Molecular Patterning along the Sea Urchin Animal-Vegetal Axis
Introduction
A century ago, Boveri, 1901a, Boveri, 1901b recognized and began experimental investigations on the animal–vegetal (A–V) polarity within the sea urchin egg. Like eggs of many animals, sea urchin eggs have a firmly established, maternally imposed A–V polarity. Axes of polarity provide a three-dimensional coordinate system that patterns the embryo. There is much variation in the timing of establishment of different axes within species and between species and also in the mechanisms that underlie polarization (Goldstein and Freeman, 1997). In sea urchins, the ectoderm forms from the animal part of the egg (the animal pole is defined as the point of polar body extrusion), whereas the mesoderm and endoderm are derived from the vegetal part (the vegetal pole is defined as the pole opposite the animal pole). An oral–aboral (O–Ab) axis orthogonal to the A–V axis is specified in the early embryo, but it is not fixed until gastrulation (Cameron et al., 1990, Hardin et al., 1992, Davidson et al., 1998). Many experiments indicate that patterning along the A–V axis depends on localized maternal determinants and cellular interactions, whereas differentiation of oral and aboral ectoderm requires interactions with vegetal cells. Although several speculative models have been advanced, the molecular mechanisms involved in axial patterning of the echinoid embryo have remained obscure until recently. New methods for assessing and modulating gene expression, combined with experimental manipulations of embryos, have provided much exciting new information. Here, we summarize the experimental results that provide our current understanding of the mechanisms of axial polarization and pattern formation in the sea urchin embryo, with an emphasis on important recent advances (Angerer and Angerer, 1999, Angerer and Angerer, 2000, Ettensohn and Sweet, 2000).
We review evidence that indicates that the establishment of the A–V axis involves cell autonomous activation of maternally specified vegetalizing and animalizing transcriptional regulatory factors. During early cleavages, maternally encoded animalizing factors, including orthologs of mammalian SoxB1, SoxB2, and Ets4, become localized exclusively to the nuclei of cells in the animal zone of the embryo destined to become ectodermal cell types. During the same time period, β-catenin becomes localized in nuclei of the cells close to the vegetal pole, cells whose descendents will produce skeletogenic and coelomic mesoderm. These animalizing and vegetalizing transcription systems overlap in nuclei of cells having an intermediate position along the A–V axis that are destined to become mesoderm, endoderm, and ectoderm. In turn, the maternally encoded transcription factors regulate the zygotic transcription of genes having spatially restricted domains of expression along the A–V axis. Some of the responding genes encode transcription factors, including those of the Krox–Krüpple class, that are involved in refining the pattern. Other responding genes encode proteins involved in intercellular signaling that have important roles in patterning the embryo. These include a Wnt8 ortholog that appears to reinforce the role of β-catenin in vegetal patterning, a BMP2/4 extracellular signal that promotes ectoderm differentiation, and a ligand for the Notch receptor that is produced by the vegetalmost blastomeres and is involved in specification of secondary mesenchyme cells, distinguishing them from endoderm.
In the intact sea urchin embryo, there is a strong correspondence between cell lineage and cell fate. Expression of “marker” genes in clones of cells sharing fates is generally activated early in development, implying that specification of fate has occurred. We use specification as defined by Logan and McClay (1999) to indicate processes that result in distinctive cellular identities that predict their fates. A stricter definition of specification requires that clonal expression of marker genes be maintained when the specified embryo fragments are cultured in isolation (Davidson, 1989). This has sometimes been confirmed for fragments of sea urchin embryos, but regulation (in this context, the expression of genes normally restricted to other lineages) is often observed as well. Most urchin embryonic cells remain uncommitted to a fate until late in development, as assayed by cell transplantation experiments. That is, the developmental potentials of most cells remain broader than their fates in the intact embryo until late stages. Micromeres are the prominent exception to this generalization.
The cellular interactions involved in specification of cell fates in the sea urchin can be either evocative or repressive. Genes whose expression in embryos is associated with cellular specification are often temporally activated by general activating transcription factors. Negative spatial regulatory transcription factors then restrict expression to appropriate embryonic territories. The late commitment to a fate by most cells and the robust and counterbalancing regulatory networks involved in patterning the sea urchin embryo account for its remarkable capacity to regulate in response to experimental perturbations that disrupt normal cellular interactions.
Section snippets
Evidence for a Maternal A–V Axis of Polarity
The eggs of most sea urchin species have no obvious morphological polarity, but in some populations of Paracentrotus lividus a pattern of pigment granules predicts the A–V polarity (Boveri, 1901b). During meiotic maturation, which occurs long before the egg is shed, the polar bodies form at the animal pole (as in many other organisms), but they usually leave the surface of the egg before it is shed (Schroeder, 1980a). In addition, a hole on the egg jelly layer, the “jelly canal,” is at the
Summary of the Molecular Events Regulating A–V Axis Patterning
Considerable progress has recently been made in understanding the molecular events that lead to the formation of cell types along the A–V axis. It is now clear that cell-autonomous, maternally derived mechanisms initiate a process that gives rise to three gene-regulatory zones. These zones are defined in the early embryo by distinct sets of vegetalizing and animalizing transcription factors and their region of overlap (Angerer and Angerer, 2000). The zones are refined as development proceeds to
Notch Signaling and the Secondary Mesenchyme–Endoderm Border
As discussed previously, macromeres can be distinguished from mesomeres and micromeres of the 16-cell embryo by their autonomous nuclear localization of both β-catenin and animalizing transcription factors such as SpEts4 and SpSoxB1. Both endoderm and secondary mesenchyme cells, the principal mesoderm of the embryo, are derived from the veg2 tier, whereas the veg1 tier produces endoderm and ectoderm. Fate mapping of the mesenchyme blastula-stage embryo indicates that following the ingression of
Initial Specification Mechanisms
The O–Ab axis arises in indirect developing sea urchin embryos following fertilization by mechanisms that are unknown. In S. purpuratus, there is a fixed relationship between the first cleavage plane and the O–Ab axis (Cameron et al., 1989), but this is not the case for some other echinoid species (Kominami, 1988, Henry, 1998). At least for S. purpuratus, this implies that the O–Ab axis is initially specified between fertilization and the first cleavage, but because isolated blastomeres at the
Cis-Regulatory Systems and Regulation of Gene Expression
Considerable effort has been made in attempting to link the initial events of axis specification and the initiation of territorial-specific gene expression. These mechanisms have been revealed in part by analysis of individual cis-regulatory regions of genes expressed in spatially restricted domains and the trans factors associated with the regulatory regions. In this review, two well-studied examples are described that are particularly relevant to A–V and O–Ab axis patterning—endo16 in the
A Simple Model for Specification along the A–V Axis
We have reviewed evidence that a simple system for patterning the sea urchin embryo along the A–V axis is initiated following fertilization by a few maternal products that act autonomously and locally within the early embryo. This system is presented in Fig. 3. Initially, translation of maternal mRNAs produces animalizing transcription factors (AFs) such as SpSoxB1 that accumulate in all nuclei. Upon the fourth cleavage, β-catenin begins to accumulate in micromere nuclei and subsequently in
Some Unresolved Issues and Future Directions
One problem with the model described in Section VII.A is that it fails to explain the distinction in fate between large and small micromeres. Both have a very high ratio of nuclear β-catenin to animalizing transcription factors (BC/AF).
The nondividing small micromeres maintain the high concentration of nuclear β-catenin, whereas it declines during cleavage in the large micromere lineages. This difference, as well as their distinctive fates, must be the result of differences autonomously
Concluding Remarks
We have reviewed the considerable recent progress in understanding axial patterning in sea urchin embryos. A striking observation is the very dynamic expression patterns of proteins such as β-catenin, Wnt8, and Notch that have important roles in the patterning of embryos. This may account, in part, for the complex regulation of genes that respond to patterning events. Although many important issues remain to be resolved, an elegantly simple model is emerging. The powerful experimental methods
Acknowledgments
We acknowledge Eric Davidson and David McClay for providing recent references and Athula Wikramanayake, Rudy Raff, Charles Ettensohn, Robert Angerer, and Lynne Angerer for communicating unpublished results. We also thank Shonan Amemiya, Eric Davidson, Jim Coffman, Koji Akasaka, Charles Ettensohn, Andy Cameron, Dave McClay, Athula Wikramanayake, Judith Venuti, Fred Wilt, Greg Wray, Catriona Logan, Hyla Sweet and Lynne and Robert Angerer for stimulating and influencing our thinking about sea
References (132)
- et al.
Animal–vegetal patterning mechanisms in the early sea urchin embryo
Dev. Biol.
(2000) - et al.
The hardwiring of development: organization and function of genomic regulatory systems
Development
(1997) - et al.
Cell type specification during sea urchin development
Trends Genet.
(1991) - et al.
The embryonic ciliated band of the sea urchin, Strongylocentrotus purpuratus, derives from both oral and aboral ectoderm territories
Dev. Biol.
(1993) - et al.
A sea urchin genome project: Sequence scan, virtual map, and additional resources
Proc. Natl. Acad. Sci. USA
(2000) - et al.
The establishment of Spemann’s organizer and patterning of the vertebrate embryo
Nature Rev. Genet.
(2000) - et al.
Patterning the early sea urchin embryo
Curr. Top. Dev. Biol.
(2000) - et al.
Spatial distribution of collagen type 1 MRNA in Paracentrotus lividus eggs and embryos
Biochem. Biophys. Res. Commun.
(1997) - et al.
Early gene expression along the animal–vegetal axis in sea urchin embryoids and grafted embryos
Development
(1996) - et al.
BMP1-related metalloproteinases promote the development of ventral mesoderm in early Xenopus embryos
Dev. Biol.
(1998)
Mass isolation and culture of sea urchin micromeres
In Vitro Cell. Dev. Biol.
The development of dorsoventral and bilateral axial properties in sea urchin embryos
Semin. Cell Dev. Biol.
Experimental Embryology of Echinoderms
SpKr1: A direct target of beta-catenin regulation required for endoderm differentiation in sea urchin embryos
Development
Interactions of different vegetal cells with mesomeres during early stages of sea urchin development
Development
A molecular mechanism for the effect of lithium on development
Proc. Natl. Acad. Sci. USA
Hp-ets, an ets-related transcription factor implicated in primary mesenchyme cell differentiation in the sea urchin embryo
Mech. Dev.
Spatial and temporal expression pattern during sea urchin embryogenesis of a gene coding for a protease homologous to the human protein BMP-1 and to the product of the Drosophila dorsal–ventral patterning gene tolloid
Development
Range and stability of cell fate determination in isolated sea urchin blastomeres
Development
The allocation of early blastomeres to the ectoderm and endoderm is variable in the sea urchin embryo
Development
The lineages that give rise to the endoderm and mesoderm in the sea urchin embryo
A micromere induction signal is activated by beta-catenin and acts through Notch to initiate specification of secondary mesenchyme cells in the sea urchin embryo
Development
Signal transduction through β-catenin and specification of cell fate during embryogenesis
Genes Dev.
A complete second gut induced by transplanted micromeres in the sea urchin embryo
Science
Whole mount in situ hybridization shows Endo16 to be a marker for the vegetal plate territory in sea urchin embryos
Mech. Dev.
Recovery of developmentally defined gene sets from high-density cDNA macroarrays
Dev. Biol.
Characterization of bep1 and bep4 antigens involved in cell interactions during Paracentrotus lividus development
Differentiation
Involvement of the cytoskeleton in localization of Paracentrotus lividus maternal BEP mRNAs and proteins
Exp. Cell Res.
Complete regulation of development though metamorphosis of sea urchin embryos devoid of macromeres
Dev. Growth Differ.
Regulative development of the sea urchin embryo: Signaling cascades and morphogen gradients
Semin. Cell Dev. Biol.
A BMP pathway regulates cell fate allocation along the sea urchin animal–vegetal embryonic axis
Development
Sea urchin goosecoid function links fate specification along the animal–vegetal and oral–aboral axes
Development
Notch signaling: Cell fate control and signal integration in development
Science
Differentiation in vitro of sea urchin micromeres on extracellular matrix in the absence of serum
J. Exp. Zool.
Die Polarität von Oocyte, Ei und Larve de Strongylocentrotus lividus
Zool. Jb. Abt. Anat. Ont.
Über die Polarität des Seeigeleies
Verb. Phys. Med. Ces. Wurzburg
Territorial specification and control of gene expression in the sea urchin embryo
Semin. Dev. Biol.
Gene expression and early cell fate specification in embryos of the simple sea urchin (Strongylocentrotus purpuratus)
Lineage and fate of each blastomere of the eight-cell sea urchin embryo
Genes Dev.
The oral–aboral axis of a sea urchin embryo is specified by first cleavage
Development
Segregation of oral from aboral ectoderm precursors is completed at fifth cleavage in the embryogenesis of Strongylocentrotus purpuratus
Dev. Biol.
Endoderm differentiation in vitro defines a transitional period for endoderm ontogeny in the sea urchin embryo
Dev. Biol.
Transient appearance of Strongylocentrotus purpuratus Otx in micromere nuclei: Cytoplasmic retention of SpOtx possibly mediated through an alpha–actinin interaction
Dev. Genet.
Oral-aboral axis specification in the sea urchin embryo. I. Axis entrainment by respiratory asymmetry
Dev. Biol.
Isolation of a transacting factor involved in localization of Paracentrotus lividus maternal mRNAs
RNA
Studies on unequal cleavage in sea urchins II. Surface differentiation and the direction of nuclear migration
Dev. Growth Differ.
Lineage-specific gene expression and the regulative capacities of the sea urchin embryo
Development
Specification of cell fate in the sea urchin embryo: Summary and some proposed mechanisms
Development
Characterization of a gene encoding a developmentally regulated winged helix transcription factor of the sea urchin Strongylocentrotus purpuratus
Gene
A common plan for dorsoventral patterning in Bilateria
Nature
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