Developmental changes of protein-bound radioactivity distribution in rat brain slices incubated with labelled leucine

Cerebral neurons and glial cell types inducing heat shock protein Hsp70 following heat stress in the rat

In this chapter, the distribution of Hsp70 in brain cell types following whole body hyperthermia is reviewed. The prevalence of Hsp70 expression in oligodendrocytes, microglia, and vascular cells in this type of stress contrasts with scarcity of Hsp70 induction in astrocytes and most neurons of the hyperthermic brain. However, a similarity between hyperthermic- and arsenite-induced brain patterns of Hsp70 expression supports the view that denaturation of specific proteins plays a major role in the selectivity of glial/vascular expression also during hyperthermia in vivo. The mechanism of neuronal Hsp70 non-responsiveness in heat stress despite their ability to use Hsc70 in a partial heat stress response remains to be elucidated.

Glutamate neurotoxicity and the inhibition of protein synthesis in the hippocampal slice

In some animal models of ischemia, neuronal degeneration can be prevented by the selective antagonism of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, suggesting that glutamate released during ischemia causes injury by activating NMDA receptors. The rat hippocampal slice preparation was used as an in vitro model to study the pharmacology of glutamate toxicity and investigate why NMDA receptors are critical in ischemic injury. Acute toxicity was assessed by quantifying the inhibition of protein synthesis, which we confirmed by autoradiography to be primarily neuronal. The effect of NMDA was prevented by the specific antagonists MK-801 and ketamine, as well as by the less selective antagonist kynurenic acid. The less selective antagonists kynurenic acid and 6,7-dinitroquinoxaline-2,3-dione antagonized the effects of quisqualate and NMDA. In contrast to previous observations with dissociated neurons in tissue culture, the toxicity of glutamate was unaffected by antagonists, regardless of the glutamate concentration, the duration of exposure, or the presence of magnesium. The high concentration of glutamate required to inhibit protein synthesis and the inability of receptor antagonists to block the effect of glutamate suggest that either glutamate acts through a non-receptor-mediated mechanism, or that the receptor-mediated nature of glutamate effects are masked in the slice preparation, perhaps by the glial uptake of glutamate. The altered physiology induced by ischemia must potentiate the neurotoxicity of glutamate, because we observed with a brain slice preparation that only high concentrations of glutamate caused neurotoxicity in the presence of oxygen and glucose and that these effects were not reversed by glutamate receptor antagonists.

Protein metabolism in an astroglial primary culture

Protein metabolism was studied in astroglial primary cultures, grown for different time periods. Removal of fetal calf-serum for two days led to a morphological differentiation consisting of retraction of cell soma and extention of processes. There was a prominent decrease in total soluble protein and a decrease in [3H]valine incorporation into soluble protein. Dibutyrylcyclic-3'-5'-adenosine monophosphate (dB-cAMP)-treatment for two days also changed morphology in a similar way, but had no effect on [3H]valine incorporation into protein. After addition of soluble brain extract to the cultures an increased [3H]valine incorporation into soluble protein was seen together with a morphological differentiation, more pronounced in the presence than in the absence of fetal calf-serum. Proteins were secreted from the cells into the incubation medium and studied by electrophoresis. The more prominent protein bands had m.w. in the region of 10,000-100,000 daltons. The amount of newly synthesized proteins released into the medium was unchanged (or decreased slightly in 14 and 16 day old cultures) after addition of dB-cAMP" or soluble brain extract, and was much reduced after removal of fetal calf-serum.

Distributive profiles of free and protein-incorporated tryptophan and valine in cerebral cortex slices from adult and 2-day-old rats

The distribution of the specific radioactivity and the incorporation into protein of [3H]tryptophan and [3H]valine at varying layers from surface to centre were measured in incubated slices of cerebral cortices from infant and adult rats. Specific radioactivity in free amino acids was in both age groups highest in the intact surface layer. Incorporation of tryptophan into protein was even in slices from adult rats but much less than the average in the surface layers in slices from infant rats. Incorporation of valine exhibited similar heterogeneity in both age groups. The results suggest in brain slice preparations a zonal compartmentation of amino acid and protein metabolism which varies for different amino acids.

Protein synthesis by astrocytes in primary cultures

Protein synthesis, measured as leucine incorporation into acid-precipitable proteins, was determined in astrocytes in primary cultures obtained from the cerebral hemispheres of newborn mice. As can be expected for eucaryotic, ribosomal protein synthesis, the incorporation was almost completely inhibited by cycloheximide (0.01 mM), but unaffected by chloramphenicol (0.03 mM). The rate of synthesis, measured during exposure to a high (0.8 mM) concentration of leucine was 5.4 nmol/hr/mg protein in mature (i.e., at least 4-week-old) cultures. This value is at least twice as high as the protein synthesis rates reported for the adult brain in vivo, suggesting that a very considerable part of the protein synthesis in the adult brain may take place in astrocytes. The molecular weight distribution of the synthesized proteins was determined by polyacrylamide gel electrophoresis, demonstrating synthesis of at least 50 different polypeptides, ranging in molecular weight between 190,000 and 27,000 daltons. The pattern of the synthesized proteins underwent considerable alteration with age in young cultures in which the total content of protein was still increasing, but it was remarkably stable after the age of two weeks. Exposure to dibutyryl cyclic AMP, which is known to alter morphology, content of glial fibrillary acidic protein (GFA), and activities of certain enzymes in the cultures in the cultured astrocytes, caused marked alterations in the pattern of the synthesized proteins.

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.