Characterization of RAG1 and IgM (mu chain) marking development of the immune system in red-spotted grouper (Epinephelus akaara).

In vertebrates, lymphoid-specific recombinase protein encoded by recombination-activating genes (RAG1/2) plays a key role in V(D)J recombination of the T-cell receptor and B-cell receptor. In this study, both RAG1 and the immunoglobulin M (IgM) mu chain were cloned to characterize their potential role in the immune defense at developmental stages of red-spotted grouper, Epinephelus akaara. The open reading frame (ORF) of E. akaara RAG1 included 2778 nucleotide residues encoding a putative protein of 925 amino acids, while the ORF of the IgM mu chain had 1734 nucleotide residues encoding 578 amino acids including variable (VH) and constant (CH1-CH2-CH3-CH4) regions. E. akaara RAG1 was composed of a zinc-binding dimerization domain (ZDD) with a RING finger and zinc finger A (ZFA) in the non-core region and a nonamer-binding region (NBR), with a zinc finger B (ZFB), the central and C-terminal domains in the core region. Tridimensional models of the ZDD and NBR of E. akaara RAG1 were constructed for the first time in fishes, while a 3D model of the E. akaara IgM mu chain was also clarified. The RAG1 mRNA was only detected in the thymus and kidney of 4-month and 1.5-year old groupers using qPCR, and the RAG1 protein was confirmed using western blotting and immunohistochemistry. The IgM mu mRNA was examined in most tissues except the gonad. RAG1 and IgM mu gene expression were observed at 15 dph (days post-hatching) and 23 dph respectively, and increased to a higher level at 37 dph. In addition, this was the first time that the morphology of the E. akaara thymus was characterized. The oval-shaped thymus of 4-month old fish was clearly seen and there were amounts of T lymphocytes present. The results suggested that the immune action of E. akaara was likely to start to develop around 15 dph to 29 dph. The transcript level of the RAG1 gene and the number of lymphocytes in the thymus between 4-month and 1.5-year old groupers indicated that age-related thymic atrophy also occurs in fishes. The similar functional structures of RAG1 and IgM protein between fish and mammals indicated that teleost species share a similar mechanism of V(D)J recombination with higher vertebrates.