【新语丝电子文库(www.xys.org)(www.xys2.org)】 ———————————————— 杨雄里院士抄袭文章全文对比 中文部分为杨雄里《Muller细胞与视网膜功能》的正文全文。原载《生理科学进展》 1998年第29卷第1期第7－10页。 英文部分为Eric Newman和Andreas Reichenbach于1996年发表于Trends in Neuroscience 第19卷第307-312页题为The Muller Cell: a functional element of the retina 的综述的相应小标题和段落，摘录尽量摘全，文中省略号表示被杨文跳过的部分。 参考文献的标记均略去。 【比较开始】 一般认为，神经胶质细胞在神经系统中起着营养和支持的作用。但是近年来， 由于许多新技术的发展（包括纯化细胞培养技术、检测细胞内Ca2+变化的成像技术， 以及膜片钳技术等），日益增多的实验证据清楚地表明，胶质细胞拥有多种离子通 道、递质受体和转运体（transporter），它们通过多种方式调制神经元活动。视网 膜的Muller细胞在胶质细胞的研究中是已种重要的模型系统。本文论述Muller细胞 在调制视网膜神经元活动中所起的重要作用。 【以上这段基本没有抄袭，个别句子雷同】 一、Muller细胞的生理特性 Muller-cell physiology Muller细胞是视网膜中主要的胶质细胞，它自外界膜纵向伸展，穿过整个视网 膜至内界膜，其胞体在内核层。附图的模式图显示Muller细胞的基本特征及其与视 网膜主要神经元间的相互关系。从胞体向近端伸展的突起终止在视网膜内界毗邻玻 璃体处，在其终端呈特有的膨大的终足（endfoot）；同时又有顶突起向远端伸展， 终止在光感受器层，其主要特征是具有众多微绒毛伸入光感受器之间和周围的空间。 此外，Muller细胞又从细胞质主干伸出许多次级突起分支，形成广泛的鞘形结构， 包围神经元胞体、树突，而在视神经层则包围神经节细胞轴突（即视神经纤维）。 Muller cells are radial glidal cells which span the entire depth of the neural retina (Fig.1). They are present in the retinae of all vertebrate species. Radiating from the soma (in the inner nuclear layer) is an inwardly directed process that terminates in an expanded endfoot at the inner border of the retina, adjacent to the vitreous humor. Also projecting from the soma is an outwardly directed process that ends in the photoreceptor layer. Microvilli project from this apical process into the subretinal space surrounding the photoreceptors...... Secondary processes branching from the main trunk of Muller cells form extensive sheaths that surround neuronal cell bodies, dendrites, and, in the optic-fiber layer, the axons of ganglion cells. 早在70年代，就有不少实验表明，Muller细胞参与了视网膜电图（ERG）b波的 形成，即光诱发的胞外K+的增加引起K+流入Muller细胞，使之去极化；而几乎所有 K+电流均从细胞终足区流出。这种K+电流是ERG b波的起源。这些结果是Muller细胞 在视网膜的信息传递中起作用的最初证据。 【以上这段没有抄袭】 近年的研究显示，Mulller细胞表达多种电压门控离子通道，特别是内向整流K+ 通道。这些通道在细胞表面分布甚不均匀，它们决定了Muller细胞的极低的膜电阻 (10 ～21 MΩ)。此外，Muller 细胞还拥有延迟整流、快失活和Ca2+激活的K+通道， Na+通道，以及钙通道，由于这些通道的电导比内向整流K+通道小得多，它们可能并 不显著调制细胞的膜电位。虽然Muller细胞表达Na+通道，但其密度太低不足以产生 动作电位。进而，Muller细胞还表达多种递质受体（GABA受体和谷氨酸受体）、相应 的高亲和性递质转运体。它们也拥有一些已证明具有重要生理功能的结构，如酸碱转 运系统，包括产电Na+-HCO-协同转运系统、阴离子交换器（exchanger）、Na+-H+交 换器，以及在pH调节中起重要作用的碳酸酐酶。上述这些生理特性正是Muller细胞 在视网膜功能中发挥作用的基础。 Muller cells, like other glidal cells, express a wide variety of voltage- gated ion channels. Their membrane conductance is dominated by inward- rectifier K+ channels, which give these cells an extremely low membrance resistance, ranging from 10 to 21 MΩ in different species. The inward- rectifier K+ channels are distributed in a highly non-uniform manner over the cell surface. ...... Muller cells of various species also possess delayed-rectifier, fast inactivating and Ca2+-activated K+ channels, Na+ channels, and Ca2+ channels. These channels probably do not modulate cell membrane potential significantly, however, as their cellular conductances are much smaller than that of the K+ inward-rectifier channel. ......Muller cells also express many types of neurotransmitter receptors, including a GABA receptor and several types of gultamate receptors. They possess high- affinity uptake carriers for glutamate and GABA. Muller cells of the salamander express a number of acid-base transport systems, including an electrogenic Na+-HCO3- co-transporter, an anion exchanger and a Na+-H+ exchanger. They also have high levels of carbonic anhydrase, and enzyme that plays an important role in pH regulation by catalysing the hydration of CO2 to H+ and HCO-. 二、神经元对Muller细胞活动的调制 Recognition of neuronal signals by Muller Cells 神经元在活动时产生许多物质，如K+、神经递质等。K+在Muller细胞与神经元 的通讯中似乎是重要的媒介质。神经元兴奋时细胞外K+浓度（[K+]o）的升高导致Muller 细胞去极化，并减缓（Na+,K+）ATP酶的激活。如上所述，已证明[K+]o增加所产生 的K+重新分布是ERG 被波的起源。此外，也已证明[K+]o的增加能触发胞内Ca2+浓度 升高。进而，神经元释放的递质也通过Muller细胞的受体调制其活动。Muller细胞 的受体通常显示高亲和性，其药理性质也与神经元的相似。递质使Muller细胞去极 化，其作用既可通过直接开启离子通道（如GABA受体），也可经由第二信使系统实 现。此外，已有实验显示，簇集在Muller细胞的小突起上的Na+通道，能为邻近的 神经元的活动所激活，从而接受后者的信号，介导与神经元间的通讯。 Many substances released from active neurons including K+, neurotransmitters and metabolites (such as CO2), can potentially modulate Muller-cell behavior. Important signaling functions have been ascribed to increases in the extracellular K+ concentration ([K+]o). Increases in [K+]o result in rapid celll depolarlization in Muller cells and in slower activation of the (Na+,K+)- ATPase. ....... Enhance [K+]o has also been shown to stimulate glycogenolysis in mammalian Muller cells, and can trigger increases in the intracellular Ca2+ concentration ([Ca2+]). Neurotransmitters released from neurons constitute a second signaling mechanism by which neuronal activity can modulate Muller cells. Muller cells express a variety of receptors, including those for amino acids, catecholamines, neuroactive peptides, hormones and growth factors. Generally, these receptors display high binding affinities and pharmacological properties similar to those described in neurons. In most cases where electrophysiological studies have been performed, ligand binding elicits cell depolarization, caused by direct opening of ion channels, as in the case of the GABA receptor in the skate and baboon, or by second- messenger systems. 晚近的研究进一步表明，神经元所产生的某些代谢产物（如CO2）可以通过激活 碳酸酐酶使Muller细胞内pH迅速降低（酸化），从而使其某些功能发生改变，包括 对谷氨酸的摄取、缝隙连接的耦合等。 Active retinal neurons (particularly photoreceptors in the dark) release CO2, leading to substantial increase in extracellular Pco2. Such increases can result in rapid intracellular acidification in Muller cells, This acidification is generated largely by the action of the enzyme carbonic anhydrase, and might modulate several pH-dependent functions of the Muller cell, including carrier-mediated glutamate uptake, acid-base transport, and gap-junctional coupling. 三、Muller细胞通过改变微环境间调制神经元活动 Control of neuronal microenvironment by Muller cells 在Muller细胞与神经元之间的通讯是双向的，即Muller细胞也主动地调制神经 元的活动。这种调制可以是间接的，也可以是直接的。间接的调制是通过改变神经 元的微环境所实现。这种微环境主要是指[K+]o和递质的浓度。 Glial cells can modulate neuronal activity by controlling the concentration of neuroactive substances in the extracellular fluid bathing CNS cells. The concentrations of neurotransmitter and K+, for example, are regulated by glial-cell homeostatic mechanisms. 就[K+]o而言，光激活视网膜神经元在两个突触层(内、外网状层) 诱发[K+]o增 高。这种增高的[K+]o必须被迅速清除，否则神经元的兴奋性会发生激烈的变动。 Muller细胞通过几种机制实施其清除多余[K+]o的作用。其一是经被动和主动过程 摄取K+，并暂时储存在细胞内。其二是[K+]o增加进入Muller细胞使之去极化，从 而使K+从其它区域流出。后者实际上是一种空间性的缓冲机制，即以重新分布的方 式来降低某些部分的[K+]o。而这种机制的运转是通过内向整流K+通道进行的。在 两栖类，这些通道多位于终足区，而终足区又紧贴玻璃体，因此神经元释放的多余 的K+大部分从终足区流至玻璃体。在哺乳类情况更复杂些，多余的K+既流至玻璃体， 又流至光感受器周围空间。这种空间缓冲机制清除K+的效率比单纯经由胞外空间扩 散要高一倍以上。 Stimulation of the retina with light results in neuronal activation and increases in [K+]o in the two retinal synaptic layers (the inner and outer plesiform layers). These light-elicited increases in [K+]o must be cleared rapidly in order to limit fluctuations in neuronal excitability. Muller cells remove excess K+ from extracellular space by several mechanisms. As in other glial cells, K+ is taken up and temporarily stored in Muller cells, with influx occuring by both passive (K+ and Cl- uptake) and active [(Na+, K+)-ATPase] processes. Potassium is also removed from extracellular space by a spatial-buffering mechanism: increases in [K+]o within the two plexiform layers depolarize Muller cells and led to K+ efflux from other Muller-cell regions. The resulting K+ spatial-buffering current effectively redistributes extracellular K+ from regions where [K+]o is initially high to regions where it is low. Spatial-buffering currents pass through inward-rectifying K+ channels, ..... .In amphibian species, a large fraction of all inward-rectifying K+ channels in Muller cells is localized to the endfoot at the retinal surface. Thus K+ spatial-buffering current preferentially exits from Muller cells at the endfoot. The result of this specialized form of spatial buffereing, termed 'K+ siphoning', is that most excess K+ released by active neurons is transferred to the viterous humor, which acts as a large K+ sink. The pattern of the spatial-buffering current is more complex in mammalian species, with excess K+ directed to both the vitreous humor and the fluid space surrounding the photoreceptors (the subtrectinal space). Modeling studies of amphibian and mammalian retinae indicate that K+ siphoning is 1.6-3.7 times more effective in clearing K+ from the retina than is K+ diffusion through extracellular space. Muller细胞所拥有的高亲和性转运体能及时地清楚胞外空间中为神经元所释放 的递质，从而使递质的作用在适当时候中止，并防止递质从突触间隙中扩布出去。 谷氨酸是视网膜中由光感受器和双极细胞所释放的主要兴奋性递质。对Muller细胞 的谷氨酸转运体已进行了深入的研究。这些转运体的运转机制相当复杂，它把一个 谷氨酸分子和两个Na+摄入，同时把一个K+和一个OH-向外转运。因为转运体是产电 的，因此谷氨酸的摄取是电压依赖的，细胞去极化使摄入减慢，甚至使之逆转（即 转运出细胞）。此外，由于OH-的外运，谷氨酸摄取也依赖于pH。 Glial cells play an important role in removing neurotransmitters from extracellular space following their release from synaptic terminals. This uptake is essential for terminating synaptic transmission as well as for preventing the spread of transmitters away from the synaptic cleft. Muller cells possess high-affinity uptake carriers for many transmitters and are believed to regulate extracellular transmitter levels in the retina. ...... The high-affinity glutamate carrier of Mullers cells has been studies extensively. ......The Muller-cell glutamate carrier has a complex stoichiometry and questions concerning its details remain. In one scheme, the inward transport of one glutamate molecule and two Na+ is coupled to the outward transport of one K+ on one OH-. Because the transporter is electrogenic, glutamate uptake is voltage-dependent; cell depolarization slows down or even reverses uptake of the excitatory amino acid. In addition, because OH- is transported along with glutamate, uptake is pH dependent and is accompanied by an extracellular alkalinization. GABA是视网膜中主要的抑制性递质，大部分由水平细胞和无长突细胞所释放。 Muller细胞也具有GABA转运体，它同样也是产电的，可能把GABA和两个Na+，一个Cl- 同时向内转运。在低等脊椎动物，水平细胞本身摄取GABA，但在某些哺乳动物，水 平细胞并不摄取GABA，清除胞外GABA主要由Muller细胞完成。 Muller cells, including those of rabbit and mouse, also possess a high-affinity uptake system for GABA. ...... The transporter is electrogenic, and is believed to have a stoichiometry of two Na+ plus one Cl- plus one GABA molecule, all transported inwardly. GABA is the primary inhibitory transmitter of the retina, and is released by horizontal cells and amacrine cells. In some mammalian species, including the rabbit, horizontal cells lack a GABA transporter, and the responsibility for removing GABA from the extracellular space presumably falls largely upon Muller cells. pH对神经元活动有重要影响。例如，在蝾螈视网膜，pH值降低0.05即能使光感 受器与第二级神经元（水平细胞和双极细胞）间的突触传递降低24%。Muller细胞 通过两种方式调节胞外的pH。一是通过它所拥有的Na+-HCO-3协同转运系统，其作用 是使胞外空间变得更酸，这有助于缓冲神经元活动所致的胞外变得更碱性。另一种 方式是，Muller细胞也能清除多余的代谢产物CO2，即它所拥有的碳酸酐酶把CO2迅 速转换为HCO3-和H+，然后被转运。 【注意，此段抄袭时前后调换位置】 This pH shift is thought to be regulated by Muller cells. Salamander Muller cells possess a Na+-HCO3- co-transport system ....... the activity of the Na+-HCO3- co-transport system will acidify extracellular space. This acidification helps to muffle extracellular pH variations by partially neutralizing the neuronally generated extracellular alkalinization. Muller cells might also regulate extracellular pH in the retina by facilitating the removal of CO, produced by neuronal activity. Excess retinal CO2 will be rapidly converted to HCO3- and H+ by the enzyme carbonic, which is found both within Muller cells and on the cell suface. The HCO3- might then be transported...... Variations in pH can modulate neuronal activity as well....... For example, and acidification of 0.05 pH units in the salamander retina produces a 24% reduction in synaptic transmission between photoreceptors and second-order neurons. 四、Muller细胞直接调制神经元活动 Muller-cell modulation of neuronal activity Muller细胞的活动也能直接影响神经元。如上所述，因为谷氨酸转运体的活动 是电压依赖的，若Muller细胞去极化足够大，谷氨酸可经转运体释放至胞外空间， 这可能在病理条件下引起神经元损伤。GABA的转运也会发生类似情况。此外，在视 网膜中谷酰胺合成酶（把谷氨酸转换成谷酰胺）只有在Muller细胞中才存在，由Muller 细胞所合成的谷酰胺再循环回视网膜神经元作为递质的前体。一旦这种合成酶被抑 制，相应神经元功能则完全丧失。 Muller cells, in addition to influencing neuronal activity by regulating the levels of substances in the neuronal microenvironment, might control neuronal activity more directly. When depolarized sufficiently, glutamate uptake by salamander Muller cells is reversed and glutamte is actually released into extra-cellular space. This release might contribute to excitotoxic damage to neurons under pathological conditions. Depolarization can also lead to the release of GABA from rat Muller cells. ...... Glutamine synthetase, and enzyme that transamidate glutamate to glutamine, is localized exclusively to Muller cells in the retina. The glutamine synthesized by Muller cells is recycled back to retinal neurons, where it serves as a precursor for the synthesis of additional neurotransmitter. Inhibition of the glial enzyme in the rabbit causes a complete loss of neuronal function, demonstrating the crucial role that Muller cells play in neurotransmission in the retina. 五、Muller细胞中的Ca2+波可能做为视网膜中信号传递的第二条通路 近年的工作表明，K+诱导的去极化可能通过电压门控Ca2+通道导致Ca2+的内流， 而谷氨酸的作用则经突触后非NMDA受体介导引起Ca2+内流。在无外钙存在的情况下， 拥ryanodine刺激Muller细胞会引起Ca2+从胞内钙库释放后流至胞内不同部位，先是 在细胞顶端Ca2+浓度升高，然后以波动的形式向终足区传播。这种Ca2+波也可以为 K+、谷氨酸、ATP和咖啡因的刺激所产生。由于Muller细胞在视网膜纵向伸展，这样 的Ca2+波能否作为信号传递的另一种方式，把信息从外层视网膜向内层视网膜传递 是一个诱人的问题【方舟子按：此处注了Newmann & Reichenbach文，为全文第17篇 参考文献】。如果这一推测被证实，那么在视网膜中信号的传递将会有两条通路， 一是通常的神经元通路（光感受器->双极细胞->神经节细胞），一是经由Muller细 胞内部的钙波而实现。 Neurotransmitters and ions released by neurons can activate second-messenger systems within Muller cells. One prominent example is the elevation of [Ca2+] within Muller cells, which can be triggered by K+-induced depolrization as well as by the activation of ligand-gated receptors. ...... glutamate elicits a Ca2+ influx through non-NMDA receptors. In the absence of external Ca2+, stimulation of salamander Muller cells leads to the release of Ca2+ from internal stores and to increases in [Ca2+] which begin in the apical end of the cell and travel in a wave-like manner towards the cell endfoot. These intracellular Ca2+ waves can be stimulated by elevated K+, glutamate and ATP, as well as by caffeine and ryanodine. It is interesting to speculate that these Ca2+ waves provide a second pathway, independent of the neuronal network, for signals to be relayed from the outer to the inner retina. 六、结语 随着对Muller细胞以及其它神经胶质细胞研究的深入，认为胶质细胞在神经系 统中只是起支持和营养作用的传统观点有必要加以修正。Muller细胞可以直接或间 接影响视网膜神经元的活动，显然在视网膜功能中起重要作用。目前的研究主要集 中在对Muller细胞本身性质的分析，今后的研究重点之一应该是细致地分析这些细 胞和各种不同的视网膜神经元之间的相互作用的特点。Muller细胞中Ca2+波作为视 网膜中信号传递的第二条通路的假设是诱人的，但尚待进一步证明。 【上面这段没有抄袭】 ———————————————— 【新语丝电子文库(www.xys.org)(www.xys2.org)】