Correlation of expression of the actin filament-bundling protein espin with stereociliary bundle formation in the developing inner ear

Authors

  • Huawei Li,

    1. Department of Otolaryngology and Program in Neuroscience, Harvard Medical School, and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114
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  • Hong Liu,

    1. Department of Otolaryngology and Program in Neuroscience, Harvard Medical School, and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114
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  • Steve Balt,

    1. Laboratory of Sensory Neuroscience and Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10021
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  • Sabine Mann,

    1. Department of Otolaryngology and Program in Neuroscience, Harvard Medical School, and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114
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  • C. Eduardo Corrales,

    1. Department of Otolaryngology and Program in Neuroscience, Harvard Medical School, and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114
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  • Stefan Heller

    Corresponding author
    1. Department of Otolaryngology and Program in Neuroscience, Harvard Medical School, and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114
    • EPL-Massachusetts Eye and Ear Infirmary, Boston, MA 02114
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    • At the outset of the investigation, S.H. was an Associate of the Howard Hughes Medical Institute.


Abstract

The vertebrate hair cell is named for its stereociliary bundle or hair bundle that protrudes from the cell's apical surface. Hair bundles mediate mechanosensitivity, and their highly organized structure plays a critical role in mechanoelectrical transduction and amplification. The prototypical hair bundle is composed of individual stereocilia, 50–300 in number, depending on the animal species and on the type of hair cell. The assembly of stereocilia, in particular, the formation during development of individual rows of stereocilia with descending length, has been analyzed in great morphological detail. Electron microscopic studies have demonstrated that stereocilia are filled with actin filaments that are rigidly cross-linked. The growth of individual rows of stereocilia is associated with the addition of actin filaments and with progressively increasing numbers of cross-bridges between actin filaments. Recently, a mutation in the actin filament-bundling protein espin has been shown to underlie hair bundle degeneration in the deaf jerker mouse, subsequently leading to deafness. Our study was undertaken to investigate the appearance and developmental expression of espin in chicken inner ear sensory epithelia. We found that the onset of espin expression correlates with the initiation and growth of stereocilia bundles in vestibular and cochlear hair cells. Intense espin immunolabeling of stereocilia was colocalized with actin filament staining in all types of hair cells at all developmental stages and in adult animals. Our analysis of espin as a molecular marker for actin filament cross-links in stereocilia is in full accordance with previous morphological studies and implicates espin as an important structural component of hair bundles from initiation of bundle assembly to mature chicken hair cells. J. Comp. Neurol. 468:125–134, 2004. © 2003 Wiley-Liss, Inc.

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