畜牧兽医学报  2024, Vol. 55 Issue (2): 846-853. DOI: 10.11843/j.issn.0366-6964.2024.02.040    PDF    
毒害艾美耳球虫谷胱甘肽过氧化物酶EnGPX的原核表达与分析
彭月梅1,2, 叶状1,2, 汪飞燕1,2, 王礼跃1,2, 冯永翠1,2, 王乐乐1,2, 候照峰1,2, 许金俊1,2, 陶建平1,2, 刘丹丹1,2     
1. 扬州大学兽医学院, 扬州 225009;
2. 扬州大学 江苏高校动物重要疫病与人兽共患病防控协同创新中心, 扬州 225009
摘要:旨在研究毒害艾美耳球虫谷胱甘肽过氧化物酶EnGPX的反应原性及其在虫体内的亚细胞定位。提取毒害艾美耳球虫(扬州株)配子体总RNA, RT-PCR扩增EnGPX的ORF编码序列, 构建原核表达质粒pET-28a(+)-EnGPX, 转化至BL21(DE3)进行体外诱导表达, 同时制备鼠抗rEnGPX多克隆抗体, 对重组蛋白进行Western blot反应原性分析和激光共聚焦免疫荧光定位分析。结果表明, EnGPX ORF序列全长753 bp, 编码250个氨基酸, 体外重组表达蛋白大小约30 ku, 主要以包涵体形式存在。该重组蛋白能被6×HIS标签单克隆抗体, 鼠抗rEnGPX多克隆抗体, 毒害艾美耳球虫、巨型艾美耳球虫和柔嫩艾美耳球虫病鸡康复血清所识别, 表明其具有较好的反应原性和交叉反应原性。在天然配子体蛋白中检测出EnGPX, 其编码蛋白主要分布于配子体内的Ⅱ型成壁体(WFBII)及卵囊壁上。本研究成功克隆表达了毒害艾美耳球虫谷胱甘肽过氧化物酶EnGPX, 验证其具有良好的反应原性, 并定位于配子体及卵囊壁上。以期为研究EnGPX参与卵囊壁形成的分子机制提供新的线索, 也为研制新型球虫亚单位疫苗提供新的靶标。
关键词毒害艾美耳球虫    EnGPX    克隆表达    反应原性    免疫荧光定位    
Procaryotic Expression and Analysis of the EnGPX of Eimeria necatrix
PENG Yuemei1,2, YE Zhuang1,2, WANG Feiyan1,2, WANG Liyue1,2, FENG Yongcui1,2, WANG Lele1,2, HOU Zhaofeng1,2, XU Jinjun1,2, TAO Jianping1,2, LIU Dandan1,2     
1. College of Veterinary Medicine, Yangzhou University, Yangzhou 225009, China;
2. Jiangsu Co-innovation Center for Prevention and Control of Important Animal Infectious Diseases and Zoonoses, Yangzhou University, Yangzhou 225009, China
Abstract: The aim of this paper was to study the antigenicity of glutathione peroxidase EnGPX and its subcellular localization in Eimeria necatrix. Total RNA was extracted from the gametophyte of E. necatrix (Yangzhou strain), and the ORF coding sequence of EnGPX was amplified using RT-PCR. The prokaryotic expression plasmid pET-28a(+)-EnGPX was constructed and transformed into BL21(DE3) for in vitro expression, additionally, a mouse anti-rEnGPX polyclonal antibody was prepared and used to analyze the recombinant protein through Western blotting, and laser confocal immunofluorescence localization analysis. The study found that the coding region of the EnGPX gene was 753 base pairs in length and encoded a protein consisting of 250 amino acids. The resulting recombinant protein had a molecular weight of around 30 ku and was predominantly present in the form of inclusion bodies. The recombinant protein exhibited favorable reactivity and cross-reactivity, as it was recognized by a 6×HIS-tagged monoclonal antibody, a mouse anti-rEnGPX polyclonal antibody, and convalescent serum from E. necatrix, E. maxima, and E. tenella. EnGPX was detected in natural gametophyte proteins, with the encoded protein primarily localized to the type II wall-forming body (WFBII) of the gametophyte and oocyst wall. This study successfully cloned and expressed the glutathione peroxidase (EnGPX) of E. necatrix. The recombinant protein exhibited good reactivity, and natural EnGPX protein was found to be localized on the gametophyte and oocyst wall. The results shed light on the molecular mechanism of EnGPX involvement in oocyst wall formation and identify potential targets for the development of novel subunit vaccines against coccidia.
Key words: Eimeria necatrix    EnGPX    cloning and expression    reactivity    immunofluorescence localization    

鸡球虫病(coccidiosis)是由艾美耳科球虫(Eimeria spp.)引起的一种寄生性原虫病[1]。毒害艾美耳球虫(E. necatrix)是致病性较强的鸡小肠球虫,主要危害8~18周龄的鸡,可引起急性小肠球虫病,严重降低家禽的生长性能,且在环境中长期存活难以消除,给家禽业造成巨大的经济损失[2-4]。目前, 鸡球虫病的防治主要是预防性使用抗球虫药物和疫苗免疫两种方法,然而耐药虫株的出现,药物残留肉蛋,以及活疫苗免疫存在毒力返强的风险,使球虫病的防治受到严峻挑战。重组亚单位疫苗相比活疫苗易于储存,通过外源抗原激发宿主的免疫反应,不会出现毒力返强,较为安全,目前重组亚单位疫苗的研究重点是筛选有效的保护性抗原,已有报道如EtMIC1[5]、EtMIC2、3-1E[6]、EmTFP250[7]、Emgam56、Emgam82[8]和Engam59[9]等重组蛋白免疫鸡群可在一定程度上提高血清抗体水平,降低卵囊产量。因此,筛选有效的保护性抗原开发高效重组亚单位疫苗可为球虫病的防控提供新策略[10-11]

球虫的卵囊壁非常坚实,是保护球虫在外界环境中长期存活并保有感染性的重要屏障,蛋白质是其主要组成成分[12]。卵囊壁蛋白是由位于配子体内成壁体(wall forming body,WFB)的前体蛋白经多种酶加工而成,已发现两类前体蛋白,一类富含半胱氨酸,证实其在二硫键交联的形成方面发挥重要作用[13];另一类富含酪氨酸,被酶解加工成富含酪氨酸的小分子多肽,再经氧化还原反应,通过二硫键的交联形成多酪氨酸聚合物,进而参与卵囊壁的形成[14-16]。谷胱甘肽过氧化物酶(glutathione peroxidase,GPX)是一种重要的过氧化物酶,预测其具有谷胱甘肽过氧化物酶活性,通过催化谷胱甘肽(glutathione,GSH)还原H2O2的氧化还原反应形成二硫键或酪氨酸交联[17],从而在卵囊壁形成中发挥重要作用[16, 18-20]。对巨型艾美耳球虫和弓形虫的卵囊壁蛋白的研究结果证实,过氧化物酶可催化酪氨酸交联,同时促使半胱氨酸形成二硫键,完成囊壁蛋白间的催化交联进而参与形成卵囊壁[18-22]。综上,通过抑制此类蛋白酶的活性,降低二硫键或酪氨酸交联的形成等氧化还原反应可为研制免疫阻断型抗球虫亚单位疫苗提供新策略。课题组前期对毒害艾美耳球虫成壁体蛋白和卵囊壁蛋白进行了比较蛋白组学分析,筛选出差异表达蛋白EnGPX[23],通过体外原核表达,制备多克隆抗体,结合Western blot和激光共聚焦免疫荧光定位等技术,以及氨基酸结构域分析,证实该蛋白在卵囊壁形成过程中发挥了重要作用,以期为研制免疫阻断型球虫亚单位疫苗提供新抗原[23]

1 材料与方法 1.1 材料

1.1.1 虫株和实验动物   毒害艾美耳球虫扬州株由扬州大学兽医学院寄生虫学教研室保存;BALB/c小鼠购自扬州大学实验动物中心。试验过程中参照扬州大学实验动物福利伦理审查表[202103058(小鼠)]对小鼠实施安乐死,动物尸体全部经高压灭菌并密封交至扬州大学动物尸体处理中心集中处理。

1.1.2 主要试剂和仪器   表达载体pET-28a(+)、兔抗rEnGAM59多克隆抗体,由笔者所在实验室制备保存;FastPure Cell/Tissue Total RNA Isolation Kit V2、HiScript III 1st Strand cDNA Synthesis Kit (+gDNA wiper)均购自南京诺唯赞生物科技股份有限公司;鼠抗6×HIS标签单克隆抗体、FITC标记山羊抗小鼠IgG(H+L)、Cy3标记山羊抗兔IgG(H+L)等均购自碧云天生物技术有限公司;Ni-NTA亲和层析介质购自金斯瑞生物科技公司;QuickAntibody-Mouse3W小鼠快速免疫佐剂购自Biodragon公司。超高分辨率激光共聚焦显微镜(TCS SP8 STED)购自Leica公司。

1.2 方法

1.2.1 EnGPX基因的克隆及序列分析   根据毒害艾美耳球虫(Houghton株)谷胱甘肽过氧化物酶(EnGPX)基因序列(GenBank登录号:XM_013581216)设计特异性引物,上游引物F:5′-ATGCTTCTGACGCGCGCGAC-3′;下游引物R:5′-CTACTGCAGCACACCCTCCTTG-3′,预计扩增目的基因大小为753 bp,引物由华大基因科技股份有限公司合成。参照叶状等[15]的方法提取毒害艾美耳球虫配子体总RNA,反转录成cDNA,PCR扩增EnGPX基因进行测序。将所获EnGPX基因序列与GenBank中已收录的毒害艾美耳球虫EnGPX和柔嫩艾美耳球虫EtGPX进行同源性比对,并预测分析其编码蛋白结构域。

1.2.2 EnGPX的原核表达、纯化及鉴定   根据EnGPX基因序列及pET-28a(+)载体上的酶切位点设计特异性引物,上游引物:5′-TCGGAATTCATGCTTCTGACGCGCGCGAC-3′,含EcoR I酶切位点及保护性碱基;下游引物:5′-TATGCGGCCGCGACTGCAGCACACCCTCCTTG-3′,含Not I酶切位点及保护性碱基。将PCR产物和pET-28a(+)载体进行Not Ⅰ、EcoRⅠ双酶切,T4 DNA Ligase连接构建重组表达质粒pET28a(+)-EnGPX,转化至感受态细胞BL21(DE3),构建重组菌pET28a(+)-EnGPX/BL21,接种至含0.5%葡萄糖的LB液体培养基中,0.2 mmol·L-1 IPTG 37 ℃诱导表达5 h,收集菌体,冰浴超声裂解(功率为30%,超声2 s,间隙3 s),进行SDS-PAGE可溶性分析。Ni-NTA亲和层析柱纯化重组蛋白。

1.2.3 鼠抗rEnGPX多克隆抗体的制备及检测   取50 μL重组蛋白(1 μμL-1)与等体积Quick Antibody-Mouse3W佐剂快速混合,参照说明书免疫6周龄BALB/c小鼠并分离血清,获得鼠抗rEnGPX多克隆抗体,间接ELISA检测抗体效价。同时取毒害艾美耳球虫天然配子体蛋白,Western blot检测鼠抗rEnGPX多克隆抗体特异性,一抗为制备的鼠抗rEnGPX多克隆抗体(1∶200稀释),二抗为HRP标记的兔抗鼠IgG(1∶20 000稀释)。

1.2.4 重组蛋白反应原性和交叉反应原性检测   以鼠抗rEnGPX多克隆抗体、毒害艾美耳球虫病鸡康复血清、巨型艾美耳球虫病鸡康复血清和柔嫩艾美耳球虫病鸡康复血清(1∶200稀释)为一抗,HRP标记的兔抗鼠IgG和HRP标记的绵羊抗鸡IgG(1∶20 000稀释)为二抗,Western blot检测重组蛋白的反应原性及种间交叉反应原性。

1.2.5 EnGPX蛋白的免疫荧光定位   参照刘丹丹[12]的方法取毒害艾美耳球虫第二代裂殖子、第三代裂殖子、配子体和未孢子化卵囊制成虫体滴片,以鼠抗rEnGPX多克隆抗体(1∶100稀释)为一抗,FITC标记山羊抗小鼠IgG(H+L)(1∶500稀释)为二抗,进行免疫荧光定位分析;同时与rEnGAM59进行共定位,其一抗为兔抗rEnGAM59多克隆抗体(1∶100稀释),二抗为Cy3标记山羊抗兔IgG(H+L)(1∶500稀释)。

2 结果 2.1 EnGPX基因的克隆与序列分析

EnGPX序列为753 bp,编码250个氨基酸,为一个完整的开放阅读框(ORF),无信号肽,预测相对分子质量为27.7 ku,GenBank登录号为MW588 202.1。与GenBank中已收录的毒害艾美耳球虫(Houghton株)EnGPX(XM_013581216)和柔嫩艾美耳球虫(Houghton株)EtGPX(XM_013376537)序列相似性分别为99.1%、96.1%。结构域预测结果显示EnGPX蛋白属于谷胱甘肽过氧化物酶家族,具有谷胱甘肽过氧化物酶活性,第70-235位氨基酸为一个硫氧还蛋白结构域,第96-111位氨基酸为GPX的活性位点。

2.2 EnGPX的原核表达、纯化鉴定及其鼠抗效价检测

重组蛋白rEnGPX大小约为30 ku,主要以包涵体形式存在,经Ni-NTA纯化后能被6×HIS标签单克隆抗体识别,间接ELISA检测其抗体效价为1∶204 800。鼠抗rEnGPX多克隆抗体能够识别天然配子体蛋白(图 1),大小约为40 ku,高于体外重组表达蛋白大小,推测其由于体外原核表达的重组蛋白与天然配子体蛋白的空间构象不同导致蛋白迁移率不同。

M. 蛋白相对分子质质量标准;1. 天然配子体蛋白 M. Protein molecular weight marker; 1. Natural proteins of gametocyte 图 1 鼠抗rEnGPX多克隆抗体对天然配子体蛋白的识别 Fig. 1 The natural proteins of gametocyte were detected by the mouse anti-rEnGPX polyclonal antibody
2.3 重组蛋白rEnGPX的反应原性和交叉反应原性检测

重组蛋白rEnGPX能够被鼠抗rEnGPX多克隆抗体(图 2a)、毒害艾美耳球虫病鸡康复血清(图 2b)、巨型艾美耳球虫病鸡康复血清(图 2c)以及柔嫩艾美耳球虫病鸡康复血清(图 2d)所识别,pET-28a(+)空质粒转化菌蛋白以及未转化的BL21菌体蛋白未发生反应,证明该重组蛋白具有较好的反应原性和交叉反应原性。

M. 蛋白质相对分子质量标准;1. pET28a(+)-EnGPX/BL21 IPTG诱导;2. pET28a(+)/BL21 IPTG诱导;3. BL21 IPTG诱导。a. 鼠抗重组蛋白多克隆抗体;b. 毒害艾美耳球虫病鸡康复血清;c. 巨型艾美耳球虫病鸡康复血清;d. 柔嫩艾美耳球虫病鸡康复血清 M. Protein molecular weight marker; 1. pET28a(+)-EnGPX/BL21 induced by IPTG; 2. pET28a(+)/BL21 induced by IPTG; 3. BL21 induced by IPTG. a. Mouse anti-recombinant proteins polyclonal antibody; b. The recovery serum from chickens infected with E. necatrix; c. The recovery serum from chickens infected with E. maxima; d. The recovery serum from chickens infected with E. tenella 图 2 重组蛋白反应原性和交叉反应原性检测 Fig. 2 Antigenicity and cross-reactivity analysis of the recombinant proteins
2.4 EnGPX蛋白的免疫荧光定位

EnGPX蛋白主要分布于配子体内的成壁体及卵囊壁上,在二代裂殖子和三代裂殖子中未定位到该蛋白(图 3)。课题组前期研究结果显示rEnGAM59主要定位于Ⅱ型成壁体(WFBⅡ)上,本研究与兔抗rEnGAM59多克隆抗体的共定位结果显示,EnGPX蛋白和rEnGAM59一样,主要分布于配子体内的Ⅱ型成壁体上(图 3A~E),并随着虫体发育整合于卵囊壁上(图 3K~O)。阴性鼠血清对照(图 3F~J图 3P~T)未检测到此蛋白(图 3H3R)。

A、F、K和P. 明场;B、G、L和Q. DAPI染色;C和M. 鼠抗rEnGPX多克隆抗体(FITC标记二抗);H和R. 阴性鼠血清(FITC标记二抗);D、I、N和S. 兔抗rEnGAM59多克隆抗体(Cy3标记二抗);E、J、O和T. DAPI、FITC和Cy3叠加;GAM. 配子体;O. 早期卵囊;标尺为5 μm A, F, K and P. Bright field; B, G, L and Q. DAPI stain; C and M. Immunofluorescence localization with FITC-conjuncted mouse anti-rEnGPX polyclonal antibodies; H and R. Immunofluorescence localization with FITC-conjuncted Negative serum; D, I, N and S. Immunofluorescence localization with Cy3-conjuncted rabbit anti-rEngam59 polyclonal antibody; E, J, O and T. Merge of DAPI, FITC and Cy3; GAM. Gametocyte; O. Early stage of oocyst; The scale is 5 μm 图 3 EnGPX激光共聚焦免疫荧光定位 Fig. 3 Results of immunofluorescence of rEnGPX
3 讨论

基于课题组前期对毒害艾美耳球虫成壁体及卵囊壁的比较蛋白组学研究,获得了差异表达蛋白EnGPX,因其在卵囊壁形成过程中被激活,所以在卵囊壁上的表达量高于配子体,体外重组表达后,证实其具有较好的反应原性和交叉反应原性,且主要存在于配子体内的II型成壁体(WFBⅡ)上,随着虫体发育参与了卵囊壁的形成。

艾美耳球虫配子体蛋白及卵囊壁蛋白主要是通过形成二酪氨酸键及二硫键在卵囊壁形成过程中发挥重要作用,而这一过程主要是在酶介导反应中完成[24]。氨基酸功能预测分析显示,本研究获得的EnGPX为一种谷胱甘肽过氧化物酶编码基因,具有谷胱甘肽过氧化物酶活性。谷胱甘肽过氧化物酶(GPX)以谷胱甘肽(glutathione,GSH)为反应底物和电子供体,催化还原型GSH转化为氧化型GSH[25],并将过氧化氢或氢过氧化物还原为水或相应醇类,其中,第96-111位氨基酸为GPX的催化位点,其内部活性位点由UGC编码的半胱氨酸残基Cys108替代了UGA编码的Sec(硒半胱氨酸),为半胱氨酸谷胱甘肽过氧化物酶(CysGPX)[26-27]。硒具有提高酶活性的功能,CysGPX的催化活性低于硒半胱氨酸谷胱甘肽过氧化物酶(SecGPX),因此CysGPX可利用H2O2参与新合成蛋白质的折叠过程[28-29],在发生反应时过氧化物将Cys108-SH氧化为Cys108-SOH,然后与另一亚基的Cys残基发生缩合反应,形成稳定的分子间二硫键[16, 30],并经硫氧还蛋白还原,完成催化循环[30],参与卵囊壁的形成过程。而在EnGPX内部含有一个硫氧还蛋白(TRX)结构域(第70—235位),该结构域的活性中心含有FAD结构域,可通过NADPH催化两个Cys-SH可逆氧化为二硫键,催化二硫键的形成和重排[31-33],此外,硫氧还蛋白结构域内含有顺式脯氨酸,可与活性位点半胱氨酸结合,维持蛋白的结构和功能[31, 34]

早在1976年, Lawrence和Burk[35]研究出不包含Sec的GPX,参与以有机过氧化物为基质催化NADPH的氧化反应。洪雅真等[36]克隆分析了扇形游仆虫GPX编码序列,其活性中心Cys(UGU编码)替代了Sec,为非硒依赖型蛋白,可通过硫氧还蛋白系统,高效还原过氧化物,这种情况在植物和啮齿类动物中也有发现[37-38]。Sztajer等[39]发现在恶性疟原虫中存在假定的的GPX基因,可作用于广谱氢过氧化物的过氧化物酶,内部含有Cys残基参与形成分子内二硫键,动力学分析显示, 其可通过硫氧还蛋白系统高效还原过氧化物[40]。线虫的表皮过氧化物酶可在角质层胶原蛋白中催化形成二酪氨酸、三酪氨酸和异酪氨酸交联残基,是形成角质层的结构外层的蛋白质复合物,其中,淋巴丝虫GPX分泌同系物gp29(表皮糖蛋白)的表达水平与生长发育阶段保持一致,同时角质层胶原蛋白交联水平增加以维持角质层的强度,且该功能与抗氧化活性高度相关[41]。以上研究结果可进一步论证本研究获得的EnGPX作为一种谷胱甘肽过氧化物酶,在球虫卵囊壁蛋白的二硫键形成中发挥重要作用。

综上可知,EnGPX蛋白存在于毒害艾美耳球虫配子体内的II型成壁体上,随着虫体发育整合到了卵囊壁上。结合氨基酸序列分析,推测其作为一种过氧化物酶在催化卵囊壁蛋白二硫键的形成发挥了重要作用。

4 结论

成功克隆了毒害艾美耳球虫谷胱甘肽过氧化物酶EnGPX ORF编码序列,预测其通过催化GSH介导的活性位点Cys残基之间的二硫键交联等一系列氧化还原反应,参与了配子体蛋白形成卵囊壁的过程。体外重组表达蛋白制备的多抗,将EnGPX成功定位于配子体内的II型成壁体和卵囊壁上。本研究结果将为研制免疫阻断型抗球虫亚单位疫苗提供重要靶点。

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