Heike Janssen (2016):
Electrophysiological analysis of the effect of neurothropic treatment in deafened cats
Hearing and speaking are crucial for communication and therefore have a meaningful impact on the mental and social development of a human. However, the percentage of people suffering from hearing loss is increasing. In cases of sensorineural hearing loss (SNHL), the main cause of deafness, the degeneration of the organ of Corti leads to a subsequent and progressive degeneration of the spiral ganglion neurons (SGN), whose axons form the auditory nerve.
The therapy of choice is the implantation of a cochlear implant (CI), which directly stimulates the SGN, therefore functionally replacing the degenerated sensory epithelium of the inner ear (i.e. the hair cells). In this context, the amount of SGN is, among other individual factors (e.g. duration of deafness) or technical issues (e.g.
number of electrode contacts), a limiting factor for CI efficacy.
Electrical stimulation (ES) through the CI and exogenous application of neurotrophic factors (NTF), as for example glial cell line-derived neurotrophic factor (GDNF), singly or combined, are a promising intervention to reduce these degenerative processes.
However, already established application methods can only be applied for a limited time (e.g. single-dose application or osmotic pumps with a drug reservoir) or are discussed with regard to bio-safety issues (gene-based treatment).
The aim of the study at hand was to assess a new long-term application method (6 month) for NTF release in combination with a CI in terms of biological safety and chronic effect (i.e. SGN survival) in an animal study as a prerequisite for subsequent human application.
The focus was to assess in how far the lack of neurotrophic support and/or the lack of hearing experience is influencing the functionality of the auditory pathway.
The new application method consisted of an encapsulated cell (EC) device, which housed cells, genetically engineered for chronic secretion of GDNF. Similar devices
with the same cell line (human retinal ARPE-19 cell line) have already been applied to the human brain in clinical studies of Parkinson’s and Alzheimer’s disease.
For the study at hand, we co-implanted CI and EC device in the scala tympani of neonatally deafened cats. Two treatment groups with either GDNF treatment alone (+GDNF), or a combined ES and GDNF treatment (+GDNF/+ES) were compared with a group which lacked any neuroprotective treatment (-GDNF). In the latter case, the EC device contained the parental ARPE-19 cell line. These experimental groups were contrasted with two comparison groups: normal hearing cats (after hair cell destruction) and congenital deaf cats (CDC), representing a group without hearing experience but with natural neurotrophic support due to longer survival of the organ of Corti compared to pharmalogical-deafened cats.
To assess the neuroprotective effect (i.e. SGN survival) in the experimental groups, cochleas were harvested for histological analysis at the end of the treatment period.
To determine the functionality of the auditory system, CIs were bilaterally implanted in an acute setting and electrically evoked auditory brainstem responses (eABR) were measured.
After the implantation time of 6 months, only one out of 9 tested EC devices (+GDNF/+ES) still showed a GDNF production, which was however markedly reduced (13 % of the pre-implantation level). The histological analyses revealed no cell survival of the ARPE-19 cells in those EC devices remaining in situ (n = 6). Additionally a tight encapsulation of the EC devices with fibrous tissue was found being an indicator for chronic inflammation. The formation of fibrous tissue was significantly enhanced in the treated ear of the +GDNF group, but not in the other experimental groups.
While the control group (-GDNF) showed more SGN in the implanted ear in 5 out of 6 subjects (33 % more SGN in the implanted ear, compared to the contralateral ear), the sole application of GDNF (+GDNF) resulted in an acceleration of SGN degeneration in the treated ear in 5 out of 6 subjects (25 % less SGN in the treated ear). Chronic ES was applied on average for a duration of 48.67 ± 35.91 hours. Despite its restriction to a short initial time period, the combined application of ES and GDNF caused a significant increase in SGN survival (66 % more SGN in the treated ear) in all subjects of the +GDNF/+ES group.
Based on the histological results, it can be proposed, that the encapsulation of the EC device with fibrous tissue led to cell death in the devices and to an accelerated SGN degeneration in the +GDNF group. The application of ES may have reduced the encapsulation of the EC device and thus allowed for a combined neuroprotective effect of ES and GDNF.
The analysis of the functionality of the hearing system revealed rarely significant differences between hearing animals and CDCs. The -GDNF as well as the +GDNF group differed significantly from the hearing group. The -GDNF group had significantly lower amplitudes of peak component IV, the +GDNF group had significantly longer latencies. Additionally, both groups had significantly lower maximal values in eABR measurements (root mean square, RMS) compared to the hearing group. The amplitudes of early and late eABR components as well as maximal RMS values of the +GDNF/+ES group were shifted towards values of normal hearing subjects but remained below the values of congenital deafened and hearing cats.
The results of the electrophysiology indicate that natural neurotrophic support has significant impact, not only on the survival, but also on the functionality of the auditory pathway, even in the absence of hearing experience. Based on histology, it can be assumed that ES was allowing for the neuroprotective effect of GDNF and an additive effect of ES and GDNF on auditory function.
Thus, exogenous application of neurotrophic factors combined with hearing experience via a CI can not only improve SGN survival, but also the functionality of the auditory system. The negative effect of GDNF which was related to enhanced fibrous tissue formation, is indicative for an additional treatment with inflammatory and anti-oxidative drugs. The application method which was presented here has the potential to act neuroprotective on both SGN survival and function over a long time period and therefore is a promising tool to increase CI treatment efficacy in the human patient.