Nutrition regulates the expression of storage proteins in Bombyx mori via insulin-like/FoxO signaling pathway

Unravelling the Role of Insulin-Like Peptide Genes in Bombyx mori: Potential Key Regulators of Insect Metabolism

Understanding gene expression in specific tissues and their modulation under environmental stimuli, such as nutritional deficiency, reveals the key physiological regulatory mechanisms of an organism. This study examined the tissue-specific expression of insulin-like peptide (ILP) genes (BmX and BmZ) in Bombyx mori larvae and their responses to hyperglycaemia, food deprivation and hormonal (20-hydroxyecdysone and bovine insulin) treatments. mRNA expression levels of BmX and BmZ were analyzed in the brain, fat body, midgut and ovary. The results revealed that BmX was highly expressed in the fat body, while both genes were abundant in the ovary. Hyperglycaemia increased BmX mRNA expression level in the midgut (3.07-fold) and brain (7.53-fold), while BmZ mRNA expression level was increased in all tissues except the midgut. Nutrient deficiency upregulated BmX mRNA expression level (1.36-fold) in the fat body while reducing it (-0.53-fold) in the midgut. Food deprivation progressively increased (0.77-fold at 24 h and 2.34-fold at 72 h) BmX mRNA expression level in the fat body, while both BmX and BmZ transcripts declined in the midgut. Insulin suppressed BmX (-0.25-fold) and BmZ (-0.91-fold) mRNA expression levels in food-deprived larvae in the fat body, whereas 20E consistently downregulated BmX, BmZ, and BmInR (insulin receptor) mRNA expression levels in all the conditions. These findings revealed the complex interaction of gene expression, tissue specificity, and environmental factors in B. mori larvae and provided insights into adaptive responses to nutritional stress and hormonal regulation in the insect with potential applications in sericulture and agricultural biotechnology.

Amino acid-mTOR pathway-associated transcription factor GATAβ4 regulates storage protein expression in Bombyx mori

Storage proteins (SPs) are hexameric macromolecular protein, an important component of insect serum protein, which plays a variety of roles in insect metamorphosis and development. However, their regulatory mechanisms remain unclear. Our previous studies revealed that the expression of SPs is regulated by nutritional signals and identified FoxO as a negative regulator of SPs in the silkworm Bombyx mori (B. mori). In this study, amino acids upregulated BmSP expression, whereas Rapamycin downregulated it in fat body cultured in vitro. Rapamycin also reduced BmSP expression in B. mori larvae. Overexpression of the nutrient transcription factor GATA family in BmE cells revealed that only BmGATAβ4 significantly upregulated BmSP expression. Furthermore, the amino acid-mTOR signaling pathway modulated BmGATAβ4 expression. Overexpression of BmGATAβ4 resulted in increased BmSP expression in B. mori larvae. Luciferase reporter assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation identified GATA-like CRE 1-1 and GATA-like CRE 2-2 of the BmSP1 promoter as binding sites for BmGATAβ4. These findings provide new insights into the regulation of nutrient protein expression in insects.

The circadian clock affects starvation resistance through the pentose phosphate pathway in silkworm, Bombyx mori

Disruption of the circadian clock can affect starvation resistance, but the molecular mechanism is still unclear. Here, we found that starvation resistance was significantly reduced in the core gene BmPer deficient mutant silkworms (Per-/-), but the mutant's starvation resistance increased with larval age. Under natural physiological conditions, the weight of mutant 5th instar larvae was significantly increased compared to wild type, and the accumulation ability of triglycerides and glycogen in the fat bodies was upregulated. However, under starvation conditions, the weight consumption of mutant larvae was increased and cholesterol utilization was intensified. Transcriptome analysis showed that beta-oxidation was significantly upregulated under starvation conditions, fatty acid synthesis was inhibited, and the expression levels of genes related to mitochondrial function were significantly changed. Further investigations revealed that the redox balance, which is closely related to mitochondrial metabolism, was altered in the fat bodies, the antioxidant level was increased, and the pentose phosphate pathway, the source of reducing power in cells, was activated. Our findings suggest that one of the reasons for the increased energy burden observed in mutants is the need to maintain a more robust redox balance in metabolic tissues. This necessitates the diversion of more glucose into the pentose phosphate pathway to ensure an adequate supply of reducing power.

Elasticity of trachea in the silkworm: A role of gene BmMuc91C

Respiration is a vital process essential for organism survival, with most terrestrial insects relying on a sophisticated tubular tracheal network. In the current study, a gene with repetitive sequence was identified within the silkworm genome. Designated as BmMuc91C, it contains a dozen repeated motifs "PSSSYGAPX" and "GGYSSGGX" in its sequence. BmMuc91C exhibits specific expression in the tracheal system of silkworm larvae, with significantly higher expression levels during the molting stage. Overexpression of BmMuc91C in individual silkworms resulted in a marked increase in tracheal diameter, particularly during the molting stage. Immunofluorescence staining using a BmMuc91C antibody revealed a noticeable thickening of the apical extracellular matrix in the trachea. Tensile testing confirmed a considerable enhancement in tracheal elasticity. Additionally, a BmMuc91C mutation strain of silkworms was generated using the clustered regularly interspaced short palindromic repeats (CRISPR) / CRISPR-associated nuclease 9 system. Although no significant differences were observed in the growth, development, and molting of BmMuc91C mutant silkworms, mechanical tests demonstrated a decrease in tracheal elasticity. Transcriptomic techniques revealed that a significant number of cuticular and chitin-binding proteins were among the differentially expressed genes between mutant and wild-type silkworms. Furthermore, the recombined BmMuc91C protein was successfully expressed using the Escherichia coli system. Cross-linking experiments with horseradish peroxidase demonstrated the formation of macromolecular complexes of BmMuc91C, which exhibited spontaneous luminescent properties under ultraviolet light. This research sheds light on the role of elastic proteins in insect tracheae and provides valuable insights for the development of elastic biomaterials.

Effects of Microbes on Insect Host Physiology and Behavior Mediated by the Host Immune System

Innate immunity is critical for insects to adjust to complicated environments. Studying the insect immune system can aid in identifying novel insecticide targets and provide insights for developing novel pest control strategies. Insects recognize environmental pathogens through pattern recognition receptors, thus activating the innate immune system to eliminate pathogens. The innate immune system of insects primarily comprises cellular immunity and humoral immunity. Toll, immune deficiency, and Janus kinase/signal transducers and activators of transcription are the main signaling pathways regulating insect humoral immunity. Nevertheless, increasing research has revealed that immune signaling activated by microbes also performs non-immune roles while exerting immune roles, and insulin signaling performs a key role in mediating the connection between the immune system and non-immune physiological activities. Therefore, this paper first briefly reviews the main innate immune signaling and insulin signaling of insects, then summarizes the relationship between immune signaling activated by microbes and insect growth and development, reproduction, pesticide resistance, chemical communication, cell turnover, lifespan, sleep, energy generation pathways and their possible underlying mechanisms. Future research directions and methodologies are also proposed, aiming to provide insights into further study on the physiological mechanism linking microbes and insect hosts.

Functional Characterization and Putative Regulatory Mechanism of an RNAi Efficiency-Related Nuclease (REase) in the Fall Armyworm, Spodoptera frugiperda

The lepidopteran-specific RNAi efficiency-related nuclease (REase) has been shown to contribute to double-strand RNA (dsRNA) degradation in several lepidopteran insects. However, little is known about its regulatory mechanism. In this study, we identified and characterized SfREase in Spodoptera frugiperda. The exposure of the third-instar larvae to dsEGFP and high temperature led to the upregulation of SfREase, whereas starvation treatment resulted in the downregulation of SfREase. Further experiments revealed that dsRNA degraded more slowly in the hemolymph or midgut fluid extracted from dsSfREase-injected or dsSfREase-ingested larvae compared with those from dsEGFP-treated larvae, and the recombinant SfREase degraded dsRNA in a concentration-dependent manner. Additionally, the knockdown of SfREase improved RNAi efficiency. Finally, both RNAi and dual-luciferase reporter assay in Sf9 cells revealed that SfREase is negatively regulated by FOXO. These data provide insights into the function and regulatory mechanism of REase and have applied implications for the development of an RNAi-based control strategy of S. frugiperda.

RNA interference against the putative insulin receptor substrate gene IRS1 affects growth and development in the pest natural enemy Pardosa pseudoannulata

BACKGROUND

Insulin signalling pathways play crucial roles in regulating growth and development in insects, but their effects on the growth and development of Arachnids, such as spiders, have rarely been studied. As a valuable pest natural enemy in agricultural fields, the molecular mechanisms of insulin signalling pathway-mediated growth and development of the wolf spider, Pardosa pseudoannulata, are of particular interest.

RESULTS

In this study, we identified and characterized six insulin signalling pathway genes - InR, InR2, IRS1, PI3K1, PI3K2, and PDK - in Pardosa pseudoannulata. Real-time quantitative polymerase chain reaction results were used to analyse the relative expression levels of the six genes in different developmental instars and tissues, and in response to starvation treatment. In addition, the function of the insulin receptor substrate (IRS1) gene was investigated using RNA interference technology, which found that IRS1 significantly influenced nutrient content, developmental duration, body weight, and gonad development.

CONCLUSION

This study revealed the roles of six key insulin signalling pathway genes in Pardosa pseudoannulata, and in particular the importance of the IRS1 gene in regulating growth and development in the spider. The results lay the foundation for further research on the internal regulation mechanisms of growth and development in Araneae species, and also provide a reference for the artificial breeding of spiders. © 2023 Society of Chemical Industry.

Comparison of the signaling pathways of wing dimorphism regulated by biotic and abiotic stress in the brown planthopper

Wing polymorphism is an evolutionary trait that is widely present in various insects and provides a model system for studying the evolutionary significance of insect dispersal. The brown planthopper (BPH, Nilaparvata lugens) can alter its wing morphs under biotic and abiotic stress. However, whether differential signaling pathways are induced by the 2 types of stress remain largely unknown. Here, we screened a number of candidate genes through weighted gene co-expression network analysis (WGCNA) and found that ornithine decarboxylase (NlODC), a key enzyme in the synthesis of polyamines, was associated with wing differentiation in BPH and mainly responded to abiotic stress stimuli. We analyzed the Kyoto Encyclopedia of Genes and Genomes enrichment pathways of differentially expressed genes under the 2 stresses by transcriptomic comparison, and found that biotic stress mainly influenced insulin-related signaling pathways while abiotic stress mainly influenced hormone-related pathways. Moreover, we found that insulin receptor 1 (NlInR1) may regulate wing differentiation of BPH by responding to both biotic and abiotic stress, but NlInR2 only responded to biotic stress. Similarly, the juvenile hormone epoxide hydrolase associated with juvenile hormone degradation and NlODC may regulate wing differentiation mainly through abiotic stress. A model based on the genes and stresses to modulate the wing dimorphism of BPH was proposed. These findings present a comprehensive molecular mechanism for wing polymorphism in BPH induced by biotic and abiotic stress.

BmINR and BmAC6 genes involve in diapause regulation via the insulin/IGF signaling pathway in the silkworm (Bombyx mori)

Diapause of the silkworm (Bombyx mori) is an important ecological adaptation strategy regulated by multiple signaling pathways. As an evolutionarily conserved signaling pathway, the insulin/IGF signaling (IIS) pathway is essential in regulating lifespan, energy accumulation, and stress resistance in diapause insects. However, the regulatory mechanism of IIS on diapause in B. mori is still not fully understood. To investigate the role of the IIS pathway in regulating diapause, we first analyzed the transcription levels of the insulin receptor (BmINR) and its downstream gene adenylate cyclase 6 (BmAC6). The diapause-terminated eggs of a bivoltine strain QiuFeng (V²-QF) were incubated at 25℃ in natural room light for preparing diapause egg producers (DEPs) and at 17℃ in total darkness for preparing non-diapause egg producers (NDEPs), respectively. Then we investigated the effects of BmINR and BmAC6 on diapause phenotype and expression of diapause-related genes by RNA interference (RNAi) and overexpression techniques. The results showed that the mRNA expression levels of BmINR and BmAC6 in the head and ovary of NDEPs were higher than those in DEPs during the early and middle pupal stages. Furthermore, when BmINR was knocked down in the NDEPs, approximately 14.43% of eggs were in light red color and subsequently changed into gray-purple color after 48 hours post-oviposition, then stayed in a diapause state. On the other hand, overexpression of BmINR or BmAC6 via recombinant baculoviruses did not cause any obvious phenotypic alterations in NDEPs, but it upregulated the expression of genes related to carbohydrate metabolism, which provides energy for embryonic growth and development. Therefore, it can be concluded that BmINR and BmAC6 genes regulate embryonic diapause in bivoltine B. mori.