Variable allelic expression of imprinted genes at the Peg13, Trappc9, Ago2 cluster in single neural cells.

Genomic imprinting is an epigenetic process through which genes are expressed in a parent-of-origin specific manner resulting in mono-allelic or strongly biased expression of one allele. For some genes, imprinted expression may be tissue-specific and reliant on CTCF-influenced enhancer-promoter interactions. The Peg13 imprinting cluster is associated with neurodevelopmental disorders and comprises canonical imprinted genes, which are conserved between mouse and human, as well as brain-specific imprinted genes in mouse. The latter consist of Trappc9, Chrac1 and Ago2, which have a maternal allelic expression bias of ∼75% in brain. Findings of such allelic expression biases on the tissue level raise the question of how they are reflected in individual cells and whether there is variability and mosaicism in allelic expression between individual cells of the tissue. Here we show that Trappc9 and Ago2 are not imprinted in hippocampus-derived neural stem cells (neurospheres), while Peg13 retains its strong bias of paternal allele expression. Upon analysis of single neural stem cells and in vitro differentiated neurons, we find not uniform, but variable states of allelic expression, especially for Trappc9 and Ago2. These ranged from mono-allelic paternal to equal bi-allelic to mono-allelic maternal, including biased bi-allelic transcriptional states. Even Peg13 expression deviated from its expected paternal allele bias in a small number of cells. Although the cell populations consisted of a mosaic of cells with different allelic expression states, as a whole they reflected bulk tissue data. Furthermore, in an attempt to identify potential brain-specific regulatory elements across the Trappc9 locus, we demonstrate tissue-specific and general silencer activities, which might contribute to the regulation of its imprinted expression bias.

Global translation during early development depends on the essential transcription factor PRDM10.

Members of the PR/SET domain-containing (PRDM) family of zinc finger transcriptional regulators play diverse developmental roles. PRDM10 is a yet uncharacterized family member, and its function in vivo is unknown. Here, we report an essential requirement for PRDM10 in pre-implantation embryos and embryonic stem cells (mESCs), where loss of PRDM10 results in severe cell growth inhibition. Detailed genomic and biochemical analyses reveal that PRDM10 functions as a sequence-specific transcription factor. We identify Eif3b, which encodes a core component of the eukaryotic translation initiation factor 3 (eIF3) complex, as a key downstream target, and demonstrate that growth inhibition in PRDM10-deficient mESCs is in part mediated through EIF3B-dependent effects on global translation. Our work elucidates the molecular function of PRDM10 in maintaining global translation, establishes its essential role in early embryonic development and mESC homeostasis, and offers insights into the functional repertoire of PRDMs as well as the transcriptional mechanisms regulating translation.

Infertility-Causing Haploinsufficiency Reveals TRIM28/KAP1 Requirement in Spermatogonia.

Spermatogenesis relies on exquisite stem cell homeostasis, the carefully balanced self-renewal and differentiation of spermatogonial stem cells (SSCs). Disturbing this equilibrium will likely manifest through sub- or infertility, a global health issue with often idiopathic presentation. In this respect, disease phenotypes caused by haploinsufficiency of otherwise vital developmental genes are of particular interest. Here, we show that mice heterozygous for Trim28, an essential epigenetic regulator, suffer gradual testicular degeneration. Contrary to previous reports we detect Trim28 expression in spermatogonia, albeit at low levels. Further reduction through Trim28 heterozygosity increases the propensity of SSCs to differentiate at the cost of self-renewal.

PRDM15 loss of function links NOTCH and WNT/PCP signaling to patterning defects in holoprosencephaly.

Holoprosencephaly (HPE) is a congenital forebrain defect often associated with embryonic lethality and lifelong disabilities. Currently, therapeutic and diagnostic options are limited by lack of knowledge of potential disease-causing mutations. We have identified a new mutation in the PRDM15 gene (C844Y) associated with a syndromic form of HPE in multiple families. We demonstrate that C844Y is a loss-of-function mutation impairing PRDM15 transcriptional activity. Genetic deletion of murine Prdm15 causes anterior/posterior (A/P) patterning defects and recapitulates the brain malformations observed in patients. Mechanistically, PRDM15 regulates the transcription of key effectors of the NOTCH and WNT/PCP pathways to preserve early midline structures in the developing embryo. Analysis of a large cohort of patients with HPE revealed potentially damaging mutations in several regulators of both pathways. Our findings uncover an unexpected link between NOTCH and WNT/PCP signaling and A/P patterning and set the stage for the identification of new HPE candidate genes.

CDK2 regulates the NRF1/Ehmt1 axis during meiotic prophase I.

Meiosis generates four genetically distinct haploid gametes over the course of two reductional cell divisions. Meiotic divisions are characterized by the coordinated deposition and removal of various epigenetic marks. Here we propose that nuclear respiratory factor 1 (NRF1) regulates transcription of euchromatic histone methyltransferase 1 (EHMT1) to ensure normal patterns of H3K9 methylation during meiotic prophase I. We demonstrate that cyclin-dependent kinase (CDK2) can bind to the promoters of a number of genes in male germ cells including that of Ehmt1 through interaction with the NRF1 transcription factor. Our data indicate that CDK2-mediated phosphorylation of NRF1 can occur at two distinct serine residues and negatively regulates NRF1 DNA binding activity in vitro. Furthermore, induced deletion of Cdk2 in spermatocytes results in increased expression of many NRF1 target genes including Ehmt1 We hypothesize that the regulation of NRF1 transcriptional activity by CDK2 may allow the modulation of Ehmt1 expression, therefore controlling the dynamic methylation of H3K9 during meiotic prophase.

The KRAB-zinc-finger protein ZFP708 mediates epigenetic repression at RMER19B retrotransposons.

Global epigenetic reprogramming is vital to purge germ cell-specific epigenetic features to establish the totipotent state of the embryo. This process transpires to be carefully regulated and is not an undirected, radical erasure of parental epigenomes. The TRIM28 complex has been shown to be crucial in embryonic epigenetic reprogramming by regionally opposing DNA demethylation to preserve vital parental information to be inherited from germline to soma. Yet the DNA-binding factors guiding this complex to specific targets are largely unknown. Here, we uncover and characterize a novel, maternally expressed, TRIM28-interacting KRAB zinc-finger protein: ZFP708. It recruits the repressive TRIM28 complex to RMER19B retrotransposons to evoke regional heterochromatin formation. ZFP708 binding to these hitherto unknown TRIM28 targets is DNA methylation and H3K9me3 independent. ZFP708 mutant mice are viable and fertile, yet embryos fail to inherit and maintain DNA methylation at ZFP708 target sites. This can result in activation of RMER19B-adjacent genes, while ectopic expression of ZFP708 results in transcriptional repression. Finally, we describe the evolutionary conservation of ZFP708 in mice and rats, which is linked to the conserved presence of the targeted RMER19B retrotransposons in these species.

From Germline to Soma: Epigenetic Dynamics in the Mouse Preimplantation Embryo.

When reflecting about cell fate commitment we think of differentiation. Be it during embryonic development or in an adult stem cell niche, where cells of a higher potency specialize and cell fate decisions are taken. Under normal circumstances this process is definitive and irreversible. Cell fate commitment is achieved by the establishment of cell-type-specific transcriptional programmes, which in turn are guided, reinforced, and ultimately locked-in by epigenetic mechanisms. Yet, this plunging drift in cellular potency linked to epigenetically restricted access to genomic information is problematic for reproduction. Particularly in mammals where germ cells are not set aside early on like in other species. Instead they are rederived from the embryonic ectoderm, a differentiating embryonic tissue with somatic epigenetic features. The epigenomes of germ cell precursors are efficiently reprogrammed against the differentiation trend, only to specialize once more into highly differentiated, sex-specific gametes: oocyte and sperm. Their differentiation state is reflected in their specialized epigenomes, and erasure of these features is required to enable the acquisition of the totipotent cell fate to kick start embryonic development of the next generation. Recent technological advances have enabled unprecedented insights into the epigenetic dynamics, first of DNA methylation and then of histone modifications, greatly expanding the historically technically limited understanding of this processes. In this chapter we will focus on the details of embryonic epigenetic reprogramming, a cell fate determination process against the tide to a higher potency.

PRDM15 safeguards naive pluripotency by transcriptionally regulating WNT and MAPK-ERK signaling.

The transcriptional network acting downstream of LIF, WNT and MAPK-ERK to stabilize mouse embryonic stem cells (ESCs) in their naive state has been extensively characterized. However, the upstream factors regulating these three signaling pathways remain largely uncharted. PR-domain-containing proteins (PRDMs) are zinc-finger sequence-specific chromatin factors that have essential roles in embryonic development and cell fate decisions. Here we characterize the transcriptional regulator PRDM15, which acts independently of PRDM14 to regulate the naive state of mouse ESCs. Mechanistically, PRDM15 modulates WNT and MAPK-ERK signaling by directly promoting the expression of Rspo1 (R-spondin1) and Spry1 (Sprouty1). Consistent with these findings, CRISPR-Cas9-mediated disruption of PRDM15-binding sites in the Rspo1 and Spry1 promoters recapitulates PRDM15 depletion, both in terms of local chromatin organization and the transcriptional modulation of these genes. Collectively, our findings uncover an essential role for PRDM15 as a chromatin factor that modulates the transcription of upstream regulators of WNT and MAPK-ERK signaling to safeguard naive pluripotency.

ELABELA deficiency promotes preeclampsia and cardiovascular malformations in mice.

Preeclampsia (PE) is a gestational hypertensive syndrome affecting between 5 and 8% of all pregnancies. Although PE is the leading cause of fetal and maternal morbidity and mortality, its molecular etiology is still unclear. Here, we show that ELABELA (ELA), an endogenous ligand of the apelin receptor (APLNR, or APJ), is a circulating hormone secreted by the placenta. Elabela but not Apelin knockout pregnant mice exhibit PE-like symptoms, including proteinuria and elevated blood pressure due to defective placental angiogenesis. In mice, infusion of exogenous ELA normalizes hypertension, proteinuria, and birth weight. ELA, which is abundant in human placentas, increases the invasiveness of trophoblast-like cells, suggesting that it enhances placental development to prevent PE. The ELA-APLNR signaling axis may offer a new paradigm for the treatment of common pregnancy-related complications, including PE.

Single Cell Restriction Enzyme-Based Analysis of Methylation at Genomic Imprinted Regions in Preimplantation Mouse Embryos.

The methylation of cytosines in DNA is a fundamental epigenetic regulatory mechanism. During preimplantation development, mammalian embryos undergo extensive epigenetic reprogramming, including the global erasure of germ cell-specific DNA methylation marks, to allow for the establishment of the pluripotent state of the epiblast. However, DNA methylation marks at specific regions, such as imprinted gene regions, escape this reprogramming process, as their inheritance from germline to soma is paramount for proper development. To study the dynamics of DNA methylation marks in single blastomeres of mouse preimplantation embryos, we devised a new approach-single cell restriction enzyme analysis of methylation (SCRAM). SCRAM allows for reliable, fast, and high-throughput analysis of DNA methylation states of multiple regions of interest from single cells. In the method described below, SCRAM is specifically used to address loss of DNA methylation at genomic imprints or other highly methylated regions of interest.

Loss of maternal Trim28 causes male-predominant early embryonic lethality.

Global DNA demethylation is a hallmark of embryonic epigenetic reprogramming. However, embryos engage noncanonical DNA methylation maintenance mechanisms to ensure inheritance of exceptional epigenetic germline features to the soma. Besides the paradigmatic genomic imprints, these exceptions remain ill-defined, and the mechanisms ensuring demethylation resistance in the light of global reprogramming remain poorly understood. Here we show that the Y-linked gene Rbmy1a1 is highly methylated in mature sperm and resists DNA demethylation post-fertilization. Aberrant hypomethylation of the Rbmy1a1 promoter results in its ectopic activation, causing male-specific peri-implantation lethality. Rbmy1a1 is a novel target of the TRIM28 complex, which is required to protect its repressive epigenetic state during embryonic epigenetic reprogramming.

A twist in zygotic reprogramming.

The first hours of mammalian embryogenesis are devoted to extensive epigenetic reprogramming. One hallmark is active demethylation of the paternal genome by Tet (ten-eleven translocation) enzymes. However, the process is now shown to be Tet-independent at first, with Tet enzymes only counteracting hitherto underappreciated de novo DNA methylation activity in later zygotic stages.

Multiplexed locus-specific analysis of DNA methylation in single cells.

This protocol details a method for measuring the DNA methylation state of multiple target sites in single cells, otherwise known as single-cell restriction analysis of methylation (SCRAM). The basic steps include isolating and lysing single cells, digesting genomic DNA with a methylation-sensitive restriction endonuclease (MSRE) and amplification of multiple targets by two rounds of PCR to determine the methylation status of target sites. The method can reliably and accurately detect the methylation status of multiple target sites in each single cell, and it can be completed in a relatively short time (<2 d) at low cost. Consequently, the method may be preferable over whole-genome methods in applications requiring highly reliable and cost-effective coverage of specific target sites in all cells from a sample and in cases when the DNA methylation states of single CpG sites are representative of the methylation status of corresponding regions of interest.

Erase-Maintain-Establish: Natural Reprogramming of the Mammalian Epigenome.

The genetic information is largely identical across most cell types in a given organism but the epigenome, which controls expression of the genome, is cell type- and context-dependent. Although most mature mammalian cells appear to have a stable, heritable epigenome, a dynamic intricate process reshapes it as these cells transition from soma to germline and back again. During normal embryogenesis, primordial germ cells, of somatic origin, are set aside to become gametes. In doing so their genome is reprogrammed-that is, the epigenome of specific regions is replaced in a sex-specific fashion as they terminally differentiate into oocytes or spermatocytes in the gonads. Upon union of these gametes, reprogramming of the new organism's epigenome is initiated, which eventually leads, through pluripotent cells, to the cell lineages required for proper embryonic development to a sexually mature adult. This never-ending cycle of birth and rebirth is accomplished through methylation and demethylation of specific genomic sites within the gametes and pluripotent cells of an organism. This enigmatic process of natural epigenomic reprogramming is now being dissected in vivo, focusing on specific genomic regions-that is, imprinted genes and retrotransposons, where TRIM28 molecular complexes appear to guide the transition from gamete to embryo.

DNA methylation dynamics during epigenetic reprogramming in the germline and preimplantation embryos.

Methylation of DNA is an essential epigenetic control mechanism in mammals. During embryonic development, cells are directed toward their future lineages, and DNA methylation poses a fundamental epigenetic barrier that guides and restricts differentiation and prevents regression into an undifferentiated state. DNA methylation also plays an important role in sex chromosome dosage compensation, the repression of retrotransposons that threaten genome integrity, the maintenance of genome stability, and the coordinated expression of imprinted genes. However, DNA methylation marks must be globally removed to allow for sexual reproduction and the adoption of the specialized, hypomethylated epigenome of the primordial germ cell and the preimplantation embryo. Recent technological advances in genome-wide DNA methylation analysis and the functional description of novel enzymatic DNA demethylation pathways have provided significant insights into the molecular processes that prepare the mammalian embryo for normal development.

Single-cell DNA-methylation analysis reveals epigenetic chimerism in preimplantation embryos.

Epigenetic alterations are increasingly recognized as causes of human cancers and disease. These aberrations are likely to arise during genomic reprogramming in mammalian preimplantation embryos, when their epigenomes are most vulnerable. However, this process is only partially understood because of the experimental inaccessibility of early-stage embryos. Here, we introduce a methodologic advance, probing single cells for various DNA-methylation errors at multiple loci, to reveal failed maintenance of epigenetic mark results in chimeric mice, which display unpredictable phenotypes leading to developmental arrest. Yet we show that mouse pronuclear transfer can be used to ameliorate such reprogramming defects. This study not only details the epigenetic reprogramming dynamics in early mammalian embryos but also suggests diagnostic and potential future therapeutic applications.

The nuage mediates retrotransposon silencing in mouse primordial ovarian follicles.

Mobilization of endogenous retrotransposons can destabilize the genome, an imminent danger during epigenetic reprogramming of cells in the germline. The P-element-induced wimpy testis (PIWI)-interacting RNA (piRNA) pathway is known to silence retrotransposons in the mouse testes. Several piRNA pathway components localize to the unique, germline structure known as the nuage. In this study, we surveyed mouse ovaries and found, for the first time, transient appearance of nuage-like structures in oocytes of primordial follicles. Mouse vasa homolog (MVH), Piwi-like 2 (PIWIL2/MILI) and tudor domain-containing 9 (TDRD9) are present in these structures, whereas aggregates of germ cell protein with ankyrin repeats, sterile alpha motif and leucine zipper (GASZ) localize separately in the cytoplasm. Retrotransposons are silenced in primordial ovarian follicles, and de-repressed upon reduction of piRNA expression in Mvh, Mili or Gasz mutants. However, these null-mutant females, unlike their male counterparts, are fertile, uncoupling retrotransposon activation from sterility.

Temporal reduction of LATS kinases in the early preimplantation embryo prevents ICM lineage differentiation.

Cellular localization of the Yes-associated protein (YAP) is dependent on large tumor suppressor (LATS) kinase activity and initiates lineage specification in the preimplantation embryo. We temporally reduced LATS activity to disrupt this early event, allowing its reactivation at later stages. This interference resulted in an irreversible lineage misspecification and aberrant polarization of the inner cell mass (ICM). Complementation experiments revealed that neither epiblast nor primitive endoderm can be established from these ICMs. We therefore conclude that precisely timed YAP localization in early morulae is essential to prevent trophectoderm marker expression in, and lineage specification of, the ICM.

A genetic and developmental pathway from STAT3 to the OCT4-NANOG circuit is essential for maintenance of ICM lineages in vivo.

Although it is known that OCT4-NANOG are required for maintenance of pluripotent cells in vitro, the upstream signals that regulate this circuit during early development in vivo have not been identified. Here we demonstrate, for the first time, signal transducers and activators of transcription 3 (STAT3)-dependent regulation of the OCT4-NANOG circuitry necessary to maintain the pluripotent inner cell mass (ICM), the source of in vitro-derived embryonic stem cells (ESCs). We show that STAT3 is highly expressed in mouse oocytes and becomes phosphorylated and translocates to the nucleus in the four-cell and later stage embryos. Using leukemia inhibitory factor (Lif)-null embryos, we found that STAT3 phosphorylation is dependent on LIF in four-cell stage embryos. In blastocysts, interleukin 6 (IL-6) acts in an autocrine fashion to ensure STAT3 phosphorylation, mediated by janus kinase 1 (JAK1), a LIF- and IL-6-dependent kinase. Using genetically engineered mouse strains to eliminate Stat3 in oocytes and embryos, we firmly establish that STAT3 is essential for maintenance of ICM lineages but not for ICM and trophectoderm formation. Indeed, STAT3 directly binds to the Oct4 and Nanog distal enhancers, modulating their expression to maintain pluripotency of mouse embryonic and induced pluripotent stem cells. These results provide a novel genetic model of cell fate determination operating through STAT3 in the preimplantation embryo and pluripotent stem cells in vivo.

Should I stay or should I go: protection and maintenance of DNA methylation at imprinted genes.

Recent findings shed light on the coordination of two fundamental, yet mechanistically opposing, processes in the early mammalian embryo. During the oocyte-to-embryo transition and early preimplantation development nuclear reprogramming occurs. This resetting of the epigenome in maternal and paternal pronuclei to a ground state is the essential step ensuring totipotency in the zygote, the first embryonic stage. Radical, global DNA demethylation, which occurs actively in the paternal and passively in the maternal genome, is a prominent feature of nuclear reprogramming; yet, this process poses a danger to a subset of methylated sequences that must be preserved for their germline to soma inheritance. Genomic imprinting and its importance were demonstrated three decades ago by a series of experiments generating non-viable mammalian uniparental embryos. Indeed, imprinted loci, gene clusters with parent-of-origin specific gene expression patterns, must retain their differential methylation status acquired during gametogenesis throughout embryogenesis and in adult tissues. It is just recently that the molecular players that protect/maintain imprinting marks during reprogramming in preimplantation embryos have been identified, in particular, an epigenetic modifier complex formed by ZFP57 and TRIM28/KAP1. The interaction of these and other molecules with the newly formed embryonic chromatin and imprinted genes is discussed and highlighted herein.