视神经再生的研究进展

视神经属于中枢神经的一部分,损伤后难以再生。视神经损伤通常伴随视网膜神经节细胞(retinal ganglioncells,RGCs)的持续性凋亡及视神经变性坏死,引起视力损害甚至完全失明。目前针对视神经再生的基础研究主要集中于保护和维持视神经损伤后RGCs的存活、促进RGCs轴突再生及重建视神经功能。本文以RGCs保护、轴突再生及视神经功能重建等为关键词,查询国内外最新视神经再生研究类文献,并分析整理,从抗氧化应激、提供外源性细胞因子、炎症刺激、抗胶质瘢痕、基因调控等方面阐述近年的视神经再生研究进展,以期对后续的基础研究开展及临床转化有所帮助。     

视网膜神经节细胞的保护和修复

视网膜神经节细胞(RGCs)的进行性死亡是许多视网膜和视神经疾病发展到最后的必经之路。长期以来一直认为，由于抑制性环境的存在，视神经损伤后不能再生和修复，现在研究证实，在特定的条件下，尽管RGCs的胞体或轴突受损，仍能免于死亡，而且变性的轴突能再生，并能与靶组织建立突触联系。对RGCs的保护和修复的研究进展作一综述。     

视神经损伤后修复与再生的研究进展

视神经损伤后的治疗和功能恢复是医学领域的历史性难题。由于作为中枢神经系统一部分的视神经损伤后缺乏神经修复和再生所需的微环境,为此,有效的神经保护、防止神经元死亡和促进神经修复至关重要。大量研究证明:视功能的恢复与视网膜神经节细胞(retinal ganglion cells,RGCs)的损伤程度、轴浆转运物质合成功能状态、自身修复和再生能力均密切相关。近10a来,随着对神经损伤机制的深入了解,各类神经保护的研究也有了很大进展,在治疗视神经损伤方面展现出诱人的前景。我们通过阅读近年国内外相关文献,针对视神经损伤后RGCs再生及视神经保护相关实验研究和临床治疗方法作一综述。     

α晶体蛋白在视神经损伤后再生中的作用研究进展

α晶体蛋白是构成晶状体的重要结构蛋白,属于小分子热休克蛋白（ sHSPs）家族,除了具有热休克蛋白（ HSPs）的共性,还具有抗细胞凋亡等作用。近年来研究还发现,α晶体蛋白可与视网膜神经节细胞（ RGCs）的细胞膜结合来促进RGCs的存活和轴突的再生,进而部分改善视功能,但其相关机制仍无统一的结论。本文就α晶体蛋白的基本结构与功能、α晶体蛋白在视神经损伤后对RGCs的保护作用及其机制展开综述。     

大鼠视神经再生的组织病理学机制

目的探讨大鼠视神经再生的组织病理学机制。方法 40只Wistar大鼠按随机数字表法随机分为正常组（n=8）、单纯视神经损伤组（n=16）和视神经损伤联合晶状体损伤组（n=16）3组。单纯视神经损伤组：单纯的视神经完全性横断性损伤并保存中央血管完好;视神经损伤联合晶状体损伤组：视神经的完全性横断性损伤并保存中央血管完好,同时损伤晶状体形成白内障。4周后处死动物,处死前3d,用毛细玻璃管注入大鼠玻璃体内4～5μL质量分数2.5%的罗丹明B异硫氰酸盐（RITC）。大鼠视神经和视网膜行常规组织病理学检查并计数视网膜神经节细胞（RGCs）的数量。结果正常视神经可以观察到神经纤维及神经胶质细胞。单纯视神经损伤组视神经断端可见呈团块状的胶质瘢痕,细胞较密集,排列紊乱。视神经损伤联合晶状体损伤组视神经断端无胶质瘢痕,且细胞排列较疏松,并有纵向排列的趋势,伴有大量巨噬细胞浸润。视神经损伤联合晶状体损伤组周边部RGCs的数量为4.06±1.45,单纯视神经损伤组为4.06±1.45,2组比较差异有统计学意义（P〈0.01）。结论视神经再生的关键是克服视神经断端作为物理性屏障的胶质瘢痕和提高RGCs的存活数量。     

晶状体损伤对视神经损伤后RGCs的保护作用及其机制

背景大鼠Miiller细胞提取液能够促进离体培养的视网膜神经节细胞（RGCs）存活及轴突的再生,伴晶状体损伤的视神经外伤眼RGCs存活率提高,但Milller细胞和晶状体损伤在促进RGCs存活方面的关系鲜见报道。目的探讨伴晶状体损伤的视神经外伤眼Mtiller细胞对RGCs存活的促进作用及其机制。方法清洁级成年Wistar大鼠48只按随机数字表法随机分为伪手术组、视神经损伤组、晶状体联合视神经损伤组。伪手术组大鼠手术中暴露但不损伤视神经,视神经损伤组大鼠行视神经横断伤,晶状体联合视神经损伤组行视神经横断伤联合晶状体针刺伤,并导致晶状体混浊。术后7d及14d各组分别取8只大鼠处死后制备视网膜标本。采用苏木精一伊红染色观察各组大鼠视网膜和RGCs的形态学改变,采用免疫组织化学法检测各组大鼠视网膜内核层胶质纤维酸性蛋白（GFAP）标记的Muller细胞,光学显微镜下计数各组大鼠RGCs数量及GFAP阳性标记的Muller细胞数量。结果术后7d及14d,伪手术组大鼠RGCs的数量分别为（52．98±1．90）个／高倍视野和（51．81±3．09）个／高倍视野,差异无统计学意义（t=0．910,P=0．378）;术后14d视神经损伤组大鼠RGCs数量为（22．67±1．94）个／高倍视野,明显少于术后7d的（36．61±1．69）个／高倍视野,差异有统计学意义（t=15．312,P=0．000）;术后14d晶状体联合视神经损伤组RGCs数量为（35．69±1．80）个／高倍视野,明显少于术后7d的（50．76±2．77）个／高倍视野,差异有统计学意义（t=12．920,P=0．000）。术后7d及14d,晶状体联合视神经损伤组存活的RGCs数量均多于视神经损伤组,差异均有统计学意义（7d：t=102．840,P=0．000;14d：t=164．020,P=0．000）;术后14d晶状体联合视神经损伤组存活的RGCs：牧量少于伪手术组,差异有统计学意义（t=187．04,P=0．034）。术后7d及14d,伪手术组大鼠视网膜内核层均未见GFAP阳性标记的Muller细胞;视神经损伤组大鼠内核层GFAP阳性标记Muller细胞数量分别为（29．38＋2．04）个／高倍视野和（19．07±2．14）个／高倍视野,差异有统计学意义（t=-9．868,P=0．000）。晶状体联合视神经损伤组大鼠内核层GFAP阳性标记的Muller细胞数量分别为（48．96±2．80）个／高倍视野和（46．73±1．50）个／高倍视野,差异无统计学意义（t=1．987,P=0．067）。术后7d及14d,晶状体联合视神经损伤组大鼠内核层GFAP阳性Muller细胞数量均较视神经损伤组增多,差异均有统计学意义（7d：t=-15．997,P=0．000;14d：t=-29．938,P=0．000）。结论在视神经损伤合并晶状体刺伤时,晶状体损伤可诱导Muller细胞活化,进而促进视神经损伤后RGCs的存活。     

Bcl-2转基因小鼠视神经损伤后再生轴突生长导向的研究

目的探讨视神经损伤后Bcl-2高表达小鼠再生的视神经如何能长入脑内正常的靶组织。方法对生后3d（P3）的C57BL／6J野生小鼠和Bcl-2转基因小鼠进行左眼视神经夹闭损伤,立刻摘除对侧眼球,4d后处死小鼠,做视神经和脑组织切片,评估视神经的再生。视神经损伤后玻璃体内注射一种顺行性标记物——带有荧光的霍乱毒素B（CTB-F）,来标记视网膜神经节细胞轴突,并在视神经切片上用抗生长相关蛋白43抗体免疫荧光染色显示再生的神经轴突。结果C57BL／6J小鼠视神经损伤后不能再生,而Bcl-2转基因小鼠的视神经活跃地再生了相当长的距离,但再生的视神经轴突不能进入两侧正常的视路,而长入前脑白质。结论生后3d的Bcl-2转基因小鼠一侧视神经损伤,同时摘除对侧眼球,去除正常视路的引导后,视神经轴突仍能再生,并可长入脑内,但再生的视神经轴突失去了视路中的引导,不能长入双侧中脑内的靶组织,而是在前脑形成异位神经分布。     

晶状体损伤与视神经轴突再生关系的研究进展

视神经损伤是眼科重要的致盲因素之一,临床中尚无有效的治疗方法.研究证实,晶状体损伤后可以成功促进视网膜神经节细胞(RGC)存活和视神经轴突再生.晶状体损伤后,晶状体上皮细胞活化,可能成为影响视神经轴突再生的潜在因素.同时,巨噬细胞活化并分泌某些因子刺激RGC轴突再生.进一步的研究发现,可溶性混合晶状体蛋白对RGC的存活和突起生长有显著的促进作用,是晶状体来源的"神经保护性物质",其作用显著强于巨噬细胞提取液.因此,研究晶状体损伤后促进视网膜神经节细胞存活和神经轴突再生的机制及其关键性物质,有利于进一步揭示视神经损伤和再生的机制,探求新的治疗方法.     

细胞因子对视神经损伤修复作用的研究进展

创伤、代谢、高眼压等多种因素均可能造成视网膜神经节细胞和或视神经损伤，而视网膜神经节细胞损伤凋亡后无法自主再生，因此往往会造成视力下降甚至丧失等严重后果。对于视神经损伤，目前临床上尚无非常有效的治疗方法。近年有研究发现细胞因子可以明显促进视网膜神经节细胞的存活和轴突再生。本文就其中睫状神经营养因子、胶质源性神经营养因子、色素上皮衍生因子、粒细胞集落刺激因子、血浆凝血因子、促红细胞生成素等几种细胞因子促进视神经损伤修复作用的研究进展进行综述。     

睫状神经营养因子对视神经损伤修复作用的研究进展

以往认为，成年哺乳动物视神经损伤后通常不能再生。视神经损伤后，其轴突很快发生变性，胞体死亡，这是个不可逆的过程。但近年来的实验研究发现，视神经轴突可长入移植的周围神经，并且认为在再生过程中，睫状神经营养因子发挥了重要作用，现就睫状神经营养因子及其受体的分子结构、组织分布、基因结构及其在视神经损伤后的作用和作用机制作一综述，为视神经损伤的治疗提供了理论基础。（中华眼底病杂志，2003，19：333-404）     

CNTF, not other trophic factors, promotes axonal regeneration of axotomized retinal ganglion cells in adult hamsters

PURPOSE: To investigate the in vivo effects of trophic factors on the axonal regeneration of axotomized retinal ganglion cells in adult hamsters. METHODS: The left optic nerve was transected intracranially or intraorbitally, and a peripheral nerve graft was apposed or sutured to the axotomized optic nerve to enhance regeneration. Trophic factors were applied intravitreally every 5 days. Animals were allowed to survive for 3 or 4 weeks. Regenerating retinal ganglion cells (RGCs) were labeled by applying the dye Fluoro-Gold to the distal end of the peripheral nerve graft 3 days before the animals were killed. RESULTS: Intravitreal application of ciliary neurotrophic factor substantially enhanced the regeneration of damaged axons into a sciatic nerve graft in both experimental conditions (intracranial and intraorbital optic nerve transections) but did not increase the survival of distally axotomized RGCs. Basic fibroblast growth factor and neurotrophins such as nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 failed to enhance axonal regeneration of distally axotomized RGCs. CONCLUSIONS: Neurons of the adult central nervous system can regenerate in response to trophic supply after injury, and ciliary neurotrophic factor is at least one of the trophic factors that can promote axonal regeneration of axotomized RGCs.     

Promoting optic nerve regeneration

Vision is the most important sense for humans and it is irreversibly impaired by axonal damage of retinal ganglion cells (RGCs) in the optic nerve due to the lack of axonal regeneration. The failure of regeneration is partially attributable to factors located in the inhibitory environment of the forming glial scar and myelin as well as an insufficient intrinsic ability for axonal regrowth. Moreover, RGCs undergo apoptotic cell death after optic nerve injury, eliminating any chance for regeneration. In this review, we discuss the different aspects that cause regenerative failure in the optic nerve. Moreover, we describe discoveries of the last two decades demonstrating that under certain circumstances mature RGCs can be transformed into an active regenerative state allowing these neurons to survive axotomy and to regenerate axons in the injured optic nerve. In this context we focus on the role of the cytokines ciliary neutrophic factor (CNTF) and leukemia inhibitory factor (LIF), their receptors and the downstream signaling pathways. Furthermore, we discuss strategies to overcome inhibitory signaling induced by molecules associated with optic nerve myelin and the glial scar as well as the regenerative outcome after combinatorial treatments. These findings are encouraging and may open the possibility that clinically meaningful regeneration may become achievable one day in the future.     

神经生长因子对成年兔视神经夹伤后 修复的影响

目的研究神经生长因子(NGF)对成年兔视神经夹伤后修复的影响。方法16只成年兔随机分成NGF组和对照组,每组8只兔。建立兔右眼视神经夹伤模型后分别将载有0.06 ml NGF（浓度:5×10-4g/L,NGF组）或等量磷酸盐缓冲液（PBS）（对照组）的组织工程化神经移植于视神经损伤处；并向右眼玻璃体腔内注入0.02 ml NGF（浓度:5×10-4 g/L ,NGF组）或等量PBS（对照组）。所有兔左眼为正常空白对照组。分别于夹伤后1 d、2周、8周进行闪光视觉诱发电位(FVEP)检查。夹伤后8周时作光学显微镜和电子显微镜检查观察视网膜神经节细胞(RGC)和视神经的改变，同时用计算机图像处理系统作视神经纤维计数。结果夹伤后2周时FVEP检查结果显示，NGF组伤眼与健眼FVEP幅值比为0.765±0.150，对照组为0.494±0.108， NGF组与对照组相比差异有统计学意义（P<0.01）。夹伤后8周时NGF组伤眼与健眼FVEP幅值比为0.581±0.138，对照组为0.409±0.119， NGF组与对照组相比差异有统计学意义（P<0.05）。夹伤后8周时的光学显微镜和电子显微镜检查结果显示：NGF组RGC、视神经纤维的退变较对照组轻。夹伤后8周时NGF组和对照组视神经纤维计数分别为（10 955±608.7）、（7 898±608.8）根/ mm²，两组间差异有统计学意义（P<0.001）。结论NGF能够在一定程度上增加RGC的存活，促进轴突的再生，因而对视神经夹伤后的修复、视功能的恢复具有一定的促进作用。(中华眼底病杂志,2005,21:253-257)     

Axonal regeneration of retinal ganglion cells depending on the distance of axotomy in adult hamsters

PURPOSE: To examine the relationship between the distance of axotomy and axonal regeneration of injured retinal ganglion cells (RGCs) systematically and the effect of a predegenerated (pretransected or precrushed) peripheral nerve (PN) graft on axonal regeneration of RGCs axotomized at a definite distance (0.5 mm from the optic disc) in comparison with a normal PN graft. METHODS: The optic nerve (ON) was transected intraorbitally at 0.5, 1, 1.5, 2, or 3 mm or intracranially at 6 to 8 mm from the optic disc, and a PN graft was transplanted onto the ocular ON stump in adult hamsters. Four weeks after grafting, the number of RGCs regenerating their injured axons into the PN graft was investigated in all animals. RESULTS: The number of regenerating RGCs decreased significantly when the distance of axotomy increased from 0.5 to 7 mm. A precrushed PN graft was shown to enhance more injured RGCs to regenerate axons than a normal or pretransected PN graft. CONCLUSIONS: The distance of axotomy on the ON of adult hamsters is critical in determining the number of regenerating RGCs. Thus, experimental strategies to repair the damaged ON by PN transplantation is to attach a precrushed PN graft as close to the optic disc as possible to obtain optimal axonal regeneration of the axotomized RGCs.     

Optic Nerve Engraftment of Neural Stem Cells

PurposeTo evaluate the integrative potential of neural stem cells (NSCs) with the visual system and characterize effects on the survival and axonal regeneration of axotomized retinal ganglion cells (RGCs).MethodsFor in vitro studies, primary, postnatal rat RGCs were directly cocultured with human NSCs or cultured in NSC-conditioned media before their survival and neurite outgrowth were assessed. For in vivo studies, human NSCs were transplanted into the transected rat optic nerve, and immunohistology of the retina and optic nerve was performed to evaluate RGC survival, RGC axon regeneration, and NSC integration with the injured visual system.ResultsIncreased neurite outgrowth was observed in RGCs directly cocultured with NSCs. NSC-conditioned media demonstrated a dose-dependent effect on RGC survival and neurite outgrowth in culture. NSCs grafted into the lesioned optic nerve modestly improved RGC survival following an optic nerve transection (593 ± 164 RGCs/mm² vs. 199 ± 58 RGCs/mm²; P < 0.01). Additionally, RGC axonal regeneration following an optic nerve transection was modestly enhanced by NSCs transplanted at the lesion site (61.6 ± 8.5 axons vs. 40.3 ± 9.1 axons, P < 0.05). Transplanted NSCs also differentiated into neurons, received synaptic inputs from regenerating RGC axons, and extended axons along the transected optic nerve to incorporate with the visual system.ConclusionsHuman NSCs promote the modest survival and axonal regeneration of axotomized RGCs that is partially mediated by diffusible NSC-derived factors. Additionally, NSCs integrate with the injured optic nerve and have the potential to form neuronal relays to restore retinofugal connections.     

Survival of axotomized retinal ganglion cells in peripheral nerve-grafted ferrets.

PURPOSE: Peripheral nerve (PN) grafting to the optic nerve stump stimulates not only axonal regeneration of the axotomized retinal ganglion cells (RGCs) into the grafted PN but also their survival. The purpose of the present study was to determine the number, distribution, and soma diameter of only surviving RGCs without regenerated axons and surviving RGCs with regenerated axons in PN-grafted mammals. METHODS: A segment of PN was grafted to the optic nerve stump of adult ferrets. Two months after the PN grafting, surviving RGCs with regenerated axons were retrogradely labeled with granular blue (GB) and stained with RGC-specific antibody C38. Surviving RGCs without regenerated axons were identified as C38-positive cells without GB labeling. RESULTS: Twenty-one percent of RGCs survived axotomy after PN grafting in the area centralis (AC), whereas 47% survived in the peripheral retina. Twenty-six percent of surviving RGCs in the AC exhibited axonal regeneration, which was higher than that in the peripheral retina. Soma diameter histograms revealed that RGCs with regenerated axons showing both GB and C38 positivity were in the large soma diameter ranges. In contrast, the soma diameter distribution of surviving RGCs that did not have regenerated axons showed a peak in the smaller soma diameter ranges. CONCLUSIONS: The present data suggest that PN grafting promotes survival of axotomized RGCs more effectively in the peripheral retina than in the AC. Among surviving RGCs, the larger cells exhibited axonal regeneration into the grafted PN, whereas the axons of smaller cells did not to regenerate in either the AC or the peripheral retina.     

Cataractogenic lens injury prevents traumatic ganglion cell death and promotes axonal regeneration both in vivo and in culture

PURPOSE: To examine and quantify neuroprotective and neurite-promoting activity on retinal ganglion cells (RGCs) after injury of the lens. METHODS: In adult albino rats, penetrating lens injury was performed by intraocular injection. To test for injury-induced neuroprotective effects in vivo, fluorescence-prelabeled RGCs were axotomized by subsequent crush of the optic nerve (ON) with concomitant lens injury to cause cataract. The numbers of surviving RGCs were determined in retinal wholemounts and compared between the different experimental and control groups. To examine axonal regeneration in vivo, the ON was cut and replaced with an autologous piece of sciatic nerve (SN). Retinal ganglion cells with axons that had regenerated within the SN under lens injury or control conditions were retrogradely labeled with a fluorescent dye and counted on retinal wholemounts. Neurite regeneration was also studied in adult retinal explants obtained either after lens injury or without injury. The numbers of axons were determined after 1 and 2 days in culture. Putative neurotrophins (NTs) were studied within immunohistochemistry and Western blot analysis. RESULTS: Cataractogenic lens injury performed at the same time as ON crush resulted in highly significant rescue of 746 +/- 126 RGCs/mm(2) (mean +/- SD; approximately 39% of total RGCs) 14 days after injury compared with controls without injury or with injection of buffer into the vitreous body (30 +/- 18 RGCs/mm(2)). When lens injury was performed with a delay of 3 days after ON crush, 49% of RGCs survived, whereas delay of 5 days still rescued 45% of all RGCs. In the grafting paradigm virtually all surviving RGCs after lens injury appeared to have regenerated an axon within the SN graft (763 +/- 114 RGCs/mm(2) versus 79 +/- 17 RGCs/mm(2) in controls). This rate of regeneration corresponds to approximately 40% of all RGCs. In the regeneration paradigm in vitro preceding lens injury and ON crush 5 days previous resulted in a maximum of regeneration of 273 +/- 39 fibers/explant after 1 day and 574 +/- 38 fibers/explant after 2 days in vitro. In comparison, in control retinal pieces without lens injury 28 +/- 13 fibers/explant grew out at 1 day, and 97 +/- 37 fibers/explant grew out at 2 days in culture. Immunohistochemical and Western blot analysis of potential NTs in the injured lens revealed no expression of ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF), NT-4, nerve growth factor (NGF), and basic fibroblast growth factor (bFGF). CONCLUSIONS: The findings indicate that the lens contains high neuroprotective and neuritogenic activity, which is not caused by NT. Compared with the data available in the literature, this neuroprotection is quantitatively among the highest ever reported within the adult rat visual system.     

Gene therapy and transplantation in CNS repair: the visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.