Differentiation of PC12 cells with v-src: comparison with nerve growth factor

Comprehensive Perspectives on Experimental Models for Parkinson's Disease

Parkinson’s disease (PD) ranks second among the most common neurodegenerative diseases, characterized by progressive and selective loss of dopaminergic neurons. Various cross-species preclinical models, including cellular models and animal models, have been established through the decades to study the etiology and mechanism of the disease from cell lines to nonhuman primates. These models are aimed at developing effective therapeutic strategies for the disease. None of the current models can replicate all major pathological and clinical phenotypes of PD. Selection of the model for PD largely relies on our interest of study. In this review, we systemically summarized experimental PD models, including cellular and animal models used in preclinical studies, to understand the pathogenesis of PD. This review is intended to provide current knowledge about the application of these different PD models, with focus on their strengths and limitations with respect to their contributions to the assessment of the molecular pathobiology of PD and identification of the therapeutic strategies for the disease.

Gynura bicolor aqueous extract attenuated H2O2 induced injury in PC12 cells

BACKGROUND

Protective effects of Gynura bicolor aqueous extract (GAE) at three concentrations upon nerve growth factor (NGF) differentiated-PC12 cells against H₂O₂ induced injury were examined.

METHODS

NGF differentiated-PC12 cells were treated with GAE at 0.25%, 0.5% or 1%. 100 μM H₂O₂ was used to treat cells with GAE pre-treatments. After incubating at 37 °C for 12 hr, experimental analyses were processed.

RESULTS

H₂O₂ exposure decreased cell viability, increased plasma membrane damage, suppressed Bcl-2 mRNA expression and enhanced Bax mRNA expression. GAE pre-treatments reversed these changes. H₂O₂ exposure reduced mitochondrial membrane potential, lowered Na⁺-K⁺-ATPase activity, and increased DNA fragmentation and Ca²⁺ release. GAE pre-treatments attenuated these alterations. H₂O₂ stimulated the production of reactive oxygen species (ROS), interleukin (IL)-1beta, IL-6 and tumor necrosis factor-alpha, lowered glutathione content, and reduced glutathione peroxidase (GPX) and catalase activities. GAE pretreatments maintained GPX and catalase activities; and concentration-dependently diminished the generation of ROS and inflammatory cytokines. H₂O₂ enhanced mRNA expression of nuclear factor kappa (NF-κ) B and p38. GAE pre-treatments decreased mRNA expression of NF-κB and p38.

CONCLUSION

These findings suggested that GAE might be a potent neuronal protective agent.

Maslinic Acid Protected PC12 Cells Differentiated by Nerve Growth Factor against β-Amyloid-Induced Apoptosis

β-Amyloid peptide (Abeta) was used to induce apoptosis in PC12 cells differentiated by nerve growth factor, and the protective activities of maslinic acid (MA) at 2-16 μM were examined. Abeta treatment lowered Bcl-2 expression, raised Bax expression, and decreased cell viability. MA pretreatments decreased Bax expression, raised the Bcl-2/Bax ratio, and increased cell viability. MA pretreatments retained glutathione content and decreased subsequent Abeta-induced release of reactive oxygen species, tumor necrosis factor-α, interleukin (IL)-1β, and IL-6. Abeta treatment up-regulated protein expression of p47(phox), gp91(phox), mitogen-activated protein kinase, advanced glycation end product receptor (RAGE), and nuclear factor-κ B (NF-κB). MA pretreatments at 2-16 μM suppressed the expression of proteins including gp91(phox), p47(phox), p-p38, and NF-κB p65, at 4-16 μM down-regulated RAGE and NF-κB p50 expression, and at 8 and 16 μM reduced p-ERK1/2 expression. These novel findings suggest that maslinic acid is a potent compound against Abeta-induced cytotoxicity.

Protosappanin B protects PC12 cells against oxygen-glucose deprivation-induced neuronal death by maintaining mitochondrial homeostasis via induction of ubiquitin-dependent p53 protein degradation

Protosappanin B (PTB) is a bioactive dibenzoxocin derivative isolated from Caesalpinia sappan L. Here, we investigated the neuroprotective effects and the potential mechanisms of PTB on oxygen-glucose deprivation (OGD)-injured PC12 cells. Results showed that PTB significantly increased cell viability, inhibited cell apoptosis and up-regulated the expression of growth-associated protein 43 (a marker of neural outgrowth). Moreover, our study revealed that PTB effectively maintained mitochondrial homeostasis by up-regulation of mitochondrial membrane potential (MMP), inhibition of cytochrome c release from mitochondria and inactivation of mitochondrial caspase-9/3 apoptosis pathway. Further study showed that PTB significantly promoted cytoplasmic component degradation of p53 protein, a key negative regulator for mitochondrial function, resulting in a release of Bcl-2 from p53-Bcl-2 complex and an enhancing translocation of Bcl-2 to mitochondrial outer membrane. Finally, we found the degradation of p53 protein was induced by PTB via activation of a MDM2-dependent ubiquitination process. Taken together, our findings provided a new viewpoint of neuronal protection strategy for anoxia and ischemic injury with natural small molecular dibenzoxocin derivative by activating ubiquitin-dependent p53 protein degradation as well as increasing mitochondrial function.

s-Ethyl cysteine and s-propyl cysteine alleviate beta-amyloid induced cytotoxicity in nerve growth factor differentiated PC12 cells

Beta-amyloid peptide (Abeta) was used to induce cytotoxicity in nerve growth factor differentiated PC12 cells, and the effects of s-ethyl cysteine (SEC) and s-propyl cysteine (SPC) on anti-inflammatory protection, DNA fragmentation, mitochondrial membrane potential (MMP), and activity of Na(+)-K(+)-ATPase and caspases were examined. Abeta treatment significantly decreased cell viability and MMP, and increased lactate dehydrogenase (LDH) activity and DNA fragmentation (P < 0.05). The pretreatments from SEC or SPC at 2.5, 5, and 10 microM significantly enhanced cell viability and MMP, and lowered LDH activity and DNA fragmentation (P < 0.05). Abeta treatment also significantly decreased Na(+)-K(+)-ATPase activity and enhanced the activity of caspase-3 and caspase-8 (P < 0.05); however, the pretreatments from SEC or SPC significantly attenuated Abeta-induced reduction in Na(+)-K(+)-ATPase activity and elevation in caspase-3 and caspase-8 activities (P < 0.05). Abeta treatment increased the protein production and mRNA expression of interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha (P < 0.05). The pretreatments from SEC at 10 microM or SPC at 2.5, 5, and 10 microM significantly suppressed mRNA expression and decreased the protein production of these cytokines. These results suggested that SEC and SPC were potent neuroprotective agents against Alzheimer's disease.

Antioxidative and anti-inflammatory neuroprotective effects of astaxanthin and canthaxanthin in nerve growth factor differentiated PC12 cells

Nerve growth factor differentiated PC12 cells were used to examine the antioxidative and anti-inflammatory effects of astaxanthin (AX) and canthaxanthin (CX). PC12 cells were pretreated with AX or CX at 10 or 20 muM, and followed by exposure of hydrogen peroxide (H(2)O(2)) or 1-methyl-4-phenylpyridinium ion (MPP(+)) to induce cell injury. H(2)O(2) or MPP(+) treatment significantly decreased cell viability, increased lactate dehydrogenase (LDH) release, enhanced DNA fragmentation, and lowered mitochondrial membrane potential (MMP) (P < 0.05). The pretreatments from AX or CX concentration-dependently alleviated H(2)O(2) or MPP(+)-induced cell death, LDH release, DNA fragmentation, and MMP reduction (P < 0.05). Either H(2)O(2) or MPP(+) treatment significantly increased malonyldialdehyde (MDA) and reactive oxygen species (ROS) formations, decreased glutathione content, and lowered glutathione peroxidase (GPX) and catalase activities (P < 0.05). The pretreatments from AX or CX significantly retained GPX and catalase activities, and decreased MDA and ROS formations (P < 0.05). H(2)O(2) or MPP(+) treatment significantly decreased Na(+)-K(+)-ATPase activity, elevated caspase-3 activity and levels of interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)-alpha (P < 0.05); and the pretreatments from these agents significantly restored Na(+)-K(+)-ATPase activity, suppressed caspase-3 activity and release of IL-1, IL-6, and TNF-alpha (P < 0.05). Based on the observed antioxidative and anti-inflammatory protection from AX and CX, these 2 compounds were potent agents against neurodegenerative disorder.

Nerve growth factor stimulates multisite tyrosine phosphorylation and activation of the atypical protein kinase C's via a src kinase pathway

Atypical protein kinase C (PKC) isoforms are required for nerve growth factor (NGF)-initiated differentiation of PC12 cells. In the present study, we report that PKC-iota becomes tyrosine phosphorylated in the membrane coincident with activation posttreatment with nerve growth factor. Tyrosine phosphorylation and activation of PKC-iota were inhibited in a dose-dependent manner by both PP2 and K252a, src and TrkA kinase inhibitors. Purified src was observed to phosphorylate and activate PKC-iota in vitro. In PC12 cells deficient in src kinase activity, both NGF-induced tyrosine phosphorylation and activation of PKC-iota were also diminished. Furthermore, we demonstrate activation of src by NGF along with formation of a signal complex including the TrkA receptor, src, and PKC-iota. Recruitment of PKC-iota into the complex was dependent on the tyrosine phosphorylation state of PKC-iota. The association of src and PKC-iota was constitutive but was enhanced by NGF treatment, with the src homology 3 domain interacting with a PXXP sequence within the regulatory domain of PKC-iota (amino acids 98 to 114). Altogether, these findings support a role for src in regulation of PKC-iota. Tyrosine 256, 271, and 325 were identified as major sites phosphorylated by src in the catalytic domain. Y256F and Y271F mutations did not alter src-induced activation of PKC-iota, whereas the Y325F mutation significantly reduced src-induced activation of PKC-iota. The functional relevance of these mutations was tested by determining the ability of each mutant to support TRAF6 activation of NF-kappaB, with significant impairment by the Y325F PKC-iota mutant. Moreover, when the Y352F mutant was expressed in PC12 cells, NGF's ability to promote survival in serum-free media was reduced. In summary, we have identified a novel mechanism for NGF-induced activation of atypical PKC involving tyrosine phosphorylation by c-Src.

Wild-type and mutant forms of v-src differentially alter neuronal migration and differentiation in vivo

The effects of three different forms of v-src on brain cell development were determined in vivo. Recombinant retroviral vectors encoding the marker lacZ (control) and either wild-type v-src or SH2 or SH3 domain-deleted forms of v-src (deltaSH2 or deltaSH3, respectively) were used to infect neuronal progenitor cells in the embryonic chicken midbrain (optic tectum; OT). Embryos were injected in the OT with retroviral concentrates on embryonic day (E) 3 and sacrificed at E6, E9, and later in development. Patterns of cell proliferation, migration, and differentiation of lacZ-marked clonal cell progeny were then analyzed. Relative to lacZ-only controls, cell clone size at E6 was significantly increased for v-src-, unchanged for deltaSH2-, and smaller for deltaSH3-injected embryos. At E9, deltaSH2 cell clones were significantly larger than controls, suggesting increased survival from normal programmed cell death. Radial neuronal migration was impaired for v-src and deltaSH3 clones, whereas tangential neuronal migration was enhanced along fiber tracts in v-src and deltaSH2 clones. Moreover, radial glial cell development and differentiation was hindered in v-src and deltaSH3 clones. These experiments demonstrate that ectopic v-src signaling alters proliferation, migration, survival, and differentiation of developing brain cells and suggest that src signaling pathways are involved in these developmental processes. Furthermore, certain effects of v-src on brain cells require specific src homology domains.

Identification of Src as a novel atypical protein kinase C-interacting protein

Atypical protein kinase C-zeta (PKC-zeta) participates in nerve growth factor (NGF) signaling and is required for NGF-induced differentiation of PC12 cells. The biological activity of PKC-zeta is likely mediated by interaction of PKC-zeta with specific proteins. Affinity column chromatography employing the PKC-zeta regulatory domain coupled to glutathione-agarose was used to search for proteins that bound PKC-zeta. Two proteins (59/60 kDa) were recovered from NGF-stimulated PC12 cell lysates that bound the matrix. Western blot analysis of pooled column fractions identified these proteins as tubulin and src, respectively. Using purified preparations of src and tubulin, PKC-zeta was shown to interact with both proteins using blot overlay. To demonstrate a functional interaction in vivo, PC12 cells expressing a temperature-sensitive v-src were shifted to the permissive temperature (37 degrees C), followed by immunoprecipitation. At the permissive temperature where src was active, PKC-zeta was tyrosine phosphorylated and coassociated with src in vivo; by comparison, at the nonpermissive temperature (40 degrees C) PKC-zeta was not tyrosine phosphorylated. Taken together, these findings support a novel role for the interaction of src and atypical PKC in vivo, which is dependent upon the activity of src and the tyrosine phosphorylation state of PKC-zeta.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Analysis of mutant platelet-derived growth factor receptors expressed in PC12 cells identifies signals governing sodium channel induction during neuronal differentiation

The mechanisms governing neuronal differentiation, including the signals underlying the induction of voltage-dependent sodium (Na+) channel expression by neurotrophic factors, which occurs independent of Ras activity, are not well understood. Therefore, Na+ channel induction was analyzed in sublines of PC12 cells stably expressing platelet-derived growth factor (PDGF) beta receptors with mutations that eliminate activation of specific signalling molecules. Mutations eliminating activation of phosphatidylinositol 3-kinase (PI3K), phospholipase C gamma (PLC gamma), the GTPase-activating protein (GAP), and Syp phosphatase failed to diminish the induction of type II Na+ channel alpha-subunit mRNA and functional Na+ channel expression by PDGF, as determined by RNase protection assays and whole-cell patch clamp recording. However, mutation of juxtamembrane tyrosines that bind members of the Src family of kinases upon receptor activation inhibited the induction of functional Na+ channels while leaving the induction of type II alpha-subunit mRNA intact. Mutation of juxtamembrane tyrosines in combination with mutations eliminating activation of PI3K, PLC gamma, GAP, and Syp abolished the induction of type II alpha-subunit mRNA, suggesting that at least partially redundant signaling mechanisms mediate this induction. The differential effects of the receptor mutations on Na+ channel expression did not reflect global changes in receptor signaling capabilities, as in all of the mutant receptors analyzed, the induction of c-fos and transin mRNAs still occurred. The results reveal an important role for the Src family in the induction of Na+ channel expression and highlight the multiplicity and combinatorial nature of the signaling mechanisms governing neuronal differentiation.

Overexpression of c-src and n-src in the developing Xenopus retina differentially impairs axonogenesis

To compare the roles of the nonreceptor tyrosine kinase c-src and its neuronal splice form n-src in developing neurons, Xenopus retinal precursors were transfected in vivo with c-src, n-src, or constitutively active mutants. Axonogenesis of retinal ganglion cells was markedly impaired by the expression of constitutively active c-src and only mildly affected by the expression of constitutively active n-src. This differential phenotype could not be accounted for by raised levels of intracellular tyrosine phosphorylation alone because the average anti-phosphotyrosine staining intensity of retinal neurons expressing mutant n-src was almost twofold greater than that of neurons expressing mutant c-src. The expression of either constitutively active isoform inhibited photoreceptor differentiation by 72% but did not influence other cell fates. These results suggest that c-src and n-src have both overlapping and distinct activities in differentiating retinal neurons.

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.

A 56,000 Mr phosphoseryl protein in PC12 cell lysates strongly associates with protein-A sepharose beads and was observed in immune complex kinase assays for PP60c-src

Using an immune complex kinase assay to measure pp60c-src kinase activity, we have identified a 56,000 Mr protein (p56) from PC12 cell lysates that co-purified with pp60c-src by strong association with protein-A sepharose beads. The p56 protein was strongly phosphorylated on serine but no tyrosine or threonine phosphorylation was evident. However, pp60c-src was strongly phosphorylated on tyrosine, weakly phosphorylated on serine with no observed threonine phosphorylation. P56 was not a proteolytic breakdown product of pp60c-src, since it was neither tyrosine phosphorylated nor was it recognized by anti-src antibody. P56 was also not recognised by other antibodies to 56kD signalling molecules such as p56lck. The identify of p56 awaits further investigation but its appearance in immunoprecipitates of pp60c-src using protein-A sepharose beads is of interest but complicates the the interpretation of results from immune complex kinase assays in PC12 cells.

NCAM-dependent neurite outgrowth is inhibited in neurons from Fyn-minus mice

Src-related nonreceptor protein tyrosine kinases in nerve growth cones (p59fyn, pp60c-src, and pp62c-yes) are potential intracellular signaling molecules for cell adhesion molecule-directed axonal growth. To determine whether src-related tyrosine kinases mediate NCAM-dependent neurite outgrowth, cultures of cerebellar and sensory neurons from fyn-, src-, and yes- minus mice were analyzed for neurite outgrowth on monolayers of NCAM140-transfected L fibroblasts. NCAM-dependent neurite outgrowth was selectively inhibited in cultures of cerebellar and dorsal root ganglion neurons from fyn-, but not src- or yes- mice. Neurite outgrowth by fyn-, src-, or yes- neurons on untransfected fibroblast monolayers was unaffected, indicating that these kinases do not contribute significantly to axon growth on at least some integrins or other adhesive substrates present on fibroblasts. This study demonstrates that p59fyn is an essential component of the NCAM signaling pathway leading to axonal growth.

Fibroblast growth factor-induced increases in calcium currents in the PC12 pheochromocytoma cell line are tyrosine phosphorylation dependent

The PC12 rat pheochromocytoma cell line is widely used to study neuronal differentiation by growth factors. In response to nerve growth factor (NGF) and basic fibroblast growth factor (bFGF), PC12 cells differentiate into sympathetic-like neurons and become electrically excitable. Using whole cell patch-clamp recording, with barium as a charge carrier, we looked at the effects of bFGF on calcium channel expression as reflected by changes in barium current amplitudes normalized to cell membrane area. Similar to the effect reported for NGF, we show that 7 day treatment with bFGF increased the barium current approximately 4-fold. The largest contributor to the increase in barium current with bFGF treatment is a 6-fold increase in the high threshold voltage activated omega-conotoxin sensitive barium current. Smaller increases in current produced by bFGF treatment of PC12 cells are observed for the dihydropyridine sensitive and dihydropyridine/conotoxin insensitive currents. The bFGF-induced increases in barium currents are dependent on tyrosine phosphorylation, since the effects of bFGF are blocked by genistein, a tyrosine kinase inhibitor. This system will ultimately be useful in understanding the signaling pathways that control calcium channel expression in response to growth factors.

Vanadate stimulates differentiation and neurite outgrowth in rat pheochromocytoma PC12 cells and neurite extension in human neuroblastoma SH-SY5Y cells

We show here that a protein tyrosine phosphatase inhibitor, sodium orthovanadate, induces rat pheochromocytoma cells to express neurites, a prominent morphological marker of neuronal phenotype. Vanadate-induced differentiation and neurite outgrowth in pheochromocytoma cells was not as extensive as that induced by the positive control employed, nerve growth factor. However, neurite outgrowth responses were comparable between nerve growth factor-treated pheochromocytoma cells and cells primed and then restimulated with vanadate. In the human neuroblastoma cell line, SH-SY5Y, a single exposure to vanadate induced neurite extension in this cell line equal to that initiated by nerve growth factor. In both cell lines vanadate treatment resulted in tyrosine phosphorylation of several high-molecular-weight proteins and using anti-phosphotyrosine antibodies, intense fluorescence was observed in the cell body and neurites of pheochromocytoma cells exposed to vanadate. Vanadate mediated differentiation and neurite outgrowth in pheochromocytoma cells could be ablated by the tyrosine kinase inhibitor erbastatin, whereas nerve growth factor-induced neurite outgrowth was only partially inhibited. In SH-SY5Y cells, erbstatin mediated partial inhibition of both vanadate and nerve growth factor-induced neurite elongation with similar kinetics. In contrast, K252b, a trk tyrosine kinase inhibitor, exhibited only a 30% reduction of neurite outgrowth in vanadate treated pheochromocytoma cells but an 80% reduction in nerve growth factor-treated cells. In SH-SY5Y cells, K252a did not have a statistically significant effect on neurite elongation induced by vanadate in contrast to a 60% reduction in nerve growth factor-treated cells. The membrane impermeable analogue K252b, had no effect on neurite elongation induced with either vanadate or nerve growth factor in these cells. The effects of vanadate were not mimicked by ouabain (0.1-50 microM) indicating that vanadate does not induce differentiation and/or neurite extension by inhibiting ion channel Na,K-ATPase, which is one of its other well-characterised inhibitory activities. Evidence for the selective action of vanadate on some but not all neuronal cell lines comes from the fact that it did not induce neurite extension in the human neuroblastoma cell line SK-N-MC. These data imply that vanadate-induced neurite outgrowth responses in pheochromocytoma and SH-SY5Y cells can be induced by the inhibition of tyrosine phosphatases and appears not to simply mimic nerve growth factor signals. The target(s) of vanadate action in the two cell lines are currently being sought.

Nerve growth factor regulates the subcellular localization of the nerve growth factor-inducible protein PC4 in PC12 cells

The immediate early gene (IEG) PC4, which encodes a protein related to gamma interferon, is activated at the onset of the neuronal differentiation induced by nerve growth factor (NGF) in PC12 cells. With an antibody raised to a bacterial beta gal-PC4 fusion protein, the PC4 protein is detected as an immunoreactive molecular species of 49 kDa, whose synthesis is rapidly induced by NGF in parallel with the induction of its mRNA. Immunofluorescence, electron microscopy and subfractionation studies indicate that the PC4 immunoreactivity is localized in the cytoplasm of PC12 cells, where it is increased transiently by NGF within 3 hr of treatment. In addition, the PC4 immunoreactivity presents an NGF-dependent pattern of intracellular localization. In fact, within 3 hr after addition of NGF, PC4 is also significantly expressed on the inner face of the plasma membrane, to which it is physically associated. After longer NGF treatment, PC4 disappears from the plasma membrane and appears in the nucleus, with reduced cytoplasmic expression. Localization in the nucleus is reversed by removal of NGF and closely parallels changes in the state of differentiation of the cell. The existence within the PC4 protein of a consensus sequence for the addition of myristic acid and of a putative sequence for the nuclear localization suggests possible mechanisms for the NGF-dependent redistribution. For an NGF-inducible IEG product, such growth factor-dependent localization of PC4 is a novel type of regulation in the pathways from the NGF receptor to the adjacent membrane proteins and to the nucleus.

Impaired neurite outgrowth of src-minus cerebellar neurons on the cell adhesion molecule L1

The nonreceptor tyrosine protein kinases pp60c-src, p59fyn, and pp62c-yes are localized in growth cones of developing neurons, but their function is undefined. To determine whether these tyrosine kinases were capable of regulating substrate-dependent axon growth, cultures of cerebellar neurons from wild-type, src-, fyn-, and yes- mice were analyzed for neurite outgrowth on the neural cell adhesion molecule L1 or the extracellular matrix protein laminin. The rate of neurite extension on L1 was reduced in src-, but not in fyn- or yes- neurons. Neurite extension on laminin was unaltered in src-, fyn-, or yes- neurons, indicating that pp60c-src, p59fyn, or pp62c-yes is not likely to participate in integrin-dependent axon growth. These results demonstrate that pp60c-src is a component of the intracellular signaling pathway in L1-mediated axonal growth and suggest that Src-related nonreceptor tyrosine kinases may have distinct, nonredundant functions in the nervous system.

The neurotrophins and their receptors: structure, function, and neuropathology

The neurotrophins are a family of polypeptides that promote differentiation and survival of select peripheral and central neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4, and neurotrophin-5 are included in this group. In recent years, tremendous advances have been made in the study of these factors. This has stimulated our review of the field, characterizing the neurotrophins from initial isolation to molecular analysis. The review also discusses their synthesis, localization, and responsive tissues, in both the periphery and CNS. The complex receptor interactions of the neurotrophins are also analyzed, as are putative signal transduction mechanisms. Discussion of the observed and postulated involvement in neuropathological disorders leads to the conclusion that the neurotrophins are involved in the function and dysfunction of the nervous system.

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.

Tyrosine phosphorylation of alpha-tubulin is an early response to NGF and pp60v-src in PC12 cells

Neuronal differentiation is accompanied by extensive reorganization of the cytoskeleton to initiate the extension of neuritic processes. We have used the rat PC12 pheochromocytoma cell line to examine the role of protein tyrosine kinase activity in the induction of these events. Immunoblotting with phosphotyrosine antibodies revealed that tyrosine phosphorylation of alpha-tubulin in PC12 cells occurred within 10 min of nerve growth factor (NGF) treatment. Tyrosine phosphorylation of alpha-tubulin also occurred on induction of pp60v-src expression in a PC12 cell line (PC12-B9) harboring an inducible v-src gene under transcriptional control of the mouse metallothionine I gene promoter. Two tyrosine phosphorylated proteins in NGF- and pp60v-src induced PC12 cells were identified as alpha-tubulin isoforms by comigration with alpha-tubulin on two-dimensional gel electrophoresis, and by immunoprecipitation with phosphotyrosine antibodies followed by immunoblotting with a monoclonal antibody specific for alpha-tubulin. These results demonstrate that alpha-tubulin is an in vivo tyrosine kinase substrate, which is phosphorylated as an early event in the neuronal differentiation pathway of PC12 cells in response to NGF or pp60v-src. Tyrosine phosphorylation of alpha-tubulin could conceivably alter microtubule dynamics during induction of neurite extension.

Enhanced expression of Ca2+ channels by nerve growth factor and the v-src oncogene in rat phaeochromocytoma cells

1. Rat phaeochromocytoma (PC12) cells were used to investigate the expression of Ca2+ channel types during neuronal differentiation. Neuronal differentiation was induced by treatment with nerve growth factor (NGF) or by activation of a temperature-sensitive tyrosine kinase (pp60v-src) in genetically modified PC12 (PC12/v-src) cells. PC12 cells differentiated morphologically in the presence of NGF. When grown at the permissive temperature of 37 degrees C which activates the kinase activity of pp60v-src, PC12/v-src cells differentiated morphologically with the extension of neurites. In contrast, PC12/v-src cells grown at the non-permissive temperature of 40 degrees C continued to divide and were morphologically indistinguishable from control PC12 cells. 2. Whole-cell Ca2+ currents were measured in PC12 cells using Ba2+ as the charge carrier. Ba2+ currents measured at the peak of the current-voltage curve from a holding potential of -80 mV were -0.28 +/- 0.04 nA (mean +/- S.E.M.) in control PC12 cells compared to -1.25 +/- 0.16 nA in NGF-differentiated cells. The current density increased from 9.4 +/- 0.7 pA/pF in control PC12 cells to 22.8 +/- 2.4 pA/pF in NGF-differentiated PC12 cells. Ba2+ currents were -0.24 +/- 0.04 nA in undifferentiated PC12/v-src cells grown at the non-permissive temperature of 40 degrees C compared to -0.95 +/- 0.16 nA in differentiated PC12/v-src cells grown at the permissive temperature of 37 degrees C. The current density increased from 4.5 +/- 0.5 pA/pF in PC12/v-src cells grown at the non-permissive temperature of 40 degrees C to 13.3 +/- 2.4 pA/pF in PC12/v-src cells grown at the permissive temperature of 37 degrees C. 3. The sensitivity of Ba2+ currents to omega-conotoxin GVIA (omega-CgTX) was determined for currents measured at the peak of the current-voltage curve (0 mV in 10 mM Ba2+) from a holding potential of -80 mV. In NGF-differentiated PC12 cells, 10 microM omega-CgTx inhibited 68.1 +/- 3.2% of the total Ba2+ current compared to 35.9 +/- 4.1% in control cells. The density of the omega-CgTX-sensitive current increased from 3.3 +/- 0.4 pA/pF in control cells to 15.7 +/- 2.0 pA/pF in NGF-differentiated cells. In differentiated PC12/v-src cells grown at 37 degrees C, omega-CgTX inhibited 52.2 +/- 4.2% of total Ba2+ current compared to 41.1 +/- 3.8% in PC12/v-src cells grown at 40 degrees C. The density of the omega-CgTX-sensitive current increased from 1.9 +/- 0.3 to 7.4 +/- 2.0 pA/pF with v-src-mediated differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Increased neurite outgrowth induced by inhibition of protein tyrosine kinase activity in PC12 pheochromocytoma cells

Genistein and other inhibitors of protein tyrosine kinases were examined for effects on neurite elongation and growth cone morphology in the rat PC12 pheochromocytoma cell line. Genistein increased the rate of neurite elongation in PC12 cells grown on a collagen/polylysine substratum after priming with nerve growth factor (NGF), but had no effect on undifferentiated cells. Steady-state levels of phosphotyrosine-modified proteins (105, 59, 52, and 46 kDa) were reduced in NGF-primed cells by genistein treatment. The target of genistein action did not appear to be the NGF receptor/trk tyrosine kinase because the presence of NGF in cultures of NGF-primed cells was not necessary for genistein-stimulated neurite outgrowth. The tyrosine kinase inhibitors tyrphostin RG508964 and herbimycin A also increased the rate of neurite elongation in NGF-primed PC12 cells. Video-enhanced differential interference contrast microscopy revealed that growth cones of genistein-treated cells had less complex morphologies and were less dynamic than untreated cells, with short filopodia restricted to the leading edge, unlike untreated cells whose growth cones exhibited longer, more numerous filopodia and lamellipodia, which remodeled continuously. These results suggest that protein tyrosine kinase activity in PC12 cells negatively regulates neurite outgrowth and directly or indirectly affects growth cone morphology.

Effect of nerve growth factor and fibroblast growth factor on PC12 cells: inhibition by orthovanadate

Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, causes increased levels of tyrosine phosphorylation and blocks, at noncytotoxic concentrations, the differentiative response of rat pheochromocytoma (PC12) cells to beta-nerve growth factor (beta NGF) and basic fibroblast growth factor (bFGF) in a reversible manner. It also prevents growth factor-induced neurite proliferation in primed cells and causes the retraction of previously formed neurites, even in the presence of beta NGF or bFGF. It is equally effective in blocking neurite proliferation by 8-Br-cAMP. Zinc chloride and ammonium molybdate, two other inhibitors of tyrosine phosphatases, also cause parallel decreases in neurite proliferation. Orthovanadate generally reduces the transcription of immediate early response genes (TIS 8 and c-fos) and secondary response genes (ornithine decarboxylase (ODC), acetyl-cholinesterase (AChE) and SCG 10) induced by beta NGF, bFGF, EGF, and PMA, albeit in a variable fashion. There was no observed effect on the kinetics of expression as judged by TIS 8 induction by beta NGF and protein kinase C (PKC) downregulation did not change the levels of inhibition by orthovanadate seen in control cells. Orthovanadate does not affect the production of diacylglycerol induced by beta NGF or bFGF. These observations are consistent with the view that growth factor stimulation of differentiation in PC12 cells involves at least one other PKC independent pathway, and that cAMP and PMA (and their active analogs) activate tyrosine kinases (albeit probably secondarily), which are at least partially responsible for their actions. Although the exact site(s) of action of orthovanadate that lead to the inhibition of growth factor-induced neurite proliferation are unknown, the results presented suggest that it prolongs tyrosine phosphorylations by nonreceptor tyrosine kinases that act downstream from the receptor kinases.

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.

Inhibition of tyrosine phosphorylation potentiates substrate-induced neurite growth

Protein tyrosine kinases (PTKs) have major roles in signal transduction and growth control. There are several lines of evidence implicating PTKs in the regulation of axon growth, and this has led to the suggestion that they are centrally involved in the transduction of neuronal growth signals. To test this idea, we assayed the effect of the compounds genistein and lavendustin, specific inhibitors of PTKs, on neurite growth. We find that genistein greatly reduces phosphotyrosine in neurons, as expected from its action on other cells. Surprisingly, administration of genistein or lavendustin potentiated substrate-induced neurite growth in at least several different neuronal types. Stimulation of neurite growth by genistein was abolished by vanadate, providing additional evidence that inhibition of PTKs is responsible for this effect. The potentiation of growth is rather general, in that it occurs on several different extracellular matrix substrates and on two different cell adhesion molecules. Both the initiation of neurite growth and the rate of neurite elongation appear to be potentiated. Our results do not provide evidence for models of substrate-induced signal transduction that involve PTKs as a positive and necessary step, but suggest that such kinases play a regulatory role in neurite elongation.

Protein tyrosine kinases in nervous system development

Protein tyrosine kinases are important mediators of intracellular signaling during nervous system development. Activation of receptor protein tyrosine kinases by neurotrophic factors are initial events in the development of discrete cell populations. The patterns of expression and characterization of substrates for nonreceptor protein tyrosine kinases indicates that they also play a crucial role in neuronal development. The observed functional redundancy among protein tyrosine kinases and their associated intracellular signaling pathways underscores the need for further characterization of these novel interactions to elucidate the mechanisms regulating nervous system development.

Testing the in vivo role of protein kinase C and c-fos in neurite outgrowth by microinjection of antibodies into PC12 cells

To define the molecular bases of growth factor-induced signal transduction pathways, antibodies known to block the activity of either protein kinase C (PKC) or the fos protein were introduced into PC12 cells by microinjection. The antibody against PKC significantly inhibited neurite outgrowth when scored 24 h after microinjection and exposure to nerve growth factor (NGF). Microinjection of antibodies to fos significantly increased the percentage of neurite-bearing cells after exposure to either NGF or basic fibroblast growth factor (bFGF) but inhibited the stimulation of DNA synthesis by serum, suggesting that in PC12 cells, fos is involved in cellular proliferation. Thus, activation of PKC is involved in the induction of neurite outgrowth by NGF, but expression of the fos protein, which is induced by both NGF and bFGF, is not necessary and inhibits neurite outgrowth.

NGF and EGF rapidly activate p21ras in PC12 cells by distinct, convergent pathways involving tyrosine phosphorylation

Activation of p21ras, demonstrated directly as an increase in p21ras-associated GTP, was induced rapidly but transiently by both nerve growth factor (NGF) and epidermal growth factor (EGF) in PC12 cells. The factors activate p21ras to equal extents and with virtually identical time courses. Growth factor-induced p21ras activation and tyrosine phosphorylation have similar time courses and sensitivities to genistein inhibition, indicating that p21ras activation is a result of tyrosine kinase activity. Furthermore, PC12 mutants lacking the Trk NGF receptor tyrosine kinase also lack NGF-inducible p21ras activation. The protein kinase inhibitor K252a and the methyltransferase inhibitor MTA abolish NGF-induced, but not EGF-induced, p21ras activation--effects correlated with inhibition only of NGF-induced tyrosine phosphorylation. In spite of differences in sensitivity to genistein, MTA, and K252a, EGF- and NGF-stimulated p21ras activation are not additive, implying that they do share at least one step in common.

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.

ras isoprenylation is required for ras-induced but not for NGF-induced neuronal differentiation of PC12 cells

We have used compactin, an inhibitor of mevalonate biosynthesis, to block p21ras posttranslational modification and membrane association in PC12 cells. Previous studies have demonstrated a requirement for isoprenylation for mitogenic effects of activated p21ras in mammalian cells and for function of RAS gene products in yeast. Immunoprecipitation of [35S]methionine-labeled p21ras from PC12 cell homogenates confirmed that the processed p21ras species is missing from compactin-treated PC12 cells. Immunoprecipitation from particulate and cytosolic fractions of PC12 cells confirmed that compactin blocks p21ras membrane association: p21ras is confined to the cytosol fraction. Induction of neuronal differentiation and ornithine decarboxylase (ODCase) transcription by oncogenic p21N-ras does not occur in compactin-treated cells indicating that activity of oncogenic p21N-ras expressed in PC12 cells is abolished by compactin treatment. Thus, p21ras isoprenylation or association with the membrane appears to be required for early responses and neuronal differentiation attributable to p21ras activation. In contrast, blockade of p21ras isoprenylation and membrane association by compactin treatment did not significantly reduce PC12 cell responses to NGF. Responses examined included rapid phosphorylation of tyrosine hydroxylase, rapid induction of ODCase expression, survival in serum-free medium and neuronal differentiation. Compactin blocked growth factor-induced rapid changes in cell surface morphology but did so whether this response was induced by NGF or by EGF. These results indicate that functional p21ras is not necessary for responses to NGF which in turn implies that if a ras-dependent NGF signal transduction pathway exists, as has been previously suggested, at least one additional ras-independent pathway must also be present.

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.

The trk proto-oncogene rescues NGF responsiveness in mutant NGF-nonresponsive PC12 cell lines

The trk tyrosine kinase proto-oncogene product gp140prototrk binds nerve growth factor (NGF) and is rapidly and selectively activated by this neurotrophic factor. To determine whether gp140prototrk is involved in transducing a functional NGF signal, PC12 cell mutants (PC12nnr) deficient in high affinity NGF binding and unresponsive to NGF were used. Northern analysis revealed that these mutant cells have greatly reduced levels of trk expression. PC12nnr cultures were transiently transfected with expression vectors encoding the full-length rat trk cDNA and assessed for responsiveness to NGF. Expression of exogenous trk rescued the capacity for NGF-promoted neurite outgrowth, cellular hypertrophy, and serum-free survival by these cells. These results indicate that gp140prototrk is necessary for functional NGF signal transduction.

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.

Nerve growth factor stimulates the tyrosine phosphorylation of MAP2 kinase in PC12 cells

NGF treatment of PC12 cells results in the rapid activation of MAP2 kinase. We report here that the induction of enzyme activity was correlated with the phosphorylation of MAP2 kinase, detected by metabolic labeling of the enzyme and with anti-phosphotyrosine antibodies. NGF stimulated the phosphorylation of MAP2 kinase on tyrosine, as well as serine and threonine residues. Western blot analysis using a polyclonal anti-phosphotyrosine antibody demonstrated that the tyrosine phosphorylation of MAP2 kinase was maximal within 2 min following NGF exposure and preceded the induction of MAP2 kinase activity. The NGF-stimulated tyrosine phosphorylation of an identified substrate provides direct evidence for the participation of a tyrosine kinase in the mechanism of action of NGF.

Differentiation of PC12 cells with K-ras: comparison with nerve growth factor

The cell line PC12, derived from a rat pheochromocytoma, has served as a model for studies on the mechanism of action of nerve growth factor, as well as for the exploration of neuronal differentiation in general. When treated with nanomolar concentrations of nerve growth factor, these neoplastic chromaffin-like cells stop dividing and acquire, for all intents and purposes, the phenotype of mature sympathetic neurons. This phenotype is characterized by the extensive outgrowth of electrically excitable neurites, the ability to form functional synapses, and the acquisition of a number of biochemical markers. Treatment of PC12 cells with retroviral vectors encoding the K-ras, the N-ras, or the v-src oncogenes also produces a marked morphological differentiation very similar to that seen upon treatment with nerve growth factor. Treated cells stop dividing and develop an extensive network of neurites. It has recently been shown that PC12 cells differentiated with v-src, while resembling, morphologically, those treated with nerve growth factor, differ substantially in the biochemical characteristics normally associated with nerve growth factor-induced differentiation. Cells infected with K-ras also develop a neurite network similar to that seen after treatment with nerve growth factor. In addition, such cells develop tetanus toxin-binding sites and saxitoxin-binding sites, as do cells treated with nerve growth factor. Decreases in the binding of epidermal growth factor and in the activity of calpain also occur and these, as well, are characteristic of nerve growth factor-treated cells. But the adhesive properties of cells infected with K-ras are different than those of nerve growth factor-treated cells, and the former do not show an increase in the NILE glycoprotein. Finally, K-252a, an inhibitor of the actions of nerve growth factor on PC12 cells, has no effect on the neurite outgrowth produced by infection with K-ras. Thus, many of the key markers of nerve growth factor-induced differentiation of PC12 cells also appear upon differentiation with K-ras, but there are, nevertheless, some crucial differences in the properties of these two sets of cells.

Nerve growth factor stimulates protein tyrosine phosphorylation in PC-12 pheochromocytoma cells

The cellular actions of nerve growth factor (NGF) and epidermal growth factor (EGF) may be mediated by changes in protein phosphorylation. The tyrosine phosphorylation of two predominant proteins of molecular mass 40 and 42 kDa is seen in PC-12 cells treated with NGF or EGF, correlating with activation of a previously identified serine/threonine protein kinase that phosphorylates microtubule-associated protein (MAP). Stimulation of phosphoprotein (pp) 40 and 42 phosphorylation and MAP kinase activity by NGF but not EGF is selectively attenuated by staurosporine and K-252A. Moreover, the time courses of pp40/42 phosphorylation and MAP kinase activation produced by NGF or EGF are identical. Chromatography of lysates from growth factor-treated cells on ion-exchange or hydrophobic-interaction HPLC resolves MAP kinase into two peaks, neither of which precisely coelutes with pp40 or pp42. One of these peaks (II) exhibits no detectable phosphotyrosine. The other peak (I) has some overlap with pp40. However, the activity residing in both peaks is almost completely inhibited after treatment with alkaline phosphatase, suggesting that, at least, serine/threonine phosphorylation is required for the activity of these enzymes. These data indicate that while tyrosine phosphorylation appears to be a critical early event in NGF action, the role of this modification in activation of MAP kinases remains unclear.

Microinjection of a p21ras antibody into PC12 cells inhibits neurite outgrowth induced by nerve growth factor and basic fibroblast growth factor

The role of p21ras in signal transduction in PC12 cells was studied using an antibody that blocks its function. Native cells were microinjected with either a control solution or a solution containing the monoclonal antibody Y13-259. Treatment of the cells with growth factors appeared to enhance the ability of the cells to survive the microinjection procedure. Of the cells microinjected with the control solution 66-69% of those treated with either nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) were still present 24 h post-injection, compared with only 57% for those not treated with growth factor after microinjection. This effect of the growth factors was inhibited by introduction of the Y13-259 antibody, suggesting that it occurs through a pathway that involves p21ras. Similarly, introduction of the Y13-259 antibody into cells also resulted in a statistically significant decrease in the percentage of neurite-bearing cells; 25-36% of the cells microinjected with the control solution had neurites, whereas 12-14% of the cells microinjected with the antibody solution had neurites. This decrease suggests that the induction of neurite outgrowth and the maintenance of established neurites by these growth factors is dependent on a functional p21ras pathway. As well as complementing the finding that p21ras is apparently involved in the mechanism of action of NGF in PC12 cells, these results further establish (1) that p21ras is also involved in the mechanism of action of bFGF, and (2) that the effect of NGF and bFGF on the number of labeled cells still present 24 h postinjection requires a functional p21ras protein.

Some neuronal properties of PC12 cells differentiated by the K-ras oncogene

When infected with a virus containing the Kirsten-ras oncogene, rat phaeochromocytoma or PC12 cells elaborated neurites and ceased mitosis, that is, they underwent neuronal differentiation. Such differentiated cells could be replaced and maintained up to 20 weeks in vitro without the need of an exogenous, continuous supply of nerve growth factor (NGF). The neurites of K-ras infected PC12 cells, filled with microtubules and actin which was concentrated within the growth cones, resembled those of primary neurons in vitro. As in the NGF-primed PC12 cells, two types of secretory vesicles were present in the K-ras-infected PC12 neurites: large (100 nm), dense core granules, and small (45 nm), clear vesicles. Compared to naive PC12 cells, K-ras infected PC12 cells had (a) higher activities of acetylcholinesterase and choline acetyltransferase, two enzymes involved in acetylcholine metabolism; (b) enhanced activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis; (c) a higher, evoked norepinephrine release; and (d) similar levels of sodium-dependent uptake of both choline and norepinephrine. Although the total content of catecholamines in K-ras-differentiated PC12 cells was less than that of naïve cells, both norepinephrine and dopamine were present in substantial amounts and norepinephrine was released after stimulation. According to their enzymatic activity, these cells can also synthesize acetylcholine and thus have potential as donors for the intracerebral replacement of either catecholaminergic or cholinergic neurotransmitters.

Functional expression of dihydropyridine-insensitive calcium channels during PC12 cell differentiation by nerve growth factor (NGF), oncogenic ras, or src tyrosine kinase

1. Recombinant retroviruses were used to introduce a temperature-sensitive v-src gene and oncogenic c-Ha-ras into PC12 cells, and stable cell lines expressing these genes were established. 2. As previously reported, expression of v-src (Alema et al., 1985) or c-Ha-ras (Noda et al., 1985) in PC12 cells results in neurite outgrowth resembling that induced by NGF. We report here that v-src but not oncogenic c-Ha-ras induces a stable morphologic neuronal differentiation similar to treatment with NGF. Oncogenic c-Ha-ras-induced neurite outgrowth is not stable with long-term culture, rather the cells revert to an undifferentiated morphology with altered cell cycle kinetics. 3. The stable neuronal phenotype induced by v-src and NGF is characterized by the functional expression of dihydropyridine-insensitive calcium currents.