Sedimentation and release properties of glial particles present in P2-fractions isolated from rat cerebral cortex

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

Synaptosomal protein synthesis is selectively modulated by learning

Synaptosomes from rat brain have long been used to investigate the properties of synaptic protein synthesis. Comparable analyses have now been made in adult male rats trained for a two-way active avoidance task to examine the hypothesis of its direct participation in brain plastic events. Using Ficoll-purified synaptosomes from neocortex, hippocampus and cerebellum, our data indicate that the capacity of synaptosomal protein synthesis and the specific activity of newly synthesized proteins were not different in trained rats in comparison with home-caged control rats. On the other hand, the synthesis of two proteins of 66.5 kDa and 87.6 kDa separated by SDS-PAGE and analyzed by quantitative densitometry was selectively enhanced in trained rats. In addition, the synthesis of the 66.5 kDa protein, but not of the 87.6 kDa protein, correlated with avoidances and escapes and inversely correlated with freezings in the neocortex, while in the cerebellum it correlated with avoidances and escapes. The data demonstrate the participation of synaptic protein synthesis in plastic events of behaving rats, and the selective, region-specific modulation of the synthesis of a synaptic 66.5 kDa protein by the newly acquired avoidance response and by the reprogramming of innate neural circuits subserving escape and freezing responses.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Subcellular distribution of receptor sites in human brain: differentiation between heavy and light structures of high and low density

Studies of the subcellular localization of neuroreceptors in the rat brain have shown that most of them are associated with light and low density subcellular fractions. In two human brain areas, quite different subcellular distributions were observed. After fractionation by differential centrifugation of frontal cortex homogenates, benzodiazepine and serotonin 5-HT2 receptors were mainly found in the heavy mitochondrial (M) fraction, whereas mu-opiate and muscarinic cholinergic receptors were mainly concentrated in the microsomal (P) fraction. In human putamen, the presynaptic markers of dopaminergic nerve terminals (neurotensin receptors, dopamine uptake sites and amine vesicular transporter-binding sites), benzodiazepine receptors and serotonin uptake sites were recovered both in the high and low density fractions, whereas the muscarinic, opiate and, to a lesser extent, dopamine D2 receptors were mostly concentrated in the microsomal fraction. In the cerebral cortex, after isopycnic centrifugation in sucrose gradients, neuroreceptors were found in the high density fractions where the peaks of cytochrome oxidase and that of nerve endings, as identified by amine uptake and by means of electron microscopy were also found. A single peak of benzodiazepine receptors was observed in high density (1.15-1.17 g/ml) fractions suggesting that these receptors are much more concentrated in the nerve terminals or dendrites rather than in the dendritic spines or vesicles. The fact that muscarinic and opiate receptors were recovered in the P fraction with plasma membrane constituents and also in M and L fractions, which is confirmed by a bimodal distribution in sucrose gradient, suggests that they are localized in both the nerve terminals or dendrites and in the small vesicles or dendritic spines. In the putamen, much of the specific binding to uptake sites for dopamine and serotonin was recovered in the high density fractions, but the existence of another peak at a lower density indicates the presence of microsomal uptake sites. The results indicate that differential and isopycnic fractionation methods performed on human brain samples, make it possible to separate tissue fractions enriched in nerve endings, dendrites, dendritic spines, plasma membranes or vesicles.

Subcellular fractionation of striatum: sedimentation properties of dopaminergic synaptosomes

Linear sucrose density gradient centrifugation of a crude synaptosomal-mitochondrial preparation of rat striatum was performed at 82,500g for 7.5, 15 and 30 min and 1, 4 and 20 h. After centrifugation various marker enzyme activities were measured throughout the gradients, viz. tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) as markers of dopaminergic synaptosomes, lactate dehydrogenase (LDH) as a general synaptosomal marker and monoamine oxidase (MAO) as a mitochondrial marker. At all centrifugation times the distribution patterns of TH and DD activity coincided almost perfectly. Notable differences were found between the sedimentation properties of these TH/DD-containing particles and LDH-containing particles: TH and DD were symmetrically distributed in the gradient much sooner than LDH, at all centrifugation times the top of the TH and DD curves was lying deeper in the gradient than the highest LDH activity, and TH and DD became enriched in the gradients to a much greater extent than LDH. It is concluded that rat striatal dopaminergic synaptosomes form a relatively homogeneous population of particles sedimenting faster into the gradients than the bulk of striatal synaptosomes does. This distinct sedimentation behaviour of the dopaminergic synaptosomes can be usefully applied for analytical purposes.

Kinetic analysis of the accumulation of gamma-aminobutyric acid by particulate fractions of rat brain: comparison of the effects of nipecotic acid and cis-3-aminocyclohexane-1-carboxylic acid

Kinetic analyses indicate that nipecotic acid and cis-3-aminocyclohexane-1-carboxylic acid (cis-3-ACHC) inhibit GABA accumulation by similar mechanisms of action. Both amino acids are competitive inhibitors of particulate GABA accumulation when GABA and the inhibitor are added simultaneously to tissue fractions. However, preincubating the tissue with either amino acid produces noncompetitive inhibition of GABA accumulation at low concentrations of inhibitor and mixed inhibition at higher concentrations. The possible roles of intrasynaptosomal mechanisms and of astroglia in producing these effects are discussed. The most notable difference between cis-3-ACHC and the other amino acid inhibitors of GABA accumulation, such as nipecotic acid, cis-4-OH-nipecotic acid, guvacine, beta-proline, homo-beta-proline, and 2,4-diaminobutyric acid (DABA), is that cis-3-ACHC is approximately 3.5 times more potent as an inhibitor following preincubation. Thus, while cis-3-ACHC does inhibit GABA transport, its major site of action in the synaptosome may be intracellular.

Lack of effect of 6-hydroxydopamine pretreatment on depolarization-induced release of ATP fron rat brain synaptosomes

Pretreatment of rats with intraventricular 6-hydroxydopamine produced considerable destruction of noradrenergic and dopaminergic nerve terminals as indicated by depletions in synaptosomal catecholamine contents. However, 6-hydroxydopamine pretreatment did not result in diminished release of ATP during depolarization of synaptosomes with either elevated extracellular K+ or veratridine. These findings suggest that most of the ATP released during the depolarization of rat brain synaptosomal preparations is not co-released with noradrenaline or dopamine but must originate from other sources.

Sedimentation and release properties of P2 fractions derived from rat cerebral cortex slices incubated with radiolabeled GABA for a short or long time period

On homogenization of rat cerebral cortex slices previously incubated with [3H] GABA or [14C]GABA for 5 or 30 min, respectively, particles were recovered in P2 fractions which exhibited similar buoyant density, but different sedimentation velocity on linear sucrose density gradient centrifugation. The K+-evoked release of [3H]GABA from particles isolated from slices previously incubated for 5 min with [3H]GABA was increased in the presence of exogenous Ca2+. In contrast, the K+-evoked release from particles isolated from slices previously incubated for 30 min with [3H]GABA, was not influenced by the presence of exogenous Ca2+. These results suggest that, depending on the incubation time of slices, exogenously applied GABA can be detected in different pools. These pools not only seem to differ in their CA2+ dependency of K+-evoked release but also in their subcellular localization.

Studies on valinomycin inhibition of synaptosome-fraction protein synthesis

The ionophore valinomycin inhibited adult and neonatal synaptosome fraction protein synthesis with half-maximal inhibition at approximately 10nM. Valinomycin had no effect on [3H]leucine uptake into synaptosomes at high or low external [K+]. Synaptosome-fraction protein synthesis was dependent on [K+]e reaching a maximum at 25mM-K+. Valinomycin inhibition of protein synthesis was not reversed at high [K+]e. Valinomycin failed to influence the intrasynaptosomal [K+] even at zero [K+]e. A significant increase in State-4 respiration of synaptosomal fractions was found at 5nM-valinomycin with a decrease in the respiratory control index. At these concentrations of valinomycin there was no inhibition of the ADP-stimulated (State 3) respiration rate. Valinomycin had no effect on cerebral microsomal protein synthesis in vitro, which was inhibited by puromycin (100 micrograms/ml) or the absence of ATP. Valinomycin, 2,4-dinitrophenol and KCN inhibition of protein synthesis was not reversed by added ATP, suggesting impermeability of the membrane to ATP. Valinomycin induced a rapid fall in synaptosome ATP content not observed with atractylate or ouabain. Valinomycin inhibition of protein synthesis under these conditions is secondary to uncoupling of mitochondrial oxidative phosphorylation with a subsequent decrease in intraterminal ATP necessary for translation.

The synthesis and possible transport of specific proteins by cells associated with brain capillaries

Vinblastine causes alterations in the subcellular distribution of certain proteins synthesized by telencephalon slices. Proteins in various subcellular fractions were separated according to their molecular weight by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and radioactive proteins were determined by autofluorography. A microvascular fraction contained very high amounts of radioactivity in proteins with a molecular weight of 71,000. At least one of these proteins accumulated in the microvascular fraction when the telencephalon slices were incubated in vinblastine. At the same time these proteins became depleted in a myelinated axon fraction, microsomal fraction, and soluble/cytosol fraction. Vinblastine also affected the subcellular distribution of some proteins with a molecular weight below 27,00, but unlike the proteins of mol. wt. 71,000 none of these were synthesized at very high rates. Vinblastine did not effect the synthesis of protein in telencephalon slices, nor did it alter the subcellular fractionation of particles and organelles from slices. It is suggested that a non-neuronal vinblastine-sensitive protein translocation system is functioning within the cells of the microvascular network in telencephalon slices, and that at least one protein of 71,000 molecular weight and one protein with a molecular weight below 27,000 are transported on this system.