Architecture of the paracellular channels formed by claudins of the blood–brain barrier tight junctions

Abstract

Tight junctions (TJs) are key players in determining tissue-specific paracellular permeability across epithelial and endothelial membranes. Claudin proteins, the primary determinants of TJs structure and functionality, assemble in paracellular spaces to form channels and pores that are charge and size selective. Here, using molecular dynamics (MD) simulations, we elucidate the molecular assembly of claudin-3 and claudin-5 proteins of blood-brain barrier TJs. Despite having a high degree of sequence and structural similarity, these two claudins form different types of cis-interactions. Molecular docking of the observed cis-interfaces into trans-forms revealed two putative pore models that were also observed in the self-assembly simulations. The observed pore structures (pore I and II) have pore-lining residues that have been previously reported in the literature. The pore I model is consistent with a previously reported claudin-15 model. The pore II model, also consistent with biochemical results, has not been reported previously. Further analysis using in silico site-directed mutations provide convincing support for the validity of the pore II model. Using steered MD and umbrella sampling, we computed the transport properties of water and α-d-glucose through pore II. The study offers new insight into the selectivity of blood-brain barrier TJs.

A refined model of claudin-15 tight junction paracellular architecture by molecular dynamics simulations

“…Strikingly, the loop region between β 1 and β 2 (residues 34 to 41) is missing in the crystal structure. The monomeric structure of Cldn15 has been employed in coarse-grained computational studies [ 23 ], and as a template to build structural models of several other claudins [ 24 , 25 ].…”

“…Interestingly however, computational studies based on coarse-grained models [ 24 ] demonstrated that for a different claudin (Cldn5), whose structure was modeled by homology with Cldn15, a cis arrangement similar to that of Suzuki et al is among those that are spontaneously formed by monomers. Morover, by docking these Cldn5 dimers [ 25 ], it was possible to reconstruct a single-pore structure similar to the Cldn15 model of Suzuki et al…”

Computational Modeling of Claudin Structure and Function

“…In particular, this model is not compatible with studies suggesting the role of TM helices in claudin assembly and strand formation through cis-interactions [37][38][39][40][41]. Based on these interactions, alternative pore models have been proposed [37,42,43]. With recent advances in computational techniques, molecular dynamics simulations can reach time scales and sizes relevant to study assembly and initial stages of protein aggregation [44][45][46][47][48][49] and can provide an insight into alternative conformations of claudins in the TJ strands.…”

“…For example, there are significant differences in how claudin-5 and claudin-3 oligomerize and this is thought to be due to differences in key residues in TM3 [39]. To investigate the role of TM3 helices in the self-assembly of claudins in the membrane, Irudayanathan et al [43] simulated the assembly of homology models of claudin-3 and claudin-5 in lipid bilayers at CG resolution. The self-assembly trajectory of claudin-3 monomers showed that the dimer with TM2-TM3 interface, detected in claudin-5 trajectories, was absent in claudin-3, while other dimer interfaces were detected.…”

“…CG simulations have also been used to study pore formation through trans-interactions between claudins in two parallel lipid bilayers. Irudayanathan et al [43] performed CG simulations of 128 claudin-5 monomers in two adjacent lipid bilayers. 20 µs of self-assembly simulation showed instances of trans-interactions between the smaller ECS2 loop of monomers in adjacent membranes.…”