An internal deletion in the cytoplasmic tail reverses the apical localization of human NGF receptor in transfected MDCK cells

PolarProtPred: Predicting apical and basolateral localization of transmembrane proteins using putative short linear motifs and deep learning

Motivation

Cell polarity refers to the asymmetric organization of cellular components in various cells. Epithelial cells are the best-known examples of polarized cells, featuring apical and basolateral membrane domains. Mounting evidence suggests that short linear motifs play a major role in protein trafficking to these domains, although the exact rules governing them are still elusive.

Results

In this study we prepared neural networks that capture recurrent patterns to classify transmembrane proteins localizing into apical and basolateral membranes. Asymmetric expression of drug transporters results in vectorial drug transport, governing the pharmacokinetics of numerous substances, yet the data on how proteins are sorted in epithelial cells is very scattered. The provided method may offer help to experimentalists to identify or better characterize molecular networks regulating the distribution of transporters or surface receptors (including viral entry receptors like that of COVID-19).

Availability

The prediction server PolarProtPred is available at http://polarprotpred.ttk.hu.

Supplementary information

Supplementary data are available at Bioinformatics online.

Listening to the Whispers in Neuroimmune Crosstalk: A Comprehensive Workflow to Investigate Neurotrophin Receptor p75NTR Under Endogenous, Low Abundance Conditions

Inflammatory conditions are critically influenced by neuroimmune crosstalk. Cytokines and neurotrophic factors shape the responses of both nervous and immune systems. Although much progress has been made, most findings to date are based on expression of recombinant (tagged) proteins. The examination of receptor interactions by immunoprecipitation (IP) at endogenous levels provides further insight into the more subtle regulations of immune responses. Here, we present a comprehensive workflow and an optimized IP protocol that provide step-by-step instructions to investigate neurotrophin receptor p75NTR at endogenous, low abundance levels: from lysate preparation and confirmation of receptor expression to antibody validation and successful detection of protein-protein interactions. We employ human melanoma cell line A375 to validate specific antibodies and IP conditions, and apply these methods to explore p75NTR interactions in human leukemic plasmacytoid dendritic cell line PMDC05 detecting 14-3-3ϵ:p75NTR interaction in this cell type. With p75NTR as an exemplary protein, our approach provides a strategy to detect specific interaction partners even under endogenous, low abundance expression conditions.

Polarized cells display asymmetric release of extracellular vesicles

Extracellular vesicles (EVs), a broad term for the lipid microparticles known as microvesicles and exosomes, are discharged by cells into their surrounding space. Microvesicles are discharged upon outward plasma membrane budding, while exosomes are secreted after multivesicular body (MVB) fusion with the plasma membrane. The majority of information regarding EV biology comes from studies performed in non-polarized cells. Here we characterize EV release in polarized cells. We found a substantial asymmetry in the number and composition of EVs produced and released from the apical membrane of epithelial cells as compared to the basolateral membrane. We showed that the quantitative difference is related to the polarized distribution of two phosphoinositide species between the two cell surfaces and that the peculiar biochemical composition of resultant EVs reflects their site of origin. In particular, apical and basolateral exosomes may derive from distinct classes of MVBs originating from and fusing with the same plasma membrane. We identify VAMP8/Endobrevin as a regulator of the basolateral release of exosomes, whereas the mechanism responsible for apical EV release requires further study.

Trafficking Ion Transporters to the Apical Membrane of Polarized Intestinal Enterocytes

Epithelial cells lining the gastrointestinal tract require distinct apical and basolateral domains to function properly. Trafficking and insertion of enzymes and transporters into the apical brush border of intestinal epithelial cells is essential for effective digestion and absorption of nutrients. Specific critical ion transporters are delivered to the apical brush border to facilitate fluid and electrolyte uptake. Maintenance of these apical transporters requires both targeted delivery and regulated membrane recycling. Examination of altered apical trafficking in patients with Microvillus Inclusion disease caused by inactivating mutations in MYO5B has led to insights into the regulation of apical trafficking by elements of the apical recycling system. Modeling of MYO5B loss in cell culture and animal models has led to recognition of Rab11a and Rab8a as critical regulators of apical brush border function. All of these studies show the importance of apical membrane trafficking dynamics in maintenance of polarized epithelial cell function.

Cystic Fibrosis, Cystic Fibrosis Transmembrane Conductance Regulator and Drugs: Insights from Cellular Trafficking

The eukaryotic cell is organized into membrane-delineated compartments that are characterized by specific cadres of proteins sustaining biochemically distinct cellular processes. The appropriate subcellular localization of proteins is key to proper organelle function and provides a physiological context for cellular processes. Disruption of normal trafficking pathways for proteins is seen in several genetic diseases, where a protein's absence for a specific subcellular compartment leads to organelle disruption, and in the context of an individual, a disruption of normal physiology. Importantly, several drug therapies can also alter protein trafficking, causing unwanted side effects. Thus, a deeper understanding of trafficking pathways needs to be appreciated as novel therapeutic modalities are proposed. Despite the promising efficacy of novel therapeutic agents, the intracellular bioavailability of these compounds has proved to be a potential barrier, leading to failures in treatments for various diseases and disorders. While endocytosis of drug moieties provides an efficient means of getting material into cells, the subsequent release and endosomal escape of materials into the cytosol where they need to act has been a barrier. An understanding of cellular protein/lipid trafficking pathways has opened up strategies for increasing drug bioavailability. Approaches to enhance endosomal exit have greatly increased the cytosolic bioavailability of drugs and will provide a means of investigating previous drugs that may have been shelved due to their low cytosolic concentration.

Disparate Regulatory Mechanisms Control Fat3 and P75NTR Protein Transport through a Conserved Kif5-Interaction Domain

Directed transport delivers proteins to specific cellular locations and is one mechanism by which cells establish and maintain polarized cellular architectures. The atypical cadherin Fat3 directs the polarized extension of dendrites in retinal amacrine cells by influencing the distribution of cytoskeletal regulators during retinal development, however the mechanisms regulating the distribution of Fat3 remain unclear. We report a novel Kinesin/Kif5 Interaction domain (Kif5-ID) in Fat3 that facilitates Kif5B binding, and determines the distribution of Fat3 cytosolic domain constructs in neurons and MDCK cells. The Kif5-ID sequence is conserved in the neurotrophin receptor P75NTR, which also binds Kif5B, and Kif5-ID mutations similarly result in P75NTR mislocalization. Despite these similarities, Kif5B-mediated protein transport is differentially regulated by these two cargos. For Fat3, the Kif5-ID is regulated by alternative splicing, and the timecourse of splicing suggests that the distribution of Fat3 may switch between early and later stages of retinal development. In contrast, P75NTR binding to Kif5B is enhanced by tyrosine phosphorylation and thus has the potential to be dynamically regulated on a more rapid time scale.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Multiple motifs regulate apical sorting of p75 via a mechanism that involves dimerization and higher-order oligomerization

The sorting signals that direct proteins to the apical surface of polarized epithelial cells are complex and can include posttranslational modifications, such as N- and O-linked glycosylation. Efficient apical sorting of the neurotrophin receptor p75 is dependent on its O-glycosylated membrane proximal stalk, but how this domain mediates targeting is unknown. Protein oligomerization or clustering has been suggested as a common step in the segregation of all apical proteins. Like many apical proteins, p75 forms dimers, and we hypothesized that formation of higher-order clusters mediated by p75 dimerization and interactions of the stalk facilitate its apical sorting. Using fluorescence fluctuation techniques (photon-counting histogram and number and brightness analyses) to study p75 oligomerization status in vivo, we found that wild-type p75-green fluorescent protein forms clusters in the trans-Golgi network (TGN) but not at the plasma membrane. Disruption of either the dimerization motif or the stalk domain impaired both clustering and polarized delivery. Manipulation of O-glycan processing or depletion of multiple galectins expressed in Madin-Darby canine kidney cells had no effect on p75 sorting, suggesting that the stalk domain functions as a structural prop to position other determinants in the lumenal domain of p75 for oligomerization. Additionally, a p75 mutant with intact dimerization and stalk motifs but with a dominant basolateral sorting determinant (Δ250 mutant) did not form oligomers, consistent with a requirement for clustering in apical sorting. Artificially enhancing dimerization restored clustering to the Δ250 mutant but was insufficient to reroute this mutant to the apical surface. Together these studies demonstrate that clustering in the TGN is required for normal biosynthetic apical sorting of p75 but is not by itself sufficient to reroute a protein to the apical surface in the presence of a strong basolateral sorting determinant. Our studies shed new light on the hierarchy of polarized sorting signals and on the mechanisms by which newly synthesized proteins are segregated in the TGN for eventual apical delivery.

PIP5KIβ selectively modulates apical endocytosis in polarized renal epithelial cells

Localized synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] at clathrin coated pits (CCPs) is crucial for the recruitment of adaptors and other components of the internalization machinery, as well as for regulating actin dynamics during endocytosis. PtdIns(4,5)P₂ is synthesized from phosphatidylinositol 4-phosphate by any of three phosphatidylinositol 5-kinase type I (PIP5KI) isoforms (α, β or γ). PIP5KIβ localizes almost exclusively to the apical surface in polarized mouse cortical collecting duct cells, whereas the other isoforms have a less polarized membrane distribution. We therefore investigated the role of PIP5KI isoforms in endocytosis at the apical and basolateral domains. Endocytosis at the apical surface is known to occur more slowly than at the basolateral surface. Apical endocytosis was selectively stimulated by overexpression of PIP5KIβ whereas the other isoforms had no effect on either apical or basolateral internalization. We found no difference in the affinity for PtdIns(4,5)P₂-containing liposomes of the PtdIns(4,5)P₂ binding domains of epsin and Dab2, consistent with a generic effect of elevated PtdIns(4,5)P₂ on apical endocytosis. Additionally, using apical total internal reflection fluorescence imaging and electron microscopy we found that cells overexpressing PIP5KIβ have fewer apical CCPs but more internalized coated structures than control cells, consistent with enhanced maturation of apical CCPs. Together, our results suggest that synthesis of PtdIns(4,5)P₂ mediated by PIP5KIβ is rate limiting for apical but not basolateral endocytosis in polarized kidney cells. PtdIns(4,5)P₂ may be required to overcome specific structural constraints that limit the efficiency of apical endocytosis.

Basolateral sorting of the coxsackie and adenovirus receptor through interaction of a canonical YXXPhi motif with the clathrin adaptors AP-1A and AP-1B

The coxsackie and adenovirus receptor (CAR) plays key roles in epithelial barrier function at the tight junction, a localization guided in part by a tyrosine-based basolateral sorting signal, (318)YNQV(321). Sorting motifs of this type are known to route surface receptors into clathrin-mediated endocytosis through interaction with the medium subunit (μ2) of the clathrin adaptor AP-2, but how they guide new and recycling membrane proteins basolaterally is unknown. Here, we show that YNQV functions as a canonical YxxΦ motif, with both Y318 and V321 required for the correct basolateral localization and biosynthetic sorting of CAR, and for interaction with a highly conserved pocket in the medium subunits (μ1A and μ1B) of the clathrin adaptors AP-1A and AP-1B. Knock-down experiments demonstrate that AP-1A plays a role in the biosynthetic sorting of CAR, complementary to the role of AP-1B in basolateral recycling of this receptor. Our study illustrates how two clathrin adaptors direct basolateral trafficking of a plasma membrane protein through interaction with a canonical YxxΦ motif.

Identification of a novel mono-leucine basolateral sorting motif within the cytoplasmic domain of amphiregulin

Epithelial cells establish apical and basolateral (BL) membranes with distinct protein and lipid compositions. To achieve this spatial asymmetry, the cell utilizes a variety of mechanisms for differential sorting, delivery and retention of cell surface proteins. The EGF receptor (EGFR) and its ligand, amphiregulin (AREG), are transmembrane proteins delivered to the BL membrane in polarized epithelial cells. Herein, we show that the cytoplasmic domain of AREG (ACD) contains dominant BL sorting information; replacement of the cytoplasmic domain of apically targeted nerve growth factor receptor with the ACD redirects the chimera to the BL surface. Using sequential truncations and site-directed mutagenesis of the ACD, we identify a novel BL sorting motif consisting of a single leucine C-terminal to an acidic cluster (EEXXXL). In adaptor protein (AP)-1B-deficient cells, newly synthesized AREG is initially delivered to the BL surface as in AP-1B-expressing cells. However, in these AP-1B-deficient cells, recycling of AREG back to the BL surface is compromised, leading to its appearance at the apical surface. These results show that recycling, but not delivery, of AREG to the BL surface is AP-1B dependent.

Basolateral sorting signals regulating tissue-specific polarity of heteromeric monocarboxylate transporters in epithelia

Many solute transporters are heterodimers composed of non-glycosylated catalytic and glycosylated accessory subunits. These transporters are specifically polarized to the apical or basolateral membranes of epithelia, but this polarity may vary to fulfill tissue-specific functions. To date, the mechanisms regulating the tissue-specific polarity of heteromeric transporters remain largely unknown. Here, we investigated the sorting signals that determine the polarity of three members of the proton-coupled monocarboxylate transporter (MCT) family, MCT1, MCT3 and MCT4, and their accessory subunit CD147. We show that MCT3 and MCT4 harbor strong redundant basolateral sorting signals (BLSS) in their C-terminal cytoplasmic tails that can direct fusion proteins with the apical marker p75 to the basolateral membrane. In contrast, MCT1 lacks a BLSS and its polarity is dictated by CD147, which contains a weak BLSS that can direct Tac, but not p75 to the basolateral membrane. Knockdown experiments in MDCK cells indicated that basolateral sorting of MCTs was clathrin-dependent but clathrin adaptor AP1B-independent. Our results explain the consistently basolateral localization of MCT3 and MCT4 and the variable localization of MCT1 in different epithelia. They introduce a new paradigm for the sorting of heterodimeric transporters in which a hierarchy of apical and BLSS in the catalytic and/or accessory subunits regulates their tissue-specific polarity.

Epithelial cell surface polarity: the early steps

Establishment and maintenance of epithelial cell surface polarity is of vital importance for the correct function of transporting epithelia. To maintain normal cell function, the distribution of apical and basal-lateral proteins is highly regulated and defects in expression levels or plasma membrane targeting can have severe consequences. It has been shown recently that initiation of cell-surface polarity occurs immediately upon cell-cell contact, and requires components of the lateral targeting patch, the Exocyst and the lateral SNARE complex to specify delivery of basolateral proteins to the site of cell-cell adhesion. The Exocyst and SNARE complex are present in the cytoplasm in single epithelial cells before adhesion. Upon initial cell-cell adhesion, E-cadherin accumulates at the forming contact between cells. Shortly hereafter, components of the lateral targeting patch, the Exocyst and the lateral SNARE complex, co-localize with E-cadherin at the forming contact, where they function in specifying the delivery of basal-lateral.

Clathrin is a key regulator of basolateral polarity

Clathrin-coated vesicles are vehicles for intracellular trafficking in all nucleated cells, from yeasts to humans. Many studies have demonstrated their essential roles in endocytosis and cellular signalling processes at the plasma membrane. By contrast, very few of their non-endocytic trafficking roles are known, the best characterized being the transport of hydrolases from the Golgi complex to the lysosome. Here we show that clathrin is required for polarity of the basolateral plasma membrane proteins in the epithelial cell line MDCK. Clathrin knockdown depolarized most basolateral proteins, by interfering with their biosynthetic delivery and recycling, but did not affect the polarity of apical proteins. Quantitative live imaging showed that chronic and acute clathrin knockdown selectively slowed down the exit of basolateral proteins from the Golgi complex, and promoted their mis-sorting into apical carrier vesicles. Our results demonstrate a broad requirement for clathrin in basolateral protein trafficking in epithelial cells.

FERM protein EPB41L5 is a novel member of the mammalian CRB-MPP5 polarity complex

Cell polarity is induced and maintained by separation of the apical and basolateral domains through specialized cell-cell junctions. The Crumbs protein and its binding partners are involved in formation and stabilization of adherens junctions. In this study, we describe a novel component of the mammalian Crumbs complex, the FERM domain protein EPB41L5, which associates with the intracellular domains of all three Crumbs homologs through its FERM domain. Surprisingly, the same FERM domain is involved in binding to the HOOK domain of MPP5/PALS1, a previously identified interactor of Crumbs. Co-expression and co-localization studies suggested that in several epithelial derived tissues Epb4.1l5 interacts with at least one Crumbs homolog, and with Mpp5. Although at early embryonic stages Epb4.1l5 is found at the basolateral membrane compartment, in adult tissues it co-localizes at the apical domain with Crumbs proteins and Mpp5. Overexpression of Epb4.1l5 in polarized MDCK cells affects tightness of cell junctions and results in disorganization of the tight junction markers ZO-1 and PATJ. Our results emphasize the importance of a conserved Crumbs-MPP5-EPB41L5 polarity complex in mammals.

Efficient electroporation of DNA and protein into confluent and differentiated epithelial cells in culture

Electroporation-mediated delivery of molecules is a procedure widely used for transfecting complementary DNA in bacteria, mammalian and plant cells. This technique has proven very efficient for the introduction of macromolecules into cells in suspension culture and even into cells in their native tissue environment, e.g. retina and embryonic tissues. However, in spite of several attempts to date, there are no well-established procedures to electroporate polarized epithelial cells adhering to a tissue culture substrate (glass, plastic or filter). We report here the development of a simple procedure that uses available commercial equipment and works efficiently and reproducibly for a variety of epithelial cell lines in culture.

Protein sorting in the Golgi complex: Shifting paradigms

The paradigms for transport along the biosynthetic route have changed dramatically over the past 15 years. Unlike the situation 15 years ago, the current paradigm involves sorting signals practically at every step of the pathway. In particular, at the exit from the Golgi complex, apical, basolateral and lysosomal targeting signals result in the generation of a variety of routes. Furthermore, it is now quite clear that not all sorting in the biosynthetic route occurs in the Golgi complex or the Trans Golgi Network (TGN). Sorting may occur distally to the Golgi, in recycling endosomes or in budded tubulosaccular structures, or it may occur proximally to the Golgi complex, at the exit from the ER. Several adaptors are candidates to sort apical and basolateral proteins but only AP1B and AP4 are currently involved. Progress is fast and future work should elucidate many of the open questions.

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.

Functional interaction between p75NTR and TrkA: the endocytic trafficking of p75NTR is driven by TrkA and regulates TrkA-mediated signalling

The topology and trafficking of receptors play a key role in their signalling capability. Indeed, receptor function is related to the microenvironment inside the cell, where specific signalling molecules are compartmentalized. The response to NGF (nerve growth factor) is strongly dependent on the trafficking of its receptor, TrkA. However, information is still scarce about the role of the cellular localization of the TrkA co-receptor, p75NTR (where NTR is neurotrophin receptor), following stimulation by NGF. It has been shown that these two receptors play a key role in epithelial tissue and in epithelial-derived tumours, where the microenvironment at the plasma membrane is defined by the presence of tight junctions. Indeed, in thyroid carcinomas, rearrangements of TrkA are frequently found, which produce TrkA mutants that are localized exclusively in the cytoplasm. We used a thyroid cellular model in which it was possible to dissect the trafficking of the two NGF receptors upon neurotrophin stimulation. In FRT (Fischer rat thyroid) cells, endogenous TrkA is localized exclusively on the basolateral surface, while transfected p75NTR is selectively distributed on the apical membrane. This cellular system enabled us to selectively stimulate either p75NTR or TrkA and to analyse the role of receptor trafficking in their signalling capability. We found that, after binding to NGF, p75NTR was co-immunoprecipitated with TrkA and was transcytosed at the basolateral membrane. We showed that the TrkA-p75NTR interaction is necessary for this relocation of p75NTR to the basolateral side. Interestingly, TrkA-specific stimulation by basolateral NGF loading also induced the TrkA-p75NTR interaction and subsequent p75NTR transcytosis at the basolateral surface. Moreover, specific stimulation of p75NTR by NGF activated TrkA and the MAPK (mitogen-activated protein kinase) pathway. Our data indicate that TrkA regulates the subcellular localization of p75NTR upon stimulation with neurotrophins, thus affecting the topology of the signal transduction molecules, driving the activation of a specific signal transduction pathway.

Basolateral localisation of KCNQ1 potassium channels in MDCK cells: molecular identification of an N-terminal targeting motif

KCNQ1 potassium channels are expressed in many epithelial tissues as well as in the heart. In epithelia KCNQ1 channels play an important role in salt and water transport and the channel has been reported to be located apically in some cell types and basolaterally in others. Here we show that KCNQ1 channels are located basolaterally when expressed in polarised MDCK cells. The basolateral localisation of KCNQ1 is not affected by co-expression of any of the five KCNE β-subunits. We characterise two independent basolateral sorting signals present in the N-terminal tail of KCNQ1. Mutation of the tyrosine residue at position 51 resulted in a non-polarized steady-state distribution of the channel. The importance of tyrosine 51 in basolateral localisation was emphasized by the fact that a short peptide comprising this tyrosine was able to redirect the p75 neurotrophin receptor, an otherwise apically located protein, to the basolateral plasma membrane. Furthermore, a di-leucine-like motif at residues 38-40 (LEL) was found to affect the basolateral localisation of KCNQ1. Mutation of these two leucines resulted in a primarily intracellular localisation of the channel.

CRB3 binds directly to Par6 and regulates the morphogenesis of the tight junctions in mammalian epithelial cells

Crumbs is an apical transmembrane protein crucial for epithelial morphogenesis in Drosophila melanogaster embryos. A protein with all the characteristics for a Crumbs homologue has been identified from patients suffering from retinitis pigmentosa group 12, but this protein (CRB1) is only expressed in retina and some parts of the brain, both in human and mouse. Here, we describe CRB3, another Crumbs homologue that is preferentially expressed in epithelial tissues and skeletal muscles in human. CRB3 shares the conserved cytoplasmic domain with other Crumbs but exhibits a very short extracellular domain without the EGF- and laminin A-like G repeats present in the other Crumbs. CRB3 is localized to the apical and subapical area of epithelial cells from the mouse and human intestine, suggesting that it could play a role in epithelial morphogenesis. Indeed, expression of CRB3 or of a chimera containing the extracellular domain of the neurotrophin receptor p75NTR and the transmembrane and cytoplasmic domains of CRB3 led to a slower development of functional tight junctions in Madin-Darby canine kidney cells. This phenotype relied on the presence of CRB3 four last amino acids (ERLI) that are involved in a direct interaction with Par6, a regulator of epithelial polarity and tight junction formation. Thus, CRB3, through its cytoplasmic domain and its interactors, plays a role in apical membrane morphogenesis and tight junction regulation.

Spinophilin stabilizes cell surface expression of alpha 2B-adrenergic receptors

The third intracellular (3i) loops of the alpha 2A- and alpha 2B-adrenergic receptor (AR) subtypes are critical for retention of these receptors at the basolateral surface of polarized Madin-Darby canine kidney (MDCKII) cells at steady state. The third intracellular loops of the alpha 2A, alpha 2B, and alpha 2C-AR subtypes interact with spinophilin, a multidomain protein that, like the three alpha 2-AR subtypes, is enriched at the basolateral surface of MDCKII cells. The present studies provide evidence that alpha 2-AR interaction with spinophilin contributes to cell surface stabilization of the receptor. We exploited the unique targeting profile of the alpha 2B-AR subtype in MDCKII cells: random delivery to apical and basolateral surfaces with rapid (t(1/2) < or = 60 min) apical versus slower (t(1/2) = 10-12 h) basolateral turnover. Apical delivery of a spinophilin subdomain containing the alpha 2-AR-interacting region (Sp151-483) by fusion with apically targeted p75NTR extended the half-life of alpha 2B-AR at the apical surface to approximately 3.6 h and eliminated the rapid phase (0-60 min) of alpha 2B-AR turnover on that surface. Furthermore, we examined alpha 2B-AR turnover at the surface of mouse embryo fibroblasts derived from wild type (Sp+/+) or spinophilin knock-out (Sp-/-) mice. Two independent experimental approaches demonstrated that agonist-evoked internalization of HA-alpha 2B-AR was accelerated in mouse embryo fibroblasts derived from Sp-/- mice. These findings are consistent with the interpretation that endogenous spinophilin contributes to the stabilization of alpha 2B-AR and presumably all three alpha2-AR subtypes at the surface of target cells and may act as a scaffold that could link alpha 2-ARs to proteins interacting with spinophilin via other domains.

Basolateral sorting of transforming growth factor-alpha precursor in polarized epithelial cells: characterization of cytoplasmic domain determinants

In polarized Madin-Darby canine kidney (MDCK) cells, newly synthesized transforming growth factor-alpha precursor (proTGFalpha) is directly sorted to the basolateral cell surface where it is sequentially cleaved and released into the basolateral conditioned medium (Dempsey, P.J., Coffey, R.J., J. Biol. Chem. 269 (1994) 16878-16889). In the present study, the role of the proTGFalpha cytoplasmic domain in basolateral sorting has been investigated using deletional and site-directed mutagenesis, as well as chimeric analyses of different TGFalpha constructs stably expressed in MDCK cells. The loss of polarized secretion of a proTGFalpha secretory mutant (TGFsec88) indicated that the proTGFalpha transmembrane and/or cytoplasmic domains contain essential basolateral sorting information. Using reporter chimeras with two apically sorted membrane proteins, p75 neurotrophin growth factor receptor and placental alkaline phosphatase, we show that the proTGFalpha cytoplasmic domain contains dominant basolateral sorting information. Analysis of proTGFalpha cytoplasmic domain truncation and internal deletion mutants, together with site-directed mutagenesis studies within the full-length proTGFalpha cytoplasmic domain, revealed redundant basolateral sorting motifs. Importantly, the C-terminal type I PDZ-binding motif was not required for basolateral sorting as determined by the integrity of basolateral sorting in deletion mutants lacking this motif. ProTGFalpha basolateral sorting may have important consequences for ligand presentation and spatial compartmentalization of epidermal growth factor receptor signaling networks in polarized epithelial cells.

Involvement of glycosylation in the intracellular trafficking of glycoproteins in polarized epithelial cells

The surface of epithelial cells is composed of apical and basolateral domains with distinct structure and function. This polarity is maintained by specific sorting mechanisms occurring in the Trans-Golgi Network. Peptidic signals are responsible for the trafficking via clathrin-coated vesicles by means of an interaction with an adaptor complex (AP). The basolateral targeting is mediated by AP-1B, which is specifically expressed in epithelial cells. In contrast, the apical targeting is proposed to occur via apical raft carriers. It is thought that apically targeted glycoproteins contain glycan signals that would be responsible for their association with rafts and for apical targeting. However, the difficulty in terms of acting specifically on a single step of glycosylation did not allow one to identify such a specific signal. The complete inhibition of the processing of N-glycans by tunicamycin often results in an intracellular accumulation of unfolded proteins in the Golgi. Similarly, inhibition of O-glycosylation can be obtained by competitive substrates which gave a complex pattern of inhibition. Therefore, it is still unknown if glycosylation acts in an indirect manner, i.e. by modifying the folding of the protein, or in a specific manner, such as an association with specific lectins.

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.

Molecular kinetics of nerve growth factor receptor trafficking and activation

A growing body of evidence indicates a close relationship between tyrosine kinase receptor trafficking and signaling. Biochemical and molecular analyses of the expression, fate, and kinetics of membrane trafficking of the nerve growth factor (NGF) receptor TrkA were performed in PC12 cells. Pulse-chase experiments indicate that TrkA is synthesized as a 110-kDa N-glycosylated precursor that leads to the mature 140-kDa form of the receptor with a half-life of conversion of approximately 24 +/- 0.5 min. Neuraminidase digestion shows that modification of the carbohydrate moiety of the receptor by sialylation occurs during maturation. The 140-kDa form is rapidly translocated to the cell surface as assessed by cell surface biotinylation performed on intact PC12 cells. Mature receptor half-life is approximately 138 +/- 4 min and is shortened to 86 +/- 8 min by NGF treatment. Flow cytometric analysis indicates that NGF induces clearing of this receptor from the cell surface within minutes of treatment. The addition of NGF decreases the half-life of cell surface gp140(TrkA) from 100 to 35 min and leads to enhanced lysosomal degradation of the receptor. The process of NGF-induced TrkA internalization is clearly affected by interfering with ligand binding to p75(NTR). An analysis of receptor activation kinetics also shows that receptor signaling primarily takes place from an intracellular location. Together, these data show that the primary effect of NGF treatment is a p75(NTR)-modulated decrease in TrkA transit time at the cell surface.

Basolateral targeting of ERBB2 is dependent on a novel bipartite juxtamembrane sorting signal but independent of the C-terminal ERBIN-binding domain

ERBB2 is a receptor tyrosine kinase present on the basolateral membrane of polarized epithelia and has important functions in organ development and tumorigenesis. Using mutagenic analyses and Madin-Darby canine kidney (MDCK) cells, we have investigated the signals that regulate basolateral targeting of ERBB2. We show that basolateral delivery of ERBB2 is dependent on a novel bipartite juxtamembrane sorting signal residing between Gln-692 and Thr-701. The signal shows only limited sequence homology to known basolateral targeting signals and is both necessary and sufficient for correct sorting of ERBB2. In addition we demonstrate that this motif can function as a dominant basolateral targeting signal by its ability to redirect the apically localized P75 neurotrophin receptor to the basolateral membrane domain of polarized epithelial cells. Interestingly, LLC-PK1 cells, which are deficient for the micro 1B subunit of the AP1B adaptor complex, missort a large proportion of ERBB2 to the apical membrane domain. This missorting can be partially corrected by the introduction of micro 1B, suggesting a possible role for AP1B in ERBB2 endosomal trafficking. Furthermore, we find that the C-terminal ERBIN binding domain of ERBB2 is not necessary for its basolateral targeting in MDCK cells.

Differential recognition of a tyrosine-dependent signal in the basolateral and endocytic pathways of thyroid epithelial cells

Trafficking of receptors is of crucial importance for the physiology of most exocrine and endocrine organs. It is not known yet if the same mechanisms are used for sorting in the exocytic and endocytic pathways in the different epithelial tissues. In this work, we have used a deletion mutant of the human neurotrophin receptor p75(hNTR) that is normally localized on the apical membrane when expressed in Madin-Darby canine kidney cells. This internal 57-amino acid deletion of the cytoplasmic tail leads to a relocation of the protein from the apical to the basolateral membrane and to rapid and efficient endocytosis. These events are mediated by a signal localized within 9 amino acids of the mutated cytoplasmic tail that is strictly dependent on a tyrosine residue (Tyr-308). We have analyzed the basolateral sorting efficiency and endocytic capacity of this signal in Fischer rat thyroid (FRT) cells, in which basolateral and endocytic determinants have not yet been identified. We found that this targeting signal can mediate efficient transport to the basolateral membrane also in FRT cells with similar tyrosine dependence as in MDCK cells. In contrast to MDCK cells, this Tyr-based signal was not able to mediate coated pits localization and endocytosis in FRT cells. These data represent the first characterization of basolateral/endocytic signals in thyroid epithelial cells. Furthermore, our results indicate that requirements for tyrosine-dependent basolateral sorting signals are conserved among cell lines from different tissues but that the recognition of the colinear endocytic signal is tissue specific.

Role of the membrane-proximal O-glycosylation site in sorting of the human receptor for neurotrophins to the apical membrane of MDCK cells

We have analyzed the respective roles of the stalk and/or the O-glycosylation sites in apical sorting by producing partially deleted mutants in this region of the human receptor for neurotrophins (P75(NTR)). The mere presence of O-glycosylations was not sufficient for efficient delivery to the apical surface since changing the stalk domain of P75(NTR) for the heavily O-glycosylated stalk from human decay-accelerating factor led to random distribution of the chimera. The presence of O-glycosylations, however, was a prerequisite for exit from the ER and protection from intracellular cleavage since a P75(NTR) containing the non O-glycosylated stalk of the human placental alkaline phosphatase was not transported to the cell surface but was cleaved and secreted from the basolateral side. Deletion of the membrane-proximal part of the stalk showed a more dramatic reversal of polarity of P75(NTR) than the deletion of the distal part. Furthermore, moving the first putative O-glycosylation site (T216) two amino acids away from the membrane resulted in a loss of apical polarity of P75(NTR), suggesting that an important clue for apical sorting resides in this part of the stalk. This loss of apical polarity paralleled a loss of association of P75(NTR) mutants with Lubrol rafts. These data indicate that the position of O-glycans in the proximal part of the stalk domain of P75(NTR) is crucial for apical sorting and may regulate association with apical rafts.

Identification of a basolateral membrane targeting signal within the cytoplasmic domain of myelin/oligodendrocyte glycoprotein

Oligodendrocytes possess two distinct membrane compartments--uncompacted plasma membrane (cell body, processes) and compact myelin. Specific targeting mechanisms must exist to establish and maintain these membrane domains. Polarized epithelial cells have the best characterized system for targeting components to apical and basolateral compartments. Since oligodendrocytes arise from neuroepithelial cells, we investigated whether they might utilize targeting paradigms similar to polarized epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is a transmembrane Ig-like molecule restricted to uncompacted oligodendroglial plasma membrane. We stably expressed MOG in Madin-Darby canine kidney (MDCK) Type II epithelial cells, which have been extensively used in protein-targeting studies. Data from surface biotinylation assays and confocal microscopy revealed that MOG sorts exclusively to the basolateral membrane of MDCK cells. Expression vectors containing progressive truncations of MOG from the cytoplasmic C-terminus were expressed in MDCK cells to localize basolateral sorting signals. A loss of only four C-terminal residues results in some MOG expression at the apical surface. More strikingly, removal of the C-terminal membrane associated hydrophobic domain from MOG results in complete loss of basolateral sorting and specific targeting to the apical membrane. These data suggest that myelinating oligodendrocytes may utilize a sorting mechanism similar to that of polarized epithelia.

The ERBB2/HER2 receptor differentially interacts with ERBIN and PICK1 PSD-95/DLG/ZO-1 domain proteins

Identification of protein complexes associated with the ERBB2/HER2 receptor may help unravel the mechanisms of its activation and regulation in normal and pathological situations. Interactions between ERBB2/HER2 and Src homology 2 or phosphotyrosine binding domain signaling proteins have been extensively studied. We have identified ERBIN and PICK1 as new binding partners for ERBB2/HER2 that associate with its carboxyl-terminal sequence through a PDZ (PSD-95/DLG/ZO-1) domain. This peptide sequence acts as a dominant retention or targeting basolateral signal for receptors in epithelial cells. ERBIN belongs to the newly described LAP (LRR and PDZ) protein family, whose function is crucial in non vertebrates for epithelial homeostasis. Whereas ERBIN appears to locate ERBB2/HER2 to the basolateral epithelium, PICK1 is thought to be involved in the clustering of receptors. We show here that ERBIN and PICK1 bind to ERBB2/HER2 with different mechanisms, and we propose that these interactions are regulated in cells. Since ERBIN and PICK1 tend to oligomerize, further complexity of protein networks may participate in ERBB2/HER2 functions and specificity.

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

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

Identification of a basolateral sorting signal for the M3 muscarinic acetylcholine receptor in Madin-Darby canine kidney cells

Muscarinic acetylcholine receptors (mAChRs) can be differentially localized in polarized cells. To identify potential sorting signals that mediate mAChR targeting, we examined the sorting of mAChRs in Madin-Darby canine kidney cells, a widely used model system. Expression of FLAG-tagged mAChRs in polarized Madin-Darby canine kidney cells demonstrated that the M(2) subtype is sorted apically, whereas M(3) is targeted basolaterally. Expression of M(2)/M(3) receptor chimeras revealed that a 21-residue sequence, Ser(271)-Ser(291), from the M(3) third intracellular loop contains a basolateral sorting signal. Substitution of sequences containing the M(3) sorting signal into the homologous regions of M(2) was sufficient to confer basolateral localization to this apical receptor. Sequences containing the M(3) sorting signal also conferred basolateral targeting to M(2) when added to either the third intracellular loop or the C-terminal cytoplasmic tail. Furthermore, addition of a sequence containing the M(3) basolateral sorting signal to the cytoplasmic tail of the interleukin-2 receptor alpha-chain caused significant basolateral targeting of this heterologous apical protein. The results indicate that the M(3) basolateral sorting signal is dominant over apical signals in M(2) and acts in a position-independent manner. The M(3) sorting signal represents a novel basolateral targeting motif for G protein-coupled receptors.

Different functional recognition of basolateral signals in Caco-2 and MDCK cells

Using the basolateral mutant PS of the normally apical neurotrophin receptor p75 (p75NTR) we have identified two cytoplasmic determinants responsible for this reversed localization in the human intestinal cell line, Caco2. These signals are based on two consecutive leucines (322-323) and a tyrosine (Y308). Truncation of the cytoplasmic tail removing the two leucines or their replacement by alanines led to a nonpolarized expression of the resulting mutants in Caco2 cells. To our surprise, the same mutations had no effect on the basolateral localization of the mutant PS in MDCK cells. In MDCK cells, the basolateral localization was entirely dependent on a cytoplasmic tyrosine Y308, while in Caco-2 cells this tyrosine signal was functional as a basolateral signal only when the cytoplasmic domain of PS was truncated shortly after it. These data indicate for the first time that there is a differential recognition of basolateral signals between MDCK and Caco-2 cells.

Sorting proteins to their target membranes

The functional polarity of epithelial cells depends upon the selective insertion of proteins and lipids into distinct plasma membrane domains, and upon the maintenance of these specialized domains once they are established during epithelial development. This polarized distribution of important categories of proteins including membrane transporters, channels, enzymes, cell adhesion molecules and junctional components allows cells to carry out the vectorial transport of fluid, ions and other molecules across the epithelial barrier. Several mechanisms are required to ensure the directed movement of membrane components within the cell, and to control their delivery to the appropriate target membrane. These include specific "targeting" cassettes in the amino acid sequence of the transported proteins (such as PDZ domains and NPXY or YRRF domains), a variety of accessory proteins (including GTP-binding proteins) that associate with carrier vesicles and membrane compartments within the cell, and cytoskeletal elements such as microtubules, microfilaments and the spectrin-ankyrin network. Incorrectly folded proteins are retained and degraded within the cell, and many "chaperones" are involved in ensuring that newly-synthesized proteins assume the correct two- and three-dimensional orientations and oligomerization prior to exiting from the endoplasmic reticulum (ER). Many of the proteins involved in neurotransmitter release (for example, synaptobrevins, syntaxins) have homologs that are found in non-neuronal cells, where they play a key role in vesicle fusion with the plasma membrane. In view of the complexity of these trafficking processes, it is not surprising that a growing number of disease pathologies have been identified that involve defective targeting and trafficking of proteins. These diseases can be grouped under the name "sorting disorders," and they result from abnormal delivery of functionally important proteins to the cell surface. In some cases, the mutated protein is retained and degraded intracellularly, while in others it may not be delivered to the cell surface after the appropriate physiological stimulation.

Detergent-insoluble GPI-anchored proteins are apically sorted in fischer rat thyroid cells, but interference with cholesterol or sphingolipids differentially affects detergent insolubility and apical sorting

In contrast to Madin-Darby canine kidney cells, Fischer rat thyroid cells deliver the majority of endogenous glycosylphosphatidyl inositol (GPI)-anchored proteins to the basolateral surface. However, we report here that the GPI proteins Placental Alkaline Phosphatase (PLAP) and Neurotrophin Receptor-Placental Alkaline Phosphatase (NTR-PLAP) are apically localized in transfected Fischer rat thyroid cells. In agreement with the "raft hypothesis," which postulates the incorporation of GPI proteins into glycosphingolipids and cholesterol-enriched rafts, we found that both of these proteins were insoluble in Triton X-100 and floated into the lighter fractions of sucrose density gradients. However, disruption of lipid rafts by removal of cholesterol did not cause surface missorting of PLAP and NTR-PLAP, and the altered surface sorting of these proteins after Fumonisin B1 treatment did not correlate with reduced levels in Triton X-100 -insoluble fractions. Furthermore, in contrast to the GPI-anchored forms of both of these proteins, the secretory and transmembrane forms (in the absence of a basolateral cytoplasmic signal) were sorted to the apical surface without association with lipid microdomains. Together, these data demonstrate that the GPI anchor is required to mediate raft association but is not sufficient to determine apical sorting. They also suggest that signals present in the ectodomain of the proteins play a major role and that lipid rafts may facilitate the recognition of these signals in the trans-Golgi network, even though they are not required for apical sorting.

Functional cooperation of two independent targeting domains in syntaxin 6 is required for its efficient localization in the trans-golgi network of 3T3L1 adipocytes

To identify the targeting domains of syntaxin 6 responsible for its localization to the trans-Golgi network (TGN), we examined the subcellular distribution of enhanced green fluorescent protein (EGFP) epitope-tagged syntaxin 6/syntaxin 4 chimerae and syntaxin 6 truncation/deletion mutants in 3T3L1 adipocytes. Expression of EGFP-syntaxin 6 resulted in a perinuclear distribution identical to endogenous syntaxin 6 as determined both by confocal fluorescence microscopy and subcellular fractionation. Furthermore, both the endogenous and the expressed EGFP-syntaxin 6 fusion protein were localized to a brefeldin A-insensitive but okadaic acid-sensitive compartment characteristic of the TGN. In contrast, EGFP-syntaxin 6 constructs lacking the H2 domain were excluded from the TGN and were instead primarily localized to the plasma membrane. Although syntaxin 4 was localized to the plasma membrane, syntaxin 6/syntaxin 4 chimerae and syntaxin 6 truncations containing the H2 domain of syntaxin 6 were predominantly directed to the TGN. Importantly, the syntaxin 6 H2 domain fused to the transmembrane domain of syntaxin 4 was also localized to the TGN, demonstrating that the H2 domain was sufficient to confer TGN localization. In addition to the H2 domain, a tyrosine-based plasma membrane internalization signal (YGRL) was identified between the H1 and H2 domains of syntaxin 6. Deletion of this sequence resulted in the accumulation of the EGFP-syntaxin 6 reporter construct at the plasma membrane. Together, these data demonstrate that syntaxin 6 utilizes two distinct domains to drive its specific subcellular localization to the TGN.

A novel clathrin adaptor complex mediates basolateral targeting in polarized epithelial cells

Although polarized epithelial cells are well known to maintain distinct apical and basolateral plasma membrane domains, the mechanisms responsible for targeting membrane proteins to the apical or basolateral surfaces have remained elusive. We have identified a novel form of the AP-1 clathrin adaptor complex that contains as one of its subunits mu1B, an epithelial cell-specific homolog of the ubiquitously expressed mu1A. LLC-PK1 kidney epithelial cells do not express mu1B and missort many basolateral proteins to the apical surface. Stable expression of mu1B selectively restored basolateral targeting, improved the overall organization of LLC-PK1 monolayers, and had no effect on apical targeting. We conclude that basolateral sorting is mediated by an epithelial cell-specific version of the AP-1 complex containing mu1B.

Mechanisms of apical protein sorting in polarized thyroid epithelial cells

The process leading to thyroid hormone synthesis is vectorial and depends upon the polarized organization of the thyrocytes into the follicular unit. Thyrocyte membrane proteins are delivered to two distinct domains of the plasma membrane using apical (AP) and basolateral (BL) sorting signals. A recent hypothesis for AP sorting proposes that apically destined proteins cluster with glycosphingolipids (GSLs) and cholesterol, into microdomains (or rafts) of the Golgi membrane from which AP vesicles originate. In MDCK cells the human neurotrophin receptor, p75hNTR, is delivered to the AP surface through a sorting signal, rich in O-glycosylated sugars, identified in its ectodomain. We have investigated whether this signal is functional in the thyroid-derived FRT cell line and whether p75hNTR clusters into lipid rafts to be sorted to the AP membrane. We found that p75hNTR is apically delivered via a direct pathway and does not associate with rafts during its transport to the surface of FRT cells. Therefore, although the same signal could be recognized by different cell types thyroid cells may possess a tissue-specific sorting machinery.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Putative O-glycosylation sites and a membrane anchor are necessary for apical delivery of the human neurotrophin receptor in Caco-2 cells

We have expressed the human neurotrophin receptor p75 (p75(NTR)) in the intestinal epithelial cell line Caco-2 as a model to study intracellular transport and subcellular sorting signals in intestinal cells. p75(NTR) was localized at the apical membrane of Caco-2 cells and reached this membrane mainly via an indirect pathway. Apical localization, intracellular routing, and basolateral to apical transcytosis were not affected by truncation of the cytoplasmic domain or replacement of the transmembrane domain by a glycosyl phosphatidylinositol anchor. Removal of membrane anchoring resulted in basolateral secretion of the ectodomain of p75(NTR) in Caco-2 cells but in apical secretion in Madin-Darby canine kidney (MDCK) cells. Substitution of potential O-glycosylation sites present in the stalk of p75(NTR) led to intracellular cleavage and secretion of the ectodomain into the basolateral medium both in Caco-2 and MDCK cells. These results suggest that the stalk of p75(NTR) carries an apical sorting information that is recognized efficiently by Caco-2 cells only when attached to the membrane. This apical sorting information is linked to the presence of predicted O-glycosylation sites in that region. These putative O-glycosylation sites also play a role in the regulation of p75(NTR) transport to the cell surface and in the prevention of rapid degradation by cleavage of the stalk domain.

A novel phenotype related to partial loss of function mutations of the follicle stimulating hormone receptor

A single natural loss of function mutation of the follicle stimulating hormone receptor (FSHR) has been described to date. Present in the Finnish population it markedly impairs receptor function, blocking follicle development at the primary stage and presenting as primary amenorrhea with atrophic ovaries. When Western European women with this phenotype were examined for FSHR mutations the result was negative, suggesting that other etiologies corresponding to this clinical pattern are markedly more frequent. We now describe a novel phenotype related to mutations provoking a partial loss of function of the FSHR. A woman with secondary amenorrhea had very high plasma gonadotropin concentrations (especially FSH), contrasting with normal sized ovaries and antral follicles up to 5 mm at ultrasonography. Histological and immunohistochemical examination of the ovaries showed normal follicular development up to the small antral stage and a disruption at further stages. The patient was found to carry compound heterozygotic mutations of the FSHR gene: Ile160Thr and Arg573Cys substitutions located, respectively, in the extracellular domain and in the third intracellular loop of the receptor. The mutated receptors, when expressed in COS-7 cells, showed partial functional impairment, consistent with the clinical and histological observations: the first mutation impaired cell surface expression and the second altered signal transduction of the receptor. This observation suggests that a limited FSH effect is sufficient to promote follicular growth up to the small antral stage. Further development necessitates strong FSH stimulation. The contrast between very high FSH levels and normal sized ovaries with antral follicles may thus be characteristic of such patients.

Human cytomegalovirus glycoprotein B contains autonomous determinants for vectorial targeting to apical membranes of polarized epithelial cells

We previously reported that human cytomegalovirus (CMV) glycoprotein B (gB) is vectorially transported to apical membranes of CMV-infected polarized human retinal pigment epithelial cells propagated on permeable filter supports and that virions egress predominantly from the apical membrane domain. In the present study, we investigated whether gB itself contains autonomous information for apical transport by expressing the molecule in stably transfected Madine-Darby canine kidney (MDCK) cells grown on permeable filter supports. Laser scanning confocal immunofluorescence microscopy and domain-selective biotinylation of surface membrane domains showed that CMV gB was transported to apical membranes independently of other envelope glycoproteins and that it colocalized with proteins in transport vesicles of the biosynthetic and endocytic pathways. Determinants for trafficking to apical membranes were located by evaluating the targeting of gB derivatives with deletions in the lumen, transmembrane (TM) anchor, and carboxyl terminus. Derivative gB(Delta717-747), with an internal deletion in the luminal juxtamembrane sequence that preserved the N- and O-glycosylation sites, retained vectorial transport to apical membranes. In contrast, derivatives that lacked the TM anchor and cytosolic domain (gBDelta646-906) or the TM anchor alone (gBDelta751-771) underwent considerable basolateral targeting. Likewise, derivatives lacking the entire cytosolic domain (gBDelta772-906) or the last 73 amino acids (gBDelta834-906) showed disrupted apical transport. Site-specific mutations that deleted or altered the cluster of acidic residues with a casein kinase II phosphorylation site at the extreme carboxyl terminus, which can serve as an internalization signal, caused partial missorting of gB to basolateral membranes. Our studies indicate that CMV gB contains autonomous information for apical targeting in luminal, TM anchor, and cytosolic domain sequences, forming distinct structural elements that cooperate in vectorial transport in polarized epithelial cells.

Apical polarity of N-CAM and EMMPRIN in retinal pigment epithelium resulting from suppression of basolateral signal recognition

Retinal pigment epithelial (RPE) cells apically polarize proteins that are basolateral in other epithelia. This reversal may be generated by the association of RPE with photoreceptors and the interphotoreceptor matrix, postnatal expansion of the RPE apical surface, and/or changes in RPE sorting machinery. We compared two proteins exhibiting reversed, apical polarities in RPE cells, neural cell adhesion molecule (N-CAM; 140-kD isoform) and extracellular matrix metalloproteinase inducer (EMMPRIN), with the cognate apical marker, p75-neurotrophin receptor (p75-NTR). N-CAM and p75-NTR were apically localized from birth to adulthood, contrasting with a basolateral to apical switch of EMMPRIN in developing postnatal rat RPE. Morphometric analysis demonstrated that this switch cannot be attributed to expansion of the apical surface of maturing RPE because the basolateral membrane expanded proportionally, maintaining a 3:1 apical/basolateral ratio. Kinetic analysis of polarized surface delivery in MDCK and RPE-J cells showed that EMMPRIN has a basolateral signal in its cytoplasmic tail recognized by both cell lines. In contrast, the basolateral signal of N-CAM is recognized by MDCK cells but not RPE-J cells. Deletion of N-CAM's basolateral signal did not prevent its apical localization in vivo. The data demonstrate that the apical polarity of EMMPRIN and N-CAM in mature RPE results from suppressed decoding of specific basolateral signals resulting in randomized delivery to the cell surface.

The basolateral localization signal of the follicle-stimulating hormone receptor

The follicle-stimulating hormone receptor (FSHR) is physiologically localized in the basolateral compartment of the membrane of Sertoli cells. This localization is also observed when the receptor is experimentally expressed in Madin-Darby canine kidney cells. We thus used in vitro mutagenesis and transfection into these polarized cells to delineate the basolateral localization signal of the receptor. The signal was localized in the C-terminal tail of the intracellular domain (amino acids 678-691) at a marked distance of the membrane. Mutation of individual amino acids highlighted the importance of Tyr684 and Leu689. The 14-amino acid sequence was grafted onto the p75 neurotrophin receptor and redirected this apical protein to the basolateral cell membrane compartment. Deletion of amino acids 677-695 did not modify the internalization of the FSHR, showing that the basolateral localization signal of the FSHR is not colinear with its internalization signal.

Structural and functional regulation of tight junctions by RhoA and Rac1 small GTPases

Tight junctions (TJ) govern ion and solute diffusion through the paracellular space (gate function), and restrict mixing of membrane proteins and lipids between membrane domains (fence function) of polarized epithelial cells. We examined roles of the RhoA and Rac1 GTPases in regulating TJ structure and function in MDCK cells using the tetracycline repressible transactivator to regulate RhoAV14, RhoAN19, Rac1V12, and Rac1N17 expression. Both constitutively active and dominant negative RhoA or Rac1 perturbed TJ gate function (transepithelial electrical resistance, tracer diffusion) in a dose-dependent and reversible manner. Freeze-fracture EM and immunofluoresence microscopy revealed abnormal TJ strand morphology and protein (occludin, ZO-1) localization in RhoAV14 and Rac1V12 cells. However, TJ strand morphology and protein localization appeared normal in RhoAN19 and Rac1N17 cells. All mutant GTPases disrupted the fence function of the TJ (interdomain diffusion of a fluorescent lipid), but targeting and organization of a membrane protein in the apical membrane were unaffected. Expression levels and protein complexes of occludin and ZO-1 appeared normal in all mutant cells, although ZO-1 was more readily solubilized from RhoAV14-expressing cells with Triton X-100. These results show that RhoA and Rac1 regulate gate and fence functions of the TJ, and play a role in the spatial organization of TJ proteins at the apex of the lateral membrane.

TGN38 cycles via the basolateral membrane of polarized Caco-2 cells

TGN38 is a heavily glycosylated, type I integral membrane protein which is predominantly localized to the trans Golgi network (TGN), but which constitutively traffics between the TGN and the cell surface. The trafficking of TGN38 has been extensively studied in non-polarized cells, and a short, tyrosine-based, peptide motif within the cytosolic domain of the protein has been shown to be necessary and sufficient for its rapid internalization from the cell surface and efficient delivery to the TGN. Such tyrosine-based motifs have also been shown to act as basolateral targeting signals, whilst N-linked glycans (as occur on the extracytosolic domain of TGN38) can act as apical targeting signals. TGN38 has previously been shown to be sorted to the basolateral surface of polarized canine MDCK cells; a polarized cell line in which biosynthetic sorting decisions concerning the eventual destination of apical or basolateral targeted plasma membrane proteins are made at the TGN. We now show that TGN38 is targeted exclusively to the basolateral domain of polarized human Caco-2 cells, a cell line in which newly synthesized membrane proteins destined for either the apical or basolateral plasma membrane may be sorted for delivery to their final destination either at the TGN or at the cell surface. These data also demonstrate that the heavily glycosylated, extracytosolic domain of TGN38 does not contain a dominant apical targeting signal.