The Neurabin/RGS4 Complex as a Novel Target for Otoprotection from Acoustic Trauma

Abstract

Noise-induced hearing loss (NIHL) occurs after acute or chronic acoustic overexposure, and is almost entirely preventable. Treatments are limited to prevention and post-trauma management (hearing aids and cochlear implants), and the latter does not repair the damaged cochlea. Therefore, there is a pressing need for pharmacological interventions to protect the cochlea from injury and repair its delicate structures. A short window of opportunity for drug treatment after noise exposure exists due to continuous oxidative stress which damages cochlear tissues after the cessation of noise exposure. Our previous studies have shown that agonists of adenosine A1 receptors (e.g. adenosine amine congener or ADAC) have the ability to mitigate NIHL, and improve the survival of sensory hair cells. Other studies have shown that the inhibition of the neurabin/RGS4 complex, which prolongs A1 receptor signalling, can enhance neuroprotection conferred by the adenosine A1 receptor in animal seizure models. The present study investigates the neurabin/RGS4 complex as a novel target for otoprotection in the Wistar rat cochlea after acoustic trauma. Methods: To achieve this, neurabin isoform expression was first determined at the transcript level using RT-PCR, followed by characterisation of neurabin I immunolocalisation in the rat cochlea. To evaluate cochlear protection from NIHL, rats were exposed to traumatic noise (8-16 kHz, 110 dB SPL) for 2 hours and treated with ADAC (10 μM, 100 μM and 1 mM), CCG-4986 (100 μM; a small molecule inhibitor of the neurabin/RGS4 complex) or a combination of the two drugs. Both compounds were delivered intratympanically 24 hours after noise exposure, using poloxamer-407 as a release medium. Hearing acuity was assessed using auditory brainstem responses (ABR) tone pips (4-28 kHz and Wave I suprathreshold responses (8-20 kHz), one day before noise exposure and 14 days after intratympanic injection. Survival of the spiral ganglion neurons was assessed in selected treatment groups. Results: Both neurabin isoforms (I and II) were expressed at the transcript level in the Wistar rat cochlea. Neurabin I immunolocalisation was observed in the spiral ganglion neurons and surprisingly in sensory and supporting cells in the organ of Corti. While strong expression in the spiral ganglion neurons and inner hair cells was observed across all cochlear turns, we found turn-related differences in immunoexpression of the outer hair cells and Deiters' cells. In vivo studies demonstrated that the intratympanic administration of CCG-4986 produced the greatest reduction in noise-induced ABR threshold shifts (17 dB averaged across all frequencies or up to 31 dB at noise band-specific frequencies). A combination of CCG-4986 with ADAC or ADAC alone, were less effective in mitigating NIHL. The effect of CCG-4986 was observed mostly in low to mid frequencies (4-20 kHz) and was minimal in high frequencies (24-28 kHz). CCG-4986 also improved ABR wave I suprathreshold responses by 30% compared to the vehicle control, and partially protected spiral ganglion neurons from acoustic injury. This study thus presents the neurabin/RGS4 complex as a novel target for pharmacological treatment of acoustic trauma.