Differential distribution of β- and γ-actin in guinea-pig cochlear sensory and supporting cells

Sensory and supporting cells of the mammalian organ of Corti have cytoskeletons containing β- and γ-actin isoforms which have been described as having differing intracellular distributions in chick cochlear hair cells. Here, we have used post-embedding immunogold labelling for β- and γ-actin to investigate semiquantitatively how they are distributed in the guinea-pig cochlea and to compare different frequency locations. Amounts of β-actin decrease and γ-actin increase in the order, outer pillar cells, inner pillar cells, Deiters' cells and hair cells. There is also more β-actin and less γ-actin in outer pillar cells in higher than lower frequency regions. In hair cells, β-actin is present in the cuticular plate but is more concentrated in the stereocilia, especially in the rootlets and towards the periphery of their shafts; labelling densities for γ-actin differ less between these locations and it is the predominant isoform of the hair-cell lateral wall. Alignments of immunogold particles suggest β-actin and γ-actin form homomeric filaments. These data confirm differential distribution of these actin isoforms in the mammalian cochlea and reveal systematic differences between sensory and supporting cells. Increased expression of β-actin in outer pillar cells towards the cochlear base may contribute to the greater stiffness of this region.