Neuregulin-1 signaling in schizophrenia

Schizophrenia is a debilitating mental illness that affects 1.1% of the general population (3 million people in the USA). It is characterized by profound disturbances in thinking, cognition and social functioning. In severe cases, schizophrenia can lead to withdrawal from society, institutionalization and even death. It imposes a huge financial burden on afflicted individuals, families and society. The overall cost of schizophrenia in the USA for 2002 was estimated to be US$63 billion [1]. Although schizophrenia is among the most prevalent of brain disorders, it continues to be one of the least understood. Unlike Alzheimer‘s disease, Parkinson‘s disease and other neurodegenerative disorders, schizophrenia lacks clear or distinct pathological lesions. Clearly, genetic factors play a role in schizophrenic pathogenesis. The risk of developing schizophrenia is significantly higher among people who have immediate relatives with a history of this or other psychiatric diseases. It is believed that schizophrenia occurs as a result of a combination of genetic predisposition, environmental exposures and/or stresses during pregnancy or childhood.

Schizophrenia is thought to be caused by impaired brain development [2–5]. This may include defects in neural genesis, neuron migration, axon pathway and synapse formation. In addition, neurotransmission and its plasticity may be altered at various neurotransmitter pathways including those of glutamate and GABA, major excitatory and inhibitory neurotransmitters, respectively, in addition to the dopaminergic pathway [6–8]. Recently, several susceptibility genes have bene identified for schizophrenia [4]. In particular, mutations or polymorphisms of neuregulin-1 (NRG-1) and ErbB4 appear to support an etiologic role [9–13]. Recent studies appear to have identified schizophrenia-relevant functions of the two proteins and provide the biological basis of the increased risk with two genes.

Neuregulin-1 & ErbB-receptor tyrosine kinases

Neuregulin-1 was identified in the early 1990s by four independent groups of scientists searching for different biological activities, and was thus named as neu-differentiation factor, heregulin, glial growth factor and acetylcholine-receptor inducible activity [14,15]. It belongs to a family of growth factors encoded by four individual genes. The prototype of neuregulin-1 is a transmembrane protein, which was cleaved to release the EGF-like domain.

In addition, neuregulin-1 has other domains that are implicated in its processing or anchoring. Neuregulin-1 acts by stimulating its receptor – a family of single transmembrane receptor tyrosine kinases (ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4) [16]. The prototype, ErbB2, was originally identified as an oncogene (type B) in the avian erythroblastosis virus (thus it was termed ErbB). ErbB proteins are homologous in structure, with an extracellular ligand-binding domain, a single transmembrane domain, a short intracellular juxtamembrane region, a tyrosine kinase domain and a carboxyl-terminal tail. Neuregulin-1 binds directly to both ErbB3 and -B4, but not -B2, and induces homo- and heterodimerization of ErbB receptors, thereby stimulating tyrosine kinase activity. On the other hand, the kinase activity of ErbB2 and -B4 can be stimulated by neuregulin-1, whereas ErbB3 is impaired by kinase activity. Thus, ErbB4 homodimers can bind to neuregulin-1 and this is sufficient to initiate downstream signal pathways, whereas ErbB2 and -B3 need to form heterodimers between them or with ErbB4.

Abnormal neuregulin-1 & ErbB4 signaling in schizophrenia

There is strong evidence that a perturbation of neuregulin-1 signaling may contribute to the etiology of schizophrenia. First, human association and linkage studies have identified the NRG-1 gene [4,9]. Several single nucleotide polymorphisms have been identified in the NRG-1 gene that are associated with altered ratios of NRG-1 isoform mRNAs, although no coding mutations have been found yet. Second, the ErbB4 gene is also a susceptibility gene for schizophrenia [10–13]. Third, increased neuregulin-1/ErbB4 signaling have been reported in brain samples of patients with schizophrenia [17].

Fourth, recent independent studies demonstrate that ErbB4 splice variants are differentially expressed in the dorsolateral prefrontal cortex (DLPFC) in schizophrenia. There is a large increase in ErbB4 isoforms CYT-1 and JMa in schizophrenic DLPFC [10,13]. These isoforms are unchanged in schizophrenic hippocampi where neuregulin-1 expression is altered [10]. Therefore, the changes in the ErbB4 isoform expression may neither be secondary to NRG-1 abnormality nor be a result of the general effect of illness state or treatment. The observations of increased ErbB4 CYT-1 expression may suggest overactivation of PI3K signaling in schizophrenia, consistent with protein studies in the human brain in schizophrenia, whereby neuregulin-1-induced stimulation of ErbB4 is accompanied by overactivation of Akt [17].

Finally, both NRG-1 and ErbB4 hypomorphic mice are, in general, hyperactive in behavioral tests such as the novel open field [9,18]. NRG-1 hypomorphic mice appear to have selective impairment in response to social novelty [19]. Remarkably, the hyperactive phenotypes in heterozygous NRG-1-mutant mice in the open-field test can be reversed by clozapine, an atypical antipsychotic drug [9]. When ErbB4 was specifically knocked out in neurons (via nestin-Cre), the conditional mutants were more active during a brief evaluation at the earlier age and less active than controls in the longer, more comprehensive evaluation at the later age [20]. Moreover, they demonstrated a lower level of spontaneous motor activity and grip strength.

Developing nervous system

Neuregulin-1 signaling is essential for development; homozygous-null embryos of NRG-1 or any of the ErbB genes die at 10.5–11 d of gestation or soon after birth. Neuregulin-1 has been implicated in differentiation of neural cells, neuron migration, neurite outgrowth and neuron survival [21–27]. It is necessary for gliogenesis, neuron–glia communication and myelination by oligodendrocytes and Schwann cells [28–31]. It was shown to regulate the expression of various neurotransmitter receptors [32]. Unusual neural development owing to abnormal neuregulin-1 signaling may underscore potential pathogenic mechanisms of schizophrenia.

Excitatory neurotransmission

Hypofunction of the glutamatergic pathway is thought to be a potential cause of schizophrenia [4,7,33]. Excitatory synaptic function in hippocampal and cortical regions was reduced in schizophrenic patients [34,35]. Individuals exposed to glutamatergic inhibitors can develop acute symptoms of schizophrenia [36], whereas drugs that promote glutamatergic activity can alleviate schizophrenic symptoms [37]. However, little is known about the underlying mechanisms of glutamatergic hypofunction in schizophrenic patients. In the developed nervous system, ErbB4 is colocalized with post-synaptic domain-95, a marker of glutamatergic post-synaptic membrane [38,39], and is localized at asymmetric glutamatergic synapses [40]. These observations suggest that neuregulin-1 may play a role in synaptic transmission and plasticity at excitatory synapses.

Indeed, we found that neuregulin-1 inhibits the induction of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses [39,40]. In support of this notion, neuregulin-1 was shown to depotentiate LTP at hippocampal CA1 synapses [41] and reduces whole-cell NMDA receptor (NMDAR), but not AMPA receptor (AMPAR), currents in prefrontal cortex (PFC) pyramidal neurons [42]. In agreement, neuregulin-1 was shown to inhibit transmission at entorhinal–hippocampal synapses [43]. Considering that neuregulin-1/ErbB4 signaling was enhanced in the brains of schizophrenic patients [17], these findings may provide a mechanism for hypofunction of the glutamatergic pathway. Interestingly, a recent study demonstrates that gain- and loss-of-function of ErbB4 enhances and reduces, respectively, glutamateric synaptic functions in hippocampal slices [44]. Moreover, neuregulin-1 was shown to control synaptic maturation and spine growth in the hippocampus. The observations suggest that the role of neuregulin-1 in regulating synaptic plasticity could be more complex than previously anticipated. More studies are needed to determine whether the reported differential effects were due to variation in neuregulin-1 (isoforms, timing and local concentration), activation of other ErbB kinases and/or experimental conditions.

GABAergic transmission

GABA is the principal inhibitory neurotransmitter essential to the proper functioning of the CNS. Patients with schizophrenia show symptoms that implicate alterations in inhibitory neurotransmission [6,33]. Intriguingly, ErbB4 mRNA is found in brain regions rich in interneurons [45] and ErbB4 is present in most, if not all, glutamic acid decarboxylase-positive neurons of the hippocampus [39]. In the PFC, a region increasingly implicated in schizophrenia, ErbB4 appears to localize to presynaptic terminals of GABAergic interneurons [46]. Remarkably, neuregulin-1 could stimulate GABA release in response to depolarization. The effects of neuregulin-1 could be blocked by an ErbB4 inhibitor and were abolished in cortical slices from ErbB4-mutant mice. Interestingly, inhibition of neuregulin-1/ErbB4 signaling could attenuate activity-dependent GABA release in the absence of exogenous neuregulin-1, suggesting that endogenous neuregulin-1 plays a role in GABA release. These results identify a novel function for neuregulin-1/ErbB4 in enhancing the stimulation-dependent release of GABA from inhibitory interneurons in the cortex. Because both NRG-1 and ErbB4 have emerged as susceptibility genes for schizophrenia, these observations raise the possibility that abnormal GABAergic function seen in schizophrenic patients may be caused by changes in neuregulin-1/ErbB4 activity.

To summarize, the identification and studies of susceptibility genes have provided intriguing leads to mechanisms underlying abnormal transmission seen in schizophrenic patients. Normal brain activity is controlled by the balance between excitatory and inhibitory pathways. Remarkably, neuregulin-1/ErbB4 signaling is involved in regulating both. Although providing links between susceptibility genes and changes in glutamatergic and GABAergic transmission, these studies left many questions unanswered. What is the physiological outcome of the neuregulin-1 regulation of both excitatory and inhibitory transmissions? What are the underlying intracellular signaling mechanisms? Are any of the physiological functions of neuregulin-1 altered by schizophrenia-associated polymorphisms in NRG-1 and ErbB4 genes? Further studies to address these questions could provide insight into the key protein networks and pathways that are potentially involved in cell biology related to synaptic plasticity and, ultimately, pathogenesis of schizophrenia.

Financial disclosure

Work performed in our laboratories was supported in part by grants from NIH and Georgia Research Alliance.