Damage-induced signalling mechanisms in the neonatal rat cochlea

Sound overstimulation and exposure to ototoxic drugs damage cochlear hair cells (HCs) and cause their death. The surrounding support cells maintain an epithelial barrier and the appropriate physiological environment for surviving HCs during pathological conditions. Coordination of this homeostatic process requires cellular signalling. However, the signalling events that are activated during damage in the mammalian cochlea, are poorly understood. Neonatal rat cochlear explants were subjected to mechanical damage or exposed to neomycin an ototoxin. Mechanical damage triggered the immediate propagation of an intercellular wave of increased intracellular Ca2+ from the lesion site into distinct cochlear regions. The properties of the Ca2+ wave and the source of Ca2+ required were specific to the cochlear region. IP3-mediated release from intracellular stores and influx of extracellular Ca2+ contribute differentially to the rise in intracellular Ca2+. The release of extracellular ATP is crucial for the propagation of the damageinduced Ca2+ wave. Gap junctions or connexin hemichannels also contribute to its formation. A subsequent damage-induced signalling event is the transient phosphorylation of ERK1/2 that arises within minutes of the insult occurring in support cells specifically. Similarly to the formation of the Ca2+ wave, release of extracellular ATP and gap junctions are critical for ERK1/2 activation. UTP-induced activation of ERK1/2 reveals the involvement of P2Y receptors. In addition, a requirement for the influx of extracellular Ca2+ also suggests a role for ion channels potentially P2X receptors. P2 X2 , 3 ,4 and P2 Y2 ,4 ,1 1 receptors were expressed in cochlear explants with P2 X2 and P2 Y2 being exclusive to support cells. Damage-induced currents were recorded from Deiters’ cells in a syncytium during mechanical damage of the cochlea. Finally, when HCs were specifically targeted using neomycin, ERK1/2 activation occurred in support cells surrounding pyknotic HC nuclei. Inhibition of ERK1/2 delayed HC death.