Cui Zhengwei, Zhao Han, Chen Xinhua
Key Laboratory of Marine Biotechnology of Fujian Province, Institute of Oceanology, College of Marine Sciences, Fujian Agriculture and Forestry University, Fuzhou, 350002, China.
Key Laboratory of Marine Biotechnology of Fujian Province, Institute of Oceanology, College of Marine Sciences, Fujian Agriculture and Forestry University, Fuzhou, 350002, China; Laboratory for Marine Biology and Biotechnology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266071, China; Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai, 519000, China.
Fish Shellfish Immunol. 2023 Mar;134:108581. doi: 10.1016/j.fsi.2023.108581. Epub 2023 Feb 6.
As the predominant immunoglobulin (Ig) isotype, IgM plays a crucial role in the acquired immunity of vertebrates. There is only one Igμ gene in mammals, except cattle, while the number of Igμ gene varies among teleost fish. In the current study, we found two functional Igμ genes (Igμ1 and Igμ2) and a pseudo Cμ gene (ψIgμ) in large yellow croaker (Larimichthys crocea). Both Igμ1 and Igμ2 genes possessed two transcript variants, which encoded the heavy chains of secreted (sIgM1 and sIgM2) and membrane-bound IgM1 and IgM2 (mIgM1 and mIgM2), respectively. Both the heavy chains of sIgM1 and sIgM2 consisted of a variable Ig domain, four constant Ig domains (CH1, CH2, CH3 and CH4) and a secretory tail, while those of mIgM1 and mIgM2 consisted of a variable Ig domain, three constant Ig domains (CH1, CH2 and CH3), a transmembrane domain and a short cytoplasmic tail. Cysteine residues that are necessary for the formation of intrachain and interchain disulfide bonds and tryptophan residues that are important for the folding of the Ig superfamily domain were well conserved in large yellow croaker IgM1 and IgM2. Interestingly, large yellow croaker IgM2 had an extra cysteine (C94) in the CH1 domain compared with IgM1, which may cause the structural difference between IgM1 and IgM2. A liquid chromatography-tandem mass spectrometry analysis revealed that both IgM1 and IgM2 were present at the protein level in large yellow croaker serum. Both the Igμ1 and Igμ2 genes were mainly expressed in systemic immune tissues, such as head kidney and spleen, but the expression level of Igμ2 was much lower than that of Igμ1. After Pseudomonas plecoglossicida infection, the expression levels of Igμ1 and Igμ2 in both the spleen and head kidney were significantly upregulated, with a higher upregulation of Igμ2 than that of Igμ1. These results suggested that Igμ1 and Igμ2 may play a differential role in the immune response of large yellow croaker against bacterial infection.
作为主要的免疫球蛋白（Ig）同种型，IgM在脊椎动物的获得性免疫中起着至关重要的作用。除牛外，哺乳动物中只有一个Igμ基因，而硬骨鱼中Igμ基因的数量各不相同。在本研究中，我们在大黄鱼（Larimichthys crocea）中发现了两个功能性Igμ基因（Igμ1和Igμ2）和一个假Cμ基因（ψIgμ）。Igμ1和Igμ2基因均具有两个转录变体，分别编码分泌型（sIgM1和sIgM2）和膜结合型IgM1和IgM2（mIgM1和mIgM2）的重链。sIgM1和sIgM2的重链均由一个可变Ig结构域、四个恒定Ig结构域（CH1、CH2、CH3和CH4）和一个分泌尾组成，而mIgM1和mIgM2的重链由一个可变Ig结构域、三个恒定Ig结构域（CH1、CH2和CH3）、一个跨膜结构域和一个短细胞质尾组成。链内和链间二硫键形成所必需的半胱氨酸残基以及对Ig超家族结构域折叠很重要的色氨酸残基在大黄鱼IgM1和IgM2中高度保守。有趣的是，与IgM1相比，大黄鱼IgM2在CH1结构域中有一个额外的半胱氨酸（C94），这可能导致IgM1和IgM2之间的结构差异。液相色谱-串联质谱分析表明，IgM1和IgM2在大黄鱼血清中均以蛋白质水平存在。Igμ1和Igμ2基因均主要在全身免疫组织如头肾和脾脏中表达，但Igμ2的表达水平远低于Igμ1。在感染鳗弧菌后，脾脏和头肾中Igμ1和Igμ2的表达水平均显著上调，其中Igμ2的上调幅度高于Igμ1。这些结果表明，Igμ1和Igμ2在大黄鱼抗细菌感染的免疫反应中可能发挥不同的作用。
