Critical Roles of Spätzle5 in Antimicrobial Peptide Production Against Escherichia coli in Tenebrio molitor Malpighian Tubules

A novel prolixicin identified in common bed bugs with activity against both bacteria and parasites

The hematophagous common bed bug, Cimex lectularius, is not known to transmit human pathogens outside laboratory settings, having evolved various immune defense mechanisms including the expression of antimicrobial peptides (AMPs). We unveil three novel prolixicin AMPs in bed bugs, exhibiting strong homology to the prolixicin of kissing bugs, Rhodnius prolixus, and to diptericin/attacin AMPs. We demonstrate for the first time sex-specific and immune mode-specific upregulation of these prolixicins in immune organs, the midgut and rest of body, following injection and ingestion of Gr+ (Bacillus subtilis) and Gr- (Escherichia coli) bacteria. Synthetic CL-prolixicin2 significantly inhibited growth of E. coli strains and killed or impeded Trypanosoma cruzi, the Chagas disease agent. Our findings suggest that prolixicins are regulated by both IMD and Toll immune pathways, supporting cross-talk and blurred functional differentiation between major immune pathways. The efficacy of CL-prolixicin2 against T. cruzi underscores the potential of AMPs in Chagas disease management.

Engineering Enhanced Antimicrobial Properties in α-Conotoxin RgIA through D-Type Amino Acid Substitution and Incorporation of Lysine and Leucine Residues

Antimicrobial peptides (AMPs), acknowledged as host defense peptides, constitute a category of predominant cationic peptides prevalent in diverse life forms. This study explored the antibacterial activity of α-conotoxin RgIA, and to enhance its stability and efficacy, D-amino acid substitution was employed, resulting in the synthesis of nine RgIA mutant analogs. Results revealed that several modified RgIA mutants displayed inhibitory efficacy against various pathogenic bacteria and fungi, including Candida tropicalis and Escherichia coli. Mechanistic investigations elucidated that these polypeptides achieved antibacterial effects through the disruption of bacterial cell membranes. The study further assessed the designed peptides' hemolytic activity, cytotoxicity, and safety. Mutants with antibacterial activity exhibited lower hemolytic activity and cytotoxicity, with Pep 8 demonstrating favorable safety in mice. RgIA mutants incorporating D-amino acids exhibited notable stability and adaptability, sustaining antibacterial properties across diverse environmental conditions. This research underscores the potential of the peptide to advance innovative oral antibiotics, offering a novel approach to address bacterial infections.

Recent trends in insect gut immunity

The gut is a crucial organ in insect defense against various pathogens and harmful substances in their environment and diet. Distinct insect gut compartments possess unique functionalities contributing to their physiological processes, including immunity. The insect gut's cellular composition is vital for cellular and humoral immunity. The peritrophic membrane, mucus layer, lumen, microvilli, and various gut cells provide essential support for activating and regulating immune defense mechanisms. These components also secrete molecules and enzymes that are imperative in physiological activities. Additionally, the gut microbiota initiates various signaling pathways and produces vitamins and minerals that help maintain gut homeostasis. Distinct immune signaling pathways are activated within the gut when insects ingest pathogens or hazardous materials. The pathway induced depends on the infection or pathogen type; include immune deficiency (imd), Toll, JAK/STAT, Duox-ROS, and JNK/FOXO regulatory pathways. These pathways produce different antimicrobial peptides (AMPs) and maintain gut homeostasis. Furthermore, various signaling mechanisms within gut cells regulate insect gut recovery following infection. Although some questions regarding insect gut immunity in different species require additional study, this review provides insights into the insect gut's structure and composition, commensal microorganism roles in Drosophila melanogaster and Tenebrio molitor life cycles, different signaling pathways involved in gut immune systems, and the insect gut post-infection recovery through various signaling mechanisms.

Innate Immune Response of TmToll-3 Following Systemic Microbial Infection in Tenebrio molitor

Although Toll-like receptors have been widely identified and functionally characterized in mammalian models and Drosophila, the immunological function of these receptors in other insects remains unclear. Here, we explored the relevant innate immune response of Tenebrio molitor (T. molitor) Toll-3 against Gram-negative bacteria, Gram-positive bacteria, and fungal infections. Our findings indicated that TmToll-3 expression was mainly induced by Candida albicans infections in the fat bodies, gut, Malpighian tubules, and hemolymph of young T. molitor larvae. Surprisingly, Escherichia coli systemic infection caused mortality after TmToll-3 knockdown via RNA interference (RNAi) injection, which was not observed in the control group. Further analyses indicated that in the absence of TmToll-3, the final effector of the Toll signaling pathway, antimicrobial peptide (AMP) genes and relevant transcription factors were significantly downregulated after E. coli challenge. Our results indicated that the expression of almost all AMP genes was suppressed in silenced individuals, whereas the expression of relevant genes was positively regulated after fungal injection. Therefore, this study revealed the immunological involvement of TmToll-3 in T. molitor in response to systematic infections.

Analysis of the Toll and Spaetzle Genes Involved in Toll Pathway-Dependent Antimicrobial Gene Induction in the Red Flour Beetle, Tribolium castaneum (Coleoptera; Tenebrionidae)

Insects rely only on their innate immune system to protect themselves from pathogens. Antimicrobial peptide (AMP) production is the main immune reaction in insects. In Drosophila melanogaster, the reaction is regulated mainly by the Toll and immune deficiency (IMD) pathways. Spaetzle proteins, activated by immune signals from upstream components, bind to Toll proteins, thus, activating the Toll pathway, which in turn, induces AMP genes. Previous studies have shown the difference in immune systems related to Toll and IMD pathways between D. melanogaster and Tribolium castaneum. In T. castaneum, nine Toll and seven spaetzle (spz) genes were identified. To extend our understanding of AMP production by T. castaneum, we conducted functional assays of Toll and spaetzle genes related to Toll-pathway-dependent AMP gene expression in T. castaneum under challenge with bacteria or budding yeast. The results revealed that Toll3 and Toll4 double-knockdown and spz7 knockdown strongly and moderately reduced the Toll-pathway-dependent expression of AMP genes, respectively. Moreover, Toll3 and Toll4 double-knockdown pupae more rapidly succumbed to entomopathogenic bacteria than the control pupae, but spz7 knockdown pupae did not. The results suggest that Toll3 and Toll4 play a large role in Toll-pathway-dependent immune reactions, whereas spz7 plays a small part.

Immunological Roles of TmToll-2 in Response to Escherichia coli Systemic Infection in Tenebrio molitor

The antimicrobial roles of Toll-like receptors have been mainly identified in mammalian models and Drosophila. However, its immunological function in other insects has yet to be fully clarified. Here, we determined the innate immune response involvement of TmToll-2 encountering Gram-negative, Gram-positive, and fungal infection. Our data revealed that TmToll-2 expression could be induced by Escherichia coli, Staphylococcus aureus, and Candida albicans infections in the fat bodies, gut, Malpighian tubules, and hemolymph of Tenebrio molitor young larvae. However, TmToll-2 silencing via RNAi technology revealed that sole E. coli systemic infection caused mortality in the double-strand RNA TmToll-2-injected group compared with that in the control group. Further investigation indicated that in the absence of TmToll-2, the final effector of Toll signaling pathway, antimicrobial peptide (AMP) genes and relevant transcription factors were significantly downregulated, mainly E. coli post-insult. We showed that the expression of all AMP genes was suppressed in the main immune organ of insects, namely, fat bodies, in silenced individuals, while the relevant expressions were not affected after fungal infection. Thus, our research revealed the immunological roles of TmToll-2 in different organs of T. molitor in response to pathogenic insults.

Emerging Evidence on Tenebrio molitor Immunity: A Focus on Gene Expression Involved in Microbial Infection for Host-Pathogen Interaction Studies

In recent years, the scientific community's interest in T. molitor as an insect model to investigate immunity and host-pathogen interactions has considerably increased. The reasons for this growing interest could be explained by the peculiar features of this beetle, which offers various advantages compared to other invertebrates models commonly used in laboratory studies. Thus, this review aimed at providing a broad view of the T. molitor immune system in light of the new scientific evidence on the developmental/tissue-specific gene expression studies related to microbial infection. In addition to the well-known cellular component and humoral response process, several studies investigating the factors associated with T. molitor immune response or deepening of those already known have been reported. However, various aspects remain still less understood, namely the possible crosstalk between the immune deficiency protein and Toll pathways and the role exerted by T. molitor apolipoprotein III in the expression of the antimicrobial peptides. Therefore, further research is required for T. molitor to be recommended as an alternative insect model for pathogen-host interaction and immunity studies.

Current Status of Immune Deficiency Pathway in Tenebrio molitor Innate Immunity

Yellow mealworm (Tenebrio molitor) is a highly beneficial beetle that serves as an excellent source of edible protein as well as a practical study model. Therefore, studying its immune system is important. Like in other insects, the innate immune response effected through antimicrobial peptides production provides the most critical defense armory in T. molitor. Immune deficiency (Imd) signaling is one of the major pathways involved in the humoral innate immune response in this beetle. However, the nature of the molecules involved in the signaling cascade of the Imd pathway, from recognition to the production of final effectors, and their mechanism of action are yet to be elucidated in T. molitor model. In this review, we present a general overview of the current literature available on the Imd signaling pathway and its identified interaction partners in T. molitor.