Lack of aquaporin-4 water transport inhibition by antiepileptics and arylsulfonamides

Abstract

Inhibitors of brain glial water channel aquaporin-4 (AQP4) are of potential clinical utility, as they are predicted to modulate brain edema, neuroexcitation and glial scarring. Recently, Huber et al. (Bioorgan. Med. Chem. 2007, 17:1270 2008, in press) reported that a series of arylsulfomamides, antiepileptics, and related small molecules strongly inhibited AQP4 water transport with IC50s down to 1 μM. We retested the compounds with greatest reported potencies, including acetylsulfanilamide, acetazolamide, 6-ethoxy-benzothiazole-2-sulfonamide, topiramate, zonisamide, phenytoin, lamotrigine and sumatriptan, in AQP4-transfected mammalian cells and primary cultures of brain glial cells, using several sensitive assays of osmotic water permeability. Contrary to the findings of Huber et al., in our studies we found no significant inhibition of AQP4 water permeability by any of the compounds at concentrations up to 100 μM.

Crystal structure of human aquaporin 4 at 1.8 Å and its mechanism of conductance

“…In 2 different cell types, each with or without expression of orthogonally arrayed AQP4, separate cell-cell adhesion and dynamic light scattering assays demonstrated that AQP4 does not have any cell-cell adhesion property (31). Because in our 1.8-Å hAQP4 structure the C loop is not involved in the crystal packing, the lack of any helix in the C loop, therefore, supports the conclusion that AQP4 does not strongly drive adhesion, and that the 3 10 helix in the C loop observed in the rAQP4 structure may be induced by crystal contact.…”

“…The rAQP4 2D crystal lattice has tetramers closer together (a ϭ 69.0 Å), and contains both in-plane and betweenplane contacts of the latticed tetramers. Based on the molecular contacts in the crystal, the interaction between the short 3 10 helices in the C loop was proposed to be a possible mechanism for AQP4-mediated cell-cell adhesion (Fig. S2b) (13).…”

“…1F). Hiroaki et al (13) proposed from the rAQP4 electron diffraction structure that the 3 10 helix in the C loop (residues 139 to 142; Fig. S4) is the main region that provides for AQP4-mediated cell-cell adhesion (13).…”

“…Certain quaternary ammonium compounds (6), antiepileptic drugs (7,8), and serotonin receptor agonists (9) have been reported to inhibit AQP4 water transport in oocytes with IC 50 values down to the low micromolar range. However, Yang et al (10) report no inhibition up to 100 M using different assay methods. Therefore, the atomic resolution structure of human AQP4 provides the means of discovering and validating therapeutic agents that might diminish damage from stroke, tumor-associated edema, epilepsy, traumatic head injury, and other CNS disorders associated with brain water imbalance.…”

AqF026 Is a Pharmacologic Agonist of the Water Channel Aquaporin-1

“…11 However, the effectiveness of acetazolamide and antiepileptics as AQP blockers has been disputed. 12 The bumetanide derivative AqB013 (Aq, aquaporin ligand; B, bumetanide scaffold) at 20 mM blocks AQP1 and AQP4 by a mechanism thought to involve physical occlusion of the intracellular water channel pore. 11 No direct pharmacologic agonist of AQPs has previously been described.…”