Delivery of Glutathione to the Lens Nucleus
Delivery of Glutathione to the Lens Nucleus
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将谷胱甘肽输送至晶状体核
DOI：
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发表时间：
2014
影响因子：
2
通讯作者：
M. Srinivas
中科院分区：
文献类型：
--
作者：
M. Srinivas
谷胱甘肽（GSH）是一种阴离子三肽，是透镜中的主要抗氧化剂，其功能是将蛋白质硫醇保持在还原状态。1 -4在透镜的上皮和外皮质中，GSH水平都很高，其中抗氧化剂通过g-谷氨酰半胱氨酸合成酶和谷胱甘肽合成酶从氨基酸半胱氨酸、谷氨酸和甘氨酸合成。外纤维细胞中的还原环境也通过谷胱甘肽还原酶从GSSG再生GSH来维持，谷胱甘肽还原酶利用经由己糖激酶旁路途径产生的NADPH作为辅因子。1 -4相反，由于参与其合成和再生的酶的低比活性，透镜核中的GSH水平低80-90%。GSH水平的这种区域差异与年龄有关。细胞核中GSH水平的年龄依赖性降低以及由此导致的GSSG增加导致蛋白质中半胱氨酸和蛋氨酸残基的氧化以及蛋白质混合二硫化物（PSSG）的形成，并最终导致白内障的丧失。1 -4 GSH水平的这种下降被认为是年龄相关性核性白内障形成的关键起始因素，这是世界上失明的主要原因。
谷胱甘肽是如何运输到透镜核的，以及为什么这个过程随着年龄的增长而中断，目前还不清楚。根据一种观点，GSH是通过透镜微循环驱动的细胞外途径转运的。5迄今为止研究的所有脊椎动物晶状体都有一个循环离子电流，从晶状体的两极进入，从赤道离开。6这种内部循环电流主要由钠离子携带，沿着细胞间的细胞外间隙进入透镜。在穿过透镜内部的纤维细胞膜后，它通过间隙连接介导的途径从细胞流向细胞表面。赤道处的高浓度间隙连接通道允许细胞内电流被引导至表面细胞，其中Na+/K+泵位于上皮细胞中，将钠转运出晶状体。6循环离子电流产生通过透镜的流体流动，最近对脊椎动物晶状体细胞内流体静压的测量表明，葡萄糖和其他营养物质通过对流进入细胞外空间在纤维细胞周围，摄取转运蛋白允许它们递送到纤维细胞中。抗氧化剂如GSH（和抗坏血酸）被认为以类似的方式被运送到透镜核心。5然而，尚未确定细胞核中GSH的特异性摄取转运蛋白。此外，还不能确定循环系统输送足够抗氧化剂的能力是否随着年龄的增长而降低。
Truscott和其他人提出的另一种观点是，GSH向细胞核的转运是通过代谢物从外皮层经间隙连接扩散而发生的。8 GSH从外周向中心扩散的屏障显示出随着年龄的增长而发展，导致GSH向透镜核的输送减少。8这一观点基于以下研究，其中晶状体与35 S半胱氨酸一起孵育，随时间跟踪标签在透镜内的移动。半胱氨酸标记物的移动发生在沿着赤道面纤维细胞的长度方向，其中间隙连接分布在赤道面。8从外皮层到细胞核的大浓度梯度有利于GSH通过间隙连接的扩散。有利于阴离子GSH扩散到核心的另一个因素是，与皮质中的细胞相比，细胞核中的纤维细胞的静息膜电位更正。我们最近使用电生理学方法的研究表明，纤维细胞中的间隙连接，这是由两个连接蛋白亚型，Cx46和Cx 50，是可渗透的大阴离子，包括GSH（PNa：PGSH ~12：1，未发表的数据），这表明这种细胞内途径的贡献可能是显着的。GSH扩散的障碍，发生与年龄的发展可能是由耦合电导的年龄依赖性减少解释。
有必要进行额外的研究来评估透镜间隙连接对GSH的渗透性的体内意义。在不存在连接蛋白介导的偶联（例如敲除晶状体中的Cx46和Cx 50）的情况下，测量细胞核中的抗氧化剂水平可能会澄清透镜核心中抗氧化剂的足够水平的维持是否取决于间隙连接偶联。细胞外途径的贡献也值得进一步研究。由于透镜微循环导致的大的流体静压梯度的存在将倾向于对抗有利于扩散的浓度梯度和电压梯度。因此，在Na+/K+泵抑制剂的存在下，检查GSH在透镜中的空间分布，这最终驱动透镜微循环，对于确定流体静压梯度和细胞外途径的作用是必要的。了解GSH转运的潜在机制将有助于开发治疗药物，增加向透镜核输送重要的抗氧化剂，并可能延迟年龄相关性核性白内障的进展。
Glutathione (GSH), an anionic tripeptide, is the principal antioxidant in the lens where it functions to maintain protein thiols in a reduced state.1-4 Levels of GSH are high in both the epithelium and the outer cortex of the lens where the anti-oxidant is synthesized from the amino acids cysteine, glutamate and glycine by g-glutamylcysteinesynthetase and glutathione synthetase. A reducing environment in the outer fiber cells is also maintained by regeneration of GSH from GSSG by the enzyme glutathione reductase which utilizes NADPH produced via the hexokinase shunt pathway as a cofactor.1-4 In contrast, levels of GSH are 80-90% lower in the nucleus of the lens owing to the low specific activity of enzymes involved in its synthesis and regeneration. This regional difference in GSH levels worsens with age. The age-dependent reduction in GSH levels in the nucleus and the resulting increase in GSSG lead to oxidation of cysteine and methionine residues in proteins and formation of protein mixed disulfides (PSSG) and eventually, loss of transparency.1-4 This decline in GSH levels is believed to be the key initiating factor in the formation of age-related nuclear cataracts, the leading cause of blindness in the world.
How GSH is transported to the lens nucleus and why this process is disrupted with age is not clear. According to one view, GSH is transported by an extracellular pathway driven by the lens microcirculation.5 All vertebrate lenses studied to date have a circulating ionic current that enters at the poles and exits at the equator of the lens.6 This internal circulating current is primarily carried by sodium ions, and enters the lens along the extracellular spaces between cells. After crossing the fiber cell membranes in the lens interior, it flows from cell to cell towards the surface through gap junction mediated pathways. The high concentration of gap junction channels at the equator allows the intracellular current to be directed to surface cells where Na+/K+ pumps are located in epithelial cells transporting sodium out of the lens.6 The circulating ionic current generates fluid flow through the lens, as indicated by recent measurements of intracellular hydrostatic pressure in vertebrate lenses.7 Glucose and other nutrients are convected into extracellular spaces surrounding fiber cells, where uptake transporters allow their delivery into fiber cells. Anti-oxidants such as GSH (and ascorbic acid) are believed to be carried to the lens core in a similar fashion.5 However, no uptake transporters specific for GSH in the nucleus have yet been identified. In addition, it is not established whether the ability of the circulation system to deliver sufficient antioxidants is reduced with age.
An alternate view advanced by Truscott and others is that the transport of GSH to the nucleus occurs by diffusion of the metabolite from the outer cortex via gap junctions.8 A barrier to diffusion of GSH from the periphery to the center was shown to develop with age, leading to a reduction in the delivery of GSH to the lens nucleus.8 This view was based on studies wherein lenses were incubated with 35S cysteine and movement of the label within the lens was followed over time. The movement of cysteine label occurred along the length of the fiber cells in the equatorial plane where gap junctions are distributed.8 Diffusion of GSH via gap junctions is favored by the large concentration gradient from the outer cortex to the nucleus. Another factor favoring diffusion of anionic GSH to the core is that the resting membrane potential of fiber cells in the nucleus is more positive as compared to that of cells in the cortex. Our recent studies using electrophysiological methods indicate that gap junctions in fiber cells, which are formed by two connexin isoforms, Cx46 and Cx50, are permeable to large anions including GSH (PNa:PGSH ~12:1, unpublished data), suggesting that the contribution of this intracellular pathway is likely to be significant. The development of a barrier to GSH diffusion that occurs with age might be explained by the age-dependent reduction in coupling conductance.
Additional studies are necessary to assess the in vivo significance of permeability of lens gap junctions to GSH. Measurement of levels of the anti-oxidant in the nucleus in the absence of connexin mediated coupling, e.g. Cx46 and Cx50 in knockout lenses, might clarify whether maintenance of adequate levels of the anti-oxidant in the lens core depends on gap junctional coupling. The contribution of the extracellular pathway also warrants additional study. The presence of a large hydrostatic pressure gradient due to the lens microcirculation would tend to oppose the concentration- and voltage-gradients favoring diffusion. Thus, examining the spatial distribution of GSH in the lens in the presence of inhibitors of the Na+/K+ pump, which ultimately drives the lens microcirculation, is necessary to define the role of hydrostatic pressure gradient and the extracellular pathway. Understanding the underlying mechanisms of GSH transport will aid in the development of therapeutic agents that increase delivery of the vital antioxidant to the lens nucleus and possibly delay the progression of age-related nuclear cataracts.
影响因子：
2.3
作者：
Giblin, FJ
通讯作者：
Giblin, FJ