miRNA-34 and miRNA-210 target hexamerin genes enhancing their differential expression during early brain development of honeybee (Apis mellifera) castes

miR-306-5p is involved in chitin metabolism in Aedes albopictus pupae via linc8338-miR-306-5p-XM_019678125.2 axis

Aedes albopictus can transmit several lethal arboviruses. This mosquito has become a sever public health threat due to its rapidly changing global distribution. Chitin, which is the major component of the cuticle and peritrophic membrane (PM), is crucial for the growth and development of insect. microRNAs (miRNAs) play important roles in the posttranscriptional level regulation of gene expression, thereby influencing many biological processes in insects. In this study, an attempt was made to evaluate the role of miR-306-5p in regulating chitin metabolism in Ae. albopictus pupae. Overexpression of miR-306-5p resulted in a significantly reduced survival rate in pupae and an increased malformation rate in adults. Both in vivo and in vitro evidence confirmed the presence of the competing endogenous RNA (ceRNA) regulatory axis (linc8338-miR-306-5p-XM_019678125.2). RNAi of linc8338 and XM_019678125.2 had effects on pupae similar to those of miR-306-5p. The highest expression level of miR-306-5p was found in the midgut, and alteration in the expression of miR-306-5p, XM_019678125.2 and linc8338 induced increased transcript levels of chitin synthase 2 (AaCHS2) and decreased chitinase 10 (AaCht10); as well as increased thickness of the midgut and enlarged midgut epithelial cells. The results of this study highlight the potential of miR-306-5p as a prospective target in mosquito control and confirm that the ceRNA mechanism is involved in chitin metabolism. These findings will provide a basis for further studies to uncover the molecular mechanisms through which ncRNAs regulate chitin metabolism.

Regulation of soldier caste differentiation by microRNAs in Formosan subterranean termite (Coptotermes formosanus Shiraki)

The soldier caste is one of the most distinguished castes inside the termite colony. The mechanism of soldier caste differentiation has mainly been studied at the transcriptional level, but the function of microRNAs (miRNAs) in soldier caste differentiation is seldom studied. In this study, the workers of Coptotermes formosanus Shiraki were treated with methoprene, a juvenile hormone analog which can induce workers to transform into soldiers. The miRNomes of the methoprene-treated workers and the controls were sequenced. Then, the differentially expressed miRNAs (DEmiRs) were corrected with the differentially expressed genes DEGs to construct the DEmiR-DEG regulatory network. Afterwards, the DEmiR-regulated DEGs were subjected to GO enrichment and KEGG enrichment analysis. A total of 1,324 miRNAs were identified, among which 116 miRNAs were screened as DEmiRs between the methoprene-treated group and the control group. A total of 4,433 DEmiR-DEG pairs were obtained. No GO term was recognized as significant in the cellular component, molecular function, or biological process categories. The KEGG enrichment analysis of the DEmiR-regulated DEGs showed that the ribosome biogenesis in eukaryotes and circadian rhythm-fly pathways were enriched. This study demonstrates that DEmiRs and DEGs form a complex network regulating soldier caste differentiation in termites.

MicroR-9c-5p and novel-mir50 co-target Akt to regulate Lasioderma serricorne reproduction

High fecundity is a common characteristic of insect pests which increases the difficulty of population control. Serine/threonine kinase Akt is an indispensable component of the insulin signaling pathway. Silencing of LsAkt severely hinders reproduction in Lasioderma serricorne, a stored product insect pest. However, the post-transcriptional pathway of LsAkt in L. serricorne remains unknown. This study identified 2 binding sites of miR-9c-5p and novel-mir50 in the coding sequences of LsAkt. The expression profiles of 2 microRNAs (miRNAs) and LsAkt displayed an opposite pattern during the adult stages. Luciferase reporter assay showed that novel-mir50 and miR-9c-5p could downregulate the expression of LsAkt. Overexpression of miR-9c-5p and novel-mir50 by injection of mimics inhibited the expression of LsAkt and reduced oviposition, decreased egg hatchability, and blocked ovarian development. It also decreased the expression of genes involved in ovarian development (LsVg and LsVgR) and the nutritional signaling pathway (LsTOR, LsS6K, and Ls4EBP), and reduced the phosphorylation of Akt. Conversely, injection of miR-9c-5p and novel-mir50 inhibitors induced the expressions of LsAkt, LsVg, LsVgR, LsTOR, LsS6K, and Ls4EBP, enhanced Akt phosphorylation level, and accelerated ovarian development. Injection of bovine insulin downregulated the expression of miR-9c-5p and novel-mir50 and upregulated the LsAkt expression. It also rescued the reproductive development defects associated with miR-9c-5p/novel-mir50 overexpression, forming a positive regulatory loop of insulin signaling. These results indicate that miR-9c-5p/novel-mir50 regulates the female reproduction of L. serricorne by targeting Akt in response to insulin signaling. The data also demonstrate the effects of the insulin/miRNA/Akt regulatory axis in insect reproduction.

Exposure to sublethal concentrations of thiacloprid insecticide modulated the expression of microRNAs in honeybees (Apis mellifera L.)

This study aimed to investigate the sublethal effects of thiacloprid on microRNA (miRNA) expression in honeybees (Apis mellifera L.) and the role of a specific miRNA, ame-miR-283-5p, in thiacloprid resistance. The high-throughput small RNA-sequencing was used to analyze global miRNA expression profile changes in honeybees orally exposed to sublethal concentrations of thiacloprid (20 mg/L and 4 mg/L) for 72 h. Thiacloprid at 20 mg/L had no observed adverse effects. In addition, bees were fed with miRNA mimics or inhibitors to increase or decrease ame-miR-283-5p expression, and its effects on P450 gene expression (CYP9Q2 and CYP9Q3) were examined. Thiacloprid susceptibility was also detected. The results showed that treatment with thiacloprid at 20 mg/L and 4 mg/L induced 11 and five differentially expressed miRNAs (DEMs), respectively. Bioinformatic analysis suggested that the DEMs are mainly involved in the regulation of growth and development, metabolism, nerve conduction, and behavior. ame-miR-283-5p was downregulated by both concentrations, which was validated using quantitative real-time reverse transcription PCR analysis. Enhancing ame-miR-283-5p expression significantly inhibited CYP9Q2 mRNA and protein expression, and increased thiacloprid toxicity, while reducing ame-miR-283-5p expression significantly promoted CYP9Q2 expression, and decreased thiacloprid susceptibility. Our study revealed a novel role of miRNAs in insecticide resistance in honeybees.

Genome-wide analysis of the chromatin sites targeted by HEX 70a storage protein in the honeybee brain and fat body

Hexamerins, the proteins massively stored in the larval haemolymph of insects, are gradually used throughout metamorphosis as a source of raw material and energy for the development of adult tissues. Such behaviour defined hexamerins as storage proteins. Immunofluorescence experiments coupled with confocal microscopy show a hexamerin, HEX 70a, in the nucleus of the brain and fat body cells from honeybee workers, an unexpected localization for a storage protein. HEX 70a colocalizes with fibrillarin, a nucleolar-specific protein and H3 histone, thus suggesting a potential role as a chromatin-binding protein. This was investigated through chromatin immunoprecipitation and high-throughput DNA sequencing (ChIP-seq). The significant HEX 70a-DNA binding sites were mainly localized at the intergenic, promoter and intronic regions. HEX 70a targeted DNA stretches mapped to the genomic regions encompassing genes with relevant functional attributes. Several HEX 70a targeted genes were associated with H3K27ac or/and H3K27me3, known as active and repressive histone marks. Brain and fat body tissues shared a fraction of the HEX 70 targeted genes, and tissue-specific targets were also detected. The presence of overrepresented DNA motifs in the binding sites is consistent with specific HEX 70a-chromatin association. In addition, a search for HEX 70a targets in RNA-seq public libraries of fat bodies from nurses and foragers revealed differentially expressed targets displaying hex 70a-correlated developmental expression, thus supporting a regulatory activity for HEX 70a. Our results support the premise that HEX 70a is a moonlighting protein that binds chromatin and has roles in the brain and fat body cell nuclei, apart from its canonical role as a storage protein.

The Potential Biological Roles of Circular RNAs in the Immune Systems of Insects to Pathogen Invasion

Circular RNAs (circRNAs) are a newly discovered class of endogenously expressed non-coding RNAs (ncRNAs). They are highly stable, covalently closed molecules that frequently exhibit tissue-specific expression in eukaryotes. A small number of circRNAs are abundant and have been remarkably conserved throughout evolution. Numerous circRNAs are known to play important biological roles by acting as microRNAs (miRNAs) or protein inhibitors ('sponges'), by regulating the function of proteins, or by being translated themselves. CircRNAs have distinct cellular functions due to structural and production differences from mRNAs. Recent advances highlight the importance of characterizing circRNAs and their targets in a variety of insect species in order to fully understand how they contribute to the immune responses of these insects. Here, we focus on the recent advances in our understanding of the biogenesis of circRNAs, regulation of their abundance, and biological roles, such as serving as templates for translation and in the regulation of signaling pathways. We also discuss the emerging roles of circRNAs in regulating immune responses to various microbial pathogens. Furthermore, we describe the functions of circRNAs encoded by microbial pathogens that play in their hosts.

ame-miR-34 Modulates the Larval Body Weight and Immune Response of Apis mellifera Workers to Ascosphara apis Invasion

MiRNAs are critical regulators of numerous physiological and pathological processes. Ascosphaera apis exclusively infects bee larvae and causes chalkbrood disease. However, the function and mechanism of miRNAs in the bee larval response to A. apis infection is poorly understood. Here, ame-miR-34, a previously predicted miRNA involved in the response of Apis mellifera larvae to A. apis invasion, was subjected to molecular validation, and overexpression and knockdown were then conducted to explore the regulatory functions of ame-miR-34 in larval body weight and immune response. Stem-loop RT-PCR and Sanger sequencing confirmed the authenticity of ame-miR-34 in the larval gut of A. mellifera. RT-qPCR results demonstrated that compared with that in the uninfected larval guts, the expression level of ame-miR-34 was significantly downregulated (p < 0.001) in the guts of A. apis-infected 4-, 5-, and 6-day-old larvae, indicative of the remarkable suppression of host ame-miR-34 due to A. apis infection. In comparison with the corresponding negative control (NC) groups, the expression level of ame-miR-34 in the larval guts in the mimic-miR-34 group was significantly upregulated (p < 0.001), while that in the inhibitor-miR-34 group was significantly downregulated (p < 0.01). Similarly, effective overexpression and knockdown of ame-miR-34 were achieved. In addition, the body weights of 5- and 6-day-old larvae were significantly increased compared with those in the mimic-NC group; the weights of 5-day-old larvae in the inhibitor-miR-34 group were significantly decreased in comparison with those in the inhibitor-NC group, while the weights of 4- and 6-day-old larvae in the inhibitor-miR-34 group were significantly increased, indicating the involvement of ame-miR-34 in modulating larval body weight. Furthermore, the expression levels of both hsp and abct in the guts of A. apis-infected 4-, 5-, and 6-day-old larvae were significantly upregulated after ame-miR-34 overexpression. In contrast, after ame-miR-34 knockdown, the expression levels of the aforementioned two key genes in the A. apis-infected 4-, 5-, and 6-day-old larval guts were significantly downregulated. Together, the results demonstrated that effective overexpression and knockdown of ame-miR-34 in both noninfected and A. apis-infected A. mellifera larval guts could be achieved by the feeding method, and ame-miR-34 exerted a regulatory function in the host immune response to A. apis invasion through positive regulation of the expression of hsp and abct. Our findings not only provide a valuable reference for the functional investigation of bee larval miRNAs but also reveal the regulatory role of ame-miR-34 in A. mellifera larval weight and immune response. Additionally, the results of this study may provide a promising molecular target for the treatment of chalkbrood disease.

Unveiling the circRNA-Mediated Immune Responses of Western Honey Bee Larvae to Ascosphaera apis Invasion

Western honey bee (Apis mellifera), a eusocial insect with a superior economic and ecological value, is widely used in the beekeeping industry throughout the world. As a new class of non-coding RNAs (ncRNAs), circular RNAs (circRNAs) participate in the modulation of considerable biological processes, such as the immune response via diverse manners. Here, the identification, characteristic investigation, and molecular verification of circRNAs in the Apis mellifera ligustica larval guts were conducted, and the expression pattern of larval circRNAs during the Ascosphaera apis infection was analyzed, followed by the exploration of the potential regulatory part of differentially expressed circRNAs (DEcircRNAs) in host immune responses. A total of 2083 circRNAs in the larval guts of A. m. ligustcia were identified, with a length distribution ranging from 106 nt to 92,798 nt. Among these, exonic circRNAs were the most abundant type and LG1 was the most distributed chromosome. Additionally, 25, 14, and 30 up-regulated circRNAs as well as 26, 25, and 62 down-regulated ones were identified in the A. apis-inoculated 4-, 5-, and 6-day-old larval guts in comparison with the corresponding un-inoculated larval guts. These DEcircRNAs were predicted to target 35, 70, and 129 source genes, which were relative to 12, 23, and 20 GO terms as well as 11, 10, and 27 KEGG pathways, including 5 cellular and humoral immune pathways containing apoptosis, autophagy, endocytosis, MAPK, Toll, and Imd signaling pathways. Furthermore, complex competing endogenous RNA (ceRNA) regulatory networks were detected to be formed among DEcircRNAs, DEmiRNAs, and DEmRNAs. The Target DEmRNAs were engaged in 24, 20, and 25 functional terms as well as 62, 80, and 159 pathways, including several vital immune defense-associated pathways, namely the lysosome, endocytosis, phagosome, autophagy, apoptosis, MAPK, Jak-STAT, Toll, and Imd signaling pathways. Finally, back-splicing sites within 15 circRNAs and the difference in the 9 DEcircRNAs' expression between un-inoculated and A. apis-inoculated larval guts were confirmed utilizing molecular methods. These findings not only enrich our understanding of bee host-fungal pathogen interactions, but also lay a foundation for illuminating the mechanism underlying the DEcircRNA-mediated immune defense of A. m. ligustica larvae against A. apis invasion.

Drosophila Relish-mediated miR-317 expression facilitates immune homeostasis restoration via inhibiting PGRP-LC

Innate immunity is the first and essential line for resisting pathogens, and the immune intensity and duration need to be strictly regulated to balance excessive or insufficient immune response. MicroRNAs (miRNAs) are crucial regulators of immune response in Drosophila, yet how immune-related miRNAs are regulated remains poorly understood. Herein, we elucidated that the involvement of miR-317 in NF-κB transcription factor Relish mediated Drosophila Imd pathway in response to Gram-negative (G-) bacteria stimulation. Remarkably, the dynamic expression profiling for immune response indicated that Relish simultaneously enhances the expression of the effector antimicrobial peptide Dpt as well as miR-317 post-infection. Upregulation of miR-317 could further down-regulate the expression of PGRP-LC, thereby forming a feedback in Drosophila Imd pathway to prevent over-activation and restore immune homeostasis. Taken together, our study not only uncovers a novel Relish/miR-317/PGRP-LC regulatory axis to attenuate Drosophila Imd immune response and facilitate immune homeostasis restoration, but also provides vital insights into the complex mechanisms of animal innate immune regulation.

Enterococcus faecium Regulates Honey Bee Developmental Genes

Honey bees provide essential pollination services to the terrestrial ecosystem and produce important agricultural products. As a beneficial lactic acid bacterium, Enterococcus faecium is often supplied as a probiotic for honey bees and other animals. However, the underlying mechanisms of its actions and possible safety risks are not well understood. We present the first complete genome sequence of E. faecium isolated from the honey bee gut using nanopore sequencing, and investigate the effects and mechanisms of interactions between E. faecium and honey bees via transcriptome and miRNA analysis. E. faecium colonization increased honey bee gut weight. Transcriptome analysis showed that developmental genes were up-regulated. In accordance, the target genes of the down-regulated miRNAs were enriched in developmental pathways. We describe how E. faecium increases honey bee gut weight at the transcriptional and post-transcriptional levels, and add insights about how miRNAs mediate host and bacteria interactions.

Molecular underpinnings of the early brain developmental response to differential feeding in the honey bee Apis mellifera

Brain differential morphogenesis in females is one of the major phenotypic manifestations of caste development in honey bees. Brain diphenism appears at the fourth larval phase as a result of the differential feeding regime developing females are submitted during early phases of larval development. Here, we used a forward genetics approach to test the early brain molecular response to differential feeding leading to the brain diphenism observed at later developmental phases. Using RNA sequencing analysis, we identified 53 differentially expressed genes (DEGs) between the brains of queens and workers at the third larval phase. Since miRNAs have been suggested to play a role in caste differentiation after horizontal and vertical transmission, we tested their potential participation in regulating the DEGs. The miRNA-mRNA interaction network, including the DEGs and the royal- and worker-jelly enriched miRNA populations, revealed a subset of miRNAs potentially involved in regulating the expression of DEGs. The interaction of miR-34, miR-210, and miR-317 with Takeout, Neurotrophin-1, Forked, and Masquerade genes was experimentally confirmed using a luciferase reporter system. Taken together, our results reconstruct the regulatory network that governs the development of the early brain diphenism in honey bees.