AP1B sorts basolateral proteins in recycling and biosynthetic routes of MDCK cells

Regulation of the apico-basolateral trafficking polarity of the homologous copper-ATPases ATP7A and ATP7B

The homologous P-type copper-ATPases (Cu-ATPases) ATP7A and ATP7B are the key regulators of copper homeostasis in mammalian cells. In polarized epithelia, upon copper treatment, ATP7A and ATP7B traffic from the trans-Golgi network (TGN) to basolateral and apical membranes, respectively. We characterized the sorting pathways of Cu-ATPases between TGN and the plasma membrane and identified the machinery involved. ATP7A and ATP7B reside on distinct domains of TGN in limiting copper conditions, and in high copper, ATP7A traffics to basolateral membrane, whereas ATP7B traverses common recycling, apical sorting and apical recycling endosomes en route to apical membrane. Mass spectrometry identified regulatory partners of ATP7A and ATP7B that include the adaptor protein-1 complex. Upon knocking out pan-AP-1, sorting of both Cu-ATPases is disrupted. ATP7A loses its trafficking polarity and localizes on both apical and basolateral surfaces in high copper. By contrast, ATP7B loses TGN retention but retained its trafficking polarity to the apical domain, which became copper independent. Using isoform-specific knockouts, we found that the AP-1A complex provides directionality and TGN retention for both Cu-ATPases, whereas the AP-1B complex governs copper-independent trafficking of ATP7B solely. Trafficking phenotypes of Wilson disease-causing ATP7B mutants that disrupts putative ATP7B-AP1 interaction further substantiates the role of AP-1 in apical sorting of ATP7B.

Cellular Release of Infectious Hepatitis C Virus Particles via Endosomal Pathways

Hepatitis C virus (HCV) is a positive-sense, single-stranded RNA virus that causes chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. The release of infectious HCV particles from infected hepatocytes is a crucial step in viral dissemination and disease progression. While the exact mechanisms of HCV particle release remain poorly understood, emerging evidence suggests that HCV utilizes intracellular membrane trafficking and secretory pathways. These pathways include the Golgi secretory pathway and the endosomal trafficking pathways, such as the recycling endosome pathway and the endosomal sorting complex required for transport (ESCRT)-dependent multivesicular bodies (MVBs) pathway. This review provides an overview of recent advances in understanding the release of infectious HCV particles, with a particular focus on the involvement of the host cell factors that participate in HCV particle release. By summarizing the current knowledge in this area, this review aims to contribute to a better understanding of endosomal pathways involved in the extracellular release of HCV particles and the development of novel antiviral strategies.

Loss of EGF receptor polarity enables homeostatic imbalance in epithelial-cell models

The polarized distribution of membrane proteins into apical and basolateral domains provides the basis for specialized functions of epithelial tissues. The EGF receptor (EGFR) plays important roles in embryonic development, adult-epithelial tissue homeostasis, and growth and survival of many carcinomas. Typically targeted to basolateral domains, there is also considerable evidence of EGFR sorting plasticity but very limited knowledge regarding domain-specific EGFR substrates. Here we have investigated effects of selective EGFR mistargeting because of inactive-basolateral sorting signals on epithelial-cell homeostatic responses to growth-induced stress in MDCK cell models. Aberrant EGFR localization was associated with multilayer formation, anchorage-independent growth, and upregulated expression of the intermediate filament-protein vimentin characteristically seen in cells undergoing epithelial-to-mesenchymal transition. EGFRs were selectively retained following their internalization from apical membranes, and a signaling pathway involving the signaling adaptor Gab1 protein and extracellular signal-regulated kinase ERK5 had an essential role integrating multiple responses to growth-induced stress. Our studies highlight the potential importance of cellular machinery specifying EGFR polarity in epithelial pathologies associated with homeostatic imbalance.

Polarized transport of membrane and secreted proteins during lumen morphogenesis

A ubiquitous feature of animal development is the formation of fluid-filled cavities or lumina, which transport gases and fluids across tissues and organs. Among different species, lumina vary drastically in size, scale, and complexity. However, all lumen formation processes share key morphogenetic principles that underly their development. Fundamentally, a lumen simply consists of epithelial cells that encapsulate a continuous internal space, and a common way of building a lumen is via opening and enlarging by filling it with fluid and/or macromolecules. Here, we discuss how polarized targeting of membrane and secreted proteins regulates lumen formation, mainly focusing on ion transporters in vertebrate model systems. We also discuss mechanistic differences observed among invertebrates and vertebrates and describe how the unique properties of the Na+/K+-ATPase and junctional proteins can promote polarization of immature epithelia to build lumina de novo in developing organs.

Sonic hedgehog is basolaterally sorted from the TGN and transcytosed to the apical domain involving Dispatched-1 at Rab11-ARE

Hedgehog proteins (Hhs) secretion from apical and/or basolateral domains occurs in different epithelial cells impacting development and tissue homeostasis. Palmitoylation and cholesteroylation attach Hhs to membranes, and Dispatched-1 (Disp-1) promotes their release. How these lipidated proteins are handled by the complex secretory and endocytic pathways of polarized epithelial cells remains unknown. We show that polarized Madin-Darby canine kidney cells address newly synthesized sonic hedgehog (Shh) from the TGN to the basolateral cell surface and then to the apical domain through a transcytosis pathway that includes Rab11-apical recycling endosomes (Rab11-ARE). Both palmitoylation and cholesteroylation contribute to this sorting behavior, otherwise Shh lacking these lipid modifications is secreted unpolarized. Disp-1 mediates first basolateral secretion from the TGN and then transcytosis from Rab11-ARE. At the steady state, Shh predominates apically and can be basolaterally transcytosed. This Shh trafficking provides several steps for regulation and variation in different epithelia, subordinating the apical to the basolateral secretion.

Basolateral Sorting of the Sodium/Iodide Symporter Is Mediated by Adaptor Protein 1 Clathrin Adaptor Complexes

Background: The sodium/iodide symporter (NIS) is a transmembrane protein located on the basolateral membrane of thyrocytes. Despite its physiological and clinical relevance, little is known about the mechanisms that mediate NIS subcellular sorting. In the present study, we examined NIS basolateral trafficking in vitro using non-thyroid and thyroid epithelial cells. Methods: Immunofluorescence and Western blotting were performed to analyze NIS subcellular location and function in cells grown in monolayers under unpolarized and/or polarized conditions. Strategic NIS residues were mutated, and binding of NIS to clathrin adaptor complexes was determined by immunoprecipitation. Results: We show that NIS reaches the plasma membrane (PM) through a thyrotropin-dependent mechanism 24 hours after treatment with the hormone. We demonstrate that NIS basolateral trafficking is a clathrin-mediated mechanism, in which the clathrin adaptor complexes AP-1 (A and B) sort NIS from the trans-Golgi network (TGN) and recycling endosomes (REs). Specifically, we show that the AP-1B μ1 subunit controls NIS basolateral sorting through common REs. In its absence, NIS is apically missorted but remains functional. Additionally, direct NIS basolateral transport from the TGN to the basolateral membrane is mediated by AP-1A through clathrin-coated vesicles that also carry the transferrin receptor. Loss of the μ1 subunit of AP-1A is functionally compensated by AP-1B. Furthermore, loss of both subunits diminishes NIS trafficking to the PM. Finally, we demonstrate that AP-1A binds to the L121 and LL562/563 residues on NIS, whereas AP-1B binds to L583. Conclusions: Our findings highlight the novel involvement of the clathrin-coated machinery in basolateral NIS trafficking. Given that AP-1A expression is reduced in tumors, and its expression correlates with that of NIS, these findings will help uncover new targets in thyroid cancer treatment.

LDLR, LRP1, and Megalin redundantly participate in the uptake of Clostridium novyi alpha-toxin

Clostridium novyi alpha-toxin (Tcnα) is a potent exotoxin that induces severe symptoms including gas gangrene, myositis, necrotic hepatitis, and sepsis. Tcnα binds to sulfated glycosaminoglycans (sGAG) for cell-surface attachment and utilizes low-density lipoprotein receptor (LDLR) for rapid entry. However, it was also shown that Tcnα may use alternative entry receptors other than LDLR. Here, we define that LRP1 and Megalin can also facilitate the cellular entry of Tcnα by employing reconstitutive LDLR family proteins. LDLR, LRP1, and Megalin recognize Tcnα via their ligand-binding domains (also known as LDL receptor type A repeats). Notably, LDLR and LRP1 have contrasting expression levels in many different cells, thus the dominant entry receptor for Tcnα could be cell-type dependent. These findings together increase our knowledge of the Tcnα actions and further help to understand the pathogenesis of C. novyi infection-associated diseases.

Clostridium novyi alpha-toxin (Tcnα) also uses LRP1 and Megalin as cellular entry receptors besides LDLR, and this might be a response to cell-type dependent receptor availability for the exotoxin.

Multifaceted control of E-cadherin dynamics by Adaptor Protein Complex 1 during epithelial morphogenesis

Intracellular trafficking regulates the distribution of transmembrane proteins including the key determinants of epithelial polarity and adhesion. The Adaptor Protein 1 (AP-1) complex is the key regulator of vesicle sorting, which binds many specific cargoes. We examined roles of the AP-1 complex in epithelial morphogenesis, using the Drosophila wing as a paradigm. We found that AP-1 knockdown leads to ectopic tissue folding, which is consistent with the observed defects in integrin targeting to the basal cell-extracellular matrix adhesion sites. This occurs concurrently with an integrin-independent induction of cell death, which counteracts elevated proliferation and prevents hyperplasia. We discovered a distinct pool of AP-1 that localizes at the subapical adherens junctions. Upon AP-1 knockdown, E-cadherin is hyperinternalized from these junctions and becomes enriched at the Golgi and recycling endosomes. We then provide evidence that E-cadherin hyperinternalization acts upstream of cell death in a potential tumor-suppressive mechanism. Simultaneously, cells compensate for elevated internalization of E-cadherin by increasing its expression to maintain cell-cell adhesion.

Self-organization of apical membrane protein sorting in epithelial cells

Polarized epithelial cells are characterized by the asymmetric distribution of proteins between apical and basolateral domains of the plasma membrane. This asymmetry is highly conserved and is fundamental to epithelial cell physiology, development, and homeostasis. How proteins are segregated for apical or basolateral delivery, a process known as sorting, has been the subject of considerable investigation for decades. Despite these efforts, the rules guiding apical sorting are poorly understood and remain controversial. Here, we consider mechanisms of apical membrane protein sorting and argue that they are largely driven by self-organization and biophysical principles. The preponderance of data to date is consistent with the idea that apical sorting is not ruled by a dedicated protein-based sorting machinery and relies instead on the concerted effects of oligomerization, phase separation of lipids and proteins in membranes, and pH-dependent glycan interactions.

AP-1 Recruits SMAP-1/SMAPs to the trans-Golgi Network to Promote Sorting in Polarized Epithelia

Coordinated AP-1 and clathrin coat assembly mediate secretory sorting on the trans-Golgi network (TGN) during conventional secretion. Here we found that SMAP-1/SMAPs deficiency caused the apical protein ERM-1 to accumulate on the basolateral side of the TGN. In contrast, the basolateral protein SLCF-1 appeared abnormally on the apical membrane. SMAP-1 colocalized with AP-1 on the TGN. The integrity of AP-1 is required for the subcellular presence of SMAP-1. Moreover, we found that the loss of SMAP-1 reduced clathrin-positive structures in the cytosol, suggesting that SMAP-1 has a regulatory role in clathrin assembly on the TGN. Functional experiments showed that overexpressing clathrin effectively alleviated exocytic defects due to the lack of SMAP-1, corroborating the role of SMAP-1 in promoting the assembly of clathrin on the TGN. Together, our results suggested that the AP-1 complex regulates the TGN localization of SMAP-1, promoting clathrin assembly to ensure polarized conventional secretion in C. elegans intestinal epithelia.

Induction of apically mistrafficked epiregulin disrupts epithelial polarity via aberrant EGFR signaling

In polarized MDCK cells, disruption of the tyrosine-based YXXΦ basolateral trafficking motif (Y156A) in the epidermal growth factor receptor (EGFR) ligand epiregulin (EREG), results in its apical mistrafficking and transformation in vivo. However, the mechanisms underlying these dramatic effects are unknown. Using a doxycycline-inducible system in 3D Matrigel cultures, we now show that induction of Y156A EREG in fully formed MDCK cysts results in direct and complete delivery of mutant EREG to the apical cell surface. Within 3 days of induction, ectopic lumens were detected in mutant, but not wild-type, EREG-expressing cysts. Of note, these structures resembled histological features found in subcutaneous xenografts of mutant EREG-expressing MDCK cells. These ectopic lumens formed de novo rather than budding from the central lumen and depended on metalloprotease-mediated cleavage of EREG and subsequent EGFR activity. Moreover, the most frequent EREG mutation in human cancer (R147stop) resulted in its apical mistrafficking in engineered MDCK cells. Thus, induction of EREG apical mistrafficking is sufficient to disrupt selective aspects of polarity of a preformed polarized epithelium. This article has an associated First Person interview with the first author of the paper.

Novel function for AP-1B during cell migration

The epithelial cell-specific clathrin adaptor protein (AP)-1B has a well-established role in polarized sorting of cargos to the basolateral membrane. Here we show that β1 integrin was dependent on AP-1B and its coadaptor, autosomal recessive hypercholesterolemia protein (ARH), for sorting to the basolateral membrane. We further demonstrate an unprecedented role for AP-1B at the basal plasma membrane during collective cell migration of epithelial sheets. During wound healing, expression of AP-1B (and ARH in AP–1B-positive cells) slowed epithelial-cell migration. We show that AP-1B colocalized with β1 integrin in focal adhesions during cell migration using confocal microscopy and total internal reflection fluorescence microscopy on fixed specimens. Further, AP-1B labeling in cell protrusions was distinct from labeling for the endocytic adaptor complex AP-2. Using stochastic optical reconstruction microscopy we identified numerous AP–1B-coated structures at or close to the basal plasma membrane in cell protrusions. In addition, immunoelectron microscopy showed AP-1B in coated pits and vesicles at the plasma membrane during cell migration. Lastly, quantitative real-time reverse transcription PCR analysis of human epithelial-derived cell lines revealed a loss of AP-1B expression in highly migratory metastatic cancer cells suggesting that AP-1B’s novel role at the basal plasma membrane during cell migration might be an anticancer mechanism.

Quantitative proteomics of MDCK cells identify unrecognized roles of clathrin adaptor AP-1 in polarized distribution of surface proteins

Epithelial cells perform critical protective, secretory, absorptive, and sensory functions, for which they require plasma membrane polarization into apical and basolateral domains. Impaired polarity causes cancer and developmental and degenerative disorders. Research on fundamental polarity mechanisms has been hindered by the paucity of model proteins and by the use of overexpression systems. Here, we introduce a high-throughput surface proteomics approach based on domain-selective biotinylation and quantitative mass spectrometry that provides candidate proteins to study polarity under normal expression levels. Using this approach, we described that clathrin adaptors mediate apical and basolateral distribution of surface proteins, expanding the traditional notion that clathrin adaptors mediate only basolateral polarity. Our results establish quantitative surface proteomics as a powerful tool to study epithelial polarity.

The current model of polarized plasma membrane protein sorting in epithelial cells has been largely generated on the basis of experiments characterizing the polarized distribution of a relatively small number of overexpressed model proteins under various experimental conditions. Thus, the possibility exists that alternative roles of various types of sorting machinery may have been underestimated or missed. Here, we utilize domain-selective surface biotinylation combined with stable isotope labeling with amino acids in cell culture (SILAC) and mass spectrometry to quantitatively define large populations of apical and basolateral surface proteins in Madin-Darby canine kidney (MDCK) cells. We identified 313 plasma membrane proteins, of which 38% were apical, 51% were basolateral, and 11% were nonpolar. Silencing of clathrin adaptor proteins (AP) AP-1A, AP-1B, or both caused redistribution of basolateral proteins as expected but also, of a large population of apical proteins. Consistent with their previously reported ability to compensate for one another, the strongest loss of polarity was observed when we silenced AP-1A and AP-1B simultaneously. We found stronger evidence of compensation in the apical pathway compared with the basolateral pathway. Surprisingly, we also found subgroups of proteins that were affected after silencing just one adaptor, indicating previously unrecognized independent roles for AP-1A and AP-1B. While AP-1B silencing mainly affected basolateral polarity, AP-1A silencing seemed to cause comparable loss of apical and basolateral polarity. Our results uncover previously overlooked roles of AP-1 in polarized distribution of apical and basolateral proteins and introduce surface proteomics as a method to examine mechanisms of polarization with a depth not possible until now.

Transcytosis maintains CFTR apical polarity in the face of constitutive and mutation-induced basolateral missorting

Apical polarity of cystic fibrosis transmembrane conductance regulator (CFTR) is essential for solute and water transport in secretory epithelia and can be impaired in human diseases. Maintenance of apical polarity in the face of CFTR non-polarized delivery and inefficient apical retention of mutant CFTRs lacking PDZ-domain protein (NHERF1, also known as SLC9A3R1) interaction, remains enigmatic. Here, we show that basolateral CFTR delivery originates from biosynthetic (∼35%) and endocytic (∼65%) recycling missorting. Basolateral channels are retrieved via basolateral-to-apical transcytosis (hereafter denoted apical transcytosis), enhancing CFTR apical expression by two-fold and suppressing its degradation. In airway epithelia, CFTR transcytosis is microtubule-dependent but independent of Myo5B, Rab11 proteins and NHERF1 binding to its C-terminal DTRL motif. Increased basolateral delivery due to compromised apical recycling and accelerated internalization upon impaired NHERF1-CFTR association is largely counterbalanced by efficient CFTR basolateral internalization and apical transcytosis. Thus, transcytosis represents a previously unrecognized, but indispensable, mechanism for maintaining CFTR apical polarity that acts by attenuating its constitutive and mutation-induced basolateral missorting.

Clathrin and clathrin adaptor AP-1 control apical trafficking of megalin in the biosynthetic and recycling routes

Megalin (gp330, LRP-2) is a protein structurally related to the low-density lipoprotein receptor family that displays a large luminal domain with multiligand binding properties. Megalin localizes to the apical surface of multiple epithelia, where it participates in endocytosis of a variety of ligands performing roles important for development or homeostasis. We recently described the apical recycling pathway of megalin in Madin–Darby canine kidney (MDCK) cells and found that it is a long-lived, fast recycling receptor with a recycling turnover of 15 min and a half-life of 4.8 h. Previous work implicated clathrin and clathrin adaptors in the polarized trafficking of fast recycling basolateral receptors. Hence, here we study the role of clathrin and clathrin adaptors in megalin’s apical localization and trafficking. Targeted silencing of clathrin or the γ1 subunit of clathrin adaptor AP-1 by RNA interference in MDCK cells disrupted apical localization of megalin, causing its redistribution to the basolateral membrane. In contrast, silencing of the γ2 subunit of AP-1 had no effect on megalin polarity. Trafficking assays we developed using FM4-HA-miniMegalin-GFP, a reversible conditional endoplasmic reticulum–retained chimera, revealed that clathrin and AP-1 silencing disrupted apical sorting of megalin in both biosynthetic and recycling routes. Our experiments demonstrate that clathrin and AP-1 control the sorting of an apical transmembrane protein.

A Carboxy-Terminal Monoleucine-Based Motif Participates in the Basolateral Targeting of the Na+/I- Symporter

The Na+/iodide (I-) symporter (NIS), a glycoprotein expressed at the basolateral plasma membrane of thyroid follicular cells, mediates I- accumulation for thyroid hormonogenesis and radioiodide therapy for differentiated thyroid carcinoma. However, differentiated thyroid tumors often exhibit lower I- transport than normal thyroid tissue (or even undetectable I- transport). Paradoxically, the majority of differentiated thyroid cancers show intracellular NIS expression, suggesting abnormal targeting to the plasma membrane. Therefore, a thorough understanding of the mechanisms that regulate NIS plasma membrane transport would have multiple implications for radioiodide therapy. In this study, we show that the intracellularly facing carboxy-terminus of NIS is required for the transport of the protein to the plasma membrane. Moreover, the carboxy-terminus contains dominant basolateral information. Using internal deletions and site-directed mutagenesis at the carboxy-terminus, we identified a highly conserved monoleucine-based sorting motif that determines NIS basolateral expression. Furthermore, in clathrin adaptor protein (AP)-1B-deficient cells, NIS sorting to the basolateral plasma membrane is compromised, causing the protein to also be expressed at the apical plasma membrane. Computer simulations suggest that the AP-1B subunit σ1 recognizes the monoleucine-based sorting motif in NIS carboxy-terminus. Although the mechanisms by which NIS is intracellularly retained in thyroid cancer remain elusive, our findings may open up avenues for identifying molecular targets that can be used to treat radioiodide-refractory thyroid tumors that express NIS intracellularly.

Viral journeys on the intracellular highways

Viruses are obligate intracellular pathogens that are dependent on cellular machineries for their replication. Recent technological breakthroughs have facilitated reliable identification of host factors required for viral infections and better characterization of the virus–host interplay. While these studies have revealed cellular machineries that are uniquely required by individual viruses, accumulating data also indicate the presence of broadly required mechanisms. Among these overlapping cellular functions are components of intracellular membrane trafficking pathways. Here, we review recent discoveries focused on how viruses exploit intracellular membrane trafficking pathways to promote various stages of their life cycle, with an emphasis on cellular factors that are usurped by a broad range of viruses. We describe broadly required components of the endocytic and secretory pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Identification of such overlapping host functions offers new opportunities to develop broad-spectrum host-targeted antiviral strategies.

Adaptor protein 1 B mu subunit does not contribute to the recycling of kAE1 protein in polarized renal epithelial cells

Mutations in the gene encoding the kidney anion exchanger 1 (kAE1) can lead to distal renal tubular acidosis (dRTA). dRTA mutations reported within the carboxyl (C)-terminal tail of kAE1 result in apical mis-targeting of the exchanger in polarized renal epithelial cells. As kAE1 physically interacts with the μ subunit of epithelial adaptor protein 1 B (AP-1B), we investigated the role of heterologously expressed μ1B subunit of the AP-1B complex for kAE1 retention to the basolateral membrane in polarized porcine LLC-PK1 renal epithelial cells that are devoid of endogenous AP-1B. We confirmed the interaction and close proximity between kAE1 and μ1B using immunoprecipitation and proximity ligation assay, respectively. Expressing the human μ1B subunit in these cells decreased significantly the amount of cell surface kAE1 at the steady state, but had no significant effect on kAE1 recycling and endocytosis. We show that (i) heterologous expression of μ1B displaces the physical interaction of endogenous GAPDH with kAE1 WT supporting that both AP-1B and GAPDH proteins bind to an overlapping site on kAE1 and (ii) phosphorylation of tyrosine 904 within the potential YDEV interaction motif does not alter the kAE1/AP-1B interaction. We conclude that μ1B subunit is not involved in recycling of kAE1.

Interactions between the Hepatitis C Virus Nonstructural 2 Protein and Host Adaptor Proteins 1 and 4 Orchestrate Virus Release. mBio. 2018;9. [PMID: 29535204 PMCID: PMC5850324 DOI: 10.1128/mbio.02233-17]

Hepatitis C virus (HCV) spreads via secreted cell-free particles or direct cell-to-cell transmission. Yet, virus-host determinants governing differential intracellular trafficking of cell-free- and cell-to-cell-transmitted virus remain unknown. The host adaptor proteins (APs) AP-1A, AP-1B, and AP-4 traffic in post-Golgi compartments, and the latter two are implicated in basolateral sorting. We reported that AP-1A mediates HCV trafficking during release, whereas the endocytic adaptor AP-2 mediates entry and assembly. We demonstrated that the host kinases AAK1 and GAK regulate HCV infection by controlling these clathrin-associated APs. Here, we sought to define the roles of AP-4, a clathrin-independent adaptor; AP-1A; and AP-1B in HCV infection. We screened for interactions between HCV proteins and the μ subunits of AP-1A, AP-1B, and AP-4 by mammalian cell-based protein fragment complementation assays. The nonstructural 2 (NS2) protein emerged as an interactor of these adaptors in this screening and by coimmunoprecipitations in HCV-infected cells. Two previously unrecognized dileucine-based motifs in the NS2 C terminus mediated AP binding and HCV release. Infectivity and coculture assays demonstrated that while all three adaptors mediate HCV release and cell-free spread, AP-1B and AP-4, but not AP-1A, mediate cell-to-cell spread. Live-cell imaging revealed HCV cotrafficking with AP-1A, AP-1B, and AP-4 and that AP-4 mediates HCV trafficking in a post-Golgi compartment. Lastly, HCV cell-to-cell spread was regulated by AAK1 and GAK and thus susceptible to treatment with AAK1 and GAK inhibitors. These data provide a mechanistic understanding of HCV trafficking in distinct release pathways and reveal a requirement for APs in cell-to-cell viral spread.

HCV spreads via cell-free infection or cell-to-cell contact that shields it from antibody neutralization, thereby facilitating viral persistence. Yet, factors governing this differential sorting remain unknown. By integrating proteomic, RNA interference, genetic, live-cell imaging, and pharmacological approaches, we uncover differential coopting of host adaptor proteins (APs) to mediate HCV traffic at distinct late steps of the viral life cycle. We reported that AP-1A and AP-2 mediate HCV trafficking during release and assembly, respectively. Here, we demonstrate that dileucine motifs in the NS2 protein mediate AP-1A, AP-1B, and AP-4 binding and cell-free virus release. Moreover, we reveal that AP-4, an adaptor not previously implicated in viral infections, mediates cell-to-cell spread and HCV trafficking. Lastly, we demonstrate cell-to-cell spread regulation by AAK1 and GAK, host kinases controlling APs, and susceptibility to their inhibitors. This study provides mechanistic insights into virus-host determinants that facilitate HCV trafficking, with potential implications for pathogenesis and antiviral agent design.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

LDL switches the LRP6 internalization route from flotillin dependent to clathrin dependent in hepatic cells

Low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) was originally identified as a co-receptor of the Wnt signalling pathway and has been shown to be involved in LDL transport. In polarized hepatocytes, many apical proteins are sorted to the basolateral membrane and then internalized and transported to the apical bile canalicular membrane - a process known as transcytosis. We show that LRP6 is transcytosed to the apical membrane of polarized hepatic HepG2 cells via a flotillin-dependent manner in the absence of LDL. LRP6 formed a complex with Niemann-Pick type C1-like 1 (NPC1L1), which is localized to the bile canalicular membrane of the liver and is involved in cholesterol absorption from the bile. LRP6 was required for apical membrane localization of NPC1L1 in the absence of LDL. Clathrin-dependent LRP6 internalization occurred in the presence of LDL, which resulted in trafficking of LRP6 to the lysosome, thereby reducing apical sorting of LRP6 and NPC1L1. These results suggest that LRP6 endocytosis proceeds by two routes, depending on the presence of LDL, and that LRP6 controls the intracellular destination of NPC1L1 in hepatocytes.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

Molluskan Hemocyanins Activate the Classical Pathway of the Human Complement System through Natural Antibodies

Molluskan hemocyanins are enormous oxygen-carrier glycoproteins that show remarkable immunostimulatory properties when inoculated in mammals, such as the generation of high levels of antibodies, a strong cellular reaction, and generation of non-specific antitumor immune responses in some types of cancer, particularly for superficial bladder cancer. These proteins have the ability to bias the immune response toward a T_h1 phenotype. However, despite all their current uses with beneficial clinical outcomes, a clear mechanism explaining these properties is not available. Taking into account reports of natural antibodies against the hemocyanin of the gastropod Megathura crenulata [keyhole limpet hemocyanin (KLH)] in humans as well as other vertebrate species, we report here for the first time, the presence, in sera from unimmunized healthy donors, of antibodies recognizing, in addition to KLH, two other hemocyanins from gastropods with documented immunomodulatory capacities: Fisurella latimarginata hemocyanin (FLH) and Concholepas concholepas hemocyanin (CCH). Through an ELISA screening, we found IgM and IgG antibodies reactive with these hemocyanins. When the capacity of these antibodies to bind deglycosylated hemocyanins was studied, no decreased interaction was detected. Moreover, in the case of FLH, deglycosylation increased antibody binding. We evaluated through an in vitro complement deposition assay whether these antibodies activated the classical pathway of the human complement system. The results showed that all three hemocyanins and their deglycosylated counterparts elicited this activation, mediated by C1 binding to immunoglobulins. Thus, this work contributes to the understanding on how the complement system could participate in the immunostimulatory properties of hemocyanins, through natural, complement-activating antibodies reacting with these proteins. Although a role for carbohydrates cannot be completely ruled out, in our experimental setting, glycosylation status had a limited effect. Finally, our data open possibilities for further studies leading to the design of improved hemocyanin-based research tools for diagnosis and immunotherapy.

Newly synthesized and recycling pools of the apical protein gp135 do not occupy the same compartments

Polarized epithelial cells sort newly synthesized and recycling plasma membrane proteins into distinct trafficking pathways directed to either the apical or basolateral membrane domains. While the trans-Golgi network is a well-established site of protein sorting, increasing evidence indicates a key role for endosomes in the initial trafficking of newly synthesized proteins. Both basolateral and apical proteins have been shown to traverse endosomes en route to the plasma membrane. In particular, apical proteins traffic through either subapical early or recycling endosomes. Here we use the SNAP tag system to analyze the trafficking of the apical protein gp135, also known as podocalyxin. We show that newly synthesized gp135 traverses the apical recycling endosome, but not the apical early endosomes (AEEs). In contrast, post-endocytic gp135 is delivered to the AEE before recycling back to the apical membrane. The pathways pursued by the newly synthesized and recycling gp135 populations do not detectably intersect, demonstrating that the biosynthetic and post-endocytic pools of this protein are subjected to distinct sorting processes.

The fast-recycling receptor Megalin defines the apical recycling pathway of epithelial cells

The basolateral recycling and transcytotic pathways of epithelial cells were previously defined using markers such as transferrin (TfR) and polymeric IgA (pIgR) receptors. In contrast, our knowledge of the apical recycling pathway remains fragmentary. Here we utilize quantitative live-imaging and mathematical modelling to outline the recycling pathway of Megalin (LRP-2), an apical receptor with key developmental and renal functions, in MDCK cells. We show that, like TfR, Megalin is a long-lived and fast-recycling receptor. Megalin enters polarized MDCK cells through segregated apical sorting endosomes and subsequently intersects the TfR and pIgR pathways at a perinuclear Rab11-negative compartment termed common recycling endosomes (CRE). Whereas TfR recycles to the basolateral membrane from CRE, Megalin, like pIgR, traffics to subapical Rab11-positive apical recycling endosomes (ARE) and reaches the apical membrane in a microtubule- and Rab11-dependent manner. Hence, Megalin defines the apical recycling pathway of epithelia, with CRE as its apical sorting station.

The carboxyl-terminally truncated kidney anion exchanger 1 R901X dRTA mutant is unstable at the plasma membrane

Mutations in the SLC4A1 gene coding for kidney anion exchanger 1 (kAE1) cause distal renal tubular acidosis (dRTA). We investigated the fate of the most common truncated dominant dRTA mutant kAE1 R901X. In renal epithelial cells, we found that kAE1 R901X is less abundant than kAE1 wild-type (WT) at the plasma membrane. Although kAE1 WT and kAE1 R901X have similar half-lives, the decreased abundance of kAE1 R901X at the surface is due to an increased endocytosis rate and a decreased recycling rate of endocytosed proteins. We propose that, in polarized renal epithelial cells, the apically mistargeted kAE1 R901X mutant is endocytosed faster than kAE1 WT and its recycling to the basolateral membrane is delayed. This resets the equilibrium, such that kAE1 R901X resides predominantly in an endomembrane compartment, thereby likely participating in development of dRTA disease.

AP1S3 is required for hepatitis C virus infection by stabilizing E2 protein

Hepatitis C virus (HCV) infects 130 million people worldwide and is a leading cause of liver cirrhosis, end-stage liver disease and hepatocellular carcinoma. The interactions between viral elements and host factors play critical role on HCV invade, replication and release. Here, we identified adaptor protein complex 1 sigma 3 subunit (AP1S3) as a dependency factor for the efficient HCV infection in hepatoma cells. AP1S3 silencing in cultivated Huh7.5.1 cells significantly reduced the production of HCV progeny particles. Immunoprecipitation analysis revealed that AP1S3 interacted with the HCV E2 protein. With this interaction, AP1S3 could protect HCV E2 from ubiquitin-mediated proteasomal degradation. Using in vivo ubiquitylation assay, we identified that E6-Associated Protein (E6AP) was associated with HCV E2. In addition, treatment with synthetic peptide that contains the AP1S3-recognized motif inhibited HCV infection in Huh7.5.1 cells. Our data reveal AP1 as a novel host network that is required by viruses during infection and provides a potential target for developing broad-spectrum anti-virus strategies.

N-linked glycans do not affect plasma membrane localization of multidrug resistance protein 4 (MRP4) but selectively alter its prostaglandin E2 transport activity

Multidrug resistance protein 4 (MRP4) is a member of subfamily C of the ATP-binding cassette superfamily of membrane transport proteins. MRP4 mediates the ATP-dependent efflux of many endogenous and exogenous solutes across the plasma membrane, and in polarized cells, it localizes to the apical or basolateral plasma membrane depending on the tissue type. MRP4 is a 170 kDa glycoprotein and here we show that MRP4 is simultaneously N-glycosylated at Asn746 and Asn754. Furthermore, confocal immunofluorescence studies showed that N-glycans do not affect MRP4's apical membrane localization in polarized LLC-PK1 cells or basolateral membrane localization in polarized MDCKI cells. However, vesicular transport assays showed that N-glycans differentially affect MRP4's ability to transport prostaglandin E2, but not estradiol glucuronide. Together these data indicate that N-glycosylation at Asn746 and Asn754 is not essential for plasma membrane localization of MRP4 but cause substrate-selective effects on its transport activity.

Dual pulse-chase microscopy reveals early divergence in the biosynthetic trafficking of the Na,K-ATPase and E-cadherin

The trafficking of newly synthesized Na,K-ATPase and E-cadherin is observed in polarized epithelial cells. E-cadherin’s exit from the Golgi complex is not susceptible to 19°C temperature block. Furthermore, these proteins exit the Golgi and are delivered to the basolateral cell surface in separate vascular carriers.

Recent evidence indicates that newly synthesized membrane proteins that share the same distributions in the plasma membranes of polarized epithelial cells can pursue a variety of distinct trafficking routes as they travel from the Golgi complex to their common destination at the cell surface. In most polarized epithelial cells, both the Na,K-ATPase and E-cadherin are localized to the basolateral domains of the plasma membrane. To examine the itineraries pursued by newly synthesized Na,K-ATPase and E-cadherin in polarized MDCK epithelial cells, we used the SNAP and CLIP labeling systems to fluorescently tag temporally defined cohorts of these proteins and observe their behaviors simultaneously as they traverse the secretory pathway. These experiments reveal that E-cadherin is delivered to the cell surface substantially faster than is the Na,K-ATPase. Furthermore, the surface delivery of newly synthesized E-cadherin to the plasma membrane was not prevented by the 19°C temperature block that inhibits the trafficking of most proteins, including the Na,K-ATPase, out of the trans-Golgi network. Consistent with these distinct behaviors, populations of newly synthesized E-cadherin and Na,K-ATPase become separated from one another within the trans-Golgi network, suggesting that they are sorted into different carrier vesicles that mediate their post-Golgi trafficking.

Structural and functional analysis of endosomal compartments in epithelial cells

Epithelial cells display segregated early endosomal compartments, termed apical sorting endosomes and basolateral sorting endosomes, that converge into a common late endosomal-lysosomal degradative compartment and common recycling endosomes (CREs). Unlike recycling endosomes of nonpolarized cells, CREs have the ability to sort apical and basolateral plasma membrane proteins into distinct apical and basolateral recycling routes, utilizing mechanisms similar to those employed by the trans Golgi network in the biosynthetic pathway. The apical recycling route includes an additional compartment, the apical recycling endosomes, consisting of multiple vesicles bundled around the basal body. Recent evidence indicates that, in addition to their role in internalizing ligands and recycling their receptors back to the cell surface, endosomal compartments act as intermediate stations in the biosynthetic routes to the plasma membrane. Here we review methods employed by our laboratory to study the endosomal compartments of epithelial cells and their multiple trafficking roles.

Analyzing the role of AP-1B in polarized sorting from recycling endosomes in epithelial cells

Epithelial cells polarize their plasma membrane into apical and basolateral domains where the apical membrane faces the luminal side of an organ and the basolateral membrane is in contact with neighboring cells and the basement membrane. To maintain this polarity, newly synthesized and internalized cargos must be sorted to their correct target domain. Over the last ten years, recycling endosomes have emerged as an important sorting station at which proteins destined for the apical membrane are segregated from those destined for the basolateral membrane. Essential for basolateral sorting from recycling endosomes is the tissue-specific adaptor complex AP-1B. This chapter describes experimental protocols to analyze the AP-1B function in epithelial cells including the analysis of protein sorting in LLC-PK1 cells lines, immunoprecipitation of cargo proteins after chemical crosslinking to AP-1B, and radioactive pulse-chase experiments in MDCK cells depleted of the AP-1B subunit μ1B.

An ACAP1 coat complex acting in endocytic recycling

A key function of coat proteins is the sorting of protein cargoes into intracellular transport pathways. For many years, however, it has been unclear whether this role of coat proteins would apply to pathways of endocytic recycling. This issue has been clarified in recent years through the identification of multiple coat complexes acting in the recycling pathways. Leading this charge have been studies on a coat complex defined by ACAP1 (adenosine diphosphate ribosylation factor GTPase-activating proteins with Coiled-coil, Ankryin repeat and PH domains 1), which acts in the sorting of cargoes at the recycling endosome for their return to the plasma membrane. This chapter describes the methods used to characterize this role of ACAP1.

Recycling of galectin-3 in epithelial cells

Galectins, a family of β-galactoside binding proteins, do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. They use a so-called unconventional secretion mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. The β-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium. The lectin re-enters the cell by non-clathrin mediated endocytosis and passages through endosomal organelles. This internalized galectin-3 plays an important role in apical protein trafficking by directing the subcellular targeting of apical glycoproteins via oligomerization into high molecular weight clusters, a process that can be fine-tuned by changes in the environmental pH. Following release at the apical plasma membrane, the lectin can reenter the cell for another round of recycling and apical protein sorting. This review will briefly address galectin-3-functions in epithelia and focus on distinct phases in apical recycling of the lectin.

Basolateral sorting of chloride channel 2 is mediated by interactions between a dileucine motif and the clathrin adaptor AP-1

ClC-2 is a ubiquitous chloride channel that regulates cell volume, ion transport, and acid-base balance. Mice knocked out for ClC-2 are blind and sterile. Basolateral localization of ClC-2 in epithelia is mediated by the interaction of a dileucine motif with a highly conserved pocket in the γ1-σ1A hemicomplex of AP-1.

In spite of the many key cellular functions of chloride channels, the mechanisms that mediate their subcellular localization are largely unknown. ClC-2 is a ubiquitous chloride channel usually localized to the basolateral domain of epithelia that regulates cell volume, ion transport, and acid–base balance; mice knocked out for ClC-2 are blind and sterile. Previous work suggested that CLC-2 is sorted basolaterally by TIFS⁸¹²LL, a dileucine motif in CLC-2's C-terminal domain. However, our in silico modeling of ClC-2 suggested that this motif was buried within the channel's dimerization interface and identified two cytoplasmically exposed dileucine motifs, ESMI⁶²³LL and QVVA⁶³⁵LL, as candidate sorting signals. Alanine mutagenesis and trafficking assays support a scenario in which ESMI⁶²³LL acts as the authentic basolateral signal of ClC-2. Silencing experiments and yeast three-hybrid assays demonstrated that both ubiquitous (AP-1A) and epithelium-specific (AP-1B) forms of the tetrameric clathrin adaptor AP-1 are capable of carrying out basolateral sorting of ClC-2 through interactions of ESMI⁶²³LL with a highly conserved pocket in their γ1-σ1A hemicomplex.

Tyrosine motifs are required for prestin basolateral membrane targeting

Prestin is targeted to the lateral wall of outer hair cells (OHCs) where its electromotility is critical for cochlear amplification. Using MDCK cells as a model system for polarized epithelial sorting, we demonstrate that prestin uses tyrosine residues, in a YXXΦ motif, to target the basolateral surface. Both Y520 and Y667 are important for basolateral targeting of prestin. Mutation of these residues to glutamine or alanine resulted in retention within the Golgi and delayed egress from the Golgi in Y667Q. Basolateral targeting is restored upon mutation to phenylalanine suggesting the importance of a phenol ring in the tyrosine side chain. We also demonstrate that prestin targeting to the basolateral surface is dependent on AP1B (μ1B), and that prestin uses transferrin containing early endosomes in its passage from the Golgi to the basolateral plasma membrane. The presence of AP1B (μ1B) in OHCs, and parallels between prestin targeting to the basolateral surface of OHCs and polarized epithelial cells suggest that outer hair cells resemble polarized epithelia rather than neurons in this important phenotypic measure.

Basolateral sorting of the Mg²⁺ transporter CNNM4 requires interaction with AP-1A and AP-1B

Ancient conserved domain protein/cyclin M (CNNM) 4 is an evolutionarily conserved Mg(2+) transporter that localizes at the basolateral membrane of the intestinal epithelia. Here, we show the complementary importance of clathrin adaptor protein (AP) complexes AP-1A and AP-1B in basolateral sorting of CNNM4. We first confirmed the basolateral localization of both endogenous and ectopically expressed CNNM4 in Madin-Darby Canine Kidney cells, which form highly polarized epithelia in culture. Single knockdown of μ1B, a cargo-recognition subunit of AP-1B, did not affect basolateral localization, but simultaneous knockdown of the μ1A subunit of AP-1A abrogated localization. Mutational analyses showed the importance of three conserved dileucine motifs in CNNM4 for both basolateral sorting and interaction with μ1A and μ1B. These results imply that CNNM4 is sorted to the basolateral membrane by the complementary function of AP-1A and AP-1B.

The Role of the Clathrin Adaptor AP-1: Polarized Sorting and Beyond

The selective transport of proteins or lipids by vesicular transport is a fundamental process supporting cellular physiology. The budding process involves cargo sorting and vesicle formation at the donor membrane and constitutes an important process in vesicular transport. This process is particularly important for the polarized sorting in epithelial cells, in which the cargo molecules need to be selectively sorted and transported to two distinct destinations, the apical or basolateral plasma membrane. Adaptor protein (AP)-1, a member of the AP complex family, which includes the ubiquitously expressed AP-1A and the epithelium-specific AP-1B, regulates polarized sorting at the trans-Golgi network and/or at the recycling endosomes. A growing body of evidence, especially from studies using model organisms and animals, demonstrates that the AP-1-mediated polarized sorting supports the development and physiology of multi-cellular units as functional organs and tissues (e.g., cell fate determination, inflammation and gut immune homeostasis). Furthermore, a possible involvement of AP-1B in the pathogenesis of human diseases, such as Crohn's disease and cancer, is now becoming evident. These data highlight the significant contribution of AP-1 complexes to the physiology of multicellular organisms, as master regulators of polarized sorting in epithelial cells.

Plasma membrane protein polarity and trafficking in RPE cells: past, present and future

The retinal pigment epithelium (RPE) comprises a monolayer of polarized pigmented epithelial cells that is strategically interposed between the neural retina and the fenestrated choroid capillaries. The RPE performs a variety of vectorial transport functions (water, ions, metabolites, nutrients and waste products) that regulate the composition of the subretinal space and support the functions of photoreceptors (PRs) and other cells in the neural retina. To this end, RPE cells display a polarized distribution of channels, transporters and receptors in their plasma membrane (PM) that is remarkably different from that found in conventional extra-ocular epithelia, e.g. intestine, kidney, and gall bladder. This characteristic PM protein polarity of RPE cells depends on the interplay of sorting signals in the RPE PM proteins and sorting mechanisms and biosynthetic/recycling trafficking routes in the RPE cell. Although considerable progress has been made in our understanding of the RPE trafficking machinery, most available data have been obtained from immortalized RPE cell lines that only partially maintain the RPE phenotype and by extrapolation of data obtained in the prototype Madin-Darby Canine Kidney (MDCK) cell line. The increasing availability of RPE cell cultures that more closely resemble the RPE in vivo together with the advent of advanced live imaging microscopy techniques provides a platform and an opportunity to rapidly expand our understanding of how polarized protein trafficking contributes to RPE PM polarity.

Global ablation of the mouse Rab11a gene impairs early embryogenesis and matrix metalloproteinase secretion

Rab11a has been conceived as a prominent regulatory component of the recycling endosome, which acts as a nexus in the endo- and exocytotic networks. The precise in vivo role of Rab11a in mouse embryonic development is unknown. We globally ablated Rab11a and examined the phenotypic and molecular outcomes in Rab11a(null) blastocysts and mouse embryonic fibroblasts. Using multiple trafficking assays and complementation analyses, we determined, among multiple important membrane-associated and soluble cargos, the critical contribution of Rab11a vesicular traffic to the secretion of multiple soluble MMPs. Rab11a(null) embryos were able to properly form normal blastocysts but died at peri-implantation stages. Our data suggest that Rab11a critically controls mouse blastocyst development and soluble matrix metalloproteinase secretion.

Galectin-4-mediated transcytosis of transferrin receptor

Some native epithelia, for example, retinal pigment epithelium (RPE) and kidney proximal tubule (KPT), constitutively lack the basolateral sorting adaptor AP-1B; this results in many basolateral plasma membrane proteins being repositioned to the apical domain, where they perform essential functions for their host organs. We recently reported the underlying apical polarity reversal mechanism: in the absence of AP-1B-mediated basolateral sorting, basolateral proteins are shuttled to the apical plasma membrane through a transcytotic pathway mediated by the plus-end kinesin KIF16B. Here, we demonstrate that this apical transcytotic pathway requires apical sorting of basolateral proteins, which is mediated by apical signals and galectin-4. Using RPE and KPT cell lines, and AP-1B-knockdown MDCK cells, we show that mutation of the N-glycan linked to N727 in the basolateral marker transferrin receptor (TfR) or knockdown of galectin-4 inhibits TfR transcytosis to apical recycling endosomes and the apical plasma membrane, and promotes TfR lysosomal targeting and subsequent degradation. Our results report a new role of galectins in basolateral to apical epithelial transcytosis.

Physical and functional links between anion exchanger-1 and sodium pump

Anion exchanger-1 (AE1) mediates chloride-bicarbonate exchange across the plasma membranes of erythrocytes and, via a slightly shorter transcript, kidney epithelial cells. On an omnivorous human diet, kidney AE1 is mainly active basolaterally in α-intercalated cells of the collecting duct, where it is functionally coupled with apical proton pumps to maintain normal acid-base homeostasis. The C-terminal tail of AE1 has an important role in its polarized membrane residency. We have identified the β1 subunit of Na(+),K(+)-ATPase (sodium pump) as a binding partner for AE1 in the human kidney. Kidney AE1 and β1 colocalized in renal α-intercalated cells and coimmunoprecipitated (together with the catalytic α1 subunit of the sodium pump) from human kidney membrane fractions. ELISA and fluorescence titration assays confirmed that AE1 and β1 interact directly, with a Kd value of 0.81 μM. GST-AE1 pull-down assays using human kidney membrane proteins showed that the last 11 residues of AE1 are important for β1 binding. siRNA-induced knockdown of β1 in cell culture resulted in a significant reduction in kidney AE1 levels at the cell membrane, whereas overexpression of kidney AE1 increased cell surface sodium pump levels. Notably, membrane staining of β1 was reduced throughout collecting ducts of AE1-null mouse kidney, where increased fractional excretion of sodium has been reported. These data suggest a requirement of β1 for proper kidney AE1 membrane residency, and that activities of AE1 and the sodium pump are coregulated in kidney.

Organization and execution of the epithelial polarity programme

Epithelial cells require apical-basal plasma membrane polarity to carry out crucial vectorial transport functions and cytoplasmic polarity to generate different cell progenies for tissue morphogenesis. The establishment and maintenance of a polarized epithelial cell with apical, basolateral and ciliary surface domains is guided by an epithelial polarity programme (EPP) that is controlled by a network of protein and lipid regulators. The EPP is organized in response to extracellular cues and is executed through the establishment of an apical-basal axis, intercellular junctions, epithelial-specific cytoskeletal rearrangements and a polarized trafficking machinery. Recent studies have provided insight into the interactions of the EPP with the polarized trafficking machinery and how these regulate epithelial polarization and depolarization.

Regulation of potassium channel trafficking in the distal nephron

PURPOSE OF REVIEW

Potassium channels in the distal nephron are precisely controlled to regulate potassium secretion in accord with physiological demands. In recent years, it has become evident that membrane trafficking processes play a fundamental role. This short review highlights recent developments in elucidating the underlying mechanisms.

RECENT FINDINGS

Novel sorting signals in the renal potassium channels, and the elusive intracellular trafficking machinery that read and act on these signals have recently been identified. These new discoveries reveal that independent signals sequentially interact with different intracellular sorting, retention and internalization machineries to appropriately ferry the channels to and from the apical and basolateral membrane domains in sufficient numbers to regulate potassium balance.

SUMMARY

A new understanding of the basic mechanisms that control potassium channel density at polarized membrane domains has emerged, providing new insights into how potassium balance is achieved and how it goes awry in disease.

Using replication defective viruses to analyze membrane trafficking in polarized epithelial cells

Epithelial cells in culture, especially once they are polarized, are extremely hard to manipulate by transient transfection methods. The use of replication defective adenoviruses for gene expression or replication defective retroviruses or lentiviruses to express shRNA for gene knockdown provides efficient tools to manipulate gene expression patterns even in hard-to-transfect cell lines. One of the advantages of using defective adenoviruses for gene expression is that once the virus has been generated, it can easily be applied to a wide variety of cells. In addition, replication defective retro- and lentiviruses are used to stably deplete proteins from cell lines, which subsequently may be used for analyzing the polarized surface delivery of receptors that may be expressed using defective adenoviruses. The latter approach is especially useful if the expressed shRNA also encodes GFP for easy assessment of shRNA-expressing cells. Thus the use of defective viruses in epithelial cell research is convenient. This makes a detailed infection protocol a research tool that would be valuable to many laboratories. Here we describe in detail how cells are infected with defective retro- or lentiviruses and subsequently selected for stable gene knockdown. We then describe how these cells may be used for infection with defective adenoviruses and the subsequent analyses.

Diverse autophagosome membrane sources coalesce in recycling endosomes

Autophagic protein degradation is mediated by autophagosomes that fuse with lysosomes, where their contents are degraded. The membrane origins of autophagosomes may involve multiple sources. However, it is unclear if and where distinct membrane sources fuse during autophagosome biogenesis. Vesicles containing mATG9, the only transmembrane autophagy protein, are seen in many sites, and fusions with other autophagic compartments have not been visualized in mammalian cells. We observed that mATG9 traffics from the plasma membrane to recycling endosomes in carriers that appear to be routed differently from ATG16L1-containing vesicles, another source of autophagosome membrane. mATG9- and ATG16L1-containing vesicles traffic to recycling endosomes, where VAMP3-dependent heterotypic fusions occur. These fusions correlate with autophagosome formation, and both processes are enhanced by perturbing membrane egress from recycling endosomes. Starvation, a primordial autophagy activator, reduces membrane recycling from recycling endosomes and enhances mATG9-ATG16L1 vesicle fusion. Thus, this mechanism may fine-tune physiological autophagic responses.

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mATG9 traffics from the plasma membrane to recycling endosomes

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mATG9 vesicles fuse with ATG16L1 vesicles in recycling endosomes

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VAMP3, Rab11, myosin Vb, and starvation regulate mATG9-ATG16L1 vesicle fusion

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mATG9-ATG16L1 vesicle fusions regulate autophagosome formation

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Aquaporin-3 and aquaporin-4 are sorted differently and separately in the trans-Golgi network

Aquaporin-3 (AQP3) and aquaporin-4 (AQP4) are homologous proteins expressed in the basolateral plasma membrane of kidney collecting duct principal cells, where they mediate the exit pathway for apically reabsorbed water. Although both proteins are localized to the same plasma membrane domain, it is unknown if they are sorted together in the Golgi, or arrive in the same or different vesicles at the plasma membrane. We addressed these questions using high resolution deconvolution imaging, spinning disk and laser scanning confocal microscopy of cells expressing AQP3 and AQP4. AQP3 and AQP4 were observed mostly in separate post-Golgi carriers, and spinning disk microscopy showed that most of AQP3 and AQP4 were delivered to the plasma membrane in separate vesicles. In contrast, VSV-G and LDL-R, two well-charcterized basolateral proteins, co-localized to a high degree in the same post-Golgi carriers, indicating that the differential sorting of AQP3 and AQP4 is specific and regulated. Significantly, a chimeric AQP3 containing the AQP4 cytoplasmic tails co-localized with AQP4 in post-Golgi vesicles. These results indicate that AQP3 and AQP4 are separated into different post-Golgi carriers based on different cytoplasmic domain sorting signals, and are then delivered separately to the plasma membrane.

The kinesin KIF16B mediates apical transcytosis of transferrin receptor in AP-1B-deficient epithelia

Polarized epithelial cells take up nutrients from the blood through receptors that are endocytosed and recycle back to the basolateral plasma membrane (PM) utilizing the epithelial-specific clathrin adaptor AP-1B. Some native epithelia lack AP-1B and therefore recycle cognate basolateral receptors to the apical PM, where they carry out important functions for the host organ. Here, we report a novel transcytotic pathway employed by AP-1B-deficient epithelia to relocate AP-1B cargo, such as transferrin receptor (TfR), to the apical PM. Lack of AP-1B inhibited basolateral recycling of TfR from common recycling endosomes (CRE), the site of function of AP-1B, and promoted its transfer to apical recycling endosomes (ARE) mediated by the plus-end kinesin KIF16B and non-centrosomal microtubules, and its delivery to the apical membrane mediated by the small GTPase rab11a. Hence, our experiments suggest that the apical recycling pathway of epithelial cells is functionally equivalent to the rab11a-dependent TfR recycling pathway of non-polarized cells. They define a transcytotic pathway important for the physiology of native AP-1B-deficient epithelia and report the first microtubule motor involved in transcytosis.

Retromer maintains basolateral distribution of the type II TGF-β receptor via the recycling endosome

After basolateral (BL) cell surface delivery, retromer promotes type II TGF-β receptor exit and recycling to the BL plasma membrane. In the absence of retromer, however, type II receptors aberrantly sort and are mislocalized such that both BL and apical expression is observed independent of the Rab11-positive apical recycling endosome.

Transforming growth factor β (TGF-β) is critical for the development and maintenance of epithelial structures. Because receptor localization and trafficking affect the cellular and organismal response to TGF-β, the present study was designed to address how such homeostatic control is regulated. To that end, we identify a new role for the mammalian retromer complex in maintaining basolateral plasma membrane expression of the type II TGF-β receptor (TβRII). Retromer and TβRII associate in the presence or absence of TGF-β ligand. After retromer knockdown, although TβRII internalization and trafficking to a Rab5-positive compartment occur as in wild-type cells, receptor recycling is inhibited. This results in TβRII mislocalization from the basolateral to both the basolateral and apical plasma membranes independent of Golgi transit and the Rab11-positive apical recycling endosome. The data support a model in which, after initial basolateral TβRII delivery, steady-state polarized TβRII expression is maintained by retromer/TβRII binding and delivery to the common recycling endosome.