Cloning and expression analysis of two alternative splicing toll-like receptor 9 isoforms A and B in large yellow croaker, Pseudosciaena crocea

Toll-like receptors (TLRs) are the archetypal pattern-recognition receptors in sensing foreign pathogens. In this report, two alternative splicing isoforms of TLR9 (named PcTLR9A and PcTLR9B) cDNA were cloned from the large yellow croaker, Pseudosciaena crocea. The full-length cDNA of PcTLR9A was of 3637 bp, including a 5′-terminal untranslated region (UTR) of 111 bp, 3′-terminal UTR of 355 bp and an open reading frame (ORF) of 3171 bp encoding a polypeptide of 1056 amino acids. However, the full-length cDNA of PcTLR9B was 119 bp longer than that of PcTLR9A from the position of 3079–3197 bp, which encoded a polypeptide of 1006 amino acids. Both of the PcTLR9A and PcTLR9B contained 12 typical structures of leucine-rich repeats (LRRs), an LRRTYP and an LRRCT in the extracellular region and a conservative Toll/IL-1R (TIR) domain in the intracellular region. However, compared to PcTLR9A, conservative Box 3 was absent in PcTLR9B TIR domain. Quantitative real-time reverse transcription PCR analysis revealed a broad expression of PcTLR9A and PcTLR9B with the highest expression in spleen and the weakest expression in muscle. Expression of PcTLR9A and PcTLR9B after stimulation with formalin-inactivated Gram-negative bacterium Vibrio parahaemolyticus was tested in blood, spleen and liver. This indicated that the highest expression was 3.3 times (at 24 h) as much as that in the control in the spleen (p < 0.05) and the lowest expression of PcTLR9A was 1/4 times (at 3 h) of that in the control in the liver (p < 0.05). PcTLR9B showed a similar expression pattern to PcTLR9A post-injection. These results suggested that both PcTLR9A and PcTLR9B might play an important role in large yellow croaker defense against pathogenic infection.