An outlook on organization of lipids in membranes: Searching for a realistic connection with the organization of biological membranes

Transbilayer phospholipids molecular imaging

Nuclear medicine has become a key part of molecular imaging. In the present review article, we focus on the transbilayer phospholipids as exquisite targets for radiolabelled probes in molecular imaging. Asymmetry of phospholipid distribution is a characteristic of mammalian cell membranes. Phosphatidylcholine and sphyngomyelin cholinophospholipids are primarily located within the external leaflet of the cell membrane. Phosphatidylserine and phosphatidylethanolamine aminophospholipids, and also phosphatidylinositol are primarily located within the internal leaflet of the cell membrane. New radiolabelled tracers have been designed in preclinical and clinical research for PET-CT and SPECT-CT molecular imaging of transbilayer phospholipids.

Lipids, curvature, and nano-medicine

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature.Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy.

Proteolipidic composition of exosomes changes during reticulocyte maturation

During the orchestrated process leading to mature erythrocytes, reticulocytes must synthesize large amounts of hemoglobin, while eliminating numerous cellular components. Exosomes are small secreted vesicles that play an important role in this process of specific elimination. To understand the mechanisms of proteolipidic sorting leading to their biogenesis, we have explored changes in the composition of exosomes released by reticulocytes during their differentiation, in parallel to their physical properties. By combining proteomic and lipidomic approaches, we found dramatic alterations in the composition of the exosomes retrieved over the course of a 7-day in vitro differentiation protocol. Our data support a previously proposed model, whereby in reticulocytes the biogenesis of exosomes involves several distinct mechanisms for the preferential recruitment of particular proteins and lipids and suggest that the respective prominence of those pathways changes over the course of the differentiation process.

Minimal model of plasma membrane heterogeneity requires coupling cortical actin to criticality

We present a minimal model of plasma membrane heterogeneity that combines criticality with connectivity to cortical cytoskeleton. The development of this model was motivated by recent observations of micron-sized critical fluctuations in plasma membrane vesicles that are detached from their cortical cytoskeleton. We incorporate criticality using a conserved order parameter Ising model coupled to a simple actin cytoskeleton interacting through point-like pinning sites. Using this minimal model, we recapitulate several experimental observations of plasma membrane raft heterogeneity. Small (r ∼ 20 nm) and dynamic fluctuations at physiological temperatures arise from criticality. Including connectivity to the cortical cytoskeleton disrupts large fluctuations, prevents macroscopic phase separation at low temperatures (T ≤ 22°C), and provides a template for long-lived fluctuations at physiological temperature (T = 37°C). Cytoskeleton-stabilized fluctuations produce significant barriers to the diffusion of some membrane components in a manner that is weakly dependent on the number of pinning sites and strongly dependent on criticality. More generally, we demonstrate that critical fluctuations provide a physical mechanism for organizing and spatially segregating membrane components by providing channels for interaction over large distances.

Changes in single K channel behavior induced by a lipid phase transition

Membrane protein activity is affected by the properties of the lipid bilayer hosting them. These properties are established by both the lipid composition and the thermodynamic state of the bilayer. In the latter case, any parameter that can alter the state of the bilayer is indirectly able to affect the activity of membrane proteins. In a recent study, we have demonstrated that the activity of the KcsA ion channel is strongly related to the thermodynamic state of the lipid bilayer. In particular, when the lipid bilayer is in its main phase transition region, the conductivity of KcsA is increased and all its characteristic times change according to the characteristic times of the lipid bilayer. We propose here that the lipid bilayer can affect the distribution among many conformational substates of the open channel, affecting the corresponding channel conductivity.

Imaging the early stages of phospholipase C/sphingomyelinase activity on vesicles containing coexisting ordered-disordered and gel-fluid domains

The binding and early stages of activity of a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa on giant unilamellar vesicles (GUV) have been monitored using fluorescence confocal microscopy. Both the lipids and the enzyme were labeled with specific fluorescent markers. GUV consisted of a mixture of phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, and cholesterol in equimolar ratios, to which 5-10 mol% of the enzyme end-product ceramide and/or diacylglycerol were occasionally added. Morphological examination of the GUV in the presence of enzyme reveals that, although the enzyme diffuses rapidly throughout the observation chamber, detectable enzyme binding appears to be a slow, random process, with new bound-enzyme-containing vesicles appearing for several minutes. Enzyme binding to the vesicles appears to be a cooperative process. After the initial cluster of bound enzyme is detected, further binding and catalytic activity follow rapidly. After the activity has started, the enzyme is not released by repeated washing, suggesting a "scooting" mechanism for the hydrolytic activity. The enzyme preferentially binds the more disordered domains, and, in most cases, the catalytic activity causes the disordering of the other domains. Simultaneously, peanut- or figure-eight-shaped vesicles containing two separate lipid domains become spherical. At a further stage of lipid hydrolysis, lipid aggregates are formed and vesicles disintegrate.

Plasmodesmata viewed as specialised membrane adhesion sites

A significant amount of work has been expended to identify the elusive components of plasmodesmata (PD) to help understand their structure, as well as how proteins are targeted to them. This review focuses on the role that lipid membranes may play in defining PD both structurally and as subcellular targeting addresses. Parallels are drawn to findings in other areas of research which focus on the lateral segregation of membrane domains and the generation of three-dimensional organellar shapes from flat lipid bilayers. We conclude that consideration of the protein-lipid interactions in cell biological studies of PD components and PD-targeted proteins may yield new insights into some of the many open questions regarding these unique structures.

Widefield microscopy for live imaging of lipid domains and membrane dynamics

Within the lateral organisation of plasma membranes of polarized cell types there exist heterogenous microdomains of distinct lipid composition, the small size of which (10–200 nm) makes them difficult to discern with traditional microscopic techniques, but which can be distinguished on the basis of lipid packing. These microdomains or rafts can be concentrated in larger more visible liquid-ordered regions, particularly by cross-linking of their constituents as in the immunological synapse or in features of the polarized cell such as pseudopodia or flagella. One technique, Laurdan fluorescence microscopy, has proven very useful for distinguishing such regions but has hitherto relied on 2-photon confocal microscopy. This has to some extent limited its utility to living systems and its widespread adoption in studying membrane dynamics on the surface of living cells. Here we describe and validate the adaptation of a standard widefield fluorescence microscope for live imaging of Laurdan stained cell membranes.