Role of Acid-Sensing Ion Channels in Glaucoma

描述（由申请人提供）：青光眼包括一组表现出特征性视神经变性（青光眼性视神经病变，GON）的眼部疾病，与伴随的视野丧失和通常的眼内压（IOP）升高相关。在许多个体中，IOP的降低改善了视力丧失，但许多其他人尽管压力降低仍继续丧失视力。没有直接靶向和预防GON的疗法。虽然GON的确切潜在机制正在阐明，但特定的病理生理变化（如机械创伤和缺血）最终导致视网膜神经节细胞及其轴突的死亡。我们已经将注意力转向了一个新发现的蛋白质家族;酸敏感离子通道（ASIC），它似乎在脑损伤中很重要。ASIC是质子激活的钠选择性阳离子通道，由六种亚型组成，在整个神经系统中表达。它们对酸和机械刺激有反应，对酸中毒和肿胀有反应，这是缺血性脑损伤的显著特征。最近的研究表明，兔视网膜神经元和大鼠脑表达ASIC信使RNA。因此，ASIC可以对眼内的机械刺激做出反应，这些机械刺激来自眼内压升高和/或视网膜和视神经内的缺血性变化，这两种机制通常与GON的发病机制有关。这些发现使得视网膜ASIC成为研究的有吸引力的细胞介质。我们的初步研究支持ASICs在视网膜中的作用：ASIC 2a亚型的免疫印迹分析显示，相对于年龄匹配的对照组，ASIC 2a亚型在青光眼患者组织的视网膜和视神经中的表达显著增加。脑ASIC 2a的上调对于细胞存活可能是重要的，因为表达ASIC 2a的细胞对损伤的反应较低。我们的实验室已经表明，在缺血性损伤中存活的神经元具有增加的ASIC 2a蛋白水平，这表明ASIC 2a具有神经保护作用。这一预测得到了以下观察结果的支持：当ASIC 2a与ASICla（一种涉及细胞损伤的亚型）复合时，ASIC 2a改变了离子通道活化特性。表达同源ASICla通道的细胞被低pH和模拟缺血损伤，而异源ASICla/2a通道不太容易受到损伤。生理学研究深入了解了这一发现; ASIC 2a通道的半最大激活pH（pHo.s）为4.35，ASIC 1a通道为6.2，而ASIC 1a/ASIC 2a异聚体通道为4.8。这在损伤的情况下可能是重要的，其中细胞内钙超载已被认为是神经元损伤的主要机制。这些研究的目的是充分表征ASIC 2a在正常和青光眼组织的视网膜和视神经中的表达，并确定ASIC 2a是否增强细胞对损伤的反应，并可能作为治疗或预防青光眼的治疗靶点。

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.