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Neuregulin-ErbB4 Signaling in Parvalbumin Interneurons: Implications for Schizophrenia and Epilepsy

16 December 2011. Signaling through neuregulin 1 (NRG1) and its receptor ErbB4 on parvalbumin (PV)-expressing neurons regulates PV neuron excitability and has an anti-epileptogenic effect, according to two studies published online December 11 in Nature Neuroscience. In addition to the clear implications for the epilepsy field, these studies may also provide insight into the pathophysiology of schizophrenia, in which both NRG1 and ErbB4 have been implicated as key players (see SRF related news story).

The studies, one from Xiao-Ming Li’s group in Zhejiang University in China and the other from the groups of Lin Mei at Georgia’s Health Sciences University, Augusta, and Zhi-Qi Xiong at the Chinese Academy of Sciences in Shanghai, probe the role of NRG1-ErbB4 signaling in rodents. NRG1 is a neurotrophic factor that binds to and activates the receptor tyrosine kinase ErbB4. Both are prominent in fast-spiking PV interneurons (see SRF related news story), involved in a variety of developmental processes and altered in schizophrenia (Mei and Xiong, 2008). For example, both NRG1 and ErbB4 are candidate susceptibility genes, and altered levels of both mRNAs and proteins have been reported in the illness (Chong et al., 2008; see also SRF related news story).

PV interneuron excitability
First author Ke-Xin Li and colleagues found that NRG1-ErbB4 signaling regulates the excitability of PV neurons. Using recordings from PV neurons in both cortical and hippocampal slices from mice, the authors observed that bath application of NRG1 decreased the average interspike interval and increased the firing frequency of PV neurons. These effects were blocked by the application of ecto-ErbB4, a peptide that blocks the action of NRG1 by preventing it from activating endogenous receptors.

In order to assess whether NRG1 modulates the excitability of PV neurons directly or acts by modulating inputs to these neurons, the researchers blocked synaptic inputs using glutamate and γ-aminobutyric acid (GABA) receptor antagonists. In the presence of these drugs, the effects of NRG1 and ecto-ErbB4 remained, suggesting that NRG1 directly affects the intrinsic properties of PV neurons. Moreover, the effect of NRG1 on PV neuron excitability was mediated by ErbB4, as there was no effect of NRG1 in mice with a selective knockout of ErbB4 in PV neurons.

Further investigation revealed that NRG1 increased the slope of action potential initiation in PV neurons, and shifted the threshold for action potentials to more hyperpolarized levels, but did not affect passive membrane properties. The voltage shift was mediated by the voltage-gated potassium channel Kv1.1, evidenced by the fact that treatment with the Kv1.1 blocker DTx-K prevented the normal ecto-ErbB4-mediated increase in current threshold and decrease in firing frequency. Using current subtraction analysis, the authors found that ecto-ErbB4 doubled the DTx-K-sensitive current amplitude, implying that NRG1 normally dampens this Kv1.1 current. This was mediated via phosphorylation, as phosphotyrosine levels increased after NRG1 treatment.

The study suggests that changes to PV neuron excitability could be one manifestation of alterations in the NRG1-ErbB4 signaling pathway in schizophrenia. This is especially relevant, given that PV neurons are known to exhibit other alterations in schizophrenia, including lower levels of both the GABA-synthesizing enzyme GAD67 and PV itself (Hashimoto et al., 2003), and because a disturbance in the balance of excitation and inhibition also produces schizophrenia-like behaviors (see SRF related news story).

Seizure prevention
Li and colleagues also showed that mice lacking ErbB4 in PV neurons have an increased susceptibility to seizures in two different models of epilepsy (using either GABA antagonist pentylenetetrazole, or the muscarinic agonist pilocarpine to induce seizures), and that treatment with NRG1 ameliorated seizures. Additionally, the authors found that ErbB4 protein levels are reduced in individuals with temporal lobe epilepsy.

In the second study, Xiong’s group showed that NRG1-ErbB4 signaling in PV neurons, but not pyramidal neurons, suppressed limbic epileptogenesis. Using both the kindling model of epilepsy, in which animals are repeatedly given an electrical stimulus that produces progressively more intense and longer seizures, as well as the pilocarpine model, first author Guo-He Tan and colleagues observed an increase in NRG1 mRNA and protein after seizures in both rats and mice. In contrast, total ErbB4 protein levels were unchanged.

Next, the authors evaluated the effect of NRG1 on seizures using the kindling model, and found that infusion of NRG1 prior to each electrical stimulation resulted in a significant delay of epileptogenesis, while infusing ecto-ErbB4 promoted it. Similar to the neuronal excitability results of Li and colleagues, Xiong’s group also observed that the anti-epileptogenic properties of NRG1 require the receptor ErbB4, as treatment with an ErbB4 inhibitor promoted epileptogenesis.

The suppressing effect of NRG1 on seizure activity seems to be specific to PV interneurons. In mice with a specific knockout of ErbB4 in PV neurons, the team observed accelerated epileptogenesis; however, mice with a knockout of ErbB4 in pyramidal neurons did not show this effect (although NRG1 treatment in these animals did inhibit epileptogenesis).

NRG1 and ErbB4: linking schizophrenia and epilepsy?
Interestingly, there seems to be a relationship between schizophrenia and epilepsy. Having epilepsy (or a family history of the disease) is associated with increased risk for developing schizophrenia or schizophrenia-like psychosis (Foong, 2006). Conversely, some epilepsy patients also exhibit psychotic symptoms (Cascella et al., 2009). Both studies elucidated mechanisms that may underlie schizophrenia and epilepsy, and together, strengthen the link between the two diseases.—Allison A. Curley.

References:
Li KX, Lu YM, Xu ZH, Zhang J, Zhu JM, Zhang JM, Cao SX, Chen XJ, Chen Z, Luo JH, Duan S, Li XM. Neuregulin 1 regulates excitability of fast-spiking neurons through Kv1.1 and acts in epilepsy. Nat Neurosci . 2011 Dec 11. Abstract

Tan GH, Liu YY, Hu XL, Yin DM, Mei L, Xiong ZQ. Neuregulin 1 represses limbic epileptogenesis through ErbB4 in parvalbumin-expressing interneurons. Nat Neurosci . 2011 Dec 11. Abstract

Comments on Related News


Related News: Neuregulin Partner ErbB4 Spices Up Genetic Associations

Comment by:  Amanda Jayne Law, SRF Advisor
Submitted 22 February 2006
Posted 22 February 2006
  I recommend the Primary Papers

The study of Ghashghaei and colleagues provides a remarkable insight into the function of neuregulin 1 (NRG1), and NRG2 in adult neurogenesis. The study demonstrates that NRG1(2)/ErbB4 signaling influences the proliferation, differentiation, organization, and migration of adult neural progenitor cells in the subventricular zone (SVZ) and rostral migratory stream (RMS), in a ligand- and cell-dependent fashion. Using immunohistochemistry, Ghashghaei and colleagues first demonstrate that NRG1, NRG2, and ErbB4 are expressed by distinct cell types in the SVZ and RMS, notably ErbB4 and NRG1 by polysialylated neural cell adhesion molecule positive (PSA-NCAM+) neuroblasts, and ErbB2/3/4 by a subset of GFAP+ cells. These observations extend the group's previous studies of NRG1 and ErbB4 in the RMS (Anton et al., 2004). In their current study, Ghashghaei went on to examine the effects of exogenous infusion of NRG1 and NRG2 on neurogenesis in the RMS of adult mice. Interestingly, NRG1 was shown to decrease the initiation of neuroblast migration from the SVZ to the RMS by inducing the rapid aggregation of cells in the SVZ. The consequence of this rise in NRG1 was a decrease in the number of PSA-NCAM+ cells in the RMS and GABA+ cells in the olfactory bulb, demonstrating that ectopic or elevated expression of NRG1 prevents differentiation and migration of neurons from the adult SVZ to the RMS.

The study is particularly interesting in terms of the role of NRG1/ErbB4 signaling in directional cell migration. Flames et al. (2004) recently reported that NRG1 (specifically the Ig containing family of isoforms, e.g., Types I, II and IV; for review, see Harrison and Law, 2006) functions as a long distance chemoattractant for ErbB4 positive GABAergic interneurons migrating from the medial ganglionic eminence to the developing cortex. The observation that NRG1 is a chemoattractant in other brain regions may appear somewhat contradictory to the findings of Ghashghaei, which suggest that in-vivo NRG1 actually inhibits migration of neurons from the SVZ (at least when introduced ectopically). However, it would seem that these two findings are actually consistent. Ghashghaei and colleagues ectopically infused NRG1 into the lateral ventricles of adult mice. The subsequent aggregation of cells in the SVZ demonstrates that NRG1 indeed acts as a chemoattractant, not in an obvious manner by inducing the cells to migrate away, but simply by "attracting" them to aggregate or "clump" where they are (subsequently preventing migration to the RMS). So in fact, both the studies of Flames and Ghashghaei show that NRG1 is chemotactic to specific populations of neurons and cells, whether it is expressed at a distance and cells preferentially migrate toward it, or in the immediate environment and cells are attracted to migrate to, or stay in its vicinity.

In the past few years, NRG1 and ErbB4 have both been identified as potential susceptibility genes for schizophrenia. The aim now is to determine the molecular and biological mechanisms by which the genes confer risk for the disease. In terms of schizophrenia, we have previously demonstrated that the Type I isoform of NRG1 is elevated in the hippocampus (and prefrontal cortex; see Hashimoto et al., 2004) in the disease and that expression of the novel Type IV isoform is related to disease-associated sequence variants within the NRG1 gene (Law et al., 2006). Furthermore, we have recently demonstrated that these changes are accompanied by altered expression of specific isoforms of the ErbB4 receptor, consistent with that of Silberberg et al., 2006 (Law et al., 2005). Ghashghaei and colleagues provide the first direct evidence that ectopic or elevated expression of NRG1 in the brain can perturb cell migration. In light of this and other evidence, our findings in schizophrenia may translate into altered neuronal migration, cortical development and possibly neurogenesis in the disease.

At present, the exact links between altered NRG1/ErbB4 signaling and the pathophysiology of schizophrenia are unknown and potentially numerous (i.e., synaptogenesis, neurotransmitter function, neuronal migration, differentiation, glia formation and function, myelination). Studies such as that of Ghashghaei et al. provide insight into the normal role of NRG1/ErbB4 signaling in neurodevelopment and the adult brain which is essential if we are to understand the pathogenic role of the NRG1 gene and its receptors in disease.

References:

Anton ES, Ghashghaei HT, Weber JL, McCann C, Fischer TM, Cheung ID, Gassmann M, Messing A, Klein R, Schwab MH, Lloyd KC, Lai C. Receptor tyrosine kinase ErbB4 modulates neuroblast migration and placement in the adult forebrain. Nat Neurosci. 2004 Dec;7(12):1319-28. Epub 2004 Nov 7. Abstract

Flames N, Long JE, Garratt AN, Fischer TM, Gassmann M, Birchmeier C, Lai C, Rubenstein JL, Marin O. Short- and long-range attraction of cortical GABAergic interneurons by neuregulin-1. Neuron. 2004 Oct 14;44(2):251-61. Abstract

Hashimoto et al., 2004, Mol. Psychiatry 9, 299-307.

Law et al (a) 2006. Neuregulin 1 (NRG1) transcripts are differentially expressed in schizophrenia and regulated by 5’ SNPs associated with the disease. PNAS

Also See SfN 2005 SRF research news: Cortical Deficits in Schizophrenia: Have Genes, Will Hypothesize

Law 2005, SNPing away at NRG1 and ErbB4 gene expression in schizophrenia Neuropsychopharmacology, vol. 30, Supplement 1.

View all comments by Amanda Jayne Law

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  William Carpenter, SRF Advisor (Disclosure)
Submitted 22 April 2006
Posted 22 April 2006
  I recommend the Primary Papers

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Stephan Heckers, SRF Advisor
Submitted 29 April 2006
Posted 29 April 2006
  I recommend the Primary Papers

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.

View all comments by Stephan Heckers

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Bryan Roth, SRF Advisor
Submitted 5 May 2006
Posted 5 May 2006
  I recommend the Primary Papers

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.

View all comments by Bryan Roth

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Patricia Estani
Submitted 9 June 2007
Posted 10 June 2007
  I recommend the Primary Papers