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The Cytopathic Activity of Cholera Toxin Requires a Threshold Quantity of Cytosolic Toxin

Bader, C;

Cholera toxin (CT), secreted from Vibrio cholerae, causes a massive fluid and electrolyte
efflux in the small intestine that results in life-threatening diarrhea and dehydration which
impacts 3-5 million people per year. CT is secreted into the intestinal lumen but acts within the
cytosol of intestinal epithelial cells. CT is an AB5 toxin that has a catalytic A1 subunit and a cell
binding B subunit. CT moves from the cell surface to the endoplasmic reticulum (ER) by
retrograde transport. Much of the toxin is transported to the lysosomes for degradation, but a
secondary pool of toxin is diverted to the Golgi apparatus and then to the ER. Here the A1
subunit detaches from the rest of the toxin and enters the cytosol. The disordered conformation
of free CTA1 facilitates toxin export to the cytosol by activating a quality control mechanism
known as ER-associated degradation. The return to a folded structure in the cytosol allows CTA1
to attain an active conformation for modification of its Gsα target through ADP-ribosylation.
This modification locks the protein in an active state which stimulates adenylate cyclase and
leads to elevated levels of cAMP. A chloride channel located in the apical enterocyte membrane
opens in response to signaling events induced by these elevated cAMP levels. The osmotic
movement of water into the intestinal lumen that results from the chloride efflux produces the
characteristic profuse watery diarrhea that is seen in intoxicated individuals.
The current model of intoxication proposes only one molecule of cytosolic toxin is
required to affect host cells, making therapeutic treatment nearly impossible. However, based on
emerging evidence, we hypothesize a threshold quantity of toxin must be present within the
cytosol of the target cell in order to elicit a cytopathic effect. Using the method of surface
plasmon resonance along with toxicity assays, I have, for the first time, directly measured the
efficiency of toxin delivery to the cytosol and correlated the levels of cytosolic toxin to toxin
activity. I have shown CTA1 delivery from the cell surface to the cytosol is an inefficient process
with only 2.3 % of the surface bound CTA1 appearing in the cytosol after 2 hours of
intoxication. I have also determined and a cytosolic quantity of more than approximately .05ng
of cytosolic CTA1 must be reached in order to elicit a cytopathic effect. Furthermore, CTA1
must be continually delivered from the cell surface to the cytosol in order to overcome the
constant proteasome-mediated clearance of cytosolic toxin. When toxin delivery to the cytosol
was blocked, this allowed the host cell to de-activate Gs, lower cAMP levels, and recover from
intoxication. Our work thus indicates it is possible to treat cholera even after the onset of disease.
These findings challenge the idea of irreversible cellular toxicity and open the possibility of postintoxication treatment options.