Mutations affecting development of the zebrafish ear

Linking genes to brain, behavior and neurological diseases: what can we learn from zebrafish?

How our brain is wired and subsequently generates functional output, ranging from sensing and locomotion to emotion, decision-making and learning and memory, remains poorly understood. Dys-regulation of these processes can lead to neurodegenerative, as well as neuro-psychiatric, disorders. Molecular genetic together with behavioral analyses in model organisms identify genes involved in the formation of neuronal circuits, the execution of behavior and mechanisms involved in neuro-pathogenesis. In this review I will discuss the current progress and future potential for study in a newly established vertebrate model organism for genetics, the zebrafish Danio rerio. Where available, schemes and results of genetic screens will be reviewed concerning the sensory, motor and neuromodulatory monoamine systems. Genetic analyses in zebrafish have the potential to provide important insights into the relationship between genes, neuronal circuits and behavior in normal as well as diseased states.

Ringing in the new ear: resolution of cell interactions in otic development

The vertebrate inner ear is a marvel of structural and functional complexity, which is all the more remarkable because it develops from such a simple structure, the otic placode. Analysis of inner ear development has long been a fascination of experimental embryologists, who sought to understand cellular mechanisms of otic placode induction. More recently, however, molecular and genetic approaches have made the inner ear a useful model system for studying a much broader range of basic developmental mechanisms, including cell fate specification and differentiation, axial patterning, epithelial morphogenesis, cytoskeletal dynamics, stem cell biology, neurobiology, physiology, etc. Of course, there has also been tremendous progress in understanding the functions and processes peculiar to the inner ear. The goal of this review is to recount how historical approaches have shaped our understanding of the signaling interactions controlling early otic development; to discuss how new findings have led to fundamental new insights; and to point out new problems that need to be resolved in future research.

Otolith formation in a mutant Medaka with a deficiency in gravity-sensing

Mutant Medaka ha exhibit spontaneous mutation that is characterized by frequent inhibition or perturbation in the formation of utricular otoliths and/or semicircular canals. Three major features of otolith morphogenesis were observed in ha strain: 1) The initial appearance of otoliths was delayed, mispositioned, and malformed compared to normal embryos. 2) No utricular otoliths appeared on macula of any ha fry just after hatching. A symmetric state of otoliths was seen only when saccular otoliths were situated on macula in both inner ears. 3) In some fry, formation of utricular otoliths was observed in their later development. However, no new utricular otoliths appeared after fish were seventy or more days old after hatching. These observations show that otolith morphogenesis in ha is very different from that of wild-type. In this study, we classified adult ha into four different phenotypes using the existence or absence of utricular otoliths as our criteria. We concluded that dysfunction of utricular otoliths and semicircular canals cause a defect that affects the gravity-sensing abilities of medaka ha.

Zebrafish as a model for hearing and deafness

The zebrafish is an especially attractive model for the study of the development and function of the vertebrate inner ear. It combines rapid and accessible embryogenesis with a host of genetic and genomic tools for systematic gene discovery and analysis. A large collection of mutations affecting development and function of the ear and a related sensory system, the lateral line, have been isolated; several of these have now been cloned, and at least five provide models for human deafness disorders. Disruption of multiple genes, using both forward and reverse genetic approaches, has established key players--both signaling molecules and autonomous factors--responsible for induction and specification of the otic placode. Vestibular and auditory defects have been detected in adult animals, making the zebrafish a useful system in which to tackle the genetic causes of late onset deafness and vestibular disease.

Fgf8 and Fgf3 are required for zebrafish ear placode induction, maintenance and inner ear patterning

The vertebrate inner ear develops from initially 'simple' ectodermal placode and vesicle stages into the complex three-dimensional structure which is necessary for the senses of hearing and equilibrium. Although the main morphological events in vertebrate inner ear development are known, the genetic mechanisms controlling them are scarcely understood. Previous studies have suggested that the otic placode is induced by signals from the chordamesoderm and the hindbrain, notably by fibroblast growth factors (Fgfs) and Wnt proteins. Here we study the role of Fgf8 as a bona-fide hindbrain-derived signal that acts in conjunction with Fgf3 during placode induction, maintenance and otic vesicle patterning. Acerebellar (ace) is a mutant in the fgf8 gene that results in a non-functional Fgf8 product. Homozygous mutants for acerebellar (ace) have smaller ears that typically have only one otolith, abnormal semi-circular canals, and behavioral defects. Using gene expression markers for the otic placode, we find that ace/fgf8 and Fgf-signaling are required for normal otic placode formation and maintenance. Conversely, misexpression of fgf8 or Fgf8-coated beads implanted into the vicinity of the otic placode can increase ear size and marker gene expression, although competence to respond to the induction appears restricted. Cell transplantation experiments and expression analysis suggest that Fgf8 is required in the hindbrain in the rhombomere 4-6 area to restore normal placode development in ace mutants, in close neighbourhood to the forming placode, but not in mesodermal tissues. Fgf3 and Fgf8 are expressed in hindbrain rhombomere 4 during the stages that are critical for placode induction. Joint inactivation of Fgf3 and Fgf8 by mutation or antisense-morpholino injection causes failure of placode formation and results in ear-less embryos, mimicking the phenotype we observe after pharmacological inhibition of Fgf-signaling. Fgf8 and Fgf3 together therefore act during induction and differentiation of the ear placode. In addition to the early requirement for Fgf signaling, the abnormal differentiation of inner ear structures and mechanosensory hair cells in ace mutants, pharmacological inhibition of Fgf signaling, and the expression of fgf8 and fgf3 in the otic vesicle demonstrate independent Fgf function(s) during later development of the otic vesicle and lateral line organ. We furthermore addressed a potential role of endomesomerm by studying mzoep mutant embryos that are depleted of head endomesodermal tissue, including chordamesoderm, due to a lack of Nodal-pathway signaling. In these embryos, early placode induction proceeds largely normally, but the ear placode extends abnormally to midline levels at later stages, suggesting a role for the midline in restricting placode development to dorsolateral levels. We suggest a model of zebrafish inner ear development with several discrete steps that utilize sequential Fgf signals during otic placode induction and vesicle patterning.

High-throughput behavioral screening method for detecting auditory response defects in zebrafish

We have developed an automated, high-throughput behavioral screening method for detecting hearing defects in zebrafish. Our assay monitors a rapid escape reflex in response to a loud sound. With this approach, 36 adult zebrafish, restrained in visually isolated compartments, can be simultaneously assessed for responsiveness to near-field 400 Hz sinusoidal tone bursts. Automated, objective determinations of responses are achieved with a computer program that obtains images at precise times relative to the acoustic stimulus. Images taken with a CCD video camera before and after stimulus presentation are subtracted to reveal a response to the sound. Up to 108 fish can be screened per hour. Over 6500 fish were tested to validate the reliability of the assay. We found that 1% of these animals displayed hearing deficits. The phenotypes of non-responders were further assessed with radiological analysis for defects in the gross morphology of the auditory system. Nearly all of those showed abnormalities in conductive elements of the auditory system: the swim bladder or Weberian ossicles.

A subtracted cDNA library from the zebrafish (Danio rerio) embryonic inner ear

A database was built that consists of 4694 sequence contigs of approximately 18,000 reads of cDNAs isolated from the microdissected otocysts of zebrafish embryos at 20-30 hour postfertilization, following subtraction with a pool of liver cDNAs from adult fish. These sequences were compared with those of public databanks. Significant similarity were recorded and organized in a relational database at http://www.genoscope.cns.fr/zie. A first group of 2067 sequences correspond to 1428 known zebrafish genes or ESTs present in the Danio rerio section of UniGene. A second group of 302 sequences encode putative proteins that showed significant similarity (50%-100%) with 302 nonzebrafish proteins in the nr databank, a public databank containing an exhaustive nonredundant collection of protein sequences from different species (ftp://ftp.ncbi.nlm.nih.gov/blast/db/nr). The remaining 2325 (49.5%) sequence contigs or singletons showed no significant similarity with sequences available in public databanks. Several genes known to be expressed in the developing inner ear were represented in the present database, in particular genes involved in hair cell differentiation or innervation The occurrence of these genes validates the outcome of this study as the first collection of ESTs preferentially expressed in the zebrafish inner ear during the period of hair cell differentiation and neuroblast delamination from the otic vesicle epithelium. Novel zebrafish genes also involved in these processes are thus likely to be represented among the sequences obtained herein, for which no homology was found in the D. rerio section of UniGene. [The sequence data from this study have been submitted to EMBL under accession nos. AL714032-AL731531].

Development of the zebrafish inner ear

Abstract Recent years have seen a renaissance of investigation into the mechanisms of inner ear development. Genetic analysis of zebrafish has contributed significantly to this endeavour, with several dramatic advances reported over the past year or two. Here, we review the major findings from recent work in zebrafish. Several cellular and molecular mechanisms have been elucidated, including the signaling pathways controlling induction of the otic placode, morphogenesis and patterning of the otic vesicle, and elaboration of functional attributes of inner ear.

Development of sensory systems in zebrafish (Danio rerio)

Zebrafish possess all of the classic sensory modalities: taste, tactile, smell, balance, vision, and hearing. For each sensory system, this article provides a brief overview of the system in the adult zebrafish followed by a more detailed overview of the development of the system. By far the majority of studies performed in each of the sensory systems of the zebrafish have involved some aspect of molecular biology or genetics. Although molecular biology and genetics are not major foci of the paper, brief discussions of some of the mutant strains of zebrafish that have developmental defects in each specific sensory system are included. The development of the sensory systems is only a small sampling of the work being done using zebrafish and provides a mere glimpse of the potential of this model for the study of vertebrate development, physiology, and human disease.

Rapid lesioning of large numbers of identified vertebrate neurons: applications in zebrafish

Establishing a causal role between the activity of specific individual nerve cells and the behaviors they produce (or the neural computations they execute) is made difficult in vertebrate animals because of the large numbers of neurons involved. Traditional techniques for establishing causal roles, such as tract cutting and electrolytic lesions, are limited because they produce damage that affects a variety of different cell types, invariably intermingled, and often of uncertain identity. We propose here an alternative lesioning technique in which large numbers of neurons are lesioned, but the lesioned neurons are specifically identified by fluorescent labeling. We use the locomotor control system of the larval zebrafish to illustrate this approach. In this example, the technique involves injection of fluorescent dextrans into far-rostral spinal cord to label descending nerve fibers. Such injections appear to interrupt the descending nerve fibers, and therefore their accompanying locomotor control signals. This protocol is shown to produce significant behavioral deficits. Because the CNS of the larval zebrafish is transparent, the entire population of lesioned cells can be imaged in vivo and reconstructed using confocal microscopy. This large-scale lesioning technique is important, even in this relatively 'simple' vertebrate animal, because the ablation of smaller numbers of neurons, using more precise laser-ablation techniques, often fails to produce observable behavioral deficits. While this technique is most readily applied in simpler and transparent vertebrate animals, the approach is general in nature and might, in principle, be applied to any vertebrate nerve tract.

The Wheels mutation in the mouse causes vascular, hindbrain, and inner ear defects

In a screen for mouse mutations with dominant behavioral anomalies, we identified Wheels, a mutation associated with circling and hyperactivity in heterozygotes and embryonic lethality in homozygotes. Mutant Wheels embryos die at E10.5-E11.5 and exhibit a host of morphological anomalies which include growth retardation and anomalies in vascular and hindbrain development. The latter includes perturbation of rhombomeric boundaries as detected by Krox20 and Hoxb1. PECAM-1 staining of embryos revealed normal formation of the primary vascular plexus. However, subsequent stages of branching and remodeling do not proceed normally in the yolk sac and in the embryo proper. To obtain insights into the circling behavior, we examined development of the inner ear by paint-filling of membranous labyrinths of Whl/+ embryos. This analysis revealed smaller posterior and lateral semicircular canal primordia and a delay in the canal fusion process at E12.5. By E13.5, the lateral canal was truncated and the posterior canal was small or absent altogether. Marker analysis revealed an early molecular phenotype in heterozygous embryos characterized by perturbed expression of Bmp4 and Msx1 in prospective lateral and posterior cristae at E11.5. We have constructed a genetic and radiation hybrid map of the centromeric portion of mouse Chromosome 4 across the Wheels region and refined the position of the Wheels locus to the approximately 1.1-cM region between D4Mit104 and D4Mit181. We have placed the locus encoding Epha7, in the Wheels candidate region; however, further analysis showed no mutations in the Epha7-coding region and no detectable changes in mRNA expression pattern. In summary, our findings indicate that Wheels, a gene which is essential for the survival of the embryo, may link diverse processes involved in vascular, hindbrain, and inner ear development.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Small molecule developmental screens reveal the logic and timing of vertebrate development

Much has been learned about vertebrate development by random mutagenesis followed by phenotypic screening and by targeted gene disruption followed by phenotypic analysis in model organisms. Because the timing of many developmental events is critical, it would be useful to have temporal control over modulation of gene function, a luxury frequently not possible with genetic mutants. Here, we demonstrate that small molecules capable of conditional gene product modulation can be identified through developmental screens in zebrafish. We have identified several small molecules that specifically modulate various aspects of vertebrate ontogeny, including development of the central nervous system, the cardiovascular system, the neural crest, and the ear. Several of the small molecules identified allowed us to dissect the logic of melanocyte and otolith development and to identify critical periods for these events. Small molecules identified in this way offer potential to dissect further these and other developmental processes and to identify novel genes involved in vertebrate development.

Harnessing the power of forward genetics--analysis of neuronal diversity and patterning in the zebrafish retina

The seven major cell classes of the vertebrate retina are organized with remarkable precision into distinct layers. The appearance of this architecture during embryogenesis raises two questions of general importance. How do individual cell classes acquire their specialized structures and functions if they all originate from a morphologically uniform cell population? What mechanisms are responsible for the formation of such a complex and exact pattern? Recent advances present an opportunity to apply the tools of forward genetic analysis to identify mutations that affect these mechanisms in zebrafish. Molecular characterization will follow, providing insight into the basis of neuronal patterning in the vertebrate CNS.

Patterning of the mammalian cochlea

The mammalian cochlea is sophisticated in its function and highly organized in its structure. Although the anatomy of this sense organ has been well documented, the molecular mechanisms underlying its development have remained elusive. Information generated from mutant and knockout mice in recent years has increased our understanding of cochlear development and physiology. This article discusses factors important for the development of the inner ear and summarizes cochlear phenotypes of mutant and knockout mice, particularly Otx and Otx2. We also present data on gross development of the mouse cochlea.

The zebrafish colourless gene regulates development of non-ectomesenchymal neural crest derivatives

Neural crest forms four major categories of derivatives: pigment cells, peripheral neurons, peripheral glia, and ectomesenchymal cells. Some early neural crest cells generate progeny of several fates. How specific cell fates become specified is still poorly understood. Here we show that zebrafish embryos with mutations in the colourless gene have severe defects in most crest-derived cell types, including pigment cells, neurons and specific glia. In contrast, craniofacial skeleton and medial fin mesenchyme are normal. These observations suggest that colourless has a key role in development of non-ectomesenchymal neural crest fates, but not in development of ectomesenchymal fates. Thus, the cls mutant phenotype reveals a segregation of ectomesenchymal and non-ectomesenchymal fates during zebrafish neural crest development. The combination of pigmentation and enteric nervous system defects makes colourless mutations a model for two human neurocristopathies, Waardenburg-Shah syndrome and Hirschsprung's disease.

alyron, an insertional mutation affecting early neural crest development in zebrafish

alyronz12 (aln) is a recessive lethal mutation that affects early stages of neural crest development in the zebrafish. alyron appears to be an insertional mutation as the mutation was generated following microinjection of plasmid DNA into one-cell embryos and the stably integrated transgenic sequences are closely linked to the mutation. The insertion site harbors multiple copies of the plasmid sequence that have experienced complex rearrangements. Host-insert junction fragments have been molecularly cloned and host sequences adjacent to the transgene have been used to map the mutation to the distal arm of linkage group 15. alyron function is required cell-autonomously in the neural crest lineage. alyron mutants have a severe but not complete deficit of premigratory neural crest as judged by reduced expression of several markers associated with early stages of neural crest development. Lack of premigratory neural crest is likely to account for the two most conspicuous characteristics of alyron mutants: the absence of body pigmentation and the inability to affect blood circulation. The neural crest phenotype of alyron mutants resembles that observed in mouse mutants that lack Pax-3 or both Wnt-1 and Wnt-3a function, and expression of the zebrafish homologues of these genes is greatly reduced in the dorsal neural keels of alyron mutants. In contrast, ventral neural keel identity appears unaffected. Given our findings that the mutation is unlinked to pax or wnt genes that have been described in the zebrafish, we propose that alyron is a novel gene function required for the specification and/or proliferative expansion of neural crest progenitors.

Development of the vertebrate ear: insights from knockouts and mutants

The three divisions of the ear (outer, middle and inner) each have an important role in hearing, while the inner ear is also crucial for the sense of balance. How these three major components arise and coalesce to form the peripheral elements of the senses of hearing and balance is now being studied using molecular-genetic approaches. This article summarizes data from studies of knockout and mutant animals in which one or more divisions of the ear are abnormal. The data confirm that development of all three divisions of the ear depends on the genes involved in hindbrain segmentation and segment identity. Genes that are regionally expressed in the inner ear can, when absent or mutated, yield selective ablation of specific inner-ear structures or cell types.

Otolith ocular reflex function of the tangential nucleus in teleost fish

In teleost fish, the tangential nucleus can be identified as a compact, separate cell group lying ventral to the VIIIth nerve near the middle of the vestibular complex. Morphological analysis of larval and adult hindbrains utilizing biocytin and fluorescent tracers showed the tangential nucleus to be located entirely within rhombomeric segment 5 with all axons projecting into the contralateral MLF. Combined single-cell electrophysiology and morphology in alert goldfish found three classes of neurons whose physiological sensitivity could be readily correlated with rotational axes about either the anterior (45 degrees), posterior (135 degrees), or horizontal (vertical axis) semicircular canals. Tangential neurons could be distinguised from those in semicircular-canal specific subnuclei by an irregular, spontaneous background of 10-15 sp/s and sustained static sensitivity after +/- 4 degrees head displacements. Each axis-specific tangential subtype terminated appropriately onto oculomotor subnuclei responsible for either vertical, torsional, or horizontal eye movements and, in a few cases, axon collaterals descended in the MLF toward the spinal cord. We hypothesize, therefore, that the tangential nucleus consists of 3 axis-specific phenotypes that process gravitoinertial signals largely responsible for controlling oculomotor function, but that also in part, maintain body posture.

Genetic analysis of tissue interactions required for otic placode induction in the zebrafish

Development of the vertebrate inner ear begins during gastrulation with induction of the otic placode. Several embryonic tissues, including cephalic mesendoderm, notochord, and hindbrain, have been implicated as potential sources of otic-inducing signals. However, the relative contributions of these tissues have not been determined, nor have any genes affecting placode induction been identified. To address these issues, we analyzed otic placode induction in zebrafish mutants that are deficient in prospective otic-inducing tissues. Otic development was monitored by examining mutant embryos for morphological changes and, in some cases, by visualizing expression patterns of dlx-3 or pax-2.1 in preotic cells several hours before otic placode formation. In cyclops (cyc-) mutants, which develop with a partial deficiency of prechordal mesendoderm, otic induction is delayed by up to 1 h. In one-eyed pinhead (oep-) mutants, which are more completely deficient in prechordal mesendoderm, otic induction is delayed by 1.5 h, and morphology of the otic vesicles is abnormal. Expression of marker genes in other regions of the neural plate is normal, suggesting that ablation of prechordal mesendoderm selectively inhibits otic induction. In contrast, the timing and morphology of otic development is not affected by mutations in no tail (ntl) or floating head (flh), which prevent notochord differentiation. Similarly, a mutation in valentino (val), which blocks early differentiation of rhombomeres 5 and 6 in the hindbrain, does not delay otic induction, although subsequent patterning of the otic vesicle is impaired. To test whether inductive signals from one tissue can compensate for loss of another, we generated double or triple mutants with various combinations of the above mutations. In none of the multiple mutants do the flh or val mutations exacerbate delays in placode induction, although val does contribute additively to defects in subsequent patterning of the otic vesicle. In contrast, mutants homozygous for both oep and ntl, which interact synergistically to disrupt differentiation of cephalic and axial mesendoderm, show a delay in otic development of about 3 h. These data suggest that cephalic mesendoderm, including prechordal mesendoderm and anterior paraxial mesendoderm, provides the first otic-inducing signals during gastrulation, whereas chordamesoderm plays no discernible role in this process. Because val- mutants are deficient for only a portion of the hindbrain, we cannot rule out a role for that tissue in otic placode induction. However, if the hindbrain does provide otic-inducing signals, they apparently differ quantitatively or qualitatively from the signals required for vesicle patterning, as val disrupts only the latter.

Delta-Notch signalling and the patterning of sensory cell differentiation in the zebrafish ear: evidence from the mind bomb mutant

Mechanosensory hair cells in the sensory patches of the vertebrate ear are interspersed among supporting cells, forming a fine-grained pattern of alternating cell types. Analogies with Drosophila mechanosensory bristle development suggest that this pattern could be generated through lateral inhibition mediated by Notch signalling. In the zebrafish ear rudiment, homologues of Notch are widely expressed, while the Delta homologues deltaA, deltaB and deltaD, coding for Notch ligands, are expressed in small numbers of cells in regions where hair cells are soon to differentiate. This suggests that the delta-expressing cells are nascent hair cells, in agreement with findings for Delta1 in the chick. According to the lateral inhibition hypothesis, the nascent hair cells, by expressing Delta protein, would inhibit their neighbours from becoming hair cells, forcing them to be supporting cells instead. The zebrafish mind bomb mutant has abnormalities in the central nervous system, somites, and elsewhere, diagnostic of a failure of Delta-Notch signalling: in the CNS, it shows a neurogenic phenotype accompanied by misregulated delta gene expression. Similar misregulation of delta; genes is seen in the ear, along with misregulation of a Serrate homologue, serrateB, coding for an alternative Notch ligand. Most dramatically, the sensory patches in the mind bomb ear consist solely of hair cells, which are produced in great excess and prematurely; at 36 hours post fertilization, there are more than ten times as many as normal, while supporting cells are absent. A twofold increase is seen in the number of otic neurons also. The findings are strong evidence that lateral inhibition mediated by Delta-Notch signalling controls the pattern of sensory cell differentiation in the ear.

The differential sensitivities of inner ear structures to retinoic acid during development

In order to examine the mechanisms that underlie development of the inner ear, the normal processes were perturbed using all-trans-retinoic acid (RA). By implanting a resin exchange bead saturated with RA into stage 16 (Hamburger and Hamilton, 1951, J. Morphol. 88, 49-92) embryonic day 2.5 chick ears, it was possible to analyze its in vivo effects on inner ear development. There is a temporal window during which the developing chick inner ear is particularly susceptible to the effects of RA (stages 16-19). This RA period of sensitivity precedes evidence of gross morphologic or histologic differentiation by at least 24 h, suggesting that mechanisms controlling formation of key inner ear structures are already in progress. There is a dose dependence on RA, with increasing doses of RA generating increasingly severe phenotypic abnormalities. Data indicate that these effects are due to differential sensitivities of the various inner ear structures to RA during their formation. In general, the vestibular structures were more susceptible to RA effects than the cochlear duct. Furthermore, nonsensory structures such as semicircular canals seemed to display a greater susceptibility to RA than their associated sensory structures (i.e., cristae). Among the three semicircular canals, the superior canal was the most susceptible to RA treatment, whereas the common crus was particularly resistant, suggesting that the molecular mechanisms for each structure's formation are different. The defect in semicircular canal formation is due to problems in the initial outgrowth of the canal plate which in turn is related to a down-regulation of early otocyst cell proliferation. This perturbation model provides valuable insight into the processes involved in producing the intricate patterning of the inner ear.

Specification of the hindbrain fate in the zebrafish

We determine the timing of neural commitment by hindbrain tissue in the zebrafish using microsurgical transplantation. When transplanted at shield stage to the ventral side of the embryo, presumptive hindbrain cells are not committed, as they can adapt to their environment and give rise to epidermis. In contrast, when transplanted at 80% epiboly, hindbrain cells retain their neural fate and express neural-specific antigens. Moreover, they are able to maintain regional fate, as is evident by the expression of the hindbrain-specific marker, Krox20. In addition, we observe that committed hindbrain tissues are able to induce presumptive ventral epidermis to form neural crest derivatives, otic vesicles, and neural tissues. We propose that hindbrain progenitors have acquired regional identity as a group at 80% epiboly even before making vertical contact with axial mesoderm. These results suggest that planar induction may constitute a significant component in the zebrafish neural patterning pathway.

Two regulatory genes, cNkx5-1 and cPax2, show different responses to local signals during otic placode and vesicle formation in the chick embryo

The early stages of otic placode development depend on signals from neighbouring tissues including the hindbrain. The identity of these signals and of the responding placodal genes, however, is not known. We have identified a chick homeobox gene cNkx5-1, which is expressed in the otic placode beginning at stage 10 and exhibits a dynamic expression pattern during formation and further differentiation of the otic vesicle. In a series of heterotopic transplantation experiments, we demonstrate that cNkx5-1 can be activated in ectopic positions. However, significant differences in otic development and cNkx5-1 gene activity were observed when placodes were transplanted into the more rostral positions within the head mesenchyme or into the wing buds of older hosts. These results indicate that only the rostral tissues were able to induce and/or maintain ear development. Ectopically induced cNkx5-1 expression always reproduced the endogenous pattern within the lateral wall of the otocyst that is destined to form vestibular structures. In contrast, cPax2 which is expressed in the medial wall of the early otic vesicle later forming the cochlea never resumed its correct expression pattern after transplantation. Our experiments illustrate that only some aspects of gene expression and presumably pattern formation during inner ear development can be established and maintained ectopically. In particular, the dorsal vestibular structures seem to be programmed earlier and differently from the ventral cochlear part.

Genetic analysis of vertebrate sensory hair cell mechanosensation: the zebrafish circler mutants

The molecular basis of sensory hair cell mechanotransduction is largely unknown. In order to identify genes that are essential for mechanosensory hair cell function, we characterized a group of recently isolated zebrafish motility mutants. These mutants are defective in balance and swim in circles but have no obvious morphological defects. We examined the mutants using calcium imaging of acoustic-vibrational and tactile escape responses, high resolution microscopy of sensory neuroepithelia in live larvae, and recordings of extracellular hair cell potentials (microphonics). Based on the analyses, we have identified several classes of genes. Mutations in sputnik and mariner affect hair bundle integrity. Mutant astronaut and cosmonaut hair cells have relatively normal microphonics and thus appear to affect events downstream of mechanotransduction. Mutant orbiter, mercury, and gemini larvae have normal hair cell morphology and yet do not respond to acoustic-vibrational stimuli. The microphonics of lateral line hair cells of orbiter, mercury, and gemini larvae are absent or strongly reduced. Therefore, these genes may encode components of the transduction apparatus.

Planar polarization of Drosophila and vertebrate epithelia

Our understanding of the actin and microtubule rearrangements that generate planar polarity in Drosophila and in vertebrate epithelia has been extended by recent discoveries. Three different Rho family proteins have been shown to mediate polarization in the wing and the eye of Drosophila. In vertebrates, the importance of myosin VIIa has been uncovered by mutations that cause defects in planar polarization in the ear. Advances in our understanding of the Frizzled pathway, which coordinates planar polarization in Drosophila, are moving the field closer to understanding the links between signal transduction and polarized cytoskeletal reorganization.

A critical period of ear development controlled by distinct populations of ciliated cells in the zebrafish

The zebrafish (Danio rerio) is a useful model system for analyzing development of the inner ear. A number of mutations affecting the inner ear have been identified. Here we investigate the initial stages of otolith morphogenesis in wild-type embryos as well as in monolith (mnl) mutant embryos, which fail to form anterior otoliths but otherwise appear normal. Otolith growth is initiated at 18-18.5 h by localized accretion of free-moving precursor particles. This process, referred to as otolith seeding, is regulated by two classes of cilia: First, kinocilia of precociously forming hair cells (tether cells) bind seeding particles, thereby localizing otolith formation. Tether cells usually occur in pairs at the anterior and posterior ends of the ear. Despite the presence of functional kinocilia, tether cells initially appear immature and do not acquire the characteristics of mature hair cells until approximately 21.5 h. Second, beating cilia distributed throughout the ear agitate seeding particles, thereby inhibiting premature agglutination. Constraining particles with laser tweezers caused them to fuse into large untethered masses. Bringing such masses into contact with tethered otoliths caused them to fuse, greatly enhancing otolith growth. Selectively enhancing one otolith greatly inhibited growth of the second, creating an imbalance that persisted for many days. Seeding particles and beating cilia disappear soon after 24 h, and the rate of otolith growth decreases by nearly 90%. In mnl mutant embryos, tethers and beating cilia are distributed normally, but anterior otoliths fail to form in 80-85% of mutant ears. The binding properties of seeding particles appear normal, as shown by their ability to fuse when entrapped by laser tweezers and their binding to posterior tethers. We infer that anterior tethers have a weakened ability to bind seeding particles in mnl embryos. Immobilizing mnl embryos with the anterior end of the ear oriented downward effectively concentrated the dense seeding particles near the anterior tethers and permitted all to form anterior otoliths. However, immobilizing mnl embryos after 24 h when seeding particles were depleted did not facilitate anterior otolith formation. Together, these data demonstrate that the ability to initiate otolith formation is limited to a critical period, from 18.5 to 24 h, and that interfering with the functions of tether cell kinocilia or beating cilia impairs otolith seeding and subsequent otolith morphogenesis.

The expression domain of two related homeobox genes defines a compartment in the chicken inner ear that may be involved in semicircular canal formation

Homeobox-containing genes encode a class of proteins that control patterning in developing systems, in some cases by acting as selector genes that define compartment identity. In an effort to demonstrate a similar role for such genes during ear development in the chicken, we present a detailed expression study of two related homeobox-containing genes, SOHo-1 and GH6, using in situ hybridization. At otocyst stages the two genes define a broad lateral domain of expression, which may represent a developmental compartment. Three-dimensional computer reconstructions of SOHo-1 expression at these and later stages revealed that the lateral domain becomes progressively restricted to the three semicircular canals. Thus, SOHo-1 and GH6 are among a small group of markers for a specific structural component of the inner ear. The gene expression domain initially includes the sensory regions of the semicircular canals, known as the cristae ampullaris, but none of the other four sensory organs which were recognizable by BMP4 expression during early morphogenesis (stages 19-24). Significantly, two of the sensory organs (the superior and posterior cristae) were found at the limits, or boundaries, of the SOHo-1/GH6 expression domain, suggesting that compartment boundaries may be involved in specifying sensory organ location as well as identity. Maintained expression at the boundaries may aid in specifying the location of canal outgrowth. These concepts are presented as a formal model which emphasizes that patterning information could be provided at the boundaries of gene expression domains in the inner ear.

Cross-interactions between two members of the Dlx family of homeobox-containing genes during zebrafish development

The Dlx homeobox genes of vertebrates are transcribed in multiple cells of the embryo with overlapping patterns but often with different onsets of expression. Here we describe the interaction between two dlx genes, dlx3 and dlx4, during zebrafish development. The observation that dlx3 expression precedes that of dlx4 in the otic vesicle led us to investigate whether dlx3 had the ability to control expression of dlx4. Truncated versions of dlx3 were overexpressed in zebrafish embryos and the expression patterns of dlx4 were examined later in development. Overexpression of truncated forms of Dlx3 or of a Dlx3-Dlx2 chimera was found to result in perturbations in dlx4 expression. In addition, cotransfection experiments indicated the ability of Dlx3 to activate transcription through a 1.7-kb fragment of the 5 prime flanking region of dlx4. These results suggest that dlx4 is one of the target genes of dlx3 in embryos and that cross-regulatory interactions between Dlx genes may be one of the mechanisms responsible for their overlapping expression.

Zebrafish: tools for investigating cellular differentiation

Two recent large-scale genetic screens in zebrafish have identified many mutations that affect differentiation in a variety of organ systems, particularly the notochord, the neural crest and the blood. The combination of these newly identified mutations and well established embryological methods makes zebrafish uniquely suited among vertebrate experimental systems to simultaneously address the roles of specific genes and specific cell-cell interactions during differentiation.

Defining the boundaries of zebrafish developmental genetics

The first systematic, functional screens to identify the genes involved in vertebrate embryogenesis have been completed in the zebrafish, Danio rerio. In an extraordinary issue of the journal Development, devoted entirely to the results of these screens, over 500 mutant loci, many with multiple alleles, are described and classified according to the phenotypes they produce. Each class defines a small number of genes that act together to determine the proper development of many features of vertebrate anatomy, from the determination of body plan to the development of discrete organs and cell types.