Expression of the novel basic-helix-loop-helix transcription factor cMespo in presomitic mesoderm of chicken embryos

Intracellular pH controls WNT downstream of glycolysis in amniote embryos

Formation of the body of vertebrate embryos proceeds sequentially by posterior addition of tissues from the tail bud. Cells of the tail bud and the posterior presomitic mesoderm, which control posterior elongation1, exhibit a high level of aerobic glycolysis that is reminiscent of the metabolic status of cancer cells experiencing the Warburg effect2,3. Glycolytic activity downstream of fibroblast growth factor controls WNT signalling in the tail bud3. In the neuromesodermal precursors of the tail bud4, WNT signalling promotes the mesodermal fate that is required for sustained axial elongation, at the expense of the neural fate3,5. How glycolysis regulates WNT signalling in the tail bud is currently unknown. Here we used chicken embryos and human tail bud-like cells differentiated in vitro from induced pluripotent stem cells to show that these cells exhibit an inverted pH gradient, with the extracellular pH lower than the intracellular pH, as observed in cancer cells6. Our data suggest that glycolysis increases extrusion of lactate coupled to protons via the monocarboxylate symporters. This contributes to elevating the intracellular pH in these cells, which creates a favourable chemical environment for non-enzymatic β-catenin acetylation downstream of WNT signalling. As acetylated β-catenin promotes mesodermal rather than neural fate7, this ultimately leads to activation of mesodermal transcriptional WNT targets and specification of the paraxial mesoderm in tail bud precursors. Our work supports the notion that some tumour cells reactivate a developmental metabolic programme.

A Gradient of Glycolytic Activity Coordinates FGF and Wnt Signaling during Elongation of the Body Axis in Amniote Embryos

Mammalian embryos transiently exhibit aerobic glycolysis (Warburg effect), a metabolic adaptation also observed in cancer cells. The role of this particular type of metabolism during vertebrate organogenesis is currently unknown. Here, we provide evidence for spatiotemporal regulation of glycolysis in the posterior region of mouse and chicken embryos. We show that a posterior glycolytic gradient is established in response to graded transcription of glycolytic enzymes downstream of fibroblast growth factor (FGF) signaling. We demonstrate that glycolysis controls posterior elongation of the embryonic axis by regulating cell motility in the presomitic mesoderm and by controlling specification of the paraxial mesoderm fate in the tail bud. Our results suggest that glycolysis in the tail bud coordinates Wnt and FGF signaling to promote elongation of the embryonic axis.

PAPC couples the segmentation clock to somite morphogenesis by regulating N-cadherin-dependent adhesion

Vertebrate segmentation is characterized by the periodic formation of epithelial somites from the mesenchymal presomitic mesoderm (PSM). How the rhythmic signaling pulse delivered by the segmentation clock is translated into the periodic morphogenesis of somites remains poorly understood. Here, we focused on the role of paraxial protocadherin (PAPC/Pcdh8) in this process. We showed that in chicken and mouse embryos, PAPC expression is tightly regulated by the clock and wavefront system in the posterior PSM. We observed that PAPC exhibits a striking complementary pattern to N-cadherin (CDH2), marking the interface of the future somite boundary in the anterior PSM. Gain and loss of function of PAPC in chicken embryos disrupted somite segmentation by altering the CDH2-dependent epithelialization of PSM cells. Our data suggest that clathrin-mediated endocytosis is increased in PAPC-expressing cells, subsequently affecting CDH2 internalization in the anterior compartment of the future somite. This in turn generates a differential adhesion interface, allowing formation of the acellular fissure that defines the somite boundary. Thus, periodic expression of PAPC in the anterior PSM triggers rhythmic endocytosis of CDH2, allowing for segmental de-adhesion and individualization of somites.

A novel gain-of-function mutation of the proneural IRX1 and IRX2 genes disrupts axis elongation in the Araucana rumpless chicken

Axis elongation of the vertebrate embryo involves the generation of cell lineages from posterior progenitor populations. We investigated the molecular mechanism governing axis elongation in vertebrates using the Araucana rumpless chicken. Araucana embryos exhibit a defect in axis elongation, failing to form the terminal somites and concomitant free caudal vertebrae, pygostyle, and associated tissues of the tail. Through whole genome sequencing of six Araucana we have identified a critical 130 kb region, containing two candidate causative SNPs. Both SNPs are proximal to the IRX1 and IRX2 genes, which are required for neural specification. We show that IRX1 and IRX2 are both misexpressed within the bipotential chordoneural hinge progenitor population of Araucana embryos. Expression analysis of BRA and TBX6, required for specification of mesoderm, shows that both are downregulated, whereas SOX2, required for neural patterning, is expressed in ectopic epithelial tissue. Finally, we show downregulation of genes required for the protection and maintenance of the tailbud progenitor population from the effects of retinoic acid. Our results support a model where the disruption in balance of mesoderm and neural fate results in early depletion of the progenitor population as excess neural tissue forms at the expense of mesoderm, leading to too few mesoderm cells to form the terminal somites. Together this cascade of events leads to axis truncation.

Active repression by RARγ signaling is required for vertebrate axial elongation

Retinoic acid receptor gamma 2 (RARγ2) is the major RAR isoform expressed throughout the caudal axial progenitor domain in vertebrates. During a microarray screen to identify RAR targets, we identified a subset of genes that pattern caudal structures or promote axial elongation and are upregulated by increased RAR-mediated repression. Previous studies have suggested that RAR is present in the caudal domain, but is quiescent until its activation in late stage embryos terminates axial elongation. By contrast, we show here that RARγ2 is engaged in all stages of axial elongation, not solely as a terminator of axial growth. In the absence of RA, RARγ2 represses transcriptional activity in vivo and maintains the pool of caudal progenitor cells and presomitic mesoderm. In the presence of RA, RARγ2 serves as an activator, facilitating somite differentiation. Treatment with an RARγ-selective inverse agonist (NRX205099) or overexpression of dominant-negative RARγ increases the expression of posterior Hox genes and that of marker genes for presomitic mesoderm and the chordoneural hinge. Conversely, when RAR-mediated repression is reduced by overexpressing a dominant-negative co-repressor (c-SMRT), a constitutively active RAR (VP16-RARγ2), or by treatment with an RARγ-selective agonist (NRX204647), expression of caudal genes is diminished and extension of the body axis is prematurely terminated. Hence, gene repression mediated by the unliganded RARγ2-co-repressor complex constitutes a novel mechanism to regulate and facilitate the correct expression levels and spatial restriction of key genes that maintain the caudal progenitor pool during axial elongation in Xenopus embryos.

Wnt-regulated dynamics of positional information in zebrafish somitogenesis

How signaling gradients supply positional information in a field of moving cells is an unsolved question in patterning and morphogenesis. Here, we ask how a Wnt signaling gradient regulates the dynamics of a wavefront of cellular change in a flow of cells during somitogenesis. Using time-controlled perturbations of Wnt signaling in the zebrafish embryo, we changed segment length without altering the rate of somite formation or embryonic elongation. This result implies specific Wnt regulation of the wavefront velocity. The observed Wnt signaling gradient dynamics and timing of downstream events support a model for wavefront regulation in which cell flow plays a dominant role in transporting positional information.

Developmental control of segment numbers in vertebrates

Segmentation or metamery in vertebrates is best illustrated by the repetition of the vertebrae and ribs, their associated skeletal muscles and blood vessels, and the spinal nerves and ganglia. The segment number varies tremendously among the different vertebrate species, ranging from as few as six vertebrae in some frogs to as many as several hundred in some snakes and fish. In vertebrates, metameric segments or somites form sequentially during body axis formation. This results in the embryonic axis becoming entirely segmented into metameric units from the level of the otic vesicle almost to the very tip of the tail. The total segment number mostly depends on two parameters: (1) the control of the posterior growth of the body axis during somitogenesis-more same-size segments can be formed in a longer axis and (2) segment size--more smaller--size segments can be formed in a same-size body axis. During evolution, independent variations of these parameters could explain the huge diversity in segment numbers observed among vertebrate species. These variations in segment numbers are accompanied by diversity in the regionalization of the vertebral column. For example, amniotes can exhibit up to five different types of vertebrae: cervical, thoracic, lumbar, sacral and caudal, the number of which varies according to the species. This regionalization of the vertebral column is controlled by the Hox family of transcription factors. We propose that during development, dissociation of the Hox- and segmentation-clock-dependent vertebral patterning systems explains the enormous diversity of vertebral formulae observed in vertebrates.

Understanding the somitogenesis clock: what's missing?

The segmentation of vertebrate embryos depends on a complex genetic network that generates highly dynamic gene expression. Many of the elements of the network have been identified, but their interaction and their influence on segmentation remain poorly understood. A few mathematical models have been proposed to explain the dynamics of subsets of the network, but the mechanistic bases remain controversial. This review focuses on outstanding problems with the generation of somitogenesis clock oscillations, and the ways they could regulate segmentation. Proposals that oscillations are generated by a negative feedback loop formed by Lunatic fringe and Notch signaling are weighed against a model based on positive feedback, and the experimental basis for models of simple negative feedback involving Her/Hes genes or Wnt targets is evaluated. Differences are then made explicit between the many 'clock and wavefront' model variants that have been proposed to explain how the clock regulates segmentation. An understanding of the somitogenesis clock will require addressing experimentally the many questions that arise from the study of simple models.

Raver2, a new member of the hnRNP family

Raver2 was identified as a novel member of the hnRNP family based on sequence homology within three RNA recognition motifs and its general domain organization reminiscent of the previously described raver1 protein. Like raver1, raver2 contains two putative nuclear localization signals and a potential nuclear export sequence, and also displays nucleo-cytoplasmic shuttling in a heterokaryon assay. In glia cells and neurons, raver2 localizes to the nucleus. Moreover, the protein interacts with polypyrimidine tract binding protein (PTB) suggesting that it may participate in PTB-mediated nuclear functions. In contrast to ubiquitously expressed raver1, raver2 exerts a distinct spatio-temporal expression pattern during embryogenesis and is essentially restricted to brain, lung, and kidney in the adult mouse.

Modulation of Notch Signaling During Somitogenesis

The Notch signaling pathway is known to govern various aspects of tissue differentiation during embryonic development by mediating local cell-cell interactions that often control cell fate. The conserved components that underlie Notch signaling have been isolated in vertebrates, leading to a biochemical delineation of a core Notch signaling pathway and functional studies of this pathway during embryogenesis. Herein we highlight recent progress in determining how Notch signaling contributes to the development of the vertebrate embryo. We first discuss the role of Notch in the process of segmentation where rapid changes have been shown to occur in both the spatial and temporal aspects of Notch signaling, which are critical for segmental patterning. Indeed, the role of Notch in segmentation re-emphasizes a recurring question in Notch biology: how are the components involved in Notch signaling regulated to ensure their dynamic properties? Second, we address this question by discussing recent work on the biochemical mechanisms that potentially regulate Notch signaling during segmentation, including those that act on the receptors, ligands, and signal transduction apparatus.

Characterization and expression of a presomitic mesoderm-specific mespo gene in zebrafish

A complete zebrafish mespo cDNA encoding a protein of 131 amino acids with a bHLH domain in the C-terminal has been isolated. The bHLH domain of zebrafish Mespo is highly similar to those in the mouse, chick and Xenopus, sharing 82.4%, 80.4% and 74.5% amino acid identity, respectively. At 50% epiboly, the zebrafish mespo is first detected in the marginal zone of the blastoderm but excluding the prospective shield. Subsequently, mespo expression is intensified in the involuting mesoderm at 60% epiboly, and then restricted to the presomitic mesoderm (PSM) at 95% epiboly. At the 1-somite stage, mespo expression becomes reduced in the most rostral PSM. During segmentation, mespo expression is gradually downregulated at the most rostral segmental plate where cells are being coalesced to form somites. In spadetail mutant embryos, most of mespo-expressing cells were missing.

Vertebrate somitogenesis

In vertebrates, the paraxial mesoderm corresponds to the bilateral strips of mesodermal tissue flanking the notochord and neural tube and which are delimited laterally by the intermediate mesoderm and the lateral plate. The paraxial mesoderm comprises the head or cephalic mesoderm anteriorly and the somitic region throughout the trunk and the tail of the vertebrates. Soon after gastrulation, the somitic region of vertebrates starts to become segmented into paired blocks of mesoderm, termed somites. This process lasts until the number of somites characteristic of the species is reached. The somites later give rise to all skeletal muscles of the body, the axial skeleton, and part of the dermis. In this review I discuss the processes involved in the formation of the paraxial mesoderm and its segmentation into somites in vertebrates.

Dynamic expression of chicken cMeso2 in segmental plate and somites

Abstract Somitogenesis in vertebrates involves prepatterning of paraxial mesoderm into somitomeres, establishing of anteroposterior polarity within somite primordia, and boundary formation between individual somites. cMeso2 is a newly identified chicken gene encoding a bHLH transcription factor, which is expressed in a transient stripe pattern in anterior presomitic mesoderm before segmentation of somites. The expression pattern overlaps with that of cMeso1 and correlates in time with the formation cycle of somites, suggesting that it may have a role in this process. Unlike its homologues in other organisms cMeso2 transcripts in chicken locate to the posterior aspects of somitomeres and constitute a marker for the caudal half of somites. Initiation of cMeso2 expression in presomitic mesoderm as well as its maintenance appears to be independent from influences by surrounding tissues, suggesting that it is part of the intrinsic program underlying segmentation. Although cMeso1 contains a C-terminal activator domain that can be transferred onto an independent DNA-binding domain, no evidence for such a transactivator domain can be found in cMeso2. In contrast, cMeso2 exerts transcriptional inhibition when coexpressed with the cMeso1 transactivator and seems to contain a repressor domain. Thus, cMeso1 and cMeso2 may function in an antagonistic manner during somitogenesis.

The winged helix transcription factor Foxc1a is essential for somitogenesis in zebrafish

Previous studies identified zebrafish foxc1a and foxc1b as homologs of the mouse forkhead gene, Foxc1. Both genes are transcribed in the unsegmented presomitic mesoderm (PSM), newly formed somites, adaxial cells, and head mesoderm. Here, we show that inhibiting synthesis of Foxc1a (but not Foxc1b) protein with two different morpholino antisense oligonucleotides blocks formation of morphological somites, segment boundaries, and segmented expression of genes normally transcribed in anterior and posterior somites and expression of paraxis implicated in somite epithelialization. Patterning of the anterior PSM is also affected, as judged by the absence of mesp-b, ephrinB2, and ephA4 expression, and the down-regulation of notch5 and notch6. In contrast, the expression of other genes, including mesp-a and papc, in the anterior of somite primordia, and the oscillating expression of deltaC and deltaD in the PSM appear normal. Nevertheless, this expression is apparently insufficient for the maturation of the presumptive somites to proceed to the stage when boundary formation occurs or for the maintenance of anterior/posterior patterning. Mouse embryos that are compound null mutants for Foxc1 and the closely related Foxc2 have no morphological somites and show abnormal expression of Notch signaling pathway genes in the anterior PSM. Therefore, zebrafish foxc1a plays an essential and conserved role in somite formation, regulating both the expression of paraxis and the A/P patterning of somite primordia.