A complete second gut induced by transplanted micromeres in the sea urchin embryo

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, species diversification. The micromere of the sea urchin embryo may serve as one of those examples: an ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, activator of G-protein signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, to species diversification. The micromere of the sea urchin embryo may serve as one of those examples: An ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, Activator of G-protein Signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

Pigment cells: Paragons of cellular development

Larvae of sea urchins have a population of conspicuous pigmented cells embedded in the outer surface epithelium. Pigment cells are a distinct mesodermal lineage that gives rise to a key component of the larval immune system. During cleavage, signaling from adjacent cells influences a small crescent of cells to initiate a network of genetic interactions that prepare the cells for morphogenesis and specializes them as immunocytes. The cells become active during gastrulation, detach from the epithelium, migrate through the blastocoel, and insert into the ectoderm where they complete their differentiation. Studies of pigment cell development have helped establish how cellular signaling controls networks of genetic interactions that bring about morphogenesis and differentiation. This review summarizes studies of pigment cell development and concludes that pigment cells are an excellent experimental model. Pigment cells provide several opportunities to further test and refine our understanding of the molecular basis of cellular development.

Micromere formation and its evolutionary implications in the sea urchin

The micromeres of the sea urchin embryo are distinct from other blastomeres. After they arise through an asymmetric cell division at the 8- to 16-cell stage, micromeres immediately function as organizers. They also commit themselves to specific cell fates such as larval skeletogenic cells and primordial germ cells, while other blastomeres remain plastic and uncommitted at the 16-cell stage. In the phylum Echinodermata, only the sea urchin (class Echinoidea) embryo forms micromeres that serve as apparent organizers during early embryogenesis. Therefore, it is considered that micromeres are the derived features and that modification(s) of the developmental system allowed evolutionary introduction of this unique cell lineage. In this chapter, we summarize the both historic and recent observations that demonstrate unique properties of micromeres and discuss how this lineage of micromeres may have arisen during echinoderm evolution.

Reprint of: Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Diversity of activator of G-protein signaling (AGS)-family proteins and their impact on asymmetric cell division across taxa

Asymmetric cell division (ACD) is a cellular process that forms two different cell types through a cell division and is thus critical for the development of all multicellular organisms. Not all but many of the ACD processes are mediated by proper orientation of the mitotic spindle, which segregates the fate determinants asymmetrically into daughter cells. In many cell types, the evolutionarily conserved protein complex of Gαi/AGS-family protein/NuMA-like protein appears to play critical roles in orienting the spindle and/or generating the polarized cortical forces to regulate ACD. Studies in various organisms reveal that this conserved protein complex is slightly modified in each phylum or even within species. In particular, AGS-family proteins appear to be modified with a variable number of motifs in their functional domains across taxa. This apparently creates different molecular interactions and mechanisms of ACD in each developmental program, ultimately contributing to developmental diversity across species. In this review, we discuss how a conserved ACD machinery has been modified in each phylum over the course of evolution with a major focus on the molecular evolution of AGS-family proteins and its impact on ACD regulation.

Maternal control of early patterning in sea urchin embryos

Sea urchin development has been studied extensively for more than a century and considered regulative since the first experimental evidence. Further investigations have repeatedly supported this standpoint by revealing the presence of inductive mechanisms that alter cell fate decisions at early cleavage stages and flexibility of development in response to environmental conditions. Some features indicate that sea urchin development is not completely regulative, but actually includes determinative events. In 16-cell embryos, mesomeres and macromeres represent multipotency, while the cell fate of most vegetal micromeres is restricted. It is known that the mature sea urchin eggs are polarized by the asymmetrical distribution of some maternal mRNAs and proteins. Spatially-distributed maternal factors are necessary for the orientation of the primary animal-vegetal axis, which is established by both maternal and zygotic mechanisms later in development. The secondary dorsal-ventral axis is conditionally specified later in development. Dorsal-ventral polarity is very liable during the early cleavages, though more recent data argue that its direction may be oriented by maternal asymmetry. In this review, we focus on the role of maternal factors in initial embryonic patterning during the first cleavages of sea urchin embryos before activation of the embryonic genome.

Evolutionary modification of AGS protein contributes to formation of micromeres in sea urchins

Evolution is proposed to result, in part, from acquisition of new developmental programs. One such example is the appearance of the micromeres in a sea urchin that form by an asymmetric cell division at the 4th embryonic cleavage and function as a major signaling center in the embryo. Micromeres are not present in other echinoderms and thus are  considered as a derived feature, yet its acquisition mechanism is unknown. Here, we report that the polarity factor AGS and its associated proteins are responsible for micromere formation. Evolutionary modifications of AGS protein seem to have provided the cortical recruitment and binding of AGS to the vegetal cortex, contributing to formation of micromeres in the sea urchins. Indeed, introduction of sea urchin AGS into the sea star embryo induces asymmetric cell divisions, suggesting that the molecular evolution of AGS protein is key in the transition of echinoderms to micromere formation and the current developmental style of sea urchins not seen in other echinoderms.

HpEtsimplicated in primary mesenchyme cell differentiation of the sea urchin (Hemicentrotus pulcherrimus) embryo

In sea urchin embryogenesis it has been suggested that the initial territories are specified by a combination of the asymmetric distribution of cytoplasmic determinants and cell-cell interactions. At the 60-cell stage blastomeres clonally originated from founder cells divide the embryo into five distinct territories: small micromeres, large micromeres, vegetal plate, oral ectoderm and aboral ectoderm. The territories are identified by the expression of specific marker genes and their cell lineages (Davidson, 1989, 1991). The large micromeres are thought to play a role as an organiser and initiate a cascade of signal transduction toward overlying cells (Davidson, 1989). In this model the large micromeres induce the overlying veg2 tier, specifying the vegetal plate (Ransick & Davidson, 1993, 1995). The veg2 tier then induces the overlying cells, which include gut cells and cells of the prospective ectodermal territories (Wikramanayakeet al., 1995; Wikramanayake & Klein, 1997). Thus, the large micromeres, which are the prospective primary mesenchyme cells (PMCs), play a key role in cell fate specification and axis determination during sea urchin embryogenesis. Previous data suggested that the large micromeres are autonomously specified to become PMCs by maternally inherited determinants (Okazaki, 1975; Kitajima & Okazaki, 1980). An important question in sea urchins embryogenesis is the identity and function of the proposed maternal determinants.

On the evolution of bilaterality

Bilaterality – the possession of two orthogonal body axes – is the name-giving trait of all bilaterian animals. These body axes are established during early embryogenesis and serve as a three-dimensional coordinate system that provides crucial spatial cues for developing cells, tissues, organs and appendages. The emergence of bilaterality was a major evolutionary transition, as it allowed animals to evolve more complex body plans. Therefore, how bilaterality evolved and whether it evolved once or several times independently is a fundamental issue in evolutionary developmental biology. Recent findings from non-bilaterian animals, in particular from Cnidaria, the sister group to Bilateria, have shed new light into the evolutionary origin of bilaterality. Here, we compare the molecular control of body axes in radially and bilaterally symmetric cnidarians and bilaterians, identify the minimal set of traits common for Bilateria, and evaluate whether bilaterality arose once or more than once during evolution.

Pre-bilaterian origin of the blastoporal axial organizer

The startling capacity of the amphibian Spemann organizer to induce naïve cells to form a Siamese twin embryo with a second set of body axes is one of the hallmarks of developmental biology. However, the axis-inducing potential of the blastopore-associated tissue is commonly regarded as a chordate feature. Here we show that the blastopore lip of a non-bilaterian metazoan, the anthozoan cnidarian Nematostella vectensis, possesses the same capacity and uses the same molecular mechanism for inducing extra axes as chordates: Wnt/β-catenin signaling. We also demonstrate that the establishment of the secondary, directive axis in Nematostella by BMP signaling is sensitive to an initial Wnt signal, but once established the directive axis becomes Wnt-independent. By combining molecular analysis with experimental embryology, we provide evidence that the emergence of the Wnt/β-catenin driven blastopore-associated axial organizer predated the cnidarian-bilaterian split over 600 million years ago.

Eric Davidson: Steps to a gene regulatory network for development

Eric Harris Davidson was a unique and creative intellectual force who grappled with the diversity of developmental processes used by animal embryos and wrestled them into an intelligible set of principles, then spent his life translating these process elements into molecularly definable terms through the architecture of gene regulatory networks. He took speculative risks in his theoretical writing but ran a highly organized, rigorous experimental program that yielded an unprecedentedly full characterization of a developing organism. His writings created logical order and a framework for mechanism from the complex phenomena at the heart of advanced multicellular organism development. This is a reminiscence of intellectual currents in his work as observed by the author through the last 30-35 years of Davidson's life.

Germ Line Versus Soma in the Transition from Egg to Embryo

With few exceptions, all animals acquire the ability to produce eggs or sperm at some point in their life cycle. Despite this near-universal requirement for sexual reproduction, there exists an incredible diversity in germ line development. For example, animals exhibit a vast range of differences in the timing at which the germ line, which retains reproductive potential, separates from the soma, or terminally differentiated, nonreproductive cells. This separation may occur during embryonic development, after gastrulation, or even in adults, depending on the organism. The molecular mechanisms of germ line segregation are also highly diverse, and intimately intertwined with the overall transition from a fertilized egg to an embryo. The earliest embryonic stages of many species are largely controlled by maternally supplied factors. Later in development, patterning control shifts to the embryonic genome and, concomitantly with this transition, the maternally supplied factors are broadly degraded. This chapter attempts to integrate these processes--germ line segregation, and how the divergence of germ line and soma may utilize the egg to embryo transitions differently. In some embryos, this difference is subtle or maybe lacking altogether, whereas in other embryos, this difference in utilization may be a key step in the divergence of the two lineages. Here, we will focus our discussion on the echinoderms, and in particular the sea urchins, in which recent studies have provided mechanistic understanding in germ line determination. We propose that the germ line in sea urchins requires an acquisition of maternal factors from the egg and, when compared to other members of the taxon, this appears to be a derived mechanism. The acquisition is early--at the 32-cell stage--and involves active protection of maternal mRNAs, which are instead degraded in somatic cells with the maternal-to-embryonic transition. We collectively refer to this model as the Time Capsule method for germ line determination.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

Ca²⁺ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos

Sea urchin embryos initiate cell specifications at the 16-cell stage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal-vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the β-catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal-vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of β-catenin. To this end, we surveyed the expression pattern of β-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of β-catenin, but also regulates a variety of cell specification events in the early sea urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.

The biology of the germ line in echinoderms

The formation of the germ line in an embryo marks a fresh round of reproductive potential. The developmental stage and location within the embryo where the primordial germ cells (PGCs) form, however, differs markedly among species. In many animals, the germ line is formed by an inherited mechanism, in which molecules made and selectively partitioned within the oocyte drive the early development of cells that acquire this material to a germ-line fate. In contrast, the germ line of other animals is fated by an inductive mechanism that involves signaling between cells that directs this specialized fate. In this review, we explore the mechanisms of germ-line determination in echinoderms, an early-branching sister group to the chordates. One member of the phylum, sea urchins, appears to use an inherited mechanism of germ-line formation, whereas their relatives, the sea stars, appear to use an inductive mechanism. We first integrate the experimental results currently available for germ-line determination in the sea urchin, for which considerable new information is available, and then broaden the investigation to the lesser-known mechanisms in sea stars and other echinoderms. Even with this limited insight, it appears that sea stars, and perhaps the majority of the echinoderm taxon, rely on inductive mechanisms for germ-line fate determination. This enables a strongly contrasted picture for germ-line determination in this phylum, but one for which transitions between different modes of germ-line determination might now be experimentally addressed.

Nuclearization of β-catenin in ectodermal precursors confers organizer-like ability to induce endomesoderm and pattern a pluteus larva

BACKGROUND

In many bilaterians, asymmetric activation of canonical Wnt (cWnt) signaling at the posterior pole is critical for anterior-posterior (AP) body axis formation. In 16-cell stage sea urchins, nuclearization of β-catenin in micromeres activates a gene regulatory network that defines body axes and induces endomesoderm. Transplanting micromeres to the animal pole of a host embryo induces ectopic endomesoderm in the mesomeres (ectoderm precursors) whereas inhibiting cWnt signaling blocks their endomesoderm-inducing activity and the micromeres become ectoderm-like. We have tested whether ectopic activation of cWnt signaling in mesomeres is sufficient to impart the cells with organizer-like abilities, allowing them to pattern normal embryonic body axes when recombined with a field of mesomeres.

RESULTS

Fertilized eggs were microinjected with constitutively active Xenopus β-catenin (actβ-cat) mRNA and allowed to develop until the 16-cell stage. Two mesomeres from injected embryos were then recombined with isolated animal halves (AH) from uninjected 16-cell stage embryos. Control chimeras produced animalized phenotypes (hollow balls of ectoderm) and rarely formed skeletogenic mesoderm (SM)-derived spicules, endoderm or pigment cells, a type of non-skeletogenic mesoderm (NSM). In contrast, over half of the 0.5 pg/pL actβ-cat mesomere/AH chimeras formed a partial or complete gut (exhibiting AP polarity), contained mesenchyme-like cells similar to SM, and produced pigment cells. At three days, chimeras formed plutei with normal embryonic body axes. When fates of the actβ-cat mRNA-injected mesomeres were tracked, we found that injected mesomeres formed mesenchyme-like and pigment cells, but endoderm was induced. Higher concentrations of actβ-cat mRNA were less likely to induce endoderm or pigment cells, but had similar mesenchyme-like cell production to 0.5 pg/pL actβ-cat mesomere/AH chimeras.

CONCLUSIONS

Our results show that nuclear β-catenin is sufficient to endow naïve cells with the ability to act as an organizing center and that β-catenin has both cell-autonomous and non-autonomous effects on cell fate specification in a concentration-dependent manner. These results are consistent with the hypothesis that a shift in the site of early cWnt signaling in cleaving embryos could have modified polarity of the main body axes during metazoan evolution.

ISWI contributes to ArsI insulator function in development of the sea urchin

Insulators are genomic elements that regulate transcriptional activity by forming chromatin boundaries. Various DNA insulators have been identified or are postulated in many organisms, and the paradigmatic CTCF-dependent insulators are perhaps the best understood and most widespread in function. The diversity of DNA insulators is, however, understudied, especially in the context of embryonic development, when many new gene territories undergo transitions in functionality. Here we report the functional analysis of the arylsulfatase insulator (ArsI) derived from the sea urchin, which has conserved insulator activity throughout the many metazoans tested, but for which the molecular mechanism of function is unknown. Using a rapid in vivo assay system and a high-throughput mega-shift assay, we identified a minimal region in ArsI that is responsible for its insulator function. We discovered a small set of proteins specifically bound to the minimal ArsI region, including ISWI, a known chromatin-remodeling protein. During embryogenesis, ISWI was found to interact with select ArsI sites throughout the genome, and when inactivated led to misregulation of select gene expression, loss of insulator activity and aberrant morphogenesis. These studies reveal a mechanistic basis for ArsI function in the gene regulatory network of early development.

Predictive computation of genomic logic processing functions in embryonic development

Gene regulatory networks (GRNs) control the dynamic spatial patterns of regulatory gene expression in development. Thus, in principle, GRN models may provide system-level, causal explanations of developmental process. To test this assertion, we have transformed a relatively well-established GRN model into a predictive, dynamic Boolean computational model. This Boolean model computes spatial and temporal gene expression according to the regulatory logic and gene interactions specified in a GRN model for embryonic development in the sea urchin. Additional information input into the model included the progressive embryonic geometry and gene expression kinetics. The resulting model predicted gene expression patterns for a large number of individual regulatory genes each hour up to gastrulation (30 h) in four different spatial domains of the embryo. Direct comparison with experimental observations showed that the model predictively computed these patterns with remarkable spatial and temporal accuracy. In addition, we used this model to carry out in silico perturbations of regulatory functions and of embryonic spatial organization. The model computationally reproduced the altered developmental functions observed experimentally. Two major conclusions are that the starting GRN model contains sufficiently complete regulatory information to permit explanation of a complex developmental process of gene expression solely in terms of genomic regulatory code, and that the Boolean model provides a tool with which to test in silico regulatory circuitry and developmental perturbations.

Morphogenesis in sea urchin embryos: linking cellular events to gene regulatory network states

Gastrulation in the sea urchin begins with ingression of the primary mesenchyme cells (PMCs) at the vegetal pole of the embryo. After entering the blastocoel the PMCs migrate, form a syncitium, and synthesize the skeleton of the embryo. Several hours after the PMCs ingress the vegetal plate buckles to initiate invagination of the archenteron. That morphogenetic process occurs in several steps. The nonskeletogenic cells produce the initial inbending of the vegetal plate. Endoderm cells then rearrange and extend the length of the gut across the blastocoel to a target near the animal pole. Finally, cells that will form part of the midgut and hindgut are added to complete gastrulation. Later, the stomodeum invaginates from the oral ectoderm and fuses with the foregut to complete the archenteron. In advance of, and during these morphogenetic events, an increasingly complex input of transcription factors controls the specification and the cell biological events that conduct the gastrulation movements.

Micron: an Actively Stabilized Handheld Tool for Microsurgery

We describe the design and performance of a hand-held actively stabilized tool to increase accuracy in micro-surgery or other precision manipulation. It removes involuntary motion such as tremor by actuating the tip to counteract the effect of the undesired handle motion. The key components are a three-degree-of-freedom piezoelectric manipulator that has 400 μm range of motion, 1 N force capability, and bandwidth over 100 Hz, and an optical position measurement subsystem that acquires the tool pose with 4 μm resolution at 2000 samples/s. A control system using these components attenuates hand motion by at least 15 dB (a fivefold reduction). By considering the effect of the frequency response of Micron on the human visual feedback loop, we have developed a filter that reduces unintentional motion, yet preserves intuitive eye-hand coordination. We evaluated the effectiveness of Micron by measuring the accuracy of the human/machine system in three simple manipulation tasks. Handheld testing by three eye surgeons and three non-surgeons showed a reduction in position error of between 32% and 52%, depending on the error metric.

Secondary embryonic axis formation by transplantation of D quadrant micromeres in an oligochaete annelid

Among spiral cleaving embryos (e.g. mollusks and annelids), it has long been known that one blastomere at the four-cell stage, the D cell, and its direct descendants play an important role in axial pattern formation. Various studies have suggested that the D quadrant acts as the organizer of the embryonic axes in annelids, although this has never been demonstrated directly. Here we show that D quadrant micromeres (2d and 4d) of the oligochaete annelid Tubifex tubifex are essential for embryonic axis formation. When 2d and 4d were ablated the embryo developed into a rounded cell mass covered with an epithelial cell sheet. To examine whether 2d and 4d are sufficient for axis formation they were transplanted to an ectopic position in an otherwise intact embryo. The reconstituted embryo formed a secondary embryonic axis with a duplicated head and/or tail. Cell lineage analyses showed that neuroectoderm and mesoderm along the secondary axis were derived from the transplanted D quadrant micromeres and not from the host embryo. However, endodermal tissue along the secondary axis originated from the host embryo. Interestingly, when either 2d or 4d was transplanted separately to host embryos, the reconstituted embryos failed to form a secondary axis, suggesting that both 2d and 4d are required for secondary axis formation. Thus, the Tubifex D quadrant micromeres have the ability to organize axis formation, but they lack the ability to induce neuroectodermal tissues, a characteristic common to chordate primary embryonic organizers.

Small micromeres contribute to the germline in the sea urchin

Many indirect developing animals create specialized multipotent cells in early development to construct the adult body and perhaps to hold the fate of the primordial germ cells. In sea urchin embryos, small micromeres formed at the fifth division appear to be such multipotent cells: they are relatively quiescent in embryos, but contribute significantly to the coelomic sacs of the larvae, from which the major tissues of the adult rudiment are derived. These cells appear to be regulated by a conserved gene set that includes the classic germline lineage genes vasa, nanos and piwi. In vivo lineage mapping of the cells awaits genetic manipulation of the lineage, but previous research has demonstrated that the germline is not specified at the fourth division because animals are fertile even when micromeres, the parent blastomeres of small micromeres, are deleted. Here, we have deleted small micromeres at the fifth division and have raised the resultant larvae to maturity. These embryos developed normally and did not overexpress Vasa, as did embryos from a micromere deletion, implying the compensatory gene regulatory network was not activated in small micromere-deleted embryos. Adults from control and micromere-deleted embryos developed gonads and visible gametes, whereas small micromere-deleted animals formed small gonads that lacked gametes. Quantitative PCR results indicate that small micromere-deleted animals produce background levels of germ cell products, but not specifically eggs or sperm. These results suggest that germline specification depends on the small micromeres, either directly as lineage products, or indirectly by signaling mechanisms emanating from the small micromeres or their descendants.

Implication of HpEts in gene regulatory networks responsible for specification of sea urchin skeletogenic primary mesenchyme cells

The large micromeres of the 32-cell stage of sea urchin embryos are autonomously specified and differentiate into primary mesenchyme cells (PMCs), giving rise to the skeletogenic cells. We previously demonstrated that HpEts, an ets-related transcription factor, plays an essential role in the specification of PMCs in sea urchin embryos. In order to clarify the function of HpEts in the gene regulatory network involved in PMC specification, we analyzed the zygotic expression pattern and the cis-regulatory region of HpEts, and examined the activity of the HpEts protein as a transcription factor. Intron-based PCR reveals that zygotic expression of HpEts starts at the cleavage stage, and that the rate of transcription reaches maximum at the unhatched blastula stage. A series of progressive deletions of the fragments from -4.2 kbp to +1206 bp of the HpEts, which directs PMC-specific expression, caused a gradual decrease in the specificity, implying that coordination of several cis-regulatory elements regulates the expression in PMCs. A minimum cis-element required for the temporal expression is located within a 10 bp from -243 bp to -234 bp. The HpEts protein remains in the cytoplasm of entire embryonic cells in the cleavage stage. At the unhatched blastula stage, the HpEts protein translocates into the nucleus in presumptive PMCs. Transactivation assays demonstrate that the HpEts protein activates a promoter of Spicule Matrix Protein 50 (SM50), which is a target of HpEts, which binds to the regulatory region of SM50.

Modularity and design principles in the sea urchin embryo gene regulatory network

The gene regulatory network (GRN) established experimentally for the pre-gastrular sea urchin embryo provides causal explanations of the biological functions required for spatial specification of embryonic regulatory states. Here we focus on the structure of the GRN which controls the progressive increase in complexity of territorial regulatory states during embryogenesis; and on the types of modular subcircuits of which the GRN is composed. Each of these subcircuit topologies executes a particular operation of spatial information processing. The GRN architecture reflects the particular mode of embryogenesis represented by sea urchin development. Network structure not only specifies the linkages constituting the genomic regulatory code for development, but also indicates the various regulatory requirements of regional developmental processes.

Cell micromanipulation with an active handheld micromanipulator

The paper describes the use of an active handheld micromanipulator, known as Micron, for micromanipulation of cells. The device enables users to manipulate objects on the order of tens of microns in size, with the natural ease of use of a fully handheld tool. Micron senses its own position using a purpose-built microscale optical tracker, estimates the erroneous or undesired component of hand motion, and actively corrects it by deflecting its own tool tip using piezoelectric actuators. Benchtop experiments in tip positioning show that active compensation can reduce positioning error by up to 51% compared to unaided performance. Preliminary experiments in bisection of sea urchin embryos exhibit an increased success rate when performed with the help of Micron.

The endoderm gene regulatory network in sea urchin embryos up to mid-blastula stage

As the result of early specification processes, sea urchin embryos eventually form various mesodermal cell lineages and a gut consisting of fore-, mid- and hindgut. The progression of specification as well as the overall spatial organization of the organism is encoded in its gene regulatory networks (GRNs). We have analyzed the GRN driving endoderm specification up to the onset of gastrulation and present in this paper the mechanisms which determine this process up to mid-blastula stage. At this stage, the embryo consists of two separate lineages of endoderm precursor cells with distinct regulatory states. One of these lineages, the veg2 cell lineage, gives rise to endoderm and mesoderm cell types. The separation of these cell fates is initiated by the spatially confined activation of the mesoderm GRN superimposed on a generally activated endoderm GRN within veg2 descendants. Here we integrate the architecture of regulatory interactions with the spatial restriction of regulatory gene expression to model the logic control of endoderm development.

Genomic control of patterning

The development of multicellular organisms involves the partitioning of the organism into territories of cells of specific structure and function. The information for spatial patterning processes is directly encoded in the genome. The genome determines its own usage depending on stage and position, by means of interactions that constitute gene regulatory networks (GRNs). The GRN driving endomesoderm development in sea urchin embryos illustrates different regulatory strategies by which developmental programs are initiated, orchestrated, stabilized or excluded to define the pattern of specified territories in the developing embryo.

Gene regulatory network interactions in sea urchin endomesoderm induction

A major goal of contemporary studies of embryonic development is to understand large sets of regulatory changes that accompany the phenomenon of embryonic induction. The highly resolved sea urchin pregastrular endomesoderm-gene regulatory network (EM-GRN) provides a unique framework to study the global regulatory interactions underlying endomesoderm induction. Vegetal micromeres of the sea urchin embryo constitute a classic endomesoderm signaling center, whose potential to induce archenteron formation from presumptive ectoderm was demonstrated almost a century ago. In this work, we ectopically activate the primary mesenchyme cell-GRN (PMC-GRN) that operates in micromere progeny by misexpressing the micromere determinant Pmar1 and identify the responding EM-GRN that is induced in animal blastomeres. Using localized loss-of -function analyses in conjunction with expression of endo16, the molecular definition of micromere-dependent endomesoderm specification, we show that the TGFbeta cytokine, ActivinB, is an essential component of this induction in blastomeres that emit this signal, as well as in cells that respond to it. We report that normal pregastrular endomesoderm specification requires activation of the Pmar1-inducible subset of the EM-GRN by the same cytokine, strongly suggesting that early micromere-mediated endomesoderm specification, which regulates timely gastrulation in the sea urchin embryo, is also ActivinB dependent. This study unexpectedly uncovers the existence of an additional uncharacterized micromere signal to endomesoderm progenitors, significantly revising existing models. In one of the first network-level characterizations of an intercellular inductive phenomenon, we describe an important in vivo model of the requirement of ActivinB signaling in the earliest steps of embryonic endomesoderm progenitor specification.

Chapter 7. Axis formation and the rapid evolutionary transformation of larval form

Marine invertebrate embryos and larvae are diverse and can evolve rapidly, providing a link between early developmental and evolutionary mechanisms. We here discuss the role of evolutionary changes in axis formation, which is a crucial part of the patterning of marine embryos and larvae. We focus on sea urchin embryos, where axial features are well defined and subject to active current investigation. The genetic control of processes of formation of the three axial systems, animal-vegetal, dorsal-ventral, and left-right, is becoming established for species that undergo development via the feeding pluteus larva. These species represent the primitive condition among living sea urchins. We compare their developmental processes to the highly modified development of a species that has evolved a nonfeeding larva. This derived form has accelerated some elements of axis formation, and eliminated or modified others. Three features of embryonic/larval evolution stand out (1) evolution of developmental features occurs rapidly over geological time; (2) upstream gene regulatory systems of axis formation are conserved, whereas downstream features evolve rapidly; and (3) heterochronies play an important role.

Twist is an essential regulator of the skeletogenic gene regulatory network in the sea urchin embryo

Recent work on the sea urchin endomesoderm gene regulatory network (GRN) offers many opportunities to study the specification and differentiation of each cell type during early development at a mechanistic level. The mesoderm lineages consist of two cell populations, primary and secondary mesenchyme cells (PMCs and SMCs). The micromere-PMC GRN governs the development of the larval skeleton, which is the exclusive fate of PMCs, and SMCs diverge into four lineages, each with its own GRN state. Here we identify a sea urchin ortholog of the Twist transcription factor, and show that it plays an essential role in the PMC GRN and later is involved in SMC formation. Perturbations of Twist either by morpholino knockdown or by overexpression result in defects in progressive phases of PMC development, including specification, ingression/EMT, differentiation and skeletogenesis. Evidence is presented that Twist expression is required for the maintenance of the PMC specification state, and a reciprocal regulation between Alx1 and Twist offers stability for the subsequent processes, such as PMC differentiation and skeletogenesis. These data illustrate the significance of regulatory state maintenance and continuous progression during cell specification, and the dynamics of the sequential events that depend on those earlier regulatory states.

Global regulatory logic for specification of an embryonic cell lineage

Explanation of a process of development must ultimately be couched in the terms of the genomic regulatory code. Specification of an embryonic cell lineage is driven by a network of interactions among genes encoding transcription factors. Here, we present the gene regulatory network (GRN) that directs the specification of the skeletogenic micromere lineage of the sea urchin embryo. The GRN now includes all regulatory genes expressed in this lineage up to late blastula stage, as identified in a genomewide survey. The architecture of the GRN was established by a large-scale perturbation analysis in which the expression of each gene in the GRN was cut off by use of morpholinos, and the effects on all other genes were measured quantitatively. Several cis-regulatory analyses provided additional evidence. The explanatory power of the GRN suffices to provide a causal explanation for all observable developmental functions of the micromere lineage during the specification period. These functions are: (i) initial acquisition of identity through transcriptional interpretation of localized maternal cues; (ii) activation of specific regulatory genes by use of a double negative gate; (iii) dynamic stabilization of the regulatory state by activation of a feedback subcircuit; (iv) exclusion of alternative regulatory states; (v) presentation of a signal required by the micromeres themselves and of two different signals required for development of adjacent endomesodermal lineages; and (vi) lineage-specific activation of batteries of skeletogenic genes. The GRN precisely predicts gene expression responses and provides a coherent explanation of the biology of specification.

Analysis of cis-regulatory elements controlling spatio-temporal expression of T-brain gene in sea urchin, Hemicentrotus pulcherrimus

In sea urchin development, micromere descendants play important roles in skeletogenesis and induction of gastrulation. We previously reported that the T-brain homolog of sea urchin Hemicentrotus pulcherrimus, HpTb expresses specifically in micromere descendants and is required for induction of gastrulation and skeletogenesis. Thus, HpTb is thought to play important roles in the function of micromere-lineage cells. To identify cis-regulatory regions responsible for spatio-temporal gene expression of HpTb, we isolated approximately 7kb genomic region of HpTb gene and showed that GFP expression driven by this region exhibits the spatio-temporal pattern corresponding substantially to that of endogenous HpTb expression. Deletion of interspecifically conserved C2 and C4 regions resulted in an increase of ectopic expression. Mutations in Hairy family and Snail family consensus sequences in C1 and C2 regions also increased ectopic expression. Furthermore, we demonstrated that C4 region functions as enhancer, and that three Ets family consensus sequences are involved in this activity but not in spatial regulation. Therefore, we concluded that expression of HpTb gene is regulated by multiple cis-regulatory elements.

The Snail repressor is required for PMC ingression in the sea urchin embryo

In metazoans, the epithelial-mesenchymal transition (EMT) is a crucial process for placing the mesoderm beneath the ectoderm. Primary mesenchyme cells (PMCs) at the vegetal pole of the sea urchin embryo ingress into the floor of the blastocoele from the blastula epithelium and later become the skeletogenic mesenchyme. This ingression movement is a classic EMT during which the PMCs penetrate the basal lamina, lose adherens junctions and migrate into the blastocoele. Later, secondary mesenchyme cells (SMCs) also enter the blastocoele via an EMT, but they accompany the invagination of the archenteron initially, in much the same way vertebrate mesenchyme enters the embryo along with endoderm. Here we identify a sea urchin ortholog of the Snail transcription factor, and focus on its roles regulating EMT during PMC ingression. Functional knockdown analyses of Snail in whole embryos and chimeras demonstrate that Snail is required in micromeres for PMC ingression. Snail represses the transcription of cadherin, a repression that appears evolutionarily conserved throughout the animal kingdom. Furthermore, Snail expression is required for endocytosis of cadherin, a cellular activity that accompanies PMC ingression. Perturbation studies position Snail in the sea urchin micromere-PMC gene regulatory network (GRN), downstream of Pmar1 and Alx1, and upstream of several PMC-expressed proteins. Taken together, our findings indicate that Snail plays an essential role in PMCs to control the EMT process, in part through its repression of cadherin expression during PMC ingression, and in part through its role in the endocytosis that helps convert an epithelial cell to a mesenchyme cell.

Evolution of the mechanisms and molecular control of endoderm formation

Endoderm differentiation and movements are of fundamental importance not only for subsequent morphogenesis of the digestive tract but also to enable normal patterning and differentiation of mesoderm- and ectoderm-derived organs. This review defines the tissues that have been called endoderm in different species, their cellular origin and their movements. We take a comparative approach to ask how signaling pathways leading to embryonic and extraembryonic endoderm differentiation have emerged in different organisms, how they became integrated and point to specific gaps in our knowledge that would be worth filling. Lastly, we address whether the gastrulation movements that lead to endoderm internalization are coupled with its differentiation.

Regional specification in the early embryo of the brittle star Ophiopholis aculeata

Early embryogenesis has been examined experimentally in several echinoderm and hemichordate classes. Although these studies suggest that the mechanisms which underlie regional specification have been highly conserved within the echinoderm + hemichordate clade, nothing is known about these mechanisms in several other echinoderm classes, including the Ophiuroidea. In this study, early embryogenesis was examined in a very little studied animal, the ophiuroid Ophiopholis aculeata. In O. aculeata, the first two cleavage planes do not coincide with the animal-vegetal axis but rather form approximately 45 degrees off this axis. A fate map of the early embryo was constructed using microinjected lineage tracers. Most significantly, this fate map indicates that there is a major segregation of ectodermal from endomesodermal fates at first cleavage. The distribution of developmental potential in the early embryo was also examined by isolating different regions of the early embryo and following these isolates though larval development. These analyses indicate that endomesodermal developmental potential segregates unequally at first, second, and third cleavage in O. aculeata. These results provide insight into the mechanisms of regional specification in O. aculeata and yield new material for the study of the evolution of echinoderm development.

The micro1 gene is necessary and sufficient for micromere differentiation and mid/hindgut-inducing activity in the sea urchin embryo

In the sea urchin embryo, micromeres have two distinct functions: they differentiate cell autonomously into the skeletogenic mesenchyme cells and act as an organizing center that induces endomesoderm formation. We demonstrated that micro1 controls micromere specification as a transcriptional repressor. Because micro1 is a multicopy gene with at least six polymorphic loci, it has been difficult to consistently block micro1 function by morpholino-mediated knockdown. Here, to block micro1 function, we used an active activator of micro1 consisting of a fusion protein of the VP16 activation domain and the micro1 homeodomain. Embryos injected with mRNA encoding the fusion protein exhibited a phenotype similar to that of micromere-less embryos. To evaluate micro1 function in the micromere, we constructed chimeric embryos composed of animal cap mesomeres and a micromere quartet from embryos injected with the fusion protein mRNA. The chimeras developed into dauerblastulae with no vegetal structures, in which the micromere progeny constituted the blastula wall. We also analyzed the phenotype of chimeras composed of an animal cap and a mesomere expressing micro1. These chimeras developed into pluteus larvae, in which the mesomere descendants ingressed as primary mesenchyme cells and formed a complete set of skeletal rods. The hindgut and a part of the midgut were also generated from host mesomeres. However, the foregut and nonskeletogenic mesoderm were not formed in the larvae. From these observations, we conclude that micro1 is necessary and sufficient for both micromere differentiation and mid/hindgut-inducing activity, and we also suggest that micro1 may not fulfill all micromere functions.

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.

Structure, regulation, and function of micro1 in the sea urchin Hemicentrotus pulcherrimus

The animal-vegetal axis of sea urchin embryos is morphologically apparent at the 16-cell stage, when the mesomeres, macromeres, and micromeres align along it. At this stage, the micromere is the only autonomously specified blastomere that functions as a signaling center. We used a subtraction PCR survey to identify the homeobox gene micro1 as a micromere-specific gene. The micro1 gene is a representative of a novel family of paired-like class homeobox genes, along with PlHbox12 from Paracentrotus lividus and pmar1 from Strongylocentrotus purpuratus. In the present study, we showed that micro1 is a multicopy gene with six or more polymorphic loci, at least three of which are clustered in a 30-kb region of the genome. The micro1 gene is transiently expressed during early cleavage stages in the micromere. Recently, nuclear beta-catenin was shown to be essential for the specification of vegetal cell fates, including micromeres, and the temporal and spatial coincidence of micro1 expression with the nuclear entry of beta-catenin is highly suggestive. We demonstrated that micro1 is a direct target of beta-catenin. In addition, we showed that micro1 is necessary and sufficient for micromere specification. These observations on the structure, regulation, and function of micro1 lead to the conclusion that micro1 and pmar1 (and potentially PlHbox12) are orthologous.

SpHnf6, a transcription factor that executes multiple functions in sea urchin embryogenesis

The Strongylocentrotus purpuratus hnf6 (Sphnf6) gene encodes a new member of the ONECUT family of transcription factors. The expression of hnf6 in the developing embryo is triphasic, and loss-of-function analysis shows that the Hnf6 protein is a transcription factor that has multiple distinct roles in sea urchin development. hnf6 is expressed maternally, and before gastrulation its transcripts are distributed globally. Early in development, its expression is required for the activation of PMC differentiation genes such as sm50, pm27, and msp130, but not for the activation of any known PMC regulatory genes, for example, alx, ets1, pmar1, or tbrain. Micromere transplantation experiments show that the gene is not involved in early micromere signaling. Early hnf6 expression is also required for expression of the mesodermal regulator gatac. The second known role of hnf6 is its participation after gastrulation in the oral ectoderm gene regulatory network (GRN), in which its expression is essential for the maintenance of the state of oral ectoderm specification. The third role is in the neurogenic ciliated band, which is foreshadowed exactly by a trapezoidal band of hnf6 expression at the border of the oral ectoderm and where it continues to be expressed through the end of embryogenesis. Neither oral ectoderm regulatory functions nor ciliated band formation occur normally in the absence of hnf6 expression.

A Raf/MEK/ERK signaling pathway is required for development of the sea urchin embryo micromere lineage through phosphorylation of the transcription factor Ets

In the sea urchin embryo, the skeleton of the larva is built from a population of mesenchymal cells known as the primary mesenchyme cells (PMCs). These derive from the large micromeres that originate from the vegetal pole at fourth cleavage. At the blastula stage, the 32 cells of this lineage detach from the epithelium and ingress into the blastocoel by a process of epithelial-mesenchymal transition. We report that shortly before ingression,there is a transient and highly localized activation of the MAP-kinase ERK in the micromere lineage. We show that ingression of the PMCs requires the activity of ERK, MEK and Raf, and depends on the maternal Wnt/β-catenin pathway. Dissociation experiments and injection of mRNA encoding a dominant-negative form of Ras indicated that this activation is probably cell autonomous. We identified the transcription factors Ets1 and Alx1 as putative targets of the phosphorylation by ERK. Both proteins contain a single consensus site for phosphorylation by the MAP kinase ERK. In addition, the Ets1 protein sequence contains a putative ERK docking site. Overexpression of ets1 by injection of synthetic mRNA in the egg caused a dramatic increase in the number of cells becoming mesenchymal at the blastula stage. This effect could be largely inhibited by treating embryos with the MEK inhibitor U0126. Moreover, mutations in the consensus phosphorylation motif substituting threonine 107 by an aspartic or an alanine residue resulted respectively in a constitutively active form of Ets1 that could not be inhibited by U0126 or in an inactive form of Ets1. These results show that the MAP kinase pathway, working through phosphorylation of Ets1, is required for full specification of the PMCs and their subsequent transition from epithelial to mesenchymal state.

Patterning mechanisms in the evolution of derived developmental life histories: the role of Wnt signaling in axis formation of the direct-developing sea urchin Heliocidaris erythrogramma

A number of echinoderm species have replaced indirect development with highly modified direct-developmental modes, and provide models for the study of the evolution of early embryonic development. These divergent early ontogenies may differ significantly in life history, oogenesis, cleavage pattern, cell lineage, and timing of cell fate specification compared with those of indirect-developing species. No direct-developing echinoderm species has been studied at the level of molecular specification of embryonic axes. Here we report the first functional analysis of Wnt pathway components in Heliocidaris erythrogramma, a direct-developing sea urchin. We show by misexpression and dominant negative knockout construct expression that Wnt8 and TCF are functionally conserved in the generation of the primary (animal/vegetal) axis in two independently evolved direct-developing sea urchins. Thus, Wnt pathway signaling is an overall deeply conserved mechanism for axis formation that transcends radical changes to early developmental ontogenies. However, the timing of expression and linkages between Wnt8, TCF, and components of the PMC-specification pathway have changed. These changes correlate with the transition from an indirect- to a direct-developing larval life history.