Protocols for Enzymatic Fluorometric Assays to Quantify Phospholipid Classes

Phospholipid biomarkers of coronary heart disease

Coronary heart disease, also known as ischemic heart disease, is induced by atherosclerosis, which is initiated by subendothelial retention of lipoproteins. Plasma lipoproteins, including high density lipoprotein, low density lipoprotein (LDL), very low density lipoprotein, and chylomicron, are composed of a surface monolayer containing phospholipids and cholesterol and a hydrophobic core containing triglycerides and cholesteryl esters. Phospholipids play a crucial role in the binding of apolipoproteins and enzymes to lipoprotein surfaces, thereby regulating lipoprotein metabolism. High LDL-cholesterol is a well-known risk factor for coronary heart disease, and statins reduce the risk of coronary heart disease by lowering LDL-cholesterol levels. In contrast, the relationships of phospholipids in plasma lipoproteins with coronary heart disease have not yet been established. To further clarify the physiological and pathological roles of phospholipids, we have developed the simple high-throughput assays for quantifying all major phospholipid classes, namely phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidic acid, phosphatidylinositol, phosphatidylglycerol + cardiolipin, and sphingomyelin, using combinations of specific enzymes and a fluorogenic probe. These enzymatic fluorometric assays will be helpful in elucidating the associations between phospholipid classes in plasma lipoproteins and coronary heart disease and in identifying phospholipid biomarkers. This review describes recent progress in the identification of phospholipid biomarkers of coronary heart disease.

Oral TNF-α siRNA delivery via milk-derived exosomes for effective treatment of inflammatory bowel disease

Oral administration facilitates the direct delivery of drugs to lesions within the small intestine and colon, making it an ideal approach for treating patients with inflammatory bowel disease. However, multiple physical barriers impede the delivery of oral RNA drugs through the gastrointestinal tract. Herein, we developed a novel oral siRNA delivery system that protects nucleic acids in extreme environments by employing exosomes derived from milk to encapsulate tumor necrosis factor-alpha (TNF-α) siRNA completely. The remarkable structural stability of milk-derived exosomes (M-Exos), as opposed to those from HEK293T cells, makes them exceptional siRNA carriers. Results demonstrate that milk exosomes loaded with TNF-α siRNA (M-Exo/siR) can effectively inhibit the expression of TNF-α-related inflammatory cytokines. Moreover, given that milk exosomes are composed of unique lipids with high bioavailability, orally administered M-Exo/siR effectively reach colonic tissues, leading to decreased TNF-α expression and successful alleviation of colitis symptoms in a dextran sulfate sodium-induced inflammatory bowel disease murine model. Hence, milk-derived exosomes carrying TNF-α siRNA can be effectively employed to treat inflammatory bowel disease. Indeed, using exosomes naturally derived from milk may shift the current paradigm of oral gene delivery, including siRNA.

Extracellular-Vesicle Catch-and-Release Isolation System Using a Net-Charge Invertible Curvature-Sensing Peptide

Extracellular vesicles (EVs) carry various informative components, including signaling proteins, transcriptional regulators, lipids, and nucleic acids. These components are utilized for cell-cell communication between donor and recipient cells. EVs have shown great promise as pharmaceutical-targeting vesicles and have attracted the attention of researchers in the fields of biological and medical science because of their importance as diagnostic and prognostic markers. However, the isolation and purification of EVs from cell-cultured media remain challenging. Ultracentrifugation is the most widely used method, but it requires specialized and expensive equipment. In the present study, we proposed a novel methodology to isolate EVs using a simple and convenient method, i.e., an EV catch-and-release isolation system (EV-CaRiS) using a net-charge invertible curvature-sensing peptide (NIC). Curvature-sensing peptides recognize vesicles by binding to lipid-packing defects on highly curved membranes regardless of the expression levels of biomarkers. NIC was newly designed to reversibly capture and release EVs in a pH-dependent manner. NIC allowed us to achieve reproducible EV isolation from three human cell lines on resin using a batch method and single-particle imaging of EVs containing the ubiquitous exosome markers CD63 and CD81 by total internal reflection fluorescence microscopy (TIRFM). EV-CaRiS was demonstrated as a simple and convenient methodology for EV isolation, and NIC is promising for applications in the single-particle analysis of EVs.

Differences in interaction of graphene/graphene oxide with bacterial and mammalian cell membranes

Graphene, a single layer, hexagonally packed two-dimensional carbon sheet is an attractive candidate for diverse applications including antibacterial potential and drug delivery. One of the knowledge gaps in biomedical application of graphene is the interaction of these materials with the cells. To address this, we investigated the interaction between graphene materials (graphene and graphene oxide) and plasma membranes of cells (bacterial and mammalian cells). The interactions of four of the most abundant phospholipids in bacteria and mammalian plasma membranes with graphene materials were studied using density functional theory (DFT) at the atomic level. The calculations showed that the mammalian phospholipids have stronger bonding to each other compared to bacterial phospholipids. When the graphene/graphene oxide sheet is approaching the phospholipid pairs, the bacterial pairs exhibit less repulsive interactions, thereby a more stable system with the sheets was found. We also assembled bacterial and mammalian phospholipids into liposomes. We further observed that the bacterial liposomes and cells let the graphene flakes penetrate the membrane. The differential scanning calorimetry measurements of liposomes revealed that the bacterial liposomes have the lowest heat capacity; this strengthens the theoretical predictions of weaker interaction between the bacterial phospholipids compared to the mammalian phospholipids. We further demonstrated that graphene oxide could be internalized into the mammalian liposomes without disrupting the membrane integrity. The results suggest that the weak bonding among bacteria phospholipids and less repulsive force when graphene materials approach, result in graphene materials interacting differently with the bacteria compared to mammalian cells.

Vegetable Oil-Peroxidation Product 'Hydroxynonenal' Causes Hepatocyte Injury and Steatosis via Hsp70.1 and BHMT Disorders in the Monkey Liver

Hsp70.1 has a dual function as a chaperone protein and lysosomal stabilizer. In 2009, we reported that calpain-mediated cleavage of carbonylated Hsp70.1 causes neuronal death by inducing lysosomal rupture in the hippocampal CA1 neurons of monkeys after transient brain ischemia. Recently, we also reported that consecutive injections of the vegetable oil-peroxidation product 'hydroxynonenal' induce hepatocyte death via a similar cascade in monkeys. As Hsp70.1 is also related to fatty acid β-oxidation in the liver, its deficiency causes fat accumulation. The genetic deletion of betaine-homocysteine S-methyltransferase (BHMT) was reported to perturb choline metabolism, inducing a decrease in phosphatidylcholine and resulting in hepatic steatosis. Here, focusing on Hsp70.1 and BHMT disorders, we studied the mechanisms of hepatocyte degeneration and steatosis. Monkey liver tissues with and without hydroxynonenal injections were compared using proteomics, immunoblotting, immunohistochemical, and electron microscopy-based analyses. Western blotting showed that neither Hsp70.1 nor BHMT were upregulated, but an increased cleavage was observed in both. Proteomics showed a marked downregulation of Hsp70.1, albeit a two-fold increase in the carbonylated BHMT. Hsp70.1 carbonylation was negligible, in contrast to the ischemic hippocampus, which was associated with ~10-fold increments. Although histologically, the control liver showed very little lipid deposition, numerous tiny lipid droplets were seen within and around the degenerating/dying hepatocytes in monkeys after the hydroxynonenal injections. Electron microscopy showed permeabilization/rupture of lysosomal membranes, dissolution of the mitochondria and rough ER membranes, and proliferation of abnormal peroxisomes. It is probable that the disruption of the rough ER caused impaired synthesis of the Hsp70.1 and BHMT proteins, while impairment of the mitochondria and peroxisomes contributed to the sustained generation of reactive oxygen species. In addition, hydroxynonenal-induced disorders facilitated degeneration and steatosis in the hepatocytes.

Determination of Phenol with Peroxidase Immobilized on CaCO3

Phenols are widely used in industries despite their toxicity, which requires governments to limit their concentration in water to 5 mg/L before discharge to the city sewer. Thus, it is essential to develop a rapid, simple, and low-cost detection method for phenol. This study explored two pathways of peroxidase immobilization to develop a phenol detection system: peroxidase encapsulation into polyelectrolyte microcapsules and peroxidase captured by CaCO3. The encapsulation of peroxidase decreased enzyme activity by 96%; thus, this method cannot be used for detection systems. The capturing process of peroxidase by CaCO3 microspherulites did not affect the maximum reaction rate and the Michaelis constant of peroxidase. The native peroxidase—Vmax = 109 µM/min, Km = 994 µM; CaCO3–peroxidase—Vmax = 93.5 µM/min, Km = 956 µM. Ultimately, a reusable phenol detection system based on CaCO3 microparticles with immobilized peroxidase was developed, capable of detecting phenol in the range of 700 ng/mL to 14 µg/mL, with an error not exceeding 5%, and having a relatively low cost and production time. The efficiency of the system was confirmed by determining the content of phenol in a paintwork product.

Application of enzymatic fluorometric assays to quantify phosphatidylcholine, phosphatidylethanolamine and sphingomyelin in human plasma lipoproteins

Phosphatidylcholine (PC), phosphatidylethanolamine (PE) and sphingomyelin (SM) are important surface components of plasma lipoproteins, including very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL). However, the pathophysiological roles of PC, PE and SM in lipoproteins have not been well characterized owing to the difficulties in quantifying phospholipid classes in lipoproteins. In this study, we assessed the precision and accuracy of the enzymatic fluorometric assays for measuring PC, PE and SM in VLDL, LDL and HDL, which were isolated from human plasma by ultracentrifugation. The within-run coefficients of variation (CV) for the measurements of PC, PE and SM in lipoproteins were 1.5-2.8 %, 1.1-2.4 % and 0.9-2.3 %, respectively, whereas the between-run CVs for the PC, PE and SM assays were 2.7-4.7 %, 2.1-4.5 % and 1.6-3.3 %, respectively. Excellent linearity and almost complete recovery were achieved for all assays measuring PC, PE and SM in VLDL, LDL and HDL. Our preliminary results using these enzymatic fluorometric assays suggested that the phospholipid compositions were different among VLDL, LDL and HDL. In conclusion, we established high-throughput enzymatic fluorometric assays to quantify PC, PE and SM in human plasma VLDL, LDL and HDL, which will be useful for further investigation of pathophysiological roles of phospholipids in lipoproteins.

Alterations in cellular and organellar phospholipid compositions of HepG2 cells during cell growth

The human hepatoblastoma cell line, HepG2, has been used for investigating a wide variety of physiological and pathophysiological processes. However, less information is available about the phospholipid metabolism in HepG2 cells. In the present report, to clarify the relationship between cell growth and phospholipid metabolism in HepG2 cells, we examined the phospholipid class compositions of the cells and their intracellular organelles by using enzymatic fluorometric methods. In HepG2 cells, the ratios of all phospholipid classes, but not the ratio of cholesterol, markedly changed with cell growth. Of note, depending on cell growth, the phosphatidic acid (PA) ratio increased and phosphatidylcholine (PC) ratio decreased in the nuclear membranes, the sphingomyelin (SM) ratio increased in the microsomal membranes, and the phosphatidylethanolamine (PE) ratio increased and the phosphatidylserine (PS) ratio decreased in the mitochondrial membranes. Moreover, the mRNA expression levels of enzymes related to PC, PE, PS, PA, SM and cardiolipin syntheses changed during cell growth. We suggest that the phospholipid class compositions of organellar membranes are tightly regulated by cell growth. These findings provide a basis for future investigations of cancer cell growth and lipid metabolism.

Comparative Lipidomic Analysis of Extracellular Vesicles Derived from Lactobacillus plantarum APsulloc 331261 Living in Green Tea Leaves Using Liquid Chromatography-Mass Spectrometry

Lactobacillus plantarum is a popular probiotic species due to its safe and beneficial effects on humans; therefore, novel L. plantarum strains have been isolated and identified from various dietary products. Given that bacteria-derived extracellular vesicles (EVs) have been considered as efficient carriers of bioactive materials and shown to evoke cellular responses effectively, L. plantarum-derived EVs are expected to efficiently elicit health benefits. Herein, we identified L. plantarum APsulloc 331261 living in green tea leaves and isolated EVs from the culture medium. We performed quantitative lipidomic analysis of L. plantarum APsulloc 331261 derived EVs (LEVs) using liquid chromatography-mass spectrometry. In comparison to L. plantarum APsulloc 331261, in LEVs, 67 of 320 identified lipid species were significantly increased and 19 species were decreased. In particular, lysophosphatidylserine(18:4) and phosphatidylcholine(32:2) were critically increased, showing over 21-fold enrichment in LEVs. In addition, there was a notable difference between LEVs and the parent cells in the composition of phospholipids. Our results suggest that the lipidomic profile of bacteria-derived EVs is different from that of the parent cells in phospholipid content and composition. Given that lipids are important components of EVs, quantitative and comparative analyses of EV lipids may improve our understanding of vesicle biogenesis and lipid-mediated intercellular communication within or between living organisms.