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 prot ective effect of p27(Kip1) in vivo. We used systemic treatment with aminogl ycosides to induce hair-cell death in the basilar papilla (BP), the auditor y organ of the avian inner ear, and characterised the expression of p27(Kip 1) with confocal and immunofluorescence microscopy. In contrast to the adul t 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 trea tment, hair cells with TUNEL positive nuclei and hair cells with pyknotic n uclei 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 spec ific protein, all dying hair cells that had been ejected from the epitheliu m were found to be myosin VIIa positive but negative for p27(Kip1) even tho ugh nuclear remnants were still visible. In the transition zone where parti al hair-cell loss occurs, freshly ejected hair cells lying immediately abov e the surface of the BP no longer expressed p27(Kip1). Damaged hair cells w ithin the epithelium in the transition zone contained p27(Kip1) in their cy toplasm but not in their nuclei. These data support recent in vitro finding s suggesting that p27(Kip1) protects cells from apoptosis and that its down regulation may be a general feature of programmed cell death.