SpFGFR, a new member of the fibroblast growth factor receptor family, is developmentally regulated during early sea urchin development

A single-cell RNA-seq analysis of Brachyury-expressing cell clusters suggests a morphogenesis-associated signal center of oral ectoderm in sea urchin embryos

Brachyury is a T-box family transcription factor and plays pivotal roles in morphogenesis. In sea urchin embryos, Brachyury, is expressed in the invaginating endoderm, and in the oral ectoderm of the invaginating mouth opening. The oral ectoderm is hypothesized to serve as a signaling center for oral (ventral)-aboral (dorsal) axis formation and to function as a ventral organizer. Our previous results of a single-cell RNA-seq (scRNA-seq) atlas of early Strongylocentrotus purpuratus embryos categorized the constituent cells into 22 clusters, in which the endoderm consists of three clusters and the oral ectoderm four clusters (Foster et al., 2020). Here we examined which clusters of cells expressed Brachyury in relation to the morphogenesis and the identity of the ventral organizer. Our results showed that cells of all three endoderm clusters expressed Brachyury in blastulae. Based on expression profiles of genes involved in the gene regulatory networks (GRNs) of sea urchin embryos, the three clusters are distinguishable, two likely derived from the Veg2 tier and one from the Veg1 tier. On the other hand, of the four oral-ectoderm clusters, cells of two clusters expressed Brachyury at the gastrula stage and genes that are responsible for the ventral organizer at the late blastula stage, but the other two clusters did not. At a single-cell level, most cells of the two oral-ectoderm clusters expressed organizer-related genes, nearly a half of which coincidently expressed Brachyury. This suggests that the ventral organizer contains Brachyury-positive cells which invaginate to form the stomodeum. This scRNA-seq study therefore highlights significant roles of Brachyury-expressing cells in body-plan formation of early sea urchin embryos, though cellular and molecular mechanisms for how Brachyury functions in these processes remain to be elucidated in future studies.

FGF and canonical Wnt signaling cooperate to induce paraxial mesoderm from tailbud neuromesodermal progenitors through regulation of a two-step epithelial to mesenchymal transition

Mesoderm induction begins during gastrulation. Recent evidence from several vertebrate species indicates that mesoderm induction continues after gastrulation in neuromesodermal progenitors (NMPs) within the posteriormost embryonic structure, the tailbud. It is unclear to what extent the molecular mechanisms of mesoderm induction are conserved between gastrula and post-gastrula stages of development. Fibroblast growth factor (FGF) signaling is required for mesoderm induction during gastrulation through positive transcriptional regulation of the T-box transcription factor brachyury We find in zebrafish that FGF is continuously required for paraxial mesoderm (PM) induction in post-gastrula NMPs. FGF signaling represses the NMP markers brachyury (ntla) and sox2 through regulation of tbx16 and msgn1, thereby committing cells to a PM fate. FGF-mediated PM induction in NMPs functions in tight coordination with canonical Wnt signaling during the epithelial to mesenchymal transition (EMT) from NMP to mesodermal progenitor. Wnt signaling initiates EMT, whereas FGF signaling terminates this event. Our results indicate that germ layer induction in the zebrafish tailbud is not a simple continuation of gastrulation events.

FGF signaling repertoire of the indirect developing hemichordate Ptychodera flava

Fibroblast growth factors (FGFs) are a group of ligands that play multiple roles during development by transducing signals through FGF receptors (FGFRs) to downstream factors. At least 22 FGF ligands and 4 receptors have been identified in vertebrates, while six to eight FGF ligands and a single FGFR are present in invertebrate chordates, such as tunicates and amphioxus. The chordate FGFs can be categorized into at least seven subfamilies, and the members of which expanded during the evolution of early vertebrates. In contrast, only one FGF and two FGFRs have been found in sea urchins. Thus, it is unclear whether the FGF subfamilies duplicated in the lineage leading to the chordates, or sea urchins lost several fgf genes. Analyses of the FGF signaling repertoire in hemichordates, which together with echinoderms form the closest group to the chordates, may provide insights into the evolution of FGF signaling in deuterostomes. In this study, we identified five FGFs and three FGFRs from Ptychodera flava, an indirect-developing hemichordate acorn worm. Phylogenetic analyses revealed that hemichordates possess a conserved FGF8/17/18 in addition to several putative hemichordate-specific FGFs. Analyses of sequence similarity and protein domain organizations suggested that the sea urchin and hemichordate FGFRs arose from independent lineage-specific duplications. Furthermore, the acorn worm fgf and fgfr genes were demonstrated to be expressed during P. flava embryogenesis. These results set the foundations for further functional studies of FGF signaling in hemichordates and provided insights into the evolutionary history of the FGF repertoire.

Logics and properties of a genetic regulatory program that drives embryonic muscle development in an echinoderm

Evolutionary origin of muscle is a central question when discussing mesoderm evolution. Developmental mechanisms underlying somatic muscle development have mostly been studied in vertebrates and fly where multiple signals and hierarchic genetic regulatory cascades selectively specify myoblasts from a pool of naive mesodermal progenitors. However, due to the increased organismic complexity and distant phylogenetic position of the two systems, a general mechanistic understanding of myogenesis is still lacking. In this study, we propose a gene regulatory network (GRN) model that promotes myogenesis in the sea urchin embryo, an early branching deuterostome. A fibroblast growth factor signaling and four Forkhead transcription factors consist the central part of our model and appear to orchestrate the myogenic process. The topological properties of the network reveal dense gene interwiring and a multilevel transcriptional regulation of conserved and novel myogenic genes. Finally, the comparison of the myogenic network architecture among different animal groups highlights the evolutionary plasticity of developmental GRNs.

DOI:http://dx.doi.org/10.7554/eLife.07343.001

Muscles, bones, and blood vessels all develop from a tissue called the mesoderm, which forms early on in the development of an embryo. Networks of genes control which parts of the mesoderm transform into different cell types. The gene networks that control the development of muscle cells from the mesoderm have so far been investigated in flies and several species of animals with backbones. However, these species are complex, which makes it difficult to work out the general principles that control muscle cell development.

Sea urchins are often studied in developmental biology as they have many of the same genes as more complex animals, but are much simpler and easier to study in the laboratory. Andrikou et al. therefore investigated the ‘gene regulatory network’ that controls muscle development in sea urchins. This revealed that proteins called Forkhead transcription factors and a process called FGF signaling are crucial for controlling muscle development in sea urchins. These are also important factors for developing muscles in other animals.

Andrikou et al. then produced models that show the interactions between the genes that control muscle formation at three different stages of embryonic development. These models reveal several important features of the muscle development gene regulatory network. For example, the network is robust: if one gene fails, the network is connected in a way that allows it to still make muscle. This also allows the network to adapt and evolve without losing the ability to perform any of its existing roles.

Comparing the gene regulatory network that controls muscle development in sea urchins with the networks found in other animals showed that many of the same genes are used across different species, but are connected into different network structures. Investigating the similarities and differences of the regulatory networks in different species could help us to understand how muscles have evolved and could ultimately lead to a better understanding of the causes of developmental diseases.

DOI:http://dx.doi.org/10.7554/eLife.07343.002

Growth factors and early mesoderm morphogenesis: insights from the sea urchin embryo

The early morphogenesis of the mesoderm is critically important in establishing the body plan of the embryo. Recent research has led to a better understanding of the mechanisms that underlie this process, and growth factor signaling pathways have emerged as key regulators of the directional movements of mesoderm cells during gastrulation. In this review, we undertake a comparative analysis of the various essential functions of growth factor signaling pathways in regulating early mesoderm morphogenesis, with an emphasis on recent advances in the sea urchin embryo. We focus on the roles of the vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) pathways in the migration of primary mesenchyme cells and the formation of the embryonic endoskeleton. We compare the functions of VEGF and FGF in sea urchins with the roles that these and other growth factors play in regulating mesoderm migration during gastrulation in Drosophila and vertebrates.

Characterization and Endocytic Internalization of Epith-2 Cell Surface Glycoprotein during the Epithelial-to-Mesenchymal Transition in Sea Urchin Embryos

The epithelial cells of the sea urchin Hemicentrotus pulcherrimus embryo express an Epith-2, uncharacterized glycoprotein, on the lateral surface. Here, we describe internalization of Epith-2 during mesenchyme formation through the epithelial-to-mesenchymal transition (EMT). Epith-2 was first expressed on the entire egg surface soon after fertilization and on the blastomeres until the 4-cell stage, but was localized to the lateral surface of epithelial cells at and after the 16-cell stage throughout the later developmental period. However, primary mesenchyme cells (PMC) and secondary mesenchyme cells (SMC) that ingress by EMT lost Epith-2 from their cell surface by endocytosis during dissociation from the epithelium, which was associated with the appearance of cytoplasmic Epith-2 dots. The cytoplasmic Epith-2 retained a similar relative molecular mass to that of the cell surface immediately after ingression through the early period of the spreading to single cells. Then, Epith-2 was completely lost from the cytoplasm. Tyrosine residues of Epith-2 were phosphorylated. The endocytic retraction of Epith-2 was inhibited by herbimycin A (HA), a protein tyrosine kinase (PTK) inhibitor, and suramin, a growth factor receptor (GFR) inhibitor, suggesting the involvement of the GFR/PTK (GP) signaling pathway. These two GP inhibitors also inhibited PMC and SMC spreading to individual cells after ingression, but the dissociation of PMC and SMC from the epithelium was not inhibited. In suramin-treated embryos, dissociated mesenchyme cells migrated partially by retaining their epithelial morphology. In HA-treated embryos, no mesenchyme cells migrated. Thus, the EMT occurs in relation to internalization of Epith-2 from presumptive PMC and SMC.

FGF signaling induces mesoderm in the hemichordate Saccoglossus kowalevskii

FGFs act in vertebrate mesoderm induction and also play key roles in early mesoderm formation in ascidians and amphioxus. However, in sea urchins initial characterizations of FGF function do not support a role in early mesoderm induction, making the ancestral roles of FGF signaling and mechanisms of mesoderm specification in deuterostomes unclear. In order to better characterize the evolution of mesoderm formation, we have examined the role of FGF signaling during mesoderm development in Saccoglossus kowalevskii, an experimentally tractable representative of hemichordates. We report the expression of an FGF ligand, fgf8/17/18, in ectoderm overlying sites of mesoderm specification within the archenteron endomesoderm. Embryological experiments demonstrate that mesoderm induction in the archenteron requires contact with ectoderm, and loss-of-function experiments indicate that both FGF ligand and receptor are necessary for mesoderm specification. fgf8/17/18 gain-of-function experiments establish that FGF8/17/18 is sufficient to induce mesoderm in adjacent endomesoderm. These experiments suggest that FGF signaling is necessary from the earliest stages of mesoderm specification and is required for all mesoderm development. Furthermore, they suggest that the archenteron is competent to form mesoderm or endoderm, and that FGF signaling from the ectoderm defines the location and amount of mesoderm. When considered in a comparative context, these data support a phylogenetically broad requirement for FGF8/17/18 signaling in mesoderm specification and suggest that FGF signaling played an ancestral role in deuterostome mesoderm formation.

Ancestral regulatory circuits governing ectoderm patterning downstream of Nodal and BMP2/4 revealed by gene regulatory network analysis in an echinoderm

Echinoderms, which are phylogenetically related to vertebrates and produce large numbers of transparent embryos that can be experimentally manipulated, offer many advantages for the analysis of the gene regulatory networks (GRN) regulating germ layer formation. During development of the sea urchin embryo, the ectoderm is the source of signals that pattern all three germ layers along the dorsal-ventral axis. How this signaling center controls patterning and morphogenesis of the embryo is not understood. Here, we report a large-scale analysis of the GRN deployed in response to the activity of this signaling center in the embryos of the Mediterranean sea urchin Paracentrotus lividus, in which studies with high spatial resolution are possible. By using a combination of in situ hybridization screening, overexpression of mRNA, recombinant ligand treatments, and morpholino-based loss-of-function studies, we identified a cohort of transcription factors and signaling molecules expressed in the ventral ectoderm, dorsal ectoderm, and interposed neurogenic ("ciliary band") region in response to the known key signaling molecules Nodal and BMP2/4 and defined the epistatic relationships between the most important genes. The resultant GRN showed a number of striking features. First, Nodal was found to be essential for the expression of all ventral and dorsal marker genes, and BMP2/4 for all dorsal genes. Second, goosecoid was identified as a central player in a regulatory sub-circuit controlling mouth formation, while tbx2/3 emerged as a critical factor for differentiation of the dorsal ectoderm. Finally, and unexpectedly, a neurogenic ectoderm regulatory circuit characterized by expression of "ciliary band" genes was triggered in the absence of TGF beta signaling. We propose a novel model for ectoderm regionalization, in which neural ectoderm is the default fate in the absence of TGF beta signaling, and suggest that the stomodeal and neural subcircuits that we uncovered may represent ancient regulatory pathways controlling embryonic patterning.

Extending the family table: Insights from beyond vertebrates into the regulation of embryonic development by FGFs

Since the discovery of fibroblast growth factors (FGFs) much focus has been placed on elucidating the roles for each vertebrate FGF ligand, receptor, and regulating molecules in the context of vertebrate development, human disorders and cancer. Studies in human, mouse, frog, chick, and zebrafish have made great contributions to our understanding of the role of FGFs in specific processes. However, in recent years, as more genomes are sequenced, information is becoming available from many non-vertebrate models and a more complete picture of the FGF superfamily as a whole is emerging. In some cases, less redundancy in these FGF signaling systems may allow for more mechanistic insights. Studies in sea anemones have highlighted how ancient FGF signaling is and helped provide insight into the evolution of the FGF gene family. Work in nematodes has shown that different splice forms can be used for functional specificity in invertebrate FGF signaling. Comparing FGFs between urochordates and vertebrates as well as between different insect species reveals important clues into the process of gene loss, duplication and subfunctionalization of FGFs throughout evolution. Finally, comparing all members of the FGF ligand superfamily reveals variability in many properties, which may point to a feature of FGFs as being highly adaptable with regards to protein structure and signaling mechanism. Further studies on FGF signaling outside of vertebrates is likely to continue to complement work in vertebrates by contributing additional insights to the FGF field and providing unexpected information that could be used for medical applications.

FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development

The sea urchin embryo is emerging as an attractive model to study morphogenetic processes such as directed migration of mesenchyme cells and cell sheet invagination, but surprisingly, few of the genes regulating these processes have yet been characterized. We present evidence that FGFA, the first FGF family member characterized in the sea urchin, regulates directed migration of mesenchyme cells, morphogenesis of the skeleton and gastrulation during early development. We found that at blastula stages, FGFA and a novel putative FGF receptor are expressed in a pattern that prefigures morphogenesis of the skeletogenic mesoderm and that suggests that FGFA is one of the elusive signals that guide migration of primary mesenchyme cells (PMCs). We first show that fgfA expression is correlated with abnormal migration and patterning of the PMCs following treatments that perturb specification of the ectoderm along the oral-aboral and animal-vegetal axes. Specification of the ectoderm initiated by Nodal is required to restrict fgfA to the lateral ectoderm, and in the absence of Nodal, fgfA is expressed ectopically throughout most of the ectoderm. Inhibition of either FGFA, FGFR1 or FGFR2 function severely affects morphogenesis of the skeleton. Furthermore,inhibition of FGFA and FGFR1 signaling dramatically delays invagination of the archenteron, prevents regionalization of the gut and abrogates formation of the stomodeum. We identified several genes acting downstream of fgfAin these processes, including the transcription factors pea3 and pax2/5/8 and the signaling molecule sprouty in the lateral ectoderm and SM30 and SM50 in the primary mesenchyme cells. This study identifies the FGF signaling pathway as an essential regulator of gastrulation and directed cell migration in the sea urchin embryo and as a key player in the gene regulatory network directing morphogenesis of the skeleton.

RTK and TGF-beta signaling pathways genes in the sea urchin genome

The Receptor Tyrosine kinase (RTK) and TGF-beta signaling pathways play essential roles during development in many organisms and regulate a plethora of cellular responses. From the genome sequence of Strongylocentrotus purpuratus, we have made an inventory of the genes encoding receptor tyrosine kinases and their ligands, and of the genes encoding cytokines of the TGF-beta superfamily and their downstream components. The sea urchin genome contains at least 20 genes coding for canonical receptor tyrosine kinases. Seventeen of the nineteen vertebrate RTK families are represented in the sea urchin. Fourteen of these RTK among which ALK, CCK4/PTK7, DDR, EGFR, EPH, LMR, MET/RON, MUSK, RET, ROR, ROS, RYK, TIE and TRK are present as single copy genes while pairs of related genes are present for VEGFR, FGFR and INSR. Similarly, nearly all the subfamilies of TGF-beta ligands identified in vertebrates are present in the sea urchin genome including the BMP, ADMP, GDF, Activin, Myostatin, Nodal and Lefty, as well as the TGF-beta sensu stricto that had not been characterized in invertebrates so far. Expression analysis indicates that the early expression of nodal, BMP2/4 and lefty is restricted to the oral ectoderm reflecting their role in providing positional information along the oral-aboral axis of the embryo. The coincidence between the emergence of TGF-beta-related factors such as Nodal and Lefty and the emergence of the deuterostome lineage strongly suggests that the ancestral function of Nodal could have been related to the secondary opening of the mouth which characterizes this clade, a hypothesis supported by functional data in the extant species. The sea urchin genome contains 6 genes encoding TGF-beta receptors and 4 genes encoding prototypical Smad proteins. Furthermore, most of the transcriptional activators and repressors shown to interact with Smads in vertebrates have orthologues in echinoderms. Finally, the sea urchin genome contains an almost complete repertoire of genes encoding extracellular modulators of BMP signaling including Chordin, Noggin, Sclerotin, SFRP, Gremlin, DAN and Twisted gastrulation. Taken together, these findings indicate that the sea urchin complement of genes of the RTK and TGF-beta signaling pathways is qualitatively very similar to the repertoire present in vertebrates, and that these genes are part of the common genetool kit for intercellular signaling of deuterostomes.

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.

Estradiol and endocrine disrupting compounds adversely affect development of sea urchin embryos at environmentally relevant concentrations

Environmental endocrine disrupting compounds (EDCs) are a wide variety of chemicals that typically exert effects, either directly or indirectly, through receptor-mediated processes, thus mimicking endogenous hormones and/or inhibiting normal hormone activities and metabolism. Little is known about the effects of EDCs on echinoderm physiology, reproduction and development. We exposed developing sea urchin embryos (Strongylocentrotus purpuratus and Lytechinus anamesus) to two known EDCs (4-octylphenol (OCT), bisphenol A (BisA)) and to natural and synthetic reproductive hormones (17beta-estradiol (E2), estrone (E1), estriol (E3), progesterone (P4) and 17alpha-ethynylestradiol (EE2)). In addition, we studied two non-estrogenic EDCs, tributyltin (TBT) and o,p-DDD. Successful development to the pluteus larval stage (96 h post-fertilization) was used to define EDC concentration-response relationships. The order of compound potency based on EC50 values for a reduction in normal development was as follows: TBT(L. anamesus)>OCT>TBT(S. purpuratus)>>E2>EE2>DDD>>BisA>P4>E1>>E3. The effect of TBT was pronounced even at concentrations substantially lower than those commonly reported in heavily contaminated areas, but the response was significantly different in the two model species. Sea urchin embryos were generally more sensitive to estrogenic EDCs and TBT than most other invertebrate larvae. Stage-specific exposure experiments were conducted to determine the most sensitive developmental periods using blastula, gastrula and post-gastrula (pluteus) stages. The stage most sensitive to E2, OCT and TBT was the blastula stage with less overall sensitivity in the gastrula stage, regardless of concentration. Selective estrogen receptor modulators (SERMs) were added to the experiments individually and in combination with estrogenic EDCs to interfere with potential receptor-mediated actions. Tamoxifen, a partial ER agonist, alone inhibited development at concentrations as low as 0.02 ng/ml and was effective at this concentration in decreasing the sensitivities of the embryos to estradiol and estrogenic EDCs. The complete antagonist ICI 182,780 inhibited development at concentrations as low as 0.03 ng/ml but increased embryo sensitivity to estradiol and estrogenic EDCs. Estradiol and estrogenic EDCs all cause developmental toxicity in sea urchins through a TAM-sensitive but an ICI-insensitive mechanism. It remains to be demonstrated whether this mechanism involves an estrogen-responsive nuclear receptor (NR), a membrane receptor (NR or non-NR-related) or a completely different mechanism of toxicity. However, early embryo sensitivity and the differential response to SERM co-incubation further suggests more than one mode of EDC action in the developing sea urchin embryo.

Role of Yes kinase during early zebrafish development

We have identified the Yes kinase in zebrafish eggs and investigated its role in development of the zebrafish embryo. In situ hybridization as well as immunofluorescence techniques demonstrated that Yes kinase is maternally expressed and is localized to the cortical region of the unfertilized egg. Fertilization resulted in concentration of Yes kinase to the blastodisc where it continued to be localized to the blastoderm cells through cleavage, gastrulation, and later development. Yes kinase activity was found to decrease abruptly at fertilization, then increase progressively during epiboly, and was maintained at high levels throughout gastrulation. The role of Yes kinase in development was tested by treating embryos with chemical protein tyrosine kinase (PTK) inhibitors such as 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP2) and by injection of antisense morpholinos. Both treatments resulted in the arrest of development at the beginning of the epiboly. Co-immunoprecipitation studies demonstrated that Yes kinase participates in a stable complex with focal adhesion kinase (FAK), which is phosphorylated in vitro. These results demonstrate that Yes kinase plays an important role in epiboly and indicate that Yes kinase participates in signaling by focal adhesion kinase during early development.

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.

Of urchins and men: evolution of an alternative splicing unit in fibroblast growth factor receptor genes

Alternative splicing of mammalian transcripts, which yields many diverse protein products from one gene, is the rule and not the exception. Although the mechanisms that govern alternative splicing are being unraveled, little is known about the evolution of this critical engine of proteome diversity. Here we present a phylogenetic analysis from a sea urchin to humans of the alternative splicing unit encoding the third Ig domain of fibroblast growth factor receptors. The remarkable conservation of intronic control elements, both in structure and function, indicates that the mechanisms that regulate this alternative splicing unit evolved over 600 million years ago.

Planarian fibroblast growth factor receptor homologs expressed in stem cells and cephalic ganglions

The strong regenerative capacity of planarians is considered to reside in the totipotent somatic stem cell called the 'neoblast'. However, the signal systems regulating the differentiation/growth/migration of stem cells remain unclear. The fibroblast growth factor (FGF)/FGF receptor (FGFR) system is thought to mediate various developmental events in both vertebrates and invertebrates. We examined the molecular structures and expression of DjFGFR1 and DjFGFR2, two planarian genes closely related to other animal FGFR genes. DjFGFR1 and DjFGFR2 proteins contain three and two immunoglobulin-like domains, respectively, in the extracellular region and a split tyrosine kinase domain in the intracellular region. Expression of DjFGFR1 and DjFGFR2 was observed in the cephalic ganglion and mesenchymal space in intact planarians. In regenerating planarians, accumulation of DjFGFR1-expressing cells was observed in the blastema and in fragments regenerating either a pharynx or a brain. In X-ray-irradiated planarians, which had lost regenerative capacity, the number of DjFGFR1-expressing cells in the mesenchymal space decreased markedly. These results suggest that the DjFGFR1 protein may be involved in the signal systems controlling such aspects of planarian regeneration as differentiation/growth/migration of stem cells.

Association of the signaling adaptor FRS2 with fibroblast growth factor receptor 1 (Fgfr1) is mediated by alternative splicing of the juxtamembrane domain

Fibroblast growth factor receptors (FGFRs) are a family of transmembrane tyrosine kinases involved in signaling via interactions with the family of fibroblast growth factors. Alternative splicing of the juxtamembrane region of FGFR1-3 leads to the inclusion or exclusion of two amino acids, valine and threonine, the VT site. The presence or absence of VT (VT+ or VT-, respectively) affects the signaling potential of the receptor. The VT+ receptor isoform is required for Erk2 phosphorylation, a component of the mitogen-activated protein kinase signaling pathway. FRS2 is an adaptor protein that links FGFRs to the mitogen-activated protein kinase signaling pathway. FRS2 interacts with a region of the juxtamembrane domain of FGFR1 that includes the alternatively spliced VT site. We investigated the interaction of FRS2 with murine Fgfr1 juxtamembrane domain. We showed the alternatively spliced VT motif, at the juxtamembrane domain of Fgfr1 is required for FRS2 interaction with Fgfr1. Activation of signaling pathways from FRS2 is likely to be regulated by controlling the Fgfr1/FRS2 interaction through alternative splicing of the VT motif of Fgfr1.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Sea urchin FGFR muscle-specific expression: posttranscriptional regulation in embryos and adults

We have shown previously by in situ hybridization that a gene encoding a fibroblast growth factor receptor (SpFGFR) is transcribed in many cell types during the initial phases of sea urchin embryogenesis (Strongylocentrotus purpuratus) (McCoon et al., J. Biol. Chem. 271, 20119-20195, 1996). Here we demonstrate by immunostaining with affinity-purified antibody that SpFGFR protein is detectable only in muscle cells of the embryo and appears at a time suggesting that its function is not in commitment to a muscle fate, but instead may be required to support the proliferation, migration, and/or differentiation of myoblasts. Surprisingly, we find that SpFGFR transcripts are enriched in embryo nuclei, suggesting that lack of processing and/or cytoplasmic transport in nonmuscle cells is at least part of the posttranscriptional regulatory mechanism. Western blots show that SpFGFR is also specifically expressed in adult lantern muscle, but is not detectable in other smooth muscle-containing tissues, including tube foot and intestine, or in coelomocytes, despite the presence of SpFGFR transcripts at similar concentrations in all these tissues. We conclude that in both embryos and adults, muscle-specific SpFGF receptor synthesis is controlled primarily at a posttranscriptional level. We show by RNase protection assays that transcripts encoding the IgS variant of the ligand binding domain of the receptor, previously shown to be enriched in embryo endomesoderm fractions, are the predominant, if not exclusive, SpFGFR transcripts in lantern muscle. Together, these results suggest that only a minority of SpFGFR transcripts are processed, exported, and translated in both adult and embryonic muscle cells and these contain predominantly, if not exclusively, IgS ligand binding domain sequences.

Two functional forms of vascular endothelial growth factor receptor-2/Flk-1 mRNA are expressed in normal rat retina

Vascular endothelial growth factor (VEGF) is an important mediator of ocular neovascularization by exerting its endothelial specific mitogenic effects through high affinity tyrosine kinase receptors. By screening a rat retina cDNA library, we have isolated a clone encoding the full-length prototypic form of the rat VEGF receptor-2/Flk-1, as well as a short form of the mRNA that encodes the complete seven N-terminal immunoglobulin-like extracellular ligand-binding domains, transmembrane region, NH2-terminal half of the intracellular kinase domain, and kinase insert domain but does not encode the COOH-terminal half of the intracellular kinase domain and carboxyl-terminal region. Both forms of mRNA are detected in rat retina, although the short form is expressed at a lower level. VEGF induced a biphasic increase of cytoplasmic calcium with both forms in HK 293 transfected cells, indicating that both forms of the VEGF receptor-2/Flk-1 are functional and that the COOH-terminal half of the intracellular kinase domain and carboxyl region of VEGF receptor-2/Flk-1 are not strictly necessary for either ligand binding or this biological activity.

The protein tyrosine kinases of the sea urchin Anthocidaris crassispina

In order to know the function of protein tyrosine kinases (PTKs) in the development of sea urchin embryos, we performed reverse transcription-polymerase chain reaction (RT-PCR) to obtain partial cDNA fragments for PTK genes using primers to highly conserved regions of the PTK family. A total of seven PTK sequences were identified, two of which represented receptor PTK (RTK1 and RTK2), and five of which were non-receptor PTKs (NRTK1-5). RTK1 was highly similar to FGF receptor and Ret kinase, while RTK2 showed features of the insulin receptor family. NRTK1 and 2 belonged to the Src family and could be involved in egg activation at fertilization. NRTK3 showed the features of the Btk family kinases, while NRTK4 seemed to be a member of the Syk/ZAP70 family. NRTK5 is the Csk-type kinase of the sea urchin, which is known to negatively regulate the Src family kinases. RTK1 was not detected in unfertilized eggs and was activated after blastula stage. All the other PTK genes were expressed both maternally in unfertilized eggs and zygotically after fertilization, though each gene showed distinct temporal patterns.

Specification of endoderm in the sea urchin embryo

Expression of the endoderm specific gene Endo16, was used to monitor endoderm specification in developmentally arrested and in dissociated embryos. beta-APN treatment halts gastrulation, however, in two species of sea urchins Endo16 mRNA is still expressed, suggesting that specification of the endodermal lineage has taken place in these developmentally arrested embryos. Endo16 mRNA is not expressed in embryos dissociated at the 4-8-cell stage unless they are reassociated shortly after the 16-cell stage. Interestingly, dissociation after the 16-cell stage also results in a lack of Endo16 expression. Lithium is unable to rescue Endo16 expression in these dissociated embryos. These results indicate that early signaling events mediated by cell-cell contact are required for the initial specification and maintenance of the endoderm in the developing embryo.