Identification of a novel function of the silkworm integument in nitrogen metabolism: Uric acid is synthesized within the epidermal cells in B. mori

Studying the role of Bombyx mori molybdenum cofactor sulfurase in Bombyx mori nucleopolyhedrovirus infection

Molybdenum cofactor sulfurase (MoCoS) is a key gene involved in the uric acid metabolic pathway that activates xanthine dehydrogenase to synthesise uric acid. Uric acid is harmful to mammals but plays crucial roles in insects, one of which is the immune responses. However, the function of Bombyx mori MoCoS in response to BmNPV remains unclear. In this study, BmMoCoS was found to be relatively highly expressed in embryonic development, gonads and the Malpighian tubules. In addition, the expression levels of BmMoCoS were significantly upregulated in three silkworm strains with different levels of resistance after virus infection, suggesting a close link between them. Furthermore, RNAi and overexpression studies showed that BmMoCoS was involved in resistance to BmNPV infection, and its antivirus effects were found to be related to the regulation of uric acid metabolism, which was uncovered by inosine- and febuxostat-coupled RNAi and overexpression. Finally, the BmMoCoS-mediated uric acid pathway was preliminarily confirmed to be a potential target to protect silkworms from BmNPV infection. Overall, this study provides new evidence for elucidating the molecular mechanism of silkworms in response to BmNPV infection and new strategies for the prevention of viral infections in sericulture.

High-throughput and genome-scale targeted mutagenesis using CRISPR in a nonmodel multicellular organism, Bombyx mori

Large-scale genetic mutant libraries are powerful approaches to interrogating genotype-phenotype correlations and identifying genes responsible for certain environmental stimuli, both of which are the central goal of life science study. We produced the first large-scale CRISPR-Cas9-induced library in a nonmodel multicellular organism, Bombyx mori We developed a piggyBac-delivered binary genome editing strategy, which can simultaneously meet the requirements of mixed microinjection, efficient multipurpose genetic operation, and preservation of growth-defect lines. We constructed a single-guide RNA (sgRNA) plasmid library containing 92,917 sgRNAs targeting promoters and exons of 14,645 protein-coding genes, established 1726 transgenic sgRNA lines following microinjection of 66,650 embryos, and generated 300 mutant lines with diverse phenotypic changes. Phenomic characterization of mutant lines identified a large set of genes responsible for visual phenotypic or economically valuable trait changes. Next, we performed pooled context-specific positive screens for tolerance to environmental pollutant cadmium exposure, and identified KWMTBOMO12902 as a strong candidate gene for breeding applications in sericulture industry. Collectively, our results provide a novel and versatile approach for functional B. mori genomics, as well as a powerful resource for identifying the potential of key candidate genes for improving various economic traits. This study also shows the effectiveness, practicality, and convenience of large-scale mutant libraries in other nonmodel organisms.

Functional characterization of Bmcap in uric acid metabolism in the silkworm

After a millennium of domestication, numerous silkworm mutants have emerged that exhibit transparent epidermis, which is caused by abnormally low levels of uric acid. We identified the Bombyx mori gene Bmcap (BMSK0003832) as the homolog of cappuccino, a subunit of the biogenesis of lysosome-related organelles complex-1 (BLOC-1) that has been extensively characterized in human, mouse, and insect species, by analyzing the amino acid sequences of putative purine metabolism genes. Using the clustered regularly interspaced palindromic repeats (CRISPR) / CRISPR-associated protein 9 system, we disrupted Bmcap, resulting in decreased uric acid levels in the silkworm epidermis and a translucent skin phenotype. In the Bmcap mutant, the purine metabolism, nitrogen metabolism, pyrimidine metabolism, and membrane system were altered compared to the wild type. Biogenesis of lysosome-related organelle complex genes play a role in the pigmentation and biogenesis of lysosome-related organelles (LROs) in platelets, melanocytes, and megakaryocytes. LROs exhibit unique morphologies and functions in various tissues and cells. Investigation of the Bmcap mutant will enhance our understanding of the uric acid metabolic pathway in silkworms, and this mutant offers a valuable silkworm model for LRO studies.

Uric acid metabolism promotes apoptosis against Bombyx mori nucleopolyhedrovirus in silkworm, Bombyx mori

The white epidermis of silkworms is due to the accumulation of uric acid crystals. Abnormal silkworm uric acid metabolism decreases uric acid production, leading to a transparent or translucent phenotype. The oily silkworm op50 is a mutant strain with a highly transparent epidermis derived from the p50 strain. It shows more susceptibility to Bombyx mori nucleopolyhedrovirus (BmNPV) infection than the wild type; however, the underlying mechanism is unknown. This study analysed the changes in 34 metabolites in p50 and op50 at different times following BmNPV infection based on comparative metabolomics. The differential metabolites were mainly clustered in six metabolic pathways. Of these, the uric acid pathway was identified as critical for resistance in silkworms, as feeding with inosine significantly enhanced larval resistance compared to other metabolites and modulated other metabolic pathways. Additionally, the increased level of resistance to BmNPV in inosine-fed silkworms was associated with the regulation of apoptosis, which is mediated by the reactive oxygen species produced during uric acid synthesis. Furthermore, feeding the industrial strain Jingsong (JS) with inosine significantly increased the level of larval resistance to BmNPV, indicating its potential application in controlling the virus in sericulture. These results lay the foundation for clarifying the resistance mechanism of silkworms to BmNPV and provide new strategies and methods for the biological control of pests.

Probiotic Bacillus subtilis contributes to the modulation of gut microbiota and blood metabolic profile of hosts

Probiotic Bacillus subtilis has beneficial efficacy on host's health. The microbiota-gut-blood system (MGBS) plays a crucial role in maintaining the homeostasis of hosts. However, the mechanism by which the probiotic B. subtilis positively acts on the MGBS of hosts remains unclear. Herein, we used an interspecies animal model to explore the causal associations between this bacterium and the micro-ecology balance and circulatory homeostasis of hosts. Results showed that the body weight of hosts significantly increased after probiotic B. subtilis supplementation (P < 0.05). Enterococcus was found to be the most important microbial marker causing the intergroup differences observed herein, and its relative abundance remarkably increased after B. subtilis supplementation. In addition, the supplementation of B. subtilis induced significant alterations in the levels of circulating metabolites, such as serine, arginine, adenine, uric acid, and pyridoxal (P < 0.05), indicating that B. subtilis modulated the metabolic profile of blood circulation in the host. The metabolisms of amino acids, purine, and vitamin B were the primary pathways modulated by B. subtilis. In conclusion, probiotic B. subtilis substantially introduced subtle but positive changes in the host's gut microbiome, and it promoted the physiological activity of the host by modulating circulating metabolites. The study provides a theoretical reference for the application of probiotic B. subtilis to improve the health state of specific populations.

Imaginal disc growth factor is involved in melanin synthesis and energy metabolism in Bombyx mori

The imaginal disc growth factor (IDGF), belonging to the glycoside hydrolase 18 family, plays an important role in various physiological processes in insects. However, the detail physiological function of IDGF is still unclear. In this study, transcriptome analysis was performed on the fatbody isolated from staged control and BmIDGF mutant silkworm larvae. Transcriptional profiling revealed that the absence of BmIDGF significantly affected differentially expressed genes involved in tyrosine and purine metabolism, as well as multiple energy metabolism pathways, including glycolysis, galactose, starch, and sucrose metabolism. The interruption of BmIDGF caused similar and specific gene expression changes to male and female fatbody. Furthermore, a genome-scale metabolic network integrating metabolomic and transcriptomic datasets revealed 11 pathways significantly altered at the transcriptional and metabolic levels, including amino acid, carbohydrate, uric acid metabolism pathways, insect hormone biosynthesis, and ABC transporters. In conclusion, this multiomics analysis suggests that IDGF is involved in gene-metabolism interactions, revealing its unique role in melanin synthesis and energy metabolism. This study provides new insights into the physiological function of IDGF in insects.

Silkworm FoxL21 plays important roles as a regulator of ovarian development in both oogenesis and ovariole development

The ovary is an important organ in reproduction. In insects, especially lepidopteran insects, the oocytes and reproductive organs develop rapidly during the pupal stage. Despite their drastic morphological changes, the molecular mechanisms of ovary development are not fully understood. In this study, it is found that forkhead box transcription factor L2, member 1 (FoxL21), which is known to be involved in ovarian differentiation and maintenance in vertebrates, is required for the development of the ovary in the silkworm, Bombyx mori. FoxL21 was expressed in the ovary and ovariole during the larval and pupal stage, respectively. In silkworms in which FoxL21 was knocked out by genome editing, multiple ovarian dysfunctions, such as, abnormal egg formation, thinning of the ovariole sheaths, and defective connection of the oviductus geminus with the ovariole were observed. Finally, ovarian transplantation experiments using the knockout silkworms revealed that FoxL21 functions in the ovariole, but not in the oviductus geminus.

Nitrogen Acquisition Strategies Mediated by Insect Symbionts: A Review of Their Mechanisms, Methodologies, and Case Studies

Simple Summary

Nitrogen acquisition strategies mediated by insect symbionts through biological nitrogen fixation (BNF) and nitrogenous waste recycling (NWR) were reviewed and compared in our paper, and a model for nitrogen provisioning in insects was then constructed. In our model, (1) insects acquired nitrogen nutrition from food stuffs directly, and the subprime channels (e.g., BNF or NWR) for nitrogen provisioning were accelerated when the available nitrogen in diets could not fully support the normal growth and development of insects; (2) the NWR strategy was more accessible to more insects due to its energy conservation and mild reaction conditions; (3) ammonia produced by different channels was used for essential nitrogenous metabolites synthesis via the glutamine synthetase and glutamate synthase pathways.

Abstract

Nitrogen is usually a restrictive nutrient that affects the growth and development of insects, especially of those living in low nitrogen nutrient niches. In response to the low nitrogen stress, insects have gradually developed symbiont-based stress response strategies—biological nitrogen fixation and nitrogenous waste recycling—to optimize dietary nitrogen intake. Based on the above two patterns, atmospheric nitrogen or nitrogenous waste (e.g., uric acid, urea) is converted into ammonia, which in turn is incorporated into the organism via the glutamine synthetase and glutamate synthase pathways. This review summarized the reaction mechanisms, conventional research methods and the various applications of biological nitrogen fixation and nitrogenous waste recycling strategies. Further, we compared the bio-reaction characteristics and conditions of two strategies, then proposed a model for nitrogen provisioning based on different strategies.

Non-molting dwarf (nm-d) as a mutant of Bombyx mori with a defect in purine synthesis

There are several known non-molting mutations of the silkworm, Bombyx mori, including non-molting dwarf (nm-d). Larvae with this mutation hatch normally and start eating leaves, but die before the completion of the first ecdysis. Genetic analysis of the nm-d mutation would contribute to the isolation of essential genes for the larval development of lepidopteran insects. To identify the causative gene of the nm-d locus, we conducted RNA-seq based rough mapping. Using two sets of RNA-seq data, one from a pooled sample of normal larvae, and one from a pooled sample of nm-d larvae, the nm-d locus was narrowed to a 500 kb region. Among the genes located in this region, a nm-d-specific exon loss was identified in the Bombyx homolog of the ATIC (5-aminoimidazole-4-carboxamide ribonucleotide transformylase/Inosine 5'-monophosphate cyclohydrolase) (BmATIC) gene, which catalyzes the final two steps of the de novo purine biosynthetic pathway in mammals. PCR and subsequent sequencing analysis revealed that a region containing exon 9 of the BmATIC gene is deleted in the nm-d larvae. A knockout allele of the BmATIC gene (BmATIC^KO), that was generated using the CRISPR/Cas9 system, revealed that first instar knockout larvae died while exhibiting the dark brown larval body that is a typical feature of mutants that lack uric acid in the integument. Lethal larvae resulted from crosses between +/BmATIC^KO moths. The uric acid content in the whole-body of the first instar was drastically reduced in the nm-d larvae compared to normal larvae. These results indicated that the BmATIC gene is responsible for the nm-d phenotype, and that nm-d larvae have a defect in purine biosynthesis, including uric acid. We also discuss the possibility that the BmATIC mRNA is maternally transmitted to eggs. Our results indicated that RNA-seq based mapping using pooled samples is a practical method for the identification of the causative genes of lethal mutations.

Mechanisms of nitrogen excretion in insects

Avoiding the toxic effects of ammonia derived from catabolism of proteins and nucleic acids typically involves synthesis of the less soluble compound uric acid in insects, although some species which are not water stressed excrete ammonia directly. Some dipterans metabolize uric acid further to allantoin or urea. Uric acid plays diverse roles as a nitrogenous waste, nitrogen store, pigment, antioxidant and possibly a signaling molecule. Multiple transporters are implicated in urate transport, including members of the ABC and SLC families. Excretion of ammonia by the Malpighian tubules, hindgut, or anal papillae involves multiple transporters, including Na⁺/K⁺-ATPase, Rhesus glycoproteins, ammonia transporters (AMTs) and possibly a hyperpolarization-activated cyclic nucleotide-gated K⁺ channel (HCN).

5'-Nucleotidase Plays a Key Role in Uric Acid Metabolism of Bombyx mori

Uric acid (UA) is the end-product in the human purine metabolism pathway. The UA that accumulates in silkworm tissues is excreted as a nitrogen waste product. Here, we first validated that Bombyx mori has a homolog of the human gene that encodes the 5′-nucleotidase (5′N) involved in purine metabolism. The B. mori gene, Bm5′N, is located upstream of other genes involved in UA metabolism in the silkworm. Disruption of Bm5′N via the CRISPR/Cas9 system resulted in decreased UA levels in the silkworm epidermis and caused a translucent skin phenotype. When Bm5′N mutant silkworms were fed with the uric acid precursor inosine, the UA levels in the epidermis increased significantly. Furthermore, the metabolomic and transcriptomic analyses of Bm5′N mutants indicated that loss of the Bm5′N affected purine metabolism and the ABC transport pathway. Taken together, these results suggest that the UA pathway is conserved between the silkworm and humans and that the Bm5′N gene plays a crucial role in the uric acid metabolism of the silkworm. Thus, the silkworm may be a suitable model for the study of UA metabolism pathways relevant to human disease.

The evolution and genetics of lepidopteran egg and caterpillar coloration

Insect colors and color patterns have fascinated biologists for centuries. While extensive research has focused on the adult colors of Drosophila and butterflies, our understanding of how colors are generated and diversified in embryonic and larval stages remains limited, especially, the genetics behind the protective coloration of the immobile embryonic and larval stages. Lepidoptera, one of the most widespread and species-rich insect orders, are extremely helpful uncovering those mechanisms due to their remarkable diverse colors in eggs and caterpillars within or among species, and these colors usually are variable in different developmental stages or in response to different environments. Here we review the recent progress on coloration of lepidopteran eggs and caterpillars, focusing on the genetic basis, developmental mechanisms, ecology, and evolution underlying the remarkable color diversity.

A defect in purine nucleotide metabolism in the silkworm, Bombyx mori, causes a translucent larval integument and male infertility

p-oily (op) is a novel mutant of Bombyx mori exhibiting translucent larval integument and male infertility. Elucidation of the causative gene of the op mutant will help understand the genetic mechanism underlying larval integument coloration and male fertility. Using polymorphisms between B. mori and B. mandarina, the op locus was narrowed down to a 375-kb region. Using RNA-seq analysis, we found that op mutants have a frameshift mutation in the KWMTBOMO13770 gene located in the 375-kb region. A database search indicated that this gene is the human cytosolic 5'-nucleotidase II gene (cN-II) homolog in Bombyx, which mediates the conversion of inosine monophosphate (IMP) to inosine, a precursor of uric acid. CRISPR/Cas9-mediated knockout mutants of the Bm-cN-II gene showed translucent integuments, and there appeared translucent larvae in the crosses between knockout moths and +/op moths. Moreover, the translucent phenotype of, and decreased uric acid content in the larval integument caused by the mutations in the Bm-cN-II gene were rescued by oral administration of inosine. These results indicated that the Bm-cN-II gene is responsible for the op phenotype and that the molecular function of the Bm-cN-II gene is the conversion of IMP to inosine. We also discuss the genetic relationship between the Bm-cN-II gene and male fertility.

Imaginal disc growth factor maintains cuticle structure and controls melanization in the spot pattern formation of Bombyx mori

The complex stripes and patterns of insects play key roles in behavior and ecology. However, the fine-scale regulation mechanisms underlying pigment formation and morphological divergence remain largely unelucidated. Here we demonstrated that imaginal disc growth factor (IDGF) maintains cuticle structure and controls melanization in spot pattern formation of Bombyx mori. Moreover, our knockout experiments showed that IDGF is suggested to impact the expression levels of the ecdysone inducible transcription factor E75A and pleiotropic factors apt-like and Toll8/spz3, to further control the melanin metabolism. Furthermore, the untargeted metabolomics analyses revealed that BmIDGF significantly affected critical metabolites involved in phenylalanine, beta-alanine, purine, and tyrosine metabolism pathways. Our findings highlighted not only the universal function of IDGF to the maintenance of normal cuticle structure but also an underexplored space in the gene function affecting melanin formation. Therefore, this study furthers our understanding of insect pigment metabolism and melanin pattern polymorphisms.

The diverse stripe patterns of animals are usually used for warning or camouflage. However, the actual mechanisms underlying diverse stripe pattern formation remains largely unknown. This study provides direct evidence that imaginal disc growth factor (IDGF) maintains cuticle structure and controls melanization in the spot pattern formation. Our exhaustive knockout experiments reveal that BmIDGF is involved in the melanin pigmentation of Bombyx mori. We demonstrate that IDGF impacts the expression levels of the 20E-inducible transcription factor E75A and pleiotropic factors apt-like and Toll8/spz3, to further affect the melanin metabolism. Furthermore, the metabolome of BmIDGF gene deletion connects metabolism to gene function. Thus, this study shed light on not only the unique function of IDGF but also the molecular mechanism of spot pattern formation.

Gas Chromatography-Mass Spectrometry Based Midgut Metabolomics Reveals the Metabolic Perturbations under NaF Stress in Bombyx mori

Fluoride tolerance is an important economic trait in sericulture, especially in some industrial development regions. Analyses of physiological changes involving structural damage to the insect body and molecular analyses of some related genes have focused on this area; however, the changes that occur at the metabolic level of silkworms after eating fluoride-contaminated mulberry leaves remain unclear. Here, metabonomic analysis was conducted using gas chromatography-mass spectrometry (GC-MS) to analyze the changes in midgut tissue after NaF stress using silkworm strains 733xin (susceptible stain) and T6 (strain resistant to fluoride), which were previously reported by our laboratory. Differential metabolomics analysis showed that both T6 and 733xin strains displayed complex responses after exposure to 200 mg/kg NaF. The purine metabolism and arginine and proline metabolic pathways of fluoride-tolerant strains reached significant levels, among which 3′-adenylic acid and hypoxanthine were significantly upregulated, whereas guanine, allantoic acid, xanthine, N-acetyl-L-glutamic acid, and pyruvate were significantly downregulated. These metabolic pathways may be related to the fluoride tolerance mechanism of NaF poisoning and tolerant strains.

iTRAQ-Based Quantitative Proteomic Analysis of Digestive Juice across the First 48 Hours of the Fifth Instar in Silkworm Larvae

The silkworm is an oligophagous insect for which mulberry leaves are the sole diet. The nutrients needed for vital activities of the egg, pupal, and adult stages, and the proteins formed in the cocoon, are all derived from the larval stages. The silkworm feeds and grows quickly during the larval stages. In particular, the amount of leaf ingested and digested quickly increases from the ecdysis to the gluttonous stage in the fifth instar period. In this study, we used the iTRAQ proteomic technique to identify and analyze silkworm larval digestive juice proteins during this period. A total of 227 proteins were successfully identified. These were primarily serine protease activity, esterase activity, binding, and serine protease inhibitors, which were mainly involved in the digestion and overcoming the detrimental effects of mulberry leaves. Moreover, 30 genes of the identified proteins were expressed specifically in the midgut. Temporal proteomic analysis of digestive juice revealed developmental dynamic features related to molecular mechanisms of the principal functions of digesting, resisting pathogens, and overruling the inhibitory effects of mulberry leaves protease inhibitors (PIs) with a dynamic strategy, although overruling the inhibitory effects has not yet been confirmed by previous study. These findings will help address the potential functions of digestive juice in silkworm larvae.

Bombyx mori monocarboxylate transporter 9 (BmMCT9) is involved in the transport of uric acid in silkworm integument

In Bombyx mori, there are more than 30 mutant strains exhibiting the translucent larval skin resulting from a decrease in the uric acid content in epidermal cell. Of these, the Chinese translucent (oc) mutant presents a moderately translucent larval skin. Previously, we narrowed the region linked to the oc phenotype to approximately 234 kb by positional cloning, and found that BmMCT9 was severely suppressed in the mutant. Here, we analyzed the mutation and potential molecular function of BmMCT9. Sequence analysis showed that a 2,624-bp fragment of BmMCT9 promoter region was replaced by a 22-bp sequence in the mutant. Luciferase reporter gene assay confirmed that BmMCT9 promoter activity in the mutant was significantly lower than that in the wild type. Knockdown of BmMCT9 caused a translucent phenotype in the first-instar silkworm larvae. Immunoblotting analysis showed that BmMCT9 expression was severely reduced in the mutant than in the wild type, and immunofluorescence showed that BmMCT9 existed mainly within the cytoplasm of epidermal cells. Together, our results suggest that reduced levels of BmMCT9 were responsible for the translucent phenotype of oc mutant, and that BmMCT9 might function in intracellular vesicles facing the cytoplasm including urate granules in silkworm integument.