The Caspase Pathway in Noise-Induced Apoptosis of the Chinchilla Cochlea

Abstract

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.

Overexpression of X-linked inhibitor of apoptosis protein protects against noise-induced hearing loss in mice

“…12 Noise drives excessive mitochondrial activity, resulting in the formation of reactive oxygen species that contribute to apoptotic death in the OC. 41 The JNK pathway, known to be redox regulated and pro-apoptotic, has been implicated in inner ear cell death produced by ototoxins (aminoglycosides) and excessive noise.…”

“…[7][8][9][10][11] Both necrosis and apoptosis have been implicated in NIHL. [12][13][14] Their relative contribution to noise-induced cochlear cell death changes with the types of acoustic exposures and the location along the cochleae relative to the center region where damage is centered on. [12][13][14][15] Nevertheless, apoptosis is thought to have a significant role in noise-induced damage of the cochlea.…”

“…18,21,22 In NIHL, apoptosis resulting from activation of the extrinsic pathway (caspase-8) or the intrinsic pathway (caspase-9) has been observed. 12,13,15,16 These two pathways converge on caspase-3, the final executioner of apoptosis. Both the intrinsic and extrinsic pathways are opposed by members of the inhibitor of apoptosis protein (IAPs) family.…”

Comparison of activated caspase detection methods in the gentamicin-treated chick cochlea

“…Two of these upstream initiator caspases, caspase-8 and caspase-9, have been shown to be activated in the cochlea following sound exposure (Nicotera et al, 2003) and following aminoglycoside treatment in vivo and in vitro (Cunningham et al, 2002;Cheng et al, 2003). Involved in the extrinsic apoptotic pathway, caspase-8 is initiated after the binding and trimerization of death receptors on the cell membrane.…”

“…Conversely, caspase-9 is involved in an intrinsic pathway associated with mitochondriamediated activation and cytochrome c release into the cytosol (Cryns and Yuan, 1998;Robertson and Orrenius, 2002). Although caspase-8 and caspase-9 represent two distinct apoptotic signaling pathways, both have been shown to activate caspase-3 (Cheng et al, 2003;Nicotera et al, 2003). However, inhibition of caspase-9 prevented the activation of downstream caspase-3, whereas the inhibition of caspase-8 did not (Cunningham et al, 2002).…”