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

Nerve growth factor regulates sodium but not potassium channel currents in sympathetic B neurons of adult bullfrogs

The TTX-sensitive and -resistant components of the voltage-gated Na(+) current (TTX-s I(Na) and TTX-r I(Na)) are increased within 2 wk of cutting the axons of B-cells in bullfrog paravertebral sympathetic ganglia (BFSG). Axotomy also increases the noninactivating, voltage-activated K(+) current (M current I(M)), whereas delayed rectifier K(+) current (I(K)) is reduced. We found that similar effects were produced when BFSG B cells were dissociated from adult bullfrogs and maintained in a defined-medium, neuron-enriched, low-density, serum-free culture. Thus the density of TTX-s I(Na), TTX-r I(Na), and I(M) were transiently increased, whereas I(K) density was decreased. Reduction in voltage-sensitive, Ca(2+)-dependent K(+) current (I(C)) was attributed to previously documented decreases in Ca(2+) channel current (I(Ca)). To test whether axotomy- or culture-induced changes in ion channel function reflect loss of retrograde influence of nerve growth factor (NGF), we examined the effect of murine beta-NGF on TTX-s I(Na), TTX-r I(Na), I(K), and I(M). Culture of neurons for 15 days in the presence of NGF (200 ng/ml), more than doubled total I(Na) density but did not enhance neurite outgrowth. The TTX-r I(Na) density was increased about threefold and the TTX-s I(Na) density increased 2.4-fold. NGF did not affect the activation or inactivation kinetics of the total Na(+) conductance. Effects of NGF were blocked by the transcription inhibitors, cordycepin (20 microM) and actinomycin D (0.01 microg/ml). I(K) and I(M) were unaffected by NGF, and although I(C) was enhanced, this likely reflected the known effect of NGF on I(Ca) in BFSG neurons. Na(+) channel synthesis and/or expression in adult sympathetic neurons is therefore subject to selective regulation by NGF. Despite this, the increase in I(Na) and I(M) as well as the decrease in I(K) seen in BFSG neurons in culture or after axotomy cannot readily be explained in terms of alterations in the availability of target-derived NGF.

Inositol 1,4,5-trisphosphate-independent calcium signalling by platelet-derived growth factor in the human SH-SY5Y neuroblastoma cell

In adherent SH-SY5Y human neuroblastoma cells, activation of G-protein-coupled muscarinic M3 receptors evoked a biphasic elevation of both intracellular [Ca(2+)] ([Ca(2+)]i) and inositol-1,4,5-trisphosphate (D-Ins(1,4,5)P3) mass. In both cases, temporal profiles consisted of rapid transient elevations followed by a decline to a lower, yet sustained level. In contrast, platelet-derived growth factor (PDGF), a receptor tyrosine kinase agonist acting via PDGF receptor b chains in these cells, elicited a slow and transient elevation of [Ca(2+)]i that returned to basal levels within 5 to 10 min with no evidence of inositol phosphate generation. Full responses for either receptor type required intracellular and extracellular Ca(2+) and mobilization of a shared thapsigargin-sensitive intracellular Ca(2+) store. Strategies that affected the ability of D-Ins(1,4,5)P3 to interact with the Ins(1,4,5)P3-receptor demonstrated an Ins(1,4,5)P3-dependency of the muscarinic receptor-mediated elevation of [Ca(2+)]i but showed that PDGF-mediated elevations of [Ca(2+)]i are Ins(1,4,5)P3-independent in these cells.

Modulation of excitability in Aplysia tail sensory neurons by tyrosine kinases

Tyrosine kinases have recently been shown to modulate synaptic plasticity and ion channel function. We show here that tyrosine kinases can also modulate both the baseline excitability state of Aplysia tail sensory neurons (SNs) as well as the excitability induced by the neuromodulator serotonin (5HT). First, we examined the effects of increasing and decreasing tyrosine kinase activity in the SNs. We found that tyrosine kinase inhibitors decrease baseline SN excitability in addition to attenuating the increase in excitability induced by 5HT. Conversely, functionally increasing cellular tyrosine kinase activity in the SNs by either inhibiting opposing tyrosine phosphatase activity or by direct injection of an active tyrosine kinase (Src) induces increases in SN excitability in the absence of 5HT. Second, we examined the interaction between protein kinase A (PKA), which is known to mediate 5HT-induced excitability changes in the SNs, and tyrosine kinases, in the enhancement of SN excitability. We found that the tyrosine kinases function downstream of PKA activation since tyrosine kinase inhibitors reduce excitability induced by activators of PKA. Finally, we examined the role of tyrosine kinases in other forms of 5HT-induced plasticity in the SNs. We found that while tyrosine kinase inhibitors attenuate excitability produced by 5HT, they have no effect on short-term facilitation (STF) of the SN-motor neuron (MN) synapse induced by 5HT. Thus tyrosine kinases modulate different forms of SN plasticity independently. Such differential modulation would have important consequences for activity-dependent plasticity in a variety of neural circuits.

Neuromodulation of Na+ channels: an unexpected form of cellular plasticity

Voltage-gated Na+ channels set the threshold for action potential generation and are therefore good candidates to mediate forms of plasticity that affect the entire neuronal output. Although early studies led to the idea that Na+ channels were not subject to modulation, we now know that Na+ channel function is affected by phosphorylation. Furthermore, Na+ channel modulation is implicated in the control of input-output relationships in several types of neuron and seems to be involved in phenomena as varied as cocaine withdrawal, hyperalgesia and light adaptation. Here we review the available evidence for the regulation of Na+ channels by phosphorylation, its molecular mechanism, and the possible ways in which it affects neuronal function.

The small Gtpase ras is involved in growth factor-regulated expression of the alpha1 integrin subunit in PC12 cells

PC12 cells interact with several growth factors (e. g. EGF, FGF, and NGF) via specific tyrosine receptor kinases, resulting in cell proliferation or neuronal differentiation. The small GTPase Ras is known to be involved in downstream signaling of these growth factor receptors. Furthermore, cell-matrix interactions mediated by integrins, as well as integrin-induced signaling, are also involved in growth factor-stimulated signal transduction in PC12 cells. In this study we determined the expression of the alpha1 integrin subunit in response to EGF and NGF in PC12 wild-type (wt) cells, and in PC12 cells overexpressing an inactive H-Ras protein (RasN17). In PC12 wt cells, alpha1 integrin expression is upregulated by EGF and NGF. Cell surface expression of alpha1beta1integrin is also enhanced in growth factor-treated cells. This upregulation leads to increased alpha1beta1-specific adhesion to collagen. In cells expressing the dominant-negative RasN17 variant, alpha1 integrin expression and alpha1beta1-specific adhesion remain unchanged in response to both growth factors.

Nerve growth factor rapidly induces prolonged acetylcholine release from cultured basal forebrain neurons: differentiation between neuromodulatory and neurotrophic influences

Long-term exposure to nerve growth factor (NGF) is well established to have neurotrophic effects on basal forebrain cholinergic neurons, but its potential actions as a fast-acting neuromodulator are not as well understood. We report that NGF (0.1-100 ng/ml) rapidly (<60 min) and robustly enhanced constitutive acetylcholine (ACh) release (148-384% of control) from basal forebrain cultures without immediate persistent increases in choline acetyltransferase activity. More ACh was released in response to NGF when exposure was coupled with a higher depolarization level, suggesting activity dependence. In a long-term potentiation-like manner, brief NGF exposure (10 ng/ml; 60 min) induced robust and prolonged increases in ACh release, a capacity that was shared with the other neurotrophins. K252a (10-100 nm), BAPTA-AM (25 microm), and Cd(2+) (200 microm) prevented NGF enhancement of ACh release, suggesting the involvement of TrkA receptors, Ca(2+), and voltage-gated Ca(2+) channels, respectively. Forskolin (10 microm), a cAMP generator, enhanced constitutive ACh release but did not interact synergistically with NGF. Tetrodotoxin (1 microm) and cycloheximide (2 microm) did not prevent NGF-induced ACh release, indicative of action at the level of the cholinergic nerve terminal and that new protein synthesis is not required for this neurotransmitter-like effect, respectively. In contrast, after a 24 hr NGF treatment, distinct protein synthesis-dependent and independent effects on choline acetyltransferase activity and ACh release were observed. These results indicate that neuromodulator/neurotransmitter-like (protein synthesis-independent) and neurotrophic (translation-dependent) actions likely make distinct contributions to the enhancement of cholinergic activity by NGF.

Serum deprivation and NGF induce and modulate voltage-gated Na(+) currents in human astrocytoma cell lines

Glial tumor cells derived from primary tissue express large voltage-gated Na(+) currents, whereas glioma cell lines usually lack this feature. We studied the effect of serum deprivation on the expression of Na(+) currents in two astrocytoma cell lines (1321N1 and A172). Serum deprivation for more than 2 days sufficed to induce large Na(+) currents in both cell lines; 300 nM of the specific blocker of voltage-gated Na(+) channels, tetrodotoxin, blocked these currents by about 85%. During serum deprivation, the cells also underwent morphological changes that were characterized by cell rounding and outgrowth of processes. Treatment with 100 ng/ml nerve growth factor (NGF) promoted these morphological changes and also accelerated the development of Na(+) currents. In 1321N1 cells, NGF increased the Na(+) current density after short serum deprivation (3--6 d) and changed several gating properties after longer serum deprivation (9--13 d). In comparison with cells from the early culture stage (3--6 d), the steady-state inactivation of the Na(+) current was shifted by -24 mV in NGF-treated cells from the late (9--13 d) culture stage. In untreated cells, this shift was only -13 mV. NGF accelerated the kinetics of inactivation and shifted the current-voltage relationship in cells from the late culture stage by -14 mV. In A172 cells, most of these effects were present already after short serum deprivation either in presence or absence of NGF. It is concluded that in astrocytoma cells, Na(+) currents are induced by serum deprivation and are modulated by NGF. This result supports the idea that NFG controls Na(+) currents in these cells by autocrine stimulation.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Tyrosine phosphatase inhibitors selectively antagonize beta-adrenergic receptor-dependent regulation of cardiac ion channels

beta-Adrenergic receptor stimulation regulates the activity of several different cardiac ion channels through an adenylate cyclase/cAMP/protein kinase A-dependent mechanism. Previous work has suggested that basal tyrosine kinase activity attenuates the beta-adrenergic responsiveness of these cardiac ion channels, supporting the idea that tyrosine phosphorylation exerts an inhibitory effect at some point in the common signaling pathway. To determine which element in the beta-adrenergic pathway is regulated by tyrosine kinase activity, we studied the effects of various protein tyrosine phosphatase (PTP) inhibitors on the cAMP-dependent regulation of the L-type Ca(2+) current in guinea pig ventricular myocytes. Three such compounds, sodium orthovanadate, peroxovanadate, and bpV(phen), had no effect on the basal Ca(2+) current, yet each caused a pronounced inhibition of the Ca(2+) current stimulated by the beta-adrenergic receptor agonist isoproterenol. These observations are consistent with the idea that basal tyrosine kinase activity is capable of inhibiting beta-adrenergic responses. However, these PTP inhibitors had no effect on cAMP-dependent stimulation of the Ca(2+) current via activation of adenylate cyclase with forskolin or activation of H(2)-histaminergic receptors with histamine. These results are consistent with the idea that inhibition of PTP activity produces an inhibitory effect involving a tyrosine kinase-dependent mechanism acting selectively at the level of the beta-adrenergic receptor. This signaling mechanism does not seem to be linked to tyrosine kinase activity associated with insulin and insulin-like growth factor receptors, because acute exposure to agonists of these receptors did not inhibit isoproterenol regulation of the Ca(2+) current.

TrkB activation by brain-derived neurotrophic factor inhibits the G protein-gated inward rectifier Kir3 by tyrosine phosphorylation of the channel

G protein-activated inwardly rectifying potassium channels (Kir3) are widely expressed throughout the brain, and regulation of their activity modifies neuronal excitability and synaptic transmission. In this study, we show that the neurotrophin brain-derived neurotrophic factor (BDNF), through activation of TrkB receptors, strongly inhibited the basal activity of Kir3. This inhibition was subunit dependent as functional homomeric channels of either Kir3.1 or Kir3.4 were significantly inhibited, whereas homomeric channels composed of Kir3.2 were insensitive. The general tyrosine kinase inhibitors genistein, Gö 6976, and K252a but not the serine/threonine kinase inhibitor staurosporine blocked the BDNF-induced inhibition of the channel. BDNF was also found to directly stimulate channel phosphorylation because Kir3.1 immunoprecipitated from BDNF-stimulated cells showed enhanced labeling by anti-phosphotyrosine-specific antibodies. The BDNF effect required specific tyrosine residues in the amino terminus of Kir3.1 and Kir3.4 channels. Mutations of either Tyr-12, Tyr-67, or both in Kir3.1 or mutation of either Tyr-32, Tyr-53, or both of Kir3. 4 channels to phenylalanine significantly blocked the BDNF-induced inhibition. The insensitive Kir3.2 was made sensitive to BDNF by adding a tyrosine (D41Y) and a lysine (P32K) upstream to generate a phosphorylation site motif analogous to that present in Kir3.4. These results suggest that neurotrophin activation of TrkB receptors may physiologically control neuronal excitability by direct tyrosine phosphorylation of the Kir3.1 and Kir3.4 subunits of G protein-gated inwardly rectifying potassium channels.

Potentiation of neuronal L calcium channels by IGF-1 requires phosphorylation of the alpha1 subunit on a specific tyrosine residue

Insulin-like growth factor 1 (IGF-1) rapidly potentiates N and L calcium channel currents in cerebellar granule neurons by an unknown mechanism. Here, we show that the L channel alpha1C subunit is tyrosine phosphorylated in response to IGF-1. Moreover, expression of kinase-dead c-Src in neurons or acute block of Src family kinases with a cell-permeable inhibitor specifically blocks L channel potentiation. Purified Src kinase phosphorylates tyrosine residue Y2122 of the C terminus of neuronal alpha1C in vitro, and c- and v-Src directly bind the C terminus. When expressed in neuroblastoma cells, point mutation of Y2122 prevents both tyrosine phosphorylation of alpha1C and IGF-1 potentiation. Our data provide a biochemical mechanism whereby phosphorylation of a single specific tyrosine residue rapidly modifies ion channel physiology.

Pituitary adenylyl cyclase-activating peptide activates multiple intracellular signaling pathways to regulate ion channels in PC12 cells

Pituitary adenylyl cyclase-activating peptide (PACAP) stimulates calcium transients and catecholamine secretion in adrenal chromaffin and PC12 cells. The PACAP type 1 receptor in these cells couples to both adenylyl cyclase and phospolipase C pathways, but although phospolipase C has been implicated in the response to PACAP, the role of adenylyl cyclase is unclear. In this study, we show that PACAP38 stimulates Ca(2+) influx in PC12 cells by activating a cation current that depends upon the dual activation of both the PLC and adenylyl cyclase signaling pathways but does not involve protein kinase C. In activating the current, PACAP38 has to overcome an inhibitory effect of Ras. Thus, in cells expressing a dominant negative form of Ras (PC12asn17-W7), PACAP38 induced larger, more rapidly activating currents. This effect of Ras could be overidden by intracellular guanosine-5'-O-3-(thio)triphosphate (GTPgammaS), suggesting that it was mediated by inhibition of downstream G proteins. Ras may also inhibit the current through a G protein-independent mechanism, because cAMP analogues activated the current in PC12asn17-W7 cells, provided GTPgammaS was present, but not in PC12 cells expressing wild type Ras. We conclude that coupling of PACAP to both adenylyl cyclase and phospholipase C is required to activate Ca(2+) influx in PC12 cells and that tonic inhibition by Ras delays and limits the response.

A sodium channel signaling complex: modulation by associated receptor protein tyrosine phosphatase beta

Voltage-gated sodium channels in brain neurons were found to associate with receptor protein tyrosine phosphatase beta (RPTPbeta) and its catalytically inactive, secreted isoform phosphacan, and this interaction was regulated during development. Both the extracellular domain and the intracellular catalytic domain of RPTPbeta interacted with sodium channels. Sodium channels were tyrosine phosphorylated and were modulated by the associated catalytic domains of RPTPbeta. Dephosphorylation slowed sodium channel inactivation, positively shifted its voltage dependence, and increased whole-cell sodium current. Our results define a sodium channel signaling complex containing RPTPbeta, which acts to regulate sodium channel modulation by tyrosine phosphorylation.

Brain insulin receptor causes activity-dependent current suppression in the olfactory bulb through multiple phosphorylation of Kv1.3

Insulin and insulin receptor (IR) kinase are found in abundance in discrete brain regions yet insulin signaling in the CNS is not understood. Because it is known that the highest brain insulin-binding affinities, insulin-receptor density, and IR kinase activity are localized to the olfactory bulb, we sought to explore the downstream substrates for IR kinase in this region of the brain to better elucidate the function of insulin signaling in the CNS. First, we demonstrate that IR is postnatally and developmentally expressed in specific lamina of the highly plastic olfactory bulb (OB). ELISA testing confirms that insulin is present in the developing and adult OB. Plasma insulin levels are elevated above that found in the OB, which perhaps suggests a differential insulin pool. Olfactory bulb insulin levels appear not to be static, however, but are elevated as much as 15-fold after a 72-h fasting period. Bath application of insulin to cultured OB neurons acutely induces outward current suppression as studied by the use of traditional whole-cell and single-channel patch-clamp recording techniques. Modulation of OB neurons is restricted to current magnitude; IR kinase activation does not modulate current kinetics of inactivation or deactivation. Transient transfection of human embryonic kidney cells with cloned Kv1.3 ion channel, which carries a large proportion of the outward current in these neurons, revealed that current suppression was the result of multiple tyrosine phosphorylation of Kv1.3 channel. Y to F single-point mutations in the channel or deletion of the kinase domain in IR blocks insulin-induced modulation and phosphorylation of Kv1.3. Neuromodulation of Kv1.3 current in OB neurons is activity dependent and is eliminated after 20 days of odor/sensory deprivation induced by unilateral naris occlusion at postnatal day 1. IR kinase but not Kv1.3 expression is downregulated in the OB ipsilateral to the occlusion, as demonstrated in cryosections of right (control) and left (sensory-deprived) OB immunolabeled with antibodies directed against these proteins, respectively. Collectively, these data support the hypothesis that the hormone insulin acts as a multiply functioning molecule in the brain: IR signaling in the CNS could act as a traditional growth factor during development, be altered during energy metabolism, and simultaneously function to modulate electrical activity via phosphorylation of voltage-gated ion channels.

Tyrosine phosphorylation modulates the osmosensitivity of volume-dependent taurine efflux from glial cells in the rat supraoptic nucleus

1. In the supraoptic nucleus, taurine, selectively released in an osmodependent manner by glial cells through volume-sensitive anion channels, is likely to inhibit neuronal activity as part of the osmoregulation of vasopressin release. We investigated the involvement of various kinases in the activation of taurine efflux by measuring [3H]taurine release from rat acutely isolated supraoptic nuclei. 2. The protein tyrosine kinase inhibitors genistein and tyrphostin B44 specifically reduced, but did not suppress, both the basal release of taurine and that evoked by a hypotonic stimulus. Inhibition of tyrosine phosphatase by orthovanadate had the opposite effect. 3. The tyrosine kinase and phosphatase inhibitors shifted the relationship between taurine release and medium osmolarity in opposite directions, suggesting that tyrosine phosphorylation modulates the osmosensitivity of taurine release, but is not necessary for its activation. 4. Genistein also increased the amplitude of the decay of the release observed during prolonged hypotonic stimulation. Potentiation of taurine release by tyrosine kinases could serve to maintain a high level of taurine release in spite of cell volume regulation. 5. Taurine release was unaffected by inhibitors and/or activators of PKA, PKC, MEK and Rho kinase. 6. Our results demonstrate a unique regulation by protein tyrosine kinase of the osmosensitivity of taurine efflux in supraoptic astrocytes. This points to the presence of specific volume-dependent anion channels in these cells, or to a specific activation mechanism or regulatory properties. This may relate to the particular role of the osmodependent release of taurine in this structure in the osmoregulation of neuronal activity.

Acute application of NGF increases the firing rate of aged rat basal forebrain neurons

Nerve growth factor (NGF) has been widely used in animal models to ameliorate age-related neurodegeneration, but it cannot cross the blood-brain barrier (BBB). NGF conjugated to an antibody against the transferrin receptor (OX-26) crosses the BBB and affects the biochemistry and morphology of NGF-deprived basal forebrain neurons. The rapid actions of NGF, including electrophysiological effects on these neurons, are not well understood. In the present study, two model systems in which basal forebrain neurons either respond dysfunctionally to NGF (aged rats) or do not have access to target-derived NGF (intraocular transplants of forebrain neurons) were tested. One group of transplanted and one group of aged animals received unconjugated OX-26 and NGF comixture as a control, while other groups received replacement NGF in the form of OX-26-NGF conjugate during the 3 months preceding the electrophysiological recording session. Neurons from animals in both the transplanted and aged control groups showed a significant increase in firing rate in response to acute NGF application, while none of the conjugate-treated groups or young intact rats showed any response. After the recordings, forebrain transplants and aged brains were immunocytochemically stained for the low-affinity NGF receptor. All conjugate treatment groups showed significantly greater staining intensity compared to controls. These data from both transplants and aged rats in situ indicate that NGF-deprived basal forebrain neurons respond to acute NGF with an increased firing rate. This novel finding may have importance even for long-term biological effects of this trophic factor in the basal forebrain.

Cellular aspects of trophic actions in the nervous system

During the past three decades the number of molecules exhibiting trophic actions in the brain has increased drastically. These molecules promote and/or control proliferation, differentiation, migration, and survival (sometimes even the death) of their target cells. In this review a comprehensive overview of small diffusible factors showing trophic actions in the central nervous system (CNS) is given. The factors discussed are neurotrophins, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin-like growth factors, ciliary neurotrophic factor and related molecules, glial-derived growth factor and related molecules, transforming growth factor-beta and related molecules, neurotransmitters, and hormones. All factors are discussed with respect to their trophic actions, their expression patterns in the brain, and molecular aspects of their receptors and intracellular signaling pathways. It becomes evident that there does not exist "the" trophic factor in the CNS but rather a multitude of them interacting with each other in a complicated network of trophic actions forming and maintaining the adult nervous system.

A dynamically regulated 14-3-3, Slob, and Slowpoke potassium channel complex in Drosophila presynaptic nerve terminals

Slob is a novel protein that binds to the carboxy-terminal domain of the Drosophila Slowpoke (dSlo) calcium-dependent potassium (K(Ca)) channel. A yeast two-hybrid screen with Slob as bait identifies the zeta isoform of 14-3-3 as a Slob-binding protein. Coimmunoprecipitation experiments from Drosophila heads and transfected cells confirm that 14-3-3 interacts with dSlo via Slob. All three proteins are colocalized presynaptically at Drosophila neuromuscular junctions. Two serine residues in Slob are required for 14-3-3 binding, and the binding is dynamically regulated in Drosophila by calcium/calmodulin-dependent kinase II (CaMKII) phosphorylation. 14-3-3 coexpression dramatically alters dSlo channel properties when wild-type Slob is present but not when a double serine mutant Slob that is incapable of binding 14-3-3 is present. The results provide evidence for a dSlo/Slob/14-3-3 regulatory protein complex.

Akt-dependent potentiation of L channels by insulin-like growth factor-1 is required for neuronal survival

The insulin-like growth factor-1 (IGF-1)/receptor tyrosine kinase recently has been shown to mediate neuronal survival and potentiate the activity of specific calcium channel subtypes; survival requires Akt, a serine/threonine kinase. We demonstrate here that Akt mediates the IGF-1-induced potentiation of L channel currents, but not that of N channels. Transient expression of wild-type, dominant-negative, and constitutively active forms of Akt in cerebellar granule neurons causes, respectively, no change in IGF-1/L channel potentiation, complete inhibition of potentiation, and a dramatic increase in basal L currents accompanied by the loss of ability to induce further increases. In no case is the IGF-1 potentiation of N currents affected. We additionally find that IGF-1 partially mediates granule neuron survival via L channel activity and that Akt-dependent L channel modulation is a necessary component. Interestingly, very brief exposure (1 min) to IGF-1 triggers nearly complete survival and requires L channel activity. These results strongly suggest that neuronal receptor tyrosine kinases can control long-term calcium-dependent processes via the rapid control of voltage-sensitive channels.

Growth factor-mediated Fyn signaling regulates alpha-amino-3- hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression in rodent neocortical neurons

Src-family protein tyrosine kinases (PTKs) transduce signals to regulate neuronal development and synaptic plasticity. However, the nature of their activators and molecular mechanisms underlying these neural processes are unknown. Here, we show that brain-derived neurotrophic factor (BDNF) and platelet-derived growth factor enhance expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor 1 and 2/3 proteins in rodent neocortical neurons via the Src-family PTK(s). The increase in AMPA receptor levels was blocked in cultured neocortical neurons by addition of a Src-family-selective PTK inhibitor. Accordingly, neocortical cultures from Fyn-knockout mice failed to respond to BDNF whereas those from wild-type mice responded. Moreover, the neocortex of young Fyn mutants exhibited a significant in vivo reduction in these AMPA receptor proteins but not in their mRNA levels. In vitro kinase assay revealed that BDNF can indeed activate the Fyn kinase: It enhanced tyrosine phosphorylation of Fyn as well as that of enolase supplemented exogenously. All of these results suggest that the Src-family kinase Fyn, activated by the growth factors, plays a crucial role in modulating AMPA receptor expression during brain development.

Expression of Ras-GRF in the SK-N-BE neuroblastoma accelerates retinoic-acid-induced neuronal differentiation and increases the functional expression of the IRK1 potassium channel

Ras-GRF, a neuron-specific Ras exchange factor of the central nervous system, was transfected in the SK-N-BE neuroblastoma cell line and stable clones were obtained. When exposed to retinoic acid, these clones showed a remarkable enhancement of Ras-GRF expression with a concomitant high increase in the level of active (GTP-bound) Ras already after 24 h of treatment. In the presence of retinoic acid, the transfected cells stopped growing and acquired a differentiated neuronal-like phenotype more rapidly than the parental ones. Cells expressing Ras-GRF also exhibited a more hyperpolarized membrane potential. Moreover, treatment with retinoic acid led to the appearance of an inward rectifying potassium channel with electrophysiological properties similar to IRK1. This current was present in a large number of cells expressing Ras-GRF, while only a small percentage of parental cells exhibited this current. However, Northern analysis with a murine cDNA probe indicated that IRK1 mRNA was induced by retinoic acid at a similar level in both kinds of cells. Brief treatment with a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway reduced the number of transfected cells showing IRK1 activity. These findings suggest that activation of the Ras pathway accelerates neuronal differentiation of this cell line. In addition, our results suggest that Ras-GRF and/or Ras-pathway may have a modulatory effect on IRK1 channel activity.

Nerve growth factor rapidly suppresses basal, NMDA-evoked, and AMPA-evoked nitric oxide synthase activity in rat hippocampus in vivo

In adult forebrain, nerve growth factor (NGF) influences neuronal maintenance and axon sprouting and is neuroprotective in several injury models through mechanisms that are incompletely understood. Most NGF signaling is thought to occur after internalization and retrograde transport of trkA receptor and be mediated through the nucleus. However, NGF expression in hippocampus is rapidly and sensitively regulated by synaptic activity, suggesting that NGF exerts local effects more dynamically than possible through signaling requiring retrograde transport to distant afferent neurons. Interactions have been reported between NGF and nitric oxide (NO). Because NO affects both neural plasticity and degeneration, and trk receptors can mediate signaling within minutes, we hypothesized that NGF might rapidly modulate NO production. Using in vivo microdialysis we measured conversion of L-[14C]arginine to L-[14C]citrulline as an accurate reflection of NO synthase (NOS) activity in adult rat hippocampus. NGF significantly reduced NOS activity to 61% of basal levels within 20 min of onset of delivery and maintained NOS activity at less than 50% of baseline throughout 3 hr of delivery. This effect did not occur with control protein (cytochrome c) and was not mediated by an effect of NGF on glutamate levels. In addition, simultaneous delivery of NGF prevented significant increases in NOS activity triggered by the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Rapid suppression by NGF of basal and glutamate-stimulated NOS activity may regulate neuromodulatory functions of NO or protect neurons from NO toxicity and suggests a novel mechanism for rapidly mediating functions of NGF and other neurotrophins.

Modulation of olfactory bulb neuron potassium current by tyrosine phosphorylation

Insulin causes a suppression of whole-cell voltage-dependent outward current in cultured neurons from the rat olfactory bulb. This suppression is time-dependent; it is mimicked by application of Src tyrosine kinase inside the cell via the whole-cell patch electrode or by treatment of the olfactory bulb neurons with the tyrosine phosphatase inhibitor pervanadate. The C-type inactivation properties of the outward current in olfactory bulb neurons resemble those of the cloned Kv1.3 potassium channel. In addition, at picomolar concentrations at which it is specific for Kv1.3, the scorpion toxin margatoxin blocks most of the olfactory bulb neuron outward current. Immunocytochemical analysis demonstrates that Kv1.3 is prominent in the cultured olfactory bulb neurons. To identify specific amino acid residues that might be important for potassium current modulation, we examined the effects of pervanadate and insulin on wild-type and mutant Kv1.3 channels expressed in human embryonic kidney (HEK 293) cells. As shown previously, treatment with either pervanadate or insulin suppresses Kv1.3 current in these cells. Mutational analysis demonstrates that at least two distinct tyrosine residues are required for current suppression by pervanadate. Insulin treatment stimulates the tyrosine phosphorylation of Kv1.3 in HEK 293 cells, and a different combination of tyrosine residues is required for the current suppression by insulin. The results suggest that complex patterns of phosphorylation may be involved in the modulation of neuronal potassium current by receptor and nonreceptor tyrosine kinases.