Zebrafish hairy/enhancer of split protein links FGF signaling to cyclic gene expression in the periodic segmentation of somites

Cellular and molecular control of vertebrate somitogenesis

Segmentation is a fundamental feature of the vertebrate body plan. This metameric organization is first implemented by somitogenesis in the early embryo, when paired epithelial blocks called somites are rhythmically formed to flank the neural tube. Recent advances in in vitro models have offered new opportunities to elucidate the mechanisms that underlie somitogenesis. Notably, models derived from human pluripotent stem cells introduced an efficient proxy for studying this process during human development. In this Review, we summarize the current understanding of somitogenesis gained from both in vivo studies and in vitro studies. We deconstruct the spatiotemporal dynamics of somitogenesis into four distinct modules: dynamic events in the presomitic mesoderm, segmental determination, somite anteroposterior polarity patterning, and epithelial morphogenesis. We first focus on the segmentation clock, as well as signalling and metabolic gradients along the tissue, before discussing the clock and wavefront and other models that account for segmental determination. We then detail the molecular and cellular mechanisms of anteroposterior polarity patterning and somite epithelialization.

Oscillatory control of embryonic development

Proper embryonic development depends on the timely progression of a genetic program. One of the key mechanisms for achieving precise control of developmental timing is to use gene expression oscillations. In this Review, we examine how gene expression oscillations encode temporal information during vertebrate embryonic development by discussing the gene expression oscillations occurring during somitogenesis, neurogenesis, myogenesis and pancreas development. These oscillations play important but varied physiological functions in different contexts. Oscillations control the period of somite formation during somitogenesis, whereas they regulate the proliferation-to-differentiation switch of stem cells and progenitor cells during neurogenesis, myogenesis and pancreas development. We describe the similarities and differences of the expression pattern in space (i.e. whether oscillations are synchronous or asynchronous across neighboring cells) and in time (i.e. different time scales) of mammalian Hes/zebrafish Her genes and their targets in different tissues. We further summarize experimental evidence for the functional role of their oscillations. Finally, we discuss the outstanding questions for future research.

Effects of microRNA-305 knockdown on brain gene expression associated with division of labor in honey bee colonies (Apis mellifera)

Division of labor in honey bee colonies is based on the behavioral maturation of adult workers that involves a transition from working in the hive to foraging. This behavioral maturation is associated with distinct task-related transcriptomic profiles in the brain and abdominal fat body that are related to multiple regulatory factors including juvenile hormone (JH) and queen mandibular pheromone (QMP). A prominent physiological feature associated with behavioral maturation is a loss of abdominal lipid mass as bees transition to foraging. We used transcriptomic and physiological analyses to study whether microRNAs (miRNAs) are involved in the regulation of division of labor. We first identified two miRNAs that showed patterns of expression associated with behavioral maturation, ame-miR-305-5p and ame-miR-375-3p. We then downregulated the expression of these two miRNAs with sequence-specific antagomirs. Neither ame-miR-305-5p nor ame-miR-375-3p knockdown in the abdomen affected abdominal lipid mass on their own. Similarly, knockdown of ame-miR-305-5p in combination with JH or QMP also did not affect lipid mass. By contrast, ame-miR-305-5p knockdown in the abdomen caused substantial changes in gene expression in the brain. Brain gene expression changes included genes encoding transcription factors previously implicated in behavioral maturation. The results of these functional genomic experiments extend previous correlative associations of microRNAs with honey bee division of labor and point to specific roles for ame-miR-305-5p.

Species-specific roles of the Notch ligands, receptors, and targets orchestrating the signaling landscape of the segmentation clock

Somitogenesis is a hallmark feature of all vertebrates and some invertebrate species that involves the periodic formation of block-like structures called somites. Somites are transient embryonic segments that eventually establish the entire vertebral column. A highly conserved molecular oscillator called the segmentation clock underlies this periodic event and the pace of this clock regulates the pace of somite formation. Although conserved signaling pathways govern the clock in most vertebrates, the mechanisms underlying the species-specific divergence in various clock characteristics remain elusive. For example, the segmentation clock in classical model species such as zebrafish, chick, and mouse embryos tick with a periodicity of ∼30, ∼90, and ∼120 min respectively. This enables them to form the species-specific number of vertebrae during their overall timespan of somitogenesis. Here, we perform a systematic review of the species-specific features of the segmentation clock with a keen focus on mouse embryos. We perform this review using three different perspectives: Notch-responsive clock genes, ligand-receptor dynamics, and synchronization between neighboring oscillators. We further review reports that use non-classical model organisms and in vitro model systems that complement our current understanding of the segmentation clock. Our review highlights the importance of comparative developmental biology to further our understanding of this essential developmental process.

Stochastic gene expression and environmental stressors trigger variable somite segmentation phenotypes

Mutations of several genes cause incomplete penetrance and variable expressivity of phenotypes, which are usually attributed to modifier genes or gene-environment interactions. Here, we show stochastic gene expression underlies the variability of somite segmentation defects in embryos mutant for segmentation clock genes her1 or her7. Phenotypic strength is further augmented by low temperature and hypoxia. By performing live imaging of the segmentation clock reporters, we further show that groups of cells with higher oscillation amplitudes successfully form somites while those with lower amplitudes fail to do so. In unfavorable environments, the number of cycles with high amplitude oscillations and the number of successful segmentations proportionally decrease. These results suggest that individual oscillation cycles stochastically fail to pass a threshold amplitude, resulting in segmentation defects in mutants. Our quantitative methodology is adaptable to investigate variable phenotypes of mutant genes in different tissues.

Ripply suppresses Tbx6 to induce dynamic-to-static conversion in somite segmentation

The metameric pattern of somites is created based on oscillatory expression of clock genes in presomitic mesoderm. However, the mechanism for converting the dynamic oscillation to a static pattern of somites is still unclear. Here, we provide evidence that Ripply/Tbx6 machinery is a key regulator of this conversion. Ripply1/Ripply2-mediated removal of Tbx6 protein defines somite boundary and also leads to cessation of clock gene expression in zebrafish embryos. On the other hand, activation of ripply1/ripply2 mRNA and protein expression is periodically regulated by clock oscillation in conjunction with an Erk signaling gradient. Whereas Ripply protein decreases rapidly in embryos, Ripply-triggered Tbx6 suppression persists long enough to complete somite boundary formation. Mathematical modeling shows that a molecular network based on results of this study can reproduce dynamic-to-static conversion in somitogenesis. Furthermore, simulations with this model suggest that sustained suppression of Tbx6 caused by Ripply is crucial in this conversion.

In vivo Functional Genomics for Undiagnosed Patients: The Impact of Small GTPases Signaling Dysregulation at Pan-Embryo Developmental Scale

While individually rare, disorders affecting development collectively represent a substantial clinical, psychological, and socioeconomic burden to patients, families, and society. Insights into the molecular mechanisms underlying these disorders are required to speed up diagnosis, improve counseling, and optimize management toward targeted therapies. Genome sequencing is now unveiling previously unexplored genetic variations in undiagnosed patients, which require functional validation and mechanistic understanding, particularly when dealing with novel nosologic entities. Functional perturbations of key regulators acting on signals' intersections of evolutionarily conserved pathways in these pathological conditions hinder the fine balance between various developmental inputs governing morphogenesis and homeostasis. However, the distinct mechanisms by which these hubs orchestrate pathways to ensure the developmental coordinates are poorly understood. Integrative functional genomics implementing quantitative in vivo models of embryogenesis with subcellular precision in whole organisms contribute to answering these questions. Here, we review the current knowledge on genes and mechanisms critically involved in developmental syndromes and pediatric cancers, revealed by genomic sequencing and in vivo models such as insects, worms and fish. We focus on the monomeric GTPases of the RAS superfamily and their influence on crucial developmental signals and processes. We next discuss the effectiveness of exponentially growing functional assays employing tractable models to identify regulatory crossroads. Unprecedented sophistications are now possible in zebrafish, i.e., genome editing with single-nucleotide precision, nanoimaging, highly resolved recording of multiple small molecules activity, and simultaneous monitoring of brain circuits and complex behavioral response. These assets permit accurate real-time reporting of dynamic small GTPases-controlled processes in entire organisms, owning the potential to tackle rare disease mechanisms.

Pairing of segmentation clock genes drives robust pattern formation

Gene expression is an inherently stochastic process ^1,2; however, organismal development and homeostasis require that cells spatiotemporally coordinate the expression of large sets of genes. Coexpressed gene pairs in metazoans often reside in the same chromosomal neighborhood, with gene pairs representing 10% - 50% of all genes depending on species ^3–6. As shared upstream regulators can ensure correlated gene expression, the selective advantage of maintaining adjacent gene pairs remains unknown ⁶. Here, using two linked zebrafish segmentation clock genes, her1 and her7, and combining single-cell transcript counting, genetic engineering, real-time imaging and computational modeling, we reveal that gene pairing boosts correlated transcription and provides phenotypic robustness for developmental pattern formation. Our results demonstrate that disrupting gene pairing disrupts oscillations and segmentation, identifying the selective pressure retaining correlated transcription to sustain a robust and rapid developmental clock. We anticipate that these findings will inspire investigating advantages of gene pairing in other systems and engineering precise synthetic clocks in embryos and organoids.

Hes5.9 Coordinate FGF and Notch Signaling to Modulate Gastrulation via Regulating Cell Fate Specification and Cell Migration in Xenopus tropicalis

Gastrulation drives the establishment of three germ layers and embryonic axes during frog embryonic development. Mesodermal cell fate specification and morphogenetic movements are vital factors coordinating gastrulation, which are regulated by numerous signaling pathways, such as the Wnt (Wingless/Integrated), Notch, and FGF (Fibroblast growth factor) pathways. However, the coordination of the Notch and FGF signaling pathways during gastrulation remains unclear. We identified a novel helix–loop–helix DNA binding domain gene (Hes5.9), which was regulated by the FGF and Notch signaling pathways during gastrulation. Furthermore, gain- and loss-of-function of Hes5.9 led to defective cell migration and disturbed the expression patterns of mesodermal and endodermal marker genes, thus interfering with gastrulation. Collectively, these results suggest that Hes5.9 plays a crucial role in cell fate decisions and cell migration during gastrulation, which is modulated by the FGF and Notch signaling pathways.

Transient activation of the Notch-her15.1 axis plays an important role in the maturation of V2b interneurons

In the vertebrate ventral spinal cord, p2 progenitors give rise to two interneuron subtypes: excitatory V2a interneurons and inhibitory V2b interneurons. In the differentiation of V2a and V2b cells, Notch signaling promotes V2b fate at the expense of V2a fate. Later, V2b cells extend axons along the ipsilateral side of the spinal cord and express the inhibitory transmitter GABA. Notch signaling has been reported to inhibit the axonal outgrowth of mature neurons of the central nervous system; however, it remains unknown how Notch signaling modulates V2b neurite outgrowth and maturation into GABAergic neurons. Here, we have investigated neuron-specific Notch functions regarding V2b axon growth and maturation into zebrafish GABAergic neurons. We found that continuous neuron-specific Notch activation enhanced V2b fate determination but inhibited V2b axonal outgrowth and maturation into GABAergic neurons. These results suggest that Notch signaling activation is required for V2b fate determination, whereas its downregulation at a later stage is essential for V2b maturation. Accordingly, we found that a Notch signaling downstream gene, her15.1, showed biased expression in V2 linage cells and downregulated expression during the maturation of V2b cells, and continuous expression of her15.1 repressed V2b axogenesis. Our data suggest that spatiotemporal control of Notch signaling activity is required for V2b fate determination, maturation and axogenesis.

Deadenylation by the CCR4-NOT complex contributes to the turnover of hairy-related mRNAs in the zebrafish segmentation clock

In the zebrafish segmentation clock, hairy/enhancer of split-related genes her1, her7, and hes6 encodes components of core oscillators. Since the expression of cyclic genes proceeds rapidly in the presomitic mesoderm (PSM), these hairy-related mRNAs are subject to strict post-transcriptional regulation. In this study, we demonstrate that inhibition of the CCR4-NOT deadenylase complex lengthens poly(A) tails of hairy-related mRNAs and increases the amount of these mRNAs, which is accompanied by defective somite segmentation. In transgenic embryos, we show that EGFP mRNAs with 3'UTRs of hairy-related genes exhibit turnover similar to endogenous mRNAs. Our results suggest that turnover rates of her1, her7, and hes6 mRNAs are differently regulated by the CCR4-NOT deadenylase complex possibly through their 3'UTRs in the zebrafish PSM.

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.

Travelling waves in somitogenesis: Collective cellular properties emerge from time-delayed juxtacrine oscillation coupling

The sculpturing of the vertebrate body plan into segments begins with the sequential formation of somites in the presomitic mesoderm (PSM). The rhythmicity of this process is controlled by travelling waves of gene expression. These kinetic waves emerge from coupled cellular oscillators and sweep across the PSM. In zebrafish, the oscillations are driven by autorepression of her genes and are synchronized via Notch signalling. Mathematical modelling has played an important role in explaining how collective properties emerge from the molecular interactions. Increasingly more quantitative experimental data permits the validation of those mathematical models, yet leads to increasingly more complex model formulations that hamper an intuitive understanding of the underlying mechanisms. Here, we review previous efforts, and design a mechanistic model of the her1 oscillator, which represents the experimentally viable her7;hes6 double mutant. This genetically simplified system is ideally suited to conceptually recapitulate oscillatory entrainment and travelling wave formation, and to highlight open questions. It shows that three key parameters, the autorepression delay, the juxtacrine coupling delay, and the coupling strength, are sufficient to understand the emergence of the collective period, the collective amplitude, and the synchronization of neighbouring Her1 oscillators. Moreover, two spatiotemporal time delay gradients, in the autorepression and in the juxtacrine signalling, are required to explain the collective oscillatory dynamics and synchrony of PSM cells. The highlighted developmental principles likely apply more generally to other developmental processes, including neurogenesis and angiogenesis.

The kinetics in mathematical models on segmentation clock genes in zebrafish

Somitogenesis is the process for the development of somites in vertebrate embryos. This process is timely regulated by synchronous oscillatory expression of the segmentation clock genes. Mathematical models expressed by delay equations or ODEs have been proposed to depict the kinetics of these genes in interacting cells. Through mathematical analysis, we investigate the parameter regimes for synchronous oscillations and oscillation-arrested in an ODE model and a model with transcriptional and translational delays, both with Michaelis-Menten type degradations. Comparisons between these regimes for the two models are made. The delay model has larger capacity to accommodate synchronous oscillations. Based on the analysis and numerical computations extended from the analysis, we explore how the periods and amplitudes of the oscillations vary with the degradation rates, synthesis rates, and coupling strength. For typical parameter values, the period and amplitude increase as some synthesis rate or the coupling strength increases in the ODE model. Such variational properties of oscillations depend also on the magnitudes of time delays in delay model. We also illustrate the difference between the dynamics in systems modeled with linear degradation and the ones in systems with Michaelis-Menten type reactions for the degradation. The chief concerns are the connections between the dynamics in these models and the mechanism for the segmentation clocks, and the pertinence of mathematical modeling on somitogenesis in zebrafish.

Delta-Notch signalling in segmentation

Modular body organization is found widely across multicellular organisms, and some of them form repetitive modular structures via the process of segmentation. It's vastly interesting to understand how these regularly repeated structures are robustly generated from the underlying noise in biomolecular interactions. Recent studies from arthropods reveal similarities in segmentation mechanisms with vertebrates, and raise the possibility that the three phylogenetic clades, annelids, arthropods and chordates, might share homology in this process from a bilaterian ancestor. Here, we discuss vertebrate segmentation with particular emphasis on the role of the Notch intercellular signalling pathway. We introduce vertebrate segmentation and Notch signalling, pointing out historical milestones, then describe existing models for the Notch pathway in the synchronization of noisy neighbouring oscillators, and a new role in the modulation of gene expression wave patterns. We ask what functions Notch signalling may have in arthropod segmentation and explore the relationship between Notch-mediated lateral inhibition and synchronization. Finally, we propose open questions and technical challenges to guide future investigations into Notch signalling in segmentation.

Enhancer activity-based identification of functional enhancers using zebrafish embryos

Chromatin immunoprecipitation (ChIP) against enhancer-associated marks with massive sequencing is a powerful approach to identify genome-wide distributions of putative enhancers. However, functional in vivo analysis is required to elucidate the activities of predicted enhancers. Using zebrafish embryos, we established a ChIP-Injection method that enables identification of functional enhancers based on their enhancer activities in embryos. Each reporter gene possessing the enhancer-associated genomic region enriched by ChIP was injected into zebrafish embryos to analyze the activity of putative enhancers. By using the ChIP-Injection, we identified 32 distinct putative enhancers that drove specific expression. Additionally, we generated transgenic lines that exhibit distributions of the EGFP signal as was observed in the screening. Furthermore, the expression pattern driven by the identified somite-specific enhancer resembled that of the gene acta2. The results indicate that ChIP-Injection provides an efficient approach for identification of active enhancers in a potentially wide variety of developmental tissues and stages.

Molecular mechanism for cyclic generation of somites: Lessons from mice and zebrafish

The somite is the most prominent metameric structure observed during vertebrate embryogenesis, and its metamerism preserves the characteristic structures of the vertebrae and muscles in the adult body. During vertebrate somitogenesis, sequential formation of epithelialized cell boundaries generates the somites. According to the "clock and wavefront model," the periodical and sequential generation of somites is achieved by the integration of spatiotemporal information provided by the segmentation clock and wavefront. In the anterior region of the presomitic mesoderm, which is the somite precursor, the orchestration between the segmentation clock and the wavefront achieves morphogenesis of somites through multiple processes such as determination of somite boundary position, generation of morophological boundary, and establishment of the rostrocaudal polarity within a somite. Recently, numerous studies using various model animals including mouse, zebrafish, and chick have gradually revealed the molecular aspect of the "clock and wavefront" model and the molecular mechanism connecting the segmentation clock and the wavefront to the multiple processes of somite morphogenesis. In this review, we first summarize the current knowledge about the molecular mechanisms underlying the clock and the wavefront and then describe those of the three processes of somite morphogenesis. Especially, we will discuss the conservation and diversification in the molecular network of the somitigenesis among vertebrates, focusing on two typical model animals used for genetic analyses, i.e., the mouse and zebrafish. In this review, we described molecular mechanism for the generation of somites based on the spatiotemporal information provided by "segmentation clock" and "wavefront" focusing on the evidences obtained from mouse and zebrafish.

Differential regulation of Hes/Hey genes during inner ear development

Notch signaling plays a crucial role during inner ear development and regeneration. Hes/Hey genes encode for bHLH transcription factors identified as Notch targets. We have studied the expression and regulation of Hes/Hey genes during inner ear development in the chicken embryo. Among several Hes/Hey genes examined, only Hey1 and Hes5 map to the sensory regions, although with salient differences. Hey1 expression follows Jag1 expression except at early prosensory stages while Hes5 expression corresponds well to Dl1 expression throughout otic development. Although Hey1 and Hes5 are direct Notch downstream targets, they differ in the level of Notch required for activation. Moreover, they also differ in mRNA stability, showing different temporal decays after Notch blockade. In addition, Bmp, Wnt and Fgf pathways also modify Hey1 and Hes5 expression in the inner ear. Particularly, the Wnt pathway modulates Hey1 and Jag1 expression. Finally, gain of function experiments show that Hey1 and Hes5 cross-regulate each other in a complex manner. Both Hey1 and Hes5 repress Dl1 and Hes5 expression, suggesting that they prevent the transition to differentiation stages, probably by preventing Atoh1 expression. In spite of its association with Jag1, Hey1 does not seem to be instrumental for lateral induction as it does not promote Jag1 expression. We suggest that, besides being both targets of Notch, Hey1 and Hes5 are subject to a rather complex regulation that includes the stability of their transcripts, cross regulation and other signaling pathways.

Caudal regulates the spatiotemporal dynamics of pair-rule waves in Tribolium

In the short-germ beetle Tribolium castaneum, waves of pair-rule gene expression propagate from the posterior end of the embryo towards the anterior and eventually freeze into stable stripes, partitioning the anterior-posterior axis into segments. Similar waves in vertebrates are assumed to arise due to the modulation of a molecular clock by a posterior-to-anterior frequency gradient. However, neither a molecular candidate nor a functional role has been identified to date for such a frequency gradient, either in vertebrates or elsewhere. Here we provide evidence that the posterior gradient of Tc-caudal expression regulates the oscillation frequency of pair-rule gene expression in Tribolium. We show this by analyzing the spatiotemporal dynamics of Tc-even-skipped expression in strong and mild knockdown of Tc-caudal, and by correlating the extension, level and slope of the Tc-caudal expression gradient to the spatiotemporal dynamics of Tc-even-skipped expression in wild type as well as in different RNAi knockdowns of Tc-caudal regulators. Further, we show that besides its absolute importance for stripe generation in the static phase of the Tribolium blastoderm, a frequency gradient might serve as a buffer against noise during axis elongation phase in Tribolium as well as vertebrates. Our results highlight the role of frequency gradients in pattern formation.

One of the most popular problems in development is how the anterior-posterior axis of vertebrates, arthropods and annelids is partitioned into segments. In vertebrates, and recently shown in the beetle Tribolium castaneum, segments are demarcated by means of gene expression waves that propagate from posterior to anterior as the embryo elongates. These waves are assumed to arise due to the regulation of a molecular clock by a frequency gradient. However, to date, neither a candidate nor a functional role has been identified for such a frequency gradient. Here we provide evidence that a static expression gradient of caudal regulates pair-rule oscillations during blastoderm stage in Tribolium. In such a static setup, a frequency gradient is essential to convert clock oscillations into a striped pattern. We further show that a frequency gradient might be essential even in the presence of axis elongation as a buffer against noise. Our work also provides the best evidence to date that Caudal acts as a type of morphogen gradient in the blastoderm of short-germ arthropods; however, Caudal seems to convey positional information through frequency regulation of pair-rule oscillations, rather than through threshold concentration levels in the gradient.

Roles of notch signalling in mandibular condylar cartilage

IMPORTANCE

Notch proteins are cell surface transmembrane spanning receptors which mediate critically important cellular functions through direct cell-cell contact. Interactions between Notch receptors and their ligands regulate cell fate decisions such differentiation, proliferation and apoptosis in numerous tissues. We have previously shown using immunohistochemistry that Notch1 is localized primarily to the prechondroblastic (chondroprogenitor) layer of the mandibular condylar cartilage (MCC).

OBJECTIVE

To test if Notch signalling changes patterns of proliferation and differentiation in the MCC and to investigate if Notch signalling acts downstream of Fibroblast Growth Factor 2 (FGF-2).

METHODS

Condylar cartilage explants were cultured over serum-free DMEM containing either 0 or 50nM DAPT, a Notch signal inhibitor. Explants were used for RNA extraction and immunohistochemistry.

RESULTS

Analysis of gene array data demonstrated that the perichondrial layer of the MCC is rich in Notch receptors (Notch 3 and 4) and Notch ligands (Jagged and Delta) as well as various downstream facilitators of Notch signalling. Disruption of Notch signalling in MCC explants decreased proliferation (Cyclin B1 expression) and increased chondrocyte differentiation (Sox9 expression). Moreover, we found that the actions of FGF-2 in MCC are mediated in part by Notch signalling.

CONCLUSION

These data suggest that Notch signalling contributes to the regulation of proliferation and differentiation in the MCC.

Modeling the zebrafish segmentation clock's gene regulatory network constrained by expression data suggests evolutionary transitions between oscillating and nonoscillating transcription

During segmentation of vertebrate embryos, somites form in accordance with a periodic pattern established by the segmentation clock. In the zebrafish (Danio rerio), the segmentation clock includes six hairy/enhancer of split-related (her/hes) genes, five of which oscillate due to negative autofeedback. The nonoscillating gene hes6 forms the hub of a network of 10 Her/Hes protein dimers, which includes 7 DNA-binding dimers and 4 weak or non-DNA-binding dimers. The balance of dimer species is critical for segmentation clock function, and loss-of-function studies suggest that the her genes have both unique and redundant functions within the clock. However, the precise regulatory interactions underlying the negative feedback loop are unknown. Here, we combine quantitative experimental data, in silico modeling, and a global optimization algorithm to identify a gene regulatory network (GRN) designed to fit measured transcriptional responses to gene knockdown. Surprisingly, we find that hes6, the clock gene that does not oscillate, responds to negative feedback. Consistent with prior in silico analyses, we find that variation in transcription, translation, and degradation rates can mediate the gain and loss of oscillatory behavior for genes regulated by negative feedback. Extending our study, we found that transcription of the nonoscillating Fgf pathway gene sef responds to her/hes perturbation similarly to oscillating her genes. These observations suggest a more extensive underlying regulatory similarity between the zebrafish segmentation clock and the mouse and chick segmentation clocks, which exhibit oscillations of her/hes genes as well as numerous other Notch, Fgf, and Wnt pathway genes.

Short-lived Her proteins drive robust synchronized oscillations in the zebrafish segmentation clock

Oscillations are prevalent in natural systems. A gene expression oscillator, called the segmentation clock, controls segmentation of precursors of the vertebral column. Genes belonging to the Hes/her family encode the only conserved oscillating genes in all analyzed vertebrate species. Hes/Her proteins form dimers and negatively autoregulate their own transcription. Here, we developed a stochastic two-dimensional multicellular computational model to elucidate how the dynamics, i.e. period, amplitude and synchronization, of the segmentation clock are regulated. We performed parameter searches to demonstrate that autoregulatory negative-feedback loops of the redundant repressor Her dimers can generate synchronized gene expression oscillations in wild-type embryos and reproduce the dynamics of the segmentation oscillator in different mutant conditions. Our model also predicts that synchronized oscillations can be robustly generated as long as the half-lives of the repressor dimers are shorter than 6 minutes. We validated this prediction by measuring, for the first time, the half-life of Her7 protein as 3.5 minutes. These results demonstrate the importance of building biologically realistic stochastic models to test biological models more stringently and make predictions for future experimental studies.

The elongation rate of RNA polymerase II in zebrafish and its significance in the somite segmentation clock

A gene expression oscillator called the segmentation clock controls somite segmentation in the vertebrate embryo. In zebrafish, the oscillatory transcriptional repressor genes her1 and her7 are crucial for genesis of the oscillations, which are thought to arise from negative autoregulation of these genes. The period of oscillation is predicted to depend on delays in the negative-feedback loop, including, most importantly, the transcriptional delay - the time taken to make each molecule of her1 or her7 mRNA. her1 and her7 operate in parallel. Loss of both gene functions, or mutation of her1 combined with knockdown of Hes6, which we show to be a binding partner of Her7, disrupts segmentation drastically. However, mutants in which only her1 or her7 is functional show only mild segmentation defects and their oscillations have almost identical periods. This is unexpected because the her1 and her7 genes differ greatly in length. We use transgenic zebrafish to measure the RNA polymerase II elongation rate, for the first time, in the intact embryo. This rate is unexpectedly rapid, at 4.8 kb/minute at 28.5°C, implying that, for both genes, the time taken for transcript elongation is insignificant compared with other sources of delay, explaining why the mutants have similar clock periods. Our computational model shows how loss of her1 or her7 can allow oscillations to continue with unchanged period but with reduced amplitude and impaired synchrony, as manifested in the in situ hybridisation patterns of the single mutants.

Single-cell-resolution imaging of the impact of Notch signaling and mitosis on segmentation clock dynamics

Vertebrate body segmentation is controlled by the segmentation clock, a molecular oscillator involving transcriptional oscillations of cyclic genes in presomitic mesoderm cells. The rapid and highly dynamic nature of this oscillating system has proved challenging for study at the single-cell level. We achieved visualization of clock activity with a cellular level of resolution in living embryos, allowing direct comparison of oscillations in neighbor cells. We provide direct evidence that presomitic mesoderm cells oscillate asynchronously in zebrafish Notch pathway mutants. By tracking oscillations in mitotic cells, we reveal that a robust cell-autonomous, Notch-independent mechanism resumes oscillations after mitosis. Finally, we find that cells preferentially divide at a certain oscillation phase, likely reducing the noise generated by cell division in cell synchrony and suggesting an intriguing relationship between the mitotic cycle and clock oscillation.

Modelling Delta-Notch perturbations during zebrafish somitogenesis

The discovery over the last 15 years of molecular clocks and gradients in the pre-somitic mesoderm of numerous vertebrate species has added significant weight to Cooke and Zeeman's 'clock and wavefront' model of somitogenesis, in which a travelling wavefront determines the spatial position of somite formation and the somitogenesis clock controls periodicity (Cooke and Zeeman, 1976). However, recent high-throughput measurements of spatiotemporal patterns of gene expression in different zebrafish mutant backgrounds allow further quantitative evaluation of the clock and wavefront hypothesis. In this study we describe how our recently proposed model, in which oscillator coupling drives the propagation of an emergent wavefront, can be used to provide mechanistic and testable explanations for the following observed phenomena in zebrafish embryos: (a) the variation in somite measurements across a number of zebrafish mutants; (b) the delayed formation of somites and the formation of 'salt and pepper' patterns of gene expression upon disruption of oscillator coupling; and (c) spatial correlations in the 'salt and pepper' patterns in Delta-Notch mutants. In light of our results, we propose a number of plausible experiments that could be used to further test the model.

HES6 enhances the motility of alveolar rhabdomyosarcoma cells

HES6, a member of the hairy-enhancer-of-split family of transcription factors, plays multiple roles in myogenesis. It is a direct target of the myogenic transcription factor MyoD and has been shown to regulate the formation of the myotome in development, myoblast cell cycle exit and the organization of the actin cytoskeleton during terminal differentiation. Here we investigate the expression and function of HES6 in rhabdomyosarcoma, a soft tissue tumor which expresses myogenic genes but fails to differentiate into muscle. We show that HES6 is expressed at high levels in the subset of alveolar rhabdomyosarcomas expressing PAX/FOXO1 fusion genes (ARMSp). Knockdown of HES6 mRNA in the ARMSp cell line RH30 reduces proliferation and cell motility. This phenotype is rescued by expression of mouse Hes6 which is insensitive to HES6 siRNA. Furthermore, expression microarray analysis indicates that the HES6 knockdown is associated with a decrease in the levels of Transgelin, (TAGLN), a regulator of the actin cytoskeleton. Knockdown of TAGLN decreases cell motility, whilst TAGLN overexpression rescues the motility defect resulting from HES6 knockdown. These findings indicate HES6 contributes to the pathogenesis of ARMSp by enhancing both proliferation and cell motility.

Topology and dynamics of the zebrafish segmentation clock core circuit

By combining biochemical, embryological, and mathematical approaches, this work uncovers an important role for protein-protein interactions in determining the dynamics of the somite-forming segmentation clock in vertebrates.

During vertebrate embryogenesis, the rhythmic and sequential segmentation of the body axis is regulated by an oscillating genetic network termed the segmentation clock. We describe a new dynamic model for the core pace-making circuit of the zebrafish segmentation clock based on a systematic biochemical investigation of the network's topology and precise measurements of somitogenesis dynamics in novel genetic mutants. We show that the core pace-making circuit consists of two distinct negative feedback loops, one with Her1 homodimers and the other with Her7:Hes6 heterodimers, operating in parallel. To explain the observed single and double mutant phenotypes of her1, her7, and hes6 mutant embryos in our dynamic model, we postulate that the availability and effective stability of the dimers with DNA binding activity is controlled in a “dimer cloud” that contains all possible dimeric combinations between the three factors. This feature of our model predicts that Hes6 protein levels should oscillate despite constant hes6 mRNA production, which we confirm experimentally using novel Hes6 antibodies. The control of the circuit's dynamics by a population of dimers with and without DNA binding activity is a new principle for the segmentation clock and may be relevant to other biological clocks and transcriptional regulatory networks.

The segmented pattern of the vertebral column, one of the defining features of the vertebrate body, is established during embryogenesis. The embryo's segments, called somites, form sequentially and rhythmically from head to tail. The periodicity of somite formation is regulated by the segmentation clock, a genetic oscillator that ticks in the posterior-most embryonic tissue: for each tick of the clock, one new bilateral pair of segments is made. The period of the clock appears to determine the number and the length of segments, but what controls this periodicity? In this article, we have investigated the interactions of three transcription factors that form the core of the clock's regulatory circuit, and have measured how the period of segmentation changes when these factors are mutated alone or in combination. We find that these three factors contribute to a “dimer cloud” that contains all possible dimeric combinations; however, only two dimers in this cloud can bind DNA, which allows them to directly regulate the oscillatory gene expression that underpins the periodicity of segment formation. Nevertheless, a mathematical model of the clock's dynamics based on our experimental findings indicates that the non-DNA-binding dimers also influence the stability, and hence the function, of the two DNA-binding dimers controlling the segmentation clock's period. Such involvement of non-DNA-binding dimers is a novel regulatory principle for the segmentation clock, which might also be a general mechanism that operates in other biological clocks.

The synchrony and cyclicity of developmental events

Many of the morphogenetic processes that occur during development in the mouse are based on cyclic events with defined time intervals, as exemplified by somitogenesis (every 2 h), hair cycles (every 25 d), and spermatogenesis (every 35 d). Among these events, somitogenesis is the most dynamic morphogenetic mechanism showing clear cyclicity during embryogenesis and is therefore a good system with which to review the synchronous and cyclic characteristics of developmental pathways. The metameric properties of the somites underpin the segmental properties along the anterior-posterior (AP) axis of the body. The periodicity of somites is controlled by the so-called segmentation clock operating in the presomitic mesoderm (PSM). This tissue contains the somite precursor cells that exist only during embryonic development. Both theoretical and experimental approaches have contributed to the understanding of the mechanism of somite segmentation. This article focuses on how the segmentation clock functions to organize the collective behavior of cells and how this information is translated into the spatial patterning of segmental somites. The interplay between signaling molecules that provides positional information and the transcription factors that respond to such positional cues are critical to the role of the segmentation clock and are discussed.

Expression of hairy/enhancer of split genes in neural progenitors and neurogenesis domains of the adult zebrafish brain

All subdivisions of the adult zebrafish brain maintain niches of constitutive neurogenesis, sustained by quiescent and multipotent progenitor populations. In the telencephalon, the latter potential neural stem cells take the shape of radial glia aligned along the ventricle and are controlled by Notch signalling. With the aim of identifying new markers of this cell type and of comparing the effectors of embryonic and adult neurogenesis, we focused on the family of hairy/enhancer of split [E(spl)] genes. We report the expression of seven hairy/E(spl) (her) genes and the new helt gene in three neurogenic areas of the adult zebrafish brain (telencephalon, hypothalamus, and midbrain) in relation to radial glia, proliferation, and neurogenesis. We show that the expression of most her genes in the adult brain characterizes quiescent radial glia, whereas only few are expressed in progenitor domains engaged in active proliferation or neurogenesis. The low proliferation status of most her-positive progenitors contrasts with the embryonic nervous system, in which her genes are expressed in actively dividing progenitors. Likewise, we demonstrate largely overlapping expression domains of a set of her genes in the adult brain, which is in striking contrast to their distinct embryonic expression profiles. Overall, our data provide a consolidated map of her expression, quiescent glia, proliferation, and neurogenesis in these various subdivisions of the adult brain and suggest distinct regulation and function of Her factors in the embryonic and adult contexts.

Oscillatory gene expression and somitogenesis

A bilateral pair of somites forms periodically by segmentation of the anterior ends of the presomitic mesoderm (PSM). This periodic event is regulated by a biological clock called the segmentation clock, which involves cyclic gene expression. Expression of her1 and her7 in zebrafish and Hes7 in mice oscillates by negative feedback, and mathematical models have been used to generate and test hypotheses to aide elucidation of the role of negative feedback in regulating oscillatory expression. her/Hes genes induce oscillatory expression of the Notch ligand deltaC in zebrafish and the Notch modulator Lunatic fringe in mice, which lead to synchronization of oscillatory gene expression between neighboring PSM cells. In the mouse PSM, Hes7 induces coupled oscillations of Notch and Fgf signaling, while Notch and Fgf signaling cooperatively regulate Hes7 oscillation, indicating that Hes7 and Notch and Fgf signaling form the oscillator networks. Notch signaling activates, but Fgf signaling represses, expression of the master regulator for somitogenesis Mesp2, and coupled oscillations in Notch and Fgf signaling dissociate in the anterior PSM, which allows Notch signaling-induced synchronized cells to express Mesp2 after these cells are freed from Fgf signaling. These results together suggest that Notch signaling defines the prospective somite region, while Fgf signaling regulates the pace of segmentation. It is likely that these oscillator networks constitute the core of the segmentation clock, but it remains to be determined whether as yet unknown oscillators function behind the scenes.

A novel zebrafish embryo xenotransplantation model to study primary human fibroblast motility in health and disease

Fibroblasts have a central role in the maintenance of tissue homeostasis and repair after injury. Currently, there are no tractable, cost-effective model systems for studying the biology of human fibroblasts in vivo. Here we demonstrate that primary human fibroblasts survive transplantation into zebrafish embryos. Transplanted cells migrate and proliferate, but do not integrate into host tissues. We used this system to study the intrinsic motility of lung fibroblasts from a prototype fibrotic lung disease, idiopathic pulmonary fibrosis (IPF). IPF fibroblasts displayed a significantly higher level of motility than did fibroblasts from nonfibrotic lungs. This is the first in vivo examination of primary human lung fibroblast motility in health and disease using zebrafish models.

The Her7 node modulates the network topology of the zebrafish segmentation clock via sequestration of the Hes6 hub

Using in vitro and in vivo assays, we define a network of Her/Hes dimers underlying transcriptional negative feedback within the zebrafish segmentation clock. Some of the dimers do not appear to be DNA-binding, whereas those dimers that do interact with DNA have distinct preferences for cis regulatory sequences. Dimerization is specific, with Hes6 serving as the hub of the network. Her1 binds DNA only as a homodimer but will also dimerize with Hes6. Her12 and Her15 bind DNA both as homodimers and as heterodimers with Hes6. Her7 dimerizes strongly with Hes6 and weakly with Her15. This network structure engenders specific network dynamics and imparts greater influence to the Her7 node. Computational analysis supports the hypothesis that Her7 disproportionately influences the availability of Hes6 to heterodimerize with other Her proteins. Genetic experiments suggest that this regulation is important for operation of the network. Her7 therefore has two functions within the zebrafish segmentation clock. Her7 acts directly within the delayed negative feedback as a DNA-binding heterodimer with Hes6. Her7 also has an emergent function, independent of DNA binding, in which it modulates network topology via sequestration of the network hub.

Aberrant expression of genes necessary for neuronal development and Notch signaling in an epileptic mind bomb zebrafish

Mutation within an ubiquitin E3 ligase gene can lead to a failure in Notch signaling, excessive neurons, and depletion of neural progenitor cells in mind bomb mutants. Using mib(hi904) zebrafish, we reported seizures and a down-regulation of γ-aminobutyric acid (GABA) signaling pathway genes. A transcriptome analysis also identified differential expression pattern of genes related to Notch signaling and neurodevelopment. Here, we selected nine of these genes (her4.2, hes5, bhlhb5, hoxa5a, hoxb5b, dmbx1a, dbx1a, nxph1, and plxnd1) and performed a more thorough analysis of expression using conventional polymerase chain reaction, real-time polymerase chain reaction and in situ hybridization. Transgenic reporter fish (Gfap:GFP and Dlx5a-6a:GFP) were used to assess early brain morphology in vivo. Down-regulation of many of these genes was prominent throughout key structures of the developing mib(hi904) zebrafish brain including, but not limited to, the pallium, ventral thalamus, and optic tectum. Brain expression of Dlx5a-6a and Gfap was also reduced. In conclusion, these expression studies indicate a general down-regulation of Notch signaling genes necessary for proper brain development and suggest that these mutant fish could provide valuable insights into neurological conditions, such as Angelman syndrome, associated with ubiquitin E3 ligase mutation.

Vertebrate segmentation: from cyclic gene networks to scoliosis

One of the most striking features of the human vertebral column is its periodic organization along the anterior-posterior axis. This pattern is established when segments of vertebrates, called somites, bud off at a defined pace from the anterior tip of the embryo's presomitic mesoderm (PSM). To trigger this rhythmic production of somites, three major signaling pathways--Notch, Wnt/β-catenin, and fibroblast growth factor (FGF)--integrate into a molecular network that generates a traveling wave of gene expression along the embryonic axis, called the "segmentation clock." Recent systems approaches have begun identifying specific signaling circuits within the network that set the pace of the oscillations, synchronize gene expression cycles in neighboring cells, and contribute to the robustness and bilateral symmetry of somite formation. These findings establish a new model for vertebrate segmentation and provide a conceptual framework to explain human diseases of the spine, such as congenital scoliosis.

Zebrafish Her8a is activated by Su(H)-dependent Notch signaling and is essential for the inhibition of neurogenesis

Understanding how diversity of neural cells is generated is one of the main tasks of developmental biology. The Hairy/E(spl) family members are potential targets of Notch signaling, which has been shown to be fundamental to neural cell maintenance, cell fate decisions, and compartment boundary formation. However, their response to Notch signaling and their roles in neurogenesis are still not fully understood. In the present study, we isolated a zebrafish homologue of hairy/E(spl), her8a, and showed this gene is specifically expressed in the developing nervous system. her8a is positively regulated by Su(H)-dependent Notch signaling as revealed by a Notch-defective mutant and injection of variants of the Notch intracellular regulator, Su(H). Morpholino knockdown of Her8a resulted in upregulation of proneural and post-mitotic neuronal markers, indicating that Her8a is essential for the inhibition of neurogenesis. In addition, markers for glial precursors and mature glial cells were down-regulated in Her8a morphants, suggesting Her8a is required for gliogenesis. The role of Her8a and its response to Notch signaling is thus similar to mammalian HES1, however this is the converse of what is seen for the more closely related mammalian family member, HES6. This study not only provides further understanding of how the fundamental signaling pathway, Notch signaling, and its downstream genes mediate neural development and differentiation, but also reveals evolutionary diversity in the role of H/E(spl) genes.

Hes6 is required for actin cytoskeletal organization in differentiating C2C12 myoblasts

Hes6 is a member of the hairy-enhancer-of-split family of transcription factors that regulate proliferating cell fate in development and is known to be expressed in developing muscle. Here we investigate its function in myogenesis in vitro. We show that Hes6 is a direct transcriptional target of the myogenic transcription factors MyoD and Myf5, indicating that it is integral to the myogenic transcriptional program. The localization of Hes6 protein changes during differentiation, becoming predominantly nuclear. Knockdown of Hes6 mRNA levels by siRNA has no effect on cell cycle exit or induction of myosin heavy chain expression in differentiating C2C12 myoblasts, but F-actin filament formation is disrupted and both cell motility and myoblast fusion are reduced. The knockdown phenotype is rescued by expression of Hes6 cDNA resistant to siRNA. These results define a novel role for Hes6 in actin cytoskeletal dynamics in post mitotic myoblasts.

The segmentation clock mechanism moves up a notch

The vertebrate segmentation clock is a molecular oscillator that regulates the periodicity of somite formation. Three signalling pathways have been proposed to underlie the molecular mechanism of the oscillator, namely the Notch, Wnt and Fgf pathways. Characterizing the roles and hierarchy of these three pathways in the oscillator mechanism is currently the focus of intense research. Recent publications report the first identification of a molecular mechanism involved in the regulation of the pace of this oscillator. We review these and other recent findings regarding the interaction between the three pathways in the oscillator mechanism that have significantly expanded our understanding of the segmentation clock.

Segment number and axial identity in a segmentation clock period mutant

A species-specific number of segments is a hallmark of the vertebrate body plan. The first segmental structures in the vertebrate embryo are the somites, which bud sequentially from the growing presomitic mesoderm (PSM). The Clock and Wavefront model for somitogenesis proposes that the total number of somites is determined by the period of an oscillator or clock operating in the PSM and the total duration of PSM growth. Furthermore, the number of oscillations of the segmentation clock has been suggested to regulate the regional identity of segments along the body axis. Here we test these two ideas in a zebrafish mutant in which the segmentation clock is specifically slowed. This reduces segment number as predicted, but hox gene expression and posterior anatomical markers align with lower segmental counts in mutants compared to the wild-type, arguing against an instructive role of the segmentation clock in determining axial identities. Our data therefore suggest that precise control of segmentation clock period in relation to axial growth ensures a species-specific segment number and that during evolution modulating the clock's period through genetic mutations may have been a relevant way to vary segment number independently of axial regionalization.

Post-transcriptional controls - adding a new layer of regulation to clock gene expression

Living organisms undergo biochemical, physiological and behavioral cycles with periods ranging from seconds to years. Cycles with intermediate periods are governed by endogenous clocks that depend on oscillating gene expression. Here we illustrate the modalities and specific functions of post-transcriptional control of gene expression (exerted on pre-mRNAs and mRNAs) in biological clocks through two examples: the circadian clock and the vertebrate somite segmentation clock, an embryonic clock with a period far below a day. We conclude that both constitutive and cyclic post-transcriptional controls underpin clock function.

Somitogenesis clock-wave initiation requires differential decay and multiple binding sites for clock protein

Somitogenesis is a process common to all vertebrate embryos in which repeated blocks of cells arise from the presomitic mesoderm (PSM) to lay a foundational pattern for trunk and tail development. Somites form in the wake of passing waves of periodic gene expression that originate in the tailbud and sweep posteriorly across the PSM. Previous work has suggested that the waves result from a spatiotemporally graded control protein that affects the oscillation rate of clock-gene expression. With a minimally constructed mathematical model, we study the contribution of two control mechanisms to the initial formation of this gene-expression wave. We test four biologically motivated model scenarios with either one or two clock protein transcription binding sites, and with or without differential decay rates for clock protein monomers and dimers. We examine the sensitivity of wave formation with respect to multiple model parameters and robustness to heterogeneity in cell population. We find that only a model with both multiple binding sites and differential decay rates is able to reproduce experimentally observed waveforms. Our results show that the experimentally observed characteristics of somitogenesis wave initiation constrain the underlying genetic control mechanisms.

The vertebral column is a characteristic structure of all vertebrates. Individual vertebrae, together with ribs and attached muscles, develop from repeated embryonic structures called somites. The somite pattern forms in the embryo during somitogenesis. We know that this process uses periodic gene expression (a biomolecular “clock”) to generate the pattern, but we do not know precisely how this expression is controlled within the cell and coordinated across multiple cells. We propose a mathematical model that incorporates experimentally confirmed features of somitogenesis. We then test four different mechanisms that may control the clock and ask if the comparison between model simulations and experimental observation can select the best model and thus suggest how the clock is controlled. We find that the model scenario with both multiple DNA binding sites and differential protein decay rates is best able to reproduce experimental observations. Because these findings can be tested experimentally, our results should help guide future experiments.

Parallel evolution of segmentation by co-option of ancestral gene regulatory networks

Different sources of data on the evolution of segmentation lead to very different conclusions. Molecular similarities in the developmental pathways generating a segmented body plan tend to suggest a segmented common ancestor for all bilaterally symmetrical animals. Data from paleontology and comparative morphology suggest that this is unlikely. A possible solution to this conundrum is that throughout evolution there was a parallel co-option of gene regulatory networks that had conserved ancestral roles in determining body axes and in elongating the anterior-posterior axis. Inherent properties in some of these networks made them easily recruitable for generating repeated patterns and for determining segmental boundaries. Phyla where this process happened are among the most successful in the animal kingdom, as the modular nature of the segmental body organization allowed them to diverge and radiate into a bewildering array of variations on a common theme.

FoxD5 mediates anterior-posterior polarity through upstream modulator Fgf signaling during zebrafish somitogenesis

The transcription factor FoxD5 is expressed in the paraxial mesoderm of zebrafish. However, the roles of FoxD5 in anterior pre-somitic mesoderm (PSM) during somitogenesis are unknown. We knocked down FoxD5 in embryos, which resulted in defects of the newly formed somites, including loss of the striped patterns of anterior-posterior polarity genes deltaC, notch2, notch3 and EphB2a, as well as the absence of mespa expression in S-I. Also, the expression of mespb exhibited a 'salt and pepper' pattern, indicating that FoxD5 is necessary for somite patterning in anterior PSM. Embryos were treated with SU5402, an Fgf receptor (FGFR) inhibitor, resulting in reduction of FoxD5 expression. This finding was consistent with results obtained from Tg(hsp70l:dnfgfr1-EGFP)pd1 embryos, whose dominant-negative form of FGFR1 was produced by heat-induction. Loss of FoxD5 expression was observed in the embryos injected with fgf3-/fgf8-double-morpholinos (MOs). Excessive FoxD5 mRNA could rescue the defective expression levels of mespa and mespb in fgf3-/fgf8-double morphants, suggesting that Fgf signaling acts as an upstream modulator of FoxD5 during somitogenesis. We concluded that FoxD5 is required for maintaining anterior-posterior polarity within a somite and that the striped pattern of FoxD5 in anterior PSM is mainly regulated by Fgf. An Fgf-FoxD5-Mesps signaling network is therefore proposed.

Notch is a critical component of the mouse somitogenesis oscillator and is essential for the formation of the somites

Segmentation of the vertebrate body axis is initiated through somitogenesis, whereby epithelial somites bud off in pairs periodically from the rostral end of the unsegmented presomitic mesoderm (PSM). The periodicity of somitogenesis is governed by a molecular oscillator that drives periodic waves of clock gene expression caudo-rostrally through the PSM with a periodicity that matches somite formation. To date the clock genes comprise components of the Notch, Wnt, and FGF pathways. The literature contains controversial reports as to the absolute role(s) of Notch signalling during the process of somite formation. Recent data in the zebrafish have suggested that the only role of Notch signalling is to synchronise clock gene oscillations across the PSM and that somite formation can continue in the absence of Notch activity. However, it is not clear in the mouse if an FGF/Wnt-based oscillator is sufficient to generate segmented structures, such as the somites, in the absence of all Notch activity. We have investigated the requirement for Notch signalling in the mouse somitogenesis clock by analysing embryos carrying a mutation in different components of the Notch pathway, such as Lunatic fringe (Lfng), Hes7, Rbpj, and presenilin1/presenilin2 (Psen1/Psen2), and by pharmacological blocking of the Notch pathway. In contrast to the fish studies, we show that mouse embryos lacking all Notch activity do not show oscillatory activity, as evidenced by the absence of waves of clock gene expression across the PSM, and they do not develop somites. We propose that, at least in the mouse embryo, Notch activity is absolutely essential for the formation of a segmented body axis.

Vertebrate animals generate their segmented body plan during embryogenesis. These embryonic segments, or somites, form one after another from tissue at the tail end of the embryo in a highly regulated process controlled by a molecular oscillator. This oscillator drives the expression of a group of genes in this tissue and determines the periodicity of somite formation. To date the genes regulated by this molecular clock comprise components of the Notch, Wnt, and FGF pathways. Recent data in the zebrafish embryo have suggested that the only role of Notch signalling in this process is to synchronise gene oscillations between neighbouring cells and that somite formation can continue in the absence of Notch activity. However, we show that mouse embryos lacking all Notch activity do not show oscillatory activity and do not develop somites. We propose that, at least in the mouse embryo, Notch activity is absolutely essential for building a segmented body axis.

Sprouty4, an FGF inhibitor, displays cyclic gene expression under the control of the notch segmentation clock in the mouse PSM

BACKGROUND

During vertebrate embryogenesis, somites are generated at regular intervals, the temporal and spatial periodicity of which is governed by a gradient of fibroblast growth factor (FGF) and/or Wnt signaling activity in the presomitic mesoderm (PSM) in conjunction with oscillations of gene expression of components of the Notch, Wnt and FGF signaling pathways.

PRINCIPAL FINDINGS

Here, we show that the expression of Sprouty4, which encodes an FGF inhibitor, oscillates in 2-h cycles in the mouse PSM in synchrony with other oscillating genes from the Notch signaling pathway, such as lunatic fringe. Sprouty4 does not oscillate in Hes7-null mutant mouse embryos, and Hes7 can inhibit FGF-induced transcriptional activity of the Sprouty4 promoter.

CONCLUSIONS

Thus, periodic expression of Sprouty4 is controlled by the Notch segmentation clock and may work as a mediator that links the temporal periodicity of clock gene oscillations with the spatial periodicity of boundary formation which is regulated by the gradient of FGF/Wnt activity.