Evaluation of red and near infrared fluorescent silver nanoclusters as potential in vivo indicators of tight junction opening

Tight junctions (TJs) play a key role in regulating permeability to liquids, ions and larger solutes through the paracellular route. To demonstrate TJ structure changes by measuring paracellular flux is challenging for understanding biological functions of tight junction and designing delivery system for highly hydrophilic macromolecular drugs. In the present study, we tested two long wavelength emitting silver nanoclusters (AgNCs), Ag2S(BSA)-NCs (lambda(ex/em) at 500/1050 nm) and Ag(GSH)-NCs (lambda(ex/em) at 488/640 nm), for their suitability as novel paracellular permeation indicators on a MDCK monolayer. Ag2S(BSA)-NCs exhibited marginal cytotoxicity and passed through the cell monolayer exclusively via the paracellular pathway. However, Ag(GSH)-NCs could be taken inside the cells possibly through endocytosis. AgNCs together with Eu-DTPA were used in the double probe strategy for detecting the change of TJ pore path parameters (i.e. pore size r and retention capacity epsilon/tau) upon TJ opening with EDTA or vanadyl acetylacetonates, respectively. The AgNCs/Eu-DTPA probe set was found to give the same results as our previous work using the short wavelength emitting AuNCs and Eu-DTPA probe set, suggesting future potential applications of AgNCs to the in vivo studies of TJ alternations upon stresses. This work reinforced the use of a double probe set for study of the TJ structure change