The major protein of pancreatic zymogen granule membranes (GP-2) is anchored via covalent bonds to phosphatidylinositol

The search for the Holy Grail: autoantigenic targets in primary sclerosing cholangitis associated with disease phenotype and neoplasia

Unlike in other autoimmune liver diseases such as autoimmune hepatitis and primary biliary cholangitis, the role and nature of autoantigenic targets in primary sclerosing cholangitis (PSC), a progressive, chronic, immune-mediated, life threatening, genetically predisposed, cholestatic liver illness, is poorly elucidated. Although anti-neutrophil cytoplasmic antibodies (ANCA) have been associated with the occurrence of PSC, their corresponding targets have not yet been identified entirely. Genome-wide association studies revealed a significant number of immune-related and even disease-modifying susceptibility loci for PSC. However, these loci did not allow discerning a clear autoimmune pattern nor do the therapy options and the male gender preponderance in PSC support a pathogenic role of autoimmune responses. Nevertheless, PSC is characterized by the co-occurrence of inflammatory bowel diseases (IBD) demonstrating autoimmune responses. The identification of novel autoantigenic targets in IBD such as the major zymogen granule membrane glycoprotein 2 (GP2) or the appearance of proteinase 3 (PR3) autoantibodies (autoAbs) have refocused the interest on a putative association of loss of tolerance with the IBD phenotype and consequently with the PSC phenotype. Not surprisingly, the report of an association between GP2 IgA autoAbs and disease severity in patients with PSC gave a new impetus to autoAb research for autoimmune liver diseases. It might usher in a new era of serological research in this field. The mucosal loss of tolerance against the microbiota-sensing GP2 modulating innate and adaptive intestinal immunity and its putative role in the pathogenesis of PSC will be elaborated in this review. Furthermore, other potential PSC-related autoantigenic targets such as the neutrophil PR3 will be discussed. GP2 IgA may represent a group of new pathogenic antibodies, which share characteristics of both type 2 and 3 of antibody-mediated hypersensitive reactions according to Coombs and Gell.

Glypican-1 and glycoprotein 2 bearing extracellular vesicles do not discern pancreatic cancer from benign pancreatic diseases

Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease that is clinically asymptomatic in its early stages of development. Non-invasive testing for pancreatic cancer biomarkers would significantly improve early detection and patient care. Extracellular vesicles (EVs) are circulating tumor fragments present in the blood and may express cancer specific biomarkers that would enable early detection of pancreatic cancer. We tested the utility of a blood test enumerating EVs positive for the pancreas-specific marker Glycoprotein 2 (GP2) and the putative pancreatic cancer marker Glypican-1 (GPC1) in patients with PDAC. Various levels of GPC1-positive and GP2/GPC1-positive EVs were detected in PDAC patients but were not significantly higher than benign pancreatic disease (BPD) patients. The sensitivity and specificity of the GPC1 EV test was 26.67% and 87.50% respectively, whereas the sensitivity and specificity for the GPC1+GP2 EV test was 23.33% and 90.00% respectively. Immunohistochemistry of GPC1 expression in a tissue microarray of PDAC and various controls also did not demonstrate specificity of GPC1 to PDAC. Hence, enumeration of GPC1-positive EVs, solely or in conjunction with GP2, was unable to effectively distinguish between BPD and pancreatic cancer.

Differential diagnosis of Crohn's disease using antibodies to glycoprotein 2 and Saccharomyces cerevisiae

BACKGROUND/AIMS

Glycoprotein 2 (GP2), the major autoantigen of Crohn's disease (CD)-specific pancreatic autoantibodies, is reportedly correlated with several characteristics of CD. We investigated this serological marker in Turkish patients with CD and assessed its utility in combination with anti-Saccharomyces cerevisiae antibodies (ASCAs) for differential diagnosis of CD.

MATERIALS AND METHODS

A total of 60 patients with CD, 62 patients with ulcerative colitis (UC), and 46 healthy controls with a definite diagnosis who were similar in age and sex were enrolled in the study conducted from November 2011 to October 2012. ASCA and anti-GP2 levels were measured using commercially available kits.

RESULTS

Anti-GP2 IgA and IgG levels were higher in patients with CD (25%) than in those with UC (5%) and controls (2%). The seroprevalence of anti-GP2 IgA was markedly higher than that of IgG in patients with CD in contrast to previous studies. The specificity and positive predictive value of seropositivity for both ASCA and anti-GP2 were 100%. ASCA IgA seropositivity was correlated with a complicated disease course and a history of surgery. There was no correlation between anti-GP2 seropositivity and disease location, disease behavior, or a history of surgery.

CONCLUSION

The combination of ASCA and anti-GP2 may enable differentiation of CD from UC. As ASCA seropositivity is associated with a more complicated disease course, patients seropositive for ASCA at the initial diagnosis should undergo more intense therapy.

Serological diagnosis and prognosis of severe acute pancreatitis by analysis of serum glycoprotein 2

BACKGROUND

Glycoprotein 2 (GP2), the pancreatic major zymogen granule membrane glycoprotein, was reported to be elevated in acute pancreatitis in animal models.

METHODS

Enzyme-linked immunosorbent assays (ELISAs) were developed to evaluate human glycoprotein 2 isoform alpha (GP2a) and total GP2 (GP2t) as specific markers for acute pancreatitis in sera of 153 patients with acute pancreatitis, 26 with chronic pancreatitis, 125 with pancreatic neoplasms, 324 with non-pancreatic neoplasms, 109 patients with liver/biliary disease, 67 with gastrointestinal disease, and 101 healthy subjects. GP2a and GP2t levels were correlated with procalcitonin and C-reactive protein in 152 and 146 follow-up samples of acute pancreatitis patients, respectively.

RESULTS

The GP2a ELISA revealed a significantly higher assay accuracy in contrast to the GP2t assay (sensitivity ≤3 disease days: 91.7%, specificity: 96.7%, positive likelihood ratio [LR+]: 24.6, LR-: 0.09). GP2a and GP2t levels as well as prevalences were significantly elevated in early acute pancreatitis (≤3 disease days) compared to all control cohorts (p<0.05, respectively). GP2a and GP2t levels were significantly higher in patients with severe acute pancreatitis at admission compared with mild cases (p<0.05, respectively). Odds ratio for GP2a regarding mild vs. severe acute pancreatitis with lethal outcome was 7.8 on admission (p=0.0222). GP2a and GP2t levels were significantly correlated with procalcitonin [Spearman's rank coefficient of correlation (ρ)=0.21, 0.26; p=0.0110, 0.0012; respectively] and C-reactive protein (ρ=0.37, 0.40; p<0.0001; respectively).

CONCLUSIONS

Serum GP2a is a specific marker of acute pancreatitis and analysis of GP2a can aid in the differential diagnosis of acute upper abdominal pain and prognosis of severe acute pancreatitis.

Autoimmunity in Crohn's Disease-A Putative Stratification Factor of the Clinical Phenotype

Inflammation in inflammatory bowel diseases (IBD) has been linked to a loss of tolerance to self-antigens suggesting the existence of autoantibodies in specific disease phenotypes. However, the lack of clearly defined autoantigenic targets has slowed down research. Genome-wide association studies have identified an impressive number of immune-related susceptibility loci for IBD with no clearly discernible pattern among them. Growing evidence supports the hypothesis that innate immune responses to a low-diversity and impaired gut microbiota may be of key importance in initiating and perpetuating chronic inflammation in IBD. Increasing evidence suggests that reduced microbial diversity and microbial-mucosal epithelium interaction (including adhesion and clearance) are critically involved in IBD pathogenesis. Along these lines the discovery of autoantigenic targets in Crohn's disease (CD) has refocused research in IBD on the possible role of autoimmune responses. The identification of the major zymogen granule membrane glycoprotein 2 (GP2) as an autoantigen in CD patients and its proposed role in the sensing of the microbiota lends credence to this trend. Loss of tolerance to GP2 occurs in up to 40% of patients with CD. Corresponding autoantibodies appear to be associated with distinct disease courses (types or phenotypes) in CD. Here, we critically review autoantibodies in CD for their impact on clinical practice and future IBD research. The immunomodulatory role of GP2 in innate and adaptive intestinal immunity is also discussed.

Autoantibodies against exocrine pancreas in Crohn's disease are directed against two antigens: the glycoproteins CUZD1 and GP2

BACKGROUND

Autoantibodies against exocrine pancreas (PAb) have been reported to be pathognomonic markers of Crohn's disease (CD). Recently, the glycoprotein GP2 has been proposed as the exclusive target for PAb but two equally prevalent binding patterns can be observed in the indirect immunofluorescence test (IIFT) using cryosections of human pancreas: a reticulogranular and a droplet pattern.

AIM

To identify autoantigens corresponding to the staining patterns.

METHODS

Different lectins were screened for their ability to immobilize PAb-reactive glycoproteins from cell free human pancreas. The glycoproteins were then purified via UEA-I affinity chromatography and identified by mass spectrometry. The two candidate autoantigens were separately expressed in HEK293 cells, and the recombinant cells applied as substrates in IIFT to analyze sera from 96 patients with CD, 89 controls and hybridoma supernatants during the generation of murine monoclonal antibodies.

RESULTS

The UEA-I eluate was able to neutralize PAb reactivity of both patterns in IIFT. It contained two major constituents which were identified as the glycoproteins CUZD1 and GP2. With the recombinant cells, 35.4% of the CD patients exhibited positive reactions (CUZD1 alone 19.8%, GP2 alone 9.4%, and both antigens 6.2%). The reaction with the CUZD1 expressing cells was strictly correlated to the reticulogranular pattern, whereas the antibodies causing the droplet pattern stained the GP2 expressing cells. Antigen-capture ELISA using the newly generated monoclonal antibodies against CUZD1 and GP2 verified this relationship.

CONCLUSIONS

The concordant reactivities of the different platforms can be regarded as a proof for the authenticity of the two identified autoantigens.

Glycoprotein 2 antibodies in Crohn's disease

The pathogenesis of Crohn's disease (CrD) and ulcerative colitis (UC), the two major inflammatory bowel diseases (IBD), remains poorly understood. Autoimmunity is considered to be involved in the triggering and perpetuation of inflammatory processes leading to overt disease. Approximately 30% of CrD patients and less than 8% of UC patients show evidence of humoral autoimmunity to exocrine pancreas, detected by indirect immunofluorescence. Pancreatic autoantibodies (PAB) were described for the first time in 1984, but the autoantigenic target(s) of PABs were identified only in 2009. Utilizing immunoblotting and matrix-assisted laser desorption ionization time-of-flight mass spectrometry, the major zymogen granule membrane glycoprotein 2 (GP2) has been discovered as the main PAB autoantigen. The expression of GP2 has been demonstrated at the site of intestinal inflammation, explaining the previously unaddressed contradiction of pancreatic autoimmunity and intestinal inflammation. Recent data demonstrate GP2 to be a specific receptor on microfold (M) cells of intestinal Peyer's patches, which are considered to be the original site of inflammation in CrD. Novel ELISAs, employing recombinant GP2 as the solid phase antigen, have confirmed the presence of IgA and IgG anti-GP2 PABs in CrD patients and revealed an association of anti-GP2 IgA as well as IgG levels with a specific clinical phenotype in CrD. Also, GP2 plays an important role in modulating innate and acquired intestinal immunity. Its urinary homologue, Tamm-Horsfall protein or uromodulin, has a similar effect in the urinary tract, further indicating that GP2 is not just an epiphenomenon of intestinal destruction. This review discusses the role of anti-GP2 autoantibodies as novel CrD-specific markers, the quantification of which provides the basis for further stratification of IBD patients. Given the association with a disease phenotype and the immunomodulating properties of GP2 itself, an important role for GP2 in the immunopathogenesis of IBD cannot be excluded.

Pancreatic-specific autoantibodies to glycoprotein 2 mirror disease location and behaviour in younger patients with Crohn's disease

Background

Glycoprotein 2 (GP2) was discovered as the major autoantigen of Crohn’s disease (CD)-specific pancreatic autoantibodies (PAB). We investigated anti-GP2 IgA and IgG antibodies as novel serological parameters in CD and assessed their association with distinct disease phenotypes.

Methods

Anti-GP2 and anti-Saccharomyces cerevisiae (ASCA) IgA and IgG were detected by ELISA employing recombinant human GP2 and phosphopeptidomannan, respectively and PAB by indirect immunofluorescence (IIF) in 271 sera, 169 with CD and 102 with ulcerative colitis (UC). As healthy controls 160 adult blood donors and 65 children were included.

Results

Anti-GP2 IgG and/or IgA were more prevalent in CD (51/169, 30.2%) than in UC (9/102, 8.9%) patients and in controls (9/225, 4%) (p < 0.001 respectively). ASCA IgG and/or IgA were present in 60/169 (35.5%) in CD and in 7/102 (6.9%) in UC patients (p < 0.001). CD patients with ileocolonic location (L3) showed a significantly higher prevalence of anti-GP2 and ASCA IgA and/or IgG (40/113 and 48/113, respectively; p < 0.05 for both comparisons), whereas CD patients with colonic location (L2) revealed a significantly diminished prevalence for these autoantibody specificities (2/32 and 5/32, respectively, p < 0.05 for both). Anti-GP2 IgG were significantly more prevalent in CD patients with stricturing behaviour (B2) and perianal disease (7/11, p < 0.02) and less prevalent in those with penetrating behaviour (B3) and perianal disease (4/31, p < 0.05). The occurrence of anti-GP2 IgA and/or IgG was significantly more prevalent in CD patients with age at diagnosis of ≤16 years (16/31, p < 0.009). Prevalence of one or more anti-GP2 or ASCA IgA and/or IgG was significantly higher in L3, B2, and A1 and lower in L2 (68/113, 27/41, 23/31, 6/32; p < 0.04, respectively).

Conclusions

Anti-GP2 IgG and IgA, constituting novel CD specific autoantibodies, appear to be associated with distinct disease phenotypes identifying patients at a younger age, with ileocolonic location, and stricturing behaviour with perianal disease.

Autoantibodies to GP2, the major zymogen granule membrane glycoprotein, are new markers in Crohn's disease

BACKGROUND

Crohn's disease (CD) is an inflammatory bowel disease (IBD) characterized by reactivity against microbial and self antigens. Zymogen granule glycoprotein 2 (GP2) was identified as the major autoantigen of CD-specific pancreatic autoantibodies (PAB).

METHODS

Human GP2 was expressed in the Spodoptera frugiperda 9 (Sf9) cell line using the baculovirus system, purified by Ni-chelate chromatography, and used as antigen for anti-GP2 IgA and IgG assessment by enzyme-linked immunosorbent assays (ELISA). Antibodies to mannan of Saccharomyces cerevisiae (ASCA), PAB, and anti-GP2 were investigated in sera of 178 CD patients, 100 ulcerative colitis (UC) patients, and 162 blood donors (BD).

RESULTS

Anti-GP2 IgG and IgA were found in 48/72 (66.7%) and 23/72 (31.9%) PAB positive and 5/106 (4.7%) and 1/106 (0.9%) PAB negative CD patients (p<0.0001), respectively. CD patients displayed significantly higher reactivity to GP2 than UC patients and BD (p<0.0001), respectively. Occurrence of anti-GP2 antibodies correlated with PAB reactivity (Spearmen's rho=0.493, p<0.00001). There was a significant relationship between the occurrence of ASCA IgG and anti-GP2 IgG (p=0.0307).

CONCLUSIONS

Anti-GP2 IgG and IgA constitute novel CD specific autoantibodies, the quantification of which could improve the serological diagnosis of IBD.

Determination of plasma glycoprotein 2 levels in patients with pancreatic disease

CONTEXT

Blood tests possessing higher diagnostic accuracy are needed for all the major pancreatic diseases. Glycoprotein 2 (GP2) is a protein that is specifically expressed by the pancreatic acinar cell and that has previously shown promise as a diagnostic marker in animal models of acute pancreatitis.

OBJECTIVE

This study describes the development of an assay for GP2, followed by the determination of plasma GP2 levels in patients with acute pancreatitis, chronic pancreatitis, and pancreatic cancer.

DESIGN

Rabbit polyclonal antisera and mouse monoclonal antibodies were generated against human GP2 and used to develop an enzyme-linked immunosorbent assay. The assay was tested in patients with an admitting diagnosis of pancreatic disease at 2 tertiary care facilities. The diagnosis of acute or chronic pancreatitis and pancreatic cancer was determined using previously established criteria that incorporated symptoms, radiology, pathology, and serology. Plasma GP2 levels were determined in 31 patients with acute pancreatitis, 16 patients with chronic pancreatitis, 36 patients with pancreatic cancer, and 143 control subjects without pancreatic disease. Amylase and lipase levels were also determined in patients with acute pancreatitis.

RESULTS

The GP2 assay's sensitivity values were 0.94 for acute pancreatitis, 0.81 for chronic pancreatitis, and 0.58 for pancreatic cancer, which were greater than the 0.71 for acute pancreatitis and 0.43 for chronic pancreatitis (P =.02) observed for amylase. The lipase assay sensitivity for acute pancreatitis was 0.66. The accuracy of the GP2 assay was greater than that of the amylase or lipase assays for acute pancreatitis (GP2 vs lipase, P =.004; GP2 vs amylase, P =.003) when analyzed using receiver operator characteristic curves. When daily serial blood samples were obtained for 13 patients with acute pancreatitis, GP2 levels remained abnormally elevated for at least 1 day longer than the amylase or lipase levels.

CONCLUSION

The GP2 assay is a useful new marker for acute and chronic pancreatitis.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

A potential endogenous ligand of annexin IV in the exocrine pancreas. Carbohydrate structure of GP-2, a glycosylphosphatidylinositol-anchored glycoprotein of zymogen granule membranes

We demonstrated previously that annexins IV, V, and VI, proteins of the calcium/phospholipid-binding annexin family, have glycosaminoglycan binding properties (Ishitsuka, R., Kojima, K., Utsumi, H., Ogawa, H., and Matsumoto, I. (1998) J. Biol. Chem. 273, 9935-9941). In this study, we investigated the endogenous ligands of annexin IV in the exocrine pancreas. Immunohistochemical study of bovine pancreas showed that annexin IV localized in the apical cytoplasmic region of pancreatic acinar cells where zymogen granules are concentrated. Because it is the major component of the zymogen granule membrane, the glycosylphosphatidylinositol-anchored glycoprotein GP-2 was suggested to play a role in apical sorting and secretion of zymogens. We isolated GP-2 from porcine pancreas extract and determined the structure of its N-linked oligosaccharides by two-dimensional mapping. The major carbohydrate structures of porcine GP-2 were trisialo-triantennary and tetrasialo-tetra-antennary complex-type oligosaccharides. Dot-blot assay showed that annexin IV interacts with GP-2 in the presence of calcium and that it recognizes the terminal sialic acid residues linked through alpha2-3 linkages to the carbohydrate of GP-2. Lectin blot assay showed that Maackia amurensis mitogen, a plant lectin specific for the trisaccharide sequence Sia(alpha)2-3Galbeta1-4GlcNAc of N-linked oligosaccharides, has strong affinity for GP-2. Thus, M. amurensis mitogen was used as a specific probe for GP-2 in the histochemical staining of the bovine pancreas. GP-2 was found to localize exclusively in the same apical cytoplasmic region of pancreatic acinar cells as annexin IV does. These results suggest that GP-2 is an endogenous ligand of annexin IV in the exocrine pancreas.

GP2, a GPI-anchored protein in the apical plasma membrane of the pancreatic acinar cell, co-immunoprecipitates with src kinases and caveolin

We previously showed that endocytosis at the apical plasma membrane (APM) of the pancreatic acinar cell is activated by the cleavage of GP2, a GPI-linked protein, from the apical cell surface. This endocytic process, as measured by horseradish peroxidase uptake into pancreatic acinar cells, is blocked by the tyrosine kinase inhibitors genistein and tyrphostin B42 as well as by disruption of actin filaments with cytochalasin. This suggests that the cleavage of GP2 from the cell membrane may activate endocytosis through a tyrosine kinase-regulated pathway. However, the mechanism by which GP2 and tyrosine kinases act together to activate endocytosis at the APM remains unknown. In this study, we demonstrate that pp60, p62yes, caveolin, and annexin, which have previously been implicated in endocytosis in other cell lines, were present in high abundance in GPI-enriched membranes by Western blot analysis. pp60, p62yes, and caveolin all co-immunoprecipitated with GP2 except annexin. An 85-kDa protein whose tyrosine-dependent phosphorylation is correlated with the activation of endocytosis in intact acinar cells also was present in these immunoprecipitates. This suggests that in pancreatic acini, GP2 may exist in a complex with src kinases, caveolin, and an 85-kDa phosphorylated substrate to regulate endocytosis at the APM.

The secretory lectin ZG16p mediates sorting of enzyme proteins to the zymogen granule membrane in pancreatic acinar cells

The recently established in vitro assay of condensation-sorting of pancreatic enzymes to the zymogen granule membrane (ZGM) (Dartsch, H., R. Kleene, H. F. Kern: In vitro condensation-sorting of enzyme proteins isolated from rat pancreatic acinar cells. Eur. J. Cell Biol. 75, 211-222 (1998)) was used to study the involvement of a novel secretory lectin, ZG16p, in the binding of aggregated proteins to ZGM. In isolated zymogen granules the lectin is predominantly associated with the membrane and can be removed to a large extent by bicarbonate treatment at pH 11.5. In the in vitro assay in which secretory proteins aggregate at pH 5.9 but only those bound to ZGM are sedimented into the pellet, ZG16p is significantly enriched in this pellet fraction, shown both by biochemical and fine structural analysis. Pretreatment of ZGM with anti-ZG16p antibody before their addition to the assay inhibits binding to the membrane by about 50%. Similarly, removal of ZG16p or prevention of its interaction with glycosaminoglycans (GAGs) in the submembranous matrix of ZGM by sodium bicarbonate treatment or chondroitinase digestion of ZGM also inhibits the binding efficiency of secretory proteins to ZGM to about the same extent. We conclude that ZG16p may act as a linker molecule between the submembranous matrix on the luminal side of ZGM and aggregated secretory proteins during granule formation in the TGN.

Membrane dipeptidase and glutathione are major components of pig pancreatic zymogen granules

Membrane proteins of highly purified porcine zymogen granules were separated by two-dimensional gel electrophoresis in order to isolate proteins which are involved in intracellular trafficking of digestive enzymes in the exocrine pancreas. A 48-kDa glycoprotein was a major component in membrane preparations washed with 0.1 M Na2CO3 and 0.5 M NaCl. By N-terminal amino acid sequencing this protein was identified as membrane dipeptidase (MDP; EC 3.4.13.19). MDP mRNA levels in rat pancreas were increased threefold by feeding rats with FOY-305, which is a known stimulus of endogenous cholecystokinin release from the gut. Cholecystokinin then stimulates secretion in pancreatic acinar cells. In another set of experiments treatment of the rat pancreatic acinar tumor cell line AR42J with dexamethasone led to an eightfold increase in the expression of MDP. Thus, the expression pattern of the MDP gene in response to hormonal stimulation in vivo and in vitro resembles those found for most of the enzymes and proteins which are involved in secretion. Since MDP has been thought to have a role in glutathione (GSH) metabolism, we also measured GSH concentration in zymogen granules and found high levels of GSH. Based on our data we propose a working model for the function of MDP. According to this model, MDP might play a pivotal role in maintaining the oxidizing conditions in the ER, which are required for the correct folding of secretory proteins.

Effect of cholecystokinin blockade on the recovery of alterations induced by acute pancreatitis in glycoconjugates of rat zymogen granules

Lectin-binding studies have been performed on rat zymogen granules to investigate alterations in the carbohydrate membrane composition that occur in acute pancreatitis induced by caerulein. The influence of treatment with hydrocortisone for seven days before inducing pancreatitis was also studied. Lectin labeling on zymogen granules was also analyzed seven days after inducing pancreatitis in rats that had previously received a hydrocortisone treatment. During this period L 364,718 (0.1 mg/kg)--specific cholecystokinin (CCK) receptor antagonist--was administered daily to some of the rats, and no treatment was applied to others. Using fluorescein-labelled T. purpureus (TP)lectin, a significant decrease in the amount of L-fucose in the granule membrane was observed in rats with caerulein-induced pancreatitis. This effect was directly caused by the pancreatitis and was not influenced by previous hydrocortisone treatment. Seven days later, the density of TP receptors in the granule membrane was similar to the controls both in L-364,718-treated and untreated rats. Therefore, we suggest that endogenous CCK is not an essential factor in the recovery of L-fucose containing glycoconjugates the granule membrane after pancreatitis. Acute pancreatitis did not alter the expression of wheat germ agglutinin (WGA) receptors in the zymogen granule membrane. WGA specifically binds N-acetyl glucosamine and sialic acids. L 364,718 administered for seven days after inducing pancreatitis significantly reduced WGA binding, untreated rats showed a normal zymogen granule membrane. Therefore, the blockade of CCK-induced alterations in membrane glycoconjugates enriched in N-acetyl glucosamine and sialic acid of newly formed granules after pancreatitis, a finding that could explain the delay in the regression of the disease.

Membrane dipeptidase in the pig exocrine pancreas. Ultrastructural localization and secretion

The GPI-anchored membrane dipeptidase is the major peptidase activity of the secretory granule membrane in the exocrine pancreas. The enzyme is also found in the granule content and in pancreatic secretions. Immunocytochemical localization confirmed its location in the granule membrane and in the acinar cell apical plasma membrane. In the endoplasmic reticulum and Golgi, membrane dipeptidase was strictly membrane-bound. There was no membrane dipeptidase in duct cells. The release of membrane dipeptidase from the membrane starts in the immature granule. To identify the mechanism responsible for its release, secretions were collected from cannulated conscious pig under basal conditions and atropine perfusion. The latter treatment caused complete inhibition of protein secretion but had a negligible effect on membrane dipeptidase activity in the secretions. In secretions, membrane dipeptidase partitioned into the detergent-rich phase on phase separation in Triton X-114, whereas treatment with bacterial phosphatidylinositol-specific phospholipase C caused the peptidase to partition into the aqueous phase, indicating that the secreted enzyme could come from shedding of membrane fragments at the apical surface or via the action of a previously characterized phospholipase A activity.

Acinar lumen pH regulates endocytosis, but not exocytosis, at the apical plasma membrane of pancreatic acinar cells

A two-step exocytosis/endocytosis protocol was used in rat pancreatic acini to study membrane trafficking events at the apical plasma membrane (APM) as a function of extracellular pH. Exocytosis, as measured by cholecystokinin (CCK)-8-induced release of amylase into the incubation medium, was relatively insensitive to changes in extracellular pH from 5.5 to 9.0. In contrast, endocytosis, as measured by temperature-dependent uptake of horseradish peroxidase (HRP), was robust at pH values between 6.5 and 8.3 but abolished at acidic pH values of 5.5 to 6.0. Energy metabolism and cell viability were maintained during pH 6-induced cessation of HRP uptake, and the vesicular block could be reversed upon raising the luminal pH to 7.4. Histochemical and morphometric studies of HRP uptake examined by electron microscopy indicated that extracellular pH regulates endocytosis at the apical plasma membrane. At pH 6.0 in prestimulated cells, HRP uptake at the APM was abolished, and acinar lumen membranes remained markedly dilated with decreased density of microvilli and "arrested" exocytic images. At pH 7.4, HRP was taken up into endolysosomal structures within the Golgi complex, and acinar lumen membranes were contracted. Cleavage of GP2, a glycosyl phosphatidylinositol-anchored protein, was associated with the pH-dependent activation of HRP uptake. These studies demonstrate that acinar lumen pH regulates endocytic but not exocytic activity at the APM and suggest that alkalinization of the acinar lumen by duct cells is required for retrieval of exocytic membranes into the acinar cell via vesicular uptake mechanisms. The role of acid-base interactions within the acinar lumen provides a novel basis for understanding the cellular and luminal defects observed within the exocrine pancreas in cystic fibrosis.

Identification of membrane dipeptidase as a major glycosyl-phosphatidylinositol-anchored protein of the pancreatic zymogen granule membrane, and evidence for its release by phospholipase A

Membrane dipeptidase (EC 3.4.13.19) enzyme activity that is inhibited by cilastatin has been detected in pancreatic zymogen granule membranes of human, porcine and rat origin. Immunoelectrophoretic blot analysis of human and porcine pancreatic zymogen granule membranes with polyclonal antisera raised against the corresponding kidney membrane dipeptidase revealed that the enzyme is a disulphide-linked homodimer of subunit mass 61 kDa in the human and 45 kDa in the pig. Although membrane dipeptidase was, along with glycoprotein-2, one of the only two major components of carbonate high pH-washed membranes, no enzyme activity or immunoreactivity was detected in the zymogen granule contents. Digestion with bacterial phosphatidylinositol-specific phospholipase C (PI-PLC), and subsequent recognition by antibodies specific for the cross-reacting determinant, revealed that membrane dipeptidase in human and porcine pancreatic zymogen granule membranes is glycosyl-phosphatidylinositol-anchored. Membrane dipeptidase was released from the pancreatic zymogen granule membranes by an endogenous hydrolase, and the released form migrated as a disulphide-linked dimer on SDS/PAGE under non-reducing conditions. Under reducing conditions it migrated with the same apparent molecular mass as the membrane-bound form, and was still a substrate for bacterial PI-PLC. Treatment of kidney microvillar membranes with phospholipase A2 resulted in the release of membrane dipeptidase in a form that demonstrated electrophoretic and cilastatin-Sepharose binding properties identical to those of the endogenously released form of the enzyme from zymogen granule membranes. These results indicate that the glycosyl-phosphatidylinositol anchor on the pancreatic membrane dipeptidase is cleaved by an endogenous hydrolase, probably a phospholipase A, and that this cleavage may promote the release of the protein from the membrane.

Identification of a glycosylphosphatidylinositol-anchored glycoprotein with nucleoside phosphatase activity on the membrane of pig pancreatic zymogen granules

The molecular events between the second messenger-mediated triggering of regulated exocytosis and the subsequent fusion of the secretory granules with the apical plasma membrane are unclear. The glycoprotein GP-2, the most abundant of the very few proteins of the pancreatic zymogen granule membrane has been cloned and sequenced in dog and rat, but no (enzymatic) function has so far been ascribed to it. Nucleoside phosphatase activities associated with the pig zymogen granule membrane were recently assumed to be related to GP-2. To identify the protein(s) carrying these activities we have used a novel combination of native and denaturing one- and two-dimensional polyacrylamide gel electrophoresis in the detergents CHAPS, Triton X-100 or SDS. Histochemical examination on the gels and incubation with lectins and phosphatidylinositol phospholipase-C have allowed characterization of the protein with the nucleoside di- and tri-phosphatase activities. SDS-PAGE of the single protein spot with nucleoside phosphatase activity excised from Triton X-100 2-dimensional gels showed the presence of 92 kDa and 67 kDa glycoproteins. The isolated protein had an isoelectric point of 5.2, formed high molecular weight complexes, was shown to be glycosylphosphatidylinositol-anchored and contained complex carbohydrate structures. It hydrolyses di- and tri-phosphate nucleotides in dependence of the glycosylphosphatidylinositol anchor and is sensitive to non-mitochondrial diphosphohydrolase inhibitors. In summary, this paper identifies GP-2 as a nucleoside phosphatase within the zymogen granule membrane, suggesting it may be involved in energy-requiring processes on the cytosolic side of the granules.

Control of gamma-glutamyl transpeptidase expression by glucocorticoids in the rat pancreas. Correlation with granule formation

Glucocorticoids are known to promote the formation of zymogen granules in acinar cells of the exocrine pancreas in vivo as well as in vitro. To gain insight into the mechanism of this regulation, we studied the effects of glucocorticoids on the synthesis of two components of the secretory granule membrane, the glycoprotein 2 (GP-2) and the gamma-glutamyl transpeptidase (GGT). It was demonstrated that following adrenalectomy, degranulation of pancreatic acinar cells is accompanied by a sharp decrease in GGT and GP-2 synthesis as measured by mRNA and protein accumulation. The decline of GGT synthesis was prevented by glucocorticoid replacement therapy, whereas GP-2 synthesis could be maintained with either glucocorticoid or estradiol treatment. These in vivo observations were corroborated and extended in an in vitro study using AR42J pancreatic cells. With this cell line, it was demonstrated that dexamethasone induces the formation of zymogen granules and the accumulation of a specific GGT transcript (mRNA III) by decreasing its degradation rate. At the same time, the GP-2 mRNA level was not modified by the hormonal treatment. These data demonstrate that glucocorticoids exert a positive control on the GGT expression in pancreatic cells at a post-transcriptional level. GGT, an enzyme of the glutathione metabolism, could play a significant role in protein packaging in secretory cells.

Nucleoside phosphatase activities on pig pancreas zymogen granule membranes analyzed by non-denaturing polyacrylamide gel electrophoresis

The membrane of the pancreatic zymogen granule plays an important part in the sequence of storage, transport and exocytosis of digestive enzymes. While much is known on stimulus-secretion coupling, very little is understood about how the storage organelles move in the cytoplasm to the luminal plasma membrane and why and how they fuse with it to release the contents. It is assumed that nucleoside phosphatases are involved in these energy consuming processes. Pancreatic zymogen granule membranes contain one major glycoprotein, GP-2, and a few minor proteins all with unknown functions. In order to identify functions we have purified zymogen granule membranes from pig pancreas, solubilized the proteins under non-denaturing conditions with the detergent CHAPS and characterized the extracted proteins by polyacrylamide gel electrophoresis, histochemistry and lectins. Three major protein bands, often fused in one broad band, revealed enzymatic activity for adenosine-, cytidine-, inositol- and guanidine- di- and triphosphates by the precipitation of liberated phosphate by Pb(NO3)2. This activity was sensitive to known ATP diphosphohydrolase inhibitors. The band with activity arises from a 92 kDa glycoprotein. A different narrow band showed monophosphatase activity for AMP, GMP, IMP and CMP. Some of the activities were inhibited by different lectins, indicating glycosyl groups near the active site. Electron microscopical cytochemistry confirmed a nucleoside phosphatase activity on granule membranes. Our results show for the first time that the nucleoside phosphatase activity of the zymogen granule membranes is carried by a 92 kDa glycoprotein, probably the known self-associating form of GP-2. The hydrolysis of tri- and diphosphate nucleotides could provide the energy required by exocytosis.

Association of some hydrolytic enzymes with the prostasome membrane and their differential responses to detergent and PIPLC treatment

Prostasomes are human prostate derived organelles that were isolated from both prostatic fluid and seminal plasma for the present study. Specific activities were determined for prostasome membrane-associated enzymes, alkaline phosphatase (ALP), 5'-nucleotidase (5'NT), and alkaline phosphodiesterase I (APD). The mode of their membranous anchoring was studied by treatment of prostasomes with phosphoinositol-specific phospholipase C (PIPLC) and different detergents. A substantial amount of ALP (50%) and 5'NT (31%) was released by incubation of prostasomes with 2 U/ml of PIPLC contrary to the small amount of APD (12%) released by the same treatment. After PIPLC treatment, the enzymes were recovered in the aqueous phase after phase repartition in Triton X-114 indicating that PIPLC removed the hydrophobic domain converting the enzymes from membrane-linked to aqueous soluble forms. Octyl glycoside was the most efficient one among different detergents to solubilize the enzymes from the prostasome membrane. Both ALP and 5'NT were resistant to the treatment with Triton X-100 and Triton X-114. These results suggest that ALP, 5'NT, and APD are more or less extensively linked to the prostasome membrane via a glycophosphoinositide anchor.

Epidermal growth factor induces the differential release of GP2 and amylase from AR4-2J cells

Epidermal growth factor (EGF) stimulates secretion of glycoprotein 2 (GP2) in a time-and concentration-dependent manner from the AR4-2J pancreatoma cell line. Cell differentiation induced by dexamethasone treatment for 3 d, however, did not significantly alter either basal or EGF-stimulated GP2 release. Basal and EGF-stimulated GP2 release were similarly unaffected by caerulein, which promotes amylase secretion by a regulated route. A brief exposure to cycloheximide profoundly blocked EGF-evoked GP2 secretion. Furthermore, EGF-stimulated GP2 release was not accompanied by significant alterations in intracellular ionic calcium levels, in contrast to the stimulatory actions of caerulein. We conclude that EGF-stimulated release of GP2 occurs via a novel secretory pathway that is neither regulated nor constitutive as currently defined.

Molecular cloning of chick beta-tectorin, an extracellular matrix molecule of the inner ear

The tectorial membrane is an extracellular matrix lying over the apical surface of the auditory epithelium. Immunofluorescence studies have suggested that some proteins of the avian tectorial membrane, the tectorins, may be unique to the inner ear (Killick, R., C. Malenczak, and G. P. Richardson. 1992. Hearing Res. 64:21-38). The cDNA and deduced amino acid sequences for chick beta-tectorin are presented. The cDNA encodes a protein of 36,902.6 D with a putative signal sequence, four potential N-glycosylation sites, 13 cysteines, and a hydrophobic COOH terminus. Western blots of two-dimensional gels using antibodies to a synthetic peptide confirm the identity of the cDNA. Southern and Northern analysis suggests that beta-tectorin is a single-copy gene only expressed in the inner ear. The predicted COOH terminus is similar to that of glycosylphosphatidylinositol-linked proteins, and antisera raised to this region react with in vitro translation products of the cDNA clone but not with mature beta-tectorin. These data suggest beta-tectorin is synthesized as a glycosylphosphatidyl-inositol-linked precursor, targeted to the apical surface of the sensory epithelium by the lipid moiety, and then further processed. Sequence analysis indicates the predicted protein possesses a zona pellucida domain, a sequence that is common to a limited number of other matrix-forming proteins and may be involved in the formation of filaments. In the cochlear duct, beta-tectorin is expressed in the basilar papilla, in the clear cells and the cuboidal cells, as well as in the striolar region of the lagena macula. The expression of beta-tectorin is associated with hair cells that have an apical cell surface specialization known as the 275-kD hair cell antigen restricted to the basal region of the hair bundle, suggesting that matrices containing beta-tectorin are required to drive this hair cell type.

Identification and characterization of carboxyl ester hydrolase as a phospholipid hydrolyzing enzyme of zymogen granule membranes from rat exocrine pancreas

Salt-washed (0.6 m NaCl) zymogen granule membranes (ZGM) of rat pancreatic acinar cells were utilized to identify and characterize membrane protein(s) responsible for phospholipase and lysophospholipase activities. Five major bands were identified in salt-washed ZGM by Coomassie Brilliant Blue. A 70-kDa protein with enzymatic activity was retained in significant quantities after several washes with 0.6 M NaCl but could be displaced from ZGM by 2 m NaCl or by 100 mg/ml heparin. By contrast, GP2, an integral membrane protein, was not displaced under these conditions. These findings suggest that the enzyme is a peripheral membrane protein of ZGM. Renaturation of ZGM proteins following electrophoresis revealed that the 70-kDa protein possessed phospholipase activity. Identification of the 70-kDa protein as a membrane-associated carboxyl ester hydrolase was based upon: (a) the use of a specific polyclonal antiserum, (b) N-terminal sequence, (c) two-dimensional gel analysis, (d) enzymatic characterization, and (e) co-localization to an area of a non-reducing gel containing significant phospholipase activity. Other ZGM proteins, namely GP2 and GP3, could not be demonstrated to possess phospholipase activity under the experimental conditions employed. Our finding that carboxyl ester hydrolase from ZGM exhibits PLA1 and lysophospholipase activities represents the first identification and characterization of a protein responsible for phospholipase activity in secretory granule membranes.

Mammalian glycosylphosphatidylinositol-anchored proteins and intracellular precursors

Glycosylphosphatidylinositol-anchored proteins can be specifically identified by several methods. PI-PLC digestion analyses, the most widely used technique, can be performed more reliably when conducted with purified protein and phase partitioning to exclude steric effects and when combined with alkaline hydrolysis to control for inositol acylation. Reductive radiomethylation not only can definitively identify a candidate protein as being GPI anchored, but also can provide information on the number of amine components (GlcN, ethanolamine) in the anchor structure. Biosynthetic labeling with anchor precursors is relatively specific when performed with [3H]ethanolamine or [3H]inositol. Incorporation of the precursors additionally can be used to (1) document anchor transfer to primary translation products, (2) identify soluble derivatives of GPI-anchored proteins that have been released from cell surfaces, and (3) localize the site of GPI anchor attachment within a GPI-anchored protein. A pathway for mammalian GP anchor assembly is depicted in Fig. 12. Initially GlcNAc is transferred to PI. The resulting GlcNAc-PI is then deacetylated to yield GlcN-PI. After that step, several points of divergence are identifiable between the mammalian and T. brucei pathways: (1) all mammalian Man-containing intermediates are built on acylated inositol phospholipids; (2) a proximal phosphoethanolamine is found in mammalian GPI anchor intermediates and is added to Man 1 prior to incorporation of Man 2 and Man 3; (3) no Gal branching substituent is added to the mammalian core glycan; and (4) the most polar mammalian GPI contains a third phosphoethanolamine substituent linked to the 6 position of Man 2.

The level of the zymogen granule protein GP2 is elevated in a rat model for acute pancreatitis

BACKGROUND/AIMS

GP2 is the major membrane protein in pancreatic zymogen granules. It is linked to the membrane via a glycosyl-phosphatidylinositol linkage. After cleavage, a significant fraction of GP2 becomes soluble. The present study assessed whether GP2 is a useful serum marker for acute pancreatitis.

METHODS

Using an anti-GP2 monoclonal antibody, an enzyme-linked immunosorbent assay was developed to measure the serum levels of GP2 in rats with cerulein-induced acute pancreatitis.

RESULTS

The anti-GP2 antibody was specific because it did not cross-react with uromodulin, a structurally similar protein to GP2, or to protein extracts from nonpancreatic tissues. Eight hours after the induction of pancreatitis, the serum levels of amylase, lipase, and GP2 peaked. Peak GP2 levels were 4.2 times higher than those of controls. At 24 hours, GP2 was still 70% of the peak level, whereas amylase and lipase were 5.5% and 0.5%, respectively, of their peak levels.

CONCLUSIONS

GP2 may serve as a potentially valuable marker for clinical acute pancreatitis.

Apical plasma membrane proteins are not obligatorily stored in secretory granules in exocrine cells

Exocrine cells are epithelial cells in which secretory granules undergo fusion with the apical plasma membrane upon secretagogue stimulation. Several apical plasma membrane proteins have been found in secretory granules in cells from pancreas and salivary glands raising the possibility that incorporation into secretory granules followed by exocytosis of the granules accounts for their insertion into the apical plasma membrane. To test this hypothesis, we have expressed the influenza hemagglutinin (HA) in pancreatic AR42J cells, which make zymogen-like granules upon incubation with dexamethasone. The influenza virus HA is known to be specifically targeted to the apical plasma membrane of epithelial cells that lack a regulated pathway and is also known to be excluded from secretory granules in virally-infected pituitary AtT20 cells. Localization of the protein by immunofluorescence microscopy revealed that it accumulated at the plasma membrane of the transfected AR42J cells. HA was not observed in the amylase-rich secretory granules. By immunolabeling of ultrathin cryosections of the transfected cells, HA was also found exclusively on the cell surface, with label over secretory granules not exceeding that seen in control, untransfected cells. In addition, in cell fractionation experiments performed on radiolabeled AR42J cell transformants, HA was not detectable in the secretory granule fractions. These results indicate that HA is not efficiently stored in mature secretory granules and is likely to reach the cell surface via constitutive transport pathways.

Two proteins associated with secretory granule membranes identified in chicken regulated secretory cells

Lately, we have identified two polypeptides of 92–94 kDa (GRL1) and 45–60 kDa (GRL2), expressed in cytoplasmic granules of chicken granulocytes and thrombocytes. Here, we report that GRL1 and GRL2 are widely distributed in all exocrine and several endocrine cell types, but not in neurons of the central nervous system, during late stages of embryonic development, as well as in newly hatched and two-month-old chickens. Immunogold studies in ultrathin frozen sections of pancreatic acinar cells show that GRL1 and GRL2 are co-localized at the periphery of zymogen granules, in granules fused with apical acinar membranes and on apical membranes of acini, while the pregranular compartments of the secretory pathway are weakly or not labeled. Semiquantitative morphometric studies indicate that GRL1 and GRL2 are equally distributed in secretory granules. A variety of physical and metabolic studies reveal that GRL2, a highly N-glycosylated polypeptide, is an intrinsic membrane protein, while GRL1 is a peripheral membrane polypeptide released by Na2CO3 treatment of granulocyte membranes. In all hematopoietic, exocrine or endocrine cells examinated, GRL1 shows identical electrophoretic patterns, while GRL2 is identified as a diffuse band, at 40–65 kDa, in hematopoietic and pancreatic cells. Taken together, the morphological and biochemical studies indicate that GRL1 and GRL2 are components of the secretory granule membrane in chicken exocrine, endocrine and hemopoietic cell types.

Acid-base interactions during exocrine pancreatic secretion. Primary role for ductal bicarbonate in acinar lumen function

The role of acid-base interactions during coordinated acinar and duct cell secretion in the exocrine pancreas is described. The sequence of acid-base events may be summarized as follows: (1) Sorting of secretory proteins and membrane components into the regulated secretory pathway of pancreatic acinar cells is triggered by acid- and calcium-induced aggregation and association mechanisms located in the trans-Golgi network. (2) Cholecystokinin-stimulated exocytosis in acinar cells releases the acidic contents of secretory granules into the acinar lumen. (3) Secretin-stimulated bicarbonate secretion from duct and duct-like cells neutralizes the acidic pH of exocytic contents, which leads to dissociation of protein aggregates and solubilization of (pro)enzymes within the acinar lumen. (4) Stimulated fluid secretion transports solubilized enzymes through the ductal system. (5) Further alkalinization of acinar lumen pH accelerates the enzymatic cleavage of the glycosyl phosphatidyl-inositol anchor associated with GP2 and thus releases the GP2/proteoglycan matrix from lumenal membranes, a process that appears to be required for vesicular retrieval of granule membranes from the apical plasma membrane and their reuse in the secretory process. We conclude that the central function of bicarbonate secretion by centroacinar and duct cells in the pancreas is to neutralize and then alkalinize the pH of the acinar lumen, sequential process that are required for (a) solubilization of secreted proteins and (b) cellular retrieval of granule membranes, respectively.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Reconstitution in vitro of the pH-dependent aggregation of pancreatic zymogens en route to the secretory granule: implication of GP-2

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. To elucidate the biogenesis of the secretory granule in the exocrine pancreas, we reconstituted in vitro the conditions of pH and ions believed to exist in the TGN using the end product of this sorting process, the zymogen granule contents. Protein aggregation was dependent on pH (acidic) and on the presence of cations (10 mM Ca2+, 150 mM K+) to reproduce the pattern of proteins found in the granule. The constitutive secretory protein IgG was excluded from these aggregates. Zymogen aggregation correlated with the relative proportion of the major granule membrane protein GP-2 in the assay. These results show that the glycosylphosphatidylinositol-anchored protein GP-2 co-aggregates with zymogens in the acidic environment believed to exist in the pancreatic TGN, and thus suggest that GP-2 would function as a membrane anchor for zymogen aggregates, facilitating their entrapment in budding vesicles directed towards the regulated secretory pathway.

Membrane-bound choline-O-acetyltransferase in rat hippocampal tissue is anchored by glycosyl-phosphatidylinositol

In an earlier study, we presented evidence to suggest that some of the particulate choline-O-acetyltransferase (ChAT) in rat hippocampal tissue might be linked to membranes by a glycosyl-phosphatidylinositol (GPI) anchor. In the present report, we attempted to determine if any of this GPI-anchored ChAT might be intracellular. Internalization of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis into rat hippocampal synaptosomes by the DMSO (dimethyl sulfoxide) freeze/thawing procedure caused an increase in cytosolic and a decrease in membrane-bound ChAT. Incubation of a plasma membrane enriched subcellular fraction at 16 degrees C relative to 4 degrees C led to a conversion of the membrane-bound, amphiphilic ChAT into hydrophilic ChAT. This conversion was blocked by zinc, an inhibitor of GPI-PLC. The cytosolic fraction of ChAT immunoreacted on western blots with an antibody directed against the cross-reacting determinant (CRD) of the GPI anchor. We suggest that some of the membrane-bound ChAT in rat hippocampal tissue is GPI-anchored intracellularly; also, that an endogenous GPI-PLC-like enzyme acts to release it into the cytosol.

Glycolipid linkage of a polyspermy blocking glycosidase to the ascidian egg surface

Ascidian eggs release N-acetylglucosaminidase rapidly into the seawater following fertilization. This glycosidase is detected seconds after fertilization, and histochemical tests suggest the cell surface as the prefertilization storage site (Lambert, C. C. (1989). Development 105, 415-420). Living eggs of Ascidia ceratodes, A. callosa, and A. paratropa all cleave a fluorogenic substrate in seawater. Following cell surface biotinylation and activation of the eggs, enzyme activity binds to streptavidin further substantiating the cell surface localization. The released glycosidase has a molecular weight of 180 kDa by size exclusion chromatography and exhibits bands at 62 and 70 kDa by SDS-PAGE, suggesting a possibly multimeric enzyme. The enzyme is released by a glycophosphatidylinositol-specific phospholipase C and HNO2 deamination, both of which are specific indicators of linkage to the cell surface via phosphatidylinositol. The enzyme from unfertilized eggs is quite hydrophobic in Triton X-114 phase partition experiments but becomes hydrophyllic after release by activation or deamination. All of these observations are consistent with the glycosidase being anchored to the cell surface via a GPI anchor that is cleaved at fertilization to yield the soluble form of the enzyme which helps protect the egg against polyspermy. We discuss the possible role of a cell surface PLC in this release.

Biogenesis of secretory granules

The biogenesis of secretory granules in endocrine, neuroendocrine, and exocrine cells is thought to involve a selective aggregation of the regulated secretory proteins into a dense-cored structure. The dense-core is then enveloped by membrane in the trans-Golgi network and buds, forming an immature secretory granule. The immature secretory granule then undergoes a maturation process which gives rise to the mature secretory granule. The recent data on the processes of aggregation, budding and maturation are summarized here. In addition, the current knowledge about the mature secretory granule is reviewed with emphasis on the biogenesis of the membrane of this organelle.

GP-2/THP gene family encodes self-binding glycosylphosphatidylinositol-anchored proteins in apical secretory compartments of pancreas and kidney

A family of homologous genes is shown to encode GP-2, the major glycosylphosphatidylinositol (GPI)-linked glycoprotein of pancreatic zymogen granule membranes, and Tamm-Horsfall protein (THP), a GPI-linked glycoprotein associated with apical vesicles in kidney thick ascending limb of Henle (TALH) cells. The C-terminal regions of GP-2 (Asp54-Phe530) and THP (Asp175-His644) from rat show 53% identity, 86% similarity, and 26 conserved cysteine residues including one epidermal growth factor motif. The unique N-terminal domain of rat THP (unique-THP, Pro29-Gln174) shows four conserved epidermal growth factor motifs, three in tandem and one in reverse orientation. GP-2 homologues are observed in a wide variety of epithelial cells, several of which contain highly regulated secretory processes. GP-2 released from zymogen granule membranes with phosphatidylinositol phospholipase C reacts with anti-cross-reactive determinant antibody (anti-CRD), confirming the GPI nature of the pancreatic homologue. In contrast, GP-2 and THP, released endogenously from pancreas and kidney, respectively, do not react with anti-cross-reactive determinant antibody, suggesting alternative enzymatic mechanisms for their physiological release. Globular domains of GP-2 and THP, but not albumin, show pH- and ion-dependent self-association in vitro. The GP-2/THP family appears to represent a newly discovered class of GPI-anchored proteins, which may utilize pH- and ion-dependent self-association mechanisms for establishing membrane (micro)domains targeted to intracellular secretory compartments.

In resting conditions, the pancreatic granule membrane protein GP-2 is secreted by cleavage of its glycosylphosphatidylinositol anchor

GP-2 is the major membrane protein of the exocrine pancreatic secretory granule. It is an integral protein which is anchored by a phosphatidylinositolglycan. In addition to being present in the soluble contents of the granule, GP-2 is also actively secreted by the pancreas. Although 93% of the GP-2 in the resting secretions of anaesthetized rats could be pelleted, Triton X-114 phase extraction showed that 70% of this GP-2 had lost its hydrophobic properties. Proteases have been postulated to release GP-2 from the membrane, but phospholipases also have the capacity to release the protein from the membrane by hydrolysis of its peculiar glycosylphosphatidylinositol membrane anchor. These studies show the presence of inositol 1,2-(cyclic)monophosphate on the secreted hydrophilic GP-2, confirming the involvement of an endogenous phospholipase C in the solubilization of GP-2 by the exocrine pancreas. It is therefore concluded that most of the GP-2 secreted by the pancreas of anaesthetized rats under resting conditions is released from the membrane by a phospholipase C which hydrolyses the phosphodiester bond linking GP-2 to its diradylglycerol anchor.