New Dogs in the Dogma: Lrp4 and Tid1 in Neuromuscular Synapse Formation

Two recent papers reported identification of a long-sought agrin coreceptor, Lrp4 (Kim et al. in Cell and Zhang et al. in Neuron). In this issue of Neuron, Linnoila et al. report the identification of a new player in the agrin-MuSK pathway, Tid1, which directly interacts with MuSK and is responsible for transducing signals from MuSK activation to AChR clustering, culminating in cross-linking to the subsynaptic cytoskeleton. These papers substantially reshape the agrin-MuSK-ACh hypothesis of neuromuscular synaptogenesis

Mechanisms of Synapse formation and Maintenance: Insights From the Developing and Diseased Nervous System

ABSTRACT MECHANISMS OF SYNAPSE FORMATION AND MAINTENANCE: INSIGHTS FROM THE DEVELOPING AND DISEASED NERVOUS SYSTEM Ethan G. Hughes Rita J. Balice-Gordon, Ph.D. The formation and maintenance of synapses is essential for the central nervous system (CNS) to function. In the developing nervous system, the assembly of synaptic circuits is a complex and dynamic process, requiring the coordinated exchange of signals between pre- and postsynaptic neurons and surrounding glia. The maintenance and modulation of synaptic connections is required for normal CNS function and ongoing plasticity. The structural and functional integrity of synaptic connections is often modified or lost in the diseased nervous system, resulting in profound cognitive and behavioral deficits. While some aspects of the mechanisms underlying the formation, maintenance and plasticity of CNS synapses in the developing and diseased nervous system have been elucidated, many more remain to be understood. In my thesis work, I have examined the role of astrocytes in the development of GABAergic hippocampal synapses in in vitro models. I have also examined the maintenance of glutamatergic synapses in in vitro and in vivo models of anti-NMDAR encephalitis, an immune-mediated disorder of memory and behavior. First, I demonstrate that secreted factors released from astrocytes specifically increase GABAergic axon length, branching, and synaptogenesis, that these effects are not mediated by several well-known candidates, and that the secreted factors from astrocytes are proteins. Second, I examined the identity of the proteins released from astrocytes that affect GABAergic neurons using size fractionation, mass spectroscopy, and computational analyses. Third, I examined the cellular and synaptic mechanisms underlying anti-NMDAR encephalitis and investigated the effects of autoantibodies from patients with this disorder on the maintenance and function of CNS excitatory synapses. Together, my work extends our understanding of how neuron-glial communication modulates the formation of synapses in the developing brain, and how the disruption of synapse maintenance may underlie cognitive deficits in the diseased nervous system

Presynaptic to postsynaptic relationships of the neuromuscular junction are held constant across age and muscle fiber type

The neuromuscular junction (NMJ) displays considerable morphological plasticity as a result of differences in activity level, as well as aging. This is true of both presynaptic and postsynaptic components of the NMJ. Yet, despite these variations in NMJ structure, proper presynaptic to postsynaptic coupling must be maintained in order for effective cell-to-cell communication to occur. Here, we examined the NMJs of muscles with different activity profiles (soleus and EDL), on both slow- and fast-twitch fibers in those muscles, and among young adult and aged animals. We used immunofluorescent techniques to stain nerve terminal branching, presynaptic vesicles, postsynaptic receptors, as well as fast/slow myosin heavy chain. Confocal microscopy was used to capture images of NMJs for later quantitative analysis. Data were subjected to a two-way ANOVA (main effects for myofiber type and age), and in the event of a significant (p \u3c 0.05) F ratio, a post hoc analysis was performed to identify pairwise differences. Results showed that the NMJs of different myofiber types routinely displayed differences in presynaptic and postsynaptic morphology (although the effect on NMJ size was reversed in the soleus and the EDL), but presynaptic to postsynaptic relationships were tightly maintained. Moreover, the ratio of presynaptic vesicles relative to nerve terminal branch length also was similar despite differences in muscles, their fiber type, and age. Thus, in the face of considerable overall structural differences of the NMJ, presynaptic to postsynaptic coupling remains constant, as does the relationship between presynaptic vesicles and the nerve terminal branches that support them. (c) 2013 Wiley Periodicals, Inc

Serial Search Based Code Acquisition in the Cooperative MIMO Aided DS-CDMA Downlink

Full text of this paper is not available in UHRAFocal blockade of postsynaptic acetylcholine receptors (AChRs) in a small region of the neuromuscular junction may cause long-term synapse elimination at that site. Blockade of the whole junction does not cause synapse loss, indicating that it is the contrast in postsynaptic activity between the blocked and unblocked regions which causes withdrawal of the synaptic terminals. This phenomenon can be explained by the dual role of calcium, both in controlling AChR gene transcription and influencing AChR aggregation. A computational model is provided and the stability of the solutions is confirmed by theoretical analysis and computer simulation

Cellular and Molecular Anatomy of the Human Neuromuscular Junction

The neuromuscular junction (NMJ) plays a fundamental role in transferring information from lower motor neuron to skeletal muscle to generate movement. It is also an experimentally accessible model synapse routinely studied in animal models to explore fundamental aspects of synaptic form and function. Here, we combined morphological techniques, super-resolution imaging, and proteomic profiling to reveal the detailed cellular and molecular architecture of the human NMJ. Human NMJs were significantly smaller, less complex, and more fragmented than mouse NMJs. In contrast to mice, human NMJs were also remarkably stable across the entire adult lifespan, showing no signs of age-related degeneration or remodeling. Super-resolution imaging and proteomic profiling revealed distinctive distribution of active zone proteins and differential expression of core synaptic proteins and molecular pathways at the human NMJ. Taken together, these findings reveal human-specific cellular and molecular features of the NMJ that distinguish them from comparable synapses in other mammalian species

Mechanisms Underlying Metabolic and Neural Defects in Zebrafish and Human Multiple Acyl-CoA Dehydrogenase Deficiency (MADD)

In humans, mutations in electron transfer flavoprotein (ETF) or electron transfer flavoprotein dehydrogenase (ETFDH) lead to MADD/glutaric aciduria type II, an autosomal recessively inherited disorder characterized by a broad spectrum of devastating neurological, systemic and metabolic symptoms. We show that a zebrafish mutant in ETFDH, xavier, and fibroblast cells from MADD patients demonstrate similar mitochondrial and metabolic abnormalities, including reduced oxidative phosphorylation, increased aerobic glycolysis, and upregulation of the PPARG-ERK pathway. This metabolic dysfunction is associated with aberrant neural proliferation in xav, in addition to other neural phenotypes and paralysis. Strikingly, a PPARG antagonist attenuates aberrant neural proliferation and alleviates paralysis in xav, while PPARG agonists increase neural proliferation in wild type embryos. These results show that mitochondrial dysfunction, leading to an increase in aerobic glycolysis, affects neurogenesis through the PPARG-ERK pathway, a potential target for therapeutic intervention

Role of Myosin Va in the Plasticity of the Vertebrate Neuromuscular Junction In Vivo

Background: Myosin Va is a motor protein involved in vesicular transport and its absence leads to movement disorders in humans (Griscelli and Elejalde syndromes) and rodents (e.g. dilute lethal phenotype in mice). We examined the role of myosin Va in the postsynaptic plasticity of the vertebrate neuromuscular junction (NMJ). Methodology/Principal Findings: Dilute lethal mice showed a good correlation between the propensity for seizures, and fragmentation and size reduction of NMJs. In an aneural C2C12 myoblast cell culture, expression of a dominant-negative fragment of myosin Va led to the accumulation of punctate structures containing the NMJ marker protein, rapsyn-GFP, in perinuclear clusters. In mouse hindlimb muscle, endogenous myosin Va co-precipitated with surface-exposed or internalised acetylcholine receptors and was markedly enriched in close proximity to the NMJ upon immunofluorescence. In vivo microscopy of exogenous full length myosin Va as well as a cargo-binding fragment of myosin Va showed localisation to the NMJ in wildtype mouse muscles. Furthermore, local interference with myosin Va function in live wildtype mouse muscles led to fragmentation and size reduction of NMJs, exclusion of rapsyn-GFP from NMJs, reduced persistence of acetylcholine receptors in NMJs and an increased amount of punctate structures bearing internalised NMJ proteins. Conclusions/Significance: In summary, our data show a crucial role of myosin Va for the plasticity of live vertebrate neuromuscular junctions and suggest its involvement in the recycling of internalised acetylcholine receptors back to th