e injected intraperitoneally with a concentration of PBA that has previously been used in animal studies: 200 mg PBA/kg body weight. CT was injected directly into the ileal loop 40 minutes after the intraperitoneal injection of PBA, and the effects of intoxication were assessed seven hours after toxin challenge. As shown in Discussion PBA is an FDA-approved drug for the treatment of urea cycle disorders. The therapeutic value of PBA in treating these disorders relates to its function as an ammonia scavenger. However, PBA is also a chemical chaperone that can stabilize protein structures. In this capacity, PBA has been evaluated as a potential therapeutic for the treatment of many genetic diseases involving the ERAD-mediated degradation of mutant proteins that appear misfolded yet retain at least some wild-type function. The PBA-mediated stabilization of these mutant proteins would, in theory, disrupt ERAD-substrate interactions and consequently result in the secretion of a functional protein. The release of a previously missing protein would correct the phenotype of the genetic defect and thus cure the disease. Since accumulating evidence suggests that CTA1 is processed as a typical ERAD substrate, we took a conceptually similar approach with CT: the PBA-mediated stabilization of CTA1 was predicted to disrupt ERAD-substrate interactions and thereby prevent the ERAD-mediated disease state. This strategy views cholera as a unique type of protein folding disease and suggests a new, antitoxin application for a TCS-OX2-29 current clinical reagent. At a concentration of 100 mM, PBA inhibited the thermal unfolding of CTA1, the ER-to-cytosol export of CTA1, and CT intoxication of cultured cells. Some inhibitory effects on CTA1 unfolding and CT intoxication were 10516638 also observed with 1 and 10 mM PBA. Retrograde toxin trafficking to the ER, holotoxin disassembly by PDI, and ERAD activity in general were not altered by 100 mM PBA. However, we found that the stabilized pool of CTA1 was not efficiently secreted from PBA-treated cells. In contrast, we previously reported that glycerol-treated cells secreted more CTA1 than untreated control cells. The different effects of PBA and glycerol on CTA1 secretion likely relates to their distinct modes of protein stabilization: PBA physically binds to the protein whereas glycerol forms a hydration shell around the protein. The additional mass resulting from PBA interaction could possibly interfere with the packaging of stabilized CTA1 into carrier vesicles for secretory transport. Alternatively, different conformations for glycerol-stabilized CTA1 vs. PBA-stabilized CTA1 could promote toxin secretion from only glycerol-treated cells. The retention of PBA-stabilized CTA1 within the endomembrane system did not reduce cell viability over a 48 hour period as assessed by MTT assay, possibly because the low levels of CT that traffic to the ER are not sufficient to overwhelm the transport and quality control functions of the secretory pathway. April 2011 | Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor PBA alters the expression of cytosolic chaperones, modulates the unfolded protein response, and inhibits chloride efflux from the cystic fibrosis transmembrane regulator. Each of these effects could influence host-CT interactions, but the concentration and duration of PBA treatment used in our studies is insufficient to elicit these off-target effects. We documented the block of CTA1 translocation and CT intoxication we injected intraperitoneally with a concentration of PBA that has previously been used in animal studies: 200 mg PBA/kg body weight. CT was injected directly into the ileal loop 40 minutes after the intraperitoneal injection of PBA, and the effects of intoxication were assessed seven hours after toxin challenge. As shown in Discussion PBA is an FDA-approved drug for the treatment of urea cycle disorders. The therapeutic value of PBA in treating these disorders relates to its function as an ammonia scavenger. However, PBA is also a chemical chaperone that can stabilize protein structures. In this capacity, PBA has been evaluated as a potential therapeutic for the treatment of many genetic diseases involving the ERAD-mediated degradation of mutant proteins that appear misfolded yet retain at least some wild-type function. The PBA-mediated stabilization of these mutant proteins would, in theory, disrupt ERAD-substrate interactions and consequently result in the secretion of a functional protein. The release of a previously missing protein would correct the phenotype of the genetic defect and thus cure the disease. Since accumulating evidence suggests that CTA1 is processed as a typical ERAD substrate, we took a conceptually similar approach with CT: the PBA-mediated stabilization of CTA1 was predicted to disrupt ERAD-substrate interactions and thereby prevent the ERAD-mediated disease state. This strategy views cholera as a unique type of protein folding disease and suggests a new, antitoxin application for a current clinical reagent. At a concentration of 100 mM, PBA inhibited the thermal unfolding of CTA1, the ER-to-cytosol export of CTA1, and CT intoxication of cultured cells. Some inhibitory effects on CTA1 unfolding and CT intoxication were also observed with 1 and 10 mM PBA. Retrograde toxin trafficking to the ER, holotoxin disassembly by PDI, and ERAD activity in general were not altered by 100 mM PBA. However, we found that the stabilized pool of CTA1 was not efficiently secreted from PBA-treated cells. In contrast, we previously reported that glycerol-treated cells secreted more CTA1 than untreated control cells. The different effects of PBA and glycerol on CTA1 secretion likely relates to their distinct modes of protein stabilization: PBA physically binds to the protein whereas glycerol forms a hydration shell around the protein. The 17942897 additional mass resulting from PBA interaction could possibly interfere with the packaging of stabilized CTA1 into carrier vesicles for secretory transport. Alternatively, different conformations for glycerol-stabilized CTA1 vs. PBA-stabilized CTA1 could promote toxin secretion from only glycerol-treated cells. The retention of PBA-stabilized CTA1 within the endomembrane system did not reduce cell viability over a 48 hour period as assessed by MTT assay, possibly because the low levels of CT that traffic to the ER are not sufficient to overwhelm the transport and quality control functions of the secretory pathway. April 2011 
| Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor PBA alters the expression of cytosolic chaperones, modulates the unfolded protein response, and inhibits chloride efflux from the cystic fibrosis transmembrane regulator. Each of these effects could influence host-CT interactions, but the concentration and duration of PBA treatment used in our studies is insufficient to elicit these off-target effects. We documented the block of CTA1 translocation and CT intoxication w