Report |
Address correspondence to Miguel A. Alonso, Centro de Biología Molecular "Severo Ochoa," Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. Tel.: 34-91-397-8037. Fax: 34-91-397-8087. E-mail: maalonso{at}cbm.uam.es
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Abstract |
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Key Words: hepatocytes; transcytosis; lipid rafts; MAL family; polarized transport
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Introduction |
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Simultaneous to the indirect transcytotic route, polarized epithelial MDCK cells transport newly synthesized proteins to the apical surface directly from the Golgi. A direct transport pathway appears to be mediated by integration of cargo protein into specialized glycolipid and cholesterol-enriched membrane microdomains or rafts that subsequently originate vesicular carriers destined for the apical surface (Simons and Wandinger-Ness, 1990). MAL is a nonglycosylated integral membrane protein of 17 kD containing four hydrophobic segments (Alonso and Weissman, 1987) that exclusively resides in rafts (Puertollano et al., 1999). Using MDCK cells whose endogenous MAL was depleted, an essential role has been demonstrated for MAL as an element of the machinery necessary for transport of apical proteins through the direct route (Cheong et al., 1999; Puertollano et al., 1999; Martín-Belmonte et al., 2000, 2001).
MAL is the founder member of a family of proteins (the MAL protein family) with structural and biochemical similarities (Pérez et al., 1997). MAL2 is a novel member of the MAL family identified recently (Wilson et al., 2001). In our initial experiments, we detected MAL2 gene expression in hepatoma HepG2 cells and in the epithelial MDCK and Caco-2 cell lines, all of which use the transcytotic pathway to a greater (HepG2) or lesser extent (MDCK and Caco-2) to target membrane proteins to the apical surface. Since we considered the hypothesis that MAL2 may act as machinery for transcytosis plausible, we undertook a study of MAL2 function in hepatoma HepG2 cells, which are a paradigm for the study of the transcytotic pathway in a cellular context deprived of other apical routes of transport for single transmembrane and glycosylphosphatidylinositol (GPI)*-anchored proteins (Bastaki et al., 2002).
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Results and discussion |
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The canalicular membrane is highly enriched with actin filaments, and so F-actin staining with fluorescent phalloidin can be used to visualize this membrane subdomain (Zegers et al., 1998). 48 h after plating, the percentage of HepG2 cells forming bile canaliculi was estimated to be 30% as assayed by F-actin staining. We have confined our analysis exclusively to this polarized cell population. Fig. 2 A shows that MAL2 was mostly detected in the canalicular membrane region as revealed by double-label immunofluorescence analysis of MAL2 and F-actin and parallel imaging under a phasecontrast microscope. In the nonpolarized cell population, MAL2 was distributed in the perinuclear region (unpublished data). Confocal microscopic analysis (Fig. 2 B) in a horizontal x,y plane shows that MAL2 mostly distributed in polarized cells beneath the actin belt that surrounds the bile canaliculus. In addition, the presence of low levels of MAL2 in perinuclear vesicular structures was observed in some of the confocal planes. The vertical x,z view shows that, although actin is also detected inside the bile canalicular space, consistent with the presence of actin in microvilli protruding in the bile canaliculi, MAL2 was only found beneath the canalicular actin cytoskeleton. Consistent with the distribution observed in polarized HepG2 cells, MAL2 was exclusively detected at the bile canalicular membrane region by immunohistochemical analysis of human liver tissue sections (Fig. 2 C).
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As a control for the specificity of the effect of oligonucleotide MAL2/AS, we designed different DNA constructs producing MAL2 transcripts with an altered 5' untranslated region and modifications in the first nucleotides of the coding region (silent substitutions, deletions, or insertions) that prevent pairing with the antisense MAL2/AS oligonucleotide. The constructs encoded an intact MAL2 protein (MAL2-e) or MAL2 proteins modified by either a fiveamino acid deletion (MAL2-N) or the addition of the Myc epitope (MAL2-Myc) in the sequence contiguous with the initial methionine residue. The modifications introduced did affect neither the glycosylation nor the incorporation of the molecule into rafts (unpublished data). HepG2 cells that stably expressed each of the MAL2 proteins were used to assay the effect of the antisense MAL2/AS oligonucleotide on the levels of exogenous and endogenous MAL2. We found MAL2 expression in the presence of antisense oligonucleotide MAL2/AS in all the transfectants as assayed by immunoblot and immunofluorescence analyses (Fig. 4 C). Although the staining and distribution of MAL2 was unaltered by transfection with the control oligonucleotide, the antisense oligonucleotide MAL2/AS rendered MAL2 levels almost undetectable in normal HepG2 cells. In the transfectants treated with control oligonucleotides, the exogenous protein presented a distribution identical to that of the endogenous protein. However, under conditions of endogenous MAL2 depletion only the transfectants expressing the intact protein (HepG2/MAL2-e cells) presented the typical subcanalicular distribution of endogenous MAL2.
MAL2 depletion blocks transcytosis of pIgR and CD59 in HepG2 cells
Fig. 5 A shows that internalization of IgA in HepG2 cells was not affected in cells treated with the antisense MAL2/AS oligonucleotide despite the reduction of MAL2 levels, since IgA accumulated in small peripheral basolateral endosome structures in the continuous presence of nocodazole, regardless of the levels of MAL2. However, whereas pIgR-IgA moved to apical endosomes and the bile canaliculi after removal of nocodazole and incubation at 37°C in the cells transfected with the control oligonucleotide, the complex mostly shifted to large endosomes with perinuclear location and accumulated in this compartment in cells with depleted levels of MAL2. The accumulated IgA colocalized partially with Tf in the same structures under conditions that allow trafficking of Tf from basolateral early endosomes to downstream compartments, including perinuclear endosomes (Fig. 5 B). Therefore, MAL2 is not required for transport of pIgR-IgA from the sinusoidal membrane to peripheral and perinuclear endosomes but is essential for exit of pIgR-IgA from perinuclear endosomes to transit to downstream apical endosome compartments.
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MAL and MAL2 as elements of the integral membrane protein machinery for raft-mediated trafficking
Although the use of direct routes of transport to the apical membrane appears to be restricted, the indirect transcytotic route constitutes a common pathway for most polarized cells. The fact that MAL and MAL2 are expressed and access rafts independently of each other suggests that the two proteins perform distinct tasks in the cell. In epithelial MDCK cells, MAL depletion causes accumulation at the Golgi region of cargo normally delivered to the apical membrane through the direct route (Cheong et al., 1999; Martín-Belmonte et al., 2001). MAL2 depletion in HepG2 cells blocks apical transport of transcytosing molecules at perinuclear endosomes. Therefore, both MAL and MAL2 are members of the machinery of polarized transport: MAL plays an essential role in the direct apical transport pathway, whereas our results presented here show that MAL2 is required for the indirect transcytotic route.
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Materials and methods |
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Cell culture conditions, DNA constructs, and transfections
Human hepatoma HepG2 cells (Sormunen et al., 1993) were grown on Petri dishes in DME supplemented with 10% FBS (GIBCO-BRL), penicillin (50 U/ml), and streptomycin (50 µg/ml) at 37°C in an atmosphere of 5% CO2/95% air. The 19-mer phosphorothioate oligonucleotide MAL2/AS (5'-GGCCGACATGCTGCCGCTG-3') (Isogen Bioscience BV) was designed to anneal to endogenous MAL2 mRNA. The phosphorothioate oligonucleotide MAL/AS used in control experiments has been described previously (Puertollano et al., 1999). Oligonucleotides (50 µM) were introduced into HepG2 cells by electroporation of 2 x 106 cells resuspended in 200 µl of culture medium buffered with 10 mM Hepes, pH 7.0, and supplemented with 10% FBS and with 20 µg of salmon sperm DNA as a carrier. The cell suspension was placed in a 4-mm gap cuvette, and the transfection was performed with the equipment set up at 200 V, 480 ohms, and 960 µF. Under these conditions, the actual pulse length, as defined by the equipment manufacturer, ranged from 40 to 50 ms. After electroporation, 1 ml of culture medium with 10% FBS was added, and the cells were left at room temperature for 10 min. Finally, cells were plated out at subconfluent levels. Parallel experiments to measure the uptake of oligonucleotides using phosphorothioate oligonucleotides labeled at the 5' end with Texas red indicated that the transfection efficiencies varied between 9599% of cells as assayed by immunofluorescence analysis (unpublished data).
To express modified MAL2 mRNA species, the cDNA containing the coding sequence of MAL2 (Wilson et al., 2001) was amplified by PCR using 5' oligonucleotide primers containing sequences designed with the appropriate modifications and an oligonucleotide primer specific for its 3' end. The amplification products were cloned into the pCR3.1 DNA eukaryotic expression vector (Invitrogen). The cDNA construct expressing human pIgR was provided by Dr. P. Brandtzaeg (University of Oslo, Oslo, Norway). Transfection of HepG2 cells with plasmid DNA was performed by electroporation using an Electro Cell Manipulator 600 (BTX). Stable transfectants were selected by treatment with 0.5 mg/ml G-418 sulfate (Sigma-Aldrich) for at least 4 wk after transfection. Cell clones were screened by immunofluorescence analysis and/or reverse transcription coupled to the PCR using oligonucleotide primers corresponding to sequences in the vector flanking the cloning sites used. The clones that proved to be positive were maintained in drug-free medium.
Preparation of monoclonal antibodies to MAL2
The peptide corresponding to amino acids 1328 of the MAL2 molecule was coupled to keyhole limpet hemocyanin. Spleen cells from mice immunized with the peptide were fused to myeloma cells and plated onto microtiter plates. The hybridoma clone 9D1, which produced antibodies that recognized MAL2 in membrane extracts from COS-7 cells transiently expressing tagged MAL2, was selected.
Northern blot analysis
20 µg total RNA from the indicated cell lines were denatured in 50% formamide and 2.2 M formaldehyde at 65°C, electrophoresed, and transferred to nylon membranes. RNA samples were hybridized under standard conditions to the indicated 32P-labeled cDNA probes. Final blot washing conditions were 0.2 x SSC/0.1% SDS (1 x SSC = 0.15 M NaCl, 0.015 M sodium citrate, pH 7.0) at 65°C.
Detergent extraction procedures and immunoblot analysis
HepG2 cells grown to confluency in 100-mm dishes were rinsed with PBS and lysed for 20 min in 1 ml of 25 mM Tris-HCl, pH 7.5, 150 mM NaCl, 5 mM EDTA, and 1% Triton X-100 at 4°C. Lipid rafts were prepared essentially as described by Brown and Rose (1992). Fractions of 1 ml were harvested from the bottom of the tube, and aliquots were subjected to immunoblot analysis. Samples were then subjected to SDS-PAGE in 15% acrylamide gels under reducing conditions and transferred to Immobilon-P membranes (Millipore). After sequential incubation with the primary and secondary antibodies, blots were developed using an ECL Western blotting kit (Amersham Biosciences).
Immunofluorescence and immunohistochemical analyses
HepG2 cells were fixed in 4% paraformaldehyde for 15 min, rinsed, and treated with 10 mM glycine for 5 min. The cells were then permeabilized with 0.2% Triton X-100, rinsed, and incubated with 3% BSA in PBS for 15 min. Cells were incubated for 1 h with the indicated primary antibodies, rinsed several times, and incubated for 1 h with the appropriate fluorescent secondary antibodies (Southern Biotech). Controls to assess labeling specificity included incubations with control primary antibodies or omission of the primary antibodies. As indicated in each case, images were obtained using either a Bio-Rad Laboratories Radiance 2000 Confocal Laser microscope or a conventional fluorescence microscope (ZEISS).
For immunohistochemical analysis, normal human liver tissue was fixed for several hours in 10% neutral buffered formalin and subjected to tissue processing and paraffin embedding. 5-µm tissue sections were blocked with a control antibody, and then sections were sequentially incubated with anti-MAL2 9D1 mAb and peroxidase-conjugated antimouse IgG antibodies. Sections were developed with 0.5 mg/ml of 3,3'-diaminobenzidine tetrahydrochloride and hydrogen peroxide and counterstained with hematoxylin.
Transcytosis assays
The polymeric human IgA stock employed consisted of a mixture of 70% dimeric IgA and 30% of heavier IgA polymers, mostly trimers and tetramers. IgA transcytosis was analyzed essentially as described by van IJzendoorn and Hoekstra (1998). Briefly, HepG2 cells stably expressing exogenous pIgR were treated twice with a 100-fold excess of asialofetuin for 30 min at 4°C to block IgA internalization through asialoglycoprotein receptors, and then incubated with 50 µg/ml IgA at 4°C for 30 min, washed extensively, and incubated at 37°C. Under these conditions HepG2 cells expressing pIgR, but not normal HepG2 cells, were able to internalize IgA. After cell fixation and permeabilization, IgA was detected using antihuman IgA antibodies coupled to fluorescein. To analyze the transcytotic transport of CD59, HepG2 cells were incubated with anti-CD59 mAb for 30 min at 4°C, washed extensively, and incubated at 37°C. Cells were fixed and permeabilized, and the antibody bound to CD59 was detected using a fluorescent antimouse IgG2a antibody.
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Footnotes |
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Acknowledgments |
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M.C. de Marco is the recipient of a predoctoral fellowship from the Ministerio de Ciencia y Tecnología. This work was supported by grants from the Comunidad de Madrid (08.3/0025/2000; 08.5/0066/2001.1), Ministerio de Ciencia y Tecnología (PM99-0092), Fondo de Investigación Sanitaria (01/0085-01/02), and Fundación Eugenio Rodríguez Pascual. We also acknowledge an institutional grant from the Fundación Ramón Areces to Centro de Biología Molecular "Severo Ochoa."
Submitted: 6 June 2002
Revised: 3 September 2002
Accepted: 5 September 2002
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References |
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