Compartmentalization of clathrin in synaptic terminals

Immunofixation of sodium lauryl sulphate (SDS)-acrylamide gels has been used to study the distribution of the major protein (clathrin) of coated vesicles in various compartments of synaptic terminals. Synaptosomal subcellular fractions were isolated and purified from pig brain homogenates by the procedure of Ueda et al. and lysed in 6 mM Tris-Cl buffer at pH 6.6, 7.8, and 8.1. The synaptosomal particulate and soluble fractions were separated by centrifugation. The synaptic junctional complex (SJC) and postsynaptic density (PSD) fractions were obtained by detergent treatment of the synaptic plasma membrane (SPM). The synaptosomal subcellular fractions and purified coated vesicle (PCV) fractions were subjected to SDS gel electrophoresis (7.5%). The resulting slabs were divided vertically into 4 segments which were stained with Coomassie blue dye, or immunofixed with preimmune and anti-clathrin serum, or affinity labeled with concanavalin A (Con A)-peroxidase. The Comassie blue stained gel indicated the presence of 180,000-molecular weight band in gels of most synaptosomal subcellular fractions. However, immunofixation of an identical gel revealed positive staining of the 180,000-molecular weight protein in PCV, synaptosomal (SF), SPM and synaptoplasmic (SS) fractions only. These findings not only support the contention that a pool of cytosolic-coated vesicle protein is localized at synaptic terminals, they also indicate that clathrin appears highly unlikely to contribute to the structural frameworks of the SJC and PSD of mature synapses.

Isolation of a presynaptic plasma membrane fraction from Torpedo cholinergic synaptosomes: evidence for a specific protein

Synaptosomal plasma membranes were isolated from Torpedo cholinergic synaptosomes which had been purified as previously described or repurified by equilibrium centrifugation. The synaptosomal plasma membrane could be distinguished from postsynaptic membranes by the absence of postsynaptic specific markers (nicotinic AChR) and by its low intramembrane particle complement after freeze fracture. In addition, the presynaptic membrane fraction contained acetylcholinesterase. Gel electrophoresis permitted the identification of a major protein component of the presynaptic membrane fraction which had a molecular weight of 67,000. This protein was not found in postsynaptic membrane or synaptic vesicle fractions. Thus it appeared to be specific to the nerve terminal plasma membrane.

Two-dimensional gel electrophoresis of human brain proteins. II. Specific proteins and brain subfractions

Brain proteins extractable with distilled water or 9 mol/L urea were subjected to two-dimensional gel electrophoresis. They are examined in relation to the identification of actin, tubulin, neurofilament proteins, glial fibrillary acidic protein; proteins of human embryonic neocortex, synaptosomes, myelin, and plasma; and rat neocortex, rat glial, and mouse neuroblastoma cell lines.

Action of oxotremorine on the sub-cellular distribution of glycine in the rat spinal cord

Intraperitoneal (i.p.) injection of 250 micrograms/kg of oxotremorine (OT) caused a 50% decrease in the glycine content of the synaptosomal-mitochondrial fraction of spinal cord homogenates prepared from rats killed 15 min after treatment. The glycine content of the supernatant fraction was correspondingly raised. In synaptosomes isolated from the spinal cord of OT-treated rats, the decrease in glycine content was 30%. Prior administration of atropine, but not of methylatropine, abolished this effect of OT on synaptosomal glycine content. Eserine exerted a potentiating effect on the action of OT in lowering the glycine content of spinal synaptosomes. Prior administration of L-DOPA, apomorphine, haloperidol, muscarine or mecamylamine had no significant effect on the action of OT on synaptosomal glycine content. OT alone or in combination with eserine, and acetylcholine (ACh) in combination with eserine, did not alter the rate of release of glycine from spinal synaptosomes of untreated rats incubated under appropriate conditions. OT was also without effect on the rate of release of glycine from normal spinal synaptosomes subjected to electrical stimulation, as well as on the eventual glycine content of the synaptosomes. On the basis of the present findings it has been suggested that (i) glycine may be released from Renshaw cells at their synapses with motoneurones in response to the muscarinic action of OT; (ii) dopaminergic modulation may not be involved in the OT-induced glycine release from Renshaw cells; and (iii) excessive release of glycine onto motoneurones may be the causative factor of the akinesia observed in OT-induced experimental Parkinsonism.

Characterization of immunoreactive somatostatin released from rat nervous system in vitro

There is considerable evidence that somatostatin is released from nerve terminals throughout the central nervous system in response to presynaptic stimulation, thus suggesting a neuromodulator role for the peptide. We here report the partial characterization of immunoreactive somatostatin released from rat nervous system in vitro (hypothalamus, spinal cord and hypothalamic, cortical, thalamic and striatal synaptosomes). Serial dilutions of released somatostatin immunoreactivity showed parallelism with dilutions of synthetic somatostatin standard. Somatostatin immunoreactivity released from all tissue areas coeluted with synthetic tetradecapeptide on Sephadex G-25 (fine grade) gel chromatography; more than 85% of this immunoreactivity bound to Sepharose-anti-somatostatin-serum immunoaffinity columns. In addition, immunoreactive material released from hypothalamus, spinal cord and hypothalamic and cortical synaptosomes inhibited somatotropin (growth hormone, 'STH', 'GH') release from perifused anterior pituitary in a dose-related manner, indicating biological similarity to synthetic somatostatin.

Morphology of developing cerebral cortical synaptosomal fractions isolated from eu- and hypothyroid rats

Synaptosomal fractions isolated from the cerebrum of eu- and of hypothyroid rats during the neonatal period were analyzed by electron microscopy. This analysis revealed 3 main components: presynaptic endings, growth cones and unidentifiable structures that may be dendritic in origin. Since the size and the percentage of presynaptic endings were not altered by hypothyroidism but the size of the fraction in comparison to cerebrum weight was altered on postnatal day 14, it could be concluded that hypothyroidism results in a transient reduced synaptic density.

Fatty acid composition of synaptosomes from normal and cobalamin deficient bat brain

1. Synaptosomes were prepared from the brain of cobalamin deficient and cobalamin supplemented fruit bats (Rousettus aegyptiacus) by a floatation method on a discontinuous sucrose gradient. 2. There were no significant differences in fatty acid composition between the deficient and supplemented bats. 3. The odd chain fatty acid 15:0 amounted to 0.2% of the total fatty acids in both groups. 4. We conclude that the fatty acids of synaptosomal lipids are not a factor in the development of neural dysfunction in the cobalamin deficient fruit bat.

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.

The organization of gangliosides and other lipid components in synaptosomal plasma membranes and modifying effects of calcium ion

Synaptosomes were prepared from bovine brain by zonal rotor sucrose density centrifugation. While a major fraction of lipid-bound sialic acid is included uniformly within the synaptosomal distribution profile, the sialoglycoproteins and some gangliosides do not follow this pattern. Exposure to extrasynaptosomal calcium results in alterations in the surface labeling properties of some gangliosides and membrane plasmalogens, suggesting that extrasynaptic Ca2+ may influence the conformation of complex lipids in synaptic plasma membranes. The level of intrinsic membrane-associated sialidase activity that liberates sialic acid from these sialoglycoconjugates parallels the synaptosomal buoyant density distribution profile, supporting a view that this enzyme resides in synaptosomal membranes in close association with a sialolipid substrate.

Subcellular distribution of coronary dilatatory protein-hormonal complexes in rat hypothalamus

We previously reported the presence of a group of coronary dilatatory protein-carriers of the cardioactive neurohormones and precursors of bioactive compounds in the hypothalamus of some animals. Investigation of the subcellular distribution of those proteins revealed their localization in neurosecretory granules and in synaptosomes of the hypothalamus. In further investigation of the regional distribution of coronary-active proteins in different parts of the brain they were found in synaptosomes of the cerebral cortex too. The main location of coronary-active proteins in synaptosomes indicates their participation in synaptic functions.

[Quantitative determination of neurospecific protein S-100 in mouse brain cortical synaptosomes]

Using monospecific antibodies against protein S-100 labelled with horseradish peroxidase (a-S-100-HP) or iodine (a-S-100-125I) two types of protein S-100 bound to the synaptosomal membrane were obtained. The first type accessible for a-S-100-HP (6.4% of total protein S-100 in the synaptosomes) and the second type accessible for a-S-100-125I (22.4%) differ in their location on the membrane surface. The total content of protein S-100 in the synaptosomes determined by solid phase radioimmune analysis is 284 +/- 0.84 ng per mg of total synaptosomal protein. Coprecipitation of the protein with the synaptosomal fraction is insignificant. After treatment of the synaptosomal membranes with 1% glutaric aldehyde or 4% paraformaldehyde, 20 and 50% of membrane-bound protein S-100 are found in the synaptosomal membrane, respectively. The immunohistochemical localization of protein S-100 in synaptosomes was established.

Disposition of gangliosides and sialosylglycoproteins in neuronal membranes

Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP-[3H]N-acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H]sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 M-KCl, 125 mM-EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of th neuraminidase-labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase-labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labelled in vivo by an intracerebral injection of N-[3H]acetylmannosamine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with behaviour of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

Perforant path lesion depletes glutamate content of fascia dentata synaptosomes

Destruction of the hippocampal perforant path fibers reduces the glutamate content of a crude synaptosomal fraction of the rat fascia dentata by about 40%, but does not reduce the aspartate content. This result supports the hypothesis that the perforant path fibers use glutamate as their transmitter. Since a perforant path lesion reduces the glutamate content of dentate homogenates to a much lesser degree, the reduction in synaptosomal glutamate may be offset by an increase in extraterminal glutamate stores. Thus, when evaluating the possible transmitter role of glutamate or aspartate, one can probably gain more definitive information by measuring the glutamate and aspartate content of a synaptosomal preparation than of the target region as a whole.

Internal K+ and Na+ concentrations and the volume of hypothalamic synaptosomes

Hypothalamic synaptosomes, prepared by discontinuous sucrose density gradient centrifugation, were suspended in media of varying osmolarity and ionic composition and the internal concentrations of K+ and Na+ were measured. The intrasynaptosomal volume was determined using [14C]inulin, 35SO4 or [14C]sucrose as extracellular markers. When the synaptosomes were suspended in 0.32 M sucrose the distribution volume of [14C]inulin and 35SO4 were similar. However, when the medium contained 140 mM NaCl and 5 mM KCl the 35SO4 space was equal to the total water space while the distribution volume of [14C]inulin corresponded to 35-51% of the total water space. [14C]sucrose distributed in a larger volume than did [14C]inulin, presumably due to intracellular permeation of sucrose. Using inulin as an extracellular marker the synaptosome volume was found to be inversely proportionate to the tonicity of the medium. [14C]inulin was considered a suitable marker of the extrasynaptosomal space and was used when determining intrasynaptosomal K+ and Na+ concentrations. The internal concentration of K+ was considerably higher than the external indicating that the synaptosomes were able to retain K+ against a concentration gradient. The synaptosomes gained Na+ when transferred to media containing 140 mM NaCl. The internal concentrations of K+ and Na+ were unaffected by glucose and elevated temperature and was only moderately changed during 90 min of incubation. The equilibrium potentials for K+ and Na+ were -68 and 13 mV respectively when the medium consisted of 0.03 M sucrose containing 140 mM NaCl, 5 mM KCl. It is concluded that hypothalamic synaptosomes can maintain a high transmembrane concentration gradient for K+ whereas the membrane is rather easily penetrated by Na+.

Somatostatin receptors: identification and characterization in rat brain membranes

We have identified and characterized specific receptors for tetradecapeptide somatostatin (SRIF; somatotropin release-inhibiting factor) in rat brain using [125I]Tyr11]SRIF as the radioligand. These receptors are present in membranes obtained from a subfraction of synaptosomes. Membranes derived from cerebral cortex bind SRIF with high affinity (Ka = 1.25 X 10(10) M-1) and have a maximum binding capacity (Bmax) of 0.155 X 10(-12) mol/mg. Neither opiates nor other neuropeptides appear to influence the binding of SRIF to brain membranes. Synthetic analogs with greater biological potency than SRIF--[D-Trp8]SRIF, [D-Cys14]SRIF, and [D-Trp8, D-Cys14]SRIF--bind to the receptors with greater avidity than SRIF, whereas inactive analogs [(2H)Ala3]SRIF and [Ala6]SRIF exhibit low binding. The ratio of receptor density to endogenous somatostatin is high in the cortex, thalamus, and striatum, low in the hypothalamus, and extremely low in the brain stem and cerebellum. Thus, SRIF receptors in the brain appear to be a distinct, new class of receptors with a regional distribution different from that of endogenous somatostatin.

Topographic studies of gangliosides of intact synaptosomes from rat brain cortex

Gangliosides in the external surface of intact synaptosomes from rat brain cortex have been studied by oxidation of exposed galactose and galactosamine groups with galactose oxidase followed by reduction with labeled sodium borohydride. Purified synaptosomes were labeled, disrupted by osmotic shock, and the particulate components fractionated on diatrizoate to give four synaptosomal membrane fractions (A-D) and a mitochondrial pellet (E). Fractions A and B represent synaptosomal plasma membranes. When intact synaptosomes were labeled, the major portion of the total radioactivity incorporated into ganglioside fraction was found to be in GM1 3 species. With isolated membrane fractions little selectivity was seen: (1) more label was present compared to intact synaptosomes, and (2) zones corresponding to GM2, GM1, GD1a, GD1b were the major gangliosides labeled. The results confirm the conclusion that membrane fractions A and B are derived from the exposed synaptosome surface and also show that GM1 is the major ganglioside species available for enzyme oxidation at the surface.

In vitro release of cholecystokinin octapeptide-like immunoreactivity from rat brain synaptosomes

Enriched synaptosome fractions prepared by differential centrifugation and ultracentrifugation of homogenates of rat cortex, striatum, thalamus and hypothalamus contained over 65% of the total immunoreactive cholecystokinin octapeptide (CCK-8) in each area. A calcium-dependent release of immunoreactive CCK-8 from these fractions in vitro in response to 2 depolarizing stimuli (60 mM KCl and 75 microM veratrine) has been demonstrated. Released CCK-8 immunoreactivity showed parallelism when dilutions were compared with the CCK-8 dose-response curve and eluted similarly to synthetic CCK-8 on Sephadex G-50 superfine chromatography. These results provide further evidence for a neurotransmitter or neuromodulator role for CCK-8 in brain.

Brain postsynaptic densities: the relationship to glial and neuronal filaments

Preparations of isolated brain postsynaptic densities (PSDs) contain a characteristic set of proteins among which the most prominent has a molecular weight of approximately 50,000. Following the suggestion that this major PSD protein might be related to a similarly sized component of neurofilaments (F. Blomberg et al., 1977, J. Cell Biol., 74:214-225), we searched for evidence of neurofilament proteins among the PSD polypeptides. This was done with a novel technique for detecting protein antigens in SDS-polyacrylamide gels (immunoblotting) and an antiserum that was selective for neurofilaments in immunohistochemical tests. As a control, an antiserum against glial filament protein (GFAP) was used because antisera against GFAP stain only glial cells in immunohistochemical tests. They would, therefore, not be expected to react with PSDs that occur only in neurons. The results of these experiments suggested that PSDs contain both neuronal and also glial filament proteins at higher concentrations than either synaptic plasma membranes, myelin, or myelinated axons. However, immunoperoxidase staining of histological sections with the same two antisera gave contradictory results, indicating that PSDs in intact brain tissue contain neither neuronal or glial filament proteins. This suggested that the intermediate filament proteins present in isolated PSD preparations were contaminants. To test this possibility, the proteins of isolated brain intermediate filaments were labeled with 125I and added to brain tissue at the start of a subcellular fractionation schedule. The results of this experiment confirmed that both neuronal and glial filament proteins stick selectively to PSDs during the isolation procedure. The stickiness of PSDs for brain cytoplasmic proteins indicates that biochemical analysis of subcellular fractions is insufficient to establish a given protein as a synaptic junctional component. An immunohistochemical localization of PSDs in intact tissue, which has now been achieved for tubulin, phosphoprotein I, and calmodulin, appears to be an essential accessory item of evidence. Our findings also corroborate recent evidence which suggests that isolated preparations of brain intermediate filaments contain both neuronal and glial filaments.

Molecular characterization, reconstitution, and "transport-specific fractionation" of the saxitoxin binding protein/Na+ gate of mammalian brain

The saxitoxin (STX) binding protein has been solubilized by sodium cholate, both from axolemma and from synaptosomal membranes of mammalian brain. On the basis of agarose gel filtration and sedimentation properties in H2O and 2H2O, the solubilized particle has the following molecular properties: Stokes radius, 120 A; partial specific volume, 0.85 cm3/g; mass, 1,020,000 daltons; frictional ratio f/fo, 1.6. The solubilized STX binding protein was incorporated into unilamellar (approximately 550-A) artificial phosphatidylcholine vesicles. Based on the expectation that the STX binding protein contains functional monovalent cation gating activity ("action potential Na+ gate") that can be activated by veratridine and inhibited by tetrodotoxin, a strategy was devised for partial purification of the reconstituted sodium gate/STX binding protein by "transport-specific fractionation." When the entire vesicle population was preloaded with 0.4 M cesium ion, addition of veratridine allowed Cs+ efflux from specifically those vesicles containing the ion gate; the concomitant reduction in intravesicular density permitted the ion gate/STX binding protein to be fractionated on density gradients. These observations demonstrate functional reconstitution and partial (30- to 50-fold) purification of the STX binding protein/Na+ gate of mammalian brain.

[Biochemical characteristics of membrane fractions from different types of cerebral cortex synaptosomes]

The activity of acetylcholinesterase (ACE) and Na, K-ATPase is distributed among three subfractions of synaptic membranes isolated from light (C) and heavy (D) synaptosomes of the optic area of the rabbit cerebral cortex. The levels of specific activity of both enzymes in C subfractions are similar to those in D subfractions. At the same time the specific activity of ACE and Na, K-ATPase in membrane fractions from both light and heavy synaptosomes is different. Such a biochemistry of subsynaptic components from certain brain structures favors studying a fine morphochemical organization of an isolated nerve terminal and its relationship with the activity of CNS in different functional states.