Apoptotic death of hair cells in mammalian vestibular sensory epithelia.

Hair cell death was examined in cultured explants of vestibular organs from mature guinea pigs and gerbils. The effects of gentamicin were compared with those of staurosporine, a membrane-permeable kinase inhibitor that induces programmed cell death in almost all cell types. Under the conditions used staurosporine killed hair cells but supporting cells appeared unaffected, and a topographic pattern of differential sensitivity to staurosporine amongst hair cells, similar to that described for aminoglycoside antibiotics, was revealed. This suggests such differential sensitivity is an inherent property of the hair cell population. Thin sectioning, and examination of whole mount preparations after application of the TUNEL procedure or after double fluorescent labelling with phalloidin and with propidium iodide, which labels nuclei, revealed that hair cells after exposure to gentamicin show features identical to those of apoptotic cells after exposure to staurosporine. Furthermore, cells showing features of apoptosis constitute a major proportion of the hair cells that are ultimately lost following exposure to gentamicin. Incubation of cultures with gentamicin in the presence of broad-spectrum inhibitors of caspases, proteases involved specifically in the cell death pathway, prevented almost all of the hair cell deaths normally triggered by gentamicin. This confirms that apoptosis is the predominant mode of hair cell death after gentamicin exposure. Hair cells exposed to gentamicin in the presence of caspase inhibitors appeared to be preserved intact. This, and the thin section observations, suggests that apoptotic death is the fate of the majority of hair cells affected by that drug and that any sub-lethal damage to hair cells exposed to gentamicin does not result in significant morphological alterations. Hair cell death was also prevented by deferoxamine which has been shown to protect cochlear hair cells in vivo from the effects of gentamicin. Explant cultures of mature vestibular organs may be, therefore, a useful model system for examining putative hair cell protecting agents.