N-glycans are not the signal for apical sorting of corticosteroid binding globulin in MDCK cells

Secretory cargo composition affects polarized secretion in MDCK epithelial cells

Polarized epithelial cells secrete proteins at either the apical or basolateral cell surface. A number of non-epithelial secretory proteins also exhibit polarized secretion when they are expressed in polarized epithelial cells but it is difficult to predict where foreign proteins will be secreted in epithelial cells. The question is of interest since secretory epithelia are considered as target tissues for gene therapy protocols that aim to express therapeutic secretory proteins. In the parathyroid gland, parathyroid hormone is processed by furin and co-stored with chromogranin A in secretory granules. To test the secretion of these proteins in epithelial cells, they were expressed in MDCK cells. Chromogranin A and a secreted form of furin were secreted apically while parathyroid hormone was secreted 60% basolaterally. However, in the presence of chromogranin A, the secretion of parathyroid hormone was 65% apical, suggesting that chromogranin can act as a "sorting escort" (sorting chaperone) for parathyroid hormone. Conversely, apically secreted furin did not affect the sorting of parathyroid hormone. The apical secretion of chromogranin A was dependent on cholesterol, suggesting that this protein uses an established cellular sorting mechanism for apical secretion. However, this sorting does not involve the N-terminal membrane-binding domain of chromogranin A. These results suggest that foreign secretory proteins can be used as "sorting escorts" to direct secretory proteins to the apical secretory pathway without altering the primary structure of the secreted protein. Such a system may be of use in the targeted expression of secretory proteins from epithelial cells.

Prominin-2 is a cholesterol-binding protein associated with apical and basolateral plasmalemmal protrusions in polarized epithelial cells and released into urine

Prominin-2 is a pentaspan membrane glycoprotein structurally related to the cholesterol-binding protein prominin-1, which is expressed in epithelial and non-epithelial cells. Although prominin-1 expression is widespread throughout the organism, the loss of its function solely causes retinal degeneration. The finding that prominin-2 appears to be restricted to epithelial cells, such as those found in kidney tubules, raises the possibility that prominin-2 functionally substitutes prominin-1 in tissues other than the retina and provokes a search for a definition of its morphological and biochemical characteristics. Here, we have investigated, by using MDCK cells as an epithelial cell model, whether prominin-2 shares the biochemical and morphological properties of prominin-1. Interestingly, we have found that, whereas prominin-2 is not restricted to the apical domain like prominin-1 but is distributed in a non-polarized fashion between the apical and basolateral plasma membranes, it retains the main feature of prominin-1, i.e. its selective concentration in plasmalemmal protrusions; prominin-2 is confined to microvilli, cilia and other acetylated tubulin-positive protruding structures. Similar to prominin-1, prominin-2 is partly associated with detergent-resistant membranes in a cholesterol-dependent manner, suggesting its incorporation into membrane microdomains, and binds directly to plasma membrane cholesterol. Finally, prominin-2 is also associated with small membrane particles that are released into the culture media and found in a physiological fluid, i.e. urine. Together, these data show that all the characteristics of prominin-1 are shared by prominin-2, which is in agreement with a possible redundancy in their role as potential organizers of plasma membrane protrusions.

Role of N- and O-glycans in polarized biosynthetic sorting

The maintenance of proper epithelial function requires efficient sorting of newly synthesized and recycling proteins to the apical and basolateral surfaces of differentiated cells. Whereas basolateral protein sorting signals are generally confined to their cytoplasmic regions, apical targeting signals have been identified that localize to luminal, transmembrane, and cytoplasmic aspects of proteins. In the past few years, both N- and O-linked glycans have been identified as apical sorting determinants. Glycan structures are extraordinarily diverse and have tremendous information potential. Moreover, because the oligosaccharides added to a given protein can change depending on cell type and developmental stage, the potential exists for altering sorting pathways by modulation of the expression pattern of enzymes involved in glycan synthesis. In this review, we discuss the evidence for glycan-mediated apical sorting along the biosynthetic pathway and present possible mechanisms by which these common and heterogeneous posttranslational modifications might function as specific sorting signals.

Conversion of proteins from a non-polarized to an apical secretory pattern in MDCK cells

Previously it was shown that fusion proteins containing the amino terminus of an apical targeted member of the serpin family fused to the corresponding carboxyl terminus of the non-polarized secreted serpin, antithrombin, are secreted mainly to the apical side of MDCK cells. The present study shows that this is neither due to the transfer of an apical sorting signal from the apically expressed proteins, since a sequence of random amino acids acts the same, nor is it due to the deletion of a conserved signal for correct targeting from the non-polarized secreted protein. Our results suggest that the polarity of secretion is determined by conformational sensitive sorting signals.

Cholesterol is required for the polarized secretion of erythropoietin in Madin-Darby canine kidney cells

It has already been reported that stably expressed exogenous human wild-type EPO (wtEPO) is preferentially secreted to the apical side and one of the three N-linked carbohydrate chains critically acts as an apical sorting determinant in Madin-Darby canine kidney (MDCK) cells. It has been suggested that lipid rafts are involved in the apical sorting of membrane and secretory proteins. To investigate the involvement of lipid rafts in the apical sorting of wtEPO, we examined the effect of cholesterol depletion with methyl-beta-cyclodextrin on the secretion polarity of EPO and analyzed Triton X-100 insoluble cell extracts by sucrose density gradients centrifugation in MDCK cells. We found that wtEPO was shifted in non-polarized direction by cholesterol depletion. Most of the wtEPO was not detectable in the raft fractions by sucrose density gradients centrifugation analysis. These results indicate that apical secretion of EPO involves a cholesterol-dependent mechanism probably not involving lipid rafts.

N-Glycans, not the GPI anchor, mediate the apical targeting of a naturally glycosylated, GPI-anchored protein in polarised epithelial cells

The glycosyl-phosphatidylinositol (GPI) anchor mediates the apical sorting of proteins in polarised epithelial cells through its interaction with lipid rafts. Here we investigated the signals required for the apical targeting of the naturally N-glycosylated and GPI-anchored membrane dipeptidase by selective point mutation to remove the GPI anchor addition signal or the sites for N-linked glycosylation, or both. Activity assays, immunoblotting and immunofluorescence microscopy revealed that the constructs lacking the GPI anchor were secreted from Madin-Darby canine kidney (MDCK) cells, whereas those retaining the GPI anchor were attached at the cell surface, irrespective of the glycosylation status. Wild-type membrane dipeptidase was expressed preferentially on the apical surface of both MDCK and CaCo-2 cells. By contrast, the GPI-anchored construct lacking the N-glycans was targeted preferentially to the basolateral surface of both cell types. In constructs lacking the GPI anchor, the N-glycans also targeted the protein to the apical surface. Both the apically targeted, glycosylated and the basolaterally targeted, unglycosylated GPI-anchored forms of the protein were located in detergent-insoluble lipid rafts. These data indicate that it is the N-glycans, not the association of the GPI anchor with lipid rafts, which determine apical targeting of an endogenously N-glycosylated, GPI-anchored protein in polarised epithelial cells.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Serpins are apically secreted from MDCK cells independently of their raft association

It has been suggested that detergent-resistant membranes (DRMs), also known as lipid rafts, are involved in vectorial transport of proteins to the apical surface. In this report we use Madin-Darby canine kidney (MDCK) cells expressing the apically secreted C1-esterase inhibitor, the non-sorted antithrombin or chimeras of serpins to study the possible connection between DRM association and apical targeting of secretory proteins. We found newly synthesised C1-esterase inhibitor associated with DRMs in MDCK cells, whereas antithrombin was not. However, two chimeric proteins, secreted mainly from the apical membrane, do not associate with DRMs. Based on these observations we suggest that apical targeting and association with DRMs are two independent events for secretory serpins.

Sphingolipid trafficking and protein sorting in epithelial cells

Sphingolipids represent a minor, but highly dynamic subclass of lipids in all eukaryotic cells. They are involved in functions that range from structural protection to signal transduction and protein sorting, and participate in lipid raft assembly. In polarized epithelial cells, which display an asymmetric apical and basolateral membrane surface, rafts have been proposed as a sorting principle for apical resident proteins, following their biosynthesis. However, raft-mediated trafficking is ubiquitous in cells. Also, sphingolipids per se, which are strongly enriched in the apical domain, are subject to sorting in polarity development. Next to the trans Golgi network, a subapical compartment called SAC or common endosome appears instrumental in regulating these sorting events.

Secretion polarity of interferon-beta in epithelial cell lines

Epithelial cells are an attractive target for local gene delivery in gene therapy for which cytokine genes such as interferon (IFN) genes are promising. However, how the secretion of the gene products is regulated in epithelial cells has been insufficiently investigated. Here, we have studied the secretion polarity of IFN-beta expressed via gene transfection in mouse epithelial Pam-T cells on a bicameral culture system. In transient expression, IFN-beta was predominantly secreted from the cell membrane side on which the transfection was carried out. Meanwhile, the secretion of constitutive IFN-beta from stable transformants was apparently unpolarized. Interestingly, the transformants displayed a polarized secretion of transiently expressed IFN-beta in a transfection-side-dependent manner, their stable IFN-beta secretion remaining unpolarized. These results suggest that epithelial cells have at least dual protein sorting-secretion pathways, transient and stable, for the same secretory proteins, such as IFNs.

Secretion of antithrombin is converted from nonpolarized to apical by exchanging its amino terminus for that of apically secreted family members

The three members of the serpin family, corticosteroid binding globulin, alpha1-antitrypsin, and C1 inhibitor are secreted apically from Madin-Darby canine kidney (MDCK) cells, whereas two homologous family members, antithrombin and plasminogen activator inhibitor-1, are secreted in a nonpolarized fashion. cDNAs coding for chimeras composed of complementary portions of an apically targeted serpin and a nonsorted serpin were generated, expressed in MDCK cells, and the ratio between apical and basolateral secretion was analyzed. These experiments identified an amino-terminal sequence of corticosteroid binding globulin (residues 1-19) that is sufficient to direct a chimera with antithrombin mainly to the apical side. A deletion/mutagenesis analysis showed that no individual amino acid is absolutely required for the apical targeting ability of amino acids 1-30 of corticosteroid binding globulin. The corresponding amino-terminal sequences of alpha1-antitrypsin and C1 inhibitor were also sufficient to confer apical sorting. Based on our results we suggest that the apical targeting ability is encoded in the conformation of the protein.

MAL mediates apical transport of secretory proteins in polarized epithelial Madin-Darby canine kidney cells

The MAL proteolipid is an integral membrane protein identified as a component of the raft machinery for apical sorting of membrane proteins in Madin-Darby canine kidney (MDCK) cells. Previous studies have implicated lipid rafts in the transport of exogenous thyroglobulin (Tg), the predominant secretory protein of thyroid epithelial cells, to the apical surface in MDCK cells. We have examined the secretion of recombinant Tg and gp80/clusterin, a major endogenous secretory protein not detected in Triton X-100 insoluble rafts, for the investigation of the involvement of MAL in the constitutive apical secretory pathway of MDCK cells. We show that MAL depletion impairs apical secretion of Tg and causes its accumulation in the Golgi. Cholesterol sequestration, which blocks apical secretion of Tg, did not alter the levels of MAL in rafts but created a block proximal to Tg entrance into rafts. Apical secretion of gp80/clusterin was also inhibited by elimination of endogenous MAL. Our results suggest a role for MAL in the transport of both endogenously and exogenously expressed apical secretory proteins in MDCK cells.

Regulation of constitutive protein transit by phospholipase D in HT29-cl19A cells

Phospholipase D (PLD) plays a central role in the control of vesicle budding and protein transit. We previously showed that in resting epithelial HT29-cl19A cells, PLD is implicated in the control of constitutive protein transit, from the trans-Golgi network to the plasma membrane, and that phorbol ester stimulation of protein transit is correlated with PLD activation (Auger, R., Robin, P., Camier, B., Vial, G., Rossignol, B., Tenu, J.-P., and Raymond, M.-N. (1999) J. Biol. Chem. 274, 28652-28659). In this paper we demonstrate that: 1) PLD is not implicated in the earliest phases of protein transit; 2) PLD controls apical but not basolateral protein transit; 3) HT29-cl19A cells express PLD1b and PLD2a mRNAs and proteins; 4) the expression of a catalytically inactive mutant of PLD2 (mPLD2-K758R) significantly inhibited apical constitutive protein transit whereas expression of a catalytically inactive mutant of PLD1 (hPLD1b-K898R) prevented increases in the rate of apical transit as triggered by phorbol esters; 5) PLD2 appears to be located in a perinuclear region containing the Golgi whereas PLD1, which is scattered in the cytoplasm in resting cells, is translocated to the plasma membrane after phorbol ester stimulation. Taken together, these data lead to the conclusion that in HT29-cl19A cells, both PLDs regulate protein transit between the trans-Golgi network and the apical plasma membrane, but that they do so at different steps in the pathway.

Intracellular traffic of the ecto-nucleotide pyrophosphatase/phosphodiesterase NPP3 to the apical plasma membrane of MDCK and Caco-2 cells: apical targeting occurs in the absence of N-glycosylation

Glycosylation was considered the major signal candidate for apical targeting of transmembrane proteins in polarized epithelial cells. However, direct demonstration of the role of glycosylation has proved difficult because non-glycosylated apical transmembrane proteins usually do not reach the cell surface. Here we were able to follow the targeting of the apical transmembrane glycoprotein NPP3 both when glycosylated and non-glycosylated. Transfected in polarized MDCK and Caco-2 cells, NPP3 was exclusively expressed at the apical membrane. The transport kinetics of the protein to the cell surface were studied after metabolic (35)S-labeling and surface immunoprecipitation. The newly synthesized protein was mainly targeted directly to the apical surface in MDCK cells, whereas 50% transited through the basolateral surface in Caco-2 cells. In both cell types, the basolaterally targeted pool was effectively transcytosed to the apical surface. In the presence of tunicamycin, NPP3 was not N-glycosylated. The non-glycosylated protein was partially retained intracellularly but the fraction that reached the cell surface was nevertheless predominantly targeted apically. However, transcytosis of the non-glycosylated protein was partially impaired in MDCK cells. These results provide direct evidence that glycosylation cannot be considered an apical targeting signal for NPP3, although glycosylation is necessary for correct trafficking of the protein to the cell surface.

Apical and non-polarized secretion of serpins from MDCK cells

Corticosteroid binding globulin, a member of the serpin family, was previously shown to be secreted mainly apically from MDCK cells in an N-glycan independent manner [Larsen et al. (1999) FEBS Lett. 451, 19-22]. Apart from N-glycosylation, serpins are not known to carry any other posttranslational modifications, suggesting the presence of a proteinaceous apical sorting signal. In the present study we have expressed four other members of the serpin family: alpha1-antitrypsin, C1 inhibitor, plasminogen activator inhibitor-1 and antithrombin in MDCK cells. Tight monolayers of transfected cells were grown on filters and the amounts of recombinantly expressed serpins in the apical and the basolateral media were determined. alpha1-Antitrypsin and C1 inhibitor were found mainly in the apical medium whereas plasminogen activator inhibitor-1 and antithrombin were found in roughly equal amounts in the apical and basolateral media. Control experiments showed that all four serpins are transported along the exocytotic pathway in an uncomplicated way that does not involve transcytosis or differences in stability on the two sides of the cells. We conclude that some members of the serpin family including corticosteroid binding globulin, alpha1-antitrypsin and C1 inhibitor are secreted mainly apically from MDCK cells whereas plasminogen activator inhibitor-1 and antithrombin are secreted in a non-polarized manner.

Polarized secretion of the regulated secretory protein chromogranin A

Bovine chromogranin A (CgA), together with secreted alkaline phosphatase (SEAP) as an external control for apical secretion were expressed in MDCK cells to test if CgA contains sorting signals for polarized secretion. CgA, SEAP, and the endogenous apical marker GP80 were secreted 75-80% apically. Basolateral secretion of SEAP was inhibited 40% by ammonium chloride. Sulfate labeling and digestion with chondroitinase ABC revealed a 120 kDa proteoglycan-CgA and 75 kDa CgA. Inhibition of proteoglycan synthesis did not affect apical secretion of CgA. As CgA is not N-glycosylated, we used tunicamycin to test if cellular N-glycosylation is required for apical sorting. Tunicamycin reversed the polarity of secretion of CgA to the basolateral side. These results suggest that CgA contains dominant apical and recessive basolateral sorting information.

Basolateral sorting signals differ in their ability to redirect apical proteins to the basolateral cell surface

Polarized sorting of membrane proteins in epithelial cells is mediated by cytoplasmic basolateral signals or by apical signals in the transmembrane or exoplasmic domains. Basolateral signals were generally found to be dominant over apical determinants. We have generated chimeric proteins with the cytoplasmic domain of either the asialoglycoprotein receptor H1 or the transferrin receptor, two basolateral proteins, fused to the transmembrane and exoplasmic segments of aminopeptidase N, an apical protein, and analyzed them in Madin-Darby canine kidney cells. Whereas both cytoplasmic sequences induced endocytosis of the chimeras, only that of the transferrin receptor mediated basolateral expression in steady state. The H1 fusion protein, although still largely sorted to the basolateral side in biosynthetic surface transport, was subsequently resorted to the apical cell surface. We tested whether the difference in sorting between trimeric wild-type H1 and the dimeric aminopeptidase chimera was caused by the number of sorting signals presented in the oligomers. Consistent with this hypothesis, the H1 signal was fully functional in a tetrameric fusion protein with the transmembrane and exoplasmic domains of influenza neuraminidase. The results suggest that basolateral signals per se need not be dominant over apical determinants for steady-state polarity and emphasize an important contribution of the valence of signals in polarized sorting.