Generation and initial characterization of tet-inducible CLDN15-expressing MDCK I monolayers. (A) No CLDN15 expression was detected without doxycycline treatment. Following doxycycline treatment, WT CLDN15^WT or CLDN15^D55E and CLDN15^D55N mutants were expressed at the tight junctions between cells (scale bar = 20 µm). (B and C) CLDN15 expression significantly reduced TER (mean ± SEM; Student’s unpaired two-tailed t test: CLDN15^WT, n = 4 technical replicates, representative of six separate experiments, P = 2.6 × 10⁻¹²; CLDN15^D55E, n = 8 technical replicates, representative of two separate experiments = 5.1 × 10⁻⁸; CLDN15^D55N, n = 4 technical replicates, representative of two separate experiments, P = 2.1 × 10⁻⁸) (C) and increased relative permeability of sodium to chloride (PNa⁺/PCl⁻) (mean ± SEM; Student’s unpaired two-tailed t test: CLDN15^WT, n = 4 technical replicates, representative of six separate experiments, P = 3.6 × 10⁻⁷; CLDN15^D55E, n = 8 technical replicates, representative of two separate experiments, P = 0.0018; CLDN15^D55N, n = 4 technical replicates, representative of two separate experiments, P = 4.2 × 10⁻⁶).