A role for the canonical nuclear factor-κB pathway in coupling neurotrophin-induced differential survival of developing spiral ganglion neurons

Protective effect of adenovirus-mediated erythropoietin expression on the spiral ganglion neurons in the rat inner ear

The aim of the present study was to evaluate the expression of erythropoietin (Epo) and the Epo receptor (Epo-R) in the spiral ganglion neurons (SGNs) of the rat inner ear, and to assess the effect of Epo adenovirus vector (Ad-Epo) on the spontaneous apoptosis of SGNs. A total of 60 ears from 30 healthy neonatal (2-3 days postnatal) Sprague-Dawley rats were used to examine the expression of Epo in the SGNs. The rats were divided into three groups: The negative control group, the vector control group [infected with a green fluorescent protein expression vector (Ad-GFP)] and the Ad-Epo group (infected with Ad-Epo). The expression of Epo and Epo-R was detected by immunohistochemistry and dual immunofluorescence staining using polyclonal antibodies directed against Epo and Epo-R, followed by confocal laser-scanning microscopy. An adenovirus vector was constructed and used to transfect the cultured SGNs. Following adenovirus infection, apoptosis of the SGNs was evaluated and Epo protein expression was assessed. Epo and Epo-R were widely expressed in the plasma membrane and the cytoplasm of the SGNs, as well as in the organ of Corti and the stria vascularis within the inner ear. Epo protein expression was upregulated in the Ad-Epo group compared with that in the other two groups (P<0.05). Apoptotic cells were seldom observed at day 4 of SGN culture in the negative control group. At day 7, marked apoptotic cells were detected in the negative control group and the vector control group. The apoptosis level in the Ad-Epo group was significantly decreased compared with that in the negative control group or the vector control group at day 7 (P<0.05). In conclusion, Epo and Epo-R are expressed in the SGNs of the inner ear of the rat, and Ad-Epo can decrease the spontaneous apoptosis of SGNs, which may provide a basis for the prevention or alleviation of sensorineural hearing loss.

High mobility group box 1 (HMGB1): dual functions in the cochlear auditory neurons in response to stress?

High mobility group box 1 (HMGB1) is a DNA-binding protein that facilitates gene transcription and may act extracellularly as a late mediator of inflammation. The roles of HMGB1 in the pathogenesis of the spiral ganglion neurons (SGNs) of the cochlea are currently unknown. In the present study, we tested the hypothesis that early phenotypical changes in the SGNs of the amikacin-poisoned rat cochlea are mediated by HMGB1. Our results showed that a marked downregulation of HMGB1 had occurred by completion of amikacin treatment, coinciding with acute damage at the dendrite extremities of the SGNs. A few days later, during the recovery of the SGN dendrites, the protein was re-expressed and transiently accumulated within the nuclei of the SGNs. The phosphorylated form of the transcription factor c-Jun (p-c-Jun) was concomitantly detected in the nuclei of the SGNs where it often co-localized with HMGB1, while the anti-apoptotic protein BCL2 was over-expressed in the cytoplasm. In animals co-treated with amikacin and the histone deacetylase inhibitor trichostatin A, both HMGB1 and p-c-Jun were exclusively found within the cytoplasm. The initial disappearance of HMGB1 from the affected SGNs may be due to its release into the external medium, where it may have a cytokine-like function. Once re-expressed and translocated into the nucleus, HMGB1 may facilitate the transcriptional activity of p-c-Jun, which in turn may promote repair mechanisms. Our study therefore suggests that HMGB1 can positively influence the survival of SGNs following ototoxic exposure via both its extracellular and intranuclear functions.

Morphometrical Analysis of Developing Cochlear Ganglion Neurons: A Light Microscopic Fetal Study

BACKGROUND AND AIM

The cochlear or spiral ganglion neurons are the initial bridge between the external world of sound and its discernment in the brain. As the developing human fetal cochlea is known to start functioning in mid gestational period, its anatomical details when compared with adults could vary with each gestational age. The aim of current study was to assess morphometrical parameter of developing human fetal cochlear ganglion neurons and comparison of data in each gestational period.

MATERIALS AND METHODS

Ten aborted human fetuses from 14th to 28th weeks of gestation were procured from Department of Obstetrics and Gynaecology of associated hospital, after obtaining ethical clearance and were processed for studying under light microscope. Area of neurons from each gestational age was measured on histophotomicrographs using image Proplus software. Standard statistical method was used to calculate area range and percentage of small and large ganglion neurons.

RESULTS

The neuronal area increased progressively in successively higher gestation age fetuses. In the fetus belonging to lowest gestational age the area ranged from 4-37μm2 while in highest gestational age fetus its range was 10-58.3μm2. The small ganglion neurons were higher in 14 weeks (65.5%) fetuses and 16-20 weeks (81.03%) fetuses, while in higher gestational age fetuses' large ganglion neuronal population was higher (62-66%).

CONCLUSION

A baseline morphometrical representation of fetal cochlear ganglion neurons could be of relevance in advanced human experimental studies on effect of neurotrophic factors in human fetuses with congenital deafness. It has been found that these factors directly influence neuronal maturation assessed by progressive increase in soma size and survival.