Subcellular Localization of the P2X4 Receptor in Cochlear Sensory Hair Cells

Our sense of hearing starts in the inner ear organ, the cochlea, which contains two types of auditory hair cells for signal transduction. Earlier research showed that the complex cochlear physiology is regulated in part by purinergic signalling through activations of purine mediated P2X, P2Y and adenosine receptors expressed in the cochlea. This study aims to extend our knowledge of purinergic signalling in the cochlea by comprehensively characterizing the expression of P2X ₄ receptor subtype. Wistar rat cochlea (embryonic day 20.5–6 weeks, both sexes) were collected and the P2X ₄ expression was examined by immunohistochemistry. Robust P2X ₄ expression was found in the organ of Corti (OoC) in the inner hair cells (IHCs) and outer hair cells (OHCs), confirmed by double-labelling with HCs marker Myosin VIIa. In IHCs, a robust cytoplasmic P2X ₄ expression occurred throughout the cell body, with the most intense signal at the medial side. In OHCs, P2X ₄ formed puncta near the apical and basal ends of the cell body. Using markers for subcellular organelles, P2X4 immunoreactivity was associated mostly with the trans-Golgi network apparatus (27%) and early endosomes (26%) in IHC, and early endosomes (42.3%) and lysosomes (32.4%) in OHC in the mature cochlea. Taken together, these observations suggest unique roles for P2X ₄ in mature IHCs and OHCs as a purinergic receptor subtype responsible for homeostatic regulation of hair cells and auditory sensory transduction.