Distribution of beta-tectorin mRNA in the early posthatch and developing avian inner ear

Cell-type identity of the avian cochlea

In contrast to mammals, birds recover naturally from acquired hearing loss, which makes them an ideal model for inner ear regeneration research. Here, we present a validated single-cell RNA sequencing resource of the avian cochlea. We describe specific markers for three distinct types of sensory hair cells, including a previously unknown subgroup, which we call superior tall hair cells. We identify markers for the supporting cells associated with tall hair cells, which represent the facultative stem cells of the avian inner ear. Likewise, we present markers for supporting cells that are located below the short cochlear hair cells. We further infer spatial expression gradients of hair cell genes along the tonotopic axis of the cochlea. This resource advances neurobiology, comparative biology, and regenerative medicine by providing a basis for comparative studies with non-regenerating mammalian cochleae and for longitudinal studies of the regenerating avian cochlea.

Structure, Function, and Development of the Tectorial Membrane: An Extracellular Matrix Essential for Hearing

The tectorial membrane is an extracellular matrix that lies over the apical surface of the auditory epithelia in the inner ears of reptiles, birds, and mammals. Recent studies have shown it is composed of a small set of proteins, some of which are only produced at high levels in the ear and many of which are the products of genes that, when mutated, cause nonsyndromic forms of human hereditary deafness. Quite how the proteins of the tectorial membrane are assembled within the lumen of the inner ear to form a structure that is precisely regulated in its size and physical properties along the length of a tonotopically organized hearing organ is a question that remains to be fully answered. In this brief review we will summarize what is known thus far about the structure, protein composition, and function of the tectorial membrane in birds and mammals, describe how the tectorial membrane develops, and discuss major events that have occurred during the evolution of this extracellular matrix.

Atoh1 expression and function during auditory hair cell regeneration in post-hatch chickens

Loss of hair cells in humans leads to irreversible hearing deficits, since auditory hair cells are not replaced. In contrast, hair cells are regenerated in the auditory epithelium of mature birds after damage by non-sensory supporting cells that transdifferentiate into hair cells by mitotic and/or non-mitotic mechanisms. Factors controlling these processes are poorly understood. The basic helix-loop-helix transcription factor ATOH1 is both necessary and sufficient for developmental hair cell differentiation, but it is unclear if it plays the same role in the mitotic and non-mitotic pathways in hair cell regeneration. We examined Atoh1 expression and function during hair cell regeneration in chickens. Atoh1 transcripts were increased in many supporting cells in the damaged auditory epithelium shortly after ototoxin administration and later became restricted to differentiating hair cells. Fate-mapping in vitro using an Atoh1 enhancer reporter demonstrated that only 56% of the supporting cells that spontaneously upregulate Atoh1 enhancer activity after damage acquired the hair cell fate. Inhibition of notch signaling using a gamma secretase antagonist stimulated an increase in Atoh1 reporter activity and induced a higher proportion of supporting cells with Atoh1 activity (73%) to differentiate as hair cells. Forced overexpression of Atoh1 in supporting cells triggered 66% of them to acquire the hair cell fate and nearly tripled their likelihood of cell cycle entry. These findings demonstrate that Atoh1 is broadly upregulated in supporting cells after damage, but a substantial proportion of supporting cells with Atoh1 activation fails to acquire hair cell features, in part due to gamma secretase-dependent activities.

Supporting cell division is not required for regeneration of auditory hair cells after ototoxic injury in vitro

In chickens, nonsensory supporting cells divide and regenerate auditory hair cells after injury. Anatomical evidence suggests that supporting cells can also transdifferentiate into hair cells without dividing. In this study, we characterized an organ culture model to study auditory hair cell regeneration, and we used these cultures to test if direct transdifferentiation alone can lead to significant hair cell regeneration. Control cultures (organs from posthatch chickens maintained without streptomycin) showed complete hair cell loss in the proximal (high-frequency) region by 5 days. In contrast, a 2-day treatment with streptomycin induced loss of hair cells from all regions by 3 days. Hair cell regeneration proceeded in culture, with the time course of supporting cell division and hair cell differentiation generally resembling in vivo patterns. The degree of supporting cell division depended upon the presence of streptomycin, the epithelial region, the type of culture media, and serum concentration. On average, 87% of the regenerated hair cells lacked the cell division marker BrdU despite its continuous presence, suggesting that most hair cells were regenerated via direct transdifferentiation. Addition of the DNA polymerase inhibitor aphidicolin to culture media prevented supporting cell division, but numerous hair cells were regenerated nonetheless. These hair cells showed signs of functional maturation, including stereociliary bundles and rapid uptake of FM1-43. These observations demonstrate that direct transdifferentiation is a significant mechanism of hair cell regeneration in the chicken auditory after streptomycin damage in vitro.

Notch regulation of progenitor cell behavior in quiescent and regenerating auditory epithelium of mature birds

Unlike mammals, birds regenerate auditory hair cells (HCs) after injury. During regeneration, mature non-sensory supporting cells (SCs) leave quiescence and convert into HCs, through non-mitotic or mitotic mechanisms. During embryogenesis, Notch ligands from nascent HCs exert lateral inhibition, restricting HC production. Here, we examined whether Notch signaling (1) is needed in mature birds to maintain the HC/SC pattern in the undamaged auditory epithelium or (2) governs SC behavior once HCs are injured. We show that Notch pathway genes are transcribed in the mature undamaged epithelium, and after HC injury, their transcription is upregulated in the region of highest mitotic activity. In vitro treatment with DAPT, an inhibitor of Notch activity, had no effect on SCs in the undamaged epithelium. Following HC damage, DAPT had no direct effect on SC division. However, after damage, DAPT caused excessive regeneration of HCs at the expense of SCs, through both mitotic and non-mitotic mechanisms. Conversely, overexpression of activated Notch in SCs after damage caused them to maintain their phenotype and inhibited HC regeneration. Therefore, signaling through Notch is not required for SC quiescence in the healthy epithelium or to initiate HC regeneration after damage. Rather, Notch prevents SCs from regenerating excessive HCs after damage.

Assembly of the otoconia complex to the macular sensory epithelium of the vestibule

In the inner ear, specificity of stimulus perception is achieved by associating the sensory epithelia of the three mechanoreceptor organs, the utricle/saccule, cristae, and cochlea, with distinct types of acellular matrices. Only the utricle and saccule have an extremely dense matrix, the otoconial complex, which overlies the sensory epithelium (macula) and provides inertial mass to generate shearing forces essential for the mechanoreceptors to sense gravity and linear acceleration. Such sensation is necessary for spatial orientation and balance. The importance of otoconia is clearly demonstrated by the impact of balance disorders upon the elderly population that involve otoconia degeneration, as well as by canalithiasis and cupulolithiasis, in which otoconia are dislocated. This underscores the need to understand how otoconia are formed and maintained and how to prevent their degeneration. To date, a number of otoconia-related proteins have been identified mostly in mice and bony fish. Although most of these proteins are also present in other structures of the inner ear, a distinct collection of proteins in the macula plus the unique ionic microenvironment of the endolymph near its epithelium likely contribute to the site-specific calcification of otoconia. Based on the current literature and ongoing research, this mini-review postulates a working model of how the otoconia complex is assembled specifically above the macular sensory epithelium of the vestibule. The central hypothesis of this model is that proteins are critical in sequestering calcium for crystallization in the calcium-poor endolymph. The review also sets forth some issues that need to be resolved in the future.

Expression of Prox1 defines regions of the avian otocyst that give rise to sensory or neural cells

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.

Extracellular matrices associated with the apical surfaces of sensory epithelia in the inner ear: molecular and structural diversity

The ultrastructure and molecular composition of the extracellular matrices that are associated with the apical surfaces of the mechanosensory epithelia in the mouse inner ear are compared. A progressive increase in molecular and structural organization is observed, with the cupula being the simplest, the otoconial membrane exhibiting an intermediate degree of complexity, and the tectorial membrane being the most elaborate of the three matrices. These differences may reflect changes that occurred in the acellular membranes of the inner ear as a mammalian hearing organ arose during evolution from a simple equilibrium receptor. A comparison of the molecular composition of the acellular membranes in the chick inner ear suggests the auditory epithelium and the striolar region of the maculae are homologous, indicating the basilar papilla may have evolved from the striolar region of an otolithic organ. A comparison of the tectorial membranes in the chick cochlear duct and the mouse cochlea reveals differences in the structure of the noncollagenous matrix in the two species that may result from differences in the stochiometry of alpha- and beta-tectorin and/or differences in the post-translational modification of alpha-tectorin. This comparison also indicates that the appearance of collagen in the mammalian tectorial membrane may have been a major step in the evolution of an electromechanically tuned vertebrate hearing organ that operates over an extended frequency range.

WDR1 colocalizes with ADF and actin in the normal and noise-damaged chick cochlea

Auditory hair cells of birds, unlike hair cells in the mammalian organ of Corti, can regenerate following sound-induced loss. We have identified several genes that are upregulated following such an insult. One gene, WDR1, encodes the vertebrate homologue of actin-interacting protein 1, which interacts with actin depolymerization factor (ADF) to enhance the rate of actin filament cleavage. We examined WDR1 expression in the developing, mature, and noise-damaged chick cochlea by in situ hybridization and immunocytochemistry. In the mature cochlea, WDR1 mRNA was detected in hair cells, homogene cells, and cuboidal cells, all of which contain high levels of F-actin. In the developing inner ear, WDR1 mRNA was detected in homogene cells and cuboidal cells by embryonic day 7, in the undifferentiated sensory epithelium by day 9, and in hair cells at embryonic day 16. We also demonstrated colocalization of WDR1, ADF, and F-actin in all three cell types in the normal and noise-damaged cochlea. Immediately after acoustic overstimulation, WDR1 mRNA was seen in supporting cells. These cells contribute to the structural integrity of the basilar papilla, the maintenance of the ionic barrier at the reticular lamina, and the generation of new hair cells. These results indicate that one of the immediate responses of the supporting cell after noise exposure is to induce WDR1 gene expression and thus to increase the rate of actin filament turnover. These results suggest that WDR1 may play a role either in restoring cytoskeletal integrity in supporting cells or in a cell signaling pathway required for regeneration.

Cellular studies of auditory hair cell regeneration in birds

A decade ago it was discovered that mature birds are able to regenerate hair cells, the receptors for auditory perception. This surprising finding generated hope in the field of auditory neuroscience that new hair cells someday may be coaxed to form in another class of warm-blooded vertebrates, mammals. We have made considerable progress toward understanding some cellular and molecular events that lead to hair cell regeneration in birds. This review discusses our current understanding of avian hair cell regeneration, with some comparisons to other vertebrate classes and other regenerative systems.

Chick alpha-tectorin: molecular cloning and expression during embryogenesis

The avian and mammalian tectorial membranes both contain two non-collagenous glycoproteins, alpha and beta-tectorin. To determine whether variations in the primary sequences of the chick and mouse alpha-tectorins account for differences in subunit composition and matrix structure of the tectorial membranes in these two species, cDNAs spanning the entire open reading frame of chick alpha-tectorin were cloned and the derived amino acid sequence was compared with that of mouse alpha-tectorin. Chick alpha-tectorin shares 73% amino acid sequence identity with mouse alpha-tectorin and, like mouse alpha-tectorin, is composed of three distinct modules: an N-terminal region similar to the G1 domain of entactin, a central region that shares identity with zonadhesin and contains three full and two partial von Willebrand factor type D repeats, and a C-terminal region containing a zona pellucida domain. The central region of chick alpha-tectorin contains fewer potential N-glycosylation sites than that of mouse alpha-tectorin and is cleaved at two additional sites. Differences in the glycosylation and proteolytic processing of chick and mouse alpha-tectorin may therefore account for the variation observed in the composition and structure of the collagenase-insensitive matrices of the avian and mammalian tectorial membranes. In situ hybridisation and Northern blot analysis of chick inner ear tissue indicate that the spatial and temporal patterns of alpha and beta-tectorin mRNA expression in the developing chick inner ear are different, suggesting the two tectorins may each form homomeric filaments.

Molecular markers for cell types of the inner ear and candidate genes for hearing disorders

To identify genes expressed in the vertebrate inner ear, we have established an assay that allows rapid analysis of the differential expression pattern of mRNAs derived from an auditory epithelium-specific cDNA library. We performed subtractive hybridization to create an enriched probe, which then was used to screen the cDNA library. After digoxigenin-labeled antisense cRNAs had been transcribed from hybridization-positive clones, we conducted in situ hybridization on slides bearing cryosections of late embryonic chicken heads, bodies, and cochleae. One hundred and twenty of the 196 clones analyzed encode 12 proteins whose mRNAs are specifically or highly expressed in the chicken's inner ear; the remainder encode proteins that occur more widely. We identified proteins that have been described previously as expressed in the inner ear, such as beta-tectorin, calbindin, and type II collagen. A second group of proteins abundant in the inner ear includes five additional types of collagens. A third group, including Coch-5B2 and an ear-specific connexin, comprises proteins whose human equivalents are candidates to account for hearing disorders. This group also includes proteins expressed in two unique cell types of the inner ear, homogene cells and cells of the tegmentum vasculosum.

Pattern formation in the basilar papilla: evidence for cell rearrangement

The avian basilar papilla is composed of hair and supporting cells arranged in a regular pattern in which the hair cells are surrounded and isolated from each other by supporting cell processes. This arrangement of cells, in which the apical borders of hair cells do not contact one another, may be generated by contact-mediated lateral inhibition. Little is known, however, about the way in which hair and supporting cells are organized during development. Whole mounts double-labeled with antibodies to the 275 kDa hair-cell antigen and the tight junction protein cingulin were therefore used to examine the development of cell patterns in the basilar papilla. Hair cells that contact each other at their apical borders are seen during early development, especially on embryonic days (E) 8 and 9, but are no longer observed after E12. Hair and supporting cell patterns were analyzed in three different areas of the papilla at E9 and E12. In two of these regions between E9 and E12, the ratio of supporting cells to hair cells does not change significantly, whereas there is an increase in both the number of supporting cells around each hair cell and the number of hair cells that each supporting cell contacts. In the third region examined, there is a dramatic rise in the number of supporting cells around each hair cell, which although accompanied by a small, significant increase in the ratio of supporting cells to hair cells cannot be accounted for by an increase in supporting cell numbers. These data show that a rearrangement of hair and supporting cells with respect to one another may be a fundamental process underlying the development of a regular pattern in the basilar papilla.

The mouse tectorins. Modular matrix proteins of the inner ear homologous to components of the sperm-egg adhesion system

The cDNA and derived amino acid sequences for the two major non-collagenous proteins of the mouse tectorial membrane, alpha- and beta-tectorin, are presented. The cDNA for alpha-tectorin predicts a protein of 239,034 Da with 33 potential N-glycosylation sites, and that of beta-tectorin a smaller protein of 36,074 Da with 4 consensus N-glycosylation sites. Southern and Northern blot analysis indicate alpha- and beta-tectorin are single copy genes only expressed in the inner ear, and in situ hybridization shows they are expressed by cells both in and surrounding the mechanosensory epithelia. Both sequences terminate with a hydrophobic COOH terminus preceded by a potential endoproteinase cleavage site suggesting the tectorins are synthesized as glycosylphosphatidylinositol-linked, membrane bound precursors, targeted to the apical surface of the inner ear epithelia by the lipid and proteolytically released into the extracellular compartment. The mouse beta-tectorin sequence contains a single zona pellucida domain, whereas alpha-tectorin is composed of three distinct modules: an NH2-terminal region similar to part of the entactin G1 domain, a large central segment with three full and two partial von Willebrand factor type D repeats, and a carboxyl-terminal region which, like beta-tectorin, contains a single zona pellucida domain. The central, high molecular mass region of alpha-tectorin containing the von Willebrand factor type D repeats has homology with zonadhesin, a sperm membrane protein that binds to the zona pellucida. These results indicate the two major non-collagenous proteins of the tectorial membrane are similar to components of the sperm-egg adhesion system, and, as such may interact in the same manner.