Vascular branching pattern and zonation of gene expression in the mammalian liver. A comparative study in rat, mouse, cynomolgus monkey, and pig

Comparative study on a novel lobule structure of the zebrafish liver and that of the mammalian liver

The mammalian liver has a lobule structure with a portal triad consisting of the portal vein, hepatic artery, and bile duct, which exhibits zonal gene expression, whereas those of teleosts do not have a portal triad. It remains to be demonstrated what kind of the unit structures they have, including their gene expression patterns. The aims of the present study were to demonstrate the unit structure of the teleost liver and discuss it in terms of evolution and adaptation in vertebrates and the use of teleosts as an alternative model for human disease. The zebrafish liver was examined as a representative of teleosts with respect to its morphological architecture and gene expression. A novel, polygonal lobule structure was detected in the zebrafish liver. In it, portal veins and central veins were distributed at the periphery and center, respectively. Sinusoids connected both veins. Anxa4-positive preductules were incorporated into the tubular lumen of two rows of hepatocytes in sections. Intrahepatic bile ducts resided randomly in the liver lobule. Zebrafish livers did not have zonal gene expression for metabolic pathways examined. The lobules of the zebrafish liver with preductules located in the tubular lumina of hepatocytes may resemble the oval cell reaction of injured livers of mammals and might convey bile to the intestine more safely than mammalian livers. The gene expression pattern in liver lobules and our liver lobule model of the zebrafish may be important to discuss data obtained in experiments using this animal as an alternative model for human disease.

Hepatic sinusoids versus central veins: Structures, markers, angiocrines, and roles in liver regeneration and homeostasis

The blood circulates through the hepatic sinusoids delivering nutrients and oxygen to the liver parenchyma and drains into the hepatic central vein, yet the structures and phenotypes of these vessels are distinctively different. Sinusoidal endothelial cells are uniquely fenestrated, lack basal lamina and possess organelles involved in endocytosis, pinocytosis, degradation, synthesis and secretion. Hepatic central veins are nonfenestrated but are also active in synthesis and secretion. Endothelial cells of sinusoids and central veins secrete angiocrines that play respective roles in hepatic regeneration and metabolic homeostasis. The list of markers for identifying sinusoidal endothelial cells is long and their terminologies are complex. Further, their uses vary in different investigations and, in some instances, could be confusing. Central vein markers are fewer but more distinctive. Here we analyze and categorize the molecular pathways/modules associated with the sinusoid-mediated liver regeneration in response to partial hepatectomy and chemical-induced acute or chronic injury. Similarly, we highlight the findings that central vein-derived angiocrines interact with Wnt/β-catenin in perivenous hepatocytes to direct gene expression and maintain pericentral metabolic zonation. The proposal that perivenous hepatocytes behave as stem/progenitor cells to provoke hepatic homeostatic cell renewal is reevaluated and newer concepts of broad zonal distribution of hepatocyte proliferation in liver homeostasis and regeneration are updated. Thus, this review integrates the structures, biology and physiology of liver sinusoids and central veins in mediating hepatic regeneration and metabolic homeostasis.

The Hepatic Central Vein: Structure, Fibrosis, and Role in Liver Biology

The hepatic central vein is a primary source of Wnt2, Wnt9b, and R-spondin3. These angiocrines activate ß-catenin signaling to regulate hepatic metabolic zonation and perivenous gene expression in mice. Little is known about the central vein ultrastructure. Here, we describe the morphological-functional correlates of the central vein and its draining and branching patterns. Central vein fibrosis occurs in liver disease and is often accompanied by perivenous perisinusoidal fibrosis, which may affect perivenous gene expression. We review the biological properties of perivenous hepatocytes and glutamine synthetase that serve as a biomarker of perivenous hepatocytes. Glutamine synthetase and P4502E1 are indicators of ß-catenin activity in centrilobular liver injury and regeneration. The Wnt/ß-catenin pathway is the master regulator of hepatic metabolic zonation and perivenous gene expression and is modulated by the R-spondin-LGR4/5-ZNRF3/RNF43 module. We examined the structures of the molecules of these pathways and their involvements in liver biology. Central vein-derived Wnts and R-spondin3 participate in the cellular-molecular circuitry of the Wnt/ß-catenin and R-spondin-LGR4/5-ZNRF3/RNF43 module. The transport and secretion of lipidated Wnts in Wnt-producing cells require Wntless protein. Secreted Wnts are carried on exosomes in the extracellular matrix to responder cells. The modes of release of Wnts and R-spondin3 from central veins and their transit in the venular wall toward perivenous hepatocytes are unknown. We hypothesize that central vein fibrosis may impact perivenous gene expression. The proposal that the central vein constitutes an anatomical niche of perivenous stem cells that subserve homeostatic hepatic renewal still needs studies using additional mouse models for validation. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1747-1767, 2020. © 2019 American Association for Anatomy.

Epidermal growth factor receptor plays a role in the regulation of liver and plasma lipid levels in adult male mice

Dsk5 mice have a gain of function in the epidermal growth factor receptor (EGFR), caused by a point mutation in the kinase domain. We analyzed the effect of this mutation on liver size, histology, and composition. We found that the livers of 12-wk-old male Dsk5 heterozygotes (+/Dsk5) were 62% heavier compared with those of wild-type controls (+/+). The livers of the +/Dsk5 mice compared with +/+ mice had larger hepatocytes with prominent, polyploid nuclei and showed modestly increased cell proliferation indices in both hepatocytes and nonparenchymal cells. An analysis of total protein, DNA, and RNA (expressed relative to liver weight) revealed no differences between the mutant and wild-type mice. However, the livers of the +/Dsk5 mice had more cholesterol but less phospholipid and fatty acid. Circulating cholesterol levels were twice as high in adult male +/Dsk5 mice but not in postweaned young male or female mice. The elevated total plasma cholesterol resulted mainly from an increase in low-density lipoprotein (LDL). The +/Dsk5 adult mouse liver expressed markedly reduced protein levels of LDL receptor, no change in proprotein convertase subtilisin/kexin type 9, and a markedly increased fatty acid synthase and 3-hydroxy-3-methyl-glutaryl-CoA reductase. Increased expression of transcription factors associated with enhanced cholesterol synthesis was also observed. Together, these findings suggest that the EGFR may play a regulatory role in hepatocyte proliferation and lipid metabolism in adult male mice, explaining why elevated levels of EGF or EGF-like peptides have been positively correlated to increased cholesterol levels in human studies.

Lymph circulation in the liver

The liver produces a large amount of lymph, which is estimated to be 25 to 50 % of lymph flowing through the thoracic duct. The hepatic lymphatic system falls into three categories depending on their locations: portal, sublobular, and superficial lymphatic vessels. It is suggested that 80 % or more of hepatic lymph drains into portal lymphatic vessels, while the remainder drains through sublobular and capsular lymphatic vessels. The hepatic lymph primarily comes from the hepatic sinusoids. Our tracer studies, together with electron microscopy, show many channels with collagen fibers traversing through the limiting plate and connecting the space of Disse with the interstitial space either in the portal tracts, or around the sublobular veins. Fluid filtered out of the sinusoids into the space of Disse flows through the channels traversing the limiting plate either independently of blood vessels or along blood vessels and enters the interstitial space of either portal tract or sublobular veins. Fluid in the space of Disse also flows through similar channels traversing the hepatocytes intervening between the space of Disse and the hepatic capsule and drains into the interstitial space of the capsule. Fluid and migrating cells in the interstitial space pass through prelymphatic vessels to finally enter the lymphatic vessels. The area of the portal lymphatic vessels increases in liver fibrosis and cirrhosis and in idiopathic portal hypertension. Lymphatic vessels are abundant in the immediate vicinity of the hepatocellular carcinoma (HCC) and liver metastasis. HCCs expressing vascular endothelial growth factor-C are more liable to metastasize, indicating that lymphangiogenesis is associated with their enhanced metastasis.

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.

Origins and pathways of fluid entering sublobular lymphatic vessels in cat livers

The liver, which produces a large volume of lymph, has a lymphatic system which can be classified into three categories: portal, sublobular, and superficial lymphatic vessels. As little is known about the origin and pathways of sublobular lymph, this study demonstrates pathways of interstitial fluid flowing into sublobular lymphatic vessels. Livers from cats whose thoracic ducts were either ligated or non-ligated were examined by light-, transmission electron- and scanning electron-microscopy (SEM). Complete ligation of the thoracic duct caused significant dilation of the hepatic sinusoids, the space of Disse, and channels passing through the limiting plate. Sublobular interstitial space and sublobular lymphatic vessels were also expanded. The channels between hepatocytes forming the limiting plate contained collagen fibers, and connected the space of Disse with a sublobular interstitial space. The alkali-water maceration/SEM confirmed that collagen fibers traversing the layer of the limiting plate independently of blood vessels connected collagen fibers in the space of Disse with those in the sublobular space. Complete ligation of the thoracic duct also showed an accumulation of mast cells and plasma cells in the sublobular interstitial space. Our data suggest that fluid in the space of Disse flows along collagen fibers in channels traversing the limiting plate as well as those along the sinusoids and central veins that drain into sublobular veins, and enters the sublobular interstitial space to finally lead into sublobular lymphatic vessels. Our study has also shown that hepatic lymphostasis causes the accumulation of mast cells and plasma cells in the sublobular interstitial space, which may be involved in lymphangiogenesis and fibrogenesis.

Three-dimensional observations of spatial arrangement of hepatic zonation and vein system in mice and house musk shrews

The three-dimensional (3D) relationship among the hepatic domains and the efferent central and afferent portal veins was investigated by macroscopy, microscopy, and computer-aided 3D reconstruction methods. To clearly distinguish the pericentral domain from the periportal, we used CCl(4)-treated mice and diabetic house musk shrews, which show typical pericentral necrosis and deposition of fat, respectively. The 3D findings obtained were verified against normal control animals using advantages of our unique observations by light and fluorescent microscopy, which made it possible to differentiate the two domains well. The pericentral domains in the mice and shrews appeared three-dimensionally as continuous branched columns, and the periportal domains exist in a sponge-like network that fills the parenchymal space among the columnar pericentral domains. The efferent central veins were concentrically surrounded by the pericentral domain, and segments of the central veins flowed into large sublobular and lobar veins. The walls of these large veins faced the pericentral domain at the confluence with the central veins; the remaining portions of the walls faced the periportal domain. The afferent portal veins were placed at the two-dimensional center of the network of the periportal domain and gave off smaller portal branches radially at the intersections of the network. Three types of liver lobules-classic, portal, and acinar-have been discussed repeatedly at the (2D) level. At the 3D level, it is reasonable to consider that the liver parenchyma consists of the two continuous domains corresponding to the distribution of the vessels that we found.

Differential expression of canalicular membrane Ca2+/Mg(2+)-ecto-ATPase in estrogen-induced and obstructive cholestasis in the rat

Extracellular adenosine triphosphate (ATP) may regulate hepatocyte and cholangiocyte functions, and under some conditions it may have deleterious effects on bile secretion and cause cholestasis. The canalicular membrane enzyme Ca2+/Mg2+-ecto-ATPase (ecto-ATPase) hydrolyzes ATP/adenosine diphosphate (ATP/ADP) and regulates hepatic extracellular ATP concentration. Changes in liver ecto-ATPase in estrogen-induced cholestasis were examined in male rats receiving 17alpha-ethinylestradiol (E groups) for 1, 3, or 5 days (5 mg/kg/day, sc) and compared with changes in rats subjected to obstructive cholestasis (O groups) for 1, 3, or 8 days. Activity of ecto-ATPase, protein mass in canalicular membranes and bile (estimated by Western blotting), steady state mRNA levels (by Northern blotting), and cellular and acinar distributions of the enzyme (histochemistry and immunocytochemistry) were assessed in these groups. Activity of ecto-ATPase, protein mass in isolated canalicular membranes, and enzyme mRNA levels were significantly increased in E group rats as compared with controls. In contrast, these parameters were markedly decreased in O group rats, and the enzyme protein was undetectable in bile. The ecto-ATPase histochemical reaction was markedly increased in the canalicular membrane of E group rats, extending from acinar zone 2 to zone 1, whereas it decreased in the O group. The ecto-ATPase immunocytochemical reaction was present in the canalicular membrane and pericanalicular vesicles in control and E group hepatocytes, but it decreased in obstructive cholestasis and was localized only to the canalicular membrane. Thus, significant changes in liver ecto-ATPase were apparent in 17alpha-ethinylestradiol-induced cholestasis that were opposite to those observed in obstructive cholestasis. Assuming that the alterations observed in obstructive cholestasis are the result of the cholestatic phenomenon, we conclude that changes in ecto-ATPase in 17alpha-ethinylestradiol-treated rats might be either primary events or part of an adaptive response in 17alpha-ethinylestradiol-induced cholestasis.

Lobular distribution pattern of lactate dehydrogenase and 6-phosphogluconate dehydrogenase activity in rat liver

Lactate dehydrogenase (LDH) and 6-phosphogluconate dehydrogenase (6-PGDH) activities were measured in lobular areas expanding between 3 portal tracts and an efferent central vein in the livers of male Wistar rats, using a Lowry technique. The maximum of LDH activity was found in a nearly uniform broad area in the lobular periphery. From that area values decreased along periportal/septal-->perivenous gradients, but only slightly within that area along the periportal-->septal axis of the vascular septum. Maximum values of 6-PGDH activity were present in an intermediate area close to the central vein demonstrating a rather inhomogeneous distribution pattern without a clear definition of zonal limits. Our data on the distribution pattern of LDH are in agreement with the concept of the metabolic lobulus and are supported by a recent evaluation of the vascular architecture in rat liver. The lobular distribution pattern of 6-PGDH cannot be interpreted without doubt in accordance with that concept.

Sublobular veins as the main site of lymphocyte adhesion/transmigration and adhesion molecule expression in the porto-sinusoidal-hepatic venous system during concanavalin A-induced hepatitis in mice

Lymphocyte infiltration is a manifest feature of hepatitis. To reveal the main site and mechanism of lymphocyte adhesion/extravasation in the hepatic vasculature during inflammation, we morphometrically and histologically analyzed these events in relation to adhesion molecule expression using a murine model of T-cell mediated hepatitis induced by concanavalin A (Con A). Although lymphocyte adhesion was restricted to the sinusoids in untreated mice, it increased in all the segments of porto-sinusoidal-hepatic venous system 8 hours after Con A injection; the number of adhering lymphocytes per unit vascular circumference was the largest in the sublobular veins, relatively large in the central veins and small hepatic veins, and relatively small in the sinusoids and negligible in the portal veins. At 20 hours, extravascular lymphocytes showed similar distribution to lymphocyte adhesion at 8 hours except in the portal veins, around which they were possibly accumulated by the translocation of extrasinusoidal lymphocytes. E-selectin and vascular cell adhesion molecule-1 (VCAM-1) were transiently expressed at 4 to 6 hours, whereas P-selectin and intercellular adhesion molecule-1 were not changed between 0 and 48 hours. In particular, E-selectin expression coincided with that of lymphocyte adhesion in distribution. Lymphocyte attachment was inhibited by pretreatment with anti-E-selectin monoclonal antibody (MAb) or anti-VCAM-1 MAb, and expression of E-selectin and VCAM-1 was suppressed by pretreatment with anti-tumor necrosis factor-alpha (TNF-alpha) MAb. Electron microscopically, lymphocytes were trapped by endothelial lamellipodia and traversed the endothelium by diapedesis. These results indicate that lymphocyte adhesion/transmigration preferentially takes place in the sublobular veins in association with TNF-alpha-induced endothelial activation, i.e., E-selectin and VCAM-1 expression and lamellipodia formation.

Expression pattern of glutamine synthetase marks transition from collecting into conducting hepatic veins

The expression of glutamine synthetase (GS) is confined to a rim of hepatocytes surrounding the efferent hepatic veins in all mammalian species investigated. In rat liver, a two- to three-cell thick layer of GS-positive (GS(+)) hepatocytes uniformly surrounds the two to four terminal branching generations of the hepatic vein that collect blood from sinusoids (central veins). With increasing diameter of the efferent vessel, this multilayered rim of GS(+) hepatocytes becomes confined to patches surrounding the decreasing number of central vein outlets. The remaining part of the wall of these sublobar hepatic veins is bordered by a one-cell thick layer of GS(+) hepatocytes. Around still larger veins, this single-cell layer of GS(+) hepatocytes gradually disappears. The expression pattern of GS is therefore a convenient biological parameter to delimit sinusoidal draining ("collecting") from nondraining ("conducting") surfaces in the wall of the efferent hepatic vessels. The hepatocytes surrounding a single tree of central veins together form a compound liver lobule. (J Histochem Cytochem 47:1507-1511, 1999)

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.

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.

Effects of diosgenin, a plant-derived steroid, on bile secretion and hepatocellular cholestasis induced by estrogens in the rat

Increased biliary secretion of cholesterol and lipid vesicles (unilamellae and multilamellae) induced by diosgenin (D), a plant-derived steroid, has cytoprotective effects in the rat liver subjected to obstructive cholestasis. In this study, our aims were to investigate the following: 1) the effects of D on the bile secretory process and on the cholestasis induced by estradiol-17beta-(beta-D-glucuronide) (E17G) or 17 alpha-ethynylestradiol (E) administration; 2) whether the potentially protective effects of D are related to D-induced increase of biliary cholesterol and lipid lamellae; and 3) whether D has other effects capable of modifying specific bile secretory processes or preventing the cholestatic effects of estrogens. Rats were fed a standard ground chow (control group) or chow containing D for 6 days. E17G was administered i.v. to control and D-fed rats and bile flow, bile salt output, and alkaline phosphatase excretion were examined. 17alpha-E was administered from days 4 to 6 to rats fed standard chow or chow plus D for 6 days and different functional parameters of the bile secretory process as well as the ultrastructure of hepatocytes and histochemistry of alkaline phosphatase and Mg2+-adenosine triphosphatase (ATPase) were examined. D-treatment markedly increased cholesterol and lamellar structures in bile and attenuated the acute cholestatic effects of E17G. D-feeding prevented the decrease of taurocholate maximum secretory rate and the increase of biliary alkaline phosphatase and Ca2+,Mg2+-EctoATPase (EctoATPase) excretion, as well as the increase of cholesterol/ phospholipids ratio, alkaline phosphatase activity, and EctoATPase content in canalicular plasma membranes induced by E. D-feeding did not prevent E-induced decrease of basal bile flow, bile salt, cholesterol, and phospholipid secretory rates nor the decrease of Na+,K+-ATPase activity and Na+-taurocholate cotransporting polypeptide (Ntcp) content in isolated sinusoidal membranes. Cholestatic alterations of canalicular domain were apparent in E-treated rats. D administration was also associated with changes of ultrastructure and histochemistry of hepatocytes. E-induced alterations in ultrastructure and acinar distribution and intensity of histochemical reaction of both enzymes were partially prevented by D-feeding. We conclude that D administration, in addition to inducing a marked increase of biliary cholesterol and lipid lamellar structures output, was associated to changes in hepatocyte morphology and plasma membrane composition, enzymes activity, and histochemistry. D-feeding attenuated the acute cholestatic effects of E17G. D-induced increase of bile cholesterol and lipid lamellae content was not apparent when D-fed rats received E. Despite this fact, D administration prevented some cholestatic effects of E, probably through different metabolic effects and/or direct membrane effects, not related to increased lipid lamellae excretion.

Role of the 5' enhancer of the glutamine synthetase gene in its organ-specific expression

In mammals, glutamine synthetase (GS) is expressed in a large number of organs, but the precise regulation of its expression is still obscure. Therefore a detailed analysis of the activity of the upstream regulatory element of the GS gene in the transcriptional regulation of its expression was carried out in transgenic mice carrying the chloramphenicol acetyltransferase (CAT) gene under the control of the upstream regulatory region of the GS gene. CAT and GS mRNA expression were compared in liver, epididymis, lung, adipocytes, testis, kidney, skeletal muscle and gastrointestinal tract, both quantitatively by ribonuclease-protection analysis and topographically by in situ hybridization. It was found that the upstream regulatory region is active with respect both to the level and to the topography of GS gene expression in liver, epididymis, gastrointestinal tract (stomach, small intestine and colon) and skeletal muscle. On the other hand, in the kidney, brain, adipocytes, spleen, lung and testis, GS gene expression is not or only partly regulated by the 5' enhancer. A second enhancer, identified within the first intron, may regulate GS expression in the latter organs. Furthermore, CAT expression in the brain did not co-localize with that of GS, showing that the 5' regulatory region of the GS gene does not direct its expression to the astrocytes.