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
Comments on News and Primary Papers
Comment by: Patricia Estani
Submitted 22 June 2006
Posted 22 June 2006
I recommend the Primary PapersComment by: Cynthia Shannon Weickert, SRF Advisor
, Victor 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.
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