Abstract
The dimeric cytokine ligand Spätzle (Spz) is responsible for Toll pathway activation and antimicrobial peptide (AMP) production upon pathogen challenge in Tenebrio molitor. Here, we indicated that TmSpz5 has a functional role in response to bacterial infections. We showed that the highest expression of TmSpz5 is induced by Candida albicans. However, TmSpz5 knockdown reduced larval survival against Escherichia coli and Staphylococcus aureus. To evaluate the molecular mechanism underlying the observed survival differences, the role of TmSpz5 in AMP production was examined by RNA interference and microbial injection. T. molitor AMPs that are active against Gram-negative and -positive bacteria, including Tmtenecins, Tmattacins, Tmcoleoptericins, Tmtaumatin-like-proteins, and Tmcecropin-2, were significantly downregulated by TmSpz-5 RNAi in the Malpighian tubules (MTs) following a challenge with E. coli and S. aureus. However, upon infection with C. albicans the mRNA levels of most AMPs in the dsTmSpz5-injected group were similar to those in the control groups. Likewise, the expression of the transcription factors NF-κB, TmDorX2, and TmRelish were noticeably suppressed in the MTs of TmSpz5-silenced larvae. Moreover, E. coli-infected TmSpz5 knockdown larvae showed decreased antimicrobial activity in the MTs and hindgut compared with the control group. These results demonstrate that TmSpz5 has a defined role in T. molitor innate immunity by regulating AMP expression in MTs in response to E. coli.
Highlights
Insects have been the largest and most diverse class over millions of years of evolution and have adapted to survive in a vast range of ecological territories [1,2,3]
The TmSpz5 full-length open reading frame (ORF) consisted of 1,062 bp, encoding a polypeptide of 353 amino acid residues (Figure 1)
As determined using InterProScan, the TmSpz5 amino acid sequence contained a cystine-knot domain at the C-terminus, one cleavage site predicted to be processed by Spz-processing enzyme (SPE), and a predicted signal peptide at the N-terminus (Figure 1)
Summary
Insects have been the largest and most diverse class over millions of years of evolution and have adapted to survive in a vast range of ecological territories [1,2,3]. Owing to their exposure to various pathogen sources including bacteria, fungi, parasites, and viruses, they have evolved several multifunctional defense mechanisms, making them an exceptional model for immunity studies. Insects do not have an adaptive immunity [2] They do possess a functional innate immune system, involving both humoral and cellular immune responses [6]. AMP production is stimulated by the activation of two major signaling pathways, the immune deficiency (Imd) and Toll pathways [6, 10]
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