**1. Introduction**

The role that cholesterol plays in cardiovascular diseases is widely known and studied [1]. However, less appreciated is the importance that cholesterol has in orchestrating other important cellular functions, such as cellular contractility and cytoskeleton organization. Cellular contractility is the ability of a cell to exert mechanical work on a substrate or on a neighboring cell due to actomyosin cytoskeleton enzymatic activity [2]. In muscle and nonmuscle cells, most of the cholesterol content is localized at the plasma membrane [3] where it can partition into microdomains called lipid rafts. Lipid rafts are highly dynamic regions of the plasma membrane

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

that contain sphingolipids and cholesterol and are responsible for compartmentalizing and regulating several intracellular signaling events [4–6]. One way of studying the importance of cholesterol for a specific cellular function is to decrease its concentration by either interfering directly with its synthesis, through the mevalonate pathway, or by chelating the molecule directly through the use of cyclodextrins [7]. However, depending on how one does the cholesterol depletion, the effects on cellular contractility can be opposite. Differences in muscle versus nonmuscle cell contractile behavior are observed upon cholesterol depletion using cyclodextrins. Muscle cells get impairment in their contractile machinery [8] whereas nonmuscle cells get more contractile [9]. This book chapter gives an overview about how cholesterol is organized at the plasma membrane and how its depletion changes cellular contractile properties.

arranged in the plasma membrane of living cells such that the polar regions are facing the aqueous phase and the hydrophobic regions are embedded on a viscous phospholipid bilayer and those proteins are able to move freely on that two-dimensional, approximately homogeneous fluid "sea" of phospholipids [18]. One year later, in 1973, Bretscher published a Science paper in which he discusses overall membrane organization based on evidences collected from experiments performed in red blood cells [19]. According to that paper, the plasma membrane of mammalian cells was not as simple as depicted by the fluid mosaic model. Some of the integral proteins span the membrane and their glycosylation is responsible for locking them at the membrane impeding their migration to the cytoplasm. Another important contribution from this paper is that proteins not only interact with the outer layer of the plasma membrane but also with the inner layer, and membrane proteins are a subtype of cytoplasmic proteins that are not secreted [19]. In the same year, Yu and collaborators, also performing experiments in red blood cells, showed that when those cells are incubated with the nonionic detergent Triton X-100, there are some fractions of the cellular proteins that are resistant to the detergent extraction and seem to form oligomeric complexes with some of the lipid components, which

Role of Membrane Cholesterol in Modulating Actin Architecture and Cellular Contractility

http://dx.doi.org/10.5772/intechopen.76532

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were preferentially composed by nonglycosylated proteins and sphingolipids [20].

as membrane trafficking and neurotransmission [26].

The idea of possible membrane microdomains started to be speculated in the early 1970s [20, 21] and experimentally demonstrated in 1982 by Karnovsky [22], who showed that there were multiple phases in the lipid environment of a membrane. One type of microdomain can be formed by cholesterol and sphingolipids [23]. These microdomains were already shown to be present in cell membranes [24]. In 1988, after several experimental demonstrations, Simons and van Meer called these microdomains as lipid rafts [25]. Thus, lipid rafts are defined as small, heterogeneous, and highly dynamic microdomains enriched in cholesterol, glycosphingolipids, and proteins that are much more organized than the surrounding lipid bilayer [26]. These membrane microdomains serve as organizing clusters capable of influencing several cellular processes such

The most striking difference between lipid rafts and the plasma membrane from which they are derived from is the lipid composition. Experiments have shown that rafts contain much more cholesterol than the surrounding bilayer [27, 28]. Cholesterol, therefore, works as a sort of "dynamic glue" that maintains the raft together [29], serving as a molecular spacer and filling the empty spaces between sphingolipids [30]. One of the main challenges when studying lipid rafts in living cells is their size. They are small microdomains ranging from 10 to 200 nm, below the classical diffraction limit of the optical microscope [28]. The first studies in the field considered methods to extract and separate rafts from the surrounding membrane. The procedure would take advantage of lipid raft resistance to nonionic detergents. When detergents are added to cells, the fluid membrane will dissolve while the lipid rafts may remain intact and could be extracted [31]; however, the validity of this methodology has been called into question due to ambiguities in the lipids and proteins obtained after extraction [32]. Other methods, based on synthetic membranes, were also used, however with many drawbacks. Firstly, synthetic membranes either lack or have lower protein concentration when compared to cell membranes [26]. Secondly, it is very difficult to simulate, in synthetic membranes, the membrane-cytoskeletal interactions that occur in cell membranes, although

**2.1. Lipid rafts**
