Expression of genes encoding proteins involved in ecdysteroidogenesis in the female mosquito, Aedes aegypti

Molecular evolution of the insect Halloween family of cytochrome P450s: phylogeny, gene organization and functional conservation

The insect molting hormone, 20-hydroxyecdysone (20E), is a major modulator of the developmental processes resulting in molting and metamorphosis. During evolution selective forces have preserved the Halloween genes encoding cytochrome P450 (P450) enzymes that mediate the biosynthesis of 20E. In the present study, we examine the phylogenetic relationships of these P450 genes in holometabolous insects belonging to the orders Hymenoptera, Coleoptera, Lepidoptera and Diptera. The analyzed insect genomes each contains single orthologs of Phantom (CYP306A1), Disembodied (CYP302A1), Shadow (CYP315A1) and Shade (CYP314A1), the terminal hydroxylases. In Drosophila melanogaster, the Halloween gene spook (Cyp307a1) is required for the biosynthesis of 20E, although a function has not yet been identified. Unlike the other Halloween genes, the ancestor of this gene evolved into three paralogs, all in the CYP307 family, through gene duplication. The genomic stability of these paralogs varies among species. Intron-exon structures indicate that D. melanogaster Cyp307a1 is a mRNA-derived paralog of spookier (Cyp307a2), which is the ancestral gene and the closest ortholog of the coleopteran, lepidopteran and mosquito CYP307A subfamily genes. Evolutionary links between the insect Halloween genes and vertebrate steroidogenic P450s suggest that they originated from common ancestors, perhaps destined for steroidogenesis, before the deuterostome-arthropod split. Conservation of putative substrate recognition sites of orthologous Halloween genes indicates selective constraint on these residues to prevent functional divergence. The results suggest that duplications of ancestral P450 genes that acquired novel functions may have been an important mechanism for evolving the ecdysteroidogenic pathway.

Larval feeding duration affects ecdysteroid levels and nutritional reserves regulating pupal commitment in the yellow fever mosquito Aedes aegypti (Diptera: Culicidae)

What little is known about the endocrine regulation of mosquito development suggests that models based on Lepidoptera and Drosophila may not apply. We report on basic parameters of larval development and the commitment to metamorphosis in the yellow fever mosquito Aedes aegypti that are affected by varying the length of feeding time for last instar larvae. A critical mass for pupal commitment was achieved after 24 h of feeding by last instars, also the age at which tissue production and hemolymph titers of ecdysteroids are increasing. A greater proportion of last instars successfully pupated and eclosed as adults as the length of their feeding time increased. Less than 24 h of feeding time resulted in last instars that were developmentally arrested; these larvae tolerated starvation conditions for up to 2 weeks and retained the capacity to pupate if re-fed. Starvation tolerance may be a common trait among container-inhabiting species, and this period is an important factor to be considered for vectorial capacity and control measures. To distinguish cues for metamorphosis related to a larva's nutritional status versus its age, newly molted last instars were fed for different periods of time but sampled at the same age; ecdysteroid levels, body mass and nutrient reserves were then measured for each group. Our data suggest that metamorphic capacity is dependent on a larva's nutritional condition and not just the age at which ecdysteroid titers increase. Last instars that have fed for a particular length of time may initiate their metamorphic molt when both threshold levels of nutrient reserves and ecdysteroid titer have been met. Future studies will lead to a conceptual model specific for the nutritional and hormonal regulation of mosquito post-embryonic development. This model should facilitate the exploitation of current and novel insect growth regulators that are among favored strategies for vector population suppression.

Development of a real-time PCR assay for measurement of yellow protein mRNA transcription in the desert locust Schistocerca gregaria: a basis for isolation of a peptidergic regulatory factor

A major unresolved issue in insect endocrinology concerns the question of whether or not insects have sex hormones. Conclusive evidence in favor of the presence of such hormones awaits the establishment of appropriate bioassays in males. The cuticle of sexually mature males of the desert locust Schistocerca gregaria turns yellow in gregarious conditions only. Neither females nor isolated males ever turn yellow. The yellowing is due to the deposition in the cuticle of a male-specific Yellow Protein (YP), of which the amino acid sequence is known. In this paper, we describe the partial cloning of the cDNA encoding this Yellow Protein. The tissue distribution and temporal expression of the YP-mRNA is studied in detail using RT-PCR. Furthermore, an RT-PCR based bioassay was developed, which may serve as a reliable tool to help identify the hormones controlling the yellowing process. In addition to juvenile hormone, we have shown that a factor present in the brain-corpora cardiaca is involved in the yellow coloration, as injection of an extract induces the expression of YP-mRNA in isolated gregarious males.

Purification, kinetic characterization, and molecular cloning of a novel enzyme, ecdysteroid 22-kinase

This is the first report succeeding in the isolation and characterization of an enzyme and its gene involved in the phosphorylation of a steroid hormone. It has been demonstrated that ecdysteroid 22-phosphates in insect ovaries, which are physiologically inactive, serve as a "reservoir" that supplies active free ecdysteroids during early embryonic development and that their dephosphorylation is catalyzed by a specific enzyme, ecdysteroid-phosphate phosphatase (Yamada, R., and Sonobe, H. (2003), J. Biol. Chem. 278, 26365-26373). In this study, ecdysteroid 22-kinase (EcKinase) was purified from the cytosol of the silkworm Bombyx mori ovaries to about 1,800-fold homogeneity in six steps of column chromatography and biochemically characterized. Results obtained indicated that the reciprocal conversion of free ecdysteroids and ecdysteroid 22-phosphates by two enzymes, EcKinase and ecdysteroid-phosphate phosphatase, plays an important role in ecdysteroid economy of the ovary-egg system of B. mori. On the basis of the partial amino acid sequence obtained from purified EcKinase, the nucleotide sequence of the cDNA encoding EcKinase was determined. The full-length cDNA of EcKinase was composed of 1,850 bp with an open reading frame encoding a protein of 386 amino acid residues. The cloned cDNA was confirmed to encode the functional EcKinase using the transformant harboring the open reading frame of EcKinase. A data base search showed that EcKinase has an amino acid sequence characteristic of phosphotransferases, in that it harbors Brenner's motif and putative ATP binding sites, but there are no functional proteins that share high identity with the amino acid sequence of EcKinase.

Effects of larval nutrition on the endocrinology of mosquito egg development

Reproduction by female mosquitoes is dependent on energy resources but modulated by hormones. Our study focused on blood-meal-dependent, anautogenous Aedes aegypti and autogenous Ochlerotatus atropalpus that rely on larval-derived nutrient stores to develop eggs. To determine how larval nutrition affects the endocrinology of egg development in these females, we manipulated the quantity of larval food and measured in vitro production of juvenile hormone (JH) by corpora allata (CA) and ecdysteroids by ovaries. Newly emerged A. aegypti contain lower larval-derived protein reserves, and their CA produce high amounts of JH, in comparison with similarly staged Oc. atropalpus. Ecdysteroid production was initiated in newly emerged Oc. atropalpus females, which have higher protein reserves and which develop eggs without a blood meal, which is required by A. aegypti females to complete egg development.

Identification, characterization and developmental expression of Halloween genes encoding P450 enzymes mediating ecdysone biosynthesis in the tobacco hornworm, Manduca sexta

The insect molting hormone 20-hydroxyecdysone (20E) plays a central role in regulating gene expression during development and metamorphosis. In many Lepidoptera, the pro-hormone 3-dehydroecdysone (3DE), synthesized from cholesterol in the prothoracic gland, is rapidly converted to ecdysone (E) by a hemolymph reductase, and E is subsequently converted to 20E in various peripheral target tissues. Recently, four Drosophila melanogaster P450 enzymes, encoded by specific Halloween genes, were cloned and functionally characterized as mediating the last hydroxylation steps leading to 20E. We extended this work to the tobacco hornworm Manduca sexta, an established model for endocrinological and developmental studies. cDNA clones were obtained for three Manduca orthologs of CYP306A1 (phantom; phm, the 25-hydroxylase), CYP302A1 (disembodied; dib, the 22-hydroxylase) and CYP315A1 (shadow; sad, the 2-hydroxylase), expressed predominantly in the prothoracic gland during the fifth (final) larval instar and during pupal-adult development, with fifth instar mRNA levels closely paralleling the hemolymph ecdysteroid titer. The data indicate that transcriptional regulation of phm, dib and sad plays a role in the developmentally varying steroidogenic capacities of the prothoracic glands during the fifth instar. The consistent expression of the Halloween genes confirms the importance of the prothoracic glands in pupal-adult development. These studies establish Manduca as an excellent model for examining the regulation of the Halloween genes.

BmStart1, a novel carotenoid-binding protein isoform from Bombyx mori, is orthologous to MLN64, a mammalian cholesterol transporter

Carotenoid-binding protein (CBP) from the silkworm Bombyx mori is an essential molecule for carotenoid dependent cocoon pigmentation. We identified a novel isoform of CBP, Start1 of B. mori (BmStart1). BmStart1 contains a membrane-spanning MENTAL domain in its N-terminus and a lipid-binding START domain in its C-terminus. This domain architecture is identical to the mammalian MLN64 and Start1 of Drosophila melanogaster (DmStart1), both of which have been implicated to function in cholesterol transport and regulation of steroidogenesis. BmStart1 is expressed in both white and yellow cocoon strains of B. mori, while CBP is only detected in the yellow cocoon strain. BmStart1 mRNA abundance in the prothoracic gland, the main ecdysteroidogenic tissue, positively correlates with changes in the hemolymph ecdysteroid level. Genomic sequence analysis revealed that BmStart1 and CBP are generated from the same gene locus by alternative splicing. Splice site comparison and homology search indicate that BmStart1 is orthologous to both MLN64 and DmStart1. This study implies that alternative splicing of the BmStart1/CBP gene generates unique protein isoforms whose endogenous ligands, sterol or carotenoid, are structurally different.