Notch signalling regulates the contribution of progenitor cells from the chick Hensen's node to the floor plate and notochord

Notch signalling influences cell fate decisions and HOX gene induction in axial progenitors

The generation of the post-cranial embryonic body relies on the coordinated production of spinal cord neurectoderm and presomitic mesoderm cells from neuromesodermal progenitors (NMPs). This process is orchestrated by pro-neural and pro-mesodermal transcription factors that are co-expressed in NMPs together with Hox genes, which are essential for axial allocation of NMP derivatives. NMPs reside in a posterior growth region, which is marked by the expression of Wnt, FGF and Notch signalling components. Although the importance of Wnt and FGF in influencing the induction and differentiation of NMPs is well established, the precise role of Notch remains unclear. Here, we show that the Wnt/FGF-driven induction of NMPs from human embryonic stem cells (hESCs) relies on Notch signalling. Using hESC-derived NMPs and chick embryo grafting, we demonstrate that Notch directs a pro-mesodermal character at the expense of neural fate. We show that Notch also contributes to activation of HOX gene expression in human NMPs, partly in a non-cell-autonomous manner. Finally, we provide evidence that Notch exerts its effects via the establishment of a negative-feedback loop with FGF signalling.

Intrinsic and extrinsic cues time somite progenitor contribution to the vertebrate primary body axis

During embryonic development, the timing of events at the cellular level must be coordinated across multiple length scales to ensure the formation of a well-proportioned body plan. This is clear during somitogenesis, where progenitors must be allocated to the axis over time whilst maintaining a progenitor population for continued elaboration of the body plan. However, the relative importance of intrinsic and extrinsic signals in timing progenitor addition at the single-cell level is not yet understood. Heterochronic grafts from older to younger embryos have suggested a level of intrinsic timing whereby later staged cells contribute to more posterior portions of the axis. To determine the precise step at which cells are delayed, we performed single-cell transcriptomic analysis on heterochronic grafts of somite progenitors in the chicken embryo. This revealed a previously undescribed cell state within which heterochronic grafted cells are stalled. The delayed exit of older cells from this state correlates with expression of posterior Hox genes. Using grafting and explant culture, we find that both Hox gene expression and the migratory capabilities of progenitor populations are intrinsically regulated at the population level. However, by grafting varied sizes of tissue, we find that small heterochronic grafts disperse more readily and contribute to more anterior portions of the body axis while still maintaining Hox gene expression. This enhanced dispersion is not replicated in explant culture, suggesting that it is a consequence of interaction between host and donor tissue and thus extrinsic to the donor tissue. Therefore, we demonstrate that the timing of cell dispersion and resulting axis contribution is impacted by a combination of both intrinsic and extrinsic cues.

Neuromesodermal Lineage Contribution to CNS Development in Invertebrate and Vertebrate Chordates

Ascidians are invertebrate chordates and the closest living relative to vertebrates. In ascidian embryos a large part of the central nervous system arises from cells associated with mesoderm rather than ectoderm lineages. This seems at odds with the traditional view of vertebrate nervous system development which was thought to be induced from ectoderm cells, initially with anterior character and later transformed by posteriorizing signals, to generate the entire anterior-posterior axis of the central nervous system. Recent advances in vertebrate developmental biology, however, show that much of the posterior central nervous system, or spinal cord, in fact arises from cells that share a common origin with mesoderm. This indicates a conserved role for bi-potential neuromesoderm precursors in chordate CNS formation. However, the boundary between neural tissue arising from these distinct neural lineages does not appear to be fixed, which leads to the notion that anterior-posterior patterning and neural fate formation can evolve independently.

Cellular processes driving gastrulation in the avian embryo

Gastrulation consists in the dramatic reorganisation of the epiblast, a one-cell thick epithelial sheet, into a multilayered embryo. In chick, the formation of the internal layers requires the generation of a macroscopic convection-like flow, which involves up to 50,000 epithelial cells in the epiblast. These cell movements locate the mesendoderm precursors into the midline of the epiblast to form the primitive streak. There they acquire a mesenchymal phenotype, ingress into the embryo and migrate outward to populate the inner embryonic layers. This review covers what is currently understood about how cell behaviours ultimately cause these morphogenetic events and how they are regulated. We discuss 1) how the biochemical patterning of the embryo before gastrulation creates compartments of differential cell behaviours, 2) how the global epithelial flows arise from the coordinated actions of individual cells, 3) how the cells delaminate individually from the epiblast during the ingression, and 4) how cells move after the ingression following stereotypical migration routes. We conclude by exploring new technical advances that will facilitate future research in the chick model system.

Key Steps in the Evolution of Mammalian Movement: A Prolegomenal Essay

Rich repertoires of movements underlie the complex social interactions of mammals. The building blocks, or syllables, of these movements are produced by spinal cord circuits that are comprised of diverse neuronal types that control musculoskeletal systems comprised of multi-segmented limbs. Together, these systems provide mammals with the evolutionary advantages of power, speed, and endurance. Here, I propose that the key steps in chordate evolution that led to these traits began with the development of the notochord and a proliferative ventricular zone (with associated Notch signalling). This step led to the production of diverse neuronal types that included the development of a sympathetic nervous system that could regulate the evolving cardiovascular system. And the sympathetic nervous system in turn led to the development of homeothermic endothermy, a requirement for motor systems to produce a combination of power, speed, and endurance. Furthermore, the evolution of the continuous structure of the spinal cord led not only to a structure fit for cartesian signalling molecules, but also to one with high processing power in which circuits for effecting movement syllables formed. These syllables are harnessed by higher regions of nervous systems to produce the complex movements required for interactions with others and with the surrounding environment.

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.

NOTCH activation interferes with cell fate specification in the gastrulating mouse embryo

NOTCH signalling is an evolutionarily conserved pathway involved in intercellular communication essential for cell fate choices during development. Although dispensable for early aspects of mouse development, canonical RBPJ-dependent NOTCH signalling has been shown to influence lineage commitment during embryonic stem cell (ESC) differentiation. NOTCH activation in ESCs promotes the acquisition of a neural fate, whereas its suppression favours their differentiation into cardiomyocytes. This suggests that NOTCH signalling is implicated in the acquisition of distinct embryonic fates at early stages of mammalian development. In order to investigate in vivo such a role for NOTCH signalling in shaping cell fate specification, we use genetic approaches to constitutively activate the NOTCH pathway in the mouse embryo. Early embryonic development, including the establishment of anterior-posterior polarity, is not perturbed by forced NOTCH activation. By contrast, widespread NOTCH activity in the epiblast triggers dramatic gastrulation defects. These are fully rescued in a RBPJ-deficient background. Epiblast-specific NOTCH activation induces acquisition of neurectoderm identity and disrupts the formation of specific mesodermal precursors including the derivatives of the anterior primitive streak, the mouse organiser. In addition, we show that forced NOTCH activation results in misregulation of NODAL signalling, a major determinant of early embryonic patterning. Our study reveals a previously unidentified role for canonical NOTCH signalling during mammalian gastrulation. It also exemplifies how in vivo studies can shed light on the mechanisms underlying cell fate specification during in vitro directed differentiation.

The zebrafish tailbud contains two independent populations of midline progenitor cells that maintain long-term germ layer plasticity and differentiate in response to local signaling cues

Vertebrate body axis formation depends on a population of bipotential neuromesodermal cells along the posterior wall of the tailbud that make a germ layer decision after gastrulation to form spinal cord and mesoderm. Despite exhibiting germ layer plasticity, these cells never give rise to midline tissues of the notochord, floor plate and dorsal endoderm, raising the question of whether midline tissues also arise from basal posterior progenitors after gastrulation. We show in zebrafish that local posterior signals specify germ layer fate in two basal tailbud midline progenitor populations. Wnt signaling induces notochord within a population of notochord/floor plate bipotential cells through negative transcriptional regulation of sox2. Notch signaling, required for hypochord induction during gastrulation, continues to act in the tailbud to specify hypochord from a notochord/hypochord bipotential cell population. Our results lend strong support to a continuous allocation model of midline tissue formation in zebrafish, and provide an embryological basis for zebrafish and mouse bifurcated notochord phenotypes as well as the rare human congenital split notochord syndrome. We demonstrate developmental equivalency between the tailbud progenitor cell populations. Midline progenitors can be transfated from notochord to somite fate after gastrulation by ectopic expression of msgn1, a master regulator of paraxial mesoderm fate, or if transplanted into the bipotential progenitors that normally give rise to somites. Our results indicate that the entire non-epidermal posterior body is derived from discrete, basal tailbud cell populations. These cells remain receptive to extracellular cues after gastrulation and continue to make basic germ layer decisions.

ProNodal acts via FGFR3 to govern duration of Shh expression in the prechordal mesoderm

The secreted glycoprotein sonic hedgehog (Shh) is expressed in the prechordal mesoderm, where it plays a crucial role in induction and patterning of the ventral forebrain. Currently little is known about how Shh is regulated in prechordal tissue. Here we show that in the embryonic chick, Shh is expressed transiently in prechordal mesoderm, and is governed by unprocessed Nodal. Exposure of prechordal mesoderm microcultures to Nodal-conditioned medium, the Nodal inhibitor CerS, or to an ALK4/5/7 inhibitor reveals that Nodal is required to maintain both Shh and Gsc expression, but whereas Gsc is largely maintained through canonical signalling, Nodal signals through a non-canonical route to maintain Shh. Further, Shh expression can be maintained by a recombinant Nodal cleavage mutant, proNodal, but not by purified mature Nodal. A number of lines of evidence suggest that proNodal acts via FGFR3. ProNodal and FGFR3 co-immunoprecipitate and proNodal increases FGFR3 tyrosine phosphorylation. In microcultures, soluble FGFR3 abolishes Shh without affecting Gsc expression. Further, prechordal mesoderm cells in which Fgfr3 expression is reduced by Fgfr3 siRNA fail to bind to proNodal. Finally, targeted electroporation of Fgfr3 siRNA to prechordal mesoderm in vivo results in premature Shh downregulation without affecting Gsc. We report an inverse correlation between proNodal-FGFR3 signalling and pSmad1/5/8, and show that proNodal-FGFR3 signalling antagonises BMP-mediated pSmad1/5/8 signalling, which is poised to downregulate Shh. Our studies suggest that proNodal/FGFR3 signalling governs Shh duration by repressing canonical BMP signalling, and that local BMPs rapidly silence Shh once endogenous Nodal-FGFR3 signalling is downregulated.

Highlighted article: In the chick prechordal mesoderm, the Nodal precursor proNodal acts via a non-canonical route to inhibit BMP signalling and thus maintain Shh expression

A conserved role for Notch signaling in priming the cellular response to Shh through ciliary localisation of the key Shh transducer Smo

Notochord-derived Sonic Hedgehog (Shh) is essential for dorsoventral patterning of the overlying neural tube. Increasing concentration and duration of Shh signal induces progenitors to acquire progressively more ventral fates. We show that Notch signalling augments the response of neuroepithelial cells to Shh, leading to the induction of higher expression levels of the Shh target gene Ptch1 and subsequently induction of more ventral cell fates. Furthermore, we demonstrate that activated Notch1 leads to pronounced accumulation of Smoothened (Smo) within primary cilia and elevated levels of full-length Gli3. Finally, we show that Notch activity promotes longer primary cilia both in vitro and in vivo. Strikingly, these Notch-regulated effects are Shh independent. These data identify Notch signalling as a novel modulator of Shh signalling that acts mechanistically via regulation of ciliary localisation of key components of its transduction machinery.

Highlighted article: Shh signalling controls dorso-ventral cell fate in the neural tube. Notch regulates ciliary architecture and localisation of key Shh pathway components, thus sensitising cells to Shh.

The route to spinal cord cell types: a tale of signals and switches

Understanding the mechanisms that control induction and elaboration of the vertebrate central nervous system (CNS) requires an analysis of the extrinsic signals and downstream transcriptional networks that assign cell fates in the correct space and time. We focus on the generation and patterning of the spinal cord. We summarize evidence that the origin of the spinal cord is distinct from the anterior regions of the CNS. We discuss how this affects the gene regulatory networks and cell state transitions that specify spinal cord cell subtypes, and we highlight how the timing of extracellular signals and dynamic control of transcriptional networks contribute to the correct spatiotemporal generation of different neural cell types.

Notochordal cell-derived therapeutic strategies for discogenic back pain

An understanding of the processes that occur during development of the intervertebral disk can help inform therapeutic strategies for discogenic pain. This article reviews the literature to identify candidates that are found in or derived from the notochord or notochordal cells and evaluates the theory that such factors could be isolated and used as biologics to target the structural disruption, inflammation, and neurovascular ingrowth often associated with discogenic back pain. A systematic review using PubMed was performed with a primary search using keywords "(notochordal OR notochord) And (nerves OR blood vessels OR SHH OR chondroitin sulfate OR notch OR CTGF) NOT chordoma." Secondary searches involved keywords associated with the intervertebral disk and pain. Several potential therapeutic candidates from the notochord and their possible targets were identified. Studies are needed to further identify candidates, explore mechanisms for effect, and to validate the theory that these candidates can promote structural restoration and limit or inhibit neurovascular ingrowth using in vivo studies.

A novel role for FOXA2 and SHH in organizing midbrain signaling centers

The floor plate (FP) is a midline signaling center, known to direct ventral cell fates and axon guidance in the neural tube. The recent identification of midbrain FP as a source of dopaminergic neurons has renewed interest in its specification and organization, which remain poorly understood. In this study, we have examined the chick midbrain and spinal FP and show that both can be partitioned into medial (MFP) and lateral (LFP) subdivisions. Although Hedgehog (HH) signaling is necessary and sufficient for LFP specification, it is not sufficient for MFP induction. By contrast, the transcription factor FOXA2 can execute the full midbrain and spinal cord FP program via HH-independent and dependent mechanisms. Interestingly, although HH-independent FOXA2 activity is necessary and sufficient for inducing MFP-specific gene expression (e.g., LMX1B, BMP7), it cannot confer ventral identity to midline cells without also turning on Sonic hedgehog (SHH). We also note that the signaling centers of the midbrain, the FP, roof plate (RP) and the midbrain-hindbrain boundary (MHB) are physically contiguous, with each expressing LMX1B and BMP7. Possibly as a result, SHH or FOXA2 misexpression can transform the MHB into FP and also suppress RP induction. Conversely, HH or FOXA2 knockdown expands the endogenous RP and transforms the MFP into a RP and/or MHB fate. Finally, combined HH blockade and FOXA2 misexpression in ventral midbrain induces LMX1B expression, which triggers the specification of the RP, rather than the MFP. Thus we identify HH-independent and dependent roles for FOXA2 in specifying the FP. In addition, we elucidate for the first time, a novel role for SHH in determining whether a midbrain signaling center will become the FP, MHB or RP.

Isolation and characterization of node/notochord-like cells from mouse embryonic stem cells

The homeobox gene Noto is expressed in the node and its derivative the notochord. Here we use a targeted Noto-GFP reporter to isolate and characterize node/notochord-like cells derived from mouse embryonic stem cells. We find very few Noto-expressing cells after spontaneous differentiation. However, the number of Noto-expressing cells was increased when using Activin A to induce a Foxa2- and Brachyury-expressing progenitor population, whose further differentiation into Noto-expressing cells was improved by simultaneous inhibition of BMP, Wnt, and retinoic acid signaling. Noto-GFP(+) cells expressed the node/notochord markers Noto, Foxa2, Shh, Noggin, Chordin, Foxj1, and Brachyury; showed a vacuolarization characteristic of notochord cells; and can integrate into midline structures when grafted into Hensen's node of gastrulating chicken embryos. The ability to generate node/notochord-like cells in vitro will aid the biochemical characterization of these developmentally important structures.

A revised model of Xenopus dorsal midline development: differential and separable requirements for Notch and Shh signaling

The development of the vertebrate dorsal midline (floor plate, notochord, and hypochord) has been an area of classical research and debate. Previous studies in vertebrates have led to contrasting models for the roles of Shh and Notch signaling in specification of the floor plate, by late inductive or early allocation mechanisms, respectively. Here, we show that Notch signaling plays an integral role in cell fate decisions in the dorsal midline of Xenopus laevis, similar to that observed in zebrafish and chick. Notch signaling promotes floor plate and hypochord fates over notochord, but has variable effects on Shh expression in the midline. In contrast to previous reports in frog, we find that Shh signaling is not required for floor plate vs. notochord decisions and plays a minor role in floor plate specification, where it acts in parallel to Notch signaling. As in zebrafish, Shh signaling is required for specification of the lateral floor plate in the frog. We also find that the medial floor plate in Xenopus comprises two distinct populations of cells, each dependent upon different signals for its specification. Using expression analysis of several midline markers, and dissection of functional relationships, we propose a revised allocation mechanism of dorsal midline specification in Xenopus. Our model is distinct from those proposed to date, and may serve as a guide for future studies in frog and other vertebrate organisms.