Proteomic analysis of Bombyx mori molting fluid: Insights into the molting process

RNAi of nuclear receptor E78 inhibits the cuticle formation in the molting process of spider mite, Tetranychus urticae

BACKGROUND

The two-spotted spider mite, Tetranychus urticae, is an important pest mite in agriculture worldwide. E78, as a member of the nuclear receptor superfamily and a downstream responsive gene of ecdysteroids, plays a crucial role in regulating physiological behaviors such as development and reproduction in insects. However, its function in mites remains unclear. The aim of this study was to explore how E78 functions in the molting process of spider mites.

RESULTS

In this study, reverse transcription quantitative polymerase chain reaction (RT-qPCR) experiments to analyze the expression pattern of TuE78 during the development of Tetranychus urticae, demonstrated that the expression level of TuE78 was higher during the molting state than that after the completion of molting, and it reached a peak expression level when the deutonymph mites entered the molting stage. RNA interference (RNAi)-mediated gene-silencing of TuE78 resulted in 95% deutonymph mite molt failure. A series of analysis under a light microscope, and scanning and transmission electron microscopy revealed that RNAi mites died within the exuvium without ecdysis, and that apolysis had started but the new cuticle was thin and the typical cuticular lamellae were absent, indicating blockage of the post-apolysial processes and explaining molt failure. Hence, transcriptome sequencing confirmed that the expression of cuticle protein and lipid metabolism-related genes was significantly affected after TuE78 silencing.

CONCLUSION

This study demonstrated that TuE78 participates in the molting process of Tetranychus urticae by regulating the post-apolysial processes with the formation of new cuticle and successful ecdysis. This in turn suggests the potential of TuE78 as a target for pest mite control and provides a theoretical basis for further exploration of the molecular regulatory mechanism of spider mite molting. © 2024 Society of Chemical Industry. Published by John Wiley & Sons Ltd.

Identification and functional characterization of the chitinase and chitinase-like gene family in Myzus persicae (Sulzer) during molting

BACKGROUND

The crucial role of insect chitinase in molting, pupation, and emergence renders it a promising target for pest control strategies. Despite the extensive investigation of chitinase genes in various pests, there is still a lack of systematic identification and functional analysis related to aphid chitinase.

RESULTS

We systematically identified a total of nine chitinase/chitinase-like genes and one ENGase gene, which included eight Cht genes, one IDGF gene, and one ENGase gene. Through phylogenetic analysis, the chitinase proteins were classified into nine distinct groups (I, II, III, V, VI, VIII, X, other, and ENGase). The expression profile revealed that the epidermis exhibited relatively high expression levels for three chitinase genes: MpCht5, MpCht7, and MpCht10. Furthermore, transcriptional levels of nine chitinase genes were up-regulated following treatment with 20-hydroxyecdysone (20E) hormone. Silencing MpCht3, MpCht5, MpCht7, MpCht10, and MpCht11-2 via RNA interference (RNAi) during the molting stage resulted in nymph shrinking, hindering normal molting and leading to death. Additionally, it was observed that silencing of MpIDGF induced the body color of the aphids to change from reddish brown to colorless after molting, culminating in eventual mortality.

CONCLUSION

Our findings suggest that chitinase/chitinase-like genes play a crucial role in the molting process of Myzus persicae. Utilizing RNAi technology, we aimed to elucidate the precise function of MpCht genes in the molting mechanism of M. persicae, this discovery establishes a significant theoretical foundation for future research on aphid control, with chitinase as the target. © 2024 Society of Chemical Industry.

Distinct enzyme activities of serine protease p37k in silkworm midgut and molting fluid

Serine proteases possess various biological functions. The serine protease p37k exhibits gelatinolytic activity in the silkworm midgut and degrades cuticular proteins in the molting fluid. In this study, we analyzed the activity changes of recombinant p37k (re-p37k) and p37k in the midgut and molting fluid of Bombyx mori. Firstly, in vitro-expressed re-p37k was activated when a 22 kDa band was observed by western blot. Re-p37k exhibits strong gelatinolytic activity, with the highest activity observed at pH 7.0-9.0 and 45 °C. Compared to p37k in the midgut, re-p37k loses thermal stability but can be restored by midgut extract or ions. E64, AEBSF, and an inhibitor cocktail inhibited the hydrolytic activity of re-p37k on epidermal proteins but did not inhibit the gelatinolytic activity. Subsequently, zymography showed that the positions of gelatinolytic band produced by p37k in the midgut and molting fluid were different, 35 kDa and 40 kDa, respectively. Finally, when heated midgut extract was added to re-p37k or molting fluid, the gelatinolytic band shifted from 40 kDa to 35 kDa, and the proteolytic activity of p37k in the molting fluid was inhibited. Collectively, our results demonstrate that p37k exhibits different activities in various tissues, suggesting its distinct tissue-specific functions during insect metamorphosis.

Functional importance of groups I and II chitinases, CHT5 and CHT10, in turnover of chitinous cuticle during embryo hatching and post-embryonic molting in the red flour beetle, Tribolium castaneum

Chitinases (CHT) comprise a large gene family in insects and have been classified into at least eleven subgroups. Many studies involving RNA interference (RNAi) have demonstrated that depletion of group I (CHT5s) and group II (CHT10s) CHT transcripts causes lethal molting arrest in several insect species including the red flour beetle, Tribolium castaneum, presumably due to failure of degradation of chitin in their old cuticle. In this study we investigated the functions of CHT5 and CHT10 in turnover of chitinous cuticle in T. castaneum during embryonic and post-embryonic molting stages. RNAi and transmission electron microscopic (TEM) analyses indicate that CHT10 is required for cuticular chitin degradation at each molting period analyzed, while CHT5 is essential for pupal-adult molting only. We further analyzed the functions of these genes during embryogenesis in T. castaneum. Real-time qPCR analysis revealed that peak expression of CHT10 occurred prior to that of CHT5 during embryonic development as has been observed at post-embryonic molting periods in several other insect species. With immunogold-labeling TEM analysis using a fluorescein isothiocyanate-conjugated chitin-binding domain protein (FITC-CBD) probe, chitin was detected in the serosal cuticle but not in any other regions of the eggshell including the chorion and vitelline membrane layers. Injection of double-stranded RNA (dsRNA) for CHT5 (dsCHT5), CHT10 (dsCHT10) or their co-injection (dsCHT5/10) into mature adult females had no effect on their fecundity and the resulting embryos developed normally inside the egg. There were no obvious differences in the morphology of the outer chorion, inner chorion and vitelline membrane among eggs from these dsRNA-treated females. However, unlike dsCHT5 eggs, dsCHT10 and dsCHT5/10 eggs exhibited failure of turnover of the serosal cuticle in which the horizontal chitinous laminae remained intact, resulting in lethal embryo hatching defects. These results indicate that group I CHT5 is essential for pupal-adult molting, whereas group II CHT10 plays an essential role in cuticular chitin degradation in T. castaneum during both embryonic hatching and all of the post-embryonic molts. CHT10 can serve in place of CHT5 in chitin degradation, except during the pupal-adult molt when both enzymes are indispensable to complete eclosion.

Proteomic Analysis of the Midgut Contents of Silkworm in the Pupal Stage

The silkworm Bombyx mori, a lepidopteran insect, possesses an 8-10-day pupal stage, during which significant changes occur in the midgut, where it first condenses into the yellow body, and then undergoes decomposition. To gain insights into this transformation process, proteomics was performed on Bombyx mori midgut contents on day 2 and day 7 after pupation. The results revealed the identification of 771 proteins with more than one unique peptide. An analysis using AgriGO demonstrated that these proteins were predominantly associated with catalytic activity. Among the identified proteins, a considerable number were found to be involved in carbohydrate metabolism, amino acid metabolism, lipid metabolism, nucleic acid degradation, and energy support. Additionally, variations in the levels of certain proteases were observed between the midgut contents on day 2 and day 7 after pupation. An in-depth analysis of the two-dimensional electrophoresis of the midgut contents on day 7 after pupation led to the identification of twelve protein spots with potential gelatinolytic activity. Among these, six proteases were identified through mass spectrometry, including the p37k protease, vitellin-degrading protease, chymotrypsin-2, etc. These proteases may be responsible for the digestion of the yellow body during the later stages of pupal development.

Serpin-1a and serpin-6 regulate the Toll pathway immune homeostasis by synergistically inhibiting the Spätzle-processing enzyme CLIP2 in silkworm, Bombyx mori

The Toll receptor signaling pathway is an important innate immune response of insects to pathogen infection; its extracellular signal transduction involves serine protease cascade activation. However, excessive or constitutive activation of the Toll pathway can be detrimental. Hence, the balance between activation and inhibition of the extracellular protease cascade must be tightly regulated to achieve favorable outcomes. Previous studies have shown that serpins-serine protease inhibitors-negatively regulate insect innate immunity by inhibiting extracellular protease cascade signaling. Although the roles of serpins in insect innate immunity are well described, the physiological mechanisms underlying their synergistic effects remain poorly understand. Here, we characterize the molecular mechanism by which serpin-1a and serpin-6 synergistically maintain immune homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Through in vitro biochemical assays and in vivo bioassays, we demonstrate that clip-domain serine protease 2 (CLIP2), as the Toll cascade-activating terminal protease, is responsible for processing proSpätzle1 to induce the expression of antimicrobial peptides. Further biochemical and genetic analyses indicate that constitutively expressed serpin-1a and inducible serpin-6 synergistically target CLIP2 to maintain homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Taken together, this study provides new insights into the precise regulation of Toll cascade activation signals in insect innate immune responses and highlights the importance and complexity of insect immune homeostasis regulation.

A molting chemical cue (N-acetylglucosamine-6-phosphate) contributes to cannibalism of Chinese mitten crab Eriocheir sinensis

Under high-density culture, cannibalism occurs frequently during the molting of the Chinese mitten crabs Eriocheir sinensis, resulting in a large reduction in production. We found that the leakage of molting fluid from sexually immature crabs informs conspecifics that they are in a molting process. This hypothesis was verified through metabolomics analyses combined with behavioral experiments. The GlcNAc-6-P was identified as a molting biomarker from the differential metabolites by non-targeted metabolomics. In addition, we found that the concentration of GlcNAc-6-P in the molting fluid was significantly higher than other molting metabolites at different molting stages, reaching 5.84 μmol L-1, indicating that the molting fluid was the source of GlcNAc-6-P. Moreover, the behavioral experiments showed that crabs were actively approached to high concentrations of GlcNAc-6-P (1 μmol L-1), but had no obvious choice tendency at different concentrations of UTP, 20-HE and low concentrations of GlcNAc-6-P (0.1 μmol L-1, 0.01 μmol L-1) compared with the control groups. In conclusion, that E. sinensis by sensing the concentration change of GlcNAc-6-P can locate the source of GlcNAc-6-P release and actively approach the high concentration GlcNAc-6-P area and attack the molting crab, causing cannibalism. Blocking the reception pathway of molting chemical cues in E. sinensis, thereby preventing the perception of signals originating from conspecifics' molting in the vicinity, could lead to a reduction in cannibalistic behavior and an increase in overall production. Additionally, this method presents a prospective solution for addressing cannibalism in other crustacean species where such behavior is prevalent.

Group h Chitinase: A Molecular Target for the Development of Lepidopteran-Specific Insecticides

Sustainable agriculture requires insecticides that are selective between insects and mammals and even between harmful and beneficial insects. Lepidoptera includes the largest number of insect pests that threaten crops, and Hymenoptera contains the natural enemies for these pests. Discovery of lepidopteran-specific molecular targets is one route to develop such selective pesticides. Group h chitinase (Chi-h) is an ideal target for lepidopteran-specific insecticides because it is only distributed in Lepidoptera and is critical to their molting processes. This minireview focuses on the latest progress in developing Chi-h as a lepidopteran-specific insecticide target. We describe the biological function, crystal structure, and small-molecule inhibitors of the enzyme. Notably, two unique pockets were discovered in the crystal structure of Chi-h for the binding of the selective inhibitors, phlegmacin B₁ and lynamicin B. Moreover, lynamicin B was found to exhibit significant insecticidal activity toward lepidopteran pests but is harmless toward their natural enemies. These findings are advancing the development of selective insecticides to meet the needs of sustainable agriculture.

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.

Molting-related proteases in the brown planthopper, Nilaparvata lugens

Digestion and absorption of old cuticles during insect molting are necessary for new cuticle formation, during which complicated enzyme catalysis is essential. To date, a few carboxypeptidases, aminopeptidases and serine proteases (mostly trypsins) connected with cuticle digestion, zymogen activation and histological differentiation during the ecdysis of lepidopteran, dipteran and hymenopteran insects have been identified. However, little is known about these proteins in hemimetabolous insects. In this study, we identified 33 candidate trypsin and trypsin-like homologs, 14 metallocarboxypeptidase and 32 aminopeptidase genes in the brown planthopper Nilaparvata lugens, a hemipteran rice pest. Among the proteins encoded by these genes, 9 trypsin-like proteases, 3 metallocarboxypeptidases and 1 aminopeptidase were selected as potential procuticle hydrolases by bioinformatics analysis and in vivo validation. RNA interference targeting these genes demonstrated that 3 trypsin-like proteases (NlTrypsin-8, NlTrypsin-29 and NlTrypsin-32) genes and 1 metallocarboxypeptidase (NlCpB) gene were found to be essential for ecdysis in N. lugens; specifically, gene silencing led to incomplete cuticle degradation and arrested ecdysis, causing lethal morphological phenotype acquisition. Spatiotemporal expression profiling by quantitative PCR and western blotting revealed their specific expression in the integument and their periodic expression during each stadium, with a peak before ecdysis and eclosion. Transmission electron microscopy demonstrated corresponding ultrastructural defects after RNAi targeting, with NlCpB-silenced specimens having the most undigested old procuticles. Immunohistochemical staining revealed that NlTrypsin-8, NlTrypsin-29 and NlCpB were predominantly located in the exuvial space. This research further adds to our understanding of proteases and its potential role in insect ecdysis.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

A Blueprint of Microstructures and Stage-Specific Transcriptome Dynamics of Cuticle Formation in Bombyx mori

Insect cuticle is critical for the environmental adaptability and insecticide resistance of insects. However, there is no clear understanding of the structure and protein components of the cuticle during each developmental stage of holometabolous insects, and knowledge about the protein components within each layer is vague. We conducted serial sectioning, cuticular structure analysis, and transcriptome sequencing of the larval, pupal, and adult cuticles of Bombyx mori. The deposition processes of epicuticle, exocuticle, and endocuticle during larval, pupal, and adult cuticle formation were similar. Transcriptome analysis showed that these cuticle formations share 74% of the expressed cuticular protein (CP) genes and 20 other structural protein genes, such as larval serum protein and prisilkin. There are seven, six, and eleven stage-specific expressed CP genes in larval, pupal, and adult cuticles, respectively. The types and levels of CP genes may be the key determinants of the properties of each cuticular layer. For example, the CPs of the RR-2 protein family with high contents of histidine (His) are more essential for the exocuticle. Functional analysis suggested that BmorCPAP1-H is involved in cuticle formation. This study not only offers an in-depth understanding of cuticle morphology and protein components but also facilitates the elucidation of molecular mechanisms underlying cuticle formation in future studies.

Homeodomain proteins POU-M2, antennapedia and abdominal-B are involved in regulation of the segment-specific expression of the clip-domain serine protease gene CLIP13 in the silkworm, Bombyx mori

Clip-domain serine proteases (CLIPs) play important roles in insect innate immunity and development. Our previous studies indicated that CLIP13, an epidermis-specific gene, was involved in cuticle remodeling during molting and metamorphosis in the silkworm, Bombyx mori. However, the transcriptional regulatory mechanism and regulatory pathways of CLIP13 remained unclear. In the present study, we investigated CLIP13 expression and the regulation pathway controlled by 20-hydroxyecdysone (20E) in the silkworm. At the transcriptional level, expression of CLIP13 exhibited pronounced spatial and temporal specificity in different regions of the epidermis; homeodomain transcription factors POU-M2, antennapedia (Antp), and abdominal-B (Abd-B) showed similar expression change trends as CLIP13 in the head capsule, thorax, and abdomen, respectively. Furthermore, results of cell transfection assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation demonstrated that POU-M2, Antp, and Abd-B were involved in the transcriptional regulation of CLIP13 by directly binding to their cis-response elements in CLIP13 promoter. RNA interference-mediated silencing of POU-M2, Antp, and Abd-B led to a decrease of CLIP13 expression in the head capsule, the epidermis of the 1st to 3rd thoracic segments and the 7th to 10th abdominal segments, respectively. Consistent with CLIP13, 20E treatment significantly upregulated expression of POU-M2, Antp, and Abd-B in the silkworm epidermis. Taken together, these data suggest that 20E positively regulates transcription of CLIP13 via homeodomain proteins POU-M2, Antp, and Abd-B in different regions of the silkworm epidermis during metamorphosis, thus affecting the molting process. Our findings provide new insight into the functions of homeodomain transcription factors in insect molting.

The pupal moulting fluid has evolved social functions in ants

Insect societies are tightly integrated, complex biological systems in which group-level properties arise from the interactions between individuals^1-4. However, these interactions have not been studied systematically and therefore remain incompletely known. Here, using a reverse engineering approach, we reveal that unlike solitary insects, ant pupae extrude a secretion derived from the moulting fluid that is rich in nutrients, hormones and neuroactive substances. This secretion elicits parental care behaviour and is rapidly removed and consumed by the adults. This behaviour is crucial for pupal survival; if the secretion is not removed, pupae develop fungal infections and die. Analogous to mammalian milk, the secretion is also an important source of early larval nutrition, and young larvae exhibit stunted growth and decreased survival without access to the fluid. We show that this derived social function of the moulting fluid generalizes across the ants. This secretion thus forms the basis of a central and hitherto overlooked interaction network in ant societies, and constitutes a rare example of how a conserved developmental process can be co-opted to provide the mechanistic basis of social interactions. These results implicate moulting fluids in having a major role in the evolution of ant eusociality.

An MD-2-related lipid-recognition protein is required for insect reproduction and integument development

The myeloid differentiation factor 2 (MD-2)-related lipid-recognition protein is involved in immune responses through recognizing bacteria lipopolysaccharide in mammals, arthropods and plants. However, the physiological roles of MD-2 in other biological processes are largely unknown. Here, we identified three homologue MD-2 genes (NlML1, NlML2 and NlML3) by searching the genome and transcriptome databases of the brown planthopper Nilaparvata lugens, a hemipteran insect species. Temporospatial analysis showed that the NlML1 gene was highly expressed in the fat body but much less so in the other tissues, while the NlML2 and NlML3 genes were highly expressed in the testis or digestive tract. RNA interference-mediated depletion of the NlML1 gene significantly downregulated the transcription of numerous integument protein genes. The NlML1 knockdown led to moulting failure and mortality at the nymph-adult transition phase, impaired egg laying and hatching, and reduced 20-hydroxyecdysone (20E) production in the nymphs. 20E could rescue the deficient moulting phenotypes derived from dsNlML1 RNAi. These novel findings indicate that NlML1 is required for nymphal moulting and female reproductive success as it plays an important role in regulating 20E synthesis, lipid and chitin metabolisms in N. lugens, thus contributing to our understanding of developmental and reproductive mechanisms in insects.

Trypsin-type serine protease p37k hydrolyzes CPAP3-type cuticle proteins in the molting fluid of the silkworm Bombyx mori

Cuticular proteins analogous to peritrophin 3 (CPAP3)-type cuticle proteins constitute a family of proteins with three chitin-binding domains (CBDs) that play an important role in cuticle formation by associating with chitin. In our previous study, we identified CPAP3-type cuticle proteins in the silkworm genome, of which we characterized CPAP3-A2 (BmCBP1), a protein highly expressed in the epidermis. In this study, to elucidate the digestion mechanism of CPAP3-type cuticle proteins, we incubated CPAP3-A2 with molting fluid in vitro and found that its hydrolysis, which was inhibited by serine and cysteine protease inhibitors, produced two major bands with a molecular weight of approximately 22 kD and 11 kD. A trypsin-type serine protease, p37k, was presumed to be responsible for hydrolyzing CPAP3-A2 based on liquid chromatography-tandem mass spectrometry analysis of naturally purified molting fluid. To verify this, p37k was subsequently expressed in Sf9 cells using the Bac-to-Bac baculovirus expression system. In its active form, the recombinant protease could successfully hydrolyze CPAP3-A2. Finally, we analyzed the CPAP3-A2 molting fluid digestion site. When arginine 169 of CPAP3-A2 was mutated to alanine, a weaker hydrolysis of mutant CPAP3-A2 was observed compared to that of normal CPAP3-A2. Collectively, we identified a trypsin-type serine protease that is involved in the degradation of CPAP3-type cuticle proteins, including CPAP3-A2, suggesting that this protease plays an important role during molting in Bombyx mori. These findings provide the basis for further elucidation of the mechanisms underlying insect molting and metamorphosis.

Comparative analysis of stalked and acorn barnacle adhesive proteomes

Barnacles interest the scientific community for multiple reasons: their unique evolutionary trajectory, vast diversity and economic impact—as a harvested food source and also as one of the most prolific macroscopic hard biofouling organisms. A common, yet novel, trait among barnacles is adhesion, which has enabled a sessile adult existence and global colonization of the oceans. Barnacle adhesive is primarily composed of proteins, but knowledge of how the adhesive proteome varies across the tree of life is unknown due to a lack of genomic information. Here, we supplement previous mass spectrometry analyses of barnacle adhesive with recently sequenced genomes to compare the adhesive proteomes of Pollicipes pollicipes (Pedunculata) and Amphibalanus amphitrite (Sessilia). Although both species contain the same broad protein categories, we detail differences that exist between these species. The barnacle-unique cement proteins show the greatest difference between species, although these differences are diminished when amino acid composition and glycosylation potential are considered. By performing an in-depth comparison of the adhesive proteomes of these distantly related barnacle species, we show their similarities and provide a roadmap for future studies examining sequence-specific differences to identify the proteins responsible for functional differences across the barnacle tree of life.

An apocrine mechanism delivers a fully immunocompetent exocrine secretion

Apocrine secretion is a recently discovered widespread non-canonical and non-vesicular secretory mechanism whose regulation and purpose is only partly defined. Here, we demonstrate that apocrine secretion in the prepupal salivary glands (SGs) of Drosophila provides the sole source of immune-competent and defense-response proteins to the exuvial fluid that lies between the metamorphosing pupae and its pupal case. Genetic ablation of its delivery from the prepupal SGs to the exuvial fluid decreases the survival of pupae to microbial challenges, and the isolated apocrine secretion has strong antimicrobial effects in "agar-plate" tests. Thus, apocrine secretion provides an essential first line of defense against exogenously born infection and represents a highly specialized cellular mechanism for delivering components of innate immunity at the interface between an organism and its external environment.

Label-free quantitative proteomic analysis of insect larval and metamorphic molts

Background

Molting is an essential biological process occurring characteristic times throughout the life cycle of holometabolous insects. However, it is not clear how insects determine the direction of molting to remain status quo or to initiate metamorphosis. To explore the functional factors that determine the direction of molts, liquid chromatography-mass spectrometry was used to identify the molecules involved in larval and metamorphic molting, and the differentially expressed proteins (DEPs) were compared in the two processes.

Results

There were 321 and 1140 DEPs identified in larval and metamorphic molting process, respectively. Bioinformatics analyses show that the amino sugar pathway was up-regulated in both processes. The up-regulated protease contributed to the metamorphosis. In addition, several proteins with different expression patterns in larval-larval and larval-pupal transitions, including Endochitinase, GRIM-19 (Genes associated with retinoid-IFN-induced mortality-19), IDE (Insulin-degrading enzyme), Sorcin (Soluble resistance related calcium binding protein), OBP (Odorant-binding protein-2 precursor), TRAP1(Tumor necrosis factor receptor associated protein-1), etc., were further identified by parallel reaction monitoring, which may play diverse functions in larval-larval and larval-pupal transitions.

Conclusions

These results provide a proteomic insight into molecules involved in larval and metamorphic molts, and will likely improve the current understanding of determination of direction of molts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-020-00227-z.

Identification of the major allergenic proteins from silkworm moth (Bombyx mori) involved in respiratory allergic diseases

INTRODUCTION AND OBJECTIVES

Moths are a significant source of indoor and outdoor aeroallergens. High prevalence of IgE-mediated sensitization was demonstrated in a group of patients with allergic respiratory diseases. There are no studies on adult stage of these moth species allergens involved in allergic respiratory reactions - the aim of this study.

MATERIAL AND METHODS

36 participants were included in an experimental study, submitted to skin prick test with Bombyx mori wing extract and six other common allergens, as well as Western blot analysis with incubated nitrocellulose membrane impregnated with silkworm moth extract and human IgE-antibody. The participants were divided into 3 groups: 1) 21 allergic patients whose skin prick test was positive to Bombyx mori wing extract, 2) eight allergic patients whose skin prick test was positive to mite and negative to Bombyx mori extract 3) seven negative non-allergic subjects.

RESULTS

Among the 21 participants from group 1, 19 serum samples reacted to Bombyx mori extract by Western blot. All of them reacted to a protein at 80 kDa and five other proteins (66, 50, 45, 37 and 30 kDa) were identified in more than 50% of the individuals tested, considered as major allergenic proteins. Sera from seven out of eight patients sensitized to house dust mite demonstrated IgE-reactivity to Bombyx mori extract by Western blot analysis. Serum samples from healthy participants did not react at all.

CONCLUSION

Six major reactive proteins by immunoblot analysis from moth's wings sensitized patients can be potential allergens. The one at 80 kDa is the major protein, seen in all IgE-reactive patients from group 1 and in none from group 2, yet to be identified. Future studies should be conducted to better characterize these proteins.

Proteolytic profile of larval developmental stages of Penaeus vannamei: An activity and mRNA expression approach

In arthropods, the cleavage of specific proteins by peptidases has pivotal roles in multiple physiological processes including oogenesis, immunity, nutrition, and parasitic infection. These enzymes are also key players in the larval development, and well-described triggers of molting and metamorphosis. In this work the peptidase complement throughout the larvae development of Penaeus vannamei was quantified at the transcript and activity level using qPCR and fluorogenic substrates designed to be hydrolyzed by class-specific peptidases respectively, providing a detailed identification of the proteolytic repertoire in P. vannamei larvae. Significant changes in the peptidase activity profile were observed. During the lecithotrophic naupliar instars, the dominant peptidase activity and expression derive from cysteine peptidases, suggesting that enzymes of this class hydrolyze the protein components of yolk as the primary amino acid source. At the first feeding instar, zoea, dominant serine peptidase activity was found where trypsin activity is particularly high, supporting previous observations that during zoea the breakdown of food protein is primarily enzymatic. At decapodid stages the peptidase expression and activity is more diverse indicating that a multienzyme network achieves food digestion. Our results suggest that proteolytic enzymes fulfill specific functions during P. vannamei larval development.

Genomic adaptations to aquatic and aerial life in mayflies and the origin of insect wings

The evolution of winged insects revolutionized terrestrial ecosystems and led to the largest animal radiation on Earth. However, we still have an incomplete picture of the genomic changes that underlay this diversification. Mayflies, as one of the sister groups of all other winged insects, are key to understanding this radiation. Here, we describe the genome of the mayfly Cloeon dipterum and its gene expression throughout its aquatic and aerial life cycle and specific organs. We discover an expansion of odorant-binding-protein genes, some expressed specifically in breathing gills of aquatic nymphs, suggesting a novel sensory role for this organ. In contrast, flying adults use an enlarged opsin set in a sexually dimorphic manner, with some expressed only in males. Finally, we identify a set of wing-associated genes deeply conserved in the pterygote insects and find transcriptomic similarities between gills and wings, suggesting a common genetic program. Globally, this comprehensive genomic and transcriptomic study uncovers the genetic basis of key evolutionary adaptations in mayflies and winged insects.

Insect Cuticular Chitin Contributes to Form and Function

Chitin contributes to the rigidity of the insect cuticle and serves as an attachment matrix for other cuticular proteins. Deficiency of chitin results in abnormal embryos, cuticular structural defects and growth arrest. When chitin is not turned over during molting, the developing insect is trapped inside the old cuticle. Partial deacetylation of cuticular chitin is also required for proper laminar organization of the cuticle and vertical pore canals, molting, and locomotion. Thus, chitin and its modifications strongly influence the structure of the exoskeleton as well as the physiological functions of the insect. Internal tendons and specialized epithelial cells called "tendon cells" that arise from the outer layer of epidermal cells provide attachment sites at both ends of adult limb muscles. Membrane processes emanating from both tendon and muscle cells interdigitate extensively to strengthen the attachment of muscles to the extracellular matrix (ECM). Protein ligands that bind to membrane-bound integrin complexes further enhance the adhesion between muscles and tendons. Tendon cells contain F-actin fiber arrays that contribute to their rigidity. In the cytoplasm of muscle cells, proteins such as talin and other proteins provide attachment sites for cytoskeletal actin, thereby increasing integrin binding and activation to mechanically couple the ECM with actin in muscle cells. Mutations in integrins and their ligands, as well as depletion of chitin deacetylases, result in defective locomotion and muscle detachment from the ECM. Thus, chitin in the cuticle and chitin deacetylases strongly influence the shape and functions of the exoskeleton as well as locomotion of insects.

Identification, characterization, and expression analysis of clip-domain serine protease genes in the silkworm, Bombyx mori

Clip-domain serine proteases (CLIPs), characterized by regulatory module clip domains, constitute an important serine protease family identified in insects and other arthropods. They participate in host immune response and embryonic development in a cascade-activated manner. Here, we present a genome-wide identification and expression analysis of CLIP genes in the silkworm, Bombyx mori. A total of 26 CLIP genes were identified in the silkworm genome. Bioinformatics analysis indicated that these CLIPs clustered into four subfamilies (CLIPA-D), and exhibit a close evolutionary relationship with CLIPs of Manduca sexta. Tissue expression profiling revealed that silkworm CLIP genes are mainly expressed in the integument, head, fat body, and hemocytes. Temporal expression profiles showed that 15 CLIP genes were predominantly expressed during the fifth-instar larval stage, early and later period of the pupal stage, and adult stage, whereas 10 CLIP genes were mainly expressed in the wandering stage and middle to later period of the pupal stage in the integument. Pathogens and 20-hydroxyecdysone (20E) induction analysis indicated that 14 CLIP genes were positively regulated by 20E, 9 were negatively regulated by 20E but positively regulated by pathogens, and 5 were positively regulated by both factors in the integument. Together, these results suggested that silkworm CLIP genes may play multiple functions in integument development, including melanization of new cuticle, molting and immune defense. Our data provide a comprehensive understanding of CLIP genes in the silkworm integument and lays a foundation for further functional studies of CLIP genes in the silkworm.

Transcriptome of pleuropodia from locust embryos supports that these organs produce enzymes enabling the larva to hatch

Background

Pleuropodia are limb-derived glandular organs that transiently appear on the first abdominal segment in embryos of insects from majority of “orders”. They are missing in the genetic model Drosophila and little is known about them. Experiments carried out on orthopteran insects 80 years ago indicated that the pleuropodia secrete a “hatching enzyme” that digests the serosal cuticle to enable the larva to hatch, but evidence by state-of-the-art molecular methods is missing.

Results

We used high-throughput RNA-sequencing to identify the genes expressed in the pleuropodia of the locust Schistocerca gregaria (Orthoptera). First, using transmission electron microscopy we studied the development of the pleuropodia during 11 stages of the locust embryogenesis. We show that the glandular cells differentiate and start secreting just before the definitive dorsal closure of the embryo and the secretion granules outside the cells become more abundant prior to hatching. Next, we generated a comprehensive embryonic reference transcriptome for the locust and used it to study genome wide gene expression across ten morphologicaly defined stages of the pleuropodia. We show that when the pleuropodia have morphological markers of functional organs and produce secretion, they are primarily enriched in transcripts associated with transport functions. They express genes encoding enzymes capable of digesting cuticular protein and chitin. These include the potent cuticulo-lytic Chitinase 5, whose transcript rises just before hatching. Unexpected finding was the enrichment in transcripts for immunity-related enzymes. This indicates that the pleuropodia are equipped with epithelial immunity similarly as barrier epithelia in postembryonic stages.

Conclusions

These data provide transcriptomic support for the historic hypothesis that pleuropodia produce cuticle-degrading enzymes and function in hatching. They may also have other functions, such as facilitation of embryonic immune defense. By the genes that they express the pleuropodia are specialized embryonic organs and apparently an important though neglected part of insect physiology.