中国水产科学  2025, Vol. 32 Issue (04): 435-444  DOI: 10.12264/JFSC2024-0336
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引用本文 

赵丽艳, 李转转, 马培振, 刘志鸿, 孙秀俊, 周丽青, 吴彪. 近江牡蛎AGL基因的分子特征及其在糖原分解中的作用[J]. 中国水产科学, 2025, 32(4): 435-444. DOI: 10.12264/JFSC2024-0336.
ZHAO Liyan, LI Zhuanzhuan, MA Peizhen, LIU Zhihong, SUN Xiujun, ZHOU Liqing, WU Biao. Molecular characterization of AGL gene in Jinjiang oyster (Crassostrea ariakensis) and its role in glyco genolysis[J]. Journal of Fishery Sciences of China, 2025, 32(4): 435-444. DOI: 10.12264/JFSC2024-0336.

基金项目

国家重点研发计划项目(2022YFD2400105);中央级公益性科研院所基本科研业务费专项资金项目(20603022024002);山东省重点研发计划项目(2022CXPT002);海水养殖生物育种与可持续产出全国重点实验室开放课题(BRESG202302);中国博士后科学基金资助项目(2024M753625);山东博士后科学基金资助项目(SDCX-2G-202400100).

作者简介

赵丽艳(1999‒),女,硕士研究生,研究方向为贝类种质资源与遗传育种. E-mail:zliyan202212@163.com

通信作者

通信作者:吴彪,研究员,研究方向为贝类种质资源与遗传育种. E-mail:wubiao@ysfri.ac.cn

文章历史

收稿日期:2024-11-08
修改日期:2024-12-31
近江牡蛎AGL基因的分子特征及其在糖原分解中的作用
赵丽艳1,2,3,李转转2,3,马培振2,3,刘志鸿2,3,孙秀俊2,3,周丽青2,3,吴彪2,3,     
1. 上海海洋大学水产科学国家级实验教学示范中心,上海 201306
2. 海水养殖生物育种与可持续产出全国重点实验室(中国水产科学研究院黄海水产研究所),山东 青岛 266071
3. 青岛海洋科技中心海洋渔业科学与食物产出过程功能实验室,山东 青岛 266237
摘要:糖原是影响牡蛎品质的关键指标,而糖原脱支酶(AGL)在糖原分解过程中发挥重要作用。为明确AGL基因对近江牡蛎(Crassostrea ariakensis)糖原代谢的影响,本研究在分析AGL基因序列特征和进化特点基础上,体外构建含有该基因的质粒并转化到HT115(DE3)大肠杆菌(Escherichia coli)中,使其表达AGL基因的双链RNA(dsRNA),通过饲喂该菌株实现RNA干扰,并对干扰前后近江牡蛎AGL基因表达量和糖原含量的相关性进行了研究。结果表明,AGL基因编码区序列长度为4719 bp,编码1572个氨基酸,含4个结构域,预测蛋白分子量为178.23 kDa,理论等电点为6.21,基因进化关系与物种传统分类关系一致;RNA干扰15 d和30 d,与对照组(饲喂含EGFP dsRNA的大肠杆菌)相比,实验组(饲喂含AGL dsRNA的大肠杆菌)牡蛎的AGL基因表达量均显著降低(P<0.05),糖原含量显著升高(P<0.05),干扰AGL基因能够显著影响糖原水平的变化;干扰30 d,实验组和对照组牡蛎AGL表达量比干扰15 d时均显著增加(P<0.05),糖原含量显著降低(P<0.05),这可能是由于此时期性腺发育需要消耗糖原;同时,进一步的相关性分析结果表明,AGL基因表达量与糖原含量呈显著负相关关系(P<0.05)。本研究明确了近江牡蛎AGL基因序列结构特征以及其表达量与糖原含量的相关性,丰富了AGL基因功能研究的科学数据,为深入研究牡蛎糖原代谢的分子机制提供了参考。
关键词近江牡蛎     AGL基因    糖原分解    RNA干扰    糖原含量    
Molecular characterization of AGL gene in Jinjiang oyster (Crassostrea ariakensis) and its role in glyco genolysis
ZHAO Liyan1,2,3,LI Zhuanzhuan2,3,MA Peizhen2,3,LIU Zhihong2,3,SUN Xiujun2,3,ZHOU Liqing2,3,WU Biao,2,3    
1. National Demonstration Center for Experimental Fisheries Science Education, Shanghai Ocean University, Shanghai 201306, China
2. State Key Laboratory of Mariculture Biobreeding and Sustainable Goods, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao, 266071, China
3. Laboratory for Marine Fisheries Science and Food Production Processes, Qingdao Marine Science and Technology Center, Qingdao 266237, China
Abstract:Glycogen serves as a crucial indicator for assessing the quality of oysters, and glycogen debranching enzyme (AGL) plays a pivotal role in glycogenolysis. The Jinjiang oyster (Crassostrea ariakensis), widely distributed along China’s coast, is highly sought after in the high-end market. However, the molecular mechanism of glycogen metabolism in the Jinjiang oyster has been inadequately investigated. This study aimed to explore the AGL gene expression during the proliferative stage by administering dsRNA, and subsequently to investigate the relationship between the mRNA expression and glycogen content, thereby elucidating the pivotal role of this gene in glycogen metabolism. First, bioinformatics tools were employed to analyze the AGL gene sequence. Subsequently, dsRNA expression vectors were constructed for inducing the dsRNA production by the HT115 (DE3) bacteria, which were then co-cultured with unicellular algae as attachment hosts. Finally, the bacterial-algal mixture was fed daily to the Jinjiang oyster for RNA interference (RNAi). Furthermore, on days 15 and 30 of RNAi treatment, gonadal tissues were collected for subsequent determination of glycogen content and gene expression analysis. In the control group, gonadal development was observed using tissue sections. The software SPSS 26 was employed to analyze the correlation between AGL gene expression and glycogen content in gonads as well as to determine differences in data. According to the findings, the coding region of the AGL gene had a sequence length of 4719 bp and encoded 1572 amino acids. It contained four structural domains and exhibited a predicted protein molecular weight of 178.23 kDa, a theoretical isoelectric point of 6.21, and an AGL sequence similarity with other selected species ranging from 49.59% to 97.26%. Phylogenetic analysis revealed that the C. ariakensis AGL gene was genetically most closely related to mollusks. Throughout the experimental period, gonads in the control group showed progressive development but remained in the proliferative stage. Following interference for 15 and 30 days, expression levels of the AGL gene were significantly lower (P<0.05) while glycogen content was significantly higher (P<0.05) in comparison to those in the control group. In addition, after RNAi for 30 days compared to 15 days, there was a significant increase (P<0.05) in AGL gene expression accompanied by a significant decrease (P<0.05) in glycogen content. Importantly, it was observed that there existed a highly significant strong negative correlation (P<0.05) between AGL gene expression and glycogen content within C. ariakensis gonads. In this study, we successfully employed the RNAi technology to downregulate the gene expression of AGL in the Jinjiang oyster and observed corresponding alterations in glycogen content, thereby confirming the critical role of the AGL gene in glycogenolysis. By manipulating the gene expression to validate its function, it is anticipated that the artificial regulation of glycogen content and other quality traits can be achieved, ultimately enhancing oyster quality and increasing market value. The present study contributes to the comprehensive understanding of the AGL gene in oyster glycogen metabolism, thereby offering valuable insights for the exploration of novel technologies aimed at regulating glycogen content.
Key words Crassostrea ariakensis      AGL gene    glycogenolysis    RNA interference    glycogen content    

近江牡蛎(Crassostrea ariakensis)隶属于双壳纲(Bivalvia)、珍珠贝目(Pterioida)、牡蛎科(Ostreidae)、巨牡蛎属(Crassostrea),具有较强的温度、盐度适应能力,多栖息在盐度10~25的河口区,主要分布于中国、朝鲜、韩国、日本、越南等海域,在我国北至中朝边境的蜊子江、南至海南岛的沿海广泛分布[1]。近江牡蛎是我国五大经济养殖牡蛎之一,不仅具有重要的经济价值,同时在近海环境系统中还具有重要的生态服务功能。

糖原是几乎存在于所有生物体中的一种支链葡萄糖聚合物,在机体中行使多种生理学功能,如能量存储、信号转导、氧化还原调节等[2]。在海洋贝类中,其糖原的含量和变化是评估贝类生理状况和营养品质的重要指标。牡蛎含有丰富的糖原、脂质和蛋白质等多种营养物质[3],其中糖原是影响其风味的重要营养成分[4],是判断牡蛎品质性状的重要标准。据已有的相关报道,牡蛎软体部糖原含量多在20%~50%之间[5-6],而近江牡蛎性腺组织糖原含量(干重)最高可达到674.36 mg/g[1]。此外,糖原是牡蛎配子发生的主要能量来源[7],在性腺组织中的变化最为明显,随性腺发育呈现季节性变化,在性腺形成期和增殖期含量较高,在成熟期糖原含量显著降低[8-10]。然而目前对牡蛎糖原代谢分子机制的研究比较匮乏,学者前期完成了高低糖原近江牡蛎的多组学分析,发现多个基因与糖原含量有关,例如糖原脱支酶基因(glycogen debranching enzyme gene, AGL)、糖原磷酸化酶B基因(glycogen phosphorylase B, PYGB)和己糖激酶2亚型X1基因(hexokinase-2 isoform X1, HEX-t2)等等[10]。糖原脱支酶作为糖原降解途径中关键的酶,其基因表达水平直接影响糖原含量[10]AGL基因与糖原分解直接相关且在高糖原(形成期)牡蛎中表达活跃,此时糖原分解活动强烈[10]。在人类中,AGL基因与糖原分解及糖原含量有关[11]。在大口黑鲈(Micropterus salmoides)的研究中发现,低蛋白高淀粉喂养导致AGL基因表达下调进而引发糖原的积累[12]。此外,在皱纹盘鲍(Haliotis discus hannai)中,观察到AGL基因的表达量与糖原含量呈负相关,同时揭示了该基因的多态性与鲍体内糖原含量密切相关[13]。这些发现进一步证实了AGL基因在糖原分解中的重要性。目前,国内外对于牡蛎糖原代谢的研究多集中于糖原含量变化及其与性腺发育[9]和环境因子的关系上[14-15],对于糖原代谢过程中关键酶的基因功能研究较少。因此,探究近江牡蛎AGL基因的功能,对于理解贝类糖原代谢的分子调控机制具有重要意义。

本研究分析了近江牡蛎AGL基因cDNA序列和基因进化特征,并利用投喂菌藻混合液的方法进行RNA干扰实验,研究其干扰前后AGL基因表达与糖原含量变化情况,阐明该基因在近江牡蛎糖原代谢过程中的关键作用,以丰富贝类糖原含量性状调控机制方面的基础资料。

1 材料与方法 1.1 实验动物

2024年4月,随机选取110枚课题组培育的速生、糖原含量高的近江牡蛎新品系开展相关实验,其壳高为(88.76±9.81) mm,壳长为(65.58±7.97) mm。取20枚个体作为空白组,解剖获得性腺并保存于−80 ℃,用于RNA提取、基因表达量和糖原含量测定,并分析AGL基因表达量和糖原含量相关性;其余90枚近江牡蛎进行RNA干扰(RNAi)实验。实验前,牡蛎在温度为14 ℃的海水中暂养7 d,期间每天投喂等鞭金藻(Isochrysis galbana)和亚心形扁藻(Platymonas subcordiformis)3次,换水1次,换水量为1/2。

1.2 RNA提取与反转录

使用FreeZol Reagent (诺唯赞,中国)试剂盒按照说明书要求提取牡蛎的性腺组织总RNA。利用琼脂糖凝胶电泳和Nanodrop 2000 (Thermo Scientific,美国)检测RNA质量,用无菌水将高质量的RNA样品浓度稀释至1000 ng/μL。参照说明书步骤,利用反转录试剂盒HiScript® III 1st Strand cDNA Synthesis Kit with gDNA wiper (诺唯赞,中国)将RNA反转录为cDNA,保存在–20 ℃备用。

1.3 AGL基因序列分析

在课题组前期已有的近江牡蛎基因组(GCA_ 020458035.1)和性腺转录组[10]数据中查找获得AGL基因的CDS序列并进行分析。分别利用SignaIP-5.0 Server (https://services.healthtech.dtu.dk/service.php?SignalP-5.0)预测信号肽,Cell-PLoc 2.0 (http://www.csbio.sjtu.edu.cn/bioinf/Cell-PLoc-2/)进行亚细胞定位,ExPASY (https://web.expasy.org/protparam/)分析蛋白理化性质,SMART (https://smart.embl.de/)分析基因结构和功能域,SOPMA (https://npsa.lyon.inserm.fr/cgi-bin/npsa_automat.pl?page=/NPSA/npsa_sopma.html)和Swiss-model (https://www.swissmodel.expasy.org/)分别预测蛋白二级结构和蛋白质三级结构,NCBI的BLAST程序进行碱基序列同源性比对分析和相似蛋白质序列搜索,MEGA 11.0软件以邻接法(Neighbor-Joining, NJ)构建系统进化树。

1.4 dsRNA获得与RNAi实验

本研究利用含有两个相反方向T7聚合酶启动子的L4440质粒,构建双链RNA(ds RNA)表达载体并转化至HT115(DE3)大肠杆菌(Escherichia coli)中,诱导菌株批量产生dsRNA。

dsRNA获得:(1)利用NCBI网站在线设计近江牡蛎AGL基因部分CDS区(扩增区:1724 bp~ 2354 bp)的扩增引物,并在两条引物的5ʹ端分别添加限制性内切酶Sal I和Acc65 I的酶切位点(序列如表1);通过PCR技术扩增该片段,并利用DNA凝胶回收试剂盒(擎科生物,中国)回收。(2)利用Sal I和Acc65 I双酶切L4440质粒,使用ClonExpress® II One Step Cloning Kit (诺唯赞,中国)连接目的片段,构建载体,并将载体转化至大肠杆菌DH5α感受态细胞(诺唯赞,中国)中,在含氨苄青霉素的LB平板上培养菌株,并挑选阳性菌落进行测序。(3)利用Plasmid Mini Kit I (Omega Bio-Tek,美国)提取含有目的片段的质粒,并将其转化至不含dsRNA 内切酶的HT115 (DE3)感受态细胞中,用LB液体培养基将其培养至OD595= 0.4。(4)用浓度为0.4 mmol/L的IPTG在37 ℃、160 r/min条件下诱导菌体4 h产生dsRNA,利用琼脂糖凝胶电泳检测确认成功诱导。实验以牡蛎缺少的增强型绿色荧光蛋白基因(EGFP)作为阴性对照基因,同上述方法诱导获得对照基因的dsRNA。

表1  实验所用的引物序列 Tab. 1  Primer sequences required for the experiment

RNAi实验:离心获得IPTG诱导的菌体,并以100 : 1的比例将其与单细胞藻(等鞭金藻和亚心形扁藻)混合形成菌藻混合液,将90枚近江牡蛎分为对照组和实验组,每组设置3个平行,每个平行放置15枚牡蛎。每天14:00投喂近江牡蛎菌藻混合液实现RNA干扰。对照组饲喂含EGFPdsRNA的菌藻混合物,实验组饲喂含AGL dsRNA的菌藻混合物。在8:00和20:00正常投喂单胞藻。在干扰第15天和第30天每组分别取6枚牡蛎的性腺组织,保存于−80 ℃备用。同时取对照组部分性腺组织,用4%多聚甲醛溶液固定以备用于制备组织切片。

1.5 近江牡蛎性腺发育阶段确定

利用组织切片确定实验过程中性腺组织的发育情况,步骤如下:将固定于4%多聚甲醛溶液中的性腺组织,用梯度酒精脱水,使用二甲苯透明;随后,将组织包埋在石蜡中,并切成4 μm的切片;脱蜡后,用苏木精染色并用伊红复染;最后,以中性树胶封片;随后使用数字切片扫描仪(Pannoramic MIDI II,匈牙利)扫描切片,并使用SlideViewer (V2.5.0.143918)观察并拍照记录。

1.6 基因表达量和糖原含量测定

β-actin作为内参基因,利用反转录实时荧光定量PCR(qRT-PCR)技术测定基因表达情况,引物如表1所示。使用SYBR Green Pro Tap Hs预混型qPCR试剂盒(艾科瑞生物,中国)在CFX 96(Bio-rad,美国)上进行qRT-PCR反应,反应体系为:2×SYBR Green Pro Taq HS Premix 5 μL,正反向引物各0.2 μL, cDNA模板1 μL, RNase Free H2O 3.6 μL。反应条件为:95 ℃预变性30 s, 95 ℃ 5 s, 60 ℃ 30 s, 40个循环。每个qRT-PCR实验进行3个重复,采用${2^{ - \Delta \Delta {C_{\rm{t}}}}}$法计算基因的相对表达量。

采用微量蒽酮比色法测定性腺组织的糖原含量[16]。每个样品重复测量3次。

1.7 数据处理

利用SPSS 26分析AGL基因表达量与糖原含量的相关性,Graphpad Prism 9.5绘制相关性散点图。数据以平均值±标准差($\bar x \pm {\rm{SD}}$)表示,采用SPSS 26进行独立样本T检验,设置P<0.05时差异显著。

2 结果与分析 2.1 AGL基因序列及进化特征

AGL基因的CDS序列长度为4719 bp,编码1572个氨基酸,蛋白无信号肽,亚细胞定位显示其存在于细胞质中。预测的蛋白质分子质量为178.23 kDa,理论等电点为6.21,分子式为C8009H12282N2166O2327S65,脂溶系数为80.75,平均亲水系数(GRAVY)为–0.374,不稳定系数为33.42<40.00,说明AGL基因编码产物为亲水性稳定蛋白。AGL存在4个结构域(图1b),分别为AGL-N-末端结构域(氨基酸30~122), AGL-葡聚糖转移酶催化结构域(氨基酸138~570), AGL-中心结构域(氨基酸715~988), AGL-C-末端结构域(氨基酸1055~1542)。利用Swiss-model在线网站预测了三维结构(图1a),发现AGL蛋白序列与同源模板的相似性为97.07%, GMQE为0.66,结果说明该蛋白与模板蛋白的匹配度较高,其中40.46%呈α-螺旋,11.32%呈β-折叠,48.22%呈无规则卷曲。

图1  AGL蛋白质三级结构及结构域预测a. AGL蛋白质三级结构预测;b. AGL蛋白质结构域预测. Fig. 1  Prediction of tertiary structure and domain of AGL proteina. AGL protein tertiary structure prediction; b. AGL protein domain prediction.

AGL序列比对发现,近江牡蛎与福建牡蛎(C. angulata)序列相似性最高,为97.26%,其次是长牡蛎(C. gigas)为96.88%;与美洲牡蛎(C. virginica)和欧洲扁牡蛎(Ostrea edulis)的相似性相近,分别为88.68%和84.49%;与虾夷扇贝(Mizuhopecten yessoensis)、紫贻贝(Mytilus galloprovincialis)、硬壳蛤(Mercenaria mercenaria)、加州海兔(Haliotis rubra)的相似性较低,在57.42%~63.38%范围内;与斑马鱼(Danio rerio)等鱼类、非洲爪蟾(Xenopus laevis)等两栖动物、智人(Homo sapiens)等哺乳动物的同源性相近且最低,为49.59%~50.19% (表2)。系统进化树如图2所示,近江牡蛎先与福建牡蛎、长牡蛎聚为一支,然后再逐步与美洲牡蛎、欧洲扁牡蛎、虾夷扇贝和紫贻贝等贝类聚为一支,与鱼类、两栖类和哺乳动物的聚类距离较远。

表2  不同物种AGL氨基酸序列号与同源性 Tab. 2  AGL amino acids sequences numbers and identity of different species
图2  不同物种AGL氨基酸序列系统进化树 Fig. 2  Phylogenetic tree of AGL amino acid sequence in different species
2.2 表达载体构建和诱导结果

构建的载体结构示意图如图3a图3c所示。琼脂糖凝胶电泳检测结果表明,经IPTG诱导的样品中检测到dsRNA (图3b图3d中的泳道3),而未经诱导的样品中则未检测到(图3b图3d中的泳道2),这表明本研究在体外成功诱导表达出了AGL基因和EGFP基因的dsRNA。

图3  外源基因重组质粒的构建与诱导的dsRNA电泳检测a. 导入AGL基因片段的L4440质粒(L4440-AGL质粒);b. dsRNA电泳检测. 1泳道为DNA marker(诺唯赞,中国),2泳道为未诱导菌体的RNA条带,3泳道为IPTG诱导菌体的RNA片段,箭头所指为AGL基因的dsRNA;c. 导入EGFP基因片段的L4440质粒(L4440-EGFP质粒);d. dsRNA电泳检测. 1泳道为DNA marker(诺唯赞,中国),2泳道为未经诱导菌体的RNA条带,3泳道为IPTG诱导菌体的RNA片段,箭头所指为EGFP基因的dsRNA. Fig. 3  Construction of recombinant plasmid with exogenous gene and induced dsRNA electrophoresis detectiona. The L4440 plasmid with AGL gene fragment was introduced (L4440-AGL plasmid); b. Induced dsRNA electrophoresis detection. Lane 1 is DNA marker (Vazyme, China), lane 2 represents the band of uninduced bacterial RNA, lane 3 represents bacterial RNA fragments induced by IPTG, and the arrow points to the dsRNA of the AGL gene; c. The L4440 plasmid with EGFP gene fragment was introduced (L4440-EGFP plasmid); d. Induced dsRNA electrophoresis detection. Lane 1 is DNA marker (Vazyme, China), lane 2 represents the band of uninduced bacterial RNA, lane 3 represents bacterial RNA fragments induced by IPTG, and the arrow points to the dsRNA of the EGFP gene.
2.3 近江牡蛎性腺发育情况

在RNAi实验期间,对照组中雌性和雄性牡蛎的性腺组织切片如图4所示。尽管性腺在实验期间均处在增殖期,但随着培养时间延续,可以明显观察到性腺发育。在实验开始时,雌性和雄性近江牡蛎中均可观察到少量精原细胞和卵原细胞,性腺中的细胞以发育中的卵母细胞和精母细胞为主,雄性中可见少量精子。在实验进行15 d时,可以观察到卵母细胞体积逐渐增大,精子逐渐增多;实验30 d,性腺中出现成熟的卵母细胞,而雄性个体精母细胞数量减少,精子数量进一步增多。

图4  不同RNAi时间点近江牡蛎性腺发育情况a,d. 对照组实验第0天的性腺组织切片图;b,e. 对照组实验第15天的性腺组织切片图;c,f. 对照组实验第30天的性腺组织切片图. CT:结缔组织;OO:卵原细胞;DO:发育中的卵母细胞;MO:成熟的卵母细胞;SPG:精原细胞;SPC:精母细胞;SPZ:精子. Fig. 4  Gonadal development of Jinjiang oysters at different RNAi time pointsa, d. The gonadal tissue slices of the control group on day 0 of the experiment; b, e. The gonad tissue slices of the control group on the 15th day of the experiment; c, f. The gonadal tissue slices of the control group on the 30th day of the experiment. CT: connective tissue; OO: oogonia; DO: developing oocyte; MO: mature oocyte; SPG: spermatogonia; SPC: spermatocyte; SPZ: spermatozoa.
2.4 RNAi后AGL基因表达的变化

qRT-PCR检测RNAi后AGL基因表达结果显示(图5),经过15 d和30 d的处理,近江牡蛎性腺中AGL 基因的表达量相对于对照组显著下降(P< 0.05)。处理15 d,其表达量降至对照组的51.79%;处理30 d降至对照组的61.27%,这说明基因敲降效果明显。30 d时,实验组和对照组AGL的表达量均显著高于15 d的表达量(P<0.05),其中对照组AGL表达量升高75.98%,实验组升高108.19%,这可能是性腺发育等原因所致。

图5  不同RNAi时间点实验组和对照组近江牡蛎AGL的相对表达量对比小写字母不同代表同一组在不同时间点有显著性差异(P< 0.05);大写字母不同代表在同一时间点不同组有显著性差异(P<0.05). Fig. 5  Comparison on the relative expression of AGL of Crassostrea ariakensis in experimental group and control group at different RNAi time pointsDifferent lowercases indicate significant difference (P<0.05) among different time points in the same group, different uppercases indicate significant difference (P<0.05) among different groups at the same time.
2.5 RNAi后糖原含量的变化

糖原含量的测定结果显示(图6),干扰AGL基因15 d和30 d时牡蛎糖原含量分别为(460.18±46.74) mg/g和(402.62±40.96) mg/g,均显著高于对照组的(356.53±67.59) mg/g和(283.65±33.22) mg/g。此外,实验30 d时,不论实验组还是对照组,牡蛎性腺中的糖原含量均显著低于实验15 d时的糖原含量(P<0.05)。

图6  不同RNAi时间点实验组和对照组近江牡蛎性腺组织糖原含量对比小写字母不同代表同一组在不同时间点有显著性差异(P<0.05);大写字母不同代表在同一时间点不同组有显著性差异(P<0.05). Fig. 6  Comparison on glycogen content in gonad tissue of Crassostrea ariakensis in experimental group and control group at different RNAi time pointsDifferent lowercases indicate significant difference (P<0.05) among different time points in the same group; different uppercases indicate significant difference (P<0.05) among different groups at the same time.
2.6 AGL基因表达量与糖原含量的相关性

空白组、实验组和对照组牡蛎的性腺组织中AGL基因表达量与糖原含量相关性分析结果如图7所示,空白组Spearman相关系数(R)为–0.7861 (P<0.05),实验组和对照组R为–0.7053 (P< 0.05),说明AGL基因表达量与糖原含量呈显著负相关,该基因可能负调控糖原含量。

图7  AGL基因表达量与糖原含量的相关系数a. 空白组AGL基因表达量和糖原含量的相关性;b. 实验组和对照组AGL基因表达量和糖原含量的相关性. Fig. 7  Correlation coefficient between AGL expression and glycogen contenta. The correlation between AGL gene expression and glycogen content in blank group; b. The correlation between AGL gene expression and glycogen content in experimental group and control group.
3 讨论

RNAi技术通过将双链RNA(dsRNA)导入细胞内以引起特定基因的mRNA降解[17]。目前,注射法[18]、饲喂法[19-21]和浸没法[22]是水产生物相关研究中运用的3种转运dsRNA的方法。饲喂法不会对动物造成身体损伤,适合进行长期实验。本研究通过此种方法对处于增殖期近江牡蛎的AGL基因进行长期干扰,即构建载体,诱导细菌产生dsRNA,再将其与单细胞藻混合饲喂牡蛎,这在贝类相关研究中具有一定创新性。RNAi实验结果表明,对近江牡蛎AGL基因进行干扰处理15 d和30 d后,AGL基因的表达量分别降低了48.21%和38.73% (P<0.05),显著低于对照组。此前,Li等[20] 用此方法研究了Pax7基因在长牡蛎黑色素合成中的作用,发现在干扰30 d后,基因表达量降低60.1%。Sun等[21]同样应用此方法分析了Foxl2Dmrt1基因在长牡蛎性别分化和性腺发育中的作用,发现干扰20 d和60 d, Foxl2分别降低了82%和62%, Dmrt1分别降低70%和58%。这些均证实在牡蛎中使用该方法有效、可行。此外,笔者还观察到处理30 d时AGL基因敲降效率低于15 d,在Sun等[21]的研究中同样观察到随着时间的延长RNA敲降效率轻微降低。这说明生物在长期RNAi处理下可能激活某些补偿机制来减弱RNAi的效果,以维持其自身的生理平衡。Olejniczak等[23]曾指出RNAi会诱发免疫脱靶效应进而影响RNAi的效果,Xie等[24]在果蝇中也发现细胞死亡可以通过阻止dsRNA转化成siRNA的方式来抑制本细胞和邻近细胞的RNAi。

糖原不仅是牡蛎风味的重要影响因素,也是牡蛎中能量储存的主要形式之一。在长牡蛎[8]、香港牡蛎[9]和近江牡蛎[10]中发现糖原随牡蛎的性腺发育呈季节性变化。牡蛎会分解糖原产生能量去支持配子发生[7],糖原脱支酶在糖原分解过程中发挥着至关重要的作用。Guin等[25]的研究表明,通过RNAi技术在体外干扰细胞AGL基因的表达会导致糖原分解减少,极限糊精增加。本研究发现AGL基因的CDS序列长度为4719 bp,编码1572个氨基酸,具有4个结构域,这一数量与先前预测的人类糖原脱支酶结构域的数量相同[26]。氨基酸序列的同源性比对和进化树表明,亲缘关系越近,氨基酸序列的同源性越高。近江牡蛎的AGL基因与其他牡蛎的同源性较高,表明AGL基因在牡蛎中相对保守。增殖期牡蛎糖原合成和分解旺盛且含量波动剧烈[10],而AGL是参与糖原分解的重要基因,此时AGL基因表达活跃,干扰AGL基因的表达可能会显著影响糖原含量。RNAi之后笔者进一步测定了两组间糖原含量的差异,结果表明,在RNAi处理15 d和30 d后,实验组的糖原含量均显著高于对照组,分别提高了103.65 mg/g和118.97 mg/g,这一结果表明降低AGL基因的表达水平会导致糖原含量增加。人[26]和马(Equus caballus)[27]AGL基因的缺乏或者突变也会导致糖原异常积累。本研究中,无论是实验组还是对照组,干扰处理30 d的牡蛎性腺糖原含量均显著低于15 d时含量,但AGL基因表达量均显著升高,这可能说明此阶段牡蛎性腺发育需要更多的糖原分解以便提供能量[10]AGL基因表达水平和糖原含量的Spearman相关性分析,进一步证实了AGL基因的表达水平与糖原含量之间存在极显著的负相关关系。

前期基因家族分析中未发现近江牡蛎AGL基因存在其他家族成员。本研究对AGL基因序列及其三维结构的分析,为深入理解其分子功能奠定了基础,同时,通过饵料投喂法进行RNAi,成功抑制了近江牡蛎AGL基因的表达,证实该方法可有效地应用在牡蛎基因研究中。而且,本研究发现近江牡蛎性腺AGL基因表达量与糖原含量呈显著负相关,且敲降AGL基因表达能显著影响糖原含量,研究结果为丰富牡蛎糖原代谢机制相关研究提供了参考资料。

参考文献
[1]
Wu B, Chen X, Hu J, et al. Combined ATAC-seq, RNA-seq, and GWAS analysis reveals glycogen metabolism regulatory network in Jinjiang oyster (Crassostrea ariakensis)[J]. Zoological Research, 2024, 45(1): 201-214..》Google Scholar
[2]
Zhang H F, Ma J W, Tang K, et al. Beyond energy storage: Roles of glycogen metabolism in health and disease[J]. The FEBS Journal, 2021, 288(12): 3772-3783..》Google Scholar
[3]
Zhu Y J, Li Q, Yu H, et al. Biochemical composition and nutritional value of different shell color strains of Pacific oyster Crassostrea gigas[J]. Journal of Ocean University of China, 2018, 17(4): 897-904..》Google Scholar
[4]
Lin H, Wang X X, Zhang B, et al. Comparison of taste components between triploid and diploid oyster[J]. Journal of Ocean University of Qingdao, 2002, 1(1): 55-58..》Google Scholar
[5]
Li B S, Song K, Meng J, et al. Integrated application of transcriptomics and metabolomics provides insights into glycogen content regulation in the Pacific oyster Crassostrea gigas[J]. BMC Genomics, 2017, 18(1): Article No.713..》Google Scholar
[6]
Liu S. Study on the genetic basis and molecular mechanism of nutritional quality traits of the Pacific oyster[D]. Qingdao: Institute of Oceanology, Chinese Academy of Sciences, 2019. [刘圣. 长牡蛎糖原等品质性状的遗传基础与分子机制研究[D]. 青岛:中国科学院大学(中国科学院海洋研究所), 2019.].》Google Scholar
[7]
Mathieu M, Lubet P. Storage tissue metabolism and reproduction in marine bivalves—a brief review[J]. Invertebrate Reproduction & Development, 1993, 23(2-3): 123-129..》Google Scholar
[8]
Dridi S, Romdhane M S, Elcafsi M. Seasonal variation in weight and biochemical composition of the Pacific oyster, Crassostrea gigas in relation to the gametogenic cycle and environmental conditions of the Bizert lagoon, Tunisia[J]. Aquaculture, 2007, 263(1-4): 238-248..》Google Scholar
[9]
Qin Y P, Li X Y, Li J, et al. Seasonal variations in biochemical composition and nutritional quality of Crassostrea hongkongensis, in relation to the gametogenic cycle[J]. Food Chemistry, 2021, 356: 129736..》Google Scholar
[10]
Li Z Z, Zhao L Y, Wang Y, et al. Glycogen variations and glycometabolism during the gametogenesis cycle of Jinjiang oyster Crassostrea (Magallana) ariakensis[J]. Aquaculture Reports, 2024, 37: 102251..》Google Scholar
[11]
Adeva-Andany M M, González-Lucán M, Donapetry-García C, et al. Glycogen metabolism in humans[J]. BBA Clinical, 2016, 5: 85-100..》Google Scholar
[12]
Wang K W, Liu Q Q, Zhu J, et al. Transcriptome analysis provides insights into the molecular mechanism of liver inflammation and apoptosis in juvenile largemouth bass Micropterus salmoides fed low protein high starch diets[J]. Comparative Biochemistry and Physiology Part D: Genomics and Proteomics, 2023, 45: 101047..》Google Scholar
[13]
Tang X Y, Li S S, Zhuo L, et al. Correlation study of SNP of glycogen degradation metabolism related genes and glycogen content in Haliotis discus Hannai[J]. Aquaculture, 2024, 578: 740022..》Google Scholar
[14]
Zhang H, Wang H, Chen H, et al. The transcriptional response of the Pacific oyster Crassostrea gigas under simultaneous bacterial and heat stresses[J]. Developmental & Comparative Immunology, 2019, 94: 1-10..》Google Scholar
[15]
Cao C, Wang W X. Copper-induced metabolic variation of oysters overwhelmed by salinity effects[J]. Chemosphere, 2017, 174: 331-341..》Google Scholar
[16]
Chen X, Wu B, Wang Y, et al. Establishment and optimization of micro-reaction system for determination of oyster glycogen content[J]. South China Fisheries Science, 2021, 17(4): 126-132. [陈夕,吴彪,王岩,等. 测定牡蛎糖原含量的微量反应体系的建立与优化[J]. 南方水产科学,2021, 17(4): 126-132.].》Google Scholar
[17]
Mello C C, Conte D. Revealing the world of RNA interference[J]. Nature, 2004, 431(7006): 338-342..》Google Scholar
[18]
Wang Y, Liu Z H, Chen X, et al. Identification and characterization of GYS and GSK3β provides insights into the regulation of glycogen synthesis in Jinjiang oyster Crassostrea ariakensis[J]. Fishes, 2023, 8(2): 65..》Google Scholar
[19]
Feng D D, Li Q, Yu H. RNA interference by ingested dsRNA-Expressing bacteria to study shell biosynthesis and pigmentation in Crassostrea gigas[J]. Marine Biotechnology, 2019, 21(4): 526-536..》Google Scholar
[20]
Li Z Z, Li Q, Xu C X, et al. Molecular characterization of Pax7 and its role in melanin synthesis in Crassostrea gigas[J]. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 2022, 260: 110720..》Google Scholar
[21]
Sun D F, Yu H, Li Q. Examination of the roles of Foxl2 and Dmrt1 in sex differentiation and gonadal development of oysters by using RNA interference[J]. Aquaculture, 2022, 548: 737732..》Google Scholar
[22]
Baum J A, Bogaert T, Clinton W, et al. Control of coleopteran insect pests through RNA interference[J]. Nature Biotechnology, 2007, 25(11): 1322-1326..》Google Scholar
[23]
Olejniczak M, Polak K, Galka-Marciniak P, et al. Recent advances in understanding of the immunological off-target effects of siRNA[J]. Current Gene Therapy, 2011, 11(6): 532-543..》Google Scholar
[24]
Xie W W, Liang C Z, Birchler J A. Inhibition of RNA interference and modulation of transposable element expression by cell death in Drosophila[J]. Genetics, 2011, 188(4): 823-834..》Google Scholar
[25]
Guin S, Pollard C, Ru Y B, et al. Role in tumor growth of a glycogen debranching enzyme lost in glycogen storage disease[J]. Journal of the National Cancer Institute, 2014, 106(5): dju062..》Google Scholar
[26]
Zmasek C M, Godzik A. Phylogenomic analysis of glycogen branching and debranching enzymatic duo[J]. BMC Evolutionary Biology, 2014, 14(1): Article No.183..》Google Scholar
[27]
Ward T L, Valberg S J, Adelson D L, et al. Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV[J]. Mammalian Genome, 2004, 15(7): 570-577..》Google Scholar