The distribution of filipin-sterol complexes in photoreceptor synaptic membranes

Lipid markers and related transcripts during excitotoxic neurodegeneration in kainate-treated mice

Lipid homeostasis is dysregulated in some neurodegenerative diseases and after brain injuries due to excess glutamate or lack of oxygen. However the kinetics and cell specificity of dysregulation in different groups of lipids during excitotoxic neuronal death are not clear. Here we examined the changes during excitotoxic neuronal death induced by injecting kainic acid (KA) into the CA1 region of mouse hippocampus. We compared neuronal loss and glial cell proliferation with changes in lipid-related transcripts and markers for different lipid groups, over 12 days after KA-treatment. As neurons showed initial signs of damage, transcripts and proteins linked to fatty acid oxidation were up-regulated. Cholesterol biosynthesis induced by transcripts controlled by the transcription factor Srebp2 seems to be responsible for a transient increase in neuronal free cholesterol at 1 to 2 days. In microglia, but not in neurons, Perilipin-2 associated lipid droplets were induced and properties of Nile red emissions suggest lipid contents change over time. After microglial expression of phagocytotic markers at 2 days, some neutral lipid deposits co-localized with lysosome markers of microglia and were detected within putative phagocytotic cups. These data delineate distinct lipid signals in neurons and glial cells during excitotoxic processes from initial neuronal damage to engagement of the lysosome-phagosome system.

CYP46A1 inhibition, brain cholesterol accumulation and neurodegeneration pave the way for Alzheimer's disease

Abnormalities in neuronal cholesterol homeostasis have been suspected or observed in several neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and Huntington's disease. However, it has not been demonstrated whether an increased abundance of cholesterol in neurons in vivo contributes to neurodegeneration. To address this issue, we used RNA interference methodology to inhibit the expression of cholesterol 24-hydroxylase, encoded by the Cyp46a1 gene, in the hippocampus of normal mice. Cholesterol 24-hydroxylase controls cholesterol efflux from the brain and thereby plays a major role in regulating brain cholesterol homeostasis. We used an adeno-associated virus vector encoding short hairpin RNA directed against the mouse Cyp46a1 mRNA to decrease the expression of the Cyp46a1 gene in hippocampal neurons of normal mice. This increased the cholesterol concentration in neurons, followed by cognitive deficits and hippocampal atrophy due to apoptotic neuronal death. Prior to neuronal death, the recruitment of the amyloid protein precursor to lipid rafts was enhanced leading to the production of β-C-terminal fragment and amyloid-β peptides. Abnormal phosphorylation of tau and endoplasmic reticulum stress were also observed. In the APP23 mouse model of Alzheimer's disease, the abundance of amyloid-β peptides increased following inhibition of Cyp46a1 expression, and neuronal death was more widespread than in normal mice. Altogether, these results suggest that increased amounts of neuronal cholesterol within the brain may contribute to inducing and/or aggravating Alzheimer's disease.

Inhibiting cholesterol degradation induces neuronal sclerosis and epileptic activity in mouse hippocampus

Elevations in neuronal cholesterol have been associated with several degenerative diseases. An enhanced excitability and synchronous firing in surviving neurons are among the sequels of neuronal death in these diseases and also in some epileptic syndromes. Here, we attempted to increase neuronal cholesterol levels, using a short hairpin RNA to suppress expression of the enzyme cytochrome P450 family 46, subfamily A, polypeptide 1 gene (CYP46A1). This protein hydroxylates cholesterol and so facilitates transmembrane extrusion. A short hairpin RNA CYP46A1construction coupled to the adeno-associated virus type 5 was injected focally and unilaterally into mouse hippocampus. It was selectively expressed first in neurons of the cornu ammonis (hippocampus) (CA)3a region. Cytoplasmic and membrane cholesterol increased, and the neuronal soma volume increased and then decreased before pyramidal cells died. As CA3a pyramidal cells died, interictal electroencephalographic (EEG) events occurred during exploration and non-rapid eye movement sleep. With time, neuronal death spread to involve pyramidal cells and interneurons of the CA1 region. CA1 neuronal death was correlated with a delayed local expression of phosphorylated tau. Astrocytes were activated throughout the hippocampus and microglial activation was specific to regions of neuronal death. CA1 neuronal death was correlated with distinct aberrant EEG activity. During exploratory behaviour and rapid eye movement sleep, EEG oscillations at 7-10 Hz (theta) could accelerate to 14-21 Hz (beta) waves. They were accompanied by low-amplitude, high-frequency oscillations of peak power at ~300 Hz and a range of 250-350 Hz. Although episodes of EEG acceleration were not correlated with changes in exploratory behaviour, they were followed in some animals by structured seizure-like discharges. These data strengthen links between increased cholesterol, neuronal sclerosis and epileptic behaviour.

Tracking quantum dot-tagged calcium channels at vertebrate photoreceptor synapses: retinal slices and dissociated cells

At synapses in the central nervous system, precisely localized assemblies of presynaptic proteins, neurotransmitter-filled vesicles, and postsynaptic receptors are required to communicate messages between neurons. Our understanding of synaptic function has been significantly advanced using electrophysiological methods, but the dynamic spatial behavior and real-time organization of synapses remains poorly understood. In this unit, we describe a method for labeling individual presynaptic calcium channels with photostable quantum dots for single-particle tracking analysis. We have used this technique to examine the mobility of L-type calcium channels in the presynaptic membrane of rod and cone photoreceptors in the retina. These channels control release of glutamate-filled synaptic vesicles at the ribbon synapses in photoreceptor terminals. This technique offers the advantage of providing a real-time biophysical readout of ion channel mobility and can be manipulated by pharmacological or electrophysiological methods. For example, the combination of electrophysiological and single-particle tracking experiments has revealed that fusion of nearby vesicles influences calcium channel mobility and changes in channel mobility can influence release. These approaches can also be readily adapted to examine membrane proteins in other systems.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

Regulation of presynaptic strength by controlling Ca2+ channel mobility: effects of cholesterol depletion on release at the cone ribbon synapse

Synaptic communication requires proper coupling between voltage-gated Ca(2+) (Ca(V)) channels and synaptic vesicles. In photoreceptors, L-type Ca(V) channels are clustered close to synaptic ribbon release sites. Although clustered, Ca(V) channels move continuously within a confined domain slightly larger than the base of the ribbon. We hypothesized that expanding Ca(V) channel confinement domains should increase the number of channel openings needed to trigger vesicle release. Using single-particle tracking techniques, we measured the expansion of Ca(V) channel confinement domains caused by depletion of membrane cholesterol with cholesterol oxidase or methyl-β-cyclodextrin. With paired whole cell recordings from cones and horizontal cells, we then determined the number of Ca(V) channel openings contributing to cone Ca(V) currents (I(Ca)) and the number of vesicle fusion events contributing to horizontal cell excitatory postsynaptic currents (EPSCs) following cholesterol depletion. Expansion of Ca(V) channel confinement domains reduced the peak efficiency of release, decreasing the number of vesicle fusion events accompanying opening of each Ca(V) channel. Cholesterol depletion also inhibited exocytotic capacitance increases evoked by brief depolarizing steps. Changes in efficiency were not due to changes in I(Ca) amplitude or glutamate receptor properties. Replenishing cholesterol restored Ca(V) channel domain size and release efficiency to control levels. These results indicate that cholesterol is important for organizing the cone active zone. Furthermore, the finding that cholesterol depletion impairs coupling between channel opening and vesicle release by allowing Ca(V) channels to move further from release sites shows that changes in presynaptic Ca(V) channel mobility can be a mechanism for adjusting synaptic strength.

Lateral mobility of presynaptic L-type calcium channels at photoreceptor ribbon synapses

At most synapses, presynaptic Ca(2+) channels are positioned near vesicle release sites, and increasing this distance reduces synaptic strength. We examined the lateral membrane mobility of presynaptic L-type Ca(2+) channels at photoreceptor ribbon synapses of the tiger salamander (Ambystoma tigrinum) retina. Movements of individual Ca(2+) channels were tracked by coupling quantum dots to an antibody against the extracellular α(2)δ(4) Ca(2+) channel subunit. α(2)δ(4) antibodies labeled photoreceptor terminals and colocalized with antibodies to synaptic vesicle glycoprotein 2 and voltage-gated Ca(2+) channel 1.4 (Ca(V)1.4) α(1) subunits. The results show that Ca(2+) channels are dynamic and move within a confined region beneath the synaptic ribbon. The size of this confinement area is regulated by actin and membrane cholesterol. Fusion of nearby synaptic vesicles caused jumps in Ca(2+) channel position, propelling them toward the outer edge of the confinement domain. Channels rebounded rapidly toward the center. Thus, although Ca(V) channels are mobile, molecular scaffolds confine them beneath the ribbon to maintain neurotransmission even at high release rates.

Localization of cholesterol, phosphocholine and galactosylceramide in rat cerebellar cortex with imaging TOF-SIMS equipped with a bismuth cluster ion source

Time-of-flight secondary-ion-mass-spectrometry (TOF-SIMS) was utilized to address the issue of co-localization of cholesterol, phosphocholine and galactosylceramide in rat cerebellar cortex. Rat cerebellum was fixed, freeze-protected by sucrose, frozen and sectioned by cryoultramicrotomy and dried at room temperature. The samples were analyzed in an imaging TOF-SIMS instrument equipped with a Bi(1-7)+-source. The cholesterol signal (m/z 369 and 385) was localized in Purkinje cells and in nuclei of granular layer cells. The phosphocholine headgroup of phosphatidylcholine and sphingomyelin was localized by imaging a specific fragment (m/z 86). This signal was localized in the molecular layer of cerebellar cortex, in Purkinje cells and in parts of the granular layer probably representing the synapse-rich glomeruli. The galactosylceramide was localized by imaging the quasi-molecular ions at m/z 835 and 851, showed a clear colocalization with cholesterol, but also a specific localization in dots (diameter <or=700 nm) in the molecular layer in the vicinity of Purkinje cells, at Purkinje cells and at cells in the granular layer. The results show a heterogeneous distribution of lipids between different cell types not previously described. In order to avoid redistribution artefacts, controls were made by a technique, based on high pressure freezing ,freeze fracturing and freeze drying of samples which were then analyzed by bombardment with a Bi3+ liquid metal ion gun. The galactosylceramide and cholesterol were found distributed as spots in the granular layer. The spots were of homogeneous size with a diameter of <700 nm. Although the galactosylceramide and cholesterol were localized to the same areas, there were clear differences in their distribution at higher resolution.

Role of cholesterol in synapse formation and function

Cholesterol is a multifaceted molecule, which serves as essential membrane component, as cofactor for signaling molecules and as precursor for steroid hormones. Consequently, defects in cholesterol metabolism cause devastating diseases. So far, the role of cholesterol in the nervous system is less well understood. Recent studies showed that cultured neurons from the mammalian central nervous system (CNS) require glia-derived cholesterol to form numerous and efficient synapses. This suggests that the availability of cholesterol in neurons limits the extent of synaptogenesis. Here, I will summarize the experimental evidence for this hypothesis, describe what is known about the structural and functional role of cholesterol at synapses, and discuss how cholesterol may influence synapse development and stability.

Filipin-induced deformations in plasma membranes of cultured bovine corneal endothelial cells with incomplete belts of tight junctions

Complete belts of intact tight junctions are thought to act as barriers to the movement of cholesterol in the plane of the plasma membrane. As cholesterol can be revealed by filipin-induced membrane deformations on freeze-fracture images, we studied the distribution of these deformation in cultured bovine corneal endothelial cells with incomplete belts of tight junctions. While the extent of filipin-induced deformations differed between individual cells, there is a homogeneous distribution of filipin-induced deformations on both sides of the incomplete junctional belt of endothelial cells. Our results suggest that cultured endothelial cells do not polarize cholesterol, possibly because of the incomplete tight-junctional barrier.

Filipin and digitonin studies of cell membrane changes during junction breakdown in the dystrophic rat retinal pigment epithelium

We have previously found that a breakdown of tight junctions in retinal pigment epithelial cells of Royal College of Surgeons' rats is associated with a redistribution of intramembrane particles and Na-K-ATPase activity. Changes in the lipid and sterol composition of membranes can alter their fluidity, permeability and enzyme activity, and may contribute to changes in cell barrier function in the dystrophic epithelium. We have now used filipin and digitonin, which bind to membrane sterols and produce membrane deformations recognizable by freeze-fracture and thin-section electron microscopy, to study the distribution of cholesterol and related 3-B-hydroxysterols in the dystrophic epithelium. The results of these studies show that in the normal pigment epithelium and prior to tight junction breakdown in the dystrophic epithelium, filipin- and digitonin-sterol complexes are rare in the membranes between tight junctions and adhering junctions, and in areas of attachment between the plasma membrane and basal lamina. Complexes are more numerous in the basal infoldings, and most densely packed in the lateral and apical microvillous membranes. During junction breakdown, complexes increase substantially in apical, basal, junctional, and nuclear membranes. Later, after the junctions disappear, complexes decrease. These results indicate that alterations in the expression of membrane sterols accompany the changes in structure and function of tight junctions in the dystrophic retinal pigment epithelium.

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 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.