Epigenetic reprogramming and development: a unique heterochromatin organization in the preimplantation mouse embryo

Adam Burton Maria-Elena Torres-Padilla Fertilization of the oocyte by the sperm results in the formation of a totipotent zygote, in which the maternal and paternal chromatin is enclosed in two pronuclei undergoing distinct programmes of transcriptional activation and chromatin remodelling. The highly packaged paternal chromatin delivered by the sperm is decondensed and acquires a number of specific epigenetic marks, but markedly remains devoid of those usually associated with constitutive heterochromatin. During this period the maternal chromatin remains relatively stable except for marks associated with transcription and/or replication such as arginine methylation and H3/H4 acetylation. The embryo then undergoes a series of mitotic divisions without significant additional growth but differentiation, resulting in the formation of a blastocyst containing distinct cell types. The chromatin remodelling events during these stages are likely to be important in establishing the nuclear foundations required for later triggers of differentiation. Overall, we summarize three important points during these earliest reprogramming events: (i) relatively stable maternal chromatin after fertilization, (ii) rapid acquisition of specific histone marks by the paternal chromatin during the hours that follow fertilization and (iii) rapid remodelling of constitutive heterochromatic marks and modifications in the core of the nucleosome from the first mitotic division. These features are likely to be required for the creation of a chromatin environment compatible with cellular reprogramming and plasticity. - INTRODUCTION During the life cycle of a mammal from a newly fertilized egg to a fully differentiated adult with over 200 different cell types, major changes in cellular specification must occur through differentiation and reprogramming events. Throughout this cycle the genetic information itself remains largely constant while epigenetic modifications undergo extensive changes. Upon fertilization in mammals, the haploid genomes of two highly differentiated cells; the sperm and oocyte combine and the first of two major reprogramming events during the life cycle occurs, resulting in the production of a totipotent zygote. This unique cell, by definition, is capable of differentiating into every specialized cell type in the organism. The zygote undergoes a series of cleavage divisions resulting in the formation of the blastocyst, by which time the first differentiation event has occurred separating the outer trophectoderm that will go on to form the placenta and extra-embryonic tissues and the inner cell mass that comprises pluripotent embryonic cells that will develop into the embryo proper. The timescale of these events varies considerably among mammalian species. In the mouse, in which the majority of studies have been conducted and will therefore form the focus of this review, the first two cycles take one and a half days in total, with consecutive divisions occurring 12 h apart until implantation of the blastocyst takes place 4.5 days after fertilization [1]. During this time, the embryo develops first under the control of maternally inherited factors during the early stages and later by embryonic gene expression, which begins at a low level in S-phase of the one-cell stage, and to a greater extent during the two-cell stage [2, 3]. The mechanisms controlling chromatin remodelling and gene expression during reprogramming and early differentiation events are the subject of ongoing research but a significant role for epigenetics is likely. Epigenetic mechanisms have attracted much interest due to their ability to regulate and interpret the DNA sequence, in a fluid yet potentially heritable manner [4]. The epigenetic dynamics during mammalian pre-implantation development are characterized by major changes in DNA methylation, histone modifications and the incorporation of histone variants [57]. Furthermore, the genetic inactivation (knockout) of many chromatin modifying enzymes results in early developmental defects, even when maternal genes are present [7]. Collectively these observations suggest that such modifications play a crucial, but still poorly understood role in the processes of reprogramming and differentiation during development. We will cover two different aspects concerning chromatin changes during the earliest phases of mouse development. The first part collects our current knowledge on the changes that characterize the epigenetic asymmetry in paternal and maternal chromatin in the hours following fertilization. The second section deals with the global changes on the chromatin during the subsequent cleavage stages and up to blastocyst formation. Although there remains significant gaps in our knowledge, the asymmetries between the paternal and maternal chromatin in the pronuclear stages have been relatively well studied compared to the epigenetic dynamics during subsequent developmental stages. There is a second major reprogramming event later in development during the formation of the germline, but we will not deal with this topic here. It should be noted that the majority of studies concerning the dynamics of histone modifications during these stages are based upon immunofluorescent labelling of specifically modified histone residues in the mouse embryo. Although the antibodies utilized are in general believed to be specific, the possibility of epitope exclusion cannot be ruled out in such experiments, particularly for modifications occurring in close vicinity to others such as the hot-spot of modifications on the histone H3 K9/S10 N-terminal tail. There are also a number of known histone modifications that have not been studied in the mouse embryo, such as histone H3K36 methylation, H3K56 acetylation and methylation and H2A ubiquitination. Finally, it should be mentioned that the generality of the mouse as a model for all mammalian species should not be assumed, particularly during these crucial early developmental events. PARENTAL CHROMATIN DYNAMICS: MARKED DISCREPANCIES AND SIMILARITIES During the first reprogramming event at the zygote stage, the two parental genomes remain physically separated as two pronuclei, and although in theory both have access to the same maternal factors they go through very distinct programmes of chromatin remodelling [8]. Indeed, it is the paternal pronucleus that experiences the most extensive reprogramming at this early stage, as the highly packaged chromatin of the sperm head undergoes decondensation cycles, likely resulting in a more permissive structure for remodelling [9]. The sperm-specific packaging protamines are replaced with histones, which are hypomethylated and hyperacteylated. As acetylation of histone H4 lysines 5 and 12 has been correlated with deposition [10], this suggests that their initial hyperacetylation is a function of their incorporation itself. The paternal DNA subsequently undergoes active global DNA demethylat (...truncated)