Uphill Transport Research Articles

Uphill directional passive transport of water droplets on axisymmetric surfaces

Spontaneous directional transport of droplets by a surface curvature gradient, adopted by many biological species such as cactus and sand moss, is particularly suitable for applications including anti-icing, self-cleaning, and water harvesting, which eliminates the need for external energy input. However, this directional droplet transport is limited to short transport distance and no maneuverability, i.e., droplets can only migrate toward a flatter region and gradually stop. Fixed structures that can regulate droplet movement, if they could be created, would significantly advance their applications in a variety of areas. In this work, we propose a method to regulate the spontaneous motion of droplets on solid surfaces using surface curvature gradients. Molecular dynamics simulations show that droplets on general bowl-shaped axisymmetric surfaces can travel in the uphill direction (from the base to the apex) and move continuously to the apex with almost a constant speed. The mechanisms governing opposite directional transport of droplets are explained, and the conditions required to guarantee the transport in the reversed direction are discussed.

Arabidopsis HAK5 under low K+ availability operates as PMF powered high-affinity K+ transporter

Plants can survive in soils of low micromolar potassium (K+) concentrations. Root K+ intake is accomplished by the K+ channel AKT1 and KUP/HAK/KT type high-affinity K+ transporters. Arabidopsis HAK5 mutants impaired in low K+ acquisition have been identified already more than two decades ago, the molecular mechanism, however, is still a matter of debate also because of lack of direct measurements of HAK5-mediated K+ currents. When we expressed AtHAK5 in Xenopus oocytes together with CBL1/CIPK23, no inward currents were elicited in sufficient K+ media. Under low K+ and inward-directed proton motive force (PMF), the inward K+ current increased indicating that HAK5 energetically couples the uphill transport of K+ to the downhill flux of H+. At extracellular K+ concentrations above 25 μM, the initial rise in current was followed by a concentration-graded inactivation. When we replaced Tyr450 in AtHAK5 to Ala the K+ affinity strongly decreased, indicating that AtHAK5 position Y450 holds a key for K+ sensing and transport. When the soil K+ concentration drops toward the range that thermodynamically cannot be covered by AKT1, the AtHAK5 K+/H+ symporter progressively takes over K+ nutrition. Therefore, optimizing K+ use efficiency of crops, HAK5 could be key for low K+ tolerant agriculture.

Probing the Chemical Space of Guanidino-Carboxylic Acids to Identify the First Blockers of the Creatine-Transporter-1.

The creatine transporter-1 (CRT-1/SLC6A8) maintains the uphill transport of creatine into cells against a steep concentration gradient. Cellular creatine accumulation is required to support the ATP-buffering by phosphocreatine. More than 60 compounds have been explored in the past for their ability to inhibit cellular creatine uptake, but the number of active compounds is very limited. Here, we show that all currently known inhibitors are full alternative substrates. We analyzed their structure-activity relation for inhibition of CRT-1 to guide a rational approach to the synthesis of novel creatine transporter ligands. Measurements of both, inhibition of [3H]creatine uptake and transport associated currents, allowed for differentiating between full and partial substrates and true inhibitors. This combined approach led to a refined understanding of the structural requirements for binding to CRT-1, which translated into the identification of three novel compounds - i.e. compound 1 (2-(N-benzylcarbamimidamido)acetic acid), and MIPA572 (=carbamimidoylphenylalanine) and MIPA573 (=carbamimidoyltryptophane) that blocked CRT-1 transport, albeit with low affinity. In addition, we found two new alternative full substrates, namely MIP574 (carbamimidoylalanine) and GiDi1257 (1-carbamimidoylazetidine-3-carboxylic acid), which was superior in affinity to all known CTR-1 ligands, and one partial substrate, namely GiDi1254 (1-carbamimidoylpiperidine-4-carboxylic acid). Significance Statement The creatine transporter-1 (CRT-1) is required to maintain intracellular creatine levels. Inhibition of CRT-1 has been recently proposed as a therapeutic strategy for cancer, but pharmacological tools are scarce. In fact, all available inhibitors are alternative substrates. We tested existing and newly synthesized guanidinocarboxylic acids for CRT-1 inhibition and identified three blockers, one partial and two full substrates of CRT-1. Our results support a refined structural understanding of ligand binding to CRT-1 and provide a proof-of-principle for blockage of CRT-1.

Dynamic role of GlyT1 as glycine sink or source: Pharmacological implications for the gain control of NMDA receptors

Glycine transporter 1 (GlyT1) mediates the termination of inhibitory glycinergic receptor signaling in the spinal cord and brainstem, and is also present diffusely in the forebrain. Here, it regulates the ambient glycine concentration and influences the ‘glycine’ site occupancy of N-methyl-d-aspartate receptors (NMDARs). GlyT1 is a reversible transporter with a substantial, but not excessive, sodium-motive force for uphill transport. This study investigates its role as a potential source of glycine supply, either by reverse uptake or heteroexchange. Indeed, glutamate alone does not induce NMDAR current in “naive” oocytes co-expressing GluN1/GluN2A and GlyT1, a previously characterized cellular model. However, after substantial intracellular glycine accumulation, GlyT1 reverses its transport mode, and begins to release glycine into the external compartment, allowing NMDAR activation by glutamate alone. These uptake-dependent glutamate currents were blocked by ALX-5407 and potentiated by sarcosine, a specific inhibitor and substrate of GlyT1, respectively, suggesting a higher occupancy of the co-agonist site when GlyT1 functions as a glycine source either by reversed-uptake or by heteroexchange. These two glycine release mechanisms can be distinguished by their voltage dependence, as the reversed-uptake cycle decreases at hyperpolarized potentials, whereas heteroexchange electroneutrality preserves glycine efflux and NMDAR activation at these potentials. These results establish GlyT1-mediated efflux as a positive regulator of NMDAR coagonist site occupancy, and demonstrate the efficacy of sarcosine heteroexchange in enhancing coagonist site occupancy. Because NMDAR facilitation by GlyT1-inhibitors and sarcosine relies on different transport mechanisms, their actions may be a source of variability in reversing NMDAR hypofunction in schizophrenia.

XPR1: a regulator of cellular phosphate homeostasis rather than a Pi exporter

Phosphate (Pi) is an essential nutrient, and its plasma levels are under tight hormonal control. Uphill transport of Pi into cells is mediated by the two Na-dependent Pi transporter families SLC34 and SLC20. The molecular identity of a potential Pi export pathway is controversial, though XPR1 has recently been suggested by Giovannini and coworkers to mediate Pi export. We expressed XPR1 in Xenopus oocytes to determine its functional characteristics. Xenopus isoforms of proteins were used to avoid species incompatibility. Protein tagging confirmed the localization of XPR1 at the plasma membrane. Efflux experiments, however, failed to detect translocation of Pi attributable to XPR1. We tested various counter ions and export medium compositions (pH, plasma) as well as potential protein co-factors that could stimulate the activity of XPR1, though without success. Expression of truncated XPR1 constructs and individual domains of XPR1 (SPX, transmembrane core, C-terminus) demonstrated downregulation of the uptake of Pi mediated by the C-terminal domain of XPR1. Tethering the C-terminus to the transmembrane core changed the kinetics of the inhibition and the presence of the SPX domain blunted the inhibitory effect. Our observations suggest a regulatory role of XPR1 in cellular Pi handling rather than a function as Pi exporter. Accordingly, XPR1 senses intracellular Pi levels via its SPX domain and downregulates cellular Pi uptake via the C-terminal domain. The molecular identity of a potential Pi export protein remains therefore elusive.

Photosynthetic-Membrane-Like Ion Translocation in Visible-Light-Harvesting Nanofluidic Channels.

The selective uphill and downhill movement of protons in and out of photosynthetic membrane enabled by ion pumps and ion channels is key to photosynthesis. Reproducing the functions of photosynthetic membranes in artificial systems has been a persistent goal. Here, a visible-light-harvesting nanofluidic channels is reported which experimentally demonstrates the ion translocation functions of photosynthetic membranes. A molecular junction consisting of photosensitive ruthenium complexes linked to TiO2 electron acceptors forms the reaction centers in the nanofluidic channels. The visible-light-triggered vectorial electron injection into TiO2 establishes a difference in transmembrane potential across the channels, which enables uphill transport of ions against a 5-fold concentration gradient. In addition, the asymmetric charge distribution across the channels enables the unidirectional downhill movement of ions, demonstrating an ion rectification effect with a ratio of 18:1. This work, for the first time, mimics both the uphill and downhill ion translocation functions of photosynthetic membranes, which lays a foundation for nanofluidic energy conversion.

Surface-Modified Pore-Filled Anion-Exchange Membranes for Efficient Energy Harvesting via Reverse Electrodialysis.

In this study, novel pore-filled anion-exchange membranes (PFAEMs) modified with polypyrrole (PPy) and reduced graphene oxide (rGO) were developed to improve the energy harvesting performance of reverse electrodialysis (RED). The surface-modified PFAEMs were fabricated by varying the contents of PPy and rGO through simple spin coating and chemical/thermal treatments. It was confirmed that the PPy and PPy/rGO layers introduced on the membrane surface did not significantly increase the electrical resistance of the membrane and could effectively control surface characteristics, such as structural tightness, hydrophilicity, and electrostatic repulsion. The PPy/rGO-modified PFAEM showed excellent monovalent ion selectivity, more than four times higher than that of the commercial membrane (AMX, Astom Corp., Tokyo, Japan). This means that the PPy/rGO layer can effectively reduce the permeation of multivalent ions with a high charge intensity and a relatively large hydration radius compared to monovalent ions. The results of evaluating the performance of the surface-modified PFAEMs by applying them to a RED cell revealed that the decrease in potential difference occurring in the membrane was reduced by effectively suppressing the uphill transport of multivalent ions. Consequently, the PPy/rGO-modified membrane exhibited a 5.43% higher power density than the AMX membrane.

Sandwich-structured covalent organic framework membranes for selective sodium ion transport

Reverse electrodialysis (RED) is a promising renewable energy technology for salinity gradient power generation, while its key component (i.e., ion exchange membranes) faces the challenge of the lower energy density caused by the uphill transport of divalent ions (e.g., Mg2+). In this study, we carried out a series of molecular dynamics (MD) simulations to investigate the permselective transport of Na+ and Mg2+ through a novel sandwich-stacked hybrid covalent organic framework (COF) membrane. The alternate set of TpHz and TpPa-SO3H constructed reduced effective pore size ranging from 5.6 to 7.5 Å, producing an offset-block effect on the ion transport. Besides, the tunable interlayer spacing of the hybrid COF membrane could manipulate the rehydration/dehydration behavior of the confined Na+ ions. A suitable set of the interlayer spacing could achieve an adorable Na+/Mg2+ permselectivity and Na+ permeance by the synergistic effect of aperture sieving and the electrostatic affinity. This work provides useful insights into the Na+/Mg2+ permselective mechanism and further helps to design COF-based membranes for realizing a high energy efficiency for the application of RED.

Sustainable power generation from salinity gradients by reverse electrodialysis: Influence of divalent ions

The performance of Reverse Electrodialysis (RED) stacks can be significantly degraded by the presence of divalent ions. To gain a deeper understanding of the influence of divalent ions, a RED model based on the Nernst-Planck framework was developed in this study. Using this model, a simulation study was performed when the feed solutions contain a mixture of NaCl and MgCl2. The impact of the divalent ion Mg2+ on RED performance was examined from the perspective of ion transport. The results indicate that divalent counterions decrease membrane permselectivity and membrane potential, while increasing membrane electrical resistance by affecting ion concentrations within the membrane. The uphill transport of divalent ions occurs only when the molar fraction of divalent ions in the feed solutions is relatively low. Specifically, when the feed solution contains only MgCl2 compared to only NaCl, the open-circuit voltage decreases by 36.2%, and the maximum gross power density decreases by 44.8%.

The Switch and Reciprocating Models for the Function of ABC Multidrug Exporters: Perspectives on Recent Research.

ATP-binding cassette (ABC) transporters comprise a large superfamily of primary active transporters, which are integral membrane proteins that couple energy to the uphill vectorial transport of substrates across cellular membranes, with concomitant hydrolysis of ATP. ABC transporters are found in all living organisms, coordinating mostly import in prokaryotes and export in eukaryotes. Unlike the highly conserved nucleotide binding domains (NBDs), sequence conservation in the transmembrane domains (TMDs) is low, with their divergent nature likely reflecting a need to accommodate a wide range of substrate types in terms of mass and polarity. An explosion in high resolution structural analysis over the past decade and a half has produced a wealth of structural information for ABCs. Based on the structures, a general mechanism for ABC transporters has been proposed, known as the Switch or Alternating Access Model, which holds that the NBDs are widely separated, with the TMDs and NBDs together forming an intracellular-facing inverted "V" shape. Binding of two ATPs and the substrate to the inward-facing conformation induces a transition to an outward conformation. Despite this apparent progress, certainty around the transport mechanism for any given ABC remains elusive. How substrate binding and transport is coupled to ATP binding and hydrolysis is not known, and there is a large body of biochemical and biophysical data that is at odds with the widely separated NBDs being a functional physiological state. An alternative Constant Contact model has been proposed in which the two NBSs operate 180 degrees out of phase with respect to ATP hydrolysis, with the NBDs remaining in close proximity throughout the transport cycle and operating in an asymmetric allosteric manner. The two models are discussed in the light of recent nuclear magnetic resonance and hydrogen-deuterium exchange mass spectrometry analyses of three ABC exporters.

Scanning Kelvin probe force microscopy study on the behavior of hydrogen transport by mobile dislocations in single crystal nickel

There was always a controversy about if the mobile dislocations generated during plastic deformation could transport hydrogen atoms. In this study, the behavior of dislocation migration hydrogen was directly observed by combining the scanning Kelvin probe force microscopy (SKPFM) technique and slow tensile test. The quantitative analysis results revealed that the behavior of dislocation transport hydrogen could not accelerate hydrogen atom migration, while could achieve the uphill transport of hydrogen atoms. In addition, in this study, SKPFM has been confirmed to be a powerful tool for measuring hydrogen diffusion coefficient in materials with a high accuracy.

Concepts and Misconceptions Concerning the Influence of Divalent Ions on the Performance of Reverse Electrodialysis Using Natural Waters.

Divalent ions have a negative effect on the obtained power and efficiency of the reverse electrodialysis (RED) process when using natural waters. These effects can largely be attributed to the interaction between the various ions and the membranes, resulting in a decreased membrane voltage, an increased membrane resistance, and uphill transport of divalent ions. The aim of this study was to investigate the causes of these differences and, if possible, to find underlying causes. The approach mainly followed that in literature articles that specifically focused on the effect of divalent ions on RED. It transpired that seven publications were useful because the methodology was well described and sufficient data was published. I found two widely shared misconceptions. The first concerns the role of the stack voltage in uphill transport of divalent ions; itis often thought that the open circuit voltage (OCV) must be taken into account, but it is plausible that the voltage under working conditions is the critical factor. The second debatable point concerns the methodology used to make a series of solutions to study the effect of divalent ions. Typically, solutions with a constant number of moles of salt are used; however, it is better to make a series with a constant ratio of equivalents of those salts. Moreover, it is plausible that the decreased voltage can be explained by the inherently lower Donnan potential of multi-charged ions and that increased resistance is caused by the fact that divalent ions-with a lower mobility there than the monovalent ions-occupy relatively much of the available space in the gel phase of the membrane. While both resistance and voltage play a decisive role in RED and probably also in other membrane processes like electrodialysis (ED), it is remarkable that there are so few publications that focus on measurements on individual membranes. The implications of these results is that research on the effect of divalent ions in RED, ED and similar processes needs to be more structured in the future. Relatively simple procedures can be developed for the determination of membrane resistance in solutions of mixtures of mono- and divalent salts. The same applies to determining the membrane potential. The challenge is to arrive at a standard method for equipment, methodology, and the composition of the test solutions.

Up-Hill Transport of Tryptophan through Polymer Inclusion Membrane Containing Aliquat 336

Up-Hill Transport of Tryptophan through Polymer Inclusion Membrane Containing Aliquat 336

Hybrid Technique for Removal of Arsenic from Drinking Water

AbstractAmalgamation of a supported liquid membrane (SLM) and electrocoagulation are carried out to remove As(III) and As(V) from drinking water to achieve a high treatment efficiency. The process is based on uphill transport of arsenic ions against their concentration gradients across phases and has the advantage of simultaneous extraction and recovery followed by electrocoagulation of arsenic. The aim of the process is selective separation of arsenic from drinking water and to reduce its concentration below 10 ppb. An iron‐arsenic precipitate is obtained by using an iron anode and the said precipitate is a value‐added saleable product. Various adsorption isotherms are evaluated to understand the removal mechanism of arsenic by the formation of iron‐arsenic precipitates. The Freundlich isotherm and Dubinin‐Radushkevich model are found to fit well with the adsorption data of the removal of arsenic ions.

Structural mechanism of SGLT1 inhibitors

Sodium glucose co-transporters (SGLT) harness the electrochemical gradient of sodium to drive the uphill transport of glucose across the plasma membrane. Human SGLT1 (hSGLT1) plays a key role in sugar uptake from food and its inhibitors show promise in the treatment of several diseases. However, the inhibition mechanism for hSGLT1 remains elusive. Here, we present the cryo-EM structure of the hSGLT1-MAP17 hetero-dimeric complex in the presence of the high-affinity inhibitor LX2761. LX2761 locks the transporter in an outward-open conformation by wedging inside the substrate-binding site and the extracellular vestibule of hSGLT1. LX2761 blocks the putative water permeation pathway of hSGLT1. The structure also uncovers the conformational changes of hSGLT1 during transitions from outward-open to inward-open states.

Influences of Divalent Ions in Natural Seawater/River Water on Nanofluidic Osmotic Energy Generation.

Besides the dominant NaCl, natural seawater/river water contains trace multivalent ions, which can provide effective screening of surface charges. Here, in both negatively and positively charged nanopores, influences from divalent ions as counterions and co-ions have been investigated with respect to the performance of osmotic energy conversion (OEC) under natural salt gradients. As counterions, trace Ca2+ ions can suppress the electric power and conversion efficiency significantly. The reduced OEC performance is due to the bivalence and low diffusion coefficient of Ca2+ ions instead of the uphill transport of divalent ions discovered in the previous work. Effectively screened charged surfaces by Ca2+ ions induce an enhanced diffusion of Cl- ions which simultaneously decreases the net ion penetration and ionic selectivity of the nanopore. As co-ions, Ca2+ ions have weak effects on the OEC performance. The promotion from charged exterior surfaces in OEC processes for ultrashort nanopores is also studied, with an effective region of ∼200 nm in width beyond pore boundaries independent of the presence of Ca2+ ions. Our results shed light on the physical details of the nanofluidic OEC process under natural seawater/river water conditions, which can provide a useful guide for high-performance osmotic energy harvesting.

Scaled-up multistage reverse electrodialysis pilot study with natural waters

A multistage reverse electrodialysis system was studied at the REDstack research facility (the Afsluitdijk, the Netherlands) for over 30 days to describe the performance of such configuration under natural water conditions. The experiments were done with two 0.22 × 0.22 m2 stacks in series comprising 32 cell pairs (3.1 m2 of membrane area) for stage 1 and 64 cell pairs (6.2 m2 membrane area) for stage 2. The total gross power density at the available salinity gradient was stable at around 0.35 W∙m−2. The total net power density, corrected for the initial pressure drop of the stacks, was 0.25 W∙m−2 at an energy efficiency of 37 %. Throughout the operation, due to increased stack pressure drop, the actual total net power density lowered to 0.1 W∙m−2. A distinct behaviour was found for multivalent ions in each stage. For stage 1, Ca2+ and SO42− were transported from the river water to the seawater side, so-called uphill transport. For stage 2, uphill transport was not found, in line with Donnan potential calculations. Stack autopsy revealed microorganisms with sizes ten times larger than the cartridge filter nominal pore size (5 µm) and biofilm covering part of the spacer open area, both contributing to the increasing pressure drop in the stacks. This study showed that stable gross power densities and high energy efficiencies were obtained from feeding natural waters to a multistage reverse electrodialysis system, independent of fouling. In addition, it emphasized the importance of maintaining pumping power losses low for a viable deployment of the technology.

Analysis of anomalous transport based on radial fractional diffusion equation

Anomalous transport in magnetically confined plasmas is investigated by radial fractional transport equations. It is shown that for fractional transport models, hollow density profiles are formed and uphill transports can be observed regardless of whether the fractional diffusion coefficients (FDCs) are radially dependent or not. When a radially dependent FDC is imposed, compared with the case under it is observed that the position of the peak of the density profile is closer to the core. Further, it is found that when FDCs at the positions of source injections increase, the peak values of density profiles decrease. The non-local effect becomes significant as the order of fractional derivative and causes the uphill transport. However, as the fractional diffusion model returns to the standard model governed by Fick’s law.

Organoplatinum Compounds as Anion‐Tuneable Uphill Hydroxide Transporters

AbstractActive transport of ions uphill, creating a concentration gradient across a cell membrane, is essential for life. It remains a significant challenge to develop synthetic systems that allow active uphill transport. Here, a transport process fuelled by organometallic compounds is reported that creates a pH gradient. The hydrolysis reaction of PtII complexes results in the formation of aqua complexes that established rapid transmembrane movement (“flip‐flop”) of neutral Pt−OH species, leading to protonation of the OH group in the inner leaflet, generating OH− ions, and so increasing the pH in the intravesicular solution. The organoplatinum complex effectively transports bound hydroxide ions across the membrane in a neutral complex. The initial net flow of the PtII complex into the vesicles generates a positive electric potential that can further drive uphill transport because the electric potential is opposed to the chemical potential of OH−. The OH− ions equilibrate with this transmembrane electric potential but cannot remove it due to the relatively low permeability of the charged species. As a result, effective hydroxide transport against its concentration gradient can be achieved, and multiple additions can continuously drive the generation of OH− against its concentration gradient up to ΔpH>2. Moreover, the external addition of different anions can control the generation of OH− depending on their anion binding affinity. When anions displayed very high binding affinities towards PtII compounds, such as halides, the external anions could dissipate the pH gradient. In contrast, a further pH increase was observed for weak binding anions, such as sulfate, due to the increase of positive electric potential.

Insights into CLC transport mechanism through single transporter flux measurements

CLC chloride-proton antiporters are part of a unique protein family that includes both secondary active transporters and ion channels. The dual-identity nature of this protein family makes it an ideal candidate for investigating the nature of coupling in transporters since the same protein fold, and nearly identical structures, result on one hand in transport proteins where multiple, tightly coupled ion movements allow thermodynamically uphill transport, but, on the other, in purely dissipative ion transfers catalyzed by ion channels.