Intracellular redirection of plasma membrane trafficking after loss of epithelial cell polarity

Lack of discreet colocalization of epithelial apoptosis to the atretic precursor in the colon of the Fibroblast growth factor receptor 2IIIb mouse and staining consistent with cellular movement suggest a revised model of atresia formation

BACKGROUND

Colonic atresias in the Fibroblast growth factor receptor 2IIIb (Fgfr2IIIb) mouse model have been attributed to increased epithelial apoptosis and decreased epithelial proliferation at embryonic day (E) 10.5. We therefore hypothesized that these processes would colocalize to the distal colon where atresias occur (atretic precursor) and would be excluded or minimized from the proximal colon and small intestine.

RESULTS

We observed a global increase in intestinal epithelial apoptosis in Fgfr2IIIb -/- intestines from E9.5 to E10.5 that did not colocalize to the atretic precursor. Additionally, epithelial proliferations rates in Fgfr2IIIb -/- intestines were statistically indistinguishable to that of controls at E10.5 and E11.5. At E11.5 distal colonic epithelial cells in mutants failed to assume the expected pseudostratified columnar architecture and the continuity of the adjacent basal lamina was disrupted. Individual E-cadherin-positive cells were observed in the colonic mesenchyme.

CONCLUSIONS

Our observations suggest that alterations in proliferation and apoptosis alone are insufficient to account for intestinal atresias and that these defects may arise from both a failure of distal colonic epithelial cells to develop normally and local disruptions in basal lamina architecture.

Soluble syntaxin 3 functions as a transcriptional regulator

Syntaxins are a conserved family of SNARE proteins and contain C-terminal transmembrane anchors required for their membrane fusion activity. Here we show that Stx3 (syntaxin 3) unexpectedly also functions as a nuclear regulator of gene expression. We found that alternative splicing creates a soluble isoform that we termed Stx3S, lacking the transmembrane anchor. Soluble Stx3S binds to the nuclear import factor RanBP5 (RAN-binding protein 5), targets to the nucleus, and interacts physically and functionally with several transcription factors, including ETV4 (ETS variant 4) and ATF2 (activating transcription factor 2). Stx3S is differentially expressed in normal human tissues, during epithelial cell polarization, and in breast cancer versus normal breast tissue. Inhibition of endogenous Stx3S expression alters the expression of cancer-associated genes and promotes cell proliferation. Similar nuclear-targeted, soluble forms of other syntaxins were identified, suggesting that nuclear signaling is a conserved, novel function common among these membrane-trafficking proteins.

Tracking Endocytosis and Intracellular Trafficking of Epitope-tagged Syntaxin 3 by Antibody Feeding in Live, Polarized MDCK Cells

The uptake and trafficking of cell surface receptors can be monitored by a technique called 'antibody-feeding' which uses an externally applied antibody to label the receptor on the surface of cultured, live cells. Here, we adapt the traditional antibody-feeding experiment to polarized epithelial cells (Madin-Darby Canine Kidney) grown on permeable Transwell supports. By adding two tandem extracellular Myc epitope tags to the C-terminus of the SNARE protein syntaxin 3 (Stx3), we provided a site where an antibody could bind, allowing us to perform antibody-feeding experiments on cells with distinct apical and basolateral membranes. With this procedure, we observed the endocytosis and intracellular trafficking of Stx3. Specifically, we assessed the internalization rate of Stx3 from the basolateral membrane and observed the ensuing endocytic route in both time and space using immunofluorescence microscopy on cells fixed at different time points. For cell lines that form a polarized monolayer containing distinct apical and basolateral membranes when cultured on permeable supports, e.g., MDCK or Caco-2, this protocol can measure the rate of endocytosis and follow the subsequent trafficking of a target protein from either limiting membrane.

Epithelial disruptions, but not immune cell invasion, induced secretory dysfunction following innate immune activation in a novel model of acute salivary gland injury

BACKGROUND

Salivary gland (SG) injurious agents are all translated into loss of salivation (xerostomia). An association has been established between activation of innate immunity and SG injury and dysfunction. However, it remains unclear how the secretory epithelia respond by halting saliva production.

METHODS

C57BL/6 submandibular glands (SMGs) were acutely challenged using a single dose of the innate immune stimulant: polyinosinic-polycytidylic acid (poly (I:C)). Secretory capacity of the infected SMGs was substantiated by assessing the flow rate in response to pilocarpine stimulation. Depletion of the acute inflammatory cells was achieved by pre-treating mice with RB6-8C5 depletion antibody. Flow cytometry, histology and immunohistochemistry were conducted to verify the immune cell depletion. Epithelial expression of saliva-driving molecules: muscarinic 3 receptor (M3R), aquaporin 5 water channel (AQP5), Na-K-CL-Cotransporter 1 (NKCC1) and transmembrane member 16A (TMEM16A), was characterized using RT-qPCR and immunohistochemistry. Tight junction (TJ) protein; zonula occludens (ZO-1) and basement membrane (BM) protein; and laminin were assessed by immunohistochemistry.

RESULTS

Innate immune challenge prompted dysfunction in the exocrine SGs. Dysregulated gene and protein expression of molecules that drive saliva secretion was substantiated. Aberrant expression of TJ and BM proteins followed innate immune activation. Hyposalivation in the current model was independent of myeloperoxidase (MPO)-positive, acute inflammatory cells.

CONCLUSIONS

In this study, we developed a novel injury model of the SGs, featuring acute secretory dysfunction and immediate structural disruptions. Our results ruled out the injurious role of aggressively infiltrating inflammatory cells.

A unique SNARE machinery for exocytosis of cytotoxic granules and platelets granules

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells target infected or transformed cells with perforin-containing cytotoxic granules through immune synapses, while platelets secrete several types of granules which contents are essential for thrombosis and hemostasis. Recent work has culminated in the notion that an exocytic SNARE complex, based on a very similar set of components, is primarily responsible for exocytosis of the diverse granules in these different cell types. Granule exocytosis is, in particular, uniquely dependent on the atypical Q-SNARE syntaxin 11, its interacting partners of the Sec/Munc (SM) family, and is regulated by Rab27a. Mutations in these exocytic components underlie disease manifestations of familial hemophagocytic lymphohistiocytosis (FHL) subtypes, characterized by hyperactivation of the immune system, as well as platelet granule secretion defects. Here we discuss the key discoveries that led to the converging notion of the syntaxin 11-based exocytosis machinery for cytotoxic granules and platelet-derived granules.

Proteomic analysis of proteins surrounding occludin and claudin-4 reveals their proximity to signaling and trafficking networks

Tight junctions are complex membrane structures that regulate paracellular movement of material across epithelia and play a role in cell polarity, signaling and cytoskeletal organization. In order to expand knowledge of the tight junction proteome, we used biotin ligase (BioID) fused to occludin and claudin-4 to biotinylate their proximal proteins in cultured MDCK II epithelial cells. We then purified the biotinylated proteins on streptavidin resin and identified them by mass spectrometry. Proteins were ranked by relative abundance of recovery by mass spectrometry, placed in functional categories, and compared not only among the N- and C- termini of occludin and the N-terminus of claudin-4, but also with our published inventory of proteins proximal to the adherens junction protein E-cadherin and the tight junction protein ZO-1. When proteomic results were analyzed, the relative distribution among functional categories was similar between occludin and claudin-4 proximal proteins. Apart from already known tight junction- proteins, occludin and claudin-4 proximal proteins were enriched in signaling and trafficking proteins, especially endocytic trafficking proteins. However there were significant differences in the specific proteins comprising the functional categories near each of the tagging proteins, revealing spatial compartmentalization within the junction complex. Taken together, these results expand the inventory of known and unknown proteins at the tight junction to inform future studies of the organization and physiology of this complex structure.

Disease-causing mutations associated with four bestrophinopathies exhibit disparate effects on the localization, but not the oligomerization, of Bestrophin-1

BEST1 encodes Bestrophin-1 (Best1), a homo-oligomeric, integral membrane protein localized to the basolateral plasma membrane of the retinal pigment epithelium. Mutations in BEST1 cause five distinct retinal degenerative diseases, including adult vitelliform macular dystrophy (AVMD), autosomal recessive bestrophinopathy (ARB), autosomal dominant vitreoretinochoroidopathy (ADVIRC), and retinitis pigmentosa (RP). The mechanisms underlying these diseases and why mutations cause one disease over another are, for the most part, unknown. To gain insights into these four diseases, we expressed 28 Best1 mutants fused to YFP in polarized MDCK monolayers and, via confocal microscopy and immunofluorescence, live-cell FRET, and reciprocal co-immunoprecipitation experiments, screened these mutants for defects in localization and oligomerization. All 28 mutants exhibited comparable FRET efficiencies to and co-immunoprecipitated with WT Best1, indicating unimpaired oligomerization. RP- and ADVIRC-associated mutants were properly localized to the basolateral plasma membrane of cells, while two AVMD and most ARB mutants were mislocalized. When co-expressed, all mislocalized mutants caused mislocalization of WT Best1 to intracellular compartments. Our current and past results indicate that mislocalization of Best1 is not an absolute feature of any individual bestrophinopathy, occurring in AVMD, BVMD, and ARB. Furthermore, some ARB mutants that do not also cause dominant disease cause mislocalization of Best1, indicating that mislocalization is not a cause of disease, and that absence of Best1 activity from the plasma membrane is tolerated. Lastly, we find that the ARB truncation mutants L174Qfs*57 and R200X can form oligomers with WT Best1, indicating that the first ∼174 amino acids of Best1 are sufficient for oligomerization to occur.

Milk secretion: The role of SNARE proteins

During lactation, polarized mammary epithelial secretory cells (MESCs) secrete huge quantities of the nutrient molecules that make up milk, i.e. proteins, fat globules and soluble components such as lactose and minerals. Some of these nutrients are only produced by the MESCs themselves, while others are to a great extent transferred from the blood. MESCs can thus be seen as a crossroads for both the uptake and the secretion with cross-talks between intracellular compartments that enable spatial and temporal coordination of the secretion of the milk constituents. Although the physiology of lactation is well understood, the molecular mechanisms underlying the secretion of milk components remain incompletely characterized. Major milk proteins, namely caseins, are secreted by exocytosis, while the milk fat globules are released by budding, being enwrapped by the apical plasma membrane. Prolactin, which stimulates the transcription of casein genes, also induces the production of arachidonic acid, leading to accelerated casein transport and/or secretion. Because of their ability to form complexes that bridge two membranes and promote their fusion, SNARE (Soluble N-ethylmaleimide-Sensitive Factor Attachment Protein Receptor) proteins are involved in almost all intracellular trafficking steps and exocytosis. As SNAREs can bind arachidonic acid, they could be the effectors of the secretagogue effect of prolactin in MESCs. Indeed, some SNAREs have been observed between secretory vesicles and lipid droplets suggesting that these proteins could not only orchestrate the intracellular trafficking of milk components but also act as key regulators for both the coupling and coordination of milk product secretion in response to hormones.

An overview and online registry of microvillus inclusion disease patients and their MYO5B mutations

Microvillus inclusion disease (MVID) is one of the most severe congenital intestinal disorders and is characterized by neonatal secretory diarrhea and the inability to absorb nutrients from the intestinal lumen. MVID is associated with patient-, family-, and ancestry-unique mutations in the MYO5B gene, encoding the actin-based motor protein myosin Vb. Here, we review the MYO5B gene and all currently known MYO5B mutations and for the first time methodologically categorize these with regard to functional protein domains and recurrence in MYO7A associated with Usher syndrome and other myosins. We also review animal models for MVID and the latest data on functional studies related to the myosin Vb protein. To congregate existing and future information on MVID geno-/phenotypes and facilitate its quick and easy sharing among clinicians and researchers, we have constructed an online MOLGENIS-based international patient registry (www.MVID-central.org). This easily accessible database currently contains detailed information of 137 MVID patients together with reported clinical/phenotypic details and 41 unique MYO5B mutations, of which several unpublished. The future expansion and prospective nature of this registry is expected to improve disease diagnosis, prognosis, and genetic counseling.

t-SNARE Syntaxin2 (STX2) is implicated in intracellular transport of sulfoglycolipids during meiotic prophase in mouse spermatogenesis

Syntaxin2 (STX2), also known as epimorphin, is a member of the SNARE family of proteins, with expression in various types of cells. We previously identified an ENU-induced mutation, repro34, in the mouse Stx2 gene. The Stx2(repro34) mutation causes male-restricted infertility due to syncytial multinucleation of spermatogenic cells during meiotic prophase. A similar phenotype is also observed in mice with targeted inactivation of Stx2, as well as in mice lacking enzymes involved in sulfoglycolipid synthesis. Herein we analyzed expression and subcellular localization of STX2 and sulfoglycolipids in spermatogenesis. The STX2 protein localizes to the cytoplasm of germ cells at the late pachytene stage. It is found in a distinct subcellular pattern, presumably in the Golgi apparatus of pachytene/diplotene spermatocytes. Sulfoglycolipids are produced in the Golgi apparatus and transported to the plasma membrane. In Stx2(repro34) mutants, sulfoglycolipids are aberrantly localized in both pachytene/diplotene spermatocytes and in multinucleated germ cells. These results suggest that STX2 plays roles in transport and/or subcellular distribution of sulfoglycolipids. STX2 function in the Golgi apparatus and sulfoglycolipids may be essential for maintenance of the constriction between neighboring developing spermatocytes, which ensures ultimate individualization of germ cells in later stages of spermatogenesis.

Adducins regulate remodeling of apical junctions in human epithelial cells

This article identifies membrane skeleton proteins, adducins, as important regulators of epithelial cell–cell adhesions that promote assembly and antagonize stimulus-induced disassembly of adherens and tight junctions.

Epithelial adherens junctions (AJs) and tight junctions (TJs) are dynamic structures that readily undergo disintegration and reassembly. Remodeling of the AJs and TJs depends on the orchestrated dynamics of the plasma membrane with its underlying F-actin cytoskeleton, and the membrane–cytoskeleton interface may play a key role in junctional regulation. Spectrin–adducin–ankyrin complexes link membranes to the actin cytoskeleton where adducins mediate specrtrin–actin interactions. This study elucidates roles of adducins in the remodeling of epithelial junctions in human SK-CO15 colonic and HPAF-II pancreatic epithelial cell monolayers. These cells expressed the α and γ isoforms of adducin that positively regulated each others protein level and colocalized with E-cadherin and β-catenin at mature, internalized and newly assembled AJs. Small interfering RNA-mediated down-regulation of α- or γ-adducin expression significantly attenuated calcium-dependent AJ and TJ assembly and accelerated junctional disassembly triggered by activation of protein kinase C. Two mechanisms were found to mediate the impaired AJ and TJ assembly in adducin-depleted cells. One mechanism involved diminished expression and junctional recruitment of βII-spectrin, and the other mechanism involved the decrease in the amount of cellular F-actin and impaired assembly of perijunctional actin bundles. These findings suggest novel roles for adducins in stabilization of epithelial junctions and regulation of junctional remodeling.

ROCK-mediated contractility, tight junctions and channels contribute to the conversion of a preapical patch into apical surface during isochoric lumen initiation

Epithelial cells assemble into three-dimensional aggregates to generate lumen-containing organ substructures. Cells therein contact the extracellular matrix with their basal surface, neighbouring cells with their contact surface and the lumen with their apical surface. We investigated the development of single MDCK cells into aggregates with lumen using quantitative live-cell imaging to identify morphogenetic rules for lumen formation. In two-cell aggregates, membrane insertion into the contact surface established a preapical patch (PAP) characterized by the presence of the apical marker gp135, microvilli and the absence of E-cadherin. This PAP originated from a compartment that had hallmarks of an apical recycling endosome, and matured through Brefeldin-A-sensitive membrane trafficking and the establishment of tight junctions around itself. As a result of the activity of water and ion channels, an optically resolvable lumen formed. Initially, this lumen enlarged without changes in aggregate volume or cell number but with decreasing cell volumes. Additionally, the ROCK1/2-myosin-II pathway counteracted PAP and lumen formation. Thus, lumen formation results from PAP establishment, PAP maturation, lumen initiation and lumen enlargement. These phases correlate with distinct cell surface and volume patterns, which suggests that such morphometric parameters are regulated by trafficking, ROCK-mediated contractility and hydrostatic pressure or vice versa.

Phylogeny of endocytic components yields insight into the process of nonendosymbiotic organelle evolution

The process by which some eukaryotic organelles, for example the endomembrane system, evolved without endosymbiotic input remains poorly understood. This problem largely arises because many major cellular systems predate the last common eukaryotic ancestor (LCEA) and thus do not provide examples of organellogenesis in progress. A model is emerging whereby gene duplication and divergence of multiple "specificity-" or "identity-" encoding proteins for the various endomembranous organelles produced the diversity of nonendosymbiotically derived cellular compartments present in modern eukaryotes. To address this possibility, we analyzed three molecular components of the endocytic membrane-trafficking machinery. Phylogenetic analyses of the endocytic syntaxins, Rab 5, and the beta-adaptins each reveal a pattern of ancestral, undifferentiated endocytic homologues in the LCEA. Subsequently, these undifferentiated progenitors independently duplicated in widely divergent lineages, convergently producing components with similar endocytic roles, e.g., beta1 and beta2-adaptin. In contrast, beta3, beta4, and all other adaptin complex subunits, as well as paralogues of the syntaxins and Rabs specific for the other membrane-trafficking organelles, all evolved before the LCEA. Thus, the process giving rise to the differentiated organelles of the endocytic system appears to have been interrupted by the major speciation event that produced the extant eukaryotic lineages. These results suggest that although many endocytic components evolved before the LCEA, other major features evolved independently and convergently after diversification into the primary eukaryotic supergroups. This finding provides an example of a basic cellular system that was simpler in the LCEA than in many extant eukaryotes and yields insight into nonendosymbiotic organelle evolution.

Par1b promotes hepatic-type lumen polarity in Madin Darby canine kidney cells via myosin II- and E-cadherin-dependent signaling

Kidney-derived Madin Darby canine kidney (MDCK) cells form lumina at their apices, and target luminal proteins to an intracellular vacuolar apical compartment (VAC) when prevented from polarizing. Hepatocytes, by contrast, organize their luminal surfaces (the bile canaliculi; BC) between their lateral membranes, and, when nonpolarized, they display an intracellular luminal compartment that is distinct from the VACs of MDCK cells. Overexpression of the serine/threonine kinase Par1b/EMK1/MARK2 induces BC-like lateral lumina and a hepatic-type intracellular luminal compartment in MDCK cells, suggesting a role for Par1b in the branching decision between kidney- and hepatic-type epithelial phenotypes. Here, we report that Par1b promotes lateral lumen polarity in MDCK cells independently of Ca(2+)-mediated cell-cell adhesion by inhibiting myosin II in a rho kinase-dependent manner. Polarization was inhibited by E-cadherin depletion but promoted by an adhesion-defective E-cadherin mutant. By contrast, apical surface formation in control MDCK cells required Ca(2+)-dependent cell-cell adhesion, but it occurred in the absence of E-cadherin. We propose that E-cadherin, when in an adhesion-incompetent state at the lateral domain, serves as targeting patch for the establishment of lateral luminal surfaces. E-cadherin depletion also reverted the hepatic-type intracellular luminal compartment in Par1b-MDCK cells to VACs characteristic of control MDCK cells, indicating a novel link between E-cadherin and luminal protein targeting.

Tight junctions and cell polarity

The tight junction is an intracellular junctional structure that mediates adhesion between epithelial cells and is required for epithelial cell function. Tight junctions control paracellular permeability across epithelial cell sheets and also serve as a barrier to intramembrane diffusion of components between a cell's apical and basolateral membrane domains. Recent genetic and biochemical studies in invertebrates and vertebrates indicate that tight junction proteins play an important role in the establishment and maintenance of apico-basal polarity. Proteins involved in epithelial cell polarization form evolutionarily conserved multiprotein complexes at the tight junction, and these protein complexes regulate the architecture of epithelia throughout the polarization process. Accumulating information regarding the regulation of these polarity proteins will lead to a better understanding of the molecular mechanisms whereby cell polarity is established.

Polarisation, key to good localisation

Polarisation of cells is crucial for vectorial transport of ions and solutes. In literature, however, proteins specifically targeted to the apical or basolateral membrane are often studied in non-polarised cells. To investigate whether these data can be extrapolated to expression in polarised cells, we studied several membrane-specific proteins. In polarised MDCK cells, the Aquaporin-2 water channel resides in intracellular vesicles and apical membrane, while the vasopressin-type 2 receptor, anion-exchanger 1 (AE1) protein and E-Cadherin mainly localise to the basolateral membrane. In non-polarised MDCK cells, however, Aquaporin-2 localises, besides plasma membrane, mainly in the Golgi complex, while the others show a dispersed staining throughout the cell. Moreover, while AQP2 mutants in dominant nephrogenic diabetes insipidus are missorted to different organelles in polarised cells, they all predominantly localise to the Golgi complex in non-polarised MDCK cells. Additionally, the maturation of V2R, and likely its missorting, is affected in transiently-transfected compared to stably-transfected cells. In conclusion, we show that the use of stably-transfected polarised cells is crucial in interpreting the processing and the localisation of membrane targeted proteins.

Syntaxins 3 and 4 are concentrated in separate clusters on the plasma membrane before the establishment of cell polarity

Syntaxins 3 and 4 localize to the apical and basolateral plasma membrane, respectively, of epithelial cells where they mediate vesicle fusion. Here, we report that before establishment of cell polarity, syntaxins 3 and 4 are confined to mutually exclusive, submicron-sized clusters. Syntaxin clusters are remarkably uniform in size, independent of expression levels, and are distinct from caveolae and clathrin-coated pits. SNAP-23 partially colocalizes with both syntaxin 3 and 4 clusters. Deletion of the apical targeting signal of syntaxin 3 does not prevent sorting into clusters away from syntaxin 4. Syntaxin 3 and 4 cluster formation depends on different mechanisms because the integrity of syntaxin 3 clusters depends on intact microtubules, whereas syntaxin 4 clusters depend on intact actin filaments. Cholesterol depletion causes dispersion of syntaxin 3 but not syntaxin 4 clusters. In migrating cells, syntaxin clusters polarize to the leading edge, suggesting a role in polarized exocytosis. These results suggest that exocytosis occurs at small fusion sites exhibiting high local concentrations of SNARE proteins that may be required for efficient membrane fusion. The establishment of separate clusters for each syntaxin suggests that the plasma membrane is inherently polarized on an ultrastructural level even before the establishment of true cell polarity.

Interferon-gamma inhibits T84 epithelial cell migration by redirecting transcytosis of beta1 integrin from the migrating leading edge

Intestinal inflammation is associated with epithelial damage and formation of mucosal wounds. Epithelial cells migration is required for wound closure. In inflammatory status, migrating epithelial cells are exposed to proinflammatory cytokines such as IFN-gamma. However, influence of such cytokines on intestinal epithelial wound closure remains unknown. The present study was designed to investigate the effect of IFN-gamma on migration of model T84 intestinal epithelial cells and recovery of epithelial wounds. IFN-gamma significantly inhibited rate of T84 cell migration and closure of epithelial wounds. This effect was accompanied by the formation of large aberrant lamellipodia at the leading edge as well as significant decrease in the number of beta(1) integrin containing focal adhesions. IFN-gamma exposure increased endocytosis of beta(1) integrin and shifted its accumulation from early/recycling endosomes at the leading edge to a yet unidentified compartment at the cell base. This redirection in beta(1) integrin transcytosis was inhibited by depolymerization of microtubules with nocodazole and was unaffected by stabilization of microtubules with docetaxel. These results suggest that IFN-gamma attenuates epithelial wound closure by microtubule-dependent redirection of beta(1) integrin transcytosis from the leading edge of migrating cells thereby inhibiting adequate turnover of focal adhesion complexes and cell migration.

Rab11a and myosin Vb are required for bile canalicular formation in WIF-B9 cells

Hepatocytes polarize by forming functionally distinct sinusoidal (basolateral) and canalicular (apical) plasma membrane domains. Two distinct routes are used for delivery of membrane proteins to the canaliculus. Proteins having glycosylphosphatidylinositol anchors or single transmembrane domains are targeted to the sinusoidal plasma membrane from where they transcytose to the canalicular domain. In contrast, apical ATP-binding-cassette (ABC) transporters, which are required for energy-dependent biliary secretion of bile acids (ABCB11), phospholipids (ABCB4), and nonbile acid organic anions (ABCC2), lack initial residence in the basolateral plasma membrane and traffic directly from Golgi membranes to the canalicular membrane. While investigating mechanisms of apical targeting in WIF-B9 cells, a polarized hepatic epithelial cell line, we observed that rab11a is required for canalicular formation. Knockdown of rab11a or overexpression of the rab11a-GDP locked form prevented canalicular formation as did overexpression of the myosin Vb motorless tail domain. In WIF-B9 cells, which lack bile canaliculi, apical ABC transporters colocalized with transcytotic membrane proteins in rab11a-containing endosomes and, unlike the transcytotic markers, did not distribute to the plasma membrane. We propose that polarization of hepatocytes (i.e., canalicular biogenesis) requires recruitment of rab11a and myosin Vb to intracellular membranes that contain apical ABC transporters and transcytotic markers, permitting their targeting to the plasma membrane. In this model, polarization is initiated upon delivery of rab11a-myosin Vb-containing membranes to the surface, which causes plasma membrane at the site of delivery to differentiate into apical domain (bile canaliculus).

Endocytosis of the apical junctional complex: mechanisms and possible roles in regulation of epithelial barriers

Tight junctions (TJ) and adherens junctions (AJ) regulate cell-cell adhesion and barrier function of simple polarized epithelia. These junctions are positioned in the apical end of the lateral plasma membrane and form the so-called apical junctional complex (AJC). Although initially seen as purely structural features, the AJC is now known to play important roles in cell differentiation and proliferation. The AJC is a highly dynamic entity, undergoing rapid remodeling during normal epithelial morphogenesis and under pathologic conditions. There is growing evidence that remodeling of the AJC is mediated by internalization of junctional proteins. This review summarizes what is known about endocytic pathways, intracellular destinations and signaling cascades involved in internalization of AJC proteins. Potential biological roles for AJC endocytosis in maintaining functional apical junctions, reversible opening of epithelial barrier and disruption of intercellular adhesion are also discussed.

Mechanism of IFN-gamma-induced endocytosis of tight junction proteins: myosin II-dependent vacuolarization of the apical plasma membrane

Disruption of epithelial barrier by proinflammatory cytokines such as IFN-gamma represents a major pathophysiological consequence of intestinal inflammation. We have previously shown that IFN-gamma increases paracellular permeability in model T84 epithelial cells by inducing endocytosis of tight junction (TJ) proteins occludin, JAM-A, and claudin-1. The present study was designed to dissect mechanisms of IFN-gamma-induced endocytosis of epithelial TJ proteins. IFN-gamma treatment of T84 cells resulted in internalization of TJ proteins into large actin-coated vacuoles that originated from the apical plasma membrane and resembled the vacuolar apical compartment (VAC) previously observed in epithelial cells that lose cell polarity. The IFN-gamma dependent formation of VACs required ATPase activity of a myosin II motor but was not dependent on rapid turnover of F-actin. In addition, activated myosin II was observed to colocalize with VACs after IFN-gamma exposure. Pharmacological analyses revealed that formation of VACs and endocytosis of TJ proteins was mediated by Rho-associated kinase (ROCK) but not myosin light chain kinase (MLCK). Furthermore, IFN-gamma treatment resulted in activation of Rho GTPase and induced expressional up-regulation of ROCK. These results, for the first time, suggest that IFN-gamma induces endocytosis of epithelial TJ proteins via RhoA/ROCK-mediated, myosin II-dependent formation of VACs.

Localization of plasma membrane t-SNAREs syntaxin 2 and 3 in intracellular compartments

BACKGROUND

Membrane fusion requires the formation of a complex between a vesicle protein (v-SNARE) and the target membrane proteins (t-SNAREs). Syntaxin 2 and 3 are t-SNAREs that, according to previous over-expression studies, are predominantly localized at the plasma membrane. In the present study we investigated localization of the endogenous syntaxin 2 and 3.

RESULTS

Endogenous syntaxin 2 and 3 were found in NRK cells in intracellular vesicular structures in addition to regions of the plasma membrane. Treatment of these cells with N-ethylmaleimide (NEM), which is known to inactivate membrane fusion, caused syntaxin 3 to accumulate in the trans-Golgi network and syntaxin 2 in perinuclear membrane vesicles. Kinetic analysis in the presence of NEM indicated that this redistribution of syntaxin 2 and 3 takes place via actin containing structures.

CONCLUSION

Our data suggest that syntaxin 2 cycles between the plasma membrane and the perinuclear compartment whereas syntaxin 3 cycles between the plasma membrane and the trans-Golgi network. It is possible that this cycling has an important role in the regulation of t-SNARE function.

PATJ regulates tight junction formation and polarity in mammalian epithelial cells

Recent studies have revealed an important role for tight junction protein complexes in epithelial cell polarity. One of these complexes contains the apical transmembrane protein, Crumbs, and two PSD95/discs large/zonula occludens domain proteins, protein associated with Lin seven 1 (PALS1)/Stardust and PALS1-associated tight junction protein (PATJ). Although Crumbs and PALS1/Stardust are known to be important for cell polarization, recent studies have suggested that Drosophila PATJ is not essential and its function is unclear. Here, we find that PATJ is targeted to the apical region and tight junctions once cell polarization is initiated. We show using RNAi techniques that reduction in PATJ expression leads to delayed tight junction formation as well as defects in cell polarization. These effects are reversed by reintroduction of PATJ into these RNAi cells. This study provides new functional information on PATJ as a polarity protein and increases our understanding of the Crumbs–PALS1–PATJ complex function in epithelial polarity.

Differential roles for actin polymerization and a myosin II motor in assembly of the epithelial apical junctional complex

Differentiation and polarization of epithelial cells depends on the formation of the apical junctional complex (AJC), which is composed of the tight junction (TJ) and the adherens junction (AJ). In this study, we investigated mechanisms of actin reorganization that drive the establishment of AJC. Using a calcium switch model, we observed that formation of the AJC in T84 intestinal epithelial cells began with the assembly of adherens-like junctions followed by the formation of TJs. Early adherens-like junctions and TJs readily incorporated exogenous G-actin and were disassembled by latrunculin B, thus indicating dependence on continuous actin polymerization. Both adherens-like junctions and TJs were enriched in actin-related protein 3 and neuronal Wiskott-Aldrich syndrome protein (N-WASP), and their assembly was prevented by the N-WASP inhibitor wiskostatin. In contrast, the formation of TJs, but not adherens-like junctions, was accompanied by recruitment of myosin II and was blocked by inhibition of myosin II with blebbistatin. In addition, blebbistatin inhibited the ability of epithelial cells to establish a columnar phenotype with proper apico-basal polarity. These findings suggest that actin polymerization directly mediates recruitment and maintenance of AJ/TJ proteins at intercellular contacts, whereas myosin II regulates cell polarization and correct positioning of the AJC within the plasma membrane.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Par-1 promotes a hepatic mode of apical protein trafficking in MDCK cells

Simple (i.e., nonstratified) epithelial cells use two different routes to target their newly synthesized luminal plasma membrane proteins to the cell surface: a direct route from the Golgi complex, as in the kidney-derived MDCK cell line, or an indirect route that involves a intermediate stop at the ab-luminal (basolateral) membrane, as in hepatocytes. The mechanisms or proteins responsible for these different protein targeting strategies are not known. Here, we show that increased expression of EMK1, a mammalian ortholog of Caenorhabditis elegans Par-1, in MDCK cells promotes a switch from a direct to a transcytotic mode of apical protein delivery and other trafficking changes typical of hepatocytes. These results, together with our recent demonstration that PAR-1 promotes morphological features of hepatocytes in MDCK cells, indicate that Par-1 modulates the developmental decision to build a columnar versus a hepatic epithelial cell. To our knowledge, Par-1 is the first gene assigned to this task in epithelial morphogenesis.

Cutting edge: selective requirement for the Wiskott-Aldrich syndrome protein in cytokine, but not chemokine, secretion by CD4+ T cells

The mechanism of cytokine secretion is not well understood, but cytokines appear to be synthesized and released in a polarized fashion toward an Ag-specific target cell. In this study, we demonstrate that the Wiskott-Aldrich syndrome protein (WASp) is an essential component of the cytokine secretory pathway in CD4(+) T cells. Murine WASp-deficient CD4(+) T cells fail to polarize cytokines toward a target and show an unexpected and striking block in cytokine secretion. In contrast, chemokine secretion and trafficking of plasma membrane proteins, transported via the constitutive secretory pathway, are unaffected by the lack of WASp. These results suggest that CD4(+) T cell cytokines require a specialized, WASp-dependent pathway for cellular traffic and/or vesicle release that is distinct from that required for chemokine release. We propose that the use of different secretory pathways for cytokines and chemokines enables CD4(+) T cell activity to be further fine-tuned to serve specialized effector functions.

Endocytosis of epithelial apical junctional proteins by a clathrin-mediated pathway into a unique storage compartment

The adherens junction (AJ) and tight junction (TJ) are key regulators of epithelial polarity and barrier function. Loss of epithelial phenotype is accompanied by endocytosis of AJs and TJs via unknown mechanisms. Using a model of calcium depletion, we defined the pathway of internalization of AJ and TJ proteins (E-cadherin, p120 and beta-catenins, occludin, JAM-1, claudins 1 and 4, and ZO-1) in T84 epithelial cells. Proteinase protection assay and immunocytochemistry revealed orchestrated internalization of AJs and TJs into a subapical cytoplasmic compartment. Disruption of caveolae/lipid rafts did not prevent endocytosis, nor did caveolin-1 colocalize with internalized junctional proteins. Furthermore, AJ and TJ proteins did not colocalize with the macropinocytosis marker dextran. Inhibitors of clathrin-mediated endocytosis blocked internalization of AJs and TJs, and junctional proteins colocalized with clathrin and alpha-adaptin. AJ and TJ proteins were observed to enter early endosomes followed by movement to organelles that stained with syntaxin-4 but not with markers of late and recycling endosomes, lysosomes, or Golgi. These results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment. Such mechanisms may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.

More than apical: Distribution of SGLT1 in Caco-2 cells

We investigated the distribution of the endogenous sodium-d-glucose cotransporter (SGLT1) in polarized Caco-2 cells, a model for enterocytes. A cellular organelle fraction was separated by free-flow electrophoresis and subjected to the analysis of endogenous and exogenous marker enzymes for various membrane vesicle components. Furthermore, the presence of SGLT1 was tested by an ELISA assay employing newly developed epitope specific antibodies. Thereby it was found that the major amount of SGLT1 resided in intracellular compartments and only a minor amount in apical plasma membranes. The distribution ratio between intracellular SGLT1 and apical membrane-associated SGLT1 was approximately 2:1. Further immunocytochemical investigation of SGLT1 distribution in fixed Caco-2 cells by epifluorescence and confocal microscopy revealed that the intracellular compartments containing SGLT1 were associated with microtubules. Elimination of SGLT1 synthesis by incubation of cells with cycloheximide did not significantly reduce the size of the intracellular SGLT1 pool. Furthermore, the half-life of SGLT1 in Caco-2 cells was determined to be 2.5 days by metabolic labeling followed by immunoprecipitation. Our data suggest that most of the intracellular SGLT1 are not transporters en route from biosynthesis to their cellular destination but represent an intracellular reserve pool. We therefore propose that intracellular compartments containing SGLT1 are involved in the regulation of SGLT1 abundance at the apical cell surface.

SNAREs and epithelial cells

SNARE proteins control the membrane fusion events of membrane trafficking pathways. Work in epithelial cells has shown that polarized trafficking to the apical and basolateral plasma membrane domains requires different sets of SNAREs, suggesting a mechanism that contributes to the overall specificity of polarized trafficking and, perhaps, the formation and maintenance of polarity itself. This article describes methods that have been designed and adapted specifically for the investigation of SNAREs in epithelial cells. The knowledge of the subcellular localization of a SNARE of interest is essential to understand its function. Unfortunately, the endogenous expression levels of SNAREs are often low which makes detection challenging. We provide guidelines for determination of the localization of SNAREs by immunofluorescence microscopy including methods for signal amplification, antigen retrieval, and suppression of antibody cross-reactivity. To define which trafficking pathway a SNARE of interest is involved in, one needs to specifically inhibit its function. We provide guidelines for SNARE inhibition by overexpression of the SNARE of interest. An alternative is to introduce inhibitors of SNARE function, such as antibodies or clostridial toxins, into cells. Two methods are presented to make this possible. The first allows the monitoring of effects on trafficking pathways by biochemical assays, and is based on plasma membrane permeabilization using the bacterial toxin streptolysin-O. The second is suitable for single-cell observations and is based on microinjection.

Syntaxin 2 and endobrevin are required for the terminal step of cytokinesis in mammalian cells

The terminal step of cytokinesis in animal cells is the abscission of the midbody, a cytoplasmic bridge that connects the two prospective daughter cells. Here we show that two members of the SNARE membrane fusion machinery, syntaxin 2 and endobrevin/VAMP-8, specifically localize to the midbody during cytokinesis in mammalian cells. Inhibition of their function by overexpression of nonmembrane-anchored mutants causes failure of cytokinesis leading to the formation of binucleated cells. Time-lapse microscopy shows that only midbody abscission but not further upstream events, such as furrowing, are affected. These results indicate that successful completion of cytokinesis requires a SNARE-mediated membrane fusion event and that this requirement is distinct from exocytic events that may be involved in prior ingression of the plasma membrane.

Polarized epithelial membrane traffic: conservation and plasticity

Most cells are polarized and have distinct plasma membrane domains, which are the result of polarized trafficking of proteins and lipids. Great progress has been made in elucidating the highly conserved polarized targeting machinery. A pre-eminent challenge now is to understand the plasticity of polarized traffic, how it is altered by differentiation and dedifferentiation during development, as well as the adaptation of differentiated cells to meet changing physiological needs.

Matrix metalloproteinase activity in urine of patients with renal cell carcinoma leads to degradation of extracellular matrix proteins: possible use as a screening assay

PURPOSE

Localized renal cell carcinoma is usually curable by nephrectomy. However, a large fraction of patients already present with metastatic disease, which results in a poor outcome. Currently no clinically relevant screening assay is available to detect early stage renal cell carcinoma. We investigated whether urinary extracellular matrix (ECM) proteins and/or matrix metalloproteinase (MMP) activity may be valuable as a noninvasive indicator of early stage renal cell carcinoma.

MATERIALS AND METHODS

Urine specimens from preoperative patients with renal cell carcinoma and healthy controls were collected. The urinary excretion of the ECM proteins collagen IV, laminin and fibronectin was investigated by immunoblotting. MMP activity was assessed by gelatin zymography and by a fluorescence based microtiter plate activity assay.

RESULTS

The full-length forms of all 3 ECM proteins investigated were significantly decreased or absent in renal cell carcinoma urine. Based on criteria established in this study this finding would lead to the correct detection of 95% of patients with renal cell carcinoma (21 of 22) with a false-positive rate of 4.5% (1 of 22 controls). All 11 nonmetastatic cases of the lowest clinical stage (T1N0M0) were correctly identified. The absence of urinary ECM proteins was due to significantly increased urinary MMP activity.

CONCLUSIONS

Analysis of decreased urinary ECM proteins and analysis of increased MMP activity may have value for the development of a sensitive, high throughput molecular screening assay to detect early stage renal cell carcinoma.

Three-dimensional analysis of post-Golgi carrier exocytosis in epithelial cells

Targeted delivery of proteins to distinct plasma membrane domains is critical to the development and maintenance of polarity in epithelial cells. We used confocal and time-lapse total internal reflection fluorescence microscopy (TIR-FM) to study changes in localization and exocytic sites of post-Golgi transport intermediates (PGTIs) carrying GFP-tagged apical or basolateral membrane proteins during epithelial polarization. In non-polarized Madin Darby Canine Kidney (MDCK) cells, apical and basolateral PGTIs were present throughout the cytoplasm and were observed to fuse with the basal domain of the plasma membrane. During polarization, apical and basolateral PGTIs were restricted to different regions of the cytoplasm and their fusion with the basal membrane was completely abrogated. Quantitative analysis suggested that basolateral, but not apical, PGTIs fused with the lateral membrane in polarized cells, correlating with the restricted localization of Syntaxins 4 and 3 to lateral and apical membrane domains, respectively. Microtubule disruption induced Syntaxin 3 depolarization and fusion of apical PGTIs with the basal membrane, but affected neither the lateral localization of Syntaxin 4 or Sec6, nor promoted fusion of basolateral PGTIs with the basal membrane.

Non-polarized targeting of AE1 causes autosomal dominant distal renal tubular acidosis

Autosomal dominant distal renal tubular acidosis (ddRTA) is caused by mutations in SLC4A1, which encodes the polytopic chloride-bicarbonate exchanger AE1 that is normally expressed at the basolateral surface of alpha-intercalated cells in the distal nephron. Here we report that, in contrast with many disorders in which mutant membrane proteins are retained intracellularly and degraded, ddRTA can result from aberrant targeting of AE1 to the apical surface.

Retinal pigment epithelial cells exhibit unique expression and localization of plasma membrane syntaxins which may contribute to their trafficking phenotype

The SNARE membrane fusion machinery controls the fusion of transport vesicles with the apical and basolateral plasma-membrane domains of epithelial cells and is implicated in the specificity of polarized trafficking. To test the hypothesis that differential expression and localization of SNAREs may be a mechanism that contributes to cell-type-specific polarity of different proteins, we studied the expression and distribution of plasma-membrane SNAREs in the retinal pigment epithelium (RPE), an epithelium in which the targeting and steady-state polarity of several plasma membrane proteins differs from most other epithelia. We show here that retinal pigment epithelial cells both in vitro and in vivo differ significantly from MDCK cells and other epithelial cells in their complement of expressed t-SNAREs that are known - or suggested - to be involved in plasma membrane trafficking. Retinal pigment epithelial cells lack expression of the normally apical-specific syntaxin 3. Instead, they express syntaxins 1A and 1B, which are normally restricted to neurons and neuroendocrine cells, on their apical plasma membrane. The polarity of syntaxin 2 is reversed in retinal pigment epithelial cells, and it localizes to a narrow band on the lateral plasma membrane adjacent to the tight junctions. In addition, syntaxin 4 and the v-SNARE endobrevin/VAMP-8 localize to this sub-tight junctional domain, which suggests that this is a region of preferred vesicle exocytosis. Altogether, these data suggest that the unique polarity of many retinal pigment epithelial proteins results from differential expression and distribution of SNAREs at the plasma membrane. We propose that regulation of the expression and subcellular localization of plasma membrane SNAREs may be a general mechanism that contributes to the establishment of distinct sorting phenotypes among epithelial cell types.

SNARE expression and localization in renal epithelial cells suggest mechanism for variability of trafficking phenotypes

The apical- and basolateral-specific distribution of target soluble N-ethylmaleimide-sensitive factor attachment protein receptors (t-SNAREs) of the syntaxin family appear to be critical for polarity in epithelial cells. To test whether differential SNARE expression and/or subcellular localization may contribute to the known diversity of trafficking phenotypes of epithelial cell types in vivo, we have investigated the distribution of syntaxins 2, 3, and 4 in epithelial cells along the renal tubule. Syntaxins 3 and 4 are restricted to the apical and basolateral domains, respectively, in all cell types, indicating that their mutually exclusive localizations are important for cell polarity. The expression level of syntaxin 3 is highly variable, depending on the cell type, suggesting that it is regulated in concert with the cellular requirement for apical exocytic pathways. While syntaxin 4 localizes all along the basal and lateral plasma membrane domains in vivo, it is restricted to the lateral membrane in Madin-Darby canine kidney (MDCK) cells in two-dimensional monolayer culture. When cultured as cysts in collagen, however, MDCK cells target syntaxin 4 correctly to the basal and lateral membranes. Unexpectedly, the polarity of syntaxin 2 is inverted between different tubule cell types, suggesting a role in establishing plasticity of targeting. The vesicle-associated (v)-SNARE endobrevin is highly expressed in intercalated cells and colocalizes with the H(+)-ATPase in alpha- but not beta-intercalated cells, suggesting its involvement in H(+)-ATPase trafficking in the former cell type. These results suggest that epithelial membrane trafficking phenotypes in vivo are highly variable and that different cell types express or localize SNARE proteins differentially as a mechanism to achieve this variability.

Nonpolarized cells selectively sort apical proteins from cell surface to a novel compartment, but lack apical retention mechanisms

Membrane trafficking is central to establishing and maintaining epithelial cell polarity. One open question is to what extent the mechanisms regulating membrane trafficking are conserved between nonpolarized and polarized cells. To answer this question, we examined the dynamics of domain-specific plasma membrane (PM) proteins in three classes of hepatic cells: polarized and differentiated WIF-B cells, nonpolarized and differentiated Fao cells, and nonpolarized and nondifferentiated Clone 9 cells. In nonpolarized cells, mature apical proteins were uniformly distributed in the PM. Surprisingly, they were also in an intracellular compartment. Double labeling revealed that the compartment contained only apical proteins. By monitoring the dynamics of antibody-labeled molecules in nonpolarized cells, we further found that apical proteins rapidly recycled between the compartment and PM. In contrast, the apical PM residents in polarized cells showed neither internalization nor return to the basolateral PM from which they had originally come. Cytochalasin D treatment of these polarized cells revealed that the retention mechanisms are actin dependent. We conclude from these data that both polarized and nonpolarized cells selectively sort apical proteins from the PM and transport them to specific, but different cellular locations. We propose that the intracellular recycling compartment in nonpolarized cells is an intermediate in apical surface formation.

Interaction of bacterial pathogens with polarized epithelium

Many pathogens must surmount an epithelial cell barrier in order to establish an infection. While much has been learned about the interaction of bacterial pathogens with cultured epithelial cells, the influence of cell polarity on these events has only recently been appreciated. This review outlines bacterial-host epithelial cell interactions in the context of the distinct apical and basolateral surfaces of the polarized epithelium that lines the lumens of our organs.