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The Journal of Neuroscience, May 1, 2003, 23(9):3616
Amiloride-Insensitive Currents of the Acid-Sensing Ion Channel-2a (ASIC2a)/ASIC2b Heteromeric Sour-Taste Receptor Channel
Shinya Ugawa1, Takashi Yamamoto3, Takashi Ueda1, Yusuke Ishida1, 2, Akira Inagaki1, 2, Makoto Nishigaki1, and Shoichi Shimada1 Departments of 1 Anatomy II and 2 Otorhinolaryngology, Nagoya City University Medical School, Nagoya 467-8601, Japan, and 3 Department of Behavioral Physiology, Graduate School of Human Sciences, Osaka University, Suita, Osaka 565-0871, Japan
Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH of
2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.
Key words: proton; sour; acid-sensing ion channel (ASIC); immunoprecipitation; electrophysiology; amiloride
Copyright © 2003 Society for Neuroscience 0270-6474/03/2393616-07$05.00/0
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