Individual leaflets of a membrane bilayer can independently regulate permeability

Peptide translocation across asymmetric phospholipid membranes

The transport of molecules across cell membranes is vital for proper cell function and effective drug delivery. While most cell membranes naturally possess an asymmetric lipid composition, research on membrane transport predominantly uses symmetric lipid membranes. The permeation through the asymmetric membrane is then calculated as a sum of the inverse permeabilities of leaflets from symmetric bilayers. In this study, we examined how two types of amphiphilic molecules translocate across both asymmetric and symmetric membranes. Using computer simulations with both coarse-grained and atomistic force fields, we calculated the free energy profiles for the passage of model amphiphilic peptides and a lipid across various membranes. Our results consistently demonstrate that while the free energy profiles for asymmetric membranes with a small differential stress concur with symmetric ones in the region of lipid headgroups, the profiles differ around the center of the membrane. In this region, the free energy for the asymmetric membrane transitions between the profiles for two symmetric membranes. In addition, we show that peptide permeability through an asymmetric membrane cannot always be predicted from the permeabilities of the symmetric membranes. This indicates that using symmetric membranes falls short in providing an accurate depiction of peptide translocation across asymmetric membranes.

Switching Cytolytic Nanopores into Antimicrobial Fractal Ruptures by a Single Side Chain Mutation

Disruption of cell membranes is a fundamental host defense response found in virtually all forms of life. The molecular mechanisms vary but generally lead to energetically favored circular nanopores. Here, we report an elaborate fractal rupture pattern induced by a single side-chain mutation in ultrashort (8-11-mers) helical peptides, which otherwise form transmembrane pores. In contrast to known mechanisms, this mode of membrane disruption is restricted to the upper leaflet of the bilayer where it exhibits propagating fronts of peptide-lipid interfaces that are strikingly similar to viscous instabilities in fluid flow. The two distinct disruption modes, pores and fractal patterns, are both strongly antimicrobial, but only the fractal rupture is nonhemolytic. The results offer wide implications for elucidating differential membrane targeting phenomena defined at the nanoscale.

Membrane association of the bacterial riboregulator Hfq and functional perspectives

Hfq is a bacterial RNA binding protein that carries out several roles in genetic expression regulation, mainly at the post-transcriptional level. Previous studies have shown its importance in growth and virulence of bacteria. Here, we provide the direct observation of its ability to interact with membranes. This was established by co-sedimentation assay, cryo-transmission electron (cryo-TEM) and atomic force (AFM) microscopies. Furthermore, our results suggest a role for its C-terminus amyloidogenic domain in membrane disruption. Precisely, AFM images of lipid bilayers in contact with Hfq C-terminus fibrils show the emergence of holes with a size dependent on the time of interaction. Cryo-TEM observations also show that liposomes are in contact with clusters of fibrils, with occasional deformation of the vesicles and afterward the apparition of a multitude of tiny vesicles in the proximity of the fibrils, suggesting peptide-induced breakage of the liposomes. Finally, circular dichroism spectroscopy demonstrated a change in the secondary structure of Hfq C-terminus upon interaction with liposomes. Altogether, these results show an unexpected property of Hfq and suggest a possible new role for the protein, exporting sRNA outside of the bacterial cell.

1H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers

We measured the transbilayer diffusion of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in large unilamellar vesicles, in both the gel (L_β') and fluid (L_α) phases. The choline resonance of headgroup-protiated DPPC exchanged into the outer leaflet of headgroup-deuterated DPPC-d13 vesicles was monitored using ¹H NMR spectroscopy, coupled with the addition of a paramagnetic shift reagent. This allowed us to distinguish between the inner and outer bilayer leaflet of DPPC, to determine the flip-flop rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited Arrhenius kinetics, from which we determined an activation energy of 122 kJ mol^-1. In gel-phase DPPC vesicles, flip-flop was not observed over the course of 250 h. Our findings are in contrast to previous studies of solid-supported bilayers, where the reported DPPC translocation rates are at least several orders of magnitude faster than those in vesicles at corresponding temperatures. We reconcile these differences by proposing a defect-mediated acceleration of lipid translocation in supported bilayers, where long-lived, submicron-sized holes resulting from incomplete surface coverage are the sites of rapid transbilayer movement.

Engineering monolayer poration for rapid exfoliation of microbial membranes

A novel mechanism of monolayer poration leading to the rapid exfoliation and lysis of microbial membranes is reported.

The spread of bacterial resistance to traditional antibiotics continues to stimulate the search for alternative antimicrobial strategies. All forms of life, from bacteria to humans, are postulated to rely on a fundamental host defense mechanism, which exploits the formation of open pores in microbial phospholipid bilayers. Here we predict that transmembrane poration is not necessary for antimicrobial activity and reveal a distinct poration mechanism that targets the outer leaflet of phospholipid bilayers. Using a combination of molecular-scale and real-time imaging, spectroscopy and spectrometry approaches, we introduce a structural motif with a universal insertion mode in reconstituted membranes and live bacteria. We demonstrate that this motif rapidly assembles into monolayer pits that coalesce during progressive membrane exfoliation, leading to bacterial cell death within minutes. The findings offer a new physical basis for designing effective antibiotics.

The Integrin Receptor in Biologically Relevant Bilayers: Insights from Molecular Dynamics Simulations

Integrins are heterodimeric (αβ) cell surface receptors that are potential therapeutic targets for a number of diseases. Despite the existence of structural data for all parts of integrins, the structure of the complete integrin receptor is still not available. We have used available structural data to construct a model of the complete integrin receptor in complex with talin F2-F3 domain. It has been shown that the interactions of integrins with their lipid environment are crucial for their function but details of the integrin/lipid interactions remain elusive. In this study an integrin/talin complex was inserted in biologically relevant bilayers that resemble the cell plasma membrane containing zwitterionic and charged phospholipids, cholesterol and sphingolipids to study the dynamics of the integrin receptor and its effect on bilayer structure and dynamics. The results of this study demonstrate the dynamic nature of the integrin receptor and suggest that the presence of the integrin receptor alters the lipid organization between the two leaflets of the bilayer. In particular, our results suggest elevated density of cholesterol and of phosphatidylserine lipids around the integrin/talin complex and a slowing down of lipids in an annulus of ~30 Å around the protein due to interactions between the lipids and the integrin/talin F2-F3 complex. This may in part regulate the interactions of integrins with other related proteins or integrin clustering thus facilitating signal transduction across cell membranes.

Water Permeability across Symmetric and Asymmetric Droplet Interface Bilayers: Interaction of Cholesterol Sulfate with DPhPC

Cellular membranes employ a variety of strategies for controlling the flow of small molecules into the cytoplasmic space, including incorporation of sterols for modulation of permeability and maintenance of lipid asymmetry to provide both sides of the membrane with differing biophysical properties. The specific case of cholesterol asymmetry, especially, is known to have profound effects in neurological cellular systems. Synthetic membrane models that can readily determine valuable physical parameters, such as water transport rates, for sterol-containing membranes of defined lipid composition remain in demand. We report the use of the droplet interface bilayer (DIB), composed of adherent aqueous droplets surrounded by a lipid monolayer and immersed in a hydrophobic medium, for measurement of water permeability across the membrane, with rapid visualization and ease of experimental setup. We studied droplet bilayer membranes composed of the prototypical synthetic membrane lipid (i.e., the archaeal lipid DPhPC) as well as of symmetric and asymmetric DIBs formed by DPhPC and sodium cholesterol sulfate (S-Chol). The presence of S-Chol in DPhPC in symmetric DIB reduced the passive water permeability rate (P(f)) at all concentrations and increased the activation energy (E(a)) to 17-18 kcal/mol. When only one side of the DIB contains S-Chol (asymmetric DIB), an E(a) of 14-15 kcal/mol was obtained, a value intermediate that of pure lipid and symmetrical DIB containing lipid and S-Chol. Our data are consistent with a capability for regulation of water transport by one leaflet independent of the other. The engineering of our various systems is believed to have implications for garnering detailed knowledge regarding the transport of small moieties across bilayers in a wide variety of lipid systems.

Multiscale modeling of droplet interface bilayer membrane networks

Droplet interface bilayer (DIB) networks are considered for the development of stimuli-responsive membrane-based materials inspired by cellular mechanics. These DIB networks are often modeled as combinations of electrical circuit analogues, creating complex networks of capacitors and resistors that mimic the biomolecular structures. These empirical models are capable of replicating data from electrophysiology experiments, but these models do not accurately capture the underlying physical phenomena and consequently do not allow for simulations of material functionalities beyond the voltage-clamp or current-clamp conditions. The work presented here provides a more robust description of DIB network behavior through the development of a hierarchical multiscale model, recognizing that the macroscopic network properties are functions of their underlying molecular structure. The result of this research is a modeling methodology based on controlled exchanges across the interfaces of neighboring droplets. This methodology is validated against experimental data, and an extension case is provided to demonstrate possible future applications of droplet interface bilayer networks.

Electrostatic modification of the lipopolysaccharide layer: competing effects of divalent cations and polycationic or polyanionic molecules

The outer membrane (OM) of Gram-negative bacteria is asymmetrical with its outer layer mainly populated with polyanionic lipopolysaccharide (LPS). Much empirical evidence shows how OM permeability can be altered electrostatically: if Mg(2+) or divalent cations are required for the integrity of the OM, antimicrobial peptides (AMPs) or ethylene-diaminetetraacetic acid (EDTA) can permeabilize it. Using a coarse-grained model of the outer LPS layer, in which the layer is viewed as forming discrete binding sites for opposite charges, we study how the LPS layer can be modified electrostatically. In particular, we capture systematically ion-pairing and lateral-charge correlations on the LPS layer. Our results offer a clear picture of (competitive) ion binding onto the LPS layer and its impact on the lateral packing of LPS molecules, similarly to what has been seen in experiments: divalent cations such as Mg(2+) not only neutralize the LPS layer but also make its planar charge distribution heterogeneous, thus tightening the LPS layer; on the other hand, polycationic AMPs or polyanionic EDTA can displace Mg(2+) ions from the LPS layer and counteract the favorable effect of Mg(2+). Our result will be useful for clarifying to what extent OM permeability can be modified electrostatically.

Using fluorescence to locate intercalants within the lipid bilayer of liposomes, bioliposomes and erythrocyte ghosts

In previous work, we have shown the utility of the "NMR technique" in locating intercalants within the lipid bilayer. We describe herein the development of a more sensitive and complementary "fluorescence technique" for this purpose and its application to liposomes, bioliposomes and erythrocyte ghosts. This technique is based on the observation in selected compounds of an excellent correlation between the emission wavelength (λ(em)) and Dimroth-Reichardt E(T)(30) polarity parameter for the solvent in which the fluorescence emission spectrum was obtained.

Ceramide channels and their role in mitochondria-mediated apoptosis

A key, decision-making step in apoptosis is the release of proteins from the mitochondrial intermembrane space. Ceramide can self-assemble in the mitochondrial outer membrane to form large stable channels capable of releasing said proteins. Ceramide levels measured in mitochondria early in apoptosis are sufficient to form ceramide channels in the outer membrane. The channels are in dynamic equilibrium with non-conducting forms of ceramide in the membrane. This equilibrium can be strongly influenced by other sphingolipids and Bcl-2 family proteins. The properties of ceramide channels formed in a defined system, planar phospholipid membranes, demonstrate that proteins are not required for channel formation. In addition, experiments in the defined system reveal structural information. The results indicated that the channels are barrel-like structures whose staves are ceramide columns that span the membrane. Ceramide channels are good candidates for the protein release pathway that initiates the execution phase of apoptosis.

Oligomeric structure and functional characterization of the urea transporter from Actinobacillus pleuropneumoniae

Urea transporters (UTs) facilitate urea permeation across cell membranes in prokaryotes and eukaryotes. Bacteria use urea as a means to survive in acidic environments and/or as a nitrogen source. The UT from Actinobacillus pleuropneumoniae, ApUT, the pathogen that causes porcine pleurisy and pneumonia, was expressed in Escherichia coli and purified. Analysis of the recombinant protein using cross-linking and blue-native gel electrophoresis established that ApUT is a dimer in detergent solution. Purified protein was reconstituted into proteoliposomes and urea efflux was measured by stopped-flow fluorometry to determine the urea transport kinetics of ApUT. The measured urea flux was saturable, could be inhibited by phloretin, and was not affected by pH. Two-dimensional crystals of the biologically active ApUT show that it is also dimeric in a lipid membrane and provide the first structural information on a member of the UT family.

Endometrial biopsy-induced gene modulation: first evidence for the expression of bladder-transmembranal uroplakin Ib in human endometrium

OBJECTIVE

To explore the possibility that endometrial injury modulates the expression of specific genes that may increase uterine receptivity.

DESIGN

Controlled clinical study.

SETTING

Clinical IVF unit and academic research center.

PATIENT(S)

IVF patients with 28- to 30-day menstrual cycles.

INTERVENTION(S)

Endometrial biopsies from two groups of patients were collected on days 20-21 of their spontaneous menstrual cycle. The experimental, but not the control, group underwent biopsies on days 11-13 and 21-24 of their preceding cycle.

MAIN OUTCOME MEASURE(S)

Global endometrial gene expression and specific analysis of uroplakin Ib (UPIb) mRNA level throughout the menstrual cycle.

RESULT(S)

Local injury modulated the expression of a wide variety of genes. One of the prominently up-regulated genes was the bladder transmembranal protein, UPIb, whose expression by the endometrium is shown here for the first time. Endometrial UPIb mRNA increases after biopsy in the same cycle wct 2with an additional elevation in the following cycle. Immunohistochemical analysis localized the UPIb protein to the glandular-epithelial cells. Genes encoding other membrane proteins such as adipose differentiation-related protein and mucin 1, transmembrane, were also up-regulated.

CONCLUSION(S)

The biopsy-induced increase in the expression of UPIb and other genes encoding membrane proteins supports the possible importance of the membrane structure and stability during implantation. The specific role of UPIb in uterine receptivity should be elucidated.

Structural determinants of water permeability through the lipid membrane

Despite intense study over many years, the mechanisms by which water and small nonelectrolytes cross lipid bilayers remain unclear. While prior studies of permeability through membranes have focused on solute characteristics, such as size, polarity, and partition coefficient in hydrophobic solvent, we focus here on water permeability in seven single component bilayers composed of different lipids, five with phosphatidylcholine headgroups and different chain lengths and unsaturation, one with a phosphatidylserine headgroup, and one with a phosphatidylethanolamine headgroup. We find that water permeability correlates most strongly with the area/lipid and is poorly correlated with bilayer thickness and other previously determined structural and mechanical properties of these single component bilayers. These results suggest a new model for permeability that is developed in the accompanying theoretical paper in which the area occupied by the lipid is the major determinant and the hydrocarbon thickness is a secondary determinant. Cholesterol was also incorporated into DOPC bilayers and X-ray diffuse scattering was used to determine quantitative structure with the result that the area occupied by DOPC in the membrane decreases while bilayer thickness increases in a correlated way because lipid volume does not change. The water permeability decreases with added cholesterol and it correlates in a different way from pure lipids with area per lipid, bilayer thickness, and also with area compressibility.

Pore formation by Vibrio cholerae cytolysin requires cholesterol in both monolayers of the target membrane

Vibrio cholerae cytolysin (VCC) forms oligomeric transmembrane pores in cholesterol-rich membranes. To better understand this process, we used planar bilayer membranes. In symmetric membranes, the rate of the channel formation by VCC has a superlinear dependency on the cholesterol membrane fraction. Thus, more than one cholesterol molecule can facilitate VCC-pore formation. In asymmetric membranes, the rate of pore formation is limited by the leaflet with the lower cholesterol content. Methyl-beta-cyclodextrin, which removes cholesterol from membranes, rapidly inhibits VCC pore formation, even when it is added to the side opposite that of VCC addition. The results suggest that cholesterol in both membrane leaflets aid VCC-pore formation and that either leaflet can function as a kinetic bottleneck with respect to the rate of pore-formation.

Lipid raft components cholesterol and sphingomyelin increase H+/OH- permeability of phosphatidylcholine membranes

H+/OH- permeation through lipid bilayers occurs at anomalously high rates and the determinants of proton flux through membranes are poorly understood. Since all life depends on proton gradients, it is important to develop a greater understanding of proton leak phenomena. We have used stopped-flow fluorimetry to probe the influence of two lipid raft components, chol (cholesterol) and SM (sphingomyelin), on H+/OH- and water permeability. Increasing the concentrations of both lipids in POPC (palmitoyl-2-oleoyl phosphatidylcholine) liposomes decreased water permeability in a concentration-dependent manner, an effect that correlated with increased lipid order. Surprisingly, proton flux was increased by increasing the concentration of chol and SM. The chol effect was complex with molar concentrations of 17.9, 33 and 45.7% giving 2.8-fold (P<0.01), 2.2-fold (P<0.001) and 5.1-fold (P<0.001) increases in H+/OH- permeability from a baseline of 2.4x10(-2) cm/s. SM at 10 mole% effected a 2.8-fold increase (P<0.01), whereas 20 and 30 mole% enhanced permeability by 3.6-fold (P<0.05) and 4.1-fold respectively (P<0.05). Supplementing membranes containing chol with SM did not enhance H+/OH- permeability. Of interest was the finding that chol addition to soya-bean lipids decreased H+/OH- permeability, consistent with an earlier report [Ira and Krishnamoorthy (2001) J. Phys. Chem. B 105, 1484-1488]. We speculate that the presence of proton carriers in crude lipid extracts might contribute to this result. We conclude that (i) chol and SM specifically and independently increase rates of proton permeation in POPC bilayers, (ii) domains enriched in these lipids or domain interfaces may represent regions with high H+/OH- conductivity, (iii) H+/OH- fluxes are not governed by lipid order and (iv) chol can inhibit or promote H+/OH- permeability depending on the total lipid environment. Theories of proton permeation are discussed in the light of these results.

Water and solute permeability of rat lung caveolae: high permeabilities explained by acyl chain unsaturation

Caveolae are invaginated membrane structures with high levels of cholesterol, sphingomyelin, and caveolin protein that are predicted to exist as liquid-ordered domains with low water permeability. We isolated a caveolae-enriched membrane fraction without detergents from rat lung and characterized its permeability properties to nonelectrolytes and protons. Membrane permeability to water was 2.85 ± 0.41 × 10−3cm/s, a value 5–10 times higher than expected based on comparisons with other cholesterol and sphingolipid-enriched membranes. Permeabilities to urea, ammonia, and protons were measured and found to be moderately high for urea and ammonia at 8.85 ± 2.40 × 10−7and 6.84 ± 1.03 × 10−2respectively and high for protons at 8.84 ± 3.06 × 10−2cm/s. To examine whether caveolin or other integral membrane proteins were responsible for high permeabilities, liposomes designed to mimic the lipids of the inner and outer leaflets of the caveolar membrane were made. Osmotic water permeability to both liposome compositions were determined and a combined inner/outer leaflet water permeability was calculated and found to be close to that of native caveolae at 1.58 ± 1.1 × 10−3cm/s. In caveolae, activation energy for water flux was high (19.4 kcal/mol) and water permeability was not inhibited by HgCl2; however, aquaporin 1 was detectable by immunoblotting. Immunostaining of rat lung with AQP1 and caveolin antisera revealed very low levels of colocalization. We conclude that aquaporin water channels do not contribute significantly to the observed water flux and that caveolae have relatively high water and solute permeabilities due to the high degree of unsaturation in their fatty acyl chains.

ENaC-membrane interactions: regulation of channel activity by membrane order

Recently, it was reported that the epithelial Na⁺ channel (ENaC) is regulated by temperature (Askwith, C.C., C.J. Benson, M.J. Welsh, and P.M. Snyder. 2001. Proc. Natl. Acad. Sci. USA. 98:6459–6463). As these changes of temperature affect membrane lipid order and lipid–protein interactions, we tested the hypothesis that ENaC activity can be modulated by membrane lipid interactions. Two approaches were used to modulate membrane anisotropy, a lipid order–dependent parameter. The nonpharmacological approach used temperature changes, while the pharmacological one used chlorpromazine (CPZ), an agent known to decrease membrane order, and Gd⁺³. Experiments used Xenopus oocytes expressing human ENaC. Methods of impedance analysis were used to determine whether the effects of changing lipid order indirectly altered ENaC conductance via changes of membrane area. These data were further corroborated with quantitative morphology on micrographs from oocytes membranes studied via electron microscopy. We report biphasic effects of cooling (stimulation followed by inhibition) on hENaC conductance. These effects were relatively slow (minutes) and were delayed from the actual bath temperature changes. Peak stimulation occurred at a calculated T_max of 15.2. At temperatures below T_max, ENaC conductance was inhibited with cooling. The effects of temperature on g_Na were distinct from those observed on ion channels endogenous to Xenopus oocytes, where the membrane conductance decreased monoexponentially with temperature (t = 6.2°C). Similar effects were also observed in oocytes with reduced intra- and extracellular [Na⁺], thereby ruling out effects of self or feedback inhibition. Addition of CPZ or the mechanosensitive channel blocker, Gd⁺³, caused inhibition of ENaC. The effects of Gd⁺³ were also attributed to its ability to partition into the outer membrane leaflet and to decrease anisotropy. None of the effects of temperature, CPZ, or Gd⁺³ were accompanied by changes of membrane area, indicating the likely absence of effects on channel trafficking. However, CPZ and Gd⁺³ altered membrane capacitance in an opposite manner to temperature, consistent with effects on the membrane-dielectric properties. The reversible effects of both Gd⁺³ and CPZ could also be blocked by cooling and trapping these agents in the rigidified membrane, providing further evidence for their mechanism of action. Our findings demonstrate a novel regulatory mechanism of ENaC.

Interaction of antimicrobial peptides with lipopolysaccharides

We study the interaction of antimicrobial peptides with lipopolysaccharide (LPS) bilayers to understand how antimicrobial peptides interact with the LPS monolayer on the outer membrane of Gram-negative bacteria. LPS in water spontaneously forms a multilamellar structure composed of symmetric bilayers. We performed X-ray lamellar diffraction and wide-angle in-plane scattering to study the physical characteristics of LPS multilayers. The multilayer alignment of LPS is comparable to phospholipids. Thus, it is suitable for the application of oriented circular dichroism (OCD) to study the state of peptides in LPS bilayers. At high hydration levels, the chain melting temperature in multilamella detected by X-ray diffraction is the same as that of LPS aqueous dispersions, as measured by calorimetry. LPS has a strong CD, but with a careful subtraction of the lipid background, the OCD of peptides in LPS is measurable. The method was tested successfully with melittin. It was then applied to two representative antimicrobial peptides, magainin and protegrin. At peptide concentrations comparable to the physiological conditions, both peptides penetrate transmembrane in LPS bilayers. The results imply that antimicrobial peptides readily penetrate the LPS monolayer of the outer membrane.

Structural basis of urothelial permeability barrier function as revealed by Cryo-EM studies of the 16 nm uroplakin particle

The apical surface of terminally differentiated mammalian urothelial umbrella cells is covered by numerous plaques consisting of two-dimensional (2D) crystals of hexagonally packed 16 nm uroplakin particles, and functions as a remarkable permeability barrier. To determine the structural basis of this barrier function, we generated, by electron cryo microscopy, a projection map of the isolated mouse urothelial plaques at 7 A and a 3D structure at 10 A resolution. Our results indicate that each 16 nm particle has a central 6 nm lipid-filled 'hole' surrounded by 6 inverted U-shaped subunits, each consisting of an inner and an outer subdomain connected via a distal joint. The transmembrane portion of each subdomain can fit about 5 helices. This finding, coupled with our STEM and EM data, suggests that uroplakin pairs Ia/II and Ib/III are associated with the inner and outer subdomains, respectively. Since the inner subdomains interconnect to form a ring, which can potentially segregate the lipids of the central hole from those outside, the 2D crystalline uroplakin network may impose an organized state and a severely restricted freedom of movement on the lipid components, thus reducing membrane fluidity and contributing to the barrier function of urothelial plaques. Our finding that distinct uroplakin substructures are in contact with the cytoplasmic and exoplasmic leaflets of the plaque suggests that the two leaflets may have different lipid composition and contribute asymmetrically to the barrier function. We propose that the crystalline lattice structure of uroplakin, through its interactions with specialized lipids, plays a major role in the remarkable permeability barrier function of urothelial apical surface. Our results also have implications for the transmembrane signal transduction in urothelial cells as induced by the binding of uropathogenic E. coli to its uroplakin receptor.

Enlargement and contracture of C2-ceramide channels

Ceramides are known to play a major regulatory role in apoptosis by inducing cytochrome c release from mitochondria. We have previously reported that ceramide, but not dihydroceramide, forms large and stable channels in phospholipid membranes and outer membranes of isolated mitochondria. C(2)-ceramide channel formation is characterized by conductance increments ranging from <1 to >200 nS. These conductance increments often represent the enlargement and contracture of channels rather than the opening and closure of independent channels. Enlargement is supported by the observation that many small conductance increments can lead to a large decrement. Also the initial conductances favor cations, but this selectivity drops dramatically with increasing total conductance. La(+3) causes rapid ceramide channel disassembly in a manner indicative of large conducting structures. These channels have a propensity to contract by a defined size (often multiples of 4 nS) indicating the formation of cylindrical channels with preferred diameters rather than a continuum of sizes. The results are consistent with ceramides forming barrel-stave channels whose size can change by loss or insertion of multiple ceramide columns.

Annexin A4 reduces water and proton permeability of model membranes but does not alter aquaporin 2-mediated water transport in isolated endosomes

Annexin A4 (Anx4) belongs to a ubiquitous family of Ca2+-dependent membrane-binding proteins thought to be involved in membrane trafficking and membrane organization within cells. Anx4 localizes to the apical region in epithelia; however, its physiological role is unclear. We show that Anx4 exhibited binding to liposomes (phosphatidylcholine:phosphatidylserine, 1:1) in the presence of Ca2+ and binding was reversible with EDTA. Anx4 binding resulted in liposome aggregation and a reduction in membrane water permeability of 29% (P < 0.001) at 25 degrees C. These effects were not seen in the presence of Ca2+ or Anx4 alone and were reversible with EDTA. Measurements of membrane fluidity made by monitoring fluorescence anisotropy of 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-HPC) demonstrated that Anx4 binding rigidified the outer leaflet of the bilayer (P < 0.001), thus providing a molecular explanation for the inhibition of water flux. To determine whether Anx4 would produce similar effects on physiological membranes we constructed liposomes which recapitulated the lipid composition of the inner leaflet of the MDCK apical membrane. These membranes exhibited reductions to water permeability upon Anx4 binding (19.5% at 25 degrees C, 31% at 37 degrees C; P < 0.01 and P < 0.001, respectively) and to proton permeability (15% at 25 degrees C, 19.5% at 37 degrees C; P < 0.05). Since our in vitro experiments indicated an effect on membrane permeability, we examined localization of Anx4 in the kidney collecting duct, a region of the nephron responsible for concentrating urine through water reabsorbtion. Anx4 was shown to colocalize apically with aquaporin 2 (AQP2) in collecting duct epithelia. To test for the existence of a functional interaction between Anx4 and AQP2 we isolated AQP2-containing endosomes and exposed them to Anx4/Ca2+. Water flux rates were unchanged, indicating Anx4 does not directly regulate AQP2. We conclude that Anx4 can alter the physical properties of membranes by associating with them and regulate passive membrane permeability to water and protons. These properties represent important new functions for Anx4.

Plant aquaporins: multifunctional water and solute channels with expanding roles

There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.

Water permeability of asymmetric planar lipid bilayers: leaflets of different composition offer independent and additive resistances to permeation

To understand how plasma membranes may limit water flux, we have modeled the apical membrane of MDCK type 1 cells. Previous experiments demonstrated that liposomes designed to mimic the inner and outer leaflet of this membrane exhibited 18-fold lower water permeation for outer leaflet lipids than inner leaflet lipids (Hill, W.G., and M.L. Zeidel. 2000. J. Biol. Chem. 275:30176-30185), confirming that the outer leaflet is the primary barrier to permeation. If leaflets in a bilayer resist permeation independently, the following equation estimates single leaflet permeabilities: 1/P(AB) = 1/P(A) + 1/P(B) (Eq. l), where P(AB) is the permeability of a bilayer composed of leaflets A and B, P(A) is the permeability of leaflet A, and P(B) is the permeability of leaflet B. Using for the MDCK leaflet-specific liposomes gives an estimated value for the osmotic water permeability (P(f)) of 4.6 x 10(-4) cm/s (at 25 degrees C) that correlated well with experimentally measured values in intact cells. We have now constructed both symmetric and asymmetric planar lipid bilayers that model the MDCK apical membrane. Water permeability across these bilayers was monitored in the immediate membrane vicinity using a Na+-sensitive scanning microelectrode and an osmotic gradient induced by addition of urea. The near-membrane concentration distribution of solute was used to calculate the velocity of water flow (Pohl, P., S.M. Saparov, and Y.N. Antonenko. 1997. Biophys. J. 72:1711-1718). At 36 degrees C, P(f) was 3.44 +/- 0.35 x 10(-3) cm/s for symmetrical inner leaflet membranes and 3.40 +/- 0.34 x 10(-4) cm/s for symmetrical exofacial membranes. From, the estimated permeability of an asymmetric membrane is 6.2 x 10(-4) cm/s. Water permeability measured for the asymmetric planar bilayer was 6.7 +/- 0.7 x 10(-4) cm/s, which is within 10% of the calculated value. Direct experimental measurement of P(f) for an asymmetric planar membrane confirms that leaflets in a bilayer offer independent and additive resistances to water permeation and validates the use of.

Molecular Mechanisms of Water and Solute Transport across Archaebacterial Lipid Membranes

Archaebacteria thrive in environments characterized by anaeobiosis, saturated salt, and both high and low extremes of temperature and pH. The bulk of their membrane lipids are polar, characterized by the archaeal structural features typified by ether linkage of the glycerol backbone to isoprenoid chains of constant length, often fully saturated, and with sn-2,3 stereochemistry opposite that of glycerolipids of Bacteria and Eukarya. Also unique to these bacteria are macrocyclic archaeol and membrane spanning caldarchaeol lipids that are found in some extreme thermophiles and methanogens. To define the barrier function of archaebacterial membranes and to examine the effects of these unique structural features on permeabilities, we investigated the water, solute (urea and glycerol), proton, and ammonia permeability of liposomes formed by these lipids. Both the macrocyclic archaeol and caldarchaeol lipids reduced the water, ammonia, urea, and glycerol permeability of liposomes significantly (6-120-fold) compared with diphytanylphosphatidylcholine liposomes. The presence of the ether bond and phytanyl chains did not significantly affect these permeabilities. However, the apparent proton permeability was reduced 3-fold by the presence of an ether bond. The presence of macrocyclic archaeol and caldarchaeol structures further reduced apparent proton permeabilities by 10-17-fold. These results indicate that the limiting mobility of the midplane hydrocarbon region of the membranes formed by macrocyclic archaeol and caldarchaeol lipids play a significant role in reducing the permeability properties of the lipid membrane. In addition, it appears that substituting ether for ester bonds presents an additional barrier to proton flux.

Reconstituting the barrier properties of a water-tight epithelial membrane by design of leaflet-specific liposomes

To define aspects of lipid composition and bilayer asymmetry critical to barrier function, we examined the permeabilities of liposomes that model individual leaflets of the apical membrane of a barrier epithelium, Madin-Darby canine kidney type 1 cells. Using published lipid compositions we prepared exofacial liposomes containing phosphatidylcholine, sphingomyelin, glycosphingolipids, and cholesterol; and cytoplasmic liposomes containing phosphatidylethanolamine, phosphatidylserine, and cholesterol. The osmotic permeability of cytoplasmic liposomes to water (P(f)), solutes, and NH(3) was 18-90-fold higher than for the exofacial liposomes (P(f(ex)) = 2.4 +/- 0.4 x 10(-4) cm/s, P(f(cy)) = 4.4 +/- 0.3 x 10(-3) cm/s; P(glycerol(ex)) = 2.5 +/- 0.3 x 10(-8) cm/s, P(glycerol(cy)) = 2.2 +/- 0.02 x 10(-6) cm/s; P(NH3(ex)) = 0. 13 +/- 0.4 x 10(-4) cm/s, P(NH3(cy)) = 7.9 +/- 1.0 x 10(-3) cm/s). By contrast, the apparent proton permeability of exofacial liposomes was 4-fold higher than cytoplasmic liposomes (P(H+(ex)) = 1.1 +/- 0. 1 x 10(-2) cm/s, P(H+(cy)) = 2.7 +/- 0.6 x 10(-3) cm/s). By adding single leaflet permeabilities, we calculated a theoretical P(f) for a Madin-Darby canine kidney apical membrane of 4.6 x 10(-4) cm/s, which compares favorably with experimentally determined values. In exofacial liposomes lacking glycosphingolipids or sphingomyelin, permeabilities were 2-7-fold higher, indicating that both species play a role in barrier function. Removal of cholesterol resulted in 40-280-fold increases in permeability. We conclude: 1) that we have reconstituted the biophysical properties of a barrier membrane, 2) that the barrier resides in the exofacial leaflet, 3) that both sphingomyelin and glycosphingolipids play a role in reducing membrane permeability but that there is an absolute requirement for cholesterol to mediate this effect, 4) that these results further validate the hypothesis that each leaflet offers an independent resistance to permeation, and 5) that proton permeation was enhanced by sphingolipid/cholesterol interactions.

The lipopolysaccharide barrier: correlation of antibiotic susceptibility with antibiotic permeability and fluorescent probe binding kinetics

Lipopolysaccharide (LPS), the primary lipid on the surface of Gram-negative bacteria, is thought to act as a permeability barrier, making the outer membrane relatively impermeable to hydrophobic antibiotics, detergents, and host proteins. Mutations in the LPS biosynthetic apparatus increase bacterial susceptibility to such agents. To determine how this increased susceptibility is mediated, we have correlated antibiotic susceptibilities of rough (antibiotic resistant) and deep rough (antibiotic susceptible) bacterial strains with antibiotic permeabilities and fluorescent probe binding kinetics for bilayers composed of LPS purified from the same strains. Bilayer permeabilities of two hydrophobic beta-lactam antibiotics were measured by encapsulating the appropriate beta-lactamases in large unilamellar vesicles. In the presence of MgCl(2), permeabilities of LPS bilayers from rough and deep rough bacteria were similar and significantly lower than those of bacterial phospholipids (BPL). Addition of BPL to the LPS bilayers increased their antibiotic permeability to approximately the level of the BPL bilayers. Binding rates of the fluorescent probe bis-aminonaphthylsulfonic acid (BANS) were 2 orders of magnitude slower for both rough and deep rough LPS bilayers compared to that of bilayers composed of BPL or mixtures of LPS and BPL. On the basis of these results and the observation that deep rough bacteria have higher levels of phospholipid on their surface than do rough bacteria (Kamio, Y., and Nikaido, H. (1976) Biochemistry 15, 2561-2569), we argue that the high susceptibility of deep rough bacteria is due to the presence of phospholipids on their surface. Experiments with phospholipid bilayers showed that the addition of PEG-lipids (containing covalently attached hydrophilic polymers) had little effect on permeability and binding rates, whereas the addition of cholesterol reduced permeability and slowed binding to levels approaching those of LPS. Therefore, we argue that the barrier provided by LPS is primarily due to its tight hydrocarbon chain packing (Snyder et al., (1999) Biochemistry 38, 10758-10767) rather than to its polysaccharide headgroup.

Role of leaflet asymmetry in the permeability of model biological membranes to protons, solutes, and gases

Bilayer asymmetry in the apical membrane may be important to the barrier function exhibited by epithelia in the stomach, kidney, and bladder. Previously, we showed that reduced fluidity of a single bilayer leaflet reduced water permeability of the bilayer, and in this study we examine the effect of bilayer asymmetry on permeation of nonelectrolytes, gases, and protons. Bilayer asymmetry was induced in dipalmitoylphosphatidylcholine liposomes by rigidifying the outer leaflet with the rare earth metal, praseodymium (Pr3+). Rigidification was demonstrated by fluorescence anisotropy over a range of temperatures from 24 to 50 degrees C. Pr3+-treatment reduced membrane fluidity at temperatures above 40 degrees C (the phase-transition temperature). Increased fluidity exhibited by dipalmitoylphosphatidylcholine liposomes at 40 degrees C occurred at temperatures 1-3 degrees C higher in Pr3+-treated liposomes, and for both control and Pr3+-treated liposomes permeability coefficients were approximately two orders of magnitude higher at 48 degrees than at 24 degrees C. Reduced fluidity of one leaflet correlated with significantly reduced permeabilities to urea, glycerol, formamide, acetamide, and NH3. Proton permeability of dipalmitoylphosphatidylcholine liposomes was only fourfold higher at 48 degrees than at 24 degrees C, indicating a weak dependence on membrane fluidity, and this increase was abolished by Pr3+. CO2 permeability was unaffected by temperature. We conclude: (a) that decreasing membrane fluidity in a single leaflet is sufficient to reduce overall membrane permeability to solutes and NH3, suggesting that leaflets in a bilayer offer independent resistances to permeation, (b) bilayer asymmetry is a mechanism by which barrier epithelia can reduce permeability, and (c) CO(2) permeation through membranes occurs by a mechanism that is not dependent on fluidity.

Modeling of P-glycoprotein-involved epithelial drug transport in MDCK cells

P-glycoprotein (P-gp) on the apical membranes of epithelial cells is known as a drug efflux pump. However, unclear is its integral quantitative role in the overall epithelial drug transfer, which also involves distinct diffusion processes in parallel and sequence. We used a simple three-compartment model to obtain kinetic parameters of each drug transfer mechanism, which can quantitatively describe the transport time courses of P-gp substrates, digoxin and vinblastine, across P-gp-expressing MDCK cell monolayers grown on permeable filters. Our results show that the model, which assumes a functionally single drug efflux pump in the apical membrane with diffusion across two membranes and intercellular junctions, is the least complex model with which to quantitatively reproduce the characteristics of the data. Interestingly, the model predicts that the MDCK apical membranes are less diffusion permeable than the basolateral membrane for both drugs and that the distribution volume of vinblastine is 10-fold higher than that of digoxin. Additional experiments verified these model predictions. The modeling approach is feasible to quantitatively describe overall kinetic picture of epithelial drug transport. Further model refinement is necessary to incorporate other modes of drug transport such as transcytosis. Also, whether P-gp solely accounts for the pump function in this model awaits more studies.

Reconstituted aquaporin 1 water channels transport CO2 across membranes

Biological membranes provide selective barriers to a number of molecules and gases. However, the factors that affect permeability to gases remain unclear because of the difficulty of accurately measuring gas movements. To determine the roles of lipid composition and the aquaporin 1 (AQP1) water channel in altering CO2 flux across membranes, we developed a fluorometric assay to measure CO2 entry into vesicles. Maximal CO2 flux was approximately 1000-fold above control values with 0.5 mg/ml carbonic anhydrase. Unilamellar phospholipid vesicles of varying composition gave widely varying water permeabilities but similar CO2 permeabilities at 25 degreesC. When AQP1 purified from human red blood cells was reconstituted into proteoliposomes, however, it increased water and CO2 permeabilities markedly. Both increases were abolished with HgCl2, and the mercurial inhibition was reversible with beta-mercaptoethanol. We conclude that unlike water and small nonelectrolytes, CO2 permeation is not significantly altered by lipid bilayer composition or fluidity. AQP1 clearly serves to increase CO2 permeation, likely through the water pore; under certain circumstances, gas permeation through membranes is protein-mediated.

K-Cl cotransporter expression in the human kidney

The K-Cl cotransporter protein KCC1 is a membrane transport protein that mediates the coupled, electroneutral transport of K and Cl across plasma membranes. The precise cell type(s) in the kidney that express the K-Cl cotransporter have remained unknown. The aim of the present investigation was to define the distribution of KCC1 mRNA in the human kidney. We used in situ hybridization with a nonradioactive digoxigenin-labeled riboprobe. We identified abundant KCC1 mRNA expression in the epithelial cells throughout the distal and proximal renal tubular epithelium. The transporter was also expressed in glomerular mesangial cells and endothelial cells of the renal vessels. These findings suggest that the K-Cl cotransporter may have an important role in transepithelial K and Cl reabsorption.

Water and solute permeabilities of medullary thick ascending limb apical and basolateral membranes

The medullary thick ascending limb (MTAL) reabsorbs solute without water and concentrates NH4+ in the interstitium without a favorable pH gradient, activities which require low water and NH3 permeabilities. The contributions of different apical and basolateral membrane structures to these low permeabilities are unclear. We isolated highly purified apical and basolateral MTAL plasma membranes and measured, by stopped-flow fluorometry, their permeabilities to water, urea, glycerol, protons, and NH3. Osmotic water permeability at 20 degrees C averaged 9.4 +/- 0.8 x 10(-4) cm/s for apical and 11.9 +/- 0.5 x 10(-4) cm/s for basolateral membranes. NH3 permeabilities at 20 degrees C averaged 0.0023 +/- 0.00035 and 0.0035 +/- 0.00080 cm/s for apical and basolateral membranes, respectively. These values are consistent with those obtained in isolated perfused tubules and can account for known aspects of MTAL function in vivo. Because the apical and basolateral membrane unit permeabilities are similar, the ability of the apical membrane to function as the site of barrier function arises from its very small surface area when compared with the highly redundant basolateral membrane.

Transbilayer movement of fully ionized taurine-conjugated bile salts depends upon bile salt concentration, hydrophobicity, and membrane cholesterol content

Taurine-conjugated bile salts mediate rapid transmembrane flux of divalent cations, irrespective of whether bile salts and divalent cations are initially on the same or opposite side of the membrane. We therefore hypothesized that ionized bile salts can equilibrate between membrane hemileaflets. We quantitated bile salt binding to large unilamellar egg yolk phosphatidylcholine (EYPC) +/- cholesterol (Ch) vesicles under conditions in which one or both hemileaflets were initially exposed to bile salts. At unbound taurodeoxycholate (TDC) concentrations >0.2 mM, the dependence of binding on TDC concentration after 30 min was indistinguishable for vesicles prepared by either method and did not change from 30 minutes to 24 h. At unbound TDC concentrations <0.1 mM, the ratio of bound/free TDC to EYPC vesicles doubled over a single exponential time course. Equilibration times were greater for the more hydrophilic bile salts taurocholate and tauroursodeoxycholate, for EYPC/Ch vesicles, and at lower temperatures. For glycine-conjugated bile salts, time-dependent changes in binding did not occur, consistent with more rapid equilibration of the small fraction of the protonated form. We conclude that fully ionized conjugated bile salts translocate between lipid bilayer hemileaflets, in contrast to previous observations that equilibration of fully ionized unconjugated bile salts occurs at a negligible rate in small unilamellar vesicles. The rate of "flip-flop" increases with increases in intramembrane bile salt concentration and hydrophobicity but decreases with cholesterol content and lower temperature. We speculate that physiologically, even in the absence of a specific membrane transporter, bile salts can gain access to intracellular compartments and mediate increases in divalent cation flux that may underlie cytotoxicity.

Functional analysis of nodulin 26, an aquaporin in soybean root nodule symbiosomes

Upon infection of soybean roots, nitrogen-fixing bacteria become enclosed in a specific organelle known as the symbiosome. The symbiosome membrane (SM) is a selectively permeable barrier that controls metabolite flux between the plant cytosol and the symbiotic bacterium inside. Nodulin 26 (NOD 26), a member of the aquaporin (AQP) water channel family, is a major protein component of the SM. Expression of NOD 26 in Xenopus oocytes gave a mercury-sensitive increase in osmotic water permeability (Pf). To define the biophysical properties of NOD 26 water channels in their native membranes, symbiosomes were isolated from soybean root nodules and the SM separated as vesicles from the bacteria. Permeabilities were measured using stopped-flow fluorimetry in SM vesicles with entrapped carboxyfluorescein. Osmotic water permeability (Pf) of SM was high, with a value of 0.05 +/- 0.003 cm/s observed at 20 degrees C (mean +/- S.E.; n = 15). Water flow exhibited a low activation energy, was inhibited by HgCl2 (0.1 mM), and exhibited a unit conductance of 3.2 +/- 1.3 x 10(-15) cm3/s, a value 30-fold lower than that of AQP 1, the red blood cell water channel. Diffusive water permeability (Pd) was 0.0024 +/- 0.0002 cm/s, and the resulting Pf to Pd ratio was 18.3, indicating that water crosses the SM in single file fashion via the NOD 26 water channel. In addition to high water permeability, SM vesicles also show high mercury-sensitive permeability to glycerol and formamide, but not urea, suggesting that NOD 26 also fluxes these solutes. Overall, we conclude that NOD 26 acts as a water channel with a single channel conductance that is 30-fold lower than AQP 1. Because the solutes that permeate NOD 26 are far larger than water, and water appears to cross the channel via a single file pathway, solute flux across NOD 26 appears to occur by a pathway that is distinct from that for water.

Autodirected insertion: preinserted VDAC channels greatly shorten the delay to the insertion of new channels

VDAC, a mitochondrial outer membrane channel, has the ability to catalyze and direct the insertion of other VDAC channels into planar phospholipid membranes. The spontaneous rate of insertion of detergent-solubilized VDAC channels into phospholipid membranes is estimated to be 1.5 x 10(-5) channels min-1 micron-2. VDAC channels already in the membrane can increase this rate by a factor of 10(9). The presence of 5 M urea on the opposite side of the membrane increases this 10-fold to 4.5 x 10(5) channels min-1 microns-2. Similar but weaker effects are observed with Triton X100 addition (10(-3)% (v/v)). These agents are not acting on uninserted channels because they do not affect the delay from sample addition to first insertion. Under the chosen conditions, this delay is long (240 s) without preinserted channels. However, the presence of a few VDAC channels in the membrane reduces this delay to 14 s, close to the diffusion limit. Therefore, urea and Triton, added to the side of the membrane opposite that to which the VDAC sample was added, likely increase the flexibility of the VDAC channels in the membrane, allowing them to be more efficient catalysts for VDAC insertion. There are obvious implications for membrane protein insertion and targeting.