Neurotransmitter receptor dynamics studied in vivo by reversible photo-unbinding of fluorescent ligands

Activity-dependent site-specific changes of glutamate receptor composition in vivo

The subunit composition of postsynaptic non-NMDA-type glutamate receptors (GluRs) determines the function and trafficking of the receptor. Changes in GluR composition have been implicated in the homeostasis of neuronal excitability and synaptic plasticity underlying learning. Here, we imaged GluRs in vivo during the formation of new postsynaptic densities (PSDs) at Drosophila neuromuscular junctions coexpressing GluRIIA and GluRIIB subunits. GluR composition was independently regulated at directly neighboring PSDs on a submicron scale. Immature PSDs typically had large amounts of GluRIIA and small amounts of GluRIIB. During subsequent PSD maturation, however, the GluRIIA/GluRIIB composition changed and became more balanced. Reducing presynaptic glutamate release increased GluRIIA, but decreased GluRIIB incorporation. Moreover, the maturation of GluR composition correlated in a site-specific manner with the level of Bruchpilot, an active zone protein that is essential for mature glutamate release. Thus, we show that an activity-dependent, site-specific control of GluR composition can contribute to match pre- and postsynaptic assembly.

alpha-Dystrobrevin isoforms differ in their colocalization with and stabilization of agrin-induced acetylcholine receptor clusters

In skeletal muscle, alpha-dystrobrevin (alphaDB) is expressed throughout the sarcolemma with high concentrations at the neuromuscular junction. Mice lacking alphaDB display a mild muscular dystrophy and perturbations at the neuromuscular junction that include disruptions to acetylcholine receptor (AChR) cluster stability and patterning. In adult skeletal muscle, three alternatively spliced isoforms (alphaDB1, alphaDB2, alphaDB3) are expressed, while two other splice variants (alphaDB1(-) and alphaDB2(-)) are expressed only during early development. alphaDB is clearly important in AChR stabilization; however, the degree to which individual alphaDB isoforms and their specific functional domains contribute to AChR cluster stability is not fully understood. To investigate this, we established a primary muscle cell culture system from alphaDB knockout mice and stably expressed individual alphaDB isoforms using retroviral infection. A comparison between wild-type and alphaDB knockout muscle cells showed that in the absence of alphaDB, fewer AChR clusters formed in response to agrin treatment, and these AChR clusters were very unstable. Retroviral expression studies revealed that the largest isoforms (alphaDB1, alphaDB1(-), alphaDB2, alphaDB2(-)) colocalized with agrin-induced AChR clusters and rescued the AChR cluster formation defects back to wild-type levels, while only the first three isoforms fully rescued AChR cluster stability back to wild-type levels. alphaDB2(-) conferred an intermediate level of stability to the AChR clusters. In contrast, alphaDB3 showed no specific colocalization with AChR clusters and little effect on AChR cluster formation or stabilization. Twice as much syntrophin was found associated with alphaDB2 compared with alphaDB2(-) in myotubes suggesting that increased recruitment of syntrophin by alphaDB may enhance the stability of AChR clusters. Taken together, these data demonstrate that different alphaDB isoforms have different functional capabilities in the formation and maintenance of AChR clusters in muscle cells, and that these differences are likely due to the presence of different functional domains in each isoform.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Neuregulins at the neuromuscular synapse: Past, present, and future

At the developing vertebrate neuromuscular junction, neuregulins are growth/differentiation factors essential for terminal Schwann cell survival. Neuregulins have also been thought as the critical signals responsible for the increased transcription of acetylcholine receptor subunit genes at the neuromuscular synapse. This latter role is now highly controversial. This article reviews the evidence that has shaped the views of the neuregulins and how these views have been challenged. The most recent experiments indicate that neuregulin signaling to postsynaptic muscle fibers may modulate, rather than determine, acetylcholine receptor expression at the neuromuscular junction. Based on findings from my lab and those of others, I propose that this modulation might involve novel posttranscriptional molecular mechanisms. Finally, I also suggest that neuregulin signaling may have an important role to play in mediating the response of adult terminal Schwann cells to denervation.

Acetylcholinesterase mobility and stability at the neuromuscular junction of living mice

Acetylcholinesterase (AChE) is an enzyme that terminates acetylcholine neurotransmitter function at the synaptic cleft of cholinergic synapses. However, the mechanism by which AChE number and density are maintained at the synaptic cleft is poorly understood. In this work, we used fluorescence recovery after photobleaching, photo-unbinding, and quantitative fluorescence imaging to investigate the surface mobility and stability of AChE at the adult innervated neuromuscular junction of living mice. In wild-type synapses, we found that nonsynaptic (perisynaptic and extrasynaptic) AChEs are mobile and gradually recruited into synaptic sites and that most of the trapped AChEs come from the perijunctional pool. Selective labeling of a subset of synaptic AChEs within the synapse by using sequential unbinding and relabeling with different colors of streptavidin followed by time-lapse imaging showed that synaptic AChEs are nearly immobile. At neuromuscular junctions of mice deficient in alpha-dystrobrevin, a component of the dystrophin glycoprotein complex, we found that the density and distribution of synaptic AChEs are profoundly altered and that the loss rate of AChE significantly increased. These results demonstrate that nonsynaptic AChEs are mobile, whereas synaptic AChEs are more stable, and that alpha-dystrobrevin is important for controlling the density and stability of AChEs at neuromuscular synapses.

Running to stand still: ionotropic receptor dynamics at central and peripheral synapses

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.

Developmental increase in the amount of rapsyn per acetylcholine receptor promotes postsynaptic receptor packing and stability

Neuromuscular synaptic transmission depends upon tight packing of acetylcholine receptors (AChRs) into postsynaptic AChR aggregates, but not all postsynaptic AChRs are aggregated. Here we describe a new confocal Fluorescence Resonance Energy Transfer (FRET) assay for semi-quantitative comparison of the degree to which AChRs are aggregated at synapses. During the first month of postnatal life the mouse tibialis anterior muscle showed increases both in the number of postsynaptic AChRs and the efficiency with which AChR was aggregated (by FRET). There was a concurrent two-fold increase in immunofluorescent labeling for the AChR-associated cytoplasmic protein, rapsyn. When 1-month old muscle was denervated, postsynaptic rapsyn immunostaining was reduced, as was the efficiency of AChR aggregation. In vivo electroporation of rapsyn-EGFP into muscle fibers increased postsynaptic rapsyn levels. Those synapses with higher ratios of rapsyn-EGFP to AChR displayed a slower metabolic turnover of AChR. Conversely, the reduction of postsynaptic rapsyn after denervation was accompanied by an acceleration of AChR turnover. Thus, a developmental increase in the amount of rapsyn targeted to the postsynaptic membrane may drive enhanced postsynaptic AChRs aggregation and AChR stability within the postsynaptic membrane.

The dynamics of the rapsyn scaffolding protein at individual acetylcholine receptor clusters

Rapsyn, a cytoplasmic receptor-associated protein, is required for the clustering of acetylcholine receptors (AChRs). Although AChR dynamics have been extensively studied, little is known about the dynamics of rapsyn. Here, we used a rapsyn-green fluorescent protein (GFP) fusion protein and quantitative fluorescent imaging to study the dynamics of rapsyn in transfected C2C12 myotubes. First, we found that rapsyn-GFP expression at clusters did not alter AChR aggregation, function, or turnover. Quantification of rapsyn immunofluorescence indicated that the expression of rapsyn-GFP proteins at clusters does not increase the overall rapsyn density compared with untransfected myotube clusters. Using time lapse imaging and fluorescence recovery after photobleaching, we demonstrated that the recovery of rapsyn-GFP fluorescence at clusters was very fast, with a halftime of about approximately 1.5 h (approximately 3 times faster than AChRs). Inhibition of protein kinase C significantly altered receptor insertion, but it had no effect on rapsyn insertion. When cells were treated with the broad spectrum kinase inhibitor staurosporine, receptor insertion was decreased even further. However, inhibition of protein kinase A had no effect on insertion of either rapsyn or receptors. Finally, when cells were treated with neural agrin, rapsyn and AChRs were both directed away from preexisting clusters and accumulated together in new small clusters. These results demonstrate the remarkable dynamism of rapsyn, which may underlie the stability and maintenance of the postsynaptic scaffold and suggest that the insertion of different postsynaptic proteins may be operating independently.

Non-NMDA-type glutamate receptors are essential for maturation but not for initial assembly of synapses at Drosophila neuromuscular junctions

The assembly of glutamatergic postsynaptic densities (PSDs) seems to involve the gradual recruitment of molecular components from diffuse cellular pools. Whether the glutamate receptors themselves are needed to instruct the structural and molecular assembly of the PSD has hardly been addressed. Here, we engineered Drosophila neuromuscular junctions (NMJs) to express none or only drastically reduced amounts of their postsynaptic non-NMDA-type glutamate receptors. At such NMJs, principal synapse formation proceeded and presynaptic active zones showed normal composition and ultrastructure as well as proper glutamate release. At the postsynaptic site, initial steps of molecular and structural assembly took place as well. However, growth of the nascent PSDs to mature size was inhibited, and proteins normally excluded from PSD membranes remained at these apparently immature sites. Intriguingly, synaptic transmission as well as glutamate binding to glutamate receptors appeared dispensable for synapse maturation. Thus, our data suggest that incorporation of non-NMDA-type glutamate receptors and likely their protein-protein interactions with additional PSD components triggers a conversion from an initial to a mature stage of PSD assembly.

Synapse development: still looking for the forest, still lost in the trees

Synapse development in the vertebrate central nervous system is a highly orchestrated process occurring not only during early stages of brain development, but also (to a lesser extent) in the mature nervous system. During development, the formation of synapses is intimately linked to the differentiation of neuronal cells, the extension of their axons and dendrites, and the course wiring of the nervous system. Subsequently, the stabilization, elimination, and strengthening of synaptic contacts is coupled to the refinement of axonal and dendritic arbors, to the establishment of functionally meaningful connections, and probably also to the day-to-day acquisition, storage, and retrieval of memories, higher order thought processes, and behavioral patterns.

Cerebellar synaptic defects and abnormal motor behavior in mice lacking alpha- and beta-dystrobrevin

The dystrobrevins (alphaDB and betaDB) bind directly to dystrophin and are components of a transmembrane dystrophin-glycoprotein complex (DGC) that links the cytoskeleton to extracellular proteins in many tissues. We show here that alphaDB, betaDB, and dystrophin are all concentrated at a discrete subset of inhibitory synapses on the somata and dendrites of cerebellar Purkinje cells. Dystrophin is depleted from these synapses in mice lacking both alphaDB and betaDB, and DBs are depleted from these synapses in mice lacking dystrophin. In dystrophin mutants and alphaDB,betaDB double mutants, the size and number of GABA receptor clusters are decreased at cerebellar inhibitory synapses, and sensorimotor behaviors that reflect cerebellar function are perturbed. Synaptic and behavioral abnormalities are minimal in mice lacking either alphaDB or betaDB. Together, our results show that the DGC is required for proper maturation and function of a subset of inhibitory synapses, that DB is a key component of this DGC, and that interference with this DGC leads to behavioral abnormalities. We suggest that motor deficits in muscular dystrophy patients, which are their cardinal symptoms, may reflect not only peripheral derangements but also CNS defects.

Induction of myasthenia by immunization against muscle-specific kinase

Muscle-specific kinase (MuSK) is critical for the synaptic clustering of nicotinic acetylcholine receptors (AChRs) and plays multiple roles in the organization and maintenance of neuromuscular junctions (NMJs). MuSK is activated by agrin, which is released from motoneurons, and induces AChR clustering at the postsynaptic membrane. Although autoantibodies against the ectodomain of MuSK have been found in a proportion of patients with generalized myasthenia gravis (MG), it is unclear whether MuSK autoantibodies are the causative agent of generalized MG. In the present study, rabbits immunized with MuSK ectodomain protein manifested MG-like muscle weakness with a reduction of AChR clustering at the NMJs. The autoantibodies activated MuSK and blocked AChR clustering induced by agrin or by mediators that do not activate MuSK. Thus MuSK autoantibodies rigorously inhibit AChR clustering mediated by multiple pathways, an outcome that broadens our general comprehension of the pathogenesis of MG.

Photoinactivation of native AMPA receptors reveals their real-time trafficking

AMPA receptors mediate the majority of the fast excitatory transmission in the central nervous system. Much evidence suggests that the fast trafficking of AMPA receptors into and out of the postsynaptic membrane underlies changes in synaptic strength thought to be necessary for higher cognitive functions such as learning and memory. Despite the abundance of research conducted in this area, a direct, real-time functional assay that measures the trafficking of native AMPA receptors has been lacking. Toward this aim, we use a photoreactive, irreversible antagonist of AMPA receptors, ANQX, to rapidly silence surface AMPA receptors and investigate directly the trafficking of native AMPA receptors in real time. We find that the most dynamic movement of AMPA receptors occurs by lateral movement across the surface of neurons. Fast cycling of surface AMPA receptors with receptors from internal stores does occur but exclusively at extrasynaptic somatic sites. The cycling of synaptic AMPA receptors only occurs on a much longer timescale with complete exchange requiring at least 16 hr. This cycling is not dependent on protein synthesis or action potential driven network activity. These data suggest a revised model of AMPA receptor trafficking wherein a large internal store of AMPA receptors exchanges rapidly with extrasynaptic somatic AMPA receptors, and these newly inserted AMPA receptors then travel laterally along dendrites to reside stably at synapses.

Visualization of integral and peripheral cell surface proteins in live Caenorhabditis elegans

To study the abundance of specific receptors and other cell surface proteins at synapses, it would be advantageous to specifically label these proteins only when inserted in the plasma membrane. We describe a method that allows to fluorescently label cell surface proteins in live and behaving animals, namely in the nematode Caenorhabditis elegans. Proteins such as subunits of the levamisole sensitive nicotinic acetylcholine receptor (nAChR) were epitope-tagged at their extracellular C-termini, and fluorescent antibodies against those tags were injected into the body fluid. These antibodies specifically labelled synaptic regions on the cell surface of muscles and neurons, and simultaneous use of different tags facilitated co-localization studies. Quantification of the fluorescence is possible, as verified by demonstrating that mutations in ric-3 and unc-38, which cause behavioural resistance to cholinergic agonists, strongly reduce or even abolish nAChR cell surface expression. We also used this method to visualize the extracellular peripheral membrane protein ODR-2, which is related to a neurotoxin-like protein regulating vertebrate neuronal nAChRs. Likewise, fluorescent alpha-bungarotoxin, when injected, bound to certain nAChRs in the pharynx and the nervous system. This showed that, theoretically, any molecular interaction of sufficient affinity may be used to specifically label cell surface structures in live nematodes.

Acetylcholinesterase dynamics at the neuromuscular junction of live animals

At cholinergic synapses, acetylcholinesterase (AChE) is critical for ensuring normal synaptic transmission. However, little is known about how this enzyme is maintained and regulated in vivo. In this work, we demonstrate that the dissociation of fluorescently-tagged fasciculin 2 (a specific and selective peptide inhibitor of AChE) from AChE is extremely slow. This fluorescent probe was used to study the removal and insertion of AChE at individual synapses of living adult mice. After a one-time blockade of AChEs with fluorescent fasciculin 2, AChEs are removed from synapses initially at a faster rate (t(1/2) of approximately 3 days) and later at a slower rate (t(1/2) of approximately 12 days). Most of the removed AChEs are replaced by newly inserted AChEs over time. However, when AChEs are continuously blocked with fasciculin 2, the removal rate increases substantially (t(1/2) of approximately 12 h), and most of the lost AChEs are not replaced by newly inserted AChE. Furthermore, complete one-time inactivation of AChE activity significantly increases the removal of postsynaptic nicotinic acetylcholine receptors (AChRs). Finally, time lapse imaging reveals that synaptic AChEs and AChRs that are removed from synapses are co-localized in the same pool after being internalized. These results demonstrate a remarkable AChE dynamism and argue for a potential link between AChE function and postsynaptic receptor lifetime.

Identification of nicotinic acetylcholine receptor recycling and its role in maintaining receptor density at the neuromuscular junction in vivo

In the CNS, receptor recycling is critical for synaptic plasticity; however, the recycling of receptors has never been observed at peripheral synapses. Using a novel imaging technique, we show here that nicotinic acetylcholine receptors (AChRs) recycle into the postsynaptic membrane of the neuromuscular junction. By sequentially labeling AChRs with biotin-bungarotoxin and streptavidin-fluorophore conjugates, we were able to distinguish recycled, preexisting, and new receptor pools at synapses in living mice. Time-lapse imaging revealed that recycled AChRs were incorporated into the synapse within hours of initial labeling, and their numbers increased with time. At fully functional synapses, AChR recycling was robust and comparable in magnitude with the insertion of newly synthesized receptors, whereas chronic synaptic activity blockade nearly abolished receptor recycling. Finally, using the same sequential labeling method, we found that acetylcholinesterase, another synaptic component, does not recycle. These results identify an activity-dependent AChR-recycling mechanism that enables the regulation of receptor density, which could lead to rapid alterations in synaptic efficacy.

The effect of agrin and laminin on acetylcholine receptor dynamics in vitro

Using optical imaging assays, we investigated the dynamics of acetylcholine receptors (AChRs) at laminin-associated clusters on cultured myotubes in the absence or presence of the nerve-derived clustering factor, agrin. Using fluorescence recovery after photobleaching (FRAP) on fluorescent bungarotoxin-labeled receptors, we found that approximately 9% of original fluorescence was recovered after 8 h as surface AChRs were recruited into clusters. By quantifying the loss of labeled receptors and the recovery of fluorescence after photobleaching, we estimated that the half-life of clustered receptors was approximately 4.5 h. Despite the rapid removal of receptors, the accumulation of new receptors at clusters was robust enough to maintain receptor density over time. We also found that the AChR half-life was not affected by agrin despite its role in inducing the aggregation of AChRs. Interestingly, when agrin was added to myotubes grown on laminin-coated substrates, most new receptors were not directed into preexisting laminin-induced clusters but instead formed numerous small aggregates on the entire muscle surface. Time-lapse imaging revealed that the agrin-induced clusters could be seen as early as 1 h, and agrin treatment resulted in the complete dissipation of laminin-associated clusters by 24 h. These results reveal that while laminin and agrin are involved in the clustering of receptors they are not critical to the regulation of receptor metabolic stability at these clusters, and further argue that agrin is able to rapidly and fully negate the laminin substrate clustering effect while inducing the rapid formation of new clusters.

Increased ratio of rapsyn to ACh receptor stabilizes postsynaptic receptors at the mouse neuromuscular synapse

The metabolic turnover of nicotinic ACh receptors (AChR) at the neuromuscular synapse is regulated over a tenfold range by innervation status, muscle electrical activity and neural agrin, but the downstream effector of such changes has not been defined. The AChR-associated protein rapsyn is essential for forming AChR clusters during development. Here, rapsyn was tagged with enhanced green fluorescent protein (EGFP) to begin to probe its influence at the adult synapse. In C2 myotubes, rapsyn-EGFP participated with AChR in agrin-induced AChR cluster formation. When electroporated into the tibialis anterior muscle of young adult mice, rapsyn-EGFP accumulated in discrete subcellular structures, many of which colocalized with Golgi markers, consistent with the idea that rapsyn assembles with AChR in the exocytic pathway. Rapsyn-EGFP also targeted directly to the postsynaptic membrane where it occupied previously vacant rapsyn binding sites, thereby increasing the rapsyn to AChR ratio. At endplates displaying rapsyn-EGFP, the metabolic turnover of AChR (labelled with rhodamine-alpha-bungarotoxin) was slowed. Thus, the metabolic half-life of receptors at the synapse may be modulated by local changes in the subsynaptic ratio of rapsyn to AChR.

A novel pharmatope tag inserted into the beta4 subunit confers allosteric modulation to neuronal nicotinic receptors

alpha-Bungarotoxin, the classic nicotinic antagonist, has high specificity for muscle type alpha1 subunits in nicotinic acetylcholine receptors. In this study, we show that an 11-amino-acid pharmatope sequence, containing residues important for alpha-bungarotoxin binding to alpha1, confers functional alpha-bungarotoxin sensitivity when strategically placed into a neuronal non-alpha subunit, normally insensitive to this toxin. Remarkably, the mechanism of toxin inhibition is allosteric, not competitive as with neuromuscular nicotinic receptors. Our findings argue that alpha-bungarotoxin binding to the pharmatope, inserted at a subunit-subunit interface diametrically distinct from the agonist binding site, interferes with subunit interface movements critical for receptor activation. Our results, taken together with the structural similarities between nicotinic and GABAA receptors, suggest that this allosteric mechanism is conserved in the Cys-loop ion channel family. Furthermore, as a general strategy, the engineering of allosteric inhibitory sites through pharmatope tagging offers a powerful new tool for the study of membrane proteins.

Distribution of calcitonin gene-related peptide at the neuromuscular junction ofmdxmice

In normal skeletal muscle, the protein dystrophin is associated with plasma membrane glycoproteins and may be involved in the stabilization of the sarcolemma. Mutant mdx mice are markedly deficient in dystrophin and show muscle fiber necrosis followed by regeneration. Changes in the distribution of acetylcholine receptors (AChRs) have been reported at the neuromuscular junction of mdx mice possibly as a result of alterations in the release or response to neural trophic factors. One such factor is calcitonin gene-related peptide (CGRP), which has been implicated in AChR synthesis and function. In this study, we used rhodamine-alpha-bungarotoxin and anti-CGRP IgG FITC to study AChR and CGRP distribution at the neuromuscular junction of mdx mice. Using laser scanning fluorescence confocal microscopy, it was possible to see that CGRP-like immunoreactivity had a presynaptic distribution, covering the AChRs. Thirty-four percent of dystrophic junctions were found to be labeled with CGRP compared to 80% of control endplates. Since CGRP-positive and -negative fibers showed similar changes in AChR distribution, it is suggested that CGRP is probably not directly involved in the altered pattern of AChR seen in dystrophin-deficient muscle fibers of mdx mice.

AMPA-receptor activation regulates the diffusion of a membrane marker in parallel with dendritic spine motility in the mouse hippocampus

Dendritic spines are the site of most excitatory connections in the hippocampus. We have investigated the diffusibility of a membrane-bound green fluorescent protein (mGFP) within the inner leaflet of the plasma membrane using Fluorescence Recovery After Photobleaching. In dendritic spines the diffusion of mGFP was significantly retarded relative to the dendritic shaft. In parallel, we have assessed the motility of dendritic spines, and found an inverse correlation between spine motility and the rate of diffusion of mGFP. We then tested the influence of glutamate receptor activation or blockade, and the involvement of the actin cytoskeleton (using latrunculin A) on spine motility and mGFP diffusion. These results show that glutamate receptors regulate the mobility of molecules in the inner leaflet of the plasma membrane through an action upon the actin cytoskeleton, suggesting a novel mechanism for the regulation of postsynaptic receptor density and composition.

In vivo mammalian brain imaging using one- and two-photon fluorescence microendoscopy

One of the major limitations in the current set of techniques available to neuroscientists is a dearth of methods for imaging individual cells deep within the brains of live animals. To overcome this limitation, we developed two forms of minimally invasive fluorescence microendoscopy and tested their abilities to image cells in vivo. Both one- and two-photon fluorescence microendoscopy are based on compound gradient refractive index (GRIN) lenses that are 350-1,000 microm in diameter and provide micron-scale resolution. One-photon microendoscopy allows full-frame images to be viewed by eye or with a camera, and is well suited to fast frame-rate imaging. Two-photon microendoscopy is a laser-scanning modality that provides optical sectioning deep within tissue. Using in vivo microendoscopy we acquired video-rate movies of thalamic and CA1 hippocampal red blood cell dynamics and still-frame images of CA1 neurons and dendrites in anesthetized rats and mice. Microendoscopy will help meet the growing demand for in vivo cellular imaging created by the rapid emergence of new synthetic and genetically encoded fluorophores that can be used to label specific brain areas or cell classes.

Nerve-independent formation of a topologically complex postsynaptic apparatus

As the mammalian neuromuscular junction matures, its acetylcholine receptor (AChR)–rich postsynaptic apparatus is transformed from an oval plaque into a pretzel-shaped array of branches that precisely mirrors the branching pattern of the motor nerve terminal. Although the nerve has been believed to direct postsynaptic maturation, we report here that myotubes cultured aneurally on matrix-coated substrates form elaborately branched AChR-rich domains remarkably similar to those seen in vivo. These domains share several characteristics with the mature postsynaptic apparatus, including colocalization of multiple postsynaptic markers, clustering of subjacent myonuclei, and dependence on the muscle-specific kinase and rapsyn for their formation. Time-lapse imaging showed that branched structures arise from plaques by formation and fusion of AChR-poor perforations through a series of steps mirroring that seen in vivo. Multiple fluorophore imaging showed that growth occurs by circumferential, asymmetric addition of AChRs. Analysis in vivo revealed similar patterns of AChR addition during normal development. These results reveal the sequence of steps by which a topologically complex domain forms on a cell and suggest an unexpected nerve-independent role for the postsynaptic cell in generating this topological complexity.

Acetylcholine receptor distribution and synapse elimination at the developing neuromuscular junction of mdx mice

The pattern of innervation of the vertebrate neuromuscular junction is established during early development, when junctions go from multiple to single innervation in the phenomenon of synapse elimination, suggesting that changes at the molecular level in the postsynaptic cell lead to the removal of nerve terminals. The mdx mouse is deficient in dystrophin and associated proteins that are part of the postsynaptic cytoskeleton. We used rhodamine-alpha-bungarotoxin and anti-neurofilament IgG-FITC to stain acetylcholine receptors and nerve terminals of the sternomastoid muscle during postnatal development in mdx and control C57BL/10 mice. Using fluorescence confocal microscopy, we observed that, 7 days after birth, 86.7% of the endplates of mdx mice were monoinnervated (n = 200) compared with 41.4% in control mice (n = 200). By the end of the second postnatal week, all endplates were innervated singly (100% mdx and 94.7% controls, n = 200 per group). These results show that dystrophic fibers achieve single innervation earlier, perhaps because dystrophin or a normal cytoskeletal complex is implicated in this phenomenon.

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.

Fluorescent ligands, antibodies, and proteins for the study of receptors

Fluorescent molecules bound to receptors can show their location and, if binding is reversible, can provide pharmacological information such as affinity and proximity between interacting molecules. The spatial precision offered by visualisation transcends the diverse localisation and low molecular concentration of receptor molecules. Consequently, the relationships between receptor location and function and life cycles of receptors have become better understood as a result of fluorescent labeling. Each of these aspects contributes new insights to drug action and potential new targets. The relationships between spatial distribution of receptor and function are largely unknown. This is particularly apparent for native receptors expressed in their normal host tissues where communication between heterogeneous cell types influences receptor distribution and function. In cultured cell systems, particularly for G-protein-coupled receptors (GPCR), fluorescence-based methods have enabled the visualisation of the cycle of agonist-stimulated receptor clustering, endocytic internalisation to the perinuclear region, degradation of the receptor-ligand complex, and recycling back to the surface membrane. Using variant forms of green fluorescent protein (GFP), antibodies, or fluorescent ligands, it is possible to detect or visualise the formation of oligomeric receptor complexes. Careful selection of fluorescent molecules based on their spectral properties enables resonance energy transfer and multilabel visualisation with colocalisation studies. Fluorescent agonist and antagonist ligands are now being used in parallel with GFP to study receptor cycling in live cells. This review covers how labeling and visualisation technologies have been applied to the study of major pharmacologically important receptors and illustrates this by giving examples of recent techniques that have relied on GFP, antibodies, or fluorescent ligands alone or in combination for the purpose of studying GPCR.

Tyrosine-phosphorylated and nonphosphorylated isoforms of alpha-dystrobrevin: roles in skeletal muscle and its neuromuscular and myotendinous junctions

alpha-Dystrobrevin (DB), a cytoplasmic component of the dystrophin-glycoprotein complex, is found throughout the sarcolemma of muscle cells. Mice lacking alphaDB exhibit muscular dystrophy, defects in maturation of neuromuscular junctions (NMJs) and, as shown here, abnormal myotendinous junctions (MTJs). In normal muscle, alternative splicing produces two main alphaDB isoforms, alphaDB1 and alphaDB2, with common NH2-terminal but distinct COOH-terminal domains. alphaDB1, whose COOH-terminal extension can be tyrosine phosphorylated, is concentrated at the NMJs and MTJs. alphaDB2, which is not tyrosine phosphorylated, is the predominant isoform in extrajunctional regions, and is also present at NMJs and MTJs. Transgenic expression of either isoform in alphaDB-/- mice prevented muscle fiber degeneration; however, only alphaDB1 completely corrected defects at the NMJs (abnormal acetylcholine receptor patterning, rapid turnover, and low density) and MTJs (shortened junctional folds). Site-directed mutagenesis revealed that the effectiveness of alphaDB1 in stabilizing the NMJ depends in part on its ability to serve as a tyrosine kinase substrate. Thus, alphaDB1 phosphorylation may be a key regulatory point for synaptic remodeling. More generally, alphaDB may play multiple roles in muscle by means of differential distribution of isoforms with distinct signaling or structural properties.