MUC1 traverses apical recycling endosomes along the biosynthetic pathway in polarized MDCK cells

Cloning, Expression, and Purification of Galectins for In Vitro Studies

Galectins are best known for their ability to bind glycoconjugates containing β-galactose, but classification of these small proteins within the galectin family is also defined by amino acid homology within structural domains and exon/intron junctions within genes. As galectins are expressed by organisms as diverse as some fungi, C. elegans, fish, birds and mammals, and biological activities attributed to galectins are equally diverse, it becomes essential to identify, clone, and characterize galectins from many sources. Glutathione S-transferase (GST) fused to the amino-terminus of galectin cDNAs has proven to be especially useful for the preparation of recombinant galectins in bacteria for use on glycan arrays, in experiments with cultured or isolated cells, and in pull-down assays with immunopurified glycoproteins. Many galectins are stabilized by reducing reagents, such that binding and elution of GST-galectins from glutathione-conjugated Sepharose with excess glutathione is both efficient and innocuous. The ability to bind and elute GST-galectins from lactose-conjugated Sepharose with excess lactose provides a relatively easy means to insure that galectins are competent for glycoconjugate binding prior to experimentation. This chapter focuses primarily on the varied approaches to use GST-galectin binding to glutathione- and lactose-conjugated Sepharose to purify recombinant galectins and then develop effective experimental protocols to characterize the specificity, interactions and function of galectins cloned from any source. We provide one example where a pull-down assay with all the GST-tagged canine galectins reveals that the C-terminal carbohydrate recognition domain of galectin-9 (Gal-9C) specifically recognizes the glycan-dependent apical targeting signal from the glycoprotein MUC1.

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.

Adenovirus entry from the apical surface of polarized epithelia is facilitated by the host innate immune response

Prevention of viral-induced respiratory disease begins with an understanding of the factors that increase or decrease susceptibility to viral infection. The primary receptor for most adenoviruses is the coxsackievirus and adenovirus receptor (CAR), a cell-cell adhesion protein normally localized at the basolateral surface of polarized epithelia and involved in neutrophil transepithelial migration. Recently, an alternate isoform of CAR, CAREx8, has been identified at the apical surface of polarized airway epithelia and is implicated in viral infection from the apical surface. We hypothesized that the endogenous role of CAREx8 may be to facilitate host innate immunity. We show that IL-8, a proinflammatory cytokine and a neutrophil chemoattractant, stimulates the protein expression and apical localization of CAREx8 via activation of AKT/S6K and inhibition of GSK3β. Apical CAREx8 tethers infiltrating neutrophils at the apical surface of a polarized epithelium. Moreover, neutrophils present on the apical-epithelial surface enhance adenovirus entry into the epithelium. These findings suggest that adenovirus evolved to co-opt an innate immune response pathway that stimulates the expression of its primary receptor, apical CAREx8, to allow the initial infection the intact epithelium. In addition, CAREx8 is a new target for the development of novel therapeutics for both respiratory inflammatory disease and adenoviral infection.

Cloning, expression, and purification of galectins for in vitro studies

Galectins are best known for their ability to bind glycoconjugates containing β-galactose, but classification of these small proteins within the galectin family is also defined by amino acid homology within structural domains and exon/intron junctions within genes. As galectins are expressed by organisms as diverse as some fungi, C. elegans, fish, birds, and mammals, and biological activities attributed to galectins are equally diverse, it becomes essential to identify, clone, and characterize galectins from many sources. Glutathione S-transferase (GST) fused to the amino-terminus of galectin cDNAs has proven to be especially useful for preparation of recombinant galectins in bacteria for use on glycan arrays, in experiments with cultured or isolated cells, and in pull-down assays with immunopurified glycoproteins. Many galectins are stabilized by reducing reagents, such that binding and elution of GST-galectins from glutathione-conjugated Sepharose with excess glutathione is both efficient and innocuous. The ability to bind and elute GST-galectins from lactose-conjugated Sepharose with excess lactose provides a relatively easy means to insure that galectins are competent for glycoconjugate binding prior to experimentation. This chapter focuses primarily on the varied approaches to use GST-galectin binding to glutathione- and lactose-conjugated Sepharose to purify recombinant galectins and then develop effective experimental protocols to characterize the specificity, interactions, and function of galectins cloned from any source. We provide one example where a pull-down assay with all the GST-tagged canine galectins reveals that the C-terminal carbohydrate recognition domain of galectin-9 (Gal-9C) specifically recognizes the glycan-dependent apical targeting signal from the glycoprotein MUC1.

DKWSLLL, a versatile DXXXLL-type signal with distinct roles in the Cu(+)-regulated trafficking of ATP7B

In the liver, the P-type ATPase and membrane pump ATP7B plays a crucial role in Cu(+) donation to cuproenzymes and in the elimination of excess Cu(+). ATP7B is endowed with a COOH-cytoplasmic (DE)XXXLL-type traffic signal. We find that accessory (Lys -3, Trp -2, Ser -1 and Leu +2) and canonical (D -4, Leu 0 and Leu +1) residues confer the DKWSLLL signal with the versatility required for the Cu(+)-regulated cycling of ATP7B between the trans-Golgi network (TGN) and the plasma membrane (PM). The separate mutation of these residues caused a disruption of the signal, resulting in different ATP7B distribution phenotypes. These phenotypes indicate the key roles of specific residues at separate steps of ATP7B trafficking, including sorting at the TGN, transport from the TGN to the PM and its endocytosis, and recycling to the TGN and PM. The distinct roles of ATP7B in the TGN and PM and the variety of phenotypes caused by the mutation of the canonical and accessory residues of the DKWSLLL signal can explain the separate or joined presentation of Wilson's cuprotoxicosis and the dysfunction of the cuproenzymes that accept Cu(+) at the TGN.

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.

Evidence for core 2 to core 1 O-glycan remodeling during the recycling of MUC1

The apical transmembrane glycoprotein MUC1 is endocytosed to recycle through the trans-Golgi network (TGN) or Golgi complex to the plasma membrane. We followed the hypothesis that not only the known follow-up sialylation of MUC1 in the TGN is associated with this process, but also a remodeling of O-glycan core structures, which would explain the previously described differential core 2- vs core 1-based O-glycosylation of secreted, single Golgi passage and recycling membrane MUC1 isoforms (Engelmann K, Kinlough CL, Müller S, Razawi H, Baldus SE, Hughey RP, Hanisch F-G. 2005. Glycobiology. 15:1111-1124). Transmembrane and secreted MUC1 probes show trafficking-dependent changes in O-glycan core profiles. To address this novel observation, we used recombinant epitope-tagged MUC1 (MUC1-M) and mutant forms with abrogated clathrin-mediated endocytosis (MUC1-M-Y20,60N) or blocked recycling (palmitoylation-defective MUC1-M-CQC/AQA). We show that the CQC/AQA mutant transits the TGN at significantly lower levels, concomitant with a strongly reduced shedding from the plasma membrane and its accumulation in endosomal compartments. Intriguingly, the O-glycosylation of the shed MUC1 ectodomain subunit changes from preponderant sialylated core 1 (MUC1-M) to core 2 glycans on the non-recycling CQC/AQA mutant. The O-glycoprofile of the non-recycling CQC/AQA mutant resembles the core 2 glycoprofile on a secretory MUC1 probe that transits the Golgi complex only once. In contrast, the MUC1-M-Y20,60N mutant recycles via flotillin-dependent pathways and shows the wild-type phenotype with dominant core 1 expression. Differential radiolabeling of protein with [(35)S]Met/Cys or glycans with [(3)H]GlcNH2 in pulse-chase experiments of surface biotinylated MUC1 revealed a significantly shorter half-life of [(3)H]MUC1 when compared with [(35)S]MUC1, whereas the same ratio for the CQC/AQA mutant was close to one. This finding further supports the novel possibility of a recycling-associated O-glycan processing from Gal1-4GlcNAc1-6(Gal1-3)GalNAc (core 2) to Gal1-3GalNAc (core 1).

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.

The MUC1 extracellular domain subunit is found in nuclear speckles and associates with spliceosomes

MUC1 is a large transmembrane glycoprotein and oncogene expressed by epithelial cells and overexpressed and underglycosylated in cancer cells. The MUC1 cytoplasmic subunit (MUC1-C) can translocate to the nucleus and regulate gene expression. It is frequently assumed that the MUC1 extracellular subunit (MUC1-N) does not enter the nucleus. Based on an unexpected observation that MUC1 extracellular domain antibody produced an apparently nucleus-associated staining pattern in trophoblasts, we have tested the hypothesis that MUC1-N is expressed inside the nucleus. Three different antibodies were used to identify MUC1-N in normal epithelial cells and tissues as well as in several cancer cell lines. The results of immunofluorescence and confocal microscopy analyses as well as subcellular fractionation, Western blotting, and siRNA/shRNA studies, confirm that MUC1-N is found within nuclei of all cell types examined. More detailed examination of its intranuclear distribution using a proximity ligation assay, subcellular fractionation, and immunoprecipitation suggests that MUC1-N is located in nuclear speckles (interchromatin granule clusters) and closely associates with the spliceosome protein U2AF65. Nuclear localization of MUC1-N was abolished when cells were treated with RNase A and nuclear localization was altered when cells were incubated with the transcription inhibitor 5,6-dichloro-1-b-d-ribofuranosylbenzimidazole (DRB). While MUC1-N predominantly associated with speckles, MUC1-C was present in the nuclear matrix, nucleoli, and the nuclear periphery. In some nuclei, confocal microscopic analysis suggest that MUC1-C staining is located close to, but only partially overlaps, MUC1-N in speckles. However, only MUC1-N was found in isolated speckles by Western blotting. Also, MUC1-C and MUC1-N distributed differently during mitosis. These results suggest that MUC1-N translocates to the nucleus where it is expressed in nuclear speckles and that MUC1-N and MUC1-C have dissimilar intranuclear distribution patterns.

Sialylation of N-linked glycans mediates apical delivery of endolyn in MDCK cells via a galectin-9-dependent mechanism

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Apical delivery of endolyn requires recognition of sialic acids on its N-glycans possibly (or likely) mediated by galectin-9.

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Along rat kidney tubules, endolyn is variously localized to the apical surface and endosomal/lysosomal compartments. Apical delivery of newly synthesized rat endolyn predominates over direct lysosomal delivery in polarized Madin–Darby canine kidney cells. Apical sorting depends on terminal processing of a subset of lumenal N-glycans. Here we dissect the requirements of N-glycan processing for apical targeting and investigate the underlying mechanism. Modulation of glycan branching and subsequent polylactosamine elongation by knockdown of N-acetylglucosaminyltransferase III or V had no effect on apical delivery of endolyn. In contrast, combined but not individual knockdown of sialyltransferases ST3Gal-III, ST3Gal-IV, and ST6Gal-I, which together are responsible for addition of α2,3- and α2,6-linked sialic acids on N-glycans, dramatically decreased endolyn surface polarity. Endolyn synthesized in the presence of kifunensine, which blocks terminal N-glycan processing, reduced its interaction with several recombinant canine galectins, and knockdown of galectin-9 (but not galectin-3, -4, or -8) selectively disrupted endolyn polarity. Our data suggest that sialylation enables recognition of endolyn by galectin-9 to mediate efficient apical sorting. They raise the intriguing possibility that changes in glycosyltransferase expression patterns and/or galectin-9 distribution may acutely modulate endolyn trafficking in the kidney.

MUC1 membrane trafficking: protocols for assessing biosynthetic delivery, endocytosis, recycling, and release through exosomes

MUC1 is normally apical in polarized epithelial cells but is aberrantly localized in tumor cells. To better understand the mechanism of this altered localization as well as the normal functions of MUC1, we are focused on characterizing the features of MUC1 that regulate the membrane trafficking of this mucin-like transmembrane protein. Previous studies using heterologous expression of MUC1 in CHO and MDCK cells revealed that trafficking to the cell surface as well as endocytosis and recycling is modulated by glycosylation, palmitoylation, and docking of adaptor protein complexes. Protocols for assessing MUC1 trafficking have utilized membrane-impermeant cell surface biotinylation and subsequent stripping with reducing reagents, such as MESNA. The cumulative data have been used for computer modeling and calculation of rate constants. As MUC1 is released through trafficking to exosomes, we have devised protocols for the affinity isolation of MUC1-containing lipid rafts from nanovesicular subpopulations to perform proteomic mapping of protein constituents in these sorting platforms. Our studies to date have shown that plasma membranous MUC1 traffics via lipid raft-associated pathways to exosomes, which are independent of caveolin-1 or dynamin, but dependent on flotillin.

Multiple biosynthetic trafficking routes for apically secreted proteins in MDCK cells

Many newly synthesized membrane proteins traverse endocytic intermediates en route to the surface in polarized epithelial cells; however, the biosynthetic itinerary of secreted proteins has not been elucidated. We monitored the trafficking route of two secreted proteins with different apical sorting signals: the N-glycan-dependent cargo glycosylated growth hormone (gGH) and Ensol, a soluble version of endolyn whose apical sorting is independent of N-glycans. Both proteins were observed to colocalize in part with apical recycling endosome (ARE) markers. Cargo that lacks an apical targeting signal and is secreted in a nonpolarized manner did not localize to the ARE. Expression of a dominant-negative mutant of myosin Vb, which disrupts ARE export of glycan-dependent membrane proteins, selectively inhibited apical release of gGH but not Ensol. Fluorescence recovery after photobleaching (FRAP) measurements revealed that gGH in the ARE was less mobile than Ensol, consistent with tethering to a sorting receptor. However, knockdown of galectin-3 or galectin-4, lectins implicated in apical sorting, had no effect on the rate or polarity of gGH secretion. Together, our results suggest that apically secreted cargoes selectively access the ARE and are exported via differentially regulated pathways.

Core-glycosylated mucin-like repeats from MUC1 are an apical targeting signal

MUC1 is efficiently delivered to the apical surface of polarized Madin-Darby canine kidney (MDCK) cells by transit through apical recycling endosomes, a route associated with delivery of apical proteins with glycan-dependent targeting signals. However, a role for glycans in MUC1 sorting has not been established. A key feature of MUC1 is a heavily O-glycosylated mucin-like domain with a variable number of nearly perfect tandem repeats and adjacent imperfect repeats. Metabolic labeling, cell surface biotinylation, immobilized lectins, and confocal immunofluorescence microscopy were used to characterize the polarized delivery of MUC1 mutants and chimeras in MDCK cells to identify the apical targeting signal. Both the interleukin-2 receptor α subunit (Tac) and a chimera where the Tac ectodomain replaced that of MUC1 were delivered primarily to the basolateral surface. Attachment of the MUC1 mucin-like domain to the N terminus of Tac enhanced apical but not basolateral delivery when compared with Tac. Conversely, deletions within the mucin-like domain in MUC1 reduced apical but not basolateral delivery when compared with MUC1. In pull-down assays with lectins, we found a notable difference in the presence of core 1 O-glycans, but not poly-N-acetyllactosamine, in apically targeted MUC1 and chimeras when compared with Tac. Consistent with these data, we found no effect on MUC1 targeting when galectin-3, with preference for poly-N-acetyllactosamine, was depleted from polarized MDCK cells. However, we did block the apical targeting activity of the mucin-like repeats when we overexpressed CMP-Neu5Ac:GalNAc-Rα2,6-sialyltransferase-1 to block core O-glycan synthesis. The cumulative data indicate that the core-glycosylated mucin-like repeats of MUC1 constitute an apical targeting signal.

Galectin-7 modulates the length of the primary cilia and wound repair in polarized kidney epithelial cells

Galectins (Gal) are β-galactoside-binding proteins that function in epithelial development and homeostasis. An overlapping role for Gal-3 and Gal-7 in wound repair was reported in stratified epithelia. Although Gal-7 was thought absent in simple epithelia, it was reported in a proteomic analysis of cilia isolated from cultured human airway, and we recently identified Gal-7 transcripts in Madin-Darby canine kidney (MDCK) cells (Poland PA, Rondanino C, Kinlough CL, Heimburg-Molinaro J, Arthur CM, Stowell SR, Smith DF, Hughey RP. J Biol Chem 286: 6780-6790, 2011). We now report that Gal-7 is localized exclusively on the primary cilium of MDCK, LLC-PK(1) (pig kidney), and mpkCCD(c14) (mouse kidney) cells as well as on cilia in the rat renal proximal tubule. Gal-7 is also present on most cilia of multiciliated cells in human airway epithelia primary cultures. Interestingly, exogenous glutathione S-transferase (GST)-Gal-7 bound the MDCK apical plasma membrane as well as the cilium, while the lectin Ulex europeaus agglutinin, with glycan preferences similar to Gal-7, bound the basolateral plasma membrane as well as the cilium. In pull-down assays, β1-integrin isolated from either the basolateral or apical/cilia membranes of MDCK cells was similarly bound by GST-Gal-7. Selective localization of Gal-7 to cilia despite the presence of binding sites on all cell surfaces suggests that intracellular Gal-7 is specifically delivered to cilia rather than simply binding to surface glycoconjugates after generalized secretion. Moreover, depletion of Gal-7 using tetracycline-induced short-hairpin RNA in mpkCCD(c14) cells significantly reduced cilia length and slowed wound healing in a scratch assay. We conclude that Gal-7 is selectively targeted to cilia and plays a key role in surface stabilization of glycoconjugates responsible for integrating cilia function with epithelial repair.

Identification and characterization of endogenous galectins expressed in Madin Darby canine kidney cells

Madin Darby canine kidney (MDCK) cells are a well characterized epithelial cell line used to study mechanisms of polarized delivery. As glycans on apically expressed proteins have been identified as targeting signals, and crosslinking by the abundant galectin-3 has been implicated in the mechanism of glycan-dependent sorting, we wanted to identify other members of the galectin (Gal) family expressed in MDCK cells. By analyzing intron-exon boundaries, we identified canine genes that were highly homologous to mammalian Gal-1, 2, 3, 4, 7, 8, 9, and 12, and galectin-related HSPC159 and GRIFIN. Transcripts for Gal-2 and -12 were not detected in MDCK cells, but we found transcript levels for Gal-3 > Gal-9 > Gal-8 > Gal-1 ⋙ Gal-4 > Gal-7. Canine Gal-1, -2, -3, -4, -7, -8, -9, and -12 were cloned and expressed in Escherichia coli as GST fusion proteins to characterize binding specificities on arrays of synthetic glycans on glass slides from Core H of the NIH Consortium for Functional Glycomics. Individual expression of the N-terminal (GST-Gal-9N) and C-terminal (GST-Gal-9C) carbohydrate recognition domains greatly improved protein yield and the ability to characterize Gal-9 binding on the array. Canine galectins differentially bound sulfated disaccharides as well as human blood groups A, B, and H on both N-glycans and linear glycan structures on the array. Analysis of GST-Gal-1, -3, -4, -7, -8, -9N, and -9C binding to immunopurified human MUC1 expressed in MDCK cells revealed a preference for binding GST-Gal-3 and -9, which interestingly reflects the two most abundant galectins expressed in MDCK cells.