The most significant finding of the present study is the novel characterization of a ROS-dependent pathway that causes calpain-dependent endothelial ZO-1 degradation in response to PM. These data represent the first evidence that calpain signaling, via calcium leakage from activated TRPM2 by ROS, plays a critical role in modulating endothelial cell barrier function, resulting in tight junction protein ZO-1 degradation (Additional file 1 Figure S7 ). The consequence of ZO-1 degradation is sustained endothelial hyperpermeability and persistent lung inflammation, both of which contribute to variety of acute or chronic cardiovascular disorders [25, 26] . These effects were also observed with other types of PM samples (1648a from Human lung microvascular ECs grown in 6-well dishes to approximately 95% confluence were treated with TRPM2 antibody or control IgG (5 μg/ml) for 4 hr, and then challenged with PM (100 μg/ml) for 6 hr. Cell lysates were analyzed by Western blotting with ZO-1 antibody. Changes in levels of ZO-1 are expressed as fold changes and normalized to β-actin. Shown are representative blots from three independent experiments. *p < 0.05 compared to control. **p < 0.05 compared to PM challenge. (B) ECs grown in 6-well dishes to approximately 80% confluence were treated with TRPM2 siRNA or control siRNA (100 ng/ml) for 48 hr, and cell lysates were analyzed by Western blotting with ZO-1 antibody. (C) ECs grown in 6-well dishes to approximately 95% confluence were treated with TRPM2 siRNA or control siRNA (100 ng/ml) for 48 hr, and then challenged with PM (100 μg/ml) for 6 hr. Cell lysates were analyzed by Western blotting with ZO-1 antibody. Changes in levels of ZO-1 are expressed as fold changes and normalized to β-actin. Shown are representative blots from three independent experiments. *p < 0.05 compared to control. **p < 0.05 compared to PM challenge. (D) ECs grown on ECIS gold electrodes were treated with TRPM2 antibody or control IgG (5 μg/ml) for 4 hr, and then challenged with PM (100 μg/ml). Changes in TER were measured with ECIS. *p < 0.05 compared to PM-challenged group. (E) ECs grown on 100 mm dishes were treated with TRPM2 siRNA or control siRNA (100 ng/ml) for 48 hr, and then plated onto gold electrodes for ECIS measurement. 24 hours after replating, the ECs were challenged with PM (100 μg/ml) and changes in TER were measured with ECIS. *p < 0.05 compared to PM-challenged group.