Proteolytic processing of rat liver membrane secretory component. Cleavage activity is localized to bile canalicular membranes

Regulation of mechanosensitive biliary epithelial transport by the epithelial Na(+) channel

Intrahepatic biliary epithelial cells (BECs), also known as cholangiocytes, modulate the volume and composition of bile through the regulation of secretion and absorption. While mechanosensitive Cl(-) efflux has been identified as an important secretory pathway, the counterabsorptive pathways have not been identified. In other epithelial cells, the epithelial Na(+) channel (ENaC) has been identified as an important contributor to fluid absorption; however, its expression and function in BECs have not been previously studied. Our studies revealed the presence of α, β, and γ ENaC subunits in human BECs and α and γ subunits in mouse BECs. In studies of confluent mouse BEC monolayers, the ENaC contributes to the volume of surface fluid at the apical membrane during constitutive conditions. Further, functional studies using whole-cell patch clamp of single BECs demonstrated small constitutive Na(+) currents, which increased significantly in response to fluid-flow or shear. The magnitude of Na(+) currents was proportional to the shear force, displayed inward rectification and a reversal potential of +40 mV (ENa+  = +60 mV), and were abolished with removal of extracellular Na(+) (N-methyl-d-glucamine) or in the presence of amiloride. Transfection with ENaCα small interfering RNA significantly inhibited flow-stimulated Na(+) currents, while overexpression of the α subunit significantly increased currents. ENaC-mediated currents were positively regulated by proteases and negatively regulated by extracellular adenosine triphosphate.

CONCLUSION

These studies represent the initial characterization of mechanosensitive Na(+) currents activated by flow in biliary epithelium; understanding the role of mechanosensitive transport pathways may provide strategies to modulate the volume and composition of bile during cholestatic conditions. (Hepatology 2016;63:538-549).

Polymeric immunoglobulin receptor

The surface of mucosal sites, such as the intestinal tract, are covered by epithelial cells. To protect the intestinal environment from invading pathogens and maintain homeostasis, the human body developed an exquisite acquired immune system, referred to as the mucosal immune system, in which epithelial cells and lymphocytes function cooperatively. The main player in this immune system is the polymeric immunoglobulins (pIgs), in particular dimeric IgA (dIgA). To exert its protective effect, dIgA produced in the lamina propria must be transported to the intestinal lumen across epithelial cells. This process is called transcytosis and is mediated by polymeric immunoglobulin receptor (pIgR), which is exclusively produced by intestinal epithelial cells (IECs). DIgA is captured by pIgR on the basolateral surface of IECs and transcytosed to the opposite side of IECs. The dIgA-pIgR complex is expressed on the apical surface of IECs and proteolytically cleaved to generate secretory IgA (SIgA). This review describes the current understanding and recent progress in this research field.

The polymeric immunoglobulin receptor: bridging innate and adaptive immune responses at mucosal surfaces

Secretory antibodies of the immunoglobulin A (IgA) class form the first line of antigen-specific immune protection against inhaled, ingested, and sexually transmitted pathogens and antigens at mucosal surfaces. Epithelial transcytosis of polymeric IgA (pIgA) is mediated by the polymeric immunoglobulin receptor (pIgR). At the apical surface, the extracellular ligand-binding region of pIgR, known as secretory component (SC), is cleaved and released in free form or as a component of secretory IgA (SIgA). SC has innate anti-microbial properties, and it protects SIgA from proteolytic degradation. Expression of pIgR is regulated by microbial products through Toll-like receptor signaling and by host factors such as cytokines and hormones. Recent studies of the structure of the extracellular ligand-binding domain of pIgR have revealed mechanisms by which it binds pIgA and other ligands. During transcytosis, pIgA has been shown to neutralize pathogens and antigens within intracellular vesicular compartments. The recent identification of disease-associated polymorphisms in human pIgR near the cleavage site may help to unravel the mystery of how pIgR is cleaved to SC. The identification of novel functions for SC and SIgA has expanded our view of the immunobiology of pIgR, a key component of the mucosal immune system that bridges innate and adaptive immune defense.

Preganglionic parasympathectomy decreases salivary SIgA secretion rates from the rat submandibular gland

Immunoglobulin A (IgA) is transported into saliva by salivary cells expressing the polymeric immunoglobulin receptor (pIgR). In rat salivary glands, autonomic nerves stimulate this process. To examine how nerves affect pIgR-mediated IgA secretion, the chorda-lingual nerve was sectioned. One week after preganglionic parasympathectomy, both the stimulated and unstimulated rates of salivary IgA secretion were reduced, despite similar glandular amounts of IgA. Biochemical analysis of cells from parasympathectomised and control glands indicated reduced membrane expression of pIgR. It appears the removal of long-term parasympathetic input has affected the routing of pIgR within salivary cells and reduced the SIgA transport into saliva.

Secretion of IgA by rat parotid and submandibular cells in response to autonomimetic stimulation in vitro

The major antibody in saliva is IgA, which is actively transported by pIgR expressed by parenchymal cells within the salivary glands. The rate of IgA secretion into saliva is regulated by the autonomic nerves supplying the glands in vivo. This study examined the mechanism of increased IgA secretion into saliva with autonomimetic stimulation. In vitro stimulation of IgA secretion from cells prepared by digestion of rat salivary glands found submandibular cell preparations responded to alpha- and beta-adrenergic stimuli whereas the parotid cells responded only to beta-adrenergic stimulation, although cells from both glands responded similarly to cholinergic stimulation. The additional responsiveness of submandibular cells to alpha-adrenergic stimulation probably reflects the presence of granular duct cells (absent in parotid glands) which are known to secrete protein in response to high frequency sympathetic stimulation. The increased secretion of IgA was not dependant upon increased plasma cell activation since isolated salivary gland plasma cells did not respond to agonists. Further evidence for the regulating role of parenchymal cells in IgA secretion into saliva was revealed by analysis of polymeric immunoglobulin receptor (pIgR) levels expressed on cells. Following in vivo nerve stimulation, there was an increased amount of pIgR expressed on the membrane surface. This was functionally demonstrated in vitro by increased uptake of human IgA by acutely prepared rat salivary cells following stimulation by adrenaline, indicating increased mobilisation of pIgR with stimulation. This study confirms that salivary cells increase the delivery of IgA into saliva by a pIgR-mediated mechanism in response to autonomic stimulation.

The Effects of Varying Oxygen Conditions and Immunoglobulin A on Barrier Defense to Bacterial Invasion

Tissue oxygenation is a critical factor in host defense against bacteria. Gut mucosal tissue oxygenation (partial pressure of O2) is normally low putting the gut at risk of invasion by luminal microbes. Secretory immunoglobulin (Ig) A (sIgA) is the principal immune defense at mucosal surfaces. The protective effect of IgA under low oxygen conditions is unknown. We studied the interaction of varying O2 environments and sIgA on protection against bacterial invasion in our in vitro model. Cell monolayers of Madin-Darby canine kidney (MDCK) cells transfected with the cDNA for polymeric immunoglobulin receptor were established in a two-chamber cell culture system. A commensal strain of Escherichia coli (108 colony-forming units) was added to the apical medium and cell cultures were placed in either a 5, 21, or 95 per cent O2 environment at 37° C. Polyclonal sIgA (100 μg/mL) was added to the apical chamber in subsets. Basal medium was sampled at intervals and bacterial translocation quantitated. The cell monolayers of MDCK transfected cells then had 100 μg/mL IgA added to the basal compartment at 4° C for 2 hours followed by various oxygen environments for 90 minutes. Afterwards apical medium was removed at one, 3, and 12 (overnight) hours. The bacterial translocation data showed a significance increase in translocation with hypoxia. Both increased oxygen and IgA abrogated these effects significantly. The transcytosis of IgA was increased during hypoxic conditions. Normal and hyperoxic conditions did not produce any significant difference in IgA transcytosis. We conclude that O2 and sIgA are protective against bacterial invasion at epithelial surfaces. Effects to either boost O2 delivery to the gut or enhance mucosal IgA production and delivery may be protective in the critically ill surgical patient.

Absence of direct delivery for single transmembrane apical proteins or their "Secretory" forms in polarized hepatic cells

The absence of a direct route to the apical plasma membrane (PM) for single transmembrane domain (TMD) proteins in polarized hepatic cells has been inferred but never directly demonstrated. The genes encoding three pairs of apical PM proteins, whose extracellular domains are targeted exclusively to the apical milieu in Madin-Darby canine kidney cells, were packaged into recombinant adenovirus and delivered to WIF-B cells in vitro and liver hepatocytes in vivo. By immunofluorescence and pulse-chase metabolic labeling, we found that the soluble constructs were overwhelmingly secreted into the basolateral milieu, which in vivo is the blood and in vitro is the culture medium. The full-length proteins were first delivered to the basolateral surface but then concentrated in the apical PM. Our results imply that hepatic cells lack trans-Golgi network (TGN)-based machinery for directly sorting single transmembrane domain apical proteins and raise interesting questions about current models of PM protein sorting in polarized and nonpolarized cells.

Carbohydrate moieties in human secretory component

Human secretory component has seven putative sites for N-linked glycosylation. From tryptic and Glu-C digests we have isolated peptides encompassing asparagines 65, 72, 117, 168, 403, 451 and 481. Analysis by on line HPLC-electrospray mass spectrometry indicated that these residues were fully glycosylated and that the major carbohydrate moieties were far less diversified in composition than expected. Fast atom bombardment mass spectrometry performed on oligosaccharides released by peptide-N-glycosidase F treatment of fractionated and unfractionated SC digests showed the following glycan compositions: Fuc(2)Hex(5)HexNAc(4), Fuc(3)Hex(5)HexNAc(4), NeuAcFucHex(5)HexNAc(4), NeuAcFuc(2)Hex(5)HexNAc(4), NeuAc(2)Hex(5)HexNAc4 and NeuAc(2)FucHex(5)HexNAc(4). Three of these oligosaccharides are the major carbohydrate moieties in human lactoferrin. A possible biological role of the secretory component glycans in the protection of mucosal surfaces is discussed.

Apical plasma membrane proteins and endolyn-78 travel through a subapical compartment in polarized WIF-B hepatocytes

We studied basolateral-to-apical transcytosis of three classes of apical plasma membrane (PM) proteins in polarized hepatic WIF-B cells and then compared it to the endocytic trafficking of basolaterally recycling membrane proteins. We used antibodies to label the basolateral cohort of proteins at the surface of living cells and then followed their trafficking at 37 degreesC by indirect immunofluorescence. The apical PM proteins aminopeptidase N, 5'nucleotidase, and the polymeric IgA receptor were efficiently transcytosed. Delivery to the apical PM was confirmed by microinjection of secondary antibodies into the bile canalicular-like space and by EM studies. Before acquiring their apical steady-state distribution, the trafficked antibodies accumulated in a subapical compartment, which had a unique tubulovesicular appearance by EM. In contrast, antibodies to the receptors for asialoglycoproteins and mannose-6-phosphate or to the lysosomal membrane protein, lgp120, distributed to endosomes or lysosomes, respectively, without accumulating in the subapical area. However, the route taken by the endosomal/lysosomal protein endolyn-78 partially resembled the transcytotic pathway, since anti-endolyn-78 antibodies were found in a subapical compartment before delivery to lysosomes. Our results suggest that in WIF-B cells, transcytotic molecules pass through a subapical compartment that functions as a second sorting site for a subset of basolaterally endocytosed membrane proteins reaching this compartment.

Free-flow electrophoretic analysis of endosome subpopulations of rat hepatocytes

The separation of functional early and late endosomes from other cellular compartments by free-flow electrophoresis (FFE) has been previously demonstrated in nonpolarized cells. Here, using 125I-labeled anti-secretory component antibodies ([125I]SC Ab) and FITC-labeled asialoorosomucoid (FITC-ASOR) as markers of the transcytotic and lysosomal pathway, respectively, we demonstrate the separation of three distinct endosome subpopulations from polarized rat hepatocytes. Internalization of both markers at 16 degrees C resulted in their accumulation in a common endosome compartment, indicating that both the transcytotic and the lysosomal pathways are arrested in the sorting early endosome at temperatures below 20 degrees C. After chase of the markers from early endosomes into the transcytotic or the degradative route at 37 degrees C, transcytotic endosomes carrying [125I]SC Ab migrated with an electrophoretic motility between early and late endosomes while late endosomes labeled with FITC-ASOR were deflected more towards the anode than early endosomes. These data indicate that in rat hepatocytes, the transcytotic and lysosomal pathways utilize a common (i.e. early endosomes) and two distinct endosome subpopulations (i.e. transcytotic endosomes, late endosomes) prior to delivering proteins for biliary secretion or lysosomal degradation, respectively.

Human free secretory component is composed of the first 585 amino acid residues of the polymeric immunoglobulin receptor

The main objective of this work was to unequivocally determine the C-terminal sequence of human milk free secretory component (SC). It was found to end at arginine-585, i.e. 33 amino acids downstream from the major heterogeneous C-terminal residue previously identified for colostrum SC. In contrast, our data showed that the C-terminal end of SC was found to be homogeneous. Conflicting assignments, Asp/Gln, a missing Asn-211, Asp/Asn, Glu/Gln were corrected and found to agree with the cDNA sequence. An Ala/Val substitution at position 562 (domain VI) was identified. Its genetic significance is uncertain at present.

Reduction in the quantity of the polymeric immunoglobulin receptor is sufficient to account for the low concentration of intestinal secretory immunoglobulin A in a weanling mouse model of wasting protein-energy malnutrition

The main objective of this investigation was to determine the influence of protein-energy malnutrition (PEM) in weanling mice on the expression of the hepatic and intestinal polymeric immunoglobulin receptor (pigR), a molecule that transports mucosal immunoglobulin A (IgA) into the intestinal lumen. An experimental system was used that produces systemic wasting (loss of approximately 1.9% of initial body weight per day) and that exhibits fidelity to human PEM in its influence on the concentration of IgA in critical biological fluids as well as in its influence on lymphoid involution and thymus-dependent immunocompetence. Male C57BL/6J mice were allocated to a zero-time control group (19 d of age) or to groups fed for 14 d as follows: free access to a complete purified diet (19% crude protein, 17 kJ/g gross energy) or free access to a low protein diet (0.5% crude protein). The concentration and total quantity per organ of the pIgR were assessed in the liver and intestine by Western immunoblotting using an antiserum raised against the secretory component portion of rat pIgR. Malnourished mice exhibited low quantities of hepatic and intestinal pIgR relative to well-nourished controls (0.4% and 36% of control, respectively) and also exhibited a low concentration (soluble-protein basis) of hepatic pIgR (2% of control). The concentration of biliary secretory component also was low in the malnourished mice (4% of the value for well-nourished controls). Finally, Western blotting revealed an eightfold increase in serum concentration of dimeric IgA in the malnourished group relative to well-nourished mice, whereas the levels of the monomeric form and of the higher order polymers of IgA were elevated by factors of three and two, respectively. In this experimental system, decreased expression of the pIgR is sufficient to account for the low concentration of IgA that is maintained in the mucous secretions of the intestine.

The polymeric immunoglobulin receptor is the major calmodulin-binding protein in an endosome fraction from rat liver enriched in recycling receptors

Rat liver endosomes contain one major high-affinity calmodulin-binding protein (CaMBP) that now has been identified as the polymeric immunoglobulin receptor (pIgR). In isolated endosomes pIgR was enriched in the receptor-recycling compartment (RRC); lesser enrichment was found in 'early' endosome (CURL) and much less in 'late' endosome fractions (multivesicular bodies, MVB). The distribution of the major CaMBP, shown by Western blotting or by overlay with I125-calmodulin in the isolated fractions, was consistent with rapid accumulation of I125-immunoglobulin A (IgA) in RRC and CURL after intravenous injection into rats. The receptor was also found in sinusoidal plasma membranes but not in cell fractions containing apical (bile canalicular) or lateral plasma membrane domains of the hepatocyte. The interaction of pIgR with calmodulin was shown by direct binding assays and by affinity chromatography. Thus, calmodulin is the first cytoplasmic protein shown to interact with the pIgR. We postulate that calmodulin regulates pIgA trafficking in rat liver. In addition, the receptor recycling fraction emerges as an endosomal subcompartment involved in pIgA transport via pIgR.

Immunoadsorption of hepatic vesicles carrying newly synthesized dipeptidyl peptidase IV and polymeric IgA receptor

Hepatocytes must transport newly synthesized apical membrane proteins from the basolateral to the apical plasma membrane. Our earlier morphological study showed that the apical proteins share a late (subapical) part of the transcytotic pathway with the well characterized polymeric immunoglobulin A receptor (Barr, V. A., and Hubbard, A. L. (1993) Gastroenterology 105, 554-571). Starting with crude microsomes from the livers of [35S]methionine-labeled rats, we sequentially immunoadsorbed first vesicles containing the endocytic asialoglycoprotein receptor and then (from the depleted supernatant) vesicles containing the polymeric IgA receptor. Biochemical characterization indicated that early basolateral and late endosomes were present in the first population but not in the second. Neither Golgi-, apical plasma membrane (PM)-, nor basolateral PM-derived vesicles were significant contaminants of either population. Both vesicle populations contained 35S-labeled receptor and 35S-labeled-dipeptidyl peptidase IV. Importantly, the elevated relative specific activity of the dipeptidyl peptidase (% of 35S-labeled/% immunoblotted) in the second population indicated that these vesicles must transport newly synthesized dipeptidyl peptidase IV. A distinct kind of vesicle was immunoadsorbed from a "carrier-vesicle fraction"; surprisingly, these vesicles contained little 35S-receptor and virtually no dipeptidyl peptidase IV. These results, together with previous kinetic data from in vivo experiments, are consistent with a computer-generated model predicting that newly synthesized dipeptidyl peptidase IV is delivered to basolateral endosomes, which also contain newly synthesized polymeric immunoglobulin A receptor. The two proteins are then transcytosed together to the subapical region.

Molecular and cellular aspects of the secretory immunoglobulin system

Adaptive immunological protection of mucous membranes is provided mainly by secretory IgA antibodies. Such "first line" defence is accomplished through a unique cooperation between the mucosal B-cell system and the secretory component (SC) expressed basolaterally on glandular epithelial cells. This transmembrane glycoprotein is quantitatively the most important receptor of the immune system because it is responsible for external transport of locally produced polymeric IgA (pIgA), which is the major product of humoral immunity. Transmembrane SC belongs to the Ig supergene family and functions as a general pIg receptor, also mediating the external translocation of pentameric IgM to form secretory IgM. The B cells responsible for local pIg production are initially stimulated in lymphoepithelial structures, particularly the Peyer's patches in the distal small intestine, from which they migrate as memory cells to exocrine tissues all over the body. Mucous membranes are thus furnished with secretory antibodies in an integrated way, ensuring a variety of specificities at every secretory site. There is currently great interest in exploiting this integrated or "common" mucosal immune system for oral vaccination against pathogenic infectious agents. However, much remains to be learned about mechanisms for antigen uptake and processing necessary to elicit mucosal immunity as well as the molecular biology and cytokine regulation of SC-dependent pIg transport. Moreover, evidence is emerging for the existence of subcompartmentalization in the mucosal immune system, particularly a dichotomy in cellular migration between the gut and the upper airway, which may complicate the design of efficient local vaccines.

Newly synthesized hepatocyte plasma membrane proteins are transported in transcytotic vesicles in the bile duct-ligated rat

BACKGROUND

Newly synthesized apical membrane proteins in hepatocytes go first to the basolateral membrane, from which they are retrieved and delivered to the apical domain. The goal of the present study was to identify the vesicular carriers of these molecules.

METHODS

The common bile duct of rats was ligated for 10-72 hours, and then various plasma membrane proteins were localized using immunofluorescence and quantitative immuno-electron microscopy of fixed liver tissue.

RESULTS

By immunofluorescence, we found intracellular punctate staining near the bile canalicular membrane of polymeric immunoglobulin A (IgA) receptor and several apical membrane proteins, but not basolateral proteins. This compartment was membrane bounded and pleiomorphic by immunoelectron microscopy. Colocalization at the electron microscopic level showed that the apical protein, dipeptidyl peptidase IV, was in the same structures as aminopeptidase N, polymeric IgA receptor, or intravenously injected horseradish peroxidase. This intracellular immunolabeling decreased after cycloheximide treatment (t1/2 = 2-2.5 hours) or reversal of the ligation for 1 hour. In the latter case, bile canalicular labeling increased. Furthermore, polymeric IgA receptor was delivered to the bile canaliculi.

CONCLUSIONS

Bile duct ligation leads to an intracellular accumulation of vesicles carrying polymeric IgA receptor, several apical membrane proteins, and a fluid phase marker. These vesicles continue to fuse with the apical membrane, even during ligation.

Ontogeny of the receptor for polymeric immunoglobulins in rat hepatocytes

Based on in vitro experiments measuring daily secretion rates in the culture media of rat hepatocytes and in vivo experiments using pulse labeling of intracellular precursors, the present study examines the ontogenic expression of the polymeric immunoglobulin receptor and secretory component by hepatocytes during growth. Our data indicate that hepatocytes from infant and suckling rats (day 5, 15) cultured in serum-free and hormone-free conditions only secreted trace amounts of secretory component. Beginning on day 20, basal secretion rate showed a marked upsurge with a 10-fold increase by day 35. The addition of dexamethasone (10(-7) mol/L) to the culture media enhanced by 2.5-fold the basal secretion of secretory component by hepatocytes from 20-, 25-, and 35-day-old rats, while addition of insulin to the media had no effect. The response to dexamethasone was dose-dependent (10(-5), 10(-6), 10(-7) mol/L) and specific. In vivo pulse labeling of receptor precursors in hepatocytes from 40-day-old rats allowed the identification of three intracellular forms: a 105-kilodalton peptide and a 116-120-kilodalton mature doublet. In 13-day-old rats, three immature precursors were detected: a 105-kilodalton peptide and a high molecular weight doublet of 185-190 kilodaltons. Sucklings (13 days) treated with corticosterone showed a pattern of precursors similar to controls. These findings support the following conclusions: (a) hepatocytes from infant and suckling rats synthesize and process immature receptor precursors whose expression is unaffected by corticosterone treatment, and (b) active secretion of secretory component is initiated at weaning independently from humoral and hormonal factors while the magnitude of its production by the liver is under the control of glucocorticoids.

Cell polarity regulates the release of secretory component, the epithelial receptor for polymeric immunoglobulins, from the surface of HT-29 colon carcinoma cells

The HT-29 human colon carcinoma cell line differentiates in glucose-free medium to an enterocytic phenotype. We previously isolated a series of HT-29 subclones selected for high levels of expression of secretory component (SC), the epithelial receptor for polymeric immunoglobulins. To develop a model system for studying effects of cell polarity on SC expression and release from the cell surface, the HT-29.74 subclone was induced to differentiate in glucose-free medium. Expression of SC was induced by glucose deprivation in both the parental HT-29 cell line and, to an even greater extent, in the HT-29.74 subclone. Prolonged glucose deprivation of HT-29.74 cells resulted in morphological changes consistent with enterocytic differentiation. Metabolic radiolabeling of SC in differentiated HT-29.74 cells indicated that proteolytic cleavage of membrane-bound to free SC occurred both on the cell surface and intracellularly, possibly in a vacuolar apical compartment or intrapeithelial lumen. To study effects of cell polarity on SC release, differentiated HT-29.74 cells were depolarized by culturing in low calcium medium. Within 2 hours after transfer of the cells into low calcium medium, a burst of SC release was observed concomitant with cell depolarization. Subsequently, release of SC declined significantly and remained low as long as cells were maintained in a depolarized state. The extent of cell depolarization could be controlled by varying the extracellular calcium concentration or by substituting the divalent cation Sr++, which partially prevents depolarization, for Ca++. In either case, the magnitude of the initial burst and subsequent decline in release of SC was proportional to the extent of cell depolarization. We conclude that cell polarity plays an important role in controlling the release of SC in intestinal epithelial cells, most likely by regulating the distribution of membrane-bound SC and SC protease, which are on the basolateral and apical cell surfaces, respectively, in differentiated cells.