A zebrafish histone variant H2A.F/Z and a transgenic H2A.F/Z:GFP fusion protein for in vivo studies of embryonic development

Abstract. We have generated transgenic zebrafish lines expressing a fusion of a histone variant, H2A.F/Z, to the green fluorescent protein (GFP) of the jellyfish Aequorea victoria. Here, we describe the molecular cloning, partial characterisation and expression of the zebrafish H2A.F/Z histone gene, as well as the construction of the transgene and its transformation into the zebrafish germ line. No abnormality can be detected in transgenic fish expressing the H2A.F/Z:GFP fusion protein. The nuclear localisation of the fusion protein correlates with the start of zygotic transcription, in that it is present in the unfertilised egg and in the cytoplasm of cells after the first cleavages, being found in some nuclei after the seventh or eighth cleavage, whereas all nuclei from the 1,000-cell stage on, i.e. after midblastula transition, contain protein. In addition to these data, we present a few examples of the many possible applications of this transgenic line for developmental studies in vivo. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00427-001-0196-x

Notch signaling is required for arterial-venous differentiation during embryonic vascular development

Recent evidence indicates that acquisition of artery or vein identity during vascular development is governed, in part, by genetic mechanisms. The artery-specific expression of a number of Notch signaling genes in mouse and zebrafish suggests that this pathway may play a role in arterial-venous cell fate determination during vascular development. We show that loss of Notch signaling in zebrafish embryos leads to molecular defects in arterial-venous differentiation, including loss of artery-specific markers and ectopic expression of venous markers within the dorsal aorta. Conversely, we find that ectopic activation of Notch signaling leads to repression of venous cell fate. Finally, embryos lacking Notch function exhibit defects in blood vessel formation similar to those associated with improper arterial-venous specification. Our results suggest that Notch signaling is required for the proper development of arterial and venous blood vessels, and that a major role of Notch signaling in blood vessels is to repress venous differentiation within developing arteries.

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An instructive function for Notch in promoting gliogenesis in the zebrafish retina

The Gal4-UAS technique has been used to misexpress a constitutively active Notch receptor variant (notch1a-intra) in the developing zebrafish retina. This is the first study to use this technique to misexpress genes and assess their function in neural development of the zebrafish. Expression of activated Notch1a either ubiquitously, driven by a heat-shock70 promoter, or in a spatially regulated manner, controlled by the deltaD promoter, causes a block in neuronal differentiation that affects all cell types. Developing cells take on either a glial fate or remain undifferentiated. A large number of cells eventually undergo apoptosis. These phenotypic effects of activated Notch1a are expressed cell autonomously. Cells within central regions of the retina adopt a glial fate if they express activated Notch1a in a time window that extends from 27 to 48 hours postfertilization. This period corresponds mainly to the time of origin of ganglion cells in the normal retina. Activation of notch1a at later stages results in defects in cell type specification that remain restricted to the ciliary marginal zone, whereas neuronal types are specified normally within the central region. These observations indicate that glial differentiation is initiated by Notch1a-intra expressing cells, which become postmitotic in the same time window. Our results strongly suggest that Notch1a instructs a certain cell population to enter gliogenesis, and keeps the remaining cells in an undifferentiated state. Some or all of these cells will eventually succumb to apoptosis.

Distinct regulatory elements direct delta1 expression in the nervous system and paraxial mesoderm of transgenic mice

The Delta1 gene encodes one of the Notch ligands in mice. Delta1 is expressed during early embryogenesis in a complex and dynamic pattern in the paraxial mesoderm and neuroectoderm, and is essential for normal somitogenesis and neuronal differentiation. Molecular components thought to act in response to ligand binding and Notch activation have been identified in different species. In contrast, little is known about the transcriptional regulation of Notch receptors and their ligands. As a first step to identify upstream factors regulating Delta1 expression in different tissues, we searched for cis-regulatory regions in the Delta1 promoter able to direct heterologous gene expression in a tissue specific manner in transgenic mice. Our results show that a 4.3 kb genomic DNA fragment of the Delta1 gene is sufficient in a lacZ reporter transgene to reproduce most aspects of Delta1 expression from the primitive streak stage to early organogenesis. Using a minimal Delta1 promoter we also show that this upstream region contains distinct regulatory modules that individually direct tissue-specific transgene expression in subdomains of the endogenous expression pattern. It appears that expression in the paraxial mesoderm depends on the interaction of multiple positive and negative regulatory elements. We also find that at least some regulatory sequences required for transgene expression in subdomains of the neural tube have been maintained during the evolution of mammals and teleost fish, suggesting that part of the regulatory network that controls expression of Delta genes may be conserved.

A recessive mutation leading to vertebral ankylosis in zebrafish is associated with amino acid alterations in the homologue of the human membrane-associated guanylate kinase DLG3

We describe the characterization of the zebrafish homologue of the human gene DLG3. The zebrafish dlg3 gene encodes a membrane-associated guanylate kinase containing a single PDZ domain. This gene was cloned using a gene-trap construct inserted in the gene's first intron. The insertion co-segregates with a viable mutation called humpback (hmp), which leads to formation of ankylotic vertebrae in adult fishes. Insertion and mutation have both been mapped to chromosome 12, in a segment which is syntenic with region p12 to q12 of human chromosome 17. The hmp mutant phenotype, however, appears to be due to two point mutations in the guanylate kinase domain rather than to the transgene insertion itself. The results of this study are discussed in the light of the possible function of the guanylate kinase domain.

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.

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.

Region-specific cell clones in the developing spinal cord of the zebrafish

To analyse the proliferative abilities of cells within particular regions of the zebrafish neural plate, injections of fluorescein-dextran were made into single cells at either medial or intermediary positions in the neural plate region of two-somite stage embryos. The resulting cell clones were analysed in 3. 5-day-old embryos. Clones with similar compositions were found among those derived from injections in both regions, and these were grouped into classes. 78 clones 29 obtained following injections in the medial region, and 22 of 59 cell clones derived from injections in the intermediary region, were classifiable into 9 and 10 different classes, respectively, each comprising a variable number of clones. Several identified cell types, as well as each of the clone classes themselves, were specific for the regions of the neural plate from which they derived, i.e. they were not represented among the clones derived from the other region. These results suggest that the composition of the lineages derived from particular cells is constant in different animals.

A new beta-globin gene from the zebrafish, betaE1, and its pattern of transcription during embryogenesis

In a search for novel, developmentally regulated genes we screened randomly picked cDNA clones, obtained from zebrafish mRNA, by in situ hybridization with digoxigenin-labelled riboprobes. Out of 150 clones tested, 1 codes for a new beta-globin gene and is expressed during embryogenesis. Here we describe its pattern of expression and its use as a marker for early zebrafish erythropoiesis.

Use of the Gal4-UAS technique for targeted gene expression in the zebrafish

The most common way to analyze the function of cloned genes in zebrafish is to misexpress the gene product or an altered variant of it by mRNA injection. However, mRNA injection has several disadvantages. The GAL4-UAS system for targeted gene expression allows one to overcome some of these disadvantages. To test the GAL4-UAS system in zebrafish, we generated two different kinds of stable transgenic lines, carrying activator and effector constructs, respectively. In the activator lines the gene for the yeast transcriptional activator GAL4 is under the control of a given promoter, while in the effectors the gene of interest is fused to the sequence of the DNA-binding motif of GAL4 (UAS). Crosses of animals from the activator and effector lines show that effector genes are transcribed with the spatial pattern of the activators. This work smoothes the way for a novel method of misexpression of gene products in zebrafish in order to analyze the function of genes in developmental processes.

The genetics of the Drosophila achaete-scute gene complex: a historical appraisal

The Drosophila achaete-scute complex consists of four genes encoding transcription factors of the bHLH family. Due to their intricate organization, these genes have occupied geneticists and developmental biologists for many years. Here, genetic studies on the complex are discussed from a historical point of view.

Asymmetic division: dynastic intricacies of neuroblast division

The cytoplasmic determinants Numb and Prospero are distributed asymmetrically into the daughter cells of Drosophila neuroblasts. The proteins encoded by the genes inscuteable, staufen and miranda are involved in the localisation of Prospero.

klumpfuss, a Drosophila gene encoding a member of the EGR family of transcription factors, is involved in bristle and leg development

The klumpfuss (klu) transcription unit in Drosophila gives rise to two different transcripts of 4.5 and 4.9 kb, both of which encode a putative transcription factor with four zinc-finger motifs of the C2H2 class. Zinc-finger 2–4 are homologous to those of the proteins of the EGR transcription factor family. As in the case of the most divergent member of the family, the Wilms' tumor suppressor gene (WT-1), klu contains an additional zinc finger, which is only distantly related. Loss of klumpfuss function is semilethal and causes a variety of defects in bristles and legs of adults, as well as in mouth hooks and brains of larvae. Analysis of the mutants indicates that klumpfuss is required for proper specification and differentiation of a variety of cells, including the sensory organ mother cells and those of the distal parts of tarsal segments.

A zebrafish Id homologue and its pattern of expression during embryogenesis

We describe the cloning, sequencing and pattern of transcript distribution during embryogenesis of a zebrafish Id homologue that we have called Id6. Transcription of the gene is spatially regulated, and its pattern of transcription shows considerable overlaps with those of other zebrafish genes with homology to Drosophila neurogenic genes, such as Notch and Delta. Since all these genes are coexpressed in particular cells, they may function together in a single genetic circuit in zebrafish as they do in Drosophila. A zebrafish homologue of Drosophila AS-C proteins can activate transcription of a CAT reporter gene by binding to an E-box in mouse 3T3 cells, either alone or in conjunction with ZfE12. The activation of transcription is inhibited in the presence of Id6. This indicates that the zebrafish gene described here is a genuine member of the Id family, and suggests that it may serve a function similar to that of the Drosophila gene emc and mammalian Ids during development.

Functional dissection of the Drosophila enhancer of split protein, a suppressor of neurogenesis

The Enhancer of split [E(spl)] gene complex of Drosophila comprises seven related genes encoding a special type of basic helix-loop-helix proteins, the function of which is to suppress the neural developmental fate. One of these proteins is E(spl) itself. To gain insight into the structural requirements for E(spl) function, we have expressed a large number of deletion variants in transgenic flies. Three protein domains were identified as essential for suppression of bristle development: the carboxyl-terminal tetrapeptide WRPW, the region comprising the putative helix III and helix IV, and the region between helix IV and the WRPW motif. Lack of the basic helix-loop-helix domain, helix III or IV, only partially inhibits the suppressor activity of the protein. Truncated variants that lack all the regions carboxyl-terminal to helix IV elicit the development of additional neural progenitors, and thus act as dominant-negative variants. All these results suggest that E(spl) suppresses neural development by direct interaction with other proteins, such as groucho and the proneural proteins.

Overexpression of a zebrafish homologue of the Drosophila neurogenic gene Delta perturbs differentiation of primary neurons and somite development

We describe here the isolation and characterization of a zebrafish Delta homologue (delta D). A PCR fragment was used to obtain overlapping cDNA clones encoding a protein of 717 amino acids with all characteristic features of proteins of this family, a signal peptide, a transmembrane domain, and an extracellular region comprising the DSL domain and eight EGF-like repeats. The gene is transcribed in a complex pattern in the developing nervous system as well as in the hypoblast. Overexpression of this gene following mRNA injections leads to a reduction in the number of islet-I positive cells, which are assumed to be primary neurons, and to various defects in the adaxial mesoderm, as well as in the somites and myotomes. This suggests that delta D, and the Notch signalling pathway are involved in the differentiation of primary neurons within the neural plate, as well as in somite development.

Lethal of scute requires overexpression of daughterless to elicit ectopic neuronal development during embryogenesis in Drosophila

Classical genetics indicates that the achaete-scute gene complex (AS-C) of Drosophila promotes development of neural progenitor cells. To further analyze the function of proneural genes, we have studied the effects of Gal4-mediated expression of lethal of scute, a member of the AS-C, during embryogenesis. Expression of lethal of scute forces progenitor cells of larval internal sensory organs, which are normally committed to this fate independently of the activity of the AS-C, to take on features of external sensory organs. Supernumerary neural cells can be induced ectopically only if daughterless is overexpressed, either alone or together with lethal of scute: cells of the amnioserosa and the hindgut then express neuronal markers. Furthermore, cells of the proctodeal anlage, which normally lack neural competence, acquire the ability to develop as neuroblasts following transplantation into the neuroectoderm. We show here that activated Notch prevents the cells of the neuroectoderm from forming extra neural tissue when they express an excess of proneural proteins. Under the present conditions, lateral inhibition is thus dominant over the activity of proneural genes.

Expression domains of a zebrafish homologue of the Drosophila pair-rule gene hairy correspond to primordia of alternating somites

her1 is a zebrafish cDNA encoding a bHLH protein with all features characteristic of members of the Drosophila HAIRY-E(SPL) family. During late gastrulation stages, her1 is expressed in the epibolic margin and in two distinct transverse bands of hypoblastic cells behind the epibolic front. After completion of epiboly, this pattern persists essentially unchanged through postgastrulation stages; the marginal domain is incorporated in the tail bud and, depending on the time point, either two or three paired bands of expressing cells are present within the paraxial presomitic mesoderm separated by regions devoid of transcripts. Labelling of cells within the her1 expression domains with fluorescein-dextran shows that the cells in the epibolic margin and the tail bud are not allocated to particular somites. However, allocation of cells to somites occurs between the marginal expression domain and the first expression band, anterior to it. Moreover, the her1 bands, and the intervening non-expressing zones, each represents the primordium of a somite. This expression pattern is highly reminiscent of that of Drosophila pair-rule genes. A possible participation of her1 in functions related to somite formation is discussed.

inscuteable, a neural precursor gene of Drosophila, encodes a candidate for a cytoskeleton adaptor protein

In Drosophila, neural precursor genes are expressed in neural progenitor cells as the neuroblasts and sense organ mother cells. These genes are thought to control the fate and/or behavior of neural progenitor cells once their fate decision has been made. We have isolated and characterized a novel neural precursor gene, inscuteable, whose expression is coincident with sites of cell shape changes or cell and tissue movement in the embryo, e.g., neuroblasts, trachea, Malphigian tubules, and in pupal wing epithelia. The Inscuteable protein is localized to the apical submembranous surface of neuroblasts and other cell types and shows certain features common to a family of putative cytoskeletal associated proteins. The inscuteable mutant phenotype, together with these other observations, suggests a possible role for the protein in cytoskeleton organization.

Persistent expression of genes of the enhancer of split complex suppresses neural development in Drosophila

The segregation of neural and epidermal progenitors in Drosophila requires the activity of transcription factors encoded by the proneural genes and the genes of the E(SPL)-C. Persistent expression of two genes of the E(SPL)-C suppresses neural development. Embryos exhibit conspicuous central neural hypoplasia and lack sensory organs; imaginal sensory organs are also affected. Suppression of neural development is associated with suppression of the activity of proneural genes. DNA binding is not essential for this effect. Large cells with characteristics of neuroblasts segregate normally in embryos, but these cells fail to express various markers, and the segregated cells and/or their progeny eventually die. These findings indicate that proneural and E(spl) proteins exert antagonistic functions.

Genetic mechanisms of early neurogenesis in Drosophila melanogaster

The neurogenic ectoderm of Drosophila melanogaster consists of the ventral neuroectoderm and the procephalic neuroectoderm. It is hypothesized that epidermal and central neural progenitor cells separate from each other in three steps: conference on the neuroectodermal cells the capability of producing neural or epidermal progenies, separation of the two classes of progenitor cells, and specification of particular types of neuroblasts and epidermoblasts. Separation of neuroblasts and epidermoblasts is controlled by proneural and neurogenic genes. Delta and Notch serve as mediators of direct protein-protein interactions. E(SPL)-C inhibits neurogenesis, creating epidermal cells. The achaete-scute complex (AS-C) controls the commitment of nonoverlapping populations of neuroblasts and leads the development of neuroectodermal cells as neuroblasts.

Lateral inhibition mediated by the Drosophila neurogenic gene delta is enhanced by proneural proteins

Cells in the neuroectoderm of Drosophila become either neural or epidermal progenitors. A critical threshold concentration of proneural gene products in a given cell causes it to develop as a neuroblast. The proteins encoded by the genes Delta (Dl) and Notch (N) act as the source and the receptor, respectively, of inhibitory signals sent by the neuroblast to neighboring cells that prevent these cells from also adopting the neural fate. We show here that proneural gene products activate transcription of Delta in the neuroectoderm by binding to specific sites in its promoter. This transcriptional activation enhances lateral inhibition and thus helps ensure that cells in the vicinity of prospective neuroblasts will themselves become epidermoblasts.

Genomic regions regulating early embryonic expression of the Drosophila neurogenic gene Delta

To identify genomic regions required for transcriptional regulation of Delta during early embryogenesis, constructs carrying promoter and gene fusions to the lacZ gene were used for germ line transformation. Most cis-regulatory sequences are dispersed throughout 6.6 kb of genomic DNA, 5' of the transcription start site; the first intron contains an enhancer element that increases the amount of RNA produced in several organs. A region was defined which drives Delta-lacZ RNA expression in clusters of neuroectodermal cells preceding and during SI neuroblast segregation. This pattern is regulated by genes of the AS-C (achaete-scute complex). To identify regulatory regions necessary for normal function of Delta during neural-epidermal lineage segregation, five minigenes consisting of fragments of the 5' genomic DNA fused to a cDNA encoding the entire protein sequence were tested for their ability to rescue the neural hyperplasia caused by a deletion of the Delta locus. Regulatory sequences required for this function are differentially distributed throughout 9 kb of genomic DNA upstream of the transcription start site. The possible significance of these findings with respect to the function of Delta during lineage segregation is discussed.

The HLH domain of a zebrafish HE12 homologue can partially substitute for functions of the HLH domain of Drosophila DAUGHTERLESS

We have identified a zebrafish homologue of the human E12 protein, which we have called ZFE12. It shows a high degree of identity to HE12 throughout its entire sequence, particularly in the bHLH domain (93%); amino acid sequence identity of the bHLH domain of ZFE12 to Drosophila DAUGHTERLESS is also very high (75%). During early embryogenesis, expression of ZfE12 is widespread. Following gastrulation, ZfE12 transcripts can be detected in all embryonic tissues with the exception of the notochord, although zones with relatively higher densities of transcripts are present in the developing brain and in the somites. To assay biological activities associated with the ZFE12 protein, two P-element constructs were made, each carrying a Drosophila daughterless gene that had been modified by replacing either the HLH domain or the entire C-terminus including the bHLH domain, by the equivalent domains of ZfE12. These constructs were used to transform flies and tested for their ability to rescue the daughterless mutant phenotype. Complete rescue of the neural phenotype and of the embryonic lethality was obtained. However, the daughterless function could not be completely restored. Although fertile flies transheterozygous for a hypomorphic and an amorphic mutation and carrying the construct encoding the zebrafish HLH domain emerged, they lacked various types of sensory organs on head and thorax and showed slight wing defects. Mutants carrying the second construct occasionally reached pupal stages but died subsequently. These data demonstrate that the HLH domain of ZFE12 can carry out most, but not all, functions performed by the corresponding region of the DAUGHTERLESS protein in Drosophila.

Neuroectodermal transcription of the Drosophila neurogenic genes E(spl) and HLH-m5 is regulated by proneural genes

The Enhancer of split gene complex (E(SPL)-C) of Drosophila comprises seven genes encoding bHLH proteins, which are required by neuroectodermal cells for epidermal development. Using promoter and gene fusions with the lacZ gene, we determined the location of cis-acting sequences necessary for normal expression of two of the genes of the E(SPL)-C, E(spl) and HLH-m5. About 0.46 kb of E(spl) and 1.9 kb of HLH-m5 upstream sequences are necessary to reproduce the normal transcription pattern of these genes. The gene products of achaete, scute and lethal of scute, together with that of ventral nervous system condensation defective, act synergistically to specify the neuroectodermal E(spl) and HLH-m5 expression domains. Negative cross- and autoregulatory interactions of the E(SPL)-C on E(spl) contribute, directly or indirectly, to this regulation. Interactions involve DNA binding, since mutagenesis of binding sites for bHLH proteins in the E(spl) promoter abolishes neuroectodermal expression and activates ectopic expression in neuroblasts. A model for activation and repression of E(spl) in the neuroectoderm and neuroblasts, respectively, is proposed.

The basic-helix-loop-helix domain of Drosophila lethal of scute protein is sufficient for proneural function and activates neurogenic genes

The development of most epidermal sensory organs in Drosophila is controlled by achaete and scute, two of the genes of the achaete-scute complex (AS-C). The genes of the AS-C encode members of the basic-helix-loop-helix (bHLH) class of transcriptional regulators, and their activity defines proneural cell clusters in the imaginal discs from which sensory organ mother cells are singled out by a process of lateral inhibition. Ectopic expression of lethal of scute, another member of the AS-C, normally dispensable for sensory organ development in the adult, promotes this process independently of the activity of the other AS-C genes. This demonstrates a high degree of functional redundancy of the products of the AS-C. Furthermore, neurogenic genes are activated in ectopic proneural clusters, allowing development of epidermal progenitor cells. Finally, the bHLH domain is necessary and sufficient to mediate the proneural function, to activate neurogenic genes, and to allow lateral inhibition.

Genetic mechanisms of early neurogenesis in Drosophila melanogaster

The neuroectoderm of insects contains an initially indifferent population of cells which during later development will give rise to the progenitor cells of the neural and epidermal lineages. Experimental evidence indicates that cellular interactions determine which cells will adopt each one of these fates. The invoked cell interactions are assumed to be mediated by the products of several genes forming a complex, not yet well understood network of interrelationships. Elements of this network are the proteins encoded by Delta and Notch, which appear to convey the regulatory signals between the cells; the proteins encoded by the achaete-scute gene complex, which regulate neural development, and the proteins encoded by the Enhancer of split gene complex, which give neuroectodermal cells access to epidermal development. The proneural genes appear to be the key elements in the regulation of the cellular decision.

Mechanisms of early neurogenesis in Drosophila melanogaster

The neuroectoderm of insects contains an initially indifferent population of cells which during later development will give rise to the progenitor cells of the neural and epidermal lineages. Experimental evidence indicates that cellular interactions determine which cells will adopt each one of these fates. Transplantation experiments suggest that a signal with neuralising character is required to stabilize the primary neural fate in 25% of all the neuroectodermal cells, which will develop as neuroblasts, and that an epidermalising signal contributes to suppress the neural fate in the remaining 75% of the cells, allowing in this way their development as epidermal progenitor cells. The invoked cell interactions are assumed to be mediated by the products of several genes forming a complex, not yet well understood network of interrelationships. Elements of this network are the proteins encoded by Delta and Notch, which appear to convey the regulatory signals between the cells; the proteins encoded by the achaete-scute gene complex, which regulate neural development; and the proteins encoded by the Enhancer of split gene complex, which give neuroectodermal cells access to epidermal development.

A zebrafish homologue of the Drosophila neurogenic gene Notch and its pattern of transcription during early embryogenesis

We describe here the primary structure of a zebrafish homologue of the Drosophila neurogenic gene Notch and its pattern of mRNA accumulation during embryogenesis. The gene produces a 8.5 kb transcript encoding a putative transmembrane protein with a high degree of sequence similarity to members of the Notch family, comprising 36 EGF-like repeats, three lin-12/Notch repeats, six cdc10/SWI6 repeats, OPA repeats and a PEST sequence. Transcription of the zebrafish Notch gene is spatially and temporally regulated. A high density of transcripts, most probably of maternal origin, can already be detected in the 2-cell stage. During pregastrulation stages, RNA is present in all cells. However, following gastrulation, transcripts accumulate in specific regions of the embryo following a rapidly changing pattern. In some of these regions, cell divisions take place at the time of Notch expression, in others processes of cell differentiation. This holds true for various mesodermal derivatives, such as the prospective notochord, and for different neural primordia, such as the neural plate and the brain vesicles. This pattern of transcript accumulation suggests a role for the zebrafish Notch homologue in processes of regionalization and cell diversification.

Single amino acid substitutions in EGF-like elements of Notch and Delta modify Drosophila development and affect cell adhesion in vitro

Notch locus EGF-like element mutations spl, altering eye development, and AxE2, affecting wing and sensilla development, are modified by mutations at Delta. It is shown that two allele-specific suppressors of spl involve single amino acid substitutions in the 4th (Dlsup5) and 9th (Dlsup4) EGF-like elements of the Delta protein. Cultured cells producing spl or AxE2 aggregate with cells producing wild-type Delta or Dlsup5 protein, and Dlsup5-producing cells adhere to cells producing wild-type Notch protein. However, spl,AxE2, and Dlsup5 are each defective in promoting these cell affinities, as none of the mutant proteins can compete with the corresponding wild-type proteins for formation of cell aggregates. Thus, widely separated EGF-like elements of Notch and Delta appear to participate in functional molecular interactions between the proteins. Dlsup5 does not improve adhesiveness of spl in vitro, so suppression in vivo may involve altered developmental signaling by spl-Dlsup5 complexes, rather than modified cell adhesion.

The Enhancer of split complex and adjacent genes in the 96F region of Drosophila melanogaster are required for segregation of neural and epidermal progenitor cells

The Enhancer of split complex [E(spl)-C] of Drosophila melanogaster is located in the 96F region of the third chromosome and comprises at least seven structurally related genes, HLH-m delta, HLH-m gamma, HLH-m beta, HLH-m3, HLH-m5, HLH-m7 and E(spl). The functions of these genes are required during early neurogenesis to give neuroectodermal cells access to the epidermal pathway of development. Another gene in the 96F region, namely groucho, is also required for this process. However, groucho is not structurally related to, and appears to act independently of, the genes of the E(spl)-C; the possibility is discussed that groucho acts upstream to the E(spl)-C genes. Indirect evidence suggests that a neighboring transcription unit (m4) may also take part in the process. Of all these genes, only gro is essential; m4 is a dispensable gene, the deletion of which does not produce detectable morphogenetic abnormalities, and the genes of the E(spl)-C are to some extent redundant and can partially substitute for each other. This redundancy is probably due to the fact that the seven genes of the E(spl)-C encode highly conserved putative DNA-binding proteins of the bHLH family. The genes of the complex are interspersed among other genes which appear to be unrelated to the neuroepidermal lineage dichotomy.

Seven genes of the Enhancer of split complex of Drosophila melanogaster encode helix-loop-helix proteins

Enhancer of split [E(spl)] is one of the neurogenic loci of Drosophila and, as such, is required for normal segregation of neural and epidermal cell progenitors. Genetic observations indicate that the E(spl) locus is in fact a gene complex comprising a cluster of related genes and that other genes of the region are also required for normal early neurogenesis. Three of the genes of the complex were known to encode helix-loop-helix (HLH) proteins and to be transcribed in nearly identical patterns. Here, we show that four other genes in the vicinity also encode HLH proteins and, during neuroblast segregation, three of them are expressed in the same pattern. We show by germ-line transformation that these three genes are also necessary to allow epidermal development of the neuroectodermal cells.

A transgene containing lacZ is expressed in primary sensory neurons in zebrafish

In order to screen for developmentally active chromosomal domains during zebrafish embryogenesis, we generated transgenic fish by microinjecting two different lacZ reporter constructs into fertilized eggs. Transgenic fish were screened among the progeny of injected fish (F0) crossed to non-injected fish. Groups of 15 to 20 progeny of each cross were tested for lacZ expression and/or transmission of injected sequences using PCR and Southern hybridizations. Progeny from 2 of 102 fish injected with supercoiled constructs containing Rous sarcoma virus promoter sequences showed apparently spatially regulated beta-galactosidase (beta-Gal) activity. However, we were not able to detect this reporter construct in DNA from fins of F1 fish. Injections of a linear reporter construct containing mouse heat-shock promoter sequences revealed transmission of injected sequences to F1 progeny in about 6% of cases (8 of 129 fish, tested with PCR). We found one lacZ-expressing line that showed a spatially and temporally restricted expression of lacZ and, therefore, features typical characteristics of “enhancer trap” lines. In this line, lacZ expression starts at 16 hours post-fertilization in trigeminal ganglion cells. At about 24 hours lacZ expression can be detected in trigeminal ganglion neurons and Rohon-Beard neurons, indicating that the development of these two cell types shows common features. The reporter gene has integrated as a single copy. The founder fish was mosaic: 19% of its offspring (3 of 16 tested animals) carried the reporter construct in their fins; about 51% (13 of 27 tested animals) of the progeny of F1 fish were beta-Gal positive indicating full hemizygosity.(ABSTRACT TRUNCATED AT 250 WORDS)

The Drosophila neurogenic gene neuralized encodes a novel protein and is expressed in precursors of larval and adult neurons

Neuralized belongs to a group of genes involved in neurogenesis in Drosophila. Loss of function mutations lead to an overproduction of neurons at the expense of epidermal tissues. We have cloned the neuralized locus and examined its expression pattern during development. Expression is initially observed during embryogenesis in the neurogenic ectoderm and later in neuroblasts. In addition, transcripts are also found in sensory precursor cells during imaginal disc development. Other tissues that express neuralized include the embryonic mesoderm and specific follicle cells in the ovary. The predicted neuralized gene product is a highly basic protein with a novel motif that bears some resemblance to those found in nucleic acid-binding proteins. The first half of this motif has sequence similarity to the first half of a homeobox whereas the second half is similar to the helix-turn-helix structure of bacterial repressors.

The distribution of transcripts of neurogenic genes in neurogenic mutants of Drosophila melanogaster

The neurogenic genes of Drosophila melanogaster are required for correct separation of neural and epidermal progenitor cells during early embryogenesis. Results from genetic analyses indicate that the neurogenic genes are functionally related. We have studied the spatial distribution of RNA from the neurogenic genes D1, neu, and m4, m5, m7 and E(spl) [four genes of the Enhancer of split complex] in various neurogenic mutant embryos by in situ hybridization. An abnormal distribution of RNA from certain of the genes is found in neurogenic mutants, suggesting that at least some of the functional interactions inferred from genetic data take place at the transcriptional level. We discuss these results in relation to the events of early neurogenesis.

The pattern of transcription of the neurogenic gene Delta of Drosophila melanogaster

The function of the Delta locus of Drosophila melanogaster is required for the correct separation of neural and epidermal cell lineages. We describe here the transcriptional organization of this locus and the spatial pattern of mRNA accumulation during embryogenesis. Delta produces three mRNAs with protein-coding capacity, which differ only at their untranslated 3′ ends and thus encode the same protein; other minor RNAs from the locus are shown not to have any protein-coding capacity and to correspond to introns. No indications were obtained for multiple translational products of the locus. In situ hybridization using digoxigenin-labelled probes confirms that Delta RNA is present at high concentration in all presumptive neurogenic territories of the embryo. Since all the constituent cells of these territories contain Delta RNA, a differential distribution of the protein among the derivatives of the neuroectodermal cells is improbable. Some time after segregation of lineages, Delta RNA reappears in neuroblasts. The possible significance of these observations with respect to the function of the Delta product during lineage segregation is discussed.

Defective neuroblast commitment in mutants of the achaete-scute complex and adjacent genes of D. melanogaster

Loss of function mutations in genes of the achaete-scute complex (ASC) or in the gene vnd of D. melanogaster result in neural hypoplasia. Two types of defects contribute to the development of the neural hypoplasic phenotype: a lower than normal proportion of neuroblasts delaminate from the neuroectoderm, and there is abundant cell death in the neural primordium during later stages. In addition, we found that increasing the copy number of ASC wild-type alleles leads to effects opposite to those caused by their deletion. All of these results indicate that the function of these genes is required for the commitment of neuroectodermal cells as neuroblasts and that the loss of these genetic functions causes the cells either to take on an epidermal fate or to die.

Molecular analysis of a cellular decision during embryonic development of Drosophila melanogaster: epidermogenesis or neurogenesis

In Drosophila melanogaster, the neuroblasts (neural progenitor cells) develop from a special region of the ectoderm, called the neuroectoderm. During early embryonic development, the neuroblasts separate from the remaining cells of the neuroectoderm, which develop as epidermoblasts (epidermal progenitor cells). The separation of these two cell types is the result of cellular interactions. The available data indicate that a signal chain formed by the products of several identified genes regulates the cell's decision to enter either neurogenesis or epidermogenesis. Various kinds of data, in particular from cell transplantation studies and from genetic and molecular analyses, suggest that the proteins encoded by the genes Notch and Delta interact at the membrane of the neuroectodermal cells to provide a regulatory signal. This signal is thought to lead, on the one hand, to epidermal development through the action of the genes of the Enhancer of split complex, a gene complex that encodes several functions related to the transduction and further processing of the signal, including the genetic regulation in the receiving cell; on the other hand, the signal is thought to lead to neural development through the participation of the genes of the achaete-scute complex and daughterless, which are members of a family of DNA-binding regulatory proteins and of the gene vnd whose molecular nature is still unknown.

The molecular genetics of early neurogenesis in Drosophila melanogaster

The extent of neurogenesis in Drosophila is under the control of the so-called neurogenic genes, named for their mutant phenotype of causing neural hyperplasia. Their wild-type products appear to be responsible for a signal chain that decides the fate of ectodermal cells in the embryo. Various kinds of data, from cell transplantation experiments as well as from genetic and molecular analyses, suggest that the proteins encoded by the genes Notch and Delta may act at the membrane of the signal-transmitting cells to provide a ligand to a still unknown receptor molecule; in contrast, the locus of Enhancer of split codes for several functions related to the transduction and further processing of the signal.

Closely related transcripts encoded by the neurogenic gene complex enhancer of split of Drosophila melanogaster

Genetic evidence suggests that E(spl), one of the neurogenic loci of Drosophila, is a gene complex comprising an as yet incompletely established number of transcription units. In order to correlate the various transcription units with E(spl) functions, wild-type flies were transformed with genomic DNA encoding the transcription unit m8 from the mutant E(spl)D, which was known to be altered in embryos carrying this mutant allele. Transformants show the same dominant enhancement of the spl phenotype as E(spl)D itself. Since m8 has a virtually identical pattern of expression as m4, m5 and m7, we have determined the sequence of these four transcripts. The deduced protein products of m5, m7 and m8 exhibit extensive sequence homology with each other. All three encode a sequence similar to one of the conserved domains of representatives of the vertebrate myc gene family which is also present in the deduced protein sequences of the Drosophila achaete-scute gene complex. Sequence analysis of the m8 transcription unit in the E(spl)D mutation revealed several DNA lesions. One of the lesions is a deletion in the region upstream of the transcription start site. Another lesion is a deletion in the coding region that leads to a shorter protein which, in addition, differs in its carboxy-terminal end from the wild-type protein by the presence of nine amino acids.

Genetic analysis of enhancer of split, a locus involved in neurogenesis in Drosophila melanogaster

Enhancer of split (E(spl)), one of the neurogenic loci of Drosophila, is uncovered by the deletion Df(3R)E(spl)(R-B251) with breakpoints at 96F8 and 96F13. We describe here the results of a genetic analysis of this chromosomal interval. Thirty-one mutations in genes of this region were recovered during various programs of mutagenesis. In addition, we included the spontaneous mutations E(spl)(D) and groucho (gro), which are known to map to this region, in our study. These 33 mutations define four lethal complementation groups, one of which includes E(spl)(D) and gro. Mutations of the E(spl) group behave as complementing and noncomplementing pseudoalleles, defining different functions. Alleles are classified according to their complementation behavior in two different ways: with respect to their viability as heterozygotes with other lethal alleles and with respect to gro and to E(spl)(D). The phenotypes of these mutations and the pattern of heteroallelic complementation speak in favor of a considerable genetic complexity of the E(spl) locus.