Mechanical properties of sensory and supporting cells in the organ of Corti of the guinea pig cochlea--study by atomic force microscopy

3D Computational Modeling of Blast Transmission through the Fluid-Filled Cochlea and Hair Cells

PURPOSE

Veterans commonly suffer from blast-induced hearing disabilities. Injury to the sensitive organ of Corti (OC) or hair cells within the cochlea can directly lead to hearing loss, but is very difficult to measure experimentally. Computational finite element (FE) models of the human ear have been used to predict blast wave transmission through the middle ear and cochlea, but these models lack a representation of the OC. This paper reports a recently developed 3D FE model of the OC to simulate the response of hair cells to blast waves and predict possible injury locations.

METHODS

Components of the OC model consist of the sensory cells, membranes, and supporting cells with endolymphatic fluid surrounding them inside the scala media. Displacement of the basilar membrane induced by a 31-kPa blast overpressure derived from the macroscale model of the human ear was applied as input to the OC model. The fluid-structure interaction coupled analysis in the time domain was conducted in ANSYS.

RESULTS

Major results derived from the FE model include the strains and displacements of the outer hair cells, stereociliary hair bundles (HBs), reticular lamina, and the tectorial membrane (TcM). The highest structural strain was concentrated around the connecting region of the HBs and the TcM, potentially indicating detachment due to blast exposure. Including the interstitial fluid in the OC created a realistic environment and improved the accuracy of the results compared to the previously published OC model without fluid.

CONCLUSION

The microscale model of OC was developed in order to simulate blast overpressure transmission through the fluid-filled cochlea and hair cells. This FE model represents a significant advancement in the study of blast wave transmission through the inner ear, and is an important step toward a comprehensive multi-scale model of the human ear that can predict blast-induced injury and hearing loss.

Inactivity of Stat3 in sensory and non-sensory cells of the mature cochlea

Signal transducer and activator of transcription 3 (Stat3) plays a role in various cellular processes such as differentiation, inflammation, cell survival and microtubule dynamics, depending on the cell type and the activated signaling pathway. Stat3 is highly expressed in the hair cells and supporting cells of the cochlea and is essential for the differentiation of mouse hair cells in the early embryonic stage. However, it is unclear how Stat3 contributes to the correct function of cells in the organ of Corti postnatally. To investigate this, an inducible Cre/loxp system was used to knock out Stat3 in either the outer hair cells or the supporting cells. The results showed that the absence of Stat3 in either the outer hair cells or the supporting cells resulted in hearing loss without altering the morphology of the organ of Corti. Molecular analysis of the outer hair cells revealed an inflammatory process with increased cytokine production and upregulation of the NF-kB pathway. However, the absence of Stat3 in the supporting cells resulted in reduced microtubule stability. In conclusion, Stat3 is a critical protein for the sensory epithelium of the cochlea and hearing and functions in a cell and function-specific manner.

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.

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.

Elastic force restricts growth of the murine utricle

Dysfunctions of hearing and balance are often irreversible in mammals owing to the inability of cells in the inner ear to proliferate and replace lost sensory receptors. To determine the molecular basis of this deficiency we have investigated the dynamics of growth and cellular proliferation in a murine vestibular organ, the utricle. Based on this analysis, we have created a theoretical model that captures the key features of the organ’s morphogenesis. Our experimental data and model demonstrate that an elastic force opposes growth of the utricular sensory epithelium during development, confines cellular proliferation to the organ’s periphery, and eventually arrests its growth. We find that an increase in cellular density and the subsequent degradation of the transcriptional cofactor Yap underlie this process. A reduction in mechanical constraints results in accumulation and nuclear translocation of Yap, which triggers proliferation and restores the utricle’s growth; interfering with Yap’s activity reverses this effect.

DOI:http://dx.doi.org/10.7554/eLife.25681.001

Dynamic characteristics of the middle ear in neonates

OBJECTIVE

Early diagnosis and treatment of hearing disorders in neonates is highly effective for realization of linguistic competence and intellectual development. To objectively and quickly evaluate the dynamic characteristics of the middle ear, a sweep frequency impedance (SFI) meter was developed, which allowed the diagnosis of middle-ear dysfunctions in adults and children. However, this SFI meter was not applicable to neonates since the size of the measurement probe was too large. In the present study, therefore, the SFI meter was improved, i.e., the diameter of the probe was reduced to that of the neonatal external ear canal. By using this newly designed SFI meter, SFI tests were performed in healthy neonates.

METHODS

A sound of the sweeping sinusoidal frequency between 0.1 kHz and 2.0 kHz in 0.02-kHz step intervals is presented to the ear canal by an SFI probe while the static pressure of the ear canal is kept constant. During this procedure, the sound pressure level (SPL) is measured. The measurements are performed at 50-daPa intervals of static pressure from 200 daPa to -200 daPa.

RESULTS

Measurements were conducted in 10 ears of 9 neonates. The SPL showed two variations at 0.26 ± 0.03 kHz and 1.13 ± 0.12 kHz. Since the SPL is known to show a variation at frequencies from 1.0 kHz to 1.6 kHz due to the resonance of the middle ear in adults and children with normal hearing, the second variation is probably related to such resonance in neonates. The measurement of gel models, which mimics the neonatal external ear canal, showed a variation in SPL at around 0.5 kHz. This implies that the source of the first variation may possibly be related to the resonance of the external ear canal wall.

CONCLUSIONS

SFI tests revealed that there were two variations in the SPL curve in neonates, one at 0.26 ± 0.03 kHz and the other at 1.13 ± 0.12 kHz, the former and the latter being possibly related to the resonance of the external ear canal wall and that of the middle ear, respectively. This result suggests that the dynamic characteristics of the middle ear in neonates are different from those in adults.

Modeling the Mechanics of Tethers Pulled From the Cochlear Outer Hair Cell Membrane

Cell membrane tethers are formed naturally (e.g., in leukocyte rolling) and experimentally to probe membrane properties. In cochlear outer hair cells, the plasma membrane is part of the trilayer lateral wall, where the membrane is attached to the cytoskeleton by a system of radial pillars. The mechanics of these cells is important to the sound amplification and frequency selectivity of the ear. We present a modeling study to simulate the membrane deflection, bending, and interaction with the cytoskeleton in the outer hair cell tether pulling experiment. In our analysis, three regions of the membrane are considered: the body of a cylindrical tether, the area where the membrane is attached and interacts with the cytoskeleton, and the transition region between the two. By using a computational method, we found the shape of the membrane in all three regions over a range of tether lengths and forces observed in experiments. We also analyze the effects of biophysical properties of the membrane, including the bending modulus and the forces of the membrane adhesion to the cytoskeleton. The model’s results provide a better understanding of the mechanics of tethers pulled from cell membranes.

Atomic force microscopy probing of cell elasticity

Atomic force microscopy (AFM) has recently provided the great progress in the study of micro- and nanostructures including living cells and cell organelles. Modern AFM techniques allow solving a number of problems of cell biomechanics due to simultaneous evaluation of the local mechanical properties and the topography of the living cells at a high spatial resolution and force sensitivity. Particularly, force spectroscopy is used for mapping mechanical properties of a single cell that provides information on cellular structures including cytoskeleton structure. This entry is aimed to review the recent AFM applications for the study of dynamics and mechanical properties of intact cells associated with different cell events such as locomotion, differentiation and aging, physiological activation and electromotility, as well as cell pathology. Local mechanical characteristics of different cell types including muscle cells, endothelial and epithelial cells, neurons and glial cells, fibroblasts and osteoblasts, blood cells and sensory cells are analyzed in this paper.

Cell mechanics: integrating cell responses to mechanical stimuli

Forces are increasingly recognized as major regulators of cell structure and function, and the mechanical properties of cells are essential to the mechanisms by which cells sense forces, transmit them to the cell interior or to other cells, and transduce them into chemical signals that impact a spectrum of cellular responses. Comparison of the mechanical properties of intact cells with those of the purified cytoskeletal biopolymers that are thought to dominate their elasticity reveal the extent to which the studies of purified systems can account for the mechanical properties of the much more heterogeneous and complex cell. This review summarizes selected aspects of current work on cell mechanics with an emphasis on the structures that are activated in cell-cell contacts, that regulate ion flow across the plasma membrane, and that may sense fluid flow that produces low levels of shear stress.

Measurement and characterization of whole-cell mechanical behavior

An understanding of whole-cell mechanical behavior can provide insight into cellular responses to mechanical loading and diseases in which such responses are altered. However, this aspect of cellular mechanical behavior has received limited attention. In this study, we used the atomic force microscope (AFM) in conjunction with several mechanical characterization methods (Hertz contact theory, an exponential equation, and a parallel-spring recruitment model) to establish a mechanically rigorous method for measuring and characterizing whole-cell mechanical behavior in the deformation range 0-500 nm. Using MC3T3-E1 osteoblasts, measurement repeatability was assessed by performing multiple loading cycles on individual cells. Despite variability in measurements, repeatability of the measurement technique was statistically confirmed. The measurement technique also proved acceptable since only 5% of the total variance across all measurements was due to variations within measurements for a single cell. The parallel-spring recruitment model, a single-parameter model, accurately described the measured nonlinear force-deformation response (R2>0.99) while providing a mechanistic explanation of whole-cell mechanical behavior. Taken together, the results should improve the capabilities of the AFM to probe whole-cell mechanical behavior. In addition, the success of the parallel-spring recruitment model provides insight into the micromechanical basis of whole-cell behavior.

The effects of morphology, confluency, and phenotype on whole-cell mechanical behavior

Emerging evidence indicates that cellular mechanical behavior can be altered by disease, drug treatment, and mechanical loading. To effectively investigate how disease and mechanical or biochemical treatments influence cellular mechanical behavior, it is imperative to determine the source of large inter-cell differences in whole-cell mechanical behavior within a single cell line. In this study, we used the atomic force microscope to investigate the effects of cell morphological parameters and confluency on whole-cell mechanical behavior for osteoblastic and fibroblastic cells. For nonconfluent cells, projected nucleus area, cell area, and cell aspect ratio were not correlated with mechanical behavior (p>or=0.46), as characterized by a parallel-spring recruitment model. However, measured force-deformation responses were statistically different between osteoblastic and fibroblastic cells (p<0.001) and between confluent and nonconfluent cells (p<0.001). Osteoblastic cells were 2.3-2.8 times stiffer than fibroblastic cells, and confluent cells were 1.5-1.8 times stiffer than nonconfluent cells. The results indicate that structural differences related to phenotype and confluency affect whole-cell mechanical behavior, while structural differences related to global morphology do not. This suggests that cytoskeleton structural parameters, such as filament density, filament crosslinking, and cell-cell and cell-matrix attachments, dominate inter-cell variability in whole-cell mechanical behavior.

Local mechanical properties of mouse outer hair cells: atomic force microscopic study

OBJECTIVES

Outer hair cells (OHCs) are capable of altering their cell length in response to changes in membrane potential. Due to this electromotility, OHCs probably subject the basilar membrane to force, resulting in cochlear amplification. To understand the mechanism of such amplification, knowledge of the mechanical properties of OHCs is required since the force produced by OHC electromotility is thought to depend on such properties. Various studies have been conducted to investigate the mechanical properties of guinea pig OHCs. With regard to mice, however, although various kinds of transgenic and knockout mice possess great potential as research models, the mechanical properties of mouse OHCs have not as yet been reported since the cells and/or tissues in the mouse hearing organ are relatively small and vulnerable to external stimuli, rendering sample preparation difficult. In this study, therefore, to establish indicators of the mechanical properties of OHCs in mice, such properties were measured by atomic force microscopy (AFM).

METHODS

CBA/JNCrj strain male mice aged 10-12 weeks (25-30 g) were used. Cochleae were dissected out from the animal and both the basilar membrane and the organ of Corti were simultaneously unwrapped from the modiolus with forceps. Dissected coiled tissue was then incubated with an enzymatic digestion medium for 15 min. After digestion, OHCs were isolated by gently triturating the coiled tissue. Local mechanical properties of the OHCs were then measured by an indentation test using an AFM.

RESULTS

Young's modulus and stiffness of the OHC in the apical turn of the mouse cochlea were 2.1+/-0.5 kPa and 4.4+/-1.2 mN/m, respectively.

CONCLUSIONS

Young's modulus of the OHC in the apical turn of the cochlea in mice was roughly the same as that in the apical turn of the cochlea in guinea pigs; however, the stiffness of the former was about two times greater than that of the latter because the cell length of the former was shorter than that of the latter.

Evidence for a highly elastic shell-core organization of cochlear outer hair cells by local membrane indentation

Cochlear outer hair cells (OHCs) are thought to play an essential role in the high sensitivity and sharp frequency selectivity of the hearing organ by generating forces that amplify the vibrations of this organ at frequencies up to several tens of kHz. This tuning process depends on the mechanical properties of the cochlear partition, which OHC activity has been proposed to modulate on a cycle-by-cycle basis. OHCs have a specialized shell-core ultrastructure believed to be important for the mechanics of these cells and for their unique electromotility properties. Here we use atomic force microscopy to investigate the mechanical properties of isolated living OHCs and to show that indentation mechanics of their membrane is consistent with a shell-core organization. Indentations of OHCs are also found to be highly nonhysteretic at deformation rates of more than 40 microm/s, which suggests the OHC lateral wall is a highly elastic structure, with little viscous dissipation, as would appear to be required in view of the very rapid changes in shape and mechanics OHCs are believed to undergo in vivo.