Elsevier

Hearing Research

Volume 380, 1 September 2019, Pages 137-149
Hearing Research

Review Article
Neurotrophin gene augmentation by electrotransfer to improve cochlear implant hearing outcomes

https://doi.org/10.1016/j.heares.2019.06.002Get rights and content

Highlights

  • Cochlear mesenchymal cells can be targeted for gene electrotransfer via electric field compression using cochlear implants.

  • Regeneration of cochlear peripheral neurites after neurotrophin gene augmentation via bionic array-based electrotransfer.

  • Local recombinant neurotrophin expression enhances neural recruitment and electrically evoked auditory brainstem responses.

  • Bionic array directed gene electrotransfer requires low voltages albeit higher than existing cochlear implant capabilities.

  • Regulatory-permissive mini-plasmids free of antibiotic resistance genes achieve efficient gene expression in the cochlea.

Abstract

This Review outlines the development of DNA-based therapeutics for treatment of hearing loss, and in particular, considers the potential to utilize the properties of recombinant neurotrophins to improve cochlear auditory (spiral ganglion) neuron survival and repair. This potential to reduce spiral ganglion neuron death and indeed re-grow the auditory nerve fibres has been the subject of considerable pre-clinical evaluation over decades with the view of improving the neural interface with cochlear implants. This provides the context for discussion about the development of a novel means of using cochlear implant electrode arrays for gene electrotransfer. Mesenchymal cells which line the cochlear perilymphatic compartment can be selectively transfected with (naked) plasmid DNA using array - based gene electrotransfer, termed ‘close-field electroporation’. This technology is able to drive expression of brain derived neurotrophic factor (BDNF) in the deafened guinea pig model, causing re-growth of the spiral ganglion peripheral neurites towards the mesenchymla cells, and hence into close proximity with cochlear implant electrodes within scala tympani. This was associated with functional enhancement of the cochlear implant neural interface (lower neural recruitment thresholds and expanded dynamic range, measured using electrically - evoked auditory brainstem responses). The basis for the efficiency of close-field electroporation arises from the compression of the electric field in proximity to the ganged cochlear implant electrodes. The regions close to the array with highest field strength corresponded closely to the distribution of bioreporter cells (adherent human embryonic kidney (HEK293)) expressing green fluorescent reporter protein (GFP) following gene electrotransfer. The optimization of the gene electrotransfer parameters using this cell-based model correlated closely with in vitro and in vivo cochlear gene delivery outcomes. The migration of the cochlear implant electrode array-based gene electrotransfer platform towards a clinical trial for neurotrophin-based enhancement of cochlear implants is supported by availability of a novel regulatory compliant mini-plasmid DNA backbone (pFAR4; plasmid Free of Antibiotic Resistance v.4) which could be used to package a ‘humanized’ neurotrophin expression cassette. A reporter cassette packaged into pFAR4 produced prominent GFP expression in the guinea pig basal turn perilymphatic scalae. More broadly, close-field gene electrotransfer may lend itself to a spectrum of potential DNA therapeutics applications benefitting from titratable, localised, delivery of naked DNA, for gene augmentation, targeted gene regulation, or gene substitution strategies.

Section snippets

Inner ear gene delivery

The translational potential of gene therapy in the inner ear is developing considerable prominence within the burgeoning domain of hearing therapeutics. This is founded upon successes in pre-clinical models that have utilised directed manipulation of gene expression to investigate developmental and physiological processes of hearing and balance at the molecular level. Modalities used for expression of recombinant proteins include ballistics (‘gene gun’-based delivery of gold particles coated

Background to gene electrotransfer

There has been a frameshift in understanding of pulsed electric field-based DNA transfer in recent years, with studies showing that electroporation is a misnomer in that plasmid DNA is too large to cross into cells through transiently generated pores in the plasma membrane, with associated electrophoretic translocation of the negatively charged DNA. Rather, it is now recognised that unlike small RNA and DNA oligonucleotide molecules, and fluorescent molecules such as propidium iodide, that

Declaration of interest

The BaDGE® registered trademark is assigned to UNSW Sydney Knowledge Exchange though New South Innovations Pty Ltd, which is also the assignee for the BaDGE®-related intellectual property.

Funding

Supported by funding from the Australian Research Council (ARC), grants (ARC DP151014754, ARC LP0992098, ARC LP140101008), the Garnett Passe and Rodney Williams Memorial Foundation, and the National Health and Medical Research Council (NHMRC) grants APP1091646, APP1122055 and GNT1142910. The research was supported by collaborative research funding from Cochlear Ltd.

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