Human melanoma/NG2 chondroitin sulfate proteoglycan is expressed in the sarcolemma of postnatal human skeletal myofibers. Abnormal expression in merosin-negative and Duchenne muscular dystrophies

NG2 is the rat homologue of the human melanoma chondroitin sulfate proteoglycan (MCSP) preferentially expressed in dividing progenitor cells of the glial and mesenchymal lineage but downregulated after differentiation. It has recently been demonstrated that MCSP/NG2 expression is not restricted to mitotic or malignant cells. We show that MCSP/NG2 expression is detectable in the sarcolemma, and in the neuromuscular junction of human postnatal skeletal muscle, and it gradually reduces with advancing age. In human and murine myogenic cell lines, we found no clear differences in MCSP/NG2 expression between myoblasts and myotubes. Reduced levels of the core protein were found in merosin-negative congenital muscular dystrophy (MDC1A). Duchenne muscular dystrophy patients muscles exhibited an overexpression of the MCSP/NG2 core protein. In gamma-sarcoglycanopathy and calpainopathy, MCSP/NG2 upregulation was restricted to regenerating myofibers. We demonstrate that MCSP/NG2 is expressed in differentiated myofibers, and appears to have a role in the pathogenesis of MDC1A and severe dystrophinopathies.

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-betas, 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.

Roles of glial cells in the formation, function, and maintenance of the neuromuscular junction

Like other vertebrate synapses, the neuromuscular junction (NMJ) has glial cells that are closely associated with the pre- and post-synaptic components. These "perisynaptic Schwann cells" (PSCs) cover nerve terminals and are in close proximity to the synapse, yet their role at the NMJ has remained mysterious for decades. In this review we explore historical perspectives on PSCs and highlight key developments in recent years that have provided novel insight into PSC functions at the NMJ. First among these developments is the generation of specific antibody probes for PSCs. Using one such antibody and the principle of complement-mediated cell lysis, we have developed a novel technique to selectively ablate PSCs en masse from frog NMJs in vivo. Applying this approach, we have shown that PSCs are essential for the long-term maintenance of synaptic structure and function. In addition, PSCs are essential for the growth and maintenance of NMJs during development. Probes for PSCs also allow us to observe in vivo that processes extended by PSCs guide nerve terminals during synapse development, remodeling, and regeneration. PSCs may therefore dictate the pattern of innervation at the NMJ. Finally, PSCs may also induce postsynaptic acetylcholine receptor expression and aggregation. This wealth of recent findings about PSCs suggests that these synapse-associated glial cells are a more integral and essential component of the NMJ than previously appreciated. New approaches currently being applied at the NMJ may further support the emerging view that glial cells help make bigger, stronger, and more stable synapses.

Mechanisms controlling axonal sprouting at the neuromuscular junction

This review considers the relative roles of sprouting stimuli, perisynaptic Schwann cells and neuromuscular activity in axonal sprouting at the neuromuscular junction in partially denervated muscles. A number of sprouting stimuli, including insulin-like growth factor II, which are generated from inactive muscle fibers in partially denervated and paralyzed skeletal muscles, has been considered. There is also evidence that perisynaptic Schwann cells induce and guide axonal sprouting in adult partially denervated muscles. Excessive neuromuscular activity significantly reduces bridging of perisynaptic Schwann cell processes between innervated and denervated endplates and thereby inhibits axonal sprouting in partially denervated adult muscles. Elimination of neuromuscular activity is also detrimental to sprouting in these muscles, suggesting that calcium influx into the nerve is crucial for axonal sprouting. The role of neuromuscular activity in axonal sprouting will be considered critically in the context of the roles of sprouting stimuli and perisynaptic Schwann cells in the process of axonal sprouting.

Erbb2 regulates neuromuscular synapse formation and is essential for muscle spindle development

Neuregulins and their Erbb receptors have been implicated in neuromuscular synapse formation by regulating gene expression in subsynaptic nuclei. To analyze the function of Erbb2 in this process, we have inactivated the Erbb2 gene in developing muscle fibers by Cre/Lox-mediated gene ablation. Neuromuscular synapses form in the mutant mice, but the synapses are less efficient and contain reduced levels of acetylcholine receptors. Surprisingly, the mutant mice also show proprioceptive defects caused by abnormal muscle spindle development. Sensory Ia afferent neurons establish initial contact with Erbb2-deficient myotubes. However, functional spindles never develop. Taken together, our data suggest that Erbb2 signaling regulates the formation of both neuromuscular synapses and muscle spindles.

Activity-dependent elimination of neuromuscular synapses

At developing neuromuscular synapses in vertebrates, different motor axon inputs to muscle fibers compete for maintenance of their synapses. Competition results in progressive changes in synaptic structure and strength that lead to the weakening and loss of some inputs, a process that has been called synapse elimination. At the same time, a single input is strengthened and maintained throughout adult life, consistently recruiting muscle fibers to contract even at rapid firing rates. Work over the last decade has led to an understanding of some of the cell biological mechanisms that underlie competition and how these culminate in synapse elimination. We discuss current ideas about how activity modulates neuromuscular synaptic competition, how competition leads to synapse loss, and how these processes are modulated by cell-cell signaling. A common feature of competition at neuromuscular as well as CNS synapses is that temporally correlated activity seems to slow or prevent competition, while uncorrelated activity seems to trigger or enhance competition. Important questions that remain to be addressed include how patterns of motor neuron activity affect synaptic strength, what is the temporal relationship between changes in synaptic strength and structure, and what cellular signals mediate synapse loss. Answers to these questions will expand our understanding of the mechanisms by which activity edits synaptic structure and function, writing permanent changes in neural circuitry.

Axonal sprouting in mdx mice and its relevance to cell and gene mediated therapies for Duchenne muscular dystrophy

We investigated whether pre-terminal axons and motor terminals retained their ability to sprout in the murine X-linked muscular dystrophy (mdx). Immunofluorescence confocal microscopy observation of nerve terminals and acetylcholine receptors in mdx muscles with crushed and non-crushed nerves showed that most of the junctions had intraterminal sprouting and that the number of junctions with extraterminal sprouting increased after the nerve crush lesion. Since new dystrophin-positive muscle fibers generated by cell-mediated therapies need to be innervated to proceed with their maturation and dystrophin production, these results suggest that the use of inducing factors to increase the sprouting capacity of nerve terminals could be an additional tool in the success of cell-mediated therapies.

Schwann cell dynamics with respect to newly formed motor—nerve terminal branches on mature (Bufo marinus) muscle fibers

A study has been made of the formation of synaptic terminals from long processes formed at the end of motor nerve branches of endplates in mature amphibian (Bufo marinus) muscle. Injection of fluorescent dyes into individual motor axons showed the full extent of their branches at single endplates. Synaptic vesicle clusters at these branches were identified with styryl dyes. Some terminal branches consisted of well separated varicosities, each possessing a cluster of functioning synaptic vesicles whilst others formed by the same axon consisted of closely spaced clusters of vesicles in a branch of approximately uniform diameter. All the varicosities gave rise to calcium transients on stimulation of their parent axon. Both types of branches sometimes possessed short processes (<5 microm long) or very long thin processes (>10 microm long) which ended in a bulb that possessed a functional synaptic vesicle cluster. These thin processes could move and form a varicosity along their length in less than 30 min. Injection of a fluorescent dye into terminal Schwann cells (TSCs) at an endplate showed that they also possessed very long thin processes (>10 microm long) which could move over relatively short times (<30 min). Injecting fluorescent dyes into both axons and their associated TSCs showed that on some occasions long TSC processes were accompanied by a long nerve terminal process and at other times they were not. It is suggested that the mature motor-nerve terminal is a dynamic structure in which the formation of processes by TSCs guides nerve terminal sprouting.

Taste, movement, and death: varying effects of new prospero mutants during Drosophila development

The PGal4 transposon inserted upstream of the pan-neural gene prospero (pros) causes several neural and behavioral defects in the Voila(1) strain. The precise excision of the transposon simultaneously rescued all these defects whereas its unprecise excision created new pros(V) alleles, including the null allele pros(V17). Here, we describe the relationship between the genetic structure of pros locus, larval locomotion, and larval gustatory response. These two behaviors showed varying degrees of variation depending upon the pros allele. We also found a good relation between behavioral alteration, the level of Pros protein in the embryo, and the degree of disorganization in the larval neuromuscular junction. These data suggest that the complete development of the nervous system requires a full complement of Pros, and that a gradual decrease in the levels of this protein can proportionally alter the development and the function of the nervous system.

GABA is dispensable for the formation of junctional GABA receptor clusters in Caenorhabditis elegans

At GABAergic synapses, GABA receptors form high-density clusters opposite GABA release sites. Whether GABA release per se plays a role in the formation of GABA receptor clusters remains uncertain. To address this question in vivo, we characterized GABA receptor clustering in the nematode Caenorhabditis elegans. In C. elegans, body wall muscles receive excitatory inputs from cholinergic motor neurons and inhibitory inputs from GABAergic neurons. Using immunohistochemistry and green fluorescent protein-tagged proteins, we observed that the muscle GABA receptor UNC-49 is precisely clustered opposite GABA release sites. During development, these clusters appear slightly after the detection of presynaptic vesicles. If motor axons are mislocalized as in unc-5 mutants, GABA receptors cluster opposite ectopic axons at GABA release sites. Together, these data imply that a motor neuron-derived factor is instructing GABA receptor clustering. Presynaptic localization of this clustering activity requires the neuronal kinesin UNC-104, suggesting that release of GABA from synaptic vesicles may represent the clustering signal. However, unc-25 mutants do not synthesize GABA but do cluster postsynaptic GABA receptors indistinguishably from the wild type. Therefore, at GABAergic neuromuscular junctions, GABA receptor clustering requires nerve-muscle interaction but not GABA neurotransmission.

Dynamics of motor nerve terminal remodeling unveiled using SNARE-cleaving botulinum toxins: the extent and duration are dictated by the sites of SNAP-25 truncation

Nerve sprouts emerge from motor nerve terminals following blockade of exo-endocytosis for more than 3 days by botulinum neurotoxin (BoNT), and form functional synapses, albeit temporary. Upon restoration of synaptic activity to the parent terminal 7 and 90 days after exposure to BoNT/F or A respectively, a concomitant retraction of the outgrowths was observed. BoNT/E caused short-term neuroparalysis, and dramatically accelerated the recovery of BoNT/A-paralyzed muscle by further truncation of SNAP-25 and its replenishment with functional full-length SNARE. The removal of 9 C-terminal residues from SNAP-25 by BoNT/A leads to persistence of the inhibitory product due to the formation of a nonproductive SNARE complex(es) at release sites, whereas deletion of a further 17 amino acids permits replenishment and a speedy recovery.

Voltage-gated sodium channels and ankyrinG occupy a different postsynaptic domain from acetylcholine receptors from an early stage of neuromuscular junction maturation in rats

Spatial segregation of membrane proteins is a feature of many excitable cells. In skeletal muscle, clusters of acetylcholine receptors (AChRs) and voltage-gated sodium channels (Na(V)1s) occupy distinct domains at the neuromuscular junction (NMJ). We used quantitative immunolabeling of developing rat soleus muscles to study the mechanism of ion channel segregation and Na(V)1 clustering at NMJs. When Na(V)1s can first be detected, at birth, they already occupy a postsynaptic domain that is distinct from that occupied by AChRs. At this time, Na(V)1s are expressed only in a diffuse area that extends 50-100 microm from the immature NMJ. However, in the region of the high-density AChR cluster at NMJ itself, Na(V)1s are actually present in lower density than in the immediately surrounding membrane. These distinctive features of the Na(V)1 distribution at birth are closely correlated with the distribution of ankyrinG immunolabeling. This suggests that an interaction with ankyrinG plays a role in the initial segregation of Na(V)1s from AChRs. Both Na(V)1 and ankyrinG become clustered at the NMJ itself 1-2 weeks after birth, coincident with the formation of postsynaptic folds. Syntrophin immunolabeling codistributes with AChRs and never resembles that for Na(V)1 or ankyrinG. Therefore, syntrophin is unlikely to play an important part in the initial accumulation of Na(V)1 at the NMJ. These findings suggest that the segregation of Na(V)1 from AChRs begins early in NMJ formation and occurs as a result of the physical exclusion of Na(V)1 and ankyrinG from the region of nerve-muscle contact rather than by a process of active clustering.

Development of Drosophila larval neuromuscular junctions: maintaining synaptic strength

In spite of the available information about the development of Drosophila neuromuscular junctions, the correlation between nerve terminal morphology and maintenance of synaptic strength has still not been systematically addressed throughout larval development. We characterized the growth of the abdominal longitudinal muscle 6 (m6) and the motor terminals Ib and Is that innervate it within segment 4. In addition, we measured the evoked excitatory junction potential (EJP) amplitudes while the Ib and Is axons were selectively recruited. Regression analysis with natural log transformation of response variables indicated that the developmental curves for m6 and the motor axons Ib and Is were best fitted as second order polynomial regressions during larval development. Initially Is terminals are longer and possess more synaptic varicosities at the first instar stage. The Is terminals also grow faster in subsequent developmental stages. The growth of nerve terminals and their target m6 are not proportional although tightly correlated. This results in a larger average muscle area innervated by a single varicosity as the animal develops. The amplitudes of the EJPs of Ib and Is neurons show no developmental difference in their amplitudes from the first to the late third larval instar. The Is axon consistently produced larger EJPs than the Ib axon at each developmental stage. The time constants for both rising and decay phases of EJPs increase exponentially throughout larval development. The results presented not only help in quantifying the normal development of Drosophila neuromuscular junctions, but also provide a framework for future investigations to properly interpret developmental abnormalities that may occur in various mutants.

Synaptogenesis: Wnt and TGF-beta take centre stage

The formation of functional synapses requires precise coordination between neurons and their synaptic targets. Recent studies have identified two signaling molecules, Wnt and TGF-beta, which are required for formation and growth of the neuromuscular synapse.

In vivo time-lapse imaging of synaptic takeover associated with naturally occurring synapse elimination

During development, competition between axons causes permanent removal of synaptic connections, but the dynamics have not been directly observed. Using transgenic mice that express two spectral variants of fluorescent proteins in motor axons, we imaged competing axons at developing neuromuscular junctions in vivo. Typically, one axon withdrew progressively from postsynaptic sites and the competing axon extended axonal processes to occupy those sites. In rare instances when the remaining axon did not reoccupy a site, the postsynaptic receptors rapidly disappeared. Interestingly, the progress and outcome of competition was unpredictable. Moreover, the relative areas occupied by the competitors shifted in favor of one axon and then the other. These results show synaptic competition is not always monotonic and that one axon's contraction in synaptic area is associated with another axon's expansion.

Effects of chronic lead exposure on the neuromuscular junction in Drosophila larvae

Long term or chronic exposure to lead is associated with cognitive and other deficits in humans, which may reflect lead-induced changes in synaptic development and function. We believe that Drosophila has great potential as a model system for studying such changes. To test this, we compared the structure of single, identified synapses between identified axons (axons 1 and 2) and muscle fibers (fibers 6 and 7) in untreated 3rd instar larvae, and in larvae reared on medium made with 100 microM lead acetate in distilled water. We used three approaches to examine the motor terminals on muscle fibers 6 and 7 in segment 2: (1) all terminals were stained with an antibody to HRP; (2) only the terminals of axon 1 were stained by injecting biotinylated Lucifer yellow into it; and (3) the regions of the terminal containing synaptic vesicles were stained with an antibody to synaptotagmin, which provides an estimate of "synaptic" terminal area. Lead burdens were determined by inductively coupled plasma mass spectrometry; hemolymph lead levels at the neuromuscular junction were likely to be micromolar. We observed that lead exposure did not significantly affect the average terminal area or the average muscle fiber area, but did significantly affect the uniformity of the matching between muscle area and motor terminal size that normally occurs during development. There was a significant positive correlation between motor terminal size and muscle area in control, but not in lead-exposed larvae. The sensitivity of Drosophila larval synaptic development to lead opens the way to using the powerful genetic and molecular tools available for this system to study the underlying mechanisms of this sensitivity. We would hope that from such an understanding may come strategies for dealing with lead-induced deficits in children.

Neuregulin-1 Isoforms Are Differentially Expressed in the Intact and Regenerating Adult Rat Nervous System

Our knowledge on Neuregulin-1 (Nrg-1) during development of the nervous system is increasing rapidly, but little is known about Nrg-1-ErbB signaling in the adult brain. Nrg-1 is involved in determination, proliferation, differentiation, and migration of neurons and glial cells in the developing brain. In the peripheral nervous system, Nrg-1 signaling is required for Schwann cell differentiation and myelination, and establishment of neuromuscular junctions (NMJs). Multiple alternative splicing of Nrg-1 was shown, but correlation of its structural and functional diversity was rarely addressed. Therefore, we investigated the expression of Nrg-1 isoforms in the rat brain and brain-derived cell types, and their involvement in regeneration of the adult brain, using immunohistochemistry, in situ hybridization, and semiquantitative RT-PCR. We found expression of at least 12 distinct Nrg-1 isoforms in the brain and altered expression of several isoforms in the facial motor nucleus after peripheral transection of the seventh cranial nerve. An upregulation of Nrg-1 type-I mRNA, probably type- I-alpha, was observed in reactive astrocytes of the facial nucleus 1 d postaxotomy. Nrg-1 type-III and the splice variants beta1 and beta5 are dramatically downregulated in axotomized motoneurons, which lack contact to their target tissue. Baseline expression levels were reestablished when the first axons reached the facial muscles and reformed NMJs. Nrg-1-beta1 and -beta5 might act in maintenance of NMJs. The splice variants beta2 and beta4 display an initial downregulation of mRNA levels, followed by an increase during the period of axon remyelination. Thus, Nrg- 1-beta2 and -beta4 might be involved in myelination.

Ectopic endplates induce localized changes in skeletal muscle ultrastructure

To investigate the processes by which motoneurons control protein synthesis, and thus the ultrastructure of the muscle fibers they innervate, ectopic endplates were induced to form on adult mouse skeletal muscle fibers by transplantation of a foreign nerve onto the muscle. In the dually innervated muscle fibers thus created, we examined two ultrastructural parameters that correlate with the expression of distinct isoforms of the myofibrillar proteins alpha-actinin and titin, specifically, Z-line width and sarcomere length. It was found that Z-lines were significantly thinner (98 vs. 128 nm) and sarcomeres were significantly shorter (1.69 vs. 2.06 microm) near the ectopic than near the original endplates. Thus, ectopic endplate formation on adult skeletal muscle fibers induces a localized alteration in myofibrillar morphology. These results may help to elucidate the role played by motoneurons in the determination and maintenance of muscle fiber properties and the processes that occur following muscle reinnervation after nerve injury.

Developmentally regulated expression of Neuro-p24 and its possible function in neurite extension

Process extension is a most marked and characteristic neuronal feature that is observed during the development, regeneration and plasticity of nervous system tissues. Neuro-p24, a novel membranous protein with a molecular weight of 24 kDa, is specifically localized in neurons, particularly in the neurites. Based on its molecular structure and distribution pattern in the brain we proposed that Neuro-p24 plays a role in neurite extension. In the present study we have made several findings that support this hypothesis; first, Neuro-p24 was abundant in motor axonal fibers, neurites of dorsal root ganglia neurons and apical dendrites of cerebral cortex neurons when their extension or arborization was proceeding very actively. Secondly, when COS-7 epithelial cells were transfected with either wild-type or deletion-mutated Neuro-p24 cDNAs, ectopic expression of wild-type cDNA caused morphological alterations resulting in a neuron-like appearance. These observations firmly support our proposal and indicate that Neuro-p24 plays an important role in the nervous tissue.

Juvenile and adult rat neuromuscular junctions: density, distribution, and morphology

Anatomical and physiological differences in neuromuscular junctions (NMJs) between juvenile and adult muscle may partially explain the variability in clinical results following chemodenervation with botulinum toxin or nerve repair in children and adults. We evaluated NMJ density, distribution, and morphometry in biceps brachii and gastrocnemius muscles from juvenile and adult rats. Motor endplates were stained with Karnovsky-Roots methods, and NMJ density (number/gram muscle tissue) was determined. The NMJ morphometry was quantitated with alpha-bungarotoxin labeling using confocal microscopy. Juvenile rats had a greater NMJ density in both muscles compared with adult rats. Juveniles and adult rats had a similar NMJ distribution in both muscles. In juvenile muscles NMJs occupied approximately 50% of the surface area and 70% of the length, width, circumference, and gutter depth compared with adult muscles. Our study demonstrates that although NMJs are smaller, juvenile muscles have a higher NMJ density than do adult muscles with similar distributions. If these age-dependent differences in NMJ density are obtained in humans, they may account, at least in part, for the better recovery that occurs in children than adults after nerve repair and also suggest that the dosage of botulinum toxin (units per kg) for chemodenervation may need to be increased in juveniles.

Differential regulation of active zone density during long-term strengthening of Drosophila neuromuscular junctions

In this study we established a transgenic Ca2+ imaging technique in Drosophila that enabled us to target the Ca2+ sensor protein yellow Cameleon-2 specifically to larval neurons. This noninvasive method allowed us to measure evoked Ca2+ signals in presynaptic terminals of larval neuromuscular junctions (NMJs). We combined transgenic Ca2+ imaging with electrophysiological recordings and morphological examinations of larval NMJs to analyze the mechanisms underlying persistently enhanced evoked vesicle release in two independent mutants. We show that persistent strengthening of junctional vesicle release relies on the recruitment of additional active zones, the spacing of which correlated with the evoked presynaptic Ca2+ dynamics of individual presynaptic terminals. Knock-out mutants of the postsynaptic glutamate receptor (GluR) subunit DGluR-IIA, which showed a reduced quantal size, developed NMJs with a smaller number of presynaptic boutons but a strong compensatory increase in the density of active zones. This resulted in an increased evoked vesicle release on single action potentials and larger evoked Ca2+ signals within individual boutons; however, the transmission of higher frequency stimuli was strongly depressed. A second mutant (pabp(P970)/+), which showed enhanced evoked vesicle release triggered by elevated subsynaptic protein synthesis, developed NMJs with an increased number of presynaptic boutons and active zones; however, the density of active zones was maintained at a value typical for wild-type animals. This resulted in wild-type evoked Ca2+ signals but persistently strengthened junctional signal transmission. These data suggest that the consolidation of strengthened signal transmission relies not only on the recruitment of active zones but also on their equal distribution in newly grown boutons.

Spinsters, synaptic defects, and amaurotic idiocy

In this issue of Neuron, Sweeney and Davis present a beautiful characterization of Drosophila mutants in a gene named spinster. The results indicate a function of the endocytic pathway in regulating transforming growth factor-beta (TGF-beta) signaling at the Drosophila motor synapse. This study provides important new information at an intersection of several disciplines, including membrane traffic, lipid organization, synaptic signaling, and neurodegenerative lysosomal storage disease.

Unrestricted synaptic growth in spinster-a late endosomal protein implicated in TGF-beta-mediated synaptic growth regulation

In a genetic screen for genes that control synapse development, we have identified spinster (spin), which encodes a multipass transmembrane protein. spin mutant synapses reveal a 200% increase in bouton number and a deficit in presynaptic release. We demonstrate that spin is expressed in both nerve and muscle and is required both pre- and postsynaptically for normal synaptic growth. We have localized Spin to a late endosomal compartment and present evidence for altered endosomal/lysosomal function in spin. We also present evidence that synaptic overgrowth in spin is caused by enhanced/misregulated TGF-beta signaling. TGF-beta receptor mutants show dose-dependent suppression of synaptic overgrowth in spin. Furthermore, mutations in Dad, an inhibitory Smad, cause synapse overgrowth. We present a model for synaptic growth control with implications for the etiology of lysosomal storage and neurodegenerative disease.

The postsynaptic glutamate receptor subunit DGluR-IIA mediates long-term plasticity in Drosophila

The developing neuromuscular junctions (NMJs) of Drosophila larvae can undergo long-term strengthening of signal transmission, a process that has been shown recently to involve local subsynaptic protein synthesis and that is associated with an elevated synaptic accumulation of the postsynaptic glutamate receptor subunit DGluR-IIA. To analyze the role of altered postsynaptic glutamate receptor expression during this form of genetically induced junctional plasticity, we manipulated the expression levels of two so far-described postsynaptic receptor subunit genes, dglur-IIA and dglur-IIB, in wild-type animals and plasticity mutants. Here we show that elevated synaptic expression of DGluR-IIA, which was achieved by direct transgenic overexpression, by genetically increased subsynaptic protein synthesis, or by a reduced dglur-IIB gene copy number, results in an increased recruitment of active zones, a corresponding enhancement in the strength of junctional signal transmission, and a correlated addition of boutons to the NMJ. Ultrastructural evidence demonstrates that active zones appear throughout NMJs at a typical density regardless of genotype, suggesting that the space requirements of active zones are responsible for the homogeneous synapse distribution and that this regulation results in the observed growth of additional boutons at strengthened NMJs. These phenotypes were suppressed by reduced or eliminated DGluR-IIA expression, which resulted from either a reduced dglur-IIA gene copy number or transgenic overexpression of DGluR-IIB. Our results demonstrate that persistent alterations of neuronal activity and subsynaptic translation result in an elevated synaptic accumulation of DGluR-IIA, which mediates the observed functional strengthening and morphological growth apparently through the recruitment of additional active zones.

Pre-existing pathways promote precise projection patterns

A large body of evidence shows that molecular cues promote specific synapse formation by guiding axons and by mediating their association with targets, but much less is known about the contribution of physical cues (such as mechanical constraints) to these processes. Here we used the peripheral motor system to investigate the latter issue. In living mice, we viewed individual motor axons bearing a fluorescent reporter, and mapped the cohort of muscle fibers that they innervated both before and after nerve damage. When gross trauma was minimized (by a nerve-crushing rather than nerve-cutting procedure), regenerating axons retraced their former pathways, bifurcated at original branch points, and formed neuromuscular junctions on the same fibers that they originally innervated. Axonal growth through tubes of non-neural cells seemed to account for this specificity, and specificity degraded when the tubes were cut. These results suggest that nonspecific guidance cues can be sufficient to generate specific synaptic circuitry.