| Abstract |
Myelinated fibers are organized into specialized domains that ensure the rapid propagation of action potentials and are characterized by specific protein complexes underlying essential interactions between glial and axonal membranes. These domains are the myelin-free node of Ranvier, the paranodes and the juxtaparanodes. TAG-1 (Transient Axonal Glycoprotein-1), a cell adhesion molecule of the immunoglobulin superfamily, is expressed by neurons as well as by myelinating glia and is concentrated at the juxtaparanodes of adult central and peripheral myelinated fibers. It is essential for the clustering of potassium channels in the juxtaparanodes that are important for the repolarization of the membrane after action potential propagation. More specifically, it forms a tripartite complex with Caspr2 and the potassium channels, whereas in its absence in Tag-1-/- mice, juxtaparanodes of the optic and the sciatic nerves are disrupted, showing diffusion of potassium channels. Morphologically, optic nerves from Tag-1-/- animals display hypomyelination as well as a severe loss of small calibre axons.
In the first part of this study we focused on the behavioral analysis of Tag-1-/- mice, examining their cognitive and motor abilities. We show that the absence of TAG-1 results in learning and memory deficits as well as motor coordination defets. We continued with a thorough analysis of the molecular organization of myelinated fibers in various areas of the central nervous system, including those implicated in the behaviors affected in Tag-1-/- mice. We demonstrate that TAG-1 absence results in the disruption of the juxtaparanodal domain of all central myelinated fibers and further causes shortening of the internodes. These molecular defects may underly the behavioral phenotype present in homozygous mutants for TAG-1.
In the second part of this study, we aimed to clarify the role of glial TAG-1 in the molecular organization of juxtaparanodes, the morphology of optic nerve fibers as well as the behaviour in mice. This analysis is important since TAG-1 is the only known component of the juxtaparanodal complex expressed by the glial cell. In our attempt to rescue the phenotype of the Tag-1 deficient mice, we generated new transgenic mice (Tag-1-/-;plpTg(rTag-1)), that exclusively express TAG-1 in oligodendrocytes and lack endogenous gene expression. Immunohistochemical analysis clearly demonstrates that glial TAG-1 is sufficient for the proper organization 11
and maintenance of the juxtaparanodal domain in the CNS. Biochemical analysis showed that glial TAG-1 physically interacts with Caspr2 and the potassium channels. Finally, subsequent ultrastructural analysis of the optic nerve as well as a thorough behavioral study of the transgenic mice showed that the expression of glial TAG-1 is sufficient to restore the axonal and myelin deficits as well as the behavioral defects observed in Tag-1-/- animals. Together, these data highlight the pivotal role of myelinating glia on axonal domain differentiation and organization.
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Μελέτη του ρόλου του μορίου κυτταρικής συνάφειας TAG-1 στην οργάνωση και λειτουργία του ενήλικου κεντρικού νευρικού συστήματος τρωκτικών
