Transcriptional regulation and chromatin architecture maintenance are decoupled functions at the Sox2 locus.

How distal regulatory elements control gene transcription and chromatin topology is not clearly defined, yet these processes are closely linked in lineage specification during development. Through allele-specific genome editing and chromatin interaction analyses of the Sox2 locus in mouse embryonic stem cells, we found a striking disconnection between transcriptional control and chromatin architecture. We traced nearly all Sox2 transcriptional activation to a small number of key transcription factor binding sites, whose deletions have no effect on promoter-enhancer interaction frequencies or topological domain organization. Local chromatin architecture maintenance, including at the topologically associating domain (TAD) boundary downstream from the Sox2 enhancer, is widely distributed over multiple transcription factor-bound regions and maintained in a CTCF-independent manner. Furthermore, partial disruption of promoter-enhancer interactions by ectopic chromatin loop formation has no effect on Sox2 transcription. These findings indicate that many transcription factors are involved in modulating chromatin architecture independently of CTCF.

Transcriptional enhancers: from prediction to functional assessment on a genome-wide scale.

Enhancers are cis-regulatory sequences located distally to target genes. These sequences consolidate developmental and environmental cues to coordinate gene expression in a tissue-specific manner. Enhancer function and tissue specificity depend on the expressed set of transcription factors, which recognize binding sites and recruit cofactors that regulate local chromatin organization and gene transcription. Unlike other genomic elements, enhancers are challenging to identify because they function independently of orientation, are often distant from their promoters, have poorly defined boundaries, and display no reading frame. In addition, there are no defined genetic or epigenetic features that are unambiguously associated with enhancer activity. Over recent years there have been developments in both empirical assays and computational methods for enhancer prediction. We review genome-wide tools, CRISPR advancements, and high-throughput screening approaches that have improved our ability to both observe and manipulate enhancers in vitro at the level of primary genetic sequences, chromatin states, and spatial interactions. We also highlight contemporary animal models and their importance to enhancer validation. Together, these experimental systems and techniques complement one another and broaden our understanding of enhancer function in development, evolution, and disease.

Enhancer-promoter interactions can bypass CTCF-mediated boundaries and contribute to phenotypic robustness.

How enhancers activate their distal target promoters remains incompletely understood. Here we dissect how CTCF-mediated loops facilitate and restrict such regulatory interactions. Using an allelic series of mouse mutants, we show that CTCF is neither required for the interaction of the Sox2 gene with distal enhancers, nor for its expression. Insertion of various combinations of CTCF motifs, between Sox2 and its distal enhancers, generated boundaries with varying degrees of insulation that directly correlated with reduced transcriptional output. However, in both epiblast and neural tissues, enhancer contacts and transcriptional induction could not be fully abolished, and insertions failed to disrupt implantation and neurogenesis. In contrast, Sox2 expression was undetectable in the anterior foregut of mutants carrying the strongest boundaries, and these animals fully phenocopied loss of SOX2 in this tissue. We propose that enhancer clusters with a high density of regulatory activity can better overcome physical barriers to maintain faithful gene expression and phenotypic robustness.

Targeted expression profiling reveals distinct stages of early canine fibroblast reprogramming are regulated by 2-oxoglutarate hydroxylases.

BACKGROUND: Ectopic expression of a defined set of transcription factors allows the reprogramming of mammalian somatic cells to pluripotency. Despite continuous progress in primate and rodent reprogramming, limited attention has been paid to cell reprogramming in domestic and companion species. Previous studies attempting to reprogram canine cells have mostly assessed a small number of presumptive canine induced pluripotent stem cell (iPSC) lines for generic pluripotency attributes. However, why canine cell reprogramming remains extremely inefficient is poorly understood. METHODS: To better characterize the initial steps of pluripotency induction in canine somatic cells, we optimized an experimental system where canine fetal fibroblasts (cFFs) are transduced with the Yamanaka reprogramming factors by Sendai virus vectors. We use quantitative PCR arrays to measure the expression of 80 target genes at various stages of canine cell reprogramming. We ask how cFF reprogramming is influenced by small molecules affecting the epigenomic modification 5-hydroxymethylcytosine, specifically L-ascorbic acid and retinoic acid (AA/RA). RESULTS: We found that the expression and catalytic output of a class of 2-oxoglutarate-dependent (2-OG) hydroxylases, known as ten-eleven translocation (TET) enzymes, can be modulated in canine cells treated with AA/RA. We further show that AA/RA treatment induces TET1 expression and facilitates early canine reprogramming, evidenced by upregulation of epithelial and pluripotency markers. Using a chemical inhibitor of 2-OG hydroxylases, we demonstrate that 2-OG hydroxylase activity regulates the expression of a subset of genes involved in mesenchymal-to-epithelial transition (MET) and pluripotency in early canine reprogramming. We identify a set of transcription factors depleted in maturing reprogramming intermediates compared to pluripotent canine embryonic stem cells. CONCLUSIONS: Our findings highlight 2-OG hydroxylases have evolutionarily conserved and divergent functions regulating the early reprogramming of canine somatic cells and show reprogramming conditions can be rationally optimized for the generation of maturing canine iPSC.

Small-Molecule Induction of Canine Embryonic Stem Cells Toward Naïve Pluripotency.

Naïve and primed pluripotent stem cells (PSCs) reflect discrete pluripotent states that approximate the inner cell mass or the progressively lineage-restricted perigastrulation epiblast, respectively. Cells that occupy primed pluripotency have distinct epigenetic landscapes, transcriptional circuitry, and trophic requirements compared with their naïve counterparts. The existence of multiple pluripotent states has not been explored in dogs, which show promise as outbred biomedical models with more than 300 inherited diseases that also afflict humans. However, our understanding of canine embryogenesis and embryo-derived stem cells is limited. Herein, we converted leukemia inhibitory factor (LIF)-dependent and fibroblast growth factor 2 (FGF2)-dependent canine embryonic stem cells (cESCs) resembling primed PSCs toward a naïve pluripotent state using LIF and inhibitors of glycogen synthase kinase 3ß and mitogen-activated protein kinase kinase 1/2 [called 2i and LIF (2iL)]. cESCs propagated in 2iL exhibited significant induction of genes associated with the naïve pluripotent state (eg, REX1, TBX3) and downregulation of primed pluripotency markers (eg, OTX2, FGF5) (P < 0.05). Differential phosphorylation of signal transducer and activator of transcription 3 (STAT3) and cell fate decisions on exposure to bone morphogenetic protein 4 (BMP4) suggested that a novel pluripotent identity has been established with 2iL. Accordingly, cESCs cultured with 2iL formed colonies at a greater efficiency than LIF-FGF2 cESCs following single-cell dissociation. Total genomic DNA methylation and histone H3 lysine 27 trimethylation signals were reduced in 2iL-treated cESCs. Our data suggest that 2iL culture conditions promote the conversion of cESCs toward an epigenetically distinct pluripotent state resembling naïve PSCs.

Canine Pluripotent Stem Cells: Are They Ready for Clinical Applications?

The derivation of canine embryonic stem cells and generation of canine-induced pluripotent stem cells are significant achievements that have unlocked the potential for developing novel cell-based disease models, drug discovery platforms, and transplantation therapies in the dog. A progression from concept to cure in this clinically relevant companion animal will not only help our canine patients but also help advance human regenerative medicine. Nevertheless, many issues remain to be resolved before pluripotent cells can be used clinically in a safe and reproducible manner.

Derivation and culture of canine embryonic stem cells.

The derivation of canine embryonic stem cells (cESCs) represents a significant achievement and opens the door to further stem cell research and therapies in the dog. Canines share a common environment with humans and exhibit a host of genetic diseases, many of which have human parallels. Thus, the canine model presents unique advantages over other currently used organisms to help develop stem cell therapies in humans. To reveal the therapeutic potential of cESCs further basic research on the molecular mechanisms controlling their pluripotency and self-renewal characteristics is needed. Herein, we present the methods for derivation and culture of cESCs. Following collection of the canine blastocyst, two derivation methods are presented; immunodissection and whole blastocyst explant. These two methods lead to cESCs differing in morphology and subculture techniques. Additional protocols for subculture of established lines, feeder-free culture, and cryopreservation protocols are also described.