Identification and expression analysis of an IL-21 receptor in large yellow croaker (Larimichthys crocea)

Interleukin-21 (IL-21) is a potent immunomodulatory cytokine with pleiotropic effects on both innate and adaptive immune responses. It signaling involves a heterodimeric receptor complex consisting of the IL-21 receptor (IL-21R) and the common γ chain (γC). In this study, we identified a homologue gene of IL-21R ( Lc IL-21R) in the large yellow croaker ( Larimichthys crocea , Lc ). The open reading frame (ORF) of Lc IL-21R gene is 1629 bp long and encodes a precursor protein of 542 amino acids (aa), with a 23-aa signal peptide and a 519-aa mature peptide containing four putative N-glycosylation sites. Lc IL-21R has two fibronectin type III (FNIII)-like domains (D1 and D2), a transmembrane domain, and a cytoplasmic region. A conserved WSXWS motif was also found in the D2 domain. The predicted structure of the extracellular region of Lc IL-21R ( Lc IL-21R-Ex) is highly similar to that of human IL-21R. Lc IL-21R was constitutively expressed in all tissues examined, and Lc IL-21R mRNA levels were increased in head kidney and spleen upon inactivated trivalent bacterial vaccine or poly(I:C) stimulation. Overall, our results suggest that Lc IL-21R shares structural and functional properties with IL-21Rs found in other vertebrates, indicating its potential involvement in the IL-21-mediated immune response to pathogenic infections. These findings contribute to our understanding of the evolutionary conservation of IL-21 signaling and its role in the immune system.