Acoustic input and efferent activity regulate the expression of molecules involved in cochlear micromechanics

Abstract

Electromotile activity in auditory outer hair cells (OHCs) is essential for sound 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 nicotinic Ach receptor (nAChR) transcripts in the cochlea, as shown by RT-Quantitative real-time PCR (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.

Keywords: acetylcholine α10 receptors; auditory cortex ablation; conductive hearing loss; descending control; prestin oligomerization.

Figure 1

Average ABR waveform obtained before and after the unilateral removal of the ossicular chain at different post surgery days (PSD), 1, 7 and 15. The stimulus onset starts at 1.7 ms and all waves were visible at 50 dB SPL in both ears before the lesion. Note that only waves from the contralateral ear to the surgery remain visible at 50 dB SPL after the lesion.

Figure 2

Changes in prestin protein expression after unilateral removal of the ossicular chain. Results from the ipsilateral (Ipsi.) and contralateral (Cont.) cochleae from three animals at 7 days after surgery (PSD7). The two tracks on the left are from two normal control animals.

Figure 3

Changes in the protein expression of prestin after unilateral ablation of AC at different post surgery days (PSD). Data represent the western blott results from both ipsilateral and contralateral ears to the surgery obtained at PSD1, 7 and 15. Ipsi = ipsilateral ear to the surgery. Contr. = contralateral ear to the surgery.

Figure 4

Changes in the mRNA levels of α-10 subunit, prestin and β-actin after unilateral AC ablations at different post surgery days, 1, 7 and 15. (A) Data of the α-10 subunit from the outer hair cell cholinergic receptor. (B) Data of prestin. (C) Data of β-actin. Results are presented by the mean ± stdev of the fold change. The statistical significance of the comparison between the fold changes of the post-surgery days and control condition is shown at the top of the bars. Bars from the same survival time that present differences between the ipsi- and contralateral ears are framed in a box, and their statistical significance is shown at the bottom. *p < 0.05; **p < 0.01; ***p < 0.001.