Browsing by Author "Nam, Jong-Hoon"

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    A Computational Study on the Structure, Dynamics and Mechanoelectric Transduction of Vestibular Hair cell
    Nam, Jong-Hoon (Virginia Tech, 2005-07-29)
    The hair cell, a specialized cell in the inner ear, is responsible for hearing and balance. The hair cell is an exquisite sensor that captures mechanical stimuli and generates neurosensory signals. A theory called gating theory has been developed and widely used to analyze the experimental data of hair cell transduction. Despite increasing knowledge about molecular structures of hair cells, the mechanical model in the gating theory remained simple. Efforts to make the most of the recent findings regarding the hair cell structures led to the development of hair cell finite element (FE) model (Cotton & Grant, 2000, 2004a, b). I have extended this approach by adding channel kinetics and structural dynamics to the FE structural model of the hair cell. I have expanded the previous static and passive model to a dynamic and active model. It is the most detailed hair cell structural model and includes up-to-date knowledge of the hair cell structure such as the stereocilia and various extracellular links. In order to observe the dynamic response of hair bundles in the endolymph fluid, I have included fluid drag in the model. Link nonlinearity has been added to reflect recent observations (Tsuprun 2003). The lateral links stiffen as they stretch and prevent contact between stereocilia when they compress. In addition to these structural features, I added channel kinetics such as the fast adaptation. In my study, the Ca²⁺ diffusion kinetics plays a key role in the hair cell adaptations. The Ca²⁺ association rate to the fast adaptation modulator is postulated to govern the fast adaptation. I assumed that two factors--the tip link tension and the Ca²⁺ concentration at the tip of stereocilia govern the hair cell mechanoelectric transduction. My dissertation comprises three parts--structure, dynamics and mechanotransduction of hair cells. First, the mechanical properties of hair bundle were sought by comparing my FE model with other experiments. The quantified Young's modulus of stereocilia and the stiffness of tip link agree well with other recent estimates. The stiffness of other structural elements (upper lateral and shaft links) was newly estimated through this effort. Second, I established equations of motion for the hair bundle in the fluid. Two possible loading conditions to the hair bundle were simulated. Two different hair bundles were subjected to a point load and a load induced by fluid flow. The results showed that some vestibular hair cells' transduction might be dominated by the fluid-induced force. Finally, I observed the hair cell transduction in various stimulus conditions. The results showed that the hair cell's sensitivity highly depends on the stimulus method. The fluid-jet stimulus activated fewer channels than the glass fiber and made the hair cell less sensitive. A faster stimulus opened more channels and made the hair cell more sensitive. The resting tension in the tip link, which is believed to be controlled by the Ca²⁺ concentration, also affected the hair cell sensitivity. A higher resting tension, equivalent to a lower Ca²⁺ concentration, tended to make the hair cell more sensitive. In conclusion, I developed a new tool to study the hair cell mechanoelectric transduction. My hair cell computational model enables us (1) to study how the hair cells' morphological variations are related to their function; (2) to investigate the hair cell mechanoelectric transduction at the single channel level, in silico, as opposed to the statistical approach; (3) to test the response of hair cells under in situ force boundary conditions.
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    Hair Bundle Stiffness in the Turtle Utricle: Structural and Regional Variations
    Spoon, Corrie E. (Virginia Tech, 2007-11-19)
    Vestibular hair cells are mechanotransducing sensory receptors in the vertebrate inner ear that detect movement and orientation of the head with respect to gravity. The morphologies of their ciliary bundles vary greatly for different species, endorgans, and within the same endorgan. Bundle morphology in the turtle utricle, like other species, demonstrates highly organized regional variations. These structural differences in bundles impact their mechanical behavior and the process of mechanotransduction. To further understanding of the mechanical behavior of hair bundles, this work experimentally measured the stiffness of bundles with differing morphology, the stiffness contribution of interciliary links and the mechanical properties of the kinocilium in the turtle utricle. The stiffness of hair bundles of varying structure and location along a medial to lateral transect of the utricle was examined. Bundle stiffness was greatest in the striola and demonstrated a systematic decline with location from the line of polarity reversal. The average stiffness of bundles in the striola and extrastriola were 82 ± 46 (n=48) and 9 ± 5 (n=25) µN/m, respectively. The stiff and weak bundles demonstrated characteristic morphologies. The stiffest bundles have short kinocilium, tall stereocilia, and ratios of kinocilium to tallest stereocilia height (KS) close to 1. In contrast, the compliant bundles have tall kinocilium, short stereocilia, and KS ratios ranging from 1.6 – 8. The stiffer bundles also tend to have longer array lengths and steeper slopes. Measurements of bundle stiffness in the turtle utricle are lower than those previously reported which may be attributed to morphological differences between species. The stiffness contributions of the interciliary links were also examined through their selective removal with exposure to the Ca²⁺ chelator BAPTA and the protease subtilisin. BAPTA treatment reportedly breaks tip, kinocilial and ankle links while subtilisin breaks the shaft and ankle links. Following BAPTA and subtilisin treatments, bundle stiffness reduced by 65 ± 10% and 63 ± 11%, respectively. The mechanical properties of the kinocilium were measured with novel techniques. Flexural rigidity (EI) was measure while the kinocilium was fixed at the height of the tallest stereocilia using a glass supporting probe. Through both force deflection and a high speed video technique, measured values of EI ranged from 1460 – 6150 pN·µm2. The rotational stiffness of the kinocilium about its apical insertion was also measured. Bundles were treated with BAPTA to break the kinocilial links and separate the kinocilium from neighboring stereocilia. Using a force deflection technique, the rotational stiffness of the kinocilium was measured as 120 ± 17 pN·µm/rad.
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    Mechanical properties and consequences of stereocilia and extracellular links in vestibular hair bundles
    Nam, Jong-Hoon; Cotton, John R.; Peterson, Ellengene H.; Grant, John Wallace (CELL PRESS, 2006-04-01)
    Although knowledge of the. ne structure of vestibular hair bundles is increasing, the mechanical properties and functional significance of those structures remain unclear. In 2004, Bashtanov and colleagues reported the contribution of different extracellular links to bundle stiffness. We simulated Bashtanov's experimental protocol using a three-dimensional finite element bundle model with geometry measured from a typical striolar hair cell. Unlike any previous models, we separately consider two types of horizontal links: shaft links and upper lateral links. Our most important results are as follows. First, we identified the material properties required to match Bashtanov's experiment: stereocilia Young's modulus of 0.74 GPa, tip link assembly (gating spring) stiffness of 5300 pN/μm, and the combined stiffness of shaft links binding two adjacent stereocilia of 750 similar to 2250 pN/μm. Second, we conclude that upper lateral links are likely to have nonlinear mechanical properties: they have minimal stiffness during small bundle deformations but stiffen as the bundle deflects further. Third, we estimated the stiffness of the gating spring based on our realistic three-dimensional bundle model rather than a conventional model relying on the parallel arrangement assumption. Our predicted stiffness of the gating spring was greater than the previous estimation.
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    A virtual hair cell, I: Addition of gating spring theory into a 3-D bundle mechanical model
    Nam, Jong-Hoon; Cotton, John R.; Grant, John Wallace (CELL PRESS, 2007-03-01)
    We have developed a virtual hair cell that simulates hair cell mechanoelectrical transduction in the turtle utricle. This study combines a full three-dimensional hair bundle mechanical model with a gating spring theory. Previous mathematical models represent the hair bundle with a single degree of freedom system which, we have argued, cannot fully explain hair bundle mechanics. In our computer model, the tip link tension and fast adaptation modulator kinetics determine the opening and closing of each channel independently. We observed the response of individual transduction channels with our presented model. The simulated results showed three features of hair cells in vitro. First, a transient rebound of the bundle tip appeared when fast adaptation dominated the dynamics. Second, the dynamic stiffness of the bundle was minimized when the response-displacement (I-X) curve was steepest. Third, the hair cell showed "polarity'', i. e., activation decreased from a peak to zero as the forcing direction rotated from the excitatory to the inhibitory direction.
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    A virtual hair cell, II: Evaluation of mechanoelectric transduction parameters
    Nam, Jong-Hoon; Cotton, John R.; Grant, John Wallace (CELL PRESS, 2007-03-01)
    The virtual hair cell we have proposed utilizes a set of parameters related to its mechanoelectric transduction. In this work, we observed the effect of such channel gating parameters as the gating threshold, critical tension, resting tension, and Ca2+ concentration. The gating threshold is the difference between the resting and channel opening tension exerted by the tip link assembly on the channel. The critical tension is the tension in the tip link assembly over which the channel cannot close despite Ca2+ binding. Our results show that 1), the gating threshold dominated the initial sensitivity of the hair cell; 2), the critical tension minimally affects the peak response, l(t), but considerably affects the time course of response, l(t), and the force-displacement, F-X, relationship; and 3), higher intracellular [Ca2+] resulted in a smaller fast adaptation time constant. Based on the simulation results we suggest a role of the resting tension: to help overcome the viscous drag of the hair bundle during the oscillatory movement of the bundle. Also we observed the three-dimensional bundle effect on the hair cell response by varying the number of cilia forced. These varying forcing conditions affected the hair cell response.