Membrane oligo- and polysialic acids.

Polysialic acid (polySia) and oligosialic acid (oligoSia) chains are linear polysaccharides composed of sialic acid monomers. The majority of biological poly/oligoSia chains are bound to membranes. There is a large diversity of membrane poly/oligoSia in terms of chain length, occurrence, biological function, and the mode of membrane attachment. Poly/oligoSia can be anchored to a membrane via a phospholipid (polySia in bacteria), a glycosphingolipid (oligoSia in gangliosides), an integral membrane glycoprotein, or a glycoprotein attached to a membrane via glycosylphosphatidylinositol. In eukaryotic cells, the attachment of a poly/oligoSia chain to the membrane anchor is usually through α-2,3-glycosidic linkage to a galactose. In prokaryotic cells this attachment is proposed to occur through glycosidic linkage to the phosphate group of a phospholipid. Both long polySia chains attached to membrane proteins and short oligoSia attached to glycosphingolipids or membrane proteins are frequently found in neural membranes. In humans, poly/oligoSia is involved in development and plasticity of the brain, pathophysiology of schizophrenic brains, cancer metastasis, neuroinvasive potential of pathogenic bacterial strains, and the immune response. Biological roles of poly/oligoSia are based on its ability to modulate repulsive and attractive interactions between two molecules, and its ability to modulate membrane surface charge density, pH at the membrane surface, and membrane potentials.