Multiple delta genes and lateral inhibition in zebrafish primary neurogenesis

Control of her1 expression during zebrafish somitogenesis by a Delta-dependent oscillator and an independent wave-front activity

Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaDsignaling is required for this oscillation.aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream ofher1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression inaei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fssbut not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading ofher1 stripes in the PSM is governed by two separate activities.

Structure and function in the saccule of the goldfish (Carassius auratus): a model of diversity in the non-amniote ear

The vertebrate inner ear is comprised of a remarkable diversity of cell types, including several types of sensory hair cells. In amniotes (reptiles, birds, and mammals), the morphological and physiological characteristics that distinguish these cell types have been well documented, while cellular variation in the ears of non-amniotes (all other vertebrate groups) has remained underrecognized. Since non-amniotes have become increasingly popular models for developmental and genetic research, a more comprehensive understanding of structure and function in the inner ears of these species is warranted. This paper first reviews the large body of data describing the morphology and physiology of hair cells and afferent neurons in the inner ear of the goldfish (Carassius auratus). In particular, we examine the structure of the goldfish saccule, an endorgan that has been the subject of numerous investigations on audition. New data on the structural variation of synaptic bodies in saccular hair cells are also presented, and the functional implications of these data are discussed. Finally, we conclude that hair cell structure varies along the length of the goldfish saccule in a manner consistent with known physiological characteristics of the endorgan. The saccule provides an excellent model for investigating structure-function relationships in the vertebrate inner ear, as well as the development of auditory and vestibular sensory epithelia.

Sequence and embryonic expression of deltaC in the zebrafish

Four genes - deltaA, deltaB, deltaC and deltaD - coding for homologues of the Notch ligand Delta have been discovered in zebrafish (Haddon et al., 1998b). We report here the cDNA sequence and expression pattern of deltaC. Its closest relatives are deltaB and Xenopus X-Delta-2. Unlike deltaA, deltaB, and deltaD, deltaC is not expressed in the majority of nascent primary neurons; but it is strongly expressed in the early retina, where it precedes other delta genes. It is also expressed in cranial ganglia, in sensory epithelia including ear and lateral line, and in scattered epidermal cells. In the mesoderm, expression is visible by 50% epiboly; it is seen subsequently in the tail bud, in stripes in the presomitic mesoderm and in the posterior half of each somite. There is expression also in notochord, blood vessels and pronephros.

Nab proteins mediate a negative feedback loop controlling Krox-20 activity in the developing hindbrain

The developing vertebrate hindbrain is transiently subdivided along the anterior-posterior axis into metameric units, called rhombomeres (r). These segments constitute units of lineage restriction and display specific gene expression patterns. The transcription factor gene Krox-20 is restricted to r3 and r5, and is required for the development of these rhombomeres. We present evidence that Krox-20 transcriptional activity is under the control of a negative feedback mechanism in the hindbrain. This regulatory loop involves two closely related proteins, Nabl and Nab2, previously identified as antagonists of Krox-20 transcriptional activity in cultured cells. Here we show that in the mouse hindbrain, Nab1 and Nab2 recapitulate the Krox-20 expression pattern and that their expression is dependent on Krox-20 function. Furthermore, misexpression of Nab1 or Nab2 in zebrafish embryos leads to alterations in the expression patterns of several hindbrain markers, consistent with an inhibition of Krox-20 activity. Taken together, these data indicate that Krox-20 positively regulates the expression of its own antagonists and raise the possibility that this negative feedback regulatory loop may play a role in the control of hindbrain development.

Lateral induction by juxtacrine signaling is a new mechanism for pattern formation

Many signaling molecules in epithelia are now known to function in a membrane-bound form, binding to receptors on immediately neighbouring cells. This "juxtacrine" mode of communication has been well studied in the case of lateral inhibition, where ligand binding at the cell surface downregulates ligand and receptor expression, and is known to generate spatial patterns with a wavelength of exactly two cells. However, recent evidence shows that a number of juxtacrine signals can lead to the opposite phenomenon of lateral induction. Here, we use mathematical modeling to show that such positive feedback, in combination with juxtacrine communication, provides a novel mechanism for the generation of spatial patterns, with wavelengths that vary with parameters and can be many cell lengths.

Autonomous and non-autonomous regulation of mammalian neurite development by Notch1 and Delta1

BACKGROUND

On the basis of experiments suggesting that Notch and Delta have a role in axonal development in Drosophila neurons, we studied the ability of components of the Notch signaling pathway to modulate neurite formation in mammalian neuroblastoma cells in vitro.

RESULTS

We observed that N2a neuroblastoma cells expressing an activated form of Notch, Notch1(IC), produced shorter neurites compared with controls, whereas N2a cell lines expressing a dominant-negative Notch1 or a dominant-negative Delta1 construct extended longer neurites with a greater number of primary neurites. We then compared the effects on neurites of contacting Delta1 on another cell and of overexpression of Delta1 in the neurite-extending cell itself. We found that N2a cells co-cultured with Delta1-expressing quail cells produced fewer and shorter neuritic processes. On the other hand, high levels of Delta1 expressed in the N2a cells themselves stimulated neurite extension, increased numbers of primary neurites and induced expression of Jagged1 and Notch1.

CONCLUSIONS

These studies show that Notch signals can antagonize neurite outgrowth and that repressing endogenous Notch signals enhances neurite outgrowth in neuroblastoma cells. Notch signals therefore act as regulators of neuritic extension in neuroblastoma cells. The response of neuritic processes to Delta1 expressed in the neurite was opposite to that to Delta1 contacted on another cell, however. These results suggest a model in which developing neurons determine their extent of process outgrowth on the basis of the opposing influences on Notch signals of ligands contacted on another cell and ligands expressed in the same cell.

The deltaA gene of zebrafish mediates lateral inhibition of hair cells in the inner ear and is regulated by pax2.1

Recent studies of inner ear development suggest that hair cells and support cells arise within a common equivalence group by cell-cell interactions mediated by Delta and Notch proteins. We have extended these studies by analyzing the effects of a mutant allele of the zebrafish deltaA gene, deltaA(dx2), which encodes a dominant-negative protein. deltaA(dx2/dx2)homozygous mutants develop with a 5- to 6-fold excess of hair cells and a severe deficiency of support cells. In addition, deltaA(dx2/dx2) mutants show an increased number of cells expressing pax2.1 in regions where hair cells are normally produced. Immunohistological analysis of wild-type and deltaA(dx2/dx2) mutant embryos confirmed that pax2.1 is expressed during the initial stages of hair cell differentiation and is later maintained at high levels in mature hair cells. In contrast, pax2.1 is not expressed in support cells. To address the function of pax2.1, we analyzed hair cell differentiation in no isthmus mutant embryos, which are deficient for pax2.1 function. no isthmus mutant embryos develop with approximately twice the normal number of hair cells. This neurogenic defect correlates with reduced levels of expression of deltaA and deltaD in the hair cells in no isthmus mutants. Analysis of deltaA(dx2/dx2); no isthmus double mutants showed that no isthmus suppresses the deltaA(dx2) phenotype, probably by reducing levels of the dominant-negative mutant protein. This interpretation was supported by analysis of T(msxB)(b220), a deletion that removes the deltaA locus. Reducing the dose of deltaA(dx2) by generating deltaA(dx2)/T(msxB)(b220)trans-heterozygotes weakens the neurogenic effects of deltaA(dx2), whereas T(msxB)(b220) enhances the neurogenic defects of no isthmus. mind bomb, another strong neurogenic mutation that may disrupt reception of Delta signals, causes a 10-fold increase in hair cell production and is epistatic to both no isthmus and deltaA(dx2). These data indicate that deltaA expressed by hair cells normally prevents adjacent cells from adopting the same cell fate, and that pax2.1 is required for normal levels of Delta-mediated lateral inhibition.

Somitogenesis in zebrafish

Both genetic and embryological studies in the zebrafish, Danio rerio, have contributed to our general understanding of how somites form and differentiate. In the zebrafish, mutants have been isolated that have specific effects on virtually every aspect of somite development. The fss-type mutants, defining 5 genes, affect somite segmentation and epithelialization. The you-type mutants, comprising 7 genes, and mutants in another 13 genes defective in notochord formation, have somites with abnormal pattern and morphology. Eighteen genes have been identified that are required for the differentiation and maintenance of the somitic musculature, and 2 genes have been identified that are involved in the development of motoneurons that innervate the somitic musculature. The true utility of the zebrafish lies in the ability to combine genetic analysis with embryological experimentation. Such analysis of somite segmentation suggests that homologues of both the Drosophila pair-rule and segment polarity genes, her1 and Sonic hedge-hog, respectively, are involved generating periodicity during somitogenesis. The Sonic hedge-hog protein secreted from the notochord also induces the formation of specific muscle types including the slow muscle fibers which are initially induced in the medial somite and undergo a series of morphological transitions including migration through the somite to the lateral surface where they complete their differentiation. The role of the notochord in patterning the somite is also demonstrated by its involvement in regulating the permissiveness of the somite to the extension of axons of primary motoneurons.

A two-step mechanism generates the spacing pattern of the ciliated cells in the skin of Xenopus embryos

The skin of Xenopus embryos contains a population of specialized ciliated cells that are distributed in an evenly spaced pattern. Here we describe two successive steps that govern the differentiation and the generation of the spacing pattern of these ciliated cells. The first step occurs in the inner or sensorial layer of the non-neural ectoderm where a subset of cells are chosen to differentiate into ciliated-cell precursors. This choice is under the control of lateral inhibition mediated by a Suppressor of Hairless-dependent Notch signaling pathway, in which X-Delta-1 is the putative ligand driving the selection process, and a new Enhancer-of-Split-related gene is an epidermal target of Notch signaling. Because nascent ciliated-cell precursors prevent neighboring cells from taking on the same fate, a scattered pattern of these precursors is generated within the deep layer of the non-neural ectoderm. Ciliated-cell precursors then intercalate into the outer layer of cells in the epidermis. We show that the intercalation event acts as a second step to regulate the spacing of the mature ciliated cells. We propose that the differentiation of the ciliated cells is not only regulated by Notch-mediated lateral inhibition, but is also an example where differentiation is coupled to the movement of cells from one cell layer to another.

Zebrafish narrowminded suggests a genetic link between formation of neural crest and primary sensory neurons

In the developing vertebrate nervous system, both neural crest and sensory neurons form at the boundary between non-neural ectoderm and the neural plate. From an in situ hybridization based expression analysis screen, we have identified a novel zebrafish mutation, narrowminded (nrd), which reduces the number of early neural crest cells and eliminates Rohon-Beard (RB) sensory neurons. Mosaic analysis has shown that the mutation acts cell autonomously suggesting that nrd is involved in either the reception or interpretation of signals at the lateral neural plate boundary. Characterization of the mutant phenotype indicates that nrd is required for a primary wave of neural crest cell formation during which progenitors generate both RB sensory neurons and neural crest cells. Moreover, the early deficit in neural crest cells in nrd homozygotes is compensated later in development. Thus, we propose that a later wave can compensate for the loss of early neural crest cells but, interestingly, not the RB sensory neurons. We discuss the implications of these findings for the possibility that RB sensory neurons and neural crest cells share a common evolutionary origin.

her1, a zebrafish pair-rule like gene, acts downstream of notch signalling to control somite development

During vertebrate embryonic development, the paraxial mesoderm becomes subdivided into metameric units known as somites. In the zebrafish embryo, genes encoding homologues of the proteins of the Drosophila Notch signalling pathway are expressed in the presomitic mesoderm and expression is maintained in a segmental pattern during somitogenesis. This expression pattern suggests a role for these genes during somite development. We misexpressed various zebrafish genes of this group by injecting mRNA into early embryos. RNA encoding a constitutively active form of notch1a (notch1a-intra) and a truncated variant of deltaD [deltaD(Pst)], as well as transcripts of deltaC and deltaD, the hairy-E(spl) homologues her1 and her4, and groucho2 were tested for their effects on somite formation, myogenesis and on the pattern of transcription of putative downstream genes. In embryos injected with any of these RNAs, with the exception of groucho2 RNA, the paraxial mesoderm differentiated normally into somitic tissue, but failed to segment correctly. Activation of notch results in ectopic activation of her1 and her4. This misregulation of the expression of her genes might be causally related to the observed mesodermal defects, as her1 and her4 mRNA injections led to effects similar to those seen with notch1a-intra. deltaC and deltaD seem to function after subdivision of the presomitic mesoderm, since the her gene transcription pattern in the presomitic mesoderm remains essentially normal after misexpression of delta genes. Whereas notch signalling alone apparently does not affect myogenesis, zebrafish groucho2 is involved in differentiation of mesodermal derivatives.

Patterning motoneurons in the vertebrate nervous system

Vertebrate motoneurons show considerable diversity in their soma locations, axonal trajectories and innervation targets. Results from studies of a variety of vertebrate species as well as fruit-flies are elucidating the mechanisms by which this diversity is generated. Motoneuron subpopulations appear to be defined by combinations of transcription factor genes expressed in distinct spatiotemporal patterns in both motoneuron progenitors and postmitotic motoneurons. Notochord-derived signals can induce motoneuron formation, paraxial-mesoderm-derived signals can pattern motoneuron subpopulations along the rostrocaudal body axis, and local signals within the neural tube can regulate the number and time at which motoneurons form. Additional, later signals can promote formation of proper central circuitry and motoneuron survival. The identification of the genes and signals responsible for regulating these processes should help to provide a more-detailed understanding of motoneuron patterning.

Heat shock produces periodic somitic disturbances in the zebrafish embryo

Environmental influences are known to produce segmental defects in a variety of organisms. In this paper we report upon segmental aberrations produced by brief heat shocks delivered to developing zebrafish embryos. The initial defects in the segmental pattern of somitic boundaries and motoneuron axon outgrowth were usually observed five somites caudal to the somite which was forming at the time of heat shock application. Segmental defects in zebrafish embryos exposed to a single heat shock treatment can occur in a periodic pattern similar to the multiple disturbances observed to occur in chick embryos. These data are discussed with regard to models involving cell cycle synchrony or 'clock and wavefront' schemes in the process of somitogenesis.

The zebrafish detour gene is essential for cranial but not spinal motor neuron induction

The zebrafish detour (dtr) mutation generates a novel neuronal phenotype. In dtr mutants, most cranial motor neurons, especially the branchiomotor, are missing. However, spinal motor neurons are generated normally. The loss of cranial motor neurons is not due to aberrant hindbrain patterning, failure of neurogenesis, increased cell death or absence of hh expression. Furthermore, activation of the Hh pathway, which normally induces branchiomotor neurons, fails to induce motor neurons in the dtr hindbrain. Despite this, not all Hh-mediated regulation of hindbrain development is abolished since the regulation of a neural gene by Hh is intact in the dtr hindbrain. Finally, dtr can function cell autonomously to induce branchiomotor neurons. These results suggest that detour encodes a component of the Hh signaling pathway that is essential for the induction of motor neurons in the hindbrain but not in the spinal cord and that dtr function is required for the induction of only a subset of Hh-mediated events in the hindbrain.

Comparison of early nerve cord development in insects and vertebrates

It is widely held that the insect and vertebrate CNS evolved independently. This view is now challenged by the concept of dorsoventral axis inversion, which holds that ventral in insects corresponds to dorsal in vertebrates. Here, insect and vertebrate CNS development is compared involving embryological and molecular data. In insects and vertebrates, neurons differentiate towards the body cavity. At early stages of neurogenesis, neural progenitor cells are arranged in three longitudinal columns on either side of the midline, and NK-2/NK-2.2, ind/Gsh and msh/Msx homologs specify the medial, intermediate and lateral columns, respectively. Other pairs of regional specification genes are, however, expressed in transverse stripes in insects, and in longitudinal stripes in the vertebrates. There are differences in the regional distribution of cell types in the developing neuroectoderm. However, within a given neurogenic column in insects and vertebrates some of the emerging cell types are remarkably similar and may thus be phylogenetically old: NK-2/NK-2.2-expressing medial column neuroblasts give rise to interneurons that pioneer the medial longitudinal fascicles, and to motoneurons that exit via lateral nerve roots to then project peripherally. Lateral column neuroblasts produce, among other cell types, nerve root glia and peripheral glia. Midline precursors give rise to glial cells that enwrap outgrowing commissural axons. The midline glia also express netrin homologs to attract commissural axons from a distance.

Neurotrophin actions during the development of the peripheral nervous system

Neurotrophins are important regulators of the development and maintenance of the vertebrate nervous system. Besides their well-established role in promoting neuronal survival during development, in vitro data suggest that they can regulate proliferation, survival, and differentiation of precursor cells. Analysis of the developing peripheral nervous system in mouse strains carrying mutations in genes encoding the neurotrophins and their receptors indicate, however, that lack of neurotrophin signalling results in specific neuronal deficits that are primarily due to neuronal death. Many of these deficits occur before final target encounter.

her4, a zebrafish homologue of the Drosophila neurogenic gene E(spl), is a target of NOTCH signalling

her4 encodes a zebrafish bHLH protein of the hairy-E(spl) family. The gene is transcribed in a complex pattern in the developing nervous system and in the hypoblast. During early neurogenesis, her4 expression domains include the regions of the neural plate from which primary neurons arise, suggesting that the gene is involved in directing their development. Indeed, misexpression of specific her4 variants leads to a reduction in the number of primary neurons formed. The amino-terminal region of her4, including the basic domain, and the region between the putative helix IV and the carboxy-terminal tetrapeptide wrpw are essential for this effect, since her4 variants lacking either of these regions are non-functional. However, the carboxy-terminal wrpw itself is dispensable. We have examined the interrelationships between deltaD, deltaA, notch1, her4 and neurogenin1 by means of RNA injections. her4 is involved in a regulatory feedback loop which modulates the activity of the proneural gene neurogenin, and as a consequence, of deltaA and deltaD. Activation of notch1 leads to strong activation of her4, to suppression of neurogenin transcription and, ultimately, to a reduction in the number of primary neurons. These results suggest that her4 acts as a target of notch-mediated signals that regulate primary neurogenesis.

NUMB localizes in the basal cortex of mitotic avian neuroepithelial cells and modulates neuronal differentiation by binding to NOTCH-1

The importance of lateral inhibition mediated by NOTCH signaling is well demonstrated to control neurogenesis both in invertebrates and vertebrates. We have identified the chicken homolog of Drosophila numb, which suppresses NOTCH signaling. We show that chicken NUMB (c-NUMB) protein is localized to the basal cortex of mitotic neuroepithelial cells, suggesting that c-NUMB regulates neurogenesis by the modification of NOTCH signaling through asymmetrical cell division. Consistent with this suggestion, we show (1) that c-NUMB interferes with the nuclear translocation of activated c-NOTCH-1 through direct binding to the PEST sequence in the cytoplasmic domain of c-NOTCH-1 and (2) that c-NUMB interferes with c-NOTCH-1-mediated inhibition of neuronal differentiation.

Delta-mediated specification of midline cell fates in zebrafish embryos

BACKGROUND

Fate mapping studies have shown that progenitor cells of three vertebrate embryonic midline structures - the floorplate in the ventral neural tube, the notochord and the dorsal endoderm - occupy a common region prior to gastrulation. This common region of origin raises the possibility that interactions between midline progenitor cells are important for their specification prior to germ layer formation.

RESULTS

One of four known zebrafish homologues of the Drosophila melanogaster cell-cell signaling gene Delta, deltaA (dlA), is expressed in the developing midline, where progenitor cells of the ectodermal floorplate, mesodermal notochord and dorsal endoderm lie close together before they occupy different germ layers. We used a reverse genetic strategy to isolate a missense mutation of dlA, dlAdx2, which coordinately disrupts the development of floorplate, notochord and dorsal endoderm. The dlAdx2 mutant embryos had reduced numbers of floorplate and hypochord cells; these cells lie above and beneath the notochord, respectively. In addition, mutant embryos had excess notochord cells. Expression of a dominant-negative form of Delta protein driven by mRNA microinjection produced a similar effect. In contrast, overexpression of dlA had the opposite effect: fewer trunk notochord cells and excess floorplate and hypochord cells.

CONCLUSION

Our results indicate that Delta signaling is important for the specification of midline cells. The results are most consistent with the hypothesis that developmentally equivalent midline progenitor cells require Delta-mediated signaling prior to germ layer formation in order to be specified as floorplate, notochord or hypochord.

Delta1 expression during avian hair cell regeneration

Postembryonic production of hair cells, the highly specialized receptors for hearing, balance and motion detection, occurs in a precisely controlled manner in select species, including avians. Notch1, Delta1 and Serrate1 mediate cell specification in several tissues and species. We examined expression of the chicken homologs of these genes in the normal and drug-damaged chick inner ear to determine if signaling through this pathway changes during hair cell regeneration. In untreated post-hatch chicks, Delta1 mRNA is abundant in a subpopulation of cells in the utricle, which undergoes continual postembryonic hair cell production, but it is absent from all cells in the basilar papilla, which is mitotically quiescent. By 3 days after drug-induced hair cell injury, Delta1 expression is highly upregulated in areas of cell proliferation in both the utricle and basilar papilla. Delta1 mRNA levels are elevated in progenitor cells during DNA synthesis and/or gap 2 phases of the cell cycle and expression is maintained in both daughter cells immediately after mitosis. Delta1 expression remains upregulated in cells that differentiate into hair cells and is downregulated in cells that do not acquire the hair cell fate. Delta1 mRNA levels return to normal by 10 days after hair cell injury. Serrate1 is expressed in both hair cells and support cells in the utricle and basilar papilla, and its expression does not change during the course of drug-induced hair cell regeneration. In contrast, Notch1 expression, which is limited to support cells in the quiescent epithelium, is increased in post-M-phase cell pairs during hair cell regeneration. This study provides initial evidence that Delta-Notch signaling may be involved in maintaining the correct cell types and patterns during postembryonic replacement of sensory epithelial cells in the chick inner ear.

Notch signalling pathway mediates hair cell development in mammalian cochlea

The mammalian cochlea contains an invariant mosaic of sensory hair cells and non-sensory supporting cells reminiscent of invertebrate structures such as the compound eye in Drosophila melanogaster. The sensory epithelium in the mammalian cochlea (the organ of Corti) contains four rows of mechanosensory hair cells: a single row of inner hair cells and three rows of outer hair cells. Each hair cell is separated from the next by an interceding supporting cell, forming an invariant and alternating mosaic that extends the length of the cochlear duct. Previous results suggest that determination of cell fates in the cochlear mosaic occurs via inhibitory interactions between adjacent progenitor cells (lateral inhibition). Cells populating the cochlear epithelium appear to constitute a developmental equivalence group in which developing hair cells suppress differentiation in their immediate neighbours through lateral inhibition. These interactions may be mediated through the Notch signalling pathway, a molecular mechanism that is involved in the determination of a variety of cell fates. Here we show that genes encoding the receptor protein Notch1 and its ligand, Jagged 2, are expressed in alternating cell types in the developing sensory epithelium. In addition, genetic deletion of Jag2 results in a significant increase in sensory hair cells, presumably as a result of a decrease in Notch activation. These results provide direct evidence for Notch-mediated lateral inhibition in a mammalian system and support a role for Notch in the development of the cochlear mosaic.

Control of neurogenesis--lessons from frogs, fish and flies

Two types of genes activated by neural inducers have been identified, those that lead to the activation of proneural genes and those that limit the activity of these genes to specific domains in the neural plate. The analysis of these genes has begun to fill gaps in our understanding of events that lead from neural induction to the generation of neurons within three longitudinal columns in the Xenopus and zebrafish neural plate.

Genetic and molecular analyses of motoneuron development

Motoneurons have distinct identities and muscle targets. Recent classical and molecular genetic studies in flies and vertebrates have begun to elucidate how motoneuron identities and target specificities are established. Many of the same molecules participate in the guidance of both vertebrate and fly motor axons. It is less clear, however, whether the same molecular mechanisms establish vertebrate and fly motoneuron identities.

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.

Cell fate choices and the expression of Notch, Delta and Serrate homologues in the chick inner ear: parallels with Drosophila sense-organ development

The sensory patches in the vertebrate inner ear are similar in function to the mechanosensory bristles of a fly, and consist of a similar set of cell types. If they are truly homologous structures, they should also develop by similar mechanisms. We examine the genesis of the neurons, hair cells and supporting cells that form the sensory patches in the inner ear of the chick. These all arise from the otic epithelium, and are produced normally even in otic epithelium cultured in isolation, confirming that their production is governed by mechanisms intrinsic to the epithelium. First, the neuronal sublineage becomes separate from the epithelial: between E2 and E3.5, neuroblasts delaminate from the otocyst. The neuroblasts then give rise to a mixture of neurons and neuroblasts, while the sensory epithelial cells diversify to form a mixture of hair cells and supporting cells. The epithelial patches where this occurs are marked from an early stage by uniform and maintained expression of the Notch ligand Serrate1. The Notch ligand Delta1 is also expressed, but transiently and in scattered cells: it is seen both early, during neuroblast segregation, where it appears to be in the nascent neuroblasts, and again later, in the ganglion and in differentiating sensory patches, where it appears to be in the nascent hair cells, disappearing as they mature. Delta-Notch-mediated lateral inhibition may thus act at each developmental branchpoint to drive neighbouring cells along different developmental pathways. Our findings indicate that the sensory patches of the vertebrate inner ear and the sensory bristles of a fly are generated by minor variations of the same basic developmental program, in which cell diversification driven by Delta-Notch and/or Serrate-Notch signalling plays a central part.

Molecular cloning of Zcoe2, the zebrafish homolog of Xenopus Xcoe2 and mouse EBF-2, and its expression during primary neurogenesis

Xcoe2 is a recently identified HLH transcription factor of the Xenopus primary neurogenesis pathway, which is necessary downstream of Neurogenin to stabilize neuroblast determination (Dubois, L. et al., 1998. Curr. Biol. 8, 199-209). We report here the embryonic expression pattern of Zcoe2, its zebrafish homolog. As observed for Xcoe2, Zcoe2 is strongly expressed in a subset of the neurogenin1- (ngn1-) positive primary neuroblasts of the spinal cord. In the anterior neural plate, in contrast, Zcoe2 is expressed earlier and more widely than ngn1. This pattern is strongly maintained in the presumptive mesencephalon and rhombomeres 1-4 until the 2-3-somite stage. This expression of Zcoe2 in the brain anlage calls for a re-analysis in zebrafish of the functional relationship demonstrated in Xenopus between Coe2 and Neurogenin factors. At later stages, Zcoe2 is expressed in early forming neurons of the anterior brain and is a marker of the olfactory placodes.

Transient expression of a novel Six3-related zebrafish gene during gastrulation and eye formation

Both the Drosophila homeobox gene sine oculis and its murine homologue Six3 have regulatory functions in eye development. In zebrafish, in addition to two previously reported homologues of murine Six3, we have identified a related gene (six7). Although the deduced Six7 protein shares less than 68% sequence identity with the other known zebrafish Six3-like proteins, the embryonic expression patterns have highly conserved features. The six7 transcripts are first detected in involuting axial mesendoderm and, subsequently, in the overlying neurectoderm from which the forebrain and optic primordia develop. Similar to the two other zebrafish Six3 homologues, the expression boundaries of six7 correspond quite closely with the edges of the optic vesicles. Hence, the partially overlapping expression domains of these three six genes probably contribute to anteroposterior specification and in defining the eye primordia.

Notch signaling: direct or what?

Notch signaling has been implicated in a wide variety of processes from cell-fate decisions, tissue patterning and morphogenesis to human diseases and cancer. A model for Notch directly regulating gene expression has been proposed and at least two signaling pathways have been identified; however, the molecular mechanism(s) by which Notch signaling produces so many outcomes remains unclear.

Characterization of neurotrophin and Trk receptor functions in developing sensory ganglia: direct NT-3 activation of TrkB neurons in vivo

Spinal sensory ganglia have been shown to contain neuronal subpopulations with different functions and neurotrophin dependencies. Neurotrophins act, in large part, through Trk receptor tyrosine kinases: nerve growth factor (NGF) via TrkA, brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) via TrkB, and neurotrophin-3 (NT-3) via TrkC. In the present paper, we use antibodies to TrkA, TrkB, and TrkC to characterize their expression patterns and to determine which subpopulations of cells are lost in mice lacking individual neurotrophins or Trk receptors. Despite previous reports of Trk receptor mRNAs in neural crest cells, we detect Trk receptor proteins only in neurons and not in neural crest cells or neuronal precursors. Comparisons of neonatal mice deficient in NT-3 or its cognate receptor TrkC have shown that there is a much greater deficiency in spinal sensory neurons in the former, suggesting that NT-3 may activate receptors in addition to TrkC. Using the same antibodies, we show that, during the major period of neurogenesis, NT-3 is required to maintain neurons that express TrkB in addition to those that express TrkC but is not essential for neurons expressing TrkA. Results also indicate that survival of cells expressing both receptors can be maintained by activation of either one alone. NT-3 can thus activate more than one Trk receptor in vivo, which when coexpressed are functionally redundant.

Regulation of neuronal specification in the zebrafish spinal cord by Delta function

The vertebrate spinal cord consists of a large number of different cell types in close proximity to one another. The identities of these cells appear to be specified largely by information acquired from their local environments. We report here that local cell-cell interactions, mediated by zebrafish homologues of the Drosophila melanogaster neurogenic gene, Delta, regulate specification of diverse neuronal types in the ventral spinal cord. We describe identification of a novel zebrafish Delta gene expressed specifically in the nervous system and show, by expressing a dominant negative form of Delta protein in embryos, that Delta proteins mediate lateral inhibition in the zebrafish spinal cord. Furthermore, we find that Delta function is important for specification of a variety of spinal cord neurons, suggesting that lateral inhibition serves to diversify neuronal fate during development of the vertebrate spinal cord.