Effect of Ceramide on Nonraft Proteins

Exploring the biophysical properties of phytosterols in the plasma membrane for novel cancer prevention strategies

Cancer is a global problem with no sign that incidences are reducing. The great costs associated with curing cancer, through developing novel treatments and applying patented therapies, is an increasing burden to developed and developing nations alike. These financial and societal problems will be alleviated by research efforts into prevention, or treatments that utilise off-patent or repurposed agents. Phytosterols are natural components of the diet found in an array of seeds, nuts and vegetables and have been added to several consumer food products for the management of cardio-vascular disease through their ability to lower LDL-cholesterol levels. In this review, we provide a connected view between the fields of structural biophysics and cellular and molecular biology to evaluate the growing evidence that phytosterols impair oncogenic pathways in a range of cancer types. The current state of understanding of how phytosterols alter the biophysical properties of plasma membrane is described, and the potential for phytosterols to be repurposed from cardio-vascular to oncology therapeutics. Through an overview of the types of biophysical and molecular biology experiments that have been performed to date, this review informs the reader of the molecular and biophysical mechanisms through which phytosterols could have anti-cancer properties via their interactions with the plasma cell membrane. We also outline emerging and under-explored areas such as computational modelling, improved biomimetic membranes and ex vivo tissue evaluation. Focus of future research in these areas should improve understanding, not just of phytosterols in cancer cell biology but also to give insights into the interaction between the plasma membrane and the genome. These fields are increasingly providing meaningful biological and clinical data but iterative experiments between molecular biology assays, biosynthetic membrane studies and computational membrane modelling improve and refine our understanding of the role of different sterol components of the plasma membrane.

Bending Rigidities and Interdomain Forces in Membranes with Coexisting Lipid Domains

To precisely quantify the fundamental interactions between heterogeneous lipid membranes with coexisting liquid-ordered (Lo) and liquid-disordered (Ld) domains, we performed detailed osmotic stress small-angle x-ray scattering experiments by exploiting the domain alignment in raft-mimicking lipid multibilayers. Performing a Monte Carlo-based analysis allowed us to determine with high reliability the magnitude and functional dependence of interdomain forces concurrently with the bending elasticity moduli. In contrast to previous methodologies, this approach enabled us to consider the entropic undulation repulsions on a fundamental level, without having to take recourse to crudely justified mean-field-like additivity assumptions. Our detailed Hamaker-coefficient calculations indicated only small differences in the van der Waals attractions of coexisting Lo and Ld phases. In contrast, the repulsive hydration and undulation interactions differed significantly, with the latter dominating the overall repulsions in the Ld phase. Thus, alignment of like domains in multibilayers appears to originate from both, hydration and undulation repulsions.

Ceramide: a simple sphingolipid with unique biophysical properties

Ceramides are involved in a variety of cellular processes and in disease. Their biological functions are thought to depend on ceramides' unique biophysical properties, which promote strong alterations of cell membrane properties and consequent triggering of signaling events. Over the last decades, efforts were made to understand the impact of ceramide on membrane biophysical features. Several studies, performed in a multitude of membrane models, address ceramides' specific interactions, the effect of their acyl chain structure and the influence of membrane lipid composition and properties on ceramide biophysical outcome. In this review, a rationale for the multiple and complex changes promoted by ceramide is provided, highlighting, on a comprehensive and critical manner, the interactions between ceramides and specific lipids and/or lipid phases. Focus is also given to the interplay between ceramide and cholesterol, particularly in lipid raft-mimicking mixtures, an issue of intense debate due to the urgent need to understand the biophysical impact of ceramide formation in models resembling the cell membrane. The implications of ceramide-induced biophysical changes on lipid-protein interactions and cell signaling are also discussed, together with the emerging evidence for the existence of ceramide-gel like domains in cellular membranes.

Monolayer spontaneous curvature of raft-forming membrane lipids

Monolayer spontaneous curvatures for cholesterol, DOPE, POPE, DOPC, DPPC, DSPC, POPC, SOPC, and egg sphingomyelin were obtained using small-angle X-ray scattering (SAXS) on inverted hexagonal phases (HII). Spontaneous curvatures of bilayer forming lipids were estimated by adding controlled amounts to a HII forming template following previously established protocols. Spontaneous curvatures of both phosphatidylethanolamines and cholesterol were found to be at least a factor of two more negative than those of phosphatidylcholines, whose J0 values are closer to zero. Interestingly, a significant positive J0 value was retrieved for DPPC. We further determined the temperature dependence of the spontaneous curvatures J0(T) in the range from 15 to 55 °C, resulting in a quite narrow distribution of -1 to -3 × 10-3 (nm °C)-1 for most investigated lipids. The data allowed us to estimate the monolayer spontaneous curvatures of ternary lipid mixtures showing liquid ordered/liquid disordered phase coexistence. We report spontaneous curvature phase diagrams for DSPC/DOPC/Chol, DPPC/DOPC/Chol and SM/POPC/Chol and discuss effects on protein insertion and line tension.

Sphingolipids and membrane domains: recent advances

There is growing evidence that cell membranes can contain domains with different lipid and protein compositions and with different physical properties. Furthermore, it is increasingly appreciated that sphingolipids play a crucial role in the formation and properties of ordered lipid domains (rafts) in cell membranes. This review describes recent advances in our understanding of ordered membrane domains in both cells and model membranes. In addition, how the structure of sphingolipids influences their ability to participate in the formation of ordered domains, as well as how sphingolipid structure alters ordered domain properties, is described. The diversity of sphingolipid structure is likely to play an important role in modulating the biologically relevant properties of "rafts" in cell membranes.

Lipid sorting by ceramide and the consequences for membrane proteins

We mimicked the effect of sphingomyelinase activity on lipid mixtures of palmitoyl-oleoyl-phosphatidylcholine, sphingomyelin, ceramide, and 10 mol % cholesterol. Using x-ray diffraction experiments in combination with osmotic stress we found, in agreement with previous studies, that ceramide induces a coexistence of L(α) and L(β) domains. A detailed structural analysis of the coexisting domains demonstrated an increase of lipid packing density and membrane thickness in the L(α) domains upon increasing overall ceramide levels. This provides evidence for a ceramide-driven accumulation of cholesterol in the L(α) domains, in support of previous reports. We further determined the bending rigidities of the coexisting domains and found that the accumulation of cholesterol in the L(α) domains stabilizes their bending rigidity, which experiences a dramatic drop in the absence of cholesterol. Deriving experimental estimates for the spontaneous curvature and Gaussian modulus of curvature, we show, using a simple geometric model for ion channels, that in this way changes in the conformational equilibrium of membrane proteins can be kept small.

Stable and unstable lipid domains in ceramide-containing membranes

We applied x-ray diffraction, calorimetry, and infrared spectroscopy to lipid mixtures of palmitoyl-oleoyl phosphatidylcholine, sphingomyelin, and ceramide. This combination of experimental techniques allowed us to probe the stability and structural properties of coexisting lipid domains without resorting to any molecular probes. In particular, we found unstable microscopic domains (compositional/phase fluctuations) in the absence of ceramide, and macroscopically separated fluid and gel phases upon addition of ceramide. We also observed phase fluctuations in the presence of ceramide within the broad phase transition regions. We compare our results with fluorescence spectroscopy data and complement the previously reported phase diagram. We also obtained electron paramagnetic resonance data to assess the possible limitations of techniques employing a single label. Our study demonstrates the necessity of applying a combination of experimental techniques to probe local/global structural and fast/slow motional properties in complex lipid mixtures.

Implication of sphingomyelin/ceramide molar ratio on the biological activity of sphingomyelinase

Sphingolipid signaling plays an important, yet not fully understood, role in diverse aspects of cellular life. Sphingomyelinase is a major enzyme in these signaling pathways, catalyzing hydrolysis of sphingomyelin to ceramide and phosphocholine. To address the related membrane dynamical structural changes and their feedback to enzyme activity, we have studied the effect of enzymatically generated ceramide in situ on the properties of a well-defined lipid model system. We found a gel-phase formation that was about four times faster than ceramide generation due to ceramide-sphingomyelin pairing. The gel-phase formation slowed down when the ceramide molar ratios exceeded those of sphingomyelin and stopped just at the solubility limit of ceramide, due to unfavorable pairwise interactions of ceramide with itself and with monounsaturated phosphatidylcholine. A remarkable correlation to in vitro experiments suggests a regulation of sphingomyelinase activity based on the sphingomyelin/ceramide molar ratio.

Applications of neutron and X-ray scattering to the study of biologically relevant model membranes

Scattering techniques, in particular electron, neutron and X-ray scattering have played a major role in elucidating the static and dynamic structure of biologically relevant membranes. Importantly, neutron and X-ray scattering have evolved to address new sample preparations that better mimic biological membranes. In this review, we will report on some of the latest model membrane results, and the neutron and X-ray techniques that were used to obtain them.