Distinct roles of Eps8 in the maturation of cochlear and vestibular hair cells
Graphical abstract
Introduction
Hair cells are the sensory receptors of the auditory-vestibular system in all vertebrates. Sound or motion is transduced into electrical signals by the hair bundles, the mechanosensory actin-packed stereocilia protruding from the apical surface of hair cells. Stereocilia of different lengths are arranged in a staircase-like structure, the number of which changes depending on the inner ear organ. Hair bundle deflection, induced by sound or head motion, modulates the open probability of mechanoelectrical transducer channels localized at the tips of the stereocilia (Beurg et al., 2009), and as such generates a receptor potential. Cell voltage responses are then shaped by different types of voltage-dependent ion channels, among which Ca2+ channels are coupled to neurotransmitter (glutamate) exocytosis. Several genetic mutations that cause deafness in mice and humans have been shown to affect the development and/or function of cochlear hair cells (see Hereditary Hearing Loss Homepage http://hereditaryhearingloss.org). Given the common embryonic origins and biology of the auditory and vestibular hair cells, it is conceivable to postulate that single gene mutations known to cause inherited hearing loss would also lead to vestibular dysfunction. Instead, vestibular function is often retained even in the case of profound deafness (Jones and Jones, 2014). One possible explanation is that deficits in vestibular hair cells have sometimes gone undetected because of compensation or adaptation by the central nervous system.
Deletion of Epidermal growth factor receptor pathway substrate 8 (Eps8), a gene involved in actin remodeling (Di Fiore and Scita, 2002), hampers normal stereocilia growth (Manor et al., 2011, Zampini et al., 2011) and ion channel expression in mouse cochlear inner hair cells (IHCs) (Zampini et al., 2011). Despite the similar expression profile of Eps8 in cochlear and vestibular hair cells (Manor et al., 2011, Zampini et al., 2011), Eps8 knockout (KO) mice are deaf but show no obvious vestibular deficits. Similarly, it has been reported that a biallelic nonsense mutation of human EPS8, presumably coding a truncated non-functional protein or none (like the Eps8-KO mouse), results in deafness but no balance defects (Behlouli et al., 2014). In order to elucidate the importance of Eps8 in vestibular hair cells, we have investigated the bundle morphology and the biophysical properties of vestibular hair cells of the Eps8-KO mouse and compare them to those of WT mice. We provide evidence that the absence of Eps8 alters the growth of vestibular hair cells stereocilia. We have also found that, different from IHCs, the receptor potential of vestibular hair cells was not affected by the absence of Eps8. The above findings could explain why Eps8 deletion, and presumably EPS8 mutation, primarily affects the auditory function.
Section snippets
Experimental procedures
All procedures used were approved by the Ministero Italiano della Salute (Rome, Italy) and animal experiments were carried out in accordance with the European Communities Council Directive of 24 November 1986 (86/609/EEC). Eps8-KO mice were obtained by breeding heterozygote mice. All animals were genotyped as previously described (Offenhäuser et al., 2006). To increase the number of controls, and particularly for the hair cells dissociation protocol (see below), wild-type (WT) mice (C57 and
Morphological features of Eps8-KO vestibular hair cells
Eps8 is expressed at the stereocilia tips of both the cochlear and the vestibular hair cells (Manor et al., 2011). The hair bundles of Eps8-KO IHCs are shorter than normal, with the first (longest) row of stereocilia being the most affected (Zampini et al., 2011). Since no clear data are available concerning Eps8-KO vestibular hair bundles, we first investigated this aspect. Fig. 1A, B show the hair bundle populations in the intermediate region of the crista ampullaris from a WT and an Eps8-KO
Discussion
We found that in the absence of Eps8 the normal growth of the stereociliary bundle was prevented in vestibular hair cells, which is consistent with previous observations from cochlear hair cells (Manor et al., 2011, Zampini et al., 2011). Despite this morphological defect, Eps8-KO mice are deaf but do not show any obvious vestibular defects (Manor et al., 2011, Zampini et al., 2011). One possible explanation for this discrepancy is that vestibular hair cells retain the kinocilium and have much
Acknowledgments
Tavazzani Elisa, Spaiardi Paolo, Contini Donatella and Manca Marco performed and analyzed the experiments on vestibular hair cells; Zampini Valeria performed and analyzed the experiments on vestibular and cochlear hair cells and helped with the writing of the manuscript; Russo Giancarlo and Prigioni Ivo helped with the design of the experiments, the figures and the writing of the manuscript; Marcotti Walter helped with the experiments and the data analysis of experiments on cochlear hair cells,
References (47)
- et al.
The precise temporal pattern of prehearing spontaneous activity is necessary for tonotopic map refinement
Neuron
(2014) - et al.
Intercellular K+ accumulation depolarizes Type I vestibular hair cells and their associated afferent nerve calyx
Neuroscience
(2012) - et al.
Eps8 in the midst of GTPases
Int J Biochem Cell Biol
(2002) - et al.
Hair cells in mammalian utricles
Otolaryngol Head Neck Surg
(1998) How are inner hair cells stimulated? Evidence for multiple mechanical drives
Hear Res
(2012)- et al.
Initial characterization of kinocilin, a protein of the hair cell kinocilium
Hear Res
(2005) - et al.
Dynamic length regulation of sensory stereocilia
Semin Cell Dev Biol
(2008) - et al.
Regulation of stereocilia length by myosin XVa and whirlin depends on the actin-regulatory protein Eps8
Curr Biol
(2011) - et al.
Increased ethanol resistance and consumption in Eps8 knockout mice correlates with altered actin dynamics
Cell
(2006) - et al.
Phase-locking in the cochlear nerve of the guinea-pig and its relation to the receptor potential of inner hair-cells
Hear Res
(1986)
A model for signal transmission in an ear having hair cells with free-standing stereocilia. III. Micromechanical stage
Hear Res
Cell potential and motility of isolated mammalian vestibular sensory cells
Hear Res
EPS8, encoding an actin-binding protein of cochlear hair cell stereocilia, is a new causal gene for autosomal recessive profound deafness
Orphanet J Rare Dis
Significance of the steady-state current voltage relationship
Localization of inner hair cell mechanotransducer channels using high-speed calcium imaging
Nat Neurosci
Vestibular hair cells and afferents: two channels for head motion signals
Annu Rev Neurosci
Genetic insights into the morphogenesis of inner ear hair cells
Nat Rev Genet
Progressive hearing loss and gradual deterioration of sensory hair bundles in the ears of mice lacking the actin-binding protein Eps8L2
Proc Natl Acad Sci USA
Developmental acquisition of voltage-dependent conductances and sensory signaling in hair cells of the embryonic mouse inner ear
J Neurosci
Hair bundle defects and loss of function in the vestibular end organs of mice lacking the receptor-like inositol lipid phosphatase PTPRQ
J Neurosci
Function and expression pattern of nonsyndromic deafness genes
Curr Mol Med
M-like K+ currents in type I hair cells and calyx afferent endings of the developing rat utricle
J Neurosci
The resting transducer current drives spontaneous activity in prehearing mammalian cochlear inner hair cells
J Neurosci
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Present address: Paris Descartes University, Biomedical and Fundamental Science Faculty, Neurophotonics Laboratory, CNRS UMR8250, 45, rue des Saints Pères, 75270 Paris Cedex 06, France.
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Present address: Department of Anatomy and Cell Biology, University of Illinois College of Medicine, 808 S. Wood St., Chicago, IL 60612, USA.