Unesterified cholesterol controls the fluidity, permeability and electrical properties of eukaryotic cell membranes. Consequently, cholesterol levels in the retina and the brain are tightly regulated whereas depletion or oversupply caused by diet or heredity contribute to neurodegenerative diseases and vision loss. Astroglia play a central role in the biosynthesis, uptake and transport of cholesterol and also drive inflammatory signaling under hypercholesterolemic conditions associated with high-fat diet (diabetes) and neurodegenerative disease. A growing body of evidence shows that unesterified membrane cholesterol modulates the ability of glia to sense and transduce ambient information. Cholesterol-dependence of Müller glia - which function as retinal sentinels for metabolic, mechanical, osmotic and inflammatory signals - is mediated in part by transient receptor potential V4 (TRPV4) channels. Cholesterol supplementation facilitates, whereas depletion suppresses, TRPV4-mediated transduction of temperature and lipid agonists in Müller cells. Acute effects of cholesterol supplementation/depletion on plasma membrane ion channels and calcium homeostasis differ markedly from the effects of chronic dyslipidemia, possibly due to differential modulation of modality-dependent energy barriers associated with the functionality of polymodal channels embedded within lipid rafts. Understanding of cholesterol-dependence of TRP channels is thus providing insight into dyslipidemic pathologi Author contributions: All authors contributed to writing and editing the manuscript. ML and DK generated the figures. 2 2018 13 2 207 207–210 6 4 2018 Copyright: © Neural Regeneration Research This is Consequently, cholesterol levels in the retina and the brain are tightly regulated whereas depletion or oversupply caused by diet or heredity contribute to neurodegenerative diseases and vision loss. Astroglia play a central role in the biosynthesis, uptake and transport of cholesterol and also drive inflammatory signaling under hypercholesterolemic conditions associated with high-fat diet (diabetes) and neurodegenerative disease. A growing body of evidence shows that unesterified membrane cholesterol modulates the ability of glia to sense and transduce ambient information. Cholesterol-dependence of Müller glia - which function as retinal sentinels for metabolic, mechanical, osmotic and inflammatory signals - is mediated in part by transient receptor potential V4 (TRPV4) channels. Cholesterol supplementation facilitates, whereas depletion suppresses, TRPV4-mediated transduction of temperature and lipid agonists in Müller cells. Acute effects of cholesterol supplementation/depletion on plasma membrane ion channels and calcium homeostasis differ markedly from the effects of chronic dyslipidemia, possibly due to differential modulation of modality-dependent energy barriers associated with the functionality of polymodal channels embedded within lipid rafts. Cholesterol Imbalance Linked to a Range of Diseases Cholesterol regulates the permeability, fluidity and bending rigidity of eukaryotic plasma membranes, serves as a precursor for steroid synthesis and regulates the function of numerous membrane proteins via specialized cholesterol-enriched membrane microdomains (lipid rafts) (Dietschy, 2009). A New Target in Dyslipidemia Pathology: Polymodal Sensory Transduction Lakk et al. (2017) found that near-total depletion of free membrane cholesterol dissolves the lipid raft domains in Müller cells without affecting Trpv4 gene expression, trafficking and localization in adult Müller cells. The absence of effects of cholesterol depletion on gene expression and TRPV4 trafficking was in contrast to its pronounced modulation of TRPV4 activation, as indicated by the effects of cyclodextrins on the amplitude of agonist- and temperature-evoked calcium signals and transmembrane cation currents. Cholesterol depletion reduced whereas supplementation increased, the amplitudes of agonist- and temperature-evoked signals. In contrast, swelling-induced signals that are also mediated by TRPV4 channels (Ryskamp et al., 2014; Jo et al., 2015) were resistant to cholesterol modulation. This suggests that cholesterol differentially modulates the polymodality of ion channel signaling. Through its regulation of membrane stiffness and TRPV4 gating, cholesterol could modulate intracellular signaling and reactive gliosis and thereby influence baseine and/or activity-dependent release of gliotransmitters and cytokines/chemokines. An interesting aspect of cholesterol-dependent chronic facilitation of TRPV4 involves the effect on polyunsaturated fatty acid metabolism downstream from the phospholipase A2 which is required for Müller cell TRPV4 activation (Ryskamp et al., 2014) ( Figure 2 ). It is also important to note that Müller cells are not the sole retinal target of systemic and local dyslipidemia, and that shifts in cholesterol availability modulate signaling in many cell types including the retinal pigment epithelium, photoreceptors, microvascular endothelial cells, trabecular meshwork cells, microglia and RGCs (Fliesler and Bretillon, 2010; Gambert et al., 2017; Hammer and Busik, 2017). It remains to be determined whether manipulation of free membrane cholesterol similarly modulates channel activation in these TRPV4-expressing cells (Jo et al., 2015; Ryskamp et al., 2016; Phuong et al., 2017). Figure 2 Model of cholesterol-dependent modulation of transient receptor potential V4 (TRPV4) polymodality in astroglia. The docked sterol enriches Cav-1-containing membrane microdomains (lipid rafts) and organizes the partitioning of CARC-CRAC domain-containing proteins such as TRPV4 into the amphiphilic microenvironment of the membrane. TRPV4 activation modulates glial excitability via changes in intracellular [Ca 2+ ] i which in turn may downstream processes such as hypertrophy, proliferation, gliosis and release of gliotransmitters and chemokines.

Effect of serum on the plasma membrane fluidity of hybridomas: an insight into its shear protective mechanism. We have previously shown that decreasing the concentration of fetal bovine serum (FBS) increased the fragility of a mouse hybridoma (HB-32) during agitated batch cultivation and that increasing the plasma membrane fluidity (PMF) increased the shear sensitivity during exposure to laminar flow. In this study, the effect of FBS concentration on the PMF of HB-32 was investigated. PMF was evaluated by steady-state fluorescence anisotropy (rs) of 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene. Increasing serum concentration increased the rs of hybridomas, indicating a decrease in their PMF. The effect of cholesterol modulation on the PMF and shear sensitivity was also evaluated. Hybridomas were exposed to turbulent fluid shear after modification of PMF by cholesterol modulation. Direct cholesterol enrichment of the plasma membranes caused a decrease in the PMF and shear sensitivity, while cholesterol depletion caused an increase in PMF and shear sensitivity.

Cholesterol modulates alkaline phosphatase activity of rat intestinal microvillus membranes. Experiments were conducted, using a nonspecific lipid transfer protein, to vary the cholesterol/phospholipid molar ratio of rat proximal small intestinal microvillus membranes in order to assess the possible role of cholesterol in modulating enzymatic activities of this plasma membrane. Cholesterol/phospholipid molar ratios from 0.71 to 1.30 were produced from a normal value of 1.05 by incubation with the transfer protein and an excess of either phosphatidylcholine or cholesterol/phosphatidylcholine liposomes for 60 min at 37 degrees C. Cholesterol loading or depletion of the membranes was accompanied by a decrease or increase, respectively, in their lipid fluidity, as assessed by steady-state fluorescence polarization techniques using the lipid-soluble fluorophore 1,6-diphenyl-1,3,5-hexatriene. Increasing the cholesterol/phospholipid molar ratio also decreased alkaline phosphatase specific activity by approximately 20-30%, whereas decreasing this ratio increased this enzymatic activity by 20-30%.

The fluidity of lipid domains can markedly affect the lateral and rotational diffusion [l] , as well as the degree of exposure [2-4], of membrane proteins. It is therefore believed that some of the control mechanisms of membrane functions involve the lipid fluidity. Under physiological conditions modulation of the fluidity of the membrane lipid domains is achieved by changes in the mole ratio of cholesterol to phospholipid (C/PL) [5,6] , changes in the mole ratio of lecithin to sphingomyelin (L/S) [6,7] , or changes in the overall degree of unsaturation of the phospholipid acyl chains. One of the main mechanisms of changing the C/PL in cell membranes is the passive partitioning of cholesterol between the serum lipoproteins and the cell plasma membrane. Under normal physiological conditions the C/PL levels in the serum and the cell membrane are at an equilibrium which is enforced by a steady-state of cholesterol exchange between these reservoirs [8] . However, alterations of the C/PL of the ambient serum will lead to translocation of cholesterol towards a new equilibrium state which will eventually alter the C/PL of the cell membrane. This mechanism is considered to be the main passive physiological process which modulates membrane fluidity in animal tissues. Essentially, all the available in vitro techniques for modulation of cellular cholesterol involve treatments with liposomes of abnormally high C/PL for cholesterol enrichment or liposomes free of cholesterol for cholester

To evaluate the effect of plasma membrane fluidity of lung endothelial cells on serotonin transport, porcine pulmonary artery endothelial cells were incubated for 3 h with either 0.1 mM cholesterol hemisuccinate, 0.1 mM cis-vaccenic acid, or vehicle (control), after which plasma membrane fluidity and serotonin transport were measured. Fluorescence spectroscopy was used to measure fluidity in the plasma membrane. Serotonin uptake was calculated from the disappearance of [14C]-serotonin from the culture medium. Cholesterol decreased fluidity in the subpolar head group and central and midacyl side-chain regions of the plasma membrane and decreased serotonin transport, whereas cis-vaccenic acid increased fluidity in the central and midacyl side-chain regions of the plasma membrane and also increased serotonin transport. Cis-vaccenic acid had no effect on fluidity in the subpolar head group region of the plasma membrane. These results provide evidence that the physical state of the central and midacyl chains within the pulmonary artery endothelial cell plasma membrane lipid bilayer modulates transmembrane transport of serotonin by these cells.

systems is cholesterol, which modulates bilayer permeability, mechanical strength, and biochemical interactions. While lipid tails primarily modulate bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell. The lipid bilayer is the T… At a given temperature a lipid bilayer can exist in either a liquid or a gel (solid) phase. All lipids have a characteristic temperature at which they transition (melt) from the gel to liquid phase. In both phases the lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange locations with its neighbor millions of times a second. This random walk exchange allows lipid to diffuse and thus wander across the surface of the membrane.Unlike liquid phase bilayers, the lipids in a gel phase bilayer have less mobility. The phase behavior of lipid bilayers is determined largely by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. Longer-tailed lipids have more area over which to interact, increasing the strength of this interaction and, as a consequence, decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid. Transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can produce a kink in the alkane chain, disrupting the lipid packing. This disruption creates extra free space within the bilayer that allows additional flexibility in the adjacent chains. An example of this effect can be noted in everyday life as butter, which has a large percentage of saturated fa The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role, even though they are only a few nanometers in width, because they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps. Biological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Phospholipids with certain head groups can alter the surface chemistry of a bilayer and can, for example, serve as signals as well as "anchors" for other molecules in the membranes of cells. Just like the heads, the tails of lipids can also affect membrane properties, for instance by determining the phase of the bilayer. The bilayer can adopt a solid gel phase state at lower temperatures but undergo phase transition to a fluid state at higher temperatures, and the chemical properties of the lipids' tails influence at which temperature this happens. The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. Many of these properties have been studied with the use At a given temperature a lipid bilayer can exist in either a liquid or a gel (solid) phase. All lipids have a characteristic temperature at which they transition (melt) from the gel to liquid phase. In both phases the lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange locations with its neighbor millions of times a second. This random walk exchange allows lipid to diffuse and thus wander across the surface of the membrane.Unlike liquid phase bilayers, the lipids in a gel phase bilayer have less mobility. The phase behavior of lipid bilayers is determined largely by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. Longer-tailed lipids have more area over which to interact, increasing the strength of this interaction and, as a consequence, decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid. Transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can produce a kink in the alkane chain, disrupting the lipid packing. This disruption creates extra free space within the bilayer that allows additional flexibility in the adjacent chains. An example of this effect can be noted in everyday life as butter, which has a large percentage of saturated fats, is solid at room temperature while vegetable oil, which is mostly unsaturated, is liquid. Most natural membranes are a complex mixture of different lipid molecules. If some of the components are liquid at a given temperature while others are in the gel phase, the two phases can coexist in spatially separated regions, rather like an iceberg floating in the ocean. This phase separation plays a critical role in biochemical phenomena because membrane components such as proteins can partition into one or the other phase and thus be locally concentrated or activated. One particularly important component of many mixed phase systems is cholesterol, which modulates bilayer permeability, mechanical strength, and biochemical interactions.