In Silico Electrophysiology of Inner-Ear Mechanotransduction Channel TMC1 Models

Abstract

Inner-ear sensory hair cells convert mechanical stimuli from sound and head movements into electrical signals during mechanotransduction. Identification of all molecular components of the inner-ear mechanotransduction apparatus is ongoing; however, there is strong evidence that TMC1 and TMC2 are pore-forming subunits of the complex. We present molecular dynamics simulations that probe ion conduction of TMC1 models built based on two different structures of related TMEM16 proteins. Unlike most channels, the TMC1 models do not show a central pore. Instead, simulations of these models in a membrane environment at various voltages reveal a peripheral permeation pathway that is exposed to lipids and that shows cation permeation at rates comparable to those measured in hair cells. Furthermore, our analyses suggest that TMC1 gating mechanisms involve protein conformational changes and tension-induced lipid-mediated pore widening. These results provide insights into ion conduction and activation mechanisms of hair-cell mechanotransduction channels essential for hearing and balance.

Mechanical gating of the auditory transduction channel TMC1 involves the fourth and sixth transmembrane helices

“…Each subunit of the dimer is thought to contain 10 TM helices and encompass a peripheral ionconduction pathway (fig. S1) (3,5,8). Channel activity of TMC homologs expressed in heterologous cell lines (9) is also consistent with the idea that TMC1 and TMC2 constitute the transduction channel pore.…”

“…We previously used cysteine mutagenesis and cysteine-modifying reagents to implicate TMC1 TM domains 4 to 7 in the ion permeation pathway (3). Other studies involving mutations in these domains, including M412K (2,43,49), D569N (25), T416K, W554L, and D528N (25), are consistent with this finding, as are detailed molecular dynamic simulations of ion permeation using predicted TMC1 structures (8). The pore opening itself, however, is less well understood.…”

“…These predicted conformations suggest that TM3 and TM4 may together move away from TM6 to open the channel (movie S1). Similarly, distancing of TM4 and TM6 enhances permeation in molecular dynamics simulations of the homology models (8).…”

“…Coverslips with adherent cochlear cultures were placed on a glassbottomed chamber. The culture media was washed away with Leibovitz's L15 medium three times and then incubated with 2 M FM1-43 in L-15 for 1 min followed by incubation with 2 mM SCAS (4-Sulfonato calix [8]arene, sodium salt) in L-15 for 3 min and washed with L-15 two additional times before imaging.…”

Identification of Druggable Binding Sites and Small Molecules as Modulators of TMC1

“…In the absence of 3D-atomic structures of the mammalian MET-channel complex, we constructed a model of the complex by assembling Mus musculus (Mm) TMC1 subunits with MmCIB2 and MmTMIE. MmCIB2 was included to increase the stability of the MET complex and preserve its influence in phospholipid dynamics and ion permiation 101,102 . We utilized the Cryo-EM structure of the CeTMC-1 complex (PDB code: 7USW) 34,35,103,104 as a reference, and employed Maestro software for structural modeling.…”