Evidence that interaction of hepatocytes with the collecting (hepatic) veins triggers position-specific transcription of the glutamine synthetase and ornithine aminotransferase genes in the mouse liver

A new glutamine synthetase index to evaluate hepatic lobular restoration in advanced fibrosis during anti-HBV therapy

Hepatic lobular architecture distortion is a deleterious turning point and a crucial histological feature of advanced liver fibrosis in chronic liver diseases. Regression of fibrosis has been documented in chronic hepatitis B (CHB) patients. However, whether lobular architecture could be restored following fibrosis regression after antiviral therapy is still unclear. Glutamine synthetase (GS) is generally expressed by perivenular hepatocytes around hepatic veins (HV). In this study, we defined abnormal lobular architecture (GSPT ) as GS expressing in the vicinity of portal tracts (PT), which denotes parenchymal extinction and lobular collapse. We defined normal lobular architecture (GSHV ) as GS positivity area not approximating PTs. Therefore, we propose a new GS-index, defined as the percentage of GSHV /(GSHV  + GSPT ), to evaluate the extent of architectural disruption and restoration. We evaluated 43 CHB patients with advanced fibrosis (Ishak stage ≥4). Posttreatment liver biopsy was performed after 78 weeks of anti-HBV therapy. The median GS-index improved from 7% (interquartile range [IQR]: 0%-23%) at baseline to 36% (IQR: 20%-57%) at Week 78 (p < 0.001). Totals of 22 patients (51%) had significant GS-index improvement from 0% (IQR: 0%-13%) to 55% (IQR: 44%-81%), while the other half had almost no change between 17% (IQR: 0%-33%) to 20% (IQR: 12%-31%). When GS-index78w  ≥ 50% was used to define hepatic lobular restoration, 37% of patients (16/43) achieved lobular restoration, with much improvement in alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels (median value of ∆/Baseline in ALT: restored vs. nonrestored was 79.1% vs. 48.8%, p = 0.018; median value of ∆/Baseline in AST: restored vs. nonrestored was 69.1% vs. 32.5%, p = 0.005). More importantly, lobular restoration correlated with fibrosis regression (median value of ∆/Baseline in Ishak stage: restored vs. nonrestored was 25.0% vs. 0%, p = 0.008). Therefore, in the era of antiviral therapy for CHB, restoration of hepatic lobular architecture is achievable in patients with advanced fibrosis. GS-index provides additional insight into fibrosis regression that goes beyond collagen degradation.

The Canonical Wnt Pathway as a Key Regulator in Liver Development, Differentiation and Homeostatic Renewal

The canonical Wnt (Wnt/β-catenin) signalling pathway is highly conserved and plays a critical role in regulating cellular processes both during development and in adult tissue homeostasis. The Wnt/β-catenin signalling pathway is vital for correct body patterning and is involved in fate specification of the gut tube, the primitive precursor of liver. In adults, the Wnt/β-catenin pathway is increasingly recognised as an important regulator of metabolic zonation, homeostatic renewal and regeneration in response to injury throughout the liver. Herein, we review recent developments relating to the key role of the pathway in the patterning and fate specification of the liver, in the directed differentiation of pluripotent stem cells into hepatocytes and in governing proliferation and zonation in the adult liver. We pay particular attention to recent contributions to the controversy surrounding homeostatic renewal and proliferation in response to injury. Furthermore, we discuss how crosstalk between the Wnt/β-catenin and Hedgehog (Hh) and hypoxia inducible factor (HIF) pathways works to maintain liver homeostasis. Advancing our understanding of this pathway will benefit our ability to model disease, screen drugs and generate tissue and organ replacements for regenerative medicine.

The many ways to mend your liver: A critical appraisal

In the latter half of the 20th century, our understanding of mammalian liver regeneration was shaped by the manner of compensatory hyperplasia occurring after a partial rat liver resection. This response involves almost all hepatocytes and thus is unlikely to be the outcome of the multiple cycling of a small stem cell population. It was most intense in the outer third of lobule, the location closest to the afferent arterial blood supply. With the advent of heritable genetic labelling techniques, usually applied to mice, hitherto unrecognized hepatocytes with clonogenic potential have been discovered, contributing to homoeostatic renewal and/or regenerative responses after tissue loss. This review combines observations from cell lineage tracing studies with other data to summarize the Four proposed anatomical locations for hepatocyte stem cells: the periportal zone, the pericentral zone, a randomized distribution and finally within the intrahepatic biliary tree. As in other endodermal-derived tissues, it appears that there are both homoeostatic stem cells and regenerative stem cells, while some normally homoeostatic stem cells can become more active to boost regeneration.

Regenerating the liver: not so simple after all?

Under normal homeostatic conditions, hepatocyte renewal is a slow process and complete turnover likely takes at least a year. Studies of hepatocyte regeneration after a two-thirds partial hepatectomy (2/3 PH) have strongly suggested that periportal hepatocytes are the driving force behind regenerative re-population, but recent murine studies have brought greater complexity to the issue. Although periportal hepatocytes are still considered pre-eminent in the response to 2/3 PH, new studies suggest that normal homeostatic renewal is driven by pericentral hepatocytes under the control of Wnts, while pericentral injury provokes the clonal expansion of a subpopulation of periportal hepatocytes expressing low levels of biliary duct genes such as Sox9 and osteopontin. Furthermore, some clarity has been given to the debate on the ability of biliary-derived hepatic progenitor cells to generate physiologically meaningful numbers of hepatocytes in injury models, demonstrating that under appropriate circumstances these cells can re-populate the whole liver.

Phenotype and growth behavior of residual β-catenin-positive hepatocytes in livers of β-catenin-deficient mice

Signaling through the Wnt/β-catenin pathway is a crucial determinant of hepatic zonal gene expression, liver development, regeneration, and tumorigenesis. Transgenic mice with hepatocyte-specific knockout of Ctnnb1 (encoding β-catenin) have proven their usefulness in elucidating these processes. We now found that a small number of hepatocytes escape the Cre-mediated gene knockout in that mouse model. The remaining β-catenin-positive hepatocytes showed approximately 25% higher cell volumes compared to the β-catenin-negative cells and exhibited a marker protein expression profile similar to that of normal perivenous hepatocytes or hepatoma cells with mutationally activated β-catenin. Surprisingly, the expression pattern was observed independent of the cell's position within the liver lobule, suggesting a malfunction of physiological periportal repression of perivenously expressed genes in β-catenin-deficient liver. Clusters of β-catenin-expressing hepatocytes lacked expression of the gap junction proteins Connexin 26 and 32. Nonetheless, β-catenin-positive hepatocytes had no striking proliferative advantage, but started to grow out on treatment with phenobarbital, a tumor-promoting agent known to facilitate the formation of mouse liver adenoma with activating mutations of Ctnnb1. Progressive re-population of Ctnnb1 knockout livers with wild-type hepatocytes was seen in aged mice with a pre-cirrhotic phenotype. In these large clusters of β-catenin-expressing hepatocytes, perivenous-specific gene expression was re-established. In summary, our data demonstrate that the zone-specificity of a hepatocyte's gene expression profile is dependent on the presence of β-catenin, and that β-catenin provides a proliferative advantage to hepatocytes when promoted with phenobarbital, or in a pre-cirrhotic environment.

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.

Tissue and subcellular distribution of glucokinase in rat liver and their changes during fasting-refeeding

The distribution of glucokinase in rat liver under both normal feeding and fasting-refeeding conditions was investigated immunohistochemically. Under normal feeding conditions, glucokinase immunoreactivity was observed in both nuclei and cytoplasm of parenchymal cells. The nuclei were stained intensely and evenly, whereas the cytoplasm showed weak immunoreactivity of different degrees of staining intensity depending on the location of the cells. The cytoplasm of perivenous hepatocytes was stained more intensely, though not so much more, than that of periportal hepatocytes. The cytoplasm of hepatocytes surrounding the terminal hepatic venule (THV), of hepatocytes surrounding the portal triad, and of some other hepatocytes showed a stronger immunoreactivity than that of residual hepatocytes. The nuclear immunoreactivity in hepatocytes surrounding the portal triad and in some other hepatocytes was weak or absent, and positive immunoreactivity was detected at the plasma membrane of some of these cells. After 72 h of fasting, glucokinase immunoreactivity was markedly decreased in all hepatocytes. After the start of refeeding, the cytoplasmic immunoreactivity began to increase first in the parenchymal cells surrounding the THV and extended to those in the intermediate zone followed by those in the periportal zone. In contrast, the increase in nuclear immunoreactivity started in hepatocytes situated in the intermediate zone adjacent to the perivenous zone and then extended to those in the perivenous zone followed by those in the periportal zone. Hepatocytes surrounding either THV or portal triad showed a distinctive change in immunoreactivity during the refeeding period. After 10 h of refeeding, strong immunoreactivity was observed in both the cytoplasm and the nuclei of all hepatocytes, and appreciable glucokinase immunoreactivity was detected at the plasma membrane of some hepatocytes. These findings are discussed from the standpoint of a functional role of glucokinase in hepatic glucose metabolism.

Differential regulation of multiple gap junction transcripts and proteins during rat liver regeneration

The mRNA and protein expression of alpha 1 (connexin 43), beta 1 (connexin 32), and beta 2 (connexin 26) gap junction genes were examined in the regenerating rat liver after 70% partial hepatectomy (PH). Expression of beta 1 and beta 2 steady-state mRNA levels changed minimally until 12 h after PH when both transcripts decreased to approximately 15% of baseline values. A similar decrease in assembled connexin levels was detected by immunoblot and indirect immunofluorescence at 18 h after PH. Both transcripts simultaneously increased between 24 and 42 h and again rapidly decreased by 48 h post-PH. beta 1 and beta 2 assembled gap junction protein expression increased at 48 h post-PH and rapidly decreased by 56 h. By 72 to 84 h post-PH, beta 1 and beta 2 mRNA and assembled protein expression returned to near baseline levels and were maintained. Interestingly, inhibition of protein synthesis with cycloheximide completely inhibited disappearance of the beta 2 transcript, in contrast to beta 1 mRNA which was unaffected. Nuclear run-on assays showed no change in transcriptional rates for either gene during the regenerative period. However, both beta 1 and beta 2 transcripts exhibited significantly decreased mRNA half-lives at 12 h post-PH (3.8 and 3.7 h, respectively) relative to those at 0 h (10.9 and 6.1 h, respectively). Surprisingly, although the transcriptional rate for alpha 1 was similar to that observed for beta 2, no alpha 1 transcripts were detectable by northern or RNase protection analysis. The results suggest that in the regenerating rat liver, beta 1 and beta 2 gap junction genes are not regulated at the transcriptional level. Rather, the cyclical modulation of their steady-state transcripts is regulated primarily by posttranscriptional events of which mRNA stability is at least one critical factor in the control process.

Periportal- and perivenous-enriched hepatocyte couplets: differences in canalicular activity and in response to oxidative stress

Unlike isolated single hepatocytes, hepatocyte couplets retain their apical polarity, and, during short-term culture form an enclosed canalicular space or vacuole between the two adjacent cells into which biliary secretion is initiated. Hepatocyte couplets were prepared after partial collagenase perfusion of rat liver. Centrifugal elutriation was used to fractionate the preparation into six couplet-containing suspensions. Image analysis was used to determine the size of cultured couplets. The size of the couplets ranged from 34.1 +/- 0.76 microns and 684 +/- 24.1 microns 2 (mean length and area respectively +/- S.E.M.) in Fraction 2, to 43.7 +/- 0.57 microns and 1033 +/- 33.8 microns 2 length and area respectively in Fraction 7. Glutamine synthetase activity was assessed in each freshly eluted fraction and was shown to be predominant in Fractions 6 and 7. Pretreatment of rats with CCl4, which selectively destroys perivenous hepatocytes, decreased the proportion of couplets in these fractions by over 67%, and their glutamine synthetase activity by over 97%. It was concluded that Fractions 2 and 3 contained predominantly couplets of Zone 1 (periportal) origin, Fractions 4 and 5 those from Zone 2, and Fractions 6 and 7 predominantly couplets of Zone 3 (perivenous) origin. The development of canalicular secretory activity was assessed in the couplets after a 15 min incubation with a fluorescent bile acid, cholyl-lysyl-fluorescein (CLF). This was sigmoidal in all fractions, but slower in the periportal couplets, taking 5.1 h for 50% to show secretory activity in Fraction 2, compared with 2.7 h for Fraction 7. Incubation of hepatocyte couplets with 1 or 10 microM taurodehydrocholate, a non-toxic bile acid analogue, did not influence the rate of development of accumulation of CLF by the couplets or the area of the canalicular vacuole in any fraction. However, it did decrease the CLF content of couplets incubated with CLF for 15 min to a greater extent in those of perivenous origin. After subjecting the couplets to oxidative stress by incubation with 20 microM menadione (2-methyl-1,4-naphthoquinone), it was evident that periportal couplets were less able to maintain canalicular secretory activity than perivenous couplets.