Functions of PI 3-kinase in development of the nervous system

Role of metalloproteinase-dependent EGF receptor activation in α1-adrenoceptor-stimulated MAP kinase phosphorylation in GT1-7 neurons

Adrenoceptors (ARs) are involved in the regulation of gonadotropin-releasing hormone (GnRH) release from native and immortalized hypothalamic (GT1-7) neurons. However, the AR-mediated signaling mechanisms and their functional significance in these cells are not known. Stimulation of GT1-7 cells with the alpha1-AR agonist, phenylephrine (Phe), causes phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinases that is mediated by protein kinase C (PKC)-dependent transactivation of the epidermal growth factor receptor (EGF-R). Phe stimulation causes shedding of the soluble ligand, heparin-binding EGF (HB-EGF), as a consequence of matrix metalloproteinase (MMP) activation. Phe-induced phosphorylation of the EGF-R, and subsequently of Shc and ERK1/2, was attenuated by inhibition of MMP or HB-EGF with the selective inhibitor, CRM197, or by a neutralizing antibody. In contrast, phosphorylation of the EGF-R, Shc and ERK1/2 by EGF and HB-EGF was independent of PKC and MMP activity. Moreover, inhibition of Src attenuated ERK1/2 responses by Phe, but not by HB-EGF and EGF, indicating that Src acts upstream of the EGF-R. Consistent with a potential role of reactive oxygen species (ROS), Phe-induced phosphorylation of EGF-R was attenuated by the antioxidant, N-acetylcysteine. These data suggest that activation of the alpha1-AR causes phosphorylation of ERK1/2 through activation of PKC, ROS and Src, and shedding of HB-EGF, which binds to and activates the EGF-R.

Activation of protein kinase B/Akt signaling pathway contributes to mechanical hypersensitivity induced by capsaicin

We investigated the involvement of the protein kinase B/Akt (PKB/Akt) signaling pathway in the mechanical hypersensitivity induced in rats by capsaicin. Intradermal injection of capsaicin results in activation of PKB/Akt in the lumbar spinal cord, most prominently in the dorsal horn, starting by 5 min after capsaicin injection and lasting at least 1h. The activated PKB/Akt in the spinal cord is in neurons, since phospho-PKB/Akt (p-PKB/Akt) colocalizes with the neuronal marker, neuronal-specific nuclear protein (NeuN). The mechanical hypersensitivity is shown by the enhanced paw withdrawal frequency to applications of von Frey filaments with different bending forces (30, 100, 200 mN) on the rat paw. Pre-treatment with several different PKB/Akt inhibitors, including SH-6, Akt inhibitor IV, and Akt inhibitor V, blocked the mechanical hypersensitivity induced by intradermal injection of capsaicin, a measure of spinal cord central sensitization. Two structurally unrelated phosphoinositide 3-Kinase (PI3K, upstream of PKB/Akt) inhibitors, Wortmannin and LY294002, also prevented the mechanical hypersensitivity induced by intradermal injection of capsaicin. Furthermore, post-treatment with the PI3K inhibitor, Wortmannin, or PKB/Akt inhibitors, such as NL-71-101, SH-6, Akt inhibitor IV, and inhibitor V significantly reduced the established mechanical hypersensitivity induced by capsaicin. The PKB/Akt signaling pathway in the spinal cord is therefore involved in pain hypersensitivity.

The challenge of brain lipidomics

After many years backstage, lipids have made a come back in the limelight of neuroscience. This renewed excitement was sparked by a series of convergent discoveries in the fields of neural development, synaptic physiology and receptor pharmacology, which have begun to reveal the roles played by lipid messengers and their receptors in brain function. Such roles extend from the development of the neocortex to the processing of complex behaviors, encompassing a territory as vast as those traditionally assigned to growth factors, neurotransmitters and neuropeptides. Along with these basic discoveries, technical advances have simplified the identification and quantification of neural lipids, achieving a degree of sensitivity and selectivity that was unthinkable only 10 years ago. Thanks to this progress, we can now resolve complex mixtures of lipid molecules and quantify each of their components, which are often present in tissues at vanishingly low concentrations. In this review, I outline several key features of brain lipid signaling and discuss the opportunities and challenges that such features impose on future lipidomic approaches.

Deregulation of the phosphatidylinositol-3 kinase signaling cascade is associated with neurodegeneration in Npc1-/- mouse brain

Niemann-Pick type C (NPC) disease is caused by mutations to genes that encode proteins critical to intracellular lipid homeostasis. The events underlying NPC progressive neurodegeneration are poorly understood but include neurofibrillary tangles of the type found in Alzheimer's disease. Here we investigated possible contributions of a phosphatidylinositol-3 kinase cascade [PI3K, Akt, glycogen synthase kinase-3beta (GSK-3beta)] that is linked to apoptosis and various degenerative conditions. Brain concentrations of phosphorylated Akt, which phosphorylates and inactivates GSK-3beta, were significantly elevated in Npc1-/- mice relative to Npc1+/+ mice. Accordingly, levels of inactive GSK-3beta were 50 to 100% higher in mutant brains than in controls. Increases in inactive GSK-3beta occurred early in postnatal development, well before neuronal loss, and were most prominent in structures with intracellular cholesterol accumulation, suggesting a contribution to subsequent degeneration. Perturbations of nuclear factor (NF)-kappaB, which is regulated by GSK-3beta, occurred in Npc1-/- mouse brains. Nuclear concentrations and DNA binding activity of NF-kappaB's transactivation subunit, p65, were significantly reduced in Npc1-/- mice compared to Npc1+/+ mice. Cytoplasmic levels of the p50 subunit and its precursor, p105, were higher in Npc1-/- mice. These results suggest that excessive activity in the PI3K-Akt pathway depresses GSK-3beta, thereby disrupting the formation and/or nuclear import of p50/p65 NF-kappaB dimers and contributing to neuronal degeneration.

The inositol polyphosphate 5-phosphatase, PIPP, Is a novel regulator of phosphoinositide 3-kinase-dependent neurite elongation

The spatial activation of phosphoinositide 3-kinase (PI3-kinase) signaling at the axon growth cone generates phosphatidylinositol 3,4,5 trisphosphate (PtdIns(3,4,5)P3), which localizes and facilitates Akt activation and stimulates GSK-3beta inactivation, promoting microtubule polymerization and axon elongation. However, the molecular mechanisms that govern the spatial down-regulation of PtdIns(3,4,5)P3 signaling at the growth cone remain undetermined. The inositol polyphosphate 5-phosphatases (5-phosphatase) hydrolyze the 5-position phosphate from phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) and/or PtdIns(3,4,5)P3. We demonstrate here that PIPP, an uncharacterized 5-phosphatase, hydrolyzes PtdIns(3,4,5)P3 forming PtdIns(3,4)P2, decreasing Ser473-Akt phosphorylation. PIPP is expressed in PC12 cells, localizing to the plasma membrane of undifferentiated cells and the neurite shaft and growth cone of NGF-differentiated neurites. Overexpression of wild-type, but not catalytically inactive PIPP, in PC12 cells inhibited neurite elongation. Targeted depletion of PIPP using RNA interference (RNAi) resulted in enhanced neurite differentiation, associated with neurite hyperelongation. Inhibition of PI3-kinase activity prevented neurite hyperelongation in PIPP-deficient cells. PIPP targeted-depletion resulted in increased phospho-Ser473-Akt and phospho-Ser9-GSK-3beta, specifically at the neurite growth cone, and accumulation of PtdIns(3,4,5)P3 at this site, associated with enhanced microtubule polymerization in the neurite shaft. PIPP therefore inhibits PI3-kinase-dependent neurite elongation in PC12 cells, via regulation of the spatial distribution of phospho-Ser473-Akt and phospho-Ser9-GSK-3beta signaling.

Neurturin has multiple neurotrophic effects on adult rat sacral parasympathetic ganglion neurons

Neurturin (NTN) is an important neurotrophic factor for parasympathetic neurons; however, no studies to date have investigated the signalling mechanisms downstream of GFRalpha2 and Ret activation underlying this neurotrophic support. This is particularly important for pelvic parasympathetic neurons, which are prone to injury during surgical procedures such as prostatectomy, and where there are no current therapies for axonal regeneration. To address this issue we have cultured dissociated adult rat pelvic ganglion neurons and also examined the structural changes in pelvic ganglion neurons after axotomy. Axotomised penile neurons deprived of target-derived support had smaller somata than intact neurons. Studies of cultured adult pelvic ganglion neurons also demonstrated that NTN stimulated soma growth. Further experiments showed that NTN reduced the up-regulation of tyrosine hydroxylase expression in cultured pelvic parasympathetic neurons. NTN stimulated the extension of neurites in cultured parasympathetic, but not sympathetic, pelvic ganglion neurons. Inhibition of phosphatidylinositol 3-kinase prevented initiation of neurite outgrowth, whereas inhibition of the mitogen-activated protein kinase and the Src family kinase pathways disrupted NTN-stimulated microtubule assembly. Surprisingly, NTN did not activate the transcription factor cAMP-response element binding protein (CREB), which is typically involved in neurotrophic signalling in sympathetic neurons. This is the first study to identify signalling pathways activated by NTN in adult parasympathetic neurons. Our results may lead to a better understanding of regenerative mechanisms in parasympathetic neurons, especially for those innervating urogenital organs. Our results also indicate that neurotrophic signalling in parasympathetic neurons is different from that in other types of peripheral neurons.

p62 modulates Akt activity via association with PKCzeta in neuronal survival and differentiation

p62 is a ubiquitously expressed phosphoprotein that interacts with a number of signaling molecules and a major component of neurofibrillary tangles in the brain of Alzheimer's disease patients. It has been implicated in important cellular functions such as cell proliferation and anti-apoptotic pathways. In this study, we have addressed the potential role of p62 during neuronal differentiation and survival using HiB5, a rat neuronal progenitor cell. We generated a recombinant adenovirus encoding T7-epitope tagged p62 to reliably transfer p62 cDNA into the neuronal cells. The results show that an overexpression of p62 led not only to neuronal differentiation, but also to decreased cell death induced by serum withdrawal in HiB5 cells. In this process p62-dependent Akt phosphorylation occurred via the release of Akt from PKCzeta by association of p62 and PKCzeta, which is known as a negative regulator of Akt activation. These findings indicate that p62 facilitates cell survival through novel signaling cascades that result in Akt activation. Furthermore, we found that p62 expression was induced during neuronal differentiation. Taken together, the data suggest p62 is a regulator of neuronal cell survival and differentiation.

Insulin-like growth factor-1 provides protection against psychosine-induced apoptosis in cultured mouse oligodendrocyte progenitor cells using primarily the PI3K/Akt pathway

Psychosine (galactosylsphingosine) is a toxic metabolite that accumulates in globoid cell leukodystrophy (GLD) due to the deficiency of galactocerebrosidase (GALC) activity. This results in subsequent programmed cell death of oligodendrocytes and demyelination in human patients and animal models. We investigated the potential role of insulin-like growth factor-1 (IGF-1) in modifying the apoptotic effect of psychosine in cultured mouse oligodendrocyte progenitor cells (OLP-II). We show that psychosine inhibits the phosphorylation of Akt and Erk1/Erk2 (Erk1/2), which are the main anti-apoptotic pathways of the IGF-1 receptor (IGF-1R). Although IGF-1 sustained phosphorylation of both of these pathways, it provided maximum protection to OLP-II cells from psychosine-induced cell death in a PI3K/Akt-dependent manner. The effects of IGF-1 were dose-dependent and resulted in increased IGF-1R autophosphorylation levels. Although relatively high concentrations of IGF-1 also resulted in the activation of the insulin receptor (IR), its effect was more significant on the IGF-1R.

Nitric oxide synthesis is required for exercise-induced increases in hippocampal BDNF and phosphatidylinositol 3' kinase expression

Previous studies have shown that running exercise, either alone or in combination with antidepressant treatment, results in increased hippocampal BDNF levels. Nitric oxide (NO) is an important signaling molecule that has neuronal survival-promoting properties and has been shown to play an important role in plasticity associated with activating interventions. Herein, we administered the NO synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (L-NAME), in conjunction with the monoamine oxidase inhibitor (MAOI) antidepressant, tranylcypromine, and voluntary wheel-running exercise to determine whether the enhancement in full-length BDNF mRNA occurring with these interventions is dependent upon NO synthesis. Our results demonstrate that both chronic exercise and chronic exercise-plus-tranylcypromine lead to enhanced hippocampal BDNF mRNA and protein expression. NOS inhibition prevents this effect of chronic exercise, but only partly prevents the effects of the exercise/antidepressant combination. Thus, the robust enhancement in BDNF mRNA occurring with exercise appears to be NO synthesis-dependent, but the intervention including antidepressant may enhance BDNF expression through alternative intracellular mechanisms. In addition, because exercise and antidepressants have both been shown to activate survival-promoting genes, we evaluated the levels of hippocampal phosphatidylinositol 3' kinase (PI-3K), an important signaling molecule within a principal neuronal survival-promoting intracellular pathway. Like BDNF mRNA and protein, exercise increases the expression of PI-3K, whereas concomitant NOS inhibition prevents this increase in PI-3K immunoreactivity above control levels. Our results are discussed in light of possible overlapping, but distinct intracellular pathways activated by exercise and antidepressant treatment to bring about enhancements in BDNF expression and other survival-promoting effects. These findings further demonstrate the potential therapeutic potential of chronic exercise to supplement pharmacotherapeutic treatment of mood disorders.

Decreases in phosphoinositide-3-kinase/Akt and extracellular signal-regulated kinase 1/2 signaling activate components of spinal motoneuron death

Motoneuron dependence on target-derived trophic factors during development is well established, with loss of trophic support leading to the death of these cells. A complete understanding of the intracellular signal transduction machinery associated with extracellular survival signals requires the examination of individual pathways in various cellular and environmental contexts. In cells deprived of trophic support, and hence compromised for survival, phosphoinositide-3-kinase (PI3K) is decreased when compared with healthy cells supplied with trophic support. Extracellular signal-regulated kinase 1/2 (ERK1/2) signaling is dramatically decreased in deprived cells. We have examined the role of these two pathways to understand how changes in their activity regulate motoneuron survival and death. Pharmacological inhibition of PI3K attenuated motoneuron survival and was important in the regulation of Bcl-2 serine phosphorylation, limited release of cytochrome c into the cytoplasm and caspase activation. Bax translocation from cytoplasm to mitochondria was not altered when PI3K was inhibited. High levels of ERK1/2 inhibition robustly attenuated motoneuron survival in cells supplied with trophic support, whereas moderate inhibition of ERK1/2 activation had little effect. ERK1/2 inhibition in these cells decreased Bcl-2 phosphorylation and resulted in release of cytochrome c from the mitochondria. Bax translocation and caspase activation were not affected by ERK1/2 inhibition. These data reveal that changes in PI3K and ERK1/2 signaling lead to individual and overlapping effects on the cell-death machinery. Characterizing the role of these pathways is critical for a fundamental understanding of the development and degeneration of specific neuronal populations.

Role of GSK-3β activity in motor neuronal cell death induced by G93A or A4V mutant hSOD1 gene

Point mutations such as G93A and A4V in the human Cu/Zn-superoxide dismutase gene (hSOD1) cause familial amyotrophic lateral sclerosis (fALS). In spite of several theories to explain the pathogenic mechanisms, the mechanism remains largely unclear. Increased activity of glycogen synthase kinase-3 (GSK-3) has recently been emphasized as an important pathogenic mechanism of neurodegenerative diseases, including Alzheimer's disease and ALS. To investigate the effects of G93A or A4V mutations on the phosphatidylinositol-3-kinase (PI3-K)/Akt and GSK-3 pathway as well as the caspase-3 pathway, VSC4.1 motoneuron cells were transfected with G93A- or A4V-mutant types of hSOD1 (G93A and A4V cells, respectively) and, 24 h after neuronal differentiation, their viability and intracellular signals, including PI3-K/Akt, GSK-3, heat shock transcription factor-1 (HSTF-1), cytochrome c, caspase-3 and poly(ADP-ribose) polymerase (PARP), were compared with those of wild type (wild cells). Furthermore, to elucidate the role of the GSK-3beta-mediated cell death mechanism, alterations of viability and intracellular signals in those mutant motoneurons were investigated after treating the cells with GSK-3beta inhibitor. Compared with wild cells, viability was greatly reduced in the G93A and A4V cells. However, the treatment of G93A and A4V cells with GSK-3beta inhibitor increased their viability by activating HSTF-1 and by reducing cytochrome c release, caspase-3 activation and PARP cleavage. However, the treatment did not affect the expression of PI3-K/Akt and GSK-3beta. These results suggest that the G93A or A4V mutations inhibit PI3-K/Akt and activate GSK-3beta and caspase-3, thus becoming vulnerable to oxidative stress, and that the GSK-3beta-mediated cell death mechanism is important in G93A and A4V cell death.

Evaluation of the chromosome 2q37.3 gene CENTG2 as an autism susceptibility gene

Autism is a highly heritable neurodevelopmental syndrome with a complex genetic etiology for which no disease genes have yet been definitively identified. We ascertained three subjects with autism spectrum disorders and chromosome 2q37.3 terminal deletions, and refined the deletion breakpoint regions using polymorphism mapping and fluorescence in situ hybridization (FISH) probes. We then genotyped polymorphic markers downstream from the breakpoint region in a sample of autism affected sibling pair families. Both the chromosomal breakpoints and linkage analyses focused our attention on the gene centaurin gamma-2 (CENTG2), an attractive candidate gene based also on its function and pattern of expression. We therefore assessed CENTG2 for its involvement in autism by (1) screening its exons for variants in 199 autistic and 160 non-autistic individuals, and (2) genotyping and assessing intra-genic polymorphisms for linkage and linkage disequilibrium (LD). The exon screen revealed a Ser --> Gly substitution in one proband, an Arg --> Gly substitution in another, and a number of additional variants unique to the autism families. No unique variants were found in the control subjects. The genotyping produced strong evidence for linkage from two intronic polymorphisms, with a maximum two-point HLOD value of 3.96 and a posterior probability of linkage (PPL) of 51%. These results were contradicted, however, by substantially weaker evidence for linkage from multi-point analyses and by no evidence of LD. We conclude, therefore, that 2q37.3 continues to be a region of interest for autism susceptibility, and that CENTG2 is an intriguing candidate gene that merits further scrutiny for its role in autism.

Essential role of protein kinase B gamma (PKB gamma/Akt3) in postnatal brain development but not in glucose homeostasis

Protein kinase B is implicated in many crucial cellular processes, such as metabolism, apoptosis and cell proliferation. In contrast to Pkb(alpha) and Pkb(beta)-deficient mice, Pkb(gamma)(-/-) mice are viable, show no growth retardation and display normal glucose metabolism. However, in adult Pkb(gamma)mutant mice, brain size and weight are dramatically reduced by about 25%. In vivo magnetic resonance imaging confirmed the reduction of Pkb(gamma)(-/-) brain volumes with a proportionally smaller ventricular system. Examination of the major brain structures revealed no anatomical malformations except for a pronounced thinning of white matter fibre connections in the corpus callosum. The reduction in brain weight of Pkb(gamma)(-/-) mice is caused, at least partially, by a significant reduction in both cell size and cell number. Our results provide novel insights into the physiological role of Pkb(gamma) and suggest a crucial role in postnatal brain development.

Role of EGF receptor transactivation in phosphoinositide 3-kinase-dependent activation of MAP kinase by GPCRs

Many G protein coupled receptors (GPCRs) cause phosphorylation of MAP kinases through transactivation of the epidermal growth factor receptor (EGF-R), leading to increased cell survival and growth, motility, and migration. Phosphoinositide 3-kinase (PI3K) is one of the important cell survival signaling molecules activated by EGF-R stimulation. However, the extent to which EGF-R transactivation is essential for GPCR agonist-stimulated PI3K activation is not known. Here we examined the mechanism of PI3K activation that elicits GPCR-mediated ERK1/2 activation by pathways dependent and/or independent of EGF-R transactivation in specific cell types. Immortalized hypothalamic neurons (GT1-7 cells) express endogenous gonadotropin-releasing hormone receptors (GnRH-R) and their stimulation causes marked phosphorylation of ERK1/2 and Akt (Ser 473) through transactivation of the EGF-R and recruitment of PI3K. In C9 hepatocytes, agonist activation of AT1 angiotensin II (AT1-R), lysophosphatidic acid (LPA), and EGF receptors caused phosphorylation of Akt through activation of the EGF-R in a PI3K-dependent manner. However, ERK1/2 activation by these agonists in these cells was independent of PI3K activation. In contrast, agonist stimulation of HEK 293 cells stably expressing AT1-R caused ERK1/2 phosphorylation that was independent of EGF-R transactivation but required PI3K activation. LPA signaling in these cells showed partial and complete dependence on EGF-R and PI3K, respectively. These data indicate that GPCR-induced ERK1/2 phosphorylation is dependent or independent of PI3K in specific cell types, and that the involvement of PI3K during ERK1/2 activation is not dependent solely on agonist-induced transactivation of the EGF-R.

Enteric neuroblasts require the phosphatidylinositol 3-kinase/Akt/Forkhead pathway for GDNF-stimulated survival

Glial cell line-derived neurotrophic factor (GDNF)/Ret signaling is required for enteric neural crest survival, proliferation, migration and process extension, but signaling pathways that mediate enteric nervous system (ENS) precursor development are poorly understood. We therefore examined GDNF effects on immunoselected ENS precursor survival and neuronal process extension in the presence of phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathway inhibitors. These studies demonstrated that GDNF promotes ENS precursor survival through phosphatidylinositol-3-kinase. Specifically, GDNF induces phosphorylation of Akt and loss of the Akt substrates FOXO1 and FOXO3a from the nucleus of ENS precursors. Furthermore, dominant negative Akt or active FOXO1 constructs promote ENS precursor cell death while a dominant negative FOXO1 construct prevents cell death. In contrast, the MAPK kinase inhibitor PD98059 did not influence ENS precursor survival or neurite extension. These data demonstrate a critical role for PI-3 kinase/Akt/FOXO signaling, but not for MAPK in ENS precursor survival and neurite extension.

Exercise activates the phosphatidylinositol 3-kinase pathway

Physical exercise is known to enhance psychological well-being and coping capacity. Voluntary physical exercise in rats also robustly and rapidly up-regulates hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels, which are potentiated following a regimen of chronic antidepressant treatment. Increased BDNF levels are associated with enhanced activity of cyclic AMP response element binding protein (CREB). So far, relatively little is known about the intracellular signaling mechanisms mediating this effect of exercise. We wished to explore the possibility that exercise and/or antidepressant treatment activate the hippocampal phosphatidylinositol-3 (PI-3) kinase pathway, which mediates cellular survival. In young male Sprague-Dawley rats, we examined the effects of 2 weeks of daily voluntary wheel-running activity and/or tranylcypromine (n = 7 per group) on the levels of the active forms of protein-dependent kinase-1 (PDK-1), PI-3 kinase, phospho-thr308-Akt, phospho-ser473-Akt, and phospho-glycogen synthase kinase-3beta (GSK3beta; inactive form), as well as BDNF, activated CREB, and the phospho-Trk receptor, in the rat hippocampus, and compared these with sedentary saline-treated controls. Immunoblotting analyses revealed that in exercising rats, there was a significant increase in PI-3 kinase expression (4.61 times that of controls, P = 0.0161) and phosphorylation of PDK-1 (2.73 times that of controls, P = 0.0454), thr308-Akt (2.857 times that of controls, P = 0.0082), CREB (60.27 times that of controls, P = 0.05), and Trk (35.3 times that of controls, P < 0.0001) in the hippocampi of exercising animals; BDNF was also increased (3.2 times that of controls), but this was not statistically significant. In rats receiving both exercise and tranylcypromine, BDNF (4.51 times that of controls, P = 0.0068) and PI-3 kinase (4.88 times that of controls, P = 0.0103), and the phospho- forms of Trk (13.67 times that of controls, P = 0.0278), thr308-Akt (3.644 times that of controls, P = 0.0004), GSK-3beta (2.93 times that of controls, P = 0.026), and CREB (88.97 times that of controls, P = 0.0053) were significantly increased. These results suggest that the exercise-induced expression of BDNF is associated with the increased expression of several key intermediates of the PI-3 kinase/Akt pathway, which is known for its role in enhancing neuronal survival.

Akt Activation Is Necessary for Growth Factor–Induced Trafficking of Functional KCaChannels in Developing Parasympathetic Neurons

The protein kinase Akt is a crucial regulator of neuronal survival and apoptosis. Here we show that Akt activation is necessary for mobilization of large-conductance KCachannels in ciliary ganglion (CG) neurons evoked by β-neuregulin-1 (NRG1) and transforming growth factor-β1 (TGFβ1). Application of NRG1 to embryonic day 9 (E9) CG neurons increased Akt phosphorylation, as observed previously for TGFβ1. NRG1- and TGFβ1-evoked stimulation of KCais blocked by inhibitors of PI3K by overexpression of a dominant-negative form of Akt, by overexpression of CTMP, an endogenous negative regulator of Akt, and by application of the Akt inhibitor 1L-6-hydroxymethyl-chiro-inositol 2-( R)-2- O-methyl-3- O-octadecylcarbonate (HIMO). Conversely, overexpression of a constitutively-active form of Akt was sufficient by itself to increase mobilization of functional KCachannels. NRG1 and TGFβ1 evoked an Akt-dependent increase in cell-surface SLO α-subunits. These procedures have no effect on voltage-activated Ca2+currents. Thus Akt plays an essential role in the developmental regulation of excitability in CG neurons.

Gene-expression analysis after alcohol exposure in the developing mouse

Exposure to alcohol in the embryonic mouse can lead to structural and neurophysiologic changes. The cause of these changes is poorly understood, but they are likely the result of numerous mechanisms. Here we investigate ethanol-induced alterations in gene expression in the fetal brain. Using complementary-DNA microarrays, we identified 25 genes that were down-regulated by prenatal ethanol exposure on days 7 and 9 of gestation. None were found to be up-regulated. Of those that were repressed, 6 (Timp4, Bmp15, Rnf25, Akt1, Tulp4, Dexras1) have been identified, and they are discussed here in the context of the developing fetus. The identified genes have been shown to be involved in cell proliferation, differentiation, and apoptosis, and they contribute to tissue growth and remodeling, as well as neuronal growth and survival. Microarray studies may be useful in the identification of a genetic marker for fetal alcohol syndrome, the discovery of novel pathways that may be involved in its origin, or both.

Inhibition of Src-like kinases reveals Akt-dependent and -independent pathways in insulin-like growth factor I-mediated oligodendrocyte progenitor survival

Insulin-like growth factor I (IGF-I) has been previously shown to promote survival of oligodendrocyte progenitors; however, the underlying mechanisms are not fully understood. Our aim was to investigate the involvement of phosphatidylinositol 3-kinase (PI3K), MEK1, and Src family tyrosine kinases in IGF-I-mediated oligodendrocyte progenitor survival. In agreement with previous studies, IGF-I promoted cell survival. We show that IGF-I prevented apoptosis induced by growth factor deprivation in a PI3K-dependent and MEK/ERK-independent manner. In addition, IGF-I activated Akt while inhibiting caspase-3 activation, and these effects were reversed by the PI3K inhibitors LY 294002 and wortmannin, but not by the MEK1 inhibitor PD 98059. Interestingly, PP2, a specific Src-like kinase inhibitor, blocked the tyrosine phosphorylation of Src, Fyn, and Lyn and IGF-I-stimulated Akt activation, yet had no significant effects on caspase-3 activation or progenitor survival. To further determine whether Akt is required for IGF-I-mediated survival, oligodendrocyte progenitors were transduced with defective Akt mutants or treated with an Akt inhibitor. Although the Akt mutants and inhibitor decreased Akt activity and reduced basal cell survival, IGF-I could partially rescue oligodendrocyte progenitors by decreasing caspase-3 activation. These results suggest that 1) PI3K is essential for IGF-I-promoted cell survival, 2) downstream activation of Akt-dependent and -independent pathways is involved, and 3) Src-like tyrosine kinases participate in IGF-I-induced Akt activation. Therefore, an unidentified effector(s) of PI3K appears to be involved in conferring complete IGF-I-mediated protection of oligodendrocyte progenitors.

Suppression subtractive hybridization and microarray identification of estrogen-regulated hypothalamic genes

The gonadal steroid estrogen is a pleiotropic hormone that has multiple effects on numerous cellular functions. One of estrogen's major targets is the brain, where the steroid not only affects growth, differentiation, and survival of neurons, but also regulates cell excitability. Because estrogen modulates multiple, overlapping signaling pathways, it has been difficult to scrutinize the transcriptional activity of the steroid. Therefore, we still lack a global picture of how different genes interact and are regulated by estrogen. Herein we report the use of suppression subtractive hybridization followed by custom microarray analysis of thousands of genes that are differentially expressed during the negative feedback phase of the female reproductive cycle. We have found a number of key transcripts that are regulated by estrogen and contribute to the alteration in synaptic transmission and hence excitability of hypothalamic neurons (e.g., GABA neurons). These include gec-1, GABA(B)R2, PI3 kinase subunit p55gamma, and a number of proteins containing pleckstrin homology domains that are critical for plasma membrane targeting. Studies are underway to refine our analysis to individual nuclei and individual cells. However, what has emerged from this highly sensitive microarray analysis is that estrogen affects neuronal plasticity in hypothalamic neurons not only by transcription of new membrane proteins (e.g., receptors and channels), but also by altering expression of downstream signaling molecules and proteins involved in neurosecretory pathways.

Estrogen increases retrograde labeling of motoneurons: evidence of a nongenomic mechanism

Estrogen has a variety of neurotrophic effects mediated via different signaling cascades, including ERK and phosphatidylinositol 3-kinase (PI3K) pathways. In this study, we investigated effects of estrogen and inhibitors for ERK and PI3K applied directly onto the cut sciatic nerve on retrograde labeling of lumbar motoneurons. A mix of retrograde tracer (Fluorogold) and 17beta-estradiol, in combination with an antagonist for estrogen receptors ICI 182,780, an inhibitor of ERK1/2 pathway (U0126), an inhibitor of PI3K (LY-294002), or a protein synthesis inhibitor (cycloheximide), was applied to the proximal stump of the transected sciatic nerve for 24 h. Coapplication of Fluorogold with 17beta-estradiol produced a significant increase in the number of retrograde-labeled lumbar motoneurons, compared with Fluorogold alone. Estrogen potentiation of retrograde labeling was inhibited by application of ICI 182,780, U0126, LY-294002, and cycloheximide. Immunohistochemical analysis of the sciatic nerve, 24 h following crush injury, revealed accumulation of phospho-ERK in regenerating nerve fibers. The data suggest a role for estrogen, ERK, PI3K, and protein synthesis in the uptake and retrograde transport of Fluorogold. We propose that estrogen action in peripheral nerve fibers is mediated via the ERK and PI3K signaling pathways and is reliant on local protein synthesis.

The Arf6 GAP centaurin α-1 is a neuronal actin-binding protein which also functions via GAP-independent activity to regulate the actin cytoskeleton

Centaurin alpha-1 is a high-affinity PtdIns(3,4,5)P3-binding protein enriched in brain. Sequence analysis indicates centaurin alpha-1 contains two pleckstrin homology domains, ankyrin repeats and an Arf GAP homology domain, placing it in the AZAP family of phosphoinositide-regulated Arf GAPs. Other members of this family are involved in actin cytoskeletal and focal adhesion organization. Recently, it was reported that centaurin alpha-1 expression diminishes cortical actin and decreases Arf6GTP levels consistent with it functioning as an Arf6 GAP in vivo. In the current report, we show that centaurin alpha-1 binds Arfs in vitro and colocalizes with Arf6 and Arf5 in vivo, further supporting an interaction with Arfs. Centaurin alpha-1 expression produces dramatic effects on the actin cytoskeleton, decreasing stress fibers, diminishing cortical actin, and enhancing membrane ruffles and filopodia. Expression of centaurin alpha-1 also enhances cell spreading and disrupts focal adhesion protein localization. The effects of centaurin alpha-1 on stress fibers and cell spreading are reminiscent of those of Arf6GTP. Consistent with this, we show that many of the centaurin alpha-1-induced effects on the actin cytoskeleton and actin-dependent activities do not require GAP activity. Thus, centaurin alpha-1 likely functions via both GAP-dependent and GAP-independent mechanisms to regulate the actin cytoskeleton. Furthermore, we demonstrate that in vitro, centaurin alpha-1 binds F-actin directly, with actin binding activity localized to the PtdIns(3,4,5)P3-binding PH domain. Our data suggest that centaurin alpha-1 may be a component of the neuronal PI 3-kinase cascade that leads to regulation of the neuronal actin cytoskeleton.

Insulin receptor substrate-2 deficiency impairs brain growth and promotes tau phosphorylation

Insulin resistance and diabetes might promote neurodegenerative disease, but a molecular link between these disorders is unknown. Many factors are responsible for brain growth, patterning, and survival, including the insulin-insulin-like growth factor (IGF)-signaling cascades that are mediated by tyrosine phosphorylation of insulin receptor substrate (IRS) proteins. Irs2 signaling mediates peripheral insulin action and pancreatic beta-cell function, and its failure causes diabetes in mice. In this study, we reveal two important roles for Irs2 signaling in the mouse brain. First, disruption of the Irs2 gene reduced neuronal proliferation during development by 50%, which dissociated brain growth from Irs1-dependent body growth. Second, neurofibrillary tangles containing phosphorylated tau accumulated in the hippocampus of old Irs2 knock-out mice, suggesting that Irs2 signaling is neuroprotective. Thus, dysregulation of the Irs2 branch of the insulin-Igf-signaling cascade reveals a molecular link between diabetes and neurodegenerative disease.

Phosphatidylinositol 3-kinase regulates the induction of long-term potentiation through extracellular signal-related kinase-independent mechanisms

Inhibitors of both phosphatidylinositol-3-kinase (PI3-kinase) and MAPK/ERK (mitogen-activate protein kinase/extracellular signal-related kinase) activation inhibit NMDA receptor-dependent long-term potentiation (LTP). PI3-kinase inhibitors also block activation of ERK by NMDA receptor stimulation, suggesting that PI3-kinase inhibitors block LTP because PI3-kinase is an essential upstream regulator of ERK activation. To examine this hypothesis, we investigated the effects of PI3-kinase inhibitors on ERK activation and LTP induction in the CA1 region of mouse hippocampal slices. Consistent with the notion that ERK activation by NMDA receptor stimulation is PI3-kinase dependent, the PI3-kinase inhibitor wortmannin partially inhibited ERK2 activation induced by bath application of NMDA and strongly suppressed ERK2 activation by high-frequency synaptic stimulation. PI3-kinase and MEK (MAP kinase kinase) inhibitors had very different effects on LTP, however. Both types of inhibitors suppressed LTP induced by theta-frequency trains of synaptic stimulation, but only PI3-kinase inhibitors suppressed the induction of LTP by high-frequency stimulation or low-frequency stimulation paired with postsynaptic depolarization. Concentrations of PI3-kinase inhibitors that inhibited LTP when present during high-frequency stimulation had no effect on potentiated synapses when applied after high-frequency stimulation, suggesting that PI3-kinase is specifically involved in the induction of LTP. Finally, we found that LTP induced by theta-frequency stimulation was MEK inhibitor insensitive but still PI3-kinase dependent in hippocampal slices from PSD-95 (postsynaptic density-95) mutant mice. Together, our results indicate that the role of PI3-kinase in LTP is not limited to its role as an upstream regulator of MAPK signaling but also includes signaling through ERK-independent pathways that regulate LTP induction.

Interleukin-2 receptors and interleukin-2-mediated signaling in myelin: activation of diacylglycerol kinase and phosphatidylinositol 3-kinase

Myelin was previously shown to possess neurotransmitter and cytokine receptors that trigger well-defined signaling mechanisms within the multilamellar structure. The present study reveals the presence of an interleukin-2 (IL-2) receptor in isolated mouse CNS myelin that responds to recombinant mouse IL-2 by activating diacylglycerol kinase (DAGK) and phosphoinositide 3-kinase (PI3K); additional evidence suggests participation by protein tyrosine kinase. Activation of myelin DAGK by IL-2 occurred in brain stem tissue mince and was blocked by chelerythrin chloride, indicating an essential role for myelin-localized protein kinase C. Two inhibitors of PI3K, wortmannin and LY294002, blocked endogenous PI3K as well as that enhanced by IL-2. Activation of PI3K by IL-2 was also blocked by tyrphostin A25, a selective inhibitor of PTK, suggesting activation of the latter by IL-2 is upstream to PI3K activation. This reaction resulted in tyrosine phosphorylation of a protein tentatively identified as the p85 subunit of PI3K. Developmental changes were noted in that receptor density and signaling activity were robust during the period of rapid myelination and declined rapidly thereafter.