Freeze-fracture cytochemistry of neuronal membranes: inhomogeneous distribution of filipin-sterol complexes in perikarya, dendrites and axons

Temporal and spatial changes of free cholesterol and neutral lipids in rat brain after transient middle cerebral artery occlusion

In order to examine lipid metabolism in relation to neural process following brain ischemia, we investigated temporal and spatial changes of free cholesterol (FC) and neutral lipids (NLs) after 90 min of transient middle cerebral artery occlusion (MCAO). Filipin and Nile Red stainings were performed to detect mainly FC and NLs, respectively. Double stainings for Nile Red plus ED1, MAP2, or GFAP were performed in order to identify cell type of positive stainings. Filipin stanining decreased during 1-7 day and lost at 21 day after transient MCAO in the ischemic core, but did not change in the penumbra. Nile Red positive droplets reached the maximum at 7 day after transient MCAO and gradually decreased in the core, while the peak time delayed in the penumbra. MAP2 immunoreactivity lost at 7 day in the core, and increased in the penumbra during 7-56 day. Most Nile Red positive droplets were double positive for ED1 in the core, and were localized within GFAP positive cells in the penumbra. These results suggest that changes of FC and NLs are different temporally and spatially between the core and penumbra in relation to degenerative and regenerative neural processes following brain ischemia.

Redistribution of cholesterol in oligodendrocyte membrane sheets after activation of distinct signal transduction pathways

Cultured oligodendrocytes produce extensive membrane sheets that contain an internal lacy network of vein-like structures composed of microtubules, actin filaments, and 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNPase). These cytoplasmic vein-like structures surround domains of myelin basic protein (MBP). Using the antibiotic filipin, that binds to cholesterol, the relationship between plasma membrane cholesterol and cytoskeleton in membrane sheets was examined. Our results show that cholesterol was relatively uniformly distributed within the plasma membranes of prefixed control oligodendrocyte membrane sheets. When live cultures were extracted with Triton X-100, however, a subpopulation of cholesterol molecules remained colocalized with cytoskeleton in the membrane sheets. Activation of two well-characterized signaling pathways that differentially affect microtubule and actin filament stability in membrane sheets resulted in an apparent massive lateral movement of cholesterol molecules away from membrane regions overlying internal MBP domains to membrane tracts directly overlying cytoplasmic cytoskeletal veins. Depolymerization of microtubules by colchicine resulted in redistribution of cholesterol directly over actin filaments, whereas depolymerization of actin filaments by cytochalasin B resulted in redistribution of cholesterol directly over CNPase/microtubular veins. These data suggest that cholesterol forms an association with cytoskeletal components or proteins associated with cytoskeleton. These data also suggest that cholesterol, via interactions with cytoskeleton, plays a role in signaling pathways in oligodendrocyte membrane sheets.

Changes in filipin-sterol binding in the rat cingulate cortex after the administration of antidepressant drugs. A freeze-fracture study

The amount of cholesterol in cell membranes of cingulate cortex nerve cells from rats treated with Imipramine (tricyclic antidepressant drug) or mianserin (non-tricyclic antidepressant) was investigated using filipin-sterol binding and freeze-fracture. The number of filipin-sterol complexes decreased within membranes of neurons from cingulate cortex chronically treated with imipramine or mianserin. These results suggest that the decreased content of cholesterol causes the cell membrane to become more fluid. This increased fluidity of the cell membrane may play an important role in regulating the interaction of transmitters and drugs with their membrane receptors.

Increase in reactive cholesterol in the presynaptic membrane of depolarized Torpedo synaptosomes: blockade by botulinum toxin type A

We have investigated the redistribution of filipin-cholesterol complexes at freeze-fractured presynaptic membrane of pure cholinergic synaptosomes isolated from Torpedo electric organ during acetylcholine release. After chemical depolarization, filipin-induced lesions increase at the presynaptic membrane. These changes do not take place when synaptosomes are stimulated in a calcium-free medium. Botulinum neurotoxin type A blocks both acetylcholine release and the rearrangement of filipin-induced lesions induced by depolarization. Since botulinum neurotoxin type A does not block either membrane depolarization or calcium entry into the nerve terminal, our results suggest that the redistribution of filipin-cholesterol complexes is linked to the acetylcholine release process.

Sexual differentiation of the neuronal plasma membrane: neonatal levels of sex steroids modulate the number of exo-endocytotic images in the developing rat arcuate neurons

Exo-endocytotic images and intramembrane protein particles (IMP) were quantitatively assessed in freeze-fracture replicas from the plasma membrane of arcuate neurons of rats aged 0 (newborns), 10, 20 and 100 days postpartum. Membranes contained significantly (P less than 0.02) more IMPs in females than in males. Exo-endocytotic images were increased in newborn and 10-day-old males when compared to adult males or to developing females (48 +/- 6 vs 6 +/- 1 images/100 micron 2 in 10-day-old male and female rats, respectively). Androgenization of females with a single injection of testosterone propionate on the day of birth resulted in an increased number of exo-endocytotic images in developing animals (75 +/- 9 images/100 micron 2, 10-day-old rats) and in the abolishment of the sex differences in the number of IMPs.

Regional membrane heterogeneity in premyelinated CNS axons: factors influencing the binding of sterol-specific probes

Binding of the sterol-specific probe filipin to developing optic nerve axonal membrane is spatially heterogeneous prior to association of glial cells with the axons. Experiments were performed using different sterol binding probes (filipin, tomatin, and saponin), at different temperatures (4 degrees C, 23 degrees C, and 37 degrees C), after incubation in different ionic conditions (10 mM Ca2+, 10 mM EGTA, and 20 mM Mg2+), to examine factors that may be responsible for this membrane heterogeneity in rat optic nerve. The patchy pattern of filipin binding is apparent with each sterol-specific probe, even prior to glial ensheathment, and is retained when membrane fluidity is increased at higher temperatures. Increased Ca2+ concentration increased membrane stability, and increased Mg2+ reduced the patchiness of filipin binding. After tannic acid staining, regions of the cytoskeleton are seen associated with the membrane via filaments extending from microtubules to the membrane, preferentially in regions where filipin interaction with the membrane is inhibited. The non-uniform interaction of filipin with the axolemma suggests an underlying heterogeneity in the sterol composition and stability of the membrane. Heterogeneity of premyelinated axonal membrane may provide an important formative influence in the differentiation of axons to their mature morphology and function.

The cellular effects of estrogens on neuroendocrine tissues

Estrogen action on sensitive neurons in the rat diencephalon has been studied by morphologic techniques; evidence of estrogen action at every level is presented, including tracts, cells, circuitry and subcellular organelles. The demonstration in the arcuate nucleus of estrogen-induced synaptic remodelling, estrogen-induced postsynaptic membrane phenotypes, changes in intracellular membranes and rapid estrogen actions on neuronal endo-exocytosis indicates that cellular estrogen actions may underlie the neuronal control of reproduction.

Filipin-cholesterol binding in CNS axons prior to myelination: evidence for microheterogeneity in premyelinated axolemma

The distribution of cholesterol in axonal membrane of developing rat optic nerves prior to myelination was studied by freeze-fracture cytochemistry. Binding of the cholesterol-specific probe, filipin, to the axolemma of premyelinated axons was heterogeneous; this suggests the presence of microdomains of axolemma with different membrane composition and/or cytoskeletal/extracellular matrix association. Although the reasons for this binding pattern have not yet been determined, heterogeneity occurs prior to association of glia with the axon, and may reflect regional differences in lipid/sterol composition of the axonal membrane bilayer, or distribution of membrane-associated cytoskeleton. The distribution of intramembranous particles was not obviously associated with the pattern of filipin binding in early developing axons, however, as might have been expected from the attending differences in fluidity of the membrane microdomains. Microheterogeneity in axonal membranes of developing axons could have an influence on several membrane properties, and may be associated with processes important for growth and differentiation of axons.

Harmaline-induced changes in plasma membrane of Purkinje cells: a trans-synaptic effect mediated by climbing fibers

Fixed cerebellar cortex slices of adult male albino rats injected with saline, were exposed to the cholesterol probe filipin and freeze-fractured. Under these conditions, the plasma membrane of Purkinje cell dendrites was labeled with only a limited number of typical, 25-30 nm diameter filipin-sterol complexes. In contrast, in rats injected with harmaline, a drug that induces a rhythmic activation of climbing fibers, dendritic membranes showed an increased number of filipin-sterol complexes. This increased filipin labeling of Purkinje cell dendrites was not observed, however, when harmaline was injected into rats after destruction of climbing fibers. These results suggest that climbing fiber activation may induce a reorganization of the lipid matrix in the plasma membrane of Purkinje cells.

Differential distribution of intermembranous particles in the plasmalemma of the migrating cerebellar granule cells

The size and distribution of intramembraneous particles in 5 different plasmalemma domains of migrating cerebellar granule cells was studied by the freeze-fracture method. The quantitative analysis in 5-20-day-old rats reveals that the membrane of the leading tip of the descending process, the soma and vertical process that are associated with Bergmann glial fibers, have a significantly higher density of small (less than 11 nm diameter) intramembraneous protein particles than the horizontal processes and their growth cones which grow along previously generated parallel fibers. This regional difference in density of membrane associated particles may be related to the selective outgrowth of the two classes of granule cell neurites along glial and neuronal surfaces.

Freeze-fracture study of filipin binding in photoreceptor outer segments and pigment epithelium of dystrophic and normal retinas

We have studied sterol distribution in the retinal pigment epithelial (RPE) microvillous and outer segment disc membranes of rats with inherited retinal degeneration (RCS; RCS-p/+) and of normal genetic controls (RCS-rdy+, RCS-rdy+-p/+) by using the polyene antibiotic filipin, which binds specifically to 3-B-hydroxy-sterols, and freeze-fracture techniques. Retinas were perfusion-fixed, incubated with filipin in the same fixative, and prepared routinely for freeze-fracture electron microscopy. In the normal retina, the distribution of filipin binding sites on both RPE microvillous and outer segment disc membranes changes during development. Prior to outer segment elongation and the onset of phagocytosis (10 days postnatal), filipin sterol complexes are homogeneously distributed in both microvillous and outer segment membranes. With the onset of phagocytosis (2 weeks postnatal and later) filipin binding in both tissues forms a proximal-to-distal gradient, and binding sites decrease as distance from the cell body increases. In the normal RPE microvillous membranes, binding sites are numerous proximally and sparse on the distal tips. In the normal outer segment disc membranes, binding sites are often present on the basal discs, but are sparse on the intact apical discs prior to shedding. As the discs are cast off and engulfed by the RPE, however, filipin binding increases on both disc and phagosome membranes. In the dystrophic retina, the distribution of filipin binding sites differs from the normal. First, in the microvillous membranes, the proximal-to-distal gradient in filipin binding is rarely present at 2 weeks postnatal and becomes prominent only after the buildup of membranous debris has begun (3-5 weeks postnatal). Second, as the photoreceptors degenerate and the membrane debris disappears (4 months postnatal), filipin binding on the microvillous membranes becomes relatively sparse and homogeneous. Third, filipin binding on the intact disc membranes does not change with outer segment elongation, and numerous filipin binding sites are present on both apical and basal outer segment disc membranes. Fourth, large aggregates of filipin binding sites occupy the vast expanses of particle-free areas of debris membranes which accumulate between the photoreceptors and the RPE. These changes in the amount and distribution of filipin binding sites in the dystrophic retina add to the evidence that the disease process involves outer segment as well as RPE membranes and suggest that alterations in cholesterol distribution could contribute to the phagocytic defect.

Absence of filipin-sterol complexes from the ciliary necklace of ependymal cells

The polyene antibiotic filipin reacts specifically with membrane cholesterol and produces membrane perturbations. Prefixed vibratome slices of cat ependymal cells were treated with a glutaraldehyde solution containing 300 microM filipin. Freeze-fracture of these cells revealed numerous 25-30 nm protuberances on fracture faces (filipin-sterol complexes) in the ciliary membrane. In the ciliary necklace, however, these filipin-sterol complexes were absent. These results suggest that the ciliary necklace has a low cholesterol content.

Sex differences and maturational changes in arcuate nucleus neuronal plasma membrane organization

Intramembrane particles (IMP) are believed to represent protein-containing structures in the membrane. Hypothalamic arcuate nucleus neuronal plasma membranes from male and female rats studied from birth to adulthood were quantitatively assessed for IMP number and size using freeze-fracture techniques. We found that newborn female rats have a significantly greater number of IMP than newborn males. There is also a progressive increase in the number of IMP during the first 20 days of postnatal life in both sexes. The rate of protein particle insertion favors females, maintaining the unequal protein particle content into adulthood with female rats having more IMP than males of the same age. The differences in IMP concentration are mainly due to greater numbers of small (less than 10 nm) particles in neuronal membranes from female rats. These data extend previous reports of sexual dimorphism in the arcuate nucleus to the level of plasma membrane organization.

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.

The distribution of filipin-sterol complexes in photoreceptor synaptic membranes

The polyene antibiotic filipin, which binds to membrane sterols, has been used to investigate the relative distribution of cholesterol at photoreceptor synaptic junctions in the chick retina. Following anesthesia and aldehyde perfusion fixation, the retina is removed and immersed in fixative solution containing the filipin for 36-48 hours. The retinas are then processed for freeze-fracture. Electron microscopy of freeze-fracture replicas demonstrates that the filipin-sterol complexes are not evident between intramembrane particles of the presynaptic and postsynaptic particle arrays that are present at ribbon and basal junctions. In contrast, the synaptic vesicle fusion zone of ribbon junctions contains large numbers of filipin-sterol complexes which are observed merging with the free margins of the presynaptic particle array. There is a scarcity of such complexes, however, around the free margins of basal junction presynaptic particle arrays. These latter sites do not contain a vesicle fusion zone. Particle-poor areas of membrane that surround postsynaptic particles arrays of ribbon and basal junctions also do not contain filipin binding sites. The nonsynaptic membrane of photoreceptor terminals contains large numbers of filipin-sterol complexes, less tightly packed than in the synaptic vesicle fusion zone. Coated vesicle fusion sites in the presynaptic membrane contain groups of intramembrane particles but the filipin-sterol complexes are excluded from these sites. The observations suggest that synaptic membrane domains which interact with cytoskeletal components, such as clathrin and pre- and postsynaptic densities contain less cholesterol than other domains such as the synaptic vesicle fusion zones.

Postsynaptic membrane domains in the molecular layer of the cerebellum: a correlation between presynaptic inputs and postsynaptic plasma membrane organization

The intramembrane particle (IMP) content of Purkinje, basket, stellate and Golgi cell plasma membrane was quantitatively assessed in freeze-fracture replicas of the cerebellum of normal rats and Weaver mutant mice. This analysis showed that, irrespective of the cell type innervated (i.e. Purkinje, stellate, basket or Golgi cells) postsynaptic membranes for parallel fibers had a relatively low IMP content in their cytoplasmic P-face (approximately equal to 750 IMP/micron2), while postsynaptic membranes for climbing, basket and stellate axons were characterized by a significantly higher IMP content (approximately equal to 1400 IMP/micron2). This difference of IMP content between the targets for parallel fibers and those for climbing, basket and stellate axons was restricted to IMP smaller than 10 nm and appeared progressively during the development of the molecular layer, suggesting a correlation between the formation of synaptic contacts and the segregation of the postsynaptic membrane in these two different domains. In addition, the study of the Weaver mice cerebellum, which is deprived of parallel fibers, but yet shows a normal IMP content in the postsynaptic membrane for the missing fibers, indicated that this characteristic IMP content is established before or during the afferent's reaching its target, and independently of whether the contact ultimately occurs.

Lateral distribution of intramembrane particles in Purkinje and granule cells of the rat cerebellar cortex

The lateral distribution of intramembrane protein particles (IMP) in the plasma membrane of Purkinje and granule cells was quantitatively assessed in freeze-fracture replicas of the rat cerebellar cortex. In the plasma membrane of each cell type this technique showed domains with statistically significant differences in the distribution of IMP. The values were highly reproducible between different animals fixed in comparable conditions. This analysis provides an additional parameter (besides the number and size of IMP) in the assessment of neuronal membrane heterogeneity.

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.

Freeze-fracture studies of photoreceptor membranes: new observations bearing upon the distribution of cholesterol

We performed electron microscopy of replicas from freeze-fractured retinas exposed during or after fixation to the cholesterol-binding antibiotic, filipin. We observed characteristic filipin-induced perturbations throughout the disk and plasma membranes of retinal rod outer segments of various species. It is evident that a prolonged exposure to filipin in fixative enhances rather than reduces presumptive cholesterol detection in the vertebrate photoreceptor cell. In agreement with the pattern seen in our previous study (Andrews, L.D., and A. I. Cohen, 1979, J. Cell Biol., 81:215-228), filipin-binding in membranes exhibiting particle-free patches seemed largely confined to these patches. Favorably fractured photoreceptors exhibited marked filipin-binding in apical inner segment plasma membrane topologically confluent with and proximate to the outer segment plasma membrane, which was comparatively free of filipin binding. A possible boundary between these differing membrane domains was suggested in a number of replicas exhibiting lower filipin binding to the apical plasma membrane of the inner segment in the area surrounding the cilium. This area contains a structure (Andrews, L. D., 1982, Freeze-fracture studies of vertebrate photoreceptors, In Structure of the Eye, J. G. Hollyfield and E. Acosta Vidrio, editors, Elsevier/North-Holland, New York, 11-23) that resembles the active zones of the nerve terminals for the frog neuromuscular junction. These observations lead us to hypothesize that these structures may function to direct vesicle fusion to occur near them, in a domain of membrane more closely resembling outer than inner segment plasma membrane. The above evidence supports the views that (a) all disk membranes contain cholesterol, but the particle-free patches present in some disks trap cholesterol from contiguous particulate membrane regions; (b) contiguous inner and outer segment membranes may greatly differ in cholesterol content; and (c) the suggested higher cholesterol in the inner segment than in the outer segment plasma membrane may help direct newly inserted photopigment molecules to the outer segment.