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