Stage and segment specificity of the secretory cell of the dermal glands of the tobacco hornworm, Manduca sexta

Gene expression in Verson's glands of the fall armyworm suggests their role in molting and immunity

Verson's glands are segmental pairs of dermal glands attached to the epidermis in lepidopteran larvae. They produce macromolecules during intermolt period and empty them during each molt. Morphological, histochemical, developmental, and protein analysis studies have been conducted to determine the functions of Verson's glands. However, the exact role of Verson's glands remains unclear. In our previous study, a strain of transgenic fall armyworm, Spdoptera frugiperda expressing green fluorescence protein (GFP) and Systemic RNA interference defective protein 1 (SID1) from Caenorhabditis elegans was established to improve RNA interference (RNAi) efficiency. Unexpectedly, we found that GFP fluorescence was significantly brighter in Verson's glands than in other tissues. Also, RNAi efficiency improved more in Verson's glands than in other tissues. We took advantage of improved RNAi efficiency to explore the function of Verson's glands. RNA-seq analysis revealed that genes highly expressed in Verson's glands code for cuticular proteins, molting fluid proteins, hemolymph proteins, and antimicrobial peptides. Injection of dsRNA targeting essential genes, inhibitor of apoptosis (IAP), Actin, and vacuolar-type ATPase (VATPase) interfered with Verson's glands growth. These results revealed that Verson's glands may contribute to hemolymph, cuticle, molting fluid, and immune response during molting. This study also provide useful tools for future research in identifying the physiological role of Verson's glands in lepidopteran insects.

Segmental pairs of dermal secretory cells release proteins into the hemolymph at the larval-pupal molt

At each molt of Manduca, the large dermal secretory cells expel the protein contents of their vacuoles into the hemocoel. The constellation of proteins expelled at the last larval-pupal molt, however, differs qualitatively from those proteins released at earlier larval-larval molts. Secretory cells at the two stages not only have different lectin staining properties but also have different proteins that separate on two-dimensional gels. Numerous physiological changes accompany the termination of the last larval instar, including increased chitin synthesis, diminished oxygen delivery, and reduced humoral immunity. Secretion of trehalase that is essential for chitin synthesis and the release of hypoxia up-regulated protein to ameliorate oxygen deprivation help ensure normal transition from larva to pupa. Proteins released by dermal secretory cells at this last molt could supplement the diminished immune defenses mediated by fat body and hemocytes at the end of larval life. Additional immune defenses provided by dermal secretory cells could help ensure a safe transition during a period of increased vulnerability for the newly molted pupa with its soft, thin cuticle and reduced mobility.

Complex steroid-peptide-receptor cascade controls insect ecdysis

Insect ecdysis sequence is composed of pre-ecdysis, ecdysis and post-ecdysis behaviors controlled by a complex cascade of peptide hormones from endocrine Inka cells and neuropeptides in the central nervous system (CNS). Inka cells produce pre-ecdysis and ecdysis triggering hormones (ETH) which activate the ecdysis sequence through receptor-mediated actions on specific neurons in the CNS. Multiple experimental approaches have been used to determine mechanisms of ETH expression and release from Inka cells and its action on the CNS of moths and flies. During the preparatory phase 1-2 days prior to ecdysis, high ecdysteroid levels induce expression of ETH receptors in the CNS and increased ETH production in Inka cells, which coincides with expression of nuclear ecdysone receptor (EcR) and transcription factor cryptocephal (CRC). However, high ecdysteroid levels prevent ETH release from Inka cells. Acquisition of Inka cell competence to release ETH requires decline of ecdysteroid levels and beta-FTZ-F1 expression few hours prior to ecdysis. The behavioral phase is initiated by ETH secretion into the hemolymph, which is controlled by two brain neuropeptides-corazonin and eclosion hormone (EH). Corazonin acts on its receptor in Inka cells to elicit low level ETH secretion and initiation of pre-ecdysis, while EH induces cGMP-mediated ETH depletion and consequent activation of ecdysis. The activation of both behaviors is accomplished by ETH action on central neurons expressing ETH receptors A and B (ETHR-A and B). These neurons produce numerous excitatory or inhibitory neuropeptides which initiate or terminate different phases of the ecdysis sequence. Our data indicate that insect ecdysis is a very complex process characterized by two principal steps: (1) ecdysteroid-induced expression of receptors and transcription factors in the CNS and Inka cells. (2) Release and interaction of Inka cell peptide hormones and multiple central neuropeptides to control consecutive phases of the ecdysis sequence.

Molecular cloning and function of ecdysis-triggering hormones in the silkwormBombyx mori

Inka cells of the epitracheal endocrine system produce peptide hormones involved in the regulation of insect ecdysis. In the silkworm Bombyx mori, injection of Inka cell extract into pharate larvae, pupae or adults activates the ecdysis behavioural sequence. In the present study, we report the identification of three peptides in these extracts, pre-ecdysis-triggering hormone (PETH), ecdysis-triggering hormone (ETH) and ETH-associated peptide(ETH-AP), which are encoded by the same cDNA precursor. Strong immunoreactivity associated with each peptide in Inka cells prior to ecdysis disappears during each ecdysis, indicating complete release of these peptides. Injection of either PETH or ETH alone is sufficient to elicit the entire ecdysis behavioural sequence through the direct action on abdominal ganglia;cephalic and thoracic ganglia are not required for the transition from pre-ecdysis to ecdysis behaviour. Our in vitro data provide evidence that these peptides control the entire ecdysis behavioural sequence through activation of specific circuits in the nervous system. Ecdysis of intact larvae is associated with the central release of eclosion hormone (EH) and elevation of cyclic 3′,5′-guanosine monophosphate (cGMP) in the ventral nerve cord. However, injection of ETH into isolated abdomens induces cGMP elevation and ecdysis behaviour without a detectable release of EH,suggesting that an additional central factor(s) may be involved in the activation of this process. Our findings provide the first detailed account of the natural and hormonally induced behavioural sequence preceding larval,pupal and adult ecdyses of B. mori and highlight significant differences in the neuro-endocrine activation of pre-ecdysis and ecdysis behaviours compared with the related moth, Manduca sexta.

Dual ecdysteroid action on the epitracheal glands and central nervous system preceding ecdysis of Manduca sexta

Initiation of the ecdysis behavioural sequence in insects requires activation of the central nervous system (CNS) by pre-ecdysis-triggering hormone (PETH) and ecdysis-triggering hormone (ETH), which are released from the Inka cells of the epitracheal glands. Here, we show that the developmental events preceding larval and pupal ecdysis of Manduca sexta involve a dual action of ecdysteroids on the epitracheal glands and CNS. The low steroid levels in freshly ecdysed and feeding larvae are associated with small-sized epitracheal glands, reduced peptide production in Inka cells and insensitivity of the CNS to ETH. The elevated ecdysteroid levels before each ecdysis lead to a dramatic enlargement of Inka cells and increased production of peptide hormones and their precursors. As blood ecdysteroids reach peak levels, the CNS becomes responsive to Inka cell peptides. These effects of natural ecdysteroid pulses can be experimentally induced by injection of 20-hydroxyecdysone or the ecdysteroid agonist tebufenozide (RH-5992) into ecdysed larvae, thus stimulating peptide production in Inka cells and inducing CNS sensitivity to ETH. A direct steroid action on the CNS is demonstrated by subsequent treatment of isolated nerve cords from ecdysed larvae with 20-hydroxyecdysone and ETH, which results in pre-ecdysis or ecdysis bursts. Our data show that ecdysteroid-induced transcriptional activity in both the epitracheal glands and the CNS are necessary events for the initiation of the ecdysis behavioural sequence.

Juvenile hormone acid and ecdysteroid together induce competence for metamorphosis of the Verson's gland in Manduca sexta

In the last larval instar of Lepidoptera, ecdysteroid in the absence of juvenile hormone (JH) is believed to cause the shift from larval to pupal development. In Manduca sexta, tissues such as the Verson's gland and crochet epidermis become pupally committed before the earliest pulse of ecdysteroid that occurs on day 2. What causes the change in commitment in these tissues? First it was necessary to determine at what stage these tissues become competent to express the pupal program. Last instar larvae of different ages were induced to molt prematurely by feeding the ecdysteroid analog RH5992 and Verson's gland proteins were analyzed by SDS-polyacrylamide gel electrophoresis. Glands became competent to make pupal proteins between 24 and 32 h after the last larval ecdysis. Next, hormonal regulation of competence was examined in ligated abdomens of 12h last instar larvae. Treatment with JH II acid or methoprene acid plus a low dose (1/50th of the molt inducing dose) of RH5992 induced competence, whereas RH5992 alone, methoprene acid alone or methoprene plus RH5992 did not. Verson's glands maintained in vitro produced pupal proteins in response to methoprene acid together with RH5992 but not with RH5992 alone. Likewise, crochet epidermis lost the ability to make crochets (metamorphic change) only in isolated abdomens treated with JH II acid or methoprene acid and low doses of RH5992. In conclusion, JH acid in the presence of basal levels of ecdysteroid induces tissue competence for metamorphosis. Metamorphic competence is followed by commitment, induced by a small pulse of ecdysteroid in the absence of JH, and finally by expression caused by a high titer of ecdysteroid. It is proposed that JH acid is an essential metamorphic hormone.

Juvenile hormone acid: evidence for a hormonal function in induction of vitellogenin in larvae of Manduca sexta

In the tobacco hornworm (Manduca sexta), vitellogenin (Vg), the major yolk protein precursor, and its mRNA are first detectable in the prepupal stage; and production of both can be enhanced by methoprene, a juvenile hormone (JH) analog. Competence to respond to methoprene is acquired after ecdysteroid-initiated commitment for metamorphosis. Here we show that acquisition of competence requires prior exposure to JH-II acid in addition to ecdysteroid. Application of 20-hydroxyecdysone or RH5992, an ecdysteroid analog, to isolated abdomens from feeding larvae (precommitment) results in exposure of the dorsal vessel (EDV), a sign of metamorphic commitment--but such abdomens do not make Vg in response to methoprene. However, injection of JH-II acid along with 20-hydroxyecdysone into isolated abdomens causes Vg production in response to methoprene. Methoprene acid similarly induces competence to respond to methoprene. Northern blot analysis confirmed that Vg transcripts are present in fat body only if isolated abdomens were pretreated with both ecdysteroid, and JH-II acid or methoprene acid. The latter two can induce competence even in precocious prepupae resulting from removal of the corpora allata (the glands that produce JH) from early penultimate larvae. JH-III acid and related metabolites such as farnesol, farnesoic acid, and methyl farnesoate do not induce competence. Hitherto, JH acids have been regarded as precursors or catabolites of JHs. Here we show for the first time that JH acid has a hormonal function that cannot be performed by JH itself.

20-Hydroxyecdysone is required for, and negatively regulates, transcription of Drosophila pupal cuticle protein genes

Transcripts of ecdysone-dependent genes (EDGs) accumulate in isolated imaginal discs with 8 hr after exposure to a pulse of the steroid hormone 20-hydroxyecdysone (20-HE; 1 microgram/ml for 6 hr) but not in discs cultured in the continuous presence or absence of the hormone. Sequence analyses show that two of the EDGs are members of gene families encoding insect cuticle proteins. We conclude that a third EDG encodes a cuticle protein because the conceptual glycine-rich protein contains sequence motifs similar to those found in insect egg shell proteins and vertebrate cytokeratins and because expression of this gene is limited to tissues that deposit the pupal cuticle. Nuclear run-on assays show that the hormone-dependent expression of each of these EDGs is due to transcriptional regulation. Readdition of hormone to imaginal discs actively synthesizing the EDG messages causes rapid repression of EDG transcription. Thus, 20-HE acts as both a positive and a negative regulator of EDG transcription. Sequences in the promoter regions of two of the EDGs are similar to an ecdysone response element and may play a role in negative regulation.

Segment-specific modifications of a neuropeptide phenotype in embryonic neurons of the moth, Manduca sexta

We have studied differences in the development of segmentally homologous neurons to identify factors that may regulate a neuropeptide phenotype. Bilaterally paired homologs of the peripheral neuron L1 were identified in the thoracic and abdominal segments in embryos of the moth Manduca: each bipolar neuron arises at a stereotyped location and, at 40% of embryogenesis, projects its major process within the transverse nerve of its own segment. Shortly after the initiation of axonogenesis (approximately 41%), L1 homologs in all but the prothoracic segment (T1) were labelled specifically by an antiserum to the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (authentic FMRFamide). Levels of peptide-immunoreactivity (IR) were comparable in all such segmental homologs up to the approximately 60% stage of embryogenesis, whereupon two distinct levels of peptide IR were displayed: homologs in the three most rostral segments (T2, T3, and A1; [abdominal segment 1]) showed high levels and were called Type I L1 neurons; homologs in the more caudal segments (A2-A8) typically showed low levels of IR and were called Type II L1 neurons. This segment-specific difference represented mature differentiated states and was retained in postembryonic stages. Intracellular dye fills of embryonic L1 neurons revealed that the morphogenesis of the Type I and II L1 neuron homologs was similar until approximately 48% of embryogenesis; thereafter it differed in two salient ways: (1) the cell bodies of Type II L1 neurons migrated approximately 150 microns laterally from their point of origin, and (2) the distal processes of the Type II L1 neurons contacted the heart, whereas those of Type I L1 neurons did not. Ultrastructural studies of both mature and developing L1 homologs showed that the FMRFamide-like antigen(s) localized specifically to secretory granules. Further, whereas the secretory granules in segmental homologs appeared similar initially (i.e., at approximately 50% of development), following the establishment of segment-specific differences, secretory granules found in mature Type I and II L1 neurons were cell type-specific.