Hisaaki Namba

Brain-derived neurotrophic factor induces mammalian target of rapamycin-dependent local activation of translation machinery and protein synthesis in neuronal dendrites.

In neurons, perisynaptic or dendritic translation is implicated in synapse-wide alterations of function and morphology triggered by neural activity. The molecular mechanisms controlling local translation activation, however, have yet to be elucidated. Here, we show that local protein synthesis and translational activation in neuronal dendrites are upregulated by brain-derived neurotrophic factor (BDNF) in a rapamycin and small interfering RNA specific for mammalian target of rapamycin (mTOR)-sensitive manner. In parallel, BDNF induced the phosphorylation of tuberin and the activation of mTOR in dendrites and the synaptoneurosome fraction. mTOR activation stimulated translation initiation processes involving both eIF4E/4E-binding protein (4EBP) and p70S6 kinase/ribosomal S6 protein. BDNF induced phosphorylation of 4EBP in isolated dendrites. Moreover, local puff application of BDNF to dendrites triggered S6 phosphorylation in a restricted area. Taken together, these data indicate that mTOR-dependent translation activation is essential for the upregulation of local protein synthesis in neuronal dendrites.

Neuregulin-1 signals from the periphery regulate AMPA receptor sensitivity and expression in GABAergic interneurons in developing neocortex.

Neuregulin-1 (NRG1) signaling is thought to contribute to both neuronal development and schizophrenia neuropathology. Here, we describe the developmental effects of excessive peripheral NRG1 signals on synaptic activity and AMPA receptor expression of GABAergic interneurons in postnatal rodent neocortex. A core peptide common to all NRG1 variants (eNRG1) was subcutaneously administered to mouse pups. Injected eNRG1 penetrated the blood–brain barrier and activated ErbB4 NRG1 receptors in the neocortex, in which ErbB4 mRNA is predominantly expressed by parvalbumin-positive GABAergic interneurons. We prepared neocortical slices from juvenile mice that were receiving eNRG1 subchronically and recorded inhibitory synaptic activity from layer V pyramidal neurons. Postnatal eNRG1 treatment significantly enhanced polysynaptic IPSCs, although monosynaptic IPSCs were not affected. Examination of excitatory inputs to parvalbumin-containing GABAergic interneurons revealed that eNRG1 treatment significantly increased AMPA-triggered inward currents and the amplitudes and frequencies of miniature EPSCs (mEPSCs). Similar effects on mEPSCs were observed in mice treated with a soluble, full-length form of NRG1 type I. Consistent with the electrophysiologic data, expression of the AMPA receptor GluA1 (i.e., GluR1, GluRA) was upregulated in the postsynaptic density/cytoskeletal fraction prepared from eNRG1-treated mouse neocortices. Cortical GABAergic neurons cultured with eNRG1 exhibited a significant increase in surface GluA1 immunoreactivity at putative synaptic sites on their dendrites. These results indicate that NRG1 circulating in the periphery influences postnatal development of synaptic AMPA receptor expression in cortical GABAergic interneurons and may play a role in conditions characterized by GABA-associated neuropathologic processes.

Brain-derived Neurotrophic Factor Enhances the Basal Rate of Protein Synthesis by Increasing Active Eukaryotic Elongation Factor 2 Levels and Promoting Translation Elongation in Cortical Neurons

The constitutive and activity-dependent components of protein synthesis are both critical for neural function. Although the mechanisms controlling extracellularly induced protein synthesis are becoming clear, less is understood about the molecular networks that regulate the basal translation rate. Here we describe the effects of chronic treatment with various neurotrophic factors and cytokines on the basal rate of protein synthesis in primary cortical neurons. Among the examined factors, brain-derived neurotrophic factor (BDNF) showed the strongest effect. The rate of protein synthesis increased in the cortical tissues of BDNF transgenic mice, whereas it decreased in BDNF knock-out mice. BDNF specifically increased the level of the active, unphosphorylated form of eukaryotic elongation factor 2 (eEF2). The levels of active eEF2 increased and decreased in BDNF transgenic and BDNF knock-out mice, respectively. BDNF decreased kinase activity and increased phosphatase activity against eEF2 in vitro. Additionally, BDNF shortened the ribosomal transit time, an index of translation elongation. In agreement with these results, overexpression of eEF2 enhanced protein synthesis. Taken together, our results demonstrate that the increased level of active eEF2 induced by chronic BDNF stimulation enhances translational elongation processes and increases the total rate of protein synthesis in neurons.

Expression of ErbB4 in substantia nigra dopamine neurons of monkeys and humans

Abnormal neuregulin-1 signaling through its receptor (ErbB4) might be associated with schizophrenia, although their neuropathological contribution remains controversial. To assess the role of neuregulin-1 in the dopamine hypothesis of schizophrenia, we used in situ hybridization and immunoblotting to investigate the cellular distribution of ErbB4 mRNA in the substantia nigra of Japanese monkeys (Macaca fuscata) and human postmortem brains. In both monkeys and humans, significant signal for ErbB4 mRNA was detected in substantia nigra dopamine neurons, which were identified by melanin deposits. The expression of ErbB4 mRNA in nigral dopamine neurons was confirmed with an independent RNA probe, as well as with combined tyrosine hydroxylase immunostaining. Immunoblotting appeared to support the observation of in situ hybridization. Immunoreactivity for ErbB4 protein was much more enriched in substantia nigra pars compacta containing dopamine neurons than in neighboring substantia nigra pars reticulata. These observations suggest that ErbB4 is expressed in the dopaminergic neurons of primate substantia nigra and ErbB4 abnormality might contribute to the dopaminergic pathology associated with schizophrenia or other brain diseases.

A Cyclooxygenase-2 Inhibitor Ameliorates Behavioral Impairments Induced by Striatal Administration of Epidermal Growth Factor

Consistent with the hypothesis that neuroinflammatory processes contribute to the neuropathology of schizophrenia, the protein levels of epidermal growth factor (EGF) and its receptor ErbB1 are abnormal in patients with schizophrenia. To evaluate neuropathological significance of this abnormality, we established an animal model for behavioral deficits by administering EGF into the striatum and evaluated the effects of cyclooxygenase-2 (Cox-2) inhibitor celecoxib. Intracranial infusion of EGF into the striatum of adult male rats activated ErbB1 and induced neurobehavioral impairments observed in several schizophrenia models. Unilateral EGF infusion to the striatum lowered prepulse inhibition (PPI) in a dose-dependent manner and impaired latent learning of active shock avoidance without affecting basal learning ability. Bilateral EGF infusion similarly affected PPI. In contrast, EGF infusion to the nucleus accumbens did not induce a behavioral deficit. Intrastriatal EGF infusion also increased Cox-2 expression, elevated tyrosine hydroxylase activity, and upregulated the levels of dopamine and its metabolites. Subchronic administration of celecoxib (10 mg/kg, p.o.) ameliorated the abnormalities in PPI and latent learning as well as normalized dopamine metabolism. We conclude that this EGF-triggered neuroinflammatory process is mediated in part by Cox-2 activity and perturbs dopamine metabolism to generate neurobehavioral abnormalities.

Transforming growth factor alpha attenuates the functional expression of AMPA receptors in cortical GABAergic neurons.

In the developing neocortex, brain-derived neurotrophic factor (BDNF) exerts a trophic activity to increase the expression and channel activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor subunits. Here, we demonstrate that the epidermal growth factor (EGF) receptor (ErbB1) ligands exert the opposite biological activity in cultured neocortical neurons. Subchronic stimulation of ErbB1 with transforming growth factor alpha (TGFalpha), EGF, or heparin-binding EGF (HB-EGF) down-regulated protein expression of the GluR1 AMPA receptor subunit in cultured neocortical neurons. In agreement, TGFalpha treatment decreased the Bmax of [3H] AMPA binding and GluR1 mRNA levels. Immunocytochemistry revealed that the decrease in GluR1 was most pronounced in multipolar GABAergic neurons. To examine the physiological consequences, we recorded AMPA-evoked currents as well as miniature excitatory postsynaptic currents in morphologically identified putative GABAergic neurons in culture. Subchronic TGFalpha treatment decreased AMPA-triggered currents as well as the amplitude and frequency of miniature excitatory postsynaptic currents. An ErbB1 tyrosine kinase inhibitor, PD153035, inhibited the TGFalpha effect. Moreover, TGFalpha counteracted the neurotrophic activity of BDNF on AMPA receptor expression. Co-application of TGFalpha with BDNF blocked the BDNF-triggered up-regulation of AMPA receptor expression and currents. These observations reveal a negative regulatory activity of the ErbB1 ligand, TGFalpha, which reduces the input sensitivity of cortical GABAergic neurons to attenuate their inhibitory function.

Brain-derived neurotrophic factor upregulates and maintains AMPA receptor currents in neocortical GABAergic neurons.

The regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors is implicated in synaptic plasticity. Although we have found that brain-derived neurotrophic factor (BDNF) triggers surface translocation of AMPA receptor proteins, the physiological significance of the BDNF effect remained to be determined. The present immunohistochemical studies revealed that cortical GABAergic neurons exhibited the most striking response to BDNF. Accordingly, we monitored AMPA-triggered currents through GABAergic neurons: Chronic BDNF treatment increased the AMPA-triggered currents but not NMDA-triggered currents in culture. In parallel, the amplitude, but not frequency, of spontaneous miniature excitatory postsynaptic currents (mEPSCs) was elevated in GABAergic neurons. In agreement, BDNF enhanced GABA release triggered by AMPA compared to the amount triggered by high potassium. Conversely, there was a significant decrease in the mEPSC amplitude of GABAergic neurons in heterozygous BDNF-knockout mice. These findings indicate that the neurotrophin enhances the input sensitivity of GABAergic neurons to facilitate their inhibitory function in the neocortex.

Pallidal hyperdopaminergic innervation underlying D2 receptor-dependent behavioral deficits in the schizophrenia animal model established by EGF.

Epidermal growth factor (EGF) is one of the ErbB receptor ligands implicated in schizophrenia neuropathology as well as in dopaminergic development. Based on the immune inflammatory hypothesis for schizophrenia, neonatal rats are exposed to this cytokine and later develop neurobehavioral abnormality such as prepulse inhibition (PPI) deficit. Here we found that the EGF-treated rats exhibited persistent increases in tyrosine hydroxylase levels and dopamine content in the globus pallidus. Furthermore, pallidal dopamine release was elevated in EGF-treated rats, but normalized by subchronic treatment with risperidone concomitant with amelioration of their PPI deficits. To evaluate pathophysiologic roles of the dopamine abnormality, we administered reserpine bilaterally to the globus pallidus to reduce the local dopamine pool. Reserpine infusion ameliorated PPI deficits of EGF-treated rats without apparent aversive effects on locomotor activity in these rats. We also administered dopamine D1-like and D2-like receptor antagonists (SCH23390 and raclopride) and a D2-like receptor agonist (quinpirole) to the globus pallidus and measured PPI and bar-hang latencies. Raclopride (0.5 and 2.0 µg/site) significantly elevated PPI levels of EGF-treated rats, but SCH23390 (0.5 and 2.0 µg/site) had no effect. The higher dose of raclopride induced catalepsy-like changes in control animals but not in EGF-treated rats. Conversely, local quinpirole administration to EGF-untreated control rats induced PPI deficits and anti-cataleptic behaviors, confirming the pathophysiologic role of the pallidal hyperdopaminergic state. These findings suggest that the pallidal dopaminergic innervation is vulnerable to circulating EGF at perinatal and/or neonatal stages and has strong impact on the D2-like receptor-dependent behavioral deficits relevant to schizophrenia.

In vivo administration of epidermal growth factor and its homologue attenuates developmental maturation of functional excitatory synapses in cortical GABAergic neurons

The ErbB1 ligand family includes epidermal growth factor (EGF), transforming growth factor‐α (TGFα), heparin‐binding EGF‐like growth factor, amphiregulin and betacellulin. Previously, we demonstrated that TGFα decreases α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA)‐type glutamate receptors in cultured neocortical γ‐aminobutyric acid (GABA) neurons. In the present study, we examined in vivo effects of EGF and TGFα in the mouse neocortex using electrophysiological and biochemical techniques. In mouse neonates, subcutaneously administered EGF penetrated the blood–brain barrier and activated ErbB1 in the neocortex. Daily administration of EGF or TGFα attenuates developmental increases in expression of the AMPA receptor subunits (GluR1 and GluR2/3) in the neocortex of postnatal mice. Immunohistochemistry revealed that the reduction in AMPA receptor expression was significant in the GABAergic neurons, especially those positive for parvalbumin. Using cortical slices prepared from EGF‐treated mice, we recorded miniature excitatory postsynaptic currents (mEPSCs) in both GABAergic and pyramidal neurons. Subchronic treatment with EGF decreased the amplitude and frequency of mEPSCs in GABAergic neurons, but its effects were negligible on pyramidal neurons. We conclude that EGF or other ErbB1 ligand(s) attenuates a developmental increase in AMPA receptor expression and function in cortical GABAergic neurons.

Intersegmental Coordination of Swimmeret Movements: Mathematical Models and Neural Circuitsa

Abstract: Swimmerets move periodically through a cycle of power‐strokes and return‐strokes. Swimmerets on neighboring segments differ in phase by ~25%, and maintain this difference even when the period of the cycle changes from < 1 to >4 Hz. We constructed a minimal cellular model of the segmental pattern‐generating circuit which incorporated its essential components, and whose dynamics were like those of the local circuit. Three different intersegmental coordinating units were known to link neighboring ganglia, but their targets are unknown. We constructed different intersegmental circuits which these units might form between neighboring cellular models, and compared their dynamics with the real system. One intersegmental circuit could maintain an ~25% phase difference through a range of periods.