Shedding light on membrane rafts structure and dynamics in living cells

Lipophilic index of serum phospholipids in patients with type 2 diabetes and atherosclerotic cardiovascular disease: links with metabolic control, vascular inflammation and platelet activation

BACKGROUND

Little is known about the mechanisms underlying the association of the serum phospholipid lipophilic index (LI) with atherosclerotic cardiovascular disease (ASCVD) in patients with type 2 diabetes (T2D). Therefore, we investigated whether the LI is associated with glucometabolic control, meta-inflammation, thrombin generation, fibrin clot properties, endothelial function and platelet activation in T2D patients with angiographically documented ASCVD.

METHODS

We studied 74 T2D patients with ASCVD, aged 65.6 ± 6.8 years, with a median diabetes duration of 10 years and median HbA1c of 7.0%. Serum phospholipid fatty acids (FAs) were measured by gas chromatography. The serum phospholipid LI was calculated as the sum of the products of the proportion (% of total FAs) with the melting points (°C) of each individual FA, divided by the sum of the proportions of all FAs. Levels of HbA1c, insulin, leptin, adiponectin, lipid profiles, inflammatory markers (hsCRP, interleukin-6, TNF-α), Lp-PLA2 (a biomarker of vascular inflammation), endothelial function (sICAM-1, sVCAM-1, FMD, NMD), thrombin generation, fibrin clot properties and platelet activation, measured by light transmission aggregometry with arachidonic acid [AA] and adenosine diphosphate [ADP], were assessed.

RESULTS

Patients with LI > 16.9 °C (median) had higher HbA1c concentrations by 5.9% compared to the remaining subjects (p = 0.035). In this group, HbA1c levels ≥ 7.0% were found more often than in individuals with LI ≤ 16.9 °C (62.2 vs. 35.1%; p = 0.020). Subjects with LI > 16.9 °C had higher levels of TCh by 17.1% (p = 0.012), LDL-Ch by 29.4% (p = 0.003), interleukin-6 by 22.2% (p = 0.031) and Lp-PLA2 by 32.4% (p = 0.040), compared to the remaining patients. Moreover, they had increased maximal platelet aggregation induced by AA (p = 0.045), but not by ADP. Serum phospholipid LI correlated with HbA1c (r = 0.24; p = 0.037), TCh (r = 0.36; p = 0.002), LDL-Ch (r = 0.38; p < 0.001), interleukin-6 (r = 0.27; p = 0.020) and Lp-PLA2 (r = 0.26; p = 0.026). There were no intergroup differences in endothelial function, thrombin generation and fibrin clot properties. Regression analysis showed that HbA1c ≥ 7.0% and serum levels of LDL-Ch, interleukin-6 and Lp-PLA2 were predictors of LI > 16.9 °C in adjusted models.

CONCLUSIONS

In well-controlled T2D patients with ASCVD, the higher serum phospholipid LI is associated with worse glucometabolic control, enhanced vascular inflammation and higher platelet reactivity during aspirin treatment at cyclooxygenase-1-selective doses.

A fatty acid-ordered plasma membrane environment is critical for Ebola virus matrix protein assembly and budding

Plasma membrane (PM) domains and order phases have been shown to play a key role in the assembly, release, and entry of several lipid-enveloped viruses. In the present study, we provide a mechanistic understanding of the Ebola virus (EBOV) matrix protein VP40 interaction with PM lipids and their effect on VP40 oligomerization, a crucial step for viral assembly and budding. VP40 matrix formation is sufficient to induce changes in the PM fluidity. We demonstrate that the distance between the lipid headgroups, the fatty acid tail saturation, and the PM order are important factors for the stability of VP40 binding and oligomerization at the PM. The use of FDA-approved drugs to fluidize the PM destabilizes the viral matrix assembly leading to a reduction in budding efficiency. Overall, these findings support an EBOV assembly mechanism that reaches beyond lipid headgroup specificity by using ordered PM lipid regions independent of cholesterol.

Line tension and line entropy associated with the monolayer/trilayer boundary of smectic liquid crystal at the air-water interface

Molecularly thin films of the smectic liquid crystal 4'-octyl-4-biphenylcarbonitrile (8CB) at the air-water interface phase separate into regions with different numbers of layers, in analogy with freestanding smectic liquid crystalline films. This paper reports the line tension associated with the boundary of coexisting trilayer and monolayer phases of in Langmuir films of 8CB at the air-water interface as a function of temperature and humidity and infers information on the boundary profile between the coexisting phases. Two complementary techniques are used to characterize the 8CB thin films: surface pressure-area isotherm and Brewster angle microscopy (BAM). We determine the line tension by stretching isolated domains from their equilibrium circular shape and analyzing the free relaxation with a hydrodynamic model. Then, we interpret the line tension vs temperature data in terms of an excess line entropy for the domain boundary, which requires careful consideration of the thermodynamics of inhomogeneous monolayer systems.

Super-resolution imaging of folate receptor alpha on cell membranes using peptide-based probes

Folate receptor alpha (FRα) is a vital membrane protein which have great association with cancers and involved in various biological processes including folate transport and cell signaling. However, the distribution and organization pattern of FRα on cell membranes remains unclear. Previous studies relied on antibodies to recognize the proteins. However, multivalent crosslinking and large size of antibodies confuse the direct observation to some extent. Fortunately, the emergence of peptide, which are small-sized and monovalent, has supplied us an unprecedented choice. Here, we applied fluorophore-conjugated peptide probe to recognize the FRα and study the distribution pattern of FRα on cell membrane using dSTORM super-resolution imaging technique. FRα were found to organized as clusters on cell surface with different sizes. And they have a higher expression level and formed larger clusters on various cancer cells than normal cells, which hinted that its specific distribution could be utilized for cancer diagnosis. Furthermore, we revealed that the lipid raft and cortical actin as restrictive factors for the FRα clustering, suggesting a potential assembly mechanism insight into FRα clustering on cell membrane. Collectively, our work clarified the morphology distribution and clustered organization of FRα with peptide probes at the nanometer scale, which paves the way for further revealing the relationship between the spatial organization and functions of membranal proteins.

Methodological Pitfalls of Investigating Lipid Rafts in the Brain: What Are We Still Missing?

The purpose of this review is to succinctly examine the methodologies used in lipid raft research in the brain and to highlight the drawbacks of some investigative approaches. Lipid rafts are biochemically and biophysically different from the bulk membrane. A specific lipid environment within membrane domains provides a harbor for distinct raftophilic proteins, all of which in concert create a specialized platform orchestrating various cellular processes. Studying lipid rafts has proved to be arduous due to their elusive nature, mobility, and constant dynamic reorganization to meet the cellular needs. Studying neuronal lipid rafts is particularly cumbersome due to the immensely complex regional molecular architecture of the central nervous system. Biochemical fractionation, performed with or without detergents, is still the most widely used method to isolate lipid rafts. However, the differences in solubilization when various detergents are used has exposed a dire need to find more reliable methods to study particular rafts. Biochemical methods need to be complemented with other approaches such as live-cell microscopy, imaging mass spectrometry, and the development of specific non-invasive fluorescent probes to obtain a more complete image of raft dynamics and to study the spatio-temporal expression of rafts in live cells.

Sphingomyelinase modulates synaptic vesicle mobilization at the mice neuromuscular junctions

AIMS

Sphingomyelin is an abundant component of the presynaptic membrane and an organizer of lipid rafts. In several pathological conditions, sphingomyelin is hydrolyzed due to an upregulation and release of secretory sphingomyelinases (SMases). Herein, the effects of SMase on exocytotic neurotransmitter release were studied in the diaphragm neuromuscular junctions of mice.

MAIN METHODS

Microelectrode recordings of postsynaptic potentials and styryl (FM) dyes were used to estimate neuromuscular transmission. Membrane properties were assessed with fluorescent techniques.

KEY FINDINGS

Application of SMase at a low concentration (0.01 U ml^-1) led to a disruption of lipid-packing in the synaptic membranes. Neither spontaneous exocytosis nor evoked neurotransmitter release (in response to single stimuli) were affected by SMase treatment. However, SMase significantly increased neurotransmitter release and the rate of fluorescent FM-dye loss from the synaptic vesicles at 10, 20 and 70 Hz stimulation of the motor nerve. In addition, SMase treatment prevented a shift of the exocytotic mode from "full-collapse" fusion to "kiss-and-run" during high-frequency (70 Hz) activity. The potentiating effects of SMase on neurotransmitter release and FM-dye unloading were suppressed when synaptic vesicle membranes were also exposed to this enzyme (i.e., stimulation occurred during SMase treatment).

SIGNIFICANCE

Thus, hydrolysis of the plasma membrane sphingomyelin can enhance mobilization of synaptic vesicles and facilitate full fusion mode of exocytosis, but SMase acting on vesicular membrane had a depressant effect on the neurotransmission. Partially, the effects of SMase can be related with the changes in synaptic membrane properties and intracellular signaling.

Super-Resolution Microscopy to Study Interorganelle Contact Sites

Interorganelle membrane contact sites (MCS) are areas of close vicinity between the membranes of two organelles that are maintained by protein tethers. Recently, a significant research effort has been made to study MCS, as they are implicated in a wide range of biological functions, such as organelle biogenesis and division, apoptosis, autophagy, and ion and phospholipid homeostasis. Their composition, characteristics, and dynamics can be studied by different techniques, but in recent years super-resolution fluorescence microscopy (SRFM) has emerged as a powerful tool for studying MCS. In this review, we first explore the main characteristics and biological functions of MCS and summarize the different approaches for studying them. Then, we center on SRFM techniques that have been used to study MCS. For each of the approaches, we summarize their working principle, discuss their advantages and limitations, and explore the main discoveries they have uncovered in the field of MCS.

Identification of a New Cholesterol-Binding Site within the IFN-γ Receptor that is Required for Signal Transduction

The cytokine interferon-gamma (IFN-γ) is a master regulator of innate and adaptive immunity involved in a broad array of human diseases that range from atherosclerosis to cancer. IFN-γ exerts it signaling action by binding to a specific cell surface receptor, the IFN-γ receptor (IFN-γR), whose activation critically depends on its partition into lipid nanodomains. However, little is known about the impact of specific lipids on IFN-γR signal transduction activity. Here, a new conserved cholesterol (chol) binding motif localized within its single transmembrane domain is identified. Through direct binding, chol drives the partition of IFN-γR2 chains into plasma membrane lipid nanodomains, orchestrating IFN-γR oligomerization and transmembrane signaling. Bioinformatics studies show that the signature sequence stands for a conserved chol-binding motif presented in many mammalian membrane proteins. The discovery of chol as the molecular switch governing IFN-γR transmembrane signaling represents a significant advance for understanding the mechanism of lipid selectivity by membrane proteins, but also for figuring out the role of lipids in modulating cell surface receptor function. Finally, this study suggests that inhibition of the chol-IFNγR2 interaction may represent a potential therapeutic strategy for various IFN-γ-dependent diseases.

Revealing local molecular distribution, orientation, phase separation, and formation of domains in artificial lipid layers: Towards comprehensive characterization of biological membranes

Lipids, together with molecules such as DNA and proteins, are one of the most relevant systems responsible for the existence of life. Selected lipids are able to assembly into various organized structures, such as lipid membranes. The unique properties of lipid membranes determine their complex functions, not only to separate biological environments, but also to participate in regulatory functions, absorption of nutrients, cell-cell communication, endocytosis, cell signaling, and many others. Despite numerous scientific efforts, still little is known about the reason underlying the variability within lipid membranes, and its biochemical significance. In this review, we discuss the structural complexity of lipid membranes, as well as the importance to simplify studied systems in order to understand phenomena occurring in natural, complex membranes. Such systems require a model interface to be analyzed. Therefore, here we focused on analytical studies of artificial systems at various interfaces. The molecular structure of lipid membranes, specifically the nanometric thickens of molecular bilayer, limits in a major extent the choice of highly sensitive methods suitable to study such structures. Therefore, we focused on methods that combine high sensitivity, and/or chemical selectivity, and/or nanometric spatial resolution, such as atomic force microscopy, nanospectroscopy (tip-enhanced Raman spectroscopy, infrared nanospectroscopy), phase modulation infrared reflection-absorption spectroscopy, sum-frequency generation spectroscopy. We summarized experimental and theoretical approaches providing information about molecular structure and composition, lipid spatial distribution (phase separation), organization (domain shape, molecular orientation) of lipid membranes, and real-time visualization of the influence of various molecules (proteins, drugs) on their integrity. An integral part of this review discusses the latest achievements in the field of lipid layer-based biosensors.

Role of Protein-Lipid Interactions in Viral Entry

The viral entry consists of several sequential events that ensure the attachment of the virus to the host cell and the introduction of its genetic material for the continuation of the replication cycle. Both cellular and viral lipids have gained a wider focus in recent years in the field of viral entry, as they are found to play key roles in different steps of the process. The specific role is summarized that lipids and lipid membrane nanostructures play in viral attachment, fusion, and immune evasion and how they can be targeted with antiviral therapies. Finally, some of the limitations of techniques commonly used for protein-lipid interactions studies are discussed, and new emerging tools are reviewed that can be applied to this field.