Elasticity of individual protocadherin 15 molecules implicates tip links as the gating springs for hearing

Abstract

Hair cells, the sensory receptors of the inner ear, respond to mechanical forces originating from sounds and accelerations. An essential feature of each hair cell is an array of filamentous tip links, consisting of the proteins protocadherin 15 (PCDH15) and cadherin 23 (CDH23), whose tension is thought to directly gate the cell’s transduction channels. These links are considered far too stiff to represent the gating springs that convert hair bundle displacement into forces capable of opening the channels, and no mechanism has been suggested through which tip-link stiffness could be varied to accommodate hair cells of distinct frequency sensitivity in different receptor organs and animals. Consequently, the gating spring’s identity and mechanism of operation remain central questions in sensory neuroscience. Using a high-precision optical trap, we show that an individual monomer of PCDH15 acts as an entropic spring that is much softer than its enthalpic stiffness alone would suggest. This low stiffness implies that the protein is a significant part of the gating spring that controls a hair cell’s transduction channels. The tip link’s entropic nature then allows for stiffness control through modulation of its tension. We find that a PCDH15 molecule is unstable under tension and exhibits a rich variety of reversible unfolding events that are augmented when the Ca2+ concentration is reduced to physiological levels. Therefore, tip link tension and Ca2+ concentration are likely parameters through which nature tunes a gating spring’s mechanical properties.

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

“…Why? Recent studies using optical tweezers and molecular dynamics simulations suggest that tip link proteins are far more elastic than initially thought 39,40 . Elasticity will allow unbound domains in the B1 state ( Fig.…”

“…Ca 2+ ions link the EC domains in each tip-link protein 8,42 , and act to maintain its structural integrity 39 . Chelation of extracellular Ca 2+ disrupts tip links in intact hair cells 25 , and destabilizes the CDH23-PCDH15 interaction 8 ( Fig.…”

“…This may slow the time course of recovery independent of tip-link kinetics. Second, Ca 2+ chelation and extreme forces lower the energy barrier to tip-link protein unfolding 39,42 , which may severely damage tip links and associated mechanotransduction components and may require more lengthy refolding before re-forming 11 . Finally, extended treatment with EGTA or BAPTA abolishes CDH23 immunoreactivity at the tips of bullfrog stereocilia 36 and mostly removes CDH23 from the tip-link region in mouse 12 .…”

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

“…Thanks to such an active process, the paired channels would close and the adaptation springs would separate them, in turn affecting tip-link tension and ultimately performing mechanical work on the hair bundle on a cycle-by-cycle basis. Other, passive properties related to fast adaptation, such as tip-link viscoelasticity (51,52), could coexist with this active process. However, they cannot replace it.…”

Structural determinants of protocadherin-15 elasticity and function in inner-ear mechanotransduction

“…Disruption of either the 2 complex between PCDH15 and CDH23 or the oligomerization of PCDH15 impairs transduction 33,34,38 . The 3 inner-ear transduction channel is gated by a soft element called the "gating spring", which is either in series with 4 the tip link or the tip link itself [46][47][48]6,8,49 . Whether cadherin tip links are elastic or rigid has been 5 controversial 11,16,42 .…”