[Show abstract][Hide abstract] ABSTRACT: Robust development of the early embryo may benefit from mechanisms that ensure that not all pluripotent cells differentiate at exactly the same time: such mechanisms would build flexibility into the process of lineage allocation. This idea is supported by the observation that pluripotent stem cells differentiate at different rates in vitro. We use a clonal commitment assay to confirm that pluripotent cells commit to differentiate asynchronously even under uniform differentiation conditions. Stochastic variability in expression of the Notch target gene Hes1 has previously been reported to influence neural vs. mesodermal differentiation through modulation of Notch activity. Here we report that Hes1 also has an earlier role to delay exit from the pluripotent state into all lineages. The early function of Hes1 to delay differentiation can be explained by an ability of Hes1 to amplify STAT3 responsiveness in a cell-autonomous manner. Variability in Hes1 expression therefore helps to explain why STAT3 responsiveness varies between individual ES cells, and this in turn helps to explain why pluripotent cells commit to differentiate asynchronously.
[Show abstract][Hide abstract] ABSTRACT: The events that prime pluripotent cells for differentiation are not well understood. Inhibitor of DNA binding/differentiation (Id) proteins, which are inhibitors of basic helix-loop-helix (bHLH) transcription factor activity, contribute to pluripotency by blocking sequential transitions toward differentiation. Using yeast-two-hybrid screens, we have identified Id-regulated transcription factors that are expressed in embryonic stem cells (ESCs). One of these, Tcf15, is also expressed in the embryonic day 4.5 embryo and is specifically associated with a novel subpopulation of primed ESCs. An Id-resistant form of Tcf15 rapidly downregulates Nanog and accelerates somatic lineage commitment. We propose that because Tcf15 can be held in an inactive state through Id activity, it may prime pluripotent cells for entry to somatic lineages upon downregulation of Id. We also find that Tcf15 expression is dependent on fibroblast growth factor (FGF) signaling, providing an explanation for how FGF can prime for differentiation without driving cells out of the pluripotent state.
[Show abstract][Hide abstract] ABSTRACT: Transgenesis is a cornerstone of molecular biology. The ability to integrate a specifically engineered piece of DNA into the
genome of a living system is fundamental to our efforts to understand life and exploit its implications for medicine, nanotechnology
and bioprospecting. However, transgenesis has been hampered by position effects and multi-copy integration problems, which
are mainly due to the use of small, plasmid-based transgenes. Large transgenes based on native genomic regions cloned into
bacterial artificial chromosomes (BACs) circumvent these problems but are prone to fragmentation. Herein, we report that contrary
to widely held notions, large BAC-sized constructs do not prohibit transposition. We also report the first reliable method
for BAC transgenesis in human embryonic stem cells (hESCs). The PiggyBac or Sleeping Beauty transposon inverted repeats were integrated into BAC vectors by recombineering, followed by co-lipofection with the corresponding
transposase in hESCs to generate robust fluorescent protein reporter lines for OCT4, NANOG, GATA4 and PAX6. BAC transposition
delivers several advantages, including increased frequencies of single-copy, full-length integration, which will be useful
in all transgenic systems but especially in difficult venues like hESCs.
Full-text · Article · Jun 2012 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Mouse models have become an invaluable tool for understanding human health and disease owing to our ability to manipulate the mouse genome exquisitely. Recent progress in genomic analysis has led to an increase in the number and type of disease-causing mutations detected and has also highlighted the importance of non-coding regions. As a result, there is increasing interest in creating 'genomically' humanized mouse models, in which entire human genomic loci are transferred into the mouse genome. The technical challenges towards achieving this aim are large but are starting to be tackled with success.
No preview · Article · Jan 2012 · Nature Reviews Genetics
[Show abstract][Hide abstract] ABSTRACT: The role of DNA sequence in determining chromatin state is incompletely understood. We have previously demonstrated that large chromosomal segments from human cells recapitulate their native chromatin state in mouse cells, but the relative contribution of local sequences versus their genomic context remains unknown. In this study, we compare orthologous chromosomal regions for which the human locus establishes prominent sites of Polycomb complex recruitment in pluripotent stem cells, whereas the corresponding mouse locus does not. Using recombination-mediated cassette exchange at the mouse locus, we establish the primacy of local sequences in the encoding of chromatin state. We show that the signal for chromatin bivalency is redundantly encoded across a bivalent domain and that this reflects competition between Polycomb complex recruitment and transcriptional activation. Furthermore, our results suggest that a high density of unmethylated CpG dinucleotides is sufficient for vertebrate Polycomb recruitment. This model is supported by analysis of DNA methyltransferase-deficient embryonic stem cells.
[Show abstract][Hide abstract] ABSTRACT: The forkhead transcription factor Foxn1 is indispensable for thymus development, but the mechanisms by which it mediates thymic epithelial cell (TEC) development are poorly understood. To examine the cellular and molecular basis of Foxn1 function, we generated a novel and revertible hypomorphic allele of Foxn1. By varying levels of its expression, we identified a number of features of the Foxn1 system. Here we show that Foxn1 is a powerful regulator of TEC differentiation that is required at multiple intermediate stages of TE lineage development in the fetal and adult thymus. We find no evidence for a role for Foxn1 in TEC fate-choice. Rather, we show it is required for stable entry into both the cortical and medullary TEC differentiation programmes and subsequently is needed at increasing dosage for progression through successive differentiation states in both cortical and medullary TEC. We further demonstrate regulation by Foxn1 of a suite of genes with diverse roles in thymus development and/or function, suggesting it acts as a master regulator of the core thymic epithelial programme rather than regulating a particular aspect of TEC biology. Overall, our data establish a genetics-based model of cellular hierarchies in the TE lineage and provide mechanistic insight relating titration of a single transcription factor to control of lineage progression. Our novel revertible hypomorph system may be similarly applied to analyzing other regulators of development.
[Show abstract][Hide abstract] ABSTRACT: Remote distal enhancers may be located tens or thousands of kilobases away from their promoters. How they control gene expression is still poorly understood. Here, we analyze the influence of a remote enhancer on the balance between repression (Polycomb-PcG) and activation (Trithorax-TrxG) of a developmentally regulated gene associated with a CpG island. We reveal its essential, nonredundant role in clearing the PcG complex and H3K27me3 from the CpG island. In the absence of the enhancer, the H3K27me3 demethylase (JMJD3) is not recruited to the CpG island. We propose a new role of long-range regulatory elements in removing repressive PcG complexes.
Full-text · Article · Aug 2011 · Genes & development
[Show abstract][Hide abstract] ABSTRACT: PI3K signaling is thought to mediate leptin and insulin action in hypothalamic pro-opiomelanocortin (POMC) and agouti-related protein (AgRP) neurons, key regulators of energy homeostasis, through largely unknown mechanisms. We inactivated either p110alpha or p110beta PI3K catalytic subunits in these neurons and demonstrate a dominant role for the latter in energy homeostasis regulation. In POMC neurons, p110beta inactivation prevented insulin- and leptin-stimulated electrophysiological responses. POMCp110beta null mice exhibited central leptin resistance, increased adiposity, and diet-induced obesity. In contrast, the response to leptin was not blocked in p110alpha-deficient POMC neurons. Accordingly, POMCp110alpha null mice displayed minimal energy homeostasis abnormalities. Similarly, in AgRP neurons, p110beta had a more important role than p110alpha. AgRPp110alpha null mice displayed normal energy homeostasis regulation, whereas AgRPp110beta null mice were lean, with increased leptin sensitivity and resistance to diet-induced obesity. These results demonstrate distinct metabolic roles for the p110alpha and p110beta isoforms of PI3K in hypothalamic energy regulation.
[Show abstract][Hide abstract] ABSTRACT: Previous studies in the mouse have shown that high levels of alpha-globin gene expression in late erythropoiesis depend on long-range, physical interactions between remote upstream regulatory elements and the globin promoters. Using quantitative chromosome conformation capture (q3C), we have now analyzed all interactions between 4 such elements lying 10 to 50 kb upstream of the human alpha cluster and their interactions with the alpha-globin promoter. All of these elements interact with the alpha-globin gene in an erythroid-specific manner. These results were confirmed in a mouse model of human alpha globin expression in which the human cluster replaces the mouse cluster in situ (humanized mouse). We have also shown that expression and all of the long-range interactions depend largely on just one of these elements; removal of the previously characterized major regulatory element (called HS -40) results in loss of all the interactions and alpha-globin expression. Reinsertion of this element at an ectopic location restores both expression and the intralocus interactions. In contrast to other more complex systems involving multiple upstream elements and promoters, analysis of the human alpha-globin cluster during erythropoiesis provides a simple and tractable model to understand the mechanisms underlying long-range gene regulation.
[Show abstract][Hide abstract] ABSTRACT: Genes on different chromosomes can be spatially associated in the nucleus in several transcriptional and regulatory situations; however, the functional significance of such associations remains unclear. Using human erythropoiesis as a model, we show that five cotranscribed genes, which are found on four different chromosomes, associate with each other at significant but variable frequencies. Those genes most frequently in association lie in decondensed stretches of chromatin. By replacing the mouse alpha-globin gene cluster in situ with its human counterpart, we demonstrate a direct effect of the regional chromatin environment on the frequency of association, whereas nascent transcription from the human alpha-globin gene appears unaffected. We see no evidence that cotranscribed erythroid genes associate at shared transcription foci, but we do see stochastic clustering of active genes around common nuclear SC35-enriched speckles (hence the apparent nonrandom association between genes). Thus, association between active genes may result from their location on decondensed chromatin that enables clustering around common nuclear speckles.
Full-text · Article · Oct 2008 · The Journal of Cell Biology
[Show abstract][Hide abstract] ABSTRACT: The p110 isoforms of phosphoinositide 3-kinase (PI3K) are acutely regulated by extracellular stimuli. The class IA PI3K catalytic subunits (p110alpha, p110beta, and p110delta) occur in complex with a Src homology 2 (SH2) domain-containing p85 regulatory subunit, which has been shown to link p110alpha and p110delta to Tyr kinase signaling pathways. The p84/p101 regulatory subunits of the p110gamma class IB PI3K lack SH2 domains and instead couple p110gamma to G protein-coupled receptors (GPCRs). Here, we show, using small-molecule inhibitors with selectivity for p110beta and cells derived from a p110beta-deficient mouse line, that p110beta is not a major effector of Tyr kinase signaling but couples to GPCRs. In macrophages, both p110beta and p110gamma contributed to Akt activation induced by the GPCR agonist complement 5a, but not by the Tyr kinase ligand colony-stimulating factor-1. In fibroblasts, which express p110beta but not p110gamma, p110beta mediated Akt activation by the GPCR ligands stromal cell-derived factor, sphingosine-1-phosphate, and lysophosphatidic acid but not by the Tyr kinase ligands PDGF, insulin, and insulin-like growth factor 1. Introduction of p110gamma in these cells reduced the contribution of p110beta to GPCR signaling. Taken together, these data show that p110beta and p110gamma can couple redundantly to the same GPCR agonists. p110beta, which shows a much broader tissue distribution than the leukocyte-restricted p110gamma, could thus provide a conduit for GPCR-linked PI3K signaling in the many cell types where p110gamma expression is low or absent.
Full-text · Article · Jul 2008 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Phosphoinositide 3-kinases (PI3Ks) signal downstream of multiple cell-surface receptor types. Class IA PI3K isoforms couple to tyrosine kinases and consist of a p110 catalytic subunit (p110alpha, p110beta or p110delta), constitutively bound to one of five distinct p85 regulatory subunits. PI3Ks have been implicated in angiogenesis, but little is known about potential selectivity among the PI3K isoforms and their mechanism of action in endothelial cells during angiogenesis in vivo. Here we show that only p110alpha activity is essential for vascular development. Ubiquitous or endothelial cell-specific inactivation of p110alpha led to embryonic lethality at mid-gestation because of severe defects in angiogenic sprouting and vascular remodelling. p110alpha exerts this critical endothelial cell-autonomous function by regulating endothelial cell migration through the small GTPase RhoA. p110alpha activity is particularly high in endothelial cells and preferentially induced by tyrosine kinase ligands (such as vascular endothelial growth factor (VEGF)-A). In contrast, p110beta in endothelial cells signals downstream of G-protein-coupled receptor (GPCR) ligands such as SDF-1alpha, whereas p110delta is expressed at low level and contributes only minimally to PI3K activity in endothelial cells. These results provide the first in vivo evidence for p110-isoform selectivity in endothelial PI3K signalling during angiogenesis.
[Show abstract][Hide abstract] ABSTRACT: PAP/HIP was first reported as an additional pancreatic secretory protein expressed during the acute phase of pancreatitis. It was shown in vitro to be anti-apoptotic and anti-inflammatory. This study aims to look at whether PAP/HIP plays the same role in vivo.
A model of caerulein-induced pancreatitis was used to compare the outcome of pancreatitis in PAP/HIP(-/-) and wild-type mice.
PAP/HIP(-/-) mice showed the normal phenotype at birth and normal postnatal development. Caerulein-induced pancreatic necrosis was, however, less severe in PAP/HIP(-/-) mice than in wild-type mice, as judged by lower amylasemia and lipasemia levels and smaller areas of necrosis. On the contrary, pancreas from PAP/HIP(-/-) mice was more sensitive to apoptosis, in agreement with the anti-apoptotic effect of PAP/HIP in vitro. Surprisingly, pancreatic inflammation was more extensive in PAP/HIP(-/-) mice, as judged from histological parameters, increased myeloperoxidase activity and increased pro-inflammatory cytokine expression. This result, in apparent contradiction with the limited necrosis observed in these mice, is, however, in agreement with the anti-inflammatory function previously reported in vitro for PAP/HIP. This is supported by the observation that activation of the STAT3/SOCS3 pathway was strongly decreased in the pancreas of PAP/HIP(-/-) mice and by the reversion of the apoptotic and inflammatory phenotypes upon administration of recombinant PAP/HIP to PAP/HIP(-/-) mice.
The anti-apoptotic and anti-inflammatory functions described in vitro for PAP/HIP have physiological relevance in the pancreas in vivo during caerulein-induced pancreatitis.
[Show abstract][Hide abstract] ABSTRACT: The expression and function of the 8 distinct catalytic isoforms of PI 3-kinase (PI3K) in the nervous system are unknown. Whereas most PI3Ks have a broad tissue distribution, the tyrosine kinase-linked p110delta isoform has previously been shown to be enriched in leukocytes. Here we report that p110delta is also highly expressed in the nervous system. Inactivation of p110delta in mice did not affect gross neuronal development but led to an increased vulnerability of dorsal root ganglia neurons to exhibit growth cone collapse and decreases in axonal extension. Loss of p110delta activity also dampened axonal regeneration following peripheral nerve injury in adult mice and impaired functional recovery of locomotion. p110delta inactivation resulted in reduced neuronal signaling through the Akt protein kinase, and increased activity of the small GTPase RhoA. Pharmacological inhibition of ROCK, a downstream effector of RhoA, restored axonal extension defects in neurons with inactive p110delta, suggesting a key role of RhoA in p110delta signaling in neurons. Our data identify p110delta as an important signaling component for efficient axonal elongation in the developing and regenerating nervous system.
[Show abstract][Hide abstract] ABSTRACT: We have devised a strategy (called recombinase-mediated genomic replacement, RMGR) to allow the replacement of large segments (>100 kb) of the mouse genome with the equivalent human syntenic region. The technique involves modifying a mouse ES cell chromosome and a human BAC by inserting heterotypic lox sites to flank the proposed exchange interval and then using Cre recombinase to achieve segmental exchange. We have demonstrated the feasibility of this approach by replacing the mouse alpha globin regulatory domain with the human syntenic region and generating homozygous mice that produce only human alpha globin chains. Furthermore, modified ES cells can be used iteratively for functional studies, and here, as an example, we have used RMGR to produce an accurate mouse model of human alpha thalassemia. RMGR has general applicability and will overcome limitations inherent in current transgenic technology when studying the expression of human genes and modeling human genetic diseases.
[Show abstract][Hide abstract] ABSTRACT: Reg2/RegIIIbeta is the murine homologue of the human secreted HIP/PAP C-type lectin. HIP/PAP transgenic mice were protected against acetaminophen-induced acute liver failure and were stimulated to regenerate post-hepatectomy. To assess the role of Reg2, we used Reg2-/- mice in a model of fulminant hepatitis induced by Fas and in the post-hepatectomy regeneration. Within 4 hours of J0-2 treatment (0.5 microg/g), only 50% of the Reg2-/- mice were alive but with an increased sensitivity to Fas-induced oxidative stress and a decreased level of Bcl-xL. In contrast, HIP/PAP transgenic mice were resistant to Fas, with HIP/PAP serving as a sulfhydryl buffer to slow down decreases in glutathione and Bcl-xL. In Reg2-/- mice, liver regeneration was markedly impaired, with 29% mortality and delay of the S-phase and the activation of ERK1/2 and AKT. Activation of STAT3 began on time at 3 hours but persisted strongly up to 72 hours despite significant accumulation of SOCS3. Thus, Reg2 deficiency induced exaggerated IL-6/STAT-3 activation and mito-inhibition. Because the Reg2 gene was activated between 6 and 24 hours after hepatectomy in wild-type mice, Reg2 could mediate the TNF-alpha/IL-6 priming signaling by exerting a negative feed-back on STAT3/IL-6 activation to allow the hepatocytes to progress through the cell cycle. In conclusion, Reg2 deficiency enhanced liver sensitivity to Fas-induced oxidative stress and delayed liver regeneration with persistent TNF-alpha/IL6/STAT3 signaling. In contrast, overexpression of human HIP/PAP promoted liver resistance to Fas and accelerated liver regeneration with early activation/deactivation of STAT3. Reg2/HIP/PAP is therefore a critical mitogenic and antiapoptotic factor for the liver.
[Show abstract][Hide abstract] ABSTRACT: Titin, a multifunctional protein that stretches from the Z-disk to the M-band in heart and skeletal muscle, contains a kinase domain, phosphorylation sites and multiple binding sites for structural and signalling proteins in the M-band. To determine whether this region is crucial for normal sarcomere development, we created mouse embryonic stem cell (ES) lines in which either one or both alleles contained a targeted deletion of the entire M-band-coding region, leaving Z-disk-binding and myosin-filament-binding sites intact. ES cells were differentiated into cardiomyocytes, and myofibrillogenesis investigated by immunofluorescence microscopy. Surprisingly, deletion of one allele did not markedly affect differentiation into cardiomyocytes, suggesting that a single intact copy of the titin gene is sufficient for normal myofibrillogenesis. By contrast, deletion of both alleles resulted in a failure of differentiation beyond an early stage of myofibrillogenesis. Sarcomeric myosin remained in non-striated structures, Z-disk proteins, such as alpha-actinin, were mainly found in primitive dot-like structures on actin stress fibres, M-band-associated proteins (myomesin, obscurin, Nbr1, p62 and MURF2) remained punctate. These results show that integration of the M-band region of titin is required for myosin filament assembly, M-band formation and maturation of the Z-disk.
Full-text · Article · Nov 2006 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: The eight catalytic subunits of the mammalian phosphoinositide-3-OH kinase (PI(3)K) family form the backbone of an evolutionarily conserved signalling pathway; however, the roles of most PI(3)K isoforms in organismal physiology and disease are unknown. To delineate the role of p110alpha, a ubiquitously expressed PI(3)K involved in tyrosine kinase and Ras signalling, here we generated mice carrying a knockin mutation (D933A) that abrogates p110alpha kinase activity. Homozygosity for this kinase-dead p110alpha led to embryonic lethality. Mice heterozygous for this mutation were viable and fertile, but displayed severely blunted signalling via insulin-receptor substrate (IRS) proteins, key mediators of insulin, insulin-like growth factor-1 and leptin action. Defective responsiveness to these hormones led to reduced somatic growth, hyperinsulinaemia, glucose intolerance, hyperphagia and increased adiposity in mice heterozygous for the D933A mutation. This signalling function of p110alpha derives from its highly selective recruitment and activation to IRS signalling complexes compared to p110beta, the other broadly expressed PI(3)K isoform, which did not contribute to IRS-associated PI(3)K activity. p110alpha was the principal IRS-associated PI(3)K in cancer cell lines. These findings demonstrate a critical role for p110alpha in growth factor and metabolic signalling and also suggest an explanation for selective mutation or overexpression of p110alpha in a variety of cancers.
[Show abstract][Hide abstract] ABSTRACT: Synopsis
ATRX belongs to a class of proteins that may modify how DNA is packaged into chromatin, altering the accessibility of other proteins in the nucleus to DNA. In this way, ATRX is thought to influence gene expression. Mutations in the ATRX gene, which is located on the female sex chromosome (X), provided the first example of a human disease (ATR-X syndrome) associated with defects in such proteins. Affected males (XMUTY) have multiple developmental abnormalities in a wide variety of systems. Currently, it is not understood how proteins like ATRX influence cell biology. To address this question, the authors deleted the version of the gene in mice, Atrx. Although affected male mice (XMUTY) started to develop normally, they died early in development because they failed to form a normal placenta. In the placenta, female mice normally inactivate the X chromosome that they inherit from their fathers (Xp), so if females inherit from their mother an X chromosome (Xm) that bears the abnormal copy of Atrx (XmMUTXp), one would predict that, like affected males, they would fail to form a normal placenta. The authors unexpectedly found this not to be so. They showed, instead, that in such females the normal, paternally derived Atrx gene is active. This study has therefore demonstrated an important facet of X-chromosome imprinting.