生长分化因子11在衰老中的研究进展

赵素洁， 陆 怡， 吴晓琰

复旦大学附属华山医院老年病科，上海 200040

·综 述·

生长分化因子11在衰老中的研究进展

赵素洁， 陆 怡， 吴晓琰*

复旦大学附属华山医院老年病科，上海 200040

生长分化因子11(growth differentiation factor 11, GDF11)是转化生长因子β(transforming growth factor-β，TGF-β)家族的一员。GDF11在多种器官和组织中表达。生长发育期间，GDF11通过可逆性地阻滞细胞周期来调节发育进程；在衰老方面，GDF11可延缓心脏、骨骼肌、大脑、骨骼、血管等多个器官和组织的衰老进程。本文就GDF11在生长发育及衰老中的作用及其机制作一综述，以期为GDF11在衰老方面的研究提供指导。

生长分化因子11；衰老；转化生长因子β

转化生长因子β(transforming growth factor-β，TGF-β)家族可以分为4个亚类：转化生长因子类(TGFs)；激活素类(activins)；骨形成蛋白类(BMPs)；生长分化因子类(GDFs)，如GDF11和其结构类似物GDF8；其他，如抗苗勒氏管激素(AMH)和GDF15[1]。与TGF-β家族其他成员一样，GDF11和GDF8的前体在细胞内合成后以成熟信号肽形式分泌到细胞外，与相应细胞膜表面的Ⅰ型、Ⅱ型受体结合后活化细胞质内的信号分子，并将信号传递给细胞核，进而调控基因的表达。GDF11和GDF8可结合Ⅰ型受体中的激活素受体B(activin typeⅠreceptor B, ActRⅠB)、转化生长因子Ⅰ型受体(TGF-β receptor Ⅰ，TβRⅠ)，Ⅱ型受体中的激活素受体A(activin typeⅡreceptor A， ActRⅡA)、激活素受体B(activin typeⅡreceptor B，ActRⅡB)。GDF11和GDF8在细胞质中激活的信号分子为Smad2/3 (smart mothers against decapentaplegic-2 and -3，Smad2/3)。细胞膜接受膜外信号分子传递的信号后选择性磷酸化Smad2/3，Smad2/3把信号传递给细胞核，从基因水平调控细胞的一系列生物活动[1-2]。

TGF-β家族在衰老方面发挥着至关重要的作用[3-4]。阻滞GDF11的同源类似物GDF8可以改善小鼠衰老相关的肌肉萎缩，并增加其胰岛素敏感性[5]。近年来，研究[6]发现TGF-β/Smad通路通过激活p21基因诱导细胞周期阻滞来干预哺乳动物胚胎发育期间的程序性衰老。

1 GDF11与GDF8的调节因子

TGF-β家族种类繁多，配体远多于受体，信号有条不紊的传递依赖多种调控因子。人GDF11前体蛋白有407个氨基酸，其二聚体由两个GDF11单体组成，每个单体由109个氨基酸组成，成熟GDF11即为酶切后的GDF11前体C端成熟区[7]。GDF11可以通过诱导机体产生卵泡抑素(follistatin，FST)来建立负反馈调节[8]。FST通过活化周期素依赖性激酶2/4(cyclin-dependent kinase2/4, cdk2/4)，上调细胞周期蛋白D1基因(cyclinD1)，下调p21基因，进而抑制Smad2/3磷酸化，减弱GDF11对肌源性祖细胞(myosphere-derived progenitor cells ，MDPCs)的抑制作用[9]。生长分化因子相关血清蛋白(growth and differentiation factor-associated serum protein，GASP)包括GASP 1和GASP2，是GDF11/GDF8的天然抑制信号分子[2]。GASP、FST与GDF11相协调共同参与机体的发育[1-2,8]。

2 GDF11对组织器官生长发育及衰老的作用

GDF11基因作为1种保守基因，广泛参与机体生长发育的调控，在衰老中的作用虽有争议，但大部分研究显示其有抗衰老作用(表1)。

表1 GDF11对生长发育及衰老的作用

2.1 GDF11对心肌、骨骼肌的作用 2013年，哈佛医学院研究团队[10]通过外科手术使高龄小鼠与低龄小鼠共用一套血液循环，发现循环中的GDF11可缓解高龄小鼠的心肌肥大。他们还发现小鼠循环中的GDF11随着年龄增长浓度降低[10,31]。在3月龄小鼠体内，GDF11的mRNA表达量由高到低分别为脾、胸腺、肾、视网膜、骨骼肌、小脑、前脑、骨髓、心脏、小肠、肺、肝，且脾的表达量明显高于其他组织[31]。按照0.1 mg/kg给高龄小鼠注射外源性重组GDF11蛋白(recombinant GDF11，rGDF11)，结果发现卫星细胞的比例明显增加，小鼠肌纤维结构得到改善[14]。与GDF11结构相似的GDF8在小鼠、牛、羊骨骼肌的生长中发挥明显的抑制作用[32]。

2.2 GDF11对神经系统的作用 GDF11对神经发育的调节大多是抑制性作用，并呈现一定的时空特异性，即在不同组织及不同发育阶段，GDF11对细胞增殖的作用不同。在胚胎视网膜发育阶段，GDF11通过调节祖细胞中影响视网膜神经节细胞(retinal ganglion cell，RGC)发生的Math5基因表达来间接影响RGC的发育[30]。在胚胎嗅神经发生过程中，GDF11则是通过上调细胞周期依赖的激酶抑制剂p27激酶抑制蛋白1(p27 kinase inhibition protein1，p27Kip1)的水平来可逆性诱导定向祖细胞的周期阻滞，从而调节RGC的发育[18]。

然而，与GDF11胚胎神经发育期间的抑制性调节作用不同，研究[19]发现，GDF11对衰老大脑则起促进神经再生的作用。22个月龄小鼠连续注射GDF11 4周后，共聚焦显微镜成像显示大脑室下区(subventricula zone，SVZ)神经细胞数量增多，此外血管数目和血容量明显增加。之后，该研究团队发现，GDF11可促进高龄小鼠嗅神经及血管重建[19]；共生模型的老年小鼠神经髓鞘得到修复和再生[16]；低龄小鼠的血液有改善高龄小鼠认知的作用[33]。研究[17]发现，通过给阿尔茨海默病(Alzheimer′s disease，AD)小鼠移植低龄小鼠富含GDF11的脾脏，可以缓解其认知和记忆能力减退。

对脊髓发育影响的研究[34]发现，GDF11可能经GDF11-Smad2通路调控同源盒基因(Hox)在脊髓中的表达区域及延喙尾轴在脊髓尾部的位置；而FST作为GDF11的抑制性因素参与靶基因的调控。

2.3 GDF11对血液循环系统的作用 应用酶联免疫吸附法(enzyme linked immunosorbent assay，ELISA)、LC-MS/MS等多种方法检测，在小鼠、大鼠、马、羊等多种动物中已检测到GDF11。但是，血液中GDF11的浓度与年龄的关系目前没有定论[10,31]。研究[26-27]发现，在血液透析患者体内，血清GDF11高浓度与低血红蛋白浓度有关；GDF11-ActRⅡB-Smad2/3 通路参与红系造血，并与血液透析患者促红细胞生成素(erythropoietin，EPO)无效性贫血有关。国内有研究[35]报道，骨髓增生异常综合征(myelodysplastic syndromes, MDS)患者血液中的GDF11浓度明显高于健康人。此外，小鼠脾脏中GDF11高表达[31]可能与脾脏本身储存大量的血小板有关。

有关GDF11与贫血关系的研究较深入。靶作用GDF11及其相关信号分子在治疗β-地中海贫血方面已经取得明显的进展[26-29]。GDF11-ActRⅡA通路与β-地中海贫血的病情进展有密切的关系，GDF11在有核红细胞中高表达，促进终末期骨髓无效造血；人体GDF11及TGF-βs阻断剂RAP-011、RAP-536(对应小鼠体内的ACE-011、ACE-536)，可以捕获ActRⅡA配体，阻滞GDF11-ActRⅡA通路，达到改善贫血的效果[28]。

2.4 GDF11对血管的作用 国内关于人脐静脉内皮细胞(human umbilical vein endothelial cells，HUVECs)的研究[36]表明，GDF11(50 ng/mL)处理细胞48 h可激活Smad1/5/8和Smad2/3信号通路，增加HUVECs中的NADPH氧化酶(NOX4)、磷酸化的c-Jun氨基末端激酶(p-JNK)和磷酸化的丝裂原活化蛋白激酶(p-AMPK)的含量；GDF11处理24 h后，噻唑蓝(MTT)实验检测显示，细胞活性增加，72 h后细胞活性降低，但GDF11没有改变细胞增殖和迁移能力。有研究[37]报道，GDF11可促进内皮祖细胞 (endothelial progenitor cells, EPCs) 分化，利于血管形成。GDF11能促进脑内毛细血管再生[19]。

2.5 GDF11对其他组织的作用 研究[38]发现，在小鼠胚胎中，NGN3+内分泌祖细胞表达GDF11；GDF11通过抑制神经原素3(neurogenin 3，NGN3)的表达来调节胰岛细胞分化。GDF11-Smad2通路参与胰岛β细胞的成熟，并影响胰岛β细胞的数目[24]。GDF11与糖尿病的发生和发展也有密切联系。2015年，骨骼衰老相关的研究[21]发现，GDF11一方面可调节间充质干细胞(MSC)向脂肪细胞和成骨细胞分化的百分比，另一方面通过降低过氧化物酶体增生物激活受体-γ(peroxisome proliferators-activated receptor-γ，PPAR-γ)及促进其类泛素化来缓解骨质疏松。

3 总结与展望

GDF11通过调控祖细胞基因表达或通过可逆性阻滞对已分化细胞周期来影响机体发育，并在多个器官的衰老进程发挥作用。多项报道提示，GDF11在胚胎神经、骨骼、肌肉发生期间通过GDF11-ActRⅡA/ActRⅡB-Smad或GDF11-TβRⅠ-Smad通路发挥反馈性抑制作用[8,20,39-40]。GDF11在神经发生、血管生成及动脉粥样硬化过程中[19,37]发挥的作用使GDF11通路有望成为治疗老年痴呆、高血压、心血管疾病、糖尿病等疾病的新靶点。干预GDF11与其类似物GDF8有望成为治疗衰老相关的心肌肥大、肌肉萎缩等疾病的新思路。

细胞衰老本质是由各种原因导致的不可逆的细胞周期阻滞，这些原因可以是端粒缩短、DNA损伤及强烈的致癌信号等，且细胞衰老也机体免于产生肿瘤的一个保护因素[41]。GDF11通过抑制细胞从G1期向S期转变的进程来抑制MDPCs增殖[9]；通过可逆性地诱导嗅神经细胞周期阻滞来调节神经发育[18]。临床研究[42]发现，GDF11高水平与结肠癌患者预后差紧密相关，结肠癌组织中高表达的GDF11可能与TGF-β有相似作用，即将肿瘤细胞周期阻滞在G1期。TGF-β激活p21通路引起的细胞周期阻滞参与哺乳动物胚胎发育期间细胞的程序性衰老[6]。

GDF11对机体的生长发育及衰老的调控贯穿自发育起始的整个生命历程。GDF11及TGF-β家族信号网络与衰老有密切的联系，干预GDF11及TGF-β家族信号通路有望延缓衰老，相关机制有待更加深入的探索。

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[本文编辑] 姬静芳

Advances on aging of growth differentiation factor 11

ZHAO Su-jie，LU Yi，WU Xiao-yan*

Department of Gerontology，Huashan Hospital，Shanghai Medical College of Fudan University, Shanghai 200040，China

Growth differentiation factor 11(GDF11), a member of the transforming growth factor β(TGF-β)family, is widely expressed in many tissues. GDF11 regulates multiple developmental processes by reversing cell cycle arrest and reverses the aging process of many tissues including heart, skeletal muscle, brain, bone and blood vessels. In this article, we review the role of GDF11 in aging considering the TGF-β family signaling pathway and the mechanism of GDF11 in growth and development.

GDF11; aging; transforming growth factor-β

2016-11-01 [接受日期] 2017-04-14

上海市科学技术委员会重点项目科技创新行动(10431904000). Supported by the Key Program of Science and Technology Commission of Shanghai (10431904000).

赵素洁, 硕士生, 住院医师. E-mail：sujiezhao2014@163.com

*通信作者(Corresponding author). Tel: 021-52887270, E-mail：wxygtj@ aliyun.com

10.12025/j.issn.1008-6358.2017.20161017

R 592

A