<i>IKK</i>β regulates antimicrobial innate immune responses in the yellow mealworm, <i>Tenebrio molitor</i>

Abstract

Toll and IMD pathways regulate antimicrobial innate immune responses in insect model systems. The transcriptional activation of antimicrobial peptides (AMPs) confers humoral immunity in the host against invaded pathogens. The IKK kinase complex (IKK alpha, IKK beta, and the regulatory subunit IKK gamma/NEMO) centrally regulates the NF-kappa B response to various stimuli. It triggers an appropriate antimicrobial immune response in the host. In this study, a TmIKK beta (or TmIrd5) homolog was screened from the RNA-seq database of the coleopteran beetle, Tenebrio molitor. A single exon characterizes the TmIKK beta gene, and the open reading frame (ORF) comprises of 2112 bp that putatively encodes a polypeptide of 703 amino acid residues. TmIKK beta contains a serine/threonine kinase domain and is phylogenetically close to Tribolium castaneum IKK beta homolog (TcIKK beta). TmIKK beta transcripts were highly expressed in the early pupal (P1) and adult (A5) stages. Among the tissues, TmIKK beta showed higher expression in the integument of the last instar larvae and the fat body and hemocytes of 5-day-old adults. TmIKK beta mRNA was upregulated post-E. coli challenge to the host. Moreover, RNAi-based TmIKK beta mRNA silencing increased host larvae' susceptibility against E. coli, S. aureus and C. albicans. TmIKK beta RNAi in the fat body led to a downregulation in mRNA expression of ten out of fourteen AMP genes, including TmTenecin1, -2, and -4; TmDefensin, and -like; TmColeoptericinA, and -B; and TmAttacin1a, -1b, and -2, suggesting the requirement of the gene in antimicrobial innate immune responses. Further, a decrease in the mRNA expression of NF-.B factors such as TmRelish, TmDorsal1, and TmDorsal2 in the fat body of T. molitor larvae was observed postmicroorganisms challenge. Thus, TmIKK beta regulates antimicrobial innate immune responses in T. molitor.