The glucocorticoid receptor modulates transcription by different modes of action | © dkfz.de

The effect of steroid hormones in target cells is mediated by specific receptors which activate or inhibit transcription of target genes. The receptors for glucocorticoids, mineralocorticoids and estradiol do recognize related but distinct response elements in DNA. They also act by protein/protein interaction in a DNA-binding-indepedent mode. To characterize the functions of the receptors for these steroid hormones during development, in physiology and in pathological processes we have extensively used the methodology of gene targeting in the mouse. A few examples will illustrate our findings:
Hepatocyte-specific inactivation of the glucocorticoid receptor leads to strongly impaired body growth. However, mice deficient for dimer formation and DNA binding show no impairment of body growth indicating that DNA-binding of the receptor is not required for this process. Unexpectedly, the receptor functions as a coactivator for the transcription factor STAT5 in the growth hormone signaling cascade. This interaction mediates a selective and unexpectedly extensive part of the transcriptional action of these molecules.
Mice with deficiency of the glucocorticoid receptor in the brain show strongly impaired regulation of the hypothalamic-pituitary-adrenal axis. To understand the role of genes in brain functions like learning and memory, it is necessary to generate mutations in the adult in order to avoid developmental effects. To achieve temporally controlled mutagenesis in the forebrain we used BAC-based transgenesis for expression of a fusion protein of the Cre recombinase with the ligand binding domains of steroid hormone receptors. This allows the inactivation of a gene once development is completed and permits to characterize the functions of these receptors in the adult. Mice with inducible ablation of glucocorticoid and mineralocorticoid function are studied for stress axis activity, behaviour, synaptic plasticity and hippocampal neurogenesis.
Estrogens modulate the output of the gonadotropin-releasing hormone and in this way control ovulation. Ablation of estradiol receptor function in neurons of the forebrain disrupts the estrogen-dependent positive feedback which is crucial for the function of GnRH neurons. Since these neurons do not contain the receptor we have definite evidence for an indirect model of estrogen action on neuronal afferents to GnRH neurons. The production and secretion of GnRH is stimulated by kisspeptins, products of the Kiss1 gene. Kisspeptin production in hypothalamic nuclei of the anterior ventral periventricular (AVPV) and the arcuate nucleus is under control of estradiol and its receptor as we could show using mice with forebrain-specific inactivation of the ERα gene. Kisspeptin is a ligand for the G protein-coupled receptor GPR54 and mutation of this receptor in the germ line leads to hypogonadotrophic hypogonadism and infertility. Using BAC-based expresssion of GPR54 selectively in GnRH neurons of the mutants, we demonstrate that expression of this receptor exclusively in GnRH neurons is necessary and sufficient for fertility.
The nuclear receptor tailless, an orphan member of the nuclear receptor family, is expressed during development in the germinal zones of the brain and the developing retina, and germ line inactivation of the tailless gene leads to defective development. In the adult brain tailless is expressed in neural stem cells of the sub-ventricular zone and an inducible mutation of the gene in the adult leads to complete loss of neurogenesis, demonstrating a crucial role of this orphan receptor in the generation and maintenance of adult neural stem cells. Using genetically modified mice we established a novel brain tumor model and defined that the tailless gene is responsible for brain tumor initiation.