A comparative calorimetric study of the effects of cholesterol and the plant sterols campesterol and brassicasterol on the thermotropic phase behavior of dipalmitoylphosphatidylcholine bilayer membranes

Robustness of heteroaggregates involving hydrophobic cholesterol and its mimetics

Exploring the self and cross aggregation affinity of cholesterol (CHL) and some of its lookalikes, e.g., cholesteryl hemisuccinate (CHM), campesterol (CAM) and arjunic acid (ARJ), provides crucial understanding towards the influence of weak forces in inducing mixed micellization through heteroaggregation. Strongly hydrophobic CHL, with a benchmark inclination towards aggregation, often forms detrimental plaques in crucial human organs that are fairly difficult to disintegrate. Traditionally known anti-dyslipidemic agents like CAM and ARJ are known to interact strongly with CHL in the gut when ingested. They further form mixed micelles along with the bile components and interfere with the CHL absorption across the epithelial cell layer of the intestine. Some invariant questions like how robust are the heteroaggregates formed between these mimetics and CHL are very important to appreciate the efficacy of such anti-dyslipidemic agents. In this work using molecular dynamics simulations and varied structural analysis, we characterize the heteroaggregates. Simulations indicate that CHL-CHM mixed assemblies are comparatively bigger and significantly stabilized by strong electrostatic and favourable vdW forces. Small and diffused CHL-ARJ aggregates are observed in our simulations with a not so favourable energetics, indicating a possible attenuation pathway of CHL aggregation in the presence of ARJ.

Sterol composition in plants is specific to pollen, leaf, pollination and pollinator

Sterols have several roles in planta, including as membrane components. Sterols are also essential nutrients for insects. Based on this, and the different functions of leaves and pollen, we tested the hypotheses that (a) the sterolome is different in leaves and pollen from the same plant, (b) pollens from wind- and insect pollinated plants comprise different sterols, and (c) sterol provision in pollen-rewarding angiosperms differs from nectar-rewarding species. A novel approach to sterolomics was developed, using LCMS to determine the sterol profile of leaf and pollen from a taxonomically diverse range of 36 plant species. Twenty-one sterols were identified unambiguously, with several more identified in trace amounts. C29 sterols dominated the sterolome in most plants. The sterol composition was significantly different in leaf and pollen and their main sterols evolved in different ways. The sterolome of pollen from animal- and wind-pollinated was also significantly different, but not between nectar- and pollen-rewarding species. Our results suggest that the sterol composition in different plant tissues is linked to their biological functions. Sterol composition in pollen might be driven by physical role rather than the nutrient needs of pollinating insects.

Sterol and lipid metabolism in bees

BACKGROUND

Bees provide essential pollination services for many food crops and are critical in supporting wild plant diversity. However, the dietary landscape of pollen food sources for social and solitary bees has changed because of agricultural intensification and habitat loss. For this reason, understanding the basic nutrient metabolism and meeting the nutritional needs of bees is becoming an urgent requirement for agriculture and conservation. We know that pollen is the principal source of dietary fat and sterols for pollinators, but a precise understanding of what the essential nutrients are and how much is needed is not yet clear. Sterols are key for producing the hormones that control development and may be present in cell membranes, where fatty-acid-containing species are important structural and signalling molecules (phospholipids) or to supply, store and distribute energy (glycerides).

AIM OF THE REVIEW

In this critical review, we examine the current general understanding of sterol and lipid metabolism of social and solitary bees from a variety of literature sources and discuss implications for bee health.

KEY SCIENTIFIC CONCEPTS OF REVIEW

We found that while eusocial bees are resilient to some dietary variation in sterol supply the scope for this is limited. The evidence of both de novo lipogenesis and a dietary need for particular fatty acids (FAs) shows that FA metabolism in insects is analogous to mammals but with distinct features. Bees rely on their dietary intake for essential sterols and lipids in a way that is dependent upon pollen availability.

The Role of Phytosterols in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease is now recognized as the most common cause of chronic liver disease with an increasing prevalence in both adults and children. Although the symptoms are absent or poorly expressed in most cases, some patients may progress to end-stage liver disease. The pathogenesis of NAFLD is known to be multifactorial. Current therapeutic recommendations focus on lifestyle changes in order to reduce the incidence of risk factors and drugs targeting major molecular pathways potentially involved in the development of this disease. Given that a pharmacological treatment, completely safe and effective, is not currently known in recent years more research has been done on the effects that some bio-active natural compounds, derived from plants, have in preventing the onset and progression of NAFLD. Numerous studies, in animals and humans, have shown that phytosterols (PSs) play an important role in this pathology. Phytosterols are natural products that are found naturally in plant. More than 250 phytosterols have been identified, but the most common in the diet are stigmasterol, β-sitosterol, and campesterol. Consumption of dietary PSs can reduce serum cholesterol levels. Due to these properties, most studies have focused on their action on lipid metabolism and the evolution of NAFLD. PSs may reduce steatosis, cytotoxicity oxidative stress, inflammation, and apoptosis. The purpose of this review is to provide an overview of the importance of dietary phytosterols, which are a window of opportunity in the therapeutic management of NAFLD.

Monolayers of Cholesterol and Cholesteryl Stearate at the Water/Vapor Interface: A Physico-Chemical Study of Components of the Meibum Layer

Langmuir monolayers containing different amounts of cholesterol and cholesteryl stearate were studied at two different temperatures (24 °C and 35 °C). The main goal was to contribute towards the understanding of how the variations in the chemical composition may affect the physico-chemical properties of these specific lipid monolayers. The model mixture was chosen considering that cholesteryl esters are present in cell membranes and some other biological systems, including human tear lipids. Therefore, an investigation into the effect of the lipid monolayer composition on their interfacial properties may elucidate some of the fundamental reasons for the deficiencies in cell membranes and tear film functioning in vivo. The experimental results have shown that the molar ratio of the mixture plays a crucial role in the modulation of the Langmuir film properties. The condensing effects of the cholesterol and the interactions between the lipids in the monolayer were the main factors altering the monolayer response to dilatational deformation. The modification of the mixture compositions leads to significant changes in the Langmuir films and the mechanical performance, altering the ability of the monolayer to reduce the surface tension and the viscoelastic properties of the monolayers. This suggests that subtle modifications of the biomembrane composition may significantly alter its physiological function.

β-Sitosterol treatment attenuates cognitive deficits and prevents amyloid plaque deposition in amyloid protein precursor/presenilin 1 mice

Alzheimer's disease (AD) is the most common neurodegenerative disorder causing dementia worldwide, and is mainly characterized by aggregated β-amyloid (Aβ). Increasing evidence has shown that plant extracts have the potential to delay AD development. The plant sterol β-Sitosterol has a potential role in inhibiting the production of platelet Aβ, suggesting that it may be useful for AD prevention. In the present study, we aimed to investigate the effect and mechanism of β-Sitosterol on deficits in learning and memory in amyloid protein precursor/presenilin 1 (APP/PS1) double transgenic mice. APP/PS1 mice were treated with β-Sitosterol for four weeks, from the age of seven months. Brain Aβ metabolism was evaluated using ELISA and Western blotting. We found that β-Sitosterol treatment can improve spatial learning and recognition memory ability, and reduce plaque load in APP/PS1 mice. β-Sitosterol treatment helped reverse dendritic spine loss in APP/PS1 mice and reversed the decreased hippocampal neuron miniature excitatory postsynaptic current frequency. Our research helps to explain and support the neuroprotective effect of β-Sitosterol, which may offer a novel pharmaceutical agent for the treatment of AD. Taken together, these findings suggest that β-Sitosterol ameliorates memory and learning impairment in APP/PS1 mice and possibly decreases Aβ deposition.

Identifying the Mechanisms and Molecular Targets of Yizhiqingxin Formula on Alzheimer's Disease: Coupling Network Pharmacology with GEO Database

Background

Yizhiqingxin formula (YZQX) is a promising formula for the treatment of Alzheimer’s disease (AD) with significant clinical effects. Here, we coupled a network pharmacology approach with the Gene Expression Omnibus (GEO) database to illustrate comprehensive mechanisms and screen for molecular targets of YZQX for AD treatment.

Methods

First, active ingredients of YZQX were screened for the Traditional Chinese Medicine Systems Pharmacology (TCMSP) database with the absorption, distribution, metabolism, and excretion (ADME) parameters. Subsequently, putative targets of active ingredients were predicted using the DrugBank database. AD-related targets were retrieved by analyzing published microarray data (accession number GSE5281). Protein–protein interaction (PPI) networks of YZQX putative targets and AD-related targets were constructed visually and merged to identify candidate targets for YZQX against AD using Cytoscape 3.7.2 software. We performed gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis to further clarify the biological functions of the candidate targets. The gene-pathway network was established to filter for key target genes.

Results

Forty-three active ingredients were identified, and 193 putative target genes were predicted. Seven hundred and ten targets related to AD were screened with |log2 FC| > 1 and P < 0.05. Based on the PPI network, 110 target genes of YZQX against AD were identified. Moreover, 32 related pathways including the PI3K-Akt signaling pathway, MAPK signaling pathway, ubiquitin-mediated proteolysis, apoptosis and the NF-kappa B signaling pathway were significantly enriched. In the gene-pathway network, MAPK1, AKT1, TP53, MDM2, EGFR, RELA, SRC, GRB2, CUL1, and MYC targets are putative core genes for YZQX in AD treatment.

Conclusion

YZQX against AD may exert its neuroprotective effect via the PI3K-Akt signaling pathway, MAPK signaling pathway, and ubiquitin-mediated proteolysis. YZQX may be a promising drug that can be used in the treatment of AD.

Naturally-occurring cholesterol analogues in lipid nanoparticles induce polymorphic shape and enhance intracellular delivery of mRNA

Endosomal sequestration of lipid-based nanoparticles (LNPs) remains a formidable barrier to delivery. Herein, structure-activity analysis of cholesterol analogues reveals that incorporation of C-24 alkyl phytosterols into LNPs (eLNPs) enhances gene transfection and the length of alkyl tail, flexibility of sterol ring and polarity due to -OH group is required to maintain high transfection. Cryo-TEM displays a polyhedral shape for eLNPs compared to spherical LNPs, while x-ray scattering shows little disparity in internal structure. eLNPs exhibit higher cellular uptake and retention, potentially leading to a steady release from the endosomes over time. 3D single-particle tracking shows enhanced intracellular diffusivity of eLNPs relative to LNPs, suggesting eLNP traffic to productive pathways for escape. Our findings show the importance of cholesterol in subcellular transport of LNPs carrying mRNA and emphasize the need for greater insights into surface composition and structural properties of nanoparticles, and their subcellular interactions which enable designs to improve endosomal escape.

The Potential of α-Spinasterol to Mimic the Membrane Properties of Natural Cholesterol

Sterols play a unique role for the structural and dynamical organization of membranes. The current study reports data on the membrane properties of the phytosterol (3β,5α,22E)-stigmasta-7,22-dien-3-β-ol (α-spinasterol), which represents an important component of argan oil and have not been investigated so far in molecular detail. In particular, the impact of α-spinasterol on the structure and organization of lipid membranes was investigated and compared with those of cholesterol. Various membrane parameters such as the molecular packing of the phospholipid fatty acyl chains, the membrane permeability toward polar molecules, and the formation of lateral membrane domains were studied. The experiments were performed on lipid vesicles using methods of NMR spectroscopy and fluorescence spectroscopy and microscopy. The results show that α-spinasterol resembles the membrane behavior of cholesterol to some degree.

Mechanical Properties of Membranes Composed of Gel-Phase or Fluid-Phase Phospholipids Probed on Liposomes by Atomic Force Spectroscopy

In many liposome applications, the nanomechanical properties of the membrane envelope are essential to ensure, e.g., physical stability, protection, or penetration into tissues. Of all factors, the lipid composition and its phase behavior are susceptible to tune the mechanical properties of membranes. To investigate this, small unilamellar vesicles (SUV; diameter < 200 nm), referred to as liposomes, were produced using either unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in aqueous buffer at pH 6.7. The respective melting temperatures of these phospholipids were -20 and 41 °C. X-ray diffraction analysis confirmed that at 20 °C DOPC was in the fluid phase and DPPC was in the gel phase. After adsorption of the liposomes onto flat silicon substrates, atomic force microscopy (AFM) was used to image and probe the mechanical properties of the liposome membrane. The resulting force-distance curves were treated using an analytical model based on the shell theory to yield the Young's modulus (E) and the bending rigidity (k_C) of the curved membranes. The mechanical investigation showed that DPPC membranes were much stiffer (E = 116 ± 45 MPa) than those of DOPC (E = 13 ± 9 MPa) at 20 °C. The study demonstrates that the employed methodology allows discrimination of the respective properties of gel- or fluid-phase membranes when in the shape of liposomes. It opens perspectives to map the mechanical properties of liposomes containing both fluid and gel phases or of biological systems.

A comparative differential scanning calorimetry study of the effects of cholesterol and various oxysterols on the thermotropic phase behavior of dipalmitoylphosphatidylcholine bilayer membranes

We have carried out a comparative differential scanning calorimetric (DSC) study of the effects of cholesterol (C) and the eight most physiologically relevant oxysterols on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes. The structures of these oxysterols differ from that of C by the presence of additional hydroxyl, keto or epoxy groups on the steroid ring system or by the presence of a hydroxyl group in the alkyl side chain. In general, the progressive incorporation of these oxysterols reduces the temperature, cooperativity and enthalpy of the pretransition of DPPC to a greater extent than C, indicating that their presence thermally destabilizes and disorders the gel states of DPPC bilayers to a greater extent than C. Similarly, the incorporation of these oxysterols either increases the temperature of the broad component of the main phase transition to a smaller extent than C or actually decreases it. Again, this indicates that the presence of these compounds is less effective at thermally stabilizing and ordering the sterol-rich domains of DPPC bilayers than is C itself. Moreover, the incorporation of these oxysterols decrease the cooperativity and enthalpy of the main phase transition of DPPC to a smaller extent than C, indicating that they are somewhat less miscible in fluid DPPC bilayers than is C. Particularly notable in this regard is 25-hydroxycholesterol, which exhibits a markedly reduced miscibility in both gel and fluid DPPC bilayers compared to C itself. In general, the effectiveness of these oxysterols in stabilizing and ordering DPPC bilayers decreases as their rate of interbilayer exchange and the polarity of the oxysterol increases. We close by providing a tentative molecular explanation for the results of our DSC studies and of those of previous biophysical studies of the effects of various oxysterol on lipid bilayer model membranes.

Membrane properties of cholesterol analogs with an unbranched aliphatic side chain

The interactions between cholesterol and other membrane molecules determine important membrane properties. It was shown that even small changes in the molecular structure of cholesterol have a crucial influence on these interactions. We recently reported that in addition to alterations in the tetracyclic ring structure, the iso-branched side chain of cholesterol also has a significant impact on membrane properties (Scheidt et al., 2013). Here we used synthetic cholesterol analogs to investigate the influence of an unbranched aliphatic side chain of different length. The (2)H NMR order parameter of the phospholipid chains and therefore the molecular packing of the phospholipid molecules shows a significant dependence on the sterol's alkyl side chain length, while, membrane permeation studied by a dithionite ion permeation assay and lateral diffusion measured by (1)H MAS pulsed field gradient NMR are less influenced. To achieve the same molecular packing effect similar to that of an iso-branched aliphatic side chain, a longer unbranched side chain (n-dodecyl instead of n-octyl) at C17 of cholesterol is required. Obviously, sterols having a branched iso-alkyl chain with two terminal methyl groups exhibit altered cholesterol-phospholipid interactions compared to analogous molecules with a straight unbranched chain.