On the Regulation of the Epithelial Paracellular Permeability

Sammanfattning: The epithelia of for instance the small intestine(s) plays a crucial role in the vectorial transport of nutrients and other solutes from the lumen to underlying body fluids, i.e. blood and lymph. However, the epithelial cells are also important in the formation and maintenance of a barrier towards harmful food components and toxic bacterial products.The research presented in this thesis aimed at further understanding theregulation of the paracellular permeability barrier in epithelia, especially in relation to structural modifications of the epithelial actin cytoskeleton and to intracellular signals.In particular, the objectives of the study were to: i) assess the associationbetween tight junctional integrity and the cytoskeleton by using cytochalasin B and dihydrocytochalasin B as modifying agents, ii) examine the involvement of phospho-inositide turnover in modulation of the paracellular barrier and the organization of the actin cytoskeleton, iii) investigate the participation of protein kinase C in the regulation of tight junctional integrity, iv) elucidate changes in the tight junction function andstructure caused by enterotoxigenic Vibrio cholerae, and v) examine the role of nitric oxide in the formation and regulation of the permeability barrier.To summarize, cytochalasin B and dihydrocytochalasin B mediate specific and dose-dependent effects on the transepithelial electrical resistance of MDCK-I epithelial cells. Furthermore, these alterations correlate well with distinct changes in the organization of filamentous actin and the tight junction-associated protein, Z0-1. Moreover, the actin-associated modulation of the epithelial barrier function appears to be dependent on phosphoinositide (PI) turnover, as elucidated by the combined effects of the PI 3-k:inase inhibitor, wortmannin, and dihydrocytochalasin B. The activation of protein kinase C with phorbol ester causes distinct changes in the transepithelial resistance and tight junction structure of MDCK-I and HT-29 cells. In addition, tight junctional function and structure also seem to be modulated by nitric oxide. A "new toxin", the hemagglutinin/protease (HNprotease) of Vibrio cho!erae was identified in the growth medium of a toxin-attenuated strain. The HA/protease causes characteristic alterations of the epithelial structure and barrier function, which were reduced in the presence of nitric oxide.