Microtubules associated with postsynaptic 'thickenings'

The synaptic life of microtubules

In neurons, control of microtubule dynamics is required for multiple homeostatic and regulated activities. Over the past few decades, a great deal has been learned about the role of the microtubule cytoskeleton in axonal and dendritic transport, with a broad impact on neuronal health and disease. However, significantly less attention has been paid to the importance of microtubule dynamics in directly regulating synaptic function. Here, we review emerging literature demonstrating that microtubules enter synapses and control central aspects of synaptic activity, including neurotransmitter release and synaptic plasticity. The pleiotropic effects caused by a dysfunctional synaptic microtubule cytoskeleton may thus represent a key point of vulnerability for neurons and a primary driver of neurological disease.

Activity-Dependent Nucleation of Dynamic Microtubules at Presynaptic Boutons Controls Neurotransmission

Control of microtubule (MT) nucleation and dynamics is critical for neuronal function. Whether MT nucleation is regulated at presynaptic boutons and influences overall presynaptic activity remains unknown. By visualizing MT plus-end dynamics at individual excitatory en passant boutons in axons of cultured hippocampal neurons and in hippocampal slices expressing EB3-EGFP and vGlut1-mCherry, we found that dynamic MTs preferentially grow from presynaptic boutons, show biased directionality in that they are almost always oriented toward the distal tip of the axon, and can be induced by neuronal activity. Silencing of γ-tubulin expression reduced presynaptic MT nucleation, and depletion of either HAUS1 or HAUS7-augmin subunits increased the percentage of retrograde comets initiated at boutons, indicating that γ-tubulin and augmin are required for activity-dependent de novo nucleation of uniformly distally oriented dynamic MTs. We analyzed the dynamics of a wide range of axonal organelles as well as synaptic vesicles (SVs) relative to vGlut1⁺ stable presynaptic boutons in a time window during which MT nucleation at boutons is promoted upon induction of neuronal activity, and we found that γ-tubulin-dependent presynaptic MT nucleation controls bidirectional (SV) interbouton transport and regulates evoked SV exocytosis. Hence, en passant boutons act as hotspots for activity-dependent de novo MT nucleation, which controls neurotransmission by providing dynamic tracks for bidirectional delivery of SVs between sites of neurotransmitter release.

Protein components of post-synaptic density lattice, a backbone structure for type I excitatory synapses

It is essential to study the molecular architecture of post-synaptic density (PSD) to understand the molecular mechanism underlying the dynamic nature of PSD, one of the bases of synaptic plasticity. A well-known model for the architecture of PSD of type I excitatory synapses basically comprises of several scaffolding proteins (scaffold protein model). On the contrary, 'PSD lattice' observed through electron microscopy has been considered a basic backbone of type I PSDs. However, major constituents of the PSD lattice and the relationship between the PSD lattice and the scaffold protein model, remain unknown. We purified a PSD lattice fraction from the synaptic plasma membrane of rat forebrain. Protein components of the PSD lattice were examined through immuno-gold negative staining electron microscopy. The results indicated that tubulin, actin, α-internexin, and Ca²⁺ /calmodulin-dependent kinase II are major constituents of the PSD lattice, whereas scaffold proteins such as PSD-95, SAP102, GKAP, Shank1, and Homer, were rather minor components. A similar structure was also purified from the synaptic plasma membrane of forebrains from 7-day-old rats. On the basis of this study, we propose a 'PSD lattice-based dynamic nanocolumn' model for PSD molecular architecture, in which the scaffold protein model and the PSD lattice model are combined and an idea of dynamic nanocolumn PSD subdomain is also included. In the model, cytoskeletal proteins, in particular, tubulin, actin, and α-internexin, may play major roles in the construction of the PSD backbone and provide linker sites for various PSD scaffold protein complexes/subdomains.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Dendritic Spine Plasticity: New Regulatory Roles of Dynamic Microtubules

Dendritic spines are small actin-rich protrusions that form the postsynaptic part of most excitatory synapses. They play critical roles in synaptic function and exhibit a striking degree of structural plasticity, which is closely linked to changes in strength of synaptic connections. Here the authors summarize recent work that has revealed an important relationship between the microtubule and actin cytoskeleton in controlling spine morphology and plasticity. Dynamic microtubules and the proteins that specifically associate with the growing microtubule plus-ends recently emerged as temporal and spatial regulators of actin organization, which controls dynamic changes in structure and function of dendritic spines.

Assembling microtubules disintegrate the postsynaptic density in vitro

The postsynaptic density (PSD), a disk-shaped protein aggregation of several hundred nm in diameter, plays important roles in the signal transduction and molecular organization of the excitatory synapses in mammalian CNS. The PSD resides in the microfilament-enriched cytoplasm of dendritic spines where the transient appearance of microtubules has been reported. When PSD isolated from porcine brain was incubated with polymerizing alpha,beta-tubulins, its turbidity became greater than that of the original PSD, suggesting that the PSD's structure was altered upon incubating with assembling microtubules. By transmission electron microscopy, smaller PSD fragments and several novel structures, including holes and finger-like extensions, were found in the PSD after incubation with assembling microtubules, but not in the original PSD or in the PSD incubated with alpha,beta-tubulins pretreated with vincristine. The results suggest that the interactions with assembling microtubules may result in the formation of holes in the PSD, and the rupture of these holes subsequently leads to the formation of smaller PSD fragments.

Heavy chain of cytoplasmic dynein is a major component of the postsynaptic density fraction

A protein with an apparent molecular size of 490 kDa was found in the postsynaptic density (PSD) fraction isolated from porcine cerebral cortices and rat forebrains, and this 490 kDa protein accounted for approximately 3% of the total protein of these samples. Matrix-assisted laser desorption ionization-time of flight mass spectrometric and Western blotting analyses consistently indicated that this 490 kDa protein consisted primarily of the heavy chain of cytoplasmic dynein (cDHC). Immunocytochemical analyses showed that cDHC was found in 92% and 89% of the phalloidin-positive protrusions that were themselves associated with discrete clusters of synaptophysin, a presynaptic terminal marker, and PSD-95, a postsynaptic marker, on neuronal processes, respectively. Quantitative Western blotting analyses of various subcellular fractions isolated from porcine cerebral cortices and rat forebrains further showed that not only the heavy but also the intermediate chains of dynein are enriched in the PSD fraction. Cytoplasmic dynein is a microtubule-associated motor protein complex that drives the movement of various cargos toward the minus ends of microtubules and plays many other diverse functions in the cell. Our results that cDHC is a major component of the PSD fraction, that both dynein heavy and intermediate chains are enriched in the PSD fraction and that cDHC is present in dendritic spines raise the possibilities that cytoplasmic dynein may play structural and functional roles in the postsynaptic terminal.

Developmental-dependent action of microtubule depolymerization on the function and structure of synaptic glycine receptor clusters in spinal neurons

Microtubules have been proposed to interact with gephyrin/glycine receptors (GlyRs) in synaptic aggregates. However, the consequence of microtubule disruption on the structure of postsynaptic GlyR/gephyrin clusters is controversial and possible alterations in function are largely unknown. In this study, we have examined the physiological and morphological properties of GlyR/gephyrin clusters after colchicine treatment in cultured spinal neurons during development. In immature neurons (5-7 DIV), disruption of microtubules resulted in a 33 +/- 4% decrease in the peak amplitude and a 72 +/- 15% reduction in the frequency of spontaneous glycinergic miniature postsynaptic currents (mIPSCs) recorded in whole cell mode. However, similar colchicine treatments resulted in smaller effects on 10-12 DIV neurons and no effect on mature neurons (15-17 DIV). The decrease in glycinergic mIPSC amplitude and frequency reflects postsynaptic actions of colchicine, since postsynaptic stabilization of microtubules with GTP prevented both actions and similar reductions in mIPSC frequency were obtained by modifying the Cl(-) driving force to obtain parallel reductions in mIPSC amplitude. Confocal microscopy revealed that colchicine reduced the average length and immunofluorescence intensity of synaptic gephyrin/GlyR clusters in immature (approximately 30%) and intermediate (approximately 15%) neurons, but not in mature clusters. Thus the structural and functional changes of postsynaptic gephyrin/GlyR clusters after colchicine treatment were tightly correlated. Finally, RT-PCR, kinetic analysis and picrotoxin blockade of glycinergic mIPSCs indicated a reorganization of the postsynaptic region from containing both alpha2beta and alpha1beta GlyRs in immature neurons to only alpha1beta GlyRs in mature neurons. Microtubule disruption preferentially affected postsynaptic sites containing alpha2beta-containing synaptic receptors.

The relationship of hydrophobic tubulin with membranes in neural tissue

Brain membrane preparations contain tubulin that can be extracted with Triton X-114. After the extract is allowed to partition, 8% of the total brain tubulin is isolated as a hydrophobic compound in the detergent-rich phase. Cytosolic tubulin does not show this hydrophobic behaviour since it is recovered in the aqueous phase. Membrane tubulin can be released by 0.1 M Na2 CO3 treatment at pH > or = 11.5 in such a way that the hydrophobic tubulin is converted into the hydrophilic form. These results suggest that tubulin exists associated with some membrane component that confers the hydrophobic behaviour to tubulin. If the tissue is homogenized in microtubule-stabilizing buffer containing Triton X-100, the hydrophobic tubulin is isolated from the microtubule fraction. This result indicates that the hydrophobic tubulin isolated from membrane preparations belongs to microtubules that in vivo are associated to membranes. Therefore, hydrophobic tubulin (tubulin-membrane component complex) can be obtained from membranes or from microtubules depending on the conditions of brain homogenization.

Vincristine-induced changes in the retina of the isolated arterially-perfused cat eye

We have attempted to determine in this study whether the arterial administration of vincristine produces in cat the functional defects associated with hereditary and vincristine-induced night blindness in man. Using the isolated perfused cat eye, it has been possible to mimic some of the essential features of human night blindness, namely, retention of normal rhodopsin chemistry and normal photoreceptor activity, with marked suppression of the ERG b-wave. In addition, we find that vincristine produces an early, rapid fall in the c-wave, a potential that arises largely in the pigment epithelium. Ultrastructurally, it appears that many classes of retinal neuron are affected by the drug, but the principal changes in cytoarchitecture are seen in the photoreceptors. Except for the outer segments, paracrystalline deposits were found in all parts of the visual cell. The disruption of the normal microtubular organization of these cells suggests that the drug interferes with the functional integrity of the transport system by which synaptic activity is maintained. By reducing the efficacy of communication between visual cells and their second-order neurons, the electrical responses of post-synaptic elements is degraded. The route by which vinca alkaloids reach the neural retina is still uncertain, but our preliminary studies using HRP indicate that the relatively high concentration of vincristine used in this study may be responsible for compromising the blood-retinal barrier.

The metabolic turnover of the major proteins of the postsynaptic density

We have used the method of Austin, Lowry, Brown and Carter, to measure the steady-state metabolic half-life of tubulin (alpha and beta individually) and actin (beta and gamma together) in the total cytosolic (S3), microsomal (P3), synaptic plasma membrane (SPM) and synaptic junction (SJ) subcellular fractions from 6-day-old and adult chicken forebrain. In the SPM and SJ fractions we also measured the steady-state metabolic half-life of the major postsynaptic density protein (mPSDp). In SPM and SJ fractions from 6-day-old chickens tubulin and actin turned over approximately twice as slowly (t1/2 approximately equal to 24 days) as tubulin and actin in the S3 fraction (t1/2 approximately equal to 13 days). This difference was unlikely merely to be due to association with membranes since the t1/2 values for the proteins were the same in P3 and S3. The estimated t1/2 values for mPSDp were similar to that for tubulin and actin in SPM and SJ fractions. Similar results were obtained in adult chickens except that all t1/2 values in all fractions were approximately 30% larger. The calculated t1/2 values did not change between labelling periods of 4 and 6.5 h suggesting that the lag phase of incorporation of newly synthesized PSD proteins is sufficiently rapid to not produce this result artefactually. When the brain from a non-labelled chicken was homogenized in the presence of the S3 fraction from a labelled chicken and sub-fractionated the relative specific activities of the SPM and SJ fractions produced were 1-2% of those from the labelled brain. These results support the notion that tubulin and actin are intrinsic components of the PSD.

New observations on the substructure of the active zone of brain synapses and motor endplates

This study offers a new concept on the origin and function of the hitherto enigmatic presynaptic dense projections (dps) of neurons and motor endplates. After a deuterium oxide-albumin pretreatment (da), brain tissue and motor endplate of rat and frog reveal an intricate association of smooth endoplasmic reticulum (ser), microtubules (mts) and synaptic vesicles (sv) at the presynaptic grid-active zone of synapses. The ser entwines the mts, which are clothed in svs, and impinges directly onto the presynaptic membrane as sacs or 'tubular-fibrillar' extensions. Since no dps are seen in these sections, whereas they do occur in conventionally processed material (i.e. without da pretreatment), it is suggested that the dps of conventional material may, in part, originate from improperly fixed ser at these points. Thus for the first time we demonstrate an in vivo system of ser which, because its 'finger' processes come into intimate contact with the presynaptic membrane, may be implicated in Ca2+ ion translocation, presumably out of the presynaptic bulb. Since no such tubular ser has been demonstrated in what are claimed to be sophisticated techniques (i.e. high-speed slam-freezing-freeze substitution) the actual sophistication of such methods is questioned.

The effects of detergents on the composition of postsynaptic densities

A method of purifying postsynaptic densities (PSD) of Cohen et al. (1977) has been modified, primarily by the substitution of octyl glucoside as the detergent used to solubilize synaptosomal fractions. Subsequent extraction with other detergents resulted in the selective removal of specific polypeptides. In particular sulphobetaine 3-14 removed most of the beta-tubulin but not alpha-tubulin. Sodium N-lauroyl sarcosinate completely destroyed the structural integrity of the PSD when the in vitro formation of intermolecular disulphide bonds was minimized. These results suggest that the structure of PSDs is more labile than previously thought and demonstrate a technique for further examining their composition.

Disassembly and reconstitution of a membrane-microtubule complex

The cell membrane of the unicellular algae Distigma proteus is associated with arrays of parallel microtubules. Fragments of the membrane-microtubule complex have been isolated and partially purified. The microtubules were stable in vitro at room temperature as well as at 0 degree C, but were specifically and rapidly disassembled by Ca2+. After removal of all endogenous microtubules, the membrane-microtubule complex could be reassembled from brain microtubule protein and denuded Distigma membrane fragments. The readded microtubules bound in a fixed orientation, and only to those regions of membrane that are normally associated with microtubules in vivo.

Three-dimensional structure of a membrane-microtubule complex

The unicellular algae Distigma proteus contain a group of aligned microtubules associated with their cell membrane. The association is maintained in isolated membrane fragments. The membrane-microtubule complex also includes a crystalline array of membrane particles. The major peptide component of this array was identified by labeling whole cells with radioiodine. The entire complex of membrane, particles, and microtubules is sufficiently well ordered to permit reconstruction from electron micrographs by Fourier techniques. A three-dimensional model of the membrane array at a nominal resolution of 2.5 nm has been calculated. Some similarities were apparent between lattice spacings in the membrane array and in microtubules. Analysis of these lattice correlations suggests a way in which the array of membrane particles may serve as scaffolding for microtubule attachment.

Organization and density of microtubules in the vestibular sensory cells in the cat

In the vestibular receptors, the cytoplasmic hair cell microtubules manifest a particular affinity for the membraneous areas juxtaposed to an afferent nerve terminal. Moreover, their number is greater in the type I cells, which possess larger neuro-epithelial surface contacts than either in the type II cells or in the supporting cells. In the discussion, two hypotheses are proposed according to the results; the first concerns the structural role of the cytoskeleton and the second suggests its participation in sensory transduction.

Isolation of postsynaptic densities retaining their membrane attachment

A method is described for the preparation of a subcellular fraction, 30-50% pure, of intact postsynaptic units from rat cerebral cortex. The isolation procedure is based on chemical dissociation of the synaptic cleft as described by Crawford, Osborne & Potter followed by sonication of the extracted membranes and separation of the postsynaptic units on a discontinuous sucrose gradient. This preparation provides the first practical procedure for the isolation of postsynaptic densities, prominent organelles of unknown function, without the use of detergents, enabling retention of the postsynaptic membrane in association with the postsynaptic density. The preparation shows enhanced binding of spiroperidol, a dopamine agonist, which, in conjunction with morphological evidence, indicates that the preparation is sufficiently intact to enable study of the interaction of the postsynaptic membrane with the postsynaptic density. Actin, alpha- and beta-tubulin and postsynaptic density protein constitute the major proteins in the preparation; they are present in amounts of 41, 54, 57 and 74 micrograms per mg protein, respectively; as compared to 54, 59, 55 and 9 micrograms per mg protein of the synaptic junctional membrane used as starting material. The utility of the preparation for a number of localization studies, including ion translocating adenosine 5'-triphosphatases, protein kinases and their substrates is discussed.

Microtubules and laminated structures in inner ear hair cells

Rat vestibular hair cells and adder (a snake) auditory and vestibular hair cells were examined by electron microscopy after various fixations. An albumin/unbuffered osmic acid/unbuffered glutaraldehyde fixation was found to preserve the microtubules well. After this fixation a narrow connection was observed between stereocilia rootlets or rootlet-like structures and apical microtubules. A laminated structure formed by rootlet filament or rootlet-like fibres was consistently found in the lower part of the cuticular structure. Similar but isolated laminated bodies (Friedmann bodies) were occasionally observed in the snake auditory organ. Bundles of microtubules from the area below the cuticular structure as well as from the dense reticular lamina stretch down in the hair cells. In the dense reticular lamina stretch down in the hair cells. In the basal part of the cells' microtubules very frequently connect with afferent synapses. A microtubular connection between the stereocilia rootlets or laminated part of the cuticular structure and the synapses is proposed. Vesicles adherent to microtubules may suggest a transport or guidance function for synaptic vesicles.

Postmortem accumulation of tubulin in postsynaptic density preparations

Postsynaptic density (PSD) preparations isolated from canine cerebral cortex that had been left at 0-37 degrees C for various times were found to become enriched in two bands in a time- but not temperature-dependent manner. The two bands were identified as tubulin subunits by gel mobility and immunology, Of all the isolated synaptic structures the increase in tubulin occurred primarily in the PSD fraction. The increase of tubulin also occurred in PSD preparations isolated from canine cerebellum and rat forebrain. Results obtained when PSD fractions were isolated from canine brain obtained as rapidly as possible after the death of the animal indicate that the maximum amount of tubulin in the PSD preparations is 2.5% of total Coomassie blue-stained protein as determined by scanning of gel electrophoretograms. These results imply that tubulin is probably not a major structural protein of the PSD as it exists in situ.

Function of a calmodulin in postsynaptic densities. III. Calmodulin-binding proteins of the postsynaptic density

A method has been developed for binding calmodulin, radioiodinated by the lactoperoxidase method, to denaturing gels and has been used to attempt to identify the calmodulin-binding proteins of cerebral cortex postsynaptic densities (PSDs). Calmodulin primarily bound to the major 51,000 Mr protein in a saturatable manner; secondarily bound to the 60,000 Mr region, 140,000 Mr region, and 230,000 Mr protein; and bound in lesser amounts to a number of other proteins. The major 51,000 Mr calmodulin-binding protein is one of unknown identity. Binding of iodinated calmodulin to these proteins was blocked by EDTA, EGTA, chlorpromazine, and preincubation with unlabeled calmodulin. Calmodulin iodinated by the chloramine-T method, which inactivates calmodulin did not bind to the PSD but bound nonspecifically to histone. Calmodulin did not bind to proteins from a variety of sources for which calmodulin interactions have not been found. Except for three proteins, all of the proteins of synaptic membranes that bind calmodulin could be accounted for by proteins of the PSD which are a part of the synaptic membrane fraction. The major 51,000 M, protein and the corresponding iodinated calmodulin binding were greatly reduced in cerebellar PSDs and this difference between cerebral cortex and cerebellar PSDs is discussed in light of the possible function of calmodulin in synaptic excitatory responses.

Microtubules, dendritic spines and spine appratuses

Using techniques for enhanced microtubular preservation, including albumin pretreatment (Gray, 1975), occipital cortex of rats was studied electron microscopically at various ages of development. A close structural relationship was seen between microtubules, sacs of SER and the postsynaptic "thickening" in primordial spines and with the dense "plate" material of spine apparatuses. Stereoscopic preparations in addition show a more complicated substructure than previously described for the "plate". Microtubules may contribute to the formation of the "plate" of the spine apparatus which in turn is associated with the postsynaptic "thickening" of the mature spine. Possible functional correlates are discussed.

Filamentous contacts containing subjunctional dense lattice and tubular smooth endoplasmic reticulum in cat lateral geniculate nuclei

Ultrastructure of filamentous contacts was examined in the cat lateral geniculate nucleus. This intercellular contact was classified into two types, asymmetrical and symmetrical, according to its fine structure. (1) The asymmetrical filamentous contact had a different fine structure on the cytoplasmic sides of its apposed membranes. This sort of contact occurred between the postsynaptic components of principal cells and the presynaptic terminals of either optic nerve or interneurons. The membrane on the presynaptic side showed a wavy outline. Plaques of cytoplasmic density were located on the crests of the wave. On the postsynaptic side cytoplasmic dense material formed a regularly arranged latticework on the membrane, 'subjunctional dense lattice'. The dense lattice was closely associated with underlying smooth surfaced endoplasmic reticulum and mitochondria, so as to form a subjunctional complex consisting of these 3 organelles. Two filamentous structures, neurofilaments and microtubules, enter and leave this subjunctional complex. (2) The symmetrical filamentous contact occurred between two principal cell components. Nearly the same fine structural arrangement as that on the postsynaptic side of the asymmetrical contact was present in the cytoplasm on the both sides of this junction.

Dendro axonic neurotransmission. II. Morphological sites for the synthesis, binding and release of neurotransmitters in dopaminergic dendrites in the substantia nigra and cholinergic dendrites in the neostriatum

Morphological evidence is presented indicating sites of synthesis, storage, and release of neurotransmitters in dendrites of dopaminergic cells of the substantia nigra and cholinergic cells of the neostriatum. Smooth endoplasmic reticulum can be identified in dopaminergic neurons touching the dendritic surface. The false transmitter for dopamine, 5-hydroxydopamine (5-OHDA), is localized to smooth endoplasmic reticulum or large vesicular structures which approach the dendritic surface. The dopamine synthesizing enzyme, tyrosine hydroxylase (TH), is localized to microtubules and smooth endoplasmic reticulum which approach the postsynaptic membrane. In the neostriatum, dopaminergic nerve endings make asymmetrical axospinous contacts. The postsynaptic spines often contain a few 'vesicles' near the postsynaptic thickenings. The surface and subsurface structures stain preferentially for choline acetyltransferase (CAT), the synthesizing enzyme for acetylcholine. It is hypothesized that neurotransmitters are released from dendrites as a general phenomenon in the CNS and that they can act upon axonal endings.