Characterization of the role of cadherin in regulating cell adhesion during sea urchin development

Gene expression patterns in a novel animal appendage: the sea urchin pluteus arm

The larval arms of echinoid plutei are used for locomotion and feeding. They are composed of internal calcite skeletal rods covered by an ectoderm layer bearing a ciliary band. Skeletogenesis includes an autonomous molecular differentiation program in primary mesenchyme cells (PMCs), initiated when PMCs leave the vegetal plate for the blastocoel, and a patterning of the differentiated skeletal units that requires molecular cues from the overlaying ectoderm. The arms represent a larval feature that arose in the echinoid lineage during the Paleozoic and offers a subject for the study of gene co-option in the evolution of novel larval features. We isolated new molecular markers in two closely related but differently developing species, Heliocidaris tuberculata and Heliocidaris erythrogramma. We report the expression of a larval arm-associated ectoderm gene tetraspanin, as well as two new PMC markers, advillin and carbonic anhydrase. Tetraspanin localizes to the animal half of blastula stage H. tuberculata and then undergoes a restriction into the putative oral ectoderm and future location of the postoral arms, where it continues to be expressed at the leading edge of both the postoral and anterolateral arms. In H. erythrogramma, its expression initiates in the animal half of blastulae and expands over the entire ectoderm from gastrulation onward. Advillin and carbonic anhydrase are upregulated in the PMCs postgastrulation and localized to the leading edge of the growing larval arms of H. tuberculata but do not exhibit coordinated expression in H. erythrogramma larvae. The tight spatiotemporal regulation of these genes in H. tuberculata along with other ontogenetic and phylogenetic evidence suggest that pluteus arms are novel larval organs, distinguishable from the processes of skeletogenesis per se. The dissociation of expression control in H. erythrogramma suggest that coordinate gene expression in H. tuberculata evolved as part of the evolution of pluteus arms, and is not required for larval or adult development.

A homologue of snail is expressed transiently in subsets of mesenchyme cells in the sea urchin embryo and is down-regulated in axis-deficient embryos

Vertebrate members of the zinc finger transcription factor family related to Drosophila snail are expressed in neural crest and paraxial mesoderm along the left-right axis of the embryo. As simple deuterostomes, echinoderms are an important sister phylum for the chordates. We have identified populations of patterned, nonskeletogenic mesenchyme in the sea urchin Lytechinus variegatus by their expression of a sea urchin member of the snail family (Lv-snail). Lv-snail mRNA and protein are detectable at the midgastrula stage within the archenteron. At the late gastrula stage, a contiguous cluster of cells on the left side of the tip of the archenteron is Lv-snail-positive. At the early prism stage, two small clusters of mesenchyme cells near the presumptive arm buds are also Lv-snail-positive. At the pluteus stage, staining is detectable in isolated mesenchyme cells and the ciliated band. Based on fate mapping of secondary mesenchyme cells (SMCs) and double-label immunostaining, these patterns are consistent with expression of SNAIL by novel subsets of SMCs that are largely distinct from skeletogenic mesenchyme. In radialized embryos lacking normal bilateral symmetry, mesenchymal expression of Lv-SNAIL is abolished. These results suggest that transient expression of Lv-snail may be important for the differentiation of a subset of axially patterned nonskeletogenic mesenchyme cells and suggest conserved functions for snail family members in deuterostome development.

The echinoderm adhesome

Although the development of sea urchin embryos has been studied extensively and clearly involves both cell adhesion and cell migration, rather little is known about the adhesion receptors and extracellular matrix molecules involved. The completion of the genome of Strongylocentrotus purpuratus allows a comprehensive survey of the complement of cell-cell and cell-matrix adhesion molecules in this organism. Furthermore, the phylogenetic position of echinoderms offers the opportunity to compare the complement of adhesion proteins between protostome and deuterostome invertebrates and between invertebrate and vertebrate deuterostomes. Many aspects of development and cell interactions differ among these different taxa and it is likely that analysis of the spectrum of adhesion receptors and extracellular matrix proteins can open up new insights into which molecules have evolved to suit particular developmental processes. In this paper, we report the results of an initial analysis along these lines. The echinoderm adhesome (complement of adhesion-related genes/proteins) is similar overall to that of other invertebrates although there are significant deuterostome-specific innovations and some interesting features previously thought to be chordate or vertebrate specific.

Epithelial-mesenchymal transition occurs after epidermal development in mouse skin

In the present study, we studied epithelial-mesenchymal transition (EMT) with fetal and postnatal serial skin sections. E-cadherin, occludin and zonula occludens 1 (ZO-1)-expressing cells appear in the dermal area from E18.5 to postnatal day 9 (P9), with highest expression from P2 to P5. The co-expression of mesenchymal marker alpha-smooth muscle (alpha-SMA), fibronectin and vimentin with E-cadherin in these dermal cells was further examined. Almost no dermal cells express alpha-SMA before P0. From P2 to P6, cells expressing both E-cadherin and alpha-SMA appear in the dermis. In contrast, fibronectin-releasing cells were detected in the dermis as early as on E15.5, although on P5, some dermal cells was found weakly expressing both fibronectin and E-cadherin, most cells strongly expressing fibronectin did not express E-cadherin. Vimentin was mainly expressed in both endothelial and blood-derived cells and did not show co-expression with E-cadherin. Confocal microscopy studies further found that during EMT, E-cadherin appears intracellularly, while the expression of alpha-SMA starts from the membrane area and moves to the cytosol of the cells. Our data are the first in vivo evidence that EMT occurs during mouse skin development. Dermal cells are derived from EMT and other origins, including blood, during skin development.

Microtubules regulate disassembly of epithelial apical junctions

Background

Epithelial tight junction (TJ) and adherens junction (AJ) form the apical junctional complex (AJC) which regulates cell-cell adhesion, paracellular permeability and cell polarity. The AJC is anchored on cytoskeletal structures including actin microfilaments and microtubules. Such cytoskeletal interactions are thought to be important for the assembly and remodeling of apical junctions. In the present study, we investigated the role of microtubules in disassembly of the AJC in intestinal epithelial cells using a model of extracellular calcium depletion.

Results

Calcium depletion resulted in disruption and internalization of epithelial TJs and AJs along with reorganization of perijunctional F-actin into contractile rings. Microtubules reorganized into dense plaques positioned inside such F-actin rings. Depolymerization of microtubules with nocodazole prevented junctional disassembly and F-actin ring formation. Stabilization of microtubules with either docetaxel or pacitaxel blocked contraction of F-actin rings and attenuated internalization of junctional proteins into a subapical cytosolic compartment. Likewise, pharmacological inhibition of microtubule motors, kinesins, prevented contraction of F-actin rings and attenuated disassembly of apical junctions. Kinesin-1 was enriched at the AJC in cultured epithelial cells and it also accumulated at epithelial cell-cell contacts in normal human colonic mucosa. Furthermore, immunoprecipitation experiments demonstrated association of kinesin-1 with the E-cadherin-catenin complex.

Conclusion

Our data suggest that microtubules play a role in disassembly of the AJC during calcium depletion by regulating formation of contractile F-actin rings and internalization of AJ/TJ proteins.

Involvement of the Ras-Ras-activated Rab5 guanine nucleotide exchange factor RIN2-Rab5 pathway in the hepatocyte growth factor-induced endocytosis of E-cadherin

E-cadherin is a key cell-cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of an extracellular signal, such as hepatocyte growth factor (HGF)/scatter factor. Rab5 small G protein has been implicated in the HGF-induced endocytosis of E-cadherin, but the molecular mechanism for the regulation of Rab5 activity remains unknown. We first studied this mechanism by using the cell-free assay system for the endocytosis of E-cadherin of the AJ-enriched fraction from rat livers. HGF induced activation of Ras small G protein, which then bound to RIN2, a Rab5 GDP/GTP exchange factor with the Vps9p-like guanine nucleotide exchange factor and Ras association domains, and activated it. Activated RIN2 then activated Rab5, eventually inducing the endocytosis of E-cadherin. We then studied whether RIN2 was involved in the HGF-induced endocytosis of E-cadherin in intact Madin-Darby canine kidney cells. RIN2 localized at the cell-cell adhesion sites, and its guanine nucleotide exchange factor activity was required for the HGF-induced endocytosis of E-cadherin in Madin-Darby canine kidney cells. These results indicate that RIN2 connects Ras to Rab5 in the HGF-induced endocytosis of E-cadherin.

Nuclear beta-catenin promotes non-neural ectoderm and posterior cell fates in amphioxus embryos

In vertebrate development, Wnt/beta-catenin signaling has an early role in specification of dorsal/anterior identity and a late one in posterior specification. To understand the evolution of these roles, we cloned beta-catenin from the invertebrate chordate amphioxus. The exon/intron organization of beta-catenin is highly conserved between amphioxus and other animals including a cnidarian, but not Drosophila. In development, amphioxus beta-catenin is concentrated in all nuclei from the 16-cell stage until the onset of gastrulation when it becomes undetectable in presumptive mesendoderm. Li(+), which up-regulates Wnt/beta-catenin signaling, had no detectable effect on axial patterning when applied before the late blastula stage, suggesting that a role for beta-catenin in specification of dorsal/anterior identity may be a vertebrate innovation. From the mid-gastrula through the neurula stage, the highest levels of nuclear beta-catenin are around the blastopore. In the early neurula, beta-catenin is down-regulated in the neural plate, but remains high in adjacent non-neural ectoderm. Embryos treated with Li(+) at the late blastula stage are markedly posteriorized and lack a neural plate. These results suggest that in amphioxus, as in vertebrates, down-regulation of Wnt/beta-catenin signaling in the neural plate is necessary for maintenance of the neuroectoderm and that a major evolutionarily conserved role of Wnt/beta-catenin signaling is to specify posterior identity and pattern the anterior/posterior axis.

A novel approach to study adhesion mechanisms by isolation of the interacting system

For decades most investigations into mechanisms of adhesive interactions have examined whole organisms or single cells. Results using whole organisms are often unclear because it may not be known if a probe used in an experiment is directly affecting the cellular interaction under study or if it is an indirect effect resulting from action on some other structure or pathway. Here we develop a novel approach to isolate the structural components of a cellular interaction by dissecting them out of the organism to study them in a pristine environment away from all confounding factors. We used the adhesion between the archenteron and blastocoel roof of the sea urchin gastrula stage embryo as a model that can be replicated in many other developmental and pathological systems. The isolated components of the cellular interaction and those in the whole organism possessed identical cell surface receptors and adhesive affinities.

Endocytosis of the apical junctional complex: mechanisms and possible roles in regulation of epithelial barriers

Tight junctions (TJ) and adherens junctions (AJ) regulate cell-cell adhesion and barrier function of simple polarized epithelia. These junctions are positioned in the apical end of the lateral plasma membrane and form the so-called apical junctional complex (AJC). Although initially seen as purely structural features, the AJC is now known to play important roles in cell differentiation and proliferation. The AJC is a highly dynamic entity, undergoing rapid remodeling during normal epithelial morphogenesis and under pathologic conditions. There is growing evidence that remodeling of the AJC is mediated by internalization of junctional proteins. This review summarizes what is known about endocytic pathways, intracellular destinations and signaling cascades involved in internalization of AJC proteins. Potential biological roles for AJC endocytosis in maintaining functional apical junctions, reversible opening of epithelial barrier and disruption of intercellular adhesion are also discussed.

Diversification of epithelial adherens junctions with independent reductive changes in cadherin form: identification of potential molecular synapomorphies among bilaterians

The adherens junction (AJ) is the most universal junction found in bilaterian epithelia and may represent one of the earliest types of cell-cell junctions. The adhesion molecules responsible for forming AJs are the classic cadherins (referred to simply as cadherins), whose extracellular domain organization displays marked variety among species examined so far. In this study, we attempted to reconstruct the evolution of cadherin by analyzing new data from several arthropods (two insects, one non-insect hexapod, three crustaceans, and one chelicerate) and previously published sequences for Drosophila melanogaster and other animals. The results of comparative analyses using the BLAST tool and immunohistochemical analyses revealed that the extracellular domain organizations of a decapod, an isopod, a spider, and a starfish cadherin, which are present at AJs in the embryonic epithelia are homologous. Independent reductive changes from the ancestral state were evident in the epithelia of hexapods+branchiopod, vertebrates+urochordates, and a cephalochordate. The form of cadherins in hexapods is more closely related to that of a branchiopod than to that of malacostracan crustaceans, and one of those of vertebrates is more closely related to that of urochordates than to that of a cephalochordate. Although the sampling of taxa is limited at this stage of research, we hypothesize that the reductive events in cadherin structure related to AJ formation in the epithelia may possess information about bilaterian relationships as molecular synapomorphies.

Unraveling signalling cascades for the Snail family of transcription factors

During development and carcinogenesis, the gradient of different molecular factors, the availability of corresponding receptors and the interplay between different signalling cascades combine to orchestrate the different stages. A good understanding of both developmental processes and oncogenesis leads to new insights into normal and aberrant regulation, processes that share some mutual key players. In this review, we will focus on the Snail family of transcription factors. These proteins, which share an evolutionarily conserved role in invertebrates and vertebrates, are implicated in several developmental processes, but are involved in carcinogenesis as well. We will highlight the different signalling cascades leading to the expression of Snail and Slug and how these factors are regulated on the transcriptional level. Then we will focus on how these factors execute their functions by repression of the numerous target genes that have been described to date.

Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium

Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium. It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron. In this review, we first illustrate the current outline of sea urchin gastrulation. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. Third, differences in the manner of gastrulation among sea urchin species will be described; in some species, the archenteron does not elongate stepwise but continuously. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate.

Gastrulation Movements: the Logic and the Nuts and Bolts

Gastrulation, the period during the early development of animals when major cell and tissue movements remodel an initially unstructured group of cells, requires coordinated control of different types of cellular activities in different cell populations. A hierarchy of genetic control mechanisms, involving cell signaling and transcriptional regulation, sets up the embryonic axes and specify the territories of the future germ layers. Cells in these territories modulate their cytoskeleton and their adhesive behavior, resulting in shape changes and movement. Similarities among different species in patterning and cell biological mechanisms are beginning to allow us to recognize general, conserved principles and speculate on possible ancestral mechanisms of gastrulation.

The ins and outs of E-cadherin trafficking

One way of controlling the activity of E-cadherin--a protein that is, simultaneously, a major cell-adhesion molecule, a powerful tumour suppressor, a determinant of cell polarity and a partner to the potent catenin signalling molecules--is to keep it on the move. During the past two decades, many insights into the fundamental role of E-cadherin in these processes have been garnered. Studies during the past five years have begun to reveal the importance of intracellular trafficking as a means of regulating the functions of E-cadherin. E-cadherin is trafficked to and from the cell surface by exocytic and multiple endocytic pathways. In this article, we survey the vesicle-trafficking machinery that is responsible for the sorting, transport, actin association and vesicle targeting of E-cadherin to regulate its movement and function during growth and development and, possibly, in cancer.

Cell adhesion and communication: a lesson from echinoderm embryos for the exploitation of new therapeutic tools

In this chapter, we summarise fundamental findings concerning echinoderms as well as research interests on this phylum for biomedical and evolutionary studies. We discuss how current knowledge of echinoderm biology, in particular of the sea urchin system, can shed light on the understanding of important biological phenomena and in dissecting them at the molecular level. The general principles of sea urchin embryo development are summarised, mainly focusing on cell communication and interactions, with particular attention to the cell-extracellular matrix and cell-cell adhesion molecules and related proteins. Our purpose is not to review all the work done over the years in the field of cellular interaction in echinoderms. On the contrary, we will rather focus on a few arguments in an effort to re-examine some ideas and concepts, with the aim of promoting discussion in this rapidly growing field and opening new routes for research on innovative therapeutic tools.

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Endocytosis of E-cadherin regulated by Rac and Cdc42 small G proteins through IQGAP1 and actin filaments

E-cadherin is a key cell–cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of extracellular signals. To elucidate the mechanism of this endocytosis, we developed here a new cell-free assay system for this reaction using the AJ-enriched fraction from rat liver. We found here that non-trans-interacting, but not trans-interacting, E-cadherin underwent endocytosis in a clathrin-dependent manner. The endocytosis of trans-interacting E-cadherin was inhibited by Rac and Cdc42 small G proteins, which were activated by trans-interacting E-cadherin or trans-interacting nectins, which are known to induce the formation of AJs in cooperation with E-cadherin. This inhibition was mediated by reorganization of the actin cytoskeleton by Rac and Cdc42 through IQGAP1, an actin filament-binding protein and a downstream target of Rac and Cdc42. These results indicate the important role of the Rac/Cdc42-IQGAP1 system in the dynamic organization and maintenance of the E-cadherin–based AJs.

Cross-regulation of Wnt signaling and cell adhesion

The complex cross-regulation between Wnt signaling, cell-cell adhesion, and cell-matrix adhesion has revealed a number of regulatory components important in development and cancer progression. In the following, we would like to highlight and summarize some of the steps where pathways converge or diverge in regulating Wnt activity, matrix-induced pathways, and cell adhesion. We would like to focus on the involvement of heparan sulfate proteoglycan-rich proteins (HSPGs), integrin-mediated outside-in signaling, and cadherin-mediated cell-cell adhesion on Wnt pathways and the transcriptional regulation of extracellular matrix components and cell adhesion molecules by Wnt signaling.

Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development

Epithelial-mesenchymal transitions (EMTs) are an important mechanism for reorganizing germ layers and tissues during embryonic development. They have both a morphogenic function in shaping the embryo and a patterning function in bringing about new juxtapositions of tissues, which allow further inductive patterning events to occur [Genesis 28 (2000) 23]. Whereas the mechanics of EMT in cultured cells is relatively well understood [reviewed in Biochem. Pharmacol. 60 (2000) 1091; Cell 105 (2001) 425; Bioessays 23 (2001) 912], surprisingly little is known about EMTs during embryonic development [reviewed in Acta Anat. 154 (1995) 8], and nowhere is the entire process well characterized within a single species. Embryonic (developmental) EMTs have properties that are not seen or are not obvious in culture systems or cancer cells. Developmental EMTs are part of a specific differentiative path and occur at a particular time and place. In some types of embryos, a relatively intact epithelium must be maintained while some of its cells de-epithelialize during EMT. In most cases de-epithelialization (loss of apical junctions) must occur in an orderly, patterned fashion in order that the proper morphogenesis results. Interestingly, we find that de-epithelialization is not always necessarily tightly coupled to the expression of mesenchymal phenotypes.Developmental EMTs are multi-step processes, though the interdependence and obligate order of the steps is not clear. The particulars of the process vary between tissues, species, and specific embryonic context. We will focus on 'primary' developmental EMTs, which are those occurring in the initial epiblast or embryonic epithelium. 'Secondary' developmental EMT events are those occurring in epithelial tissues that have reassembled within the embryo from mesenchymal cells. We will review and compare a number of primary EMT events from across the metazoans, and point out some of the many open questions that remain in this field.

The repressor function of snail is required for Drosophila gastrulation and is not replaceable by Escargot or Worniu

Mesoderm formation in the Drosophila embryo depends on the maternal Toll signaling pathway. The Toll pathway establishes the Dorsal nuclear gradient, which regulates many zygotic genes to establish the mesodermal fate and promote the invagination of ventral cells. An important target gene of Dorsal is snail, which is required for proper mesoderm invagination. The Snail protein contains five zinc fingers and is a transcriptional repressor. However, it is not clear whether repressing target genes is a requirement for Snail to control ventral invagination. To examine such requirement, we conducted a series of genetic rescue experiments in snail mutant embryos. Snail, Worniu, and Escargot are closely related zinc-finger proteins and have equal functions during neuroblast development. However, among these three proteins, only Snail can rescue the mesoderm invagination phenotype. Moreover, the ability of various Snail mutant constructs to repress gene expression correlates with their ability to control invagination. This unique property of Snail in mesoderm formation can be attributed mostly to the CtBP co-repressor interaction motifs in the N-terminus, not to the C-terminal DNA-binding zinc fingers. Ectopic expression of Snail outside the ventral domain is not sufficient to induce cell movement even though repression of target genes still occurs. Together, the results show that the repressor function of Snail is essential for gastrulation. The repression of target genes by Snail may permit other factors in the ventral cells to positively promote mesoderm invagination.

Two classic cadherin-related molecules with no cadherin extracellular repeats in the cephalochordate amphioxus: distinct adhesive specificities and possible involvement in the development of multicell-layered structures

We previously reported the existence of Bb-cadherin, a molecule related to classic cadherin, in the cephalochordate amphioxus (Branchiostoma belcheri). The structure of Bb-cadherin is unique in that it lacks the cadherin extracellular repeats, although its cytoplasmic domain shows close similarities to those of typical classic cadherins. The extracellular region of Bb-cadherin consists of laminin globular domains and a cysteine-rich EGF-like domain that are similar to domains in nonchordate classic cadherins. In this study, we identified a second amphioxus cadherin. It was designated Bb2-cadherin (Bb2C) while the previously reported cadherin has been renamed Bb1-cadherin (Bb1C). Bb2C is very similar to Bb1C in its overall structure and amino acid sequence. Genomic BLAST searches and phylogenetic analyses suggested that these two amphioxus genes have been generated through a gene duplication that occurred after separation of the cephalochordates from the other animals. They also bear distinct adhesive specificities. Immunohistochemical analyses showed that Bb1C and Bb2C, together with β-catenin, appear to function as adherens junction constituents in the epithelia of different germ layers of the amphioxus embryo. Differential expression of the two cadherins was also observed in the developing, multicell-layered notochord. These observations suggest that, despite their unique structures, the functions and developmental roles of Bb1C and Bb2C are comparable to those of the classic cadherins characterized to date in other animal groups, such as the vertebrate E- and N-cadherins and the Drosophila DE- and DN-cadherins. The possible involvement of Bb1C and Bb2C in the development of multicell-layered structures characteristic of the cephalochordate body plan is presented.

VE-cadherin: adhesion at arm's length

VE-cadherin was first identified in the early 1990s and quickly emerged as an important endothelial cell adhesion molecule. The past decade of research has revealed key roles for VE-cadherin in vascular permeability and in the morphogenic events associated with vascular remodeling. The details of how VE-cadherin functions in adhesion became apparent with structure-function analysis of the cadherin extracellular domain and with the identification of the catenins, a series of cytoplasmic proteins that bind to the cadherin tail and mediate interactions between cadherins and the cytoskeleton. Whereas early work focused on the armadillo family proteins beta-catenin and plakoglobin, more recent investigations have identified p120-catenin (p120(ctn)) and a related group of armadillo family members as key binding partners for the cadherin tail. Furthermore, a series of new studies indicate a key role for p120(ctn) in regulating cadherin membrane trafficking in mammalian cells. These recent studies place p120(ctn) at the hub of a cadherin-catenin regulatory mechanism that controls cadherin plasma membrane levels in cells of both epithelial and endothelial origin.

Role for actin filament turnover and a myosin II motor in cytoskeleton-driven disassembly of the epithelial apical junctional complex

Disassembly of the epithelial apical junctional complex (AJC), composed of the tight junction (TJ) and adherens junction (AJ), is important for normal tissue remodeling and pathogen-induced disruption of epithelial barriers. Using a calcium depletion model in T84 epithelial cells, we previously found that disassembly of the AJC results in endocytosis of AJ/TJ proteins. In the present study, we investigated the role of the actin cytoskeleton in disassembly and internalization of the AJC. Calcium depletion induced reorganization of apical F-actin into contractile rings. Internalized AJ/TJ proteins colocalized with these rings. Both depolymerization and stabilization of F-actin inhibited ring formation and disassembly of the AJC, suggesting a role for actin filament turnover. Actin reorganization was accompanied by activation (dephosphorylation) of cofilin-1 and its translocation to the F-actin rings. In addition, Arp3 and cortactin colocalized with these rings. F-actin reorganization and disassembly of the AJC were blocked by blebbistatin, an inhibitor of nonmuscle myosin II. Myosin IIA was expressed in T84 cells and colocalized with F-actin rings. We conclude that disassembly of the AJC in calcium-depleted cells is driven by reorganization of apical F-actin. Mechanisms of such reorganization involve cofilin-1-dependent depolymerization and Arp2/3-assisted repolymerization of actin filaments as well as myosin IIA-mediated contraction.

Neural crest and the origin of ectomesenchyme: neural fold heterogeneity suggests an alternative hypothesis

The striking similarity between mesodermally derived fibroblasts and ectomesenchyme cells, which are thought to be derivatives of the neural crest, has long been a source of interest and controversy. In mice, the gene encoding the alpha subunit of the platelet-derived growth factor receptor (PDGFRalpha) is expressed both by mesodermally derived mesenchymal cells and by ectomesenchyme. Whole-mount immunostaining previously revealed that PDGFRalpha is present in the cephalic neural fold epithelium of early murine embryos (Takakura et al. [1997] J Histochem Cytochem 45:883-893). We now show that, within the neural fold, a sharp boundary exists between E-cadherin-expressing non-neural epithelium and the neural epithelium of the dorsal ridge. In addition, we found that cells coexpressing E-cadherin and PDGFRalpha are present in the non-neural epithelium of the neural folds. These observations raise the possibility that at least some PDGFRalpha(+) ectomesenchyme originates from the lateral non-neural domain of neural fold epithelium. This inference is consistent with previous reports (Nichols [ 1981] J Embryol Exp Morphol 64:105-120; Nichols [ 1986] Am J Anat 176:221-231) that mesenchymal cells emerge precociously from an epithelial neural fold domain resembling the primitive streak in the early embryonic epiblast. Therefore, we propose the name "metablast" for this non-neural epithelial domain to indicate that it is the site of a delayed local delamination of mesenchyme similar to involution of mesoderm during gastrulation. We further propose the testable hypothesis that neural crest and ectomesenchyme are developmentally distinct progenitor populations and that at least some ectomesenchyme is metablast-derived rather than neural crest-derived tissue. Developmental Dynamics 229:118-130, 2004.

Identification of a nonchordate-type classic cadherin in vertebrates: Chicken Hz-cadherin is expressed in horizontal cells of the neural retina and contains a nonchordate-specific domain complex

Classic cadherins mediate calcium-dependent cell-cell adhesion in a variety of animals, but there are marked differences in their domain structures between chordate and nonchordate animals. The extracellular domain of chordate-type classic cadherins (type I and type II classic cadherins) consists of five tandem repeats of conserved sequences called EC domains, whereas that of nonchordate-type classic cadherins (designated as type III classic cadherin) contains a variable number of EC domains, followed by a characteristic domain complex made of laminin-A globular domains and EGF-like repeats. In the present study, we identified a novel vertebrate type III cadherin showing high sequence similarity to Drosophila N-cadherin, and named this molecule chicken Hz-cadherin (cHz-cadherin), because of the distinct expression in horizontal cells of the neural retina. cHz-cadherin functioned as an adhesion molecule when introduced into cultured cells. Database search revealed one cHz-cadherin homologue in zebrafish and two in puffer fish, but none in other vertebrate species examined. These observations indicate that type III classic cadherins have been conserved in vertebrate species, being expressed by limited cells types, but lost in particular phylogenic groups of the vertebrates.

Molecular networks controlling epithelial cell polarity in development

During embryonic development, polarized epithelial cells are either formed during cleavage or formed from mesenchymal cells. Because the formation of epithelia during embryogenesis has to occur with high fidelity to ensure proper development, embryos allow a functional approach to study epithelial cell polarization in vivo. In particular, genetic model organisms have greatly advanced our understanding of the generation and maintenance of epithelial cell polarity. Many novel and important polarity genes have been identified and characterized in invertebrate systems, like Drosophila melanogaster and Caenorhabditis elegans. With the rapid identification of mammalian homologues of these invertebrate polarity genes, it has become clear that many important protein domains, single proteins and even entire protein complexes are evolutionarily conserved. It is to be expected that the field of epithelial cell polarity is just experiencing the 'top of the iceberg' of a large protein network that is fundamental for the specific adhesive, cell signalling and transport functions of epithelial cells.

Endocytosis of epithelial apical junctional proteins by a clathrin-mediated pathway into a unique storage compartment

The adherens junction (AJ) and tight junction (TJ) are key regulators of epithelial polarity and barrier function. Loss of epithelial phenotype is accompanied by endocytosis of AJs and TJs via unknown mechanisms. Using a model of calcium depletion, we defined the pathway of internalization of AJ and TJ proteins (E-cadherin, p120 and beta-catenins, occludin, JAM-1, claudins 1 and 4, and ZO-1) in T84 epithelial cells. Proteinase protection assay and immunocytochemistry revealed orchestrated internalization of AJs and TJs into a subapical cytoplasmic compartment. Disruption of caveolae/lipid rafts did not prevent endocytosis, nor did caveolin-1 colocalize with internalized junctional proteins. Furthermore, AJ and TJ proteins did not colocalize with the macropinocytosis marker dextran. Inhibitors of clathrin-mediated endocytosis blocked internalization of AJs and TJs, and junctional proteins colocalized with clathrin and alpha-adaptin. AJ and TJ proteins were observed to enter early endosomes followed by movement to organelles that stained with syntaxin-4 but not with markers of late and recycling endosomes, lysosomes, or Golgi. These results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment. Such mechanisms may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.

Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo

In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through beta-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein.

Activation of pmar1 controls specification of micromeres in the sea urchin embryo

pmar1 is a transcription factor in the paired class homeodomain family that was identified and found to be transcribed in micromeres beginning at the fourth cleavage of sea urchin development [Dev. Biol. 246 (2002), 209]. Based on in situ data, molecular perturbation studies, and QPCR data, the recently published gene regulatory network (GRN) model for endomesoderm specification [Science 295 (2002) 1669; Dev. Biol. 246 (2002), 162] places pmar1 early in the micromere specification pathway, and upstream of two important micromere induction signals. The goal of this study was to test these three predictions of the network model. A series of embryo chimeras were produced in which pmar1 activity was perturbed in one cell that was transplanted to control hosts. At the fourth cleavage, micromeres bearing altered pmar1 activity were combined with a normal micromereless host embryo. If beta-catenin signaling is blocked, the micromeres remain unspecified and are unable to signal to the host cells. When such beta-catenin-blocked micromeres also express Pmar1, all observed micromere functions are rescued. The rescue includes expression of the primary mesenchyme cell (PMC) differentiation program, expression and execution of the Delta signal to induce secondary mesoderm cell (SMC) specification in macromere progeny, and expression of the early endomesoderm induction signal necessary for full specification of the endoderm. Additionally, Pmar1 expressed mosaically from inserted DNA constructs causes induction of ectopic Endo 16 in adjacent cells, demonstrating further that Pmar1 controls expression of the early endomesoderm induction signal. Based on these experiments, Pmar1 is an important transcription factor necessary for initiating the micromere specification program and for the expression of two inductive signals produced by micromeres. Each of the tests we describe supports the placement and function of Pmar1 in the endomesoderm GRN model.

Mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells

VE-cadherin is an endothelial-specific cadherin that plays important roles in vascular morphogenesis and growth control. To investigate the mechanisms by which endothelial cells regulate cadherin cell surface levels, a VE-cadherin mutant containing the non-adhesive interleukin-2 (IL-2) receptor extracellular domain and the VE-cadherin cytoplasmic tail (IL-2R-VE-cadcyto) was expressed in microvascular endothelial cells. Expression of the IL-2R-VE-cadcyto mutant resulted in the internalization of endogenous VE-cadherin and in a dramatic decrease in endogenous VE-cadherin levels. The internalized VE-cadherin co-localized with early endosomes, and the lysosomal inhibitor chloroquine dramatically inhibited the down-regulation of VE-cadherin in cells expressing the IL-2R-VE-cadcyto mutant. Chloroquine treatment also resulted in the accumulation of a VE-cadherin fragment lacking the beta-catenin binding domain of the VE-cadherin cytoplasmic tail. The formation of the VE-cadherin fragment could be prevented by treating endothelial cells with proteasome inhibitors. Furthermore, inhibition of the proteasome prevented VE-cadherin internalization and inhibited the disruption of endothelial intercellular junctions by the IL-2RVE-cadcyto mutant. These results provide new insights into the mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells.

[Morphogenetic aspects of colorectal cancer]

Patterning is a spatial and temporal process by which ordered arrangements of cells and tissue structure are attained. The term is mostly applied to the morphogenesis in developmental pathology, but it can also be useful for the neomorphogenesis in tumor biology. Despite increasing data on the proliferation and differentiation of tumor cells, processes of tumor patterning are rarely studied and poorly understood. A fundamental embryonic process of patterning is the embryonic gastrulation and a basic patterning is found in the colonic adenoma-carcinoma sequence. Both processes exhibit distinct nuclear translocation and expression of beta-catenin, which is considered to be a decisive transcriptional regulator. Our recent studies demonstrated striking analogies of patterning and nuclear beta-catenin expression between the colonic adenoma-carcinoma sequence and the steps of gastrulation. The shared patterns are dissociation, reassembly, tubular reconstruction and branching of neoplastic cells in association with nuclear beta-catenin expression. These findings establish patterning as a relevant concept for tumor formation and link the neoplastic morphogenesis with embryogenesis.

ARF6-GTP recruits Nm23-H1 to facilitate dynamin-mediated endocytosis during adherens junctions disassembly

ARF6-regulated endocytosis of E-cadherin is essential during the disassembly of adherens junctions in epithelial cells. Here, we show that activation of ARF6 promotes clathrin-dependent internalization of E-cadherin and caveolae at the basolateral cell surface. Furthermore, we demonstrate that ARF6-GTP, a constitutively activate form of ARF6, interacts with and recruits Nm23-H1, a nucleoside diphosphate (NDP) kinase that provides a source of GTP for dynamin-dependent fission of coated vesicles during endocytosis. Finally, we show that ARF6-mediated recruitment of Nm-23-H1 to cell junctions is accompanied by a decrease in the cellular levels of Rac1-GTP, consistent with previous findings that Nm23-H1 down-regulates activation of Rac1. These studies provide a molecular basis for ARF6 function in polarized epithelia during adherens junction disassembly.

A novel amphioxus cadherin that localizes to epithelial adherens junctions has an unusual domain organization with implications for chordate phylogeny

Although data are available from only vertebrates, urochordates, and three nonchordate animals, there are definite differences in the structures of classic cadherins between vertebrates plus urochordates and nonchordates. In this study we examined structural diversity of classic cadherins among bilaterian animals by obtaining new data from an amphioxus (Cephalochordata, Chordata), an acorn worm (Hemichordata), a sea star (Echinodermata), and an oyster (Mollusca). The structures of newly identified nonchordate cadherins are grouped together with those of the known sea urchin and Drosophila cadherins, whereas the structure of an amphioxus (Branchiostoma belcheri) cadherin, designated BbC, is differently categorized from those of other known chordate cadherins. BbC is identified as a cadherin by its cytoplasmic domain whose sequence is highly related to the cytoplasmic sequences of all known classic cadherins, but it lacks all of the five repeats constituting the extracellular homophilic-binding domain of other chordate cadherins. The ectodomains of BbC match the ectodomains found in nonchordate cadherins but not present in other chordate cadherins. We show that the BbC functions as a cell-cell adhesion molecule when expressed in Drosophila S2 cells and localizes to adherens junctions in the ectodermal epithelia in amphioxus embryos. We argue that BbC is the amphioxus homologue of the classic cadherins involved in the formation of epithelial adherens junctions. The structural relationships of the cadherin molecules allow us to propose a possibility that cephalochordates might be basal to the sister-groups vertebrates and urochordates.

Urodeles remove mesoderm from the superficial layer by subduction through a bilateral primitive streak

Urodeles begin gastrulation with much of their presumptive mesoderm in the superficial cell layer, all of which must move into the deep layers during development. We studied the morphogenesis of superficial mesoderm in the urodeles Ambystoma maculatum, Ambystoma mexicanum, and Taricha granulosa. In all three species, somitic, lateral, and ventral mesoderm move into the deep layer during gastrulation, ingressing through a "bilateral primitive streak" just inside the blastopore. The mesodermal epithelium appears to slide under the endodermal epithelium by a mechanism we term "subduction." Subduction removes the large expanse of superficial presumptive somitic and lateral-ventral mesoderm that initially separates the sub-blastoporal endoderm from the notochord, leaving the endoderm bounding the still epithelial notochord along the gastrocoel roof. Subduction may be a common feature of urodele gastrulation, differing in this regard from anurans. Subducting cells constrict their apices and become bottle-shaped as they approach the junction of the mesodermal and endodermal epithelia. Subducting bottle cells endocytose apical membrane and withdraw the tight junctional component cingulin from the contracting circumferential tight junctions. Either in conjunction with or immediately after subducting, the mesodermal cells undergo an epithelial-to-mesenchymal transition. The mechanism by which epithelial cells release their apical junctions to become mesenchymal, without disrupting the integrity of the epithelium, remains mysterious, but this system should prove useful in understanding this process in a developmental context.

Protein kinase C regulates endocytosis and recycling of E-cadherin

E-cadherin is a major component of adherens junctions in epithelial cells. We showed previously that a pool of cell surface E-cadherin is constitutively internalized and recycled back to the surface. In the present study, we investigated the potential role of protein kinase C (PKC) in regulating the trafficking of surface E-cadherin in Madin-Darby canine kidney cells. Using surface biotinylation and immunofluorescence, we found that treatment of cells with phorbol esters increased the rate of endocytosis of E-cadherin, resulting in accumulation of E-cadherin in apically localized early or recycling endosomes. The recycling of E-cadherin back to the surface was also decreased in the presence of phorbol esters. Phorbol ester-induced endocytosis of E-cadherin was blocked by specific inhibitors, implicating novel PKC isozymes, such as PKC-epsilon in this pathway. PKC activation led to changes in the actin cytoskeleton facilitating E-cadherin endocytosis. Depolymerization of actin increased endocytosis of E-cadherin, whereas the PKC-induced uptake of E-cadherin was blocked by the actin stabilizer jasplakinolide. Our findings show that PKC regulates vital steps of E-cadherin trafficking, its endocytosis, and its recycling.

Cell movements during gastrulation: snail dependent and independent pathways

The morphogenetic process of gastrulation requires multiple inputs and intricate coordination. Genetic analyses demonstrate critical roles of vertebrate and invertebrate Snail proteins in this process. Together with other regulatory molecules including Wnt and BMP, the Snail pathways specify cell fate and reorganize cellular machineries to coordinate morphological changes and cell movements during gastrulation.

Galpha12 and Galpha13 negatively regulate the adhesive functions of cadherin

Cadherins function to promote adhesion between adjacent cells and play critical roles in such cellular processes as development, tissue maintenance, and tumor suppression. We previously demonstrated that heterotrimeric G proteins of the G12 subfamily comprised of Galpha12 and Galpha13 interact with the cytoplasmic domain of cadherins and cause the release of the transcriptional activator beta-catenin (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). Because of the importance of beta-catenin in cadherin-mediated cell-cell adhesion, we examined whether G12 subfamily proteins could also regulate cadherin function. The introduction of mutationally activated G12 proteins into K562 cells expressing E-cadherin blocked cadherin-mediated cell adhesion in steady-state assays. Also, in breast cancer cells, the introduction of activated G12 proteins blocked E-cadherin function in a fast aggregation assay. Aggregation mediated by a mutant cadherin that lacks G12 binding ability was not affected by activated G12 proteins, indicating a requirement for direct G12-cadherin interaction. Furthermore, in wound-filling assays in which ectopic expression of E-cadherin inhibits cell migration, the expression of activated G12 proteins reversed the inhibition via a mechanism that was independent of G12-mediated Rho activation. These results validate the G12-cadherin interaction as a potentially important event in cell biology and suggest novel roles for G12 proteins in the regulation of cadherin-mediated developmental events and in the loss of cadherin function that is characteristic of metastatic tumor progression.

New early zygotic regulators expressed in endomesoderm of sea urchin embryos discovered by differential array hybridization

Genes that are upregulated by LiCl treatment of sea urchin embryos and/or downregulated by injection into the egg of mRNA encoding an internal fragment of cadherin (Cad) were detected in a differential macroarray screen. The method was that recently described by J. P. Rast et al. (2000, Dev. Biol. 228, 270-296). Almost 10(5) clones from a 12-h cDNA library were screened. Measurements on internal standards showed that the screening procedure was sufficiently sensitive to afford detection of differentially expressed mRNAs of the most rare class, those present in only a few copies per average cell. The injection of Cad mRNA, which specifically blocks nuclearization of beta-catenin, resulted in many-fold decreases in the levels of transcripts of a suite of marker genes expressed zygotically during endomesoderm specification. These measurements substantiated the use of Cad mRNA as the basis for a differential screen for discovery of new endomesodermal genes. By use of the newly developed BioArray software for analysis of macroarray screens, 1106 clones representing differentially expressed genes and yielding useful sequence were recovered. The 367 clones that gave significant BLASTX matches to known cellular proteins fell into 264 nonredundant sequence classes. Those of particular interest for this work were clones encoding DNA-binding transcription factors, signal transduction pathway components, proteases, kinases, and phosphatases. Quantitative PCR analysis of 66 such selected clones revealed that the large majority of these clones had been selected because they are upregulated by LiCl treatment, which affects the expression of a much greater diversity and number of genes than are involved in endomesoderm specification. Seven transcript species were identified that responded sharply to injection of Cad mRNA, and that are not represented in maternal mRNA. Six of those encode transcription factors. We focused on three transcription factor genes of this set that were previously unknown in sea urchin embryos. By whole-mount in situ hybridization, these genes are expressed in specific domains of the endomesodermal territory. They are: (1) Speve, an evenskipped orthologue expressed very early in all vegetal blastomeres and then gradually shifting to veg(1) derivatives by the mesenchyme blastula stage; (2) Spgcm, an orthologue of the fruit fly gene glial cells missing, which is first expressed specifically and exclusively in part of the prospective secondary mesenchyme (mesodermal) domain at late-cleavage blastula stage; and (3) Spfoxc, which is first expressed in the early blastula only in the four small micromeres, and later only expressed in that coelomic pouch which gives rise to the mesoderm of the ventral surface of the adult rudiment.

LvDelta is a mesoderm-inducing signal in the sea urchin embryo and can endow blastomeres with organizer-like properties

Signals from micromere descendants play a critical role in patterning the early sea urchin embryo. Previous work demonstrated a link between the induction of mesoderm by micromere descendants and the Notch signaling pathway. In this study, we demonstrate that these micromere descendants express LvDelta, a ligand for the Notch receptor. LvDelta is expressed by micromere descendants during the blastula stage, a time when signaling has been shown to occur. By a combination of embryo microsurgery, mRNA injection and antisense morpholino experiments, we show that expression of LvDelta by micromere descendants is both necessary and sufficient for the development of two mesodermal cell types, pigment cells and blastocoelar cells. We also demonstrate that LvDelta is expressed by macromere descendants during mesenchyme blastula and early gastrula stages. Macromere-derived LvDelta is necessary for blastocoelar cell and muscle cell development. Finally, we find that expression of LvDelta is sufficient to endow blastomeres with the ability to function as a vegetal organizing center and to coordinate the development of a complete pluteus larva.

Cell polarity and gastrulation inC. elegans

Gastrulation in C. elegans embryos involves formation of a blastocoel and the ingression of surface cells into the blastocoel. Mutations in the par-3 gene cause abnormal separations between embryonic cells, suggesting that the PAR-3 protein has a role in blastocoel formation. In normal development, PAR proteins localize to either the apical or basal surfaces of cells prior to blastocoel formation; we demonstrate that this localization is determined by cell contacts. Cells that ingress into the blastocoel undergo an apical flattening associated with an apical concentration of non-muscle myosin. We provide evidence that ingression times are determined by genes that control cell fate, though interactions with neighboring cells can prevent ingression.

Left-right positioning of the adult rudiment in sea urchin larvae is directed by the right side

Indirect-developing sea urchins eventually form an adult rudiment on the left side through differential left-right development in the late larval stages. Components of the adult rudiment, such as the hydropore canal, the hydrocoel and the primary vestibule, all develop on the left side alone, and are the initial morphological traits that exhibit left-right differences. Although it has previously been shown that partial embryos dissected in cleavage stages correctly determine the normal left-right placement of the adult rudiment, the timing and the mechanism that determine left-right polarity during normal development remain unknown. In order to determine these, we have carried out a series of regional operations in two indirect-developing sea urchin species. We excised all or a part of tissue on the left or right side of the embryos during the early gastrula stage and the two-armed pluteus stage, and examined the left-right position of the adult rudiment, and of its components. Excisions of tissues on the left side of the embryos, regardless of stage, resulted in formation of a left adult rudiment, as in normal development. By contrast, excisions on the right side of the embryos resulted in three different types of impairment in the left-right placement of the adult rudiment in a stage-dependent manner. Generally, when the adult rudiment was definitively formed only on the right side of the larvae, no trace of basic development of the components of the adult rudiment was found on the left side, indicating that a right adult rudiment results from reversal of the initial left-right polarity but not from a later inhibitory effect on the development of an adult rudiment. Thus, we suggest that determination of the left-right placement of the adult rudiment depends on a process, which is directed by the right side, of polarity establishment during the gastrula and the prism stages; however, but commitment of the cell fate to initiate formation of the adult rudiment occurs later than the two-armed pluteus stage.

The role of Brachyury (T) during gastrulation movements in the sea urchin Lytechinus variegatus

The studies described here sought to identify and characterize genes involved in the gastrulation and morphogenetic movements that occur during sea urchin embryogenesis. An orthologue of the T-box family transcription factor, Brachyury, was cloned through a candidate gene approach. Brachyury (T) is the founding member of this T-box transcription factor family and has been implicated in gastrulation movements in Xenopus, zebrafish, and mouse embryogenesis. Polyclonal serum was generated to LvBrac in order to characterize protein expression. LvBrac initially appears at mesenchyme blastula stage in two distinct regions with embryonic expression perduring until pluteus stage. Vegetally, LvBrac expression is in endoderm and lies circumferentially around the blastopore. This torus-shaped area of LvBrac expression remains constant in size as endoderm cells express LvBrac upon moving into that circumference and cease LvBrac expression as they leave the circumference. Vegetal expression remains around the anus through pluteus stage. The second domain of LvBrac expression first appears broadly in the oral ectoderm at mesenchyme blastula stage and at later embryonic stages is refined to just the stomodael opening. Vegetal LvBrac expression depends on autonomous beta-catenin signaling in macromeres and does not require micromere or veg2-inductive signals. It was then determined that LvBrac is necessary for the morphogenetic movements occurring in both expression regions. A dominant-interfering construct was generated by fusing the DNA binding domain of LvBrac to the transcriptional repression module of the Drosophila Engrailed gene in order to perturb gene function. Microinjection of mRNA encoding this LvBrac-EN construct resulted in a block in gastrulation movements but not expression of endoderm and mesoderm marker genes. Furthermore, injection of LvBrac-EN into one of two blastomeres resulted in normal gastrulation movements of tissues derived from the injected blastomere, indicating that LvBrac downstream function may be nonautonomous during sea urchin gastrulation.

A large-scale analysis of mRNAs expressed by primary mesenchyme cells of the sea urchin embryo

The primary mesenchyme cells (PMCs) of the sea urchin embryo have been an important model system for the analysis of cell behavior during gastrulation. To gain an improved understanding of the molecular basis of PMC behavior, a set of 8293 expressed sequenced tags (ESTs) was derived from an enriched population of mid-gastrula stage PMCs. These ESTs represented approximately 1200 distinct proteins, or about 15% of the mRNAs expressed by the gastrula stage embryo. 655 proteins were similar (P<10−7 by BLAST comparisons) to other proteins in GenBank, for which some information is available concerning expression and/or function. Another 116 were similar to ESTs identified in other organisms, but not further characterized. We conservatively estimate that sequences encoding at least 435 additional proteins were included in the pool of ESTs that did not yield matches by BLAST analysis. The collection of newly identified proteins includes many candidate regulators of primary mesenchyme morphogenesis, including PMC-specific extracellular matrix proteins, cell surface proteins, spicule matrix proteins and transcription factors. This work provides a basis for linking specific molecular changes to specific cell behaviors during gastrulation. Our analysis has also led to the cloning of several key components of signaling pathways that play crucial roles in early sea urchin development.

Bep4 Protein Is Involved in Patterning along the Animal–Vegetal Axis in the Paracentrotus lividus Embryo

In sea urchin embryos, the initial animal-vegetal (AV) axis is specified during oogenesis but the mechanism is largely unknown. By using chemical reagents such as lithium, it is possible to shift the principal embryonic territories toward a vegetal fate. We have investigated the possibility of obtaining the same morphological effect as with lithium by utilizing Fabs against the maternal Bep4 protein that is localized in the animal part of Paracentrotus lividus egg and embryos. Incubation of fertilized eggs with Fabs against Bep4 protein causes exogastrulation at 48 h of development of P. lividus embryos, similar to embryos treated with lithium. This vegetalizing effect was ascertained by utilizing territorial markers such as EctoV, EndoI, and Ig8. The effect of Fabs against Bep4 on gene expression was observed by monitoring spatial expression of the hatching enzyme gene. A decreased expression domain compared to its normal spatial distribution was detected and this effect was again comparable to those obtained with lithium treatment. Association of Bep4 with a cadherin was demonstrated by immunoprecipitation and immunostaining experiments, and an involvement in cell signaling is discussed. In addition, treatment of embryos with anti-Bep4 Fabs causes an enhancement in the level and an expansion in the pattern of nuclear beta-catenin. Moreover, this treatment also provokes a decrease of beta-catenin in adherens junctions. Together, these data indicate that anti-Bep4 Fabs provoke a shift of the animal-vegetal boundary toward the animal pole and suggest an active role of Bep4 protein in patterning along the AV axis.

Ectoderm exerts the driving force for gastrulation in the sand dollar Scaphechinus mirabilis

How the ectodermal layer relates to the invagination processes was examined in the sand dollar Scaphechinus mirabilis. When the turgor pressure of blastocoele was increased, invagination was completely blocked. In contrast, an increase in turgor pressure did not affect elongation of the gut rudiment in the regular echinoid Hemicentrotus pulcherrimus. Rhodamine-phalloidin staining showed that the distribution of actin filaments was different between two species of embryos. In S. mirabilis gastrulating embryos, abundant actin filaments were seen at the basal cortex of ectoderm in addition to archenteron cells, while the intense signal was restricted to the archenteron in H. pulcherrimus. To investigate whether actin filaments contained in the ectodermal layer exert the force of invagination, a small part of the ectodermal layer was aspirated with a micropipette. If S. mirabilis embryos were aspirated from the onset of gastrulation, invagination did not occur at all, irrespective of the suction site. Even after the archenteron had invaginated to one-half of its full length, further elongation of the archenteron was severely blocked by suction of the lateral ectoderm. In contrast, suction of the ectodermal layer did not affect the elongation processes in H. pulcherrimus. These results strongly suggest that the ectodermal layer, especially in the vegetal half, exerts the driving force of invagination in S. mirabilis.

Involvement of IQGAP1, an effector of Rac1 and Cdc42 GTPases, in cell-cell dissociation during cell scattering

We have previously proposed that IQGAP1, an effector of Rac1 and Cdc42, negatively regulates cadherin-mediated cell-cell adhesion by interacting with beta-catenin and by causing the dissociation of alpha-catenin from cadherin-beta-catenin-alpha-catenin complexes and that activated Rac1 and Cdc42 positively regulate cadherin-mediated cell-cell adhesion by inhibiting the interaction of IQGAP1 with beta-catenin. However, it remains to be clarified in which physiological processes the Rac1-Cdc42-IQGAP1 system is involved. We here examined whether the Rac1-IQGAP1 system is involved in the cell-cell dissociation of Madin-Darby canine kidney II cells during 12-O-tetradecanoylphorbol-13-acetate (TPA)- or hepatocyte growth factor (HGF)-induced cell scattering. By using enhanced green fluorescent protein (EGFP)-tagged alpha-catenin, we found that EGFP-alpha-catenin decreased prior to cell-cell dissociation during cell scattering. We also found that the Rac1-GTP level decreased after stimulation with TPA and that the Rac1-IQGAP1 complexes decreased, while the IQGAP1-beta-catenin complexes increased during action of TPA. Constitutively active Rac1 and IQGAP1 carboxyl terminus, a putative dominant-negative mutant of IQGAP1, inhibited the disappearance of alpha-catenin from sites of cell-cell contact induced by TPA. Taken together, these results indicate that alpha-catenin is delocalized from cell-cell contact sites prior to cell-cell dissociation induced by TPA or HGF and suggest that the Rac1-IQGAP1 system is involved in cell-cell dissociation through alpha-catenin relocalization.

Change in the adhesive properties of blastomeres during early cleavage stages in sea urchin embryo

Blastomeres of sea urchin embryo change their shape from spherical to columnar during the early cleavage stage. It is suspected that this cell shape change might be caused by the increase in the adhesiveness between blastomeres. By cell electrophoresis, it was found that the amount of negative cell surface charges decreased during the early cleavage stages, especially from the 32-cell stage. It was also found that blastomeres formed lobopodium-like protrusions if the embryos were dissociated in the presence of Ca2+. Interestingly, a decrease in negative cell surface charges and pseudopodia formation first occurred in the descendants of micromeres and then in mesomeres, and last in macromeres. By examining the morphology of cell aggregates derived from the isolated blastomeres of the 8-cell stage embryo, it was found that blastomeres derived from the animal hemisphere (mesomere lineage) increased their adhesiveness one cell cycle earlier than those of the vegetal hemisphere (macromere lineage). The timing of the initiation of close cell contact in the descendants of micro-, meso- and macromeres was estimated to be 16-, 32- and 60-cell stage, respectively. Conversely, the nucleus-to-cell-volume ratios, which are calculated from the diameters of the nucleus and cell, were about 0.1 when blastomeres became adhesive, irrespective of the lineage.

Disappearance of an epithelial cell surface-specific glycoprotein (Epith-1) associated with epithelial-mesenchymal conversion in sea urchin embryogenesis

Cell surface modification during mesenchyme ingression was examined using a monoclonal antibody (mAb), anti-Epith-1 mAb, raised against a protein (Epith-1) that was confined to the lateral surface of the epithelial cells in embryo of the sea urchin, Temnopleurus hardwicki. The mAb epitope was N-glycosylated oligosaccharides of 160 kDa monomeric Epith-1 protein. The glycoprotein was negatively charged, and its isoelectric point (IP) was 4.98. The mAb, however, is not immunologically cross-reactive with other sea urchin embryos including Hemicentrotus pulcherrimus, Strongylocentrotus nudus, and Scaphechinus mirabilis. Epith-1 is present initially in the cytoplasm of unfertilized eggs. Cytoplasmic Epith-1 shifted to the cell surface to be integrated in plasma membrane during the first cleavage, and remained there during early embryogenesis by retaining the same relative molecular mass (Mr). During primary and secondary mesenchyme ingression periods, however, Epith-1 disappears from the presumptive mesenchyme cell surface that was associated with internalization of the protein. In plutei, an additional anti-Epith-1 mAb-positive protein appears at the 142 kDa region, which was not associated with any visible alteration of the histologic localization of the protein in larvae. Anti-Epith-1 mAb IgG did not inhibit the reaggregation of epithelial cells in vitro, which suggests that either the protein is not involved in cell-cell adhesion or that the mAb is not recognizing the active site of the protein.

Recent progress in protocadherin research

Protocadherins constitute a large family belonging to the cadherin superfamily and function in different tissues of a wide variety of multicellular organisms. Protocadherins have unique features that are not found in classic cadherins. Expression of protocadherins is spatiotemporally regulated and they are localized at synapses in the CNS. Although protocadherins have Ca(2+)-dependent homophilic interaction activity, the activities are relatively weak. Some protocadherins have heterophilic interaction activity and the cytoplasmic domains associate with the unique cytoplasmic proteins, which are essential for their biological functions. Given the characteristic properties, the large size, and the diversity of members of the protocadherin family, protocadherins may participate in various biological processes. In particular, protocadherins seem to play a central role(s) in the CNS as related to synaptic function.

Drosophila grain encodes a GATA transcription factor required for cell rearrangement during morphogenesis

The genetic mechanisms controlling organ shape are largely unknown. We show that the Drosophila grain gene is required during development for shaping the adult legs and the larval posterior spiracles. Mutant legs are short and wide rather than long and thin, while the spiracles are flat instead of dome-shaped. We demonstrate that grain encodes the GATAc transcription factor. Analysis of loss-of-function mutations at the cellular level indicates that grain affects organ shape by locally controlling cell rearrangement. Ectopic grain expression causes major morphogenetic movements, resulting in the invagination of the posterior segments into the embryo. This is the first gene that has been shown to affect epithelial morphogenesis by controlling cell rearrangements, and suggests a novel function for GATA transcription factors.