Characterization of the Aquaporin-9 Inhibitor RG100204 In Vitro and in db/db Mice

Aquaporin-9 (AQP9) is a facilitator of glycerol and other small neutral solute transmembrane diffusion. Identification of specific inhibitors for aquaporin family proteins has been difficult, due to high sequence similarity between the 13 human isoforms, and due to the limited channel surface areas that permit inhibitor binding. The few AQP9 inhibitor molecules described to date were not suitable for in vivo experiments. We now describe the characterization of a new small molecule AQP9 inhibitor, RG100204 in cell-based calcein-quenching assays, and by stopped-flow light-scattering recordings of AQP9 permeability in proteoliposomes. Moreover, we investigated the effects of RG100204 on glycerol metabolism in mice. In cell-based assays, RG100204 blocked AQP9 water permeability and glycerol permeability with similar, high potency (~5 × 10-8 M). AQP9 channel blocking by RG100204 was confirmed in proteoliposomes. After oral gavage of db/db mice with RG100204, a dose-dependent elevation of plasma glycerol was observed. A blood glucose-lowering effect was not statistically significant. These experiments establish RG100204 as a direct blocker of the AQP9 channel, and suggest its use as an experimental tool for in vivo experiments on AQP9 function

The Aquaporin Splice Variant NbXIP1;1α Is Permeable to Boric Acid and Is Phosphorylated in the N-terminal Domain

Aquaporins (AQPs) are membrane channel proteins that transport water and uncharged solutes across different membranes in organisms in all kingdoms of life. In plants, the AQPs can be divided into seven different subfamilies and five of these are present in higher plants. The most recently characterized of these subfamilies is the XIP subfamily, which is found in most dicots but not in monocots. In this article, we present data on two different splice variants (α and β) of NbXIP1;1 from Nicotiana benthamiana. We describe the heterologous expression of NbXIP1;1α and β in the yeast Pichia pastoris, the subcellular localization of the protein in this system and the purification of the NbXIP1;1α protein. Furthermore, we investigated the functionality and the substrate specificity of the protein by stopped-flow spectrometry in P. pastoris spheroplasts and with the protein reconstituted in proteoliposomes. The phosphorylation status of the protein and localization of the phosphorylated amino acids were verified by mass spectrometry. Our results show that NbXIP1;1α is located in the plasma membrane when expressed in P. pastoris, that it is not permeable to water but to boric acid and that the protein is phosphorylated at several amino acids in the N-terminal cytoplasmic domain of the protein. A growth assay showed that the yeast cells expressing the N-terminally His-tagged NbXIP1;1α were more sensitive to boric acid as compared to the cells expressing the C-terminally His-tagged isoform. This might suggest that the N-terminal His-tag functionally mimics the phosphorylation of the N-terminal domain and that the N-terminal domain is involved in gating of the channel

Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

BACKGROUND: Aquaporins (AQPs) are integral membrane proteins that facilitate transport of water and/or other small neutral solutes across membranes in all forms of life. The X Intrinsic Proteins (XIPs) are the most recently recognized and the least characterized aquaporin subfamily in higher plants. XIP1s have been shown to be impermeable to water but permeable to boric acid, glycerol, hydrogen peroxide and urea. However, uncertainty regarding the determinants for selectivity and lack of an activity that is easy to quantify have hindered functional investigations. In an effort to resolve these issues, we set out to introduce water permeability in Nicotiana benthamiana XIP1;1α (NbXIP1;1α), by exchanging amino acid residues of predicted alternative aromatic/arginine (ar/R) selectivity filters of NbXIP1;1α for residues constituting the water permeable ar/R selectivity filter of AtTIP2;1.RESULTS: Here, we present functional results regarding the amino acid substitutions in the putative filters as well as deletions in loops C and D of NbXIP1;1α. In addition, homology models were created based on the high resolution X-ray structure of AtTIP2;1 to rationalize the functional properties of wild-type and mutant NbXIP1;1α. Our results favour Thr 246 rather than Val 242 as the residue at the helix 5 position in the ar/R filter of NbXIP1;1α and indicate that the pore is not occluded by the loops when heterologously expressed in Pichia pastoris. Moreover, our results show that a single amino acid substitution in helix 1 (L79G) or in helix 2 (I102H) is sufficient to render NbXIP1;1α water permeable. Most of the functional results can be rationalized from the models based on a combination of aperture and hydrophobicity of the ar/R filter.CONCLUSION: The water permeable NbXIP1;1α mutants imply that the heterologously expressed proteins are correctly folded and offer means to explore the structural and functional properties of NbXIP1;1α. Our results support that Thr 246 is part of the ar/R filter. Furthermore, we suggest that a salt bridge to an acidic residue in helix 1, conserved among the XIPs in clade B, directs the orientation of the arginine in the ar/R selectivity filter and provides a novel approach to tune the selectivity of AQPs

Additional file 5: Table S4. of Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

Results of unpaired t-test for comparisons indicated in Fig. 5a. (PDF 44 kb

Additional file 3: Table S2. of Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

Estimated protein amounts, rate constants and specific activities for the first set of NbXIP1;1α mutants. (PDF 88 kb

Additional file 2: Figure S1. of Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

Expression of N-terminally His-tagged NbXIP1;1α mutants in P. pastoris. Western blots showing the expression levels of N-terminally His-tagged NbXIP1;1 mutants in P. pastoris X-33 clones. Blots were developed by enhanced chemiluminiscence in a Syngene PXi touch instrument. a. The first set of NbXIP1;1 α mutants. NbXIP1;1αst is an N-terminally truncated construct of NbXIP1;1α used as control for the western blot. b. The second set of NbXIP1;1α mutants. The control is 2 μg of purified NbXIP1;1αwt. (PDF 207 kb

Additional file 6: Table S5. of Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

Results of unpaired t-test for comparisons indicated in Fig. 5b. (PDF 45 kb

Additional file 7: Figure S2. of Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

Cartoon representations of the homology models of NbXIP1;1αwt and NbXIP1;1α mutants. The pores in the models are shown in mesh representation from blue (widest) to red (narrowest). The HOLE program [38] was used to estimate the radius of the pore in the models. Starting from the top left to the bottom right; NbXIP1;1αwt, NbXIP1;1αL79G/I102H/V242I (mutant 1), NbXIP1;1αL79G/I102H/T246I (mutant 2), NbXIP1;1αL79G (mutant 3), NbXIP1;1αI102H (mutant 4), NbXIP1;1αV242I (mutant 5), NbXIP1;1αL79G/I102H (mutant 6), NbXIP1;1αI102H/V242I (mutant 7) and NbXIP1;1αL79G/V242I (mutant 8), respectively. See also Fig. 7. (PDF 187 kb