Organization of vesicular trafficking in epithelia

Key Points

  • At present, our understanding of the mechanisms that control the establishment and maintenance of polarized vesicular-transport routes in epithelial cells can be traced to the introduction of the Madin–Darby canine kidney (MDCK) model system almost three decades ago.

  • Studies in this cell line have identified a multiplicity of sorting signals (hierarchically arranged, with basolateral usually dominant over apical signals) that guide proteins along biosynthetic, endocytic, recycling and transcytotic routes, to and from apical and basolateral membranes.

  • The basolateral sorting mechanisms are better understood owing to the simpler nature of basolateral signals — short peptide motifs that are similar to endocytic motifs — and to the discovery of several potential interacting adaptors that function in this pathway. An epithelial-specific adaptor (AP1B) sorts basolateral proteins in a post-Golgi compartment at the crossroads of the biosynthetic and recycling routes.

  • Recent work indicates that the clustering of small lipid rafts into larger lipid rafts, through protein oligomerization, might be an important determinant of apical targeting.

  • The multiplicity of signals and adaptors probably accounts for the variability in polarized transport routes and in the final localization of proteins at the cell surface in different epithelial cell types (this is known as 'flexible epithelial phenotype').

  • Apical and basolateral routes, which were originally defined by biochemical approaches, are, at present, being more precisely defined by live-cell-imaging experiments using green fluorescent protein (GFP)-tagged apical and basolateral markers.

  • These studies indicate that the junctional area is a 'hot spot' for the delivery of basolateral proteins and of some apical proteins that use the transcytotic route.

  • Live-cell-imaging techniques are facilitating the study of the cytoskeleton and its contribution to polarized trafficking routes. Recent work indicates that the actin and microtubule cytoskeletons cooperate at various levels: first, by organizing the assembly of apical and basolateral vesicular and tubular transporters from intracellular sorting organelles (trans-Golgi network, recycling endosomes); second, by facilitating their transport across the viscous cytoplasm; and third, by organizing the docking and fusion machinery at specific sites in the plasma membrane.

  • Studies in Drosophila melanogaster and Caenorhabditis elegans have identified a number of 'polarity genes' that are responsible for the 'identity' of the apical and basolateral domains. A challenge for the future is to identify trafficking roles for these genes.

Abstract

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.

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Figure 1: Trafficking routes and sorting mechanisms in epithelial cells.
Figure 2: Machinery that controls polarized vesicular trafficking in epithelial cells.
Figure 3: The exocytic machinery of MDCK cells.
Figure 4: Microtubules organize vesicular trafficking to the apical pole.
Figure 5: PAR1 controls epithelial microtubule organization and lumen morphogenesis.

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Acknowledgements

We gratefully acknowledge the support of the National Institutes of Health (E.R.-B.), a Jules and Doris Stein Professorship from the Research to Prevent Blindness Foundation (E.R.-B.) and a Career Development Award from the American Heart Association (A.M.). We are grateful to A. Gonzalez, T. McGraw, J. Nelson and V. Malhotra for useful comments on the manuscript. Our goal to cite mostly primary references is not free of arbitrariness given the limited number of references allowed by the format of the review and the rapid growth of the field. We apologize that several excellent papers could not be cited individually and could only be discussed indirectly or through reviews.

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DATABASES

Swiss-Prot

annexin 13b

μ1B

caveolin-1

Cdc42

GFP

HA

KIFC3

LDL receptor

p75NTR

PAR1

PAR3

Rab8

Rab11

RalA

rhodopsin

SNAP23

STRAD

TCTEX

ti-VAMP

VIP-36

VSVG

Glossary

TIGHT JUNCTION

The most apical intercellular junctions in mammalian epithelial cells, which function as selective (semi-permeable) diffusion barriers between individual cells. They are identified as a belt-like region in which two lipid-apposing membranes lie close together.

ADHERENS JUNCTION

A cell–cell adhesion complex that contains cadherins and catenins that are attached to cytoplasmic actin filaments.

DESMOSOME

A patch-like adhesive intercellular junction found in vertebrate tissues that is linked to intermediate filaments.

APICAL SURFACE

The surface of an epithelial or endothelial cell that faces the lumen of a cavity or tube or the outside of the organism.

BASOLATERAL SURFACE

The surface of an epithelial cell that adjoins underlying tissue.

TRANS-GOLGI NETWORK

(TGN). Membranous compartment from which vesicles bud to deliver proteins and other materials to the cell surface or to the late endosomes for delivery to lysosomes.

DOCKING/FUSION SITE

Vesicles and tubules that are targeted to the plasma membrane from the Golgi complex or from endosomes possess mechanisms by which to dock and fuse at the cell surface. These include tethering factors, Rab proteins and SNARE proteins.

GPI-ANCHOR

The function of this post-translational modification is to attach proteins to the exoplasmic leaflet of membranes, possibly to specific domains therein. The anchor is made of one molecule of phosphatidylinositol to which a carbohydrate chain is linked through the C-6 hydroxyl of the inositol. This anchor is linked to the protein through an ethanolamine phosphate moiety.

EXOPLASMIC

Facing the outside of the cell or the topologically equivalent lumen of organelles in the secretory pathway; the opposite of cytoplasmic.

GLYCAN

A polymer that consists of several monosaccharide residues (polysaccharide). In the case of GPI-anchored proteins, the basic unit is composed of glucosamine and three mannose residues.

LIPID RAFTS

Membrane microdomains that are enriched in cholesterol, sphingolipids and lipid-modified proteins such as GPI-linked proteins and palmitoylated proteins. These microdomains often function as platforms for signalling events.

LECTIN

A protein that can bind to carbohydrates with high selectivity. For example, concanavalin A is a lectin with affinity for mannose residues in glycoproteins.

FRAP

(fluorescence recovery after photobleaching). A live-cell-imaging technique used to study the mobility of fluorescent molecules. A pulse of high-intensity light is used to irreversibly photobleach a population of fluorophores in a target region. Recovery of fluorescence in the bleached region represents movement of fluorophores into that region.

FRET

(fluorescence resonance energy transfer). The non-radiative transfer of energy from a donor fluorophore to an acceptor fluorophore that is typically <80 Å away. FRET will only occur between fluorophores in which the emission spectrum of the donor has a significant overlap with the excitation of the acceptor.

CAVEOLAE

Flask-shaped invaginations of the plasma membrane that are coated with the protein caveolin. Caveolae are endocytosed in a clathrin-independent manner.

CHOROID PLEXUS

A capillary bed that is covered by transporting epithelial cells, and that protrudes on the cerebral ventricles. The cells are responsible for producing cerebral spinal fluid.

PDZ DOMAIN

A protein-interaction domain that often occurs in scaffolding proteins, and is named after the founding members of this protein family (Post-synaptic density protein of 95 kDa, Discs large and Zona occludens-1).

RETROMER COMPLEX

A protein complex that consists of Vps35, Vps26, Vps29, Vps17 and Vps5, which was discovered through genetic screens in Saccharomyces cerevisiae. It functions in the retrieval of proteins from the prevacuolar compartment and transport to the Golgi.

ADAPTIN

A tetrameric (for example, AP1, AP2, AP3 and AP4) or monomeric (such as GGAs) protein that promotes the formation of coated vesicles. Adaptins might interact with clathrin, small GTPases (such as Arf1) and microtubule-based motor proteins.

CLATHRIN

The main component of the coat that is associated with clathrin-coated vesicles, which are involved in membrane transport both in the endocytic and biosynthetic pathways.

RAB

A small protein with GTPase activity that is involved in the formation and delivery of vesicles.

TRANSCYTOSIS

Transport of macromolecules across a cell, which consists of the endocytosis of a macromolecule at one side of a monolayer and its exocytosis at the other side.

PRENYLATION

The enzymatic addition of prenyl moieties (geranyl, farnesyl or geranylgeranyl groups) to a protein as a post-translational modification.

SNARE

(soluble N-ethylmaleimide-sensitive fusion protein attachment protein (SNAP) receptor). SNARE proteins are a family of membrane-tethered coiled-coil proteins that regulate fusion reactions and target specificity in vesicle trafficking. They can be divided into v-SNAREs and t-SNAREs on the basis of their localization.

TOTAL-INTERNAL-REFLECTION MICROSCOPY

Fluorescence-microscopy technique with significant depth discrimination that can selectively excite only those fluorescent molecules within 100 nm of the interface between a cell and a coverslip.

BILE CANALICULI

Tiny channels on the surface of liver cells that collect the bile that they produce.

14-3-3 PROTEIN

A scaffolding protein that regulates the localization of other proteins by binding to conserved phosphotyrosine-containing motifs in a phosphorylation-dependent manner.

MICROVILLI

Small, finger-like projections (1–2-μm long and 100-nm wide) that occur on the exposed surfaces of epithelial cells to maximize the surface area.

MICROTUBULE-ORGANIZING CENTRE

(MTOC). Also called the centrosome or spindle-pole body, this structure nucleates and organizes microtubules.

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Rodriguez-Boulan, E., Kreitzer, G. & Müsch, A. Organization of vesicular trafficking in epithelia. Nat Rev Mol Cell Biol 6, 233–247 (2005). https://doi.org/10.1038/nrm1593

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