Methionine sulfoxide reductase A provides age-dependent protection from early-onset hearing loss in the mouse cochlea: anatomical study.

Background: Methionine sulfoxide reductase A (MsrA) protects the biological activity of proteins from oxidative stress damage and fights onset of age-related hearing loss (Alqudah, ARO 2015 and 2016). In the current study we attempted to determine the defect in the protein expression of Msrs in adults MsrA knockout mice and the role of MsrA in preventing loss of the cellular structures from aging. Method: Mid-modiolar sections from cochlea were incubated overnight at room temperature with the primary antibody for MsrA (Abcam, cat# ab16803, diluted 1:100). Next day, the slices were incubated with a biotinylated goat anti-rabbit IgG secondary antibody (Vector, cat # BA-1100 diluted1:400) for 3 h at room temperature, then with a streptavidin Alexa 488 (Thermo Fisher, cat# A20000 diluted1:1000) for 1 h. The stained sections were examined under a fluorescence microscope. For histology analysis, 6-month-old wild-type and MsrA knockout mice were perfused intracardially with a fixative solution containing 2.5% glutaraldehyde and 1.5% paraformaldehyde. Both inner ears were dissected, postfixed in osmium, embedded in Araldite, and sectioned at 20 µm on a microtome. Sections were then mounted on microscope slides and spiral ganglion neurons counted. Results: Although both mutants and controls showed the same staining intensity of the cochlear tissues, there was a clear difference in the immunolabeling of the stained brain sections. This observation suggests a relatively low abundance of MsrA protein within the cochlea. Semiquantitative analysis of plastic sections of cochlea from 6-month old MsrA knockout mice showed loss of cochlear spiral ganglion neurons by 15% in the 16-26 kHz frequencies region, and 60-70% loss of fibrocyte type IV cells when compared to age-matched control mice. Conclusion: In this study, we raise the possibility that MsrA represents one component of the protective mechanism that may be responsible for cochlear protection from aging