Cholera Toxin - Pathogenesis

Pathogenesis

Cholera toxin acts by, first, binding its B subunit ring to GM1 gangliosides on the surface of target cells. After binding takes place, the entire toxin complex is endocytosed by the cell and the cholera toxin A1 chain is released by the reduction of a disulfide bridge. The endosome is moved to the Golgi apparatus, where the A1 protein is recognized by the endoplasmic reticulum chaperon, protein disulfide isomerase. The A1 chain is then unfolded and delivered to the membrane, where the ER-vcoxidase - Ero1 triggers the release of the A1 protein by oxidation of protein disulfide isomerase complex. As the A1 protein moves from the ER into the cytoplasm by the Sec61 channel, it refolds and avoids deactivation as a result of ubiquitination.

CTA1 is then free to bind with a human partner protein called ADP-ribosylation factor 6 (Arf6); binding to Arf6 drives a change in the shape of CTA1 which exposes its active site and enables its catalytic activity. The CTA1 fragment catalyses ADP-ribosylation of the regulatory component (G-protein) of adenylate cyclase, using NAD. The ADP-ribosylation causes the Gα_s subunit to lose its catalytic activity in hydrolyzing GTP to GDP + P_i, which therefore remains continually activated. Increased adenylate cyclase activity increases the intracellular concentration of cAMP to more than 100-fold than normal, which massively activates cytosolic PKA. These active PKA then open up the cystic fibrosis transmembrane conductance regulator (CFTR) proteins, which will lead to the efflux of chloride ions, which in turn leads to secretion of H₂O, Na+, K+, and HCO₃- into the intestinal lumen. In addition, The entry of Na+ and consequently the entry of water into enterocytes are diminished. The combined effects result in rapid fluid loss from the intestine, up to 2 liters per hour, leading to severe dehydration and other factors associated with cholera, including a rice-water stool.

Interestingly, the pertussis toxin (also an AB₅ protein) produced by Bordetella pertussis acts in a similar manner with the exception that it ADP-ribosylates the Gα_i subunit, rendering it inactive and unable to inhibit adenylyl cyclase production of cAMP (leading to constitutive production).