Hypervulnerability to Sound Exposure through Impaired Adaptive Proliferation of Peroxisomes

A deficiency in pejvakin, a protein of unknown function, causes a strikingly heterogeneous form of human deafness. Pejvakin-deficient (Pjvk(-/-)) mice also exhibit variable auditory phenotypes. Correlation between their hearing thresholds and the number of pups per cage suggest a possible harmful effect of pup vocalizations. Direct sound or electrical stimulation show that the cochlear sensory hair cells and auditory pathway neurons of Pjvk(-/-) mice and patients are exceptionally vulnerable to sound. Subcellular analysis revealed that pejvakin is associated with peroxisomes and required for their oxidative-stress-induced proliferation. Pjvk(-/-) cochleas display features of marked oxidative stress and impaired antioxidant defenses, and peroxisomes in Pjvk(-/-) hair cells show structural abnormalities after the onset of hearing. Noise exposure rapidly upregulates Pjvk cochlear transcription in wild-type mice and triggers peroxisome proliferation in hair cells and primary auditory neurons. Our results reveal that the antioxidant activity of peroxisomes protects the auditory system against noise-induced damage

Data from: LHFPL5 is a key element to the gating spring of cochlear hair cells: comparison of mechanotransduction channel activation in presence or absence of LHFPL5

<p>During auditory transduction, sound-evoked vibrations of the hair cell stereociliary bundles open mechanotransducer (MET) ion channels via tip links extending from one stereocilium to its neighbor. How tension in the tip link is delivered to the channel is not fully understood.  The MET channel comprises a pore-forming subunit, transmembrane channel-like protein (TMC1 or TMC2), aided by several accessory proteins, including LHFPL5 (lipoma HMGIC fusion partner-like 5). We investigated the role of LHFPL5 in transduction by comparing MET channel activation in outer hair cells of <em>Lhfpl5-/- </em>knockout mice with those in <em>Lhfpl5+/-</em> heterozygotes. The 10-90 percent working range of transduction in <em>Tmc1+/+; Lhfpl5+/-</em> was 52 nm, from which the single-channel gating force, Z was evaluated as 0.34 pN. However, in <em>Tmc1+/+; Lhfpl5‑/- </em>mice,<em> </em>the<em> </em>working range increased to 123 nm and Z more than halved to 0.13 pN, indicating reduced sensitivity. Tip link tension is thought to activate the channel via a gating spring, whose stiffness is inferred from the stiffness change on tip link destruction. The gating stiffness was ~40 percent of the total bundle stiffness in wild-type but was virtually abolished in <em>Lhfpl5-/-,</em> implicating LHFPL5 as a principal component of the gating spring.  The mutation <em>Tmc1 </em>p.D569N reduced the LHFPL5 immunolabeling in the stereocilia and like <em>Lhfpl5-/-</em> doubled the MET working range but other deafness mutations had no effect on the dynamic range.  We conclude that tip-link tension is transmitted to the channel primarily via LHFPL5; residual activation without LHFPL5 may occur by direct interaction between PCDH15 and TMC1.</p><p>Funding provided by: National Institute on Deafness and Other Communication Disorders<br>Crossref Funder Registry ID: https://ror.org/04mhx6838<br>Award Number: R01 DC01362</p><p>MET currents were recorded from outer hair cells (OHCs) in isolated organs of Corti of mice between P2 and P7. All whole cell recordings were performed at room temperature, ~23ºC.</p> <p>All data included in the published article are presented in an Excel file, giving all values obtained to construct the mean +/- 1 SD, for every recorded hair cell in all the different mouse genotypes used in the study.</p> <p>The Excel file is organized in 3 sheets, describing the MET current bundle displacement results, hair bundle stiffness data, and the hair bundle height measurements.</p> <p>Stereociliary bundles were stimulated with a fluid jet, and the bundle deflections calibrated by projecting the bundle image onto a pair of photodiodes and measuring the change in photocurrent. The Excel file gives all values obtained for the relationship between the MET current, I, and bundle displacement, X. The I-X relationship was fitted with a Boltzmann equation: I = I<sub>MAX</sub> /(1 + exp((X<sub>O </sub>-X)/X<sub>S</sub>), where I<sub>MAX </sub>is the maximum current, X<sub>O </sub>the half-saturating displacement and X<sub>S</sub> the slope factor.  The 10 to 90 percent working range of transduction (WR) is 4.4*X<sub>S.</sub></p> <p>Hair bundle stiffness was determined by calibrating the force generated by the fluid jet against a flexible glass fiber of 1 mN/m and plotting against bundle displacement. The Excel file shows all hair bundle stiffness measurements used to construct our mean +/- SD, for all cells and mouse genotype used in the study.</p> <p>Bundle height measurement was performed on solitary outer hair cells and the hair bundle length was measured from the root to the tip of the longest stereocilia.</p&gt

Atypical Tuning and Amplification Mechanisms in Gecko Auditory Hair Cells

The auditory papilla of geckos contains two zones of sensory hair cells, one covered by a continuous tectorial membrane affixed to the hair bundles and the other by discrete tectorial sallets each surmounting a transverse row of bundles. Gecko papillae are thought to encode sound frequencies up to 5 kHz, but little is known about the hair cell electrical properties or their role in frequency tuning. We recorded from hair cells in the isolated auditory papilla of the crested gecko, Correlophus ciliatus, and found that in both the nonsalletal region and part of the salletal region, the cells displayed electrical tuning organized tonotopically. Along the salletal zone, occupying the apical two-thirds of the papilla, hair bundle length decreased threefold and stereociliary complement increased 1.5-fold. The two morphological variations predict a 13-fold gradient in bundle stiffness, confirmed experimentally, which, when coupled with salletal mass, could provide passive mechanical resonances from 1 to 6 kHz. Sinusoidal electrical currents injected across the papilla evoked hair bundle oscillations at twice the stimulation frequency, consistent with fast electromechanical responses from hair bundles of two opposing orientations across the papilla. Evoked bundle oscillations were diminished by reducing Ca2+ influx, but not by blocking the mechanotransduction channels or inhibiting prestin action, thereby distinguishing them from known electromechanical mechanisms in hair cells. We suggest the phenomenon may be a manifestation of an electromechanical amplification that augments the passive mechanical tuning of the sallets over the high-frequency region

Calcium- and Otoferlin-Dependent Exocytosis by Immature Outer Hair Cells

International audienceImmature cochlear outer hair cells (OHCs) make transient synaptic contacts (ribbon synapses) with type I afferent nerve fibers, but direct evidence of synaptic vesicle exocytosis is still missing. We thus investigated calcium-dependent exocytosis in murine OHCs at postnatal day 2 (P2)–P3, a developmental stage when calcium current maximum amplitude was the highest. By using time-resolved patch-clamp capacitance measurements, we show that voltage step activation of L-type calcium channels triggers fast membrane capacitance increase. Capacitance increase displayed two kinetic components, which are likely to reflect two functionally distinct pools of synaptic vesicles, a readily releasable pool (RRP; τ = 79 ms) and a slowly releasable pool (τ = 870 ms). The RRP size and maximal release rate were estimated at ∼1200 vesicles and ∼15,000 vesicles/s, respectively. In addition, we found a linear relationship between capacitance increase and calcium influx, like in mature inner hair cells (IHCs). These results give strong support to the existence of efficient calcium-dependent neurotransmitter release in immature OHCs. Moreover, we show that immature OHCs, just like immature IHCs, are able to produce regenerative calcium-dependent action potentials that could trigger synaptic exocytosis in vivo . Finally, the evoked membrane capacitance increases were abolished in P2–P3 OHCs from mutant Otof −/− mice defective for otoferlin, despite normal calcium currents. We conclude that otoferlin, the putative major calcium sensor at IHC ribbon synapses, is essential to synaptic exocytosis in immature OHCs too