The maternal to zygotic transition in mammals

Abstract

Prior to activation of the embryonic genome, the initiating events of mammalian development are under maternal control and include fertilization, the block to polyspermy and processing sperm DNA. Following gamete union, the transcriptionally inert sperm DNA is repackaged into the male pronucleus which fuses with the female pronucleus to form a 1-cell zygote. Embryonic transcription begins during the maternal to zygotic transfer of control in directing development. This transition occurs at species-specific times after one or several rounds of blastomere cleavage and is essential for normal development. However, even after activation of the embryonic genome, successful development relies on stored maternal components without which embryos fail to progress beyond initial cell divisions. Better understanding of the molecular basis of maternal to zygotic transition including fertilization, the activation of the embryonic genome and cleavage-stage development will provide insight into early human development that should translate into clinical applications for regenerative medicine and assisted reproductive technologies.

Keywords: 5-carboxylcytosine; 5-formylcytosine; 5caC; 5fC; ART; DMR; DNA methylation; DNA methyltransferases; DNMTs; Gamete recognition; Histone modification; ICR; MZT; Maternal effect genes; Maternal to zygotic transition (MZT); Mouse fertilization; Piwi-interacting RNA; RNA interference; RNAi; SCMC; SCNT; Subcortical maternal complex (SCMC); TDG; TRC; ZGA; Zygotic genome activation (ZGA); assisted reproductive technology; differentially methylated regions; double strand RNA; dsRNA; endo-siRNA; endo-small interfering RNA; imprinting control regions; maternal to zygotic transition; miRNA; microRNA; piRNA; somatic cell nuclear transfer; subcortical maternal complex; thymine-DNA glycosylase; transcription required complex; zygotic genome activation.

Figure 1

The maternal to zygotic transition in mice. (A) Ovulated eggs are fertilized in the ampulla of the oviduct to form 1-cell zygotes. After three cell divisions (cleavage stage development), the embryo undergoes compaction to form the morula. A fluid filled blastocoel forms in the blastocyst that escapes from the zona pellucida to implant on the wall of the uterus. (B) During gametogenesis, primordial germ cells (PGCs) develop into haploid sperm and oocytes after two rounds of meiotic division. The maternal to zygotic transition (MZT) is initiated when sperm and egg fuse at the time of fertilization. Each haploid gamete forms a pronucleus and after syngamy develops into a 1-cell zygote that divides to form the 2-cell embryo. (C) During oogenesis, oocytes accumulate a large pool of maternal RNAs essential for fertilization, the maternal to zygotic transition and preimplantation embryogenesis. After fertilization, maternal RNAs are gradually degraded in early embryonic stages. Transcription of the embryonic genome is initiated at the late 1-cell stage (minor ZGA) and robustly activated at 2- and 4-cell stages (major ZGA). (D) During gametogenesis, the genome of the sperm and oocyte are methylated de novo to obtain sex-specific patterns in which DNA methylation is slightly higher in sperm than in oocytes (Smith et al., 2012). After fertilization and replacement of protamines with hyperacetylated maternal histones, the male pronucleus is actively demethylated and initiates zygotic gene transcription. Following syngamy, the zygotic genome undergoes passive demethylation in a DNA replication dependent manner, but preserves the methylation status of imprinting regions.

Figure 2

Fertilization, the initiation of development. (A) The ~7 μm wide mouse zona pellucida surrounds a ~80 μm egg. As observed by scanning electron microscopy, the zona matrix has multiple pores which may facilitate sperm penetration (Familiari et al., 2008). Mouse sperm are ~125 μm long with a thin acrosome overlying a distinctive falciform (hook-like) head. (B) The thicker (~ 15 μm) human zona pellucida surrounds a larger (~120 μm) egg and yet its three-dimensional structure is similar to mouse (Familiari et al., 2006). Human sperm are half as long (~ 60 μm) as mouse sperm with a smaller, flattened, spatulate head. Of note, human sperm are fastidious and will not bind to mouse eggs, although mouse sperm bind to human (and most other) eggs. (C) Glycan release models postulate a carbohydrate ligand attached to a zona pellucida protein that interacts with a sperm surface receptor. Although N-glycans have been proposed, most attention has been focused on O-glycans attached to ZP3. Following fertilization, the removal of the glycan by a cortical granule glycosidase would account for the inability of sperm to bind to 2-cell embryos. However, recent biochemical investigations and genetic ablation studies have not confirmed the candidacy of proposed glycan ligands or receptors as essential for sperm-egg recognition. (D) The ZP2 cleavage model proposes that sperm bind with taxon-specificity to the N-terminus of ZP2 and the cleavage status of ZP2 is crucial for sperm-zona recognition and binding. Following fertilization, ovastacin, a metalloendoprotease, is exocytosed from egg cortical granules to cleave the N-terminus of ZP2 and prevent post-fertilization sperm binding. Implicit in the model is the presence of a cognate receptor(s) on the sperm surface which has not as yet been molecularly defined.

Figure 3

The Subcortical Maternal Complex. (A) A model of the SCMC (subcortical maternal complex). The SCMC includes at least four proteins: MATER, FLOPED, TLE6, Filia and, most likely, PADI6. The first three proteins interacts each other, while Filia only binds to MATER (modified from (Li et al., 2008a). (B) Protein domains in the members of the SCMC. MATER includes a novel 5X N- terminal repeat, a NACHT domain and a 13-fold leucine-rich domain. FLOPED contains an atypical KH domain and PADI6 has a deiminase domain that converts arginine to citrulline. TLE6 belongs to the Groucho co-repressor family and has a nuclear localization signal (NLS) and a WD domain, but lacks the N-terminal Q domain required for DNA binding. Filia has an atypical KH domain in its N- terminal and a novel 23 amino acid 10-fold repeat. (C) Localization of the SCMC in egg and early embryos. Mouse eggs and early embryos were stained with phalloidin labeled with fluorescence (F-actin, green), DAPI (DNA, blue) and rabbit anti-FLOPED (red) and imaged by confocal microscopy (modified from Li et al., 2008a).