How Photoswitchable Lipids Affect the Order and Dynamics of Lipid Bilayers and Embedded Proteins.

Altering the properties of phospholipid membranes by light is an attractive option for the noninvasive manipulation of membrane proteins and cellular functions. Lipids with an azobenzene group within their acyl chains such as AzoPC are suitable tools for manipulating lipid order and dynamics through a light-induced trans-to-cis isomerization. However, the action of these photoswitchable lipids at the atomic level is still poorly understood. Here, liposomes containing AzoPC, POPE, and POPG have been characterized by solid-state NMR through chemical shift and dipolar CH order parameter measurements. Upon UV-light illumination, an efficient trans-to-cis conversion can be achieved resulting in a localized reduction of the CH order parameter within the bulk lipid acyl chains. This effect is even more pronounced in liposomes containing the integral membrane protein E. coli diacylglycerol kinase. The protein responds to the light-induced trans-to-cis isomerization by a site-specific increase in the molecular dynamics as observed by altered cross peak intensities in NCA spectra. This study represents a proof-of-concept demonstration for the use of photoswitchable lipids to modulate membrane properties by light for inducing dynamic changes within an embedded membrane protein.

Determination of n-alkane partitioning within phosphatidylethanolamine Lα/HII phases.

Alkanes are known to promote the fluid lamellar (Lα)-to-inverted hexagonal (HII) phase transition of different phospholipids. In this work, we studied the interaction of decane and tetradecane with self-assemblies formed of 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine (POPE), using sequential 2H and 31P solid-state NMR spectroscopy. This technique allowed calculating the partitioning constant of the alkanes between the Lα and HII phases of POPE. Our results show that both alkanes are preferentially distributed in the HII phase compared to the Lα phase. In the HII phase, both alkanes display a very high conformational disorder, consistent with their location in the intercylinder voids. This preferential partitioning in the HII phase is more pronounced for tetradecane than for decane. This finding is proposed to be associated with a less energetically favored insertion (limited solubility) of tetradecane in the lamellar phase. This proposition is supported by the observation that tetradecane experiences more conformational freedom than its shorter analog in POPE bilayers, suggesting that it is located, on average, closer to the middle of the bilayers. It is also established that the increase in size of the intercylinder voids, by the addition of 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) in the HII cylinders, enhances the partitioning of decane in the HII phase compared to the Lα phase. These findings propose handles to modulate the balance of the relative Lα/HII phase stability.

Design and Characterization of a Therapeutic Non-phospholipid Liposomal Nanocarrier with Osteoinductive Characteristics To Promote Bone Formation.

Sterosomes are recently developed types of non-phospholipid liposomes formed from single-chain amphiphiles and high content of sterols. Although sterosomes presented significantly increased stability compared to conventional phospholipid liposomes, current sterosome biomaterials are not truly bioactive and have no intrinsic therapeutic effects. The purpose of this study was to develop a sterosome formulation with osteoinductive properties by an effective selection of sterol, one of the sterosome components. Oxysterols are oxidized derivatives of cholesterol and are known to stimulate osteogenesis and bone formation. Thus, 20S-hydroxycholesterol (Oxy), one of the most potent oxysterols for bone regeneration, was examined as a promising candidate molecule to form fluid lamellar phases with a single-chain amphiphile, namely, stearylamine (SA). First, the optimal composition was identified by investigating the phase behavior of SA/Oxy mixtures. Next, the capacity of the optimized SA/Oxy sterosomes to promote osteogenic differentiation of bone marrow stromal cells was assessed in vitro in a hydrogel environment. Furthermore, we explored the effects of osteogenic oxysterol sterosomes in vivo with the mouse critical-sized calvarial defect model. Our results showed that SA/Oxy sterosomes induced osteogenic differentiation in vitro and enhanced calvarial healing without delivery of additional therapeutic agents, indicating their intrinsic bone-forming potential. This study suggests a promising non-phospholipid liposomal platform with osteoinductive properties for delivery of small molecular drugs and/or other therapeutic genes for enhanced bone formation.

Ceramide-C16 Is a Versatile Modulator of Phosphatidylethanolamine Polymorphism.

Ceramide-C16 (CerC16) is a sphingolipid associated with several diseases like diabetes, obesity, Parkinson disease, and certain types of cancers. As a consequence, research efforts are devoted to identify the impact of CerC16 on the behavior of membranes, and to understand how it is involved in these diseases. In this work, we investigated the impacts of CerC16 (up to 20 mol %) on the lipid polymorphism of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), using differential scanning calorimetry, and sequential 2H and 31P solid-state nuclear magnetic resonance spectroscopy. A partial phase diagram is proposed. The results indicate that the presence of CerC16 leads to an upshift of the temperature of the gel-to-liquid crystalline (Lß - Lα) phase transition, leading to a large Lß/Lα phase coexistence region where gel-phase domains contain ∼35 mol % CerC16. It also leads to a downshift of the temperature of the lamellar-to-inverted hexagonal (L - HII) phase transition of POPE. The opposite influence on the two-phase transitions of POPE brings a three-phase coexistence line when the two transitions overlap. The resulting HII phase can be ceramide enriched, coexisting with a Lα phase, or ceramide depleted, coexisting with a Lß phase, depending on the CerC16 proportions. The uncommon capability of CerC16 to modulate the membrane fluidity, its curvature propensity, and the membrane interface properties highlights its potential as a versatile messenger in cell membrane events.