Induction of chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI ) gene expression is mediated by ETS factor binding sites

Zbtb20 modulates the sequential generation of neuronal layers in developing cortex

Background

During corticogenesis, genetic programs encoded in progenitor cells at different developmental stages and inherited in postmitotic neurons specify distinct layer and area identities. Transcription factor Zbtb20 has been shown to play a role for hippocampal development but whether it is implicated in mammalian neocortical morphogenesis remains unknown.

Results

Here, we report that during embyogenesis transcription factor Zbtb20 has a dynamic spatio-temporal expression pattern in mitotic cortical progenitors through which it modulates the sequential generation of cortical neuronal layer identities. Zbtb20 knock out mice exhibited enhanced populations of early born L6-L4 neuronal subtypes and a dramatic reduction of the late born L3/L2 neurons. This defect was due to a temporal misbalance in the production of earlier versus later born neurons, leading to a progressive diminishing of the progenitor pool for the generation of L3-L2 neurons. Zbtb20 implements these temporal effects in part by binding to promoter of the orphan nuclear receptor CoupTF1/Nr2f1. In addition to its effects exerted in cortical progenitors, the postmitotic expression of Zbtb20 in L3/L2 neurons starting at birth may contribute to their proper differentiation and migration.

Conclusions

Our findings reveal Zbtb20 as a novel temporal regulator for the generation of layer-specific neuronal identities.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0242-2) contains supplementary material, which is available to authorized users.

Cis-regulatory control of the nuclear receptor Coup-TF gene in the sea urchin Paracentrotus lividus embryo

Coup-TF, an orphan member of the nuclear receptor super family, has a fundamental role in the development of metazoan embryos. The study of the gene's regulatory circuit in the sea urchin embryo will facilitate the placement of this transcription factor in the well-studied embryonic Gene Regulatory Network (GRN). The Paracentrotus lividus Coup-TF gene (PlCoup-TF) is expressed throughout embryonic development preferentially in the oral ectoderm of the gastrula and the ciliary band of the pluteus stage. Two overlapping λ genomic clones, containing three exons and upstream sequences of PlCoup-TF, were isolated from a genomic library. The transcription initiation site was determined and 5′ deletions and individual segments of a 1930 bp upstream region were placed ahead of a GFP reporter cassette and injected into fertilized P.lividus eggs. Module a (−532 to −232), was necessary and sufficient to confer ciliary band expression to the reporter. Comparison of P.lividus and Strongylocentrotus purpuratus upstream Coup-TF sequences, revealed considerable conservation, but none within module a. 5′ and internal deletions into module a, defined a smaller region that confers ciliary band specific expression. Putative regulatory cis-acting elements (RE1, RE2 and RE3) within module a, were specifically bound by proteins in sea urchin embryonic nuclear extracts. Site-specific mutagenesis of these elements resulted in loss of reporter activity (RE1) or ectopic expression (RE2, RE3). It is proposed that sea urchin transcription factors, which bind these three regulatory sites, are necessary for spatial and quantitative regulation of the PlCoup-TF gene at pluteus stage sea urchin embryos. These findings lead to the future identification of these factors and to the hierarchical positioning of PlCoup-TF within the embryonic GRN.

The nuclear receptors COUP-TF: a long-lasting experience in forebrain assembly

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are nuclear receptors belonging to the superfamily of the steroid/thyroid hormone receptors. Members of this family are internalized to the nucleus both in a ligand-dependent or -independent manner and act as strong transcriptional regulators by binding to the DNA of their target genes. COUP-TFs are defined as orphan receptors, since ligands regulating their activity have not so far been identified. From the very beginning of metazoan evolution, these molecules have been involved in various key events during embryonic development and organogenesis. In this review, we will mainly focus on their function during development and maturation of the central nervous system, which has been well characterized in various animal classes ranging from ctenophores to mammals. We will start by introducing the current knowledge on COUP-TF mechanisms of action and then focus our discussion on the crucial processes underlying forebrain ontogenesis, with special emphasis on mammalian development. Finally, the conserved roles of COUP-TFs along phylogenesis will be highlighted, and some hypotheses, worth exploring in future years to gain more insight into the mechanisms controlled by these factors, will be proposed.

Polycomb recruitment attenuates retinoic acid-induced transcription of the bivalent NR2F1 gene

Polycomb proteins play key roles in mediating epigenetic modifications that occur during cell differentiation. The Polycomb repressive complex 2 (PRC2) mediates the tri-methylation of histone H3 lysine 27 (H3K27me3). In this study, we identify a distinguishing feature of two classes of PRC2 target genes, represented by the Nr2F1 (Coup-TF1) and the Hoxa5 gene, respectively. Both genes are transcriptionally activated by all-trans retinoic acid (RA) and display increased levels of the permissive H3K9/K14ac and tri-methylated histone H3 lysine 4 epigenetic marks in response to RA. However, while in response to RA the PRC2 and H3K27me3 marks are greatly decreased at the Hoxa5 promoter, these marks are initially increased at the Nr2F1 promoter. Functional depletion of the essential PRC2 protein Suz12 by short hairpin RNA (shRNA) technology enhanced the RA-associated transcription of Nr2F1, Nr2F2, Meis1, Sox9 and BMP2, but had no effect on the Hoxa5, Hoxa1, Cyp26a1, Cyp26b1 and RARβ2 transcript levels in wild-type embryonic stem cells. We propose that PRC2 recruitment attenuates the RA-associated transcriptional activation of a subset of genes. Such a mechanism would permit the fine-tuning of transcriptional networks during differentiation.

Gradients in the brain: the control of the development of form and function in the cerebral cortex

In the developing brain, gradients are commonly used to divide neurogenic regions into distinct functional domains. In this article, we discuss the functions of morphogen and gene expression gradients in the assembly of the nervous system in the context of the development of the cerebral cortex. The cerebral cortex is a mammal-specific region of the forebrain that functions at the top of the neural hierarchy to process and interpret sensory information, plan and organize tasks, and to control motor functions. The mature cerebral cortex is a modular structure, consisting of anatomically and functionally distinct areas. Those areas of neurons are generated from a uniform neuroepithelial sheet by two forms of gradients: graded extracellular signals and a set of transcription factor gradients operating across the field of neocortical stem cells. Fgf signaling from the rostral pole of the cerebral cortex sets up gradients of expression of transcription factors by both activating and repressing gene expression. However, in contrast to the spinal cord and the early Drosophila embryo, these gradients are not subsequently resolved into molecularly distinct domains of gene expression. Instead, graded information in stem cells is translated into discrete, region-specific gene expression in the postmitotic neuronal progeny of the stem cells.

The regulation of COUP-TFII gene expression by Ets-1 is enhanced by the steroid receptor co-activators

Recent phenotypic analysis of orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) [NR2F2] knockout mice shows that COUP-TFII is involved in the angiogenic process in the developing embryos. Since Ets-1 expression is also correlated with angiogenesis, and both Ets-1 and COUP-TFII mRNA are present in mesenchymal cells, we have sought to determine whether Ets-1 is a potential regulator of COUP-TFII gene expression. For this purpose, we performed transient transfection experiments using a luciferase reporter construct containing the mouse COUP-TFII promoter. We found that the COUP-TFII promoter activity is indeed regulated by Ets-1. We have identified two identical inverted potential ETS-binding sites located 47 nucleotides downstream of the start site. Mutation of both sites reduced the ability of Ets-1 to enhance the COUP-TFII promoter activity. Furthermore, other members of the ETS family such as Ets-2 or ETV1 are also potent regulators of the COUP-TFII promoter. Finally, the induction of the COUP-TFII gene is strongly enhanced by the expression of steroid receptor co-activator factors through a direct interaction with Ets-1. These results indicate that COUP-TFII is a potential downstream target of Ets-1 and it may partially mediate the Ets-1 function in angiogenesis.