Effects of juvenile hormone, ecdysteroids and nutrition on larval hemolymph protein gene expression in Galleria mellonella

Characterization and expression of the Hex 110 gene encoding a glutamine-rich hexamerin in the honey bee, Apis mellifera

An N-terminal amino acid sequence of a previously reported honey bee hexamerin, HEX 110 [Danty et al., Insect Biochem Mol Biol 28:387-397 (1998)], was used as reference to identify the predicted genomic sequence in a public GenBank database. In silico analysis revealed an ORF of 3,033 nucleotides that encompasses eight exons. The conceptual translation product is a glutamine-rich polypeptide with a predicted molecular mass of 112.2 kDa and pI of 6.43, which contains the conserved M and C hemocyanin domains. Semiquantitative and quantitative RT-PCR with specific primers allowed for an analysis of mRNA levels during worker bee development and under different physiological conditions. Concomitantly, the abundance of the respective polypeptide in the hemolymph was examined by SDS-PAGE. Hex 110 transcripts were found in high levels during the larval stages, then decreased gradually during the pupal stage, and increased again in adults. HEX 110 subunits were highly abundant in larval hemolymph, decreased at the spinning-stage, and remained at low levels in pupae and adults. In 5th instar larvae, neither starvation nor supplementation of larval food with royal jelly changed the Hex 110 transcript levels or the amounts of HEX 110 subunit in hemolymph. In adult workers, high levels of Hex 110 mRNA, but not of the respective subunit, were related to ovary activation, and also to the consumption of a pollen-rich diet.

Larval nutrition affects lipid storage and growth, but not protein or carbohydrate storage in newly eclosed adults of the grasshopper Schistocerca americana

Nitrogen availability from dietary protein can have profound effects on the physiology and evolutionary ecology of insect herbivores. While many studies consider the effects of nutrition on consumption and gross body composition of protein and other important nutrients, few consider partitioning to storage for future use. I used chemically defined artificial diets to quantitatively manipulate the amount of dietary carbohydrates and proteins available to growing larvae of the grasshopper Schistocerca americana to determine how larval nutrient availability affects growth and all three classes of stored nutrients (proteins, lipids, and carbohydrates) carried over from larval feeding into adulthood. Larvae on poor diets increased consumption, but could not compensate for diet quality, eclosing small and containing no significant nutrient stores at adulthood. Individuals fed intermediate to high nutrient content diets as larvae were significantly larger and contained a significantly greater proportion of lipid stores at adult eclosion, but not protein or carbohydrate stores than individuals fed low nutrient content diets. This suggests that larvally derived lipid stores may be more important to adult fitness than carbohydrate or protein stores. This result is contrary to previous studies performed on the role of larval nutrition and allocation to protein stores, and this difference is likely due to variation in the relative availability of protein in adult diets across species.

Molecular cloning and expression of a hexamerin cDNA from the honey bee, Apis mellifera

A cDNA encoding a hexamerin subunit of the Africanized honey bee (Apis mellifera) was isolated and completely sequenced. In the deduced translation product we identified the N-terminal sequence typical of the honey bee HEX 70b hexamerin. The genomic sequence consists of seven exons flanked by GT/AT exon/intron splicing sites, which encode a 683 amino acid polypeptide with an estimated molecular mass of 79.5 kDa, and pI value of 6.72. Semi-quantitative RT-PCR revealed high levels of Hex 70b message in larval stages, followed by an abrupt decrease during prepupal-pupal transition. This coincides with decaying titers of juvenile hormone (JH) and ecdysteroids that is the signal for the metamorphic molt. To verify whether the high Hex 70b expression is dependent on high hormone levels, we treated 5th instar larvae with JH or 20-hydroxyecdysone (20E). In treated larvae, Hex 70b expression was maintained at high levels for a prolonged period of time than in the respective controls, thus indicating a positive hormone regulation at the transcriptional level. Experiments designed to verify the influence of the diet on Hex 70b expression showed similar transcript amounts in adult workers fed on a protein-enriched diet or fed exclusively on sugar. However, sugar-fed workers responded to the lack of dietary proteins by diminishing significantly the amount of HEX 70b subunits in hemolymph. Apparently, they use HEX 70b to compensate the lack of dietary proteins.

Response of storage protein levels to variation in dietary protein levels

Storage proteins have been found to play a major role in insect metamorphosis and egg production and are accumulated during the actively feeding larval stage. Yet few studies have focused on how nutrition affects storage protein levels. Three storage proteins were identified in male and female Heliothis virescens pupae, one arylphorin and two putative high-methionine hexamers. Storage proteins were quantified in early pupae and in pharate adults. Storage protein levels peaked in 48-h pupae and were more abundant in females across all stages. Both male and female pharate adults retained a portion of total storage protein levels and females retained greater levels overall. In females, post-eclosion protein reserves will likely be used toward egg manufacturing, while the role of protein reserves in males remains speculative. In our previous study of H. virescens larvae, we found that protein-derived growth in females progressively increased as dietary protein levels increased. Our present data show that levels of storage protein also increased progressively along with dietary protein levels. This suggests that females allocated protein, in excess of adult tissue formation needs, toward storage protein. Our study is the first to demonstrate how responsive storage protein levels can be in face of varying levels of dietary protein.

Ligand regulation of juvenile hormone binding protein mRNA in mutant Manduca sexta

Insect hemolymph juvenile hormone binding protein (hJHBP) regulates peripheral titers of its ligands, the juvenile hormones. In larvae of the black (bl) strain of the tobacco hornworm, Manduca sexta, treatment with small doses of juvenile hormone I (JH I) can also regulate titers of hJHBP. To further investigate this regulation, responsiveness of hJHBP mRNA expression to JH I was characterized in vivo. RNA analyzes revealed that transcript levels in fat body, the site of hJHBP synthesis, increased fivefold within several hours of treatment with physiological doses of hormone and remained elevated for approximately 16 h. Sensitivity to JH treatment was found to vary temporally. To ensure transcript identity, a wild-type cDNA clone and a bl RT-PCR fragment were sequenced and found to be 99% homologous. Together, these results suggest that JH participates in regulating expression of its transport protein in bl larvae by modifying the in vivo abundance of hJHBP's mRNA transcript.

Molecular cloning and expression of two hexamerin cDNAs from the mosquito, Aedes aegypti

Fourth-instar larvae of Aedes aegypti synthesize two types of hexamerins, Hexamerin-1 (AaHex-1) and Hexamerin-2 (AaHex-2), whose subunits are distinguished by different methionine and aromatic amino acid contents. In early female pupae only the methionine-rich AaHex-1gamma subunit accumulates to two-fold higher levels than in males. To investigate the relationship between hexamerin structure and the roles of Hex-1 and Hex-2 during mosquito development and reproduction, we have cloned and sequenced cDNAs encoding the AaHex-2alpha, -2beta and AaHex-1gamma subunits. Comparison with other insect hexamerins revealed that the Aedes Hex-1 and Hex-2 proteins belong, respectively, to the two hexamerin subfamilies previously defined for brachyceran Diptera. Probes specific for the Hex-2alpha and Hex-1gamma transcripts showed that expression of both genes follows the same developmental timetable. However, greater Hex-1gamma mRNA accumulation may contribute to the higher levels of Hex-1 gamma protein in early female pupae.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

Differential accumulation and tissue distribution of mosquito hexamerins during metamorphosis

The pupal hexamerins were characterized for two mosquitoes representative of the culicine and anopheline families, Aedes aegypti and Anopheles gambiae. Like higher Diptera, both mosquito species express two types of hexamerins, Hex-1 and Hex-2, whose subunits are distinguished by different levels of methionine and aromatic amino acids. In A. aegypti there are two heterohexamers, AaHex-1 and AaHex-2. In A. gambiae there are two homohexamers, AgHex-1.1 and AgHex-1.2, and one heterohexamer, AgHex-2. These hexamerins are rich in aromatic residues, with 18-23% Phe + Tyr for Hex-1 subunits and 13-17% Phe + Tyr for Hex-2 subunits. In addition, both mosquito species synthesize methionine-rich Hex-1 subunits: Aedes AaHex-1 gamma (8% met) and Anopheles AgHex-1.1 (3.9% met). Aedes Hex-1 and Hex-2 proteins exhibit different, stage-specific tissue distributions: AaHex-2 is the primary hexamerin of late larval hemolymph whereas AaHex-1 is the most important non-hemolymph protein of early pupae. Although both proteins are stored in the pupal fat body, peak AaHex-1 levels are 2-fold higher. Both pupal protein levels decline rapidly between 25 and 36 h after pupation. Furthermore, AaHex-1 not only reaches peak values in female Aedes pupae later than in males, but the methionine-rich AaHex-1 gamma subunit level is specifically higher in females. These observations suggest different roles for Hex-1 and Hex-2 during mosquito development.

Nucleotide sequence, structure and developmental regulation of LHP82, a juvenile hormone-suppressible hexamerin gene from the waxmoth, Galleria mellonella

We cloned and sequenced a composite cDNA corresponding to the 2.6 kb last instar-specific, juvenile hormone-suppressible Lhp82 mRNA from Galleria mellonella. The identity of the cDNA was confirmed by N-terminal amino acid sequencing of the purified Lhp82 subunit. In addition, we sequenced all coding regions and most of the intronic DNA as well as 1300 nucleotides of 5' flanking DNA from the Lhp82 gene. The eight exons of the Lhp82 gene specify a pre-protein of 706 residues, including the signal peptide of 18 amino acids. The deduced amino acid sequence of Lhp82 contains four potential N-glycosylation sites, and the calculated isoelectric point and molecular weight of secreted Lhp82 are pI = 6.43 and 79.9 kDa, respectively. Inspection of the 5' flanking and intronic regions of Lhp82 DNA revealed a 301 nucleotide cassette in intron 6 that appears to be a recently inserted repetitive element. We also performed Northern blot and nuclear run-off transcription experiments in order to determine the basis for Lhp82 gene inactivity after day 2 of the pupal stage. The results of these studies indicate that Lhp82 transcription is permanently shut off by the ecdysteroid pulse which occurs in the absence of juvenile hormone beginning 24 h post-pupation.

Juvenile hormone action to suppress gene transcription and influence message stability

Proteins normally expressed in high abundance only at larval-pupal metamorphosis in Trichoplusia ni were examined in a comparative analysis of the role and level of hormonal control of their expression. Some related proteins in the hemocyanin-superfamily (i.e., an acidic protein [AJHSP1] and two basic proteins [BJHSP1, BJHSP2]) were shown by nuclear run-on analysis to be specifically transcriptionally suppressed by juvenile hormone (JH), while transcription of another member of that family which is also metamorphosis-associated (arylphorin) was not specifically sensitive to JH. The stability of the mRNA for those members transcriptionally down-regulated by JH appeared to decrease under high JH conditions. While each protein was resorbed to some extent by the prepupal fat body, only the two basic proteins were quantitatively cleared from prepupal hemolymph. The JH-sensitive proteins studied appear to be encoded in single copy genes not immediately juxtaposed in the genome. These and previous studies now permit a more comprehensive understanding of the different combinations of mechanisms involving transcription, mRNA stability, translation, and protein clearance that operate to regulate these metamorphosis-associated proteins.

Regulation of expression of arylphorin and female-specific protein mRNAs in the tobacco hornworm, Manduca sexta

Two non-cross-hybridizing cDNA clones were isolated from a lambda gt11 cDNA library prepared from Day 2 fifth instar female fat body of Manduca sexta and shown by hybrid selection to code respectively for the two storage proteins arylphorin and female-specific protein (FSP). Analysis of the developmental expression of arylphorin showed its presence during the feeding phases of the penultimate (fourth) and final (fifth) larval instars and its absence during the molt. Abdominal ligation of larvae followed by infusion of Grace's medium showed that this amino acid-rich medium was able to maintain arylphorin expression in fourth instar larvae, but not continued high expression in fifth instar larvae. This nutrient medium however was sufficient to allow initiation of expression in newly ecdysed fifth larval abdomens. Infusion of 5 micrograms 20-hydroxyecdysone (20HE) caused a significant reduction of arylphorin RNA in ligated fourth larval abdomens, whereas 50 micrograms was required in Day 2 fifth larval abdomens to suppress this RNA. Thus, both the lack of incoming nutrients and the rising titer of ecdysteroid contribute to the loss of arylphorin mRNA at the molts and at wandering. By contrast, FSP mRNA was first detected in females on Day 2 of the fifth instar, but not in males until wandering, and then was present throughout the prepupal period. In females allatectomy caused the precocious appearance of FSP mRNA which was prevented by application of 10 micrograms methoprene, a juvenile hormone analog. Expression of FSP mRNA in males however appeared to be independent of hormonal milieu.

Synthesis of two storage proteins during larval development of the tobacco hornworm, Manduca sexta

Studies of synthesis and accumulation of the two storage proteins arylphorin and female-specific protein (FSP) during the final two larval instars of the tobacco hornworm showed both stage and temporal specificity. Arylphorin was present in both stages, but its synthesis ceased during the molt, during starvation, and at the wandering stage, and then resumed about 24 hr after the onset of feeding. During the larval molt about 25% of injected iodinated arylphorin was incorporated into the newly forming fifth instar cuticle. The cessation of arylphorin synthesis was mimicked by exposure of the fat body to 1 microgram/ml 20-hydroxyecdysone (20HE) in complete Grace's medium or to dilutions of Grace's medium greater than 50%. Lower concentrations of 20HE were ineffective, indicating that the cessation of synthesis in vivo was likely due to a combination of lack of excess nutrients and the hormonal milieu. The female-specific protein was not synthesized until the final larval instar, appearing first in females on Day 2 and later in males at the time of wandering, with synthesis continuing throughout the prepupal period. In vitro studies showed that this protein was synthesized as a 620-kDa protein, and then during secretion a 730-kDa immunoreactive form also appeared. Synthesis of FSP was inhibited by exposure of Day 2 fat body to 1 microgram/ml 20HE for 24 hr. Ligation followed by 20HE infusion showed that the disappearance of FSP from the hemolymph during the prepupal period was controlled by the rising ecdysteroid titer.

Role of hormones in starvation-induced delay in larval hemolymph protein gene expression in Galleria mellonella

Developmental Northern analysis of larval hemolymph protein gene transcripts in Galleria mellonella showed that Lhp76 is expressed all through the larval life, while Lhp82 is expressed only during the last larval stadium. Neither transcript was detectable in pharate adults and adult moths. In addition, these transcripts were not detectable during the first 48 h of the last larval stadium. Experimental analyses of the effects of the nutritional state and the hormonal levels of the last instar larvae on LHP gene expression suggests that the nutritional state of the larva has no direct effect, but nutrition acts through its effect on the hormone titers. Larvae starved after the first 24 h of the stadium and those fed on a non-protein diet produced both the transcripts on schedule on day 3. However, starvation during the first 24 or 48 h or feeding them on agar caused a marked delay in activation of Lhp82. These starved larvae pupated 4 to 12 days later than controls. Furthermore, JH titers in the starved larvae remain high even 5 days after ecdysis into the last instar. Prothorax-ligation of the starved and the starved-refed larvae accelerated production of the transcripts, thereby suggesting that the nutritional state does not directly affect LHP gene expression. Application of JH 1 to the ligated preparations resulted in selective blocking of Lhp82, while 20-OH ecdysone affected both the genes, thereby supporting the view that the insect hormones play an important role in stage-specific expression of LHP genes.