The organization of synaptic axcplasm in the lamprey (petromyzon marinus) central nervous system

Lumenal components of cytoplasmic microtubules

The lumen of cytoplasmic microtubules is a poorly explored expanse of intracellular space. Although typically represented in textbooks as a hollow tube, studies over several decades have shown that the microtubule lumen is occupied by a range of morphologically diverse components. These are predominantly globular particles of varying sizes which appear to exist either in isolation, bind to the microtubule wall, or form discontinuous columns that extend through the lumenal space. Actin filaments with morphologies distinct from the canonical cytoplasmic forms have also now been found within the microtubule lumen. In this review, we examine the historic literature that observed these lumenal components in tissues from diverse species and integrate it with recent cryo-electron tomography studies that have begun to identify lumenal proteins. We consider their cell and tissue distribution, possible mechanisms of incorporation, and potential functions. It is likely that continuing work in this area will open a new frontier in cytoskeletal biology.

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.

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

Microtubule and Actin Differentially Regulate Synaptic Vesicle Cycling to Maintain High-Frequency Neurotransmission

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial.

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial. At the calyx of Held in rats of either sex, confocal and high-resolution microscopy revealed that MTs enter deep into presynaptic terminal swellings and partially colocalize with a subset of synaptic vesicles (SVs). Electrophysiological analysis demonstrated that depolymerization of MTs specifically prolonged the slow-recovery time component of EPSCs from short-term depression induced by a train of high-frequency stimulation, whereas depolymerization of F-actin specifically prolonged the fast-recovery component. In simultaneous presynaptic and postsynaptic action potential recordings, depolymerization of MTs or F-actin significantly impaired the fidelity of high-frequency neurotransmission. We conclude that MTs and F-actin differentially contribute to slow and fast SV replenishment, thereby maintaining high-frequency neurotransmission.

SIGNIFICANCE STATEMENT The presence and functional role of MTs in the presynaptic terminal are controversial. Here, we demonstrate that MTs are present near SVs in calyceal presynaptic terminals and that MT depolymerization specifically prolongs the slow-recovery component of EPSCs from short-term depression. In contrast, F-actin depolymerization specifically prolongs fast-recovery component. Depolymerization of MT or F-actin has no direct effect on SV exocytosis/endocytosis or basal transmission, but significantly impairs the fidelity of high-frequency transmission, suggesting that presynaptic cytoskeletal filaments play essential roles in SV replenishment for the maintenance of high-frequency neurotransmission.

The Effect of Axon Resealing on Retrograde Neuronal Death after Spinal Cord Injury in Lamprey

Failure of axon regeneration in the central nervous system (CNS) of mammals is due to both extrinsic inhibitory factors and to neuron-intrinsic factors. The importance of intrinsic factors is illustrated in the sea lamprey by the 18 pairs of large, individually identified reticulospinal (RS) neurons, whose axons are located in the same spinal cord tracts but vary greatly in their ability to regenerate after spinal cord transection (TX). The neurons that are bad regenerators also undergo very delayed apoptosis, signaled early by activation of caspases. We noticed that the neurons with a low probability of axon regeneration tend to be larger than the good regenerators. We postulate that the poorly regenerating larger neurons have larger caliber axons, which reseal more slowly, allowing more prolonged entry of toxic signals (e.g., Ca++) into the axon at the injury site. To test this hypothesis, we used a dye-exclusion assay, applying membrane-impermeable dyes to the cut ends of spinal cords at progressively longer post-TX intervals. Axons belonging to the very small neurons (not individually identified) of the medial inferior RS nucleus resealed within 15 min post-TX. Almost 75% of axons belonging to the medium-sized identified RS neurons resealed within 3 h. At this time, only 36% of the largest axons had resealed, often taking more than 24 h to exclude the dye. There was an inverse relationship between an RS neuron's size and the probability that its axon would regenerate (r = -0.92) and that the neuron would undergo delayed apoptosis, as indicated by staining with a fluorescently labeled inhibitor of caspases (FLICA; r = 0.73). The artificial acceleration of resealing with polyethylene glycol (PEG) reduced retrograde neuronal apoptosis by 69.5% at 2 weeks after spinal cord injury (SCI), suggesting that axon resealing is a critical determinant of cell survival. Ca++-free Ringer's solution with EGTA prolonged the sealing time and increased apoptotic signaling, suggesting that factors other than Ca++ diffusion into the injured tip contribute to retrograde death signaling. A longer distance of the lesion from the cell body reduced apoptotic signaling independent of the axon sealing time.

The Presynaptic Microtubule Cytoskeleton in Physiological and Pathological Conditions: Lessons from Drosophila Fragile X Syndrome and Hereditary Spastic Paraplegias

The capacity of the nervous system to generate neuronal networks relies on the establishment and maintenance of synaptic contacts. Synapses are composed of functionally different presynaptic and postsynaptic compartments. An appropriate synaptic architecture is required to provide the structural basis that supports synaptic transmission, a process involving changes in cytoskeletal dynamics. Actin microfilaments are the main cytoskeletal components present at both presynaptic and postsynaptic terminals in glutamatergic synapses. However, in the last few years it has been demonstrated that microtubules (MTs) transiently invade dendritic spines, promoting their maturation. Nevertheless, the presence and functions of MTs at the presynaptic site are still a matter of debate. Early electron microscopy (EM) studies revealed that MTs are present in the presynaptic terminals of the central nervous system (CNS) where they interact with synaptic vesicles (SVs) and reach the active zone. These observations have been reproduced by several EM protocols; however, there is empirical heterogeneity in detecting presynaptic MTs, since they appear to be both labile and unstable. Moreover, increasing evidence derived from studies in the fruit fly neuromuscular junction proposes different roles for MTs in regulating presynaptic function in physiological and pathological conditions. In this review, we summarize the main findings that support the presence and roles of MTs at presynaptic terminals, integrating descriptive and biochemical analyses, and studies performed in invertebrate genetic models.

Presynaptic microtubules and their association with synaptic vesicles

When fragments of rat or frog brain are teased in albumin solution before fixation, the synapses show microtubules focused on the presynaptic membrane and in close association with synaptic vesicles.

Giant terminals in the dorsal octavolateralis nucleus of lampreys

The dorsal octavolateralis nucleus of lampreys is a primary nucleus for electroreceptive stimuli in the medulla. In Lampetra japonica, the rostral and caudal thirds of this nucleus are exclusively occupied by giant terminals, which become evident when the primary fibers of an electrosensory nerve (recurrent branch of the anterior lateral line nerve) are labeled with horseradish peroxidase. We studied the ultrastructure of these terminals. They contain neurofilaments, mitochondria, microtubules, and tubular membranous structures. Many synapses, all of the chemical type, are located around the neck region of the terminal swellings. Many vesicular structures, which are clear, round, and uniform in size, and most of which are probably synaptic vesicles, are densely clustered in a single large mass in the neck region of the terminals. Some of the tubular structures may serve as a membrane reservoir for the large number of synaptic vesicles required in the giant terminals.

Neurofilaments are prominent in bullfrog olfactory axons but are rarely seen in those of the tiger salamander, Ambystoma tigrinum

Bullfrog olfactory axons show variable numbers (0-29) of structurally typical neurofilaments (NFs) about 10 nm in diameter. In studies tracking these NFs through serial sections of axons in cross section, they were found to be discontinuous, with a calculated average length of about 118 microns. In contrast to olfactory axons in bullfrogs, those of the tiger salamander Ambystoma trigrinum rarely show NFs. To be certain that the absence of NFs is a specific characteristic of olfactory axons, pieces of salamander spinal cord, optic nerve, and sciatic nerve were examined and found to contain typical NFs. To minimize the possibility that NFs in salamander olfactory axons were degraded or poorly fixed during preparation for electron microscopy, samples were fixed by using a variety of fixative and buffer combinations. To exclude the possibility that proteases degraded NFs during processing, prior to fixation some pieces of olfactory nerve were incubated in physiological saline containing protease inhibitors. Regardless of the preparation method, NFs were generally not seen in salamander olfactory axons. Extracts of salamander olfactory nerve were subjected to SDS-polyacrylamide gel electrophoresis (PAGE) and immunoblotting studies with monoclonal antibodies to the three NF subunit proteins. The immunoblots showed negligible or trace amounts of NF-L (light) and NF-H (heavy), while an NF-M (medium) protein having a molecular mass (Mr) of 160 kD was present in abundance. Extracts of salamander spinal cord, on the other hand, showed all three subunit proteins (with Mrs of 230, 160, and 77 kD). If one assumes that cells assemble structural elements to provide for a given function, the findings suggest that NFs in olfactory neurons of bullfrogs provide a function that may be missing in olfactory neurons of the salamander; the evidence also suggests that the absence of NFs in the salamander may be due to a deficiency in two of the three NF subunit proteins.

Substructure of sea urchin egg cytoplasmic dynein

The substructure of the cytoplasmic dynein molecule was studied using the quick-freeze, deep-etch technique. Cytoplasmic dynein purified as a 12 S form from the eggs of the sea urchin Hemicentrotus pulcherrimus was composed of a single high molecular weight polypeptide. Rotary shadowing images of cytoplasmic dynein either sprayed on to a mica surface or quick-frozen on mica flakes demonstrated a single-headed molecule, in contrast to the two-headed molecule of sea urchin sperm flagellar 21 S dynein. More detailed substructure was visualized by rotary shadowing after quick-freeze deep-etching. Cytoplasmic dynein consisted of a head and a stem. The head was pear-shaped (16 nm X 11 nm) and a little smaller than the pear-shaped head of 21 S dynein (18 nm X 14 nm). The form of the stem was irregular, and its apparent length varied from 0 to 32 nm. Binding of cytoplasmic dynein to brain microtubule in the solution was observed by negative staining, and that in the precipitate was examined by the quick-freeze, deep-etch method as well. Both methods revealed the presence of two kinds of microtubules, one a fully decorated microtubule and the other a non-decorated microtubule. Cytoplasmic dynein bound to microtubule also appeared as a globular particle. Neither the periodic binding nor the crossbridges that were observed with 21 S dynein were formed by cytoplasmic dynein, although cytoplasmic dynein appeared to bind to microtubules co-operatively.

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.

Characterization of microtubule-associated proteins isolated from bovine adrenal gland

We investigated the biochemical characteristics of microtubule-associated proteins (MAPs) of both the adrenal medulla and the cortex. The major constituents of the adrenal MAPs isolated by the taxol-dependent procedure [Vallee, R. B. (1982) J. Cell. Biol. 92, 435-442] were several polypeptides in the high-molecular-mass region (high-Mr MAPs) and a 190000-Mr polypeptide (190-kDa MAP). In the cortex MAP fraction, the most prominent component was 190-kDa MAP, while the medulla MAP fraction was rich in high-Mr MAPs. Twice-cycled microtubule proteins prepared without taxol from the same sources also contained high-Mr MAPs and 190-kDa MAP. High-Mr MAPs contained protein species identical to MAP1 and MAP2 of mammalian brain as judged from electrophoretic mobility, heat-stability and immunoreactivity. 190-kDa MAP was classified as MAP subspecies distinct from high-Mr MAPs by several criteria. The MAP fractions had the ability to polymerize purified tubulin into microtubules, and the major MAP species (high-Mr MAPs and 190-kDa MAP) were found to cosediment with reconstituted microtubules. Tau factor, one of the major MAPs in the mammalian brain, appeared to be a minor species in the adrenal gland.

Influence of translocation track on the motion of intra-axonally transported organelles in human nerve

The mechanism by which organelles are transported bidirectionally in axoplasm is still unknown; however, evidence of a key role for microtubules in many nonmammalian models has been established. We have observed common or shared tracks within the axoplasm of human nerves along which multiple organelles of varying size and shape are bidirectionally transported. Organelles traveling anterogradely and retrogradely were visualized by video-enhanced differential interference contrast optics and analyzed with the aid of computer-image-processing techniques. Speeds of translocating organelles were determined at eight to 16 translocation points along a path or "track." Each translocation speed was plotted against its corresponding position on the track to develop a "speed/position diagram." Regardless of mean organelle speed or direction of motion, organelles sharing a common track exhibited similar patterns of "speeding up" and "slowing down" relative to position along the track. Speed position data for organelles translocating the local axonal region of a common track showed no unique patterns (not different from a uniform distribution, p less than 0.05). The unique speed/position patterns exhibited by common tracks were not necessarily related to the patterns of other tracks in the immediate vicinity (distance between tracks of less than 0.50 micron). These findings suggest that there are "common tracks" shared by organelles moving retrogradely and anterogradely; both the organelles and the "track" associated with its translocation play a role in the resultant motion of that organelle; the influence exerted by a common track on the motion of an organelle results in a pattern of speed changes related to position along the track.

Cultured neurons contain a variety of microtubule-associated proteins

The non-tubulin proteins associated with microtubules (MAPs) in cultures of pure sympathetic neurons have been identified using a variety of biochemical and immunochemical methods. MAPs of cultured sympathetic neurons include proteins corresponding to brain MAP-1 (consisting of MAP-1a and MAP-1b species), MAP-2, MAP-3, tau, 4 proteins that range in molecular weight from 60,000 to 76,000, and proteins with molecular weights of 210,000, 130,000 and 32,000. Many of the MAPs are phosphorylated in situ. MAP-2 and tau of cultured sympathetic neurons differ from their counterparts of brain in electrophoretic mobility. The observed variety of MAPs in sympathetic neurons together with the differences in MAPs of brain and sympathetic neurons are discussed in terms of microtubule heterogeneity in the nervous system.

Virus-like particles in human vestibular ganglion cells

We found intracytoplasmic aggregates of virus-like particles in human vestibular ganglion cells. These particles were always observed in the peripheral area of the cytoplasm. Morphological characteristics of the cytoplasm are similar to those of other ganglion cells. The inclusion bodies are round and measure about 1.7 micron in maximum diameter. They consist of a porous convoluted dense material and virus-like particles. The overall diameter of 118 randomly selected particles varies from 36 nm to 73 nm, and the mean value is 53 nm. Most of the particles are spherical while a few possess a hexagonal or semicircular profile. The particles exhibit a double external membrane or vesiculated external layer. Most of the particles are empty. There are, however, some particles which show vesicular structures in their content. Although our present data are insufficient to determine these particles as viral, their hexagonal shape and size are similar to true virus. With these data in mind, we suggest that these particles might be a dormant form of virus and may possibly produce infectious disease in the inner ear.

Gliding movement of and bidirectional transport along single native microtubules from squid axoplasm: evidence for an active role of microtubules in cytoplasmic transport

Native microtubules prepared from extruded and dissociated axoplasm have been observed to transport organelles and vesicles unidirectionally in fresh preparations and more slowly and bidirectionally in older preparations. Both endogenous and exogenous (fluorescent polystyrene) particles in rapid Brownian motion alight on and adhere to microtubules and are transported along them. Particles can switch from one intersecting microtubule to another and move in either direction. Microtubular segments 1 to 30 microns long, produced by gentle homogenization, glide over glass surfaces for hundreds of micrometers in straight lines unless acted upon by obstacles. While gliding they transport particles either in the same (forward) direction and/or in the backward direction. Particle movement and gliding of microtubule segments require ATP and are insensitive to taxol (30 microM). It appears, therefore, that the mechanisms producing the motive force are very closely associated with the native microtubule itself or with its associated proteins. Although these movements appear irreconcilable with several current theories of fast axoplasmic transport, in this article we propose two models that might explain the observed phenomena and, by extension, the process of fast axoplasmic transport itself. The findings presented and the possible mechanisms proposed for fast axoplasmic transport have potential applications across the spectrum of microtubule-based motility processes.

A model for fast axonal transport

A model for fast axonal transport is developed in which the essential features are that organelles may interact with mechanochemical cross-bridges that in turn interact with microtubules, forming an organelle-engine-microtubule complex which is transported along the microtubules. Computer analysis of the equations derived to describe such a system show that most of the experimental observations on fast axonal transport can be simulated by the model, indicating that the model is useful for the interpretation and design of experiments aimed at clarifying the mechanism of fast axonal transport.

Luminal material in microtubules of frog olfactory axons: structure and distribution

The substructure and distribution of luminal material in microtubules of olfactory axons were studied in the bullfrog, Rana catesbeiana. By using numerous fixation methods, with and without osmium tetroxide, the luminal component was shown not to be an artifact of fixation. The material consists of globular elements 4-5 nm in diameter loosely arranged within the lumen in a discontinuous column. Counts of microtubules showing luminal material were obtained for axons in the proximal and distal ends of the olfactory nerve, and it was found that 16-18% more of the microtubules in the distal regions showed the luminal component. This raises the possibility that the material might be translocated within the microtubule lumen and tends to accumulate as it moves distally toward the axon terminal. In contrast to those of the olfactory axons, microtubules assembled in vitro from frog brain tubulin did not show luminal material. When microtubules in olfactory axons were depolymerized in situ by cold and calcium treatment and then induced to reassemble, most of those that were formed de novo showed empty lumina. Such evidence suggests that the luminal material is not an integral component of the microtubule. The hypothesis is discussed that material may be translocated within the lumina of microtubules. Furthermore, in the case of neuronal microtubules, the possibility is raised that they may serve as conduits for their own wall subunits.

Counts of axons in electron microscopic sections of ventral roots in lampreys

The number of motoneurons in the lamprey spinal cord was estimated by counting axons in electron microscopic sections of ventral roots. Physiologically identified and marked motoneurons had one axon each in an ipsilateral ventral root. The corresponding axons in sections were unmyelinated, individually ensheathed, and 2-20 micron in diameter. Lampreys lack an organized sympathetic nervous system, but a minor and variable population of small axons with dense-cored vesicles was also present in ventral and dorsal roots. The estimated numbers of motor axons per ventral root in adult Ichthyomyzon unicupis and I. castaneus were 70.6 +/- 9.3 (mean +/- SD), in adult landlocked Petromyzon marinus 100.1 +/- 15.0, and in large ammocoetes of anadromous Petromyzon marinus 106.8 +/- 14.3. The total estimated numbers of myotomal and fin motoneurons were as many as 13,000 in the spinal cord of adult Ichthymyzon and 24,000 in adult Petromyzon.

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.

Studies on the motility of the foraminifera. II. The dynamic microtubular cytoskeleton of the reticulopodial network of Allogromia laticollaris

Lamellipodia have been induced to form within the reticulopodial networks of Allogromia laticollaris by being plated on positively charged substrata. Video-enhanced, polarized light, and differential interference contrast microscopy have demonstrated the presence of positively birefringent fibrils within these lamellipodia. The fibrils correspond to the microtubules and bundles of microtubules observed in whole-mount transmission electron micrographs of lamellipodia. Microtubular fibrils exhibit two types of movements within the lamellipodia: lateral and axial translocations. Lateral movements are often accompanied by reversible lateral associations between adjacent fibrils within a lamellipodium. This lateral association-dissociation of adjacent fibrils has been termed 'zipping' and 'unzipping'. Axial translocations are bidirectional. The axial movements of the microtubular fibrils can result in the extension of filopodia by pushing against the plasma membrane of the lamellipodia. Shortening, or complete withdrawal, of such filopodia is accomplished by the reversal of the direction of the axial movement. The bidirectional streaming characteristic of the reticulopodial networks also occurs within the lamellipodia. In these flattened regions the streaming is clearly seen to occur exclusively in association with the intracellular fibrils. Transport of both organelles and bulk hyaline cytoplasm occurs bidirectionally along the fibrils.

Immunocytochemical evidence for tubulin in the presynaptic terminal of synaptosomes

Ultrastructural studies of intact tissue rarely show presynaptic microtubules, and immunocytochemical studies on tissue sections have previously been unable to demonstrate tubulin in the nerve terminal. In contrast, a microtubular coil can be readily detected in the presynaptic nerve terminal of synaptosomes. We have developed an immunocytochemical procedure on the synaptosome preparation and demonstrated, using monoclonal antibodies, that in the presynaptic terminal alpha and beta tubulin subunits are specifically restricted to the equatorial microtubular coil.

Neurotransmitter release mechanisms and microtubules

The morphological mechanisms involved in translocation of the synaptic vesicle to the presynaptic membrane, release of transmitter from the vesicle and recycling of the vesicle membrane are still far from understood. However, there is strong evidence that vesicles move along the surfaces of a specific set of highly labile presynaptic microtubules that direct the vesicles to the active zones. These microtubules are focused in a precise geometrical array, which is in register with and in contact with presynaptic dense projections of the central nervous system synapse or presynaptic dense bars of the motor endplate. These dense complexes constitute the presynaptic grid or active zones. The regular arrays of dense projections or bars are in turn coincident with rings or chains of synaptic vesicles mobilized at release sites on the presynaptic membrane (having arrived at these precise points by microtubule translocation). Thus it is suggested that the presynaptic microtubules not only translocate synaptic vesicles, but because of their ordered arrays determine, in ontogeny, the ordered structure of the presynaptic grid.

Firm structural associations between migratory pigment granules and microtubules in crayfish retinula cells

The morphology of associations between mobile pigment granules and microtubules of the crayfish retinula cells was examined with transmission electron microscopy. Many pigment granules were found associated with microtubules through linkages of fuzzy appearance in thin sections. The linkages were revealed as discrete strands of variable shape in rotary-shadowed replicas of freeze-fractured and deep-etched specimens. The only feature of constant morphology among these connections consisted of 2-4-nm filaments projecting laterally from the microtubules. The firmness of the pigment granule-microtubule associations was judged by their ability to hold up during cell disruption procedures of increasing disaggregation effects in a low-Ca++ stabilization buffer. The results of these tests were inspected with scanning electron microscopy and with transmission electron microscopy of negatively stained preparations. Numerous pigment granules remained associated with a stable microtubule framework after the plasma membrane had been stripped away. Moreover, granule-microtubule attachments survived breakdown of this framework into free fascicles of microtubules. The pigment granules were associated with the free microtubules either individually or as clusters entangled in a fibrous material interwoven with 10-nm filaments. These findings attest that many pigment granules are bound to microtubules through linkages that constitute effective attachments. Further, it is demonstrated that a highly cohesive substance associates the pigment granules with one another. These conclusions are discussed in terms of a pigment transport mechanism in which a network of interconnected granules would establish firm transient interactions with a supporting skeleton of microtubules.

Association between coated vesicles and microtubules

In this study, a possible functional association between microtubules and coated vesicles is described. We have found that our preparations of microtubules contained coated vesicles in quantities of usually above 10%. These coated vesicles were identified both by immunological methods using anticoat antibodies and by electron microscopy of negatively stained specimens. In the immune replica, two components of coated vesicles, i.e., heavy (clathrin) and light chains, were recognized as constituents of the preparations. In the electron microscope, it was found that coated vesicles were attached predominantly along the length of microtubules. Furthermore, projections from the microtubules to the triskelion centers of the clathrin lattice were identified and thus seem to serve as linkers between the cytoskeletal structure of the organelle. A similar type of association was detected in tissue culture cells; bridges between coated vesicles and microtubules were clearly identified by electron microscopy of thin sections.

The enigma of microtubule coils in brain synaptosomes

When synaptosomes are prepared from rat brain and incubated in Krebs solution, the presynaptic bulb develops a coil of microtubules (mts). Various considerations indicate that the coil does not have a cytoskeletal supportive function. Synaptosome coil mts show certain peculiarities, e.g. they thrive during incubation in Krebs solution (dendritic mts are depolymerized in Krebs solution) and they show no protofilament molecular substructure with tannic acid. Dendritic mts show clearly a 13 protofilament substructure when processed in the same way. Synaptosomal coil mts are sensitive to micromolar calcium and are depolymerized by treatment of the synaptosomes with veratridine or A23187. Our evidence indicates that coil mts of synaptosome and synaptic vesicle clothed mts of 'intact' albumin-treated synapses are different morphological and functional entities. As mentioned above, the function of coil mts remains enigmatic, while the mts seen in albumin-treated synapses could well have a role in synaptic vesicle translocation.

Synaptic organisation and neuron microtubule distribution

The microtubules in different parts of the neuron and synaptosomes were examined with respect to their stability, structure and orientation. On the basis of distribution, different labilities and differences in protofilament substructure seen by tannic acid staining, we have classified microtubules into eight major categories. Functional involvements in vesicle translocation, cytoskeletal support and the regulation of assembly/disassembly are considered.

Cytoskeleton-secretory vesicle interactions during the docking of secretory vesicles at the cell membrane in Paramecium tetraurelia cells

Stationary-phase cells of Paramecium tetraurelia have most of their many secretory vesicles ("trichocysts") attached to the cell surface. Log-phase cells contain numerous unoccupied potential docking sites for trichocysts and many free trichocysts in the cytoplasm. To study the possible involvement of cytoskeletal elements, notably of microtubules, in the process of positioning of trichocysts at the cell surface, we took advantage of these stages. Cells were stained with tannic acid and subsequently analyzed by electron microscopy. Semithin sections allowed the determination of structural connections over a range of up to 10 micrometer. Microtubules emanating from ciliary basal bodies are seen in contact with free trichocysts, which appear to be transported, with their tip first, to the cell surface. (This can account for the saltatory movement reported by others). It is noteworthy that the "rails" represented by the microtubules do not directly determine the final attachment site of a trichocyst. Unoccupied attachment sites are characterized by a "plug" of electron-dense material just below the plasma membrane; the "plug" seems to act as a recognition or anchoring site; this material is squeezed out all around the trichocyst attachment zone, once a trichocyst is inserted (Westphal and Plattner, in press. [53]). Slightly below this "plug" we observed fasciae of microfilaments (identified by immunocytochemistry using peroxidase labeled F(ab) fragments against P. tetraurelia actin). Their arrangement is not altered when a trichocyst is docked. These fasciae seem to form a loophole for the insertion of a trichocyst. Trichocyst remain attached to the microtubules originating from the ciliary basal bodies--at least for some time--even after they are firmly installed in the preformed attachment sites. Evidently, the regular arrangement of exocytotic organelles is controlled on three levels: one operating over a long distance from the exocytosis site proper (microtubules), one over a short distance (microfilament bundles), and one directly on the exocytosis site ("plug").

Taxol induces postmitotic myoblasts to assemble interdigitating microtubule-myosin arrays that exclude actin filaments

Taxol has the following effects on myogenic cultures: (a) it blocks cell replication of presumptive myoblasts and fibroblasts. (b) It induces the aggregation of microtubules into sheets or massive cables in presumptive myoblasts and fibroblasts, but not in postmitotic, mononucleated myoblasts. (c) It induces normally elongated postmitotic myoblasts to form stubby, star-shaped cells. (d) It reversibly blocks the fusion of the star-shaped myoblasts into multinucleated myotubes. (e) It augments the number of microtubules in postmitotic myoblasts, and these are assembled into interdigitating arrays of microtubules and myosin filaments. (f) Actin filaments are largely excluded from these interdigitating microtubule-myosin complexes. (g) The myosin filaments in the interdigitating microtubule-myosin arrays are aligned laterally, forming A-bands approximately 1.5 micrometers long.

Synaptic regeneration and glial reactions in the transected spinal cord of the lamprey

We have examined axonal growth and synaptic regeneration in identified giant neurons of the transected lamprey spinal cord using intracellular injection of horseradish peroxidase. Wholemounts together with serial section light and electron microscopy, show that axons from identified Müller and Mauthner reticulospinal neurons grow across the lesion and regenerate new synaptic contacts. Relatively normal swimming returns in these animals by 3-4 weeks after spinal transection. This occurs despite the formation of regenerated synapses in regions of the cord that are not usually occupied by these neurons. The regenerating axons branch profusely in contrast to their unbranched state in the normal animal. In addition to showing the two synaptic configuration found normally, synapses may be formed by slender sprouts from the growing giant axon. These 'sprout' type synaptic contacts appear unique to the regenerating neuron. Only regenerated chemical synapses were seen; the morphologically mixed chemical and electrical (gap junction) synaptic complex common in the normal animal was not observed at regenerated synapses. The site of spinal transection in the functionally recovered animal shows an increase in the number of ependymal and glial cells. Ependymal-like cells appear in regions away from the central canal. The expanded ependymal and glial processes covering the peripheral surface of the injured cord become convoluted, in contrast to their normal smooth configuration. There is no collagen within the cord at the site of transection but a considerable deposition is seen external to the cord surface. Axonal growth across a spinal lesion and subsequent synaptic regeneration can be examined in single identifiable giant interneurons in the spinal cord of the larval lamprey. This preparation may be used as an assay to investigate factors that could contribute to functional recovery following central nervous system injury in the higher vertebrates.

Microtrabecular structure of the axoplasmic matrix: visualization of cross-linking structures and their distribution

Axoplasmic transport is a dramatic example of cytoplasmic motility. Constituents of axoplasm migrate as far as 400 mm/d or at approximately 5 micron/s. Thin-section studies have identified the major morphological elements within the axoplasm as being microtubules, neurofilaments (100-A filaments), an interconnected and elongated varicose component of smooth endoplasmic reticulum (SER), more dilated and vesicular organelles resembling portions of SER, multivesicular bodies, mitochondria, and, finally, a matrix of ground substance in which the tubules, filaments, and vesicles are suspended. In the ordinary thin-section image, the ground substance is comprised of wispy fragments which, in not being noticeably tied together, do not give the impression of representing more than a condensation of what might be a homogeneous solution of proteins. With the high-voltage microscope on thick (0.5-micron) sections, we have noticed, however, that the so-called wispy fragments are part of a three-dimensional lattice. We contend that this lattice is not an artifact of aldehyde fixation, and our contention is supported by its visability after rapid-freezing and freeze-substitution. This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane. We propose that formation and deformation of this system are involved in rapid axonal transport. To facilitate electron microscope visualization of the trabecular connections between elements of axoplasm, the following three techniques were used: first, the addition of tannic acid to the primary fixative, OsO4 postfixation, then en bloc staining in uranyl acetate for conventional transmission electron microscope (TEM); second, embedding tissue in polyethylene glycol for thin sectioning, dissolving out the embedding medium from the sections and blocks, critical-point-drying (J. J. Wolosewick, 1980, J. Cell Biol., 86:675-681.), and then observing the matrix-free sections with TEM or the blocks with a scanning electron microscope; and third, rapid freezing of fixed tissue followed by freeze-etching and rotary-shadowing with replicas observed by TEM. All of these procedures yielded images of cross-linking elements between neurofilaments and organelles of the axoplasm. These improvements in visualization should enable us to examine the distribution of trabecular links on motile axonal organelles.

Secretory mechanism of fibroin, a silk protein, in the posterior silk gland cells of Bombyx mori

There are two microtubule-microfilament systems in the posterior silk gland cells of Bombyx mori. One is a radial microtubule system; the other is a circular microtubule-microfilament system. These two systems are presumably concerned with the intracellular transport of secretory granules of fibroin and the secretion of fibroin into the lumen, respectively. Conventional and scanning electron microscopic observations of the two microtubule-microfilament systems in the posterior silk gland cells are reported. Scanning electron micrographs showed that a number of parallel linear cytoplasmic processes ran circularly on the luminal surface of the posterior silk gland cells. These processes were assumed to correspond to the circular microtubule-microfilament systems. The effects of cytochalasin (B or D), a secretion stimulating agent of fibroin, on the intracellular recording of membrane potential from the posterior silk gland cells are also reported. Exposure to cytochalasin resulted in depolarization of the membrane potential of the gland cells. Possible functional roles of the two microtubule-microfilament systems in the secretory mechanism of fibroin are discussed with reference to the effects of antimitotic reagents and cytochalasin on these two systems.

Projections of lamprey spinal neurons determined by the retrograde axonal transport of horseradish peroxidase

The spinal cords of larval sea lampreys (Petromyzon marinus) and adult river lampreys (Ichthyomyzon unicuspis) were injected with horseradish peroxidase through a transection 1 cm caudal to the last gill. Some animals also had a spinal hemisection 1 cm caudal to the injection. After recovery periods of 1 to 52 days, the spinal cords were treated with diaminobenzidene and hydrogen peroxide, and the projections of various cell types determined in wholemount slides. From these observations the following conclusions were drawn. Most dorsal cells (primary sensory cells) are bipolar with a long rostral projection and a short caudal projection of no more than 5-10 mm. Both processes travel in the ipsilateral dorsal column. Their peripheral processes enter the dorsal roots as branches of their central axons. Some dorsal cells send processes out three or more dorsal roots both rostral and caudal to the cell body. Myotomal motoneurons have characteristic locations in the medial gray column and send prominent transversely oriented dendrites into the lateral columns. A few motoneurons are unusually large. In addition to giant interneurons the majority of smaller rostrally projecting interneurons also have decussating axons. A recently described cell type, the oblique bipolar cell, appears to have an exclusively crossed rostral projection. Although most edge cells project rostrally, as many as 20% may have a caudal projection or both rostral and caudal projections. Edge cells project equally to the ipsilateral and contralateral spinal hemicord, but their processes do not extend more than about 18 mm in sea lamprey larvae and 37 mm in adult river lampreys. Lateral cells project exclusively to the ipsilateral caudal hemicord. A few cells which resemble lateral cells in location and in possessing large lateral dendrites, project rostrally. However, these have atypical morphologic features which probably distinguish them from true lateral cells. Thus far, regardless of cell type, all decussating axons seem to pass ventral to the central canal, while decussating medial dendrites pass dorsally.

Synaptic regeneration in identified neurons of the lamprey spinal cords

Identified reticulospinal neurons whose giant axons were severed after spinal cord transection were filled with horseradish peroxidase. Whole mounts and serial-section light and electron micrographs show axon regeneration across the spinal lesion and the formation of new synapses. Normal swimming activity returns in the spinally transected animals, although the regenerated synapses are in atypical regions of the spinal cord.

Microtrabecular lattice of the cytoplasmic ground substance. Artifact or reality

The cytoplasmic ground substance of cultured cells prepared for high voltage transmission electron microscopy (glutaraldehyde/osmium fixed, alcohol or acetone dehydrated, critical-point dried) consists of slender (3-6 nm Diam) strands--the microtrabeculae (55)--that form an irregular three-dimensional lattice (the microtrabecular lattice). The microtrabeculae interconnect the membranous and nonmembranous organelles and are confluent with the cortices of the cytoplast. The lattice is found in all portions of the cytoplast of all cultured cells examined. The possibility that the lattice structure is an artifact of specimen preparation has been tested by (a) subjecting whole cultured cells (WI-38, NRK, chick embryo fibroblasts) to various chemical (aldehydes, osmium tetroxide) and nonchemical (freezing) fixation schedules, (b) examination of model systems (erythrocytes, protein solutions), (c) substantiating the relaibility of critical-point drying, and (d) comparing images of whole cells with conventionally prepared (plastic-embedded) cells. The lattice structure is preserved by chemical and nonchemical fixation, though alterations in ultrastructure can occur especially after prolonged exposure to osmium tetroxide. The critical-point method for drying specimens appears to be reliable as is the freeze-drying method. The discrepancies between images of plastic-embedded and sectioned cells, and images of whole, critical-point dried cells appear to be related, in part, to the electron-scattering properties of the embedding resin. The described observations indicate that the microtrabecular lattice seen in electron micrographs closely represents the nonrandom structure of the cytoplasmic ground substance of living cultured cells.

Evoked depolarizing and hyperpolarizing potentials in reticulospinal axons of lamprey

1. Intracellular recordings were made from reticulospinal axons (Müller axons) in the lamprey spinal cord. Electrical stimuli applied to the spinal cord surface elicited depolarizing and hyperpolarizing 'synaptic-like' potentials in Müller axons. The physiological basis of these evoked potentials was investigated. 2. The depolarizing response was not the result of increased extracellular K, as demonstrated by the constancy of the undershoot of the axonal action potential during the depolarization, by the failure of the response to summate during repetitive stimulation and by the failure of the response amplitude to vary as predicted when the [K] of the saline was varied. 3. When the membrane potential of the axon was varied by passing current through a micro-electrode, the amplitude of the depolarizing evoked potential decreased at membrane potentials positive to the resting potential and increased up to a maximum when the axon was hyperpolarized by about 10 mV. The extrapolated 'reversal potential' for the depolarizing response was about 15 mV positive to the normal -80 mV resting potential of the axon. However, the amplitude of the response did not continue to grow with hyperpolarizations greater than 10 mV, and, thus, the response did not behave as would a normal depolarizing synaptic potential. 4. Müller axons make numerous electrical synapses with spinal motoneurones and interneurones, and this suggested that the depolarizing response might be a coupling potential. In agreement with this idea, quantitative correspondence was found between changes in the input resistance of the axon produced by the depolarizing response and the variation in the depolarizing response amplitude. Thus, although the depolarizing response mimicked in some ways the behaviour of an excitatory synaptic potential, we conclude that it is a coupling potential. 5. The hyperpolarizing response also appeared to be a coupling potential. Its amplitude was not changed by hyperpolarizing the axon up to 30 mV and was decreased by depolarizing the axon sufficiently to decrease the axon's input resistance. 6. It is proposed that both depolarizing and hyperpolarizing evoked potentials in lamprey Müller axons are a result of passive flow of current from cells activated by the spinal cord stimulus and electrically coupled to Müller axons.

Physiological and anatomical characteristics of reticulospinalneurones in lamprey

1. Intracellular records were obtained from giant reticulospinal cells (Müller cells) in the brain of adult lamprey. The cells had maximum resting potentials of -80 mV and action potentials with overshoots of 30 mV. Input resistances varied from 2 to 8 MOmega.2. Individual spontaneous excitatory and inhibitory synaptic potentials (e.p.s.p.s and i.p.s.p.s) were observed, as well as occasional high frequency bursts of excitatory potentials. Much of the spontaneous synaptic activity could be eliminated by elevating the Ca(2+) concentration in the bathing solution to 10-15 mM, suggesting that the synaptic potentials were due to spike activity in elements presynaptic to Müller cells.3. Electrical stimulation of cranial nerves produced synaptic responses in Müller cells. Ipsilateral vestibular nerve stimulation produced i.p.s.p.s; contralateral stimulation, e.p.s.p.s. Stimulation of either optic nerve produced mixed synaptic responses with e.p.s.p.s dominating in cells with large resting potentials. Trigeminal nerve stimulation produced mixed responses. Olfactory nerve stimulation produced excitation. Spinal cord stimulation produced e.p.s.p.s and i.p.s.p.s, the dominant effect being inhibition.4. In favourable preparations strong electrical stimulation of cranial nerves produced afterdisharges in Müller cells, lasting from a few seconds after stimulation of the olfactory and vestibular nerves to as long as several minutes after optic, trigeminal or spinal cord stimulation.5. Natural stimulation of tactile, visual and vestibular receptors resulted in synaptic responses similar to those produced by electrical stimulation of the cranial nerves. Fish odour applied to the olfactory mucosa produced no response.6. Iontophoretic application of L-glutamate to Müller cells produced depolarization accompanied by a decrease in input resistance. In addition, glutamate produced bursts of inhibitory and excitatory synaptic potentials, presumably by depolarizing excitatory or inhibitory nerve terminals or nearby cell bodies.7. Iontophoretic application of gamma-aminobutyric acid (GABA) resulted in a slight hyperpolarization, accompanied by a large reduction in input resistance. The reversal point both of the hyperpolarizations and of the spontaneous inhibitory post-synaptic potentials was about 6 mV greater than the resting potential.8. There were two types of synaptic ending on Müller cell bodies, one type containing round vesicles and the other containing ellipsoidal vesicles. These terminals were intermixed over the surface of the cell bodies and dendrites with no readily apparent segregation.9. Intracellular records from the spinal axons of Müller cells during electrical stimulation of cranial nerves and spinal cord showed, in addition to the normal propagating action potential activity which normally originates in the cell bodies, depolarizing, hyperpolarizing and biphasic evoked potentials. These membrane responses were grossly similar in appearance to synaptic potentials except that the large depolarizing potentials had unusually long decay times. The physiological basis of these potentials remains unclear.10. Electron microscopic examination showed very few synaptic endings afferent to Müller axons, a finding in contrast to the abundance of synaptic-like potentials recorded. However, the occasional synapses afferent to Müller axons were invariably located near an efferent synaptic region of the axon itself. This raises the possibility that a very limited number of synaptic regions of Müller axons may be subject to presynaptic modulation of transmitter release.11. The observations reported here support the idea that Müller cells in lamprey are an important motor outflow from the brain and serve to coordinate the lamprey's trunk responses to external sensory stimulation.

Microtubules in the neurosecretory neurones of the posterior pituitary of the rat

The microtubules in the neurosecretory neurones of the posterior pituitary were studied using different electron microscopical techniques. Tannic acid staining indicated that the microtubules had a 13 protofilament substructure similar to that described for microtubules from other tissues and organisms; the dimensions of the microtubules were also similar to that previously reported. Albumen pretreatment clearly showed the microtubules running across axonal swellings, but not continuing across the nerve endings. The only organelles showing possible association with the microtubules were small vesicles and smooth endoplasmic reticulum, no association between hormone granules and microtubules could be seen.