A nerve growth factor-sensitive S6 kinase in cell-free extracts from PC12 cells

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

Nerve growth factor specifically stimulates translation of eukaryotic elongation factor 1A-1 (eEF1A-1) mRNA by recruitment to polyribosomes in PC12 cells

During postnatal brain development the level of peptide elongation factor-1A (eEF1A-1) expression declines and that of the highly homologous isoform, eEF1A-2, increases in neurons. eEF1A-1 is implicated in cytoskeletal interactions, tumorigenesis, differentiation, and the absence of eEF1A-2 is implicated in neurodegeneration in the mouse mutant, wasted. The translation of eEF1A-1 mRNA is up-regulated via mitogenic stimulation. However, it is not known if eEF1A-1 mRNA translation is regulated by neurotrophins or if its synthesis is differentially regulated than that of the neuronal isoform, eEF1A-2. Regulated translation of these factors by neurotrophins, particularly by the Trk class of neurotrophin receptors, would implicate them in differentiation, survival, and neuronal plasticity. In this study, we investigated the effect of nerve growth factor (NGF) stimulation on the synthesis of eEF1A-1 and eEF1A-2. We found that NGF stimulation causes a preferential synthesis of eEF1A-1 over eEF1A-2 in PC12 cells. We analyzed the co-sedimentation of eEF1A-1 mRNA with polyribosome fractions in sucrose gradients, and found that NGF stimulation enriched the presence of eEF1A-1 mRNA in polyribosomes, indicating that the translation of eEF1A-1 mRNA is regulated by NGF. Inhibitors of phosphatidylinositol 3-kinase (LY 294002), mammalian target of rapamycin (rapamycin), and the NGF receptor, TrkA (K-252a), but not of mitogen-activated protein kinase (PD 98059), prevented the recruitment of eEF1A-1 mRNA to polyribosomes. The mobilization of eEF1A-1 mRNA to polyribosomes was rapamycin-sensitive in both proliferating and differentiated PC12 cells, indicating the importance of this pathway during differentiation. Our data shows that after growth factor withdrawal, an NGF-signaling pathway stimulates eEF1A-1 mRNA translation in proliferating and differentiated PC12 cells. Therefore, eEF1A-1 mRNA is a specific translational target of TrkA signaling.

NGF and other growth factors induce an association between ERK1 and the NGF receptor, gp140prototrk

As detected by coimmunoprecipitation from PC12 cells, NGF induces rapid association between ERK1 (a growth factor-activated serine/threonine protein kinase) and gp140prototrk NGF receptors. In contrast, no such association is found with the closely related ERK2. Anti-trk immunocomplexes generated from NGF-treated cells also contain protein kinase activity that shares many properties with soluble ERK1. The association of both ERK1 protein and ERK-like kinase activity with gp140prototrk is maximal by 5 min of NGF treatment, persists for approximately 1 hr, and subsequently declines by 18 hr. Treatment with either basic fibroblast growth factor, epidermal growth factor, or orthovanadate also leads to association of ERK1 with gp140prototrk without tyrosine phosphorylation of the latter. The interaction between ERK1 and gp140prototrk may prove relevant to the NGF mechanism.

Differential responses of the phosphorylation of ribosomal protein S6 to nerve growth factor and epidermal growth factor in PC12 cells

Previous studies from this laboratory have shown that the phosphorylation of the S6 protein of the ribosomes is catalyzed by at least two different and separable kinase activities in PC12 cells. One of these activities is increased by treatment of the cells with nerve growth factor, the other by treatment of the cells with epidermal growth factor. The present work shows that these two factors stimulate the phosphorylation of S6 with quite different kinetics, and that both the number of phosphates incorporated into S6 and the phosphopeptide pattern of S6 are different in cells treated with nerve growth factor than in cells treated with epidermal growth factor. The characteristics of the nerve growth factor-sensitive S6 kinase and of the epidermal growth factor-sensitive kinase were also clearly different. Substrate specificity and inhibitor studies indicated that neither was identical to cyclic AMP-dependent kinase, kinase C, or the calcium/calmodulin-dependent kinases. However, two major phosphopeptides produced by S6 phosphorylation in nerve growth factor-treated cells were also seen on phosphorylation of S6 by cyclic AMP-dependent kinase in vitro. In addition, when rat liver 40S ribosomal subunits were pretreated with cyclic AMP-dependent kinase in vitro, the action of the nerve growth factor-sensitive S6 kinase was increased about twofold.

Inhibition of nerve growth factor-induced neurite outgrowth of PC12 cells by a protein kinase inhibitor which does not permeate the cell membrane

K-252a, a protein kinase inhibitor of microbial origin, has proven to be a specific inhibitor of nerve growth factor. In this study, the effects of K-252b, the 9-carboxylic acid derivative of K-252a, on nerve growth factor-induced neurite outgrowth in PC12 cells was examined. K-252b is hydrophilic and does not permeate the cell membrane of PC12 cells, whereas K-252a clearly does. K-252b is, however, as potent as K-252a itself in inhibiting the nerve growth factor-induced neurite outgrowth. These results can be interpreted to suggest that effects of K-252b may be through surface-bound/anchored K-252b-sensitive molecules on PC12 cells.

NGF-induction of the metalloproteinase-transin/stromelysin in PC12 cells: involvement of multiple protein kinases

In previous work, we found that nerve growth factor (NGF) induced expression of the mRNA transcript encoding the metalloproteinase transin/stromelysin in PC12 cells. Transin was found, moreover, to be a "late" gene product whose expression correlated with neurites extension. In this study, various aspects of the NGF intracellular signaling pathway in PC12 cells are investigated. We show that the protein kinase inhibitor staurosporine, but not various other kinase inhibitors, specifically blocked the NGF induction of transin. Preliminary characterization of this staurosporine-sensitive kinase suggest that it does not correspond to a tyrosine kinase, nor various serine kinases, and that it is involved both at the transcriptional and posttranscriptional levels of transin gene regulation. In contrast to these effects of staurosporine, various activators of protein kinases C and A augmented the NGF induction of transin. Similar effects of these kinase inhibitors and activators were also observed with the expression of various immediate-early genes that have been proposed to mediate the transcriptional effects of NGF, including c-fos and c-jun. These data suggest, therefore, that the NGF induction of transin mRNA expression involves multiple protein kinases acting at a number of postreceptor regulatory steps in the NGF signaling pathway.

Nerve growth factor stimulation of arachidonic acid release from PC12 cells: independence from phosphoinositide turnover

The effect of nerve growth factor on the metabolism of arachidonic acid and the hydrolysis of phosphatidylinositol in PC12 cells was examined. Addition of nerve growth factor to PC12 cells isotopically labeled with [3H]arachidonic acid caused an increased release of radioactivity. In a similar manner, treatment of PC12 cells prelabeled with [3H]inositol increased inositol monophosphate accumulation in the presence of LiCl. Stimulation of [3H]arachidonic acid release by nerve growth factor was concentration dependent, attaining a maximum at 0.5 nM. Concentrations of nerve growth factor above 0.5 nM caused less than maximal stimulation. In contrast, nerve growth factor-stimulated accumulation of [3H]inositol monophosphate exhibited a sigmoidal dose-response curve with an apparent maximum at 8 nM. Increased accumulation of [3H]inositol monophosphate could be detected as early as 60 s after nerve growth factor addition, whereas nerve growth factor-stimulated release of [3H]arachidonic acid was not observed until 5 min after nerve growth factor treatment. The nerve growth factor-stimulated release of [3H]arachidonic acid was independent of extracellular calcium concentration. Increased [3H]inositol monophosphate accumulation elicited by nerve growth factor was dependent on the presence of extracellular calcium. These results suggest that the increased metabolism of arachidonic acid and the enhanced hydrolysis of phosphatidylinositol are separately regulated by nerve growth factor.

Nerve growth factor-induced transient increase in the phosphorylation of ribosomal protein S6 mediated through a mechanism independent of cyclic AMP-dependent protein kinase and protein kinase C

Treatment of PC12h cells with nerve growth factor (NGF) induced a transient increase in the phosphorylation of a 35,000-dalton protein. This transient increase was observed also when extracts of NGF-treated cells were incubated with [gamma-32P]ATP. In the intact-cell phosphorylation system, treatment with N,2'-dibutyryladenosine 3',5'-cyclic monophosphate (dBcAMP) or 12-O-tetradecanoylphorbol 13-acetate (TPA) also induced a transient increase in the phosphorylation of the 35,000-dalton protein, but the effect was less than that of NGF. An effect comparable to that of NGF was obtained by the combination of dBcAMP and TPA. Pretreatment of PC12h cells with dBcAMP plus TPA for 3 days, which deprived the cells of their ability to respond to a rechallenge with dBcAMP, TPA, or dBcAMP plus TPA by increasing the rate of 35,000-dalton protein phosphorylation, caused only a slight attenuation of the NGF effect, directly indicating a minimal role of cyclic AMP (cAMP)-dependent protein kinase and protein kinase C in the mechanism of the NGF action. Pretreatment of the cells with K-252a, a protein kinase inhibitor, at a concentration of 300 nM almost completely blocked the action of NGF, but scarcely affected the action of dBcAMP, TPA, or dBcAMP plus TPA in intact-cell phosphorylation experiments. This NGF-sensitive 35,000-dalton protein was a ribosomal protein and identified as ribosomal protein S6. The results lead us to conclude that NGF activates some NGF-sensitive component(s), probably some specific protein kinase(s) other than cAMP-dependent protein kinase or protein kinase C, which is suppressed by K-252a and directly or indirectly activates a 35,000-dalton protein kinase(s) [S6 kinase(s)] to increase the rate of phosphorylation of the 35,000-dalton ribosomal protein (S6).

Possible involvements of intracellular Ca2+ and Ca2+ -dependent protein phosphorylation in cholinergic differentiation of clonal rat pheochromocytoma cells (PC12) induced by glioma-conditioned medium and retinoic acid

It is known that nerve growth factor (NGF) induces neurite outgrowth and elevation of the activity of adrenergic marker enzyme, tyrosine hydroxylase (TH) in clonal rat pheochromocytoma cells (PC12), whereas glioma-conditioned medium (GCM) induces neurite outgrowth and elevation of the activity of cholinergic marker enzyme, choline acetyltransferase (ChAT) in PC12 cells. In the previous study we have shown that retinoic acid (RA) induces specific elevation of ChAT activity and depression of TH activity without morphological differentiation (Matsuoka, I. et al., Brain Res., 502 (1989]. In the present study, we compared the effects of NGF, GCM and RA on the intracellular signalings in PC12 cells in relation to the mechanism of cholinergic differentiation. Addition of NGF, GCM or RA to the culture medium of PC12 cells caused a rapid rise in intracellular Ca2+ concentration [( Ca2+]i) reaching the level of almost 2.5-fold the resting condition within 3-18 h. Thereafter, [Ca2+]i of NGF-treated cells were decreased to the resting level within 12 h. On the other hand, [Ca2+]i of GCM-and RA-treated cells decreased to a level which was 1.8- to 2-fold the resting condition within 24-48 h and stayed at this level for up to 4-7 days. When homogenates of GCM- and RA-treated PC12 cells were incubated with [gamma-32P]ATP, phosphorylation of a protein with molecular mass of 27 kDa (27 K-protein) was specifically enhanced. The phosphorylation of the 27 K-protein was not seen in the homogenate of the NGF-treated cells. The phosphorylation of the 27 K-protein was dependent on Ca2+ and inhibited by inhibitors of Ca2+-dependent protein kinase, H-7 and W-7. Addition of H-7 and W-7 to the culture medium of PC12 cells abolished the elevation of ChAT activity specifically induced by GCM and RA. These observations suggested that the sustained increase of [Ca2+]i and Ca2+-dependent protein phosphorylation are involved in the intracellular signaling mechanism required for the cholinergic differentiation of PC12 cells induced by GCM and RA.

Nerve growth factor regulates expression of neuropeptide genes in adult sensory neurons

Nerve growth factor (NGF) is a trophic molecule essential for the survival of sympathetic and sensory neurons during ontogeny. The extent to which NGF is involved in the maintenance or regulation of the differentiated phenotypes of mature peripheral neurons is much less clear, however. Biochemical analysis of the actions of NGF upon peripheral neurons has been hampered by the lack of a preparation of neuronal cells that are responsive to NGF but do not require it for survival. We report here that in adult dorsal root ganglion neurons, which can be isolated, enriched and maintained in culture in the absence of neuronal growth factors, the expression of mRNAs encoding the precursors of two neuropeptides, substance P and calcitonin gene-related peptide is regulated by NGF. Our results provide the first direct evidence of a continuous dynamic role for NGF in regulation of peptide neurotransmitter/neuromodulator levels in mature sensory neurons.

Activation of S6 kinase in astroglial cells by FGFa and FGFb

Basic (b) and acidic (a) forms of the fibroblast growth factor (FGF) promoted a rapid increase of the cytosolic S6 kinase activity in astroglial cells. S6 kinase activity was maximal 10 min after addition of the factors to cell cultures and remained at this level for at least 30 min. Half-activation of the enzyme was obtained with 3 ng/ml FGFa. Heparin (100 micrograms/ml) potentiated the response to suboptimal concentrations of FGFa. This growth factor appeared to stimulate an astroglial S6 kinase resembling that stimulated by insulin, IGF1, TPA and cAMP. Although FGFb is more potent than FGFa in stimulating proliferation of Chinese hamster lung fibroblasts (CCL39), it was not more efficient than FGFa in stimulating the S6 kinase activity of astroglial cells.

Properties of the 12-O-tetradecanoylphorbol-13-acetate-stimulated S6 kinase from rat astroglial cells

The S6 kinase activity of astroglial cells in primary culture stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) has been studied. This activity was eluted as a single peak at 0.15 M NaCl from a DEAE-Sephacel column. The chromatography of this peak on phosphocellulose revealed an activity eluted at 0.15 M NaCl. This partially purified enzyme had a sedimentation coefficient of 3.7S; Km values were 2 X 10(-5) M for ATP and 10(-6) M for 40S ribosomal subunits. The optimal Mg2+ concentration requirement was 2-3 mM. Mn2+ and Co2+ could substitute for Mg2+ (optimum concentrations 1.5 and 0.8 mM, respectively), but these cations were strong inhibitors in the presence of Mg2+. The enzyme was inhibited by N-ethylmaleimide, indicating that it contained thiol groups. This S6 kinase used ATP, but not GTP, as a phosphate donor, and exhibited great specificity for S6 as phosphate acceptor. Whole histones and protamine were slightly phosphorylated whereas phosvitin, histone H1, and surprisingly the peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala were not phosphorylated. The TPA-stimulated S6 kinase resembles the insulin-, fibroblast growth factor- and cyclic AMP-stimulated enzymes, suggesting that several pathways might activate the same entity.

K-252a, a potent protein kinase inhibitor, blocks nerve growth factor-induced neurite outgrowth and changes in the phosphorylation of proteins in PC12h cells

Nerve growth factor (NGF) promotes neuronal differentiation of PC12 pheochromocytoma cells. One of the most prominent and distinguishing features of neuronal differentiation is neurite outgrowth. The mechanism by which NGF causes the cells to elaborate neurites is unknown. This study shows that K-252a, a potent protein kinase inhibitor, blocks NGF-induced neurite outgrowth and the changes in protein phosphorylation elicited by NGF. In the experiment with intact cells phosphorylated with 32P-orthophosphoric acid, an exposure of PC12h cells to NGF (50 ng/ml) caused an increase in the phosphorylation of tyrosine hydroxylase and a 35,000-D protein and a decrease in a 36,500-D protein. Pretreatment of PC12h cells with K-252a (100 nM) inhibited the effects of NGF on the phosphorylation of these three proteins. In the phosphorylation of cell-free extracts with [gamma-32P] ATP, treatment of PC12h cells with NGF (50 ng/ml) caused a decrease in the phosphorylation of Nsp100. Pretreatment of the cells with K-252a (30 nM) almost completely blocked the NGF effect on the phosphorylation of Nsp100 elicited by subsequent treatment of the cells with NGF. Treatment of PC12h cells with NGF promoted outgrowth of neurites. The addition of K-252a (100 nM) into the culture almost completely blocked the generation of neurites elicited by NGF. Earlier studies demonstrated that NGF-induced neurite outgrowth in PC12 cells involves at least two components: the first of these is transcription-dependent and the second is transcription-independent. To determine the component on which K-252a acts, experiments were carried out on NGF-induced priming or regeneration of neurites. When K-252a was present in the priming step, NGF induced only actinomycin D-sensitive neurites, showing that K-252a interferes with the transcription-dependent actions of NGF. When already primed cells were treated with NGF, actinomycin D-resistant neurites were formed and these were blocked by K-252a, showing that the inhibitor interferes with the transcription-independent actions of NGF as well. Although the exact mechanism of inhibition of NGF-promoted neurite formation by K-252a is unknown, the most probable explanation is that both transcription-dependent and -independent components are involved in at least one step of the activation of some specific protein kinase(s) that can be suppressed by K-252a.

The mode of action of nerve growth factor in PC12 cells

This review deals with the mechanism of nerve growth factor action. In view of the many and diversified effects of this growth factor, and since it could utilize different mechanism(s) in distinct types of cells, we have confined our analysis to the best characterized and more extensively studied target, the clonal cell line PC12. When exposed to NGF in vitro, these neoplastic cells recapitulate the last major steps of neuronal differentiation, i.e., the commitment to become a neuron and the acquisition of the neuronal phenotype. This is characterized by electrically excitable neurites, a display of a highly organized cytoskeleton, and the specific chemical and molecular neuronal properties. These effects are elicited upon the interaction of NGF with a receptor whose gene has been cloned and whose kinetic properties are now relatively well characterized. It is not yet clear, on the contrary, if and which of the several potential second messengers (cAMP, Ca, or phosphoinositides) that undergo marked fluctuations following NGF binding, transduce and amplify the NGF message. Among both the early and late effects of NGF is the modulation of expression of several genes. Some of the products of these genes are mainly restricted to nerve cells and others appear to play a crucial role in regulating the proper assembly of cytoskeletal elements. It is hypothesized that this complex array of chemical, molecular, and ultrastructural changes is triggered by NGF, not through activation of a single pathway, but more likely via combinatorial processes whereby several intracellular signals interplay before the irreversible commitment of becoming a neuron is undertaken.

Inhibition by K-252a, a new inhibitor of protein kinase, of nerve growth factor-induced neurite outgrowth of chick embryo dorsal root ganglion cells

The effect of K-252a, a new alkaloid-like kinase inhibitor from the culture broth of Nocardiopsis sp., on the action of nerve growth factor on normal nerve cells was examined. Nerve cells were isolated from the dorsal root ganglia of chick embryo by trituration without using digesting enzymes, and were seeded on collagen- or poly-L-lysine-coated plastic dishes containing serum-free medium. K-252a at 50-100 nM reversibly inhibited the formation of neurites by dorsal root ganglion cells elicited by the treatment with nerve growth factor.

Activation of an S6 kinase from rat astroglial cells by cAMP

Forskolin and isoproterenol, agonists of adenylate cyclase activity, and dibutyryl cyclic AMP, stimulated an S6 kinase activity in astroglial cells. This activity was insensitive to the thermostable inhibitor of cyclic AMP-dependent protein kinase and had the same behaviour on a DEAE-Sephacel column as the mitogen stimulated S6 kinase. These observations support the idea that the cyclic AMP cascade, as well as various growth factors, can activate S6 kinase.