Acetylcholine receptors direct rapsyn clusters to the neuromuscular synapse in zebrafish

A zebrafish gene with sequence similarities to human uromodulin and GP2 displays extensive evolutionary diversification among teleost and confers resistance to bacterial infection

In the process of investigating synaptic changes happening to mutants lacking postsynaptic receptors in the neuromuscular junction, we focused on a hitherto uncharacterized zebrafish gene zgc153932 whose expression was increased in the RNAseq and droplet digital PCR (ddPCR) analysis of a paralyzed mutant sofa potato. The zgc153932 gene which we named omcin5 (omc5) showed amino acid sequence similarity to human uromodulin and GP2, which are expressed in epithelial cells of the kidney and the gut respectively and bind to bacteria pili. omc5 had 14 paralogues in a ∼400 KB region on the chromosome 12 of the zebrafish genome. These genes, named omcin1 through 15, constitute a gene cluster which presumably arose from recent gene duplications in the zebrafish lineage. An antibody raised against the epitope common to 6-9 genes in the omcin family revealed expression in the cloaca of 1 day post fertilization (dpf) embryos which broadened to the urinary and digestive tracts by 5 dpf. Expression of omc5 was increased by exposure of embryos to Escherichia coli (E. coli). Survival of omc5 mutant embryos was shortened in the presence of E. coli, or when they were not maintained in germ-free conditions. Adults omc5 mutants also exhibited susceptibility to infection. Other teleost species which had omcin-like genes in their genomes showed a range of gene duplication, resulting in clusters of 1 to >15 omcin-like genes. We hereby identified a new gene family specific to teleost that include a microbial induced gene which confers resistance to bacterial infection.

Allosteric inhibition of muscle-type nicotinic acetylcholine receptors by a neuromuscular blocking agent pancuronium

Muscle relaxants are indispensable for surgical anesthesia. Early studies suggested that a classical non-depolarizing muscle relaxant pancuronium competitively binds to the ligand binding site to block nicotinic acetylcholine receptors (nAChR). Our group recently showed that nAChR which has two distinct subunit combinations are expressed in zebrafish muscles, αβδε and αβδ, for which potencies of pancuronium are different. Taking advantage of the distinct potencies, we generated chimeras between two types of nAChRs and found that the extracellular ACh binding site is not associated with the pancuronium sensitivity. Furthermore, application of either 2 μM or 100 μM ACh in native αβδε or αβδ subunits yielded similar IC50 of pancuronium. These data suggest that pancuronium allosterically inhibits the activity of zebrafish nAChRs.

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.

Building neuromuscular junctions in vitro

The neuromuscular junction (NMJ) has been the model of choice to understand the principles of communication at chemical synapses. Following groundbreaking experiments carried out over 60 years ago, many studies have focused on the molecular mechanisms underlying the development and physiology of these synapses. This Review summarizes the progress made to date towards obtaining faithful models of NMJs in vitro We provide a historical approach discussing initial experiments investigating NMJ development and function from Xenopus to mice, the creation of chimeric co-cultures, in vivo approaches and co-culture methods from ex vivo and in vitro derived cells, as well as the most recent developments to generate human NMJs. We discuss the benefits of these techniques and the challenges to be addressed in the future for promoting our understanding of development and human disease.

AChRs Are Essential for the Targeting of Rapsyn to the Postsynaptic Membrane of NMJs in Living Mice

Rapsyn, a 43 kDa scaffold protein, is required for the clustering of acetylcholine receptors (AChRs) at synaptic sites between mammalian motor neurons and muscle cells. However, the mechanism by which rapsyn is inserted and retained at postsynaptic sites at the neuromuscular junction (NMJ) in vivo remains largely unknown. We found that neither the N-terminal myristoylation nor the cysteine-rich RING H2 domain of rapsyn is required for its stable association with the postsynaptic membrane of NMJs. When N-myristoylation-defective rapsyn-EGFP mutant (G2A) and RING-H2 domain truncated rapsyn-EGFP were electroporated into sternomastoid muscles, a strong rapsyn fluorescent signal was observed selectively at synapses, similar to WT rapsyn-EGFP. The targeting of rapsyn-EGFP (WT and mutants) is independent of synaptic activity because they were inserted at denervated NMJs. However, when the coiled-coil domain (the AChR-binding domain of rapsyn) is deleted, rapsyn fails to associate with AChRs at NMJs of living mice. In cultured myoblasts (in which AChRs are absent), myristoylated WT rapsyn mostly localizes to lysosomes and is not associated with the plasma membrane. However, in the presence of AChR subunits, rapsyn molecules were targeted to the cell surface and formed aggregates with AChRs. The targeting of AChRs to the cell membrane, in contrast, does not require rapsyn because expressed AChRs are visible on the cell membranes of rapsyn-deficient myoblasts. These results provide evidence for an active role of AChRs in the targeting of rapsyn to the NMJ in vivo

SIGNIFICANCE STATEMENT

Rapsyn is required for the clustering of acetylcholine receptors (AChRs) at postsynaptic sites. However, the mechanism by which rapsyn is targeted to synaptic sites at the vertebrate neuromuscular junction remains unclear. In this study, we showed that the coiled-coil domain of rapsyn is required for its targeting to the cell surface via its interaction with AChRs. In contrast, the targeting of AChRs to the cell membrane does not require rapsyn. These results indicate that AChRs play a critical role in the insertion and/or association of rapsyn with the plasma membrane of synaptic sites.

Expressing acetylcholine receptors after innervation suppresses spontaneous vesicle release and causes muscle fatigue

The formation and function of synapses are tightly orchestrated by the precise timing of expression of specific molecules during development. In this study, we determined how manipulating the timing of expression of postsynaptic acetylcholine receptors (AChRs) impacts presynaptic release by establishing a genetically engineered zebrafish line in which we can freely control the timing of AChR expression in an AChR-less fish background. With the delayed induction of AChR expression after an extensive period of AChR-less development, paralyzed fish displayed a remarkable level of recovery, exhibiting a robust escape response following developmental delay. Despite their apparent behavioral rescue, synapse formation in these fish was significantly altered as a result of delayed AChR expression. Motor neuron innervation determined the sites for AChR clustering, a complete reversal of normal neuromuscular junction (NMJ) development where AChR clustering precedes innervation. Most importantly, among the three modes of presynaptic vesicle release, only the spontaneous release machinery was strongly suppressed in these fish, while evoked vesicle release remained relatively unaffected. Such a specific presynaptic change, which may constitute a part of the compensatory mechanism in response to the absence of postsynaptic AChRs, may underlie symptoms of neuromuscular diseases characterized by reduced AChRs, such as myasthenia gravis.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

The knockdown of αkap alters the postsynaptic apparatus of neuromuscular junctions in living mice

A muscle-specific nonkinase anchoring protein (αkap), encoded within the calcium/calmodulin kinase II (camk2) α gene, was recently found to control the stability of acetylcholine receptor (AChR) clusters on the surface of cultured myotubes. However, it remains unknown whether this protein has any effect on receptor stability and the maintenance of the structural integrity of neuromuscular synapses in vivo. By knocking down the endogenous expression of αkap in mouse sternomastoid muscles with shRNA, we found that the postsynaptic receptor density was dramatically reduced, the turnover rate of receptors at synaptic sites was significantly increased, and the insertion rates of both newly synthesized and recycled receptors into the postsynaptic membrane were depressed. Moreover, we found that αkap shRNA knockdown impaired synaptic structure as postsynaptic AChR clusters and their associated postsynaptic scaffold proteins within the neuromuscular junction were completely eliminated. These results provide new mechanistic insight into the role of αkap in regulating the stability of the postsynaptic apparatus of neuromuscular synapses.

Rem uncouples excitation-contraction coupling in adult skeletal muscle fibers

The RGK protein Rem uncouples the voltage sensors of Ca_V1.1 from RYR1-mediated sarcoplasmic reticulum Ca²⁺ release via its ability to interact with the auxiliary β_1a subunit.

In skeletal muscle, excitation–contraction (EC) coupling requires depolarization-induced conformational rearrangements in L-type Ca²⁺ channel (Ca_V1.1) to be communicated to the type 1 ryanodine-sensitive Ca²⁺ release channel (RYR1) of the sarcoplasmic reticulum (SR) via transient protein–protein interactions. Although the molecular mechanism that underlies conformational coupling between Ca_V1.1 and RYR1 has been investigated intensely for more than 25 years, the question of whether such signaling occurs via a direct interaction between the principal, voltage-sensing α_1S subunit of Ca_V1.1 and RYR1 or through an intermediary protein persists. A substantial body of evidence supports the idea that the auxiliary β_1a subunit of Ca_V1.1 is a conduit for this intermolecular communication. However, a direct role for β_1a has been difficult to test because β_1a serves two other functions that are prerequisite for conformational coupling between Ca_V1.1 and RYR1. Specifically, β_1a promotes efficient membrane expression of Ca_V1.1 and facilitates the tetradic ultrastructural arrangement of Ca_V1.1 channels within plasma membrane–SR junctions. In this paper, we demonstrate that overexpression of the RGK protein Rem, an established β subunit–interacting protein, in adult mouse flexor digitorum brevis fibers markedly reduces voltage-induced myoplasmic Ca²⁺ transients without greatly affecting Ca_V1.1 targeting, intramembrane gating charge movement, or releasable SR Ca²⁺ store content. In contrast, a β_1a-binding–deficient Rem triple mutant (R200A/L227A/H229A) has little effect on myoplasmic Ca²⁺ release in response to membrane depolarization. Thus, Rem effectively uncouples the voltage sensors of Ca_V1.1 from RYR1-mediated SR Ca²⁺ release via its ability to interact with β_1a. Our findings reveal Rem-expressing adult muscle as an experimental system that may prove useful in the definition of the precise role of the β_1a subunit in skeletal-type EC coupling.

Zebrafish mutants of the neuromuscular junction: swimming in the gene pool

This review provides an overview of zebrafish mutants with dysfunctional acetylcholine receptors or related proteins at the neuromuscular junction (NMJ). The NMJ, which has served as the classical model of the chemical synapse, uses acetylcholine as the neurotransmitter, and mutations of proteins involved in the signaling cascade lead to a variety of behavioral phenotypes. Mutants isolated after random chemical mutagenesis screening are summarized, and advances in the field resulting from these mutants are discussed.

A single mutation in the acetylcholine receptor δ-subunit causes distinct effects in two types of neuromuscular synapses

Mutations in AChR subunits, expressed as pentamers in neuromuscular junctions (NMJs), cause various types of congenital myasthenic syndromes. In AChR pentamers, the adult ε subunit gradually replaces the embryonic γ subunit as the animal develops. Because of this switch in subunit composition, mutations in specific subunits result in synaptic phenotypes that change with developmental age. However, a mutation in any AChR subunit is considered to affect the NMJs of all muscle fibers equally. Here, we report a zebrafish mutant of the AChR δ subunit that exhibits two distinct NMJ phenotypes specific to two muscle fiber types: slow or fast. Homozygous fish harboring a point mutation in the δ subunit form functional AChRs in slow muscles, whereas receptors in fast muscles are nonfunctional. To test the hypothesis that different subunit compositions in slow and fast muscles underlie distinct phenotypes, we examined the presence of ε/γ subunits in NMJs using specific antibodies. Both wild-type and mutant larvae lacked ε/γ subunits in slow muscle synapses. These findings in zebrafish suggest that some mutations in human congenital myasthenic syndromes may affect slow and fast muscle fibers differently.

eIF4EBP3L acts as a gatekeeper of TORC1 in activity-dependent muscle growth by specifically regulating Mef2ca translational initiation

Muscle activity promotes muscle growth through the TOR-4EBP pathway by controlling the translation of specific mRNAs, including Mef2ca, a muscle transcription factor required for normal growth.

Muscle fiber size is activity-dependent and clinically important in ageing, bed-rest, and cachexia, where muscle weakening leads to disability, prolonged recovery times, and increased costs. Inactivity causes muscle wasting by triggering protein degradation and may simultaneously prevent protein synthesis. During development, muscle tissue grows by several mechanisms, including hypertrophy of existing fibers. As in other tissues, the TOR pathway plays a key role in promoting muscle protein synthesis by inhibition of eIF4EBPs (eukaryotic Initiation Factor 4E Binding Proteins), regulators of the translational initiation. Here, we tested the role of TOR-eIF4EBP in a novel zebrafish muscle inactivity model. Inactivity triggered up-regulation of eIF4EBP3L (a zebrafish homolog of eIF4EBP3) and diminished myosin and actin content, myofibrilogenesis, and fiber growth. The changes were accompanied by preferential reduction of the muscle transcription factor Mef2c, relative to Myod and Vinculin. Polysomal fractionation showed that Mef2c decrease was due to reduced translation of mef2ca mRNA. Loss of Mef2ca function reduced normal muscle growth and diminished the reduction in growth caused by inactivity. We identify eIF4EBP3L as a key regulator of Mef2c translation and protein level following inactivity; blocking eIF4EBP3L function increased Mef2ca translation. Such blockade also prevented the decline in mef2ca translation and level of Mef2c and slow myosin heavy chain proteins caused by inactivity. Conversely, overexpression of active eIF4EBP3L mimicked inactivity by decreasing the proportion of mef2ca mRNA in polysomes, the levels of Mef2c and slow myosin heavy chain, and myofibril content. Inhibiting the TOR pathway without the increase in eIF4EBP3L had a lesser effect on myofibrilogenesis and muscle size. These findings identify eIF4EBP3L as a key TOR-dependent regulator of muscle fiber size in response to activity. We suggest that by selectively inhibiting translational initiation of mef2ca and other mRNAs, eIF4EBP3L reprograms the translational profile of muscle, enabling it to adjust to new environmental conditions.

Most genes are transcribed into mRNA and then translated into proteins that function in various cellular processes. Initiation of mRNA translation is thus a fundamental control point in gene expression. Working in a zebrafish model, we have found that muscle activity (or inactivity) can differentially regulate the translation of specific mRNAs and thereby control the growth of skeletal muscle. Emerging evidence suggests that control of translational initiation of particular mRNAs by an intracellular signaling pathway acting through TORC1 is a major regulator of cell growth and function. We show here that muscle activity both activates the TORC1 pathway and suppresses the expression of a downstream TORC1 target—the translational inhibitor eIF4EBP3L. This removes a brake on translation of certain mRNAs. Conversely, we show that muscle inactivity can up-regulate this translational inhibitor, thereby causing reduced translation of these mRNAs. One of the mRNAs targeted in this manner by eIF4EBP3L is Mef2ca, which encodes a transcription factor that promotes assembly of muscle contractile apparatus. Our work thus reveals a mechanism by which muscle growth can be differentially influenced depending on the context of muscle activity (or lack thereof). If this pathway operates in people, it may help explain how exercise regulates muscle growth and performance.

Zebrafish muscular disease models towards drug discovery

BACKGROUND

Zebrafish is an amenable vertebrate model useful for the study of development and genetics. Small molecule screenings in zebrafish have successfully identified several drugs that affect developmental process.

OBJECTIVE

This review covers the basics of zebrafish muscle system such as muscle development and muscle defects. It also reviews the potential use of zebrafish for chemical screening with regards to muscle disorders.

CONCLUSION

During embryogenesis, zebrafish start to coil their body by contracting trunk muscles 17 h postfertilization, indicating that a motor circuit and skeletal muscle are functionally developed at early stages. Mutagenesis screens in zebrafish have identified many motility mutants that display morphological or functional defects in the CNS, clustering defects of acetylcholine receptors at the neuromuscular junctions or pathological defects of muscles. Most of the muscular mutants are useful as animal models of human muscle disease such as muscle dystrophy. As zebrafish live in water, pharmacological drugs are easily assayable during development, and thus zebrafish may be used to determine novel drugs that mitigate muscle disease.

Synuclein accumulation is associated with cell-specific neuronal death after spinal cord injury

Spinal cord injury axotomizes neurons and induces many of them to die, whereas others survive. Therefore, it is important to identify factors that lead to neuronal death after injury as a first step toward developing better strategies for increasing neuronal survival and functional recovery. However, the intrinsic molecular pathways that govern whether an injured neuron lives or dies remain surprisingly unclear. To address this question, we took advantage of the large size of giant reticulospinal (RS) neurons in the brain of the lamprey, Petromyzon marinus. We report that axotomy of giant RS neurons induces a select subset of them to accumulate high levels of synuclein, a synaptic vesicle-associated protein whose abnormal accumulation is linked to Parkinson's disease. Injury-induced synuclein accumulation occurred only in neurons that were classified as "poor survivors" by both histological and Fluoro-Jade C staining. In contrast, post-injury synuclein immunofluorescence remained at control levels in neurons that were identified as "good survivors." Synuclein accumulation appeared in the form of aggregated intracellular inclusions. Cells that accumulated synuclein also exhibited more ubiquitin-containing inclusions, similar to what occurs during disease states. When synuclein levels and cell vitality were measured in the same neurons, it became clear that synuclein accumulation preceded and strongly correlated with subsequent neuronal death. Thus, synuclein accumulation is identified as a marker and potential risk factor for forthcoming neuronal death after axotomy, expanding its implications beyond the neurodegenerative diseases.

Acetylcholine receptors enable the transport of rapsyn from the Golgi complex to the plasma membrane

The accumulation of acetylcholine receptors (AChRs) at nerve terminals is critical for signal transmission at the neuromuscular junction, and rapsyn is essential for this process. Previous studies suggest that AChRs might direct rapsyn self-clusters to the synapse. In vivo experiments with fluorescently tagged AChR or rapsyn in zebrafish larvae revealed that rapsyn self-clusters separate from AChRs did not exist before synapse formation. Examination of rapsyn in the AChR-less mutant sofa potato revealed that rapsyn in the absence of AChR was localized in the Golgi complex. Expression of muscle-type AChR in sofa potato restored synaptic clustering of rapsyn, while neuronal type AChR had no effect. To determine whether this requirement of protein interaction is reciprocal, we examined the mutant twitch once, which has a missense mutation in rapsyn. While the AChRs distributed nonsynaptically on the plasma membrane in twitch once, mutant rapsyn was retained in the Golgi complex. We conclude that AChRs enable the transport of rapsyn from the Golgi complex to the plasma membrane through a molecule-specific interaction.

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.

Localization of Cadm2a and Cadm3 proteins during development of the zebrafish nervous system

Members of the Cadm/SynCAM/Necl/IGSF/TSLC family of cell adhesion molecules are known to have diverse functions during development of the nervous system, but information regarding their role during central nervous system (CNS) development in vivo is scarce. The rapid development of a relatively simple nervous system in larval zebrafish makes them a highly tractable model organism for studying gene function during nervous system development. An essential prerequisite for functional studies is a description of protein localization. To address this we have generated subtype-specific antibodies to two members of the zebrafish cell adhesion molecule family: cadm2a and cadm3. Using these novel antibodies we show that cadm3 and cadm2a are expressed throughout the nervous system of larval stage zebrafish. Particularly striking, and largely nonoverlapping expression of cadm2a and cadm3 is observed in the developing retina and spinal cord. Using in vitro binding assays we show that cadm2a and cadm3 bind heterophilically and preferentially to cadm1 and cadm4, respectively. These binding preferences are very similar to those seen for tetrapod Cadms but our study of protein localization suggests novel and diverse functions of cadms during nervous system development.

An acetylcholine receptor lacking both γ and ε subunits mediates transmission in zebrafish slow muscle synapses

Fast and slow skeletal muscle types in larval zebrafish can be distinguished by a fivefold difference in the time course of their synaptic decay. Single-channel recordings indicate that this difference is conferred through kinetically distinct nicotinic acetylcholine receptor (AChR) isoforms. The underlying basis for this distinction was explored by cloning zebrafish muscle AChR subunit cDNAs and expressing them in Xenopus laevis oocytes. Measurements of single-channel conductance and mean open burst duration assigned α₂βδε to fast muscle synaptic current. Contrary to expectations, receptors composed of only αβδ subunits (presumed to be α₂βδ₂ receptors) recapitulated the kinetics and conductance of slow muscle single-channel currents. Additional evidence in support of γ/ε-less receptors as mediators of slow muscle synapses was reflected in the inward current rectification of heterologously expressed α₂βδ₂ receptors, a property normally associated with neuronal-type nicotinic receptors. Similar rectification was reflected in both single-channel and synaptic currents in slow muscle, distinguishing them from fast muscle. The final evidence for α₂βδ₂ receptors in slow muscle was provided by our ability to convert fast muscle synaptic currents to those of slow muscle by knocking down ε subunit expression in vivo. Thus, for the first time, muscle synaptic function can be ascribed to a receptor isoform that is composed of only three different subunits. The unique functional features offered by the α₂βδ₂ receptor likely play a central role in mediating the persistent contractions characteristic to this muscle type.

Na(v)1.6a is required for normal activation of motor circuits normally excited by tactile stimulation

A screen for zebrafish motor mutants identified two noncomplementing alleles of a recessive mutation that were named non-active (nav(mi89) and nav(mi130)). nav embryos displayed diminished spontaneous and touch-evoked escape behaviors during the first 3 days of development. Genetic mapping identified the gene encoding Na(V)1.6a (scn8aa) as a potential candidate for nav. Subsequent cloning of scn8aa from the two alleles of nav uncovered two missense mutations in Na(V)1.6a that eliminated channel activity when assayed heterologously. Furthermore, the injection of RNA encoding wild-type scn8aa rescued the nav mutant phenotype indicating that scn8aa was the causative gene of nav. In-vivo electrophysiological analysis of the touch-evoked escape circuit indicated that voltage-dependent inward current was decreased in mechanosensory neurons in mutants, but they were able to fire action potentials. Furthermore, tactile stimulation of mutants activated some neurons downstream of mechanosensory neurons but failed to activate the swim locomotor circuit in accord with the behavioral response of initial escape contractions but no swimming. Thus, mutant mechanosensory neurons appeared to respond to tactile stimulation but failed to initiate swimming. Interestingly fictive swimming could be initiated pharmacologically suggesting that a swim circuit was present in mutants. These results suggested that Na(V)1.6a was required for touch-induced activation of the swim locomotor network.

Paired patch clamp recordings from motor-neuron and target skeletal muscle in zebrafish

Larval zebrafish represent the first vertebrate model system to allow simultaneous patch clamp recording from a spinal motor-neuron and target muscle. This is a direct consequence of the accessibility to both cell types and ability to visually distinguish the single segmental CaP motor-neuron on the basis of morphology and location. This video demonstrates the microscopic methods used to identify a CaP motor-neuron and target muscle cells as well as the methodologies for recording from each cell type. Identification of the CaP motor-neuron type is confirmed by either dye filling or by the biophysical features such as action potential waveform and cell input resistance. Motor-neuron recordings routinely last for one hour permitting long-term recordings from multiple different target muscle cells. Control over the motor-neuron firing pattern enables measurements of the frequency-dependence of synaptic transmission at the neuromuscular junction. Owing to a large quantal size and the low noise provided by whole cell voltage clamp, all of the unitary events can be resolved in muscle. This feature permits study of basic synaptic properties such as release properties, vesicle recycling, as well as synaptic depression and facilitation. The advantages offered by this in vivo preparation eclipse previous neuromuscular model systems studied wherein the motor-neurons are usually stimulated by extracellular electrodes and the muscles are too large for whole cell patch clamp. The zebrafish preparation is amenable to combining electrophysiological analysis with a wide range of approaches including transgenic lines, morpholino knockdown, pharmacological intervention and in vivo imaging. These approaches, coupled with the growing number of neuromuscular disease models provided by mutant lines of zebrafish, open the door for new understanding of human neuromuscular disorders.

Regenerated synapses in lamprey spinal cord are sparse and small even after functional recovery from injury

Despite the potential importance that synapse regeneration plays in restoring neuronal function after spinal cord injury (SCI), even the most basic questions about the morphology of regenerated synapses remain unanswered. Therefore, we set out to gain a better understanding of central synapse regeneration by examining the number, distribution, molecular composition, and ultrastructure of regenerated synapses under conditions in which behavioral recovery from SCI was robust. To do so, we used the giant reticulospinal (RS) neurons of lamprey spinal cord because they readily regenerate, are easily identifiable, and contain large synapses that serve as a classic model for vertebrate excitatory neurotransmission. Using a combination of light and electron microscopy, we found that regenerated giant RS synapses regained the basic structures and presynaptic organization observed at control giant RS synapses at a time when behavioral recovery was nearly complete. However, several obvious differences remained. Most strikingly, regenerated giant RS axons produced very few synapses. In addition, presynaptic sites within regenerated axons were less complex, had fewer vesicles, and had smaller active zones than normal. In contrast, the densities of presynapses and docked vesicles were nearly restored to control values. Thus, robust functional recovery from SCI can occur even when the structures of regenerated synapses are sparse and small, suggesting that functional recovery is due to a more complex set of compensatory changes throughout the spinal network.

Acetylcholine negatively regulates development of the neuromuscular junction through distinct cellular mechanisms

Emerging evidence suggests that the neurotransmitter acetylcholine (ACh) negatively regulates the development of the neuromuscular junction, but it is not clear if ACh exerts its effects exclusively through muscle ACh receptors (AChRs). Here, we used genetic methods to remove AChRs selectively from muscle. Similar to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs increased motor axon branching and expanded innervation territory, suggesting that ACh negatively regulates synaptic growth through postsynaptic AChRs. However, in contrast to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs in agrin-deficient mice failed to restore deficits in pre- and postsynaptic differentiation, suggesting that ACh negatively regulates synaptic differentiation through nonpostsynaptic receptors. Consistent with this idea, the ACh agonist carbachol inhibited presynaptic specialization of motorneurons in vitro. Together, these data suggest that ACh negatively regulates axon growth and presynaptic specialization at the neuromuscular junction through distinct cellular mechanisms.

Defective glycinergic synaptic transmission in zebrafish motility mutants

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem. Recently, in vivo analysis of glycinergic synaptic transmission has been pursued in zebrafish using molecular genetics. An ENU mutagenesis screen identified two behavioral mutants that are defective in glycinergic synaptic transmission. Zebrafish bandoneon (beo) mutants have a defect in glrbb, one of the duplicated glycine receptor (GlyR) beta subunit genes. These mutants exhibit a loss of glycinergic synaptic transmission due to a lack of synaptic aggregation of GlyRs. Due to the consequent loss of reciprocal inhibition of motor circuits between the two sides of the spinal cord, motor neurons activate simultaneously on both sides resulting in bilateral contraction of axial muscles of beo mutants, eliciting the so-called 'accordion' phenotype. Similar defects in GlyR subunit genes have been observed in several mammals and are the basis for human hyperekplexia/startle disease. By contrast, zebrafish shocked (sho) mutants have a defect in slc6a9, encoding GlyT1, a glycine transporter that is expressed by astroglial cells surrounding the glycinergic synapse in the hindbrain and spinal cord. GlyT1 mediates rapid uptake of glycine from the synaptic cleft, terminating synaptic transmission. In zebrafish sho mutants, there appears to be elevated extracellular glycine resulting in persistent inhibition of postsynaptic neurons and subsequent reduced motility, causing the 'twitch-once' phenotype. We review current knowledge regarding zebrafish 'accordion' and 'twitch-once' mutants, including beo and sho, and report the identification of a new alpha2 subunit that revises the phylogeny of zebrafish GlyRs.

Postsynaptic development of the neuromuscular junction in mice lacking the gamma-subunit of muscle nicotinic acetylcholine receptor

The mammalian muscle nicotinic acetylcholine receptor (AChR) is composed of five membrane-spanning subunits and its composition differs between embryonic and adult muscles. In embryonic muscles, it is composed of two alpha-, one beta-, one delta-, and one gamma-subunit; the gamma-subunit is later replaced by the epsilon-subunit during postnatal development. This unique temporal expression pattern of the gamma-subunit suggests it may play specific roles in embryonic muscles. To address this issue, we examined the formation and function of the neuromuscular junction in mouse embryos deficient in the gamma-subunit. At embryonic day 15.5, AChR clusters were absent and the spontaneous miniature endplate potentials were undetectable in the mutant muscles. However, electrical stimulation of the nerves triggered muscle contraction and elicited postsynaptic endplate potential (EPP) in the mutant muscles, although the magnitude of the muscle contraction and the amplitudes of the EPPs were smaller in the mutant compared to the wild-type muscles. Reintroducing a wild-type gamma-subunit into the mutant myotubes restored the formation of AChR clusters in vitro. Together, these results have demonstrated that functional AChRs were present in the mutant muscle membrane, but at reduced levels. Thus, in the absence of the gamma-subunit, a combination of alpha, beta, and delta subunits may assemble into functional receptors in vivo. These results also suggest that the gamma-subunit maybe involved in interacting with rapsyn, a cytoplasmic protein required for AChR clustering.

Secondary motoneurons in juvenile and adult zebrafish: axonal pathfinding errors caused by embryonic nicotine exposure

Nicotine is a drug of abuse that has been reported to have many adverse effects on the developing nervous system. We previously demonstrated that embryonic exposure to nicotine alters axonal pathfinding of spinal secondary motoneurons in zebrafish. We hypothesize that these changes will persist into adulthood. The Tg(isl1:GFP) line of zebrafish, which expresses green fluorescent protein (GFP) in a subtype of spinal secondary motoneurons, was used to investigate potential long-term consequences of nicotine exposure on motoneuron development. Anatomical characterization of Tg(isl1:GFP) zebrafish ranging between 3 and 30 days postfertilization (dpf) was initially performed in fixed tissue to characterize axonal trajectories in larval and juvenile fish. Tg(isl1:GFP) embryos were transiently exposed to 5–30 μM nicotine. They were then rescued from nicotine and raised into later stages of life (3–30 dpf) and fixed for microscopic examination. Morphological analysis revealed that nicotine-induced abnormalities in secondary motoneuron anatomy were still evident in juvenile fish. Live imaging of Tg(isl1:GFP) zebrafish using fluorescent stereomicroscopy revealed that the nicotine-induced changes in motoneuron axonal pathfinding persisted into adulthood. We detected abnormalities in 37-dpf fish that were transiently exposed to nicotine as embryos. These fish were subsequently imaged over a 7-week period of time until they were ≈3 months of age. These pathfinding errors of spinal secondary motoneuron axons detected at 37 dpf persisted within the same fish until 86 dpf, the latest age analyzed. These findings indicate that exposure to nicotine during embryonic development can have permanent consequences for motoneuron anatomy in zebrafish. J. Comp. Neurol. 512:305–322, 2009. © 2008 Wiley-Liss, Inc.

A modified acetylcholine receptor delta-subunit enables a null mutant to survive beyond sexual maturation

The contraction of skeletal muscle is dependent on synaptic transmission through acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). The lack of an AChR subunit causes a fetal akinesia in humans, leading to death in the first trimester and characteristic features of Fetal Akinesia Deformation Sequences (FADS). A corresponding null mutation of the delta-subunit in zebrafish (sofa potato; sop) leads to the death of embryos around 5 d postfertilization (dpf). In sop(-/-) mutants, we expressed modified delta-subunits, with one (delta1YFP) or two yellow fluorescent protein (delta2YFP) molecules fused at the intracellular loop, under the control of an alpha-actin promoter. AChRs containing these fusion proteins are fluorescent, assemble on the plasma membrane, make clusters under motor neuron endings, and generate synaptic current. We screened for germ-line transmission of the transgene and established a line of sop(-/-) fish stably expressing the delta2YFP. These delta2YFP/sop(-/-) embryos can mount escape behavior close to that of their wild-type siblings. Synaptic currents in these embryos had a smaller amplitude, slower rise time, and slower decay when compared with wild-type fish. Remarkably, these embryos grow to adulthood and display complex behaviors such as feeding and breeding. To the best of our knowledge, this is the first case of a mutant animal corresponding to first trimester lethality in human that has been rescued by a transgene and survived to adulthood. In the rescued fish, a foreign promoter drove the transgene expression and the NMJ had altered synaptic strength. The survival of the transgenic animal delineates requirements for gene therapies of NMJ.

Synaptic homeostasis in a zebrafish glial glycine transporter mutant

Truncated escape responses characteristic of the zebrafish shocked mutant result from a defective glial glycine transporter (GlyT1). In homozygous GlyT1 mutants, irrigating brain ventricles with glycine-free solution rescues normal swimming. Conversely, elevating brain glycine levels restores motility defects. These experiments are consistent with previous studies that demonstrate regulation of global glycine levels in the CNS as a primary function of GlyT1. As GlyT1 mutants mature, their ability to mount an escape response naturally recovers. To understand the basis of this recovery, we assay synaptic transmission in primary spinal motor neurons by measuring stimulus-evoked postsynaptic potentials. At the peak of the motility defect, inhibitory synaptic potentials are both significantly larger and more prolonged indicating a prominent role for GlyT1 in shaping fast synaptic transmission. However, as GlyT1 mutants naturally regain their ability to swim, the amplitude of inhibitory potentials decreases to below wild-type levels. In parallel with diminishing synaptic potentials, the glycine concentration required to evoke the mutant motility defect increases 61-fold during behavioral recovery. Behavioral recovery is also mirrored by a reduction in the levels of both glycine receptor protein and transcript. These results suggest that increased CNS glycine tolerance and reduced glycine receptor expression in GlyT1 mutants reflect compensatory mechanisms for functional recovery from excess nervous system inhibition.

Function of neuromuscular synapses in the zebrafish choline-acetyltransferase mutant bajan

We have identified a zebrafish mutant line, bajan, in which compromised motility and fatigue result from a point mutation in the gene coding choline acetyltransferase (ChAT), the enzyme responsible for acetylcholine (ACh) synthesis. Although the mutation predicts loss of ChAT function, bajan inexplicably retains low levels of neuromuscular transmission. We exploited this residual activity and determined the consequences for synaptic function. The attenuated synaptic responses were a direct consequence of a decrease in both resting mean quantal size and quantal content. To replicate behavioral fatigue in swimming, motorneurons were stimulated at high frequencies. A prominent reduction in quantal content, reflecting vesicle depletion, was coincident with a small additional reduction in quantal size. In humans, defective ChAT leads to episodic apnea, a form of congenital myasthenic syndrome characterized by use-dependent fatigue. In contrast to bajan, however, afflicted individuals exhibit a normal resting quantal size and quantal content. The fatigue in humans results from a pronounced long-lasting drop in quantal size with little or no change in quantal content. These differences have important implications for interpreting fatigue as well as on understanding the impact of ACh availability on vesicle filling and recycling.

Essential roles of the acetylcholine receptor gamma-subunit in neuromuscular synaptic patterning

Formation of the vertebrate neuromuscular junction (NMJ) takes place in a stereotypic pattern in which nerves terminate at select sarcolemmal sites often localized to the central region of the muscle fibers. Several lines of evidence indicate that the muscle fibers may initiate postsynaptic differentiation independent of the ingrowing nerves. For example, nascent acetylcholine receptors (AChRs) are pre-patterned at select regions of the muscle during the initial stage of neuromuscular synaptogenesis. It is not clear how these pre-patterned AChR clusters are assembled, and to what extent they contribute to pre- and post-synaptic differentiation during development. Here, we show that genetic deletion of the AChR gamma-subunit gene in mice leads to an absence of pre-patterned AChR clusters during initial stages of neuromuscular synaptogenesis. The absence of pre-patterned AChR clusters was associated with excessive nerve branching, increased motoneuron survival, as well as aberrant distribution of acetylcholinesterase (AChE) and rapsyn. However, clustering of muscle specific kinase (MuSK) proceeded normally in the gamma-null muscles. AChR clusters emerged at later stages owing to the expression of the AChR epsilon-subunit, but these delayed AChR clusters were broadly distributed and appeared at lower level compared with the wild-type muscles. Interestingly, despite the abnormal pattern, synaptic vesicle proteins were progressively accumulated at individual nerve terminals, and neuromuscular synapses were ultimately established in gamma-null muscles. These results demonstrate that the gamma-subunit is required for the formation of pre-patterned AChR clusters, which in turn play an essential role in determining the subsequent pattern of neuromuscular synaptogenesis.

CNS-derived glia ensheath peripheral nerves and mediate motor root development

Motor function requires that motor axons extend from the spinal cord at regular intervals and that they are myelinated by Schwann cells. Little attention has been given to another cellular structure, the perineurium, which ensheaths the motor nerve, forming a flexible, protective barrier. Consequently, the origin of perineurial cells and their roles in motor nerve formation are poorly understood. Using time-lapse imaging in zebrafish, we show that perineurial cells are born in the CNS, arising as ventral spinal-cord glia before migrating into the periphery. In embryos lacking perineurial glia, motor neurons inappropriately migrated outside of the spinal cord and had aberrant axonal projections, indicating that perineurial glia carry out barrier and guidance functions at motor axon exit points. Additionally, reciprocal signaling between perineurial glia and Schwann cells was necessary for motor nerve ensheathment by both cell types. These insights reveal a new class of CNS-born glia that critically contributes to motor nerve development.

Overexpression of rapsyn in rat muscle increases acetylcholine receptor levels in chronic experimental autoimmune myasthenia gravis

The primary autoantigen in myasthenia gravis, the acetylcholine receptor (AChR), is clustered and anchored in the postsynaptic membrane of the neuromuscular junction by rapsyn. Previously, we found that overexpression of rapsyn by cDNA transfection protects AChRs in rat muscles from antibody-mediated loss in passive transfer experimental autoimmune myasthenia gravis (EAMG). Here, we determined whether rapsyn overexpression can reduce or even reverse AChR loss in muscles that are already damaged by chronic EAMG, which mimics the human disease. Active immunization against purified AChR was performed in female Lewis rats. Rapsyn overexpression resulted in an increase in total muscle membrane AChR levels, with some AChR at neuromuscular junctions but much of it in extrasynaptic membrane regions. At the ultrastructural level, most endplates in rapsyn-treated chronic EAMG muscles showed increased damage to the postsynaptic membrane. Although rapsyn overexpression stabilized AChRs in intact or mildly damaged endplates, the rapsyn-induced increase of membrane AChR enhanced autoantibody binding and membrane damage in severe ongoing disease. Thus, these results show the complexity of synaptic stabilization of AChR during the autoantibody attack. They also indicate that the expression of receptor-associated proteins may determine the severity of autoimmune diseases caused by anti-receptor antibodies.

Mutation in the delta-subunit of the nAChR suppresses the muscle defects caused by lack of Dystrophin

Normal motility of the zebrafish embryo requires a large number of gene loci, many of which have human orthologues implicated in myasthenias and other myopathies. We have identified a mutation in the zebrafish that abolishes body motility. Embryos have narrower myofibrils and lack clusters of nicotinic acetylcholine receptors (nAChRs) on the surface of the somitic muscle. We mapped the mutation to the delta-subunit of the nAChR, showing this mutant to be a new allele of the previously named sofa potato (sop). The mutant allele carries a missense mutation in the extracellular domain altering the cysteine at position 150 to an arginine. The delta-subunit is expressed in all striated muscles in embryonic and early larval stages together with the alpha1, beta1, epsilon, and gamma-subunits of nAChR. In contrast to mammals that show switching from the gamma embryonic to the adult epsilon-subunit, the two subunits are coexpressed in zebrafish embryos. We, furthermore, demonstrated that the sop/delta-nAChR mutation is a suppressor of the myopathy caused by lack of Dystrophin. The myofiber detachment phenotype of Dystroglycan-deficient embryos was not suppressed, suggesting that Dystrophin and Dystroglycan play distinct roles in muscle formation and maintenance of muscle integrity.

Neuromuscular synaptogenesis in wild-type and mutant zebrafish

Genetic screens for synaptogenesis mutants have been performed in many organisms, but few if any have simultaneously screened for defects in pre- and postsynaptic specializations. Here, we report the results of a small-scale genetic screen, the first in vertebrates, for defects in synaptogenesis. Using zebrafish as a model system, we identified seven mutants that affect different aspects of neuromuscular synapse formation. Many of these mutant phenotypes have not been previously reported in zebrafish and are distinct from those described in other organisms. Characterization of mutant and wild-type zebrafish, from the time that motor axons first arrive at target muscles through adulthood, has provided the new information about the cellular events that occur during neuromuscular synaptogenesis. These include insights into the formation and dispersal of prepatterned AChR clusters, the relationship between motor axon elongation and synapse size, and the development of precise appositions between presynaptic clusters of synaptic vesicles in nerve terminals and postsynaptic receptor clusters. In addition, we show that the mechanisms underlying synapse formation within the myotomal muscle itself are largely independent of those that underlie synapse formation at myotendinous junctions and that the outgrowth of secondary motor axons requires at least one cue not necessary for the outgrowth of primary motor axons, while other cues are required for both. One-third of the mutants identified in this screen did not have impaired motility, suggesting that many genes involved in neuromuscular synaptogenesis were missed in large scale motility-based screens. Identification of the underlying genetic defects in these mutants will extend our understanding of the cellular and molecular mechanisms that underlie the formation and function of neuromuscular and other synapses.

Rhythmic motor activity evoked by NMDA in the spinal zebrafish larva

We have examined the localization and activity of the neural circuitry that generates swimming behavior in developing zebrafish that were spinalized to isolate the spinal cord from descending brain inputs. We found that addition of the excitatory amino acid agonist N-methyl-d-aspartate (NMDA) to spinalized zebrafish at 3 days in development induced repeating episodes of rhythmic tail beating activity reminiscent of slow swimming behavior. The neural correlate of this activity, monitored by extracellular recording comprised repeating episodes of rhythmic, rostrocaudally progressing peripheral nerve discharges that alternated between the two sides of the body. Motoneuron recordings revealed an activity pattern comprising a slow oscillatory and a fast synaptic component that was consistent with fictive swimming behavior. Pharmacological and voltage-clamp analysis implicated glycine and glutamate in generation of motoneuron activity. Contralateral alternation of motor activity was disrupted with strychnine, indicating a role for glycine in coordinating left-right alternation during NMDA-induced locomotion. At embryonic stages, while rhythmic synaptic activity patterns could still be evoked in motoneurons, they were typically lower in frequency. Kinematic recordings revealed that prior to 3 days in development, NMDA was unable to reliably generate rhythmic tail beating behavior. We conclude that NMDA induces episodes of rhythmic motor activity in spinalized developing zebrafish that can be monitored physiologically in paralyzed preparations. Therefore as for other vertebrates, the zebrafish central pattern generator is intrinsic to the spinal cord and can operate in isolation provided a tonic source of excitation is given.

Muscular contractions in the zebrafish embryo are necessary to reveal thiuram-induced notochord distortions

Dithiocarbamates form a large group of chemicals that have numerous uses in agriculture and medicine. It has been reported that dithiocarbamates, including thiuram (tetramethylthiuram disulfide), cause wavy distortions of the notochord in zebrafish and other fish embryos. In the present study, we investigated the mechanism underlying the toxicity of thiuram in zebrafish embryos. When embryos were exposed to thiuram (2-1000 nM: 0.48-240 microg/L) from 3 h post fertilization (hpf) (30% epiboly) until 24 hpf (Prim-5), all embryos develop wavy notochords, disorganized somites, and have shortened yolk sac extensions. The thiuram response was specific and did not cause growth retardation or mortality at 24 hpf. The thiuram-dependent responses showed the same concentration dependence with a waterborne EC50 values of approximately 7 nM. Morphometric measurements revealed that thiuram does not affect the rate of notochord lengthening. However, the rate of overall body lengthening was significantly reduced in thiuram-exposed animals. Other dithiocarbamates, such as ziram, caused similar malformations to thiuram. While expression of genes involved in somitogenesis was not affected, the levels of notochord-specific transcripts were altered after the onset of malformations. Distortion of the notochord started precisely at 18 hpf, which is concomitant with onset of spontaneous rhythmic trunk contractions. Abolishment of spontaneous contractions using tricaine, alpha-bungarotoxin, and a paralytic mutant sofa potato, resulted in normal notochord morphology in the presence of thiuram. These results indicate that muscle activity is necessary to reveal the underlying functional deficit and suggest that the developmental target of dithiocarbamates impairs trunk plasticity through an unknown mechanism.

A mutation in serca underlies motility dysfunction in accordion zebrafish

Zebrafish acquire the ability for fast swimming early in development. The motility mutant accordion (acc) undergoes exaggerated and prolonged contractions on both sides of the body, interfering with the acquisition of patterned swimming responses. Our whole cell recordings from muscle indicate that the defect is not manifested in neuromuscular transmission. However, imaging of skeletal muscle of larval acc reveals greatly prolonged calcium transients and associated contractions in response to depolarization. Positional cloning of acc identified a serca mutation as the cause of the acc phenotype. SERCA is a sarcoplasmic reticulum transmembrane protein in skeletal muscle that mediates calcium re-uptake from the myoplasm. The mutation in SERCA, a serine to phenylalanine substitution, is likely to result in compromised protein function that accounts for the observed phenotype. Indeed, direct evidence that mutant SERCA causes the motility dysfunction was provided by the finding that wild type fish injected with an antisense morpholino directed against serca, exhibited accordion-like contractions and impaired swimming. We conclude that the motility dysfunction in embryonic and larval accordion zebrafish stems directly from defective calcium transport in skeletal muscle rather than defective CNS drive.

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 transmembrane protein required for acetylcholine receptor clustering in Caenorhabditis elegans

Clustering neurotransmitter receptors at the synapse is crucial for efficient neurotransmission. Here we identify a Caenorhabditis elegans locus, lev-10, required for postsynaptic aggregation of ionotropic acetylcholine receptors (AChRs). lev-10 mutants were identified on the basis of weak resistance to the anthelminthic drug levamisole, a nematode-specific cholinergic agonist that activates AChRs present at neuromuscular junctions (NMJs) resulting in muscle hypercontraction and death at high concentrations. In lev-10 mutants, the density of levamisole-sensitive AChRs at NMJs is markedly reduced, yet the number of functional AChRs present at the muscle cell surface remains unchanged. LEV-10 is a transmembrane protein localized to cholinergic NMJs and required in body-wall muscles for AChR clustering. We also show that the LEV-10 extracellular region, containing five predicted CUB domains and one LDLa domain, is sufficient to rescue AChR aggregation in lev-10 mutants. This suggests a mechanism for AChR clustering that relies on extracellular protein-protein interactions. Such a mechanism is likely to be evolutionarily conserved because CUB/LDL transmembrane proteins similar to LEV-10, but lacking any assigned function, are expressed in the mammalian nervous system and might be used to cluster ionotropic receptors in vertebrates.

accordion, a zebrafish behavioral mutant, has a muscle relaxation defect due to a mutation in the ATPase Ca2+ pump SERCA1

When wild-type zebrafish embryos are touched at 24 hours post-fertilization (hpf), they typically perform two rapid alternating coils of the tail. By contrast, accordion (acc) mutants fail to coil their tails normally but contract the bilateral trunk muscles simultaneously to shorten the trunk, resulting in a pronounced dorsal bend. Electrophysiological recordings from muscles showed that the output from the central nervous system is normal in mutants, suggesting a defect in muscles is responsible. In fact, relaxation in acc muscle is significantly slower than normal. In vivo imaging of muscle Ca2+ transients revealed that cytosolic Ca2+ decay was significantly slower in acc muscle. Thus, it appears that the mutant behavior is caused by a muscle relaxation defect due to the impairment of Ca2+ re-uptake. Indeed, acc mutants carry a mutation in atp2a1 gene that encodes the sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1), a Ca2+ pump found in the muscle sarcoplasmic reticulum (SR) that is responsible for pumping Ca2+ from the cytosol back to the SR. As SERCA1 mutations in humans lead to Brody disease, an exercise-induced muscle relaxation disorder, zebrafish accordion mutants could be a useful animal model for this condition.