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

Abstract

156 Words) 2 , an atypical member of the cadherin superfamily, is essential for vertebrate hearing 3 and its dysfunction has been associated with deafness and progressive blindness. The PCDH15 ectodomain, 4 made of eleven extracellular cadherin (EC1-11) repeats and a membrane adjacent domain (MAD12), assembles 5 as a parallel homodimer that interacts with cadherin-23 (CDH23) to form the tip link, a fine filament necessary 6 for inner-ear mechanotransduction. Here we report X-ray crystal structures of a PCDH15 + CDH23 7 heterotetrameric complex and ten PCDH15 fragments that were used to build complete high-resolution models 8 of the monomeric PCDH15 ectodomain. Using molecular dynamics (MD) simulations and validated crystal 9 contacts we propose models for complete PCDH15 parallel homodimers and the tip-link bond. Steered MD 10 simulations of these models predict their strength and suggest conditions in which a multimodal PCDH15 11 ectodomain can act as a stiff or soft gating spring. These results provide a detailed view of the first molecular 12 steps in inner-ear sensory transduction.

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.…”

“…6c). As the concentration of Ca 2+ bathing the tip link decreases, the junctions between each EC domain become more flexible due to decreased occupancy of Ca 2+ 8 , and the series elasticity of EC domains is increased 40 . We showed that protein elasticity modifies the extent of avidity under force ( Fig.…”

“…9). Since CDH23 and PCDH15 are relatively stiff proteins 7,40,42 , are complexed with membrane and intracellular proteins 29,80,81 , and appear to be constrained at their transmembrane domains 11,12,82 , their N-termini may be constrained to a smaller volume and the effective concentration significantly higher, further increasing the re-forming rate. A more reasonable sphere of 50 nm gives an effective concentration of ~25 µM and a reforming time of just 0.29 s, 29 times faster than the unbinding rate at a resting tension of 10 pN.…”

Species-Dependent Heterophilic and Homophilic Cadherin Interactions in Intestinal Intermicrovillar Links

“…Interestingly, alignments of EC repeat sequences across up to 20 species for intermicrovillar and tip-link cadherins reveal poor conservation for PCDH24 and CDHR5 when compared to CDH23 and PCDH15. Average percent identity is 51.6% for CDH23 EC repeats and 45.5% for PCDH15 repeats, with the N- and C-terminal ends being more conserved than the middle EC repeats (Choudhary et al, 2019; Jaiganesh et al, 2018). In contrast, average percent identity is 16.4% for PCDH24 and 11% for CDHR5, considerably lower when compared to values for CDH23 and PCDH15.…”

“…The strength and dynamics of cadherin heterophilic bonds, which are present in systems subjected to mechanical stimuli, might also differ from and provide specific advantages over weak homophilic cadherin complexes that cluster to provide strong adhesiveness. Whether PCDH24 and CDHR5 engage in cis interactions that can lead to more mechanically robust intermicrovillar links, as observed for tip links (Choudhary et al, 2019; Dionne et al, 2018; Ge et al, 2018), or to the formation of larger adhesive complexes in non-microvillar membranes, as observed for classical cadherins and clustered protocadherins (Brasch et al, 2019, 2012; Schreiner and Weiner, 2010), remains to be explored.…”