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

Differential neurotrophic regulation of sodium and calcium channels in an adult sympathetic neuron

Adult neuronal phenotype is maintained, at least in part, by the sensitivity of individual neurons to a specific selection of neurotrophic factors and the availability of such factors in the neurons' environment. Nerve growth factor (NGF) increases the functional expression of Na(+) channel currents (I(Na)) and both N- and L-type Ca(2+) currents (I(Ca,N) and I(Ca,L)) in adult bullfrog sympathetic ganglion (BFSG) B-neurons. The effects of NGF on I(Ca) involve the mitogen-activated protein kinase (MAPK) pathway. Prolonged exposure to the ganglionic neurotransmitter luteinizing hormone releasing hormone (LHRH) also increases I(Ca,N) but the transduction mechanism remains to be elucidated as does the transduction mechanism for NGF regulation of Na(+) channels. We therefore exposed cultured BFSG B-neurons to chicken II LHRH (0.45 microM; 6-9 days) or to NGF (200 ng/ml; 9-10 days) and used whole cell recording, immunoblot analysis, and ras or rap-1 pulldown assays to study effects of various inhibitors and activators of transduction pathways. We found that 1) LHRH signals via ras-MAPK to increase I(Ca,N), 2) this effect is mediated via protein kinase C-beta (PKC-beta-IotaIota), 3) protein kinase A (PKA) is necessary but not sufficient to effect transduction, 4) NGF signals via phosphatidylinositol 3-kinase (PI3K) to increase I(Na), and 5) long-term exposure to LHRH fails to affect I(Na). Thus downstream signaling from LHRH has access to the ras-MAPK pathway but not to the PI3K pathway. This allows for differential retrograde and anterograde neurotrophic regulation of sodium and calcium channels in an adult sympathetic neuron.

Genistein-induced neuronal differentiation is associated with activation of extracellular signal-regulated kinases and upregulation of p21 and N-cadherin

Neuronal differentiation in the mammalian CNS is driven by multiple events. When treated with retinoic acid (RA), hNTera-2 (NT-2) cells undergo postmitotic neuronal differentiation. Here, we show that a prolonged exposure of NT-2 cells with non-cytotoxic doses of genistein, a protein tyrosine kinase (PTK) inhibitor, induced differentiation of NT-2 cells. Additionally, genistein enhanced RA-induced neuronal differentiation by increasing the activation of extracellular signal-related kinase 1/2 (ERK1/2) via phosphorylation at Thr183 and Tyr185 in 3-7 days. Meanwhile, genistein also upregulated N-cadherin and p21 (a Cdk inhibitor), but downregulated proliferating cell nuclear antigen protein (PCNA). MEK1/2 inhibitors, such as PD98059 and U0126, reduced RA-induced ERK1/2 activity, but could not block the genistein effects. Our observations indicate that genistein-induced neuronal differentiation is not dependent of the MEK-ERK signaling cascade. Instead, genistein-upregulated ERK activation is likely due to this chemical's direct effect on chromosome and gene transcription, rather than its inhibition on tyrosine kinases. Failure of inhibition of ERK1/2 activation by the MEK1/2 inhibitors PD98059 and U0126 suggests presence of an unknown activator for ERK1/2 in neuronal cells.

Additive effects of PDGF receptor beta signaling pathways in vascular smooth muscle cell development

The platelet-derived growth factor β receptor (PDGFRβ) is known to activate many molecules involved in signal transduction and has been a paradigm for receptor tyrosine kinase signaling for many years. We have sought to determine the role of individual signaling components downstream of this receptor in vivo by analyzing an allelic series of tyrosine–phenylalanine mutations that prevent binding of specific signal transduction components. Here we show that the incidence of vascular smooth muscle cells/pericytes (v/p), a PDGFRβ-dependent cell type, can be correlated to the amount of receptor expressed and the number of activated signal transduction pathways. A decrease in either receptor expression levels or disruption of multiple downstream signaling pathways lead to a significant reduction in v/p. Conversely, loss of RasGAP binding leads to an increase in this same cell population, implicating a potential role for this effector in attenuating the PDGFRβ signal. The combined in vivo and biochemical data suggest that the summation of pathways associated with the PDGFRβ signal transduction determines the expansion of developing v/p cells.

Using both in vivo and biochemical approaches, the summation of pathways associated with the PDGFRβ signal transduction is shown to determine the expansion of a specific PDGFRβ-dependent cell type

Calcium channel upregulation in response to activation of neurotrophin and surrogate neurotrophin receptor tyrosine kinases

Modulation of calcium channel expression and function in the context of neurotrophin induced neuronal differentiation remains incompletely understood at a mechanistic level. We addressed this issue in the PC12 model neuronal system using patch clamp electrophysiology combined with ectopic expression of the human beta platelet-derived growth factor (betaPDGF) receptor as a surrogate neurotrophin receptor system. PC12 cells ectopically expressing the human betaPDGF receptor were treated with PDGF or nerve growth factor (NGF) for up to 7 days, and Ca2+ channel subtype expression was analyzed using selective pharmacological agents in both whole-cell and cell-attached single channel patch clamp configurations. PDGF-induced upregulation of N- and P/Q-type Ca2+ channel currents completely mimicked upregulation of these currents caused by NGF stimulation of the endogenous TrkA receptor tyrosine kinase (RTK). Neither PDGF nor NGF significantly altered L- or R-type currents. Single channel recordings together with immunocytochemistry implied that growth factor-induced increases in whole-cell Ca2+ currents were a result of synthesis of new channels, and that whereas increased N channel density was apparent in the soma, additional P/Q channels distributed preferentially to extrasomal locations, most likely the proximal neurites. Finally, specific signaling-deficient mutant forms of the betaPDGF receptor were used to show that activation of Src, PI3-kinase, RasGAP, PLCgamma or SHP-2 (some of which are implicated in certain other aspects of PC12 cell differentiation) by RTKs is not required for growth factor-induced Ca2+ channel upregulation. In contrast, activation of the Ras-related G-protein Rap1 was found critical to this process.

Upregulation of swelling-activated Cl- channel sensitivity to cell volume by activation of EGF receptors in murine mammary cells

Whole-cell recordings showed that, in mouse mammary C127 cells transfected with the full genome of the bovine papilloma virus (BPV), a hypotonic challenge induced the activation of outwardly rectifying Cl- currents with a peak amplitude 2.7 times greater than that in control C127 cells. Cell-attached single-channel recordings showed that BPV-induced augmentation of the peak amplitude of the whole-cell current could not chiefly be explained by a small increase (1.2 times) in unitary conductance. There was no difference between control and BPV-transfected cells in the osmotic cell swelling rate, and hence, osmotic water permeability. However, a plot of the whole-cell current density as a function of cell volume, which was measured simultaneously, showed that the BPV-transfected cells had a strikingly greater volume sensitivity than control cells. Since the E5 protein of BPV has been reported to induce constitutive activation of the epidermal growth factor (EGF) receptor and platelet-derived growth factor (PDGF) receptor in a variety of cell lines including C127 cells, effects of the growth factors on volume-sensitive outwardly rectifying (VSOR) Cl- currents were examined in C127 cells. Application of PDGF peptides failed to affect the Cl- currents in control and BPV-transfected cells, although C127 cells are known to endogenously express PDGF receptors. In contrast, EGF peptides significantly increased the VSOR Cl- current in control cells. However, they failed to induce further augmentation of the current in BPV-transfected cells. VSOR Cl- currents were inhibited by tyrphostin B46, an inhibitor of the EGF receptor tyrosine kinase, in both control and BPV-transfected cells. The IC50 value in BPV-transfected cells (12 micro M) was lower than that in control cells (31 micro M). However, the VSOR Cl- currents in both cell types were insensitive to tyrphostin AG1296, an inhibitor of the PDGF receptor tyrosine kinase. The rate of regulatory volume decrease (RVD) was markedly diminished by tyrphostin B46 but not significantly affected by tyrphostin AG1296. We thus conclude that the EGF receptor tyrosine kinase upregulates the activity of the VSOR Cl- channel, mainly by enhancing the volume sensitivity.

PC12 cell activation by epidermal growth factor receptor: role of autophosphorylation sites

PC12 cells have been used as a model system for neuronal differentiation due to their ability to alter their phenotype to a sympathetic neuron-like cell in response to nerve growth factor or fibroblast growth factor. Under some conditions, epidermal growth factor (EGF) can also induce PC12 cells to differentiate. To study signaling from the EGF receptor without the confounding effects of endogenous EGF receptors we generated a chimeric receptor comprised of the ectodomain of platelet-derived growth factor (PDGF) receptor in-frame with the transmembrane and cytoplasmic domains of EGF receptor, termed PER. Expression of PER in PC12 cells confers the ability of PDGF to induce differentiation whereas PDGF has no effect on untransfected PC12 cells. This response is kinase activity-dependent since a kinase-deficient mutant (K721M) fails to induce differentiation in response to PDGF. Mutation of five tyrosine residues that are autophosphorylated in response to EGF either individually or in combination had minimal effects on the ability of these receptors to induce morphological PC12 cell differentiation. The PER mutant with all five autophosphorylation sites mutated to phenylalanine (5YF) was equivalently capable of interacting with several important signaling molecules, including Shc, Grb2, Gab1, phospholipase Cgamma, and Cbl. Furthermore, both the phosphatidylinositol 3-kinase (PI3K)/Akt and Ras/Erk pathways were activated in a sustained manner when PER or 5YF-expressing cells were stimulated with PDGF. Our results show that the five autophosphorylation sites in the extra-kinase C-terminal domain of EGFR are not required for the ability of EGFR to induce morphological differentiation of PC12 cells.

Platelet-derived growth factor regulates K-Cl cotransport in vascular smooth muscle cells

Platelet-derived growth factor (PDGF), a potent serum mitogen for vascular smooth muscle cells (VSMCs), plays an important role in membrane transport regulation and in atherosclerosis. K-Cl cotransport (K-Cl COT/KCC), the coupled-movement of K and Cl, is involved in ion homeostasis. VSMCs possess K-Cl COT activity and the KCC1 and KCC3 isoforms. Here, we report on the effect of PDGF on K-Cl COT activity and mRNA expression in primary cultures of rat VSMCs. K-Cl COT was determined as the Cl-dependent Rb influx and mRNA expression by semiquantitative RT-PCR. Twenty four-hour serum deprivation inhibited basal K-Cl COT activity. Addition of PDGF increased total protein content and K-Cl COT activity in a time-dependent manner. PDGF activated K-Cl COT in a dose-dependent manner, both acutely (10 min) and chronically (12 h). AG-1296, a selective inhibitor of the PDGF receptor tyrosine kinase, abolished these effects. Actinomycin D and cycloheximide had no effect on the acute PDGF activation of K-Cl COT, suggesting posttranslational regulation by the drug. Furthermore, PDGF increased KCC1 and decreased KCC3 mRNA expression in a time-dependent manner. These results indicate that chronic activation of K-Cl COT activity by PDGF may involve regulation of the two KCC mRNA isoforms, with KCC1 playing a dominant role in the mechanism of PDGF-mediated activation.

Functional analysis of aortic endothelial cells expressing mutant PDGF receptors with respect to expression of matrix metalloproteinase-3

Platelet-derived growth factor (PDGF) stimulates expression of matrix metalloproteinases (MMPs), including stromelysin-1 (MMP-3). Induction of these expressions is known to occur during the course of atherosclerosis, tumor invasion, and metastasis. We investigated PDGF-alpha receptor (alphaR)- and beta receptor (betaR)-mediated signaling pathways for the expression of MMP-3 and invasion activity using porcine aortic endothelial (PAE) cells with stable expression of normal or mutated PDGF receptors. RT-PCR and Western blot analyses revealed that PDGF-BB induces MMP-3 expression in PAE cells that exclusively express either the PDGF-alphaR or the -betaR, but not in non-transfected control cells. To identify the signals necessary for PDGF receptor-mediated induction of MMP-3 expression, several lines of PAE cells expressing mutant PDGF receptors were further analyzed. Cells expressing mutant PDGF receptors unable to associate with Src or PLCgamma, retained the ability to induce MMP-3 expression as a result of PDGF-BB stimulation. However, incubation with PDGF-BB did not induce MMP-3 expression in cells expressing a mutant PDGF-betaR unable to associate with phosphatidylinositol 3(')-kinase (PI3K). LY294002, a PI3K inhibitor, reduced PDGF-BB-stimulated MMP-3 expression in PAE cells expressing wild-type PDGF receptors. In contrast, PDGF-BB induced MMP-3 expression in the presence of U-73122, a PLCgamma inhibitor. Moreover, PDGF-BB enhanced the invasiveness of cells expressing wild type PDGF-beta receptors, but not of cells expressing mutant PDGF-betaRs impaired in their ability to associate with PI3K. In light of these results, it appears that PDGF-BB is capable of inducing MMP-3 expression through both the PDGF-alphaR and the -betaR, and the effects are contributed by the PI3K-mediated transduction pathways.

Collaboration of JNKs and ERKs in nerve growth factor regulation of the neurofilament light chain promoter in PC12 cells

Nerve growth factor (NGF) induces transcription-dependent neural differentiation of PC12 cells, and the ERK family of MAPKs has been implicated as the dominant signal pathway that mediates this response. We employed a neurofilament light chain (NFLC) promoter-luciferase (NFLC-Luc) reporter to define the role of the ERKs as well as additional MAPK pathways in NGF induction of this neural specific gene. Constitutive active forms of c-Raf-1, MEKK1 and MKK6, proximal regulators of the ERKs, JNKs, and p38 MAPKs, respectively, all stimulated NFLC-Luc activity. NFLC-Luc activity stimulated by NGF, however, was partially (approximately 50%) inhibited by the MEK inhibitor, PD098059, or by co-transfection of kinase-inactive MEK1 but not by the p38 MAPK inhibitor, SB203580, indicating a role for the ERKs, but not the p38 MAPKs, in NGF regulation of the NFLC promoter. Importantly, a gain-of-function MKK7-JNK3 fusion protein stimulated NFLC-Luc and synergized with gain-of-function c-Raf-1 to activate the NFLC promoter. In addition, transfection of kinase-inactive forms of MEK1 and MKK7 produced an additive inhibition of NGF-stimulated NFLC-Luc relative to either inhibitor alone. These findings indicate that the ERK and JNK pathways collaborate downstream of the NGF receptor for regulation of the NFLC promoter. Truncation analysis and electromobility shift assays established the requirement for a cAMP-response element/activating transcription factor-like site in the NFLC promoter that minimally interacts with constitutively expressed cAMP-response element-binding protein and JunD as well as c-Jun which is induced by NGF in an ERK-dependent manner. Cumulatively, these findings indicate that the ERK pathway requires collaboration with the JNK pathway for maximal activation of the NFLC gene in PC12 cells through the integrated control of c-Jun function.

Erythropoietin activates two distinct signaling pathways required for the initiation and the elongation of c-myc

Erythropoietin (Epo) stimulation of erythroid cells results in the activation of several kinases and a rapid induction of c-myc expression. Protein kinase C is necessary for Epo up-regulation of c-myc by promoting elongation at the 3'-end of exon 1. PKCepsilon mediates this signal. We now show that Epo triggers two signaling pathways to c-myc. Epo rapidly up-regulated Myc protein in BaF3-EpoR cells. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 blocked Myc up-regulation in a concentration-dependent manner but had no effect on the Epo-induced phosphorylation of ERK1 and ERK2. LY294002 also had no effect on Epo up-regulation of c-fos. MEK1 inhibitor PD98059 blocked both the c-myc and the c-fos responses to Epo. PD98059 and the PKC inhibitor H7 also blocked the phosphorylation of ERK1 and ERK2. PD98059 but not LY294002 inhibited Epo induction of ERK1 and ERK2 phosphorylation in normal erythroid cells. LY294002 blocked transcription of c-myc at exon 1. PD98059 had no effect on transcription from exon 1 but, rather, blocked Epo-induced c-myc elongation at the 3'-end of exon 1. These results identify two Epo signaling pathways to c-myc, one of which is PI3K-dependent operating on transcriptional initiation, whereas the other is mitogen-activated protein kinase-dependent operating on elongation.

The TrkB receptor tyrosine kinase regulates cellular proliferation via signal transduction pathways involving SHC, PLCgamma, and CBL

The TrkB protein tyrosine kinase is a high affinity receptor for brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4). TrkB autophosphorylation occurs on five cytoplasmic tyrosines: Y484, Y670, Y674, Y675, and Y785. Using site directed mutagenesis, we have assessed the importance of TrkB tyrosines 484 and 785 in affecting TrkB-mediated signaling events leading to NIH 3T3 cell mitogenesis and survival. Mutation of TrkB tyrosine 484, while having no affect on BDNF-inducible PLCgamma and Cbl tyrosine phosphorylation, is essential for the phosphorylation of Shc, the complete activation of extracellular regulated kinase 1/2 (ERK1/2) and the induction of c-fos protein synthesis. In contrast, mutation of Y785 does not significantly affect BDNF-inducible Shc phosphorylation, ERK1/2 activation, or c-fos protein synthesis, but completely inhibits the tyrosine phosphorylation of PLCgamma and Cbl. These data indicate that both ERK-dependent and ERK-independent signaling pathways lead to BDNF-inducible mitogenesis and survival.

The selective and inducible activation of endogenous PI 3-kinase in PC12 cells results in efficient NGF-mediated survival but defective neurite outgrowth

The Trk/Nerve Growth Factor receptor mediates the rapid activation of a number of intracellular signaling proteins, including phosphatidylinositol 3-kinase (PI 3-kinase). Here, we describe a novel, NGF-inducible system that we used to specifically address the signaling potential of endogenous PI 3-kinase in NGF-mediated neuronal survival and differentiation processes. This system utilizes a Trk receptor mutant (Trk(def)) lacking sequences Y490, Y785 and KFG important for the activation of the major Trk targets; SHC, PLC-gammal, Ras, PI 3-kinase and SNT. Trk(def) was kinase active but defective for NGF-induced responses when stably expressed in PC12nnr5 cells (which lack detectable levels of TrkA and are non-responsive to NGF). The PI 3-kinase consensus binding site, YxxM (YVPM), was introduced into the insert region within the kinase domain of Trk(def). NGF-stimulated tyrosine phosphorylation of the Trk(def)+PI 3-kinase addback receptor, resulted in the direct association and selective activation of PI 3-kinase in vitro and the production of PI(3,4)P2 and PI(3,4,5)P3 in vivo (comparable to wild-type). PC12nnr5 cells stably expressing Trk(def) + PI 3-kinase, initiated neurite outgrowth but failed to stably extend and maintain these neurites in response to NGF as compared to PC12 parental cells, or PC12nnr5 cells overexpressing wild-type Trk. However, Trk(def) + PI 3-kinase was fully competent in mediating NGF-induced survival processes. We propose that while endogenous PI 3-kinase can contribute in part to neurite initiation processes, its selective activation and subsequent signaling to downstream effectors such as Akt, functions mainly to promote cell survival in the PC12 system.

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

Disruption of gap junctional communication by the platelet-derived growth factor is mediated via multiple signaling pathways

The platelet-derived growth factor (PDGF) mediates its cellular functions via activation of its receptor tyrosine kinase followed by the recruitment and activation of several signaling molecules. These signaling molecules then initiate specific signaling cascades, finally resulting in distinct physiological effects. To delineate the PDGF signaling pathway responsible for the disruption of gap junctional communication (GJC), wild-type PDGF receptor beta (PDGFRbeta) and a series of PDGFRbeta mutants were expressed in T51B rat liver epithelial cells. In cells expressing wild-type PDGFRbeta, PDGF induced disruption of GJC and phosphorylation of a gap junctional protein, connexin-43 (Cx43), which required activation of mitogen-activated protein kinase, although involvement of additional factors was also evident. In the F5 mutant lacking binding sites for phosphatidylinositol 3-kinase, GTPase-activating protein, SHP-2, and phospholipase Cgamma1 (PLCgamma1), PDGF induced mitogen-activated protein kinase, but failed to affect GJC or Cx43, indicating involvement of additional signals presumably initiated by one or more of the mutated binding sites. Examination of the single-site mutants revealed that PDGF effects were not mediated via a single signaling component. This was confirmed by the "add-back" mutants, which showed that restoration of either SHP-2 or PLCgamma1 binding was sufficient to propagate the GJC inhibitory actions of PDGF. Further analysis showed that activation of PLCgamma1 is involved in Cx43 phosphorylation, which surprisingly failed to correlate with GJC blockade. The results of our study demonstrate that PDGF-induced disruption of GJC can be mediated by multiple signaling pathways and requires participation of multiple components.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Growth factor receptor tyrosine kinases acutely regulate neuronal sodium channels through the src signaling pathway

Growth factor receptor tyrosine kinase (RTK)-activated signaling pathways are well established regulators of neuronal growth and development, but whether these signals provide mechanisms for acute modulation of neuronal activity is just beginning to be addressed. We show in pheochromocytoma (PC12) cells that acute application of ligands for both endogenous RTKs [trkA, basic FGF (bFGF) receptor, and epidermal growth factor (EGF) receptor] and ectopically expressed platelet-derived growth factor (PDGF) receptors rapidly inhibits whole-cell sodium channel currents, coincident with a hyperpolarizing shift in the voltage dependence of inactivation. Sodium channel inhibition by trkA and PDGF receptors is mutually occlusive, suggestive of a common signal transduction mechanism. Furthermore, specific inhibitors for trkA and PDGF RTK activities abrogate sodium channel inhibition in response to NGF and PDGF, respectively, showing that the intrinsic RTK activity of these receptors is necessary for sodium channel inhibition. Use of PDGF receptor mutants deficient for specific signaling activities demonstrated that this inhibition is dependent on RTK interaction with Src but not with other RTK-associated signaling molecules. Inhibition was also compromised in cells expressing dominant-negative Ras. These results suggest a possible mechanism for acute physiological actions of RTKs, and they indicate regulatory functions for Ras and Src that may complement the roles of these signaling proteins in long-term neuronal regulation.

Growth factor receptor tyrosine kinases acutely regulate neuronal sodium channels through the src signaling pathway

Growth factor receptor tyrosine kinase (RTK)-activated signaling pathways are well established regulators of neuronal growth and development, but whether these signals provide mechanisms for acute modulation of neuronal activity is just beginning to be addressed. We show in pheochromocytoma (PC12) cells that acute application of ligands for both endogenous RTKs [trkA, basic FGF (bFGF) receptor, and epidermal growth factor (EGF) receptor] and ectopically expressed platelet-derived growth factor (PDGF) receptors rapidly inhibits whole-cell sodium channel currents, coincident with a hyperpolarizing shift in the voltage dependence of inactivation. Sodium channel inhibition by trkA and PDGF receptors is mutually occlusive, suggestive of a common signal transduction mechanism. Furthermore, specific inhibitors for trkA and PDGF RTK activities abrogate sodium channel inhibition in response to NGF and PDGF, respectively, showing that the intrinsic RTK activity of these receptors is necessary for sodium channel inhibition. Use of PDGF receptor mutants deficient for specific signaling activities demonstrated that this inhibition is dependent on RTK interaction with Src but not with other RTK-associated signaling molecules. Inhibition was also compromised in cells expressing dominant-negative Ras. These results suggest a possible mechanism for acute physiological actions of RTKs, and they indicate regulatory functions for Ras and Src that may complement the roles of these signaling proteins in long-term neuronal regulation.