Keynote review: epithelial and endothelial barriers in human disease.

There is a spectrum of distinct disease states that have in common their effect of breaking down epithelial and/or endothelial barrier function. Fluid compartmentalization goes awry, with profound implications for epithelial and stromal homeostasis, fluid and/or electrolyte balance, generation of inflammatory states, and even tumor microenvironment. Specific effects on the tight junction are found to be integral to bacterial invasion and tumor progression

Ras Mutation Impairs Epithelial Barrier Function to a Wide Range of Nonelectrolytes

Although ras mutations have been shown to affect epithelial architecture and polarity, their role in altering tight junctions remains unclear. Transfection of a valine-12 mutated ras construct into LLC-PK(1) renal epithelia produces leakiness of tight junctions to certain types of solutes. Transepithelial permeability of d-mannitol increases sixfold but transepithelial electrical resistance increases >40%. This indicates decreased paracellular permeability to NaCl but increased permeability to nonelectrolytes. Permeability increases to d-mannitol (M(r) 182), polyethylene glycol (M(r) 4000), and 10,000-M(r) methylated dextran but not to 2,000,000-M(r) methylated dextran. This implies a “ceiling” on the size of solutes that can cross a ras-mutated epithelial barrier and therefore that the increased permeability is not due to loss of cells or junctions. Although the abundance of claudin-2 declined to undetectable levels in the ras-overexpressing cells compared with vector controls, levels of occludin and claudins 1, 4, and 7 increased. The abundance of claudins-3 and -5 remained unchanged. An increase in extracellular signal-regulated kinase-2 phosphorylation suggests that the downstream effects on the tight junction may be due to changes in the mitogen-activated protein kinase signaling pathway. These selective changes in permeability may influence tumorigenesis by the types of solutes now able to cross the epithelial barrier