Structure of GM1 ganglioside

A ganglioside is a molecule composed of a glycosphingolipid (ceramide and oligosaccharide) with one or more sialic acids (e.g. N-acetylneuraminic acid, NANA) linked on the sugar chain. NeuNAc, an acetylated derivative of the carbohydrate sialic acid, makes the head groups of gangliosides anionic at pH 7, which distinguishes them from globosides.

The name ganglioside was first applied by the German scientist Ernst Klenk in 1942 to lipids newly isolated from ganglion cells of the brain.^[1] More than 60 gangliosides are known, which differ from each other mainly in the position and number of NANA residues. It is a component of the cell plasma membrane that modulates cell signal transduction events, and appears to concentrate in lipid rafts.

Recently, gangliosides have been found to be highly important molecules in immunology. Natural and semisynthetic gangliosides are considered possible therapeutics for neurodegenerative disorders.^[2]

Location

Gangliosides are present and concentrated on cell surfaces, with the two hydrocarbon chains of the ceramide moiety embedded in the plasma membrane and the oligosaccharides located on the extracellular surface, where they present points of recognition for extracellular molecules or surfaces of neighboring cells. They are found predominantly in the nervous system where they constitute 6% of all lipids.^[3]

Function

The oligosaccharide groups on gangliosides extend well beyond the surfaces of the cell membranes, and act as distinguishing surface markers that can serve as specific determinants in cellular recognition and cell-to-cell communication. These carbohydrate head groups also act as specific receptors for certain pituitary glycoprotein hormones and certain bacterial protein toxins such as cholera toxin.

The functions of gangliosides as specific determinants suggest its important role in the growth and differentiation of tissues as well as in carcinogenesis. It has been found that tumor formation can induce the synthesis of a new complement of ganglioside, and very low concentrations of a specific ganglioside can induce differentiation of cultured neuronal tumor cells.^[4]

Common gangliosides

Structures of GM1, GM2, GM3

• One NANA ("M")
  • GM1
  • GM2
  • GM3
• Two NANAs ("D")
  • GD1a
  • GD1b
  • GD2
  • GD3
• Three NANAs ("T")
  • GT1b
  • GT3
• Four NANAs ("Q")
  • GQ1

Pathology

Gangliosides are continuously synthesized and degraded in cells. They are degraded to ceramides by sequential removal of sugar units in the oligosaccharide group, catalyzed by a set of highly specific lysosomal enzymes. Mutations in genes coding for these enzymes leads to the accumulation of partially broken down gangliosides in lysosomes, which results in a group of diseases called gangliosidosis. For example, the fatal Tay–Sachs disease arises as a genetic defect which leads to no functional hexosaminidase A produced, causing GM2 to accumulate in lysosomes. Ultimately the ganglion cells in the nervous system swell enormously, disturbing the normal functions of neurons.^[3]

A summary showing the causes of sphingolipidosis, including gangliosidosis.

Gangliosides are also involved in several diseases:

• Influenza, in which haemagglutinin of influenza virus exploits certain gangliosides to enter and infect the cells expressing them.
• Guillain–Barré syndrome, which has been linked to the production of anti-ganglioside antibodies.^[5]
• Cholera
• Tetanus
• Botulism
• Leprosy^[6]
• Obesity, where inadequate ganglioside expression in mediobasal hypothalamic neurons deregulates neuronal leptin^[7] and insulin signaling.^[8]

References

External links

• Gangliosides at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Overview of gangliosides at lipidlibrary.co.uk
• Overview of gangliosides at cyberlipid.org