Ciona intestinalis notochord as a new model to investigate the cellular and molecular mechanisms of tubulogenesis

The ion channel Anoctamin 10/TMEM16K coordinates organ morphogenesis across scales in the urochordate notochord

During embryonic development, tissues and organs are gradually shaped into their functional morphologies through a series of spatiotemporally tightly orchestrated cell behaviors. A highly conserved organ shape across metazoans is the epithelial tube. Tube morphogenesis is a complex multistep process of carefully choreographed cell behaviors such as convergent extension, cell elongation, and lumen formation. The identity of the signaling molecules that coordinate these intricate morphogenetic steps remains elusive. The notochord is an essential tubular organ present in the embryonic midline region of all members of the chordate phylum. Here, using genome editing, pharmacology and quantitative imaging in the early chordate Ciona intestinalis we show that Ano10/Tmem16k, a member of the evolutionarily ancient family of transmembrane proteins called Anoctamin/TMEM16 is essential for convergent extension, lumen expansion, and connection during notochord morphogenesis. We find that Ano10/Tmem16k works in concert with the plasma membrane (PM) localized Na+/Ca2+ exchanger (NCX) and the endoplasmic reticulum (ER) residing SERCA, RyR, and IP3R proteins to establish developmental stage specific Ca2+ signaling molecular modules that regulate notochord morphogenesis and Ca2+ dynamics. In addition, we find that the highly conserved Ca2+ sensors calmodulin (CaM) and Ca2+/calmodulin-dependent protein kinase (CaMK) show an Ano10/Tmem16k-dependent subcellular localization. Their pharmacological inhibition leads to convergent extension, tubulogenesis defects, and deranged Ca2+ dynamics, suggesting that Ano10/Tmem16k is involved in both the "encoding" and "decoding" of developmental Ca2+ signals. Furthermore, Ano10/Tmem16k mediates cytoskeletal reorganization during notochord morphogenesis, likely by altering the localization of 2 important cytoskeletal regulators, the small GTPase Ras homolog family member A (RhoA) and the actin binding protein Cofilin. Finally, we use electrophysiological recordings and a scramblase assay in tissue culture to demonstrate that Ano10/Tmem16k likely acts as an ion channel but not as a phospholipid scramblase. Our results establish Ano10/Tmem16k as a novel player in the prevertebrate molecular toolkit that controls organ morphogenesis across scales.

Loss of collagen gene expression in the notochord of the tailless tunicate Molgula occulta

In tunicates, several species in the Molgulidae family have convergently lost the tailed, swimming larval body plan, including the morphogenesis of the notochord, a major chordate-defining trait. Through the comparison of tailless M. occulta and a close relative, the tailed species M. oculata, we show that notochord-specific expression of the Collagen Type I/II Alpha (Col1/2a) gene appears to have been lost specifically in the tailless species. Using CRISPR/Cas9-mediated mutagenesis in the tailed laboratory model tunicate Ciona robusta, we demonstrate that Col1/2a plays a crucial role in the convergent extension of notochord cells during tail elongation. Our results suggest that the expression of Col1/2a in the notochord, although necessary for its morphogenesis in tailed species, is dispensable for tailless species. This loss is likely a result of the accumulation of cis-regulatory mutations in the absence of purifying selective pressure. More importantly, the gene itself is not lost, likely due to its roles in other developmental processes, including during the adult stage. Our study further confirms the Molgulidae as an interesting family in which to study the evolutionary loss of tissue-specific expression of indispensable genes.

Molecular mechanisms of tubulogenesis revealed in the sea star hydro-vascular organ

A fundamental goal in the organogenesis field is to understand how cells organize into tubular shapes. Toward this aim, we have established the hydro-vascular organ in the sea star Patiria miniata as a model for tubulogenesis. In this animal, bilateral tubes grow out from the tip of the developing gut, and precisely extend to specific sites in the larva. This growth involves cell migration coupled with mitosis in distinct zones. Cell proliferation requires FGF signaling, whereas the three-dimensional orientation of the organ depends on Wnt signaling. Specification and maintenance of tube cell fate requires Delta/Notch signaling. Moreover, we identify target genes of the FGF pathway that contribute to tube morphology, revealing molecular mechanisms for tube outgrowth. Finally, we report that FGF activates the Six1/2 transcription factor, which serves as an evolutionarily ancient regulator of branching morphogenesis. This study uncovers distinct mechanisms of tubulogenesis in vivo and we propose that cellular dynamics in the sea star hydro-vascular organ represents a key comparison for understanding the evolution of vertebrate organs.

Quantitative Phosphoproteomics Reveals the Requirement of DYRK1-Mediated Phosphorylation of Ion Transport- and Cell Junction-Related Proteins for Notochord Lumenogenesis in Ascidian

The dual-specificity tyrosine phosphorylation-regulated kinase (DYRK1) phosphorylates diverse substrates involved in various cellular processes. Here, we found that blocking the kinase activity of DYRK1 inhibited notochord development and lumenogenesis in ascidian Ciona savignyi. By performing phosphoproteomics in conjunction with notochord-specific proteomics, we identified 1065 notochord-specific phosphoproteins that were present during lumen inflation, of which 428 differentially phosphorylated proteins (DPPs) were identified after inhibition of DYRK1 kinase activity. These DPPs were significantly enriched in metal ion transmembrane transporter activity, protein transport and localization, and tight junction. We next analyzed the downregulated phosphoproteins and focused on those belonging to the solute carrier (SLC), Ras-related protein (RAB), and tight junction protein (TJP) families. In vivo phospho-deficient study showed that alanine mutations on the phosphosites of these proteins resulted in defects of lumenogenesis during Ciona notochord development, demonstrating the crucial roles of phosphorylation of transmembrane transport-, vesicle trafficking-, and tight junction-related proteins in lumen formation. Overall, our study provides a valuable data resource for investigating notochord lumenogenesis and uncovers the molecular mechanisms of DYRK1-mediated notochord development and lumen inflation.

DYRK1-mediated phosphorylation of endocytic components is required for extracellular lumen expansion in ascidian notochord

BACKGROUND

The biological tube is a basal biology structure distributed in all multicellular animals, from worms to humans, and has diverse biological functions. Formation of tubular system is crucial for embryogenesis and adult metabolism. Ascidian Ciona notochord lumen is an excellent in vivo model for tubulogenesis. Exocytosis has been known to be essential for tubular lumen formation and expansion. The roles of endocytosis in tubular lumen expansion remain largely unclear.

RESULTS

In this study, we first identified a dual specificity tyrosine-phosphorylation-regulated kinase 1 (DYRK1), the protein kinase, which was upregulated and required for ascidian notochord extracellular lumen expansion. We demonstrated that DYRK1 interacted with and phosphorylated one of the endocytic components endophilin at Ser263 that was essential for notochord lumen expansion. Moreover, through phosphoproteomic sequencing, we revealed that in addition to endophilin, the phosphorylation of other endocytic components was also regulated by DYRK1. The loss of function of DYRK1 disturbed endocytosis. Then, we demonstrated that clathrin-mediated endocytosis existed and was required for notochord lumen expansion. In the meantime, the results showed that the secretion of notochord cells is vigorous in the apical membrane.

CONCLUSIONS

We found the co-existence of endocytosis and exocytosis activities in apical membrane during lumen formation and expansion in Ciona notochord. A novel signaling pathway is revealed that DYRK1 regulates the endocytosis by phosphorylation that is required for lumen expansion. Our finding thus indicates a dynamic balance between endocytosis and exocytosis is crucial to maintain apical membrane homeostasis that is essential for lumen growth and expansion in tubular organogenesis.

ELMOD3-Rab1A-Flotillin2 cascade regulates lumen formation via vesicle trafficking in Ciona notochord

Lumen development is a crucial phase in tubulogenesis, although its molecular mechanisms are largely unknown. In this study, we discovered an ELMO domain-containing 3 (ELMOD3), which belongs to ADP-ribosylation factor GTPase-activating protein family, was necessary to form the notochord lumen in Ciona larvae. We demonstrated that ELMOD3 interacted with lipid raft protein Flotillin2 and regulated its subcellular localization. The loss-of-function of Flotillin2 prevented notochord lumen formation. Furthermore, we found that ELMOD3 also interacted with Rab1A, which is the regulatory GTPase for vesicle trafficking and located at the notochord cell surface. Rab1A mutations arrested the lumen formation, phenocopying the loss-of-function of ELMOD3 and Flotillin2. Our findings further suggested that Rab1A interactions influenced Flotillin2 localization. We thus identified a unique pathway in which ELMOD3 interacted with Rab1A, which controlled the Flotillin2-mediated vesicle trafficking from cytoplasm to apical membrane, required for Ciona notochord lumen formation.

Nuclear Factor of Activated T Cells-5 Regulates Notochord Lumenogenesis in Chordate Larval Development

Osmoregulation is essential for organisms to adapt to the exterior environment and plays an important role in embryonic organogenesis. Tubular organ formation usually involves a hyperosmotic lumen environment. The mechanisms of how the cells respond and regulate lumen formation remain largely unknown. Here, we reported that the nuclear factor of activated T cells-5 (NFAT5), the only transcription factor in the NFAT family involved in the cellular responses to hypertonic stress, regulated notochord lumen formation in chordate Ciona. Ciona NFAT5 (Ci-NFAT5) was expressed in notochord, and its expression level increased during notochord lumen formation and expansion. Knockout and expression of the dominant negative of NFAT5 in Ciona embryos resulted in the failure of notochord lumen expansion. We further demonstrated that the Ci-NFAT5 transferred from the cytoplasm into nuclei in HeLa cells under the hyperosmotic medium, indicating Ci-NFAT5 can respond the hypertonicity. To reveal the underly mechanisms, we predicted potential downstream genes of Ci-NFAT5 and further validated Ci-NFAT5-interacted genes by the luciferase assay. The results showed that Ci-NFAT5 promoted SLC26A6 expression. Furthermore, expression of a transport inactivity mutant of SLC26A6 (L421P) in notochord led to the failure of lumen expansion, phenocopying that of Ci-NFAT5 knockout. These results suggest that Ci-NFAT5 regulates notochord lumen expansion via the SLC26A6 axis. Taken together, our results reveal that the chordate NFAT5 responds to hypertonic stress and regulates lumen osmotic pressure via an ion channel pathway on luminal organ formation.

Xbp1 and Brachyury establish an evolutionarily conserved subcircuit of the notochord gene regulatory network

Gene regulatory networks coordinate the formation of organs and structures that compose the evolving body plans of different organisms. We are using a simple chordate model, the Ciona embryo, to investigate the essential gene regulatory network that orchestrates morphogenesis of the notochord, a structure necessary for the proper development of all chordate embryos. Although numerous transcription factors expressed in the notochord have been identified in different chordates, several of them remain to be positioned within a regulatory framework. Here, we focus on Xbp1, a transcription factor expressed during notochord formation in Ciona and other chordates. Through the identification of Xbp1-downstream notochord genes in Ciona, we found evidence of the early co-option of genes involved in the unfolded protein response to the notochord developmental program. We report the regulatory interplay between Xbp1 and Brachyury, and by extending these results to Xenopus, we show that Brachyury and Xbp1 form a cross-regulatory subcircuit of the notochord gene regulatory network that has been consolidated during chordate evolution.

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.

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.

Tunicate gastrulation

Tunicates are a diverse group of invertebrate marine chordates that includes the larvaceans, thaliaceans, and ascidians. Because of their unique evolutionary position as the sister group of the vertebrates, tunicates are invaluable as a comparative model and hold the promise of revealing both conserved and derived features of chordate gastrulation. Descriptive studies in a broad range of tunicates have revealed several important unifying traits that make them unique among the chordates, including invariant cell lineages through gastrula stages and an overall morphological simplicity. Gastrulation has only been studied in detail in ascidians such as Ciona and Phallusia, where it involves a simple cup-shaped gastrula driven primarily by endoderm invagination. This appears to differ significantly from vertebrate models, such as Xenopus, in which mesoderm convergent extension and epidermal epiboly are major contributors to involution. These differences may reflect the cellular simplicity of the ascidian embryo.

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.

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.

T-Box Genes and Developmental Gene Regulatory Networks in Ascidians

Ascidians are invertebrate chordates with a biphasic life cycle characterized by a dual body plan that displays simplified versions of chordate structures, such as a premetamorphic 40-cell notochord topped by a dorsal nerve cord and postmetamorphic pharyngeal slits. These relatively simple chordates are characterized by rapid development, compact genomes and ease of transgenesis, and thus provide the opportunity to rapidly characterize the genomic organization, developmental function, and transcriptional regulation of evolutionarily conserved gene families. This review summarizes the current knowledge on members of the T-box family of transcription factors in Ciona and other ascidians. In both chordate and nonchordate animals, these genes control a variety of morphogenetic processes, and their mutations are responsible for malformations and developmental defects in organisms ranging from flies to humans. In ascidians, T-box transcription factors are required for the formation and specialization of essential structures, including notochord, muscle, heart, and differentiated neurons. In recent years, the experimental advantages offered by ascidian embryos have allowed the rapid accumulation of a wealth of information on the molecular mechanisms that regulate the expression of T-box genes. These studies have also elucidated the strategies employed by these transcription factors to orchestrate the appropriate spatial and temporal deployment of the numerous target genes that they control.

Transcriptional Control of Developmental Cell Behaviors

Tissue-specific transcription regulators emerged as key developmental control genes, which operate in the context of complex gene regulatory networks (GRNs) to coordinate progressive cell fate specification and tissue morphogenesis. We discuss how GRNs control the individual cell behaviors underlying complex morphogenetic events. Cell behaviors classically range from mesenchymal cell motility to cell shape changes in epithelial sheets. These behaviors emerge from the tissue-specific, multiscale integration of the local activities of universal and pleiotropic effectors, which underlie modular subcellular processes including cytoskeletal dynamics, cell-cell and cell-matrix adhesion, signaling, polarity, and vesicle trafficking. Extrinsic cues and intrinsic cell competence determine the subcellular spatiotemporal patterns of effector activities. GRNs influence most subcellular activities by controlling only a fraction of the effector-coding genes, which we argue is enriched in effectors involved in reading and processing the extrinsic cues to contextualize intrinsic subcellular processes and canalize developmental cell behaviors. The properties of the transcription-cell behavior interface have profound implications for evolution and disease.

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.

Cell behaviors and dynamics during angiogenesis

Vascular networks are formed and maintained through a multitude of angiogenic processes, such as sprouting, anastomosis and pruning. Only recently has it become possible to study the behavior of the endothelial cells that contribute to these networks at a single-cell level in vivo. This Review summarizes what is known about endothelial cell behavior during developmental angiogenesis, focusing on the morphogenetic changes that these cells undergo.

p114RhoGEF governs cell motility and lumen formation during tubulogenesis through a ROCK-myosin-II pathway

Tubulogenesis is fundamental to the development of many epithelial organs. Although lumen formation in cysts has received considerable attention, less is known about lumenogenesis in tubes. Here, we utilized tubulogenesis induced by hepatocyte growth factor (HGF) in MDCK cells, which form tubes enclosing a single lumen. We report the mechanism that controls tubular lumenogenesis and limits each tube to a single lumen. Knockdown of p114RhoGEF (also known as ARHGEF18), a guanine nucleotide exchange factor for RhoA, did not perturb the early stages of tubulogenesis induced by HGF. However, this knockdown impaired later stages of tubulogenesis, resulting in multiple lumens in a tube. Inhibition of Rho kinase (ROCK) or myosin IIA, which are downstream of RhoA, led to formation of multiple lumens. We studied lumen formation by live-cell imaging, which revealed that inhibition of this pathway blocked cell movement, suggesting that cell movement is necessary for consolidating multiple lumens into a single lumen. Lumen formation in tubules is mechanistically quite different from lumenogenesis in cysts. Thus, we demonstrate a new pathway that regulates directed cell migration and formation of a single lumen during epithelial tube morphogenesis.

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

Regulation by a TGFβ-ROCK-actomyosin axis secures a non-linear lumen expansion that is essential for tubulogenesis

Regulation of lumen growth is crucial to ensure the correct morphology, dimensions and function of a tubular structure. How this is controlled is still poorly understood. During Ciona intestinalis notochord tubulogenesis, single extracellular lumen pockets grow between pairs of cells and eventually fuse into a continuous tube. Here, we show that lumen growth exhibits a lag phase, during which the luminal membranes continue to grow but the expansion of the apical/lateral junction pauses for ∼30 min. Inhibition of non-muscle myosin II activity abolishes this lag phase and accelerates expansion of the junction, resulting in the formation of narrower lumen pockets partially fusing into a tube of reduced size. Disruption of actin dynamics, conversely, causes a reversal of apical/lateral junction expansion, leading to a dramatic conversion of extracellular lumen pockets to intracellular vacuoles and a tubulogenesis arrest. The onset of the lag phase is correlated with a de novo accumulation of actin that forms a contractile ring at the apical/lateral junctions. This actin ring actively restricts the opening of the lumen in the transverse plane, allowing sufficient time for lumen growth via an osmotic process along the longitudinal dimension. The dynamics of lumen formation is controlled by the TGFβ pathway and ROCK activity. Our findings reveal a TGFβ-ROCK-actomyosin contractility axis that coordinates lumen growth, which is powered by the dynamics of luminal osmolarity. The regulatory system may function like a sensor/checkpoint that responds to the change of luminal pressure and fine-tunes actomyosin contractility to effect proper tubulogenesis.

Islet is a key determinant of ascidian palp morphogenesis

The anterior-most ectoderm of ascidian larvae contains the adhesive papillae, or palps, which play an important role in triggering the metamorphosis of swimming tadpoles. In Ciona intestinalis, the palps consist of three conical protrusions within a field of thickened epithelium that form late in embryogenesis, as tailbuds mature into larvae. The palp protrusions express the LIM-homeodomain transcription factor Islet. Protrusion occurs through differential cell elongation, probably mediated by Islet, as we find that ectopic expression of Islet is sufficient to promote cell lengthening. FGF signaling is required for both Islet expression and palp morphogenesis. Importantly, we show that Islet expression can rescue the palp-deficient phenotype that results from inhibition of FGF signaling. We conclude that Islet is a key regulatory factor governing morphogenesis of the palps. It is conceivable that Islet is also essential for the cellular morphogenesis of placode-derived sensory neurons in vertebrates.

Single continuous lumen formation in the zebrafish gut is mediated by smoothened-dependent tissue remodeling

The formation of a single lumen during tubulogenesis is crucial for the development and function of many organs. Although 3D cell culture models have identified molecular mechanisms controlling lumen formation in vitro, their function during vertebrate organogenesis is poorly understood. Using light sheet microscopy and genetic approaches we have investigated single lumen formation in the zebrafish gut. Here we show that during gut development multiple lumens open and enlarge to generate a distinct intermediate, which consists of two adjacent unfused lumens separated by basolateral contacts. We observed that these lumens arise independently from each other along the length of the gut and do not share a continuous apical surface. Resolution of this intermediate into a single, continuous lumen requires the remodeling of contacts between adjacent lumens and subsequent lumen fusion. We show that lumen resolution, but not lumen opening, is impaired in smoothened (smo) mutants, indicating that fluid-driven lumen enlargement and resolution are two distinct processes. Furthermore, we show that smo mutants exhibit perturbations in the Rab11 trafficking pathway and demonstrate that Rab11-mediated trafficking is necessary for single lumen formation. Thus, lumen resolution is a distinct genetically controlled process crucial for single, continuous lumen formation in the zebrafish gut.

An equatorial contractile mechanism drives cell elongation but not cell division

A cytokinesis-like contractile mechanism is co-opted in a different developmental scenario to achieve cell elongation instead of cell division in Ciona intestinalis.

Cell shape changes and proliferation are two fundamental strategies for morphogenesis in animal development. During embryogenesis of the simple chordate Ciona intestinalis, elongation of individual notochord cells constitutes a crucial stage of notochord growth, which contributes to the establishment of the larval body plan. The mechanism of cell elongation is elusive. Here we show that although notochord cells do not divide, they use a cytokinesis-like actomyosin mechanism to drive cell elongation. The actomyosin network forming at the equator of each notochord cell includes phosphorylated myosin regulatory light chain, α-actinin, cofilin, tropomyosin, and talin. We demonstrate that cofilin and α-actinin are two crucial components for cell elongation. Cortical flow contributes to the assembly of the actomyosin ring. Similar to cytokinetic cells, membrane blebs that cause local contractions form at the basal cortex next to the equator and participate in force generation. We present a model in which the cooperation of equatorial actomyosin ring-based constriction and bleb-associated contractions at the basal cortex promotes cell elongation. Our results demonstrate that a cytokinesis-like contractile mechanism is co-opted in a completely different developmental scenario to achieve cell shape change instead of cell division. We discuss the occurrences of actomyosin rings aside from cell division, suggesting that circumferential contraction is an evolutionally conserved mechanism to drive cell or tissue elongation.

The actomyosin cytoskeleton is the primary force that drives cell shape changes. These fibers are organized in elaborate structures that form sarcomeres in the muscle and the contractile ring during cytokinesis. In cytokinesis, the establishment of an equatorial actomyosin ring is preceded and regulated by many cell cycle events, and the ring itself is a complex and dynamic structure. Here we report the presence of an equatorial circumferential actomyosin structure with remarkable similarities to the cytokinetic ring formed in postmitotic notochord cells of sea squirt Ciona intestinalis. The notochord is a transient rod-like structure found in all embryos that belong to the phylum Chordata, and in Ciona, a simple chordate, it consists of only 40 cylindrical cells arranged in a single file, which elongate individually during development. Our study shows that the activity of the equatorial actomyosin ring is required for the elongation of the notochord cells. We also find that cortical flow contributes significantly to the formation of the ring at the equator. Similar to cytokinetic cells, we observe the formation of membrane blebs outside the equatorial region. Our analyses suggest that cooperation of actomyosin ring-based circumferential constriction and bleb-associated contractions drive cell elongation in Ciona. We conclude that cells can utilize a cytokinesis-like force generation mechanism to promote cell shape change instead of cell division.

Cis-regulatory timers for developmental gene expression

How does a fertilized egg decode its own genome to eventually develop into a mature animal? Each developing cell must activate a battery of genes in a timely manner and according to the function it will ultimately perform, but how? During development of the notochord—a structure akin to the vertebrate spine—in a simple marine invertebrate, an essential protein called Brachyury binds to specific sites in its target genes. A study just published in PLOS Biology reports that if the target gene contains multiple Brachyury-binding sites it will be activated early in development but if it contains only one site it will be activated later. Genes that contain no binding site can still be activated by Brachyury, but only indirectly by an earlier Brachyury-dependent gene product, so later than the directly activated genes. Thus, this study shows how several genes can interpret the presence of a single factor differently to become active at distinct times in development.

Ascidians as excellent models for studying cellular events in the chordate body plan

The larvae of non-vertebrate chordate ascidians consist of countable numbers of cells. With this feature, ascidians provide us with excellent models for studying cellular events in the construction of the chordate body. This review discusses the recent observations of morphogenetic movements and cell cycles and divisions along with tissue specifications during ascidian embryogenesis. Unequal cleavages take place at the posterior blastomeres during the early cleavage stages of ascidians, and the structure named the centrosome-attracting body restricts the position of the nuclei near the posterior pole to achieve the unequal cleavages. The most-posterior cells differentiate into the primordial germ cells. The gastrulation of ascidians starts as early as the 110-cell stage. During gastrulation, the endodermal cells show two-step changes in cell shape that are crucial for gastrulation. The ascidian notochord is composed of only 40 cells. The 40 cells align to form a single row by an event named the convergent extension, and then the notochord cells undergo vacuolation to transform the notochord into a single hollowed tube. The strictly restricted number of notochord cells is achieved by the regulated number of cell divisions coupled with the differentiation of the cells conducted by a key transcription factor, Brachyury. The dorsally located neural tube is a characteristic of chordates. During the closure of the ascidian neural tube, the epidermis surrounding the neural plate moves toward the midline to close the neural fold. This morphogenetic movement is allowed by an elongation of interphase in the epidermal cell cycles.

Tubulogenesis in a simple cell cord requires the formation of bi-apical cells through two discrete Par domains

Apico-basal polarization is a crucial step in the de novo formation of biological tubes. In Ciona notochord, tubulogenesis occurs in a single file of cells in the absence of cell proliferation. This configuration presents a unique challenge for the formation of a continuous lumen. Here, we show that this geometric configuration is associated with a novel polarization strategy: the generation of bipolar epithelial cells possessing two apical/luminal domains instead of one, as in the conventional epithelium. At the molecular level, cells establish two discrete Par3/Par6/aPKC patches, and form two sets of tight junctions, in opposite points of the cells. The key molecule controlling the formation of both domains is Par3. Changing the position of the cells within the organ fundamentally changes their polarity and the number of apical domains they develop. These results reveal a new mechanism for tubulogenesis from the simplest cell arrangement, which occurs in other developmental contexts, including vertebrate vascular anastomosis.