Genetic and Epigenetic Regulators of Pluripotency
2007; Cell Press; Volume: 128; Issue: 4 Linguagem: Inglês
10.1016/j.cell.2007.02.010
ISSN1097-4172
AutoresM. Azim Surani, Katsuhiko Hayashi, Petra Hájková,
Tópico(s)Cancer-related gene regulation
ResumoGenetic and epigenetic mechanisms regulate the transition from the totipotent zygote to pluripotent primitive ectoderm cells in the inner cell mass of mouse blastocysts. These pluripotent cells can be propagated indefinitely in vitro, underpinned by a unique epigenetic state. Following implantation of the blastocyst, diverse epigenetic modifiers control differentiation of pluripotent epiblast cells into somatic cells, while specification of germ cells requires repression of the somatic program. Regenerating totipotency during development of germ cells entails re-expression of pluripotency-specific genes and extensive erasure of epigenetic modifications. Increasing knowledge of key underlying mechanisms heightens prospects for creating pluripotent cells directly from adult somatic cells. Genetic and epigenetic mechanisms regulate the transition from the totipotent zygote to pluripotent primitive ectoderm cells in the inner cell mass of mouse blastocysts. These pluripotent cells can be propagated indefinitely in vitro, underpinned by a unique epigenetic state. Following implantation of the blastocyst, diverse epigenetic modifiers control differentiation of pluripotent epiblast cells into somatic cells, while specification of germ cells requires repression of the somatic program. Regenerating totipotency during development of germ cells entails re-expression of pluripotency-specific genes and extensive erasure of epigenetic modifications. Increasing knowledge of key underlying mechanisms heightens prospects for creating pluripotent cells directly from adult somatic cells. Development and cell fate determination require close coordination between genetic and epigenetic programs. These are in turn regulated by signaling molecules, which together with interactions among neighboring cells induce appropriate transcriptional and epigenetic responses that are essential for cell fate determination. In addition, epigenetic mechanisms contribute to the repression of inappropriate developmental programs in time and space while ensuring heritability of existing or newly acquired phenotypic states. These extrinsic and intrinsic regulators determine the developmental origin and subsequent propagation of pluripotent states in vivo and in vitro. Totipotency and pluripotency are two quite distinct epigenetic states with different developmental potentials. The zygote and to some extent early blastomeres are totipotent, as they are self-contained entities that can give rise to the whole organism. As these cells undergo cleavage divisions, they lack the capacity for self-renewal. Pluripotent cells are established from totipotent blastomeres within the inner cell mass (ICM) of blastocysts. As these cells cease cleavage divisions and acquire properties of normal cell division, they become responsive to external signals and acquire the capacity for self-renewal when cultured in vitro. Germ cells, while highly specialized, are unique because the end product of the lineage is the totipotent zygote. Furthermore, early germ cells repress the somatic program, and their epigenetic and transcriptional statuses share features that are compatible with pluripotency, although they cannot differentiate into diverse cell types. However, pluripotent stem cells can be derived in vitro from both the ICM and germ cells. Here we discuss the relationship between all of these different developmental states and their in vitro derivatives. We confine our discussion to chromatin- and DNA-based epigenetic changes and the transcription factors that contribute to their inherent states. When development commences, the totipotent zygote contains key maternally inherited transcriptional and epigenetic factors that regulate early development. The switch from the zygotic to embryonic program occurs when transcription starts at the late zygote and at the two-cell stage (Solter et al., 2004Solter D. Hiiragi T. Evsikov A.V. Moyer J. De Vries W.N. Peaston A.E. Knowles B.B. Epigenetic mechanisms in early mammalian development.Cold Spring Harb. Symp. Quant. Biol. 2004; 69: 11-17Crossref Scopus (12) Google Scholar). This is followed by preimplantation development involving about six cleavage divisions to form a blastocyst (Chazaud et al., 2006Chazaud C. Yamanaka Y. Pawson T. Rossant J. Early lineage segregation between epiblast and primitive endoderm in mouse blastocysts through the Grb2-MAPK pathway.Dev. Cell. 2006; 10: 615-624Abstract Full Text Full Text PDF PubMed Scopus (290) Google Scholar, Niwa et al., 2005Niwa H. Toyooka Y. Shimosato D. Strumpf D. Takahashi K. Yagi R. Rossant J. Interaction between Oct3/4 and Cdx2 determines trophectoderm differentiation.Cell. 2005; 123: 917-929Abstract Full Text Full Text PDF PubMed Scopus (450) Google Scholar, Yamanaka et al., 2006Yamanaka Y. Ralston A. Stephenson R.O. Rossant J. Cell and molecular regulation of the mouse blastocyst.Dev. Dyn. 2006; 235: 2301-2314Crossref PubMed Scopus (126) Google Scholar), a unique developmental stage in mammals without parallel in other organisms. Blastocysts consist of approximately 60 cells, with an ICM containing the pluripotent primitive ectoderm (PEct) cells, and the specialized outer trophectoderm cells that are required for implantation and development of the placenta. The ICM is the foundation of all somatic tissues and germ cells in adults. Following implantation, the ICM commences development to form the epiblast cells of the early egg cylinder, which are also pluripotent as judged by the expression of pluripotent cell-specific genes such as Oct4. These cells respond to signals from the surrounding extraembryonic tissues that direct differentiation and initiation of gastrulation (see Figure 1). One of the earliest developmental events at the onset of gastrulation is the establishment of the founder germ cells (Surani et al., 2004Surani M.A. Ancelin K. Hajkova P. Lange U.C. Payer B. Western P. Saitou M. Mechanism of mouse germ cell specification: a genetic program regulating epigenetic reprogramming.Cold Spring Harb. Symp. Quant. Biol. 2004; 69: 1-9Crossref Google Scholar). Germ cells are highly specialized cells established by a specific transcriptional program that includes repression of the somatic fate. Importantly, this is the only lineage that exhibits expression of pluripotency-specific genes after gastrulation. The transcriptional program involved in generating germ cells must also regulate the extensive epigenetic reprogramming of the genome, including genome-wide erasure of existing epigenetic modifications, which is evidently unique to this lineage and an essential step toward the eventual totipotent state. The ICM and primordial germ cells (PGCs) are in turn the precursors of pluripotent embryonic stem (ES) and embryonic germ (EG) cells, respectively, which are derived and maintained only in culture in vitro (Durcova-Hills et al., 2006Durcova-Hills G. Adams I.R. Barton S.C. Surani M.A. McLaren A. The role of exogenous fibroblast growth factor-2 on the reprogramming of primordial germ cells into pluripotent stem cells.Stem Cells. 2006; 24: 1441-1449Crossref PubMed Scopus (64) Google Scholar, Matsui et al., 1992Matsui Y. Zsebo K. Hogan B.L. Derivation of pluripotential embryonic stem cells from murine primordial germ cells in culture.Cell. 1992; 70: 841-847Abstract Full Text PDF PubMed Scopus (702) Google Scholar, Resnick et al., 1992Resnick J.L. Bixler L.S. Cheng L. Donovan P.J. Long-term proliferation of mouse primordial germ cells in culture.Nature. 1992; 359: 550-551Crossref PubMed Scopus (522) Google Scholar, Ying et al., 2003Ying Q.L. Nichols J. Chambers I. Smith A. BMP induction of Id proteins suppresses differentiation and sustains embryonic stem cell self-renewal in collaboration with STAT3.Cell. 2003; 115: 281-292Abstract Full Text Full Text PDF PubMed Scopus (1073) Google Scholar). More recently, pluripotent stem cells have been derived from spermatogonial stem cells (Kanatsu-Shinohara et al., 2005Kanatsu-Shinohara M. Ogonuki N. Iwano T. Lee J. Kazuki Y. Inoue K. Miki H. Takehashi M. Toyokuni S. Shinkai Y. et al.Genetic and epigenetic properties of mouse male germline stem cells during long-term culture.Development. 2005; 132: 4155-4163Crossref PubMed Scopus (105) Google Scholar). This suggests that the transcriptional network and epigenetic regulators capable of supporting pluripotency may be maintained during germ cell development. The ES cells can exhibit a perpetual pluripotent state in vitro, which may correspond to but is not identical to the transient pluripotent state of PEct cells in vivo. For example, specific cytokines promote the derivation and maintenance of ES cells. Leukemia inhibitory factor (LIF) and BMP4 are key factors that may not only modify PEct and evoke appropriate responses during the derivation of ES cells but also sustain pluripotency indefinitely in culture (Ying et al., 2003Ying Q.L. Nichols J. Chambers I. Smith A. BMP induction of Id proteins suppresses differentiation and sustains embryonic stem cell self-renewal in collaboration with STAT3.Cell. 2003; 115: 281-292Abstract Full Text Full Text PDF PubMed Scopus (1073) Google Scholar, Chambers and Smith, 2004Chambers I. Smith A. Self-renewal of teratocarcinoma and embryonic stem cells.Oncogene. 2004; 23: 7150-7160Crossref PubMed Scopus (314) Google Scholar). When released from the influence of these cytokines in vitro or following their introduction back into the blastocyst, ES cells undergo differentiation, just like PEct cells. These observations stress the transient nature of the pluripotency of PEct cells, as they progress quickly to the next developmental stage in vivo but can be maintained indefinitely as ES cells in vitro. Because ES (and EG) cells have no strict equivalents in vivo, theirs is a unique epigenetic state (Figure 1). First, we consider the critical events that occur within the totipotent zygote and the origin of pluripotent cells during early development. At fertilization, when the parental genomes come together in the oocyte cytoplasm to form the totipotent zygote, the paternal genome has a very different developmental history from the resident maternal genome and must acquire an appropriate epigenetic state to participate in development (Arney et al., 2001Arney K.L. Erhardt S. Drewell R.A. Surani M.A. Epigenetic reprogramming of the genome–from the germ line to the embryo and back again.Int. J. Dev. Biol. 2001; 45: 533-540Google Scholar). Initially, the paternal genome is highly condensed, partly through its binding by protamines, which are rapidly replaced by histones. As this replacement occurs prior to S phase, a particular histone variant, H3.3, is selectively incorporated, probably by the histone chaperone Hira, into the paternal genome (Torres-Padilla et al., 2006Torres-Padilla M.E. Bannister A.J. Hurd P.J. Kouzarides T. Zernicka-Goetz M. Dynamic distribution of the replacement histone variant H3.3 in the mouse oocyte and preimplantation embryos.Int. J. Dev. Biol. 2006; 50: 455-461Crossref PubMed Scopus (104) Google Scholar, van der Heijden et al., 2005van der Heijden G.W. Dieker J.W. Derijck A.A. Muller S. Berden J.H. Braat D.D. van der Vlag J. de Boer P. Asymmetry in histone H3 variants and lysine methylation between paternal and maternal chromatin of the early mouse zygote.Mech. Dev. 2005; 122: 1008-1022Crossref PubMed Scopus (155) Google Scholar). Interestingly, the canonical histone H3.1 is absent from the paternal pronucleus before DNA replication (van der Heijden et al., 2005van der Heijden G.W. Dieker J.W. Derijck A.A. Muller S. Berden J.H. Braat D.D. van der Vlag J. de Boer P. Asymmetry in histone H3 variants and lysine methylation between paternal and maternal chromatin of the early mouse zygote.Mech. Dev. 2005; 122: 1008-1022Crossref PubMed Scopus (155) Google Scholar). This initial epigenetic asymmetry between the parental genomes is further manifested by differences in histone modifications and localization of numerous epigenetic modifiers such as Ezh2 (Erhardt et al., 2003Erhardt S. Su I.H. Schneider R. Barton S. Bannister A.J. Perez-Burgos L. Jenuwein T. Kouzarides T. Tarakhovsky A. Surani M.A. Consequences of the depletion of zygotic and embryonic enhancer of zeste 2 during preimplantation mouse development.Development. 2003; 130: 4235-4248Crossref PubMed Scopus (160) Google Scholar). The paternal pronucleus also features a specific pattern of histone modifications. While H3K4me1, H3K9me1, and H3K27me1 are detected at fertilization, H3K4me3, H3K9me2, and H3K27me3 become detectable only after DNA replication (although H3K9me2 is detected very weakly) (Arney et al., 2001Arney K.L. Erhardt S. Drewell R.A. Surani M.A. Epigenetic reprogramming of the genome–from the germ line to the embryo and back again.Int. J. Dev. Biol. 2001; 45: 533-540Google Scholar, Lepikhov and Walter, 2004Lepikhov K. Walter J. Differential dynamics of histone H3 methylation at positions K4 and K9 in the mouse zygote.BMC Dev. Biol. 2004; 4: 12Crossref PubMed Scopus (61) Google Scholar, Santos et al., 2005Santos F. Peters A.H. Otte A.P. Reik W. Dean W. Dynamic chromatin modifications characterise the first cell cycle in mouse embryos.Dev. Biol. 2005; 280: 225-236Crossref PubMed Scopus (173) Google Scholar; see also the Review by A. Groth et al., page 721 of this issue). The epigenetic status of the paternal pronucleus changes in other respects as well. There is extensive and rapid genome-wide DNA demethylation of the paternal genome (Mayer et al., 2000Mayer W. Niveleau A. Walter J. Fundele R. Haaf T. Demethylation of the zygotic paternal genome.Nature. 2000; 403: 501-502Crossref PubMed Google Scholar, Oswald et al., 2000Oswald J. Engemann S. Lane N. Mayer W. Olek A. Fundele R. Dean W. Reik W. Walter J. Active demethylation of the paternal genome in the mouse zygote.Curr. Biol. 2000; 10: 475-478Abstract Full Text Full Text PDF PubMed Scopus (477) Google Scholar). The molecular mechanism of this global DNA demethylation is currently unknown, but correct epigenetic configuration of the paternal chromatin is likely to be important given the fact that the maternal genome escapes this process. As histone methylation can direct DNA methylation, at least in particular genomic regions, the differences in histone modifications between parental pronuclei may explain the protection of the maternal genome from undergoing DNA demethylation. More recently, Stella was shown to be required for preventing DNA demethylation of the maternal genome; in Stella-deficient oocytes, the maternal genome is massively demethylated (Nakamura et al., 2007Nakamura T. Arai Y. Umehara H. Masuhara M. Kimura T. Taniguchi H. Sekimoto T. Ikawa M. Yoneda Y. Okabe M. et al.PGC7/Stella protects against DNA demethylation in early embryogenesis.Nat. Cell Biol. 2007; 9 (Published online December 3, 2006): 64-71https://doi.org/10.1038/ncb1519Crossref Scopus (199) Google Scholar). However, as Stella is found in both maternal and paternal pronuclei, additional factors must cooperate to protect the maternal genome from DNA demethylation. The zygote contains a number of key maternally inherited transcription factors, including some that are essential for pluripotency, such as Oct3/4 and Sox2, as well as epigenetic factors for histone modifications including Polycomb group (PcG) proteins such as Ezh2 and Eed, proteins of histone metabolism (Padi4), and chromatin remodelers such as Brg1 (see the Review by B. Schuettengruber et al., page 735 of this issue). As the key requirement at this stage of development is to convert the quiescent genome into a transcriptionally competent one, this must be accomplished by maternally inherited factors in the oocyte. Among the maternal factors whose function has been well defined is Brg1, a component of the SWI/SNF chromatin-remodeling complex (Bultman et al., 2006Bultman S.J. Gebuhr T.C. Pan H. Svoboda P. Schultz R.M. Magnuson T. Maternal BRG1 regulates zygotic genome activation in the mouse.Genes Dev. 2006; 20: 1744-1754Crossref Scopus (125) Google Scholar). Loss of Brg1 results in reduced transcription and arrest at the two-cell stage. Another example is Npm2, whose presence in the oocyte is essential for histone deacetylation and heterochromatin formation surrounding the nucleoli (Burns et al., 2003Burns K.H. Viveiros M.M. Ren Y. Wang P. DeMayo F.J. Frail D.E. Eppig J.J. Matzuk M.M. Roles of NPM2 in chromatin and nucleolar organization in oocytes and embryos.Science. 2003; 300: 633-636Crossref PubMed Scopus (165) Google Scholar; Table 1). The role of key epigenetic modifiers has been established by genetic experiments. Deletion of many of these genes also causes a failure of the ICM to give rise to ES cells in vitro, suggesting a direct role for these factors in the establishment or maintenance of pluripotency. ND, not determined; E, embryonic day; MBD, methylcytosine binding domain; HAT, histone acetyltransferase; HMTase, histone methyltransferase; DNA MTase, DNA methyltransferase; ES viable, viability of ES cells when the second allele or both alleles are deleted from established ES cells in vitro. From the late zygote to the two-cell stage, when the embryonic genome becomes activated, the epigenetic status of the parental genomes starts to become less distinct, with the exception of DNA methylation. The overall differences in DNA methylation persist for one to two cleavage divisions, followed by a passive and steady decline through preimplantation development (Mayer et al., 2000Mayer W. Niveleau A. Walter J. Fundele R. Haaf T. Demethylation of the zygotic paternal genome.Nature. 2000; 403: 501-502Crossref PubMed Google Scholar). This change is accompanied by a gradual increase in H3K9me2 (Santos et al., 2003Santos F. Zakhartchenko V. Stojkovic M. Peters A. Jenuwein T. Wolf E. Reik W. Dean W. Epigenetic marking correlates with developmental potential in cloned bovine preimplantation embryos.Curr. Biol. 2003; 13: 1116-1121Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar, Yeo et al., 2005Yeo S. Lee K.K. Han Y.M. Kang Y.K. Methylation changes of lysine 9 of histone H3 during preimplantation mouse development.Mol. Cells. 2005; 20: 423-428Google Scholar). Notably, examination of cloned mammalian embryos has revealed that the levels of methylated H3K9 in preimplantation embryos are important for further development (Santos et al., 2003Santos F. Zakhartchenko V. Stojkovic M. Peters A. Jenuwein T. Wolf E. Reik W. Dean W. Epigenetic marking correlates with developmental potential in cloned bovine preimplantation embryos.Curr. Biol. 2003; 13: 1116-1121Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar). This provides further evidence for the importance of the chromatin configuration during this developmental stage. The early blastomeres, until about the eight-cell stage, are essentially identical and totipotent and retain considerable plasticity. However, individual blastomeres at the four-cell stage may have some bias in their contribution to the ICM and trophectoderm lineages in an unperturbed embryo (Zernicka-Goetz, 2005Zernicka-Goetz M. Developmental cell biology: cleavage pattern and emerging asymmetry of the mouse embryo.Nat. Rev. Mol. Cell Biol. 2005; 6: 919-928Crossref Scopus (46) Google Scholar). Although the descendents of a single eight-cell-stage blastomere may give rise only to trophectoderm cells, no descendents of a single blastomere at this stage can give rise only to pluripotent PEct cells. At the eight-cell stage, each blastomere becomes polarized and divides either symmetrically to generate two polar outer cells (OCs) or asymmetrically to generate an apolar inner cell (IC) and a polar OC. Thus, between the eight- and 16-cell stage, the first distinct group of ICs and OCs are generated, which are the precursors of the pluripotent PEct cells in the ICM and the trophectoderm cells, respectively (Johnson and Ziomek, 1981Johnson M.H. Ziomek C.A. The foundation of two distinct cell lineages within the mouse morula.Cell. 1981; 24: 71-80Abstract Full Text PDF PubMed Google Scholar, Yamanaka et al., 2006Yamanaka Y. Ralston A. Stephenson R.O. Rossant J. Cell and molecular regulation of the mouse blastocyst.Dev. Dyn. 2006; 235: 2301-2314Crossref PubMed Scopus (126) Google Scholar). The “permissive” epigenetic state generated in the zygote allows a number of key transcription factors to play a critical role during development of the blastocyst. Among these factors are Oct4 and Cdx2, which are essential for development of the ICM and trophectoderm, respectively (Niwa et al., 2005Niwa H. Toyooka Y. Shimosato D. Strumpf D. Takahashi K. Yagi R. Rossant J. Interaction between Oct3/4 and Cdx2 determines trophectoderm differentiation.Cell. 2005; 123: 917-929Abstract Full Text Full Text PDF PubMed Scopus (450) Google Scholar). In the early morula stage, both of these factors are expressed in all blastomeres. In the late morula stage, when the IC and OC are formed, Oct4 is detected in the IC whereas Cdx2 is confined to the OC (Niwa et al., 2005Niwa H. Toyooka Y. Shimosato D. Strumpf D. Takahashi K. Yagi R. Rossant J. Interaction between Oct3/4 and Cdx2 determines trophectoderm differentiation.Cell. 2005; 123: 917-929Abstract Full Text Full Text PDF PubMed Scopus (450) Google Scholar). The ICM itself comprises the inner PEct and the outer primitive endoderm (PEnd). An additional feature of the ICs in a morula is the expression of Nanog, a homeodomain protein (Chambers et al., 2003Chambers I. Colby D. Robertson M. Nichols J. Lee S. Tweedie S. Smith A. Functional expression cloning of Nanog, a pluripotency sustaining factor in embryonic stem cells.Cell. 2003; 113: 643-655Abstract Full Text Full Text PDF PubMed Scopus (1649) Google Scholar, Mitsui et al., 2003Mitsui K. Tokuzawa Y. Itoh H. Segawa K. Murakami M. Takahashi K. Maruyama M. Maeda M. Yamanaka S. The homeoprotein Nanog is required for maintenance of pluripotency in mouse epiblast and ES cells.Cell. 2003; 113: 631-642Abstract Full Text Full Text PDF PubMed Scopus (1559) Google Scholar). Recent studies show that the expression of nanog in early blastomeres may be regulated by the histone arginine methyltransferase Carm1 (Torres-Padilla et al., 2007Torres-Padilla M.E. Parfitt D.E. Kouzarides T. Zernicka-Goetz M. Histone arginine methylation regulates pluripotency in the early mouse embryo.Nature. 2007; 445: 214-218Crossref PubMed Scopus (218) Google Scholar), although the expression of Carm1 does not commit the cell to develop exclusively as an IC. The role of Nanog is apparently to promote development of PEct, as the ICM of the E3.5 blastocyst shows a mutually exclusive mosaic pattern of expression of Nanog and Gata6 in individual cells. (Expression of the latter is essential for the development of PEnd cells.) If Gata6 expression is eliminated from the ICM, all of the cells show expression of Nanog (Chazaud et al., 2006Chazaud C. Yamanaka Y. Pawson T. Rossant J. Early lineage segregation between epiblast and primitive endoderm in mouse blastocysts through the Grb2-MAPK pathway.Dev. Cell. 2006; 10: 615-624Abstract Full Text Full Text PDF PubMed Scopus (290) Google Scholar). No distinct epigenetic differences between ICs and OCs in the morula have been reported, but we cannot exclude a possibility that such differences could dictate mutually exclusive expression of the key transcription factors described above. In any event, the ICs at the 16-cell stage are not yet fully committed and can develop into trophectoderm cells if extracted and exposed to the outside environment during subsequent development. It is likely, however, that once distinct cell fate decisions are made, appropriate gene- or locus-specific epigenetic modifications may ensure that the identities of the pluripotent PEct cells, as well as those of the trophectoderm and PEnd cells in the blastocyst, are maintained. The PEct cells in particular may be constrained from undergoing differentiation into extraembryonic tissues by epigenetic regulators. Nevertheless, at the blastocyst stage, there are clear epigenetic differences between the ICM and trophectoderm cells. This is evident from the analysis of X inactivation in these tissues, which may be indicative of other differences between them. In female embryos, the “imprinted” paternal X chromosome is preferentially inactivated during preimplantation development. The initial event involves expression of the noncoding RNA Xist from the paternal X chromosome, which is followed by histone modifications including loss of H3K4me2 and H3K4me3 and the gain of H3K9me2 and H3K27me3, as well as the ubiquitination of H2A (Heard, 2004Heard E. Recent advances in X-chromosome inactivation.Curr. Opin. Cell Biol. 2004; 16: 247-255Crossref PubMed Scopus (179) Google Scholar; see also the Review by P.K. Yang and K.I. Kuroda, page 777 of this issue). Notably, all of the cells of blastocysts initially show epigenetic marks that are consistent with the inactivated paternal X chromosome. In the late blastocyst, however, the epigenetic marks associated with the inactive paternal X chromosome are preferentially erased in the PEct cells, where both X chromosomes become potentially active (Mak et al., 2004Mak W. Nesterova T.B. de Napoles M. Appanah R. Yamanaka S. Otte A.P. Brockdorff N. Reactivation of the paternal X chromosome in early mouse embryos.Science. 2004; 303: 666-669Crossref PubMed Scopus (220) Google Scholar, Okamoto et al., 2004Okamoto I. Otte A.P. Allis C.D. Reinberg D. Heard E. Epigenetic dynamics of imprinted X inactivation during early mouse development.Science. 2004; 303: 644-649Crossref PubMed Scopus (361) Google Scholar). The paternal X chromosome stays imprinted and inactivated only in the extraembryonic trophectoderm and PEnd cells. The erasure of the imprint on the paternal X chromosome occurs in the ICM, where the pluripotent PEct cells reside, and this event may signify the establishment of the pluripotent state. Subsequently, there is random X inactivation in the developing embryo when PEct cells commence differentiation (Figure 2). The zygote contains maternally inherited factors that, together with the embryonic transcripts, regulate cleavage divisions. At the morula stage, two distinct cell populations, inner cells (ICs) and outer cells (OCs), are formed. The ICs are the precursors of the pluripotent primitive ectoderm cells (PEct) within the ICM. The ICM also contains the outer layer of primitive endoderm cells (PEnd). Cleavage divisions are replaced by cell divisions as the primitive ectoderm cells within the ICM undergo final epigenetic reprogramming to generate pluripotent cells. These cells can be propagated indefinitely under appropriate conditions as pluripotent ES cells in vitro, where they exhibit a unique epigenetic state, and can differentiate into all of the diverse cell types upon reintroduction into host blastocysts. Profound epigenetic differences between the ICM and trophectoderm cells appear not only at the level of histone modifications but also at the level of DNA methylation. Following passive DNA demethylation, which is characteristic for preimplantation development, the ICM of the blastocyst starts to reacquire DNA methylation marks, which is coupled with the restricted expression of Dnmt3b in the ICM (Watanabe et al., 2002Watanabe D. Suetake I. Tada T. Tajima S. Stage- and cell-specific expression of Dnmt3a and Dnmt3b during embryogenesis.Mech. Dev. 2002; 118: 187-190Crossref Scopus (88) Google Scholar). By contrast, trophectoderm cells stay relatively hypomethylated. These differences are also reflected in the mechanisms used for the maintenance of genomic imprints, which involves modifications of histones and DNA methylation in the placenta and the embryo, respectively (Lewis et al., 2004Lewis A. Mitsuya K. Umlauf D. Smith P. Dean W. Walter J. Higgins M. Feil R. Reik W. Imprinting on distal chromosome 7 in the placenta involves repressive histone methylation independent of DNA methylation.Nat. Genet. 2004; 36: 1291-1295Crossref PubMed Scopus (265) Google Scholar). This is perhaps an evolutionary adaptation, given that the placenta exists for a relatively short duration compared to the embryo, which develops into an adult. Significant epigenetic events such as the erasure of the epigenetic marks associated with the paternal inactive X chromosome occur specifically in PEct cells within the ICM, which may be indicative of other epigenetic reprogramming events. It is possible that X reactivation observed in the PEct cells is a consequence of epigenetic reprogramming, which may be essential in these cells for them to acquire pluripotency. Currently, little is known about the precise mechanisms that trigger these epigenetic changes in PEct cells and what other epigenetic changes occur in these cells that could be critical for pluripotency. It is possible that the ICM provides a “niche” where signaling molecules from the surrounding cells may regulate the erasure of some of the epigenetic modifications as PEct cells acquire pluripotency. The reactivation of the inactive paternal X chromosome includes uplifting of chromatin marks, such as H3K27me3, which is introduced by Ezh2, a member of the PcG complex (Mak et al., 2004Mak W. Nesterova T.B. de Napoles M. Appanah R. Yamanaka S. Otte A.P. Brockdorff N. Reactivation of the paternal X chromosome in early mouse embryos.Science. 2004; 303: 666-669Crossref PubMed Scopus (220) Google Scholar, Okamoto et al., 2004Okamoto I. Otte A.P. Allis C.D. Reinberg D. Heard E. Epigenetic dynamics of imprinted X inactivation during early mouse development.Science. 2004; 303: 644-649Crossref PubMed Scopus (361) Google Scholar). It is possible that the underlying mechanism could have some similarities with the molecular processes that are associated with the transdetermination phenomenon in Drosophila. In the imaginal discs, signaling molecules drive the
Referência(s)