Published online before print July 7, 2004
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* Departments of Hematology, C14, and
Cell and Molecular Biology, B14, BMC, Lund, Sweden; and
The Netherlands Cancer Institute, Amsterdam
1Correspondence: Department of Hematology, C14, BMC, SE-221 84 Lund, Sweden. E-mail: Inge.Olsson{at}hematologi.lu.se
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Key Words: inflammation • secretory lysosome • endosome pathway
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Cytolytic proteins are packed together with hydrolases in secretory lysosomes (lytic granules) [8 ]. These organelles contain dense cores of tightly packed proteins and internal membrane-bound vesicles and are able to combine storage, regulated secretion, and lysosome function. tm protein targeting to the secretory lysosome requires a sorting signal. Granule matrix protein-targeting mechanisms are largely unknown [3 ] with the exception of targeting lysosome hydrolases and granzyme by the mannose-6-phosphate receptor (MPR) system [9 ], which navigates cargo from the trans-Golgi network (TGN) along the endosome pathway to secretory lysosomes, after which, empty MPR is recycled. In contrast, a nonendosome pathway is a major route for regulated secretory granule formation in endocrine, neuroendocrine, and exocrine cells. In the latter cells, proteins are packed in dense secretory vesicles within the TGN, and mis-sorted proteins are removed before granule maturation [10 11 12 ]. It is not known whether hematopoietic cells have a direct, nonendosome, secretory lysosome-targeting pathway in addition to an endosome pathway. Separate pathways could target separate secretory lysosome subcompartments.
Hematopoietic cells must strictly regulate secretory lysosome delivery of soluble and membrane proteins at sites of inflammation [13 ]. Cell-surface receptor activation induces Ca2+mobilization within the cell and triggers degranulation. During degranulation, fusion of the limiting membrane with the plasma membrane releases the matrix content of the secretory lysosomes. Furthermore, internal vesicles of the secretory lysosomes, designated exosomes, can be released intact [14 ]. Specific protein components such as synaptotagmin are required for the regulation of the exocytosis pathway of hematopoietic cells. These components may act as Ca2+ sensors for secretory lysosomes [15 ]. The membrane fusion processes are mediated by vesicle and target soluble N-ethylmaleimide-sensitive factor attachment protein receptors [16 , 17 ] as well as by small isoforms of GTPases of the Rab protein family [18 ]. A common, critical mechanism for regulated secretion from secretory lysosomes is suggested by observations in the Chediak-Higashi syndrome. In this syndrome, the immune function of cells with secretory lysosomes is impaired, and the nonsecretory function of conventional lysosomes is normal [8 , 13 ].
We have now evaluated further a concept of using hematopoietic granules as delivery vehicles of therapeutic agents. We posed the question as to whether exogenous, secretory lysosome-targeted proteins could be secreted. To address this, RBL cells stably transfected with sTNFR1–tm–Y or sTNFR1–tm–Y–enhanced green fluorescent protein (egfp) were investigated. Our results showed direct targeting of secretory lysosomes with sTNFR1–tm–Y but suggested indirect targeting of sTNFR1–tm–Y–egfp after plasma membrane translocation and endocytosis. Regulated secretion of secretory lysosome-generated sTNFR1 could be triggered by a Ca2+ signal.
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Buffer systems
Lysis buffer for cells [1 M NaCl, 50 mM Tris-HCl, pH 8.0, 0.5% Triton X-100 (v/v)], radioimmune precipitation buffer [radio immunoprecipitation assay (RIPA); 0.75 M NaCl, 0.15 M HEPES, pH 7.3, 0.5% sodium dodecyl sulfate (SDS; v/v), 5% Triton X-100 (v/v), 5% sodium deoxycholate (w/v)], and lysis buffer for immunoblotting [electrophoresis sample buffer; 86 mM Tris, pH 6.8, 11% (v/v) glycerol, 2.3% (w/v) SDS, 1.2% (v/v) ß-ME, 0.005% (v/v) bromophenol blue] were used. Protease inhibitors were added to all buffers before use.
Construction of expression vectors
The cDNA fusion constructs sTNFR1–tm–Y and sTNFR1–tm–Y–egfp were used. tm corresponds to the tm domain of human TNFR1 with flanking sequences (VKGTEDSGTTVLLPLVIFFGLCLLSLLFIGLMYRYORWKSKLYSIV), and Y corresponds to the cytosol tyrosine-sorting signal (SIRSGYEVM) for secretory lysosomes of CD63 [8
]. All polymerase chain reactions (PCRs) were performed by 20-cycle reactions in a Perkin-Elmer 480 thermal cycler using Pfu polymerase (Stratagene, La Jolla, CA), per the manufacturer’s instructions. In the design of the primers, the Kozak consensus leader sequence for maximal translational efficiency [20
] was introduced 5' to the ATG initiation codon. The constructs were cloned into the pcDNA3 vector.
The generation of sTNFR1–tm–Y cDNA was described previously [1 ]. To generate sTNFR1–tm–Y–egfp cDNA, two extension PCRs were performed. The first PCR used the sTNFR1–tm–Y cDNA as a template with the following primers: upstream primer, 5'-TTCGGAGGATCCGCCACCATGGGCCTCTCCACCGTG (A1); downstream primer, 3'-TTCGGAGAATTCTCAATCGATCATCACCTCGTAGCCACTTCT (A6 short), leading to the creation of sTNFR1–tm–Y (A6 short) with BamHI, ClaI, and EcoRI restriction-enzyme sites. The second PCR used the pEGFP-N1 as a template with the following primers: upstream primer, 5'-TTCGGAATCGATGCCACCGTGAGCAAGGGCGAGGAGCTG (A14); downstream primer, 3'- TTCGGAGAATTCTCACTTGTACAGCTCGTCCATGCC (A15). The product was cut by ClaI and EcoRI and ligated into the sTNFR1–tm–Y (A6 short) cDNA, leading to the creation of sTNFR1–tm–Y–egfp with BamHI and EcoRI restriction enzyme sites.
Transfections
RBL-1 cells [7
] were stably transfected by using the Bio-Rad Gene PulserTM (Bio-Rad, Hercules, CA) with electrical settings of 960 µF and 260–300 V as described previously [21
, 22
]. RBL-2H3 cells were transfected by using SuperFect®. Recombinant clones were selected with 1 mg/ml geneticin and were screened for the expression of the transfected protein by biosynthetic radiolabeling and immunoprecipitation.
Biosynthetic radiolabeling
Biosynthetic radiolabeling of newly synthesized protein was carried out using [35S]methionine/[35S]cysteine, as described previously [23
].
Subcellular fractionation
A cell homogenate was centrifuged in a continuous Percoll gradient, and nine fractions were collected as described previously [21
]. All the cytosol was contained in fraction 9. The distribution of secretory lysosomes and Golgi elements was determined by assaying for ß-hexosaminidase and galactosyl transferase, respectively [24
, 25
]. The peak activities of ß-hexosaminidase and galactosyl transferase in Percoll fractions of RBL cells were localized in fractions 2 and 6, respectively [21
]. Furthermore, the distribution of the secretory lysosome marker RMCP-II was determined by immunoprecipitation of radiolabeled cells.
Immunoprecipitation
Cells (2x106/ml) were disrupted in lysis buffer for cells, frozen, and thawed. Percoll-containing subcellular fractions were diluted with 1 vol H2O and one-half of a volume of fivefold-concentrated RIPA. Samples were cleared by centrifugation at 32,000 g for 60 min. Antiserum, protein A-Sepharose, and protein G-Agarose were added, the mixture was rotated overnight, and the pellet was analyzed by SDS-polyacrylamide gel electrophoresis (PAGE) followed by gel exposure to X-ray film at –80°C. Densitometry was performed in a Molecular Imager® FX (Bio-Rad).
Western blotting
Cell extracts were frozen and sonicated in lysis buffer for immunoblotting, boiled for 5 min, and cleared by centrifugation. A lysate aliquot was separated by SDS-PAGE and electroblotted onto Hybond-P nitrocellulose membranes and exposed to HyperfilmTM ECLTM for 10–30 s.
Immunoelectron microscopy
RBL cells were fixed for 24 h in 4% paraformaldehyde (PFA) in 0.1 M PHEM buffer (pH 6.9) containing 240 mM piperazine diethane sulfonic acid (PIPES), 100 mM HEPES, 8 mM MgCl2, and 40 mM ethylene glycol tetra-acetic acid (EGTA) and then processed for ultrathin cryosectioning as described before [26
]. Cryosections (45 nm) were cut at –125°C using diamond knives (Drukker Cuijk, The Netherlands) in an ultracryomicrotome (Leica Aktiengesellschaft, Vienna, Austria) and transferred with a mixture of sucrose and methylcellulose onto formvar-coated copper grids. The grids were placed on 35-mm petri dishes containing 2% gelatin. Double-immunolabeling was performed with the procedure described by Slot et al. [27
] with 10- and 15-nm protein A-conjugated colloidal gold probes (EM Lab, Utrecht University, The Netherlands). After immunolabeling, the cryosections were embedded in a mixture of methylcellulose and uranyl acetate and examined with a Philips CM 10 electron microscope (Eindhoven, The Netherlands). For the controls, the primary antibody was replaced by a nonrelevant rabbit or mouse antiserum.
Immunofluorescence microscopy
For colocalization experiments, fixed RBL-2H3 cells (cultured overnight on glass coverslips) were permeabilized in 1 ml cytoskeletal buffer containing 100 mM KOH, 2 mM MgCl2, 5 mM EGTA, 0.05% (v/v) Triton X-100, and 100 mM piperazine-bis 2-ethane sulfonic acid (pH 6.8) for 10 min on ice and thereafter, were incubated in blocking solution [phosphate-buffered saline (PBS) containing 0.1% bovine serum albumin (BSA; w/v), 0.2% Tween 20 (v/v), and 5% (v/v) goat serum] for 30 min at room temperature. Next, cells were incubated overnight at 8°C with the primary antibody (dilution 1:800–1:1000) in blocking solution. Following washing, cells were incubated with secondary antibody (goat anti-rabbit Alexa FluorR 488 F(ab)2 fragment, dilution 1:500; or goat anti-mouse Alexa FluorR 594 F(ab)2 fragment, dilution 1:500; Molecular Probes, Eugene, OR) for 1 h in blocking solution. After washing twice, the samples were then overlaid with ProLong Antifade reagent (Molecular Probes) and were mounted. Images were recorded on a Nicon Eclipse TE300 inverted fluorescence microscope equipped with a Hamamatsu C4742-95 cooled charged-coupled device camera using a Plan Apochromat 100x objective and a high numerical aperture oil-condenser.
Plasma membrane-localized sTNFR1–tm–Y–egfp was labeled with red fluorescence by incubating transfected cells with a mouse mAb against the sTNFR1 (1:1000, 5 min, room temperature) and subsequently, with an Alexa FluorR 594 F(ab)2 fragment-labeled anti-mouse secondary antibody (1:200, 5 min). After several rinses, experiments were performed. Thereafter, the cells were immediately observed in the microscope or fixed using 1 ml 2% (v/v) PFA solution (Becton Dickinson, Franklin Lakes, NJ) for 15 min on ice and 45 min at room temperature. Fixed cells in suspension (RBL-1) were adhered to poly-L-lysine-coated coverslips. The samples were then mounted, and images were recorded as described above.
Secretion
RBL-1 cells (20x106) or RBL-2H3 cells (10x106) were biosynthetically radiolabeled for 1 h followed by radiolabel chase for 2 h without or with 1 µM Ca2+ ionophore or 10 ng/mL phorbol myristate acetate (PMA), added 30 and 60 min before end of chase, respectively. Secretion was also induced by cross-linking the immunoglobulin (Ig)E receptors. For this, cells were biosynthetically radiolabeled for 45 min followed by a 90-min radiolabel chase in the presence of mouse monoclonal anti-dinitrophenyl (DNP) IgE (1 µg/106 cells/ml). After washing, the cells were incubated 30 min with DNP-albumin conjugate (10 µg/106cells/ml) at 37°C to achieve IgE receptor cross-linking. Cell lysates and supernatants were immunoprecipitated and analyzed by SDS-PAGE.
Biotinylation
After washing with cold PBS, surface proteins of 3 x 106 RBL-2H3 cells expressing sTNFR1–tm–Y–egfp were labeled with 0.5 mg/ml NHS-SS-biotin in PBS with Ca2+ and Mg2+ for 60 min at 4°C and were then washed once with ice-cold 10 mM Tris, pH 7.0, 150 mM NaCl, and 50 mM glycin and twice with the same buffer without glycin. Internalization was initiated by incubation of the cells in Eagle’s minimal essential medium with 20% FBS at 37°C and was terminated by placing the cultures on ice. Remaining biotinylated surface proteins were debiotinylated by incubation with 50 mM 2-ME sulfonic acid in 50 mM Tris, pH 8.7, 100 mM NaCl, 2.5 mM CaCl2 for 2 x 20 min at 4°C. After additional washing twice with 10 mM Tris, pH 7.0, 150 mM NaCl, the cells were lysed in 50 mM Tris, pH 7.0, 1% Triton X-100 with protease inhibitors. The internalized, biotinylated proteins were precipitated by 50 µl streptavidin-agarose beads overnight, separated by SDS-PAGE, and analyzed by Western blotting.
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Processing and targeting sTNFR1–tm–Y
Results from Western blotting of the adherent RBL-2H3 cells transfected with sTNFR1–tm–Y showed full-length 37 kDa protein and a 29-kDa processed form consisting of sTNFR1 (data not shown). Results from immunofluorescence microscopy and immunoelectron microscopy demonstrated targeting of sTNFR1–tm–Y to secretory lysosomes (data not shown) similar to what has been shown before for RBL-1 cells [1
]. Thus, the sTNFR1 signal colocalized with the rat lgp120 and RMCP-II, a major constituent of RBL cell granules. In conclusion, the data demonstrate targeting of sTNFR1–tm–Y to the secretory lysosomes of the adherent RBL-2H3 cells.
Secretion of sTNFR1 from sTNFR1–tm–Y-expressing cells
Physiological Ca2+ stimulation can promote regulated secretion from secretory lysosomes in RBL cells [28
, 29
]. We tried to induce secretion with Ca2+ ionophore, IgE receptor activation, or PMA in RBL-1 (Fig. 1A
) and RBL-2H3 cells (Fig. 1B)
expressing sTNFR1–tm–Y. Using the endogenous secretory lysosome constituent RMCP-II as an indicator of secretory lysosome release, RBL-1 cells seemed to lack secretory capacity (Fig. 1A)
, although constitutive RMCP-II secretion was observed. This was consistent with the observed lack of sTNFR1 secretion in these cells upon incubation with Ca2+ionophore (Fig. 1A)
or IgE receptor cross-linking (data not shown). Furthermore, the endogenous RMCP-II as well as sTNFR1, which accumulated in the densest fractions, corresponding to the secretory lysosomes of sTNFR1–tm–Y-transfected cells, were unaffected by PMA (Fig. 1A)
. PMA is unable to stimulate regulated secretion from secretory lysosomes unless Ca2+influx is stimulated with a Ca2+ionophore [30
]. In contrast, RBL-2H3 cells have a secretory capability. Thus, upon incubation with a Ca2+ionophore, the sTNFR1 that accumulated in the densest fractions and the endogenous RMCP-II were secreted (Fig. 1B)
. In addition, IgE receptor activation, which is a more physiological stimulus for mast cells, induced regulated secretion, although to a lower degree than Ca2+ionophore (Fig. 1B)
. Our results therefore show that secretory lysosome-targeted sTNFR1 was subject to regulated secretion in RBL-2H3-cells but not in RBL-1 cells.
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Figure 1. Ca2+-induced secretion of sTNFR1 in RBL-2H3 but not RBL-1 cells expressing sTNFR1–tm–Y. RBL-1 cells (A) and RBL-2H3 cells (B) transfected with sTNFR1–tm–Y were biosynthetically radiolabeled for 1 h followed by radiolabel chase for 2 h without or with 1 µM Ca2+ionophore or 10 ng/mL PMA added 30 or 60 min before the end of chase, respectively. For induction of secretion by IgE receptor activation, cells were biosynthetically radiolabeled for 45 min followed by 90 min radiolabel chase in the presence of mouse monoclonal anti-DNP IgE. After washing, the cells were incubated 30 min with DNP-albumin conjugate at 37°C for IgE receptor cross-linking. Cell lysates and medium were immunoprecipitated for detection of sTNFR1 and RMCP-II as described in Materials and Methods. The position of sTNFR1–tm–Y, sTNFR1, and RMCP-II is indicated. The flurogram of the IgE receptor activation experiment is overexposed, as secretion was lower as compared with the Ca2+-ionophore experiment.
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Secretory lysosome targeting was investigated by pulse-chase radiolabeling combined with subcellular fractionation. After radiolabeling, sTNFR1–tm–Y–egfp and sTNFR1–tm–Y were visible in the light-density fractions corresponding to the ER, Golgi, and plasma membrane elements (Fig. 2 ). During radiolabel chase, most of the sTNFR1–tm–Y–egfp disappeared from the light-density fractions, and processed sTNFR1–tm–Y remained in these fractions (Fig. 2) . Concomitantly, proteolytically generated sTNFR1 was detected in the densest fractions, suggesting that secretory lysosome targeting occurred prior to release of sTNFR1. The density gradient distribution of secretory lysosomes was confirmed by pulse-chase radiolabeling of RMCP-II. After pulse, RMCP-II was present in the light fractions, and after subsequent radiolabel chase, a proportion of RMCP-II shifted to the densest fractions, indicating the location of the secretory lysosomes. The generated sTNFR1 showed a similar distribution to RMCP-II consistent with secretory lysosome targeting (Fig. 2) .
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Figure 2. Subcellular distribution of radiolabeled sTNFR1–tm–Y–egfp and processed forms as well as endogenous RMCP-II in RBL-1 cells. Cells expressing sTNFR1–tm–Y–egfp were biosynthetically radiolabeled for 1 h (Pulse), followed by chase of the label for 2 h. At the pulse and chase intervals, 100 x 106 cells were removed and homogenized, and the postnuclear supernatant was subcellularly fractionated by centrifugation in Percoll. The fractions were immunoprecipitated and analyzed as described in Materials and Methods. The position of sTNFR1–tm–Y–egfp and the processed sTNFR1–tm–Y and sTNFR1 forms are indicated with arrows. The position of endogenous RMCP-II is also indicated. Molecular weight markers are shown to the left in kDa.
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Figure 3. Colocalization of sTNFR1 with a matrix protein (RMCP) and a membrane-associated protein (lgp120) in secretory lysosomes of RBL-1 cells expressing sTNFR1–tm–Y–egfp. Ultrathin cryosections were labeled with rabbit anti-TNFR1 and protein A gold (10 nm; A, B), double-labeled with anti-TNFR1 (10 nm) and mouse anti-RMCP followed by protein A gold (15 nm; C), or double-labeled with anti-TNFR1 (10 nm) and rabbit anti-lgp120 (15 nm; D). (A) Area showing that the Golgi (G) and the TGN are labeled for sTNFR1. (B) Area showing that the plasma membrane from several cells is also labeled. (C) Area near the Golgi (G) with two granules (g) double-labeled for sTNFR1 and RMCP, and (D) area with two granules (g) double-labeled for lgp120 and sTNFR1. n, Nucleus. Original bar = 200 nm.
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Figure 4. Partial colocalization of egfp with a membrane-associated protein (lgp120) of secretory lysosomes in RBL-1 cells expressing sTNFR1–tm–Y–egfp. Direct localization of egfp and indirect immunolocalization of lgp120 are shown. The cells were fixed, permeabilized, and stained, as described in Materials and Methods. Briefly, the cells were incubated with a rabbit anti-lgp120 antibody. Thereafter, the cells were stained with Alexa FluorTM-labeled anti-rabbit secondary antibody. The localization of egfp and lgp120 is shown in green (A) and red (B), respectively, whereas an overlay of the green and red staining pattern is seen (D), demonstrating partial colocalization (yellow). The corresponding Nomarski image is shown (C). Original size bar = 10 µm.
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Figure 5. PMA-induced shedding of sTNFR1 in RBL-1 cells expressing sTNFR1–tm–Y–egfp. Cells transfected with sTNFR1–tm–Y–egfp were incubated 5 min at room temperature with a mouse mAb against the sTNFR1 (1:1000). Thereafter, the cells were stained with an Alexa FluorTM 594-labeled anti-mouse secondary antibody (1:200, 5 min). Following several rinses to remove unbound antibody, cells were incubated with PMA at 37°C and then fixed using 2% PFA and attached to poly-L-lysine-coated coverslips. The cells shown (A–C) were incubated with 50 nM PMA for less than 1 min, whereas the cells shown (D–F) were incubated 30 min with PMA. The insets show cells incubated with PMA for 15 min at 37°C prior to analysis. The egfp-derived green fluorescence of sTNFR1–tm–Y–egfp is shown (A and D); the red staining patterns of bound sTNFR1 antibody are shown (B and E). The corresponding Nomarski images are shown (C and F). PMA induced a loss of egfp signal from the cell surface and cell interior (D). Prelabeling of cell surface with anti-sTNFR1 (B) was combined with a rapid internalization of the signal (E). PMA incubation prior to antibody incubation resulted in loss of antibody reactivity with the cell surface (E, inset). Original size bar = 10 µm.
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Figure 6. Ca2+-induced secretion of sTNFR1 in RBL-1 expressing sTNFR1–tm–Y–egfp. RBL-1 cells transfected with sTNFR1–tm–Y–egfp were radiolabeled biosynthetically for 1 h followed by radiolabel chase for 2 h without or with 1 µM Ca2+ionophore or 10 ng/mL PMA added 30 or 60 min before end of chase, respectively. Lysates of cells and medium were immunoprecipitated for detection of sTNFR1 as described in Materials and Methods. The position of sTNFR1–tm–Y–egfp and the processed sTNFR1–tm–Y and sTNFR1 forms are indicated with arrows.
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Figure 7. Subcellular distribution of radiolabeled sTNFR1–tm–Y–egfp and processed forms after induced secretion. RBL-1 cells transfected with sTNFR1–tm–Y–egfp were radiolabeled biosynthetically for 1 h, followed by chase of the label for 2 h. During chase, the cells were incubated with or without PMA (A and B) or Ca2+ionophore (C and D). For each condition, 100 x 106 cells were removed and homogenized, and the postnuclear supernatant was fractionated subcellularly by centrifugation in Percoll. The fractions were immunoprecipitated and analyzed as described in Materials and Methods. The SDS-PAGE bands corresponding to sTNFR1–tm–Y–egfp (I; A and C) and the processed forms sTNFR1–tm–Y (II) and sTNFR1 (III; B and D) are shown. The results from densitometry (relative density) of these bands are shown. The sTNFR1–tm–Y and sTNFR1 bands were combined for densitometry. PMA promoted a decrease in sTNFR1–tm–Y–egfp (A) and sTNFR1–tm–Y (B), in particular, of the light-density fractions. Ca2+ionophore gave a slight decrease of sTNFR1–tm–Y–egfp (C) but an extensive elimination of sTNFR1–tm–Y and sTNFR1 (D).
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Endosome reuptake of sTNFR1–tm–Y–egfp
Results from fluorescence microscopy using egfp fluorescence and immunofluorescence with anti-sTNFR1 demonstrated the presence of sTNFR1–tm–Y–egfp at the cell surface in RBL-1 cells (Fig. 8A
and 8B
). In contrast, cells transfected with sTNFR1–tm–Y did not show any sTNFR1 signal at this location (data not shown). Furthermore, antibody labeling of cell-surface sTNFR1–tm–Y–egfp of living cells at room temperature (Fig. 8B)
was followed by rapid internalization of the antibody during subsequent chase at 37°C (Fig. 8D)
. These results implied that internalization of sTNFR1–tm–Y–egfp took place. Rapid internalization was confirmed by a time-course experiment (Fig. 9
).
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Figure 8. Internalization of plasma membrane-located sTNFR1–tm–Y–egfp in RBL-1 cells. Unfixed cells transfected with sTNFR1–tm–Y–egfp were incubated for 5 min at room temperature with a mouse mAb against the sTNFR1 (1:1000). Thereafter, the cells were stained with an Alexa FluorTM 594 -labeled anti-mouse secondary antibody (1:200, 5 min). Following several rinses to remove unbound antibody, images were recorded immediately (A and B) or after a 45-min chase period at 37°C (C and D). The green fluorescence of sTNFR1–tm–Y–egfp is shown (A and C), whereas the images to the right show the red-staining patterns resulting from bound sTNFR1 antibody. If the primary antibody was omitted, no staining was observed. The red fluorescence is initially observed at the cell surface (B) but is transferred intracellularly after chase (D), indicating endosome reuptake. Original size bar = 10 µm.
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Figure 9. Time-course of the endosomal reuptake of sTNFR1–tm–Y–egfp in RBL-1 cells. Cells transfected with sTNFR1–tm–Y–egfp were incubated for 5 min at room temperature with a mouse mAb against the sTNFR1 (1:1000). Thereafter, the cells were stained with an Alexa FluorTM 594 -labeled anti-mouse secondary antibody (1:200, 5 min). Following several rinses to remove unbound antibody, cells were chased for various times at 37°C and then fixed using 2% PFA and attached to poly-L-lysine-coated coverslips. The staining pattern of the bound sTNFR1 antibody after a 1-min (D), 3-min (E), and 10-min (F) chase is shown, together with the corresponding Nomarski images. Original size bar = 10 µm.
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Figure 10. Internalization of sTNFR1–tm–Y–egfp determined by surface biotinylation and Western blotting. Surface proteins of 3 x 106 RBL-2H3 cells expressing sTNFR1–tm–Y–egfp were labeled at 4°C by sulfo-NHS-SS-biotin before initiation of constitutive sTNFR1–tm–Y–egfp internalization at 37°C. Total cell-surface, biotinylated sTNFR1–tm–Y–egfp was determined in cells held at 4°C to prevent internalization (A). Background internalization (at 4°C) of sTNFR1–tm–Y–egfp was measured in cells debiotinylated without prior initiation of internalization (B). After allowing internalization for 10 (C) and 30 min (D) at 37°C, debiotinylation was carried out before Western blotting analysis. Molecular weight markers are shown to the left in kDa.
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Targeting signals
When the tyrosine-based, cytosol-sorting signal from CD63 (a normal tm secretory lysosome constituent) was placed at the C-terminus, secretory lysosome targeting of exogenous proteins expressed in RBL cells was successfully achieved. This signal conforms to the tm protein-sorting sequence YXXØ necessary for coat recruitment, membrane invagination, and transport vesicle formation at the TGN and plasma membrane [31
]. Y corresponds to tyrosine and X to any amino acid, and the Ø position can accommodate residues with bulky hydrophobic side-chains. The distance from the tm domain of the sorting motif is of importance in lysosome-targeting and endocytosis [32
]. First, a carboxy-terminal YXXØ sequence close to the tm domain serves as a lysosome-targeting signal at the TGN and/or endosome. Second, a hidden YXXØ signal that was not placed carboxy-terminally might still serve as a signal for endocytosis. In the latter case, the hidden YXXØ motif prevents interactions with the sorting machinery at the TGN, thus leading to plasma membrane targeting but permitting endocytosis. Our results are consistent with this general model for targeting. The sTNFR1–tm–Y was targeted directly to secretory lysosomes by a carboxy-terminal YXXØ sequence placed close to the tm domain. The sTNFR1–tm–Y–egfp, conversely, was targeted through an indirect pathway to the secretory lysosome, presumably because the YXXØ motif was not carboxy-terminal but hidden amino-terminal to egfp.
Internalization of sTNFR1–tm–Y–egfp via the plasma membrane agreed with an indirect pathway for secretory lysosome targeting. Endocytosis might be followed by the generation of sTNFR1–tm–Y after proteolytic release of egfp from the endosomes and/or secretory lysosomes. As a result, after egfp removal, Ø of YXXØ would be offered at a carboxy-terminal position and have the ability to signal for secretory lysosome targeting at an endosome-sorting station. Thus, newly synthesized sTNFR1–tm–Y–egfp is, at least in part, rapidly moved to the cell surface after exiting the TGN. sTNFR1–tm–Y–egfp then enters the endocytic system, presumably via activated protein (AP)-2-dependent sorting at the plasma membrane [31 ], with the subsequent generation of sTNFR1–tm–Y. We assume that a second sorting event in endosomes, possibly through the AP-3 complex, could be involved in secretory lysosome targeting. AP-3 has been localized to an endocytic compartment [33 ]. The reason that sTNFR1–tm–Y–egfp might not interact with the AP-3 complex at the TGN in the first place was thought to be insufficient exposure of the sorting sequence. In conclusion, our results can be explained on the basis of direct targeting of sTNFR1–tm–Y to secretory lysosomes and indirect targeting of sTNFR1–tm–Y–egfp to secretory lysosomes after plasma membrane targeting and endocytosis.
Generation of sTNFR1 for secretion
The sTNFR1–tm–Y was processed into a lower molecular weight protein. As the processed protein was located to the densest subcellular fractions, it was likely to consist of sTNFR1 released into the secretory lysosomes as a result of the limited proteolysis of sTNFR1–tm–Y. Furthermore, immunoelectron microscopy verified colocalization of sTNFR1–tm–Y and/or its processed form with endogenous secretory lysosome constituents such as RMCP-II and lysosome-associated membrane protein-1 (LAMP-1). The sTNFR1 signal was, however, located mostly in the interior of the secretory lysosome in contrast to LAMP-1, which was on the limiting membrane. It can therefore be concluded that sTNFR1–tm–Y was first targeted to the limiting membrane followed by the transfer to internal vesicles in a similar manner to CD63 [34
]. Transfer to internal vesicles has been reported to require a mono-ubiquitin tag on the cytosol tail and binding by an escorting complex, endosomal sorting complex required for transport 1 [35
]. However, CD63 transfer to internal vesicles does not require ubiquitination of the cytosol-sorting sequence [36
]. The cytosol-sorting motif (Y) at the C terminus of sTNFR1–tm–Y was derived from CD63. Consequently, the internal vesicle transfer of sTNFR1–tm–Y was likely similar to the transfer of CD63 and independent of ubiquitination. As a result of internal vesicle transfer, the major N-terminal region of the sTNFR1–tm–Y would be on the outside of the internal vesicle. At this position, proteolysis would generate sTNFR1 for release into the lumen of the secretory lysosome. As a consequence, the sTNFR1 formed would be deposited in the secretory lysosome matrix and/or dense cores.
Regulated secretion
We found a discrepancy in the secretory capacity between RBL-2H3 cells, which grow in adherent layers, and RBL-1 cells, which grow in suspension. RBL-2H3 was able to secrete endogenous secretory lysosome constituents, but RBL-1 cells lacked such a capacity. Results from experiments with RBL-2H3 cells clearly showed support for our concept in that regulated secretion of secretory lysosome-targeted sTNFR1 is induced by a Ca2+ ionophore or more physiologically, by IgE receptor activation. Furthermore, sTNFR1–tm–Y seemed to be rather stable in the secretory lysosome environment, with the exception of when the tm–Y fragment was cleaved by proteolysis, resulting in generation of membrane-free sTNFR1. The Ca2+-triggered secretion induced the release of the sTNFR1, consistent with discharge from the granule matrix and dense cores of the secretory lysosome. In contrast, neither RMCP-II nor sTNFR1 was, however, secreted upon Ca2+influx in sTNFR1–tm–Y-transfected RBL-1 cells. This lack of Ca2+-triggered degranulation indicated a secretion deficiency in RBL-1 cells. In spite of this, Ca2+ionophore promoted release and secretion of sTNFR1 in RBL-1 cells transfected with sTNFR1–tm–Y–egfp. This may correspond to the regulated secretion from secretory lysosomes or ectodomain shedding at the cell surface. The discrepancy in Ca2+-dependent secretion between sTNFR1–tm–Y- and sTNFR1–tm–Y–egfp-transfected RBL-1 cells may reflect a difference in compartmentalization of the targeted products corresponding to the differences in intracellular transport discussed above.
PMA activation of PKC is a strong signal for shedding of sTNFR at the cell surface [19 ]. In support of this, PMA promoted sTNFR1 release from sTNFR1–tm–Y–egfp-transfected RBL-1 cells. However, PMA did not promote secretory lysosome secretion (Fig. 1) . This is consonant with an inability of PMA to stimulate granule release unless Ca2+influx is generated with a Ca2+ionophore [30 ].
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Received November 26, 2003; revised May 19, 2004; accepted June 1, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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