Sustained signaling by phospholipase C-gamma mediates nerve growth factor-triggered gene expression

In contrast to conventional signaling by growth factors that requires their continual presence, a 1-min pulse of nerve growth factor (NGF) is sufficient to induce electrical excitability in PC12 cells due to induction of the peripheral nerve type 1 (PN1) sodium channel gene. We have investigated the mechanism for this triggered signaling pathway by NGF in PC12 cells. Mutation of TrkA at key autophosphorylation sites indicates an essential role for the phospholipase C-gamma (PLC-gamma) binding site, but not the Shc binding site, for NGF-triggered induction of PN1. In concordance with results with Trk mutants, drug-mediated inhibition of PLC-gamma activity also blocks PN1 induction by NGF. Examination of the kinetics of TrkA autophosphorylation indicates that triggered signaling does not result from sustained activation and autophosphorylation of the TrkA receptor kinase, whose phosphorylation state declines rapidly after NGF removal. Rather, TrkA triggers an unexpectedly prolonged phosphorylation and activation of PLC-gamma signaling that is sustained for up to 2 h. Prevention of the elevation of intracellular Ca2+ levels using BAPTA-AM results in a block of PN1 induction by NGF. Sustained signaling by PLC-gamma provides a means for differential neuronal gene induction after transient exposure to NGF.

Fibroblast growth factor receptor 3 induces gene expression primarily through Ras-independent signal transduction pathways

Fibroblast growth factor receptors (FGFR) are widely expressed in many tissues and cell types, and the temporal expression of these receptors and their ligands play important roles in the control of development. There are four FGFR family members, FGFR-1-4, and understanding the ability of these receptors to transduce signals is central to understanding how they function in controlling differentiation and development. We have utilized signal transduction by FGF-1 in PC12 cells to compare the ability of FGFR-1 and FGFR-3 to elicit the neuronal phenotype. In PC12 cells FGFR-1 is much more potent in the induction of neurite outgrowth than FGFR-3. This correlated with the ability of FGFR-1 to induce robust and sustained activation of the Ras-dependent mitogen-activated protein kinase pathways. In contrast, FGFR-3 could not induce strong sustained Ras-dependent signals. In this study, we analyzed the ability of FGFR-3 to induce the expression of sodium channels, peripherin, and Thy-1 in PC12 cells because all three of these proteins are known to be induced via Ras-independent pathways. We determined that FGFR-3 was capable of inducing several Ras-independent gene expression pathways important to the neuronal phenotype to a level equivalent of that induced by FGFR-1. Thus, FGFR-3 elicits phenotypic changes primarily though activation of Ras-independent pathways in the absence of robust Ras-dependent signals.

Identification of the cytoplasmic regions of fibroblast growth factor (FGF) receptor 1 which play important roles in induction of neurite outgrowth in PC12 cells by FGF-1

Fibroblast growth factor 1 (FGF-1) induces neurite outgrowth in PC12 cells. Recently, we have shown that the FGF receptor 1 (FGFR-1) is much more potent than FGFR-3 in induction of neurite outgrowth. To identify the cytoplasmic regions of FGFR-1 that are responsible for the induction of neurite outgrowth in PC12 cells, we took advantage of this difference and prepared receptor chimeras containing different regions of the FGFR-1 introduced into the FGFR-3 protein. The chimeric receptors were introduced into FGF-nonresponsive variant PC12 cells (fnr-PC12 cells), and their ability to mediate FGF-stimulated neurite outgrowth of the cells was assessed. The juxtamembrane (JM) and carboxy-terminal (COOH) regions of FGFR-1 were identified as conferring robust and moderate abilities, respectively, for induction of neurite outgrowth to FGFR-3. Analysis of FGF-stimulated activation of signal transduction revealed that the JM region of FGFR-1 conferred strong and sustained tyrosine phosphorylation of several cellular proteins and activation of MAP kinase. The SNT/FRS2 protein was demonstrated to be one of the cellular substrates preferentially phosphorylated by chimeras containing the JM domain of FGFR-1. SNT/FRS2 links FGF signaling to the MAP kinase pathway. Thus, the ability of FGFR-1 JM domain chimeras to induce strong sustained phosphorylation of this protein would explain the ability of these chimeras to activate MAP kinase and hence neurite outgrowth. The role of the COOH region of FGFR-1 in induction of neurite outgrowth involved the tyrosine residue at amino acid position 764, a site required for phospholipase C gamma binding and activation, whereas the JM region functioned primarily through a non-phosphotyrosine-dependent mechanism. In contrast, assessment of the chimeras in the pre-B lymphoid cell line BaF3 for FGF-1-induced mitogenesis revealed that the JM region did not play a role in this cell type. These data indicate that FGFR signaling can be regulated at the level of intracellular interactions and that signaling pathways for neurite outgrowth and mitogenesis use different regions of the FGFR.

Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons

Membrane excitability in different tissues is due, in large part, to the selective expression of distinct genes encoding the voltage-dependent sodium channel. Although the predominant sodium channels in brain, skeletal muscle, and cardiac muscle have been identified, the major sodium channel types responsible for excitability within the peripheral nervous system have remained elusive. We now describe the deduced primary structure of a sodium channel, peripheral nerve type 1 (PN1), which is expressed at high levels throughout the peripheral nervous system and is targeted to nerve terminals of cultured dorsal root ganglion neurons. Studies using cultured PC12 cells indicate that both expression and targeting of PN1 is induced by treatment of the cells with nerve growth factor. The preferential localization suggests that the PN1 sodium channel plays a specific role in nerve excitability.

Activation of codependent transcription factors is required for transcriptional induction of the vgf gene by nerve growth factor and Ras

Nerve growth factor (NGF) treatment of PC12 cells leads to the elaboration of a neuronal phenotype, including the induction of neuronally expressed genes such as vgf. To study vgf transcription, we have created chimeric vgf/beta-globin genes in which vgf promoter sequences drive the expression of the beta-globin reporter gene or of a chimeric beta-globin gene fused to 3' untranslated vgf gene sequences. We have found that the level of inducibility of the latter construct by NGF resembles that of the endogenous vgf gene. Using transient transfection of the chimeric reporter genes into PC12 cells, into PC12 subclones expressing activated or dominantly interfering mutant Ras proteins, and into PC12 variants expressing specific NGF receptor/Trk mutants, we show that transcriptional regulation of the vgf promoter by NGF is mediated through a Ras-dependent signaling pathway. By mutational analysis of the vgf promoter, we have identified three promoter elements involved in mediating transcriptional induction by NGF and Ras. In addition to the cyclic AMP-responsive element (CRE), which binds to ATF-1, ATF-2, and CRE-binding protein in PC12 nuclear extracts, a novel CCAAT element and its binding proteins were identified, which, like the CRE, is necessary but not sufficient for the Ras-dependent induction of the vgf gene by NGF. We also identify a G(S)G element unusually located between the TATA box and transcriptional start site, which binds the NGF- and Ras-induced transcription factor, NGFI-A, and amplifies the transcriptional response. Integrating data from studies of vgf promoter regulation and NGF signal transduction, we present a model for vgf gene induction in which transcriptional activation is achieved through the persistent, direct activation of multiple interacting transcription factors binding to CRE and CCAAT elements, coordinated with the delayed transcription factor action at a G(S)G element resulting from the induced expression of NGFI-A.

The fibroblast growth factor receptor-1 is necessary for the induction of neurite outgrowth in PC12 cells by aFGF

The PC12 subclone, fnr-PC12 cells, is defective in neurite outgrowth in response to acidic fibroblast growth factor (aFGF); however, its response to nerve growth factor (NGF) is normal. Examination of the expression of FGF receptors (FGFRs) revealed that although PC12 cells express FGFR-1, -3, and -4, fnr-PC12 cells have a reduced level of expression of FGFR-1 but not FGFR-3 and -4. Transfection of FGFR-1 into fnr-PC12 cells efficiently restored aFGF-induced neurite outgrowth, whereas transfection of FGFR-3 was much less efficient. Transfection of a chimeric receptor consisting of the extracellular domain of FGFR-3 fused to the transmembrane and intracellular domain of FGFR-1, termed FR31b, efficiently restored aFGF-induced neurite outgrowth. This demonstrates that the difference between these two receptors in their ability to induce neurite outgrowth is attributable to differences in the signaling capacity of their cytoplasmic domains. Activation of the chimeric receptor by aFGF induced a stronger and more persistent increase in the tyrosine phosphorylation of cellular proteins than did activation of FGFR-3 alone. In particular, the activation of MAP kinase by FR31b was more persistent than when activated by FGFR-3. This difference in signaling potential of FGFR-1 and -3 in fnr-PC12 cells may account for the difference in the potential for induction of neurite outgrowth. These results demonstrate that FGF-induced neurite outgrowth in PC12 cells occurs mainly via FGFR-1 and not via the other FGFRs expressed in these cells.

Calcium influx induces neurite growth through a Src-Ras signaling cassette

We find that calcium influx through voltage-dependent calcium channels causes extensive neurite outgrowth in PC12 cells. The calcium signal transduction pathway promoting neurite outgrowth causes the rapid activation of protein tyrosine kinases, which include Src. Protein tyrosine phosphorylation results in the formation of an Shc/Grb2 complex, leading to Ras activation, MAP kinase activation, and the subsequent induction of the immediate early gene NGFI-A. Protein tyrosine phosphorylation, gene induction, and neurite outgrowth are inhibited by the expression of dominant negative forms of both Src and Ras, indicating a requirement for both proto-oncoproteins in calcium signaling. Our results suggest that a signaling cassette which includes Src and Ras is likely to underlie a broad range of calcium of actions in the nervous system.

A single pulse of nerve growth factor triggers long-term neuronal excitability through sodium channel gene induction

The continuous presence of nerve growth factor (NGF) is thought to be required for the elaboration of neuronal-like traits in PC12 cells. Surprisingly, we find that a 1 min exposure to NGF is sufficient to engage a longer-term genetic program leading to the acquisition of membrane excitability. Whereas continuous exposure to NGF causes the induction of a family of sodium channels, the effect of a brief exposure is to induce selectively expression of the peripheral nerve-type sodium channel gene PN1, through a distinct signaling pathway requiring immediate-early genes. A 1 min exposure of PC12 cells to interferon-gamma also causes PN1 gene induction, suggesting that the "triggered" NGF and interferon-gamma signaling pathways share common molecular intermediates.

Nerve growth factor regulates the abundance and distribution of K+ channels in PC12 cells

We examined the effect of nerve growth factor (NGF) treatment on expression of a neuronal delayed rectifler K+ channel subtype, Kv2.1 (drk1), in PC12 cells. Anti-Kv2.1 antibodies recognized a single polypeptide population of M(r) = 132 kD in PC12 cell membranes, distinct from the more heterogeneous population found in adult rat brain. In response to NGF treatment, levels of Kv2.1 polypeptide in PC12 membranes increased fourfold. This increase in polypeptide levels could be seen within 12 h, and elevated levels were maintained for at least 6 d of continuous NGF treatment. RNase protection assays indicate that this increase in Kv2.1 protein occurs without an increase in steady state levels of Kv2.1 mRNA following NGF treatment. Immunofluorescent localization of the Kv2.1 polypeptide revealed plasma membrane-associated staining of cell bodies in both untreated and NGF-treated PC12 cells. In undifferentiated cells, intense staining is seen at sites of cell-cell and cell-substratum contact. In differentiated cells the most intense Kv2.1 staining is observed in neuritic growth cones. These studies show that in PC12 cells both the abundance and distribution of the Kv2.1 k+ channel are regulated by NGF, and suggest that PC12 cells provide a model for the selective expression of Kv2.1 in neuritic endings.

Neuronal growth factor regulation of two different sodium channel types through distinct signal transduction pathways

Neuronal growth factors regulate the expression of voltage-activated sodium current in differentiating sympathetic neurons and PC12 cells. We show that, in PC12 cells, the NGF- and FGF-induced sodium current results from increased expression of two distinct sodium channel types. Sodium current results from the rapid induction of a novel sodium channel transcript, also found in peripheral neurons, and from the long term induction of brain type II/IIA mRNA. Expression of the type II/IIA sodium channel requires activation of the cyclic AMP-dependent protein kinase (A-kinase), whereas induction of the peripheral neuron type sodium channel occurs through an A-kinase-independent signal transduction pathway. These findings suggest that the two sodium channel types act in concert to ensure the generation of action potentials during neuronal differentiation.

The cytoplasmic raf oncogene induces a neuronal phenotype in PC12 cells: a potential role for cellular raf kinases in neuronal growth factor signal transduction

The neuron-like differentiation of PC12 cells is induced by nerve growth factor (NGF) through stimulation of a membrane-bound protooncoprotein signaling pathway containing the NGF receptor Trk, the tyrosine kinase Src, and the GTP-binding protein Ras. The Raf-1 and B-raf protooncogenes encode cytoplasmic serine/threonine kinases that are stimulated by NGF in a Ras-dependent manner. To investigate the possible roles of cytoplasmic Raf kinases in eliciting neuronal differentiation, we have expressed the activated Raf-1 oncogene in PC12 cells. Expression of the raf oncogene results in the elaboration of a neuron-like phenotype, including neurite growth and the induction of the NGF-responsive genes NGFI-A and transin. The actions of activated Raf-1 and NGF are not additive. Furthermore, activated Raf-1 oncoprotein can prime cells for transcription-independent neurite growth by NGF and can elicit rapid neurite growth from NGF-primed cells. Our data indicate that the pathways utilized by NGF and activated raf to effect PC12 differentiation overlap and lead to the suggestion that cellular raf kinase activities play significant roles in transducing the differentiating signals of neuronal growth factors.

A branched signaling pathway for nerve growth factor is revealed by Src-, Ras-, and Raf-mediated gene inductions

A myriad of gene induction events underlie nerve growth factor (NGF)-induced differentiation of PC12 cells. To dissect the signal transduction pathways which lead to NGF actions, we have assessed the relative roles of NGF receptor, Src, Ras, and Raf activities in mediating specific gene inductions. We have used the PC12 cell line as well as sublines which inducibly express activated forms of either Src, Ras, or Raf or a dominant inhibitory form of Ras (p21N17 Ras) to study the expression of multiple NGF-inducible mRNAs. The NGF induction of NGFI-A, transin, and VGF mRNAs was mimicked by activated forms of Src, Ras, or Raf and was blocked by p21N17 Ras. The NGF induction of SCG10 mRNA was mimicked only by activated Src and Ras and was blocked by p21N17 Ras, while the induction of Thy-1 mRNA was mimicked only by activated Src and was not blocked by p21N17 Ras. The NGF induction of mRNAs for two sodium channel types was neither mimicked by any activated oncoprotein nor blocked by p21N17 Ras. From these and previous results, we suggest a model in which a linear order of NGF receptor, Src, Ras, and Raf activities is used by NGF to elicit gene inductions. These signaling components define branchpoints in the pathway to specific gene induction events, providing a mechanism for generating a host of diverse NGF actions.

Signal transduction pathways in neuronal differentiation

New insights into the signal transduction pathways for neuronal growth factors and cell adhesion molecules are affording us a better understanding of the intracellular mechanisms for neuronal differentiation, and of the ways in which the various signals are integrated during this process.

Rapid fibroblast growth factor-induced increases in protein phosphorylation and ornithine decarboxylase activity: regulation by heparin and comparison to nerve growth factor-induced increases

Fibroblast growth factors (FGFs), like nerve growth factor (NGF), induce morphological differentiation of PC12 cells. This activity of FGF is regulated by glycosaminoglycans. To further understand the mechanisms of FGF and glycosaminoglycan actions in PC12 cells, we studied the regulation of protein phosphorylation and ornithine decarboxylase (ODC) activity by FGF in the presence and absence of heparin. As with NGF, aFGF and bFGF increased the incorporation of radioactive phosphate into the protein tyrosine hydroxylase (TH). The increase in TH phosphorylation was localized to the tryptic peptide, T3. Both T3 and T1 phosphorylations occur in response to NGF, but there was no evidence that aFGF or bFGF stimulated the phosphorylation of the T1 peptide. This result suggests differential regulation of second messenger systems by NGF and FGF in PC12 cells. Heparin, at a concentration that potentiated aFGF-induced neurite outgrowth 100-fold (100 micrograms/ml), did not alter the ability of aFGF to increase S6 phosphorylation or ODC activity. One milligram per milliliter of heparin, a concentration that inhibited bFGF-induced neurite outgrowth, also inhibited bFGF-induced increases in S6 phosphorylation and ODC activity. These observations suggest (i) that acidic and basic FGF activate a protein kinase, possibly protein kinase C, resulting in the phosphorylation of peptide T3 of TH; (ii) that the FGFs and NGF share some but not all second messenger systems; (iii) that heparin potentiates aFGF actions and inhibits bFGF actions in PC12 cells via distinct mechanisms; (iv) that heparin does not potentiate the neurite outgrowth promoting activity of aFGF by enhancing binding to its PC12 cell surface receptor; and (v) that heparin may coordinately regulate several activities of bFGF (induction of protein phosphorylation, ODC and neurite outgrowth) via a common mechanism, most likely by inhibiting the productive binding of bFGF to its PC12 cell surface receptor.

Ras is essential for nerve growth factor- and phorbol ester-induced tyrosine phosphorylation of MAP kinases

Treatment of PC12 cells with nerve growth factor (NGF) induces a rapid increase in tyrosine phosphorylation of multiple cellular proteins. Expression of a dominant inhibitory Ras mutant specifically blocked NGF- and TPA-induced tyrosine phosphorylation of two proteins of approximately 42 and 44 kd. Conversely, expression of an oncogenic variant of Ras induced tyrosine phosphorylation of the same 42 and 44 kd proteins. The 44 kd protein was immunoprecipitated with an antibody directed against extracellular signal-regulated kinase 1/mitogen-activated protein kinase (MAPK) and the 42 kd protein comigrated with a 42 kd MAPK, indicating that at least one and probably both Ras-regulated phosphoproteins are MAPKs. In addition, MAPK activation, as measured by in vitro phosphorylation of myelin basic protein, was also regulated by Ras. Ras was not required for NGF-induced activation of Trk or tyrosine phosphorylation of PLC-gamma 1. Thus, NGF-induced tyrosine phosphorylation occurs both prior to and following Ras action, and Ras plays a critical role in the NGF- and TPA-induced tyrosine phosphorylation of MAPKs.

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.

Induction of neurite outgrowth by v-src mimics critical aspects of nerve growth factor-induced differentiation

PC12 cells treated with nerve growth factor (NGF) or infected with Rous sarcoma virus differentiate into sympathetic, neuronlike cells. To compare the differentiation programs induced by NGF and v-src, we have established a PC12 cell line expressing a temperature-sensitive v-src protein. The v-src-expressing PC12 cell line was shown to elaborate neuritic processes in a temperature-inducible manner, indicating that the differentiation process was dependent on the activity of the v-src protein. Further characterization of this cell line, in comparison with NGF-treated PC12 cells, indicated that the events associated with neurite outgrowth induced by these two agents shared features but could be distinguished by others. Both NGF- and v-src-induced neurite outgrowths were reversible. In addition, NGF and v-src could prime PC12 cells for NGF-induced neurite outgrowth, and representative early and late NGF-responsive genes were also induced by v-src. However, unlike NGF-induced neurite growth, v-src-induced neurite outgrowth was not blocked at high cell density. A comparison of phosphotyrosine containing-protein profiles showed that v-src and NGF each increase tyrosine phosphorylation of multiple cellular proteins. There was overlap in substrates; however, both NGF-specific and v-src-specific tyrosine phosphorylations were observed. One protein which was found to be phosphorylated in both the NGF- and v-src-induced PC12 cells was phospholipase C-gamma 1. Taken together, these results suggest that v-src's ability to function as an inducing agent may be a consequence of its ability to mimic critical aspects of the NGF differentiation program and raise the possibility that Src-like tyrosine kinases are involved in mediating some of the events triggered by NGF.

Changes in the phosphorylation and distribution of vinculin during nerve growth factor induced neurite outgrowth

The mechanism of neurite initiation and elongation was studied using nerve growth factor (NGF) treatment of PC12 cells. The distribution of focal adhesion sites and of the cytoskeletal protein vinculin was determined in large, fused, multinucleated PC12 cells. In the absence of NGF, focal adhesion sites as seen by interference reflection microscopy were restricted to the cell periphery in a regular distribution. Vinculin assemblies (foci), observed by indirect immunofluorescence microscopy using affinity purified anti-vinculin antibodies, were restricted to the cell periphery at focal adhesion sites. Within 4 hr after NGF treatment of the cells, the distribution of both vinculin and focal adhesion sites began to change. Focal adhesion sites became restricted to discrete protruding portions of the cell periphery. Larger, brighter vinculin foci appeared at the tips of the cell margin extensions, concomitant with the loss of foci at locations between the protrusions. As neurites elongated focal adhesion sites and vinculin foci remained with the tips of the growth cone extensions. Both focal adhesion sites and vinculin foci were rarely seen in the perikarya of cells with elongating neurites, and these were always confined to extended portions of the cell body margin. Occasionally, vinculin foci could be seen at the proximal portion of the neurite, at bending elbows, and at discrete expansions along the length. By immunoprecipitation of vinculin from 32P-labeled cells, vinculin phosphorylation was found to be increased within 1 hr of NGF treatment. The role of vinculin phosphorylation and assembly in the formation and directional elongation of neuritic processes in response to NGF is discussed.

Nerve growth factor action is mediated by cyclic AMP- and Ca+2/phospholipid-dependent protein kinases

Nerve growth factor (NGF) mediates the phosphorylation of tyrosine hydroxylase in PC12 cells on two distinct peptide fragments, separable by two-dimensional tryptic phosphopeptide mapping (phosphopeptides T1 and T3). Phorbol diester derivatives capable of activating Ca+2/phospholipid-dependent protein kinase (C-kinase) cause a specific phosphorylation of peptide T3 in a dose-dependent, saturable manner. Derivatives of the endogenous C-kinase activator diacylglycerol, also cause the phosphorylation of tyrosine hydroxylase on peptide T3. The C-kinase inhibitors chlorpromazine and trifluoperazine inhibit the phorbol diester stimulated phosphorylation of site T3 in a dose-dependent manner. These agents inhibit the phosphorylation of T3 in response to NGF, but have no effect on NGF's ability to cause T1 phosphorylation. In a PC12 mutant deficient in cAMP-dependent protein kinase activity, NGF mediates the phosphorylation of tyrosine hydroxylase on peptide T3 but not on T1. We conclude that NGF mediates the activation of both the cAMP-dependent protein kinase and the C-kinase to phosphorylate substrate proteins. These kinases can act independently to phosphorylate tyrosine hydroxylase, each at a different site, and each of which results in the enzyme activation. A molecular framework is thus provided for events underlying NGF action.

Inhibition of growth factor-induced differentiation of PC12 cells by microinjection of antibody to ras p21

The protein products (p21) of the ras cellular proto-oncogenes are thought to transduce membrane signals necessary for the induction of cell division. However, there is uncertainty as to the precise role of ras p21 in mediating ligand-membrane receptor signals leading to cell differentiation. Treatment of rat phaeochromocytoma cells (PC12) with nerve growth factor (NGF) results in the induction of a number of phenotypic characteristics of sympathetic neurones, including cessation of cell division and outgrowth of neuronal processes (neurites). Here we report that microinjection of antibody to ras p21 into PC12 cells inhibited neurite formation and resulted in temporary regression of partially extended neurites, an effect which was observed up to 36 h after initiation of NGF treatment. Neurite formation induced by cyclic AMP was unaffected by injection of anti-p21 antibody. These results indicate that p21 is involved in the initiation phase of NGF-induced neurite formation in PC12 cells and has a role in hormone-mediated cellular responses distinct from cell proliferation.

Nerve growth factor and other agents mediate phosphorylation and activation of tyrosine hydroxylase. A convergence of multiple kinase activities

A rapid phosphorylation of tyrosine hydroxylase occurs in the PC12 nerve-like clonal cell line in response to nerve growth factor (NGF), epidermal growth factor (EGF), dibutyryl-cAMP, cholera toxin, phorbol- 12-myristate-13-acetate (PMA), or potassium depolarization in the presence of calcium ions. Complete tryptic digestion and two-dimensional peptide mapping reveals four available sites of phosphorylation in the enzyme. Phosphoamino acid analysis demonstrates that serine is the amino acid residue phosphorylated in each peptide. Specific phosphorylation of each of the four sites is achieved by different subsets of the above agents. One peptide site is phosphorylated in response to EGF alone. A second site is phosphorylated only in response to NGF, cholera toxin or dibutyryl-cAMP. A third site is phosphorylated only in response to potassium depolarization and requires the presence of extracellular Ca2+. The fourth site is the only site phosphorylated in response to PMA. These data indicate that at least 4 distinct kinase systems can act to phosphorylate tyrosine hydroxylase in PC12 cells. The PMA-stimulated peptide site is also phosphorylated in response to every one of the other agents. Further proteolytic digestions and phosphopeptide mapping of this common peptide, using Staphylococcus V8 protease and thermolysin, did not generate different phosphopeptides resulting from the different agents. These data suggest that the phosphorylation of this common peptide in response to all of the agents may be mediated by a common kinase, and, hence, that tyrosine hydroxylase phosphorylation by some agents may be mediated by two kinases. Although phosphopeptide maps of tyrosine hydroxylase resulting from cAMP elevation or NGF are qualitatively similar, quantitative differences exist, suggesting differential regulation of the same kinases by these agents. Tyrosine hydroxylase was found to be activated 2--4-fold in response to each phosphorylating agent. Thus, NGF and EGF present novel, natural means of regulating the activation state of tyrosine hydroxylase in responsive neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Nerve growth factor mediates phosphorylation of specific proteins

Nerve growth factor (NGF), epidermal growth factor (EGF), insulin, cholera toxin (CT) and cAMP all stimulate the phosphorylation of proteins in the PC12 nerve-like cell line. NGF, CT and cAMP enhance phosphorylation of the same set of proteins including tyrosine hydroxylase, ribosomal protein S6, histones H1 and H3, and the nonhistone chromosomal and cytoplasmic high mobility group (HMG) 17 protein, and reduce phosphorylation of H2A. EGF but not insulin enhances the phosphorylation of tyrosine hydroxylase. Insulin but not EGF enhances the phosphorylation of histone H3 and decreases the phosphorylation of H2A. EGFD and insulin each enhance phosphorylations of both ribosomal protein S6 and histone H1, but neither hormone induces phosphorylation of HMG 17. The extent of these effects depends upon the ligand concentration and is half-maximal at physiological concentrations of the hormones (beta-NGF, 2 ng/ml; EGF, 1 ng/ml. insulin, 0.5 microunits/ml). Maximal effects of NGF are seen within 15 min and persist even after 3 days of culture in the presence of NGF. When phosphorylation of ribosomal protein S6 is maximally stimulated by NGF, no further stimulation can be achieved by adding saturating quantities of either cAMP or CT. However, simultaneous addition of saturating quantities of NGF and either EGF or insulin results in an enhancement of phosphorylation that is equal to the sum of that achieved when the two ligands are added separately. These results suggest that the enhanced phosphorylation of S6 achieved by NGF or cAMP occurs through a common mechanism which differs from those which mediate EGF or insulin-enhanced phosphorylation. The data also provide strong evidence that the action of NGF included protein phosphorylation mediated by cAMP-dependent protein kinase. The phosphorylation of each of these proteins in response to NGF may play an important role in NGF action.

Secretion and membrane localization of proteins in Escherichia coli

The envelope of Escherichia coli consists of two distinct membranes, the outer membrane and the cytoplasmic membrane. The space between the two membranes is called the periplasmic space, and each fraction contains its own specific proteins. In this review, it is discussed how proteins are localized in their final locations in the envelope. Proteins localized in the outer membrane and the periplasmic space as well as transmembranous proteins in the cytoplasmic membranes appear to be produced from their precursors which have peptide extensions of about 20 amino acid residues at the amino terminal ends. General features for the peptide extension are deduced from the known sequences of the peptide extensions, and, based on their known properties, a hypothesis (loop model) is proposed to explain the possible functions of the peptide extension during the mechanism of secretion across the cytoplasmic membrane.

A new form of structural lipoprotein of outer membrane of Escherichia coli

Among the membrane proteins synthesized in toluene-treated cells of Escherichia coli were two distinct membrane proteins of different molecular weights, which were cross-reactive with antiserum against a structural lipoprotein of the outer membrane. One was thought to be the known membrane lipoprotein since it migrated to the same position as that of the lipoprotein (Mr = 7,200) in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, the other protein migrated slower than the lipoprotein. No protein corresponding to the slower-migrating species was detected in the membrane proteins synthesized in vivo. The apparent molecular weight of the protein at the new peak was estimated to be between 10,000 and 15,000. Both the new protein and the lipoprotein were found to be synthesized from stable mRNA(s) in the toluene-treated cells. The synthesis of the new protein as well as the lipoprotein was sensitive to chloramphenicol, indicating that both proteins were synthesized on ribosomes. Peptides mapping of the new protein revealed the same COOH-terminal sequence as in the lipoprotein. This indicates that the new protein has an extra sequence at the NH2-terminal end. This hypothesis is supported by the finding that the NH2 terminus of the new lipoprotein is methionine, while that of the lipoprotein is a substituted cysteine. From double label experiments with each of 17 different amino acids and arginine, the amino acid composition of the extra region was deduced. The new protein was found to contain at least 18 to 19 extra amino acid residues over the lipoprotein, if it is assumed that the new protein has no extra arginine residues. It was found that 4 out of the 5 amino acids which were deficient in the lipoprotein (phenylalanine, tryptophan, proline, and histidine) were also deficient in the new protein, but the fifth one, glycine, was present in the new protein. From these results, it seems possible that this new form of the lipoprotine is a precursor of the lipoprotein (prolipoprotein) in the process of biosynthesis and assembly of the lipoprotein in the outer membrane.

Amino acid sequence for the peptide extension on the prolipoprotein of the Escherichia coli outer membrane

The messenger RNA for the lipoprotein of the E. coli outer membrane was found to code for a putative precursor, prolipoprotein, which has 20 additional amino acid residues extending from the amino terminus of the lipoprotein. Using the prolipoprotein synthesized in an E. coli cell-free system directed by purified messenger RNA for the lipoprotein, the complete amino acid sequence of the amino-terminal precursor region was determined to be as follows: (formula: see text). It was also found that the prolipoprotein that accumulates in toluene-treated cells has the same sequence. The significance of the amino acid sequence is discussed in terms of the mechanism of biosynthesis and assembly of the lipoprotein in the E. coli outer membrane.

Puromycin-resistant biosynthesis of a specific outer-membrane lipoprotein of Escherichia coli

The reported puromycin resistance of the in vivo biosynthesis of a specific outer-membrane lipoprotein of Escherichia coli was further investigated. The biosynthetic machinery making the lipoprotein was made more accessible to puromycin by disruption of the cell structure using ethylenediaminetetracetate or toluene, and finally in an in vitro protein biosynthesis system using polyribosomes. Puromycin sensitivity of overall protein synthesis increased by about 10-fold for each method of disruption of the cell structure; 50% inhibitions were obtained at 330, 35, 2.7, and 0.22 mug of puromycin per ml for intact cells, ethylenediaminetetraacetate-treated cells, toluene-treated cells, and the polyribosome system, respectively. However, the lipoprotein biosynthesis remained more resistant to puromycin than the biosynthesis of other proteins in all systems tested. These results strongly suggest that puromycin resistance of the lipoprotein biosynthesis is due to an intrinsic property of the lipoprotein biosynthetic machinery.

Lysozymes from bacteriophages T3 and T5

Lysozymes produced in host cells infected with bacteriophages T3 and T5 were found to have the same enzymatic specificity toward the peptidoglycan from Escherichia coli as T7 phage lysozyme, which has been shown to be an N-acetylmuramyl-L-alanine amidase.

Existence of a free form of a specific membrane lipoprotein in gram-negative bacteria

The existence of a free form of a specific lipoprotein of molecular weight 7,200 was examined in the envelopes of several gram-negative bacteria. When the envelope proteins were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, distinct peaks were observed in Salmonella typhimurium, Serratia marcescens, and Pseudomonas aeruginosa at the same position as the free form of the lipoprotein of Escherichia coli. However, the peak was not observed in Proteus mirabilis. The protein at the peak in S. typhimurium was shown to contain little or no histidine as expected from the amino acid composition of the lipoprotein. Furthermore, antiserum against the highly purified lipoprotein from E. coli was shown to react with the proteins from S. typhimurium and S. marcescens and to form the specific immunoprecipitates. In contrast, the protein from P. aeruginosa did not react with the antiserum at all. Thus, it is concluded that S. typhimurium and S. marcescens have the free form of the lipoprotein in their envelopes as does E. coli. P. aeruginosa contains a protein of the same size as the lipoprotein, but it is not certain whether the protein is the same structural protein as the lipoprotein from E. coli. P. mirabilis may not have any free form of the lipoprotein, may have it in a very small amount, or may have a lipoprotein of different molecular weight serving the same function.