Opposing microRNA families regulate self-renewal in mouse embryonic stem cells. Melton C et al. When embryonic stem cells (ESCs) differentiate, they must both silence the ESC self-renewal program and activate new tissue-specific programs. In the absence of DGCR8 (Dgcr8(-/-)), a protein required for microRNA (miRNA) biogenesis, mouse ESCs are unable to silence self-renewal. Here we show that the introduction of let-7 miRNAs-a family of miRNAs highly expressed in somatic cells-can suppress self-renewal in Dgcr8(-/-) but not wild-type ESCs. Introduction of ESC cell cycle regulating (ESCC) miRNAs into the Dgcr8(-/-) ESCs blocks the capacity of let-7 to suppress self-renewal. Profiling and bioinformatic analyses show that let-7 inhibits whereas ESCC miRNAs indirectly activate numerous self-renewal genes. Furthermore, inhibition of the let-7 family promotes de-differentiation of somatic cells to induced pluripotent stem cells. Together, these findings show how the ESCC and let-7 miRNAs act through common pathways to alternatively stabilize the self-renewing versus differentiated cell fates.
Cellular localization
Nuclear
Comment
stemness.////////A gene expression signature shared by human mature oocytes and embryonic stem cells. Assou S et al. ABSTRACT: BACKGROUND: The first week of human pre-embryo development is characterized by the induction of totipotency and then pluripotency. The understanding of this delicate process will have far reaching implication for in vitro fertilization and regenerative medicine. Human mature MII oocytes and embryonic stem (ES) cells are both able to achieve the feat of cell reprogramming towards pluripotency, either by somatic cell nuclear transfer or by cell fusion, respectively. Comparison of the transcriptome of these two cell types may highlight genes that are involved in pluripotency initiation. Results: Based on a microarray compendium of 205 samples, we compared the gene expression profile of mature MII oocytes and human ES cells (hESC) to that of somatic tissues. We identified a common oocyte/hESC gene expression profile, which included a strong cell cycle signature, genes associated with pluripotency such as LIN28 and TDGF1, a large chromatin remodelling network (TOP2A, DNMT3B, JARID2, SMARCA5, CBX1, CBX5), 18 different zinc finger transcription factors, including ZNF84, and several still poorly annotated genes such as KLHL7, MRS2, or the Selenophosphate synthetase 1 (SEPHS1). Interestingly, a large set of genes was also found to code for proteins involved in the ubiquitination and proteasome pathway. Upon hESC differentiation into embryoid bodies, the transcription of this pathway declined. In vitro, we observed a selective sensitivity of hESC to the inhibition of the activity of the proteasome. Conclusions: These results shed light on the gene networks that are concurrently overexpressed by the two human cell types with somatic cell reprogramming properties.
Ovarian function
Early embryo development
Comment
Expression regulated by
AIRE
Comment
Aire promotes the self-renewal of embryonic stem cells through Lin28. Gu B et al. Autoimmune regulator (Aire) is one of the most well-characterized molecules in autoimmunity, but its function outside the immune system is largely unknown. The recent discovery of Aire expression in stem cells and early embryonic cells and its function in the self-renewal of embryonic stem(ES) cells highlight the importance of Aire in these cells. In this study, we present evidence that Aire promotes the expression of the pluripotent factor Lin28 and the self-renewal of ES cells. We presented the first evidence that the let-7 microRNA family contributed to the self-renewal promoting effect of Aire in ES cells. Moreover, we showed that Aire and Lin28 are co-expressed in the genital ridge, oocytes and cleavage stage embryos, and the expression level of Lin28 is correlated with expression level of Aire. Although it is widely considered to be a promiscuous gene expression activator, these results indicated that Aire promotes the self-renewal of embryonic stem cells through a specific pathway (i.e., the activation of Lin28 and inhibition of the let-7 microRNA family). The correlation between Aire and Lin28 expression in germ cells and early embryos indicated an in vivo function for Aire in toti- and pluripotent stem cells. This study presents the first molecular pathway that incorporates Aire into the pluripotency network. Moreover, it presents the first evidence that microRNAs contributed to the regulatory function of Aire and highlights a novel function of Aire in stem cell biology and reproduction. These functions reveal novel perspectives for studying the molecular mechanisms of pluripotent identity establishment and sustenance.
Ovarian localization
Primordial Germ Cell, Oocyte
Comment
LIN28 is selectively expressed by primordial and pre-meiotic germ cells in the human fetal ovary. Childs A et al. Germ cell development requires timely transition from primordial germ cell (PGC) self-renewal to meiotic differentiation. This is associated with widespread changes in gene expression including down-regulation of stem cell-associated genes such as OCT4 and KIT, and upregulation of markers of germ cell differentiation and meiosis, including VASA, STRA8 and SYCP3. The stem cell-expressed RNA-binding protein Lin28 has recently been demonstrated to be essential for PGC specification in mice, and LIN28 is expressed in human germ cell tumours with phenotypic similarities to human fetal germ cells. We have therefore examined the expression of LIN28 during normal germ cell development in the human fetal ovary, from the PGC stage, through meiosis to the initiation of follicle formation. LIN28 transcript levels were highest when the gonad contained only PGCs, and decreased significantly with increasing gestation, coincident with the onset of germ cell differentiation. Immunohistochemistry revealed LIN28 protein expression to be germ cell-specific at all stages examined. All PGCs expressed LIN28, but at later gestations expression was restricted to a sub-population of germ cells, which we demonstrate to be primordial and pre-meiotic germ cells based on immunofluorescent co-localisation of LIN28 and OCT4, and absence of overlap with the meiosis marker SYCP3. We also demonstrate the expression of the LIN28 target precursor pri-microRNA transcripts pri-LET7a/f/d and pri-LET-7g in the human fetal ovary, and that expression of these is highest at the PGC stage, mirroring that of LIN28. The spatial and temporal restriction of LIN28 expression, and co-incident peaks of expression of LIN28 and target pri-microRNAs, suggest important roles for this protein in the maintenance of the germline stem cell state and the regulation of microRNA activity in the developing human ovary.
A role for Lin28 in primordial germ-cell development and germ-cell malignancy. West JA et al. The rarity and inaccessibility of the earliest primordial germ cells (PGCs) in the mouse embryo thwart efforts to investigate molecular mechanisms of germ-cell specification. stella (also called Dppa3) marks the rare founder population of the germ lineage. Here we differentiate mouse embryonic stem cells carrying a stella transgenic reporter into putative PGCs in vitro. The Stella(+) cells possess a transcriptional profile similar to embryo-derived PGCs, and like their counterparts in vivo, lose imprints in a time-dependent manner. Using inhibitory RNAs to screen candidate genes for effects on the development of Stella(+) cells in vitro, we discovered that Lin28, a negative regulator of let-7 microRNA processing, is essential for proper PGC development. Furthermore, we show that Blimp1 (also called Prdm1), a let-7 target and a master regulator of PGC specification, can rescue the effect of Lin28 deficiency during PGC development, thereby establishing a mechanism of action for Lin28 during PGC specification. Overexpression of Lin28 promotes formation of Stella(+) cells in vitro and PGCs in chimaeric embryos, and is associated with human germ-cell tumours. The differentiation of putative PGCs from embryonic stem cells in vitro recapitulates the early stages of gamete development in vivo, and provides an accessible system for discovering novel genes involved in germ-cell development and malignancy.
Follicle stages
Comment
Phenotypes
Mutations
1 mutations
Species: mouse
Mutation name: type: None fertility: fertile Comment: Lin28a Is Dormant, Functional, and Dispensable During Mouse Oocyte-to-Embryo Transition. Flemr M 2014 et al.
The oocyte-to-embryo transition (OET) denotes transformation of a highly differentiated oocyte into totipotent blastomeres of the early mammalian embryo. OET exclusively depends on maternal RNAs and proteins accumulated during oocyte growth, which implies importance of post-transcriptional control of gene expression. OET includes replacement of abundant maternal microRNAs (miRNAs), enriched also in differentiated cells and exemplified by the Let-7 family, with embryonic miRNAs common in pluripotent stem cells (the miR-290 family in the mouse). Lin28a and its homolog Lin28b encode RNA-binding proteins, which interfere with Let-7 maturation and facilitate reprogramming of induced pluripotent stem cells. Both Lin28a and Lin28b transcripts are abundant in mouse oocytes. To test the role of maternal expression of Lin28a and Lin28b during oocyte-to-zygote reprogramming, we generated mice with oocyte-specific knock-down of both genes using transgenic RNAi. Lin28a and Lin28b are dispensable during oocyte growth since their knock-down has no effect on Let-7a levels in fully-grown germinal vesicle (GV)-intact oocytes. Furthermore, transgenic females were fertile, produced healthy offspring and their overall breeding performance was comparable to wild type mice. At the same time, 2-cell embryos derived from transgenic females showed up-regulation of mature Let-7 suggesting that maternally provided LIN28A and LIN28B function during zygotic genome activation. Consistent with this conclusion is increased translation of Lin28a transcripts upon resumption of meiosis. Our data imply dual repression Let-7 during OET in the mouse model--the selective suppression of Let-7 biogenesis by Lin28 homologs superimposed on previously reported global suppression of miRNA activity.
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