Neuregulin and ErbB receptor signaling pathways in the nervous system

The neuregulins are a complex family of factors that perform many functions during neural development. Recent experiments have shown that neuregulins promote neuronal migration and differentiation, and regulate the selective expression of neurotransmitter receptors in neurons and at the neuromuscular junction. They also regulate glial commitment, proliferation, survival and differentiation. At interneuronal synapses, neuregulin ErbB receptors associate with PDZ-domain proteins at postsynaptic densities where they can modulate synaptic plasticity. How this combinatorial network - comprising many neuregulin ligands that signal through distinct combinations of dimeric ErbB receptors - elicits its multitude of biological effects is beginning to be resolved.     

Synaptic development: insights from Drosophila

In Drosophila, the larval neuromuscular junction is particularly tractable for studying how synapses develop and function. In contrast to vertebrate central synapses, each presynaptic motor neuron and postsynaptic muscle cell is unique and identifiable, and the wiring circuit is invariant. Thus, the full power of Drosophila genetics can be brought to bear on a single, reproducibly identifiable, synaptic terminal. Each individual neuromuscular junction encompasses hundreds of synaptic neurotransmitter release sites housed in a chain of synaptic boutons. Recent advances have increased our understanding of the mechanisms that shape the development of both individual synapses--that is, the transmitter release sites including active zones and their apposed glutamate receptor clusters--and the whole synaptic terminal that connects a pre- and post-synaptic cell.     

Wnt signaling during synaptic development and plasticity

The formation of synaptic connections requires a dialogue between pre and postsynaptic cells to coordinate the assembly of the presynaptic release machinery and the postsynaptic receptive complexes. Signaling molecules of the Wnt family of proteins are central to this trans-synaptic dialogue. At the neuromuscular junction and central synapses, Wnts promote synaptic assembly by signaling to the developing pre and postsynaptic compartments. In addition, new studies reveal that expression of Wnt proteins and localization of their Fz receptors are regulated by neuronal activity. Importantly, Wnts mediates the synaptic changes induced by patterned neuronal activity or sensory experience in mature neurons. Here we review recent findings into the function of Wnt signaling at the synapse and its link to activity-dependent synaptic growth and function.     

Increased neuromuscular activity causes axonal defects and muscular degeneration

Before establishing terminal synapses with their final muscle targets, migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twister mutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor alpha-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.     

Lrp4 is a receptor for Agrin and forms a complex with MuSK

Neuromuscular synapse formation requires a complex exchange of signals between motor neurons and skeletal muscle fibers, leading to the accumulation of postsynaptic proteins, including acetylcholine receptors in the muscle membrane and specialized release sites, or active zones in the presynaptic nerve terminal. MuSK, a receptor tyrosine kinase that is expressed in skeletal muscle, and Agrin, a motor neuron-derived ligand that stimulates MuSK phosphorylation, play critical roles in synaptic differentiation, as synapses do not form in their absence, and mutations in MuSK or downstream effectors are a major cause of a group of neuromuscular disorders, termed congenital myasthenic syndromes (CMS). How Agrin activates MuSK and stimulates synaptic differentiation is not known and remains a fundamental gap in our understanding of signaling at neuromuscular synapses. Here, we report that Lrp4, a member of the LDLR family, is a receptor for Agrin, forms a complex with MuSK, and mediates MuSK activation by Agrin.     

Kismet Positively Regulates Glutamate Receptor Localization and Synaptic Transmission at the Drosophila Neuromuscular Junction

The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse that is structurally and functionally similar to mammalian glutamatergic synapses. These synapses can, as a result of changes in activity, alter the strength of their connections via processes that require chromatin remodeling and changes in gene expression. The chromodomain helicase DNA binding (CHD) protein, Kismet (Kis), is expressed in both motor neuron nuclei and postsynaptic muscle nuclei of the Drosophila larvae. Here, we show that Kis is important for motor neuron synaptic morphology, the localization and clustering of postsynaptic glutamate receptors, larval motor behavior, and synaptic transmission. Our data suggest that Kis is part of the machinery that modulates the development and function of the NMJ. Kis is the homolog to human CHD7, which is mutated in CHARGE syndrome. Thus, our data suggest novel avenues of investigation for synaptic defects associated with CHARGE syndrome.     

Neurotrophic regulation of the development and function of the neuromuscular synapses

Recent studies have established that one of the major functions of neurotrophic factors is to regulate synaptic development and plasticity. This owes a great deal to the studies using the neuromuscular junction (NMJ) as a model system. In this review, we summarize the effects of various neurotrophic factors on the development and function of the neuromuscular synapses. We describe experiments addressing the role of neurotrophins, as well as that of other factors (GFLs, TGF-βs, and Wnts). The synaptic effects of neurotrophic factors are divided into two categories: acute effects on synaptic transmission and plasticity occurring within seconds or minutes after cells are exposed to a particular factor, and long-term regulation of synaptic structure and function that takes days to accomplish. We consider the presynaptic effects on the release of the neurotransmitter ACh, as well as the postsynaptic effects on the clustering of ACh receptors. Further studies of the mechanisms underlying these regulatory effects will help us better understand how neurotrophic factors can achieve diverse and synapse-specific modulation in the brain.     

Lipid rafts in neuregulin signaling at synapses

Neuregulins are a family of EGF domain-containing factors that play an important role in development. In the nervous system, they promote glial differentiation, induce neurotransmitter receptor expression, and regulate synaptic plasticity. Recent studies indicate that ErbB protein tyrosine kinases, neuregulin receptors, translocate to lipid raft microdomains in the plasma membrane in response to neuregulin. Localization of ErbB proteins in lipid rafts appeared to be necessary for neuregulin signaling and regulation of synaptic plasticity. We will review recent studies of lipid rafts and neuregulin function and discuss possible roles of lipid rafts in compartmentalized neuregulin signaling and translocation of ErbB proteins to synapses.     

Running to stand still

For synapses to form and function, neurotransmitter receptors must be recruited to a location on the postsynaptic cell in direct apposition to presynaptic neurotransmitter release. However, once receptors are inserted into the postsynaptic membrane, they are not fixed in place but are continually exchanged between synaptic and extrasynaptic regions, and they cycle between the surface and intracellular compartments. This article highlights and compares the current knowledge about the dynamics of acetylcholine receptors at the vertebrate peripheral neuromuscular junction and AMPA, N-methyl-D-aspartate, and gamma-aminobutyric acid receptors in central synapses.     

AMPA receptor trafficking at excitatory synapses

Excitatory synapses in the CNS release glutamate, which acts primarily on two sides of ionotropic receptors: AMPA receptors and NMDA receptors. AMPA receptors mediate the postsynaptic depolarization that initiates neuronal firing, whereas NMDA receptors initiate synaptic plasticity. Recent studies have emphasized that distinct mechanisms control synaptic expression of these two receptor classes. Whereas NMDA receptor proteins are relatively fixed, AMPA receptors cycle synaptic membranes on and off. A large family of interacting proteins regulates AMPA receptor turnover at synapses and thereby influences synaptic strength. Furthermore, neuronal activity controls synaptic AMPA receptor trafficking, and this dynamic process plays a key role in the synaptic plasticity that is thought to underlie aspects of learning and memory.     

Molecules involved in the formation of synaptic connections in muscle and brain.

Synapses are highly specialized structures designed to guarantee precise and efficient communication between neurons and their target cells. Molecules of the extracellular matrix have an instructive role in the formation of the neuromuscular junction, the best-characterized synapse. In this review, the molecular mechanisms underlying these instructive signals will be discussed with particular emphasis on the receptors involved. Additionally, recent evidence for the involvement of specific adhesion complexes in the formation and modulation of synapses in the central nervous system will be reviewed. Synapses are specialized junctions between neurons and their target cells where information is transferred from the pre- to the postsynaptic cell. At most vertebrate synapses, this transfer is accomplished by the release of a specific neurotransmitter from the presynaptic nerve terminal. The release of neurotransmitter is initiated by the action potential and the subsequent influx of Ca(2+) into the presynaptic nerve terminal. This results in the rapid fusion of vesicles with the nerve membrane and the release of the neurotransmitter into the synaptic cleft. The neurotransmitter then diffuses across the cleft and binds to specific postsynaptic receptors, resulting in a change in the membrane potential of the postsynaptic cell. This can result in the generation of an action potential. The high precision of synaptic transmission requires that pre- and postsynaptic structures are both highly organized and in juxtaposition to each other. In addition, alterations in synaptic transmission are the basis of learning and memory and are likely to be accompanied by the remodeling of synaptic structures (Toni et al., 1999). Thus, the study of how synapses are formed during development is also of relevance for the understanding of the cellular and molecular processes involved in learning and memory. This review focuses on the molecular mechanisms involved in the formation and the function of synapses.     

Transmission,Development, and Plasticity of Synapses

Chemical synapses are sites of contact and information transfer between a neuron and its partner cell. Each synapse is a specialized junction, where the presynaptic cell assembles machinery for the release of neurotransmitter, and the postsynaptic cell assembles components to receive and integrate this signal. Synapses also exhibit plasticity, during which synaptic function and/or structure are modified in response to activity. With a robust panel of genetic, imaging, and electrophysiology approaches, and strong evolutionary conservation of molecular components, Drosophila has emerged as an essential model system for investigating the mechanisms underlying synaptic assembly, function, and plasticity. We will discuss techniques for studying synapses in Drosophila, with a focus on the larval neuromuscular junction (NMJ), a well-established model glutamatergic synapse. Vesicle fusion, which underlies synaptic release of neurotransmitters, has been well characterized at this synapse. In addition, studies of synaptic assembly and organization of active zones and postsynaptic densities have revealed pathways that coordinate those events across the synaptic cleft. We will also review modes of synaptic growth and plasticity at the fly NMJ, and discuss how pre- and postsynaptic cells communicate to regulate plasticity in response to activity.     

Local presynaptic activity gates homeostatic changes in presynaptic function driven by dendritic BDNF synthesis

Homeostatic synaptic plasticity is important for maintaining stability of neuronal function, but heterogeneous expression mechanisms suggest that distinct facets of neuronal activity may shape the manner in which compensatory synaptic changes are implemented. Here, we demonstrate that local presynaptic activity gates a retrograde form of homeostatic plasticity induced by blockade of AMPA receptors (AMPARs) in cultured hippocampal neurons. We show that AMPAR blockade produces rapid (<3 hr) protein synthesis-dependent increases in both presynaptic and postsynaptic function and that the induction of presynaptic, but not postsynaptic, changes requires coincident local activity in presynaptic terminals. This "state-dependent" modulation of presynaptic function requires postsynaptic release of brain-derived neurotrophic factor (BDNF) as a retrograde messenger, which is locally synthesized in dendrites in response to AMPAR blockade. Taken together, our results reveal a local crosstalk between active presynaptic terminals and postsynaptic signaling that dictates the manner by which homeostatic plasticity is implemented at synapses.     

Activity coregulates quantal AMPA and NMDA currents at neocortical synapses

AMPA and NMDA receptors are coexpressed at many central synapses, but the factors that control the ratio of these two receptors are not well understood. We recorded mixed miniature or evoked synaptic currents arising from coactivation of AMPA and NMDA receptors and found that long-lasting changes in activity scaled both currents up and down proportionally through changes in the number of postsynaptic receptors. The ratio of NMDA to AMPA current was similar at different synapses onto the same neuron, and this relationship was preserved following activity-dependent synaptic scaling. These data show that AMPA and NMDA receptors are tightly coregulated by activity at synapses at which they are both expressed and suggest that a mechanism exists to actively maintain a constant receptor ratio across a neuron's synapses.     

Neuregulin-heparan-sulfate proteoglycan interactions produce sustained erbB receptor activation required for the induction of acetylcholine receptors in muscle

Neuregulins bind to and activate members of the EGF receptor family of tyrosine kinases that initiate a signaling cascade that induces acetylcholine receptor synthesis in the postsynaptic membrane of neuromuscular synapses. In addition to an EGF-like domain, sufficient for receptor binding and tyrosine auto-phosphorylation, many spliced forms also have an IG-like domain that binds HSPGs and maintains a high concentration of neuregulin at synapses. Here, we show that the IG-like domain functions to keep the EGF-like domain at sufficiently high concentrations for a sufficiently long period of time necessary to induce acetylcholine receptor gene expression in primary chick myotubes. Using recombinant neuregulins with and without the IG-like domain, we found that IG-like domain binding to endogenous HSPGs produces a 4-fold increase in receptor phosphorylation. This enhancement of activity was blocked by soluble heparin or by pretreatment of muscle cells with heparitinase. We show that at least 12-24 h of neuregulin exposure was required to turn on substantial acetylcholine receptor gene expression and that the erbB receptors need to be kept phosphorylated during this time. The need for sustained erbB receptor activation may be the reason why neuregulins are so highly concentrated in the extracellular matrix of synapses.     

Bidirectional plasticity gated by hyperpolarization controls the gain of postsynaptic firing responses at central vestibular nerve synapses

Linking synaptic plasticity with behavioral learning requires understanding how synaptic efficacy influences postsynaptic firing in neurons whose role in behavior is understood. Here, we examine plasticity at a candidate site of motor learning: vestibular nerve synapses onto neurons that mediate reflexive movements. Pairing nerve activity with changes in postsynaptic voltage induced bidirectional synaptic plasticity in vestibular nucleus projection neurons: long-term potentiation relied on calcium-permeable AMPA receptors and postsynaptic hyperpolarization, whereas long-term depression relied on NMDA receptors and postsynaptic depolarization. Remarkably, both forms of plasticity uniformly scaled synaptic currents evoked by pulse trains, and these changes in synaptic efficacy were translated into linear increases or decreases in postsynaptic firing responses. Synapses onto local inhibitory neurons were also plastic but expressed only long-term depression. Bidirectional, linear gain control of vestibular nerve synapses onto projection neurons provides a plausible mechanism for motor learning underlying adaptation of vestibular reflexes.     

Diffusional trapping of GluR1 AMPA receptors by input-specific synaptic activity

Synaptic activity regulates the postsynaptic accumulation of AMPA receptors over timescales ranging from minutes to days. Indeed, the regulated trafficking and mobility of GluR1 AMPA receptors underlies many forms of synaptic potentiation at glutamatergic synapses throughout the brain. However, the basis for synapse-specific accumulation of GluR1 is unknown. Here we report that synaptic activity locally immobilizes GluR1 AMPA receptors at individual synapses. Using single-molecule tracking together with the silencing of individual presynaptic boutons, we demonstrate that local synaptic activity reduces diffusional exchange of GluR1 between synaptic and extraynaptic domains, resulting in postsynaptic accumulation of GluR1. At neighboring inactive synapses, GluR1 is highly mobile with individual receptors frequently escaping the synapse. Within the synapse, spontaneous activity confines the diffusional movement of GluR1 to restricted subregions of the postsynaptic membrane. Thus, local activity restricts GluR1 mobility on a submicron scale, defining an input-specific mechanism for regulating AMPA receptor composition and abundance.     

ATP potentiates spontaneous transmitter release at developing neuromuscular synapses

Extracellular application of ATP, a substance co-stored and co-released with acetylcholine in peripheral nervous systems, potentiates the spontaneous secretion of acetylcholine at developing neuromuscular synapses in Xenopus cell culture, as shown by a marked increase in the frequency of spontaneous synaptic currents recorded in the postsynaptic muscle cell. The effect of ATP is apparently mediated by the activation of cytosolic protein kinases and requires the influx of Ca2+ through the plasma membrane. Since spontaneous acetylcholine release is known to regulate the development of contractile properties of the postsynaptic muscle cell, extracellular ATP may serve as a positive trophic factor at developing neuromuscular synapses.