Evidence for changes in beta- and gamma-actin proportions during inner ear hair cell life.

Cytoplasmic actin isoforms beta (β-) and gamma (γ-) perform crucial physiological roles in inner ear hair cells (HC). The stereocilium, which is structured by parallel actin filaments composed of both isoforms, is the responsive organelle to mechanical stimuli such as sound, gravity and head movements. Modifications in isoform proportions affect the function of the stereocilia as previously shown in genetic studies of mutant mice. Here, immunogold labeling TEM studies in mice showed that both β- and γ-actin isoforms colocalize throughout stereocilia actin filaments, adherens junctions and cuticular plates as early as embryonic stage 16.5. Gold-particle quantification indicated that there was 40% more γ- actin than β-actin at E16.5. In contrast, β- and γ-actin were equally concentrated in adult stereocilia of cochlear and vestibular HC. Interestingly, all actin-based structures presented almost five-fold more β-actin than γ-actin in 22 month- old mice, suggesting that γ-actin is probably under-expressed during the aging process. These data provide evidence of dynamic modifications of the actin isoforms in stereocilia, cuticular plates and cell junctions during the whole HC life.