The Neuroscientist - Semantic Scholar

Thehttp://nro.sagepub.com/ Neuroscientist

Neuregulins and the Shaping of Synapses Miwako Ozaki Neuroscientist 2001 7: 146 DOI: 10.1177/107385840100700209 The online version of this article can be found at: http://nro.sagepub.com/content/7/2/146

Published by: http://www.sagepublications.com

Additional services and information for The Neuroscientist can be found at: Email Alerts: http://nro.sagepub.com/cgi/alerts Subscriptions: http://nro.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Citations: http://nro.sagepub.com/content/7/2/146.refs.html

>> Version of Record - Apr 1, 2001 What is This?

Downloaded from nro.sagepub.com at DREXEL UNIV LIBRARIES on November 1, 2011

Functional Multiplicity of Neuregulin

Volume 7, Number 2, 2001THE NEUROSCIENTIST

REVIEW

Neuregulins and the Shaping of Synapses MIWAKO OZAKI Laboratory for Cellular Information Processing, Brain Science Institute The Institute of Physical and Chemical Research (RIKEN) Japan

In developing and mature neural circuits, neural electrical activity controls the correct formation of connections and their state. Neuregulins (NRGs) mediate between the electrical neural activity and molecular components by regulating the expression of ion channel receptors or transmitter release in synapses. Furthermore, NRGs may be signaling factors involved in tuning locomotion or other higher functions by coordinating excitatory and inhibitory neurons. NEUROSCIENTIST 7(2):146–154, 2001 KEY WORDS Neuregulin, ErbB, NMDA receptor, GABAA receptor, Synapse, Activity-dependent

The neuregulins (NRGs), a large group of structurally related signaling proteins, are likely to have important biological functions. Four related NRG genes (NRG1, NRG2, NRG3, and NRG4) have been identified (Busfield and others 1997; Carraway and others 1997; Chang and others 1997; Fischbach and Rosen 1997; Zhang and others 1997; Harari and others 1999). The NRG family (Fig. 1) displays differential tissue distributions and receptor interactions for these ligands (Zhang and others 1998). In particular, the functions of NRG1 have been studied in detail. Products of the NRG1 gene have been implicated in such diverse cellular processes as heart formation (Peles and Yarden 1993), lung formation (Patel and others 2000), mammalian gland formation (Burden and Yarden 1997), milk production (Burden and Yarden 1997), the development and differentiation of neurons and glial cells (Peles and Yarden 1993; Burden and Yarden 1997, Fischbach and Rosen 1997), cell survival (Peles and Yarden 1993; Burden and Yarden 1997), and cell migration (Rio and others 1997). At first, NRG was referred to as acetylcholine receptor-inducing activity (ARIA), heregulin (HRG), neu differentiation factor (NDF), glial growth factor (GGF), and sensorimotor-derived factor (SMDF) (Peles and Yarden 1993; Burden and Yarden 1997; Fischbach and Rosen 1997). These names were changed to neruregulins, which are compounds that regulate especially neural properties. These molecules were independently studied as different isoforms of NRG1, but finally, it was revealed that these factors were produced from a single gene. Alternative splicing of at least 15 exons generates at least 14 different NRG1 mRNAs (Marchionni and others 1993; Fischbach and Rosen 1997; Shinoda and others 1997). Most of the NRG isoforms include an epithelial growth factor-like (EGF-like) domain, a C2-type immunoglobulin-like Address correspondence to: Miwako Ozaki, Laboratory for Cellular Information Processing, Brain Science Institute, RIKEN, Wako-shi, Saitama 351-0198, Japan; phone: +81 48 462 1111, ext. 7264; fax: +81 48 467 9691 (e-mail: [email protected]).

146

(Ig-like) domain, a transmembrane (TM) domain, and a cytoplasmic domain (Fischbach and Rosen 1997). Some neuregulin mRNAs encode a stop codon at the 3’-terminal of the EGF-like domain coding region and produce proteins lacking TM, in addition to cytoplasmic domains. Soluble forms of NRG can be produced in two ways: One soluble form is produced from mRNA and another from a membrane-anchored form of NRG through proteolytic cleavage (Loeb and others 1998; Han and Fischbach 1999; Loeb and others 1999; Ozaki and others 2000). The cytoplasmic tail of NRG, which mediates signals from presynaptic cells or bidirectional signaling by a subset of NRG isoforms, is responsible for the release of a soluble form of NRG via NRG-LIMK1 (nonreceptor kinase)-protein kinase C interaction (Liu and others 1998a, 1998b; Wang and others 1998; Ozaki and others 2000). In a similar manner to NRG1, several alternatively spliced forms of NRG2 have been reported. A similar domain structure is exhibited by both NRG1 and NRG2: an N-terminal domain that resembles GGF, a C2-like Ig-like domain, an EGF-like domain that determines the receptor-binding specificity, a TM domain, and a large intracellular domain. An EGF-like domain is included in NRG3, but it does not contain an Ig-like domain or a kringel domain in its extracellular region. Instead, NRG3 has a Ser/Thr rich domain with a number of potential sites for glycosylation. NRG3 also contains a large intracellular domain but it is not well conserved in NRGs 1 and 2. Within the nervous system, NRG1, NRG2, and NRG3 are all expressed, and NRG4 is also a transmembrane protein carrying a unique EGF-like motif and a short cytoplasmic domain. The mRNA is detected in the adult pancreas, in a small amount within muscle, and is undetectable in other tissues. Different NRG gene products that have distinct biochemical properties have been demonstrated in vitro, but may possibly mediate different biological functions in vivo (Yang and others 1998; Wolpowitz and others 2000). During embryogenesis, NRG1 is expressed in spatial and temporal patterns, suggesting that the

THE NEUROSCIENTIST Copyright © 2001 Sage Publications, Inc. ISSN 1073-8584



Fig. 1. Neuregulin and its receptor. The NRG family is composed of four members, NRG1, NRG2, NRG3, and NRG4. Each NRG displays a different tissue distribution and receptor interaction for these ligands. Neuregulin receptors, which fall within the ErbB family, belong to a subfamily of tyrosine kinases. There are four types: ErbB1 (EGF receptor), ErB2 (Neu), ErB3, and ErB4. A receptor functions by forming a homodimer or heterodimer, as shown, with a receptor of known combination. The cellular distribution and binding affinities for each distinct NRG isoform, and the tyrosine kinase activities, vary between ErbB receptors. The ErbB2 receptor has a high kinase activity but fails to directly bind NRG1-4. The ErbB3 receptor binds NRG1 and NRG2 but is a kinase defective protein. As shown here, NRG1 is observed to bind ErbB3 and ErbB4 and to sequester ErbB2 by receptor dimerization. Also, NRG2 is a pan-ErbB ligand. NRG2 is a ligand of ErbB4, and NRG4 acts through the ErbB4 receptor tyrosine kinase.

different NRG1 isoforms might mediate distinct biological signals, even though NRGs show substantial functional multiplicity (Mayer and others 1997; Covello and others 1998; Pinkas-Kramarski and others 1998; Zhang and others 1998). The functions of NRGs in the nervous system are summarized in Figure 2. NRGs are involved in synapse f ormation and maintenance at the neuromuscular junction (NMJ) (Fischbach and Rosen 1997; Sandrock and others 1997) and in the central nervous system (Ozaki and others 1997; Rieff and others 1999), in neural cell migration (Rio and others 1997), the differentiation of glial cells, myelination (Jessen and Mirsky 1999; Adlkofer and Lai 2000), and apoptosis (Jessen and Mirsky 1999; Adlkofer and Lai 2000). In addition, the expression of NRGs, and an ErbB4 from NRG receptors, has been induced by stimulation that is activity dependent (Eilam and others 1998). Recently, all identified NRG1 isoforms have been classified into two broad categories: one isoform contains an Ig-like domain (Ig-NRG) and the other isoform has a cysteine-rich domain (CRD-NRG). The isoform that was originally referred to as SMDF (Fischbach and Rosen 1997) has a cysteine-rich region that is hydrophobic. In the NMJ, Ig-NRG controls acetylcholine

receptor (AChR) synthesis in muscle, whereas CRD-NRG induces a nicotinic acetylcoline receptor (nAChR). This suggests that CRD-NRG may be a regulator in synapse formation between neurons, not in neuromusclar synapse. CRD-NRGs and Ig-NRGs have significant differences as well as similarities (Yang and others 1998). There have been several investigations of NRG gene disruption. Homozygous mice of all NRG1 isoforms (NRG1–/–), all Ig-NRG1 isoforms (Ig-NRG1–/–), and all isoforms containing a cytoplasmic tail are observed to die at embryonic day 10.5 (E10.5) from heart defects (Meyer and Birchmeier 1995; Sanes 1997). Studies of neuromuscular transmission in heterozygous Ig-NRG+/– mice reveal that for the initial induction, and/or maintenance, of the mature level of AChRs Ig-NRG isoform is required in neuromuscular synapses (Sandrock and others 1997). In addition, knockout mice of CRD-NRG have been analyzed. The homozygous mice (CRD-NRG–/–) can survive past postnatal day 0, but the pups lack detectable limb movement (Wolpowitz and others 2000). The mice show abnormalities in peripheral projections, and the motor nerve terminals are transiently associated with broad bands of

Volume 7, Number 2, 2001

THE NEUROSCIENTIST


147

Fig. 2. The roles of neuregulin in the nervous system. The illustration is a summary of the functions of neuregulin in the nervous system. Blue denotes cells that provide NRG. Synaptic states are controlled by regulating the expression of ion channel receptors in peripheral neurons and central neurons. Interestingly, NRG and ErbB4 receptor expression are controlled in an activity-dependent manner. Accordingly, kinate stimulation, LTP, and forced locomotion cause up-regulation NRG and ErbB4 expression in the selected areas where the stimulation is received. The observations suggest that NRG expression is regulated in a use-dependent manner. Other functions, including myelination, cell migration, glial cell differentiation, and regulation of neuron and glial cell death have been reported.

postsynaptic AChR clusters. There are essential roles of CRD-NRG1. The first is to mediate signaling for coordinating nerve, target, and Schwann cell interactions in the normal maintenance of the peripheral synapses. The second role ultimately is in the survival of CRD-NRGexpressing neurons. Neuregulin Receptors The neuregulin receptors fall within the ErbB family, which is a group belonging to one of the tyrosine kinase subfamilies. There are four types: the EGF receptor (ErbB1), ErbB2 (Neu), ErbB3, and ErbB4. The ErbB2, ErbB3, and ErbB4 are neuregulin receptors, and recently ErbB1 has also been reported to function as neuregulin’s receptor (Zhang and others 1998). The receptors function by f orming a homodimer or heterodimer from the four types. All of the receptors, except ErbB3, show autophosphorylation activity. However, the cellular distribution, binding affinities for distinct NRG isoforms, and tyrosine kinase activities vary between the ErbB receptors (Lemke 1996; Burden and Yarden 1997; Gassman and Lemke 1997; Ozaki and others 1998). Theoretically, the four ErbB proteins can form all of the 10 possible homo- and heterodimeric complexes. However, the ErbB2 receptor (the ligandless

148

receptor) has a high kinase activity but fails to directly bind NRG1-4. Furthermore, ErbB3 (the kinase-defective receptor) binds NRG1 and NRG2 but has low kinase activity. To function, ErbB2 and ErbB3 require heterodimerization (Lemke 1996; Gassmann and Lemke 1997). Figure 1 illustrates that NRG1 is observed to bind ErbB3 and ErbB4 and to sequester ErbB2 by receptor dimerization (Burden and Yarden 1997; Fischbach and Rosen 1997). The figure shows that NRG2 is a pan-ErbB ligand (Busfield and others 1997; Carraway and others 1997; Chang and others 1997; Zhang and others 1998) and that NRG3 is a ligand of ErbB4 (Zhang and others 1997). Moreover, NRG4 acts through the ErbB4 receptor tyrosine kinase (Harari and others 1999). Because ErbB4 binds NRG either as a homodimer or a heterodimer, it differs from ErbB2 and ErbB3. Furthermore, the ErbB4 receptor is predominantly expressed in the nervous system. The ErbB2, ErbB3, and ErbB4 are localized in the postsynaptic density (basically, ErbB3 is a receptor for glial cells). However, the ErbB4 interacts with three membraneassociated guanylate kinases (MAGUKs), which are PSD95/SAP90, PSD93/chapsin110 and SAP102. It also interacts with β-syntrophin in the postsynaptic density (Garcia and others 2000). The β-syntrophin has a single

THE NEUROSCIENTIST



PDZ domain, and the remaining three molecules harbor three PDZ domains. The ErbB4 c-terminal contains a sequence that binds to a protein motif, which is known as the PDZ domain. The downstream targets of NRG action in skeletal muscle are well understood and have been studied intensively. During development, NRG1 accumulates with the ErbB2-4 receptors at the NMJ. By studying AChR gene expression, the NRG signaling pathway has been made increasingly clear. Binding of the NRG results in receptor dimerization and autophosphorylation of tyrosine residues, signaling by means of the Ras/Raf/MEK/MAPK pathway (Tansey and others 1996; Altiok and others 1997). Several studies have implicated Ets proteins (known targets of the MAP kinase pathway) as components of the activated transcriptional machinery (Fromm and Burden 1998). A potential Ets-protein-binding site has been identified as a neuregulin-response element (NRE), and some NRE binding proteins have been identified (Fromm and Burden 1998; Sapru and others 1998). The NRG and ErbB proteins localized in the adult NMJ are required for synapse- specific transcription. Current evidence supports NRG as the best candidate for the primary signal of synapse-specific transcription. Knockout mice for ErbB2, ErbB3, and ErbB4 have been established. Homozygous mice of ErbB2 and -4 (ErbB2–/–, ErbB4–/–) also die from cardiac defects at E10.5 (Gassman and others 1995; Lee and others 1995; Sanes 1997; Lin and others 2000). With respect to ErbB3, mice live until the early postnatal days beyond E13.5 (Erickson and others 1997; Riethmacher and others 1997). However, abnormalities are displayed in formation of the cerebellum and in development of dorsal root ganglion neurons with an absence of Schwann cells. Animals that are ErbB2–/– and ErbB3–/– and in which cardiac expression is rescued, can survive until birth (Morris and others 1999; Woldeyesus and others 1999). But the mutant mice display a severe loss of both motor and sensory neurons. The motor and sensory axons are severely defasciculated and aberrantly projected within their final target tissues, and Schwann cells are completely absent in the peripheral nerve. The NRG/ErbB signaling system participates in fundamental processes underlying the development of components of both the peripheral and central nervous system. The Function of Neuregulin in Synapses The NRG molecule retains various functional aspects even in synapse formation. Neuregulin is also a key molecule in studying dynamic changes in adult synapses that occur in an activity-dependent manner. Chronic disorders of an adult may result from a variety of early developmental events. For example, any disturbance that locally perturbs regulation of NMDA receptors, or the temporal correlations in synaptic activity that drive these receptors, has the potential to alter the normal de-

velopment of local circuitry and the critical balance of inhibition and excitation in the brain. In the NMJ, Ig-NRG regulates the expression of the AChR subunit genes in muscles. Recently, it has been reported that CRD-NRG regulates the channel activity and expression of nAChR subunit genes at developing interneural synapses. The effect of CDR-NRG is confined to nAChR expression, as there is no effect on a response evoked with GABA (γ -aminobutyric acid). On the other hand, an analysis of Ig-NRG effects on ion channels reveals a very different result: After 2 to 4 days of treatment with equivalent Ig-NRG activity, there is a marked increase in the magnitude of GABA-evoked currents and a slight decrease in the magnitude of the nAChR-mediated currents (Yang and others 1998). Both CRD-NRG and Ig-NRG regulate the coordination of excitatory and inhibitory signals. Furthermore, disruption of CRD-NRG causes withdrawal and degeneration of sensory and motor nerves. This demonstrates the importance of inhibitory signals for a normal maintenance of peripheral nerves (Wolpowitz and others 2000). In the cerebellum, the expression of glutamate-gated NMDA receptors at the mossy fiber-granule cell synapse has been observed to be controlled by Ig-NRG and NMDA receptor (Ozaki and others 1997). Developmental changes in NMDA receptor subunits are most evident in cerebellar granule cells. The mossy fiber-granule cell synapse further matures by switching subunits of the NMDA receptor subunit from NR2B to NR2C. If the receptor subunit characteristics in vivo are similar to those found by expression in vitro (Monyer and others 1994), most of these developmental changes should have a common functional effect. Young animals should express NMDA receptors having NR2B subunits that flux large currents, and the receptor subtypes having NR2C subunits expressed in older animals should flux smaller currents. A change of NMDA receptor function is paralleled by activity-dependent development of receptor subunit expression in cerebellar neurons. In an intact cerebellum, the relative contribution of NMDA receptors to postsynaptic potentials in granule cells decreases during the second and third postnatal weeks, as the cells finish migrating and become innervated by mossy fibers (Farrant and others 1994; Monyer and others 1994). In a cerebellar slice culture, Ig-NRG regulates the expression of the NMDA receptor subunit NR2C as neurally derived factors. This occurs after 7 days culture under a condition of chronic depolarization and NMDA receptor stimulation (Ozaki and others 1997; Ozaki and others 2000; M. Ozaki and others, personal communication). Survival and differentiation of the cultured neurons appear to require NMDA receptor activation after about 5 days in culture, because cells grown in physiologic concentrations of potassium begin to die at that time (Gallo and others 1987). Treatment with low doses of NMDA or glutamate, or a depolarizing concentration of potassium (which causes release of endogenous


THE NEUROSCIENTIST


149

Fig. 3. A model for controlling the balance between excitatory and inhibitory signals. In the cerebellum, Ig-NRG trans-synaptically regulates the expression of the NMDA receptor subunit NR2C as neurally derived factors. On the other hand, Ig-NRG also controls GABAA receptor β2 subunit expression in granule cells. In the case of induction of GABAA receptor, it is not known which cells provide NRG. When neurons do not have any neural activity, NR2CmRNA is not induced (I). If the electrical activity, or a specific pattern of neural impulses, is retained, induction of NR2C may occur in granule cells (II). An alternative possibility is that the signal from GABAA receptor may affect NR2C expression (III). Furthermore, the fact that the NRG receptor (ErbB4) and NMDA receptor interact with PSD95 suggests that the involvement of some PSD components in balancing excitatory and inhibitory signals.

glutamate), promotes survival and differentiation of the neurons in a dose-dependent manner (Moran and Patel 1989; Burgoyne and others 1993). These treatments also increase NMDA receptor currents in cultured cells (Van der Valk and others 1991). Treatment such as chronic depolarization or NMDA stimulation may reproduce the effect of mossy fiber innervation on immature granule cells during normal cerebellar development. On the other hand, Ig-NRG also controls GABAA receptor expression in granule cells (Rieff and others 1999). In cerebellar glomeruli, there are inhibitory circuits such as Golgi cell-granule cell interactions. The Golgi cells release GABA transmitter and are believed to contribute to reduction and filtering of mossy fiber excitatory input. In the granule cells, the number of GABAA receptor complexes and the stoichiometry of these complexes is unknown, but the α1, β2, and γ 2 subunits are thought to contribute to an abundant receptor type (Benke and others 1991). Control of expression of the GABAA β2 subunit by Ig-NRG is observed in a relatively early time course (2 days in vitro) by using a dissociation culture. These data may suggest that Ig-NRG directly controls expression of the GABAA receptor subunit. Thus far, some groups have reported that a pattern of neural activity carries various information to nerve cells

150

and decides the fate of cells through gene expression, protein phospholyration, and other processes (Buonanno and Fields 1999). When the electrical activity, or a specific pattern of neural impulses, is retained (as shown in Fig. 3), induction of NR2C may occur in granule cells. In the case of NR2C, its distinct expression by NRG is observed only in an organotypic culture after 7 days culture. Induction of NR2C is not observed with a dissociation culture and treatment with NMDA receptor antagonist or tetrodotoxin (TTX), a blocker for spontaneous activity (Ozaki and others 1997; Rieff and others 1999). The common factor is that in all conditions the neurons are inactive in basal neural activities (M. Ozaki, personal communication). There are many possibilities for the difference in response of dissociation and organotypic cultures. However, a definite difference between them may be the neuronal electrical activity. Another possibility is that the signal from the GABAA receptor may affect NR2C expression. Furthermore, the fact that the NRG receptor (ErbB4) and the NMDA receptor interact with PSD95 suggests the involvement of some PSD components in the balance between excitatory and inhibitory signals. A balance of NMDA receptor activation and GABA inhibition is essential for motor coordination (Watanabe and others 1998), or the fine tuning of neuronal excitability after synaptic input

THE NEUROSCIENTIST



in the cerebellum (Mitchell and Silver 2000), in the olfactory bulb (Chen and others 2000) and in the retina (Wong 1999). Neural Networks in the Cerebellum Afferent mossy fibers are mainly composed of trigeminal mossy fibers, spinocerebellar mossy fibers, vestibular mossy fibers, reticular mossy fibers, and pontocerebellar mossy fibers (Altman and Bayer 1997; Voogd and Glickstein 1998). So that a global mechanism of information processing in the cerebellar neural network can be understood, it is necessary to clarify the synaptic organization of the mossy fiber-granule cell synapse. A requirement is to determine how these different groups of mossy fiber display a distinct functional heterogeneity, and how information processing occurs within mossy fiber systems. It is believed that NR2C expression is controlled by mossy fiber innervation, but the actual relationship of the innervation of each mossy fiber system to NR2C expression is unclear. From only a few studies, it appears that the pontocerebellar mossy fiber system is more likely to be involved in the NRG-related innervation (Ozaki and others 2000). Moreover, the pontine nuclei are the relay points between the cerebrum, cerebellar cortex, and peripheral regions. When motor coordination is considered, the pontocerebellar mossy fiber system should play a specific role in information processing, and not just be involved as an information relay system. In addition, the pontocerebellar mossy fibers play an important role in formation of the somatotopic map, which is revealed by electrophysiological mapping of mossy fiber responses in the cerebellum (Voogd and Glickstein 1998). Consequently, the role of NRG in the pontocerebellar mossy fiber system may be to form and/or maintain the structure of the somatotopic map. Investigations using lipophilic tracer DiI labeling in adults have shown that (our observations) the terminals of pontocerebellar mossy fibers expand on the central lobe as a center of distribution. During development, the innervation starts from lobule IX, not from lobule X, to the anterior lobe. In contrast, NR2C mRNA distribution expands zonally with a lobule unit from the posterior lobule, that is, from lobule X (Watanabe and others 1992). The relationship between the innervation of pontocerebellar mossy fibers and NR2C expression may be a coincidence, but it cannot be ruled out entirely from anatomical studies only (Ozaki and others 1999; M. Ozaki and others, personal communication). For additional evidence, a reporter gene, β-galactosidase gene (LacZ), was inserted into NR2C genome, and the knock-in mouse was established by using transgenic mouse techniques. In experiments on an organotypic coculture of pontine nuclei and granule cells of the transgenic mouse, NR2C expression was observed by mossy fiber contact throughout the β-galactosidase expression (Buonanno and others 1998). This result also supports the evidence that NRG is provided from mossy fibers. Mossy fiber innervation may mean maturation of

a substantial synaptic contact or circuit and not physical contact of one synapse only. Further investigation is required of the pontocerebellar mossy fiber system, its interaction with other mossy fiber systems, and its relationship with the development of granule cells. An additional function of NRG is that the membraneanchored form displays a homophilic binding activity that is similar to an adhesion molecule in the pontocerebellar system (Ozaki and others 2000). Our observations suggest, on the basis of a period-specific (postnatal days 7-9) trans-synaptic cell labeling with DiI (M. Ozaki and T. Hashikawa, personal communication), that when a mossy fiber contacts a granule cell, and the formation of a synaptic junction commences, information may be exchanged between the two. At this stage, the membrane- anchored form of NRG may operate as an adhesion molecule for synaptic recognition or junction formation. In addition, because the isoforms of NRG show wide diversity, the possibility of heterophilic interaction cannot be excluded during synaptogenesis in other mossy fiber systems (excluding the pontocerebellar mossy fiber system). Another possibility is that the membrane-anchored form of NRG may undergo stimulation-dependent proteolytic cleavage after the exchange of molecular information through bidirectional signaling and/or reception of a signal at the presynaptic neuron to shape the synapses. The specific period from postnatal days 7 to 9 may be a critical period for establishment of mossy fiber-granule cell synapse. Many neural circuits may be determined by competition involving electrical activity within the critical period, and this may influence the properties of adult brains. Neuregulin-Related Neural Diseases and Possibility of Remedies The functional multiplicity of NRG suggests a high level of involvement in diseases of the nervous system. NRG1 is located on human chromosome 8p11-22, whereas NRG2 is located on human chromosome 5q23-33 (Zhang and others 1998). NRG3 has been mapped to human chromosome 10q22 (Zhang and others 1998). Because NRGs can control demyelination, remyelination, neural degeneration, and neural regeneration, the NRG family may be a candidate for some human disease-related genes. Considering this field overall, NRG is probably involved in synaptic tuning in addition to neural coordination, by mediating responses to environmental change. One of the important roles of NRG at synapses is to control ion channel gene expression. Electrophysiological and genetic studies indicate that ion channel abnormalities cause several disorders. Heterozygous mice that are NRG-deficient, through the deletion of NRG isoforms containing an Ig-like domain, are myasthenic because of abnormalities in the AChR (Sandrock and others 1997). In the central nervous system, the expression of NMDA and GABA receptors is controlled by NRG. An NMDA receptor subtype is involved in activity-dependent


THE NEUROSCIENTIST


151

synaptic plasticity in many developing and adult systems. A blockade of NMDA receptors with the competitive antagonist D-AP5 interferes with developmental synapse organization in the primary visual cortex (Bear and others 1990), the cerebellar Purkinje cells (Rabacchi and others 1992), the somatosensory cortex (Schlaggar and others 1993), and both the superficial and deep layers of the superior colliculus (Schnupp and others 1995). In adults, the NMDA receptor subtype appears more likely to be related to more compound movements. Nakanishi’s group has demonstrated that NR2A and NR2C double knockout mice walk normally and can manage simple coordinated tasks, but they fail to perform more complicated movements (Kadotani and others 1996). In addition, the movement disorders of Golgi cell-ablated transgenic mice closely resemble those of NR2A/NR2C double knockout mice (Watanabe and others 1998). From this evidence, abnormalities of the NMDA receptor or of a critical balance between excitatory and inhibitory signals could cause certain movement disorders by loss of synaptic integration (Watanabe and others 1998). Furthermore, cognitive disorders could be caused by a similar mechanism. One example of a human cognitive disorder is Williams’ syndrome, in which the specific cognitive profile is characterized by a pronounced weakness in visuospatial constructive cognition. Neuregulin may be involved in cognitive disorders as well as motor disorders (Ozaki and others 2000). The study of activity-dependent synaptic development provides many scenarios for treatment of neurological disorders including the following: 1) Disruption of early activity patterns might result in a spreading imbalance between excitation and inhibition; 2) because some late-developing chronic seizure disorders (e.g., epilepsy) are associated with transient childhood seizures (Sagar and Oxbury 1987), disorders that occur in adults might be corrected by changing neural activity; 3) even neuronal degeneration in aging might be treated by changing the patterns of neural activity. With specific neural activity, NRG induces NR2C but NRG itself is also activity dependent (Eilam and others 1998). NRG and ErbB4 expression are induced at transcriptional and translational levels in a region-selective manner, by activity such as forced locomotion, epileptic seizures, and pathway-specific tetanic stimulation. The multiple regulatory mechanisms provided by NRG at the synapse may have an advantage for translating use-dependent signals into altered molecules. Neuregulins may represent one of the best families of molecules for studying the molecular mechanisms underlying activity-dependent plasticity, not only during synaptogenesis such as target recognition and synaptic maturation but also during dynamic change of synapses in mature neurons. We have examined the mechanisms of synapse formation and maintenance from an overall viewpoint. Current studies have revealed that a particular pattern of neural activitiy, for example, frequency, could be

152

effective in the retinal surface of blind from end-stage retinitis pigmentosa or age-related macular degeneration (Humayun and others 1999; Weiland and others 1999). In some cases, a particular pattern of neural activity may decide neural fate and be able to control brain function. As a further possibility, human brain tissue may be able to be replaced by an artificial brain having appropriate programmed activity patterns. Such a replacement could allow repair of the brain or treatment of a disorder by developing suitable devices having different function frequencies. A recent study of application to humans suggests that there is a high possibility of this in the near future. Acknowledgments I thank Drs. R. D. Fields and R. Yano for critically reading the manuscript. References Adlkofer K, Lai C. 2000. Role of neuregulins in glial cell development. Glia 29(2):104–11. Altiok N, Altiok S, Changeux JP. 1997. Heregulin-stimulated acetylcholine receptor gene expression in muscle: requirement for MAP kinase and evidence for a parallel inhibitory pathway independent of electrical activity. EMBO J 16:717–25. Altman J, Bayer SA. 1997. In: Development of the cerebellar system. Boca Raton, FL: CRC Press. p 44–53. Bear M, Kleinschmidt A, Gu Q, Singer W. 1990. Disruption of experience-dependent synaptic modifications in striate cortex by infusion of an NMDA receptor antagonist. J Neurosci 10:909–24. Benke D, Merthens S, Trzeciak A, Gillessen D, Mohler H. 1991. GABAA receptors display association of gamma2-subunit with alpha1- and beta2/3-subunits. J Biol Chem 266:4478–83. Buonanno A, Cheng J, Wynshaw BT, Sasner M. 1998. Differential regulation of NMDA receptors by activity and neuregulin. Neurosci Abstr 28:111.8.2. Buonanno A, Fields RD. 1999. Gene regulation by patterned electrical activity during neural and skeletal muscle development. Curr Opin Neurobiol 9:110–20. Burden S, Yarden Y. 1997. Neuregulins and their receptors: a versatile signaling module in organogenesis and oncogenesis. Neuron 18:847–55. Burgoyne R, Graham M, Cambray-Deakin M. 1993. Neurotrophic effects of NMDA receptor activation ion developing cerebellar granule cells. J Neurocytol 22:687–95. Busfield SJ, Michnick DA, Chickering TW, Revett TL, Woolf EA, Comrack, CA and others. 1997. Characterization of a neuregulin-related gene, Don-1, that is highly expressed in restricted regions of the cerebellum and hippocampus. Mol Cell Biol 17:4007–14. Carraway KL III, Weber JL, Unger MJ, Ledesma J, Yu N, Gassmann M, Lai C. 1997. Neuregulin-2, a new ligand of ErbB3/ErbB4receptor tyrosine kinases. Nature 387:512–6. Chang H, Riese DJ II, Gilbert W, Stern DF, McMahan UJ. 1997. Ligands for ErbB-family receptors encoded by a neuregulin-like gene. Nature 387:509–12. Chen WR, Xiong W, Shepherd GM. 2000. Analysis of relations between NMDA receptors and GABA release at olfactory bulb reciprocal synapses. Neuron 25:625–33. Covello CS, Lai C, Cantley LC, Carraway KL III.. 1998. Differential signaling by the epidermal growth factor-like growth factors neuregulin-1 and neuregulin-2. J Biol Chem 273:26954–61. Eilam R, Pinkas-Kramarski R, Ratzkin BJ, Segal M, Yarden Y. 1998. Activity-dependent regulation of Neu differentiation factor/neuregulin expression in rat brain. Proc Natl Acad Sci U S A 95:1888–93.

THE NEUROSCIENTIST



Erickson SL, O’Shea KS, Ghaboosi N, Loverro L, Frantz G, Bauer M, and others. 1997. ErbB3 is required for normal cerebellar and cardiac development: a comparison with ErbB2- and heregulin-deficient mice. Development 124:4999–5011. Farrant M, Feldmeyer D, Takahashi T, Cull-Candy, SG. 1994. NMDA-receptor channel diversity in the developing cerebellum. Nature 368:335–9. Fischbach GD, Rosen KM. 1997. ARIA: a neuromuscular junction neuregulin. Annu Rev Neurosci 20:429–58. Fromm L, Burden SJ. 1998. Synaptic-specific and neuregulin-induced transcription require an Ets site that binds GABPα/GABPβ. Genes Dev 12:3074–83. Gallo V, Kingsbury A, Balazs R, Jorgensen OS. 1987. The role of depolarization in the survival and differentiation of cerebellar granule cells in culture. J Neurosci 7:2203–13. Garcia RA, Vasudevan K, Buonanno A. 2000. The neuregulin receptor ErbB-4 interacts with PDZ-containing proteinsat neuronal synapses. Proc Natl Acad Sci U S A, 97(7):3596–3601. Gassmann M, Casagranda F, Orioli D, Simon H, Lai C, Klain R, Lemke G. 1995. Aberrant neural and cardiac development in mice lacking the ErbB4 neuregulin receptor. Nature 378:390–4. Gassmann M, Lemke G. 1997. Neuregulins and neuregulin receptors in neural development. Curr Opin Neurobil 7:87–92. Han B, Fischbach. 1999. Processing of ARIA and release from isolated nerve terminals. Phil Trans R Soc Lond B 354:411–6. Harari D, Tzahar E, Romano J, Shelly M, Pierce JH, Andrews GC, and others. 1999. Neuregulin-4: a novel growth factor that acts through the ErbB-4 receptor tyrosine kinase. Oncogene 18:2681–9. Humayun MS, de Juan E Jr, Weiland JD, Dagnelie G, Katona S, Greenberg R, Suzuki S. 1999. Pattern electrical stimulation of the human retina. Vision Research 39:2569–76. Jessen KR, Mirsky R. 1999. Why do Schwann cells survive in the absence of axons? Ann N Y Acad Sci 883:109–15. Kadotani H, Hirano T, Masugi M, Nakamura K, Nakao K, Katsuki M, and others. 1996. Motor discoordination results from combined gene disruption of the NMDA receptor NR2A and NR2C subunits, but not from single disruption of the NR2A or NR2C. J Neurosci 16:7859–67. Lee KF, Simon H, Chen H, Bates B, Hung MC, Hause C. 1995. Requirement for neuregulin receptor erbB2 in neural and cardiac development. Nature 378:394–8. Lemke G. 1996. Neuregulins in development. Mol Cell Neurosci 7:247–62. Lin W, Sanchez HB, Deerinck T, Morris JK, Ellisman M, Lee KF. 2000. Aberrant development of motor axons and neuromuscular synapses in erbB2-deficient mice. Proc Natl Acad Sci U S A 97(3):1299–304. Liu X, Hwang H, Cao L, Buckland M, Cunningham A, Chen J, and others. 1998a. Domain-specific gene disruption reveals critical regulation ofneuregulin signaling by its cytoplasmic tail. Proc Natl Acad Sci U S A 95(22):13024–9. Liu X, Hwang H, Cao L, Wen D, Liu N, Graham RM, Zhou M. 1998b. Release of the neuregulin functional polypeptide requires its cytoplasmic tail. J Biol Chem 273(51):34335–40. Loeb JA, Khurana TS, Robbins JT, Yee AG, Fischbach GD. 1999. Expression patterns of transmembrane and released form of neuregulin during spinal cord and neuromuscular synapse development. Development 126:781–91. Loeb JA, Susanto ET, Fischbach GD. 1998. The neuregulin precursor proARIA is processed to ARIA after expression on the cell surface by a protein kinase C-enhanced mechanism. Mol Cell Neurosci 11 (1/2):77–91. Marchionni MA, Goodearl AD, Chen MS, McDonogh BO, Kirk C, Hendrichs M, and others. 1993. Glial growth factors are alternatively spliced erbB2 ligands expressed in the nervous system. Nature 362:312–8. Mayer D, Yamaai, Garratt A, Riethmacher-Sonnenberg E, Kane D, Theill LE, Birchmeier C. 1997. Isoform-specific expression and function of neuregulin. Development 124:3575–86. Meyer D, Birchmeier C. 1995. Multiessential functions of neuregulin in development. Nature 378:386–90. Mitchell SJ, Silver RA. 2000. Glutamate spillover suppresses inhibition by activating presynaptic m GluRs. Nature 404:498–502.

Monyer H, and others. 1994. Developmental and regional expression in the rat brain and functional properties of four NMDA receptors. Neuron 12:529–40. Moran J, Patel A. 1989. Stimulation of the N-methyl-D-aspartate receptor promotes the biochemical differentiation of cerebellar granule neurons and not astrocytes. Brain Res 486:15–25. Morris JK, Lin W, Hauser C, Marchuk Y, Getman D, Lee KF. 1999. Rescue of the cardiac defect in ErbB2 mutant mice reveals essential roles of ErbB2 in peripheral nervous system development. Neuron 23(2):273–83. Ozaki M. 2000. Roles of neuregulin in synaptogenesis between mossy fibers and cerebellar granule cells. J Neurosci Res 59(5):612–23. Ozaki M, Hashikawa T, Yano R. 1999. The connection between granule cells and mossy fibers changes dynamically during postnatal cerebellar development. In: Uyemura K, Kawamura K, Yazaki T, editors. Neural development. Tokyo: Springer. p 425–9. Ozaki M, Kishigami S, Yano R. 1998. Expression of receptors for neuregulins, ErbB2, ErbB3 and ErbB4 in developing mouse cerebellum. Neurosci Res 30(4):351–4. Ozaki M, Sasner M, Yano R, Lu HS, Buonanno A. 1997. Neurgulin-β induces expression of an NMDA receptor subunit. Nature 390:691–4. Patel NV, Acarregui MJ, Snyder JM, Klein JM, Sliwkowski MX, Kern JA, and others. 2000. Neuregulin-1 and human epidermal growth factor receptors 2 and 3 play a role in human lung development in vitro. Cell Mol Biol 22(4):432–40. Peles E, Yarden Y. 1993. Neu and its ligands: from an oncogene to neural factors. BioEssays 15:815–24. Pinkas-Kramarski R, Shelly M, Guarino BC, Wang LM, Lyass L, Alroy I, and others. 1998. ErbB tyrosine kinases and the two neuregulin families constitute a ligand-receptor network. Mol Cell Biol 18:6090–101. Rabacchi S, Bailly Y, Delhaye-Bouchaud N, Mariani J. 1992. Involvement of the N-methyl-D-aspartate (NMDA) receptor in synapse elimination during cerebellar development. Science 256:1823–5. Rieff HI, Raetzman LT, Sapp DW, Yeh HH, Siegel RE, Corfas G. 1999. Neuregulin induces GABA(A) receptor subunit expression and neuriteoutgrowth in cerebellar granule cells. J Neurosci 19(24):10757–66. Riethmacher D, Riethmacher SE, Brinkmann V, Yamaai T, Lewin GR, Birchmeier C. 1997. Severe neuropathies in mice with targeted mutations in the ErbB3 receptor. Nature 389:725–30. Rio C, Rieff HI, Qi P, Corfas G. 1997. Neuregulin and erbB receptors play a critical role in neuronal migration. Neuron 19:39–50. Sagar H, Oxbury J. 1987. Hippocampal neuron loss in temporal lobe epilepsy: correlation with early childhood convulsions. Ann Neurol 22:334–40. Sandrock AW Jr, Dryer SE, Rosen KM, Gozani SN, Kramer R, Theill LE, and others. 1997. Maintenance of acetylcholine receptor number by neuregulins at the neuromuscular junction in vivo. Science 276:599–603. Sanes JR. 1997. Genetic analysis of postsynaptic differentiation at the vertebrate neuromuscular junction. Curr Opin Neurobiol 7:93–100. Sapru MK, Florance SK, Kirk C, Goldman D. 1998. Identification of a neuregulin and protein-tyrosine phosphatase response element in the nicotinic acetylcholine receptor ε subunit gene: regulatory role of an ets transcription factor. Proc Natl Acad Sci U S A 95:1289–94. Schlaggar B, Fox K, O’Leary D. 1993. Postsynaptic control of plasticity in developing somatosensory cortex. Nature 364:623–6. Schnupp JW, King AJ, Smith AL, Thompson ID. 1995. NMDA-receptor antagonists disrupt the formation of auditory space map in the mammalian superior colliculus. J Neurosci 15:1516–31. Shinoda J, Nakao J, Iizuka Y, Taba Y, Yazaki T, Kawase T, and others. 1997. Multiple isoforms of neuregulin are expressed in developing rat dorsal root ganglia. J Neurosci Res 50:673–83. Tansey M, Chu GC, Merlie JP. 1996. ARIA/HRG regulates AchR ε subunit gene expression at the neuromuscular synapse via activation of phosphatidylinositol 3-kinase and Ras/MAPK pathway. J Cell Biol, 134(2):465–76. Van der Valk J, Resink A, Balazs R. 1991. Membrane depolarization and the expression of glutamate receptors in cerebellar granule cells. Eur J Pharmacol 201:247–50.


THE NEUROSCIENTIST


153

Voogd J, Glickstein M. 1998. The anatomy of the cerebellum. Trends Neurosci 21:370–5. Wang JY, Frenzel KE, Wen D, Falls DL. 1998. Transmembrane neuregulins interact with LIM kinase 1, a cytoplasmic protein kinase implicated in development of visuospatial cognition. J Biol Chem 273(32):20525–34. Watanabe D, Inokawa H, Hashimoto K, Suzuki N, Kano M, Shigemoto R, and others. 1998. Ablation of cerebellar Golgi cells disrupts synaptic integration involving GABA inhibition and NMDA receptor activation in motor coordination. Cell 95(1):17–27. Watanabe M, Inoue Y, Sakimura K, Mishina M. 1992. Developmental changes in distribution of NMDA receptor channel subunit mRNAs. NeuroReport 3:1138–40. Weiland JD, Humayun MS, Dagnelie G, de Juan E Jr, Greenberg RJ, Illiff NT. 1999. Understanding the origin of visual percepts elicited by electrical stimulation of the human retina. Graefe’s Arch Clin Exp Ophthalmol 237:1007–13. Woldeyesus MT, Birtsch S, Riethmacher D, Xu L, Sonnenberg Riethmacher E, Abou-Rebyeh F, and others. 1999. Peripheral

154

nervous system defects in erbB2 mutants following genetic rescue of heart development. Genes Dev 13:2538–48. Wolpowitz D, Mason TB, Dietrich P, Mendelsohn M, Talmage DA, Role LW. 2000. Cysteine-rich domain isoforms of the neuregulin-1 gene are required for maintenance of peripheral synapses. Neuron 25(1):79–91. Wong ROL. 1999. Retinal waves and visual ayatem development. Anu Rev Neurosci 22:29–47. Yang X, Kuo Y, Devay P, Yu C, Role L. 1998. A cysteine-rich isoform of neuregulin controls the level of expression of neuronal nicotinic receptor channels during synaptogenesis. Neuron 20:255–70. Zhang D, Sliwkowski MX, Mark M, Frantz G, Akita R, Sun Y, and others. 1997. Neuregulin-3 (NRG-3): a novel neural tissue-enriched protein that binds and activates ErbB4. Proc Natl Acad Sci U S A 94:9562–7. Zhang D, Frantz G, Godowski PJ. 1998. New branches on the neuregulin family tree. Mol Psychiatry 3(2):112–5.

THE NEUROSCIENTIST

