Comparison of ephrin-A ligand and EphA receptor distribution in the developing inner ear

Interactions between Guidance Cues and Neuronal Activity: Therapeutic Insights from Mouse Models

Topographic mapping of neural circuits is fundamental in shaping the structural and functional organization of brain regions. This developmentally important process is crucial not only for the representation of different sensory inputs but also for their integration. Disruption of topographic organization has been associated with several neurodevelopmental disorders. The aim of this review is to highlight the mechanisms involved in creating and refining such well-defined maps in the brain with a focus on the Eph and ephrin families of axon guidance cues. We first describe the transgenic models where ephrin-A expression has been manipulated to understand the role of these guidance cues in defining topography in various sensory systems. We further describe the behavioral consequences of lacking ephrin-A guidance cues in these animal models. These studies have given us unexpected insight into how neuronal activity is equally important in refining neural circuits in different brain regions. We conclude the review by discussing studies that have used treatments such as repetitive transcranial magnetic stimulation (rTMS) to manipulate activity in the brain to compensate for the lack of guidance cues in ephrin-knockout animal models. We describe how rTMS could have therapeutic relevance in neurodevelopmental disorders with disrupted brain organization.

Kölliker’s organ-supporting cells and cochlear auditory development

The Kölliker’s organ is a transient cellular cluster structure in the development of the mammalian cochlea. It gradually degenerates from embryonic columnar cells to cuboidal cells in the internal sulcus at postnatal day 12 (P12)–P14, with the cochlea maturing when the degeneration of supporting cells in the Kölliker’s organ is complete, which is distinct from humans because it disappears at birth already. The supporting cells in the Kölliker’s organ play a key role during this critical period of auditory development. Spontaneous release of ATP induces an increase in intracellular Ca²⁺ levels in inner hair cells in a paracrine form via intercellular gap junction protein hemichannels. The Ca²⁺ further induces the release of the neurotransmitter glutamate from the synaptic vesicles of the inner hair cells, which subsequently excite afferent nerve fibers. In this way, the supporting cells in the Kölliker’s organ transmit temporal and spatial information relevant to cochlear development to the hair cells, promoting fine-tuned connections at the synapses in the auditory pathway, thus facilitating cochlear maturation and auditory acquisition. The Kölliker’s organ plays a crucial role in such a scenario. In this article, we review the morphological changes, biological functions, degeneration, possible trans-differentiation of cochlear hair cells, and potential molecular mechanisms of supporting cells in the Kölliker’s organ during the auditory development in mammals, as well as future research perspectives.

Hearing loss genes reveal patterns of adaptive evolution at the coding and non-coding levels in mammals

Background

Mammals possess unique hearing capacities that differ significantly from those of the rest of the amniotes. In order to gain insights into the evolution of the mammalian inner ear, we aim to identify the set of genetic changes and the evolutionary forces that underlie this process. We hypothesize that genes that impair hearing when mutated in humans or in mice (hearing loss (HL) genes) must play important roles in the development and physiology of the inner ear and may have been targets of selective forces across the evolution of mammals. Additionally, we investigated if these HL genes underwent a human-specific evolutionary process that could underlie the evolution of phenotypic traits that characterize human hearing.

Results

We compiled a dataset of HL genes including non-syndromic deafness genes identified by genetic screenings in humans and mice. We found that many genes including those required for the normal function of the inner ear such as LOXHD1, TMC1, OTOF, CDH23, and PCDH15 show strong signatures of positive selection. We also found numerous noncoding accelerated regions in HL genes, and among them, we identified active transcriptional enhancers through functional enhancer assays in transgenic zebrafish.

Conclusions

Our results indicate that the key inner ear genes and regulatory regions underwent adaptive evolution in the basal branch of mammals and along the human-specific branch, suggesting that they could have played an important role in the functional remodeling of the cochlea. Altogether, our data suggest that morphological and functional evolution could be attained through molecular changes affecting both coding and noncoding regulatory regions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01170-6.

Eph/ephrin signalling in the development and function of the mammalian cochlea

In mammals, the functional development of the cochlea requires the tight regulation of multiple molecules and signalling pathways including fibroblast growth factors, bone morphogenetic proteins, Wnt and Notch signalling pathways. Over the last decade, the Eph/ephrin system also emerged as a key player of the development and function of the mammalian cochlea. In this review, we discuss the recent advances on the role of Eph/ephrin signalling in patterning the cochlear sensory epithelium and the complex innervation of mechanosensory hair cells by spiral ganglion neurons. Finally, we address the issue of a syndromic form of hearing loss caused by a deficient member of the Eph/ephrin family.

Ephrin-A2 and ephrin-A5 guide contralateral targeting but not topographic mapping of ventral cochlear nucleus axons

Background

In the auditory brainstem, ventral cochlear nucleus (VCN) axons project to the contralateral, but not ipsilateral, medial nucleus of trapezoid body (MNTB), terminating in the calyx of Held. Dorsal VCN neurons, representing high frequencies, synapse with medial MNTB neurons, while low frequency-coding ventral VCN neurons synapse with lateral MNTB neurons, reflecting tonotopic organization. The mechanisms that ensure strictly contralateral targeting and topographic ordering are incompletely understood. Here we examined the roles of ephrin-A signaling in both types of targeting.

Results

Ephrin-A2 and ephrin-A5 are expressed in VCN cells during late embryonic and early postnatal development. At these ages ephrin-A2 is expressed in axons surrounding MNTB and ephrin-A5 is expressed in MNTB principal neurons. Ephrin-A2/A5 double knockout mice displayed axon targeting errors in which VCN axons project to MNTB on both sides of the brainstem, where they terminate in calyceal endings. Ephrin-A2 and ephrin-A5 single knockout mice showed a similar phenotype. In contrast to effects on contralateral targeting, ephrin-A2/A5 double knockout mice showed no defects in formation of tonotopically ordered projections from VCN to MNTB.

Conclusions

These findings demonstrate that distinct mechanisms regulate targeting of VCN axons to the contralateral MNTB and targeting to appropriate tonotopic locations. Ephrin-A signaling plays a similar role to ephrin-B signaling in the VCN-MNTB pathway, where both classes normally prevent formation of calyceal projections to ipsilateral MNTB. These classes may rely in part on common signaling pathways.

EphA7 regulates spiral ganglion innervation of cochlear hair cells

During the development of periphery auditory circuitry, spiral ganglion neurons (SGNs) form a spatially precise pattern of innervation of cochlear hair cells (HCs), which is an essential structural foundation for central auditory processing. However, molecular mechanisms underlying the developmental formation of this precise innervation pattern remain not well understood. Here, we specifically examined the involvement of Eph family members in cochlear development. By performing RNA-sequencing for different types of cochlear cell, in situ hybridization, and immunohistochemistry, we found that EphA7 was strongly expressed in a large subset of SGNs. In EphA7 deletion mice, there was a reduction in the number of inner radial bundles originating from SGNs and projecting to HCs as well as in the number of ribbon synapses on inner hair cells (IHCs), as compared with wild-type or heterozygous mutant mice, attributable to fewer type I afferent fibers. The overall activity of the auditory nerve in EphA7 deletion mice was also reduced, although there was no significant change in the hearing intensity threshold. In vitro analysis further suggested that the reduced innervation of HCs by SGNs could be attributed to a role of EphA7 in regulating outgrowth of SGN neurites as knocking down EphA7 in SGNs resulted in diminished SGN fibers. In addition, suppressing the activity of ERK1/2, a potential downstream target of EphA7 signaling, either with specific inhibitors in cultured explants or by knocking out Prkg1, also resulted in reduced SGN fibers. Together, our results suggest that EphA7 plays an important role in the developmental formation of cochlear innervation pattern through controlling SGN fiber ontogeny. Such regulation may contribute to the salience level of auditory signals presented to the central auditory system.

Sensory afferent segregation in three-eared frogs resemble the dominance columns observed in three-eyed frogs

The formation of proper sensory afferent connections during development is essential for brain function. Activity-based competition is believed to drive ocular dominance columns (ODC) in mammals and in experimentally-generated three-eyed frogs. ODC formation is thus a compromise of activity differences between two eyes and similar molecular cues. To gauge the generality of graphical map formation in the brain, we investigated the inner ear projection, known for its well-defined and early segregation of afferents from vestibular and auditory endorgans. In analogy to three eyed-frogs, we generated three-eared frogs to assess to what extent vestibular afferents from two adjacent ears could segregate. Donor ears were transplanted either in the native orientation or rotated by 90 degrees. These manipulations should result in either similar or different induced activity between both ears, respectively. Three-eared frogs with normal orientation showed normal swimming whereas those with a rotated third ear showed aberrant behaviors. Projection studies revealed that only afferents from the rotated ears segregated from those from the native ear within the vestibular nucleus, resembling the ocular dominance columns formed in three-eyed frogs. Vestibular segregation suggests that mechanisms comparable to those operating in the ODC formation of the visual system may act on vestibular projection refinements.

The role of ephrin-A2 and ephrin-A5 in sensorimotor control and gating

Many factors influence neurodevelopment. However, their contribution to adult neural function is often unclear. This is often due to complex expression profiles, cell signalling, neuroanatomy, and a lack of effective tests to assess the function of neural circuits in vivo. Ephrin-A2 and ephrin-A5 are cell surface proteins implicated in multiple aspects of neurodevelopment. While the role of ephrin-As in visual, auditory and learning behaviours has been explored, little is known about their role in dopaminergic and neuromotor pathways, despite expression in associated brain regions. Here we probe the function of ephrin-A2 and ephrin-A5 in the development of the dopaminergic and neuromotor pathways using counts of tyrosine hydroxylase (TH) positive cells in the substantia nigra pars compacta (SNpc) and the ventral tegmental area (VTA), the acoustic startle reflex (ASR), and a measure of sensorimotor gating, prepulse inhibition (PPI). Analysis of the ASR and PPI in ephrin-A2 and/or ephrin-A5 knock-out mice revealed that both genes play distinct roles in mediating ASR circuits, but are unlikely to play a role in PPI. Knock-out of either gene resulted in robust changes in startle response magnitude and measures of startle onset and peak latencies. However, ephrin-A2 and ephrin-A5 regulate aspects of the ASR differently: ephrin-A2 KO mice have increased startle amplitude, increased sensitivity and reduced latency to startle, whilst ephrin-A5 KO mice show opposite effects. Neither of the gene knock outs affected PPI, despite ephrin-A5 KO mice showing changes in dopamine cell numbers in nuclei thought to regulate PPI. We propose that majority of the changes observed ephrin-A2 and ephrin-A5 KO mice appear to be mediated by the effects on motor neurons and their muscle targets, rather than changes in auditory sensitivity.

Auditory brainstem responses of ephrin-A2, ephrin-A5(-/-) and ephrin-A2A5(-/-) mice

Eph receptors and ephrin ligands are large families of cell surface proteins which have established roles in axonal growth and guidance. These are well characterized in the visual and somatosensory systems but are less well documented in the auditory pathway. We examined the possible functional role of two ephrin genes (ephrin-A2 and ephrin-A5) in the auditory system by measuring auditory brainstem responses (ABR) to tone bursts from 6 to 30 kHz in ephrin-A2(-/-), ephrin-A5(-/-) and ephrin-A2A5(-/-) (knockout) mice. At high frequencies, the ephrin-A2A5(-/-) mice exhibited thresholds that were significantly lower than in wild-type mice by approximately 20 dB, suggesting ephrin-A2 and ephrin-A5 may have frequency-specific effects on the auditory system. There were also alterations in ABR wave peak amplitudes that were specific to each mouse strain which suggested both peripheral and central involvement of EphA-ephrin-A signalling in auditory function.

Differential roles for EphA and EphB signaling in segregation and patterning of central vestibulocochlear nerve projections

Auditory and vestibular afferents enter the brainstem through the VIIIth cranial nerve and find targets in distinct brain regions. We previously reported that the axon guidance molecules EphA4 and EphB2 have largely complementary expression patterns in the developing avian VIIIth nerve. Here, we tested whether inhibition of Eph signaling alters central targeting of VIIIth nerve axons. We first identified the central compartments through which auditory and vestibular axons travel. We then manipulated Eph-ephrin signaling using pharmacological inhibition of Eph receptors and in ovo electroporation to misexpress EphA4 and EphB2. Anterograde labeling of auditory afferents showed that inhibition of Eph signaling did not misroute axons to non-auditory target regions. Similarly, we did not find vestibular axons within auditory projection regions. However, we found that pharmacologic inhibition of Eph receptors reduced the volume of the vestibular projection compartment. Inhibition of EphB signaling alone did not affect auditory or vestibular central projection volumes, but it significantly increased the area of the auditory sensory epithelium. Misexpression of EphA4 and EphB2 in VIIIth nerve axons resulted in a significant shift of dorsoventral spacing between the axon tracts, suggesting a cell-autonomous role for the partitioning of projection areas along this axis. Cochlear ganglion volumes did not differ among treatment groups, indicating the changes seen were not due to a gain or loss of cochlear ganglion cells. These results suggest that Eph-ephrin signaling does not specify auditory versus vestibular targets but rather contributes to formation of boundaries for patterning of inner ear projections in the hindbrain.

Roles of EphA2 in Development and Disease

The Eph family of receptor tyrosine kinases (RTKs) has been implicated in the regulation of many aspects of mammalian development. Recent analyses have revealed that the EphA2 receptor is a key modulator for a wide variety of cellular functions. This review focuses on the roles of EphA2 in both development and disease.

Making connections in the inner ear: recent insights into the development of spiral ganglion neurons and their connectivity with sensory hair cells

In mammals, auditory information is processed by the hair cells (HCs) located in the cochlea and then rapidly transmitted to the CNS via a specialized cluster of bipolar afferent connections known as the spiral ganglion neurons (SGNs). Although many anatomical aspects of SGNs are well described, the molecular and cellular mechanisms underlying their genesis, how they are precisely arranged along the cochlear duct, and the guidance mechanisms that promote the innervation of their hair cell targets are only now being understood. Building upon foundational studies of neurogenesis and neurotrophins, we review here new concepts and technologies that are helping to enrich our understanding of the development of the nervous system within the inner ear.

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.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

Disruption of ephrin B/Eph B interaction results in abnormal cochlear innervation patterns

OBJECTIVE

To determine the expression patterns of B ephrins and Ephs in the cochlea and identify functional consequences of disruption of ephrin B/Eph B interactions in both cultured spiral ganglion neurons and in the cochlea of live animals.

STUDY DESIGN

The expression patterns of various B ephrins and Ephs were determined in mice with Lac-Z mutation. Mice with null function of individual B ephrin and Eph proteins and those with multiple knockouts were studied for cochlear innervation patterns.

METHODS

Mice with B ephrins and Ephs disrupted with the β-galactosidase gene were sacrificed at P6, and their cochleae isolated and processed for Lac-Z staining to determine expression of these proteins in cochlear tissue. Spiral ganglion cells from wild-type as well as ephrin B1 knockout mice were isolated and cocultured with Eph B2 expressing Cos1 cells and neurite lengths were determined. Fluorescent lipophillic dyes were used to label spiral ganglion cell nerve fibers to determine cochlear innervation patterns in wild-type and knockout mice.

RESULTS

Eph B1, B2, and ephrin B2 but not B3 was expressed in the cochlea. Eph B2 inhibited outgrowth of spiral ganglion cell axons from wild-type mice, but not from ephrin B1 knockout mice in culture. Knockout mice with null function of ephrin B1 alone or Eph B1, Eph B2, Eph B3 in combination demonstrated abnormal innervation patterns in the organ of Corti.

CONCLUSIONS

Disruption of B ephrins and Ephs results in functional consequences in spiral ganglion cells, suggesting that these proteins play a role in establishing normal innervation patterns in the 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.

Expression of Wnt receptors in adult spiral ganglion neurons: frizzled 9 localization at growth cones of regenerating neurites

Little is known about signaling pathways, besides those of neurotrophic factors, that are operational in adult spiral ganglion neurons. In patients with sensorineural hearing loss, such pathways could eventually be targeted to stimulate and guide neurite outgrowth from the remnants of the spiral ganglion towards a cochlear implant, thereby improving the fidelity of sound transmission. To systematically identify neuronal receptors for guidance cues in the adult cochlea, we conducted a genome-wide cDNA microarray screen with 2-month-old CBA/CaJ mice. A meta-analysis of our data and those from older mice in two other studies revealed the presence of neuronal transmembrane receptors that represent all four established guidance pathways--ephrin, netrin, semaphorin, and slit--in the mature cochlea as late as 15 months. In addition, we observed the expression of all known receptors for the "wingless-related MMTV integration site" (Wnt) morphogens, whose neuronal guidance function has only recently been recognized. In situ hybridizations located the mRNAs of the Wnt receptors frizzled 1, 4, 6, 9, and 10 specifically in adult spiral ganglion neurons. Finally, frizzled 9 protein was found in the growth cones of adult spiral ganglion neurons that were regenerating neurites in culture. We conclude from our results that adult spiral ganglion neurons are poised to respond to neurite damage, owing to the constitutive expression of a large and diverse collection of guidance receptors. Wnt signaling, in particular, emerges as a candidate pathway for guiding neurite outgrowth towards a cochlear implant after sensorineural hearing loss.

The molecular phylogeny of eph receptors and ephrin ligands

Background

The tissue distributions and functions of Eph receptors and their ephrin ligands have been well studied, however less is known about their evolutionary history. We have undertaken a phylogenetic analysis of Eph receptors and ephrins from a number of invertebrate and vertebrate species.

Results

Our findings indicate that Eph receptors form three major clades: one comprised of non-chordate and cephalochordate Eph receptors, a second comprised of urochordate Eph receptors, and a third comprised of vertebrate Eph receptors. Ephrins, on the other hand, fall into either a clade made up of the non-chordate and cephalochordate ephrins plus the urochordate and vertebrate ephrin-Bs or a clade made up of the urochordate and vertebrate ephrin-As.

Conclusion

We have concluded that Eph receptors and ephrins diverged into A and B-types at different points in their evolutionary history, such that primitive chordates likely possessed an ancestral ephrin-A and an ancestral ephrin-B, but only a single Eph receptor. Furthermore, ephrin-As appear to have arisen in the common ancestor of urochordates and vertebrates, whereas ephrin-Bs have a more ancient bilaterian origin. Ancestral ephrin-B-like ligands had transmembrane domains; as GPI anchors appear to have arisen or been lost at least 3 times.

Axon guidance cues in auditory development

The innervation of the cochlear sensory epithelium is intricately organized, allowing the tonotopy established by the auditory hair cells to be maintained along the ascending auditory pathways. These auditory projections are patterned by several gene families that regulate neurite attraction and repulsion, known as axon guidance cues. In this review, the roles of various axon guidance molecules, including fibroblast growth factor, ephs, semaphorins, netrins and slits, are examined in light of their known contribution to auditory development. Additionally, morphogens are discussed in the context of their recently described influence on axonal pathfinding in other sensory systems. The elucidation of these various mechanisms may guide the development of therapies aimed at maximizing the connectivity of auditory neurons in the context of congenital or acquired sensorineural hearing loss, especially as pertains to cochlear implants. Further afield, improved understanding of the molecular processes which regulate innervation of the organ of Corti during normal development may prove useful in connecting regenerated hair cells to the central nervous system.

Environmental Micropatterning for the Study of Spiral Ganglion Neurite Guidance

The projection of neuronal processes is guided by a variety of soluble and insoluble factors, which are sensed by a fiber's growth cone. It is the differential distribution of such guidance cues that determine the direction in which neurites grow. The growth cone senses these cues on a fine scale, using extensible filopodia that range from a few to tens of mum in length. In order to study the effects of guidance cues on spiral ganglion (SG) neurites, we have used methods for distributing both soluble and insoluble cues on a scale appropriate for sensing by growth filopodia. The scale of these methods are at the micro, rather than nano, level to match the sensing range of the growth cone. Microfluidics and transfected cells were used to spatially localize tropic factors within the fluid environment of extending neurites. Micro-patterning was used to present neurites with stripes of insoluble factors. The results indicate that differentially distributed permissive, repulsive and stop signals can control the projection of SG neurites. Implications for future micro-patterning studies, for SG development and for the growth of deafferented SG dendrites toward a cochlear implant are discussed.

Mutant mice reveal the molecular and cellular basis for specific sensory connections to inner ear epithelia and primary nuclei of the brain

We review the in vivo evidence for afferent fiber guidance to the inner ear sensory epithelia and the central nuclei of termination. Specifically, we highlight our current molecular understanding for the role of hair cells and sensory epithelia in guiding afferents, how disruption of certain signals can alter fiber pathways, even in the presence of normal hair cells, and what role neurotrophins play in fiber guidance of sensory neurons to hair cells. The data suggest that the neurotrophin BDNF is the most important molecule known for inner ear afferent fiber guidance to hair cells in vivo. This suggestion is based on experiments on Ntf3 transgenic mice expressing BDNF under Ntf3 promoter that show deviations of fiber growth in the ear to areas that express BDNF but have no hair cells. However, fiber growth can occur in the absence of BDNF as demonstrated by double mutants for BDNF and Bax. We directly tested the significance of hair cells or sensory epithelia for fiber guidance in mutants that lose hair cells (Pou4f3) or do not form a posterior crista (Fgf10). While these data emphasize the role played by BDNF, normally released from hair cells, there is some limited capacity for directed growth even in the absence of hair cells, BDNF, or sensory epithelia. This directed growth may rely on semaphorins or other matrix proteins because targeted ablation of the sema3 docking site on the sema receptor Npn1 results in targeting errors of fibers even in the presence of hair cells and BDNF. Overall, our data support the notion that targeting of the afferent processes in the ear is molecularly distinct from targeting processes in the central nuclei. This conclusion is derived from data that show no recognizable central projection deviation, even if fibers are massively rerouted in the periphery, as in Ntf3(tgBDNF) mice in which vestibular fibers project to the cochlea.

Localization of a novel autosomal recessive non-syndromic hearing impairment locus DFNB55 to chromosome 4q12-q13.2

Hereditary hearing impairment (HI) is the most genetically heterogeneous trait known in humans. So far, 54 autosomal recessive non-syndromic hearing impairment (ARNSHI) loci have been mapped, and 21 ARNSHI genes have been identified. Here is reported the mapping of a novel ARNSHI locus, DFNB55, to chromosome 4q12-q13.2 in a consanguineous Pakistani family. A maximum multipoint LOD score of 3.5 was obtained at marker D4S2638. The region of homozygosity and the 3-unit support interval are flanked by markers D4S2978 and D4S2367. The region spans 8.2 cm on the Rutgers combined linkage-physical map and contains 11.5 Mb. DFNB55 represents the third ARNSHI locus mapped to chromosome 4.

Differential expression of Eph receptors and ephrins in the cochlear ganglion and eighth cranial nerve of the chick embryo

The cochleovestibular ganglion of the chick differentiates at early embryonic stages as VIIIth nerve axons enter the brainstem. The tonotopic organization of the auditory portion of the VIIIth nerve can be discerned at the time axons initially reach their brainstem targets. The mechanisms underlying this early organization are not known. Eph receptor tyrosine kinases and their ligands, the ephrins, have a demonstrated role in guiding axons to topographically appropriate locations in other areas of the nervous system. In order to begin to test whether Eph proteins have a similar role in the auditory system, we investigated the tonotopic expression of several Eph receptors and ephrins in the VIIIth nerve during embryonic ages corresponding to the initial innervation of the auditory brainstem. Expression patterns of EphA4, EphB2, EphB5, ephrin-A2, and ephrin-B1 were evaluated immunohistochemically at embryonic days 4 through 10. Protein expression was observed in the cochlear ganglion and VIIIth nerve axons at these ages. EphB5, ephrin-A2, and ephrin-B1 were expressed throughout the nerve. EphA4 and EphB2 had complementary expression patterns within the nerve, with EphA4 expression higher in the dorsolateral part of the nerve and EphB2 expression higher in the ventromedial part of the nerve. These regions may correspond to auditory and vestibular components, respectively. Moreover, EphA4 expression was higher toward the low-frequency region of both the centrally and peripherally projecting branches of cochlear ganglion cells. Regional variation of Eph protein expression may influence the target selection and topography of developing VIIIth nerve projections.

EphA4 provides repulsive signals to developing cochlear ganglion neurites mediated through ephrin-B2 and -B3

The ephrins and Eph receptors make up two large families of bi-directional signaling molecules that are known to play a role in the development of the nervous system. Recently, expression of EphA4 in the developing cochlea was shown, with strong expression in cells lining the osseous spiral lamina (OSL) through which afferent dendrites must pass to reach the organ of Corti (OC). It was also demonstrated that ephrin-B2 and -B3, both of which are known to interact with EphA4, are expressed by spiral ganglion (SG) neurons. To investigate the functional role of EphA4 in the development of inner ear neurons, neonatal rat SG explants were cultured for 72 hours on uniformly coated surfaces or near stripes of EphA4/IgG-Fc-chimera. Control explants were cultured on or near IgG-Fc and EphA1/IgG-Fc-chimera. To assess the roles of ephrin-B2 and -B3 in EphA4 signaling, SG explants were cultured with or without anti-ephrin-B2 and/or -B3 blocking antibodies. Growth patterns of SG neurites at the border of EphA4 receptor stripes showed repulsion, characterized by turning, stopping and/or reversal. In the case of IgG-Fc and EphA1, the neurites grew straight onto the stripes. Treatment with either anti-ephrin-B2 or -B3 blocking antibodies significantly reduced the repulsive effect of an EphA4 stripe. Moreover, when both antibodies were used together, neurites crossed onto EphA4 stripes with no evidence of repulsion. The results suggest that EphA4 provides repulsive signals to SG neurites in the developing cochlea, and that ephrin-B2 and -B3 together mediate this response.

Development of vestibular afferent projections into the hindbrain and their central targets

In contrast to most other sensory systems, hardly anything is known about the neuroanatomical development of central projections of primary vestibular neurons and how their second order target neurons develop. Recent data suggest that afferent projections may develop not unlike other sensory systems, forming first the overall projection by molecular means followed by an as yet unspecified phase of activity mediated refinement. The latter aspect has not been tested critically and most molecules that guide the initial projection are unknown. The molecular and topological origin of the vestibular and cochlear nucleus neurons is also only partially understood. Auditory and vestibular nuclei form from several rhombomeres and a given rhombomere can contribute to two or more auditory or vestibular nuclei. Rhombomere compartments develop as functional subdivisions from a single column that extends from the hindbrain to the spinal cord. Suggestions are provided for the molecular origin of these columns but data on specific mutants testing these proposals are not yet available. Overall, the functional significance of both overlapping and segregated projections are not yet fully experimentally explored in mammals. Such lack of details of the adult organization compromises future developmental analysis.

Development of inner ear afferent connections: forming primary neurons and connecting them to the developing sensory epithelia

The molecular and cellular origin of the primary neurons of the inner ear, the vestibular and spiral neurons, is reviewed including how they connect to the specific sensory epithelia and what the molecular nature of their survival is. Primary neurons of the ear depend on a single basic Helix-Loop-Helix (bHLH) protein for their formation, neurogenin 1 (ngn1). An immediate downstream gene is the bHLH gene neuronal differentiation (NeuroD). Targeted null mutations of ngn1 results in absence of primary neuron formation; targeted null mutation of NeuroD results in loss of almost all spiral and many vestibular neurons. NeuroD and a later expressed gene, Brn3a, play a role in pathfinding to and within sensory epithelia. The molecular nature of this pathfinding property is unknown. Reduction of hair cells in ngn1 null mutations suggests a clonal relationship with primary neurons. This relationship may play some role in specifying the identity of hair cells and the primary neurons that connect with them. Primary neuron neurites growth to sensory epithelia is initially independent of trophic factors released from developing sensory epithelia, but becomes rapidly dependent on those factors. Null mutations of specific neurotrophic factors lose distinct primary neuron populations which undergo rapid embryonic cell death.

Eph receptor deficiencies lead to altered cochlear function

Ephrins and Eph receptors are a family of molecules that have been implicated in many developmental processes including neuronal network formation, guidance of cell migration, and axonal pathfinding. These molecules exhibit the ability to send bidirectional signals following ligand-receptor interactions resulting from cell-cell contacts. Gene-targeted knockout mice of B-class ephrins and Eph receptors have been shown to display phenotypic responses that correlate with anatomical defects. For example, disruption of the EphB2 receptor leads to defects of the vestibular system, including pathfinding abnormalities in efferent axons and reduced endolymph production. Such developmental distortions lead to deficiencies in ionic homeostasis and repetitive circling behaviors. The present study demonstrates that B-class ephrins and Eph receptors are expressed in cochlear tissues, suggesting that they may play some role in auditory function. To determine whether ephrins and Eph receptors have a functional role in the peripheral auditory system, distortion-product otoacoustic emission (DPOAE) levels, collected across a broad frequency range, were compared between groups of mice expressing different Eph receptor genotypes. In particular, EphB1 and EphB3 receptor knockout mice exhibited significantly diminished DPOAE levels as compared to wild-type littermates, indicating that these specific Eph receptors are necessary for normal cochlear function.

Expression of Ephs and ephrins in developing mouse inner ear

Levels of expression of mRNAs encoding the different Ephs and ephrins were measured by semi-quantitative reverse-transcription polymerase chain reaction in developing mouse whole inner ears, and in dissected fractions of the neonatal mouse inner ear. Nineteen of the 24 known Ephs and ephrins were surveyed. The results showed that between embryonic age (E) 11.5 days and E12.5, levels increased 10-300 times per unit of tissue. In neonatal mice, the fraction containing combined organ of Corti and spiral ganglion showed relatively strong expression of EphA4, EphB3, ephrin-A3, ephrin-B2 and ephrin-B3. In the lateral wall, EphA4, ephrin-A3 and ephrin-B2 were strongly expressed, while ephrin-A3 was particularly strongly expressed in utricular and saccular sensory epithelia. The results suggest that the Ephs and ephrins are likely to play a part in the differentiation of the structures of the inner ear, and show which Ephs and ephrins are most likely to play important roles in the different structures.

Molecular conservation and novelties in vertebrate ear development

Evolution shaped the vertebrate ear into a complicated three-dimensional structure and positioned the sensory epithelia so that they can extract specific aspects of mechanical stimuli to govern vestibular and hearing-related responses of the whole organism. This information is conducted from the ear via specific neuronal connections to distinct areas of the hindbrain for proper processing. During development, the otic placode, a simple sheet of epidermal cells, transforms into a complicated system of ducts and recesses. This placode also generates the mechanoelectrical transducers, the hair cells, and sensory neurons of the vestibular and cochlear (spiral) ganglia of the ear. We argue that ear development can be broken down into dynamic processes that use a number of known and unknown genes to govern the formation of the three-dimensional labyrinth in an interactive fashion. Embedded in this process, but in large part independent of it, is an evolutionary conserved process that induces early the development of the neurosensory component of the ear. We present molecular data suggesting that this later process is, in its basic aspects, related to the mechanosensory cell formation across phyla and is extremely conserved at the molecular level. We suggest that sensory neuron development and maintenance are vertebrate or possibly chordate novelties and present the molecular data to support this notion.

Lac z Histochemistry and immunohistochemistry reveal ephrin-B ligand expression in the inner ear

Immunostaining in transgenic mice carrying the lac z gene can be used to map gene and protein distribution in a single tissue. In this study, we examined inner ears from ephrin-B3 homozygous and ephrin-B2 heterozygous mice. Ephrin-B3 lac z expression was limited in these mice. However, immunostaining revealed ephrin-B3 throughout cochlear and vestibular regions. Immunoreactivity was absent in ephrin-B3-homozygous null mutants, demonstrating the specificity of the antibody. Ephrin-B2 lac z reactivity was detected in a limited number of cells in cochlear and vestibular regions. Different immunostaining patterns were found with different antibodies. Comparison with lac z expression indicated which antibody was specific for the transmembrane-bound ephrin-B2 ligand.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

EphB receptors influence growth of ephrin-B1-positive statoacoustic nerve fibers

The Eph family of receptors and ligands has been implicated in a variety of developmental processes, including the provision of inhibitory guidance cues to developing nerve fibers. A unique property of the B class of receptors is that they are able to phosphorylate ephrin-B ligands, allowing for bi-directional, or reverse signalling. While most of the studies to date have focused on central nerve fibers, little is known about the role of Eph molecules in guiding nerve fibers of the peripheral nervous system. In the present study, ephrin-B1 was found to be highly expressed on developing peripheral nerve fibers including auditory and vestibular (statoacoustic) and dorsal root ganglion nerve fibers. In vitro assays revealed that EphB-Fc receptors inhibited further growth of statoacoustic nerve fibers. In contrast, EphA7-Fc and ephrin-B2-Fc did not prevent further growth of SAG. Together, these results suggest a role for EphB receptors in providing guidance signals to ephrin-B1-positive SAG nerve fibers.

Complementary and layered expression of Ephs and ephrins in developing mouse inner ear

The distributions of the Eph-class receptors EphA4 and EphB1, and their ligands ephrin-A2, ephrin-B1, and ephrin-B2, were analysed by immunostaining in the mouse inner ear. Complementary patterns of EphA4 and its potential ligand ephrin-A2 were found, with ephrin-A2 in many of the structures lining the cochlear duct and within the cochlear nerve cells, and EphA4 in the deeper structures underlying the cochlear duct and in the cells lining the nerve pathway. EphB1 and its potential ligands ephrin-B1 and ephrin-B2 showed a segregated layered expression in the lateral wall of the cochlear duct (the external sulcus), which together with EphA4 expressed in the area, form a four-layered structure with an alternating pattern of receptors and ligands in the different layers. This arrangement gives the potential for different bidirectional Eph-mediated interactions between each of the layers. The results suggest that the Eph system in the cochlea may have a role in maintaining cell segregation during phases of cochlear development.

ENU mutagenesis reveals a highly mutable locus on mouse Chromosome 4 that affects ear morphogenesis

Chemical mutagenesis followed by screening for abnormal phenotypes in the mouse holds much promise as a method for revealing gene function. This method is particularly well-suited for discovering genes involved in hearing or balance function, as these defects are relatively easy to screen for in the mouse. We report here the inner ear abnormalities and genetic localization of seven new dominant mutations created by ENU mutagenesis. All seven mutant stocks were identified because of circling and/or head-weaving behavior, which is an indication of balance dysfunction. Investigation of the inner ears of the seven mutant stocks revealed very similar lateral and posterior semicircular canal defects. Studies of the development of the canals in one mutant stock revealed that the affected canals showed reduced outgrowth and delayed canal fusion. Physiological studies performed in one mutant stock showed raised average compound-action-potential thresholds of approximately 10-20 dB sound pressure level (SPL) (depending on frequency), indicating a mild hearing impairment, although scanning electron microscopy performed in several of the mutant stocks revealed no obvious structural defects in the organ of Corti. All seven mutations mapped to the proximal portion of Chromosome (Chr) 4, near the centromere. On the basis of their similar phenotype and map location, we suggest that the seven mutant genes may be allelic and represent a highly mutable locus on Chr 4 that may be particularly susceptible to ENU-induced mutation on the BALB/c genetic background.

Cryoembedding and sectioning of cochleas for immunocytochemistry and in situ hybridization

Current emphasis on biochemical and molecular aspects of cochlear anatomy underscores the necessity for high quality cryostat sections of the inner ear. The large volume of fluid space within the cochlea makes cryoembedding and sectioning of the organ more problematic than that of other, more homogeneous tissues. Our method for cryoembedding of cochleas for immunocytochemistry and in situ hybridization uses slow infiltration with increasing concentrations of sucrose followed by degassed embedding medium before final orientation and freezing. This method permits high quality cryosections to be cut which preserve overall structure and cellular resolution.

EphB2 guides axons at the midline and is necessary for normal vestibular function

Mice lacking the EphB2 receptor tyrosine kinase display a cell-autonomous, strain-specific circling behavior that is associated with vestibular phenotypes. In mutant embryos, the contralateral inner ear efferent growth cones exhibit inappropriate pathway selection at the midline, while in mutant adults, the endolymph-filled lumen of the semicircular canals is severely reduced. EphB2 is expressed in the endolymph-producing dark cells in the inner ear epithelium, and these cells show ultrastructural defects in the mutants. A molecular link to fluid regulation is provided by demonstrating that PDZ domain-containing proteins that bind the C termini of EphB2 and B-ephrins can also recognize the cytoplasmic tails of anion exchangers and aquaporins. This suggests EphB2 may regulate ionic homeostasis and endolymph fluid production through macromolecular associations with membrane channels that transport chloride, bicarbonate, and water.

Expression of EphA4 in developing inner ears of the mouse and guinea pig

The expression of EphA4, an Eph-class receptor tyrosine kinase, was determined by immunohistochemistry in developing inner ears of the mouse and the guinea pig. In the mouse, EphA4 expression was visible in the fibroblasts of the spiral ligament and in the structures that were to become the osseous spiral lamina. Cochlear nerve ganglion cells expressed ephrin-B2, and the modiolus expressed mRNA coding for ephrin-B3, both transmembrane ligands for EphA4. In contrast, in the guinea pig, cells of the cochlear nerve ganglion expressed EphA4, as did supporting cells of the organ of Corti (Hensen's cells and inner pillar cells). There was also some expression in fibroblasts of the spiral ligament but none in the structures that were to become the osseous spiral lamina. It is suggested that in the mouse, EphA4 may help direct the cochlear innervation towards the organ of Corti by a repulsive interaction, but that this is highly species dependent.