Role of Acid-Sensing Ion Channels in Glaucoma

描述(申请人提供)：青光眼包括一组眼部疾病，表现为典型的视神经变性(青光眼视神经病变，GON)，与伴随的视野丧失相关，通常还会导致眼压升高(LOP)。在许多人中，LOP的减少可以改善视力损失，但许多人在压力降低的情况下仍会继续失去视力。没有直接针对和预防Gon的治疗方法。虽然Gon的确切机制还在阐明，但特定的病理生理变化(如机械损伤和缺血)最终会导致视网膜神经节细胞及其轴突的死亡。我们将注意力集中在一个新发现的蛋白质家族上：酸敏感离子通道(ASICs)，它似乎在脑损伤中具有重要意义。ASIC是质子激活的钠离子选择性阳离子通道，由六种亚型组成，表达于整个神经系统。它们对酸性和机械刺激做出反应，对酸中毒和肿胀做出反应，这是缺血性脑损伤的显著特征。最近的研究表明，兔视网膜神经元和GILA表达ASIC信使RNA。因此，ASICs可以通过眼压升高和/或视网膜和视神经内的缺血性改变来对眼内的机械刺激做出反应，这两种机制通常与GON的发病有关。这些发现使视网膜ASIC吸引了研究的细胞介质。我们的初步研究支持ASICs在视网膜中的作用：对ASIC2a亚型的免疫印迹分析显示，与年龄匹配的对照组相比，青光眼患者组织中视网膜和视神经的表达显著增加。脑ASIC2a的上调对细胞生存可能很重要，因为表达ASIC2a的细胞对损伤的反应较弱。我们的实验室已经证明，在缺血性损伤中幸存下来的神经元可以增加ASIC2a蛋白的水平，这表明ASIC2a具有神经保护作用。这一预测得到了以下观察的支持：当ASIC2a与ASIC1a复合时，它改变了离子通道的激活属性，ASIC1a是与细胞损伤有关的一个亚型。表达同源ASICla通道的细胞在低pH和模拟缺血时受到损伤，而异构体ASICla/2a通道较不容易受到损伤。生理学研究证实了这一发现：ASIC2a通道的半最大激活(pHo.s)为4.35，ASIC1a通道为6.2，而ASIC1a/ASIC2a异构体通道为4.8。这在损伤的背景下可能是重要的，因为细胞内钙超载被认为是神经元损伤的主要机制。这些研究的目的是全面研究ASIC2a在正常和青光眼组织的视网膜和视神经中的表达，并确定ASIC2a是否可以提高细胞在损伤后的存活，并可能成为治疗或预防青光眼的靶点。

DESCRIPTION (provided by applicant): Glaucoma comprises a group of ocular disorders that exhibit characteristic optic nerve degeneration (glaucomatous optic neuropathy, GON), associated with concomitant visual field loss, and often, increased intraocular pressure (lOP). In many individuals, reduction of lOP ameliorates vision loss, but many others continue to lose vision despite pressure lowering. There are no therapies that directly target and prevent GON. While the exact underlying mechanisms of GON are being elucidated, specific patho-physiological changes (such as mechanical trauma and ischemia) ultimately lead to the death of retinal ganglion cells and their axons. We have directed our attention to a newly discovered family of proteins; acid-sensing ion channels (ASICs), which appear to be significant in brain injury. The ASICs are proton-activated sodium selective cation channels comprised of six subtypes that are expressed throughout the nervous system. They respond to acidic and mechanical stimuli activating in response to acidosis and swelling, notable features of ischemic brain injury. Recent studies reveal that rabbit retinal neurons and gila express ASIC messenger RNA. Thus ASICs could respond to mechanical stimulation in the eye from increased intraocular pressure and/or ischemic changes within the retina and optic nerve, two mechanisms commonly implicated in the pathogenesis of GON. These findings make retinal ASICs attractive cellular mediators for investigation. Our preliminary studies support a role of ASICs in the retina: immunoblot analysis of the ASIC2a subtype reveals a dramatic increase in expression in the retina and optic nerve of glaucomatous human tissue relative to age-matched controls. Upregulation of brain ASIC2a may be important for cell survival as cells expressing ASIC2a are less responsive to injury. Our laboratory has shown that neurons that survive an ischemic insult have increased levels of ASIC2a protein, suggesting a neuroprotective role for ASIC2a. This prediction is supported by the observation that ASIC2a alters the ion channel activation properties when it is complexed with ASICla, a subtype that is implicated in cell injury. Cells expressing homomeric ASICla channels are injured by low pH and modeled ischemia, while heteromeric ASICla/2a channels are less vulnerable to injury. Physiological studies lend insight into this finding; the pH of half-maximal activation (pHo.s) for ASIC2a channels is 4.35, for ASICla channels is 6.2, whereas ASICla/ASIC2a heteromeric channels is 4.8. This could be significant in the context of injury, where intracellular calcium overload has been implicated as a primary mechanism of neuronal injury. The goal of these studies is to characterize fully the expression of ASIC2a in the retina and optic nerve of normal and glaucomatous tissue, and to determine whether ASIC2a enhances cell survival in response to injury and could potentially serve as a therapeutic target for the treatment or prevention of glaucoma.