Identification and functional characterization of three TLR signaling pathway genes in Cyclina sinensis

• CsTLR4, CsTLR13 and CsMyD88 were identified and characterized from Cyclina sinensis.
• They were universally distributed in six tissues, and mainly expressed in the hemocytes.
• Only CsTLR13 and CsMyD88 were up-regulated by poly (I:C) challenge.
• CsMyD88 plays an important role inducing the expression of two immune-related genes in C. sinensis.

Toll-like receptors (TLRs) are an ancient family of pattern recognition receptors that play a critical role in initiating and activating the innate immune system. In this study, we identified two TLR genes (CsTLR4 and CsTLR13) and the MyD88 (CsMyD88) gene using a transcriptome library from Cyclina sinensis. The sequence features and mRNA expression profiles of the genes were characterized, and their functions in the immune response were investigated to validate the TLR signaling pathway and its potential role in immune defense. The expression patterns of CsTLR4, CsTLR13 and CsMyD88 were detected in all the tissues examined from healthy clams and were primarily expressed in the hemocytes (P < 0.05), as shown by real-time PCR. Upon challenge with Vibrio anguillarum and Micrococcus luteus, they were significantly increased in hemocytes (P < 0.01), whereas only CsTLR13 and CsMyD88 were up-regulated (P < 0.01) by poly (I:C) challenge. In addition, the mRNA expression level of CsC-LYZ and CsAMP was down-regulated at 72 h (P < 0.01) after injection with CsMyD88 RNAi. These findings might be valuable for understanding the innate immune signaling pathways of C. sinensis and enabling future studies on host–pathogen interactions.