Concentration Dependent Effects of Human Cometin on Spiral Ganglion Neuron Survival and Neurite Outgrowth

Introduction: Neurotrophic factors are widely known for their protective effect on spiral ganglion neurons (SGN) and the protection of these neurons is of great importance to optimize cochlear implants, which directly stimulate SGN in deaf patients. Previous studies have identified Cometin - also known as Meteorin-like - to be neuroprotective and beneficial for metabolic disorders. The aim of our study was to investigate the effects of different concentrations of recombinant human Cometin (hCometin) on SGN in regard to neuroprotection and neurite outgrowth and to evaluate its neurite guidance potential using a neurite outgrowth chamber.

hCometin was initially tested in two separate dosing experiments: 5, 10, and 15 μg/mL (medium dose group) and 10, 25, and 50 μg/mL (high dose group). The hCometin was added to dissociated neonatal murine SGN. The number, morphology, and neurite length of SGN treated with hCometin were compared to untreated (negative control, NC) and brain-derived neurotrophic factor treated (BDNF, 50 ng/mL) (positive control, PC) cells. Subsequently, to investigate a potential effect on neurite guidance, 10 μg/mL hCometin was delivered via osmotic pumps to neonatal murine spiral ganglion explants (SGE) cultured in a neurite outgrowth chamber to experimentally mimic the scala tympani and the Rosenthal's canal. The amount of pump-released hCometin was measured by enzyme-linked immunosorbent assay and neurite growth was quantified and compared to a Cometin-free NC.

All medium dose group concentrations of hCometin resulted in significant neuronal protection, whereas high dose group concentrations (25 and 50 μg/mL) were neurotoxic. The medium dose concentrations significantly increased the number of monopolar neurons compared to NC, and 10 and 15 μg/mL hCometin increased the number of neurons with a physiological bipolar morphology to an even greater extent than BDNF. For neurite length, 5 and 10 μg/mL hCometin had the greatest effect, which was comparable with the BDNF-PC. The osmotic-pump based delivery of 10 μg/mL hCometin to SGE had no positive effect on the number, extent, or orientation of outgrowing neurites in the culture set up used.

A concentration of 10 μg/mL hCometin significantly protects dissociated SGN from degeneration and significantly increases the outgrowth of neurites, which is favourable in view of induced neurite outgrowth towards cochlear electrode arrays for future optimisation of the nerve-electrode-interface. The study failed to detect a guided neurite outgrowth by pump-based drug release, which may be due to the experimental set up, which could be improved in future studies.