Congruity of acetylcholine receptor, acetylcholinesterase, and Dolichos biflorus lectin binding glycoprotein in postsynaptic-like sarcolemmal specializations in noninnervated regenerating rat muscles

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

Muscle activity-resistant acetylcholine receptor accumulation is induced in places of former motor endplates in ectopically innervated regenerating rat muscles

Expression of acetylcholine receptors (AChRs) in the extrajunctional muscle regions, but not in the neuromuscular junctions, is repressed by propagated electric activity in muscle fibers. During regeneration, subsynaptic-like specializations accumulating AChRs are induced in new myotubes by agrin attached to the synaptic basal lamina at the places of former motor endplates even in the absence of innervation. We examined whether AChRs still accumulated at these places when the regenerating muscles were ectopically innervated and the former synaptic places became extrajunctional. Rat soleus muscles were injured by bupivacaine and ischemia to produce complete myofiber degeneration. The soleus muscle nerve was permanently severed and the muscle was ectopically innervated by the peroneal nerve a few millimeters away from the former junctional region. After 4 weeks of regeneration, the muscles contracted upon nerve stimulation, showed little atrophy and the cross-section areas of their fibers were completely above the range in non-innervated regenerating muscles, indicating successful innervation. Subsynaptic-like specializations in the former junctional region still accumulated AChRs (and acetylcholinesterase) although no motor nerve endings were observed in their vicinity and the cross-section area of their fibers clearly demonstrated that they were ectopically innervated. We conclude that the expression of AChRs at the places of the former neuromuscular junctions in the ectopically innervated regenerated soleus muscles is activity-independent.

Acetylcholinesterase in the neuromuscular junction

New findings regarding acetylcholinesterase (AChE) in the neuromuscular junction (NMJ), obtained in the last decade, are briefly reviewed. AChE is highly concentrated in the NMJs of vertebrates. Its location remains stable after denervation in mature rat muscles but not in early postnatal muscles. Agrin in the synaptic basal lamina is able to induce sarcolemmal differentiations accumulating AChE even in the absence of a nerve ending. Asymmetric A12 AChE form is the major molecular form of AChE in vertebrate NMJs. Extrajunctional suppression of this form is a developmental phenomenon. Motor nerve is able to reinduce expression of the A12 AChE form in the ectopic NMJs even in muscles with complete extrajunctional suppression of this form. The 'tail' of the A12 AChE form is made of collagen Q. It contains domains for binding AChE to basal lamina with ionic and covalent interactions. Muscle activity is required for normal AChE expression in muscles and its accumulation in the NMJs. In addition, the pattern of muscle activation also regulates AChE activity in the NMJs, demonstrating that the pattern of synaptic transmission is able to modulate one of the key synaptic components.

Role for a synapse-specific carbohydrate in agrin-induced clustering of acetylcholine receptors

Lectins such as VVA-B4, which bind N-acetylgalactosaminyl (GalNAc)-terminated saccharides, selectively stain the neuromuscular junction, thus defining a synapse-specific carbohydrate. In seeking roles for this carbohydrate, we asked whether VVA-B4 affected the distribution of acetylcholine receptors (AChRs) on cultured muscle cells. We found that incubation of myotubes with VVA-B4 induced formation of AChR clusters and potentiated the effect of a nerve-derived clustering factor, agrin. Additional experiments implicated GalNAc-terminated glycoconjugates as modulators of agrin-induced AChR clustering. Enzymatic removal of GalNAc residues or treatment with a multivalent protein-GalNAc conjugate blocked agrin-induced clustering, whereas enzymatic unmasking of additional GalNAc residues induced clustering in the absence of added agrin. Moreover, incubation with agrin led to redistribution of VVA-B4-binding material on myotubes. Together, these results suggest that agrin-induced clustering of AChRs involves a GalNAc-dependent step.