Effects of protein kinase inhibitors on hair bundle structure

The sensory hair bundles of the cochlea are complex structures that are vital for hearing. Phosphorylation plays an important role in regulating actin in many cell types, but the role it plays in regulating the development and maintenance of the hair bundle has yet to be fully elucidated. Furthermore, the extent to which damaged hair bundles can be repaired in the mammalian cochlea remains to be determined. The aims of this study were twofold: 1) to assess the role of phosphorylation in hair bundle development and 2) to ascertain whether hair bundles could recover from any damage caused by protein kinase inhibition or the aminoglycoside neomycin. Post-natal mouse cochlear cultures were treated with a range of protein kinase inhibitors. Of those tested, the broadspectrum protein kinase inhibitor staurosporine was found to cause hair bundle collapse and detachment of the apical cell membrane from the cuticular plate without causing cell death. Further investigation suggests that staurosporine may have its effects through inhibiting the function of the Ezrin-Radixin-Moesin proteins, an important family of proteins that cross link actin to the plasma membrane. An inhibitor of c-Jun NH2-terminal kinase, JNK inhibitor I, was also found to cause the loss of hair bundles, but in contrast to staurosporine, also caused hair cell death. This was unexpected, as previous studies have shown that inhibition of c-Jun NH2-terminal kinase confers protection on hair cells from noise and aminoglycoside damage. Recovery of hair bundles was not observed following brief treatment with either staurosporine or neomycin. The current study shows that protein phosphorylation is vital for the proper maintenance of the hair bundle, and hair that bundles show little sign of recovery from limited damage in vitro.