Role of tilt order in the asymmetric ripple phase of phospholipid bilayers

We present the electron density map of the asymmetric ripple phase of dilauroylphosphatidylcholine. We find that the primary feature characterizing the “asymmetry” of the rippled bilayers is the difference in the bilayer thickness in the two arms, and not the asymmetry of the bilayer height profile as is generally assumed. This difference in the bilayer thickness can be attributed to a mean tilt of the hydrocarbon chains of the lipid molecules along the direction of the ripple wave vector. We propose a Landau theory for this phase which takes into account the anisotropic elastic properties of a bilayer with tilt order

Structure of the ripple phase in chiral and racemic dimyristoylphosphatidylcholine multibilayers

We present electron density maps of the ripple phase of chiral and racemic dimyristoylphosphatidylcholine. The structures of the two systems are found to be identical within experimental errors, thus unambiguously showing that the chirality of the lipid molecules does not influence the structure of this phase

Signature of a non-harmonic potential as revealed from a consistent shape and fluctuation analysis of an adherent membrane

The interaction of fluid membranes with a scaffold, which can be a planar surface or a more complex structure, is intrinsic to a number of systems - from artificial supported bilayers and vesicles to cellular membranes. In principle, these interactions can be either discrete and protein mediated, or continuous. In the latter case, they emerge from ubiquitous intrinsic surface interaction potentials as well as nature-designed steric contributions of the fluctuating membrane or from the polymers of the glycocalyx. Despite the fact that these nonspecific potentials are omnipresent, their description has been a major challenge from experimental and theoretical points of view. Here we show that a full understanding of the implications of the continuous interactions can be achieved only by expanding the standard superposition models commonly used to treat these types of systems, beyond the usual harmonic level of description. Supported by this expanded theoretical framework, we present three independent, yet mutually consistent, experimental approaches to measure the interaction potential strength and the membrane tension. Upon explicitly taking into account the nature of shot noise as well as of finite experimental resolution, excellent agreement with the augmented theory is obtained, which finally provides a coherent view of the behavior of the membrane in a vicinity of a scaffold.Comment: 15 pages, 12 figures, accepted by Physical Review

T Cells on Engineered Substrates: The Impact of TCR Clustering Is Enhanced by LFA-1 Engagement

We created APC-mimetic synthetic substrates to study the impact of ligand clustering on T cell activation and spreading. The substrates exhibit antibodies directed against the TCR-complex in the form of a patterned array of sub micrometric dots surrounded by a fluid supported lipid bilayer (SLB) which may itself be functionalized with another bio-molecule. We show that for T cell adhesion mediated by T cell receptor (TCR) alone, in the patterned, but not in the corresponding homogeneous controls, the TCR, ZAP-70 and actin are present in the form of clusters or patches that co-localize with the ligand-dots. However, global cell scale parameters like cell area and actin distribution are only weakly impacted by ligand clustering. In presence of ICAM-1 - the ligand of the T cell integrin LFA-1 - on the SLB, the TCR is still clustered due to the patterning of its ligands, but now global parameters are also impacted. The actin organization changes to a peripheral ring, resembling the classical actin distribution seen on homogeneous substrates, the patterned membrane topography disappears and the membrane is flat, whereas the cell area increases significantly. These observations taken together point to a possible pivotal role for LFA-1 in amplifying the effect of TCR-clustering. No such effect is evident for co-engagement of CD28, affected via its ligand B7.2. Unlike on ICAM-1, on B7.2 cell spreading and actin organization are similar for homogeneous and patterned substrates. However, TCR and ZAP-70 clusters are still formed in the patterned case. These results indicate complementary role for LFA-1 and CD28 in the regulation and putative coupling of TCR micro-clusters to actin. The engineered substrates presented here clearly have the potential to act as platform for fundamental research in immune cell biology, as well as translational analyses in immunotherapy, for example to screen molecules for their role in T cell adhesion/activation

Physics of Organelle Membrane Bridging via Cytosolic Tethers is Distinct From Cell Adhesion

Tremendous progress has been made recently in imaging the contacts between intra-cellular organelles, which are thought to be mediated by soluble tethers. However, they are still difficult to study in cellulo, and reconstituting them in vitro is a standing challenge. Here we take a mimetic approach to study Giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) interacting via single- (or double-) stranded DNA sequences of two different lengths. Like intra-cellular tethers which may reside in the cytosol when unbound, the DNA-tethers are soluble, but can insert into the membrane with the help of cholesterol moieties found at their extremities. Tether-exchange between the bulk “cytosol” and the GUV/SLB membrane leads to a novel statistical ensemble in which the entire system equilibrates together, rather than individual GUVs behaving as separate closed systems. As a consequence, adhesion between the GUV and the SLB is driven by collective entropic effects amplified by tether shape changes associated with membrane bridging. A direct experimental consequence is an unusual dependence on tether-concentration, which becomes an important control parameter at low concentrations, while tether length/flexibility are less important. The establishment of this fundamentally different interaction between two membranes suggests that in physiological conditions, the regulation of contact formation inside cells may be very different from the case of the much studied ligand-receptor mediated cell adhesion

Quantitative reflection interference contrast microscopy (RICM) in soft matter and cell adhesion.: RICM in Soft Matter and Cell Adhesion

International audienceAdhesion can be quantified by measuring the distance between the interacting surfaces. Reflection interference contrast microscopy (RICM), with its ability to measure inter-surface distances under water with nanometric precision and milliseconds time resolution, is ideally suited to studying the dynamics of adhesion in soft systems. Recent technical developments, which include innovative image analysis and the use of multi-coloured illumination, have led to renewed interest in this technique. Unambiguous quantitative measurements have been achieved for colloidal beads and model membranes, thus revealing new insights and applications. Quantification of data from cells shows exciting prospects. Herein, we review the basic principles and recent developments of RICM applied to studies of dynamical adhesion processes in soft matter and cell biology and provide practical hints to potential users

Structure of the ripple phase of phospholipid multibilayers

We present electron density maps (EDMs) of the ripple phase formed by phosphorylcholine lipids such as dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dihexadecyl phosphatidylcholine, and dilauroyl phosphatidylcholine (DLPC). With the exception of DLPC, the rippled bilayers have a sawtooth shape in all the systems, with one arm being almost twice as long as the other. For DMPC and POPC bilayers, EDMs have been obtained at different temperatures at a fixed relative humidity, and the overall shape of the ripples and the ratio of the lengths of the two arms are found to be insensitive to temperature. EDMs of all the systems with saturated hydrocarbon chains suggest the existence of a mean chain tilt along the ripple wave vector. In the literature it is generally assumed that the asymmetry of the rippled bilayers (absence of a mirror plane normal to the ripple wave vector) arises from a sawtoothlike height profile. However, in the case of DLPC, the height profile is found to be almost symmetric and the asymmetry results mainly from different bilayer thicknesses in the two arms of the ripple. We also present EDMs of the metastable ripple phase of dipalmitoyl phosphatidylcholine, formed on cooling from the Lalpha phase.NRC publication: Ye