Development of the zebrafish inner ear

Physiological recordings from zebrafish lateral-line hair cells and afferent neurons

Sensory signal transduction, the process by which the features of external stimuli are encoded into action potentials, is a complex process that is not fully understood. In fish and amphibia, the lateral-line organ detects water movement and vibration and is critical for schooling behavior and the detection of predators and prey. The lateral-line system in zebrafish serves as an ideal platform to examine encoding of stimuli by sensory hair cells. Here, we describe methods for recording hair-cell microphonics and activity of afferent neurons using intact zebrafish larvae. The recordings are performed by immobilizing and mounting larvae for optimal stimulation of lateral-line hair cells. Hair cells are stimulated with a pressure-controlled water jet and a recording electrode is positioned next to the site of mechanotransduction in order to record microphonics--extracellular voltage changes due to currents through hair-cell mechanotransduction channels. Another readout of the hair-cell activity is obtained by recording action currents from single afferent neurons in response to water-jet stimulation of innervated hair cells. When combined, these techniques make it possible to probe the function of the lateral-line sensory system in an intact zebrafish using controlled, repeatable, physiological stimuli.

Induction of otic structures by canonical Wnt signalling in medaka

The Wnt family of signalling proteins is known to participate in multiple developmental decisions during embryogenesis. We misexpressed Wnt1 in medaka embryos and observed anterior truncations, similar to those described for ectopic activation of canonical Wnt signalling in other species. Interestingly, when we induced a heat-shock Wnt1 transgenic line exactly at 30% epiboly, we observed multiple ectopic otic vesicles in the truncated embryos. The vesicles then fused, forming a single large ear structure. These "cyclopic ears" filled the complete anterior region of the embryos. The ectopic induction of otic development can be explained by the juxtaposition of hindbrain tissue with anterior ectoderm. Fibroblast growth factor (Fgf) ligands are thought to mediate the otic-inducing properties of the hindbrain. However, signals different from Fgf3 and Fgf8 are necessary to explain the formation of the ectopic ear structures, suggesting that Wnt signalling is involved in the otic induction process in medaka.

Proteomic and functional analysis of NCS-1 binding proteins reveals novel signaling pathways required for inner ear development in zebrafish

BACKGROUND

The semicircular canals, a subdivision of the vestibular system of the vertebrate inner ear, function as sensors of angular acceleration. Little is currently known, however, regarding the underlying molecular mechanisms that govern the development of this intricate structure. Zebrafish represent a particularly tractable model system for the study of inner ear development. This is because the ear can be easily visualized during early embryogenesis, and both forward and reverse genetic techniques are available that can be applied to the discovery of novel genes that contribute to proper ear development. We have previously shown that in zebrafish, the calcium sensing molecule neuronal calcium sensor-1 (NCS-1) is required for semicircular canal formation. The function of NCS-1 in regulating semicircular canal formation has not yet been elucidated.

RESULTS

We initiated a multistep functional proteomic strategy to identify neuronal calcium sensor-1 (NCS-1) binding partners (NBPs) that contribute to inner ear development in zebrafish. By performing a Y2H screen in combination with literature and database searches, we identified 10 human NBPs. BLAST searches of the zebrafish EST and genomic databases allowed us to clone zebrafish orthologs of each of the human NBPs. By investigating the expression profiles of zebrafish NBP mRNAs, we identified seven that were expressed in the developing inner ear and overlapped with the ncs-1a expression profile. GST pulldown experiments confirmed that selected NBPs interacted with NCS-1, while morpholino-mediated knockdown experiments demonstrated an essential role for arf1, pi4kbeta, dan, and pink1 in semicircular canal formation.

CONCLUSION

Based on their functional profiles, the hypothesis is presented that Ncs-1a/Pi4kbeta/Arf1 form a signaling pathway that regulates secretion of molecular components, including Dan and Bmp4, that are required for development of the vestibular apparatus. A second set of NBPs, consisting of Pink1, Hint2, and Slc25a25, are destined for localization in mitochondria. Our findings reveal a novel signalling pathway involved in development of the semicircular canal system, and suggest a previously unrecognized role for NCS-1 in mitochondrial function via its association with several mitochondrial proteins.

Plasma membrane calcium ATPase required for semicircular canal formation and otolith growth in the zebrafish inner ear

Fish otoliths consist of >90% calcium carbonate, the accretion of which depends on acellular endolymph. This study confirms the presence of plasma membrane calcium ATPase 1a isoform (Atp2b1a) in the auditory and vestibular system of a teleost fish. As shown by in situ hybridization,zebrafish atp2b1a is expressed mainly in larval otic placode and lateral-line neuromast as well as in the hair cells within the adult zebrafish inner ear chamber. Zebrafish atp2b1a knockdown by antisense morpholinos reduced the number of hair cells and produced malformation of semicircular canals and smaller otoliths. These defects coincide with unbalanced body orientation. The formation of smaller otoliths in atp2b1a morphants may stem from an impairment of calcium supply in the endolymph. However, otolith formation persists in most morphants,suggesting that other zebrafish Atp2b isoforms or paracellular pathways may also transport calcium into the endolymph. These results suggest that Atp2b1a plays an important role for normal development of the auditory and vestibular system as well as calcium transport in the inner ear of zebrafish.

Rapid identification of PAX2/5/8 direct downstream targets in the otic vesicle by combinatorial use of bioinformatics tools

A novel bioinformatics pipeline is used to discover PAX2/5/8 direct downstream targets involved in inner ear development.

Background

The pax2/5/8 genes belonging to the PAX family of transcription factors are key developmental regulators that are involved in the patterning of various embryonic tissues. More particularly, their function in inner ear specification has been widely described. However, little is known about the direct downstream targets and, so far, no global approaches have been performed to identify these target genes in this particular tissue.

Results

Here we present an original bioinformatics pipeline composed of comparative genomics, database querying and text mining tools, which is designed to rapidly and specifically discover PAX2/5/8 direct downstream targets involved in inner ear development. We provide evidence supported by experimental validation in medaka fish that brain 2 (POU domain, class 3, transcription factor 2), claudin-7, secretory pathway component sec31-like and meteorin-like precursor are novel direct downstream targets of PAX2/5/8.

Conclusions

This study illustrates the power of extensive mining of public data repositories using bioinformatics methods to provide answers for a specific biological question. It furthermore demonstrates how the usage of such a combinatorial approach is advantageous for the biologist in terms of experimentation time and costs.

Eya4 regulation of Na+/K+-ATPase is required for sensory system development in zebrafish

To investigate the mechanisms by which mutations in the human transcriptional co-activator EYA4 gene cause sensorineural hearing loss that can occur in association with dilated cardiomyopathy, we studied eya4 expression during zebrafish development and characterized eya4 deficiency. eya4 morphant fish embryos had reduced numbers of hair cells in the otic vesicle and lateral line neuromasts with impaired sensory responses. Analyses of candidate genes that are known to be expressed in a temporal and spatial pattern comparable to eya4 focused our analyses on atp1b2b, which encodes the beta2b subunit of the zebrafish Na+/K+-ATPase. We demonstrate atp1b2b levels are reduced in eya4 morphant fish and that morpholino oligonucleotides targeting the atp1b2b gene recapitulated the eya4 deficiency phenotypes, including heart failure, decreased sensory hair cell numbers in the otic vesicle and neuromasts, and abnormal sensory responses. Furthermore, atp1b2b overexpression rescued these phenotypes in eya4 morphant fish. We conclude that eya4 regulation of Na+/K+-ATPase is crucial for the development of mechanosensory cells and the maintenance of cardiac function in zebrafish.

Sparc protein is required for normal growth of zebrafish otoliths

Otoliths and the homologous otoconia in the inner ear are essential for balance. Their morphogenesis is less understood than that of other biominerals, such as bone, and only a small number of their constituent proteins have been characterized. As a novel approach to identify unknown otolith proteins, we employed shotgun proteomics to analyze crude extracts from trout and catfish otoliths. We found three proteins that had not been associated previously with otolith or otoconia formation: 'Secreted acidic cysteine rich glycoprotein' (Sparc), an important bone protein that binds collagen and Ca(2+); precerebellin-like protein, which contains a C1q domain and may associate with the collagenous otolin-1 during its assembly into a framework; and neuroserpin, a serine protease inhibitor that may regulate local protease activity during framework assembly. We then used the zebrafish to investigate whether Sparc plays a role in otolith morphogenesis. Immunodetection demonstrated that Sparc is a true constituent of otoliths. Knockdown of Sparc expression in morphant zebrafish resulted in four principal types of defective otoliths: smaller, extra and ectopic, missing and fused, or completely absent. Smaller size was the predominant phenotype and independent of the severity of otic-vesicle defects. These results suggested that Sparc is directly required for normal otolith growth.

Semicircular canal size determines the developmental onset of angular vestibuloocular reflexes in larval Xenopus

Semicircular canals have been sensors of angular acceleration for 450 million years. This vertebrate adaptation enhances survival by implementing postural and visual stabilization during motion in a three-dimensional environment. We used an integrated neuroethological approach in larval Xenopus to demonstrate that semicircular canal dimensions, and not the function of other elements, determines the onset of angular acceleration detection. Before angular vestibuloocular function in either the vertical or horizontal planes, at stages 47 and 48, respectively, each individual component of the vestibuloocular system was shown to be operational: extraocular muscles could be activated, central neural pathways were complete, and canal hair cells were capable of evoking graded responses. For Xenopus, a minimum semicircular canal lumen radius of 60 microm was necessary to permit endolymph displacement sufficient for sensor function at peak accelerations of 400 degrees /s(2). An intra-animal comparison demonstrated that this size is reached in the vertical canals earlier in development than in the horizontal canals, corresponding to the earlier onset of vertical canal-activated ocular motor behavior. Because size constitutes a biophysical threshold for canal-evoked behavior in other vertebrates, such as zebrafish, we suggest that the semicircular canal lumen and canal circuit radius are limiting the onset of vestibular function in all small vertebrates. Given that the onset of gravitoinertial acceleration detection precedes angular acceleration detection by up to 10 d in Xenopus, these results question how the known precise spatial patterning of utricular and canal afferents in adults is achieved during development.

The transmembrane inner ear (tmie) gene contributes to vestibular and lateral line development and function in the zebrafish (Danio rerio)

The inner ear is a complex organ containing sensory tissue, including hair cells, the development of which is not well understood. Our long-term goal is to discover genes critical for the correct formation and function of the inner ear and its sensory tissue. A novel gene, transmembrane inner ear (Tmie), was found to cause hearing-related disorders when defective in mice and humans. A homologous tmie gene in zebrafish was cloned and its expression characterized between 24 and 51 hours post-fertilization. Embryos injected with morpholinos (MO) directed against tmie exhibited circling swimming behavior (approximately 37%), phenocopying mice with Tmie mutations; semicircular canal formation was disrupted, hair cell numbers were reduced, and maturation of electrically active lateral line neuromasts was delayed. As in the mouse, tmie appears to be required for inner ear development and function in the zebrafish and for hair cell maturation in the vestibular and lateral line systems as well.

Identification of a novel matrix protein contained in a protein aggregate associated with collagen in fish otoliths

In the biomineralization processes, proteins are thought to control the polymorphism and morphology of the crystals by forming complexes of structural and mineral-associated proteins. To identify such proteins, we have searched for proteins that may form high-molecular-weight (HMW) aggregates in the matrix of fish otoliths that have aragonite and vaterite as their crystal polymorphs. By screening a cDNA library of the trout inner ear using an antiserum raised against whole otolith matrix, a novel protein, named otolith matrix macromolecule-64 (OMM-64), was identified. The protein was found to have a molecular mass of 64 kDa, and to contain two tandem repeats and a Glu-rich region. The structure of the protein and that of its DNA are similar to those of starmaker, a protein involved in the polymorphism control in the zebrafish otoliths [Söllner C, Burghammer M, Busch-Nentwich E, Berger J, Schwarz H, Riekel C & Nicolson T (2003) Science302, 282-286]. (45)Ca overlay analysis revealed that the Glu-rich region has calcium-binding activity. Combined analysis by western blotting and deglycosylation suggested that OMM-64 is present in an HMW aggregate with heparan sulfate chains. Histological observations revealed that OMM-64 is expressed specifically in otolith matrix-producing cells and deposited onto the otolith. Moreover, the HMW aggregate binds to the inner ear-specific short-chain collagen otolin-1, and the resulting complex forms ring-like structures in the otolith matrix. Overall, OMM-64, by forming a calcium-binding aggregate that binds to otolin-1 and forming matrix protein architectures, may be involved in the control of crystal morphology during otolith biomineralization.

Expression of zebrafish aldh1a3 (raldh3) and absence of aldh1a1 in teleosts

The vitamin A-derived morphogen retinoic acid (RA) plays important roles during the development of chordate animals. The Aldh1a-family of RA-synthesizing enzymes consists of three members, Aldh1a1-3 (Raldh1-3), that are dynamically expressed throughout development. We have searched the known teleost genomes for the presence of Raldh family members and have found that teleost fish possess orthologs of Aldh1a2 and Aldh1a3 only. Here we describe the expression of aldh1a3 in the zebrafish, Danio rerio. Whole mount in situ hybridization shows that aldh1a3 is expressed during eye development in the retina flanking the optic stalks and later is expressed ventrally, opposite the expression domain of aldh1a2. During inner ear morphogenesis, aldh1a3 is expressed in developing sensory epithelia of the cristae and utricular macula and is specifically up-regulated in epithelial projections throughout the formation of the walls of the semicircular canals and endolymphatic duct. In contrast to the mouse inner ear, which expresses all three Raldhs, we find that only aldh1a3 is expressed in the zebrafish otocyst, while aldh1a2 is present in the periotic mesenchyme. During larval stages, additional expression domains of aldh1a3 appear in the anterior pituitary and the swim bladder. Our analyses provide a starting point for genetic studies to examine the role of RA in these organs and emphasize the suitability of the zebrafish inner ear in dissecting the contribution of RA signaling to the development of the vestibular system.

Expression of marker genes during otolith development in medaka

Little is known about the genes and processes involved in the development of otoliths. In this study, we isolated the biomineralization-related genes otolin and chondromodulin-1 (chm1) from medaka, and examined their spatiotemporal expression pattern as well as that of two other genes also related to biomineralization, i.e., sparc/osteonectin and type II collagen (col2a), during otic development in medaka. Our results demonstrated that all the tested genes were expressed in the otic vesicle, and that chm1 was exclusively expressed in the semicircular canal of the otic vesicle.

Role of Bone morphogenetic protein 4 in zebrafish semicircular canal development

Bone morphogenetic proteins (BMPs) are known to play roles in inner ear development of higher vertebrates. In zebrafish, there are several reports showing that members of the BMP family are expressed in the otic vesicle. We have isolated a novel zebrafish mutant gallery, which affects the development of the semicircular canal. Gallery merely forms the lateral and the immature anterior protrusion, and does not form posterior and ventral protrusions. We found that the expression of bmp2b and bmp4, both expressed in the normal optic vesicle at the protrusion stage, are extremely upregulated in the otic vesicle of gallery. To elucidate the role of BMPs in the development of the inner ear of zebrafish, we have applied excess BMP to the wild-type otic vesicle. The formation of protrusions was severely affected, and in some cases, they were completely lost in BMP4-treated embryos. Furthermore, the protrusions in gallery treated with Noggin were partially rescued. These data indicate that BMP4 plays an important role in the development of protrusions to form semicircular canals.

Changes in retinoic acid signaling alter otic patterning

Retinoic acid (RA) has pleiotropic functions during embryogenesis. In zebrafish, increasing or blocking RA signaling results in enlarged or reduced otic vesicles, respectively. Here we elucidate the mechanisms that underlie these changes and show that they have origins in different tissues. Excess RA leads to ectopic foxi1 expression throughout the entire preplacodal domain. Foxi1 provides competence to adopt an otic fate. Subsequently, pax8, the expression of which depends upon Foxi1 and Fgf, is also expressed throughout the preplacodal domain. By contrast, loss of RA signaling does not affect foxi1 expression or otic competence, but instead results in delayed onset of fgf3 expression and impaired otic induction. fgf8 mutants depleted of RA signaling produce few otic cells, and these cells fail to form a vesicle, indicating that Fgf8 is the primary factor responsible for otic induction in RA-depleted embryos. Otic induction is rescued by fgf8 overexpression in RA-depleted embryos, although otic vesicles never achieve a normal size, suggesting that an additional factor is required to maintain otic fate. fgf3;tcf2 double mutants form otic vesicles similar to RA-signaling-depleted embryos, suggesting a signal from rhombomere 5-6 may also be required for otic fate maintenance. We show that rhombomere 5 wnt8b expression is absent in both RA-signaling-depleted embryos and in fgf3;tcf2 double mutants, and inactivation of wnt8b in fgf3 mutants by morpholino injection results in small otic vesicles, similar to RA depletion in wild type. Thus, excess RA expands otic competence, whereas the loss of RA impairs the expression of fgf3 and wnt8b in the hindbrain, compromising the induction and maintenance of otic fate.

Distinct roles of Fgf8, Foxi1, Dlx3b and Pax8/2 during otic vesicle induction and maintenance in medaka

The development of the vertebrate inner ear is a complex process that has been investigated in several model organisms. In this work, we examined genetic interactions regulating early development of otic structures in medaka. We demonstrate that misexpression of Fgf8, Dlx3b and Foxi1 during early gastrulation is sufficient to produce ectopic otic vesicles. Combined misexpression strongly increases the appearance of this phenotype. By using a heat-inducible promoter we were furthermore able to separate the regulatory interactions among Fgf8, Foxi1, Dlx3b, Pax8 and Pax2 genes, which are active during different stages of early otic development. In the preplacodal stage we suggest a central position of Foxi1 within a regulatory network of early patterning genes including Dlx3b and Pax8. Different pathways are active after the placodal stage with Dlx3b playing a central role. There Dlx3b regulates members of the Pax-Six-Eya-Dach network and also strongly affects the early dorsoventral marker genes Otx1 and Gbx2.

Fgf-dependent otic induction requires competence provided by Foxi1 and Dlx3b

Background

The inner ear arises from a specialized set of cells, the otic placode, that forms at the lateral edge of the neural plate adjacent to the hindbrain. Previous studies indicated that fibroblast growth factors (Fgfs) are required for otic induction; in zebrafish, loss of both Fgf3 and Fgf8 results in total ablation of otic tissue. Furthermore, gain-of-function studies suggested that Fgf signaling is not only necessary but also sufficient for otic induction, although the amount of induced ectopic otic tissue reported after misexpression of fgf3 or fgf8 varies among different studies. We previously suggested that Foxi1 and Dlx3b may provide competence to form the ear because loss of both foxi1 and dlx3b results in ablation of all otic tissue even in the presence of a fully functional Fgf signaling pathway.

Results

Using a transgenic line that allows us to misexpress fgf8 under the control of the zebrafish temperature-inducible hsp70 promoter, we readdressed the role of Fgf signaling and otic competence during placode induction. We find that misexpression of fgf8 fails to induce formation of ectopic otic vesicles outside of the endogenous ear field and has different consequences depending upon the developmental stage. Overexpression of fgf8 from 1-cell to midgastrula stages leads to formation of no or small otic vesicles, respectively. Overexpression of fgf8 at these stages never leads to ectopic expression of foxi1 or dlx3b, contrary to previous studies that indicated that foxi1 is activated by Fgf signaling. Consistent with our results we find that pharmacological inhibition of Fgf signaling has no effect on foxi1 or dlx3b expression, but instead, Bmp signaling activates foxi1, directly and dlx3b, indirectly. In contrast to early activation of fgf8, fgf8 overexpression at the end of gastrulation, when otic induction begins, leads to much larger otic vesicles. We further show that application of a low dose of retinoic acid that does not perturb patterning of the anterior neural plate leads to expansion of foxi1 and to a massive Fgf-dependent otic induction.

Conclusion

These results provide further support for the hypothesis that Foxi1 and Dlx3b provide competence for cells to respond to Fgf and form an otic placode.

Zebrafishatoh1genes: classic proneural activity in the inner ear and regulation by Fgf and Notch

Hair cells of the inner ear develop from an equivalence group marked by expression of the proneural gene Atoh1. In mouse, Atoh1 is necessary for hair cell differentiation, but its role in specifying the equivalence group (proneural function) has been questioned and little is known about its upstream activators. We have addressed these issues in zebrafish. Two zebrafish homologs, atoh1a and atoh1b, are together necessary for hair cell development. These genes crossregulate each other but are differentially required during distinct developmental periods, first in the preotic placode and later in the otic vesicle. Interactions with the Notch pathway confirm that atoh1 genes have early proneural function. Fgf3 and Fgf8 are upstream activators of atoh1 genes during both phases,and foxi1, pax8 and dlx genes regulate atoh1b in the preplacode. A model is presented in which zebrafish atoh1 genes operate in a complex network leading to hair cell development.

Zebrafish pax5 regulates development of the utricular macula and vestibular function

The zebrafish otic vesicle initially forms with only two sensory epithelia, the utricular and saccular maculae, which primarily mediate vestibular and auditory function, respectively. Here, we test the role of pax5, which is preferentially expressed in the utricular macula. Morpholino knockdown of pax5 disrupts vestibular function but not hearing. Neurons of the statoacoustic ganglion (SAG) develop normally. Utricular hair cells appear to form normally but a variable number subsequently undergo apoptosis and are extruded from the otic vesicle. Dendrites of the SAG persist in the utricle but become disorganized after hair cell loss. Hair cells in the saccule develop and survive normally. Otic expression of pax5 requires pax2a and fgf3, mutations in which cause vestibular defects, albeit by distinct mechanisms. Thus, pax5 works in conjunction with fgf3 and pax2a to establish and/or maintain the utricular macula and is essential for vestibular function.

Localization of anosmin-1a and anosmin-1b in the inner ear and neuromasts of zebrafish

Anosmin-1, encoded by the KAL-1 gene, is the protein defective in the X-linked form of Kallmann syndrome. This human developmental disorder is characterized by defects in cell migration and axon target selection. Anosmin-1 is an extracellular matrix protein that plays a role, in vitro, in processes such as cell adhesion, neurite outgrowth, axon guidance, and axon branching. The zebrafish possesses two orthologues of the KAL-1 gene: kal1a and kal1b, which encode anosmin-1a and anosmin-1b, respectively. Previous in situ hybridization studies have shown that kal1a and kal1b mRNAs are expressed in undetermined cells of the inner ear but not in neuromast cells. Using specific antibodies against anosmin-1a and anosmin-1b, we report here that both proteins are expressed in sensory hair cells of the inner ear cristae ampullaris and the lateral line neuromasts. Accumulation of these proteins was observed mainly at the level of the hair bundle and also at the cell membrane. In neuromast hair cells, immunogold scanning electronmicroscopy demonstrated that anosmin-1a and anosmin-1b were present at the surface of the stereociliary bundle. In addition, anosmin-1a, but not anosmin-1b, was detected on the track of the ampullary nerve. This is the first report of anosmin-1 expression in sensory hair cells of the inner ear and lateral line, and along the ampullary nerve track.

Cadherin-2 participates in the morphogenesis of the zebrafish inner ear

Molecular mechanisms that control inner ear morphogenesis from the placode to the three-dimensional functional organ are not well understood. We hypothesize that cell-cell adhesion, mediated by cadherin molecules, contributes significantly to various stages of inner ear formation. Cadherin-2 (Cdh2) function during otic vesicle morphogenesis was investigated by examining morpholino antisense oligonucleotide knockdown and glass onion (glo) (Cdh2 mutant) zebrafish embryos. Placode formation, vesicle cavitation and specification occurred normally, but morphogenesis of the otic vesicle was affected by Cdh2 deficiency: semicircular canals were reduced or absent. Phalloidin staining of the hair cell stereocillia demonstrated that cadherin-2 (cdh2) loss-of-function did not affect hair cell number, but acetylated tubulin labeling showed that hair cell kinocilia were shorter and irregularly shaped. Statoacoustic ganglion size was significantly reduced, which suggested that neuron differentiation or maturation was affected. Furthermore, cdh2 loss-of-function did not cause a general developmental delay, since differentiation of other tissues, including eye, proceeded normally. These findings demonstrate that Cdh2 selectively affects epithelial morphogenetic cell movements, particularly semicircular canal formation, during normal ear mophogenesis.

The transcription factor six1 inhibits neuronal and promotes hair cell fate in the developing zebrafish (Danio rerio) inner ear

The developmental processes leading to the differentiation of mechanosensory hair cells and statoacoustic ganglion neurons from the early otic epithelium remain unclear. Possible candidates include members of the Pax-Six-Eya-Dach (paired box-sine oculis homeobox-eyes absent-dachshund) gene regulatory network. We cloned zebrafish six1 and studied its function in inner ear development. Gain- and loss-of-function experiments show that six1 has opposing roles in hair cell and neuronal lineages. It promotes hair cell fate and, conversely, inhibits neuronal fate by differentially affecting cell proliferation and cell death in these lineages. By independently targeting hair cells with atoh1a (atonal homolog 1a) knockdown or neurons with neurog1 (neurogenin 1) knockdown, we showed that the remaining cell population, neurons or hair cells, respectively, is still affected by gain or loss of six1 function. six1 interacts with other members of the Pax-Six-Eya-Dach regulatory network, in particular dacha and dachb in the hair cell but not neuronal lineage. Unlike in mouse, six1 does not appear to be dependent on eya1, although it seems to be important for the regulation of eya1 and pax2b expression in the ventral otic epithelium. Furthermore, six1 expression appears to be regulated by pax2b and also by foxi1 (forkhead box I1) as expected for an early inducer of the otic placode. Our results are the first to demonstrate a dual role for a member of the Pax-Six-Eya-Dach regulatory network in inner ear development.

Role of the hindbrain in patterning the otic vesicle: a study of the zebrafish vhnf1 mutant

The vertebrate inner ear develops from an ectodermal placode adjacent to rhombomeres 4 to 6 of the segmented hindbrain. The placode then transforms into a vesicle and becomes regionalised along its anteroposterior, dorsoventral and mediolateral axes. To investigate the role of hindbrain signals in instructing otic vesicle regionalisation, we analysed ear development in zebrafish mutants for vhnf1, a gene expressed in the caudal hindbrain during otic induction and regionalisation. We show that, in vhnf1 homozygous embryos, the patterning of the otic vesicle is affected along both the anteroposterior and dorsoventral axes. First, anterior gene expression domains are either expanded along the whole anteroposterior axis of the vesicle or duplicated in the posterior region. Second, the dorsal domain is severely reduced, and cell groups normally located ventrally are shifted dorsally, sometimes forming a single dorsal patch along the whole AP extent of the otic vesicle. Third, and probably as a consequence, the size and organization of the sensory and neurogenic epithelia are disturbed. These results demonstrate that, in zebrafish, signals from the hindbrain control the patterning of the otic vesicle, not only along the anteroposterior axis, but also, as in amniotes, along the dorsoventral axis. They suggest that, despite the evolution of inner ear structure and function, some of the mechanisms underlying the regionalisation of the otic vesicle in fish and amniotes have been conserved.

Expression of marker genes during early ear development in medaka

Induction of the otic placode involves a number of regulatory interactions. Early studies revealed that the induction of this program is initiated by instructive signals from the mesendoderm as well as from the adjacent hindbrain. Further investigations on the molecular level identified in zebrafish Fgf3, Fgf8, Foxi1, Pax8, Dlx3b and Dlx4b genes as key players during the induction phase. Thereafter an increasing number of genes participates in the regulatory interactions finally resulting in a highly structured sensory organ. Based on data from zebrafish we selected medaka genes with presumptive functions during early ear development for an expression analysis. In addition we isolated Foxi1 and Dlx3b gene fragments from embryonic cDNA. Altogether we screened the spatio-temporal distribution of more than 20 representative marker genes for otic development in medaka embryos, with special emphasis on the early phases. Whereas the spatial distribution of these genes is largely conserved between medaka and zebrafish, our comparative analysis revealed several differences, in particular for the timing of expression.

Mutation of the atrophin2 gene in the zebrafish disrupts signaling by fibroblast growth factor during development of the inner ear

The development of the vertebrate inner ear depends on the precise expression of fibroblast growth factors. In a mutagenesis screen for zebrafish with abnormalities of inner-ear development and behavior, we isolated a mutant line, ru622, whose phenotypic characteristics resembled those of null mutants for the gene encoding fibroblast growth factor 8 (Fgf8): an inconsistent startle response, circular swimming, fused otoliths, and abnormal semicircular canals. Positional cloning disclosed that the mutant gene encodes the transcriptional corepressor Atrophin2. Both the Fgf8 protein and zebrafish "similar expression to fgf genes" protein (Sef), an antagonist of fibroblast growth factors induced by Fgf8 itself, were found to be overexpressed in ru622 mutants. We therefore hypothesized that an excess of Sef eliminates Fgf8 signals and produces an fgf8 null phenotype in ru622 mutants. In support of this idea, we could rescue larvae whose atrophin2 expression had been diminished with morpholinos by reducing the expression of Sef as well. We propose that Atrophin2 plays a role in the feedback regulation of Fgf8 signaling. When mutation of the atrophin2 gene results in the overexpression of both Fgf8 and Sef, the excessive Sef inhibits Fgf8 signaling. The resultant imbalance of Fgf8 and Sef signals then underlies the abnormal aural development observed in ru622.

Genetics of graviperception in animals

Gravity is a constant stimulus for life on Earth and most organisms have evolved structures to sense gravitational force and incorporate its influence into their behavioral repertoire. Here we focus attention on animals and their diverse structures for perceiving and responding to the gravitational vector-one of the few static reference stimuli for any mobile organism. We discuss vertebrate, arthropod, and nematode models from the perspective of the role that genetics is playing in our understanding of graviperception in each system. We describe the key sensory structures in each class of organism and present what is known about the genetic control of development of these structures and the molecular signaling pathways operating in the mature organs. We also discuss the role of large genetic screens in identifying specific genes with roles in mechanosensation and graviperception.

Gene duplication and functional divergence of the zebrafish insulin-like growth factor 1 receptors

Insulin-like growth factor (IGF) 1 receptor (IGF1R)-mediated signaling plays key roles in growth, development, and physiology. Recent studies have shown that there are two distinct ig f1r genes in zebrafish, termed ig f1ra and ig f1rb. In this study, we tested the hypothesis that zebrafish ig f1ra and ig f1rb resulted from a gene duplication event at the ig f1r locus and that this has led to their functional divergence. The genomic structures of zebrafish ig f1ra and ig f1rb were determined and their loci mapped. While zebrafish ig f1ra has 21 exons and is located on linkage group (LG) 18, zebrafish ig f1rb has 22 exons and mapped to LG 7. There is a strong syntenic relationship between the two zebrafish genes and the human IG F1R gene. Using a MO-based loss-of-function approach, we show that both Igf1ra and Igf1rb are required for zebrafish embryo viability and proper growth and development. Although Igf1ra and Igf1rb demonstrated a large degree of functional overlap with regard to cell differentiation in the developing eye, inner ear, heart, and muscle, they also exhibited functional distinction involving a greater requirement for Igf1rb in spontaneous muscle contractility. These findings suggest that the duplicated zebrafish ig f1r genes play largely overlapping but not identical functional roles in early development and provide novel insight into the functional evolution of the IGF1R/insulin receptor gene family.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Eya1 is required for lineage-specific differentiation, but not for cell survival in the zebrafish adenohypophysis

The homeodomain transcription factor Six1 and its modulator, the protein phosphatase Eya1, cooperate to promote cell differentiation and survival during mouse organ development. Here, we studied the effects caused by loss of eya1 and six1 function on pituitary development in zebrafish. eya1 and six1 are co-expressed in all adenohypophyseal cells. Nevertheless, eya1 (aal, dog) mutants show lineage-specific defects, defining corticotropes, melanotropes, and gonadotropes as an Eya1-dependent lineage, which is complementary to the Pit1 lineage. Furthermore, eya1 is required for maintenance of pit1 expression, leading to subsequent loss of cognate hormone gene expression in thyrotropes and somatotropes of mutant embryos, whereas prolactin expression in lactotropes persists. In contrast to other organs, adenohypophyseal cells of eya1 mutants do not become apoptotic, and the adenohypophysis remains at rather normal size. Also, cells do not trans-differentiate, as in the case of pit1 mutants, but display morphological features characteristic for nonsecretory cells. Some of the adenohypophyseal defects of eya1 mutants are moderately enhanced in combination with antisense-mediated loss of Six1 function, which per se does not affect pituitary cell differentiation. In conclusion, this is the first report of an essential role of Eya1 during pituitary development in vertebrates. Eya1 is required for lineage-specific differentiation of adenohypophyseal cells, but not for their survival, thereby uncoupling the differentiation-promoting and anti-apoptotic effects of Eya proteins seen in other tissues.

Competence of cranial ectoderm to respond to Fgf signaling suggests a two-step model of otic placode induction

Vertebrate craniofacial sensory organs derive from ectodermal placodes early in development. It has been suggested that all craniofacial placodes arise from a common ectodermal domain adjacent to the anterior neural plate, and a number of genes have been recently identified that mark such a 'pre-placodal' domain. However, the functional significance of this pre-placodal domain is still unclear. In the present study, we show that Fgf signaling is necessary and sufficient to directly induce some, but not all, markers of the otic placode in ectoderm taken from the pre-placodal domain. By contrast, ectoderm from outside this domain is not competent to express otic markers in response to Fgfs. Grafting naïve ectoderm into the pre-placodal domain causes upregulation of pre-placodal markers within 8 hours, together with the acquisition of competence to respond to Fgf signaling. This suggests a two-step model of craniofacial placode induction in which ectoderm first acquires pre-placodal region identity, and subsequently differentiates into particular craniofacial placodes under the influence of local inducing signals.

Formation of the digestive system in zebrafish: III. Intestinal epithelium morphogenesis

Recent analysis of a novel strain of transgenic zebrafish (gutGFP) has provided a detailed description of the early morphological events that occur during the development of the liver and pancreas. In this paper, we aim to complement these studies by providing an analysis of the morphological events that shape the zebrafish intestinal epithelium. One of our goals is to provide a framework for the future characterization of zebrafish mutant phenotypes in which intestinal epithelial morphogenesis has been disrupted. Our analysis encompasses the period between 26 and 126 h post-fertilization (hpf) and follows the growth, lumen formation and differentiation of a continuous layer of endoderm into a functional intestinal epithelium with three morphologically distinct segments: the intestinal bulb, mid-intestine and posterior intestine. Between 26 hpf and 76 hpf, the entire intestinal endoderm is a highly proliferative organ. To make a lumen, the zebrafish endoderm cells undergo apical membrane biogenesis, adopt a bilayer configuration and form small cavities that coalesce without cell death. Thereafter, the endoderm cells polarize and differentiate into distinct cell lineages. Enteroendocrine cells are distinguished first at 52 hpf in the caudal region of the intestine in a new stable transgenic line, Tg[nkx2.2a:mEGFP]. The differentiation of mucin-containing goblet cells is first evident at 100 hpf and is tightly restricted to a middle segment of the intestine, designated the mid-intestine, that is also demarcated by the presence of enterocytes with large supranuclear vacuoles. Meanwhile, striking expansion of the lumen in the rostral intestine forms the intestinal bulb. Here the epithelium elaborates folds and proliferating cells become progressively restricted to a basal compartment analogous to the crypts of Lieberkühn in mammals. At 126 hpf, the posterior intestine remains an unfolded monolayer of simple columnar epithelium.

The Genetics of Hearing and Balance in Zebrafish

The zebrafish is an excellent model system for studying the molecular basis of inner ear development and function. The eggs develop ex utero and the ear is transparent for the first few weeks of life. Forward genetic screens and antisense technology have helped to elucidate the signaling pathways and molecules required for inner ear development and function. This review addresses the most recent advances in our understanding of how the ear forms and discusses the molecules in hair cells that are essential for sensing sound and movement in the zebrafish.

High-affinity peptide transporter PEPT2 (SLC15A2) of the zebrafish Danio rerio: functional properties, genomic organization, and expression analysis

Solute carrier 15 (SLC15) membrane proteins PEPT1 (SLC15A1) and PEPT2 (SLC15A2) have been described in great detail in mammals. In contrast, information in lower vertebrates is limited. We characterized the functional properties of a novel zebrafish peptide transporter orthologous to mammalian and avian PEPT2, described its gene (pept2) structure, and determined mRNA tissue distribution. An expressed sequence tag (EST) cDNA (Integrated Molecular Analysis of Gene Expression; IMAGE) corresponding to zebrafish pept2 was completed by inserting a stretch of 75 missing nucleotides in the coding sequence to obtain a 3,238-bp functional clone. The complete open reading frame (ORF) was 2,160 bp and encoded a 719-amino acid protein. Electrophysiological analysis after cRNA injection in Xenopus laevis oocytes suggested that zebrafish PEPT2 is a high-affinity/low-capacity transporter (K(0.5) for glycyl-L-glutamine approximately 18 microM at -120 mV and pH 7.5). Zebrafish pept2 gene was 19,435 kb, thus being the shortest vertebrate pept2 fully characterized so far. Also, zebrafish pept2 exhibited 23 exons and 22 introns, whereas human and rodent pept2 genes contain 22 exons and 21 introns only. Zebrafish pept2 mRNA was mainly detected in brain, kidney, gut, and, interestingly, otic vesicle, the embryonic structure that develops into the auditory/vestibular organ, homolog to the higher vertebrate inner ear, of the adult fish. Characterization of zebrafish pept2 will contribute to the investigation of peptide transporters using a well-established genetic model and will allow the elucidation of the evolutionary and functional relationships among vertebrate peptide transporters. Moreover, it can represent a useful marker to screen mutations that affect choroid plexus and inner ear development.

Pax2 expression patterns in the developing chick inner ear

The fate specification of the developing vertebrate inner ear could be determined by complex regulatory genetic pathways involving the Pax2/5/8 genes. Pax2 expression has been reported in the otic placode and vesicle of all vertebrates that have been studied. Loss-of-function experiments suggest that the Pax2 gene plays a key role in the development of the cochlear duct and acoustic ganglion. Despite all these data, the role of Pax2 gene in the specification of the otic epithelium is still only poorly defined. In the present work, we report a detailed study of the spatial and temporal Pax2 expression patterns during the development of the chick inner ear. In the period analysed, Pax2 is expressed only in some presumptive sensory patches, but not all, even though all sensory patches show the scattered Pax2 expression pattern later on. We also show that Pax2 is also expressed in several non-sensory structures.

Directional asymmetry of the zebrafish epithalamus guides dorsoventral innervation of the midbrain target

The zebrafish epithalamus, consisting of the pineal complex and flanking dorsal habenular nuclei, provides a valuable model for exploring how left-right differences could arise in the vertebrate brain. The parapineal lies to the left of the pineal and the left habenula is larger, has expanded dense neuropil, and distinct patterns of gene expression from the right habenula. Under the influence of Nodal signaling, positioning of the parapineal sets the direction of habenular asymmetry and thereby determines the left-right origin of habenular projections onto the midbrain target, the interpeduncular nucleus (IPN). In zebrafish with parapineal reversal, neurons from the left habenula project to a more limited ventral IPN region where right habenular axons would normally project. Conversely, efferents from the right habenula adopt a more extensive dorsoventral IPN projection pattern typical of left habenular neurons. Three members of the leftover-related KCTD (potassium channel tetramerization domain containing) gene family are expressed differently by the left and right habenula, in patterns that define asymmetric subnuclei. Molecular asymmetry extends to protein levels in habenular efferents, providing additional evidence that left and right axons terminate within different dorsoventral regions of the midbrain target. Laser-mediated ablation of the parapineal disrupts habenular asymmetry and consequently alters the dorsoventral distribution of innervating axons. The results demonstrate that laterality of the dorsal forebrain influences the formation of midbrain connections and their molecular properties.

How can teleostean inner ear hair cells maintain the proper association with the accreting otolith?

The perception of equilibrium and sound in fish depends on the deflection of hair bundles of hair cell by the otolith. However, the accreting nature of teleostean otoliths poses a problem for maintenance of proper contact between the hair bundle and the otolith surface. Immunocytochemical staining localizes abundant proton-secreting H(+)-ATPase in the apical membrane of the hair cells. The H(+)-ATPase-mediated proton secretion into the endolymph causes an approximately 0.4-unit pH decrease, which was quantified by an H(+)-selective microelectrode. Thus, the hair cells maintain the proper distance from the otolith by neutralizing the alkaline endolymph to retard CaCO(3) deposition on the otolith opposite the sensory macula. Carbonic anhydrase, which hydrolyses CO(2) and produces HCO(3) (-) and H(+), was also localized in the hair cells. Ionocytes showed prominent immunostaining of carbonic anhydrase and Na(+)-K(+)-ATPase, indicating its role in transepithelial transport of HCO(3) (-) across the membranous labyrinth into the endolymph. Ionocytes form a ring closely surrounding the sensory macula. HCO(3) (-) secreted from the ionocytes may serve as a barrier to neutralize H(+) diffused from the sensory macula while keeping the endolymph alkaline outside the sensory macula. The ingenious arrangement of ionocytes and hair cells results in a unique sculptured groove, which is a common feature on the proximal surface of all teleostean otoliths.

Groucho corepressor proteins regulate otic vesicle outgrowth

The Groucho/Tle family of corepressor proteins is known to regulate multiple developmental pathways. Applying the dominant-negative effect of the short member Aes, we demonstrate here a critical role of this gene family also for ear development. Misexpression of Aes in medaka embryos resulted in reduced size or loss of otic vesicles, whereas overexpression of the full-length Groucho protein Tle4 gave the opposite phenotype. These results are in close agreement with phenotypes observed for eye formation, suggesting a similar role for Groucho/Tle proteins in the developmental pathways of both sensory organs. Furthermore, by using the heat-inducible HSE promoter, we observed reversible branching of the embryonic axis upon Aes misexpression, indicating a transient duplication of the organizer. Groucho proteins, therefore, are critical for organizer maintenance.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Expression of zebrafish six1 during sensory organ development and myogenesis

Drosophila sine oculis homologous genes in vertebrates are homeobox-containing transcription factors functioning within the Pax-Six-Eya-Dach regulatory network during development. In this study, we describe the cloning and expression of a zebrafish homolog of sine oculis, six1. The reverse transcription-polymerase chain reaction demonstrated accumulation of six1 transcripts at mid-gastrula, and in situ hybridization showed their subsequent expression in the cranial placode and later in the olfactory, otic, and lateral line placodes, inner ear, and neuromasts. In addition, six1 is expressed in the pituitary, branchial arches, somites, pectoral fin, ventral abdomen muscle, and the cranial muscles of the eye and lower jaw. An increase of six1 expression was observed in the lateral line, muscles, and inner ear of the mind bomb mutant, illustrating a regulatory effect of the Notch pathway on expression of Six genes.

Expression patterns of three estrogen receptor genes during zebrafish (Danio rerio) development: evidence for high expression in neuromasts

The estrogen receptor (ER) genes encode a group of nuclear enhancer proteins, which are important ligand-activated transcription factors, modulating estrogen-target gene transcription. In this study we analyzed expression patterns of three zebrafish ER genes, esr1, esr2a, and esr2b, during development using whole-mount in situ hybridization. High levels of esr2a and esr2b of maternal origin are inherited and segregated to the blastomers. After the mid-blastula transition, the three genes exhibit similar spatio-temporal patterns of expression. In 24 h postfertilization (hpf) embryos, high levels of esr2a and esr2b and low levels of esr1 mRNAs are detected in the epidermis, pectoral fin buds, hatching gland and, to a lesser extent, developing brain. From 24 hpf onward, the expression of the three genes is down-regulated in the epidermis. By 60 hpf, esr2a mRNA is abundant in mature primary neuromasts of the anterior line system and by 3 days postfertilization (dpf), all mature primary neuromasts in both the anterior and posterior lateral line systems express significant levels of esr2a and esr2b transcripts. Histological sections show a high level of esr2a transcripts in both mechanoreceptive hair cells and supporting cells. The transcripts are still detected after neomycin-induced hair cell death, consistent with the presence of esr2a transcripts in supporting cells. From 6 dpf onward, esr2a and esr2b transcripts are robustly co-expressed in primary neuromasts, branchial arches, pectoral fins, and anal papilla, while slight labeling is observed for esr1 transcripts.

Role of the hindbrain in dorsoventral but not anteroposterior axial specification of the inner ear

An early and crucial event in vertebrate inner ear development is the acquisition of axial identities that in turn dictate the positions of all subsequent inner ear components. Here, we focus on the role of the hindbrain in establishment of inner ear axes and show that axial specification occurs well after otic placode formation in chicken. Anteroposterior (AP) rotation of the hindbrain prior to specification of this axis does not affect the normal AP orientation and morphogenesis of the inner ear. By contrast, reversing the dorsoventral (DV) axis of the hindbrain results in changing the DV axial identity of the inner ear. Expression patterns of several ventrally expressed otic genes such as NeuroD, Lunatic fringe (Lfng) and Six1 are shifted dorsally, whereas the expression pattern of a normally dorsal-specific gene, Gbx2, is abolished. Removing the source of Sonic Hedgehog (SHH) by ablating the floor plate and/or notochord, or inhibiting SHH function using an antibody that blocks SHH bioactivity results in loss of ventral inner ear structures. Our results indicate that SHH, together with other signals from the hindbrain, are important for patterning the ventral axis of the inner ear. Taken together, our studies suggest that tissue(s) other than the hindbrain confer AP axial information whereas signals from the hindbrain are necessary and sufficient for the DV axial patterning of the inner ear.

Prostaglandin E Synthases in Zebrafish

Objective— Prostaglandin E synthases (PGESs) are being explored as antiinflammtory drug targets as alternatives to cyclooxygenase (COX)-2. Located downstream of the cyclooxygenases, PGESs catalyze PGE 2 formation, and deletion of microsomal (m)-PGES-1 abrogates inflammation. We sought to characterize the developmental expression of COX and PGES in zebrafish.

Methods and Results— We cloned zebrafish cytosolic (c) and m-PGES orthologs and mapped them to syntenic regions of chromosomes 23 and 5. cPGES was widely expressed during development and was coordinately regulated with zCOX-1 in the inner ear, the pronephros, and intestine. COX-2 and mPGES-1 exhibited restricted expression, dominantly in the vasculature of the aortic arch. However, the enzymes were anatomically segregated within the vessel wall. Experiments with antisense morpholinos and with nonsteroidal antiinflammatory drugs suggest that these genes may not be critical for development.

Conclusions— mPGES-1 is developmentally coregulated with COX-2 in vasculature. Given the high fecundidity and translucency of the zebrafish, this model may afford a high throughput system for characterization of novel PGES inhibitors.

Two contrasting roles for Notch activity in chick inner ear development:specification of prosensory patches and lateral inhibition of hair-cell differentiation

Lateral inhibition mediated by Notch is thought to generate the mosaic of hair cells and supporting cells in the inner ear, but the effects of the activated Notch protein itself have never been directly tested. We have explored the role of Notch signalling by transiently overexpressing activated Notch (NICD) in the chick otocyst. We saw two contrasting consequences, depending on the time and site of gene misexpression: (1)inhibition of hair-cell differentiation within a sensory patch; and (2)induction of ectopic sensory patches. We infer that Notch signalling has at least two functions during inner ear development. Initially, Notch activity can drive cells to adopt a prosensory character, defining future sensory patches. Subsequently, Notch signalling within each such patch mediates lateral inhibition, restricting the proportion of cells that differentiate as hair cells so as to generate the fine-grained mixture of hair cells and supporting cells.

The zebrafish dog-eared mutation disrupts eya1, a gene required for cell survival and differentiation in the inner ear and lateral line

To understand the molecular basis of sensory organ development and disease, we have cloned and characterized the zebrafish mutation dog-eared (dog) that is defective in formation of the inner ear and lateral line sensory systems. The dog locus encodes the eyes absent-1 (eya1) gene and single point mutations were found in three independent dog alleles, each prematurely truncating the expressed protein within the Eya domain. Moreover, morpholino-mediated knockdown of eya1 gene function phenocopies the dog-eared mutation. In zebrafish, the eya1 gene is widely expressed in placode-derived sensory organs during embryogenesis but Eya1 function appears to be primarily required for survival of sensory hair cells in the developing ear and lateral line neuromasts. Increased levels of apoptosis occur in the migrating primordia of the posterior lateral line in dog embryos and as well as in regions of the developing otocyst that are mainly fated to give rise to sensory cells of the cristae. Importantly, mutation of the EYA1 or EYA4 gene causes hereditary syndromic deafness in humans. Determination of eya gene function during zebrafish organogenesis will facilitate understanding the molecular etiology of human vestibular and hearing disorders.

Pax8 and Pax2a function synergistically in otic specification, downstream of the Foxi1 and Dlx3b transcription factors

The vertebrate inner ear arises from an ectodermal thickening, the otic placode, that forms adjacent to the presumptive hindbrain. Previous studies have suggested that competent ectodermal cells respond to Fgf signals from adjacent tissues and express two highly related paired box transcription factors Pax2a and Pax8 in the developing placode. We show that compromising the functions of both Pax2a and Pax8 together blocks zebrafish ear development, leaving only a few residual otic cells. This suggests that Pax2a and Pax8 are the main effectors downstream of Fgf signals. Our results further provide evidence that pax8 expression and pax2a expression are regulated by two independent factors, Foxi1 and Dlx3b, respectively. Combined loss of both factors eliminates all indications of otic specification. We suggest that the Foxi1-Pax8 pathway provides an early 'jumpstart' of otic specification that is maintained by the Dlx3b-Pax2a pathway.

Zebrafish pax8 is required for otic placode induction and plays a redundant role with Pax2 genes in the maintenance of the otic placode

Vertebrate Pax2 and Pax8 proteins are closely related transcription factors hypothesized to regulate early aspects of inner ear development. In zebrafish and mouse, Pax8 expression is the earliest known marker of otic induction, and Pax2 homologs are expressed at slightly later stages of placodal development. Analysis of compound mutants has not been reported. To facilitate analysis of zebrafish pax8, we completed sequencing of the entire gene, including the 5' and 3' UTRs. pax8 transcripts undergo complex alternative splicing to generate at least ten distinct isoforms. Two different subclasses of pax8 splice isoforms encode different translation initiation sites. Antisense morpholinos (MOs) were designed to block translation from both start sites, and four additional MOs were designed to target different exon-intron boundaries to block splicing. Injection of MOs, individually and in various combinations, generated similar phenotypes. Otic induction was impaired, and otic vesicles were small. Regional ear markers were expressed correctly, but hair cell production was significantly reduced. This phenotype was strongly enhanced by simultaneously disrupting either of the co-inducers fgf3 or fgf8, or another early regulator, dlx3b, which is thought to act in a parallel pathway. In contrast, the phenotype caused by disrupting foxi1, which is required for pax8 expression, was not enhanced by simultaneously disrupting pax8. Disrupting pax8, pax2a and pax2b did not further impair otic induction relative to loss of pax8 alone. However, the amount of otic tissue gradually decreased in pax8-pax2a-pax2b-deficient embryos such that no otic tissue was detectable by 24 hours post-fertilization. Loss of otic tissue did not correlate with increased cell death, suggesting that otic cells dedifferentiate or redifferentiate as other cell type(s). These data show that pax8 is initially required for normal otic induction, and subsequently pax8, pax2a and pax2b act redundantly to maintain otic fate.

Otopetrin 1 is required for otolith formation in the zebrafish Danio rerio

Orientation with respect to gravity is essential for the survival of complex organisms. The gravity receptor is one of the phylogenetically oldest sensory systems, and special adaptations that enhance sensitivity to gravity are highly conserved. The fish inner ear contains three large extracellular biomineral particles, otoliths, which have evolved to transduce the force of gravity into neuronal signals. Mammalian ears contain thousands of small particles called otoconia that serve a similar function. Loss or displacement of these structures can be lethal for fish and is responsible for benign paroxysmal positional vertigo (BPPV) in humans. The distinct morphologies of otoconial particles and otoliths suggest divergent developmental mechanisms. Mutations in a novel gene Otopetrin 1 (Otop1), encoding multi-transmembrane domain protein, result in nonsyndromic otoconial agenesis and a severe balance disorder in mice. Here we show that the zebrafish, Danio rerio, contains a highly conserved gene, otop1, that is essential for otolith formation. Morpholino-mediated knockdown of zebrafish Otop1 leads to otolith agenesis without affecting the sensory epithelium or other structures within the inner ear. Despite lack of otoliths in early development, otolith formation partially recovers in some fish after 2 days. However, the otoliths are malformed, misplaced, lack an organic matrix, and often consist of inorganic calcite crystals. These studies demonstrate that Otop1 has an essential and conserved role in the timing of formation and the size and shape of the developing otolith.

nlz gene family is required for hindbrain patterning in the zebrafish

This study describes the conserved nlz gene family whose members encode unusual zinc finger proteins. In the zebrafish neurectoderm, both nlz1 and the newly isolated nlz2 are expressed in the presumptive hindbrain and midbrain/hindbrain boundary, where expression of nlz1 is dependent on pax2a. In addition, nlz2 is uniquely expressed more anteriorly, in the presumptive midbrain and diencephalon. Overexpression of Nlz proteins during gastrula stages inhibits hindbrain development. In particular, ectopically expressed Nlz1 inhibits formation of future rhombomeres 2 and 3 (r2, r3), whereas neighboring r1 and r4 are not affected. Conversely, simultaneous reduction of Nlz1 and Nlz2 protein function by expression of antisense morpholino-modified oligomers leads to expansion of future r3 and r5, with associated loss of r4. These data indicate that one function of the nlz gene family is to specify or maintain r4 identity, and to limit r3 and r5 during hindbrain formation.