Phase separation of Polycomb-repressive complex 1 is governed by a charged disordered region of CBX2.

Mammalian development requires effective mechanisms to repress genes whose expression would generate inappropriately specified cells. The Polycomb-repressive complex 1 (PRC1) family complexes are central to maintaining this repression. These include a set of canonical PRC1 complexes, each of which contains four core proteins, including one from the CBX family. These complexes have been shown previously to reside in membraneless organelles called Polycomb bodies, leading to speculation that canonical PRC1 might be found in a separate phase from the rest of the nucleus. We show here that reconstituted PRC1 readily phase-separates into droplets in vitro at low concentrations and physiological salt conditions. This behavior is driven by the CBX2 subunit. Point mutations in an internal domain of Cbx2 eliminate phase separation. These same point mutations eliminate the formation of puncta in cells and have been shown previously to eliminate nucleosome compaction in vitro and generate axial patterning defects in mice. Thus, the domain of CBX2 that is important for phase separation is the same domain shown previously to be important for chromatin compaction and proper development, raising the possibility of a mechanistic or evolutionary link between these activities.

Lncing epigenetic control of transcription to cardiovascular development and disease.

Transcriptional and epigenetic regulation is critical for proper heart development, cardiac homeostasis, and pathogenesis. Long noncoding RNAs have emerged as key components of the transcriptional regulatory pathways that govern cardiac development as well as stress response, signaling, and remodeling in cardiac pathologies. Within the past few years, studies have identified many long noncoding RNAs in the context of cardiovascular biology and have begun to reveal the key functions of these transcripts. In this review, we discuss the growing roles of long noncoding RNAs in different aspects of cardiovascular development as well as pathological responses during injury or disease. In addition, we discuss diverse mechanisms by which long noncoding RNAs orchestrate cardiac transcriptional programs. Finally, we explore the exciting potential of this novel class of transcripts as biomarkers and novel therapeutic targets for cardiovascular diseases.

The evolutionarily conserved longevity determinants HCF-1 and SIR-2.1/SIRT1 collaborate to regulate DAF-16/FOXO.

The conserved DAF-16/FOXO transcription factors and SIR-2.1/SIRT1 deacetylases are critical for diverse biological processes, particularly longevity and stress response; and complex regulation of DAF-16/FOXO by SIR-2.1/SIRT1 is central to appropriate biological outcomes. Caenorhabditis elegans Host Cell Factor 1 (HCF-1) is a longevity determinant previously shown to act as a co-repressor of DAF-16. We report here that HCF-1 represents an integral player in the regulatory loop linking SIR-2.1/SIRT1 and DAF-16/FOXO in both worms and mammals. Genetic analyses showed that hcf-1 acts downstream of sir-2.1 to influence lifespan and oxidative stress response in C. elegans. Gene expression profiling revealed a striking 80% overlap between the DAF-16 target genes responsive to hcf-1 mutation and sir-2.1 overexpression. Subsequent GO-term analyses of HCF-1 and SIR-2.1-coregulated DAF-16 targets suggested that HCF-1 and SIR-2.1 together regulate specific aspects of DAF-16-mediated transcription particularly important for aging and stress responses. Analogous to its role in regulating DAF-16/SIR-2.1 target genes in C. elegans, the mammalian HCF-1 also repressed the expression of several FOXO/SIRT1 target genes. Protein-protein association studies demonstrated that SIR-2.1/SIRT1 and HCF-1 form protein complexes in worms and mammalian cells, highlighting the conservation of their regulatory relationship. Our findings uncover a conserved interaction between the key longevity determinants SIR-2.1/SIRT1 and HCF-1, and they provide new insights into the complex regulation of FOXO proteins.

High-content synaptic phenotyping in human cellular models reveals a role for BET proteins in synapse assembly.

Resolving fundamental molecular and functional processes underlying human synaptic development is crucial for understanding normal brain function as well as dysfunction in disease. Based upon increasing evidence of species-divergent features of brain cell types, coupled with emerging studies of complex human disease genetics, we developed the first automated and quantitative high-content synaptic phenotyping platform using human neurons and astrocytes. To establish the robustness of our platform, we screened the effects of 376 small molecules on presynaptic density, neurite outgrowth, and cell viability, validating six small molecules that specifically enhanced human presynaptic density in vitro. Astrocytes were essential for mediating the effects of all six small molecules, underscoring the relevance of non-cell-autonomous factors in synapse assembly and their importance in synaptic screening applications. Bromodomain and extraterminal (BET) inhibitors emerged as the most prominent hit class and global transcriptional analyses using multiple BET inhibitors confirmed upregulation of synaptic gene expression. Through these analyses, we demonstrate the robustness of our automated screening platform for identifying potent synaptic modulators, which can be further leveraged for scaled analyses of human synaptic mechanisms and drug discovery efforts.

Caenorhabditis elegans HCF-1 functions in longevity maintenance as a DAF-16 regulator.

The transcription factor DAF-16/forkhead box O (FOXO) is a critical longevity determinant in diverse organisms, however the molecular basis of how its transcriptional activity is regulated remains largely unknown. We report that the Caenorhabditis elegans homolog of host cell factor 1 (HCF-1) represents a new longevity modulator and functions as a negative regulator of DAF-16. In C. elegans, hcf-1 inactivation caused a daf-16-dependent lifespan extension of up to 40% and heightened resistance to specific stress stimuli. HCF-1 showed ubiquitous nuclear localization and physically associated with DAF-16. Furthermore, loss of hcf-1 resulted in elevated DAF-16 recruitment to the promoters of its target genes and altered expression of a subset of DAF-16-regulated genes. We propose that HCF-1 modulates C. elegans longevity and stress response by forming a complex with DAF-16 and limiting a fraction of DAF-16 from accessing its target gene promoters, and thereby regulates DAF-16-mediated transcription of selective target genes. As HCF-1 is highly conserved, our findings have important implications for aging and FOXO regulation in mammals.

Regulation of histone deposition proteins Asf1/Hir1 by multiple DNA damage checkpoint kinases in Saccharomyces cerevisiae.

CAF-1, Hir proteins, and Asf1 are histone H3/H4 binding proteins important for chromatin-mediated transcriptional silencing. We explored genetic and physical interactions between these proteins and S-phase/DNA damage checkpoint kinases in the budding yeast Saccharomyces cerevisiae. Although cells lacking checkpoint kinase Mec1 do not display defects in telomeric gene silencing, silencing was dramatically reduced in cells lacking both Mec1 and the Cac1 subunit of CAF-1. Silencing was restored in cac1Delta and cac1Delta mec1Delta cells upon deletion of Rad53, the kinase downstream of Mec1. Restoration of silencing to cac1Delta cells required both Hir1 and Asf1, suggesting that Mec1 counteracts functional sequestration of the Asf1/Hir1 complex by Rad53. Consistent with this idea, the degree of suppression of silencing defects by rad53 alleles correlated with effects on Asf1 binding. Furthermore, deletion of the Dun1 kinase, a downstream target of Rad53, also suppressed the silencing defects of cac1Delta cells and reduced the levels of Asf1 associated with Rad53 in vivo. Loss of Mec1 and Rad53 did not alter telomere lengths or Asf1 protein levels, nuclear localization, or chromosome association. We conclude that the Mec1 and Dun1 checkpoint kinases regulate the Asf1-Rad53 interaction and therefore affect the activity of the Asf1/Hir complex in vivo.

Direct conversion of mouse embryonic fibroblasts into functional keratinocytes through transient expression of pluripotency-related genes.

The insufficient ability of specialized cells such as neurons, cardiac myocytes, and epidermal cells to regenerate after tissue damage poses a great challenge to treat devastating injuries and ailments. Recent studies demonstrated that a diverse array of cell types can be directly derived from embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), or somatic cells by combinations of specific factors. The use of iPSCs and direct somatic cell fate conversion, or transdifferentiation, holds great promise for regenerative medicine as these techniques may circumvent obstacles related to immunological rejection and ethical considerations. However, producing iPSC-derived keratinocytes requires a lengthy two-step process of initially generating iPSCs and subsequently differentiating into skin cells, thereby elevating the risk of cellular damage accumulation and tumor formation. In this study, we describe the reprogramming of mouse embryonic fibroblasts into functional keratinocytes via the transient expression of pluripotency factors coupled with directed differentiation. The isolation of an iPSC intermediate is dispensable when using this method. Cells derived with this approach, termed induced keratinocytes (iKCs), morphologically resemble primary keratinocytes. Furthermore they express keratinocyte-specific markers, downregulate mesenchymal markers as well as the pluripotency factors Oct4, Sox2, and Klf4, and they show important functional characteristics of primary keratinocytes. iKCs can be further differentiated by high calcium administration in vitro and are capable of regenerating a fully stratified epidermis in vivo. Efficient conversion of somatic cells into keratinocytes could have important implications for studying genetic skin diseases and designing regenerative therapies to ameliorate devastating skin conditions.

Discovery and validation of sub-threshold genome-wide association study loci using epigenomic signatures.

Genetic variants identified by genome-wide association studies explain only a modest proportion of heritability, suggesting that meaningful associations lie 'hidden' below current thresholds. Here, we integrate information from association studies with epigenomic maps to demonstrate that enhancers significantly overlap known loci associated with the cardiac QT interval and QRS duration. We apply functional criteria to identify loci associated with QT interval that do not meet genome-wide significance and are missed by existing studies. We demonstrate that these 'sub-threshold' signals represent novel loci, and that epigenomic maps are effective at discriminating true biological signals from noise. We experimentally validate the molecular, gene-regulatory, cellular and organismal phenotypes of these sub-threshold loci, demonstrating that most sub-threshold loci have regulatory consequences and that genetic perturbation of nearby genes causes cardiac phenotypes in mouse. Our work provides a general approach for improving the detection of novel loci associated with complex human traits.

Model-Based Tumor Growth Dynamics and Therapy Response in a Mouse Model of De Novo Carcinogenesis.

Tumorigenesis is a complex, multistep process that depends on numerous alterations within the cell and contribution from the surrounding stroma. The ability to model macroscopic tumor evolution with high fidelity may contribute to better predictive tools for designing tumor therapy in the clinic. However, attempts to model tumor growth have mainly been developed and validated using data from xenograft mouse models, which fail to capture important aspects of tumorigenesis including tumor-initiating events and interactions with the immune system. In the present study, we investigate tumor growth and therapy dynamics in a mouse model of de novo carcinogenesis that closely recapitulates tumor initiation, progression and maintenance in vivo. We show that the rate of tumor growth and the effects of therapy are highly variable and mouse specific using a Gompertz model to describe tumor growth and a two-compartment pharmacokinetic/ pharmacodynamic model to describe the effects of therapy in mice treated with 5-FU. We show that inter-mouse growth variability is considerably larger than intra-mouse variability and that there is a correlation between tumor growth and drug kill rates. Our results show that in vivo tumor growth and regression in a double transgenic mouse model are highly variable both within and between subjects and that mathematical models can be used to capture the overall characteristics of this variability. In order for these models to become useful tools in the design of optimal therapy strategies and ultimately in clinical practice, a subject-specific modelling strategy is necessary, rather than approaches that are based on the average behavior of a given subject population which could provide erroneous results.

Host cell factor 1 inhibits SKN-1 to modulate oxidative stress responses in Caenorhabditis elegans.

Host cell factor-1 (HCF-1) is a conserved regulator of the longevity and stress response functions of DAF-16/FOXO. SKN-1 transcription factor is an evolutionarily conserved xenobiotic stress regulator and a pro-longevity factor. Here, we demonstrate that SKN-1 contributes to the enhanced oxidative stress resistance incurred by hcf-1 mutation in C. elegans. HCF-1 prevents the nuclear accumulation of SKN-1 and represses the transcriptional activation of SKN-1 specifically at target genes involved in cellular detoxification pathways. Our findings reveal a novel and context-specific regulatory relationship between two highly conserved longevity and stress response factors HCF-1 and SKN-1.

Mice fed a lipogenic methionine-choline-deficient diet develop hypermetabolism coincident with hepatic suppression of SCD-1.

Lipogenic diets that are completely devoid of methionine and choline (MCD) induce hepatic steatosis. MCD feeding also provokes systemic weight loss, for unclear reasons. In this study, we found that MCD feeding causes profound hepatic suppression of the gene encoding stearoyl-coenzyme A desaturase-1 (SCD-1), an enzyme whose regulation has significant effects on metabolic rate. Within 7 days of MCD exposure, hepatic SCD-1 mRNA decreased to nearly undetectable levels. By day 21, SCD-1 protein was absent from hepatic microsomes and fatty acids showed a decrease in monounsaturated species. These changes in hepatic SCD-1 were accompanied by signs of hypermetabolism. Calorimetry revealed that MCD-fed mice consumed 37% more energy than control mice (P = 0.0003). MCD feeding also stimulated fatty acid oxidation, although fatty oxidation genes were not significantly upregulated. Interestingly, despite their increased metabolic rate, MCD-fed mice did not increase their food consumption, and as a result, they lost 26% of their body weight in 21 days. In summary, MCD feeding suppresses SCD-1 in the liver, which likely contributes to hypermetabolism and weight loss. MCD feeding also induces hepatic steatosis, by an independent mechanism. Viewed together, these two disparate consequences of MCD feeding (weight loss and hepatic steatosis) give the appearance of an unusual form of lipodystrophy.