Broken force dispersal network in tip-links by the mutations induces hearing-loss

Abstract

Tip-link as force-sensor in the hearing conveys the mechanical force originating from sound to ion-channels while maintaining the integrity of the entire sensory assembly in inner-ear. This delicate balance between structure and function of tip-links is regulated by Ca 2+ -ions present in endolymph. Mutations at the Ca 2+ -binding sites of tip-links often lead to congenital deafness, sometimes syndromic defects impairing vision along with hearing.Although such mutations are already identified, it is still not clear how the mutants alter the structure-function properties of the force-sensors associated with diseases. With an aim to decipher the differences in force-conveying properties of the force-sensors in molecular details, we identified the conformational variability of mutant and wild-type tip-links at the single-molecule level using FRET at the endolymphatic Ca 2+ concentrations and subsequently measured the force-responsive behavior using single-molecule force spectroscopy with an AFM. AFM allowed us to mimic the high and wide range of force ramps (10 3 -10 6 pN.s -1 ) as experienced in the inner ear. We performed in silico network analyses to learn that alterations in the conformations of the mutants interrupt the natural force-propagation paths through the sensors and make the mutant tip-links vulnerable to input forces from sound stimuli. We also demonstrated that a Ca 2+ rich environment can restore the force-response of the mutant tip-links which may eventually facilitate the designing of better therapeutic strategies to the hearing loss. Significance StatementForce-sensors in inner ear are the key components in the hearing. Mutations in forcesensors often lead to congenital hearing loss. Loss of hearing has become a threat to humanity, with over 5% of world population suffering from deafness and 40% of which is congenital, primarily due to mutations in the sensory machinery in inner-ear. A better understanding of the molecular mechanism of the underlined hearing loss due to mutations is, therefore, necessary for better therapeutics to deaf. Here with a zoomed region of the force-sensors, we pointed out the differences in the force-propagation properties of the mutant and wild-type force-sensors. Our observation on restoring of functions of mutants

Tip-links serve as force-pass filter to fulfil the role of gating-springs

“…For Cdh23, such lateral coupling is formed between EC1 domain, however, only when in close-proximity. It is, thus, apparent that the cis-dimer cross-links are key to diffuse the applied forces orthogonally along cross-links(Arora et al, 2021; Hazra et al, 2019; Ott et al, 2015). We, therefore, propose that the heterotetrameric geometry of tip-links is designed and fabricated by nature on a purpose to serve as simplest gating-springs.…”