Topography of synaptosomal high affinity uptake systems

Inhibition by trypsin of the high-affinity acidic amino acid transport system in C6 glioma cells

The high-affinity uptake of the acidic amino acid D-aspartate was inhibited in a dose- and time-dependent manner, when C6 cells were exposed to trypsin. The protease decreased the maximal velocity for uptake but not its Km, consistent with a reduction in the number of competent carriers at the plasma membrane. Cellular energy production and [K+]i were unaffected, indicating that the transporter itself was the site of trypsin action. Maximum inhibition of uptake was 50%, which suggests the presence of a heterogeneous population of transporters, only half of which is sensitive to trypsin. These results support our earlier postulate that in glial cells, the high-affinity transporter for acidic amino acids is a transmembrane protein, part of which extends into the external environment.

Effects of trypsin and protein modification on the renal transporter ofp-aminohippurate

Basal-lateral membranous vesicles prepared from rabbit renal cortex exhibited Mg2+-stimulated, probenecid-inhibitable transport of p-aminohippurate (PAH). This uptake could be completely eliminated by incubating the membranes with trypsin at a weight ratio of 1:700 (trypsin/membrane protein). The loss of PAH uptake activity occurred in two stages. Over the first ten minutes of the vesicles' exposure to trypsin, there was a nearly linear loss, with respect to time, of about 80% of the PAH uptake activity. The remaining 20% of activity was resistant to further trypsin digestion for the next ten minutes, but by twenty-five minutes a total inactivation of the uptake activity occurred. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of normal and trypsin-treated vesicles showed very little degradation of proteins. However, two minor polypeptides (Mr - 410,000 and 388,000) were degraded during the first ten minutes of the membranes' exposure to trypsin. After twenty minutes of exposure, two other polypeptides (Mr = 94,500 and 87,500) were degraded. Chymotrypsin and clostripain also caused a loss of PAH transport activity. However, compared to the effects of trypsin, the effects of these two proteases were less complete, slower in onset, and for clostripain, a much higher concentration of enzyme was required. Other functions or properties of the vesicles including morphological appearance, degree of vesiculation, glucose space or Na+-dependent L-glutamate transport and Na+,K+-ATPase activity were not altered by the concentration of trypsin which abolished 80% of the transport of PAH. Thus, it is possible that one or more of the degraded polypeptides detected by polyacrylamide gel electrophoresis comprises the PAH transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of anti-synaptosomal membrane antibodies of neurotransmitter uptake

Antibodies raised against synaptosomal plasma membranes of rat hippocampus (anti-HPC IgG) caused inhibition of [3H]noradrenaline, [3H]5-hydroxytryptamine, [3H]GABA and [3H]aspartate uptake into S1 fractions and slices of hippocampus and cerebral cortex, but not those of caudate nucleus and hypothalamus. Similar inhibition was not observed on using antibodies against synaptosomal membranes of rat caudate nucleus. Anti-HPC IgG raised against synaptosomal membranes of hippocampus failed to alter both spontaneous and K+-evoked release of [3H]noradrenaline. They did not interfere with the binding of [3H]desipramine (the potent noradrenaline-uptake inhibitor) and with the binding of [3H]dihydroalprenolol, thus excluding any interaction of the antibodies with drug receptors which are located on either the pre- or postsynaptic membrane. The anti-HPC IgG inhibit the enzymatic activity of [Na+-K+-]ATPase by 30% upon incubation of the antibodies with crude membrane preparations. A comparison of their inhibitory effects with those of the neurotoxin 6-hydroxydopamine suggests that the corresponding hippocampal specific antigens are located at a presynaptic site.

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.