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

Plasticity and structural alterations of mitochondria and sarcoplasmic organelles in muscles of mice deficient in α-dystrobrevin, a component of the dystrophin-glycoprotein complex

The dystrophin-glycoprotein complex (DGC) plays a crucial role in maintaining the structural integrity of the plasma membrane and the neuromuscular junction. In this study, we investigated the impact of the deficiency of α-dystrobrevin (αdbn), a component of the DGC, on the homeostasis of intracellular organelles, specifically mitochondria and the sarcoplasmic reticulum (SR). In αdbn deficient muscles, we observed a significant increase in the membrane-bound ATP synthase complex levels, a marker for mitochondria in oxidative muscle fiber types compared to wild-type. Furthermore, examination of muscle fibers deficient in αdbn using electron microscopy revealed profound alterations in the organization of mitochondria and the SR within certain myofibrils of muscle fibers. This included the formation of hyper-branched intermyofibrillar mitochondria with extended connections, an extensive network spanning several myofibrils, and a substantial increase in the number/density of subsarcolemmal mitochondria. Concurrently, in some cases, we observed significant structural alterations in mitochondria, such as cristae loss, fragmentation, swelling, and the formation of vacuoles and inclusions within the mitochondrial matrix cristae. Muscles deficient in αdbn also displayed notable alterations in the morphology of the SR, along with the formation of distinct anomalous concentric SR structures known as whorls. These whorls were prevalent in αdbn-deficient mice but were absent in wild-type muscles. These results suggest a crucial role of the DGC αdbn in regulating intracellular organelles, particularly mitochondria and the SR, within muscle cells. The remodeling of the SR and the formation of whorls may represent a novel mechanism of the unfolded protein response (UPR) in muscle cells.

Ravulizumab pharmacokinetics and pharmacodynamics in patients with generalized myasthenia gravis

INTRODUCTION

The terminal complement C5 inhibitor ravulizumab has a long elimination half-life, allowing maintenance dosing every 8 weeks. In the 26-week, double-blind, randomized, placebo-controlled period (RCP) of the CHAMPION MG study, ravulizumab provided rapid and sustained efficacy and was well tolerated in adults with anti-acetylcholine receptor antibody-positive (AChR Ab+) generalized myasthenia gravis (gMG). This analysis evaluated the pharmacokinetics (PK), pharmacodynamics (PD), and potential immunogenicity of ravulizumab in adults with AChR Ab+ gMG.

METHODS

Data were analyzed from 86 patients who received ravulizumab in the CHAMPION MG RCP. Ravulizumab dosing was weight-based: initial loading dose of 2400/2700/3000 mg on Day 1 and maintenance doses of 3000/3300/3600 mg on Day 15 and then every 8 weeks. PK parameters were estimated from serum ravulizumab concentrations determined pre- and post-dose; PD effects of ravulizumab on serum free C5 concentrations were measured; and immunogenicity was assessed using anti-drug antibody and neutralizing-antibody assays.

RESULTS

Target serum ravulizumab concentrations (> 175 µg/mL) were achieved immediately after the first ravulizumab dose (within 30 min of infusion completion) and maintained throughout the 26-week treatment period irrespective of patient body weight. Following the final maintenance dose, mean C_max was 1548 µg/mL and C_trough 587 µg/mL; no meaningful differences were noted among body-weight categories. Inhibition of serum free C5 was immediate, complete (< 0.5 μg/mL), and sustained throughout treatment in all patients. No treatment-emergent anti-drug antibodies were observed.

CONCLUSIONS

PK/PD evidence supports the use of ravulizumab every 8 weeks for immediate, complete, and sustained inhibition of terminal complement C5 in adults with AChR Ab+ gMG.

TRIAL REGISTRATION

ClinicalTrials.gov ID: NCT03920293 (April 18, 2019).

Distinct roles of the dystrophin-glycoprotein complex: α-dystrobrevin and α-syntrophin in the maintenance of the postsynaptic apparatus of the neuromuscular synapse

α-syntrophin (α-syn) and α-dystrobrevin (α-dbn), two components of the dystrophin-glycoprotein complex, are essential for the maturation and maintenance of the neuromuscular junction (NMJ) and mice deficient in either α-syn or α-dbn exhibit similar synaptic defects. However, the functional link between these two proteins and whether they exert distinct or redundant functions in the postsynaptic organization of the NMJ remain largely unknown. We generated and analyzed the synaptic phenotype of double heterozygote (α-dbn+/-, α-syn+/-), and double homozygote knockout (α-dbn-/-; α-syn-/-) mice and examined the ability of individual molecules to restore their defects in the synaptic phenotype. We showed that in double heterozygote mice, NMJs have normal synaptic phenotypes and no signs of muscular dystrophy. However, in double knockout mice (α-dbn-/-; α-syn-/-), the synaptic phenotype (the density, the turnover and the distribution of AChRs within synaptic branches) is more severely impaired than in single α-dbn-/- or α-syn-/- mutants. Furthermore, double mutant and single α-dbn-/- mutant mice showed more severe exercise-induced fatigue and more significant reductions in grip strength than single α-syn-/- mutant and wild-type. Finally, we showed that the overexpression of the transgene α-syn-GFP in muscles of double mutant restores primarily the abnormal extensions of membrane containing AChRs that extend beyond synaptic gutters and lack synaptic folds, whereas the overexpression of α-dbn essentially restores the abnormal dispersion of patchy AChR aggregates in the crests of synaptic folds. Altogether, these data suggest that α-syn and α-dbn act in parallel pathways and exert distinct functions on the postsynaptic structural organization of NMJs.

The Metabolic Stability of the Nicotinic Acetylcholine Receptor at the Neuromuscular Junction

The clustering and maintenance of nicotinic acetylcholine receptors (AChRs) at high density in the postsynaptic membrane is a hallmark of the mammalian neuromuscular junction (NMJ). The regulation of receptor density/turnover rate at synapses is one of the main thrusts of neurobiology because it plays an important role in synaptic development and synaptic plasticity. The state-of-the-art imaging revealed that AChRs are highly dynamic despite the overall structural stability of the NMJ over the lifetime of the animal. This review highlights the work on the metabolic stability of AChRs at developing and mature NMJs and discusses the role of synaptic activity and the regulatory signaling pathways involved in the dynamics of AChRs.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

Neuromuscular reinnervation efficacy using a YFP model

INTRODUCTION

The gold standard reconstruction for facial reanimation is the functional muscle transfer. The reinnervation of a muscle is never complete, and clinical results are variable with 20% not achieving a satisfactory outcome. We hypothesise that this may be due to a mismatch between the characteristics of the donor nerve and transferred muscle.

METHOD

81 YFP-16 and 14 YFP-H mice were studied in three intervention groups over three time periods. Two parameters were investigated: the number and surface area of reinnervated neuromuscular junctions and regenerating axons. An assessment was made of motor unit proportions.

RESULTS

All cases of nerve repair and nerve graft, the neuromuscular junctions (NMJ) were completely reinnervated by regenerating axons. The number and calibre of the regenerating axons were significantly different from controls for both intervention groups. The motor units were smaller in both intervention groups.

DISCUSSION

Reinnervation occurs after nerve repair or graft; however, the arbour was reinnervated by large numbers of much smaller axons. These axons showed some evidence of remodelling in the repair group, but not in the graft group. Neither group achieved the parameters of the control group. There were persistent qualitative changes to the morphology of both axons and junctions. Imaging documented both synkinesis and alterations that resemble those seen in ageing.

CONCLUSION

Overall, the efficacy of reinnervation is very high with all NMJ reoccupied by regenerating axons. The way small axons are remodelled is different in the nerve repairs compared with the nerve grafts.

Illuminating the Path to Target GPCR Structures and Functions

G-Protein-coupled receptors (GPCRs) are ubiquitous within eukaryotes, responsible for a wide array of physiological and pathological processes. Indeed, the fact that they are the most drugged target in the human genome is indicative of their importance. Despite the clear interest in GPCRs, most information regarding their activity has been so far obtained by analyzing the response from a "bulk medium". As such, this Perspective summarizes some of the common methods for this indirect observation. Nonetheless, by inspecting approaches applying super-resolution imaging, we argue that imaging is perfectly situated to obtain more detailed structural and spatial information, assisting in the development of new GPCR-targeted drugs and clinical strategies. The benefits of direct optical visualization of GPCRs are analyzed in the context of potential future directions in the field.

Phosphorylation of α-dystrobrevin is essential for αkap accumulation and acetylcholine receptor stability

The maintenance of a high density of the acetylcholine receptor (AChR) is the hallmark of the neuromuscular junction. Muscle-specific anchoring protein (αkap) encoded within the calcium/calmodulin-dependent protein kinase IIα (CAMK2A) gene is essential for the maintenance of AChR clusters both in vivo and in cultured muscle cells. The underlying mechanism by which αkap is maintained and regulated remains unknown. Here, using human cell lines, fluorescence microscopy, and pulldown and immunoblotting assays, we show that α-dystrobrevin (α-dbn), an intracellular component of the dystrophin glycoprotein complex, directly and robustly promotes the stability of αkap in a concentration-dependent manner. Mechanistically, we found that the phosphorylatable tyrosine residues of α-dbn are essential for the stability of α-dbn itself and its interaction with αkap, with substitution of three tyrosine residues in the α-dbn C terminus with phenylalanine compromising the αkap-α-dbn interaction and significantly reducing both αkap and α-dbn accumulation. Moreover, the αkap-α-dbn interaction was critical for αkap accumulation and stability. We also found that the absence of either αkap or α-dbn markedly reduces AChRα accumulation and that overexpression of α-dbn or αkap in cultured muscle cells promotes the formation of large agrin-induced AChR clusters. Collectively, these results indicate that the stability of αkap and α-dbn complex plays an important role in the maintenance of high-level expression of AChRs.

Multiple MuSK signaling pathways and the aging neuromuscular junction

The neuromuscular junction (NMJ) is the vehicle for fast, reliable and robust communication between motor neuron and muscle. The unparalleled accessibility of this synapse to morphological, electrophysiological and genetic analysis has yielded an in depth understanding of many molecular components mediating its formation, maturation and stability. However, key questions surrounding the signaling pathways mediating these events and how they play out across the lifetime of the synapse remain unanswered. Such information is critical since the NMJ is necessary for normal movement and is compromised in several settings including myasthenia gravis, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), muscular dystrophy, sarcopenia and aging. Muscle specific kinase (MuSK) is a central player in most if not all contexts of NMJ formation and stability. However, elucidating the function of this receptor in this range of settings is challenging since MuSK participates in at least three signaling pathways: as a tyrosine kinase-dependent receptor for agrin-LRP4 and Wnts; and, as a kinase-independent BMP co-receptor. Here we focus on NMJ stability during aging and discuss open questions regarding the molecular mechanisms that govern active maintenance of the NMJ, with emphasis on MuSK and the potential role of its multiple signaling contexts.

The disassembly of the neuromuscular synapse in high-fat diet-induced obese male mice

Objective

A sustained high fat diet in mice mimics many features of human obesity. We used male and female Non-Swiss albino mice to investigate the impact of short and long-term high-fat diet-(HFD)-induced obesity on the peripheral neuromuscular junction (NMJ) and whether obesity-related synaptic structural alterations were reversible after switching obese mice from HFD to a standard fat diet (SD).

Methods

HFD-induced obese and age-matched control mice fed SD were used. We carried out in vivo time lapse imaging to monitor changes of synapses over time, quantitative fluorescence imaging to study the regulation of acetylcholine receptor number and density at neuromuscular junctions, and high resolution confocal microscope to study structural alterations in both the pre- and postsynaptic apparatus.

Results

Time-lapse imaging in vivo over a 9 month period revealed that NMJs of HFD obese male mice display a variety of obesity-related structural alterations, including the disappearance of large synaptic areas, significant reduction in the density/number of nicotinic acetylcholine receptor (AChRs), abnormal distribution of AChRs, high turnover rate of AChRs, retraction of axons from lost postsynaptic sites, and partially denervated synapses. The severity of these synaptic alterations is associated with the duration of obesity. However, no substantial alterations were observed at NMJs of age-matched HFD obese female mice or male mice fed with a standard or low fat diet. Intriguingly, when obese male mice were switched from HFD to a standard diet, receptor density and the abnormal pattern of AChR distribution were completely reversed to normal, whereas lost synaptic structures were not restored.

Conclusions

These results show that the obese male mice are more vulnerable than female mice to the impacts of long-term HFD on the NMJ damage and provide evidence that diet restriction can partially reverse obesity-related synaptic changes.

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Neuromuscular junctions of High-fat induced obese male mice display a variety of obesity-related structural alterations.

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The severity of alterations in neuromuscular junction morphology is associated with the duration of obesity.

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Neuromuscular junctions of High-fat diet induced obese female mice display no substantial morphological changes.

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Not all obesity-related synaptic alterations were reversible after switching male mice from High-fat diet to standard diet.

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Obese male mice are more vulnerable than female mice to the impacts of long-term HFD on the neuromuscular junction damage.

mTORC1 and PKB/Akt control the muscle response to denervation by regulating autophagy and HDAC4

Loss of innervation of skeletal muscle is a determinant event in several muscle diseases. Although several effectors have been identified, the pathways controlling the integrated muscle response to denervation remain largely unknown. Here, we demonstrate that PKB/Akt and mTORC1 play important roles in regulating muscle homeostasis and maintaining neuromuscular endplates after nerve injury. To allow dynamic changes in autophagy, mTORC1 activation must be tightly balanced following denervation. Acutely activating or inhibiting mTORC1 impairs autophagy regulation and alters homeostasis in denervated muscle. Importantly, PKB/Akt inhibition, conferred by sustained mTORC1 activation, abrogates denervation-induced synaptic remodeling and causes neuromuscular endplate degeneration. We establish that PKB/Akt activation promotes the nuclear import of HDAC4 and is thereby required for epigenetic changes and synaptic gene up-regulation upon denervation. Hence, our study unveils yet-unknown functions of PKB/Akt-mTORC1 signaling in the muscle response to nerve injury, with important implications for neuromuscular integrity in various pathological conditions.

Denervation leads to muscle atrophy and neuromuscular endplate remodeling. Here, the authors show that a balanced activation of mTORC1 contributes to the dynamic regulation of autophagic flux in denervated muscle and that activation of PKB/Akt promotes the nuclear import of HDAC4, which is essential for endplate maintenance upon nerve injury

Balanced Rac1 activity controls formation and maintenance of neuromuscular acetylcholine receptor clusters

Rac1, an important Rho GTPase that regulates the actin cytoskeleton, has long been suggested to participate in acetylcholine receptor (AChR) clustering at the postsynaptic neuromuscular junction. However, how Rac1 is regulated and how it influences AChR clusters have remained unexplored. This study shows that breaking the balance of Rac1 regulation, by either increasing or decreasing its activity, led to impaired formation and maintenance of AChR clusters. By manipulating Rac1 activity at different stages of AChR clustering in cultured myotubes, we show that Rac1 activation was required for the initial formation of AChR clusters, but its persistent activation led to AChR destabilization, and uncontrolled hyperactivation of Rac1 even caused excessive myotube fusion. Both AChR dispersal and myotube fusion induced by Rac1 were dependent on its downstream effector Pak1. Two Rac1 GAPs and six Rac1 GEFs were screened and found to be important for normal AChR clustering. This study reveals that, although general Rac1 activity remains at low levels during terminal differentiation of myotubes and AChR cluster maintenance, tightly regulated Rac1 activity controls normal AChR clustering.

Agrin and LRP4 antibodies as new biomarkers of myasthenia gravis

Myasthenia gravis (MG) is a common disorder that affects the neuromuscular junction. It is caused by antibodies against acetylcholine receptor and muscle-specific tyrosine kinase; however, some MG patients do not have antibodies against either of the proteins. Recent studies have revealed antibodies against agrin and its receptor LRP4-both critical for neuromuscular junction formation and maintenance-in MG patients from various populations. Results from experimental autoimmune MG animal models indicate that anti-LRP4 antibodies are causal to MG. Clinical studies have begun to reveal the significance of the new biomarkers. With their identification, MG appears to be a complex disease entity that can be classified into different subtypes with different etiology, each with unique symptoms. Future systematic studies of large cohorts of well-diagnosed MG patients are needed to determine whether each subtype of patients would respond to different therapeutic strategies. Results should contribute to the goal of precision medicine for MG patients. Anti-agrin and anti-LRP4 antibodies are also detectable in some patients with amyotrophic lateral sclerosis or Lou Gehrig's disease; however, whether they are a cause or response to the disorder remains unclear.

Spatial distribution and molecular dynamics of dystrophin glycoprotein components at the neuromuscular junction in vivo

A bimolecular fluorescence complementation (BiFC) approach was used to study the molecular interactions between different components of the postsynaptic protein complex at the neuromuscular junction of living mice. We show that rapsyn forms complex with both α-dystrobrevin and α-syntrophin at the crests of junctional folds. The linkage of rapsyn to α-syntrophin and/or α-dystrobrevin is mediated by utrophin, a protein localized at acetylcholine receptor (AChR)-rich domains. In mice deficient in α-syntrophin, in which utrophin is no longer present at the synapse, rapsyn interaction with α-dystrobrevin was completely abolished. This interaction was completely restored when either utrophin or α-syntrophin was introduced into muscles deficient in α-syntrophin. However, in neuromuscular junctions deficient in α-dystrobrevin, in which utrophin is retained, complex formation between rapsyn and α-syntrophin was unaffected. Using fluorescence recovery after photobleaching, we found that α-syntrophin turnover is 5-7 times faster than that of AChRs, and loss of α-dystrobrevin has no effect on rapsyn and α-syntrophin half-life, whereas the half-life of AChR was significantly altered. Altogether, these results provide new insights into the spatial distribution of dystrophin glycoprotein components and their dynamics in living mice.

Nonmechanical Roles of Dystrophin and Associated Proteins in Exercise, Neuromuscular Junctions, and Brains

Dystrophin-glycoprotein complex (DGC) is an important structural unit in skeletal muscle that connects the cytoskeleton (f-actin) of a muscle fiber to the extracellular matrix (ECM). Several muscular dystrophies, such as Duchenne muscular dystrophy, Becker muscular dystrophy, congenital muscular dystrophies (dystroglycanopathies), and limb-girdle muscular dystrophies (sarcoglycanopathies), are caused by mutations in the different DGC components. Although many early studies indicated DGC plays a crucial mechanical role in maintaining the structural integrity of skeletal muscle, recent studies identified novel roles of DGC. Beyond a mechanical role, these DGC members play important signaling roles and act as a scaffold for various signaling pathways. For example, neuronal nitric oxide synthase (nNOS), which is localized at the muscle membrane by DGC members (dystrophin and syntrophins), plays an important role in the regulation of the blood flow during exercise. DGC also plays important roles at the neuromuscular junction (NMJ) and in the brain. In this review, we will focus on recently identified roles of DGC particularly in exercise and the brain.

Insights from reconstitution reactions of COPII vesicle formation using pure components and low mechanical perturbation

As shape transformations of membranes are vital for intracellular trafficking, it is crucial to understand both the mechanics and the biochemistry of these processes. The interplay of these two factors constitutes an experimental challenge, however, because biochemical experiments are not tailored to the investigation of mechanical processes, and biophysical studies using model membranes are not capable of emulating native biological complexity. Reconstituted liposome-based model systems have been widely used for investigating the formation of transport vesicles by the COPII complex that naturally occurs at the endoplasmic reticulum. Here we have revisited these model systems, to address the influence of lipid composition, GTP hydrolyzing conditions and mechanical perturbation on the experimental outcome. We observed that the lipid-dependence of COPII-induced membrane remodeling differs from that predicted based on the lipid-dependence of COPII membrane binding. Under GTP non-hydrolyzing conditions, a structured coat was seen while GTP-hydrolyzing conditions yielded uncoated membranes as well as membranes coated by a thick protein coat of rather unstructured appearance. Detailed up-to-date protocols for purifications of Saccharomyces cerevisiae COPII proteins and for reconstituted reactions using these proteins with giant liposomes are also provided.

CLASP2-dependent microtubule capture at the neuromuscular junction membrane requires LL5β and actin for focal delivery of acetylcholine receptor vesicles

A novel mechanism is described for the agrin-mediated focal delivery of acetylcholine receptors (AChRs) to the postsynaptic membrane of the neuromuscular junction. Microtubule capture mediated by CLASP2 and its interaction partner, LL5β, and an intact subsynaptic actin cytoskeleton are both required for focal AChR transport to the synaptic membrane.

A hallmark of the neuromuscular junction (NMJ) is the high density of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane. The postsynaptic apparatus of the NMJ is organized by agrin secreted from motor neurons. The mechanisms that underlie the focal delivery of AChRs to the adult NMJ are not yet understood in detail. We previously showed that microtubule (MT) capture by the plus end–tracking protein CLASP2 regulates AChR density at agrin-induced AChR clusters in cultured myotubes via PI3 kinase acting through GSK3β. Here we show that knockdown of the CLASP2-interaction partner LL5β by RNAi and forced expression of a CLASP2 fragment blocking the CLASP2/LL5β interaction inhibit microtubule capture. The same treatments impair focal vesicle delivery to the clusters. Consistent with these findings, knockdown of LL5β at the NMJ in vivo reduces the density and insertion of AChRs into the postsynaptic membrane. MT capture and focal vesicle delivery to agrin-induced AChR clusters are also inhibited by microtubule- and actin-depolymerizing drugs, invoking both cytoskeletal systems in MT capture and in the fusion of AChR vesicles with the cluster membrane. Combined our data identify a transport system, organized by agrin through PI3 kinase, GSK3β, CLASP2, and LL5β, for precise delivery of AChR vesicles from the subsynaptic nuclei to the overlying synaptic membrane.

Muscle-specific kinase (MuSK) autoantibodies suppress the MuSK pathway and ACh receptor retention at the mouse neuromuscular junction

Muscle-specific kinase (MuSK) autoantibodies from myasthenia gravis patients can block the activation of MuSK in vitro and/or reduce the postsynaptic localization of MuSK. Here we use a mouse model to examine the effects of MuSK autoantibodies upon some key components of the postsynaptic MuSK pathway and upon the regulation of junctional ACh receptor (AChR) numbers. Mice became weak after 14 daily injections of anti-MuSK-positive patient IgG. The intensity and area of AChR staining at the motor endplate was markedly reduced. Pulse-labelling of AChRs revealed an accelerated loss of pre-existing AChRs from postsynaptic AChR clusters without a compensatory increase in incorporation of (newly synthesized) replacement AChRs. Large, postsynaptic AChR clusters were replaced by a constellation of tiny AChR microaggregates. Puncta of AChR staining also appeared in the cytoplasm beneath the endplate. Endplate staining for MuSK, activated Src, rapsyn and AChR were all reduced in intensity. In the tibialis anterior muscle there was also evidence that phosphorylation of the AChR β-subunit-Y390 was reduced at endplates. In contrast, endplate staining for β-dystroglycan (through which rapsyn couples AChR to the synaptic basement membrane) remained intense. The results suggest that anti-MuSK IgG suppresses the endplate density of MuSK, thereby down-regulating MuSK signalling activity and the retention of junctional AChRs locally within the postsynaptic membrane scaffold.

PKC and PKA regulate AChR dynamics at the neuromuscular junction of living mice

The steady state of the acetylcholine receptor (AChR) density at the neuromuscular junction (NMJ) is critical for efficient and reliable synaptic transmission. However, little is known about signaling molecules involved in regulating the equilibrium between the removal and insertion of AChRs that establishes a stable postsynaptic receptor density over time. In this work, we tested the effect of activities of two serine/threonine kinases, PKC and PKA, on the removal rate of AChRs from and the re-insertion rate of internalized recycled AChRs into synaptic sites of innervated and denervated NMJs of living mice. Using an in vivo time-lapse imaging approach and various pharmacological agents, we showed that PKC and PKA activities have antagonistic effects on the removal and recycling of AChRs. Inhibition of PKC activity or activation of PKA largely prevents the removal of pre-existing AChRs and promotes the recycling of internalized AChRs into the postsynaptic membrane. In contrast, stimulation of PKC or inactivation of PKA significantly accelerates the removal of postsynaptic AChRs and depresses AChR recycling. These results indicate that a balance between PKA and PKC activities may be critical for the maintenance of the postsynaptic receptor density.

Structure and superorganization of acetylcholine receptor-rapsyn complexes

The scaffolding protein at the neuromuscular junction, rapsyn, enables clustering of nicotinic acetylcholine receptors in high concentration and is critical for muscle function. Patients with insufficient receptor clustering suffer from muscle weakness. However, the detailed organization of the receptor-rapsyn network is poorly understood: it is unclear whether rapsyn first forms a wide meshwork to which receptors can subsequently dock or whether it only forms short bridges linking receptors together to make a large cluster. Furthermore, the number of rapsyn-binding sites per receptor (a heteropentamer) has been controversial. Here, we show by cryoelectron tomography and subtomogram averaging of Torpedo postsynaptic membrane that receptors are connected by up to three rapsyn bridges, the minimum number required to form a 2D network. Half of the receptors belong to rapsyn-connected groups comprising between two and fourteen receptors. Our results provide a structural basis for explaining the stability and low diffusion of receptors within clusters.

Regulation of muscle acetylcholine receptor turnover by β subunit tyrosine phosphorylation

At the neuromuscular junction (NMJ), the postsynaptic localization of muscle acetylcholine receptor (AChR) is regulated by neural signals and occurs via several processes including metabolic stabilization of the receptor. However, the molecular mechanisms that influence receptor stability remain poorly defined. Here, we show that neural agrin and the tyrosine phosphatase inhibitor, pervanadate slow the degradation of surface receptor in cultured muscle cells. Their action is mediated by tyrosine phosphorylation of the AChR β subunit, as agrin and pervandate had no effect on receptor half-life in AChR-β(3F/3F) muscle cells, which have targeted mutations of the β subunit cytoplasmic tyrosines. Moreover, in wild type AChR-β(3Y) muscle cells, we found a linear relationship between average receptor half-life and the percentage of AChR with phosphorylated β subunit, with half-lives of 12.7 and 23 h for nonphosphorylated and phosphorylated receptor, respectively. Surprisingly, pervanadate increased receptor half-life in AChR-β(3Y) myotubes in the absence of clustering, and agrin failed to increase receptor half-life in AChR-β(3F/3F) myotubes even in the presence of clustering. The metabolic stabilization of the AChR was mediated specifically by phosphorylation of βY390 as mutation of this residue abolished β subunit phosphorylation but did not affect δ subunit phosphorylation. Receptor stabilization also led to higher receptor levels, as agrin increased surface AChR by 30% in AChR-β(3Y) but not AChR-β(3F/3F) myotubes. Together, these findings identify an unexpected role for agrin-induced phosphorylation of β(Y390) in downregulating AChR turnover. This likely stabilizes AChR at developing synapses, and contributes to the extended half-life of AChR at adult NMJs.

Nicotinic acetylcholine receptor stability at the NMJ deficient in α-syntrophin in vivo

α-Syntrophin (α-syn), a scaffold protein, links signaling molecules to the dystrophin-glycoprotein complex. Absence of α-syn from the DGC is known to lead to structurally aberrant neuromuscular junctions (NMJs) with few acetylcholine receptors (AChRs) clustered at synaptic sites. Using α-syn knock-out mice, we show that during the first postnatal week, α-syn is not required for synapse formation. However, at postnatal day 6 (P6)-P7, the structural integrity of the postsynaptic apparatus is altered, the turnover rate of AChRs increases significantly, and the number/density of AChRs is impaired. At the adult α-syn(-/-) NMJ, the turnover rate of AChRs is ∼ 4 times faster than wild-type synapses, and most removed receptors are targeted to degradation as few AChRs recycled to synaptic sites. Biochemical analyses show that in muscle cells of adult knock-out α-syn mice, total AChRs and scaffold protein rapsyn are significantly reduced, the 89 kDa and 75 kDa isoforms of tyrosine phosphorylated α-dystrobrevin (α-dbn) 1 (which are required for the maintenance and stability of AChR in α-dbn(-/-) synapses) are barely detectable. Electroporation of GFP-α-dbn1 in α-syn(-/-) muscle cells partially restored receptor density, turnover rate, and the structural integrity of the postsynaptic apparatus, whereas expression of rapsyn-GFP failed to rescue the α-syn(-/-) synaptic phenotype. These results demonstrate that α-syn is required for the maturation and stability of the postsynaptic apparatus and suggest that α-syn may act via α-dbn1.

Neuregulin/ErbB regulate neuromuscular junction development by phosphorylation of α-dystrobrevin

Neuregulin/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin (NRG)/ErbB signaling is involved in numerous developmental processes in the nervous system, including synapse formation and function in the central nervous system. Although intensively investigated, its role at the neuromuscular synapse has remained elusive. Here, we demonstrate that loss of neuromuscular NRG/ErbB signaling destabilized anchoring of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane and that this effect was caused by dephosphorylation of α-dystrobrevin1, a component of the postsynaptic scaffold. Specifically, in mice in which NRG signaling to muscle was genetically or pharmacologically abolished, postsynaptic AChRs moved rapidly from the synaptic to the perisynaptic membrane, and the subsynaptic scaffold that anchors the AChRs was impaired. These defects combined compromised synaptic transmission. We further show that blockade of NRG/ErbB signaling abolished tyrosine phosphorylation of α-dystrobrevin1, which reduced the stability of receptors in agrin-induced AChR clusters in cultured myotubes. Our data indicate that NRG/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Photounbinding of calmodulin from a family of CaM binding peptides

Background

Recent studies have shown that fluorescently labeled antibodies can be dissociated from their antigen by illumination with laser light. The mechanism responsible for the photounbinding effect, however, remains elusive. Here, we give important insights into the mechanism of photounbinding and show that the effect is not restricted to antibody/antigen binding.

Methodology/Principal Findings

We present studies of the photounbinding of labeled calmodulin (CaM) from a set of CaM-binding peptides with different affinities to CaM after one- and two-photon excitation. We found that the photounbinding effect becomes stronger with increasing binding affinity. Our observation that photounbinding can be influenced by using free radical scavengers, that it does not occur with either unlabeled protein or non-fluorescent quencher dyes, and that it becomes evident shortly after or with photobleaching suggest that photounbinding and photobleaching are closely linked.

Conclusions/Significance

The experimental results exclude surface effects, or heating by laser irradiation as potential causes of photounbinding. Our data suggest that free radicals formed through photobleaching may cause a conformational change of the CaM which lowers their binding affinity with the peptide or its respective binding partner.

Calcium/calmodulin kinase II-dependent acetylcholine receptor cycling at the mammalian neuromuscular junction in vivo

At the mammalian skeletal neuromuscular junction, cycling of nicotinic ACh receptors (nAChRs) is critical for the maintenance of a high postsynaptic receptor density. However, the mechanisms that regulate nAChRs recycling in living animals remain unknown. Using in vivo time-lapse imaging, fluorescence recovery after photobleaching, and biochemical pull down assays, we demonstrated that recycling of internalized nAChRs into fully functional and denervated synapses was promoted by both direct muscle stimulation and pharmacologically induced intracellular calcium elevations. Most of internalized nAChRs are recycled directly into synaptic sites. Chelating of intracellular calcium below resting level drastically decreased cycling of nAChRs. Furthermore we found that calcium-dependent AChR recycling is mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Inhibition of CaMKII selectively blocked recycling and caused intracellular accumulation of internalized nAChRs, whereas internalization of surface receptors remained unaffected. Electroporation of CaMKII-GFP isoforms into the sternomastoid muscle showed that muscle-specific CaMKIIβm isoform is highly expressed at the neuromuscular junction (NMJ) and precisely colocalized with nAChRs at crests of synaptic folds while the CaMKIIγ and δ isoforms are poorly expressed in synaptic sites. These results indicate that Ca(2+) along with CaMKII activity are critical for receptor recycling and may provide a mechanism by which the postsynaptic AChR density is maintained at the NMJ in vivo.

Lateral mobility of nicotinic acetylcholine receptors on neurons is determined by receptor composition, local domain, and cell type

The lateral mobility of surface receptors can define the signaling properties of a synapse and rapidly change synaptic function. Here we use single-particle tracking with Quantum Dots to follow nicotinic acetylcholine receptors (nAChRs) on the surface of chick ciliary ganglion neurons in culture. We find that both heteropentameric alpha3-containing receptors (alpha3*-nAChRs) and homopentameric alpha7-containing receptors (alpha7-nAChRs) access synaptic domains by lateral diffusion. They have comparable mobilities and display Brownian motion in extrasynaptic space but are constrained and move more slowly in synaptic space. The two receptor types differ in the nature of their synaptic restraints. Disruption of lipid rafts, PDZ-containing scaffolds, and actin filaments each increase the mobility of alpha7-nAChRs in synaptic space while collapse of microtubules has no effect. The opposite is seen for alpha3*-nAChRs where synaptic mobility is increased only by microtubule collapse and not the other manipulations. Other differences are found for regulation of alpha3*-nAChR and alpha7-nAChR mobilities in extrasynaptic space. Most striking are effects on the immobile populations of alpha7-nAChRs and alpha3*-nAChRs. Disruption of either lipid rafts or PDZ scaffolds renders half of the immobile alpha3*-nAChRs mobile without changing the proportion of immobile alpha7-nAChRs. Similar results were obtained with chick sympathetic ganglion neurons, though regulation of receptor mobility differed in at least one respect from that seen with ciliary ganglion neurons. Control of nAChR lateral mobility, therefore, is determined by mechanisms that are domain specific, receptor subtype dependent, and cell-type constrained. The outcome is a system that could tailor nicotinic signaling capabilities to specific needs of individual locations.

TGF-beta2 alters the characteristics of the neuromuscular junction by regulating presynaptic quantal size

The amount of neurotransmitter released from a presynaptic terminal is the product of the quantal content (number of vesicles) and the presynaptic quantal size (QSpre, amount of transmitter per vesicle). QSpre varies with synaptic use, but its regulation is poorly understood. The motor nerve terminals at the neuromuscular junction (NMJ) contain TGF-beta receptors. We present evidence that TGF-beta2 regulates QSpre at the NMJ. Application of TGF-beta2 to the rat diaphragm NMJ increased the postsynaptic response to both spontaneous and evoked release of acetylcholine, whereas antibodies to TGF-beta2 or its receptor had the converse effect. L-vesamicol and bafilomycin blocked the actions of TGF-beta2, indicating that TGF-beta2 acts by altering the extent of vesicular filling. Recordings of the postsynaptic currents from the diaphragm were consistent with TGF-beta2 having this presynaptic action and a lesser postsynaptic effect on input resistance. TGF-beta2 also decreased quantal content by an atropine-sensitive pathway, indicating that this change is secondary to cholinergic feedback on vesicular release. Consequently, the net actions of TGF-beta2 at the NMJ were to amplify the postsynaptic effects of spontaneous transmission and to diminish the number of vesicles used per evoked stimulus, without diminishing the amount of acetylcholine released.

Dystrophins, utrophins, and associated scaffolding complexes: role in mammalian brain and implications for therapeutic strategies

Two decades of molecular, cellular, and functional studies considerably increased our understanding of dystrophins function and unveiled the complex etiology of the cognitive deficits in Duchenne muscular dystrophy (DMD), which involves altered expression of several dystrophin-gene products in brain. Dystrophins are normally part of critical cytoskeleton-associated membrane-bound molecular scaffolds involved in the clustering of receptors, ion channels, and signaling proteins that contribute to synapse physiology and blood-brain barrier function. The utrophin gene also drives brain expression of several paralogs proteins, which cellular expression and biological roles remain to be elucidated. Here we review the structural and functional properties of dystrophins and utrophins in brain, the consequences of dystrophins loss-of-function as revealed by numerous studies in mouse models of DMD, and we discuss future challenges and putative therapeutic strategies that may compensate for the cognitive impairment in DMD based on experimental manipulation of dystrophins and/or utrophins brain expression.

Searching for signaling balance through the identification of genetic interactors of the Rab guanine-nucleotide dissociation inhibitor gdi-1

Background

The symptoms of numerous diseases result from genetic mutations that disrupt the homeostasis maintained by the appropriate integration of signaling gene activities. The relationships between signaling genes suggest avenues through which homeostasis can be restored and disease symptoms subsequently reduced. Specifically, disease symptoms caused by loss-of-function mutations in a particular gene may be reduced by concomitant perturbations in genes with antagonistic activities.

Methodology/Principal Findings

Here we use network-neighborhood analyses to predict genetic interactions in Caenorhabditis elegans towards mapping antagonisms and synergisms between genes in an animal model. Most of the predicted interactions are novel, and the experimental validation establishes that our approach provides a gain in accuracy compared to previous efforts. In particular, we identified genetic interactors of gdi-1, the orthologue of GDI1, a gene associated with mental retardation in human. Interestingly, some gdi-1 interactors have human orthologues with known neurological functions, and upon validation of the interactions in mammalian systems, these orthologues would be potential therapeutic targets for GDI1-associated neurological disorders. We also observed the conservation of a gdi-1 interaction between different cellular systems in C. elegans, suggesting the involvement of GDI1 in human muscle degeneration.

Conclusions/Significance

We developed a novel predictor of genetic interactions that may have the ability to significantly streamline the identification of therapeutic targets for monogenic disorders involving genes conserved between human and C. elegans.

Dynamics of the rapsyn scaffolding protein at the neuromuscular junction of live mice

The efficacy of synaptic transmission depends on the maintenance of a high density of neurotransmitter receptors and their associated scaffold proteins in the postsynaptic membrane. While the dynamics of receptors has been extensively studied, the dynamics of the intracellular scaffold proteins that make up the postsynaptic density are largely unknown in vivo. Here, we focused on the dynamics of rapsyn, a protein required for the clustering and maintenance of acetylcholine receptor (AChR) density at postsynaptic sites. Using time-lapse imaging, we demonstrated that rapsyn is remarkably dynamic compared to AChRs at functional synapses, turning over 4-6 times more rapidly than AChRs. In addition we found that the rapid turnover of rapsyn is insensitive to alterations in synaptic activity, whereas AChR turnover is profoundly affected, illustrating that rapsyn and receptor dynamics are controlled by distinct mechanisms. These data indicate that individual postsynaptic components are in permanent exchange despite the overall stability of synaptic structure, which may play a role in synaptic plasticity.

The roles of the dystrophin-associated glycoprotein complex at the synapse

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene and is characterized by progressive muscle wasting. A number of Duchenne patients also present with mental retardation. The dystrophin protein is part of the highly conserved dystrophin-associated glycoprotein complex (DGC) which accumulates at the neuromuscular junction (NMJ) and at a variety of synapses in the peripheral and central nervous systems. Many years of research into the roles of the DGC in muscle have revealed its structural function in stabilizing the sarcolemma. In addition, the DGC also acts as a scaffold for various signaling pathways. Here, we discuss recent advances in understanding DGC roles in the nervous system, gained from studies in both vertebrate and invertebrate model systems. From these studies, it has become clear that the DGC is important for the maturation of neurotransmitter receptor complexes and for the regulation of neurotransmitter release at the NMJ and central synapses. Furthermore, roles for the DGC have been established in consolidation of long-term spatial and recognition memory. The challenges ahead include the integration of the behavioral and mechanistic studies and the use of this information to identify therapeutic targets.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

Formation of complex AChR aggregates in vitro requires alpha-dystrobrevin

Efficient function at the neuromuscular junction requires high-density aggregates of acetylcholine receptors (AChRs) to be precisely aligned with the motor nerve terminal. A collaborative effort between the motor neuron and muscle intrinsic factors drives the formation and maintenance of these AChR aggregates. alpha-Dystrobrevin (alpha DB), a cytoplasmic protein found at the postsynaptic membrane, has been implicated in the regulation of AChR aggregate density and patterning. To investigate the contribution of alpha DB to the muscle intrinsic program regulating AChR aggregate development, we analyzed the formation of complex, pretzel-like AChR aggregates on primary muscle cell cultures derived from alpha DB knockout (alpha DB-KO) mice in the absence of nerve or agrin. In myotubes lacking alpha DB, complex AChR aggregates failed to form, whereas aggregates formed readily in wildtype myotubes. Five major isoforms of alpha DB are expressed in skeletal muscle: alpha DB1, alpha DB1(-), alpha DB2, alpha DB2(-), and alpha DB3. Expression of alpha DB1 or alpha DB1(-) in alpha DB-KO myotubes restored formation of complex AChR aggregates similar to those in wildtype myotubes. In contrast, individual expression of alpha DB2, alpha DB2(-), alpha DB3, or an alpha DB1 phosphorylation mutant resulted in the formation of few, if any, complex AChR aggregates. Collectively, these data suggest that alpha DB is a significant component of the muscle intrinsic program that mediates the formation of complex AChR aggregates and that alpha DB's tyrosine phosphorylation sites are of particular functional importance to this program. Although the muscle intrinsic program appears to influence synaptogenesis, the formation of complex mature AChR aggregates in alpha DB-KO mice (with the motor neuron present) suggests the motor neuron, not the muscle intrinsic program, is the major stimulus driving the maturation of AChRs from plaque to pretzel in vivo.

Postsynaptic scaffolds for nicotinic receptors on neurons

Complex postsynaptic scaffolds determine the structure and signaling capabilities of glutamatergic synapses. Recent studies indicate that some of the same scaffold components contribute to the formation and function of nicotinic synapses on neurons. PDZ-containing proteins comprising the PSD-95 family co-localize with nicotinic acetylcholine receptors (nAChRs) and mediate downstream signaling in the neurons. The PDZ-proteins also promote functional nicotinic innervation of the neurons, as does the scaffold protein APC and transmembrane proteins such as neuroligin and the EphB2 receptor. In addition, specific chaperones have been shown to facilitate nAChR assembly and transport to the cell surface. This review summarizes recent results in these areas and raises questions for the future about the mechanism and synaptic role of nAChR trafficking.

[Structural and molecular organization, development and maturation of the neuromuscular junction]

The neuromuscular junction is made up of the apposition of highly differentiated domains of three types of cell: the motor neuronal ending, the terminal Schwann cell and the muscle postsynaptic membrane. These three components are surrounded by a basal lamina, dedicated to molecular signal exchanges controlling neuromuscular formation, maturation and maintenance. This functional and structural differentiated complex conducts synaptic neurotransmission to the skeletal muscle fiber. Nerve and muscle have distinct roles in synaptic compartment differentiation. The initial steps of this differentiation and the motor endplate formation require several postsynaptic molecular agents including agrin, the tyrosine kinase receptor MuSK. Neuregulin is essentially involved in Schwann cell survival and guidance for axonal growth.

Mutations in contactin-1, a neural adhesion and neuromuscular junction protein, cause a familial form of lethal congenital myopathy

We have previously reported a group of patients with congenital onset weakness associated with a deficiency of members of the syntrophin-alpha-dystrobrevin subcomplex and have demonstrated that loss of syntrophin and dystrobrevin from the sarcolemma of skeletal muscle can also be associated with denervation. Here, we have further studied four individuals from a consanguineous Egyptian family with a lethal congenital myopathy inherited in an autosomal-recessive fashion and characterized by a secondary loss of beta2-syntrophin and alpha-dystrobrevin from the muscle sarcolemma, central nervous system involvement, and fetal akinesia. We performed homozygosity mapping and candidate gene analysis and identified a mutation that segregates with disease within CNTN1, the gene encoding for the neural immunoglobulin family adhesion molecule, contactin-1. Contactin-1 transcripts were markedly decreased on gene-expression arrays of muscle from affected family members compared to controls. We demonstrate that contactin-1 is expressed at the neuromuscular junction (NMJ) in mice and man in addition to the previously documented expression in the central and peripheral nervous system. In patients with secondary dystroglycanopathies, we show that contactin-1 is abnormally localized to the sarcolemma instead of exclusively at the NMJ. The cntn1 null mouse presents with ataxia, progressive muscle weakness, and postnatal lethality, similar to the affected members in this family. We propose that loss of contactin-1 from the NMJ impairs communication or adhesion between nerve and muscle resulting in the severe myopathic phenotype. This disorder is part of the continuum in the clinical spectrum of congenital myopathies and congenital myasthenic syndromes.

Receptor-associated proteins and synaptic plasticity

Changes in synaptic strength are important for synaptic development and synaptic plasticity. Most directly responsible for these synaptic changes are alterations in synaptic receptor number and density. Although alterations in receptor density mediated by the insertion, lateral mobility, removal, and recycling of receptors have been extensively studied, the dynamics and regulators of intracellular scaffolding proteins have only recently begun to be illuminated. In particular, a closer look at the receptor-associated proteins, which bind to receptors and are necessary for their synaptic localization and clustering, has revealed broader functions than previously thought and some rather unexpected thematic similarities. More than just "placeholders" or members of a passive protein "scaffold," receptor-associated proteins in every synapse studied have been shown to provide a number of signaling roles. In addition, the most recent state-of-the-art imaging has revealed that receptor-associated proteins are highly dynamic and are involved in regulating synaptic receptor density. Together, these results challenge the view that receptor-associated proteins are members of a static and stable scaffold and argue that their dynamic mobility may be essential for regulating activity-dependent changes in synaptic strength.

Destabilization of the dystrophin-glycoprotein complex without functional deficits in alpha-dystrobrevin null muscle

α-Dystrobrevin is a component of the dystrophin-glycoprotein complex (DGC) and is thought to have both structural and signaling roles in skeletal muscle. Mice deficient for α-dystrobrevin (adbn^−/−) exhibit extensive myofiber degeneration and neuromuscular junction abnormalities. However, the biochemical stability of the DGC and the functional performance of adbn^−/− muscle have not been characterized. Here we show that the biochemical association between dystrophin and β-dystroglycan is compromised in adbn^−/− skeletal muscle, suggesting that α-dystrobrevin plays a structural role in stabilizing the DGC. However, despite muscle cell death and DGC destabilization, costamere organization and physiological performance is normal in adbn^−/− skeletal muscle. Our results demonstrate that myofiber degeneration alone does not cause functional deficits and suggests that more complex pathological factors contribute to the development of muscle weakness in muscular dystrophy.

Rapid and modifiable neurotransmitter receptor dynamics at a neuronal synapse in vivo

Synaptic plasticity underlies the adaptability of the mammalian brain, but has been difficult to study in living animals. Here we imaged the synapses between pre- and postganglionic neurons in the mouse submandibular ganglion in vivo, focusing on the mechanisms that maintain and regulate neurotransmitter receptor density at postsynaptic sites. Normally, synaptic receptor densities were maintained by rapid exchange of receptors with nonsynaptic regions (over minutes) and by continual turnover of cell surface receptors (over hours). However, after ganglion cell axons were crushed, synaptic receptors showed greater lateral mobility and there was a precipitous decline in insertion. These changes led to near-complete loss of synaptic receptors and synaptic depression. Disappearance of postsynaptic spines and presynaptic terminals followed this acute synaptic depression. Therefore, neurotransmitter receptor dynamism associated with rapid changes in synaptic efficacy precedes long-lasting structural changes in synaptic connectivity.

Localized acetylcholine receptor clustering dynamics in response to microfluidic focal stimulation with agrin

Agrin is a proteoglycan secreted by the motor neuron's growing axon terminal upon contact with the muscle during embryonic development. It was long thought that agrin's role was to trigger the clustering of acetylcholine receptors (AChRs) to nascent synapse sites. However, agrin-predating, protosynaptic AChR clusters are present well before innervation in the embryo and in myotube cultures, yet no role has been conclusively ascribed to agrin. We used a microfluidic device to focally deliver agrin to protosynaptic AChR clusters in micropatterned myotube cultures. The distribution of AChRs labeled with fluorescent bungarotoxin was imaged at various time points over >24 h. We find that a 4-h focal application of agrin (100 nM) preferentially reduces AChR loss at agrin-exposed clusters by 17% relative to the agrin-deprived clusters on the same myotube. In addition, the focal application increases the addition of AChRs preferentially at the clusters by 10% relative to the agrin-exposed, noncluster areas. Taken together, these findings suggest that a focal agrin stimulus can play a key stabilizing role in the aggregation of AChRs at the early stages of synapse formation. This methodology is generally applicable to various developmental processes and cell types, including neurons and stem cells.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.