Link between Fluorescent Probe Partitioning and Molecular Order of Liquid Ordered-Liquid Disordered Membranes

Laurdan in living cells: Where do we stand?

Laurdan is a valuable tool for analyzing phase transitions and general behavior in synthetic lipid membranes. Its use is very straightforward, thus, its application in cells has expanded rapidly in recent years. It has been demonstrated that Laurdan is very useful for analyzing membrane trends when cells are subjected to some treatment, or when different cell mutations are compared. However, a deep interpretation of the data is not as straightforward as in synthetic lipid bilayers. In this review, we complied results found in mammalian and bacterial cells and noted that the use of Laurdan could be improved if a comparison between publications could be done. At the moment this is not easy, mainly due to the lack of complete information in the publications, and to the different methodologies employed in the data recording and processing. We conclude that research in cell membrane topics would benefit from a better use of the Laurdan probe.

Coupling liquid phases in 3D condensates and 2D membranes: Successes, challenges, and tools

This review describes the major experimental challenges researchers meet when attempting to couple phase separation between membranes and condensates. Although it is well known that phase separation in a 2D membrane could affect molecules capable of forming a 3D condensate (and vice versa), few researchers have quantified the effects to date. The scarcity of these measurements is not due to a lack of intense interest or effort in the field. Rather, it reflects significant experimental challenges in manipulating coupled membranes and condensates to yield quantitative values. These challenges transcend many molecular details, which means they impact a wide range of systems. This review highlights recent exciting successes in the field, and it lays out a comprehensive list of tools that address potential pitfalls for researchers who are considering coupling membranes with condensates.

Quantitative assessment of lipophilic membrane dye-based labelling of extracellular vesicles by nano-flow cytometry

Although lipophilic membrane dyes (LMDs) or probes (LMPs) are widely used to label extracellular vesicles (EVs) for detection and purification, their labelling performance has not been systematically characterized. Through concurrent side scattering and fluorescence detection of single EVs as small as 40 nm in diameter by a laboratory-built nano-flow cytometer (nFCM), present study identified that (1) PKH67 and PKH26 could maximally label ∼60%-80% of EVs isolated from the conditioned cell culture medium (purity of ∼88%) and ∼40%-70% of PFP-EVs (purity of ∼73%); (2) excessive PKH26 could cause damage to the EV structure; (3) di-8-ANEPPS and high concentration of DiI could achieve efficient and uniform labelling of EVs with nearly 100% labelling efficiency for di-8-ANEPPS and 70%-100% for DiI; (4) all the four tested LMDs can aggregate and form micelles that exhibit comparable side scatter and fluorescence intensity with those of labelled EVs and thus hardly be differentiate from each other; (5) as the LMD concentration went up, the particle number of self-aggregates increased while the fluorescence intensity of aggregates remained constant; (6) PKH67 and PKH26 tend to form more aggregated micelles than di-8-ANEPPS and DiI, and the effect of LMD self-aggregation can be negligible at optimal staining conditions. (7) All the four tested LMDs can label almost all the very-low-density lipoprotein (VLDL) particles, indicating potential confounding factor in plasma-EV labelling. Besides, it was discovered that DSPE-PEG2000 -biotin can only label ∼50% of plasma-EVs. The number of LMP inserted into the membrane of single EVs was measured for the first time and it was confirmed that membrane labelling by lipophilic dyes did not interfere with the immunophenotyping of EVs. nFCM provides a unique perspective for a better understanding of EV labelling by LMD/LMP.

Hybrid lipid/block copolymer vesicles display broad phase coexistence region

The fluidity and polar environment of ~100 nm hybrid vesicles combining dipalmitoylphosphatidylcholine (DPPC) and poly(1,2-butadiene)-block-polyethylene oxide (PBd-PEO, average molecular weight 950 g/mol) were studied upon vesicle heating using the fluorescence spectroscopy techniques of DPH anisotropy and laurdan generalized polarization (GP). These techniques indicated PBd-PEO membranes are less ordered than solid DPPC, but slightly more ordered than fluid DPPC or dioleoylphosphatidylcholine (DOPC) membranes. We find the DPH anisotropy values are less than expected from additivity of the components' anisotropies in the fluid phase mixture of DPPC and PBd-PEO, inferring that DPPC strongly fluidizes the PBd-PEO. We use transitions in DPH anisotropy and laurdan GP to create a temperature/composition phase diagram for DPPC/PBd-PEO which we find displays a significantly broader solid/fluid phase coexistence region than DPPC/DOPC, showing that DPPC partitions less readily into fluid PBd-PEO than into fluid DOPC. The existence of a broad solid/fluid phase coexistence region in DPPC/PBd-PEO vesicles is verified by Förster resonance energy transfer results and the visualization of phase separation in giant unilamellar vesicles containing up to 95% PBd-PEO and a single phase in 100% PBd-PEO vesicles at room temperature. These results add to the limited knowledge of phase behavior and phase diagrams of hybrid vesicles, and should be useful in understanding and tailoring membrane surface architecture toward biomedical applications such as drug delivery or membrane protein reconstitution.

The influence of lipid membranes on fluorescent probes' optical properties

BACKGROUND

Organic fluorophores embedded in lipid bilayers can nowadays be described by a multiscale computational approach. Combining different length and time scales, a full characterization of the probe localization and optical properties led to novel insight into the effect of the environments.

SCOPE OF REVIEW

Following an introduction on computational advancements, three relevant probes are reviewed that delineate how a multiscale approach can lead to novel insight into the probes' (non) linear optical properties. Attention is paid to the quality of the theoretical description of the optical techniques.

MAJOR CONCLUSIONS

Computation can assess a priori novel probes' optical properties and guide the analysis and interpretation of experimental data in novel studies. The properties can be used to gain information on the phase and condition of the surrounding biological environment.

GENERAL SIGNIFICANCE

Computation showed that a canonical view on some of the probes should be revisited and adapted.

Bicelles and nanodiscs for biophysical chemistry

Membrane nanoobjects are very important tools to study biomembrane properties. Two types are described herein: Bicelles and Nanodiscs. Bicelles are obtained by thorough water mixing of long chain and short chain lipids and may take the form of membranous discs of 10-50 nm. Temperature-composition-hydration diagrams have been established for Phosphatidylcholines and show limited domains of existence. Bicelles can be doped with charged lipids, surfactants or with cholesterol and offer a wide variety of membranous platforms for structural biology. Internal dynamics as measured by solid-state NMR is very similar to that of liposomes in their fluid phase. Because of the magnetic susceptibility anisotropy of the lipid chains, discs may be aligned along or perpendicular to the magnetic field. They may serve as weak orienting media to provide distance information in determining the 3D structure of soluble proteins. In different conditions they show strong orienting properties which may be used to study the 3D structure, topology and dynamics of membrane proteins. Lipid Bicelles with biphenyl chains or doped with lanthanides show long lasting remnant orientation after removing the magnetic field due to smectic-like properties. An alternative to pure lipid Bicelles is provided by nanodiscs where the half torus composed by short chain lipids is replaced by proteins. This renders the nano-objects less fragile as they can be used to stabilize membrane protein assemblies to be studied by electron microscopy. Internal dynamics is again similar to liposomes except that the phase transition is abolished, possibly due to lateral constrain imposed by the toroidal proteins limiting the disc size. Advantages and drawbacks of both nanoplatforms are discussed.

Principles and Applications of Biological Membrane Organization

Many critical biological events, including biochemical signaling, membrane traffic, and cell motility, originate at membrane surfaces. Each such event requires that members of a specific group of proteins and lipids rapidly assemble together at a specific site on the membrane surface. Understanding the biophysical mechanisms that stabilize these assemblies is critical to decoding and controlling cellular functions. In this article, we review progress toward a quantitative biophysical understanding of the mechanisms that drive membrane heterogeneity and organization. We begin from a physical perspective, reviewing the fundamental principles and key experimental evidence behind each proposed mechanism. We then shift to a biological perspective, presenting key examples of the role of heterogeneity in biology and asking which physical mechanisms may be responsible. We close with an applied perspective, noting that membrane heterogeneity provides a novel therapeutic target that is being exploited by a growing number of studies at the interface of biology, physics, and engineering.

Conformational Changes as Driving Force for Phase Recognition: The Case of Laurdan

The development of a universal probe to assess the phase of a lipid membrane is one of the most ambitious goals for fluorescence spectroscopy. The ability of a well-known molecule as Laurdan to reach this aim is here exploited as the behavior of the probe is fully characterized in a dipalmitoylphosphatidylcholine (DPPC) solid gel (So) phase by means of molecular dynamics simulations. Laurdan can take two conformations, depending on whether the carbonyl oxygen points toward the β-position of the naphthalene core (Conf-I) or to the α-position (Conf-II). We observe that Conf-I has an elongated form in this environment, whereas Conf-II takes an L-shape. Interestingly, our theoretical calculations show that these two conformations behave in an opposite way from what is reported in the literature for a DPPC membrane in a liquid disordered (Ld) phase, where Conf-I assumes an L-shape and Conf-II is elongated. Moreover, our results show that in DPPC (So) no intermixing between the conformations is present, whereas it has been seen in a fluid environment such as DOPC (Ld). Through a careful analysis of angle distributions and by means of the rotational autocorrelation function, we predict that the two conformers of Laurdan behave differently in different membrane environments.

Liquid-liquid phase coexistence in lipid membranes observed by natural abundance 1H-13C solid-state NMR

We demonstrate that 1H-13C solid-state MAS NMR is suitable to detect liquid disordered/liquid ordered phase coexistence in a DOPC/DPPC/cholesterol mixture with natural abundance of isotopes as an alternative to 2H NMR. Such methodology is potentially applicable to study lipid phase coexistence phenomena in biological matter with high lipid content, e.g. lung surfactant or myelin, for which isotopic labeling is not possible.

Critical effects of polar fluorescent probes on the interaction of DHA with POPC supported lipid bilayers

The understanding of lipid bilayer structure and function has been advanced by the application of molecular fluorophores. However, the effects of these probe molecules on the physicochemical properties of membranes being studied are poorly understood. A quartz crystal microbalance with dissipation monitoring instrument was used in this work to investigate the impact of two commonly used fluorescent probes, 1‑palmitoyl‑2‑{12‑[(7‑nitro‑2‑1,3‑benzoxadiazol‑4‑yl)amino]dodecanoyl}‑sn‑glycero‑3‑phosphocholine (NBD-PC) and 1,2‑dipalmitoyl‑sn‑glycero‑3‑phosphoethanolamine‑n‑(lissamine rhodamine‑B‑sulfonyl) (Lis-Rhod PE), on the formation and physicochemical properties of a 1‑palmitoyl‑2‑oleoyl‑sn‑glycero‑3‑phosphocholine supported lipid bilayer (POPC-SLB). The interaction of the POPC-SLB and fluorophore-modified POPC-SLB with docosahexaenoic acid, DHA, was evaluated. The incorporation of DHA into the POPC-SLB was observed to significantly decrease in the presence of the Lis-Rhod PE probe compared with the POPC-SLB. In addition, it was observed that the small concentration of DHA incorporated into the POPC:NBD-PC SLB can produce rearrangement processes followed by the lost not only of DHA but also of POPC or NBD-PC molecules or both during the washing step. This work has significant implications for the interpretation of data employing fluorescent reporter molecules within SLBs.

n-Alcohol Length Governs Shift in Lo-Ld Mixing Temperatures in Synthetic and Cell-Derived Membranes

A persistent challenge in membrane biophysics has been to quantitatively predict how membrane physical properties change upon addition of new amphiphiles (e.g., lipids, alcohols, peptides, or proteins) in order to assess whether the changes are large enough to plausibly result in biological ramifications. Because of their roles as general anesthetics, n-alcohols are perhaps the best-studied amphiphiles of this class. When n-alcohols are added to model and cell membranes, changes in membrane parameters tend to be modest. One striking exception is found in the large decrease in liquid-liquid miscibility transition temperatures (Tmix) observed when short-chain n-alcohols are incorporated into giant plasma membrane vesicles (GPMVs). Coexisting liquid-ordered and liquid-disordered phases are observed at temperatures below Tmix in GPMVs as well as in giant unilamellar vesicles (GUVs) composed of ternary mixtures of a lipid with a low melting temperature, a lipid with a high melting temperature, and cholesterol. Here, we find that when GUVs of canonical ternary mixtures are formed in aqueous solutions of short-chain n-alcohols (n ≤ 10), Tmix increases relative to GUVs in water. This shift is in the opposite direction from that reported for cell-derived GPMVs. The increase in Tmix is robust across GUVs of several types of lipids, ratios of lipids, types of short-chain n-alcohols, and concentrations of n-alcohols. However, as chain lengths of n-alcohols increase, nonmonotonic shifts in Tmix are observed. Alcohols with chain lengths of 10-14 carbons decrease Tmix in ternary GUVs of dioleoyl-PC/dipalmitoyl-PC/cholesterol, whereas 16 carbons increase Tmix again. Gray et al. observed a similar influence of the length of n-alcohols on the direction of the shift in Tmix. These results are consistent with a scenario in which the relative partitioning of n-alcohols between liquid-ordered and liquid-disordered phases evolves as the chain length of the n-alcohol increases.