Y regulators of lipid submicrometric domains in biological membranes (for reviews, please see [181-184]). These include cholesterol, complex SLs and Cer, a.o.. Cholesterol is the most abundant lipid in several PMs, with up to 45mol in RBCs (see Table 3). This lipid emerges as a major regulator of submicrometric domain biogenesis and/or maintenance in living cells, as illustrated by the following studies. Depletion of cholesterol from living fibroblasts or CHO cells labeled by fluorescent SM analogs induces the formation of submicrometric domains or increases their size, indicating a restricting role of cholesterol for domain formation/maintenance in these cells [30, 173]. In contrast, slight cholesterol depletion of the RBC PM decreases the abundance of PC- and SM- but not GSLs-enriched submicrometric domains [26, 27] as well as lipid packing, as revealed by Laurdan [185]. Moreover, cholesterol influences the shape of submicrometric domains. For example, lowering cholesterol levels in native pulmonary surfactant membranes induces a transition from circular to fluctuating borderline micrometric domains, typical of gel-ordered like phases [16]. The fine and ambivalent effect of cholesterol on submicrometric domains in different cells may be related to differences in membrane composition. Indeed, cholesterol has been proposed to either promote lipid mixing by converting gel and Ld phases into an intermediate Lo phase or, conversely, to favor SL coalescence into SL- and cholesterol-rich Lo domains that separate from Ld domains [186]. Supporting the importance of SLs for domain organization, we have shown that cholesterolenriched submicrometric domains at the PM of RBCs are abrogated by SM depletion [29] (Fig. 7b). Takamori and coll. showed that signal translation associated submicrometric domains are only formed in a neutrophil cell line expressing long fatty acyl chain lactosylceramide (LacCer) [187]. In line with this evidence, natural D-erythro-LacCer is more prone to form highly-enriched submicrometric domains than the artificial L-threoLacCer [188]. These two studies suggest that both the fatty acyl chain length and the overall conformation of the SL play a role in domain formation and/or maintenance. Whereas Cer levels are extremely low in resting PMs, Cer significantly increases in stress conditions and in response to stimuli by the hydrolytic action of SMase on SM, playing key roles in a variety of cellular processes and diseases ([60, 172]; see also Section 6.4). Interestingly, the extent of Cer-induced alterations is influenced by the interplay between cholesterol and SM ratios: Cer-enriched domains are formed in conditions with low but not high cholesterol levels. For more details, please see [60]. Depending on their lipid composition (especially cholesterol, SL and Cer contents), lipid domain biophysical properties can strongly vary. Among others, one can cite: (i) membrane fluidity, a property highly influenced by the 4F-Benzoyl-TN14003 web nature of lipids and the degree of unsaturation of fatty acyl chains; (ii) membrane asymmetry resulting from differences in composition of the two membrane leaflets and the slight area excess in the outer layer (bilayer couple hypothesis) [189]; and (iii) membrane curvature and the bending energy due to the resultant bilayer rigidity and the line tension on domain edges [190, 191].Prog Lipid Res. Author manuscript; Pemafibrate web available in PMC 2017 April 01.Carquin et al.Page5.2. Protein-based mechanismsAuthor Manuscript Au.Y regulators of lipid submicrometric domains in biological membranes (for reviews, please see [181-184]). These include cholesterol, complex SLs and Cer, a.o.. Cholesterol is the most abundant lipid in several PMs, with up to 45mol in RBCs (see Table 3). This lipid emerges as a major regulator of submicrometric domain biogenesis and/or maintenance in living cells, as illustrated by the following studies. Depletion of cholesterol from living fibroblasts or CHO cells labeled by fluorescent SM analogs induces the formation of submicrometric domains or increases their size, indicating a restricting role of cholesterol for domain formation/maintenance in these cells [30, 173]. In contrast, slight cholesterol depletion of the RBC PM decreases the abundance of PC- and SM- but not GSLs-enriched submicrometric domains [26, 27] as well as lipid packing, as revealed by Laurdan [185]. Moreover, cholesterol influences the shape of submicrometric domains. For example, lowering cholesterol levels in native pulmonary surfactant membranes induces a transition from circular to fluctuating borderline micrometric domains, typical of gel-ordered like phases [16]. The fine and ambivalent effect of cholesterol on submicrometric domains in different cells may be related to differences in membrane composition. Indeed, cholesterol has been proposed to either promote lipid mixing by converting gel and Ld phases into an intermediate Lo phase or, conversely, to favor SL coalescence into SL- and cholesterol-rich Lo domains that separate from Ld domains [186]. Supporting the importance of SLs for domain organization, we have shown that cholesterolenriched submicrometric domains at the PM of RBCs are abrogated by SM depletion [29] (Fig. 7b). Takamori and coll. showed that signal translation associated submicrometric domains are only formed in a neutrophil cell line expressing long fatty acyl chain lactosylceramide (LacCer) [187]. In line with this evidence, natural D-erythro-LacCer is more prone to form highly-enriched submicrometric domains than the artificial L-threoLacCer [188]. These two studies suggest that both the fatty acyl chain length and the overall conformation of the SL play a role in domain formation and/or maintenance. Whereas Cer levels are extremely low in resting PMs, Cer significantly increases in stress conditions and in response to stimuli by the hydrolytic action of SMase on SM, playing key roles in a variety of cellular processes and diseases ([60, 172]; see also Section 6.4). Interestingly, the extent of Cer-induced alterations is influenced by the interplay between cholesterol and SM ratios: Cer-enriched domains are formed in conditions with low but not high cholesterol levels. For more details, please see [60]. Depending on their lipid composition (especially cholesterol, SL and Cer contents), lipid domain biophysical properties can strongly vary. Among others, one can cite: (i) membrane fluidity, a property highly influenced by the nature of lipids and the degree of unsaturation of fatty acyl chains; (ii) membrane asymmetry resulting from differences in composition of the two membrane leaflets and the slight area excess in the outer layer (bilayer couple hypothesis) [189]; and (iii) membrane curvature and the bending energy due to the resultant bilayer rigidity and the line tension on domain edges [190, 191].Prog Lipid Res. Author manuscript; available in PMC 2017 April 01.Carquin et al.Page5.2. Protein-based mechanismsAuthor Manuscript Au.
