Neuromuscular junctions (NMJs) are specialized synapses that connect motor neurons to skeletal muscle fibers and orchestrate proper signal transmission from the nervous system to muscles. The efficient formation and maintenance of the postsynaptic machinery that contains acetylcholine receptors (AChR) are indispensable for proper NMJ function. Abnormalities in the organization of synaptic components often cause severe neuromuscular disorders, such as muscular dystrophy. The dystrophin-associated glycoprotein complex (DGC) was shown to play an important role in NMJ development. We recently identified liprin-α-1 as a novel binding partner for one of the cytoplasmic DGC components, α-dystrobrevin-1. In the present study, we performed a detailed analysis of localization and function of liprin-α-1 at the murine NMJ. We showed that liprin-α-1 localizes to both pre- and postsynaptic compartments at the NMJ, and its synaptic enrichment depends on the presence of the nerve. Using cultured muscle cells, we found that liprin-α-1 plays an important role in AChR clustering and the organization of cortical microtubules. Our studies provide novel insights into the function of liprin-α-1 at vertebrate neuromuscular synapses.

Motor neurons form specialized synapses with skeletal muscle fibers, called neuromuscular junctions (NMJs). Cultured myotubes are used as a simplified in vitro system to study the postsynaptic specialization of muscles. The stimulation of myotubes with the glycoprotein agrin or laminin-111 induces the clustering of postsynaptic machinery that contains acetylcholine receptors (AchRs). When myotubes are grown on laminin-coated surfaces, AchR clusters undergo developmental remodeling to form topologically complex structures that resemble mature nMJs. needing further exploration are the molecular processes that govern AchR cluster assembly and its developmental maturation. Here, we describe an improved protocol for culturing muscle cells to promote the formation of complex AchR clusters. We screened various laminin isoforms and showed that laminin-221 was the most potent for inducing AChR clusters, whereas laminin-121, laminin-211, and laminin-221 afforded the highest percentages of topologically complex assemblies. Human primary myotubes that were formed by myoblasts obtained from patient biopsies also assembled AchR clusters that underwent remodeling in vitro. collectively, these results demonstrate an advancement of culturing myotubes that can facilitate high-throughput screening for potential therapeutic targets for neuromuscular disorders. Vertebrate neuromuscular junctions (NMJs) are synapses between motor neurons and skeletal muscle fibers. The function of this type of synapse is to transmit signals from the central nervous system to muscles and thus stimulate their contraction. The nerve terminal releases the neurotransmitter acetylcholine (ACh), which binds to postsynaptic ACh receptors (AChRs) that are located on the surface of muscle fibers. For efficient synaptic transmission, muscle fibers need to accumulate a high density of AChRs in their postsynaptic membrane 1-4. Rodent muscles cluster components of postsynaptic machinery around day 12 of embryonic development (i.e., before motor neuron axons approach muscle fibers) 5,6. Innervation leads to the dispersion of preexisting AChR clusters 2,7. A single postsynaptic machinery is formed per muscle fiber directly below the nerve terminal. The nerve plays a crucial role in organizing NMJ postsynaptic machinery by secreting signaling molecules, such as agrin and ACh, which regulate the clustering and dispersion of AChRs, respectively 2,8-12. Nerve-derived agrin binds to the surface of low-density lipoprotein receptor-related protein (Lrp4), which activates muscle-specific kinase (MuSK) 13. This triggers an intracellular signaling cascade that activates AChR clustering by the scaffold protein rapsyn 2. In mice during the first postnatal weeks, NMJs grow in size and undergo developmental remodeling from simple plaque-shaped structures to topologically complex assemblies 2,14. During this process, postsynaptic machinery becomes perforated with scattered openings that, with time, become more numerous and fuse with each other to form indentations between AChR-rich...

The neuromuscular junctions (NMJs) connect muscle fibers with motor neurons and enable the coordinated contraction of skeletal muscles. The dystrophin-associated glycoprotein complex (DGC) is an essential component of the postsynaptic machinery of the NMJ and is important for the maintenance of NMJ structural integrity. To identify novel proteins that are important for NMJ organization, we performed a mass spectrometry-based screen for interactors of α-dystrobrevin 1 (aDB1), one of the components of the DGC. The guanidine nucleotide exchange factor (GEF) Arhgef5 was found to be one of the aDB1 binding partners that is recruited to Tyr-713 in a phospho-dependent manner. We show here that Arhgef5 localizes to the NMJ and that its genetic depletion in the muscle causes the fragmentation of the synapses in conditional knockout mice. Arhgef5 loss in vivo is associated with a reduction in the levels of active GTP-bound RhoA and Cdc42 GTPases, highlighting the importance of actin dynamics regulation for the maintenance of NMJ integrity.

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