Biology subjects

Schulz, E. G.

Publications and source records attributed to Schulz, E. G..

9 recordsLinked to original sources

bioRxiv2026

Transcription-dependent heterochromatin at the Xist promoter shapes the random choice of the inactive X chromosome

In female mammals, Xist, the master regulator of X-chromosome inactivation (XCI), is expressed monoallelically. This pattern is established during early embryonic development, when the active Xist allele is chosen at random in each cell. How this choice is made remains incompletely understood. Combining knock-down and overexpression strategies in differentiating mouse embryonic stem cells, which recapitulate the onset of random XCI, we identify a role for the repressive chromatin mark H3K9me3 in the XCI initiation. We show that H3K9me3 accumulates at the promoter-proximal region of the silent Xist allele in female cells as monoallelic expression is established. Unexpectedly, this accumulation requires prior transcription of Xist itself, likely during the initial phase of upregulation, when Xist is frequently transcribed in male cells and from both X chromosomes in females. A repressive function of Xist-dependent H3K9me3 accumulation is supported by our finding that premature, transient Xist overexpression primes an allele for future silencing and skews the choice of the inactive X. Xist-dependent H3K9me3 recruitment does not require its antisense transcript Tsix, which can nonetheless enhance subsequent maintenance of the mark. In addition, the X-linked Xist activator RNF12 counteracts H3K9me3 formation, independently of its known target REX1. Our results thus point to facultative heterochromatin formation as a key contributor to choice at the onset of XCI, where activating and repressing mechanisms are intertwined to establish monoallelic Xist expression.

genetics↗

bioRxiv2024

Reporter CRISPR screens decipher cis- and trans-regulatory principles at the Xist locus

Developmental genes are controlled by an ensemble of cis-acting regulatory elements (REs), which in turn respond to multiple trans-acting transcription factors (TFs). Understanding how a cis-regulatory landscape integrates information from many dynamically expressed TFs has remained a challenge. We develop a combined CRISPR-screening approach using endogenous RNA and RE-reporters as readouts. Applied to the Xist locus, crucial for X-chromosome inactivation in females, this method allows us to comprehensively identify Xist-controlling TFs and map their TF-RE wiring. We find a group of transiently expressed TFs that regulate proximal REs, driving the binary activation of Xist expression. These basal activators are more highly expressed in cells with two X chromosomes, potentially driving female-specific Xist upregulation. A second set of developmental TFs is upregulated later during differentiation and targets distal REs. This regulatory axis is crucial to achieve high levels of Xist RNA, which is necessary for X-chromosome inactivation. Our findings support a model for developmental gene regulation in which factors targeting proximal REs drive binary ON-OFF decisions, while factors interacting with distal REs control the transcription output.

genetics↗

bioRxiv2024

Antisense transcription can induce expression memory via stable promoter repression

The capacity of cells to retain a memory of previous signals enables them to adopt unique cell fates and adjust to their surrounding environment. The underlying gene expression memory can arise from mutual repression of two genes, forming a toggle switch. Such mutual repression may occur at antisense loci, where two convergently oriented genes repress each other in cis. Under which conditions antisense transcription can generate expression memory remains poorly understood. To address this question, we combine mathematical modeling, genomics and a synthetic biology approach. Through simulations we show that stable memory can emerge, if both genes in an antisense pair transcribe through the convergent promoter and induce a stable repressive chromatin state. Genome-wide analysis of nascent transcription further supports antisense-mediated promoter repression with promoter-overlapping antisense gene pairs exhibiting mutually exclusive expression. Through constructing a synthetic antisense locus in mouse embryonic stem cells (mESCs) we then show that such a locus architecture can indeed maintain a memory of a transient stimulus. Mutual repression and the capacity for memory formation are elevated, when mESCs differentiate, showing that epigenetic memory is a cell type-specific property. Our finding that stem cells adapt their ability to remember stimuli as they differentiate might help to elucidate how stemness is maintained.

genetics↗

bioRxiv2022

GATA transcription factors drive initial Xist upregulation after fertilization through direct activation of a distal enhancer element

To ensure dosage compensation for X-linked genes between the sexes, one X chromosome is silenced during early embryonic development of female mammals. This process of X-chromosome inactivation (XCI) is initiated through upregulation of the RNA Xist from one X chromosome shortly after fertilization. Xist then mediates chromosome-wide gene silencing in cis and remains expressed in all cell types except the germ line and the pluripotent state, where XCI is reversed. The factors that drive Xist upregulation and thereby initiate XCI remain however unknown. We identify GATA transcription factors as potent Xist activators and demonstrate that they are essential for the activation of Xist in mice following fertilization. Through a pooled CRISPR activation screen we find that GATA1 can drive ectopic Xist expression in murine embryonic stem cells (mESCs). We demonstrate that all GATA factors can activate Xist directly via a GATA-responsive regulatory element (RE79) positioned 100 kb upstream of the Xist promoter. Additionally, GATA factors are essential for the induction of XCI in mouse preimplantation embryos, as simultaneous deletion of three members of the GATA family (GATA1/4/6) in mouse zygotes effectively prevents Xist upregulation. Thus, initiation of XCI and possibly its maintenance in distinct lineages of the preimplantation embryo is ensured by the combined activity of different GATA family members, and the absence of GATA factors in the pluripotent state likely contributes to X reactivation. We thus describe a form of regulation in which the combined action of numerous tissue-specific factors can achieve near-ubiquitous expression of a target gene.

genetics↗

bioRxiv2021

Distal and proximal cis-regulatory elements sense X-chromosomal dosage and developmental state at the Xist locus

Developmental genes such as Xist, the master regulator of X-chromosome inactivation (XCI), are controlled by complex cis-regulatory landscapes, which decode multiple signals to establish specific spatio-temporal expression patterns. Xist integrates information on X-chromosomal dosage and developmental stage to trigger XCI at the primed pluripotent state in females only. Through a pooled CRISPR interference screen in differentiating mouse embryonic stem cells, we identify functional enhancer elements of Xist during the onset of random XCI. By quantifying how enhancer activity is modulated by X-dosage and differentiation, we find that X-dosage controls the promoter-proximal region in a binary switch-like manner. By contrast, differentiation cues activate a series of distal elements and bring them into closer spatial proximity of the Xist promoter. The strongest distal element is part of an enhancer cluster [~]200 kb upstream of the Xist gene which is associated with a previously unannotated Xist-enhancing regulatory transcript, we named Xert. Developmental cues and X-dosage are thus decoded by distinct regulatory regions, which cooperate to ensure female-specific Xist upregulation at the correct developmental time. Our study is the first step to disentangle how multiple, functionally distinct regulatory regions interact to generate complex expression patterns in mammals.

genomics↗

bioRxiv2020

Integrated analysis of Xist upregulation and gene silencing at the onset of random X-chromosome inactivation at high temporal and allelic resolution

To ensure dosage compensation between the sexes, one randomly chosen X chromosome is silenced in each female cell in the process of X-chromosome inactivation (XCI). XCI is initiated during early development through upregulation of the long non-coding RNA Xist, which mediates chromosome-wide gene silencing. Cell differentiation, Xist upregulation and silencing are thought to be coupled at multiple levels to ensure inactivation of exactly one out of two X chromosomes. Here we perform an integrated analysis of all three processes through allele-specific single-cell RNA-sequencing. Specifically, we assess the onset of random XCI with high temporal resolution in differentiating mouse embryonic stem cells, and develop dedicated analysis approaches. By exploiting the inter-cellular heterogeneity of XCI onset, we identify Nanog downregulation as its main trigger and discover additional putative Xist regulators. Moreover, we confirm several predictions of the stochastic model of XCI where monoallelic silencing is thought to be ensured through negative feedback regulation. Finally, we show that genetic variation modulates the XCI process at multiple levels, providing a potential explanation for the long-known Xce effect, which leads to preferential inactivation of a specific X chromosome in inter-strain crosses. We thus draw a detailed picture of the different levels of regulation that govern the initiation of XCI. The experimental and computational strategies we have developed here will allow us to profile random XCI in more physiological contexts, including primary human cells in vivo.

genomics↗

bioRxiv2020

Identification of X-chromosomal genes that drive global X-dosage effects in mammals

X-chromosomal genes contribute to sex differences, in particular during early development, when both X chromosomes are active in females. Here, double X-dosage shifts female pluripotent cells towards the naive stem cell state by increasing pluripotency factor expression, inhibiting the differentiation-promoting MAP kinase (MAPK) signalling pathway and delaying differentiation. To identify the genetic basis of these sex differences, we have performed a series of CRISPR knockout screens in murine embryonic stem cells to comprehensively identify X-linked genes that cause the female pluripotency phenotype. We found multiple genes that act in concert, among which Klhl13 plays a central role. We show that this E3 ubiquitin ligase substrate adaptor protein promotes pluripotency factor expression, delays differentiation and represses MAPK target genes, and we identify putative substrates. We thus elucidate the mechanisms that drive sex-induced differences in pluripotent cells with implications for gender medicine in the context of induced pluripotent stem cell based therapies.

genetics↗

bioRxiv2019

Kinetics of Xist-induced gene silencing can be predicted from combinations of epigenetic and genomic features

To initiate X-chromosome inactivation (XCI), the long non-coding RNA Xist mediates chromosome-wide gene silencing of one X chromosome in female mammals to equalize gene dosage between the sexes. The efficiency of gene silencing, however is highly variable across genes, with some genes even escaping XCI in somatic cells. A genes susceptibility to Xist-mediated silencing appears to be determined by a complex interplay of epigenetic and genomic features; however, the underlying rules remain poorly understood. We have quantified chromosome-wide gene silencing kinetics at the level of the nascent transcriptome using allele-specific Precision nuclear Run-On sequencing (PRO-seq). We have developed a Random Forest machine learning model that can predict the measured silencing dynamics based on a large set of epigenetic and genomic features and tested its predictive power experimentally. While the genomic distance to the Xist locus is the prime determinant of the speed of gene silencing, we find that also pre-marking of gene promoters with polycomb complexes is associated with fast silencing. Moreover, a series of features associated with active transcription and the O-GlcNAc transferase Ogt are enriched at rapidly silenced genes. Our machine learning approach can thus uncover the complex combinatorial rules underlying gene silencing during X inactivation.

genomics↗

bioRxiv2017

Two coupled feedback loops explain random mono-allelic Xist upregulation at the onset of X-chromosome inactivation

Gene-regulatory networks control establishment and maintenance of alternative gene expression states during development. A particular challenge is the acquisition of opposing states by two copies of the same gene, as it is the case in mammals for Xist at the onset of random X-chromosome inactivation (XCI). The regulatory principles that lead to stable mono-allelic expression of Xist remain unknown. Here, we uncovered the minimal Xist regulatory network, by combining mathematical modeling and experimental validation of central model predictions. We identified a symmetric toggle switch as the basis for random mono-allelic Xist up-regulation, which reproduces data from several mutant, aneuploid and polyploid murine cell lines with various Xist expression patterns. Moreover, this toggle switch explains the diversity of strategies employed by different species at the onset of XCI. In addition to providing a unifying conceptual framework to explore X-chromosome inactivation across mammals, our study sets the stage for identifying the molecular mechanisms required to initiate random XCI.

systems biology↗