Understanding the role of cholesterol in cellular biomechanics and regulation of vesicular trafficking: The power of imaging
Biomedical Spectroscopy and Imaging
Cholesterol is an important component of cell plasma membrane. Due to its chemical composition (long rigid hydrophobic chain and a small polar hydroxyl group), it fits most of its structure into the lipid bilayer, where its steroid rings are in close proximity and attracted to the hydrocarbon chains of neighboring lipids. This gives a condensing effect on the packing of lipids in cell membranes creating cholesterol-enriched regions called membrane rafts, which also congregate a lot of specific
... roteins. Membrane rafts have been shown to work as platforms involved with signaling in diverse cellular processes, such as immune regulation, cell cycle control, membrane trafficking and fusion events. A series of studies in the last two decades have linked many of these functions with the effects of membrane cholesterol content and rafts integrity on actin cytoskeleton organization, as well as its consequences in cellular biomechanics. This was possible by using microscopy techniques before and after manipulation of cholesterol content from cell plasma membrane, using agents that are able to sequester these molecules, such as cyclodextrins. In this review we'll give a personal perspective on these studies and how microscopy techniques were important to unravel the effects of cholesterol on actin and cellular mechanics. We will also discuss how actin and cholesterol contributes to control cell secretion and vesicular trafficking. L. de Oliveira Andrade / The power of imaging Fig. 1. Cholesterol chemical structure. about cholesterol and membrane rafts and their effects on actin organization, cellular biomechanics and consequently membrane trafficking events. Cholesterol and membrane rafts Cholesterol displays a very important function as a component of cellular membranes, specially the cell plasma membrane where it is found in higher concentrations. Its positioning into the lipid bilayer and interaction with other lipids have a significant role in membrane fluidity together with other lipid components, such as the amount of sphingomyelin or the degree of saturation of the phospholipid acyl chains [55, 96, 104, 111] . Cholesterol fits most of its structure into the lipid bilayer and only the small hydroxyl group faces the external environment. As a consequence, its steroid rings are in close proximity and attracted to the hydrocarbon chains of neighboring lipids. This gives a condensing effect on the packing of lipids in cell membranes  . However this effect seems to depend on the type of lipid it interacts with. As cholesterol hydrocarbon chain is rigid it tends to segregate together with fatty acids with saturated long acyl chains, especially sphingomyelin, leading to the formation of more compact liquid ordered and less fluid phases [88, 91] (reviewed by  ). Cholesterol-enriched membrane regions, also called membrane rafts, were first identified as liquid ordered domains, containing high amounts of cholesterol and sphingomyelin and usually resistant to detergent solubilization (reason why they were first called detergent resistant membranes or DRMs). They were also known for its interaction with GPI-anchored proteins [94, 113] . Despite its packaged structure, rafts were also shown to be dynamic structures, since their components still retained some lateral and rotational mobility  . In fact, one specific protein can display different partition coefficients with rafts, residing temporarily inside or outside these regions  . Detergent resistance was used as an important criterion to define these domains. However, it has been shown that other characteristics are important to designate a membrane raft  . Therefore, after a Keystone Symposium on Lipid Rafts and Cell function, a set of parameters is now used to determine the so-called membrane rafts region. The membrane rafts are identified as small heterogeneous and dynamic membrane domains, of approximately 10-200 nm, which are capable of compartmentalizing specific cell processes  .