Chongmague reveals an essential role for laminin-mediated boundary formation in chordate convergence and extension movements

Prickle1-driven basement membrane deposition of the iPSC-derived embryoid bodies is separable from the establishment of apicobasal polarity

Basement membrane (BM) component deposition is closely linked to the establishment of cell polarity. Previously, we showed that Prickle1 is crucial for BM deposition and cell polarity events in tear duct elongation. To gain a deeper understanding of the intimate relationship between BM formation and cell polarity, we generated induced pluripotent stem cells (iPSCs)-derived embryoid bodies (EBs) with a basement membrane separating the visceral endoderm (VE) and inner EB cell mass. We found that Prickle1 was highly expressed in VE of the normal EBs, and the Prickle1 mutant EBs displayed severely impaired BM. Notably, the formation of the basement membrane appeared to rely on the proper microtubule network of the VE cells, which was disrupted in the Prickle1 mutant EBs. Moreover, disruption of vesicle trafficking in the VE hindered BM secretion. Furthermore, reintroducing Prickle1 in the mutant EBs completely rescued BM formation but not the apicobasal cell polarity of the VE. Our data, in conjunction with studies by others, highlight the conserved role of Prickle1 in directing the secretion of BM components of the VE cells during embryonic germ layer differentiation, even in the absence of established general polarity machinery. Our study introduces a novel system based on iPSCs-derived EBs for investigating cellular and molecular events associated with cell polarity.

Quantitative proteome dynamics across embryogenesis in a model chordate

The evolution of gene expression programs underlying the development of vertebrates remains poorly characterized. Here, we present a comprehensive proteome atlas of the model chordate Ciona, covering eight developmental stages and ∼7,000 translated genes, accompanied by a multi-omics analysis of co-evolution with the vertebrate Xenopus. Quantitative proteome comparisons argue against the widely held hourglass model, based solely on transcriptomic profiles, whereby peak conservation is observed during mid-developmental stages. Our analysis reveals maximal divergence at these stages, particularly gastrulation and neurulation. Together, our work provides a valuable resource for evaluating conservation and divergence of multi-omics profiles underlying the diversification of vertebrates.

Temporal dynamics of BMP/Nodal ratio drive tissue-specific gastrulation morphogenesis

Anteroposterior (AP) elongation of the vertebrate body plan is driven by convergence and extension (C&E) gastrulation movements in both the mesoderm and neuroectoderm, but how or whether molecular regulation of C&E differs between tissues remains an open question. Using a zebrafish explant model of AP axis extension, we show that C&E of the neuroectoderm and mesoderm can be uncoupled ex vivo, and that morphogenesis of individual tissues results from distinct morphogen signaling dynamics. Using precise temporal manipulation of BMP and Nodal signaling, we identify a critical developmental window during which high or low BMP/Nodal ratios induce neuroectoderm- or mesoderm-driven C&E, respectively. Increased BMP activity similarly enhances C&E specifically in the ectoderm of intact zebrafish gastrulae, highlighting the in vivo relevance of our findings. Together, these results demonstrate that temporal dynamics of BMP and Nodal morphogen signaling activate distinct morphogenetic programs governing C&E gastrulation movements within individual tissues.

A collagen-rich arch in the urochordate notochord coordinates cell shaping and multi-tissue elongation

Cell and tissue reshaping is crucial for coordinating three-dimensional pattern formation, in which the size and shape of the cells must be accurately regulated via signal transport and communication among tissues. However, the identity of signaling and transportation mechanisms in this process remains elusive. In our study, we identified an extracellular matrix (ECM) structure with a vertebra-like shape surrounding the central notochord tissue in the larval tail of the urochordate Ciona. Additionally, we verified that the ECM structure was formed de novo, mainly from collagens secreted by notochord cells. Fluorescence recovery after photobleaching and simulation results revealed that this structure was formed via diffusional collagen flow from a notochord that was restricted and molded in the spaces among tail tissues. We revealed that the collagen structure was essential for notochord cell arrangement and elongation. Furthermore, we observed that the central notochord connects with the epidermis through this ECM structure. The disruption of this structure by collagen knockdown and loss-of-collagen function caused the failure of notochord elongation. More importantly, the epidermis could not elongate proportionally with notochord, indicating that the collagen-rich structure serves as a scaffold to coordinate the concurrent elongation of the tail tissues. These findings provide insights into how the central tissue forms and molds its surrounding ECM structure, by not only regulating its own morphogenesis but also functioning as a scaffold for signal transmission to orchestrate the coordinated morphologic reshaping of the surrounding tissues.

Diverse logics and grammar encode notochord enhancers

The notochord is a defining feature of all chordates. The transcription factors Zic and ETS regulate enhancer activity within the notochord. We conduct high-throughput screens of genomic elements within developing Ciona embryos to understand how Zic and ETS sites encode notochord activity. Our screen discovers an enhancer located near Lama, a gene critical for notochord development. Reversing the orientation of an ETS site within this enhancer abolishes expression, indicating that enhancer grammar is critical for notochord activity. Similarly organized clusters of Zic and ETS sites occur within mouse and human Lama1 introns. Within a Brachyury (Bra) enhancer, FoxA and Bra, in combination with Zic and ETS binding sites, are necessary and sufficient for notochord expression. This binding site logic also occurs within other Ciona and vertebrate Bra enhancers. Collectively, this study uncovers the importance of grammar within notochord enhancers and discovers signatures of enhancer logic and grammar conserved across chordates.

The Degenerate Tale of Ascidian Tails

Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the "notoball." We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

Single-cell analysis of cell fate bifurcation in the chordate Ciona

BACKGROUND

Inductive signaling interactions between different cell types are a major mechanism for the further diversification of embryonic cell fates. Most blastomeres in the model chordate Ciona robusta become restricted to a single predominant fate between the 64-cell and mid-gastrula stages. The deeply stereotyped and well-characterized Ciona embryonic cell lineages allow the transcriptomic analysis of newly established cell types very early in their divergence from sibling cell states without the pseudotime inference needed in the analysis of less synchronized cell populations. This is the first ascidian study to use droplet scRNAseq with large numbers of analyzed cells as early as the 64-cell stage when major lineages such as primary notochord first become fate restricted.

RESULTS AND CONCLUSIONS

We identify 59 distinct cell states, including new subregions of the b-line neural lineage and the early induction of the tail tip epidermis. We find that 34 of these cell states are directly or indirectly dependent on MAPK-mediated signaling critical to early Ciona patterning. Most of the MAPK-dependent bifurcations are canalized with the signal-induced cell fate lost upon MAPK inhibition, but the posterior endoderm is unique in being transformed into a novel state expressing some but not all markers of both endoderm and muscle. Divergent gene expression between newly bifurcated sibling cell types is dominated by upregulation in the induced cell type. The Ets family transcription factor Elk1/3/4 is uniquely upregulated in nearly all the putatively direct inductions. Elk1/3/4 upregulation together with Ets transcription factor binding site enrichment analysis enables inferences about which bifurcations are directly versus indirectly controlled by MAPK signaling. We examine notochord induction in detail and find that the transition between a Zic/Ets-mediated regulatory state and a Brachyury/FoxA-mediated regulatory state is unexpectedly late. This supports a "broad-hourglass" model of cell fate specification in which many early tissue-specific genes are induced in parallel to key tissue-specific transcriptional regulators via the same set of transcriptional inputs.

Ciona Brachyury proximal and distal enhancers have different FGF dose-response relationships

Many genes are regulated by two or more enhancers that drive similar expression patterns. Evolutionary theory suggests that these seemingly redundant enhancers must have functionally important differences. In the simple ascidian chordate Ciona, the transcription factor Brachyury is induced exclusively in the presumptive notochord downstream of lineage specific regulators and FGF-responsive Ets family transcription factors. Here we exploit the ability to finely titrate FGF signaling activity via the MAPK pathway using the MEK inhibitor U0126 to quantify the dependence of transcription driven by different Brachyury reporter constructs on this direct upstream regulator. We find that the more powerful promoter-adjacent proximal enhancer and a weaker distal enhancer have fundamentally different dose-response relationships to MAPK inhibition. The Distal enhancer is more sensitive to MAPK inhibition but shows a less cooperative response, whereas the Proximal enhancer is less sensitive and more cooperative. A longer construct containing both enhancers has a complex dose-response curve that supports the idea that the proximal and distal enhancers are moderately super-additive. We show that the overall expression loss from intermediate doses of U0126 is not only a function of the fraction of cells expressing these reporters, but also involves graded decreases in expression at the single-cell level. Expression of the endogenous gene shows a comparable dose-response relationship to the full length reporter, and we find that different notochord founder cells are differentially sensitive to MAPK inhibition. Together, these results indicate that although the two Brachyury enhancers have qualitatively similar expression patterns, they respond to FGF in quantitatively different ways and act together to drive high levels of Brachyury expression with a characteristic input/output relationship. This indicates that they are fundamentally not equivalent genetic elements.

When and where genes are expressed is controlled by regulatory DNA regions known as enhancers. Genes often have multiple enhancers that control expression in different cell types or embryonic regions, but there are also genes that have multiple enhancers that control near-identical expression patterns. These ‘shadow’ enhancers are common features of many animal genomes, but it is unclear to what extent they are truly identical in function. Here we studied a pair of shadow enhancers for the notochord-specific gene Brachyury in the simple model chordate Ciona that are both directly activated by the same signaling pathway. We titrated the activity of this pathway with graded doses of a pharmacological inhibitor and measured the effects in quantitative enhancer assays. We found that the two enhancers had significant differences in sensitivity and cooperativity to the same shared regulator and are thus not identical in function when assessed quantitatively. We also identified subtle differences in sensitivity to this upstream signal between different notochord precursor cells.

Ciona embryonic tail bending is driven by asymmetrical notochord contractility and coordinated by epithelial proliferation

Ventral bending of the embryonic tail within the chorion is an evolutionarily conserved morphogenetic event in both invertebrates and vertebrates. However, the complexity of the anatomical structure of vertebrate embryos makes it difficult to experimentally identify the mechanisms underlying embryonic folding. This study investigated the mechanisms underlying embryonic tail bending in chordates. To further understand the mechanical role of each tissue, we also developed a physical model with experimentally measured parameters to simulate embryonic tail bending. Actomyosin asymmetrically accumulated at the ventral side of the notochord, and cell proliferation of the dorsal tail epidermis was faster than that in the ventral counterpart during embryonic tail bending. Genetic disruption of actomyosin activity and inhibition of cell proliferation dorsally caused abnormal tail bending, indicating that both asymmetrical actomyosin contractility in the notochord and the discrepancy of epidermis cell proliferation are required for tail bending. In addition, asymmetrical notochord contractility was sufficient to drive embryonic tail bending, whereas differential epidermis proliferation was a passive response to mechanical forces. These findings showed that asymmetrical notochord contractility coordinates with differential epidermis proliferation mechanisms to drive embryonic tail bending.

This article has an associated ‘The people behind the papers’ interview.

Using migrating cells as probes to illuminate features in live embryonic tissues

The biophysical and biochemical properties of live tissues are important in the context of development and disease. Methods for evaluating these properties typically involve destroying the tissue or require specialized technology and complicated analyses. Here, we present a novel, noninvasive methodology for determining the spatial distribution of tissue features within embryos, making use of nondirectionally migrating cells and software we termed "Landscape," which performs automatized high-throughput three-dimensional image registration. Using the live migrating cells as bioprobes, we identified structures within the zebrafish embryo that affect the distribution of the cells and studied one such structure constituting a physical barrier, which, in turn, influences amoeboid cell polarity. Overall, this work provides a unique approach for detecting tissue properties without interfering with animal's development. In addition, Landscape allows for integrating data from multiple samples, providing detailed and reliable quantitative evaluation of variable biological phenotypes in different organisms.

Ontogenesis of the tear drainage system requires Prickle1-driven polarized basement membrane deposition

In terrestrial animals, the lacrimal drainage apparatus evolved to serve as conduits for tear flow; however, little is known about the ontogenesis of this system. Here, we define the anatomy of the fully formed tear duct in mice, characterize crucial morphogenetic events for the development of tear duct components and identify the site for primordial tear duct (PTD) initiation. We report that the PTD originates from the orbital lacrimal lamina, a junction formed by the epithelia of the maxillary and lateral nasal processes. We demonstrate that Prickle1, a key component of planar cell polarity signaling, is expressed in progenitors of the PTD and throughout tear duct morphogenesis. Disruption of Prickle1 stalls tear duct elongation; in particular, the loss of basement membrane deposition and aberrant cytoplasmic accumulation of laminin are salient. Altered cell adhesion, cytoskeletal transport systems, vesicular transport systems and cell axis orientation in Prickle1 mutants support the role of Prickle1 in planar cell polarity. Taken together, our results highlight a crucial role of Prickle1-mediated polarized basement membrane secretion and deposition in PTD elongation.

Polarity Establishment and Maintenance in Ascidian Notochord

Cell and tissue polarity due to the extracellular signaling and intracellular gene cascades, in turn, signals the directed cell behaviors and asymmetric tissue architectures that play a crucial role in organogenesis and embryogenesis. The notochord is a characteristic midline organ in chordate embryos that supports the body structure and produces positioning signaling. This review summarizes cellular and tissue-level polarities during notochord development in ascidians. At the early stage, planar cell polarity (PCP) is initialized, which drives cell convergence extension and migration to form a rod-like structure. Subsequently, the notochord undergoes a mesenchymal-epithelial transition, becoming an unusual epithelium in which cells have two opposing apical domains facing the extracellular lumen deposited between adjacent notochord cells controlled by apical-basal (AB) polarity. Cytoskeleton distribution is one of the main downstream events of cell polarity. Some cytoskeleton polarity patterns are a consequence of PCP: however, an additional polarized cytoskeleton, together with Rho signaling, might serve as a guide for correct AB polarity initiation in the notochord. In addition, the notochord's mechanical properties are associated with polarity establishment and transformation, which bridge signaling regulation and tissue mechanical properties that enable the coordinated organogenesis during embryo development.

Morphogenetic mechanisms forming the notochord rod: The turgor pressure-sheath strength model

The notochord is a defining feature of chordates. During notochord formation in vertebrates and tunicates, notochord cells display dynamic morphogenetic movement, called convergent extension, in which cells intercalate and align at the dorsal midline. However, in cephalochordates, the most basal group of chordates, the notochord is formed without convergent extension. It is simply developed from mesodermal cells at the dorsal midline. This suggests that convergent extension movement of notochord cells is a secondarily acquired developmental attribute in the common ancestor of olfactores (vertebrates + tunicates), and that the chordate ancestor innovated the notochord upon a foundation of morphogenetic mechanisms independent of cell movement. Therefore, this review focuses on biological features specific to notochord cells, which have been well studied using clawed frogs, zebrafish, and tunicates. Attributes of notochord cells, such as vacuolation, membrane trafficking, extracellular matrix formation, and apoptosis, can be understood in terms of two properties: turgor pressure of vacuoles and strength of the notochord sheath. To maintain the straight rod-like structure of the notochord, these parameters must be counterbalanced. In the future, the turgor pressure-sheath strength model, proposed in this review, will be examined in light of quantitative molecular data and mathematical simulations, illuminating the evolutionary origin of the notochord.

The notochord gene regulatory network in chordate evolution: Conservation and divergence from Ciona to vertebrates

The notochord is a structure required for support and patterning of all chordate embryos, from sea squirts to humans. An increasing amount of information on notochord development and on the molecular strategies that ensure its proper morphogenesis has been gleaned through studies in the sea squirt Ciona. This invertebrate chordate offers a fortunate combination of experimental advantages, ranging from translucent, fast-developing embryos to a compact genome and impressive biomolecular resources. These assets have enabled the rapid identification of numerous notochord genes and cis-regulatory regions, and provide a rather unique opportunity to reconstruct the gene regulatory network that controls the formation of this developmental and evolutionary chordate landmark. This chapter summarizes the morphogenetic milestones that punctuate notochord formation in Ciona, their molecular effectors, and the current knowledge of the gene regulatory network that ensures the accurate spatial and temporal orchestration of these processes.

Ascidian notochord elongation

The elongation of embryo and tissue is a key morphogenetic event in embryogenesis and organogenesis. Notochord, a typical chordate organ, undergoes elongation to perform its regulatory roles and to form the structural support in the embryo. Notochord elongation is morphologically similar across all chordates, but ascidian has evolved distinct molecular and cellular processes. Here, we summarize the current understanding of ascidian notochord elongation. We divide the process into three phases and discuss the underlying molecular mechanisms in each phase. In the first phase, the notochord converges and extends through invagination and mediolateral intercalation, and partially elongates to form a single diameter cell column along the anterior-posterior axis. In the second phase, a cytokinesis-like actomyosin ring is constructed at the equator of each cell and drives notochord to elongate approximately two-fold. The molecular composition and architecture of the ascidian notochord contractile ring are similar to that of the cytokinetic ring. However, the notochord contractile ring does not impose cell division but only drives cell elongation followed by disassembly. We discuss the self-organizing property of the circumferential actomyosin ring, and why it disassembles when certain notochord length is achieved. The similar ring structures are also present in the elongation process of other organs in evolutionarily divergent animals such as Drosophila and C. elegans. We hereby propose that actomyosin ring-based circumferential contraction is a common mechanism adopted in diverse systems to drive embryo and tissue elongation. In the third phase, the notochord experiences tubulogenesis and the endothelial-like cells crawl bi-directionally on the notochord sheath to further lengthen the notochord. In this review, we also discuss extracellular matrix proteins, notochord sheath, and surrounding tissues that may contribute to notochord integrity and morphogenesis.

Dynamic interplay of cell fate, polarity and force generation in ascidian embryos

A fundamental challenge in developmental biology is to understand how forces produced by individual cells are patterned in space and time and then integrated to produce stereotyped changes in tissue-level or embryo-level morphology. Ascidians offer a unique opportunity to address this challenge by studying how small groups of cells collectively execute complex, but highly stereotyped morphogenetic movements. Here we highlight recent progress and open questions in the study of ascidian morphogenesis, emphasizing the dynamic interplay of cell fate determination, cellular force generation and tissue-level mechanics.

Gon4l regulates notochord boundary formation and cell polarity underlying axis extension by repressing adhesion genes

Anteroposterior (AP) axis extension during gastrulation requires embryonic patterning and morphogenesis to be spatiotemporally coordinated, but the underlying genetic mechanisms remain poorly understood. Here we define a role for the conserved chromatin factor Gon4l, encoded by ugly duckling (udu), in coordinating tissue patterning and axis extension during zebrafish gastrulation through direct positive and negative regulation of gene expression. Although identified as a recessive enhancer of impaired axis extension in planar cell polarity (PCP) mutants, udu functions in a genetically independent, partially overlapping fashion with PCP signaling to regulate mediolateral cell polarity underlying axis extension in part by promoting notochord boundary formation. Gon4l limits expression of the cell–cell and cell–matrix adhesion molecules EpCAM and Integrinα3b, excesses of which perturb the notochord boundary via tension-dependent and -independent mechanisms, respectively. By promoting formation of this AP-aligned boundary and associated cell polarity, Gon4l cooperates with PCP signaling to coordinate morphogenesis along the AP embryonic axis.

Anteroposterior axis extension during gastrulation is dynamically coordinated, but how this is regulated at a molecular level is unclear. Here, the authors show in zebrafish that the chromatin factor Gon4l, encoded by ugly duckling, coordinates axis extension by modulating EpCAM and Integrinα3b expression.

Functional and evolutionary insights from the Ciona notochord transcriptome

The notochord of the ascidian Ciona consists of only 40 cells, and is a longstanding model for studying organogenesis in a small, simple embryo. Here, we perform RNAseq on flow-sorted notochord cells from multiple stages to define a comprehensive Ciona notochord transcriptome. We identify 1364 genes with enriched expression and extensively validate the results by in situ hybridization. These genes are highly enriched for Gene Ontology terms related to the extracellular matrix, cell adhesion and cytoskeleton. Orthologs of 112 of the Ciona notochord genes have known notochord expression in vertebrates, more than twice as many as predicted by chance alone. This set of putative effector genes with notochord expression conserved from tunicates to vertebrates will be invaluable for testing hypotheses about notochord evolution. The full set of Ciona notochord genes provides a foundation for systems-level studies of notochord gene regulation and morphogenesis. We find only modest overlap between this set of notochord-enriched transcripts and the genes upregulated by ectopic expression of the key notochord transcription factor Brachyury, indicating that Brachyury is not a notochord master regulator gene as strictly defined.

Architectural delineation and molecular identification of extracellular matrix in ascidian embryos and larvae

The extracellular matrix (ECM) not only provides essential physical scaffolding for cellular constituents but also initiates crucial biochemical and biomechanical cues that are required for tissue morphogenesis. In this study, we utilized wheat germ agglutinin (WGA) staining to characterize the ECM architecture in ascidian embryos and larvae. The results showed three distinct populations of ECM presenting in Ciona embryogenesis: the outer layer localized at the surface of embryo, an inner layer of notochord sheath and the apical ECM secreted by the notochord. To further elucidate the precise structure of Ciona embryonic ECM, we employed scanning and transmission electron microscopy, and found that the outer membrane was relatively thick with short fibres, whereas the ECM layer in notochord sheath was not as thick as the outer membrane but more regular arranged; the lumen between notochord cells was hydrostatic and sticky. Then, we used the RNA sequencing data from the embryos and larvae of Ciona savignyi to identify ECM genes and acquire their expression patterns. We identified 115 unigenes as 67 ECM genes, and 77 unigenes showed dynamic expression changes between different stages. Our results reveal the architecture, molecular composition and dynamic expression profile of ECM in ascidian embryogenesis, and may increase understanding of the function of the ECM in chordate development.

Summary: This study reveals the architecture, molecular composition and dynamic expression profile of the extracellular matrix in ascidian embryos and larvae, providing clues for its function in chordate development.

Role of the ECM in notochord formation, function and disease

The notochord is a midline structure common to all chordate animals; it provides mechanical and signaling cues for the developing embryo. In vertebrates, the notochord plays key functions during embryogenesis, being a source of developmental signals that pattern the surrounding tissues. It is composed of a core of vacuolated cells surrounded by an epithelial-like sheath of cells that secrete a thick peri-notochordal basement membrane made of different extracellular matrix (ECM) proteins. The correct deposition and organization of the ECM is essential for proper notochord morphogenesis and function. Work carried out in the past two decades has allowed researchers to dissect the contribution of different ECM components to this embryonic tissue. Here, we will provide an overview of these genetic and mechanistic studies. In particular, we highlight the specific functions of distinct matrix molecules in regulating notochord development and notochord-derived signals. Moreover, we also discuss the involvement of ECM synthesis and its remodeling in the pathogenesis of chordoma, a malignant bone cancer that originates from remnants of notochord remaining after embryogenesis.

Regulation of gastrulation movements by emergent cell and tissue interactions

It is during gastrulation that the primordial germ layers are specified, embryonic axes become morphologically manifest, and the embryonic body plan begins to take shape. As morphogenetic movements push and pull nascent tissues into position within the gastrula, new interactions are established between neighboring cells and tissues. These interactions represent an emergent property within gastrulating embryos, and serve to regulate and promote ensuing morphogenesis that establishes the next set of cell/tissue contacts, and so on. Several recent studies demonstrate the critical roles of such interactions during gastrulation, including those between germ layers, along embryonic axes, and at tissue boundaries. Emergent tissue interactions result from - and result in - morphogen signaling, cell contacts, and mechanical forces within the gastrula. Together, these comprise a dynamic and complex regulatory cascade that drives gastrulation morphogenesis.

Fibronectin contributes to notochord intercalation in the invertebrate chordate, Ciona intestinalis

BACKGROUND

Genomic analysis has upended chordate phylogeny, placing the tunicates as the sister group to the vertebrates. This taxonomic rearrangement raises questions about the emergence of a tunicate/vertebrate ancestor.

RESULTS

Characterization of developmental genes uniquely shared by tunicates and vertebrates is one promising approach for deciphering developmental shifts underlying acquisition of novel, ancestral traits. The matrix glycoprotein Fibronectin (FN) has long been considered a vertebrate-specific gene, playing a major instructive role in vertebrate embryonic development. However, the recent computational prediction of an orthologous "vertebrate-like" Fn gene in the genome of a tunicate, Ciona savignyi, challenges this viewpoint suggesting that Fn may have arisen in the shared tunicate/vertebrate ancestor. Here we verify the presence of a tunicate Fn ortholog. Transgenic reporter analysis was used to characterize a Ciona Fn enhancer driving expression in the notochord. Targeted knockdown in the notochord lineage indicates that FN is required for proper convergent extension.

CONCLUSIONS

These findings suggest that acquisition of Fn was associated with altered notochord morphogenesis in the vertebrate/tunicate ancestor.

Dynamics of cell polarity in tissue morphogenesis: a comparative view from Drosophila and Ciona

Tissues in developing embryos exhibit complex and dynamic rearrangements that shape forming organs, limbs, and body axes. Directed migration, mediolateral intercalation, lumen formation, and other rearrangements influence the topology and topography of developing tissues. These collective cell behaviors are distinct phenomena but all involve the fine-grained control of cell polarity. Here we review recent findings in the dynamics of polarized cell behavior in both the Drosophila ovarian border cells and the Ciona notochord. These studies reveal the remarkable reorganization of cell polarity during organ formation and underscore conserved mechanisms of developmental cell polarity including the Par/atypical protein kinase C (aPKC) and planar cell polarity pathways. These two very different model systems demonstrate important commonalities but also key differences in how cell polarity is controlled in tissue morphogenesis. Together, these systems raise important, broader questions on how the developmental control of cell polarity contributes to morphogenesis of diverse tissues across the metazoa.

T-type Calcium Channel Regulation of Neural Tube Closure and EphrinA/EPHA Expression

A major class of human birth defects arise from aberrations during neural tube closure (NTC). We report on a NTC signaling pathway requiring T-type calcium channels (TTCCs) that is conserved between primitive chordates (Ciona) and Xenopus. With loss of TTCCs, there is a failure to seal the anterior neural folds. Accompanying loss of TTCCs is an upregulation of EphrinA effectors. Ephrin signaling is known to be important in NTC, and ephrins can affect both cell adhesion and repulsion. In Ciona, ephrinA-d expression is downregulated at the end of neurulation, whereas, with loss of TTCC, ephrinA-d remains elevated. Accordingly, overexpression of ephrinA-d phenocopied TTCC loss of function, while overexpression of a dominant-negative Ephrin receptor was able to rescue NTC in a Ciona TTCC mutant. We hypothesize that signaling through TTCCs is necessary for proper anterior NTC through downregulation of ephrins, and possibly elimination of a repulsive signal.

Reciprocal and dynamic polarization of planar cell polarity core components and myosin

The Ciona notochord displays planar cell polarity (PCP), with anterior localization of Prickle (Pk) and Strabismus (Stbm). We report that a myosin is polarized anteriorly in these cells and strongly colocalizes with Stbm. Disruption of the actin/myosin machinery with cytochalasin or blebbistatin disrupts polarization of Pk and Stbm, but not of myosin complexes, suggesting a PCP-independent aspect of myosin localization. Wash out of cytochalasin restored Pk polarization, but not if done in the presence of blebbistatin, suggesting an active role for myosin in core PCP protein localization. On the other hand, in the pk mutant line, aimless, myosin polarization is disrupted in approximately one third of the cells, indicating a reciprocal action of core PCP signaling on myosin localization. Our results indicate a complex relationship between the actomyosin cytoskeleton and core PCP components in which myosin is not simply a downstream target of PCP signaling, but also required for PCP protein localization.

DOI:http://dx.doi.org/10.7554/eLife.05361.001

Animal cells that form flat layers of a tissue, such as the skin or the lining of internal cavities, are often orientated in the same direction. The same is true for structures such as hairs or feathers, which are attached to the skin. This phenomenon is known as ‘planar cell polarity’ (or ‘PCP’ for short).

Many different organisms use similar mechanisms to establish this kind of tissue pattern. The best-studied mechanism involves the so-called ‘core PCP pathway’. Signaling proteins in this pathway coordinate the polarity of neighboring cells. Other ‘global signaling pathways’ are thought to first ensure that tissues are correctly orientated within the embryo as a whole, and to do this, the global pathways are thought to align a network of filament-like structures within the cells in a particular direction. Once correctly orientated, these filaments—known as microtubules—have been proposed to help position the components of the core PCP pathway such that they can correctly orientate the rest of the cell.

Now Newman-Smith, Kourakis et al. have identified another network of filaments within cells that interacts with components of the core PCP pathway in a sea squirt called Ciona savignyi. This organism begins life as a tadpole-like larva that has a flexible rod-shaped structure, called a ‘notochord’, running along the length of its body. The cells of the notochord become polarized as they develop. When microtubules are disrupted, their planar polarity remains unaffected. However, when another network of filaments—called the actomyosin network––is chemically disrupted, the polarity of certain core PCP components is lost.

The findings of Newman-Smith, Kourakis et al. reveal that the core PCP components and the actomyosin network in this sea squirt reinforce each other's polarity. This represents an alternative to the previously described models of planar polarity in which the core PCP components are thought to drive the polarization of the actomyosin network. Whether this model extends to planar cell polarity mechanisms in other organisms, such humans and other animals with backbones, remains a question for future work.

DOI:http://dx.doi.org/10.7554/eLife.05361.002

Quantitative and in toto imaging in ascidians: working toward an image-centric systems biology of chordate morphogenesis

Developmental biology relies heavily on microscopy to image the finely controlled cell behaviors that drive embryonic development. Most embryos are large enough that a field of view with the resolution and magnification needed to resolve single cells will not span more than a small region of the embryo. Ascidian embryos, however, are sufficiently small that they can be imaged in toto with fine subcellular detail using conventional microscopes and objectives. Unlike other model organisms with particularly small embryos, ascidians have a chordate embryonic body plan that includes a notochord, hollow dorsal neural tube, heart primordium and numerous other anatomical details conserved with the vertebrates. Here we compare the size and anatomy of ascidian embryos with those of more traditional model organisms, and relate these features to the capabilities of both conventional and exotic imaging methods. We review the emergence of Ciona and related ascidian species as model organisms for a new era of image-based developmental systems biology. We conclude by discussing some important challenges in ascidian imaging and image analysis that remain to be solved.

Chondroitin 4-O-sulfotransferases are required for cell adhesion and morphogenesis in the Ciona intestinalis embryo

Chondroitin sulfate (CS) is a carbohydrate component of proteoglycans. Several types of sulfotransferases determine the pattern of CS sulfation, and thus regulate the biological functions of proteoglycans. The protochordate ascidians are the closest relatives of vertebrates, but the functions of their sulfotransferases have not been investigated. Here, we show that two chondroitin 4-O-sulfotransferases (C4STs) play important roles in the embryonic morphogenesis of the ascidian Ciona intestinalis. Ci-C4ST-like1 is predominantly expressed in the epidermis and muscle. Epidermal and muscle cells became spherical upon the injection of a Ci-C4ST-like1-specific morpholino oligo (MO), thus suggesting weakened cell adhesion. Co-injection of a Ci-C4ST-like1-expressing transgene rescued the phenotype, suggesting that the effects of the MO were specific. Ci-C4ST-like3 was expressed in the central nervous system, muscle, and mesenchyme. A specific MO appeared to affect cell adhesion in the epidermis and muscle. Convergent extension movement of notochordal cells was also impaired. Forced expression of Ci-C4ST-like3 restored normal morphogenesis, suggesting that the effects of the MO were specific. The present study suggests that Ci-C4ST-like1 and Ci-C4ST-like3 are required for cell adhesion mainly in the epidermis and muscle.

Chondroitin 6-O-sulfotransferases are required for morphogenesis of the notochord in the ascidian embryo

BACKGROUND

Chondroitin sulfate (CS) is a sulfated polysaccharide chain that binds to various core proteins to form proteoglycans. The amount and position of sulfate groups in CS are variable among different tissues, and are determined by specific sulfotransferases. Although the ascidians are the closest relatives of vertebrates, the functions of their sulfotransferases have not been studied.

RESULTS

The genome of the ascidian Ciona intestinalis contains eight genes encoding proteins similar to chondroitin 6-O-sulfotransferases (C6STs), which appear to have independently diverged in the ascidian lineage during evolution. Among them, Ci-C6ST-like1 and Ci-C6ST-like7 were predominantly expressed in the developing notochord. In addition, they were weakly expressed in the neural tube. The disruption of either one of them affected the convergent extension movement of notochordal cells. Presumptive notochord cells coming from both sides of the embryo did not intercalate. The results suggest that both of them are necessary. In some cases, the anterior neural tube failed to close. Forced expression of Ci-C6ST-like1 or Ci-C6ST-like7 in the notochord restored the normal intercalation of notochordal cells, indicating that the effects of morpholino oligos are specific.

CONCLUSIONS

Ci-C6ST-like1 and Ci-C6ST-like7 are required for the morphogenesis of the notochord in the ascidian embryo.

Mutation studies in ascidians: a review

Historically, mutations have had a significant impact on the study of developmental processes and phenotypic evolution. Lesions in DNA are created by artificial methods or detected by natural genetic variation. Random mutations are then ascribed to genetic change by direct sequencing or positional cloning. Tunicate species of the ascidian genus Ciona represent nearly fully realized model systems in which gene function can be investigated in depth. Additionally, tunicates are valuable organisms for the study of naturally occurring mutations due to the capability to exploit genetic variation down to the molecular level. Here, we summarize the available information about how mutations are studied in ascidians with examples of insights that have resulted from these applications. We also describe notions and methodologies that might be useful for the implementation of easy and tight procedures for mutations studies in Ciona.

Stochasticity and stereotypy in the Ciona notochord

Fate mapping with single cell resolution has typically been confined to embryos with completely stereotyped development. The lineages giving rise to the 40 cells of the Ciona notochord are invariant, but the intercalation of those cells into a single-file column is not. Here we use genetic labeling methods to fate map the Ciona notochord with both high resolution and large sample sizes. We find that the ordering of notochord cells into a single column is not random, but instead shows a distinctive signature characteristic of mediolaterally-biased intercalation. We find that patterns of cell intercalation in the notochord are somewhat stochastic but far more stereotyped than previously believed. Cell behaviors vary by lineage, with the secondary notochord lineage being much more constrained than the primary lineage. Within the primary lineage, patterns of intercalation reflect the geometry of the intercalating tissue. We identify the latest point at which notochord morphogenesis is largely stereotyped, which is shortly before the onset of mediolateral intercalation and immediately after the final cell divisions in the primary lineage. These divisions are consistently oriented along the AP axis. Our results indicate that the interplay between stereotyped and stochastic cell behaviors in morphogenesis can only be assessed by fate mapping experiments that have both cellular resolution and large sample sizes.

An adhesome comprising laminin, dystroglycan and myosin IIA is required during notochord development in Xenopus laevis

Dystroglycan (Dg) is a transmembrane receptor for laminin that must be expressed at the right time and place in order to be involved in notochord morphogenesis. The function of Dg was examined in Xenopus laevis embryos by knockdown of Dg and overexpression and replacement of the endogenous Dg with a mutated form of the protein. This analysis revealed that Dg is required for correct laminin assembly, for cell polarization during mediolateral intercalation and for proper differentiation of vacuoles. Using mutations in the cytoplasmic domain, we identified two sites that are involved in cell polarization and are required for mediolateral cell intercalation, and a site that is required for vacuolation. Furthermore, using a proteomic analysis, the cytoskeletal non-muscle myosin IIA has been identified for the first time as a molecular link between the Dg-cytoplasmic domain and cortical actin. The data allowed us to identify the adhesome laminin-Dg-myosin IIA as being required to maintain the cortical actin cytoskeleton network during vacuolation, which is crucial to maintain the shape of notochordal cells.

Exploiting the extraordinary genetic polymorphism of ciona for developmental genetics with whole genome sequencing

Studies in tunicates such as Ciona have revealed new insights into the evolutionary origins of chordate development. Ciona populations are characterized by high levels of natural genetic variation, between 1 and 5%. This variation has provided abundant material for forward genetic studies. In the current study, we make use of deep sequencing and homozygosity mapping to map spontaneous mutations in outbred populations. With this method we have mapped two spontaneous developmental mutants. In Ciona intestinalis we mapped a short-tail mutation with strong phenotypic similarity to a previously identified mutant in the related species Ciona savignyi. Our bioinformatic approach mapped the mutation to a narrow interval containing a single mutated gene, α-laminin3,4,5, which is the gene previously implicated in C. savignyi. In addition, we mapped a novel genetic mutation disrupting neural tube closure in C. savignyi to a T-type Ca(2+) channel gene. The high efficiency and unprecedented mapping resolution of our study is a powerful advantage for developmental genetics in Ciona, and may find application in other outbred species.

Cell intercalation from top to bottom

Animal development requires a carefully orchestrated cascade of cell fate specification events and cellular movements. A surprisingly small number of choreographed cellular behaviours are used repeatedly to shape the animal body plan. Among these, cell intercalation lengthens or spreads a tissue at the expense of narrowing along an orthogonal axis. Key steps in the polarization of both mediolaterally and radially intercalating cells have now been clarified. In these different contexts, intercalation seems to require a distinct combination of mechanisms, including adhesive changes that allow cells to rearrange, cytoskeletal events through which cells exert the forces needed for cell neighbour exchange, and in some cases the regulation of these processes through planar cell polarity.

Tbx2/3 is an essential mediator within the Brachyury gene network during Ciona notochord development

T-box genes are potent regulators of mesoderm development in many metazoans. In chordate embryos, the T-box transcription factor Brachyury (Bra) is required for specification and differentiation of the notochord. In some chordates, including the ascidian Ciona, members of the Tbx2 subfamily of T-box genes are also expressed in this tissue; however, their regulatory relationships with Bra and their contributions to the development of the notochord remain uncharacterized. We determined that the notochord expression of Ciona Tbx2/3 (Ci-Tbx2/3) requires Ci-Bra, and identified a Ci-Tbx2/3 notochord CRM that necessitates multiple Ci-Bra binding sites for its activity. Expression of mutant forms of Ci-Tbx2/3 in the developing notochord revealed a role for this transcription factor primarily in convergent extension. Through microarray screens, we uncovered numerous Ci-Tbx2/3 targets, some of which overlap with known Ci-Bra-downstream notochord genes. Among the Ci-Tbx2/3 notochord targets are evolutionarily conserved genes, including caspases, lineage-specific genes, such as Noto4, and newly identified genes, such as MLKL. This work sheds light on a large section of the notochord regulatory circuitry controlled by T-box factors, and reveals new components of the complement of genes required for the proper formation of this structure.

A linear program formulation for the segmentation of Ciona membrane volumes

We address the problem of cell segmentation in confocal microscopy membrane volumes of the ascidian Ciona used in the study of morphogenesis. The primary challenges are non-uniform and patchy membrane staining and faint spurious boundaries from other organelles (e.g. nuclei). Traditional segmentation methods incorrectly attach to faint boundaries producing spurious edges. To address this problem, we propose a linear optimization framework for the joint correction of multiple over-segmentations obtained from different methods. The main idea motivating this approach is that multiple over-segmentations, resulting from a pool of methods with various parameters, are likely to agree on the correct segment boundaries, while spurious boundaries are methodor parameter-dependent. The challenge is to make an optimized decision on selecting the correct boundaries while discarding the spurious ones. The proposed unsupervised method achieves better performance than state of the art methods for cell segmentation from membrane images.

A transiently expressed connexin is essential for anterior neural plate development in Ciona intestinalis

A forward genetic screen in the ascidian Ciona intestinalis identified a mutant line (frimousse) with a profound disruption in neural plate development. In embryos with the frimousse mutation, the anteriormost neural plate cells, which are products of an FGF induction at the blastula and gastrula stages, initially express neural plate-specific genes but fail to maintain the induced state and ultimately default to epidermis. The genetic lesion in the frimousse mutant lies within a connexin gene (cx-11) that is transiently expressed in the developing neural plate in a temporal window corresponding to the period of a-lineage neural induction. Using a genetically encoded calcium indicator we observed multiple calcium transients throughout the developing neural plate in wild-type embryos, but not in mutant embryos. A series of treatments at the gastrula and neurula stages that block the calcium transients, including gap junction inhibition and calcium depletion, were also found to disrupt the development of the anterior neural plate in a similar way to the frimousse mutation. The requirement for cx-11 for anterior neural fate points to a crucial role for intercellular communication via gap junctions, probably through mediation of Ca(2+) transients, in Ciona intestinalis neural induction.

Whole-organ cell shape analysis reveals the developmental basis of ascidian notochord taper

Here we use in toto imaging together with computational segmentation and analysis methods to quantify the shape of every cell at multiple stages in the development of a simple organ: the notochord of the ascidian Ciona savignyi. We find that cell shape in the intercalated notochord depends strongly on anterior-posterior (AP) position, with cells in the middle of the notochord consistently wider than cells at the anterior or posterior. This morphological feature of having a tapered notochord is present in many chordates. We find that ascidian notochord taper involves three main mechanisms: Planar Cell Polarity (PCP) pathway-independent sibling cell volume asymmetries that precede notochord cell intercalation; the developmental timing of intercalation, which proceeds from the anterior and posterior towards the middle; and the differential rates of notochord cell narrowing after intercalation. A quantitative model shows how the morphology of an entire developing organ can be controlled by this small set of cellular mechanisms.

Genetic and genomic toolbox of the chordate Ciona intestinalis

The experimental malleability and unique phylogenetic position of the sea squirt Ciona intestinalis as part of the sister group to the vertebrates have helped establish these marine chordates as model organisms for the study of developmental genetics and evolution. Here we summarize the tools, techniques, and resources available to the Ciona geneticist, citing examples of studies that employed such strategies in the elucidation of gene function in Ciona. Genetic screens, germline transgenesis, electroporation of plasmid DNA, and microinjection of morpholinos are all routinely employed, and in the near future we expect these to be complemented by targeted mutagenesis, homologous recombination, and RNAi. The genomic resources available will continue to support the design and interpretation of genetic experiments and allow for increasingly sophisticated approaches on a high-throughput, whole-genome scale.

Regulation of retinal progenitor expansion by Frizzled receptors: implications for microphthalmia and retinal coloboma

Nineteen Wnt ligands and 10 Frizzled (Fz) receptors mediate multiple distinct cellular events during neuronal development. However, their precise roles in cell-type specification and organogenesis are poorly delineated because of overlapping functions and expression profiles. Here, we have explored the role of two closely related Frizzled receptors, Fz5 and Fz8, in mouse retinal development. We previously showed that Fz5(-/-) mice exhibit mild coloboma and microphthalmia at ~50% penetrance. Fz8 expression overlaps with Fz5 in the neural retina and optic fissure/disc. Mice lacking Fz8 show minimal eye and retinal defects. The embryos lacking both Fz5 and Fz8 die early in development, but a majority of triallelic Fz5(-/-);Fz8(+/-) mutants survive until birth. The triallelic mutant develops severe retinal coloboma and microphthalmia with full penetrance. At the cellular level, impaired neurogenesis is indicated by increased early-born retinal neurons that result from accelerated cell cycle exit of progenitors. Deficiency of apical retinal neuroepithelium is indicated by altered localization of apical junction markers, such as atypical protein kinase C, RhoA and β-catenin. Hes1 expression, which is critical for retinal progenitor expansion, is down-regulated in the triallelic mutant mouse. Furthermore, blocking Frizzled receptors in cultured retinal explants led to basally shifted divisions of retinal progenitors. Together, our studies suggest a dose-dependent regulation of signaling by Fz5 and Fz8 in optic fissure/disc formation and progenitor expansion.

The identification of transcription factors expressed in the notochord of Ciona intestinalis adds new potential players to the brachyury gene regulatory network

The notochord is the distinctive characteristic of chordates; however, the knowledge of the complement of transcription factors governing the development of this structure is still incomplete. Here we present the expression patterns of seven transcription factor genes detected in the notochord of the ascidian Ciona intestinalis at various stages of embryonic development. Four of these transcription factors, Fos-a, NFAT5, AFF and Klf15, have not been directly associated with the notochord in previous studies, while the others, including Spalt-like-a, Lmx-like, and STAT5/6-b, display evolutionarily conserved expression in this structure as well as in other domains. We examined the hierarchical relationships between these genes and the transcription factor Brachyury, which is necessary for notochord development in all chordates. We found that Ciona Brachyury regulates the expression of most, although not all, of these genes. These results shed light on the genetic regulatory program underlying notochord formation in Ciona and possibly other chordates.

Evolutionary changes in the notochord genetic toolkit: a comparative analysis of notochord genes in the ascidian Ciona and the larvacean Oikopleura

Background

The notochord is a defining feature of the chordate clade, and invertebrate chordates, such as tunicates, are uniquely suited for studies of this structure. Here we used a well-characterized set of 50 notochord genes known to be targets of the notochord-specific Brachyury transcription factor in one tunicate, Ciona intestinalis (Class Ascidiacea), to begin determining whether the same genetic toolkit is employed to build the notochord in another tunicate, Oikopleura dioica (Class Larvacea). We identified Oikopleura orthologs of the Ciona notochord genes, as well as lineage-specific duplicates for which we determined the phylogenetic relationships with related genes from other chordates, and we analyzed their expression patterns in Oikopleura embryos.

Results

Of the 50 Ciona notochord genes that were used as a reference, only 26 had clearly identifiable orthologs in Oikopleura. Two of these conserved genes appeared to have undergone Oikopleura- and/or tunicate-specific duplications, and one was present in three copies in Oikopleura, thus bringing the number of genes to test to 30. We were able to clone and test 28 of these genes. Thirteen of the 28 Oikopleura orthologs of Ciona notochord genes showed clear expression in all or in part of the Oikopleura notochord, seven were diffusely expressed throughout the tail, six were expressed in tissues other than the notochord, while two probes did not provide a detectable signal at any of the stages analyzed. One of the notochord genes identified, Oikopleura netrin, was found to be unevenly expressed in notochord cells, in a pattern reminiscent of that previously observed for one of the Oikopleura Hox genes.

Conclusions

A surprisingly high number of Ciona notochord genes do not have apparent counterparts in Oikopleura, and only a fraction of the evolutionarily conserved genes show clear notochord expression. This suggests that Ciona and Oikopleura, despite the morphological similarities of their notochords, have developed rather divergent sets of notochord genes after their split from a common tunicate ancestor. This study demonstrates that comparisons between divergent tunicates can lead to insights into the basic complement of genes sufficient for notochord development, and elucidate the constraints that control its composition.

A single GATA factor plays discrete, lineage specific roles in ascidian heart development

GATA family transcription factors are core components of the vertebrate heart gene network. GATA factors also contribute to heart formation indirectly through regulation of endoderm morphogenesis. However, the precise impact of GATA factors on vertebrate cardiogenesis is masked by functional redundancy within multiple lineages. Early heart specification in the invertebrate chordate Ciona intestinalis is similar to that of vertebrates but only one GATA factor, Ci-GATAa, is expressed in the heart progenitor cells and adjacent endoderm. Here we delineate precise, tissue specific contributions of GATAa to heart formation. Targeted repression of GATAa activity in the heart progenitors perturbs their transcriptional identity. Targeted repression of endodermal GATAa function disrupts endoderm morphogenesis. Subsequently, the bilateral heart progenitors fail to fuse at the ventral midline. The resulting phenotype is strikingly similar to cardia bifida, as observed in vertebrate embryos when endoderm morphogenesis is disturbed. These findings indicate that GATAa recapitulates cell-autonomous and non-cell-autonomous roles performed by multiple, redundant GATA factors in vertebrate cardiogenesis.

Ciona genetics

Ascidians, such as Ciona, are invertebrate chordates with simple embryonic body plans and small, relatively non-redundant genomes. Ciona genetics is in its infancy compared to many other model systems, but it provides a powerful method for studying this important vertebrate outgroup. Here we give basic methods for genetic analysis of Ciona, including protocols for controlled crosses both by natural spawning and by the surgical isolation of gametes; the identification and propagation of mutant lines; and strategies for positional cloning.

Eph regulates dorsoventral asymmetry of the notochord plate and convergent extension-mediated notochord formation

BACKGROUND

The notochord is a signaling center required for the patterning of the vertebrate embryonic midline, however, the molecular and cellular mechanisms involved in the formation of this essential embryonic tissue remain unclear. The urochordate Ciona intestinalis develops a simple notochord from 40 specific postmitotic mesodermal cells. The precursors intercalate mediolaterally and establish a single array of disk-shaped notochord cells along the midline. However, the role that notochord precursor polarization, particularly along the dorsoventral axis, plays in this morphogenetic process remains poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that the notochord preferentially accumulates an apical cell polarity marker, aPKC, ventrally and a basement membrane marker, laminin, dorsally. This asymmetric accumulation of apicobasal cell polarity markers along the embryonic dorsoventral axis was sustained in notochord precursors during convergence and extension. Further, of several members of the Eph gene family implicated in cellular and tissue morphogenesis, only Ci-Eph4 was predominantly expressed in the notochord throughout cell intercalation. Introduction of a dominant-negative Ci-Eph4 to notochord precursors diminished asymmetric accumulation of apicobasal cell polarity markers, leading to defective intercalation. In contrast, misexpression of a dominant-negative mutant of a planar cell polarity gene Dishevelled preserved asymmetric accumulation of aPKC and laminin in notochord precursors, although their intercalation was incomplete.

CONCLUSIONS/SIGNIFICANCE

Our data support a model in which in ascidian embryos Eph-dependent dorsoventral polarity of notochord precursors plays a crucial role in mediolateral cell intercalation and is required for proper notochord morphogenesis.

Integration of planar cell polarity and ECM signaling in elongation of the vertebrate body plan

The shaping of the vertebrate embryonic body plan depends heavily on the narrowing and lengthening (convergence and extension) of embryonic tissues by cell intercalation, a process by which cells actively crawl between one another along the axis of convergence to produce a narrower, longer array. We discuss recent evidence that the vertebrate non-canonical Wnt/Planar Cell Polarity (PCP) pathway, known to directly function in polarizing the movements of intercalating cells, is also involved in the localized assembly of extracellular matrix (ECM). These cell-ECM interactions, in turn, are necessary for expression of the oriented, polarized cell intercalation. The mechanism of PCP/ECM interactions, their molecular signaling, and their mechanical consequences for morphogenesis are discussed with the goal of identifying important unsolved issues.

doublesex/mab3 related-1 (dmrt1) is essential for development of anterior neural plate derivatives in Ciona

Ascidian larvae have a hollow, dorsal central nervous system that shares many morphological features with vertebrate nervous systems yet is composed of very few cells. We show here that a null mutation in the gene dmrt1 in the ascidian Ciona savignyi results in profound abnormalities in the development of the sensory vesicle (brain), as well as other anterior ectodermal derivatives, including the palps and oral siphon primordium (OSP). Although the phenotype of the mutant embryos is variable, the majority have a complete loss of the most anterior structures (palps and OSP) and extensive disruption of sensory structures, such as the light-sensitive ocellus, in the sensory vesicle. dmrt1 is expressed early in the blastula embryo in a small group of presumptive ectodermal cells as they become restricted to anterior neural, OSP and palp fates. Despite the early and restricted expression of dmrt1, we were unable, using several independent criteria, to observe a defect in the mutant embryos until the early tailbud stage. We speculate that the variability and late onset in the phenotype may be due to partially overlapping activities of other gene products.