Organ-specific activity of the 5' regulatory region of the glutamine synthetase gene in developing mice

Pericentral activity of alpha-fetoprotein enhancer 3 and glutamine synthetase upstream enhancer in the adult liver are regulated by β-catenin in mice

We previously showed that mouse alpha-fetoprotein (AFP) enhancer 3 activity is highly restricted to pericentral hepatocytes in the adult liver. Here, using transgenic mice, we show that the upstream enhancer of the rat glutamine synthetase gene is also active, specifically in pericentral regions. Activity of both enhancers is lost in the absence of β-catenin, a key regulator of zonal gene expression in the adult liver. Both enhancers contain a single, highly conserved T-cell factor/lymphoid enhancer factor binding site that is required for responsiveness to β-catenin. We also show that endogenous AFP messenger RNA levels in the perinatal liver are lower when β-catenin is reduced.

CONCLUSION

These data identify the first distinct zonally active regulatory regions required for β-catenin responsiveness in the adult liver, and suggest that postnatal AFP repression and the establishment of zonal regulation are controlled, at least in part, by the same factors.

Glutamine synthetase is essential in early mouse embryogenesis

Glutamine synthetase (GS) is expressed in a tissue-specific and developmentally controlled manner, and functions to remove ammonia or glutamate. Furthermore, it is the only enzyme that can synthesize glutamine de novo. Since congenital deficiency of GS has not been reported, we investigated its role in early development. Because GS is expressed in embryonic stem (ES) cells, we generated a null mutant by replacing one GS allele in-frame with a beta-galactosidase-neomycine fusion gene. GS(+/LacZ) mice have no phenotype, but GS(LacZ/LacZ) mice die at ED3.5, demonstrating GS is essential in early embryogenesis. Although cells from ED2.5 GS(LacZ/LacZ) embryos and GS(GFP/LacZ) ES cells survive in vitro in glutamine-containing medium, these GS-deficient cells show a reduced fitness in chimera analysis and fail to survive in tetraploid-complementation assays. The survival of heavily (>90%) chimeric mice up to at least ED16.5 indicates that GS deficiency does not entail cell-autonomous effects and that, after implantation, GS activity is not essential until at least the fetal period. We hypothesize that GS-deficient embryos die when they move from the uterine tube to the harsher uterine environment, where the embryo has to catabolize amino acids to generate energy and, hence, has to detoxify ammonia, which requires GS activity.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Cellular concentrations of glutamine synthetase in murine organs

Glutamine synthetase (GS) is the only enzyme that can synthesize glutamine, but it also functions to detoxify glutamate and ammonia. Organs with high cellular concentrations of GS appear to function primarily to remove glutamate or ammonia, whereas those with a low cellular concentration appear to primarily produce glutamine. To validate this apparent dichotomy and to clarify its regulation, we determined the GS concentrations in 18 organs of the mouse. There was a >100-fold difference in GS mRNA, protein, and enzyme-activity levels among organs, whereas there was only a 20-fold difference in the GS protein:mRNA ratio, suggesting extensive transcriptional and posttranscriptional regulation. In contrast, only small differences in the GS enzyme activity : protein ratio were found, indicating that posttranslational regulation is of minor importance. The cellular concentration of GS was determined by relating the relative differences in cellular GS concentration, detected using image analysis of immunohistochemically stained tissue sections, to the biochemical data. There was a >1000-fold difference in cellular concentrations of GS between GS-positive cells in different organs, and cellular concentrations were up to 20x higher in subpopulations of cells within organs than in whole organs. GS activity was highest in pericentral hepatocytes (approximately 485 micromol.g(-1).min-(1), followed in descending order by epithelial cells in the epididymal head, Leydig cells in the testicular interstitium, epithelial cells of the uterine tube, acid-producing parietal cells in the stomach, epithelial cells of the S3 segment of the proximal convoluted tubule of the kidney, astrocytes of the central nervous tissue, and adipose tissue. GS activity in muscle amounted to only 0.4 micromol.g(-1).min(-1). Our findings confirmed the postulated dichotomy between cellular concentration and GS function.

Upstream and intronic regulatory sequences interact in the activation of the glutamine synthetase promoter

Glutamine synthetase (GS) is expressed at high levels in subsets of cells in some tissues and at low levels in all cells of other tissues, suggesting that the GS gene is surrounded by multiple regulatory elements. We searched for such elements in the 2.5-kb upstream region and in the 2.6-kb first intron of the GS gene, using FTO-2B hepatoma and C2/7 muscle cells as representatives of both cell types and transient transfection assays as our tools. In addition to the entire upstream region and entire intron, an upstream enhancer module at -2.5 kb, and 5', middle and 3' modules of the first intron were tested. The main effects of the respective modules and their combinatorial interactions were quantified using the analysis of variance (anova) technique. The upstream enhancer was strongly stimulatory, the middle intron module strongly inhibitory, and the 3'-intron module weakly stimulatory in both hepatoma and muscle cells. The 5'-intron module was strongly stimulatory in muscle cells only. The major new finding was that in both cell types, the upstream enhancer and 5'-intron module needed to be present simultaneously to fully realize their transactivational potencies. This interaction was responsible for a pronounced inhibitory effect of the 5'-intron module in the absence of the upstream enhancer in hepatoma cells, and for a strong synergistic effect of these two modules, when present simultaneously in muscle cells. The main difference between hepatoma and muscle cells therefore appeared to reside in tissue-specific differences in activity of the respective regulatory elements due to interactions rather than in the existence of tissue-specific regulatory elements.

CAAT/enhancer binding protein activates an enhancer in the glutamine synthetase distal 5'-flanking sequence

The glutamine synthetase (GS) gene is expressed at high levels in several cell types, including astrocytes, pericentral hepatocytes, and adipocytes. During hormone-mediated adipocyte differentiation of 3T3-L1 cells, GS gene expression increases several hundred fold. We previously reported that elements in the distal 5'-flanking sequence and intron-1 participate in establishing the temporal pattern of GS transcription during adipocyte differentiation. To examine the role of the distal 5'-flanking region in regulating adipocyte-specific GS expression, GS-CAT fusion genes were constructed and analyzed in transiently transfected 3T3-L1 cells. In this way, adipocyte differentiation-responsive enhancer activity was localized to a 422-bp sequence that occurs about 3.5 kb upstream from the transcription start site. This sequence includes several putative C/EBP binding sites and is activated by ectopic expression of C/EBPalpha in NIH-3T3 cells. Thus, our data indicate that C/EBPalpha has the capacity to activate functional C/EBP sites in the GS gene distal 5'-flanking region.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.