Evolution of development: diversified dorsoventral patterning

Learning a conserved mechanism for early neuroectoderm morphogenesis

Morphogenesis is the process whereby the body of an organism develops its target shape. The morphogen BMP is known to play a conserved role across bilaterian organisms in determining the dorsoventral (DV) axis. Yet, how BMP governs the spatio-temporal dynamics of cytoskeletal proteins driving morphogenetic flow remains an open question. Here, we use machine learning to mine a morphodynamic atlas of Drosophila development, and construct a mathematical model capable of predicting the coupled dynamics of myosin, E-cadherin, and morphogenetic flow. Mutant analysis shows that BMP sets the initial condition of this dynamical system according to the following signaling cascade: BMP establishes DV pair-rule-gene patterns that set-up an E-cadherin gradient which in turn creates a myosin gradient in the opposite direction through mechanochemical feedbacks. Using neural tube organoids, we argue that BMP, and the signaling cascade it triggers, prime the conserved dynamics of neuroectoderm morphogenesis from fly to humans.

BMP Signaling: Lighting up the Way for Embryonic Dorsoventral Patterning

One of the most significant events during early embryonic development is the establishment of a basic embryonic body plan, which is defined by anteroposterior, dorsoventral (DV), and left-right axes. It is well-known that the morphogen gradient created by BMP signaling activity is crucial for DV axis patterning across a diverse set of vertebrates. The regulation of BMP signaling during DV patterning has been strongly conserved across evolution. This is a remarkable regulatory and evolutionary feat, as the BMP gradient has been maintained despite the tremendous variation in embryonic size and shape across species. Interestingly, the embryonic DV axis exhibits robust stability, even in face of variations in BMP signaling. Multiple lines of genetic, molecular, and embryological evidence have suggested that numerous BMP signaling components and their attendant regulators act in concert to shape the developing DV axis. In this review, we summarize the current knowledge of the function and regulation of BMP signaling in DV patterning. Throughout, we focus specifically on popular model animals, such as Xenopus and zebrafish, highlighting the similarities and differences of the regulatory networks between species. We also review recent advances regarding the molecular nature of DV patterning, including the initiation of the DV axis, the formation of the BMP gradient, and the regulatory molecular mechanisms behind BMP signaling during the establishment of the DV axis. Collectively, this review will help clarify our current understanding of the molecular nature of DV axis formation.

BMP signaling plays a role in anterior-neural/head development, but not organizer activity, in the gastropod Crepidula fornicata

BMP signaling is involved in many aspects of metazoan development, with two of the most conserved functions being to pattern the dorsal-ventral axis and to specify neural versus epidermal fates. An active area of research within developmental biology asks how BMP signaling was modified over evolution to build disparate body plans. Animals belonging to the superclade Spiralia/Lophotrochozoa are excellent experimental subjects for studying the evolution of BMP signaling because a highly conserved, stereotyped early cleavage program precedes the emergence of distinct body plans. In this study we examine the role of BMP signaling in one representative, the slipper snail Crepidula fornicata. We find that mRNAs encoding BMP pathway components (including the BMP ligand decapentaplegic, and BMP antagonists chordin and noggin-like proteins) are not asymmetrically localized along the dorsal-ventral axis in the early embryo, as they are in other species. Furthermore, when BMP signaling is perturbed by adding ectopic recombinant BMP4 protein, or by treating embryos with the selective Activin receptor-like kinase-2 (ALK-2) inhibitor Dorsomorphin Homolog 1 (DMH1), we observe no obvious effects on dorsal-ventral patterning within the posterior (post-trochal) region of the embryo. Instead, we see effects on head development and the balance between neural and epidermal fates specifically within the anterior, pre-trochal tissue derived from the 1q1 lineage. Our experiments define a window of BMP signaling sensitivity that ends at approximately 44-48 ​hours post fertilization, which occurs well after organizer activity has ended and after the dorsal-ventral axis has been determined. When embryos were exposed to BMP4 protein during this window, we observed morphogenetic defects leading to the separation of the anterior, 1q lineage from the rest of the embryo. The 1q-derived organoid remained largely undifferentiated and was radialized, while the post-trochal portion of the embryo developed relatively normally and exhibited clear signs of dorsal-ventral patterning. When embryos were exposed to DMH1 during the same time interval, we observed defects in the head, including protrusion of the apical plate, enlarged cerebral ganglia and ectopic ocelli, but otherwise the larvae appeared normal. No defects in shell development were noted following DMH1 treatments. The varied roles of BMP signaling in the development of several other spiralians have recently been examined. We discuss our results in this context, and highlight the diversity of developmental mechanisms within spiral-cleaving animals.

A deuterostome origin of the Spemann organiser suggested by Nodal and ADMPs functions in Echinoderms

During development of chordates, establishment of the body plan relies on the activity of an organizing centre located on the dorsal side of the embryo that patterns the embryo and induces neural tissue. Intriguingly, the evolutionary origin of this crucial signalling centre remains unclear and whether analogous organizers regulate D/V patterning in other deuterostome or protostome phyla is not known. Here we provide evidence that the ventral ectoderm of the sea urchin embryo is a long-range organizing centre that shares several fundamental properties with the Spemann organizer: the ability to induce duplicated embryonic axes when ectopically induced, the ability to induce neural fate in neighbouring tissues and the ability to finely regulate the level of BMP signalling by using an autoregulatory expansion–repression mechanism. These findings suggest that the evolutionary origin of the Spemann organizer is more ancient than previously thought and that it may possibly be traced back to the common ancestor of deuterostomes.

Establishment of the body plan in chordates is determined by an organizing centre located on the dorsal side of the embryo. Here, the authors show that the ventral ectoderm of the sea urchin embryo is an organizing centre that shares several fundamental properties with the amphibian Spemann organizer.

A Bmp Reporter Transgene Mouse Embryonic Stem Cell Model as a Tool to Identify and Characterize Chemical Teratogens

Embryonic stem cells (ESCs) were first isolated from mouse embryos more than 30 years ago. They have proven invaluable not only in generating genetically modified mice that allow for analysis of gene function in tissue development and homeostasis but also as models for genetic disease. In addition, ESCs in vitro are finding inroads in pharmaceutical and toxicological testing, including the identification of teratogenic compounds. Here, we describe the use of a bone morphogenetic protein (Bmp)-reporter ESC line, isolated from a well-characterized transgenic mouse line, as a new tool for the identification of chemical teratogens. The Bmp-mediated expression of the green fluorescent protein enabled the quantification of dose- and time-dependent effects of valproic acid as well as retinoic acid. Significant effects were detectable at concentrations that were comparable to the ones observed in the classical embryonic stem cell test, despite the fact that the reporter gene is expressed in distinct cell types, including endothelial and endodermal cells. Thus these cells provide a valuable new tool for the identification and characterization of relevant mechanisms of embryonic toxicity.

RGM regulates BMP-mediated secondary axis formation in the sea anemone Nematostella vectensis

Patterning of the metazoan dorsoventral axis is mediated by a complex interplay of BMP signaling regulators. Repulsive guidance molecule (RGM) is a conserved BMP coreceptor that has not been implicated in axis specification. We show that NvRGM is a key positive regulator of BMP signaling during secondary axis establishment in the cnidarian Nematostella vectensis. NvRGM regulates first the generation and later the shape of a BMP-dependent Smad1/5/8 gradient with peak activity on the side opposite the NvBMP/NvRGM/NvChordin expression domain. Full knockdown of Smad1/5/8 signaling blocks the formation of endodermal structures, the mesenteries, and the establishment of bilateral symmetry, while altering the gradient through partial NvRGM or NvBMP knockdown shifts the boundaries of asymmetric gene expression and the positioning of the mesenteries along the secondary axis. These findings provide insight into the diversification of axis specification mechanisms and identify a previously unrecognized role for RGM in BMP-mediated axial patterning.

BMPs regulate msx gene expression in the dorsal neuroectoderm of Drosophila and vertebrates by distinct mechanisms

In a broad variety of bilaterian species the trunk central nervous system (CNS) derives from three primary rows of neuroblasts. The fates of these neural progenitor cells are determined in part by three conserved transcription factors: vnd/nkx2.2, ind/gsh and msh/msx in Drosophila melanogaster/vertebrates, which are expressed in corresponding non-overlapping patterns along the dorsal-ventral axis. While this conserved suite of “neural identity” gene expression strongly suggests a common ancestral origin for the patterning systems, it is unclear whether the original regulatory mechanisms establishing these patterns have been similarly conserved during evolution. In Drosophila, genetic evidence suggests that Bone Morphogenetic Proteins (BMPs) act in a dosage-dependent fashion to repress expression of neural identity genes. BMPs also play a dose-dependent role in patterning the dorsal and lateral regions of the vertebrate CNS, however, the mechanism by which they achieve such patterning has not yet been clearly established. In this report, we examine the mechanisms by which BMPs act on cis-regulatory modules (CRMs) that control localized expression of the Drosophila msh and zebrafish (Danio rerio) msxB in the dorsal central nervous system (CNS). Our analysis suggests that BMPs act differently in these organisms to regulate similar patterns of gene expression in the neuroectoderm: repressing msh expression in Drosophila, while activating msxB expression in the zebrafish. These findings suggest that the mechanisms by which the BMP gradient patterns the dorsal neuroectoderm have reversed since the divergence of these two ancient lineages.

The trunk nervous system of both vertebrates and invertebrates develops from three primary rows of neural stem cells whose fate is determined by neural identity genes expressed in an evolutionarily conserved dorso-ventral pattern. Establishment of this pattern requires a shared signaling pathway in both groups of animals. Previous studies suggested that a shared signaling pathway functions in opposite ways in vertebrates and invertebrates, despite the final patterning outcomes having remained the same. Here, we employ bioinformatics, biochemistry, and transgenic animal technology to elucidate the genetic mechanism by which this pathway can engage the same components to generate opposite instructions and yet arrive at similar outcomes in patterning of the nervous system. Our findings highlight how natural selection can act to conserve a particular output pattern despite changes during evolution in the genetic mechanisms underlying the formation of this pattern.