Revealing the topography of cellular membrane domains by combined atomic force microscopy/fluorescence imaging

Abstract

Simultaneous atomic force microscopy (AFM) and confocal fluorescence imaging were used to observe in aqueous buffer the three-dimensional landscape of the inner surface of membrane sheets stripped from fixed tumor mast cells. The AFM images reveal prominent, irregularly shaped raised domains that label with fluorescent markers for both resting and activated immunoglobin E receptors (FcepsilonRI), as well as with cholera toxin-aggregated GM1 and clathrin. The latter suggests that coated pits bud from these regions. These features are interspersed with flatter regions of membrane and are frequently surrounded and interconnected by cytoskeletal assemblies. The raised domains shrink in height by approximately 50% when cholesterol is extracted with methyl-beta-cyclodextrin. Based on composition, the raised domains seen by AFM correspond to the cholesterol-enriched dark patches observed in transmission electron microscopy (TEM). These patches were previously identified as sites of signaling and endocytosis based on their localization of activated FcepsilonRI, at least 10 associated signaling molecules, and the presence of clathrin-coated pits. Overall the data suggest that signaling and endocytosis occur in mast cells from raised membrane regions that depend on cholesterol for their integrity and may be organized in specific relationship with the cortical cytoskeleton.

FIGURE 1

Schematic view of the membrane sheet preparation for AFM analysis. Whole cells are dispensed onto substrates (step 1). A poly-L-lysine-coated coverslip is lowered (step 2) onto the dorsal surface of lightly fixed cells to make a “sandwich” that can be separated (step 3), producing a monolayer of membrane sheets, all oriented with the cytoplasmic face-up for simultaneous AFM/fluorescence imaging (lower left). A very similar procedure generates cytoplasmic face-up membrane sheets on nickel grids for TEM imaging (lower right).

FIGURE 2

TEM images of FcɛRI IgE receptor distributions in membrane sheets, as revealed by 10 nm anti-FcɛRI β-gold labels. The membranes were prepared from IgE-primed cells, without (A) or with (B) 5 min of antigen (DNP-BSA) activation. Immunogold labeling is performed after membrane harvesting. In resting membranes (A), the receptor is distributed uniformly in small clusters. In activated cells (B), it forms large clusters localized in dark regions. Clathrin-coated pits are visible and tend to occur on the edges of the dark regions.

FIGURE 3

Simultaneous AFM and confocal fluorescence images of FcɛRI IgE receptor distributions in membrane sheets. The membranes were prepared from Alexa-488-labeled IgE-primed cells, without (A–C) or with (D–F) 5 min of antigen (DNP-BSA) activation. In the resting membrane (A–C), the white arrows point to examples of correlation between bright fluorescent IgE spots (B) marking small clusters of resting receptors near the edge of the membrane sheet that correlate with raised domains in the AFM image (A). In C, we have overlaid the topographic domain edges from A onto the fluorescence image (B) to confirm that the tagged IgE receptors cluster in “raised” membrane regions. The clustering is much more pronounced for activated receptors (images D and E), where the bright regions in the IgE fluorescence (E) map clearly with the raised domains in the AFM image (D). In F, we have overlaid the topographic domain edges from D onto the fluorescence image (E) to confirm the coincidence of receptors and raised domains. The pseudocolor scales indicate the relative height of membrane features in the AFM images (A and D).

FIGURE 4

Simultaneous AFM (A) and confocal fluorescence (B) images of resting RBL cell membrane sheets where the GM1 ganglioside is aggregated by Alexa-488-labeled CTX-B before fixation. In B, the upper right corner is dark due to photobleaching. Arrows in the paired images point to examples of correlation between fluorescent label in B and raised domains in A. In C, we have overlaid the topographic domain edges from A onto the fluorescence image (B) to confirm the colocalization. The pseudocolor scale indicates the relative height of membrane features in the AFM image (A).

FIGURE 5

Simultaneous AFM (A) and confocal fluorescence (B) images of resting RBL cell membrane sheets where clathrin is labeled with mouse monoclonal anti-clathrin heavy chain and Alexa 488 F(ab′)₂ goat anti-mouse IgG. The raised domains in A correlate strongly with the bright regions in B, thus clearly indicating the presence of clathrin. In C, we have overlaid the topographic domain edges from A onto the fluorescence image (B) to confirm the colocalization. The pseudocolor scale indicates the relative height of membrane features in the AFM image (A). In this particular sheet, the raised domains are reduced in height by treatment with 10 μM MβCD (see Fig. 7).

FIGURE 6

(A) AFM image at higher resolution of a resting RBL cell membrane sheet (clathrin labeled) showing several raised domains. (B) One domain (0.4 μm box) was selected for 3-D representation to highlight possible clathrin pits within the domain.

FIGURE 7

(A) Example of AFM topographic line profiles of raised domains for both control RBL cell membrane sheets (B) and those treated with 10 μM MβCD (C). The measurements show that the height of individual domains is substantially lower in cholesterol-depleted membranes (black line) than in control membranes (red line). The data for all the sheets studied are summarized in Table 1 and in histograms of Fig. 9. The data are independent of labeling. Image B was from a resting cell and image C was from an activated cell. A representative TEM image of cholesterol-depleted membranes is shown in D, where “flattened” clathrin arrays can be observed.

FIGURE 8

Distribution of PFO conjugated to 5 nm gold nanoparticles on RBL cell membrane sheets seen by TEM. On sheets from control cells (A), the abundant PFO label associates preferentially, but not exclusively, with darkened membrane (arrow). After treatment with 10 μM MβCD (B), only a few gold particles remain, demonstrating specificity of the PFO binding.

FIGURE 9

Histogram display of distributions of raised domain heights and widths as measured by cross section (illustrated in Fig. 7). The data incorporate all the membrane sheets in this study and were independent of cell labeling by IgE, CTX-B, or clathrin. The clathrin label (mouse monoclonal anti-clathrin heavy chain and Alexa 488 F(ab′)₂ goat anti-mouse IgG) adds ∼13 nm to the domain height. Histogram A(C) shows the distribution of domain heights(widths) for activated (shaded bars) and resting (open bars) membrane sheets. Histogram B(D) is the distribution of heights(widths) for membranes subjected to cholesterol extraction by MβCD.

FIGURE 10

(A and B) AFM topography of RBL cell membrane sheets where cytoskeleton cables (white arrows) appear to link numerous raised domains. The maximum height in A is 54 nm and the maximum height in B is 45 nm. (C) Cytoskeleton cable linkages (black arrows) appear in a TEM image.