Elastic properties of the inner-ear tip link explored using all-atom molecular dynamics simulations.

Abstract

Vertebrate sound and balance perception relies on a process known as mechanotransduction, or the transformation of mechanical sensory information such as sound and head movements into electrochemical signals. Mechanical force sensation occurs in the apical region of inner-ear hair cells, where tensile forces applied to a protein complex called the tip link causes activation of nearby ion channels through a force-gated mechanism. The tip link is a filamentous complex formed by cadherin-23 (CDH23) and protocadherin-15 (PCDH15), proteins with large ectodomains that likely assemble as heterotetrameric structures in vivo. While the identity of the putative mechanotransduction ion channel has been revealed, it remains unclear how the tip link propagates force to it at the molecular level. Studying the mechanical properties of the tip link in situ has been challenging, while in vitro and in silico studies dealing with this large complex have been inconclusive. Using existing and new x-ray crystal structures combined with AlphaFold2 predictions for subdomains of the CDH23 and PCDH15 ectodomains, we constructed an atomistic model of the entire extracellular tip link for in silico force application. Here, we present all-atom equilibrium and steered molecular dynamics simulations of the entire inner-ear tip link ectodomain with predictions of strength and structural domains relevant for its mechanical response. We conclude that the presence of dimerization and membrane adjacent domains (MAD) account for rigid and elastic responses of the tip link responsible for modulation of force propagation. With these results, we begin to understand how the tip-link complex may convey force to ion channels and how deafness mutations on the CDH23 and PCDH15 ectodomains affect their function.