Juvenile hormone receptors in insect larval epidermis: identification by photoaffinity labeling

A Life's Journey Through Insect Metamorphosis

This autobiographical article describes the research career of Lynn M. Riddiford from its early beginnings in a summer program for high school students at Jackson Laboratory to the present "retirement" at the Friday Harbor Laboratories. The emphasis is on her forays into many areas of insect endocrinology, supported by her graduate students and postdoctoral associates. The main theme is the hormonal regulation of metamorphosis, especially the roles of juvenile hormone (JH). The article describes the work of her laboratory first in the elucidation of the endocrinology of the tobacco hornworm, Manduca sexta, and later in the molecular aspects of the regulation of cuticular and pigment proteins and of the ecdysone-induced transcription factor cascade during molting and metamorphosis. Later studies utilized Drosophila melanogaster to answer further questions about the actions of JH.

Juvenile hormone regulation of Drosophila Epac--a guanine nucleotide exchange factor

Real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to characterize the effects of juvenile hormone (JH) on Epac (Exchange Protein directly Activated by Cyclic AMP; NM_001103732), a guanine nucleotide exchange factor for Rap1 in Drosophila S2 cells. JH treatment led to a rapid, dose-dependent increase in Epac relative expression ratio (RER) when compared to treatment with methyl linoleate (MLA) that lacks biological activity. The minimal level of hormone needed to elicit a response was 100 ng/ml. Time-course studies indicated a significant rise in the RER 1h after treatment. S2 cells were challenged with 20-hydroxyecdysone and a series of compounds similar in structure to JH to determine the specificity of the response. Methoprene and JH III displayed the greatest increases in RER. Late third instar (96 h) Drosophila were exposed to diet containing methoprene (500 ng/g diet); significantly higher RERs for Epac were observed 12h after exposure. JH had no effect on Epac RERs in the human cell line HEK-293.

The isolation of two juvenile hormone-inducible genes in Drosophila melanogaster

Juvenile hormone (JH) is an important regulator of both insect development and reproductive maturation. Although the molecular mechanism of JH action is not yet known, there is growing circumstantial evidence that JH directly regulates gene expression. In the absence of a JH target gene, however, this suggestion has remained speculative. Cultured Drosophila S2 cells have been used to identify genes whose expression is regulated by JH. Employing differential display we identified several genes whose transcripts accumulate in cells treated with the JH agonist methoprene. Two of the genes-JhI-1 and JhI-26-were cloned and characterized in detail. For both genes, transcripts showed rapid and specific induction in the presence of either methoprene or JHIII, but not in the presence of other biologically inactive compounds of similar chemical structure. Accumulation of JhI-1 and JhI-26 RNAs requires continuous hormone presence. The developmental expression of the two JH-inducible genes corresponds to the abundance profile of JH in vivo. Furthermore, topical methoprene application to pupae leads to the ectopic accumulation of JhI-1 and JhI-26 transcripts.

Insect pheromones--an overview of biosynthesis and endocrine regulation

This overview describes, compares, and attempts to unify major themes related to the biosynthetic pathways and endocrine regulation of insect pheromone production. Rather than developing and dedicating an entirely unique set of enzymes for pheromone biosynthesis, insects appear to have evolved to add one or a few tissue-specific auxiliary or modified enzymes that transform the products of "normal" metabolism to pheromone compounds of high stereochemical and quantitative specificity. This general understanding is derived from research on model species from one exopterygote insect order (Blattodea) and three endopterygote insect orders (Coleoptera, Diptera, and Lepidoptera). For instance, the ketone hydrocarbon contact sex pheromone of the female German cockroach, Blattella germanica, derives its origins from fatty acid biosynthesis, arising from elongation of a methyl-branched fatty acyl-CoA moiety followed by decarboxylation, hydroxylation, and oxidation. Coleopteran sex and aggregation pheromones also arise from modifications of fatty acid biosynthesis or other biosynthetic pathways, such as the isoprenoid pathway (e.g. Cucujidae, Curculionidae, and Scolytidae), or from simple transformations of amino acids or other highly elaborated host precursors (e.g. Scarabaeidae and Scolytidae). Like the sex pheromone of B. germanica, female-produced dipteran (e.g. Drosophilidae and Muscidae) sex pheromone components originate from elongation of fatty acyl-CoA moieties followed by loss of the carbonyl carbon and the formation of the corresponding hydrocarbon. Female-produced lepidopteran sex pheromones are also derived from fatty acids, but many moths utilize a species-specific combination of desaturation and chain-shortening reactions followed by reductive modification of the carbonyl carbon. Carbon skeletons derived from amino acids can also be used as chain initiating units and elongated to lepidopteran pheromones by this pathway (e.g. Arctiidae and Noctuidae). Insects utilize at least three hormonal messengers to regulate pheromone biosynthesis. Blattodean and coleopteran pheromone production is induced by juvenile hormone III (JH III). In the female common house fly, Musca domestica, and possibly other species of Diptera, it appears that during hydrocarbon sex pheromone biosynthesis, ovarian-produced ecdysteroids regulate synthesis by affecting the activities of one or more fatty acyl-CoA elongation enzyme(s) (elongases). Lepidopteran sex pheromone biosynthesis is often mediated by a 33 or 34 amino acid pheromone biosynthesis activating neuropeptide (PBAN) through alteration of enzyme activities at one or more steps prior to or during fatty acid synthesis or during modification of the carbonyl group. Although a molecular level understanding of the regulation of insect pheromone biosynthesis is in its infancy, in the male California fivespined ips, Ips paraconfusus (Coleoptera: Scolytidae), JH III acts at the transcriptional level by increasing the abundance of mRNA for 3-hydroxy-3-methylglutaryl-CoA reductase, a key enzyme in de novo isoprenoid aggregation pheromone biosynthesis.

Juvenile hormone: the status of its "status quo" action

Juvenile hormone (JH) allows larval molting in response to ecdysteroids but prevents the switching of gene expression necessary for metamorphosis. I first review our efforts to isolate the nuclear receptor for JH in the larval epidermis of Manduca sexta using photoaffinity analogs and our recent findings that the molecule isolated does not bind JH I with high affinity. The reported apparent high affinity binding of JH I by the recombinant 29 kDa protein (rJP29) was artifactual due to the presence of contaminating esterases. Purified rJP29 bound little detectable JH I, but its binding of the photoaffinity analog was prevented by JH I as well as other isoprenoids, indicating a low affinity for these compounds. Our recent studies focus on the effects of JH on the early molecular events induced by 20-hydroxyecdysone (20E). Culture of day 2 5th larval epidermis with 10(-6)M 20E for 24 h caused first pupal commitment, then the onset of the predifferentiative events necessary for pupation. Biphasic increases in the mRNAs of the two isoforms of the ecdysone receptor (EcR-A and EcR-B1) and of E75A, an ecdysteroid-induced transcription factor, coincided with these two phases. The mRNAs for Ultraspiracle (USP) and the metamorphosis-specific Broad-Complex (BR-C) increased only during the second phase. The presence of JH had no effect on the initial increases in EcR mRNAs but caused an increased accumulation of E75A mRNA. This JH also prevented the later changes in EcR, USP, and BR-C mRNAs. Thus, JH influences only certain of the early actions of 20E which then result in its preservation of the "status quo."

Ecdysteroid-dependent protein synthesis in caste-specific development of the larval honey bee ovary

In the honey bee, Apis mellifera, the fifth larval instar is a critical period for caste differentiation. During this premetamorphic phase the hormonal milieu shows pronounced caste differences and several organs, particularly the ovaries, enter different developmental pathways leading to highly fertile queens and nearly sterile workers. Developmental profiles of total protein synthesis in larval ovaries showed marked caste differences starting with the early fifth instar. By two-dimensional electrophoresis, caste-specific patterns could be detected in the synthesis of a 29 kDa/pI 4.6 and two 24 kDa/pI 5.2-5.5. proteins (pI=isoelectric point). A marked decrease in the expression of these proteins was found to coincide with caste-specific differences in the haemolymph ecdysteroid titer. In vitro exposure of larval worker ovaries to physiological (10^-7 M) concentrations of synthetic makisterone A elicited an identical response. Juvenile hormone did not affect protein synthesis patterns in larval ovaries, and also did not inhibit or reverse the ecdysteroid-induced effects. Heat shock experiments revealed that the 29 kDa/pI 4.6 ecdysteroid-regulated protein belongs to the class of small heat shock proteins.

A nuclear juvenile hormone-binding protein from larvae of Manduca sexta: a putative receptor for the metamorphic action of juvenile hormone

A 29-kDa nuclear juvenile hormone (JH)-binding protein from the epidermis of Manduca sexta larvae was purified by using the photoaffinity analog for JH II ([3H]epoxyhomofarnesyldiazoacetate) and partially sequenced. A 1.1-kb cDNA was isolated by using degenerate oligonucleotide primers for PCR based on these sequences. The cDNA encoded a 262-amino acid protein that showed no similarity with other known proteins, except for short stretches of the interphotoreceptor retinoid-binding protein, rhodopsin, and human nuclear protein p68. Recombinant baculovirus containing this cDNA made a 29-kDa protein that was covalently modified by [3H]epoxyhomofarnesyldiazoacetate and specifically bound the natural enantiomer of JH I (Kd = 10.7 nM). This binding was inhibited by the natural JHs but not by methoprene. Immunocytochemical analysis showed localization of this 29-kDa protein to epidermal nuclei. Both mRNA and protein are present during the intermolt periods; during the larval molt, the mRNA disappears but the protein persists. Later when cells become pupally committed, both the mRNA and protein disappear with a transient reappearance near pupal ecdysis. The properties of this protein are consistent with its being the receptor necessary for the antimetamorphic effects of JH.

Ligand binding by a recombinant insect juvenile hormone binding protein

A cDNA for the hemolymph juvenile hormone binding protein (JHBP) of larval Manduca sexta has been isolated, sequenced, and expressed in an insect cell line. A recombinant baculovirus, containing the JHBP cDNA fused to the p10 promoter of Autographa californica nuclear polyhedrosis virus, was constructed. Insect cells (Sf9) infected with this virus secreted recombinant JHBP (rJHBP) into the medium (> 50 micrograms/mL), and cotranslational removal of an 18 amino acid leader sequence was observed. rJHBP was cross-reactive with an antiserum prepared to the hemolymph JHBP and was specifically labeled by [3H]EHDA, a photoaffinity analog of JH II, demonstrating that rJHBP was an isoform of the previously reported 32-kDa JHBP [Lerro, K. A., & Prestwich, G.D. (1990) J. Biol. Chem. 265, 19800-19806]. rJHBP was purified from insect cell medium to homogeneity by ion-exchange and gel-filtration chromatography. The purified rJHBP had a higher affinity (KD = 11 nM for JH I and KD = 42 nM for JH II) than that reported for crude hemolymph JHBP (KD = 80 nM for JH I). The circular dichroism (CD) spectrum of purified rJHBP indicated 34% alpha-helix and 23% beta-sheet. The CD spectra of rJHBP in the presence and absence of JH II were the same, indicating no change in secondary structure induced by ligand binding. Thus, the rJHBP expressed in insect cells binds JHs and is suitable for structural and functional analysis.

Nucleotide sequence of an adult-specific cuticular protein gene from the beetle Tenebrio molitor: effects of 20-hydroxyecdysone on mRNA accumulation

The accumulation of transcripts from two adult-specific cuticular genes (ACP-20 and ACP-22) is shown to be modified after addition of exogenous 20-hydroxyecdysone. In the continuous presence of high levels of the hormone, the expression of ACP-20 gene is significantly weaker than that of untreated controls, while ACP-22 expression is 2.5-fold increased. During active synthesis of the ACP messages, a 0.5 microg 20-hydroxyecdysone injection causes a rapid 2-fold increase in ACP-22 mRNA and is not able to repress ACP-20 mRNA accumulation. We conclude that these genes whose transcripts appear in an almost coordinated manner in epidermal cells during the moulting cycle are regulated by ecdysteroids in a different way. In order to undertake a functional dissection of the promoter regions of ACP-22 gene, we have isolated and sequenced a genomic clone. The sequence similarities with other cuticular protein genes are discussed.

Regulation of cricket phonotaxis through hormonal control of the threshold of an identified auditory neuron

1. The phonotactic threshold of 3 to 5-day-old adult female Acheta domesticus and the threshold of the L1 auditory neuron drop progressively (Fig. 1). 2. Application of juvenile hormone III (JHIII) to 1-day-old females caused both the female's threshold for phonotaxis and the threshold of the L1 auditory neuron to drop 20 or more dB over the next 12 h (Figs. 3-4). 3. JHIII's effect on phonotactic threshold could be blocked by injection with a transcription (alpha-amanitin) or a translation blocker (emetine, Fig. 3). 4. Injection of emetine also prevented the JHIII induced drop in L1's threshold (Fig. 4). 5. Application of JHIII to the surface of, or microinjection of JHIII into one prothoracic hemiganglion caused the female to circle phonotactically away from the side of hormone addition at thresholds 25 to 35 dB lower than the pre-JHIII addition threshold within 2 h (Fig. 6). 6. Application of JHIII to the surface of both prothoracic hemiganglia, accompanied by microinjection of emetine into one hemiganglion resulted in the female emetine into one hemiganglion resulted in the female circling phonotactically toward the side receiving emetine injection, with a 25 to 35 dB drop in threshold (Fig. 6).