Role of Acid–Sensing Ion Channels in Glaucoma

Abstract

Abstract: : Purpose: Acid–sensing ion channels (ASICs) are a newly discovered family of cation channels that respond to acidic and mechanical stimuli. Brain injury studies show increased expression of the ASIC2a subtype in surviving neurons, suggesting a neuroprotective role for this ASIC. ASIC2a messenger RNA is detected in retina, thus ASIC2a could respond to ischemic and/or mechanical stimuli from increased intraocular pressure, two mechanisms implicated in the pathogenesis of glaucomatous optic neuropathy. The purpose of this study was to characterize functional ASIC2a responses in retinal ganglion cells, and to determine whether ASIC2a protein levels were altered in glaucomatous eyes. Methods: Rat retinal ganglion cells were purified by Thy–1 attachment as described (Barres et al., 1998) and analyzed by whole–cell patch–clamp recording for acid–activated currents (Chu et al., 2002). ASIC2a immunocytochemistry was performed on retinal sections of normal (n=5) and glaucomatous (n=13) human donor eyes using an ASIC2a rabbit polyclonal antibody (Alomone) and expression was quantified using the Bioquant image analysis system. Results: Patch–clamp recording of 52 purified rat retinal ganglion cells revealed that all had amiloride–sensitive acid–activated currents, while 18 had acid–activated currents that were potentiated by extracellular Zn²⁺ (100–300 µM), a feature unique to ASIC2a. ASIC2a expression was significantly higher in the retinal nerve fiber layer of human glaucomatous retina relative to age–matched controls. Analysis of patient history data shows no correlation between increased ASIC2a expression and any measurable clinical data. Conclusions: Of the six ASIC subtypes, ASIC2a is the most resistant to activation by acidosis and ASIC2a–containing protein complexes are less responsive to stimuli. The finding that ASIC2a is expressed in retinal ganglion cells and that expression is increased in glaucomatous retina suggests that ischemic or mechanical injury leads to an increase in ASIC2a–containing protein complexes in the retina, and that ASIC2a may influence cell survival in glaucomatous optic neuropathy.

Keywords: ganglion cells • ion channels • neuroprotection