Expression of P2X4 Receptors in the Auditory Sensory Hair cells

Abstract

Hearing loss is a global problem with huge impacts on society and economics. Hearing starts in the cochlea, our sensory organ for hearing, where auditory hair cells are stimulated by sound waves and transmit the information to the auditory neurons. Damage to sensory hair cells and/or auditory neurons leads to sensorineural hearing loss, an irreversible condition with no pharmacological treatment available to date. Homeostasis within the cochlea is maintained by many different cell types working together. Purinergic signalling mediated by P2X, P2Y and adenosine receptors play a pivotal role in regulating cellular physiology of hair cells and neurons (e.g. P2X2, A1, A2A). One purinergic receptor subtype whose expression and function have not been investigated is P2X4. Therefore, the aim of this study was to comprehensively characterize the expression of P2X4 isoform in the cochlea. Wistar rat (6-week-old, both genders) cochleae were collected and the P2X4 expression at the protein level was examined by immunohistochemistry. Robust P2X4 expression was found throughout the inner hair cell (IHCs) and at the apical and basal parts of the outer hair cells (OHCs) of the organ of Corti (ooC). P2X4 in both OHCs and IHCs were predominately cytoplasmic but with different subcellular distribution patterns. Notably in OHCs, P2X4 expression was concentrated as clusters under the cuticular plate. Using markers for subcellular organelles, large proportions of P2X4 immunolabelling was associated with Golgi apparatus (27%) and early endosomes (26%). In OHCs, much of P2X4 immunolabelling was associated with early endosomes (42.3%) and lysosomes (32.4%). This subcellular distribution of P2X4 featured in mature hair cells, but was not evident at younger ages (E21 – P8). Our preliminary findings showed similar P2X4 expression in the IHCs of sheep, mouse and human cochlea. This study provides a foundation for future work to understand the role of the P2X4 receptor in the cochlea which may inform novel pharmacological therapies for sensorineural hearing loss.