Regulation of p27Kip1 during gentamicin mediated hair cell death.

The INK4 and Kip/Cip families of Cyclin Dependent Kinase inhibitors (CKIs) are regulators of the cell cycle. In addition, CKIS including p27(Kip1) can protect cells from apoptosis in vitro. However, little is known about protective effect of p27(Kip1) in vivo. We used systemic treatment with aminoglycosides to induce hair-cell death in the basilar papilla (BP), the auditory organ of the avian inner ear, and characterised the expression of p27(Kip1) with confocal and immunofluorescence microscopy. In contrast to the adult mammalian cochlea where p27(Kip1) is expressed only in supporting cells, p27(Kip1) is found in the nuclei of both hair cells and supporting cells in the BP of the normal, mature bird. Forty-eight hours after gentamicin treatment, hair cells with TUNEL positive nuclei and hair cells with pyknotic nuclei were both detected, suggesting many hair cells die by apoptosis. When the BP was double labelled for p27(Kip1) and myosin VIIa, a hair-cell specific protein, all dying hair cells that had been ejected from the epithelium were found to be myosin VIIa positive but negative for p27(Kip1) even though nuclear remnants were still visible. In the transition zone where partial hair-cell loss occurs, freshly ejected hair cells lying immediately above the surface of the BP no longer expressed p27(Kip1). Damaged hair cells within the epithelium in the transition zone contained p27(Kip1) in their cytoplasm but not in their nuclei. These data support recent in vitro findings suggesting that p27(Kip1) protects cells from apoptosis and that its downregulation may be a general feature of programmed cell death.