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

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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Single Molecule Force Spectroscopy of Hair-Cell Tip-Link Proteins

Biophysical Journal ◽

10.1016/j.bpj.2015.11.1090 ◽

2016 ◽

Vol 110 (3) ◽

pp. 196a

Author(s):

Mounir A. Koussa ◽

Andrew Ward ◽

Marcos Sotomayor ◽

Wesley P. Wong ◽

David P. Corey

Keyword(s):

Hair Cell ◽

Single Molecule ◽

Force Spectroscopy ◽

Single Molecule Force Spectroscopy ◽

Tip Link

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

10.1101/763847 ◽

2019 ◽

Author(s):

Eric M. Mulhall ◽

Andrew Ward ◽

Darren Yang ◽

Mounir A. Koussa ◽

David P. Corey ◽

...

Keyword(s):

Hair Cell ◽

Inner Ear ◽

Single Molecule ◽

Circuit Breaker ◽

Dynamic Strength ◽

Mechanoelectric Transduction ◽

Tip Link ◽

Molecular Machinery ◽

Chemical Conditions ◽

Mechanical Circuit

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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