Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

MCB Department, Brown University, 185 Meeting Street, BOX-GL173, Providence, RI 02912, USA.
Development (Impact Factor: 6.27). 10/2012; 139(20):3786-94. DOI: 10.1242/dev.082230
Source: PubMed

ABSTRACT The process of germ line determination involves many conserved genes, yet is highly variable. Echinoderms are positioned at the base of Deuterostomia and are crucial to understanding these evolutionary transitions, yet the mechanism of germ line specification is not known in any member of the phyla. Here we demonstrate that small micromeres (SMics), which are formed at the fifth cell division of the sea urchin embryo, illustrate many typical features of primordial germ cell (PGC) specification. SMics autonomously express germ line genes in isolated culture, including selective Vasa protein accumulation and transcriptional activation of nanos; their descendants are passively displaced towards the animal pole by secondary mesenchyme cells and the elongating archenteron during gastrulation; Cadherin (G form) has an important role in their development and clustering phenotype; and a left/right integration into the future adult anlagen appears to be controlled by a late developmental mechanism. These results suggest that sea urchin SMics share many more characteristics typical of PGCs than previously thought, and imply a more widely conserved system of germ line development among metazoans.

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    • "In summary, with a significance cutoff of 0.05 (false discovery rate, FDR), we identified a union set of 230 differentially expressed transcripts (both sMic enriched and depleted) between these comparisons (Fig. 1E; supplementary material Fig. S1I,J; Tables S7-S9). The sMic-enriched transcripts included Nanos and Delta, which have previously been identified as sMic-localized, as well as SpGcadherin , which is required for sMic fate in S. purpuratus and is enriched in the sMics of Lytechinus variegatus (Juliano et al., 2010; Miller and McClay, 1997; Oliveri et al., 2002; Yajima and Wessel, 2012). The sMic-enriched transcripts fell into diverse functional categories, but transcriptional regulation and RNA binding were overrepresented according to a gene set enrichment analysis (supplementary material Table S1). "
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    ABSTRACT: A crucial event in animal development is the specification of primordial germ cells (PGCs), which become the stem cells that create sperm and eggs. How PGCs are created provides a valuable paradigm for understanding stem cells in general. We find that the PGCs of the sea urchin Strongylocentrotus purpuratus exhibit broad transcriptional repression, yet enrichment for a set of inherited mRNAs. Enrichment of several germline determinants in the PGCs requires the RNA-binding protein Nanos to target the transcript that encodes CNOT6, a deadenylase, for degradation in the PGCs, thereby creating a stable environment for RNA. Misexpression of CNOT6 in the PGCs results in their failure to retain Seawi transcripts and Vasa protein. Conversely, broad knockdown of CNOT6 expands the domain of Seawi RNA as well as exogenous reporters. Thus, Nanos-dependent spatially restricted CNOT6 differential expression is used to selectively localize germline RNAs to the PGCs. Our findings support a 'time capsule' model of germline determination, whereby the PGCs are insulated from differentiation by retaining the molecular characteristics of the totipotent egg and early embryo.
    Development 08/2014; 141(16). DOI:10.1242/dev.110395 · 6.27 Impact Factor
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    • "sea stars) depend on the fertilization envelope to constrain the blastomeres, but the relatively small sMics may require an extra layer of constraint in the embryo to aid in their retention. The loss of a blastomere to even an 8-cell embryo is easily surmountable, but the loss of the PGCs at an early stage could lead to sterility (Yajima and Wessel 2011). "
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    ABSTRACT: 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 as 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. Mol. Reprod. Dev. © 2013 Wiley Periodicals, Inc.
    Molecular Reproduction and Development 07/2014; 81(8). DOI:10.1002/mrd.22223 · 2.68 Impact Factor
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    • "Surprisingly, it is clear that PGC development in the animal kingdom is initiated from a diverse array of upstream pathways. For example, in frogs, PGCs are of endodermal origin, whereas, in sea urchins, PGCs develop in a cell-autonomous manner from small micromeres (Yajima and Wessel, 2012). In this study, we demonstrate that axolotl PGCs develop downstream of the same pathway that initiates mesoderm development. "
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    ABSTRACT: A common feature of development in most vertebrate models is the early segregation of the germ line from the soma. For example, in Xenopus and zebrafish embryos primordial germ cells (PGCs) are specified by germ plasm that is inherited from the egg; in mice, Blimp1 expression in the epiblast mediates the commitment of cells to the germ line. How these disparate mechanisms of PGC specification evolved is unknown. Here, in order to identify the ancestral mechanism of PGC specification in vertebrates, we studied PGC specification in embryos from the axolotl (Mexican salamander), a model for the tetrapod ancestor. In the axolotl, PGCs develop within mesoderm, and classic studies have reported their induction from primitive ectoderm (animal cap). We used an axolotl animal cap system to demonstrate that signalling through FGF and BMP4 induces PGCs. The role of FGF was then confirmed in vivo. We also showed PGC induction by Brachyury, in the presence of BMP4. These conditions induced pluripotent mesodermal precursors that give rise to a variety of somatic cell types, in addition to PGCs. Irreversible restriction of the germ line did not occur until the mid-tailbud stage, days after the somatic germ layers are established. Before this, germline potential was maintained by MAP kinase signalling. We propose that this stochastic mechanism of PGC specification, from mesodermal precursors, is conserved in vertebrates.
    Development 06/2014; 141(12):2429-2440. DOI:10.1242/dev.105346 · 6.27 Impact Factor
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