Role of Acid-Sensing Ion Channels in Glaucoma

描述（由申请人提供）：青光眼包括一组眼部疾病，表现为特征性视神经变性（青光眼性视神经病变，GON），伴有视野丧失，通常伴有眼压升高（lOP）。在许多个体中，降低lOP可改善视力丧失，但仍有许多人在血压降低后继续丧失视力。目前还没有直接针对和预防肾上腺素瘤的治疗方法。虽然神经节细胞的确切潜在机制正在阐明，但特定的病理生理变化（如机械性创伤和缺血）最终导致视网膜神经节细胞及其轴突死亡。我们把注意力转向了一个新发现的蛋白质家族；酸感离子通道（asic），这在脑损伤中似乎很重要。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。它们对酸性和机械刺激作出反应，对酸中毒和肿胀作出反应，这是缺血性脑损伤的显著特征。近年来的研究表明，兔视网膜神经元和胶质细胞表达ASIC信使RNA。因此，asic可以对眼内眼压升高和/或视网膜和视神经内缺血性改变的机械刺激作出反应，这两种机制通常与GON的发病机制有关。这些发现使视网膜asic有吸引力的细胞介质的研究。我们的初步研究支持asic在视网膜中的作用：ASIC2a亚型的免疫印迹分析显示，相对于年龄匹配的对照组，ASIC2a亚型在青光眼人体组织的视网膜和视神经中的表达显著增加。脑ASIC2a的上调可能对细胞存活很重要，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，在缺血损伤中存活下来的神经元ASIC2a蛋白水平增加，这表明ASIC2a具有神经保护作用。这一预测得到了ASIC2a与ASICla（一种与细胞损伤有关的亚型）复合时改变离子通道激活特性的观察结果的支持。表达同质ASICla通道的细胞受到低pH和模拟缺血的损伤，而异质ASICla/2a通道不易受到损伤。生理学研究为这一发现提供了见解；ASIC2a通道的半最大激活pH值为4.35，ASICla通道的半最大激活pH值为6.2，ASICla/ASIC2a异质通道的半最大激活pH值为4.8。这在损伤的背景下可能是重要的，其中细胞内钙超载被认为是神经元损伤的主要机制。这些研究的目的是充分表征ASIC2a在正常和青光眼组织视网膜和视神经中的表达，并确定ASIC2a是否能增强细胞在损伤反应中的存活，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.