Schizophrenia Research Forum - A Catalyst for Creative Thinking

Neuroscience 2008—Exploring the Neuregulin-ErbB Pathway

5 January 2009. Despite the failures of some recent studies to support the genes for neuregulin or its receptor ErbB4 as schizophrenia susceptibility factors (e.g., Sanders et al., 2008), many geneticists still refer to NRG1 as a top candidate based on the strength of the original finding (Stefansson et al., 2002) and the low probability that subsequent replications could be due to chance. As with other questionable candidate genes identified by their biological plausibility and location in top linkage regions, it is proposed that different SNPs within Nrg1 could underlie the disease in different populations, or even that Nrg1 might be a schizophrenia susceptibility gene in some populations and not others.

Because of the continuing interest in NRG1 from both geneticists and basic scientists, Joshua A Gordon, of Columbia University in New York City, and Amelia L. Gallitano, of the University of Arizona in Phoenix, organized a mini-symposium on "Integrative Approaches to Candidate Gene Models of Schizophrenia: Exploring the Neuregulin/ErbB4 Pathway," on Monday, October 17, at the Neuroscience 2009 meeting.

Organizers' description
Characterizing cellular pathways that incorporate multiple schizophrenia candidate genes may help elucidate the pathophysiology of this devastating illness. Multiple components of the signaling pathway acting downstream of the candidate gene, neuregulin1, and its receptor, ErbB4, have been linked to schizophrenia. This mini-symposium will explore the consequences of manipulating individual elements of this pathway, addressing their functional and clinical relevance.

Figure 1. Pictorial overview of symposium (by Amelia Gallitano; based in part on Mei and Xiong, 2008). (View larger image.)

In her introductory remarks, Gallitano noted some of the fundamental aspects of this ligand receptor duo: Nrg1 is a neurotrophic factor that begins its functional life after it has been inserted as a transmembrane protein. The extracellular domain can be cleaved to interact with the ErbB family of receptor tyrosine kinases on postsynaptic membranes, but there is also evidence that Nrg1 can "back-signal" to its home cell.

Bo Li of Cold Spring Harbor Laboratory in New York gave the first talk: "The schizophrenia-linked protein neuregulin1 and its receptor ErbB4 control glutamatergic synapse maturation and plasticity." Li reported evidence that synaptic activity in glutamatergic synapses regulates the distribution of ErbB4 receptors on the surface of dendritic spines, and that Nrg1 is the ligand for this activity (see SRF related news story). He and his colleagues have also found evidence that the relationship is a two-way street—Nrg1-ErbB4 signaling in turn regulates synaptic signaling. RNAi knockdown experiments of ErbB4 indicated that without Nrg1-ErbB4 signaling, synapses do not mature properly, there are changes in dendritic spine morphology, and long-term potentiation is reduced. The stabilization of AMPA receptors appears to be one mechanism through which this regulation is achieved, according to Li.

In the next talk, Lin Mei of the Medical College of Georgia in Augusta discussed "Neuregulin1/ErbB4 in neurotransmission and synaptic plasticity." Mei first surveyed some of the very complex signaling of Nrg1 isoforms (for review, see Mei and Xiong, 2008), and noted that the candidate neuregulin risk SNPs are in the promoter and suggested that any resultant changes in expression would lead to a gain of function in Nrg1-ErbB4 signaling overall.

Mei then turned to his group's work in inhibitory GABAergic neurons in prefrontal cortex, where they have identified a role for Nrg1-ErbB4 signaling in controlling the release of GABA (some of this work is described in an SRF related news story). The synaptic terminals of these inhibitory neurons are enriched in ErbB4, and Mei's group has found that Nrg1 enhances the release of GABA via activation of ErbB4.

Moving their analysis over to the other side of the inhibitory synapse, Mei and colleagues have found that Nrg1 regulates pyramidal cell activity, leading them to hypothesize that the apparent hypofunction of glutamatergic pathways in schizophrenia could be in part due to an increase in Nrg1 activity leading to increased GABA release onto pyramidal neurons. In support of this, Mei and colleagues' experiments indicate that the major target of Nrg1 in prefrontal cortex is parvalbumin-positive interneurons (see SRF interview with David Lewis). When ErbB4 is knocked out specifically in these neurons, mice show putative schizophrenic phenotypes.

The session shifted gears from electrophysiology to gene expression research, with a talk by Tsuyoshi Miyakawa, of Fujita Health University, Toyoake, Japan, on "Loss of α-CamKII results in dentate gyrus immaturity, a candidate endophenotype for schizophrenia." He and his colleagues have done large-scale behavioral testing of 90 strains of genetically engineered mice, and most show some behavioral variation. In earlier work with Susumu Tonegawa at MIT, he had found that mice with forebrain-specific calcineurin deficits displayed possible schizophrenia-like phenotypes (Miyakawa et al., 2003). This group subsequently reported an association of the calcineurin gene, PPP3CC, with schizophrenia susceptibility in humans (Gerber et al., 2003). These results have led Miyakawa et al. to take a closer look at the behavior of mice with mutations in calcineurin pathway genes, and they have found behavioral abnormalities with mutations in the gene for αCAMKII, a protein kinase that phosphorylates serine or threonine residues. The link to the theme of the symposium is that αCAMKII and calcineurin are NMDA receptor targets regulated by NRG1/ErbB4 signaling.

In a recently published study, the researchers report that mice with a heterozygous knockout of αCAMKII have a number of abnormalities in hippocampus, including apparent deficits in adult neurogenesis, evidence Miyakawa and colleagues have dubbed an "immature" dentate gyrus (Yamasaki et al., 2008; see also editorial: Frankland et al., 2008). In more recent work, they have looked for other mouse models with "immature" dentate gyri, finding similar phenotypes in mice lacking the gene for the zinc-finger protein Schnurri-2 (SHN2). Hippocampal gene expression patterns in these mice parallel those of the αCAMKII mice.

Is there any evidence for "immature" hippocampus in human disorders? In their recent paper, Miyakawa's group report that gene expression pattern analysis, predicated on the patterns seen in hippocampi of αCAMKII mutant mice, clustered 16 of 18 schizophrenic hippocampal brain samples together. Many of the abnormally expressed genes are involved in neurodevelopmental processes.

Session co-organizer Amelia Gallitano, followed with the talk, "Dysfunction of the immediate early gene transcription factor Egr3 is implicated in schizophrenia." In her introduction, Gallitano positioned her research within the quest for biological mediators of the environmental component of schizophrenia risk (e.g., in utero infections, traumatic birth, or stressful life events), which are estimated to account for about half the etiology of the disorder. Immediate early genes (activated within 30 min. of a stimulus) are a prime candidate, as they are known to be activated by environmental stressors.

Gallitano has been investigating the potential role of the transcription factor Egr3, which is activated by immediate early genes, in schizophrenia based on a number of different avenues of evidence for several years. Her interest in the gene received further support from the research reported by Tonegawa and colleagues on calcineurin, which regulates Egr3. Egr3 is also regulated by NMDA receptors and by Nrg1.

Gallitano has found that Egr3 knockout mice show some of the phenotypes proposed as animal models of schizophrenia psychopathology, including hyperactivity in a novel environment, an abnormal startle response, and working memory deficits, as well as hypersensitivity to stress. Some of these behavioral abnormalities are rescued by antipsychotic drugs; of particular interest is the finding that Egr3 knockout mice are relatively resistant to the sedating effects of clozapine, as are schizophrenia patients.

While hippocampal long-term potentiation is apparently normal in Egr3 knockout mice, long-term depression cannot be induced normally, a finding that may be traceable to the NR2B subunit of the NMDA receptor. These results and others, Gallitano suggested, may place Egr3 at "the convergence of stress and genetic susceptibility to schizophrenia."

In the next talk, Malcolm Nason, State University of New York, Stony Brook, discussed "Neuregulin-1 Type-III heterozygous mice as a model for schizophrenia: behavior, anatomy and physiology." He reviewed several recent studies from the laboratories of Lorna Role and David Talmage (several of which are described in detail in this SRF overview of recent Nrg1 studies). Briefly, these mice have behavioral deficits and regional neurobiological abnormalities. More recently, Nason has found evidence of a "disorganized" electrophysiologic response in the nucleus accumbens in response to input from the ventral hippocampus. Specifically, accumbens neuron activity is less synchronized to ventral hippocampal oscillatory input.

Session co-organizer Joshua Gordon finished the session with a talk on his collaborations with the Role and Talmage groups: "Neural correlates of cognitive dysfunction in Nrg1 heterozygous mice." These mice have deficits in spatial working memory, which requires an intact hippocampus. In awake behaving recordings from Nrg1 heterozygote and wild-type mice during working memory tasks, Gordon and colleagues have noted decreased gamma band oscillations in the nucleus accumbens in the mutants. However, in contrast to Nason's findings, the hippocampal modulation of the accumbens neurons appears to be normal. What the researchers do find is an increase in hippocampal activity, suggesting a compensatory hyperactivity in the hippocampus of Nrg1 heterozygous knockout mice that may be able to overcome the deficits in the hippocampal-accumbens circuit noted by Nason and colleagues.—Hakon Heimer.

Comments on Related News


Related News: Genetics and Schizophrenia—Calcineurin Connection Grows

Comment by:  Mary Reid
Submitted 26 February 2007
Posted 27 February 2007

Tom Fagan mentions that calcineurin regulates phosphorylations elicited by both glutamatergic and dopaminergic signaling. The activity of D-amino acid oxidase is increased in schizophrenia, and this also affects signaling through both these pathways. Is there any clinical benefit with the use of sodium benzoate which inhibits DAO activity?

He also mentions that EGR3 can be regulated by the activity of neuregulin. Interestingly, Roberts et al. suggest that BDNF, which is decreased in first-episode psychosis (Buckley et al., 2007), induces synthesis of EGR3 to regulate activity of GABRA4. Ma and colleagues (Ma et al., 2005) conclude that GABRA4 is involved in the etiology of autism and it has also has been implicated in nicotine dependence (Saccone et al., 2007).

Glorioso and colleagues (Glorioso et al., 2006) report changes in genes encoding early-immediate genes such as EGR1 and EGR2 and RGS4 which is involved in cellular signaling and has been implicated in schizophrenia following BDNF gene ablation. Several studies report that zinc increases BDNF expression. Does this give support to the zinc deficiency theory of schizophrenia?

View all comments by Mary Reid

Related News: DISC1 Players Gird For Adult Neurodevelopment

Comment by:  Kevin J. Mitchell
Submitted 8 October 2009
Posted 8 October 2009

The seminal identification of mutations in DISC1 associated with schizophrenia and other psychiatric disorders raises several obvious questions: what does the DISC1 protein normally do? What are its biochemical and cellular functions, and what processes are affected by its mutation? How do defects in these cellular processes ultimately lead to altered brain function and psychopathology? Which brain systems are affected and how? Similar questions could be asked for the growing number of other genes that have been implicated by the identification of putatively causal mutations, including NRG1, ERBB4, NRXN1, CNTNAP2, and many copy number variants. Finding the points of biochemical or phenotypic convergence for these proteins or mutations may be key to understanding how mutations in so many different genes can lead to a similar clinical phenotype and to suggesting points of common therapeutic intervention.

The papers by Kim et al. and Enomoto et al. add more detail to the complex picture of the biochemical interactions of DISC1 and its diverse cellular functions. The links with Akt and PTEN signaling are especially interesting, given the previous implication of these proteins in schizophrenia and autism. Akt, in particular, may provide a link between Nrg1/ErbB4 signaling and DISC1 intracellular functions.

These studies also reinforce the importance of DISC1 and its interacting partners in neurodevelopment, specifically in cell migration and axonal extension. In particular, they highlight the roles of these proteins in postnatal hippocampal development and adult hippocampal neurogenesis. They also raise the question of which extracellular signals and receptors regulate these processes through these signalling pathways. The Nrg1/ErbB4 pathway has already been implicated, but there are a multitude of other cell migration and axon guidance cues known to regulate hippocampal development, some of which, for example, semaphorins, signal through the PTEN pathway.

Whether or how disruptions in these developmental processes contribute to psychopathology also remains unclear. It seems likely that the effects of mutations in any of these genes will be highly pleiotropic and have effects in many brain systems. The reported pathology in schizophrenia is not restricted to hippocampus but extends to cortex, thalamus, cerebellum, and many other regions. Similarly, while the cognitive deficits receive a justifiably large amount of attention, given that they may have the most clinical impact, motor and sensory deficits are also a stable and consistent part of the syndrome that must be explained. Pleiotropic effects on prenatal and postnatal development, as well as on adult processes, may actually be the one common thread characterizing the genes so far implicated. These new papers represent the first steps in the kinds of detailed biological studies that will be required to make explanatory links from mutations, through biochemical and cellular functions, to effects on neuronal networks and ultimately psychopathology.

View all comments by Kevin J. Mitchell

Related News: DISC1 Players Gird For Adult Neurodevelopment

Comment by:  Peter PenzesMichael Cahill
Submitted 8 October 2009
Posted 8 October 2009

DISC1 disruption by chromosomal translocation cosegregates with several neuropsychiatric disorders, including schizophrenia (Blackwood et al., 2001; Millar et al., 2000). Recent attention has focused on the effects of DISC1 on the structure and function of the dentate gyrus, one of the few brain regions that exhibit neurogenesis throughout life. The downregulation of DISC1 has several deleterious effects on the dentate gyrus, including aberrant neuronal migration (Duan et al., 2007). However, the mechanisms through which DISC1 regulates the structure and function of the dentate gyrus remain unknown. The dentate gyrus and its output to the CA3 area, the mossy fiber, show several abnormalities in schizophrenia and other neuropsychiatric diseases (Kobayashi, 2009). Thus, understanding how a gene associated with neuropsychiatric disease, DISC1, mechanistically impacts the dentate gyrus is an important question with much clinical relevance.

The recent papers by Kim et al. and Enomoto et al. characterize an interaction between DISC1 and girdin (also known as KIAA1212), and reveal how girdin, and the interaction between DISC1 and girdin, impact axon development, dendritic development, and the proper positioning of newborn neurons in the dentate gyrus. Girdin normally stimulates the function of AKT (Anai et al., 2005), and Kim et al. show that DISC1 binds to girdin and inhibits its function. Thus, the loss of DISC1 leaves girdin unopposed, resulting in excessive AKT signaling. Indeed, the developmental defects in neurons lacking DISC1 can be rescued by pharmacologically blocking the activation of an AKT downstream target. However, as shown by Enomoto et al., the loss of girdin produces deleterious effects on neuronal morphology, suggesting that a proper balance of girdin function is crucial.

Collectively, these studies thoroughly characterize the interaction between DISC1 and girdin, and shed much light on the consequences of this interaction on neuronal morphology as well as on the positioning of neurons in the dentate gyrus. The role of girdin in the pathology of neuropsychiatric diseases is unknown, and remains an interesting question for the future. Characterizing the molecules that act up- or downstream of DISC1 remains an important area of investigation and could aid the development of pharmacological interventions in the future. It’s intriguing that DISC1 acting through girdin regulates the activity of AKT as AKT1 was previously identified as a schizophrenia risk gene (Emamian et al., 2004). This suggests a convergence of multiple schizophrenia-associated genes in a shared pathway, and thus it will be important to determine if the DISC1-girdin-AKT1 pathway is particularly vulnerable in neuropsychiatric disorders.

References:

Blackwood DH, Fordyce A, Walker MT, St Clair DM, Porteous DJ, Muir WJ. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet . 2001 Aug 1 ; 69(2):428-33. Abstract

Millar JK, Christie S, Semple CA, Porteous DJ. Chromosomal location and genomic structure of the human translin-associated factor X gene (TRAX; TSNAX) revealed by intergenic splicing to DISC1, a gene disrupted by a translocation segregating with schizophrenia. Genomics . 2000 Jul 1 ; 67(1):69-77. Abstract

Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell . 2007 Sep 21 ; 130(6):1146-58. Abstract

Kobayashi K. Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders. Mol Neurobiol . 2009 Feb 1 ; 39(1):24-36. Abstract

Anai M, Shojima N, Katagiri H, Ogihara T, Sakoda H, Onishi Y, Ono H, Fujishiro M, Fukushima Y, Horike N, Viana A, Kikuchi M, Noguchi N, Takahashi S, Takata K, Oka Y, Uchijima Y, Kurihara H, Asano T. A novel protein kinase B (PKB)/AKT-binding protein enhances PKB kinase activity and regulates DNA synthesis. J Biol Chem . 2005 May 6 ; 280(18):18525-35. Abstract

Emamian ES, Hall D, Birnbaum MJ, Karayiorgou M, Gogos JA. Convergent evidence for impaired AKT1-GSK3beta signaling in schizophrenia. Nat Genet . 2004 Feb 1 ; 36(2):131-7. Abstract

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Related News: Unkind Cuts of NRG3 May Lead to Schizophrenia

Comment by:  Assen Jablensky
Submitted 15 September 2010
Posted 15 September 2010

Common or rare genetic variation in NRG3 influences risk for schizophrenia?
Emerging evidence implicating NRG3 as a likely susceptibility gene in population samples as diverse as the Ashkenazi Jews, Han Chinese, Australians of Anglo-Irish ancestry, and white Americans is certainly a “noteworthy” occurrence in schizophrenia genetics. The latest addition to the evidence (Kao et al., 2010) provides considerable support to earlier (Fallin et al., 2003; Wang et al., 2008) and recent findings of association of several polymorphisms (rs10883866, rs6584400, rs10748842) within a conserved linkage disequilibrium (LD) block in intron 1 of the NRG3 gene with a delusion-laden factor and a neurocognitive quantitative trait in the schizophrenia phenotype (Chen et al., 2009; Morar et al., 2010).

A fundamental contribution of the present study is the cloning and detailed characterization of full-length NRG3 transcripts from postmortem fetal, child, adolescent, and adult brain samples (whole brain, hippocampus, and dorsolateral prefrontal cortex). Sequencing of the cDNA clones and expression analysis revealed a complex picture of alternative splicing, abundance of developmentally regulated transcripts in schizophrenia brains, and, notably, increased expression of a fetal brain-derived clone (hFBNRG3), which introduces a premature stop codon resulting in a truncated protein and a possibly destabilized NRG3-ErbB4 signalling pathway. In their clinical collections (a family-based sample and a partially independent case-control sample), the authors report significant associations of rs10748842 (representing 12 SNPs located in the LD block within intron 1) with schizophrenia, with the PANSS (Positive and Negative Syndrome Scale) subscale score on delusion severity, as well as with the PANSS negative symptom load.

Overall, the findings from this investigation and the earlier studies appear to be in a broad agreement, converging on a plausible role of NRG3 in schizophrenia pathogenesis. However, there is a fly in the ointment: The associations found in the present study exhibit a risk allele reversal compared to previously reported results; namely, all significant associations are with the major, common alleles, rather than with the minor alleles, as in Chen et al. (2009) and Morar et al. (2010). While many reasons for genuine allele flipping can be invoked (multi-locus interactions, variation in local patterns of LD, environmental exposures, ethnic background differences—see Clarke and Cardon, 2010), the explanation for the flip in this particular context is not obvious, and NRG3 should remain on the examination bench. Even in the GWAS era, studies proceeding from biologically and clinically anchored hypotheses remain rewarding and potentially productive.

References:

Chen PL, Avramopoulos D, Lasseter VK, McGrath JA, Fallin MD, Liang K-Y, Nestadt G, Feng N, Steel G, Cutting AS, Wolyniec P, Pulver AE, Valle D. Fine mapping on chromosome 10q22-q23 implicates Neuregulin 3 in schizophrenia. Am J Hum Genet. 2009;84:21-34. Abstract

Clarke GM, Cardon LR. Aspects of observing and claiming allele flips in association studies. Genet Epidemiol. 2010;34:266-74. Abstract

Fallin MD, Lasseter VK, Wolyniec PS, McGrath JA, Nestadt G, Valle D, Liang KY, Pulver AE. Genomewide linkage scan for schizophrenia susceptibility loci among Ashkenazi Jewish families shows evidence of linkage on chromosome 10q22. Am J Hum Genet. 2003;73:601-11. Abstract

Kao WT, Wang Y, Kleinman JE, Lipska BK, Hyde TM, Weinberger DR, Law AJ. Common genetic variation in Neuregulin 3 (NRG3) influences risk for schizophrenia and impacts NRG3 expression in human brain. Proc Natl Acad Sci U S A. 2010 Aug 31;107(35):15619-24. Abstract

Morar B, Dragovic M, Waters FAV, Chandler D, Kalaydjieva L, Jablensky A. Neuregulin 3 (NRG3) as a susceptibility gene in a schizophrenia subtype with florid delusions and relatively spared cognition. Mol Psychiatry. 2010 June 15. Abstract

Wang YC, Chen JY, Chen ML, Chen CH, Lai IC, Chen TT, Hong CJ, Tsai SJ, Liou YL. Neuregulin 3 genetic variations and susceptibility to schizophrenia in a Chinese population. Biol Psychiatry. 2008;64:1093-6. Abstract

View all comments by Assen Jablensky