Chick alpha-tectorin: molecular cloning and expression during embryogenesis

Regeneration in the Auditory Organ in Cuban and African Dwarf Crocodiles (Crocodylus rhombifer and Osteolaemus tetraspis) Can We Learn From the Crocodile How to Restore Our Hearing?

Background: In several non-mammalian species, auditory receptors undergo cell renewal after damage. This has raised hope of finding new options to treat human sensorineural deafness. Uncertainty remains as to the triggering mechanisms and whether hair cells are regenerated even under normal conditions. In the present investigation, we explored the auditory organ in the crocodile to validate possible ongoing natural hair cell regeneration.

Materials and Methods: Two male Cuban crocodiles (Crocodylus rhombifer) and an adult male African Dwarf crocodile (Osteolaemus tetraspis) were analyzed using transmission electron microscopy and immunohistochemistry using confocal microscopy. The crocodile ears were fixed in formaldehyde and glutaraldehyde and underwent micro-computed tomography (micro-CT) and 3D reconstruction. The temporal bones were drilled out and decalcified.

Results: The crocodile papilla basilaris contained tall (inner) and short (outer) hair cells surrounded by a mosaic of tightly connected supporting cells coupled with gap junctions. Afferent neurons with and without ribbon synapses innervated both hair cell types. Supporting cells occasionally showed signs of trans-differentiation into hair cells. They expressed the MAFA and SOX2 transcription factors. Supporting cells contained organelles that may transfer genetic information between cells, including the efferent nerve fibers during the regeneration process. The tectorial membrane showed signs of being replenished and its architecture being sculpted by extracellular exosome-like proteolysis.

Discussion: Crocodilians seem to produce new hair cells during their life span from a range of supporting cells. Imposing efferent nerve fibers may play a role in regeneration and re-innervation of the auditory receptors, possibly triggered by apoptotic signals from wasted hair cells. Intercellular signaling may be accomplished by elaborate gap junction and organelle systems, including neural emperipolesis. Crocodilians seem to restore and sculpt their tectorial membranes throughout their lives.

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.

Zona pellucida domain-containing protein β-tectorin is crucial for zebrafish proper inner ear development

Background

The zona pellucida (ZP) domain is part of many extracellular proteins with diverse functions from structural components to receptors. The mammalian β-tectorin is a protein of 336 amino acid residues containing a single ZP domain and a putative signal peptide at the N-terminus of the protein. It is 1 component of a gel-like structure called the tectorial membrane which is involved in transforming sound waves into neuronal signals and is important for normal auditory function. β-Tectorin is specifically expressed in the mammalian and avian inner ear.

Methodology/Principal Findings

We identified and cloned the gene encoding zebrafish β-tectorin. Through whole-mount in situ hybridization, we demonstrated that β-tectorin messenger RNA was expressed in the otic placode and specialized sensory patch of the inner ear during zebrafish embryonic stages. Morpholino knockdown of zebrafish β-tectorin affected the position and number of otoliths in the ears of morphants. Finally, swimming behaviors of β-tectorin morphants were abnormal since the development of the inner ear was compromised.

Conclusions/Significance

Our results reveal that zebrafish β-tectorin is specifically expressed in the zebrafish inner ear, and is important for regulating the development of the zebrafish inner ear. Lack of zebrafish β-tectorin caused severe defects in inner ear formation of otoliths and function.

Development of tonotopy in the auditory periphery

Acoustic frequency analysis plays an essential role in sound perception, communication and behavior. The auditory systems of most vertebrates that perceive sounds in air are organized based on the separation of complex sounds into component frequencies. This process begins at the level of the auditory sensory epithelium where specific frequencies are distributed along the tonotopic axis of the mammalian cochlea or the avian/reptilian basilar papilla (BP). Mechanical and electrical mechanisms mediate this process, but the relative contribution of each mechanism differs between species. Developmentally, structural and physiological specializations related to the formation of a tonotopic axis form gradually over an extended period of time. While some aspects of tonotopy are evident at early stages of auditory development, mature frequency discrimination is typically not achieved until after the onset of hearing. Despite the importance of tonotopic organization, the factors that specify unique positional identities along the cochlea or basilar papilla are unknown. However, recent studies of developing systems, including the inner ear provide some clues regarding the signalling pathways that may be instructive for the formation of a tonotopic axis.

Mid-frequency DFNA8/12 hearing loss caused by a synonymous TECTA mutation that affects an exonic splice enhancer

Autosomal dominant hearing loss is highly heterogeneous. Hearing impairment mainly involves the mid-frequencies (500-2000 Hz) in only a low percentage of the cases. In a Dutch family with autosomal dominant mid-frequency/flat hearing loss, genome-wide SNP analysis combined with fine mapping using microsatellite markers mapped the defect to the DFNA8/12 locus, with a maximum two-point LOD score of 3.52. All exons and intron-exon boundaries of the TECTA gene, of which mutations are causative for DFNA8/12, were sequenced. Only one heterozygous synonymous change in exon 16 (c.5331G>A; p.L1777L) was found to segregate with the hearing loss. This change was predicted to cause the loss of an exonic splice enhancer (ESE). RT-PCR using primers flanking exon 16 revealed, besides the expected PCR product from the wild-type allele, a smaller fragment only in the affected individual, representing part of an aberrant TECTA transcript lacking exon 16. The aberrant splicing is predicted to result in a deletion of 37 amino acids (p.S1758Y/G1759_N1795del) in alpha-tectorin. Subsequently, the same mutation was detected in two out of 36 individuals with a comparable phenotype. Owing to the position of the protein deletion just N-terminal of the zona pellucida (ZP) domain of alpha-tectorin, it is likely that the deletion of 37 amino acids may affect the proteolytic processing, structure and/or function of this domain, which results in a clinical phenotype comparable to that of missense mutations in the ZP domain. In addition, this is the first report of a synonymous mutation that affects an ESE and causes hereditary hearing loss.

Spatial and temporal expression pattern of a novel gene in the frog Xenopus laevis: correlations with adult intestinal epithelial differentiation during metamorphosis

We report the cloning of a novel gene (ID14) and its expression pattern in tadpoles and adults of Xenopus laevis. ID14 encodes a 315-amino acid protein that has a signal peptide and a nidogen domain. Even though several genes have a nidogen domain, ID14 is not the homolog of any known gene. ID14 is a late thyroid hormone (TH)-regulated gene in the tadpole intestine, and its expression in the intestine does not begin until the climax of metamorphosis, correlating with adult intestinal epithelial differentiation. In contrast, ID14 is expressed in tadpole skin and tail and is not regulated by TH. In situ hybridization revealed that this putative extracellular matrix protein is expressed in the epithelia of the tadpole skin and tail and in the intestinal epithelium after metamorphosis. In the adult, ID14 is found predominantly in the intestine with weak expression in the stomach, lung, and testis. Its exclusive expression in the adult intestinal epithelial cells makes it a useful marker for developmental studies and may give insights into cell/cell interactions in intestinal metamorphosis and adult intestinal stem cell maintenance.

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.

Thyroid hormone-deficient period prior to the onset of hearing is associated with reduced levels of beta-tectorin protein in the tectorial membrane: implication for hearing loss

The genes for alpha- and beta-tectorin encode the major non-collagenous proteins of the tectorial membrane. Recently, a targeted deletion of the mouse alpha-tectorin gene was found to cause loss of cochlear sensitivity (). Here we describe that mRNA levels for beta-tectorin, but not alpha-tectorin, are significantly reduced in the cochlear epithelium under constant hypothyroid conditions and that levels of beta-tectorin protein in the tectorial membrane are lower. A delay in the onset of thyroid hormone supply prior to onset of hearing, recently described to result in permanent hearing defects and loss of active cochlear mechanics (), can also lead to permanently reduced beta-tectorin protein levels in the tectorial membrane. beta-Tectorin protein levels remain low in the tectorial membrane up to one year after the onset of thyroid hormone supply has been delayed until postnatal day 8 or later and are associated with an abnormally structured tectorial membrane and the loss of active cochlear function. These data indicate that a simple delay in thyroid hormone supply during a critical period of development can lead to low beta-tectorin levels in the tectorial membrane and suggest for the first time that beta-tectorin may be required for development of normal hearing.

Spatiotemporal expression of otogelin in the developing and adult mouse inner ear

Using a PCR-based subtractive method on cDNA from 2-day-old mouse cochlea, we identified a gene encoding otogelin, Otog, an inner ear specific glycoprotein expressed in all acellular structures. Here, we provide evidence that otogelin is detected as early as embryonic day 10 in the otic vesicle. At this stage, otogelin is detected in the epithelial cells which do not overlap with the myosin VIIA-expressing cells, namely the precursors of the hair cells, thus arguing for an early commitment of the two cell populations. Analysis of otogelin spatiotemporal cell distribution allows a molecular tracing for the contribution of the cochlear and vestibular inner ear supporting cells to the formation of the acellular structures. Throughout embryonic and adult life, the expression of the otogelin gene as monitored by LacZ inserted into Otog, and the abundance of the protein are greater in the vestibule than in the cochlea. In adult, otogelin is still produced by the vestibular supporting cells, which argues for a continuous process of otogelin renewal in the otoconial membranes and cupulae. In contrast, in the tectorial membrane, otogelin should be a long-lasting protein since both the otogelin gene and protein were almost undetectable in adult cochlear cells. The data are consistent with the requirement for otogelin in the attachment of the otoconial membranes and cupulae to their corresponding sensory epithelia as revealed in Otog -/- mice.

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.

The major chicken egg envelope protein ZP1 is different from ZPB and is synthesized in the liver

The extracellular matrix surrounding vertebrate oocytes is called the zona pellucida in mammals and perivitelline membrane (pvm) in birds. We have analyzed this structure in chicken follicles and laid eggs and have identified a 95-kDa component of the pvm, which, by protein sequencing, shows homology to mammalian zona pellucida proteins. Surprisingly, we could not detect this protein in ovarian granulosa cells or oocytes but instead found high levels in the liver of the laying hen. In contrast, it is absent in rooster liver but can be efficiently induced by estrogen treatment of the animal. An immunoscreen of a liver lambda-ZAP library yielded a cDNA coding for a protein of 934 amino acids. It displayed significant homology to members of the ZP1/ZPB family from other species, notably to mouse and rat ZP1, and was therefore designated chkZP1. It is clearly different from a protein designated chkZPB that had been deposited in the data base previously. Alignment of the known members of the ZP1/ZPB family demonstrated the existence of at least three subgroups, with representatives of both the ZP1 and the ZPB sequence homology group occurring in vertebrates. Northern blot analysis of liver extracts revealed the presence of a single 3. 2-kilobase mRNA coding for chkZP1, distinct from the chkZPB transcript detectable in follicles. Immunohistochemical analysis of follicle sections demonstrates that chkZP1 can be found in the blood vessels of the theca cell layer as well as in the pvm surrounding the oocyte. Thus, in the chicken, at least one of the major pvm components is synthesized in the liver and is transported via the bloodstream to the follicle.

The supporting-cell antigen: a receptor-like protein tyrosine phosphatase expressed in the sensory epithelia of the avian inner ear

After noise- or drug-induced hair-cell loss, the sensory epithelia of the avian inner ear can regenerate new hair cells. Few molecular markers are available for the supporting-cell precursors of the hair cells that regenerate, and little is known about the signaling mechanisms underlying this regenerative response. Hybridoma methodology was used to obtain a monoclonal antibody (mAb) that stains the apical surface of supporting cells in the sensory epithelia of the inner ear. The mAb recognizes the supporting-cell antigen (SCA), a protein that is also found on the apical surfaces of retinal Müller cells, renal tubule cells, and intestinal brush border cells. Expression screening and molecular cloning reveal that the SCA is a novel receptor-like protein tyrosine phosphatase (RPTP), sharing similarity with human density-enhanced phosphatase, an RPTP thought to have a role in the density-dependent arrest of cell growth. In response to hair-cell damage induced by noise in vivo or hair-cell loss caused by ototoxic drug treatment in vitro, some supporting cells show a dramatic decrease in SCA expression levels on their apical surface. This decrease occurs before supporting cells are known to first enter S-phase after trauma, indicating that it may be a primary rather than a secondary response to injury. These results indicate that the SCA is a signaling molecule that may influence the potential of nonsensory supporting cells to either proliferate or differentiate into hair cells.