Odd paired transcriptional activation of decapentaplegic in the Drosophila eye/antennal disc is cell autonomous but indirect

Odd-Paired is Involved in Morphological Divergence of Snail-Feeding Beetles

Body shape and size diversity and their evolutionary rates correlate with species richness at the macroevolutionary scale. However, the molecular genetic mechanisms underlying the morphological diversification across related species are poorly understood. In beetles, which account for one-fourth of the known species, adaptation to different trophic niches through morphological diversification appears to have contributed to species radiation. Here, we explored the key genes for the morphological divergence of the slender to stout body shape related to divergent feeding methods on large to small snails within the genus Carabus. We show that the zinc-finger transcription factor encoded by odd-paired (opa) controls morphological variation in the snail-feeding ground beetle Carabus blaptoides. Specifically, opa was identified as the gene underlying the slender to stout morphological difference between subspecies through genetic mapping and functional analysis via gene knockdown. Further analyses revealed that changes in opa cis-regulatory sequences likely contributed to the differences in body shape and size between C. blaptoides subspecies. Among opa cis-regulatory sequences, single nucleotide polymorphisms on the transcription factor binding sites may be associated with the morphological differences between C. blaptoides subspecies. opa was highly conserved in a wide range of taxa, especially in beetles. Therefore, opa may play an important role in adaptive morphological divergence in beetles.

Odd-paired is a pioneer-like factor that coordinates with Zelda to control gene expression in embryos

Pioneer factors such as Zelda (Zld) help initiate zygotic transcription in Drosophila early embryos, but whether other factors support this dynamic process is unclear. Odd-paired (Opa), a zinc-finger transcription factor expressed at cellularization, controls the transition of genes from pair-rule to segmental patterns along the anterior-posterior axis. Finding that Opa also regulates expression through enhancer sog_Distal along the dorso-ventral axis, we hypothesized Opa's role is more general. Chromatin-immunoprecipitation (ChIP-seq) confirmed its in vivo binding to sog_Distal but also identified widespread binding throughout the genome, comparable to Zld. Furthermore, chromatin assays (ATAC-seq) demonstrate that Opa, like Zld, influences chromatin accessibility genome-wide at cellularization, suggesting both are pioneer factors with common as well as distinct targets. Lastly, embryos lacking opa exhibit widespread, late patterning defects spanning both axes. Collectively, these data suggest Opa is a general timing factor and likely late-acting pioneer factor that drives a secondary wave of zygotic gene expression.

Zygotic pioneer factor activity of Odd-paired/Zic is necessary for late function of the Drosophila segmentation network

Because chromatin determines whether information encoded in DNA is accessible to transcription factors, dynamic chromatin states in development may constrain how gene regulatory networks impart embryonic pattern. To determine the interplay between chromatin states and regulatory network function, we performed ATAC-seq on Drosophila embryos during the establishment of the segmentation network, comparing wild-type and mutant embryos in which all graded maternal patterning inputs are eliminated. While during the period between zygotic genome activation and gastrulation many regions maintain stable accessibility, cis-regulatory modules (CRMs) within the network undergo extensive patterning-dependent changes in accessibility. A component of the network, Odd-paired (opa), is necessary for pioneering accessibility of late segmentation network CRMs. opa-driven changes in accessibility are accompanied by equivalent changes in gene expression. Interfering with the timing of opa activity impacts the proper patterning of expression. These results indicate that dynamic systems for chromatin regulation directly impact the reading of embryonic patterning information.

Link between the causative genes of holoprosencephaly: Zic2 directly regulates Tgif1 expression

One of the causal genes for holoprosencephaly (HPE) is ZIC2 (HPE5). It belongs to the zinc finger protein of the cerebellum (Zic) family of genes that share a C2H2-type zinc finger domain, similar to the GLI family of genes. In order to clarify the role of Zic2 in gene regulation, we searched for its direct target genes using chromatin immunoprecipitation (ChIP). We identified TGIF1 (HPE4), another holoprosencephaly-causative gene in humans. We identified Zic2-binding sites (ZBS) on the 5′ flanking region of Tgif1 by in vitro DNA binding assays. ZBS were essential for Zic2-dependent transcriptional activation in reporter gene assays. Zic2 showed a higher affinity to ZBS than GLI-binding sequences. Zic2-binding to the cis-regulatory element near the Tgif1 promoter may be involved in the mechanism underlying forebrain development and incidences of HPE.

Odd-Paired: The Drosophila Zic Gene

Zinc finger in the cerebellum (Zic) proteins are a family of transcription factors with multiple roles during development, particularly in neural tissues. The founding member of the Zic family is the Drosophila odd-paired (opa) gene. The Opa protein has a DNA binding domain containing five Cys2His2-type zinc fingers and has been shown to act as a sequence-specific DNA binding protein. Opa has significant homology to mammalian Zic1, Zic2, and Zic3 within the zinc finger domain and in two other conserved regions outside that domain. opa was initially identified as a pair-rule gene, part of the hierarchy of genes that establish the segmental body plan of the early Drosophila embryo. However, its wide expression pattern during embryogenesis indicates it plays additional roles. Embryos deficient in opa die before hatching with aberrant segmentation but also with defects in larval midgut formation. Post-embryonically, opa plays important roles in adult head development and circadian rhythm. Based on extensive neural expression, opa is predicted to be involved in many aspects of neural development and behavior, like other proteins of the Zic family. Consensus DNA binding sites have been identified for Opa and have been shown to activate transcription in vivo. However, there is evidence Opa may serve as a transcriptional regulator in the absence of direct DNA binding, as has been seen for other Zic proteins.

The Zic family homologue Odd-paired regulates Alk expression in Drosophila

The Anaplastic Lymphoma Kinase (Alk) receptor tyrosine kinase (RTK) plays a critical role in the specification of founder cells (FCs) in the Drosophila visceral mesoderm (VM) during embryogenesis. Reporter gene and CRISPR/Cas9 deletion analysis reveals enhancer regions in and upstream of the Alk locus that influence tissue-specific expression in the amnioserosa (AS), the VM and the epidermis. By performing high throughput yeast one-hybrid screens (Y1H) with a library of Drosophila transcription factors (TFs) we identify Odd-paired (Opa), the Drosophila homologue of the vertebrate Zic family of TFs, as a novel regulator of embryonic Alk expression. Further characterization identifies evolutionarily conserved Opa-binding cis-regulatory motifs in one of the Alk associated enhancer elements. Employing Alk reporter lines as well as CRISPR/Cas9-mediated removal of regulatory elements in the Alk locus, we show modulation of Alk expression by Opa in the embryonic AS, epidermis and VM. In addition, we identify enhancer elements that integrate input from additional TFs, such as Binou (Bin) and Bagpipe (Bap), to regulate VM expression of Alk in a combinatorial manner. Taken together, our data show that the Opa zinc finger TF is a novel regulator of embryonic Alk expression.

The Alk receptor tyrosine kinase is employed repeatedly during Drosophila development to drive signaling events in a variety of tissues. The spatial and temporal expression pattern of the Alk gene is tightly regulated. Identifying factors that influence the expression of Alk is important to better understand how Alk signaling is controlled. In this paper we characterize cis-regulatory sequences in the Alk locus and the transcription factors that bind them to govern Alk expression in the Drosophila embryo. Using a robotic protein-DNA interaction assay, we identified the Zic family transcription factor Odd-paired as a factor that binds to regulatory elements in the Alk locus. Binding of Odd-paired to Alkcis-regulatory elements varies spatially, revealing a requirement for additional transcription factors such as the NK3 and FoxF orthologues Bagpipe and Biniou in a subset of Alk-expressing tissues. Our findings provide new insight into the dynamics underlying temporal and spatial regulation of the Alk receptor during embryogenesis.

Odd-paired controls frequency doubling in Drosophila segmentation by altering the pair-rule gene regulatory network

The Drosophila embryo transiently exhibits a double-segment periodicity, defined by the expression of seven 'pair-rule' genes, each in a pattern of seven stripes. At gastrulation, interactions between the pair-rule genes lead to frequency doubling and the patterning of 14 parasegment boundaries. In contrast to earlier stages of Drosophila anteroposterior patterning, this transition is not well understood. By carefully analysing the spatiotemporal dynamics of pair-rule gene expression, we demonstrate that frequency-doubling is precipitated by multiple coordinated changes to the network of regulatory interactions between the pair-rule genes. We identify the broadly expressed but temporally patterned transcription factor, Odd-paired (Opa/Zic), as the cause of these changes, and show that the patterning of the even-numbered parasegment boundaries relies on Opa-dependent regulatory interactions. Our findings indicate that the pair-rule gene regulatory network has a temporally modulated topology, permitting the pair-rule genes to play stage-specific patterning roles.

Jun N-terminal kinase signaling makes a face

decapentaplegic (dpp), the Drosophila ortholog of BMP 2/4, directs ventral adult head morphogenesis through expression in the peripodial epithelium of the eye-antennal disc. This dpp expressing domain exerts effects both on the peripodial epithelium, and the underlying disc proper epithelium. We have uncovered a role for the Jun N-terminal kinase (JNK) pathway in dpp-mediated ventral head development. JNK activity is required for dpp's action on the disc proper, but in the absence of dpp expression, excessive JNK activity is produced, leading to specific loss of maxillary palps. In this review we outline our hypotheses on how dpp acts by both short range and longer range mechanisms to direct head morphogenesis and speculate on the dual role of JNK signaling in this process. Finally, we describe the regulatory control of dpp expression in the eye-antennal disc, and pose the problem of how the various expression domains of a secreted protein can be targeted to their specific functions.

Diverse patterns of genomic targeting by transcriptional regulators in Drosophila melanogaster

Annotation of regulatory elements and identification of the transcription-related factors (TRFs) targeting these elements are key steps in understanding how cells interpret their genetic blueprint and their environment during development, and how that process goes awry in the case of disease. One goal of the modENCODE (model organism ENCyclopedia of DNA Elements) Project is to survey a diverse sampling of TRFs, both DNA-binding and non-DNA-binding factors, to provide a framework for the subsequent study of the mechanisms by which transcriptional regulators target the genome. Here we provide an updated map of the Drosophila melanogaster regulatory genome based on the location of 84 TRFs at various stages of development. This regulatory map reveals a variety of genomic targeting patterns, including factors with strong preferences toward proximal promoter binding, factors that target intergenic and intronic DNA, and factors with distinct chromatin state preferences. The data also highlight the stringency of the Polycomb regulatory network, and show association of the Trithorax-like (Trl) protein with hotspots of DNA binding throughout development. Furthermore, the data identify more than 5800 instances in which TRFs target DNA regions with demonstrated enhancer activity. Regions of high TRF co-occupancy are more likely to be associated with open enhancers used across cell types, while lower TRF occupancy regions are associated with complex enhancers that are also regulated at the epigenetic level. Together these data serve as a resource for the research community in the continued effort to dissect transcriptional regulatory mechanisms directing Drosophila development.

Divergent transcriptional regulatory logic at the intersection of tissue growth and developmental patterning

The Yorkie/Yap transcriptional coactivator is a well-known regulator of cellular proliferation in both invertebrates and mammals. As a coactivator, Yorkie (Yki) lacks a DNA binding domain and must partner with sequence-specific DNA binding proteins in the nucleus to regulate gene expression; in Drosophila, the developmental regulators Scalloped (Sd) and Homothorax (Hth) are two such partners. To determine the range of target genes regulated by these three transcription factors, we performed genome-wide chromatin immunoprecipitation experiments for each factor in both the wing and eye-antenna imaginal discs. Strong, tissue-specific binding patterns are observed for Sd and Hth, while Yki binding is remarkably similar across both tissues. Binding events common to the eye and wing are also present for Sd and Hth; these are associated with genes regulating cell proliferation and “housekeeping” functions, and account for the majority of Yki binding. In contrast, tissue-specific binding events for Sd and Hth significantly overlap enhancers that are active in the given tissue, are enriched in Sd and Hth DNA binding sites, respectively, and are associated with genes that are consistent with each factor's previously established tissue-specific functions. Tissue-specific binding events are also significantly associated with Polycomb targeted chromatin domains. To provide mechanistic insights into tissue-specific regulation, we identify and characterize eye and wing enhancers of the Yki-targeted bantam microRNA gene and demonstrate that they are dependent on direct binding by Hth and Sd, respectively. Overall these results suggest that both Sd and Hth use distinct strategies – one shared between tissues and associated with Yki, the other tissue-specific, generally Yki-independent and associated with developmental patterning – to regulate distinct gene sets during development.

The Hippo tumor suppressor pathway controls proliferation in a tissue-nonspecific fashion in Drosophila epithelial progenitor tissues via the transcriptional coactivator Yorkie (Yki). However, despite the tissue-nonspecific role that Yki plays in tissue growth, the transcription factors that recruit Yki to DNA, most notably Scalloped (Sd) and Homothorax (Hth), are important regulators of developmental patterning with many tissue-specific functions. Thus, these three transcriptional regulators – Yki, Sd, and Hth – provide a model for exploring the properties of protein-DNA interactions that regulate both tissue-shared and tissue-specific functions. With this goal in mind, we identified the positions in the fly genome that are bound by Yki, Sd, and Hth in the progenitors of the wing and eye-antenna structures of the fly. These data not only provide a global view of the Yki gene regulatory network, they reveal an unusual amount of tissue specificity in the genomic regions targeted by Sd and Hth, but not Yki. The data also reveal that tissue-specific binding is very likely to overlap tissue-specific enhancer regions, provide important clues for how tissue-specific Sd and Hth binding occurs, and support the idea that gene regulatory networks are plastic, with spatial differences in binding significantly impacting network structures.

Hox proteins coordinate peripodial decapentaplegic expression to direct adult head morphogenesis in Drosophila

The Drosophila BMP, decapentaplegic (dpp), controls morphogenesis of the ventral adult head through expression limited to the lateral peripodial epithelium of the eye-antennal disc by a 3.5 kb enhancer in the 5' end of the gene. We recovered a 15 bp deletion mutation within this enhancer that identified a homeotic (Hox) response element that is a direct target of labial and the homeotic cofactors homothorax and extradenticle. Expression of labial and homothorax are required for dpp expression in the peripodial epithelium, while the Hox gene Deformed represses labial in this location, thus limiting its expression and indirectly that of dpp to the lateral side of the disc. The expression of these homeodomain genes is in turn regulated by the dpp pathway, as dpp signalling is required for labial expression but represses homothorax. This Hox-BMP regulatory network is limited to the peripodial epithelium of the eye-antennal disc, yet is crucial to the morphogenesis of the head, which fate maps suggest arises primarily from the disc proper, not the peripodial epithelium. Thus Hox/BMP interactions in the peripodial epithelium of the eye-antennal disc contribute inductively to the shape of the external form of the adult Drosophila head.

Regulation of a duplicated locus: Drosophila sloppy paired is replete with functionally overlapping enhancers

In order to investigate regulation and redundancy within the sloppy paired (slp) locus, we analyzed 30 kilobases of DNA encompassing the tandem, coordinately regulated slp1 and slp2 transcription units. We found a remarkable array of stripe enhancers with overlapping activities surrounding the slp1 transcription unit, and, unexpectedly, glial cell enhancers surrounding slp2. The slp stripe regulatory region generates 7 stripes at blastoderm, and later 14 stripes that persist throughout embryogenesis. Phylogenetic analysis among drosophilids suggests that the multiplicity of stripe enhancers did not evolve through recent duplication. Most of the direct integration among cis-regulatory modules appears to be simply additive, with one notable exception. Despite the apparent redundancy among stripe enhancers, transgenic rescue suggests that most are required for full function, to maintain wingless expression and parasegment boundaries throughout embryogenesis. Transgenic rescue also reveals indirect positive autoregulation by the 7 early stripes, without which alternate stripes within the 14-stripe pattern are lost, leading to embryos with a pair-rule phenotype.

Transcription factor Zic2 inhibits Wnt/β-catenin protein signaling

The Zic transcription factors play critical roles during embryonic development. Mutations in the ZIC2 gene are associated with human holoprosencephaly, but the etiology is still unclear. Here, we report a novel function for ZIC2 as a regulator of β-catenin·TCF4-mediated transcription. We show that ZIC2 can bind directly to the DNA-binding high mobility group box of TCF4 via its zinc finger domain and inhibit the transcriptional activity of the β-catenin·TCF4 complex. However, the binding of TCF4 to DNA was not affected by ZIC2. Zic2 RNA injection completely inhibited β-catenin-induced axis duplication in Xenopus embryos and strongly blocked the ability of β-catenin to induce expression of known Wnt targets in animal caps. Moreover, Zic2 knockdown in transgenic Xenopus Wnt reporter embryos led to ectopic Wnt signaling activity mainly at the midbrain-hindbrain boundary. Together, our results demonstrate a previously unknown role for ZIC2 as a transcriptional regulator of the β-catenin·TCF4 complex.