-methylcholanthrene, phenobarbital, aminopyrine, rifampicin, prochloraz, and clofibrate) known to induces genes from the families CYP1, CYP2, CYP3, CYP4, and CYP6. Metabolisation of all the substrates by P450 from flies microsomes was observed. The chemicals had different effects on these activities, ranging from induction to inhibition. The effects of these chemicals varied with the strains as most of them were ineffective on the RDDTR strain. The results showed that P450-dependent activities are numerous in Drosophila. Regulation features of these activities are complex. The availability of mutant strains as RDDTR should allow fundamental studies of P450 in insects.

A cytochrome P450 terpenoid hydroxylase linked to the suppression of insect juvenile hormone synthesis

A cDNA encoding a cytochrome P450 enzyme was isolated from a cDNA library of the corpora allata (CA) from reproductively active Diploptera punctata cockroaches. This P450 from the endocrine glands that produce the insect juvenile hormone (JH) is most closely related to P450 proteins of family 4 and was named CYP4C7. The CYP4C7 gene is expressed selectively in the CA; its message could not be detected in the fat body, corpora cardiaca, or brain, but trace levels of expression were found in the midgut and caeca. The levels of CYP4C7 mRNA in the CA, measured by ribonuclease protection assays, were linked to the activity cycle of the glands. In adult females, CYP4C7 expression increased immediately after the peak of JH synthesis, reaching a maximum on day 7, just before oviposition. mRNA levels then declined after oviposition and during pregnancy. The CYP4C7 protein was produced in Escherichia coli as a C-terminal His-tagged recombinant protein. In a reconstituted system with insect NADPH cytochrome P450 reductase, cytochrome b5, and NADPH, the purified CYP4C7 metabolized (2E,6E)-farnesol to a more polar product that was identified by GC-MS and by NMR as (10E)-12-hydroxyfarnesol. CYP4C7 converted JH III to 12-trans-hydroxy JH III and metabolized other JH-like sesquiterpenoids as well. This omega-hydroxylation of sesquiterpenoids appears to be a metabolic pathway in the corpora allata that may play a role in the suppression of JH biosynthesis at the end of the gonotrophic cycle.

Neuropeptide regulation of biosynthesis of the juvenoid, methyl farnesoate, in the edible crab, Cancer pagurus

The neuropeptide mandibular organ (MO)-inhibiting hormone (MO-IH), synthesized and secreted from the X-organ-sinus-gland complex of the eyestalk, regulates the biosynthesis of the putative crustacean juvenile hormone, methyl farnesoate (MF). Using radiolabelled acetate as a precursor for isoprenoid biosynthesis, farnesoic acid (FA), farnesol, farnesal, MF and geranyl geraniol were detected in MOs cultured for 24 h. Treatment of MOs with extract of sinus gland inhibited the final step of biosynthesis of MF, catalysed by FA O-methyltransferase. Additionally, treatment of MOs with purified MO-IH exhibited a dose-dependent inhibition of this final step of MF synthesis. The extent of this inhibition was dependent on the ovary stage of the MO-donor animal, being maximal in MOs from animals in the early stages of ovarian development. Assay of FA O-methyltransferase activity, using [3H]FA in the presence of S-adenosyl-l-methionine, demonstrated that the enzyme was located in the cytosolic fraction of MOs and was inhibited by incubation of MOs with MO-IH prior to preparation of subcellular fractions. For cytosolic preparations taken from vitellogenic animals, both Vmax and Km were appreciably lower than for those taken from non-vitellogenic animals. Conversely, eyestalk ablation of early-vitellogenic animals, which removes the source of MO-IH in vivo, resulted in enhancement of the cytosolic FA O-methyltransferase activity. Although both Vmax and Km show an appreciable increase upon eyestalk ablation, the increased enzyme activity is probably reflected by the fact that Vmax/Km (an approximate indication of kcat) has increased 5-fold. The combined evidence demonstrates that MO-IH inhibits FA O-methyltransferase, the enzyme which catalyses the final step of MF biosynthesis in MOs.

Hydroxy juvenile hormones: new putative juvenile hormones biosynthesized by locust corpora allata in vitro

The in vitro production of sesquiterpenoids was investigated by using corpora allata (CA) of the African locust Locusta migratoria migratorioides. Labeled products from unstimulated biosynthesis were extracted, purified by normal phase HPLC, and derivatized to determine the functional groups present. An extra hydroxyl group was detected in each of two juvenile hormone (JH) biosynthetic products. One compound, NP-8, was found to co-migrate with a chemically-synthesized (Z)-hydroxymethyl isomer, 12'-OH JH-III, but not with the (E)-hydroxymethyl isomer, 12-OH JH III. Mass spectral analyses further supported the identity of the synthetic material with that biosynthesized by the corpora allata. A second compound was identified as the 8'-OH JH-III based on spectroscopic analyses. 12'-OH JH-III exhibited morphogenetic activity when tested on the heterospecific Tenebrio test. These data suggest that 12'-OH JH-III and 8'-OH JH-III are additional biosynthetically-produced and biologically-active juvenile hormones, and constitute the first known members of the class of hydroxy juvenile hormones (HJHs).

Photoaffinity labeling of methyl farnesoate epoxidase in cockroach corpora allata

The last enzyme in the biosynthetic pathway to juvenile hormone III in the corpora allata of hemimetabolous insects is methyl farnesoate epoxidase, a cytochrome P450 monooxygenase. Assays with intact glands incubated in vitro and with gland homogenates have identified a series of 1,5-disubstituted imidazoles as potent inhibitors of the enzyme. We have designed, synthesized and tested two imidazoles, diazirine-Ice T and benzophenone-Ice T, in which a radiolabeled and photoactivatable diazirine or benzophenone group was introduced to label the hydrophobic substrate binding site of the enzyme. Our results show that these bifunctional compounds inhibit JH III synthesis by intact glands as well as methyl farnesoate epoxidation by gland homogenates. Moreover both compounds selectively label a protein of ca. 55 kDa in corpora allata of the cockroach, Diploptera punctata. These photoaffinity labels, which use an imidazole to coordinate to the heme iron and a photoreactive group to modify the hydrophobic substrate binding pocket, are specific and effective probes for the molecular analysis of methyl farnesoate epoxidase.

Molecular analysis of a cytochrome P450 gene of family 4 on the Drosophila X chromosome

A member of the cytochrome P450 superfamily has been identified within region 2D of the Drosophila X chromosome. The sequence of this gene shows strongest homology with P450 family 4 genes, and has thus been named CYP4D1 in accordance with convention. This P450 gene is expressed throughout Drosophila development, with the highest levels of transcript accumulation occurring during late larval stages. A fragment of Drosophila genomic DNA including the CYP4D1 gene has been reintroduced into the germ line by P-element-mediated transformation. This transduced fragment does not rescue any of the lethal mutations that have been identified in this genetically well-characterized region of the Drosophila genome.

Separation of cytochrome P-450 containing vesicles from the midgut microsomal fraction of Manduca sexta

1. Microsomes prepared from midgut tissue of Manduca sexta by differential centrifugation are a heterogeneous population of vesicles. 2. Upon centrifugation of microsomes in a sucrose gradient of 30%, 15% and 10%, specific activity of cytochrome P-450 catalyzed O-demethylation of p-nitroanisole (a marker enzyme for endoplasmic reticulum) was increased 2.5 to 3.5-fold in the 15% fraction. 3. Specific activities of alkaline phosphatase and leucine aminopeptidase (marker enzymes for brush border membranes) were increased 3.1 to 5.7-fold in the pellet. 4. Differential centrifugation is a useful step in the purification of cytochrome P-450 enzymes.

Cytochrome P-450 monooxygenases in insects

1. The insect monooxygenase system is involved in the oxidative metabolism of both endogenous and exogenous substrates. 2. Monooxygenases appear to be important in insect growth and development, in adaptation to multiple food plants in phytophagous insects and in pesticide resistance. 3. The genetics, purification and properties of cytochrome P-450 isozymes of the house fly (Musca domestica) are discussed in relation to their role in resistance to xenobiotics. 4. Induction of insect cytochrome P-450 isozymes is discussed in comparison to induction in mammals and with reference to polyphagy.

Isolation and sequence of cDNA encoding a cytochrome P-450 from an insecticide-resistant strain of the house fly, Musca domestica

A cDNA expression library from phenobarbital-treated house fly (Musca domestica) was screened with rabbit antisera directed against partially purified house fly cytochrome P-450. Two overlapping clones with insert lengths of 1.3 and 1.5 kilobases were isolated. The sequence of a 1629-base-pair (bp) cDNA was obtained, with an open reading frame (nucleotides 81-1610) encoding a P-450 protein of 509 residues (Mr = 58,738). The insect P-450 protein contains a hydrophobic NH2 terminus and a 22-residue cysteine-containing fragment near the COOH terminus that is known to bind the heme; both of these features have been found in the more than five dozen vertebrate P-450 proteins of which the sequences are presently known. Interestingly, the termination codon UAA may be contained in a putative polyadenylylation signal (AAUAAA) of the mRNA. This P-450 protein exhibits the most similarity (27% amino acid positional identity) with mammalian proteins of the P450III family but qualifies as a member of another family, which we propose to designate the P450VI gene family. This cDNA and deduced protein sequence should provide important information in the study of evolution of the P-450 gene superfamily, as well as provide an important probe for studying the regulation of insect P-450 and understanding the molecular genetics of insecticide resistance.

Characterization and regulation of HMG-CoA reductase during a cycle of juvenile hormone synthesis

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase was characterized in cockroach corpora allata which produce insect juvenile hormone III (methyl-(10R)10,11-epoxy-3,7,11-tri-methyl-2E,6E-dodecadienoate ). HMG-CoA reductase is a microsomal enzyme dependent on NADPH and dithiothreitol (or glutathione) for activity. The enzyme selectively reduced (3S)-HMG-CoA to (3R)-mevalonate with an apparent KM of 7.6 microM. Mevinolin was a competitive inhibitor of HMG-CoA reductase with a KI of 2.4 nM. No evidence for a modulation of enzyme activity by phosphorylation was obtained. Levels of HMG-CoA reductase were not altered after incubation of the corpora allata with either mevinolin (to decrease isoprenoid flux) or with mevalonate or farnesol (to increase isoprenoid flux). Split pairs of corpora allata were used to compare JH III synthetic activity with HMG-CoA reductase activity during the cycle of JH III synthesis that controls vitellogenesis and oocyte growth in adult females. Both activities changed over 10-fold and peaked on day 5 after emergence/mating, but JH III synthesis did not parallel HMG-CoA reductase activity precisely thereafter. The half-life of HMG-CoA reductase measured in the presence of cycloheximide was significantly different between low and high activity glands and was not related to the half-life of JH III synthesis. The results suggest that HMG-CoA reductase should not be considered 'the rate-limiting enzyme' in juvenile hormone synthesis by Diploptera punctata corpora allata.

Purification and characterization of hyoscyamine 6 beta-hydroxylase from root cultures of Hyoscyamus niger L. Hydroxylase and epoxidase activities in the enzyme preparation

Hyoscyamine 6 beta-hydroxylase, a 2-oxoglutarate-dependent dioxygenase that catalyzes the hydroxylation of l-hyoscyamine to 6 beta-hydroxyhyoscyamine in the biosynthetic pathway leading to scopolamine [Hashimoto, T. & Yamada, Y. (1986) Plant Physiol. 81, 619-625] was purified 310-fold from root cultures of Hyoscyamus niger L. The enzyme has an average Mr of 41,000 as determined by gel filtration on Superose 12 and exhibited maximum activity at pH 7.8 l-Hyoscyamine and 2-oxoglutarate are required for the enzyme activity, with respective Km values of 35 microM and 43 microM. Fe2+, catalase and a reductant such as ascorbate significantly activated the enzyme. 2-Oxoglutarate was not replaced by any of ten other oxo acids tested, nor was Fe2+ by nine other divalent cations tested. The enzyme was inhibited moderately by EDTA, Tiron and various oxo acids and aliphatic dicarboxylic acids, and strongly by nitroblue tetrazolium and divalent cations Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+. Several pyridine dicarboxylates and o-dihydroxyphenyl derivatives inhibited the hydroxylase. Pyridine 2,4-dicarboxylate and 3,4-dihydroxybenzoate are competitive inhibitors with respect to 2-oxoglutarate with the respective Ki values of 9 microM and 90 microM. Several alkaloids with structures similar to l-hyoscyamine were hydroxylated by the enzyme at the C-6 position of the tropane moiety. The enzyme preparation also epoxidized 6,7-dehydrohyoscyamine, a hypothetical precursor of scopolamine, to scopolamine (Km 10 microM). This epoxidation reaction required the same co-factors as the hydroxylation reaction and the epoxidase activities were found in the same fractions with the hydroxylase activities during purification. Two possible pathways for scopolamine biosynthesis are discussed in the light of the hydroxylase and epoxidase activities found in the partially purified preparation of hyoscyamine 6 beta-hydroxylase.

camR, a negative regulator locus of the cytochrome P-450cam hydroxylase operon

A 4.27-kilobase insert from a HindIII DNA library of Pseudomonas putida carrying the CAM plasmid allowed coordinate expression of genes camD and camC under control of camR, an upstream regulator. The camC gene specifies cytochrome P-450cam, and camD specifies the 5-exo-alcohol dehydrogenase. A 1.38-kilobase deletion from the insert results in the constitutive expression of genes camC and camD; transformation in trans restores the substrate control, indicating that camR is a negative regulator.

Ultrastructure of corpora allata of known activity during the vitellogenic cycle in the cockroach Diploptera punctata

Ultrastructure was correlated with rates of juvenile hormone synthesis in corpora allata from females of the viviparous cockroach Diploptera punctata at seven daily intervals during the first vitellogenic cycle. Synthetic activity of the glands was determined by in vitro radiochemical assay before the glands were fixed for electron microscopic analysis. The cycle in rates of juvenile hormone synthesis progressed from about 20 pmol h-1 per gland pair (oocytes 0.60 mm long) to a maximum mean rate of 140 pmol h-1 per pair (oocytes 1.40-1.47 mm long) and declined to about 20 pmol h-1 per pair at ovulation (oocytes about 1.65 mm long). Conspicuous ultrastructural changes occurred with changing synthetic rates. In glands with increasing rates of synthesis, mitochondria showed less electron-dense matrix, greater diameter and more irregular shape. Smooth endoplasmic reticulum changed from easily seen to obscure tubules, networks, and vesicles. Rough endoplasmic reticulum appeared in longer, more curved segments. Newly formed autophagic vacuoles appeared in all glands of highest activity rates. In glands with decreasing rates of synthesis, the mitochondrial matrix became denser, width smaller, and shapes less irregular. Smooth endoplasmic reticulum again appeared tubular and distinct. Golgi complexes were more conspicuous. Rough endoplasmic reticulum in whorls and large numbers of autophagic vacuoles continued to be present.