Trans-Mediated, Cis-Inhibited Paradoxal Activity of <i>Clostridium perfringens</i> Enterotoxin (c-CPE) in Modulating Epithelial Permeability

In the context of transdermal delivery, favoring the drug permeability of epithelia through convenient formulations would open new opportunities for local versus systemic drug delivery, envisaging higher patient comfort and an enhanced therapeutic effect. Ligands of tight junctions are interesting agents that enhance epithelial permeability by relaxing the protein complexes that form them. The C-terminal domain of <i>Clostridium perfringens</i> enterotoxin (c-CPE), which binds claudins, one of the tight junction (TJ) components, has been explored here as a functional domain in modular recombinant proteins, to evaluate its ability to self-promote its paracellular epithelial passage in a Caco-2 cell monolayer model. c-CPE-containing fusion proteins bind cells in the absence of internalization and cytotoxicity and support the passage, in trans, of other fusion proteins devoid of c-CPE. However, c-CPE-carrying proteins fail to cross the epithelia by themselves, probably because their affinity for TJs immobilizes them in the intercellular space. Therefore, while recombinant c-CPE versions have been here confirmed as convenient epithelial-permeabilizing agents, a paradoxical behavior has been observed where this effect is only successful when applied in trans, specifically on entities that lack c-CPE. Then, c-CPE itself inhibits the paracellular mobility of carrier molecules, not being suited as a self-driver (in c-CPE-drug complexes) for drug delivery through epithelia.