Transbilayer Distribution of Cholesterol Is Modified in Brain Synaptic Plasma Membranes of Knockout Mice Deficient in the Low‐Density Lipoprotein Receptor, Apolipoprotein E, or Both Proteins

Abstract

Abstract:Both apolipoprotein E (apoE) and the low-density lipoprotein (LDL) receptor are present in brain; however, little is known regarding the function of these proteins in brain, in particular with respect to brain cholesterol. The role of apoE and the LDL receptor in modulating the transbilayer or asymmetric distribution of cholesterol in the exofacial and cytofacial leaflets of synaptic plasma membranes (SPM5) was examined in mutant mice deficient in apoE, the LDL receptor, or both proteins by using the fluorescent sterol dehydroergosterol and fluorescent quenching procedures. Fluidity of the exofacial and cytofacial leaflets was also measured. Cholesterol asymmetry of SPMs was altered in the mutant mice, with the largest effect observed in the LDL receptor-deficient mice. There was an approximately twofold increase in the percent distribution of cholesterol in the exofacial leaflet of the LDL receptor-deficient mice (32%) compared with C57BL/6J mice (15%). Mice deficient in apoE or both proteins also showed a significantly higher percent distribution of cholesterol (23 and 26%, respectively) in the exofacial leaflet compared with the C57BL/6J mice. Although the percent distribution of cholesterol was highest in the exofacial leaflet of the LDL receptor-deficient mice, fluidity of the exofacial leaflet of that group was significantly lower. However, the cholesterol-to-phospholipid ratio of SPMs of the LDL receptor-deficient mice was significantly lower, and this difference was largely the result of a significant increase in the total amount of SPM phospholipid. This study demonstrates for the first time that SPM lipid structure is altered in mice deficient in apoE or the LDL receptor. Although the mechanism that maintains the asymmetric distribution of cholesterol in plasma membranes is not well understood, data of the present experiments indicate that both apoE and the LDL receptor are involved in maintaining the transbilayer distribution of cholesterol. Key Words: Apolipoprotein ELow-density lipoprotein receptor-CholesterolBrain-Fluidity.

Cholesterol-dependent Localization of NAP-22 on a Neuronal Membrane Microdomain (Raft)

“…In the synaptosome membrane, over 85% of cholesterol exists in the inner leaflet of the membrane (50). Assuming that this partition ratio of cholesterol is true to the raft membrane and the cholesterol is evenly extracted from both leaflets of membrane by MCD, our results indicate that one molecule of NAP-22 covers 390 molecules of cholesterol in the inner leaflet of raft.…”

Role of apolipoprotein E and estrogen in mossy fiber sprouting in hippocampal slice cultures

“…5,33 A model for this role has been described 5,52,53,59 where glia phagocytosing degenerating terminals esterify cholesterol from scavenged membrane lipid, repackaged it with ApoE and secrete it, facilitating delivery of cholesterol to neurons via their ApoE receptors for neurite growth. In support of this model, (i) ApoE synthesis increases in the deafferented ML after ECL 53,54 possibly due to up-regulation by intracellular free cholesterol and oxidized low-density lipoprotein (LDL); 7,66,67 (ii) ApoE-KO fails to clear degenerating terminals in the ML after ECL, 17 an activity attributed to both astrocytes and microglia; 3 (iii) ApoE-KO and LDL receptor-knockout mice show altered synaptic plasma membrane cholesterol distribution and phospholipid content, 29 a phenomenon also observed in aged wild-type mice; 30 and (iv) ApoE stimulates neurite outgrowth. 16,48,56,73a The regional restriction of the defective sprouting response may reflect the differences in developmental age between dorsal and ventral DG.…”

Fluorescence Techniques Using Dehydroergosterol to Study Cholesterol Trafficking

“…in (12,144,145). DHE revealed that the cytofacial leaflet of plasma membranes from L-cell fibroblasts (12,24,146,147), erythrocytes (144), and brain synaptosomes (145,(148)(149)(150)(151). While other methods (filipin staining, exchange, cholesterol oxidase accessibility, neutron diffraction, etc) have also been used to measure transbilayer cholesterol distribution, many have significant limitations including accessibility of membrane cholesterol (filipin, cholesterol oxidase), production of a potential perturbant (cholesterol oxidase produces cholesterone), and poor dynamic resolution (neutron diffraction) (rev.…”

“…While measurements of transbilayer sterol distribution by different methods may therefore not necessarily agree, multiple methods (DHE, NBD-cholesterol quenching, and exchange assays) indicate that the cytofacial leaflet of erythrocytes has more cholesterol than the exofacial leaflet (144,152). Likewise, several methods (DHE, cholesterol oxidase) indicate that the synaptosomal plasma membrane has more cholesterol in the cytofacial leaflet than the exofacial leaflet (12,145,(148)(149)(150). The transbilayer distribution of sterol is a major determinant of the transbilayer fluidity gradient present in these membranes, the cytofacial cholesterol-rich leaflet is more rigid (less fluid) than the relatively cholesterol-poor exofacial leaflet (rev.…”

“…The finding of rapid transbilayer sterol movement suggested that the asymmetric transbilayer distribution in the plasma membrane was not due to restricted movement of cholesterol from one leaflet to the other, but was instead a property of the lipid bilayer itself. DHE transbilayer distribution has broad applicability in studies of the action of anesthetics, cholesterol lowering drugs (statins), alcohol (chronic and acute), lupus erythematosus, aging, apoE, and sterol carrier proteins all of which can alter plasma membrane or synaptosomal plasma membrane transbilayer sterol distribution and/or transbilayer fluidity (26,135,136,145,149,150,154,(161)(162)(163)(164)(165)(166)(167)(168)(169)(170)(171)(172).Second, the DHE lateral distribution in both model and biological membranes was not uniform, but instead reflected sterol-rich and poor domains (rev. in (54,55).…”