Many interactions between tight junction proteins have been described. Yet, understanding of how these interactions are regulated and how they impact tight junction barrier function is limited. This work examined effects of occludin phosphorylation on tight junction molecular structure and barrier function.
Casein kinase 2 (CK2) is an occludin kinase in vivo and in vitro, but the structural and functional consequences of occludin phosphorylation were unknown prior to this study. The results elucidate a previously unrecognized mechanism by which CK2-mediated phosphorylation of a specific residue, serine 408, within the occludin cytoplasmic tail regulates interactions between occludin and ZO-1.
Dephosphorylation of serine 408 stabilizes interactions between occludin and ZO-1 and, in turn, ZO-1 interactions with claudin-2. Assembly of this stable tripartite complex de-anchors tight junction-associated occludin and inactivates the paracellular channels formed by claudin-2.
In vitro, cytokine-induced increases in claudin-2 expression lead to enhanced Na+ and water conductance across the tight junction. CK2 inhibition restores the barrier without affecting claudin-2 expression.
The data therefore indicate that occludin S408 is a molecular switch by which CK2, and, potentially other occludin kinases, regulates molecular remodeling, protein interactions, and paracellular pore function at the tight junction. Subsequent in vivo work has shown this CK2 inhibitor-induced barrier restoration has potential as therapeutic intervention in inflammatory bowel disease.