Myong-Hee Sung

Extensive chromatin remodelling and establishment of transcription factor ‘hotspots’ during early adipogenesis

Adipogenesis is tightly controlled by a complex network of transcription factors acting at different stages of differentiation. Peroxisome proliferator‐activated receptor γ (PPARγ) and CCAAT/enhancer‐binding protein (C/EBP) family members are key regulators of this process. We have employed DNase I hypersensitive site analysis to investigate the genome‐wide changes in chromatin structure that accompany the binding of adipogenic transcription factors. These analyses revealed a dramatic and dynamic modulation of the chromatin landscape during the first hours of adipocyte differentiation that coincides with cooperative binding of multiple early transcription factors (including glucocorticoid receptor, retinoid X receptor, Stat5a, C/EBPβ and ‐δ) to transcription factor ‘hotspots’. Our results demonstrate that C/EBPβ marks a large number of these transcription factor ‘hotspots’ before induction of differentiation and chromatin remodelling and is required for their establishment. Furthermore, a subset of early remodelled C/EBP‐binding sites persists throughout differentiation and is later occupied by PPARγ, indicating that early C/EBP family members, in addition to their well‐established role in activation of PPARγ transcription, may act as pioneering factors for PPARγ binding.

Expansion and Functional Relevance of High-Avidity Myelin-Specific CD4+ T Cells in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease in which myelin-specific T cells are believed to play a crucial pathogenic role. Nevertheless, so far it has been extremely difficult to demonstrate differences in T cell reactivity to myelin Ag between MS patients and controls. We believe that by using unphysiologically high Ag concentrations previous studies have missed a highly relevant aspect of autoimmune responses, i.e., T cells recognizing Ag with high functional avidity. Therefore, we focused on the characterization of high-avidity myelin-specific CD4+ T cells in a large cohort of MS patients and controls that was matched demographically and with respect to expression of MHC class II alleles. We demonstrated that their frequency is significantly higher in MS patients while the numbers of control T cells specific for influenza hemagglutinin are virtually identical between the two cohorts; that high-avidity T cells are enriched for previously in vivo-activated cells and are significantly skewed toward a proinflammatory phenotype. Moreover, the immunodominant epitopes that were most discriminatory between MS patients and controls differed from those described previously and were clearly biased toward epitopes with lower predicted binding affinities to HLA-DR molecules, pointing at the importance of thymic selection for the generation of the autoimmune T cell repertoire. Correlations between selected immunological parameters and magnetic resonance imaging markers indicate that the specificity and function of these cells influences phenotypic disease expression. These data have important implications for autoimmunity research and should be considered in the development of Ag-specific therapies in MS.

In silico identification of the key components and steps in IFN-γ induced JAK-STAT signaling pathway

Systems biology efforts are increasingly adopting quantitative, mechanistic modeling to study cellular signal transduction pathways and other networks. However, it is uncertain whether the particular set of kinetic parameter values of the model closely approximates the corresponding biological system. We propose that the parameters be assigned statistical distributions that reflect the degree of uncertainty for a comprehensive simulation analysis. From this analysis, we globally identify the key components and steps in signal transduction networks at a systems level. We investigated a recent mathematical model of interferon gamma induced Janus kinase‐signal transducers and activators of transcription (JAK‐STAT) signaling pathway by applying multi‐parametric sensitivity analysis that is based on simultaneous variation of the parameter values. We find that suppressor of cytokine signaling‐1, nuclear phosphatases, cytoplasmic STAT1, and the corresponding reaction steps are sensitive perturbation points of this pathway.

Kinetic Complexity of the Global Response to Glucocorticoid Receptor Action

We have characterized the kinetic response of gene targets throughout the murine genome to transcriptional modulation by the glucocorticoid receptor (GR). In contrast to a model in which multiple genes are either repressed or activated during the GR response, the vast majority of responsive genes are subject to complex regulation profiles, frequently with alternate activation and repression phases. We also observe that GR binding at response elements does not always correlate with the target gene response profile. Thus, the cellular response to GR stimulation involves a highly orchestrated series of regulatory actions and not simply a binary response to hormone.

A Macrohistone Variant Links Dynamic Chromatin Compaction to BRCA1-Dependent Genome Maintenance

SUMMARY Appropriate DNA double-strand break (DSB) repair factor choice is essential for ensuring accurate repair outcome and genomic integrity. The factors that regulate this process remain poorly understood. Here, we identify two repressive chromatin components, the macrohistone variant macroH2A1 and the H3K9 methyltransferase and tumor suppressor PRDM2, which together direct the choice between the antagonistic DSB repair mediators BRCA1 and 53BP1. The macroH2A1/PRDM2 module mediates an unexpected shift from accessible to condensed chromatin that requires the ataxia telangiectasia mutated (ATM)-dependent accumulation of both proteins at DSBs in order to promote DSB-flanking H3K9 dimethylation. Remarkably, loss of macroH2A1 or PRDM2, as well as experimentally induced chromatin decondensation, impairs the retention of BRCA1, but not 53BP1, at DSBs. As a result, mac-roH2A1 and/or PRDM2 depletion causes epistatic defects in DSB end resection, homology-directed repair, and the resistance to poly(ADP-ribose) polymerase (PARP) inhibition—all hallmarks of BRCA1-deficient tumors. Together, these findings identify dynamic, DSB-associated chromatin reorganization as a critical modulator of BRCA1-dependent genome maintenance.

Overlapping chromatin-remodeling systems collaborate genome wide at dynamic chromatin transitions

ATP-dependent chromatin remodeling is an essential process required for the dynamic organization of chromatin structure. Here we describe the genome-wide location and activity of three remodeler proteins with diverse physiological functions in the mouse genome: Brg1, Chd4 and Snf2h. The localization patterns of all three proteins substantially overlap with one another and with regions of accessible chromatin. Furthermore, using inducible mutant variants, we demonstrate that the catalytic activity of these proteins contributes to the remodeling of chromatin genome wide and that each of these remodelers can independently regulate chromatin reorganization at distinct sites. Many regions require the activity of more than one remodeler to regulate accessibility. These findings provide a dynamic view of chromatin organization and highlight the differential contributions of remodelers to chromatin maintenance in higher eukaryotes.

DNase footprint signatures are dictated by factor dynamics and DNA sequence.

Genomic footprinting has emerged as an unbiased discovery method for transcription factor (TF) occupancy at cognate DNA in vivo. A basic premise of footprinting is that sequence-specific TF-DNA interactions are associated with localized resistance to nucleases, leaving observable signatures of cleavage within accessible chromatin. This phenomenon is interpreted to imply protection of the critical nucleotides by the stably bound protein factor. However, this model conflicts with previous reports of many TFs exchanging with specific binding sites in living cells on a timescale of seconds. We show that TFs with short DNA residence times have no footprints at bound motif elements. Moreover, the nuclease cleavage profile within a footprint originates from the DNA sequence in the factor-binding site, rather than from the protein occupying specific nucleotides. These findings suggest a revised understanding of TF footprinting and reveal limitations in comprehensive reconstruction of the TF regulatory network using this approach.

Wnt pathway mutations selected by optimal β-catenin signaling for tumorigenesis

Mutations in components of the Wnt/β‐catenin pathway are observed to be the earliest initiating event for most colorectal tumors. The majority of the mutations occur in the tumor suppressor adenomatous polyposis coli (APC), even though there are other genes that are capable of modulating the pathway activity. Moreover, the specific APC mutations associated in colon cancer indicate the possibility that the tumor selects for certain truncated forms of APC that partially retain its function, namely, inhibition of β‐catenin. We estimated the effects of various mutations in APC and other known mutations using a recent mathematical model of the Wnt pathway that was constructed to represent the conserved core molecular events. We provide evidence that APC mutations are selected not based on the maximal level of β‐catenin but rather based on distinct state of activity that appears to be optimal for the tissue‐specific tumorigenesis. This optimal level is determined by balancing β‐catenin signaling and the induction of Axin2 that acts as a potent negative feedback. The predominant pattern of APC mutations may provide synergistic oncogenic effects that promote colorectal tumorigenesis: the optimal signaling for cell survival and renewal, disrupted cell adhesion, chromosomal instability, and altered asymmetric division of stem cells.

Switching of the Relative Dominance Between Feedback Mechanisms in Lipopolysaccharide-Induced NF-κB Signaling

Dueling feedback loops set the threshold for mounting an effective innate immune response to infection. Rewiring NF-κB Signaling The bacterial product lipopolysaccharide (LPS) stimulates nuclear factor κB (NF-κB) signaling, which results in the production of proinflammatory cytokines, such as tumor necrosis factor–α (TNF-α), as part of the immune response. NF-κB target genes also include those encoding proteins that inhibit NF-κB signaling through negative feedback loops. By simultaneously studying the dynamics of the nuclear translocation of the NF-κB subunit RelA and the activity of a Tnf-driven reporter in a mouse macrophage cell line, Sung et al. found that the gene encoding RelA was also a target of NF-κB. Synthesis of RelA occurred only at higher concentrations of LPS and constituted a positive feedback loop that dominated over existing negative feedback mechanisms. Genes expressed in response to a high concentration of LPS were enriched for those involved in innate immune responses. Together, these data suggest that the RelA-mediated positive feedback loop enables macrophages to mount an effective immune response only above a critical concentration of LPS. A fundamental goal in biology is to gain a quantitative understanding of how appropriate cell responses are achieved amid conflicting signals that work in parallel. Through live, single-cell imaging, we monitored both the dynamics of nuclear factor κB (NF-κB) signaling and inflammatory cytokine transcription in macrophages exposed to the bacterial product lipopolysaccharide (LPS). Our analysis revealed a previously uncharacterized positive feedback loop involving induction of the expression of Rela, which encodes the RelA (p65) NF-κB subunit. This positive feedback loop rewired the regulatory network when cells were exposed to LPS above a distinct concentration. Paradoxically, this rewiring of NF-κB signaling in macrophages (a myeloid cell type) required the transcription factor Ikaros, which promotes the development of lymphoid cells. Mathematical modeling and experimental validation showed that the RelA positive feedback overcame existing negative feedback loops and enabled cells to discriminate between different concentrations of LPS to mount an effective innate immune response only at higher concentrations. We suggest that this switching in the relative dominance of feedback loops (“feedback dominance switching”) may be a general mechanism in immune cells to integrate opposing feedback on a key transcriptional regulator and to set a response threshold for the host.

Reprogramming the Chromatin Landscape: Interplay of the Estrogen and Glucocorticoid Receptors at the Genomic Level

Cross-talk between estrogen receptors (ER) and glucocorticoid receptors (GR) has been shown to contribute to the development and progression of breast cancer. Importantly, the ER and GR status in breast cancer cells is a significant factor in determining the outcome of the disease. However, mechanistic details defining the cellular interactions between ER and GR are poorly understood. We investigated genome-wide binding profiles for ER and GR upon coactivation and characterized the status of the chromatin landscape. We describe a novel mechanism dictating the molecular interplay between ER and GR. Upon induction, GR modulates access of ER to specific sites in the genome by reorganization of the chromatin configuration for these elements. Binding to these newly accessible sites occurs either by direct recognition of ER response elements or indirectly through interactions with other factors. The unveiling of this mechanism is important for understanding cellular interactions between ER and GR and may represent a general mechanism for cross-talk between nuclear receptors in human disease.