Membrane lipid heterogeneity associated with acetylcholine receptor particle aggregates in Xenopus embryonic muscle cells

Golgi sorting regulates organization and activity of GPI proteins at apical membranes

Here we combined classical biochemistry with new biophysical approaches to study the organization of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) with high spatial and temporal resolution at the plasma membrane of polarized epithelial cells. We show that in polarized MDCK cells, after sorting in the Golgi, each GPI-AP reaches the apical surface in homoclusters. Golgi-derived homoclusters are required for their subsequent plasma membrane organization into cholesterol-dependent heteroclusters. By contrast, in nonpolarized MDCK cells, GPI-APs are delivered to the surface as monomers in an unpolarized manner and are not able to form heteroclusters. We further demonstrate that this GPI-AP organization is regulated by the content of cholesterol in the Golgi apparatus and is required to maintain the functional state of the protein at the apical membrane. Thus, in contrast to fibroblasts, in polarized epithelial cells, a selective cholesterol-dependent sorting mechanism in the Golgi regulates both the organization and function of GPI-APs at the apical surface.

Probing cytoskeleton modulation by optical biosensors

This paper reported the use of resonant waveguide grating biosensors for studying the cytoskeleton structure in cells. This was achieved by measuring the changes in mass within the bottom portion of cells upon exposure to saponin in the absence and presence of cytoskeleton modulators. Treatment of Chinese hamster ovary cells with saponin led to a dose-dependent and dynamic mass changes. When a higher concentration of saponin (> 60 microg/ml) was used, a net loss in mass was observed. This is probably resulted from the diffusion of soluble intracellular materials away from the bottom portion of cells after pore formation in the cell plasma membranes by saponin. The pretreatment of cells with actin disruption agents, cytochalasin B and latrunculin A, led to significantly increased loss in cell mass induced by either 75 or 125 microg/ml saponin. These results suggested that optical biosensors provide an attractive means to study the cytoskeleton structure and screen modulators that affect the cytoskeleton structure.

Specific interaction between tetrandrine and Quillaja saponins in promoting permeabilization of plasma membrane in human leukemic HL-60 cells

Spontaneous Ni2+ entry (leak), measured as fluorescence quench in fura-2-loaded HL-60 cells at the excitation wavelength of 360 nm, was strongly inhibited by tetrandrine (TET, 100 microM), a Ca2+ antagonist of Chinese herbal origin. Exposure of the cells for 5 min to saponins from Quillaja saponaria (QS, 30 microg/ml), surfactants well known to permeabilize the plasma membrane by complexing with cholesterol, promoted Ni2+ entry without causing fura-2 leak-out. Unexpectedly, TET caused an immediate (within 2.5 min) augmentation of QS-promoted Ni2+ entry; and a 5-min treatment with both TET and QS resulted not only in an enhanced Ni2+ entry, but also a fura-2 leak-out. Ginseng saponins (100 microg/ml) alone or together with TET did not cause such a permeabilization. Permeabilization induced by 1-3 microM digitonin, another cholesterol-complexing glycoside, could not be enhanced by TET. TET did not affect permeabilization induced by Triton X-100 (0.01%), a detergent which non-specifically disrupts the hydrophobic interaction at the plasma membrane. TET also did not enhance Ni2+ entry triggered by ionomycin (0.35 microM) or SK&F 96365 (20 microM). Further, it did not augment Ni2+ entry when the plasma membrane fluidity was modulated by changes of temperature (27-47 degrees C) or treatment with 5% ethanol. This QS-promoted Ni2+ entry could not be amplified by other lipophilic Ca2+ antagonists, such as diltiazem (100 microM) and verapamil (100 microM). The results hence indicate that TET enhanced Ni2+ entry (or permeabilization) elicited by QS treatment, but not other perturbations of the plasma membrane. We suggest that pore formation at the plasma membrane, a consequence of QS-cholesterol interaction, can be specifically enhanced by TET. Also, a comparative study of the effects of TET and its very close analogues, hernandezine and berbamine, reveals that the methoxyl group at the R2 position of TET appears to be crucial in enhancing QS-promoted Ni2+ entry.

Growth cone enrichment and cytoskeletal association of non-receptor tyrosine kinases

Fetal rat brain (E18) expresses at least three c-src-like, membrane-associated non-receptor tyrosine kinases: c-src, fyn, and lyn. c-src and fyn are the most abundant and are highly enriched in a subcellular fraction of nerve growth cones (GCPs). To study the cytoskeletal association of these tyrosine kinases, Triton X-100-resistant fractions were prepared from GCPs. All three non-receptor tyrosine kinases are associated with the cytoskeleton to a significant degree with the relative affinities: fyn > c-src > lyn. The binding is sensitive to ionic strength and to phosphotyrosine, but not to phosphoserine or phosphothreonine. To investigate the regulation of this association we used phosphatase inhibitors to increase phosphotyrosine levels in GCPs. This resulted in the release of c-src from the cytoskeleton. Under these conditions tyrosine phosphorylation was increased selectively in released c-src and primarily on tyrosine 527. Cytoskeletally bound c-src had a higher specific kinase activity than Triton X-100-soluble c-src. These findings indicate that src family members interact in a regulated manner with the cytoskeleton in non-transformed cells. This regulation is explained by a model in which c-src binds to the cytoskeleton via its SH2 domain and is released when phosphorylated tyrosine-527 binds to this domain intramolecularly, inhibiting kinase activity.

Identification of subunits of acetylcholine receptor that interact with a cholesterol photoaffinity probe

All four subunits of the acetylcholine receptor in membrane vesicles isolated from Torpedo californica have been labeled with [3H]cholesteryl diazoacetate. As this probe incorporates into lipid bilayers analogously to cholesterol, this result indicates that acetylcholine receptor interacts with cholesterol. This investigation also demonstrates that this probe is a useful reagent for studying the interaction of cholesterol with membrane proteins.

Absence of sterol-specific complexes at active zones of degenerating and regenerating frog neuromuscular junctions

Freeze-fracture combined with filipin treatment has been used as a cytochemical probe for membrane cholesterol. As previously shown at the frog neuromuscular junction, distinctive sterol-specific complexes were formed on the presynaptic membrane after filipin treatment, except at active zones. The absence of sterol-specific complexes from active zones was confirmed using two other cytochemical agents--digitonin and saponin. We also studied the maintenance and differentiation of the presynaptic membrane heterogeneity revealed by membrane cholesterol probes at degenerating and regenerating neuromuscular junctions. During degeneration, active zones in frog nerve terminals were disorganized, but still lacked sterol-specific complexes. After engulfing the degenerating nerve terminals, Schwann cells occupied the synaptic gutters and displayed a uniform distribution of sterol-specific complexes. Schwann cell ridges opposite the postjunctional folds also had prominent sterol-specific complexes in regions formerly occupied by active zones. By 2 weeks after nerve crush, nerve terminals reinvaded the endplate region and active zones began to regenerate. While the intramembrane particles of the early regenerating active zones were not arranged in the normal double-rowed organization, filipin-sterol complexes were nevertheless excluded from these primitive active zones. Areas of nerve terminal membrane opposite to junctional folds but lacking active zones were covered with filipin-sterol complexes. These results show that the normal double-rowed organization is not required for the expression of the membrane heterogeneity associated with the active zone. In addition, the absence of sterol-specific complexes is closely associated with the active zone particles and not simply the membrane regions opposite to the postjunctional folds. The membrane heterogeneity does not seem to be directly linked with the functional state of the active zone since it is still associated with degenerating active zones after transmission failure has occurred.

Freeze-fracture cytochemistry of sympathetic ganglia. Distribution of filipin and tomatin induced membrane deformations in neurons and satellite cells

Application of filipin to sympathetic ganglia results in membrane deformations in both the neurons and the satellite cells. The plasma membranes of the principal ganglion cells show a non-homogeneous distribution of filipin induced deformations with fewer deformations in the perikaryal plasma membrane than in the nerve fiber membrane. The filipin induced membrane lesions are correlated to the number of IMPs of the neuronal membrane i.e. a high density of intramembrane particles (IMP) gives fewer deformations and vice versa. The membrane of the satellite cells contain a higher density of probe induced lesions than the neuronal membrane. The filipin induced deformations in the satellite cells are not correlated to the number of IMPs or to the number of orthogonal arrays of small particles (OAP). Specialized membrane areas such as the gap junction is always devoided of filipin induced lesions. A similar distribution of membrane lesions was found when tomatin was used instead of filipin. These results indicate a possible difference in lipid content between various parts of the neurons and between the neuronal and satellite cell plasma membrane in guinea pig sympathetic ganglia.

Organization of acetylcholine receptor clusters in cultured rat myotubes is calcium dependent

The effect of extracellular Ca2+ concentration and myasthenic globulin on the distribution and appearance of acetylcholine receptor (AChR) clusters on rat myotubes was studied with tetramethyl-rhodamine-labeled alpha BTX. Low Ca2+ medium (2.5 X 10(-5) M) caused a time-dependent loss of AChR clusters, and a concomitant increase in small punctate areas of fluorescence. High Ca2+ concentrations (1.5 X 10(-2) M) increased the size of AChR clusters without altering AChR synthesis. These changes were not observed with other divalent ions. In the presence of myasthenic globulin, the rate of AChR turnover increases, and AChR clusters are rapidly dispersed. High Ca2+ concentration partially protects the AChR clusters from dispersal and decreases the rate of receptor turnover.

Filipin as a flow microfluorometry probe for cellular cholesterol

The polyene antibiotic filipin, which forms specific complexes with 3 beta-hydroxysterols, displays spectral properties compatible with its use in flow microfluorometry (FMF). The purpose of this study was to test the suitability of filipin as an FMF probe for unesterified cellular cholesterol. The following experimental conditions appeared optimal for cells with an average unesterified cholesterol content of less than 3 nmol per 10(6) cells: 2 X 10(6) fixed cells (1-4% p-formaldehyde, 30 min, 21 degrees C) stained for 2-4 h with 100 micrograms/ml filipin and excited at 350.7/356.7 nm. Fluorescence emission (Em) was measured above 510 nm. Less suitable conditions involved excitation at 488 nm or using cells which had not been fixed. Fixation preserved the live-dead cell discrimination provided by forward light scatter measurements, so that dead cells could be excluded from the FMF analysis of cellular cholesterol. Under the above conditions FMF analysis of a variety of murine cell types showed that in all cases the fluorescence intensity of filipin-stained cells was clearly increased above autofluorescence levels of the unstained control cells. The increase in fluorescence signal in different filipin stained cell types correlated (P less than or equal to .001) with the cellular content of unesterified cholesterol determined by an independent enzymatic assay. The sensitivity of the FMF assay was in the femtomole (10(-15) ) range. Mixing experiments with cells of different cholesterol levels showed that the technique distinguishes cell populations with distinctive levels of unesterified cholesterol. We therefore concluded that filipin is a useful FMF probe for determining relative levels of unesterified cholesterol in cells.

Lipid domains of acetylcholine receptor clusters detected with saponin and filipin

The acetylcholine receptor (AChR) clusters of cultured rat myotubes contain two distinct, interdigitating, membrane domains, one enriched in AChR, the other poor in AChR but associated with sites of myotube-substrate contact (Bloch, R.J., and B. Geiger, 1980, Cell, 21:25-35). We have used two cholesterol-specific cytochemical probes, saponin and filipin, to investigate the lipid nature of these membrane domains. When studied with freeze-fracture electron microscopy or fluorescence microscopy, these reagents reacted moderately and preferentially with the AChR-rich domains of AChR clusters. Little or no reaction with the membrane in "contact" domains was seen. In contrast, membrane regions surrounding the AChR clusters reacted extensively with filipin. These results suggest that, in rat myotubes, the composition or the state of the lipids differs between the two membrane domains of the AChR clusters, and between clusters and surrounding membrane. In chick myotubes, AChR clusters do not appear to react with filipin or saponin, although surrounding membrane reacts extensively with these reagents.

Distribution of sterol-specific complexes in a continually shearing region of a plasma membrane and at procaryotic-eucaryotic cell junctions

A narrow zone of plasma membrane between the head and body of a protozoan from termites undergoes continual in-plane shear because the head rotates continuously in the same direction relative to the cell body (Tamm, S.L., and S. Tamm, 1974, Proc. Natl. Acad. Sci. USA 71:4589-4593). Using filipin and digitonin as cytochemical probes for cholesterol and related 3-beta-hydroxysterols, we found a high level of sterol-specific complexes, visible as membrane lesions in thin sections, in both shearing and nonshearing regions of the membrane, indicating no difference in sterol content. This confirmed previous observations that any region of the fluid membrane can undergo shear, but that this occurs only at certain locations due to cell geometry and proximity to rotating cytoskeletal structures. Filipin and digitonin did not disrupt the plasma membrane at the junctions with ectosymbiotic rod and fusiform bacteria (i.e., membrane pockets and ridges). However, pepsin degradation of dense material coating the junctional membranes resulted in a positive response of these regions to filipin. Fluorescence microscopy revealed a bright halo around each rod bacterium, due to filipin-sterol binding in the sides of the membrane pockets, but no fluorescence at the bottom of the pockets; the same fluorescence pattern was found in pepsin-treated cells despite the presence of sterols throughout the pocket membrane, as shown by electron microscopy. These findings indicate that (a) regional constraints may restrict the ability of filipin to interact with sterols or form visible membrane lesions, and (b) a negative response to filipin, assayed by either electron or fluorescence microscopy, is not sufficient to demonstrate low membrane sterol concentration, particularly in membrane domains characterized by closely associated proteins.

Distribution of filipin-sterol complexes on cultured muscle cells: cell-substratum contact areas associated with acetylcholine receptor clusters

Specialized areas within broad, close, cell-substratum contacts seen with reflection interference contrast microscopy in cultures of Xenopus embryonic muscle cells were studied. These areas usually contained a distinct pattern of light and dark spots suggesting that the closeness of apposition between the membrane and the substratum was irregular. They coincided with areas containing acetylcholine receptor clusters identified by fluorescence labeled alpha-bungarotoxin. Freeze-fracture of the cells confirmed these observations. The membrane in these areas was highly convoluted and contained aggregates of large P-face intramembrane particles (probably representing acetylcholine receptors). If cells were fixed and then treated with the sterol-specific antibiotic filipin before fracturing, the pattern of filipin-sterol complex distribution closely followed the pattern of cell-substratum contact. Filipin-sterol complexes were in low density in the regions where the membrane contained clustered intramembrane particles. These membrane regions were away from the substratum (bright white areas in reflection interference contrast; depressions of the P-face in freeze-fracture). Filipin-sterol complexes were also in reduced density where the membrane was very close to the substratum (dark areas in reflection interference contrast; bulges of the P-face in freeze-fracture). These areas were not associated with clustered acetylcholine receptors (aggregated particles). This result suggests that filipin treatment causes little or no artefact in either acetylcholine receptor distribution or membrane topography of fixed cells and that the distribution of filipin-sterol complexes may closely parallel the microheterogeneity of membranes that exist in living cells.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.

Distribution of cytochemically detectable cholesterol in the electric organ of Torpedo marmorata

A cytochemical probe for cholesterol, the polyene antibiotic filipin, was applied to aldehyde-fixed samples of the electric organ of Torpedo marmorata to identify filipin-binding sites in the various membrane components of the organ and, hence, the probable cholesterol content at these levels. In both thin-sectioned and freeze-fractured samples, filipin-cholesterol complexes appeared numerous and homogeneously distributed on the Schwann cell plasma membrane. On the presynaptic membrane, filipin-cholesterol complexes occurred in patches alternating with unlabeled membrane segments. The postsynaptic, acetylcholine receptor-rich plasma membrane of the electroplax showed no or few filipin-cholesterol complexes in the flat region and upper part of the invaginations (both areas characterized by a lattice of small intramembrane particles); however, the membrane of the bottom part of the postsynaptic invaginations contained several complexes. The ventral, noninnervated plasma membrane of the electroplax showed a moderate, homogeneous filipin labeling. These data suggest that the distribution of cholesterol among membranes of the electroplax is not homogeneous and that the acetylcholine receptor-rich region of the postsynaptic membrane (as characterized by the lattice of small intramembrane particles) may contain little cholesterol.