Abstract
As one of the neurotrophic factors and insulin family, insulin like growth factor-1(IGF-1) can promote cell synthesis and metabolism in tissues and organs, activate cell growth, proliferation and differentiation, and inhibit cell apoptosis.Sensorineural hearing loss is the most common manifestation of the inner ear disease, which is mainly caused by the damage of cochlear hair cells.The lack of IGF-1 directly affects the growth, development and differentiation of cochlear hair cells, thus IGF-1 participates in the maintenance of cell survival and repair during inner ear cell injury.This article systematically reviews the recent research progress on the protective mechanism of IGF-1 on the inner ear.
Keywords: insulin like growth factor-1, hair cell, spiral ganglion, sensorineural hearing loss
感音神经性聋(sensorineural hearing loss,SNHL)是造成听力损伤的主要发病类型。在我国,每年有5/100 000~27/100 000因SNHL导致听力减退或听力丧失,在美国每年约有6.6万的SNHL新发病例[1]。SNHL主要由耳蜗毛细胞损伤引起,由于耳蜗毛细胞和支持细胞在哺乳动物出生后失去增殖能力,SNHL被认为是一种难治性疾病,因此,耳蜗损伤后毛细胞和支持细胞数量的维持对SNHL的治疗具有重要意义[2]。目前除了人工耳蜗植入或糖皮质激素治疗[3-4],尚无有效的治疗方法。随着21世纪再生医学的进步,实现内耳耳蜗毛细胞和支持细胞再生的几种创新方法已经成为可能。目前药物诱导支持细胞向耳蜗毛细胞转化[5]、基因[6]和干细胞[7]等再生方法正在研究阶段。除毛细胞损伤可致耳聋之外,耳蜗螺旋神经节(SGNs)缺失也会导致不可逆的听力损伤。因此,对于神经存活、生长和功能调节的生物学机制上的研究也是目前重点研究内容[8]之一。
1. 胰岛素样生长因子-1与内耳损伤修复密切相关
胰岛素样生长因子-1(insulin like growth factor-1,IGF-1)是参与内耳发育的生长因子之一,可促进耳蜗毛细胞的生长、发育和分化,以及在内耳细胞损伤时维持细胞的存活与修复。IGF-1对多种损伤的保护作用已在体内外得到证实,如IGF-1保护耳蜗外植体培养的耳蜗毛细胞不受氨基糖苷的影响[9-10];从电子耳蜗的水凝胶涂层电极上释放IGF-1可保护耳蜗免受电极插入时造成的损伤[11];英国的一个临床队列研究发现高水平的IGF-1可减少发生听力减退的风险[12];亦有流行病学研究证实IGF-1可以保护听力缺失[13],是治疗全身糖皮质激素耐受突发性聋合并2型糖尿病的新疗法[14]。IGF-1与毛细胞损伤、再生和修复的调控机制研究将为治疗听力损失提供新方向。
2. 毛细胞的再生、修复
哺乳动物耳蜗毛细胞有三种可能的再生方式,包括支持细胞的有丝分裂替代增殖、耳蜗毛细胞的自我修复和支持细胞的直接转分化。支持细胞的有丝分裂替代增殖主要是支持细胞在有丝分裂过程中直接产生新的耳蜗毛细胞。耳蜗毛细胞的自我修复是细胞内的一种机制,当耳蜗毛细胞失去其顶端结构(角质层和立体纤毛束)时,将自行修复这些结构,例如在热应激下,椭圆囊中的支持细胞通过分泌热休克蛋白70(HSP70)以促进耳蜗毛细胞的自我修复[15];HSP70能保护耳蜗毛细胞免受热应激以外的各种耳毒性损伤。而在支持细胞分化中,支持细胞可不经分裂直接转化为耳蜗毛细胞,Bramhall等[16]研究发现出生后小鼠的耳蜗支持细胞虽然有限,但在受到损伤时仍有向耳蜗毛细胞转化的能力;同时,支持细胞向耳蜗毛细胞的转化受多种因素的影响,比如细胞周期激活因子MYC与内耳前体基因Notch1的共同激活被证实可诱导多种成人耳蜗感觉上皮细胞的增殖,短暂的MYC和Notch1活性使成熟的支持细胞对转录因子Atoch1产生反应,并有效地转分化为耳蜗毛细胞[17];Zhang等[18]在新生小鼠耳蜗的研究中发现支持细胞中Foxg1基因的下调增加了支持细胞向耳蜗毛细胞的转化。可见,在毛细胞再生和修复过程中,支持细胞发挥着重要作用。
3. SGNs损伤、修复与保护
在哺乳动物中,听觉系统的功能依赖于两种神经感觉组织:由感觉耳蜗毛细胞和支持细胞组成Corti器官和感觉螺旋神经节的SGNs投射出放射状神经纤维来支配Corti器官。其中,人类SGNs分为Ⅰ型和Ⅱ型两种。Ⅰ型神经元主要通过突触与Corti器官的内毛细胞相联系,占总数的90%~95%;Ⅱ型神经元通过突触向外毛细胞发出外周投射,占总量的5%~10%[19]。Ⅰ型神经元一个胞体只能与一个树突或轴突形成突触,具有一对一的特点;而Ⅱ型神经元一个胞体可与多个树突或轴突形成突触,具有一对多的特点。除了毛细胞损伤导致的听力损伤,SGNs的损伤同样可引起不同程度的听力损伤,例如使用氨基糖苷类抗生素、哇巴因和顺铂等耳毒性药物可引发SGNs丢失,导致哺乳动物永久性SNHL;耳毒性药物可通过刺激耳蜗螺旋神经元产生活性氧(ROS)引发氧化应激反应,使SGNs氧化损伤,导致听力下降[20]。外周突触虽在听觉系统功能发挥中占比较低,但Liang等[21]发现电刺激在体外以电流强度和持续时间依赖的方式诱导SGNs外周纤维和内毛细胞突触的退化,导致听觉下降;而神经营养素3可以调节耳蜗的带状突触密度,诱导耳蜗听觉损伤后突触的再生[22],促进听觉功能恢复。可见,耳蜗毛细胞损伤后神经营养支持的丧失是SGNs降解的一个主要原因,IGF-1作为一种神经营养因子,可用于挽救SGNs。
4. IGF-1对内耳听力损伤保护机制的研究
4.1. IGF-1通过多种通路抑制耳蜗毛细胞凋亡并诱导支持细胞增殖
IGF-1通过与胰岛素样生长因子受体(IGF-1R)结合来控制细胞的增殖、分化和凋亡。IGF-1R是一种膜受体酪氨酸激酶,通过多条通路传递信号,如PI3K/AKT和MEK/ERK等[23-24]。在哺乳动物细胞中,PI3K/ AKT信号对新生毛囊的再生至关重要[25]。为了研究IGF-1在耳蜗中激活的下游级联以及IGF-1维持耳蜗毛细胞的机制,Hayashi等[9]利用新生小鼠耳蜗的外植体培养,发现IGF-1同时激活PI3K/AKT和MEK/ERK通路,维持受损耳蜗毛细胞数量。其中,PI3K/AKT通路通过抑制细胞凋亡、特异性保护耳蜗内毛细胞而维持耳蜗毛细胞数量;而MEK/ERK通路促进Hensen's和Claudius'细胞(位于耳蜗毛细胞外侧的支持细胞)的细胞周期增加支持细胞数量,从而维持了耳蜗毛细胞数量。利用Akt抑制剂仅能显著减弱IGF-1诱导的耳蜗毛细胞内维持,而ERK抑制剂U0126则能显著减弱IGF-1诱导的耳蜗毛细胞外维持和内维持。Liu等[26]证明Wnt信号的过表达通过抑制耳蜗毛细胞中ROS的积累,保护小鼠耳蜗免受新霉素引起的耳蜗毛细胞损伤;随后又发现Wnt信号通过激活凋亡调节剂,抑制SGNs的氧化应激和凋亡,保护毛细胞避免顺铂诱导的听力损伤[27]。Wang等[28]证明Wnt信号在体外可保护小鼠耳蜗免受紫杉醇诱导的损伤来保护毛细胞。典型的Wnt信号还可以负调控炎症通路,如NF-κB和MAPK通路[29]。可见IGF-1和Wnt信号通路均参与了耳蜗毛细胞的增殖、损伤、修复和凋亡,但IGF-1是否可能通过Wnt通路抑制毛细胞凋亡的具体机制尚不明确。IGF-1是否通过Wnt通路抑制耳蜗毛细胞中ROS的积累,或者激活凋亡调节剂来发挥作用值得进一步研究。
4.2. IGF-1信号介质在转录水平上发挥调节作用
信使核糖核酸(mRNA)的调控已经成为一种调控信号通路的方法[30]。为了研究IGF-1信号介质在转录水平上的调节,Hayashi等[31]采用新生小鼠耳蜗外植体培养的新霉素损伤模型,用定量逆转录聚合酶链反应方法比较了IGF-1信号介导基因AKT1、MAPK3和MAPK1在外植体中不同时间点的转录水平,发现IGF-1可上调AKT1、MAPK3和MAPK1的mRNA水平,这种调节依赖于每个基因编码蛋白的磷酸化,而PI3K/AKT或MEK/ERK通路抑制剂的加入可减弱各基因转录的上调;同时还观察到AKT1的上调在中间时间点达到高峰,而MAPK3和MAPK1的表达持续增加,表明与AKT1相比,MAPK3和MAPK1在耳蜗毛细胞保护中对IGF-1反应更灵敏,然而他们没有评估编码AKT蛋白的另外两种亚型AKT2和AKT3的表达,这两个基因对IGF-1的反应可能类似于MAPK3和MAPK1,同样,AKT2和AKT3可协同参与保护耳蜗毛细胞免受氨基糖苷的侵袭[32]。探索其转录水平应使用与探索AKT1、MAPK3和MAPK1相似的条件,将扩大IGF-1信号介质在转录水平上发挥调节作用的范围,挖掘更多可能治疗SNHL的药物。
4.3. IGF-1可促进兴奋性损伤后耳蜗突触的再生
在哺乳动物中,内毛细胞利用复杂的带状突触[33]将声音信息传递给螺旋神经节神经元,即听觉系统的初级传入神经元。声音刺激引起耳蜗感觉上皮的振动,引起内毛细胞立体纤毛的倾斜,内毛细胞中的机械敏感离子通道被打开,导致内毛细胞去极化,去极化的内毛细胞从突触前膜释放谷氨酸传入SGNs树突的突触后受体。这种内毛细胞与SGNs之间的突触联系是听觉加工的重要组成部分[34]。在耳蜗毛细胞和SGNs中,脑源性神经营养因子条件性缺失后,内毛细胞上的传入突触活性和数量减少,表明神经营养支持可能在维持耳蜗传入突触功能和存活方面起着关键作用[35]。Hayashi等[36]发现IGF-1通过PI3K/Akt和MEK/ERK通路上调相关蛋白GAP43和神经生长因子NTN1对新生小鼠耳蜗毛细胞起到保护作用。其中,NTN1作为IGF-1在耳蜗毛细胞保护过程中的信号效应分子,主要通过IGF1激活支持细胞后释放,释放出的NTN1结合其在耳蜗毛细胞上表达的典型受体之一UNC5B,通过抑制耳蜗毛细胞凋亡,保护耳蜗毛细胞免受毒性作用[37];GAP43是一种神经系统特异性蛋白,是整合质膜和细胞骨架对细胞外信号反应的细胞内信号通路的主要组成部分,在神经系统的正常发育和损伤神经的再生过程中起重要作用,但在耳蜗毛细胞保护过程中,GAP43介导IGF-1对耳蜗毛细胞的保护作用的具体机制尚不清楚。GAP43是否和NTN1一样作为一种信号效应分子结合耳蜗毛细胞特异性受体发挥作用需要进一步研究。
IGF-1在小鼠耳蜗发育期中的SGNs均有表达,且表达量随着出生年龄增加而逐渐减少。在噪声损伤引起的听力下降中,内耳微循环障碍、代谢紊乱可导致耳蜗毛细胞的死亡[38],同时发现内毛细胞-螺旋神经节(IHC-SGN)突触会发生变性[39]。Yamahara等[40]通过体外培养外植体研究IGF-1对IHC-SGN突触再生的影响,发现外源性IGF-1(rhIGF-1)处理后IHC-SGN突触数量增加;同时用IGF-1R竞争性拮抗剂JB1和IGF-1R酪氨酸激酶抑制剂OSI-906对IGF-1R进行药理抑制,发现其减弱了rhIGF-1对IHC-SGN突触的修复作用。以上证明IHC-SGN突触数量不能完全恢复导致感觉输入的累积损失,可能导致听觉功能障碍,包括延迟发作性听力损失和噪声环境中的听力损伤。Chao等提供了从小鼠胚胎早期(E15.5)到成年期(P30)5个年龄段的SGN转录组数据库。Scrt2基因和Celf4基因在Ⅰ型和Ⅱ型SGNs中均有表达,它们产生了两个敲除小鼠株:Scrt2-tdTomato/+和Celf4-iCreER/+。以上为我们今后研究内耳SGNs提供了有利的实验支持[41]。然而,在目前的研究中,IGF-1对IHC-SGN突触再生的影响仅在年轻小鼠耳蜗外植体培养中得到证实,年轻小鼠听觉功能尚未成熟,且IGF-1R在年轻成年小鼠耳蜗中的表达也受到了抑制,未来,我们应该在成年动物体内探索IGF-1对IHC-SGN突触再生的影响。
5. IGF-1在电子耳蜗植入术后对听力的影响
对于严重的SNHL患者,人工耳蜗技术能提高听力、改善生活质量[3]。但插入人工耳蜗时有时需要进行耳蜗开窗,有时因为电刺激、热损伤以及植入电极时粗暴操作等因素也会导致残余听力的丧失。虽然动物实验和对人类颞骨的研究提出致耳蜗纤维化的炎症反应[42]、电刺激产生兴奋性毒性致螺旋神经节细胞变性[43]和延迟毛细胞变性均可引起听力减弱,但人工耳蜗植入术后残余听力损失的病因学仍未得到很好的解释。已知电极插入会导致外毛细胞凋亡的机制在于电极插入激活纤维细胞或巨噬细胞释放炎性细胞因子如白细胞介素1β和肿瘤坏死因子α等,这些炎性细胞因子可引起氧化应激并激活由丝裂原激活蛋白激酶/c-Jun-N-末端激酶(MAPK/JNK)介导的促凋亡信号级联,导致外毛细胞凋亡[44]。此外,活化的纤维细胞和大型噬菌体释放生长因子促进了纤维化的炎症增殖反应,使电极周围形成瘢痕组织[45],进而导致听力下降。Yamahara等[46]研究成年豚鼠发现IGF-1治疗组的外毛细胞数量在低频听力的耳蜗区(上中基底节)保持数量的维持,以及电极周围的纤维组织形成较少,表明IGF-1可以减轻人工耳蜗植入术后的低频听力损失。相关文献报道,IGF-1在体外可促进兴奋性损伤后耳蜗突触的再生[40],同时IGF-1治疗可诱导新生小鼠耳蜗中Netrin1的表达[37],而Netrin1参与轴突引导和突触形成。这与耳蜗植入术后听力下降的可能原因的解释机制密切相关。临床上保留低频听力尤为重要,IGF-1在治疗低频听力上可能比皮质类固醇更为有效。比较皮质激素和IGF-1等不同药物的作用机制和特点,可能为我们提供更有效的药物联合治疗方案。
6. 展望
近年来,通过显微外科手术,经耳蜗或半规管等内耳方式给药的方法相对成熟,IGF-1可能是一种值得探索的药物。我们应更多地研究IGF-1保护内耳的具体作用机制,在动物模型和临床实验中去验证IGF-1的安全性和可靠性。
综上所述,IGF-1对内耳的保护机制应作为一种新型有效的SNHL治疗方法和治疗SNHL的潜在药物来深入探讨,基于IGF-1对IHC-SGN突触再生的影响,需要进一步的研究来确定更完整地修复IHC-SGN突触的策略。类固醇与IGF-1联合运用于动物模型能否最大可能地修复IHC-SGN突触也需要我们进一步去探索。同时,我们应该关注IGF-1诱导的突触再生相关的细胞内信号,探索IHC-SGN与细胞内信号的关系,以及对IGF-1下游信号作用靶点的研究可能都有助于SNHL治疗方法的发展。这些治疗方法是对包括类固醇治疗在内的现有方法的补充。未来真正的挑战是IGF-1药物载体的构建,如何将IGF-1药物具体点对点运用于毛细胞中治疗SNHL将是一个具体的挑战。
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