SeMoVi - April 14th, 2010 — BioSyL - Réseau de Biologie Systémique de l'Université de Lyon

14h00 - 15h00	Pascal Martin, Institut Curie Nonlinear cellular oscillators as sensory receptors and mechanical amplifiers for hearing More information on the Pascal Martin's website
15h00 - 15h30	Pause café
15h30 - 16h30	Bénédicte Durand, CGMC RFX transcription factors: key regulators of the ciliogenic program Manuel Théry, CEA Grenoble Mechanical and geometrical regulation of cell polarisation and ciliogenesis
16h30 - 17h00	Discussion générale

Pascal Martin, Institut Curie
Nonlinear cellular oscillators as sensory receptors and mechanical amplifiers for hearing

The vertebrate ear benefits from nonlinear mechanical amplification to operate over a vast range of sound intensities. The amplificatory process is thought to emerge from active force production by sensory hair cells in the inner ear. The hair bundle, a tuft of actin-filled cylindrical processes that protrudes from the apical surface of each hair, can function both as a mechano-sensory organelle and as a force generator. The hair cell can indeed produce active hair-bundle movements, including spontaneous oscillations. Spontaneous oscillations could arise from the interplay between negative hair-bundle stiffness, the activity of a myosin-based molecular motor and Ca2+ feedback on the force that the motor can produce. An oscillatory hair bundle functions as a frequency-selective, nonlinear amplifier that is ideally suited for auditory detection. Intrinsic fluctuations, however, jostle the response of a single hair bundle to weak stimuli and seriously limit amplification. In vivo, most hair bundles are mechanically coupled by overlying gelatinous structures. We have recently assayed the effects of mechanical coupling on the hair-bundle amplifier by combining dynamic force clamp of a hair bundle from the bullfrog's sacculus with real-time stochastic simulations of hair-bundle mechanics. This setup couples the hair bundle to two virtual hair bundles, called "cyber clones", and mimics a situation in which the hair bundle is elastically linked to two neighbors with similar characteristics. We found that coupling increased the coherence of spontaneous hair-bundle oscillations. By effectively reducing noise, the synergic interplay between the hair bundle and its cyber clones also enhanced amplification of sinusoidal stimuli. All observed effects of coupling were in quantitative agreement with simulations. We argue that the auditory amplifier relies on hair-bundle cooperation at a multi-cellular scale to overcome intrinsic noise limitations and achieve high sensitivity and sharp frequency selectivity.

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SeMoVi - April 14th, 2010