Geometry of domains and domain boundaries in monolayers and bilayers

Atomic Force Microscopy: Interaction Forces Measured in Phospholipid Monolayers, Bilayers and Cell Membranes

Atomic force microscopy (AFM) is a powerful technique which is commonly used to image surfaces at the nanoscale and single-molecule level, as well as to investigate physical properties of the sample surface using a technique known as force spectroscopy. In this chapter, we review our recent research where we used AFM to investigate physical properties of phospholipid monolayers, bilayers, and cell membranes. We describe the experimental procedures for AFM imaging, force measurements, and theoretical models to analyze force spectroscopy data. The data obtained allowed correlations between AFM topography and local adhesion and mechanoelastic properties of supported lipid bilayers in water, supported pulmonary surfactant films in air, and the plasma membrane of epithelial type II cells. Finally, AFM was applied to help elucidate the effect of anesthetics and cholesterol present in the lipid films.

Effects of HMG-CoA reductase inhibitors on endothelial function: role of microdomains and oxidative stress

Certain pleiotropic activities reported for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are related to reductions in cellular cholesterol biosynthesis and isoprenoid levels. In endothelial cells, these metabolic changes contribute to favorable effects on nitric oxide (NO) bioavailability. Given the essential role of NO in preserving vascular structure and function, this effect of statins is of considerable therapeutic importance. Statins have been demonstrated to restore endothelial NO production by several mechanisms, including upregulating endothelial NO synthase (eNOS) protein expression and blocking formation of reactive oxygen species. In this article, we will discuss additional ways in which statins restore endothelial NO production and improve endothelial function. (1) Statins modulate membrane microdomain formation, resulting in reduced expression of proteins that specifically inhibit eNOS activation. (2) Statins reduce sterol biosynthesis, thus interfering with the formation of pathologic microdomains, including cholesterol crystalline structures. This observation has important implications for plaque stabilization, as these microdomains contribute to cholesterol crystal formation and endothelial apoptosis. Finally, (3) statins improve endothelial function by interfering with oxidative stress pathways through both enzymatic and nonenzymatic mechanisms. The relationships between membrane microdomains, cholesterol biosynthesis, and endothelial function will be discussed.

Role of cholesterol in lipid raft formation: lessons from lipid model systems

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid 'rafts' and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.

Direct evidence for cholesterol crystalline domains in biological membranes: role in human pathobiology

This review will discuss the use of small-angle X-ray diffraction approaches to study the organization of lipids in plasma membranes derived from two distinct mammalian cell types: arterial smooth muscle cells and ocular lens fiber cells. These studies indicate that cholesterol at an elevated concentration can self-associate and form immiscible domains in the plasma membrane, a phenomenon that contributes to both physiologic and pathologic cellular processes, depending on tissue source. In plasma membrane samples isolated from atherosclerotic smooth muscle cells, the formation of sterol-rich domains is associated with loss of normal cell function, including ion transport activity and control of cell replication. Analysis of meridional diffraction patterns from intact and reconstituted plasma membrane samples indicates the presence of an immiscible cholesterol domain with a unit cell periodicity of 34 A, consistent with a cholesterol monohydrate tail-to-tail bilayer, under disease conditions. These cholesterol domains were observed in smooth muscle cells enriched with cholesterol in vitro as well as from cells obtained ex vivo from an animal model of atherosclerosis. By contrast, well-defined cholesterol domains appear to be essential to the normal physiology of fiber cell plasma membranes of the human ocular lens. The organization of cholesterol into separate domains underlies the role of lens fiber cell plasma membranes in maintaining lens transparency. These domains may also interfere with cataractogenic aggregation of soluble lens proteins at the membrane surface. Taken together, these analyses provide examples of both physiologic and pathologic roles that sterol-rich domains may have in mammalian plasma membranes. These findings support a model of the membrane in which cholesterol aggregates into structurally distinct regions that regulate the function of the cell membrane.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

The differential miscibility of lipids as the basis for the formation of functional membrane rafts

The formation of sphingolipid-cholesterol microdomains in cellular membranes has been proposed to function in sorting and transport of lipids and proteins as well as in signal transduction. An increasing number of cell biological and biochemical studies now supports this concept. Here we discuss the structural properties of lipids in a cell biological context. The sphingolipid-cholesterol microdomains or rafts are described as dispersed liquid ordered phase domains. These domains are dynamic assemblies to which specific proteins are selectively sequestered while others are excluded. The proteins associated to rafts can act as organizers and can modulate raft size and function.