Muscle-like contractile proteins and tubulin in synaptosomes

The morphology of microtubules in incubated synaptosomes. Effect of low temperature and vinblastine

A time-dependent rise in the percentage of microtubule-containing synaptosomes was observed reaching a 41--47% peak after 30 min incubation at room temperature (22--25 degrees C) in a saline medium. Fixation at low temperature or incubation with vinblastine decreased markedly the number of synpatosomes containing microtubules. It was concluded that incubation with cations and processing at room temperature were prerequisites of optimal preservation of synaptosomal microtubules. Their spatial orientation relative to the synaptic cleft was deduced from statistical examination of profiles in different orientations.

The effect of morphine tolerance on the incorporation of 3H-leucine into proteins of rat synaptic membranes

The rate of incorporation of 3H-leucine into proteins of the synaptic junctional and nonjunctional membranes and synaptic vesicles of rat brain has been examined in control and morphine-tolerant rats. There are no discernible differences between control and tolerant animals in amount of protein as measured by densitometric tracings of dye-stained proteins separated by acrylamide gel electrophoresis from the three membrane fractions of whole brain areas. However, there are differences in the turnover of membrane protein: three vesicle protein bands and one junctional-membrane protein band are significantly more highly labeled, and one junctional-membrane protein is significantly less highly labeled by 3H-leucine in samples from tolerant rats. Of these, the two junctional-membrane proteins can be tentatively identified as components of the post synaptic densities, and one of the vesicle proteins as tubulin.

Fibroblasts and macrophages of mice with the Chediak-Higashi-like syndrome have microtubules and actin cables

Cells of the beige mouse contain abnormally large lysosomes and show enhanced capping of concanavalin A. It has been suggested that these phenomena may be secondary to a defect in microtubule polymerization. We have examined the cytoskeleton of beige mouse cells by indirect immunofluorescence and find the number and distribution of microtubules and actin cables to be indistinguishable from those of normal control cells.

Chemical composition of cholinergic synaptic vesicles from Torpedo marmorata based on improved purification

Cholinergic synaptic vesicles from the electric organ of Torpedo marmorata have been purified to a constant composition and a higher transmitter content than previously reported. By optimising the extraction conditions and using a two-step purification on discontinuous and continuous sucrose density gradients, 10-fold higher acetylcholine and ATP values per weight of protein were obtained. The purity of the vesicle preparation was confirmed by electronmicroscopy, absence of marker enzymes, behaviour in density gradient centrifugation, as well as by a specific and reproducible protein composition. Vesicles contain 6.9 mumol acetylcholine and 1.0 mumol ATP per mg protein. The lipid/protein ratio of 3.5 (w/w) indicates a lipid-rich membrane. The value suggests the absence of a proteinaceous core. Upon dodecylsulphate gel electrophoresis a distinct protein pattern is obtained with components ranging from 20000 to 160000 in molecular weight. Vesiculin, reported earlier to be a low-molecular-weight vesicle protein, is not detected. One of the major bands comigrates with muscle actin from the same animal. Further characterisation of this protein by two-dimensional gel electrophoresis suggested that it is an actin-like polypeptide. Evidence for a specific association of this actin-like protein with vesicles and its possible involvement in the neurosecretory process is discussed.

beta-Bungarotoxin. The binding of [3H]pyridoxylated beta-bungarotoxin to a high-molecular-weight protein receptor

beta-Bungarotoxin was labelled with pyridoxal 5'-phosphate (incorporating 3H). The kinetics of beta-bungarotoxin binding to several tissue subfragments of nervous tissue was studied. The dissociation constant of 3H-pyridoxylated beta-bungarotoxin in this reaction was 0.21-0.37 micron and that of unlabelled beta-bungarotoxin was 25 nM. Hill [(1910) J. Physiol. (London) 40, iv-vii] and Scatchard [(1949) Ann. N.Y. Acad. Sci. 51, 660-672] analyses demonstrated no co-operativity of binding and only a single class of receptor sites, consistent with a bimolecular association of beta-bungarotoxin and its receptor. The iodinated toxin was physiologically inactive. Toxin was bound in non-specific unsaturable fashion by glass and/or plastic. This low-affinity binding was corrected by addition of bovine serum albumin to a final concentration of 30 mg/ml. A soluble protein receptor of beta-bungarotoxin was isolated and the mol.wt. is approx. 200000.

Nerve growth factor potentiates actomyosin adenosinetriphosphatase

The nerve growth factor protein (NGF) favors polymerization of brain actin and induces its organization to form paracrystalline structures that activate myosin ATPase (ATP phosphohydrolase, EC 3.6.1.3) to an extent greater than actin alone. Binding studies show that the initial 1:1 stoichiometry of NGF-G-actin complexes decreases to 1:7-10 when polymerization is ended and paracrystalline structures are formed. The ratio becomes even lower when heavy meromyosin is added in the absence of ATP, suggesting that heavy meromyosin displaces NGF bound to actin microfilaments. This conclusion is supported by the finding that when heavy meromyosin is added to NGF-microfilament complexes, under conditions for "decorating" microfilaments, the usual paracrystalline structure of the complexes disappears. The NGF-mediated organization of actin and activation of myosin ATPase is visualized as a self-regulatory and self-propagating mechanism, because progressive displacement of the growth factor induced by heavy meromyosin binding to F actin as ATP consumption proceeds renders an increasingly higher amount of NGF free for new interactions. These findings are discussed in the light of the mechanism of action of NGF in the target cells.

An association between mitochondria and microtubules in synaptosomes and axon terminals of cerebral cortex

A substantial number of synaptosomes prepared from rat cerebral cortex were found by electron microscopy to contain a horseshoe-shaped mitochondrion flanked by an arc of three to ten microtubules opposite the synaptic membrane specializations. The microtubules were replaced by characteristic paracrystals following the incubation of synaptosomes with vinblastine sulfate. A similar spatial organization of microtubules and mitochondria was also observed in some axon terminals of the cerebral cortex in situ. The significance of this novel observation is discussed with regard to the role of microtubules in axonal transport at the synaptic terminal and previous reports on the identification of tubulin in synaptosomes and subsynaptic structures.

The structure of postsynaptic densities isolated from dog cerebral cortex. I. Overall morphology and protein composition

A postsynaptic density (PSD) fraction, including some adherent subsynaptic web material, has been isolated from dog cerebral cortex by a short-procedure modification of methods of Davis and Bloom (21, 22) and Cotman and Taylor (20), using Triton X-100. The fraction has been visualized by thin-section, replica, and negative (phosphotungstic acid) staining electron microscopy and its proteins separated by high-resoltuion SDS gel electrophoresis. Morphologically, the preparation seems to be quite pure, with very little membrane contamination. The density is composed of protein, no nuclei acids, and very little phospholipids being detectable. The fraction had no ATPase or GTPase activity, but it did have a very small amount of cytochrome c oxidase activity (of a specific activity less than 0.5 percent that of a mitochondrial fraction) and a small amount of 5'- nucleotidase activity (of a specific activity between 6 and 7 percent that of a synaptic membrane fraction). Electron micrographs reveal cup-shaped structures approximately 400nm long and approximately 40nm wide, made up of apparent particles 13-28nm in diameter. However, en face views, and particularly micrographs of replicas and PTA-stained preparations, reveal a disk-shaped structure, outside diameter approximately 400 nm, in which filaments are seen to extend from the central part of the density. High resolution gel electrophoresis studies indicated some 15 major proteins and perhaps 10 or more minor ones; the predominant protein had a mol wt of 51,000, followed by ones at 45,000, 40,000, 31,000, 26,000, and several at 100,000. A comparison by gel electrophoresis of density fraction proteins with those of a lysed synaptosomal membrane fraction containing some adherent densities indicated some comigrating proteins, but the major membrane fraction protein, mol wt 52,000, was not found in the density fraction. Antibodies raised against the density fraction reacted with a preparation of solubilized synaptic membrane proteins. By both these criteria, it was considered that the density and the synaptic membrane have some proteins in common. By separately mixing (125)I-labeled myelin, synaptic vesicle, and mitochondrial fraction proteins with synaptosomes, and then isolating the density fraction from the mixture, it was concluded that a major 26,000 mol wt density fraction protein was common to both mitochondria and density, that none of the proteins of the density were contaminants from the mitochondrial fraction, that a minor approximately 150,000 band was a contaminant from the synaptic vesicle fraction, and that the moderately staining PSD fraction protein of 17,000 mol wt band was the result of contamination by the major basic protein of myelin. On the basis of the marker enzymatic assays and the mixing experiments, it is considered that the density fraction is moderately pure biochemically, and that its protein composition, aside from a few exceptions noted above, reflects its in situ character.

The structure of postsynaptic densities isolated from dog cerebral cortex. II. Characterization and arrangement of some of the major proteins within the structure

An attempt was made to identify some of the proteins of the postsynaptic density (PSD) fraction isolated from dog cerebral cortex. The major protein has been tentatively labeled "neurofilament" protein, on the basis of its 51,000 mol wt correspondence to a protein found in neurofilament preparations. Other proteins are akin to some dog myofibrillar proteins, on the basis if immunological crossreaction and equal sodium dodecyl sulfate (SDS)-gel electrophoretic mobilities. While a protein similar to dog muscle myosin is not present in the PSD fraction, a major protein present is actin, as evident from reactivity with antiactin serum, from SDS-gel mobility, and from amino acid composition. Only very little tubulin may be present in the PSD fraction, as determined by gel electrophoresis. Various treatments of the PSD fraction were attempted in order to extract some proteins, as revealed by gel electrophoresis, and to observe the structural changes of the PSD fraction residue after extraction of these proteins. The PSD is remarkably resistant to various extraction conditions, with only 4 M guanidine being found to extract most of the proteins, except the 51,000 mol wt protein. Disulfide reducing agents such as dithiothreitol (DTT), blocking agents such as p-chloromercuribenzoate (PCMB) (both in the presence of deoxycholate [DOC]), a Ca++ extractor, ethylene glycol-bis (beta- aminoethyl ether) N,N,N',N'-tetraacetate (EGTA), and guanidine caused an opening up of the native dense PSD structure, revealing approximately 10-nm filaments, presumably consisting of "neurofilament" protein. Both DTT-DOC and PCMB-DOC removed chiefly actin but also some other proteins. EGTA, in greatly opening up the structure, as observed in the electron microscope, revealed both 10-nm and 3- to 5-nm filaments; the later could be composed of actin, since actin was still in the residue after the treatment. EGTA removed a major 18,000 mol wt component and two minor proteins of 68,000 and 73,000 mol wt. Based on the morphological and biochemical evidence, a picture is presented of the PSD as a structure partly made up of 10-nm and 3- to 5-nm filaments, held together through Ca++ interaction and by bonds amendable to breakage by sulfhydrylblocking and disulfide-reducing reagents; either removal of Ca++ and/or rupture of these disulfide bonds opens up the structure. On the basis of the existence of filamentous proteins and the appearance of the PSD after certain treatments as a closed or open structure, a theory is presented with envisages the PSD to function as a modulator in the conduction of the nerve impulse, by movements of its protein relative.

The buoyant density of synaptic plasma membranes from the cerebral cortex of neonatal rats

A fraction enriched in synaptic plasma membranes was prepared from neonatal (5-6 day old) rat cerebral cortex. The procedure was based on a method used to prepare synaptic plasma membranes from adult cerebral cortex. Critical steps were monitored by electron microscopy. Synaptic plasma membranes from neonatal cerebral cortex sedimented as a broad peak between 0.9 M and 1.1 M sucrose. In contrast, the majority of adult synaptic plasma membranes have been reported to sediment to 1.2 M sucorose. The activities of various enzyme markers were determined in subfractions of neonatal preparations in order to estimate contamination. The specific activities of these markers indicated substantial contamination of the neonatal synaptic plasma membrane fractions by microsomes and glia.

Isolation, characterization and localization of bovine adrenal medullary myosin

Myosin was isolated in high purity from the bovine adrenal medulla by gel filtration and ion exchange chromatography. The purified myosin was analyzed by electrophoresis in gels containing SDS and found to contain a 200,000 molecular weight heavy chain and major light chains of molecular weights 20,000 and 17,000 in a 1:1:1 molar ratio. At high ionic strength the myosin had high Ca-ATPase and K-EDTA-ATPase activities and low Mg-ATPase activity. At low ionic strength, the Mg-ATPase was activated to a low level by rabbit muscle actin. The myosin was found to decorate F-actin in the absence, but not the presence of ATP. In low ionic strength solutions, the myosin assembled into characteristic bipolar filaments. The distribution of this myosin in the adrenal medulla and of cross-reacting myosin in several other bovine tissues was determined with the use of anti-medullary myosin immunoglobulin G as a specific stain that was detected by direct and indirect immunofluorescence. In the medulla strong staining was seen between the chords of chromaffin cells indicating that presence of a highly muscular vasculature that may perform functions analogous to those of the myoepithelium of exocrine glands. The chromaffin cells showed weak positive staining around the nuclei and in a pattern radiating toward adjacent blood vessels. Cells of the inner zone of the adrenal cortex showed strong staining in the peripheral cytoplasm while cells in the intermediate and outer zones did not stain. In a blood smear, platelets and the cytoplasm of leukocytes stained strongly while erythrocytes did not stain. In striated muscle and the gray and white matter of the cerebrum only the capillaries and larger vessels stained. In the liver the phagocytic cells bordering vascular sinuses staine strongly while the hepatocytes were separated from one another by a 2 micron trilaminar band possibly representing the microfilament web surrounding the bile canaliculi and associated with junctional complexes. The results suggest that myosin is present in several highly differentiated, non-motile tissue cells where it may play a role in secretion or other specialized functions.

Identification of a protein related to tubulin in the postsynaptic density

The postsynaptic density is a unique subcellular organelle associated with the synaptic complex and appears as an electron-dense area immediately subjacent to the postsynaptic plasma membrane. The postsynaptic density was isolated from the synaptosomal fraction and the protein constituents were analyzed by polyacrylamide gel electrophoresis. Polypeptides closely related to tubulin were identified as a major component of the postsynaptic density on the basis of molecular weight, subunit structure, and peptide map criteria.

Microtubular proteins in pigeon erythrocyte membranes

[3H]Colchicine binds in a concentration and temperature dependent, saturatable and noncooperative manner to cytoplasmic proteins from pigeon erythrocytes: Kd = 3.5 x 10(-7) M at 37 degrees C. Binding of [3H] colchicine at 0 degrees C and of [3H]lumicolchicine at 37 degrees C was significantly reduced. Hence microtubular proteins are present in the cytoplasm of pigeon erythrocytes. Antibody against bovine brain tubulin was raised in rabbits and confirmed by immunodiffusion, passive immunohaemolysis and in radioimmunoassay. Pigeon erythrocyte membrane proteins solubilized with 2% sodium cholate competed with 125I-labelled tubulin in the radioimmununoassay although much higher concentrations of membrane proteins than of purified bovine brain tubulin were required for effective competition. No binding to antibody occurred with boiled solubilized membrane preparations. Similar results were obtained with antitubulin-dependent passive immunohaemolysis of tubulin-coated sheep erythrocytes in the presence of complement. The presence of tubulin in membranes was verified by binding intact pigeon erythrocytes to colchicine-Sepharose beads at 37 degrees C. Free colchicine (5mM) or incubation at 0 degrees C prevented binding. Lumicolchicine-Sepharose beads did not attach to erythrocytes at 37 degrees S. Thus pigeon erythrocyte membranes contain microtubular protein.

Isolation of synaptic junctional complexes of high structural integrity from rat brain

A new method has been developed for isolating synaptic junctional complexes (SJC) of high structural integrity. The major step in the isolation involves homogenization of a synaptosomal membrane (SM) fraction in a biphasic system consisting of Freon 113 and an aqueous phase containing 0.2% Triton X-100. Well-preserved SJCs, along with membrane vesicles, were recovered in the aqueous phase after low-speed centrifugation of the homogenate. The membranes were subsequently separated from the SJCs by centrifugation on a discontinuous sucrose density gradient. The purity and identity of subcellular fractions were monitored by thin sectioning electron microscopy, using specific and nonspecific staining methods. From the electron microscope studies we conclude that SJCs and their components occupy about 65% of the area covered by structures in this fraction. The assay of enzyme activities indicates that homogenization in Triton-Freon and subsequent steps of the isolation procedure affect the activities of Na, K-ATPase, cytochrome oxidase, and acid phosphatase to different extents, but do not cause total inactivation. Electrophoresis of the SJC-enriched fraction on sodium dodecyl sulfate-polyacrylamide gels has demonstrated that a polypeptide which co-migrates with tubulin is the major component in this fraction, and that a polypeptide co-migrating with actin is also present.

Topography of the synaptosomal membrane

The composition and disposition of the constituent polypeptides of rat cerebral cortical synaptosomal membranes were analyzed on SDS acrylamide gels. Of 20 bands readily detected, 11 account for greater than 93% of the total protein analyzed. These are: (molecu25); 3 (175); 4 (doublet, 137); 5 (doublet, 97); 6 (68); 7 (61); 8 (54); 9 (44); 10 (37); and 11 (33). Bands 5 and 8-10 are the most prominent and account for greater than 60% of the protein mass or 0.67 of its molecular fraction. By lactoperoxidase iodination, the bulk of the proteins in bands 3, 5, 6, and 8 and a portion of band 11 appear to be located on the external (junctional) face of the membrane of intact synaptosomes; proteins in bands 1, 2, 7, 9, and 10 appear to be localized on the internal (synaptoplasmic) face and become labeled only when synaptosomes are lysed. Further confirmation of the topographical distribution is provided by evidence that bands 3-6, 8, and 11 contain glycoproteins susceptible to labeling in intact synaptosomes by oxidation with galactose oxidase or periodate followed by reduction with NaB3H4. Evidence is provided for significant contributions by tubulin- and actin-like molecules to bands 8 and 9, respectively, suggesting that a substantial fraction of the tubulin in the synaptosomal membrane is disposed externally (accessible to iodination) whereas most, if not all, of the actin appears to exhibit the opposite topography. Similar though weaker inferences can also be drawn with regard to the location of tropomyosin and troponin. Preliminary evidence is provided that postsynaptic densities exhibit a protein and iodination profile distinct from that of the synpatosomal membrane.

Coated-vesicle shells, particle/chain material, and tubulin in brain synaptosomes. An electron microscope and biochemical study

Coated vesicles (CVs), plain synaptic vesicles (PSVs), and nonvesicular flocculent material were isolated from synaptosomes and examined with goniometry and high-resolution electron microscopy after either negative staining or various biochemical procedures. The flocculent material (i.e. the presynaptic matrix material except CV shells) is largely composed of particulate or elongated (chainlike) structures; some of this material (here referred to as particle/chain material) is attached to PSVs. The results obtained were: (a) the proteinaceous properties of the CV coat (also referred to as CV shell) and the particle/chain material were demonstrated with chymotrypsin; (b) the CV shell, studied with various negative-staining techniques, differs from the particle/chain material since it has no 3-4-nm globular subunits and reacts differently to alkaline pH; (c) the particle/chain material consists of aggregates of 3-4-nm globular subunits, four of which yield 8-10-nm fine particles; and these particles can be further aggregated into chains 8-10 nm wide and up to 30-60 nm long showing a "hollow" core; (d) vinblastine sulfate induced ringlike or helical crystalloid precipitates closely resembling the vinblastine-induced microtubule crystals reported in the literature, but vinblastine had no effect on either the CV shell material or the particle/chain material.

Involvement of vesicle coat material in casein secretion and surface regeneration

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied with emphasis on vesicle coat structures. Typical 40-60 nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles", were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heabily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.

Enhanced labeling of a retinal protein during regeneration of optic nerve in goldfish

Goldfish retinas were examined for changes in the labeling pattern of protein during regeneration of retinal ganglion cell axons following unilateral optic nerve crush. At various times after optic nerve crush the normal retinas were incubated in vitro with [3H] methionine and retinas from the opposite side of the treated fish were inculbated with [35S] methionine. The incubated retinas were combined and a soluble protein fraction was isolated and separated by sodium dodecyl sulfate-urea polyacrylamide gel electrophoresis. A band on the gel showing an elevated 35S to 3H ratio appeared at 5 days following optic nerve crush, increased to a maximum at 15 days, and was barely detectable at 45 days, during which time vision is known to return. On the basis of several criteria, the protein fraction showing the increased incorporation of methionine in retina after optic nerve crush appears to be of the tubulin class.

An ultrastructural examination of the action of vinblastine on microtubules, neurofilaments and muscle filaments in vitro

In vitro incubation of autonomic nerves with vinblastine sulphate (1 times 10(-4) M) caused a disappearance of microtubules within 15 min; during the following 15 min paracrystalline arrays appeared within the axons. An increase in the abundance of microfilaments was also observed, but these did not appear to arise from disaggregated microtubules since the increase in microfilament numbers was noted at an incubation time when crystal formation was extensive. Pretreatment of autonomic nerves with colchicine (2.5 times 10(-4) M) caused a reduction of approximately 80% in the numbers of microtubules, but did not prevent the formation of crystals on subsequent exposure to vinblastine. No ultrastru"tural changes were observed in myofilaments on incubation with vinblastine.

Brain colchicine-binding protein (tubulin) during postnatal development in the rat

A time-decay colchicine-binding assay was used under optimal conditions for measuring tubulin concentration in the cerebral cortex and cerebellum during postnatal development. In both brain areas soluble tubulin concentration (pmoles colchicine bound/mg tissue) declined by about 35% between 4 and 12 days of age after which the decrease was more gradual up to adulthood. Colchicine-binding protein concentration of particulate fractions, on the other hand, increased between 4 and 12 days of age followed by relatively little change. The concentration of tubulin in the soluble and particulate fractions of the cerebral cortex was 45% and 75% greater, respectively, than corresponding fractions of the cerebellum. The expression of tubulin on the basis of DNA, revealed marked regional differences which can be considered a reflection of differences in the relative extent of morphological differentiation in these areas during the postnatal period. Tubulin in both soluble and particulate fractions exhibited a marked increase in lability for colchicine binding from approximately 12 days of age. The half-life for loss of colchicine-binding activity was 2,5-fold greater in particulate compared to soluble fractions. An analysis of the characteristics of the particulate colchicine-binding reaction provides additional support for the hypothesis that it represents a functionally significant association of tubulin with the membranous components.