cytochrome-c-t and Hearing-Loss--Noise-Induced

We tested and characterized the therapeutic value of round window membrane-delivered (RWM) d-JNKI-1 peptide (Bonny et al., 2001) against sound trauma-induced hearing loss. Morphological characteristics of sound-damaged hair cell nuclei labeled by Hoechst staining show that apoptosis is the predominant mode of cell death after sound trauma. Analysis of the events occurring after sound trauma demonstrates that c-Jun N-terminal kinase (JNK)/stress-activated protein kinase activates a mitochondrial cell death pathway (i.e., activation of Bax, release of cytochrome c, activation of procaspases, and cleavage of fodrin). Fluorescein isothiocyanate (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 demonstrates 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 microM). The therapeutic window for protection of the cochlea from sound trauma with RWM delivery of d-JNKI-1 extended out to 12 h after sound exposure. These results show that the mitogen-activated protein kinase/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.

Topics: Animals; Apoptosis; bcl-2-Associated X Protein; Carrier Proteins; Caspases; Cytochromes c; Guinea Pigs; Hair Cells, Auditory; Hearing Loss, Noise-Induced; JNK Mitogen-Activated Protein Kinases; MAP Kinase Signaling System; Microfilament Proteins; Mitochondria; Necrosis; Peptides; Phosphorylation; Protein Transport; Proto-Oncogene Proteins c-jun; Round Window, Ear

Free radical and calcium buffering mechanisms are implicated in cochlear cell damage that has been induced by sound trauma. Thus in this study we evaluated the therapeutic effect of a novel dual inhibitor of calpains and of lipid peroxidation (BN 82270) on the permanent hearing and hair cell loss induced by sound trauma. Perfusion of BN 82270 into the scala tympani of the guinea pig cochlea prevented the formation of calpain-cleaved fodrin, translocation of cytochrome c, DNA fragmentation and hair cell degeneration caused by sound trauma. This was confirmed by functional tests in vivo, showing a clear dose-dependent reduction of permanent hearing loss (ED50 = 4.07 microM) with almost complete protection at 100 microM. Furthermore, BN82270 still remained effective even when applied onto the round window membrane after sound trauma had occurred, within a therapeutic window of 24 h. This indicates that BN 82270 may be of potential therapeutic value in treating the cochlea after sound trauma.

Topics: Action Potentials; Animals; Apoptosis; Calpain; Carrier Proteins; Cochlea; Cysteine Proteinase Inhibitors; Cytochromes c; Dipeptides; DNA Fragmentation; Electrophysiology; Female; Guinea Pigs; Hair Cells, Auditory; Hearing Loss, Noise-Induced; Immunohistochemistry; Lipid Peroxidation; Microfilament Proteins; Microscopy, Electron, Scanning; Microscopy, Electron, Transmission; Round Window, Ear; Tympanic Membrane

We previously reported that intense noise exposure causes outer hair cell (OHC) death primarily through apoptosis. Here we investigated the intracellular signal pathways associated with apoptotic OHC death. Chinchillas were exposed to a 4 kHz narrowband noise at 110 dB SPL for 1 h. After the noise exposure, the cochleas were examined for the activity of each of three caspases, including caspase-3, -8, or -9 with carboxyfluorescein-labeled fluoromethyl ketone (FMK)-peptide inhibitors. The cochleas were further examined for cytochrome c release from mitochondria by immunohistology and for DNA degradation by the TUNEL method. The results showed that the noise exposure triggered activation of caspase-3, an important mediator of apoptosis. The noise exposure also caused the activation of caspase-8 and caspase-9, each of which is associated with a distinct signaling pathway that leads to activation of caspase-3. Caspase activation occurred only in the apoptotic OHCs and not in the necrotic OHCs. These results indicate that multiple signaling pathways leading to caspase-3 activation take place simultaneously in the apoptotic OHCs. In addition to caspase activation, noise exposure caused the release of cytochrome c from mitochondria, resulting in a punctate fluorescence in the cytosol. In contrast to activation of caspases, the release of cytochrome c took place in both apoptotic and necrotic OHCs. Moreover, the release of cytochrome c in a subpopulation of OHCs took place early in the cell death process, prior to any outward signs of necrosis or apoptosis. These data suggest that in this subpopulation there exists a common step that is shared by cell death pathways before entering either necrosis or apoptosis. Lastly, use of the TUNEL assay in combination with PI labeling provides a more accurate discrimination between apoptosis and necrosis.

Topics: Animals; Caspase 3; Caspase 8; Caspase 9; Caspases; Chinchilla; Cytochromes c; DNA Fragmentation; Hair Cells, Auditory, Outer; Hearing Loss, Noise-Induced; In Situ Nick-End Labeling; Noise; Staining and Labeling