Spatzle4 gene of silkworm, Bombyx mori: identification, immune response, and the effect of RNA interference on the antimicrobial peptides’ expression in the integument

Insects, including silkworms, can protect themselves from exotic invasions by innate immune responses. In the immune response of Drosophila, the activation of Toll receptors is strictly dependent on the product of Spatzle. In this study, we cloned the 1567 bp BmSpz4 cDNA which contained a complete 1386 bp open reading frame, encoding 461 amino acids, out of which the forgoing 19 residues were signal peptide. The result of the cDNA sequencing showed that we found a longer transcript than the one included in large scale full-Length cDNA sequencing data by Yoshitaka Suetsugu et al. in 2013. Several spliced variants of BmSpz4 have been found, based on our preliminary results. It was shown by the RT-PCR that BmSpz4 was expressed highest in the head, lower in the integument and testis. The expression of BmSpz4 in the integument of silkworm was upregulated by formalin-inactivated Gram-positive bacteria and fungi but not by Gram-negative bacteria, when compared to the control group of PBS injection. This phenomenon was the same as the one found in the Toll signaling pathway of Drosophila. In addition, the result of double-stranded RNA interference of BmSpz4 also demonstrated that it had a corresponding interference effect on the expression of the integument antimicrobial peptides induced by bacillus and yeast. Thus, it may be concluded that BmSpz4 plays an important role in the innate immunity against microbe infection in the integument of silkworm, Bombyx mori.