A <i>Tmc1</i> mutation reduces calcium permeability and expression of mechanoelectrical transduction channels in cochlear hair cells.

Abstract

Mechanoelectrical transducer (MET) currents were recorded from cochlear hair cells in mice with mutations of transmembrane channel-like protein TMC1 to study the effects on MET channel properties. We characterized a <i>Tmc1</i> mouse with a single-amino-acid mutation (D569N), homologous to a dominant human deafness mutation. Measurements were made in both <i>Tmc2</i> wild-type and <i>Tmc2</i> knockout mice. By 30 d, <i>Tmc1</i> pD569N heterozygote mice were profoundly deaf, and there was substantial loss of outer hair cells (OHCs). MET current in OHCs of <i>Tmc1</i> pD569N mutants developed over the first neonatal week to attain a maximum amplitude one-third the size of that in <i>Tmc1</i> wild-type mice, similar at apex and base, and lacking the tonotopic size gradient seen in wild type. The MET-channel Ca²⁺ permeability was reduced 3-fold in <i>Tmc1</i> pD569N homozygotes, intermediate deficits being seen in heterozygotes. Reduced Ca²⁺ permeability resembled that of the <i>Tmc1 p</i>M412K <i>Beethoven</i> mutant, a previously studied semidominant mouse mutation. The MET channel unitary conductance, assayed by single-channel recordings and by measurements of current noise, was unaffected in mutant apical OHCs. We show that, in contrast to the <i>Tmc1</i> M412K mutant, there was reduced expression of the TMC1 D569N channel at the transduction site assessed by immunolabeling, despite the persistence of tip links. The reduction in MET channel Ca²⁺ permeability seen in both mutants may be the proximate cause of hair-cell apoptosis, but changes in bundle shape and protein expression in <i>Tmc1</i> D569N suggest another role for TMC1 apart from forming the channel.