Selection of elements to test for silencer activity in Drosophila embryos 

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Screening a library of elements for silencer activity in whole Drosophila embryos

We adapted our previously published enhancer-FACS-Seq technology, for highly parallel screening of elements for enhancer activity in Drosophila embryos (Gisselbrecht et al., 2013), into ‘silencer-FACS-Seq’ (sFS) technology, which enriches for elements that tissue-specifically silence reporter gene expression (see Methods). We generated a reporter vector, pSFSdist, which drives GFP expression under the control of a strong, ubiquitous enhancer and an element from a library of candidate silencers, in a genomically integrated context (Fig. 1a,b, Supplementary Fig. 5). The reporter construct is integrated in a single location in the haploid genome via phiC31 integrase (Gisselbrecht et al., 2013). Flies carrying single insertions from the reporter library are crossed to a strain in which expression of the exogenous marker protein CD2 is driven in a tissue or cell type of interest, and the resulting informative embryos dissociated to produce a single cell suspension. By sorting CD2+ cells in which GFP expression is reduced from the level driven by the strong ubiquitous enhancer in the absence of silencing activity, we enrich for cells containing silencers active in the cell type of interest, which can be recovered and identified by high-throughput sequencing.

Selection of elements to test for silencer activity in Drosophila embryos

We designed a first library of 576 genomic elements (Supplementary Table 1), chosen to represent a variety of chromatin states or enhancer activities, to test for silencer activity. Since the general features of silencers are unknown, we pursued a few different strategies to identify elements that might correspond to repressive elements. The availability of genome-wide chromatin immunoprecipitation data for well-characterized transcriptional corepressors (Celniker, 2009) provided one source of candidate silencers to test. Next, we reasoned that some CRMs might function as enhancers in one context and as silencers in other contexts, as two such bifunctional CRMs had been identified previously in Drosophila (Jiang et al., 1993; Stathopoulos and Levine, 2005). Therefore, since we were designing a library to screen for silencing activity in the mesoderm, we selected CRMs from the REDfly and CAD2 databases (Gallo et al., 2011; Bonn et al., 2012) that exhibited no or highly restricted mesodermal expression at embryonic stage 11. We furthermore filtered out elements associated with genes that show widespread mesodermal expression at this stage. Another potential source of such bifunctional elements that we selected was genomic regions associated with markers of both active and repressed chromatin structure in whole-mesoderm or whole-embryo experiments (Bonn et al., 2012; Rosenbloom et al., 2015; Thomas et al., 2011). All sequences identified from genome-wide ChIP methods were associated with nearby genes (see Methods) and filtered for absence of widespread mesodermal expression. Finally, we included three positive control sequences previously shown to have mesodermal silencing activity, and two types of negative controls: broadly active mesodermal enhancers, and length-matched regions of E. coli genomic sequence. Using sFS, we screened our library of genomic elements for silencer activity in embryonic mesoderm. Testing of this library yielded a readily detectable population of mesodermal cells in which green fluorescent protein (GFP) expression is reduced (Fig. 1b,c). Of the 576 sequence elements chosen for inclusion in this library, we detected 372 in cells derived from transgenic flies. We found 79 elements significantly enriched (see Methods) in the reduced-GFP cell population in either of two biological replicate screens (Fig. 2a, Supplementary Fig. 2, Supplementary Table 3).

Promoter competition in sFS-positive elements

The most enriched feature among elements that were identified as ‘hits’ in our silencer screen is overlap with regions surrounding the transcriptional start sites of genes, which likely reveals the presence of promoter competition. Promoter competition previously has been observed to restrict the enhancer-driven activity of reporter gene promoters (Ohtsuki et al., 1998). Consistent with this interpretation, three of the sequences that caused reduced GFP expression were broadly active mesodermal enhancers that we included in our library as negative controls; all three of these sequences contain promoters. Overall, the set of 41 ‘hits’ that overlapped promoter regions was significantly enriched for mapped instances of the TATA box (Zhu et al., 2009). While these are technical positives in our silencer screen, since our goal was to analyze CRMs that silence gene expression by other means, we omitted any identified ‘hits’ that overlapped promoter regions from further analysis, resulting in a filtered set of 262 detected library elements, of which 38 are genomic regions showing silencer activity in our assay. In the second step of our silencer identification strategy, we then generated pure reporter lines and individually tested each of the 38 non-promoter sFS ‘hits’ for silencer activity by fluorescence-activated cell sorting (FACS) (see Methods). This resulted in a final, high-confidence set of 15 validated, mesodermal silencers (Fig. 2b,c, supplementary figure 5), which we considered a rather low validation rate.

Characterization of silencing activity in the context of distinct enhancers

To confirm that the observed silencing activity does not represent an artifact of the FACS-based assay, we tested the ability of one of the newly discovered silencer elements to suppress activation by two different mesodermally restricted enhancers and visualized reporter gene expression in the resulting embryos. Both constructs showed silencing by the tested element relative to a negative control sequence (Fig. 2d–g). Interestingly, we observed different patterns of silencing in the context of these two mesodermal enhancers, which are active at different times in development; silencing activity was much weaker in the posterior germband at embryonic stage 12 (Fig. 2f,g), a pattern not observed in the earlier embryo (Fig. 2d,e). Thus, the repressive activity of silencers may exhibit complex spatiotemporal regulation similar to that of many enhancers.

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All validated silencers act as transcriptional enhancers in other cellular contexts

We analyzed these 15 validated mesodermal silencers to determine which genomic features that we explicitly sampled in the design of our element library were predictive of silencer activity. Despite the inclusion in our library of ChIP peaks for two well-known transcriptional corepressors (Groucho, CtBP) (Mani-Telang and Arnosti, 2007; Orian et al., 2007) and for the repressive chromatin mark trimethylated Lys27 of histone H3 (H3K27me3) (Kharchenko et al., 2011), the only screened element type significantly enriched among active mesodermal silencers was non-mesodermal enhancers (Fig. 3a; P = 0.0147, Fisher’s exact test). In fact, all but two of the 15 high-confidence mesodermal silencers were previously reported to have enhancer activity. Testing of the remaining two silencers for enhancer activity revealed that they both also function as non-mesodermal enhancers in the embryo (Fig. 3b,c). Thus, our results suggest that most if not all mesodermal silencers are also enhancers in other cellular contexts.

Transcription factor compositional complexity at silencers

We hypothesized that, as these elements are more functionally complex than CRMs that act only as enhancers, they may exhibit a more complex suite of TF interactions across various tissues. Indeed, we observed that validated silencers are strongly enriched for overlap with highly occupied target (HOT) regions, defined by Roy et al. as exceeding a TF complexity score threshold of ~10 overlapping, bound factors (Roy et al., 2010) (P = 0.00026, Fisher’s exact test).
To investigate DNA sequence features that might be important for silencer activity, we inspected the enrichment of combinations of DNA binding site motifs for annotated repressors (Supplementary Table 8) among the validated silencers (see Methods). A motif or motif combination was considered significantly enriched if it targets ≥50% of the foreground sequences and has an aera under the receiver operating characteristic curve [AUROC] ≥ 0.65 and an q-value ≤ 0.1 (see methods). Five motif combinations were significantly enriched among mesodermal silencers (Fig. 4a); all shared a core combination of motifs for Snail (sna) and Defective proventriculus (dve). Sna is a well-known repressor of non-mesodermal genes in the developing mesoderm (Nieto, 2002). The dve motif is characteristic of the Bicoid group of homeodomain proteins (Noyes et al., 2008), which are not known to be involved specifically in mesodermal repression. Mutation of the dve motif sequence 5′-TAATCC-3′ in four validated silencers did not detectably impair their silencer activity (Supplementary Fig. 4); however, three of these four mutated elements exhibited consistent alterations of their enhancer activity (Fig. 4b-d, Supplementary Fig. 4), further emphasizing the bifunctional nature of this class of cis-regulatory elements.

Chromatin features of active silencers

Various post-translational modifications of histones have been associated with different functional elements in the genome (Ernst and Kellis, 2010; Filion et al., 2010; Heintzman et al., 2007; Roy et al., 2010); however, little is currently known about the chromatin features of active silencers (Riel, 2014). We found that mean H3K27me3 ChIP signal is mildly enriched (area under the receiver operating characteristic curve [AUROC] ~ 0.60, P < 0.03, two-tailed Wilcoxon test) (see Methods) among validated mesodermal silencers. Considering additional marks beyond H3K27me3, analysis of a broad panel of whole-embryo ChIP-chip and ChIP-seq data for histone modifications, TFs, coactivators, and corepressors (see Methods) suggested a variety of other marks enriched or depleted among silencers, but none – including H3K27me3 – was statistically significant in this broader analysis after correction for multiple hypothesis testing (Supplementary Table 9). Thus, these results suggest that H3K27me3 may be a potential feature of active, tissue-specific silencers, but it alone is not sufficient to distinguish silencers from other genomic elements.

Table of contents :

Acknowledgements
Introduction
Regulation of Transcription
Transcription factors and cis regulatory modules
Enhancers
Silencers
Predictions of CRMs
Predictions using motifs and conservation
Predictions from ChIP-seq
Histone marks and chromatin accessibility
Spatial proximity between genomic regions
Experimental identification of enhancers
Current challenges
Goals of the dissertation
References
Chapter 1: bifunctionality of CRMs
Author contributions
Abstract
Acknowledgments
Introduction
Initial library and first experiment
Screening a library of elements for silencer activity in whole Drosophila embryos .
Selection of elements to test for silencer activity in Drosophila embryos
Promoter competition in sFS-positive elements
Characterization of silencing activity in the context of distinct enhancers .
All validated silencers act as transcriptional enhancers in other cellular contexts
Transcription factor compositional complexity at silencers
Chromatin features of active silencers
Second library and ongoing analyses
Second library
Results and validations
TF compositional complexity and chromatin features of active silencers
Current and future experiments and analyses
Genome editing: CRM knock-out
Hi-C: mesoderm specific interactions
Spatiotemporal activity of silencers
Discussion
Methods
Generation of reporter vector pSFSdist
Design of the candidate silencer libraries
Performing silencer-FACS-Seq experiments
Statistical analysis of sFS sequencing reads.
Validation of sFS results
Assessing CRM bifunctionality
Downstream analysis of the validated silencers
Cell sorting and fixation with formaldehyde
Guide-RNA and primer design
Cas9 and gRNA preparation and microinjection
In situ hybridization: probes primer design
References
Supplementary figures
Supplementary References:
Chapter 2: rare cell purification
Introduction
Cell panning approach
Preliminary results
pCD8 vector
Cell panning
Future directions
Methods
Generation of vector pCD8
Creation of dpp_VRR:CD8 vector and fly lines
Cell purification protocol – a work in progress
Supplementary figure
References
Conclusion and Future directions
Summary
Limitations
Future directions
Concluding remarks
References
Annexes and supplementary tables
Annex 1: full sequence of the pCD8 plasmid
Annex 2: Protocol for positive panning from Drosophila embryos
Annex 3 : gRNA list for bifunctional element knockout
Supplementary table 1
Supplementary table 2
Supplementary table 3
Supplementary table 4
Supplementary table 5
Supplementary table 6
Supplementary table 7
Supplementary table 8
Supplementary table 9

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