Analysis of perinatal gene expression: hormone response elements mediate activation of a lacZ reporter gene in liver of transgenic mice

In Vivo Imaging of Nuclear Receptor Transcriptional Activity

Nuclear receptors drive key processes during development, reproduction, metabolism, and disease. In order to understand and analyze, as well as manipulate, their actions it is imperative that we are able to study them in whole animals and in a spatiotemporal manner. The increasing repertoire of transgenic animals, expressing reporter genes driven by a specific nuclear receptor, enables us to do this. Use of luciferase reporter genes is the method of choice of many researchers as it is well tolerated, relatively easy to use, and robust. Further, luciferase lends itself to the process as it can penetrate tissue and can be manipulated to degrade rapidly thus allowing a dynamic response. However, limited resolution, lack of quantitation, and the largely two-dimensional images acquired make it desirable to support results using ex vivo imaging and enzymatic and/or immunohistochemical analysis of dissected tissue. As well as enabling the visualization of nuclear receptor signaling in wild-type animals, crossing these mouse models with models of disease will provide invaluable information on how such signaling is dysregulated during disease progression, and how we may manipulate nuclear receptor signaling in therapy. The use of in vivo imaging therefore provides the power to determine where and when in development, aging, and disease nuclear receptors are active and how ligands or receptor modulators affect this.

Epigenetic Modulators Enhance Constitutive and Liver-Specific Reporter Expression in Murine Liver Progenitor Cell Lines

Stem cells expressing reporter constructs are extremely useful for their tracking in vivo or for determining cell lineage fate in vivo and in vitro. We generated liver progenitor cell (LPC) lines from actin-EGFP and TAT-GRE-lacZ mice. LPCs from the actin-EGFP mouse facilitate cell tracing following transplant as the reporter is constitutively expressed. LPCs from the TAT-GRE-lacZ mouse express β-galactosidase under the control of the tyrosine aminotransferase (TAT) promoter and are only active in mature hepatocytes. We found that the utility of such LPC lines becomes severely limited by downregulation of transgene expression following extended culture. We show that epigenetic mechanisms are responsible for suppressing expression of both transgenes. Enhancement of transgene expression in both LPC lines was achieved by treating the cell lines with either the histone acetylating agent sodium butyrate or the DNA demethylating agent 5-azacytidine.

In vivo dynamics of estrogen receptor activity: the ERE-Luc model

In recent years several studies demonstrated the presence of estrogen receptors in mammalian tissues and significantly improved our understanding of their ability to control biological processes in reproductive as well as non-reproductive organs. Considering the manifold mechanisms and organs that are involved in estrogen action and the implication of estrogens in human female physiology, innovative approaches are required to shed light on the widespread activities of estrogen receptors in woman physiology. This is particularly relevant for the definition of novel, more efficacious hormonal replacement therapies or for the evaluation of the risk associated with the exposure to endocrine disruptors. The introduction of genetic engineering and the development and application of in vivo imaging techniques offer new tools for pre-clinical studies. The generation of the ERE-Luc mouse, a reporter animal developed for in vivo studies of the estrogen receptor activity, allows assessing the activity state of the ER signaling pathway in all target tissues and organs at once, under physiological stimuli or as a result of a pharmacological treatment. This review summarizes the main steps in the generation and appraisal of the estrogen receptor reporter mouse ERE-Luc, designed for in vivo molecular imaging studies, and describes examples demonstrating the suitability of the ERE-Luc model for drug development and for the investigation of the effects of endogenous, environmental, and dietary estrogens in vivo. This article is part of a Special Issue entitled 'Phytoestrogens'.

Epigenetic Control of Circadian Clock Operation during Development

The molecular players of circadian clock oscillation have been identified and extensively characterized. The epigenetic mechanisms behind the circadian gene expression control has also been recently studied, although there are still details to be illucidated. In this review, we briefly summarize the current understanding of the mammalian clock. We also provide evidence for the lack of circadian oscillation in particular cell types. As the circadian clock has intimate interaction with the various cellular functions in different type of cells, it must have plasticity and specicity in its operation within different epigenetic environments. The lack of circadian oscillation in certain cells provide an unique opportunity to study the required epigenetic environment in the cell that permit circadian oscillation and to idenfify key influencing factors for proper clock function. How epigenetic mechansims, including DNA methylaiton and chromatin modifications, participate in control of clock oscillation still awaits future studies at the genomic scale.

Protein reporter bioassay systems for the phenotypic screening of candidate drugs: a mouse platform for anti-aging drug screening

Recent drug discovery efforts have utilized high throughput screening (HTS) of large chemical libraries to identify compounds that modify the activity of discrete molecular targets. The molecular target approach to drug screening is widely used in the pharmaceutical and biotechnology industries, because of the amount of knowledge now available regarding protein structure that has been obtained by computer simulation. The molecular target approach requires that the structure of target molecules, and an understanding of their physiological functions, is known. This approach to drug discovery may, however, limit the identification of novel drugs. As an alternative, the phenotypic- or pathway-screening approach to drug discovery is gaining popularity, particularly in the academic sector. This approach not only provides the opportunity to identify promising drug candidates, but also enables novel information regarding biological pathways to be unveiled. Reporter assays are a powerful tool for the phenotypic screening of compound libraries. Of the various reporter genes that can be used in such assays, those encoding secreted proteins enable the screening of hit molecules in both living cells and animals. Cell- and animal-based screens enable simultaneous evaluation of drug metabolism or toxicity with biological activity. Therefore, drug candidates identified in these screens may have increased biological efficacy and a lower risk of side effects in humans. In this article, we review the reporter bioassay systems available for phenotypic drug discovery.

Stressing zebrafish for behavioral genetics

The stress response is a normal reaction to a real or perceived threat. However, stress response systems that are overwhelmed or out of balance can increase both the incidence and severity of diseases including addiction and mood and anxiety disorders. Using an animal model with both genetic diversity and large family size can help discover the specific genetic and environmental contributions to these behavioral diseases. The stress response has been studied extensively in teleosts because of their importance in food production. The zebrafish (Danio rerio) is a major model organism with a strong record for use in developmental biology, genetic screening, and genomic studies. More recently, the stress response of larval and adult zebrafish has been documented. High-throughput automated tracking systems make possible behavioral readouts of the stress response in zebrafish. This non-invasive measure of the stress response can be combined with mutagenesis methods to dissect the genes involved in complex stress response behaviors in vertebrates. Understanding the genetic and epigenetic basis for the stress response in vertebrates will help to develop advanced screening and therapies for stress-aggravated diseases such as addiction and mood and anxiety disorders.

Exploitation of herpesvirus immune evasion strategies to modify the immunogenicity of human mesenchymal stem cell transplants

BACKGROUND

Mesenchymal stem cells (MSCs) are multipotent cells residing in the connective tissue of many organs and holding great potential for tissue repair. In culture, human MSCs (hMSCs) are capable of extensive proliferation without showing chromosomal aberrations. Large numbers of hMSCs can thus be acquired from small samples of easily obtainable tissues like fat and bone marrow. MSCs can contribute to regeneration indirectly by secretion of cytokines or directly by differentiation into specialized cell types. The latter mechanism requires their long-term acceptance by the recipient. Although MSCs do not elicit immune responses in vitro, animal studies have revealed that allogeneic and xenogeneic MSCs are rejected.

METHODOLOGY/PRINCIPAL FINDINGS

We aim to overcome MSC immune rejection through permanent down-regulation of major histocompatibility complex (MHC) class I proteins on the surface of these MHC class II-negative cells through the use of viral immune evasion proteins. Transduction of hMSCs with a retroviral vector encoding the human cytomegalovirus US11 protein resulted in strong inhibition of MHC class I surface expression. When transplanted into immunocompetent mice, persistence of the US11-expressing and HLA-ABC-negative hMSCs at levels resembling those found in immunodeficient (i.e., NOD/SCID) mice could be attained provided that recipients' natural killer (NK) cells were depleted prior to cell transplantation.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate the potential utility of herpesviral immunoevasins to prevent rejection of xenogeneic MSCs. The observation that down-regulation of MHC class I surface expression renders hMSCs vulnerable to NK cell recognition and cytolysis implies that multiple viral immune evasion proteins are likely required to make hMSCs non-immunogenic and thereby universally transplantable.

Prox1 expression patterns in the developing and adult murine brain

Prox1, a homeobox gene related to the Drosophila gene prospero, is necessary for retina, lens, liver, pancreas, and lymphatics development. However, not much is yet known about Prox1 expression during central nervous system development. Here we provide a detailed analysis of Prox1 mRNA and protein expression during prenatal and postnatal murine brain development. Prenatally, Prox1 is expressed in the subventricular zone or in early differentiating regions of the brain. At these stages, Prox1 mRNA, but not Prox1 protein, was also detected in several regions of the prethalamus and hypothalamus. At an early postnatal stage, Prox1 expression is mainly detected in several nuclei of the thalamus, the cerebellum, and the hippocampus. In adulthood, Prox1 expression remains only in the hippocampus and cerebellum. These complex patterns of expression suggest that Prox1 activity is differentially required during brain development and adulthood.

Jekyll and Hyde: evolving perspectives on the function and potential of the adult liver progenitor (oval) cell

The liver progenitor cell (LPC) has enormous potential for use in cell therapy to treat liver disease. Since liver regenerates readily from pre-existing hepatocytes, a role for LPCs and, indeed, their existence have been questioned. Research during the last decade has established that LPCs are an important alternative source of cells for liver regeneration. Their utility for cell therapy lies in their ability to generate both hepatocytes and cholangiocytes. However, they are observed in liver diseases that often lead to cancer and there is experimental evidence that implicates LPCs as the source of tumours. This article provides a brief history of the studies that established the functional importance of LPCs in liver disease. It focuses on mouse models that have led to the identification of factors that regulate LPC growth and differentiation and discusses LPCs derived from different sources. Recent promising results from both in vitro and vivo studies suggest that LPCs could be useful for cell therapy. In the context of liver disease, LPCs may indeed be the cell of the future and understandably "our favourite cell".

Reporter mice and drug discovery and development

In vivo reporter gene and imaging technologies have the potential to contribute to the drug discovery pipeline in several areas. They provide systems that enable the study of the biochemical activity of a target in disease, and in response to a drug, to be monitored over periods of time, and offer more accurate methods of measuring pharmacodynamics and toxicity. Although reporter-gene technology is in its infancy, with further refinement reporter animals could become a valuable tool in the early stages of target and lead identification and preclinical drug development.

Transcriptional regulation of agouti-related protein (Agrp) in transgenic mice

Agouti-related protein (Agrp) encodes a hypothalamic neuropeptide that promotes positive energy balance by stimulating food intake and reducing energy expenditure. Agrp expression in the brain is restricted to neurons within the arcuate nucleus of the hypothalamus, and expression levels are elevated as a consequence of food deprivation. We tested a series of bacterial artificial chromosome reporter constructs with varying amounts of sequence flanking the Agrp transcription unit in transgenic mice to identify and refine a region of DNA capable of recapitulating characteristics of Agrp expression. We report that a 42.5-kb region upstream of Agrp, containing three distinct regions that are evolutionarily conserved between mouse and human, is necessary and sufficient to consistently drive reporter expression specifically within AgRP neurons in a fasting-responsive manner. In addition, we demonstrate that this region allows for the stable expression of Cre recombinase in transgenic mice, providing a genetic tool for studying anabolic neural circuits that control energy balance.

A Transgenic Mouse Model with Inducible Tyrosinase Gene Expression Using the Tetracycline (Tet-on) System Allows Regulated Rescue of Abnormal Chiasmatic Projections Found in Albinism

Congenital defects in retinal pigmentation, as in oculocutaneous albinism Type I (OCA1), where tyrosinase is defective, result in visual abnormalities affecting the retina and pathways into the brain. Transgenic animals expressing a functional tyrosinase gene on an albino genetic background display a correction of all these abnormalities, implicating a functional role for tyrosinase in normal retinal development. To address the function of tyrosinase in the development of the mammalian visual system, we have generated a transgenic mouse model with inducible expression of the tyrosinase gene using the tetracycline (TET-ON) system. We have produced two types of transgenic mice: first, mice expressing the transactivator rtTA chimeric protein under the control of mouse tyrosinase promoter and its locus control region (LCR), and; second, transgenic mice expressing a mouse tyrosinase cDNA construct driven by a minimal promoter inducible by rtTA in the presence of doxycycline. Inducible experiments have been carried out with selected double transgenic mouse lines. Tyrosinase expression has been induced from early embryo development and its impact assessed with histological and biochemical methods in heterozygous and homozygous double transgenic individuals. We have found an increase of tyrosinase activity in the eyes of induced animals, compared with littermate controls. However, there was significant variability in the activation of this gene, as reported in analogous experiments. In spite of this, we could observe corrected uncrossed chiasmatic pathways, decreased in albinism, in animals induced from their first gestational week. These mice could be instrumental in revealing the role of tyrosinase in mammalian visual development.

Identification and functional validation of a 5' upstream regulatory sequence in the human tyrosinase gene homologous to the locus control region of the mouse tyrosinase gene

Comparison analysis of the sequences of the mouse and human genomes has proven a powerful approach in identifying functional regulatory elements within the non-coding regions that are conserved through evolution between homologous mammalian loci. Here, we applied computational analysis to identify regions of homology in the 5' upstream sequences of the human tyrosinase gene, similar to the locus control region (LCR) of the mouse tyrosinase gene, located at -15 kb. We detected several stretches of homology within the first 30 kb 5' tyrosinase gene upstream sequences of both species that include the proximal promoter sequences, the genomic region surrounding the mouse LCR, and further upstream segments. We cloned and sequenced a 5' upstream regulatory sequence found between -8 and -10 kb of the human tyrosinase locus (termed h5'URS) homologous to the mouse LCR sequences, and confirmed the presence of putative binding sites at -9 kb, homologous to those described in the mouse tyrosinase LCR core. Finally, we functionally validated the presence of a tissue-specific enhancer in the h5'URS by transient transfection analysis in human and mouse cells, as compared with homologous DNA sequences from the mouse tyrosinase locus. Future experiments in cells and transgenic animals will help us to understand the in vivo relevance of this newly described h5'URS sequence as a potentially important regulatory element for the correct expression of the human tyrosinase gene.

Techniques: Reporter mice – a new way to look at drug action

During the past decade remarkable progress in molecular genetics and the possibility of manipulating cells so that the expression of genes can directly 'report' on drug activity has produced major changes in drug development strategies. The recent description and pharmacological validation of reporter mice for in vivo analysis of hormone receptor activity opens new horizons for drug discovery. These novel animal models, in association with in vivo imaging technologies, provide a global view of the target tissues of drug action following acute and repeated drug treatment, thus enabling the prediction of potential side-effects in the early phase of preclinical studies. It is anticipated that further improvements of transgene architecture will lead to models that combine pharmacokinetic, pharmacodynamic and toxicological studies in a single step, which should provide a tremendous saving in time and, paradoxically, the number of animals to be sacrificed in the development of novel pharmacologically active molecules.

Functional dissection of the mouse tyrosinase locus control region identifies a new putative boundary activity

Locus control regions (LCRs) are complex high-order chromatin structures harbouring several regulatory elements, including enhancers and boundaries. We have analysed the mouse tyrosinase LCR functions, in vitro, in cell lines and, in vivo, in transgenic mice and flies. The LCR-core (2.1 kb), located at -15 kb and carrying a previously described tissue-specific DNase I hypersensitive site, operates as a transcriptional enhancer that efficiently transactivates heterologous promoters in a cell-specific orientation-independent manner. Furthermore, we have investigated the boundary activity of these sequences in transgenic animals and cells. In mice, the LCR fragment (3.7 kb) rescued a weakly expressed reference construct that displays position effects. In Drosophila, the LCR fragment and its core insulated the expression of a white minigene reporter construct from chromosomal position effects. In cells, sequences located 5' from the LCR-core displayed putative boundary activities. We have obtained genomic sequences surrounding the LCR fragment and found a LINE1 repeated element at 5'. In B16 melanoma and L929 fibroblast mouse cells, this element was found heavily methylated, supporting the existence of putative boundary elements that could prevent the spreading of condensed chromatin from the LINE1 sequences into the LCR fragment, experimentally shown to be in an open chromatin structure.

Artificial chromosome transgenesis in pigmentary research

Pigmentary genes were among the first mammalian genes to be studied, mostly because of the obvious phenotypes associated with their mutations. In 1990, tyrosinase, encoding the limiting enzyme in the melanin synthesis pathway, was eventually assigned to the c (albino) locus by classical rescue experiments driven by functional constructs in transgenic mice. These pioneer reports triggered the study of the regulation of endogenous tyrosinase gene expression by combining different amounts of upstream regulatory and promoter regions and testing their function in vivo in transgenic animals. However, faithful and reproducible transgenic expression was not achieved until the entire tyrosinase expression domain was transferred to the germ-line of mice using artificial-chromosome-type transgenes. The use of these large tyrosinase transgenic constructs and the ease with which they could be manipulated in vitro enabled the discovery of previously unknown but fundamental regulatory regions, such as the tyrosinase locus control region (LCR), whose presence was required in order to guarantee position-independent and copy-number-dependent expression of tyrosinase transgenes, with an expression level, per copy, comparable to that of an endogenous wild-type allele. Subsequently, functional dissection of elements present within this LCR through the generation of new artificial-chromosome type tyrosinase transgenes has revealed the existence of different regulatory activities. The existence of some of these units had been suggested previously by standard-type transgenic analyses. In this review, we will discuss both independent approaches and conclude that optimal tyrosinase transgene expression requires the use of its complete expression domain.

A single regulatory module of the carbamoylphosphate synthetase I gene executes its hepatic program of expression

A 469-base pair (bp) upstream regulatory fragment (URF) and the proximal promoter of the carbamoylphosphate synthetase I (CPS) gene were analyzed for their role in the regulation of spatial, developmental, and hormone-induced expression in vivo. The URF is essential and sufficient for hepatocyte-specific expression, periportal localization, perinatal activation and induction by glucocorticoids, and cAMP in transgenic mice. Before birth, the transgene is silent but can be induced by cAMP and glucocorticoids, indicating that these compounds are responsible for the activation of expression at birth. A 102-bp glucocorticoid response unit within the URF, containing binding sites for HNF3, C/EBP, and the glucocorticoid receptor, is the main determinant of the hepatocyte-specific and hormone-controlled activity. Additional sequences are required for a productive interaction between this minimal response unit and the core CPS promoter. These results show that the 469-bp URF, and probably only the 102-bp glucocorticoid response unit, functions as a regulatory module, in that it autonomously executes a correct spatial, developmental and hormonal program of CPS expression in the liver.

A mechanistic model for the development and maintenance of portocentral gradients in gene expression in the liver

In the liver, genes are expressed along a portocentral gradient. Based on their adaptive behavior, a gradient versus compartment type, and a dynamic versus stable type of gradient have been recognized. To understand at least in principle the development and maintenance of these gradients in gene expression in relation to the limited number of signal gradients, we propose a simple and testable model. The model uses portocentral gradients of signal molecules as input, while the output depends on two gene-specific variables, viz., the affinity of the gene for its regulatory factors and the degree of cooperativity that determines the response in the signal-transduction pathways. As a preliminary validity test for its performance, the model was tested on control and hormonally induced expression patterns of phosphoenolpyruvate carboxykinase (PCK), carbamoylphosphate synthetase I (CPS), and glutamine synthetase (GS). Affinity was found to determine the overall steepness of the gradient, whereas cooperativity causes these gradients to steepen locally, as is necessary for a compartment-like expression pattern. Interaction between two or more different signal gradients is necessary to ensure a stable expression pattern under different conditions. The diversity in sequence and arrangement of related DNA-response elements of genes appears to account for the gene-specific shape of the portocentral gradients in expression. The feasibility of testing the function of hepatocyte-specific DNA-response units in vivo is demonstrated by integrating such units into a ubiquitously active promoter/enhancer and analyzing the pattern of expression of these constructs in transgenic mice.

Glucocorticoids are insufficient for neonatal gene induction in the liver

Glucocorticoids and their receptor (GR) play a key role in perinatal gene induction. In the liver, the GR is essential for the neonatal induction of a number of genes, including that coding for tyrosine aminotransferase (TAT). To assess the function of the GR in the perinatal period, we have compared the activity of two types of glucocorticoid responsive elements in transgenic mice; one is the Tat gene glucocorticoid-responsive unit (GRU), an assembly of numerous binding sites for transcription factors, including the GR; the other is a simple dimer of high-affinity GR binding sites (GREs). Both elements confer strong glucocorticoid response in the adult liver. However, only the Tat GRUs are able to promote neonatal induction; the GRE dimer is unresponsive. Because this dimer is responsive to glucocorticoid administration in the neonate, the absence of neonatal induction is not due to the inactivity of the GR at this stage. At birth, the neonate has to withstand a brief period of starvation and hypoglycemia, a nutritional and hormonal situation that resembles fasting in the adult. In transgenic mice, the responses at birth and after fasting in the adult are similar: the Tat GRUs but not the dimeric GREs are activated. Our results show that, in rodents, glucocorticoids are not sufficient for neonatal gene induction in the liver and support the conclusion that the hypoglycemia at birth is the main trigger for expression.

An alternative splice variant of the tissue specific transcription factor HNF4alpha predominates in undifferentiated murine cell types

The transcription factor hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue specific transcription factor mainly expressed in the liver, kidney, intestine and the endocrine pancreas, but is also an essential regulator for early embryonic events. Based on its protein structure HNF4alpha is classified as an orphan member of the nuclear receptor superfamily. Comparing HNF4alpha transcription factors in the differentiated and dedifferentiated murine hepatocyte cell line MHSV-12 we identified in dedifferentiated cells the novel splice variant HNF4alpha7. This variant is characterized by an alternative first exon and has a lower transactivation potential in transient transfection assays using HNF4 dependent reporter genes. HNF4alpha7 mRNA and the corresponding protein are expressed in the undifferentiated pluripotent embryonal carcinoma cell line F9, whereas HNF4alpha1 only appears after differentiation of F9 cells to visceral endoderm. HNF4alpha7 mRNA is also found in totipotent embryonic stem cells. However, the function of HNF4alpha7 seems not to be restricted to embryonic cells as the HNF4alpha7 mRNA is also present in adult tissues, most notably the stomach. All these features suggest that the presence of distinct splice variants of HNF4alpha modulates the activity of HNF4alphain a cell type specific way.

Regulated production of mature insulin by non-beta-cells

Rat hepatoma cells were engineered to express, in a regulated manner, mature human insulin as an approach to the development of artificial beta-cells for insulin-dependent diabetes mellitus (IDDM) gene therapy. A chimeric gene obtained by linking a 2.4-kb fragment of the P-enolpyruvate carboxykinase (PEPCK) gene promoter to a human proinsulin gene (PEPCK/Insm), containing genetically engineered furin endoprotease cleavage sites, was stably transfected into FTO-2B rat hepatoma cells. The FTOInsm cells expressed high levels of insulin mRNA and protein after Northern blot or immunocytochemical analysis. High-performance liquid chromatography (HPLC) fractionation of culture medium and cell extracts revealed that about 90% of the proinsulin was processed to mature insulin. Insulin secretion was very fast, and 15 min after induction with dibutyryl cyclic AMP (Bt2cAMP) plus dexamethasone significant amounts of the hormone were released. Moreover, during the first hour, the rise in insulin concentration in the medium was 10-fold that detected in nontreated FTOInsm cells. Insulin produced by FTOInsm cells was biologically active because it blocked endogenous PEPCK gene expression and induced glucose uptake and lactate production. Thus, our results showed that genetically engineered FTOInsm hepatoma cells synthesized, processed, and secreted active insulin. The implantation of encapsulated engineered FTOInsm cells might provide a safe and practical therapeutic approach for IDDM treatment.

5' sequences direct developmental expression and hormone responsiveness of tyrosine aminotransferase in primary cultures of fetal rat hepatocytes

Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and N6,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP), whereas less mature hepatocytes (<E15) do not [Shelly, L. L. & Yeoh, G. C. T. (1991) Eur. J. Biochem. 199, 475-481]. Therefore, we consider >E15 hepatocytes, and not <E15 hepatocytes, to be determined. This study reports that 11.1 kb of sequences upstream of the TyrAT transcription start site, which include a cAMP-responsive element (CRE) and a glucocorticoid-responsive element (GRE), are required for correct developmental regulation of gene expression in determined fetal hepatocytes. In contrast, the TyrAT CRE alone does not have this capability. Dexamethasone augments basal and Bt2cAMP-stimulated activity of the TyrAT CRE alone, suggesting that synergism may be due to interaction between the glucocorticoid and cAMP-signaling pathways. However, Bt2cAMP does not further increase dexamethasone-induced activity of the 11.1 kb 5' sequences when the TyrAT CRE is removed, thus excluding interaction of Bt2cAMP with the glucocorticoid pathway. Finally, insulin inhibition of dexamethasone-induced gene transcription is shown to be conferred by TyrAT 5' sequences. This study shows that cellular components, other than those which mediate hormonal regulation of genes, are required for determination of hepatocytes with respect to TyrAT. Since this phenomenon is observed with transient transfections, it is unlikely to involve higher-order chromatin structure.

Targeted disruption of the glucocorticoid receptor gene blocks adrenergic chromaffin cell development and severely retards lung maturation

The role of the glucocorticoid receptor (GR) in glucocorticoid physiology and during development was investigated by generation of GR-deficient mice by gene targeting. GR -/- mice die within a few hours after birth because of respiratory failure. The lungs at birth are severely atelectatic, and development is impaired from day 15.5 p.c. Newborn livers have a reduced capacity to activate genes for key gluconeogenic enzymes. Feedback regulation via the hypothalamic-pituitary-adrenal axis is severely impaired resulting in elevated levels of plasma adrenocorticotrophic hormone (15-fold) and plasma corticosterone (2.5-fold). Accordingly, adrenal glands are enlarged because of hypertrophy of the cortex, resulting in increased expression of key cortical steroid biosynthetic enzymes, such as side-chain cleavage enzyme, steroid 11 beta-hydroxylase, and aldosterone synthase. Adrenal glands lack a central medulla and synthesize no adrenaline. They contain no adrenergic chromaffin cells and only scattered noradrenergic chromaffin cells even when analyzed from the earliest stages of medulla development. These results suggest that the adrenal medulla may be formed from two different cell populations: adrenergic-specific cells that require glucocorticoids for proliferation and/or survival, and a smaller noradrenergic population that differentiates normally in the absence of glucocorticoid signaling.