Neuregulin 1 (NRG1) is a trophic factor that acts by stimulating ErbB receptor tyrosine kinases and has been implicated in neural development and synaptic plasticity. In this study, we investigated mechanisms of its suppression of long-term potentiation (LTP) in the hippocampus. We found that NRG1 did not alter glutamatergic transmission at SC-CA1 synapses but increased the GABA A receptormediated synaptic currents in CA1 pyramidal cells via a presynaptic mechanism. Inhibition of GABA A receptors blocked the suppressing effect of NRG1 on LTP and prevented ecto-ErbB4 from enhancing LTP, implicating a role of GABAergic transmission. To test this hypothesis further, we generated parvalbumin (PV)-Cre;ErbB4 −/− mice in which ErbB4, an NRG1 receptor in the brain, is ablated specifically in PV-positive interneurons. NRG1 was no longer able to increase inhibitory postsynaptic currents and to suppress LTP in PV-Cre; ErbB4 −/− hippocampus. Accordingly, contextual fear conditioning, a hippocampus-dependent test, was impaired in PV-Cre;ErbB4 −/− mice. In contrast, ablation of ErbB4 in pyramidal neurons had no effect on NRG1 regulation of hippocampal LTP or contextual fear conditioning. These results demonstrate a critical role of ErbB4 in PV-positive interneurons but not in pyramidal neurons in synaptic plasticity and support a working model that NRG1 suppresses LTP by enhancing GABA release. Considering that NRG1 and ErbB4 are susceptibility genes of schizophrenia, these observations contribute to a better understanding of how abnormal NRG1/ErbB4 signaling may be involved in the pathogenesis of schizophrenia. N euregulin 1 (NRG1) is a trophic factor that acts by activating ErbB receptor tyrosine kinases, including ErbB4. NRG1 signaling has been implicated in various steps in neural development, including neuron migration, axon guidance, synapse formation, and expression of neurotransmitter receptors (1). Studies of NRG1 have attracted much attention because both NRG1 and ErbB4 were identified as susceptibility genes of schizophrenia and NRG1 and ErbB4 mutant mice show schizophrenia-relevant behaviors (1-4).Recent studies suggest that NRG1 plays a role in neurotransmission and synaptic plasticity (1). NRG1 has been shown to suppress the induction of LTP acutely at Schaffer collateral (SC)-CA1 synapses in adult rodent hippocampus (5-8), but it has no effect on basal synaptic transmission (5, 7, 9). NRG1 regulation of long-term potentiation (LTP) requires ErbB4 (8); however, underlying mechanisms remain unclear. In vitro studies suggest that NRG1 may alter functions of pyramidal neurons and glutamatergic transmission. For example, it could suppress NMDA receptor (NMDAR) currents in prefrontal cortical (PFC) neurons in culture (10). NRG1 was shown to stimulate internalization of surface AMPA receptors (AMPARs) in dissociated hippocampal neurons (11). Moreover, changes in ErbB4 levels in neonatal hippocampal slices alter dendritic spine size and AMPA synaptic currents (12). Conversely, ErbB4 expression is largely restricted to ...

Synapse formation requires proper interaction between pre- and postsynaptic cells. In anterograde signaling, neurons release factors to guide postsynaptic differentiation. However, less is known about how postsynaptic targets retrogradely regulate presynaptic differentiation or function. We found that muscle-specific conditional knockout of beta-catenin (Ctnnb1, also known as beta-cat) in mice caused both morphologic and functional defects in motoneuron terminals of neuromuscular junctions (NMJs). In the absence of muscle beta-catenin, acetylcholine receptor clusters were increased in size and distributed throughout a wider region. Primary nerve branches were mislocated, whereas secondary or intramuscular nerve branches were elongated and reduced in number. Both spontaneous and evoked neurotransmitter release was reduced at the mutant NMJs. Furthermore, short-term plasticity and calcium sensitivity of neurotransmitter release were compromised in beta-catenin-deficient muscle. In contrast, the NMJ was normal in morphology and function in motoneuron-specific beta-catenin-deficient mice. Taken together, these observations indicate a role for muscle beta-catenin in presynaptic differentiation and function, identifying a previously unknown retrograde signaling in the synapse formation and synaptic plasticity.

Neuregulin 1 (NRG1) and its receptor ErbB4 are both susceptibility genes of schizophrenia. However, little is known about the underlying mechanisms of their malfunction. Although ErbB4 is enriched in GABAergic interneurons, the role of NRG1 in excitatory synapse formation in these neurons remains poorly understood. We showed that NRG1 increased both the number and size of PSD-95 puncta and the frequency and amplitude of miniature EPSCs (mEPSCs) in GABAergic interneurons, indicating that NRG1 stimulates the formation of new synapses and strengthens existing synapses. In contrast, NRG1 treatment had no effect on either the number or size of excitatory synapses in glutamatergic neurons, suggesting its synaptogenic effect is specific to GABAergic interneurons. Ecto-ErbB4 treatment diminished both the number and size of excitatory synapses, suggesting that endogenous NRG1 may be critical for basal synapse formation. NRG1 could stimulate the stability of PSD-95 in the manner that requires tyrosine kinase activity of ErbB4. Finally, deletion of ErbB4 in parvalbumin-positive interneurons led to reduced frequency and amplitude of mEPSCs, providing in vivo evidence that ErbB4 is important in excitatory synaptogenesis in interneurons. Together, our findings suggested a novel synaptogenic role of NRG1 in excitatory synapse development, possibly via stabilizing PSD-95, and this effect is specific to GABAergic interneurons. In light of the association of the genes of both NRG1 and ErbB4 with schizophrenia and dysfunction of GABAergic system in this disorder, these results provide insight into its potential pathological mechanism.

Perturbations in neuregulin-1 (NRG1)/ErbB4 function have been associated with schizophrenia. Affected patients exhibit altered levels of these proteins and display hypofunction of glutamatergic synapses as well as altered neuronal circuitry. However, the role of NRG1/ErbB4 in regulating synapse maturation and neuronal process formation has not been extensively examined. Here we demonstrate that ErbB4 is expressed in inhibitory interneurons at both excitatory and inhibitory postsynaptic sites. Overexpression of ErbB4 postsynaptically enhances size but not number of presynaptic inputs. Conversely, knockdown of ErbB4 using shRNA decreases the size of presynaptic inputs, demonstrating a specific role for endogenous ErbB4 in synapse maturation. Using ErbB4 mutant constructs, we demonstrate that ErbB4-mediated synapse maturation requires its extracellular domain, whereas its tyrosine kinase activity is dispensable for this process. We also demonstrate that depletion of ErbB4 decreases the number of primary neurites and that stimulation of ErbB4 using a soluble form of NRG1 results in exuberant dendritic arborization through activation of the tyrosine kinase domain of ErbB4 and the phosphoinositide 3-kinase pathway. These findings demonstrate that NRG1/ErbB4 signaling differentially regulates synapse maturation and dendritic morphology via two distinct mechanisms involving trans-synaptic signaling and tyrosine kinase activity, respectively.Although central nervous system synapses utilize a variety of brain-specific molecules to mediate contact formation and maturation, some of the proteins implicated in this process are also major players in neuromuscular junction development. Among these shared molecules are NRG1 3 and its receptor, ErbB4, which are expressed in both the developing and adult brain. Neuregulins comprise a family of four related genes (nrg1-4), each producing a large number of isoforms via differential promoter usage and alternative splicing (1, 2). NRGs contain EGF-like repeats, which enable them to bind to and activate EGF family receptors (ErbB2-4). Previous studies showed that NRG1 is initially synthesized as a transmembrane protein, which then undergoes proteolytic processing, whereby the extracellular EGF-containing fragment is released into the extracellular environment. The remaining intracellular fragment has been shown to translocate into the nucleus, where it regulates neuronal survival and transcription of PSD-95 (3, 4). Proteolytic processing of NRG1 is also regulated by neuronal activity and by interaction with ErbB receptors (3, 5). NRG1 is widely expressed throughout development and adulthood, with the highest expression in nervous tissue (6) and is essential for survival. In the central nervous system NRG1 is also required for differentiation, migration, and development of neurons and glia as well as for axonal myelination and pathfinding, dendritic development, and neurotransmitter receptor maintenance. During development, NRG1-ErbB signaling mediates radial glia maintenance and elonga...

SUMMARY Rapsyn, an acetylcholine receptor (AChR)-interacting protein, is essential for synapse formation at the neuromuscular junction (NMJ). Like many synaptic proteins, rapsyn turns over rapidly at synapses. However, little is known about molecular mechanisms that govern rapsyn stability. Using a differential mass-spectrometry approach, we identified heat-shock protein 90β (HSP90β) as a component in surface AChR clusters. The HSP90β-AChR interaction required rapsyn and was stimulated by agrin. Inhibition of HSP90β activity or expression, or disruption of its interaction with rapsyn attenuated agrin-induced formation of AChR clusters in vitro and impaired the development and maintenance of the NMJ in vivo. Finally, we showed that HSP90β was necessary for rapsyn stabilization and regulates its proteasome-dependent degradation. Together, these results indicate a role of HSP90β in NMJ development by regulating rapsyn turnover and subsequent AChR cluster formation and maintenance.