Subcellular Localization of Large Yellow Croaker (Larimichthys crocea) TLR21 and Expression Profiling of Its Gene in Immune Response

Toll-like receptor 21 (TLR21) is a non-mammalian type TLR, and plays an important role in innate immune response in fish. In this paper, the full-length cDNA sequence of TLR21 gene was identified and characterized from large yellow croaker, Larimichthys crocea and was termed as Lc TLR21. It consists of 3365 bp, including a 5’-terminal untranslated region (UTR) of 97 bp, a 3’-terminal UTR of 331 bp, and an open reading frame (ORF) of 2937 bp encoding a polypeptide of 978 amino acid residues. The deduced Lc TLR21 contains a signal peptide domain at N-terminal, 12 leucine-rich repeats (LRRs) at the extracellular region, a transmembrane domain and a cytoplasmic toll-interleukin-1 receptor (TIR) domain at the C-terminal. Subcellular localization analysis revealed that the Lc TLR21-GFP was constitutively expressed in cytoplasm. Tissue expression analysis indicated that Lc TLR21 gene broadly expressed in most of the examined tissues, with the most predominant abundance in spleen, followed by head-kidney and liver, while the weakest expression was detected in brain. The expression level of Lc TLR21 after LPS, poly I:C and Vibrio parahaemolyticus challenges was investigated in spleen, head-kidney and liver. Lc TLR21 gene transcripts increased significantly in all examined tissues after the challenges, and the highest expression level was detected in liver at 24 h after poly I:C stimulation ( P < 0.05), suggesting that Lc TLR21 might play a crucial role in fish resistance to viral and bacterial infections.