Schizophrenia Research Forum - A Catalyst for Creative Thinking

Neuregulin and ErbB4 Mutant Mice Reveal Myelin and Synaptic Deficits

2 May 2007. Genetic association and expression studies have implicated the neuregulin1 growth factor (NRG1) and its ErbB receptors, particularly ErbB4, in raising the risk of schizophrenia (Stefansson et al., 2002). A paper out May 1 in PNAS online from Gabriel Corfas and colleagues from Harvard Medical School in Boston, Massachusetts, adds substantial evidence to previous suggestions that defects in NRG1/ErbB4 signaling could underlie white matter defects in schizophrenia. The study finds that blocking NRG1/ErbB4 signaling in oligodendrocytes in mice causes a defect in myelination and slower nerve transmission in the brain. Other changes that they report, including enhanced dopaminergic signaling, decreased activity, impaired social interactions, and sensitivity to amphetamine, may model some of the manifestations of schizophrenia.

A second paper, led by the Icelandic company that originally linked neuregulin to schizophrenia, looks at how the loss of the NRG1/ErbB pathway affects neurons and synaptic function. In the April 25 Journal of Neuroscience, a research team led by Mark Gurney at deCODE Genetics in Reykjavik, Iceland, and their collaborators at Roche Palo Alto in California report that NRG1 regulates the function of NMDA-type glutamate receptors, thereby exerting a direct effect on neurotransmission through glutamatergic pathways, another locus of aberration in schizophrenia (see Current Hypothesis paper by B. Moghaddam).

Myelin matters
To find out if NRG1/ErbB signaling contributes to CNS myelination, and whether that might play a role in psychiatric disorders, joint first authors Kristine Roy and Joshue Murtie and the Harvard team created transgenic mice with a targeted block of ErbB signaling only in oligodendrocytes (OL), the cells that wrap CNS axons with myelin. The ErbB family of receptors features extracellular ligand binding domains linked to cytoplasmic tyrosine kinase signaling domains. Expression of a truncated receptor lacking the cytoplasmic domain has been shown to block signaling through endogenous receptors in a dominant negative fashion. By making transgenic mice with a truncated ErbB4 gene under control of an OL-specific promoter, the investigators produced mice that expressed the dominant negative protein only in OLs and their precursors. Because the truncated form can dimerize with other ErbB family members, Roy and colleagues detected inhibition of signaling not just through the ErbB4 receptor, but the related -B2 and -B3 receptors as well.

Since NRG1/ErbB was believed to be required to maintain the OL lineage, the investigators thought they might find fewer OLs in the transgenic mice, but that was not the case. OL number actually increased by 40 percent, but morphology was not abnormal. The cells were smaller, and their processes had fewer branches. These simplified, smaller OLs would be expected to cover less axon area, and indeed the researchers found thinner myelin sheaths in the optic nerve and the corpus callosum of transgenic mice.

In young mice, the changes in myelination resulted in slower nerve conduction speeds and changes in behavior. Transgenic mice explored in the open field less, displayed increased anxiety, and had abnormal social interactions. The mice were also hypersensitive to repeated doses of amphetamine, a sign of defects in dopamine function seen in schizophrenia. The researchers took a closer look at the dopamine system in the transgenic mice, and found that levels of dopamine transporters and dopamine receptor type 1-like binding increased in several regions of the brain. Moreover, stimulation with either a dopamine receptor agonist or amphetamine induced greater responses in gene expression, dopamine release, and behavioral measures than in control mice.

“Our findings indicate that defects in OL structure/function can cause alterations in neurotransmission that are relevant to psychiatric diseases,” the authors conclude. The study may help to resolve an outstanding question from NRG1/ErbB4 genetics work, that is, whether the loss of the pathway or a gain of function contributes to the features of schizophrenia. This study suggests a loss-of-function model for these genes. It also suggests that one lesion could contribute to both positive symptoms (as evidenced by increased sensitivity to amphetamines in this study) and negative ones (as evidenced by hypoactivity, and social withdrawal).

In a press release accompanying the paper, Corfas also speculated that the involvement of white matter could help explain why schizophrenia often sets in during adolescence or early adulthood, a time of active myelination of the prefrontal cortex.

NRG1 and neurons
The NRG1/ErbB pathway could contribute to some of the symptoms of schizophrenia through effects in neurons as well, says the second report. In that paper, the researchers show that interfering with NRG1 signaling (they use NRG1 or ErbB knockout mice) leads to reductions in NMDA receptor phosphorylation and changes in its function (see SRF related news story). Treatment with the atypical antipsychotic drug clozapine reversed the receptor hypophosphorylation and improved behavioral abnormalities in the mice. The results indicate that alterations in the NRG1/ErbB pathway and resulting alterations of NMDA receptor function might cause some of the pathophysiology of schizophrenia.

Joint first authors Maria Bjarnadottir and Dinah Misner started out looking for downstream effectors of the NRG1/ErbB4 pair. A yeast two-hybrid assay with the ErbB4 cytoplasmic tail and a mass spectrometry analysis of ErbB4 protein complexes immunoprecipitated from brain identified the cytoplasmic tyrosine kinases Fyn and Pyk2, respectively, as binding partners for the receptor. In cells overexpressing ErbB4 and Fyn, the researchers found that NRG1 caused Fyn activation.

Both kinases can phosphorylate a regulatory site on the NR2A subunit of the NMDA receptor, and the researchers determined that NRG1 treatment of human neuroblastoma cells resulted in an increase in NR2A phosphorylation at Y1472. Conversely, the same site is hypophosphorylated in either NRG1 or ErbB4 heterozygous knockout mice, suggesting the pathway functions in vivo.

Consistent with the idea that NRG1/ErbB4 regulates NMDA receptor activity, the knockout mice showed changes in some measures of synaptic plasticity. Depending on the stimulus, they had defects in LTP and changes in short-term synaptic plasticity. Treating hippocampal slices from NRG1+/- animals with exogenous neuregulin reversed some effects. The dose-response curves were complicated, suggesting that there may be an optimal level of NRG1 required for proper synapse formation.

The deCODE group previously showed that their NRG1+/- mice have behavioral abnormalities that could be reversed by treatment with clozapine (Stefansson et al., 2002). The current study extends this by showing that the same treatment also restores normal levels of NMDA receptor phosphorylation.

“These data suggest to us that NRG1-associated susceptibility to schizophrenia is at least partly associated with hypofunction of NRG1 signaling through ErbB4, Fyn, and other associated kinases such as Pyk2, that phosphorylate regulatory sites on NMDAR subunits, resulting in abnormal modulation of excitatory glutamatergic neurotransmission,” the authors write. These data are consistent, they say, with the glutamatergic hypothesis of schizophrenia, and studies implicating several other genes involved in glutamatergic signaling in the disease.

Both reports suggest potential new avenues for treatment, including restoring oligodendrocyte function by augmenting the NRG1/ErbB4 signaling pathway, or normalizing NMDA receptor phosphorylation and function.—Pat McCaffrey.

References:
Roy K, Murtie JC, El-Khodor BF, Edgar N, Sardi SP, Hooks BM, Benoit-Marand M, Chen C, Moore H, O'Donnell P, Brunner D, Corfas G. Loss of erbB signaling in oligodendrocytes alters myelin and dopaminergic function, a potential mechanism for neuropsychiatric disorders. 2007 May 1; PNAS Early Edition.

Bjarnadottir et al. Neuregulin1 (NRG1) signaling through Fyn modulates NMDA receptor phosphorylation: Differential synaptic function in NRG+/- knock-outs compared with wild-type mice. J. Neurosci. 2007 April 25; 27:4519-4529. Abstract

Comments on News and Primary Papers
Comment by:  Daniel StewartKenneth Davis
Submitted 3 May 2007
Posted 3 May 2007

Comment by Daniel Stewart and Kenneth Davis
The Corfas results are intriguing. Their findings confirm much of what we have either found or suspect in schizophrenia relating to white matter involvement. Demonstrations of OLIG2 interactions with ErbB4 in the cortex and with CNP in the striatum in schizophrenia from our team (Georgieva et al., 2006) fit well with this investigation in providing evidence for a link between a variety of potential etiologic oligodendrocyte-related mechanisms in schizophrenia. While in our study, we did not find interaction with NRG1 and OLIG2, it is important to note that differential expression of NRG1 might be found only at certain points in the timeline of disease development. Other recent support from our team for white matter involvement in schizophrenia comes from an investigation in which an SNP associated with CNP was found to be significantly correlated with schizophrenia (Peirce et al., 2006). Interestingly, Corfas’s group reports that when ErbB signaling is abolished in oligodendrocytes, myelin structure appears normal, but the myelin sheath is significantly thinner. This is in line with some of the ultrastructural findings of Uranova’s group and in rodent studies looking at MAG-deficient mice (both reviewed in Davis et al., 2003)—another downregulated myelin-related gene found in brains of schizophrenia patients.

Reductions in oligodendrocyte number on the order of 20 percent have been demonstrated in the brains of schizophrenia patients (Hof et al., 2002). Although this finding does not precisely parallel the findings in this investigation, the authors’ adroitly point out that this may be because the abnormalities they induced were during early oligodendrocyte and myelin expression, while it is possible that the abnormalities seen in the brains of schizophrenia patients occur relatively later in development, more likely during the second large wave of cortical myelination at the end of the second decade of life. The authors also point out that “defects in ErbB signaling in different cell types may contribute to different aspects of psychiatric symptoms.” This might also be the case in schizophrenia, giving rise to the myriad presentations of the disease, as might the fact that expression of both NRG1 and ErbB4 are susceptible to environmental insult.

Other important similarities between the authors’ findings and schizophrenia include that, even in light of these myelin abnormalities, gross brain volumes, as well as several other measures, remained normal. This buttresses the idea that in schizophrenia, myelin abnormalities might be at the root of the often unimpressive brain changes noted in schizophrenia on gross structural imaging. And finally, although speculative, the authors do note an intriguing set of behavioral abnormalities, some of which could mimic the social isolation and poor relatedness of schizophrenia, which is particularly remarkable given the increased susceptibility to amphetamines and the trends seen in DAT, D1, and D2 expression in this investigation.

Corfas’s findings are indeed exciting, and we commend his team on an eloquently designed and implemented investigation.

View all comments by Daniel Stewart
View all comments by Kenneth DavisComment by:  Akira Sawa, SRF Advisor
Submitted 4 May 2007
Posted 4 May 2007

Neuregulin1 (NRG1) is the most promising risk factor for schizophrenia, and the study of the signaling of NRG1 and its receptor ErbB4 is very important in understanding the pathophysiology of the disease. Like other promising risk factors for schizophrenia, NRG1/ErbB4 is multifunctional with many molecular isoforms. NRG1/ErbB signaling plays a role both before and after birth. Furthermore, ErbB4 is expressed not only in neurons but also in other types of cells, such as oligodendrocytes.

To address context-dependent functions one by one, dominant-negative transgenic mice can be very useful. The advantage of dominant-negative transgenics is that we can knock down the endogenous function of our target molecules (in this work, ErbB4) in a temporally and spatially specific manner by utilizing a well-characterized promoter. In this outstanding study by Corfas and colleagues, they used the CNP promoter that confirms dominant-negative ErbB4 selectively in oligodendrocytes (but not in astrocytes and neurons) only after birth. This approach will be very useful in schizophrenia research.

The remarkable finding is that they observed alterations in dopamine-mediated neuronal networks and associated behaviors by disturbing NRG1/ErbB4 selectively in cells of oligodendrocyte lineage. Three important paradigms for schizophrenia (white matter pathology, dopamine, and a susceptibility gene) converge in this paper, and in this sense, I find it very exciting.

View all comments by Akira SawaComment by:  Mary Reid
Submitted 3 May 2007
Posted 5 May 2007

Does the effect of NRG1/ErbB4 signaling on myelination occur downstream of purinergic signaling? Fields suggests that adenosine is of primary importance in regulating early development of OPCs, where it stimulates differentiation and myelination (Fields, 2006). It's of interest that cAMP stimulates expression of neuregulin and cAMP levels in the lung are decreased in A2A adenosine receptor (22q11.2)-deficient mice (Tokita et al., 2001; Nadeem et al., 2007). Do you see reduced neuregulin levels in 22q11 deletion syndrome? Of particular interest is the study by Desai and colleagues reporting that signaling via the adenosine A2A receptor downregulates thrombospondin 1 (Desai et al., 2005). Perhaps overexpression of thrombospondin 1 may help explain the occular abnormalities in this syndrome (Wu et al., 2006; Forbes et al., 2007; Stalmans, 2005). Thrombospondins are also involved in synaptogenesis (Christopherson et al., 2005).

References:

Fields RD. Nerve impulses regulate myelination through purinergic signalling. Novartis Found Symp. 2006;276:148-58; discussion 158-61, 233-7, 275-81.

Tokita Y, Keino H, Matsui F, Aono S, Ishiguro H, Higashiyama S, Oohira A. Regulation of neuregulin expression in the injured rat brain and cultured astrocytes. J Neurosci. 2001 Feb 15;21(4):1257-64.

Nadeem A, Fan M, Ansari HR, Ledent C, Mustafa SJ. Enhanced airway reactivity and inflammation in A2A adenosine receptor deficient allergic mice. Am J Physiol Lung Cell Mol Physiol. 2007 Feb 9; [Epub ahead of print]

Desai A, Victor-Vega C, Gadangi S, Montesinos MC, Chu CC, Cronstein BN. Adenosine A2A receptor stimulation increases angiogenesis by down-regulating production of the antiangiogenic matrix protein thrombospondin 1. Mol Pharmacol. 2005 May;67(5):1406-13. Epub 2005 Jan 26. Comment in: Mol Pharmacol. 2005 May;67(5):1385-7.

Wu Z, Wang S, Sorenson CM, Sheibani N. Attenuation of retinal vascular development and neovascularization in transgenic mice over-expressing thrombospondin-1 in the lens. Dev Dyn. 2006 Jul;235(7):1908-20.

Forbes BJ, Binenbaum G, Edmond JC, Delarato N, McDonald-McGinn DM, Zackai EH. Ocular findings in the chromosome 22q11.2 deletion syndrome. J AAPOS. 2007 Apr;11(2):179-182. Epub 2006 Nov 30.

Stalmans I. Role of the vascular endothelial growth factor isoforms in retinal angiogenesis and DiGeorge syndrome. Verh K Acad Geneeskd Belg. 2005;67(4):229-76.

Christopherson KS, Ullian EM, Stokes CC, Mullowney CE, Hell JW, Agah A, Lawler J, Mosher DF, Bornstein P, Barres BA. Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis. Cell. 2005 Feb 11;120(3):421-33. Comment in: Cell. 2005 Feb 11;120(3):292-3.

View all comments by Mary ReidComment by:  Patricia Estani
Submitted 6 May 2007
Posted 6 May 2007
  I recommend the Primary Papers

Comments on Related News


Related News: CNP Findings Strengthen Oligodendrocyte Link to Schizophrenia

Comment by:  Hans W. Moises
Submitted 24 January 2006
Posted 24 January 2006
  I recommend the Primary Papers

This is another important study supporting the glial growth factors deficiency and synaptic destabilization hypothesis of schizophrenia we proposed in 2002 (Moises et al., 2002). The glial synaptic destabilization hypothesis is based on the landmark 1997 paper by Pfrieger and Barres and the tripartite synapse model suggested by Philip Haydon and coworkers (Araque et al., 1999; Pascual et al., 2005). In reference to its underlying principle, the glial growth factors deficiency and synaptic destabilization hypothesis might also more conveniently and briefly be designated as the weakened tripartite-synapse hypothesis of schizophrenia.

References:
Moises HW, Zoega T, Gottesman II. The glial growth factors deficiency and synaptic destabilization hypothesis of schizophrenia. BMC Psychiatry. 2002;2:8. Abstract

Moises HW, Gottesman II. Does glial asthenia predispose to schizophrenia? Arch Gen Psychiatry 2004; 61:1170. Abstract

Pfrieger FW, Barres BA. Synaptic efficacy enhanced by glial cells in vitro. Science. 1997;277:1684-7. Abstract

Araque A, Parpura V, Sanzgiri RP, Haydon PG. Tripartite synapses: glia, the unacknowledged partner. Trends Neurosci. 1999; 22:208-15. Abstract

Pascual O, Casper KB, Kubera C, Zhang J, Revilla-Sanchez R, Sul JY, Takano H, Moss SJ, McCarthy K, Haydon PG. Astrocytic purinergic signaling coordinates synaptic networks. Science 2005; 310: 113-6. Abstract

View all comments by Hans W. Moises

Related News: CNP Findings Strengthen Oligodendrocyte Link to Schizophrenia

Comment by:  Daniel StewartKenneth Davis
Submitted 31 January 2006
Posted 31 January 2006

Peirce's paper is an exciting addition to the white matter hypothesis in schizophrenia. (Note: many of the authors of this paper are colleagues of ours at the Conte Center investigating white matter in schizophrenia at Mount Sinai.) As noted in the news story, findings from a number of different areas are beginning to come together in support of the white matter hypothesis in schizophrenia. Genetic findings in myelin-related genes, as outlined and referenced above, are demonstrating increased susceptibility to schizophrenia. Imaging findings from diffusion tensor studies are demonstrating abnormalities across multiple brain areas (reviewed in Kubicki et al., 2005), with more recent studies showing that specific white matter tracts are not only abnormal in schizophrenia, but are associated with symptomatology and cognitive deficits (Kubicki et al., 2002; Kubicki et al., 2003; Nestor et al., 2004). Postmortem examination is revealing that oligodendrocytes are decreased in number and abnormally spaced in patients with schizophrenia (Hof et al., 2002; Hof et al., 2003). These converging data argue strongly for the involvement of myelin, oligodendrocytes, and white matter in schizophrenia.

We continue to examine various aspects of white matter involvement in schizophrenia with the hope of providing both translational data (i.e., the relationship between symptom severity or independent living and white matter coherence) and further basic science data that may shed some light on upstream events that contribute to myelin and oligodendrocyte deficits. These new data by the Owen and O'Donovan group are a valuable contribution.

References:
Hof PR, Haroutunian V, Copland C, Davis KL, Buxbaum JD. Molecular and cellular evidence for an oligodendrocyte abnormality in schizophrenia. Neurochem Res. 2002 Oct;27(10):1193-200. Abstract

Hof PR, Haroutunian V, Friedrich VL Jr, Byne W, Buitron C, Perl DP, Davis KL. Loss and altered spatial distribution of oligodendrocytes in the superior frontal gyrus in schizophrenia. Biol Psychiatry. 2003 Jun 15;53(12):1075-85. Abstract

Kubicki M, McCarley R, Westin CF, Park HJ, Maier S, Kikinis R, Jolesz FA, Shenton ME. A review of diffusion tensor imaging studies in schizophrenia. J Psychiatr Res. 2005 Jul 13; [Epub ahead of print] Abstract

Kubicki M, Westin CF, Maier SE, Frumin M, Nestor PG, Salisbury DF, Kikinis R, Jolesz FA, McCarley RW, Shenton ME. Uncinate fasciculus findings in schizophrenia: a magnetic resonance diffusion tensor imaging study. Am J Psychiatry. 2002 May;159(5):813-20. Abstract

Kubicki M, Westin CF, Nestor PG, Wible CG, Frumin M, Maier SE, Kikinis R, Jolesz FA, McCarley RW, Shenton ME. Cingulate fasciculus integrity disruption in schizophrenia: a magnetic resonance diffusion tensor imaging study. Biol Psychiatry. 2003 Dec 1;54(11):1171-80. Erratum in: Biol Psychiatry. 2004 Mar 15;55(6):661. Abstract

Nestor PG, Kubicki M, Gurrera RJ, Niznikiewicz M, Frumin M, McCarley RW, Shenton ME. Neuropsychological correlates of diffusion tensor imaging in schizophrenia. Neuropsychology. 2004 Oct;18(4):629-37. Abstract

View all comments by Daniel Stewart
View all comments by Kenneth Davis

Related News: CNP Findings Strengthen Oligodendrocyte Link to Schizophrenia

Comment by:  William Honer
Submitted 4 March 2006
Posted 5 March 2006
  I recommend the Primary Papers

The Peirce et al. paper represents an important contribution to understanding the possible mechanisms through which genetic risk factors could contribute to the pathophysiology of schizophrenia. Studies of SNPs in candidate genes for schizophrenia are most clearly related to mechanism when the SNP changes amino acid sequence (rarely), or when the SNP changes mRNA expression (commonly postulated, but less often demonstrated). Studies combining SNP and mRNA analyses are challenging, and Peirce et al. provide a novel approach—by measuring the relative amount of mRNA expressed from the variant and the wild-type alleles in brain tissue from heterozygotes. They demonstrated relatively reduced expression from the variant allele. It must be noted however, that these studies were carried out in brain tissue from individuals described as being “free from psychiatric or neurological disorder at time of death” (not schizophrenia samples as suggested by the SRF news story [Editor's note: since corrected]), and the total expression of CNP mRNA was not determined. While CNP mRNA expression is reported to be lower in schizophrenia, and Peirce et al. demonstrate the variant allele is a risk factor for schizophrenia in studies of genetic association, it remains uncertain to what extent the lower CNP mRNA expression in schizophrenia is related to genetic variation or to other factors. CNP mRNA differences in expression between schizophrenia and control samples appear to be of different magnitude in different brain regions from the same cases (Katsel et al., 2005). This could represent non-genetic effects. However, genetic variation in CNP could also be more or less likely to be expressed in different brain regions. In this regard, the samples used in the Peirce et al. study were mixed, coming from frontal, parietal, or temporal cortex. Studies with larger sample sizes, and of schizophrenia as well as control tissues, will be needed to test these possibilities.

View all comments by William Honer

Related News: Neuregulin, ErbB4—Levels Normal but Signaling Strengthened in Schizophrenia

Comment by:  Patricia Estani
Submitted 22 June 2006
Posted 22 June 2006
  I recommend the Primary Papers

Related News: OLIG2 Gene Supports Notion of Myelin Abnormalities in Schizophrenia

Comment by:  William Honer
Submitted 4 August 2006
Posted 4 August 2006

This paper demonstrates several important shifts in research strategies for schizophrenia. Many previous studies of candidate genes in the illness have chosen their targets based on concepts of the mechanism of action of antipsychotic drugs, or by virtue of the proximity of a gene to a genetic linkage site defined with anonymous markers. The choice of candidate gene here is based on a wide range of neurobiological evidence, including studies of gene expression and protein levels. As well, the authors do not limit their study to one gene; instead, they expand their investigation to include plausibly interacting gene targets. Analysis of complex disorders will likely need more than simple models, and the approach here is worth noting.

The gap still remains between the DNA-mRNA approaches and protein analysis. Gene expression is one factor determining mRNA levels. However, especially in human brain tissue samples, many other antemortem and postmortem factors contribute to the measured level of mRNA. The meaning of gene expression measures obtained for oligodendrocyte/myelination-related genes from samples comprising largely gray matter is not entirely certain. The role of oligodendrocytes in gray matter may deserve more attention.

The genetic evidence presented here for an interaction between OLIG2 and ErbB4 is intriguing. A recent paper from Steve Arnold’s group indicated the neuregulin-1−ErbB4 signaling pathway appears to be overactive in schizophrenia, with consequences for NMDA receptor function (Hahn et al., 2006). Although the analysis in their paper focused on neurons, if a similarly overactive pathway was operative in oligodendrocytes, inhibition of myelination might be a predicted outcome (Sussman et al., 2005), with downregulation of a host of oligodendrocyte/myelination-related genes as a consequence. Of further interest, NMDA receptors may have important roles in oligodendrocytes as well as in neurons (Matute, 2006).

References:

Hahn CG, Wang HY, Cho DS, Talbot K, Gur RE, Berrettini WH, Bakshi K, Kamins J, Borgmann-Winter KE, Siegel SJ, Gallop RJ, Arnold SE. Altered neuregulin 1-erbB4 signaling contributes to NMDA> receptor hypofunction in schizophrenia. Nat Med. 2006 Jul;12(7):824-8. Epub 2006 Jun 11. Abstract

Sussman CR, Vartanian T, Miller RH. The ErbB4 neuregulin receptor mediates suppression of oligodendrocyte maturation. J Neurosci. 2005 Jun 15;25(24):5757-62. Abstract

Matute C. Oligodendrocyte NMDA receptors: a novel therapeutic target. Trends Mol Med. 2006 Jul;12(7):289-92. Epub 2006 Jun 5. Abstract

View all comments by William Honer

Related News: Neuregulin, ErbB4—Levels Normal but Signaling Strengthened in Schizophrenia

Comment by:  Cynthia Shannon Weickert, SRF AdvisorVictor Chong
Submitted 8 August 2006
Posted 8 August 2006

In contrast to its once barren form, the table of potential causative genes for schizophrenia is now stocked to feast level (Straub and Weinberger, 2006). In keeping with the culinary theme, we suggest that this recent paper by Chang-Gyu Hahn and Hoau-Yan Wang is “a full course meal”!

Appetizer: An Important Biological Problem
If one assumes that alterations in NRG-1 account for at least some of the liability to developing schizophrenia, then it is only reasonable to look to the NRG-1 receptors for clues as to how and where NRG-1 may be acting. However, there are three known NRG-1 receptors that mediate a myriad of biological functions, almost all of which could be argued to be relevant to schizophrenia pathology. This paper draws our attention to one NRG-1 receptor, ErbB4, showing this receptor to be a probable NRG-1 partner in mediating this pathology. Recent studies provide further support that ErbB4 may be integral to the development of schizophrenia by demonstrating its gene to be a potential susceptibility gene (Norton et al., 2006; Silberberg et al., 2006; Nicodemus et al., in press). So, genetic and neurobiological evidence suggest the authors selected their NRG-1 receptor wisely.

Main Course: A New Approach
The novel postmortem-stimulation approach used by Hahn and colleagues represents an important advance in the field of schizophrenia research. Through extensive validation of this protocol, this research group has paved the way for future experimentation into the molecular activation of proteins within the schizophrenic brain. More specifically, while previous studies have only been able to draw conclusions about the static state of the schizophrenic brain, this article has introduced a novel method for examining dynamic signaling systems in postmortem brains of patients with schizophrenia. For example, based on the finding that certain splice variant ErbB4 mRNAs are elevated in the prefrontal cortex of these individuals (Silberberg et al., 2006), one would assume that ErbB4 protein should also be elevated in these patients. But Hahn et al. demonstrate that schizophrenic individuals show only marginal increases in prefrontal cortical ErbB4 protein levels, which could suggest that ErbB4 protein plays little role in the pathology of schizophrenia. However, using the more dynamic postmortem-stimulation approach, the authors showed that ErbB4 signaling is, in fact, greatly enhanced in the prefrontal cortex of patients with this disease, leading to the alternative interpretation that ErbB4 protein may play significant roles in schizophrenia. In other words, this postmortem-stimulation protocol extends the examination of human postmortem brain protein from quantification to the functional level. We view this method as a powerful approach that will be important in translating genetic susceptibility into molecular mechanisms of the disease process. The postmortem-stimulation approach also gave rise to the observation that schizophrenic patients exhibit reduced prefrontal cortical NMDA receptor signaling capacity. This finding is highly significant because it is the first evidence directly linking reduced prefrontal cortical NMDA receptor function to schizophrenia. However, whether NRG-1-ErbB4 signaling is a major contributor to NMDA receptor hypofunction is debatable since the attenuation of NMDA receptor phosphorylation by NRG-1 appears proportionally similar between controls and schizophrenic patients.

Side Dish: Dealing with Antipsychotic Drugs
Since most patients with schizophrenia have received antipsychotic drugs and these agents can have profound impact on brain systems, it is essential to determine whether changes observed in the brains of patients with schizophrenia are secondary to antipsychotic drug exposure. To address this issue, the authors took two important steps. Firstly, Hahn et al. examined whether antipsychotic drug exposure affected prefrontal cortical ErbB4 expression or signaling in their human study group and found no correlation between antipsychotic drug treatment and either of these measured variables. Secondly, the authors examined antipsychotic drug effects on prefrontal cortical ErbB4 signaling in mice implanted with a haloperidol-containing bioabsorbable polymer, which has a number of advantages. For example, it allows for long-term treatment of the animals (12 weeks) while minimizing handling. This duration of exposure is arguably more appropriate than some schedules used to examine chronic effects of antipsychotic drugs in rodents. Remarkably, haloperidol treatment caused a reduction in ErbB4 signaling in the mice, suggesting that a decrease in ErbB4 signaling is associated with the therapeutic effects of antipsychotic agents. What may have been more informative is to show whether haloperidol had any effect on ErbB4 protein levels without NRG-1 treatment. In addition, the authors could have considered examining antipsychotic drug effect in mice whose ages were more reflective of those of the investigated human cohort, which consisted of elderly individuals (65-92 years). Furthermore, while their analysis of antipsychotic drugs on ErbB4 expression and signaling in postmortem brain was noteworthy, the authors only examined the effects of antipsychotic drugs taken in the final month before death in a very aged sample population. Thus, it is difficult to ascertain whether ErbB4 expression or signaling is not affected by lifetime antipsychotic drug treatment, which can result in cellular and molecular consequences that can remain long after termination of therapy.

Dessert: Challenging the Field
Of course, the first thing the field needs to do is attempt to replicate these findings in another cohort of patients with schizophrenia compared to controls. Careful attention to matching for age, PMI, and gender, etc., as was done in this study, is critical. We suggest that using a young cohort of patients would help rule out potential confounds such as associated dementia and interaction with the aging process. However, it is recognized that many other potential confounds will still remain in most studies comparing schizophrenics to unaffected controls. These confounds include suffering from years of an unremitting illness that compromises normal social and environmental stimulation, increased incidence of cigarette smoking among patients with schizophrenia, and years of antipsychotic drug exposure. When the finding of schizophrenia-associated increased ErbB4 signaling capacity is replicated, then the task at hand will be to determine how possible genetic changes in the DNA at the NRG-1 or ErbB4 locus (representing one etiological route) could lead to a “hyperactivatable” ErbB4.

Doggie Bag: Nagging Questions
One of the caveats we would like to raise in attempting to link molecular neurobiological changes found in schizophrenic brain tissue with possible changes in DNA is that causative variants in any one susceptibility gene are expected to occur only in a minority of schizophrenic patients. Most measures performed on postmortem schizophrenic brains are made on small sample sizes, which likely show much heterogeneity in terms of etiology. In other words, only a handful of patients in this study would be expected to have a faulty NRG-1 gene; yet this subpopulation shows alterations in ErbB4 signaling as a group. The logical extension of this observation may be that there are multiple routes by which ErbB4 could be “hyperactivatable” (i.e., not solely through NRG-1 genetic liability). To sort this out, we need to work from the gene forward, and thus there is a need to identify causative variants in susceptibility genes and to use these as starting points for basic mechanistic molecular and cellular studies.

View all comments by Cynthia Shannon Weickert
View all comments by Victor Chong

Related News: OLIG2 Gene Supports Notion of Myelin Abnormalities in Schizophrenia

Comment by:  Patricia Estani
Submitted 22 August 2006
Posted 23 August 2006
  I recommend the Primary Papers

Related News: Down to BACE-ics—Old Mouse a New Model for Schizophrenia?

Comment by:  Victor ChongCynthia Shannon Weickert (SRF Advisor)
Submitted 23 May 2008
Posted 23 May 2008

The findings of Savonenko et al. (2008) are an impressive addition to the growing evidence supporting a role for neuregulin-1 (NRG1) in schizophrenia pathology. The authors not only revealed a novel relationship between schizophrenia-like behavior and the loss of BACE1 proteolytic function, but also showed that this association results from disruption of BACE1-mediated NRG1 cleavage. These observations support the notion that aberrant processing of NRG1 may contribute to the development of schizophrenia-like phenotypes, providing a basis for examining other NRG1-cleaving pathways in the context of schizophrenia. Savonenko et al. were thorough in their behavioral assessment of the BACE1 mutant mice, convincingly showing that these animals exhibit schizophrenia-related behaviors that could be exacerbated by psychostimulants and improved by antipsychotic drug treatment.

What remains unclear, however, is the relationship between the NRG1/ErbB4 protein findings in the BACE1 mutant mouse brain and those previously reported in the schizophrenic human brain. For example, the authors reported reductions in ErbB4-PSD95 coupling in the BACE1 mutant mouse, whereas Hahn et al. (2006) demonstrated increased ErbB4-PSD95 interaction in the prefrontal cortices of schizophrenic patients. In addition, our recent investigation found elevated prefrontal cortical levels of both NRG1 C-terminal fragment (ICD) and full-length ErbB4 protein in schizophrenic subjects (Chong et al., 2008), while Savonenko et al. showed decreased NRG1 C-terminal fragment levels with no alterations in ErbB4 protein in the BACE1 mutant mouse cortex. On the other hand, the lack of variations in overall cortical ErbB4 in these mice may correspond to the findings of Hahn et al. (2006) who reported no alterations in prefrontal cortical ErbB4 protein levels in schizophrenic subjects.

These seemingly conflicting results could suggest that any imbalance in cortical NRG1 signaling, whether increased or diminished, may lead to schizophrenia. Indeed, studies have suggested that improper tuning of other cortical signaling systems, particularly those of dopamine, can contribute to cognitive deficits associated with this disease (Vijayraghavan et. al, 2007). Optimal synaptic function may display “inverted-U” shaped response to NRG1-ErbB4 activity as proposed by Role and Talmage (2007). Alternatively, the authors speculated that some of the discrepancies between the findings in the BACE1 mutant mice and those observed in the schizophrenic humans may be due to differences in the duration of NRG1 signaling modification between the animals and the patients, who had a lifetime of mental illness. One way to examine the validity of this suggestion is to look at cortical ErbB4-PSD95 coupling and NRG1/ErbB4 protein levels in the BACE1 mutant mice at different developmental and adult time points. This approach could test whether these animals at later stages in life display alterations in cortical ErbB4-PSD95 interactions and/or in NRG1/ErbB4 protein levels comparable to those seen in schizophrenic subjects of the human studies, which primarily consisted of adults beyond middle age. Also of interest would be to create NRG1 and ErbB4 gain-of-function mutants where the timing of over-expression could be controlled.

Given the significance of NRG1 signaling/cleavage in the BACE1 mutant mouse schizophrenia-like phenotypes, it may also be important to consider pathways leading to changes in ErbB4 C-terminal fragment levels in schizophrenia etiology. A recent paper by Walsh et al. (2008) demonstrated that at least one schizophrenic patient in their study has a gene deletion encompassing the C-terminal intracellular kinase domain of ErbB4, and we have found decreases in ErbB4 C-terminal fragments relative to full-length ErbB4 in the frontal cortex of schizophrenic subjects (Chong et al., 2008). These observations together with those of Savonenko et al. raise interesting questions regarding how molecular alterations in NRG1 signaling and cleavage may impact ErbB4 signaling and cleavage and whether changes in NRG1 and/or ErbB4 could be primary or secondary to the schizophrenia disease process.

In summary, Savonenko et al. have provided a novel avenue to probe NRG1 function and processing in relation to schizophrenia pathology. They have also introduced BACE1 as a potentially important schizophrenia susceptibility molecule that to our knowledge has not been directly investigated in subjects with schizophrenia and may be worth studying in the brain tissues of these patients. In addition, it would be interesting to examine how the schizophrenia-related traits of the BACE1 mutant mice compare with those of other NRG1 mutant mice such as the heterozygous NRG1 transmembrane knock-out mice (Stefansson et al., 2002). Such an investigation could provide insight into whether similar NRG1 signaling deficiencies underlie the schizophrenia-like phenotypes of these animal models.

References:

Hahn CG, Wang HY, Cho DS, Talbot K, Gur RE, Berrettini WH, Bakshi K, Kamins J, Borgmann-Winter KE, Siegel SJ, Gallop RJ, Arnold SE. (2006) Altered neuregulin 1-erbB4 signaling contributes to NMDA receptor hypofunction in schizophrenia. Nat Med. 12:824-8. Abstract

Chong VZ, Thompson M, Beltaifa S, Webster MJ, Law AJ, Weickert CS. (2008) Elevated neuregulin-1 and ErbB4 protein in the prefrontal cortex of schizophrenic patients. Schizophr Res. 100:270-80. Abstract

Vijayraghavan S, Wang M, Birnbaum SG, Williams GV, Arnsten AF. (2007) Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory. Nat Neurosci. 10:376-84. Abstract

Role LW, Talmage DA (2007) Neurobiology: new order for thought disorders. Nature. 448:263-5. Abstract

Walsh T, McClellan JM, McCarthy SE, Addington AM, Pierce SB, Cooper GM, Nord AS, Kusenda M, Malhotra D, Bhandari A, Stray SM, Rippey CF, Roccanova P, Makarov V, Lakshmi B, Findling RL, Sikich L, Stromberg T, Merriman B, Gogtay N, Butler P, Eckstrand K, Noory L, Gochman P, Long R, Chen Z, Davis S, Baker C, Eichler EE, Meltzer PS, Nelson SF, Singleton AB, Lee MK, Rapoport JL, King MC, Sebat J. (2008) Rare structural variants disrupt multiple genes in neurodevelopmental pathways in schizophrenia. Science. 320:539-43. Abstract

Stefansson H, Sigurdsson E, Steinthorsdottir V, Bjornsdottir S, Sigmundsson T, Ghosh S, Brynjolfsson J, Gunnarsdottir S, Ivarsson O, Chou TT, Hjaltason O, Birgisdottir B, Jonsson H, Gudnadottir VG, Gudmundsdottir E, Bjornsson A, Ingvarsson B, Ingason A, Sigfusson S, Hardardottir H, Harvey RP, Lai D, Zhou M, Brunner D, Mutel V, Gonzalo A, Lemke G, Sainz J, Johannesson G, Andresson T, Gudbjartsson D, Manolescu A, Frigge ML, Gurney ME, Kong A, Gulcher JR, Petursson H, Stefansson K. (2002) Neuregulin 1 and susceptibility to schizophrenia. Am J Hum Genet. 71:877-92. Abstract

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Related News: Mice Dispense With Neuregulin/ErbB Pathway in CNS Myelination

Comment by:  David Talmage
Submitted 22 September 2008
Posted 22 September 2008

To the extent that animal models can represent the human condition, this paper argues against a direct connection between changes in Nrg1/ErbB function and the glial hypothesis that is based on postmortem evidence implicating Nrg1/ErbB3 signaling in the etiology of schizophrenia. However, there is a clear difference from the results in this paper with both data from Taveggia, Salzer et al. (on CNS myelination in Nrg1 heterozygotes) and Corfas and colleagues (in animals in which ErbB function is blocked in oligodendrocytes) that needs resolution before any real conclusion can be made. At this point, I think the basic conclusion is that we have a lot more precise (at the cellular level) work to do before we can make really strong predictions on which disease-associated phenotypes relate to disruptions in normal Nrg1/ErbB signaling.

View all comments by David Talmage