The treatment of epithelial cells with interferon-γ (IFN-γ) and tumour-necrosis factor-α (TNF) causes changes to barrier function and leads to endocytosis of the tight junction protein occludin. In their Research Article, Christina Van Itallie and colleagues (Van Itallie et al., 2018) set out to identify proteins that localise near to occludin and might be associated with the cytokine-induced changes in the MDCK cells. Unexpectedly, the authors identify MARCKS-related protein (MRP) as one of the most highly enriched occludin-proximal proteins in the cytokine-treated cells. MRP has been implicated in the regulation of integrin-dependent adhesion and control of cell migration in non-epithelial cells, but its role in epithelial cells and cell junctions is unclear. Interestingly, the authors show that MRP–GFP predominantly colocalises with lateral actin, and its overexpression potentiates the physiological response of the tight junction barrier to cytokines. Deletion of MRP does not alter the occludin-dependent cytokine-induced changes in the MDCK cells, but does result in changes to the organisation of multiple actin-based structures. The authors observe dramatic changes in focal adhesion organisation and basal actin stress fibres in MRP-knockout cells, similar to changes seen in FAK-knockout cells. Moreover, they find that α-actinin and vinculin are increased at cell contacts in knockout cells, suggesting changes in cell–cell interactions and overall monolayer tension. These findings identify MRP as a crucial regulatory protein in the organisation of focal adhesions and cell–cell contacts in epithelial cells.
