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Noise-induced hearing loss (NIHL) is one of the most prevalent disabilities for which effective therapeutic treatment is currently lacking. Auditory injury caused by excessive or constant noise exposure damages the sensory elements of the ear, leading to metabolic or mechanical damage to hair cells and subsequent degeneration of spiral ganglion neurons (SGNs). To improve hearing in the NIHL patient population, maintenance and regeneration of inner hair cell-SGN synapses is critical. Although effective compounds for the treatment of NIHL have been suggested, challenges in delivery of the compounds to the inner hair cells (IHCs) hinder translation of these therapies to a clinical setting. Current routes of administration are ineffective in reaching IHCs or are invasive and may damage the cochlea. In this study, we examined less invasive delivery routes, via intrathecal (cerebrospinal fluid delivery) or intravenous injection, to administer adeno-associated virus serotype 9 (AAV9) expressing green fluorescent protein (GFP) to target IHCs and SGNs. To induce auditory injury, mice were exposed to 100 dB sound pressure level (SPL) octave band noise for 2 hours. Mice were then injected at various time points post noise injury to determine if noise exposure influenced targeting efficiency. Three weeks post injection, cochleae were processed and analyzed for GFP expression using immunofluorescence. Our results demonstrate that intravenous delivery was not successful in targeting of IHCs or SGNs at any time point measured. However, intrathecally injected AAV9 was highly effective in targeting cells in both healthy and noise-damaged cochleae at multiple time points post-noise exposure. We conclude that gene therapy techniques can be utilized to efficiently deliver therapeutic transgenes to the cochlea using cerebrospinal fluid as a delivery route. These findings determine a novel route of delivery and open a new avenue of therapeutic treatment for auditory injury and cochlear disorders.

Batten Disease is a fatal, lysosomal storage disorder characterized by cognitive and motor deficits, vision impairments, and seizures. Loss of vision is a hallmark of 10 of the 13 Batten Disease subtypes. Our group has pioneered AAV9 gene therapy treatments achieving widespread transduction of the brain and spinal cord. Two clinical trials are currently ongoing at Nationwide Children’s Hospital delivering this vector via cerebrospinal fluid (CSF) to the brain and spinal cord for treatment of the lethal neurodegenerative aspects of Batten Disease. However, AAV9 transduction of the retina after CSF delivery is limited and there is a critical need for a solution that prevents vision loss and further improves quality of life for Batten Disease patients. Similar to most genetic ocular diseases, photoreceptor degeneration is the most commonly cited pathology in patients. However, recent studies suggest that in some subtypes of Batten Disease, expression must be rescued also within the deeper layers of the retina that are difficult to reach with therapeutic vectors (inner nuclear layer, INL). We performed single cell RNA sequencing of mice and non-human primate retinas in collaboration with Dr. Fischer (OSU) and concluded that Batten Disease vision specific gene therapy needs to target a wide range of cells, including the INL, which is a major challenge for translation of a vision specific therapy to the clinic. We have recently discovered that administration of neuraminidase (NA), a sialidase enzyme, prior to or in combination with AAV9.GFP, drastically increases transduction throughout the murine C57Bl/6 retina including the INL and all the way through to the photoreceptor layer. Our preliminary data indicates GFP expression in almost all, if not all, retinal cell types using this method. Importantly, we have confirmed successful targeting of INL bipolar cells, a notoriously difficult cell-type to transduce, and up to 40% increase in Muller glia transduction. Preliminary histological examination indicates no damage or alterations in retinal integrity. While additional testing in large animal models is required and scheduled to occur in a WT pig model in Autumn 2020, this remarkable discovery suggests that it may now be possible to target every cell-type of the retina with a single AAV vector. This is especially important in regard to treatment of Batten Disease but has many additional implications. Current retinal therapies that require a more invasive subretinal delivery to photoreceptors could now opt for the safer intravitreal delivery strategy. In addition, this strategy would be highly useful in the field of optogenetics, where investigators continue to struggle to find an efficient way to express light-sensitive opsins in cells of the INL to restore sight in individuals that have already lost their photoreceptors.