Lysosomal fusion and SNARE function are impaired by cholesterol accumulation in lysosomal storage disorders

Abstract

The function of lysosomes relies on the ability of the lysosomal membrane to fuse with several target membranes in the cell. It is known that in lysosomal storage disorders (LSDs), lysosomal accumulation of several types of substrates is associated with lysosomal dysfunction and impairment of endocytic membrane traffic. By analysing cells from two severe neurodegenerative LSDs, we observed that cholesterol abnormally accumulates in the endolysosomal membrane of LSD cells, thereby reducing the ability of lysosomes to efficiently fuse with endocytic and autophagic vesicles. Furthermore, we discovered that soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs), which are key components of the cellular membrane fusion machinery are aberrantly sequestered in cholesterol-enriched regions of LSD endolysosomal membranes. This abnormal spatial organization locks SNAREs in complexes and impairs their sorting and recycling. Importantly, reducing membrane cholesterol levels in LSD cells restores normal SNARE function and efficient lysosomal fusion. Our results support a model by which cholesterol abnormalities determine lysosomal dysfunction and endocytic traffic jam in LSDs by impairing the membrane fusion machinery, thus suggesting new therapeutic targets for the treatment of these disorders.

Figure 1

Lysosomal fusion is impaired in LSD cells. (A) EGFR degradation was followed in MSD, MPS-IIIA and WT MEFs by treating the cells with EGF for the indicated time to stimulate EGFR internalization. The cells were then immediately lysed and subjected to anti-EGFR blotting. The amount of remaining EGFR was quantified by densitometry analysis (Image J) of the blot and expressed in the chart as % of the EGFR amount present at time T₀ (100%). The values in the chart represent the mean±s.e.m. values of three independent experiments. (B) MSD, MPS-IIIA and WT MEFs cells loaded with dextran (alexafluor-594 conjugated) were labelled with anti-LAMP1 antibody and the percentage of dextran co-localizing with LAMP1 was evaluated. The chart displays merge values (means±s.e.m.) that represent the percentage of dextran co-localizing with LAMP1 measured in 15 different cells of triplicate experiments. (C) The rate of lysosome fusion with autophagosomes was monitored in MSD, MPS-IIIA and WT MEFs transfected with a tandem fluorescently tagged LC3 (Kimura et al, 2007). The rate of autophagosome maturation reflected the percentage of the LC3 ‘unfused' (green/red fluorescence ratio) at each time (1 and 3 h) after bafilomycin removal (T₀). The percentage of LC3 ‘unfused' was displayed versus the value at T₀ (assumed to be 100%). Values are represented as means±s.e.m. of triplicate experiments. ^*P<0.05, Student's t-test: (A): WT versus MSD and WT versus MPS-IIIA; (B, C): WT versus MSD and WT versus MPS-IIIA at each time point. Scale bar: 10 μm (B, C).

Figure 2

Alterations in lipid composition of endolysosomal membranes from LSD cells. (A) Cholesterol levels were measured in the indicated endolysosomal membrane samples containing equal amount of proteins and expressed as ng of cholesterol per μg of protein (see Materials and methods section for details). (B) Filipin staining showing cholesterol accumulation in the endolysosomal compartment of MSD and MPS-IIIA MEFs (arrowheads and enlarged images). (C, D) Total lipids extracted from the indicated endolysosomal membrane samples (30 μg of proteins) were either (C) subjected to a phosphate assay to quantify the bulk of phospholipids or (D) separated by TLC. Phospholipids and cholesterol on TLC plates were revealed by molybdenum blue staining. CHOL, cholesterol; LBPA lysobisphosphatidic acid; PC, phosphatidylcoline; PE phosphatidylethanolamine; PI phosphatidyinositol; SM sphingomyelin. (A, C) Values represent the mean±s.e.m. values of three independent experiments. ^*P<0.05, Student's t-test: (A, C): WT versus MSD and WT versus MPS-IIIA. Scale bar, 10 μm (B).

Figure 3

Cholesterol accumulation inhibits lysosomal fusion. Endolysosomal membrane cholesterol measurements and Filipin staining were carried out in either (A) WT MEFs loaded with cholesterol or in MSD and (D) MPS-IIIA MEFs treated with MβCD. Arrowheads and enlarged images show cholesterol accumulation in endolysosomes of cholesterol-loaded WT MEFs. After treatments the rate of autophagosome maturation (B, E) and the transport of fluorescent dextran to lysosomes (C, F) were also analysed as in Figure 1. WT controls for autophagosome maturation and dextran transport experiments were performed as shown in Figure 1. (A–F) Values represent the mean±s.e.m. values of three independent experiments. ^*P<0.05, Student's t-test: (A, C): WT versus WT+cholesterol; (B): WT versus WT+cholesterol for each time point; (D, F): MSD versus MSD+MβCD and MPS-IIIA versus MPS-IIIA+MβCD; (E): MSD versus MSD+MβCD and MPS-IIIA versus MPS-IIIA+MβCD for each time point. Scale bar: 10 μm (A, C, D, F).

Figure 4

The LSD endolysosomal membrane contains increased amount of cholesterol-enriched regions. (A) Endolysosomal membranes from MSD, MPS-IIIA and WT MEFs were treated with 1% Triton X-114 and loaded on a sucrose gradient. Immunoblots with Flotillin-1 identified DRMs in fractions 2, 3 and 4 (arrows). The fractions at the bottom of the gradient (12 and 13) correspond to high-density detergent soluble fractions, whereas the remaining ones were defined as intermediate fractions (intermediate-I: 5, 6, 7 8; intermediate-II: 9, 10 and 11). The percentage of Flotillin-1 in DRMs was calculated from the densitometric quantification of immunoblots. (B) Immuno-EM of GM1 lipid was carried out in WT, MSD and MPS-IIIA MEFs by staining cells with anti-cholera toxin B antibodies (see Materials and methods section). The number of GM1-positive dots was measured in 25 cells from three independent experiments and displayed as fold to WT. (C) Endolysosomal membranes from MSD, MPS-IIIA and WT MEFs were stained with C-laurdan and subsequently analysed by fluorescence spectrophotometry to calculate the GP value (see Materials and methods section for details). Distribution of cholesterol was also measured throughout the gradient ad expressed as percentage of total cholesterol in raft (DRMs) and soluble fractions. (D) Equal aliquots from either DRMs or soluble fractions were pooled, the protein content determined and displayed as percentage of total protein in DRM and soluble gradient regions. (E) Immunoblotting profiles of the transferrin receptor and LAMP1 in the sucrose gradient. Values represent the mean±s.e.m. values of three experiments (A–D). ^*P<0.05, Student's t-test: (A–C): WT versus MSD and WT versus MPS-IIIA; (D): WT versus MSD and WT versus MPS-IIIA for each fraction. Scale bar: 0.3 μm (B).

Figure 5

SNAREs are sequestered by cholesterol within endolysosomal membranes. (A) VAMP7, Vti1b and syntaxin 7 distributions in endo-lysosomal membranes from MSD, MPSIIIA and WT MEFs was evaluated by immonoblotting analysis of gradient fractions. To simplify the analysis the DRM, the intermediate-I, the intermediate-II and soluble fractions of the gradient were pooled separately and then subjected to immunoblotting. SNARE distribution was also analysed after loading WT cells with cholesterol and after MβCD treatment. In MSD and MPS-IIIA MEFs, all analysed SNAREs abnormally accumulate in DRMs of lysosomal membranes. Cholesterol modulation results in a change of SNARE distribution. (B) The percentage of each analysed SNARE observed in DRM fractions was quantified from blots (ImageJ densitometry analysis) and displayed as relative amount versus WT. (C, E) Immunoblots and relative quantification showing SNARE protein levels along with LAMP1 protein levels in (C) endolysosomal membranes and (E) total cell lysates from WT (untreated and cholesterol treated) and MSD MEFs (untreated and MβCD treated). In the graphs, the protein levels were displayed as relative amount versus WT. (D) WT and MSD MEFs transfected with GFP–VAMP7 were stained with anti-GFP for immuno-EM. The enlarged image shows internalization of GFP–VAMP7 particles (arrow). (F) Syntaxin 5, Sec22 and SNAP23 distribution in total membrane derived from control WT and MSD MEFs. Syntaxin 5 immunoblot shows two bands (^*, 35 kDa and ^**, 42 kDa) corresponding to the two isoforms of the protein. (G) Distribution profile of Rab7 in WT and MSD lysosomal membranes. Values represent the mean±s.e.m. values of three independent experiments (B, C, E). ^*P<0.05, Student's t-test: (B): WT versus MSD, WT versus MPS-IIIA, WT versus WT+cholesterol, MSD versus MSD+MβCD, and MPS-IIIA versus MβCD for each analysed SNARE; (C, E): WT versus MSD, WT versus WT+cholesterol and MSD versus MSD+MβCD for each analysed protein. Scale bar: 0.3 μm (D).

Figure 6

SNAREs are locked in an assembled form in LSD endolysosomal membranes. (A) SDS-resistant complexes containing Vti1b were detected by immunoblotting analysis of nonboiled samples corresponding to total, detergent insoluble (DRM) and detergent soluble (Sol.) endo-lysosomal membrane fractions derived from MSD and WT MEFs. The SDS-resistant complexes were also visualized after loading WT MEFs with cholesterol and after treating MSD MEFs with MβCD. Immunoblots revealed the presence of low molecular weight complexes (^*, 50–60 kDa) and high molecular weight complexes (^**, >80 kDa). The percentage of Vti1b in SDS-resistant complexes in total endolysosomal membranes (bottom-left chart) and the amount of Vti1b-containing SDS-resistant complexes in DRM and soluble fractions (bottom-right chart) were calculated by the densitometric quantification of the correspondent immunoblots (ImageJ). Values represent the mean±s.e.m. values of three independent measurements. ^*P<0.05, Student's t-test: WT versus MSD, WT versus WT+cholesterol and MSD versus MSD+MβCD (bottom-left chart); WT versus MSD, WT versus WT+cholesterol and MSD versus MSD+MβCD for each fraction (bottom-right chart). (B) Syntaxin 7 and VAMP7 were co-immunoprecipitated with Vti1b using anti-Vti1b antibodies in WT (untreated or cholesterol treated) and in MSD (not treated or MβCD treated) MEFs. The amount of Vti1b precipitated in each cell line is also shown. (C) SDS-resistant complexes are decorated by anti-syntaxin 7 antibodies in total endolysosomal membrane fraction from WT and MSD MEFs. (D) Membrane-associated α-SNAP and its release in the cytosol were evaluated by western blot analysis on total cell lysates (total), intracellular membranes recovered after centrifugation from a post-nuclear supernatant fraction (membrane associated) and cell lysates devoid of membranes (cytosolic released) derived from MSD (untreated or MβCD treated) and WT (untreated or cholesterol treated) MEFs.

Figure 7

Cholesterol levels affect SNARE trafficking. (A) MSD, MPS-IIIA and WT MEFs were subjected to a triple labelling with anti-VAMP7, anti-Vti1b and anti-LAMP1 antibodies. The merges between VAMP7 and Vti1b (double merges in yellow) and between VAMP7, Vti1b and LAMP1 (triple merges in white) are shown (see also enlarged images showing the extent of co-localization in different regions of the cells). The VAMP7–Vti1b co-localization was quantified in 15 different cells and displayed as % of Vti1b co-localizing with VAMP7 (means±s.e.m.). (B) Co-localization of Vti1b with epsinR was quantified by double-labelling experiments in MSD and MPS-IIIA (untreated and MβCD treated) and in control WT (untreated and cholesterol treated) MEFs. The chart displays merge values (means±s.e.m.) that represent the percentage of Vti1b co-localizing with epsinR measured in 15 different cells. (C) Vti1b trafficking was monitored by FRAP analysis in WT (untreated and cholesterol treated) and MSD (untreated and MβCD treated) MEFs transfected with GFP–Vti1b (see Materials and methods section for details). FRAP data are displayed as percentage of recovery with respect to the fluorescence before bleach (100%) and are representative of 10 recordings from different cells. A summary of t_1/2 values is also shown. ^*P<0.05, Student's t-test: (A): WT versus MSD and WT versus MPS-IIIA; (B): WT versus MSD, WT versus MPS-IIIA, WT versus WT+cholesterol, MSD versus MSD+MβCD and MPS-IIIA versus MPS-IIIA+MβCD); (C): WT versus MSD, WT versus WT+cholesterol, MSD versus MSD+MβCD. Scale bar: 10 μm (A, B).

Figure 8

A working model. Cholesterol-enriched regions accumulate in the endolysosomal membranes of LSDs sequestering SNAREs in defined domains. This locks SNAREs in assembled complexes and reduces their availability to interact with dedicated adaptor components of vesicle coat, a process needed for SNARE re-distribution between membranes. Consequently, the dysfunction of SNARE cycle decreases the rate of new fusion events.