Hair cell death in the avian basilar papilla:: Characterization of the in vitro model and caspase activation

Caspases are a family of proteases that have been implicated as key mediators of cell death. Although nonspecific inhibition of caspase activation has been reported to prevent mammalian sensory hair cell death, the exact roles of individual caspases during hair cell death are unclear. In other systems, the activation of initiator caspases, such as caspase-8 and caspase-9, can lead to the activation of the effector caspase-3. We have begun to systematically characterize hair cell death in an in vitro system by examining the activation of these specific caspases in degenerating hair cells after acutely damaging the whole avian basilar papilla with gentamicin. Basilar papillae (BP) displayed a dose-dependent hair cell loss after a 24-h treatment with gentamicin at concentrations of 0.1, 0.5, and 2.0 mM. When treated with 0.5 mM gentamicin for 6, 12, or 24 h, hair cells first began to degenerate in the basal third of the BP and damage progressed apically. Supplementation of z-VAD-fmk, a general caspase inhibitor, provided short-term protection against gentamicin-induced hair cell death. Treatment with gentamicin for 6 or 12 h promoted the expression of active caspase-3 and active caspase-9 in many hair cells along the BP as shown by immunohistochemistry. At these time-points, specific fluorescent-labeled peptide substrates detected more active caspase-3, caspase-8, and caspase-9 in gentamicin-treated hair cells relative to controls. Our data indicate that auditory hair cells degenerate as a result of gentamicin exposure in a caspase-dependent manner. Specifically, the upstream caspases, caspase-8 and caspase-9, and the downstream caspase-3 are activated in aminoglycoside-damaged hair cells.