Activation of m1 Muscarinic Acetylcholine Receptor Regulates τ Phosphorylation in Transfected PC12 Cells

Abstract: Hyperphosphorylated τ proteins are the principal fibrous component of the neurofibrillary tangle pathology in Alzheimer's disease. The possibility that τ phosphorylation is controlled by cell surface neurotransmitter receptors was examined in PC12 cells transfected with the gene for the rat m₁ muscarinic acetylcholine receptor. Stimulation of m₁ receptor in these cells with two acetylcholine agonists, carbachol and AF102B, decreased τ phosphorylation, as indicated by specific τ monoclonal antibodies that recognize phosphorylation-dependent epitopes and by alkaline phosphatase treatment. The muscarinic effect was both time and dose dependent. In addition, a synergistic effect on τ phosphorylation was found between treatments with muscarinic agonists and nerve growth factor. These studies provide the first evidence for a link between the cholinergic signal transduction system and the neuronal cytoskeleton that can be mediated by regulated phosphorylation of τ microtubule-associated protein.     

Suramin Induces Phosphorylation of the High-Affinity Nerve Growth Factor Receptor in PC12 Cells and Dorsal Root Ganglion Neurons

Abstract: Suramin is a polysulfonated naphthylurea with demonstrated antineoplastic activity. Toxicity includes adrenal insufficiency and peripheral neuropathy. Although the mechanism of antitumor activity is unknown, inhibition of binding of growth factors to their receptors has been suggested. Growth factors inhibited by suramin include platelet-derived growth factor, fibroblast growth factor, transforming growth factor, epidermal growth factor, insulin-like growth factor, and nerve growth factor (NGF). In these studies, suramin was shown to be cytotoxic to PC12 cells in a dose-dependent manner. At lower doses and in surviving cells, we observed the induction of neurite outgrowth. To determine the mechanism of suramin-induced neurite outgrowth, PC12 cells were exposed to suramin and/or NGF for various time periods and treated cells were analyzed, by western blot analysis, for expression of tyrosine phosphoproteins. There was a similarity in the pattern of tyrosine-phosphorylated proteins in PC12 cells stimulated with suramin or NGF. Of particular interest was the rapid phosphorylation (by 1 min) of the high-affinity NGF (TrkA) receptor. Activation of other members of the signal-transduction cascade (Shc, p21 ^ras , Raf-1, ERK-1) revealed similar phosphorylation levels induced by suramin and NGF. Parallel studies were performed in rat dorsal root ganglion cultures; suramin potentiated neurite outgrowth and activated the NGF receptor on these cells. This finding of specific patterns of tyrosine phosphorylation of cellular proteins in response to suramin treatment demonstrated that suramin is a partial agonist for the NGF receptor in both PC12 cells and dorsal root ganglion neurons.     

Nerve Growth Factor-Stimulated Nuclear S6 Kinase in PC12 Cells

Abstract: Previous work has shown that nerve growth factor (NGF) stimulates the phosphorylation of the ribosomal protein S6 in PC12 cells. In this study, we show that S6 kinase activity is also present in purified PC12 cell nuclei. This activity was increased by treatment of the cells with NGF and, to a lesser extent, by treatment with epidermal growth factor. The NGF-stimulated activity was obtained from nuclear extracts and some of its characteristics described. The increase in activity was prevented by treatment of the cells with rapamycin or with wortmannin, and the overall activity could be precipitated by antibodies directed against the p85^S6K. These data indicate that p85^S6K is the NGF-stimulated S6 kinase in PC12 cell nuclei. The presence of S6 protein in the nucleus of PC12 cells has been confirmed and evidence is presented that suggests that it is identical to a protein called SMP reported some years ago.     

Prosaposin Facilitates Sciatic Nerve Regeneration In Vivo

Abstract: Prosaposin, a multifunctional protein, is the precursor of saposins, which activate sphingolipid hydrolases. In addition to acting as a precursor for saposins, prosaposin has been shown to rescue hippocampal CA1 neurons from lethal ischemic damage in vivo and to promote neurite extension of neuroblastoma cells in vitro. Here we show that prosaposin, when added to a collagen-filled nerve guide after sciatic nerve transection in guinea pigs, increased dramatically the number of regenerating nerve fibers within the guide. To identify the target neurons of prosaposin during peripheral nerve regeneration, we determined the degree of atrophy and chromatolysis of neurons in the spinal anterior horn and dorsal root ganglia on the prosaposin-treated and untreated side. The effect of prosaposin on large spinal neurons and small neurons of the dorsal root ganglion was more conspicuous. Subsequent immunohistochemistry demonstrated that the atrophy of cholinergic large neurons in the anterior horn is prevented to significant extent by prosaposin treatment. These findings suggest that prosaposin promotes peripheral nerve regeneration by acting on α-motor neurons in the anterior horn and on small sensory neurons in the dorsal root ganglion. The present study raises the possibility of using prosaposin as a tool for the treatment of peripheral nerve injuries.     

K252a Modulates the Expression of Nerve Growth Factor-Dependent Capsaicin Sensitivity and Substance P Levels in Cultured Adult Rat Dorsal Root Ganglion Neurones

Abstract: K252a, an inhibitor of trk phosphorylation and nerve growth factor signal transduction in PC12 cells, blocked nerve growth factor-induced responses in cultured adult rat dorsal root ganglion sensory neurones. The nerve growth factor-dependent appearance of capsaicin sensitivity and accumulation of the neuropeptide substance P were inhibited when dorsal root ganglion neurones were grown in the presence of low concentrations (100 n M ) of K252a. At higher concentrations (3 µ M ), however, K252a stimulated the development of capsaicin sensitivity and the accumulation of substance P even in the absence of nerve growth factor. By using a wide dose range, therefore, we showed that K252a could either inhibit or mimic nerve growth factor's actions on sensory neurones. These results may explain the apparent paradox in the literature that some groups show a blocking effect of K252a on nerve growth factor-dependent survival of dorsal root ganglion sensory neurones, whereas others report that K252a can substitute for nerve growth factor or other trophic factors and promote neuronal survival.     

Ras-Regulated Hypophosphorylation of the Retinoblastoma Protein Mediates Neuronal Differentiation in PC12 Cells

Abstract: To investigate the role of the retinoblastoma protein pRB in neuronal differentiation, we have measured the accumulation of hypophosphorylated pRB in PC12 cells stimulated by nerve growth factor (NGF). NGF induced the accumulation of hypophosphorylated pRB within 30 min and the level peaked after 12 h. Viral Kiras, cyclic AMP (cAMP), and 12- O -tetradecanoylphorbol 13-acetate (TPA) also induced the hypophosphorylation of pRB, but epidermal growth factor and interleukin-6 did not. The extent of hypophosphorylation of pRB correlated well with the capacity of these factors to stimulate neurite outgrowth. The constitutively activated Ras induced persistent shift of the phosphorylation state of pRB toward hypophosphorylation. A dominant negative form of cHa-Ras suppressed significantly induction of the hypophosphorylation of pRB by NGF, but not by cAMP. Taken together, these results suggest that the hypophosphorylation of pRB triggered by NGF is mediated by a Ras-dependent pathway. Furthermore, microinjection of a monoclonal antibody specific for the hypophosphorylated form of pRB blocked the neurite outgrowth initiated by NGF. These results suggest a crucial role of pRB in withdrawal of cells from the cell cycle and in neuronal differentiation of PC12 cells.     

Regulation by Androgen of Levels of the β Subunit of Nerve Growth Factor and Its mRNA in Selected Regions of the Mouse Brain

Abstract: Our previous studies showed that the concentration of the β subunit of nerve growth factor (β-NGF) in nervous tissues is higher in male than in female mice. To identify the brain regions that are affected by androgens, the amounts of β-NGF protein and its mRNAs were measured in male, female, and castrated male CD-1 mice and testicular feminization mice at 3–4 months of age. Among tissues examined, the hypophysis of males contained the highest average concentration of β-NGF protein. In most regions of the brain, individual levels were more variable in males than in females. However, after the castration, such variations in β-NGF levels disappeared. Average levels of β-NGF protein in males were higher in the cerebellum (eightfold higher), olfactory bulb (12-fold higher), hypothalamus (sixfold higher), and hypophysis (72-fold higher) than thope in corresponding regions of females. No significant differences were observed in levels of β-NGF protein in the hippocampus, cerebral cortex, striatum, septum, and brainstem. The castration of male mice caused a reduction in levels of β-NGF protein in the hypothalamus and hypophysis, but not in the cerebellum and olfactory bulb, to the femgle levels. The concentrations of β-NGF protein in testicular feminization mice were similar to those in female CD-1 mice in all regions. The concentrations of mRNA for β-NGF in the olfactory bulb and hypophysis from males were higher than those from females. By contrast, northern blots showed no remarkable differences in the amounts of brain-derived neurotrophic factor and neurotrophin-3 between the two sexes. Thus, in some regions of the brain, the production of β-NGF appears to be regulated by testosterone, but the regulatory mechanisms do not appear to be simple. Our present results indicate that the binding of testosterone to its receptor is an important step in the regulation of the level of β-NGF in these region.     

Effects of Nerve Growth Factor on Glutathione Peroxidase and Catalase in PC 12 Cells

Abstract: Nerve growth factor (NGF) is a member of the neuro- trophin family and is required for the survival and maintenance of peripheral sympathetic and sensory ganglia. In the CNS, NGF regulates cholinergic expression by basal forebrain cholinergic neurons. NGF also stimulates cellular resistance to oxidative stress in the PC12 cell line and protects PC12 cells from the toxic effects of reactive oxygen species. The hypothesis that NGF protection involves changes in antioxidant enzyme expression was tested by measuring its effects on catalase and glutathione per- oxidase (GSH Px) mRNA expression in PC12 cells. NGF increased catalase and GSH Px mRNA levels in PC 12 cells in a time- and dose-dependent manner. There was also a corresponding increase in the enzyme activities of catalase and GSH Px. Thus, NGF can provide cytoprotection to PC12 cells by inducing the free radical scavenging enzymes catalase and GSH Px.