Molecular insights of a novel cephalopod toll-like receptor homologue in Sepiella japonica, revealing its function under the stress of aquatic pathogenic bacteria

Abstract

Toll-like receptors (TLRs) play an important role in defense response to pathogens in mollusk. In this study the first TLR from Sepiella japonica (named as SjTLR) was functionally characterized, and its full-length cDNA consisted of 3914bp (GenBank accession no. AQY56780.1) including an open reading frame of 3582bp, encoding a putative protein of 1193 amino acids. Its theoretical molecular weight was 137.87 KDa and the predicted isoelectric point was 3.69. The derived amino acids sequence comprised of an extracellular domain including 26 amino acids signal peptide and eleven leucine-rich repeats (LRR), capped with LRRCT and LRRNT followed by transmembrane domain and cytoplasmic Toll/IL-1R domain (TIR). In addition, 12 potential N-linked glycosylation sites were present in the ectodomain to influence protein trafficking, surface presentation and ligand recognition. Multiple sequence alignment and phylogenetic analysis revealed that SjTLR shared the highest similarity to that of Euprymna scolopes and they fell into the same clade. Real-time PCR showed SjTLR expressed constitutively in all tested tissues, including gill, liver, brain, muscle, intestine, heart, lobus opticus and stomach, but showed different expression levels with genders. The highest expression was in the liver, and the lowest was in stomach for both genders. The functional domain region sequences encoding LRRs domain protein and TIR domain containing protein (TcpB) were expressed in BL21(DE3) respectively and purified with Ni-NAT Superflow resin conforming to the expected molecular weight. The cellular localization of SjTLR in HEK293 cells was conducted and plasma membrane localization was detected. SjLRRs internalization upon the activation of LPS was also observed, and dramatic redistribution of SjLRRs in the cytoplasm with distinct perinuclear accumulation was found. After SjTLR transfection Toll/NF-κB signaling pathway was active in HEK293 treated with LPS and TNFɑ. The nuclear related genes may also be activated by NF-κB in the nucleus, and the corresponding mRNA was transferred through the intracellular signal transduction pathway, so that IL-6 cytokines could be synthesized and released. After infection by Vibrio parahemolyticus and Aeromonas hydrophila the expression of SjTLR were upregulated with time-dependent manner. These findings might be valuable for understanding the innate immune signaling pathways of S.japonica and enabling future studies on host-pathogen interactions.