SHIGA TOXIN MODE OF ACTION IN E. COLI O157:H7 DISEASE

Tom G. Obrig

Department of Microbiology and Immunology, University of Rochester, Box 672, 601 Elmwood Avenue, Rochester, NY 14642

Received 12/2/97 Accepted 12/8/97

5. STX RECEPTORS EXPRESSED ON EUKARYOTIC CELLS

5.1 The Stx receptor is a glycosphingolipid, Gb3

The receptor for Stx, Stx1 and Stx2 on the cell surface of eukaryotic cells is the neutral glycolipid, globotriaosylceramide, Gb3 (26). A comprehensive review of this topic exits (27). Gb3 consists of a ceramide long chain fatty acid embedded in the plasma membrane, and a short extracellular trisaccharide chain terminated by a digalactose residue. The B-subunits of Shiga toxins facilitate high-affinity (Kd= 0.1 nM) binding of holotoxin to the terminal digalactose residue of Gb3. This terminal disaccharide contains the unusual carbohydrate linkage, galactose(alpha 1,4)-galactose. Gb3 is also known as cell differentiation marker, CD77 and is a tumor marker for a B-cell lymphoma. Gb3 trisaccharide also occurs on red blood cells of the Pk blood group.

5.2 Specificity of Stx-receptor interaction

Recently, the interaction between Stx molecules and Gb3 has been elucidated. Combined information from the crystal structure of Stx B-subunit and site-directed mutations in the subunit has led to the concept of two sites within the B-subunit that interact with the digalactose of Gb3 (27,28). Another important finding is that the chemical structure of the ceramide portion of Gb3 strongly influences which Stx isotypes bind to Gb3 (29). More exactly, the length of the ceramide fatty acids and their chemical substitutions were shown to dictate binding of Stx isotypes to Gb3. This phenomenon appears to be due to conformational changes in the extracellular carbohydrate portion of Gb3 which are dictated, in part, by how the ceramide is arranged in the plasma membrane of the target cell.

5.3 Potential receptor-based therapeutics for EHEC disease

Therapeutic intervention for EHEC-associated disease does not exist at this time. Traditional antibiotics are largely ineffective against EHEC, and as the disease is a toxemia rather than a bacteremia, a different type of intervention is required. Current treatment which has been effective in sharply reducing the death rate due to EHEC infections has been limited to renal-based supportive care aimed at the symptoms of HUS, i.e. hemolytic uremic syndrome (30). However, there exists a real need for a method of treatment early in the EHEC infection process. Indeed, a novel treatment currently in phase III trials in the US and Canada is a toxin receptor-based agent consisting of the carbohydrate moiety of Gb3 attached to the surface of silica particles administered orally to potential victims of EHEC on admission. This approach is aimed at binding of the free Stx when released by the pathogenic bacteria in the distal small intestine and the large intestine. The results of this "SYNSORB" clinical study should be available in late 1998.

Stx receptor does exist on cells within the human kidney and human endothelial cells are capable of producing large quantities of cell surface Gb3 (24,31,32). Whether receptor-based intervention would be feasible once Stx enters the bloodstream is highly questionable, but systemic receptor-based treatments for other diseases do exist, and more are being developed for future use.