Cholera toxin inhibits SNX27-retromer mediated delivery of cargo proteins to the plasma membrane

Abstract

Cholera toxin (CT) causes severe diarrhea by increasing intracellular cAMP leading to a PKA dependent increase in Cl− secretion through CFTR and decreased Na+ absorption by inhibition of Na+/H+ exchanger 3 (NHE3). The mechanism(s) by which CT inhibits NHE3 is partially understood, although no drug therapy has successfully been targeted at reversing this inhibition. We now describe that CT phosphorylates an amino acid in the PDZ domain of SNX27, which inhibits SNX27-mediates trafficking of NHE3 from the early endosomes to plasma membrane and contributes to reduced basal NHE3 activity that involves reduced plasma membrane (PM) expression, and reduced endocytic recycling. Importantly, mutagenesis studies (Ser to Asp) showed that this phosphorylation of SNX27 phenocopies loss of SNX27 function affecting plasma membrane trafficking of cargo proteins that bind SNX27-retromer. Additionally, CT destabilizes retromer function by decreasing the amount of core retromer proteins. These effects of CT can be partially rescued by enhancing retromer stability using “pharmacological chaperones”. Moreover, “pharmacological chaperones” can be used to increase basal and cholera toxin inhibited NHE3 activity and fluid absorption by intestinal epithelial cells.