Interactome comparison of human embryonic stem cell lines with the inner cell mass and trophectoderm

Networks of interacting co-regulated genes distinguish the inner cell mass (ICM) from the differentiated trophectoderm (TE) in the preimplantation blastocyst, in a species specific manner. In mouse the ground state pluripotency of the ICM appears to be maintained in murine embryonic stem cells (ESCs) derived from the ICM. This is not the case for human ESCs. In order to gain insight into this phenomenon, we have used quantitative network analysis to identify how similar human (h)ESCs are to the human ICM. Using the hESC lines MAN1, HUES3 and HUES7 we have shown that all have only a limited overlap with ICM specific gene expression, but that this overlap is enriched for network properties that correspond to key aspects of function including transcription factor activity and the hierarchy of network modules. These analyses provide an important framework which highlights the developmental origins of hESCs

Transcriptionally active HERV-H retrotransposons demarcate topologically associating domains in human pluripotent stem cells.

Chromatin architecture has been implicated in cell type-specific gene regulatory programs, yet how chromatin remodels during development remains to be fully elucidated. Here, by interrogating chromatin reorganization during human pluripotent stem cell (hPSC) differentiation, we discover a role for the primate-specific endogenous retrotransposon human endogenous retrovirus subfamily H (HERV-H) in creating topologically associating domains (TADs) in hPSCs. Deleting these HERV-H elements eliminates their corresponding TAD boundaries and reduces the transcription of upstream genes, while de novo insertion of HERV-H elements can introduce new TAD boundaries. The ability of HERV-H to create TAD boundaries depends on high transcription, as transcriptional repression of HERV-H elements prevents the formation of boundaries. This ability is not limited to hPSCs, as these actively transcribed HERV-H elements and their corresponding TAD boundaries also appear in pluripotent stem cells from other hominids but not in more distantly related species lacking HERV-H elements. Overall, our results provide direct evidence for retrotransposons in actively shaping cell type- and species-specific chromatin architecture

Quantitative methods for profiling dynamic chromatin features

Living systems, from entire organisms down to the single cells constituting them are dynamic entities that continuously adapt and respond to their local environment. Cells achieve this through gene expression programs derived from static information encoded in the DNA made dynamic through chemical modifications at the chromatin level, collectively termed the epigenome. Numerous epigenetic regulators have been implicated in early embryonic developmental transitions and pluripotency. Ex vivo, the different states of pluripotency can be recapitulated by embryonic stem cells (ESCs) grown in defined media conditions. Many developmental gene promoters in ESCs display co-occurrence of the activating histone H3 lysine 4 trimethylation (H3K4me3) mark and the repressive H3K27me3 mark. This distinctive bivalent signature is considered to poise expression, allowing timely resolution to an active or inactive state depending on the signal. The distribution of histone modifications and chromatin-associated factors across the genome can be mapped using chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq). However, traditional ChIP-seq methods fail to quantitatively profile the nuanced global and local epigenetic rewiring that takes place in key developmental stages. This thesis addresses this limitation through the development of a quantitative multiplexed ChIP-seq technology: MINUTE (multiplexed indexed unique molecule T7 amplification end to end sequencing) ChIP. Across the three papers included in this thesis, we reveal the underpinnings of chromatin state dynamics in early mouse and human embryonic development by employing MINUTE ChIP. In Paper I, we first show that MINUTE ChIP enables accurate quantitative comparisons over a wide linear range. By employing it to characterize mouse ESCs grown in 2i and serum conditions, we find that the 2i naïve state is characterized by high global levels of H3K27me3 and low H3K4me3. At bivalent promoters, we observe that while H3K27me3 levels are stably maintained between serum and 2i, H3K4me3 levels are higher in the serum condition. Through quantitative epigenome profiling, in Paper II we find that naïve human ESCs also have broad global gain of Polycomb repressive complex 2 (PRC2)-mediated H3K27me3 and define a previously unrecognized, naïve-specific set of bivalent promoters. Bulk and single-cell transcriptomics confirmed that naïve bivalency maintains key trophectoderm and mesoderm transcription factors in a transcriptionally poised state which is resolved to an active state upon depletion of H3K27me3. Therefore, we discovered that PRC2-mediated repression provides a highly adaptive mechanism to restrict lineage potential during early human development. In paper III we show how quantitative RNA polymerase II occupancy profiles generated by MINUTE ChIP can be integrated with transient transcriptomics data to unravel genome wide transcriptional kinetics in three mESCs pluripotent states: naïve, ground and paused. Taken together, this thesis provides compelling evidence for a broad H3K27me3 hypermethylation of the genome in both naïve mouse and human ESCs and the basis for substantially revising the model for bivalency during embryonic developmen

Human Primordial Germ Cells Are Specified from Lineage-Primed Progenitors.

In vitro gametogenesis is the process of making germline cells from human pluripotent stem cells. The foundation of this model is the quality of the first progenitors called primordial germ cells (PGCs), which in vivo are specified during the peri-implantation window of human development. Here, we show that human PGC (hPGC) specification begins at day 12 post-fertilization. Using single-cell RNA sequencing of hPGC-like cells (hPGCLCs) differentiated from pluripotent stem cells, we discovered that hPGCLC specification involves resetting pluripotency toward a transitional state with shared characteristics between naive and primed pluripotency, followed by differentiation into lineage-primed TFAP2A+ progenitors. Applying the germline trajectory to TFAP2C mutants reveals that TFAP2C functions in the TFAP2A+ progenitors upstream of PRDM1 to regulate the expression of SOX17. This serves to protect hPGCLCs from crossing the Weismann's barrier to adopt somatic cell fates and, therefore, is an essential mechanism for successfully initiating in vitro gametogenesis

Single-cell entropy for accurate estimation of differentiation potency from a cell's transcriptome

The ability to quantify differentiation potential of single cells is a task of critical importance. Here we demonstrate, using over 7,000 single-cell RNA-Seq profiles, that differentiation potency of a single cell can be approximated by computing the signalling promiscuity, or entropy, of a cell's transcriptome in the context of an interaction network, without the need for feature selection. We show that signalling entropy provides a more accurate and robust potency estimate than other entropy-based measures, driven in part by a subtle positive correlation between the transcriptome and connectome. Signalling entropy identifies known cell subpopulations of varying potency and drug resistant cancer stem-cell phenotypes, including those derived from circulating tumour cells. It further reveals that expression heterogeneity within single-cell populations is regulated. In summary, signalling entropy allows in silico estimation of the differentiation potency and plasticity of single cells and bulk samples, providing a means to identify normal and cancer stem-cell phenotypes

Identification of SALL4 Expressing Islet-1+ Cardiovascular Progenitor Cell Clones

The utilization of cardiac progenitor cells (CPC) has been shown to induce favorable regenerative effects. While there are various populations of endogenous CPCs in the heart, there is no consensus regarding which population is the most ideal for cell-based regenerative therapy. Identifying an Islet-1+ (Isl-1+) early-stage progenitor population with enhanced stemness, multipotency and differentiation potential would be beneficial for regenerative therapy. Spalt-like transcription factor 4 (SALL4) plays a role in embryonic development as well as proliferation and expansion of hematopoietic progenitor cells. We hypothesize that SALL4 will be co-expressed in Isl-1+ cardiac progenitor cell clones isolated from human cardiac tissue and represent a pre-mesendodermal progenitor population. Ingenuity Pathway Analysis revealed Isl-1+ human neonates exhibit enhanced stemness properties compared to Isl-1+ adult CPCs. We compared RNA-seq based transcriptomic data from human neonatal Isl-1+ CPC clones with that of published time-course specific RNA-Seq based transcriptomic data collected from various stages of cardiac differentiation from human pluripotent stem cells (hPSCs). This approach elucidated genes that are highly expressed at different stages of cardiac development giving insight into the developmental state of SALL4 expressing Isl-1+ neonatal CPCs. In addition to SALL4, evidence suggests that SOX2, EpCAM and TBX5 are expressed early along the cardiovascular pathway. CPCs were previously derived from human cardiac tissue discarded at surgery, clonally expanded, and screened for expression of Isl-1. RNA was isolated from individual human neonatal (n=10) Isl-1+ CPCs, cDNA was synthesized, and real time PCR was done. We utilized RT-PCR to identify the expression of SALL4, SOX2, EpCAM and TBX5 in individual neonatal Isl-1+ CPC clones. Gel electrophoresis of the PCR products was used to confirm that transcripts of the correct size were amplified. Results demonstrate that 9 out of 10 Isl-1+ neonatal CPC clones tested expressed SALL4. To further substage neonatal Isl-1+ CPC clones that expressed SALL4, SOX2, EpCAM and TBX5, clones were tested for expression of TFAP2C. These features will allow for the identification and isolation of an optimal early-stage CPC clone that may be of value in regenerative therapeutic applications

Multi-omic Profiling Reveals Dynamics of the Phased Progression of Pluripotency

Pluripotency is highly dynamic and progresses through a continuum of pluripotent stem cell states. The two states that bookend the pluripotency continuum, naive and primed, are well characterized, but our understanding of the intermediate states and transitions between them remains incomplete. Here, we dissect the dynamics of pluripotent state transitions underlying pre- to post-implantation epiblast differentiation. Through comprehensive mapping of the proteome, phosphoproteome, transcriptome, and epigenome of embryonic stem cells transitioning from naive to primed pluripotency, we find that rapid, acute, and widespread changes to the phosphoproteome precede ordered changes to the epigenome, transcriptome, and proteome. Reconstruction of the kinase-substrate networks reveals signaling cascades, dynamics, and crosstalk. Distinct waves of global proteomic changes mark discrete phases of pluripotency, with cell-state-specific surface markers tracking pluripotent state transitions. Our data provide new insights into multi-layered control of the phased progression of pluripotency and a foundation for modeling mechanisms regulating pluripotent state transitions (www.steamcellatlas.org)

Long Noncoding RNA Moderates MicroRNA Activity to Maintain Self-Renewal in Embryonic Stem Cells

Of the thousands of long noncoding RNAs expressed in embryonic stem cells (ESCs), few have known roles and fewer have been functionally implicated in the regulation of self-renewal and pluripotency, or the reprogramming of somatic cells to the pluripotent state. In ESCs, Cyrano is a stably expressed long intergenic noncoding RNA with no previously assigned role. We demonstrate that Cyrano contributes to ESC maintenance, as its depletion results in the loss of hallmarks of self-renewal. Delineation of Cyrano's network through transcriptomics revealed widespread effects on signaling pathways and gene expression networks that contribute to ESC maintenance. Cyrano shares unique sequence complementarity with the differentiation-associated microRNA, mir-7, and mir-7 overexpression reduces expression of a key self-renewal factor to a similar extent as Cyrano knockdown. This suggests that Cyrano functions to restrain the action of mir-7. Altogether, we provide a view into the multifaceted function of Cyrano in ESC maintenance