Development of an electrode for the artificial cochlear sensory epithelium.

An artificial cochlear sensory epithelium has been developed on the basis of a new concept that the piezoelectric membrane, which converts mechanical distortion into electricity, can mimic the function of the inner hair cell and basilar membrane of the mammalian cochlea. Our previous research demonstrated that the piezoelectric membrane generated electrical outputs in response to the sound stimulation after implantation into the guinea pig cochlea, whereas electrodes for the stimulation of spiral ganglion neurons have not been fabricated, and a method to fix the device in the cochlea is also required to show proof-of-concept. In the present study, to achieve proof-of-concept of hearing recovery by implantation of the artificial cochlear sensory epithelium, we fabricated new electrodes that stick into the cochlear modiolus, which also play a role in the fixation of the device in the cochlea. The efficacy of new electrodes for fixation of the device in the cochlea and for the stimulation of spiral ganglion neurons was estimated in guinea pigs. Four weeks after implantation, we confirmed that the devices were in place. Histological analysis of the implanted cochleae revealed inconspicuous fibrosis and scar formation compared with the sham-operated specimens (n = 5 for each). The terminal deoxynucleotidyl transferase dUTP nick end labeling method was used to assess cell death due to surgical procedures in the cochleae that were harvested after 1 day (n = 6) and 7 days (n = 6) of implantation; there was no significant increase in apoptotic cell death in the implanted cochleae compared with sham-operated cochleae. In seven animals, serial measurements of electrically evoked auditory brainstem responses were obtained, with the electrode positioned in the scala tympani and with the electrode inserted into the cochlear modiolus. With the insertion of electrodes into the cochlear modiolus, significant reduction was achieved in the thresholds of electrically evoked auditory brainstem responses compared with those placed in the scala tympani (p = 0.028). These findings indicated that the new electrodes efficiently fixed the device in the cochlea and were able to stimulate spiral ganglion neurons.

Piezoelectric materials mimic the function of the cochlear sensory epithelium.

Cochlear hair cells convert sound vibration into electrical potential, and loss of these cells diminishes auditory function. In response to mechanical stimuli, piezoelectric materials generate electricity, suggesting that they could be used in place of hair cells to create an artificial cochlear epithelium. Here, we report that a piezoelectric membrane generated electrical potentials in response to sound stimuli that were able to induce auditory brainstem responses in deafened guinea pigs, indicating its capacity to mimic basilar membrane function. In addition, sound stimuli were transmitted through the external auditory canal to a piezoelectric membrane implanted in the cochlea, inducing it to vibrate. The application of sound to the middle ear ossicle induced voltage output from the implanted piezoelectric membrane. These findings establish the fundamental principles for the development of hearing devices using piezoelectric materials, although there are many problems to be overcome before practical application.

Adipose tissue-derived stromal cells protect hair cells from aminoglycoside.

BACKGROUND: Previous studies have demonstrated the therapeutic paracrine activity of adipose tissue-derived stromal cells (ADSCs). This study aimed to examine the ADSC potential for protecting auditory hair cells from aminoglycoside toxicity via paracrine of multiple growth factors and cytokines. STUDY DESIGN: Experimental study. METHODS: We assessed hair cell protection from neomycin toxicity by ADSC-derived factors using an explant culture system, in which cochlear explants and ADSCs were separated by a culture mesh insert to avoid direct contact. We measured the levels of growth factors and cytokines in ADSC culture media using an enzyme-linked immunosorbent assay (ELISA). RESULTS: Neomycin induced severe degeneration of auditory hair cells in cochlear explants, but co-culture with ADSCs significantly increased the number of surviving hair cells in explants. ELISA analysis revealed that ADSCs secreted insulin-like growth factor-1, nerve growth factor, vascular epithelial growth factor, transforming growth factor ß1, monocyte chemotactic protein-1, and most prominently hepatocyte growth factor. CONCLUSIONS: These findings demonstrate that ADSCs have the capacity to protect auditory hair cells, and can be a useful strategy to develop therapy for deafness in the clinic. The multiple paracrine growth factors and cytokines secreted by ADSCs might be involved in this effect. Laryngoscope, 2011.

Transplantation of bone marrow-derived neurospheres into guinea pig cochlea.

OBJECTIVES/HYPOTHESIS: To investigate the potential of neurally induced bone marrow stromal cells (BMSCs) as transplants for replacement of spiral ganglion neurons. METHODS: BMSCs were harvested from the femurs and tibias of adult guinea pigs. BMSCs were cultured with neural induction media and formed spheres. The capacity of BMSC-derived spheres for neural differentiation was examined by immunocytochemistry in vitro. BMSC-derived spheres were injected into the modiolus of the intact cochleae or those locally damaged by ouabain, followed by histological and functional analyses. RESULTS: In vitro analysis revealed a high capacity of BMSC-derived spheres for neural differentiation. After transplantation into the cochlear modiolus, the survival and neural differentiation of BMSC-derived spheres was observed in both the intact and damaged cochleae. In intact cochleae, transplants settled in various portions of the cochlea, including the cochlear modiolus, whereas in damaged cochleae, transplants were predominantly observed in the internal auditory meatus. Transplantation of BMSC-derived spheres resulted in no functional recovery of the cochlea or protection of host spiral ganglion neurons. CONCLUSIONS: The present findings indicate that BMSC-derived spheres can be a source for replacement of spiral ganglion neurons, although further manipulations are required for functional recovery.

Hepatocyte growth factor protects auditory hair cells from aminoglycosides.

OBJECTIVES/HYPOTHESIS: To examine the effect of hepatocyte growth factor (HGF) for protection of auditory hair cells against aminoglycosides and its molecular mechanisms. STUDY DESIGN: Experimental study. METHODS: We quantitatively assessed protective effects of HGF on mouse cochlear hair cells against neomycin toxicity using explant culture systems. To understand mechanisms of hair cell protection by HGF, we examined the expression of c-Met, HGF receptor, and 4-hydroxynonenal (a lipid peroxidation marker) in the cochlea by means of immunohistochemistry and Western blotting. RESULTS: The application of HGF to cochlear explant cultures significantly reduced the hair cell loss induced by neomycin. Immunohistochemistry showed c-Met expression in normal auditory hair cells, and its increase in response to neomycin-induced damage. Immunostaining for 4-hydroxynonenal suggested that HGF acted by attenuating the lipid peroxidation of auditory epithelia induced by neomycin. CONCLUSIONS: These findings demonstrate that a functional HGF/c-Met coupling is present in the cochlea, and HGF application exerts protective effects on hair cells, indicating the potential of HGF as a therapeutic agent for sensorineural hearing loss.

Local application of hepatocyte growth factor using gelatin hydrogels attenuates noise-induced hearing loss in guinea pigs.

CONCLUSION: Local application of hepatocyte growth factor using biodegradable gelatin hydrogels attenuates noise-induced hearing loss in guinea pigs. OBJECTIVES: To develop an inner ear drug delivery system using gelatin hydrogels that is capable of a sustained delivery of growth factors to the cochlea. We examined the efficacy of the local application of gelatin hydrogels containing hepatocyte growth factor (HGF) in protecting cochlear hair cells from noise-induced damage. MATERIALS AND METHODS: A piece of gelatin hydrogel previously immersed in either HGF or saline was placed on the round window membrane of a guinea pig 1 h after noise exposure (4 kHz octave band noise at 120 dB sound pressure level for 3 h). Auditory function was monitored using auditory brainstem responses (ABRs), and the loss of hair cells was evaluated quantitatively. RESULTS: Local HGF treatment significantly reduced the noise exposure-caused ABR threshold shifts and the loss of outer hair cells in the basal portion of the cochleae.

Surgical invasiveness of cell transplantation into the guinea pig cochlear modiolus.

OBJECTIVE: Previous studies have demonstrated the potential of cell transplantation for regeneration of spiral ganglion neurons (SGNs). However, the effect of surgical invasion on host cochleae has yet to be evaluated. The present study investigated the efficiency and invasiveness of our surgical procedure using a fine glass pipette for injections into the cochlear modiolus. METHODS: We examined the survival of transplanted embryonic stem cell-derived neurons in the cochlear modiolus of guinea pigs. Surgical invasiveness was assessed by measurements of electrically evoked auditory brainstem responses (eABRs) and SGN densities after an injection of 5 microl of saline into the cochlear modiolus. RESULTS: All of the transplanted animals exhibited localization of transplanted cells in the cochlear modiolus. No significant alterations in the eABR thresholds or SGN densities were found following surgery. CONCLUSION: These findings indicate that our procedure is a viable method for testing the potential of transplants for SGN replacement.