The Sodium Glucose Cotransporter SGLT1 Is an Extremely Efficient Facilitator of Passive Water Transport

Approaching Ideal Selectivity with Bioinspired and Biomimetic Membranes

The applications of polymeric membranes have grown rapidly compared to traditional separation technologies due to their energy efficiency and smaller footprint. However, their potential is not fully realized due, in part, to their heterogeneity, which results in a "permeability-selectivity" trade-off for most membrane applications. Inspired by the intricate architecture and excellent homogeneity of biological membranes, bioinspired and biomimetic membranes (BBMs) aim to emulate biological membranes for practical applications. This Review highlights the potential of BBMs to overcome the limitations of polymeric membranes by utilizing the "division of labor" between well-defined permeable pores and impermeable matrix molecules seen in biological membranes. We explore the exceptional performance of membranes in biological organisms, focusing on their two major components: membrane proteins (biological channels) and lipid matrix molecules. We then discuss how these natural materials can be replaced with artificial mimics for enhanced properties and how macro-scale BBMs are developed. We highlight key demonstrations in the field of BBMs that draw upon the factors responsible for transport through biological membranes. Additionally, current state-of-the-art methods for fabrication of BBMs are reviewed with potential challenges and prospects for future applications. Finally, we provide considerations for future research that could enable BBMs to progress toward scale-up and enhanced applicability.

SWEET family transporters act as water conducting carrier proteins in plants

Dedicated water channels are involved in the facilitated diffusion of water molecules across the cell membrane in plants. Transporter proteins are also known to transport water molecules along with substrates, however the molecular mechanism of water permeation is not well understood in plant transporters. Here, we show plant sugar transporters from the SWEET (Sugar Will Eventually be Exported Transporter) family act as water-conducting carrier proteins via a variety of passive and active mechanisms that allow diffusion of water molecules from one side of the membrane to the other. This study provides a molecular perspective on how plant membrane transporters act as water carrier proteins, a topic that has not been extensively explored in literature. Water permeation in membrane transporters could occur via four distinct mechanisms which form our hypothesis for water transport in SWEETs. These hypothesis are tested using molecular dynamics simulations of the outward-facing, occluded, and inward-facing state of AtSWEET1 to identify the water permeation pathways and the flux associated with them. The hydrophobic gates at the center of the transport tunnel act as a barrier that restricts water permeation. We have performed in silico single and double mutations of the hydrophobic gate residues to examine the changes in the water conductivity. Surprisingly, the double mutant allows the water permeation to the intracellular half of the membrane and forms a continuous water channel. These computational results are validated by experimentally examining the transport of hydrogen peroxide molecules by the AtSWEET family of transporters. We have also shown that the transport of hydrogen peroxide follows the similar mechanism as water transport in AtSWEET1. Finally, we conclude that similar water-conduction states are also present in other SWEET transporters due to the high sequence and structure conservation exhibited by this transporter family.

Photolipid excitation triggers depolarizing optocapacitive currents and action potentials

Optically-induced changes in membrane capacitance may regulate neuronal activity without requiring genetic modifications. Previously, they mainly relied on sudden temperature jumps due to light absorption by membrane-associated nanomaterials or water. Yet, nanomaterial targeting or the required high infrared light intensities obstruct broad applicability. Now, we propose a very versatile approach: photolipids (azobenzene-containing diacylglycerols) mediate light-triggered cellular de- or hyperpolarization. As planar bilayer experiments show, the respective currents emerge from millisecond-timescale changes in bilayer capacitance. UV light changes photolipid conformation, which awards embedding plasma membranes with increased capacitance and evokes depolarizing currents. They open voltage-gated sodium channels in cells, generating action potentials. Blue light reduces the area per photolipid, decreasing membrane capacitance and eliciting hyperpolarization. If present, mechanosensitive channels respond to the increased mechanical membrane tension, generating large depolarizing currents that elicit action potentials. Membrane self-insertion of administered photolipids and focused illumination allows cell excitation with high spatiotemporal control.

Photolipid excitation triggers depolarizing optocapacitive currents and action potentials

Optically-induced changes in membrane capacitance may regulate neuronal activity without requiring genetic modifications. Previously, they mainly relied on sudden temperature jumps due to light absorption by membrane-associated nanomaterials or water. Yet, nanomaterial targeting or the required high infrared light intensities obstruct broad applicability. Now, we propose a very versatile approach: photolipids (azobenzene-containing diacylglycerols) mediate light-triggered cellular de- or hyperpolarization. As planar bilayer experiments show, the respective currents emerge from millisecond-timescale changes in bilayer capacitance. UV light changes photolipid conformation, which awards embedding plasma membranes with increased capacitance and evokes depolarizing currents. They open voltage-gated sodium channels in cells, generating action potentials. Blue light reduces the area per photolipid, decreasing membrane capacitance and eliciting hyperpolarization. If present, mechanosensitive channels respond to the increased mechanical membrane tension, generating large depolarizing currents that elicit action potentials. Membrane self-insertion of administered photolipids and focused illumination allows cell excitation with high spatiotemporal control.

Influence of SGLT1 Sugar Uptake Inhibitors on Water Transport

Sodium glucose cotransporters (SGLTs) are cotransporters located in the cell membrane of various epithelia that uptake glucose or galactose and sodium into the cell. Its founding member, SGLT1, represents a major pharmaceutically relevant target protein for development of new antidiabetic drugs, in addition to being the target protein of the oral rehydration therapy. Previous studies focused primarily on the transport of substrates and ions, while our study focuses on the effect of water transport. SGLT1 is implicated in the absorption of water, yet the exact mechanism of how the water absorption occurs or how inhibitors of SGLT1, such as phlorizin, are able to inhibit it is still unclear. Here we present a comprehensive study based on molecular dynamics simulations with the aim of determining the influence of the energetic and dynamic properties of SGLT1, which are influenced by selected sugar uptake inhibitors on water permeation.

Entropic barrier of water permeation through single-file channels

Facilitated water permeation through narrow biological channels is fundamental for all forms of life. Despite its significance in health and disease as well as for biotechnological applications, the energetics of water permeation are still elusive. Gibbs free energy of activation is composed of an enthalpic and an entropic component. Whereas the enthalpic contribution is readily accessible via temperature dependent water permeability measurements, estimation of the entropic contribution requires information on the temperature dependence of the rate of water permeation. Here, we estimate, by means of accurate activation energy measurements of water permeation through Aquaporin-1 and by determining the accurate single channel permeability, the entropic barrier of water permeation through a narrow biological channel. Thereby the calculated value for [Formula: see text] = 2.01 ± 0.82 J/(mol·K) links the activation energy of 3.75 ± 0.16 kcal/mol with its efficient water conduction rate of ~1010 water molecules/second. This is a first step in understanding the energetic contributions in various biological and artificial channels exhibiting vastly different pore geometries.

Collective Domain Motion Facilitates Water Transport in SGLT1

The human sodium-glucose cotransporter protein (SGLT1) is an important representative of the sodium solute symporters belonging to the secondary active transporters that are critical to the homeostasis of sugar, sodium, and water in the cell. The underlying transport mechanism of SGLT1 is based on switching between inward- and outward-facing conformations, known as the alternating access model, which is crucial for substrate transport, and has also been postulated for water permeation. However, the nature of water transport remains unclear and is disputed along the passive and active transport, with the latter postulating the presence of the pumping effect. To better examine the water transport in SGLT1, we performed a series of equilibrium all-atom molecular dynamics simulations, totaling over 6 μs of sample representative conformational states of SGLT1 and its complexes, with the natural substrates, ions, and inhibitors. In addition to elucidating the basic physical factors influencing water permeation, such as channel openings and energetics, we focus on dynamic flexibility and its relationship with domain motion. Our results clearly demonstrate a dependence of instantaneous water flux on the channel opening and local water diffusion in the channel, strongly supporting the existence of a passive water transport in SGLT1. In addition, a strong correlation found between the local water diffusion and protein domain motion, resembling the "rocking-bundle" motion, reveals its facilitating role in the water transport.

Trapped Pore Waters in the Open Proton Channel HV 1

The voltage-gated proton channel, H_V 1, is crucial for innate immune responses. According to alternative hypotheses, protons either hop on top of an uninterrupted water wire or bypass titratable amino acids, interrupting the water wire halfway across the membrane. To distinguish between both hypotheses, the water mobility for the putative case of an uninterrupted wire is estimated. The predicted single-channel water permeability 2.3 × 10^-12 cm³ s^-1 reflects the permeability-governing number of hydrogen bonds between water molecules in single-file configuration and pore residues. However, the measured unitary water permeability does not confirm the predicted value. Osmotic deflation of reconstituted lipid vesicles reveals negligible water permeability of the H_V 1 wild-type channel and the D174A mutant open at 0 mV. The conductance of 1400 H⁺ s^-1 per wild-type channel agrees with the calculated diffusion limit for a ≈2 Å capture radius for protons. Removal of a charged amino acid (D174) at the pore mouth decreases H⁺ conductance by reducing the capture radius. At least one intervening amino acid contributes to H⁺ conductance while interrupting the water wire across the membrane.

Increased intestinal permeability and downregulation of absorptive ion transporters Nhe3, Dra, and Sglt1 contribute to diarrhea during Clostridioides difficile infection

BACKGROUND & AIM

Clostridioides difficile infection (CDI) is the leading cause of hospital-acquired diarrhea and pseudomembranous colitis. Two protein toxins, TcdA and TcdB, produced by C. difficile are the major determinants of disease. However, the pathophysiological causes of diarrhea during CDI are not well understood. Here, we investigated the effects of C. difficile toxins on paracellular permeability and apical ion transporters in the context of an acute physiological infection.

METHODS

We studied intestinal permeability and apical membrane transporters in female C57BL/6J mice. Üssing chambers were used to measure paracellular permeability and ion transporter function across the intestinal tract. Infected intestinal tissues were analyzed by immunofluorescence microscopy and RNA-sequencing to uncover mechanisms of transporter dysregulation.

RESULTS

Intestinal permeability was increased through the size-selective leak pathway in vivo during acute CDI in a 2-day-post infection model. Chloride secretory activity was reduced in the cecum and distal colon during infection by decreased CaCC and CFTR function, respectively. SGLT1 activity was significantly reduced in the cecum and colon, accompanied by ablated SGLT1 expression in colonocytes and increased luminal glucose concentrations. SGLT1 and DRA expression was ablated by either TcdA or TcdB during acute infection, but NHE3 was decreased in a TcdB-dependent manner. The localization of key proteins that link filamentous actin to the ion transporters in the apical plasma membrane was unchanged. However, Sglt1, Nhe3, and Dra were drastically reduced at the transcript level, implicating downregulation of ion transporters in the mechanism of diarrhea during CDI.

CONCLUSIONS

CDI increases intestinal permeability and decreases apical abundance of NHE3, SGLT1, and DRA. This combination likely leads to dysfunctional water and solute absorption in the large bowel, causing osmotic diarrhea. These findings provide insights into the pathophysiological mechanisms underlying diarrhea and may open novel avenues for attenuating CDI-associated diarrhea.

Honey, I shrunk the extracellular space: Measurements and mechanisms of astrocyte swelling

Astrocyte volume fluctuation is a physiological phenomenon tied closely to the activation of neural circuits. Identification of underlying mechanisms has been challenging due in part to use of a wide range of experimental approaches that vary between research groups. Here, we first review the many methods that have been used to measure astrocyte volume changes directly or indirectly. While the field has recently shifted towards volume analysis using fluorescence microscopy to record cell volume changes directly, established metrics corresponding to extracellular space dynamics have also yielded valuable insights. We then turn to analysis of mechanisms of astrocyte swelling derived from many studies, with a focus on volume changes tied to increases in extracellular potassium concentration ([K+ ]o ). The diverse methods that have been utilized to generate the external [K+ ]o environment highlight multiple scenarios of astrocyte swelling mediated by different mechanisms. Classical potassium buffering theories are tempered by many recent studies that point to different swelling pathways optimized at particular [K+ ]o and that depend on local/transient versus more sustained increases in [K+ ]o .

Modeling of SGLT1 in Reconstituted Systems Reveals Apparent Ion-Dependencies of Glucose Uptake and Strengthens the Notion of Water-Permeable Apo States

The reconstitution of secondary active transporters into liposomes shed light on their molecular transport mechanism. The latter are either symporters, antiporters or exchangers, which use the energy contained in the electrochemical gradient of ions to fuel concentrative uptake of their cognate substrate. In liposomal preparations, these gradients can be set by the experimenter. However, due to passive diffusion of the ions and solutes through the membrane, the gradients are not stable and little is known on the time course by which they dissipate and how the presence of a transporter affects this process. Gradient dissipation can also generate a transmembrane potential (VM). Because it is the effective ion gradient, which together with VM fuels concentrative uptake, knowledge on how these parameters change within the time frame of the conducted experiment is key to understanding experimental outcomes. Here, we addressed this problem by resorting to a modelling approach. To this end, we mathematically modeled the liposome in the assumed presence and absence of the sodium glucose transporter 1 (SGLT1). We show that 1) the model can prevent us from reaching erroneous conclusions on the driving forces of substrate uptake and we 2) demonstrate utility of the model in the assignment of the states of SGLT1, which harbor a water channel.

Effect of Ingested Fluid Volume and Solution Osmolality on Intestinal Drug Absorption: Impact on Drug Interaction with Beverages

It was recently shown that osmolality-dependent fluid movement is a significant factor causing the clinically observed apple juice (AJ)-atenolol interaction. Here we examined whether osmolality-dependent fluid movement may also explain the AJ volume dependence of the AJ-atenolol interaction. In Wistar rats, the luminal fluid volume after administration of different volumes of purified water (0.5 and 1.0 mL) gradually reduced to a similar steady-state level, while that after administration of different volumes of AJ (0.5 and 1.0 mL) increased and attained different apparent steady-state levels. It was hypothesized that osmolality-dependent fluid secretion would account for the volume dependence of the apparent steady-state. Indeed, the luminal concentration of FD-4, a non-permeable compound, after administration in AJ was attenuated depending upon the ingested volume, whereas that after administration in purified water was independent of the ingested fluid volume. An in vivo pharmacokinetic study in rats showed that co-administration of AJ and hyperosmotic solution (adjusted to the osmolality of AJ) with atenolol volume-dependently reduced the AUC and C_max of atenolol significantly. These results show that osmolality-dependent variations in luminal fluid volume may indirectly influence the absorption characteristics of drugs, and can account for the observed volume dependence of beverage-drug interactions.

Scattering versus fluorescence self-quenching: more than a question of faith for the quantification of water flux in large unilamellar vesicles?

The endeavors to understand the determinants of water permeation through membrane channels, the effect of the lipid or polymer membrane on channel function, the development of specific water flow inhibitors, the design of artificial water channels and aquaporins for the use in industrial water filtration applications all rely on accurate ways to quantify water permeabilities (P f). A commonly used method is to reconstitute membrane channels into large unilamellar vesicles (LUVs) and to subject these vesicles to an osmotic gradient in a stopped-flow device. Fast recordings of either scattered light intensity or fluorescence self-quenching signals are taken as a readout for vesicle volume change, which in turn can be recalculated to accurate P f values. By means of computational and experimental data, we discuss the pros and cons of using scattering versus self-quenching experiments or subjecting vesicles to hypo- or hyperosmotic conditions. In addition, we explicate for the first time the influence of the LUVs size distribution, channel distribution between vesicles and remaining detergent after protein reconstitution on P f values. We point out that results such as the single channel water permeability (p f) depend on the membrane matrix or on the direction of the applied osmotic gradient may be direct results of the measurement and analysis procedure.

Native-like membrane models of E. coli polar lipid extract shed light on the importance of lipid composition complexity

BACKGROUND

Lipid-protein interactions stabilize protein oligomers, shape their structure, and modulate their function. Whereas in vitro experiments already account for the functional importance of lipids by using natural lipid extracts, in silico methods lack behind by embedding proteins in single component lipid bilayers. However, to accurately complement in vitro experiments with molecular details at very high spatio-temporal resolution, molecular dynamics simulations have to be performed in natural(-like) lipid environments.

RESULTS

To enable more accurate MD simulations, we have prepared four membrane models of E. coli polar lipid extract, a typical model organism, each at all-atom (CHARMM36) and coarse-grained (Martini3) representations. These models contain all main lipid headgroup types of the E. coli inner membrane, i.e., phosphatidylethanolamines, phosphatidylglycerols, and cardiolipins, symmetrically distributed between the membrane leaflets. The lipid tail (un)saturation and propanylation stereochemistry represent the bacterial lipid tail composition of E. coli grown at 37∘C until 3/4 of the log growth phase. The comparison of the Simple three lipid component models to the complex 14-lipid component model Avanti over a broad range of physiologically relevant temperatures revealed that the balance of lipid tail unsaturation and propanylation in different positions and inclusion of lipid tails of various length maintain realistic values for lipid mobility, membrane area compressibility, lipid ordering, lipid volume and area, and the bilayer thickness. The only Simple model that was able to satisfactory reproduce most of the structural properties of the complex Avanti model showed worse agreement of the activation energy of basal water permeation with the here performed measurements. The Martini3 models reflect extremely well both experimental and atomistic behavior of the E. coli polar lipid extract membranes. Aquaporin-1 embedded in our native(-like) membranes causes partial lipid ordering and membrane thinning in its vicinity. Moreover, aquaporin-1 attracts and temporarily binds negatively charged lipids, mainly cardiolipins, with a distinct cardiolipin binding site in the crevice at the contact site between two monomers, most probably stabilizing the tetrameric protein assembly.

CONCLUSIONS

The here prepared and validated membrane models of E. coli polar lipids extract revealed that lipid tail complexity, in terms of double bond and cyclopropane location and varying lipid tail length, is key to stabilize membrane properties over a broad temperature range. In addition, they build a solid basis for manifold future simulation studies on more realistic lipid membranes bridging the gap between simulations and experiments.

Pathophysiology of SARS-CoV-2 in Lung of Diabetic Patients

Novel coronavirus disease (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Its impact on patients with comorbidities is clearly related to fatality cases, and diabetes has been linked to one of the most important causes of severity and mortality in SARS-CoV-2 infected patients. Substantial research progress has been made on COVID-19 therapeutics; however, effective treatments remain unsatisfactory. This unmet clinical need is robustly associated with the complexity of pathophysiological mechanisms described for COVID-19. Several key lung pathophysiological mechanisms promoted by SARS-CoV-2 have driven the response in normoglycemic and hyperglycemic subjects. There is sufficient evidence that glucose metabolism pathways in the lung are closely tied to bacterial proliferation, inflammation, oxidative stress, and pro-thrombotic responses, which lead to severe clinical outcomes. It is also likely that SARS-CoV-2 proliferation is affected by glucose metabolism of type I and type II cells. This review summarizes the current understanding of pathophysiology of SARS-CoV-2 in the lung of diabetic patients and highlights the changes in clinical outcomes of COVID-19 in normoglycemic and hyperglycemic conditions.

A comprehensive overview of substrate specificity of glycoside hydrolases and transporters in the small intestine : "A gut feeling"

The human body is able to process and transport a complex variety of carbohydrates, unlocking their nutritional value as energy source or as important building block. The endogenous glycosyl hydrolases (glycosidases) and glycosyl transporter proteins located in the enterocytes of the small intestine play a crucial role in this process and digest and/or transport nutritional sugars based on their structural features. It is for these reasons that glycosidases and glycosyl transporters are interesting therapeutic targets to combat sugar related diseases (such as diabetes) or to improve drug delivery. In this review we provide a detailed overview focused on the molecular structure of the substrates involved as a solid base to start from and to fuel research in the area of therapeutics and diagnostics.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Role of Glucose Transporters in Drug Membrane Transport

BACKGROUND

Glucose is the main energy component of cellular activities. However, as a polar molecule, glucose cannot freely pass through the phospholipid bilayer structure of the cell membrane. Thus, glucose must rely on specific transporters in the membrane. Drugs with a similar chemical structure to glucose may also be transported through this pathway.

METHODS

This review describes the structure, distribution, action mechanism and influencing factors of glucose transporters and introduces the natural drugs mediated by these transporters and drug design strategies on the basis of this pathway.

RESULTS

The glucose transporters involved in glucose transport are of two major types, namely, Na+-dependent and Na+-independent transporters. Glucose transporters can help some glycoside drugs cross the biological membrane. The transmembrane potential is influenced by the chemical structure of drugs. Glucose can be used to modify drugs and improve their ability to cross biological barriers.

CONCLUSION

The membrane transport mechanism of some glycoside drugs may be related to glucose transporters. Glucose modification may improve the oral bioavailability of drugs or achieve targeted drug delivery.

Micropipette Aspiration-Based Assessment of Single Channel Water Permeability

Measurements of the unitary hydraulic conductivity of membrane channels, p_f , may be hampered by difficulties in producing sufficient quantities of purified and reconstituted proteins. Low yield expression, the purely empiric choice of detergents, as well as protein aggregation and misfolding during reconstitution may result in an average of less than one reconstituted channel per large unilamellar vesicle. This limits their applicability for p_f measurements, independent of whether light scattering or fluorescence quenching of encapsulated dyes is monitored. Here the micropipette aspiration technique is adopted because its superb sensitivity allows resolving p_f values for one order of magnitude smaller protein densities in sphingomyelin and cholesterol rich giant unilamellar vesicles (GUVs). Protein density is derived from intensity fluctuations that fluorescently labeled channels in the aspirated GUV induce by diffusing through the diffraction limited spot. A perfusion system minimizes unstirred layers in the immediate membrane vicinity as demonstrated by the distribution of both encapsulated and extravesicular aqueous dyes. p_f amounted to 2.4 ± 0.1 × 10^-13 cm³ s^-1 for aquaporin-1 that served as a test case. The new assay paves the way for directly monitoring the effect that interaction of aquaporins with other proteins or inhibitors may have on p_f on a single sample.

PRDM9 forms a trimer by interactions within the zinc finger array

PRDM9 is a trans-acting factor directing meiotic recombination to specific DNA-binding sites by its zinc finger (ZnF) array. It was suggested that PRDM9 is a multimer; however, we do not know the stoichiometry or the components inducing PRDM9 multimerization. In this work, we used in vitro binding studies and characterized with electrophoretic mobility shift assays, mass spectrometry, and fluorescence correlation spectroscopy the stoichiometry of the PRDM9 multimer of two different murine PRDM9 alleles carrying different tags and domains produced with different expression systems. Based on the migration distance of the PRDM9-DNA complex, we show that PRDM9 forms a trimer. Moreover, this stoichiometry is adapted already by the free, soluble protein with little exchange between protein monomers. The variable ZnF array of PRDM9 is sufficient for multimerization, and at least five ZnFs form already a functional trimer. Finally, we also show that only one ZnF array within the PRDM9 oligomer binds to the DNA, whereas the remaining two ZnF arrays likely maintain the trimer by ZnF-ZnF interactions.

The mechanisms of pharmacokinetic food-drug interactions - A perspective from the UNGAP group

The simultaneous intake of food and drugs can have a strong impact on drug release, absorption, distribution, metabolism and/or elimination and consequently, on the efficacy and safety of pharmacotherapy. As such, food-drug interactions are one of the main challenges in oral drug administration. Whereas pharmacokinetic (PK) food-drug interactions can have a variety of causes, pharmacodynamic (PD) food-drug interactions occur due to specific pharmacological interactions between a drug and particular drinks or food. In recent years, extensive efforts were made to elucidate the mechanisms that drive pharmacokinetic food-drug interactions. Their occurrence depends mainly on the properties of the drug substance, the formulation and a multitude of physiological factors. Every intake of food or drink changes the physiological conditions in the human gastrointestinal tract. Therefore, a precise understanding of how different foods and drinks affect the processes of drug absorption, distribution, metabolism and/or elimination as well as formulation performance is important in order to be able to predict and avoid such interactions. Furthermore, it must be considered that beverages such as milk, grapefruit juice and alcohol can also lead to specific food-drug interactions. In this regard, the growing use of food supplements and functional food requires urgent attention in oral pharmacotherapy. Recently, a new consortium in Understanding Gastrointestinal Absorption-related Processes (UNGAP) was established through COST, a funding organisation of the European Union supporting translational research across Europe. In this review of the UNGAP Working group "Food-Drug Interface", the different mechanisms that can lead to pharmacokinetic food-drug interactions are discussed and summarised from different expert perspectives.

Intrinsic Membrane Permeability to Small Molecules

Spontaneous solute and solvent permeation through membranes is of vital importance to human life, be it gas exchange in red blood cells, metabolite excretion, drug/toxin uptake, or water homeostasis. Knowledge of the underlying molecular mechanisms is the sine qua non of every functional assignment to membrane transporters. The basis of our current solubility diffusion model was laid by Meyer and Overton. It correlates the solubility of a substance in an organic phase with its membrane permeability. Since then, a wide range of studies challenging this rule have appeared. Commonly, the discrepancies have their origin in ill-used measurement approaches, as we demonstrate on the example of membrane CO₂ transport. On the basis of the insight that scanning electrochemical microscopy offered into solute concentration distributions in immediate membrane vicinity of planar membranes, we analyzed the interplay between chemical reactions and diffusion for solvent transport, weak acid permeation, and enzymatic reactions adjacent to membranes. We conclude that buffer reactions must also be considered in spectroscopic investigations of weak acid transport in vesicular suspensions. The evaluation of energetic contributions to membrane translocation of charged species demonstrates the compatibility of the resulting membrane current with the solubility diffusion model. A local partition coefficient that depends on membrane penetration depth governs spontaneous membrane translocation of both charged and uncharged molecules. It is determined not only by the solubility in an organic phase but also by other factors like cholesterol concentration and intrinsic electric membrane potentials.

Single-file transport of water through membrane channels

Water at interfaces governs many processes on the molecular scale from electrochemical and enzymatic reactions to protein folding. Here we focus on water transport through proteinaceous pores that are so narrow that the water molecules cannot overtake each other in the pore. After a short introduction into the single-file transport theory, we analyze experiments in which the unitary water permeability, pf, of water channel proteins (aquaporins, AQPs), potassium channels (KcsA), and antibiotics (gramicidin-A derivatives) has been obtained. A short outline of the underlying methods (scanning electrochemical microscopy, fluorescence correlation spectroscopy, measurements of vesicle light scattering) is also provided. We conclude that pf increases exponentially with a decreasing number NH of hydrogen bond donating or accepting residues in the channel wall. The variance in NH is responsible for a more than hundredfold change in pf. The dehydration penalty at the channel mouth has a smaller effect on pf. The intricate link between pf and the Gibbs activation energy barrier, ΔG‡t, for water flow suggests that conformational transitions of water channels act as a third determinant of pf.

Molecular Dynamics Simulation for All

The impact of molecular dynamics (MD) simulations in molecular biology and drug discovery has expanded dramatically in recent years. These simulations capture the behavior of proteins and other biomolecules in full atomic detail and at very fine temporal resolution. Major improvements in simulation speed, accuracy, and accessibility, together with the proliferation of experimental structural data, have increased the appeal of biomolecular simulation to experimentalists-a trend particularly noticeable in, although certainly not limited to, neuroscience. Simulations have proven valuable in deciphering functional mechanisms of proteins and other biomolecules, in uncovering the structural basis for disease, and in the design and optimization of small molecules, peptides, and proteins. Here we describe, in practical terms, the types of information MD simulations can provide and the ways in which they typically motivate further experimental work.

Quantification of Water Flux in Vesicular Systems

Water transport across lipid membranes is fundamental to all forms of life and plays a major role in health and disease. However, not only typical water facilitators like aquaporins facilitate water flux, but also transporters, ion channels or receptors represent potent water pathways. The efforts directed towards a mechanistic understanding of water conductivity determinants in transmembrane proteins, the development of water flow inhibitors, and the creation of biomimetic membranes with incorporated membrane proteins or artificial water channels depend on reliable and accurate ways of quantifying water permeabilities Pf. A conventional method is to subject vesicles to an osmotic gradient in a stopped-flow device: Fast recordings of scattered light intensity are converted into the time course of vesicle volume change. Even though an analytical solution accurately acquiring Pf from scattered light intensities exists, approximations potentially misjudging Pf by orders of magnitude are used. By means of computational and experimental data we point out that erroneous results such as that the single channel water permeability pf depends on the osmotic gradient are direct results of such approximations. Finally, we propose an empirical solution of which calculated permeability values closely match those calculated with the analytical solution in the relevant range of parameters.

Positively charged residues at the channel mouth boost single-file water flow

Positively charged residues in the vicinity of the channel entrance or exit accelerate single-file water flow.

Water molecules lose two of their four bulk neighbours when entering single-file channels. This process may be sensitive to the presence of positive and negative charges at the channel mouth, since the costs for dehydrating cations and anions differ by a large margin. However, it is not known whether entrance charges affect the single channel water permeability (p_f). So far, p_f is only known to be governed by H-bond formation between permeating water molecules and wall-lining residues. Here we compare the p_f values of five different aquaporin species (AQP1, AQPZ, AQP4 wild type, and two phosphorylation mimicking AQP4 mutants) that offer the same number of hydrogen bond donating and receiving residues in their single-file region but display different entrance charges. The p_f measurements were performed with reconstituted lipid vesicles. We assessed (i) the osmotically induced vesicle deflation from the light scattering intensity in a stopped-flow device and (ii) the aquaporin abundance by fluorescence correlation spectroscopy. Substitution of serine at positions 111 and 180 in AQP4 for aspartic acid showed only a marginal effect on p_f, suggesting that negative entrance charges are of minor importance. In contrast, the total number of positively charged amino acid side chains at entrances and exits correlates with p_f: a total of three, four and seven charges of AQP4, AQPZ, and AQP1 translate into p_f values of 1.1, 1.8, and 3.2 × 10^–13 cm³ s^–1, respectively. Thus, positive interfacial charges boost the p_f value of AQP1 to three times the value of AQP4. Nevertheless, the number of hydrogen bond donating and receiving residues in the single-file region remains the major determinant of p_f. Their effect on p_f may be a hundredfold larger than that of interfacial charges.

Chloride Dysregulation, Seizures, and Cerebral Edema: A Relationship with Therapeutic Potential

Pharmacoresistant seizures and cytotoxic cerebral edema are serious complications of ischemic and traumatic brain injury. Intraneuronal Cl^- concentration ([Cl^-]_i) regulation impacts on both cell volume homeostasis and Cl^--permeable GABA_A receptor-dependent membrane excitability. Understanding the pleiotropic molecular determinants of neuronal [Cl^-]_i - cytoplasmic impermeant anions, polyanionic extracellular matrix (ECM) glycoproteins, and plasmalemmal Cl^- transporters - could help the identification of novel anticonvulsive and neuroprotective targets. The cation/Cl^- cotransporters and ECM metalloproteinases may be particularly druggable targets for intervention. We establish here a paradigm that accounts for recent data regarding the complex regulatory mechanisms of neuronal [Cl^-]_i and how these mechanisms impact on neuronal volume and excitability. We propose approaches to modulate [Cl^-]_i that are relevant for two common clinical sequela of brain injury: edema and seizures.

Structural and functional significance of water permeation through cotransporters

Membrane transporters, in addition to their major role as specific carriers for ions and small molecules, can also behave as water channels. However, neither the location of the water pathway in the protein nor their functional importance is known. Here, we map the pathway for water and urea through the intestinal sodium/glucose cotransporter SGLT1. Molecular dynamics simulations using the atomic structure of the bacterial transporter vSGLT suggest that water permeates the same path as Na+ and sugar. On a structural model of SGLT1, based on the homology structure of vSGLT, we identified and mutated residues lining the sugar transport pathway to cysteine. The mutants were expressed in Xenopus oocytes, and the unitary water and urea permeabilities were determined before and after modifying the cysteine side chain with reversible methanethiosulfonate reagents. The results demonstrate that water and urea follow the sugar transport pathway through SGLT1. The changes in permeability, increases or decreases, with side-chain modifications depend on the location of the mutation in the region of external or internal gates, or the sugar binding site. These changes in permeability are hypothesized to be due to alterations in steric hindrance to water and urea, and/or changes in protein folding caused by mismatching of side chains in the water pathway. Water permeation through SGLT1 and other transporters bears directly on the structural mechanism for the transport of polar solutes through these proteins. Finally, in vitro experiments on mouse small intestine show that SGLT1 accounts for two-thirds of the passive water flow across the gut.

Functional kinomics establishes a critical node of volume-sensitive cation-Cl- cotransporter regulation in the mammalian brain

Cell volume homeostasis requires the dynamically regulated transport of ions across the plasmalemma. While the ensemble of ion transport proteins involved in cell volume regulation is well established, the molecular coordinators of their activities remain poorly characterized. We utilized a functional kinomics approach including a kinome-wide siRNA-phosphoproteomic screen, a high-content kinase inhibitor screen, and a kinase trapping-Orbitrap mass spectroscopy screen to systematically identify essential kinase regulators of KCC3 Thr991/Thr1048 phosphorylation - a key signaling event in cell swelling-induced regulatory volume decrease (RVD). In the mammalian brain, we found the Cl--sensitive WNK3-SPAK kinase complex, required for cell shrinkage-induced regulatory volume decrease (RVI) via the stimulatory phosphorylation of NKCC1 (Thr203/Thr207/Thr212), is also essential for the inhibitory phosphorylation of KCC3 (Thr991/Thr1048). This is mediated in vivo by an interaction between the CCT domain in SPAK and RFXV/I domains in WNK3 and NKCC1/KCC3. Accordingly, genetic or pharmacologic WNK3-SPAK inhibition prevents cell swelling in response to osmotic stress and ameliorates post-ischemic brain swelling through a simultaneous inhibition of NKCC1-mediated Cl- uptake and stimulation of KCC3-mediated Cl- extrusion. We conclude that WNK3-SPAK is an integral component of the long-sought "Cl-/volume-sensitive kinase" of the cation-Cl- cotransporters, and functions as a molecular rheostat of cell volume in the mammalian brain.

Inhibition of sodium glucose cotransporters following status epilepticus induced by intrahippocampal pilocarpine affects neurodegeneration process in hippocampus

Temporal lobe epilepsy (TLE) is characterized by spontaneous recurrent seizures, starting from secondary functional disorders due to several insults, including self-sustaining continuous seizures identified as status epilepticus (SE). Although hypoglycemia has been associated with SE, the effect of inhibition of the Na(+)/glucose cotransporters (SGLTs) on hippocampus during SE is still unknown. Here we evaluated the functional role of SGLT in the pattern of limbic seizures and neurodegeneration process after pilocarpine (PILO)-induced SE. Vehicle (VEH, 1μL) or phlorizin, a specific SGLT inhibitor (PZN, 1μL, 50μg/μL), was administered in the hippocampus of rats 30min before PILO (VEH+PILO or PZN+PILO, respectively). The limbic seizures were classified using the Racine's scale, and the amount of wet dog shakes (WDS) was quantified before and during SE. Neurodegeneration process was evaluated by Fluoro-Jade C (FJ-C), and FJ-C-positive neurons (FJ-C+) were counted 24h and 15days after SE. The PZN-treated rats showed higher (p<0.05) number of WDS when compared with VEH+PILO. There was no difference in seizure severity between PZN+PILO and VEH+PILO groups. However, the pattern of limbic seizures significantly changed in PZN+PILO. Indeed, the class 5 seizures repeated themselves more times (p<0.05) than the other classes in the PZN group at 50min after SE induction. The PZN+PILO animals had a higher (p<0.05) number of FJ-C+ cells in the dentate gyrus (DG), hilus, and CA3 and CA1 of hippocampus, when compared with VEH+PILO. The PZN+PILO animals had a decreased number (p<0.05) of FJ-C+ cells in CA1 compared with VEH+PILO 15days after SE induction. Taken together, our data suggest that SGLT inhibition with PZN increased the severity of limbic seizures during SE and increased neurodegeneration in hippocampus 24h after SE, suggesting that SGLT1 and SGLT2 could participate in the modulation of earlier stages of epileptogenic processes.