Abstract

The aim of this study was to investigate whether peptides from the extracellular loops of the tight junction protein occludin could be used as a new principle for tight junction modulation. Peptides of 4 to 47 amino acids in length and covering the two extracellular loops of the tight junction protein occludin were synthesized, and their effect on the tight junction permeability in Caco-2 cells was investigated using [¹⁴C]mannitol as a para-cellular marker. Lipopeptide derivatives of one of the active occludin peptides (OPs), synthesized by adding a lipoamino acid containing 14 carbon atoms (C₁₄-) to the N terminus of the peptide, were also investigated. Peptides corresponding to the N terminus of the first extracellular loop of occludin increased the permeability of the tight junctions without causing short-term toxicity. However, the peptides had an effect only when added to the basolateral side of the cells, which could be partly explained by degradation by apical peptidases and aggregate formation. By contrast, the lipopeptide C₁₄-OP_90-103, which protects the peptide from degradation and aggregation, displayed a rapid apical effect. The l- and d-diastereomers of C₁₄-OP_90-103 had distinctly different effects. The d-isomer, which releases intact OP_90-103 from the lipoamino acid, displayed a rapid and transient increase in tight junction permeability. The l-isomer, which releases OP_90-103 more rapidly, gave a more sustained increase in tight junction permeability. In conclusion, C₁₄-OP_90-103 represents a prototype of a new class of tight junction modulators that act on the extracellular domains of tight junction proteins.