Article

The α10 nicotinic acetylcholine receptor subunit is required for normal synaptic function and integrity of the olivocochlear system

Department of Neuroscience, Tufts University School of Medicine, Boston, MA 02111, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 01/2008; 104(51):20594-9. DOI: 10.1073/pnas.0708545105
Source: PubMed

ABSTRACT Although homomeric channels assembled from the alpha9 nicotinic acetylcholine receptor (nAChR) subunit are functional in vitro, electrophysiological, anatomical, and molecular data suggest that native cholinergic olivocochlear function is mediated via heteromeric nAChRs composed of both alpha9 and alpha10 subunits. To gain insight into alpha10 subunit function in vivo, we examined olivo cochlear innervation and function in alpha10 null-mutant mice. Electrophysiological recordings from postnatal (P) days P8-9 inner hair cells revealed ACh-gated currents in alpha10(+/+) and alpha10(+/-) mice, with no detectable responses to ACh in alpha10(-/-) mice. In contrast, a proportion of alpha10(-/-) outer hair cells showed small ACh-evoked currents. In alpha10(-/-) mutant mice, olivocochlear fiber stimulation failed to suppress distortion products, suggesting that the residual alpha9 homomeric nAChRs expressed by outer hair cells are unable to transduce efferent signals in vivo. Finally, alpha10(-/-) mice exhibit both an abnormal olivocochlear morphology and innervation to outer hair cells and a highly disorganized efferent innervation to the inner hair cell region. Our results demonstrate that alpha9(-/-) and alpha10(-/-) mice have overlapping but nonidentical phenotypes. Moreover, alpha10 nAChR subunits are required for normal olivocochlear activity because alpha9 homomeric nAChRs do not support maintenance of normal olivocochlear innervation or function in alpha10(-/-) mutant mice.

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    • "We tested changes in Prestin protein expression by Western blot after unilateral conductive hearing loss as a measure of regulation by acoustic input imbalance, and compared this with changes in Prestin protein expression after unilateral AC ablation. In parallel, because OHC electromotility regulation by MOC involves specialized cholinergic neurotransmission, we tested by qRT-PCR whether the expression of the alpha10 nAChR gene mRNA, a staple of cholinergic neurotransmission at the MOC-OHC synapse (Dallos et al., 1997; Maison et al., 2002, 2007; Batta et al., 2004; Vetter et al., 2007), changes along with the expression of Prestin and β-actin genes, as cell markers of the effects of MOC inactivation on OHC electromotility. "
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    ABSTRACT: amplification. It relies on the highly specialized membrane motor protein prestin, and its interactions with the cytoskeleton. It is believed that the expression of prestin and related molecules involved in OHC electromotility may be dynamically regulated by signals from the acoustic environment. However little is known about the nature of such signals and how they affect the expression of molecules involved in electromotility in OHCs. We show evidence that prestin oligomerization is regulated, both at short and relatively long term, by acoustic input and descending efferent activity originating in the cortex, likely acting in concert. Unilateral removal of the middle ear ossicular chain reduces levels of trimeric prestin, particularly in the cochlea from the side of the lesion, whereas monomeric and dimeric forms are maintained or even increased in particular in the contralateral side, as shown in Western blots. Unilateral removal of the auditory cortex (AC), which likely causes an imbalance in descending efferent activity on the cochlea, also reduces levels of trimeric and tetrameric forms of prestin in the side ipsilateral to the lesion, whereas in the contralateral side prestin remains unaffected, or even increased in the case of trimeric and tetrameric forms. As far as efferent inputs are concerned, unilateral ablation of the AC up-regulates the expression of α10 nAChR transcripts in the cochlea, as shown by RT-qPCR. This suggests that homeostatic synaptic scaling mechanisms may be involved in dynamically regulating OHC electromotility by medial olivocochlear efferents. Limited, unbalanced efferent activity after unilateral AC removal, also affects prestin and β-actin mRNA levels. These findings support that the concerted action of acoustic and efferent inputs to the cochlea is needed to regulate the expression of major molecules involved in OHC electromotility, both at the transcriptional and posttranscriptional levels.
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    • "The expression of these genes and the distribution of the proteins in the cytoplasm or in the plasma membrane (but not in the nucleus) must therefore be regarded as a consequence, not a cause for the HC type specific differentiation. Consistent with this suggestion is that null mutants of these genes have either no effect on HC type specific differentiation (Vetter et al., 2007) or cause only morphologic alterations in HC stereocilia followed by the eventual loss of all HCs (Friedman et al., 1999). More interesting are expressions of TFs, or diffusible substances such as Fgf8, selectively in one specific HC type. "
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    ABSTRACT: Basic helix-loop-helix (bHLH) transcription factors (TFs) are crucial for inner ear neurosensory development. The proneural TF Atoh1 regulates the differentiation of hair cells (HCs) whereas Neurog1 and Neurod1 regulate specification and differentiation of neurons, respectively, but also affect HC development. Expression of Delta and Jagged ligands in nascent HCs and Notch receptors in supporting cells induce supporting cell differentiation through the regulation of neurogenic bHLH TFs (such as Hes1, Hes5) and suppression of limited Atoh1 expression. In sensorineural hearing loss, HCs are lost followed by supporting cells and progressive degeneration of neurons, at least in rodents. Regaining complete hearing may require reconstituting the organ of Corti (OC) from scratch, including the two types of HCs, inner (IHC) and outer (OHC) hair cells with the precise sorting of two types of afferent (type I and II) and efferent (lateral, LOC and medial, MOC olivo-cochlear) innervation. We review effects of bHLH TF dosage and their cross-regulation to differentiate HC types in the OC. We categorize findings of specific gene expressions in HCs: 1. as markers without meaning for the regeneration task, 2. as stabilizers who are needed to maintain or complete differentiation, and 3. as decision making genes, expressed and acting early enough to be useful in this process. Only one TF has been characterized that fits the last aspect: Atoh1. We propose that temporal and intensity variations of Atoh1 are naturally modulated to differentiate specific types of HCs. Importantly, the molecular means to modify the Atoh1 expression are at least partially understood and can be readily implemented in the attempts to regenerate specific types of HCs.
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    • "Although the peripheral effects of both systems are mediated by ACh, the receptors on skeletal muscles, such as the stapedius, and those on the OHCs differ in their pharmacology: muscle ACh receptors are blocked by the paralytic curare (Fig. 7), whereas ACh receptors on OHCs, ␣9 and ␣10 subunits of the nicotinic type, are most effectively blocked by strychnine (Fig. 3) (Elgoyhen et al. 2001; Rothlin et al. 1999). Because these OHC receptors are not widely expressed in the body, they can be targeted for genetic deletion without embryonic lethality: prior studies have shown that MOC effects are eliminated by deletion of either the ␣9 or ␣10 subunit gene (Vetter et al. 1999, 2007). Middle-ear muscle activation can also be inferred from changes in the ear's input impedance, which, when present, change ipsilateral ear canal sound pressure synchronously with the onset (and offset) of the contranoise stimuli. "
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