RPE Lactate Transporters: A Role in Retinal Survival

描述（由申请人提供）：视网膜色素上皮（RPE）是外层视网膜的守门人，控制外层视网膜和脉络膜血液供应之间的离子、代谢物和液体的运动。光转导的能量需求通过糖酵解和氧化途径的葡萄糖代谢来满足。神经视网膜是体内代谢最活跃的组织之一，通过有氧糖酵解产生大量乳酸，并通过呼吸产生CO2。由于感光细胞功能依赖于严格的pH调节，因此必须从视网膜下腔（SRS）中清除乳酸盐。我们的研究表明，RPE表达两种质子偶联单羧酸转运蛋白，其促进乳酸从视网膜下腔到脉络膜的跨上皮运动;顶膜中的MCT 1和基底外侧膜中的MCT 3。这两种转运蛋白都与CD 147相连，CD 147是一种辅助蛋白，用于正确转运至质膜。靶向缺失编码CD 147（Bsg）基因的小鼠由于视网膜中MCT 1、MCT 3和MCT 4表达缺失而导致视觉功能和视网膜完整性降低。这些转运蛋白的丢失抑制糖酵解并破坏pH稳态，从而导致视网膜电图（ERG）降低和感光细胞死亡。在Mct 3-/-小鼠中，虽然表型不如Bsg-/-小鼠严重，但观察到视网膜乳酸水平增加和光刺激感光器响应减少。总体而言，这些结果表明，MCT通过调节SRS的pH值和渗透压在维持视觉功能方面发挥重要作用。因此，我们建议：1）检验MCT有助于外视网膜代谢和pH稳态的假设，2）检验碳酸氢盐转运蛋白增强RPE中跨上皮乳酸盐转运的假设，3）检验MCT的C末端胞质尾具有结合基序的假设，该结合基序将其组装成大分子复合物并稳定其极化分布、稳定性和转运蛋白活性。这些研究结果将阐明调节RPE中代谢转运蛋白的表达、极化运输和协调活动的分子机制。这些研究结果应该进一步了解代谢转运蛋白在维持正常视觉功能中发挥的多方面作用，以及这些转运蛋白的表达或活性变化如何导致眼睛的退行性异常。 公共卫生相关性：视网膜具有极高的新陈代谢率，并产生大量的乳酸盐。由于酸性pH抑制了感光细胞对光的反应能力，因此乳酸盐必须被运输出视网膜以维持正常的视觉功能。视网膜色素上皮形成外部血-视网膜屏障，并执行视网膜的许多基本功能。其中之一是将乳酸盐、质子和水运输出视网膜以通过脉络膜血管移除。我们实验室以前的研究已经确定了RPE中的两种乳酸转运蛋白，它们可能控制乳酸从视网膜中的运动。目前研究的目的是了解这些转运蛋白的功能活性是如何调节的，因为不能有效地从视网膜中清除乳酸会导致视觉细胞变性和失明。

DESCRIPTION (provided by applicant): The retinal pigment epithelium (RPE) is the gatekeeper of the outer retina and controls the movement of ions, metabolites, and fluid between the outer retina and the choroidal blood supply. The energy demands of phototransduction are met through glucose metabolism via glycolytic and oxidative pathways. The neural retina is one of the most metabolically active tissues in the body and produces large quantities of lactate via aerobic glycolysis and CO2 via respiration. Because photoreceptor cell function is dependent on strict pH regulation, it is essential that lactate be removed from the subretinal space (SRS). Our studies have shown that the RPE expresses two proton-coupled monocarboxylate transporters that facilitate the trans-epithelial movement of lactate from the subretinal space to the choroid; MCT1 in the apical membrane and MCT3 in the basolateral membrane. Both transporters are linked to CD147, an accessory protein required for proper trafficking to the plasma membrane. Mice with targeted deletion of the gene encoding CD147 (Bsg) have reduced visual function and retinal integrity due to a loss of expression of MCT1, MCT3, and MCT4 in the retina. Loss of these transporters inhibits glycolysis and disrupts pH homeostasis accounting for the reduced electroretinograms (ERGs) and photoreceptor cell death. In the Mct3-/- mouse, while the phenotype is less severe than that of the Bsg-/- mouse, increased retinal lactate levels and reduced light-stimulated photoreceptor responses are observed. Overall, these findings demonstrate that MCTs play an essential role in maintaining visual function by regulating the pH and the osmolarity of the SRS. Therefore, we propose: 1) To test the hypothesis that MCTs contribute to metabolic and pH homeostasis of the outer retina, 2) To test the hypothesis that bicarbonate transporters enhance trans-epithelial lactate transport in the RPE, 3) To test the hypothesis that C-terminal cytoplasmic tails of MCTs have binding motifs that orchestrate their assembly into macromolecular complexes and stabilize their polarized distribution, stability and transporter activity. The results of these studies will elucidate the molecular mechanisms that regulate the expression, polarized trafficking and coordinated activities of metabolic transporters in the RPE. These findings should further our understanding of the multifaceted roles that metabolic transporters play in maintaining normal visual function and how changes in expression or activity of these transporters could lead to degenerative abnormalities in the eye. PUBLIC HEALTH RELEVANCE: The retina has an extremely high rate of metabolism and produces large quantities of lactate. Since acidic pH inhibits the ability of photoreceptor cells to respond to light, lactate must be transported out of the retina to maintain normal visual function. The retinal pigment epithelium forms the outer blood-retinal barrier and performs many essential functions for the retina. One of these is to transport lactate, protons and water out of the retina to be removed by the choroidal blood vessels. Previous studies from our laboratory have identified two lactate transporters in the RPE that are likely to control lactate movement out of the retina. The purpose of the current studies is to understand how the functional activity of these transporters is regulated since failure to efficiently remove lactate from the retina leads to degeneration of the visual cells and blindness.