The Dynamic Strength of the Hair-Cell Tip Link Reveals Mechanisms of Hearing and Deafness

AbstractOur senses of hearing and balance rely on the extraordinarily sensitive molecular machinery of the inner ear to convert deflections as small as the width of a single carbon atom1,2 into electrical signals that the brain can process3. In humans and other vertebrates, transduction is mediated by hair cells4, where tension on tip links conveys force to mechanosensitive ion channels5. Each tip link comprises two helical filaments of atypical cadherins bound at their N-termini through two unique adhesion bonds6–8. Tip links must be strong enough to maintain a connection to the mechanotransduction channel under the dynamic forces exerted by sound or head movement—yet might also act as mechanical circuit breakers, releasing under extreme conditions to preserve the delicate structures within the hair cell. Previous studies have argued that this connection is exceptionally static, disrupted only by harsh chemical conditions or loud sound9–12. However, no direct mechanical measurements of the full tip-link connection have been performed. Here we describe the dynamics of the tip-link connection at single-molecule resolution and show how avidity conferred by its double stranded architecture enhances mechanical strength and lifetime, yet still enables it to act as a dynamic mechanical circuit breaker. We also show how the dynamic strength of the connection is facilitated by strong cis-dimerization and tuned by extracellular Ca2+, and we describe the unexpected etiology of a hereditary human deafness mutation. Remarkably, the connection is several thousand times more dynamic than previously thought, challenging current assumptions about tip-link stability and turnover rate, and providing insight into how the mechanotransduction apparatus conveys mechanical information. Our results reveal fundamental mechanisms that underlie mechanoelectric transduction in the inner ear, and provide a foundation for studying multi-component linkages in other biological systems.

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Single-molecule force spectroscopy reveals the dynamic strength of the hair-cell tip-link connection

Nature Communications ◽

10.1038/s41467-021-21033-6 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Eric M. Mulhall ◽

Andrew Ward ◽

Darren Yang ◽

Mounir A. Koussa ◽

David P. Corey ◽

...

Keyword(s):

Hair Cell ◽

Single Molecule ◽

Force Spectroscopy ◽

Dynamic Strength ◽

Single Strand ◽

Mechanical Stimuli ◽

Static Structure ◽

Single Molecule Force Spectroscopy ◽

Hereditary Deafness ◽

Tip Link

AbstractThe conversion of auditory and vestibular stimuli into electrical signals is initiated by force transmitted to a mechanotransduction channel through the tip link, a double stranded protein filament held together by two adhesion bonds in the middle. Although thought to form a relatively static structure, the dynamics of the tip-link connection has not been measured. Here, we biophysically characterize the strength of the tip-link connection at single-molecule resolution. We show that a single tip-link bond is more mechanically stable relative to classic cadherins, and our data indicate that the double stranded tip-link connection is stabilized by single strand rebinding facilitated by strong cis-dimerization domains. The measured lifetime of seconds suggests the tip-link is far more dynamic than previously thought. We also show how Ca2+ alters tip-link lifetime through elastic modulation and reveal the mechanical phenotype of a hereditary deafness mutation. Together, these data show how the tip link is likely to function during mechanical stimuli.

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The development of cooperative channels explains the maturation of hair cell’s mechanotransduction

10.1101/654764 ◽

2019 ◽

Author(s):

Francesco Gianoli ◽

Thomas Risler ◽

Andrei S. Kozlov

Keyword(s):

Ion Channels ◽

Hair Cell ◽

Inner Ear ◽

Hair Cells ◽

Hair Bundle ◽

Mechanical Stimuli ◽

Biophysical Properties ◽

Tip Link ◽

Fast Adaptation ◽

Kinetics Of

ABSTRACTHearing relies on the conversion of mechanical stimuli into electrical signals. In vertebrates, this process of mechano-electrical transduction (MET) is performed by specialized receptors of the inner ear, the hair cells. Each hair cell is crowned by a hair bundle, a cluster of microvilli that pivot in response to sound vibrations, causing the opening and closing of mechanosensitive ion channels. Mechanical forces are projected onto the channels by molecular springs called tip links. Each tip link is thought to connect to a small number of MET channels that gate cooperatively and operate as a single transduction unit. Pushing the hair bundle in the excitatory direction opens the channels, after which they rapidly reclose in a process called fast adaptation. It has been experimentally observed that the hair cell’s biophysical properties mature gradually during postnatal development: the maximal transduction current increases, sensitivity sharpens, transduction occurs at smaller hair-bundle displacements, and adaptation becomes faster. Similar observations have been reported during tip-link regeneration after acoustic damage. Moreover, when measured at intermediate developmental stages, the kinetics of fast adaptation varies in a given cell depending on the magnitude of the imposed displacement. The mechanisms underlying these seemingly disparate observations have so far remained elusive. Here, we show that these phenomena can all be explained by the progressive addition of MET channels of constant properties, which populate the hair bundle first as isolated entities, then progressively as clusters of more sensitive, cooperative MET channels. As the proposed mechanism relies on the difference in biophysical properties between isolated and clustered channels, this work highlights the importance of cooperative interactions between mechanosensitive ion channels for hearing.SIGNIFICANCEHair cells are the sensory receptors of the inner ear that convert mechanical stimuli into electrical signals transmitted to the brain. Sensitivity to mechanical stimuli and the kinetics of mechanotransduction currents change during hair-cell development. The same trend, albeit on a shorter timescale, is also observed during hair-cell recovery from acoustic trauma. Furthermore, the current kinetics in a given hair cell depends on the stimulus magnitude, and the degree of that dependence varies with development. These phenomena have so far remained unexplained. Here, we show that they can all be reproduced using a single unifying mechanism: the progressive formation of channel pairs, in which individual channels interact through the lipid bilayer and gate cooperatively.

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