Role and mechanism of retinoic acid in axonal regeneration

doi:10.3969/j.issn.2095-4344.2014.15.025

Abstract

Abstract:

BACKGROUND: Retinoic acid signaling pathways is very important in the formation pf nervous system, specialization of neurons and outgrowth of axons. The recent studies show that, retinoic acid plays an important role in the process of axonal regeneration, but few research reports its exact molecular mechanism.
OBJECTIVE: To analyze and summarize the mechanism underlying retinoic acid signaling pathways in the process of axonal regeneration.
METHODS: A computer-based online research was conducted among the VIP, CNKI, PubMed, BioMed Centeral, Springer, The Free Medical Journals, EBSCO and Foreign Journals Integration System between January 2000 and December 2013, with the key words of “retinoic acid, the central nervous system, nerve damage, axon regeneration, and mechanism” in Chinese and English. A total of 43 studies addressing the molecular mechanism of retinoic acid in axonal regeneration were screened. According to the supplementary documents, another five references were added. Repetitive research and atypical reports were excluded.
RESULTS AND CONCLUSION: Following acute central nervous system injury, axonal regeneration and functional recovery are extremely limited. For proper functionality following injury, axons must regrow, reinnervate their targets, and remyelinate their axons. When the central nervous system injuries occur, retinoic acid signaling pathways express transcription factor retinoic acid receptor β2 to induce axonal regeneration following injury; in dorsal root ganglion neurons, cAMP levels are greatly increased by lentiviral retinoic acid receptor β2 expression and contribute to neurite outgrowth. More recently, retinoic acid-retinoic acid receptor β2 pathways directly transcriptionally repress a member of the inhibitory Nogo receptor complex, Lingo-1, under an axonal growth inhibitory environment in vitro as well as following spinal cord injury in vivo. Through these molecular mechanisms, retinoic acid signaling pathways play its important role in the process of axonal regeneration.

中国组织工程研究杂志出版内容重点：组织构建；骨细胞；软骨细胞；细胞培养；成纤维细胞；血管内皮细胞；骨质疏松；组织工程

全文链接：

Key words: tretinoin, central nervous system, axons, nerve regeneration, spinal cord injuries

CLC Number:

R318

Wu Shi-xing, Yu Zhen-hai, Liu Fang, Lin Hai-yan, Zhang Zhi-ying, Zhang Chuan-sen. Role and mechanism of retinoic acid in axonal regeneration [J]. Chinese Journal of Tissue Engineering Research, 2014, 18(15): 2450-2454.

Figures/Tables 1

2.1 纳入文献基本情况初次检索得到89篇文献，包括中文文献23篇和英文文献66篇。由通讯作者按纳入及排除标准筛选后，排除与本文无关文献，共纳入34篇进入结果分析。其中中枢神经系统损伤概述6篇[1-6]；维甲酸代谢途径及其信号通路概述16篇[7-22]；维甲酸在轴突再生中作用机制的研究12篇[23-34]。 2.2 维甲酸信号通路概述维甲酸受体属于核类配体激活的转录调节因子超家族，该家族还包括甾类激素受体、甲状腺素受体、维生素D3受体以及多种尚未发现配体的孤儿受体等[11]。维甲酸可直接穿越细胞膜，与细胞质内的维甲酸结合蛋白结合，再通过相应受体的转移而进入细胞核调节基因的HRE区(Hormone Responsive Elements，HERE)，以便指导mRNA的转录、合成诱导蛋白，进而启动成形基因[12]。研究表明，维甲酸受体可分为两大类：即维甲酸受体(Retinoic Acid Receptor，RAR)和维甲类X受体(Retinoid X Receptor，RXR)，每种受体又可分为不同的亚型[13]，因此，维甲酸信号通路的调节具有多样性。维甲酸通过CRABP2的协助进入胞核后，与一个包含由RAR-RXR异质二聚体组成的配体活化转录因子的转录复合体结合，3个RAR基因(RARA、RARB、RARG)和3个RXR基因(RXRA、RXRB、RXRG)排列在一起，异质二聚体对与DNA片段结合后形成维甲酸反应原件(Retinoic acid-response element，RARE)。维甲酸信号通路与膜受体信号通路的相互作用，以及RARs和RXRs与多种转录介导因子(包括辅活化因子和辅抑制因子)的相互作用，更增加了维甲酸信号途径的复杂性，成为介导其多样效应的分子基础，从而使得受维甲酸信号调控的基因的表达呈现多样性[14]。因此，维甲酸作为一种重要的信号分子在调控多种组织和细胞的形态发生、增殖、生长发育、代谢以及维持内环境稳定等方面具有广泛的生物学作用，是维持生长发育不可缺少的物质。 2.3 维甲酸在神经轴突生长中的作用维甲酸信号通路在神经系统发育、神经元分化、腹侧神经形成和运动神经元特化过程中有重要作用[15]。最近的研究表明，维甲酸在诱导神经元轴突生长的过程中起重要作用。在胚胎以及成体背根神经节神经元和小脑颗粒神经元中，维甲酸可以上调RARβ2的表达，从而诱导神经轴突的生长[16]。而且体外实验已经证实，添加外源性维甲酸和/或RARβ2可以诱导背根神经节神经元、成体大脑皮质神经元和小脑颗粒神经元轴突的再生[17]。此外，RARβ的激动剂也可以诱导胚胎背根神经节神经元神经轴突的生长[18]；在成年大鼠的皮质层和背根神经节外植体在经过RARβ的激动剂处理14 d后，也可以诱导神经轴突生长，而未经过上述处理的实验动物却不会出现以上改变[19]。在出生后的小脑颗粒神经元中，当RARβ信号通路被阻断后，即使有维甲酸的作用，也不能诱导小脑颗粒神经元轴突的生长[20]。由此可以证实维甲酸是通过专一性的激活RARβ信号通路来诱导神经轴突生长的。另外，胚胎脊髓外植体在维甲酸的作用下可以诱导神经轴突生长，而成体脊髓外植体却不能。这是由于和胚胎脊髓相比，成体脊髓中RARβ的表达有限，而这可能就是导致两者之间对于维甲酸产生不同反应的原因[21]。此外，当用病毒转导的方法把RARβ2转导进入成体脊髓中时，就可以诱导神经轴突的生长[22]。因此，可以得出结论，维甲酸可以在RARβ信号通路中发挥特殊的作用，从而诱导胚胎和成体神经元轴突的生长。 2.4 甲酸与轴突再生 2.4.1 维甲酸在轴突再生中的作用在出生后个体或成体中，轴突在物理或病理性损伤后的再生长属于再生的范畴，周围神经系统在神经移植后可以再生，但中枢神经系统却不行[23]，因此，如果能加速周围神经系统再生速度或诱导中枢神经系统再生将会有很大的治疗意义。越来越多的证据表明，通过激活成体中的发育程序，维甲酸将会成为调节再生的重要因子。研究表明维甲酸具有显著的抗炎作用，并且能够通过诱导细胞因子信号转导抑制因子(Suppressors of cytokine signaling，SOCS3)，抑制JAK、STAT3的活性，而抑制γ干扰素引起的原代大鼠脑的星形胶质细胞的炎症反应[24]。另外，RA-RARβ信号通路与硫酸软骨素酶ABC(Chondroitinase ABC，ChABC)结合后，可以减轻髓磷脂抑制环境的影响，并可以有效的促进轴突的再生。因此，在中枢神经系统损伤后，应用维甲酸治疗能够通过减轻炎症反应来减少胶质瘢痕的形成，为轴突的再生提供有利的环境。另外在脊髓损伤后，增强的RA-RARβ信号通路与ChABC结合后，可以有效的减轻髓磷脂抑制环境的影响，有利于促进轴突的再生[25]。有研究显示，当成年大鼠背根神经损伤后，经慢病毒转染RARβ2的成体背根神经节神经元进入脊髓后根入髓区后可以诱导轴突再生和促进损伤的感觉神经元功能的恢复[26]。另外，在成年大鼠脊髓损伤后的3周内用慢病毒在感觉运动区中转染RARβ2后，也可以诱导皮质脊髓束中轴突的再生和促进功能的恢复[27]。同时在大鼠脊柱病变后，大脑通过增加RARβ的特定受体激动剂-CD2019的表达来促进了皮质脊髓束轴突的生长和前肢功能的恢复[28]。以上的研究结果表明维甲酸是通过激活RA-RARβ信号通路在神经突起生长和轴突再生过程中起作用的。 2.4.2 维甲酸信号通路在轴突再生中的分子机制虽然维甲酸信号通路在神经生长和轴突再生中起重要作用，但是却鲜有关于其确切作用分子机制的研究报道。最近研究结果显示，成体背根神经节神经元经慢病毒转染RARβ2后可以诱导神经突起的生长并能增加cAMP的水平，但是当应用腺苷酸环化酶抑制剂-2,5-二脱氧腺苷酸(2，5-dideoxyadenosine，DDA)或cAMP依赖性的细胞永生化抑制剂蛋白激酶A(Protein kinase A，PKA)时，RARβ2促进神经生长的作用将大大减弱[29]。这表明cAMP 虽然存在于维甲酸信号中，但是它的作用却没有那么确切。比如在成体的脊髓后根入髓区的动物模型中，当成体背根神经节神经元中应用双丁酰环磷酸时，其促进功能恢复的作用就不如单独应用RARβ2的作用明显。事实上，在出生后的小脑颗粒神经元中已经证实，RARβ受体的激动剂-CD2019在神经轴突生长中的正性作用可以被 PI3K抑制剂减弱。另外，实验证实仅有神经元损伤时，RARβ可以诱导体内小脑颗粒神经元中的AKT的磷酸化[30]。最近的研究发现，在小脑颗粒神经元中，维甲酸信号可以通过RARβ的作用减少RhoA的激活，并在髓鞘抑制环境中抑制Lingo-1基因及其蛋白的表达，从而促进神经轴突的生长。当应用维甲酸后，组蛋白去乙酰化酶3(HDAC3)与Lingo-1启动子的RARE中RARβ紧密结合后，可置换出PDAF，而降低3-组蛋白乙酰化赖氨酸-9(AcH3K9)的水平。同时，还可使赖氨酸-27的H3的甲基化水平升高，从而抑制Lingo-1的表达[31]。有人认为上述过程是为了取代NogoR复合体，并且阻遏RhoA的活化，以促进轴突的再生。此外，通过荧光素酶分析发现，当发生突变而机体对维甲酸信号无反应时，RARE开始行使其功能。同时在脊髓的抑制环境中，当用维甲酸处理小脑颗粒神经元时，增加Lingo-1，可以降低RA-RARβ诱导神经轴突生长的能力。最后，在体实验中，脊髓损伤野生型小鼠经过维甲酸处理后可出现Lingo-1蛋白的表达降低，但是在RARβ缺失的小鼠却没有这种现象[32]，这为体外实验得结果提供了生理相关性。重要的是，先前的研究已证明在红核脊髓或皮质脊髓束横断后，Lingo-1受体拮抗剂能够减少Rho-A的激活，从而促进轴突生长、改善功能恢复，并增加胶质细胞和神经细胞的存活[33]。如果这种依赖维甲酸信号可以降低Lingo-1表达的机制仅仅存在于神经元中或者也同时存在于少突胶质细胞中的话，将会有很大的意义"

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