The Central Region of the Drosophila Co-repressor Groucho as a Regulatory Hub

Redox-inactive CC-type glutaredoxins interfere with TGA transcription factor-dependent repression of target promoters in roots

Changes in nitrogen (N) availability in the soil trigger transcriptional responses in plants to optimize N acquisition, allocation, and remobilization. In roots of N-starved Arabidopsis (Arabidopsis thaliana) plants, transcriptional activation of genes encoding, for example, low-affinity nitrate transporters, depends on 4 related C-TERMINALLY ENCODED PEPTIDE DOWNSTREAM (CEPD) proteins, also known as ROXY6, ROXY7, ROXY8, and ROXY9. All 21 ROXYs found in A. thaliana interact with members of the TGACG-binding (TGA) family of transcription factors. Here, we demonstrate that 2 Clade I TGAs (TGA1, TGA4) serve as molecular links between CEPDs and their target promoters in roots. In the roxy6 roxy7 roxy8 roxy9 quadruple mutant (named cepd in this manuscript), transcriptional activation of N-starvation-inducible genes is impaired, most likely due to the association of Clade I TGAs with a repressive complex at their target promoters. In wild-type plants, this repressive complex is nonfunctional, and gene expression may be regulated by the N supply-regulated ratio of CEPDs over opposing ROXYs containing the TOPLESS-interacting ALWL motif. Although CEPDs resemble glutaredoxins with glutathione-dependent oxidoreductase activity, a ROXY9 variant with a mutation in the catalytic cysteine in its putative active site can confer wild-type-like regulation of target genes. This finding demonstrates that ROXY9 does not function through redox-dependent mechanisms.

Roles of transducin-like enhancer of split (TLE) family proteins in tumorigenesis and immune regulation

Mammalian transducin-like enhancer of split family proteins (TLEs) are homologous to Drosophila Groucho (Gro) and are essential transcriptional repressors. Seven TLE family members, TLE1-7, have been identified to date. These proteins do not bind DNA directly; instead, they bind a set of transcription factors and thereby inhibit target gene expression. Loss of TLEs in mice usually leads to defective early development; however, TLE functions in developmentally mature cells are unclear. Recent studies have revealed that TLEs are dysregulated in certain human cancer types and may function as oncogenes or tumor suppressors in different contexts. TLE levels also affect the efficacy of cancer treatments and the development of drug resistance. In addition, TLEs play critical roles in the development and function of immune cells, including macrophages and lymphocytes. In this review, we provide updates on the expression, function, and mechanism of TLEs; discuss the roles played by TLEs in tumorigenesis and the inflammatory response; and elaborate on several TLE-associated signaling pathways, including the Notch, Wnt, and MAPK pathways. Finally, we discuss potential strategies for targeting TLEs in cancer therapy.

Ras-Erk signaling induces phosphorylation of human TLE1 and downregulates its repressor function

Signaling mediated by the Ras-extracellular signal-regulated kinase (Erk) pathway often leads to the phosphorylation of transcriptional regulators, thereby modulating their activity and causing concerted changes in gene expression. In Drosophila, the induction of multiple Ras-Erk pathway target genes depends on prior phosphorylation of the general co-repressor Groucho, a modification that downregulates its repressive function. Here, we show that TLE1, one of the four human Groucho orthologs, is similarly phosphorylated in response to Ras-Erk pathway activation, and that this modification attenuates its capacity to repress transcription. Specifically, unphosphorylated TLE1 dominantly suppresses the induction of Ras-Erk pathway target genes in cultured human cells, and the expression of an unphosphorylatable TLE1 derivative causes severe phenotypes in a transgenic Drosophila model system, whereas a phosphomimetic variant of TLE1 exerts only negligible effects. We present data indicating that TLE1 is rapidly excluded from the nucleus following epidermal growth factor receptor pathway activation, an effect that likely accounts for its inability to mediate effective repression under such conditions. Significantly, we find that unphosphorylated TLE1 blocks oncogenic phenotypes induced by mutated H-Ras in human mammary cells, both in vitro and following their implantation in mice. Collectively, our data strongly indicate that phosphorylation of TLE family members and the consequent downregulation of their repressor function is a key conserved step in the transcriptional responses to Ras-Erk signaling, and possibly a critical event in the tumorigenic effects caused by excessive Ras-Erk pathway activity.

Drosophila Protein Kinase CK2: Genetics, Regulatory Complexity and Emerging Roles during Development

CK2 is a Ser/Thr protein kinase that is highly conserved amongst all eukaryotes. It is a well-known oncogenic kinase that regulates vital cell autonomous functions and animal development. Genetic studies in the fruit fly Drosophila are providing unique insights into the roles of CK2 in cell signaling, embryogenesis, organogenesis, neurogenesis, and the circadian clock, and are revealing hitherto unknown complexities in CK2 functions and regulation. Here, we review Drosophila CK2 with respect to its structure, subunit diversity, potential mechanisms of regulation, developmental abnormalities linked to mutations in the gene encoding CK2 subunits, and emerging roles in multiple aspects of eye development. We examine the Drosophila CK2 "interaction map" and the eye-specific "transcriptome" databases, which raise the prospect that this protein kinase has many additional targets in the developing eye. We discuss the possibility that CK2 functions during early retinal neurogenesis in Drosophila and mammals bear greater similarity than has been recognized, and that this conservation may extend to other developmental programs. Together, these studies underscore the immense power of the Drosophila model organism to provide new insights and avenues to further investigate developmentally relevant targets of this protein kinase.

The C-Terminal SynMuv/DdDUF926 Domain Regulates the Function of the N-Terminal Domain of DdNKAP

NKAP (NF-κB activating protein) is a highly conserved SR (serine/arginine-rich) protein involved in transcriptional control and splicing in mammals. We identified DdNKAP, the Dictyostelium discoideum ortholog of mammalian NKAP, as interacting partner of the nuclear envelope protein SUN-1. DdNKAP harbors a number of basic RDR/RDRS repeats in its N-terminal domain and the SynMuv/DUF926 domain at its C-terminus. We describe a novel and direct interaction between DdNKAP and Prp19 (Pre mRNA processing factor 19) which might be relevant for the observed DdNKAP ubiquitination. Genome wide analysis using cross-linking immunoprecipitation-high-throughput sequencing (CLIP-seq) revealed DdNKAP association with intergenic regions, exons, introns and non-coding RNAs. Ectopic expression of DdNKAP and its domains affects several developmental aspects like stream formation, aggregation, and chemotaxis. We conclude that DdNKAP is a multifunctional protein, which might influence Dictyostelium development through its interaction with RNA and RNA binding proteins. Mutants overexpressing full length DdNKAP and the N-terminal domain alone (DdN-NKAP) showed opposite phenotypes in development and opposite expression profiles of several genes and rRNAs. The observed interaction between DdN-NKAP and the DdDUF926 domain indicates that the DdDUF926 domain acts as negative regulator of the N-terminus.