Lipids have long been known to play key roles in maintaining the plasma membrane’s structure and functionality in mammalian cells. Studying the distribution of these lipids, across the inner and the outer leaflets (sides) of the membrane, is crucial to understanding how cells regulate the biosynthesis of lipids. Cholesterol, one of the major lipids in the mammalian plasma
membrane, has been extensively studied for its various roles in cell structure and function. The lack of accurate quantification methods, however, has hindered any further analysis. Shu-Lin Liu et al., nonetheless, in an article published recently in Nature, report their development of promising new methods that allow for in situ quantification of cholesterol in the plasma membrane is mammalian cells with high sensitivity and accuracy. The quantification of the distribution of plasma membrane cholesterol, and being able to modulate its transbilayer distribution can potentially lead to treatment of hypercholesterolemia-linked breast cancer.
The spatiotemporal, asymmetric, distribution of cholesterol in the membrane is maintained by special transporters that respond to stimulants and feedback loops. Due to the complexity of these processes, it is crucial to be able to study the membrane of a living cell, without disrupting its function. To achieve this, Shu-Lin Liu et al. isolated cholesterol-specific bacterial domains (D4) that had been mutated to bind to membrane cholesterol at variable concentrations. These domains were then tagged with solvatochromic fluorophores (dyes) to create tunable sensors that have ranging affinities for cholesterol.
To confirm the accuracy of the sensors, the authors conducted a quantitative mass spectroscopy experiment in which they were able to obtain the concentration of all major lipids found in the membrane of the same HeLa cell, under the same cellular conditions. Upon separation of the lipids, it was found that the total concentration of cholesterol in the membrane agreed with the sum of inner and outer leaflet concentrations obtained via sensor quantification.
Using those sensors to quantify membrane cholesterol, the authors found the concentration in the outer side of the membrane to be significantly higher than that of the inner leaflet. This is consistent with previous findings that high outer-membrane cholesterol concentration determined membrane permeability and assisted in membrane domain formation in the outer leaflet, and the inner membrane cholesterol functions in cell signalling when in low concentrations. This finding further confirms the accuracy of the results obtained from the sensors.
The method that the authors of this paper developed can be a model to later development of sensors that can potentially quantify any membrane lipid, in any cellular membrane. This clear and accurate quantification of a membrane lipid will have an enormous impact on many scientific fields, including cancer research. Being able to regulate and moderate the concentrations of membrane cholesterol can lead to possible detection, and treatment options for hypercholesterolemia-linked breast cancer. As the research of cholesterol-linked cancer grows, it is possible to stumble upon other membrane lipids that participate in the reactions, and it would be interesting to see how the development of new sensors that can detect various lipids will have an affect on the field.
1. Liu, S.-L. et al. Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol. Nat Chem Biol advance online publication, (2016).
2. Sheng, R. et al. Cholesterol modulates cell signaling and protein networking by specifically interacting with PDZ domain-containing scaffold proteins. Nat. Commun.3, 1249 (2012).
Powered by WPeMatico