Differential expression of β tubulin isotypes in the adult gerbil cochlea

Tubulin, the principal component of microtubules, exists as two polypeptides, termed α and β. Seven isotypes of β tubulin are known to exist in mammals. The distributions of four β tubulin isotypes, β(I), β(II), β(III), and β(IV), have been examined in the adult cochlea by indirect immunofluorescence using isotype-specific antibodies. In the organ of Corti, outer hair cells contained only β(I) and β(IV), while inner hair cells contained only β(I) and β(II). Inner and outer pillar cells contained β(II) and β(IV), but Deiters cells contained those isotypes plus β(I). Fine fibers in the inner spiral bundle, tunnel crossing fibers, and outer spiral fibers, probably efferent in character, contained β(I), β(II), and β(III), but not β(IV). In the spiral ganglion, the somas and axons of neurons contained all four isotypes, and the myelination of ganglion cells also contained β(I). Fibers of the intraganglionic spiral bundle contained β(I), β(II), and β(III). No antibody labeled the dendritic processes of spiral ganglion neurons. The differences in isotype distribution in organ of Corti and neurons described here are consistent with and support the multi- tubulin hypothesis, which states that tubulin isotypes are expressed specifically in different cell types and may therefore have different functions. (C) 2000 Elsevier Science B.V.