Muscarinic acetylcholine receptors (mAChRs) have a crucial role in epithelial cells where they regulate the transport of water, ions and solutes, and their activation leads to increased secretion through the transcellular pathway. It has been suggested that mAChRs also regulate transport through the tight-junction-based paracellular route, but there are only limited data regarding the underlying mechanisms. In their work on page , Li-Ling Wu, Guang-Yan Yu and co-workers now set out to investigate in detail the effects of mAChR activation on tight junctions using rat salivary epithelial cells as a model system. They observed that carbachol, which activates the M3 subtype, increased the paracellular permeability and found that this was caused by a selective loss of claudin-4 from tight junctions, whereas other tight junction proteins were unaffected. In addressing the underlying mechanism, the authors demonstrated here that carbachol promoted the phosphorylation of claudin-4 in an ERK1/2-dependent manner, which resulted in its binding to β-arrestin2 and subsequent clathrin-dependent endocytosis. Internalisation of claudin-4 was then followed by its ubiquitylation and proteasome-dependent degradation. Taken together, the data presented here clearly identify claudin-4 as the target of mAChRs that mediates the regulation of paracellular permeability in epithelial cells, and describe a new molecular pathway through which the properties of tight junctions can be modulated.
