Neuregulin-1 Enhances Depolarization-Induced GABA Release

Association between a Missense Polymorphism (rs3924999, Arg253Gln) of Neuregulin 1 and Schizophrenia in Korean Population

Neuregulin 1 (NRG1) is associated with the pathogenesis of schizophrenia through controlling activation and signaling of neurotransmitter receptors. Influence to schizophrenia development by the NRG1 gene may differ in individuals, and genetic polymorphism is one of the factors affecting their differences. Association between three single nucleotide polymorphisms (SNPs) (rs7014762, -1174 A/T; rs11998176, -788 A/T; rs3924999, Arg253Gln) of NRG1 and the development of schizophrenia was analyzed in 221 schizophrneia and 359 control subjects. Polymerase chain reaction and direct sequencing were performed to obtain genotype data of NRG1 SNPs of the subjects. In analysis of genetic data, multiple logistic regression models (codominant1, codominant2, dominant, recessive, and log-additive model) were applied. SNPStats and SPSS 18.0 were used to calculate odds ratio (OR), 95% confidence interval (CI), and p-value of each model. The genotype distributions of rs3924999 were associated with schizophrenia development (OR=0.67, 95% CI=0.47-0.95, p=0.022 in the dominant model and OR=0.69, 95% CI=0.51-0.93, p=0.013 in the log-addtive model) and allelic distributions also showed significant association (OR=0.70, 95% CI=0.52-0.93, p=0.014). The results suggest that rs3924999 of the NRG1 gene may be associated with schizophrenia susceptibility.

Losing your inhibition: linking cortical GABAergic interneurons to schizophrenia

GABAergic interneurons of the cerebral cortex (cINs) play crucial roles in many aspects of cortical function. The diverse types of cINs are classified into subgroups according to their morphology, intrinsic physiology, neurochemical markers and synaptic targeting. Recent advances in mouse genetics, imaging and electrophysiology techniques have greatly advanced our efforts to understand the role of normal cIN function and its dysfunction in neuropsychiatric disorders. In schizophrenia (SCZ), a wealth of data suggests that cIN function is perturbed, and that interneuron dysfunction may underlie key symptoms of the disease. In this review, we discuss the link between cINs and SCZ, focusing on the evidence for GABAergic signaling deficits from both SCZ patients and mouse models.

Processing of neuregulin-1 by neuropsin regulates GABAergic neuron to control neural plasticity of the mouse hippocampus

Protease-mediated signaling is an important modulator of the nervous system. However, identifying the specific signaling substrates of such proteases is limited by the rapidity with which intermediate substrate forms are cleaved and released. Here, a screening method to detect noncleaved enzyme-bound forms was developed and used to identify a novel neuropsin/neuregulin-1 (NRG-1) proteolytic signaling system, which is specifically localized in the microdomain of synaptic cleft, in the mouse hippocampus. The extracellular protease, neuropsin, cleaved mature NRG-1 (comprising the extracellular domain of the NRG-1) at three newly identified sites to remove the heparin-binding domain of NRG-1. This released the ligand moiety from the matrix-glycosaminoglycan pool and enabled it to trigger the phosphorylation of NRG-1 receptor, p185 (ErbB4). Proteolysis of mature NRG-1 by neuropsin led to colocalization of the processed NRG-1 with ErbB4 in parvalbumin-positive hippocampal interneurons and consequent phosphorylation of tyrosine residues of proteins in the cells. Moreover, neuropsin knock-out mice exhibited impairments in Schaffer collateral early phase long-term potentiation, and application of the recombinant NRG-1 lacking heparin-binding activity reversed the effects through the activation of ErbB4 and GABA(A) receptors. Thus, ErbB4 signaling induced by neuropsin-dependent processing of NRG-1 contributes to the modulation of synaptic plasticity via regulation of GABAergic transmission. This signaling system may be involved in human cognition and mental disorders, such as schizophrenia and bipolar disorder, by its dysfunction.

The Integration of the Glutamatergic and the White Matter Hypotheses of Schizophrenia's Etiology

BACKGROUND

schizophrenia's endophenotipic profile is not only generally complex, but often varies from case to case. The perspective of trying to define specific anatomic correlates of the syndrome has led to disappointing results. In that context, neurophysiologic hypotheses (e.g. glutamatergic hypothesis) and connectivity hypotheses became prominent. Nevertheless, despite their commitment to the principle of denying 'localist' views and approaching the syndrome's endophenotype from a whole brain perspective, efforts to integrate both have not flourished at this moment in time.

OBJECTIVES

This paper aims to introduce a new etiological model that integrates the glutamatergic and the WM (WM) hypotheses of schizophrenia's etiology. This model proposes to serve as a framework in order to relate to patterns of brain abnormalities from the onset of the syndrome to stages of advanced chronification.

HIGHLIGHTS

Neurotransmitter abnormalities forego noticeable WM abnormalities. The former, chiefly represented by NMDAR hypo-function and associated molecular cascades, is related to the first signs of cell loss. This process is both directly and indirectly integrated to the underpinning of WM structural abnormalities; not only is the excess of glutamate toxic to the WM, but its disruption is associated to the expression of known genetic risk factors (e.g., NRG-1). A second level of the model develops the idea that abnormal neurotransmission within specific neural populations ('motifs') impair particular cognitive abilities, while subsequent WM structural abnormalities impair the integration of brain functions and multimodality. As a result of this two-stage dynamic, the affected individual progresses from experiencing specific cognitive and psychological deficits, to a condition of cognitive and existential fragmentation, linked to hardly reversible decreases in psychosocial functioning.

Reversal of impaired hippocampal long-term potentiation and contextual fear memory deficits in Angelman syndrome model mice by ErbB inhibitors

BACKGROUND

Angelman syndrome (AS) is a human neuropsychiatric disorder associated with autism, mental retardation, motor abnormalities, and epilepsy. In most cases, AS is caused by the deletion of the maternal copy of UBE3A gene, which encodes the enzyme ubiquitin ligase E3A, also termed E6-AP. A mouse model of AS has been generated and these mice exhibit many of the observed neurological alterations in humans. Because of clinical and neuroanatomical similarities between AS and schizophrenia, we examined AS model mice for alterations in the neuregulin-ErbB4 pathway, which has been implicated in the pathophysiology of schizophrenia. We focused our studies on the hippocampus, one of the major brain loci impaired in AS mice.

METHODS

We determined the expression of neuregulin 1 and ErbB4 receptors in AS mice and wild-type littermates (ages 10-16 weeks) and studied the effects of ErbB inhibition on long-term potentiation in hippocampal area cornu ammonis 1 and on hippocampus-dependent contextual fear memory.

RESULTS

We observed enhanced neuregulin-ErbB4 signaling in the hippocampus of AS model mice and found that ErbB inhibitors could reverse deficits in long-term potentiation, a cellular substrate for learning and memory. In addition, we found that an ErbB inhibitor enhanced long-term contextual fear memory in AS model mice.

CONCLUSIONS

Our findings suggest that neuregulin-ErbB4 signaling is involved in synaptic plasticity and memory impairments in AS model mice, suggesting that ErbB inhibitors have therapeutic potential for the treatment of AS.

A common variant in ERBB4 regulates GABA concentrations in human cerebrospinal fluid

The neuregulin 1 (NRG1) receptor ErbB4 is involved in the development of cortical inhibitory GABAergic circuits and NRG1-ErbB4 signaling has been implicated in schizophrenia (SCZ). A magnetic resonance spectroscopy ((1)H-MRS) study has demonstrated that a single-nucleotide polymorphism in ERBB4, rs7598440, influences human cortical GABA concentrations. Other work has highlighted the significant impact of this genetic variant on expression of ERBB4 in the hippocampus and dorsolateral prefrontal cortex in human post mortem tissue. Our aim was to examine the association of rs7598440 with cerebrospinal fluid (CSF) GABA levels in healthy volunteers (n=155). We detected a significant dose-dependent association of the rs7598440 genotype with CSF GABA levels (G-allele standardized β=-0.23; 95% CIs: -0.39 to -0.07; P=0.0066). GABA concentrations were highest in A homozygous, intermediate in heterozygous, and lowest in G homozygous subjects. When excluding subjects on psychotropic medication (three subjects using antidepressants), the results did not change (G-allele standardized β=-0.23; 95% CIs: -0.40 to -0.07; P=0.0051). The explained variance in CSF GABA by rs7598440 in our model is 5.2% (P=0.004). The directionality of our findings agrees with the aforementioned (1)H-MRS and gene expression studies. Our observation therefore strengthens the evidence that the A-allele of rs7598440 in ERBB4 is associated with increased GABA concentrations in the human central nervous system (CNS). To our knowledge, our finding constitutes the first confirmation that CSF can be used to study genotype-phenotype correlations of GABA levels in the CNS. Such quantitative genetic analyses may be extrapolated to other CSF constituents relevant to SCZ in future studies.

Monosynaptic inputs to ErbB4-expressing inhibitory neurons in mouse primary somatosensory cortex

Previous reports have described inputs to the somatosensory cortex (S1) in mouse or rat using retrograde or anterograde tracers. Such studies do not, however, reveal which particular cell types within the S1 cortex receive direct monosynaptic connections from these input sources. Here we describe the monosynaptic inputs to a subpopulation of mouse S1 inhibitory neurons that express ErbB4. We used a previously described "bridge protein," composed of the ErbB4 ligand, neuregulin (NRG1), fused to the avian viral receptor TVB (TVB-NRG1), along with EnvB pseudotyped lentivirus (LV) and rabies virus (RV), to selectively coinfect ErbB4-expressing neurons (Choi et al. [2010] Proc Natl Acad Sci U S A 107:16703-16708). The RV had its glycoprotein gene deleted and replaced with mCherry, so that infected cells express mCherry and the virus cannot spread without provision of rabies glycoprotein (RG) by transcomplementation. The LV encoded and expressed RG to allow transcomplementation in coinfected neurons, so that the RV could spread transsynaptically and label their direct monosynaptic inputs. The RV could not spread beyond the direct inputs, due to the lack of RG in presynaptic cells. This method revealed long-range connections from thalamus, nucleus basalis, raphe, and distant cortical areas, including ipsilateral motor, secondary somatosensory, retrosplenial, and perirhinal cortex and contralateral S1. In addition, local connections from ipsilateral pyramidal neurons within S1 were labeled. These input sources account for all of the known inputs to S1 described with standard tracers, suggesting that the subpopulation of ErbB4-positive inhibitory neurons infected using the TVB-NRG1 bridge protein receives inputs indiscriminately from S1 input sources.

Synaptic cell adhesion

Chemical synapses are asymmetric intercellular junctions that mediate synaptic transmission. Synaptic junctions are organized by trans-synaptic cell adhesion molecules bridging the synaptic cleft. Synaptic cell adhesion molecules not only connect pre- and postsynaptic compartments, but also mediate trans-synaptic recognition and signaling processes that are essential for the establishment, specification, and plasticity of synapses. A growing number of synaptic cell adhesion molecules that include neurexins and neuroligins, Ig-domain proteins such as SynCAMs, receptor phosphotyrosine kinases and phosphatases, and several leucine-rich repeat proteins have been identified. These synaptic cell adhesion molecules use characteristic extracellular domains to perform complementary roles in organizing synaptic junctions that are only now being revealed. The importance of synaptic cell adhesion molecules for brain function is highlighted by recent findings implicating several such molecules, notably neurexins and neuroligins, in schizophrenia and autism.

Multifaceted genomic risk for brain function in schizophrenia

Recently, deriving candidate endophenotypes from brain imaging data has become a valuable approach to study genetic influences on schizophrenia (SZ), whose pathophysiology remains unclear. In this work we utilized a multivariate approach, parallel independent component analysis, to identify genomic risk components associated with brain function abnormalities in SZ. 5157 candidate single nucleotide polymorphisms (SNPs) were derived from genome-wide array based on their possible connections with SZ and further investigated for their associations with brain activations captured with functional magnetic resonance imaging (fMRI) during a sensorimotor task. Using data from 92 SZ patients and 116 healthy controls, we detected a significant correlation (r=0.29; p=2.41 × 10(-5)) between one fMRI component and one SNP component, both of which significantly differentiated patients from controls. The fMRI component mainly consisted of precentral and postcentral gyri, the major activated regions in the motor task. On average, higher activation in these regions was observed in participants with higher loadings of the linked SNP component, predominantly contributed to by 253 SNPs. 138 identified SNPs were from known coding regions of 100 unique genes. 31 identified SNPs did not differ between groups, but moderately correlated with some other group-discriminating SNPs, indicating interactions among alleles contributing toward elevated SZ susceptibility. The genes associated with the identified SNPs participated in four neurotransmitter pathways: GABA receptor signaling, dopamine receptor signaling, neuregulin signaling and glutamate receptor signaling. In summary, our work provides further evidence for the complexity of genomic risk to the functional brain abnormality in SZ and suggests a pathological role of interactions between SNPs, genes and multiple neurotransmitter pathways.

Acute neuregulin-1 signaling influences AMPA receptor mediated responses in cultured cerebellar granule neurons

Neuregulin-1 (NRG1) is a trophic and differentiation factor that signals through ErbB receptor tyrosine kinases to regulate nervous system development. Previous studies have demonstrated that NRG1 affects plasticity at glutamatergic synapses in principal glutamatergic neurons of the hippocampus and frontal cortex; however, immunohistochemical and genetic analyses strongly suggest these effects are indirect and mediated via ErbB4 receptors on GABAergic interneurons. Here, we used cultured cerebellar granule cells (CGCs) that express ErbB4 to analyze the cell-autonomous effects of NRG1 stimulation on glutamatergic function. These cultures have the advantage that they are relatively homogenous and consist primarily of granule neurons that express ErbB4. We show that acute NRG1 treatment does not affect whole-cell AMPA or NMDA receptor (NMDAR) mediated currents in CGCs at 10-12 days in vitro. NRG1 also does not affect the frequency or amplitude of spontaneous AMPAR or NMDAR mediated miniature excitatory post-synaptic currents (mEPSCs). To further investigate the effects of NRG1 on activity-dependent plasticity of glutamatergic synapses in CGCs, we characterized the effects of high-glyine/0 Mg(2+) (which activates synaptic NMDARs) on AMPAR-mEPSC frequency and amplitude. We show that high-glycine induces a form of chemical long-term potentiation (chemLTP) in CGCs characterized by an increase in AMPAR-mEPSC frequency but not amplitude. Moreover, NRG1 induces a decrease in AMPAR-mEPSC frequency following chemLTP, but does not affect AMPAR-mEPSC amplitude. CGCs in our cultures conditions express low levels of GluR1, in contrast to dissociated hippocampal cultures, but do express the long isoform of GluR4. This study provides first evidence that (1) high-glycine can induce plasticity at glutamatergic synapses in CGCs, and (2) that acute NRG1/ErbB-signaling can regulate glutamatergic plasticity in CGCs. Taken together with previous reports, our results suggest that, similar to Schaeffer collateral to CA1 synapses, NRG1 effects are activity dependent and mediated via modulation of synaptic AMPARs.

Synaptic dysfunction in schizophrenia

Schizophrenia alters basic brain processes of perception, emotion, and judgment to cause hallucinations, delusions, thought disorder, and cognitive deficits. Unlike neurodegeneration diseases that have irreversible neuronal degeneration and death, schizophrenia lacks agreeable pathological hallmarks, which makes it one of the least understood psychiatric disorders. With identification of schizophrenia susceptibility genes, recent studies have begun to shed light on underlying pathological mechanisms. Schizophrenia is believed to result from problems during neural development that lead to improper function of synaptic transmission and plasticity, and in agreement, many of the susceptibility genes encode proteins critical for neural development. Some, however, are also expressed at high levels in adult brain. Here, we will review evidence for altered neurotransmission at glutamatergic, GABAergic, dopaminergic, and cholinergic synapses in schizophrenia and discuss roles of susceptibility genes in neural development as well as in synaptic plasticity and how their malfunction may contribute to pathogenic mechanisms of schizophrenia. We propose that mouse models with precise temporal and spatial control of mutation or overexpression would be useful to delineate schizophrenia pathogenic mechanisms.

Neuregulin/ErbB regulate neuromuscular junction development by phosphorylation of α-dystrobrevin

Neuregulin/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin (NRG)/ErbB signaling is involved in numerous developmental processes in the nervous system, including synapse formation and function in the central nervous system. Although intensively investigated, its role at the neuromuscular synapse has remained elusive. Here, we demonstrate that loss of neuromuscular NRG/ErbB signaling destabilized anchoring of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane and that this effect was caused by dephosphorylation of α-dystrobrevin1, a component of the postsynaptic scaffold. Specifically, in mice in which NRG signaling to muscle was genetically or pharmacologically abolished, postsynaptic AChRs moved rapidly from the synaptic to the perisynaptic membrane, and the subsynaptic scaffold that anchors the AChRs was impaired. These defects combined compromised synaptic transmission. We further show that blockade of NRG/ErbB signaling abolished tyrosine phosphorylation of α-dystrobrevin1, which reduced the stability of receptors in agrin-induced AChR clusters in cultured myotubes. Our data indicate that NRG/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin-1 protects against neurotoxicities induced by Swedish amyloid precursor protein via the ErbB4 receptor

Neuregulin-1 (NRG1) plays an important role in the development and plasticity of the brain and exhibits potent neuroprotective properties. However, little information on its role in Alzheimer's disease (AD) is known. The neuroprotective effect and mechanisms of NRG1 in SH-SY5Y cells overexpressing the Swedish mutant form of amyloid precursor protein (Swe-APP) and primary cortical neuronal cells treated with amyloid beta peptide(1-42) (Aβ(1-42)) were investigated in this study. NRG1 attenuated Swe-APP- or Aβ(1-42)-induced lactate dehydrogenase (LDH) release in a concentration-dependent manner. The mitigating effects of NRG1 on neuronal cell death were blocked by ErbB4 inhibition, a key NRG1 receptor, which suggests a role of ErbB4 in the neuroprotective function of NRG1. Moreover, NRG1 reduced the number of Swe-APP- and Aβ(1-42)-induced TUNEL-positive SH-SY5Y cells and primary cortical neurons, respectively. NRG1 reduced the accumulation of reactive oxygen species and attenuated Swe-APP-induced mitochondrial membrane potential loss. NRG1 also induced the upregulation of the expression of the anti-apoptotic protein, Bcl-2, and decreased caspase-3 activation. Collectively, our results demonstrate that NRG1 exerts neuroprotective effects via the ErbB4 receptor, which suggests the neuroprotective potential of NRG1 in AD.

Neuregulin-1/ErbB signaling in rostral ventrolateral medulla is involved in blood pressure regulation as an antihypertensive system

OBJECTIVES

Neuregulin-1 (NRG-1), located in the central nervous system (CNS), plays an important role in synaptic function, neurite outgrowth, and survival of neurons and glia acting on the ErbB receptor family. However, the functional role of NRG-1/ErbB signaling in the CNS and blood pressure regulation is unknown, particularly in the rostral ventrolateral medulla (RVLM), a major vasomotor center. Thus, we investigated whether NRG-1/ErbB signaling in the RVLM is involved in blood pressure regulation.

METHODS AND RESULTS

Microinjection of NRG-1 into the RVLM decreased arterial blood pressure, heart rate (HR), and renal sympathetic nerve activity (RSNA) in Wistar rats. In contrast, microinjection of an ErbB2 or ErbB4 inhibitor into the RVLM increased arterial pressure, HR, and RSNA. ErbB2 expression levels in the brainstem were significantly lower in spontaneously hypertensive rats (SHRs) than in Wistar-Kyoto (WKY) rats. Depressor responses to NRG-1 and pressor responses to the ErbB2 inhibitor were significantly smaller in SHRs than in WKY rats (P < 0.05). Furthermore, the inhibition of ErbB2 expression in the RVLM by RNA interference significantly increased arterial pressure, HR, and urinary norepinephrine excretion in conscious WKY rats (P < 0.01).

CONCLUSION

Our findings indicate that the NRG-1/ErbB signaling in the RVLM has depressor and sympathoinhibitory effects. Reduced NRG/ErbB2 signaling in the RVLM may contribute to the neural mechanisms of hypertension.

AAK1 identified as an inhibitor of neuregulin-1/ErbB4-dependent neurotrophic factor signaling using integrative chemical genomics and proteomics

Target identification remains challenging for the field of chemical biology. We describe an integrative chemical genomic and proteomic approach combining the use of differentially active analogs of small molecule probes with stable isotope labeling by amino acids in cell culture-mediated affinity enrichment, followed by subsequent testing of candidate targets using RNA interference-mediated gene silencing. We applied this approach to characterizing the natural product K252a and its ability to potentiate neuregulin-1 (Nrg1)/ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4)-dependent neurotrophic factor signaling and neuritogenesis. We show that AAK1 (adaptor-associated kinase 1) is a relevant target of K252a, and that the loss of AAK1 alters ErbB4 trafficking and expression levels, providing evidence for a previously unrecognized role for AAK1 in Nrg1-mediated neurotrophic factor signaling. Similar strategies should lead to the discovery of novel targets for therapeutic development.

Expression of the neuregulin receptor ErbB4 in the brain of the rhesus monkey (Macaca mulatta)

We demonstrated recently that frontal cortical expression of the Neuregulin (NRG) receptor ErbB4 is restricted to interneurons in rodents, macaques, and humans. However, little is known about protein expression patterns in other areas of the brain. In situ hybridization studies have shown high ErbB4 mRNA levels in various subcortical areas, suggesting that ErbB4 is also expressed in cell types other than cortical interneurons. Here, using highly-specific monoclonal antibodies, we provide the first extensive report of ErbB4 protein expression throughout the cerebrum of primates. We show that ErbB4 immunoreactivity is high in association cortices, intermediate in sensory cortices, and relatively low in motor cortices. The overall immunoreactivity in the hippocampal formation is intermediate, but is high in a subset of interneurons. We detected the highest overall immunoreactivity in distinct locations of the ventral hypothalamus, medial habenula, intercalated nuclei of the amygdala and structures of the ventral forebrain, such as the islands of Calleja, olfactory tubercle and ventral pallidum, and medium expression in the reticular thalamic nucleus. While this pattern is generally consistent with ErbB4 mRNA expression data, further investigations are needed to identify the exact cellular and subcellular sources of mRNA and protein expression in these areas. In contrast to in situ hybridization in rodents, we detected only low levels of ErbB4-immunoreactivity in mesencephalic dopaminergic nuclei but a diffuse pattern of immunofluorescence that was medium in the dorsal striatum and high in the ventral forebrain, suggesting that most ErbB4 protein in dopaminergic neurons could be transported to axons. We conclude that the NRG-ErbB4 signaling pathway can potentially influence many functional systems throughout the brain of primates, and suggest that major sites of action are areas of the “corticolimbic” network. This interpretation is functionally consistent with the genetic association of NRG1 and ERBB4 with schizophrenia.

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.

Conserved interneuron-specific ErbB4 expression in frontal cortex of rodents, monkeys, and humans: implications for schizophrenia

BACKGROUND

Neuregulin-1 and ErbB4 are genetically associated with schizophrenia, and detailed knowledge of the cellular and subcellular localization of ErbB4 is important for understanding how neuregulin-1 regulates neuronal network activity and behavior. Expression of ErbB4 is restricted to interneurons in the rodent hippocampus and cortex. However, controversy remains about the cellular expression pattern in primate brain and its subcellular distribution in postsynaptic somatodendritic locations versus presynaptic terminals.

METHODS

ErbB4 expression was analyzed in pyramidal cells and interneurons in the frontal cortex of five species: C57BL6 mice (n = 3), ErbB4⁻/⁻ mice (n = 2), Sprague-Dawley rats (n = 3), two macaque species (n = 3 + 2), and humans (normal control subjects, n = 2). We investigated 1) messenger RNA in mice, macaques, and humans; 2) protein expression in all species using highly specific monoclonal antibodies; and 3) specificity tests of several ErbB4 antibodies on brain samples (mouse, macaque, human).

RESULTS

ErbB4 RNA is restricted to interneurons in the frontal cortex of mice. ErbB4 protein is undetectable in pyramidal cells of rodents, macaques, and human frontal cortex, whereas most interneurons positive for parvalbumin, calretinin, or cholecystokinin, but only a minority of calbindin-positive cells, co-express ErbB4 in macaques. Importantly, no presynaptic ErbB4 expression was detected in any species.

CONCLUSIONS

The interneuron-selective somatodendritic expression of ErbB4 is consistent with a primary role of neuregulin-ErbB4 signaling in the postsynaptic modulation of gamma-aminobutyric acidergic function in rodents and primates. Our data validate the use of rodents to analyze effects of abnormal ErbB4 function as a means to model endophenotypes of psychiatric disorders.

Specific regulation of NRG1 isoform expression by neuronal activity

Neuregulin 1 (NRG1) is a trophic factor that has been implicated in neural development, neurotransmission, and synaptic plasticity. NRG1 has multiple isoforms that are generated by usage of different promoters and alternative splicing of a single gene. However, little is known about NRG1 isoform composition profile, whether it changes during development, or the underlying mechanisms. We found that each of the six types of NRG1 has a distinct expression pattern in the brain at different ages, resulting in a change in NRG1 isoform composition. In both human and rat, the most dominant are types III and II, followed by either type I or type V, while types IV and VI are the least abundant. The expression of NRG1 isoforms is higher in rat brains at ages of E13 and P5 (in particular type V), suggesting roles in early neural development and in the neonatal critical period. At the cellular level, the majority of NRG1 isoforms (types I, II, and III) are expressed in excitatory neurons, although they are also present in GABAergic neurons and astrocytes. Finally, the expression of each NRG1 isoform is distinctly regulated by neuronal activity, which causes significant increase in type I and IV NRG1 levels. Neuronal activity regulation of type IV expression requires a CRE cis-element in the 5' untranslated region (UTR) that binds to CREB. These results indicate that expression of NRG1 isoforms is regulated by distinct mechanisms, which may contribute to versatile functions of NRG1 and pathologic mechanisms of brain disorders such as schizophrenia.

Transgenic overexpression of the type I isoform of neuregulin 1 affects working memory and hippocampal oscillations but not long-term potentiation

Neuregulin 1 (NRG1) is a growth factor involved in neurodevelopment and plasticity. It is a schizophrenia candidate gene, and hippocampal expression of the NRG1 type I isoform is increased in the disorder. We have studied transgenic mice overexpressing NRG1 type I (NRG1^{tg-type I}) and their wild-type littermates and measured hippocampal electrophysiological and behavioral phenotypes. Young NRG1^{tg-type I} mice showed normal memory performance, but in older NRG1^{tg-type I} mice, hippocampus-dependent spatial working memory was selectively impaired. Hippocampal slice preparations from NRG1^{tg-type I} mice exhibited a reduced frequency of carbachol-induced gamma oscillations and an increased tendency to epileptiform activity. Long-term potentiation in NRG1^{tg-type I} mice was normal. The results provide evidence that NRG1 type I impacts on hippocampal function and circuitry. The effects are likely mediated via inhibitory interneurons and may be relevant to the involvement of NRG1 in schizophrenia. However, the findings, in concert with those from other genetic and pharmacological manipulations of NRG1, emphasize the complex and pleiotropic nature of the gene, even with regard to a single isoform.

Genetics and function of neocortical GABAergic interneurons in neurodevelopmental disorders

A dysfunction of cortical and limbic GABAergic circuits has been postulated to contribute to multiple neurodevelopmental disorders in humans, including schizophrenia, autism, and epilepsy. In the current paper, I summarize the characteristics that underlie the great diversity of cortical GABAergic interneurons and explore how the multiple roles of these cells in developing and mature circuits might contribute to the aforementioned disorders. Furthermore, I review the tightly controlled genetic cascades that determine the fate of cortical interneurons and summarize how the dysfunction of genes important for the generation, specification, maturation, and function of cortical interneurons might contribute to these disorders.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.

Neuregulin links dopaminergic and glutamatergic neurotransmission to control hippocampal synaptic plasticity

Neuregulin-1 (NRG-1) and its receptor ErbB4 are genetically associated with schizophrenia, a complex developmental disorder of high heritability but unknown etiology that has been proposed to result from deficits in functional connectivity and synaptic plasticity. Based on pharmacological evidence, imbalances in dopaminergic and glutamatergic transmission systems are believed to contribute to its pathophysiology, but genetic data supporting a causative role for either are sparse. Stimulation of NRG-1/ErbB4 signaling inhibits or reverts hippocampal long-term potentiation (LTP) at glutamatergic synapses between Schaeffer collateral afferents and CA1 pyramidal neurons (SC-->CA1). We have recently demonstrated that NRG-1 regulates glutamatergic plasticity by rapidly increasing extracellular hippocampal dopamine levels and activation of D4 dopamine receptors.7 These new findings position the NRG-1/ErbB4 signaling pathway at the crossroads between dopaminergic and glutamatergic neurotransmission and offer novel ways to consolidate genetic, functional and pharmacological data toward a better understanding of the etiological processes underlying schizophrenia, and the role of NRG-1 for normal synaptic function and plasticity. The currently available data suggest that hippocampal interneurons might play a crucial role in mediating NRG-1 induced depotentiation. This interpretation is in line with other evidence pointing towards an involvement of GABAergic cells in the etiology of schizophrenia.

Expression of ErbB4 in the neurons of Alzheimer's disease brain and APP/PS1 mice, a model of Alzheimer's disease

Neuregulin-1 (NRG1) plays important roles in the development and plasticity of the brain, and has also been reported to exhibit potent neuroprotective properties. Although ErbB4, a key NRG1 receptor, is expressed in multiple regions in the adult animal brain, little is known about its role in Alzheimer's disease (AD). AD is characterized by progressive impairment of cognition and behavioral disturbance that strongly correlate with degeneration and death of neurons in the cerebral cortex and limbic brain areas, such as the hippocampus and the amygdala. Here, we show that the ErbB4 and phospho-ErbB4 immunoreactivities were higher intensity in the neurons of the CA1-2 transitional field of AD brains as compared to age-matched controls. Also, ErbB4 expression was increased in the neurons of the cortico medial nucleus amygdala, human basal forebrain and superior frontal gyrus of AD brains. In cerebral cortex and hippocampus of amyloid precursor protein/presenilin 1 double transgenic mice, ErbB4 immunoreactivity significantly increased in comparison to age-matched wild type control. These results suggest that up-regulating of ErbB4 immunoreactivity may involve in the progression of pathology of AD.

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.

Antipsychotic treatment and neuregulin 1-ErbB4 signalling in schizophrenia

Evidence from genetic, transgenic and post-mortem studies has strongly supported the critical role that neuregulin 1 (NRG1) and its ErbB4 receptor plays in the pathophysiology of schizophrenia. This article aims to review current evidence regarding the effects of antipsychotic treatment on NRG1-ErbB4 signalling. NRG1 and ErbB4 knockout mice display abnormal behaviours relevant to certain features of schizophrenia, which could be improved by antipsychotic (clozapine/haloperidol) treatment. In contrast to most NRG1/ErbB4 knockout mice with a decreased NRG1-ErbB4 signalling, the majority post-mortem studies showed an increased NRG1-ErbB4 signalling in schizophrenic patients. These differences could be due to degrees of alteration in risk genes (subtle variations in patients vs pronounced alteration in mutant mice) or the duration of the modification on NRG1 signalling. Various antipsychotics have different effects on NRG1 and ErbB4 expression and signalling that are dependent on treatment duration. Current evidence suggests that a chronic (12weeks) antipsychotic treatment, at least in animal models, could downregulate NRG1-ErbB4 signalling, although an upregulation is seen for a short-term treatment. These effects may be due to multiple binding profiles with various G-coupled protein receptors (e.g. dopamine, and serotonin receptors) of antipsychotics. Studies are needed to investigate the interactions between NRG1-ErbB4 and the other signalling pathways (such as glutamatergic, GABAergic and dopaminergic). Furthermore, the interactions between NRG1/ErbB4 and other schizophrenia suspensibility genes under antipsychotic treatment also require investigation.

Increased expression of receptor phosphotyrosine phosphatase-β/ζ is associated with molecular, cellular, behavioral and cognitive schizophrenia phenotypes

Schizophrenia is a serious and chronic mental disorder, in which both genetic and environmental factors have a role in the development of the disease. Neuregulin-1 (NRG1) is one of the most established genetic risk factors for schizophrenia, and disruption of NRG1 signaling has been reported in this disorder. We reported previously that NRG1/ErbB4 signaling is inhibited by receptor phosphotyrosine phosphatase-β/ζ (RPTP β/ζ) and that the gene encoding RPTPβ/ζ (PTPRZ1) is genetically associated with schizophrenia. In this study, we examined the expression of RPTPβ/ζ in the brains of patients with schizophrenia and observed increased expression of this gene. We developed mice overexpressing RPTPβ/ζ (PTPRZ1-transgenic mice), which showed reduced NRG1 signaling, and molecular and cellular changes implicated in the pathogenesis of schizophrenia, including altered glutamatergic, GABAergic and dopaminergic activity, as well as delayed oligodendrocyte development. Behavioral analyses also demonstrated schizophrenia-like changes in the PTPRZ1-transgenic mice, including reduced sensory motor gating, hyperactivity and working memory deficits. Our results indicate that enhanced RPTPβ/ζ signaling can contribute to schizophrenia phenotypes, and support both construct and face validity for PTPRZ1-transgenic mice as a model for multiple schizophrenia phenotypes. Furthermore, our results implicate RPTPβ/ζ as a therapeutic target in schizophrenia.

Schizophrenia susceptibility pathway neuregulin 1-ErbB4 suppresses Src upregulation of NMDA receptors

Hypofunction of the N-methyl D-aspartate subtype of glutamate receptor (NMDAR) is hypothesized to be a mechanism underlying cognitive dysfunction in individuals with schizophrenia. For the schizophrenia-linked genes NRG1 and ERBB4, NMDAR hypofunction is thus considered a key detrimental consequence of the excessive NRG1-ErbB4 signaling found in people with schizophrenia. However, we show here that neuregulin 1β-ErbB4 (NRG1β-ErbB4) signaling does not cause general hypofunction of NMDARs. Rather, we find that, in the hippocampus and prefrontal cortex, NRG1β-ErbB4 signaling suppresses the enhancement of synaptic NMDAR currents by the nonreceptor tyrosine kinase Src. NRG1β-ErbB4 signaling prevented induction of long-term potentiation at hippocampal Schaffer collateral-CA1 synapses and suppressed Src-dependent enhancement of NMDAR responses during theta-burst stimulation. Moreover, NRG1β-ErbB4 signaling prevented theta burst-induced phosphorylation of GluN2B by inhibiting Src kinase activity. We propose that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of synaptic NMDAR function.

Dual transplantation of human neural stem cells into cervical and lumbar cord ameliorates motor neuron disease in SOD1 transgenic rats

Stem cells provide novel sources of cell therapies for motor neuron disease that have recently entered clinical trials. In the present study, we transplanted human neural stem cells (NSCs) into the ventral horn of both the lumbar (L4-L5) and cervical (C4-C5) protuberance of SOD1 G93A rats, in an effort to test the feasibility and general efficacy of a dual grafting paradigm addressing several muscle groups in the front limbs, hind limbs and the respiratory apparatus. Transplantation was done prior to the onset of motor neuron disease. Compared with animals that had received dead NSC grafts (serving as controls), rats with live NSCs grafted at the two spinal levels lived 17 days longer. Disease onset in dually grafted animals was delayed by 10 days compared to control animals. Disease duration in NSC-grafted animals was longer by 7 days compared to controls. Our results support the potential of NSC grafts at multiple levels of spinal cord as future cellular therapy for motor neuron disease.

The ERBB4 intracellular domain (4ICD) regulates NRG1-induced gene expression in hippocampal neurons

The NRG1 growth factor and ERBB4 receptor have been identified as leading schizophrenia risk genes. Although NRG1 and ERBB4 have been shown to modulate neuronal functions involved in schizophrenia, including both GABAergic and glutamatergic synapses, the exact molecular mechanisms remain poorly understood. Here we investigated ERBB4 intracellular domain, 4ICD, transactivator function in rat hippocampal cultures by inhibiting γ-secretase mediated ERBB4 regulated intramembrane proteolysis (RIP). NRG1 stimulation resulted in a dramatic increase in the number of hippocampal cells displaying nuclear 4ICD which was abolished in cultures pretreated with the γ-secretase inhibitor compound E (CE). To identify NRG1-4ICD transactivated genes we compared global gene expression profiles of hippocampal cultures stimulated with NRG1 in the absence or presence of CE. In concordance with the contribution of NRG1-ERBB4 signaling to dendritic spine maturation and schizophrenia, global gene expression analysis followed by Ingenuity Pathway Analysis of the dataset identified NRG1-4ICD regulated genes significantly represented in semaphorin signaling and actin cytoskeletal plasticity and multiple genes with confirmed roles in dendritic spine morphogenesis. Using the power of global gene expression analysis our data provides a proof-of-concept supporting a role for non-canonical NRG1-4ICD signaling in the regulation of gene expression contributing to normal and schizophrenic neuronal function.

Neuregulin 1 promotes excitatory synapse development and function in GABAergic interneurons

Neuregulin 1 (NRG1) and its receptor ErbB4 are both susceptibility genes of schizophrenia. However, little is known about the underlying mechanisms of their malfunction. Although ErbB4 is enriched in GABAergic interneurons, the role of NRG1 in excitatory synapse formation in these neurons remains poorly understood. We showed that NRG1 increased both the number and size of PSD-95 puncta and the frequency and amplitude of miniature EPSCs (mEPSCs) in GABAergic interneurons, indicating that NRG1 stimulates the formation of new synapses and strengthens existing synapses. In contrast, NRG1 treatment had no effect on either the number or size of excitatory synapses in glutamatergic neurons, suggesting its synaptogenic effect is specific to GABAergic interneurons. Ecto-ErbB4 treatment diminished both the number and size of excitatory synapses, suggesting that endogenous NRG1 may be critical for basal synapse formation. NRG1 could stimulate the stability of PSD-95 in the manner that requires tyrosine kinase activity of ErbB4. Finally, deletion of ErbB4 in parvalbumin-positive interneurons led to reduced frequency and amplitude of mEPSCs, providing in vivo evidence that ErbB4 is important in excitatory synaptogenesis in interneurons. Together, our findings suggested a novel synaptogenic role of NRG1 in excitatory synapse development, possibly via stabilizing PSD-95, and this effect is specific to GABAergic interneurons. In light of the association of the genes of both NRG1 and ErbB4 with schizophrenia and dysfunction of GABAergic system in this disorder, these results provide insight into its potential pathological mechanism.

Ras and Rap signaling in synaptic plasticity and mental disorders

The Ras family GTPases (Ras, Rap1, and Rap2) and their downstream mitogen-activated protein kinases (ERK, JNK, and p38MAPK) and PI3K signaling cascades control various physiological processes. In neuronal cells, recent studies have shown that these parallel cascades signal distinct forms of AMPA-sensitive glutamate receptor trafficking during experience-dependent synaptic plasticity and adaptive behavior. Interestingly, both hypo- and hyperactivation of Ras/ Rap signaling impair the capacity of synaptic plasticity, underscoring the importance of a "happy-medium" dynamic regulation of the signaling. Moreover, accumulating reports have linked various genetic defects that either up- or down-regulate Ras/Rap signaling with several mental disorders associated with learning disability (e.g., Alzheimer's disease, Angelman syndrome, autism, cardio-facio-cutaneous syndrome, Coffin-Lowry syndrome, Costello syndrome, Cowden and Bannayan-Riley-Ruvalcaba syndromes, fragile X syndrome, neurofibromatosis type 1, Noonan syndrome, schizophrenia, tuberous sclerosis, and X-linked mental retardation), highlighting the necessity of happy-medium dynamic regulation of Ras/Rap signaling in learning behavior. Thus, the recent advances in understanding of neuronal Ras/Rap signaling provide a useful guide for developing novel treatments for mental diseases.

Disrupted activity in the hippocampal-accumbens circuit of type III neuregulin 1 mutant mice

Neuregulin 1 (Nrg1), a schizophrenia susceptibility gene, is involved in fundamental aspects of neurodevelopment. Mice lacking any one of the several isoforms of Nrg1 have a variety of schizophrenia-related phenotypes, including deficits in working memory and sensorimotor gating, loss of spines in pyramidal neurons in the ventral subiculum, loss of dendrites in cortical pyramidal cells, loss of parvalbumin-positive interneurons in the prefrontal cortex, and altered plasticity in corticolimbic synapses. Mice heterozygous for a disruption in exon 7 of the Nrg1 gene lack Type III (cysteine-rich-domain-containing) isoforms and have sensorimotor gating deficits that may involve changes in the activity of a circuit involving projections from the ventral hippocampus (vHPC) to medium spiny neurons in the nucleus accumbens (nACC). To explore the neural basis of these deficits, we examined electrophysiological activity in the nACC and vHPC of these mice. Under urethane anesthesia, bursts of spontaneous activity propagated from the vHPC to the nACC in both wild-type and mutant mice. However, these bursts were weaker in mutant nACC, with reduced local field potential amplitude and spiking activity. Single units in mutant nACC fired less frequently within the bursts, and more frequently outside of the bursts. Moreover, within-burst nACC spiking was less modulated by vHPC activity, as determined by phase-locking to the low-frequency oscillatory components of the bursts. These data suggest that the efficacy of vHPC input to the nACC is reduced in the Type III Nrg1 heterozygotes, supporting a role for Nrg1 in the functional profile of hippocampal-accumbens synapses.

Neuroplasticity signaling pathways linked to the pathophysiology of schizophrenia

Schizophrenia is a severe mental illness that afflicts nearly 1% of the world's population. One of the cardinal pathological features of schizophrenia is perturbation in synaptic connectivity. Although the etiology of schizophrenia is unknown, it appears to be a developmental disorder involving the interaction of a potentially large number of risk genes, with no one gene producing a strong effect except rare, highly penetrant copy number variants. The purpose of this review is to detail how putative schizophrenia risk genes (DISC-1, neuregulin/ErbB4, dysbindin, Akt1, BDNF, and the NMDA receptor) are involved in regulating neuroplasticity and how alterations in their expression may contribute to the disconnectivity observed in schizophrenia. Moreover, this review highlights how many of these risk genes converge to regulate common neurotransmitter systems and signaling pathways. Future studies aimed at elucidating the functions of these risk genes will provide new insights into the pathophysiology of schizophrenia and will likely lead to the nomination of novel therapeutic targets for restoring proper synaptic connectivity in the brain in schizophrenia and related disorders.

Expression of ErbB4 in the apoptotic neurons of Alzheimer's disease brain

Neuregulin-1 (NRG1) signaling participates in the synaptic plasticity, maintenance or regulation of adult brain. Although ErbB4, a key NRG1 receptor, is expressed in multiple regions in the adult animal brain, little is known about its localization in Alzheimer's disease (AD) brains. We previously reported that ErbB4 immunoreactivity showed regional difference in the hippocampus of age-matched control. In the present paper, immunohistochemical characterization of the distribution of ErbB4 receptor in the hippocampus relative to pathology staging were performed in age-matched control (Braak stage 0, n=6) and AD (Braak stage I/V, n=10). Here, we found that ErbB4 immunoreactivity was significantly increased in apoptotic hippocampal pyramidal neurons in the brains of AD patients, compared to those of age-matched control subjects. In AD brains, ErbB4 immunoreactivity was demonstrated to colocalize with the apoptotic signal Bax in apoptotic hippocampal pyramidal neurons. These results suggest that up-regulation of ErbB4 immunoreactivity in apoptotic neuron may involve in the progression of pathology of AD.

ErbB4 in parvalbumin-positive interneurons is critical for neuregulin 1 regulation of long-term potentiation

Neuregulin 1 (NRG1) is a trophic factor that acts by stimulating ErbB receptor tyrosine kinases and has been implicated in neural development and synaptic plasticity. In this study, we investigated mechanisms of its suppression of long-term potentiation (LTP) in the hippocampus. We found that NRG1 did not alter glutamatergic transmission at SC-CA1 synapses but increased the GABA(A) receptor-mediated synaptic currents in CA1 pyramidal cells via a presynaptic mechanism. Inhibition of GABA(A) receptors blocked the suppressing effect of NRG1 on LTP and prevented ecto-ErbB4 from enhancing LTP, implicating a role of GABAergic transmission. To test this hypothesis further, we generated parvalbumin (PV)-Cre;ErbB4(-/-) mice in which ErbB4, an NRG1 receptor in the brain, is ablated specifically in PV-positive interneurons. NRG1 was no longer able to increase inhibitory postsynaptic currents and to suppress LTP in PV-Cre;ErbB4(-/-) hippocampus. Accordingly, contextual fear conditioning, a hippocampus-dependent test, was impaired in PV-Cre;ErbB4(-/-) mice. In contrast, ablation of ErbB4 in pyramidal neurons had no effect on NRG1 regulation of hippocampal LTP or contextual fear conditioning. These results demonstrate a critical role of ErbB4 in PV-positive interneurons but not in pyramidal neurons in synaptic plasticity and support a working model that NRG1 suppresses LTP by enhancing GABA release. Considering that NRG1 and ErbB4 are susceptibility genes of schizophrenia, these observations contribute to a better understanding of how abnormal NRG1/ErbB4 signaling may be involved in the pathogenesis of schizophrenia.

The neuregulin signaling pathway and schizophrenia: from genes to synapses and neural circuits

Numerous genetic linkage and association studies implicate members of the Neuregulin-ErbB receptor (NRG-ErbB) signaling pathway as schizophrenia "at risk" genes. An emphasis of this review is to propose plausible neurobiological mechanisms, regulated by the Neuregulin-ErbB signaling network, that may be altered in schizophrenia and contribute to its etiology. To this end, the distinct neurotransmitter pathways, neuronal subtypes and neural network systems altered in schizophrenia are initially discussed. Next, the review focuses on the possible significance of genetic studies associating NRG1 and ErbB4 with schizophrenia, in light of the functional role of this signaling pathway in regulating glutamatergic, GABAergic and dopaminergic neurotransmission, as well as modulating synaptic plasticity and gamma oscillations. The importance of restricted ErbB4 receptor expression in GABAergic interneurons is emphasized, particularly their expression at glutamatergic synapses of parvalbumin-positive fast-spiking interneurons where modulation of inhibitory drive could account for the dramatic effects of NRG-ErbB signaling on gamma oscillations and pyramidal neuron output. A case is made for reasons that the NRG-ErbB signaling pathway constitutes a "biologically plausible" system for understanding the pathogenic mechanisms that may underlie the complex array of positive, negative and cognitive deficits associated with schizophrenia during development.

Selective viral vector transduction of ErbB4 expressing cortical interneurons in vivo with a viral receptor-ligand bridge protein

Both treatment of disease and basic studies of complex tissues can benefit from directing viral vector infection to specific cell types. We have used a unique cell targeting method to direct viral vector transduction to cerebral cortical neurons expressing the neuregulin (NRG) receptor ErbB4; both NRG and ErbB4 have been implicated in schizophrenia, and ErbB4 expression in cerebral cortex is known to be restricted to inhibitory neurons. We find that a bridge protein composed of the avian viral receptor TVB fused to NRG, along with EnvB-pseudotyped virus, is able to direct infection selectively to ErbB4-expressing inhibitory cortical neurons in vivo. Interestingly, although ErbB4 is expressed in a broad range of cortical inhibitory cell types, NRG-dependent infection is restricted to a more selective subset of inhibitory cell types. These results demonstrate a tool that can be used for further studies of NRG and ErbB receptors in brain circuits and demonstrate the feasibility for further development of related bridge proteins to target gene expression to other specific cell types in complex tissues.

Intramembranous valine linked to schizophrenia is required for neuregulin 1 regulation of the morphological development of cortical neurons

Neuregulin 1 (NRG1) signaling is critical to various aspects of neuronal development and function. Among different NRG1 isoforms, the type III isoforms of NRG1 are unique in their ability to signal via the intracellular domain after gamma-secretase-dependent intramembranous processing. However, the functional consequences of type III NRG1 signaling via its intracellular domain are mostly unknown. In this study, we have identified mutations within type III NRG1 that disrupt intramembranous proteolytic processing and abolish intracellular domain signaling. In particular, substitutions at valine 321, previously linked to schizophrenia risks, result in NRG1 proteins that fail to undergo gamma-secretase-mediated nuclear localization and transcriptional activation. Using processing-defective mutants of type III NRG1, we demonstrate that the intracellular domain signaling is specifically required for NRG1 regulation of the growth and branching of cortical dendrites but not axons. Consistent with the role of type III NRG1 signaling via the intracellular domain in the initial patterning of cortical dendrites, our findings from pharmacological and genetic studies indicate that type III NRG1 functions in dendritic development independent of ERBB kinase activity. Together, these results support the proposal that aberrant intramembranous processing and defective signaling via the intracellular domain of type III NRG1 impair a subset of NRG1 functions in cortical development and contribute to abnormal neuroconnectivity implicated in schizophrenia.

Glial growth factor 2 promotes functional recovery with treatment initiated up to 7 days after permanent focal ischemic stroke

Neuregulins are a family of growth factors essential for normal cardiac and nervous system development. The EGF-like domain of neuregulins contains the active site which binds and activates signaling cascades through ErbB receptors. A neuregulin-1 gene EGF-like fragment demonstrated neuroprotection in the transient middle cerebral artery occlusion (MCAO) stroke model and drastically reduced infarct volume (Xu et al., 2004). Here we use a permanent MCAO rat model to initially compare two products of the neuregulin-1 gene and also assess levels of recovery with acute versus delayed time to treatment. In the initial study full-length glial growth factor 2 (GGF2) and an EGF-like domain fragment were compared with acute intravenous delivery. In a second study GGF2 only was delivered starting at 24h, 3 days or 7 days after permanent ischemia was induced. In both studies daily intravenous administration continued for 10 days. Recovery of neurological function was assessed using limb placing and body swing tests. GGF2 had similar functional improvements compared to the EGF-like domain fragment at equimolar doses, and a higher dose of GGF2 demonstrated more robust functional improvements compared to a lower dose. GGF2 improved sensorimotor recovery with all treatment paradigms, even enhancing recovery of function with a delay of 7 days to treatment. Histological assessments did not show any associated reduction in infarct volume at either 48 h or 21 days post-ischemic event. Neurorestorative effects of this kind are of great potential clinical importance, given the difficulty of delivering neuroprotective therapies within a short time after an ischemic event in human patients. If confirmed by additional work including additional data on mechanism(s) of improved outcome with verification in other stroke models, one can make a compelling case to bring GGF2 to clinical trials as a neurorestorative approach to improving outcome following stroke injury.