少突胶质细胞和视神经损伤

少突胶质细胞在中枢神经系统中具有重要和广泛的生理功能。视神经损伤后,出现髓鞘脱失、少突胶质细胞死亡和髓鞘再生等病理改变,产生的髓鞘碎片能抑制视神经轴索再生。少突胶质细胞的抑制特性由特定的抑制分子介导,目前已鉴定的抑制分子主要有Nogo、髓鞘相关糖蛋白（myelin—associated glycoprotein,MAG）、少突胶质细胞髓鞘糖蛋白（oligodendrocyte myelin glycoprotein,OMgp）等,它们通过同一受体复合体传导抑制信号。阻滞抑制分子及其受体,或调整神经元的内在生长状态以克服抑制分子的抑制作用,可以促进视神经损伤后再生。本文就这方面的进展作一综述。     

髓鞘相关糖蛋白与神经系统的髓鞘发育和轴突生长

髓鞘相关糖蛋白（myelin-associated glycoprotein,MAG）是免疫球蛋白超家族成员,它由中枢神经系统的少突胶质细胞和外周神经系统的施万细胞表达。MAG定位于直接和轴突相接触的髓鞘膜的最里层,它通过介导胶质细胞与轴突的相互作用参与髓鞘的形成及其完整性的维持。同时MAG也是髓鞘来源的神经生长抑制因子的主要成分。在神经系统发育的不同阶段,MAG显示不同的功能：即发育期促进轴突生长,成熟期抑制轴突生长。其抑制作用主要由髓鞘来源的抑制分子的共同受体NgR介导,在神经营养因子受体p75NTR以及小GTP酶Rho等信号分子的共同参与下完成。     

Nogo在中枢神经损伤再生中的作用机制

Nogo是中枢神经系统（CNS）少突胶质细胞分泌的一种髓磷脂蛋白,它的主要功能是抑制损伤后轴突的再生,它含有两个完全独立的具有抑制活性的结构域：位于细胞内的amino—Nogo和位于细胞表面的Nogo-66。Nogo-66是通过与受体复合体NgR／p75／Lingo—1结合,触发Rho信号通路来发挥作用。Nogo及其信号转导机制日益成为CNS损伤再生的研究热点,就Nogo在CNS损伤再生中的作用机制作一综述。     

中枢神经再生抑制因子及免疫治疗研究

成体哺乳动物中枢神经损伤后早期轴突再生失败的一个主要原因是由于髓磷脂抑制分子的存在。Nogo、髓磷脂相关糖蛋白以及少突胶质细胞髓磷脂糖蛋白等神经再生抑制因子的发现,大大促进了中枢神经再生分子机制的研究。它们均能独立通过Nogo-66受体产生对轴突再生的抑制效应,髓磷脂抑制分子及其信号转导机制的研究日益成为中枢神经再生的研究热点,髓磷脂及其信号转导分子特别是Nogo-66受体、p75神经营养素受体成为损伤后促进轴突再生、抑制生长锥塌陷的主要治疗靶点。抑制上述抑制因子及相关受体NgR或p75NTR可能有助于中枢神经损伤的修复,围绕这些抑制因子及其相关受体介导的信号转导途径,人们提出了多种治疗中枢神经损伤的新思路,其中免疫学方法尤其受到关注。     

TAT-NEP1-40——神经再生的新型药物

在受损的中枢神经系统中,Nogo-A蛋白、髓鞘蛋白和少突髓鞘蛋白是抑制中枢神经轴突再生的主要物质,它们通过一个共同受体——Nogo蛋白受体(Nogo receptor,NgR)介导中枢神经轴突抑制。NEP1-40是NgR受体的竞争性抑制剂,是治疗中枢神经损伤是一个具有潜在性的候选药物。TAT蛋白质转导序列是HIV反式转录激活因子,是目前已知具有有效蛋白质转导功能的序列。利用蛋白质重组技术构建并表达的含有TAT结构域和NEP1-40肽的融合蛋白质TAT-NEP1-40可能成为一种治疗中枢神经系统损伤如中风、脑缺氧、脑出血、脑外伤和脊髓损伤的新颖的候选药物。     

Nogo与Nogo受体研究

nogo是新近发现的一种基因,编码3种蛋白质：Nogo-A、Nogo-B和Nogo-C.迄今为止,已证明它有抑制成熟中枢神经系统(CNS)神经元轴突再生及诱导细胞凋亡的作用.Nogo受体是一种糖基醇磷脂结合蛋白.对Nogo和Nogo受体的研究,对于CNS再生障碍及肿瘤的认识和治疗有重要意义.     

LINGO-1:新发现的脑内神经再生抑制因子

在成年动物和人中枢神经系统 ,髓鞘内的神经再生抑制因子 (MAG、OMgp、Nogo等 )通过与神经元上的特异性受体复合体相互作用 ,启动对神经轴突再生的抑制 ;“Nogo 6 6受体”(Nogo 6 6receptor,即Nogoreceptor 1,NgR1)和“p75神经生长因子受体”是组成此受体复合体的两个关键亚单位 ;被Nogo等激活的受体复合体能活化“胞内骨架调节因子”———RhoA ,RhoA最终实现对轴突延长的抑制。美国学者最近发现 ,在转染后成功表达NgR1和p75的非神经细胞 (COS 7细胞 ) ,神经再生抑制因子OMgp不能激活NgR1和p75复合体、亦不能活化RhoA ,暗示神经…     

Nogo受体相关研究进展

中枢神经系统损伤后其再生能力较弱已被人们所熟知,原因在于髓磷脂抑制物如Nogo、MAG、Omgp等抑制因子的作用,这些抑制因子通过与神经元上的Nogo受体(NgR)特异性结合,发挥对神经轴突再生的抑制作用。Nogo是一种存在于中枢神经系统少突胶质细胞上的髓磷脂蛋白,其作用主要在于神经细胞损伤后抑制其突触再生,这同时也是对损伤部位其他细胞免于进一步损伤的保护作用。存在于细胞表面的Nogo-66结构是与NgR特异性结合的功能域。NgR是一种存在于神经元表面,传递抑制轴突生长信号的复合共受体。近年来随着对NgR、Nogo及其下游信号通路其他相关蛋白研究的深入,提示多种神经系统疾病与之相关。我们简要综述近年来关于NgR的研究进展。     

Nogo-A氨基末端：通过特异整合素机制抑制细胞黏附及轴突生长

成年哺乳动物的中枢神经系统（CNS）受损后,解剖学上的修复水平非常有限。因神经纤维再生明显受阻,往往造成神经损伤后永久性的功能缺陷。在成年CNS抑制轴突生长的因子中,有一类是髓磷脂蛋白（myelin）,而Nogo是这类蛋白中的一种,由少突神经胶质细胞产生,抑制轴突的生长。通过不同的启动子和差别剪接,nogo基因会产生三种主要的转录产物Nogo-A、-B和-C。     

克罗米芬促进髓鞘发育并改善缺氧性运动功能障碍

目的 研究选择性雌激素受体调节剂克罗米芬在促进白质损伤模型动物大脑少突胶质前体细胞分化和髓鞘形成中的作用和对运动功能障碍的影响。方法 离体少突胶质前体细胞纯化培养;新生3 d小鼠连续缺氧(10%O₂)7 d,模拟新生儿脑白质损伤;采用免疫荧光染色、运动协调功能检测等方法,观察克罗米芬对大脑皮质和胼胝体区域少突胶质细胞和髓鞘发育与运动功能的影响。结果 克罗米芬可促进纯化培养的少突胶质前体细胞分化为成熟少突胶质细胞,显著增加脑白质损伤模型小鼠脑组织2种髓鞘标志物——髓鞘碱性蛋白和髓鞘蛋白脂蛋白的表达,也显著增加成熟少突胶质细胞标志物腺瘤性结肠息肉病蛋白的表达;平衡杆实验证明克罗米芬治疗能够改善低氧导致的小鼠远期运动协调功能障碍。结论 克罗米芬能有效促进慢性缺氧诱导的白质损伤模型小鼠髓鞘形成和改善神经功能异常,为治疗脑白质损伤提供可能的临床药物。     

Nogo on the go

Growth inhibition in the central nervous system (CNS) is a major barrier to axon regeneration. Recent findings indicate that three distinct myelin proteins, myelin-associated glycoprotein (MAG), Nogo, and oligodendrocyte-myelin glycoprotein (OMgp), inhibit axon growth by binding a common receptor, the Nogo66 receptor (NgR), and likely converge on a common signaling cascade.     

Multiple signals regulate axon regeneration through the nogo receptor complex

Several myelin-derived proteins have been identified as components of central nervous system (CNS) myelin, which prevents axonal regeneration in the adult vertebrate CNS. The discovery of the receptor for these proteins was a major step toward understanding the failure of axon regeneration. The receptor complex consists of at least three elements: the p75 receptor (p75^NTR), the Nogo receptor and LINGO-1. Downstream from the receptor complex, RhoA activation has been shown to be a key element of the signaling mechanism of these proteins. Rho activation arrests axon growth, and blocking Rho activation promotes axon regeneration in vivo. Recent studies have identified conventional protein kinase C as an additional necessary component for axon growth inhibition. Possible crosstalk downstream of these signals should be explored to clarify all the inhibitory signals and may provide an efficient molecular target against injuries to the CNS.     

AMIGO3 Is an NgR1/p75 Co-Receptor Signalling Axon Growth Inhibition in the Acute Phase of Adult Central Nervous System Injury

Axon regeneration in the injured adult CNS is reportedly inhibited by myelin-derived inhibitory molecules, after binding to a receptor complex comprised of the Nogo-66 receptor (NgR1) and two transmembrane co-receptors p75/TROY and LINGO-1. However, the post-injury expression pattern for LINGO-1 is inconsistent with its proposed function. We demonstrated that AMIGO3 levels were significantly higher acutely than those of LINGO-1 in dorsal column lesions and reduced in models of dorsal root ganglion neuron (DRGN) axon regeneration. Similarly, AMIGO3 levels were raised in the retina immediately after optic nerve crush, whilst levels were suppressed in regenerating optic nerves, induced by intravitreal peripheral nerve implantation. AMIGO3 interacted functionally with NgR1-p75/TROY in non-neuronal cells and in brain lysates, mediating RhoA activation in response to CNS myelin. Knockdown of AMIGO3 in myelin-inhibited adult primary DRG and retinal cultures promoted disinhibited neurite growth when cells were stimulated with appropriate neurotrophic factors. These findings demonstrate that AMIGO3 substitutes for LINGO-1 in the NgR1-p75/TROY inhibitory signalling complex and suggests that the NgR1-p75/TROY-AMIGO3 receptor complex mediates myelin-induced inhibition of axon growth acutely in the CNS. Thus, antagonizing AMIGO3 rather than LINGO-1 immediately after CNS injury is likely to be a more effective therapeutic strategy for promoting CNS axon regeneration when combined with neurotrophic factor administration.     

Generation of an OMgp allelic series in mice

The very limited ability to regenerate axons after injury in the mature mammalian central nervous system (CNS) has been partly attributed to the growth restrictive nature of CNS myelin. Oligodendrocyte myelin glycoprotein (OMgp) was identified as a major myelin‐derived inhibitor of axon growth. However, its role in axon regeneration in vivo is poorly understood. Here we describe the generation and molecular characterization of an OMgp allelic series. With a single gene targeting event and Cre/FLP mediated recombination, we generated an OMgp null allele with a LacZ reporter, one without a reporter gene, and an OMgp conditional allele. This allelic series will aid in the study of OMgp in adult CNS axon regeneration using mouse models of spinal cord injury. The conditional allele will overcome developmental compensation when employed with an inducible Cre, and allows for the study of temporal and tissue/cell type‐specific roles of OMgp in CNS injury‐induced axonal plasticity. genesis 47:751–756, 2009. © 2009 Wiley‐Liss, Inc.     

A TNF receptor family member, TROY, is a coreceptor with Nogo receptor in mediating the inhibitory activity of myelin inhibitors

A major obstacle for successful axon regeneration in the adult central nervous system (CNS) arises from inhibitory molecules in CNS myelin, which signal through a common receptor complex on neurons consisting of the ligand-binding Nogo-66 receptor (NgR) and two transmembrane coreceptors, p75 and LINGO-1. However, p75 expression is only detectable in subpopulations of mature neurons, raising the question of how these inhibitory signals are transduced in neurons lacking p75. In this study, we demonstrate that TROY (also known as TAJ), a TNF receptor family member selectively expressed in the adult nervous system, can form a functional receptor complex with NgR and LINGO-1 to mediate cellular responses to myelin inhibitors. Also, both overexpressing a dominant-negative TROY or presence of a soluble TROY protein can efficiently block neuronal response to myelin inhibitors. Our results implicate TROY in mediating myelin inhibition, offering new insights into the molecular mechanisms of regeneration failure in the adult nervous system.     

Repulsive guidance molecule b inhibits neurite growth and is increased after spinal cord injury

Neuronal axons are guided by attractive and repulsive cues in their local environment. Since the identification of the repulsive guidance molecule (RGM) a (RGMa) as an axon repellent in the visual system, diverse functions, as part of the developing and adult central nervous system (CNS), have been ascribed to it. The binding of RGMa to its receptor neogenin has been shown to induce RhoA activation, leading to inhibitory/repulsive behavior and the collapse of the neuronal growth cone. In this paper, we provide evidence to suggest the involvement of RGMb, another member of the RGM family, in the rat CNS. RGMb inhibits neurite outgrowth in postnatal cerebellar granule neurons (CGNs) in vitro. RGMb is expressed by oligodendrocytes and neurons in the adult rat CNS, and the expression of this molecule is upregulated around the site of spinal cord injury. RGMb is present in myelin isolated from an adult rat brain. RGMb and neogenin are coexpressed in CGNs and entorhinal cortex neurons. These findings suggest that RGMb is a myelin-derived inhibitor of axon growth in the CNS. Inhibition of RGMb may provide an alternative approach for the treatment of spinal injuries.     

MHC-I and PirB Upregulation in the Central and Peripheral Nervous System following Sciatic Nerve Injury

Major histocompatibility complex class one (MHC-I) antigen-presenting molecules participate in central nervous system (CNS) synaptic plasticity, as does the paired immunoglobulin-like receptor B (PirB), an MHC-I ligand that can inhibit immune-cells and bind to myelin axon growth inhibitors. Based on the dual roles of both molecules in the immune and nervous systems, we evaluated their expression in the central and peripheral nervous system (PNS) following sciatic nerve injury in mice. Increased PirB and MHC-I protein and gene expression is present in the spinal cord one week after nerve transection, PirB being mostly expressed in the neuropile region. In the crushed nerve, MHC-I protein levels increased 2 weeks after lesion (wal) and progressively decreased over the next eight weeks. The same kinetics were observed for infiltrating cytotoxic T lymphocytes (CTLs) but not for PirB expression, which continuously increased. Both MHC-I and PirB were found in macrophages and Schwann cells but rarely in axons. Interestingly, at 8 wal, PirB was mainly restricted to the myelin sheath. Our findings reinforce the participation of MHC-I and PirB in CNS plasticity events. In contrast, opposing expression levels of these molecules were found in the PNS, so that MHC-I and PirB seem to be mostly implicated in antigen presentation to CTLs and axon myelination, respectively.     

Glial inhibition of CNS axon regeneration

Damage to the adult CNS often leads to persistent deficits due to the inability of mature axons to regenerate after injury. Mounting evidence suggests that the glial environment of the adult CNS, which includes inhibitory molecules in CNS myelin as well as proteoglycans associated with astroglial scarring, might present a major hurdle for successful axon regeneration. Here, we evaluate the molecular basis of these inhibitory influences and their contributions to the limitation of long-distance axon repair and other types of structural plasticity. Greater insight into glial inhibition is crucial for developing therapies to promote functional recovery after neural injury.     

Soluble NgR fusion protein modulates the proliferation of neural progenitor cells via the Notch pathway

NogoA, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein are CNS myelin molecules that bind to the neuronal Nogo-66 receptor (NgR) and inhibit axon growth. The NgR antagonist, soluble NgR1-Fc protein (sNgR-Fc), facilitates axon regeneration by neutralizing the inhibitory effects of myelin proteins in experimental models of CNS injury. Here we aim to investigate the effect of sNgR-Fc on the proliferation of neural progenitor cells (NPCs). The hippocampus cells of embryonic rats were isolated and cultured in vitro. The expression of nestin, βIII-Tubulin, GFAP and Nogo-A on these cells was observed using immunocytochemistry. In order to investigate the effect on proliferation of NPCs, sNgR-Fc, MAG-Fc chimera and Notch1 blocker were added respectively. The total cell number for the proliferated NPCs was counted. BrdU was applied and the rate of proliferating cells was examined. The level of Notch1 was analyzed using Western blotting. We identified that NogoA is expressed in NPCs. sNgR-Fc significantly enhanced the proliferation of NPCs in vitro as indicated by BrdU labeling and total cell count. This proliferation effect was abolished by the administration of MAG suggesting specificity. In addition, we demonstrate that sNgR-Fc is a potent activator for Notch1 and Notch1 antagonist reversed the effect of sNgR-Fc on NPC proliferation. Our results suggest that sNgR-Fc may modulate Nogo activity to induce NPC proliferation via the Notch pathway.