Hearing Improvement In A/J Mice By Mouse Nerve Growth Factor Running Title:Hearing Improvement In A/J Mice

Background: Age related hearing loss (AHL) is the most common sensory deficit of elderly population which adversely affects the quality of life for these individuals15.Mouse models play a crucial role in understanding the pathogenesis associated with genetic hearing disorders.A/J mice are a typical mouse model of age related hearing loss16. This project was carried out to investigate the otoprotective effects of mouse nerve growth factor (mNGF) on A/J mice. Nerve growth factor (NGF) is a small secreted protein that is essential for viability, differentiation, and maintenance of nerve cell,It was the first neurotrophin to have a protective capacity in the inner ears in neomycin-induced auditory neural degeneration in guinea pigs17—. Methods: A/J mouse pups at postnatal day 7 (P7) were randomly chosen and divided into a mNGF treated group and an injection water treated group (control). Mice in the test group received mouse nerve growth factor in injection water (9 x 10-3 ?g/?l) by intramuscular injection in hips (3.6 x 10-3 ?g/g body weight) once every other day. The dosages of mNGF were calculated following the methods described previously(Reagan-Shaw et al.,2008). Mice in the control group received 0.4 ?l injection water per gram of body weight at the same time points as the mNGF treated group. ? ABR and DPOAE were measured at various intervals at 3, 4, 6, 8weeks for a Mngf treated group and an injection water treated group (control),and then all of the mice from each group were sacrificed for histological investigation.? The time points selected were 3weeks,4 weeks, 6 weeks, and 8 weeks. Sections (5 ?m) mounted on glass slides were counterstained in hematoxylinand eosin (HE)and . ?Observing of OHC in the cochleae at 8weeks,10 weeks. ?Assays for gene transcription of apoptosis related genes at 3 weeks,4 weeks,6 weeks and 8 weeks. ?Observe the expression of caspase-3 by western blotting of 4 weeks and 8 weeks.Results : Hearing loss occurs much earlier in A/J mice at about 4 weeks of age. The results showed that NGF significantly decreased ABR thresholds in A/J mice as compared with those of the ddH20 treated or untreated mice at stimulus frequencies of click, 8-, 16- and 32- kHz at 6 or 8 weeks of age. Moreover, NGF preserved hair cells and SGNs in the cochleae at 8 weeks of age. Further experiment showed that gene expression profiles in the inner ears of A/J mice were detected at 2, 4 and 8 weeks of age. mRNA levels of Bak, Bax, Caspase-3, Caspase-9 and Caspase-12 in mNGF treated mice were lower than those of control mice at age of 2 weeks. The amplitudes decreased for caspase mRNA levels at 4 weeks were more than other time points. Caspase-3 expression was confirmed by Western blotting at 4 weeks and 8 weeks of age.Conclusion : NGF shows otoprotective effects by preserving SGNs and hair cells in the cochleae. Inhibition of apoptotic related pathways is likely involved in the otoprotective process of NGF.