The development of the cochlear implant has overcome the severe to profound sensorineural hearing loss that results from hair cell degeneration. The cochlear implant bypasses the nonfunctional hair cells using an electrode interface that directly stimulates auditory neurons, replacing the need for a hair cell synapse.
However, interaction between the implant electrode and remaining neurons is crude, as the electric current spread only allows for a relatively small number of electrodes to be present on the implant ( J Neural Eng 2009;6:055002 http://iopscience.iop.org/1741-2552/6/5/055002).
Furthermore, retracting nerve fibers create an anatomical gap between the implant and the neuron that must be bridged in order to restore the fidelity of the sound signal transmission. The promotion of peripheral nerve fiber regrowth toward the electrode, as described in the following study, offers one possible solution.
Close-Field Electroporation Gene Delivery Using the Cochlear Implant Electrode Array Enhances the Bionic Ear
Pinyon JL, Tadros SF, Froud KE, et al
Sci Transl Med
Following ototoxic damage or noise trauma, hair cells are destroyed and connections with neurons are lost, leading to sensorineural hearing loss.
Neurons retract their finger-like projections, called neurites, away from the hair cells, and the remaining neurons ultimately degenerate because of missing neurotrophic support from hair cells ( J Neurosci 2013;33:13042-13052 http://www.jneurosci.org/content/33/32/13042.short; J Assoc Res Otolaryngol 2013;14:187-211 http://link.springer.com/article/10.1007/s10162-013-0372-5).
Previous animal studies have demonstrated that the application of outside neurotrophic factors could partially reverse the retraction of nerve fibers and prevent subsequent neuronal degeneration ( Exp Neurol 2010;223:464-472 http://www.sciencedirect.com/science/article/pii/S0014488610000166).
However, continuous release of neurotrophins is needed in order to maintain the effect ( PLoS One 2012;7:e52338 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0052338). Earlier studies using gene delivery of these factors often faced the challenge of limited neurotrophin viability ( Sci Rep 2014;4:4733 http://www.nature.com/srep/2014/140422/srep04733/full/srep04733.html).
The goal of the present study was to create a self-sustaining depot of neurotrophins in vivo that would maintain neuron survival, promote outgrowth of fibers, and improve effectiveness of the cochlear implant.
Recent studies have used viruses to deliver genes to inner ear cells, resulting in the production of neurotrophins, successful regeneration of hair cells and neurons, and prevention of cell death ( Hear Res 2014;309:124-135 http://www.sciencedirect.com/science/article/pii/S0378595513002827; J Neurosci 2003;23:4395-4400 http://www.jneurosci.org/content/23/11/4395.full).
An alternative to the use of a viral vector is electroporation, whereby an electrical pulse creates openings in cell membranes, allowing for uptake of DNA material and subsequent expression of a gene.
This technique has been used to introduce the key hair cell gene, ATOH1, into the developing inner ear and manipulate hair cell fate in progenitors ( Nature 2008;455:537-541 http://www.nature.com/nature/journal/v455/n7212/full/nature07265.html). Successful electroporation requires the source of the electrical impulse to be close to the cells of interest, however.
This new study is the first to combine technical advances of the cochlear implant with a gene therapy approach. The investigators used the electric current of the cochlear implant as an electroporation device to introduce the gene encoding brain-derived neurotrophic factor (BDNF) into the guinea pig cochlea.
The first part of the study tested the ability of the cochlear implant as an electroporation device ex vivo.
DNA for BDNF was injected into the perilymphatic space of guinea pig cochleas, and an eight-node electrode array was implanted into the basal turn. Local electric fields were generated by using alternating or tandem electrode configuration of anodes and cathodes to facilitate voltage pulses that electroporated cells close to the electrode.
In the next set of experiments, this method was repeated in vivo. Cochleas of anesthetized guinea pigs were injected and then implanted. The implant was used to generate five tandem pulses that successfully electroporated surrounding non-sensory cells with BDNF.
BDNF expression could still be validated several weeks after electroporation. By six to 10 weeks post-electroporation, expression had ceased.
In the final step of the experiment, chemically deafened guinea pigs were injected, implanted, and electroporated. As seen previously, a small amount of cells successfully took up the gene and expressed BDNF.
Strikingly, immunohistochemical analysis of these animals’ inner ears showed regeneration of peripheral neurites, enhanced outgrowth toward the implant, remyelination of neurons, and prevention of further spiral ganglion neuron death compared with controls.
Improved cochlear implant function was demonstrated by better auditory brainstem responses (ABRs) in animals that had BDNF gene therapy compared with implanted animals that did not have concurrent BDNF gene therapy.
It seems surprising that a relatively small number of transfected cells were able to elicit such a significant effect on neurite regeneration and hearing improvement. It is not known whether the current yield of transfected cells would be sufficient for regrowth of neurites in humans.
Mesenchymal cells transfected with and expressing BDNF were located in non-sensory regions surrounding the organ of Corti. As a result, neurotrophic signals came from different areas and promoted neurite outgrowth not only toward the implant, but also away from the implant.
Outgrowth toward the electrode is key, and transfection of cells close to the implant is needed to improve connectivity, especially when the yield of transfected cells is low.
To this point, it is believed that a fibrous scar forms in place of former neurons after hair cell death and neural degeneration. In cases of early implantation, limited fibrous tissue may serve as a scaffold for outgrowing neurites, as claimed in this study. However, after long-standing hearing loss with substantial degeneration, scarring may no longer allow for directed neural outgrowth, even in the presence of BDNF.
One of the most important challenges to overcome with this approach is the short-term expression of BDNF observed in these experiments. While expression in guinea pigs for up to 10 weeks is very encouraging, this is not a permanent solution for hearing restoration. Future work is needed to engineer stable DNA constructs that are resistant to molecular modifications in the final cell.
LOOKING TO THE FUTURE
At a time when personalized medicine and regeneration strategies are on the rise, the approach outlined in this study represents an advance toward merging exogenous technology and endogenous resources of the inner ear.
While stem cell therapies for the regeneration of hair cells and neurons are still highly experimental and have not yet overcome the hurdle of animal research, the cochlear implant is a well-established surgical option for restoring human hearing.
This study provides a glimpse of the future of auditory prostheses, in which the cochlear implant can more closely integrate with the human peripheral nervous system and, potentially, yield more robust and physiological sound and speech perception.
The research opens up a fundamental new treatment method that is not limited to the application of BDNF. Other genes, such as neurotrophin-3 (NT-3), could be delivered instead or at the same time.
While cochlear implants may ultimately be replaced or augmented by biological approaches, such as drug delivery or cell transplantation for inner ear regeneration, it may take many years before such approaches become available ( J Neurosci 2013;33:13042-13052 http://www.jneurosci.org/content/33/32/13042.full: Neuron 2013;77:58-69 http://www.cell.com/neuron/fulltext/S0896-6273(12)00953-1).
This technique, though still in the animal model, could be implemented in cochlear implant surgery provided that successful and stable delivery of genetic material into the cochlea can be achieved, the safety of using brief but high-voltage electric currents to the auditory nerve is demonstrated, and the electric current levels needed for successful electroporation can be delivered via the cochlear implant electrode.