An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation

Evolution of the mechanisms that establish the embryonic axes

A long-standing debate in developmental biology concerns the extent to which embryos are largely 'mosaic' (cell fates are allocated by localization of maternal determinants that are inherited differentially) or 'regulative' (cell interactions determine cell fates). Generally, it has been thought that amniotes, especially birds and mammals, are at the extreme regulative end of the spectrum, whereas most invertebrates, lower chordates and anamnia are more mosaic. Various studies have identified additional differences, including egg size, the timing of zygotic transcription and the speed of development. However, new research is starting to reveal among the vertebrate classes an astonishing degree of conservation in the intercellular signalling mechanisms that regulate cell fate and embryonic polarity before gastrulation.

Developmental functions of the P120-catenin sub-family

For more than a decade, cell, developmental and cancer investigators have brought about a wide interest in the biology of catenin proteins, an attraction being their varied functions within differing cellular compartments. While the diversity of catenin localizations and roles has been intriguing, it has also posed a challenge to the clear interpretation of loss- or gain-of-function developmental phenotypes. The most deeply studied member of the larger catenin family is beta-catenin, whose contributions span areas including cell adhesion and intracellular signaling/ transcriptional control. More recently, attention has been directed towards p120-catenin, which in conjunction with the p120-catenin sub-family members ARVCF- and delta-catenins, are the subjects of this review. Although the requirement for vertebrate versus invertebrate p120-catenin are at variance, vertebrate p120-catenin sub-family members may each inter-link cadherin, cytoskeletal and gene regulatory functions in embryogenesis and disease.

Asymmetric expression of the BMP antagonists chordin and gremlin in the sea anemone Nematostella vectensis: implications for the evolution of axial patterning

The evolutionary origin of the anterior-posterior and the dorsoventral body axes of Bilateria is a long-standing question. It is unclear how the main body axis of Cnidaria, the sister group to the Bilateria, is related to the two body axes of Bilateria. The conserved antagonism between two secreted factors, BMP2/4 (Dpp in Drosophila) and its antagonist Chordin (Short gastrulation in Drosophila) is a crucial component in the establishment of the dorsoventral body axis of Bilateria and could therefore provide important insight into the evolutionary origin of bilaterian axes. Here, we cloned and characterized two BMP ligands, dpp and GDF5-like as well as two secreted antagonists, chordin and gremlin, from the basal cnidarian Nematostella vectensis. Injection experiments in zebrafish show that the ventralizing activity of NvDpp mRNA is counteracted by NvGremlin and NvChordin, suggesting that Gremlin and Chordin proteins can function as endogenous antagonists of NvDpp. Expression analysis during embryonic and larval development of Nematostella reveals asymmetric expression of all four genes along both the oral-aboral body axis and along an axis perpendicular to this one, the directive axis. Unexpectedly, NvDpp and NvChordin show complex and overlapping expression on the same side of the embryo, whereas NvGDF5-like and NvGremlin are both expressed on the opposite side. Yet, the two pairs of ligands and antagonists only partially overlap, suggesting complex gradients of BMP activity along the directive axis but also along the oral-aboral axis. We conclude that a molecular interaction between BMP-like molecules and their secreted antagonists was already employed in the common ancestor of Cnidaria and Bilateria to create axial asymmetries, but that there is no simple relationship between the oral-aboral body axis of Nematostella and one particular body axis of Bilateria.

Dorso/ventral genes are asymmetrically expressed and involved in germ-layer demarcation during cnidarian gastrulation

Cnidarians (corals, sea anemones, hydroids, and jellyfish) are a basal taxon closely related to bilaterally symmetrical animals and have been characterized as diploblastic and as radially symmetrical around their longitudinal axis. We show that some orthologs of key bilaterian dorso/ventral (D/V) patterning genes, including the TGFbeta signaling molecules NvDpp and NvBMP5-8 and their antagonist NvChordin, are initially expressed asymmetrically at the onset of gastrulation in the anthozoan sea anemone Nematostella vectensis. Surprisingly, unlike flies and vertebrates, the TGFbeta ligands and their antagonist are colocalized at the onset of gastrulation but then segregate by germ layer as gastrulation proceeds. TGFbeta ligands, their extracellular enhancer, NvTolloid, and components of their downstream signaling pathway (NvSmad1/5 and NvSmad4) are all coexpressed in presumptive endoderm, indicating that only planar TGFbeta signaling operates at these stages. NvChordin expression forms a boundary between TGFbeta-expressing endodermal cells and aboral ectoderm. Manipulation of nuclear beta-catenin localization affects TGFbeta ligand and antagonist expression, suggesting that the ancestral role of the dpp/chordin antagonism during gastrulation may have been in germ-layer segregation and/or epithelial patterning rather than dorsal/ventral patterning.

Animal pole determinants define oral–aboral axis polarity and endodermal cell-fate in hydrozoan jellyfish Podocoryne carnea

Cnidarians, in contrast with bilaterians, are generally considered to exhibit radial symmetry around a single body axis (oral-aboral) throughout their life-cycles. We have investigated how the oral-aboral axis is established in the hydrozoan jellyfish Podocoryne carnea. Vital labeling experiments showed that the oral end of the blastula derives from the animal pole region of the egg as has been demonstrated for other cnidarian species. Gastrulation is restricted to the oral pole such that the oral 20% of blastula cells give rise to endoderm. Unexpectedly, bisection experiments at the 8-cell stage showed that animal regions are able to develop into normally polarized larvae, but that vegetal (aboral) blastomeres completely fail to develop endoderm or to elongate. These vegetal-derived larvae also failed to polarize, as indicated by a lack of oral-specific RFamide-positive nerve cells and a disorganized tyrosinated tubulin-positive nerve net. A different result was obtained following bisection of the late blastula stage: aboral halves still lacked the capacity to develop endoderm but retained features of axial polarity including elongation of the larva and directional swimming. These results demonstrate for the first time in a cnidarian the presence of localized determinants responsible for axis determination and endoderm formation at the animal pole of the egg. They also show that axial polarity and endoderm formation are controlled by separable pathways after the blastula stage.

StellaBase: the Nematostella vectensis Genomics Database

StellaBase, the Nematostella vectensis Genomics Database, is a web-based resource that will facilitate desktop and bench-top studies of the starlet sea anemone. Nematostella is an emerging model organism that has already proven useful for addressing fundamental questions in developmental evolution and evolutionary genomics. StellaBase allows users to query the assembled Nematostella genome, a confirmed gene library, and a predicted genome using both keyword and homology based search functions. Data provided by these searches will elucidate gene family evolution in early animals. Unique research tools, including a Nematostella genetic stock library, a primer library, a literature repository and a gene expression library will provide support to the burgeoning Nematostella research community. The development of StellaBase accompanies significant upgrades to CnidBase, the Cnidarian Evolutionary Genomics Database. With the completion of the first sequenced cnidarian genome, genome comparison tools have been added to CnidBase. In addition, StellaBase provides a framework for the integration of additional species-specific databases into CnidBase. StellaBase is available at .

Gastrulation in the sea anemone Nematostella vectensis occurs by invagination and immigration: an ultrastructural study

The sea anemone Nematostella vectensis has recently been established as a new model system for the understanding of the evolution of developmental processes. In particular, the evolutionary origin of gastrulation and its molecular regulation are the subject of intense investigation. However, while molecular data are rapidly accumulating, no detailed morphological data exist describing the process of gastrulation. Here, we carried out an ultrastructural study of different stages of gastrulation in Nematostella using transmission electron microscope and scanning electron microscopy techniques. We show that presumptive endodermal cells undergo a change in cell shape, reminiscent of the bottle cells known from vertebrates and several invertebrates. Presumptive endodermal cells organize into a field, the pre-endodermal plate, which undergoes invagination. In parallel, the endodermal cells decrease their apical cell contacts but remain loosely attached to each other. Hence, during early gastrulation they display an incomplete epithelial-mesenchymal transition (EMT). At a late stage of gastrulation, the cells eventually detach and fill the interior of the blastocoel as mesenchymal cells. This shows that gastrulation in Nematostella occurs by a combination of invagination and late immigration involving EMT. The comparison with molecular expression studies suggests that cells expressing snailA undergo EMT and become endodermal, whereas forkhead/brachyury expressing cells at the ectodermal margin of the blastopore retain their epithelial integrity throughout gastrulation.

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.

Specification of embryonic axis and mosaic development in ascidians

Setting up future body axes is the first important event before and at the beginning of embryogenesis. The ascidian embryo is a classic model that has been used to gain insight into developmental processes for over a century. This review summarizes advances made in this decade in our understanding of the developmental processes involved in the specification of the embryonic axes and cell fates during early ascidian embryogenesis. Maternal factors, including mRNAs, are translocated to specific regions of the egg by cytoplasmic and cortical reorganization, so-called ooplasmic segregation, and specify the animal-vegetal axis and the one perpendicular to it, which is defined as the anteroposterior axis in ascidians. Some postplasmic/PEM RNAs that are anchored to cortical endoplasmic reticulum are brought to the future posterior pole of fertilized eggs, and play crucial roles in posterior development. Following specification of the animal-vegetal axis, nuclear localization of beta-catenin takes place in the vegetal blastomeres; this occurrence is important for the acquisition of the vegetal character of the blastomeres in later development. Positioning of these maternal factors lead to subsequent cell interactions and zygotic gene expression responsible for axis establishment and for cell fate specification. We describe how endoderm blastomeres in the vegetal pole region emanate inductive signals mainly attributable to fibroblast growth factor. Marginal blastomeres next to endoderm blastomeres respond differently in ways that are determined by intrinsic competence factors. Expression patterns of developmentally important genes, including key transcription factors of each tissue type, are also summarized.

Complex colony-level organization of the deep-sea siphonophoreBargmannia elongata(Cnidaria, Hydrozoa) is directionally asymmetric and arises by the subdivision of pro-buds

Siphonophores are free-swimming colonial hydrozoans (Cnidaria) composed of asexually produced multicellular zooids. These zooids, which are homologous to solitary animals, are functionally specialized and arranged in complex species-specific patterns. The coloniality of siphonophores provides an opportunity to study the major transitions in evolution that give rise to new levels of biological organization, but siphonophores are poorly known because they are fragile and live in the open ocean. The organization and development of the deep-sea siphonophore Bargmannia elongata is described here using specimens collected with a remotely operated underwater vehicle. Each bud gives rise to a precise, directionally asymmetric sequence of zooids through a stereotypical series of subdivisions, rather than to a single zooid as in most other hydrozoans. This initial description of development in a deep-sea siphonophore provides an example of how precise colony-level organization can arise, and illustrates that the morphological complexity of cnidarians is greater than is often assumed.

An evolutionary conserved role of Wnt signaling in stem cell fate decision

Wnt/Frizzled/ss-catenin-based signaling systems play diverse roles in metazoan development, being involved not only in the establishment of body axes in embryogenesis but also in regulating stem cell fate in mammalian post-embryonic development. We have studied the role the canonical Wnt cascade plays in stem cell fate determination in Hydractinia, a member of the ancient metazoan phylum Cnidaria, by analyzing two key molecules in this pathway, frizzled and ss-catenin, and blocking GSK-3. Generally, frizzled was expressed in cells able to divide but absent in post-mitotic, terminally differentiated cells such as nerve cells and nematocytes. Transcripts of frizzled were identified in all embryonic stages beginning with maternal transcripts in the oocyte. Following gastrulation and in the planula larva, frizzled expression concentrated in the central endodermal mass from which the first interstitial stem cells and their derivatives arise. In post-metamorphic development, high levels of frizzled transcripts were detected in interstitial stem cells. Activating downstream events of the Wnt-cascade in the post-metamorphic life phase by blocking GSK-3 with paullones induced recruitment of nematocytes and nerve cells from the pool of interstitial stem cells. Terminal differentiation was preceded by an initial burst of proliferation of frizzled-positive i-cells. In activated i-cells, ss-catenin appeared in the cytoplasm, later in the nucleus. It was subsequently again observed in the cytoplasm and eventually faded out during terminal differentiation. Our results suggest an ancient role of Wnt signaling in stem cell fate determination.

The evolution of metazoan axial properties

Renewed interest in the developmental basis of organismal complexity, and the emergence of new molecular tools, is improving our ability to study the evolution of metazoan body plans. The most substantial changes in body-plan organization occurred early in metazoan evolution; new model systems for studying basal metazoans are now being developed, and total-genome-sequencing initiatives are underway for at least three of the four most important taxa. The elucidation of how the gene networks that are involved in axial organization, germ-layer formation and cell differentiation are used differently during development is generating a more detailed understanding of the events that have led to the current diversity of multicellular life.

Canonical Wnt Signaling and Its Antagonist Regulate Anterior-Posterior Axis Polarization by Guiding Cell Migration in Mouse Visceral Endoderm

The mouse embryonic axis is initially formed with a proximal-distal orientation followed by subsequent conversion to a prospective anterior-posterior (A-P) polarity with directional migration of visceral endoderm cells. Importantly, Otx2, a homeobox gene, is essential to this developmental process. However, the genetic regulatory mechanism governing axis conversion is poorly understood. Here, defective axis conversion due to Otx2 deficiency can be rescued by expression of Dkk1, a Wnt antagonist, or following removal of one copy of the beta-catenin gene. Misexpression of a canonical Wnt ligand can also inhibit correct A-P axis rotation. Moreover, asymmetrical distribution of beta-catenin localization is impaired in the Otx2-deficient and Wnt-misexpressing visceral endoderm. Concurrently, canonical Wnt and Dkk1 function as repulsive and attractive guidance cues, respectively, in the migration of visceral endoderm cells. We propose that Wnt/beta-catenin signaling mediates A-P axis polarization by guiding cell migration toward the prospective anterior in the pregastrula mouse embryo.

Genomic inventory and expression of Sox and Fox genes in the cnidarian Nematostella vectensis

The Sox and Forkhead (Fox) gene families are comprised of transcription factors that play important roles in a variety of developmental processes, including germ layer specification, gastrulation, cell fate determination, and morphogenesis. Both the Sox and Fox gene families are divided into subgroups based on the amino acid sequence of their respective DNA-binding domains, the high-mobility group (HMG) box (Sox genes) or Forkhead domain (Fox genes). Utilizing the draft genome sequence of the cnidarian Nematostella vectensis, we examined the genomic complement of Sox and Fox genes in this organism to gain insight into the nature of these gene families in a basal metazoan. We identified 14 Sox genes and 15 Fox genes in Nematostella and conducted a Bayesian phylogenetic analysis comparing HMG box and Forkhead domain sequences from Nematostella with diverse taxa. We found that the majority of bilaterian Sox groups have clear Nematostella orthologs, while only a minority of Fox groups are represented, suggesting that the evolutionary pressures driving the diversification of these gene families may be distinct from one another. In addition, we examined the expression of a subset of these genes during development in Nematostella and found that some of these genes are expressed in patterns consistent with roles in germ layer specification and the regulation of cellular behaviors important for gastrulation. The diversity of expression patterns among members of these gene families in Nematostella reinforces the notion that despite their relatively simple morphology, cnidarians possess much of the molecular complexity observed in bilaterian taxa.

Ciona intestinalis: chordate development made simple

Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been a model system for embryological studies for over a century. Recently, ascidians have entered the postgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecular resources that rival those available for fruit flies and mice. One strength of ascidians as a model system is their close similarity to vertebrates. Literature reporting molecular homologies between vertebrate and ascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However, it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 million years ago. Here, we review the main similarities and differences so far identified, at the molecular level, between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.

Formation of the head organizer in hydra involves the canonical Wnt pathway

Stabilization of β-catenin by inhibiting the activity of glycogen synthase kinase-3β has been shown to initiate axis formation or axial patterning processes in many bilaterians. In hydra, the head organizer is located in the hypostome, the apical portion of the head. Treatment of hydra with alsterpaullone, a specific inhibitor of glycogen synthase kinase-3β,results in the body column acquiring characteristics of the head organizer, as measured by transplantation experiments, and by the expression of genes associated with the head organizer. Hence, the role of the canonical Wnt pathway for the initiation of axis formation was established early in metazoan evolution.

Common mutations of beta-catenin in adamantinomatous craniopharyngiomas but not in other tumours originating from the sellar region

Dysregulation of the Wnt signalling pathway contributes to developmental abnormalities and carcinogenesis of solid tumours. Here, we examined beta-catenin and adenomatous polyposis coli (APC) by mutational analysis in pituitary adenomas (n=60) and a large series of craniopharyngiomas (n=41). Furthermore, the expression pattern of beta-catenin was immunohistochemically analysed in a cohort of tumours and cysts of the sellar region including pituitary adenomas (n=58), craniopharyngiomas (n=57), arachnoidal cysts (n=8), Rathke's cleft cysts (n=10) and xanthogranulomas (n=6). Whereas APC mutations were not detectable in any tumour entity, beta-catenin mutations were present in 77% of craniopharyngiomas, exclusively of the adamantinomatous subtype. All mutations affected exon 3, which encodes the degradation targeting box of beta-catenin compatible with an accumulation of nuclear beta-catenin protein. In addition, a novel 81-bp deletion of this exonic region was detected in one case. Immunohistochemical analysis confirmed a shift from membrane-bound to nuclear accumulation of beta-catenin in 94% of the adamantinomatous tumours. Aberrant distribution patterns of beta-catenin were never observed in the other tumour entities under study. We conclude that beta-catenin mutations and/or nuclear accumulation serve as diagnostic hallmarks of the adamantinomatous variant, setting it apart from the papillary variant of craniopharyngioma.

Rising starlet: the starlet sea anemone, Nematostella vectensis

In recent years, a handful of model systems from the basal metazoan phylum Cnidaria have emerged to challenge long-held views on the evolution of animal complexity. The most-recent, and in many ways most-promising addition to this group is the starlet sea anemone, Nematostella vectensis. The remarkable amenability of this species to laboratory manipulation has already made it a productive system for exploring cnidarian development, and a proliferation of molecular and genomic tools, including the currently ongoing Nematostella genome project, further enhances the promise of this species. In addition, the facility with which Nematostella populations can be investigated within their natural ecological context suggests that this model may be profitably expanded to address important questions in molecular and evolutionary ecology. In this review, we explore the traits that make Nematostella exceptionally attractive as a model organism, summarize recent research demonstrating the utility of Nematostella in several different contexts, and highlight a number of developments likely to further increase that utility in the near future.

Maternal Wnt11 Activates the Canonical Wnt Signaling Pathway Required for Axis Formation in Xenopus Embryos

Wnt signaling pathways play essential roles in patterning and proliferation of embryonic and adult tissues. In many organisms, this signaling pathway directs axis formation. Although the importance of intracellular components of the pathway, including beta-catenin and Tcf3, has been established, the mechanism of their activation is uncertain. In Xenopus, the initiating signal that localizes beta-catenin to dorsal nuclei has been suggested to be intracellular and Wnt independent. Here, we provide three lines of evidence that the pathway specifying the dorsal axis is activated extracellularly in Xenopus embryos. First, we identify Wnt11 as the initiating signal. Second, we show that activation requires the glycosyl transferase X.EXT1. Third, we find that the EGF-CFC protein, FRL1, is also essential and interacts with Wnt11 to activate canonical Wnt signaling.

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 canonical Wnt pathway in early mammalian embryogenesis and stem cell maintenance/differentiation

The Wnt signaling pathway plays essential roles in embryogenesis of higher eukaryotes from diploblastic, radially symmetrical cnidarians to mice and humans. Whereas studies in model organisms such as Drosophila and Xenopus continue to elucidate how the Wnt pathway is regulated, studies of mouse mutants and cultured mammalian cells start to reveal how the Wnt pathway controls development and differentiation in mammals. Here we review some of the recent progresses in our understanding of the Wnt pathway. We focus on how this pathway, through regulating transcription of its downstream target genes, specifies pattern formation during mammalian embryogenesis and functions in the differentiation and maintenance of stem cells both in vivo and in vitro.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.

Analysis of forkhead and snail expression reveals epithelial-mesenchymal transitions during embryonic and larval development of Nematostella vectensis

The winged helix transcription factor Forkhead and the zinc finger transcription factor Snail are crucially involved in germ layer formation in Bilateria. Here, we isolated and characterized a homolog of forkhead/HNF3 (FoxA/group 1) and of snail from a diploblast, the sea anemone Nematostella vectensis. We show that Nematostella forkhead expression starts during late Blastula stage in a ring of cells that demarcate the blastopore margin during early gastrulation, thereby marking the boundary between ectodermal and endodermal tissue. snail, by contrast, is expressed in a complementary pattern in the center of forkhead-expressing cells marking the presumptive endodermal cells fated to ingress during gastrulation. In a significant portion of early gastrulating embryos, forkhead is expressed asymmetrically around the blastopore. While snail-expressing cells form the endodermal cell mass, forkhead marks the pharynx anlage throughout embryonic and larval development. In the primary polyp, forkhead remains expressed in the pharynx. The detailed analysis of forkhead and snail expression during Nematostella embryonic and larval development further suggests that endoderm formation results from epithelial invagination, mesenchymal immigration, and reorganization of the endodermal epithelial layer, that is, by epithelial-mesenchymal transitions (EMT) in combination with extensive morphogenetic movements. snail also governs EMT at different processes during embryonic development in Bilateria. Our data indicate that the function of snail in Diploblasts is to regulate motility and cell adhesion, supporting that the triggering of changes in cell behavior is the ancestral role of snail in Metazoa.

Combinatorial peptide library binding of mammalian spermatozoa identifies a ligand (HIPRT) in the axin protein: putative identification of a sperm surface axin binding protein and intriguing developmental implications

The identification of components in cell-cell interactions is an important research goal in reproductive and developmental biology. Such interactions are critical to gamete development, fertilization, implantation and basic development. Several proteins involved with sperm-oocyte interaction and other developmentally important phenomena have been identified. However, these are obviously only a subset of the molecular components involved in such complex cell-cell interactions. One method that has been used to identify binding partners for particular molecular targets is the use of combinatorial libraries accessible on phage surfaces. For the most part, this technique has mainly been applied to screen specific target moieties. However, in some cases whole-cell screening has been attempted. This study describes the first report of screening intact, living mammalian gametes using a proprietary whole-cell combinatorial library binding and analysis protocol. Results from the first screening protocol of mouse spermatozoa strongly identified a putative sperm-binding ligand using proprietary bioinformatic analysis. This amino acid sequence (HIPRT) precisely corresponds with a previously characterized highly conserved protein-protein interaction site in the axin protein. This sequence is found within the binding site for a known sperm surface protein, glycogen synthase kinase-3. This result not only provides proof of the utility of this technique to identify cell surface ligands in mammalian gametes, but it also suggests a potential role for spermatozoa in facilitating developmental axis formation in mammalian embryos.

The dawn of bilaterian animals: the case of acoelomorph flatworms

The origin of the bilaterian metazoans from radial ancestors is one of the biggest puzzles in animal evolution. A way to solve it is to identify the nature and main features of the last common ancestor of the bilaterians (LCB). Recent progress in molecular phylogeny has shown that many platyhelminth flatworms, regarded for a long time as basal bilaterians, now belong to the lophotrochozoan protostomates. In contrast, the LCB is now considered a complex organism bearing several features of modern bilaterians. Here we discuss an alternative view, in which acoelomorph (Acoela + Nemertodermatida) flatworms, which do not belong to the Platyhelminthes, represent the earliest extant bilaterian clade. Sequences from ribosomal and other nuclear genes, Hox cluster genes, and reinterpretation of some morphological features strongly support the basal position of acoelomorphs arguing against a complex LCB. This reconstruction backs the old planuloid-acoeloid hypothesis and may help our understanding of the evolution of body axes, Hox genes and the Cambrian explosion.

Differential stability of β-catenin along the animal-vegetal axis of the sea urchin embryo mediated by dishevelled

β-Catenin has a central role in the early axial patterning of metazoan embryos. In the sea urchin, β-catenin accumulates in the nuclei of vegetal blastomeres and controls endomesoderm specification. Here, we use in-vivo measurements of the half-life of fluorescently tagged β-catenin in specific blastomeres to demonstrate a gradient in β-catenin stability along the animal-vegetal axis during early cleavage. This gradient is dependent on GSK3β-mediated phosphorylation of β-catenin. Calculations show that the difference in β-catenin half-life at the animal and vegetal poles of the early embryo is sufficient to produce a difference of more than 100-fold in levels of the protein in less than 2 hours. We show that dishevelled (Dsh), a key signaling protein, is required for the stabilization of β-catenin in vegetal cells and provide evidence that Dsh undergoes a local activation in the vegetal region of the embryo. Finally, we report that GFP-tagged Dsh is targeted specifically to the vegetal cortex of the fertilized egg. During cleavage, Dsh-GFP is partitioned predominantly into vegetal blastomeres. An extensive mutational analysis of Dsh identifies several regions of the protein that are required for vegetal cortical targeting, including a phospholipid-binding motif near the N-terminus.

The great escape; phosphorylation of Ena/VASP by PKA promotes filopodial formation

The Ena/VASP family of proteins consists of adaptor molecules that localize to subcellular sites of actin polymerization. The role of Ena/VASP proteins in the regulation of cell motility and axon outgrowth has been controversial. Recently, these proteins have been proposed to function as "anticapping" factors, which may have differential effects on filopodial versus lammelipodial actin-based protrusions. A study by Lebrand et al. in this issue of Neuron supports this model and identifies PKA as a key regulator of Ena/VASP function downstream of the chemoattractant Netrin.

Convergence of Wnt, beta-catenin, and cadherin pathways

The specification and proper arrangements of new cell types during tissue differentiation require the coordinated regulation of gene expression and precise interactions between neighboring cells. Of the many growth factors involved in these events, Wnts are particularly interesting regulators, because a key component of their signaling pathway, beta-catenin, also functions as a component of the cadherin complex, which controls cell-cell adhesion and influences cell migration. Here, we assemble evidence of possible interrelations between Wnt and other growth factor signaling, beta-catenin functions, and cadherin-mediated adhesion.

The Cnidarian and the Canon: the role of Wnt/?-catenin signaling in the evolution of metazoan embryos

In a recent publication, Wikramanayake and colleagues have implicated the canonical Wnt/beta-catenin signaling pathway as a mediator of axial polarity and germ-layer specification in embryos of the cnidarian Nematostella. In this anthozoan, beta-catenin is localized in nuclei of blastomeres in one region of the 16- to 32-cell embryo whose descendants subsequently form the entoderm of the embryo. They claim that the pattern of nuclear localization is significant for two reasons: (1) when nuclear localization of beta-catenin was inhibited, gastrulation does not occur, and (2) when localization of beta-catenin took place in all cells of the pregastrula embryo, the number of entodermal cells increases. Since the Wnt/beta-catenin signaling pathway also plays a role in establishing axial polarity and specifying endoderm and mesoderm in a number of bilaterians, Wikramanayake et al. imply that this developmental mechanism is an evolutionary inheritance from a radially symmetrical ancestor. Some of the gaps in the current evidence, which must be filled to evaluate their interpretation, are discussed.