Structural Basis for Mutations of Human Aquaporins Associated to Genetic Diseases (original) (raw)
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Febs Lett, 2003
Although water is the major component of all biological £uids, the molecular pathways for water transport across cell membranes eluded identi¢cation until the discovery of the aquaporin family of water channels. The atomic structure of mammalian AQP1 illustrates how this family of proteins is freely permeated by water but not protons (hydronium ions, H 3 O +). De¢nition of the subcellular sites of expression predicted their physiological functions and potential clinical disorders. Analysis of several human disease states has con¢rmed that aquaporins are involved in multiple di¡erent illnesses including abnormalities of kidney function, loss of vision, onset of brain edema, starvation, and arsenic toxicity.
A refined structure of human aquaporin-1
FEBS Letters, 2001
A re ned structure of the human water channel aquaporin 1 AQP1 is presented. The model rests on a the high resolution x-ray structure of the homologous bacterial glycerol transporter GlpF, b electron-crystallographic data of AQP1 at 3.8 A resolution and c a multiple sequence alignment of the aquaporin superfamily. The computed crystallographic R and free R values 36.7 and 37.8 for the re ned structure are signi cantly lower in comparison to two previous AQP1 models. Together with improved geometrical normality scores and an enhanced stability in molecular dynamics simulations, this implicates a signi cant improvement of the AQP1 structure. Comparisons with the previous models of the AQP1 structure show signi cant di erences, not only in the loop regions where the experimental densities are relatively weak, but also in the transmembrane region facing the core of the water channel. Implications of these di erences for the structural integrity of the protein as well as for the water permeability are discussed.
The Hidden Intricacies of Aquaporins: Remarkable Details in a Common Structural Scaffold
Small
Evolution turned aquaporins (AQPs) into the most efficient facilitators of passive water flow through cell membranes at no expense of solute discrimination. In spite of a plethora of solved AQP structures, many structural details remain hidden. Here, by combining extensive sequence-and structural-based analysis of a unique set of 20 non-redundant high-resolution structures and molecular dynamics simulations of 4 representatives, we identify key aspects of AQP stability, gating, selectivity, pore geometry and oligomerization, with a potential impact on channel functionality. We challenge the general view of AQPs possessing a continuous open water pore and depict that AQPs selectivity is not exclusively shaped by pore lining residues but also by the relative arrangement of transmembrane helices. Moreover, our analysis reveals that hydrophobic interactions constitute the main determinant of protein thermal stability. Finally, we establish a novel numbering scheme of the conserved AQP scaffold facilitating direct comparison and prediction of potential structural effects of e.g. disease-causing mutations. Additionally, our results pave the way for the design of optimized AQP water channels to be utilized in biotechnological applications. 1. Introduction Aquaporins (AQPs), part of a larger family of major intrinsic proteins, are one of the best studied protein families with currently 20 non-redundant high-resolution structures (≤3,70 Å) solved. Since their first discovery in 1992 by Peter Agre and coworkers (1, 2), thirteen different types of aquaporins (AQP0-12) were discovered in mammals (3). The narrow AQP pores combine enormous permeability, conducting water in a single-file manner close to the diffusion limit of water in bulk, with exceptional selectivity (4). A subset of AQPs, the aquaglyceroporins (AQGPs), paralogs of AQPs, are also able to conduct glycerol and other small neutral solutes (5, 6). Bacteria also express members of AQPs and AQGPs, generally functioning with one copy of each paralog and, interestingly, some lacking both. Unicellular eukaryotes and fungi follow a similar pattern, with a clear division between AQPs or AQGPs and a heterogeneous distribution in the number of copies of each paralog in the different genera (7). So far, no archaea has been found that possesses both paralogs concurrently. Plants exhibit the highest AQP diversity, with five main subfamilies (plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin-26 like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) and X intrinsic proteins (XIPs)), which are each further divided into subgroups (7). Furthermore, in primitive plant species, two additional subfamilies, GLPF-like intrinsic proteins (GIPs) and hybrid intrinsic proteins (HIPs), have been found (8). However, the full diversity of AQ(G)Ps is still not represented by the numerous high-resolution structures, as exemplified by only three plant aquaporin structures and none of the unorthodox AQ(G)Ps (represented by AQP11 and 12 in mammals).
Biophysical Journal, 2013
Aquaporins are protein channels located across the cell membrane with the role of conducting water or other small sugar alcohol molecules (aquaglyceroporins). The high-resolution X-ray structure of the human aquaporin 5 (HsAQP5) shows that HsAQP5, as all the other known aquaporins, exhibits tetrameric structure. By means of molecular dynamics simulations we analyzed the role of spontaneous fluctuations on the structural behavior of the human AQP5. We found that different conformations within the tetramer lead to a distribution of monomeric channel structures, which can be characterized as open or closed. The switch between the two states of a channel is a tap-like mechanism at the cytoplasmic end which regulates the water passage through the pore. The channel is closed by a translation of the His67 residue inside the pore. Moreover, water permeation rate calculations revealed that the selectivity filter, located at the other end of the channel, regulates the flow rate of water molecules when the channel is open, by locally modifying the orientation of His173. Furthermore, the calculated permeation rates of a fully open channel are in good agreement with the reported experimental value.
Aquaporin water channels: molecular mechanisms for human diseases
FEBS Letters, 2003
Although water is the major component of all biological £uids, the molecular pathways for water transport across cell membranes eluded identi¢cation until the discovery of the aquaporin family of water channels. The atomic structure of mammalian AQP1 illustrates how this family of proteins is freely permeated by water but not protons (hydronium ions, H 3 O +). De¢nition of the subcellular sites of expression predicted their physiological functions and potential clinical disorders. Analysis of several human disease states has con¢rmed that aquaporins are involved in multiple di¡erent illnesses including abnormalities of kidney function, loss of vision, onset of brain edema, starvation, and arsenic toxicity.
Reconstitution of water channel function and 2D-crystallization of human aquaporin 8
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2012
Among the thirteen human aquaporins (AQP0-12), the primary structure of AQP8 is unique. By sequence alignment it is evident that mammalian AQP8s form a separate subfamily distinct from the other mammalian aquaporins. The constriction region of the pore determining the solute specificity deviates in AQP8 making it permeable to both ammonia and H 2 O 2 in addition to water. To better understand the selectivity and gating mechanism of aquaporins, high-resolution structures are necessary. So far, the structure of three human aquaporins (HsAQP1, HsAQP4, and HsAQP5) have been solved at atomic resolution. For mammalian aquaporins in general, high-resolution structures are only available for those belonging to the water-specific subfamily (including HsAQP1, HsAQP4 and HsAQP5). Thus, it is of interest to solve structures of other aquaporin subfamily members with different solute specificities. To achieve this the aquaporins need to be overexpressed heterologously and purified. Here we use the methylotrophic yeast Pichia pastoris as a host for the overexpression. A wide screen of different detergents and detergent-lipid combinations resulted in the solubilization of functional human AQP8 protein and in well-ordered 2D crystals. It also became evident that removal of amino acids constituting affinity tags was crucial to achieve highly ordered 2D crystals diffracting to 3 Å.
NPA motifs play a key role in plasma membrane targeting of aquaporin-4
IUBMB Life, 2010
The two highly conserved NPA motifs (asparagine-prolinealanine, NPA) are the most important structural domains that play a crucial role in water-selective permeation in aquaporin water channels. However, the functions of NPA motifs in aquaporin (AQP) biogenesis remain largely unknown. Few AQP members with variations in NPA motifs such as AQP11 and AQP12 do not express in the plasma membrane, suggesting an important role of NPA motifs in AQP plasma membrane targeting. In this study, we examined the role of the two NPA motifs in AQP4 plasma membrane targeting by mutagenesis. We constructed a series of AQP4 mutants with NPA deletions or single amino acid substitutions in AQP4-M1 and AQP4-M23 isoforms and analyzed their expression patterns in transiently transfected FRT and COS-7 cells. Western blot analysis showed similar protein bands of all the AQP4 mutants and the wild-type AQP4. AQP4 immunofluorescence indicated that deletion of one or both NPA motifs resulted in defective plasma membrane targeting, with apparent retention in endoplasmic reticulum (ER). The A99T mutant mimicking AQP12 results in ER retention, whereas the A99C mutant mimicking AQP11 expresses normally in plasma membrane. Furthermore, the AQP4-M1 but not the M23 isoform with P98A substitution in the first NPA motif can target to the plasma membrane, indicating an interaction of Nterminal sequence of AQP4-M1 with the first NPA motif. These results suggest that NPA motifs play a key role in plasma membrane expression of AQP4 but are not involved in AQP4 protein synthesis and degradation. The NPA motifs may interact with other structural domains in the regulation of membrane trafficking during aquaporin biogenesis. 2010 IUBMB IUBMB Life, 62(3): 222-226, 2010
Role of Aquaporins in Diseases and Drug Discovery
Aquaporins as water channels in transportation of water in and out of the cells due to their water permeability property play an important role in maintaining water's constancy which result in normal human physiology. Any mutation of the genes encoding them result in causing many diseases which are life threatening and dangerous like hyperinsulinemia,Sjogren's syndrome,vasogenic brain edema, glaucoma,nephrogenic diabetes insipidus, carcinoma ,lymph node metastatic carcinoma and tumor growth, etc . Recent studies and discoveries has shown that aquaporins have fundamental role in drug discovery and they serves as attractive targets for different diseases. Several types of aquaporins are discovered which play important role in different types of diseases and drug discovery .In this paper we focused on role of aquaporins , structure- function relationship , types of aquaporins and their related diseases and strategies for identification of modulators of these drug targets for discovery of novel therapies and recent discoveries on different types of aquaporins and introduction of them as attractive targets. These paper showed significant role of aquaporins in normal human physiology and pathophysiology and give insights for deserving attention for them to effectively treat some of life threatening diseases.
Human Aquaporin-4 and Molecular Modeling: Historical Perspective and View to the Future
International Journal of Molecular Sciences, 2016
Among the different aquaporins (AQPs), human aquaporin-4 (hAQP4) has attracted the greatest interest in recent years as a new promising therapeutic target. Such a membrane protein is, in fact, involved in a multiple sclerosis-like immunopathology called Neuromyelitis Optica (NMO) and in several disorders resulting from imbalanced water homeostasis such as deafness and cerebral edema. The gap of knowledge in its functioning and dynamics at the atomistic level of detail has hindered the development of rational strategies for designing hAQP4 modulators. The application, lately, of molecular modeling has proved able to fill this gap providing a breeding ground to rationally address compounds targeting hAQP4. In this review, we give an overview of the important advances obtained in this field through the application of Molecular Dynamics (MD) and other complementary modeling techniques. The case studies presented herein are discussed with the aim of providing important clues for computational chemists and biophysicists interested in this field and looking for new challenges.