Mechanics of microtubule bundles in pillar cells from the inner ear

TUBB4B is essential for the cytoskeletal architecture of cochlear supporting cells and motile cilia development

Microtubules are essential for various cellular processes. The functional diversity of microtubules is attributed to the incorporation of various α- and β-tubulin isotypes encoded by different genes. In this work, we investigated the functional role of β4B-tubulin isotype (TUBB4B) in hearing and vision as mutations in TUBB4B are associated with sensorineural disease. Using a Tubb4b knockout mouse model, our findings demonstrate that TUBB4B is essential for hearing. Mice lacking TUBB4B are profoundly deaf due to defects in the inner and middle ear. Specifically, in the inner ear, the absence of TUBB4B lead to disorganized and reduced densities of microtubules in pillar cells, suggesting a critical role for TUBB4B in providing mechanical support for auditory transmission. In the middle ear, Tubb4b-/- mice exhibit motile cilia defects in epithelial cells, leading to the development of otitis media. However, Tubb4b deletion does not affect photoreceptor function or cause retinal degeneration. Intriguingly, β6-tubulin levels increase in retinas lacking β4B-tubulin isotype, suggesting a functional compensation mechanism. Our findings illustrate the essential roles of TUBB4B in hearing but not in vision in mice, highlighting the distinct functions of tubulin isotypes in different sensory systems.

3D Computational Modeling of Blast Wave Transmission in Human Ear From External Ear to Cochlear Hair Cells: A Preliminary Study

INTRODUCTION

Auditory disabilities like tinnitus and hearing loss caused by exposure to blast overpressures are prevalent among military service members and veterans. The high-pressure fluctuations of blast waves induce hearing loss by injuring the tympanic membrane, ossicular chain, or sensory hair cells in the cochlea. The basilar membrane (BM) and organ of Corti (OC) behavior inside the cochlea during blast remain understudied. A computational finite element (FE) model of the full human ear was used by Bradshaw et al. (2023) to predict the motion of middle and inner ear tissues during blast exposure using a 3-chambered cochlea with Reissner's membrane and the BM. The inclusion of the OC in a blast transmission model would improve the model's anatomy and provide valuable insight into the inner ear response to blast exposure.

MATERIALS AND METHODS

This study developed a microscale FE model of the OC, including the OC sensory hair cells, membranes, and structural cells, connected to a macroscale model of the ear to form a comprehensive multiscale model of the human peripheral auditory system. There are 5 rows of hair cells in the model, each row containing 3 outer hair cells (OHCs) and the corresponding Deiters' cells and stereociliary hair bundles. BM displacement 16.75 mm from the base induced by a 31 kPa blast overpressure waveform was derived from the macroscale human ear model reported by Bradshaw et al. (2023) and applied as input to the center of the BM in the OC. The simulation was run for 2 ms as a structural analysis in ANSYS Mechanical.

RESULTS

The FE model results reported the displacement and principal strain of the OHCs, reticular lamina, and stereociliary hair bundles during blast transmission. The movement of the BM caused the rest of the OC to deform significantly. The reticular lamina displacement and strain amplitudes were highest where it connected to the OHCs, indicating that injury to this part of the OC may be likely due to blast exposure.

CONCLUSIONS

This microscale model is the first FE model of the OC to be connected to a macroscale model of the ear, forming a full multiscale ear model, and used to predict the OC's behavior under blast. Future work with this model will incorporate cochlear endolymphatic fluid, increase the number of OHC rows to 19 in total, and use the results of the model to reliably predict the sensorineural hearing loss resulting from blast exposure.

Structure and dynamics of motor-driven microtubule bundles

Connecting the large-scale emergent behaviors of active cytoskeletal materials to the microscopic properties of their constituents is a challenge due to a lack of data on the multiscale dynamics and structure of such systems. We approach this problem by studying the impact of depletion attraction on bundles of microtubules and kinesin-14 molecular motors. For all depletant concentrations, kinesin-14 bundles generate comparable extensile dynamics. However, this invariable mesoscopic behavior masks the transition in the microscopic motion of microtubules. Specifically, with increasing attraction, we observe a transition from bi-directional sliding with extension to pure extension with no sliding. Small-angle X-ray scattering shows that the transition in microtubule dynamics is concurrent with a structural rearrangement of microtubules from an open hexagonal to a compressed rectangular lattice. These results demonstrate that bundles of microtubules and molecular motors can display the same mesoscopic extensile behaviors despite having different internal structures and microscopic dynamics. They provide essential information for developing multiscale models of active matter.

Asymmetric vibrations in the organ of Corti by outer hair cells measured from excised gerbil cochlea

Pending questions regarding cochlear amplification and tuning are hinged upon the organ of Corti (OoC) active mechanics: how outer hair cells modulate OoC vibrations. Our knowledge regarding OoC mechanics has advanced over the past decade thanks to the application of tomographic vibrometry. However, recent data from live cochlea experiments often led to diverging interpretations due to complicated interaction between passive and active responses, lack of image resolution in vibrometry, and ambiguous measurement angles. We present motion measurements and analyses of the OoC sub-components at the close-to-true cross-section, measured from acutely excised gerbil cochleae. Specifically, we focused on the vibrating patterns of the reticular lamina, the outer pillar cell, and the basilar membrane because they form a structural frame encasing active outer hair cells. For passive transmission, the OoC frame serves as a rigid truss. In contrast, motile outer hair cells exploit their frame structures to deflect the upper compartment of the OoC while minimally disturbing its bottom side (basilar membrane). Such asymmetric OoC vibrations due to outer hair cell motility explain how recent observations deviate from the classical cochlear amplification theory.

Behavioral characteristics in sensing mechanism of the Corti

Some experiments can't be realized because the cochlea's Corti is the most delicate and complex sensory organ. In this paper, some typical and special behavioral characteristics in the process of sensation were found in medical clinic. Based on the interdisciplinary principles of medicine, physics and biology, a real numerical simulation model of Corti is established. On the basis of verifying the correctness of the model, the mechanism corresponding to these typical and special behavior characteristics in the process of sensation is explored through simulation calculation and analysis. This study provides theoretical and applied basis for people to better understand the sound sensing mechanism, and provides a numerical simulation platform for further analyzing Corti's sensing mechanism and good clinical application.

β-II tubulin isotype directs stiffness and differentiation of neuroblastoma SH-SY5Y cells

β-tubulin isotypes regulate the structure and bundling of microtubule (MT) lattice, its dynamics, and resulting functions. They exhibit differential tissue expression, varying due to physical and biochemical cues. In this work, we investigated the effect of transient heat shock at 42 °C on the nuclear and cytoplasmic stiffness of SH-SY5Y neuroblastoma cells through atomic force microscopy. Moreover, the variations in the expression of β-tubulin isotypes as a heat shock response were also monitored. The heat-exposed cells endured a recovery at 37 °C for 24 h and they manifested an increase of cytoplasmic stiffness by 130 ± 25% with respect to untreated controls. The expression of β-II tubulin isotype in heat-recovered cells is augmented by 51 ± 5% whereas the levels of total tubulin and β-III tubulin isotype remain unaltered. Upon depletion of β-II tubulin isotype using shRNA, the increase in cytoplasmic stiffness was dampened. However, it remained unaffected upon depletion with β-III tubulin isotype shRNA. This features the role of the β-II tubulin isotype in regulating cellular stiffness. In addition, neuroblastoma SH-SY5Y cells undergo differentiation by initiating neuritogenesis and prior evidence suggests the indispensable role of β-II tubulin isotype in this process. The heat-recovered cells which expressed higher levels of β-II tubulin isotype expedited the differentiation process in 3-day which was around 5-day for control cells, however, upon depletion of β-II tubulin isotype, the cells almost lost their differentiation potential. Altogether, this work highlights the role of β-II tubulin isotype as a biomarker for cellular stiffness.

Effect of Strain Rate on Single Tau, Dimerized Tau and Tau-Microtubule Interface: A Molecular Dynamics Simulation Study

Microtubule-associated protein (MAP) tau is a cross-linking molecule that provides structural stability to axonal microtubules (MT). It is considered a potential biomarker for Alzheimer's disease (AD), dementia, and other neurological disorders. It is also a signature protein for Traumatic Brain Injury (TBI) assessment. In the case of TBI, extreme dynamic mechanical energies can be felt by the axonal cytoskeletal members. As such, fundamental understandings of the responses of single tau protein, polymerized tau protein, and tau-microtubule interfaces under high-rate mechanical forces are important. This study attempts to determine the high-strain rate mechanical behavior of single tau, dimerized tau, and tau-MT interface using molecular dynamics (MD) simulation. The results show that a single tau protein is a highly stretchable soft polymer. During deformation, first, it significantly unfolds against van der Waals and electrostatic bonds. Then it stretches against strong covalent bonds. We found that tau acts as a viscoelastic material, and its stiffness increases with the strain rate. The unfolding stiffness can be ~50-500 MPa, while pure stretching stiffness can be >2 GPa. The dimerized tau model exhibits similar behavior under similar strain rates, and tau sliding from another tau is not observed until it is stretched to >7 times of original length, depending on the strain rate. The tau-MT interface simulations show that very high strain and strain rates are required to separate tau from MT suggesting Tau-MT bonding is stronger than MT subunit bonding between themselves. The dimerized tau-MT interface simulations suggest that tau-tau bonding is stronger than tau-MT bonding. In summary, this study focuses on the structural response of individual cytoskeletal components, namely microtubule (MT) and tau protein. Furthermore, we consider not only the individual response of a component, but also their interaction with each other (such as tau with tau or tau with MT). This study will eventually pave the way to build a bottom-up multiscale brain model and analyze TBI more comprehensively.

Geometrical nonlinear elasticity of axon under tension: A coarse-grained computational study

Axon bundles cross-linked by microtubule (MT) associate proteins and bounded by a shell skeleton are critical for normal function of neurons. Understanding effects of the complexly geometrical parameters on their mechanical properties can help gain a biomechanical perspective on the neurological functions of axons and thus brain disorders caused by the structural failure of axons. Here, the tensile mechanical properties of MT bundles cross-linked by tau proteins are investigated by systematically tuning MT length, axonal cross-section radius, and tau protein spacing in a bead-spring coarse-grained model. Our results indicate that the stress-strain curves of axons can be divided into two regimes, a nonlinear elastic regime dominated by rigid-body like inter-MT sliding, and a linear elastic regime dominated by affine deformation of both tau proteins and MTs. From the energetic analyses, first, the tau proteins dominate the mechanical performance of axons under tension. In the nonlinear regime, tau proteins undergo a rigid-body like rotating motion rather than elongating, whereas in the nonlinear elastic regime, tau proteins undergo a flexible elongating deformation along the MT axis. Second, as the average spacing between adjacent tau proteins along the MT axial direction increases from 25 to 125 nm, the Young's modulus of axon experiences a linear decrease whereas with the average space varying from 125 to 175 nm, and later reaches a plateau value with a stable fluctuation. Third, the increment of the cross-section radius of the MT bundle leads to a decrease in Young's modulus of axon, which is possibly attributed to the decrease in MT numbers per cross section. Overall, our research findings offer a new perspective into understanding the effects of geometrical parameters on the mechanics of MT bundles as well as serving as a theoretical basis for the development of artificial MT complexes potentially toward medical applications.

Cochlear supporting cells require GAS2 for cytoskeletal architecture and hearing

In mammals, sound is detected by mechanosensory hair cells that are activated in response to vibrations at frequency-dependent positions along the cochlear duct. We demonstrate that inner ear supporting cells provide a structural framework for transmitting sound energy through the cochlear partition. Humans and mice with mutations in GAS2, encoding a cytoskeletal regulatory protein, exhibit hearing loss due to disorganization and destabilization of microtubule bundles in pillar and Deiters' cells, two types of inner ear supporting cells with unique cytoskeletal specializations. Failure to maintain microtubule bundle integrity reduced supporting cell stiffness, which in turn altered cochlear micromechanics in Gas2 mutants. Vibratory responses to sound were measured in cochleae from live mice, revealing defects in the propagation and amplification of the traveling wave in Gas2 mutants. We propose that the microtubule bundling activity of GAS2 imparts supporting cells with mechanical properties for transmitting sound energy through the cochlea.

Mechanics and kinetics of dynamic instability

During dynamic instability, self-assembling microtubules (MTs) stochastically alternate between phases of growth and shrinkage. This process is driven by the presence of two distinct states of MT subunits, GTP- and GDP-bound tubulin dimers, that have different structural properties. Here, we use a combination of analysis and computer simulations to study the mechanical and kinetic regulation of dynamic instability in three-dimensional (3D) self-assembling MTs. Our model quantifies how the 3D structure and kinetics of the distinct states of tubulin dimers determine the mechanical stability of MTs. We further show that dynamic instability is influenced by the presence of quenched disorder in the state of the tubulin subunit as reflected in the fraction of non-hydrolysed tubulin. Our results connect the 3D geometry, kinetics and statistical mechanics of these tubular assemblies within a single framework, and may be applicable to other self-assembled systems where these same processes are at play.

Reduced postnatal expression of cochlear Connexin26 induces hearing loss and affects the developmental status of pillar cells in a dose-dependent manner

Mutations in the GJB2 gene (which encodes Connexin26 (Cx26)) are the most common cause of non-syndromic deafness. Previous studies showed that an extensive knockout of the Gjb2 gene in cochlear epithelium can cause severe deafness, significant hair cell (HC) loss and failure of pillar cells (a type of supporting cell, PCs) to differentiate in mice. This study aimed to establish different mouse models with gradient reductions of cochlear Cx26 expression and to investigate the effect of different reduced levels of cochlear Cx26 expression on hearing and development of PCs. According to the reduction in the levels of cochlear Cx26, these models were named high knockdown (KD), middle KD and low KD group. In the low KD group, the mice showed normal hearing and well-developed PCs. In the high KD group, up to 90 percent of supporting cells (SCs) lost Cx26 expression. These mice exhibited severe deafness, rapid hair cell degeneration and juvenile PCs. In the middle KD group, nearly half of SCs lost Cx26 expression. However, these mice showed a moderate deafness and a late-onset hair cell loss. Moreover, nearly all the PCs in mice of this group were in a partially differentiated state. These results indicated that reduction of postnatal expression of cochlear Cx26 induces hearing loss in a dose-dependent manner. Null Cx26 in a few SCs affects the developmental status of PCs and the hair cell degeneration pattern. The abnormal developmental status of PCs may be a potential cause of Gjb2-related hearing loss.

Elastic, dipole-dipole interaction and viscosity impact on vibrational properties of anisotropic hexagonal microtubule lattice

The paper investigates microtubules lattice properties taking into consideration elastic, dipole-dipole interaction of tubulins and viscosity. A microtubule is modeled as a system of bound tubulins, forming a skewed hexagonal two-dimensional lattice. Wave frequencies and group velocities have been calculated. Calculations have been performed for various directions of wave front propagation: helix, along the protofilament, and anti-helix. Three different wave polarization directions have been considered. It has been shown that the direction of the wave polarization influences the frequency and wave group velocity values in the lattice considerably. The impact of dipole-dipole interaction greatly depends on the direction of the wave polarization; thus, it is only moderate for the longitudinally (LA) polarized waves while it is sufficient for the transversely (TA), and out-of-plane (ZA) polarized waves. Moreover dipole-dipole interaction may result in the waves which are able to cause the rupture of microtubules. With viscosity considered, lattice oscillations become harmonically damped only over a certain wavelength range when longitudinal polarization occurs. Out of this range as well as for the other polarization directions, lattice deviations from equilibrium are dampened exponentially. Taking viscosity into consideration also results in a noticeable decrease in frequency and increase in the group wave velocity when the waves are longitudinally polarized. Reverse wave domains which may be associated with a possible phenomenon of negative refraction have been determined for hexagonal microtubule lattice.

A microtubule bestiary: structural diversity in tubulin polymers

Microtubules are long, slender polymers of αβ-tubulin found in all eukaryotic cells. Tubulins associate longitudinally to form protofilaments, and adjacent protofilaments associate laterally to form the microtubule. In the textbook view, microtubules are 1) composed of 13 protofilaments, 2) arranged in a radial array by the centrosome, and 3) built into the 9+2 axoneme. Although these canonical structures predominate in eukaryotes, microtubules with divergent protofilament numbers and higher-order microtubule assemblies have been discovered throughout the last century. Here we survey these noncanonical structures, from the 4-protofilament microtubules of Prosthecobacter to the 40-protofilament accessory microtubules of mantidfly sperm. We review the variety of protofilament numbers observed in different species, in different cells within the same species, and in different stages within the same cell. We describe the determinants of protofilament number, namely nucleation factors, tubulin isoforms, and posttranslational modifications. Finally, we speculate on the functional significance of these diverse polymers. Equipped with novel tubulin-purification tools, the field is now prepared to tackle the long-standing question of the evolutionary basis of microtubule structure.

The counterbend dynamics of cross-linked filament bundles and flagella

Cross-linked filament bundles, such as in cilia and flagella, are ubiquitous in biology. They are considered in textbooks as simple filaments with larger stiffness. Recent observations of flagellar counterbend, however, show that induction of curvature in one section of a passive flagellum instigates a compensatory counter-curvature elsewhere, exposing the intricate role of the diminutive cross-linking proteins at large scales. We show that this effect, a material property of the cross-linking mechanics, modifies the bundle dynamics and induces a bimodal L² - L³ length-dependent material response that departs from the Euler-Bernoulli theory. Hence, the use of simpler theories to analyse experiments can result in paradoxical interpretations. Remarkably, the counterbend dynamics instigates counter-waves in opposition to driven oscillations in distant parts of the bundle, with potential impact on the regulation of flagellar bending waves. These results have a range of physical and biological applications, including the empirical disentanglement of material quantities via counterbend dynamics.

Hydrostatic measurement and finite element simulation of the compliance of the organ of Corti complex

In the mammalian cochlea, the geometrical and mechanical properties of the organ of Corti complex (OCC, consisting of the tectorial membrane, the organ of Corti, and the basilar membrane) have fundamental consequences for understanding the physics of hearing. Despite efforts to correlate the mechanical properties of the OCC with cochlear function, experimental data of OCC stiffness are limited due to difficulties in measurement. Modern measurements of the OCC stiffness use microprobes exclusively, but suffer ambiguity when defining the physiologically relevant stiffness due to the high nonlinearity in the force-displacement relationship. The nonlinearity stems from two sources. First, microprobes apply local force instead of fluid pressure across the OCC. Second, to obtain the functionally relevant stiffness, the OCC is deformed well beyond in vivo levels (>10 μm). The objective of this study was to develop an alternative technique to overcome challenges intrinsic to the microprobe method. Using a custom-designed microfluidic chamber system, hydrostatic pressures were applied to the excised gerbil cochlea. Deformations of the OCC due to hydrostatic pressures were analyzed through optical-axis image correlation. The pressure-displacement relationship was linear within nanoscale displacement ranges (<1 μm). To compare the results in this paper with existing measurements, a three-dimensional finite element model was used.

Static length changes of cochlear outer hair cells can tune low-frequency hearing

The cochlea not only transduces sound-induced vibration into neural spikes, it also amplifies weak sound to boost its detection. Actuators of this active process are sensory outer hair cells in the organ of Corti, whereas the inner hair cells transduce the resulting motion into electric signals that propagate via the auditory nerve to the brain. However, how the outer hair cells modulate the stimulus to the inner hair cells remains unclear. Here, we combine theoretical modeling and experimental measurements near the cochlear apex to study the way in which length changes of the outer hair cells deform the organ of Corti. We develop a geometry-based kinematic model of the apical organ of Corti that reproduces salient, yet counter-intuitive features of the organ's motion. Our analysis further uncovers a mechanism by which a static length change of the outer hair cells can sensitively tune the signal transmitted to the sensory inner hair cells. When the outer hair cells are in an elongated state, stimulation of inner hair cells is largely inhibited, whereas outer hair cell contraction leads to a substantial enhancement of sound-evoked motion near the hair bundles. This novel mechanism for regulating the sensitivity of the hearing organ applies to the low frequencies that are most important for the perception of speech and music. We suggest that the proposed mechanism might underlie frequency discrimination at low auditory frequencies, as well as our ability to selectively attend auditory signals in noisy surroundings.

Data-driven reduced-order model of microtubule mechanics

A beam element is constructed for microtubules based upon data reduction of the results from atomistic simulation of the carbon backbone chain of αβ-tubulin dimers. The database of mechanical responses to various types of loads from atomistic simulation is reduced to dominant modes. The dominant modes are subsequently used to construct the stiffness matrix of a beam element that captures the anisotropic behavior and deformation mode coupling that arises from a microtubule's spiral structure. In contrast to standard Euler-Bernoulli or Timoshenko beam elements, the link between forces and node displacements results not from hypothesized deformation behavior, but directly from the data obtained by molecular scale simulation. Differences between the resulting microtubule data-driven beam model (MTDDBM) and standard beam elements are presented, with a focus on coupling of bending, stretch, shear deformations. The MTDDBM is just as economical to use as a standard beam element, and allows accurate reconstruction of the mechanical behavior of structures within a cell as exemplified in a simple model of a component element of the mitotic spindle.

Adaptive Response of Actin Bundles under Mechanical Stress

Actin is one of the main components of the architecture of cells. Actin filaments form different polymer networks with versatile mechanical properties that depend on their spatial organization and the presence of cross-linkers. Here, we investigate the mechanical properties of actin bundles in the absence of cross-linkers. Bundles are polymerized from the surface of mDia1-coated latex beads, and deformed by manipulating both ends through attached beads held by optical tweezers, allowing us to record the applied force. Bundle properties are strikingly different from the ones of a homogeneous isotropic beam. Successive compression and extension leads to a decrease in the buckling force that we attribute to the bundle remaining slightly curved after the first deformation. Furthermore, we find that the bundle is solid, and stiff to bending, along the long axis, whereas it has a liquid and viscous behavior in the transverse direction. Interpretation of the force curves using a Maxwell visco-elastic model allows us to extract the bundle mechanical parameters and confirms that the bundle is composed of weakly coupled filaments. At short times, the bundle behaves as an elastic material, whereas at long times, filaments flow in the longitudinal direction, leading to bundle restructuring. Deviations from the model reveal a complex adaptive rheological behavior of bundles. Indeed, when allowed to anneal between phases of compression and extension, the bundle reinforces. Moreover, we find that the characteristic visco-elastic time is inversely proportional to the compression speed. Actin bundles are therefore not simple force transmitters, but instead, complex mechano-transducers that adjust their mechanics to external stimulation. In cells, where actin bundles are mechanical sensors, this property could contribute to their adaptability.

Optomechanical proposal for monitoring microtubule mechanical vibrations

Microtubules provide the mechanical force required for chromosome separation during mitosis. However, little is known about the dynamic (high-frequency) mechanical properties of microtubules. Here, we theoretically propose to control the vibrations of a doubly clamped microtubule by tip electrodes and to detect its motion via the optomechanical coupling between the vibrational modes of the microtubule and an optical cavity. In the presence of a red-detuned strong pump laser, this coupling leads to optomechanical-induced transparency of an optical probe field, which can be detected with state-of-the art technology. The center frequency and line width of the transparency peak give the resonance frequency and damping rate of the microtubule, respectively, while the height of the peak reveals information about the microtubule-cavity field coupling. Our method opens the new possibilities to gain information about the physical properties of microtubules, which will enhance our capability to design physical cancer treatment protocols as alternatives to chemotherapeutic drugs.

Genetic defects in β-spectrin and tau sensitize C. elegans axons to movement-induced damage via torque-tension coupling

Our bodies are in constant motion and so are the neurons that invade each tissue. Motion-induced neuron deformation and damage are associated with several neurodegenerative conditions. Here, we investigated the question of how the neuronal cytoskeleton protects axons and dendrites from mechanical stress, exploiting mutations in UNC-70 β-spectrin, PTL-1 tau/MAP2-like and MEC-7 β-tubulin proteins in Caenorhabditis elegans. We found that mechanical stress induces supercoils and plectonemes in the sensory axons of spectrin and tau double mutants. Biophysical measurements, super-resolution, and electron microscopy, as well as numerical simulations of neurons as discrete, elastic rods provide evidence that a balance of torque, tension, and elasticity stabilizes neurons against mechanical deformation. We conclude that the spectrin and microtubule cytoskeletons work in combination to protect axons and dendrites from mechanical stress and propose that defects in β-spectrin and tau may sensitize neurons to damage.

DOI:http://dx.doi.org/10.7554/eLife.20172.001

NUMERICAL SIMULATION ON THE MECHANICAL BEHAVIOR OF OUTER STEREOCILIA IN CORTI

In this paper, a two-dimensional model of the organ of Corti (OC) which includes basilar membrane (BM), tectorial membrane (TM), inner and outer hair cells and reticular lamina (RL) is established by Comsol. Based on experimental data that sinusoidal excitation was applied on the pectinate zone of the BM, the incentives are added to the corresponding position of the model. Transient analysis is made and the displacement of six different positions is achieved. The results are in good agreement with experimental data, which confirms the validity of the FE model. Based on time-domain and frequency-domain analysis, the relative motion between stereocilia and TM and RL is studied under sinusoidal excitation. The results show that, under time-domain analysis, the whole trend of relative displacement and velocity difference of Inner is similar to that of Outer, while there is little different when comparing with Middle. The relative velocity difference of Middle and Inner lag behind Outer with roughly 0.01[Formula: see text]s. Under frequency-domain analysis, at characteristic frequency, the deviation of each stereocilium is the largest. In the meantime, the frequency that the maximum value of outer stereocilia achieved is different. This new finding may cause the difference of lateral vibration of BM and indicate the frequency sensitivity of BM.

Tubulin acetylation: responsible enzymes, biological functions and human diseases

Microtubules have important functions ranging from maintenance of cell morphology to subcellular transport, cellular signaling, cell migration, and formation of cell polarity. At the organismal level, microtubules are crucial for various biological processes, such as viral entry, inflammation, immunity, learning and memory in mammals. Microtubules are subject to various covalent modifications. One such modification is tubulin acetylation, which is associated with stable microtubules and conserved from protists to humans. In the past three decades, this reversible modification has been studied extensively. In mammals, its level is mainly governed by opposing actions of α-tubulin acetyltransferase 1 (ATAT1) and histone deacetylase 6 (HDAC6). Knockout studies of the mouse enzymes have yielded new insights into biological functions of tubulin acetylation. Abnormal levels of this modification are linked to neurological disorders, cancer, heart diseases and other pathological conditions, thereby yielding important therapeutic implications. This review summarizes related studies and concludes that tubulin acetylation is important for regulating microtubule architecture and maintaining microtubule integrity. Together with detyrosination, glutamylation and other modifications, tubulin acetylation may form a unique 'language' to regulate microtubule structure and function.

All Three Rows of Outer Hair Cells Are Required for Cochlear Amplification

In the mammalian auditory system, the three rows of outer hair cells (OHCs) located in the cochlea are thought to increase the displacement amplitude of the organ of Corti. This cochlear amplification is thought to contribute to the high sensitivity, wide dynamic range, and sharp frequency selectivity of the hearing system. Recent studies have shown that traumatic stimuli, such as noise exposure and ototoxic acid, cause functional loss of OHCs in one, two, or all three rows. However, the degree of decrease in cochlear amplification caused by such functional losses remains unclear. In the present study, a finite element model of a cross section of the gerbil cochlea was constructed. Then, to determine effects of the functional losses of OHCs on the cochlear amplification, changes in the displacement amplitude of the basilar membrane (BM) due to the functional losses of OHCs were calculated. Results showed that the displacement amplitude of the BM decreases significantly when a single row of OHCs lost its function, suggesting that all three rows of OHCs are required for cochlear amplification.

Microtubule teardrop patterns

Several strategies for controlling microtubule patterns are developed because of the rigidity determined from the molecular structure and the geometrical structure. In contrast to the patterns in co-operation with motor proteins or associated proteins, microtubules have a huge potential for patterns via their intrinsic flexural rigidity. We discover that a microtubule teardrop pattern emerges via self-assembly under hydrodynamic flow from the parallel bundles without motor proteins. In the growth process, the bundles ultimately bend according to the critical bending curvature. Such protein pattern formation utilizing the intrinsic flexural rigidity will provide broad understandings of self-assembly of rigid rods, not only in biomolecules, but also in supramolecules.

Mechanical systems biology of C. elegans touch sensation

The sense of touch informs us of the physical properties of our surroundings and is a critical aspect of communication. Before touches are perceived, mechanical signals are transmitted quickly and reliably from the skin's surface to mechano-electrical transduction channels embedded within specialized sensory neurons. We are just beginning to understand how soft tissues participate in force transmission and how they are deformed. Here, we review empirical and theoretical studies of single molecules and molecular ensembles thought to be involved in mechanotransmission and apply the concepts emerging from this work to the sense of touch. We focus on the nematode Caenorhabditis elegans as a well-studied model for touch sensation in which mechanics can be studied on the molecular, cellular, and systems level. Finally, we conclude that force transmission is an emergent property of macromolecular cellular structures that mutually stabilize one another.

A model for the nonlinear mechanism responsible for cochlear amplification

A nonlinear model for the mechanism responsible for the amplification of the sound wave in the ear is derived using the geometric and material properties of the system. The result is a nonlinear beam equation, with the nonlinearity appearing in a coefficient of the equation. Once derived, the beam problem is analyzed for various loading conditions. Based on this analysis it is seen that the mechanism is capable of producing a spatially localized gain, as required by any amplification mechanism, but it is also capable of increasing the spatial contrast in the signal.

Structure and mechanical characterization of DNA i-motif nanowires by molecular dynamics simulation

We studied the structure and mechanical properties of DNA i-motif nanowires by means of molecular dynamics computer simulations. We built up to 230 nm-long nanowires, based on a repeated TC5 sequence from crystallographic data, fully relaxed and equilibrated in water. The unusual C⋅C(+) stacked structure, formed by four ssDNA strands arranged in an intercalated tetramer, is here fully characterized both statically and dynamically. By applying stretching, compression, and bending deformations with the steered molecular dynamics and umbrella sampling methods, we extract the apparent Young's and bending moduli of the nanowire, as well as estimates for the tensile strength and persistence length. According to our results, the i-motif nanowire shares similarities with structural proteins, as far as its tensile stiffness, but is closer to nucleic acids and flexible proteins, as far as its bending rigidity is concerned. Furthermore, thanks to its very thin cross section, the apparent tensile toughness is close to that of a metal. Besides their yet to be clarified biological significance, i-motif nanowires may qualify as interesting candidates for nanotechnology templates, due to such outstanding mechanical properties.

The counterbend phenomenon in flagellar axonemes and cross-linked filament bundles

Recent observations of flagellar counterbend in sea urchin sperm show that the mechanical induction of curvature in one part of a passive flagellum induces a compensatory countercurvature elsewhere. This apparent paradoxical effect cannot be explained using the standard elastic rod theory of Euler and Bernoulli, or even the more general Cosserat theory of rods. Here, we develop a geometrically exact mechanical model to describe the statics of microtubule bundles that is capable of predicting the curvature reversal events observed in eukaryotic flagella. This is achieved by allowing the interaction of deformations in different material directions, by accounting not only for structural bending, but also for the elastic forces originating from the internal cross-linking mechanics. Large-amplitude static configurations can be described analytically, and an excellent match between the model and the observed counterbend deformation was found. This allowed a simultaneous estimation of multiple sperm flagellum material parameters, namely the cross-linking sliding resistance, the bending stiffness, and the sperm head junction compliance ratio. We further show that small variations on the empirical conditions may induce discrepancies for the evaluation of the flagellar material quantities, so that caution is required when interpreting experiments. Finally, our analysis demonstrates that the counterbend emerges as a fundamental property of sliding resistance in cross-linked filamentous polymer bundles, which also suggests that cross-linking proteins may contribute to the regulation of the flagellar waveform in swimming sperm via counterbend mechanics.

Selective ablation of pillar and deiters' cells severely affects cochlear postnatal development and hearing in mice

Mammalian auditory hair cells (HCs) are inserted into a well structured environment of supporting cells (SCs) and acellular matrices. It has been proposed that when HCs are irreversibly damaged by noise or ototoxic drugs, surrounding SCs seal the epithelial surface and likely extend the survival of auditory neurons. Because SCs are more resistant to damage than HCs, the effects of primary SC loss on HC survival and hearing have received little attention. We used the Cre/loxP system in mice to specifically ablate pillar cells (PCs) and Deiters' cells (DCs). In Prox1CreER(T2)+/-;Rosa26(DTA/+) (Prox1DTA) mice, Cre-estrogen receptor (CreER) expression is driven by the endogenous Prox1 promoter and, in presence of tamoxifen, removes a stop codon in the Rosa26(DTA/+) allele and induces diphtheria toxin fragment A (DTA) expression. DTA produces cell-autonomous apoptosis. Prox1DTA mice injected with tamoxifen at postnatal days 0 (P0) and P1 show significant DC and outer PC loss at P2-P4, that reaches ∼70% by 1 month. Outer HC loss follows at P14 and is almost complete at 1 month, while inner HCs remain intact. Neural innervation to the outer HCs is disrupted in Prox1DTA mice and auditory brainstem response thresholds in adults are 40-50 dB higher than in controls. The hearing deficit correlates with loss of cochlear amplification. Remarkably, in Prox1DTA mice, the auditory epithelium preserves the ability to seal the reticular lamina and spiral ganglion neuron counts are normal, a key requirement for cochlear implant success. In addition, our results show that cochlear SC pools should be appropriately replenished during HC regeneration strategies.

Cdc42-dependent structural development of auditory supporting cells is required for wound healing at adulthood

Cdc42 regulates the initial establishment of cytoskeletal and junctional structures, but only little is known about its role at later stages of cellular differentiation. We studied Cdc42′s role in vivo in auditory supporting cells, epithelial cells with high structural complexity. Cdc42 inactivation was induced early postnatally using the Cdc42^loxP/loxP;Fgfr3-iCre-ER^T2 mice. Cdc42 depletion impaired elongation of adherens junctions and F-actin belts, leading to constriction of the sensory epithelial surface. Fragmented F-actin belts, junctions containing ectopic lumens and misexpression of a basolateral membrane protein in the apical domain were observed. These defects and changes in aPKCλ/ι expression suggested that apical polarization is impaired. Following a lesion at adulthood, supporting cells with Cdc42 loss-induced maturational defects collapsed and failed to remodel F-actin belts, a process that is critical to scar formation. Thus, Cdc42 is required for structural differentiation of auditory supporting cells and this proper maturation is necessary for wound healing in adults.

Structure and mechanics of supporting cells in the guinea pig organ of Corti

The mechanical properties of the mammalian organ of Corti determine its sensitivity to sound frequency and intensity, and the structure of supporting cells changes progressively with frequency along the cochlea. From the apex (low frequency) to the base (high frequency) of the guinea pig cochlea inner pillar cells decrease in length incrementally from 75–55 µm whilst the number of axial microtubules increases from 1,300–2,100. The respective values for outer pillar cells are 120–65 µm and 1,500–3,000. This correlates with a progressive decrease in the length of the outer hair cells from >100 µm to 20 µm. Deiters'cell bodies vary from 60–50 µm long with relatively little change in microtubule number. Their phalangeal processes reflect the lengths of outer hair cells but their microtubule numbers do not change systematically. Correlations between cell length, microtubule number and cochlear location are poor below 1 kHz. Cell stiffness was estimated from direct mechanical measurements made previously from isolated inner and outer pillar cells. We estimate that between 200 Hz and 20 kHz axial stiffness, bending stiffness and buckling limits increase, respectively,∼3, 6 and 4 fold for outer pillar cells, ∼2, 3 and 2.5 fold for inner pillar cells and ∼7, 20 and 24 fold for the phalangeal processes of Deiters'cells. There was little change in the Deiters'cell bodies for any parameter. Compensating for effective cell length the pillar cells are likely to be considerably stiffer than Deiters'cells with buckling limits 10–40 times greater. These data show a clear relationship between cell mechanics and frequency. However, measurements from single cells alone are insufficient and they must be combined with more accurate details of how the multicellular architecture influences the mechanical properties of the whole organ.

Anomalous flexural behaviors of microtubules

Apparent controversies exist on whether the persistence length of microtubules depends on its contour length. This issue is particularly challenging from a theoretical point of view due to the tubular structure and strongly anisotropic material property of microtubules. Here we adopt a higher order continuum orthotropic thin shell model to study the flexural behavior of microtubules. Our model overcomes some key limitations of a recent study based on a simplified anisotropic shell model and results in a closed-form solution for the contour-length-dependent persistence length of microtubules, with predictions in excellent agreement with experimental measurements. By studying the ratio between their contour and persistence lengths, we find that microtubules with length at ~1.5 μm show the lowest flexural rigidity, whereas those with length at ~15 μm show the highest flexural rigidity. This finding may provide an important theoretical basis for understanding the mechanical structure of mitotic spindles during cell division. Further analysis on the buckling of microtubules indicates that the critical buckling load becomes insensitive to the tube length for relatively short microtubules, in drastic contrast to the classical Euler buckling. These rich flexural behaviors of microtubules are of profound implication for many biological functions and biomimetic molecular devices.

The doublecortin-related gene zyg-8 is a microtubule organizer in Caenorhabditis elegans neurons

Doublecortin-domain containing (DCDC) genes play key roles in the normal and pathological development of the human brain cortex. The origin of the cellular specialisation and the functional redundancy of these microtubule (MT)-associated proteins (MAPs), especially those of Doublecortin (DCX) and Doublecortin-like kinase (DCLKs) genes, is still unclear. The DCX domain has the ability to control MT architecture and bundling. However, the physiological significance of such properties is not fully understood. To address these issues, we sought post-mitotic roles for zyg-8, the sole representative of the DCX-DCLK subfamily of genes in C. elegans. Previously, zyg-8 has been shown to control anaphase-spindle positioning in one-cell stage embryos, but functions of the gene later in development have not been investigated. Here we show that wild-type zyg-8 is required beyond early embryonic divisions for proper development, spontaneous locomotion and touch sensitivity of adult worms. Consistently, we find zyg-8 expression in the six touch receptor neurons (TRNs), as well as in a subset of other neuronal and non-neuronal cells. In TRNs and motoneurons, zyg-8 controls cell body shape/polarity and process outgrowth and morphology. Ultrastructural analysis of mutant animals reveals that zyg-8 promotes structural integrity, length and number of individual MTs, as well as their bundled organisation in TRNs, with no impact on MT architecture.

Cytoskeletal changes in actin and microtubules underlie the developing surface mechanical properties of sensory and supporting cells in the mouse cochlea

Correct patterning of the inner ear sensory epithelium is essential for the conversion of sound waves into auditory stimuli. Although much is known about the impact of the developing cytoskeleton on cellular growth and cell shape, considerably less is known about the role of cytoskeletal structures on cell surface mechanical properties. In this study, atomic force microscopy (AFM) was combined with fluorescence imaging to show that developing inner ear hair cells and supporting cells have different cell surface mechanical properties with different developmental time courses. We also explored the cytoskeletal organization of developing sensory and non-sensory cells, and used pharmacological modulation of cytoskeletal elements to show that the developmental increase of hair cell stiffness is a direct result of actin filaments, whereas the development of supporting cell surface mechanical properties depends on the extent of microtubule acetylation. Finally, this study found that the fibroblast growth factor signaling pathway is necessary for the developmental time course of cell surface mechanical properties, in part owing to the effects on microtubule structure.

In vivo outer hair cell length changes expose the active process in the cochlea

Background

Mammalian hearing is refined by amplification of the sound-evoked vibration of the cochlear partition. This amplification is at least partly due to forces produced by protein motors residing in the cylindrical body of the outer hair cell. To transmit power to the cochlear partition, it is required that the outer hair cells dynamically change their length, in addition to generating force. These length changes, which have not previously been measured in vivo, must be correctly timed with the acoustic stimulus to produce amplification.

Methodology/Principal Findings

Using in vivo optical coherence tomography, we demonstrate that outer hair cells in living guinea pigs have length changes with unexpected timing and magnitudes that depend on the stimulus level in the sensitive cochlea.

Conclusions/Significance

The level-dependent length change is a necessary condition for directly validating that power is expended by the active process presumed to underlie normal hearing.

Computational modeling of axonal microtubule bundles under tension

Microtubule bundles cross-linked by tau protein serve a variety of neurological functions including maintaining mechanical integrity of the axon, promoting axonal growth, and facilitating cargo transport. It has been observed that axonal damage in traumatic brain injury leads to bundle disorientation, loss of axonal viability, and cognitive impairment. This study investigates the initial mechanical response of axonal microtubule bundles under uniaxial tension using a discrete bead-spring representation. Mechanisms of failure due to traumatic stretch loading and their impact on the mechanical response and stability are also characterized. This study indicates that cross-linked axonal microtubule bundles in tension display stiffening behavior similar to a power-law relationship from nonaffine network deformations. Stretching of cross-links and microtubule bending were the primary deformation modes at low stresses. Microtubule stretch was negligible up to tensile stresses of ∼1 MPa. Bundle failure occurred by failure of cross-links leading to pull-out of microtubules and loss of bundle integrity. This may explain the elongation, undulation, and delayed elasticity of axons following traumatic stretch loading. More extensively cross-linked bundles withstood higher tensile stresses before failing. The bundle mechanical behavior uncovered by these computational techniques should guide future experiments on stretch-injured axons.

Effects of microgravity on osteoclast bone resorption and osteoblast cytoskeletal organization and adhesion

Exposure to microgravity has been associated with several physiological changes in astronauts, including an osteoporosis-like loss in bone mass. Despite many in vivo and in vitro studies in both microgravity and simulated microgravity conditions, the mechanism for bone loss is still not clear. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to assess bone cell responses. In this work, we conduct a unique investigation of the effects of microgravity on bone-producing osteoblasts and, in parallel, on bone-resorbing osteoclasts. An increase in total number of discrete resorption pits is observed in osteoclasts that experienced microgravity versus ground controls. We further show that osteoblasts exposed to 5 days of microgravity have shorter and wavier microtubules (MTs), smaller and fewer focal adhesions, and thinner cortical actin and stress fibers. Space-flown osteoblasts present extended cell shapes as well as significantly more disrupted and often fragmented or condensed nuclei. The absence of gravitational forces therefore causes both an increase in bone resorption by osteoclasts, and a decrease in osteoblast cellular integrity. The observed effects on both major bone cell types likely accelerate bone loss in microgravity environments, and additionally offer a potential explanation to the development of disuse osteoporosis on Earth.

Force transmission in the organ of Corti micromachine

Auditory discrimination is limited by the performance of the cochlea whose acute sensitivity and frequency tuning are underpinned by electromechanical feedback from the outer hair cells. Two processes may underlie this feedback: voltage-driven contractility of the outer hair cell body and active motion of the hair bundle. Either process must exert its mechanical effect via deformation of the organ of Corti, a complex assembly of sensory and supporting cells riding on the basilar membrane. Using finite element analysis, we present a three-dimensional model to illustrate deformation of the organ of Corti by the two active processes. The model used available measurements of the properties of structural components in low-frequency and high-frequency regions of the rodent cochlea. The simulations agreed well with measurements of the cochlear partition stiffness, the longitudinal space constant for point deflection, and the deformation of the organ of Corti for current injection, as well as displaying a 20-fold increase in passive resonant frequency from apex to base. The radial stiffness of the tectorial membrane attachment was found to be a crucial element in the mechanical feedback. Despite a substantial difference in the maximum force generated by hair bundle and somatic motility, the two mechanisms induced comparable amplitudes of motion of the basilar membrane but differed in the polarity of their feedback on hair bundle position. Compared to the hair bundle motor, the somatic motor was more effective in deforming the organ of Corti than in displacing the basilar membrane.

Compliance profiles derived from a three-dimensional finite-element model of the basilar membrane

A finite-element analysis is used to explore the impact of elastic material properties, boundary conditions, and geometry, including coiling, on the spatial characteristics of the compliance of the unloaded basilar membrane (BM). It is assumed that the arcuate zone is isotropic and the pectinate zone orthotropic, and that the radial component of the effective Young's modulus in the pectinate zone decreases exponentially with distance from base to apex. The results concur with tonotopic characteristics of compliance and neural data. Moreover, whereas the maximum compliance in a radial profile is located close to the boundary between the two zones in the basal region, it shifts to the midpoint of the pectinate zone for the apical BM; the width of the profile also expands. This shift begins near the 1 kHz characteristic place for guinea pig and the 2.4 kHz place for gerbil. Shift and expansion are not observed for linear rather than exponential decrease of the radial component of Young's modulus. This spatial change of the compliance profile leads to the prediction that mechanical excitation in the apical region of the organ of Corti is different to that in the basal region.

Torsional elastic deformations of microtubules within continuous sheet model

This paper develops a rigorous analysis of the microtubule elastic deformations in terms of the torsional degrees of freedom using the helix-based cylindrical structure of this biopolymer. Methods of differential geometry and the theory of elasticity are employed in our analysis. We find equilibrium conditions and constitutive equations in the linear regime. We estimate the value of torsional rigidity for microtubules based on their structure and some experimentally known elastic properties. The paper concludes with the derivation of a bulk modulus formula for a microtubule in solution. Both the entropy change and the fluctuation of the twist angle are obtained.

A MULTISCALE MODEL OF THE ORGAN OF CORTI

The organ of Corti is the sensory epithelium in the cochlea of the inner ear. It is modeled as a shell-of-revolution structure with continuous and discrete components. Our recent work has been on the inclusion of the viscous fluid. Measurements from various laboratories provide the opportunity to refocus on the elastic properties. The current detailed model for the organ of Corti is reasonably consistent with diverse measurements. Most components have little stiffness in the propagation direction. However, the isotropic stiffness of the pillar heads is found to offer an explanation for the difference in point load and pressure measurements. The individual rows of inner hair cell stereocilia with tip links and the Hensen stripe are included, since these details are important for the determination of the neural excitation. The results for low frequency show a phase of tip link tension similar to auditory nerve measurements. The nonlinearity of fluid in the small gaps is considered. A result is that as amplitude increases, because of the near contact with the Hensen stripe, the excitation changes polarity, similar to the peak-splitting neural behavior sometimes observed.

Nanoscale organization of the MEC-4 DEG/ENaC sensory mechanotransduction channel in Caenorhabditis elegans touch receptor neurons

Hearing, touch and proprioception are thought to involve direct activation of mechano-electrical transduction (MeT) channels. In Caenorhabditis elegans touch receptor neurons (TRNs), such channels contain two pore-forming subunits (MEC-4 and MEC-10) and two auxiliary subunits (MEC-2 and MEC-6). MEC-4 and MEC-10 belong to a large superfamily of ion channel proteins (DEG/ENaCs) that form nonvoltage-gated, amiloride-sensitive Na+ channels. In TRNs, unique 15-protofilament microtubules and an electron-dense extracellular matrix have been proposed to serve as gating tethers critical for MeT channel activation. We combined high-pressure freezing and serial-section immunoelectron microscopy to determine the position of MeT channels relative to putative gating tethers. MeT channels were visualized using antibodies against MEC-4 and MEC-2. This nanometer-resolution view of a sensory MeT channel establishes structural constraints on the mechanics of channel gating. We show here that MEC-2 and MEC-5 collagen, a putative extracellular tether, occupy overlapping but distinct domains in TRN neurites. Although channels decorate all sides of TRN neurites; they are not associated with the distal endpoints of 15-protofilament microtubules hypothesized to be gating tethers. These specialized microtubules, which are unique to TRNs, assemble into a cross-linked bundle connected by a network of kinked filaments to the neurite membrane. We speculate that the microtubule bundle converts external point loads into membrane stretch which, in turn, facilitates MeT channel activation.

Cytoskeletal bundle mechanics

The mechanical properties of cytoskeletal actin bundles play an essential role in numerous physiological processes, including hearing, fertilization, cell migration, and growth. Cells employ a multitude of actin-binding proteins to actively regulate bundle dimensions and cross-linking properties to suit biological function. The mechanical properties of actin bundles vary by orders of magnitude depending on diameter and length, cross-linking protein type and concentration, and constituent filament properties. Despite their importance to cell function, the molecular design principles responsible for this mechanical behavior remain unknown. Here, we examine the mechanics of cytoskeletal bundles using a molecular-based model that accounts for the discrete nature of constituent actin filaments and their distinct cross-linking proteins. A generic competition between filament stretching and cross-link shearing determines three markedly different regimes of mechanical response that are delineated by the relative values of two simple design parameters, revealing the universal nature of bundle-bending mechanics. In each regime, bundle-bending stiffness displays distinct scaling behavior with respect to bundle dimensions and molecular composition, as observed in reconstituted actin bundles in vitro. This mechanical behavior has direct implications on the physiological bending, buckling, and entropic stretching behavior of cytoskeletal processes, as well as reconstituted actin systems. Results are used to predict the bending regimes of various in vivo cytoskeletal bundles that are not easily accessible to experiment and to generate hypotheses regarding implications of the isolated behavior on in vivo bundle function.

Statistical mechanics of semiflexible bundles of wormlike polymer chains

We demonstrate that a semiflexible bundle of wormlike chains exhibits a state-dependent bending stiffness that alters fundamentally its scaling behavior with respect to the standard wormlike chain. We explore the equilibrium conformational and mechanical behavior of wormlike bundles in isolation, in cross-linked networks, and in solution.

Hearing development and spiral ganglion neurite growth in VASP deficient mice

Vasodilator-stimulated phosphoprotein (VASP) has been found to be involved in intracellular signalling pathways and to play an important role in the actin associated organization and formation of the cytoskeleton. Since differential VASP expression was noted in inner ear tissues, the present study was performed to investigate the hearing development in VASP deficient mice. Hearing development in VASP-/- mice and wild type animals was investigated by auditory brain stem (ABR) measurements. In addition, inner ear tissues of wild type animals were tested for VASP expression using PCR, Western blot analysis, in situ hybridisation, and immunohistochemistry. To compare spiral ganglion (SG) neurite growth, SG explants from VASP-/- and wild type mice were analyzed under cell culture conditions. The electroacoustical results of the present study indicate that VASP deficient mice present with a later onset of hearing during postnatal development compared to wild type animals. Transient VASP expression was detected in neonatal SG of wild type mice. Tissue culture experiments with SG explants from VASP-/- animals revealed significant alterations in SG neurite extension compared to wild types. The present findings suggest a role for VASP during neonatal development of the mammalian cochlea and allow speculation on a possible delayed innervation of cochlear hair cells due to changes in SG neurite growth in VASP-deficient mice. Temporary VASP deficits in the neonatal inner ear may be compensated by related proteins like MENA leading to a delayed but complete development of hearing function in VASP-/- animals.

Sound-evoked radial strain in the hearing organ

The hearing organ contains sensory hair cells, which convert sound-evoked vibration into action potentials in the auditory nerve. This process is greatly enhanced by molecular motors that reside within the outer hair cells, but the performance also depends on passive mechanical properties, such as the stiffness, mass, and friction of the structures within the organ of Corti. We used resampled confocal imaging to study the mechanical properties of the low-frequency regions of the cochlea. The data allowed us to estimate an important mechanical parameter, the radial strain, which was found to be 0.1% near the inner hair cells and 0.3% near the third row of outer hair cells during moderate-level sound stimulation. The strain was caused by differences in the motion trajectories of inner and outer hair cells. Motion perpendicular to the reticular lamina was greater at the outer hair cells, but inner hair cells showed greater radial vibration. These differences led to deformation of the reticular lamina, which connects the apex of the outer and inner hair cells. These results are important for understanding how the molecular motors of the outer hair cells can so profoundly affect auditory sensitivity.