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Nat Med. Author manuscript; available in PMC Jan 23, 2012.
Published in final edited form as:
Published online Mar 27, 2011. doi:  10.1038/nm.2315
PMCID: PMC3264662
CAMSID: CAMS1842

Schizophrenia susceptibility pathway neuregulin 1–ErbB4 suppresses Src upregulation of NMDA receptors

Abstract

Hypofunction of the N-methyl D-aspartate subtype of glutamate receptor (NMDAR) is hypothesized to be a mechanism underlying cognitive dysfunction in individuals with schizophrenia. For the schizophrenia-linked genes NRG1 and ERBB4, NMDAR hypofunction is thus considered a key detrimental consequence of the excessive NRG1-ErbB4 signaling found in people with schizophrenia. However, we show here that neuregulin 1β–ErbB4 (NRG1β-ErbB4) signaling does not cause general hypofunction of NMDARs. Rather, we find that, in the hippocampus and prefrontal cortex, NRG1β-ErbB4 signaling suppresses the enhancement of synaptic NMDAR currents by the nonreceptor tyrosine kinase Src. NRG1β-ErbB4 signaling prevented induction of long-term potentiation at hippocampal Schaffer collateral–CA1 synapses and suppressed Src-dependent enhancement of NMDAR responses during theta-burst stimulation. Moreover, NRG1β-ErbB4 signaling prevented theta burst–induced phosphorylation of GluN2B by inhibiting Src kinase activity. We propose that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of synaptic NMDAR function.

The NMDAR, a major excitatory ligand-gated ion channel in the central nervous system and a principal mediator of synaptic plasticity, is a multiprotein complex whose core is a heterotetramer comprising two obligate GluN1 subunits and two GluN2(A–D) subunits1,2. Proteins associated with the core NMDAR have key roles in trafficking, stability, subunit composition and function of NMDARs3. A key process regulating NMDAR activity is phosphorylation by NMDAR-associated kinases. Certain NMDAR-dependent functions, such as ventilation and locomotion, may be independent of NMDAR phosphorylation4, whereas phosphorylation-induced upregulation of NMDARs is critical for synaptic plasticity57.

A prominent hypothesis for the pathophysiology of schizophrenia is that core symptoms such as hallucinations and cognitive deficits may be due to hypofunction of the NMDAR810. In support of this hypothesis, administering NMDAR-blocking drugs to otherwise normal adults elicits hallucinations and cognitive deficits11. However, NMDAR-blocking drugs do not discriminate between the phosphorylation-dependent and phosphorylation-independent states of the NMDAR. Thus, the pathophysiology of schizophrenia may involve reduction of basal NMDAR function or of phosphorylation-mediated enhancement of NMDAR function. In support of the latter, tyrosine phosphorylation of NMDAR subunit proteins is reduced in schizophrenic subjects as compared with controls12. The predominant tyrosine kinase mediating NMDAR tyrosine phosphorylation is Src, which enhances NMDAR channel activity13. These findings suggest the possibility that, in schizophrenia, Src-mediated enhancement of NMDAR function is suppressed specifically, rather than there being general hypofunction of NMDARs.

Here we investigated this possibility by determining whether there is a link between glutamatergic dysfunction and the candidate schizophrenia genes, Nrg1 and Erbb41424, which encode the ligand-receptor pair neuregulin 1 (NRG1) and ErbB4, respectively. In mouse models, behavioral signs of schizophrenia are found in mice heterozygous for Nrg1 or Erbb4 (refs. 14,2527) and also in mice overexpressing NRG1 selectively in the brain28,29. In studies of schizophrenic individuals, NRG1 expression is increased in both the cortex30 and hippocampus31, where NRG1-ErbB4 signaling is excessive12,31. As NRG1β blocks NMDAR-dependent long-term potentiation (LTP) at hip-pocampal Schaffer collateral–CA1 synapses3237, a form of LTP also dependent on Src activity5,38,39, we determined the effect of NRG1β-ErbB4 signaling on Src-mediated enhancement of NMDAR function, tyrosine phosphorylation of NMDARs and the resultant potentiation of synaptic transmission. We examined neuronal responses not only in the hippocampus but also in the prefrontal cortex (PFC); both of these brain regions are crucial in the pathobiology of cognitive dysfunction in schizophrenia21,4043.

RESULTS

NRG1β-ErbB4 blocks Src enhancement of NMDAR EPSCs in CA1

To determine whether NRG1-ErbB4 signaling affects Src-mediated enhancement of NMDAR function, we made whole-cell recordings from visually identified neurons in the CA1 pyramidal layer in acute hippocampal slices from adult animals. We evoked synaptic responses by stimulating the Schaffer collateral afferent input to CA1, and we pharmacologically isolated NMDAR-mediated excitatory postsynaptic currents (EPSCs) by bathing the slices with extracellular solution containing the AMPA receptor antagonist CNQX (10 μM). To prevent potential effects on GABAA-mediated inhibition, the GABAA receptor antagonist bicuculline methochloride (10 μM) was present in all experiments. We activated Src by using the phosphopeptide EPQ(pY)EEIPIA, which binds the SH2 domain of the kinase, preventing the binding of the C-terminal inhibitory phosphotyrosine5,13.

During recordings in which EPQ(pY)EEIPIA was administered intracellularly, we found that NMDAR EPSC amplitude progressively increased over 10–15 min to reach 218 ± 16% (mean ± s.e.m.) of the initial level, whereas NMDAR EPSCs were stable during recordings without EPQ(pY)EEIPIA (117 ± 7% of the initial level; P < 0.01 compared with EPQ(pY)EEIPIA; Fig. 1a,b). However, when we bath-applied a soluble form of NRG1, NRG1β (2 nM), 20 min before recordings in which EPQ(pY)EEIPIA was intracellularly administered, NMDAR EPSCs did not increase during 30 min of whole-cell recording (Fig. 1a,b). EPQ(pY)EEIPIA potentiated NMDAR EPSCs in neurons from wild-type (WT) mice (Src+/+) but not in neurons from Src-null mutant mice (Src−/−; Supplementary Fig. 1a). Thus, NRG1β prevented the enhancement of NMDAR currents by Src.

Figure 1
NRG1β-ErbB4 signaling prevents endogenous Src activation–induced potentiation of NMDAR-mediated synaptic responses in the hippocampal CA1. (a) Scatter plot of NMDAR EPSC peak amplitude over time (min) from three rat CA1 neurons recorded ...

NRG1β might have caused a change in the blockade of NMDARs by Mg2+ or reversal potential that could have masked an increase in amplitude of NMDAR EPSCs if examined only at a single holding potential. We found, however, that in neurons recorded with EPQ(pY)EEIPIA and treated with NRG1β, the current-voltage relationship was not different from that of neurons recorded with EPQ(pY)EEIPIA without NRG1β or of control neurons without EPQ(pY)EEIPIA (Fig. 1c). Thus, NRG1β prevented the EPQ(pY)EEIPIA-induced increase in NMDAR EPSCs but did not affect the voltage-dependent blockade of NMDARs by extracellular Mg2+ or the NMDAR EPSC reversal potential.

To determine whether the suppression of the Src-mediated enhancement of NMDAR currents by NRG1β requires ErbB4, we investigated the effect of NRG1β on the EPQ(pY)EEIPIA-induced potentiation of NMDAR EPSCs in hippocampal neurons lacking ErbB4. We used mutant mice in which Erbb4 had been deleted but in which human ErbB4 expression was driven in the heart by the α-myosin heavy chain promoter (Erbb4−/−HER4heart), which rescued the otherwise lethal cardiac defect44. In NRG1β-treated hippocampal slices from adult Erbb4−/−HER4heart mice, we found that EPQ(pY)EEIPIA-induced potentiation of NMDAR EPSCs reached 241 ± 49%, whereas NMDAR EPSCs remained stable in NRG1β-treated hippocampal slices from WT mice (115 ± 14% of the initial level; P < 0.001 compared with Erbb4−/−HER4heart; Fig. 1d,e). Thus, Erbb4 is necessary for suppression of Src-dependent enhancement of synaptic NMDAR currents by NRG1β.

We also determined the effect of acute ErbB4 inhibition on Src-dependent enhancement of synaptic NMDAR currents in adult WT neurons to find whether the lack of effect of NRG1β on the enhancement in Erbb4−/−HER4heart neurons was due to the absence of ErbB4 signaling in the adult or during development. To this end, we used an ErbB4 inhibitor, AG1478, which prevents ErbB4 signaling45,46. We found that bath-applied AG1478 prevented NRG1β-mediated suppression of the Src enhancement of synaptic NMDAR currents (Fig. 1d,e). Considering our findings together, we conclude that ErbB4 mediates the blockade of Src-induced enhancement of NMDAR EPSCs by NRG1β.

We next applied NRG1β in recordings with EPQ(pY)EEIPIA at a time point when the enhancement of NMDAR EPSCs had developed and found that the amplitude of the currents returned toward the initial level (Supplementary Fig. 1b), indicating that EPQ(pY)EEIPIA-induced enhancement of NMDAR EPSCs could be reversed by NRG1β. In contrast, we found that NRG1β had no effect on the amplitude (Fig. 2a), time course (Fig. 2b) or voltage dependence (Supplementary Fig. 1d) of NMDAR ESPCs in neurons in which EPQ(pY)EEIPIA was not administered. Moreover, in such recordings NMDAR EPSCs were not affected by bath-applying a broad-spectrum ErbB kinase inhibitor, PD158780 (Fig. 2c). Thus, basal NMDAR synaptic responses were not affected by NRG1β-ErbB4 signaling, consistent with evidence that synaptic NMDARs are not tonically upregulated by Src in CA1 neurons5,47,48. Our findings indicate that NRG1β prevents and reverses the enhancement of synaptic NMDAR currents by Src but does not affect basal NMDAR function in CA1 neurons.

Figure 2
NRG1β has no effect on basal NMDAR-mediated synaptic responses in hippocampal CA1 or in prefrontal cortex but prevents endogenous Src activation-induced potentiation of NMDAR EPSCs at prefrontal cortex synapses. (a) Summary scatter plot of peak ...

NRG1β blocks Src enhancement of NMDAR EPSCs in PFC

To investigate whether NRG1β affects Src-induced enhancement of synaptic NMDARs in another brain region implicated in the pathogenesis of schizophrenia, we studied the prefrontal cortex43,49,50. In acute slices from the medial prefrontal cortex, we evoked NMDAR EPSCs in layer V pyramidal neurons by stimulating the corticocortical neurons and afferents in layers II and III while pharmacologically blocking AMPA and GABA receptors. During recordings in which EPQ(pY)EEIPIA was administered intra-cellularly, NMDAR EPSC amplitude progressively increased over 10–15 min to reach 144 ± 19% of the initial level, whereas NMDAR EPSCs were stable during recordings without EPQ(pY)EEIPIA (Fig. 2d,e). However, we found that in the presence of NRG1β (6 nM), intracellular administration of EPQ(pY)EEIPIA did not cause an increase in NMDAR EPSCs. In layer V pyramidal neurons treated with NRG1β, but in which EPQ(pY)EEIPIA was not administered, NMDAR EPSCs were stable (Fig. 2d,e). Therefore, similarly to hippocampal CA1 neurons, layer V pyramidal cells in prefrontal cortex show a Src-mediated enhancement of synaptic NMDAR currents that is prevented by NRG1β.

Src potentiation of EPSPs is prevented by NRG1β

Src-enhanced NMDAR currents initiate Ca2+-mediated potentiation of AMPAR synaptic responses5,7,51. Here we found that during recordings in which EPQ(pY)EEIPIA was delivered into CA1 neurons through a patch pipette, the slope of the excitatory postsynaptic potential (EPSP) gradually increased to 163 ± 14% of the initial level (Fig. 3a,b). However, the enhancement of EPSPs by EPQ(pY)EEIPIA was suppressed in a concentration-dependent manner when NRG1β was administered 20 min before whole-cell recording. In contrast to the effect of NRG1β administered before EPQ(pY)EEIPIA, there was no subsequent change in EPSP slope when NRG1β (2 nM) was administered after the potentiation by EPQ(pY)EEIPIA had been established (Fig. 3c,d).

Figure 3
NRG1β prevents but does not reverse endogenous Src-induced synaptic potentiation. (a) Scatter plot of EPSP slope over time from three rat CA1 neurons recorded with control ICS, ICS containing EPQ(pY)EEIPIA or ICS containing EPQ(pY)EEIPIA with ...

The potentiation of EPSPs by EPQ(pY)EEIPIA was prevented by the NMDAR antagonist D-amino-phosphonovaleric acid (D-APV) and the potentiated EPSPs were blocked by CNQX, indicating that EPQ(pY)EEIPIA had initiated NMDAR-dependent potentiation of synaptic AMPAR responses (Supplementary Fig. 2). Moreover, EPSPs were potentiated by EPQ(pY)EEIPIA in neurons from Src+/+ mice but not in Src−/− neurons (Supplementary Fig. 2a,b). Considering our findings together, we conclude that by suppressing Src-mediated enhancement of NMDAR currents, NRG1β prevented the potentiation of AMPAR-mediated synaptic transmission. In contrast to the enhancement of NMDAR synaptic responses, the potentiation of AMPAR synaptic responses was not reversed by NRG1β, indicating that, once established, AMPAR potentiation became independent of regulation by NRG1β signaling.

NRG1β prevents LTP induced by theta-burst stimulation

Src-mediated enhancement of NMDAR currents is crucial for inducing but not maintaining the long-term potentiation of synaptic transmission induced by high-frequency activation of Schaffer collateral inputs5. Here we used theta-burst patterned stimulation (TBS) to generate TBS-induced LTP (tbLTP) that was prevented by Src inhibitors (Supplementary Fig. 3). We found that bath-applying NRG1β starting 20 min before TBS suppressed the increase in the slope of field EPSPs (f EPSP) that characterized tbLTP (Fig. 4a). In contrast, NRG1β had no effect on basal fEPSP slope before TBS (Fig. 4a) nor did it affect f EPSP slope when applied starting 30 min after TBS, a time when synaptic responses had already been potentiated (Fig. 4b). Thus, NRG1β suppressed the potentiation of AMPAR-mediated synaptic transmission induced by TBS, as well as that induced by the direct activation of Src.

Figure 4
NRG1β prevents but does not reverse TBS-induced LTP in CA1 hippocampus and has no effect in Src−/− mice. (a) Scatter plot of normalized fEPSP slope over time for a rat hippocampal slice treated with NRG1β (2 nM) or another ...

NRG1β suppression of tbLTP is prevented by lack of Src

As our findings indicate that NRG1β suppresses Src-dependent enhancement of synaptic transmission, we considered that NRG1β might not have an effect on Src-independent forms of synaptic potentiation. Because Src-null mutant mice show residual LTP, which by definition is Src independent, we tested the effect of NRG1β on LTP induction in hippocampal slices from Src+/+ mice versus Src−/− mice. Basal synaptic transmission, input-output relationship and paired-pulse facilitation were not different between Src+/+ and Src−/− slices, and NRG1β had no effect on basal synaptic transmission in either genotype (Supplementary Fig. 4a–c). However, NRG1β significantly decreased tbLTP in slices from Src+/+ mice but had no effect on tbLTP in slices from Src−/− mice (Fig. 4c,d). As the level of tbLTP in Src−/− mice was reduced compared with that in Src+/+ mice, we conclude that the effect of NRG1β was occluded in Src-null mutant mice. The loss of effect of NRG1β in Src−/− slices could not be accounted for by lack of ErbB4 because the amount of ErbB4 in the hippocampus of Src−/− mice was not different from that in Src+/+ mice (Fig. 4e and Supplementary Fig. 4d). Moreover, Src−/− mice were not different from Src+/+ mice in the amount of the following prominent synaptic proteins: the NMDAR subunits GluN1, GluN2A and GluN2B; the AMPAR subunits GluA1 and GluA2 and/or GluA3 (the antibody used detects both); postsynaptic density protein-95 (PSD-95); α-calcium calmodulin-dependent kinase II; or the Src-family kinase Fyn (Supplementary Fig. 4d,e). Thus, the suppression of the induction of tbLTP by NRG1β requires Src. On the basis of our findings, we conclude that NRG1β suppresses activity-dependent potentiation of synaptic transmission in CA1 hippocampus by preventing Src-mediated enhancement of NMDAR function.

NRG1β suppresses burst EPSPs during TBS

Our findings that NRG1β prevents induction of tbLTP but does not affect basal synaptic transmission led us to consider that NRG1β might affect transmission during the period of TBS itself. We therefore made current-clamp recordings from CA1 neurons and monitored the membrane potential throughout the TBS, which was composed of a train of 15 bursts of four stimuli delivered every 200 ms (Fig. 5). We observed that each of the four-stimuli bursts elicited a characteristic change in membrane potential, a response we call the ‘burst EPSP’ (Fig. 5a): the membrane potential depolarized progressively with each stimulus, peaking after the fourth and then decaying toward the resting membrane potential until the next burst in the train. In control cells, the peak amplitude of the first-burst EPSP was 67 ± 3 mV, and the peak amplitude declined gradually to 20 ± 3 mV with the subsequent 14 bursts in the train (n = 28 cells; Fig. 5b,c). In addition, the membrane potential did not return to the resting membrane potential between the bursts but only did so ~400 ms after the final burst in the TBS train.

Figure 5
NRG1β reduces depolarization of CA1 neurons during the period of TBS. (a) The first four pulse-induced burst EPSP of TBS for control mouse (Src+/+) slices (n = 28), slices (from Src+/+ mice) pretreated with D-APV (n = 17), slices from Src−/− ...

Before investigating the possible effects of NRG1β during TBS, we determined whether components of the membrane potential response during the train depended on NMDARs or Src. We found that D-APV reduced the peak amplitude of the burst EPSPs and accelerated the recovery of membrane potential at the end of the burst EPSPs (Fig. 5a and Supplementary Fig. 5a,d). Thus, a component of the burst EPSPs depends on NMDARs. For EPSPs evoked by a single stimulus, we found that D-APV had no effect on the rising phase or the peak amplitude but shortened the decay of the single EPSP (Fig. 5a and Supplementary Fig. 5f). Hence, both the single EPSPs and burst EPSPs had an NMDAR-dependent component. The burst EPSP amplitude was reduced in Src−/− neurons, as compared with Src+/+, whereas the single EPSPs did not differ between the two genotypes (Fig. 5a and Supplementary Fig. 5b,e,g).

We found that applying NRG1β did not alter the rising phase, peak amplitude or decay of single-stimulus–evoked EPSPs (Fig. 5a and Supplementary Fig. 5h). In contrast, NRG1β caused a reduction in the peak amplitude of the first-burst EPSP (54 ± 4 mV, P < 0.05 compared with control without NRG1β; Fig. 5a–c). The NRG1β-induced reduction in the first-burst EPSP amplitude was less than that produced by D-APV (38 ± 4 mV; P < 0.01 compared with NRG1β) but was not different from that in Src−/− mice (53 ± 3 mV; P > 0.5 compared with NRG1β). Furthermore, we found that AG1478 had no effect on single stimulus–evoked EPSPs (Fig. 5e and Supplementary Fig. 5i) but prevented the NRG1β-induced suppression of burst EPSPs (Fig. 5e,f and Supplementary Fig. 5c). Thus, NRG1β-ErbB4 signaling reduced responses of CA1 neurons during the period of TBS itself. Notably, although both single and burst EPSPs showed NMDAR-dependent components, the burst EPSPs but not the single EPSPs were reduced by NRG1β-ErbB4 signaling or by lack of Src.

NRG1β suppresses Src and GluN2B tyrosine phosphorylation

NRG1β did not alter the level of Src within the NMDAR complex in CA1 hippocampus (Fig. 6a), but we found that Src activity in tissue from slices treated with NRG1β was significantly lower than that in untreated slices (Fig. 6b). In NRG1β-treated slices, AG1478 increased Src activity (data not shown), indicating that the suppression of Src function by NRG1β required ErbB4 signaling.

Figure 6
NRG1β does not alter Src association with the NMDAR but reduces Src tyrosine kinase activity and prevents TBS-induced GluN2B phosphorylation in hippocampal CA1. (a) Immunoprecipitation (IP) of GluN2 subunits carried out from hippocampal proteins ...

LTP-inducing tetanic stimulation increases tyrosine phosphorylation of the GluN2B subunit of the NMDAR in the hippocampus52,53. Here we found that TBS caused an increase in GluN2B tyrosine phosphorylation that depended on Src (Fig. 6c). TBS increased tyrosine phosphorylation in the GluN2B band in untreated slices but not in slices treated with NRG1β (Fig. 6d). Furthermore, the suppression of TBS-induced GluN2B tyrosine phosphorylation by NRG1β was prevented by AG1478 (Fig. 6e). Collectively, these findings indicate that NRG1β-ErbB4 signaling inhibits Src kinase activity, leading to blockade of TBS-induced tyrosine phosphorylation of GluN2B.

DISCUSSION

We have discovered that NRG1β suppresses the enhancement of synaptic NMDAR currents by Src kinase in CA1 hippocampus and in prefrontal cortex. The most parsimonious model to account for our findings is that signaling by NRG1, through its cognate receptor ErbB4, leads to inhibition of the catalytic activity of Src kinase, thereby blocking the enhancement of NMDAR function. Consequently, NRG1-ErbB4 signaling prevents downstream events that require Src-mediated enhancement of NMDARs; in the case of CA1 neurons, this is the prevention of long-term potentiation at Schaffer collateral synapses. Thus, our study identifies Src as a downstream target of NRG1-ErbB4 signaling and inhibition of Src as an essential step by which this signaling pathway regulates synaptic plasticity in the hippocampus. Because the dominant paradigm for signaling by the family of ErbB receptors is that receptor stimulation leads to activation, rather than to suppression, of Src and other Src-family kinases54,55, we have discovered a function of ErbB receptors unexpected from previous work.

Our findings that basal NMDAR currents and voltage dependence are unaffected by NRG1β indicate that NRG1-ErbB4 signaling does not generally suppress NMDAR channel function. Nor does NRG1-ErbB4 signaling generally affect AMPAR function, as indicated by the lack of effect of NRG1 on basal EPSPs or on EPSPs potentiated after the administration of EPQ(pY)EEIPIA or TBS. NRG1β also did not seem to affect presynaptic glutamate release, as paired-pulse responses were unaffected. Rather, our evidence that NRG1β prevents the post-synaptic upregulation of synaptic NMDAR and AMPAR responses by EPQ(pY)EEIPIA, the tyrosine phosphorylation of NMDAR subunits and the LTP induced by TBS indicates that NRG1-ErbB4 signaling interferes with Src, as demonstrated by NRG1β-induced reduction of Src activity. The effects of reduced Src activity were evident during the induction of LTP, as the Src- and NMDAR-dependent components of the burst EPSPs were reduced by NRG1β. Thus, NRG1-ErbB4 signaling preferentially suppresses Src-dependent upregulation of NMDARs and, in turn, the induction of persistent synaptic plasticity in CA1 without affecting basal synaptic transmission or short-term plasticity, both of which are Src independent.

NRG1-ErbB4 signaling suppresses responses during TBS but does not have an effect on responses to individual, isolated stimuli, indicating that CA1 neuron responses are differentially sensitive to patterned synaptic input. This differential sensitivity is apparent even during the first-burst EPSP, indicating that responses to bursts of as few as three or four stimuli are suppressed by NRG1-ErbB4 signaling, with the degree of suppression increasing with later bursts in the train. The stimulation train we used corresponds to the pattern of theta rhythm activity that normally occurs in the hippocampus56. Thus, our findings suggest that oscillatory network activity in the brain at theta frequency may be suppressed by NRG1-ErbB4 signaling, whereas nonoscillatory, irregular activity may be unaffected. Therefore, functions that depend on theta rhythm may be disrupted under conditions when NRG1-ErbB4 signaling is increased. NRG1 disruption of responses of CA1 neurons to theta-patterned input could contribute to the alterations in oscillatory brain network activity observed in schizophrenia56,57. Because alteration of oscillatory activity may underlie disorders of thought56,57 and hallucinations57, our findings suggest that suppression of Src enhancement of NMDAR activity by excessive NRG1-ErbB4 signaling may contribute to cognitive dysfunction and positive symptoms in individuals with schizophrenia.

Decreased Src catalytic activity by NRG-ErbB4 may be caused by suppressing activators, or by facilitating inhibitors, of kinase function. Within the NMDAR complex, potential mediators of NRG1-ErbB4 signaling include the Src regulators Csk48 and protein tyrosine phosphatase-α58. Another potential mediator is PSD-95, which suppresses Src activity in the NMDAR complex through binding of a sequence in the PSD-95 N-terminal region to the SH2 domain of Src59.

Our finding that acute pharmacological inhibition of ErbB4 did not affect basal NMDAR responses suggests that NMDARs are not tonically suppressed by NRG1-ErbB4 signaling. In contrast, the magnitude of LTP is increased by acute ErbB4 inhibition, or in mice lacking ErbB4 (ref. 36), indicating that a physiological role of NRG1-ErbB4 signaling is to suppress LTP at Schaffer collateral–CA1 synapses. The differential effect of NRG1-ErbB4 signaling on LTP induction but not on basal NMDAR currents is consistent with increasing neuronal activity causing NRG1 release in the hippocampus60. Alternatively, there might be tonic release of NRG1, which suppresses Src-enhanced but not basal NMDAR currents. Our findings indicate that NRG1-ErbB4 functions physiologically to inhibit Src-mediated upregulation of NMDAR function. This physiological function of NRG1-ErbB4 is not normally saturated because LTP can be further suppressed by increasing this signaling through the administration of NRG1β32,36. Thus, the normal physiological function could be exaggerated under conditions of increased expression of NRG1 or elevated NRG1-ErbB4 signaling.

NMDAR EPSCs in layer V prefrontal cortex neurons were increased by intracellular administration of EPQ(pY)EEIPIA, indicating that synaptic NMDAR-mediated responses in these neurons are enhanced by the activation of Src kinase7. This enhancement of NMDAR function in layer V prefrontal cortex neurons was suppressed by NRG1β, as in hippocampal CA1 neurons. Together, these findings show similarities in the effect of NRG1β on neurons that are major output pathways for the prefrontal cortex and the hippocampus, respectively. Suppressing Src enhancement of NMDAR activity in these pathways could thereby impair brain functions that depend on these outputs, including learning, memory and executive function. Impairment of these functions is linked to dysfunction of cognition and positive symptoms in schizophrenia. Overall, our work identifies a specific molecular mechanism, suppression of the upregulation of NMDARs by Src, that links gain of function of NRG1-ErbB4 signaling in schizophrenia and hypofunction of NMDARs. Our findings suggest that strategies to normalize Src-mediated enhancement of NMDARs could be new therapeutic approaches in the treatment of schizophrenia.

METHODS

Methods and any associated references are available in the online version of the paper at http://www.nature.com/naturemedicine/.

Supplementary Material

Supplementary Text and Figures

Acknowledgments

We thank J.F. MacDonald, P.A. Frankland, L.Y. Wang, M.F. Jackson and M.H. Pitcher for critical reading of the manuscript, and Genentech and L. Mei for providing NRG1β. Src knockout mice were obtained as a gift from B.F. Boyce, University of Rochester Medical Center. This study was supported by grants from the Canadian Institutes of Health Research (CIHR) to M.W.S. (MT-12682) and to E.K.L. (MOP-89825), and from the Deafness Research Foundation to K.T.Y. M.W.S. holds a Canada research chair (tier I) in neuroplasticity and pain, and is an International Research Scholar of the Howard Hughes Medical Institute. E.K.L. holds a Canada research chair (tier II) in developmental cortical physiology. G.M.P. was a CIHR postdoctoral fellow and was supported by a Merck Frosst Fellowship award. D.N. held a CIHR doctoral research award, L.V.K. was a CIHR MD-PhD student, and N.M.G. holds a CIHR Canada Graduate Scholarship doctoral award. We thank D. Wong, J. Hicks and S. Singhroy for technical support.

Footnotes

Note: Supplementary information is available on the Nature Medicine website.

AUTHOR CONTRIBUTIONS

G.M.P. designed the project, conducted the hippocampal experiments, analyzed the data and wrote the manuscript. L.V.K. and D.N. carried out and analyzed biochemical experiments. E.K.L. oversaw and analyzed the prefrontal cortex experiments. N.M.G. carried out and analyzed the prefrontal cortex experiments. K.T.Y. maintained, housed and provided ErbB4 knockout mice. All authors participated in revising the manuscript and agreed to the final version. M.W.S. conceived the study, analyzed data, supervised the overall project and wrote the manuscript.

COMPETING FINANCIAL INTERESTS

The authors declare no competing financial interests.

Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/.

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