ATP-gated ion channels mediate adaptation to elevated sound levels

Abstract

The sense of hearing is remarkable for its auditory dynamic range, which spans more than 10 12 in acoustic intensity. The mechanisms that enable the cochlea to transduce high sound levels without damage are of key interest, particularly with regard to the broad impact of industrial, military, and recreational auditory overstimulation on hearing disability. We show that ATP-gated ion channels assembled from P2X 2 receptor subunits in the cochlea are necessary for the development of temporary threshold shift (TTS), evident in auditory brainstem response recordings as sound levels rise. In mice null for the P2RX2 gene (encoding the P2X 2 receptor subunit), sustained 85-dB noise failed to elicit the TTS that wild-type (WT) mice developed. ATP released from the tissues of the cochlear partition with elevation of sound levels likely activates the broadly distributed P2X 2 receptors on epithelial cells lining the endolymphatic compartment. This purinergic signaling is supported by significantly greater noise-induced suppression of distortion product otoacoustic emissions derived from outer hair cell transduction and decreased suprathreshold auditory brainstem response input/output gain in WT mice compared with P2RX2-null mice. At higher sound levels (≥95 dB), additional processes dominated TTS, and P2RX2-null mice were more vulnerable than WT mice to permanent hearing loss due to hair cell synapse disruption. P2RX2-null mice lacked ATP-gated conductance across the cochlear partition, including loss of ATP-gated inward current in hair cells. These data indicate that a significant component of TTS represents P2X 2 receptordependent purinergic hearing adaptation that underpins the upper physiological range of hearing.noise-induced hearing loss | acoustic overstimulation | permanent threshold shift | auditory neurotransmission | sound transduction S ensory systems are characterized by adaptation processes that sustain transduction as stimulus intensity increases. The mammalian auditory system operates across an acoustic power range of 120 dB, measured on the logarithmic decibel scale. The mechanism for the extraordinary acuity of the cochlea (recalling the ageold adage of "hearing a pin drop") arises from the commitment of 75% of the sensory hair cells, the outer hair cells, to electromechanical (reverse) transduction, driving a "cochlear amplifier." The nonlinear outer hair cell reverse transduction provides an ∼40-dB gain at hearing threshold, reducing to zero as sound levels rise (1). A major challenge for auditory physiology is to understand how hearing is preserved in the face of acoustic overstimulation, as noise can damage the cochlea and can greatly exacerbate hearing loss with aging (2). Given the recent propensity for direct delivery of high-level recreational sound to the ear canals by personal music players and, more broadly, the impact on our hearing of noise from industrial and military environments, there is an imperative to better understand the intrinsic mechanisms that enable the cochlea to accommodate lo...

Purinergic control of lysenin’s transport and voltage-gating properties

“…Burnstock's early hypothesis that ATP, the major intracellular molecule providing the energy required for multiple biochemical and biophysical processes, may actually function as an extracellular non-adrenergic and noncholinergic signaling molecule [12,13] was received with great skepticism [14,15]. After several decades of extensive work, the scientific community came to the realization that ATP is widely employed as a signaling molecule in multiple biological processes in both normal and pathophysiological conditions [9,11,[16][17][18][19][20][21][22][23][24]. The rapid progress in understanding and deciphering multiple molecular mechanisms of signaling revealed that ATP is a potent mediator of multiple signaling cascades, which may act through binding to, and nonhydrolytic activation of, P2X ionotropic receptors or G Electronic supplementary material The online version of this article (doi:10.1007/s11302-016-9520-9) contains supplementary material, which is available to authorized users.…”

“…Although multiple past studies focused on understanding the implications of ATP-controlled signaling with respect to endogenous transmembrane transporters such as ion channels [17,19,20,24,[28][29][30][31], there is a recent interest in understanding how ATP controls the lytic action of pore-forming toxins (PFTs) [22,26,27,[32][33][34]. PFTs introduce unregulated conducting pathways into the host cell plasmalemma [35][36][37][38][39], which is expected to yield direct cytolysis.…”

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

“…Whole-cell patch clamp analysis showed that the sustained inward currents elicited by ATP application was absent from P2rX2 −/− inner and outer hair cells as well as RM epithelial cells, indicating that this conductance arose from a homomeric P2X 2 trimeric subunit configuration of the ATP-gated ion channels. Extending from this, the P2rX2 null mouse also lacked the extracellular fall in endocochlear potential and associated decrease in cochlear partition resistance that is evident when picolitres of ATP is injected into the endolymphatic compartment [7]. This finding suggests that all of the ATP-activated membrane conductance in cell lining that cochlear compartment is attributable to P2X 2 -type channels.…”

“…In the cochlea, ATP influences sound transduction, the endocochlear potential and neurotransmission, and underlies an intrinsic purinergic P2X 2 receptor-based humoral hearing adaptation mechanism that reduces hearing sensitivity as sustained sound levels increase [7]. However, native ATPinduced responses in many cochlear cells have not correlated well with known recombinant P2X receptor combinations [8][9][10][11][12][13].…”

“…We have previously reported that the ATP-gated conductance in the cochlear partition arising from a range of epithelial cells facing scala media (endolymphatic compartment) was absent in the P2rX2 null mouse [7]. Whole-cell patch clamp analysis showed that the sustained inward currents elicited by ATP application was absent from P2rX2 −/− inner and outer hair cells as well as RM epithelial cells, indicating that this conductance arose from a homomeric P2X 2 trimeric subunit configuration of the ATP-gated ion channels.…”