The activation and nuclear translocation of extracellular signal-regulated kinases (ERK-1 and -2) appear not to be required for elongation of neurites in PC12D cells

The pseudophosphatase MK-STYX induces neurite-like outgrowths in PC12 cells

The rat pheochromocytoma PC12 cell line is a widely used system to study neuronal differentiation for which sustained activation of the extracellular signaling related kinase (ERK) pathway is required. Here, we investigate the function of MK-STYX [MAPK (mitogen-activated protein kinase) phosphoserine/threonine/tyrosine-binding protein] in neuronal differentiation. MK-STYX is a member of the MAPK phosphatase (MKP) family, which is generally responsible for dephosphorylating the ERKs. However, MK-STYX lacks catalytic activity due to the absence of the nucleophilic cysteine in the active site signature motif HC(X₅)R that is essential for phosphatase activity. Despite being catalytically inactive, MK-STYX has been shown to play a role in important cellular pathways, including stress responses. Here we show that PC12 cells endogenously express MK-STYX. In addition, MK-STYX, but not its catalytically active mutant, induced neurite-like outgrowths in PC12 cells. Furthermore, MK-STYX dramatically increased the number of cells with neurite extensions in response to nerve growth factor (NGF), whereas the catalytically active mutant did not. MK-STYX continued to induce neurites in the presence of a MEK (MAP kinase kinase) inhibitor suggesting that MK-STYX does not act through the Ras-ERK/MAPK pathway but is involved in another pathway whose inactivation leads to neuronal differentiation. RhoA activity assays indicated that MK-STYX induced extensions through the Rho signaling pathway. MK-STYX decreased RhoA activation, whereas RhoA activation increased when MK-STYX was down-regulated. Furthermore, MK-STYX affected downstream players of RhoA such as the actin binding protein cofilin. The presence of MK-STYX decreased the phosphorylation of cofilin in non NGF stimulated cells, but increased its phosphorylation in NGF stimulated cells, whereas knocking down MK-STYX caused an opposite effect. Taken together our data suggest that MK-STYX may be a regulator of RhoA signaling, and implicate this pseudophosphatase as a regulator of neuronal differentiation.

LRP1 assembles unique co-receptor systems to initiate cell signaling in response to tissue-type plasminogen activator and myelin-associated glycoprotein

In addition to functioning as an activator of fibrinolysis, tissue-type plasminogen activator (tPA) interacts with neurons and regulates multiple aspects of neuronal cell physiology. In this study, we examined the mechanism by which tPA initiates cell signaling in PC12 and N2a neuron-like cells. We demonstrate that enzymatically active and inactive tPA (EI-tPA) activate ERK1/2 in a biphasic manner. Rapid ERK1/2 activation is dependent on LDL receptor-related protein-1 (LRP1). In the second phase, ERK1/2 is activated by tPA independently of LRP1. The length of the LRP1-dependent phase varied inversely with the tPA concentration. Rapid ERK1/2 activation in response to EI-tPA and activated α2-macroglobulin (α2M*) required the NMDA receptor and Trk receptors, which assemble with LRP1 into a single pathway. Assembly of this signaling system may have been facilitated by the bifunctional adapter protein, PSD-95, which associated with LRP1 selectively in cells treated with EI-tPA or α2M*. Myelin-associated glycoprotein binds to LRP1 with high affinity but failed to induce phosphorylation of TrkA or ERK1/2. Instead, myelin-associated glycoprotein recruited p75 neurotrophin receptor (p75NTR) into a complex with LRP1 and activated RhoA. p75NTR was not recruited by other LRP1 ligands, including EI-tPA and α2M*. Lactoferrin functioned as an LRP1 signaling antagonist, inhibiting Trk receptor phosphorylation and ERK1/2 activation in response to EI-tPA. These results demonstrate that LRP1-initiated cell signaling is ligand-dependent. Proteins that activate cell signaling by binding to LRP1 assemble different co-receptor systems. Ligand-specific co-receptor recruitment provides a mechanism by which one receptor, LRP1, may trigger different signaling responses.

Forkhead family transcription factor FoxO and neural differentiation

The Forkhead Box subgroup O (FoxO) transcription factor family is one of the most important downstream targets of the phosphatidylinositol 3-kinase/protein kinase B signaling pathway playing an important role in many biological functions including transcriptional regulation of cellular differentiation. Neuronal differentiation is a complex process which involves many signaling pathways and molecular mechanisms. Interestingly, recent studies indicate that the FoxO family is involved in a number of signaling pathways regulating cell differentiation. The actions occur at different stages in the differentiation process and by differing mechanisms. This review will focus on FoxO as a novel transcription factor in neural differentiation.

A quantitative image cytometry technique for time series or population analyses of signaling networks

Background

Modeling of cellular functions on the basis of experimental observation is increasingly common in the field of cellular signaling. However, such modeling requires a large amount of quantitative data of signaling events with high spatio-temporal resolution. A novel technique which allows us to obtain such data is needed for systems biology of cellular signaling.

Methodology/Principal Findings

We developed a fully automatable assay technique, termed quantitative image cytometry (QIC), which integrates a quantitative immunostaining technique and a high precision image-processing algorithm for cell identification. With the aid of an automated sample preparation system, this device can quantify protein expression, phosphorylation and localization with subcellular resolution at one-minute intervals. The signaling activities quantified by the assay system showed good correlation with, as well as comparable reproducibility to, western blot analysis. Taking advantage of the high spatio-temporal resolution, we investigated the signaling dynamics of the ERK pathway in PC12 cells.

Conclusions/Significance

The QIC technique appears as a highly quantitative and versatile technique, which can be a convenient replacement for the most conventional techniques including western blot, flow cytometry and live cell imaging. Thus, the QIC technique can be a powerful tool for investigating the systems biology of cellular signaling.

Structure-based discovery of low molecular weight compounds that stimulate neurite outgrowth and substitute for nerve growth factor

Olanzapine, an atypical antipsychotic drug, was previously shown to protect neuronal cells against nutrient deprivation and to enhance neurite outgrowth. In an effort to identify small molecules with greater potency, the structure of olanzapine was used as a template to search commercially available chemical inventories for compounds with similar features. These compounds were evaluated for their ability to protect cells against glutamine deprivation and low-serum conditions. Positive compounds, 'hits' from initial screening, were then tested for stimulation of neurite outgrowth, alone and in combination with suboptimum concentrations of nerve growth factor (NGF). Numerous neuroprotective compounds (mw < 550 Da) were identified that significantly stimulated neurite outgrowth in PC12 cells. These included 4', 6'-diamidino-2-phenylindole, a nuclear stain; staurosporine, an antibiotic and kinase inhibitor; and 2-phenylamino-adenosine, an adenosine analog. The small molecules were comparable with NGF, and in fact, replaced NGF in outgrowth assays. Pharmacophore analysis of the hits led to the design and synthesis of an active compound, LSU-D84, which represented an initial lead for drug discovery efforts.

Targeting the PI3K and MAPK pathways to treat Kaposi's-sarcoma-associated herpes virus infection and pathogenesis

Cells require the ability to appropriately respond to signals in their extracellular environment. To initiate, inhibit and control these processes, the cell has developed a complex network of signaling cascades. The phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways regulate several responses including mitosis, apoptosis, motility, proliferation, differentiation and many others. It is not surprising, therefore, that many viruses target the PI3K and MAPK pathways as a means to manipulate cellular function. Recently, Kaposi's sarcoma-associated herpes virus (KSHV) has been added to the list. KSHV manipulates the PI3K and MAPK pathways to control such divergent processes as cell survival, cellular migration, immune responses, and to control its own reactivation and lytic replication. Manipulation of the PI3K and MAPK pathways also plays a role in malignant transformation. Here, the authors review the potential to target the PI3K and MAPK signaling pathways to inhibit KSHV infection and pathogenesis.

Pheophytin a, a low molecular weight compound found in the marine brown alga Sargassum fulvellum, promotes the differentiation of PC12 cells

We identified and characterized a neurodifferentiation compound from the marine brown alga Sargassum fulvellum collected from the Japanese coastline. Several instrumental analyses revealed the compound to be pheophytin a. Pheophytin a did not itself promote neurite outgrowth of PC12 cells. However, when PC12 cells were treated with a low concentration of pheophytin a (3.9 microg/ml) in the presence of a low level of nerve growth factor (10 ng/ml), the compound produced neurite outgrowth similar to that produced by a high level of nerve growth factor (50 ng/ml). Pheophytin a also enhanced signal transduction in the mitogen-activated protein kinase signaling pathway, which is also induced by nerve growth factor. The effect of pheophytin a on neurite outgrowth of PC12 cells was completely blocked by U0126, a representative mitogen-activated protein kinase kinase inhibitor. These results suggest that pheophytin a enhances the neurodifferentiation of PC12 cells in the presence of a low level of nerve growth factor and that this effect is mediated by activation of a mitogen-activated protein kinase signaling pathway.

Differential activation of extracellular signal-regulated kinase 1 and a related complex in neuronal nuclei

The extracellular signal-regulated kinases 1 and 2 (ERKs 1/2) are known to participate in regulating transcription in response to moderate depolarization, such as synaptic stimulation, but how the same active enzyme can differentially regulate distinct transcriptional programs induced with abnormal depolarization (high potassium) is unknown. We hypothesized that ERK1 or 2 accomplishes this differential nuclear response through close association with other proteins in stable complexes. In support of this hypothesis, we have found that immunoreactivity for an apparent high molecular weight complex containing phospho-ERK1 increased in response to synaptic stimulation, but decreased in response to high potassium; p-ERK immunoreactivity at 44/42 kDa increased in both cases. Evidence supporting the conclusion that the band of interest contained ERK1 in a complex, as opposed to it being an unrelated protein crossreacting with antibodies against p-ERK, is that ERK1 (p44 MAPK) and 14-3-3 protein were electroeluted from the 160-kDa band cut from a gel. We also found the nuclear complexes to be exceptionally durable, suggesting a role for the crosslinking enzyme, transglutaminase, in its stabilization. In addition, we found other components of the ERK pathway, including MEK, ERK2, p90RSK, and Elk-1, migrating at higher-than-expected weights in brain nuclei. These results describe a novel stable complex of ERK1 in neuronal nuclei that responds differentially to synaptic and depolarizing stimulation, and thus may be capable of mediating gene transcription in a way distinct from the monomeric protein.

MAP-quest: could we produce constitutively active variants of MAP kinases?

Constitutively active mutants that acquired intrinsic activity and escaped regulation, serve as powerful tools for revealing the biochemical, biological and pathological functions of proteins. Such mutants are not available for mitogen-activated protein kinases (MAPKs). It is not known how to mimic the unusual mode of MAPK activation and to enforce, by mutations, their active conformation. In this review we describe the strategies employed in attempts to overcome this obstacle. We focus on a recent breakthrough with the p38 family that suggests that active variants of all MAPKs will soon be available.

Protection by puerarin against MPP+-induced neurotoxicity in PC12 cells mediated by inhibiting mitochondrial dysfunction and caspase-3-like activation

Puerarin, a main isoflavone glycoside distributed in Pueraria lobata (Willd.) Ohwi, showed inhibitory activity on H2O2-induced PC12 cells damage in our previous work. However, there is insufficient evidence in protective mechanism of puerarin, especially that relating to the mitochondrial function. In this study, when cells were pretreated with puerarin prior to 0.4 mM MPP+, protective roles were accompanied by a reduction of cell viability loss, morphological changes of apoptosis and apoptotic rate. To explore the protective mechanism of puerarin in MPP+-induced PC12 cells, mitochondrial function and caspase-3-like activity were measured. The results indicated that puerarin inhibited the release of mitochondrial cytochrome c to cytosol and the loss of mitochondrial membrane potentials. In addition, puerarin also reduced MPP+-induced caspase-3-like activation. Taken together, the above results suggest that pretreatment of PC12 cells with puerarin could block MPP+-mediated apoptosis by mitochondria-dependent caspase cascade.

Nardosinone enhances nerve growth factor-induced neurite outgrowth in a mitogen-activated protein kinase- and protein kinase C-dependent manner in PC12D cells

The mechanism to enhance nerve growth factor (NGF, 2 ng/ml)-induced neurite outgrowth from PC12D cells by nardosinone isolated from Nardostachys chinensis was examined. It was shown that the potentiation of the NGF-induced neurite outgrowth by nardosinone was mitogen-activated protein (MAP) kinase-dependent, but was not accompanied by stimulation of NGF-induced increase in MAP kinase phosphorylation. Furthermore, this augmentation of NGF-induced neurite outgrowth was abolished by GF109203X, a protein kinase C (PKC) inhibitor. These results suggest that the enhancement of NGF-induced neurite outgrowth from PC12D cells by nardosinone involves activation of a down-stream step of the MAP kinase-dependent cascade of NGF coupled with PKC.

Catalpol inhibits apoptosis in hydrogen peroxide-induced PC12 cells by preventing cytochrome c release and inactivating of caspase cascade

In the present study, using a rat pheochromocytoma (PC12) cell line, the effect of catalpol on H2O2-induced apoptosis was studied. The apoptosis in H2O2-induced PC12 cells was accompanied by down-regulation of Bcl-2, up-regulation of Bax, the release of mitochondrial cytochrome c to cytosol and sequential activation of caspase-1 and caspase-3 then leading to cleavage of poly-ADP-ribose polymerase (PARP). Catalpol not only suppressed the down-regulation of Bcl-2, up-regulation of Bax and the release of mitochondrial cytochrome c to cytosol, but also attenuated caspase-3 activation, PARP cleavage, and eventually protected against H2O2-induced apoptosis. Taken together, these results suggest that treatment of PC12 cells with catalpol can block H2O2-induced apoptosis by the regulation of Bcl-2 family members, as well as suppression of cytochrome c release and caspase cascade activation.

Nardosinone, the first enhancer of neurite outgrowth-promoting activity of staurosporine and dibutyryl cyclic AMP in PC12D cells

Nardosinone was isolated as an enhancer of nerve growth factor (NGF) from Nardostachys chinensis [Neurosci. Lett. 273 (1999) 53]. Nardosinone (0.1-100 microM) enhanced dibutyryl cyclic AMP (dbcAMP, 0.3 mM)- and staurosporine (10 nM)-induced neurite outgrowth from PC12D cells in a concentration-dependent manner. PD98059 (20 microM), a potent mitogen-activated protein (MAP) kinase kinase inhibitor, partially blocked enhancements of dbcAMP (0.3 mM)- or staurosporine (10 nM)-induced neurite outgrowth by nardosinone. Nardosinone alone had no effect on the phosphorylation of MAP kinase. The dbcAMP-induced increase in phosphorylation of MAP kinase was not affected by nardosinone. Staurosporine almost unaffected the phosphorylation of MAP kinase, and nardosinone potentiated the staurosporine-induced neurite outgrowth without stimulation of the phosphorylation of MAP kinase. Since it is known that MAP kinase signaling is required for neurite outgrowth in PC12D cells, these results suggest that nardosinone enhances staurosporine- or dbcAMP-induced neurite outgrowth from PC12D cells, probably by amplifying both the MAP kinase-dependent and -independent signaling pathways of dbcAMP and staurosporine. It is also suggested that nardosinone enhances a downstream step of MAP kinase in the MAP kinase-dependent signaling pathway. Nardosinone is the first enhancer of the neuritogenic action of dbcAMP and staurosporine and may become a useful pharmacological tool for studying the mechanism of action of not only NGF but also both the neuritogenic substances.

Examining the mechanism of Erk nuclear translocation using green fluorescent protein

In neuronal cells, the mitogen-activated protein kinase (MAP kinase) cascade is an important mediator of neurotrophin signaling from cell surface receptors to the nucleus, resulting in changes in gene expression. Nuclear localization of Erk is thought to be required for these effects. To examine the mechanism and regulation of Erk nuclear translocation, we have created a green fluorescent protein (GFP)-labeled Erk2 construct, which provides a sensitive means to follow the movement of Erk from the cytoplasm to the nucleus following receptor-mediated MAP kinase activation. Using this system in PC12 cells, we have examined a number of mechanisms that have been implicated in regulating the translocation of Erk. In PC12 cells, NGF and EGF induce a rapid translocation of GFP-Erk that requires Ras and Mek. We have found that prolonged phosphorylation of Erk is not required for the rapid and early influx of Erk into the nucleus following growth factor stimulation. Furthermore, following influx, GFP-Erk rapidly returned to the cytoplasm regardless of its phosphorylation state. The release of Erk from its cytoplasmic activator, Mek, followed by the dimerization of Erk, was sufficient to stimulate nuclear uptake, whereas Erk kinase activity was dispensable. PKA activity has been reported to be required for Erk translocation in PC12 cells. However, PKA activity was also not necessary for the early translocation of Erk into the nucleus by NGF or Ras, but it was able to induce a small influx of Erk that could be measured with GFP-Erk2.

Picrosides I and II, selective enhancers of the mitogen-activated protein kinase-dependent signaling pathway in the action of neuritogenic substances on PC12D cells

Picrosides I and II caused a concentration-dependent (> 0.1 microM) enhancement of basic fibroblast growth factor (bFGF, 2 ng/ml)-, staurosporine (10 nM)- and dibutyryl cyclic AMP (dbcAMP, 0.3 mM)-induced neurite outgrowth from PC12D cells. PD98059 (20 microM), a potent mitogen-activated protein (MAP) kinase kinase inhibitor, blocked the enhancement of bFGF (2 ng/ml)-, staurosporine (10 nM)- or dbcAMP (0.3 mM)-induced neurite outgrowth by picrosides, suggesting that picrosides activate MAP kinase-dependent signaling pathway. However, PD98059 did not affect the bFGF (2 ng/ml)-, staurosporine (10 nM)- and dbcAMP (0.3 mM)-induced neurite outgrowth in PC12D cells, indicating the existence of two components in neurite outgrowth induced by bFGF, staurosporine and dbcAMP, namely the MAP kinase-independent and the masked MAP kinase-dependent one. Furthermore, picrosides-induced enhancements of the bFGF-action were markedly inhibited by GF109203X (0.1 microM), a protein kinase C inhibitor. The expression of phosphorylated MAP kinase was markedly increased by bFGF (2 ng/ml) and dbcAMP (0.3 mM), whereas that was not enhanced by staurosporine (10 nM). Picrosides had no effect on the phosphorylation of MAP kinase induced by bFGF or dbcAMP and also unaffected it in the presence of staurosporine. These results suggest that picrosides I and II enhance bFGF-, staurosporine- or dbcAMP-induced neurite outgrowth from PC12D cells, probably by amplifying a down-stream step of MAP kinase in the intracellular MAP kinase-dependent signaling pathway. Picrosides I and II may become selective pharmacological tools for studying the MAP kinase-dependent signaling pathway in outgrowth of neurites induced by many kinds of neuritogenic substances including bFGF.

p21(ras) stimulates pathways in addition to ERK, p38, and Akt to induce elongation of neurites in PC12 cells

Initiation and elongation of neurites in PC12 cells has been shown to be stimulated by nerve growth factor (NGF). Initiation of NGF-stimulated neurites in a PC12 subclone (PC12-N09) is rapid, giving rise to short neurites that do not elongate after 1 day. To determine whether increasing activation of p21(ras) could restore neurite elongation in these cells and whether it would affect the phosphorylation of signaling proteins, the subclone PC12-N09 was transfected with constitutively active p21(ras61L) (PC12-N09ras61L) and neurite outgrowth with or without NGF was determined. Overexpression of wild-type p21(ras) (PC12-N09rasWT) did not lead to spontaneous neurite initiation but restored the ability of NGF to stimulate continuous neurite elongation. However, NGF-stimulated phosphorylation of ERK, p38, and Akt in PC12-N09rasWT cells is similar in duration to that in PC12-N09 cells, indicating that the p21(ras) signaling through ERK, p38, and Akt was not involved in the restoration of normal neurite elongation in PC12-N09 cells. These results show that p21(ras)-activated pathways other than ERK, p38, and Akt are necessary for appropriate NGF-stimulated neurite elongation in PC12 cells.

Pike. A nuclear gtpase that enhances PI3kinase activity and is regulated by protein 4.1N

While cytoplasmic PI3Kinase (PI3K) is well characterized, regulation of nuclear PI3K has been obscure. A novel protein, PIKE (PI3Kinase Enhancer), interacts with nuclear PI3K to stimulate its lipid kinase activity. PIKE encodes a 753 amino acid nuclear GTPase. Dominant-negative PIKE prevents the NGF enhancement of PI3K and upregulation of cyclin D1. NGF treatment also leads to PIKE interactions with 4.1N, which has translocated to the nucleus, fitting with the initial identification of PIKE based on its binding 4.1N in a yeast two-hybrid screen. Overexpression of 4.1N abolishes PIKE effects on PI3K. Activation of nuclear PI3K by PIKE is inhibited by the NGF-stimulated 4.1N translocation to the nucleus. Thus, PIKE physiologically modulates the activation by NGF of nuclear PI3K.

Regulation of retinal neurite growth by alterations in MAPK/ERK kinase (MEK) activity

Activation of the extracellular-signal regulated kinase (ERK) cascade may be involved in the promotion of neurite outgrowth by a variety of stimuli. For example, we have previously shown that laminin (LN) and N-cadherin activate ERK2 in chick retinal neurons, and that pharmacological inhibition of MAPK/ERK kinase (MEK), the major upstream ERK2 activator, severely impairs neurite growth induced by these proteins. We have therefore hypothesized that ERK activation through MEK is required for optimal induction of neurite growth by these proteins. Here we show that expression of mutant MEK in transfected retinal neurons alters neuronal responses to LN in a manner consistent with this hypothesis. Neurons expressing a constitutively active MEK construct extended longer neurites on LN than controls, while neurons transfected with a dominant negative construct extended shorter neurites. Further, experiments in which transfected neurons were replated onto polylysine substrates suggest that activation of MEK is sufficient for neurite promotion on a non-inducing substrate, and neurons replated onto LN confirm the pharmacological data that inhibition of MEK activation inhibits LN-induced neurite growth. We conclude that ERK activation plays a direct role in the promotion of neurite outgrowth from retinal neurons by LN.

Distinct neurite outgrowth signaling pathways converge on ERK activation

Several distinct classes of proteins positively regulate axonal growth; some of these are known to activate the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling cascade, at least in nonneuronal cells. We have found that N-cadherin, as well as laminin (LN) and basic fibroblast growth factor (bFGF), can activate ERK in embryonic chick retinal neurons. Additionally, adhesion of retinal neurons to LN or N-cadherin substrates induced a redistribution of ERK from the cytoplasm toward the plasma membrane. Neurite outgrowth induced by bFGF, LN, or N-cadherin was strongly inhibited by treatment with inhibitors of ERK kinase activation, but not by an inhibitor of p38 MAPK. We conclude (1) that N-cadherin and LN can activate ERK in retinal neurons and (2) that activation of ERK is required for full neurite outgrowth induced by these proteins. Our results suggest that ERK activation is one point of convergence for signaling pathways generated by a variety of axon growth inducers.

A new factor derived from 1321N1 human astrocytoma cells causes differentiation of PC-12 cells mediated through mitogen-activated protein kinase cascade

Glial cells play an important role in maintaining neural function. In the present study, we examined the effects of a factor derived from human astrocytoma cells (1321N1) on differentiation of rat pheochromocytoma cells (PC-12). The conditioned medium which had been used for culture of 1321N1 cells caused the differentiation of PC-12 cells, suggesting that 1321N1 cells release a neurotrophic factor. The factor was apparently distinct from well-known neurotrophic factors, such as nerve growth factor (NGF), since it was resistant to boiling and trypsin treatment. The molecular size of the factor was assumed to be below 1000 through dialysis and ultrafiltration experiments. Furthermore, PC-12 cells were differentiated synergistically by the combined addition of NGF and the conditioned medium of 1321N1 cells. Partially purified fraction of the factor by Sephadex G-15 gel filtration column caused the prolonged activation of mitogen-activated protein kinase (MAPK). The differentiation of PC-12 cells induced by the fraction or NGF disappeared after the treatment with PD98059, a specific inhibitor of MAPK kinase (MEK), suggesting the involvement of MAPK in the differentiation. These results suggest that the new low-molecular factor derived from glial cells causes differentiation of PC-12 cells mediated through an activation of MAPK.

Activation of mitogen-activated protein kinases is not required for the extension of neurites from PC12D cells triggered by nerve growth factor

Numerous studies with PC12 cells have suggested that the mitogen-activated protein (MAP) kinase pathway might play a major role in the neuronal differentiation that is induced by nerve growth factor (NGF). Cells of the PC12D subline extend neurites within several hours in response to NGF in the presence of inhibitors of the synthesis of RNA and protein. We examined the effects of a specific inhibitor 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) of the MAP kinase kinase (MEK)/MAP kinase pathway on the NGF-induced outgrowth of neurites in PC12D cells. The increase in MAP kinase activity in response to NGF was reduced by 80% upon treatment of PC12D cells with 50 microM PD98059, whereas the NGF-dependent formation of ruffles and the subsequent outgrowth of neurites were not blocked by PD98059 at this concentration. The outgrowth of neurites from conventional PC12 cells by NGF was suppressed by the addition of 50 microM PD98059 as reported by Pang et al. [L. Pang, T. Sawada, J. Stuart,S.J. Decker, A.R. Saltiel, Inhibition of MAP kinase kinase blocks the differentiation of PC12 cells induced by nerve growth factor, J. Biol. Chem. 270 (1995) 13585-13588]. In contrast, the rapid regeneration of neurites from PC12 cells primed with NGF, was not altered in the presence of the same dose of the inhibitor of MEK. It appeared, therefore, that the activation of the MAP kinase pathway was not necessarily required for the NGF-dependent extension of neurites. When PC12D cells were transfected with the dominant inhibitory Ha-ras Asn-17 gene, the induction of the mutant Ras protein led the suppression of the rapid outgrowth of neurites in response to NGF but not to dibutyryl cyclic AMP (dbcAMP). The result implies a direct involvement of Ras protein in the NGF-induced signal transduction that lead to the formation of neurites in PC12D cells. We can conclude that the activation of MAP kinase and selective gene expression are required for the differentiation of conventional PC12 cells to sympathetic neuron-like cells and that activation of Ras protein and, subsequently, of a MAP kinase-independent pathway might be involved in the extension of neurites from PC12D cells or in the regeneration of neurites from primed PC12 cells in response to NGF.

Mitogen-activated protein kinase pathways

Nearly all cell surface receptors utilize one or more of the mitogen-activated protein kinase cascades in their repertoire of signal transduction mechanisms. Recent advances in the study of such cascades include the cloning of genes encoding novel members of the cascades, further definition of the roles of the cascades in responses to extracellular signals, and examination of cross-talk between different cascades.

Inhibition of the nerve growth factor-induced outgrowth of neurites by trichostatin A requires protein synthesis de novo in PC12D cells

Trichostatin A (TSA) inhibits the activity of histone deacetylase and blocks both oncogenic ras-induced and nerve growth factor-induced (NGF-induced) outgrowth of neurites from PC12 cells. Cells of the PC12D subline extend neurites very rapidly in response to NGF, basic fibroblast growth factor (bFGF), dibutyryl cAMP (dbcAMP) and to staurosporine, even in the presence of an inhibitor of RNA synthesis, as do primed PC12 cells or cultured sympathetic neurons. TSA at 100 nM selectively blocked the NGF- and bFGF-induced outgrowth of neurites from PC12D cells, but not the outgrowth induced by dbcAMP or staurosporine. The NGF-induced changes in morphology with the relocalization of F-actin, were not inhibited by TSA. However, the subsequent formation of growth cones and the outgrowth of neurites was blocked. The activation of mitogen-activated protein (MAP) kinases in NGF-stimulated cells was also unaffected by TSA. When TSA was added to cells that were extending neurites in response to NGF, the number of neurite-bearing cells decreased after a lag period. In the presence of inhibitors of RNA or protein synthesis namely, actinomycin D, cordycepin, and cycloheximide, TSA no longer blocked the NGF- and bFGF-dependent outgrowth of neurites from PC12D cells. Regardless of the effect of TSA, the rapid outgrowth of neurites from PC12D cells was unaffected by the presence of cycloheximide, which inhibited protein synthesis by 97%, as determined by monitoring the incorporation of [35S]methionine/cysteine. This study provides proof that the NGF-induced elongation of neurites does not require protein synthesis de novo. These observations suggest that TSA might not inhibit the early signal-transduction pathway of NGF, but might block the late pathway, which is related to the formation of growth cones and/or neurites. Cellular conditions that no longer allow the NGF- and bFGF-mediated elongation of neurites might be produced by TSA via synthesis of some specific protein(s) due to changes in RNA(s) synthesis de novo.

Re-examination of the local control by nerve growth factor of the outgrowth of neurites in PC12D cells

We have examined the local control by nerve growth factor (NGF) of the outgrowth of neurites from clonal cells, PC12D, a subline whose phenotype resembles that of the parent PC12 cell line in the NGF-primed state. We show here that (i) the outgrowth of neurites, and their survival can be induced by NGF in enucleated PC12D cells (ii) individual neurites of a single 'giant cell', produced by cell fusion of PC12D cells, can respond independently to the NGF in the local environment, (iii) dissected neurites from giant cells survive for longer in medium that contains NGF than in medium that does not, (iv) in PC12D cells, the rapid formation of ruffles in response to NGF, which appears to be based on increased cell-substratum adhesion, leads to the subsequent formation of neurites, and (v) upon addition of NGF, the movement of short processes displaces polylysine-coated beads in the vicinity of neurites. These observations suggest that the NGF-dependent maintenance or extension of neurites might be controlled within the neurites themselves and might not require the direct involvement of the cell body, even in PC12 cells. It seems possible that any NGF-induced changes that promote an increase in cell-substratum adhesion might be responsible for the initiation and elongation of neurites. It also seems possible that the growth of neurites towards a source of NGF might be based on repeated rounds of extension and retraction of filopodia and neurites in a manner that depends on the concentration of NGF.