Molecular mechanisms involved in cochlear implantation trauma and the protection of hearing and auditory sensory cells by inhibition of c-jun-N-terminal kinase signaling

Mechanisms of programmed cell death signaling in hair cells and support cells post-electrode insertion trauma

CONCLUSION

Programmed cell death (PCD) initially starts in the support cells (SCs) after electrode insertion trauma (EIT), followed by PCD in hair cells (HCs). Activation of caspase-3 was observed only in SCs. Protecting both SCs and HCs with selective otoprotective drugs at an early stage post implantation may help to preserve residual hearing.

OBJECTIVES

Cochlear implant EIT can initiate sensory cell losses via necrosis and PCD within the organ of Corti, which can lead to a loss of residual hearing. PCD appears to be a major factor in HC loss post-EIT. The current study aimed to: (1) determine the onset of PCD in both SCs and HCs within the traumatized organ of Corti; and (2) identify the molecular mechanisms active within the HCs and SCs that are undergoing PCD.

METHODS

Adult guinea pigs were assigned to one of two groups: (1) EIT and (2) unoperated contralateral ears as controls. Immunostaining of dissected organ of Corti surface preparations for phosphorylated-Jun, cleaved caspase-3, and 4-hydroxy-2,3-nonenal (HNE) were performed at 6, 12, and 24 h post-EIT and for contralateral control ears.

RESULTS

At 6 h post-EIT the SCs immunolabeled for the presence of phosphorylated-Jun and activated caspase-3. Phosphorylated p-Jun labeling was observed at 12 h in both the HCs and SCs of middle and basal cochlear turns. Cleaved caspase-3 was not observed in HCs of any cochlear turn at up to 24 h post-EIT. Lipid peroxidation (HNE immunostaining) was first observed at 12 h post-EIT in both the HCs and SCs of the basal turn, and reached the apical turn by 24 h post-EIT.

Transforming growth factor β1 inhibition protects from noise-induced hearing loss

Excessive exposure to noise damages the principal cochlear structures leading to hearing impairment. Inflammatory and immune responses are central mechanisms in cochlear defensive response to noise but, if unregulated, they contribute to inner ear damage and hearing loss. Transforming growth factor β (TGF-β) is a key regulator of both responses and high levels of this factor have been associated with cochlear injury in hearing loss animal models. To evaluate the potential of targeting TGF-β as a therapeutic strategy for preventing or ameliorating noise-induced hearing loss (NIHL), we studied the auditory function, cochlear morphology, gene expression and oxidative stress markers in mice exposed to noise and treated with TGF-β1 peptidic inhibitors P17 and P144, just before or immediately after noise insult. Our results indicate that systemic administration of both peptides significantly improved both the evolution of hearing thresholds and the degenerative changes induced by noise-exposure in lateral wall structures. Moreover, treatments ameliorated the inflammatory state and redox balance. These therapeutic effects were dose-dependent and more effective if the TGF-β1 inhibitors were administered prior to inducing the injury. In conclusion, inhibition of TGF-β1 actions with antagonistic peptides represents a new, promising therapeutic strategy for the prevention and repair of noise-induced cochlear damage.

Factors associated with hearing loss in a normal-hearing guinea pig model of Hybrid cochlear implants

The Hybrid cochlear implant (CI), also known as Electro-Acoustic Stimulation (EAS), is a new type of CI that preserves residual acoustic hearing and enables combined cochlear implant and hearing aid use in the same ear. However, 30-55% of patients experience acoustic hearing loss within days to months after activation, suggesting that both surgical trauma and electrical stimulation may cause hearing loss. The goals of this study were to: 1) determine the contributions of both implantation surgery and EAS to hearing loss in a normal-hearing guinea pig model; 2) determine which cochlear structural changes are associated with hearing loss after surgery and EAS. Two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no direct acoustic or electric stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem response thresholds were followed over time at 1, 2, 6, and 16 kHz. At the end of the study, the following cochlear measures were quantified: hair cells, spiral ganglion neuron density, fibrous tissue density, and stria vascularis blood vessel density; the presence or absence of ossification around the electrode entry was also noted. After surgery, implanted animals experienced a range of 0-55 dB of threshold shifts in the vicinity of the electrode at 6 and 16 kHz. The degree of hearing loss was significantly correlated with reduced stria vascularis vessel density and with the presence of ossification, but not with hair cell counts, spiral ganglion neuron density, or fibrosis area. After 10 weeks of stimulation, 67% of implanted, stimulated animals had more than 10 dB of additional threshold shift at 1 kHz, compared to 17% of implanted, non-stimulated animals and 0% of non-implanted animals. This 1-kHz hearing loss was not associated with changes in any of the cochlear measures quantified in this study. The variation in hearing loss after surgery and electrical stimulation in this animal model is consistent with the variation in human patients. Further, these findings illustrate an advantage of a normal-hearing animal model for quantification of hearing loss and damage to cochlear structures without the confounding effects of chemical- or noise-induced hearing loss. Finally, this study is the first to suggest a role of the stria vascularis and damage to the lateral wall in implantation-induced hearing loss. Further work is needed to determine the mechanisms of implantation- and electrical-stimulation-induced hearing loss.

ROS-containing endosomal compartments: implications for signaling

The endogenous generation of reactive oxygen species (ROS), previously perceived as a detrimental by-product of cellular processes, is now recognized as a critical component of intracellular signaling. Exploration of these biological signaling functions requires understanding the complex redox biochemistry and recognizing the compartment-specific elements of ROS generation. The endosomal compartment is increasingly recognized as a source for NADPH oxidase (NOX)-generated signaling ROS. Despite this growing understanding, there are significant limitations to the available detection and measurement systems for endogenous ROS. This chapter provides information about specific methodologies and redox-sensitive probes to guide the investigator and define the critical limitations for many of the available approaches. Although measurement continues to be challenging, the rapid growth and development of new detection systems suggests that our capacity to assign specific signaling roles to endosomal ROS will expand markedly in the next several years.