Channel geometry governs the unitary osmotic water channel permeability, pf, according to classical hydrodynamics. Yet, pf varies by several orders of magnitude for membrane channels with a constriction zone that is one water molecule in width and four to eight molecules in length. We show that both the pf of those channels and the diffusion coefficient of the single-file waters within them are determined by the number NH of residues in the channel wall that may form a hydrogen bond with the single-file waters. The logarithmic dependence of water diffusivity on NH is in line with the multiplicity of binding options at higher NH densities. We obtained high-precision pf values by (i) having measured the abundance of the reconstituted aquaporins in the vesicular membrane via fluorescence correlation spectroscopy and via high-speed atomic force microscopy, and (ii) having acquired the vesicular water efflux from scattered light intensities via our new adaptation of the Rayleigh-Gans-Debye equation.

The permeability of lipid membranes for metabolic molecules or drugs is routinely estimated from the solute's oil/water partition coefficient. However, the molecular determinants that modulate the permeability in different lipid compositions have remained unclear. Here, we combine scanning electrochemical microscopy and molecular-dynamics simulations to study the effect of cholesterol on membrane permeability, because cholesterol is abundant in all animal membranes. The permeability of membranes from natural lipid mixtures to both hydrophilic and hydrophobic solutes monotonously decreases with cholesterol concentration [Chol]. The same is true for hydrophilic solutes and planar bilayers composed of dioleoyl-phosphatidylcholine or dioleoyl-phosphatidyl-ethanolamine. However, these synthetic lipids give rise to a bell-shaped dependence of membrane permeability on [Chol] for very hydrophobic solutes. The simulations indicate that cholesterol does not affect the diffusion constant inside the membrane. Instead, local partition coefficients at the lipid headgroups and at the lipid tails are modulated oppositely by cholesterol, explaining the experimental findings. Structurally, these modulations are induced by looser packing at the lipid headgroups and tighter packing at the tails upon the addition of cholesterol.

Background: The tightness of various membrane barriers to CO2 is of unknown molecular origin.Results: The bladder tissue lacks carbonic anhydrase. The resulting low intra-epithelial CO2 concentration gives rise to the apparent CO2 impermeability.Conclusion: Uroplakins do not act to decrease transepithelial CO2 flux.Significance: Enzymatic regulation of CO2 abundance rules out that aquaporins significantly contribute to the maintenance of acid base homeostasis.

Mechanical forces generated by cells mediate shape changes that occur during essential life processes including polarization, division and spreading. While the cell cytoskeleton is recognized for its myriad contributions to force generation, the mechanisms by which the cell membrane may also generate forces are often overlooked. Therefore, we explore the potential that membrane generates mechanical tension on cellular length scales by measuring the traction stresses generated during liposome adhesion and spreading on compliant substrates. We find that giant liposomes devoid of a cytoskeleton generate contractile traction stresses on par with those exerted by living cells. These stresses result from the equilibration of internal, hydrostatic pressures elevated by the membrane tension built by strong adhesion to the substrate. These results highlight the active role of membranes in the generation of mechanical stresses on cellular length scales. Furthermore, it uncovers that the modulation of hydrostatic pressure via membrane tension and adhesion can be channeled to perform mechanical work on the environment, providing a more comprehensive description of cell contractility and force generation. Accumulating evidence indicates that diverse physiological processes are influenced by the lipid composition of the membrane and by its material properties. This has notably been shown for the function of diverse proteins and their oligomerization, and processes on larger scales such as membrane reshaping and fusion. Determination of the elastic properties of lipidic membranes is therefore of great importance to our understanding of these processes. Experimental approaches to determine the material properties of lipids remain challenging and usually rely on their study in a relaxed environment or in flat bilayers, although it is widely accepted that cell membranes can be under considerable stress and frustration as well as high local curvature. Whether this impacts the measured properties is a matter of debate so that studying membranes under more realistic conditions is key for our understanding how these material properties impact different physiological processes. In this context, we propose a computational method to determine the elastic properties of lipid assemblies of arbitrarily shaped interfaces and use it to study the impact of the curvature of a membrane on its elastic properties. Specifically, we apply the methodology to mixtures of DOPE (dioleoylphosphatidylethanolamine) lipids and cholesterol in the inverted-hexagonal and lamellar phases and find that the bending rigidity for a particular lipid composition critically depends on the geometry of the lipidic system. This dependence correlates on the molecular level to the changes in lipid chain order imposed by the membrane curvature, implying that these results should pertain to other situations where the membrane is deformed, stressed or frustrated that notably emerge around integral membrane proteins or during membrane remodeling processes such as budding. 916-...

The routes water takes through membrane barriers is still a matter of debate. Although aquaporins only allow transmembrane water movement along an osmotic gradient, cotransporters are believed to be capable of water transport against the osmotic gradient. Here we show that the renal potassium-chloride-cotransporter (KCC1) does not pump a fixed amount of water molecules per movement of one K(+) and one Cl(-), as was reported for the analogous transporter in the choroid plexus. We monitored water and potassium fluxes through monolayers of primary cultured renal epithelial cells by detecting tiny solute concentration changes in the immediate vicinity of the monolayer. KCC1 extruded K(+) ions in the presence of a transepithelial K(+) gradient, but did not transport water. KCC1 inhibition reduced epithelial osmotic water permeability P(f) by roughly one-third, i.e., the effect of inhibitors was small in resting cells and substantial in hormonal stimulated cells that contained high concentrations of aquaporin-2 in their apical membranes. The furosemide or DIOA (dihydroindenyl-oxy-alkanoic acid)-sensitive water flux was much larger than expected when water passively followed the KCC1-mediated ion flow. The inhibitory effect of these drugs on water flux was reversed by the K(+)-H(+) exchanger nigericin, indicating that KCC1 affects water transport solely by K(+) extrusion. Intracellular K(+) retention conceivably leads to cell swelling, followed by an increased rate of endocytic AQP2 retrieval from the apical membrane.