A histochemical study about the zonal distribution of the galactose-binding protein in rat liver

Introduction to glycopathology: the concept, the tools and the perspectives

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Analyzing the flow of biological information is a fundamental challenge for basic sciences. The emerging results will then lend themselves to the development of new approaches for medical applications. Toward this end, the products of protein/lipid glycosylation deserve special attention. The covalent attachment of sugars to these carriers means much more than just a change of the carriers’ physicochemical properties. In principle, the ubiquitous presence of glycoconjugates and the close inspection of the particular structural ‘talents’ of carbohydrates provide suggestive evidence for information coding by sugars. In fact, the theoretical number of ‘words’ (oligomers) formed by ‘letters’ (monosaccharides) is by far higher than by using nucleotides or amino acids. In other words, glycans harbor an unsurpassed coding capacity. The cyto- and histochemical detection of dynamic changes in the profile of cellular glycans (glycome, the equivalent of the proteome) by sugar receptors such as antibodies used as tools underscores the suitability of carbohydrates for such a task. The resulting staining patterns can be likened to a molecular fingerprint. By acting as ligand (counterreceptor) for endogenous receptors (tissue lectins), glycan epitopes become partners in a specific recognition pair, and the sugar-encoded information can then be translated into effects, e.g. in growth regulation. Of note, expression of both sides of such a pair, i.e. lectin and cognate glycan, can physiologically be orchestrated for optimal efficiency. Indeed, examples how to prevent autoimmune diseases by regulatory T cells and restrict carcinoma growth by a tumor suppressor attest occurrence of co-regulation. In consequence, these glycans have potential to establish a new class of functional biomarkers, and mapping presence of their receptors is warranted. In this review, the cyto- and histochemical methods, which contribute to explore information storage and transfer within the sugar code, are described. This introduction to the toolbox is flanked by illustrating the application of each type of tool in histopathology, with focus on adhesion/growth-regulating galectins. Together with an introduction to fundamental principles of the sugar code, the review is designed to guide into this field and to inspire respective research efforts.

Glycohistochemistry: the why and how of detection and localization of endogenous lectins

The central dogma of molecular biology limits the downstream flow of genetic information to proteins. Progress from the last two decades of research on cellular glycoconjugates justifies adding the enzymatic production of glycan antennae with information-bearing determinants to this famous and basic pathway. An impressive variety of regulatory processes including cell growth and apoptosis, folding and routing of glycoproteins and cell adhesion/migration have been unravelled and found to be mediated or modulated by specific protein (lectin)-carbohydrate interactions. The conclusion has emerged that it would have meant missing manifold opportunities not to recruit the sugar code to cellular information transfer. Currently, the potential for medical applications in anti-adhesion therapy or drug targeting is one of the major driving forces fuelling progress in glycosciences. In histochemistry, this concept has prompted the introduction of carrier-immobilized carbohydrate ligands (neoglycoconjugates) to visualize the cells' capacity to be engaged in oligosaccharide recognition. After their isolation these tissue lectins will be tested for ligand analysis. Since fine specificities of different lectins can differ despite identical monosaccharide binding, the tissue lectins will eventually replace plant agglutinins to move from glycan profiling and localization to functional considerations. Namely, these two marker types, i.e. neoglycoconjugates and tissue lectins, track down accessible binding sites with relevance for involvement in interactions in situ. The documented interplay of synthetic organic chemistry and biochemistry with cyto- and histochemistry nourishes the optimism that the application of this set of innovative custom-prepared tools will provide important insights into the ways in which glycans can act as hardware in transmitting information during normal tissue development and pathological situations.

Pathogenesis of autoimmune hepatitis

Autoimmune hepatitis (AIH) is an idiopathic disorder affecting the hepatic parenchyma. There are no morphological features that are pathognomonic of the condition but the characteristic histological picture is that of an interface hepatitis without other changes that are more typical of other liver diseases. It is associated with hypergammaglobulinaemia, high titres of a wide range of circulating auto-antibodies, often a family history of other disorders that are thought to have an autoimmune basis, and a striking response to immunosuppressive therapy. The pathogenetic mechanisms are not yet fully understood but there is now considerable circumstantial evidence suggesting that: (a) there is an underlying genetic predisposition to the disease; (b) this may relate to several defects in immunological control of autoreactivity, with consequent loss of self-tolerance to liver auto-antigens; (c) it is likely that an initiating factor, such as a hepatotropic viral infection or an idiosyncratic reaction to a drug or other hepatotoxin, is required to induce the disease in susceptible individuals; and, (d) the final effector mechanism of tissue damage probably involves auto-antibodies reacting with liver-specific antigens expressed on hepatocyte surfaces, rather than direct T-cell cytotoxicity against hepatocytes.

Beyond plant lectin histochemistry: preparation and application of markers to visualize the cellular capacity for protein-carbohydrate recognition

Oligosaccharides can store biological information. In this respect, their capacity even outmatches that of oligo- and polymeric structures of nucleotides and amino acids. Protein-carbohydrate interactions are thus considered to be involved in the regulation of diverse cellular activities. Over decades, plant lectins have proven valuable for assessing structural aspects of the enormous variety of carbohydrate epitopes and for monitoring spatially and/or temporally restricted patterns of expression. If the presence of these epitopes and the alterations in their occurrence bear physiological relevance, one reasonable possibility is that the visualized saccharides serve as ligands in an operative protein-carbohydrate recognition system. To support the validity of this hypothesis, receptor sites for a sugar compound must be localized. Carrier-immobilized carbohydrates (neoglycoconjugates) are adequate for this purpose. Chemical synthesis gains access to such probes. In the first stage, the presence of binding sites such as lectins in the tissue is ascertained. The next step toward proving the outlined hypothesis is the application of the first localized then purified endogenous receptors as glycohistochemical markers. It is essential to point out that the fine specificities of plant and animal lectins can differ, although they share an identical monosaccharide specificity. Thus, neoglycoconjugates for localizing sugar ligand-binding proteins and endogenous lectins to detect suitable binding partners are promising probes to enhance our knowledge about the capacities of cells to be engaged in protein-carbohydrate recognition in situ.

Different zonal distribution of the asialoglycoprotein receptor, the alpha 2-macroglobulin receptor/low-density-lipoprotein receptor-related protein and the lipoprotein-remnant receptor of rat liver parenchymal cells

Periportal and perivenous parenchymal cells were isolated by the digitonin-pulse perfusion method. The digitonin-pulse perfusion was shown to lead to selective lysis of the correct zone with a straight and sharp border of two to three cells. The mean ratios of alanine aminotransferase activity (a marker for periportal parenchymal cells) and glutamine synthetase activity (a perivenous marker) of periportal to perivenous parenchymal cells were 1.76 and 0.025 respectively. Cells were incubated in vitro with 125I-asialo-orosomucoid (ASOR), 125I-trypsin-activated alpha 2-macroglobulin (alpha 2M-T) or 125I-beta-migrating very-low-density lipoprotein (beta-VLDL), in order to determine the zonal distribution of the asialoglycoprotein receptor (ASGPr), the alpha 2-macroglobulin receptor/low-density-lipoprotein receptor-related protein (alpha 2Mr/LRP) and the lipoprotein-remnant receptor, respectively. Maximum binding capacity for 125I-ASOR on parenchymal cells showed a periportal/perivenous ratio of 0.70. The periportal/perivenous ratio of Bmax. values of binding of 125I-alpha 2M-T to parenchymal cells was 1.51. The Bmax. values of binding of 125I-beta-VLDL, however, were about equal for both cell populations. It is concluded that the maximum binding capacity of the ASGPr on isolated periportal parenchymal cells is 0.70 times that of perivenous parenchymal cells. The 1.51-fold higher expression of the alpha 2Mr/LRP on periportal cells, compared with perivenous parenchymal cells, indicates a zonal specialization for the uptake of the suggested multiple ligands. In contrast, the observed homogeneous distribution of the lipoprotein-remnant receptor is in accordance with the suggestion that lipoprotein remnants bind to a specific receptor, which is different from the alpha 2Mr/LRP. The zonal heterogeneity in the expression of receptors suggests that receptor-dependent uptake pathways are under zonal control, leading to intrahepatic heterogeneity in the removal of ligands from the blood circulation.

Interpretation and clinical significance of alkaline phosphatase isoenzyme patterns

Alkaline phosphatase (ALP, EC 3.1.3.1) is a membrane-bound metalloenzyme that consists of a group of true isoenzymes, all glycoproteins, encoded for by at least four different gene loci: tissue-nonspecific, intestinal, placental, and germ-cell ALP. Through posttranslational modifications of the tissue-nonspecific gene, for example, through differences in carbohydrate composition, bone and liver ALP are formed. Nowadays, most commercially available methods for separating or measuring ALP isoenzymes are easy to perform and sensitive and allow for reproducible and quantitative results. As more isoenzymes and isoforms have been characterized, confusion has arisen due to the many different names they were given. For the sake of simplicity and because of structural analogies, we propose an alternative nomenclature for the ALP isoenzymes and isoforms based on their structural characteristics: soluble, dimeric (Sol), anchor-bearing (Anch), and membrane-bound (Mem) liver, bone, intestinal, and placental ALP. Together with lipoprotein-bound liver ALP and immunoglobulin-bound ALP, these names largely fit the many forms of ALP one can encounter in human serum and tissues. The clinically relevant isoenzymes are sol-liver, Mem-liver, lipoprotein-bound liver, and Sol-intestinal ALP in liver diseases, and Sol-bone and Anch-bone ALP in bone diseases. Many different isoenzyme patterns can be found in malignancies and renal diseases. This test provides the clinician with valuable information for diagnostic purposes as well as for follow-up of patients and monitoring of treatment. However, ALP isoenzyme determination will only provide clinically useful information if the patterns are correctly interpreted. In this respect, care should be taken to use the proper reference ranges, taking into account the age and sex of the patient. A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children. Separating ALP into its isoenzymes adds considerable value to the mere assay of total ALP activity.

Immunopathology of alkaline phosphatase-induced granulomatous hepatitis in rats

Granulomatous inflammation is a specific type of chronic inflammation in which macrophages and T-cell-mediated immunity to the inciting agent play a pivotal role. In the present study, granulomatous hepatitis was induced in rats by the administration of a single intravenous dose of porcine intestinal alkaline phosphatase. The cellular composition of the hepatic granulomas was analyzed in-situ with a number of recently developed mouse anti-rat monoclonal antibodies to cells of the monocyte-macrophage lineage and lymphocyte subsets. Well-developed granulomas consisted of aggregates of macrophages with central modification into epithelioid cells, a peripheral rim of T- and B-lymphoid cells, including considerable numbers of immunoblasts and plasma cells. In addition, the periphery of the granulomas contained many fat storing cells, a sinusoidal cell type thought to play a central role in hepatic fibrosis. Moreover, intense immunostaining for the extracellular matrix proteins fibronectin and collagen type III was observed at the periphery of the lesions. The granulomas persisted for long periods without eliciting liver cirrhosis. Alkaline phosphatase induced hepatic granulomas in the rat may help to elucidate the contribution of cells of the B-lineage to chronic granulomatous inflammation.

Antibodies against the hepatic asialoglycoprotein receptor perfused in situ preferentially attach to periportal liver cells in the rat

Autoantibodies reacting with the galactose-specific hepatic asialoglycoprotein receptor--a liver-specific component expressed on the surfaces of hepatocytes--are often found in patients with chronic active hepatitis of presumed autoimmune origin. As part of an investigation into whether these anti-asialoglycoprotein receptor antibodies might be involved in the development of periportal liver damage in chronic active hepatitis, livers of ether-anesthetized rats were perfused in situ with polyclonal guinea pig anti-rabbit asialoglycoprotein receptor or murine monoclonal anti-human galactose-specific hepatic asialoglycoprotein receptor antibodies in excess at less than 8 degrees C or, as a control, with guinea pig anti-human plasma protein antibodies or normal guinea pig serum. Rapid (1 min) antegrade (by way of portal vein) or retrograde (through hepatic veins by way of vena cava) perfusions were performed in a nonrecirculating (once-through) mode in Ca+(+)-free medium. Blocks of liver tissue were immediately snap-frozen and the distribution of the antibody examined in cryostat sections by using an avidin-biotin immunohistochemical technique. In all of the perfusions with anti-asialoglycoprotein receptor (six antegrade, seven retrograde), the antibodies were found to be prominently and almost exclusively deposited on liver cells in the periportal areas. No deposition of immunoglobulins was detected in livers perfused with the control guinea pig sera. The findings suggest that the asialoglycoprotein receptor is expressed at high density mainly on cells in zone 1 of the hepatic lobule, and this may have implications for the development of periportal liver damage in chronic active hepatitis.

Neoglycoenzymes: a versatile tool for lectin detection in solid-phase assays and glycohistochemistry

Carbodiimide-mediated coupling of p-aminophenyl glycosides to a naturally nonglycosylated enzyme yields a neoglycoenzyme. This compound combines inherent enzymatic activity with synthetically conferred ligand properties to lectins. Appropriate choice of the ligand allows custom-made synthesis to reliably detect various types of lectins. To exemplify practical applications of this class of compounds, glycosylated bacterial beta-galactosidase has been employed to quantitate plant lectins, immobilized on plastic surfaces as well as on nitrocellulose. Competitive inhibition by specific sugar ascertained the dependence of binding on protein--carbohydrate interactions. In view of lectins as tools, a sandwich lectin-binding assay for high mannose-type glycoprotein detection has been modified to principally facilitate wide application to other lectin-reactive sugar chains by introducing the neoglycoenzyme. In addition to lectin determination in solid-phase assays, neoglycoenzymes allow one to glycohistochemically localize endogenous lectins in tissue prints and tissue sections with a minimum number of steps. This nonradioactive, rapid, sensitive, and convenient assay concept, based on conjugation of a ligand to an enzyme with maintenance of its receptor-binding activity, may find extended application beyond lectinology in receptor analysis.

Heterogeneous acinar localization of the asialoglycoprotein internalization system in rat hepatocytes

Desialylated glycoprotein is rapidly cleared from plasma by a receptor-mediated endocytic mechanism located on hepatocytes. We studied the hepatic acinar distribution of this asialoglycoprotein transport system with the ligand 125I-asialoorosomucoid using rat liver perfused in either antegrade or retrograde direction in combination with quantitative light microscopic autoradiography. Grain distribution along the acinus appeared dependent on the perfusion direction. A rather shallow zone 1 to zone 3 gradient was observed if livers were perfused in the normal direction. However, a statistically significantly steeper zone 3 to zone 1 gradient was detected in retrograde perfusions. Kinetic analysis of perfusate clearance profiles yielded a hepatic clearance of 21.6 +/- 1.3 ml per min in antegradely perfused liver. Hepatic extraction was calculated to be 60.1 +/- 7.4%. Biliary secretion of radioactivity amounted to 1.89 +/- 0.18% of the dose within 1 hr after injection and consisted of intact material (1.39 +/- 0.25%) and radioactive low-molecular-weight degradation products (0.52 +/- 0.08%), of which more than 90% could be accounted for by 125I-. Apart from a minor difference regarding biliary secretion of an unidentified glycopeptide (less than 0.1% of the injected dose), transport data for the retrogradely perfused livers were identical to those obtained with livers perfused in antegrade direction, emphasizing the functional equivalence of both groups of livers. The autoradiographic data indicate that zone 3 hepatocytes take up 125I-asialoorosomucoid more avidly than zone 1 cells. The kinetic and biochemical data indicate that further processing in the hepatocytes is virtually similar in the two zones.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the mechanism of binding and uptake of immune complexes by various cell types of rat liver in vivo

Soluble heterologous immune complexes (IC) were used to study the mechanism of IC binding to rat liver in vivo. Binding of IC to the various cell types of the liver, endothelial cells, hepatocytes, and Kupffer cells, was only inhibited by aggregated swine immunoglobulins. Binding was not inhibited by the absence of complement components. Intravenous injection of asialoglycoproteins, to block the galactose receptor, could not prevent IC binding. We conclude that Fc receptors play an important role in the binding of soluble heterologous IC to hepatocytes, endothelial cells, and Kupffer cells.

Zonal heterogeneity of rat hepatocytes in the in vivo uptake of 17 nm colloidal gold granules

The in vivo uptake in hepatocytes of intravenously injected colloidal gold granules with a diameter of 17 nm or 79 nm and coated with bovine serum albumin or with polyvinyl-pyrrolidone was studied. Irrespective of coating only the 17 nm granules were taken up in hepatocytes. Perivenous hepatocytes did take up much more gold granules than periportal hepatocytes. The gold granules were found in lysosomes around bile canaliculi. Two hours after injection hepatocytes contained the maximal amount of granules. At least a portion of the granules was discharged into the bile. The observed zonal gradient in the uptake of 17 nm gold granules might be caused by the greater supply of granules to the perivenous hepatocytes as a combined result of the higher porosity of the endothelial lining and the smaller number of Kupffer cells with a low endocytic activity in this zone.

Cell biology of the asialoglycoprotein receptor system: a model of receptor-mediated endocytosis

Substantial information about the ASGP-R has accumulated in the 10 years following the initial studies of this receptor by Ashwell and Morell. Many of its biochemical properties, its structure, and its orientation within the plasma membrane are now known. The pathways of ASGP ligand and receptor, with the CURL organelle being a central component, are summarized in Fig. 18. The major pathway of the ligand through the cell, beginning with binding at the cell surface and ending with degradation in lysosomes, has been investigated in detail. Recently, alternate routes of the ligand such as the ligand recycling pathway have been observed. With regard to the itinerary of the receptor, there is now biochemical, kinetic, and morphological evidence to support receptor recycling. The new concept of CURL as an important intracellular organelle has originated from studies of ASGP-R recycling. Its importance in the dissociation and segregation of ligand and receptor as well as in receptor recycling is now evident. In addition, there has been a concurrent investigation of other receptor systems that participate in receptor-mediated endocytosis, providing parallels and contrasts to the ASGP-R of hepatocytes. Many critical issues still exist in the cell biology of the ASGP-R. What are the structural requirements of the receptor for ligand binding and subsequent endocytosis of the receptor-ligand complex? Very little is known about the interactions between the receptor and the lipid bilayer in which it resides. How does the receptor move laterally in the plasma membrane? Are there proteins or glycolipids closely associated with the ASGP-R and, if so, what is their function? What is the mechanism that causes receptor clustering into coated pits? Although the existence of a pathway for ligand recycling has been demonstrated, there are still many issues to be addressed. What signals a particular ligand molecule for recycling? Is it a stochastic process? What is the function of this route of ligand movement? How are the various ligand pathways coordinated and regulated? In addition, there are many unanswered questions regarding the receptor pathway. How does CURL mediate the sorting of ASGP-R from ligand? How are receptors with different destinations (e.g., ASGP-R and IgA receptor) sorted in CURL? What is the mechanism of ASGP-R degradation and how is it regulated? Finally, how does the Golgi function in the ASGP system and what is the relationship between the Golgi and CURL? Future investigation of these issues will require further observations with existing techniques as well as new approaches.(ABSTRACT TRUNCATED AT 400 WORDS)

The hepatic asialoglycoprotein receptor

Asialo- (i.e., galactose-terminal) glycoproteins are specifically and avidly recognized by a mammalian hepatic parenchymal cell receptor. This receptor, itself a glycoprotein, binds ligand molecules and directs their delivery to lysosomes for catabolism. The receptor is reutilized during this process of receptor-mediated endocytosis. Ligand specificity is conferred by galactose or N-acetyl-galactosamine at the nonreducing termini of the oligosaccharide chains. The receptor appears to be a transmembrane protein and is localized both to the cell surface as well as to several membranous intracellular compartments.

Heterogeneity of rat hepatocytes in transport and hepatic binding of asialoalkaline phosphatase studied after induction of selective acinar damage by N-hydroxy-2-acetylaminofluorene and carbon tetrachloride

In order to investigate rat hepatocyte heterogeneity in asialoglycoprotein transport, rats were pretreated with N-hydroxy-2-acetylaminofluorene (N-OH-AAF, 90 mumol/kg, i.v.) to damage zone 1 hepatocytes, or with carbon tetrachloride (CCl4, 2.1 mmole/kg, p.o.) to damage zone 3 hepatocytes. Twenty-four hours after pretreatment, the asialoglycoprotein dog intestinal alkaline phosphatase (As-ALPase) was injected and plasma disappearance and biliary excretion were measured. In addition, the acinar distribution of the hepatic binding of As-ALPase 10 min after injection in vivo, or after incubation of fixed liver sections with As-ALPase in vitro, was determined by enzyme histochemistry. In control rats, a rapid biexponential plasma disappearance was observed and 6.4 +/- 1.5% of the dose was excreted into bile after 60 min. Hepatocyte binding occurred predominantly in zone 3, both after administration in vivo and after incubation with liver sections in vitro. In rats with zone 1 liver damage, both the half-lives of the first and of the second phase were strongly increased, but biliary excretion did not change significantly. Both in vivo and in vitro the relatively weak binding of As-ALPase in zone 1 of the liver was abolished, whereas binding to zone 3 cells was normal or only slightly decreased. After CCl4-pretreatment histochemically detectable binding to zone 3 cells was completely abolished, leaving only the relatively weak binding in zone 1. Unexpectedly, a normal plasma disappearance and biliary excretion rate were found in these rats. The discrepancy between the pharmacokinetic results, which point to a predominant involvement of zone 1 cells in As-ALPase transport, and the enzyme histochemical studies, which show preferential binding of As-ALPase in zone 3, is discussed.

The role of the liver in clearance of glycoproteins from the general circulation, with special reference to intestinal alkaline phosphatase

Glycoproteins represent a wide variety of macromolecules with important physiological functions. Characteristic variations in carbohydrate composition and plasma concentration of these proteins may occur during pathological conditions. Steady-state plasma concentrations are determined by release from normal or diseased tissues and simultaneous clearance from the general circulation. The liver occupies a central position in the production but also clearance and catabolism of such glycoproteins. A number of specialized receptor-mediated transport processes for different types of glycoproteins in this organ is reviewed. Membrane recognition is generally followed by absorptive endocytosis and vesicle transport to lysosomes, Golgi system and/or bile canaliculis. The charge of the protein, the nature of the terminal sugar residue or complex formation with other glycoproteins may determine the extent of uptake in the various cell types of the liver. By means of these transport processes the liver is able to remove potentially dangerous macromolecules such as denatured proteins, aggressive enzymes and immunocomplexes from the general circulation. Drugs can bind to some of these proteins or may interact with the hepatic transport or catabolism processes. Special attention is paid to the hepatic clearance of asialoglycoproteins with terminal galactose groups. Intestinal alkaline phosphatase is used as a model compound to characterize the pharmacokinetic profiles of hepatic uptake and biliary excretion in the rat in vivo and isolated perfused rat livers. Histochemical and electron-microscopic studies demonstrated a galactose-specific, receptor-mediated endocytotic process, mainly but not exclusively localized in centrolobular hepatocytes. Drug interactions with these processes will be the subject of further investigations.

Cytochemical detection of mannose-specific receptors for glycoproteins with horseradish peroxidase as a ligand

Horseradish peroxidase (HRP), a glycoprotein rich in mannose groups, was used as a ligand to detect receptors for glycoproteins in formalin-fixed, frozen sections of rat liver. Specific binding of HRP occurred to surface membranes of sinusoidal cells but not to those of parenchymal cells. The binding sites were visualized after the peroxidatic reaction in erythrocytes had been suppressed by methanol-H2O2 and phenylhydrazine, the latter reagent also decreasing the nonspecific background adsorption of HRP. Several factors influencing the reaction were studied systematically. The specific binding of HRP to sinusoidal cells was greatly decreased or abolished when tissue blocks were fixed for longer than 1-2 h in a cold 4% formaldehyde solution and the frozen sections subsequently treated for 30 min in cold methanol. The specific binding of HRP increased when the concentration of HRP in the medium was increased from 10 microgram/ml to 40 microgram/ml, when the time of incubation with HRP was increased from 1 h to 4 h, or when the temperature of incubation with HRP was increased from 4 degrees C to 22 degrees C, the pH of the incubation medium was increased from 7.0 to 10.0. Little or no specific binding of HRP was observed in the absence of added Ca++. The binding of HRP was suppressed by 10 mM mannose or 0.004% mannan whereas the suppression of the binding reaction by galactose or galactan required 30-40 ties higher concentrations.