TmSpz4 Plays an Important Role in Regulating the Production of Antimicrobial Peptides in Response to Escherichia coli and Candida albicans Infections

Abstract

Spätzle family proteins activate the Toll pathway and induce antimicrobial peptide (AMP) production against microbial infections. However, the functional importance of <i>Tm</i>spätzle4 (<i>TmSpz4</i>) in the immune response of <i>Tenebrio molitor</i> has not been reported. Therefore, here, we have identified and functionally characterized the role of <i>TmSpz4</i> against bacterial and fungal infections. We showed that <i>TmSpz4</i> expression was significantly induced in hemocytes at 6 h post-injection with <i>Escherichia coli</i>, <i>Staphylococcus aureus</i>, and <i>Candida albicans</i>. <i>TmSpz4</i> knock-down significantly reduced larval survival against <i>E. coli</i> and <i>C. albicans.</i> To understand the reason for the survivability difference, the role of <i>TmSpz4</i> in AMP production was examined in <i>TmSpz4</i>-silenced larvae following microbe injection. The AMPs that are active against Gram-negative bacteria, including <i>TmTenecin-2</i>, <i>TmTenecin-4</i>, <i>TmAttacin-1a</i>, <i>TmDefensin-2</i>, and <i>TmCecropin-2</i>, were significantly downregulated in response to <i>E. coli</i> in <i>TmSpz4</i>-silenced larvae. Similarly, the expression of <i>TmTenecin-1</i>, <i>TmTenecin-3</i>, <i>TmThaumatin-like protein-1</i> and <i>-2</i>, <i>TmDefensin-1</i>, <i>TmDefensin-2</i>, and <i>TmCecropin-2</i> were downregulated in response to <i>C. albicans</i> in <i>TmSpz4</i>-silenced larvae. In addition, the transcription factor NF-κB (<i>TmDorX1</i> and <i>TmDorX2)</i> expression was significantly suppression in <i>TmSpz4</i>-silenced larvae. In conclusion, these results suggest that <i>TmSpz4</i> plays a key role in regulating immune responses of <i>T. molitor</i> against to <i>E. coli</i> and <i>C. albicans</i>.