Comparative molecular dynamics study of neuromyelitis optica-immunoglobulin G binding to aquaporin-4 extracellular domains

Abstract

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system in which most patients have serum autoantibodies (called NMO-IgG) that bind to astrocyte water channel aquaporin-4 (AQP4). A potential therapeutic strategy in NMO is to block the interaction of NMO-IgG with AQP4. Building on recent observation that some single-point and compound mutations of the AQP4 extracellular loop C prevent NMO-IgG binding, we carried out comparative Molecular Dynamics (MD) investigations on three AQP4 mutants, TP^137-138AA, N¹⁵³Q and V¹⁵⁰G, whose 295-ns long trajectories were compared to that of wild type human AQP4. A robust conclusion of our modeling is that loop C mutations affect the conformation of neighboring extracellular loop A, thereby interfering with NMO-IgG binding. Analysis of individual mutations suggested specific hydrogen bonding and other molecular interactions involved in AQP4-IgG binding to AQP4.

Keywords: Aquaporin-4; Molecular Dynamics; Mutations; Neuromyelitis Optica.

Fig. 1

Cartoon representation of the hAQP4 extracellular loops A (purple) and C (green). The mutated residues are rendered as sticks (left). Sketches of the residues involved in the studied mutations (right).

Fig. 2

Top view (left) and lateral view (right) of the investigated systems. Water molecules and membrane bilayer are rendered as sticks while the AQP4 tetramer is depicted in cartoon representation.

Fig. 3

A) Computed C-alpha_AV for the residues of the WT and of the three MTs calculated over the MD trajectories. B) Computed C-alpha_AV for the loop A residues. Standard errors are calculated through the block average method.

Fig. 4

Normalized distribution of the C-alpha distances computed for the residue T62.

Fig. 5

A) RMSF of the residues of the WT and of the three investigated MTs calculated over the MD trajectories averaged over the four monomers. B) RMSF of the loop A residues. C) RMSF of the loop C residues. The errors are calculated as standard errors over the RMSF values of the single monomers.

Fig. 6

Selected frames showing the intra-monomeric H-bond between T137 and L133, the inter-monomeric H-bond between T137 and Y207 in WT and the intermonomeric interaction between Y207 and G54 in TP^137-138AA (monomer B is depicted in green cartoons while monomer C in purple cartoons). In TP^137-138AA the A137 side chain is unable to engage H-bond interactions and, as a consequence, Y207 side chain interacts with G54 backbone. All key residues are depicted in sticks representation.

Fig. 7

Selected snapshots showing the hydrophobic interaction among W59, A138 (monomer C in purple cartoon representation) and L154 (monomer B in green cartoon representation) in TP^137-138AA. In WT this interaction are absent. All key residues are depicted in licorice representation.

Fig. 8

Normalized distribution of the distance between the center of mass of the side chains of residues P138 and W59 in WT and between A138 and W59 in TP^137-138AA.

Fig. 9

Selected snapshots showing the H-bond between N153 and H151 (dotted line) and the inter-monomeric 90° π-π interaction between H151 (monomer D in purple cartoons) and W59 (monomer B in green cartoons) in WT (continuous line). In N¹⁵³Q these interaction are absent presenting the hydrophobic interaction among W59, P138 and L154. All key residues are depicted in sticks representation. Loop A is depicted in black cartoon representation.

Fig. 10

Normalized distribution of the distance between the center of mass of W59 and the two residues of the pocket P138 and L154 (d_W59-pocket) in both WT and N¹⁵³Q.

Fig. 11

Selected snapshots showing the H-bond between T56 (side chain) and L53 (backbone) and the hydrophobic interaction between V150, L154 and G159 in WT. In V¹⁵⁰G G150 loses the interactions with V150 and G159 due its reduced side chain thus destabilizing loop C. As a consequence, G146 shifts towards T56 and L53 generating a steric hindrance that weakens the T56-L53 interaction thus affecting the conformation of loop A (in black cartoon representation). AQP4 monomer is represented in green cartoon and all the key residues are depicted in sticks.

Fig. 12

Normalized distribution of the distances between the c. o. m. of the segment 146–150 and T56 side chains (d_{(146–150)-T56}) in both WT and V¹⁵⁰G.