Converting an Activator Into a Repressor

View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector Y. TONY IP TRANSCRIPTIONAL REGULATION Converting an activator into a repressor Dorsoventral patterning in Drosophila requires the Dorsal morphogen to act as both an activator and a repressor of transcription: an HMG1 -like protein may serve to switch Dorsal from one to the other.

Higher eukaryotic cells have to cope with the ever The expression of these genes is therefore restricted to changing environment, and need to make very different the dorsal half of the embryo, where they direct the cells decisions at various times. One example of this is cell- to differentiate as ectoderm. fate determination during metazoan development, which frequently requires genes to be turned on or off at The signaling pathway regulating Dorsal nuclear localiza- specific times and places. Such regulation of transcription is strikingly similar to that regulating the mammalian tion involves sequence-specific DNA-binding proteins transcription factor NF-KB. The intracellular domain of that interact with control elements located near or Toll is homologous to that of the IL-1 receptor (which is within their target genes. These protein-DNA inter- an upstream regulator of NF-KB); both Cactus and IKB actions may result in an increase or decrease in both (which binds NF-KB and prevents it from entering the transcription rate and, consequently, level of target-gene nucleus) contain multiple ankyrin repeats; Pelle is a product. It is possible that, for each specific signal, cells kinase, and different kinases have been implicated in the use a specific factor to generate the desired response. regulation of NF-KB activity; Dorsal and NF-KB both The lesson of the last decade, however, is that combina- belong to the Rel family of transcription factors. torial interactions of multiple factors are often used to make the final on or off decision. This may reflect evolu- How can Dorsal function as both an activator and a tionary selection for the economical use of a limited repressor? A number of different mechanisms can be number of regulatory factors and for flexible mechanisms envisaged. First, Dorsal could be differentially modified. of controlling gene expression. But as the activation of twist, say, and the repression of zen take place in the same cells, it is unlikely that specific Dorsoventral patterning in early Drosophila embryos is modifications are responsible for the opposite effects. governed by a group of twelve maternal gene products Second, Dorsal could turn on another factor which then [1]. These proteins constitute a signal transduction path- represses zen expression. This indirect approach is ruled way. Three of the seven extracellular components of the out by experimental results showing that, at least in vitro, pathway are serine proteases, which function in a cascade Dorsal can interact directly with silencer elements on the and ultimately cleave a protein called Spatzle [2]. The all the genetically identified target promoters [5-7]. proteolytic Spatzle product then interacts with the trans- Third, the exact sequence of the binding site could dic- membrane receptor Toll. Two proteins, Tube and Pelle, tate Dorsal's function, for example by inducing a non- are required to relay the message in the cytoplasm. The productive conformation. Extensive promoter analyses, last step of the signaling pathway is the transcription fac- however, argue against this mechanism. For instance, a tor Dorsal. Dorsal is originally localized in the cytoplasm, minimal Dorsal binding site from the zen promoter can through its interaction with the anchoring protein induce ventral activation of an adjacent basal promoter Cactus. The activated signal somehow modifies the [8,9]. Conversely, when the binding sites in the zen ven- Dorsal-Cactus complex so that Dorsal is released to enter tral repression element (VRE) are mutated so to resemble the nucleus. that of the twist promoter, the element still functions as a silencer [9]. Once in the nucleus, Dorsal interacts directly with the target promoters and regulates the expression of zygotic Thus, Dorsal is intrinsically an activator when bound genes. As the activity that stimulates the Toll receptor to DNA. It is the promoter context that converts the originates only from the ventral regions of the embryo, protein into a repressor. The involvement of a 'co-repres- more Dorsal enters the nuclei in the ventral regions and sor' is supported by the observation that mutagenizing the levels gradually decrease towards more lateral regions. the DNA sequence surrounding the Dorsal binding sites This nuclear Dorsal gradient activates different sets of in the zen promoter led to derepression of a reporter zygotic genes, including twist, snail and rhomboid, in the gene [10,11]. Furthermore, the Dorsal binding site then ventral regions with different thresholds [3,4]. The local- becomes an enhancer and activates the reporter gene in ized zygotic gene expression establishes the presumptive the ventral-most regions. These molecular analyses mesoderm and neuroectoderm (Fig. 1). Both high and strongly suggest that proteins binding to sites next to low levels of Dorsal in the ventral half of the embryo are Dorsal can convert it into a repressor. As Dorsal and its also able to repress another set of zygotic genes, such as co-repressor can, when bound to a silencer element, zerknullt (zen), decapentaplegic (dpp) and tolloid (tld) [5-7]. dominantly repress a number of heterologous enhancers

Fig. 1. Activation and repression by Dorsal in the early Drosophila embryo. On the left is a schematic cross section of a blastoderm stage embryo. The nuclear Dorsal gradient extends from the ventral regions to the lateral regions, while the dorsal regions contain no nuclear Dorsal. This nuclear gradient patterns the embryo into three basic regions: mesoderm, neuroectoderm and dorsal ectoderm. Dorsal control elements are shown schematically on the right. In ventral regions, nuclear Dorsal interacts with target promoters, such as twist and snail, to activate gene expression. Dorsal also binds to a zen ventral repression element, interacts with DSP1 and other co-repressor proteins that bind AT-rich sequences (AT1-3) and silences the neighboring promoter. In dorsal regions, the ubiquitously distributed co-repressors can interact with the silencer, but as there is no nuclear Dorsal in these regions, the co-repressors alone cannot function and zen is still activated.

[5-10], it is possible that the Dorsal-co-repressor and of p50/p50 homodimers nine-fold, but has no effect complex loops back to block the basal transcription on p65/p65 homodimers. machinery at the transcriptional initiation site. DSPl-mediated repression in HeLa cells is dependent The search for such a co-repressor is being conducted on a binding site called negative regulatory element fiercely in several laboratories. While biochemical and (NRE), with the sequence TCTGAA. This sequence molecular methods have not been fruitful, a yeast genetic was originally identified in the human interferon-P approach has recently shed some light on the possible (IFN-3) gene promoter [13,14]. Virus induction of mechanism. Based on the known properties of the zen IFN-3 depends on a proximal promoter element that silencer element, Lehming and colleagues designed a yeast contains binding sites for proteins such as NF-KB and genetic screen for the co-repressor [12]. The prediction ATF-2. Recent experiments have shown that another was that the co-repressor, when bound to a site near HMG-like protein, HMG I(Y), enhances the assembly Dorsal, would abolish reporter gene expression. As the of NF-KB and ATF-2 on the virus-inducible element zen VRE functions as a Dorsal-dependent enhancer in [13]. The NRE is located immediately downstream of, yeast, the microorganism does not contain the co-repres- and overlapping with, the NF-KB and HMG I(Y) sites. sor. It was therefore possible to identify proteins that Dorsal-DSP1 complex formation is reminiscent of the inhibit the activation by co-transforming the yeast with an NF-KB-HMG I(Y) interaction. These combinations, early embryonic cDNA library and selecting for loss of however, have opposite biological outcomes. Whereas activation. Such a screen yielded two proteins. The first DSP1 binding converts Dorsal and NF-KB to repressors, turned out to be Cactus, the cytoplasmic inhibitor of HMG I(Y) enhances the activators' activities. It might Dorsal. The second was a high mobility group (HMG)- be that HMG I(Y) interacts productively, while the like protein, named Dorsal switch protein (DSP1). This DSP1-Dorsal/NF-KB interaction results in a non-pro- has all the predicted properties, when assayed in HeLa ductive conformation that locks up the basal transcrip- cells, of a co-repressor. Not only can it inhibit Dorsal/ tion machinery. None of the known mammalian HMG NF-KB activation, it also silences the heterologous thymi- proteins can interact with the NRE and mediate repres- dine kinase gene promoter in a distance- and orientation- sion. It is therefore probable that the mammalian analog independent manner. Furthermore, in vitro experiments of Drosophila DSP1 has yet to be identified. In vitro assays show that bacterially expressed DSP1 increases the bind- have identified two factors of molecular weights 95 and ing affinity of NF-KB (a p50/p65 heterodimer) three-fold 100 kD that can bind to the NRE [14]. The relationship DISPATCH 3 between these factors and DSP1, however, remains to Acknowledgements: I would like to thank Li Zeng for critical be determined. reading of the manuscript.

References Interestingly, the zen VRE also contains an NRE 1. St. Johnston D, Nusslein-Volhard C: The origin of pattern and polar- sequence, next to one of the Dorsal binding sites [10-12] ity inthe Drosophilaembryo. Cell 1992, 68:201-219. (Fig. 1). Although this NRE functions well in the HeLa 2. Morisato D, Anderson KV: The spatzle gene encodes a component of the extracellular pathway establishing the dorsal-ventral pattern cell assays [12], it has not been tested in fly embryos. of the Drosophila embryo. Cell 1994, 76:677-688. Previous experiments focused, instead, on other regions 3. Ip YT, Park RE, Kosman D, Bier E, Levine M: The dorsal gradient of the VRE [10,11]. In particular, three AT-rich sequen- morphogen regulates stripes of rhomboid expression in the presumptive neuroectoderm of the Drosophila embryo. Genes Dev ces have been implicated in ventral repression (Fig. 1). 1992, 6:1728-1 739. Mutations of the AT1 and AT2 sites convert the repres- 4. Jiang J, Levine M: Binding affinities and cooperative interactions sion element to an enhancer. This is expected as, if the with bHLH activators delimit threshold responses to the dorsal gradient morphogen. Cell 1993, 72:741-752. co-repressor no longer binds to the element, Dorsal 5. Ip YT, Kraut R, Levine M, Rushlow CA: The dorsal morphogen isa alone should be free to activate the reporter gene. In sequence-specific DNA-binding protein that interacts with a long- addition, distinct nuclear factors in crude embryonic range repression element inDrosophila. Cell 1991, 64:439-446. 6. Huang JD, Schwyter DH, Shirokawa JM, Courey AJ: The interplay extracts were found to interact with AT1 and AT2 sites. between multiple enhancer and silencer elements defines the pat- Further biochemical characterization of these factors, tern of decapentaplegic expression. Genes Dev 1993, 7:694-704. 7. Kirov N, Childs S, O'Connor M, Rushlow C: The Drosophila dorsal however, has not been reported. morphogen represses the tolloidgene by interacting with a silencer element. Mol Cell Biol 1994, 14:713-722. It is essential to test whether mutation of the TCTGAA 8. Pan D,Courey AJ: The same dorsal binding site mediates both activation and repression in a context-dependent manner. EMBO J sequence in the VRE abolishes ventral repression. Also, 1992, 11:1837-1842. genetic analysis of DSP1 could provide definitive proof 9. Jiang J,Rushlow CA, Zhou Q, Small S, Levine M: Individual dorsal that it can function as a co-repressor in early embryos. It morphogen binding sites mediate activation and repression in the Drosophila embryo. EMBOJ 1992, 11:3147-3154. is likely that the AT-binding proteins are distinct from 10. Kirov N, Zhelnin L,Shah J, Rushlow C: Conversion of a silencer into DSP1. Whether these proteins interact with each other an enhancer: evidence for a co-repressor in dorsalmediated repres- to constitute the 'holo-co-repressor' remains to be deter- sion inDrosophila. EMBOJ 1993, 12:3193-3199. 11. Jiang J, Cai H, Zhou Q, Levine M: Conversion of a dorsal-dependent mined. The repression of dpp and td by Dorsal seems to silencer into an enhancer: evidence for dorsal corepressors. EMBO have similar requirements, and yet their control elements J 1993, 12:3201-3209. are not the same as that of zen. In particular, the dpp 12. Lehming N, Thanos D, Brickman JM, Ma J, Maniatis T, Ptashne M: An HMG-like protein that can switch a transcriptional activator to repression element contains multiple, low-affinity Dorsal a repressor. Nature 1994, 371:175-179. binding sites in the first intron [6]. It would be interest- 13. Du W, Thanos D, Maniatis T: Mechanisms of transcriptional syner- ing to find out whether the same gism between distinct virus-inducible enhancer elements. Cell co-repressor acts on 1993, 74:887-898. all the elements. Another noteworthy result is that the 14. Nourbakhsh M, Hoffmann K, Hauser H: Interferon-3 promoters IFN-3 NRE itself can function as a silencer when iso- contain a DNA element that acts as a position-independent silencer lated from the neighboring NF-KB site [14], whereas on the NF-kB site. EMBOJ 1993, 12:451-459. ventral repression in early embryos seems to- depend on the Dorsal-co-repressor interaction. The story has come Y. Tony Ip, Program in Molecular Medicine, University a long way, but there are more chapters to go before the of Massachusetts Medical Center, 373 Plantation Street, end is reached. Worcester, Massachusetts 01605, USA.