Investigating the molecular mechanism of slow adaptation

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Project Summary/Abstract The mechano-electrical transduction (MET) process allows the transduction of mechanical information from sound and head movements into electrical signals, and it is a fundamental step in cochlear and vestibular system function. MET takes place at the level of the hair bundle and is mediated by tip links, extracellular proteins connecting shorter stereocilia to adjacent taller stereocilia. A positive deflection of the hair bundle (toward the tallest row of stereocilia) increases tip-link tension, which increases the open probability of MET channels. During a sustained displacement, the receptor current peaks then decays, indicating a gradual decrease in MET channel open probability. This particular process is called "adaptation" and is extremely important because it shifts the operating range of the MET process to preserve the sensitivity of the system. A decades-old hypothesis proposed that slow adaptation, which operates with a time constant on the order of 10 ms or more, requires Ca2+ entry through the MET channels and the activity of myosin motors to modulate the tip-link position on taller stereocilia. The major piece of evidence for the motor model is the presence, during the stimulation, of a creep (a continued movement in the direction of a step-like force stimulus) in the hair bundle motion with a similar time course as slow adaptation. However, methodological difficulties have contributed to the limited experiments that test the motor model hypothesis. Using cochlear and vestibular hair cells of mice, rats, and gerbils, we confirmed that in mammals, slow adaptation requires Ca2+ and myosin motors, and we assessed that modulating adaptation does not affect hair-bundle creep. Therefore, adaptation does not involve the movement of the upper tip-link insertion challenging a critical piece of evidence upholding the motor model. Using electrophysiological recording in vestibular and cochlear hair cells, I will test a new hypothesis where phospholipids are essential for slow adaptation. In particular, studies in rats and frogs have shown that the phospholipid PIP2 affects MET channel proprieties, and recent data demonstrate that TMIE is an essential subunit of the MET channel and mediates interactions with PIP2 to modulate channel function. I will test if PIP2 is necessary for slow adaptation in cochlear and vestibular hair cells, and I will test its interplay with myosin motors. My results will allow me to determine the underlying molecular mechanism of slow adaptation in mammals, the key process that preserves the sensitivity of the system and allows us to detect a wide range of sound intensities with extremely high precision.