Mammalian ASIC2a and ASIC3 subunits co-assemble into heteromeric proton-gated channels sensitive to Gd3+

Proton receptors of the acid-sensing ion channel (ASIC) family are expressed in sensory neurons and thus could play a critical role in the detection of noxious acidosis. To investigate the subunit composition of native ASICs in peripheral and central neurons, we coinjected human as well as rodent ASIC2a and ASIC3 subunits in Xenopus oocytes, The amplitudes of acid-induced biphasic responses mediated by co-expressed ASIC2a and ASIC3 subunits were much larger (as much as 20-fold) than the currents mediated by the respective homomers, clearly indicating functional association. The reversal potential of the ASIC2a+3 current (greater than or equal to+20 mV) reflected a cationic current mainly selective for sodium, The sensitivity to pH or amiloride of single versus co-expressed ASIC subunits was not significantly different; however, gadolinium ions inhibited ASICS and ASIC2a+3 responses with much higher potency (IC50 similar to 40 mu M) than the ASIC2a response (IC50 greater than or equal to 1 mM). Biochemical interaction between ASIC2a and ASIC3 subunits was demonstrated by co-purification from transfected human embryonic kidney (HEK293) cells and Xenopus oocytes. Our in situ hybridization data showed that rat ASIC2a and ASIC3 transcripts are colocalized centrally, whereas reverse transcription-polymerase chain reaction data led us to detect co-expression of human ASIC2a and ASIC3 subunits in trigeminal sensory ganglia, brain, and testis where they might coassemble into a novel subtype of proton-gated channels sensitive to gadolinium.