The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

Modulation of agrin binding and activity by the CT and related carbohydrate antigens

Agrin is a nerve-derived signal that is essential for the proper organization of postsynaptic acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction. It is likely that carbohydrates play a significant role in regulating agrin activity, as agrin binds multiple glycan structures and is itself a highly glycosylated protein. Here we provide support for this contention by showing that agrin can be modified with the CT antigen, a carbohydrate structure expressed at the neuromuscular junction, and by describing the resulting changes in agrin binding to neoglycoconjugates and cultured myotubes, as well as changes in agrin-dependent AChR clustering. Glycosylation of agrin with the CT antigen required the mucin domain and the dystroglycan/heparin-binding domain. The presence of the mucin domain lowered agrin binding to several N-acetyllactosaminyl-containing saccharides and C2 myotubes and lowered agrin activity in AChR clustering. Glycosylation of agrin with the CT antigen, by contrast, increased agrin binding to myotubes and potentiated its AChR clustering activity at subsaturating concentrations. Last, sialylated and nonsialylated variants of N-acetyllactosamine differentially modulated AChR clustering and agrin activity, and these changes correlated with the ability of MuSK, an agrin-stimulated kinase, to bind to these structures. These experiments demonstrate that the glycosylation state of agrin affects its activity and suggest a role for the CT antigen in modulating agrin function.

Overexpression of the CT GalNAc transferase in skeletal muscle alters myofiber growth, neuromuscular structure, and laminin expression

Carbohydrates have been shown to mediate or modulate a number of important events in the development of the nervous system; however, there is little evidence that they participate directly in the development of synapses. One carbohydrate structure that is likely to be important in synaptic development of the neuromuscular junction is the CT carbohydrate antigen [GalNAcbeta1,4[NeuAcalpha2,3]Galbeta1(-3GalNAc or -4GlcNAc)]. The synaptic localization of the CT antigen is due to the presence of the terminal beta1,4 GalNAc linkage, and such linkages are localized to the neuromuscular junction in many species. Here we show that an enzyme that can create the synaptic CT structure, the CT GalNAc transferase, is also confined to the neuromuscular junction in mice. Using transgenic mice, we show that overexpression of the CT GalNAc transferase in extrasynaptic regions in skeletal myofibers caused as much as a 60% reduction in the diameter of adult myofibers and an order of magnitude increase in satellite cells. Neuromuscular junctions of transgenic mice had severely reduced numbers of secondary folds, Schwann cell processes were present in the synaptic cleft, and secondary folds were often misaligned with active zones. In addition, multiple presynaptic specializations occurred on individual myofibers. In addition, some normally synaptic proteins, including laminin alpha4, laminin alpha5, utrophin, and NCAM, were expressed along extrasynaptic regions of myofibers. One of the muscle proteins that displayed increased glycosylation with the CT antigen in the transgenic mice was alpha-dystroglycan. These experiments provide the first in vivo evidence that a synaptic carbohydrate antigen has important roles in the development of the neuromuscular synapse and suggest that the CT antigen is involved in controlling the expression of synaptic molecules.

Roles, regulation, and mechanism of polysialic acid function during neural development

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) appeared during the evolution of vertebrates as a new mechanism for regulation of cell interactions. This large and abundant glycoprotein can exert steric effects at the cell surface that lead to the attenuation of cell-cell bonds mediated not only by NCAM but also a variety of other adhesion receptors. PSA-NCAM expression changes both as a result of developmental programs and physiological inputs. This global modulation of cell-cell attachment has been shown to facilitate cell migration, axon pathfinding and targeting, and plastic changes in the embryonic and adult nervous system.