a SciTechnol journal Research Article Wise et al., J Regen Med 2015, 4:1 http://dx.doi.org/10.4172/2325-9620.1000121 International Publisher of Science, Technology and Medicine All articles published in Journal of Regenerative Medicine are the property of SciTechnol, and is protected by copyright laws. Copyright © 2015, SciTechnol, All Rights Reserved. Journal of Regenerative Medicine Regeneration of Cochlear Hair Cells with Atoh1 Gene Therapy after Noise-Induced Hearing Loss Andrew K Wise 1-3 , Brianna O Flynn 1 , Patrick J Atkinson 4 , James B Fallon 1-3 , Madeline Nicholson 1 and Rachael T Richardson 1-3* Abstract Background: Degeneration of hair cells in the mammalian cochlea results in irreversible hearing loss with no current treatment options to regain lost hair cell function. The Atoh1 gene is necessary for hair cell development and recent research has shown that Atoh1 gene therapy promotes new hair cell formation and hearing restoration in adult rodent deafness models. Objective: The aim of this study was to examine new hair cell formation via Atoh1 gene therapy in noise-deafened adult guinea pigs. Methods: Guinea pigs were deafened by noise exposure (130 dB, 11-13 kHz, 2 hours). After two weeks, the left cochleae were injected with an adenoviral vector containing the Atoh1 gene. Control animals were injected with a control adenoviral vector. Three weeks after injection cochleae were assessed for hair cell density, maturity and hair cell synaptogenesis with auditory neurons. Hearing thresholds were assessed throughout. Results: There were signiicantly more myosin VIIa-positive hair cells in cochleae that received Atoh1 gene therapy compared to contralateral cochleae and compared to cochleae that received control gene therapy (p<0.05 one way ANOVA). However, the number of hair cells in Atoh1-treated animals was far below normal. Expression of Atoh1 had a signiicant preservation effect on the cytoarchitecture of the sensory epithelium compared to controls (p<0.001 one way ANOVA). Expression of the synaptic protein CtBP2 was present in some transfected cells from Atoh1- injected guinea pigs but at a reduced density compared to normal cochleae. There was evidence of auditory neuron preservation near transfected hair cells in Atoh1-injected cochleae (p<0.05 one way ANOVA), but there were no improvements in hearing thresholds. Conclusion: This study supports growing evidence that new hair cell formation is possible in mature cochleae that have been severely damaged, in this case by noise, and demonstrates a protective inluence of Atoh1 gene therapy on the immediate surrounding cellular environment. Keywords Atoh1; Cochlea; Deafness; Gene therapy; Hair cells; Noise; Regeneration; Supporting cells *Corresponding author: Dr. Rachael T Richardson, Bionics Institute, 384 Albert Street, East Melbourne, Victoria, 3002, Australia, Tel: +613 9667 7594; Fax: +613 9667 7518; E-mail: rrichardson@bionicsinstitute.org Received: January 07, 2015 Accepted: June 12, 2015 Published: June 18, 2015 Introduction Cochlear hair cells (HCs) convert acoustic sound waves into electrical signals that activate spiral ganglion neurons (SGNs) to convey acoustic information to the auditory brain. Noise, infection or ototoxic drugs can trigger sensorineural hearing loss as a result of degeneration of HCs, supporting cells and SGNs, or the loss of the synapse between the HC and SGN. herapeutic restoration of hearing ater sensorineural hearing loss would therefore require a multi- faceted approach that includes protection of residual sensory cells and supporting cells, regeneration of lost sensory cells and reconnection of denervated HCs with the peripheral ibres of the SGNs. Inner hair cells (IHCs) and outer hair cells (OHCs) reside in the sensory epithelium of the cochlea known as the organ of Corti along with supporting cells (border cells, pillar cells, phalangeal cells, Deiters’ cells, Hensen’s cells and Claudius’ cells). IHCs receive innervation from SGNs, with a single IHC being innervated by 20- 30 type I aferent SGN ibres. Unmyelinated type II SGNs directly synapse OHCs, with a single aferent nerve ibre innervating many OHCs [1]. IHCs are primarily responsible for the excitation of SGNs whereas OHCs have been implicated in the mechanical ampliication of sound, enhancing the responsiveness of the sensory epithelium to selected frequencies and the detection of low intensity sounds [2]. he cochlea is tonotopically organised such that low frequency sounds primarily activate IHCs in the apical turns of the cochlea while high frequency sounds primarily activate IHCs in the basal turns of the cochlea. Noise induced hearing loss is the most common form of environmental hearing loss. A noise component was associated with 37% of the population with a hearing loss [3]. Acoustic overstimulation causes massive swelling of the IHC nerve terminals 24-48 hours ater acoustic overexposure [4,5] due to glutamate toxicity [6,7]. he swelling disappears ater a few days and thresholds can return to normal [8], a phenomenon known as a temporary threshold shit (TTS). However, even with a TTS, permanent structural changes have occurred to the IHC-SGN synapse [9]. Permanent threshold shit (PTS) results from destruction of cochlear HCs or damage to their stereociliary bundles [10]. OHCs at the base of the cochlea (high frequency) are the most sensitive, but IHCs can also be afected. he HC lesion spreads basally and apically over time stabilising at about two weeks [11-13]. However, neural loss ater noise exposure is delayed by months and can continue for years [14,15]. Viral gene therapy to the cochlea has been widely investigated as a means to introduce genes that protect HCs, SGNs and even trans- diferentiate supporting cells into new HCs. he luid illed scalae of the cochlea provide an ideal environment for localised gene delivery. Transgene delivery to the scala tympani results in gene expression in a broad range of cells (e.g. mesenchymal cells lining the scala tympani, SGNs, supporting cells, HCs) with expression typically observed in all turns of the cochlea [16-21]. Delivery to the scala media, the compartment that houses the sensory epithelium, results in a more localised gene expression pattern (e.g. HCs, supporting cells, interdental cells) that is also more localised to the basal turn of the cochlea [16,17,22-25]. But when supporting cells are the target for gene therapy, the progression of deafness-induced degeneration