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

Geminin Orchestrates Somite Formation by Regulating Fgf8 and Notch Signaling

During somitogenesis, Fgf8 maintains the predifferentiation stage of presomitic mesoderm (PSM) cells and its retraction gives a cue for somite formation. Delta/Notch initiates the expression of oscillation genes in the tail bud and subsequently contributes to somite formation in a periodic way. Whether there exists a critical factor coordinating Fgf8 and Notch signaling pathways is largely unknown. Here, we demonstrate that the loss of function of geminin gave rise to narrower somites as a result of derepressed Fgf8 gradient in the PSM and tail bud. Furthermore, in geminin morphants, the somite boundary could not form properly but the oscillation of cyclic genes was normal, displaying the blurry somitic boundary and disturbed somite polarity along the AP axis. In mechanism, these manifestations were mediated by the disrupted association of the geminin/Brg1 complex with intron 3 of mib1. The latter interaction was found to positively regulate mib1 transcription, Notch activity, and sequential somite segmentation during somitogenesis. In addition, geminin was also shown to regulate the expression of deltaD in mib1-independent way. Collectively, our data for the first time demonstrate that geminin regulates Fgf8 and Notch signaling to regulate somite segmentation during somitogenesis.

Notch signaling in the division of germ layers in bilaterian embryos

Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/β-catenin pathway.

Ketamine induces motor neuron toxicity and alters neurogenic and proneural gene expression in zebrafish

Ketamine, a noncompetitive antagonist of N-methyl-d-aspartate-type glutamate receptors, is a pediatric anesthetic that has been shown to be neurotoxic in rodents and nonhuman primates when administered during the brain growth spurt. Recently, the zebrafish has become an attractive model for toxicity assays, in part because the predictive capability of the zebrafish model, with respect to chemical effects, compares well with that from mammalian models. In the transgenic (hb9:GFP) embryos used in this study, green fluorescent protein (GFP) is expressed in the motor neurons, facilitating the visualization and analysis of motor neuron development in vivo. In order to determine whether ketamine induces motor neuron toxicity in zebrafish, embryos of these transgenic fish were treated with different concentrations of ketamine (0.5 and 2.0 mm). For ketamine exposures lasting up to 20 h, larvae showed no gross morphological abnormalities. Analysis of GFP-expressing motor neurons in the live embryos, however, revealed that 2.0 mm ketamine adversely affected motor neuron axon length and decreased cranial and motor neuron populations. Quantitative reverse transcriptase-polymerase chain reaction analysis demonstrated that ketamine down-regulated the motor neuron-inducing zinc finger transcription factor Gli2b and the proneural gene NeuroD even at 0.5 mm concentration, while up-regulating the expression of the proneural gene Neurogenin1 (Ngn1). Expression of the neurogenic gene, Notch1a, was suppressed, indicating that neuronal precursor generation from uncommitted cells was favored. These results suggest that ketamine is neurotoxic to motor neurons in zebrafish and possibly affects the differentiating/differentiatedneurons rather than neuronal progenitors. Published 2011. This article is a US Government work and is in the public domain in the USA.

Morpholino artifacts provide pitfalls and reveal a novel role for pro-apoptotic genes in hindbrain boundary development

Morpholino antisense oligonucleotides (MOs) are widely used as a tool to achieve loss of gene function, but many have off-target effects mediated by activation of Tp53 and associated apoptosis. Here, we re-examine our previous MO-based loss-of-function studies that had suggested that Wnt1 expressed at hindbrain boundaries in zebrafish promotes neurogenesis and inhibits boundary marker gene expression in the adjacent para-boundary regions. We find that Tp53 is highly activated and apoptosis is frequently induced by the MOs used in these studies. Co-knockdown of Tp53 rescues the decrease in proneural and neuronal marker expression, which is thus an off-target effect of MOs. While loss of gene expression can be attributed to cell loss through apoptotic cell death, surprisingly we find that the ectopic expression of hindbrain boundary markers is also dependent on Tp53 activity and its downstream apoptotic effectors. We examine whether this non-specific activation of hindbrain boundary gene expression provides insight into the endogenous mechanisms underlying boundary cell specification. We find that the pro-apoptotic Bcl genes puma and bax-a are required for hindbrain boundary marker expression, and that gain of function of the Bcl-caspase pathway leads to ectopic boundary marker expression. These data reveal a non-apoptotic role for pro-apoptotic genes in the regulation of gene expression at hindbrain boundaries. In light of these findings, we discuss the precautions needed in performing morpholino knockdowns and in interpreting the data derived from their use.

Delta1 family members are involved in filopodial actin formation and neuronal cell migration independent of Notch signaling

Delta family proteins are transmembrane molecules that bind Notch receptors and activate downstream signaling events in neighboring cells. In addition to serving as Notch ligands, Notch-independent roles for Delta have been suggested but are not fully understood. Here, we demonstrate a previously unrecognized role for Delta in filopodial actin formation. Delta1 and Delta4, but not Delta3, exhibit filopodial protrusive activity, and this activity is independent of Notch signaling. The filopodial activity of Delta1 does not depend on the PDZ-binding domain at the C-terminus; however, the intracellular membrane-proximal region that is anchored to the plasma membrane plays an important role in filopodial activity. We further identified a Notch-independent role of DeltaD in neuronal cell migration in zebrafish. These findings suggest a possible functional link between Notch-independent filopodial activity of Delta and the control of cell motility.

DeltaA mRNA and protein distribution in the zebrafish nervous system

Physical interaction between the transmembrane proteins Delta and Notch allows only a subset of neural precursors to become neurons, as well as regulating other aspects of neural development. To examine the localization of Delta protein during neural development, we generated an antibody specific to zebrafish Delta A (Dla). Here, we describe for the first time the subcellular localization of Dla protein in distinct puncta at cell cortex and/or membrane, supporting the function of Dla in direct cell-cell communication. In situ RNA hybridization and immunohistochemistry revealed dynamic, coordinated expression patterns of dla mRNA and Dla protein in the developing and adult zebrafish nervous system. Dla expression is mostly excluded from differentiated neurons and is maintained in putative precursor cells at least until larval stages. In the adult brain, dla mRNA and Dla protein are expressed in proliferative zones normally associated with stem cells.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Temporal Notch activation through Notch1a and Notch3 is required for maintaining zebrafish rhombomere boundaries

In vertebrates, hindbrain is subdivided into seven segments termed rhombomeres and the interface between each rhombomere forms the boundary. Similar to the D/V boundary formation in Drosophila, Notch activation has been shown to regulate the segregation of rhombomere boundary cells. Here we further explored the function of Notch signaling in the formation of rhombomere boundaries. By using bodipy ceramide cell-labeling technique, we found that the hindbrain boundary is formed initially in mib mutants but lost after 24 hours post-fertilization (hpf). This phenotype was more severe in mib^ta52b allele than in mib^tfi91 allele. Similarly, injection of su(h)-MO led to boundary defects in a dosage-dependent manner. Boundary cells were recovered in mib^ta52b mutants in the hdac1-deficient background, where neurogenesis is inhibited. Furthermore, boundary cells lost sensitivity to reduced Notch activation from 15 somite stage onwards. We also showed that knockdown of notch3 function in notch1a mutants leads to the loss of rhombomere boundary cells and causes neuronal hyperplasia, indicating that Notch1a and Notch3 play a redundant role in the maintenance of rhombomere boundary.

Notch receptor and Notch ligand expression in developing avian cartilage

The development of limb cartilage involves complex signalling pathways allowing the formation of distinct segments of cartilage that are maintained in the fully developed joint. In this study, we investigated the Notch signalling pathway and its role in cartilage development. The differential distribution of the Notch signalling family of receptors and their corresponding ligands in developing avian (gallus gallus) cartilage revealed expression of Notch 1, Delta 1, Jagged 1 and Jagged 2 in all limb mesenchyme cells at the early stages of cartilage anlagen development, which were subsequently restricted to the developing cartilage element. Expression of both Notch 1 and Jagged 1 became increasingly restricted to the surface cartilage once joint cavity formation had occurred. Delta 1 and Jagged 1 were restricted to a layer of cells underneath the surface cartilage and were also observed in the hypertrophic chondrocytes, where Notch 1 expression was evident in stage 40-44 limbs. Notch 2, Notch 3 and Notch 4 were not evident in early stage limbs but were present after cavitation, although expression was lost in late stage limbs (stage 40-44). We also demonstrated that inhibition of the Notch pathway leads to altered Notch receptor expression, disrupting cartilage differentiation. From these data it is clear that Notch signalling is a necessary and critical factor in regulating cell fate decisions allowing controlled chondrogenesis, elongation and subsequent maintenance of limb cartilage.

Interaction with Notch determines endocytosis of specific Delta ligands in zebrafish neural tissue

Mind bomb1 (Mib1)-mediated endocytosis of the Notch ligand DeltaD is essential for activation of Notch in a neighboring cell. Although most DeltaD is localized in cytoplasmic puncta in zebrafish neural tissue, it is on the plasma membrane in mib1 mutants because Mib1-mediated endocytosis determines the normal subcellular localization of DeltaD. Knockdown of Notch increases cell surface DeltaA and DeltaD, but not DeltaC, suggesting that, like Mib1, Notch regulates endocytosis of specific ligands. Transplant experiments show that the interaction with Notch, both in the same cell (in cis) and in neighboring cells (in trans), regulates DeltaD endocytosis. Whereas DeltaD endocytosis following interaction in trans activates Notch in a neighboring cell, endocytosis of DeltaD and Notch following an interaction in cis is likely to inhibit Notch signaling by making both unavailable at the cell surface. The transplantation experiments reveal a heterogeneous population of progenitors: in some, cis interactions are more important; in others, trans interactions are more important; and in others, both cis and trans interactions are likely to contribute to DeltaD endocytosis. We suggest that this heterogeneity represents the process by which effective lateral inhibition leads to diversification of progenitors into cells that become specialized to deliver or receive Delta signals, where trans and cis interactions with Notch play differential roles in DeltaD endocytosis.

Relationship between Delta-like and proneural bHLH genes during chick retinal development

Notch signaling in the retina maintains a pool of progenitor cells throughout retinogenesis. However, two Notch-ligands from the Delta-like gene family, Dll1 and Dll4, are present in the developing retina. To understand their relationship, we characterized Dll1 and Dll4 expression with respect to proliferating progenitor cells and newborn neurons in the chick retina. Dll4 matched the pattern of neural differentiation. By contrast, Dll1 was primarily expressed in progenitor cells. We compared Dll1 and Dll4 kinetic profiles with that of the transiently up-regulated cascade of proneural basic helix-loop-helix (bHLH) genes after synchronized progenitor cell differentiation, which suggested a potential role for Ascl1 in the regulation of Delta-like genes. Gain-of-function assays demonstrate that Ascl1 does influence Delta-like gene expression and Notch signaling activity. These data suggest that multiple sources of Notch signaling from newborn neurons and progenitors themselves coordinate retinal histogenesis.

MiRNA expression analysis during normal zebrafish development and following inhibition of the Hedgehog and Notch signaling pathways

microRNAs (miRNAs) are small (approximately 22 nucleotide) non-coding RNAs that regulate gene expression at the post-transcriptional level, typically by inhibiting translation. The genes encoding these small RNAs are estimated to comprise approximately 2-3% of animal genomes yet potentially regulate a majority of protein-coding genes including those involved in cell specification and development. A key remaining question is to identify target mRNAs regulated by microRNAs. As a means to identify potential targets, we designed a sensitive microarray to analyze global miRNA expression patterns at twelve developmental stages in zebrafish. Further, we conducted arrays on zebrafish embryos treated with small molecule inhibitors of the Hedgehog and Notch signaling pathways to enable identification of differentially expressed miRNAs that target genes controlling key developmental pathways during early embryogenesis.

A novel function of DELTA-NOTCH signalling mediates the transition from proliferation to neurogenesis in neural progenitor cells

A complete account of the whole developmental process of neurogenesis involves understanding a number of complex underlying molecular processes. Among them, those that govern the crucial transition from proliferative (self-replicating) to neurogenic neural progenitor (NP) cells remain largely unknown. Due to its sequential rostro-caudal gradients of proliferation and neurogenesis, the prospective spinal cord of the chick embryo is a good experimental system to study this issue. We report that the NOTCH ligand DELTA-1 is expressed in scattered cycling NP cells in the prospective chick spinal cord preceding the onset of neurogenesis. These Delta-1-expressing progenitors are placed in between the proliferating caudal neural plate (stem zone) and the rostral neurogenic zone (NZ) where neurons are born. Thus, these Delta-1-expressing progenitors define a proliferation to neurogenesis transition zone (PNTZ). Gain and loss of function experiments carried by electroporation demonstrate that the expression of Delta-1 in individual progenitors of the PNTZ is necessary and sufficient to induce neuronal generation. The activation of NOTCH signalling by DELTA-1 in the adjacent progenitors inhibits neurogenesis and is required to maintain proliferation. However, rather than inducing cell cycle exit and neuronal differentiation by a typical lateral inhibition mechanism as in the NZ, DELTA-1/NOTCH signalling functions in a distinct manner in the PNTZ. Thus, the inhibition of NOTCH signalling arrests proliferation but it is not sufficient to elicit neuronal differentiation. Moreover, after the expression of Delta-1 PNTZ NP continue cycling and induce the expression of Tis21, a gene that is upregulated in neurogenic progenitors, before generating neurons. Together, these experiments unravel a novel function of DELTA–NOTCH signalling that regulates the transition from proliferation to neurogenesis in NP cells. We hypothesize that this novel function is evolutionary conserved.

Amphioxus AmphiDelta: evolution of Delta protein structure, segmentation, and neurogenesis

The amphioxus genome has a single Delta gene (AmphiDelta) encoding a protein 766 amino acids long. Comparison of Delta proteins of amphioxus and other animals indicates that AmphiDelta retains features of a basal bilaterian Delta protein--in having nine epidermal growth factor (EGF) repeats and also in having characteristic numbers of amino acids separating successive cysteines between and within EGF repeats. During development, AmphiDelta is expressed in the forming somites, in some regions of pharyngeal endoderm, and in cells (presumably differentiating neurons) scattered in both the neural plate and ectoderm. Expression is strongly associated with cells initiating movements to separate themselves from parent epithelia, either en masse by evagination (endoderm and mesoderm) or by delamination as isolated cells (ectoderm). The AmphiDelta-expressing cells delaminating from the ectoderm apparently migrate beneath it as they begin differentiating into probable sensory neurons, suggesting a scenario for the evolutionary origin of the placode-derived neurons of vertebrate cranial ganglia.

The genetics and embryology of zebrafish metamerism

Somites are the most obvious metameric structures in the vertebrate embryo. They are mesodermal segments that form in bilateral pairs flanking the notochord and are created sequentially in an anterior to posterior sequence concomitant with the posterior growth of the trunk and tail. Zebrafish somitogenesis is regulated by a clock that causes cells in the presomitic mesoderm (PSM) to undergo cyclical activation and repression of several notch pathway genes. Coordinated oscillation among neighboring cells manifests as stripes of gene expression that pass through the cells of the PSM in a posterior to anterior direction. As axial growth continually adds new cells to the posterior tail bud, cells of the PSM become relatively less posterior. This gradual assumption of a more anterior position occurs over developmental time and constitutes part of a maturation process that governs morphological segmentation in conjunction with the clock. Segment morphogenesis involves a mesenchymal to epithelial transition as prospective border cells at the anterior end of the mesenchymal PSM adopt a polarized, columnar morphology and surround a mesenchymal core of cells. The segmental pattern influences the development of the somite derivatives such as the myotome, and the myotome reciprocates to affect the formation of segment boundaries. While somites appear to be serially homologous, there may be variation in the segmentation mechanism along the body axis. Moreover, whereas the genetic architecture of the zebrafish, mouse, and chick segmentation clocks shares many common elements, there is evidence that the gene networks have undergone independent modification during evolution.

Notch signaling in development and cancer

Notch is an evolutionarily conserved local cell signaling mechanism that participates in a variety of cellular processes: cell fate specification, differentiation, proliferation, apoptosis, adhesion, epithelial-mesenchymal transition, migration, and angiogenesis. These processes can be subverted in Notch-mediated pathological situations. In the first part of this review, we will discuss the role of Notch in vertebrate central nervous system development, somitogenesis, cardiovascular and endocrine development, with attention to the mechanisms by which Notch regulates cell fate specification and patterning in these tissues. In the second part, we will review the molecular aspects of Notch-mediated neoplasias, where Notch can act as an oncogene or as a tumor suppressor. From all these studies, it becomes evident that the outcome of Notch signaling is strictly context-dependent and differences in the strength, timing, cell type, and context of the signal may affect the final outcome. It is essential to understand how Notch integrates inputs from other signaling pathways and how specificity is achieved, because this knowledge may be relevant for future therapeutic applications.

Notch signaling regulates neural precursor allocation and binary neuronal fate decisions in zebrafish

Notch signaling plays a well-described role in regulating the formation of neurons from proliferative neural precursors in vertebrates but whether, as in flies, it also specifies sibling cells for different neuronal fates is not known. Ventral spinal cord precursors called pMN cells produce mostly motoneurons and oligodendrocytes, but recent lineage-marking experiments reveal that they also make astrocytes, ependymal cells and interneurons. Our own clonal analysis of pMN cells in zebrafish showed that some produce a primary motoneuron and KA' interneuron at their final division. We investigated the possibility that Notch signaling regulates a motoneuron-interneuron fate decision using a combination of mutant, transgenic and pharmacological manipulations of Notch activity. We show that continuous absence of Notch activity produces excess primary motoneurons and a deficit of KA' interneurons, whereas transient inactivation preceding neurogenesis results in an excess of both cell types. By contrast, activation of Notch signaling at the neural plate stage produces excess KA' interneurons and a deficit of primary motoneurons. Furthermore, individual pMN cells produce similar kinds of neurons at their final division in mib mutant embryos, which lack Notch signaling. These data provide evidence that, among some postmitotic daughters of pMN cells, Notch promotes KA' interneuron identity and inhibits primary motoneuron fate, raising the possibility that Notch signaling diversifies vertebrate neuron type by mediating similar binary fate decisions.

Priming, initiation and synchronization of the segmentation clock by deltaD and deltaC

Zebrafish somitogenesis is governed by a segmentation clock that generates oscillations in expression of several Notch pathway genes, including her1, her7 and deltaC. Using a combination of pharmacological inhibition and Mendelian genetics, we show that DeltaD and DeltaC, two Notch ligands, represent functionally distinct signals within the segmentation clock. Using high-resolution fluorescent in situ hybridization, the oscillations were divided into phases based on eight distinct subcellular patterns of mRNA localization for 140,000 cells. her1, her7 and deltaC expression was examined in wild-type, deltaD(-/-) and deltaC(-/-) embryos. We identified areas within the tailbud where the clock is set up in the progenitor cells (priming), where the clock starts running (initiation), and where the clocks of neighbouring cells are entrained (synchronization). We find that the clocks of motile cells are primed by deltaD in a progenitor zone in the posterior tailbud and that deltaD is required for cells to initiate oscillations on exiting this zone. Oscillations of adjacent cells are synchronized and amplified by deltaC in the posterior presomitic mesoderm as cell movement subsides and cells maintain stable neighbour relationships.

Appropriate suppression of Notch signaling by Mesp factors is essential for stripe pattern formation leading to segment boundary formation

Mesp1 and Mesp2 are homologous transcription factors that are co-expressed in the anterior presomitic mesoderm (PSM) during mouse somitogenesis. The loss of Mesp2 alone in our conventional Mesp2-null mice results in the complete disruption of somitogenesis, including segment border formation, rostro-caudal patterning and epithelialization of somitic mesoderm. This has led us to interpret that Mesp2 is solely responsible for somitogenesis. Our novel Mesp2 knock-in alleles, however, exhibit a remarkable upregulation of Mesp1. Removal of the pgk-neo cassette from the new allele leads to localization of Mesp1 and several gene expression, and somite formation in the tail region. Moreover, a reduction in the gene dosage of Mesp1 by one copy disrupts somite formation, confirming the involvement of Mesp1 in the rescue events. Furthermore, we find that activated Notch1 knock-in significantly upregulates Mesp1 expression, even in the absence of a Notch signal mediator, Psen1. This indicates that the Psen1-independent effects of activated Notch1 are mostly attributable to the induction of Mesp1. However, we have also confirmed that Mesp2 enhances the expression of the Notch1 receptor in the anterior PSM. The activation and subsequent suppression of Notch signaling might thus be a crucial event for both stripe pattern formation and boundary formation.

A positive regulatory loop between foxi3a and foxi3b is essential for specification and differentiation of zebrafish epidermal ionocytes

BACKGROUND

Epidermal ionocytes play essential roles in the transepithelial transportation of ions, water, and acid-base balance in fish embryos before their branchial counterparts are fully functional. However, the mechanism controlling epidermal ionocyte specification and differentiation remains unknown.

METHODOLOGY/PRINCIPAL FINDINGS

In zebrafish, we demonstrated that Delta-Notch-mediated lateral inhibition plays a vital role in singling out epidermal ionocyte progenitors from epidermal stem cells. The entire epidermal ionocyte domain of genetic mutants and morphants, which failed to transmit the DeltaC-Notch1a/Notch3 signal from sending cells (epidermal ionocytes) to receiving cells (epidermal stem cells), differentiates into epidermal ionocytes. The low Notch activity in epidermal ionocyte progenitors is permissive for activating winged helix/forkhead box transcription factors of foxi3a and foxi3b. Through gain- and loss-of-function assays, we show that the foxi3a-foxi3b regulatory loop functions as a master regulator to mediate a dual role of specifying epidermal ionocyte progenitors as well as of subsequently promoting differentiation of Na(+),K(+)-ATPase-rich cells and H(+)-ATPase-rich cells in a concentration-dependent manner.

CONCLUSIONS/SIGNIFICANCE

This study provides a framework to show the molecular mechanism controlling epidermal ionocyte specification and differentiation in a low vertebrate for the first time. We propose that the positive regulatory loop between foxi3a and foxi3b not only drives early ionocyte differentiation but also prevents the complete blockage of ionocyte differentiation when the master regulator of foxi3 function is unilaterally compromised.

Jagged2a-notch signaling mediates cell fate choice in the zebrafish pronephric duct

Pronephros, a developmental model for adult mammalian kidneys (metanephros) and a functional kidney in early teleosts, consists of glomerulus, tubule, and duct. These structural and functional elements are responsible for different kidney functions, e.g., blood filtration, waste extraction, salt recovery, and water balance. During pronephros organogenesis, cell differentiation is a key step in generating different cell types in specific locations to accomplish designated functions. However, it is poorly understood what molecules regulate the differentiation of different cell types in different parts of the kidney. Two types of epithelial cells, multi-cilia cells and principal cells, are found in the epithelia of the zebrafish distal pronephric duct. While the former is characterized by at least 15 apically localized cilia and expresses centrin2 and rfx2, the latter is characterized by a single primary cilium and sodium pumps. Multi-cilia cells and principal cells differentiate from 17.5 hours post-fertilization onwards in a mosaic pattern. Jagged2a-Notch1a/Notch3-Her9 is responsible for specification and patterning of these two cell types through a lateral inhibition mechanism. Furthermore, multi-cilia cell hyperplasia was observed in mind bomb mutants and Mind bomb was shown to interact with Jagged2a and facilitate its internalization. Taken together, our findings add a new paradigm of Notch signaling in kidney development, namely, that Jagged2a-Notch signaling modulates cell fate choice in a nephric segment, the distal pronephric duct.

The kidney is a complex organ that regulates blood homeostasis through the maintenance of fluid and ion balance and disposal of metabolic waste. We used zebrafish pronephros, a primordial vertebrate kidney, to address how a kidney tissue acquires its cell types and pattern. Two types of epithelial cells were found in the pronephric duct: multi-cilia cells and principal cells, which could be distinguished based on morphology and expression of different marker genes. In the pronephric duct, the multi-cilia cells and principal cells form a “salt and pepper,” or mosaic, pattern. Using existing zebrafish mutants and a knockdown technique, we demonstrated that the mosaic pattern and differentiation of these two cell types are controlled through a Notch-dependent lateral inhibition mechanism. Notch signaling has been shown to be essential for other aspects of kidney development, such as formation of the glomerulus and the tubule. Here, to our knowledge for the first time, we show that the same signaling pathway is required for the differentiation of two different epithelial cells in a kidney segment known as the distal pronephric duct. The same mechanism is very likely to be employed by other similar developmental processes in the same context to generate distinct cell types in a tissue.

Zebrafish Mib and Mib2 are mutual E3 ubiquitin ligases with common and specific delta substrates

It was already known that both mind bomb (mib) and mind bomb-2 (mib2) encode E3 ubiquitin ligases that target Delta in Notch activation. Here we further demonstrated that zebrafish Mib and Mib2, similar to their mouse orthologs, have a C-terminal-most RING finger-dependent E3 ubiquitin ligase activity. Mib and Mib2 are reciprocal E3 ubiquitin ligases and substrates. They function similarly in Notch signaling by using DeltaC as a common substrate. However, Mib2 behaves differently from Mib in DeltaD internalization. In addition, Mib and Mib2 bind differently to extracellular and intracellular parts of DeltaA and DeltaC. Finally, mutant Mibs, Mib(ta52b) with a missense mutation in the C-terminal-most RING finger (M1013R) and Mib(m132) with a premature stop codon that leads to a deletion of three RING fingers (C785stop), act dominant-negatively and compete with Mib2 in DeltaC ubiquitylation and internalization, suggesting a molecular basis for the antimorphic phenotypes (stronger than the null phenotypes) observed in zebrafish mib(ta52b) and mib(m132) alleles.

Coordination of symmetric cyclic gene expression during somitogenesis by Suppressor of Hairless involves regulation of retinoic acid catabolism

Vertebrate embryos faithfully produce bilaterally symmetric somites that give rise to repetitive body structures such as vertebrae and skeletal muscle. Body segmentation is regulated by a cyclic gene expression system, containing the Delta-Notch pathway and targets, which generates bilaterally symmetric oscillations across the Pre-Somitic Mesoderm (PSM). The position of the forming somite boundary is controlled by interaction of this oscillator with a determination front comprised of opposing gradients of FGF and retinoic acid (RA) signalling. Disruption of RA production leads to asymmetries in cyclic gene expression, but the link between RA and the oscillator is unknown. In somitogenesis, Notch signalling activates target genes through the transcription factor Suppressor of Hairless (Su(H)). Here, we report that two Su(H) genes coordinate bilaterally symmetric positioning of somite boundaries in the zebrafish embryo. Combined Su(H) gene knockdown caused defects in visceral left/right asymmetry, neurogenic lateral inhibition, and symmetrical failure of the segmentation oscillator. However, by selectively down-regulating Su(H)2 or Su(H)1 function using specific antisense morpholinos, we observed asymmetric defects in anterior or posterior somite boundaries, respectively. These morphological abnormalities were reflected by underlying asymmetric cyclic gene expression waves in the presomitic mesoderm, indicating a key role for Su(H) in coordinating the left-right symmetry of this process. Strikingly, expression of the RA-degrading enzyme cyp26a1 in the tailbud was controlled by Su(H) activity, and morpholino knockdown of cyp26a1 alone caused asymmetric cyclic dlc expression, suggesting that excess RA in the tailbud may contribute to the cyclic asymmetries. Indeed, exogenous RA was sufficient to generate asymmetric expression of all cyclic genes. Our observations indicate that one element of the Notch signalling pathway, Su(H), is required for control of RA metabolism in the tailbud and that this regulation is involved in bilateral symmetry of cyclic gene expression and somitogenesis.

Zebrafish Angiotensin II Receptor-like 1a (agtrl1a) is expressed in migrating hypoblast, vasculature, and in multiple embryonic epithelia

The human gene AGTRL1 is an angiotensin II receptor-like gene expressed in vasculature, which acts as the receptor for the small peptide APELIN, and a co-receptor for Human Immunodeficiency Virus. Mammalian AGTRL1 has been shown to modulate cardiac contractility, venous and arterial dilation, and endothelial cell migration in vitro, but no role in the development of the vasculature, or other tissues, has been described. We report the identification and expression of the zebrafish ortholog of the human gene AGTRL1. Zebrafish agtrl1a is first expressed before epiboly in dorsal precursors. During epiboly it is expressed in the enveloping layer, yolk syncytial layer and migrating mesendoderm. During segmentation stages, expression is observed in epithelial structures such as adaxial cells, border cells of the newly formed somites, developing lens, otic vesicles and venous vasculature.

Notch signaling regulates midline cell specification and proliferation in zebrafish

Notochord and floor plate cells are sources of molecules that pattern tissues near the midline, including the spinal cord. Hypochord cells are also found at the midline of anamniote embryos and are important for aorta development. Delta-Notch signaling regulates midline patterning in the dorsal organizer by inhibiting notochord formation and promoting hypochord and possibly floor plate development, but the precise mechanisms by which this regulation occurs are unknown. We demonstrate here that floor plate and hypochord cells arise from distinct regions of the zebrafish shield. Blocking Notch signaling during gastrulation entirely prevented hypochord specification but only reduced the number of floor plate cells that developed compared to control embryos. In contrast, elevation of Notch signaling at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not affected. A cell proliferation assay revealed that Notch signaling maintains dividing floor plate progenitors. Together, our results indicate that Notch signaling regulates allocation of appropriate numbers of different midline cells by different mechanisms.

Genetic analysis of early neurogenesis: dedicated to the scientific contributions of Jose A. Campos-Ortega (1940-2004)

Jose Campos-Ortega stands out as one of the pioneers of developmental-genetic studies of early neurogenesis. He also liked to reflect about the history of science: how one discovery leads to the next, and what role individuals play in the progress of science. He had indeed started to work on a book describing the history of developmental genetics during the last year of his life. His goal in this book was to "explain how developmental genetics originated, how it transformed developmental biology and, while doing so, how it contributed to achieve the biological synthesis." In the following, I would like to reflect on the origin and growth of the field Campos-Ortega contributed so much. In doing so, it is of particular interest to consider his scientific roots, and the manner in which he entered the stage of developmental genetics. I believe that Campos-Ortega's unusual scientific background influenced in an important manner the way in which he shaped the study of early neurogenesis.

The Enigma of the Numb-Notch Relationship during Mammalian Embryogenesis

Asymmetric cell division is an attractive means to diversify cell fates during development and for stem cells to balance self-renewal and differentiation. In Drosophila, two signaling pathways, one mediated by Numb and the other by Notch, play essential but antagonistic roles in enabling the two daughters to adopt different fates after a wide variety of asymmetric cell divisions. However, recent studies show that mutating mammalian Numb homologues, m-Numb and Numblike (Numbl or Nbl), and eliminating Notch signaling in the developing nervous system both lead to premature depletion of neural stem/progenitor cells in mice. These findings raise an interesting question as to whether and how the antagonistic roles of Numb and Notch signaling are conserved in vertebrates. Here we provide evidence that loss of m-Numb and Numbl outside the embryonic nervous system also causes phenotypes similar to those exhibited by mice with defective Notch signaling. We further show that very little Numb protein is necessary for embryogenesis and that the presence of different m-Numb isoforms is unlikely to provide a molecular basis for differential regulation of Notch signaling in mammals, as these isoforms are functionally indistinguishable in cell fate specification. We discuss possible mechanisms by which the antagonistic roles of Numb and Notch are evolutionarily conserved to meet the needs of stem cell maintenance during mammalian neurogenesis.

Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event

The first morphological sign of vertebrate postcranial body segmentation is the sequential production from posterior paraxial mesoderm of blocks of cells termed somites. Each of these embryonic structures is polarized along the anterior/posterior axis, a subdivision first distinguished by marker gene expression restricted to rostral or caudal territories of forming somites. To better understand the generation of segment polarity in vertebrates, we have studied the zebrafish mutant fused somites (fss), because its paraxial mesoderm lacks segment polarity. Previously examined markers of caudal half-segment identity are widely expressed, whereas markers of rostral identity are either missing or dramatically down-regulated, suggesting that the paraxial mesoderm of the fss mutant embryo is profoundly caudalized. These findings gave rise to a model for the formation of segment polarity in the zebrafish in which caudal is the default identity for paraxial mesoderm, upon which is patterned rostral identity in an fss-dependent manner. In contrast to this scheme, the caudal marker gene ephrinA1 was recently shown to be down-regulated in fss embryos. We now show that notch5, another caudal identity marker and a component of the Delta/Notch signaling system, is not expressed in the paraxial mesoderm of early segmentation stage fss embryos. We use cell transplantation to create genetic mosaics between fss and wild-type embryos in order to assay the requirement for fss function in notch5 expression. In contrast to the expression of rostral markers, which have a cell-autonomous requirement for fss, expression of notch5 is induced in fss cells at short range by nearby wild-type cells, indicating a cell-non-autonomous requirement for fss function in this process. These new data suggest that segment polarity is created in a three-step process in which cells that have assumed a rostral identity must subsequently communicate with their partially caudalized neighbors in order to induce the fully caudalized state.

beamter/deltaC and the role of Notch ligands in the zebrafish somite segmentation, hindbrain neurogenesis and hypochord differentiation

The Tübingen large-scale zebrafish genetic screen completed in 1996 identified a set of five genes required for orderly somite segmentation. Four of them have been molecularly identified and three were found to code for components of the Notch pathway, which are required for the coordinated oscillation of gene expression, known as the segmentation clock, in the presomitic mesoderm (PSM). Here, we show that the final member of the group, beamter (bea), codes for the Notch ligand DeltaC, and we present and characterize two new alleles, including one allele encoding for a protein truncated in the 7th EGF repeat and an allele deleting only the DSL domain which was previously shown to be necessary for ligand function. Interestingly however, when we over-express any of the mutant deltaC mRNAs, we observe antimorphic effects on both hindbrain neurogenesis and hypochord formation. Expression of bea/deltaC oscillates in the PSM, and a triple fluorescent in situ analysis of its oscillation in relation to that of other oscillating genes in the PSM reveals differences in subcellular localization of the oscillating mRNAs in individual cells in different oscillation phases. Mutations in aei/deltaD and bea/deltaC differ in the way they disrupt the oscillating expression of her1 and deltaC. Furthermore, we find that the double mutants have significantly stronger defects in hypochord formation but not in somitogenesis or hindbrain neurogenesis, indicating genetically that the two delta's may function either semi-redundantly or distinctly, depending upon context.

Notch in the pathway: the roles of Notch signaling in neural crest development

Here, we review recent studies that suggest that Notch signaling has two roles during neural crest development: first in establishing the neural crest domain within the ectoderm via lateral induction and subsequently in diversifying the fates of cells that arise from the neural crest via lateral inhibition. The first of these roles, specification of neural crest via lateral induction, has been explored primarily in the cranial neural folds from which the cranial neural crest arises. Evidence for such a role has thus far only been obtained from chick and frog; results from these two species differ, but share the feature that Notch signaling regulates genes that are expressed by cranial neural crest through effects on expression of Bmp family members. The second of these roles, diversification of neural crest progeny via lateral inhibition, has been identified thus far only in trunk neural crest. Evidence from several species suggests that Notch-mediated lateral inhibition functions in multiple episodes in this context, in each case inhibiting neurogenesis. In the 'standard' mode of lateral inhibition, Notch promotes proliferation and in the 'instructive' mode, it promotes specific secondary fates, including cell death or glial differentiation. We raise the possibility that a single molecular mechanism, inhibition of so-called proneural bHLH genes, underlies both modes of lateral inhibition mediated by Notch signaling.

Cooperative function of deltaC and her7 in anterior segment formation

Segmentation of paraxial mesoderm in vertebrates is regulated by a genetic oscillator that manifests as a series of wavelike or cyclic gene expression domains in the embryo. In zebrafish, this oscillator involves members of the Delta/Notch intercellular signaling pathway, and its down-stream targets, the Her family of transcriptional repressors. Loss of function of any one of the genes of this system, such as her7, gives rise to segmentation defects in the posterior trunk and tail, concomitant with a disruption of cyclic expression domains, indicating that the oscillator is required for posterior segmentation. Control of segmentation in the anterior trunk, and its relationship to that of the posterior is, however, not yet well understood. A combined loss of the cyclic Her genes her1 and her7 disrupts segmentation of both anterior and posterior paraxial mesoderm, indicating that her genes function redundantly in anterior segmentation. To test whether this anterior redundancy is specific to the her gene family, or alternatively is a more global feature of the segmentation oscillator, we looked at anterior segmentation after morpholino knock down of the cyclic cell-surface Notch ligand deltaC (dlc), either alone or in combination with her7, or other Delta/Notch pathway genes. We find that dlc is required for coherence of wavelike expression domains of cyclic genes her1 and her7 and maintenance of their expression levels, as well as for cyclic transcription of dlc itself, confirming that dlc is a component of the segmentation oscillator. Dose dependent, posteriorly-restricted segmentation defects were seen in the dlc knock down, and in combination with the deltaD or notch1a mutants. However, combined reduction of function of dlc and her7 results in defective segmentation of both anterior and posterior paraxial mesoderm, and a failure of cyclic expression domains to initiate, similar to loss of both her genes. Thus, anterior segmentation requires the functions of both her and delta family members in a parallel manner, suggesting that the segmentation oscillator operates in paraxial mesoderm along the entire vertebrate axis.

Conserved and acquired features of neurogenin1 regulation

The telencephalon shows vast morphological variations among different vertebrate groups. The transcription factor neurogenin1(ngn1) controls neurogenesis in the mouse pallium and is also expressed in the dorsal telencephalon of the evolutionary distant zebrafish. The upstream regions of the zebrafish and mammalian ngn1 loci harbour several stretches of conserved sequences. Here, we show that the upstream region of zebrafish ngn1 is capable of faithfully recapitulating endogenous expression in the zebrafish and mouse telencephalon. A single conserved regulatory region is essential for dorsal telencephalic expression in the zebrafish, and for expression in the dorsal pallium of the mouse. However, a second conserved region that is inactive in the fish telencephalon is necessary for expression in the lateral pallium of mouse embryos. This regulatory region, which drives expression in the zebrafish diencephalon and hindbrain, is dependent on Pax6 activity and binds recombinant Pax6 in vitro. Thus, the regulatory elements of ngn1 appear to be conserved among vertebrates, with certain differences being incorporated in the utilisation of these enhancers, for the acquisition of more advanced features in amniotes. Our data provide evidence for the co-option of regulatory regions as a mechanism of evolutionary diversification of expression patterns, and suggest that an alteration in Pax6expression was crucial in neocortex evolution.

The role of Suppressor of Hairless in Notch mediated signalling during zebrafish somitogenesis

Suppressor of Hairless (Su(H)) codes for a protein that interacts with the intracellular domain of Notch to activate the target genes of the Delta-Notch signalling pathway. We have cloned the zebrafish homologue of Su(H) and have analysed its function by morpholino mediated knockdown. While there are at least four notch and four delta homologues in zebrafish, there appears to be only one complete Su(H) homologue. We have analysed the function of Su(H) in the somitogenesis process and its influence on the expression of notch pathway genes, in particular her1, her7, deltaC and deltaD. The cyclic expression of her1, her7 and deltaC in the presomitic mesoderm is disrupted by the Su(H) knockdown mimicking the expression of these genes in the notch1a mutant deadly seven. deltaD expression is similarly affected by Su(H) knockdown like deltaC but shows in addition an ectopic expression in the developing neural tube. The inactivation of Su(H) in a fss/tbx24 mutant background leads furthermore to a clear breakdown of cyclic her1 and her7 expression, indicating that the Delta-Notch pathway is required for the creation of oscillation and not only for the synchronisation between neighbouring cells. The strongest phenotypes in the Su(H) knockdown embryos show a loss of all somites posterior to the first five to seven ones. This phenotype is stronger than the known amorphic phenotypes for notch1 (des) or deltaD (aei) in zebrafish, but mimicks the knockout phenotype of RBP-Jkappa gene in the mouse, which is the homologue of Su(H). This suggests that there is some functional redundancy among the Notch and Delta genes. This fact that the first five to seven somites are only weakly affected by Su(H) knockdown indicates that additional genetic pathways may be active in the specification of the most anterior somites.

her3, a zebrafish member of the hairy-E(spl) family, is repressed by Notch signalling

her3 encodes a zebrafish bHLH protein of the Hairy-E(Spl) family. During embryogenesis, the gene is transcribed exclusively in the developing central nervous system, according to a fairly simple pattern that includes territories in the mesencephalon/rhombencephalon and the spinal cord. In all territories, the her3 transcription domain encompasses regions in which neurogenin 1 (neurog1) is not transcribed, suggesting regulatory interactions between the two genes. Indeed, injection of her3 mRNA leads to repression of neurog1 and to a reduction in the number of primary neurones, whereas her3 morpholino oligonucleotides cause ectopic expression of neurog1 in the rhombencephalon. Fusions of Her3 to the transactivation domain of VP16 and to the repression domain of Engrailed show that Her3 is indeed a transcriptional repressor. Dissection of the Her3 protein reveals two possible mechanisms for transcriptional repression: one mediated by the bHLH domain and the C-terminal WRPW tetrapeptide; and the other involving the N-terminal domain and the orange domain. Gel retardation assays suggest that the repression of neurog1 transcription occurs by binding of Her3 to specific DNA sequences in the neurog1 promoter. We have examined interrelationships of her3 with members of the Notch signalling pathway by the Gal4-UAS technique and mRNA injections. The results indicate that Her3 represses neurog1 and, probably as a consequence of the neurog1 repression, deltaA, deltaD and her4. Moreover, Her3 represses its own transcription as well. Surprisingly, and in sharp contrast to other members of the E(spl) gene family, transcription of her3 is repressed rather than activated by Notch signalling.

Histone deacetylase 1 is required to repress Notch target gene expression during zebrafish neurogenesis and to maintain the production of motoneurones in response to hedgehog signalling

Histone deacetylases (Hdacs) are widely implicated as key components of transcriptional silencing mechanisms. Here, I show that hdac1 is specifically required in the zebrafish embryonic CNS to maintain neurogenesis. In hdac1 mutant embryos, the Notch-responsive E(spl)-related neurogenic gene her6 is ectopically expressed at distinct sites within the developing CNS and proneural gene expression is correspondingly reduced or eliminated. Using an hdac1-specific morpholino, I show that this requirement for hdac1 is epistatic to the requirement for Notch signalling. Consequently, hdac1-deficient embryos exhibit several defects of neuronal specification and patterning, including a dramatic deficit of hedgehog-dependent branchiomotor neurones that is refractory to elevated levels of hedgehog signalling. Thus, in the zebrafish embryo, hdac1 is an essential component of the transcriptional silencing machinery that supports the formation and subsequent differentiation of neuronal precursors.

Notch function in the vasculature: insights from zebrafish, mouse and man

Vascular development entails multiple cell-fate decisions to specify a diverse array of vascular structures. Notch proteins are signaling receptors that regulate cell-fate determination in a variety of cell types. The finding that Notch genes are robustly expressed in the vasculature suggests roles for Notch in guiding endothelial and associated mural cells through the myriad of cell-fate decisions needed to form the vasculature. In fact, mice with defects in genes encoding Notch, Notch ligands, and components of the Notch signaling cascade invariably display vascular defects. Human Notch genes are linked to Alagille's Syndrome, a developmental disorder with vascular defects, and CADASIL, a cerebral arteriopathy. Studies in zebrafish, mice and humans indicate that Notch works in conjunction with other angiogenic pathways to pattern and stabilize the vasculature. Here, we will focus on established functions for Notch in vascular remodeling and arterial/venous specification and more speculative roles in vascular homeostasis and organ-specific angiogenesis.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Notch promotes epithelial-mesenchymal transition during cardiac development and oncogenic transformation

Epithelial-to-mesenchymal transition (EMT) is fundamental to both embryogenesis and tumor metastasis. The Notch intercellular signaling pathway regulates cell fate determination throughout metazoan evolution, and overexpression of activating alleles is oncogenic in mammals. Here we demonstrate that Notch activity promotes EMT during both cardiac development and oncogenic transformation via transcriptional induction of the Snail repressor, a potent and evolutionarily conserved mediator of EMT in many tissues and tumor types. In the embryonic heart, Notch functions via lateral induction to promote a selective transforming growth factor-beta (TGFbeta)-mediated EMT that leads to cellularization of developing cardiac valvular primordia. Embryos that lack Notch signaling elements exhibit severely attenuated cardiac snail expression, abnormal maintenance of intercellular endocardial adhesion complexes, and abortive endocardial EMT in vivo and in vitro. Accordingly, transient ectopic expression of activated Notch1 (N1IC) in zebrafish embryos leads to hypercellular cardiac valves, whereas Notch inhibition prevents valve development. Overexpression of N1IC in immortalized endothelial cells in vitro induces EMT accompanied by oncogenic transformation, with corresponding induction of snail and repression of VE-cadherin expression. Notch is expressed in embryonic regions where EMT occurs, suggesting an intimate and fundamental role for Notch, which may be reactivated during tumor metastasis.

Characterization of hey bHLH genes in teleost fish

Hairy-related basic helix-loop-helix (bHLH) transcription factors are targets of Delta-Notch signaling and represent essential components for a number of cell fate decisions during vertebrate embryogenesis. Hey genes encode a subfamily of hairy-related proteins that have been implicated in processes like somitogenesis, blood vessel and heart development. We have identified and characterized hey genes in three teleost fish lineages using degenerate PCR and database searches. Phylogenetic analysis of Hey proteins suggests a complex pattern of evolution with high divergence of hey2 in Takifugu rubripes (Fugu, Japanese pufferfish) and possibly loss in the related Tetraodon nigroviridis (the freshwater pufferfish). In addition, duplication of hey1 in both pufferfishes, Fugu and Tetraodon, was observed. Conversely, zebrafish (Danio rerio) has the same complement of three hey genes as known from mammals. All three hey genes show much more restricted gene expression profiles in zebrafish when compared to mouse. Importantly, while all three murine Hey genes are expressed in overlapping patterns in the presomitic mesoderm (PSM) and somites, in zebrafish only hey1 shows PSM and somite expression in a highly dynamic fashion. Therefore, while overlapping expression might account for redundancy of hey function in higher vertebrates, this is unlikely to be the case in zebrafish. In deltaD (dlD) deficient after-eight zebrafish mutants, the dynamic expression of hey1 in the PSM is impaired and completely lost in newly formed somitomeres. Overexpression of dlD on the other hand results in the ectopic expression of hey1 in the axial mesoderm. Hence, hey1 represents a target of Delta-Notch signaling dynamically expressed during somite formation in zebrafish.

Feedback loops comprising Dll1, Dll3 and Mesp2, and differential involvement of Psen1 are essential for rostrocaudal patterning of somites

Elaborate metamerism in vertebrate somitogenesis is based on segmental gene expression in the anterior presomitic mesoderm (PSM). Notch signal pathways with Notch ligands Dll1 and Dll3, and the transcription factor Mesp2 are implicated in the rostrocaudal patterning of the somite. We have previously shown that changes in the Mesp2 expression domain from a presumptive one somite into a rostral half somite results in differential activation of two types of Notch pathways, dependent or independent of presenilin 1 (Psen1), which is a Notch signal mediator. To further refine our hypothesis, we have analyzed genetic interactions between Dll1, Dll3, Mesp2 and Psen1, and elucidated the roles of Dll1- and Dll3-Notch pathways, with or without Psen1, in rostrocaudal patterning. Dll1 and Dll3 are co-expressed in the PSM and so far are considered to have partially redundant functions. We find in this study that positive and negative feedback loops comprising Dll1 and Mesp2 appear to be crucial for this patterning, and Dll3 may be required for the coordination of the Dll1-Mesp2 loop. Additionally, our epistatic analysis revealed that Mesp2 affects rostrocaudal properties more directly than Dll1 or Dll3. Finally, we find that Psen1 is involved differently in the regulation of rostral and caudal genes. Psen1 is required for Dll1-Notch signaling for activation of Dll1, while the Psen1-independent Dll3-Notch pathway may counteract the Psen1-dependent Dll1-Notch pathway. These observations suggest that Dll1 and Dll3 may have non-redundant, even counteracting functions. We conclude from our analyses that Mesp2 functions as a central mediator of such Notch pathways and regulates the gene expression required for rostrocaudal patterning of somites.

Anterior and posterior waves of cyclic her1 gene expression are differentially regulated in the presomitic mesoderm of zebrafish

Somite formation in vertebrates depends on a molecular oscillator in the presomitic mesoderm (PSM). In order to get a better insight into how oscillatory expression is achieved in the zebrafish Danio rerio, we have analysed the regulation of her1 and her7, two bHLH genes that are co-expressed in the PSM. Using specific morpholino oligonucleotide mediated inhibition and intron probe in situ hybridisation, we find that her7 is required for initiating the expression in the posterior PSM, while her1 is required to propagate the cyclic expression in the intermediate and anterior PSM. Reporter gene constructs with the her1 upstream sequence driving green fluorescent protein (GFP) expression show that separable regulatory regions can be identified that mediate expression in the posterior versus intermediate and anterior PSM. Our results indicate that the cyclic expression is generated at the transcriptional level and that the resulting mRNAs have a very short half-life. A specific degradation signal for her1 mRNA must be located in the 5'-UTR, as this region also destabilises the GFP mRNA such that it mimics the dynamic pattern of the endogenous her1 mRNA. In contrast to the mRNA, GFP protein is stable and we find that all somitic cells express the protein, proving that her1 mRNA is transiently expressed in all cells of the PSM.

Delta-Notch signaling regulates oligodendrocyte specification

Oligodendrocytes, the myelinating cell type of the central nervous system, arise from a ventral population of precursors that also produces motoneurons. Although the mechanisms that specify motoneuron development are well described, the mechanisms that generate oligodendrocytes from the same precursor population are largely unknown. By analysing mutant zebrafish embryos, we found that Delta-Notch signaling is required for spinal cord oligodendrocyte specification. Using a transgenic, conditional expression system, we also learned that constitutive Notch activity could promote formation of excess oligodendrocyte progenitor cells (OPCs). However, excess OPCs are induced only in ventral spinal cord at the time that OPCs normally develop. Our data provide evidence that Notch signaling maintains subsets of ventral spinal cord precursors during neuronal birth and, acting with other temporally and spatially restricted factors, specifies them for oligodendrocyte fate.

bHLH transcription factor Her5 links patterning to regional inhibition of neurogenesis at the midbrain-hindbrain boundary

The midbrain-hindbrain (MH) domain of the vertebrate embryonic neural plate displays a stereotypical profile of neuronal differentiation, organized around a neuron-free zone ('intervening zone', IZ) at the midbrain-hindbrain boundary (MHB). The mechanisms establishing this early pattern of neurogenesis are unknown. We demonstrate that the MHB is globally refractory to neurogenesis, and that forced neurogenesis in this area interferes with the continued expression of genes defining MHB identity. We further show that expression of the zebrafish bHLH Hairy/E(spl)-related factor Her5 prefigures and then precisely delineates the IZ throughout embryonic development. Using morpholino knock-down and conditional gain-of-function assays, we demonstrate that Her5 is essential to prevent neuronal differentiation and promote cell proliferation in a medial compartment of the IZ. We identify one probable target of this activity, the zebrafish Cdk inhibitor p27Xic1. Finally, although the her5 expression domain is determined by anteroposterior patterning cues, we show Her5 does not retroactively influence MH patterning. Together, our results highlight the existence of a mechanism that actively inhibits neurogenesis at the MHB, a process that shapes MH neurogenesis into a pattern of separate neuronal clusters and might ultimately be necessary to maintain MHB integrity. Her5 appears as a partially redundant component of this inhibitory process that helps translate early axial patterning information into a distinct spatiotemporal pattern of neurogenesis and cell proliferation within the MH domain.

Anatomy of neurogenesis in the early zebrafish brain

The detailed architecture of postembryonic (i.e. secondary, as opposed to primary) neurogenesis in the zebrafish brain at 2 days postfertilization was investigated by studying expression domains of various proneural basic helix-loop-helix genes (i.e. neuroD=nrd, neurogenin1=ngn1, Zash-1b) and neurogenic genes (i.e. Notch-1a, deltaA) on the level of in situ-hybridized sectioned material and compared with brain sections of the same age immunostained for PCNA (a proliferation marker) or for Hu-proteins (marker for early neuronal differentiation). Whereas both Notch-1a and deltaA domains are present in all proliferative zones of the brain, only the more diffuse and scattered deltaA expression appears to extend additionally into the adjacent postmitotic gray matter. Zash-1b is the first achaete-scute orthologue shown here to be expressed exclusively in all proliferative central nervous zones (except for the eye). The ngn1 and nrd genes are typically both expressed in overlapping fashion in many-but not all-brain regions, with ngn1 being more restricted towards the ventricular proliferative zones and nrd extending more laterally. This fact as well as comparisons with PCNA- and Hu-immunostains indicate that a great proportion of ngn1-positive cells are mitotic, but some appear to extend into the postmitotic gray matter where the nrd-domains lie. A comparison of the relative extents of PCNA- (proliferative), nrd- (freshly determined) and Hu-positive (differentiating) cell populations allows to determine the relative maturation state of a given brain part. Similar to findings in late embryonic amniote brains, expression of nrd is absent (from 2 to 5 days) in the zebrafish subpallium, ventral preoptic region, ventral thalamus and hypothalamus. These four regions are also free of ngn1 expression (with the exception of an unusual peripheral ngn1 domain in the preoptic region), indicating that a neurogenetic network not involving nrd and ngn1 is at work there. Our characterization of locally distinct patterns and dynamics of secondary neurogenesis in the entire early (2 dpf) postembryonic zebrafish brain delivers the blueprint for more specialized functional studies.