Interplay between positive and negative activities that influence the role of Bicoid in transcription

Quantitative dissection of transcription in development yields evidence for transcription-factor-driven chromatin accessibility

Thermodynamic models of gene regulation can predict transcriptional regulation in bacteria, but in eukaryotes, chromatin accessibility and energy expenditure may call for a different framework. Here, we systematically tested the predictive power of models of DNA accessibility based on the Monod-Wyman-Changeux (MWC) model of allostery, which posits that chromatin fluctuates between accessible and inaccessible states. We dissected the regulatory dynamics of hunchback by the activator Bicoid and the pioneer-like transcription factor Zelda in living Drosophila embryos and showed that no thermodynamic or non-equilibrium MWC model can recapitulate hunchback transcription. Therefore, we explored a model where DNA accessibility is not the result of thermal fluctuations but is catalyzed by Bicoid and Zelda, possibly through histone acetylation, and found that this model can predict hunchback dynamics. Thus, our theory-experiment dialogue uncovered potential molecular mechanisms of transcriptional regulatory dynamics, a key step toward reaching a predictive understanding of developmental decision-making.

Spatially varying cis-regulatory divergence in Drosophila embryos elucidates cis-regulatory logic

Spatial patterning of gene expression is a key process in development, yet how it evolves is still poorly understood. Both cis- and trans-acting changes could participate in complex interactions, so to isolate the cis-regulatory component of patterning evolution, we measured allele-specific spatial gene expression patterns in D. melanogaster × simulans hybrid embryos. RNA-seq of cryo-sectioned slices revealed 66 genes with strong spatially varying allele-specific expression. We found that hunchback, a major regulator of developmental patterning, had reduced expression of the D. simulans allele specifically in the anterior tip of hybrid embryos. Mathematical modeling of hunchback cis-regulation suggested a candidate transcription factor binding site variant, which we verified as causal using CRISPR-Cas9 genome editing. In sum, even comparing morphologically near-identical species we identified surprisingly extensive spatial variation in gene expression, suggesting not only that development is robust to many such changes, but also that natural selection may have ample raw material for evolving new body plans via changes in spatial patterning.

Patterns of chromatin accessibility along the anterior-posterior axis in the early Drosophila embryo

As the Drosophila embryo transitions from the use of maternal RNAs to zygotic transcription, domains of open chromatin, with relatively low nucleosome density and specific histone marks, are established at promoters and enhancers involved in patterned embryonic transcription. However it remains unclear how regions of activity are established during early embryogenesis, and if they are the product of spatially restricted or ubiquitous processes. To shed light on this question, we probed chromatin accessibility across the anterior-posterior axis (A-P) of early Drosophila melanogaster embryos by applying a transposon based assay for chromatin accessibility (ATAC-seq) to anterior and posterior halves of hand-dissected, cellular blastoderm embryos. We find that genome-wide chromatin accessibility is highly similar between the two halves, with regions that manifest significant accessibility in one half of the embryo almost always accessible in the other half, even for promoters that are active in exclusively one half of the embryo. These data support previous studies that show that chromatin accessibility is not a direct result of activity, and point to a role for ubiquitous factors or processes in establishing chromatin accessibility at promoters in the early embryo. However, in concordance with similar works, we find that at enhancers active exclusively in one half of the embryo, we observe a significant skew towards greater accessibility in the region of their activity, highlighting the role of patterning factors such as Bicoid in this process.

Concentration dependent chromatin states induced by the bicoid morphogen gradient

In Drosophila, graded expression of the maternal transcription factor Bicoid (Bcd) provides positional information to activate target genes at different positions along the anterior-posterior axis. We have measured the genome-wide binding profile of Bcd using ChIP-seq in embryos expressing single, uniform levels of Bcd protein, and grouped Bcd-bound targets into four classes based on occupancy at different concentrations. By measuring the biochemical affinity of target enhancers in these classes in vitro and genome-wide chromatin accessibility by ATAC-seq, we found that the occupancy of target sequences by Bcd is not primarily determined by Bcd binding sites, but by chromatin context. Bcd drives an open chromatin state at a subset of its targets. Our data support a model where Bcd influences chromatin structure to gain access to concentration-sensitive targets at high concentrations, while concentration-insensitive targets are found in more accessible chromatin and are bound at low concentrations. This may be a common property of developmental transcription factors that must gain early access to their target enhancers while the chromatin state of the genome is being remodeled during large-scale transitions in the gene regulatory landscape.

Structure-Function Analysis of the Drosophila melanogaster Caudal Transcription Factor Provides Insights into Core Promoter-preferential Activation

Regulation of RNA polymerase II transcription is critical for the proper development, differentiation, and growth of an organism. The RNA polymerase II core promoter is the ultimate target of a multitude of transcription factors that control transcription initiation. Core promoters encompass the RNA start site and consist of functional elements such as the TATA box, initiator, and downstream core promoter element (DPE), which confer specific properties to the core promoter. We have previously discovered that Drosophila Caudal, which is a master regulator of genes involved in development and differentiation, is a DPE-specific transcriptional activator. Here, we show that the mouse Caudal-related homeobox (Cdx) proteins (mCdx1, mCdx2, and mCdx4) are also preferential core promoter transcriptional activators. To elucidate the mechanism that enables Caudal to preferentially activate DPE transcription, we performed structure-function analysis. Using a systematic series of deletion mutants (all containing the intact DNA-binding homeodomain) we discovered that the C-terminal region of Caudal contributes to the preferential activation of the fushi tarazu (ftz) Caudal target gene. Furthermore, the region containing both the homeodomain and the C terminus of Caudal was sufficient to confer core promoter-preferential activation to the heterologous GAL4 DNA-binding domain. Importantly, we discovered that Drosophila CREB-binding protein (dCBP) is a co-activator for Caudal-regulated activation of ftz. Strikingly, dCBP conferred the ability to preferentially activate the DPE-dependent ftz reporter to mini-Caudal proteins that were unable to preferentially activate ftz transcription themselves. Taken together, it is the unique combination of dCBP and Caudal that enables the co-activation of ftz in a core promoter-preferential manner.

Genomic occupancy of the transcriptional co-activators p300 and CBP

The p300 and CBP co-activators are histone acetylases and central regulators of transcription in metazoans. The genomic occupancy of p300/CBP detected by ChIP-seq experiments can be used to identify transcriptional enhancers. However, studies in Drosophila embryos suggest that there is a preference for some transcription factors in directing p300/CBP to the genome. Although p300/CBP occupancy in general correlates with gene activation, they can also be found at silent genomic regions, which does not result in histone acetylation. Polycomb-mediated H3K27me3 is associated with repression, but does not preclude p300/CBP binding. An antagonism between H3K27ac and H3K27me3 indicates that p300/CBP may be involved in switching between repressed and active chromatin states.

Preferential genome targeting of the CBP co-activator by Rel and Smad proteins in early Drosophila melanogaster embryos

CBP and the related p300 protein are widely used transcriptional co-activators in metazoans that interact with multiple transcription factors. Whether CBP/p300 occupies the genome equally with all factors or preferentially binds together with some factors is not known. We therefore compared Drosophila melanogaster CBP (nejire) ChIP-seq peaks with regions bound by 40 different transcription factors in early embryos, and we found high co-occupancy with the Rel-family protein Dorsal. Dorsal is required for CBP occupancy in the embryo, but only at regions where few other factors are present. CBP peaks in mutant embryos lacking nuclear Dorsal are best correlated with TGF-ß/Dpp-signaling and Smad-protein binding. Differences in CBP occupancy in mutant embryos reflect gene expression changes genome-wide, but CBP also occupies some non-expressed genes. The presence of CBP at silent genes does not result in histone acetylation. We find that Polycomb-repressed H3K27me3 chromatin does not preclude CBP binding, but restricts histone acetylation at CBP-bound genomic sites. We conclude that CBP occupancy in Drosophila embryos preferentially overlaps factors controlling dorso-ventral patterning and that CBP binds silent genes without causing histone hyperacetylation.

Fates-shifted is an F-box protein that targets Bicoid for degradation and regulates developmental fate determination in Drosophila embryos

Bicoid (Bcd) is a morphogenetic protein that instructs patterning along the anterior-posterior (A-P) axis in Drosophila embryos. Despite extensive studies, what controls the formation of a normal concentration gradient of Bcd remains an unresolved and controversial question. In this report we show that Bcd protein degradation is mediated by the ubiquitin-proteasome pathway. We identify a novel F-box protein, encoded by fates-shifted (fsd), that plays an important role in Bcd protein degradation by targeting it for ubiquitination. Embryos from females lacking fsd have an altered Bcd gradient profile, resulting in a shift of the fatemap along the A-P axis. Our study represents a first experimental demonstration that, contrary to an alternative hypothesis, Bcd protein degradation is required for normal gradient formation and developmental fate determination.

Shaping a morphogen gradient for positional precision

Morphogen gradients, which provide positional information to cells in a developing tissue, could in principle adopt any nonuniform profile. To our knowledge, how the profile of a morphogen gradient affects positional precision has not been well studied experimentally. Here, we compare the positional precision provided by the Drosophila morphogenetic protein Bicoid (Bcd) in wild-type (wt) embryos with embryos lacking an interacting cofactor. The Bcd gradient in the latter case exhibits decreased positional precision around mid-embryo compared with its wt counterpart. The domain boundary of Hunchback (Hb), a target activated by Bcd, becomes more variable in mutant embryos. By considering embryo-to-embryo, internal, and measurement fluctuations, we dissect mathematically the relevant sources of fluctuations that contribute to the error in positional information. Using this approach, we show that the defect in Hb boundary positioning in mutant embryos is directly reflective of an altered Bcd gradient profile with increasing flatness toward mid-embryo. Furthermore, we find that noise in the Bcd input signal is dominated by internal fluctuations but, due to time and spatial averaging, the spatial precision of the Hb boundary is primarily affected by embryo-to-embryo variations. Our results demonstrate that the positional information provided by the wt Bcd gradient profile is highly precise and necessary for patterning precision.

Organization of developmental enhancers in the Drosophila embryo

Most cell-specific enhancers are thought to lack an inherent organization, with critical binding sites distributed in a more or less random fashion. However, there are examples of fixed arrangements of binding sites, such as helical phasing, that promote the formation of higher-order protein complexes on the enhancer DNA template. Here, we investigate the regulatory ‘grammar’ of nearly 100 characterized enhancers for developmental control genes active in the early Drosophila embryo. The conservation of grammar is examined in seven divergent Drosophila genomes. Linked binding sites are observed for particular combinations of binding motifs, including Bicoid–Bicoid, Hunchback–Hunchback, Bicoid–Dorsal, Bicoid–Caudal and Dorsal–Twist. Direct evidence is presented for the importance of Bicoid–Dorsal linkage in the integration of the anterior–posterior and dorsal–ventral patterning systems. Hunchback–Hunchback interactions help explain unresolved aspects of segmentation, including the differential regulation of the eve stripe 3 + 7 and stripe 4 + 6 enhancers. We also present evidence that there is an under-representation of nucleosome positioning sequences in many enhancers, raising the possibility for a subtle higher-order structure extending across certain enhancers. We conclude that grammar of gene control regions is pervasively used in the patterning of the Drosophila embryo.

The Drosophila Smad cofactor Schnurri engages in redundant and synergistic interactions with multiple corepressors

In Drosophila a large zinc finger protein, Schnurri, functions as a Smad cofactor required for repression of brinker and other negative targets in response to signaling by the transforming growth factor beta ligand, Decapentaplegic. Schnurri binds to the silencer-bound Smads through a cluster of zinc fingers located near its carboxy-terminus and silences via a separate repression domain adjacent to this zinc-finger cluster. Here we show that this repression domain functions through interaction with two corepressors, dCtBP and dSin3A, and that either interaction is sufficient for repression. We also report that Schnurri contains additional repression domains that function through interaction with dCtBP, Groucho, dSin3A and SMRTER. By testing for the ability to rescue a shn RNAi phenotype we provide evidence that these diverse repression domains are both cooperative and partially redundant. In addition we find that Shn harbors a region capable of transcriptional activation, consistent with evidence that Schnurri can function as an activator as well as a repressor.

Transcription factors bind thousands of active and inactive regions in the Drosophila blastoderm

Identifying the genomic regions bound by sequence-specific regulatory factors is central both to deciphering the complex DNA cis-regulatory code that controls transcription in metazoans and to determining the range of genes that shape animal morphogenesis. We used whole-genome tiling arrays to map sequences bound in Drosophila melanogaster embryos by the six maternal and gap transcription factors that initiate anterior-posterior patterning. We find that these sequence-specific DNA binding proteins bind with quantitatively different specificities to highly overlapping sets of several thousand genomic regions in blastoderm embryos. Specific high- and moderate-affinity in vitro recognition sequences for each factor are enriched in bound regions. This enrichment, however, is not sufficient to explain the pattern of binding in vivo and varies in a context-dependent manner, demonstrating that higher-order rules must govern targeting of transcription factors. The more highly bound regions include all of the over 40 well-characterized enhancers known to respond to these factors as well as several hundred putative new cis-regulatory modules clustered near developmental regulators and other genes with patterned expression at this stage of embryogenesis. The new targets include most of the microRNAs (miRNAs) transcribed in the blastoderm, as well as all major zygotically transcribed dorsal-ventral patterning genes, whose expression we show to be quantitatively modulated by anterior-posterior factors. In addition to these highly bound regions, there are several thousand regions that are reproducibly bound at lower levels. However, these poorly bound regions are, collectively, far more distant from genes transcribed in the blastoderm than highly bound regions; are preferentially found in protein-coding sequences; and are less conserved than highly bound regions. Together these observations suggest that many of these poorly bound regions are not involved in early-embryonic transcriptional regulation, and a significant proportion may be nonfunctional. Surprisingly, for five of the six factors, their recognition sites are not unambiguously more constrained evolutionarily than the immediate flanking DNA, even in more highly bound and presumably functional regions, indicating that comparative DNA sequence analysis is limited in its ability to identify functional transcription factor targets.

Embryonic pattern scaling achieved by oppositely directed morphogen gradients

Morphogens are proteins, often produced in a localized region, whose concentrations spatially demarcate regions of differing gene expression in developing embryos. The boundaries of gene expression are typically sharp and the genes can be viewed as abruptly switching from on to off or vice versa upon crossing the boundary. To ensure the viability of the organism these boundaries must be set at certain fractional positions within the corresponding developing field. Remarkably this can be done with high precision despite the fact that the size of the developing field itself can vary widely from embryo to embryo. How this scaling is accomplished is unknown but it is clear that a single morphogen gradient is insufficient. Here we show how a pair of morphogens A and B, produced at opposite ends of a one-dimensional developing field, can solve the pattern-scaling problem. In the most promising scenario the morphogens interact via an effective annihilation reaction A + B --> slashed circle and the switch occurs according to the absolute concentration of A or B. We define a scaling criterion and show that morphogens coupled in this way can set embryonic markers across the entire developing field in proportion to the field size. This scaling occurs at developing-field sizes of a few times the morphogen decay length. The scaling criterion is not met if instead the gradients couple combinatorially such that downstream genes are regulated by the ratio A/B of the morphogen concentrations.