A novel sterol-binding protein reveals heterogeneous cholesterol distribution in neurite outgrowth and in late endosomes/lysosomes

Maistero-2, a Novel Probe for Sterols: Application for Visualizing Cellular Cholesterol

Maistero-2 is a novel, non-toxic cholesterol-binding protein derived from an edible mushroom Grifola frondosa mRNA. Maistero-2 specifically binds to lipid membranes containing 3-hydroxy sterols with a lower cholesterol concentration threshold than cholesterol-binding domain 4 (D4) of perfringolysin O (PFO) and anthrolysin O (ALO). Maistero-2 binding is particularly sensitive to the size and conformation of the A-, B-, and D-ring of sterols but not very sensitive to modifications of the isooctyl side chain commonly found in phytosterols. EGFP-maistero-2 has been demonstrated to be a suitable tool to visualize the increase of cell surface cholesterol during the differentiation of Neuro 2a cells and heterogeneous cholesterol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes in HeLa cells. This chapter describes the detailed protocols for examining the binding of maistero-2 to lipids and model membranes and the labeling of cell surface and intracellular cholesterol by EGFP-maistero-2.

Methods for Visualizing and Quantifying Cholesterol Distribution in Mammalian Cells Using Filipin and D4 Probes

Cholesterol is a key component of biological membranes and, like many cellular lipids, is unevenly distributed among organelles. Disruptions in cholesterol trafficking are associated with various pathologies, including lysosomal lipid storage disorders, often characterized by intracellular cholesterol accumulation. A significant challenge in studying cholesterol trafficking is the lack of easy methods to trace this molecule in situ. Fluorescent probes that specifically bind cholesterol have enabled the visualization and imaging of cholesterol distribution within cells. This chapter details optimized methods for visualizing and quantifying free cholesterol at the plasma membrane and intracellulaly, both in individual cells and in large cell populations. These methods use two fluorescent probes: the D4 fragment of perfringolysin O fused to monomeric EGFP (mEGFP-D4 and the more sensitive mutant mEGFP-D4H) and the polyene macrolide filipin. We describe robust methods for quantifying plasma membrane cholesterol by flow cytometry and to visualize intracellular cholesterol pools by light microscopy. Furthermore, we introduce a refined filipin staining protocol that enhances intracellular cholesterol detection. For precise quantification, we developed an automated image analysis pipeline. This chapter provides a comprehensive guide for staining and quantifying cellular cholesterol, offering valuable tools for studying cholesterol dynamics in mammalian cells.

Exploring lipid-protein interactions in plant membranes

Once regarded as mere membrane building blocks, lipids are now recognized as diverse and intricate players that mold the functions, identities, and responses of cellular membranes. Although the interactions of lipids with integral and peripheral membrane proteins are crucial for their localization, activity, and function, how proteins bind lipids is still far from being thoroughly explored. Describing and characterizing these dynamic protein-lipid interactions is thus essential to understanding the membrane-associated processes. Here we review the current range of experimental techniques employed to study plant protein-lipid interactions, integrating various methods. We summarize the principles, advantages, and limitations of classical in vitro biochemical approaches, including protein-lipid overlays and various liposome binding assays, and complement them with in vivo microscopic techniques centered around the use of genetically encoded lipid sensors and pharmacological or genetic membrane lipid manipulation tools. We also highlight several emerging techniques still awaiting their advancement into plant membrane research and emphasize the need to use complementary experimental strategies as key for elucidating the mechanistic roles of protein-lipid interactions in plant cell biology.

Using lipid binding proteins and advanced microscopy to study lipid domains

Sphingomyelin is postulated to form clusters with glycosphingolipids, cholesterol and other sphingomyelin molecules in biomembranes through hydrophobic interaction and hydrogen bonds. These clusters form submicron size lipid domains. Proteins that selectively binds sphingomyelin and/or cholesterol are useful to visualize the lipid domains. Due to their small size, visualization of lipid domains requires advanced microscopy techniques in addition to lipid binding proteins. This Chapter describes the method to characterize plasma membrane sphingomyelin-rich and cholesterol-rich lipid domains by quantitative microscopy. This Chapter also compares different permeabilization methods to visualize intracellular lipid domains.

Downregulation of dermatopontin in cholangiocarcinoma cells suppresses CCL19 secretion of macrophages and immune infiltration

OBJECTIVE

The tumor microenvironment (TME) in cholangiocarcinoma (CHOL) is typically characterized by a low level of immune infiltration, which accounts for the dismal prognosis of this patient population. This study sought to investigate the mechanisms underlying the reduced infiltration of immune cells into the CHOL TME.

METHODS

We constructed a Least Absolute Shrinkage and Selection Operator (LASSO) regression model to identify prognosis-related differentially expressed genes (DEGs). The 'Corrplot' package was employed to analyze the correlation between dermatopontin (DPT) and immune infiltration in CHOL. The Tumor and Immune System Interaction Database (TISIDB) was used to evaluate the association between DPT and immunology. Single-cell analysis was conducted to localize CCL19 secretions. Western blot and qPCR were utilized to detect DPT expression, while immunofluorescence was performed to investigate the cellular localization of DPT. Additionally, ELISA analysis was employed to assess the alteration in CCL19 secretion in cancer-associated fibroblasts (CAFs) and macrophages.

RESULTS

Our findings revealed that CHOL patients with low DPT expression had a poorer prognosis. Enrichment analysis demonstrated a positive correlation between DPT levels and the infiltration of immunomodulators and immune cells. Moreover, high DPT levels were associated with enhanced anti-PD-1/PD-L1 immunotherapeutic responses. Furthermore, DPT expression impacted the landscape of gene mutations, showing a negative association with tumor grade, stage, and lymph node metastasis. Based on the results of protein peptides analysis and cell experiments, it was inferred that the downregulation of DPT in CHOL cells effectively suppressed the secretion of CCL19 in macrophages.

CONCLUSIONS

DPT is a novel prognosis-related biomarker for CHOL patients, and this study provides preliminary insights into the mechanism by which DPT promotes the infiltration of immune cells into the CHOL TME.

Cell density-dependent membrane distribution of ganglioside GM3 in melanoma cells

Monosialoganglioside GM3 is the simplest ganglioside involved in various cellular signaling. Cell surface distribution of GM3 is thought to be crucial for the function of GM3, but little is known about the cell surface GM3 distribution. It was shown that anti-GM3 monoclonal antibody binds to GM3 in sparse but not in confluent melanoma cells. Our model membrane study evidenced that monoclonal anti-GM3 antibodies showed stronger binding when GM3 was in less fluid membrane environment. Studies using fluorescent GM3 analogs suggested that GM3 was clustered in less fluid membrane. Moreover, fluorescent lifetime measurement showed that cell surface of high density melanoma cells is more fluid than that of low density cells. Lipidomics and fatty acid supplementation experiment suggested that monounsaturated fatty acid-containing phosphatidylcholine contributed to the cell density-dependent membrane fluidity. Our results indicate that anti-GM3 antibody senses GM3 clustering and the number and/or size of GM3 cluster differ between sparse and confluent melanoma cells.

A basic model for the association of ligands with membrane cholesterol: application to cytolysin binding

Almost all the cholesterol in cellular membranes is associated with phospholipids in simple stoichiometric complexes. This limits the binding of sterol ligands such as filipin and Perfringolysin O (PFO) to a small fraction of the total. We offer a simple mathematical model that characterizes this complexity. It posits that the cholesterol accessible to ligands has two forms: active cholesterol, which is that not complexed with phospholipids; and extractable cholesterol, that which ligands can capture competitively from the phospholipid complexes. Simulations based on the model match published data for the association of PFO oligomers with liposomes, plasma membranes and the isolated endoplasmic reticulum. The model shows how the binding of a probe greatly underestimates cholesterol abundance when its affinity for the sterol is so weak that it competes poorly with the membrane phospholipids. Two examples are the under-staining of plasma membranes by filipin and the failure of domain D4 of PFO to label their cytoplasmic leaflets. Conversely, the exaggerated staining of endolysosomes suggests that their cholesterol, being uncomplexed, is readily available. The model is also applicable to the association of cholesterol with intrinsic membrane proteins. For example, it supports the hypothesis that the sharp threshold in the regulation of homeostatic ER proteins by cholesterol derives from the cooperativity of their binding to the sterol weakly held by the phospholipid. § Thus, the model explicates the complexity inherent in the binding of ligands like PFO and filipin to the small accessible fraction of membrane cholesterol.