Gangliosides and autoimmune diabetes.
Gangliosides are sialic acid-containing glycolipids which are formed by a hydrophobic portion, the ceramide, and a hydrophilic part, i.e. the oligosaccharide chain. First described in neural tissue, several studies have shown that gangliosides are almost ubiquitous molecules expressed in all vertebrate tissues. Within cells, gangliosides are usually associated with plasma membranes, where they can act as receptors for a variety of molecules and have been shown to take part in cell-to-cell interaction and in signal transduction. In addition, gangliosides are expressed in cytosol membranes like those of secretory granules of some endocrine cells (adrenal medulla, pancreatic islets). As far as the role of gangliosides in diseases is concerned, there are some cases in which an aberrant ganglioside expression plays a crucial role in the disease pathogenetic process. These diseases include two major forms of ganglioside storage, namely GM2-gangliosidosis (Tay-Sachs and its beta-hexosaminidase deficiency) and GM1-gangliosidosis (beta-galactosidase deficiency), where the most prominent pathological characteristic is the lysosomal ganglioside accumulation in neurons. Other inflammatory or degenerative diseases both within and outside the nervous system have been shown to be associated with an altered pattern of ganglioside expression in the target organ. Since monoclonal antibodies have been discovered and used in immunology, a large variety of ganglioside antigens has been described both as blood group antigens and as tumour-related antigens. Several studies have also indicated that gangliosides can act not only as antigens, but also as autoantigens. As a matter of fact, auto-antibodies to gangliosides, detected by immunostaining methods performed directly on TLC plates or by ELISA, have been described in several autoimmune disorders such as Guillain-Barré syndrome, multiple sclerosis, lupus erythematosus, Hashimoto's thyroiditis and, last but not least, insulin-dependent (type 1) diabetes mellitus. This last disease is caused by the autoimmune destruction of insulin-producing pancreatic islet cells in genetically predisposed individuals. Autoantibodies and T lymphocytes directed towards multiple islet autoantigens have been detected in the circulation, well before the clinical onset of the disease, in a prodromal phase during which pancreatic islet beta-cells are presumably destroyed. Among the target autoantigens, some are of protein nature but others are acidic glycolipids such as sulphatides158 and the gangliosides GT3, GD3 and especially GM2-1. This last component is specifically expressed in pancreatic islets and has been shown to represent a target of IgG autoantibodies highly associated with diabetes development in first-degree relatives of type 1 diabetic individuals. In addition, the GM2-1 ganglioside appears to be one of the antigens recognized by cytoplasmic ICA, a heterogeneous group of antibodies which specifically react with islets on pancreatic frozen sections. In conclusion, studies performed in the last decade have clearly indicated that gangliosides represent a heterogeneous class of molecules that are involved in several cellular processes that are of crucial importance in physiological as well as in pathological conditions. Interestingly, these molecules, despite their small size, have been shown to represent not only important antigens in tumour immunology but are also able to elicit a specific autoimmune response, thus representing important autoantigens in some autoimmune disorders. It is of interest that, in addition to neurological autoimmune disorders where autoimmunity to gangliosides is frequent and usually of considerable magnitude, an autoimmune response to this class of molecules has been observed in autoimmune diabetes. (ABSTRACT TRUNCATED)