Chemistry Reference
In-Depth Information
cesses. The nervous system contains the same general categories of glycoconju-
gates as other tissues, but with many unique features. These arise to a large extent
from unique structural elements, including excitable cells (neurons), required to
communicate rapidly, and supporting cells (glia) that produce specialized ele-
ments (myelin, astrocytic processes) to aid neuronal function. It is noteworthy that
some glycoconjugates, such as CSPGs, tenascin-C and -R, and the MAG protein
of myelin, that aid in development can prove inhibitory at later stages involving
damage repair and regeneration.
Summary Box
Glycolipids are prominent glycan carriers of the nervous system. They consist
primarily of gangliosides, the major sialoglycoconjugates of brain present in
all neural cells, and galactocerebrosides plus sulfatides, key components of
CNS and PNS myelin. Molecular mechanisms by which GM1 and other gan-
gliosides modulate neural protein activities involve direct protein- ganglioside
association and also cross-linking of GM1 leading to signaling and ion channel
activation. Glycoproteins in the form of recognition molecules have major roles
in the various phases of neural differentiation. These are characterized by the
presence of polysialic acid, HNK-1 and/or oligomannosidic glycans. The myelin
sheath, which insulates CNS axons and aids saltatory conduction, contains
glycoproteins such as MAG and OMgp which promote myelination but also
block axonal regeneration. These inhibitory mechanisms are known to involve
the GPI-linked NgR and ganglioside GT1b on the axonal membrane. Glycopro-
teins of PNS myelin are more abundant, but much less inhibitory than those
of the CNS. The ECM is rich in proteoglycans which have dual roles in both
promoting and blocking neuronal differentiation. CSPGs are especially promi-
nent in the glial scar and contribute to the failure of damaged CNS neurons to
regenerate. PNS neurons, lacking these inhibitory elements, are able to regen-
erate damaged axons. Glycoconjugates of neural stem cells are subjects of
interest and will likely have a prominent role in this rapidly growing fi eld.
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