5 research outputs found
The nuclear entry of the aryl hydrocarbon receptor (AHR) relies on the first nuclear localization signal and can be negatively regulated through IMPα/ÎČ specific inhibitors
The human aryl hydrocarbon receptor (AHR) undergoes continuous shuttling between nucleus and cytoplasm. Binding to exogenous or endogenous ligands promotes its rapid nuclear import. The proposed mechanism for the ligand-dependent import is based on exposing the bipartite nuclear localisation signal (NLS) to members of the importin (IMP) superfamily. Among this, the molecular interactions involved in the basal import still need to be clarified. Utilizing fluorescently fused AHR variants, we recapitulated and characterized AHR localization and nucleo-cytoplasmic shuttling in living cells. Analysis of AHR variants carrying NLS point mutations demonstrated a mandatory role of first (13RKRRK17) and second (37KR-R40) NLS segments on the basal import of AHR. Further experiments indicated that ligand-induced import is mainly regulated through the first NLS, while the second NLS is supportive but not essential. Additionally, applying IMPα/ÎČ specific inhibitors, ivermectin (IVM) and importazole (IPZ), slowed down the ligand-induced import and, correspondingly, decreased the basal nuclear accumulation of the receptor. In conclusion, our data show that ligand-induced and basal nuclear entry of AHR rely on the same mechanism but are controlled uniquely by the two NLS components
Eine Machbarkeitsstudie
1\. Introduction 1-1 1.1 Teratology 1-1 1.1.1 General Aspects and present
testing strategies 1-1 1.1.2 In vitro alternatives and recent developments 1-3
1.2 Embryonic stem cells 1-5 1.2.1 Characteristics and usage 1-5 1.2.2
Differentiation of ESCs into cardiomyocytes 1-9 1.3 Signaling Cascades 1-12
1.3.1 General aspects 1-12 1.3.2 Bmp signaling Pathway 1-15 1.3.3 Wnt
Signaling Pathway 1-19 1.4 Substances 1-22 2\. Objective 2-27 3\. Results 3-29
3.1 A Bmp reporter transgene mouse embryonic stem cell model as a tool to
identify and characterize chemical teratogens 3-29 3.2 Identification and
characterization of teratogenic chemicals using embryonic stem cells isolated
from a Wnt/ÎČ-Catenin-reporter transgenic mouse line 3-57 3.3 Transgenic mouse
models transferred into the test tube: New perspectives for toxicity testing
in vitro? 3-85 4\. Discussion 4-105 5\. Conclusion and Outlook 5-111 6\.
Summary 6-113 7\. Zusammenfassung 7-115 8\. Literature 8-117 9\. Abbreviations
9-129 10\. List of Publications 10-133 11\. Acknowledgment - Danksagung 11-135The assessment of a teratogenic potential of substances and compounds
currently relies on time and cost intensive in vivo tests, although recent
changes in legislation increase the need for in vitro alternatives. My work is
a proof of concept study, analyzing whether highly conserved, essential
signaling cascades can be used as predictive tool for teratogenic activity.
Signaling cascades regulate embryonic development in vivo as well as the
differentiation of embryonic stem cells (ESCs) in vitro and the latter already
had been shown to be suitable as a predictive tool in the embryonic stem cell
test (EST). For my studies I focused on two different cascades, the canonical
Wnt and Bmp signaling pathways. The used transgenic ESCs were generated from
well described transgenic mouse lines, each harboring an easily detectable
reporter gene under the control of one essential signaling pathway. The
canonical Wnt pathway is essential for the mesoderm induction and the reporter
gene in the BAT-Gal-ESCs was expressed in mesodermal cells and cells
undergoing epithelial to mesenchymal transition. During cardiac
differentiation, the Wnt pathway has a biphasic role, which could be monitored
in vitro as well. For the detection of an increase and/or a decrease in
reporter gene activity, two independent time points were analyzed. Increased
as well as decreased reporter gene activity was associated with impaired
cardiac differentiation in vitro such that the BAT-Gal-ESCs could indeed be
used as predictive tool for teratogenic activities. The BRE-ESCs, expressing
the green fluorescence protein (EGFP) under the control of Bmp signaling,
showed an endogenous induction of EGFP during the differentiation into
cardiomyocytes. Especially endodermal and epithelial cells expressed high
amounts of EGFP and the endodermal cells proved to be essential for the
specification of cardiac mesoderm. Upon treatment with model substances a
dose-dependent decrease in EGFP expression and cardiomyocyte differentiation
was detectable, demonstrating suitability of this cell line for the testing of
teratogenic activities as well. In general, the data strongly supports the
working hypothesis that highly conserved, essential signaling cascades can be
used to predict teratogenic substances during the differentiation of ESCs. And
along with the general inhibition of differentiation, these cell systems
provide mechanistic information and can even shorten assay duration.WĂ€hrend der Embryonalentwicklung entstehen aus einer befruchteten Eizelle
sÀmtliche Gewebe, Organe und Strukturen des spÀteren Individuums. Die
KomplexitĂ€t des Vorganges bedingt, dass Ă€uĂere EinflĂŒsse dramatische
Auswirkungen auf diesen streng regulierten Prozess haben können und die
Identifizierung in dieser Weise wirkenden Substanzen im Besonderen erfolgt mit
Hilfe von zeit- und kostenintensiven Tierversuchen. Verschiedene gesetzliche
Ănderungen der letzten Jahre haben dazu gefĂŒhrt, dass in vitro Alternativen
zum Tierversuch eine zunehmenden Bedeutung erlangten. In dieser Arbeit sollte
eine in vitro Alternative, der embryonale Stammzelltest (EST), zur Testung von
Teratogenen im Hinblick auf neue Endpunkte weiter entwickelt werden. Der EST
betrachtet die Differenzierung der ESCs in spontan schlagende
Herzmuskelzellen. Dieser Vorgang ist Àhnlich strikt reguliert wie die in vivo
Entwicklung und hÀufig genutzt zur AufklÀrung der zugrunde liegenden Prozesse
in vitro. Dazu nutzte ich murine, embryonale Stammzellen (ESCs), die aus
transgenen Mauslinien gewonnen wurden und ein leicht zu detektierendes
Reportergen unter dem Einfluss von hoch konservierten, essentiellen
Signalkaskaden besitzen. Dabei habe ich mich auf den Bmp und den Wnt/ÎČ-Catenin
Signalweg konzentriert. Die BRE-ESCs, die ein grĂŒn-fluoreszierendes Protein
(EGFP) unter der Kontrolle des Bmp Signalweges exprimieren, zeigten eine
endogene Induction der EGFP Expression wÀhrend der Herzmuskeldifferenzierung.
Die Analyse der EGFP-positiven Zellen ergab, dass es sich um endodermale und
endotheliale Zellen handelt, wobei die endodermalen Zellen fĂŒr die
Spezifikation des Mesoderms in kardiogenes Mesoderm essentiell sind. Die
Analyse teratogen wirkender Substanzen zeigte eine dosisabhÀngige Reduktion
der EGFP Expression, die gut mit der Verminderung der
Herzmuskeldifferenzierung korrelierte. Das zweite Zellsystem, die BAT-Gal-
ESCs, exprimieren ihr Reportergen unter dem Einfluss des kanonischen Wnt-
Signalweges. Auch dieser Signalweg wird wÀhrend der Differenzierung endogen
induziert und ist eng mit der Mesoderminduktion assoziiert. Dies konnte ich
bestÀtigen, da die reportergenpositiven Zellen mesodermalen Ursprungs waren.
ZusÀtzlich zeigte sich, dass Zellen, die eine epithelial-mesenchymale
Transition durchlaufen ebenfalls ĂŒber eine erhöhte AktivitĂ€t des Wnt
Signalweges verfĂŒgen. In diesem Testsystem konnte gezeigt werden, dass sowohl
eine kĂŒnstliche Induktion als auch eine Reduktion der ReportergenaktivitĂ€t mit
einer verminderten Herzmuskeldifferenzierung einhergehen kann und auch diese
Zellen sich als prÀdiktives Testsystem eignen. Damit konnte in der
vorliegenden Arbeit die Arbeitshypothese bestÀtigt werden, dass eine Analyse
von essentiellen, hoch konservierten Signalkaskaden sich fĂŒr eine Vorhersage
eines teratogenen Potentials einer Substanz eignen. ZusÀtzlich können durch
die Erfassung mehrerer Endpunkte in einem Zellsystem auch mechanistische
Informationen gewonnen werden, so dass eventuell die Wirkungsweise der
verschiedenen, potentiell teratogenen Stoffe genauer charakterisiert werden
kann
The role of DNA-binding and ARNT dimerization on the nucleo-cytoplasmic translocation of the aryl hydrocarbon receptor
The human aryl hydrocarbon receptor (AHR) is predominantly located in the cytoplasm, while activation depends on its nuclear translocation. Binding to endogenous or xenobiotic ligands terminates the basal nucleo-cytoplasmic shuttling and stabilizes an exclusive nuclear population. The precise mechanisms that facilitate such stable nuclear accumulation remain to be clarified as essential step in the activation cascade. In this study, we have tested whether the sustained nuclear compartmentalization of ligand-bound or basal AHR might further require heterodimerization with the AHR-nuclear translocator (ARNT) and binding to the cognate XRE-motif. Mutagenesis of the DNA-binding motif or of selected individual residues in the ARNT-binding motif did not lead to any variation in AHRâs nucleo-cytoplasmic distribution. In response to ligands, all mutants were retained in the nucleus demonstrating that the stable compartmentalization of activated AHR in the nucleus is neither dependent on interactions with DNA, nor ARNT. Knocking down the ARNT gene using small interfering RNA confirmed that ARNT does not play any role in the intracellular trafficking of AHR