Table 6.
Function of claudin 16 according to heterologous expression studies in cell lines.
| Claudin | Cell Line | Transfection | TER | PNa/PCl | PNa | PCl | PMg | PCa | Mg2+ Flux | Ca2+ Flux | Ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Rat Cldn16 | MDCK | stable | ↗ | ↗ *∫◊ | [129] | ||||||
| Rat? Cldn16 | MDCK | stable | ↗ | ↘ | ↘ | NS | ↗ §ø | [130] | |||
| Rat Cldn16 | MDCK Tet-OFF | Inducible expression | ↗ | ↗ §* | [132] | ||||||
| Human ∆70 CLDN16 | LLC-PK1 | Stable? | ↘ | ↗ | ↗ | NS | ↗ † | [46] | |||
| Human ∆70 CLDN16 | MDCK II | Stable? | NS | NS | NS | [46] | |||||
| Full length Human CLDN16 | LLC-PK1 | Stable? | ↘ | ↗ | ↗ | NS | [47] | ||||
| Full length Human CLDN16 | MDCK II | Stable? | NS | NS | [47] | ||||||
| Short and long version of human CLDN16 | MDCK-C7 | Stable | *** | ↗ £ | NS ∫ | [48] | |||||
| Long version of human CLDN16 | MDCK-C7 | Stable | NS | ↗ ∞ | NS ∞ | [48] | |||||
| Full-length human CLDN16 | MDCK-C7 | stable | NS | ↗ ¥ | [131] |
Relative epithelial permeabilities (e.g., PNa/PCl) are calculated using the Goldman-Hodgkin-Katz equation and the diffusion potential caused by the application of distinct solutions at the apical and basolateral compartment. Absolute permeabilities can be calculated if the transepithelial conductance is known using Kimizuka-Koketsu equation [41]. Measuring ion flux can be performed using either radioactive isotopes or fluorescent compounds or non-radioactive ions. Flux measurement include trans- and paracellular transport [41]. TER: trans epithelial resistance; NS: not significantly different from control; Px: permeability to ion X; ∫: Transepithelial transport of 45Ca2+ measurement; §: The transepithelial transport of Mg2+ was measured using Xylidyl Blue-I.; †: The permeability of Mg2+ across monolayers was determined according to Tang and Goodenough [133]; £: Mg2+ flux was measured employing atomic absorption spectrometry; ∞: Mg2+ and Ca2+ permeabilities, calculated from dilution potential/bionic potential measurements; ¥: Measurement of unidirectional fluxes from the basolateral to the apical side was performed under short-circuit conditions with MgSO4. The atomic absorption of Mg2+ was measured in an oxidizing air-acetylene flame at 285.2 nm. PMg was calculated from resulting fluxes (PMg = flux/concentration); ∂Permeability PC = Flux/Substrate concentration in cis compartment. This was then corrected for the permeability of blank filters, PB, to obtain the true transepithelial permeability (PT), using the following equation PT = [(1/PC) & (1/PB)]−1; ***: Increased basolateral Mg2+ concentration induces a short circuit current (may activate a transcellular Cl− current); *: from apical to basal compartment, without affecting transport from basal to apical compartments; ø: from apical to basal compartment; ◊: The apical to basolateral flux was competitively inhibited by Mg2+.