畜牧兽医学报  2020, Vol. 51 Issue (3): 443-451. DOI: 10.11843/j.issn.0366-6964.2020.03.005    PDF    
hnRNPUL1基因克隆及变异剪接体鉴定
宋艳芳1, 张彩霞1, 杜芳芳1, 马骏杰1, 刘东宇1, 刘娣2, 杨秀芹1     
1. 东北农业大学动物科学技术学院, 哈尔滨 150030;
2. 黑龙江省农业科学院, 哈尔滨 150086
摘要:旨在克隆民猪hnRNPUL1基因全长编码区(complete coding sequence,CDS)序列并进行可变剪接分析,研究其组织表达和亚细胞定位情况。本研究以3头3月龄民猪为试验材料,利用RT-PCR技术克隆hnRNPUL1基因CDS及可变剪接体,应用qRT-PCR检测其在心、肝、脾、肺、肾、胃、大肠、小肠、背肌、腿肌等组织中的表达情况,同时,在生物信息学预测的基础上,通过融合表达绿色荧光蛋白的方法鉴定核定位序列。研究结果表明,猪hnRNPUL1基因(GenBank accession No.:MN399154)CDS全长2 580 bp,预期编码的多肽链存在着hnRNPs家族的特征性结构域——SAP、SPRY和AAA;猪hnRNPUL1普遍表达于所检测的各组织中,其中在脾脏中表达量最高,在腿肌中表达量最低;核定位序列(NLS)位于多肽链的133~430 aa处,与生物学信息学预测的结果存在着偏差;同时获得了2个变异剪接体和11种可变剪接片段。本研究结果对进一步揭示猪hnRNPUL1基因功能及转录调控机制提供了基础。
关键词    hnRNPUL1    克隆    变异剪接体    
Cloning and Identification of Splice Variants of the Porcine hnRNPUL1 Gene
SONG Yanfang1, ZHANG Caixia1, DU Fangfang1, MA Junjie1, LIU Dongyu1, LIU Di2, YANG Xiuqin1     
1. College of Animal Science and Technology, Northeast Agricultural University, Harbin 150030, China;
2. Heilongjiang Academy of Agricultural Sciences, Harbin 150086, China
Abstract: The study was conducted to clone the complete coding sequence (CDS) and transcript variants of porcine hnRNPUL1, and to analyze its tissue expression and subcellular localization. In this study, three 3-month-old Min pigs were used. The CDS and transcript variants of porcine hnRNPUL1 were cloned using RT-PCR method, and the expression was detected in tissues including heart, liver, spleen, lung, kidney, stomach, large intestine, small intestine, back muscle and leg muscle by qRT-PCR. At the same time, the subcellular localization was analyzed by inserting the fragments of CDS into pEGFP-N1 vector on the basis of bioinformatics prediction. It was found that the CDS of porcine hnRNPUL1 gene (GenBank accession No.:MN399154) was 2 580 bp of length, and the predicted polypeptide contained SAP, SPRY, and AAA domains, the characteristic domains of hnRNPs family. Porcine hnRNPUL1 was ubiquitously expressed in all detected tissues, with the highest level in the spleen but the lowest in leg muscle. The nuclear localization sequence(NLS) was located at 133-430 aa of the polypeptide chain, which was different from the result predicted by the bioinformatics methods. In addition, 2 splice variants and 11 alternative splicing fragments were identified. The results have provided the basis for further revealing the function and transcriptional regulation mechanism of porcine hnRNPUL1.
Key words: pig    hnRNPUL1    cloning    splice variants    

核不均一核糖核蛋白(heterogeneous nuclear ribonucleoprotein, hnRNPs)是一个保守的核蛋白超家族,在动植物中广泛分布,目前为止,已发现该家族成员有20多种,命名分别从A1至U[1]。大量的研究表明,hnRNPs家族成员在哺乳动物发育中起着至关重要的作用[2],可参与pre-RNA剪接、mRNA运输、定位及翻译等过程[3-5]。hnRNPUL1是hnRNPs家族新成员。hnRNPUL1可与mRNA结合影响RNA的转运和加工[6-7],也可以通过激活多种信号通路促进DNA损伤修复[8-11],还有研究表明,hnRNPUL1与疾病(如早发型心肌梗死)发生相关[12]

目前已在多物种中对hnRNPUL1基因进行了研究,人hnRNPUL1基因位于19号染色体上,由18个外显子组成,预计编码856个氨基酸残基;牛hnRNPUL1基因位于18号染色体上,由16个外显子组成,预计编码858个氨基酸残基;小鼠hnRNPUL1基因位于7号染色体上,由17个外显子组成,预计编码859个氨基酸残基;这些物种hnRNPUL1均具有保守结构域SAP、SPRY和AAA。但目前尚未见到有关猪hnRNPUL1基因的报道。为进一步揭示猪hnRNPUL1基因功能,本研究以民猪为试验材料,通过RT-PCR方法克隆了猪hnRNPUL1的全长编码区(complete coding sequence, CDS),利用生物信息学方法分析其序列和结构,确定其组织表达谱和亚细胞定位情况,并对其存在的变异剪接体进行鉴定。

1 材料与方法 1.1 试验动物

3头3月龄民猪(体重约15 kg,母猪)由黑龙江省农业科学院畜牧研究所科研基地提供,屠宰后立即采集心、肝、脾、肺、肾、胃、大肠、小肠、背肌、腿肌等10个组织样,于液氮中速冻后,-80 ℃冰箱长期保存。

1.2 引物设计与合成

通过分析GenBank数据库提供的猪EST、电子预测序列及人、鼠等动物的相应序列,设计H引物,用于扩增猪hnRNPUL1基因的全长编码区。根据克隆测序结果设计引物M1~4、M5、V1~4,分别用于分析变异剪接体、组织表达谱及亚细胞定位情况(详细信息见表 1)。所有引物均利用Premier 5.0软件自行设计,由北京六合华大基因科技股份有限公司合成。

表 1 引物信息 Table 1 Primer information
1.3 民猪组织RNA提取和cDNA合成

利用Trizol(Invitrogen, Carlsbad, California, USA)提取各组织总RNA,用琼脂糖凝胶电泳和紫外分光光度计检测总RNA的质量和浓度,稀释成1 μg·μL-1备用。按照Prime Script RT reagent Kit(TaKaRa, Dalian, China)说明书进行反转录。将合成的cDNA置于-20 ℃中保存备用。

1.4 hnRNPUL1基因克隆及序列分析

以多种组织混合cDNA为模板,利用引物H(表 1)通过PCR方法扩增猪hnRNPUL1基因编码区。反应体系:2×Taq Master Mix 5 μL(Vazyme, Nanjing, China),上下游引物(10 μmol·L-1)各0.4 μL,模板1 μL,ddH2O 3.2 μL,总计10 μL;反应程序:95 ℃ 5 min,(95 ℃ 30 s,60 ℃ 30 s,72 ℃ 1 min 20 s)×30循环,72 ℃ 7 min,16 ℃ 30 s。目的片段胶回收后,插入pMD-18-T载体,转化至大肠杆菌DH5α菌,对平板上的阳性菌落进行菌液PCR和质粒双酶切鉴定后,送到北京六合华大基因科技股份有限公司测序鉴定。

利用SMART程序(http://smart.embl-heidelberg.de/)对猪hnRNPUL1编码的氨基酸序列和蛋白结构域进行分析;利用ProtCompv 9.0 (http://linux1.softberry.com/berry.phtml?group=programs&subgroup=proloc&topic=protcompan)、NLS_Mapper (http://nls-mapper.iab.keio.ac.jp/cgi-bin/NLS_Mapper_form.cgi#opennewwindow)和NetNES (http://www.cbs.dtu.dk/services/NetNES/)软件分析猪hnRNPUL1蛋白亚细胞定位情况。

为了更好地分析猪hnRNPUL1基因的可变剪接现象,利用彼此重叠的4对引物(M1~M4,表 1)通过RT-PCR方法进行可变剪接分析,具体方法同上。

1.5 猪hnRNPUL1基因组织表达谱构建

将3头猪的同一种组织cDNA进行等量混合后,作为模板,通过Real-time PCR方法构建组织表达谱。利用2×SYBR Green qPCR Master Mix试剂(Bimake, Houston, TX, USA)在ABI QuantStudio3仪器上进行反应。反应体系:SYBR Green Master Mix 10 μL,上下游引物(10 μmol·L-1)各1 μL,cDNA 1 μL,ddH2O 7 μL,总体系为20 μL;反应程序:95 ℃ 5 min,(95 ℃ 15 s,60 ℃ 45 s)×40个循环,95 ℃ 15 s,60 ℃ 60 s,95 ℃ 15 s。每个样品设置3个重复。以β-actin基因为内参,利用2-ΔΔCt方法进行相对表达量计算。

1.6 猪hnRNPUL1亚细胞定位分析

以全长编码区的克隆质粒为模板,利用引物V1、V2、V3、V4(表 1)进行PCR扩增后,扩增产物克隆入pMD18-T载体,再通过BamH I和EcoR I酶切位点转接入pEGFP-N1(TaKaRa, Dalian, China)质粒,测序鉴定。

用含10% FBS的DMEM/F12培养基培养PK15细胞,过夜培养待其细胞密度达到70%~80%时,使用脂质体Lipofectamine 2000(Invitrogen, Carlsbad, California, USA)将重组质粒转染至PK15细胞。设置空载体转染组作为阴性对照。转染24 h后使用DAPI(Vazyme, Nanjing, China)染细胞核,在倒置荧光显微镜下观察融合蛋白在PK15细胞中的定位情况。

2 结果 2.1 猪hnRNPUL1基因克隆及序列分析

通过RT-PCR和克隆、测序获得了猪hnRNPUL1的全长编码区序列,长度为2 580 bp(图 1图 2),预期编码859个氨基酸残基的多肽链,分子量为98.06 ku,等电点为8.26,平均亲水性为-0.986(亲水性)。其与人(NP_008971.2)、鼠(NP_659171.1)、牛(NP_001076935.1)hnRNPUL1蛋白的同源性分别为96.97%、89.56%、95.47%。利用SMART程序预测,发现其存在着SAP、SPRY和AAA结构域,分别位于多肽链的3~37、257~390和426~571 aa处。通过BLAT程序(https://genome.ucsc.edu/cgi-bin/hgBlat)分析,确定其含有15个外显子。利用核定位序列预测软件发现,在多肽链的2~30和498~507 aa处存在2个核定位序列(nuclear location sequence, NLS)。将该序列已经提交到GenBank数据库得到序列登录号:MN399154。

M. DNA相对分子质量标准,条带长度分别为5 000、3 000、2 000、1 500、1 000、750、500、250、100 bp;1.扩增产物 M. DL5000 marker, composed of 5 000, 3 000, 2 000, 1 500, 1 000, 750, 500, 250 and 100 bp DNA fragments; 1. Amplified products 图 1 RT-PCR电泳检测结果 Fig. 1 Electrophoresis pattern of RT-PCR
E表示外显子,小写字母为引物序列,斜体加粗表示预测的核定位序列,单下划线表示可变剪接体H-1和H-2共有序列,双下划线表示H-2特有序列,边框表示起始密码子和终止密码子,其中TAG为全长扩增产物H的终止密码子,TAA为可变剪接体H-1和H-2的终止密码子 The E represent the exons, the lowercase letters are the primer sequences, the bold italic are putative NLS, the common sequences of transcript variant H-1 and H-2 are underlined, the sequence unique to H-2 is double underlined. The start and stop codons are boxed, of which TAG is the stop codon of H, while TAA is the stop codon of alternative splicing variant H-1 and H-2 图 2 hnRNPUL1核苷酸序列 Fig. 2 Nucleotide sequence of porcine hnRNPUL1
2.2 猪hnRNPUL1组织表达谱分析

实时荧光定量PCR分析表明,hnRNPUL1基因在所检测的所有组织中均表达,在脾脏中表达量最高,心、肝和肾中的表达量较高,但在肺、胃、大肠、小肠、背肌中表达量较低,在腿肌中表达量最低(图 3)。

腿肌组织中的相对表达量作为1,数据为“平均数±标准误” The relative expression level in the leg muscle tissue is used as 1, data is "mean ± standard error" 图 3 hnRNPUL1组织表达谱 Fig. 3 Tissue expression profile of porcine hnRNPUL1 gene
2.3 猪hnRNPUL1亚细胞定位分析

根据亚细胞定位预测结果,首先构建了V1(1~155 aa)和V3(417~738 aa)2个片段的真核表达质粒,转染后发现V1在核质中都有表达,V3仅表达于细胞质中,说明二者不含有NLS。在此基础上,又构建了V2(133~430 aa)和V4(717~859 aa)2个片段的真核表达质粒,发现V2仅表达于细胞核,V4仅表达于细胞质(图 4),说明V2片段存在着NLS。

图 4 hnRNPUL1基因亚细胞定位 Fig. 4 Subcellular localization of porcine hnRNPUL1 gene
2.4 猪hnRNPUL1基因可变剪接分析

在克隆测序时,通过菌液PCR对平板上的菌落进行初步鉴定,发现了两个较短的扩增产物,测序后证明是hnRNPUL1基因的可变剪接体,命名为H-1和H-2。与上述克隆的全长CDS相比,二者均缺失外显子2~12的完整序列以及外显子1、13的部分序列;此外,H-2在外显子13末端插入了111 bp(图 2)。H-1和H-2都存在着编码提前终止现象且终止密码子相同,预期编码多肽链长分别为128和166个氨基酸残基,其中前70 aa完全一致,在多肽链的3~37 aa处存在着经典结构域SAP。

为了更好地分析猪hnRNPUL1基因的可变剪接现象,利用彼此重叠的4对引物(M1~4)通过RT-PCR方法进行可变剪接分析。除M4引物外,M1~M3引物均成功扩增得到了可变剪接片段,3对引物分别获得了2、5和4个可变剪接片段(图 5)。与全长CDS的相应片段比对发现,新发现的可变剪接片段都存在着外显子缺失,剪接方式有外显子跳跃、可变5′剪接位点和可变3′剪接位点,其中外显子8和13存在着较多的可变3′剪接位点,外显子3存在着较多的可变5′剪接位点。M1引物得到的两个可变剪接片段还存在着内含子滞留现象。

a. M1引物扩增结果;b. M2引物扩增结果;c. M3引物扩增结果。E表示外显子,折线表示外显子跳跃,灰色阴影方框表示内含子滞留 a. Amplification results of M1 primer; b. Amplification results of M2 primer; c. Amplification results of M3 primer. E indicate exons, the broken lines represent exon skipping, the closed boxes indicate intron retention 图 5 M1、M2、M3扩增区域变异剪接体结构 Fig. 5 Mutant splice variants structure of M1, M2 and M3 amplifying region
3 讨论

最近的研究表明,hnRNPUL1与聚ADP核糖聚合酶1(poly ADP-ribose polymerase-1, PARP1)有关的DNA损伤修复密切相关,可通过与PARP1的相互作用进而调节PARP1表达的转录调控[13],而PARP1是维持基因组稳定所必需的重要蛋白质之一[14-16]。DNA损伤后不能有效修复会导致损伤积聚和基因组稳定性下降,进而会引起相应遗传性疾病和散发性癌症的发展[17-19]。因此,hnRNPUL1在维持动物机体基因组完整性中发挥作用。

本研究成功克隆了猪hnRNPUL1基因全长CDS序列,发现其在组织中广泛表达,并通过生物信息学分析确定了其编码的多肽链含有hnRNPs家族的经典结构域——SAP、SPRY和AAA。SAP基序通常存在于多种参与转录、DNA修复、RNA处理或凋亡染色质降解的核蛋白中[20-21];具有SPRY结构域的蛋白多参与RNA代谢、细胞内钙释放、调控和发育过程,可能还参与细胞因子信号调控[22-23];AAA基序则分布广泛,参与各种细胞过程,包括膜融合、蛋白水解和DNA复制等[24]。因此,推测猪hnRNPUL1基因同样参与以上多种生理生化进程,对维持动物机体正常的理化反应及功能具有重要意义。

亚细胞定位是分子细胞生物学和蛋白组学一个重要的研究课题,目前多使用绿色荧光蛋白GFP构建融合载体来实现目标蛋白的定位观察[25-28]。通过网站预测发现,该基因具有两个核定位信号,分别位于2~30、498~507 aa处,对应编码区的6~90和1 494~1 521 bp。为鉴定核定位信号位置,首先构建了V1(1~155 aa)和V3(417~738 aa)2个片段的真核表达质粒。发现V1融合蛋白在细胞核与细胞质中均有表达,V3融合蛋白在细胞质中表达。表明这两个区域不存在核定位序列,软件预测结果存在着偏差。作为hnRNPs家族成员,细胞核内定位表达是其发挥功能的基础[29-32]。本研究又继续构建了V2(133~430 aa)和V4(717~859 aa)2个片段的真核表达质粒。结果发现,V2融合蛋白仅在细胞核中表达;V4融合蛋白的表达量比较弱,但能清晰地看到其表达于细胞质中,这可能是由于截短蛋白稳定性降低导致的。

可变剪接现象在真核生物中普遍存在,约有90%人类基因存在可变剪接,是机体在转录后水平调控基因表达的重要手段[33]。研究表明,几乎所有人类基因都存在选择性剪接[34-38]。选择性剪接在控制分化和发育中起着关键作用[39-41],许多重大疾病都与选择性剪接失调有关[42-45]。本试验在鉴定猪hnRNPUL1基因的全长编码区时,获得了2个新的变异剪接体H-1和H-2。为了更好地分析猪hnRNPUL1基因的可变剪接现象,利用彼此重叠的4对引物(M1~M4)通过RT-PCR方法进行鉴定。本试验在其编码区发现了11种新的剪接现象,剪接方式有外显子跳跃、可变5′剪接位点、可变3′剪接位点和内含子滞留。本研究通过菌液PCR结合琼脂糖凝胶电泳检测方法对RT-PCR产物进行筛选,选择长度不同的条带进行测序。由于琼脂糖凝胶电泳的灵敏性较低,无法鉴定长度相同但剪接方式不同的RT-PCR产物,所以不能全面反映猪hnRNPUL1基因的可变剪接情况,但说明了猪hnRNPUL1基因编码区存在着丰富的可变剪接现象。

4 结论

通过RT-PCR方法克隆了猪hnRNPUL1基因的全长编码区,确定了其组织表达和亚细胞定位情况,并发现其存在着复杂的可变剪接现象,为研究猪hnRNPUL1基因的生物学功能提供了基础。

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