Effect of the ostreolysin A/pleurotolysin B pore-forming complex on neuroblastoma cell morphology and intracellular Ca²⁺ activity

Characterization of two distinct immortalized endothelial cell lines, EA.hy926 and HMEC-1, for in vitro studies: exploring the impact of calcium electroporation, Ca2+ signaling and transcriptomic profiles

BACKGROUND

Disruption of Ca2+ homeostasis after calcium electroporation (CaEP) in tumors has been shown to elicit an enhanced antitumor effect with varying impacts on healthy tissue, such as endothelium. Therefore, our study aimed to determine differences in Ca2+ kinetics and gene expression involved in the regulation of Ca2+ signaling and homeostasis, as well as effects of CaEP on cytoskeleton and adherens junctions of the established endothelial cell lines EA.hy926 and HMEC-1.

METHODS

CaEP was performed on EA.hy926 and HMEC-1 cells with increasing Ca2+ concentrations. Viability after CaEP was assessed using Presto Blue, while the effect on cytoskeleton and adherens junctions was evaluated via immunofluorescence staining (F-actin, α-tubulin, VE-cadherin). Differences in intracellular Ca2+ regulation ([Ca2+]i) were determined with spectrofluorometric measurements using Fura-2-AM, exposing cells to DPBS, ionomycin, thapsigargin, ATP, bradykinin, angiotensin II, acetylcholine, LaCl3, and GdCl3. Molecular distinctions were identified by analyzing differentially expressed genes and pathways related to the cytoskeleton and Ca2+ signaling through RNA sequencing.

RESULTS

EA.hy926 cells, at increasing Ca2+ concentrations, displayed higher CaEP susceptibility and lower survival than HMEC-1. Immunofluorescence confirmed CaEP-induced, time- and Ca2+-dependent morphological changes in EA.hy926's actin filaments, microtubules, and cell-cell junctions. Spectrofluorometric Ca2+ kinetics showed higher amplitudes in Ca2+ responses in EA.hy926 exposed to buffer, G protein coupled receptor agonists, bradykinin, and angiotensin II compared to HMEC-1. HMEC-1 exhibited significantly higher [Ca2+]i changes after ionomycin exposure, while responses to thapsigargin, ATP, and acetylcholine were similar in both cell lines. ATP without extracellular Ca2+ ions induced a significantly higher [Ca2+]i rise in EA.hy926, suggesting purinergic ionotropic P2X and metabotropic P2Y receptor activation. RNA-sequencing analysis showed significant differences in cytoskeleton- and Ca2+-related gene expression, highlighting upregulation of ORAI2, TRPC1, TRPM2, CNGA3, TRPM6, and downregulation of TRPV4 and TRPC4 in EA.hy926 versus HMEC-1. Moreover, KEGG analysis showed upregulated Ca2+ import and downregulated export genes in EA.hy926.

CONCLUSIONS

Our finding show that significant differences in CaEP response and [Ca2+]i regulation exist between EA.hy926 and HMEC-1, which may be attributed to distinct transcriptomic profiles. EA.hy926, compared to HMEC-1, displayed higher susceptibility and sensitivity to [Ca2+]i changes, which may be linked to overexpression of Ca2+-related genes and an inability to mitigate changes in [Ca2+]i. The study offers a bioinformatic basis for selecting EC models based on research objectives.

What Can Mushroom Proteins Teach Us about Lipid Rafts?

The lipid raft hypothesis emerged as a need to explain the lateral organization and behavior of lipids in the environment of biological membranes. The idea, that lipids segregate in biological membranes to form liquid-disordered and liquid-ordered states, was faced with a challenge: to show that lipid-ordered domains, enriched in sphingomyelin and cholesterol, actually exist in vivo. A great deal of indirect evidence and the use of lipid-binding probes supported this idea, but there was a lack of tools to demonstrate the existence of such domains in living cells. A whole new toolbox had to be invented to biochemically characterize lipid rafts and to define how they are involved in several cellular functions. A potential solution came from basic biochemical experiments in the late 1970s, showing that some mushroom extracts exert hemolytic activities. These activities were later assigned to aegerolysin-based sphingomyelin/cholesterol-specific cytolytic protein complexes. Recently, six sphingomyelin/cholesterol binding proteins from different mushrooms have been identified and have provided some insight into the nature of sphingomyelin/cholesterol-rich domains in living vertebrate cells. In this review, we dissect the accumulated knowledge and introduce the mushroom lipid raft binding proteins as molecules of choice to study the dynamics and origins of these liquid-ordered domains in mammalian cells.

Electron Paramagnetic Resonance Gives Evidence for the Presence of Type 1 Gonadotropin-Releasing Hormone Receptor (GnRH-R) in Subdomains of Lipid Rafts

This study investigated the effect of type 1 gonadotropin releasing hormone receptor (GnRH-R) localization within lipid rafts on the properties of plasma membrane (PM) nanodomain structure. Confocal microscopy revealed colocalization of PM-localized GnRH-R with GM₁-enriched raft-like PM subdomains. Electron paramagnetic resonance spectroscopy (EPR) of a membrane-partitioned spin probe was then used to study PM fluidity of immortalized pituitary gonadotrope cell line αT3-1 and HEK-293 cells stably expressing GnRH-R and compared it with their corresponding controls (αT4 and HEK-293 cells). Computer-assisted interpretation of EPR spectra revealed three modes of spin probe movement reflecting the properties of three types of PM nanodomains. Domains with an intermediate order parameter (domain 2) were the most affected by the presence of the GnRH-Rs, which increased PM ordering (order parameter (S)) and rotational mobility of PM lipids (decreased rotational correlation time (τc)). Depletion of cholesterol by methyl-β-cyclodextrin (methyl-β-CD) inhibited agonist-induced GnRH-R internalization and intracellular Ca²⁺ activity and resulted in an overall reduction in PM order; an observation further supported by molecular dynamics (MD) simulations of model membrane systems. This study provides evidence that GnRH-R PM localization may be related to a subdomain of lipid rafts that has lower PM ordering, suggesting lateral heterogeneity within lipid raft domains.

Recent developments in natural products for white adipose tissue browning

Excess accumulation of white adipose tissue (WAT) causes obesity which is an imbalance between energy intake and energy expenditure. Obesity is a serious concern because it has been the leading causes of death worldwide, including diabetes, stroke, heart disease and cancer. Therefore, uncovering the mechanism of obesity and discovering anti-obesity drugs are crucial to prevent obesity and its complications. Browning, inducing white adipose tissue to brown or beige (brite) fat which is brown-like fat emerging in WAT, becomes an appealing therapeutic strategy for obesity and metabolic disorders. Due to lack of efficacy or intolerable side-effects, the clinical trials that promote brown adipose tissue (BAT) thermogenesis and browning of WAT have not been successful in humans. Obviously, more specific means still need to be developed to activate browning of white adipose tissue. In this review, we summarized seven kinds of natural products (alkaloids, flavonoids, terpenoids, long chain fatty acids, phenolic acids, else and extract) promoting white adipose tissue browning which can ameliorate the metabolic disorders, including obesity, dislipidemia, insulin resistance and diabetes. Since natural products are important drug sources and the browning property plays a significant role in not only obesity treatment but also in type 2 diabetes (T2DM) improvement, natural products of inducing browning may be an irreplaceable drug discovery orientation for obesity, diabetes and even other metabolic disorders.

Aegerolysins from the fungal genus Pleurotus - Bioinsecticidal proteins with multiple potential applications

The aegerolysin proteins ostreolysin A6, pleurotolysin A2 and erylysin A are produced by mushrooms of the genus Pleurotus. These aegerolysins can interact specifically with sphingolipid-enriched membranes. In particular, they strongly bind insect cells and to artificial lipid membranes that contain physiologically relevant concentrations of the main invertebrate-specific sphingolipid, ceramide phosphoethanolamine. Moreover, the aegerolysins permeabilise these membranes when combined with their protein partner pleurotolysin B, which contains a membrane-attack-complex/perforin domain. These aegerolysin/ pleurotolysin B complexes show strong and selective toxicity towards western corn rootworm larvae and adults and Colorado potato beetle larvae. Their insecticidal activities arise through aegerolysin binding to ceramide phosphoethanolamine in the insect midgut. This mode of membrane binding is different from those described for similar aegerolysin-based complexes of bacterial origin (e.g., Cry34Ab1/Cry35Ab1), or other Bacillus thuringiensis proteinaceous crystal toxins, which associate with protein receptors. The ability of Pleurotus aegerolysins to specifically interact with sphingolipid-enriched domains in mammalian cells can be further exploited to visualize lipid rafts in living cells, and to treat certain types of tumours and metabolic disorders. Finally, these proteins can strongly enhance fruiting initiation of P. ostreatus even when applied externally. In this review, we summarise the current knowledge of the potential biotechnological and biomedical applications of the Pleurotus aegerolysins, either alone or when complexed with pleurotolysin B, with special emphasis on their bioinsecticidal effects.

Aegerolysins: Lipid-binding proteins with versatile functions

Proteins of the aegerolysin family span many kingdoms of life. They are relatively widely distributed in bacteria and fungi, but also appear in plants, protozoa and insects. Despite being produced in abundance in cells at specific developmental stages and present in secretomes, only a few aegerolysins have been studied in detail. In particular, their organism-specific physiological roles are intriguing. Here, we review published findings to date on the distribution, molecular interactions and biological activities of this family of structurally and functionally versatile proteins, the aegerolysins.

Ostreolysin A/Pleurotolysin B and Equinatoxins: Structure, Function and Pathophysiological Effects of These Pore-Forming Proteins

Acidic ostreolysin A/pleurotolysin B (OlyA/PlyB, formerly known as ostreolysin (Oly), and basic 20 kDa equinatoxins (EqTs) are cytolytic proteins isolated from the edible mushroom Pleurotus ostreatus and the sea anemone Actinia equina, respectively. Both toxins, although from different sources, share many similar biological activities: (i) colloid-osmotic shock by forming pores in cellular and artificial membranes enriched in cholesterol and sphingomyelin; (ii) increased vascular endothelial wall permeability in vivo and perivascular oedema; (iii) dose-dependent contraction of coronary vessels; (iv) haemolysis with pronounced hyperkalaemia in vivo; (v) bradycardia, myocardial ischemia and ventricular extrasystoles accompanied by progressive fall of arterial blood pressure and respiratory arrest in rodents. Both types of toxins are haemolytic within nanomolar range concentrations, and it seems that hyperkalaemia plays an important role in toxin cardiotoxicity. However, it was observed that the haemolytically more active EqT III is less toxic than EqT I, the most toxic and least haemolytic EqT. In mice, EqT II is more than 30 times more toxic than OlyA/PlyB when applied intravenously. These observations imply that haemolysis with hyperkalaemia is not the sole cause of the lethal activity of both toxins. Additional mechanisms responsible for lethal action of the two toxins are direct effects on heart, coronary vasoconstriction and related myocardial hypoxia. In this review, we appraise the pathophysiological mechanisms related to the chemical structure of OlyA/PlyB and EqTs, as well as their toxicity.

Characterisation of plasmalemmal shedding of vesicles induced by the cholesterol/sphingomyelin binding protein, ostreolysin A-mCherry

Ostreolysin A (OlyA) is a 15-kDa protein that binds selectively to cholesterol/sphingomyelin membrane nanodomains. This binding induces the production of extracellular vesicles (EVs) that comprise both microvesicles with diameters between 100nm and 1μm, and larger vesicles of around 10-μm diameter in Madin-Darby canine kidney cells. In this study, we show that vesiculation of these cells by the fluorescent fusion protein OlyA-mCherry is not affected by temperature, is not mediated via intracellular Ca2+ signalling, and does not compromise cell viability and ultrastructure. Seventy-one proteins that are mostly of cytosolic and nuclear origin were detected in these shed EVs using mass spectroscopy. In the cells and EVs, 218 and 84 lipid species were identified, respectively, and the EVs were significantly enriched in lysophosphatidylcholines and cholesterol. Our collected data suggest that OlyA-mCherry binding to cholesterol/sphingomyelin membrane nanodomains induces specific lipid sorting into discrete patches, which promotes plasmalemmal blebbing and EV shedding from the cells. We hypothesize that these effects are accounted for by changes of local membrane curvature upon the OlyA-mCherry-plasmalemma interaction. We suggest that the shed EVs are a potentially interesting model for biophysical and biochemical studies of cell membranes, and larger vesicles could represent tools for non-invasive sampling of cytosolic proteins from cells and thus metabolic fingerprinting.

Effect of the ostreolysin A/pleurotolysin B pore-forming complex on intracellular Ca2+ activity in the vascular smooth muscle cell line A10

Ostreolysin A/pleurotolysin B (OlyA/PlyB) is a binary pore-forming protein complex that produces a rapid cardiorespiratory arrest. Increased tonus of the coronary vascular wall produced by OlyA/PlyB may lead to ischemia, arrhythmias, the hypoxic injury of cardiomyocytes and cardiotoxicity. We evaluated the effects of OlyA/PlyB in cultured vascular smooth muscle A10 cells. Fluorometric measurements using the Ca(2+) indicator Fluo-4 AM and Fura-2 AM revealed that nanomolar concentrations of OlyA/PlyB increased the intracellular Ca(2+) activity [Ca(2+)]i in A10 cells. This effect was absent in a Ca(2+)-free medium, indicating that OlyA/PlyB-induced [Ca(2+)]i increase was dependent on Ca(2+) influx into cells. The increase in [Ca(2+)]i by OlyA/PlyB was partially prevented by: i) the calcium channel blockers verapamil and La(3+), ii) the inhibitor of the sodium-calcium exchanger (NCX) benzamil, and iii) the iso-osmotic replacement of NaCl by sucrose. The pre-treatment of cells with the Ca(2+)-ATPase inhibitor thapsigargin reduced the [Ca(2+)]i increase evoked by OlyA/PlyB, whereas the plasma membrane depolarization with high K(+) in the medium did not prevent OlyA/PlyB-induced [Ca(2+)]i. In summary, our data could suggest that the OlyA/PlyB-induced increase in [Ca(2+)]i is due to an influx of Ca(2+) through a variety of co-existing plasma membrane Ca(2+)-permeable channels, Ca(2+) entry through non-selective ion permeable pores formed de novo by OlyA/PlyB in the plasma membrane and calcium-induced intracellular Ca(2+) release, altogether leading to disturbed Ca(2+) homeostasis in A10 cells.