Expression of the catalytic domain of myosin light chain kinase increases paracellular permeability

Contractile events resulting from phosphorylation of the 20-kDa myosin light chain (MLC20) have been implicated in the regulation of epithelial tight junction permeability. To address this question, Madin-Darby canine kidney cells were transfected with a murine leukemia retroviral vector containing DNA encoding either the catalytic domain of myosin light chain kinase (tMK) or the β-galactosidase gene (β-gal). Autoradiograms of sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of myosin immunoprecipitated from ³²P(i)-labeled transfected cells demonstrated that MLC20 phosphorylation was increased 3.1 ± 0.9-fold in cells expressing tMK compared with cells expressing β-gal. Phosphopeptide mapping confirmed that myosin light chain kinase was responsible for the increased MLC20 phosphorylation. Transepithelial electrical resistance, a measurement of barrier function, of tMK cell monolayers was consistently <10% (123 ± 20 Ω · cm²) of that of monolayers comprised of wild-type cells (1,456 ± 178 Ω · cm²) or cells expressing β-gal (1,452 ± 174 Ω · cm²). Dual ²²Na⁺ and [³H]mannitol flux studies indicated that the decrease in resistance in tMK cells was attributable to increased paracellular flow. These data support the idea that MLC20 phosphorylation by myosin light chain kinase is involved in regulating epithelial tight junction permeability.