Yuki Miyamoto

ErbB2 directly activates the exchange factor Dock7 to promote Schwann cell migration

The cellular events that precede myelination in the peripheral nervous system require rapid and dynamic morphological changes in the Schwann cell. These events are thought to be mainly controlled by axonal signals. But how signals on the axons are coordinately organized and transduced to promote proliferation, migration, radial sorting, and myelination is unknown. We describe that the axonal signal neuregulin-1 (NRG1) controls Schwann cell migration via activation of the atypical Dock180-related guanine nucleotide exchange factor (GEF) Dock7 and subsequent activation of the Rho guanine triphosphatases (GTPases) Rac1 and Cdc42 and the downstream c-Jun N-terminal kinase. We show that the NRG1 receptor ErbB2 directly binds and activates Dock7 by phosphorylating Tyr-1118. Dock7 knockdown, or expression of Dock7 harboring the Tyr-1118–to–Phe mutation in Schwann cells, attenuates the effects of NRG1. Thus, Dock7 functions as an intracellular substrate for ErbB2 to promote Schwann cell migration. This provides an unanticipated mechanism through which ligand-dependent tyrosine phosphorylation can trigger the activation of Rho GTPase-GEFs of the Dock180 family.

Cdk5 phosphorylation of WAVE2 regulates oligodendrocyte precursor cell migration through nonreceptor tyrosine kinase Fyn.

Myelin formation of the CNS is a complex and dynamic process. Before the onset of myelination, oligodendrocytes (OLs), the myelin-forming glia of the CNS, proliferate and migrate along axons. Little is known about the molecular mechanisms underlying the early myelination processes. Here, we show that platelet-derived growth factor (PDGF), the crucial physiological ligand in early OL development, controls the migration of oligodendrocyte precursor cells (OPCs) through cyclin-dependent kinase 5 (Cdk5). PDGF stimulates Cdk5 activity in a time-dependent manner, whereas suppression of Cdk5 by the specific inhibitor roscovitine or by the retrovirus encoding short-hairpin RNA for Cdk5 impairs PDGF-dependent OPC migration. The activation of Cdk5 by PDGF is mediated by the phosphorylation of the nonreceptor tyrosine kinase, Fyn, whose inhibition reduces PDGF-dependent OPC migration. Furthermore, Cdk5 regulates PDGF-dependent OPC migration through the direct phosphorylation of WASP (Wiskott–Aldrich syndrome protein)-family verprolin-homologous protein 2 (WAVE2). Cdk5 phosphorylates WAVE2 at Ser-137 in vitro. Infection of the WAVE2 construct harboring the Ser-137-to-Ala reduces PDGF-dependent migration. Together, PDGF regulates OPC migration through an as-yet-unidentified signaling cascade coupling Fyn kinase to Cdk5 phosphorylation of WAVE2. These results provide new insights into both the role of Cdk5 in glial cells and the molecular mechanisms controlling the early developmental stage of OLs.

Valproic acid-inducible Arl4D and cytohesin-2/ARNO, acting through the downstream Arf6, regulate neurite outgrowth in N1E-115 cells.

The mood-stabilizing agent valproic acid (VPA) potently promotes neuronal differentiation. As yet, however, little is known about the underlying molecular mechanism. Here, we show that VPA upregulates cytohesin-2 and mediates neurite outgrowth in N1E-115 neuroblastoma cells. Cytohesin-2 is the guanine-nucleotide exchange factor (GEF) for small GTPases of the Arf family; it regulates many aspects of cellular functions including morphological changes. Treatment with the specific cytohesin family inhibitor SecinH3 or knockdown of cytohesin-2 with its siRNA results in blunted induction of neurite outgrowth in N1E-115 cells. The outgrowth is specifically inhibited by siRNA knockdown of Arf6, but not by that of Arf1. Furthermore, VPA upregulates Arl4D, an Arf-like small GTPase that has recently been identified as the regulator that binds to cytohesin-2. Arl4D knockdown displays an inhibitory effect on neurite outgrowth resulting from VPA, while expression of constitutively active Arl4D induces outgrowth. We also demonstrate that the addition of cell-permeable peptide, coupling the cytohesin-2-binding region of Arl4D into cells, reduces the effect of VPA. Thus, Arl4D is a previously unknown regulator of neurite formation through cytohesin-2 and Arf6, providing another example that the functional interaction of two different small GTPases controls an important cellular function.

The mood stabilizer valproic acid upregulates neurofibromatosis 2 tumor suppressor (merlin) and JNK to induce aberrant neurite extension

s / Neuroscience Research 68S (2010) e223–e334 e317 P2-p12 The mood stabilizer valproic acid upregulates neurofibromatosis 2 tumor suppressor (merlin) and JNK to induce aberrant neurite extension Tomohiro Torii 1 , Yuki Miyamoto 1, Junji Yamauchi 1,2, Akito Tanoue 1 1 Department of Pharmacol., NICHD, Tokyo 2 Department of Biosci., Tokyo Inst. of Technol., Kanagawa Valproic acid (VPA), the drug for bipolar disorder and epilepsy, has a potent ability to induce neuronal differentiation, yet comparatively little is presently known about the underlying mechanism. We previously demonstrated that c-Jun N-terminal kinase (JNK) phosphorylation of the focal adhesion protein paxillin mediates differentiation in N1E-115 neuroblastoma cells. Here, we show that VPA up-regulates the neurofibromatosis type 2 (NF2) tumor suppressor, merlin, to regulate neurite outgrowth through the interaction with paxillin. The inhibition of merlin function by its knockdown or expression of merlin harboring the Gln-538-to-Pro mutation, a naturally occurring NF2 missense mutation deficient in linking merlin to the actin cytoskeleton, decreases VPA-induced neurite outgrowth. Importantly, the expression of merlin by itself is not sufficient to induce neurite outgrowth, which requires co-expression with paxillin, the binding partner of merlin. In fact, the missense mutation Trp-60-to-Cys or Phe-62-to-Ser, that is deficient in binding to paxillin, reduces neurite outgrowth induced by VPA. In addition, co-expression of a paxillin construct harboring the mutation at the JNK phosphorylation site with merlin results in blunted induction of the outgrowth. We also find that the first LIM domain of paxillin is a major binding region with merlin and that expression of the isolated first LIM domain blocks the effects of VPA. Furthermore, similar findings that merlin regulates neurite outgrowth through the interaction with paxillin have been observed in several kinds of neuronal cells. These results suggest that merlin is an as yet unknown regulator of neurite outgrowth through the interaction with paxillin, providing a possibly common mechanism regulating neurite formation. doi:10.1016/j.neures.2010.07.1406 P2-p13 Mood stabilizers-mediated gene expression in human astrocytes Zhiqian Yu , Chiaki Ono, Yoichiro Tanabe, Ichiro Sora, Hiroaki Tomita Department of Biological Psychiatry Graduate School of Medicine Tohoku University Bipolar disorder (BD) is a recurrent disorder characterized by episodes of mania and depression. Lithium (Li), valproic acid (VPA), carbamazepine (CBZ), and lamotrigine (LTG) are the four most often prescribed mood stabilizers (MD) to prevent recurrence as well as treat manic and depressive symptoms of the disease. MD have significant effects on characteristics of neuronal culture cell, including cell survival, morphology and chemotaxis. At least some of these effects are due to the inhibition of GSK3. Another direct target for VPA is HDAC. However molecular mechanisms underlying the actions of MD as well as the pathogeneses of the illness itself remain largely unknown. Astrocytes take a major roles in regulation of extracellular ion concentrations, modification of synaptic efficacy, maintenance and mediation of the blood–brain barrier (BBB) and supply of nutrients and trophic factors for neurons and oligodendrocytes. In order to study the common effects of the four MD on gene expression profiles of astrocytes, astrocyte-derived cell line UG-87 MG cells was suspended in MEM, and divided into 6 plastic T75 Falcon flasks. Li (0.75 mM) and VPA (0.5 mM), CBZ (50 M) and LTG (5 M) were then suspended into MEM at therapeutic concentrations. After 7 days, the cells were collected with PBS. Extracted samples were applied on Illumina Human-6 V2 microarray to analyze global changes in mRNA expression, compared with non-treated samples. Genome-wide gene expression analyses indicate that four major mood stabilizers commonly suppress development and regulation related genes, which play important roles in cellular survival, cell morphogenesis and cell migration. MD may exert preventive effects on recurrence of manic and depressive episodes by regulating transcriptional activities of these genes in astrocytes. doi:10.1016/j.neures.2010.07.1407 P2-p14 Antidepressants increase GDNF gene transcription through the histone modifications by HDAC4 Koji Otsuki , Shusaku Uchida, Hirotaka Yamagata, Yoshifumi Watanabe Dept Neuroscience, Univ of Yamaguchi, Ube Antidepressants have been indicated to induce the expression of a variety of neurotrophic factors which play important roles in the neural and structural plasticity. We have previously developed an animal model of depression: BALB/c mice exhibited the increase in depression-like and anxiety behaviors and the decreased expression level of glial cell line-derived neurotrophic factor (GDNF) mRNA in the striatum after a 6-wk of chronic ultra-mild stress episode. Importantly, the reduction of GDNF mRNA in the stressed BALB/c mice was reversed by chronic treatment with antidepressant imipramine. Therefore in this study, we aimed to clarify the molecular mechanism underlying the changes in the GDNF expression by antidepressants. First, we examined the effects of antidepressants, mood stabilizers, an anxiolytic drug and an antipsychotic drug on the expression of GDNF mRNA in C6 glial cells. The expression of GDNF mRNA was increased by all antidepressants used and valproate, whereas other drugs did not induce the GDNF expression. Next, since the valproate has been known to act as a histone deacetylase inhibitor, we examined the effects of several histone deacetylase inhibitors on the expression of GDNF mRNA in C6 glial cells. The expression of GDNF mRNA was increased by all histone deacetylase inhibitors used. Therefore we hypothesized that the induction of GDNF mRNA by antidepressants could be mediated by the histone modification at the GDNF gene promoter. Actually, ChIP assay revealed that the levels of histone H3 and H4 acetylation at the GDNF promoter were enhanced by antidepressants, whereas lithium and diazepam did not affect the levels of these histone acetylations. In addition, HDAC4 significantly decreased the transcriptional activity of GDNF promoter and inhibited the induction of GDNF expression by antidepressants. Thus, our data suggest that antidepressants increase transcriptional activity of GDNF gene through the modulation of histone acetylation by HDAC4. doi:10.1016/j.neures.2010.07.1408 P2-p15 Comprehensive gene expression analysis of SK-NSH cells after Lamotrigine treatment Yoichiro Tanabe 1,2 , Zhiqian Yu 1, Chiaki Ono 1, Ai Hato 1, Hiroo Matsuoka 2, hiroaki Tomita 1 1 Department of Biological Psychiatry, Tohoku University Graduate school of medicine 2 Department of Psychiatry, Tohoku University Graduate school of Medicine Objection: Bipolar disorder is serious illness that afflicts around 1% of populations. Lamotrigine is effective for depressive state of bipolar disorder and prophylaxis of the disease. However, the mechanisms of actions are still unknown. In order to elucidate these mechanisms, we conducted microarray gene expression analyses of human neuronal cells with or without theraputic concentrations of Lamotrigine. Method: SK-N-SH cells, derived from human neuronal cells, were cultured with or without therapeutic concentration<5 M,50 M> of Lamotrigine for five days. After RNA extraction, cDNA was synthesized from each sample. Following a protocol provided from Illumina company, biotinylated cRNA was synthesized and hybridized to Human-6 V2 microarray for 16 hours. After washing out non-specific attachment, signal intensity for each probe was measured with a Illumina scanner. Overrepresentations of categories of the Lamotrigine-mediated genes were analyzed using David Bioinfomatics Resources 6.7. Dose dependent manner of gene expression changes were evaluated using qRT-PCR. Results & conclusion: Lamotrigine induced expression levels of 781 genes and suppressed 802 genes by 20% in SK-N-SH cells. DAVID overrepresentation analyses suggest that Lamotrigine regulates transcriptional levels of genes involved in cytoskeleton, cell cycle, intracellular non-membrane-bounded organelle. These genes may be relevant to the phamacological action of Lamotrigine in the treatment of bipolar disorder. doi:10.1016/j.neures.2010.07.1409

Neurofibromatosis 2 tumor suppressor, the gene induced by valproic acid, mediates neurite outgrowth through interaction with paxillin

s S63 neurons. Because these microcolumns are found in both young and adult animals, and over a fairly wide area in the neocortex, they may be important in neocortical function. In this paper, detailed analyses of the organization of the microcolumns and functional investigations using molecular and physiological methods will be presented. doi:10.1016/j.neures.2009.09.184 O2-J3-2 Periodic microcolumns in layer V of binocular visual cortex Hisato Maruoka, Shun Tsuruno, Rumi Kurokawa, Toshihiko Hosoya Hosoya Reserch Unit, RIKEN BSI, Japan Our previous study demonstrated that layer V of the mouse neocortex has a periodically repeated structure composed of microcolumns of subcerebral projection neurons (SCPNs). Here, in order to analyze in vivo functions of the microcolumns and their possible involvement in plasticity, we confirmed the existence of the periodic microcolumnar arrangement in the binocular visual cortex of adult mice. In situ hybridization for marker genes revealed a periodical arrangement of SCPNs in slices of the binocular visual cortex prepared with a specific cutting angle and a thickness. When SCPNs in the binocular visual cortex were labeled by injecting retrograde tracers into the superior colliculus, they were found in a periodic microcolumnar arrangement. Moreover, we also found periodic structures in slices of the binocular visual cortex prepared from BAC transgenic mice which specifically express eGFP in SCPNs. These results strongly suggest that SCPNs form a periodic microcolumnar arrangement in the binocular visual cortex. It would therefore be intriguing to characterize the response properties of these microcolumns in vivo. doi:10.1016/j.neures.2009.09.185 O2-J3-3 Neuroprotection and cell death in the retina Monica L. Acosta, Michael Kalloniatis University of Auckland, New Zealand Little progress has occurred in the provision of useful strategies to delay/intervene the programmed cell death (apoptotic) pathways that typify many ocular diseases. In this study it is evaluated the neuroprotective effect of vinpocetine on anoxic and ischemic retinas and its relationship with channel permeability properties and metabolism of the retina. We investigated the mechanisms of apoptosis that result in cell death in retinal ischemia, anoxia and hypoglycemia. The study focused on analysis of apoptosis in the inner retina. Simultaneous protein labelling combined with visualization of cell permeability markers allowed to infer the spatial display of cell death in the retina. Vinpocetine facilitated metabolism of the retina by blocking calcium-channel and voltage-gated sodium channels activity. We analyzed the effect of vinpocetine on lactate dehydrogenase activity and in the permeability of agmatine into population of cells known to be affected by anoxia and ischemia. The results indicated that there is differential cell population susceptibility to this drug. doi:10.1016/j.neures.2009.09.186 O2-J3-4 Open channels in gap junctions between homologous retinal neurons Soh Hidaka Dept. Physiol., Fujita Health Univ., Japan Gap junctions of retinal horizontal, amacrine and ganglion cells (Hidaka et al., J. Neurosci., 2004) were addressed to establish the relationship between connexin distribution and electrical coupling measured by simultaneous recordings. Channel opening in electrical synapses was investigated with protocols using multiplex arrays of methodologies: dual patch clamp, connexin immunocytochemistry, freeze fracturing and high-voltage electron microscopy (09HVEM-502, NIPS, Okazaki) of the cells. Junctional conductance, and the size and number of gap junctions were examined in pairs of cells. Measuring the packing density of connexons in replicated tissue of the intramembrane distribution, a total number of connexons in a pair was estimated. When a single channel conductance of the connexin is present, a total of gap junction conductance in a pair was evaluated. The measured junctional conductance could allow us to identify the fraction of active channels as 0.3% for horizontal cells, 0.1% for amacrine cells, and 0.6% for ganglion cells, respectively. The small fraction suggests that the presence of positive neuromodulators could give rise to a greater proportion of active channels. doi:10.1016/j.neures.2009.09.187 O2-J4-1 Pathophysiological functions of DISC1 as a component of RNA granule Daisuke Tsuboi1,2, Keisuke Kuroda1,2, Yasutaka Fujino1,2, Kozo Kaibuchi1,2 1 Department Cell Pharmacology, University of Nagoya, Nagoya, Japan; 2 JST, CREST, Tokyo, Japan Disrupted-In-Schizophrenia 1 (DISC1) is a candidate gene for susceptibility to schizophrenia. We previously found that DISC1 regulated the axonal transport of NUDEL/LIS1/14-3-3 complex (Taya et al., J. Neurosci, 27, 2007) and Grb2 (Shinoda et al., J. Neurosci, 27, 2007) through Kinesin-1 in rat hippocampal neurons. Here, we found that several messenger ribonucleoproteins (mRNP) including Hzf were identified as DISC1-interacting molecules. mRNP regulates the localization and translation of mRNA in dendrites. Hzf is thought to regulate the dendritic localization and translation of inositol 1, 4, 5-triphosphate receptor 1 (IP3R1) mRNA. IP3R1 is an intracellular calcium release channel, involving in synaptic transmission and plasticity. DISC1 directly interacted with Hzf in vitro and in vivo. DISC1 partly colocalized with Hzf and IP3R1 mRNA in hippocampal dendrites. The overexpression and knockdown experiments indicated that DISC1 regulated the dendritic localization of IP3R1 mRNA. I will discuss the roles of DISC1 in the regulation of mRNP functions. doi:10.1016/j.neures.2009.09.188 O2-J4-2 Disturbance of Disrupted-in-Schizophrenia-1 during neurodevelopment develops phenotypes of psychiatric disorders related to dysfunction of the dopaminergic system Minae Niwa1,2, Atsushi Kamiya3, Rina Murai2, Ken-ichiro Kubo4, Hanna Jaaro-Peled3, Lingling Lu2, Norio Ozaki1, Kazunori Nakajima4, Yukihiro Noda5, Akira Sawa3, Toshitaka Nabeshima2 1 Psychiatry, Nagoya University Grad. Sch. Med., Nagoya, Japan; 2 Chem. Pharmacol., Meijo University Grad. Sch. Pharmacal. Sci., Nagoya, Japan; 3 Psychiatry, Johns Hopkins University Sch. Med., Baltimore, USA; 4 Anat, Keio University Sch. Med., Tokyo, Japan; 5 Clin Sci. Neuropsychopharmacol., Meijo University Grad. Sch. Pharmacal. Sci., Nagoya, Japan We generated the mice in which selective knockdown of DISC1 is achieved in a lineage for pyramidal neurons in the frontal cortex (Fc) during the development via in utero gene transfer (KD mice). Dopamine contents, and immunoreactivities against tyrosine hydroxylase of KD mice were decreased compared with those of control on postnatal day 56 (P56). KD mice on P56, but not P28, exhibited deficits in prepulse inhibition and novel object recognition task compared with control. Treatment of clozapine improved these deficits in KD mice. These results indicate that disturbance of DISC1 during development is sufficient to generate several core endophenotypes underlying schizophrenia and mood disorders. doi:10.1016/j.neures.2009.09.189 O2-J4-3 Neurofibromatosis 2 tumor suppressor, the gene induced by valproic acid, mediates neurite outgrowth through interaction with paxillin Junji Yamauchi1,2, Tomohiro Torii 1, Shinji Kusakawa1, Atsushi Sanbe1, Yuki Miyamoto1, Akito Tanoue1 1 Dept. Pharmacol., NICHD of Japan; 2 Dept. Biol. Sci., Tokyo Institute of

Control of myelination by oligodendrocytes through the novel phosphorylation mechanism of cytoskeletal proteins

s S13 SY1-F1-5 Adaptability of the biped robot Koh Hosoda Osaka University, Japan Body compliance of organims is supposed to play a crucial role for adaptive behavior. If robots are designed to have rigid bodies, their upper limitation of controllability is the control bandwidth. On the other hand, if the robots are compliant, it can react to environmental changes with no time-delay. In the talk, we will pick up biped locomotion driven by pneumatic artifitial muscles which is essentially compliant, and discuss on the adaptability. doi:10.1016/j.neures.2009.09.1545 SY1-F2-1 Neuronal development and neurodegeneration on Cdk5 and related signaling Shin-ichi Hisanaga, Taro Saito Tokyo Metropolitan University, Japan Cdk5, a member of cyclin-dependent kinases, is a Ser/Thr protein kinase activated by binding a neuron specific activator p35 or p39. Cdk5/p35 functions in both brain formation and neurodegeneration. These functions are closely related to the kinase activity and cellular localization of Cdk5. In healthy neurons, Cdk5 associates with membranes via myristoylation of p35, and membrane association maintains the kinase activity to moderate levels. The kinase activity of Cdk5 is also regulated by degradation and cleavage of p35. p35 is a short life protein degraded by proteasome on membranes and cleaved to p25 by calpain in neurons undergoing cell death. The cleavage induces abnormal activation, resulting in neuronal cell death of neurodegenerative diseases. Degradation of p35 is regulated developmentally by phosphorylation. The phosphorylated form in embryonic or juvenile brains is easily degraded and the dephosphorylated form in matured neurons has a propensity to be cleaved. How Cdk5 kinase activity is regulated and involved in brain development and pathology will be further discussed. doi:10.1016/j.neures.2009.09.1546 SY1-F2-2 The role of Dock family Rho-GEFs in dendrite morphogenesis Hironori Katoh, Shuhei Ueda, Kazuya Kuramoto, Satoshi Fujimoto, Manabu Negishi Grad Sch Biostudies, Kyoto University, Japan Rho family small GTPases play important roles in various aspects of neuronal development including dendritic growth and spine formation, although the molecular mechanisms that control their activities are not fully understood. Recent studies have identified an evolutionarily conserved superfamily of Dock180-related proteins that function as novel types of guanine nucleotide exchange factors (GEFs) for Rho family GTPases, and disruption of genes encoding members of this family has been associated with human neurological diseases such as mental retardation. We found that Dock4, a Rac-specific GEF, regulates dendrite branching, whereas Dock9/Zizimin1, a Cdc42-specific GEF, promotes dendrite growth in hippocampal neurons. In addition, Dock4 localizes to dendritic spines at late developmental stages in hippocampal neurons and regulates spine formation. We are also studying the molecular mechanisms of the regulation of dendrite development by Dock proteins. I will discuss the diverse roles of Dock family GEFs in the regulation of dendrite morphogenesis and neuronal development. doi:10.1016/j.neures.2009.09.1547 SY1-F2-3 Homeostasis of nigro-striatal dopaminergic projection – compensatory regulation of tyrosine hydroxylase and dopamine content Hirofumi Tokuoka, Hiroshi Ichinose Grad Schl Biosci & Biotech, Tokyo Tech, Tokyo, Japan Nigro-striatal dopaminergic transmission is important for regulation of voluntary movement. Homeostatic change of nigro-striatal dopaminergic projection has been suggested from studies on Parkinson’s disease and its models. However, it is still unclear how dopaminergic neurons are homeostatically regulated. To further investigate this issue, we generated floxed tyrosine hydroxylase (TH) mice and used an adeno-associated viral vector expressing Cre recombinase (AAV-Cre). Stereotaxic microinjection of AAV-Cre into the substantia nigra pars compacta (SNc) of adult floxed TH mice efficiently ablated the TH gene. We found that, the rate of decrease in TH protein level is slower in the striatum (axon terminals) than in SNc (cell bodies). Moreover, in the striatum, while TH protein level decreased to about 20% eight weeks after injection, dopamine contents decreased to only about 50%. These results suggest that both TH protein level and activity are compensatory regulated in response to TH gene inactivation. doi:10.1016/j.neures.2009.09.1548 SY1-F2-4 Control of myelination by oligodendrocytes through the novel phosphorylation mechanism of cytoskeletal proteins Yuki Miyamoto1, Akito Tanoue1, Shin-ichi Hisanaga2, Junji Yamauchi1,3 1 Dept Pharmacol, Nat Res Inst for Child Health and Dev, Setagaya, Tokyo, Japan; 2 Dept of Biol Sci, Grad School of Sci, Tokyo Metropolitan Univ, Hachiohji, Tokyo, Japan; 3 Dept of Biol Sci, Tokyo Inst of Technology, Yokohama,

Gadd45a, the gene induced by the mood stabilizer valproic acid, regulates neurite outgrowth through JNK and the substrate Paxillin

Valproic acid (VPA), a mood stabilizer and anticonvulsant, has a variety of neurotrophic functions; however, less is known about how VPA regulates neurite outgrowth. Here, using N1E-115 neuroblastoma cells as the model, we show that VPA upregulates Gadd45a to trigger activation of the downstream JNK cascade controlling neurite outgrowth. VPA induces the phosphorylation of c-Jun N-terminal kinase (JNK) and the substrate paxillin, while VPA induction of neurite outgrowth is inhibited by JNK inhibitors (SP600125 and the small JNK-binding peptide) or a paxillin construct harboring a Ser 178-to-Ala mutation at the JNK phosphorylation. Transfection of Gadd45a, acting through the effector MEKK4, leads to the phosphorylation of the JNK cascade. Conversely, knockdown of Gadd45a with siRNA reduces the effect of VPA. Taken together, these results suggest that upregulation of Gadd45a explains one of the mechanisms whereby VPA induces the neurotrophic effect, providing a new role of Gadd45a in neurite outgrowth.

Ras activation of Rac1 exchange factor Tiam1 mediates neurotrophin-3-induced Schwann cell migration

Endogenous neurotrophins positively and negatively regulate migration of premyelinating Schwann cells before the initiation of myelination. Neurotrophin-3 (NT3) acting through the TrkC receptor tyrosine kinase stimulates Schwann cell migration via the Rho GTPases Rac1 and Cdc42. We previously demonstrated that TrkC directly phosphorylates and activates Dbs, the guanine-nucleotide exchange factor (GEF) for Cdc42, to partially mediate Schwann cell migration. Here, we identify T lymphoma invasion and metastasis (Tiam) 1 as the Rac1-specific guanine-nucleotide exchange factor involved in NT3-induced Schwann cell migration. Furthermore, the interaction between the small GTPase Ras and Tiam1 plays an essential role in the activation of Rac1. Taken together, these results suggest that NT3 activation of TrkC stimulates Schwann cell migration through two parallel signaling units, Ras/Tiam1/Rac1 and Dbs/Cdc42, and that Schwann cell migration is uniquely regulated in the case of Ras and Rac1, by two different types of small GTPases.