No abstract

We studied the effects of insulin, nerve growth factor (NGF), and tetrodotoxin (TTX) on cellular metabolism and the activity of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) in neuron-rich cultures prepared from embryonic day 15 rat striatum. Insulin (5 micrograms/ml) increased glucose utilization, protein synthesis, and GAD activity in cultures plated over a range of cell densities (2,800-8,400 cells/mm2). TTX reduced GAD activity; NGF had no effect on GAD activity. Insulin treatment reversibly reduced ChAT activity in cultures plated at densities of greater than 4,000 cells/mm2, and the extent of this reduction increased with increasing cell density. The number of acetylcholinesterase-positive neurons was not reduced by insulin, suggesting that insulin acts by down-regulating ChAT rather than by killing cholinergic neurons. Insulin-like growth factor-1 (IGF-1) reduced ChAT activity at concentrations 10-fold lower than insulin, suggesting that insulin's effect on ChAT may involve the IGF-1 receptor. NGF increased ChAT activity; TTX had no effect on ChAT activity. These results suggest that striatal cholinergic and GABAergic neurons are subject to differential trophic control.

The total extracellular proteins and the most abundant 960 intracellular proteins of clonal CNS nerve and glial cell lines were examined by quantitative 2-dimensional acrylamide gel electrophoresis. While less than 0.2% of the intracellular proteins differ among the 5 nerve and 4 glial cell lines studied, over 65% of the extracellular proteins vary in distribution between the 2 major classes of CNS cells. These data indicate that the phenotypic complexity of nerve and glia populations is similar and that most of the protein complexity is in extracellular molecules.It has been argued on the basis of RNA hybridization studies that the mammalian CNS has a severalfold higher number of unique mRNA sequences than other tissues (Brown and Church, 1972;Chikaraishi et al., 1983; Hahn and Laird, 197 1). Assuming that these mRNAs are translated, there are 2 alternatives that could explain the apparent increase in protein complexity within the brain; they are not mutually exclusive. Each cell within the CNS might make many more species of proteins than cells of other tissues, or there might be a greater number of cell types, each making a few unique species that contribute to total tissue complexity but whose overall protein complexity is similar to cells of different tissues. To distinguish between these alternatives, protein synthesis in a series of clonal CNS nerve and glial cell lines was examined by 2-dimensional gel electrophoresis. Total cellular protein synthesis and the extracellular proteins released into the culture medium were analyzed by computer-assisted methods. It is shown that there is a great deal of variability in the protein species synthesized by cells from the rat CNS and that the variability within the glial population is as great as that between the nerve cells. The majority of this protein complexity is associated with the extracellular proteins, which are more abundant in nerve and glia than in mesodermally derived cells. The total number of cellular proteins synthesized by clonal CNS cell lines is, however, indistinguishable from that of mesodermal cells. It follows that the higher protein complexity in the CNS is due to both the large number of unique phenotypes and the increased number of extracellular proteins relative to other tissues. Materials and MethodsCell lines

When spinal cord cultures from embryonic day 12 rats were cultured at low density, both acidic and basic fibroblast growth factors significantly increased neuronal survival and neurite outgrowth in a dose-dependent manner. The effects of acidic fibroblast growth factor were independent of heparin, in contrast to its mitogenic effects on both NIH3T3 cells and cerebral cortical astrocytes. In high-density cultures, acidic fibroblast growth factor increased choline acetyltransferase activity by 57%, glutamic acid decarboxylase activity by 58%, and aspartate aminotransferase activity by 65%. Basic fibroblast growth factor increased choline acetyltransferase activity by 73% and glutamic acid decarboxylase activity by 200% but decreased aspartate aminotransferase activity by 40%. Growing these cultures in the presence of a mitotic inhibitor did not significantly alter the effect of acidic or basic fibroblast growth factor on these enzyme activities. These results demonstrate that acidic and basic fibroblast growth factors differentially affect neurotransmitter enzyme levels of multiple classes of neurons, rather than having effects on a single neuronal population.

During neuromuscular junction formation ACh receptors accumulate at the nerve-contact region. It has been shown that this is at least partly due to lateral migration of existing receptors in the membrane (Anderson et al., 1977). Randomly diffusing ACh receptor molecules in the membrane may be trapped at the nerve-contact region to form a high receptor density area. If this were the major mechanism, cross-linking ACh receptors by tetravalent concanavalin A (Con A) should immobilize receptors and prevent nerve-induced receptor accumulation. We examined the effect of Con A on nerve-induced receptor accumulation and on the mobility of ACh receptors in cultured Xenopus muscle cells. ACh receptors were stained with tetramethyl rhodamine conjugated alpha-bungarotoxin. The cells were then treated briefly with Con A, and neural tube cells were added to these cultures. The mobility of ACh receptors was measured by the fluorescence photobleaching recovery method. The Con A treatment prevented rapid diffusion of ACh receptors as well as nerve-induced receptor accumulation. Functional synapse formation was not inhibited by this treatment. In contrast, divalent succinyl Con A did not affect the mobility of ACh receptors nor prevent nerve-induced ACh receptor accumulation. When the Con A concentration was varied, the blocking effect on the nerve-induced receptor accumulation changed in parallel with the mobile fraction of receptors. Newly inserted ACh receptors after the Con A treatment were found to be mobile and to accumulate at the nerve-contact region. In these cultures, new receptors accumulated around old, immobilized receptors in some areas along the nerve contact. This observation suggests that new receptors were inserted elsewhere and migrated to the nerve-contact region surrounding immobilized old ones. In addition to the accumulation of receptors, the nerve disperses preexisting receptor clusters prior to induction of high-density regions along the contact area, and, at this early stage, denervation disperses nerve-induced receptor clusters in Xenopus cultures (Kuromi and Kidokoro, 1984a, b). When cultures were treated with Con A, neither of these events occurred, suggesting that these are also diffusion-mediated.