As both NRG1 and ErbB4 have been linked to schizophrenia, the biological function of this signaling pathway in the CNS has been an area of intensive study. Recent studies indicate that NRG1-ErbB4 signaling may play an important role in the development of glutamatergic synapses (e.g., Fazzari et al., 2010). Since ErbB4 is highly expressed in a subset of GABAergic interneurons in the cortex and hippocampus, including parvalbumin (PV) positive interneurons, it is important to understand the role of NRG1-ErbB4 in synapse development specifically in GABAergic interneurons.

The recent study by Ting et al., a group led by Dr. Lin Mei in the Medical College of Georgia, provides new evidence that the NRG1-ErbB4 signaling pathway plays an important role in regulating the development and function of excitatory synapses in cortical and hippocampal GABAergic interneurons. By applying NRG1 to cultured neurons, they found that NRG1 increases the frequency and amplitude of miniature EPSCs, which is accompanied by an increase in the number...  Read more

As both NRG1 and ErbB4 have been linked to schizophrenia, the biological function of this signaling pathway in the CNS has been an area of intensive study. Recent studies indicate that NRG1-ErbB4 signaling may play an important role in the development of glutamatergic synapses (e.g., Fazzari et al., 2010). Since ErbB4 is highly expressed in a subset of GABAergic interneurons in the cortex and hippocampus, including parvalbumin (PV) positive interneurons, it is important to understand the role of NRG1-ErbB4 in synapse development specifically in GABAergic interneurons.

The recent study by Ting et al., a group led by Dr. Lin Mei in the Medical College of Georgia, provides new evidence that the NRG1-ErbB4 signaling pathway plays an important role in regulating the development and function of excitatory synapses in cortical and hippocampal GABAergic interneurons. By applying NRG1 to cultured neurons, they found that NRG1 increases the frequency and amplitude of miniature EPSCs, which is accompanied by an increase in the number and size of PSD-95 punctas, a marker of excitatory synapses. Furthermore, deletion of ErbB4 selectively in PV positive neurons causes a reduction of mEPSC frequency and amplitude in these neurons.

This study confirms and extends the major findings in the Fazzari et al., 2010 study, and they together make a strong case that NRG1-ErbB4 signaling is critical in regulating excitatory synapse maturation in GABAergic interneurons, and also suggest that NRG1-ErbB4 may regulate the wiring of cortical microcircuits during development. Given that both glutamatergic synapses and cortical GABAergic interneurons are thought to be involved in the pathophysiology of schizophrenia, the new findings by Ting and colleagues provide additional information for understanding the possible link between NRG1/ErbB4, synapses, and schizophrenia.

The study by Pitcher and colleagues incorporates a series of well-designed in-vitro experiments aimed at investigating the proposed link between NRG1-ErbB4 signaling, NMDAR activity, and synaptic function. Using wild-type and ErbB4 mutant mice (ErbB4-/-Her4^heart), plus pharmacological inhibition of ErbB receptors, the study demonstrates that NRG1 signaling, via ErbB4, suppresses enhancement of NMDA receptors (NMDAR) via inhibition of the Src kinase, Src. Consistent with previous studies (see references 32-37 of Pitcher et al.), the authors demonstrate that NRG1 prevents induction of theta burst-induced LTP (tbLTP), and provide novel data to suggest that this requires Src. Mechanistically, the authors demonstrate that NRG1 is inhibitory to NMDAR EPSCs via blockade of Src. Activation of Src, via EPQ stimulation, results in increased NMDAR EPSC amplitudes; however, pretreatment of slices with soluble NRG1 prior to EPQ blocks the effects of EPQ on NMDAR EPSCs. Convincingly, the authors show that ErbB4 genetic deletion prevents the inhibitory effects of NRG1 on EPQ...  Read more

The study by Pitcher and colleagues incorporates a series of well-designed in-vitro experiments aimed at investigating the proposed link between NRG1-ErbB4 signaling, NMDAR activity, and synaptic function. Using wild-type and ErbB4 mutant mice (ErbB4-/-Her4^heart), plus pharmacological inhibition of ErbB receptors, the study demonstrates that NRG1 signaling, via ErbB4, suppresses enhancement of NMDA receptors (NMDAR) via inhibition of the Src kinase, Src. Consistent with previous studies (see references 32-37 of Pitcher et al.), the authors demonstrate that NRG1 prevents induction of theta burst-induced LTP (tbLTP), and provide novel data to suggest that this requires Src. Mechanistically, the authors demonstrate that NRG1 is inhibitory to NMDAR EPSCs via blockade of Src. Activation of Src, via EPQ stimulation, results in increased NMDAR EPSC amplitudes; however, pretreatment of slices with soluble NRG1 prior to EPQ blocks the effects of EPQ on NMDAR EPSCs. Convincingly, the authors show that ErbB4 genetic deletion prevents the inhibitory effects of NRG1 on EPQ induced NMDAR EPSCs, suggesting that ErbB4 is necessary for suppression of Src-mediated enhancement of synaptic NMDAR currents. Additional data suggest that NRG1 inhibitory effects on NMDAR activity, via Src, are mediated via inhibition of NR2B phosphorylation. However, no effect of NRG1 on general NMDAR function was observed. The sum conclusion of the authors is that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of NMDAR function.

Overall, this is an important paper, and it is noteworthy that this is not the first demonstration of a mechanistic link between NRG1-ErbB4 signaling, the Src family of kinases, NR2B phosphorylation, and synaptic function (see Bjarnadottir et al., 2007). Src kinases are a large family of protein tyrosine kinases that include Src, Fyn, Yes, Lck, Lyn, Hck, Fgr, and Blk. In 2007, Bjarnadottir and colleagues reported that ErbB4 directly interacts with the Src-kinase Fyn in hippocampal neurons, and that stimulation of cells with NRG1 results in the formation of a Fyn-ErbB4 complex. However, in contrast to the findings of Pitcher et al., Bjarnadottir et al. identified that NRG1, via ErbB4, “activates” Fyn (a finding recently confirmed by Cahill et al., 2011), and that this activation results in NR2B phosphorylation. Bjarnadottir et al. also demonstrated that NRG1+/- mutant mice have reduced tbLTP, and that NRG1+/- and ErbB4+/- mutants have reduced NR2B phosphorylation, suggesting that attenuated (rather than increased) NRG1 signaling results in NMDAR hypofunction and schizophrenia-like behaviors in rodents.

At present, it is unclear how the findings of these studies reconcile. Several possible explanations exist for the apparent inconsistencies relating to NRG1-mediated regulation of NMDAR function. First, context-dependent effects of NRG1-ErbB4 signaling on specific Src-family kinase protein members (Fyn vs. Src activation) may occur. Nevertheless, all kinase-mediated effects would be expected to be altered in NRG1/ErbB4 mutant mice. Second, an “inverted U” model of NRG1-mediated signaling, i.e., too much or too little NRG1 results in NMDAR hypofunction, may account for the observed differences. This model has been previously suggested for NRG1-mediated regulation of LTP (see Role and Talmage, 2007), but further work is needed to determine if this applies to the direct regulation of NMDAR-mediated synaptic function. Third, experimental particulars related to in-vitro versus in-vivo study of NRG1 signaling (i.e., exogenous application vs. transgenic manipulation of NRG1, in-vitro physiologically relevant doses of NRG1, and differential effects of NRG1 on NMDAR phosphorylation depending on the type of stimulus used—i.e., tetanic vs. TBS stimulation) may also contribute. Irrespective, these factors are important considerations when interpreting exactly how NRG1-ErbB signaling regulates NMDAR function in-vivo, the directionality of this effect, and its relationship to schizophrenia.

Of final note, Pitcher et al. base the foundation of their work on the hypothesis that “excessive NRG1-ErbB4 signaling is found in patients with schizophrenia.” Although some studies suggest increased expression of NRG1 and ErbB4 mRNAs and proteins (Law et al., 2006; Law et al., 2007) in relation to risk for schizophrenia and one example of increased NRG1 signaling has been reported (Hahn et al., 2006), caution must be exercised when concluding that measures of gene expression or ex-vivo postmortem signaling translate into elevated signaling in the brains of patients with schizophrenia. Substantially more data are needed to determine whether there is hypo- or hyperfunction of NRG1-ErbB4 signaling in the disorder.

In conclusion, the study of Pitcher et al. represents an important advance in characterizing the molecular components of the NRG1 signaling pathway downstream of ErbB4 and how this relates to NMDAR function and synaptic plasticity. Further work is needed to determine how this translates at the level of brain and behavior in schizophrenia.

References:

Bjarnadottir M, Misner DL, Haverfield-Gross S, Bruun S, Helgason VG, Stefansson H, Sigmundsson A, Firth DR, Nielsen B, Stefansdottir R, Novak TJ, Stefansson K, Gurney ME, Andresson T. Neuregulin1 (NRG1) signaling through Fyn modulates NMDA receptor phosphorylation: differential synaptic function in NRG1+/- knock-outs compared with wild-type mice. J Neurosci . 2007 Apr 25 ; 27(17):4519-29. Abstract

Cahill ME, Jones KA, Rafalovich I, Xie Z, Barros CS, Müller U, Penzes P. Control of interneuron dendritic growth through NRG1/erbB4-mediated kalirin-7 disinhibition. Mol Psychiatry . 2011 Apr 12. Abstract

Role LW, Talmage DA. Neurobiology: new order for thought disorders. Nature . 2007 Jul 19 ; 448(7151):263-5. Abstract

Law AJ, Lipska BK, Weickert CS, Hyde TM, Straub RE, Hashimoto R, Harrison PJ, Kleinman JE, Weinberger DR. Neuregulin 1 transcripts are differentially expressed in schizophrenia and regulated by 5' SNPs associated with the disease. Proc Natl Acad Sci U S A . 2006 Apr 25 ; 103(17):6747-52. Abstract

Law AJ, Kleinman JE, Weinberger DR, Weickert CS. Disease-associated intronic variants in the ErbB4 gene are related to altered ErbB4 splice-variant expression in the brain in schizophrenia. Hum Mol Genet . 2007 Jan 15 ; 16(2):129-41. Abstract

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 1 ; 12(7):824-8. Abstract

The recent study by Pitcher et al. provides a novel mechanism linking NRG1/ErbB4 activity to the suppression of NMDAR activity in a manner requiring Src kinase inhibition. The study uses biochemical manipulation of Src activation, as well as studies on cells lacking Src, to examine the role for Src kinase on the effects of NRG1 on NMDAR responses in pyramidal neurons. Overall, the study provides convincing evidence indicating that Src inhibition by NRG1 is an important contributor to the effects of NRG1 on NMDAR pyramidal neuronal hypofunction. The effect of NRG1 and ErbB4 on Src family kinase activation remains complex. Previous studies have found that NRG1 can activate Src, and that inhibition of Src family kinases can block some of the effects of NRG1 on cells, including cellular migration and proliferation (Eckert et al., 2009; Grossmann et al., 2009). Moreover, ErbB4 activity is able to activate fyn when overexpressed in heterologous cells, and NRG1 treatment activates fyn in...  Read more

The recent study by Pitcher et al. provides a novel mechanism linking NRG1/ErbB4 activity to the suppression of NMDAR activity in a manner requiring Src kinase inhibition. The study uses biochemical manipulation of Src activation, as well as studies on cells lacking Src, to examine the role for Src kinase on the effects of NRG1 on NMDAR responses in pyramidal neurons. Overall, the study provides convincing evidence indicating that Src inhibition by NRG1 is an important contributor to the effects of NRG1 on NMDAR pyramidal neuronal hypofunction. The effect of NRG1 and ErbB4 on Src family kinase activation remains complex. Previous studies have found that NRG1 can activate Src, and that inhibition of Src family kinases can block some of the effects of NRG1 on cells, including cellular migration and proliferation (Eckert et al., 2009; Grossmann et al., 2009). Moreover, ErbB4 activity is able to activate fyn when overexpressed in heterologous cells, and NRG1 treatment activates fyn in cells expressing endogenous ErbB4 (Bjarnadottir et al., 2007). Recent findings have similarly found that ErbB4 can activate Src kinases in heterologous cells and indicate that Src family kinase activation, particularly that of fyn, has a role in regulating the effects of NRG1 on interneuron morphology through RhoGEF activity (Cahill et al., 2011).

The findings of Picher et al. indicating that NRG1 can suppress Src kinase are not incompatible with these previously discussed studies, however. Indeed, studies have found that Src kinases can be both activated and inhibited by NRG1 treatment in a cyclic manner (Eckert et al., 2009), suggesting that the duration of NRG1 activity is an important consideration. The effects of NRG1 on pyramidal neuronal structure and/or function also seem to differ depending on the length of NRG1 treatment, as studies have found that chronic NRG1 or ErbB4 activity can promote synaptic structure and/or function (e.g., Barros et al., 2010; Li et al., 2007), whereas short-term NRG1 treatment is detrimental to pyramidal neuronal function (e.g., Wen et al., 2010), indicative of the importance of treatment duration to the functional consequences on neurons. The location of the examined effects is also an important consideration, as biochemical and morphological effects in pyramidal neurons and interneurons might differ following NRG1 treatment, potentially due to differences in ErbB4 expression profiles in these cells (Vullhorst et al., 2009). Given the links of NRG1/ErbB4 to schizophrenia, understanding how short-term and long-term activity of these molecules regulates both interneuron and pyramidal neuron function is of special importance, and merits further studies.

References:

Eckert JM, Byer SJ, Clodfelder-Miller BJ, Carroll SL. Neuregulin-1 beta and neuregulin-1 alpha differentially affect the migration and invasion of malignant peripheral nerve sheath tumor cells. Glia . 2009 Nov 1 ; 57(14):1501-20. Abstract

Grossmann KS, Wende H, Paul FE, Cheret C, Garratt AN, Zurborg S, Feinberg K, Besser D, Schulz H, Peles E, Selbach M, Birchmeier W, Birchmeier C. The tyrosine phosphatase Shp2 (PTPN11) directs Neuregulin-1/ErbB signaling throughout Schwann cell development. Proc Natl Acad Sci U S A . 2009 Sep 29 ; 106(39):16704-9. Abstract

Bjarnadottir M, Misner DL, Haverfield-Gross S, Bruun S, Helgason VG, Stefansson H, Sigmundsson A, Firth DR, Nielsen B, Stefansdottir R, Novak TJ, Stefansson K, Gurney ME, Andresson T. Neuregulin1 (NRG1) signaling through Fyn modulates NMDA receptor phosphorylation: differential synaptic function in NRG1+/- knock-outs compared with wild-type mice. J Neurosci . 2007 Apr 25 ; 27(17):4519-29. Abstract

Cahill ME, Jones KA, Rafalovich I, Xie Z, Barros CS, Müller U, Penzes P. Control of interneuron dendritic growth through NRG1/erbB4-mediated kalirin-7 disinhibition. Mol Psychiatry . 2011 Apr 12. Abstract

Eckert JM, Byer SJ, Clodfelder-Miller BJ, Carroll SL. Neuregulin-1 beta and neuregulin-1 alpha differentially affect the migration and invasion of malignant peripheral nerve sheath tumor cells. Glia . 2009 Nov 1 ; 57(14):1501-20. Abstract

Barros CS, Calabrese B, Chamero P, Roberts AJ, Korzus E, Lloyd K, Stowers L, Mayford M, Halpain S, Müller U. Impaired maturation of dendritic spines without disorganization of cortical cell layers in mice lacking NRG1/ErbB signaling in the central nervous system. Proc Natl Acad Sci U S A . 2009 Mar 17 ; 106(11):4507-12. Abstract

Li B, Woo RS, Mei L, Malinow R. The neuregulin-1 receptor erbB4 controls glutamatergic synapse maturation and plasticity. Neuron . 2007 May 24 ; 54(4):583-97. Abstract

Wen L, Lu YS, Zhu XH, Li XM, Woo RS, Chen YJ, Yin DM, Lai C, Terry AV, Vazdarjanova A, Xiong WC, Mei L. Neuregulin 1 regulates pyramidal neuron activity via ErbB4 in parvalbumin-positive interneurons. Proc Natl Acad Sci U S A . 2010 Jan 19 ; 107(3):1211-6. Abstract

Vullhorst D, Neddens J, Karavanova I, Tricoire L, Petralia RS, McBain CJ, Buonanno A. Selective expression of ErbB4 in interneurons, but not pyramidal cells, of the rodent hippocampus. J Neurosci . 2009 Sep 30 ; 29(39):12255-64. Abstract

The gene Neuregulin 1 (NRG1) on chromosome 8p has been identified as one of the risk genes for schizophrenia. It is unclear how the DNA sequence variation linked to schizophrenia leads to abnormalities of mRNA expression. This would be important to know, in order to understand the downstream effects of the neuregulin gene on neuronal functioning in schizophrenia.

Law and colleagues explored this question in post-mortem specimens of the hippocampus of control subjects and patients with schizophrenia. This elegant study of the expression of four types of NRG1 mRNA (types I-IV) is exactly what we need to translate findings from the field of human genetics into the field of schizophrenia neuropathology. The findings are complex and cannot be translated easily into a model of neuregulin dysfunction in schizophrenia. I would like to highlight two findings.

First, the level of NRG1 type I mRNA expression was increased in the hippocampus of schizophrenia patients. This confirms an earlier study of NRG1 mRNA expression in schizophrenia. It remains to be seen how this change in...  Read more

The gene Neuregulin 1 (NRG1) on chromosome 8p has been identified as one of the risk genes for schizophrenia. It is unclear how the DNA sequence variation linked to schizophrenia leads to abnormalities of mRNA expression. This would be important to know, in order to understand the downstream effects of the neuregulin gene on neuronal functioning in schizophrenia.

Law and colleagues explored this question in post-mortem specimens of the hippocampus of control subjects and patients with schizophrenia. This elegant study of the expression of four types of NRG1 mRNA (types I-IV) is exactly what we need to translate findings from the field of human genetics into the field of schizophrenia neuropathology. The findings are complex and cannot be translated easily into a model of neuregulin dysfunction in schizophrenia. I would like to highlight two findings.

First, the level of NRG1 type I mRNA expression was increased in the hippocampus of schizophrenia patients. This confirms an earlier study of NRG1 mRNA expression in schizophrenia. It remains to be seen how this change in NRG1 type I mRNA expression relates to the finer details of neuregulin dysfunction in schizophrenia.

Second, one single nucleotide polymorphism (SNP8NRG243177) of the risk haplotype linked to schizophrenia in earlier studies predicts NRG1 type IV mRNA expression. The SNP determines a binding site for transcription factors, providing clues for how DNA sequence variation may lead, via modulation of mRNA expression, to neuronal dysfunction in schizophrenia. It is exciting to see that we can now test specific hypotheses of molecular mechanisms in the brains of patients who have suffered from schizophrenia. The study by Law et al. is an encouraging step in the right direction.

I think this is a very interesting and potentially significant paper. It is important to point out, however, that it deals with changes in mRNA abundance rather than alterations in neuregulin protein expression. No measures of isoform protein expression were performed, and it is conceivable that neuregulin isoform protein expression could be increased, decreased, or not changed. A second point is that although statistically significant changes in mRNA were measured, they are modest.

Finally, although multiple comparisons were performed, the authors chose not to perform Bonferroni corrections, noting in the primary paper that, "Correction for random effects, such as Bonferroni correction, would be an excessively conservative approach, particularly given that we have restricted our primary analyses to planned comparisons (based on strong prior clinical association and physical location of the SNPs) of four SNPs and a single haplotype comprised of these SNPs. Because the SNPs are in moderate LD, the degree of independence between markers is low and, therefore, correcting for...  Read more

I think this is a very interesting and potentially significant paper. It is important to point out, however, that it deals with changes in mRNA abundance rather than alterations in neuregulin protein expression. No measures of isoform protein expression were performed, and it is conceivable that neuregulin isoform protein expression could be increased, decreased, or not changed. A second point is that although statistically significant changes in mRNA were measured, they are modest.

Finally, although multiple comparisons were performed, the authors chose not to perform Bonferroni corrections, noting in the primary paper that, "Correction for random effects, such as Bonferroni correction, would be an excessively conservative approach, particularly given that we have restricted our primary analyses to planned comparisons (based on strong prior clinical association and physical location of the SNPs) of four SNPs and a single haplotype comprised of these SNPs. Because the SNPs are in moderate LD, the degree of independence between markers is low and, therefore, correcting for multiple testing would result in a high type II error rate. The prior probability and the predictable association between the deCODE haplotype and expression of NRG1 isoforms (especially type IV, which is its immediate physical neighbor) combined with the LD between SNPs in this haplotype makes statistical correction for these comparisons inappropriate. Nevertheless, our finding regarding type IV expression and the deCODE haplotype and SNP8NRG243177 requires independent replication."

It will thus be important to determine if these changes in neuregulin mRNA isoform abundance are mirrored by significant changes in neuregulin isoform protein expression and if the findings can be independently replicated with other cohorts.

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...  Read more

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.