Influence of the conformational state on the isoelectric points of rat brain synaptosomes, mitochondria and mitoplasts

Isolation of functional mitochondria by inertial microfluidics – a new method to sort intracellular organelles from a small scale biological sample

Isolation of subcellular fractions from a limited amount of clinical sample is imperative for the continuous advancement of biological and clinical research.

Capillary isoelectric focusing of individual mitochondria

Mitochondria are highly heterogeneous organelles that likely have unique isoelectric points (pI), which are related to their surface compositions and could be exploited in their purification and isolation. Previous methods to determine pI of mitochondria report an average pI. This article is the first report of the determination of the isoelectric points of individual mitochondria by capillary isoelectric focusing (cIEF). In this method, mitochondria labeled with the mitochondrial-specific probe 10-N-nonyl acridine orange (NAO) are injected into a fused-silica capillary in a solution of carrier ampholytes at physiological pH and osmolarity, where they are focused then chemically mobilized and detected by laser-induced fluorescence (LIF). Fluorescein-derived pI markers are used as internal standards to assign a pI value to each individually detected mitochondrial event, and a mitochondrial pI distribution is determined. This method provides reproducible distributions of individual mitochondrial pI, accurate determination of the pI of individual mitochondria by the use of internal standards, and resolution of 0.03 pH units between individual mitochondria. This method could also be applied to investigate or design separations of organelle subtypes (e.g., subsarcolemmal and interfibrillar skeletal muscle mitochondria) and to determine the pIs of other biological or nonbiological particles.

RNA synthesis in isolated mitochondria from brain cortex, cerebellum and stem: evidence of different transcriptional rates

1. A system for studying RNA synthesis in isolated sheep brain mitochondria was set up to investigate the transcriptional activity of different brain regions (cortex, cerebellum and brain stem). In this system, mitochondrial DNA is transcribed and RNA processed in a way that faithfully reproduces the in vivo process. 2. The comparison of the electrophoretic patterns of the mitochondrial DNA transcription products showed that although they were qualitatively similar, there were large differences in the rate of mitochondrial DNA transcription of the three regions studied, cerebellum and brain stem showing transcriptional rates which were 34 and 18% respectively of that of cerebral cortex.

Detection of barbiturate-induced surface changes in cerebrocortical synaptosomes by phase partitioning in an aqueous two-phase system

The barbiturates tested in this work (barbital, phenobarbital, thiopental and pentobarbital) modify the partition of synaptosomes in a Dextran T500-poly(ethylene glycol) 4000 two-phase system. Under adequate experimental conditions, the drugs increase the partition into the upper phase and this effect appears to be due to an action on the biological material and not on the interface potential of the system. This conclusion can be drawn from the fact that synaptosomes preincubated with low concentrations (0.1 mM) of barbital and pentobarbital maintained an increased partition into the upper phase. The extent of the effect observed appeared to be inversely proportional to the hydrophobicity of the drugs since phenobarbital and barbital showed a higher effect than thiopental and pentobarbital. Dithionite-induced anoxia, rotenone and ouabain also induced a similar increase of partition of synaptosomes into the upper phase, suggesting that the surface changes detected by phase partitioning modification of synaptosomes could be somehow related to the bioenergetic maintenance of the membrane ATPase.

pH-induced reorganization of synaptic membrane as revealed by fluorescence anisotropy and energy transfer

Two fluorescent probes, 2-(9-anthroyloxy)stearate and 12-(9-anthroyloxy)stearate, were used to investigate the effects of the neutralization of membrane charges on the organization of synaptic plasma membrane. Steady state fluorescence anisotropy measurements showed that a pH decrease provoked a rigidification of the synaptic membrane surface, whereas the bilayer core remained unaffected. The same effect was observed with negatively charged lipid vesicles. The relative distribution of proteins and the probes was estimated by fluorescence energy transfer from protein tryptophans to fluorescent probes: a pH decrease provoked an increase of the energy transfer, which was most pronounced with the surface probe, indicating an average closer packing between proteins and the probes. The modifications induced by a pH decrease were temperature dependent and were most marked at low temperatures. The results suggest that neutralization of the membrane charges provoked a redistribution of both membrane lipids and proteins. These findings are discussed in terms of a heterogeneous distribution of these membrane components.

Reduced synthesis of mtRNA in isolated mitochondria of senescent rat brain

A system for studying RNA synthesis in isolated mitochondria from rat brain was set up to investigate the mechanisms responsible for the age-dependent reduction of mtRNA content. In the presence of an appropriate incubation buffer both synaptic and non-synaptic mitochondria from cerebral hemispheres were able to synthesize and process mtRNA in a way quantitatively and qualitatively similar to the in vivo transcription. The comparison of the electrophoretic pattern of mtRNAs synthesized by adult and senescent rat showed, in the senescent rat, a 50% reduction in the mtRNA synthesis rate relative to the adult value. This indicates that the age-dependent decrease of the mtRNA content is linked to a lower efficiency of the mt transcription.

Rat brain synaptosomes prepared by phase partition

Synaptosomes from rat forebrain can easily be isolated by combining centrifugation with partition in an aqueous two-phase system composed of dextran T500 and polyethylene glycol 4000 in which synaptosomes have an extreme affinity for the upper phase. The fraction thus obtained has been characterized by electron microscopy and biochemical markers for synaptosomes and some other cell components. The contamination by microsomes, free mitochondria, and myelin was 4.4, 3.2, and 0.1%, respectively. The morphometric analysis of the electron micrographs shows that greater than 60% of the structures are synaptosomes. This preparation of the isolation procedure is remarkably short (less than 1 h), formance as assayed by their respiratory activities and ATP level in the absence and presence of depolarizing agents. Synaptosomes prepared by phase partition release the neurotransmitter glutamate in a Ca2(+)-dependent manner. The duration of the isolation procedure is remarkably short (less than 1 h), no ultracentrifuge is required, and the method can be applied for small- or large-scale preparations.

Analysis of polyadenylated RNA from brain synaptosomes and mitochondria

We have isolated RNA from sheep brain synaptosomes and mitochondria separated by an aqueous two-phase system composed of dextran and poly(ethylene glycol). RNA was fractionated through oligo(dT)-cellulose columns and analyzed by electrophoresis through agarose slab gels containing methylmercuric hydroxide and stained with ethidium bromide. The electrophoretic patterns of the poly(A)-containing RNA fraction from synaptosomes and mitochondria are very similar although some high molecular weight RNA species, clearly visible in the synaptosomal fraction, are scarcely detected in the mitochondrial preparations. The electrophoretic analysis of a cleaner RNA preparation from digitonin-treated free mitochondria (mitoplasts) showed that all the poly (A)-RNA species of the synaptosomal preparation are also present in mitoplast. These results strongly suggest that all the discrete poly(A)-RNA species identified in brain synaptosomes are of mitochondrial origin.

Isolation of rat liver lysosomes by a single two-phase partition on dextran/polyethylene glycol

Crude mitochondria from liver rats were added to a two-phase system containing dextran and polyethylene glycol. The polymer and ionic concentration values of the two-phase system were changed in order to separate lysosomes from mitochondria. The best separation of lysosomes and mitochondria was obtained at 6.6-6.6% (w/w) dextran-polyethylene glycol and 5 mmol/kg ammonium chloride as shown by enzyme assays. This procedure showed good reproducibility, and lysosomes were never contaminated with more than 16% mitochondria, as determined by succinate dehydrogenase activity, and beta-D-galactosidase and acid phosphatase activities were enriched five- to sixfold. The lipid composition profile of lysosomes was quite similar to that obtained by means of free carrier electrophoresis, considered a reference method.

Quantitation of the beta-bungarotoxin-induced release of lactate dehydrogenase from cerebrocortical synaptosomes

Treatment with beta-bungarotoxin for 1 h induces lactate dehydrogenase release from cerebrocortical synaptosomes. The effect is Ca2+-dependent as suggested by the inhibition observed with EGTA. An inhibition of the effect can be also obtained by addition of Sr2+ ions, suggesting that the phospholipase A2 activity associated to the toxin is involved in the efflux of the enzyme. The beta-bungarotoxin-induced release of synaptoplasmic lactate dehydrogenase is a saturable effect, showing a half-maximal effect concentration of 32 nM and a maximal efflux of 25% of the total synaptosomal enzyme as calculated by double-reciprocal plot.

Occurrence of a large Ca2+-independent release of glutamate during anoxia in isolated nerve terminals (synaptosomes)

Isolated rat cerebral cortical synaptosomes made anoxic by addition of cyanide developed an inhibition of the Ca2+-dependent release of glutamate 2 min after the addition of the metabolic inhibitor when the intrasynaptosomal ATP/ADP ratio decreased below 1.7. In contrast, cyanide induced a continuous efflux of glutamate through a Ca2+-independent pathway that accounted for the release of 25% of total intrasynaptosomal glutamate in 5 min. The results suggest that a Ca2+-independent release of glutamate could be implicated in the neurotoxic action of this amino acid during anoxia.