中国水产科学  2024, Vol. 31 Issue (11): 1351-1364  DOI: 10.12264/JFSC2024-0105
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引用本文 

余子豪, 陆俊杰, 孙佳敏, 胡文静, 徐烨彤, 刘其根. 兴国红鲤幼鱼对高温胁迫与复温恢复的氧化应激和适应机制[J]. 中国水产科学, 2024, 31(11): 1351-1364. DOI: 10.12264/JFSC2024-0105.
YU Zihao, LU Junjie, SUN Jiamin, HU Wenjing, XU Yetong, LIU Qigen. Analysis of oxidative stress and acclimatization mechanism of gill tissues of Xingguo red carp juvenile fish (Cyprinus carpio var. singuonensis) by high temperature stress and cooling recovery[J]. Journal of Fishery Sciences of China, 2024, 31(11): 1351-1364. DOI: 10.12264/JFSC2024-0105.

基金项目

国家自然科学基金项目(32172995);上海科技馆长三角城市群生态安全与生物多样性保护实验室资助项目(FW-2024-079).

作者简介

余子豪(1999‒),男,硕士研究生,研究方向为鱼类环境适应. E-mail:18070222037@163.com

通信作者

通信作者:刘其根,教授,研究方向为生态养殖技术与综合养殖生态工程. E-mail:qgliu@shou.edu.cn

文章历史

收稿日期:2024-06-14
修改日期:2024-08-19
兴国红鲤幼鱼对高温胁迫与复温恢复的氧化应激和适应机制
余子豪,陆俊杰,孙佳敏,胡文静,徐烨彤,刘其根,     
上海海洋大学,农业农村部鱼类营养与环境生态研究中心,农业农村部稻渔综合种养生态重点实验室,农业农村部淡水水产遗传资源重点实验室,上海 201306
摘要:为了解析兴国红鲤(Cyprinus carpio var. singuonensis)在热应激下的生理调控特征,本研究通过模拟高温环境和复温环境,研究了在不同温度水平(28 ℃和38 ℃)下兴国红鲤幼鱼鳃组织的相关酶活性指标和转录组变化。酶活指标结果显示,高温胁迫后丙二醛(MDA)含量显著上升(P<0.05),过氧化氢酶(CAT)活性显著下降(P<0.05),而谷胱甘肽过氧化物酶(GSH-PX)和超氧化物歧化酶(SOD)活性无显著差异(P>0.05);复温后,SOD活性显著下降(P<0.05),其余均无显著差异(P>0.05)。通过转录组测序分析,共检测出5752个差异表达基因(DEGs),KEGG分析揭示这些基因主要富集在免疫和凋亡相关通路,包括内质网蛋白加工、IL-17信号通路、NF-κB信号通路、p53信号通路和细胞凋亡等。在这些通路中,热休克蛋白(HSPs)、白细胞介素-1β(IL-1β)、白细胞介素-8(IL-8)、P53肿瘤抑制基因(p53)、半胱天冬氨酸蛋白酶(Caspases)、缺氧诱导因子-1α(HIF-1α)等关键信号因子表现出不同的表达趋势。研究结果表明,在高温环境下,兴国红鲤幼鱼的细胞免疫和细胞凋亡被抑制,导致脂质过氧化损伤,并且机体处于缺氧状态;复温后,机体解除缺氧状态,但机体损伤不可逆,细胞免疫与细胞凋亡依旧被抑制。该研究为揭示水产养殖物种适应高温环境的分子机制提供了理论依据。
关键词兴国红鲤    高温应激    氧化应激    转录组学        
Analysis of oxidative stress and acclimatization mechanism of gill tissues of Xingguo red carp juvenile fish (Cyprinus carpio var. singuonensis) by high temperature stress and cooling recovery
YU Zihao,LU Junjie,SUN Jiamin,HU Wenjing,XU Yetong,LIU Qigen    
Shanghai Ocean University; Centre for Research on Environmental Ecology and Fish Nutrition of the Ministry of Agriculture and Rural Affours; Key Laboratory of Integrated Rice-fish Farming, Ministry of Agriculture and Rural Affairs; Key Laboratory of Freshwater Aquatic Genetic Resources, Ministry of Agriculture and Rural Affairs, Shanghai 201306, China
Abstract:Increasing global temperature is causing water temperatures to increase, potentially leading to metabolic disorders in fish. The Xingguo red carp (Cyprinus carpio var. singuonensis) is an economically important fish in China; however, studies on its adaptation mechanism to high temperatures are relatively few. To analyze the changes in physiological regulation under heat stress, the present study simulated an acute high-temperature environment and cooling environment, and gill tissues of Xingguo red carp at different temperature levels (28 ℃ and 38 ℃) were examined. The results of enzyme activity indices showed that malondialdehyde (MDA) increased significantly and catalase (CAT) content decreased significantly after high temperature stress, whereas no significant difference was observed in glutathione peroxidase (GSH-PX) and superoxide dismutase (SOD). The SOD content decreased significantly after cooling, and the rest of the indicators were not significantly different. Using transcriptome sequencing analysis, 2643 differentially expressed genes (DEGs) were screened for differential genes between the high- and normal-temperature groups, of which 1,401 DEGs were upregulated and 1,242 were downregulated in Xingguo Red Carp. A total of 2396 DEGs were screened to compare the cooled and high-temperature groups, including 936 upregulated DEGs and 1460 downregulated DEGs. In comparing the cooled and normal-temperature groups, a total of 908 DEGs were screened, of which 300 DEGs were upregulated and 608 DEGs were downregulated. Meanwhile, the activation and involvement of DEGs in immune and apoptosis-related pathways were identified using the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, such as endoplasmic reticulum protein processing, IL-17 signaling pathway, NF-κB signaling pathway, p53 signaling pathway, and apoptosis. Within these pathways, key signaling factors such as HSPs, IL-1β, IL-8, p53, Caspase-3, and HIF-1α showed different expression trends. The results showed that Xingguo red carp responds to high-temperature stress stimuli during high temperature stress by positively upregulating gene expression. However, the expression of Xingguo red carp immune-related genes and pathways were downregulated or suppressed, leading to suppression of the immune system, while apoptosis regulation was blocked, preventing the effective removal of abnormal and damaged cells. Furthermore, no signs of hypoxia existed. Following cooling recovery, the stress of the Xingguo red carp decreased. The immune response of Xingguo red carp was activated, and the inflammatory symptoms were reduced, but they did not recover completely. The symptoms of hypoxia returned to normal. These two levels were used to systematically investigate the physiological regulation mechanism of Xingguo red carp in high temperature environment. This study provides valuable information on how aquatic species adapt to high-temperature environments.
Key words Cyprinus carpio var. singuonensis    heat stress    oxidative stress    transcriptomics    gill    

全球变暖和温室效应导致全球气温持续升高[1-2]。适宜的环境条件是水生生物维持正常生命活动的必要条件[3]。温度作为关键的环境因子之一,其变化可对鱼类的生理稳态和生物学功能产生重要影响[4]。研究发现红鲑(Oncorhynchus nerka)会因温度上升致使耗氧量增加而导致缺氧[5]。此外,全球气温每升高3.2 ℃将导致近1/3淡水鱼类的生态栖息地受到破坏[6],进而对水产养殖行业造成巨大打击[7-8]。高温不仅带来经济损失,还会削弱鱼类的活力和免疫力,增加疾病感染的概率[9]。鳃作为重要的气体交换和免疫器官,是抵御病原体的第一道防线[10],也是环境胁迫最直接的靶器官[11-12]。因此,本研究选择鳃组织作为研究材料,对鱼类应对高温环境的适应性机制展开研究。

兴国红鲤(Cyprinus carpio var. singuonensis)属鲤属(Cyprinidae),既可作为观赏鱼,也可作为食用鱼,具有极高的经济价值,因具有生长速度快、繁殖能力高、抗逆境强和杂交亲和性好等特点,被广泛运用于品种改良和新品种选育。兴德鲤(Cyprinus carpio var. singdenensis)、芙蓉鲤(Cyprinus carpio var. furongnensis)、丰鲤(Cyprinus carpio var. fengnensis)、异育银鲫(Carassius auratus gibelio)等优良品种均以兴国红鲤为杂交亲本[13]。因此,兴国红鲤在我国水产养殖中具有重要意义。然而,关于兴国红鲤的研究大多是对杂交品种的培育,对其高温适应性机制的研究相对较少。目前了解到兴国红鲤的适宜生长温度为18~32 ℃,对于其最高热临界值尚未可知。为此本研究以青田鲤(Cyprinus carpio var. qingtianensis)的最大热临界值38 ℃为参考[14],利用RNA-Seq转录组测序技术,系统揭示兴国红鲤在高温和复温期间的转录组变化,旨在找到关键的基因表达调控网络,深入了解兴国红鲤在高温与复温下的适应性调节机制。

1 材料和方法 1.1 实验材料与暂养管理

实验用鱼取自江西省赣州市兴国县方太乡富坑村合群组,个体规格相近,体重(74.28±9.08) g,体长(13.54±1.32) cm,生命力旺盛且健康无损。正式实验前在室内空调房中暂养14 d,将兴国红鲤幼鱼置于水箱中(250 L),每个水箱正中间吊放一个500 W的加热棒加热,水温控制在(28±1) ℃,溶氧保持在>5.5 mg/L,每两天进行一次投喂,正式实验前一天停止投喂。在每日9:00和19:00分别用曝气自来水置换1/2的养殖用水,保障养殖用水的更新。

1.2 实验设计和样品采集

本实验对兴国红鲤幼鱼进行高温胁迫与复温恢复,以青田鲤(Cyprinus carpio var. qingtianensis)的最大热临界值38 ℃为参考[14],共设置3个实验组:对照组(28 ℃)、高温组(38 ℃)、复温组(28 ℃),每个处理组3个重复。暂养14 d后,以1 ℃/h的速度,将水温从暂养温度28 ℃升直至38 ℃,胁迫6 h。胁迫后自然降温至28 ℃并维持6 h。采集样品时,从每个重复中随机挑选3尾鱼,每个处理组共取9尾鱼。采用MS-222 (300 mg/L)麻醉后取出鳃组织于液氮中速冻后,置于−80 ℃超低温冰箱中保存待用。

1.3 鳃的氧化应激参数

冻存的鳃组织寄送至南京建成生物工程研究所,对超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH-PX)、过氧化氢酶(CAT)活性和丙二醛(MDA)含量进行检测和分析。利用SPSS22.0统计分析软件对试验所得的数据进行单因素方差分析(one-way, ANOVA),运用邓肯(Duncan)多重比较对均值进行差异显著性检验,显著水平设为P< 0.05。所得数据用平均数±标准差($\bar x \pm {\rm{SD}}$)表示。

1.4 RNA提取、文库制备和测序

从−80 ℃冰箱中取出样品,在提前预冷的研钵中加入液氮研磨成粉末,使用Trizol法提取总RNA。运用琼脂糖凝胶电泳(1%琼脂糖)和NanoDrop2000对提取的总RNA进行完整性、浓度和纯度等方面的质量检测。通过混样各重复的3尾鱼鳃组织样品来消除个体间差异,每组取1 μg RNA用于cDNA文库构建及Illumina测序。本次研究中的文库构建和Illumina测序均由美吉生物科技有限公司完成,建库使用试剂为TruseqTM RNA sample prep Kit (Illumina, CA, USA),测序系统为Illumina Novaseq 6000,测序模式为2×150。

1.5 质量控制和序列比对

使用SeqPrep和Sickle软件去除掉原始序列(raw reads)中接头、含N (N:模块碱基)的序列和过滤掉低质量序列(质量值<20),得到质控后高质量测序序列(clean reads)。统计质控后clean reads的错配率(error rate)、Q20、Q30和GC含量,以评估测序质量。

基于改进的BWT (Burrows-heeler transform)算法,使用HISAT2 v.2.1.0软件将clean reads与2014年公布的鲤基因组信息[15]进行比对获得比对序列(mapped reads)。

1.6 转录组组装和注释

以鲤基因组为参考基因,使用Cufflinks v.2.2.1软件将获得的mapped reads进行拼接,最终组装成转录本(unigene)。通过Gffcompare软件进行组装后转录本与已知转录本的比较,发现未知基因,优化转录本并完成功能注释。使用BLAST+ v.2.9.0软件将获得的所有基因和转录本与Swiss-Port、Nr、Pfam、COG、GO和KEGG数据库进行比对,为后续获得的基因和转录本的功能信息进行统计注释。

1.7 基因差异表达及富集分析

使用RSEM1.3.1软件通过TPM (Transcripts Per Million)定量标准,对基因长度进行均一化后对测序深度均一化,统一不同样本的总表达量获得read counts分析样本之间的基因差异表达情况。使用DEGseq2 v.1.24.0软件进行read counts的标准化处理并筛选差异表达基因(differentially expressed genes, DEGs),以校正P值(P-adjust)< 0.05与|log2(Fold Change)|≥1作为差异表达的阈值。在差分表达式的统计计算中,采用多重统计检验的校正方法对P值进行校正,得到P-adjust。使用Goatools v.0.6.5软件和KOBAS v.2.1软件,运用Fisher检验分别对DEGs进行GO功能富集分析和对KEGG Pathway代谢通路进行富集分析。

1.8 实时荧光定量PCR验证

为确认转录组的结果精确度,根据获得的RNA-Seq数据选取5个显著不同的基因进行了RT-qPCR测试以进一步证实其正确性。利用Primer 6.0软件进行引物设计,以β-action基因为内参。在实时荧光定量PCR仪(ABI7300,美国)上进行试验。每个反应进行3个生物学重复和技术重复,使用2−ΔΔCt法分析结果。使用SPSS 22.0软件进行单因素方差分析。

2 结果与分析 2.1 不同温度条件下鳃的抗氧化酶活性

兴国红鲤幼鱼对高温胁迫和复温恢复氧化应激的鳃组织酶活指标见表1。高温胁迫对兴国红鲤幼鱼抗氧化系统产生了显著影响。在P=0.05的置信水平下,高温组的MDA含量显著高于对照组(P<0.05), CAT活性显著低于对照组(P<0.05), SOD及GSH-PX活性无显著差异(P>0.05)。另外,复温组的SOD活性显著低于对照组和高温组(P< 0.05),其MDA含量显著高于常温组(P<0.05), CAT和GSH-PX活性无显著差异(P>0.05)。

表1  3个实验组兴国红鲤幼鱼鳃组织中氧化应激指标 Tab. 1  Oxidative stress indices in the gill tissues of juvenile Xingguo red carp (Cyprinus carpio var. singuonensis) in three experimental groups n=3; $\bar x \pm {\rm{SD}}$
2.2 转录组测序数据的质量评估

采用Illumina测序平台对3组9个独立cDNA文库完成的转录组测序,共产生523189978个原始序列(Raw reads),过滤后其中有509446572个序列通过质量控制。GC含量为47.33%~48.64%, Q20值均超过97%, Q30值均超过94%,错配率仅有0.024%~0.025%,表明数据可靠,能够进一步开展相关研究和分析。具体详情如表2所示。

表2  兴国红鲤幼鱼鳃组织RNA-seq文库原始序列和质控序列的统计值 Tab. 2  Statistics of the raw sequence and quality control sequences of RNA-seq libraries from the gill tissues of juvenile Xingguo red carp (Cyprinus carpio var. singuonensis) in three experimental groups
2.3 转录组组装和注释

将构建的86185种转录本用6个公用数据平台(GO、KEGG、COG、NR、Swiss-Prot、Pfam)进行注释,分别成功注释到56980、61784、76440、81851、71513、55901条转录本,总计注释了81883条转录本。将获取的69065个基因与6个数据库进行注释,分别成功注释到47112、49617、61010、65233、57099、42984条转录本,共计注释65263条转录本(表3)。

表3  转录本和基因注释统计 Tab. 3  Statistics of annotation for unigenes and genes
2.4 差异基因(DEG)表达分析

从3个实验组筛选DEGs (differentially expressed genes): XGSH vs XGSC、XGRW vs XGSC和XGRW vs XGSH。将差异基因进行聚类热图分析和韦恩分析(图1)。从3组中分别得到了2643、908和2396个DEGs,其中分别包括了1401、300和936个上调表达的基因,以及1242、608和1460个下调表达的基因(图1, 2)。

图1  各实验组间基因差异表达(DEGs)统计图a. 基因差异表达聚类热图;b. 基因差异表达Venn图. Fig. 1  Statistical figure of differentially expressed genes among different experimental groupsa. The heat map of clustering analysis on DEGs among groups; b. The Veen diagram of DEGs among groups.
图2  3组差异表达基因统计图 Fig. 2  Volcano plot of DEGs in 3 groups
2.5 DEGs的GO功能富集分析

通过Fisher检验对差异表达基因进行富集分析。当P-adjust<0.05时,认为DEGs在此两类中表现出显著的富集现象。将GO功能分为分子功能(MF)、生物过程(BP)和细胞组分(CC),其富集分析结果如下:

在XGSH vs XGSC组有1931个DEGs在GO功能中显著富集,有377个GO二级条目,包括BP 227个、CC 55个和MF 95个(图3)。在XGRW vs XGSH组中,有1792个DEGs显著地富集在GO功能上,其中包含了199个GO二级条目,包括BP 103个、CC 27个、MF 69个(图4)。在XGRW vs XGSC组有645个DEGs在GO功能中显著富集,其中有150个GO二级条目,包括BP 92个、CC 34个、MF 24个(图5)。生物过程主要涉及DNA复制、细胞凋亡、昼夜节律的调控等;分子功能主要涉及细胞信号转导、免疫调节或细胞内蛋白质的折叠修复等;细胞组分主要涉及细胞的生长、分裂、信号转导等。

图3  XGSH vs XGSC组中GO富集分析结果(富集排名前20) Fig. 3  Results of GO function enrichment analysis in XGSH vs XGSC groups (top20)
图4  XGRW vs XGSH组中GO富集分析(富集排名前20) Fig. 4  GO function enrichment analysis in XGRW vs XGSH groups (top20)
图5  XGRW vs XGSC组中GO富集分析结果(富集排名前20) Fig. 5  Results of GO function enrichment analysis in XGRW vs XGSC groups (top20)
2.6 DEGs的KEGG通路富集分析

在XGSH vs XGSC组中有2016个DEGs在KEGG通路显著富集,有效富集到53条KEGG通路上,其中主要包括DNA复制(36个基因)、内质网中的蛋白质加工(84个基因)、细胞周期(61个基因)、p53信号通路(38个基因)、IL-17信号通路(38个基因)、病毒致癌(76个基因)、系统性红斑狼疮(52个基因)等,主要涉及细胞免疫系统和细胞凋亡方面(图6)。

图6  XGSH vs XGSC组中KEGG pathway富集分析结果(富集排名前20) Fig. 6  Results of KEGG pathway function enrichment analysis in XGSH vs XGSC groups (top20)

在XGRW vs XGSH组之间共有1786个DEGs在KEGG通路被发现显著富集,有效富集到41条KEGG通路上,其中主要包括内质网中的蛋白质加工(89个基因)、昼夜节律(29个基因)、抗原处理和呈递(41个基因)、细胞凋亡(49个基因)、系统性红斑狼疮(53个基因)、IL-17信号通路(28个基因)等,这些通路主要涉及到治疗相关疾病方面(图7)。

图7  XGRW vs XGSH组中KEGG pathway富集分析结果(富集排名前20) Fig. 7  Results of KEGG pathway function enrichment analysis in XGRW vs XGSH groups (top20)

在XGRW vs XGSC组之间发现有675个DEGs在KEGG通路显著富集,其中有效富集到15条KEGG通路,主要包括昼夜节律(Circadian rhythm, 16个基因)、细胞周期(Cell cycle, 25个基因)、卵母细胞减数分裂(Oocyte meiosis, 22个基因)、孕激素介导的卵母细胞成熟(Progesterone-mediated oocyte maturation, 17个基因)、p53信号通路(p53 signaling pathway, 13个基因)、近曲小管的重吸收碳酸盐(Proximal tubule bicarbonate reclamation, 9个基因)等,这些通路主要涉及到治疗疾病以及相关恢复等方面(图8)。

图8  XGRW vs XGSC组中KEGG pathway富集分析结果(富集排名前20) Fig. 8  Results of KEGG pathway function enrichment analysis in XGRW vs XGSC groups (top20)
2.7 RT-qPCR验证RNA-seq结果

在XGSH vs XGSC组选取了5个DEGs (HSP90aHIF-1αIL-1βCASP3IL8)进行RT-qPCR试验来验证RNA-seq的准确性。这些基因的表达与转录组测序获得的结果显示了相似的表达水平且升降趋势相同,表明转录组测序结果准确可信(图9)。

图9  RT-qPCR与RNA-seq基因表达量的比较 Fig. 9  Comparison of gene expression between RT-qPCR and RNA-seq
3 讨论 3.1 高温胁迫与复温恢复下氧化应激指标变化

Billard等[16]发现,鲤作为一种变温动物,可以在接近0 ℃到超过30 ℃的宽温度范围内生存。Vinagre等[17]的研究结果中表明,鱼类的脂质过氧化对环境温度非常敏感,温度超过一定阈值下会发生氧化应激反应[18]。本研究发现热胁迫对兴国红鲤幼鱼的氧化应激水平具有显著影响。在高温胁迫阶段,兴国红鲤幼鱼的MDA含量显著增加。MDA是脂质过氧化的二次产物,可作为氧化应激的标志物[19],产生氧化应激时,其活性氧(ROS)大量积累,当ROS积累超过抗氧化防御系统的能力时,会引发脂质过氧化[20]。抗氧化酶则是中和或清除ROS的第一防线,包括CAT、SOD和GSH-PX,这些酶在鱼类中已被广泛发现[21],王涛等[22]的研究发现,在急性铜胁迫下河鲀(Takifugu fasciatus)通过提升CAT、SOD和GSH-PX来清除ROS。本研究中兴国红鲤幼鱼在高温胁迫下CAT活性显著下调,SOD与GSH-PX活性无显著变化,这与Liu等[23]的研究结果相似,即武昌鱼(Megalobrama amblycephala)在热应激下抗氧化酶活性显著下降,这种现象是由于高温导致抗氧化酶的失活和破坏[24]。在复温恢复阶段,兴国红鲤幼鱼的SOD活性显著下降,其余无明显差异,在其他鱼类的研究中也出现相似的结果,如双带鲷(Diplodus vulgaris)[25]和虹鳟(‌Oncorhynchus mykiss)[26]‌,这些结果表明,在高温胁迫下,鱼体的抗氧化酶很可能已经失活,导致其抗氧化系统受到了抑制。此外,过量ROS的形成会导致DNA损伤,出现抗氧化酶降解的现象,这也可能是抗氧化酶减少的原因之一[27]。综上所述,高温胁迫降低了兴国红鲤幼鱼的抗氧化防御能力并扰乱生理平衡,致使脂质过氧化损伤,并在复温后也未能及时恢复。

3.2 高温胁迫与复温对细胞免疫与细胞凋亡相关通路和基因表达的影响

在Makrinos等[28]和Valero等[29]的研究中发现,水温会影响鱼类的生理机能,包括免疫反应。尤其是在夏季的,高温环境会抑制鱼类机体的免疫系统。Ndong等[30]研究发现莫桑比克罗非鱼(Oreochromis mossambicus)被转移至高温环境后免疫力会降低,细胞凋亡增加,染病风险升高。Guijarro等[31]曾表明,鱼类对各种病原体的免疫反应也会因温度影响而降低。由此可见,温度是变温脊椎动物细胞免疫、凋亡的重要影响因素。

在本研究中,通过KEGG分析发现,显著富集的与细胞免疫相关的信号通路有内质网蛋白加工、NF-kappa B信号通路和IL-17信号通路,筛选到与细胞免疫相关的差异表达基因有HSPs (HSP40、HSP70、HSP90)、IL-8、IL-1β等。Chen等[32]发现分子伴侣HSPs在维持细胞内稳态中起到关键作用,并且通过Calderwood等[33]证明每一类的HSP在炎症和免疫平衡方面发挥重要作用。对鮸(Miichthys miiuy)的研究中发现,NF-kappa B信号通路受到抑制会影响到免疫反应[34],同时Kim等[35]在研究褐牙鲆(Paralichthys olivaceus)时证明NF-kappa B信号通路受到IL-8趋化因子的激活。在草鱼(Ctenopharyngodon idella)的研究中发现,通过激活IL-1β基因的表达来促进炎症反应[36],而IL-1β基因主要由IL-17信号通路激活。对高温组和对照组进行转录组测序,结果显示高温组的HSPs (HSP40、HSP70、HSP90)和IL-1β基因以及IL-17信号通路表达显著上调或激活,IL-8基因和NF-kappa B信号通路表达显著下调或抑制。表明在高温胁迫后,兴国红鲤幼鱼虽然通过HSPs的上调表达来维持机体稳定,但是激活了IL-17信号通路,产生了炎症反应,同时下调了NF-kappa B信号通路,抑制免疫系统。在对大口黑鲈(Micropterus salmoides)鳃组织和肝脏组织的研究发现它们的变化趋势一致,高温下调了鳃组织和肝脏组织免疫相关基因的表达(如IL-8),抑制了机体的免疫系统[37],与本研究结果相似,为本研究用鳃组织分析兴国红鲤幼鱼的氧化应激和适应机制提供了支持。而复温恢复后与高温组相比,细胞免疫相关基因表达显著下调,表明热应激对兴国红鲤幼鱼产生了不可逆的损伤,导致免疫应答功能下降。

通过KEGG分析发现,显著富集的与细胞凋亡相关的信号通路主要是细胞凋亡和P53信号通路,筛选出与细胞凋亡相关的关键差异表达基因Caspases (Caspase3、Caspase6、Caspase7Caspase8)和P53。在细胞凋亡过程中,每一类的Caspase相互配合以达到促进细胞凋亡,例如Caspase3会被Caspase8直接或间接激活,并催化一些关键细胞蛋白的切割[38]。在对河鲀的研究中发现,P53信号通路可能在热应激诱导的细胞凋亡中发挥重要作用[39]。对高温组和对照组进行转录组测序,结果显示高温组的4种Caspases和P53信号通路表达显著下调或抑制。表明在高温胁迫后兴国红鲤幼鱼鳃组织的细胞凋亡受到抑制,这一结果一方面与Cheng等[14]研究结果一致,会增强细胞的存活能力,另一方面可能会导致无法有效清除异常和受损细胞,促使炎症的发生。根据机体的氧化损伤,本研究结果更倾向于后者。复温恢复后与高温组相比,细胞凋亡相关基因无显著变化,表明热应激对兴国红鲤幼鱼产生了不可逆的损伤,或者是需要更多的时间进行恢复,导致细胞凋亡调控功能下降。

3.3 高温胁迫与复温对缺氧调节相关通路和基因表达的影响

在Stehfest等[40]的研究中发现,较高的温度会降低氧气的溶解度,也会增加鱼类新陈代谢率,从而增加需氧量。对大西洋鲑(Salmo salar)的相关研究中发现热耐受性和缺氧耐受性在功能上相关[41], Remen等[42]表明极限氧饱和度会随着水温的上升而增加,因此在高温下,氧气的供给和需求不匹配会导致组织缺氧。在本研究中,经KEGG富集分析发现,与缺氧调节相关的HIF-1信号通路被显著富集,筛选到HIF-1αHIF-1β关键亚基显著差异表达。当生物处于低氧环境时,HIF-1α被激活,与HIF-1β一起作用,直接调节参与缺氧反应特定基因的转录[43]。大量研究表明,HIF-1α显著差异表达,起到血管生成、能量代谢改变以及炎症调节等作用[44-46],但其主要的作用是应对缺氧环境,如武昌鱼(Megalobrama amblycephala)[47]、石首鱼(Leiostomus xanthurus)[48]和胭脂鱼(Myxocyprinus asiaticus)[49]等处于缺氧环境时,其不同组织中的HIF-1α均显著富集上调表达。本研究高温胁迫后,兴国红鲤幼鱼鳃组织的HIF-1αHIF-1β显著上调,表明在高温胁迫下兴国红鲤幼鱼的鳃组织出现了缺氧,通过上调基因的表达来响应低氧环境刺激,但这仅仅代表着鳃组织,其他组织是否也发生了缺氧呢?在青鳉(Oryzias latipes)缺氧的研究中表明,虽然不同组织(鳃、脑和肝脏)的基因表达差异有所不同,但是导致的代谢趋势趋于一致,不同组织HIF-1的靶基因均显著表达[50],表明鳃、脑和肝脏组织均出现缺氧情况,这也为兴国红鲤幼鱼的其他组织缺氧提供一定的佐证。复温恢复后,HIF-1α显著下调,与常温组无显著差异,表明机体不再缺氧。根据氧限热耐受模型,变温动物会在最佳温度范围之外时出现功能性缺氧[51]。在本研究中,笔者验证热应激诱导缺氧信号假设的同时,也证明了38 ℃超出了兴国红鲤幼鱼的适宜温度范围。

4 结论

本研究通过转录组测序技术对高温胁迫和复温恢复的兴国红鲤幼鱼鳃组织进行了转录组学分析,揭示了该过程中细胞免疫、细胞凋亡和缺氧因子的变化。基因表达情况表明,在高温胁迫后,其细胞免疫与细胞凋亡受到抑制,产生脂质过氧化损伤,同时机体出现缺氧症状;复温后,缺氧症状消除,但机体细胞免疫与细胞凋亡依旧受到抑制。该研究为揭示水产养殖物种适应高温环境的分子机制提供了理论依据。

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