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

Neuregulin1 Nuclear Signaling Influences Adult Neurogenesis and Regulates a Schizophrenia Susceptibility Gene Network within the Mouse Dentate Gyrus

Neuregulin1 (Nrg1) signaling is critical for neuronal development and function from fate specification to synaptic plasticity. Type III Nrg1 is a synaptic protein which engages in bidirectional signaling with its receptor ErbB4. Forward signaling engages ErbB4 phosphorylation, whereas back signaling engages two known mechanisms: (1) local axonal PI3K-AKT signaling and (2) cleavage by γ-secretase resulting in cytosolic release of the intracellular domain (ICD), which can traffic to the nucleus (Bao et al., 2003; Hancock et al., 2008). To dissect the contribution of these alternate signaling strategies to neuronal development, we generated a transgenic mouse with a missense mutation (V321L) in the Nrg1 transmembrane domain that disrupts nuclear back signaling with minimal effects on forward signaling or local back signaling and was previously found to be associated with psychosis (Walss-Bass et al., 2006). We combined RNA sequencing, retroviral fate mapping of neural stem cells, behavioral analyses, and various network analyses of transcriptomic data to investigate the effect of disrupting Nrg1 nuclear back signaling in the dentate gyrus (DG) of male and female mice. The V321L mutation impairs nuclear translocation of the Nrg1 ICD and alters gene expression in the DG. V321L mice show reduced stem cell proliferation, altered cell cycle dynamics, fate specification defects, and dendritic dysmorphogenesis. Orthologs of known schizophrenia (SCZ)-susceptibility genes were dysregulated in the V321L DG. These genes coordinated a larger network with other dysregulated genes. Weighted gene correlation network analysis and protein interaction network analyses revealed striking similarity between DG transcriptomes of V321L mouse and humans with SCZ.

Nrg1 intracellular signaling regulates the development of interhemispheric callosal axons in mice

Schizophrenia is associated with altered cortical circuitry. Although the schizophrenia risk gene NRG1 is known to affect the wiring of inhibitory interneurons, its role in excitatory neurons and axonal development is unclear. Here, we investigated the role of Nrg1 in the development of the corpus callosum, the major interhemispheric connection formed by cortical excitatory neurons. We found that deletion of Nrg1 impaired callosal axon development in vivo. Experiments in vitro and in vivo demonstrated that Nrg1 is cell-autonomously required for axonal outgrowth and that intracellular signaling of Nrg1 is sufficient to promote axonal development in cortical neurons and specifically in callosal axons. Furthermore, our data suggest that Nrg1 signaling regulates the expression of Growth Associated Protein 43, a key regulator of axonal growth. In conclusion, our study demonstrates that NRG1 is involved in the formation of interhemispheric callosal connections and provides a novel perspective on the relevance of NRG1 in excitatory neurons and in the etiology of schizophrenia.

Aberrant expression of FBXO22 is associated with propofol-induced synaptic plasticity and cognitive dysfunction in adult mice

Recent observation demonstrated that prolonged anesthesia modifies brain synaptic architecture in all ages, including adult. Propofol is the most commonly utilized anesthetics at clinic. Whether repeated administration of propofol modulates cognitive impairment in adults and changes synaptic plasticity remains, however, to be explored. In this study, we first discovered that repeated and prolonged exposure to propofol-induced cognitive impairment in adult rodents. Then, we examined the property of hippocampal primary neurons and slices after propofol treatment in mice, including synaptic protein profile, dendritic spine density, as well as synaptic transmission. We found the distinctive change of the F-box only protein 22 (FBXO22), an F-box E3 ligase, during this process and further explored its role. Knockdown experiments showed the downregulation of FBXO22 restored the changes by propofol treatment on hippocampal primary neurons and attenuated propofol-induced hippocampal dependent cognitive dysfunction. Our results showed that FBXO22 is involved in the regulation of repeated propofol treatment induced changes of synaptic plasticity and cognitive dysfunction in adult mice. Repeated propofol treatment leads to cognitive dysfunction by regulating FBXO22 in adult rodents.

Nrg1 haploinsufficiency alters inhibitory cortical circuits

Neuregulin 1 (NRG1) and its receptor ERBB4 are schizophrenia (SZ) risk genes that control the development of both excitatory and inhibitory cortical circuits. Most studies focused on the characterization ErbB4 deficient mice. However, ErbB4 deletion concurrently perturbs the signaling of Nrg1 and Neuregulin 3 (Nrg3), another ligand expressed in the cortex. In addition, NRG1 polymorphisms linked to SZ locate mainly in non-coding regions and they may partially reduce Nrg1 expression. Here, to study the relevance of Nrg1 partial loss-of-function in cortical circuits we characterized a recently developed haploinsufficient mouse model of Nrg1 (Nrg1^tm1Lex). These mice display SZ-like behavioral deficits. The cellular and molecular underpinnings of the behavioral deficits in Nrg1^tm1Lex mice remain to be established. With multiple approaches including Magnetic Resonance Spectroscopy (MRS), electrophysiology, quantitative imaging and molecular analysis we found that Nrg1 haploinsufficiency impairs the inhibitory cortical circuits. We observed changes in the expression of molecules involved in GABAergic neurotransmission, decreased density of Vglut1 excitatory buttons onto Parvalbumin interneurons and decreased frequency of spontaneous inhibitory postsynaptic currents. Moreover, we found a decreased number of Parvalbumin positive interneurons in the cortex and altered expression of Calretinin. Interestingly, we failed to detect other alterations in excitatory neurons that were previously reported in ErbB4 null mice suggesting that the Nrg1 haploinsufficiency does not entirely phenocopies ErbB4 deletions. Altogether, this study suggests that Nrg1 haploinsufficiency primarily affects the cortical inhibitory circuits in the cortex and provides new insights into the structural and molecular synaptic impairment caused by NRG1 hypofunction in a preclinical model of SZ.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

The ketogenic diet increases Neuregulin 1 expression via elevating histone acetylation and its anti-seizure effect requires ErbB4 kinase activity

Background

The ketogenic diet (KD)has been considered an effective treatment for epilepsy, whereas its underlying mechanisms remain obscure. We have previously reported that the KD feeding increased Neuregulin 1 (NRG1) expression in the hippocampus; disruption of NRG1 signaling by genetically deleting its receptor-ErbB4 abolished KDs effects on inhibitory synaptic activity and seizures. However, it is still unclear about the mechanisms underlying the effect of KD on NRG1 expression and whether the effects of KD require ErbB4 kinase activity.

Methods

The effects of the KD on NRG1 expression were assessed via western blotting and real-time PCR. Acetylation level at the Nrg1 promoter locus was examined using the chromatin immunoprecipitation technique. Kainic acid (KA)-induced acute seizure model was utilized to examine the effects of KD and histone deacetylase inhibitor-TSA on seizures. Synaptic activities in the hippocampus were recorded with the technique of electrophysiology. The obligatory role of ErbB4 kinase activity in KDs effects on seizures and inhibitory synaptic activity was evaluated by using ErbB kinase antagonist and transgenic mouse-T796G.

Results

We report that KD specifically increases Type I NRG1 expression in the hippocampus. Using the chromatin immunoprecipitation technique, we observe increased acetylated-histone occupancy at the Nrg1 promoter locus of KD-fed mice. Treatment of TSA dramatically elevates NRG1 expression and diminishes the difference between the effects of the control diet (CD) and KD. These data indicate that KD increases NRG1 expression via up-regulating histone acetylation. Moreover, both pharmacological and genetic inhibitions of ErbB4 kinase activity significantly block the KDs effects on inhibitory synaptic activity and seizure, suggesting an essential role of ErbB4 kinase activity.

Conclusion

These results strengthen our understanding of the role of NRG1/ErbB4 signaling in KD and shed light on novel therapeutic interventions for epilepsy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00611-7.

Overexpression of neuregulin 1 in GABAergic interneurons results in reversible cortical disinhibition

Cortical disinhibition is a common feature of several neuropsychiatric diseases such as schizophrenia, autism and intellectual disabilities. However, the underlying mechanisms are not fully understood. To mimic increased expression of Nrg1, a schizophrenia susceptibility gene in GABAergic interneurons from patients with schizophrenia, we generated gtoNrg1 mice with overexpression of Nrg1 in GABAergic interneurons. gtoNrg1 mice showed cortical disinhibition at the cellular, synaptic, neural network and behavioral levels. We revealed that the intracellular domain of NRG1 interacts with the cytoplasmic loop 1 of Na_v1.1, a sodium channel critical for the excitability of GABAergic interneurons, and inhibits Na_v currents. Intriguingly, activation of GABAergic interneurons or restoring NRG1 expression in adulthood could rescue the hyperactivity and impaired social novelty in gtoNrg1 mice. These results identify mechanisms underlying cortical disinhibition related to schizophrenia and raise the possibility that restoration of NRG1 signaling and GABAergic function is beneficial in certain neuropsychiatric disorders.

The molecular and cellular mechanisms of cortical disinhibition as a common feature of many psychiatric diseases are not fully understood. The authors identify an interaction between NRG1 and Nav1.1 sodium channel as a mechanism of how NRG1 modulates the excitability of GABAergic interneurons.

Neuregulins 1, 2, and 3 Promote Early Neurite Outgrowth in ErbB4-Expressing Cortical GABAergic Interneurons

The neuregulins (Nrgs 1-4) are a family of signaling molecules that play diverse roles in the nervous system. Nrg1 has been implicated in the formation of synapses and in synaptic plasticity. Previous studies have shown Nrg1 can affect neurite outgrowth in several neuronal populations, while the role of Nrg2 and Nrg3 in this process has remained understudied. The Nrgs can bind and activate the ErbB4 receptor tyrosine kinase which is preferentially expressed in GABAergic interneurons in the rodent hippocampus and cerebral cortex. In the present study, we evaluated the effects of Nrgs 1, 2, and 3 on neurite outgrowth of dissociated rat cortical ErbB4-positive (⁺)/GABA⁺ interneurons in vitro. All three Nrgs were able to promote neurite outgrowth during the first 2 days in vitro, with increases detected for both the axon (116-120%) and other neurites (100-120%). Increases in the average number of primary and secondary neurites were also observed. Treatment with the Nrgs for an additional 3 days promoted an increase in axonal length (86-96%), with only minimal effects on the remaining neurites (8-13%). ErbB4 expression persisted throughout the dendritic arbor and cell soma at all stages examined, while its expression in the axon was transient and declined with cell maturation. ErbB4 overexpression in GABAergic neurons promoted neurite outgrowth, an effect that was potentiated by Nrg treatment. These results show that Nrgs 1, 2, and 3 are each capable of influencing dendritic and axonal growth at early developmental stages in GABAergic neurons grown in vitro.

Proteolytic Processing of Neuregulin 2

Neuregulin 2 (NRG2) belongs to the EGF family of growth factors. Most of this family members require proteolytic cleavage to liberate their ectodomains capable of binding and activating their cognate ErbB receptors. To date, most of the studies investigating proteolytic processing of neuregulins focused on NRG1, which was shown to undergo ectodomain shedding by several ADAM proteases and BACE1 and the remaining fragment was further cleaved by γ-secretase. Recently, NRG2 attracted more attention due to its role in the neurogenesis and modulation of behaviors associated with psychiatric disorders. In this study, we used genetic engineering methods to identify proteases involved in proteolytic processing of murine NRG2. Using non-neuronal cell lines as well as cultures of primary hippocampal neurons, we demonstrated that the major proteases responsible for releasing NRG2 ectodomain are ADAM10 and BACE2. Co-expression of NRG2 and BACE2 in neurons of certain brain structures including medulla oblongata and cerebellar deep nuclei was confirmed via immunohistochemical staining. The cleavage of NRG2 by ADAM10 or BACE2 generates a C-terminal fragment that serves as a substrate for γ-secretase. We also showed that murine NRG2 is subject to post-translational modifications, substantial glycosylation of its extracellular part, and phosphorylation of the cytoplasmic tail.

Nrg1 Intracellular Signaling Is Neuroprotective upon Stroke

The schizophrenia risk gene NRG1 controls the formation of excitatory and inhibitory synapses in cortical circuits. While the expression of different NRG1 isoforms occurs during development, adult neurons primarily express the CRD-NRG1 isoform characterized by a highly conserved intracellular domain (NRG1-ICD). We and others have demonstrated that Nrg1 intracellular signaling promotes dendrite elongation and excitatory connections during neuronal development. However, the role of Nrg1 intracellular signaling in adult neurons and pathological conditions remains largely unaddressed. Here, we investigated the role of Nrg1 intracellular signaling in neuroprotection and stroke. Our bioinformatic analysis revealed the evolutionary conservation of the NRG1-ICD and a decrease in NRG1 expression with age in the human frontal cortex. Hence, we first evaluated whether Nrg1 signaling may affect pathological hallmarks in an in vitro model of neuronal senescence; however, our data failed to reveal a role for Nrg1 in the activation of the stress-related pathway p38 MAPK and DNA damage. Previous studies demonstrated that the soluble EGF domain of Nrg1 alleviated brain ischemia, a pathological process involving the generation of free radicals, reactive oxygen species (ROS), and excitotoxicity. Hence, we tested the hypothesis that Nrg1 intracellular signaling could be neuroprotective in stroke. We discovered that Nrg1 expression significantly increased neuronal survival upon oxygen-glucose deprivation (OGD), an established in vitro model for stroke. Notably, the specific activation of Nrg1 intracellular signaling by expression of the Nrg1-ICD protected neurons from OGD. Additionally, time-lapse experiments confirmed that Nrg1 intracellular signaling increased the survival of neurons exposed to OGD. Finally, we investigated the relevance of Nrg1 intracellular signaling in stroke in vivo. Using viral vectors, we expressed the Nrg1-ICD in cortical neurons and subsequently challenged them by a focal hemorrhagic stroke; our data indicated that Nrg1 intracellular signaling improved neuronal survival in the infarcted area. Altogether, these data highlight Nrg1 intracellular signaling as neuroprotective upon ischemic lesion both in vitro and in vivo. Given the complexity of the neurotoxic effects of stroke and the involvement of various mechanisms, such as the generation of ROS, excitotoxicity, and inflammation, further studies are required to determine the molecular bases of the neuroprotective effect of Nrg1 intracellular signaling. In conclusion, our research highlights the stimulation of Nrg1 intracellular signaling as a promising target for cortical stroke treatment.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

Elevated cleavage of neuregulin-1 by beta-secretase 1 in plasma of schizophrenia patients

Neuregulin 1 (NRG1) is a key candidate susceptibility gene for schizophrenia. It is reported that the function of NRG1 can be regulated by cleavage via the β-Secretase (BACE1), particularly during early development. While current knowledge suggested that schizophrenia might have different phenotypes, it is unknown whether BACE1-cleaved-NRG1 (BACE1-NRG1) activity is related to clinical phenotypes of schizophrenia. In the current study, we used a newly developed enzymatic assay to detect BACE1-NRG1 activity in the human plasma and investigated the levels of cleavage of NRG1 by BACE1 in the plasma from schizophrenia patients. Our results are the first to demonstrate that the level of plasma BACE1-NRG1 activity was significantly increased in subjects affected with schizophrenia compared with healthy controls. Interestingly, the elevated BACE1-NRG1 activity was correlated with the disease severity and duration of schizophrenia, such as patients suffering from shorter-term course and worse disease status expressed higher BACE1-NRG1 activity levels compared to whom with longer duration and less severity of the disease. Furthermore, this is also the first report that the alternation of BACE1-NRG1 activity was a substrate -specific event in schizophrenia. Together, our findings suggested that the plasma BACE1-NRG1 activity can be a potential biomarker for the early diagnosis of schizophrenia.

Antibody-mediated stabilization of NRG1 induces behavioral and electrophysiological alterations in adult mice

Neuregulin 1 (NRG1) is required for development of the central and peripheral nervous system and regulates neurotransmission in the adult. NRG1 and the gene encoding its receptor, ERBB4, are risk genes for schizophrenia, although how alterations in these genes disrupt their function has not been fully established. Studies of knockout and transgenic mice have yielded conflicting results, with both gain and loss of function resulting in similar behavioral and electrophysiological phenotypes. Here, we used high affinity antibodies to NRG1 and ErbB4 to perturb the function of the endogenous proteins in adult mice. Treatment with NRG1 antibodies that block receptor binding caused behavioral alterations associated with schizophrenia, including, hyper-locomotion and impaired pre-pulse inhibition of startle (PPI). Electrophysiological analysis of brain slices from anti-NRG1 treated mice revealed reduced synaptic transmission and enhanced paired-pulse facilitation. In contrast, mice treated with more potent ErbB4 function blocking antibodies did not display behavioral alterations, suggesting a receptor independent mechanism of the anti-NRG1-induced phenotypes. We demonstrate that anti-NRG1 causes accumulation of the full-length transmembrane protein and increases phospho-cofilin levels, which has previously been linked to impaired synaptic transmission, indicating enhancement of non-canonical NRG1 signaling could mediate the CNS effects.

An essential role for neuregulin-4 in the growth and elaboration of developing neocortical pyramidal dendrites

Neuregulins, with the exception of neuregulin-4 (NRG4), have been shown to be extensively involved in many aspects of neural development and function and are implicated in several neurological disorders, including schizophrenia, depression and bipolar disorder. Here we provide the first evidence that NRG4 has a crucial function in the developing brain. We show that both the apical and basal dendrites of neocortical pyramidal neurons are markedly stunted in Nrg4^-/- neonates in vivo compared with Nrg4^+/+ littermates. Neocortical pyramidal neurons cultured from Nrg4^-/- embryos had significantly shorter and less branched neurites than those cultured from Nrg4^+/+ littermates. Recombinant NRG4 rescued the stunted phenotype of embryonic neocortical pyramidal neurons cultured from Nrg4^-/- mice. The majority of cultured wild type embryonic cortical pyramidal neurons co-expressed NRG4 and its receptor ErbB4. The difference between neocortical pyramidal dendrites of Nrg4^-/- and Nrg4^+/+ mice was less pronounced, though still significant, in juvenile mice. However, by adult stages, the pyramidal dendrite arbors of Nrg4^-/- and Nrg4^+/+ mice were similar, suggesting that compensatory changes in Nrg4^-/- mice occur with age. Our findings show that NRG4 is a major novel regulator of dendritic arborisation in the developing cerebral cortex and suggest that it exerts its effects by an autocrine/paracrine mechanism.

Assessment of a combination of Serum Proteins as potential biomarkers to clinically predict Schizophrenia

Schizophrenia (SZ) is a devastating psychiatric disorder. Validation of potential serum biomarkers during first-episode psychosis (FEP) is especially helpful to understand the onset and prognosis of this disorder. To address this question, we examined multiple blood biomarkers and assessed the efficacy to diagnose SZ. The expression levels of Neuregulin1 (NRG1), ErbB4, brain-derived neurotrophic factor (BDNF), DNA methyltransferases 1 (DNMT1) and ten-eleven translocation 1 (TET1) proteins in peripheral blood of 53 FEP patients and 57 healthy controls were determined by enzyme-linked immunosorbent assay (ELISA). Multivariable logistic regression including biomarker concentration as covariates was used to predict SZ. Differentiating performance of these five serum protein levels was analyzed by Receiver Operating Characteristic (ROC) curve analysis. We found that patients with SZ present a higher concentration of DNMT1, and TET1 in peripheral blood, but a lower concentration of NRG1, ErbB4 and BDNF than controls. Multivariable logistic regression showed that ErbB4, BDNF and TET1 were independent predictors of SZ, and when combined, provided high diagnostic accuracy for SZ. Together, our findings highlight that altered expression of NRG1, ErbB4, BDNF, DNMT1 and TET1 are involved in schizophrenia development and they may serve as potential biomarkers for the diagnosis of the schizophrenia. Therefore, our study provides evidence that combination of ErbB4, BDNF and TET1 biomarkers could greatly improve the diagnostic performance.

BACE1-Dependent Neuregulin-1 Signaling: An Implication for Schizophrenia

Schizophrenia is a chronic psychiatric disorder with a lifetime prevalence of about 1% in the general population. Recent studies have shown that Neuregulin-1 (Nrg1) is a candidate gene for schizophrenia. At least 15 alternative splicing of NRG1 isoforms all contain an extracellular epidermal growth factor (EGF)-like domain, which is sufficient for Nrg1 biological activity including the formation of myelin sheaths and the regulation of synaptic plasticity. It is known that Nrg1 can be cleaved by β-secretase (BACE1) and the resulting N-terminal fragment (Nrg1-ntf) binds to receptor tyrosine kinase ErbB4, which activates Nrg1/ErbB4 signaling. While changes in Nrg1 expression levels in schizophrenia still remain controversial, understanding the BACE1-cleaved Nrg1-ntf and Nrg1/ErbB4 signaling in schizophrenia neuropathogenesis is essential and important. In this review paper, we included three major parts: (1) Nrg1 structure and cleavage pattern by BACE1; (2) BACE1-dependent Nrg1 cleavage associated with schizophrenia in human studies; and (3) Animal studies of Nrg1 and BACE1 mutations with behavioral observations. Our review will provide a better understanding of Nrg1 in schizophrenia and a potential strategy for using BACE1 cleavage of Nrg1 as a unique biomarker for diagnosis, as well as a new therapeutic target, of schizophrenia.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Bidirectional Signaling of Neuregulin-2 Mediates Formation of GABAergic Synapses and Maturation of Glutamatergic Synapses in Newborn Granule Cells of Postnatal Hippocampus

Expression of neuregulin-2 (NRG2) is intense in a few regions of the adult brain where neurogenesis persists; however, little is understood about its role in developments of newborn neurons. To study the role of NRG2 in synaptogenesis at different developmental stages, newborn granule cells in rat hippocampal slice cultures were labeled with retrovirus encoding tetracycline-inducible microRNA targeting NRG2 and treated with doxycycline (Dox) at the fourth or seventh postinfection day (dpi). The developmental increase of GABAergic postsynaptic currents (GPSCs) was suppressed by the early Dox treatment (4 dpi), but not by late treatment (7 dpi). The late Dox treatment was used to study the effect of NRG2 depletion specific to excitatory synaptogenesis. The Dox effect on EPSCs emerged 4 d after the impairment in dendritic outgrowth became evident (10 dpi). Notably, Dox treatment abolished the developmental increases of AMPA-receptor mediated EPSCs and the AMPA/NMDA ratio, indicating impaired maturation of glutamatergic synapses. In contrast to GPSCs, Dox effects on EPSCs and dendritic growth were independent of ErbB4 and rescued by concurrent overexpression of NRG2 intracellular domain. These results suggest that forward signaling of NRG2 mediates GABAergic synaptogenesis and its reverse signaling contributes to dendritic outgrowth and maturation of glutamatergic synapses.

SIGNIFICANCE STATEMENT

The hippocampal dentate gyrus is one of special brain regions where neurogenesis persists throughout adulthood. Synaptogenesis is a critical step for newborn neurons to be integrated into preexisting neural network. Because neuregulin-2 (NRG2), a growth factor, is intensely expressed in these regions, we investigated whether it plays a role in synaptogenesis and dendritic growth. We found that NRG2 has dual roles in the development of newborn neurons. For GABAergic synaptogenesis, the extracellular domain of NRG2 acts as a ligand for a receptor on GABAergic neurons. In contrast, its intracellular domain was essential for dendritic outgrowth and glutamatergic synapse maturation. These results imply that NRG2 may play a critical role in network integration of newborn neurons.

Altered resonance properties of somatosensory responses in mice deficient for the schizophrenia risk gene Neuregulin 1

To reveal the neuronal underpinnings of sensory processing deficits in patients with schizophrenia, previous studies have investigated brain activity in response to sustained sensory stimulation at various frequencies. This paradigm evoked neural activity at the stimulation frequency and harmonics thereof. During visual and auditory stimulation that elicited enhanced or 'resonant' responses in healthy controls, patients with schizophrenia displayed reduced activity. The present study sought to elucidate the cellular basis of disease-related deficits in sensory resonance properties using mice heterozygous for the schizophrenia susceptibility gene Neuregulin 1 (NRG1). We applied repetitive whisker stimulation at 1-15 Hz, a range relevant to whisking behavior in mice, and measured cellular activity in the primary somatosensory cortex. At frequencies where control mice displayed enhancements in measures of response magnitude and precision, NRG1 (+/-) mutants showed reductions. Our results demonstrate for the first time a link between a mutation of a schizophrenia risk gene and altered neuronal resonance properties in sensory cortex.

Common mechanisms of excitatory and inhibitory imbalance in schizophrenia and autism spectrum disorders

Autism Spectrum Disorders (ASD) and Schizophrenia (SCZ) are cognitive disorders with complex genetic architectures but overlapping behavioral phenotypes, which suggests common pathway perturbations. Multiple lines of evidence implicate imbalances in excitatory and inhibitory activity (E/I imbalance) as a shared pathophysiological mechanism. Thus, understanding the molecular underpinnings of E/I imbalance may provide essential insight into the etiology of these disorders and may uncover novel targets for future drug discovery. Here, we review key genetic, physiological, neuropathological, functional, and pathway studies that suggest alterations to excitatory/inhibitory circuits are keys to ASD and SCZ pathogenesis.

Proteolytic Processing of Neuregulin 1 Type III by Three Intramembrane-cleaving Proteases

Numerous membrane-bound proteins undergo regulated intramembrane proteolysis. Regulated intramembrane proteolysis is initiated by shedding, and the remaining stubs are further processed by intramembrane-cleaving proteases (I-CLiPs). Neuregulin 1 type III (NRG1 type III) is a major physiological substrate of β-secretase (β-site amyloid precursor protein-cleaving enzyme 1 (BACE1)). BACE1-mediated cleavage is required to allow signaling of NRG1 type III. Because of the hairpin nature of NRG1 type III, two membrane-bound stubs with a type 1 and a type 2 orientation are generated by proteolytic processing. We demonstrate that these stubs are substrates for three I-CLiPs. The type 1-oriented stub is further cleaved by γ-secretase at an ϵ-like site five amino acids N-terminal to the C-terminal membrane anchor and at a γ-like site in the middle of the transmembrane domain. The ϵ-cleavage site is only one amino acid N-terminal to a Val/Leu substitution associated with schizophrenia. The mutation reduces generation of the NRG1 type III β-peptide as well as reverses signaling. Moreover, it affects the cleavage precision of γ-secretase at the γ-site similar to certain Alzheimer disease-associated mutations within the amyloid precursor protein. The type 2-oriented membrane-retained stub of NRG1 type III is further processed by signal peptide peptidase-like proteases SPPL2a and SPPL2b. Expression of catalytically inactive aspartate mutations as well as treatment with 2,2'-(2-oxo-1,3-propanediyl)bis[(phenylmethoxy)carbonyl]-l-leucyl-l-leucinamide ketone inhibits formation of N-terminal intracellular domains and the corresponding secreted C-peptide. Thus, NRG1 type III is the first protein substrate that is not only cleaved by multiple sheddases but is also processed by three different I-CLiPs.

Amygdala NRG1-ErbB4 is critical for the modulation of anxiety-like behaviors

Anxiety disorder is related to the pathophysiology of psychiatric diseases, including major depression, substance abuse, and schizophrenia. The amygdala is important for manifestation and modulation of anxiety. However, relatively little is known regarding the mechanisms that control the amygdala inhibitory activity that is involved in anxiety. We found that almost all ErbB4, which is the only autonomous receptor of neuregulin 1 (NRG1) in the basolateral amygdala (BLA), was expressed in GABAergic neurons. Endogenous NRG1-ErbB4 signaling pathway in the BLA could modulate anxiety-like behaviors and GABA release, whereas it had no effect on glutamatergic transmission. The administration of NRG1 into the BLA of high-anxiety mice alleviated their anxiety and enhanced GABAergic neurotransmission. Moreover, exogenous NRG1 also produced an anxiolytic effect in the stressed mice. Together, these observations indicated that NRG1-ErbB4 signaling is critical to maintaining GABAergic activity in the amygdala and thus to modulating anxiety-like behaviors. Because NRG1 and ErbB4 are susceptibility genes of schizophrenia, our findings might also help to explain the potential mechanism of emotional abnormality in schizophrenia.

Prostaglandin D2 synthase/GPR44: a signaling axis in PNS myelination

Neuregulin 1 type III is processed following regulated intramembrane proteolysis, which allows communication from the plasma membrane to the nucleus. We found that the intracellular domain of neuregulin 1 type III upregulated the prostaglandin D2 synthase (L-pgds, also known as Ptgds) gene, which, together with the G protein-coupled receptor Gpr44, forms a previously unknown pathway in PNS myelination. Neuronal L-PGDS is secreted and produces the PGD2 prostanoid, a ligand of Gpr44. We found that mice lacking L-PGDS were hypomyelinated. Consistent with this, specific inhibition of L-PGDS activity impaired in vitro myelination and caused myelin damage. Furthermore, in vivo ablation and in vitro knockdown of glial Gpr44 impaired myelination. Finally, we identified Nfatc4, a key transcription factor for myelination, as one of the downstream effectors of PGD2 activity in Schwann cells. Thus, L-PGDS and Gpr44 are previously unknown components of an axo-glial interaction that controls PNS myelination and possibly myelin maintenance.

Genomics and epigenomics in novel schizophrenia drug discovery: translating animal models to clinical research and back

INTRODUCTION

Schizophrenia is a major psychiatric disorder that afflicts about 1% of the world's population, falling into the top 10 medical disorders causing disability. Existing therapeutic strategies have had limited success; they have poor effects on core cognitive impairment and long-term disability. They are also burdened by relevant side effects. Although new antipsychotic medications have been launched in the past decades, there has been a general lack of significant innovation over the past 60 years. This lack of significant progress in the pharmacotherapy of schizophrenia is a reflection of the complexity and heterogeneity of its etiopathogenetic mechanisms.

AREAS COVERED

In this article, the authors briefly review genetic models of schizophrenia, focusing on examples of how new therapeutic strategies have been developed from them. They report on the evidence of epigenetic alterations in schizophrenia and their relevance to pharmacological studies. Further, they describe the implications of epigenetic mechanisms in the etiopathogenesis of the disease and the effects of current antipsychotic drugs on epigenetic processes. Finally, they provide their perspective of using epigenetic drugs for treating schizophrenia.

EXPERT OPINION

Current genetic and epigenetic studies are finally shedding light on the biomolecular mechanisms linked to the core pathogenetic alterations in schizophrenia, rather than just their symptoms. These advancements in the understanding of the physiopathology of schizophrenia provide exciting new perspectives for treatments. Indeed, the possibility of looking directly at the biomolecular level allows us to bypass the age-old issues of animal studies pertaining to their questionable validity as behavioral models.

Neuregulin-ERBB signaling in the nervous system and neuropsychiatric diseases

Neuregulins (NRGs) comprise a large family of growth factors that stimulate ERBB receptor tyrosine kinases. NRGs and their receptors, ERBBs, have been identified as susceptibility genes for diseases such as schizophrenia (SZ) and bipolar disorder. Recent studies have revealed complex Nrg/Erbb signaling networks that regulate the assembly of neural circuitry, myelination, neurotransmission, and synaptic plasticity. Evidence indicates there is an optimal level of NRG/ERBB signaling in the brain and deviation from it impairs brain functions. NRGs/ERBBs and downstream signaling pathways may provide therapeutic targets for specific neuropsychiatric symptoms.

Function, therapeutic potential and cell biology of BACE proteases: current status and future prospects

The β-site APP cleaving enzymes 1 and 2 (BACE1 and BACE2) were initially identified as transmembrane aspartyl proteases cleaving the amyloid precursor protein (APP). BACE1 is a major drug target for Alzheimer's disease because BACE1-mediated cleavage of APP is the first step in the generation of the pathogenic amyloid-β peptides. BACE1, which is highly expressed in the nervous system, is also required for myelination by cleaving neuregulin 1. Several recent proteomic and in vivo studies using BACE1- and BACE2-deficient mice demonstrate a much wider range of physiological substrates and functions for both proteases within and outside of the nervous system. For BACE1 this includes axon guidance, neurogenesis, muscle spindle formation, and neuronal network functions, whereas BACE2 was shown to be involved in pigmentation and pancreatic β-cell function. This review highlights the recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer's disease. The protease BACE1 is a major drug target in Alzheimer disease. Together with its homolog BACE2, both proteases have an increasing number of functions within and outside of the nervous system. This review highlights recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer disease.

Cell autonomous regulation of hippocampal circuitry via Aph1b-γ-secretase/neuregulin 1 signalling

Neuregulin 1 (NRG1) and the γ-secretase subunit APH1B have been previously implicated as genetic risk factors for schizophrenia and schizophrenia relevant deficits have been observed in rodent models with loss of function mutations in either gene. Here we show that the Aph1b-γ-secretase is selectively involved in Nrg1 intracellular signalling. We found that Aph1b-deficient mice display a decrease in excitatory synaptic markers. Electrophysiological recordings show that Aph1b is required for excitatory synaptic transmission and plasticity. Furthermore, gain and loss of function and genetic rescue experiments indicate that Nrg1 intracellular signalling promotes dendritic spine formation downstream of Aph1b-γ-secretase in vitro and in vivo. In conclusion, our study sheds light on the physiological role of Aph1b-γ-secretase in brain and provides a new mechanistic perspective on the relevance of NRG1 processing in schizophrenia.

DOI:http://dx.doi.org/10.7554/eLife.02196.001

Schizophrenia affects around 1% of the world's population, with symptoms including hallucinations and delusions, apathy and cognitive impairments. Multiple genes and environmental factors interact to increase the risk of schizophrenia, making the causes of the disease—which can differ between individuals—difficult to disentangle. However, Schizophrenia is known to be associated with a reduction in the number of dendritic spines, the small protrusions that allow brain cells to receive inputs from other brain cells.

One gene that has repeatedly been implicated in schizophrenia is neuregulin 1 (NRG1), which encodes a signalling protein with more than thirty different variants. One of these variants, type III NRG1, is located on the cell membrane. An enzyme called γ-secretase can cleave the 'tail' of this protein, which means that the tail becomes free to move to the nucleus of the cell, where it can alter the expression of genes.

Fazzari et al. have now studied how different γ-secretases interact with type III NRG1 by using genetic techniques to remove a specific part of the enzymes in the brains of mice. The brain cells of these mutant mice contained fewer dendritic spines than mice with normal γ-secretases. However, the number of dendritic spines in the mutant mice could be restored by introducing γ-secretase.

These results are consistent with a model in which mutations that remove the ability of γ-secretases to cleave NRG1 lead to some of the structural and functional changes in the brain that are associated with schizophrenia. An improved understanding of the properties of the various γ-secretases could also lead to the design of safer versions of drugs called γ-secretase modulators that are used to treat Alzheimer's disease.

DOI:http://dx.doi.org/10.7554/eLife.02196.002

Global signaling effects of a schizophrenia-associated missense mutation in neuregulin 1: an exploratory study using whole genome and novel kinome approaches

Aberrant neuregulin 1-ErbB4 signaling has been implicated in schizophrenia. We previously identified a novel schizophrenia-associated missense mutation (valine to leucine) in the NRG1 transmembrane domain. This variant inhibits formation of the NRG1 intracellular domain (ICD) and causes decreases in dendrite formation. To assess the global effects of this mutation, we used lymphoblastoid cell lines from unaffected heterozygous carriers (Val/Leu) and non-carriers (Val/Val). Transcriptome data showed 367 genes differentially expressed between the two groups (Val/Val N = 6, Val/Leu N = 5, T test, FDR (1 %), α = 0.05, -log10 p value >1.5). Ingenuity pathway (IPA) analyses showed inflammation and NRG1 signaling as the top pathways altered. Within NRG1 signaling, protein kinase C (PKC)-eta (PRKCH) and non-receptor tyrosine kinase (SRC) were down-regulated in heterozygous carriers. Novel kinome profiling (serine/threonine) was performed after stimulating cells (V/V N = 6, V/L N = 6) with ErbB4, to induce release of the NRG1 ICD, and revealed significant effects of treatment on the phosphorylation of 35 peptides. IPA showed neurite outgrowth (six peptides) as the top annotated function. Phosphorylation of these peptides was significantly decreased in ErbB4-treated Val/Val but not in Val/Leu cells. These results show that perturbing NRG1 ICD formation has major effects on cell signaling, including inflammatory and neurite formation pathways, and may contribute significantly to schizophrenia pathophysiology.

Distinct phenotypes of new transmembrane-domain neuregulin 1 mutant mice and the rescue effects of valproate on the observed schizophrenia-related cognitive deficits

Accumulating evidence suggests that neuregulin 1 (NRG1) might be involved in the neurodevelopment, neural plasticity, GABAergic neurotransmission, and pathogenesis of schizophrenia. NRG1 is abundantly expressed in the hippocampus, and emerging studies have begun to reveal the link between NRG1 signaling and cognitive deficits in schizophrenic patients. Because the transmembrane domain of NRG1 is vital for both forward and reverse signaling cascades, new Nrg1-deficient mice that carry a truncation of the transmembrane domain of the Nrg1 gene were characterized and used in this study to test a NRG1 loss-of-function hypothesis for schizophrenia. Both male and female Nrg1 heterozygous mutant mice and their wild-type littermates were used in a series of 4 experiments to characterize the impact of Nrg1 on behavioral phenotypes and to determine the importance of Nrg1 in the regulation of hippocampal neuromorphology and local GABAergic interneurons. First, a comprehensive battery of behavioral tasks indicated that male Nrg1-deficient mice exhibited significant impairments in cognitive functions. Second, pharmacological challenges were conducted and revealed that Nrg1 haploinsufficiency altered GABAergic activity in males. Third, although no genotype-specific neuromorphological alterations were found in the hippocampal CA1 pyramidal neurons, significant reductions in the hippocampal expressions of GAD67 and parvalbumin were revealed in the Nrg1-deficient males. Fourth, chronic treatment with valproate rescued the observed behavioral deficits and hippocampal GAD67 reduction in Nrg1-deficient males. Collectively, these results indicate the potential therapeutic effect of valproate and the importance of Nrg1 in the regulation of cognitive functions and hippocampal GABAergic interneurons, especially in males.

Postmortem brain: an underutilized substrate for studying severe mental illness

We propose that postmortem tissue is an underutilized substrate that may be used to translate genetic and/or preclinical studies, particularly for neuropsychiatric illnesses with complex etiologies. Postmortem brain tissues from subjects with schizophrenia have been extensively studied, and thus serve as a useful vehicle for illustrating the challenges associated with this biological substrate. Schizophrenia is likely caused by a combination of genetic risk and environmental factors that combine to create a disease phenotype that is typically not apparent until late adolescence. The complexity of this illness creates challenges for hypothesis testing aimed at understanding the pathophysiology of the illness, as postmortem brain tissues collected from individuals with schizophrenia reflect neuroplastic changes from a lifetime of severe mental illness, as well as treatment with antipsychotic medications. While there are significant challenges with studying postmortem brain, such as the postmortem interval, it confers a translational element that is difficult to recapitulate in animal models. On the other hand, data derived from animal models typically provide specific mechanistic and behavioral measures that cannot be generated using human subjects. Convergence of these two approaches has led to important insights for understanding molecular deficits and their causes in this illness. In this review, we discuss the problem of schizophrenia, review the common challenges related to postmortem studies, discuss the application of biochemical approaches to this substrate, and present examples of postmortem schizophrenia studies that illustrate the role of the postmortem approach for generating important new leads for understanding the pathophysiology of severe mental illness.

Type III neuregulin 1 is required for multiple forms of excitatory synaptic plasticity of mouse cortico-amygdala circuits

The amygdala plays an important role in the formation and storage of memories associated with emotional events. The cortical glutamatergic inputs onto pyramidal neurons in the basolateral nucleus of the amygdala (BLA) contribute to this process. As the interaction between neuregulin 1 (Nrg1) and its ErbB receptors has been implicated in the pathological mechanisms of schizophrenia, loss of Nrg1 may disrupt cortical-amygdala neural circuits, resulting in altered processing of salient memories. Here we show that Nrg1 is critical in multiple forms of plasticity of cortical projections to pyramidal neurons of the BLA. The miniature EPSCs in Nrg1 heterozygous animals have a faster time constant of decay and evoked synaptic currents have a smaller NMDA/AMPA ratio than those recorded in wild-type (WT) littermates. Both high-frequency electrical stimulation of cortical inputs and θ burst stimulation combined with nicotine exposure results in long-lasting potentiation in WT animals. However, the same manipulations have little to no effect on glutamatergic synaptic plasticity in the BLA from Nrg1 heterozygous mice. Comparison of WT, Nrg1 heterozygous animals and α7 nicotinic receptor heterozygous mice reveals that the sustained phase of potentiation of glutamatergic transmission after θ burst stimulation with or without nicotine only occurs in the WT mice. Together, these findings support the idea that type III Nrg1 is essential to multiple aspects of the modulation of excitatory plasticity at cortical-BLA synapses.

The adhesion-GPCR BAI3, a gene linked to psychiatric disorders, regulates dendrite morphogenesis in neurons

Adhesion-G protein-coupled receptors (GPCRs) are a poorly studied subgroup of the GPCRs, which have diverse biological roles and are major targets for therapeutic intervention. Among them, the Brain Angiogenesis Inhibitor (BAI) family has been linked to several psychiatric disorders, but despite their very high neuronal expression, the function of these receptors in the central nervous system has barely been analyzed. Our results, obtained using expression knockdown and overexpression experiments, reveal that the BAI3 receptor controls dendritic arborization growth and branching in cultured neurons. This role is confirmed in Purkinje cells in vivo using specific expression of a deficient BAI3 protein in transgenic mice, as well as lentivirus driven knockdown of BAI3 expression. Regulation of dendrite morphogenesis by BAI3 involves activation of the RhoGTPase Rac1 and the binding to a functional ELMO1, a critical Rac1 regulator. Thus, activation of the BAI3 signaling pathway could lead to direct reorganization of the actin cytoskeleton through RhoGTPase signaling in neurons. Given the direct link between RhoGTPase/actin signaling pathways, neuronal morphogenesis and psychiatric disorders, our mechanistic data show the importance of further studying the role of the BAI adhesion-GPCRs to understand the pathophysiology of such brain diseases.

Developmental vulnerability of synapses and circuits associated with neuropsychiatric disorders

Psychiatric and neurodegenerative disorders, including intellectual disability, autism spectrum disorders (ASD), schizophrenia (SZ), and Alzheimer's disease, pose an immense burden to society. Symptoms of these disorders become manifest at different stages of life: early childhood, adolescence, and late adulthood, respectively. Progress has been made in recent years toward understanding the genetic substrates, cellular mechanisms, brain circuits, and endophenotypes of these disorders. Multiple lines of evidence implicate excitatory and inhibitory synaptic circuits in the cortex and hippocampus as key cellular substrates of pathogenesis in these disorders. Excitatory/inhibitory balance--modulated largely by dopamine--critically regulates cortical network function, neural network activity (i.e. gamma oscillations) and behaviors associated with psychiatric disorders. Understanding the molecular underpinnings of synaptic pathology and neuronal network activity may thus provide essential insight into the pathogenesis of these disorders and can reveal novel drug targets to treat them. Here, we discuss recent genetic, neuropathological, and molecular studies that implicate alterations in excitatory and inhibitory synaptic circuits in the pathogenesis of psychiatric disorders across the lifespan.

Dual cleavage of neuregulin 1 type III by BACE1 and ADAM17 liberates its EGF-like domain and allows paracrine signaling

Proteolytic shedding of cell surface proteins generates paracrine signals involved in numerous signaling pathways. Neuregulin 1 (NRG1) type III is involved in myelination of the peripheral nervous system, for which it requires proteolytic activation by proteases of the ADAM family and BACE1. These proteases are major therapeutic targets for the prevention of Alzheimer's disease because they are also involved in the proteolytic generation of the neurotoxic amyloid β-peptide. Identification and functional investigation of their physiological substrates is therefore of greatest importance in preventing unwanted side effects. Here we investigated proteolytic processing of NRG1 type III and demonstrate that the ectodomain can be cleaved by three different sheddases, namely ADAM10, ADAM17, and BACE1. Surprisingly, we not only found cleavage by ADAM10, ADAM17, and BACE1 C-terminal to the epidermal growth factor (EGF)-like domain, which is believed to play a pivotal role in signaling, but also additional cleavage sites for ADAM17 and BACE1 N-terminal to that domain. Proteolytic processing at N- and C-terminal sites of the EGF-like domain results in the secretion of this domain from NRG1 type III. The soluble EGF-like domain is functionally active and stimulates ErbB3 signaling in tissue culture assays. Moreover, the soluble EGF-like domain is capable of rescuing hypomyelination in a zebrafish mutant lacking BACE1. Our data suggest that NRG1 type III-dependent myelination is not only controlled by membrane-retained NRG1 type III, but also in a paracrine manner via proteolytic liberation of the EGF-like domain.

Neuregulin-1 signalling and antipsychotic treatment: potential therapeutic targets in a schizophrenia candidate signalling pathway

Identifying the signalling pathways underlying the pathophysiology of schizophrenia is an essential step in the rational development of new antipsychotic drugs for this devastating disease. Evidence from genetic, transgenic and post-mortem studies have strongly supported neuregulin-1 (NRG1)-ErbB4 signalling as a schizophrenia susceptibility pathway. NRG1-ErbB4 signalling plays crucial roles in regulating neurodevelopment and neurotransmission, with implications for the pathophysiology of schizophrenia. Post-mortem studies have demonstrated altered NRG1-ErbB4 signalling in the brain of schizophrenia patients. Antipsychotic drugs have different effects on NRG1-ErbB4 signalling depending on treatment duration. Abnormal behaviours relevant to certain features of schizophrenia are displayed in NRG1/ErbB4 knockout mice or those with NRG1/ErbB4 over-expression, some of these abnormalities can be improved by antipsychotic treatment. NRG1-ErbB4 signalling has extensive interactions with the GABAergic, glutamatergic and dopaminergic neurotransmission systems that are involved in the pathophysiology of schizophrenia. These interactions provide a number of targets for the development of new antipsychotic drugs. Furthermore, the key interaction points between NRG1-ErbB4 signalling and other schizophrenia susceptibility genes may also potentially provide specific targets for new antipsychotic drugs. In general, identification of these targets in NRG1-ErbB4 signalling and interacting pathways will provide unique opportunities for the development of new generation antipsychotics with specific efficacy and fewer side effects.

Synapse-specific contributions in the cortical pathology of schizophrenia

Schizophrenia (SZ) is often described as a disease of neuronal connectivity. Cognitive processes such as working memory, which are particularly dependent on the proper functioning of complex cortical circuitry, are disturbed in the disease. Reciprocal connections between pyramidal neurons and interneurons, as well as dopaminergic innervations, form the basis for higher cognition in the cortex. Nonetheless, only a few review articles are available which address how each synapse operates, and is possibly disturbed in SZ, at least in part by the mechanisms involving genetic susceptibility factors for SZ. In this review, we provide an overview of cortical glutamatergic, GABAergic, and dopaminergic circuitry, review SZ-associated deficits at each of these synapses, and discuss how genetic factors for SZ may contribute to SZ-related phenotype deficits in a synapse-specific manner. Pinpointing the spatially and temporally distinct sites of action of putative SZ susceptibility factors may help us better understand the pathological mechanisms of SZ, especially those associated with synaptic functioning and neuronal connectivity.

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.

Differential neuregulin 1 cleavage in the prefrontal cortex and hippocampus in schizophrenia and bipolar disorder: preliminary findings

Background

Neuregulin 1 (NRG1) is a key candidate susceptibility gene for both schizophrenia (SCZ) and bipolar disorder (BPD). The function of the NRG1 transmembrane proteins is regulated by cleavage. Alteration of membrane bound-NRG1 cleavage has been previously shown to be associated with behavioral impairments in mouse models lacking expression of NRG1-cleavage enzymes such as BACE1 and gamma secretase. We sought to determine whether alterations in NRG1 cleavage and associated enzymes occur in patients with SCZ and BPD.

Methodology/Principal Findings

Using human postmortem brain, we evaluated protein expression of NRG1 cleavage products and enzymes that cleave at the external (BACE1, ADAM17, ADAM19) and internal (PS1-gamma secretase) sides of the cell membrane. We used three different cohorts (Controls, SCZ and BPD) and two distinct brain regions: BA9-prefrontal cortex (Controls (n = 6), SCZ (n = 6) and BPD (n = 6)) and hippocampus (Controls (n = 5), SCZ (n = 6) and BPD (n = 6)). In BA9, the ratio of the NRG1 N-terminal fragment relative to full length was significantly upregulated in the SCZ cohort (Bonferroni test, p = 0.011). ADAM17 was negatively correlated with full length NRG1 levels in the SCZ cohort (r = –0.926, p = 0.008). In the hippocampus we found significantly lower levels of a soluble 50 kDa NRG1 fragment in the two affected groups compared the control cohort (Bonferroni test, p = 0.0018). We also examined the relationship of specific symptomatology criteria with measures of NRG1 cleavage using the Bipolar Inventory of Signs and Symptoms Scale (BISS) and the Montgomery Åsberg Depression Rating Scale (MADRS). Our results showed a positive correlation between ADAM19 and psychosis (r = 0.595 p = 0.019); PS1 and mania (r = 0.535, p = 0.040); PS1 and depression (r = 0.567, p = 0.027) in BA9, and BACE1 with anxiety (r = 0.608, p = 0.03) in the hippocampus.

Conclusion/Significance

Our preliminary findings suggest region-specific alterations in NRG1 cleavage in SCZ and BPD patients. These changes may be associated with specific symptoms in these psychiatric disorders.

Schizophrenia-associated HapICE haplotype is associated with increased NRG1 type III expression and high nucleotide diversity

Excitement and controversy have followed neuregulin (NRG1) since its discovery as a putative schizophrenia susceptibility gene; however, the mechanism of action of the associated risk haplotype (HapICE) has not been identified, and specific genetic variations, which may increase risk to schizophrenia have remained elusive. Using a postmortem brain cohort from 37 schizophrenia cases and 37 controls, we resequenced upstream of the type I-IV promoters, and the HapICE repeat regions in intron 1. Relative abundance of seven NRG1 mRNA transcripts in the prefrontal cortex were determined and compared across diagnostic and genotypic groups. We identified 26 novel DNA variants and showed an increased novel variant load in cases compared with controls (χ(2)=7.815; P=0.05). The average nucleotide diversity (θ = 10.0 × 10(-4)) was approximately twofold higher than that previously reported for BDNF, indicating that NRG1 may be particularly prone to genetic change. A greater nucleotide diversity was observed in the HapICE linkage disequilibrium block in schizophrenia cases (θ((case)) = 13.2 × 10(-4); θ((control)) = 10.0 × 10(-4)). The specific HapICE risk haplotype was associated with increased type III mRNA (F = 3.76, P = 0.028), which in turn, was correlated with an earlier age of onset (r = -0.343, P = 0.038). We found a novel intronic five-SNP haplotype ~730 kb upstream of the type I promoter and determined that this region functions as transcriptional enhancer that is suppressed by SRY. We propose that the HapICE risk haplotype increases expression of the most brain-abundant form of NRG1, which in turn, elicits an earlier clinical presentation, thus providing a novel mechanism through which this genetic association may increase risk of schizophrenia.

Development of disease-modifying treatment of schizophrenia

Development of disease-modifying therapies requires an innovative approach to drug development where novel drugs are designed to target mechanisms of interest rather than to produce preclinical effects similar to those of currently used antipsychotics. Application of such novel strategy will undoubtedly require a very deep understanding of the disease biology that is just starting to emerge. Alternatively, one may let environmental experiences of the diseased individual guide the repair process and use drugs only to facilitate the effects of experience. Such an approach would bring together functional experience that is age-, environment- and disease-dependent with the plasticity resources that may otherwise not be available. There are currently no preclinical drug-environment interaction models that can be claimed to have significant degrees of validity. Therefore, from a drug development perspective, principles that combine acute symptomatic and disease-modifying properties are clearly preferred. The question arises then how such treatments can be differentiated from those that have only symptomatic effects (i.e., most currently used antipsychotic medications). One expectation is that the former will show superior and broader efficacy (especially with longer treatment duration). Another possibility is that disease-modifying drugs will be particularly useful at the very earliest stages of the disease. Society and medical communities may not be ready yet to initiate the treatment as early as during the prodromal phase, but the situation may change by the time the science advances enough to bring a convincing case of a drug with disease-modification potential.

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.

Type III neuregulin 1 regulates pathfinding of sensory axons in the developing spinal cord and periphery

Sensory axons must develop appropriate connections with both central and peripheral targets. Whereas the peripheral cues have provided a classic model for neuron survival and guidance, less is known about the central cues or the coordination of central and peripheral connectivity. Here we find that type III Nrg1, in addition to its known effect on neuron survival, regulates axon pathfinding. In type III Nrg1(-/-) mice, death of TrkA(+) nociceptive/thermoreceptive neurons was increased, and could be rescued by Bax elimination. In the Bax and type III Nrg1 double mutants, axon pathfinding abnormalities were seen for TrkA(+) neurons both in cutaneous peripheral targets and in spinal cord central targets. Axon guidance phenotypes in the spinal cord included penetration of axons into ventral regions from which they would normally be repelled by Sema3A. Accordingly, sensory neurons from type III Nrg1(-/-) mice were unresponsive to the repellent effects of Sema3A in vitro, which might account, at least in part, for the central projection phenotype, and demonstrates an effect of type III Nrg1 on guidance cue responsiveness in neurons. Moreover, stimulation of type III Nrg1 back-signaling in cultured sensory neurons was found to regulate axonal levels of the Sema3A receptor neuropilin 1. These results reveal a molecular mechanism whereby type III Nrg1 signaling can regulate the responsiveness of neurons to a guidance cue, and show that type III Nrg1 is required for normal sensory neuron survival and axon pathfinding in both central and peripheral targets.

The role of neuregulin-1 in the response to nerve injury

Axons and Schwann cells exist in a highly interdependent relationship: damage to one cell type invariably leads to pathophysiological changes in the other. Greater understanding of communication between these cell types will not only give insight into peripheral nerve development, but also the reaction to and recovery from peripheral nerve injury. The type III isoform of neuregulin-1 (NRG1) has emerged as a key signaling factor that is expressed on axons and, through binding to erbB2/3 receptors on Schwann cells, regulates multiple phases of their development. In adulthood, NRG1 is dispensable for the maintenance of the myelin sheath; however, this factor is required for both axon regeneration and remyelination following nerve injury. The outcome of NRG1 signaling depends on interactions with other pathways within Schwann cells such as Notch, integrin and cAMP signaling. In certain circumstances, this signaling pathway may be maladaptive; for instance, direct binding of Mycobacterium leprae onto erbB2 receptors produces excessive activation and can actually promote demyelination. Attempts to modulate this pathway in order to promote nerve repair will therefore need to give consideration to the exact isoform used, as well as how it is processed and the context in which it is presented to the Schwann cell.

The NMDA receptor co-agonists, D-serine and glycine, regulate neuronal dendritic architecture in the somatosensory cortex

There is substantial evidence, both pharmacological and genetic, that hypofunction of the N-methyl-d-aspartate receptor (NMDAR) is a core pathophysiological feature of schizophrenia. There are morphological brain changes associated with schizophrenia, including perturbations in the dendritic morphology of cortical pyramidal neurons and reduction in cortical volume. Our experiments investigated whether these changes in dendritic morphology could be recapitulated in a genetic model of NMDAR hypofunction, the serine racemase knockout (SR-/-) mouse. Pyramidal neurons in primary somatosensory cortex (S1) of SR-/- mice had reductions in the complexity, total length, and spine density of apical and basal dendrites. In accordance with reduced cortical neuropil, SR-/- mice also had reduced cortical volume as compared to wild type mice. Analysis of S1 mRNA by DNA microarray and gene expression analysis revealed gene changes in SR-/- that are associated with psychiatric and neurologic disorders, as well as neurodevelopment. The microarray analysis also identified reduced expression of brain derived neurotrophic factor (BDNF) in SR-/- mice. Follow-up analysis by ELISA confirmed a reduction of BDNF protein levels in the S1 of SR-/- mice. Finally, S1 pyramidal neurons in glycine transporter heterozygote (GlyT1+/-) mutants, which display enhanced NMDAR function, had increased dendritic spine density. These results suggest that proper NMDAR function is important for the arborization and spine density of pyramidal neurons in cortex. Moreover, they suggest that NMDAR hypofunction might, in part, be contributing to the dendritic and synaptic changes observed in schizophrenia and highlight this signaling pathway as a potential target for therapeutic intervention.

Type III Nrg1 back signaling enhances functional TRPV1 along sensory axons contributing to basal and inflammatory thermal pain sensation

Type III Nrg1, a member of the Nrg1 family of signaling proteins, is expressed in sensory neurons, where it can signal in a bi-directional manner via interactions with the ErbB family of receptor tyrosine kinases (ErbB RTKs). Type III Nrg1 signaling as a receptor (Type III Nrg1 back signaling) can acutely activate phosphatidylinositol-3-kinase (PtdIns3K) signaling, as well as regulate levels of α7* nicotinic acetylcholine receptors, along sensory axons. Transient receptor potential vanilloid 1 (TRPV1) is a cation-permeable ion channel found in primary sensory neurons that is necessary for the detection of thermal pain and for the development of thermal hypersensitivity to pain under inflammatory conditions. Cell surface expression of TRPV1 can be enhanced by activation of PtdIns3K, making it a potential target for regulation by Type III Nrg1. We now show that Type III Nrg1 signaling in sensory neurons affects functional axonal TRPV1 in a PtdIns3K-dependent manner. Furthermore, mice heterozygous for Type III Nrg1 have specific deficits in their ability to respond to noxious thermal stimuli and to develop capsaicin-induced thermal hypersensitivity to pain. Cumulatively, these results implicate Type III Nrg1 as a novel regulator of TRPV1 and a molecular mediator of nociceptive function.

Avoiding mouse traps in schizophrenia genetics: lessons and promises from current and emerging mouse models

Schizophrenia is one of the most common psychiatric disorders, but despite progress in identifying the genetic factors implicated in its development, the mechanisms underlying its etiology and pathogenesis remain poorly understood. Development of mouse models is critical for expanding our understanding of the causes of schizophrenia. However, translation of disease pathology into mouse models has proven to be challenging, primarily due to the complex genetic architecture of schizophrenia and the difficulties in the re-creation of susceptibility alleles in the mouse genome. In this review we highlight current research on models of major susceptibility loci and the information accrued from their analysis. We describe and compare the different approaches that are necessitated by diverse susceptibility alleles, and discuss their advantages and drawbacks. Finally, we discuss emerging mouse models, such as second-generation pathophysiology models based on innovative approaches that are facilitated by the information gathered from the current genetic mouse models.