Identification and analysis of genes from the mouse otic vesicle and their association with developmental subprocesses through in situ hybridization

Pearls of Temporal Bone Imaging in Children with Hearing Loss

Hearing loss is one of the most common indications for temporal bone imaging in children. Hearing loss may be congenital or acquired, and it may be conductive, sensorineural, or mixed audiologically. Temporal bone imaging plays an important role in the assessment and management of this condition. An understanding of the embryology of ear structures better enables the radiologist to interpret abnormalities on imaging of the temporal bone. Here, we provide a general review of ear development and a description of known genetic defects that contribute to congenital ear anomalies associated with hearing loss. We provide appropriate imaging techniques for the temporal bone depending on the clinical presentation and a systematic approach to imaging for children with hearing loss. Diagnostic imaging for developmental anomalies of the ear and cholesteatoma will be discussed.

Gene expression profiling of the inner ear

The identification of transcriptional differences has served as an important starting point in understanding the molecular mechanisms behind biological processes and systems. The developmental biology of the inner ear, the biology of hearing and of course the pathology of deafness are all processes that warrant a molecular description if we are to improve human health. To this end, technological innovation has meant that larger scale analysis of gene transcription has been possible for a number of years now, extending our molecular analysis of genes to beyond those that are currently in vogue for a given system. In this review, some of the contributions gene profiling has made to understanding developmental, pathological and physiological processes in the inner ear are highlighted.

SoxC transcription factors are essential for the development of the inner ear

Hair cells, the mechanosensory receptors of the inner ear, underlie the senses of hearing and balance. Adult mammals cannot adequately replenish lost hair cells, whose loss often results in deafness or balance disorders. To determine the molecular basis of this deficiency, we investigated the development of a murine vestibular organ, the utricle. Here we show that two members of the SoxC family of transcription factors, Sox4 and Sox11, are down-regulated after the epoch of hair cell development. Conditional ablation of SoxC genes in vivo results in stunted sensory organs of the inner ear and loss of hair cells. Enhanced expression of SoxC genes in vitro conversely restores supporting cell proliferation and the production of new hair cells in adult sensory epithelia. These results imply that SoxC genes govern hair cell production and thus advance these genes as targets for the restoration of hearing and balance.

Identification and characterization of mouse otic sensory lineage genes

Vertebrate embryogenesis gives rise to all cell types of an organism through the development of many unique lineages derived from the three primordial germ layers. The otic sensory lineage arises from the otic vesicle, a structure formed through invagination of placodal non-neural ectoderm. This developmental lineage possesses unique differentiation potential, giving rise to otic sensory cell populations including hair cells, supporting cells, and ganglion neurons of the auditory and vestibular organs. Here we present a systematic approach to identify transcriptional features that distinguish the otic sensory lineage (from early otic progenitors to otic sensory populations) from other major lineages of vertebrate development. We used a microarray approach to analyze otic sensory lineage populations including microdissected otic vesicles (embryonic day 10.5) as well as isolated neonatal cochlear hair cells and supporting cells at postnatal day 3. Non-otic tissue samples including periotic tissues and whole embryos with otic regions removed were used as reference populations to evaluate otic specificity. Otic populations shared transcriptome-wide correlations in expression profiles that distinguish members of this lineage from non-otic populations. We further analyzed the microarray data using comparative and dimension reduction methods to identify individual genes that are specifically expressed in the otic sensory lineage. This analysis identified and ranked top otic sensory lineage-specific transcripts including Fbxo2, Col9a2, and Oc90, and additional novel otic lineage markers. To validate these results we performed expression analysis on select genes using immunohistochemistry and in situ hybridization. Fbxo2 showed the most striking pattern of specificity to the otic sensory lineage, including robust expression in the early otic vesicle and sustained expression in prosensory progenitors and auditory and vestibular hair cells and supporting cells.

Establishment of mice expressing EGFP in the placode-derived inner ear sensory cell lineage and FACS-array analysis focused on the regional specificity of the otocyst

In this study, we established a novel enhanced green fluorescent protein (EGFP) reporter mouse line that enables the visualization of the placode-derived inner ear sensory cell lineage. EGFP was initially expressed in the otic placode and throughout its differentiation process into the inner ear sensory patches. At embryonic day 10.5 (E10.5), EGFP was expressed in the ventral and dorsomedial region of the otocyst. These regions could mainly give rise to the cochlea, including the organ of Corti, and the saccule, including the macula and the endolymphatic duct. The region could also give rise to cells that will develop as either prosensory cells or statoacoustic ganglion neuroblasts. By using this line and fluorescence-activated cell sorting (FACS)-array technology, we developed a new gene expression profile of the regional specificity of the otocyst. EGFP-positive regions include the Otx1-positive region, which could be clearly distinguished from EGFP-negative regions. The signal log ratio of microarray data showed high efficiency in predicting the genes expressed mainly in the ventral and/or dorsomedial otocyst and the data could be mined to uncover many novel genes involved in inner ear morphogenesis and cell fate regulation. Additionally, these data suggest that some novel genes enriched in EGFP-positive regions may be potentially involved in human congenital sensorineural hearing loss. This reporter line could play important roles in the use of animal models for detailed analysis of the differentiation process into the sensory patches and the identification of regional-specific gene networks and novel gene functions in the developing inner ear.

Serial analysis of gene expression in the chicken otocyst

The inner ear arises from multipotent placodal precursors that are gradually committed to the otic fate and further differentiate into all inner ear cell types, with the exception of a few immigrating neural crest-derived cells. The otocyst plays a pivotal role during inner ear development: otic progenitor cells sub-compartmentalize into non-sensory and prosensory domains, giving rise to individual vestibular and auditory organs and their associated ganglia. The genes and pathways underlying this progressive subdivision and differentiation process are not entirely known. The goal of this study was to identify a comprehensive set of genes expressed in the chicken otocyst using the serial analysis of gene expression (SAGE) method. Our analysis revealed several hundred transcriptional regulators, potential signaling proteins, and receptors. We identified a substantial collection of genes that were previously known in the context of inner ear development, but we also found many new candidate genes, such as SOX4, SOX5, SOX7, SOX8, SOX11, and SOX18, which previously were not known to be expressed in the developing inner ear. Despite its limitation of not being all-inclusive, the generated otocyst SAGE library is a practical bioinformatics tool to study otocyst gene expression and to identify candidate genes for developmental studies.

DNA repair in mammalian embryos

Mammalian cells have developed complex mechanisms to identify DNA damage and activate the required response to maintain genome integrity. Those mechanisms include DNA damage detection, DNA repair, cell cycle arrest and apoptosis which operate together to protect the conceptus from DNA damage originating either in parental gametes or in the embryo's somatic cells. DNA repair in the newly fertilized preimplantation embryo is believed to rely entirely on the oocyte's machinery (mRNAs and proteins deposited and stored prior to ovulation). DNA repair genes have been shown to be expressed in the early stages of mammalian development. The survival of the embryo necessitates that the oocyte be sufficiently equipped with maternal stored products and that embryonic gene expression commences at the correct time. A Medline based literature search was performed using the keywords 'DNA repair' and 'embryo development' or 'gametogenesis' (publication dates between 1995 and 2006). Mammalian studies which investigated gene expression were selected. Further articles were acquired from the citations in the articles obtained from the preliminary Medline search. This paper reviews mammalian DNA repair from gametogenesis to preimplantation embryos to late gestational stages.

Conditional and inducible gene recombineering in the mouse inner ear

Genetically engineered mice have greatly improved our understanding of gene functions and disease mechanisms. Nevertheless, the traditional knock-out approach has limitations in the overall viability of mutants. The application of the Cre/loxP system in the inner ear can help bypass this difficulty by generation of conditional gene recombineering. However, to do so requires an expression system that allows ear-specific temporally inducible, gene abrogation of one or more of the increasingly available floxed genes. To date, three approaches have been successfully used to create murine inner ear-specific Cre lines: conventional transgenesis, BAC transgenesis, and gene knock-in. Unfortunately, timing of conditional Cre activity does not extend beyond the regulatory range of the gene controlling Cre expression. Rectification of this problem requires the generation of tamoxifen or tetracycline inducible systems in the inner ear. Examination of integrase expression at different loci will facilitate studies on the expression of exogenous transgenes. These genetic applications for the mouse genome will dramatically advance in vivo gene function studies.

FGF signalling controls expression of vomeronasal receptors during embryogenesis

Fibroblast growth factors (FGFs) have been shown to control formation and differentiation of multiple organ systems in the developing vertebrate embryo. The analysis of differential gene expression during embryogenesis is, therefore, a potent tool to identify novel target genes regulated by FGF signalling. Here, we have applied microarray analysis to identify differentially regulated genes in FGF mutant mouse embryos. Surprisingly, transcripts corresponding to vomeronasal receptors (VRs), which so far have been only detected in the vomeronasal organ (VNO), were found to be downregulated in FGF mutant embryos. VR expression was detected in the developing olfactory pit and the anlage of the VNO. Interestingly, several FGFs can be detected in the developing olfactory pit during mouse embryogenesis [Bachler, M., Neubuser, A. 2001. Expression of members of the Fgf family and their receptors during midfacial development. Mech. Dev. 100, 313-316]. FGF signalling may thus control expression of VRs in the olfactory pit and formation of the VNO. Moreover, VR expression was detected in unexpected locations within the developing embryo including retina, dorsal root ganglia and neural tube. The relevance of VR expression in these structures and for formation of the VNO is discussed.

Keynote review: The auditory system, hearing loss and potential targets for drug development

There is a huge potential market for the treatment of hearing loss. Drugs are already available to ameliorate predictable, damaging effects of excessive noise and ototoxic drugs. The biggest challenge now is to develop drug-based treatments for regeneration of sensory cells following noise-induced and age-related hearing loss. This requires careful consideration of the physiological mechanisms of hearing loss and identification of key cellular and molecular targets. There are many molecular cues for the discovery of suitable drug targets and a full range of experimental resources are available for initial screening through to functional analysis in vivo. There is now an unparalleled opportunity for translational research.