Identification of a novel hemolymph peptide that modulates silkworm feeding motivation

Involvement of a silkworm D2-like dopamine receptor in the promotion of feeding and related behaviors

The biogenic amine dopamine (DA) regulates various physiological and behavioral processes in insects through binding with specific receptors. Three types of typical receptors are known to date. Previously, we achieved functional and pharmacological characterization of the three DA receptors in the silkworm Bombyx mori (BmDopR1-3). BmDopR1 and BmDopR2 are functionally classified as D1-like DA receptors, and BmDopR3 as D2-like. The present pharmacological data and our previous studies suggested that bromocriptine (Bro), which acts as an agonist on the DA D2 receptors and also interacts with various serotonin and adrenergic receptors in vertebrates, is an agonist that also acts specifically on BmDopR3, with little effect on BmDopR1 and BmDopR2 in silkworms. Exploiting this subtype specificity of Bro, to offer clues on the involvement of DA and its receptors in silkworm feeding behavior, Bro was injected into fifth instar larvae and subsequent feeding and related behaviors (feeding amount, excretion amount, mandibular movement, and feeding behavior observation) were quantitatively evaluated. Bro injection increased feeding and excretion amounts but did not affect mandibular chewing speed. Visual observation of feeding behavior for 1 h revealed the prolongation of feeding and related moving time in Bro-injected larvae. Collectively, these results suggest that Bro directly acted on BmDopR3 as an agonist and promoted feeding and related behaviors in silkworm larvae.

High-resolution analysis of baculovirus-induced host manipulation in the domestic silkworm, Bombyx mori

Many parasites manipulate host behaviour to enhance their transmission. Baculoviruses induce enhanced locomotory activity (ELA) combined with subsequent climbing behaviour in lepidopteran larvae, which facilitates viral dispersal. However, the mechanisms underlying host manipulation system are largely unknown. Previously, larval locomotion during ELA was summarized as the distance travelled for a few minutes at several time points, which are unlikely to characterize ELA precisely, as ELA typically persists for several hours. In this study, we modified a recently developed method using time-lapse recording to characterize locomotion of Bombyx mori larvae infected with B. mori nucleopolyhedrovirus (BmNPV) for 24 h at 3 s resolution. Our data showed that the locomotion of the mock-infected larvae was restricted to a small area, whereas the BmNPV-infected larvae exhibited a large locomotory area. These results indicate that BmNPV dysregulates the locomotory pattern of host larvae. Furthermore, both the mock- and BmNPV-infected larvae showed periodic cycles of movement and stationary behaviour with a similar frequency, suggesting the physiological mechanisms that induce locomotion are unaffected by BmNPV infection. In contrast, the BmNPV-infected larvae exhibited fast and long-lasting locomotion compared with mock-infected larvae, which indicates that locomotory speed and duration are manipulated by BmNPV.

Feeding modulation in insects through factors in the hemolymph

In insect hemolymph, many factors are present that can influence feeding motivation, such as lipids, carbohydrates, and other metabolites. Levels of these hemolymph factors fluctuate according to metabolic, nutrient and feeding states, eventually affecting feeding motivation and consequent regularly occurring feeding cycles. Such fluctuations contribute to energy homeostasis and innate feeding behavior in insects possibly by endocrine systems. Ultimately, orchestration of bioactive factors in the hemolymph modulate feeding motivation and nutrient selective behavior in insects.

Glucose, some amino acids and a plant secondary metabolite, chlorogenic acid induce the secretion of a regulatory hormone, tachykinin-related peptide, from the silkworm midgut

Enteroendocrine cells in the insect midgut are thought to secrete peptide hormones in response to the nutritional state. However, the role of dietary compounds in inducing peptide hormone secretion from enteroendocrine cells in insects remains unknown. In the present study, we demonstrated that several dietary compounds from mulberry leaves, including glucose, amino acids, and the secondary metabolite chlorogenic acid, induced significant secretion of tachykinin-related peptides from isolated silkworm midguts at the luminal concentrations measured in fed larvae. This study provides evidence that the insect midgut senses a non-nutritious secondary metabolite in addition to nutrient metabolites to monitor luminal food status and secretes a feeding regulatory hormone, suggesting that a unique dietary sensory system modulates insect feeding via enteroendocrine control.

Calcitonin-like peptide hormone (CT/DH) in the frontal ganglia as a feeding regulatory peptide of the silkworm, Bombyx mori

The frontal ganglion (FrG) in insects contributes to the modulation of feeding behavior via the regulation of foregut contraction and other neural networks. Profiling the peptides of the FrG is important to understand endocrine regulation of feeding behavior in insects. High-resolution spiral matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) identified an ion peak, corresponding to calcitonin-like diuretic hormone 31 (CT/DH) in the FrG of silkworm Bombyx mori larvae. RT-PCR confirmed that CT/DH is expressed in the FrG, as are other peptide hormones, including allatoregulatory peptides. A feeding latency assay using synthetic CT/DH revealed that it increases the time to the initiation of feeding in a dose-dependent manner. These data indicate that CT/DH is a candidate regulatory peptide that modulates the feeding behavior of B. mori.

Effects of starvation on brain short neuropeptide F-1, -2, and -3 levels and short neuropeptide F receptor expression levels of the silkworm, Bombyx mori

In our previous report, we demonstrated the possibility that various regulatory neuropeptides influence feeding behavior in the silkworm, Bombyx mori. Among these feeding-related neuropeptides, short neuropeptide F (sNPF) exhibited feeding-accelerating activity when injected into B. mori larvae. Like other insect sNPFs, the deduced amino acid sequence of the cDNA encoding the sNPF precursor appears to produce multiple sNPF and sNPF-related peptides in B. mori. The presence of three sNPFs, sNPF-1, sNPF-2, and sNPF-3, in the brain of B. mori larvae was confirmed by direct MALDI-TOF mass spectrometric profiling. In addition, all three sNPFs are present in other larval ganglia. The presence of sNPF mRNA in the central nervous system (CNS) was also confirmed by Reverse transcription-polymerase chain reaction. Semi-quantitative analyses of sNPFs in the larval brain using matrix-assisted laser desorption ionization time-of-flight mass spectrometry further revealed that brain sNPF levels decrease in response to starvation, and that they recover with the resumption of feeding. These data suggest that sNPFs were depleted by the starvation process. Furthermore, food deprivation decreased the transcriptional levels of the sNPF receptor (BNGR-A10) in the brain and CNS, suggesting that the sNPF system is dependent on the feeding state of the insect and that the sNPF system may be linked to locomotor activity associated with foraging behavior. Since the injection of sNPFs accelerated the onset of feeding in B. mori larvae, we concluded that sNPFs are strongly related to feeding behavior. In addition, semi-quantitative MS analyses revealed that allatostatin, which is present in the larval brain, is also reduced in response to starvation, whereas the peptide level of Bommyosuppressin was not affected by different feeding states.

A hemolymph major anionic peptide, HemaP, motivates feeding behavior in the sweetpotato hornworm, Agrius convolvuli

We recently identified a novel feeding-modulating peptide, hemolymph major anionic peptide (HemaP), designated Bommo-HemaP (B-HemaP), from hemolymph of the silkworm Bombyx mori. B-HemaP has a unique biological activity in modulating the regular frequency of feeding motivation, which is accompanied by increased foraging behaviors. To confirm the conservation of the HemaP-regulated feeding mechanism in lepidopteran species, we purified and sequenced two candidate peptides from the hemolymph of larvae of the sweet potato hornworm Agrius convolvuli. Unlike B. mori, A. convolvuli had two forms of HemaP, which were designated Agrco-HemaP-1 (A-HemaP-1) and Agrco-HemaP-2 (A-HemaP-2). The amino acid sequence of A-HemaP-2 was identical with that of A-HemaP-1, except for O-glycosylation on the fifth amino acid, threonine, within the N-terminal region. The amino acid sequence of A-HemaP-1/A-HemaP-2 had only 32% identity with B-HemaP. Structural analysis revealed that the carbohydrate moiety of A-HemaP-2 was an α-GalNAc residue. Injection of A-HemaP-1, A-HemaP-2 and recombinant A-HemaP-1 (rA-HemaP-1) individually caused a significant increase in foraging behaviors in A. convolvuli larvae, and no significant differences were observed among these three A-HemaPs. The CD spectra of these three A-HemaPs were quite similar, and all had α-helix-rich secondary structures. Although A-HemaP-1 and B-HemaP did not exhibit cross-reactivity at any injection doses examined, HemaP might be a conserved molecule among lepidopteran species that can modulate feeding motivation through the fluctuation of peptide levels in hemolymph.