畜牧兽医学报  2018, Vol. 49 Issue (12): 2558-2566. DOI: 10.11843/j.issn.0366-6964.2018.12.004    PDF    
microRNA对哺乳动物卵泡发育的影响
李琴1,2     
1. 重庆市畜牧科学院, 重庆 402460;
2. 重庆市肉鹅遗传改良工程技术研究中心, 重庆 402460
摘要:卵巢是动态变化的实质性器官,一个发情周期内卵巢上的卵泡始终经历着募集、选择、优势化等生理过程。卵母细胞以及外周的颗粒细胞和膜细胞共同组成卵巢上功能性单位-卵泡,卵泡发育受卵泡生殖细胞和体细胞间内分泌、自分泌、旁分泌的共同调控。microRNA(miRNA)是一种重要的转录后调控方式,通过与靶基因的3'-UTR结合转录后抑制或降解靶基因的方式,在调控动物卵巢卵泡的发生、发育、颗粒细胞增殖、凋亡及类固醇激素合成分泌等方面发挥重要作用。本文主要对miRNA在哺乳动物卵泡发育中的最新研究进展,包括miRNA对卵泡发育、颗粒细胞、膜细胞功能的影响及其在卵巢上的表达分析等进行综述。通过了解miRNA在卵泡发育中的作用,为进一步解析哺乳动物繁殖性状调控的分子机制奠定基础。
关键词miRNA    卵巢    卵泡发育    颗粒细胞    膜细胞    增殖    凋亡    类固醇激素    
Roles of microRNA on Mammalian Follicle Development
LI Qin1,2     
1. Chongqing Academy of Animal Science, Chongqing 402460, China;
2. Chongqing Engineering Research Center of Goose Genetic Improvement, Chongqing 402460, China
Abstract: The mammalian ovary is a dynamic parenchyma organ. The ovarian follicle occurs continually physiological process of recruitment, selection and dominance during mammalian estrous cycle. Ovarian follicle is mainly composed of the oocyte, granulosa layers and theca layer. The follicular development is co-regulated by endocrine factors, autocrine and paracrine communication among the oocyte, granulosa and theca cells. microRNAs (miRNAs) post-transcriptionally regulate gene expression through base pairing with the 3'-untranslated region of target mRNA, leading to either mRNA cleavage or translational repression. miRNAs have been acknowledged to play an important role in ovarian reproduction and endocrinology, including oogenesis and follicular growth, as well as proliferation, apoptosis, differentiation and steroidogenesis of granulosa cells. This review summarized the recent advances in the fields of identification of miRNAs related to the regulation of ovarian function, the role of miRNAs in folliculogenesis, follicle development, as well as proliferation, differentiation, apoptosis, steroidogenesis of granulosa cell and theca cell. This review will lay the foundation for understanding the regulatory mechanism of mammalian reproduction based on summing up the role of miRNAs on follicle development.
Key words: microRNA     ovary     follicle development     granulosa cell     theca layer     proliferation     apoptosis     steroidogenesis    

卵巢是一个动态发育的生殖器官,卵巢上的卵泡由颗粒细胞、膜细胞和卵母细胞构成,3种不同类型细胞之间的相互作用是维持卵泡正常发育的前提。随着卵母细胞体积增加和发育成熟,其周围的颗粒细胞和膜细胞也随之增殖和分化,为分泌或应答性激素做准备[1],而颗粒细胞与膜细胞的分化是促使体内卵泡快速生长的主要原因[2]。在下丘脑-垂体-卵巢轴(hypothalamo-pituitary-ovarian axis,HPG)调控下,颗粒细胞和膜细胞能够合成多种激素(包括雄激素、雌激素等)及生长因子,并表达受体,通过间隙连接,为卵母细胞的生长发育提供营养物质,促进卵母细胞的发育成熟[3]。颗粒细胞决定着卵泡最终命运,卵泡的闭锁首先由颗粒细胞启动,当大多数颗粒细胞发生凋亡时,卵泡正在或已经闭锁[4]。这些研究表明,颗粒细胞在卵泡发育、增殖、凋亡及类固醇激素合成中发挥着重要作用。miRNA是一类短序列的非编码RNA,其主要调控方式是通过互补结合其靶基因3′UTR,从而达到转录后调控的作用[5]。研究表明,miRNA在卵泡发育[6-8]、卵泡募集和选择[9-10]、颗粒细胞增殖凋亡[11-16],类固醇激素合成分泌[17-19]等方面发挥着重要作用。目前研究主要集中在miRNA对颗粒细胞增殖、凋亡以及类固醇激素合成分泌的影响。本文对miRNA与哺乳动物卵泡发育最新研究进展,包括miRNA表达谱筛选鉴定、miRNA调控卵泡发育、miRNA对颗粒细胞和膜细胞增殖、凋亡、类固醇激素合成分泌等方面的研究进行综述。

1 卵巢miRNAs表达谱鉴定

miRNAs是卵巢中最为丰富的内源性小RNAs分子[20-21]。得益于基因芯片、二代测序等高通量测序技术,人们鉴定了不同物种卵巢组织中大量的miRNAs,包括鼠[22]、人[23]、牛[24]、羊[25]、猪[26]、鸡[27]、鹅[28]等物种,发现miRNAs在卵巢组织中可能发挥着重要的作用。Li等[26]通过深度测序的方法,鉴定了猪卵巢和睾丸组织的miRNAs,检测到582个前体miRNAs编码732个成熟的miRNAs,其中224个是卵巢和睾丸差异表达miRNAs,miRNAs主要位于X染色体上(X-连锁miRNAs),这些X-连锁miRNAs偏好表达在睾丸中。Bonnet等[29]利用激光捕获显微切割(laser capture microdissection, LCM)的方法分离了初生绵羊的颗粒细胞和卵母细胞,从颗粒细胞和卵母细胞中分别鉴定了1 050和759个特异组织表达的miRNAs,采用qRT-PCR方法鉴定了卵母细胞和颗粒细胞特异表达的基因,此项研究为了解早期卵泡亚功能单位的基因调控提供较全面的视野。Gebremedhn等[30]采用Illumina测序技术,对牛发情周期后期排卵前优势卵泡和从属卵泡中颗粒细胞的miRNAs表达谱进行了分析,分别在优势卵巢和从属卵泡中发现了315、323个已知的miRNAs和11个新的miRNAs。其中,64个miRNAs显著表达在排卵前优势卵泡中,60个在优势卵泡中显著下降。通过基因富集分析表明,这些miRNAs可能参与肿瘤生成、细胞黏附、细胞增殖、细胞凋亡和代谢,并利用双荧光素报告系统分析和验证了miR-183与靶基因FOXO 1的作用。Yu等[28]通过高通量测序方法,分别对产蛋和就巢卵巢和等级前卵泡microRNA进行表达谱的分析鉴定,发现let-7家族、miR-10家族和miR-143家族在卵泡发生中起决定作用。对差异表达miRNA靶基因进行GO和KEGG分析表明,这些基因参与了轴突导向、癌症通路、细胞黏附、MAPK通路以及细胞与细胞因子受体作用。

目前,关于卵巢microRNAs表达谱研究在各个物种上已全面展开,从对整个卵巢组织到以颗粒细胞、膜细胞以及卵母细胞为对象,从整体水平上更加细致和深入地了解miRNA在卵巢、卵泡发育以及卵泡亚功能单位的作用;以miRNA表达谱数据及功能分析为基础,通过分子生物学技术,验证关键候选miRNA在卵泡发育中的功能及调控机制,是miRNAs后续研究的主要内容和方向。

2 miRNA在卵泡发育中的作用 2.1 miRNA在早期卵泡发生中的作用

目前,已初步阐明,miR-143、miR-145、miR-376a对鼠胎儿期和初生期卵巢原始卵泡的形成和维持起重要作用[6-8]。miR-143能够增加鼠胎儿期(从交配后15.5 d~初生后4 d)原始卵泡的数量。通过研究miR-143在体外培养鼠胎儿卵巢中的作用,发现miR-143通过降低前颗粒细胞增殖和减少细胞周期相关基因的表达,抑制原始卵泡的形成[8]TGFβ在调控原始卵泡~初级卵泡发育的过程中起重要作用,miR-145可通过促进初生鼠卵巢中TGFβ受体(TGFβR)调控卵泡的发生[31]。抑制miR-145的表达能够降低原始卵泡的比例和数量,增加生长卵泡数量。miR-145可通过与靶基因TGFβR 2作用,调控原始卵泡的发育,并维持原始卵泡的静止[31]。将miR-376a转染培养的新生小鼠卵巢中,发现原始卵泡的数量增加,卵母细胞凋亡降低。卵母细胞凋亡的降低是miR-376a与靶基因PCNA(proliferating cell nuclear antigen)作用而实现[6-7]。在鼠胎儿期(交配后18.5 d)过表达miR-376,降低促凋亡基因(BaxTnfTnfr 2)的表达,促进抗凋亡基因(Bcl 2)和卵母细胞存活基因(Pard6aLhx 8)的表达。以上研究可知,miR-143、miR-145、miR-376a均在在卵泡发生中具有重要的作用[32]

2.2 miRNA在卵泡发育和优势卵泡选择中的作用

通过高通量测序技术,对哺乳动物从属卵泡到优势卵泡过渡期(或健康卵泡和凋亡卵泡)的膜细胞和颗粒细胞[10, 30]、卵丘-卵母细胞复合体[33]、卵泡液[34]及黄体测序[35-36]表明,miRNA在卵泡生长和优势卵泡选择中起重要作用。Salilew-Wondim等[10]在牛健康大卵泡和凋亡大卵泡中发现523个差异miRNA分子。其中,miR-144、miR-202和miR-873在健康大卵泡中表达量高于凋亡大卵泡,推测这3个miRNA分子可能与优势卵泡选择有关;miR-873表达量最高,暗示其可能在优势卵泡的选择中起重要作用。Schauer等[36]研究表明,在马的优势卵泡、从属卵泡中,miR-132、miR-212、miR-21、miR-145、miR-224和miR-378差异表达,暗示这些miRNAs与卵泡选择和排卵有关。Salilew-Wondim等[10]研究表明,牛发情期3~7 d,从属卵泡和优势卵泡中有244个miRNAs分子差异表达。其中, let-7家族、miR-10b、miR-26a、miR-27b和miR-99b在整个发情周期都高表达,在发情第4~7天中,从属卵泡中miRNAs表达量几乎没有变化,而优势卵泡中miRNA表达量变化较大。通过比较马排卵期和非排卵期卵泡液miRNA分子发现,miR-21、miR-23b、miR-378和miR-202高表达在排卵期卵泡,而miR-145则高表达在非排卵期卵泡液中[37]。通过miRNA测序和分析表明,上述miRNA可能在卵泡发育和优势卵泡的选择中有重要作用,但其作用机制还需要进一步研究。

2.3 miRNA在颗粒细胞中的作用 2.3.1 miRNA对颗粒细胞增殖的影响

颗粒细胞的增殖和功能对于卵泡发育、成熟和凋亡有重要的作用。miRNA在促进颗粒细胞增殖、存活、凋亡和功能维持起重要的作用(表 12)。Dang等[38]研究表明,miR-379-5通过互补结合靶基因RARP 1、XRCC 6,抑制颗粒细胞增殖,并降低DNA修复效率。而miRNA 17-92 cluster (miR-17-5p、miR-19a、miR-20a、miR-92a)通过靶向结合PTEMBMPR 2基因,调控颗粒细胞增殖分化[39]。Yin等[40]研究表明,在鼠体内、外颗粒细胞中过表达miR-320,均能抑制颗粒细胞的增殖;miR-320的表达和对颗粒细胞增殖的抑制作用能够被miR-383进一步促进。多囊卵巢综合征中过表达miR-93,能够通过与靶基因周期依赖抑制酶(CDKN1A)作用,增加人颗粒癌细胞的增殖,高浓度的胰岛素也能够诱导miR-93表达,增加KGN增殖,降低CDKN1A表达[41]

表 1 miRNA在卵泡发育中的作用 Table 1 Roles of miRNA in granulosal cell, theca cell, corpus luteum and occyte
2.3.2 miRNA对颗粒细胞凋亡的影响

研究表明,miRNA对颗粒细胞有抗凋亡和促凋亡作用(表 1表 2)。在抗凋亡方面,Yan等[12]发现,miR-145能够通过与靶向作用于激活素受体ⅠB (activin receptorⅠB, Acvr1b)基因,抑制颗粒细胞凋亡。miR-21在抗凋亡中起作用,抑制miR-21能够诱导老鼠颗粒细胞和卵巢凋亡,降低排卵率[11]; miR-21也能通过与靶基因Smad 7作用,抑制鼠由去甲肾上腺素介导的颗粒细胞凋亡[15]。Liu等[13]发现,miR-92a能够通过作用Smad 7抑制猪颗粒细胞凋亡,miR-182可通过与靶基因Smad 7结合,抑制鼠多囊卵泡综合征卵巢中颗粒细胞凋亡[42]。在促进颗粒细胞凋亡方面,人颗粒细胞中过表达miR-23a能够通过作用XIAPCASP 3靶基因,促进颗粒细胞凋亡[43]。miR-26b通过Smad和Mad-related protein 4,促进猪颗粒细胞的凋亡[44],miR-26b通过作用靶基因Smad 4,促进猪颗粒细胞凋亡,可以直接和间接方式通过WSP9X调节Smad 4泛素化[45]。Cao等[46]利用microRNA芯片技术鉴定出let-7g在凋亡卵泡中显著升高,并将let-7g过表达转染猪颗粒细胞,发现let-7g能够极显著促进细胞凋亡,并通过MAP3K 1信号途径诱导颗粒细胞凋亡。miR-34c通过与靶基因FOXO3a作用,促进猪颗粒细胞凋亡,降低增殖能力[14]。而一些miRNA分子对增殖和凋亡相关基因的表达均有作用。例如,Let-7家族((let-7b/c/d/g)miRNA能够降低与增殖和凋亡相关蛋白质表达[47]。在凋亡卵泡中,let-7a/b/c/i的表达量下降,而let-7g表达量上升[48]。在培养的猪颗粒细胞转染let-7a/b/c/i或let-7g类似物时,let-7a/b/c/i表达量下降,而let-7g表达量上升[46],以上研究表明,let-7家族在颗粒细胞增殖、存活、凋亡等方面均有作用。

表 2 miRNA在颗粒细胞中的作用 Table 2 Roles of miRNA in granulosa cells
2.3.3 miRNA对颗粒细胞类固醇分泌激素的影响

颗粒细胞和膜细胞能够产生雌激素,以维持子宫的功能,调节激素的释放,从而表现出繁殖行为。miRNA在类固醇生成中的作用见表 1表 2。miR-378是类固醇生成的抑制因子,能够抑制芳香酶的表达和雌激素的产生,抑制培养的猪颗粒细胞PGR的表达[18]。miR-320作为类固醇合成的正调控因子,能够通过E2F 1和SF 1两个靶基因作用,抑制鼠雌激素合成和颗粒细胞增殖。而在卵巢中注射miR-320,能够抑制雌激素的释放,但却促进睾酮和孕酮的产生[41]。miR-383能够促进颗粒细胞中雌激素的产生,但不影响孕酮的释放[19]。miR-34a和miR-320分别能够抑制人颗粒细胞和鼠卵巢雌激素的释放[40, 49];而miR-132能够抑制NURR1(CYP19A1的负调控因子)的转录后表达,促进雌激素的合成[50]

2.4 miRNA在膜细胞中的作用

卵泡的发生、发育受促性腺激素LH和FSH共同调控。在哺乳动物上,颗粒细胞在FSH作用下,将雄激素转化生成雌二醇。而膜细胞是在LH作用下合成雄激素[51]。与颗粒细胞相比,卵泡膜细胞miRNA研究则相对较少(表 1表 2)。对绵羊卵泡-黄体过渡期卵泡膜miRNA测序,通过Northern杂交发现,miR-199a-3p、miR-145、miR-31、miR-125b、miR-503、miR-21在膜细胞中高表达,提示这些基因可能在膜细胞中起重要作用[52]。过表达miR-26a-5p能够通过靶基因TNRC6A促进卵泡膜细胞增殖[53]。Robinson[54]研究发现,miR-221在牛从属卵泡中的表达量显著高于在优势卵泡中的表达量,且FGF 9处理后,miR-221在膜细胞中的表达量增加,表明,miR-221可能在膜细胞中发挥作用。在卵泡中,膜细胞主要生成雄激素,并在促卵泡血管发生中具有重要作用[55]

3 小结和展望

随着高通量测序技术的不断发展,人们已从不同物种卵巢中筛选鉴定了大量miRNA分子。这些miRNA分子差异表达在不同类型、不同时期或不同生理条件下的卵泡细胞中。对miRNA进行功能研究表明,miRNA在早期卵泡发生、颗粒细胞增殖、凋亡及类固醇合成分泌中发挥重要作用。通过高通量测序发现,大量的miRNA差异表达在卵泡选择过程中,但其作用的分子调控机制还需要进一步研究。在卵泡选择过程中,卵泡组织形态学、类固醇激素合成分泌及相关的内分泌因子等方面发生着重要的变化[65-67]。因此,要更好地理解miRNA在卵泡选择中的作用机制,需要从不同的角度研究miRNA在卵泡中的分子机制。膜细胞在雄激素生成、卵泡血管发生中发挥着重要作用,但目前关于miRNA在膜细胞中的研究报道较少,因此这也是今后研究的一个方向。

由于miRNA与靶基因间互作以及分子调控的复杂性,对miRNA功能研究变得复杂且具有挑战性。虽然一部分miRNA单独作用于某些特异的信号通路,但大多数miRNA成簇且精细地调控着细胞功能[32, 68]。因此,如果仅对某个miRNA分子进行功能研究,可能不会导致生物性状的显著变化。一个比较好的解决方式是通过过表达同簇miRNA分子估计miRNA的功能,以及对其他miRNA分子的影响[32]。除此之外,miRNA发挥功能还受激素和细胞周期的影响,为了更加深入地理解miRNA在卵泡发育中的作用机制,需要在不同的激素环境和生长因子作用下,探索miRNA与靶基因的整体网络关系。因此,利用高通量测序技术,测定不同时空条件下卵巢中miRNA表达谱,对揭示miRNA与卵泡发育的关系起重要作用。

另外,miRNA功能的研究主要基于模式生物。虽然对模式生物的研究能够为人们了解miRNA在卵泡中的发育作用提供帮助,但物种间的差异也可能会造成miRNA分子功能上的差异。此外,miRNA调控也受动物生理状态的影响。因此,对除模式动物外的物种miRNA分子进行功能研究也非常必要。基因编辑CRISPR等技术为在其他物种卵巢中深入研究miRNA功能提供了可能。这也将对了解动物繁殖性状调控的分子机制提供帮助。另外,miRNA可能在卵巢疾病临床诊断、治疗方面有较大的应用前景。因此,在今后的研究中,利用新的生物技术对miRNA在动物体内的生理功能研究尤为重要。

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