The synaptic vesicle and the cytoskeleton

Multiomics of synaptic junctions reveals altered lipid metabolism and signaling following environmental enrichment

Membrane lipids and their metabolism have key functions in neurotransmission. Here we provide a quantitative lipid inventory of mouse and rat synaptic junctions. To this end, we developed a multiomics extraction and analysis workflow to probe the interplay of proteins and lipids in synaptic signal transduction from the same sample. Based on this workflow, we generate hypotheses about novel mechanisms underlying complex changes in synaptic connectivity elicited by environmental stimuli. As a proof of principle, this approach reveals that in mice exposed to an enriched environment, reduced endocannabinoid synthesis and signaling is linked to increased surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) in a subset of Cannabinoid-receptor 1 positive synapses. This mechanism regulates synaptic strength in an input-specific manner. Thus, we establish a compartment-specific multiomics workflow that is suitable to extract information from complex lipid and protein networks involved in synaptic function and plasticity.

Terminal Alkynes as Raman Probes of α-Synuclein in Solution and in Cells

Conformational changes in α-synuclein (α-syn) are central to its biological function and Parkinson's disease pathology. Here, terminal alkynes (homopropargylglycine) were employed as environmentally sensitive Raman probes at residues 1, 5, 116, and 127 to characterize soluble (disordered), micelle-bound (α-helical), and fibrillar (β-sheet) α-syn. Along with the full-length protein, a disease-related C-terminal truncation (1-115) was also studied. For the first time, β-sheet α-syn amyloid structure was detected by the amide-I band in N27 dopaminergic rat cells, where a reciprocal relationship between levels of fibrils and lipids was seen. Site-specific spectral features of the terminal alkynes also revealed the heterogeneity of the cellular environment. This work shows the versatility of Raman microspectroscopy and the power of unnatural amino acids in providing structural and residue-level insights in solution and in cells.

Advantages of Acute Brain Slices Prepared at Physiological Temperature in the Characterization of Synaptic Functions

Acute brain slice preparation is a powerful experimental model for investigating the characteristics of synaptic function in the brain. Although brain tissue is usually cut at ice-cold temperature (CT) to facilitate slicing and avoid neuronal damage, exposure to CT causes molecular and architectural changes of synapses. To address these issues, we investigated ultrastructural and electrophysiological features of synapses in mouse acute cerebellar slices prepared at ice-cold and physiological temperature (PT). In the slices prepared at CT, we found significant spine loss and reconstruction, synaptic vesicle rearrangement and decrease in synaptic proteins, all of which were not detected in slices prepared at PT. Consistent with these structural findings, slices prepared at PT showed higher release probability. Furthermore, preparation at PT allows electrophysiological recording immediately after slicing resulting in higher detectability of long-term depression (LTD) after motor learning compared with that at CT. These results indicate substantial advantages of the slice preparation at PT for investigating synaptic functions in different physiological conditions.

Interplay between α-synuclein amyloid formation and membrane structure

Amyloid formation is a pathological hallmark of many neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's. While it is unknown how these disorders are initiated, in vitro and cellular experiments confirm the importance of membranes. Ubiquitous in vivo, membranes induce conformational changes in amyloidogenic proteins and in some cases, facilitate aggregation. Reciprocally, perturbations in the bilayer structure can be induced by amyloid formation. Here, we review studies in the last 10 years describing α-synuclein (α-syn) and its interactions with membranes, detailing the roles of anionic and zwitterionic lipids in aggregation, and their contribution to Parkinson's disease. We summarize the impact of α-syn - comparing monomeric, oligomeric, and fibrillar forms - on membrane structure, and the effect of membrane remodeling on amyloid formation. Finally, perspective on future studies investigating the interplay between α-syn aggregation and membranes is discussed. This article is part of a Special Issue entitled: Amyloids.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

Presynaptic GABAB-and gamma-hydroxybutyric acid-mediated mechanisms in generalized absence seizures

gamma-Hydroxybutyric acid (GHB) is a naturally occurring compound which has the ability to induce generalized absence seizures when given to animals. This effect of GHB may be blocked by either GHB or GABAB receptor antagonists. We sought to test the hypothesis that pre-synaptic GHB- and GABAB-mediated mechanisms in thalamus and cortex are operative in the GHB model of generalized absence seizures. Presynaptic Ca(2+)-dependent K+ efflux was determined using Ca(2+)-stimulated Rb86 efflux in synaptosomes prepared from thalamus and cortex in the presence of GHB, a specific GHB receptor antagonist, the specific GABAB agonist (-)baclofen, or the specific GABAB antagonists, phaclofen and CGP 35348. The effect of these compounds was determined also on basal and K(+)-stimulated 45Ca2+ uptake and basal and K(+)-stimulated synaptosomal cytosolic Ca2+([Ca2]i) in synaptosomes prepared from thalamus and cortex and on [125I] omega-conotoxin binding in thalamus and cortex using autoradiographic binding techniques. There was no demonstrable change in Ca(2+)-stimulated Rb86 efflux in any experimental condition studied; however GHB and (-)baclofen both suppressed K(+)-stimulated 45Ca2+ uptake and [Ca2]i in synaptosomes and were associated with a decrease in [125I] omega-conotoxin binding which achieved statistical significance only in frontal cortex, a brain region selectively involved in the genesis of GHB-induced absence seizures. The effects of GHB and (-)baclofen on K(+)-stimulated 45Ca2+ uptake and [Ca2]i in synaptosomes were additive. The effects of GHB in this regard were attenuated by the GHB antagonist and phaclofen while that of (-)baclofen was attenuated by CGP 35348. These data do not support the hypothesis that the GHB and GABAB receptor are one and the same. Rather, they raise the possibility that a presynaptic GHB/GABAB receptor complex might be involved in the pathogenesis of GHB-induced generalized absence seizures.

Evidence for the association of protein 4.1 immunoreactive forms with neurofibrillary tangles in Alzheimer's disease brains

The formation of neurofibrillary tangles (NFTs) and paired-helical filaments (PHFs) in Alzheimer's disease (AD) reflects a major disorganization of the cytoskeleton. The role of the neuronal membrane skeleton in the development of these abnormalities has not previously been investigated. In this study, we used 9 antibodies raised against the erythrocyte membrane skeleton protein 4.1 (P4.1) for immunocytochemical and immunoblot analyses to investigate whether or not the brain homologues of this protein were constituents of NFTs or PHFs. Our results show that 7 of the 9 monospecific antibodies against the human and pig erythrocyte P4.1 stained NFTs in the prefrontal cortex and hippocampus of AD brains. The P4.1 antibodies used here did not cross-react with tau protein isolated from AD brain, and preabsorption of these antibodies with tau protein did not cause loss of NFT staining. In age-matched control brains, these P4.1 antibodies stained neuronal cell bodies or nuclei. Six of the antibodies also stained isolated NFTs but the SDS-insoluble NFTs were immunostained only by two of the P4.1 antibodies. By using inositol hexaphosphate affinity chromatography and immunoblot analysis, we identified a 68-kDa protein as the most likely brain analogue of P4.1. When SDS-extracted proteins from the isolated NFTs were immunoblotted, a 50-kDa band was immunostained. The 68-kDa and 50-kDa proteins were not stained by tau protein and neurofilament subunit NF-H antibodies, that strongly stained NFTs. We conclude that brain protein 4.1 isoform(s) are constituents of NFTs in AD.

Frequency dependence of vasoactive intestinal polypeptide release and vagally induced tachycardia in the canine heart

We evaluated the frequency dependence of vasoactive intestinal polypeptide (VIP) release from the parasympathetic nerves to the canine heart. In intact animals in the presence of beta-adrenergic receptor blockade (propranolol, 0.5 mg/kg), the cervical vagosympathetic trunks were stimulated at various frequencies before and after the administration of atropine (0.1 mg/kg). The stimulations before atropine produced a classical bradycardia that progressed to cardiac arrest when the stimulation frequency was raised above 10 to 15 Hz. After atropine, vagal stimulation at various frequencies increased heart rate. The heart rate reached a maximum increase of 21 +/- 3 beats per min at a stimulation frequency of 20 Hz. In an isolated atrial preparation in which the VIP outflow was measured, the tachycardia elicited after atropine had a frequency dependence similar to that obtained in vivo. The peak increase of 23 +/- 3% above the basal rate (95 +/- 8 beats per min) occurred at a stimulation frequency of 20 Hz. The VIP outflow paralleled the tachycardia response (r = 0.95); the maximum outflow of VIP was 172 +/- 54 pg/(min . 100 g wet wt) and was evoked at a stimulation frequency of 20 Hz. This suggests that the vagally induced tachycardia is mediated, at least partly, by VIP.

The role of synaptic proteins in the pathogenesis of disorders of the central nervous system

Complex sets of nervous system functions are dependent on proper working of the synaptic apparatus, and these functions are regulated by diverse synaptic proteins that are distributed in various subcellular compartments of the synapse. The most extensively studied synaptic proteins are synaptophysin, the synapsins, growth associated protein 43 (GAP-43), SV-2, and p65. Moreover, synaptic terminals contain a great number of other proteins involved in calcium transport, neurotransmission, signaling, growth and plasticity. Probes against various synaptic proteins have recently been used to study synaptic alterations in human disease, as well as in experimental models of neurological disorders. Such probes are useful markers of synaptic function and synaptic population density in the nervous system. For the present, we will review the role of synaptic proteins in the following conditions: Alzheimer's disease (AD) and other disorders including ischemia, disorders where synapse-associated proteins are abnormally accumulated in the nerve terminals, synaptic proteins altered after denervation, and synaptic proteins as markers in neoplastic disorders. The study of the molecular alterations of the synapses and of plasticity might yield important clues as to the mechanisms of neurodegeneration in AD, and of the patterns of presynaptic and dendritic damage under diverse pathological conditions.

Activation of protein kinase C suppresses the gamma-aminobutyric acidB receptor-mediated inhibition of the vesicular release of noradrenaline and acetylcholine

Modulation of the gamma-aminobutyric acidB (GABAB) receptor-mediated response by protein kinase C (PKC) was examined with regard to inhibition by stimulation of the GABAB receptor of stimulation-evoked release of noradrenaline (NA) from slices of cerebellar cortex and of acetylcholine (ACh) from strips of ileum. 12-O-Tetradecanoylphorbol 13-acetate (TPA) potentiated the high K(+)-evoked Ca2+-dependent release of NA and ACh, but not the ouabain-evoked release, even in the presence of external Ca2+. The potentiating effect was antagonized by sphingosine, thereby suggesting that PKC participates in the exocytotic-vesicular release of neurotransmitters, but does not do so in case of a nonvesicular release. GABA inhibited the high K(+)-evoked release of NA and ACh, but not the ouabain-evoked Ca(2+)-independent release. The effect of GABA was mimicked by baclofen and was antagonized by phaclofen, thereby suggesting that stimulation of the GABAB receptor inhibits the vesicular but not the nonvesicular release of neurotransmitters. TPA suppressed the GABAB receptor-mediated inhibition of high K(+)-evoked release of NA and ACh. The effect of TPA was antagonized by sphingosine. These results indicate that stimulation of the GABAB receptor inhibits the stimulation-evoked Ca(2+)-dependent release of neurotransmitters and that activation of PKC suppresses the GABAB receptor-mediated response.

The spatial pattern of the synaptic vesicular apparatus as a correlate of transmitter storage models

A statistical stereological approach which allows one to derive a three-dimensional pattern of synaptic vesicle accumulation in relation to the active zone from an analysis of electron micrograms (random sections) of synapses is described. This approach is illustrated with a study of presynaptic terminals from the dorsal horn of the cat spinal cord, based on the morphometrical treatment of 105 micrographs containing 5190 synaptic vesicles. The spatial pattern obtained was found to have a bimodal shape, which can be considered a possible structural correlate of the two-pool model of transmitter storage. The connection of similar quantitative estimates with physiological data is discussed.

A synaptic vesicle specific GTP-binding protein from ray electric organ

A cDNA encoding a synaptic vesicle associated GTP-binding protein was identified by screening a lambda gt11 expression library derived from the electric lobe of Discopyge ommata with polyclonal antibodies recognizing vesicle-specific proteins of Mr 25,000. Nucleotide sequence analysis defines an open reading frame of 218 amino acids. The protein belongs to the ras superfamily and shares about 75% amino acid identity with smg-25A, B and C identified in bovine brain and rab3A characterized in rat brain. Northern blot analysis revealed a 4.5 kb transcript present only in neural tissues, the highest level of expression being observed in electric lobe. Western blot analysis of total tissue homogenates derived from D. ommata detected the protein in electric organ, forebrain and to a lesser extent in electric lobe and spinal cord. No immunoreactivity was detected in non-neuronal tissues. Blotting of subcellular fractions derived from electric ray electric organ revealed that the GTP-binding protein co-purifies with synaptic vesicles. The neural specific expression and the localization to synaptic vesicles suggest a role of this protein in synaptic vesicle trafficking and targeting.

Monoclonal antibodies identify two novel proteins associated with vasopressin secretory granules of the rat neurohypophysis

Immunocytochemical and immunoblotting technique have been used to characterize the antigens recognized by two monoclonal antibodies (MAbs C6 and D5) produced against dissociated cells from punches of neonatal supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei of the rat. Peroxidase immunocytochemistry revealed that both MAbs label magnocellular perikarya in the adult and neonatal SON and PVN as well as smaller neurons in the suprachiasmatic nucleus. Axons of the hypothalamo-neurohypophysial tract are also immunolabeled within the hypothalamus and zona interna of the median eminence, and C6 and D5 each bind specifically to both the adult and neonatal neurohypophysis. Dual-label immunofluorescence experiments employing C6 or D5 simultaneously with rabbit antisera specific for either oxytocin, neurophysin or vasopressin neurophysin revealed that C6 binds only to vasopressinergic magnocellular perikarya in the SON, while D5 labels both vasopressinergic and a small subset of oxytocinergic magnocellular neurons. Post-embedding immunogold analysis of MAb binding to the neurohypophysis at the ultrastructural level showed that both C6 and D5 recognize antigens associated with large dense core neurosecretory granules in a subset of neurosecretory axons. Initial biochemical characterization of the antigens recognized by C6 and D5 was performed using SDS-PAGE and Western immunoblotting. MAbs C6 and D5 label single protein bands with apparent molecular weights of 38 and 68 kDa, resp., in blots of reduced extracts from the adult neurointermediate lobe. No cross-reactivity between C6 and D5 and the neurophysins was apparent, nor did anti-neurophysin sera recognize the bands identified by C6 and D5. We have therefore designated these novel antigens as VPGP38 and VPGP68 for VasoPressin Granule Proteins.

Microtubule-dissociating drugs and A23187 reveal differences in the inhibition of synaptosomal transmitter release by botulinum neurotoxins types A and B

The inhibitory effects of botulinum neurotoxins types A and B on Ca2(+)-dependent evoked release of [3H]noradrenaline from rat cerebrocortical synaptosomes were compared and their molecular basis investigated. A23187, a Ca2+ ionophore, proved more efficacious in reversing the blockade produced by type A than that by B, whereas the actions of neither were changed by increasing intraterminal cyclic GMP levels using 8-bromo-cyclic GMP of nitroprusside. Disruption of the actin-based cytoskeleton with cytochalasin D did not alter the inhibition seen subsequently with either toxin. However, prior disassembly of microtubules with colchicine, nocodazole, or griseofulvin reduced the potency of type B toxin, but not that of type A toxin; stabilization of the microtubules with taxol counteracted this effect of colchicine. Because colchicine treatment of synaptosomes did not interfere with the measurable binding of type B toxin or its apparent uptake, it appears to act intracellularly. Collectively, these data suggest that botulinum neurotoxins types A and B inactivate transmitter release by interaction at different sites in the process. Based on the consistent results observed with four different drugs known to affect selectively microtubules, their involvement in the action of the type B neurotoxin is proposed.

H(+)-ATPase, a primary pump for accumulation of neurotransmitters, is a major constituent of brain synaptic vesicles

Upon treatment with sodium carbonate, rat brain synaptic vesicles lost ATP-dependent H+ transport and released major polypeptide components (about 72, 57, 41, 34 and 33 kDa). These polypeptides, consisting about 15% of the total protein, were identified as subunits of H(+)-ATPase by immunoblotting with antibodies against H(+)-ATPase from chromaffin granules. The same treatment also abolished the ATP-dependent, bafilomycin-sensitive uptakes of glutamate, serotonin and gamma-aminobutyrate by the synaptic vesicles. These results indicated that H(+)-ATPase is a major constituent of the vesicles (consisting about 20% of their total protein) and is a primary pump for accumulation of neurotransmitters.

Synaptophysin binds to physophilin, a putative synaptic plasma membrane protein

We have developed procedures for detecting synaptic vesicle-binding proteins by using glutaraldehyde-fixed or native vesicle fractions as absorbent matrices. Both adsorbents identify a prominent synaptic vesicle-binding protein of 36 kD in rat brain synaptosomes and mouse brain primary cultures. The binding of this protein to synaptic vesicles is competed by synaptophysin, a major integral membrane protein of synaptic vesicles, with half-maximal inhibition seen between 10(-8) and 10(-7) M synaptophysin. Because of its affinity for synaptophysin, we named the 36-kD synaptic vesicle-binding protein physophilin (psi nu sigma alpha, greek = bubble, vesicle; psi iota lambda os, greek = friend). Physophilin exhibits an isoelectric point of approximately 7.8, a Stokes radius of 6.6 nm, and an apparent sedimentation coefficient of 5.6 S, pointing to an oligomeric structure of this protein. It is present in synaptic plasma membranes prepared from synaptosomes but not in synaptic vesicles. In solubilization experiments, physophilin behaves as an integral membrane protein. Thus, a putative synaptic plasma membrane protein exhibits a specific interaction with one of the major membrane proteins of synaptic vesicles. This interaction may play a role in docking and/or fusion of synaptic vesicles to the presynaptic plasma membrane.

Partial purification and characterization of the vacuolar H(+)-ATPase of mammalian synaptic vesicles

Several major proteins of synaptic vesicles from rat or cow brain sediment as a large complex on sucrose density gradients when solubilized in nonionic detergents. A vacuolar H(+)-ATPase identified by sensitivity to bafilomycin A1 appears to be associated with this oligomeric protein complex. Two subunits of this complex, synaptic vesicle proteins S and U, correspond to the 57-kDa (B) and 39-kDa accessory (Ac39) subunits, respectively, of bovine chromaffin granule vacuolar H(+)-ATPase as shown by Western immunoblot analysis. The five subunits of the oligomeric complex constitute approximately 20% of the total protein of rat brain synaptic vesicles. Taken together, these results strongly suggest that the abundant, multisubunit complex partially purified from brain synaptic vesicles by density gradient centrifugation is a vacuolar H(+)-ATPase. Bafilomycin A1 completely blocks proton pumping in rat brain synaptic vesicles as measured by [14C]methylamine uptake and also blocks catecholamine accumulation measured by [3H]dopamine uptake. Moreover, ATPase activity, [14C]methylamine uptake, and [3H]dopamine uptake are inhibited by bafilomycin A1 at similar I50 values of approximately 1.7 nmol/mg of protein. These findings indicate that the vacuolar H(+)-ATPase is essential for proton pumping as well as catecholamine uptake by mammalian synaptic vesicles.

Identification and localization of low-molecular-mass GTP-binding proteins associated with synaptic vesicles and other membranes

GTP-binding proteins were studied in synaptic vesicles prepared from bovine brain by differential centrifugation and separated further from plasma membranes using gel permeation chromatography. Following separation by SDS-PAGE of proteins from the different fractions, and transfer to nitrocellulose sheets, the presence and localization of low-molecular-mass GTP-binding proteins were assessed by [alpha-32 P]GTP binding. The vesicle-membrane fraction (SV) was enriched in synaptophysin (p38, a synaptic vesicle marker) and contained low-molecular-mass GTP-binding proteins; these consisted of a major 27 kDa protein and minor components (Mr 26 and 24 kDa) which were trypsin-sensitive and immunologically distinguishable from ras p21 protein. GTP-binding proteins of low molecular mass, but displaying less sensitivity to trypsin, were also found in the plasma membrane fraction (PM; enriched in Na+/K(+)-ATPase). In addition, the PM fraction contained GTP-binding proteins with higher Mr (Gi alpha and G0 alpha), together with another GTP-binding protein, ras p21. Putative function(s) of these GTP-binding proteins with low mass are discussed.

Diurnal changes in actin mRNA levels and incorporation of 35S-methionine into actin in the rat hypothalamus

1. The in vitro incorporation of 35S-methionine into actin and total soluble proteins, as well as the levels of actin mRNA, were studied in the hypothalamus and frontal cerebral cortex of adult male rats killed at six different time intervals during a 24-hr cycle. 2. The specific activity of total soluble proteins after labeled methionine incubations did not vary as a function of time of day in any of the examined brain regions. 3. The incorporation of 35S-methionine into a 43-kDa protein, corresponding to the electrophoretic mobility of actin, varied diurnally in the hypothalamus, exhibiting a maximum at 1200 hr. Such a diurnal variation was not found in frontal cerebral cortex. 4. Similar results were obtained when labeled methionine incorporation into actin was assessed in hypothalamus and cerebral cortex by an immunoprecipitation procedure. 5. An increase in actin hypothalamic mRNA levels, quantitated by dot-blot analysis, was found at 0800, 4 hr in advance to the maximum in 35S-methionine incorporation to actin. 6. The levels of actin mRNA did not vary significantly as a function of time of day in the frontal cerebral cortex.

Characterization of 3'-phosphoadenosine 5'-phospho[35S]sulfate transport carrier from rat brain microsomes

3'-Phosphoadenosine 5'-phospho[35S]sulfate [( 35S]PAPS) specific binding properties of rat brain tissue were studied. [35S]PAPS specific binding was optimal at pH 5.8 in either Tris-maleate or potassium phosphate buffers. Association was maximal at low temperature, reaching equilibrium in 20 min. Dissociation was rapid, with a dissociation time of 80 s. Scatchard analysis of [35S]PAPS specific binding was consistent with a single site having a KD of 0.46 +/- 0.06 microM and a Bmax of 20.8 +/- 2.0 pmol/mg of protein. Low concentrations of Triton X-100 (0.025%) were effective in increasing the number of binding sites to a Bmax of 44.5 +/- 4.6 pmol/mg of protein without affecting the affinity. [35S]PAPS specific binding was enriched in crude synaptic membranes (P2) and microsomes (P3). Regional distribution of [35S]PAPS specific binding was quite homogeneous in all brain structures studied. The pharmacological profile of [35S]PAPS specific binding in rat brain microsomes was consistent with a membrane protein having a high selectivity for the 3'-O-phosphoryl group substitution on the ribose moiety. Thus, 3'-phosphoadenosine 5'-phosphate was more potent than 2'-phosphoadenosine 5'-phosphate in competing for [35S]PAPS specific binding. Adenosine 5'-phosphosulfate was a good inhibitor of [35S]PAPS specific binding. ATP and ADP were also good displacers. Dipyridamole, a highly selective marker for adenosine uptake sites, was ineffective. 4,4-Diisothiocyanostilbene-2,2-disulfonic acid, the chloride transporter inhibitor, showed an IC50 of 36 +/- 5.1 microM for inhibition of [35S]PAPS specific binding. 2,6-Dichloro-4-nitrophenol had a low selectivity in competing for the [35S]PAPS binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of a gelsolin-like 74,000 mol. wt protein in neural and endocrine tissues

A Ca2(+)-dependent actin binding protein with a molecular weight of 74,000, was purified from bovine adrenal medulla by using deoxyribonuclease I affinity chromatography followed by ion-exchange chromatography and gel filtration. This protein broke actin filaments into fragments and promoted nucleation of actin polymerization in a Ca2(+)-dependent manner as effectively as gelsolin. Proteolytic and immunological comparison with gelsolin which is widely distributed actin-severing protein, indicated that the 74,000 mol. wt protein is a distinct protein, but its domain structure resembles that of gelsolin. Immunoblotting using antibody against this protein showed a tissue-specific distribution. The protein was detected in various endocrine, neuroendocrine and nervous tissues, but not in muscle tissues and plasma which contained relatively large amounts of cytoplasmic and plasma gelsolin. This fact might indicate that this actin-severing protein is involved in the regulation of the secretory process of endocrine and nervous tissues. In the exocytotic process regulated by Ca2+, this protein probably plays a role to free secretory organelles like vesicles from the cytoskeletal network, mainly F-actin, which prevents the movement of secretory vesicles in the resting state.

Time-dependent effect of melatonin on actin mRNA levels and incorporation of 35S-methionine into actin and proteins by the rat hypothalamus

The synthesis of the cytoskeletal protein actin exhibits, in the rat hypothalamus, a diurnal variation with maxima during morning hours. The objective of the present study was to assess whether melatonin injection could affect the in vitro incorporation of 35S-methionine into actin, as well as the levels of actin mRNA, in the hypothalamus of adult male rats treated either acutely or chronically with the hormone at 10:00 or 18:00. Injection of 100 micrograms/kg of melatonin for ten days at either time induced a significant depression in the incorporation of 35S-methionine into a 43 kDa protein with the electrophoretic mobility of actin. The specific activity of total soluble proteins after labeled methionine incubations decreased only after evening melatonin administration (100 micrograms/kg, ten days). Hypothalamic actin mRNA levels, quantitated by dot-blot analysis, decreased only after the injection of 100 micrograms/kg melatonin for ten days at 10:00. Neither a 10-micrograms/kg dose of melatonin, nor a single injection of 100 micrograms/kg melatonin, caused any significant change in the parameters examined. Melatonin (100 micrograms/kg for ten days) did not modify hypothalamic somatostatin or H-Ras mRNA concentration. These results suggest the existence of an inhibitory effect of melatonin on hypothalamic actin synthesis.

Role of adenylate cyclase in modulatory effect of neuropeptide Y on [3H]noradrenaline release in guinea-pig vas deferens

1. The effect of neuropeptide Y (NPY) on [3H]noradrenaline ([3H]NA) release-evoked by 5-Hz electrical stimulation or 5 microns calcium ionophore A23187 was studied in vitro in guinea-pig vas deferens. 2. The evoked tritium overflow (which reflected [3H]NA release) was determined by liquid scintillation spectrometry. 3. NPY, 1 microM, reduced electrically-evoked tritium overflow. NPY reduction was more pronounced upon 20-sec, 3 msec continuous stimulation (73.2 +/- 4.4%) and upon 5-min, 1 msec intermittent stimulation (47.8 +/- 2.4%) as compared to the reduction upon 5-min, 1 msec continuous stimulation (24.3 +/- 3.8%). Forskolin (0.1-1 microM) and theophylline (0.65-1.25 mM) dose-dependently diminished this NPY reducing effect. 4. NPY, 1 microM, reduced A23187-evoked tritium overflow by 52.3 +/- 7.1%. Forskolin (5 microM) and theophylline (1.25 mM) significantly decreased the effect of NPY. 5. It is concluded that in guinea-pig vas deferens NPY reduces [3H]NA release through affecting adenylate cyclase and the processes responsible for calcium mobilization.

An improved method for Lowicryl K4M electron microscopic embedding of brain tissue

The Lowicryl K4M low-temperature embedding technique was modified for brain tissue to facilitate antigen localization in neuronal structures by using postembedding immunoelectron microscopy. Rat brains fixed by perfusion with a mixture of 1% glutaraldehyde and 1% paraformaldehyde, were sectioned into 200-microns-thick coronal sections, and the appropriate area of the sections further microdissected into small samples (200 x 700 x 700 microns). These samples were processed for K4M embedding using prolonged incubation times at each step. This resulted in a homogeneously infiltrated tissue with good preservation of ultrastructure and antigen reactivity. Despite the lack of osmium fixation, the morphology of the synapses was clearly identifiable.

Purification and characterization of a cortical secretory vesicle membrane fraction

A membrane fraction has been prepared by sucrose density gradient fractionation of purified cortical secretory vesicles from the eggs of the sea urchin Strongylocentrotus purpuratus. The purified cortical vesicle membrane fraction has a phospholipid to protein ratio of 1.76 and exhibits a morphology typical of biological membranes as seen by electron microscopy. The protein composition of the purified membranes was analyzed by SDS-polyacrylamide gel electrophoresis and shown to be distinct from that of eggs, cell surface complex, cortical vesicles, fertilization product, and yolk platelets. Alkaline extraction (pH 11.0) of peripheral membrane proteins increased the phospholipid to protein ratio to 2.55 and removed several polypeptides. Immunoblot analysis of the isolated cortical vesicle membrane fraction revealed low levels of contamination with two major cortical vesicle content proteins. Fractions enriched in egg plasma membranes and yolk platelet membranes also have been isolated and compared with the cortical vesicle membranes by SDS-polyacrylamide gel electrophoresis. The protein compositions of the three membrane fractions were found to contain very little overlap, indicating that the cortical vesicle membrane preparation is relatively free of contamination from these likely noncortical vesicle sources of membrane. Both the plasma membrane and cortical vesicle membrane samples were found by immunoblotting to contain actin.

Isolation and characterization of secretory granules storing a vasoactive intestinal polypeptide-like peptide in Torpedo cholinergic electromotor neurones

Previous immunocytochemical work showed that the cholinergic electromotor neurones of Torpedo marmorata contain a vasoactive intestinal polypeptide-like immunoreactivity (VIPLI) that is conveyed to the terminals by axonal transport from the cell bodies where it is presumably synthesized. In extension of this work, we have now succeeded in isolating the VIPLI storage granules from both the terminals and the axons of these neurones and characterizing them morphologically and biochemically. They were readily separated from synaptic vesicles but contained several components in common that had previously been regarded as specific for synaptic vesicles. Among these were a heparan sulphate type of proteoglycan, synaptophysin, and a Mg2+-dependent ATPase. The VIPLI concentration in lobe tissue and the amount of tissue available were both insufficient to permit the isolation of granules from the electromotor cell bodies by the same technique but it was possible to establish the presence of such granules by particle-exclusion chromatography, using the stable markers mentioned above. In contrast to the VIPLI-containing granules, axonal synaptic vesicles differed from their terminal counterparts in having a very low acetylcholine content relative to stable vesicle markers: they presumably fill up on reaching the terminal where they are exposed to higher concentrations of cytoplasmic acetylcholine.

Calcium uptake and protein phosphorylation in myenteric neurons, like the release of vasoactive intestinal polypeptide and acetylcholine, are frequency dependent

The mechanism of the electrical-to-chemical decoding involved in the preferential release of the transmitters acetylcholine and vasoactive intestinal polypeptide (VIP) by electrical field stimulation at low (5 Hz) and high (50 Hz) frequencies was studied in superfused myenteric neurons. The stimulation-induced uptake of 45Ca2+ accompanying high frequency stimulation was markedly reduced by 10 microM nifedipine, a specific blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), as was also the preferential high-frequency release of VIP. By contrast, the 45Ca2+ uptake during low-frequency stimulation was somewhat lower per pulse, and neither this uptake nor the preferential release of acetylcholine occurring at this frequency was significantly reduced by nifedipine. These findings suggest that the release of acetylcholine and VIP involve different VSCCs. The pattern of in vitro protein thiophosphorylation in tissue extracts of differentially stimulated myenteric neurons involved polypeptides of 205, 173, 86, 73, 57, 54, 46, 32, 28, and 24 kDa and was also markedly stimulus and nifedipine dependent. This suggests that different phosphoproteins are involved during the frequency-dependent activation of the different Ca2+ channels and exocytotic mechanisms.

Most synaptic vesicles isolated from rat brain carry three membrane proteins, SV2, synaptophysin, and p65

We have prepared highly purified synaptic vesicles from rat brain by subjecting vesicles purified by our previous method to a further fractionation step, i.e., equilibrium centrifugation on a Ficoll gradient. Monoclonal antibodies to three membrane proteins enriched in synaptic vesicles--SV2, synaptophysin, and p65--each were able to immunoprecipitate specifically approximately 90% of the total membrane protein from Ficoll-purified synaptic vesicle preparations. Anti-SV2 precipitated 96% of protein, anti-synaptophysin 92%, and anti-p65 83%. These results demonstrate two points: (1) Ficoll-purified synaptic vesicles appear to be greater than 90% pure, i.e., less than 10% of membranes in the preparation do not carry synaptic vesicle-associated proteins. These very pure synaptic vesicles may be useful for direct biochemical analyses of mammalian synaptic vesicle composition and function. (2) SV2, synaptophysin, and p65 coexist on most rat brain synaptic vesicles. This result suggests that the functions of these proteins are common to most brain synaptic vesicles. However, if SV2, synaptophysin, or p65 is involved in synaptic vesicle dynamics, e.g., in vesicle trafficking or exocytosis, separate cellular systems are very likely required to modulate the activity of such proteins in a temporally or spatially specific manner.

Intracellular localization of secretable cAMP in relaying Dictyostelium discoideum cells

Dictyostelium discoideum cells synthesize and secrete the chemoattractant cAMP within minutes after chemotactic stimulation. During development, this signal-relay process is instrumental in cell aggregation, pattern formation, and differentiation. Cyclic AMP is known to accumulate inside the cell before secretion. In this study we investigated the subcellular localization of the nascent cAMP. After chemotactic stimulation at 0 degrees C and subsequent accumulation of intracellular cAMP, the newly synthesized chemoattractant could be released by gently opening cells in two different ways. Both methods make the cytosolic compartment accessible, whereas intracellular compartments surrounded by a membrane remain largely intact. The first method involved rapid lysis by forced passage through a 5-micron pore-size Nuclepore filter. The second technique was electropermeabilization under carefully controlled conditions that ensured the formation of small, stable pores in the plasma membrane. These pores allowed the passage of small molecules, such as cAMP, but not of macromolecules. To confirm the selectivity for the plasma membrane of both methods, we showed that a typical vesicular cell compartment, the lysosome, remained intact. Both procedures immediately released all intracellularly accumulated cAMP. We interpret our results as strong evidence for accumulation of nascent cAMP in the cytosolic compartment rather than in a vesicular compartment before it is secreted. This implies that cAMP secretion takes place via a trans-membrane transport mechanism, rather than by exocytosis.

Intraneuronal distribution of a synaptic vesicle membrane protein: antibody binding sites at axonal membrane compartments and trans-Golgi network and accumulation at nodes of Ranvier

The distribution of a cholinergic synaptic vesicle-specific transmembrane glycoprotein (Buckley and Kelly, 1985, J. Cell Biol. 100, 1284-1294) was investigated in the entire electromotor neuron of Torpedo marmorata using a monoclonal antibody and immunocytochemistry at the light- and electron-microscopical level (immunoperoxidase, colloidal gold). In the nerve, terminal binding of immunogold particles is restricted to synaptic vesicles. In the axon a number of additional membrane compartments like multivesicular bodies, vesiculotubular structures, lamellar bodies and electron-dense granules share the surface located synaptic vesicle-specific transmembrane glycoprotein-epitope. Membranous structures likely to represent the axoplasmic reticulum inside axons and nerve terminals are not labelled. Antibody-binding membrane compartments are accumulated at nodes of Ranvier. In the perikaryon the tubules of the trans-Golgi network as well as multivesicular bodies, lamellar bodies, electron-lucent vesicles, granules with electron-dense core and peroxisomes are labelled. Immunotransfer blots of isolated synaptic vesicles and tissue extracts of electric organ display a 100,000 mol. wt band of broad electrophoretic mobility typical of the synaptic vesicle-specific transmembrane glycoprotein. Extracts of electromotor nerve and electric lobe contain in addition a strong band at 85,000 mol. wt and a few lower molecular weight bands. We suggest that the synaptic vesicle originates directly from the trans-Golgi network. The endoplasmic reticulum is not involved in vesicle formation or retrieval. On retrograde transport the vesicle membrane compartment is likely to fuse with other intra-axonal (endosomal?) organelles.