Inhibition of protein kinase C increases Prdm14 level to promote self-renewal of embryonic stem cells through reducing Suv39h-induced H3K9 methylation.

Inhibition of protein kinase C (PKC) efficiently promoted the self-renewal of embryonic stem cells (ESCs). However, information about the function of PKC inhibition remains lacking. Here, RNA-sequencing showed that the addition of Go6983 significantly inhibited the expression of de novo methyltransferases (Dnmt3a and Dnmt3b) and their regulator Dnmt3l, resulting in global hypomethylation of DNA in mouse ESCs. Mechanistically, PR domain-containing 14 (Prdm14), a site-specific transcriptional activator, partially contributed to Go6983-mediated repression of Dnmt3 genes. Administration of Go6983 increased Prdm14 expression mainly through the inhibition of PKCδ. High constitutive expression of Prdm14 phenocopied the ability of Go6983 to maintain` mouse ESC stemness in the absence of self-renewal-promoting cytokines. In contrast, the knockdown of Prdm14 eliminated the response to PKC inhibition and substantially impaired the Go6983-induced resistance of mouse ESCs to differentiation. Furthermore, liquid chromatography-mass spectrometry profiling and Western blotting revealed low levels of Suv39h1 and Suv39h2 in Go6983-treated mouse ESCs. Suv39h enzymes are histone methyltransferases that recognize dimethylated and trimethylated histone H3K9 specifically and usually function as transcriptional repressors. Consistently, the inhibition of Suv39h1 by RNA interference or the addition of the selective inhibitor chaetocin increased Prdm14 expression. Moreover, chromatin immunoprecipitation assay showed that Go6983 treatment led to decreased enrichment of dimethylation and trimethylation of H3K9 at the Prdm14 promoter but increased RNA polymerase Ⅱ binding affinity. Together, our results provide novel insights into the pivotal association between PKC inhibition-mediated self-renewal and epigenetic changes, which will help us better understand the regulatory network of stem cell pluripotency.

SETDB1/NSD-dependent H3K9me3/H3K36me3 dual heterochromatin maintains gene expression profiles by bookmarking poised enhancers.

Gene silencing by heterochromatin plays a crucial role in cell identity. Here, we characterize the localization, the biogenesis, and the function of an atypical heterochromatin, which is simultaneously enriched in the typical H3K9me3 mark and in H3K36me3, a histone mark usually associated with gene expression. We identified thousands of dual regions in mouse embryonic stem (ES) cells that rely on the histone methyltransferases SET domain bifurcated 1 (SETDB1) and nuclear set domain (NSD)-containing proteins to generate H3K9me3 and H3K36me3, respectively. Upon SETDB1 removal, dual domains lose both marks, gain signatures of active enhancers, and come into contact with upregulated genes, suggesting that it might be an important pathway by which genes are controlled by heterochromatin. In differentiated tissues, a subset of these dual domains is destabilized and becomes enriched in active enhancer marks, providing a mechanistic insight into the involvement of heterochromatin in the maintenance of cell identity.

MK2 promotes Tfcp2l1 degradation via ß-TrCP ubiquitin ligase to regulate mouse embryonic stem cell self-renewal.

Tfcp2l1 can maintain mouse embryonic stem cell (mESC) self-renewal. However, it remains unknown how Tfcp2l1 protein stability is regulated. Here, we demonstrate that ß-transducin repeat-containing protein (ß-TrCP) targets Tfcp2l1 for ubiquitination and degradation in a mitogen-activated protein kinase (MAPK)-activated protein kinase 2 (MK2)-dependent manner. Specifically, ß-TrCP1 and ß-TrCP2 recognize and ubiquitylate Tfcp2l1 through the canonical ß-TrCP-binding motif DSGDNS, in which the serine residues have been phosphorylated by MK2. Point mutation of serine-to-alanine residues reduces ß-TrCP-mediated ubiquitylation and enhances the ability of Tfcp2l1 to promote mESC self-renewal while repressing the speciation of the endoderm, mesoderm, and trophectoderm. Similarly, inhibition of MK2 reduces the association of Tfcp2l1 with ß-TrCP1 and increases the self-renewal-promoting effects of Tfcp2l1, whereas overexpression of MK2 or ß-TrCP genes decreases Tfcp2l1 protein levels and induces mESC differentiation. Collectively, our study reveals a posttranslational modification of Tfcp2l1 that will expand our understanding of the regulatory network of stem cell pluripotency.

DUSP9, a Dual-Specificity Phosphatase with a Key Role in Cell Biology and Human Diseases.

Mitogen-activated protein kinases (MAPKs) are essential for proper cell functioning as they regulate many molecular effectors. Careful regulation of MAPKs is therefore required to avoid MAPK pathway dysfunctions and pathologies. The mammalian genome encodes about 200 phosphatases, many of which dephosphorylate the MAPKs and bring them back to an inactive state. In this review, we focus on the normal and pathological functions of dual-specificity phosphatase 9 (DUSP9)/MAP kinase phosphatases-4 (MKP-4). This cytoplasmic phosphatase, which belongs to the threonine/tyrosine dual-specific phosphatase family and was first described in 1997, is known to dephosphorylate ERK1/2, p38, JNK and ASK1, and thereby to control various MAPK pathway cascades. As a consequence, DUSP9 plays a major role in human pathologies and more specifically in cardiac dysfunction, liver metabolic syndromes, diabetes, obesity and cancer including drug response and cell stemness. Here, we recapitulate the mechanism of action of DUSP9 in the cell, its levels of regulation and its roles in the most frequent human diseases, and discuss its potential as a therapeutic target.

Effects of 2,2',4,4'-tetrabromodiphenyl ether on the development of mouse embryonic stem cells.

2,2',4,4'-Tetrabromodiphenyl ether (BDE47) poses potential risks to reproduction and development, but the mechanism of its toxicity has not yet been elucidated. To explore the developmental toxicity of BDE47, mouse embryonic stem cells (mESCs), which are ideal models for testing the developmental toxicity of environmental contaminants in vitro, were exposed to BDE47 (0.04 µM, 1 µM, 25 µM, or 100 µM) for 24 h or 48 h in this study. Our results indicated that BDE47 treatment changed the morphology of mESCs, inhibited cell viability and increased apoptosis. In addition, alkaline phosphatase (AP) staining in mESCs was significantly decreased after BDE47 treatment (25 µM and 100 µM), indicating that BDE47 treatment affected the pluripotency of mESCs. Through a cell immunofluorescence assay, we found that the fluorescence intensities of Oct4, Sox2 and Nanog were all significantly lower in the group treated with the highest BDE47 concentration (100 µM) than in the control group, consistent with the qRT-PCR and Western blot results. The levels of miR-145 and miR-34a, which regulate genes related to cell differentiation, were significantly increased in BDE47-treated mESCs, further clarifying the potential mechanism. Overall, our findings demonstrate that BDE47 exposure upregulates the expression of miR-145 and miR-34a and in turn downregulates the expression of Oct4, Sox2 and Nanog, thereby affecting apoptosis and pluripotency and causing toxicity during embryonic development.

The Lysine Methylase SMYD3 Modulates Mesendodermal Commitment during Development.

SMYD3 (SET and MYND domain containing protein 3) is a methylase over-expressed in cancer cells and involved in oncogenesis. While several studies uncovered key functions for SMYD3 in cancer models, the SMYD3 role in physiological conditions has not been fully elucidated yet. Here, we dissect the role of SMYD3 at early stages of development, employing mouse embryonic stem cells (ESCs) and zebrafish as model systems. We report that SMYD3 depletion promotes the induction of the mesodermal pattern during in vitro differentiation of ESCs and is linked to an upregulation of cardiovascular lineage markers at later stages. In vivo, smyd3 knockdown in zebrafish favors the upregulation of mesendodermal markers during zebrafish gastrulation. Overall, our study reveals that SMYD3 modulates levels of mesendodermal markers, both in development and in embryonic stem cell differentiation.

SIRT1 regulates sphingolipid metabolism and neural differentiation of mouse embryonic stem cells through c-Myc-SMPDL3B.

Sphingolipids are important structural components of cell membranes and prominent signaling molecules controlling cell growth, differentiation, and apoptosis. Sphingolipids are particularly abundant in the brain, and defects in sphingolipid degradation are associated with several human neurodegenerative diseases. However, molecular mechanisms governing sphingolipid metabolism remain unclear. Here, we report that sphingolipid degradation is under transcriptional control of SIRT1, a highly conserved mammalian NAD+-dependent protein deacetylase, in mouse embryonic stem cells (mESCs). Deletion of SIRT1 results in accumulation of sphingomyelin in mESCs, primarily due to reduction of SMPDL3B, a GPI-anchored plasma membrane bound sphingomyelin phosphodiesterase. Mechanistically, SIRT1 regulates transcription of Smpdl3b through c-Myc. Functionally, SIRT1 deficiency-induced accumulation of sphingomyelin increases membrane fluidity and impairs neural differentiation in vitro and in vivo. Our findings discover a key regulatory mechanism for sphingolipid homeostasis and neural differentiation, further imply that pharmacological manipulation of SIRT1-mediated sphingomyelin degradation might be beneficial for treatment of human neurological diseases.

All cells in the brain start life as stem cells which are yet to have a defined role in the body. A wide range of molecules and chemical signals guide stem cells towards a neuronal fate, including a group of molecules called sphingolipids. These molecules sit in the membrane surrounding the cell and play a pivotal role in a number of processes which help keep the neuronal cell healthy. Various enzymes work together to break down sphingolipids and remove them from the membrane. Defects in these enzymes can result in excess levels of sphingolipids, which can lead to neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's disease. But how these enzymes are used and controlled during neuronal development is still somewhat of a mystery. To help answer this question, Fan et al. studied an enzyme called SIRT1 which has been shown to alleviate symptoms in animal models of neurodegenerative diseases. Stem cells were extracted from a mouse embryo lacking the gene for SIRT1 and cultured in the laboratory. These faulty cells were found to have superfluous amounts of sphingolipids, which made their membranes more fluid and reduced their ability to develop into neuronal cells. Further investigation revealed that SIRT1 regulates the degradation of sphingolipids by promoting the production of another enzyme called SMPDL3B. Fan et al. also found that when female mice were fed a high-fat diet, this caused sphingolipids to accumulate in their embryos which lacked the gene for SIRT1; this, in turn, impaired the neural development of their offspring. These findings suggest that targeting SIRT1 may offer new strategies for treating neurological diseases. The discovery that embryos deficient in SIRT1 are sensitive to high-fat diets implies that activating this enzyme might attenuate some of the neonatal complications associated with maternal obesity.

Targeting the histone demethylase LSD1 prevents cardiomyopathy in a mouse model of laminopathy.

LMNA mutations in patients are responsible for a dilated cardiomyopathy. Molecular mechanisms underlying the origin and development of the pathology are unknown. Herein, using mouse pluripotent embryonic stem cells (ESCs) and a mouse model both harboring the p.H222P Lmna mutation, we found early defects in cardiac differentiation of mutated ESCs and dilatation of mutated embryonic hearts at E13.5, pointing to a developmental origin of the disease. Using mouse ESCs, we demonstrated that cardiac differentiation of LmnaH222P/+ was impaired at the mesodermal stage. Expression of Mesp1, a mesodermal cardiogenic gene involved in epithelial-to-mesenchymal transition of epiblast cells, as well as Snai1 and Twist expression, was decreased in LmnaH222P/+ cells compared with WT cells in the course of differentiation. In turn, cardiomyocyte differentiation was impaired. ChIP assay of H3K4me1 in differentiating cells revealed a specific decrease of this histone mark on regulatory regions of Mesp1 and Twist in LmnaH222P/+ cells. Downregulation or inhibition of LSD1 that specifically demethylated H3K4me1 rescued the epigenetic landscape of mesodermal LmnaH222P/+ cells and in turn contraction of cardiomyocytes. Inhibition of LSD1 in pregnant mice or neonatal mice prevented cardiomyopathy in E13.5 LmnaH222P/H222P offspring and adults, respectively. Thus, LSD1 appeared to be a therapeutic target to prevent or cure dilated cardiomyopathy associated with a laminopathy.

Selective eradication of pluripotent stem cells by inhibiting DHODH activity.

Pluripotent stem cells (PSCs), such as embryonic stem cells and induced pluripotent stem cells, give rise to all kinds of functional cells, making them promising for successful application in regenerative medicine. However, there is concern that a PSC-derived differentiated cell population may form teratomas when used for cell therapy if the population contains undifferentiated PSCs. Therefore, for the success of regenerative medicine, it is crucial to establish methods that induce complete PSC differentiation and eliminate the contamination of PSCs. Here, I show that the dihydroorotate dehydrogenase (DHODH) inhibitor brequinar (BRQ) induced cell cycle arrest, cell death, and stemness loss in mouse PSCs (mPSCs), whereas it was less toxic against normal tissue-specific stem cells and differentiating cells. I demonstrate that BRQ-pretreated mPSCs did not form teratomas after being transplanted into NOD/SCID mice. Moreover, BRQ administration to teratoma-bearing mice prevented tumor growth and decreased PSC marker levels in the tumor without any visible effects in the differentiated germ layer cells and the mice. Collectively, these data suggested that DHODH inhibitors such as BRQ can be indispensable in the fundamental methods of PSC-based therapy.

Cathepsin A regulates pluripotency, proliferation and differentiation in mouse embryonic stem cells.

Mouse embryonic stem cells (mESCs) are pluripotent cells that possess the ability to self-renew and differentiate into three germ layers. Owing to these characteristics, mESCs act as important models for stem cell research and are being used in many clinical applications. Among the many cathepsins, cathepsin A (Ctsa), a serine protease, affects the function and properties of stem cells. However, studies on the role of Ctsa in stem cells are limited. Here, we observed a significant increase in Ctsa expression during mESC differentiation at protein levels. Furthermore, we established Ctsa knockdown mESCs. Ctsa knockdown led to Erk1/2 phosphorylation, which in turn inhibited the pluripotency of mESCs and induced G2/M cell cycle arrest to inhibit mESC proliferation. The knockdown also induced abnormal differentiation in mESCs and aberrant expression of differentiation markers. Furthermore, we identified inhibition of teratoma formation in nude mice. Our results suggested that Ctsa affects mESC pluripotency, proliferation, cell cycle and differentiation, and highlighted the potential of Ctsa to act as a core factor that can regulate various mESC properties. SIGNIFICANCE OF THE STUDY: Our results indicate that cathepsin A (Ctsa) affects the properties of mESCs. Inhibition of Ctsa resulted in a decrease in the pluripotency of mouse embryonic stem cells (mESCs). Further, Ctsa suppression resulted in decreased proliferation via cell cycle arrest. Moreover, Ctsa inhibition reduced differentiation abilities and formation of teratoma in mESCs. Our results demonstrated that Ctsa is an important factor controlling mESC abilities.

Inhibition of protein kinase D by CID755673 promotes maintenance of the pluripotency of embryonic stem cells.

The identification of novel mechanisms to maintain embryonic stem cell (ESC) pluripotency is of crucial importance, because the currently used culture conditions are not suitable for ESCs from all species. In this study, we show that the protein kinase D (PKD) inhibitor CID755673 (CID) is able to maintain the undifferentiated state of mouse ESCs in combination with the mitogen-activated protein kinase kinase (MEK) inhibitor. The expression levels of PKD members, including PKD1, PKD2 and PKD3, were low in mouse ESCs but significantly increased under differentiation conditions. Therefore, depletion of three PKD genes was able to phenocopy PKD inhibition. Mechanistically, PKD inhibition activated PI3K/AKT signaling by increasing the level of AKT phosphorylation, and the addition of a PI3K/AKT signaling pathway inhibitor partially reduced the cellular response to PKD inhibition. Importantly, the self-renewal-promoting effect of CID was maintained in human ESCs. Simultaneous knockdown of the three human PKD isoforms enabled short-term self-renewal in human ESCs, whereas PI3K/AKT signaling inhibition eliminated this self-renewal ability downstream of the PKD inhibitor. These findings expand our understanding of the gene regulatory network of ESC pluripotency.

Activation of transcription factor circuity in 2i-induced ground state pluripotency is independent of repressive global epigenetic landscapes.

Mouse embryonic stem cells (mESCs) cultured with MEK/ERK and GSK3ß (2i) inhibitors transition to ground state pluripotency. Gene expression changes, redistribution of histone H3K27me3 profiles and global DNA hypomethylation are hallmarks of 2i exposure, but it is unclear whether epigenetic alterations are required to achieve and maintain ground state or occur as an outcome of 2i signal induced changes. Here we show that ESCs with three epitypes, WT, constitutively methylated, or hypomethylated, all undergo comparable morphological, protein expression and transcriptome changes independently of global alterations of DNA methylation levels or changes in H3K27me3 profiles. Dazl and Fkbp6 expression are induced by 2i in all three epitypes, despite exhibiting hypermethylated promoters in constitutively methylated ESCs. We identify a number of activated gene promoters that undergo 2i dependent loss of H3K27me3 in all three epitypes, however genetic and pharmaceutical inhibition experiments show that H3K27me3 is not required for their silencing in non-2i conditions. By separating and defining their contributions, our data suggest that repressive epigenetic systems play minor roles in mESC self-renewal and naïve ground state establishment by core sets of dominant pluripotency associated transcription factor networks, which operate independently from these epigenetic processes.

The rRNA m6A methyltransferase METTL5 is involved in pluripotency and developmental programs.

Covalent chemical modifications of cellular RNAs directly impact all biological processes. However, our mechanistic understanding of the enzymes catalyzing these modifications, their substrates and biological functions, remains vague. Amongst RNA modifications N6-methyladenosine (m6A) is widespread and found in messenger (mRNA), ribosomal (rRNA), and noncoding RNAs. Here, we undertook a systematic screen to uncover new RNA methyltransferases. We demonstrate that the methyltransferase-like 5 (METTL5) protein catalyzes m6A in 18S rRNA at position A1832 We report that absence of Mettl5 in mouse embryonic stem cells (mESCs) results in a decrease in global translation rate, spontaneous loss of pluripotency, and compromised differentiation potential. METTL5-deficient mice are born at non-Mendelian rates and develop morphological and behavioral abnormalities. Importantly, mice lacking METTL5 recapitulate symptoms of patients with DNA variants in METTL5, thereby providing a new mouse disease model. Overall, our biochemical, molecular, and in vivo characterization highlights the importance of m6A in rRNA in stemness, differentiation, development, and diseases.

Pten-mediated Gsk3ß modulates the naïve pluripotency maintenance in embryonic stem cells.

Mouse embryonic stem cells (ESCs) are isolated from the inner cell mass of blastocysts, and they exist in different states of pluripotency-naïve and primed states. Pten is a well-known tumor suppressor. Here, we generated Pten-/- mouse ESCs with the CRISPR-Cas9 system and verified that Pten-/- ESCs maintained naïve pluripotency by blocking Gsk3ß activity. Serum/LIF and 2i (MAPK and GSK3 inhibitors) conditions are commonly used for ESC maintenance. We show that the Pten-inhibitor SF1670 contributed to sustaining mouse ESCs and that Pten activation by the S380A, T382A, and T383A mutations (Pten-A3) suppressed the pluripotency of ESCs. The in vivo teratoma formation ability of SF1670-treated ESCs increased, while the Pten-A3 mutations suppressed teratoma formation. Furthermore, the embryoid bodies derived from Pten-deficient ESCs or SF1670-treated wild-type ESCs showed greater expression of ectoderm and pluripotency markers. These results suggest that Pten-mediated Gsk3ß modulates the naïve pluripotency of ESCs and that Pten ablation regulates the lineage-specific differentiation.

Single cell transcriptome dynamics from pluripotency to FLK1+ mesoderm.

Hemangiogenic progenitors generating blood and endothelial cells are specified from FLK1-expressing (FLK1+) mesoderm by the transcription factor ETV2. FLK1+ mesoderm also contributes to smooth muscle and cardiomyocytes. However, the developmental process of FLK1+ mesoderm generation and its allocation to various cell fates remain obscure. Recent single cell RNA-sequencing studies of early embryos or in vitro-differentiated human embryonic stem (ES) cells have provided unprecedented information on the spatiotemporal resolution of cells in embryogenesis. These snapshots, however, lack information on continuous dynamic developmental processes. Here, we performed single cell RNA sequencing of in vitro-differentiated mouse ES cells to capture the continuous developmental process leading to hemangiogenesis. We found that hemangiogenic progenitors from ES cells develop through intermediate gastrulation stages, which are gradually specified by 'relay'-like highly overlapping transcription factor modules. Moreover, the transcriptional program of the Flk1+ mesoderm was maintained in the smooth muscle lineage, suggesting that smooth muscle is the default fate of Flk1+ mesoderm. We also identified the SRC kinase contributing to ETV2-mediated activation of the hemangiogenic program. This continuous transcriptome map will facilitate both basic and applied studies of mesoderm development.

KDM2 proteins constrain transcription from CpG island gene promoters independently of their histone demethylase activity.

CpG islands (CGIs) are associated with the majority of mammalian gene promoters and function to recruit chromatin modifying enzymes. It has therefore been proposed that CGIs regulate gene expression through chromatin-based mechanisms, however in most cases this has not been directly tested. Here, we reveal that the histone H3 lysine 36 (H3K36) demethylase activity of the CGI-binding KDM2 proteins contributes only modestly to the H3K36me2-depleted state at CGI-associated gene promoters and is dispensable for normal gene expression. Instead, we discover that KDM2 proteins play a widespread and demethylase-independent role in constraining gene expression from CGI-associated gene promoters. We further show that KDM2 proteins shape RNA Polymerase II occupancy but not chromatin accessibility at CGI-associated promoters. Together this reveals a demethylase-independent role for KDM2 proteins in transcriptional repression and uncovers a new function for CGIs in constraining gene expression.

DNA (de)methylation in embryonic stem cells controls CTCF-dependent chromatin boundaries.

Coordinated changes of DNA (de)methylation, nucleosome positioning, and chromatin binding of the architectural protein CTCF play an important role for establishing cell-type-specific chromatin states during differentiation. To elucidate molecular mechanisms that link these processes, we studied the perturbed DNA modification landscape in mouse embryonic stem cells (ESCs) carrying a double knockout (DKO) of the Tet1 and Tet2 dioxygenases. These enzymes are responsible for the conversion of 5-methylcytosine (5mC) into its hydroxymethylated (5hmC), formylated (5fC), or carboxylated (5caC) forms. We determined changes in nucleosome positioning, CTCF binding, DNA methylation, and gene expression in DKO ESCs and developed biophysical models to predict differential CTCF binding. Methylation-sensitive nucleosome repositioning accounted for a significant portion of CTCF binding loss in DKO ESCs, whereas unmethylated and nucleosome-depleted CpG islands were enriched for CTCF sites that remained occupied. A number of CTCF sites also displayed direct correlations with the CpG modification state: CTCF was preferentially lost from sites that were marked with 5hmC in wild-type (WT) cells but not from 5fC-enriched sites. In addition, we found that some CTCF sites can act as bifurcation points defining the differential methylation landscape. CTCF loss from such sites, for example, at promoters, boundaries of chromatin loops, and topologically associated domains (TADs), was correlated with DNA methylation/demethylation spreading and can be linked to down-regulation of neighboring genes. Our results reveal a hierarchical interplay between cytosine modifications, nucleosome positions, and DNA sequence that determines differential CTCF binding and regulates gene expression.

ITPase deficiency causes a Martsolf-like syndrome with a lethal infantile dilated cardiomyopathy.

Typical Martsolf syndrome is characterized by congenital cataracts, postnatal microcephaly, developmental delay, hypotonia, short stature and biallelic hypomorphic mutations in either RAB3GAP1 or RAB3GAP2. Genetic analysis of 85 unrelated "mutation negative" probands with Martsolf or Martsolf-like syndromes identified two individuals with different homozygous null mutations in ITPA, the gene encoding inosine triphosphate pyrophosphatase (ITPase). Both probands were from multiplex families with a consistent, lethal and highly distinctive disorder; a Martsolf-like syndrome with infantile-onset dilated cardiomyopathy. Severe ITPase-deficiency has been previously reported with infantile epileptic encephalopathy (MIM 616647). ITPase acts to prevent incorporation of inosine bases (rI/dI) into RNA and DNA. In Itpa-null cells dI was undetectable in genomic DNA. dI could be identified at a low level in mtDNA without detectable mitochondrial genome instability, mtDNA depletion or biochemical dysfunction of the mitochondria. rI accumulation was detectable in proband-derived lymphoblastoid RNA. In Itpa-null mouse embryos rI was detectable in the brain and kidney with the highest level seen in the embryonic heart (rI at 1 in 385 bases). Transcriptome and proteome analysis in mutant cells revealed no major differences with controls. The rate of transcription and the total amount of cellular RNA also appeared normal. rI accumulation in RNA-and by implication rI production-correlates with the severity of organ dysfunction in ITPase deficiency but the basis of the cellulopathy remains cryptic. While we cannot exclude cumulative minor effects, there are no major anomalies in the production, processing, stability and/or translation of mRNA.

Protein Methyltransferase Inhibition Decreases Endocrine Specification Through the Upregulation of Aldh1b1 Expression.

Understanding the mechanisms that promote the specification of pancreas progenitors and regulate their self-renewal and differentiation will help to maintain and expand pancreas progenitor cells derived from human pluripotent stem (hPS) cells. This will improve the efficiency of current differentiation protocols of hPS cells into ß-cells and bring such cells closer to clinical applications for the therapy of diabetes. Aldehyde dehydrogenase 1b1 (Aldh1b1) is a mitochondrial enzyme expressed specifically in progenitor cells during mouse pancreas development, and we have shown that its functional inactivation leads to accelerated differentiation and deficient ß-cells. In this report, we aimed to identify small molecule inducers of Aldh1b1 expression taking advantage of a mouse embryonic stem (mES) cell Aldh1b1 lacZ reporter line and a pancreas differentiation protocol directing mES cells into pancreatic progenitors. We identified AMI-5, a protein methyltransferase inhibitor, as an Aldh1b1 inducer and showed that it can maintain Aldh1b1 expression in embryonic pancreas explants. This led to a selective reduction in endocrine specification. This effect was due to a downregulation of Ngn3, and it was mediated through Aldh1b1 since the effect was abolished in Aldh1b1 null pancreata. The findings implicated methyltransferase activity in the regulation of endocrine differentiation and showed that methyltransferases can act through specific regulators during pancreas differentiation. Stem Cells 2019;37:640-651.

Transcriptional Burst Initiation and Polymerase Pause Release Are Key Control Points of Transcriptional Regulation.

Transcriptional regulation occurs via changes to rates of different biochemical steps of transcription, but it remains unclear which rates are subject to change upon biological perturbation. Biochemical studies have suggested that stimuli predominantly affect the rates of RNA polymerase II (Pol II) recruitment and polymerase release from promoter-proximal pausing. Single-cell studies revealed that transcription occurs in discontinuous bursts, suggesting that features of such bursts like frequency and intensity could also be regulated. We combined Pol II chromatin immunoprecipitation sequencing (ChIP-seq) and single-cell transcriptional measurements to show that an independently regulated burst initiation step is required before polymerase recruitment can occur. Using a number of global and targeted transcriptional regulatory perturbations, we showed that biological perturbations regulated both burst initiation and polymerase pause release rates but seemed not to regulate polymerase recruitment rate. Our results suggest that transcriptional regulation primarily acts by changing the rates of burst initiation and polymerase pause release.