Membrane proteins: Aquaporins--channels without ions

Artificial water channels: inspiration, progress, and challenges

Developing bioinspired artificial water channels may lead to the next-generation filtration membranes with ultra-high pore density and exclusive water permeability.

Cell Specificity of Aquaporins 0, 3, and 10 Expressed in the Testis, Efferent Ducts, and Epididymis of Adult Rats

Aquaporins (AQPs) are transmembrane protein channels that allow the rapid passage of water across an epithelium at a low energy requirement, though some also transport glycerol, urea, and solutes of various sizes. At present, 11 members of the AQP family of proteins have been described in mammals, with several being localized to the testis (AQP-7 and AQP-8), efferent ducts (AQP-1 and AQP-9), and epididymis (AQP-1 and AQP-9) of adult rats. With the discovery of expression of multiple AQPs in different tissues, we undertook a systematic analysis of several other members of the AQP family on Bouin-fixed tissues of the male reproductive tract employing light microscope immunocytochemistry. In the testis, AQP-0 expression in the seminiferous epithelium was restricted to Sertoli cells and to Leydig cells of the interstitial space; no reaction was observed in the efferent ducts or epididymis. In Sertoli cells, a semicircular pattern of staining was noted, with only one fourth or one half of the Sertoli cells of a given tubule showing a reaction product. Furthermore, while Sertoli cells at stages VI-VIII of the cycle showed intense staining, those at stages IX-XIV were least reactive, with Sertoli cells at stages I-V showing intermediate levels of reaction product. The epithelial expression of AQP-10 was restricted to the microvilli of the nonciliated cells and the cilia of the ciliated cells of the efferent ducts; however, the endothelial cells of vascular channels of the efferent ducts and epididymis were also intensely reactive. AQP-3 expression was localized exclusively to the epididymis, where intense staining was noted exclusively over basal cells. Examination of orchidectomized rats revealed that AQP-3 expression was abolished over basal cells and that it was greatly diminished after efferent duct ligation. As the reaction was not fully restored in orchidectomized animals supplemented with high levels of testosterone, we suggest that AQP-3 expression in basal cells is regulated in part by testosterone, in addition to a luminal factor emanating from the testis. Together, the data indicate a cell- and tissue-specific expression for AQP-0, AQP-3, and AQP-10 in the testis, efferent ducts, and epididymis, as well as differential regulating factors for the expression of AQP-3 in basal cells.

Effect of the position of constriction on water permeation across a single-walled carbon nanotube

The transportation of water across a cell membrane facilitated by water channel proteins is fundamental to the normal water metabolism in all forms of life. It is understood that the narrow region in a water channel is responsible for gating or selectivity. However, the influence of the position of the narrow region on water transportation is still not thoroughly understood. By choosing a single-walled carbon nanotube (SWNT) as a simplified model and using molecular dynamics simulation, we have found that the water flux through the nanotube would change significantly if the narrow location moves away from the middle region along the tube. Simulation results show that the flux reaches the maximum when the deformation occurs in the middle part of nanotube and decreases as the deformation location moves toward the ends of the nanotube. However, the decrease of water flux is not monotonic and the flux gets the minimum near the ends. These interesting phenomena can be explained in terms of water-water interactions and water-SWNT interactions. It can be concluded that the regulation of water transportation through nanopores depends sensitively on the location of the narrow region, and these findings are helpful in devising high flux nanochannels and nanofiltration as well.

Membrane domain specificity in the spatial distribution of aquaporins 5, 7, 9, and 11 in efferent ducts and epididymis of rats

Water content within the epididymis of the male reproductive system is stringently regulated to promote sperm maturation. Several members of the aquaporin (AQP) family of water channel-forming integral membrane proteins have been identified in epididymal cells, but expression profiling for this epithelium is presently incomplete, and no AQP isoform has yet been identified on basolateral plasma membranes of these cells. In this study, we explored AQP expression by RT-PCR and light microscopy immunolocalizations using peroxidase and wide-field fluorescence techniques. The results indicate that several AQPs are coexpressed in the epididymis including AQP 5, 7, 9, and 11. Immunolocalizations suggested complex patterns in the spatial distribution of these AQPs. In principal cells, AQP 9 and 11 were present mainly on microvilli, whereas AQP 7 was localized primarily to lateral and then to basal plasma membranes in a region-specific manner. AQP 5 was also expressed regionally but was associated with membranes of endosomes. Additionally, AQPs were expressed by some but not all basal (AQP 7 and 11), clear (AQP 7 and 9), and halo (AQP 7 and 11) cells. These findings indicate unique associations of AQPs with specific membrane domains in a cell type- and region-specific manner within the epididymis of adult animals.

Topology of NGEP, a prostate-specific cell:cell junction protein widely expressed in many cancers of different grade level

New gene expressed in prostate (NGEP) is a prostate-specific polytopic membrane protein found at high concentrations at cell:cell contact regions. To determine if NGEP is a useful target for antibody-based therapy of prostate cancer, we performed an immunohistochemical analysis of 126 human prostate carcinoma samples using polyclonal anti-NGEP sera and found that 91% of the cancers express NGEP protein. To elucidate the topology of NGEP and guide the development of monoclonal antibodies (mAb) reacting with the extracellular regions of NGEP, a hemagglutinin epitope tag was inserted at several positions within the NGEP sequence. The tagged proteins were expressed in 293T cells and locations of the tags were determined by immunofluorescence in intact or permeabilized cells. The results indicate that NGEP contains eight transmembrane domains with both the NH(2) and COOH termini of NGEP located inside the cell. We produced mAb to three regions that are predicted to be intracellular based on the epitope tag data (amino acids 1-352, 441-501, and 868-933), and as predicted, the mAb only detected the protein in permeabilized cells. NGEP is a glycoprotein with predicted glycosylation sites at N809 and N824. When these residues were converted to glutamine, glycosylation was abolished, confirming that the residues are extracellular. Our findings on the expression and the orientation of the NGEP protein serve as an important framework for the development of mAb targeting the extracellular regions of NGEP that could be used for prostate cancer immunotherapy.

Bubbles, gating, and anesthetics in ion channels

We suggest that bubbles are the bistable hydrophobic gates responsible for the on-off transitions of single channel currents. In this view, many types of channels gate by the same physical mechanism-dewetting by capillary evaporation-but different types of channels use different sensors to modulate hydrophobic properties of the channel wall and thereby trigger and control bubbles and gating. Spontaneous emptying of channels has been seen in many simulations. Because of the physics involved, such phase transitions are inherently sensitive, unstable threshold phenomena that are difficult to simulate reproducibly and thus convincingly. We present a thermodynamic analysis of a bubble gate using morphometric density functional theory of classical (not quantum) mechanics. Thermodynamic analysis of phase transitions is generally more reproducible and less sensitive to details than simulations. Anesthetic actions of inert gases-and their interactions with hydrostatic pressure (e.g., nitrogen narcosis)-can be easily understood by actions on bubbles. A general theory of gas anesthesia may involve bubbles in channels. Only experiments can show whether, or when, or which channels actually use bubbles as hydrophobic gates: direct observation of bubbles in channels is needed. Existing experiments show thin gas layers on hydrophobic surfaces in water and suggest that bubbles nearly exist in bulk water.

Expression of aquaporins in the efferent ductules, sperm counts, and sperm motility in estrogen receptor-alpha deficient mice fed lab chow versus casein

Estrogens play an important role in the male reproductive tract, and this is especially so for the efferent ductules, where alpha-estrogen receptors (ERalpha) have been localized. Mice deficient in ERalpha (alphaERKO mice) are infertile, and the effect appears to be due in part to retention of water at the level of the efferent ductules. In the present study, we examined the consequences of ERalpha deletion on the distribution of certain aquaporins (AQPs), water protein channels, in the efferent ductules and on sperm numbers and motility. In addition, the effects of feeding mice a regular lab chow diet, which contains phytoestrogens, known to affect male reproductive tract functions, and a casein diet, which lacks phytoestrogens, were also assessed. Light microscope immunolocalizations of AQP-1 and AQP-9 revealed dramatic reduction and patchier staining in alphaERKO mice with distal areas of the efferent ductules being more affected than proximal areas. No other changes in immunolocalizations were noted as a consequence of diet. Computer-assisted sperm analyses demonstrated a 62% reduction in cauda epididymal sperm/ml in alphaERKO mice fed lab chow, whereas 87% fewer sperm/ml were observed in alphaERKO mice fed casein, suggesting an enhanced role for sperm production and concentration in a diet containing phytoestrogens. All sperm motility parameters were altered to some degree in alphaERKO mice fed lab chow. Alterations in sperm motility parameters were also detected, but were less dramatic in alphaERKO mice fed casein. These data suggest that the decrease in AQP expression in the efferent ductules of alphaERKO mice contributes in part to water retention in this tissue, eventually leading to backflow of water into the testis, with subsequent decreases in sperm concentration and motility. The data also suggest that phytoestrogens, which are present in regular lab chow, can influence the male reproductive tract with and without the presence of ERalpha, promoting efferent ductule and epididymal functions when ERalpha is expressed, but inhibiting these same functions when ERalpha is missing. Taken together the data underscore the importance of estrogens and ERalpha in maintaining sperm maturation and preventing male infertility.

What Makes an Aquaporin a Glycerol Channel? A Comparative Study of AqpZ and GlpF

The recent availability of high-resolution structures of two structurally highly homologous, but functionally distinct aquaporins from the same species, namely Escherichia coli AqpZ, a pure water channel, and GlpF, a glycerol channel, presents a unique opportunity to understand the mechanism of substrate selectivity in these channels. Comparison of the free energy profile of glycerol conduction through AqpZ and GlpF reveals a much larger barrier in AqpZ (22.8 kcal/mol) than in GlpF (7.3 kcal/mol). In either channel, the highest barrier is located at the selectivity filter. Analysis of substrate-protein interactions suggests that steric restriction of AqpZ is the main contribution to this large barrier. Another important difference is the presence of a deep energy well at the periplasmic vestibule of GlpF, which was not found in AqpZ. The latter difference can be attributed to the more pronounced structural asymmetry of GlpF, which may play a role in attracting glycerol.

Studies of proton translocations in biological systems: simulating proton transport in carbonic anhydrase by EVB-based models

Proton transport (PTR) processes play a major role in bioenergetics and thus it is important to gain a molecular understanding of these processes. At present the detailed description of PTR in proteins is somewhat unclear and it is important to examine different models by using well-defined experimental systems. One of the best benchmarks is provided by carbonic anhydrase III (CA III), because this is one of the few systems where we have a clear molecular knowledge of the rate constant of the PTR process and its variation upon mutations. Furthermore, this system transfers a proton between several water molecules, thus making it highly relevant to a careful examination of the "proton wire" concept. Obtaining a correlation between the structure of this protein and the rate of the PTR process should help to discriminate between alternative models and to give useful clues about PTR processes in other systems. Obviously, obtaining such a correlation requires a correct representation of the "chemistry" of PTR between different donors and acceptors, as well as the ability to evaluate the free energy barriers of charge transfer in proteins, and to simulate long-time kinetic processes. The microscopic empirical valence bond (Warshel, A., and R. M. Weiss. 1980. J. Am. Chem. Soc. 102:6218-6226; and Aqvist, J., and A. Warshel. 1993. Chem. Rev. 93:2523-2544) provides a powerful way for representing the chemistry and evaluating the free energy barriers, but it cannot be used with the currently available computer times in direct simulation of PTR with significant activation barriers. Alternatively, one can reduce the empirical valence bond (EVB) to the modified Marcus' relationship and use semimacroscopic electrostatic calculations plus a master equation to determine the PTR kinetics (Sham, Y., I. Muegge, and A. Warshel. 1999. Proteins. 36:484-500). However, such an approximation does not provide a rigorous multisite kinetic treatment. Here we combine the useful ingredients of both approaches and develop a simplified EVB effective potential that treats explicitly the chain of donors and acceptors while considering implicitly the rest of the protein/solvent system. This approach can be used in Langevin dynamics simulations of long-time PTR processes. The validity of our new simplified approach is demonstrated first by comparing its Langevin dynamics results for a PTR along a chain of water molecules in water to the corresponding molecular dynamics simulations of the fully microscopic EVB model. This study examines dynamics of both models in cases of low activation barriers and the dependence of the rate on the energetics for cases with moderate barriers. The study of the dependence on the activation barrier is next extended to the range of higher barriers, demonstrating a clear correlation between the barrier height and the rate constant. The simplified EVB model is then examined in studies of the PTR in carbonic anhydrase III, where it reproduces the relevant experimental results without the use of any parameter that is specifically adjusted to fit the energetics or dynamics of the reaction in the protein. We also validate the conclusions obtained previously from the EVB-based modified Marcus' relationship. It is concluded that this approach and the EVB-based model provide a reliable, effective, and general tool for studies of PTR in proteins. Finally in view of the behavior of the simulated result, in both water and the CA III, we conclude that the rate of PTR in proteins is determined by the electrostatic energy of the transferred proton as long as this energy is higher than a few kcal/mol.

Realistic Simulations of Proton Transport along the Gramicidin Channel: Demonstrating the Importance of Solvation Effects

The nature of proton transduction (PTR) through a file of water molecules, along the gramicidin A (gA) channel, has long been considered as being highly relevant to PTR in biological systems. Previous attempts to model this process implied that the so-called Grotthuss mechanism and the corresponding orientation of the water file plays a major role. The present work reexamines the PTR in gA by combining a fully microscopic empirical valence bond (EVB) model and a recently developed simplified EVB-based model with Langevin dynamics (LD) simulations. The full model is used first to evaluate the free energy profile for a stepwise PTR process. The corresponding results are then used to construct the effective potential of the simplified EVB. This later model is then used in Langevin dynamics simulations, taking into account the correct physics of possible concerted motions and the effect of the solvent reorganization. The simulations reproduce the observed experimental trend and lead to a picture that is quite different from that assumed previously. It is found that the PTR in gA is controlled by the change in solvation energy of the transferred proton along the channel axis. Although the time dependent electrostatic fluctuations of the channel and water dipoles play their usual role in modulating the proton-transfer process (Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 444), the PTR rate is mainly determined by the free energy profile. Furthermore, the energetics of the reorientation of the unprotonated water file do not appear to provide a consistent way of assessing the activation barrier for the PTR process. It seems to us that in the case of gA, and probably other systems with significant electrostatic barriers for the transfer of the proton charge, the PTR rate is controlled by the electrostatic barrier. This finding has clear consequences with regards to PTR processes in biological systems.

Fluctuation-driven molecular transport through an asymmetric membrane channel

Channel proteins that selectively conduct molecules across cell membranes often exhibit an asymmetric structure. By means of a stochastic model, we argue that channel asymmetry in the presence of nonequilibrium fluctuations, fueled by the cell's metabolism as observed recently, can dramatically influence the transport through such channels by a ratchetlike mechanism. For an aquaglyceroporin that conducts water and glycerol, we show that a previously determined asymmetric glycerol potential leads to enhanced inward transport of glycerol, but for unfavorably high glycerol concentrations also to enhanced outward transport that protects a cell against poisoning.

A new effector pathway links ATM kinase with the DNA damage response

The related kinases ATM (ataxia-telangiectasia mutated) and ATR (ataxia-telangiectasia and Rad3-related) phosphorylate a limited number of downstream protein targets in response to DNA damage. Here we report a new pathway in which ATM kinase signals the DNA damage response by targeting the transcriptional cofactor Strap. ATM phosphorylates Strap at a serine residue, stabilizing nuclear Strap and facilitating formation of a stress-responsive co-activator complex. Strap activity enhances p53 acetylation, and augments the response to DNA damage. Strap remains localized in the cytoplasm in cells derived from ataxia telangiectasia individuals with defective ATM, as well as in cells expressing a Strap mutant that cannot be phosphorylated by ATM. Targeting Strap to the nucleus reinstates protein stabilization and activates the DNA damage response. These results indicate that the nuclear accumulation of Strap is a critical regulator in the damage response, and argue that this function can be assigned to ATM through the DNA damage-dependent phosphorylation of Strap.

What really prevents proton transport through aquaporin? Charge self-energy versus proton wire proposals

The nature of the control of water/proton selectivity in biological channels is a problem of a fundamental importance. Most studies of this issue have proposed that an interference with the orientational requirements of the so-called proton wire is the source of selectivity. The elucidation of the structures of aquaporins, which have evolved to prevent proton transfer (PT), provided a clear benchmark for exploring the selectivity problem. Previous simulations of this system have not examined, however, the actual issue of PT, but only considered the much simpler task of the transfer of water molecules. Here we take aquaporin as a benchmark and quantify the origin of the water/proton selectivity in this and related systems. This is done by evaluating in a consistent way the free energy profile for transferring a proton along the channel and relating this profile to the relevant PT rate constants. It is found that the water/proton selectivity is controlled by the change in solvation free energy upon moving the charged proton from water to the channel. The reason for the focus on the elegant concept of the proton wire and the related Grotthuss-type mechanism is also considered. It is concluded that these mechanisms are clearly important in cases with flat free energy surfaces (e.g., in bulk water, in gas phase water chains, and in infinitely long channels). However, in cases of biological channels, the actual PT mechanism is much less important than the energetics of transferring the proton charge from water to different regions in the channels.

Aquaporin proteins in murine trophectoderm mediate transepithelial water movements during cavitation

Mammalian blastocyst formation is dependent on establishment of trophectoderm (TE) ion and fluid transport mechanisms. We have examined the expression and function of aquaporin (AQP) water channels during murine preimplantation development. AQP 3, 8, and 9 proteins demonstrated cell margin-associated staining starting at the 8-cell (AQP 9) or compacted morula (AQP 3 and 8) stages. In blastocysts, AQP 3 and 8 were detected in the basolateral membrane domains of the trophectoderm, while AQP3 was also observed in cell margins of all inner cell mass (ICM) cells. In contrast, AQP 9 was predominantly observed within the apical membrane domains of the TE. Murine blastocysts exposed to hyperosmotic culture media (1800 mOsm; 10% glycerol) demonstrated a rapid volume decrease followed by recovery to approximately 80% of initial volume over 5 min. Treatment of blastocysts with p-chloromercuriphenylsulfonic acid (pCMPS, > or =100 microM) for 5 min significantly impaired (P < 0.05) volume recovery, indicating the involvement of AQPs in fluid transport across the TE. Blastocysts exposure to an 1800-mOsm sucrose/KSOMaa solution did not demonstrate volume recovery as observed following treatment with glycerol containing medium, indicating glycerol permeability via AQPs 3 and 9. These findings support the hypothesis that aquaporins mediate trans-trophectodermal water movements during cavitation.

Marked heterogeneity of human skeletal muscle lipolysis at rest

In this study, variations in lipolysis among different muscle groups were examined by measuring local net glycerol release in vivo in healthy, normal-weight subjects (n = 11) during rested, postabsorptive conditions. Microdialysis of the gastrocnemius, deltoid, and vastus lateralis muscle regions revealed that extracellular glycerol concentrations of these three muscle regions were 84.7 +/- 6.7, 59.7 + 7.3, and 56.4 +/- 7.5 micro mol/l, respectively, and the arterial plasma glycerol concentration was 44.8 +/- 2.3 micro mol/l (P = 0.0003-0.006, gastrocnemius vs. others). Local tissue blood flow, as measured by Xe clearance, did not differ among the regions. Net glycerol release was significantly higher in gastrocnemius muscle than in the two other regions. There were no regional differences in glycerol uptake when studied during glycerol infusion. Gastrocnemius muscle showed a dominance of type 1 fibers (70%), whereas the vastus lateralis muscle had equal distribution of fiber types (P = 0.02). No differences in intramuscular triaclyceride content, perimuscular fat, or the adipocyte-specific protein perilipin were observed among the muscle regions. Triglyceride turnover in the gastrocnemius muscle was 3.3 + 1.4% over 24 h, which is about 10 times more rapid than the turnover rate in subcutaneous adipose tissue (P < 0.01). Thus there were marked differences in lipolytic activity among skeletal muscle groups at rest, possibly reflecting variations in fiber type.

Water transporters: how so fast yet so selective?

A high-resolution X-ray structure of an aquaporin has revealed water molecules bound within the transmembrane pore and provided new clues to the mechanisms of rapid water transport and high selectivity in this important class of membrane proteins.

Characterization of plant proteins that interact with cowpea mosaic virus '60K' protein in the yeast two-hybrid system

Cowpea mosaic virus (CPMV) replication occurs in close association with small membranous vesicles in the host cell. The CPMV RNA1-encoded 60 kDa nucleotide-binding protein ('60K') plays a role in the formation of these vesicles. In this study, five cellular proteins were identified that interacted with different domains of 60K using a yeast two-hybrid search of an Arabidopsis cDNA library. Two of these host proteins (termed VAP27-1 and VAP27-2), with high homology to the VAP33 family of SNARE-like proteins from animals, interacted specifically with the C-terminal domain of 60K and upon transient expression colocalized with 60K in CPMV-infected cowpea protoplasts. eEF1-beta, picked up using the central domain of 60K, was also found to colocalize with 60K. The possible role of these host proteins in the viral replicative cycle is discussed.