Reconstitution of enhancer function in paternal pronuclei of one-cell mouse embryos

STAT3 Is an Upstream Regulator of Granzyme G in the Maternal-To-Zygotic Transition of Mouse Embryos

The maternal-to-zygotic transition (MZT), which controls maternal signaling to synthesize zygotic gene products, promotes the preimplantation development of mouse zygotes to the two-cell stage. Our previous study reported that mouse granzyme g (Gzmg), a serine-type protease, is required for the MZT. In this study, we further identified the maternal factors that regulate the Gzmg promoter activity in the zygote to the two-cell stage of mouse embryos. A full-length Gzmg promoter from mouse genomic DNA, FL-pGzmg (−1696~+28 nt), was cloned, and four deletion constructs of this Gzmg promoter, Δ1-pGzmg (−1369~+28 nt), Δ2-pGzmg (−939~+28 nt), Δ3-pGzmg (−711~+28 nt) and Δ4-pGzmg (−417~+28 nt), were subsequently generated. Different-sized Gzmg promoters were used to perform promoter assays of mouse zygotes and two-cell stage embryos. The results showed that Δ4-pGzmg promoted the highest expression level of the enhanced green fluorescent protein (EGFP) reporter in the zygotes and two-cell embryos. The data suggested that time-specific transcription factors upregulated Gzmg by binding cis-elements in the −417~+28-nt Gzmg promoter region. According to the results of the promoter assay, the transcription factor binding sites were predicted and analyzed with the JASPAR database, and two transcription factors, signal transducer and activator of transcription 3 (STAT3) and GA-binding protein alpha (GABPα), were identified. Furthermore, STAT3 and GABPα are expressed and located in zygote pronuclei and two-cell nuclei were confirmed by immunofluorescence staining; however, only STAT3 was recruited to the mouse zygote pronuclei and two-cell nuclei injected with the Δ4-pGzmg reporter construct. These data indicated that STAT3 is a maternal transcription factor and may upregulate Gzmg to promote the MZT. Furthermore, treatment with a STAT3 inhibitor, S3I-201, caused mouse embryonic arrest at the zygote and two-cell stages. These results suggest that STAT3, a maternal protein, is a critical transcription factor and regulates Gzmg transcription activity in preimplantation mouse embryos. It plays an important role in the maternal-to-zygotic transition during early embryonic development.

Roles of heat shock factors in gametogenesis and development

Heat shock factors form a family of transcription factors (four in mammals), which were named according to the first discovery of their activation by heat shock. As a result of the universality and robustness of their response to heat shock, the stress-dependent activation of heat shock factor became a ‘paradigm’: by binding to conserved DNA sequences (heat shock elements), heat shock factors trigger the expression of genes encoding heat shock proteins that function as molecular chaperones, contributing to establish a cytoprotective state to various proteotoxic stress and in several pathological conditions. Besides their roles in the stress response, heat shock factors perform crucial roles during gametogenesis and development in physiological conditions. First, during these process, in stress conditions, they are either proactive for survival or, conversely, for apoptotic process, allowing elimination or, inversely, protection of certain cell populations in a way that prevents the formation of damaged gametes and secure future reproductive success. Second, heat shock factors display subtle interplay in a tissue- and stage-specific manner, in regulating very specific sets of heat shock genes, but also many other genes encoding growth factors or involved in cytoskeletal dynamics. Third, they act not only by their classical transcription factor activities, but are necessary for the establishment of chromatin structure and, likely, genome stability. Finally, in contrast to the heat shock gene paradigm, heat shock elements bound by heat shock factors in developmental process turn out to be extremely dispersed in the genome, which is susceptible to lead to the future definition of ‘developmental heat shock element’.

Developmental regulation of transcription initiation: more than just changing the actors

The traditional model of transcription initiation nucleated by the TFIID complex has suffered significant erosion in the last decade. The discovery of cell-specific paralogs of TFIID subunits and a variety of complexes that replace TFIID in transcription initiation of protein coding genes have been paralleled by the description of diverse core promoter sequences. These observations suggest an additional level of regulation of developmental and tissue-specific gene expression at the core promoter level. Recent work suggests that this regulation may function through specific roles of distinct TBP-type factors and TBP-associated factors (TAFs), however the picture emerging is still far from complete. Here we summarize the proposed models of transcription initiation by alternative initiation complexes in distinct stages of developmental specialization during vertebrate ontogeny.

Abnormal expression of REST/NRSF and Myc in neural stem/progenitor cells causes cerebellar tumors by blocking neuronal differentiation

Medulloblastoma, one of the most malignant brain tumors in children, is thought to arise from undifferentiated neural stem/progenitor cells (NSCs) present in the external granule layer of the cerebellum. However, the mechanism of tumorigenesis remains unknown for the majority of medulloblastomas. In this study, we found that many human medulloblastomas express significantly elevated levels of both myc oncogenes, regulators of neural progenitor proliferation, and REST/NRSF, a transcriptional repressor of neuronal differentiation genes. Previous studies have shown that neither c-Myc nor REST/NRSF alone could cause tumor formation. To determine whether c-Myc and REST/NRSF act together to cause medulloblastomas, we used a previously established cell line derived from external granule layer stem cells transduced with activated c-myc (NSC-M). These immortalized NSCs were able to differentiate into neurons in vitro. In contrast, when the cells were engineered to express a doxycycline-regulated REST/NRSF transgene (NSC-M-R), they no longer underwent terminal neuronal differentiation in vitro. When injected into intracranial locations in mice, the NSC-M cells did not form tumors either in the cerebellum or in the cerebral cortex. In contrast, the NSC-M-R cells did produce tumors in the cerebellum, the site of human medulloblastoma formation, but not when injected into the cerebral cortex. Furthermore, the NSC-M-R tumors were blocked from terminal neuronal differentiation. In addition, countering REST/NRSF function blocked the tumorigenic potential of NSC-M-R cells. To our knowledge, this is the first study in which abnormal expression of a sequence-specific DNA-binding transcriptional repressor has been shown to contribute directly to brain tumor formation. Our findings indicate that abnormal expression of REST/NRSF and Myc in NSCs causes cerebellum-specific tumors by blocking neuronal differentiation and thus maintaining the "stemness" of these cells. Furthermore, these results suggest that such a mechanism plays a role in the formation of human medulloblastoma.

Activation of REST/NRSF target genes in neural stem cells is sufficient to cause neuronal differentiation

REST/NRSF is a transcriptional repressor that acts at the terminal stage of the neuronal differentiation pathway and blocks the transcription of several differentiation genes. REST/NRSF is generally downregulated during induction of neuronal differentiation. The recombinant transcription factor REST-VP16 binds to the same DNA binding site as does REST/NRSF but functions as an activator instead of a repressor and can directly activate the transcription of REST/NRSF target genes. However, it is not known whether REST-VP16 expression is sufficient to cause formation of functional neurons from neural stem cells (NSCs). Here we show that regulated expression of REST-VP16 in a physiologically relevant NSC line growing under cycling conditions converted the cells rapidly to the mature neuronal phenotype. Furthermore, when grown in the presence of retinoic acid, REST-VP16-expressing NSCs activated their target, as well as other differentiation genes that are not their direct target, converting them to the mature neuronal phenotype and enabling them to survive in the presence of mitotic inhibitors, which is a characteristic of mature neurons. In addition, these neuronal cells were physiologically active. These results showed that direct activation of REST/NRSF target genes in NSCs with a single transgene, REST-VP16, is sufficient to cause neuronal differentiation, and the findings suggested that direct activation of genes involved in the terminal stage of differentiation may cause neuronal differentiation of NSCs.

Isolation of nascent messenger RNA from mouse preimplantation embryos

The expression of the zygotic genome starts at the late one-cell stage in mouse embryos, and its regulation changes dynamically until the late two-cell stage. To understand this process, it is important to accumulate the profiles of the genes transcribed at any given instant at each stage of development. However, because large amounts of maternal mRNA accumulate in embryos to sustain early development, it is difficult to determine the profile of newly synthesized mRNA just after gene activation. To overcome this difficulty, we established a novel method of isolating nascent mRNA from the large pool of preexisting mRNA. Briefly, the procedure was as follows. Embryos were electrically permeabilized and loaded with 5-bromouridine-5'-triphosphate (BrUTP). Nascent mRNA with incorporated BrU was isolated by immunoprecipitation with an antibody recognizing BrU. The cDNA was synthesized from the isolated mRNA, and its abundance was evaluated using semiquantitative real-time PCR. Using this method, we examined the amounts of newly synthesized eIF-1A, MuERV-L, and cyclin-A2 transcripts in two-cell mouse embryos and compared them with the quantities of these transcripts present in the total mRNA pool. The amount of each transcript in the nascent mRNA fraction and in the total mRNA pool changed differently over time, demonstrating that this method can be used to obtain profiles of genes transcribed during development.

Conversion of myoblasts to physiologically active neuronal phenotype

Repressor element 1 (RE1)-silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF) can repress several terminal neuronal differentiation genes by binding to a specific DNA sequence (RE1/neuron-restrictive silencer element [NRSE]) present in their regulatory regions. REST-VP16 binds to the same RE1/NRSE, but activates these REST/NRSF target genes. However, it is unclear whether REST-VP16 expression is sufficient to cause formation of functional neurons either from neural stem cells or from heterologous stem cells. Here we show that the expression of REST-VP16 in myoblasts grown under muscle differentiation conditions blocked entry into the muscle differentiation pathway, countered endogenous REST/NRSF-dependent repression, activated the REST/NRSF target genes, and, surprisingly, activated other neuronal differentiation genes and converted the myoblasts to a physiologically active neuronal phenotype. Furthermore, in vitro differentiated neurons produced by REST-VP16-expressing myoblasts, when injected into mouse brain, survived, incorporated into the normal brain, and did not form tumors. This is the first instance in which myoblasts were converted to a neuronal phenotype. Our results suggest that direct activation of REST/NRSF target genes with a single transgene, REST-VP16, is sufficient to activate other terminal neuronal differentiation genes and to override the muscle differentiation pathways, and they suggest that this approach provides an efficient way of triggering neuronal differentiation in myoblasts and possibly other stem cells.

Early transcriptional activation of the hsp70.1 gene by osmotic stress in one-cell embryos of the mouse

In fertilized mouse eggs, de novo transcription of embryonic genes is first observed during the S phase of the one-cell stage. This transcription, however, is mostly limited to the male pronucleus and possibly uncoupled from translation, making the functional meaning obscure. We found that one-cell mouse embryos respond to the osmotic shock of in vitro isolation with migration of HSF1, the canonical stress activator of mammalian heat shock genes, to pronuclei and by transient transcription of the hsp70.1, but not hsp70.3 and hsp90, heat shock genes. Isolated growing dictyate oocytes also display a nuclear HSF1 localization, but, in contrast with embryos, they transcribe both hsp70.1 and hsp70.3 genes only after heat shock. Intranuclear injection of double-stranded oligodeoxyribonucleotides containing HSE, GAGA box or GC box consensus sequences, and antibodies raised to transcription factors HSF1, HSF2, Drosophila melanogaster GAGA factor, or Sp1 demonstrated that hsp70.1 transcription depends on HSF1 in both oocytes and embryos and that Sp1 is dispensable in oocytes and inhibitory in the embryos. Hsp70.1 thus represents the first endogenous gene so far identified to be physiologically activated and tightly regulated after fertilization in mammals.

Remodelling the paternal chromatin at fertilization in mammals

At fertilization, the highly condensed and transcriptionally inert chromatin of the spermatozoa becomes remodelled into the decondensed and transcriptionally competent chromatin of the male pronucleus. The chromatin initially becomes dispersed and then transiently recondenses into a small mass upon entry into the ooplasm. This morphological change is coincident with and likely dependent on the replacement of the sperm-specific protamines by oocyte-supplied histones and the organization of the chromatin into nucleosomes. The chromatin then extensively decondenses within the male pronucleus and acquires many of the proteins that are associated with the maternal chromatin. Nonetheless, the paternal chromatin manifests distinct characteristics, including transient hyperacetylation of histone H4, increased transcription of endogenous and microinjected genes, and replication-independent demethylation of DNA. Sperm chromatin remodelling is controlled by an oocyte activity that appears during meiotic maturation and disappears approximately 3 h after activation (release from metaphase II arrest), and which requires factors associated with the germinal vesicle of the oocyte. The molecular components of this activity remain largely unknown. In frogs, nucleoplasmin is required to assemble histones H2A and H2B onto the paternal chromatin. Evidence is presented that related proteins may perform similar functions in mammals. Identifying the mechanisms that underlie sperm chromatin remodelling at fertilization may be relevant for understanding reprogramming of somatic cell nuclei after transfer into oocytes.