The Intricate Role of Ecdysis Triggering Hormone Signaling in Insect Development and Reproductive Regulation

Insect growth is interrupted by molts, during which the insect develops a new exoskeleton. The exoskeleton confers protection and undergoes shedding between each developmental stage through an evolutionarily conserved and ordered sequence of behaviors, collectively referred to as ecdysis. Ecdysis is triggered by Ecdysis triggering hormone (ETH) synthesized and secreted from peripheral Inka cells on the tracheal surface and plays a vital role in the orchestration of ecdysis in insects and possibly in other arthropod species. ETH synthesized by Inka cells then binds to ETH receptor (ETHR) present on the peptidergic neurons in the central nervous system (CNS) to facilitate synthesis of various other neuropeptides involved in ecdysis. The mechanism of ETH function on ecdysis has been well investigated in holometabolous insects such as moths Manduca sexta and Bombyx mori, fruit fly Drosophila melanogaster, the yellow fever mosquito Aedes aegypti and beetle Tribolium castaneum etc. In contrast, very little information is available about the role of ETH in sequential and gradual growth and developmental changes associated with ecdysis in hemimetabolous insects. Recent studies have identified ETH precursors and characterized functional and biochemical features of ETH and ETHR in a hemimetabolous insect, desert locust, Schistocerca gregaria. Recently, the role of ETH in Juvenile hormone (JH) mediated courtship short-term memory (STM) retention and long-term courtship memory regulation and retention have also been investigated in adult male Drosophila. Our review provides a novel synthesis of ETH signaling cascades and responses in various insects triggering diverse functions in adults and juvenile insects including their development and reproductive regulation and might allow researchers to develop sustainable pest management strategies by identifying novel compounds and targets.

The Ecdysis Triggering Hormone System, via ETH/ETHR-B, Is Essential for Successful Reproduction of a Major Pest Insect, Bactrocera dorsalis (Hendel)

Ecdysis triggering hormone (ETH), released by the Inka cells, is a master hormone in regulating the ecdysis process in insect. Here we investigated the presence and role of the ETH signaling in the female adult of the oriental fruit fly, Bactrocera dorsalis (Hendel) that is one of the most important invasive pest insects in agriculture worldwide. In the female adult, ETH was confirmed in the Inka cells at the tracheae by immunostaining and also in vitro exposure to ETH stimulated the isolated corpora allata of adult in activity. Then we prepared cDNA of females at 0, 5, 10, 15, and 20 days after adult eclosion, and RT-qPCR showed that the expression pattern of ETH and its receptor ETHR-B started from a peak at the day of adult eclosion (day 0), then dropped to basal levels and increased again between day 10 and 15 which is also the period corresponding to ovary growth. In contrast, ETHR-A was absent with Ct values of >33. The expression patterns of the ecdysteroid-producing Halloween genes Spook and Shade, and the vitellogenin genes Vg1, Vg2, and Vg3 co-occurred with peak levels at days 10–15, and also juvenile hormone acid methyltransferase (JHAMT) showed increased levels on day 15. Further in RNAi assays to better understand the role of ETH and ETHR, dsRNA was injected to adult and this led to a respective decrease in expression of 62 and 56% for ETH and ETHR-B, while ETHR-A stayed absent with Ct values of 33. In these RNAi-females, there was an apparently decreased expression for JHAMT and Vg2, together with a significant decrease of the JH titer and egg production. Injection of the JH mimetic methoprene could rescue Vg2 expression and egg production. Upstream, in dsETH/dsETHR-injected females, 20-hydroxyecdysone (20E) injection rescued the transcriptions of ETH and ETHR and also egg production. In summary, our results shed more light on the pivotal role that the ETH peptide hormone and its receptor ETHR-B play an essential role in the reproduction of the female adult of B. dorsalis, via the regulation of JH and vitellogenin, which are controlled by a pulse of 20E.

Endocrine regulation of airway clearance in Drosophila

Fluid clearance from the respiratory system during developmental transitions is critically important for achieving optimal gas exchange in animals. During insect development from embryo to adult, airway clearance occurs episodically each time the molt is completed by performance of the ecdysis sequence, coordinated by a peptide-signaling cascade initiated by ecdysis-triggering hormone (ETH). We find that the neuropeptide Kinin (also known as Drosokinin or Leukokinin) is required for normal respiratory fluid clearance or "tracheal air-filling" in Drosophila larvae. Disruption of Kinin signaling leads to defective air-filling during all larval stages. Such defects are observed upon ablation or electrical silencing of Kinin neurons, as well as RNA silencing of the Kinin gene or the ETH receptor in Kinin neurons, indicating that ETH targets Kinin neurons to promote tracheal air-filling. A Kinin receptor mutant fly line (Lkr^f02594 ) also exhibits tracheal air-filling defects in all larval stages. Targeted Kinin receptor silencing in tracheal epithelial cells using breathless or pickpocket (ppk) drivers compromises tracheal air-filling. On the other hand, promotion of Kinin signaling in vivo through peptide injection or Kinin neuron activation through Drosophila TrpA1 (dTrpA1) expression induces premature tracheal collapse and air-filling. Moreover, direct exposure of tracheal epithelial cells in vitro to Kinin leads to calcium mobilization in tracheal epithelial cells. Our findings strongly implicate the neuropeptide Kinin as an important regulator of airway clearance via intracellular calcium mobilization in tracheal epithelial cells of Drosophila.

Ecdysis Triggering Hormone Signaling (ETH/ETHR-A) Is Required for the Larva-Larva Ecdysis in Bactrocera dorsalis (Diptera: Tephritidae)

Insects must undergo ecdysis for successful development and growth, and the ecdysis triggering hormone (ETH), released by the Inka cells, is a master hormone in this process. In this study, we determined the sequence of the ETH precursor and receptors in an agriculturally important pest insect, the oriental fruit fly Bactrocera dorsalis (Hendel). We identified two functionally distinct splice receptor isoforms: BdETH-R-A and BdETH-R-B, and when expressed in Chinese hamster ovary (CHO-WTA11) cells, they exhibited a high sensitivity to the two mature peptides BdETH1 and BdETH2. The BdETH transcript was detected in the tracheal tissue of the larvae. Inka cells were identified with immunohistochemical antibody staining against Drosophila melanogaster ETH1, and in situ hybridization with specific DNA probes. Selective RNA silencing of BdETH or BdETH-R-A, but not of BdETH-R-B, caused developmental failure at ecdysis. The dsRNA-treated larvae displayed tracheal defects and could not shed the old cuticle followed by death. Our results demonstrated that BdETH, via activation of BdETH-R-A but not ETH-R-B, plays an essential role in regulating the process of larva-larva ecdysis in B. dorsalis.