Bursicon-α subunit modulates dLGR2 activity in the adult Drosophila melanogaster midgut independently to Bursicon-β

Rnai-based functional analysis of bursicon genes related to cuticle pigmentation in a ladybird beetle

In arthropods, the binding of a bursicon (encoded by burs and pburs) heterodimer or homodimer to a leucine-rich repeat-containing G protein coupled receptor LGR2 (encoded by rk) can activate many physiological processes, especially cuticle pigmentation during insect ecdysis. In the current paper, we intended to ascertain whether bursicon signaling mediates body coloration in the 28-spotted larger potato ladybird, Henosepilachna vigintioctomaculata, and if so, by which way bursicon signal governs the pigmentation. The high expression of Hvburs, Hvpburs and Hvrk occurred in the young larvae, pupae and adults, especially in the head and ventral nerve cord. RNA interference (RNAi) aided knockdown of Hvburs, Hvpburs or Hvrk in the prepupae caused similar phenotypic defects. The pigmentation of the resultant adults was affected, with significantly reduced dark areas on the sternums. Moreover, the accumulated mRNA levels of two sclerotin biosynthesis genes, aspartate 1-decarboxylase gene Hvadc and N-β-alanyldopamine synthase gene Hvebony, were significantly increased in the Hvburs, Hvpburs or Hvrk RNAi beetles. Furthermore, depletion of either Hvadc or Hvebony could completely rescue the impaired coloration on the sternums of Hvpburs RNAi adult. Our results supported that bursicon heterodimer-mediated signal regulate cuticle pigmentation. The bursicon signaling may tune the ratio of melanins (dark/black, brown) to sclerotins (light yellow, colorless) exerting its regulative role in the pigmentation of H. vigintioctomaculata sternums.

Behavioral events and functional analysis of bursicon signal during adult eclosion in the 28-spotted larger potato ladybird

To accommodate growth, insects must periodically shed their exoskeletons. In Manduca sexta, Drosophila melanogaster and Tribolium castaneum, Bursicon (Burs)/ Partner of bursicon (Pburs)-LGR2 signal is an indispensable component for the proper execution of ecdysis behavior during adult eclosion. Nevertheless, the behavioral events and the roles of bursicon signaling in other insects deserve further exploration. In the current paper, we found that the pupal-adult ecdysis in Henosepilachna vigintioctomaculata could be divided into three distinct stages, preecdysis, ecdysis and postecdysis. Preecdysis behavioral sequences included abdomen twitches, dorsal-ventral contractions and air filling that function to loosen the old cuticle. Ecdysis events began with anterior-posterior contractions that gradually split the old integument along the dorsal body midline, followed by freeing of legs and mouthparts, and culminated in detachment from pupal cuticle. Postecdysis behavioral processes contained three actions: perch selection and stretching of elytra and hindwings. RNA interference for HvBurs, HvPburs or Hvrk (encoding LGR2) strongly impaired wing expansion actions, and slightly influenced preecdysis and ecdysis behaviors. The RNAi beetles failed to extend their elytra and hindwings. In addition, injected with dsrk also caused kinked femurs and tibia. Our findings establish that bursicon pathway is involved in regulation of adult eclosion behavior, especially wing expansion motor programs. Given that wings facilitate food foraging, courtship, predator avoidance, dispersal and migration, our results provide a potential target for controlling H. vigintioctomaculata.

Partner of neuropeptide bursicon homodimer pburs mediates a novel antimicrobial peptide Ten3LP via Dif/Dorsal2 in Tribolium castaneum

Bursicon is a cystine knot family neuropeptide, composed of two subunits, bursicon (burs) and partner of burs (pburs). The subunits can form heterodimers to regulate cuticle tanning and wing maturation and homodimers to signal different biological functions in innate immunity, midgut stem cell proliferation and energy homeostasis, and reproductive physiology in the model insects Drosophila melanogaster or Tribolium castaneum. Here, we report on the role of the pburs homodimer in signaling innate immunity in T. castaneum larvae. Through transcriptome analysis we identified a set of immune-related genes that respond to pburs RNAi. Treating larvae with recombinant-pburs protein led to up-regulation of antimicrobial peptide (AMP) genes in vivo and in vitro. The upregulation of most AMP genes was dependent on the NF-κB transcription factor Relish. Most importantly, we identified a novel AMP, Tenecin 3-like peptide (Ten3LP), regulated by pburs via NF-κB transcription factor Dorsal-related immunity factor (Dif)/Dorsal2, but not Relish. We conducted Ten3LP RNAi, synthesized recombinant Ten3LP protein for microbial inhibition assays and functionally characterized Ten3LP as an AMP specific for fungi and Gram-positive bacteria. We demonstrate that expression of Ten3LP is activated by pburs via the Toll pathway. These findings identify new molecular targets for development of potential antibiotics for treating microbial infections and perhaps for RNAi based pest management technology.

RNAi-based functional analysis of bursicon genes related to wing expansion in gypsy moths

Bursicon is a heterodimeric neuropeptide composed of Burs-α and Burs-β subunits that plays an important role in cuticle tanning and wing expansion in insects. In this study, full-length cDNAs of Burs-α (LdBurs-α) and Burs-β (LdBurs-β) genes were identified in gypsy moth (Lymantria dispar) and cloned. The 480 bp and 420 bp open reading frames (ORFs) encode 159 and 129 amino acid polypeptides, respectively. LdBurs-α and LdBurs-β have 11 conserved cysteine residues, and LdBurs-α and LdBurs-β genes were expressed during all developmental stages according to quantitative reverse transcription PCR (qRT-PCR), with highest expression in the egg stage. High expression levels were also detected in the haemolymph, cuticle and head. To explore the physiological functions of LdBurs-α and LdBurs-β, the genes were knocked down in larvae and pupae using RNA interference (RNAi), and expression levels of LdBurs-α and LdBurs-β were decreased by 42.26-80.09%. Wing defects were observed in L. dispar pupae following Ldbursion silencing, with a phenotypic percentage ranging from 10.17% to 15.00%. RNAi-mediated knockdown of Ldbursicon prevented the expansion of male and female L. dispar adult wings, with malformation rates ranging from 6.38% and 30.00% to 57.69% and 69.23%, but no cuticle tanning defects were observed in pupae or adults. The results indicate that bursicon plays a key role in wing expansion in L. dispar adults, making it a potentially novel molecular target for insecticide-based control of this pest species.

Neuropeptide Bursicon Influences Reproductive Physiology in Tribolium Castaneum

Bursicon is a neuropeptide belonging to the cystine knot family and is composed of burs and partner of burs (pburs) subunits. It can form heterodimers or homodimers to execute different biological functions. Bursicon heterodimers regulate cuticle sclerotization and wing maturation, whereas bursicon homodimers mediate innate immunity and midgut stem cell proliferation. A recent study has shown that bursicon potentially induces the expression of vitellogenin (Vg) in the black tiger shrimp Penaeus monodon; however, the underlying mechanism remains unknown. In this study, we investigated the role of bursicon in the reproductive physiology of the red flour beetle, Tribolium castaneum. The knockdown of burs, pburs, or its receptor T. castaneum rickets (Tcrk) in 2-day pupae significantly downregulated the expression levels of Vg1, Vg2, and Vg receptor (VgR) genes in females 3- and 5-day post-adult emergence, leading to abnormal oocytes with limited Vg content. The silencing of burs repressed the number of eggs laid and completely inhibited egg hatch, whereas the silencing of pburs dramatically decreased the number of eggs laid, hatch rate, and offspring larval size, and this RNA interference (RNAi) effects persisted to the next generation. Furthermore, the knockdown of burs or pburs downregulated the expression of the insulin/insulin-like signaling/target of rapamycin (TOR) signaling genes encoding insulin receptor (InR), protein kinase B (Akt), TOR, and ribosomal protein S6 kinase (S6K). Most importantly, the injection of recombinant pburs (r-pburs) protein was able to upregulate the expression of Vg, VgR, InR, Akt, TOR, S6K, JH synthesis (JHAMT), Methoprene-tolerant (Met), and Taiman (Tai) in normal females and rescue the expression of Vg and VgR in pburs RNAi females but failed to rescue Vg and VgR in Tcrk knockdown females. We infer that bursicon homodimers influence Vg expression via the receptor Tcrk, possibly by mediating the expression of the juvenile hormone (JH) and IIS/TOR pathway genes, thereby regulating reproduction in T. castaneum.

Ecdysis-related neuropeptide expression and metamorphosis in a non-ecdysozoan bilaterian

Ecdysis-related neuropeptides (ERNs), including eclosion hormone, crustacean cardioactive peptide, myoinhibitory peptide, bursicon alpha, and bursicon beta regulate molting in insects and crustaceans. Recent evidence further revealed that ERNs likely play an ancestral role in invertebrate life cycle transitions, but their tempo-spatial expression patterns have not been investigated outside Arthropoda. Using RNA-seq and in situ hybridization, we show that ERNs are broadly expressed in the developing nervous system of a mollusk, the polyplacophoran Acanthochitona fascicularis. While some ERN-expressing neurons persist from larval to juvenile stages, others are only present during settlement and metamorphosis. These transient neurons belong to the "ampullary system," a polyplacophoran-specific larval sensory structure. Surprisingly, however, ERN expression is absent from the apical organ, another larval sensory structure that degenerates before settlement is completed in A. fascicularis. Our findings thus support a role of ERNs in A. fascicularis metamorphosis but contradict the common notion that the apical organ-like structures shared by various aquatic invertebrates (i.e., cnidarians, annelids, mollusks, echinoderms) are of general importance for this process.

Metabolism and growth adaptation to environmental conditions in Drosophila

Organisms adapt to changing environments by adjusting their development, metabolism, and behavior to improve their chances of survival and reproduction. To achieve such flexibility, organisms must be able to sense and respond to changes in external environmental conditions and their internal state. Metabolic adaptation in response to altered nutrient availability is key to maintaining energy homeostasis and sustaining developmental growth. Furthermore, environmental variables exert major influences on growth and final adult body size in animals. This developmental plasticity depends on adaptive responses to internal state and external cues that are essential for developmental processes. Genetic studies have shown that the fruit fly Drosophila, similarly to mammals, regulates its metabolism, growth, and behavior in response to the environment through several key hormones including insulin, peptides with glucagon-like function, and steroid hormones. Here we review emerging evidence showing that various environmental cues and internal conditions are sensed in different organs that, via inter-organ communication, relay information to neuroendocrine centers that control insulin and steroid signaling. This review focuses on endocrine regulation of development, metabolism, and behavior in Drosophila, highlighting recent advances in the role of the neuroendocrine system as a signaling hub that integrates environmental inputs and drives adaptive responses.

Bursicon homodimers induce the innate immunity via Relish in Procambarus clarkii

Bursicon (burs) is a neuropeptide hormone consisting of two cystine-knot proteins (burs α and burs β), and burs α-β is responsible for cuticle tanning in insects. Further studies show that burs homodimers induce prophylactic immunity. Here, we investigated the hypothesis that burs homodimers act in regulating immunity in the red swamp crayfish Procambarus clarkii. We found that burs α and burs β are expressed in neural system of crayfish. Treating crayfish with recombinant burs-homodimer proteins led to up-regulation of several anti-microbial peptide (AMP) genes, and RNAi-mediated knockdown of burs led to decreased expression of AMP genes. The burs proteins also facilitated bacterial clearance and decreased crayfish mortality upon bacterial infection. Furthermore, burs proteins activated the transcriptional factor Relish, and knockdown of Relish abolished the influence of recombinant burs homodimers on AMP induction. We infer the burs homodimers induce expression of AMP genes via Relish in crayfish and this study extends this immune signaling pathway from insects to crustaceans.

A Neuronal Relay Mediates a Nutrient Responsive Gut/Fat Body Axis Regulating Energy Homeostasis in Adult Drosophila

The control of systemic metabolic homeostasis involves complex inter-tissue programs that coordinate energy production, storage, and consumption, to maintain organismal fitness upon environmental challenges. The mechanisms driving such programs are largely unknown. Here, we show that enteroendocrine cells in the adult Drosophila intestine respond to nutrients by secreting the hormone Bursicon α, which signals via its neuronal receptor DLgr2. Bursicon α/DLgr2 regulate energy metabolism through a neuronal relay leading to the restriction of glucagon-like, adipokinetic hormone (AKH) production by the corpora cardiaca and subsequent modulation of AKH receptor signaling within the adipose tissue. Impaired Bursicon α/DLgr2 signaling leads to exacerbated glucose oxidation and depletion of energy stores with consequent reduced organismal resistance to nutrient restrictive conditions. Altogether, our work reveals an intestinal/neuronal/adipose tissue inter-organ communication network that is essential to restrict the use of energy and that may provide insights into the physiopathology of endocrine-regulated metabolic homeostasis.

Relish2 mediates bursicon homodimer-induced prophylactic immunity in the mosquito Aedes aegypti

Bursicon is a neuropeptide hormone consisting of two cystine-knot proteins (burs α and burs β), responsible for cuticle tanning and other developmental processes in insects. Recent studies show that each bursicon subunit forms homodimers that induce prophylactic immunity in Drosophila melanogaster. Here, we investigated the hypothesis that bursicon homodimers act in prophylactic immunity in insects, and possibly arthropods, generally, using the mosquito, Aedes aegypti. We found that burs α and burs β are expressed in larvae, pupae and newly emerged adults. Treating newly emerged Ae. aegypti and D. melanogaster adults with recombinant bursicon (r-bursicon) heterodimer led to cuticle tanning in both species. Treating larvae and adults with r-bursicon homodimers led to up-regulation of five anti-microbial peptide (AMP) genes, noting the possibility that bursicon heterodimers also lead to up-regulation of these genes can not been excluded. The induced AMPs effectively suppressed the growth of bacteria in vitro. RNAi knock-down of the transcriptional factor Relish2 abolished the influence of r-bursicon homodimers on AMP production. We infer the bursicon homodimers induce expression of AMP genes via Relish2 in Ae. aegypti, as prophylactic immunity to protect mosquitoes during the vulnerable stages of each molt.