Raft-dependent endocytic movement and intracellular cluster formation during T cell activation triggered by concanavalin A

Phase-Separated Structures of Sake Flavors-Containing Cell Model Membranes

Sake (a traditional Japanese alcoholic beverage) contains ethyl caproate (EC), which enhances its economic value. Isovaleraldehyde (IVA) is also a well-known flavoring agent in alcoholic beverages, which some people enjoy. Recently, studies revealed that EC decreased the size of homogenous 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes whereas IVA increased their size. Cholesterol (Chol) and ergosterol were previously referred to as animal and fungus sterols. For the first time, this study demonstrated the phase behavior of the membrane in cell-sized liposomes containing EC and IVA. After adding EC, the solid ordered/liquid disordered (Ld) and liquid ordered (Lo)/Ld phase separation in DOPC/dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol or ergosterol ternary membranes decreased, but the Lo/Ld phase separation decreased after adding IVA. Biophysics, physiological, and application aspects of EC and IVA evaluation were discussed. The findings of this study not only enhance our understanding of the function of flavors but also provide rapid and cost effective performance for the measurement.

Charged Lipids Influence Phase Separation in Cell-Sized Liposomes Containing Cholesterol or Ergosterol

Positively charged ion species and charged lipids play specific roles in biochemical processes, especially those involving cell membranes. The cell membrane and phase separation domains are attractive research targets to study signal transduction. The phase separation structure and functions of cell-sized liposomes containing charged lipids and cholesterol have been investigated earlier, and the domain structure has also been studied in a membrane model, containing the yeast sterol ergosterol. The present study investigates phase-separated domain structure alterations in membranes containing charged lipids when cholesterol is substituted with ergosterol. This study finds that ergosterol increases the homogeneity of membranes containing charged lipids. Cholesterol-containing membranes are more sensitive to a charged state, and ergosterol-containing liposomes show lower responses to charged lipids. These findings may improve our understanding of the differences in both yeast and mammalian cells, as well as the interactions of proteins with lipids during signal transduction.

The Flavonoid Molecule Procyanidin Reduces Phase Separation in Model Membranes

Procyanidin extracted from fruits, such as apples, has been shown to improve lipid metabolization. Recently, studies have revealed that procyanidin interacts with lipid molecules in membranes to enhance lipid metabolism; however, direct evidence of the interaction between procyanidin and lipid membranes has not been demonstrated. In this study, the phase behaviors and changes in the membrane fluidity of cell-sized liposomes containing apple procyanidin, procyanidin B2 (PB2), were demonstrated for the first time. Phase separation in 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol ternary membranes significantly decreased after the addition of PB2. The prospect of applying procyanidin content measurements, using the results of this study, to commercial apple juice was also assessed. Specifically, the PB2 concentrations were 50%, 33%, and 0% for pure apple juice, 2-fold diluted apple juice, and pure water, respectively. The results of the actual juice were correlated with PB2 concentrations and phase-separated liposomes ratios, as well as with the results of experiments involving pure chemicals. In conclusion, the mechanism through which procyanidin improves lipid metabolism through the regulation of membrane fluidity was established.

Phase Separation in Liposomes Determined by Ergosterol and Classified Using Machine Learning

Recent studies indicated that ergosterol (Erg) helps form strongly ordered lipid domains in membranes that depend on their chemical characters. However, direct evidence of concentration-dependent interaction of Erg with lipid membranes has not been reported. We studied the Erg concentration-dependent changes in the phase behaviors of membranes using cell-sized liposomes containing 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). We observed the concentration range of phase separation in ternary membranes was significantly wider when Erg rather than cholesterol (Chol) was used as the sterol component. We used machine learning for the first time to analyze microscopic images of cell-sized liposomes and identify phase-separated structures. The automated method was successful in identifying homogeneous membranes but performance remained data-limited for the identification of phase separation domains characterized by more complex features.

Artificial Palmitoylation of Proteins Controls the Lipid Domain-Selective Anchoring on Biomembranes and the Raft-Dependent Cellular Internalization

Protein palmitoylation, a post-translational modification, is universally observed in eukaryotic cells. The localization of palmitoylated proteins to highly dynamic, sphingolipid- and cholesterol-rich microdomains (called lipid rafts) on the plasma membrane has been shown to play an important role in signal transduction in cells. However, this complex biological system is not yet completely understood. Here, we used a combined approach where an artificial lipidated protein was applied to biomimetic model membranes and plasma membranes in cells to illuminate chemical and physiological properties of the rafts. Using cell-sized giant unilamellar vesicles, we demonstrated the selective partitioning of enhanced green fluorescent protein modified with a C-terminal palmitoyl moiety (EGFP-Pal) into the liquid-ordered phase consisting of saturated phospholipids and cholesterol. Using Jurkat T cells treated with an immunostimulant (concanavalin A), we observed the vesicular transport of EGFP-Pal. Further cellular studies with the treatment of methyl β-cyclodextrin revealed the cholesterol-dependent internalization of EGFP-Pal, which can be explained by a raft-dependent, caveolae-mediated endocytic pathway. The present synthetic approach using artificial and natural membrane systems can be further extended to explore the potential utility of artificially lipidated proteins at biological and artificial interfaces.

Protein phosphatase 2A catalytic subunit β suppresses PMA/ionomycin-induced T-cell activation by negatively regulating PI3K/Akt signaling

The precise regulation of the T-cell activation process is critical for overall immune homeostasis. Although protein phosphatase 2A (PP2A) is required for T-cell development and function, the role of PPP2CB, which is the catalytic subunit β isoform of PP2A, remains unknown. In the present study, using a T cell-specific knockout mouse of PPP2CB (PPP2CB^fl/fl Lck-Cre⁺ ), we demonstrated that PPP2CB was dispensable for T-cell development in the thymus and peripheral lymphoid organs. Furthermore, PPP2CB deletion did not affect T-cell receptor (TCR)-induced T-cell activation or cytokine-induced T-cell responses; however, it specifically enhanced phorbol myristate acetate (PMA) plus ionomycin-induced T-cell activation with increased cellular proliferation, elevated CD69 and CD25 expression, and enhanced cytokine production (inteferon-γ, interleukin-2 and tumor necrosis factor). Mechanistic analyses suggested that the PPP2CB deletion enhanced activation of the phosphoinositide 3-kinase/Akt signaling pathway and Ca²⁺ flux following stimulation with PMA plus ionomycin. Moreover, the specific PI3K inhibitor rescued the augmented cell activation in PPP2CB-deficient T cells. Using mass spectrometry-based phospho-peptide analysis, we identified potential substrates of PPP2CB during PMA plus ionomycin-induced T-cell activation. Collectively, our study provides evidence of the specific role of PPP2CB in controlling PMA plus ionomycin-induced T-cell activation.

Enrichment of membrane curvature-sensing proteins from Escherichia coli using spherical supported lipid bilayers

Bacteria display dynamically organized curved membrane structures, especially during cell division. The importance of membrane curvature-sensing (MCS) proteins for the recognition and regulation of biological membrane morphologies has predominately been investigated in eukaryotic cells. Recently, a technique for screening MCS proteins from solutions that contain peripheral membrane proteins was developed, and MCS protein candidates were identified from mammalian cells. The technique uses differently sized spherical supported lipid bilayers (SSLBs), which consist of spherical SiO₂ particles covered with a lipid bilayer. To discriminate between proteins possessing the MCS property, SSLBs with the same surface area were used in a comparative sedimentation assay with shotgun proteome analysis. In this study, to prove that the technique could be applied to other samples, MCS proteins in Escherichia coli were investigated. Through a comparative proteomic study, 35 and 47 proteins were enriched as candidate MCS proteins preferentially bound to SSLBs of 100 nm and 1000 nm, respectively. Among the identified MCS candidate proteins, FtsZ and SecA were further examined for their MCS properties using the two SSLB sizes, which revealed a high binding affinity for the low membrane curvature (large SSLB). This is the first study to explore MCS proteins in prokaryotic cells and the MCS property of the SecA protein. The results demonstrate a method to enrich MCS proteins that could be utilized to better elucidate membrane dynamics and protein function expression on curved membrane structures in prokaryotic cells.

Loureirin B Exerts its Immunosuppressive Effects by Inhibiting STIM1/Orai1 and KV1.3 Channels

Loureirin B (LrB) is a constituent extracted from traditional Chinese medicine Resina Draconis. It has broad biological functions and an impressive immunosuppressive effect that has been supported by numerous studies. However, the molecular mechanisms underlying Loureirin B-induced immune suppression are not fully understood. We previously reported that Loureirin B inhibited K_V1.3 channel, calcium ion (Ca²⁺) influx, and interleukin-2 (IL-2) secretion in Jurkat T cells. In this study, we applied CRISPR/Cas9 to edit K_V1.3 coding gene KCNA3 and successfully generated a K_V1.3 knockout (KO) cell model to determine whether K_V1.3 KO was sufficient to block the Loureirin B-induced immunosuppressive effect. Surprisingly, we showed that Loureirin B could still inhibit Ca²⁺ influx and IL-2 secretion in the Jurkat T cells in the absence of K_V1.3 although KO K_V1.3 reduced about 50% of Ca²⁺ influx and 90% IL-2 secretion compared with that in the wild type cells. Further experiments showed that Loureirin B directly inhibited STIM1/Orai1 channel in a dose-dependent manner. Our results suggest that Loureirin B inhibits Ca²⁺ influx and IL-2 secretion in Jurkat T cells by inhibiting both K_V1.3 and STIM1/Orai1 channels. These studies also revealed an additional molecular target for Loureirin B-induced immunosuppressive effect, which makes it a promising leading compound for treating autoimmune diseases.

Development of a compound oral liquid containing herbal extracts and its effect on immunity and gastric mucosa

Nowadays, consumers have an increasing demand for health products. In this study, an oral liquid was developed using a compound extract consisting of three herbal extracts (Dendrobium nobile Lindl., Lycium barbarum, and Puerariae lobatae Radix) because the compound extract (a combination of all three extracts) was superior to every single extract in promoting the phagocytic capacity of RAW264.7 macrophages and the proliferation ability of GES-1 cells. In this oral liquid, the dosage of the stabilizer and the sweetener was selected using a stability test and sensory quality evaluation. When 0.30% (m/v) xanthan gum and 0.20% (m/v) mogroside were added, the oral liquid had not only a good stability but also the highest sensory score for overall acceptability. The chemical composition analysis showed that the oral liquid had various functional ingredients including polysaccharides, phenols, alkaloids, and so forth. The immune-enhancing efficacy of the oral liquid was evaluated in BALB/c mice by measuring the levels of different immune indicators. The results indicated that the oral liquid obviously enhanced nonspecific and specific immunity. A rat model with ethanol-induced gastric ulcer was used to examine the protective effect of the oral liquid on the gastric mucosa and to explore the related mechanisms. The oral administration of the oral liquid for days significantly prevented the formation of gastric ulcer. This study provided an effective oral liquid that could enhance immunity and protect gastric mucosa.

Effect of temperature on raft-dependent endocytic cluster formation during activation of Jurkat T cells by concanavalin A

Temperature plays an important role in the immune response. Acclimatization occurs when there are changes in ambient temperature over a long period. In this study, we used the human leukemic Jurkat T cell line to study the effect of temperature on the immune system using concanavalin A (ConA), a plant-derived immunostimulant, as a trigger for T-cell activation. Previously, we have reported endocytic intracellular cluster formation during T-cell activation by ConA with the aid of rafts and polymerization of the cytoskeleton (actin and microtubules). Here, we investigated the effect of temperature on cluster formation (with the aid of three-dimensional images of the cells) and on the stability of rafts, actin, and microtubules. When the temperature was changed between 23°C and 37°C (physiological temperature), clusters could be observed throughout this temperature range. Raft structure was stabilized at lower temperatures but destabilized at higher temperatures. Actin was stable when the temperature was higher than 27°C. When actin was depolymerized, clustering was not observed at 37°C but could be observed at 23°C. There were no changes in microtubules within this temperature range. Thus, raft clustering may be associated with raft stability at lower temperatures (<27°C) and with actin at higher temperatures (≥27°C). Hence, we provided insight into the associations between temperature, rafts, actin, and microtubules in the immune response.