Involvement of extracellular matrix in the formation of the inner ear

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

Varying mechanical forces drive sensory epithelium formation

The mechanical cues of the external microenvironment have been recognized as essential clues driving cell behavior. Although intracellular signals modulating cell fate during sensory epithelium development is well understood, the driving force of sensory epithelium formation remains elusive. Here, we manufactured a hybrid hydrogel with tunable mechanical properties for the cochlear organoids culture and revealed that the extracellular matrix (ECM) drives sensory epithelium formation through shifting stiffness in a stage-dependent pattern. As the driving force, moderate ECM stiffness activated the expansion of cochlear progenitor cell (CPC)-derived epithelial organoids by modulating the integrin α3 (ITGA3)/F-actin cytoskeleton/YAP signaling. Higher stiffness induced the transition of CPCs into sensory hair cells (HCs) through increasing the intracellular Ca2+ signaling mediated by PIEZO2 and then activating KLF2 to accomplish the cell specification . Our results identify the molecular mechanism of sensory epithelium formation guided by ECM mechanical force and contribute to developing therapeutic approaches for HC regeneration.

Spatially distinct otic mesenchyme cells show molecular and functional heterogeneity patterns before hearing onset

The cochlea consists of diverse cellular populations working in harmony to convert mechanical stimuli into electrical signals for the perception of sound. Otic mesenchyme cells (OMCs), often considered a homogeneous cell type, are essential for normal cochlear development and hearing. Despite being the most numerous cell type in the developing cochlea, OMCs are poorly understood. OMCs are known to differentiate into spatially and functionally distinct cell types, including fibrocytes of the lateral wall and spiral limbus, modiolar osteoblasts, and specialized tympanic border cells of the basilar membrane. Here, we show that OMCs are transcriptionally and functionally heterogeneous and can be divided into four distinct populations that spatially correspond to OMC-derived cochlear structures. We also show that this heterogeneity and complexity of OMCs commences during early phases of cochlear development. Finally, we describe the cell-cell communication network of the developing cochlea, inferring a major role for OMC in outgoing signaling.

Recent advancements in understanding the role of epigenetics in the auditory system

Sensorineural deafness in mammals is most commonly caused by damage to inner ear sensory epithelia, or hair cells, and can be attributed to genetic and environmental causes. After undergoing trauma, many non-mammalian organisms, including reptiles, birds, and zebrafish, are capable of regenerating damaged hair cells. Mammals, however, are not capable of regenerating damaged inner ear sensory epithelia, so that hair cell damage is permanent and can lead to hearing loss. The field of epigenetics, which is the study of various phenotypic changes caused by modification of genetic expression rather than alteration of DNA sequence, has seen numerous developments in uncovering biological mechanisms of gene expression and creating various medical treatments. However, there is a lack of information on the precise contribution of epigenetic modifications in the auditory system, specifically regarding their correlation with development of inner ear (cochlea) and consequent hearing impairment. Current studies have suggested that epigenetic modifications influence differentiation, development, and protection of auditory hair cells in cochlea, and can lead to hair cell degeneration. The objective of this article is to review the existing literature and discuss the advancements made in understanding epigenetic modifications of inner ear sensory epithelial cells. The analysis of the emerging epigenetic mechanisms related to inner ear sensory epithelial cells development, differentiation, protection, and regeneration will pave the way to develop novel therapeutic strategies for hearing loss.

A Protocol for Decellularizing Mouse Cochleae for Inner Ear Tissue Engineering

In mammals, mechanosensory hair cells that facilitate hearing lack the ability to regenerate, which has limited treatments for hearing loss. Current regenerative medicine strategies have focused on transplanting stem cells or genetic manipulation of surrounding support cells in the inner ear to encourage replacement of damaged stem cells to correct hearing loss. Yet, the extracellular matrix (ECM) may play a vital role in inducing and maintaining function of hair cells, and has not been well investigated. Using the cochlear ECM as a scaffold to grow adult stem cells may provide unique insights into how the composition and architecture of the extracellular environment aids cells in sustaining hearing function. Here we present a method for isolating and decellularizing cochleae from mice to use as scaffolds accepting perfused adult stem cells. In the current protocol, cochleae are isolated from euthanized mice, decellularized, and decalcified. Afterward, human Wharton's jelly cells (hWJCs) that were isolated from the umbilical cord were carefully perfused into each cochlea. The cochleae were used as bioreactors, and cells were cultured for 30 days before undergoing processing for analysis. Decellularized cochleae retained identifiable extracellular structures, but did not reveal the presence of cells or noticeable fragments of DNA. Cells perfused into the cochlea invaded most of the interior and exterior of the cochlea and grew without incident over a duration of 30 days. Thus, the current method can be used to study how cochlear ECM affects cell development and behavior.

Exploiting decellularized cochleae as scaffolds for inner ear tissue engineering

Background

Use of decellularized tissues has become popular in tissue engineering applications as the natural extracellular matrix can provide necessary physical cues that help induce the restoration and development of functional tissues. In relation to cochlear tissue engineering, the question of whether decellularized cochlear tissue can act as a scaffold and support the incorporation of exogenous cells has not been addressed. Investigators have explored the composition of the cochlear extracellular matrix and developed multiple strategies for decellularizing a variety of different tissues; however, no one has investigated whether decellularized cochlear tissue can support implantation of exogenous cells.

Methods

As a proof-of-concept study, human Wharton’s jelly cells were perfused into decellularized cochleae isolated from C57BL/6 mice to determine if human Wharton’s jelly cells could implant into decellularized cochlear tissue. Decellularization was verified through scanning electron microscopy. Cocheae were stained with DAPI and immunostained with Myosin VIIa to identify cells. Perfused cochleae were imaged using confocal microscopy.

Results

Features of the organ of Corti were clearly identified in the native cochleae when imaged with scanning electron microscopy and confocal microscopy. Acellular structures were identified in decellularized cochleae; however, no cellular structures or lipid membranes were present within the decellularized cochleae when imaged via scanning electron microscopy. Confocal microscopy revealed positive identification and adherence of cells in decellularized cochleae after perfusion with human Wharton’s jelly cells. Some cells positively expressed Myosin VIIa after perfusion.

Conclusions

Human Wharton’s jelly cells are capable of successfully implanting into decellularized cochlear extracellular matrix. The identification of Myosin VIIa expression in human Wharton’s jelly cells after implantation into the decellularized cochlear extracellular matrix suggest that components of the cochlear extracellular matrix may be involved in differentiation.

Histone demethylase KDM4B regulates otic vesicle invagination via epigenetic control of Dlx3 expression

In vertebrate embryos, the histone demethylase KDM4B affects otic placode proliferation, intercellular adhesion, and invagination by directly regulating Dlx3 expression.

In vertebrates, the inner ear arises from the otic placode, a thickened swathe of ectoderm that invaginates to form the otic vesicle. We report that histone demethylase KDM4B is dynamically expressed during early stages of chick inner ear formation. A loss of KDM4B results in defective invagination and striking morphological changes in the otic epithelium, characterized by abnormal localization of adhesion and cytoskeletal molecules and reduced expression of several inner ear markers, including Dlx3. In vivo chromatin immunoprecipitation reveals direct and dynamic occupancy of KDM4B and its target, H3K9me3, at regulatory regions of the Dlx3 locus. Accordingly, coelectroporations of DLX3 or KDM4B encoding constructs, but not a catalytically dead mutant of KDM4B, rescue the ear invagination phenotype caused by KDM4B knockdown. Moreover, a loss of DLX3 phenocopies a loss of KDM4B. Collectively, our findings suggest that KDM4B play a critical role during inner ear invagination via modulating histone methylation of the direct target Dlx3.

Nonviral Reprogramming of Human Wharton's Jelly Cells Reveals Differences Between ATOH1 Homologues

The transcription factor atonal homolog 1 (ATOH1) has multiple homologues that are functionally conserved across species and is responsible for the generation of sensory hair cells. To evaluate potential functional differences between homologues, human and mouse ATOH1 (HATH1 and MATH-1, respectively) were nonvirally delivered to human Wharton's jelly cells (hWJCs) for the first time. Delivery of HATH1 to hWJCs demonstrated superior expression of inner ear hair cell markers and characteristics than delivery of MATH-1. Inhibition of HES1 and HES5 signaling further increased the atonal effect. Transfection of hWJCs with HATH1 DNA, HES1 siRNA, and HES5 siRNA displayed positive identification of key hair cell and support cell markers found in the cochlea, as well as a variety of cell shapes, sizes, and features not native to hair cells, suggesting the need for further examination of other cell types induced by HATH1 expression. In the first side-by-side evaluation of HATH1 and MATH-1 in human cells, substantial differences were observed, suggesting that the two atonal homologues may not be interchangeable in human cells, and artificial expression of HATH1 in hWJCs requires further study. In the future, this line of research may lead to engineered systems that would allow for evaluation of drug ototoxicity or potentially even direct therapeutic use.

Cranial placodes: models for exploring the multi-facets of cell adhesion in epithelial rearrangement, collective migration and neuronal movements

Key to morphogenesis is the orchestration of cell movements in the embryo, which requires fine-tuned adhesive interactions between cells and their close environment. The neural crest paradigm has provided important insights into how adhesion dynamics control epithelium-to-mesenchyme transition and mesenchymal cell migration. Much less is known about cranial placodes, patches of ectodermal cells that generate essential parts of vertebrate sensory organs and ganglia. In this review, we summarise the known functions of adhesion molecules in cranial placode morphogenesis, and discuss potential novel implications of adhesive interactions in this crucial developmental process. The great repertoire of placodal cell behaviours offers new avenues for exploring the multiple roles of adhesion complexes in epithelial remodelling, collective migration and neuronal movements.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Wnt signals mediate a fate decision between otic placode and epidermis

The otic placode, the anlagen of the inner ear, develops from an ectodermal field characterized by expression of the transcription factor Pax2. Previous fate mapping studies suggest that these Pax2+cells will give rise to both otic placode tissue and epidermis, but the signals that divide the Pax2+ field into placodal and epidermal territories are unknown. We report that Wnt signaling is normally activated in a subset of Pax2+ cells, and that conditional inactivation of β-catenin in these cells causes an expansion of epidermal markers at the expense of the otic placode. Conversely, conditional activation of β-catenin in Pax2+ cells causes an expansion of the otic placode at the expense of epidermis, and the resulting otic tissue expresses exclusively dorsal otocyst markers. Together, these results suggest that Wnt signaling acts instructively to direct Pax2+cells to an otic placodal, rather than an epidermal, fate and promotes dorsal cell identities in the otocyst.

Dlx gene expression during chick inner ear development

Members of the Dlx gene family play essential roles in the development of the zebrafish and mouse inner ear, but little is known regarding Dlx genes and avian inner ear development. We have examined the inner ear expression patterns of Dlx1, Dlx2, Dlx3, Dlx5, and Dlx6 during the first 7 days of chicken embryonic development. Dlx1 and Dlx2 expression was seen only in nonneuronal cells of the cochleovestibular ganglion and nerves from stage 21 to stage 32. Dlx3 marks the otic placode beginning at stage 9 and becomes limited to epithelium adjacent to the hindbrain as invagination of the placode begins. Dlx3 expression then resolves to the dorsal otocyst and gradually becomes limited to the endolymphatic sac by stage 30. Dlx5 and Dlx6 expression in the developing inner ear is first seen at stages 12 and 13, respectively, in the rim of the otic pit, before spreading throughout the dorsal otocyst. As morphogenesis proceeds, Dlx5 and Dlx6 expression is seen throughout the forming semicircular canals and endolymphatic structures. During later stages, both genes are seen to mark the distal surface of the forming canals and display expression complementary to that of BMP4 in the vestibular sensory regions. Dlx5 expression is also seen in the lagena macula and the cochlear and vestibular nerves by stage 30. These findings suggest important roles for Dlx genes in the vestibular and neural development of the avian inner ear.

Assessing the contributions of gene products to the form-shaping events of neurulation: A transgenic approach in chick

Most of our current knowledge on the tissue and cellular basis of neurulation in amniotes has been gained using the chick embryo as an experimental model system. Gene manipulation during chick neurulation has been difficult, greatly limiting our ability to assess the contribution of gene products to the tissue and cellular behaviors of neurulation. Using electroporation, we have developed a simple and reliable method for expressing transgenes in the ectoderm of the neural folds of chick embryos developing in whole-embryo culture. Sense- or antisense-expressing plasmids are electroporated, resulting in gain or loss of gene function, respectively. The morphogenesis of transgenic tissues was compared to the morphogenesis of contralateral wildtype tissues as neurulation was taking place. As a proof of principle, we present a functional analysis of the chick gene encoding Cartilage Linking Protein 1 (CRTL1), identified as a candidate neurulation gene using subtractive hybridization. This experimental approach provides a much-needed innovation for studying the mechanisms by which genes influence neurulation and reveals here important contributions of CRTL1 to the formation of the neural folds.

Heterogeneity of chondroitin sulfate glycosaminoglycan localization during early development of the striped bass (Morone saxatilis)

Recent studies have suggested important functions for proteoglycan-associated chondroitin sulfate glycosaminoglycans (GAGs) during embryonic and larval development in numerous organisms, including the teleost. Little is known, however, about the specific distribution of different chondroitin sulfate GAGs during early development. The present study utilized immunohistochemistry to localize chondroitin sulfate GAG antigens during development of the striped bass (Morone saxatilis). Immunoreagents utilized were monoclonal antibodies (MAbs) TC2, d1C4, and CS-56, which recognize, respectively, native epitopes on glycosaminoglycan chains enriched in chondroitin-4-, chondroitin-6-, and both chondroitin-4- and -6-sulfate. Little or no immunoreactivity was observed in gastrulating embryos at 18 hr postfertilization with any MAb tested. By 24 hr (8 somites), the CS-56 epitope was localized around the notochord. At hatching (48 hr) and early larval (72 hr) stages, d1C4 and CS-56 antigens codistributed in some sites (e.g., the notochord and myosepta), but a striking heterogeneity of chondroitin sulfate GAG localization was observed in other developing tissues, including the eye and specific subsets of basement membrane. At these latter time points, TC2 reacted primarily with the extracellular matrix of the developing heart, particularly the ventricular and conotruncal segments. Heterogeneous patterning of these chondroitin sulfate GAG epitopes suggests dynamic regulation of proteoglycan function during critical morphogenetic events in early development of the striped bass.

Perturbation of extracellular matrix prevents association of the otic primordium with the posterior rhombencephalon and inhibits subsequent invagination

In the avian embryo, the otic primordia become visible by Hamburger and Hamilton stage 10 as a pair of thickened regions of head ectoderm. In contrast to other epithelial primordia, invagination occurs by means of formation of a series of folds in distinct areas of the primordium, giving the otic vesicle a box-like appearance. Because previous work has shown that otic invagination is ATP and calcium independent, it is unlikely that cytoskeletal changes are the primary mechanism responsible for invagination as in other epithelial primordia. Interaction of the primordium with surrounding tissues may provide the force for otic invagination. These extracellular forces may be transduced through extracellular matrix macromolecules and their cell surface receptors. This investigation tests the hypothesis that fusion of the otic and hindbrain basal laminae between stages 11 and 13 is necessary for normal invagination. Perturbation of binding of the otic primordium to the neural tube was accomplished by means of microinjection of antibodies to various extracellular matrix components and integrin subunits into the head mesenchyme in the otic region at stage 10. Only antibodies to laminin and integrins caused detachment of the otic primordium from the hindbrain. These experiments suggest that fusion of the otic and hindbrain basal laminae is required for subsequent invagination and, furthermore, that this event is mediated by components of the extracellular matrix.

Chondroitin sulphate proteoglycan is involved in lens vesicle morphogenesis in chick embryos

Proteoglycans have been implicated in the invagination and formation of various embryonal cavitied primordia. In this paper the expression of chondroitin sulphate proteoglycan (CSPG) is analysed in the lens primordium during lens vesicle formation, and demonstrate that this proteoglycan has a specific distribution pattern with regard to invagination and fusion processes in the transformation of placode into lens vesicle. More specifically, CSPG was detected in: (1) the apical surface of lens epithelial cells, where early CSPG expression was observed in the whole of the lens placode whilst in the vesicle phase it was restricted to the posterior epithelium; (2) intense CSPG expression in the basal lamina, which remained constant for the entire period under study; (3) CSPG expression in the intercellular spaces of the lens primordium epithelium, which increased during the invagination of the primordium and which at the vesicle stage was more evident in the posterior epithelium; and (4) CSPG expression on the edges of the lens placode both prior to and during fusion. Treatment with beta- D -xyloside causes significant CSPG depletion in the lens primordium together with severe alterations in the invagination and fusion of the lens vesicle; this leads to the formation of lens primordia which in some cases remain practically flat or show partial invagination defects or fusion disruption. Similar results were obtained by enzyme digestion with chondroitinase AC but not with type II heparinase, which indicates that alterations induced by beta- D -xyloside were due to interference in CSPG synthesis. The findings demonstrate that CSPG is a common component of the lens primordium at the earliest developmental stages during which it undergoes specific modifications. It also includes experimental evidence to show that 'in vivo' CSPG plays an important role in the invagination and fusion processes of the lens primordium.

A role for neural cell adhesion molecule in the formation of the avian inner ear

The inner ear forms by a series of folds within an ectodermal placode. Previous work has shown that changes in surrounding tissues play a more prominent role in invagination than changes in the cytoskeleton of the primordium. Interference with the integrity of the extracellular matrix causes abnormalities in the folding process, primarily related to abnormalities in the paraxial mesoderm which lies ventral to the placode. In this study, the role of the neural cell adhesion molecule (N-CAM) was investigated, based on the expression of this component of the plasmalemma at the time the otic placode begins to fold. Microinjection of blocking antibodies to N-CAM into the paraxial mesoderm adjacent to the otic placode resulted in two major classes of defects, detachment of the primordium from the neural tube and interference with formation of the folds. Microinjection of saline, control immunoglobulin, or antibody against cytoplasmic domain had no effect. These defects correlate with the pattern of N-CAM expression at the time of injection, along the neural ectoderm and otic epithelium and the mesenchyme cells ventral to the primordium. It seems likely that N-CAM is playing a role in heterophilic associations rather than through the homophilic binding domain during formation of the otic vesicle.

Morphological and quantitative studies in the otic region of the neural tube in chick embryos suggest a neuroectodermal origin for the otic placode

Careful histological observation of the development of the anlage of the inner ear in chicken embryos led us to question the traditional view of otic placode (OP) formation. First, morphological studies in the cephalic region carried out on stages preceding the appearance of the placodal epithelium revealed that the medial placodal cells are continuous temporally and spatially with cells belonging to the neural fold (NF). Second, both the formation of the basal lamina between the dorsal region of the neural tube (NT) and ectoderm and the pattern of formation of the neural crest present distinctive characteristics between otic levels and regions located anteriorly and posteriorly. Third, numerical comparisons of parameters for the NT and the OP between different levels of the rhombencephalon allowed us to assign a differential behaviour in the growth pattern of the otic region. These results indicated that the medial part of the OP is not derived from already independent ectoderm that increases in thickness under the influence of the NT (as previously accepted) but that it develops directly from the NFs. Although we do not exclude other possibilities, we propose that at least a proportion of the OP cells originate directly from cells committed to be neural crest. After this incorporation, basal laminal formation would delimit the NT from the OP without transition of the otic cells to ectoderm. This hypothesis would imply that part of the otic cells originate directly from neuroepithelial cells having a neuroectodermal (rather than the previously established ectodermal) origin.

Factors modulating supernumerary hair cell production in the postnatal rat cochlea in vitro

It has been shown in the past that extra hair cells or supernumerary cells can be produced when neonatal cochleae are maintained in vitro. In this report, we investigated the effects of the culture methods, molecules and growth factors that are thought to be involved in cell proliferation. Quantitative studies of supernumerary hair cells were made by measuring the cell density over the entire spiral lamina at two postnatal stages: birth and 3 days after birth. With a standard feeding solution without serum, a difference in cell density was observed between the two methods of culture. Cochlear explants in a standard feeding solution supplemented with serum showed an increase of cell density only when the explantation is made at birth. Retinoic acid added to the standard feeding solution did not increase the hair cell density, while insulin induced an increase, especially at 5 micrograms/ml. Several growth factors were tested. Epidermal growth factor (EGF) presented a dose dependent effect with an increase of up to 30% of hair cell density that was observed in the basal region when the explantation was made at birth. Transforming growth factor-alpha did not induce an increase of cell density, whereas transforming growth factor-beta presented an effect on hair cell density, with a dose dependent effect reaching 37.4% for the basal inner hair cells. Interpretation of these results is limited because of the lack of data concerning the presence of specific membrane receptors. One possibility is that insulin stimulates hair cell differentiation from existing undifferentiated cells. Another hypothesis may be related to the EGF and transforming growth factor-beta, where these molecules might induce transdifferentiation of cells by acting on the transmembrane molecules and the extracellular matrix.