In Search of a Solution to the Sphinx-Like Riddle of GM1 |
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Authors: | Robert W Ledeen Gusheng Wu |
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Institution: | (1) Department of Neurology and Neurosciences, New Jersey Medical School, UMDNJ, 185 So. Orange Ave., MSB-H506, Newark, NJ 07103, USA |
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Abstract: | Among the many glycoconjugates contributing to the sugar code, gangliosides have drawn special attention owing to their predominance
as the major sialoglycoconjugate category within the nervous system. However, their occurrence, albeit at lower levels, appears
ubiquitous in vertebrate cells and even some invertebrate tissues. Now that over 100 gangliosides have been structurally characterized,
their diverse physiological functions constitute a remaining enigma. This has been especially true of GM1, for which a surprising
array of functions has already been revealed. Our current research has focused on two areas of GM1 function: (a) signaling
induced in neural and immune cells by cross-linking of GM1 in the plasma membrane that leads to activation of TRPC5 (transient
receptor potiential, canonical form 5) channels, a process important in neuritogenesis and autoimmune suppression; (b) activation
by GM1 of a sodium-calcium exchanger (NCX) in the inner membrane of the nuclear envelope (NE) with resulting modulation of
nuclear and cellular calcium. The latter has a role in maintaining neuronal viability, loss of which renders neurons vulnerable
to Ca2+ overload. Pathological manifestations in mutant mice and their cultured neurons lacking GM1 have shown dramatic rescue with
a membrane permeable derivative of GM1 that enters the nucleus and restores NCX activity. Nuclear function of GM1 is related
to the presence of neuraminidase in the NE, an enzyme that generates GM1 through hydrolysis of GD1a. A different isoform of
this enzyme was found in each of the two membranes of the NE. |
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