Biomedical subjects

Christina Zechel

Publications and source records attributed to Christina Zechel.

5 recordsLinked to original sources

PubMed2005

Requirement of retinoic acid receptor isotypes alpha, beta, and gamma during the initial steps of neural differentiation of PCC7 cells.

Retinoic acid (RA) is indispensable for morphogenesis and differentiation of several tissues, including the nervous system. The requirement of the RA receptor (RAR) isotypes alpha, beta, and gamma and the putative role of retinoid X receptor-(RXR) signaling in RA-induced neural differentiation, was analyzed. For this compound-selective retinoids and the murine embryonal carcinoma cell line PCC7, a model system for RA-dependent neural differentiation was used. The present paper shows that proliferating PCC7 cells primarily express RXRalpha and RARalpha, lower levels of RXRbeta, and barely detectable amounts of RARbeta, RARgamma, and RXRgamma. At receptor-selective concentrations, only a RARalpha or RARgamma agonist induced the typical tissue-like differentiation pattern consisting of neuronal and nonneuronal cells. Differentiation-associated processes, such as the down-regulation of Oct4, up-regulation of certain nuclear receptors and proneuronal genes, and the induction of neuronal markers could be triggered by receptor-selective concentrations of a RARalpha-, beta-, or gamma-selective agonist, although with distinct efficacy. The differences are only partially explained by the distinct RARalpha, beta, and gamma expression levels and the dissociation constants for the bound retinoids, suggesting differential requirement of RAR isotypes during the initial stages of neural differentiation of PCC7 cells.

Animals↗

PubMed2005

The germ cell nuclear factor (GCNF).

The germ cell nuclear factor (GCNF), which is also known as RTR (retinoid receptor-related testis-associated receptor) is a member of the nuclear receptor superfamily. As a natural ligand remains to be discovered, GCNF is referred to as an orphan receptor. Owing to GCNF's unique features and its distant relation to any other known nuclear receptor it has been classified as the only member of the subgroup six and designated NR6A1 by the Receptor Nomenclature Committee (Duarte et al., 2002: Nucleic Acids Res 30: 364-368). To date, GCNF has been cloned from distinct vertebrate species, including zebrafish, Xenopus laevis, mouse, rat, and human. Cloning and characterization of the gene, domain organization and DNA binding properties of the protein, as well as the differential expression of mRNA splice variants or the protein during development and in the adult animal have been comprehensively reviewed by others (Greschik and Schüle, 1998: J Mol Med 76:800-810; Cooney et al., 1999: Am Zool 39:796-806). In this minireview I focus on the pleiotropic function of GCNF in embryogenesis and germ cell differentiation, and discuss novel concepts about its putative role in neurogenesis.

Animals↗

PubMed2004

The expression level of the orphan nuclear receptor GCNF (germ cell nuclear factor) is critical for neuronal differentiation.

The germ cell nuclear factor (GCNF) is essential for normal embryonic development and gametogenesis. To test the prediction that GCNF is additionally required for neuronal differentiation, we used the mouse embryonal carcinoma cell line PCC7-Mz1, which represents an advantageous model to study neuronal cells from the stage of fate choice until the acquirement of functional competence. We generated stable transfectants that express gcnf sense or antisense RNA under the control of a tetracycline-regulated promoter. After retinoic acid-induced withdrawal from the cell cycle, sense clones developed a neuron network with changed properties, and the time course of neuron maturation was shortened. Consistent with these data, differentiation of neuronal precursor cells was impaired in antisense cultures. This involved a delay in 1) the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells and stem cells of the central nervous system, and 2) up-regulation of the somatodendritic protein microtubule-associated protein 2 and the synaptic vesicle protein synaptophysin. Neuronal cells in the antisense cultures acquired functional competence, although with a significant delay. Our data propose that the level of GCNF is critical for differentiation and maturation of neuronal precursor cells.

Animals↗

PubMed2002

Co-regulator recruitment and the mechanism of retinoic acid receptor synergy.

Crystal structure and co-regulator interaction studies have led to a general mechanistic view of the initial steps of nuclear receptor (NR) action. Agonist-induced transconformation of the ligand-binding domain (holo-LBD) leads to the formation of co-activator complexes, and destabilizes the co-repressor complexes bound to the ligand-free (apo) LBD. However, the molecular basis of retinoid-X receptor (RXR) 'subordination' in heterodimers, an essential mechanism to avoid signalling pathway promiscuity, has remained elusive. RXR, in contrast to its heterodimer partner, cannot autonomously induce transcription on binding of cognate agonists. Here we show that RXR can bind ligand and recruit co-activators as a heterodimer with apo-retinoic-acid receptor (apo-RAR). However, in the usual cellular environment co-repressors do not dissociate and they prohibit co-activator access because co-regulator binding is mutually exclusive. Accordingly, RXR subordination can be overcome in heterodimers that bind co-repressor weakly or in cells with a high co-activator content. We identify two types of RAR antagonists that differentially modulate co-regulator interaction, and we demonstrate that synergy between RAR ligands and RXR agonists results from increased interaction efficiency of a single p160 with the heterodimer, requiring two intact receptor-binding surfaces on the co-activator.

DNA-Binding Proteins↗

PubMedDate not supplied

Synthetic retinoids dissociate coactivator binding from corepressor release.

The ligand-activated retinoid receptors RXR and RAR control development, homeostasis and disease by regulating transcription of retinoic acid (RA) responsive target genes or crosstalk with other signalling pathways. According to the current model ligand-binding triggers an exchange between corepressor- and coactivator-complexes that inhibit or potentiate transcription by deacetylating and acetylating nucleosomal histones, respectively. Additional cofactors may modify the transcriptional regulatory process by linking liganded retinoid receptors to structural components of chromatin or protein degradation. The desire to specifically influence defined events in RA-signalling, while others are left unaffected, motivated the synthesis of retinoid X receptors (RXR)- and retinoid acid receptors (RAR) isoform-selective retinoids. The present study investigates the potential of RARalpha isotype-specific synthetic agonists and antagonists to separate the processes of coactivator recruitment and corepressor release. The synthetic retinoids studied fall into four categories, two of which work according the above model, since they induce surfaces within the RARalpha ligand binding domain (LBD) suitable for either corepressor or coactivator interaction; these retinoids act as pure antagonists and pure agonists, respectively. In contrast, another type of retinoid induces a structure that allows for both, the interaction with corepressors and coactivators (partial RARalpha agonist), and exerts a cell context-specific (ant)agonistic activity. Finally, another type of retinoid, which cannot activate transcription itself but renders heterodimeric RARalpha permissive for signaling by RXR agonists inhibits both, corepressor and coactivator interaction (partial antagonist). Moreover, this retinoid discriminates between the nuclear corepressors SMRT and NCoR, since it efficiently dissociates SMRT but not NCoR from the RARalpha LBD.

Amino Acid Motifs↗