Properties of ATP-gated ion channels assembled from P2X2 subunits in mouse cochlear Reissner’s membrane epithelial cells

Abstract

In the cochlea, Reissner’s membrane separates the scala media endolymphatic compartment that sustains the positive endocochlear potential and ion composition necessary for sound transduction, from the scala vestibuli perilymphatic compartment. It is known that with sustained elevated sound levels, adenosine 5ʹ-triphosphate (ATP) is released into the endolymph and ATP-gated ion channels on the epithelial cells lining the endolymphatic compartment shunt the electrochemical driving force, contributing to protective purinergic hearing adaptation. This study characterises the properties of epithelial cell P2X2-type ATP-activated membrane conductance in the mouse Reissner’s membrane, which forms a substantial fraction of the scale media surface. The cells were found to express two isoforms (a and b) of the P2X2 subunit arising from alternative splicing of the messenger RNA (mRNA) transcript that could contribute to the trimeric subunit assembly. The ATP-activated conductance demonstrated both immediate and delayed desensitisation consistent with incorporation of the combination of P2X2 subunit isoforms. Activation by the ATP analogue 2meSATP had equipotency to ATP, whereas α,β-meATP and adenosine 5′-diphosphate (ADP) were ineffective. Positive allosteric modulation of the P2X2 channels by protons was profound. This native conductance was blocked by the P2X2-selective blocker pyridoxal-phosphate-6-azophenyl-2ʹ,4ʹ-disulphonic acid (PPADS) and the conductance was absent in these cells isolated from mice null for the P2rX2 gene encoding the P2X2 receptor subunit. The activation and desensitisation properties of the Reissner’s membrane epithelial cell ATP-gated P2X2 channels likely contribute to the sensitivity and kinetics of purinergic control of the electrochemical driving force for sound transduction invoked by noise exposure.