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Early embryonic development depends on proper utilization and clearance of maternal transcriptomes. How these processes are spatiotemporally regulated remains unclear. Here we show that nuclear RNA‐binding protein Rbm14 and maternal mRNAs co‐phase separate into cytoplasmic condensates to facilitate vertebrate blastula‐to‐gastrula development. In zebrafish, Rbm14 condensates were highly abundant in blastomeres and markedly reduced after prominent activation of zygotic transcription. They concentrated at spindle poles by associating with centrosomal γ‐tubulin puncta and displayed mainly asymmetric divisions with a global symmetry across embryonic midline in 8‐ and 16‐cell embryos. Their formation was dose‐dependently stimulated by m6A, but repressed by m5C modification of the maternal mRNA. Furthermore, deadenylase Parn co‐phase separated with these condensates, and this was required for deadenylation of the mRNAs in early blastomeres. Depletion of Rbm14 impaired embryonic cell differentiations and full activations of the zygotic genome in both zebrafish and mouse and resulted in developmental arrest at the blastula stage. Our results suggest that cytoplasmic Rbm14 condensate formation regulates early embryogenesis by facilitating deadenylation, protection, and mitotic allocation of m6A‐modified maternal mRNAs, and by releasing the poly(A)‐less transcripts upon regulated disassembly to allow their re‐polyadenylation and translation or clearance. Synopsis: The mechanisms mediating the spatiotemporal control of maternal mRNA translational competence and decay remain largely unclear. In this study, maternal mRNAs in zebrafish and mouse embryos are shown to form condensates with the RNA binding protein Rbm14 for their deadenylation, sequestration, mitotic allocation, and timely release during early embryogenesis. Rbm14 co‐phase‐separates with deadenylase Parn and maternal m6A‐modified mRNAs into cytoplasmic condensates in early embryos.The condensates are segregated into daughter blastomeres through associations with centrosomal γ‐tubulin‐positive puncta.Rbm14 is required for correct maternal mRNA deadenylation, sequestration, and release from the condensates during embryonic development for timely re‐activation or clearance.Depletion of Rbm14 represses maternal‐to‐zygotic transition and blastula‐to‐gastrula development in both zebrafish and mouse. [ABSTRACT FROM AUTHOR]

The goal of preimplantation development is to establish the fates of the embryonic and extra‐embryonic cells. However, when and how cell fates are determined during early mammalian embryonic development remains unclear. We report that the high mobility group (HMG) protein family member HMGA1 was distributed differentially in mouse two‐cell blastomeres. Knockdown of Hmga1 expression in one of the two cells reduced the number of cells contributing to the inner cell mass (ICM), suggesting that differential distribution of HMGA1 in the blastomeres in two‐cell mouse embryos affected the selection of embryonic cell lineages. Mechanistically, HMGA1 promotes the expression of the ICM‐specific gene Sox2. The results of this study show that mouse embryos demonstrate heterogeneity as early as the two‐cell stage, and that these differences are related to cell‐fate differentiation in early mouse embryos. [ABSTRACT FROM AUTHOR]

Since their discovery, the study of maternal mRNAs has led to the identification of mechanisms underlying their spatiotemporal regulation within the context of oogenesis and early embryogenesis. Following synthesis in the oocyte, maternal mRNAs are translationally silenced and sequestered into storage in cytoplasmic granules. At the same time, their unique distribution patterns throughout the oocyte and embryo are tightly controlled and connected to their functions in downstream embryonic processes. At certain points in oogenesis and early embryogenesis, maternal mRNAs are translationally activated to perform their functions in a timely manner. The cytoplasmic polyadenylation machinery is responsible for the translational activation of maternal mRNAs, and its role in initiating the maternal to zygotic transition events has recently come to light. Here, we summarize the current knowledge on maternal mRNA regulation, with particular focus on cytoplasmic polyadenylation as a mechanism for translational regulation. [ABSTRACT FROM AUTHOR]

The oocyte-to-embryo transition (OET) is thought to be mainly driven by post-transcriptional gene regulation. However, expression of both RNAs and proteins during the OET has not been comprehensively assayed. Furthermore, specific molecular mechanisms that regulate gene expression during OET are largely unknown. Here, we quantify and analyze transcriptome-wide, expression of mRNAs and thousands of proteins in Caenorhabditis elegans oocytes, 1-cell, and 2-cell embryos. This represents a first comprehensive gene expression atlas during the OET in animals. We discovered a first wave of degradation in which thousands of mRNAs are cleared shortly after fertilization. Sequence analysis revealed a statistically highly significant presence of a polyC motif in the 30 untranslated regions of most of these degraded mRNAs. Transgenic reporter assays demonstrated that this polyC motif is required and sufficient for mRNA degradation after fertilization. We show that orthologs of human polyC-binding protein specifically bind this motif. Our data suggest a mechanism in which the polyC motif and binding partners direct degradation of maternal mRNAs. Our data also indicate that endogenous siRNAs but not miRNAs promote mRNA clearance during the OET. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are generated from primary transcripts through sequential processing by two RNase III enzymes, Drosha and Dicer, in association with other proteins. This maturation is essential for their function as post-transcriptional regulators. Notably, Dicer is also a component of RNA-induced silencing complexes, which incorporate either miRNA or small interfering RNA (siRNA) as guides to target specific mRNAs. In zebrafish, processed miRNAs belonging to the miR-430 family have previously been shown to promote deadenylation and degradation of maternal mRNAs during early embryogenesis. We show that injection of one-cell-stage zebrafish embryos with siRNA causes a significant reduction in the endogenous levels of processed miR-430 and other miRNAs, leading to unspecific developmental defects. Coinjection of siRNA with preprocessed miR-430 efficiently rescued development. This indicates that the abnormalities generally observed in siRNA-treated zebrafish embryos could be due to inhibition of miR-430 processing and/or activity. Our results also suggest that the miRNA pathway in mammals, under some experimental or therapeutic conditions, may be affected by siRNA. [ABSTRACT FROM AUTHOR]

Here we show that heterogeneous nuclear ribonucleoprotein K (hnRNP K), a member of the K homology domain-containing proteins, is expressed in Xenopus immature oocytes, unfertilized eggs, and early embryos. Fertilization or egg activation treatment involving upregulation of the egg tyrosine kinase Src promotes a rapid and transient tyrosine phosphorylation of hnRNP K. HnRNP K is also phosphorylated on serine/threonine residues in unfertilized eggs, dephosphorylated after fertilization, and re-phosphorylated during the premitotic phase of early embryogenesis. In vitro, Src and mitogen-activated protein kinase (MAPK) were capable of phosphorylating hnRNP K on tyrosine and serine/threonine residues, respectively. In support of this, pretreatment of oocytes, eggs, or embryos with inhibitors for Src (PP2) and MAPK (U0126) blocked effectively the phosphorylation of hnRNP K. We also identify some maternal mRNAs that coimmunoprecipitate with hnRNP K in unfertilized eggs. Specific binding of these mRNAs to hnRNP K was verified by reverse transcriptase–polymerase chain reaction (RT–PCR). In addition, real-time PCR analyses revealed a subset of the mRNAs whose binding to hnRNP K might be up or downregulated in activated eggs. In vitro binding assay with the use of poly U monopolymeric RNA-coupled beads demonstrated that the RNA-binding property of hnRNP K is negatively regulated by tyrosine phosphorylation and positively or neutrally regulated by serine/threonine phosphorylation. Taken together, it is attractive to suggest that hnRNP K is in association with certain pools of maternal mRNAs whose translational activation are modulated by the Src/MAPK phosphorylation of hnRNP K during oocyte-egg-embryo transition. [ABSTRACT FROM AUTHOR]

Early steps of embryo development are directed by maternal gene products and trace levels of zygotic gene activity in vertebrates. A major activation of zygotic transcription occurs together with degradation of maternal mRNAs during the midblastula transition in several vertebrate systems. How these processes are regulated in preparation for the onset of differentiation in the vertebrate embryo is mostly unknown. Here, we studied the function of TATA-binding protein (TBP) by knock down and DNA microarray analysis of gene expression in early embryo development. We show that a subset of polymerase II-transcribed genes with ontogenic stage-dependent regulation requires TBP for their zygotic activation. TBP is also required for limiting the activation of genes during development. We reveal that TBP plays an important role in the degradation of a specific subset of maternal mRNAs during late blastulation/early gastrulation, which involves targets of the miR-430 pathway. Hence, TBP acts as a specific regulator of the key processes underlying the transition from maternal to zygotic regulation of embryogenesis. These results implicate core promoter recognition as an additional level of differential gene regulation during development. [ABSTRACT FROM AUTHOR]

An ascidian Y-box protein gene was cloned, designated as CiYB, which consists of a highly conserved cold shock domain and an auxiliary tail domain with alternating modules of acidic and basic amino acids. CiYB is a single copy gene in the ascidian genome. During oogenesis and early development, CiYB produces three different transcripts (CiYB1, CiYB2 and CiYB3) by alternate splicing. CiYB1 and CiYB2 were expressed during oogenesis, suggesting that they are recruited into maternal ribonucleoprotein particles. According to gel mobility shift assay, the CiYB1 protein has the ability to bind RNA. The sequence preference of RNA binding is similar to that of the Xenopus Y-box protein (FRGY2), which is a major component of the maternal messenger ribonucleoprotein particles (mRNP) in the oocyte. These results suggest that the ascidian Y-box protein may have an important role for masking and translational regulation of maternal mRNA. Furthermore, CiYB1, CiYB2 and CiYB3 were expressed zygotically in a tissue restricted manner. CiYB1 was expressed specifically in muscle precursor blastomeres and tail muscle cells suggesting its important role in muscle differentiation. [ABSTRACT FROM AUTHOR]