The laminins in the murine inner ear: developmental transitions and expression in cochlear basement membranes

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.

Dual inhibition of the endothelin and angiotensin receptor ameliorates renal and inner ear pathologies in Alport mice

Alport syndrome (AS), a type IV collagen disorder, leads to glomerular disease and, in some patients, hearing loss. AS is treated with inhibitors of the renin-angiotensin system; however, a need exists for novel therapies, especially those addressing both major pathologies. Sparsentan is a single-molecule dual endothelin type-A and angiotensin II type 1 receptor antagonist (DEARA) under clinical development for focal segmental glomerulosclerosis and IgA nephropathy. We report the ability of sparsentan to ameliorate both renal and inner ear pathologies in an autosomal-recessive Alport mouse model. Sparsentan significantly delayed onset of glomerulosclerosis, interstitial fibrosis, proteinuria, and glomerular filtration rate decline. Sparsentan attenuated glomerular basement membrane defects, blunted mesangial filopodial invasion into the glomerular capillaries, increased lifespan more than losartan, and lessened changes in profibrotic/pro-inflammatory gene pathways in both the glomerular and the renal cortical compartments. Notably, treatment with sparsentan, but not losartan, prevented accumulation of extracellular matrix in the strial capillary basement membranes in the inner ear and reduced susceptibility to hearing loss. Improvements in lifespan and in renal and strial pathology were observed even when sparsentan was initiated after development of renal pathologies. These findings suggest that sparsentan may address both renal and hearing pathologies in Alport syndrome patients. © 2023 Travere Therapeutics, Inc and The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Epithelial-mesenchymal transition, and collective and individual cell migration regulate epithelial changes in the amikacin-damaged organ of Corti

Characterizing the microenvironment of a damaged organ of Corti and identifying the basic mechanisms involved in subsequent epithelial reorganization are critical for improving the outcome of clinical therapies. In this context, we studied the expression of a variety of cell markers related to cell shape, cell adhesion and cell plasticity in the rat organ of Corti poisoned with amikacin. Our results indicate that, after severe outer hair cell losses, the cytoarchitectural reorganization of the organ of Corti implicates epithelial-mesenchymal transition mechanisms and involves both collective and individual cell migratory processes. The results also suggest that both root cells and infiltrated fibroblasts participate in the homeostasis of the damaged epithelium, and that the flat epithelium that may emerge offers biological opportunities for late regenerative therapies.

A 18p11.23-p11.31 microduplication in a boy with psychomotor delay, cerebellar vermis hypoplasia, chorioretinal coloboma, deafness and GH deficiency

Background

Rearrangements involving the short arm of chromosome 18 have been extensively described. Here we report a microduplication of 320.5–431.5 Kb at 18p11.31-p11.23 in a 10 year-old boy.

Case presentation

In a 10 year-old boy with moderate psychomotor delay, hypoplasia of the cerebellar vermis, chorioretinal coloboma, deafness and growth hormone deficiency (GHD), an interstitial microduplication at 18p11.31-p11.23 was identified by array-CGH. This maternally inherited microduplication, encompasses three genes, namely ARHGAP28, LINC00668 and LAMA1 (a gene involved in cerebellum and retinal development).

Conclusions

The genotype-phenotype is discussed with particular attention to the LAMA1 gene, although it is difficult, as in many other similar situations, to assess the causality of the detected duplication in the absence of further studies aiming to explore the presence of co-occurring variants that could explain the incomplete penetrance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13039-016-0298-9) contains supplementary material, which is available to authorized users.

Biofunctionalized peptide-based hydrogels provide permissive scaffolds to attract neurite outgrowth from spiral ganglion neurons

Cochlear implants (CI) allow for hearing rehabilitation in patients with sensorineural hearing loss or deafness. Restricted CI performance results from the spatial gap between spiral ganglion neurons and the CI, causing current spread that limits spatially restricted stimulation and impairs frequency resolution. This may be substantially improved by guiding peripheral processes of spiral ganglion neurons towards and onto the CI electrode contacts. An injectable, peptide-based hydrogel was developed which may provide a permissive scaffold to facilitate neurite growth towards the CI. To test hydrogel capacity to attract spiral ganglion neurites, neurite outgrowth was quantified in an in vitro model using a custom-designed hydrogel scaffold and PuraMatrix^®. Neurite attachment to native hydrogels is poor, but significantly improved by incorporation of brain-derived neurotrophic factor (BDNF), covalent coupling of the bioactive laminin epitope IKVAV and the incorporation a full length laminin to hydrogel scaffolds. Incorporation of full length laminin protein into a novel custom-designed biofunctionalized hydrogel (IKVAV-GGG-SIINFEKL) allows for neurite outgrowth into the hydrogel scaffold. The study demonstrates that peptide-based hydrogels can be specifically biofunctionalized to provide a permissive scaffold to attract neurite outgrowth from spiral ganglion neurons. Such biomaterials appear suitable to bridge the spatial gap between neurons and the CI.

The expression pattern and inhibitory influence of Tenascin-C on the growth of spiral ganglion neurons suggest a regulatory role as boundary formation molecule in the postnatal mouse inner ear

Sensorineural hearing loss, as a consequence of acoustic trauma, aging, genetic defects or ototoxic drugs, is highly associated with irreversible damage of cochlear hair cells (HCs) and secondary degeneration of spiral ganglion (SG) cells. Cochlear implants (CIs), which bypass the lost HC function by direct electrical stimulation of the remaining auditory neurons, offer an effective therapy option. Several studies imply that components of the extracellular matrix (ECM) have a great impact on the adhesion and growth of spiral ganglion neurons (SGNs) during development. Based on these findings, ECM proteins might act as bioactive CI substrates to optimize the electrode-nerve interface and to improve efficacy of these implants. In the present study, we focused on the ECM glycoproteins Tenascin-C (TN-C), Laminin (LN), and Fibronectin (FN), which show a prominent expression along the growth route of SGNs and the niche around HCs during murine postnatal development in vivo. We compared their influence on adhesion, neurite length, and neurite number of SGNs in vitro. Moreover, we studied the expression of the chondroitin sulfate proteoglycan (CSPG) dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG), an interaction partner of TN-C. In sum, our in vitro data suggest that TN-C acts as an anti-adhesive and inhibitory factor for the growth of SGNs. The DSD-1 carbohydrate epitope is specifically localized to HC stereocilia and SG fibers. Interestingly, TN-C and the DSD-1-PG exhibit a mutually exclusive expression pattern, with the exception of a very restricted region beneath the habenula perforata, where SG neurites grow through the basilar membrane (BM) toward the HCs. The complementary expression of TN-C, LN, FN, and the DSD-1 epitope suggests that TN-C may act as an important boundary formation molecule in the developing postnatal mouse inner ear, which makes it a promising candidate to regulate neurite outgrowth in the light of CIs.

The pre- and post-somatic segments of the human type I spiral ganglion neurons--structural and functional considerations related to cochlear implantation

Human auditory nerve afferents consist of two separate systems; one is represented by the large type I cells innervating the inner hair cells and the other one by the small type II cells innervating the outer hair cells. Type I spiral ganglion neurons (SGNs) constitute 96% of the afferent nerve population and, in contrast to other mammals, their soma and pre- and post-somatic segments are unmyelinated. Type II nerve soma and fibers are unmyelinated. Histopathology and clinical experience imply that human SGNs can persist electrically excitable without dendrites, thus lacking connection to the organ of Corti. The biological background to this phenomenon remains elusive. We analyzed the pre- and post-somatic segments of the type I human SGNs using immunohistochemistry and transmission electron microscopy (TEM) in normal and pathological conditions. These segments were found surrounded by non-myelinated Schwann cells (NMSCs) showing strong intracellular expression of laminin-β2/collagen IV. These cells also bordered the perikaryal entry zone and disclosed surface rugosities outlined by a folded basement membrane (BM) expressing laminin-β2 and collagen IV. It is presumed that human large SGNs are demarcated by three cell categories: (a) myelinated Schwann cells, (b) NMSCs and (c) satellite glial cells (SGCs). Their BMs express laminin-β2/collagen IV and reaches the BM of the sensory epithelium at the habenula perforata. We speculate that the NMSCs protect SGNs from further degeneration following dendrite loss. It may give further explanation why SGNs can persist as electrically excitable monopolar cells even after long-time deafness, a blessing for the deaf treated with cochlear implantation.

Headbobber: a combined morphogenetic and cochleosaccular mouse model to study 10qter deletions in human deafness

The recessive mouse mutant headbobber (hb) displays the characteristic behavioural traits associated with vestibular defects including headbobbing, circling and deafness. This mutation was caused by the insertion of a transgene into distal chromosome 7 affecting expression of native genes. We show that the inner ear of hb/hb mutants lacks semicircular canals and cristae, and the saccule and utricle are fused together in a single utriculosaccular sac. Moreover, we detect severe abnormalities of the cochlear sensory hair cells, the stria vascularis looks severely disorganised, Reissner's membrane is collapsed and no endocochlear potential is detected. Myo7a and Kcnj10 expression analysis show a lack of the melanocyte-like intermediate cells in hb/hb stria vascularis, which can explain the absence of endocochlear potential. We use Trp2 as a marker of melanoblasts migrating from the neural crest at E12.5 and show that they do not interdigitate into the developing strial epithelium, associated with abnormal persistence of the basal lamina in the hb/hb cochlea. We perform array CGH, deep sequencing as well as an extensive expression analysis of candidate genes in the headbobber region of hb/hb and littermate controls, and conclude that the headbobber phenotype is caused by: 1) effect of a 648 kb deletion on distal Chr7, resulting in the loss of three protein coding genes (Gpr26, Cpmx2 and Chst15) with expression in the inner ear but unknown function; and 2) indirect, long range effect of the deletion on the expression of neighboring genes on Chr7, associated with downregulation of Hmx3, Hmx2 and Nkx1.2 homeobox transcription factors. Interestingly, deletions of the orthologous region in humans, affecting the same genes, have been reported in nineteen patients with common features including sensorineural hearing loss and vestibular problems. Therefore, we propose that headbobber is a useful model to gain insight into the mechanisms underlying deafness in human 10qter deletion syndrome.

Prospects for replacement of auditory neurons by stem cells

Sensorineural hearing loss is caused by degeneration of hair cells or auditory neurons. Spiral ganglion cells, the primary afferent neurons of the auditory system, are patterned during development and send out projections to hair cells and to the brainstem under the control of largely unknown guidance molecules. The neurons do not regenerate after loss and even damage to their projections tends to be permanent. The genesis of spiral ganglion neurons and their synapses forms a basis for regenerative approaches. In this review we critically present the current experimental findings on auditory neuron replacement. We discuss the latest advances with a focus on (a) exogenous stem cell transplantation into the cochlea for neural replacement, (b) expression of local guidance signals in the cochlea after loss of auditory neurons, (c) the possibility of neural replacement from an endogenous cell source, and (d) functional changes from cell engraftment.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

Connecting the ear to the brain: Molecular mechanisms of auditory circuit assembly

Our sense of hearing depends on precisely organized circuits that allow us to sense, perceive, and respond to complex sounds in our environment, from music and language to simple warning signals. Auditory processing begins in the cochlea of the inner ear, where sounds are detected by sensory hair cells and then transmitted to the central nervous system by spiral ganglion neurons, which faithfully preserve the frequency, intensity, and timing of each stimulus. During the assembly of auditory circuits, spiral ganglion neurons establish precise connections that link hair cells in the cochlea to target neurons in the auditory brainstem, develop specific firing properties, and elaborate unusual synapses both in the periphery and in the CNS. Understanding how spiral ganglion neurons acquire these unique properties is a key goal in auditory neuroscience, as these neurons represent the sole input of auditory information to the brain. In addition, the best currently available treatment for many forms of deafness is the cochlear implant, which compensates for lost hair cell function by directly stimulating the auditory nerve. Historically, studies of the auditory system have lagged behind other sensory systems due to the small size and inaccessibility of the inner ear. With the advent of new molecular genetic tools, this gap is narrowing. Here, we summarize recent insights into the cellular and molecular cues that guide the development of spiral ganglion neurons, from their origin in the proneurosensory domain of the otic vesicle to the formation of specialized synapses that ensure rapid and reliable transmission of sound information from the ear to the brain.

Immunocytochemical distribution of WARP (von Willebrand A domain-related protein) in the inner ear

The basic components of the epithelial, perineural, and perivascular basement membranes in the inner ear have been well-documented in several animal models and in the human inner ear. The von Willebrand A domain-related protein (WARP) is an extracellular matrix molecule with restricted expression in cartilage, and a subset of basement membranes in peripheral nerves, muscle, and central nervous system vasculature. It has been suggested that WARP has an important role in maintaining the blood-brain barrier. To date no studies on WARP distribution have been performed in the inner ear, which is equipped with an intricate vasculature network. In the present study, we determined the distribution of WARP by immunocytochemistry in the human inner ear using auditory and vestibular endorgans microdissected from human temporal bones obtained at autopsy. All subjects (n=5, aged 55-87years old) had documented normal auditory and vestibular function. We also determined the WARP immunolocalization in the mouse inner ear. WARP immunoreactivity localized to the vasculature throughout the stroma of the cristae ampullaris, the maculae utricle, and saccule in the human and mouse. In the human and mouse inner ear, WARP immunoreactivity delineated blood vessels located in the stria vascularis, spiral ligament, sub-basilar region, stromal tissue, and the spiral and vestibular ganglia. The distinct localization of WARP in the inner ear vasculature suggests an important role in maintaining its integrity. In addition, WARP allows delineation of microvessels in the inner ear allowing the study of vascular pathology in the development of otological diseases.

Immunohistochemical distribution of basement membrane proteins in the human inner ear from older subjects

The immunolocalization of several basement membrane (BM) proteins was investigated in vestibular endorgans microdissected from temporal bones obtained from subjects with a documented normal auditory and vestibular function (n=5, average age=88 years old). Fluorescent immunostaining using antibodies directed at collagen IV alpha 2, nidogen-1, laminin-beta2, alpha-dystroglycan, and tenascin-C was applied to cryosections from human cochlea, cristae ampullares, utricular and saccular maculae. Collagen IV alpha 2, nidogen-1, and laminin-beta2 localized to all subepithelial cochlear BMs, Reissner's membrane, strial and spiral ligamental perineural and perivascular BMs, and the spiral limbus. Tenascin-C localized to the basilar membrane and the osseous spiral lamina. alpha-Dystroglycan localized to most cochlear BMs except those in the spiral ligament, basilar membrane and spiral limbus. Collagen IV, nidogen-1, and laminin-beta2 localized to the subepithelial BMs of the maculae and cristae ampullares, and the perineural and perivascular BMs within the underlying stroma. The BM underlying the transitional and dark cell region of the cristae ampullares also expressed collagen IV, nidogen-1, and laminin-beta2. Tenascin-C localized to the subepithelial BMs of the utricular maculae and cristae ampullares, and to calyx-like profiles throughout the vestibular epithelium, but not to the perineural and perivascular BMs. alpha-Dystroglycan colocalized with aquaporin-4 in the basal vestibular supporting cell, and was also expressed in the subepithelial BMs, as well as perivascular and perineural BMs. This study provides the first comprehensive immunolocalization of these ECM proteins in the human inner ear. The validity of the rodent models for inner ear disorders secondary to BM pathologies was confirmed as there is a high degree of conservation of expression of these proteins in the human inner ear. This information is critical to begin to unravel the role that BMs may play in human inner ear physiology and audiovestibular pathologies.

Laminin expression in juvenile angiofibroma indicates vessel's early developmental stage

CONCLUSION

This study confirms the wide range of vascular architecture in juvenile angiofibromas. Proof of laminin alpha2 expression in tumour vessels is suggested to indicate presence of vessels of early developmental stage in juvenile angiofibromas, supporting the concept that plexus remnants of the first branchial arch artery contribute to the vascular tumour component.

OBJECTIVES

Laminins, one of the major components of vascular wall basement membranes, have been implicated in tumour growth and have been shown to have developmentally regulated expression patterns. The goal of this study was to analyse the expression of laminins in juvenile angiofibromas.

MATERIALS AND METHODS

A detailed analysis of the laminin isoform expression was performed by immunofluorescence staining for laminin chains alpha1, alpha2, alpha3, alpha4, alpha5, beta1, beta2, beta3, gamma1, gamma2, and gamma3 on cryosections of 10 juvenile angiofibromas and inferior nasal turbinate tissue for control.

RESULTS

Vascular staining of the different laminin chains revealed areas of differential vessel density in juvenile angiofibromas and irregularities in vessel size, configuration and architecture. Similar to vessels in nasal turbinates, laminins alpha4, alpha5, beta1, beta2 and gamma1 were found to be expressed in juvenile angiofibroma vessels. In contrast to vessels of nasal turbinates, staining for alpha2 and alpha3 chains was only detected in vessels of juvenile angiofibromas.

Developmental changes in cell-extracellular matrix interactions limit proliferation in the mammalian inner ear

Hair cell losses can produce severe hearing and balance deficits in mammals and nonmammals alike, but nonmammals recover after epithelial supporting cells divide and give rise to replacement hair cells. Here, we describe cellular changes that appear to underlie the permanence of hair cell deficits in mammalian vestibular organs. In sensory epithelia isolated from the utricles of embryonic day 18 (E18) mice, supporting cells readily spread and proliferated, but spreading and proliferation were infrequent in supporting cells from postnatal day 6 (P6) mice. Cellular spreading and proliferation were dependent on alpha6 integrin, which disappeared from lateral cell membranes by P6 and colocalized with beta4 integrin near the basement membrane at both ages. In the many well-spread, proliferating E18 supporting cells, beta4 was localized at cell borders, but it was localized to hemidesmosome-like structures in the columnar, nondividing supporting cells that were prevalent in P6 cultures. We treated cultures with phorbol myristate acetate (PMA) to activate protein kinase C (PKC) in an initial test of the possibility that maturational changes in supporting cell cytoskeletons or their anchorage might restrict the proliferation of these progenitor cells in the developing mammalian inner ear. That treatment triggered the disassembly of the hemidesmosome-like beta4 structures and resulted in significantly increased cellular spreading and S-phase entry in the P6 epithelia. The results suggest that maturational changes in cytoskeletal organization and anchorage restrict proliferation of mammalian supporting cells whose counterparts are the progenitors of replacement hair cells in nonmammals, thereby leaving mammals vulnerable to persistent sensory deficits caused by hair cell loss.

Laminin and fibronectin modulate inner ear spiral ganglion neurite outgrowth in anin vitro alternate choice assay

Extracellular matrix (ECM) molecules have been shown to function as cues for neurite guidance in various populations of neurons. Here we show that laminin (LN) and fibronectin (FN) presented in stripe micro-patterns can provide guidance cues to neonatal (P5) inner ear spiral ganglion (SG) neurites. The response to both ECM molecules was dose-dependent. In a LN versus poly-L-lysine (PLL) assay, neurites were more often observed on PLL at low coating concentrations (5 and 10 microg/mL), while they were more often on LN at a high concentration (80 microg/mL). In a FN versus PLL assay, neurites were more often on PLL than on FN stripes at high coating concentrations (40 and 80 microg/mL). In a direct competition between LN and FN, neurites were observed on LN significantly more often than on FN at both 10 and 40 microg/mL. The data suggest a preference by SG neurites for LN at high concentrations, as well as avoidance of both LN at low and FN at high concentrations. The results also support a potential model for neurite guidance in the developing inner ear in vivo. LN, in the SG and osseus spiral lamina may promote SG dendrite growth toward the organ of Corti. Within the organ of Corti, lower concentrations of LN may slow neurite growth, with FN beneath each row of hair cells providing a stop or avoidance signal. This could allow growth cone filopodia increased time to sample their cellular targets, or direct the fibers upward toward the hair cells.

Abnormal basement membrane in the inner ear and the kidney of the Mpv17-/- mouse strain: ultrastructural and immunohistochemical investigations

The loss of the function of the peroxisomal Mpv17-protein and associated imbalanced radical oxygen species (ROS) homeostasis leads to an early onset of focal segmental glomerulosclerosis and sensorineural deafness associated with severe degeneration of cochlear structures. An excessive enlargement of basal laminae of the stria vascularis capillaries and glomeruli indicates numerous changes in their molecular composition. The basement membrane (BM) of the glomeruli and the stria vascularis are simultaneously affected in early stages of the disease and the lamination, splitting of the membrane and formation of the "basket weaving" seen at the onset of the disease in the kidney are similar to the ultrastructural alterations characteristic for Alporta9s syndrome. The progressive alteration of the BMs is accompanied by irregularity in the distribution of the collagen IV subunits and by an accumulation of the laminin B2(gamma1) in the inner ear and B(beta1) in the kidney. Since Mpv17 protein contributes to ROS homeostasis, further studies are necessary to elucidate downstream signaling molecules activated by ROS. These studies explain the cellular responses to missing Mpv17-protein, such as accumulation of the extracellular matrix, degeneration, and apoptosis in the inner ear.

Matrix metalloproteinase dysregulation in the stria vascularis of mice with Alport syndrome: implications for capillary basement membrane pathology

Alport syndrome results from mutations in genes encoding collagen alpha3(IV), alpha4(IV), or alpha5(IV) and is characterized by progressive glomerular disease associated with a high-frequency sensorineural hearing loss. Earlier studies of a gene knockout mouse model for Alport syndrome noted thickening of strial capillary basement membranes in the cochlea, suggesting that the stria vascularis is the primary site of cochlear pathogenesis. Here we combine a novel cochlear microdissection technique with molecular analyses to illustrate significant quantitative alterations in strial expression of mRNAs encoding matrix metalloproteinases-2, -9, -12, and -14. Gelatin zymography of extracts from the stria vascularis confirmed these findings. Treatment of Alport mice with a small molecule inhibitor of these matrix metalloproteinases exacerbated strial capillary basement membrane thickening, demonstrating that alterations in basement membrane metabolism result in matrix accumulation in the strial capillary basement membranes. This is the first demonstration of true quantitative analysis of specific mRNAs for matrix metalloproteinases in a cochlear microcompartment. Further, these data suggest that the altered basement membrane composition in Alport stria influences the expression of genes involved in basement membrane metabolism.

Targeting hearing genes in mice

The rate of identification of genes for hearing has clearly outpaced the rate of determination of the functions of these genes' products. The use of transgenic and knock-out mouse models is a powerful approach to the elucidation of gene function in the ear. A large number of gene-targeted mice with auditory defects have recently been created and characterized, and nine independent mouse lines in which Cre recombinase activity begins to be expressed during early embryonic development of the ear or is specifically expressed in hair cells during postnatal development will be useful for ear-specific gene manipulation when combined with mouse lines that have loxP sites flanking the genes of interest. Existing gene-trapped embryonic stem (ES) cells and existing targeting constructs are readily available; new targeting constructs can easily be created by modifying bacterial artificial chromosomes and using them to directly transfect and screen ES cells; and N-ethyl-N-nitrosourea mutagenesis of ES cells can create point mutations in specific genes. To minimize variation in hearing phenotypes and avoid undesired hearing defects, mutant mice in the common gene-targeting background strains (129 and C57BL/6) should be transferred into congenic CBA/CaJ, a strain with "gold standard" normal hearing. Valuable mutant strains can be maintained, distributed, and cryopreserved in one of four NIH-sponsored Mutant Mouse Regional Resource Centers. Targeting hearing genes in mice will provide unprecedented opportunities for collaboration and new directions in the hearing research community.

Autoimmune Mouse Antibodies Recognize Multiple Antigens Proposed in Human Immune-Mediated Hearing Loss

HYPOTHESIS

Autoimmune diseased mice with hearing loss will have autoantibodies against the various cochlear antigens proposed in clinical autoimmune inner ear disease.

BACKGROUND

Serum antibodies of patients with hearing loss recognize several proteins that are proposed as possible antigenic targets in the ear. This often leads to a clinical diagnosis of autoimmune inner ear disease, although it is not clear how these antibodies cause inner ear disease. Therefore, to better understand the relationship of autoantibodies and ear disease, an examination was made of serum autoantibodies in the MRL/MpJ-Fas(lpr) autoimmune mouse with hearing loss. Similar antibody patterns in the mouse would provide an animal model in which to investigate potential autoimmune mechanisms of this clinical ear disorder.

METHODS

Sera from MRL/MpJ-Fas(lpr) autoimmune mice and normal C3H mice were tested by the enzyme-linked immunosorbent assay technique for reactivity against various reported cochlear antigens: heat shock protein 70 (bovine, human, bacterial), laminin, heparan sulfate proteoglycan, cardiolipin, and collagen types II and IV.

RESULTS

The autoimmune mouse sera showed significantly greater antibody reactivity against all of the antigens when compared with normal mouse sera.

CONCLUSIONS

Serum antibodies from autoimmune mice recognized several putative autoantigens reported for patients with hearing loss, suggesting that comparable antigen-antibody mechanisms might be operating. However, the recognition of multiple antigens did not identify any one as being the specific target in autoimmune hearing loss. The correlation of antibodies in the MRL/MpJ-Fas(lpr) autoimmune mouse and human studies indicates this animal model should aid further investigations into potential cochlear antigens in autoimmune hearing loss.

An emilin family extracellular matrix protein identified in the cochlear basilar membrane

The precise movement of the cochlear basilar membrane (BM) stimulates the sensory hair cells during auditory transduction. However, the molecular composition of the BM that confers its specialized properties of support and elasticity is poorly understood. A differential screen of cochlear RNA from deaf mice lacking thyroid hormone receptor beta was used to identify a sequence encoding a secreted protein, which is abundant in the BM and is expressed at low levels in the heart, lung, and brain. The protein possesses several domains for protein interactions and is related to emilin (elastin microfibril interface-located protein) previously isolated from aorta. This cochlear emilin-2 mRNA is expressed in the tympanic border cells underlying the BM and an antibody detected protein in the extracellular matrix surrounding the collagenous fibers in the BM. These results identify emilin-2 as a major BM component and suggest that it contributes to the developmental assembly or function of the BM.

Dystroglycan expression in the developing and senescent gerbil cochlea

Dystroglycan (DG) forms part of a cell surface laminin receptor complex and is believed to play a critical role in the assembly and homeostasis of basement membranes (BM). The receptor complex is made up of alpha- and beta-DG subunits and is found in muscle, epithelial and nerve tissue. In the cochlea, DG may be involved in the abnormal accumulation of laminin seen in the thickened BM of strial capillaries with age. This excess deposition of laminin is thought to lead to capillary necrosis and contribute to degeneration of the stria vascularis (SV). Here we assessed the presence and distribution of DG in the developing, mature and senescent gerbil cochlea in order to ascertain whether altered patterns of expression are a factor in age-related pathology. Western blots of proteins isolated from the entire cochlea demonstrated the presence of the alpha-DG subunit. mRNA encoding DG was identified in microdissected specimens of the lateral wall and the combined organ of Corti/modiolus by RT-PCR analysis. Immunohistochemical experiments localized alpha-DG in epithelial BMs and regions of epithelial cell-cell contact with no intervening BM in the developing and mature cochlea. Immunoreactive alpha-DG was present in the BM underlying strial capillaries and in vessels of the central portion of the auditory nerve, but was not detected in any other vessels in the cochlea. Age-related changes in alpha-DG expression were observed only in the SV where a marked decrease in alpha-DG immunoreactivity was seen in the BM of strial capillaries as well as throughout the SV. The results demonstrate the selective expression of alpha-DG in both BM and non-BM sites in the mature cochlea and suggests its involvement in both developmental and aging processes.

Hearing loss in the laminin-deficient dy mouse model of congenital muscular dystrophy

Sensorineural hearing loss is found in many inherited forms of muscular dystrophy. We investigated the dy mouse model, which has congenital muscular dystrophy due to a defect in laminin alpha 2, for evidence of cochlear dysfunction. Auditory brainstem response (ABR) audiometry to pure tones was used to evaluate 3-month-old homozygous dy/dy and age-matched C57 control mice. The average ABR thresholds to tone-burst stimuli for four frequencies (4, 8, 16, and 32 kHz) were determined and statistically compared by ANOVA. The dy/dy mice demonstrated elevated auditory thresholds ranging from 25 to 27 dB at each frequency tested (p<0.0001). Anatomic evaluations of the ears revealed pathology ranging from extensive connective tissue infiltration within the inner ear to possible minor defects in the cells of the organ of Corti. These anatomic and physiologic observations suggest that the extracellular matrix protein laminin plays a crucial role in normal cochlear function. Furthermore, the dy congenital muscular dystrophy mouse offers a novel model for evaluation of sensorineural hearing loss associated with muscular dystrophy.

Differential expression of alpha 3 and alpha 6 integrins in the developing mouse inner ear

The development of the mammalian inner ear involves a complex series of cell-cell and cell-extracellular matrix interactions. These interactions are likely to be mediated by families of adhesion molecules, including the integrins. We have studied the expression of three integrin subunits known to be expressed on epithelia in a number of tissues (namely, alpha3, alpha6, and beta4) during the development of the murine inner ear. At E10.5, both alpha3 and alpha6 were expressed in the epithelial layers of the otocyst. The expression of alpha6 was concentrated in an anterioventral region of the epithelium and in a proportion of the cells forming the cochlear-vestibular and facial ganglia. By E12.5, alpha6 showed a more restricted expression, confined mainly to the pro-sensory epithelia and the neural processes from the cochlear-vestibular ganglion. In contrast, alpha3 was expressed in epithelia adjacent to the pro-sensory areas. This reciprocal expression pattern was maintained until birth. Between birth and P6, a switch in expression occurred such that alpha3 was upregulated and alpha6 was downregulated in the sensory epithelia of both the auditory and vestibular systems. At this stage, alpha3 was expressed in all the epithelia lining the scala media, thus defining the endolymph compartment. The expression of beta4 was restricted to epithelial/mesenchymal borders throughout the developmental stages studied, suggesting that alpha6 expression observed within the epithelium and neuronal tissue was alpha6beta1. The early expression and changing pattern of alpha3 and alpha6 integrins during development of the mammalian inner ear suggests that they may be involved in the molecular processes that define epithelial boundaries and guide sensory innervation.

Strial marginal cells play a role in basement membrane homeostasis: in vitro and in vivo evidence

The interaction of extracellular matrix and receptors plays a role in tissue homeostasis. The thickened strial capillary basement membrane (SCBM) reported in animal models of presbycusis and Alport's syndrome might be secondary to elevated synthesis and/or decreased turnover of specific basement membrane (BM) components. In this study, expression of specific BM proteins, integrin receptors and mediators of matrix turnover in the murine lateral wall were determined using cDNA probes and antibodies. The presence of collagen alpha1 and alpha2(IV) and laminin-8 in the SCBM was verified. The integrin subunits alpha3, alphav and beta1, cell surface receptors for the BM proteins, localized primarily to the SCBM and/or the strial marginal cells as did TIMP-3, a tissue inhibitor of matrix metalloproteinase. The epithelial cell line SV-k1, derived from the lateral wall of the 'immortomouse', showed expression of the same BM proteins as well as demonstrating the presence of markers specific to strial marginal cells, namely Na,K-ATPase alpha1 and beta2 subunits. Thus, the cultured cells are identified as deriving from marginal cells of the stria vascularis. Moreover, these data suggest that a culture system using this marginal cell line will be useful to delineate mechanisms underlying the pathologic accumulation of extracellular matrix in the SCBM.

The molecular architecture of the inner ear

The inner ear is structurally complex. A molecular description of its architecture is now emerging from the use of contemporary methods of cell and molecular biology, and from studies of ontogenetic development. With the application of clinical and molecular genetics, it has now become possible to identify genes associated with inherited, non-syndromic deafness and balance dysfunction in humans and in mice. This work is providing new insights into how the tissues of the inner ear are built to perform their tasks, and into the pathogenesis of a range of inner ear disorders.