Comparison of the D. melanogaster Nos 3'UTR with that of D. virilis reveals a conservation of 9 regions throughout the 800+ bases with a similar arrangement of sequences. The sequence does not resemble that of other posteriorly localized RNAs (Gavis, 1996).
The mechanism by which Nanos acts in Drosophila is a common developmental strategy in Dipteran insects. Nanos activity is found in the posterior poleplasm of five diverse Dipteran species. Genes homologous to nanos were identified from D. virilis, the housefly Musca domestica, and the midge Chironomus samoensis. Each gene encodes RNAs that are localized, like Nanos, to the embryonic posterior pole. The homologs can functionally substitute for nanos in D. melanogaster. Thus nanos acts in a similar pathway for axis determination in other insects. Comparison of the Nanos sequences reveals only 19% overall protein sequence similarity; high conservation of a novel zinc finger near the carboxy terminus of the protein defines a region critical for nanos gene function (Curtis, 1995).
Axial patterning is a fundamental event in early development, and molecules involved in determining the body axes provide a coordinate system for subsequent patterning. While orthologs of Drosophila bicoid and nanos play a conserved role in anteroposterior (AP) patterning within at least a subset of Diptera, conservation of this process has not yet been demonstrated outside of the flies. Indeed, it has been argued that bicoid, an instrumental 'anterior' factor in Drosophila melanogaster, acquired this role during the evolution of more-derived dipterans. Interestingly, the interaction of Drosophila maternal nanos and maternal hunchback provides a system for patterning the AP axis that is partially redundant to the anterior system. Previous studies in grasshoppers suggest that hunchback may play a conserved role in axial patterning in this insect, but this function may be supplied solely by the zygotic component of hunchback expression. Evidence suggests that the early pattern of zygotic grasshopper Hunchback expression is achieved through translational repression that may be mediated through the action of grasshopper nanos. This is consistent with the notion that an anterior gradient system is not necessary in all insects and that the posterior pole probably conveys longitudinal polarity on the ensuing germ anlage (Lall, 2003)
The results indicate that nanos mRNA and protein are expressed asymmetrically at several stages of development. Within the germarium, Nanos protein is asymmetrically distributed within the developing oocytes. During early oogenesis, hunchback mRNA and protein are expressed in the same pattern of cells, suggesting that there is no translational repression of hunchback at this stage (there is also little or no Nanos protein in the Hunchback-expressing stage oocytes). Later in oogenesis, and in newly laid eggs, nanos mRNA is localized to the posterior pole of the egg. When cellularization begins, Nanos protein is found in cells toward the posterior (but not anterior) end of the egg. While this superficially resembles the asymmetry of Nanos protein in syncytial Drosophila embryos, it is important to remember that it is not possible, at least with the current data, to correlate this expression pattern to the future AP axis of the grasshopper embryo. It should be noted that the seeming lack of correspondence of the AP egg axis with the AP embryo axis may be a derived situation in the grasshopper, since the correlation is obvious in most other insects (Lall, 2003)
Nevertheless analysis of grasshopper nanos expression in the germ anlage indicates that this phase of asymmetric expression may underlie formation of the embryonic AP axis and posterior patterning of the embryo via Hunchback regulation. This suggests that an axial patterning mechanism involving translational repression of hb mRNA may be an ancestral feature of insect pattern formation (at least as far back as the common ancestor of Schistocerca and Drosophila). However, since maternal S. americana Hb is provided as protein, the target of translational repression in grasshopper would appear to be zygotic hunchback mRNA and not maternal hunchback mRNA as in Drosophila. It is currently unclear whether S. americana Nanos is acting as a switch that specifies some cells as posterior or whether it is acting in a graded fashion to permit the differentiation of different posterior identities. It is also interesting to note that work in Tribolium suggests that caudal may act as an activator of hunchback transcription and that S. gregaria caudal is expressed during condensation of the germ disc and in the early germ anlage. On the basis of these data, it is suggested that grasshopper caudal (and, possibly, maternally inherited Hunchback protein) could act to promote zygotic hunchback transcription throughout the entire embryonic primordium and that nanos acts to prevent translation of zygotic hunchback mRNA in the posterior of the grasshopper embryo (Lall, 2003)
Drosophila nanos also has a well-studied role in germline development, and it has been suggested that the ancestral role of nanos in metazoans was in germline function. Data presented in the current paper indicate, however, that the role of nanos in both axial patterning and germline development is probably ancestral to at least the insects. Furthermore, Cnnos2 is expressed in a manner consistent with a role in axial patterning of the growing buds and regenerating head, but not foot, of the cnidarian, H. magnipapillata. Thus, the function of nanos in both axial patterning (not necessarily via hb regulation) and germline development may be ancient. Indeed nanos may function in situations where a specific set of cells must be set aside and protected from patterning factors. This is entirely consistent with the role of nanos in germline specification as well as its role in protecting cells from anterior patterning factors, such as hunchback, within the insects (Lall, 2003)
The Drosophila gene nanos encodes two particular zinc finger motifs that are also found in germ-line-associated factors from nematodes to vertebrates. Two nanos (nos)-related genes, Cnnos1 and Cnnos2 have been cloned from Hydra magnipapillata. Using whole-mount in situ hybridization, the expression of Cnnos1 and Cnnos2 was examined. Cnnos1 is specifically expressed in multipotent stem cells and germline cells, but not in somatic cells. Cnnos2 is weakly expressed in germline cells and more specifically in the endoderm of the hypostome where it appears to be involved in head morphogenesis. In addition to structural conservation in the zinc finger domain of nanos-related genes, functional conservation of Cnnos1 is also demonstrated by the finding that a Cnnos1 transgene can partially rescue the nosRC phenotype that is defective in the egg production of Drosophila. Thus, the function of nanos-related genes in the germline appears to be well conserved from primitive to highly evolved metazoans (Mochizuki, 2000).
In Drosophila, the posterior determinant nanos is required for embryonic patterning and for primordial germ cell (PGC) development. Three genes have been identified in C. elegans that contain a putative zinc-binding domain similar to the one found in nanos: two of these genes function during PGC development. Like Drosophila nanos, C. elegans nos-1 and nos-2 are not generally required for PGC fate specification, but instead regulate specific aspects of PGC development. nos-2 is expressed in PGCs around the time of gastrulation from a maternal RNA associated with P granules, and is required for the efficient incorporation of PGCs into the somatic gonad. nos-1 is expressed in PGCs after gastrulation, and is required redundantly with nos-2 to prevent PGCs from dividing in starved animals and to maintain germ cell viability during larval development. In the absence of nos-1 and nos-2, germ cells cease proliferation at the end of the second larval stage, and die in a manner that is partially dependent on the apoptosis gene ced-4. These results also indicate that putative RNA-binding proteins related to Drosophila Pumilio are required for the same PGC processes as nos-1 and nos-2. These studies demonstrate that evolutionarily distant organisms utilize conserved factors to regulate early germ cell development and survival, and that these factors include members of the nanos and pumilio gene families (Subramaniam, 1999).
How do Nanos and NOS-1/NOS-2 function in PGCs at the molecular level? In early Drosophila embryos, Nanos controls embryonic patterning by repressing the translation of Hunchback mRNA. This repression requires the activity of Pumilio, a sequence-specific RNA-binding protein and translational regulator that recognizes and binds to nanos response elements in the 3'UTR of hunchback mRNA. Whether Nanos also functions with Pumilio in PGCs has not yet been reported. Like Nanos, Pumilio is required for germline stem cell development in adults, but its function there appears distinct from that of Nanos, raising the possibility that Nanos and Pumilio can function independent from one another in the germline. There are at least eight pumilio-like genes in the C. elegans genome, including four single-copy genes (puf-3, puf-4, puf-5 and puf-9), and two highly identical gene pairs (fbf-1/fbf-2 and puf-6/puf-7). fbf-1/fbf-2 have been implicated in the translational control of fem-3, coding a factor required for the sperm/oocyte switch in hermaphrodites. Simultaneous disruption of fbf-1/fbf-2, puf-6/puf-7 and puf-8 by RNA-mediated interference causes PGC defects identical to those observed in nos-1(-);nos-2(-) animals. These results suggest that nos-1 and nos-2 function in PGCs much as nanos does in embryonic patterning: by regulating the translation of specific mRNAs with the help of RNA-binding proteins related to Drosophila Pumilio. It is proposed that translational control by members of the nanos and pumilio gene families is a commonly used mechanism to regulate the development and survival of early germ cells (Subramaniam, 1999 and references therein).
The Caenorhabditis elegans FBF protein and its Drosophila relative, Pumilio, define a large family of eukaryotic RNA-binding proteins. By binding regulatory elements in the 3' untranslated regions (UTRs) of their cognate RNAs, FBF and Pumilio have key post-transcriptional roles in early developmental decisions. In C. elegans, FBF is required for repression of fem-3 mRNA to achieve the hermaphrodite switch from spermatogenesis to oogenesis. FBF and NANOS-3 (NOS-3), one of three C. elegans Nanos homologs, interact with each other in both yeast two-hybrid and in vitro assays. The portions of each protein required for this interaction have been delineated. Worms lacking nanos function were derived either by RNA-mediated interference (nos-1 and nos-2) or by use of a deletion mutant (nos-3). The roles of the three nos genes overlap during germ-line development. In certain nos-deficient animals, the hermaphrodite sperm-oocyte switch is defective, leading to the production of excess sperm and no oocytes. In other nos-deficient animals, the entire germ line dies during larval development. This germ-line death does not require CED-3, a protease required for apoptosis. The data suggest that NOS-3 participates in the sperm-oocyte switch through its physical interaction with FBF, forming a regulatory complex that controls fem-3 mRNA. NOS-1 and NOS-2 also function in the switch, but do not interact directly with FBF. The three C. elegans nanos genes, like Drosophila nanos, are also critical for germ-line survival. It is proposed that this may have been the primitive function of nanos genes (Kraemer, 1999).
Maintenance of the stem cell population in the C. elegans germline requires GLP-1/Notch signaling. This signaling inhibits the accumulation of the KH domain-containing RNA binding protein GLD-1, homolog of Drosophila How. In a genetic screen to identify other genes involved in regulating GLD-1 activity, mutations were identified in the nos-3 gene, the protein product of which is similar to the Drosophila translational regulator Nanos. The data demonstrate that nos-3 promotes GLD-1 accumulation redundantly with gld-2, coding for the catalytic portion of a poly(A) polymerase, and that nos-3 functions genetically downstream or parallel to fbf, an inhibitor of GLD-1 translation. The GLD-1 accumulation pattern is important in controlling the proliferation versus meiotic development decision, with low GLD-1 levels allowing proliferation and increased levels promoting meiotic entry (Hansen, 2003).
This study shows that a major mechanism by which GLP-1/Notch signaling maintains the stem cell population is by inhibiting GLD-1 protein accumulation in the distal end of the germline, thereby restricting its activity to more proximal regions. Not only does low GLD-1 allow proliferation, but high GLD-1 promotes meiosis. The position of the rise in GLD-1 levels determines the size of the stem cell population and the location where germ cells begin meiotic development. nos-3, whose role was identified in a mutant screen, functions redundantly with gld-2 to promote the rise in GLD-1 that is necessary for entry into meiosis. Genetic experiments indicate that repression of GLD-1 accumulation by FBF is acting through nos-3, while regulation of gld-2 in this processes is likely by something other than, or in addition to, FBF. The data suggest a model in which GLP-1 signaling regulates the size of the stem cell population by regulating GLD-1 levels, at least in part, through antagonism between the repressive activity of fbf and the positive activities of nos-3 and gld-2 (Hansen, 2003).
The PAR proteins have an essential and conserved function in establishing polarity in many cell types and organisms. However, their key upstream regulators remain to be identified. In C. elegans, regulators of the PAR proteins can be identified by their ability to suppress the lethality of par-2 mutant embryos. This study shows that a loss of function mutant in a Nanos homolog nos-3 suppresses the lethality of par-2 mutants by regulating PAR-6 protein levels. The suppression requires the activity of the sex determination genes fem-1/2/3 and of the cullin cul-2. FEM-1 is a substrate-specific adaptor for a CUL-2-based ubiquitin ligase (CBCFEM-1). Interestingly, CUL-2 is required for the regulation of PAR-6 levels and that PAR-6 physically interacts with FEM-1. These data strongly suggest that PAR-6 levels are regulated by the CBCFEM-1 ubiquitin ligase thereby uncovering a novel role for the FEM proteins and cullin-dependent degradation in regulating PAR proteins and polarity processes (Pacquelet, 2007).
Hro-nos, a gene from the glossiphoniid leech Helobdella robusta (phylum Annelida) has been cloned and found to be homologous to the Drosophila gene nanos. Hro-nos, like nanos, is a maternal transcript that decays rapidly during early development. The HRO-NOS protein is first detectable in 2-cell embryos (4-6 hours of development) and exhibits a transient expression peaking during fourth cleavage (9-12 cells; 8-14 hours of development). The HRO-NOS protein exhibits a graded distribution along the primary embryonic axis and is partitioned unequally between the sister cells DNOPQ and DM, progeny of macromere D' at fourth cleavage: DNOPQ is the segmental ectoderm precursor cell and exhibits levels of HRO-NOS protein that are at least two-fold higher than in cell DM, the segmental mesoderm precursor cell. The observed expression pattern suggests that Hro-nos plays a role in the decision between ectodermal and mesodermal cell fates in leech. It is suggested that a nos-class gene was part of an ancient mechanism for establishing early embryonic polarity; it appears that this gene has been co-opted in the course of evolutionary tinkering to play different roles in different embryos (Pilon, 1997).
The nanos-class gene of the leech Helobdella robusta (Hro-nos) is present as a maternal transcript whose levels decay during cleavage: HRO-NOS protein is more abundant in the D quadrant cells relative to the A, B, and C quadrants, and HRO-NOS is more abundant in the ectodermal precursor cell (DNOPQ) than in its sister mesodermal precursor (DM). Using in situ hybridization, it has been shown that Hro-nos mRNA is broadly distributed throughout the zygote, is concentrated in both animal and vegetal teloplasm during stage 1 and is at higher levels in DNOPQ than in DM at stage 4b. Hro-nos expression increases after stage 7, as judged by in situ hybridization, developmental RT-PCR, and Western blots; this increase must therefore represent later zygotic expression. Of particular interest, during stages 9 and 10, each of 11 mid-body segments (M8-M18) has a pair of Hro-nos positive 'spots' comprising one or two large cells each. These spots later disappear in an anteroposterior progression. These Hro-nos-expressing cells are of mesodermal origin, arising in a segmentally iterated manner from the M lineage, and correspond to cells previously proposed as primordial germ cells (PGCs). These results support the proposal that nanos-class genes functioned in the specification of germline cells in the ancestral bilaterian and possibly in a separate process related to embryonic polarity in the ancestral protostome (Kang, 2002).
The embryonic origins of the reproductive system organs and PGCs in leech (and other annelids) is unclear. In contrast to Drosophila or Caenorhabditis, for example, there is no obvious segregation of PGC precursors in early development. Leeches are hermaphroditic, with bilaterally paired testes and ovaries. The testes connect to the exterior through segment M5 and in many species occur as segmentally iterated testisacs, linked by a sperm duct. It is unknown whether this apparently segmental organization of the testisacs is real in the sense that these organs arise from blast cells, as opposed to reflecting a secondarily imposed patterning of organs derived by migration from some other source (Kang, 2002).
The finding that PGCs do not segregate from other lineages until after more than 20 rounds of zygotic mitosis suggests that in contrast to Drosophila, Caenorhabditis, or Xenopus, leeches contain no germ plasm that uniquely specifies one set of cells to be set aside as the exclusive germ cell lineage early in embryogenesis. Further evidence for this conclusion comes from observations that some oligochaete species can regenerate intact worms, complete with gonads, from body fragments that do not contain gonads. For example, it was found that in the oligochaete Criodilus lacuum, which normally contains two pairs of testes and one pair of ovaries, the regeneration of gonads is irregular with respect to both location and number of gonads regenerated, up to a total of 12 pairs, similar to what has been proposed as the ancestral number for leech pre-PGCs. Whether this similarity reflects some feature of the ancestral clitellate remains to be determined (Kang, 2002).
The gene nanos (nos) is a maternal posterior group gene required for normal development of abdominal segments and the germ line in Drosophila. Expression of nos-related genes is associated with the germ line in a broad variety of other taxa, including the leech Helobdella robusta, where zygotically expressed Hro-nos appears to be associated with primordial germ cells. The function of maternally inherited Hro-nos transcripts remains to be determined, however. In this study the function of maternal Hro-nos was examined using an antisense morpholino (MO) knockdown strategy, as confirmed by immunostaining and western blot analysis. HRO-NOS knockdown embryos exhibit abnormalities in the distribution of micromeres during cleavage. Subsequently, their germinal bands are positioned abnormally with respect to the embryonic midline and the micromere cap, epiboly fails, and the HRO-NOS knockdown embryos die. This lethality can be rescued by injection of mRNA encoding an eGFP::HRO-NOS fusion protein. HRO-NOS knockdown embryos make their normal complements of mesodermal and ectodermal teloblasts, and the progeny of these teloblasts segregate into distinct mesodermal and ectodermal layers. These results suggest that maternal Hro-nos is required for embryonic development. However, contrary to previous suggestions, maternal inherited Hro-nos does not appear necessary for ectoderm specification (Agee, 2006).
During animal development, blast cell lineages are generated by repeated divisions of a mother cell into a series of daughter cells, often with a specific series of distinct fates. Nanos is a translational regulator that is involved in germline development in diverse animals and also involved in somatic patterning in insects. Nanos is required for maintenance of stem cell divisions in the Drosophila germline. This study found that in the mollusk Ilyanassa obsoleta, Nanos messenger RNA and protein are specifically localized in the mesendodermal blast cell lineage derived from the strongly conserved 4d cell. Nanos activity is required for differentiation of multiple tissues that are derived from the 4d cell, showing that IoNanos is required for somatic development in this embryo. At the cellular level, IoNanos activity is required for the highly stereotyped cleavage pattern of the 4d lineage, the proliferative capacity of the blast cells, and the marked asymmetry of the blast cell divisions. These results suggest that IoNanos is involved in regulating blast cell behaviors in the 4d lineage (Rabinowitz, 2008).
The origin of germline cells was a crucial step in animal evolution. Therefore, in both developmental biology and evolutionary biology, the mechanisms of germline specification have been extensively studied over the past two centuries. However, in many animals, the process of germline specification remains unclear. This study shows that in the cephalochordate amphioxus Branchiostoma floridae, the germ cell-specific molecular markers Vasa and Nanos become localized to the vegetal pole cytoplasm during oogenesis and are inherited asymmetrically by a single blastomere during cleavage. After gastrulation, this founder cell gives rise to a cluster of progeny that display typical characters of primordial germ cells (PGCs). Blastomeres separated at the two-cell stage grow into twin embryos, but one of the twins fails to develop this Vasa-positive cell population, suggesting that the vegetal pole cytoplasm is required for the formation of putative PGCs in amphioxus embryos. Contrary to the hypothesis that cephalochordates may form their PGCs by epigenesis, these data strongly support a preformation mode of germ cell specification in amphioxus. In addition to the early localization of their maternal transcripts in the putative PGCs, amphioxus Vasa and Nanos are also expressed zygotically in the tail bud, which is the posterior growth zone of amphioxus. Thus, in addition to PGC specification, amphioxus Vasa and Nanos may also function in highly proliferating somatic stem cells (Wu, 2011).
Asymmetrically distributed cytoplasmic determinants collectively termed germ plasm have been shown to play an essential role in the development of primordial germ cells (PGCs). The identification of a nanos-like (nanos1) gene, which is expressed in the germ plasm and in the PGCs of the zebrafish, is reported. Several mechanisms act in concert to restrict the activity of Nanos1 to the germ cells, including RNA localization and control over the stability and translatability of the RNA. Reducing the level of Nanos1 in zebrafish embryos reveals an essential role for the protein in ensuring proper migration and survival of PGCs in this vertebrate model organism (Köprunner, 2001).
In order to restrict Nos1 to the PGCs of zebrafish, several control mechanisms are operating in concert at the level of asymmetric RNA localization, differential RNA stability and translation. Interestingly, in Drosophila the spatial distribution of Nanos is similarly controlled. Since other PGC-specific RNA molecules in zebrafish exhibit similar spatial distribution, it is conceivable that the mechanisms described here for nanos serve to restrict the function of other proteins to the PGCs as well. Taken together, these findings show that key steps in PGC development in invertebrates and in vertebrates, that is, PGC migration and maintenance of the PGC fate, require the function of related molecules, which are regulated by similar mechanisms (Köprunner, 2001).
In Xenopus, localization of a rare class of mRNAs during oogenesis is believed to initiate pattern formation in the early embryo. The pattern of RNA localization was determined for one of these RNAs, Xcat-2, which encodes a putative RNA-binding protein related to Drosophila Nanos. Xcat-2 is exclusively localized to the mitochondrial cloud in stage I oocytes. It moves with this body into the vegetal cortex during stage II and later partitions into islands consistent with it being a component of the germ plasm. Differential RNA binding to a cytoskeletal component(s) in the vegetal cortex determines the pattern of inheritance for that RNA in the embryo (Forristall, 1995).
In Xenopus, the inheritance of germ plasm by a small subset of blastomeres during early development is thought to direct these cells into the germ cell lineage. Xcat2 RNA, related to Drosophila nanos, is a germ plasm component that is translationally repressed during oogenesis. Xcat2 protein is not detected in oocytes at times either prior to, or after its RNA is localized in germ plasm, suggesting Xcat2 RNA is functionally sequestered soon after transcription. Indeed, Xcat2 RNA is found in a dense non-polysomal compartment in oocytes. Repression of translation is not relieved by substituting the Xcat2 3'UTR with that of beta-globin. Immunodetection of Xcat2 protein during blastula and gastrula stages coincides with the time of symmetric segregation of the germ plasm and a net increase in the number of primordial germ cells. Xcat2 is capable of binding RNA in vitro and it is proposed that Xcat2 may function to translationally regulate other RNAs specific to primordial germ cells (MacArthur, 1999).
Translational activation of dormant cyclin B1 mRNA stored in oocytes is a prerequisite for the initiation or promotion of oocyte maturation in many vertebrates. Using a monoclonal antibody against the domain highly homologous to that of Drosophila Pumilio, it has been shown for the first time in any vertebrate that a homolog of Pumilio is expressed in Xenopus oocytes. This 137-kDa protein binds to the region including the sequence UGUA at nucleotides 1335-1338 in the 3'-untranslated region of cyclin B1 mRNA, which is close to but does not overlap the cytoplasmic polyadenylation elements (CPEs). Physical in vitro association of Xenopus Pumilio with a Xenopus homolog of Nanos (Xcat-2) was demonstrated by a protein pull-down assay. The results of immunoprecipitation experiments have shown in vivo interaction between Xenopus Pumilio and CPE-binding protein (CPEB: Drosophila homolog Orb), a key regulator of translational repression and activation of mRNAs stored in oocytes. This evidence provides a new insight into the mechanism of translational regulation through the 3'-end of mRNA during oocyte maturation. These results also suggest the generality of the function of Pumilio as a translational regulator of dormant mRNAs in both invertebrates and vertebrates (Nakahata, 2001).
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, transgenic mouse lines were generated that express Nanos2 under the control of the Oct4DeltaPE promoter and Nanos2 function was examined in a Nanos3-null genetic background. 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, the current data show that Nanos3 does not rescue the defects in Nanos2-null mice. The 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 (Suzuki, 2007).
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. This study shows that Nanos2 is continuously expressed in male germ cells from fetal gonocytes to postnatal spermatogonial stem cells. 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. NANOS2 interacts with PUM2, and these two proteins colocalize in the ribonucleoparticle and polysomal fractions on sucrose gradients, supporting the notion that they bind RNA. Finally, it was found that recombinant NANOS2 binds to two spermatogonial mRNAs, Gata2 and Taf7l, which are involved in germ-cell differentiation (Barrios, 2010).
The RNA-binding proteins of the Nanos family play an essential role in germ cell development and survival in a wide range of metazoan species. They function by suppressing the expression of target mRNAs through the recruitment of effector complexes, which include the CCR4-NOT deadenylase complex. This study shows that the three human Nanos paralogs (Nanos1-3) interact with the CNOT1 C-terminal domain and determine the structural basis for the specific molecular recognition. Nanos1-3 bind CNOT1 through a short CNOT1-interacting motif (NIM) that is conserved in all vertebrates and some invertebrate species. The crystal structure of the human Nanos1 NIM peptide bound to CNOT1 reveals that the peptide opens a conserved hydrophobic pocket on the CNOT1 surface by inserting conserved aromatic residues. The substitutions of these aromatic residues in the Nanos1-3 NIMs abolish binding to CNOT1 and abrogate the ability of the proteins to repress translation. These findings provide the structural basis for the recruitment of the CCR4-NOT complex by vertebrate Nanos, indicate that the NIMs are the major determinants of the translational repression mediated by Nanos, and identify the CCR4-NOT complex as the main effector complex for Nanos function (Bhandari, 2014).
date revised: 12 September 98
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