David P. Corey, Ph.D.
Department of Neurobiology, Harvard Medical School
We use a variety of biophysical, structural, genetic and imaging methods to understand the mechanism of sensory transduction by the receptor cells of the inner ear.
We are interested in the gating of mechanically sensitive ion channels, which open in response to force on the channel proteins. We study these channels primarily in vertebrate hair cells—the receptor cells of the inner ear, which are sensitive to sounds or accelerations. Hair cells are epithelial cells, with a bundle of actin-rich stereocilia rising from their apical surfaces. Mechanical deflection of the bundles changes the tension in fine "tip links" that stretch between the stereocilia; these filaments are thought to pull directly on the mechanically-gated transduction channels to regulate their opening.
Tip links are made of two unusual cadherins with long extracellular domains--cadherin 23 and protocadherin 15—whose N-termini join to complete the link. We are interested in the tip link’s biophysical properties and how the two cadherins join. We have determined the crystal structure of the cadherin-23 and protocadherin 15 N-termini bound to each other, and have used steered molecular dynamics to determine their elastic properties and unbinding force. The crystal structures and molecular dynamics together have helped explain how deafness-producing mutations in the tip link disrupt its structure. These simulations are being confirmed in vitro by pulling apart single bound complexes with laser tweezer forces.
The molecular composition of the mechanotransduction complex—the transduction channels and associated proteins—is beginning to be solved. Two members of a new protein family, TMC1 and TMC2, are most likely the channels, and two others, TMHS and TMIE, are essential as well. All four, as well as cadherin 23 and protocadherin 15, are the products of deafness genes. We are using high-resolution electron microscopy to localize these proteins, X-ray crystallography and cryo-EM to solve their structure, and site-directed mutagenesis to understand functional domains.
Transduction channels open during a mechanical stimulus, but then adapt with a timecourse of milliseconds. One phase of adaptation was shown to be mediated by a motor complex of myosin-1c molecules relaxing tension on the channels, but a faster phase apparently results from Ca2+ that enters through the channels immediately binding to close them. We are characterizing the site of Ca2+ action within the transduction complex and how Ca2+ binding closes channels. Channel closure produces forces that feed back on the dynamics of the whole cochlea and may drive amplification and frequency tuning, so understanding fast adaptation may elucidate the exquisite sensitivity of the ear.
Delling, M., Indzhykulian, A.A., Liu, X., Li, Y., Xie, T., Corey, D.P.† and Clapham, D. E.† (2016) Primary cilia are not calcium-responsive mechanosensors. Nature 531:656-60.
Scheffer, D.I., Shen, J., Corey, D.P. †, Chen, Z-Y.† (2015) Gene expression by mouse inner ear hair cells during development. J. Neurosci. 35:6366-6380
Sotomayor, M., Weihofen, W.A., Gaudet, R. and Corey, D.P. (2012) Structure of a force-conveying cadherin bond essential for inner-ear mechanotransduction. Nature 492:128-32
Zhang, D-Z., Piazza, V., Perrin, B.J., Rzadzinska, A.K., Poczatek, J.C., Wang, M., Prosser, H.M., Ervasti, J.M., Corey, D.P.† and Lechene, C.P.† (2012) Multi-isotope imaging mass spectrometry reveals slow protein turnover in hair-cell stereocilia. Nature 481:520-524.
Sotomayor, M., Weihofen, W.A., Gaudet, R. and Corey, D.P. (2010) Structural determinants of cadherin-23 function in hearing and deafness. Neuron 66:85-100.
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