Cryo-electron tomography reveals paracellular claudin-15 pores at the tight junction.

Tight junctions (TJs) are composed of anastomosing strands between epithelial cells. Members of the claudin family of proteins reside within TJ strands and either seal the paracellular space or assemble into charge and size-selective pathways. Functional studies suggest that claudin-mediated conductance pathways resemble traditional ion channels. However, such postulated pores have not been directly visualized. Using a model claudin deficient epithelium where exogenously introduced EGFP-CLDN15 is the only claudin family member expressed, our investigation sheds light on the arrangement and structure of the postulated claudin pores. Following correlative light and electron microscopical identification of TJs and cryo-electron tomography, we identified series of linearly distributed electron lucent features that locate between two closely apposed plasma membranes of adjacent cells. At these sites, the median spacing between adjacent features is 2.25 nm (IQR = 1.83), with a median 1.66 nm (IQR = 0.92) diameter. In contrast, such features were not observed in claudin deficient model epithelium with exogenous mCherry-ZO-1 expression. These findings agree with the postulated and extensively modeled claudin pores formed within the simple columnar epithelium. This provides the first direct evidence of paracellular pore organization and paves way for future biophysical investigation.