Transient expression of the t-isoform of plastins/fimbrin in the stereocilia of developing auditory hair cells.

The transduction of auditory signals by cochlear hair cells depends upon the integrity of hair cell stereociliary bundles. Stereocilia contain a central core of actin filaments, cross-linked by actin bundling proteins. In the cochlea, the two proteins described to date as responsible for the spatial arrangement of actin filaments in sterocilia are fimbrin and the recently discovered espin. Fimbrin (the chick homolog of human I-plastin) belongs to the plastins/fimbrin family that includes two additional isoforms of plastins, T- and L-plastin. In the present study, we used isoform specific antibodies to investigate the presence of the T- and L-isoforms of plastin/fimbrin in the adult and developing rat cochlea. We found that T-plastin, but not L-plastin, is expressed in the rat cochlea. During postnatal development of the rat organ of Corti, T-plastin can be detected in the core of stereocilia from early stages of hair cell differentiation, and its expression gradually increases in stereocilia as hair cells mature. However, as opposed to other actin-binding proteins expressed in stereocilia, T-plastin is absent from the stereocilia of mature hair cells. Such temporally restricted expression strengthens the idea of functional differences between plastins isoforms, and suggests that T-plastin could have a specific role in stereocilia formation.