Calpain is a regulator of endothelial integrity which helps control fundamental cellular processes including cytoskeletal remodeling, membrane fusion, cell proliferation and differentiation, and activation of proteolytical cascades leading to apoptosis [44, 45] . Under oxidative stress, activated calpain cleaves eNOS and cytoskeletal proteins and induces apoptosis [21, [46] [47] [48] . Particulate matter induces endothelial cell intracellular oxidative stress, which leads to the activation of calpain, one of the major cytoskeletal regulators. Here we describe the cleavage of tight junction protein ZO-1 by activated calpain both in vitro and in vivo, indicating that calpain plays a central role in PM-induced endothelial barrier disruption and vascular hyperpermeability. In addition, as activated calpain cleaves other critical cytoskeletal proteins including ezrin and MARCKS protein, the contribution of the other cytoskeletal proteins to the EC hyperpermeability induced by PM needs to be further investigated.