Received for publication July 18, 2006.
Revised November 21, 2006.
Accepted for publication November 21, 2006.

Inhibition of the JNK-mediated mitochondrial cell death pathway restores auditory function in sound exposed animals

Abstract

The therapeutic value of round window membrane delivered (RWM) D-JNKI-1 peptide (Bonny et al., 2001) against sound trauma-induced hearing loss is tested and characterized. Morphological characteristics of sound-damaged hair cell nuclei labeled by Hoechst staining show that apoptosis is the predominant mode of cell death following sound trauma. Analysis of the events occurring after sound trauma demonstrates that JNK/SAPK activates a mitochondrial cell death pathway (i.e. activation of Bax, release of cytochrome c, activation of procaspases and cleavage of fodrin). FITC-conjugated D-JNKI-1 peptide applied onto an intact cochlear RWM diffuses through this membrane and penetrates cochlear tissues with the exception of the stria vascularis. A time-sequence of fluorescence measurements demonstrate that FITC labeled-D-JNKI-1 remains in cochlear tissues for as long as 3 weeks. In addition to blocking JNK-mediated activation of a mitochondrial cell death pathway, RWM delivered D-JNKI-1 prevents hair cell death and development of a permanent shift in hearing threshold that is caused by sound trauma in a dose-dependent manner (EC₅₀ = 2.05 µM). The therapeutic window for protection of the cochlea from sound trauma with RWM delivery of D-JNKI-1 extended out to 12 hours post- sound exposure. These results show that the MAPK/JNK signaling pathway plays a crucial role in sound trauma-initiated hair cell death. Blocking this signaling pathway with RWM delivery of D-JNKI-1 may have significant therapeutic value as a therapeutic intervention to protect the human cochlea from the effects of sound trauma.

Key words: MAP Kinase, Jun Kinase, Apoptosis, Oxidative stress/antioxidants, Protein targets, Mechanisms of cell killing/apoptosis, Protein synthesis inhibitors, Excitotoxicity, neurodegeneration