Developmental expression of HpNanos, the Hemicentrotus pulcherrimus homologue of nanos

The diversity of nanos expression in echinoderm embryos supports different mechanisms in germ cell specification

Specification of the germ cell lineage is required for sexual reproduction in all animals. However, the timing and mechanisms of germ cell specification is remarkably diverse in animal development. Echinoderms, such as sea urchins and sea stars, are excellent model systems to study the molecular and cellular mechanisms that contribute to germ cell specification. In several echinoderm embryos tested, the germ cell factor Vasa accumulates broadly during early development and is restricted after gastrulation to cells that contribute to the germ cell lineage. In the sea urchin, however, the germ cell factor Vasa is restricted to a specific lineage by the 32-cell stage. We therefore hypothesized that the germ cell specification program in the sea urchin/Euechinoid lineage has evolved to an earlier developmental time point. To test this hypothesis we determined the expression pattern of a second germ cell factor, Nanos, in four out of five extant echinoderm clades. Here we find that Nanos mRNA does not accumulate until the blastula stage or later during the development of all other echinoderm embryos except those that belong to the Echinoid lineage. Instead, Nanos is expressed in a restricted domain at the 32-128 cell stage in Echinoid embryos. Our results support the model that the germ cell specification program underwent a heterochronic shift in the Echinoid lineage. A comparison of Echinoid and non-Echinoid germ cell specification mechanisms will contribute to our understanding of how these mechanisms have changed during animal evolution.

The effects of Nanos2 on Boule and Stra8 in male germline stem cells (mGSCs)

The mitosis-meiosis switch is a key event in the differentiation of germ cells. Meiosis is important in development biology, however, it has not been clear what is the regulation mechanism in mammals. Our previous study showed that Boule could activate Stra8 directly and result in the meiosis initiation of dairy goat male germline stem cells (mGSCs). Nanos2, a RNA-binding protein, plays critical roles in the suppression of meiosis by preventing Stra8 expression and maintain the male germ cell development. The main purpose of this study was to explore whether Nanos2 represses Stra8 transcription through Boule or not. We found ectopic over-expression of Nanos2 in GC-1 and mGSCs down-regulated Stra8 transcription and translation, and Boule expression was not affected. It was in consistent with our expectation that RA could up-regulate Boule and Stra8 expression, but down-regulate Nanos2 expression in mGSCs. In dairy goat, the expression levels of Boule and Stra8 would rise with the increase of age, but the expression level of Nanos2 in 90 dpp and adult testis had not shown a clear change. In conclusion, Nanos2 represses Stra8 expression but not through Boule in dairy goat mGSCs.

The 3'UTR of nanos2 directs enrichment in the germ cell lineage of the sea urchin

Nanos is a translational regulator required for the survival and maintenance of primordial germ cells during embryogenesis. Three nanos homologs are present in the genome of the sea urchin Strongylocentrotus purpuratus (Sp), and each nanos mRNA accumulates specifically in the small micromere (sMic) lineage. We found that a highly conserved element in the 3' UTR of nanos2 is sufficient for reporter expression selectively in the sMic lineage: microinjection into a Sp fertilized egg of an RNA that contains the GFP open reading frame followed by Sp nanos2 3'UTR leads to selective reporter enrichment in the small micromeres in blastulae. The same result was seen with nanos2 from the sea urchin Hemicentrotus pulcherrimus (Hp). In both species, the 5'UTR alone is not sufficient for the sMic localization but it always increased the sMic reporter enrichment when present with the 3'UTR. We defined an element conserved between Hp and Sp in the nanos2 3'UTR which is necessary and sufficient for protein enrichment in the sMic, and refer to it as GNARLE (Global Nanos Associated RNA Lability Element). We also found that the nanos2 3'UTR is essential for the selective RNA retention in the small micromeres; GNARLE is required but not sufficient for this process. These results show that a combination of selective RNA retention and translational control mechanisms instills nanos accumulation uniquely in the sMic lineage.

Molecular and expression characterization of a nanos1 homologue in Chinese sturgeon, Acipenser sinensis

The nanos gene family was essential for germ line development in diverse organisms. In the present study, the full-length cDNA of a nanos1 homologue in A. sinensis, Asnanos1, was isolated and characterized. The cDNA sequence of Asnanos1 was 1489 base pairs (bp) in length and encoded a peptide of 228 amino acid residues. Multiple sequence alignment showed that the zinc-finger motifs of Nanos1 were highly conserved in vertebrates. By RT-PCR analysis, Asnanos1 mRNAs were ubiquitously detected in all tissues examined except for the fat, including liver, spleen, heart, ovary, kidney, muscle, intestines, pituitary, hypothalamus, telencephalon, midbrain, cerebellum, and medulla oblongata. Moreover, a specific polyclonal antibody was prepared from the in vitro expressed partial AsNanos1 protein. Western blot analysis revealed that the tissue expression pattern of AsNanos1 was not completely coincided with that of its mRNAs, which was not found in fat, muscle and intestines. Additionally, by immunofluoresence localization, it was observed that AsNanos1 protein was in the cytoplasm of primary oocytes and spermatocytes. The presented results indicated that the expression pattern of Asnanos1 was differential conservation and divergence among diverse species.

Opposing effects of retinoic acid and FGF9 on Nanos2 expression and meiotic entry of mouse germ cells

In the mouse, three genes that are homologous to the Drosophila Nanos (Nos) gene have been identified. Deletion of one of these genes, Nanos2, results in male sterility, owing to loss of germ cells during fetal life. Before apoptosis, Nanos2-null gonocytes enter meiosis, suggesting that Nanos2 functions as a meiotic repressor. Here, we show that Nanos2 is continuously expressed in male germ cells from fetal gonocytes to postnatal spermatogonial stem cells. We observed that the promeiotic factor AtRA, an analog of retinoic acid (RA), downregulates NANOS2 levels, in both fetal and postnatal gonocytes, while promoting meiosis. Interestingly, FGF9, a growth factor crucial for sex differentiation and survival of fetal gonocytes, upregulates levels of NANOS2 in both male and female primordial germ cells (PGCs) and in premeiotic spermatogonia. This effect was paralleled by an impairment of meiotic entry, suggesting that FGF9 acts as an inhibitor of meiosis through the upregulation of Nanos2. We found that NANOS2 interacts with PUM2, and that these two proteins colocalize in the ribonucleoparticle and polysomal fractions on sucrose gradients, supporting the notion that they bind RNA. Finally, we found that recombinant NANOS2 binds to two spermatogonial mRNAs, Gata2 and Taf7l, which are involved in germ-cell differentiation.

Role of the nanos homolog during sea urchin development

The nanos genes play important roles in the development of primordial germ cells in animal species. In the sea urchin, Hemicentrotus pulcherrimus, small micromere descendants specifically express HpNanos mRNA and this expression continues in the left coelomic pouch, which produces the major component of the adult rudiment. In this study, we showed that morpholino knockdown of HpNanos resulted in a delay of primary mesenchyme cell ingression and a decrease in the number of cells comprising the left coelomic pouch. Knockdown analysis in chimeras and whole embryos revealed the disappearance of small micromere descendants from the archenteron tip. Furthermore, the expression of HpNanos mRNA was induced in other cell lineages in the HpNanos-knockdown and micromere-deleted embryos. Taken together, our results suggest that HpNanos is involved in the inductive interaction of small micromere descendants with other cell lineages, and that HpNanos is required for the survival of small micromere descendants.

Vasa protein expression is restricted to the small micromeres of the sea urchin, but is inducible in other lineages early in development

Vasa is a DEAD-box RNA helicase that functions in translational regulation of specific mRNAs. In many animals it is essential for germ line development and may have a more general stem cell role. Here we identify vasa in two sea urchin species and analyze the regulation of its expression. We find that vasa protein accumulates in only a subset of cells containing vasa mRNA. In contrast to vasa mRNA, which is present uniformly throughout all cells of the early embryo, vasa protein accumulates selectively in the 16-cell stage micromeres, and then is restricted to the small micromeres through gastrulation to larval development. Manipulating early embryonic fate specification by blastomere separations, exposure to lithium, and dominant-negative cadherin each suggest that, although vasa protein accumulation in the small micromeres is fixed, accumulation in other cells of the embryo is inducible. Indeed, we find that embryos in which micromeres are removed respond by significant up-regulation of vasa protein translation, followed by spatial restriction of the protein late in gastrulation. Overall, these results support the contention that sea urchins do not have obligate primordial germ cells determined in early development, that vasa may function in an early stem cell population of the embryo, and that vasa expression in this embryo is restricted early by translational regulation to the small micromere lineage.

From oocyte to 16-cell stage: Cytoplasmic and cortical reorganizations that pattern the ascidian embryo

The dorsoventral and anteroposterior axes of the ascidian embryo are defined before first cleavage by means of a series of reorganizations that reposition cytoplasmic and cortical domains established during oogenesis. These domains situated in the periphery of the oocyte contain developmental determinants and a population of maternal postplasmic/PEM RNAs. One of these RNAs (macho-1) is a determinant for the muscle cells of the tadpole embryo. Oocytes acquire a primary animal-vegetal (a-v) axis during meiotic maturation, when a subcortical mitochondria-rich domain (myoplasm) and a domain rich in cortical endoplasmic reticulum (cER) and maternal postplasmic/PEM RNAs (cER-mRNA domain) become polarized and asymmetrically enriched in the vegetal hemisphere. Fertilization at metaphase of meiosis I initiates a series of dramatic cytoplasmic and cortical reorganizations of the zygote, which occur in two major phases. The first major phase depends on sperm entry which triggers a calcium wave leading in turn to an actomyosin-driven contraction wave. The contraction concentrates the cER-mRNA domain and myoplasm in and around a vegetal/contraction pole. The precise localization of the vegetal/contraction pole depends on both the a-v axis and the location of sperm entry and prefigures the future site of gastrulation and dorsal side of the embryo. The second major phase of reorganization occurs between meiosis completion and first cleavage. Sperm aster microtubules and then cortical microfilaments cause the cER-mRNA domain and myoplasm to reposition toward the posterior of the zygote. The location of the posterior pole depends on the localization of the sperm centrosome/aster attained during the first major phase of reorganization. Both cER-mRNA and myoplasm domains localized in the posterior region are partitioned equally between the first two blastomeres and then asymmetrically over the next two cleavages. At the eight-cell stage the cER-mRNA domain compacts and gives rise to a macroscopic cortical structure called the Centrosome Attracting Body (CAB). The CAB is responsible for a series of unequal divisions in posterior-vegetal blastomeres, and the postplasmic/PEM RNAs it contains are involved in patterning the posterior region of the embryo. In this review, we discuss these multiple events and phases of reorganizations in detail and their relationship to physiological, cell cycle, and cytoskeletal events. We also examine the role of the reorganizations in localizing determinants, postplasmic/PEM RNAs, and PAR polarity proteins in the cortex. Finally, we summarize some of the remaining questions concerning polarization of the ascidian embryo and provide comparisons to a few other species. A large collection of films illustrating the reorganizations can be consulted by clicking on "Film archive: ascidian eggs and embryos" at http://biodev.obs-vlfr.fr/recherche/biomarcell/.

Functional redundancy among Nanos proteins and a distinct role of Nanos2 during male germ cell development

The mouse Nanos proteins, Nanos2 and Nanos3, are required for germ cell development and share a highly conserved zinc-finger domain. The expression patterns of these factors during development, however, differ from each other. Nanos3 expression in the mouse embryo commences in the primordial germ cells (PGCs) just after their formation, and a loss of this protein results in the germ cell-less phenotype in both sexes. By contrast, Nanos2 expression begins only in male PGCs after their entry into the genital ridge and a loss of this protein results in a male germ cell deficiency, irrespective of the co-expression of Nanos3 in these cells. These results indicate that these two Nanos proteins have distinct functions, which depend on the time and place of their expression. To further elucidate this, we have generated transgenic mouse lines that express Nanos2 under the control of the Oct4DeltaPE promoter and examined Nanos2 function in a Nanos3-null genetic background. We find that ectopically produced Nanos2 protein rescues the Nanos3-null defects, because the germ cells fully develop in both sexes in the transgenic mice. This result indicates that Nanos2 can substitute for Nanos3 during early PGC development. By contrast, our current data show that Nanos3 does not rescue the defects in Nanos2-null mice. Our present findings thus indicate that there are redundant functions of the Nanos proteins in early PGC development, but that Nanos2 has a distinct function during male germ cell development in the mouse.

Germ line determinants are not localized early in sea urchin development, but do accumulate in the small micromere lineage

Two distinct modes of germ line determination are used throughout the animal kingdom: conditional-an inductive mechanism, and autonomous-an inheritance of maternal factors in early development. This study identifies homologs of germ line determinants in the sea urchin Strongylocentrotus purpuratus to examine its mechanism of germ line determination. A list of conserved germ-line associated genes from diverse organisms was assembled to search the S. purpuratus genome for homologs, and the expression patterns of these genes were examined during embryogenesis by whole mount in situ RNA hybridization and QPCR. Of the 14 genes tested, all transcripts accumulate uniformly during oogenesis and Sp-pumilio, Sp-tudor, Sp-MSY, and Sp-CPEB1 transcripts are also uniformly distributed during embryonic development. Sp-nanos2, Sp-seawi, and Sp-ovo transcripts, however, are enriched in the vegetal plate of the mesenchyme blastula stage and Sp-vasa, Sp-nanos2, Sp-seawi, and Sp-SoxE transcripts are localized in small micromere descendents at the tip of the archenteron during gastrulation and are then enriched in the left coelomic pouch of larvae. The results of this screen suggest that sea urchins conditionally specify their germ line, and support the hypothesis that this mechanism is the basal mode of germ line determination amongst deuterostomes. Furthermore, accumulation of germ line determinants selectively in small micromere descendents supports the hypothesis that these cells contribute to the germ line.