Regulation Of Membrane Fluidity Research Articles

Cholesterol mediates the potential adverse influence of graphene quantum dots on placental lipid membrane model

Nanomaterial-biomembrane interactions constitute a critical biological process in assessing the toxicity of such materials in theoretical studies. However, many investigations simplify these interactions by using membrane models containing only one or a few lipid types, deviating significantly from the complexity of real membrane compositions. In particular, cholesterol, a ubiquitous lipid essential for regulating membrane fluidity and closely linked to various diseases, is often overlooked. Consequently, the role of cholesterol in nanomaterial-biomembrane interactions remains poorly understood. In this study, we employ molecular dynamics (MD) simulations to explore the effect of graphene quantum dots (GQDs) on a realistic placental lipid membrane model, aiming to elucidate the role of cholesterol in these interactions. Our MD results reveal that both GQD monomers and clusters can spontaneously insert into the placental lipid membrane model, driven by strong van der Waals interaction energy. Further analyses indicate that cholesterol and POPC lipids primarily contribute to interfacial interactions. Notably, cholesterol can be squeezed into the bilayer interface, forming a unique structure where it is sandwiched between the GQD cluster and the membrane’s bottom leaflet. More significantly, cholesterol, together with the GQD cluster, exhibits free lateral movement, suggesting a strong affinity of cholesterol for GQD clusters. These findings highlight the critical role of cholesterol in mediating GQD insertion into the biomembrane. Structural analyses of the membrane further demonstrate deformation of the placental lipid membrane model during GQD penetration. Finally, free energy calculations confirm that the insertion of both GQD monomers and clusters into the placental lipid membrane model is energetically favorable. Overall, this study not only sheds new light on the potential harmful effects of GQDs on realistic placental membranes but also provides the first theoretical evidence of the pivotal role of cholesterol in nanomaterial-biomembrane interactions, contributing to a deeper understanding of nanomaterial-cell membrane interactions.

24S-Hydroxycholesterol in Neuropsychiatric Diseases: Schizophrenia, Autism Spectrum Disorder, and Bipolar Disorder.

Neuropsychiatric diseases (NPDs) are severe, debilitating psychiatric conditions that affect the nervous system. These are among the most challenging disorders in medicine. Some examples include Alzheimer's, anxiety disorders, autism spectrum disorder, bipolar disorder, and schizophrenia. NPDs represent an ever-increasing burden on public health and are prevalent throughout the world. For most of these diseases, the particular etiopathogeneses are still enigmatic. NPDs are also associated with structural and functional changes in the brain, along with altered neurotransmitter and neuroendocrine systems.Approximately 25% of the total human body cholesterol is located in the brain. Its involvement in neuronal functions starts in the early growth stages and remains important throughout adulthood. It is also an integral part of the neuronal membrane, ensuring membrane lipid organization and regulating membrane fluidity. The main mechanism for removing cholesterol from the brain is cholesterol 24-hydroxylation by cytochrome P450 46A1 (CYP46A1), an enzyme specifically found in the central nervous system. Although research on 24S-OHC and its role in neuropsychiatric diseases is still in its early stages, this oxidized cholesterol metabolite is thought to play a crucial role in the etiology of NPDs. 24S-OHC can affect neurons, astrocytes, oligodendrocytes, and vascular cells. In addition to regulating the homeostasis of cholesterol in the brain, this oxysterol is involved in neurotransmission, oxidative stress, and inflammation. The role of 24S-OHC in NPDs has been found to be controversial in terms of the findings so far. There are several intriguing discrepancies in the data gathered so far regarding 24S-OHC and NPDs. In fact, 24S-OHC levels were reported to have decreased in a number of NPDs and increased in others.Hence, in this chapter, we first summarize the available data regarding 24S-OHC as a biomarker in NPDs, including schizophrenia, autism spectrum disorder, and bipolar disorder. Then, we present a brief synopsis of the pharmacological targeting of 24S-OHC levels through the modulation of CYP46A1 activity.

Omega-3 PUFA and the fitness and cognition of the nematode Caenorhabditis elegans under different environmental conditions

Many invertebrate species possess the metabolic ability to synthesize long-chain ω3 polyunsaturated fatty acids (PUFA) de novo. Due to their diverse effects on membrane architecture, neuroplasticity, growth and reproduction, PUFA have a high potential to positively influence the fitness of an organism. But how and when do these supposed advantages actually come into play? Other species, that are often closely related, pass natural selection without this special metabolic ability. The ω3-PUFA rich model organism Caenorhabditis elegans (Nematoda) and its mutant fat-1(wa9), lacking these PUFA, are a suitable test system. We analyzed potential impairments in reproduction and growth in a soil assay. Further, chemotaxis after aversive olfactory, associative learning and integration of a second sensory signal were assessed on agar plates. Moreover, we analyzed the phospholipid pattern of both C. elegans strains and further free-living nematodes species at different temperatures. While the phenotypic effects were rather small under standard conditions, lowering the temperature to 15 or even 10 °C or reducing the soil moisture, led to significant limitations, with the investigated parameters for neuroplasticity being most impaired. The ω3-PUFA free C. elegans mutant strain fat-1 did not adapt the fatty acid composition of its phospholipids to a decreasing temperature, while ω3-PUFA containing nematodes proportionally increased this PUFA group. In contrats, other ω3-PUFA free nematode species produced significantly more ω6-PUFA. Thus, the ability to synthesize long-chain ω3-PUFA de novo likely is fundamental for an increase in neuroplasticity and an efficient way for regulating membrane fluidity to maintain their functionality.

Comparative chromatin dynamics reveal differential thermal tolerance mechanisms between two congeneric oyster species

Oysters, being globally farmed bivalve of significant economic and ecological value, are increasingly confronted with heat stress induced by global warming, such as the frequent occurrence of mass summer mortality events worldwide. The role of epigenetics in adapting to high-temperature environments has garnered growing attention; however, its application in oysters remains poorly understood. In this study, we conducted a comparative analysis between two closely related oyster species (the relatively thermosensitive Crassostrea gigas and the relatively thermotolerant Crassostrea angulata) using ATAC-Seq and RNA-Seq to reveal the differential thermal tolerance mechanisms at the chromatin dynamics level in response to high-temperature stress. The comparative analysis showed that the inhibitor of apoptosis protein (IAP) family and its associated apoptosis pathways were the major divergent pathway between the two species. The promoter regions of differentially expressed genes in C. gigas under heat stress were found to be enriched with motifs such as Foxp1, Foxk2, Myc, Cebpd, Foxo3, and Hsf1 motifs and expressed genes related to molecular chaperones, including heat shock protein family. And C. angulata activated the genes related to anti-apoptosis, DNA damage repair, and fatty acid synthesis (decreasing fatty acid unsaturation of membrane to regulate membrane fluidity). These findings suggest that C. angulata may have evolved a more centralized and energy-efficient transcriptional regulatory network at the genomic level, thereby facilitating its long-term adaptation to relatively high temperature environments. Furthermore, the key transcription factor Hsf1, involved in heat shock responses, may contribute to the differential heat response patterns in C. gigas and C. angulata through divergent expression levels of its alternatively spliced isoforms (Hsf1a and Hsf1d). Our work will contribute to predicting the adaptive potential of oysters and other marine invertebrates in facing future global warming, and provide a theoretical foundation for genetic improvement of resistance traits and sustainable development of oyster industry.

Phospholipid tail asymmetry allows cellular adaptation to anoxic environments

Membrane biophysical properties are critical to cell fitness and depend on unsaturated phospholipid acyl tails. These can only be produced in aerobic environments since eukaryotic desaturases require molecular oxygen. This raises the question of how cells maintain bilayer properties in anoxic environments. Using advanced microscopy, molecular dynamics simulations, and lipidomics by mass spectrometry we demonstrated the existence of an alternative pathway to regulate membrane fluidity that exploits phospholipid acyl tail length asymmetry, replacing unsaturated species in the membrane lipidome. We show that the fission yeast, Schizosaccharomyces japonicus, which can grow in aerobic and anaerobic conditions, is capable of utilizing this strategy, whereas its sister species, the well-known model organism Schizosaccharomyces pombe, cannot. The incorporation of asymmetric-tailed phospholipids might be a general adaptation to hypoxic environmental niches.

Oxidative stress and protection against it in bacteria

Microorganisms are exposed to reactive oxygen species (ROS) that are formed in various ways, in particular, as a result of respiration or other intracellular processes, during metal-catalyzed Fenton reactions, as a result of the action of UV- and X-radiation, under the influence of some antimicrobial drugs, or during the host immune oxidative-burst response against infection agents. In this review, we take a look at the mechanisms of microbial cell damage, including damage of lipids and proteins. Lipid peroxidation (LPO) is one of the main molecular mechanisms involved in oxidative damage to cellular structures. A variety of products are formed during LPO reactions: alkoxyl radicals, peroxyl radicals, hydroperoxides, diene conjugates, carbonyl compounds, aldehyde adducts with biopolymers, alcohols, esters, etc. These products include cytotoxic and highly reactive compounds. Free radical reactions of protein damage occur via hydrogen atom abstraction from α-carbon or SH-, NH2-groups of aminoacids and electron abstraction from nucleophile centers of proteins resulting in the fragmentation of proteins, their denaturation and the formation of amino acid radicals. Bacteria show a significant adaptive potential to the influence of stress agents, including ROS. We summarized the data on bacterial antioxidant protection, ROS redox sensors, and regulators of bacterial cell response to ROS exposure, focusing on the features of anaerobic microorganisms, as their responses to the oxidative damage are the least studied, and many problems remain unsolved. This review contains information about changes in fatty acid composition of lipids of the plasma membrane to maintain the necessary fluidity, and, thus, counteract the effects of various stressing agents, including ROS. The main modifications of the fatty acid composition of lipids important for the regulation of membrane fluidity are described, in particular, via changes in the degree of lipid saturation, cis/trans isomerization, and synthesis of cyclopropane fatty acids.

Growth characteristics and fatty acids of the thermotolerant green alga Desmodesmus sp. isolated from a freshwater puddle in Ibaraki, Japan

We investigated the growth, thermotolerance, and fatty acid composition of the green alga Desmodesmus sp., which was isolated from a freshwater puddle in Ibaraki, Japan. Incubation experiments on the isolated strain were performed at a range of temperatures (20–40°C) with continuous illumination (110 µmol photons m−2 s−1) and nutrient-replete medium. High specific growth rates of 1.14–1.62 day−1 were observed at 20–35°C, but growth rates declined to 0.59 day−1 at 40°C. Lipid contents were 13.2%–14.4% (% dry weight) at 20–35°C and increased to 21.3% at 40°C. Some microalgae are known to regulate membrane fluidity by changing their fatty acid compositions in response to changes in ambient temperature. We found that the major polyunsaturated fatty acid (PUFA; C18:3 ω3) of Desmodesmus sp. was negatively correlated with water temperature. By contrast, no significant relationship was identified between temperature and the ratio of saturated to unsaturated fatty acids. The negative relationship between ω3 PUFA composition and temperature was stronger among thermotolerant as opposed to non-thermotolerant microalgae. This suggests that thermotolerant Desmodesmus sp. can grow at high temperatures by altering its fatty acid composition to affect membrane fluidity.

Dynamic Regulation of Order Lipid Meso-domains in the Capillary Endothelial Cell Membrane

The structure and function of ion channels are sensitive to direct interactions with lipids of the membrane and to the biophysical properties of the membrane microenvironment, such that membrane fluidity, lateral pressure profile, and thickness can modulate the channels. Cholesterol plays a significant role in regulating membrane fluidity and is capable of phase separation where cholesterol will spontaneously separate into its own domain, which is partially responsible for developing two distinct domains within the membrane. These ordered domains are regions of high cholesterol and low fluidity, and the disordered domains are regions of low cholesterol and high fluidity. Here we examine and provide evidence of the presence of ordered cholesterol membrane domains in capillary endothelial cells. We will test the hypothesis that capillary endothelial cell ordered domains function as a signaling hub for different phospholipids regulating ion channel function. Using the isolated retinal vasculature, confocal microscopy, and fluorescent Bodipy-cholesterol, a marker of ordered lipid domains, we observed large regions approximately 2.3± 0.9 μm long of cholesterol ordered meso-domains within capillary endothelial cells and not the feeding arteriole. To determine the dynamic nature of these meso-domains, we used membrane-disordering drugs to disrupt the organization of membrane lipids. The ordered meso-domains display dynamic membrane disordering by ethanol and can re-cluster into the large ordered meso-domains. In addition, we observed that the critical signaling phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) co-localized with the ordered meso-domains. To determine the cooperativity between these lipid molecules, we examined the effects of ethanol-dependent disordering of cholesterol on the membrane localization of PIP2. In addition, we examined the effects of PIP2 depletion with the G-protein couple receptor agonist, U46619, on the disordered state of the cholesterol meso-domains. These data suggest that ordered meso-domains might play a role as a signaling hub affecting the membrane fluidity and bioavailability of signaling lipids modulating ion channel function in capillary endothelial cell. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Allylamines, Benzylamines, and Fungal Cell Permeability: A Review of Mechanistic Effects and Usefulness against Fungal Pathogens.

Allylamines, naftifine and terbinafine, and the benzylamine, butenafine, are antifungal agents with activity on the fungal cell membrane. These synthetic compounds specifically inhibit squalene epoxidase, a key enzyme in fungal sterol biosynthesis. This results in a deficiency in ergosterol, a major fungal membrane sterol that regulates membrane fluidity, biogenesis, and functions, and whose damage results in increased membrane permeability and leakage of cellular components, ultimately leading to fungal cell death. With the fungal cell membrane being predominantly made up of lipids including sterols, these lipids have a vital role in the pathogenesis of fungal infections and the identification of improved therapies. This review will focus on the fungal cell membrane structure, activity of allylamines and benzylamines, and the mechanistic damage they cause to the membrane. Furthermore, pharmaceutical preparations and clinical uses of these drugs, mainly in dermatophyte infections, will be reviewed.

The Flavonoid Molecule Procyanidin Reduces Phase Separation in Model Membranes.

Procyanidin extracted from fruits, such as apples, has been shown to improve lipid metabolization. Recently, studies have revealed that procyanidin interacts with lipid molecules in membranes to enhance lipid metabolism; however, direct evidence of the interaction between procyanidin and lipid membranes has not been demonstrated. In this study, the phase behaviors and changes in the membrane fluidity of cell-sized liposomes containing apple procyanidin, procyanidin B2 (PB2), were demonstrated for the first time. Phase separation in 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol ternary membranes significantly decreased after the addition of PB2. The prospect of applying procyanidin content measurements, using the results of this study, to commercial apple juice was also assessed. Specifically, the PB2 concentrations were 50%, 33%, and 0% for pure apple juice, 2-fold diluted apple juice, and pure water, respectively. The results of the actual juice were correlated with PB2 concentrations and phase-separated liposomes ratios, as well as with the results of experiments involving pure chemicals. In conclusion, the mechanism through which procyanidin improves lipid metabolism through the regulation of membrane fluidity was established.

Regulation of membrane fluidity by RNF145‐triggered degradation of the lipid hydrolase ADIPOR2

The regulation of membrane lipid composition is critical for cellular homeostasis. Cells are particularly sensitive to phospholipid saturation, with increased saturation causing membrane rigidification and lipotoxicity. How mammalian cells sense membrane lipid composition and reverse fatty acid (FA)‐induced membrane rigidification is poorly understood. Here we systematically identify proteins that differ between mammalian cells fed saturated versus unsaturated FAs. The most differentially expressed proteins were two ER‐resident polytopic membrane proteins: the E3 ubiquitin ligase RNF145 and the lipid hydrolase ADIPOR2. In unsaturated lipid membranes, RNF145 is stable, promoting its lipid‐sensitive interaction, ubiquitination and degradation of ADIPOR2. When membranes become enriched in saturated FAs, RNF145 is rapidly auto‐ubiquitinated and degraded, stabilising ADIPOR2, whose hydrolase activity restores lipid homeostasis and prevents lipotoxicity. We therefore identify RNF145 as a FA‐responsive ubiquitin ligase which, together with ADIPOR2, defines an autoregulatory pathway that controls cellular membrane lipid homeostasis and prevents acute lipotoxic stress.

Atorvastatin Modulates the Function and Trafficking of Dopamine Transporters

IntroductionDopaminergic signaling is involved in learning, memory, reward, movement, and emotion. The dopamine transporter (DAT) is the primary mechanism for terminating this signaling via DA uptake. A few lines of evidence indicate that DAT function is regulated by membrane cholesterol. Cholesterol is a critical component of plasma membranes and plays an essential role in regulating membrane fluidity, protein compartmentalization, and function. Statins are cholesterol‐lowering medications that have been linked to mild cognitive impairment. The goal of this project was to examine the effects of atorvastatin‐mediated reductions in cholesterol content on DAT function and trafficking.MethodsCultured cells & cholesterol measurement: HA‐tagged human DAT (hDAT) was stably expressed in neuroblastoma 2a (N2A) cells using G418 as selection. Cells were treated with atorvastatin (0.1, 0.3, 1.0, or 10 μM) and membrane cholesterol content was measured using Amplex Red.[3H]DA Uptake: After 24hr atorvastatin or vehicle treatment, cells were incubated with 15 nM [3H]DA in the presence of various concentrations of non‐labeled DA and [3H]DA uptake was measured to determine Vmax and Km.Cocaine inhibition of DA uptake & cholesterol replenishment: Cells were treated for 24hr with atorvastatin (1 μM) or vehicle and incubated with 15 nM [3H]DA and increasing concentrations of cocaine (100 nM‐10 μM). To determine if there was a causal effect of reduced cholesterol content and cocaine inhibition of DA uptake, atorvastatin‐treated cells were replenished with water soluble cholesterol (~40 mg per gram solid; MβCD) prior to cocaine inhibition.Amphetamine‐induced, DAT‐mediated DA efflux: Cells treated for 24hrs with either atorvastatin (1 μM) or vehicle were preloaded with 30 nM of [3H]DA. Then, cells were incubated with amphetamine (1 or 10 μM). AMPH‐induced DA efflux was determined by measuring DA content in the medium across time.Surface DAT Biotinylation: Basal surface levels of DAT in atorvastatin and vehicle treated cells, as well as DA‐induced DAT internalization, were measured by surface biotinylation using membrane non‐permeable sulfo‐NHS‐SS biotin.ImmunocytochemistryWe measured levels of F‐actin and G‐actin to determine the effects of atorvastatin treatment on actin dynamics using immunocytochemistry.ResultsWe found that atorvastatin dose dependently reduced membrane cholesterol content. This reduction was accompanied by decreases in the maximal rate of DA uptake and by decreases in the potency of cocaine to inhibit [3H]DA uptake. In contrast, atorvastatin treatment did not significantly alter AMPH‐induced DA efflux. Importantly, the reduced ability of cocaine to inhibit DA uptake was reversed by replenishing cholesterol in atorvastatin‐treated cells, suggesting a direct causal effect of reduced cholesterol content and cocaine inhibition. Lastly, we found that atorvastatin disrupted actin dynamics and abolished DA‐induced DAT internalization. The present study provides evidence for the first time that atorvastatin dysregulates DAT function and trafficking. Furthermore, this dysregulation may be associated with the cognitive deficits observed in patients receiving atorvastatin treatment.

Bacterial Homologs of Progestin and AdipoQ Receptors (PAQRs) Affect Membrane Energetics Homeostasis but Not Fluidity.

ABSTRACTMembrane potential homeostasis is essential for cell survival. Defects in membrane potential lead to pleiotropic phenotypes, consistent with the central role of membrane energetics in cell physiology. Homologs of the progestin and AdipoQ receptors (PAQRs) are conserved in multiple phyla of Bacteria and Eukarya. In eukaryotes, PAQRs are proposed to modulate membrane fluidity and fatty acid (FA) metabolism. The role of bacterial homologs has not been elucidated. Here, we use Escherichia coli and Bacillus subtilis to show that bacterial PAQR homologs, which we name “TrhA,” have a role in membrane energetics homeostasis. Using transcriptional fusions, we show that E. coli TrhA (encoded by yqfA) is part of the unsaturated fatty acid biosynthesis regulon. Fatty acid analyses and physiological assays show that a lack of TrhA in both E. coli and B. subtilis (encoded by yplQ) provokes subtle but consistent changes in membrane fatty acid profiles that do not translate to control of membrane fluidity. Instead, membrane proteomics in E. coli suggested a disrupted energy metabolism and dysregulated membrane energetics in the mutant, though it grew similarly to its parent. These changes translated into a disturbed membrane potential in the mutant relative to its parent under various growth conditions. Similar dysregulation of membrane energetics was observed in a different E. coli strain and in the distantly related B. subtilis. Together, our findings are consistent with a role for TrhA in membrane energetics homeostasis, through a mechanism that remains to be elucidated.IMPORTANCE Eukaryotic homologs of the progestin and AdipoQ receptor family (PAQR) have been shown to regulate membrane fluidity by affecting, through unknown mechanisms, unsaturated fatty acid (FA) metabolism. The bacterial homologs studied here mediate small and consistent changes in unsaturated FA metabolism that do not seem to impact membrane fluidity but, rather, alter membrane energetics homeostasis. Together, the findings here suggest that bacterial and eukaryotic PAQRs share functions in maintaining membrane homeostasis (fluidity in eukaryotes and energetics for bacteria with TrhA homologs).

Ether Lipids in Obesity: From Cells to Population Studies.

Ether lipids are a unique class of glycero- and glycerophospho-lipid that carry an ether or vinyl ether linked fatty alcohol at the sn-1 position of the glycerol backbone. These specialised lipids are important endogenous anti-oxidants with additional roles in regulating membrane fluidity and dynamics, intracellular signalling, immunomodulation and cholesterol metabolism. Lipidomic profiling of human population cohorts has identified new associations between reduced circulatory plasmalogen levels, an abundant and biologically active sub-class of ether lipids, with obesity and body-mass index. These findings align with the growing body of work exploring novel roles for ether lipids within adipose tissue. In this regard, ether lipids have now been linked to facilitating lipid droplet formation, regulating thermogenesis and mediating beiging of white adipose tissue in early life. This review will assess recent findings in both population studies and studies using cell and animal models to delineate the functional and protective roles of ether lipids in the setting of obesity. We will also discuss the therapeutic potential of ether lipid supplementation to attenuate diet-induced obesity.

Response to Cold: A Comparative Transcriptomic Analysis in Eight Cold-Adapted Yeasts

Microorganisms have evolved to colonize all biospheres, including extremely cold environments, facing several stressor conditions, mainly low/freezing temperatures. In general, terms, the strategies developed by cold-adapted microorganisms include the synthesis of cryoprotectant and stress-protectant molecules, cold-active proteins, especially enzymes, and membrane fluidity regulation. The strategy could differ among microorganisms and concerns the characteristics of the cold environment of the microorganism, such as seasonal temperature changes. Microorganisms can develop strategies to grow efficiently at low temperatures or tolerate them and grow under favorable conditions. These differences can be found among the same kind of microorganisms and from the same cold habitat. In this work, eight cold-adapted yeasts isolated from King George Island, subAntarctic region, which differ in their growth properties, were studied about their response to low temperatures at the transcriptomic level. Sixteen ORFeomes were assembled and used for gene prediction and functional annotation, determination of gene expression changes, protein flexibilities of translated genes, and codon usage bias. Putative genes related to the response to all main kinds of stress were found. The total number of differentially expressed genes was related to the temperature variation that each yeast faced. The findings from multiple comparative analyses among yeasts based on gene expression changes and protein flexibility by cellular functions and codon usage bias raise significant differences in response to cold among the studied Antarctic yeasts. The way a yeast responds to temperature change appears to be more related to its optimal temperature for growth (OTG) than growth velocity. Yeasts with higher OTG prepare to downregulate their metabolism to enter the dormancy stage. In comparison, yeasts with lower OTG perform minor adjustments to make their metabolism adequate and maintain their growth at lower temperatures.

Pathophysiological role of 27-hydroxycholesterol in human diseases.

Oxysterols are oxygenated cholesterol derivatives and important regulators of cholesterol metabolism, lipid homeostasis, the immune system, and membrane fluidity regulation. Although the detailed mechanism of action of oxysterols remains unclear, activation of some nuclear receptors, such as liver X receptor α (LXRα) and RAR-related orphan receptors, have been believed to be critical for the regulation of various physiological processes in multiple tissues. 27-Hydroxycholesterol (27-OHC) is an endogenous oxysterol, which has an intermediate function in cholesterol catabolism to bile acid synthesis. According to previous studies, however, there are opposing opinions on whether 27-OHC activates human LXR. Recently, several studies have shown that 27-OHC can activate or inhibit the function of estrogen receptors ERα and ERβ in a tissue-specific manner, indicating that the understanding of 27-OHC-mediated biological output is very complicated. This review summarizes the pathophysiological relevance of 27-OHC in various tissues, with a special discussion on their functions in human diseases.

Bicarbonate-Stimulated Membrane Reorganization in Stallion Spermatozoa.

Classical in vitro fertilization (IVF) is still poorly successful in horses. This lack of success is thought to be due primarily to inadequate capacitation of stallion spermatozoa under in vitro conditions. In species in which IVF is successful, bicarbonate, calcium, and albumin are considered the key components that enable a gradual reorganization of the sperm plasma membrane that allows the spermatozoa to undergo an acrosome reaction and fertilize the oocyte. The aim of this work was to comprehensively examine contributors to stallion sperm capacitation by investigating bicarbonate-induced membrane remodelling steps, and elucidating the contribution of cAMP signalling to these events. In the presence of capacitating media containing bicarbonate, a significant increase in plasma membrane fluidity was readily detected using merocyanine 540 staining in the majority of viable spermatozoa within 15 min of bicarbonate exposure. Specific inhibition of soluble adenylyl cyclase (sAC) in the presence of bicarbonate by LRE1 significantly reduced the number of viable sperm with high membrane fluidity. This suggests a vital role for sAC-mediated cAMP production in the regulation of membrane fluidity. Cryo-electron tomography of viable cells with high membrane fluidity revealed a range of membrane remodelling intermediates, including destabilized membranes and zones with close apposition of the plasma membrane and the outer acrosomal membrane. However, lipidomic analysis of equivalent viable spermatozoa with high membrane fluidity demonstrated that this phenomenon was neither accompanied by a gross change in the phospholipid composition of stallion sperm membranes nor detectable sterol efflux (p > 0.05). After an early increase in membrane fluidity, a significant and cAMP-dependent increase in viable sperm with phosphatidylserine (PS), but not phosphatidylethanolamine (PE) exposure was noted. While the events observed partly resemble findings from the in vitro capacitation of sperm from other mammalian species, the lack of cholesterol removal appears to be an equine-specific phenomenon. This research will assist in the development of a defined medium for the capacitation of stallion sperm and will facilitate progress toward a functional IVF protocol for horse gametes.

An Stomatin, Prohibitin, Flotillin, and HflK/C-Domain Protein Required to Link the Phage-Shock Protein to the Membrane in Bacillus subtilis.

Membrane surveillance and repair is of utmost importance to maintain cellular integrity and allow cellular life. Several systems detect cell envelope stress caused by antimicrobial compounds and abiotic stresses such as solvents, pH-changes and temperature in bacteria. Proteins containing an Stomatin, Prohibitin, Flotillin, and HflK/C (SPFH)-domain, including bacterial flotillins have been shown to be involved in membrane protection and membrane fluidity regulation. Here, we characterize a bacterial SPFH-domain protein, YdjI that is part of a stress induced complex in Bacillus subtilis. We show that YdjI is required to localize the ESCRT-III homolog PspA to the membrane with the help of two membrane integral proteins, YdjG/H. In contrast to classical flotillins, YdjI resides in fluid membrane regions and does not enrich in detergent resistant membrane fractions. However, similarly to FloA and FloT from B. subtilis, deletion of YdjI decreases membrane fluidity. Our data reveal a hardwired connection between phage shock response and SPFH proteins.

Cold Shock Response in Bacteria

Bacteria often encounter temperature fluctuations in their natural habitats and must adapt to survive. The molecular response of bacteria to sudden temperature upshift or downshift is termed the heat shock response (HSR) or the cold shock response (CSR), respectively. Unlike the HSR, which activates a dedicated transcription factor that predominantly copes with heat-induced protein folding stress, the CSR is mediated by a diverse set of inputs. This review provides a picture of our current understanding of the CSR across bacteria. The fundamental aspects of CSR involved in sensing and adapting to temperature drop, including regulation of membrane fluidity, protein folding, DNA topology, RNA metabolism, and protein translation, are discussed. Special emphasis is placed on recent findings of a CSR circuitry in Escherichia coli mediated by cold shock family proteins and RNase R that monitors and modulates messenger RNA structure to facilitate global translation recovery during acclimation.

Methyl-Beta-Cyclodextrin Restores KIR Channel Function in Brain Endothelium of Female Alzheimer's Disease Mice.

Background:As the sixth-leading cause of death in the United States, Alzheimer’s disease (AD) entails deteriorating endothelial control of blood flow throughout the brain. In particular, reduced inward-rectifying K+ (KIR) channel function in animal models of aging and AD compromises endothelial function and optimal perfusion of brain parenchyma. Deficient endothelial KIR channels may result from aberrant interaction with plasma membrane cholesterol as a primary regulator of membrane fluidity and ion channels.Objective:We tested the hypothesis that mild methyl-β-cyclodextrin (MβCD) treatment to reduce membrane cholesterol may restore endothelial KIR channel function in brain endothelium of old AD mice.Methods:Membrane potential was continuously measured in isolated endothelial tubes from posterior cerebral arteries of young (1 to 3 months) and old (16 to 19 months) female 3xTg-AD mice before and after mild treatment with the cholesterol-removing agent MβCD (1 mmol/L). Elevated extracellular potassium ([K+]E; 15 mmol/L) and NS309 (1μmol/L) activated KIR and Ca2+-activated K+ (SKCa/IKCa) channels respectively before and after MβCD treatment.Results:SKCa/IKCa channel function for producing hyperpolarization remained stable regardless of age group and MβCD treatment (ΔVm: ∼–33 mV). However, as deficient during AD, KIR channel function was restored (ΔVm: –9±1 mV) versus pre-MβCD conditions (–5±1 mV); a progressive effect that reached –14±1 mV hyperpolarization at 60 min following MβCD washout.Conclusion:In female animals, MβCD treatment of brain endothelium selectively restores KIR versus SKCa/IKCa channel function during AD. Thus, the endothelial cholesterol-KIR channel interface is a novel target for ameliorating perfusion of the AD brain.