The SWI/SNF nucleosome remodeler constrains enhancer activity during Drosophila wing development

Chromatin remodeling is central to the dynamic changes in gene expression that drive cell fate determination. During development, the sets of enhancers that are accessible for use change globally as cells transition between stages. While transcription factors and nucleosome remodelers are known to work together to control enhancer accessibility, it is unclear how the short stretches of DNA that they individually unmask yield the kilobase-sized accessible regions characteristic of active enhancers. Here, we performed a genetic screen to investigate the role of nucleosome remodelers in control of dynamic enhancer activity. We find that the Drosophila Switch/Sucrose Non-Fermenting complex, BAP, is required for repression of a temporally dynamic enhancer, brdisc. Contrary to expectations, we find that the BAP-specific subunit Osa is dispensable for mediating changes in chromatin accessibility between the early and late stages of wing development. Instead, we find that Osa is required to constrain the levels of brdisc activity when the enhancer is normally active. Genome-wide profiling reveals that Osa directly binds brdisc as well as thousands of other developmentally dynamic regulatory sites, including multiple genes encoding components and targets of the Notch signaling pathway. Transgenic reporter analyses demonstrate that Osa is required for activation and for constraint of different sets of target enhancers in the same cells. Moreover, Osa loss results in hyperactivation of the Notch ligand Delta and development of ectopic sensory structures patterned by Notch signaling early in development. Together, these findings indicate that proper constraint of enhancer activity is necessary for regulation of dose-dependent developmental events.

Widespread priming of transcriptional regulatory elements by incipient accessibility or RNA polymerase II pause in early embryos of the sea urchin Strongylocentrotus purpuratus

Transcriptional regulatory elements (TREs) are the primary nodes that control developmental gene regulatory networks. In embryo stages, larvae, and adult differentiated red spherule cells of the sea urchin Strongylocentrotus purpuratus, transcriptionally engaged TREs are detected by Precision Run-On Sequencing (PRO-seq), which maps genome-wide at base pair resolution the location of paused or elongating RNA polymerase II (Pol II). In parallel, TRE accessibility is estimated by the Assay for Transposase-Accessible Chromatin using Sequencing (ATAC-seq). Our analysis identifies surprisingly early and widespread TRE accessibility in 4-cell cleavage embryos that is not necessarily followed by concurrent or subsequent transcription. TRE transcriptional differences identified by PRO-seq provide more contrast among embryonic stages than ATAC-seq accessibility differences, in agreement with the apparent excess of accessible but inactive TREs during embryogenesis. Global TRE accessibility reaches a maximum around the 20-hour late blastula stage, which coincides with the consolidation of major embryo regionalizations and peak histone variant H2A.Z expression. A transcriptional potency model based on labile nucleosome TRE occupancy driven by DNA sequences and the prevalence of histone variants is proposed in order to explain the basal accessibility of transcriptionally inactive TREs during embryogenesis. However, our results would not reconcile well with labile nucleosome models based on simple A/T sequence enrichment. In addition, a large number of distal TREs become transcriptionally disengaged during developmental progression, in support of an early Pol II paused model for developmental gene regulation that eventually resolves in transcriptional activation or silencing. Thus, developmental potency in early embryos may be facilitated by incipient accessibility and transcriptional pause at TREs.

Repression and 3D-restructuring resolves regulatory conflicts in evolutionarily rearranged genomes

Regulatory landscapes drive complex developmental gene expression, but it remains unclear how their integrity is maintained when incorporating novel genes and functions during evolution. Here, we investigated how a placental mammal-specific gene, Zfp42, emerged in an ancient vertebrate topologically associated domain (TAD) without adopting or disrupting the conserved expression of its gene, Fat1. In ESCs, physical TAD partitioning separates Zfp42 and Fat1 with distinct local enhancers that drive their independent expression. This separation is driven by chromatin activity and not CTCF/cohesin. In contrast, in embryonic limbs, inactive Zfp42 shares Fat1's intact TAD without responding to active Fat1 enhancers. However, neither Fat1 enhancer-incompatibility nor nuclear envelope-attachment account for Zfp42's unresponsiveness. Rather, Zfp42's promoter is rendered inert to enhancers by context-dependent DNA methylation. Thus, diverse mechanisms enabled the integration of independent Zfp42 regulation in the Fat1 locus. Critically, such regulatory complexity appears common in evolution as, genome wide, most TADs contain multiple independently expressed genes.

How Does the Regulatory Genome Work?

The regulatory genome controls genome activity throughout the life of an organism. This requires that complex information processing functions are encoded in, and operated by, the regulatory genome. Although much remains to be learned about how the regulatory genome works, we here discuss two cases where regulatory functions have been experimentally dissected in great detail and at the systems level, and formalized by computational logic models. Both examples derive from the sea urchin embryo, but assess two distinct organizational levels of genomic information processing. The first example shows how the regulatory system of a single gene, endo16, executes logic operations through individual transcription factor binding sites and cis-regulatory modules that control the expression of this gene. The second example shows information processing at the gene regulatory network (GRN) level. The GRN controlling development of the sea urchin endomesoderm has been experimentally explored at an almost complete level. A Boolean logic model of this GRN suggests that the modular logic functions encoded at the single-gene level show compositionality and suffice to account for integrated function at the network level. We discuss these examples both from a biological-experimental point of view and from a computer science-informational point of view, as both illuminate principles of how the regulatory genome works.

Mechanisms of Action of Hematopoietic Transcription Factor PU.1 in Initiation of T-Cell Development

PU.1 is an ETS-family transcription factor that plays a broad range of roles in hematopoiesis. A direct regulator of myeloid, dendritic-cell, and B cell functional programs, and a well-known antagonist of terminal erythroid cell differentiation, it is also expressed in the earliest stages of T-cell development of each cohort of intrathymic pro-T cells. Its expression in this context appears to give T-cell precursors initial, transient access to myeloid and dendritic cell developmental competence and therefore to represent a source of antagonism or delay of T-cell lineage commitment. However, it has remained uncertain until recently why T-cell development is also intensely dependent upon PU.1. Here, we review recent work that sheds light on the molecular biology of PU.1 action across the genome in pro-T cells and identifies the genes that depend on PU.1 for their correct regulation. This work indicates modes of chromatin engagement, pioneering, and cofactor recruitment (“coregulator theft”) by PU.1 as well as gene network interactions that not only affect specific target genes but also have system-wide regulatory consequences, amplifying the impact of PU.1 beyond its own direct binding targets. The genes directly regulated by PU.1 also suggest a far-reaching transformation of cell biology and signaling potential between the early stages of T-cell development when PU.1 is expressed and when it is silenced. These cell-biological functions can be important to distinguish fetal from adult T-cell development and have the potential to illuminate aspects of thymic function that have so far remained the most mysterious.

Cell-specific transcription of the smooth muscle gamma-actin gene requires both positive- and negative-acting cis elements

We have characterized the function of putative regulatory sequences upon the smooth muscle transcription of the SMGA gene, using promoter deletion analyses. We demonstrate that the SMGA promoter contains four domains: a basal promoter (-1 to -100), a smooth muscle specifier sequence (-100 to -400), a negative regulator (-400 to -1000), and a smooth muscle-specific modulator (-1000 to -2000). The basal or core promoter supports equivalent transcription in both smooth and skeletal muscle cells. Addition of sequences containing a CArG motif juxtaposed to an E-box element stimulates smooth muscle transcription by five- to sixfold compared to skeletal muscle. This smooth muscle-specific segment is maintained for about 200 bp, after which is a segment of DNA that appears to inhibit the transcriptional capacity of the SMGA promoter in smooth muscle cells. Within the boundary between the smooth muscle specifier and negative regulatory sequences (-400 to -500) are three E-box elements. The smooth muscle modulator domain contains two CArG elements and multiple E-boxes. When added to the SMGA promoter it causes an additional three- to fivefold increase in smooth muscle-specific transcription over that stimulated by the smooth muscle specifier domain. Thus, our studies show that the appropriate cell-specific transcription of the SMGA gene involves complex interactions directed by multiple cis-acting elements. Moreover, our characterization of a cell culture system employing embryonic gizzard smooth muscle cells lays the foundation for further molecular analyses of factors that regulate or control SMGA and other smooth muscle genes during differentiation.

The RNA helicase DHH1 is central to the correct expression of many developmentally regulated mRNAs in trypanosomes

In trypanosomes, the predominant mechanisms of regulation of gene expression are post-transcriptional. The DEAD-box RNA helicase DHH1 was identified in a screen for gene products that are necessary for the instability of the GPI-PLC mRNA in insect-stage trypanosomes. Expression of an ATPase-deficient dhh1 mutant caused a rapid growth arrest associated with a decrease in polysomes, an increase in P-bodies and a slight decrease in average mRNA levels. However, the effect of dhh1 mutant expression on both turnover and translational repression of mRNAs was selective. Whereas there was little effect on the stability of constitutive mRNAs, the control of a large cohort of developmentally regulated mRNAs was reversed; many mRNAs normally downregulated in insect-stage trypanosomes were stabilized and many mRNAs normally upregulated decreased in level. One stabilised mRNA, ISG75, was characterised further. Despite the overall decrease in polysomes, the proportion of the ISG75 mRNA in polysomes was unchanged and the result was ISG75 protein accumulation. Our data show that specific mRNAs can escape DHH1-mediated translational repression. In trypanosomes, DHH1 has a selective role in determining the levels of developmentally regulated mRNAs.

The smooth muscle gamma-actin gene is androgen responsive in prostate epithelia

Nkx 3.1 is an evolutionarily conserved vertebrate homolog of the Drosophila Nk-3 homeodomain gene bagpipe that is expressed by a variety of cells during early mammalian development and has been shown to be a critical factor for prostate development and function. Previous studies utilizing a heterologous cell transfection strategy from our laboratory identified the smooth muscle gamma-actin (SMGA) gene as a novel molecular target of Nkx 3.1 regulatory activity. In the studies presented here, SMGA gene activity and regulation were evaluated in normal and cancerous prostate epithelial cells. SMGA transcripts were demonstrated in prostate epithelia and SMGA mRNA levels were increased in androgen-responsive LNCaP cancer and normal prostate epithelial cells. SMGA gene transcriptional activity was androgen responsive in these cells and required a segment of the human SMGA promoter containing NKE and SRF (serum response factor) binding elements. This region of the human SMGA proximal promoter is well conserved across species and is synergistically activated by coexpression of Nkx 3.1 and SRF in heterologous CV-1 cells. SMGA transcription was not responsive to steroid in PC-3 prostate epithelial cancer cells, which do not express Nkx 3.1. However, SMGA transcription was influenced by expression of androgen receptor in these cells, a situation that allows the androgen-dependent expression of Nkx 3.1. Furthermore, SMGA gene activity was influenced by direct Nkx 3.1 expression in the PC-3 cells. Thus, SMGA gene activity in prostate epithelia is due, in part, to the androgen-dependent expression of Nkx 3.1. As such, our studies provide the initial description of Nkx 3.1 target gene regulatory activity in the prostate.