Transduction of basolateral-to-apical signals across epithelial cells: ligand-stimulated transcytosis of the polymeric immunoglobulin receptor requires two signals

Transepithelial Secretion of Mucosal IgM Mediated by Polymeric Immunoglobulin Receptor of Flounder (Paralichthys olivaceus): In-Vivo and In-Vitro Evidence

Secretory immunoglobulin (SIg) is crucial for mucosal surface defenses, but the transepithelial secretion of SIg mediated by polymeric immunoglobulin receptor (pIgR) is not clarified in fish. We previously found that flounder (Paralichthys olivaceus) pIgR (fpIgR) and secretory IgM (SIgM) increased in gut mucus post-vaccination. Here, the fpIgR-positive signal was mainly observed in the intestinal epithelium, whereas the IgM-positive signal was mainly distributed in the lamina propria, before immunization. IgM signals increased in the lamina propria and then in the epithelium after immunization with inactivated Vibrio anguillarum, and co-localization between IgM and fpIgR in the epithelium was determined, while the presence of EdU⁺IgM⁺ cells in the lamina propria identified the proliferative B cells, revealing that the secretion and transepithelial transport of SIgM locally occurred in the gut of flounder. Subsequently, we established an in-vitro model of transfected MDCK cells that stably expressed the fpIgR. After a recombinant eukaryotic expression plasmid (pCIneoEGFP-fpIgR) was constructed and transfected into MDCK cells, stable expression of the fpIgR in transfected MDCK-fpIgR cells was confirmed, and the tightness and integrity of the polarized cell monolayers grown on Transwells were evaluated. Afterward, the serum IgM of flounder was purified as a binding ligand and placed in the lower compartment of Transwells. An ~800-kDa protein band in the upper compartment was shown to be IgM- and fpIgR-positive, and IgM-positive fluorescence was seen in MDCK-fpIgR cells but not in MDCK-mock cells. Hence, the fpIgR helped polymeric IgM to pass across MDCK-fpIgR cells via transcytosis in a basolateral-to-apical fashion. These new findings provide a better understanding of the pathways shaping mucosal IgM responses and the local mucosal immune mechanisms in teleosts.

A transcytotic transport mechanism across the tympanic membrane

Drug treatments for middle ear diseases are currently delivered systemically, or locally after opening the impermeable tympanic membrane (TM). We previously used bacteriophage display to discover novel peptides that are actively transported across the intact TM, with a variety of transport rates. Peptide structures were analyzed for evidence regarding the mechanism for this unexpected transport, which was then tested by the application of chemical inhibitors. Primary sequences indicated that trans-TM peptides share one of two amino acid motifs. Secondary structures revealed that linear configurations associate with higher transport rates than coiled structures. Tertiary analysis indicated that the shared sequence motifs are prominently displayed at the free ends of rapidly transported peptide phage. The shared motifs were evaluated for similarity to known motifs. The highest probability matches were for protein motifs involved in transmembrane transport and exosomes. Overall, structural findings suggest that the shared motifs represent binding sequences. They also implicate transcytosis, a polarized cell transport mechanism consisting of endocytosis, transcellular transport, and exocytosis. Inhibitor studies indicated that macropinocytosis, retrograde transport through Golgi and exocytosis participate in transport across the TM, consistent with transcytosis. This process can be harnessed to noninvasively deliver therapeutics to the middle ear.

Comparison of polymeric immunoglobulin receptor between fish and mammals

Polymeric immunoglobulin receptor (pIgR) functions in transporting polymeric immunoglobulin across epithelial cells into external secretion in animals. During animal evolution, fish was situated at a transition point on the phylogenetic spectrum between species possessing only innate immunity (i.e., invertebrates) and species depending heavily on adaptive immunity (i.e., mammals). Previous studies reported that fish and mammals significantly differ in pIgR. This review summarized the differences in pIgR structure, function, and transcriptional regulation between fish and mammals. A model of the transcriptional regulation of the pIgR gene was suggested. In this model, microbes could activate Toll-like receptor, trigger the cascade reactions in the signaling pathway, and then activate transcription factors that regulate pIgR expression through combining with the pIgR promoter. This review provides some suggestions for further studies on the function and regulatory mechanism of pIgR in fish and other animals.

Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities

Vesicle-mediated transcellular transport or simply "transcytosis" is a cellular process used to shuttle macromolecules such as lipoproteins, antibodies, and albumin from one surface of a polarized cell to the other. This mechanism is in contrast to the transit of small molecules such as anions, cations and amino acids that occur via uptake, diffusion through the cytosol and release and is also distinct from paracellular leak between cells. Importantly, transcytosis has evolved as a process to selectively move macromolecules between 2 neighboring yet unique microenvironments within a multicellular organism. Examples include the movement of lipoproteins out of the circulatory system and into tissues and the delivery of immunoglobulins to mucosal surfaces. Regardless of whether the transport is conducted by endothelial or epithelial cells, the process often involves receptor-mediated uptake of a ligand into an endocytic vesicle, regulated transit of the carrier through the cytoplasm and release of the cargo via an exocytic event. While transcytosis has been examined in detail in epithelial cells, for both historical and technical reasons, the process is less understood in endothelial cells. Here, we spotlight aspects of epithelial transcytosis including recent findings and review the comparative dearth of knowledge regarding the process in endothelial cells highlighting the opportunity for further study.

Characterization of IgM-binding protein: A pIgR-like molecule expressed by intestinal epithelial cells in the common carp (Cyprinus carpio L.)

The adaptive mucosal immune system seems to be an important defence mechanism for fish, but the binding of immunoglobulin M (IgM) in mucosal organs has yet to be clarified in fish. The present study was designed to search for the protein that binds IgM in the intestinal epithelium and determine its distribution in mucosa-associated lymphoid tissues of the common carp (Cyprinus carpio L.). The serum-derived carp IgM fraction was isolated by Sephadex G-200 and assessed for purity by SDS-PAGE under reducing conditions. Serum IgM was subsequently used in affinity chromatography of IgM-sepharose for isolation of a specific binding protein from the intestinal epithelium. The resultant adsorbed protein (IgM-binding protein) demonstrated a single band using SDS-PAGE, with a relative molecular mass of 43.5 kDa. These results demonstrate for the first time that IgM-sepharose can be used as affinity chromatography to purify membrane proteins that bind IgM in fish. Using immunohistochemistry, we found that the distribution of IgM-binding protein in intestinal tissues was abundant, while that of splenic leukocytes were undetectable. Our study indicates that IgM-binding protein might be involved in transportation of IgM in intestine tissues, which is distinct from the IgM receptor on splenocytes.

Establishment of a functional secretory IgA transcytosis model system in vitro for functional food screening

A characteristic mucosal immune response involves the production of antigen-specific secretory immunoglobulin A (SIgA) antibodies. In order to study transcytosis by mimicking the SIgA secretion and to screen for SIgA secretion-promoting substances, we developed a model system of a transfected Madin-Darby canine kidney (MDCK) cell line that expresses the human polymeric immunoglobulin receptor (pIgR). We thus isolated the human dIgA (dimeric IgA)/pIgA (polymeric IgA) complex as the binding ligands. In the present study, a recombinant vector encoding the human pIgR gene was constructed and infected into MDCK cells. Following reverse transcription polymerase chain reaction (RT-PCR), immunoblotting, and immunofluorescence staining, we confirmed that pIgR was expressed in the transfectant MDCK-pIgR cells and was located at the basolateral side of the cell surface. We also confirmed the coexistence of the dIgA/pIgA complex in the IgA myeloma serum. The covalent dIgA/pIgA complex was then isolated from the serum of an IgA multiple myeloma patient using an ÄKTA purifier operation system with a HiPrep 16/60 Sephacryl S-300 HR column, in order to utilize the complex as transcytosis ligands for human pIgR. Finally, we confirmed the uptake of the isolated human dIgA/pIgA complex into MDCK-pIgR cells. We demonstrated that the human dIgA/pIgA complex was transcytosed into the apical side of the monolayer cells. Therefore, our MDCK-human pIgR cell transcytosis model is an operational system and can be used for screening functional food components that promote dIgA/pIgR transcytosis as well as SIgA secretion.

Enhancement of polymeric immunoglobulin receptor transcytosis by biparatopic VHH

The polymeric immunoglobulin receptor (pIgR) ensures the transport of dimeric immunoglobulin A (dIgA) and pentameric immunoglobulin M (pIgM) across epithelia to the mucosal layer of for example the intestines and the lungs via transcytosis. Per day the human pIgR mediates the excretion of 2 to 5 grams of dIgA into the mucosa of luminal organs. This system could prove useful for therapies aiming at excretion of compounds into the mucosa. Here we investigated the use of the variable domain of camelid derived heavy chain only antibodies, also known as VHHs or Nanobodies®, targeting the human pIgR, as a transport system across epithelial cells. We show that VHHs directed against the human pIgR are able to bind the receptor with high affinity (∼1 nM) and that they compete with the natural ligand, dIgA. In a transcytosis assay both native and phage-bound VHH were only able to get across polarized MDCK cells that express the human pIgR gene in a basolateral to apical fashion. Indicating that the VHHs are able to translocate across epithelia and to take along large particles of cargo. Furthermore, by making multivalent VHHs we were able to enhance the transport of the compounds both in a MDCK-hpIgR and Caco-2 cell system, probably by inducing receptor clustering. These results show that VHHs can be used as a carrier system to exploit the human pIgR transcytotic system and that multivalent compounds are able to significantly enhance the transport across epithelial monolayers.

The epithelia-specific membrane trafficking factor AP-1B controls gut immune homeostasis in mice

BACKGROUND & AIMS

Epithelial cells that cover the intestinal mucosal surface maintain immune homeostasis and tolerance in the gastrointestinal tract. However, little is known about the molecular mechanisms that regulate epithelial immune functions. Epithelial cells are distinct in that they are highly polarized; this polarity is, at least in part, established by the epithelium-specific polarized sorting factor adaptor protein (AP)-1B. We investigated the role of AP-1B-mediated protein sorting in the maintenance of gastrointestinal immune homeostasis.

METHODS

The role of AP-1B in intestinal immunity was examined in AP-1B-deficient mice (Ap1m2(-/-)) by monitoring their phenotypes, intestinal morphology, and epithelial barrier functions. AP-1B-mediated protein sorting was examined in polarized epithelial cells from AP-1B knockdown and Ap1m2(-/-) mice.

RESULTS

Ap1m2(-/-) mice developed spontaneous chronic colitis, characterized by accumulation of interleukin-17A-producing, T-helper 17 cells. Deficiency of AP-1B caused epithelial immune dysfunction, such as reduced expression of antimicrobial proteins and impaired secretion of immunoglobulin A. These defects promoted intestinal dysbiosis and increased bacterial translocation within the mucosa. Importantly, AP-1B deficiency led to mistargeting of a subset of basolateral cytokine receptors to the apical plasma membrane in a polarized epithelial cell line and in colonic epithelial cells from mice. AP1M2 expression was reduced significantly in colonic epithelium samples from patients with Crohn's disease.

CONCLUSIONS

AP-1B is required for proper localization of a subset of cytokine receptors in polarized epithelial cells, which allows them to respond to cytokine signals from underlying lamina propria cells. The AP-1B-mediated protein sorting machinery is required for maintenance of immune homeostasis and prevention of excessive inflammation.

Polymeric immunoglobulin receptor-mediated invasion of Streptococcus pneumoniae into host cells requires a coordinate signaling of SRC family of protein-tyrosine kinases, ERK, and c-Jun N-terminal kinase

Streptococcus pneumoniae are commensals of the human nasopharynx with the capacity to invade mucosal respiratory cells. PspC, a pneumococcal surface protein, interacts with the human polymeric immunoglobulin receptor (pIgR) to promote bacterial adherence to and invasion into epithelial cells. Internalization of pneumococci requires the coordinated action of actin cytoskeleton rearrangements and the retrograde machinery of pIgR. Here, we demonstrate the involvement of Src protein-tyrosine kinases (PTKs), focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinases (MAPK) in pneumococcal invasion via pIgR. Pharmacological inhibitors of PTKs and MAPKs and genetic interference with Src PTK and FAK functions caused a significant reduction of pIgR-mediated pneumococcal invasion but did not influence bacterial adhesion to host cells. Furthermore, pneumococcal ingestion by host cells induces activation of ERK1/2 and JNK. In agreement with activated JNK, its target molecule and DNA-binding protein c-Jun was phosphorylated. We also show that functionally active Src PTK is essential for activation of ERK1/2 upon pneumococcal infections. In conclusion, these data illustrate the importance of a coordinated signaling between Src PTKs, ERK1/2, and JNK during PspC-pIgR-mediated uptake of pneumococci by host epithelial cells.

Cdc42 and the phosphatidylinositol 3-kinase-Akt pathway are essential for PspC-mediated internalization of pneumococci by respiratory epithelial cells

The pneumococcal surface protein C (PspC) is a major adhesin of Streptococcus pneumoniae, the cause of lobar pneumonia and invasive diseases. PspC interacts in a human-specific manner with the ectodomain of the human polymeric immunoglobulin receptor (pIgR) produced by respiratory epithelial cells. By adopting the retrograde machinery of human pIgR, this protein-protein interaction promotes colonization and transcytosis across the epithelial layer. Here, we explored the role of Rho family guanosine triphosphatases (GTPases), phosphatidylinositol 3-kinase (PI3K) and protein kinase B (Akt) for ingestion of pneumococci via the human pIgR. Inhibition experiments suggested that the host-cell actin microfilaments and microtubules are essential for this pneumococcal uptake mechanism. By using specific GTPase-modifying toxins, inhibitors, and GTPase expression constructs we demonstrate that Cdc42, but not Rac1 and RhoA are involved in PspC-mediated invasion of pneumococci into host cells. Accordingly, Cdc42 is time-dependently activated during ingestion of pneumococci. In addition, PI3K and Akt are essential for ingestion of pneumococci by respiratory epithelial cells via the PspC-pIgR interaction. The subunit p85alpha of PI3K and Akt was activated during the infection process. Moreover, Akt activation upon pneumococcal invasion depends on PI3K. In conclusion, our results illustrate for the first time key signaling molecules of host cells that are required for PspC-pIgR-mediated invasion of pneumococci into epithelial cells. This unique and specific bacterial entry process is dependent on the cooperation and activation of Rho family GTPase Cdc42, PI3K, and Akt.

Identification of a cytoplasmic signal for apical transcytosis

Polarized epithelial cells contain apical and basolateral surfaces with distinct protein compositions. To establish and maintain this asymmetry, newly made plasma membrane proteins are sorted in the trans Golgi network for delivery to apical or basolateral surfaces. Signals for basolateral sorting are generally located in the cytoplasmic domain of the protein, whereas signals for apical sorting can be in any part of the protein and can depend on N-linked glycosylation of the protein. Signals for constitutive transcytosis to the apical surface have not been reported. In this study, we used the polymeric immunoglobulin receptor (pIgR), which is biosynthetically delivered to the basolateral surface. There the pIgR can bind a ligand and, with or without bound ligand, the pIgR can then be transcytosed to the apical surface. We found that the glycosylation of the pIgR did not affect the biosynthetic transport of the pIgR. However, glycosylation had an effect on pIgR apical transcytosis. Importantly, analysis of the cytoplasmic tail of the pIgR suggested that a short peptide segment was sufficient to transcytose the pIgR or a neutral reporter from the basolateral to the apical surface. This apical transcytosis sorting signal was not involved in polarized biosynthetic traffic of the pIgR.

Receptor complexes cotransported via polarized endocytic pathways form clusters with distinct organizations

Previously, FRET confocal microscopy has shown that polymeric IgA-receptor (pIgA-R) is distributed in a clustered manner in apical endosomes. To test whether different membrane-bound components form clusters during membrane trafficking, live-cell quantitative FRET was used to characterize the organization of pIgA-R and transferrin receptor (TFR) in endocytic membranes of polarized MDCK cells upon internalization of donor- and acceptor-labeled ligands. We show that pIgA-R and TFR complexes form increasingly organized clusters during cotransport from basolateral to perinuclear endosomes. The organization of these receptor clusters in basolateral versus perinuclear/apical endosomes is significantly different; the former showing a mixed random/clustered distribution while the latter highly organized clusters. Our results indicate that although both perinuclear and apical endosomes comprise pIgA-R and TFR clusters, their E% levels are significantly different suggesting that these receptors are packed into clusters in a distinct manner. The quantitative FRET-based assay presented here suggests that different receptor complexes form clusters, with diverse levels of organization, while being cotransported via the polarized endocytic pathways.

Cholesterol-sensitive modulation of transcytosis

Cholesterol-rich membrane domains (e.g., lipid rafts) are thought to act as molecular sorting machines, capable of coordinating the organization of signal transduction pathways within limited regions of the plasma membrane and organelles. The significance of these domains in polarized postendocytic sorting is currently not understood. We show that dimeric IgA stimulates the incorporation of its receptor into cholesterol-sensitive detergent-resistant membranes confined to the basolateral surface/basolateral endosomes. A fraction of human transferrin receptor was also found in basolateral detergent-resistant membranes. Disrupting these membrane domains by cholesterol depletion (using methyl-beta-cyclodextrin) before ligand-receptor internalization caused depolarization of traffic from endosomes, suggesting that cholesterol in basolateral lipid rafts plays a role in polarized sorting after endocytosis. In contrast, cholesterol depletion performed after ligand internalization stimulated cargo transcytosis. It also stimulated caveolin-1 phosphorylation on tyrosine 14 and the appearance of the activated protein in dimeric IgA-containing apical organelles. We propose that cholesterol depletion stimulates the coupling of transcytotic and caveolin-1 signaling pathways, consequently prompting the membranes to shuttle from endosomes to the plasma membrane. This process may represent a unique compensatory mechanism required to maintain cholesterol balance on the cell surface of polarized epithelia.

Dynamics of MAL2 During Glycosylphosphatidylinositol-Anchored Protein Transcytotic Transport to the Apical Surface of Hepatoma HepG2 Cells

Delivery of glycosylphosphatidylinositol (GPI)-anchored proteins to the apical surface takes place by transcytosis in hepatocytes and also probably in epithelial Madin-Darby canine cells. The integral protein MAL2 was demonstrated to be essential for basolateral-to-apical transcytosis in hepatoma HepG2 cells. Reduction of endogenous MAL2 levels impedes cargo delivery to the apical membrane, but, paradoxically, cargo does not accumulate in the subapical compartment where MAL2 predominantly resides but in distant endosome elements. To understand how transcytosis can be apparently mediated at a distance, we have analyzed the dynamics of machinery and cargo by live-cell imaging of MAL2 and transcytosing CD59, a GPI-anchored protein, in HepG2 cells. MAL2 was revealed as being a highly dynamic protein. Soon after basolateral endocytosis of CD59, a fraction of MAL2 redistributed into peripheral vesicular clusters that concentrated CD59 and that were accessible to transferrin (Tf) receptor, a basolateral recycling protein. Following Tf receptor segregation, the clusters fused in a MAL2(+)globular structure and moved toward the apical surface for CD59 delivery. All these processes were impaired in cells with reduced MAL2 content. Other GPI-anchored proteins examined behave similarly. As MAL2 is expressed by many types of epithelia, the sorting events described herein are probably of quite general utility.

Antibodies to the polymeric immunoglobulin receptor with different binding and trafficking patterns

The polymeric immunoglobulin receptor (pIgR) has been proposed as a therapeutic target, but its potential depends on the efficiency of uptake and trafficking of the receptor ligand. Mouse monoclonal antibodies (Mabs) directed against pIgR, selected for strong binding to secretory component (SC) and secretory IgA (sIgA), were tested in a transcytosis assay in 16HBEo--cells (human bronchial epithelial cell line) transfected with human pIgR. Intracellular trafficking was followed by confocal microscopy. Mabs fell into two classes. For two Mabs, transcytosis from basolateral to apical surface is rapid, unidirectional, and little Mab is retained in the cell. For three Mabs, basolateral to apical transcytosis occurs to a significantly lesser extent, reverse transcytosis is permitted, and some of the Mab is retained in the perinuclear region even after 24 h. When tested for their ability to recognize and immunoprecipitate pIgR with systematic truncations and deletions of the five immunoglobulin (Ig)-like domains, all Mabs bound to the fifth Ig-like domain, but three of them also bound to the C-terminal region of pIgR near the plasma membrane. Different binding sites probably account for the different trafficking of these Mabs and may predict differential therapeutic utility.

A functional polymeric immunoglobulin receptor in chicken (Gallus gallus) indicates ancient role of secretory IgA in mucosal immunity

Animals are continuously threatened by pathogens entering the body through natural openings. Here we show that in chicken ( Gallus gallus ), secretory IgA (sIgA) protects the epithelia lining these natural cavities. A gene encoding a chicken polymeric Ig receptor ( GG-pIgR ), a key component of sIgA, was identified, and shown to be expressed in the liver, intestine and bursa of Fabricius. All motifs involved in pIgR function are present, with a highly conserved Ig-binding motif in the first Ig-like domain. Physical association of GG-pIgR with pIgA in bile and intestine demonstrates that this protein is a functional receptor. Thus, as shown for mammals, this receptor interacts with J-chain-containing polymeric IgA (pIgA) at the basolateral epithelial cell surface resulting in transcytosis and subsequent cleavage of the pIgR, releasing sIgA in the mucosal lumen. Interestingly, the extracellular portion of GG-pIgR protein comprises only four Ig-like domains, in contrast with the five domain structure found in mammalian pIgR genes. The second Ig-like domain of mammalian pIgR does not have an orthologous domain in the chicken gene. The presence of pIgR in chicken suggests that this gene has evolved before the divergence of birds and reptiles, indicating that secretory Igs may have a prominent role in first line defence in various non-mammalian species.

Freshly isolated rat alveolar type I cells, type II cells, and cultured type II cells have distinct molecular phenotypes

We used microarray analysis with Affymetrix rat chips to determine gene expression profiles of freshly isolated rat type I (TI) and TII cells and cultured TII cells. Our goals were 1) to describe molecular phenotypic "fingerprints" of TI and TII cells, 2) to gain insight into possible functional differences between the two cell types through differentially expressed genes, 3) to identify genes that might indicate potential functions of TI cells, since so little is known about this cell type, and 4) to ascertain the similarities and differences in gene expression between cultured TII cells and freshly isolated TI cells. For these experiments, we used preparations of isolated TI and TII cells that contained <2% cross-contamination. With a false discovery rate of 1%, 601 genes demonstrated over twofold different expression between TI and TII cells. Those genes with very high levels of differential expression may be useful as markers of cell phenotype and in generating novel hypotheses about functions of TI and TII cells. We found similar numbers of differentially expressed genes between freshly isolated TI or TII cells and cultured TII cells (698, 637 genes) and freshly isolated TI and TII cells (601 genes). Tests of sameness/difference including cluster dendrograms and log/log identity plots indicated major differences between the phenotypes of freshly isolated TI cell and cultured type II cell populations. The latter results suggest that experiments with TII cells cultured under these conditions should be interpreted with caution with respect to biological relevance to TI or TII cells.

Caspase induction by IgA antimitochondrial antibody: IgA-mediated biliary injury in primary biliary cirrhosis

Anti-mitochondrial antibodies (AMAs) have long been recognized as a serological hallmark of primary biliary cirrhosis (PBC). Although high titers of immunoglobulin (Ig)A AMAs are found in bile, saliva, and urine of patients, a pathogenic role for this antibody has remained elusive. Functional studies of this IgA in general have been impeded by low quantities of antibody and the inability to recover antigen-specific IgA in dimeric form. Using a newly defined synthetic group A. Streptococcus derived peptide, we purified large quantities of dimeric and monomeric IgA from patient sera. The purified IgA was incubated with Madine-Darby canine kidney (MDCK) cells transfected with the human polymeric Ig receptor (pIgR) and the cells studied by flow cytometric analysis for binding of carboxyfluorescein conjugated VAD-fmk peptide to activated caspase enzymes. A total of 87% of PBC patients that were anti-PDC-E2 positive had serum IgA that increased caspase activation in MDCK-pIgR+ cells compared to serum-derived IgA from controls with a maximum reaction 48 hours after addition of IgA. The titer of anti-PDC-E2 IgA among the PBC patients strongly correlated with caspase activation (cc = 0.88). Pre-absorption of the IgA using recombinant 2-oxo-acid dehydrogenase complex significantly diminished this activation. IgG from the same PBC patients did not induce caspase activation. These data suggest that during transcytosis through pIgR-positive cells, exposure to PDC-E2-specific dimeric IgA results in the initiation of caspase activation. In conclusion, we propose that due to an even greater concentration of dimeric IgA in biliary and mucosal secretions, constant transcytosis would render the exposed cells more susceptible to apoptosis resulting in subsequent bile duct damage.

The COOH-terminal tail of the GAT-2 GABA transporter contains a novel motif that plays a role in basolateral targeting

The ability of polarized epithelia to perform vectorial transport depends on the asymmetrical distribution of transmembrane proteins among their plasma membrane domains. The establishment and maintenance of these polar distributions relies on molecular signals embedded in the proteins themselves and the interpretation of these signals by cellular sorting machinery. Using Madin-Darby canine kidney (MDCK) cells as an in vitro model of polarized epithelia, our laboratory has previously shown that the COOH-terminal cytoplasmic 22 amino acids of the GAT-2 isoform of the gamma-amino butyric acid (GABA) transporter are necessary for its basolateral distribution. We demonstrate that the COOH-terminal tail of the transporter can function as an autonomous basolateral distribution signal, independently of the rest of the transporter. We find that the three-amino acid PDZ domain-interacting motif at the COOH-terminus of GAT-2 is not necessary for its basolateral distribution. Instead, the more proximal seven amino acids are necessary both for targeting and for steady-state distribution. Because this sequence resembles no other known basolateral sorting information, we conclude that these seven amino acids contain a novel basolateral targeting and distribution motif.

Ectodomains 3 and 4 of human polymeric Immunoglobulin receptor (hpIgR) mediate invasion of Streptococcus pneumoniae into the epithelium

Streptococcus pneumoniae binds to the ectodomain of the human polymeric Ig receptor (pIgR), also known as secretory component (SC), via a hexapeptide motif in the choline-binding protein SpsA. The SpsA-pIgR interaction mediates adherence and internalization of the human pathogen into epithelial cells. In this study the results of SpsA binding to human, mouse, and chimeric SC strongly supported the human specificity of this unique interaction and suggested that binding sites in the third and fourth Ig-like domain of human SC (D3 and D4, respectively) are involved in SpsA-pIgR complex formation. Binding of SpsA to SC-derived synthetic peptides indicated surface-located potential binding motifs in D3 and D4. Adherence and uptake of pneumococci or SpsA-coated latex beads depended on the SpsA hexapeptide motif as well as SpsA-binding sites in D3 and D4 of human pIgR. The involvement of D3 and D4 in adherence and invasion was demonstrated by the lack of binding of SpsA-coated latex beads to transfected epithelial cells expressing mutated pIgR. Finally, blocking experiments with chimeric human-mouse SC as well as synthetic peptides indicated the participation of D3 and a key role of D4 in pneumococcal invasion.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Novel functions of the polymeric Ig receptor: well beyond transport of immunoglobulins

The polymeric Ig receptor (pIgR) ensures efficient secretion of polymeric IgA (pIgA) at mucosal surfaces. On basal to apical transport across epithelial cells, the pIgR extracellular domain is cleaved, releasing secretory component (SC) in association with pIgA. This finds its raison d'être in the recent observation that SC is directly involved in the protective function of secretory IgA. In addition, free SC exhibits scavenger properties with respect to enteric pathogens. However, although pIgR dedicates its life to mucosal protection, it also seems to permit pathogen entrance through the epithelial barrier. The multiple mechanisms that they are involved in make pIgR and SC instrumental to mucosal immunity.

Role of Rab Proteins in Epithelial Membrane Traffic

Small GTPase rab proteins play an important role in various aspects of membrane traffic, including cargo selection, vesicle budding, vesicle motility, tethering, docking, and fusion. Recent data suggest also that rabs, and their divalent effector proteins, organize organelle subdomains and as such may define functional organelle identity. Most rabs are ubiquitously expressed. However, some rabs are preferentially expressed in epithelial cells where they appear intimately associated with the epithelial-specific transcytotic pathway and/or tight junctions. This review discusses the role of rabs in epithelial membrane transport.

Immunoglobulin transport across polarized epithelial cells

IgA, IgG and IgM are transported across epithelial cells in a receptor-mediated process known as transcytosis. In addition to neutralizing pathogens in the lumen of the gastrointestinal, respiratory and urogenital tracts, these antibody-receptor complexes are now known to mediate intracellular neutralization of pathogens and might also be important in immune activation and tolerance. Recent studies on the intracellular transport pathways of antibody-receptor complexes and antibody-stimulated receptor-mediated transcytosis are providing new insight into the nature and regulation of endocytic pathways.

Up-regulation of polymeric immunoglobulin receptor mRNA in mammary epithelial cells by IFN-gamma

As shown in previous in vivo experiment, the amount of polymeric immunoglobulin receptor (pIgR), which mediates the transcytosis of pIgA across epithelial cells, is regulated by lactogenic hormones (PRL and cortisol) during the development of the mammary gland. In the present in vitro study, it appeared that these hormones were insufficient to induce the strong expression of the gene that we observed in vivo. Several papers have shown that IFN-gamma is a strong stimulator of pIgR gene expression in different models. In contrast, nothing is known of the effects of IFN-gamma on pIgR gene expression in the mammary gland. We report here that IFN-gamma strongly increased pIgR mRNA levels through a direct effect on mammary epithelial cells. We show that IFN-gamma activated not only Stat1 but also Stat5 and that expression of the pIgR and IRF-1 genes was strongly correlated following IFN-gamma stimulation in mammary epithelial cells. In conclusion, these experiments enabled the analysis of different types of regulation of pIgR gene expression in the mammary gland and suggest possible co-operation between circulating hormones and locally produced cytokines, leading to pIgR gene expression in the mammary gland.

Nonreceptor tyrosine kinase c-Yes interacts with occludin during tight junction formation in canine kidney epithelial cells

Occludin is an integral membrane protein that is tyrosine phosphorylated when localized at tight junctions. When Ca(2+) was depleted from the culture medium, occludin tyrosine phosphorylation was diminished from Madin-Darby canine kidney epithelial cells in 2 min. This dephosphorylation was correlated with a significant reduction in transepithelial electrical resistance (TER), indicating a global loss of the tight junction barrier function. Reconstitution of Ca(2+) resulted in a robust tyrosine rephosphorylation of occludin that was temporally associated with an increase in TER. Moreover, we demonstrate in this study that occludin was colocalized with the nonreceptor tyrosine kinase c-Yes at cell junction areas and formed an immunoprecipitable complex with c-Yes in vivo. This complex dissociated when the cells were incubated in medium without Ca(2+) or treated with a c-Yes inhibitor, CGP77675. In the presence of CGP77675 after Ca(2+) repletion, occludin tyrosine phosphorylation was completely abolished and both tight junction formation and the increase of the TER were inhibited. Our study thus provides strong evidence that occludin tyrosine phosphorylation is tightly linked to tight junction formation in epithelial cells, and that the nonreceptor tyrosine kinase c-Yes is involved in the regulation of this process.

Direct interaction between Rab3b and the polymeric immunoglobulin receptor controls ligand-stimulated transcytosis in epithelial cells

We have examined the role of rab3b in epithelial cells. In MDCK cells, rab3b localizes to vesicular structures containing the polymeric immunoglobulin receptor (pIgR) and located subjacent to the apical surface. We found that GTP-bound rab3b directly interacts with the cytoplasmic domain of pIgR. Binding of dIgA to pIgR causes a dissociation of the interaction with rab3b, a process that requires dIgA-mediated signaling, Arg657 in the cytoplasmic domain of pIgR, and possibly GTP hydrolysis by rab3b. Binding of dIgA to pIgR at the basolateral surface stimulates subsequent transcytosis to the apical surface. Overexpression of GTP-locked rab3b inhibits dIgA-stimulated transcytosis. Together, our data demonstrate that a rab protein can bind directly to a specific cargo protein and thereby control its trafficking.

Interaction with mLin-7 alters the targeting of endocytosed transmembrane proteins in mammalian epithelial cells

To investigate the targeting mechanism for proteins bound to the mammalian Lin-7 (mLin-7) PDZ domain, we created receptor protein chimeras composed of the carboxyl-terminal amino acids of LET-23 fused to truncated nerve growth factor receptor/P75. mLin-7 bound to the chimera with a wild-type LET-23 carboxyl-terminal tail (P75t-Let23WT), but not a mutant tail (P75t-Let23MUT). In Madin-Darby canine kidney (MDCK) cells, P75t-Let23WT localized to the basolateral plasma membrane domain, whereas P75t-Let23MUT remained apical. Furthermore, mutant mLin-7 constructs acted as dominant interfering proteins and inhibited the basolateral localization of P75t-Let23WT. The mechanisms for this differential localization were examined further, and, initially, we found that P75t-Let23WT and P75t-Let23MUT were delivered equally to the apical and basolateral plasma membrane domains. Although basolateral retention of P75t-Let23WT, but not P75t-Let23MUT, was observed, the greatest difference in receptor localization was seen in the rapid trafficking of P75t-Let23WT to the basolateral plasma membrane domain after endocytosis, whereas P75t-Let23MUT was degraded in lysosomes, indicating that mLin-7 binding can alter the fate of endocytosed proteins. Altogether, these data support a model for basolateral protein targeting in mammalian epithelial cells dependent on protein-protein interactions with mLin-7, and also suggest a dynamic role for mLin-7 in endosomal sorting.

Effects of mutations in potential phosphorylation sites on transcytosis of FcRn

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across intestinal epithelial cells of suckling rats and mice from the lumenal surface to the serosal surface. In cell culture models FcRn transports IgG bidirectionally, but there are differences in the mechanisms of transport in the two directions. We investigated the effects of mutations in the cytoplasmic domain of FcRn on apical to basolateral and basolateral to apical transport of Fc across rat inner medullary collecting duct (IMCD) cells. Basolateral to apical transport did not depend upon determinants in the cytoplasmic domain. In contrast, an essentially tailless FcRn was markedly impaired in apical to basolateral transport. Using truncation and substitution mutants, we identified serine-313 and serine-319 as phosphorylation sites in the cytoplasmic domain of FcRn expressed in Rat1 fibroblasts. Mutations at Ser-319 did not affect transcytosis across IMCD cells. FcRn-S313A was impaired in apical to basolateral transcytosis to the same extent as tailless FcRn, whereas FcRn-S313D transported at wild-type levels. FcRn-S313A recycled more Fc to the apical medium than the wild-type receptor, suggesting that Ser-313 is required to allow FcRn to be diverted from an apical recycling pathway to a transcytotic pathway.

IgA and the IgA Fc receptor

IgA has traditionally been regarded a non-inflammatory antibody. This might indeed be true for secretory IgA (SIgA), which exerts its function at mucosal surfaces where commensal microorganisms and dietary antigens prevail. Serum IgA, however, potently triggers (pro)-inflammatory activity upon binding to the myeloid IgA receptor, FcalphaRI. Here, new insights in the roles of IgA and FcalphaRI are addressed and a model integrating the various functions of IgA in immunity is discussed.

Caveolin-1 binding to endoplasmic reticulum membranes and entry into the regulated secretory pathway are regulated by serine phosphorylation. Protein sorting at the level of the endoplasmic reticulum

Caveolin-1 serves as the main coat protein of caveolae membranes, as an intracellular cholesterol shuttle, and as a regulator of diverse signaling molecules. Of the 12 residues conserved across all caveolin isoforms from all species examined to date, only Ser(80) and Ser(168) could serve as phosphorylation sites. We show here that mimicking chronic phosphorylation of Ser(80) by mutation to Glu (i.e. Cav-1(S80E)), blocks phosphate incorporation. However, Cav-1(S168E) is phosphorylated to the same extent as wild-type caveolin-1. Cav-1(S80E) targets to the endoplasmic reticulum membrane, remains oligomeric, and maintains normal membrane topology. In contrast, Cav-1(S80A), which cannot be phosphorylated, targets to caveolae membranes. Some exocrine cells secrete caveolin-1 in a regulated manner. Cav-1(S80A) is not secreted by AR42J pancreatic adenocarcinoma cells even in the presence of dexamethasone, an agent that induces the secretory phenotype. Conversely, Cav-1(S80E) is secreted to a greater extent than wild-type caveolin-1 following dexamethasone treatment. We conclude that caveolin-1 phosphorylation on invariant serine residue 80 is required for endoplasmic reticulum retention and entry into the regulated secretory pathway.

Polymeric IgA binding to the human pIgR elicits intracellular signalling, but fails to stimulate pIgR-transcytosis

The intracellular pathway of polymeric immunoglobulin receptor (pIgR) is governed by multiple signals that lead to constitutive transcytosis. In addition, in transfected polarized MDCK cells, polymeric immunoglobulin A (pIgA) binding stimulates rabbit pIgR-transcytosis, owing to phospholipase-C gamma 1 activation and increase of intracellular calcium. Transcytosis of rat pIgR across hepatocytes is similarly accelerated by pIgA injection. In contrast we show here that human Madrin-Darby Canine Kidney (pIgR)-transcytosis, in human Calu-3 and human pIgR-transfected MDCK cells, is not promoted by pIgA, as monitored by a continuous apical release of its secreted ectodomain. However, the incubation of cells expressing human or rabbit pIgR with pIgA induces a comparable IP3 production, and pIgR-transcytosis of either species is accelerated by the protein kinase C (PKC)-activator phorbol myristate acetate. Without pIgA, mimicking phospholipase-C activation by combining low concentrations of phorbol myristate acetate with ionomycin, or high concentrations of ionomycin alone, stimulates the rabbit, but not the human, pIgR transcytosis. These data suggest that the species difference in pIgA-induced pIgR-transcytosis does not stem from the defective production of second messengers, but from a different sensitivity of pIgR to intracellular calcium. Our results outline the danger of extrapolating to humans the abundant data obtained from mucosal vaccination of laboratory animals.

The visual display of regulatory information and networks

Cell regulation and signal transduction are becoming increasingly complex, with reports of new cross-signalling, feedback, and feedforward regulations between pathways and between the multiple isozymes discovered at each step of these pathways. However, this information, which requires pages of text for its description, can be summarized in very simple schemes, although there is no consensus on the drawing of such schemes. This article presents a simple set of rules that allows a lot of information to be inserted in easily understandable displays.

Detergent insoluble microdomains are not involved in transcytosis of polymeric Ig receptor in FRT and MDCK cells

In polarized epithelial cells, sorting of proteins and lipids to the apical or basolateral domain of the plasma membrane can occur via direct or indirect (transcytotic) pathways from the trans Golgi network (TGN). The 'rafts' hypothesis postulates that the key event for direct apical sorting of some transmembrane proteins and the majority of GPI-anchored proteins depends on their association with glycosphingolipid and cholesterol enriched microdomains (rafts). However, the mechanism of indirect sorting to the apical membrane is not clear. The polyimmunoglobulin receptor (pIgR) is one of the best studied proteins that follow the transcytotic pathway. It is normally delivered from the TGN to the basolateral surface of polarized Madin-Darby Canine Kidney (MDCK) cells from where it transports dIgA or dIgM to the apical surface. We have studied the intracellular trafficking of pIgR in Fischer rat thyroid cells (FRT), and have investigated the sorting machinery involved in transcytosis of this receptor in both FRT and MDCK cells. We found that, in contrast with MDCK cells, a significant amount (approximately 30%) of pIgR reaches the apical surface by a direct pathway. Furthermore, in both cell lines it does not associate with Triton X-100 insoluble microdomains, suggesting that at least in these cells 'rafts' are not involved in basolateral to apical transcytosis.

Membrane traffic in polarized epithelial cells

Epithelial cells contain apical and basolateral surfaces with distinct compositions. Sorting of certain proteins to the basolateral surface involves the epithelial-specific mu 1b clathrin adaptor subunit. Recent results have shown that targeting to the basolateral surface utilizes the exocyst, whereas traffic to the apical surface uses syntaxin 3. Endocytosis at the apical surface is regulated by ARF6. Transcytosis of IgA is regulated by the p62Yes tyrosine kinase.

The subapical compartment and its role in intracellular trafficking and cell polarity

In polarized epithelial cells and hepatocytes, apical and basolateral plasma membrane surfaces are maintained, each displaying a distinct molecular composition. In recent years, it has become apparent that a subapical compartment, referred to as SAC, plays a prominent if not crucial role in the domain-specific sorting and targeting of proteins and lipids that are in dynamic transit between these plasma membrane domains. Although the molecular identity of the traffic-regulating devices is still obscure, the organization of SAC in distinct subcompartments and/or subdomains may well be instrumental to such functions. In this review, we will focus on the potential subcompartmentalization of the SAC in terms of regulation of membrane traffic, on how SAC relates to the endosomal system, and on how this compartment may operate in the context of other intracellular sorting organelles such as the Golgi complex, in generating and maintaining cell polarity.

Covalent homodimers of murine secretory component induced by epitope substitution unravel the capacity of the polymeric Ig receptor to dimerize noncovalently in the absence of IgA ligand

Recombinant secretory immunoglobulin A containing a bacterial epitope in domain I of the secretory component (SC) moiety can serve as a mucosal delivery vehicle triggering both mucosal and systemic responses (Corthésy, B., Kaufmann, M., Phalipon, A., Peitsch, M., Neutra, M. R., and Kraehenbuhl, J.-P. (1996) J. Biol. Chem. 271, 33670-33677). To load recombinant secretory IgA with multiple B and T epitopes and extend its biological functions, we selected, based on molecular modeling, five surface-exposed sites in domains II and III of murine SC. Loops predicted to be exposed at the surface of SC domains were replaced with the DYKDDDDK octapeptide (FLAG). Another two mutants were obtained with the FLAG inserted in between domains II and III or at the carboxyl terminus of SC. As shown by mass spectrometry, internal substitution of the FLAG into four of the mutants induced the formation of disulfide-linked homodimers. Three of the dimers and two of the monomers from SC mutants could be affinity-purified using an antibody to the FLAG, mapping them as candidates for insertion. FLAG-induced dimerization also occurred with the polymeric immunoglobulin receptor (pIgR) and might reflect the so-far nondemonstrated capacity of the receptor to oligomerize. By co-expressing in COS-7 cells and epithelial Caco-2 cells two pIgR constructs tagged at the carboxyl terminus with hexahistidine or FLAG, we provide the strongest evidence reported to date that the pIgR dimerizes noncovalently in the plasma membrane in the absence of polymeric IgA ligand. The implication of this finding is discussed in terms of IgA transport and specific antibody response at mucosal surfaces.

The SRC family protein tyrosine kinase p62yes controls polymeric IgA transcytosis in vivo

Transcytosis of polymeric immunoglobulin A (pIgA) across epithelial cells is mediated by the polymeric immunoglobulin receptor (pIgR). Binding of pIgA to pIgR stimulates transcytosis of the pIgA-pIgR complex via a signal transduction pathway that is dependent on a protein tyrosine kinase (PTK) of the SRC family. Here we identify the PTK as p62yes. We demonstrate the specific physical and functional association of the pIgR with p62yes in rodent liver. Analysis of p62yes knockout mice revealed a dramatic reduction in the association of tyrosine kinase activity with the pIgR and in transcytosis of pIgA. We conclude that p62yes controls pIgA transcytosis in vivo.