Effect of Phosphatidylcholine Unsaturation on the Lateral Segregation of Palmitoyl Ceramide and Palmitoyl Dihydroceramide in Bilayer Membranes

Stiffening Effect of Ceramide on Lipid Membranes Provides Non-Sacrificial Protection against Potent Chemical Damage

Ceramide is a sphingolipid that constitutes only a small fraction of membrane biomolecules but plays a central role in regulating many biological processes. The ceramide level in cell membranes can drastically increase in response to external damage, which has been hypothesized to involve ceramide's biophysical role that increases the membrane packing density and lowers the membrane permeability. However, direct observation of the consequent influence on membrane chemistry resulting from these ceramide-induced physical properties has been absent. Using our unique field-induced droplet ionization mass spectrometry technique combined with molecular dynamics simulations, here we report that the addition of ceramide to POPC monolayer membranes at the air-water interface greatly reduces the chemical damage from potent chemicals, ^•OH radicals, and HCl vapor, by stiffening the membrane packing and lowering the permeability. These results shed new light on the underlying chemoprotective role of ceramide in lipid membranes, which might serve as a previously unknown protection mechanism in response to external stimuli that cause cell stress or death.

Canonical and 1-Deoxy(methyl) Sphingoid Bases: Tackling the Effect of the Lipid Structure on Membrane Biophysical Properties

Compared to the canonical sphingoid backbone of sphingolipids (SLs), atypical long-chain bases (LCBs) lack C1-OH (1-deoxy-LCBs) or C1-CH₂OH (1-deoxymethyl-LCBs). In addition, when unsaturated, they present a cis-double bond instead of the canonical  Δ4-5 trans-double bond. These atypical LCBs are directly correlated with the development and progression of hereditary sensory and autonomic neuropathy type 1 and diabetes type II through yet unknown mechanisms. Changes in membrane properties have been linked to the biological actions of SLs. However, little is known about the influence of the LCB structure, particularly 1-deoxy(methyl)-LCB, on lipid-lipid interactions and their effect on membrane properties. To address this question, we used complementary fluorescence-based methodologies to study membrane model systems containing POPC and the different LCBs of interest. Our results show that 1-deoxymethyl-LCBs have the highest ability to reduce the fluidity of the membrane, while the intermolecular interactions of 1-deoxy-LCBs were found to be weaker, leading to the formation of less-ordered domains compared to their canonical counterparts-sphinganine and sphingosine. Furthermore, while the presence of a trans-double bond at the Δ4-5 position of the LCB increased the fluidity of the membrane compared to a saturated LCB, a cis-double bond completely disrupted the ability of the LCB to segregate into ordered domains. In conclusion, even small changes on the structure of the LCB, as seen in 1-deoxy(methyl)-LCBs, strongly affects lipid-lipid interactions and membrane fluidity. These results provide evidence that altered balance between species with different LCBs affect membrane properties and may contribute to the pathobiological role of these lipids.

Lateral Segregation of Palmitoyl Ceramide-1-Phosphate in Simple and Complex Bilayers

Ceramide-1-phosphate is a minor sphingolipid with important functions in cell signaling. In this study, we examined the propensity of palmitoyl ceramide-1-phosphate (Cer-1P) to segregate laterally into ordered domains in different bilayer compositions at 23 and 37°C and compared this with segregation of palmitoyl ceramide (PCer) and palmitoyl sphingomyelin (PSM). The ordered-domain formation in the fluid phosphatidylcholine bilayers was determined using the emission lifetime changes of trans-parinaric acid and from differential scanning calorimetry thermograms. The lateral segregation of Cer-1P was examined when hydrated to bilayers in Tris buffer (50 mM Tris, 140 mM NaCl (pH 7.4)). At this pH, Cer-1P was negatively charged. The lateral segregation propensity of Cer-1P in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers was intermediate between PCer and PSM. Based on differential scanning calorimetry analysis, we observed that the gel domains formed by Cer-1P in POPC bilayers (POPC:Cer-1P 70:30 by mol) were less stable (melting interval 16-37°C) than the corresponding POPC and PCer gel domains at equal composition (melting interval 20-55°C). The gel-phase melting enthalpy was also much lower in Cer-1P (1.5 kcal/mol) than in the PCer-containing POPC bilayers (9 kcal/mol). Cer-1P appeared to be at least partially miscible with PCer domains in POPC bilayers. Cer-1P domains were stabilized in the presence of PSM (POPC:PSM 85:15), similarly as seen with PCer-rich domains. In bilayers at 37°C, with an approximate outer-leaflet cell membrane composition (sphingomyelin and cholesterol enriched, aminophospholipid poor), Cer-1P segregation did not lead to the formation of ordered domains, at least when compared with PCer segregation. In bilayers with an approximate inner-leaflet composition (sphingomyelin poor, cholesterol and aminophospholipid enriched), Cer-1P also failed to form ordered domains. PCer segregated into ordered domains only after the PCer/cholesterol ratio exceeded an approximate equimolar ratio.

Nonlamellar-Phase-Promoting Colipids Enhance Segregation of Palmitoyl Ceramide in Fluid Bilayers

Ceramide is an important intermediate in sphingolipid homeostasis. We examined how colipids, with negative intrinsic curvature and which may induce curvature stress in the bilayers, affected the segregation of palmitoyl ceramide (PCer). Such colipids include 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and tetra-linoleoyl cardiolipin (CL). In 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers, PCer formed ordered, gel-like domains at concentrations above 10 mol% at 23°C, as evidenced by the change in the average lifetime of the trans-parinaric acid emission. When POPE or DOPE were included in the DOPC bilayer (at 20:80 or 40:60 POPE or DOPE to DOPC, by mol), the lateral segregation of PCer was facilitated in a concentration-dependent manner, and less PCer was required for the formation of the ordered ceramide-rich domains. Inclusion of CL in the DOPE bilayer (at 10:90 or 20:80 CL to PC, by mol) also caused a similar facilitation of the lateral segregation of PCer. The PCer-rich domains formed in the presence of POPE, DOPE, or CL in DOPC bilayers were slightly more thermostable (by 2-10°C) when compared to PCer-rich domains in DOPC-only bilayers. Nonlamellar phases were not present in bilayers in which the effects of POPE or DOPE on PCer segregation were the largest, as verified by ³¹P NMR. When palmitoyl sphingomyelin was added to the different bilayer compositions at 5 mol%, relative to the phospholipids, PCer segregated into gel domains at lower concentrations (2-3 mol% PCer), and the effect of POPE on PCer segregation was eliminated. We suggest that the effects of POPE, DOPE, and CL on PCer segregation was in part influenced by their effects on membrane curvature stress and in part because of unfavorable interactions with PCer due to their unsaturated acyl chains. These lipids are abundant in mitochondrial membranes and are likely to affect functional properties of saturated ceramides in them.

Natural Ceramides and Lysophospholipids Cosegregate in Fluid Phosphatidylcholine Bilayers

The mode of interactions between palmitoyl lysophosphatidylcholine (palmitoyl lyso-PC) or other lysophospholipids (lyso-PLs) and palmitoyl ceramide (PCer) or other ceramide analogs in dioleoylphosphatidylcholine (DOPC) bilayers has been examined. PCer is known to segregate laterally into a ceramide-rich phase at concentrations that depend on the nature of the ceramides and the co-phospholipids. In DOPC bilayers, PCer forms a ceramide-rich phase at concentrations above 10 mol%. In the presence of 20 mol% palmitoyl lyso-PC in the DOPC bilayer, the lateral segregation of PCer was markedly facilitated (segregation at lower PCer concentrations). The thermostability of the PCer-rich phase in the presence of palmitoyl lyso-PC was also increased compared to that in the absence of palmitoyl lyso-PC. Other saturated lyso-PLs (e.g., palmitoyl lyso-phosphatidylethanolamine and lyso-sphingomyelin) also facilitated the lateral segregation of PCer in a similar manner as palmitoyl lyso-PC. When examined in the DOPC bilayer, it appeared that the association between palmitoyl lyso-PC and PCer was equimolar in nature. It is proposed that the interaction of PCer with lyso-PLs was driven by the need of ceramide to obtain a large-headgroup co-lipid, and saturated lyso-PLs were preferred co-lipids over DOPC because of the nature of their acyl chain. Structural analogs of PCer (1- or 3-deoxy-PCer) were also associated with palmitoyl lyso-PC, similarly to PCer, suggesting that the ceramide/lyso-PL interaction was not sensitive to structural alterations in the ceramide molecule. Binary complexes containing palmitoyl lyso-PC and ceramide were prepared, and these had a bilayer structure as ascertained by transmission electron microscopy. It is concluded that ceramides and lyso-PLs associated with each other both in binary bilayers and in ternary systems based on the DOPC bilayers. This association may have biological relevance under conditions in which both sphingomyelinases and phospholipase A₂ enzymes are activated, such as during inflammatory processes.

On the Importance of the C(1)-OH and C(3)-OH Functional Groups of the Long-Chain Base of Ceramide for Interlipid Interaction and Lateral Segregation into Ceramide-Rich Domains

Ceramides are important intermediates in sphingolipid biosynthesis (and degradation) and are normally present in only small amounts in unstressed cells. However, following the receptor-mediated activation of neutral sphingomyelinase, sphingomyelin can acutely give rise to substantial amounts of ceramides, which dramatically alter membrane properties. In this study, we have examined the role of the 1-OH and 3-OH functional groups of ceramide for its membrane properties. We have specifically examined how the oxidation of the primary alcohol to COOH or COOMe in palmitoyl ceramide (PCer) or the removal of either the primary alcohol or C(3)-OH (deoxy analogs) affected ceramides' interlipid interactions in fluid phosphatidylcholine bilayers. Measuring the time-resolved fluorescence emission of trans-parinaric acid, or its steady-state anisotropy, we have obtained information about the propensity of the ceramide analogs to form ceramide-rich domains and the thermostability of the formed domains. We observed that the oxidation of the primary alcohol to COOH shifted the ceramide's gel-phase onset concentration to slightly higher values in 1-palmitoyl-2-oleoyl- sn-3- glycero-3-phosphocholine (POPC) bilayers. Methylation of the COOH function of the ceramide did not change the segregation tendency further. The complete removal of the primary alcohol dramatically reduced the ability of 1-deoxy-PCer to form ceramide-rich ordered domains. However, the removal 3-OH (in 3-deoxy-PCer) had only small effects on the lateral segregation of the ceramide analog. The thermostability of the ceramide-rich domains in the POPC bilayers decreased in the following order: 1-OH > COOH > COOMe = 3-deoxy > 1-deoxy. We conclude that ceramide needs a hydrogen-bonding-competent functional group in the C(1) position to be able to form laterally segregated ceramide-rich domains of high packing density in POPC bilayers. The presence or absence of 3-OH was not functionally critical for ceramide's lateral segregation properties.

Bilayer Interactions among Unsaturated Phospholipids, Sterols, and Ceramide

Using differential scanning calorimetry and lifetime analysis of trans-parinaric acid fluorescence, we have examined how cholesterol and cholesteryl phosphocholine (CholPC) affect gel-phase properties of palmitoyl ceramide (PCer) in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC) bilayers. By ²H NMR, we also measured fluid-phase interactions among these lipids using deuterated analogs of POPC, PCer, and cholesterol. The PCer-rich gel phase in POPC bilayers (9:1 molar ratio of POPC to PCer) was partially and similarly dissolved (and thermostability decreased) by both cholesterol and CholPC (sterol was present equimolar to PCer, or in fourfold excess). In DOPC bilayers (4:1 DOPC/PCer molar ratio), CholPC was much more efficient in dissolving the PCer-rich gel phase when compared to cholesterol. This can be interpreted as indicating that PCer interaction with POPC was stronger than PCer interaction with DOPC. PCer-CholPC interactions were also more favored in DOPC bilayers compared to POPC bilayers. In the fluid POPC-rich phase, cholesterol increased the order of the acyl chain of d₂-PCer much more than did CholPC. In DOPC-rich fluid bilayers, both cholesterol and CholPC increased d₂-PCer acyl chain order, and the ordering induced by CholPC was more efficient in DOPC than in POPC bilayers. In fluid POPC bilayers, the ordering of 3-d1-cholesterol by PCer was weak. In summary, we found that in the gel phase, sterol effects on the PCer-rich gel phase were markedly influenced by the acyl chain composition of the fluid PC. The same was true for fluid-phase interactions involving the sterols. Our results further suggest that PCer did not display high affinity toward either of the sterols used. We conclude that the nature of unsaturated phospholipids (POPC versus DOPC) in bilayers has major effects on the properties of ceramide gel phases and on sterol-ceramide-phospholipid interactions in such complex bilayers.

The Long-Chain Sphingoid Base of Ceramides Determines Their Propensity for Lateral Segregation

We examined how the length of the long-chain base or the N-linked acyl chain of ceramides affected their lateral segregation in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers. Lateral segregation and ceramide-rich phase formation was ascertained by a lifetime analysis of trans-parinaric acid (tPA) fluorescence. The longer the length of the long-chain base (d16:1, d17:1, d18:1, d19:1, and d20:1 in N-palmitoyl ceramide), the less ceramide was needed for the onset of lateral segregation and ceramide-rich phase formation. A similar but much weaker trend was observed when sphingosine (d18:1)-based ceramide had N-linked acyl chains of increasing length (14:0 and 16:0-20:0 in one-carbon increments). The apparent lateral packing of the ceramide-rich phase, as determined from the longest-lifetime component of tPA fluorescence, also correlated strongly with the long-chain base length, but not as strongly with the N-acyl chain length. Finally, we compared two ceramide analogs with equal carbon numbers (d16:1/17:0 or d20:1/13:0) and observed that the analog with a longer sphingoid base segregated at lower bilayer concentrations to a ceramide-rich phase compared with the shorter sphingoid base analog. The gel phase formed by d20:1/13:0 ceramide also was more thermostable than the gel phase formed by d16:1/17:0 ceramide. ²H NMR data for 10 mol % stearoyl ceramide in POPC also showed that the long-chain base was more ordered than the acyl chain at comparable chain positions and temperatures. We conclude that the long-chain base length of ceramide is more important than the acyl chain length in determining the lateral segregation of the ceramide-rich gel phase and intermolecular interactions therein.