Progressive hearing loss and gradual deterioration of sensory hair bundles in the ears of mice lacking the actin-binding protein Eps8L2

Abstract

Mechanotransduction in the mammalian auditory system depends on mechanosensitive channels in the hair bundles that project from the apical surface of the sensory hair cells. Individual stereocilia within each bundle contain a core of tightly packed actin filaments, whose length is dynamically regulated during development and in the adult. We show that the actin-binding protein epidermal growth factor receptor pathway substrate 8 (Eps8)L2, a member of the Eps8-like protein family, is a newly identified hair bundle protein that is localized at the tips of stereocilia of both cochlear and vestibular hair cells. It has a spatiotemporal expression pattern that complements that of Eps8. In the cochlea, whereas Eps8 is essential for the initial elongation of stereocilia, Eps8L2 is required for their maintenance in adult hair cells. In the absence of both proteins, the ordered staircase structure of the hair bundle in the cochlea decays. In contrast to the early profound hearing loss associated with an absence of Eps8, Eps8L2 nullmutant mice exhibit a late-onset, progressive hearing loss that is directly linked to a gradual deterioration in hair bundle morphology. We conclude that Eps8L2 is required for the long-term maintenance of the staircase structure and mechanosensory function of auditory hair bundles. It complements the developmental role of Eps8 and is a candidate gene for progressive age-related hearing loss.deafness | sensory system | ion channel H earing and balance depend on the transduction of mechanical stimuli into electrical signals. Transduction involves activation of mechanically gated ion channels near the tips of the stereocilia, specialized microvilli that form the hair bundles that project from the surface of sensory hair cells (1). Stereocilia have a cytoskeletal core composed of tightly packed, cross-linked, and uniformly polarized actin filaments (2, 3). Stereociliary length is regulated to ensure the characteristic staircase-like structure of each bundle, whose overall size and shape depends on location along the sensory organ (4). In the mammalian cochlea, hair bundles usually include three rows of stereocilia coupled by several types of extracellular links (2, 5). The embryonic and postnatal development of the bundle involves elongation and thickening of stereocilia, as well as elimination of redundant stereocilia (4, 5).In the adult cochlea, the height of stereocilia within each row is similar, not only within a single hair bundle but also between the bundles of adjacent hair cells, indicating a sophisticated level of control over growth (5, 6). Stereociliary growth and maintenance involves actin-binding proteins such as espin (7,8), plastin (9), twinfilin 2 (10), gelsolin (11), and unconventional myosin motors including myosin XVa (12) and myosin IIIa (13). Currently, we do not have a complete molecular understanding of hair bundle structure or how its growth and maintenance are controlled. Recently, we showed that epidermal growth factor receptor pathway substrate 8 (Eps8) (14) is located ...

Gαi Proteins are Indispensable for Hearing

“…Deletion of the Eps8L2, a member of the Eps8-like protein family, causes deafness [55,69] similarly to Gnai2/i3 cKO. Strikingly, the deletion of Eps8 induced progressive high-frequency hearing loss [75] similar to the phenotype observed here in Gnai3 KO mice. Further studies will clarify whether Gα i protein expression and/or signaling depends on Eps8 or vice versa.…”

Development of the inner ear

“…A key player in hair cell differentiation is the autoregulatory basic helix-loop-helix factor Atoh1, which induces expression of Dl1, and is regulated by both Sox2 31]. Work in the zebrafish has generated new models of Usher syndrome, demonstrating that ER stress is likely to underlie the hair cell death in this disorder [32].…”

Stereocilia morphogenesis and maintenance through regulation of actin stability

“…Some of these, such as myosin XVa ( Myo15 ), whirlin ( Whrn) , Epidermal growth factor receptor pathway substrate 8 ( Eps8 ), myosin IIIa and IIIb ( Myo3a and Myo3b) and espin-1 (splice isoform of Espn ) are important for growth of the actin core [16–22]. Others, including CapZ, Eps8-like 2 ( Eps8L2 ) and Twinfilin-2 ( Twf2 ) have actin filament capping activity and likely regulate actin filament elongation [2,23–25]. Interestingly, the protein composition of stereocilia tips differs in the tallest row of the bundle compared to those in shorter rows.…”

“…Interestingly, the protein composition of stereocilia tips differs in the tallest row of the bundle compared to those in shorter rows. In particular, EPS8 is enriched in row 1 while EPS8L2 and TWF2 are largely restricted to rows 2 and 3 along with the long splice isoform of myosin XVa [2,24,26,27]. This localization pattern is likely important for generating or maintaining the staircase morphology of the bundle.…”