Neuregulin and ErbB receptor signaling pathways in the nervous system

Neuregulin-1 reverses long-term potentiation at CA1 hippocampal synapses

Neuregulin-1 (NRG-1) has been identified genetically as a schizophrenia susceptibility gene, but its function in the adult brain is unknown. Here, we show that NRG-1beta does not affect basal synaptic transmission but reverses long-term potentiation (LTP) at hippocampal Schaffer collateral-->CA1 synapses in an activity- and time-dependent manner. Depotentiation by NRG-1beta is blocked by two structurally distinct and selective ErbB receptor tyrosine kinase inhibitors. Moreover, ErbB receptor inhibition increases LTP at potentiated synapses and blocks LTP reversal by theta-pulse stimuli. NRG-1beta selectively reduces AMPA, not NMDA, receptor EPSCs and has no effect on paired-pulse facilitation ratios. Live imaging of hippocampal neurons transfected with receptors fused to superecliptic green fluorescent protein, as well as quantitative analysis of native receptors, show that NRG-1beta stimulates the internalization of surface glutamate receptor 1-containing AMPA receptors. This novel regulation of LTP by NRG-1 has important implications for the modulation of synaptic homeostasis and schizophrenia.

Aberrant motor axon projection, acetylcholine receptor clustering, and neurotransmission in cyclin-dependent kinase 5 null mice

Cyclin-dependent kinase (Cdk)5 is a key regulator of neural development. We have previously demonstrated that Cdk5/p35 are localized to the postsynaptic muscle and are implicated in the regulation of neuregulin/ErbB signaling in myotube culture. To further elucidate whether Cdk5 activity contributes to neuromuscular junction (NMJ) development in vivo, the NMJ of Cdk5-/- mice was examined. Consistent with our previous demonstration that Cdk5 phosphorylates ErbB2/3 to regulate its tyrosine phosphorylation, we report here that the phosphorylation of ErbB2 and ErbB3 and the ErbB2 kinase activity are reduced in Cdk5-deficient muscle. In addition, Cdk5-/- mice also display morphological abnormalities at the NMJ pre- and postsynaptically. Whereas the outgrowth of the main nerve trunk is grossly normal, the intramuscular nerve projections exhibit profuse and anomalous branching patterns in the Cdk5-/- embryos. The central band of acetylcholine receptor (AChR) clusters is also wider in Cdk5-/- diaphragms, together with the absence of S100 immunoreactivity along the phrenic nerve during late embryonic stages. Moreover, we unexpectedly discovered that the agrin-induced formation of large AChR clusters is significantly increased in primary muscle cultures prepared from Cdk5-null mice and in C2C12 myotubes when Cdk5 activity was suppressed. These abnormalities are accompanied by elevated frequency of miniature endplate potentials in Cdk5-null diaphragm. Taken together, our findings reveal the essential role of Cdk5 in regulating the development of motor axons and neuromuscular synapses in vivo.

Neuregulin-1 enhances survival of human astrocytic glioma cells

Malignant astrocytic gliomas, referred to as astrocytomas, represent the most commonly diagnosed adult primary brain tumor. These tumors are characterized by unrelenting growth that is often resistant to chemotherapy and radiation therapy. Tumor expansion into the healthy surrounding brain tissue produces severe and often fatal consequences. In this study, we examine the potential for the neuregulin-1/erbB receptor signaling cascade to contribute to this process by modulating glioma cell growth. Using antibodies specific for the erbB receptors, we demonstrate the expression patterns for the erbB2, erbB3, and erbB4 receptors in human glioma biopsy samples. We then verify receptor expression in a panel of human glioma cell lines. Next, we investigate the status of the erbB2 and erbB3 receptors in the human glioma cell lines and find that they are constitutively tyrosine-phosphorylated and heterodimerized. Subsequently, we demonstrate that theses same cell lines express membrane bound and released forms of neuregulins, the erbB receptor ligands, suggesting a possible autocrine or paracrine signaling network. Furthermore, we show that exogenous activation of erbB2 and erbB3 receptors in U251 glioma cells by recombinant Nrg-1beta results in enhanced glioma cell growth under conditions of serum-deprivation. This enhancement is due to an increase in cell survival rather than an increase in cell proliferation and is dependent on the activation of erbB2 and phosphatidylinositol-3 kinase (PI3K). Moreover, Nrg-1beta activates an inhibitor of apoptosis, Akt, implying a possible role for this kinase in mediating Nrg-1beta effects in gliomas. This data suggests that glioma cells may use autocrine or paracrine neuregulin-1/erbB receptor signaling to enhance cell survival under conditions where growth would otherwise be limited.

The origin and development of glial cells in peripheral nerves

During the development of peripheral nerves, neural crest cells generate myelinating and non-myelinating glial cells in a process that parallels gliogenesis from the germinal layers of the CNS. Unlike central gliogenesis, neural crest development involves a protracted embryonic phase devoted to the generation of, first, the Schwann cell precursor and then the immature Schwann cell, a cell whose fate as a myelinating or non-myelinating cell has yet to be determined. Embryonic nerves therefore offer a particular opportunity to analyse the early steps of gliogenesis from transient multipotent stem cells, and to understand how this process is integrated with organogenesis of peripheral nerves.

Neuregulin-1 type III determines the ensheathment fate of axons

The signals that determine whether axons are ensheathed or myelinated by Schwann cells have long been elusive. We now report that threshold levels of neuregulin-1 (NRG1) type III on axons determine their ensheathment fate. Ensheathed axons express low levels whereas myelinated fibers express high levels of NRG1 type III. Sensory neurons from NRG1 type III deficient mice are poorly ensheathed and fail to myelinate; lentiviral-mediated expression of NRG1 type III rescues these defects. Expression also converts the normally unmyelinated axons of sympathetic neurons to myelination. Nerve fibers of mice haploinsufficient for NRG1 type III are disproportionately unmyelinated, aberrantly ensheathed, and hypomyelinated, with reduced conduction velocities. Type III is the sole NRG1 isoform retained at the axon surface and activates PI 3-kinase, which is required for Schwann cell myelination. These results indicate that levels of NRG1 type III, independent of axon diameter, provide a key instructive signal that determines the ensheathment fate of axons.

Activation of ErbB2 by Overexpression or by Transmembrane Neuregulin Results in Differential Signaling and Sensitivity to Herceptin

The ligands of the epidermal growth factor family and their receptors, the ErbB proteins, have been linked to the development of different types of cancer. Particular attention has focused on ErbB2, whose activation may occur by receptor overexpression or by ligand-induced oligomerization with other ErbB receptors. Whether these two modes of ErbB2 activation cause the same biological responses is unknown. Here, we uncovered important differences in the signaling, proliferation rates, and the response to anti-ErbB2 antibodies when comparing MCF7 cells expressing the ligand neuregulin, to MCF7 cells overexpressing ErbB2. Expression of neuregulin caused higher proliferation than ErbB2 overexpression. Transmembrane neuregulin expression was accompanied by constitutive activation of ErbB2, ErbB3, and ErbB4 receptors. ErbB2 overexpression caused tyrosine phosphorylation of ErbB2, whereas ErbB3 and ErbB4 were only slightly tyrosine phosphorylated. Autocrine transmembrane neuregulin also caused constitutive activation of several signaling pathways, such as the Erk1/2, Erk5, and Akt routes, which have been linked to breast cancer cell proliferation. Interestingly, expression of neuregulin increased p21 levels and this was required for the proliferation of MCF7 cells. Treatment with the anti-ErbB2 receptor antibody Herceptin had an inhibitory effect on proliferation only in cells expressing neuregulin but not on cells overexpressing ErbB2, and its inhibitory activity was accompanied by a decrease in p21. These results suggest that Herceptin may also be of help in the treatment of tumors in which neuregulin feeds the tumoral tissue.

High-level activation of cyclic AMP signaling attenuates bone morphogenetic protein 2-induced sympathoadrenal lineage development and promotes melanogenesis in neural crest cultures

The intensity of cyclic AMP (cAMP) signaling is a differential instructive signal in neural crest (NC) cell specification. By an unknown mechanism, sympathoadrenal lineage specification is suppressed by high-level activation of cAMP signaling. In NC cultures, high-level activation of cAMP signaling mediates protein kinase A (PKA)-dependent Rap1-B-Raf-ERK1/2 activation, leading to cytoplasmic accumulation of phospho-Smad1, thus terminating bone morphogenetic protein 2 (BMP2)-induced sympathoadrenal cell development. Concurrently, cAMP signaling induces transcription of the melanocyte-determining transcription factor Mitf and melanogenesis. dnACREB and E1A inhibit Mitf expression and melanogenesis, supporting the notion that CREB activation is necessary for melanogenesis. However, constitutively active CREB(DIEDML) without PKA activation is insufficient for Mitf expression and melanogenesis, indicating PKA regulates additional aspects of Mitf transcription. Thus, high-level activation of cAMP signaling plays a dual role in NC cell differentiation: attenuation of BMP2-induced sympathoadrenal cell development and induction of melanogenesis. We conclude the intensity of activation of signal transduction cascades determines cell lineage segregation mechanisms.

Peripheral glia: Schwann cells in motion

Neuregulin signaling through ErbB receptors is known to play an essential role in Schwann cell proliferation, survival and myelination. Recent studies in zebrafish provide a peek at living Schwann cells migrating along axons in vivo and suggest that ErbB signaling, while not required for cell movement per se, is required to maintain the directed migration of these cells.

Axon and Schwann Cell Partnership During Nerve Regrowth

Regeneration of peripheral nerve involves an essential contribution by Schwann cells (SCs) in collaboration with regrowing axons. We examined such collaboration between new axons and Schwann cells destined to reform peripheral nerve trucks in a regeneration chamber bridging transected rat sciatic nerves. There was a highly intimate "dance" between axons that followed outgrowing and proliferating SCs. Axons without SCs only grew short distances and almost all axon processes had associated SC processes. When regeneration chambers were infused through an external access port with local mitomycin, a mitosis inhibitor, SC proliferation, migration and subsequent axon regrowth were dramatically reduced. Adding laminin to mitomycin did not reverse this regenerative lag and indicated that SCs provide more than laminin synthesis alone. Laminin infused alone supplemented endogenous laminin and facilitated first SC then axon regrowth. "Wrong way" misdirected axons were associated with misdirected SC processes and were more numerous in bridges exposed to mitomycin, but were fewer in laminin supplemented bridges. Later, by 21 days, there was myelinated axon repopulation of regenerative bridges but those exposed to mitomycin alone at early time points had substantial impairments in axon investment. Reforming peripheral nerve trucks involves a very close and intimate relationship between axons and SCs that must proliferate and migrate, facilitated by laminin.

Neuregulin-1 increases the proliferation of neuronal progenitors from embryonic neural stem cells

Neuregulins are a family of proteins expressed in the developing brain and in brain regions that continue to undergo neurogenesis in adult animals. We investigated the effects of neuregulins on embryonic neural stem cells (NSCs) isolated from E11 mouse telencephalon. Treatment of basic fibroblast growth factor (bFGF)-expanded neurosphere cultures with the EGF-like domain of neuregulin1-beta1 (NRG-1(177-244)) resulted in a 4-fold increase of bromodeoxyuridine (BrDU)-labeled cells, suggesting that NRG-1 stimulated proliferation. The majority of the BrdU-positive cells co-labeled with an antibody against MAP2, indicating that the proliferating cells were neuronal. No BrDU labeling was seen in GFAP- or O4-positive cells. In NRG-1-treated cultures, many of the MAP2-positive cells co-labeled with an anti-nestin antibody, suggesting that these cells are neuron-restricted progenitors (NRPs). Few MAP2/nestin-positive cells were seen in control cultures. The increase in the number of neuronal cells in NRG-1-treated cultures was due to increased proliferation of MAP2-positive cells rather than the regulation of cell survival or fate determination. These results suggest that neuregulins are mitogenic to NRPs, thus endogenous neuregulins may play important roles during CNS neurogenesis.

The acetylcholine receptor γ-to-ε switch occurs in individual endplates

Maturation of the neuromuscular junction is accompanied by molecular switching of acetylcholine receptor (AChR) channels from embryonic types with gamma-subunits to adult ones with epsilon-subunits after birth. As a step toward understanding the molecular mechanisms of the gamma-to-epsilon switch, we addressed the question of whether embryonic- and adult-type AChRs constitute different endplates during the transitional period. From analyses with double- or triple-staining with anti-gamma- and/or anti-epsilon-antibodies together with alpha-bungarotoxin, which binds to alpha-subunits, we demonstrated that during neonatal stages in mice, adult-type AChRs are incorporated into individual endplates expressing embryonic-AChRs and replace these embryonic-AChRs gradually. The main period of AChR transition in the mouse diaphragm was between postnatal days 5 (P5) and P7, similar to the period described previously in which endplates shift from multi-axon to single-axon innervation. This finding will help our understanding of the mechanisms of the gamma-to-epsilon switch during establishment of the neuromuscular junction.

Integrative Proteomics: Functional and Molecular Characterization of a Particular Glutamate-Related Neuregulin Isoform

Glutamate is the major excitatory neurotransmitter in the mammalian brain and is related to memory by calcium-conducting receptors. Neuregulins have emerged as long-term modulating molecules of synaptic signaling by glutamate receptors, playing a role in some cognition/memory-related disorders and moreover being part of transient functional microdomains, called lipid rafts. Here we characterize one specific isoform of neuregulin as a central biomarker for glutamate-related signaling, integrating results from in vitro and in vivo models by a differential functional and proteomic approach.

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.

Regulation of NMDA receptors by neuregulin signaling in prefrontal cortex

Recent linkage studies have identified a significant association of the neuregulin gene with schizophrenia, but how neuregulin is involved in schizophrenia is primarily unknown. Aberrant NMDA receptor functions have been implicated in the pathophysiology of schizophrenia. Therefore, we hypothesize that neuregulin, which is present in glutamatergic synaptic vesicles, may affect NMDA receptor functions via actions on its ErbB receptors enriched in postsynaptic densities, hence participating in emotional regulation and cognitive processes that are impaired in schizophrenia. To test this, we examined the regulation of NMDA receptor currents by neuregulin signaling pathways in prefrontal cortex (PFC), a prominent area affected in schizophrenia. We found that bath perfusion of neuregulin significantly reduced whole-cell NMDA receptor currents in acutely isolated and cultured PFC pyramidal neurons and decreased NMDA receptor-mediated EPSCs in PFC slices. The effect of neuregulin was mainly blocked by application of the ErbB receptor tyrosine kinase inhibitor, phospholipase C (PLC) inhibitor, IP3 receptor (IP3R) antagonist, or Ca2+ chelators. The neuregulin regulation of NMDA receptor currents was also markedly attenuated in cultured neurons transfected with mutant forms of Ras or a dominant-negative form of MEK1 (mitogen-activated protein kinase kinase 1). Moreover, the neuregulin effect was prevented by agents that stabilize or disrupt actin polymerization but not by agents that interfere with microtubule assembly. Furthermore, neuregulin treatment increased the abundance of internalized NMDA receptors in cultured PFC neurons, which was also sensitive to agents affecting actin cytoskeleton. Together, our study suggests that both PLC/IP3R/Ca2+ and Ras/MEK/ERK (extracellular signal-regulated kinase) signaling pathways are involved in the neuregulin-induced reduction of NMDA receptor currents, which is likely through enhancing NR1 internalization via an actin-dependent mechanism.

Neuregulin-1 immunoglobulin-like domain mutant mice: clozapine sensitivity and impaired latent inhibition

Genetic and behavioral studies in humans and mouse mutants have implicated the gene encoding neuregulin-1 (Nrg-1) as a candidate susceptibility gene for schizophrenia. We examined the behavior of mice heterozygous for a mutation in neuregulin-1's immunoglobulin (Ig)-like domain (Ig-nrg-1 mice). We found that these animals displayed behaviors related to a schizophrenia-like phenotype, such as clozapine suppression of open-field and running wheel activity and impaired latent inhibition. Contrary to findings with other nrg-1 mutants, Ig-nrg-1 mice did not exhibit significantly elevated locomotion relative to littermate controls. These results suggest that Ig-Nrg-1's contribute to some - but not all - aspects of the schizophrenia-like phenotype of nrg-1 mutants, and further support nrg-1 as a candidate gene for schizophrenia.

Dilated cardiomyopathy in Erb-b4-deficient ventricular muscle

The neuregulin receptor tyrosine kinase Erb-b4, initially linked to early cardiac development, is shown here to play a critical role in adult cardiac function. In wild-type mice, Erb-b4 protein localized to Z lines and to intercalated disks, suggesting a role in subcellular and intercellular communications of cardiomyocytes. Conditional inactivation of erb-b4 in ventricular muscle cells led to a severe dilated cardiomyopathy, characterized by thinned ventricular walls with eccentric hypertrophy, reduced contractility, and delayed conduction. This cardiac dysfunction may account for premature death in adult erb-b4-knockout mice. This study establishes a critical role for Erb-b4 in the maintenance of normal postnatal cardiac structure and function.

Denervation-induced alterations in gene expression in mouse skeletal muscle

Motoneurons are important for regulating the function and properties of skeletal muscle. In the present study high-density oligonucleotide arrays have been used to compare gene expression in innervated and six-days denervated NMRI mouse skeletal muscle. To avoid looking at genes mainly participating in the process of atrophy, both hind-limb muscles (atrophic after denervation) and hemidiaphragm muscle (transiently hypertrophic after denervation) were used. Only genes previously not known to respond to denervation and with potential roles in DNA/RNA interactions/transcription and/or cellular communication/signalling are presented. Data for additional genes are provided as supplementary material. Thirty-two genes, up-regulated by a factor of two or down-regulated to the same extent after denervation, are presented. These include genes that may act through chromatin remodelling and/or as transcription factors/regulators (Cdkn1a, Cdr2, Hrmt1l2, Idb2, Myc/c-myc, L-myc1, Rb1, Sap30 and Tgif), genes possibly involved in the regulation of muscle membrane properties and/or excitation-contraction coupling (Cacng1, Camk2d, Hrmt1l2, Kcnj12, Kcna7 and Rrad) and genes potentially involved in neuromuscular interactions and/or receptor signalling (Acvr2b, Adam19, D0H4S114, Kai1, Maged1, Mt2, Prkcabp, Ptp4a3, Ramp1, Rras, Timp1, Vegfa and Zfp145). A set of five genes with altered expression after denervation (Fzd9, Nr4a1, Frat2, Ctgf and Cyr61) indicate that Wnt signalling may be reduced in denervated skeletal muscle.

Regulation of the Drosophila epidermal growth factor-ligand vein is mediated by multiple domains

Vein (Vn), a ligand for the Drosophila epidermal growth factor receptor (Egfr), has a complex structure including a PEST, Ig, and EGF domain. We analyzed the structure-function relationships of Vn by assaying deletion mutants. The results show that each conserved domain influences Vn activity. A PEST deletion increases Vn potency and genetic evidence suggests that Vn is regulated by proteasomal degradation. The Ig deletion causes toxic effects not seen following expression of native Vn, but the Ig domain is not required for Vn localization or for the activation of Egfr signaling in wing vein patterning. Remarkably, when the EGF domain is deleted, Vn functions as a dominant negative ligand, implying that Vn normally physically interacts with another factor to promote its activity. We identified additional highly conserved sequences and found several regions that affect Vn potency and one that may mediate the effect of dominant negative Vn molecules. Together the results show that the activity of Vn is controlled both positively and negatively, demonstrating the existence of additional levels at which Egfr signaling can be regulated.

Tissue interaction mediated by neuregulin-1 and ErbB receptors regulates epithelial morphogenesis of mouse embryonic submandibular gland

Dimerization and activation of ErbB receptors by their ligands play crucial roles in organogenesis. Epithelial morphogenesis of embryonic mouse submandibular gland (SMG) has been shown to depend on intraepithelial signaling mediated by the epidermal growth factor (EGF) family of molecules and the EGF receptor (ErbB1). Here, we report on the neuregulin (NRG) -1 protein and its receptors ErbB2 and ErbB3 in the developing SMG. The expression of these molecules was demonstrated by reverse transcriptase-polymerase chain reaction and Western blot analysis. Immunofluorescence microscopy showed that the two ErbB receptors as well as ErbB1 were expressed mainly in the epithelium, whereas NRG-1 was exclusively found in the mesenchyme. Epithelial morphogenesis was retarded by anti-NRG-1 neutralizing antibody and promoted by recombinant NRG-1 protein. We suggest that, in the developing SMG, both mesenchyme-derived NRG molecules and epithelium-derived EGF molecules regulate ErbB signaling in the epithelium to participate in tissue morphogenesis.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.

A real-time quantitative PCR comparative study between rat optic and sciatic nerves: determination of neuregulin-1 mRNA levels

Injured axons from peripheral nervous system (PNS) possess the ability to regenerate. In contrast, regeneration of injured axons does not occur in the central nervous system (CNS) or occurs to a limited extent. Previous works have shown that rat sciatic nerve conditioned medium (CM) produced PC12 cells neuronal-like differentiation and neurite outgrowth. In the present work, we compared the expression of neuregulin-1s (NRG-1s) from rat sciatic and optic nerves as members of the PNS and CNS, respectively. Sciatic nerve CM showed a higher neurotrophic activity on PC12 cells than rat optic nerve CM. RT-PCR analysis verified the presence of all three types of NRG-1 mRNAs and their receptors in both types of nerves. Real-time quantitative PCR (QPCR) assays showed that the relative expression levels of all three types of NRG-1 mRNAs were higher in optic nerves than in sciatic nerves. Eleven-day cultured optic nerves showed an increased in NDF and SMDF when compared to freshly isolated optic nerves, whereas GGF decreased. However, 11-day-cultured sciatic nerves only showed an increase in SMDF mRNA. Western blots corroborated the differences in NRG-1 expression profile for both types of nerves and their CMs. Incubation of both CMs with the anti-pan-NRG-1 antibody showed that the neurotrophic activity of the optic nerve CM increased, whereas the sciatic nerve CM remained unchanged. These results indicated that different NRG-1 levels are expressed upon nerve degeneration and the balance between those levels and other neurotrophic factors could have an important role on nerve regeneration.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Neuregulin potentiates agrin-induced acetylcholine receptor clustering in myotubes

Agrin and neuregulin (HRG-beta1) play complementary roles in synapse formation. While HRG-beta1 induces transcriptional up-regulation of postsynaptic proteins, here we present evidence that it can potentiate agrin-induced acetylcholine receptor (AChR) clustering in C2 myotubes. Agrin induced maximal AChR clustering in 4 h. HRG-beta1 treatment for 4 h produced no increase over basal AChR cluster numbers. When myotubes were treated for 4 h with 100 pM agrin, HRG-beta1 augmented AChR cluster numbers by 2-fold compared to myotubes treated with 100 pM agrin alone. Thus, HRG-beta1 can potentiate agrin-induced AChR clustering.

Neuregulin-1beta induces neurite extension and arborization in cultured hippocampal neurons

Neuregulin-1 (NRG-1) growth and differentiation factors and their erbB receptors are hypothesized to promote embryonic hippocampal neuron differentiation via as yet unknown mechanisms. We have found that NRG-1beta increases the outgrowth of primary neurites, neuronal area, total neurite length, and neuritic branching in E18 hippocampal neurons. NRG-1beta effects on neurite extension and arborization are similar to, but not additive with, those of brain-derived neurotrophic factor and reflect direct NRG-1 action on hippocampal neurons as these cells express the NRG-1 receptors erbB2 and erbB4, the erbB-specific inhibitor PD158780 decreases NRG-1beta induced neurite outgrowth, and NRG-1beta stimulation induces p42/44 ERK phosphorylation. Pharmacological inhibition of p42/44 ERK and protein kinase C (PKC), but not PI3K or p38 MAP kinase, inhibits NRG-1beta-induced neurite extension and elaboration. We conclude that NRG-1beta stimulates hippocampal neurite extension and arborization via a signaling pathway that involves erbB membrane tyrosine kinases (erbB2 and/or erbB4), p42/44 ERK, and PKC.

The neuregulin receptor, ErbB4, is not required for normal development and adult maintenance of the substantia nigra pars compacta

Degeneration of dopaminergic neurons in the substantia nigra is associated with one of the most prominent human neurological disorders, Parkinson's disease. It is therefore of high interest to identify molecules with trophic effects on this neuronal population. We show here that the neuregulin receptor ErbB4 is differentially expressed in mesencephalic dopaminergic neurons, found in the substantia nigra and in a subregion of the ventral tegmentum but not in the retrorubral field. Early developmental onset and continued expression of ErbB4 into the adult and the presence of two high affinity ligands, neuregulin-1 and betacellulin, in the basal ganglia, suggested that these molecules might participate in the differentiation and/or maintenance of the nigrostriatal system. In order to address this hypothesis, we used a loxP flanked ErbB4 allele in combination with a nestin-Cre transgene and generated brain-specific ErbB4 null mice. These mutant animals survived into adulthood. The distribution of dopaminergic cell bodies in the midbrain, the expression of numerous genes specific to mesencephalic dopaminergic neurons, and the axonal projection to the basal ganglia all appeared normal. Finally, an assessment of their motor function revealed no behavioral deficits. The apparent lack of any mutant phenotype suggests the presence of a strong compensatory mechanism.

Schizophrenia genes, gene expression, and neuropathology: on the matter of their convergence

This review critically summarizes the neuropathology and genetics of schizophrenia, the relationship between them, and speculates on their functional convergence. The morphological correlates of schizophrenia are subtle, and range from a slight reduction in brain size to localized alterations in the morphology and molecular composition of specific neuronal, synaptic, and glial populations in the hippocampus, dorsolateral prefrontal cortex, and dorsal thalamus. These findings have fostered the view of schizophrenia as a disorder of connectivity and of the synapse. Although attractive, such concepts are vague, and differentiating primary events from epiphenomena has been difficult. A way forward is provided by the recent identification of several putative susceptibility genes (including neuregulin, dysbindin, COMT, DISC1, RGS4, GRM3, and G72). We discuss the evidence for these and other genes, along with what is known of their expression profiles and biological roles in brain and how these may be altered in schizophrenia. The evidence for several of the genes is now strong. However, for none, with the likely exception of COMT, has a causative allele or the mechanism by which it predisposes to schizophrenia been identified. Nevertheless, we speculate that the genes may all converge functionally upon schizophrenia risk via an influence upon synaptic plasticity and the development and stabilization of cortical microcircuitry. NMDA receptor-mediated glutamate transmission may be especially implicated, though there are also direct and indirect links to dopamine and GABA signalling. Hence, there is a correspondence between the putative roles of the genes at the molecular and synaptic levels and the existing understanding of the disorder at the neural systems level. Characterization of a core molecular pathway and a 'genetic cytoarchitecture' would be a profound advance in understanding schizophrenia, and may have equally significant therapeutic implications.

Neurodevelopment, neuroplasticity, and new genes for schizophrenia

Schizophrenia is a complex, debilitating neuropsychiatric disorder. Epidemiological, clinical, neuropsychological, and neurophysiological studies have provided substantial evidence that abnormalities in brain development and ongoing neuroplasticity play important roles in the pathogenesis of the disorder. Complementing these clinical studies, a range of cytoarchitectural, morphometric, ultrastructural, immunochemical, and gene expression methods have been applied in investigations of postmortem brain tissues to characterize the cellular and molecular profile of putative developmental and plastic abnormalities in schizophrenia. While findings have been diverse and many are in need of replication, investigations focusing on higher cortical and limbic brain regions are increasingly demonstrating abnormalities in the structural and molecular integrity of the synaptic complex as well as glutamate-related receptors and signal transduction pathways that play critical roles in brain development, synaptogenesis, and synaptic plasticity. Most exciting have been recent associations of schizophrenia with specific genes, such as neuregulin-1, dysbindin-1, and AKT-1, which are vital to synaptic development, neurotransmission, and plasticity.

Epigen, the last ligand of ErbB receptors, reveals intricate relationships between affinity and mitogenicity

Four ErbB receptors and multiple growth factors sharing an epidermal growth factor (EGF) motif underlie transmembrane signaling by the ErbB family in development and cancer. Unlike other ErbB proteins, ErbB-2 binds no known EGF-like ligand. To address the existence of a direct ligand for ErbB-2, we applied algorithms based on genomic and cDNA structures to search sequence data bases. These searches reidentified all known EGF-like growth factors including Epigen (EPG), the least characterized ligand, but failed to identify novel factors. The precursor of EPG is a widely expressed transmembrane glycoprotein that undergoes cleavage at two sites to release a soluble EGF-like domain. A recombinant EPG cannot stimulate cells singly expressing ErbB-2, but it acts as a mitogen for cells expressing ErbB-1 and co-expressing ErbB-2 in combination with the other ErbBs. Interestingly, soluble EPG is more mitogenic than EGF, although its binding affinity is 100-fold lower. Our results attribute the anomalous mitogenic power of EPG to evasion of receptor-mediated depletion of ligand molecules, as well as to inefficient receptor ubiquitylation and down-regulation. In conclusion, EPG might represent the last EGF-like growth factor and define a category of low affinity ligands, whose bioactivity differs from the more extensively studied high affinity ligands.

Upregulation of erbB receptors in rat brain after middle cerebral arterial occlusion

We have previously demonstrated that neuregulin-1 (NRG-1) is upregulated and is neuroprotective in ischemic brain injury, however the expression and localization of its receptors during ischemia has not been investigated. Therefore, we used a rat middle cerebral artery occlusion (MCAO) model to examine the distribution of erbB receptors following ischemic stroke. Like neuregulin-1, we observed a dramatic induction of erbB4 in the peri-infarct regions of the ipsilateral cortex 24 h following MCAO. Using Fluoro-Jade B (FJB) staining as a marker of neurodegeneration, erbB4 was upregulated in FJB-positive cells, suggesting that erbB receptors are induced in injured neurons. The increase in erbB receptors was seen in neurons and a subpopulation of macrophages/microglia. There was no erbB co-localization with GFAP-positive astrocytes. These results demonstrate that erbB receptors are upregulated in neurons and macrophages/microglia following ischemic stroke and may be involved in neuroprotection and repair.

Short- and long-range attraction of cortical GABAergic interneurons by neuregulin-1

Most cortical interneurons arise from the subcortical telencephalon, but the molecules that control their migration remain largely unidentified. Here, we show that different isoforms of Neuregulin-1 are expressed in the developing cortex and in the route that migrating interneurons follow toward the cortex, whereas a population of the migrating interneurons express ErbB4, a receptor for Neuregulin-1. The different isoforms of Neuregulin-1 act as short- and long-range attractants for migrating interneurons, and perturbing ErbB4 function in vitro decreases the number of interneurons that tangentially migrate to the cortex. In vivo, loss of Neuregulin-1/ErbB4 signaling causes an alteration in the tangential migration of cortical interneurons and a reduction in the number of GABAergic interneurons in the postnatal cortex. These observations provide evidence that Neuregulin-1 and its ErbB4 receptor directly control neuronal migration in the nervous system.

Receptor tyrosine kinase ErbB4 modulates neuroblast migration and placement in the adult forebrain

Neural progenitor proliferation, differentiation and migration are continually active in the rostral migratory stream of the adult brain. Here, we show that the receptor tyrosine kinase ErbB4 is expressed prominently by the neuroblasts present in the subventricular zone and the rostral migratory stream. The neuregulins (NRG1-NRG3), which have been identified as ErbB4 ligands, are detected either in the stream or in adjacent regions. Mice deficient in ErbB4 expressed under the control of either the nestin or the hGFAP promoter have altered neuroblast chain organization and migration and deficits in the placement and differentiation of olfactory interneurons. These findings suggest that ErbB4 activation helps to regulate the organization of neural chains that form the rostral migratory stream and influences the differentiation of olfactory interneuronal precursors.

Neuregulin genotype and medication response in Finnish patients with schizophrenia

Neuregulin 1 is involved both in neurodevelopment and neurotransmitter mechanisms in the brain. There is evidence of an association between neuregulin 1 genotype and schizophrenia. We compared neuregulin 1 genotypes in patients with schizophrenia (n=94) and control subjects (n=395) of Finnish origin by using one SNP (SNP8NRG221533) as a genetic marker. We also analyzed NRG1 genotype with regard to age at onset and between responders and non-responders to conventional antipsychotics. The NRG1 genotype or allele frequencies showed similar distributions between patient and control groups. Age at onset was not associated with NRG1 genotype. The TT genotype was overrepresented in the non-responders group compared with the responders (p=0.013). Further studies are needed to ascertain the significance of neuregulin genotype in medication response to schizophrenia.

Supranigral injection of neuregulin1-β induces striatal dopamine overflow

Previous studies have provided anatomical evidence that the functional neuregulin receptor, ErbB4, is present within the ventral midbrain where it is co-localized to dopamine neurons of the substantia nigra and ventral tegmental area. In this study, we provide evidence that neuregulin1-beta (a.k.a. heregulin1-beta), a neuregulin-1 gene isoform that preferentially binds to and activates the ErbB4 receptor, evokes an almost immediate overflow of striatal dopamine when injected into a region just dorsal to the ipsilateral substantia nigra. These data are indicative that neuregulins can modulate the activity of mesostriatal dopaminergic neurons.

Neuregulin-1 is neuroprotective and attenuates inflammatory responses induced by ischemic stroke

Recent work from our laboratory demonstrated that the expression neuregulin-1 in neurons was induced in the ischemic penumbra by focal stroke in the rat. Here, we show that a single intravascular injection of neuregulin-1beta (approximately 2.5 ng/kg) reduced cortical infarct volume by >98% when given immediately before middle cereral artery occlusion. Subcortical infarct volume was reduced by approximately 40%. Analysis of DNA fragmentation in brain tissues indicated that neuregulin-1 blocked apoptosis in cortical neurons in the penumbra. Neuregulin-1 prevented macrophage/microglial infiltration and astrocytic activation following focal ischemia. The neuroprotective effect of neuregulin-1 was also associated with a suppression of interleukin-1beta mRNA levels. These data suggest that neuregulin-1 protects neurons from delayed, ischemia-induced apoptotic cell death in the cortex by inhibiting pro-inflammatory responses. Neuregulins represent a novel, potent neuroprotective strategy that has potential therapeutic value in treating individuals after acute ischemic stroke.

Activity-dependent transcription regulation of PSD-95 by neuregulin-1 and Eos

Neuregulin-1 (Nrg-1) contains an intracellular domain (Nrg-ICD) that translocates into the nucleus, where it may regulate gene expression upon neuronal depolarization. However, the identity of its target promoters and the mechanisms by which it regulates transcription have been elusive. Here we report that, in the mouse cochlea, synaptic activity increases the level of nuclear Nrg-ICD and upregulates postsynaptic density protein-95 (PSD-95), a scaffolding protein that is enriched in post-synaptic structures. Nrg-ICD enhances the transcriptional activity of the PSD-95 promoter by binding to a zinc-finger transcription factor, Eos. The Nrg-ICD-Eos complex induces endogenous PSD-95 expression in vivo through a signaling pathway that is mostly independent of gamma-secretase regulation. This upregulation of PSD-95 expression by the Nrg-ICD-Eos complex provides a molecular basis for activity-dependent synaptic plasticity.

Lipid rafts in neuregulin signaling at synapses

Neuregulins are a family of EGF domain-containing factors that play an important role in development. In the nervous system, they promote glial differentiation, induce neurotransmitter receptor expression, and regulate synaptic plasticity. Recent studies indicate that ErbB protein tyrosine kinases, neuregulin receptors, translocate to lipid raft microdomains in the plasma membrane in response to neuregulin. Localization of ErbB proteins in lipid rafts appeared to be necessary for neuregulin signaling and regulation of synaptic plasticity. We will review recent studies of lipid rafts and neuregulin function and discuss possible roles of lipid rafts in compartmentalized neuregulin signaling and translocation of ErbB proteins to synapses.

Generation and characterization of neuregulin-2-deficient mice

The neuregulins (NRGs) are a family of four structurally related growth factors that are expressed in the developing and adult brain. NRG-1 is essential for normal heart formation and has been implicated in the development and maintenance of both neurons and glia. NRG-2 was identified on the basis of its homology to NRG-1 and, like NRG-1, is expressed predominantly by neurons in the central nervous system. We have generated mice with the active domain of NRG-2 deleted in an effort to characterize the biological function of NRG-2 in vivo. In contrast to the NRG-1 knockout animals, NRG-2 knockouts have no apparent heart defects and survive embryogenesis. Mutant mice display early growth retardation and reduced reproductive capacity. No obvious histological differences were observed in the major sites of NRG-2 expression. Our results indicate that in vivo NRG-2 activity differs substantially from that of NRG-1 and that it is not essential for normal development in utero.

Protein processing and releases of neuregulin-1 are regulated in an activity-dependent manner

Identification of the key molecules that bridge presynaptic neuronal events and long-term modification of the postsynaptic process is an important challenge which will have to be met in order to further our understanding of the mechanisms for learning and memory. This study is focused on neuregulin-1 (NRG-1), a neurotrophic factor, that is known to regulate the development of various tissues and/or the life/death of cells through activation of the ErbB family receptor tyrosine kinases. It was discovered that the soluble form of NRG-1 (sNRG-1) is produced from the transmembrane form of NRG through proteolytic cleavage during electrical stimulation of either cultured cerebellar granule cells (GCs) or pontine nucleus neurons (PNs) that are presynaptic to GCs. sNRG-1 was assayed by measuring the phosphorylation of both the ErbB receptors and cyclic AMP-responsive element-binding protein (CREB), and by means of antibodies to sNRG-1. The cleavage and release of NRG-1 depended on the frequency of electrical stimulation; the peak effect was at 50 Hz in both GCs and PNs. Activation of protein kinase C (PKC) mimicked this effect. The culture apparatus provided along with the mass-electrical stimulation that was employed proved to be a powerful tool for combining neuronal electrical events and chemical events. We conclude from the results that, in mossy fibre (PN axon)-GC synapses, electrical activity controls the proteolytic processing of NRG-1 in a frequency-dependent fashion and involves PKC. Furthermore, cleaved sNRG-1 plays an important functional role in regulating transmission across these synapses.

Evidence of novel neuronal functions of dysbindin, a susceptibility gene for schizophrenia

Genetic variation in dysbindin (DTNBP1: dystrobrevin-binding protein 1) has recently been shown to be associated with schizophrenia. The dysbindin gene is located at chromosome 6p22.3, one of the most promising susceptibility loci in schizophrenia linkage studies. We attempted to replicate this association in a Japanese sample of 670 patients with schizophrenia and 588 controls. We found a nominally significant association with schizophrenia for four single nucleotide polymorphisms and stronger evidence for association in a multi-marker haplotype analysis (P = 0.00028). We then explored functions of dysbindin protein in primary cortical neuronal culture. Overexpression of dysbindin induced the expression of two pre-synaptic proteins, SNAP25 and synapsin I, and increased extracellular basal glutamate levels and release of glutamate evoked by high potassium. Conversely, knockdown of endogenous dysbindin protein by small interfering RNA (siRNA) resulted in the reduction of pre-synaptic protein expression and glutamate release, suggesting that dysbindin might influence exocytotic glutamate release via upregulation of the molecules in pre-synaptic machinery. The overexpression of dysbindin increased phosphorylation of Akt protein and protected cortical neurons against neuronal death due to serum deprivation and these effects were blocked by LY294002, a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor. SiRNA-mediated silencing of dysbindin protein diminished Akt phosphorylation and facilitated neuronal death induced by serum deprivation, suggesting that dysbindin promotes neuronal viability through PI3-kinase-Akt signaling. Genetic variants associated with impairments of these functions of dysbindin could play an important role in the pathogenesis of schizophrenia.

Neuregulin induces the expression of transcription factors and myosin heavy chains typical of muscle spindles in cultured human muscle

Neuregulin (NRG) (also known as ARIA, GGF, and other names) is a heparin sulfate proteoglycan secreted into the neuromuscular junction by innervating motor and sensory neurons. An integral part of synapse formation, we have analyzed NRG-induced changes in gene expression over 48 h in primary human myotubes. We show that in addition to increasing the expression of acetylcholine receptors on the myotube surface, NRG treatment results in a transient increase of several members of the early growth response (Egr) family of transcription factors. Three Egrs, Egr1, -2, and -3, are induced within the first hour of NRG treatment, with Egr1 and -3 RNA levels showing the most significant increases of approximately 9- and 16-fold, respectively. Also noted was a corresponding increase in protein levels for both of these transcription factors. Previous literature indicates that Egr3 expression is required for the formation of muscle spindle fibers, sensory organs that are distinct from skeletal muscle contractile fibers. At the molecular level, muscle spindle fibers express a unique subset of myosin heavy chains. Two isoforms of the myosin heavy chain, the slow development and neonatal, were found to be increased in our myotube cultures after 48 h of treatment with NRG. Taken together, these results indicate that not only can NRG induce the expression of a transcription factor key to spindle fiber development (Egr3), but that a portion of this developmental process can be replicated in vitro.

Involvement of cell surface HSP90 in cell migration reveals a novel role in the developing nervous system

Heat shock protein HSP90 plays important roles in cellular regulation, primarily as a chaperone for a number of key intracellular proteins. We report here that the two HSP90 isoforms, alpha and beta, also localize on the surface of cells in the nervous system and are involved in their migration. A 94-kDa surface antigen, the 4C5 antigen, which was previously shown to be involved in migration processes during development of the nervous system, is shown to be identical to HSP90alpha using mass spectrometry analysis. This identity is further confirmed by immunoprecipitation experiments and by induction of 4C5 antigen expression in heat shock-treated embryonic rat brain cultures. Moreover, immunocytochemistry on live cerebellar rat cells reveals cell surface localization of both HSP90alpha and -beta. Cell migration from cerebellar and sciatic nerve explants is inhibited by anti-HSP90alpha and anti-HSP90beta antibodies, similarly to the inhibition observed with monoclonal antibody 4C5. Moreover, immunostaining with rhodamine-phalloidin of migrating Schwann cells cultured in the presence of antibodies against both alpha and beta isoforms of HSP90 reveals that HSP90 activity is associated with actin cytoskeletal organization, necessary for lamellipodia formation.

No evidence for linkage or association of neuregulin-1 (NRG1) with disease in the Irish study of high-density schizophrenia families (ISHDSF)

The neuregulin-1 gene (NRG1) at chromosome 8p21-22 has been implicated as a schizophrenia susceptibility gene in Icelandic, Scottish, Irish and mixed UK populations. The shared ancestry between these populations led us to investigate the NRG1 polymorphisms and appropriate marker haplotypes for linkage and/or association to schizophrenia in the Irish study of high-density schizophrenia families (ISHDSF). Neither single-point nor multi-point linkage analysis of NRG1 markers gave evidence for linkage independent of our pre-existing findings telomeric on 8p. Analysis of linkage disequilibrium (LD) across the 252 kb interval encompassing the 7 marker core Icelandic/Scottish NRG1 haplotype revealed two separate regions of modest LD, comprising markers SNP8NRG255133, SNP8NRG249130 and SNP8NRG243177 (telomeric) and microsatellites 478B14-428, 420M9-1395, D8S1810 and 420M9-116I12 (centromeric). From single marker analysis by TRANSMIT and FBAT we found no evidence for association with schizophrenia for any marker. Haplotype analysis for the three SNPs in LD region 1 and, separately, the four microsatellites in LD region 2 (analyzed in overlapping 2-marker windows), showed no evidence for overtransmission of specific haplotypes to affected individuals. We therefore conclude that if NRG1 does contain susceptibility alleles for schizophrenia, they impact quite weakly on risk in the ISHDSF.

Synergistic effects of neuregulin and agrin on muscle acetylcholine receptor expression

The proper function of neuromuscular junctions requires an extremely high density of acetylcholine receptors (AChRs) that may be achieved from neuron-derived factors including agrin and neuregulin. Here, we show that neuregulin-1 and agrin co-localize at neuromuscular junctions in vivo and form complexes when co-transfected into COS-7 cells. When these COS-7 cells are cultured with myotubes, synergistic effects are observed for AChR clustering, membrane insertion of new AChRs, and induction of AChR mRNA. Even a muscle form of agrin that lacks intrinsic clustering activities by itself, significantly enhances neuregulin-induced clustering and insertion of AChRs. While the heparin-binding (A) domain of agrin is required for agrin localization in the extracellular matrix adjacent to AChR clusters, the heparan sulfate-containing domain of agrin is needed for the synergistic effects and co-localization with neuregulin-1. These results suggest that matrix interactions between exogenously supplied agrin and neuregulin-1 on the muscle surface provide a localized source of signaling factors needed to produce high densities of AChRs at neuromuscular junctions.

Neuregulin 1-erbB signaling and the molecular/cellular basis of schizophrenia

Schizophrenia is a devastating psychiatric disease that affects 0.5-1% of the world's adult population. The hypothesis that this disease is a developmental disorder of the nervous system with late onset of its characteristic symptoms has been gaining acceptance in past years. However, the anatomical, cellular and molecular bases of schizophrenia remain unclear. Numerous studies point to alterations in different aspects of brain development as possible causes of schizophrenia, including defects in neuronal migration, neurotransmitter receptor expression and myelination. Recently, the gene that encodes neuregulin-1 (NRG1) has been identified as a potential susceptibility gene for schizophrenia, and defects in the expression of erbB3, one of the NRG1 receptors, have been shown to occur in the prefrontal cortex of schizophrenic patients, suggesting that NRG1-erbB signaling is involved in the pathogenesis of schizophrenia. These findings open new approaches to defining the molecular and cellular basis of schizophrenia in more mechanistic terms.

Neuregulin Inhibits Acetylcholine Receptor Aggregation in Myotubes

The high local concentration of acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction results from their aggregation by the agrin/MuSK signaling pathway and their synthetic up-regulation by the neuregulin/ErbB pathway. Here, we show a novel role for the neuregulin/ErbB pathway, the inhibition of AChR aggregation on the muscle surface. Treatment of C2C12 myotubes with the neuregulin epidermal growth factor domain decreased the number of both spontaneous and agrin-induced AChR clusters, in part by increasing the rate of cluster disassembly. Upon cluster disassembly, AChRs were internalized into caveolae (as identified by caveolin-3). Time-lapse microscopy revealed that individual AChR clusters fragmented into puncta, and application of neuregulin accelerated the rate at which AChR clusters decreased in area without affecting the density of AChRs remaining in individual clusters (as measured by the fluorescence intensity/unit area). We propose that this novel action of neuregulin regulates synaptic competition at the developing neuromuscular junction.

Essential roles of Meltrin beta (ADAM19) in heart development

Morphogenesis of the heart requires development of the endocardial cushion tissue that gives rise to the membranous septa and valves. Here we show that Meltrin beta/ADAM19, a novel metalloprotease-disintegrin, participates in the development of the endocardial cushion. Mice lacking Meltrin beta exhibit ventricular septal defect (VSD) and immature valves, and most of the animals die soon after birth. During development of the endocardial cushion, epithelial-mesenchymal transformation (EMT) of endocardial epithelial cells generates most of the cushion mesenchymes that constitute the main components of the septa and valves. Meltrin beta is expressed in both the epithelia and the mesenchymes of the endocardial cushion. In the absence of Meltrin beta, the cushion is small or thin in the septum-forming region and show poor remodeling of cardiac jelly components; both of these characteristics suggest impaired growth and differentiation of the endocardial cushion. When embryonic fibroblasts are cultured sparsely, Meltrin beta-lacking cells exhibit aberrant ectodomain shedding of type I Neuregulin, one of the ErbB ligands expressed in endocardial cells. These results suggest the necessity of proteolytic regulation of ErbB ligands by Meltrin beta for proper heart development.

The NMDA receptor glycine modulatory site: a therapeutic target for improving cognition and reducing negative symptoms in schizophrenia

Numerous clinical studies demonstrate that subanaesthetic doses of dissociative anaesthetics, which are non-competitive antagonists at the NMDA receptor, replicate in normal subjects the cognitive impairments, negative symptoms and brain functional abnormalities of schizophrenia. Post-mortem and genetic studies have identified several abnormalities associated with schizophrenia that would interfere with the activation of the glycine modulatory site on the NMDA receptor. Placebo controlled clinical trials with agents that directly or indirectly activate the glycine modulatory site consistently reduce negative symptoms and frequently improve cognition in patients with chronic schizophrenia, who are receiving concurrent typical antipsychotics. Thus, there is convincing evidence that the glycine modulatory site on the NMDA receptor is a valid therapeutic target for improving cognition and associated negative symptoms in schizophrenia.

Cyclin-dependent kinase 5 phosphorylates signal transducer and activator of transcription 3 and regulates its transcriptional activity

The activity of cyclin-dependent kinase 5 (Cdk5) depends on the association with one of its activators, p35 and p39, which are prominently expressed in the nervous system. Studies on the repertoire of protein substrates for Cdk5 have implicated the involvement of Cdk5 in neuronal migration and synaptic plasticity. Our recent analysis of the sequence of signal transducer and activator of transcription (STAT)3, a key transcription factor, reveals the presence of potential Cdk5 phosphorylation site. We report here that the Cdk5/p35 complex associates with STAT3 and phosphorylates STAT3 on the Ser-727 residue in vitro and in vivo. Intriguingly, whereas the Ser phosphorylation of STAT3 can be detected in embryonic and postnatal brain and muscle of wild-type mice, it is essentially absent from those of Cdk5-deficient embryos. In addition, treatment of cultured myotubes with neuregulin enhances the Ser phosphorylation of STAT3 and transcription of STAT3 target genes, such as c-fos and junB, in a Cdk5-dependent manner. Both the DNA-binding activity of STAT3 and the transcription of specific target genes, such as fibronectin, are reduced in Cdk5-deficient muscle. Taken together, these results reveal a physiological role of Cdk5 in regulating STAT3 phosphorylation and modulating its transcriptional activity.

Lipid rafts identified as locations of ectodomain shedding mediated by Meltrin beta/ADAM19

Meltrin beta (Mel beta, also called ADAM19) is a member of the ADAM (adisintegrin and metalloprotease) family, which are membrane-anchored glycoproteins that play crucial roles in various biological processes. Many intercellular signaling molecules are membrane-anchored proteins, which are proteolytically processed after becoming membrane-bound, to liberate their extracellular domains (ectodomain shedding). Genetic and biochemical studies have shown that some ADAMs participate in these events. We found previously that Mel beta can cleave the extracellular region of the membrane-anchored beta-exon-containing Neuregulin-1 (NRG beta1) protein, which is one of the main ligands for the neural ErbB receptor. Mel beta-deficient mice showed developmental defects in the nervous system. These observations raise the possibility that the NRG ectodomain shedding mediated by Mel beta is closely related to the neural development. Here we show that Mel beta-mediated ectodomain shedding of NRG beta1 takes place in the lipid rafts of neurons. The lipid rafts localization of Mel beta requires its membrane-anchoring region, and NRG beta1 ectodomain shedding is not enhanced if Mel beta cannot reach the lipid rafts. These results indicate that localization of Mel beta in lipid rafts is critical for its ectodomain shedding.