Regulation of lipid saturation without sensing membrane fluidity

VPS13 and bridge-like lipid transporters, mechanisms, and mysteries

Bridge-like lipid transporters (BLTPs) have recently been revealed as key regulators of intraorganellar lipid trafficking, with their loss being associated with defective synaptic signalling and congenital neurological diseases. This group consists of five protein subfamilies [BLTP1-3, autophagy-related 2 (ATG2), and vacuolar protein sorting 13 (VPS13)], which mediate minimally selective lipid transfer between cellular membranes. Deceptively simple in both structure and presumed function, this review addresses open questions as to how bridge-like transporters work, the functional consequences of bulk lipid transfer on cellular signalling, and summarises some recent studies that have shed light on the surprising level of regulation and specificity found in this family of transporters.

Proximity-activated DNA scanning encoded sequencing for massive access to membrane proteins nanoscale organization

Cellular structure maintenance and function regulation critically depend on the composition and spatial distribution of numerous membrane proteins. However, current methods face limitations in spatial coverage and data scalability, hindering the comprehensive analysis of protein interactions in complex cellular nanoenvironment. Herein, we introduce proximity-activated DNA scanning encoded sequencing (PADSE-seq), an innovative technique that utilizes flexible DNA probes with adjustable lengths. These dynamic probes are anchored at a single end, enabling free swings within a nanoscale range to perform global scanning, recording, and accumulating of information on diverse proximal proteins in random directions along unrestricted paths. PADSE-seq leverages the autonomous cyclic cleavage of single-stranded DNA to sequentially activate encoded probes distributed throughout the local area. This process triggers strand displacement amplification and bidirectional extension reactions, linking proteins barcodes with molecular barcodes in tandem and further generating millions to billions of amplicons embedded with the combinatorial identifiers for next-generation sequencing analysis. As a proof of concept, we validated PADSE-seq for mapping the distribution of over a dozen kinds of proteins, including HER1, EpCAM, and PDL1, in proximity to HER2 in breast cancer cell lines, demonstrating its ability to decode multiplexed protein proximities at the nanoscale. Notably, we observed that the spatial distribution of proximal proteins around low-abundance target proteins exhibited greater diversity across regions with variable proximity ranges. This method offers a massive access for high-resolution and comprehensive mapping of cellular molecular interactions, paving the way for deeper insights into complex biological processes and advancing the field of precision medicine.

50 years since the concept of homeoviscous adaptation

This mini review focuses on the phenomenon of homeoviscous adaptation (HVA). The concept, which dominated for decades, had a significant impact on membrane and lipid research. It includes the functional characterization of biological membranes and their domains, the role of lipids and fatty acids in cell metabolic control, and the characterization of fatty acid desaturases and their roles in membrane properties modulation. This hypothesis led to the discovery of a feed-back manner of desaturase expression and membrane-associated temperature sensors in bacteria.

Alanine Scanning to Define Membrane Protein-Lipid Interaction Sites Using Native Mass Spectrometry

Lipids surrounding membrane proteins interact with different sites on the protein with varying specificities, ranging from highly specific to weak interactions. These interactions can modulate the structure, function, and stability of membrane proteins. Thus, to better understand membrane protein structure and function, it is important to identify the locations of lipid binding and the relative specificities of lipid binding at these sites. In our previous native mass spectrometry (MS) study, we developed a single and double mutant analysis approach to profile the contribution of specific residues toward lipid binding. Here, we extend this method by screening a broad range of mutants of AqpZ to identify specific lipid binding sites and by measuring binding of different lipid types to measure the selectivity of different lipids at selected binding sites. We complemented these native MS studies with molecular dynamics (MD) simulations to visualize lipid interactions at selected sites. We discovered that AqpZ is selective toward cardiolipins (CL) but only at specific sites. Specifically, CL orients with its headgroup facing the cytoplasmic side, and its acyl chains interact with a hydrophobic pocket located at the monomeric interface within the lipid bilayer. Overall, this integrative approach provides unique insights into lipid binding sites and the selectivity of various lipids toward AqpZ, enabling us to map the AqpZ protein structure based on the lipid affinity.

Three-Dimensional Quantitative Structure-Activity Relationship-Based Molecular Design through a Side Arm Strategy to Synthesize Phenylpyrazole Oxime Derivatives and Improve Their Insecticidal Activity and Photoself-Degradation

Three-dimensional quantitative structure-activity relationship (3D-QSAR) serves as one of the most important and effective tools to guide molecular design for the development of new pesticides. According to the principle of structural splicing, only changing a small group may lead to a great increase in activity while maintaining the active center unchanged. Under the guidance of 3D-QSAR, three series of phenylpyrazole oxime fluorescent insecticides acting on the GABA receptor, namely, esters (POEs), ethers (POETs), and triazoles (POTs), were designed through a side arm strategy and synthesized by an ultrasonic bath reaction, which were fully characterized and crystal-analyzed. The preliminary bioassay results indicated that the insecticidal activities of POE12 and POT2 against Plutella xylostella were 4.2 and 2.7 times higher than that of fipronil and better than that of the isolated Mythimna separata. Through the trend of insecticidal activity, the introduction of an aryl ring and an electron-withdrawing group in the substituted functional group of the side arm can enhance the insecticidal activity. Reversed-phase HPLC also confirmed that POEs and POTs had good lipid solubility, which was beneficial to improve their fluidity in the cell membrane. Through molecular packing, molecular docking, and Hirshfeld surface, the intermolecular interaction brought by side arms of POEs, POETs, and POTs and the strong interaction with GABA receptors were preliminarily verified. Photophysical tests revealed that the introduction of the side arm expanded the conjugated system and improved its light absorption and fluorescence. Under the irradiation of simulated sunlight, it was found that they had photoself-degradation and could be retransformed into the parent fragment of phenylpyrazole, thereby improving its biological activity and reducing residues.

Lipid packing and local geometry influence septin curvature sensing

Septins assemble into scaffolds that direct cell growth and morphology that are often localized to the plasma membrane. While septins preferentially bind convex membranes via amphipathic helices, their assembly on varied geometries in cells suggests additional localization cues. We tested the hypothesis that lipid composition directs septin assembly through lipid packing properties. Lipid mixtures varying in lipid packing were designed by molecular dynamics simulations and incorporated onto supported lipid bilayers to measure septin adsorption in vitro. Septins strongly favor loosely-packed, disordered lipid bilayers but additional geometry cues act in conjunction with this membrane property. Introducing tighter lipid packing in cells disrupted septin structures in a curvature dependent manner, specifically limiting septin assembly and retention along flat regions of the plasma membrane. This work demonstrates that packing defects and geometry jointly regulate septin localization and highlights how multiple membrane properties are integrated to control organization of the septin cytoskeleton.

Summary

Localization of the septin cytoskeleton is controlled by regulatory factors, membrane curvature, and charge. In this study, changes to lipid composition that modulate lipid packing defects are found to impact septin assemblies in vitro and in cells.

Alanine Scanning to Define Membrane Protein-Lipid Interaction Sites Using Native Mass Spectrometry

Lipids surrounding membrane proteins interact with different sites on the protein at varying specificities, ranging from highly specific to weak interactions. These interactions can modulate the structure, function, and stability of membrane proteins. Thus, to better understand membrane protein structure and function, it is important to identify the locations of lipid binding and the relative specificities of lipid binding at these sites. In our previous native mass spectrometry (MS) study, we developed a single and double mutant analysis approach to profile the contribution of specific residues toward lipid binding. Here, we extend this method by screening a broad range of mutants of AqpZ to identify specific lipid binding sites and by measuring binding of different lipid types to measure the selectivity of different lipids at selected binding sites. We complemented these native MS studies with molecular dynamics (MD) simulations to visualize lipid interactions at selected sites. We discovered that AqpZ is selective towards cardiolipins (CL) but only at specific sites. Specifically, CL orients with its headgroup facing the cytoplasmic side, and its acyl chains interact with a hydrophobic pocket located at the monomeric interface within the lipid bilayer. Overall, this integrative approach provides unique insights into lipid binding sites and the selectivity of various lipids towards AqpZ, enabling us to map the AqpZ protein structure based on the lipid affinity.

Effects of lipid membranes on RNA catalytic activity and stability

BACKGOUND INFORMATION

RNA plays crucial roles in cellular organization and metabolism, and modulating its activity is essential for maintaining cellular functions. RNA activity, involving both catalytic (ribozymes) and translation processes, is controlled via myriad mechanisms involving different binding partners such as proteins and smaller polar solutes. We previously reported that lipid membranes can directly interact with the artificial R3C ribozyme changing its activity, however, the effect of lipids on naturally occurring ribozymes remains unknown.

RESULTS

Here, we report that both catalytic activity as well as RNA integrity can be controlled by the presence of different lipid membranes. Gel-phase lipid membranes decreased the activity of hepatitis delta virus ribozyme and increased the activity of a hammerhead ribozyme. The presence of lipid liquid membrane surfaces triggered RNA degradation with greater degradation occurring in the single-stranded regions of RNA.

CONCLUSION

The interplay between RNA activity and stability in the presence of different lipid membranes introduces multiple possibilities, where different combinations of ribozyme and lipid membrane composition could produce different effects on activity.

SIGNIFICANCE

Taken together, these observations support the hypothesis that the activity of both natural and artificial RNAs can be modulated by lipid membranes which, in turn, provides a foundation for the development of novel riboswitch-like molecules, and lipid membrane-based RNA-biosensors.

Lipids and chromatin: a tale of intriguing connections shaping genomic landscapes

Recent studies in yeast reveal an intricate interplay between nuclear envelope (NE) architecture and lipid metabolism, and between lipid signaling and both epigenome and genome integrity. In this review, we highlight the reciprocal connection between lipids and histone modifications, which enable metabolic reprogramming in response to nutrients. The endoplasmic reticulum (ER)-NE regulates the compartmentalization and temporal availability of epigenetic metabolites and its lipid composition also impacts nuclear processes, such as transcriptional silencing and the DNA damage response (DDR). We also discuss recent work providing mechanistic insight into lipid droplet (LD) formation and sterols in the nucleus, and the collective data showing Opi1 as a central factor in both membrane sensing and transcriptional regulation of lipid-chromatin interrelated processes.

Omega-3 Fatty Acids and Chronic Lung Diseases: A Narrative Review of Impacts from Womb to Tomb

The lungs are a mucosal organ constantly exposed to potentially harmful compounds and pathogens. Beyond their role in gas exchange, they must perform a well-orchestrated protective response against foreign invaders. The lungs identify these foreign compounds, respond to them by eliciting an inflammatory response, and restore tissue homeostasis after inflammation to ensure the lungs continue to function. In addition, lung function can be affected by genetics, environmental exposures, and age, leading to pulmonary diseases that infringe on quality of life. Recent studies indicate that diet can influence pulmonary health including the incidence and/or severity of lung diseases. Specifically, long-chain omega-3 polyunsaturated fatty acids (n-3 PUFAs) have gained attention because of their potential to reduce inflammation and promote resolution of inflammation. Docosahexaenoic acid and eicosapentaenoic acid are 2 potentially beneficial n-3 PUFAs primarily acquired through dietary intake. Here we review current literature examining the role of n-3 PUFAs and the biological mechanisms by which these fatty acids alter the incidence and pathologies of chronic lung diseases including asthma, chronic obstructive pulmonary disease, and interstitial lung disease. We also highlight the role of n-3 PUFAs in vulnerable populations such as pre/postnatal children, those with obesity, and the elderly. Lastly, we review the impact of n-3 PUFA intake and supplementation to evaluate if increasing consumption can mitigate mechanisms driving chronic lung diseases.

Diet-Induced Obesity Modulates Close-Packing of Triacylglycerols in Lipid Droplets of Adipose Tissue

Adipose-derived lipid droplets (LDs) are rich in triacylglycerols (TAGs), which regulate essential cellular processes, such as energy storage. Although TAG accumulation and LD expansion in adipocytes occur during obesity, how LDs dynamically package TAGs in response to excessive nutrients remains elusive. Here, we found that LD lipidomes display a remarkable increase in TAG acyl chain saturation under calorie-dense diets, turning them conducive to close-packing. Using high-resolution X-ray diffraction, solid-state NMR, and imaging, we show that beyond size expansion LDs from mice under varied obesogenic diets govern fat accumulation by packing TAGs in different crystalline polymorphs. Consistently, LDs and tissue stiffen for high-calorie-fed mice with more than a 2-fold increase in elastic moduli compared to normal diet. Our data suggest that in addition to expanding, adipocyte LDs undergo structural remodeling by close-packing rigid and highly saturated TAGs in response to caloric overload, as opposed to liquid TAGs in a low-calorie diet. This work provides insights into how lipid packing within LDs can allow for the rapid and optimal expansion of fat during the initial stages of obesity.

The Interactions of Anti-HIV Pronucleotides with a Model Phospholipid Membrane

Pronucleotides, after entering the cell, undergo chemical or enzymatic conversion into nucleotides with a free phosphate residue, and the released nucleoside 5'-monophosphate is then phosphorylated to the biologically active form, namely nucleoside 5'-triphosphate. The active form can inhibit HIV virus replication. For the most effective therapy, it is necessary to improve the transport of prodrugs into organelles. The introduction of new functional groups into their structure increases lipophilicity and, as a result, facilitates the interaction of pronucleotide molecules with components of biological membranes. Studies of these interactions were performed using the Langmuir technique. The prototype of the biological membrane was a thin monolayer composed of phospholipid molecules, DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine). The pronucleotides were 3'-azido-3'-deoxythymidine (AZT) analogs, formed by the phosphorylation of AZT to monophosphate (AZTMP) and containing various masking moieties that could increase their lipophilicity. Our results show the influence of the pronucleotide's chemical structure on the fluidization of the model biomembrane. Changes in monolayer morphology in the presence of prodrugs were investigated by BAM microscopy. It was found that the incorporation of new groups into the structure of the drug as well as the concentration of AZT derivatives have a significant impact on the surface properties of the formed DPPC monolayer.

Phonons reveal coupled cholesterol-lipid dynamics in ternary membranes

Experimental studies of collective dynamics in lipid bilayers have been challenging due to the energy resolution required to observe these low-energy phonon-like modes. However, inelastic x-ray scattering (IXS) measurements-a technique for probing vibrations in soft and biological materials-are now possible with sub-meV resolution, permitting direct observation of low-energy, phonon-like modes in lipid membranes. Here, IXS measurements with sub-meV energy resolution reveal a low-energy optic-like phonon mode at roughly 3 meV in the liquid-ordered (Lo) and liquid-disordered phases of a ternary lipid mixture. This mode is only observed experimentally at momentum transfers greater than 5 nm-1 in the Lo system. A similar gapped mode is also observed in all-atom molecular dynamics (MD) simulations of the same mixture, indicating that the simulations accurately represent the fast, collective dynamics in the Lo phase. Its optical nature and the Q range of the gap together suggest that the observed mode is due to the coupled motion of cholesterol-lipid pairs, separated by several hydrocarbon chains within the membrane plane. Analysis of the simulations provides molecular insight into the origin of the mode in transient, nanoscale substructures of hexagonally packed hydrocarbon chains. This nanoscale hexagonal packing was previously reported based on MD simulations and, later, by NMR measurements. Here, however, the integration of IXS and MD simulations identifies a new signature of the Lo substructure in the collective lipid dynamics, thanks to the recent confluence of IXS sensitivity and MD simulation capabilities.

Mechanisms underlying the low-temperature adaptation of 17β-estradiol-degrading bacterial strain Rhodococcus sp. RCBS9: insights from physiological and transcriptomic analyses

The 17β-estradiol (E2)-degrading bacterium Rhodococcus sp.RCBS9 previously showed remarkable resistance to the combined stresses of low temperature and E2. In this study, physiological experiments and transcriptomic analysis were performed to investigate the mechanisms underlying the strain's low-temperature adaptation and briefly analyze how it maintains its ability to degrade E2 at low temperature. The results showed that the strain's signal transduction functions, adaptive changes in cell membrane and cell wall structure, gene repair functions, and synthesis of antioxidants and compatible solutes are key to its ability to adapt to low temperature. In addition, its stress proteins in response to low temperature were not typical cold shock proteins, but rather universal stress proteins (USPs) and heat shock proteins (HSPs), among others. The strain also upregulated biofilm production, transporter proteins for carbon source uptake, and proteins for fatty acid degradation to ensure energy generation. The strain's multiple stress responses work synergistically to resist low-temperature stress, ensuring its adaptability to low-temperature environments and ability to degrade E2. Finally, six genes related to survival at low temperature (identified in the transcriptome analysis) were expressed in E. coli BL21, and they were found to contribute to recombinant E. coli growth at low temperature.

ER remodeling via lipid metabolism

Unlike most other organelles found in multiple copies, the endoplasmic reticulum (ER) is a unique singular organelle within eukaryotic cells. Despite its continuous membrane structure, encompassing more than half of the cellular endomembrane system, the ER is subdivided into specialized sub-compartments, including morphological, membrane contact site (MCS), and de novo organelle biogenesis domains. In this review, we discuss recent emerging evidence indicating that, in response to nutrient stress, cells undergo a reorganization of these sub-compartmental ER domains through two main mechanisms: non-destructive remodeling of morphological ER domains via regulation of MCS and organelle hitchhiking, and destructive remodeling of specialized domains by ER-phagy. We further highlight and propose a critical role of membrane lipid metabolism in this ER remodeling during starvation.

A temperature-sensitive metabolic valve and a transcriptional feedback loop drive rapid homeoviscous adaptation in Escherichia coli

All free-living microorganisms homeostatically maintain the fluidity of their membranes by adapting lipid composition to environmental temperatures. Here, we quantify enzymes and metabolic intermediates of the Escherichia coli fatty acid and phospholipid synthesis pathways, to describe how this organism measures temperature and restores optimal membrane fluidity within a single generation after a temperature shock. A first element of this regulatory system is a temperature-sensitive metabolic valve that allocates flux between the saturated and unsaturated fatty acid synthesis pathways via the branchpoint enzymes FabI and FabB. A second element is a transcription-based negative feedback loop that counteracts the temperature-sensitive valve. The combination of these elements accelerates membrane adaptation by causing a transient overshoot in the synthesis of saturated or unsaturated fatty acids following temperature shocks. This strategy is comparable to increasing the temperature of a water bath by adding water that is excessively hot rather than adding water at the desired temperature. These properties are captured in a mathematical model, which we use to show how hard-wired parameters calibrate the system to generate membrane compositions that maintain constant fluidity across temperatures. We hypothesize that core features of the E. coli system will prove to be ubiquitous features of homeoviscous adaptation systems.

Pegylated gold nanoparticles interact with lipid bilayer and human serum albumin and transferrin

Gold nanoparticles (AuNPs) are potentially applicable in drug/nucleic acid delivery systems. Low toxicity, high stability, and bioavailability are crucial for the therapeutic use of AuNPs and they are mainly determined by their interactions with proteins and lipids on their route to the target cells. In this work, we investigated the interaction of two pegylated gold nanoparticles, AuNP14a and AuNP14b, with human serum proteins albumin (HSA) and transferrin (Tf) as well as dimyristoyl-phosphatidylcholine (DMPC) liposomes, which can be a representative of biomembranes. We showed that AuNP14a/b interacted with HSA and Tf changing their electrical, thermodynamic, and structural properties as evidenced by dynamic light scattering, zeta potential, transmission electron microscopy, circular dichroism, fluorescence quenching, and isothermal titration calorimetry. These nanoparticles penetrated the DMPC membrane suggesting their ability to reach a target inside the cell. In most of the effects, AuNP14b was more effective than AuNP14a, which might result from its more positive charge. Further studies are needed to evaluate whether the interaction of AuNP14a/b with HSA and Tf is safe for the cell/organism and whether they may safely penetrate natural membranes.

Stabilized Cell Membrane-Derived Vesicles by Lipid Anchoring for Enhanced Drug Delivery

A cell membrane-derived vesicle (MV) that has cell-mimicking features with characteristic functionalities holds vast appeal for biomimetic nanomedicine and drug delivery but suffers from a major limitation of innate fragility and poor stability. Herein, we report a lipid-anchoring strategy for stabilizing MV for enhanced drug delivery. An array of amphiphilic mono-acyl phosphatidylcholines (MPCs) with specific hydrophobic moieties are synthesized and readily engineered on MV based on their commendable aqueous solubility and efficient membrane insertability. Incorporation of MPCs containing rigid ring structures in the hydrophobic segment demonstrates the potency of stabilizing MV by the combined ordering and condensing effects. The optimized MPC-stabilized MV exhibits prolonged circulation in the bloodstream, elevated accumulation within a tumor, and enhanced therapeutic effects of chemotherapeutic and photothermal drugs. Moreover, doxorubicin-loaded MV, engineered with mono-all-trans retinoyl phosphatidylcholine as an MV stabilizer and a therapeutic prodrug, potently suppresses growth and metastasis of high-stemness tumors.

The AAA+ protein Msp1 recognizes substrates by a hydrophobic mismatch

An essential aspect of protein quality control is enzymatic removal of membrane proteins from the lipid bilayer. Failures in this essential cellular process are associated with neurodegenerative diseases and cancer. Msp1 is a AAA+ (ATPases Associated with diverse cellular Activities) protein that removes mistargeted proteins from the outer mitochondrial membrane (OMM). How Msp1 selectively recognizes and extracts substrates within the complex OMM ecosystem, and the role of the lipid bilayer on these processes is unknown. Here, we describe the development of fully defined, rapid, and quantitative extraction assay that retains physiological substrate selectivity. Using this new assay, we systematically modified both substrates and the lipid environment to demonstrate that Msp1 recognizes substrates by a hydrophobic mismatch between the substrate TMD and the lipid bilayer. We further demonstrate that the rate limiting step in Msp1 activity is extraction of the TMD from the lipid bilayer. Together, these results provide foundational insights into how the lipid bilayer influences AAA+ mediated membrane protein extraction.

Extreme makeover: the incredible cell membrane adaptations of extremophiles to harsh environments

The existence of life beyond Earth has long captivated humanity, and the study of extremophiles-organisms surviving and thriving in extreme environments-provides crucial insights into this possibility. Extremophiles overcome severe challenges such as enzyme inactivity, protein denaturation, and damage of the cell membrane by adopting several strategies. This feature article focuses on the molecular strategies extremophiles use to maintain the cell membrane's structure and fluidity under external stress. Key strategies include homeoviscous adaptation (HVA), involving the regulation of lipid composition, and osmolyte-mediated adaptation (OMA), where small organic molecules protect the lipid membrane under stress. Proteins also have direct and indirect roles in protecting the lipid membrane. Examining the survival strategies of extremophiles provides scientists with crucial insights into how life can adapt and persist in harsh conditions, shedding light on the origins of life. This article examines HVA and OMA and their mechanisms in maintaining membrane stability, emphasizing our contributions to this field. It also provides a brief overview of the roles of proteins and concludes with recommendations for future research directions.

A comprehensive study of the colloidal properties, biocompatibility, and synergistic antioxidant actions of Antarctic krill phospholipids

Excipient selection is crucial to address the oxidation and solubility challenges of bioactive substances, impacting their safety and efficacy. AKPL, a novel ω-3 polyunsaturated fatty acids (PUFAs) esterified phospholipid derived from Antarctic krill, demonstrates unique antioxidant capabilities and synergistic effects. It exhibits pronounced surface activity and electronegativity at physiological pH, as evidenced by a critical micelle concentration (CMC) of 0.15 g/L and ζ-potential of -49.9 mV. In aqueous environments, AKPL self-assembles into liposomal structures, offering high biocompatibility and promoting cell proliferation. Its polyunsaturated bond-rich structure provides additional oxidation sites, imparting antioxidant properties superior to other phospholipids like DSPC and DOPC. Additionally, AKPL augments the efficacy of lipophilic antioxidants, such as alpha-tocopherol and curcumin, in aqueous media through both intermolecular and intramolecular interactions. In sum, AKPL emerges as an innovative unsaturated phospholipid, offering new strategies for encapsulating and delivering oxygen-sensitive agents.

On the Structure, Stability, and Cell Uptake of Nanostructured Lipid Carriers for Drug Delivery

The efficacy of nanostructured lipid carriers (NLC) for drug delivery strongly depends on their stability and cell uptake. Both properties are governed by their compositions and internal structure. To test the effect of the lipid composition of NLC on cell uptake and stability, three kinds of liquid lipids with different degrees of unsaturation are employed. After ensuring homogeneous size distributions, the thermodynamic characteristics, stability, and mixing properties of NLC are characterized. Then the rates and predominant pathways of cell uptake are determined. Although the same surfactant is used in all cases, different uptake rates are observed. This finding contradicts the view that the surface properties of NLC are dominated by the surfactant. Instead, the uptake rates are explained by the structure of the nanocarrier. Depending on the mixing properties, some liquid lipids remain inside the nanocarrier, while other liquid lipids are present on the surface. Nanocarriers with liquid lipids on the surface are taken up more readily by the cells. This shows that the engineering of efficient lipid nanocarriers requires a delicate balance of interactions between all components of the nanocarrier on the molecular level.

Selectively superior production of docosahexaenoic acid in Schizochytrium sp. through engineering the fatty acid biosynthetic pathways

BACKGROUND

Schizochytrium sp. is commercially used for production of docosahexaenoic acid (DHA). Schizochytrium sp. utilizes the polyketide synthase complex (PKS) and a single type I fatty acid synthase (FAS) to synthesize polyunsaturated fatty acids and saturated fatty acids, respectively. The acyl carrier protein (ACP) domains of FAS or PKS are used to load acyl groups during fatty acids biosynthesis. Phosphopantetheinyl transferase (PPTase) transfers the pantetheine moiety from Coenzyme A to the conserved serine residue of an inactive ACP domain to produce its active form.

RESULTS

In this study, in order to improve production and content of DHA, we decreased the expression of fas, strengthened the expression of the PKS pathway, and enhanced the supply of active ACP in Schizochytrium sp. ATCC20888. Weakening the expression of fas or disruption of orfA both led to growth defect and reduction of lipid yields in the resulting strains WFAS and DPKSA, indicating that both FAS and PKS were indispensable for growth and lipid accumulation. Although WFAS had a higher DHA content in total fatty acids than the wild-type strain (WT), its growth defect and low DHA yield hinders its use for DHA production. Overexpression of the orfAB, orfC, orfC-DH (truncated orfC), or ppt promoted DHA and lipid production, respectively. The yields and contents of DHA were further increased by combined overexpression of these genes. Highest values of DHA yield (7.2 g/L) and DHA content (40.6%) were achieved in a recombinant OPKSABC-PPT, ⁓56.5% and 15.3% higher than the WT values, respectively.

CONCLUSIONS

This study demonstrates that genetic engineering of the fatty acid biosynthetic pathways provides a new strategy to enhance DHA production in Schizochytrium.

MUFA synthesis and stearoyl-CoA desaturase as a new pharmacological target for modulation of lipid and alpha-synuclein interaction against Parkinson's disease synucleinopathy

Protein pathology spreading within the nervous system, accompanies neurodegeneration and a spectrum of motor and cognitive dysfunctions. Currently available therapies against Parkinson's disease and other synucleinopathies are mostly symptomatic and fail to slow the disease progression in the long term. Modification of α-synuclein (αS) aggregation and toxicity of its pathogenic forms is one of the main goals in neuroprotective approach. Since the discovery of lipid component of Lewy bodies, fatty acids became a crucial, yet little explored target for research. MUFAs (monounsaturated fatty acids) are substrates for lipids, such as phospholipids, triglycerides and cholesteryl esters. They regulate membrane fluidity, take part in signal transduction, cellular differentiation and other fundamental processes. αS and MUFA interactions are essential for Lewy body pathology. αS increases levels of MUFAs, mainly oleic acid, which in turn can enhance αS toxicity and aggregation. Thus, reduction of MUFAs synthesis by inhibition of stearoyl-CoA desaturase (SCD) activity could be the new way to prevent aggravation of αS pathology. Due to the limited distribution in peripheral tissues, SCD5 is a potential target in novel therapies and therefore could be an important starting point in search for disease-modifying neuroprotective therapy. Here we summarize facts about physiology and pathology of αS, explain recently discovered lipid-αS interactions, review SCD function and involved mechanisms, present available SCD inhibitors and discuss their pharmacological potential in disease management. Modulation of MUFA synthesis, decreasing αS and lipid toxicity is clearly essential, but unexplored avenue in pharmacotherapy of Parkinson's disease and synucleinopathies.

The ubiquitous role of ubiquitination in lipid metabolism

Lipids are essential molecules that play key roles in cell physiology by serving as structural components, for storage of energy, and in signal transduction. Hence, efficient regulation and maintenance of lipid homeostasis are crucial for normal cellular and tissue function. In the past decade, increasing research has shown the importance of ubiquitination in regulating the stability of key players in different aspects of lipid metabolism. This review describes recent insights into the regulation of lipid metabolism by ubiquitin signaling, discusses how ubiquitination can be targeted in diseases characterized by lipid dysregulation, and identifies areas that require further research.

MemPrep, a new technology for isolating organellar membranes provides fingerprints of lipid bilayer stress

Biological membranes have a stunning ability to adapt their composition in response to physiological stress and metabolic challenges. Little is known how such perturbations affect individual organelles in eukaryotic cells. Pioneering work has provided insights into the subcellular distribution of lipids in the yeast Saccharomyces cerevisiae, but the composition of the endoplasmic reticulum (ER) membrane, which also crucially regulates lipid metabolism and the unfolded protein response, remains insufficiently characterized. Here, we describe a method for purifying organelle membranes from yeast, MemPrep. We demonstrate the purity of our ER membrane preparations by proteomics, and document the general utility of MemPrep by isolating vacuolar membranes. Quantitative lipidomics establishes the lipid composition of the ER and the vacuolar membrane. Our findings provide a baseline for studying membrane protein biogenesis and have important implications for understanding the role of lipids in regulating the unfolded protein response (UPR). The combined preparative and analytical MemPrep approach uncovers dynamic remodeling of ER membranes in stressed cells and establishes distinct molecular fingerprints of lipid bilayer stress.

Bis(Monoacylglycero)Phosphate Promotes Membrane Fusion Facilitated by the SARS-CoV-2 Fusion Domain

Membrane fusion is a critical component of the viral lifecycle. For SARS-CoV-2, fusion is facilitated by the spike glycoprotein and can take place via either the plasma membrane or the endocytic pathway. The fusion domain (FD), which is found within the spike glycoprotein, is primarily responsible for the initiation of fusion as it embeds itself within the target cell's membrane. A preference for SARS-CoV-2 to fuse at low pH akin to the environment of the endocytic pathway has already been established; however, the impact of the target cell's lipid composition on the FD has yet to be explored. Here, we have shown that the SARS-CoV-2 FD preferentially initiates fusion at the late endosomal membrane over the plasma membrane, on the basis of lipid composition alone. A positive, fusogenic relationship with anionic lipids from the plasma membrane (POPS: 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine) and endosomal membrane (BMP: bis(monoacylglycero)phosphate) was established, with a large preference demonstrated for the latter. When comparing the binding affinity and secondary structure of the FD in the presence of different anionic lipids, little deviation was evident while the charge was maintained. However, it was discovered that BMP had a subtle, negative impact on lipid packing in comparison to that of POPS. Furthermore, an inverse relationship between lipid packing and the fusogenecity of the SARS-CoV-2 FD was witnessed. In conclusion, the SARS-CoV-2 FD preferentially initiates fusion at a membrane resembling that of the late endosomal compartment, predominately due to the presence of BMP and its impact on lipid packing.

Single-Cell Unsaturated Lipid Profiling for Studying Chemoresistance Heterogeneity of Triple-Negative Breast Cancer Cells

Chemoresistance to triple-negative breast cancer (TNBC) is a critical issue in clinical practice. Lipid metabolism takes a unique role in breast cancer cells; especially, unsaturated lipids involving cell membrane fluidity and peroxidation are highly remarked. At present, for the lack of a high-resolution molecular recognition platform at the single-cell level, it is still hard to systematically study chemoresistance heterogeneity based on lipid unsaturation proportion. By designing a single-cell mass spectrometry workflow based on CyESI-MS, we profiled the unsaturated lipids of TNBC cells to evaluate lipidomic remodeling under platinum stress. Profiling revealed the heterogeneity of the polyunsaturated lipid proportion of TNBC cells under cisplatin treatment. A cluster of cells identified by polyunsaturated lipid accumulation was found to be involved in platinum sensitivity. Furthermore, we found that the chemoresistance of TNBC cells could be regulated by fatty acid supplementation, which determinates the composition of unsaturated lipids. These discoveries provide insights for monitoring and controlling cellular unsaturated lipid proportions to overcome chemoresistance in breast cancer.

Phospholipid Epitaxial Assembly Behavior on a Hydrophobic Highly Ordered Pyrolytic Graphite Surface

Supported lipid bilayers (SLBs) are excellent models of cell membranes. However, most SLBs exist in the form of phospholipid molecules standing on a substrate, making it difficult to have a side view of the phospholipid membranes. In this study, the phospholipid striped lamella with the arrangement of their alkane tails lying on highly ordered pyrolytic graphite (HOPG) was constructed by a spin coating method. Atomic force microscopy and molecular dynamics simulations are utilized to study the self-assembly of phospholipids on HOPG. Results show that various phospholipids with different packing parameters and electrical property are able to epitaxially adsorb on HOPG. 0.1 mg/mL Plasm PC (0.1 mg/mL) could form a striped monolayer with a width of 5.93 ± 0.21 nm and form relatively stable four striped layers with the concentration increasing to 1 mg/mL. The width of the DOPS multilayer is more than that of electroneutral lipids due to the static electrical repulsion force. This universal strategy sheds light on direct observation of the membrane structure from the side view and modification of 2D materials with amphiphilic biomolecules.

Cristae formation is a mechanical buckling event controlled by the inner mitochondrial membrane lipidome

Cristae are high-curvature structures in the inner mitochondrial membrane (IMM) that are crucial for ATP production. While cristae-shaping proteins have been defined, analogous lipid-based mechanisms have yet to be elucidated. Here, we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions dictate IMM morphology and ATP generation. When modulating phospholipid (PL) saturation in engineered yeast strains, we observed a surprisingly abrupt breakpoint in IMM topology driven by a continuous loss of ATP synthase organization at cristae ridges. We found that cardiolipin (CL) specifically buffers the inner mitochondrial membrane against curvature loss, an effect that is independent of ATP synthase dimerization. To explain this interaction, we developed a continuum model for cristae tubule formation that integrates both lipid and protein-mediated curvatures. This model highlighted a snapthrough instability, which drives IMM collapse upon small changes in membrane properties. We also showed that cardiolipin is essential in low-oxygen conditions that promote PL saturation. These results demonstrate that the mechanical function of cardiolipin is dependent on the surrounding lipid and protein components of the IMM.

Anomalous lateral diffusion of lipids during the fluid/gel phase transition of a lipid membrane

A lipid membrane undergoes a phase transition from fluid to gel phase upon changing external thermodynamic conditions, such as decreasing temperature and increasing pressure. Extremophilic organisms face the challenge of preventing this deleterious phase transition. The main focus of their adaptive strategy is to facilitate effective temperature sensing through sensor proteins, relying on the drastic changes in packing density and membrane fluidity during the phase transition. Although the changes in packing density parameters due to the fluid/gel phase transition are studied in detail, the impact on membrane fluidity is less explored in the literature. Understanding the lateral diffusive dynamics of lipids in response to temperature, particularly during the fluid/gel phase transition, is albeit crucial. Here we have simulated the phase transition of a single component lipid membrane composed of dipalmitoylphosphatidylcholine (DPPC) lipids using a coarse-grained (CG) model and studied the changes of the structural and dynamical properties. It is observed that near the phase transition point, both fluid and gel phase domains coexist together. The dynamics remains highly non-Gaussian for a long time even when the mean square displacement reaches the Fickian regime at a much earlier time. This Fickian yet non-Gaussian diffusion (FnGD) is a characteristic of a highly heterogeneous system, previously observed for the lateral diffusion of lipids in raft mimetic membranes having liquid-ordered and liquid-disordered phases co-existing together. We have analyzed the molecular trajectories and calculated the jump-diffusion of the lipids, stemming from sudden jump translations, using a translational jump-diffusion (TJD) approach. An overwhelming contribution of the jump-diffusion of the lipids is observed suggesting anomalous diffusion of lipids during fluid/gel phase transition of the membrane. These results are important in unravelling the intricate nature of lipid diffusion during the phase transition of the membrane and open up a new possibility of investigating the most significant change of membrane properties during phase transition, which can be effectively sensed by proteins.

Patterns of Variation in the Usage of Fatty Acid Chains among Classes of Ester and Ether Neutral Lipids and Phospholipids in the Queensland Fruit Fly

Modern lipidomics has the power and sensitivity to elucidate the role of insects' lipidomes in their adaptations to the environment at a mechanistic molecular level. However, few lipidomic studies have yet been conducted on insects beyond model species such as Drosophila melanogaster. Here, we present the lipidome of adult males of another higher dipteran frugivore, Bactrocera tryoni. We describe 421 lipids across 15 classes of ester neutral lipids and phospholipids and ether neutral lipids and phospholipids. Most of the lipids are specified in terms of the carbon and double bond contents of each constituent hydrocarbon chain, and more ether lipids are specified to this degree than in any previous insect lipidomic analyses. Class-specific profiles of chain length and (un)saturation are broadly similar to those reported in D. melanogaster, although we found fewer medium-length chains in ether lipids. The high level of chain specification in our dataset also revealed widespread non-random combinations of different chain types in several ester lipid classes, including deficits of combinations involving chains of the same carbon and double bond contents among four phospholipid classes and excesses of combinations of dissimilar chains in several classes. Large differences were also found in the length and double bond profiles of the acyl vs. alkyl or alkenyl chains of the ether lipids. Work on other organisms suggests some of the differences observed will be functionally consequential and mediated, at least in part, by differences in substrate specificity among enzymes in lipid synthesis and remodelling pathways. Interrogation of the B. tryoni genome showed it has comparable levels of diversity overall in these enzymes but with some gene gain/loss differences and considerable sequence divergence from D. melanogaster.

Membrane homeostasis beyond fluidity: control of membrane compressibility

Biomembranes are complex materials composed of lipids and proteins that compartmentalize biochemistry. They are actively remodeled in response to physical and metabolic cues, as well as during cell differentiation and stress. The concept of homeoviscous adaptation has become a textbook example of membrane responsiveness. Here, we discuss limitations and common misconceptions revolving around it. By highlighting key moments in the life cycle of a transmembrane protein, we illustrate that membrane thickness and a finely regulated membrane compressibility are crucial to facilitate proper membrane protein insertion, function, sorting, and inheritance. We propose that the unfolded protein response (UPR) provides a mechanism for endoplasmic reticulum (ER) membrane homeostasis by sensing aberrant transverse membrane stiffening and triggering adaptive responses that re-establish membrane compressibility.

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Cristae formation is a mechanical buckling event controlled by the inner membrane lipidome

Cristae are high curvature structures in the inner mitochondrial membrane (IMM) that are crucial for ATP production. While cristae-shaping proteins have been defined, analogous mechanisms for lipids have yet to be elucidated. Here we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions dictate IMM morphology and ATP generation. When modulating phospholipid (PL) saturation in engineered yeast strains, we observed a surprisingly abrupt breakpoint in IMM topology driven by a continuous loss of ATP synthase organization at cristae ridges. We found that cardiolipin (CL) specifically buffers the IMM against curvature loss, an effect that is independent of ATP synthase dimerization. To explain this interaction, we developed a continuum model for cristae tubule formation that integrates both lipid and protein-mediated curvatures. The model highlighted a snapthrough instability, which drives IMM collapse upon small changes in membrane properties. We also showed that CL is essential in low oxygen conditions that promote PL saturation. These results demonstrate that the mechanical function of CL is dependent on the surrounding lipid and protein components of the IMM.

Lipid saturation controls nuclear envelope function

The nuclear envelope (NE) is a spherical double membrane with elastic properties. How NE shape and elasticity are regulated by lipid chemistry is unknown. Here we discover lipid acyl chain unsaturation as essential for NE and nuclear pore complex (NPC) architecture and function. Increased lipid saturation rigidifies the NE and the endoplasmic reticulum into planar, polygonal membranes, which are fracture prone. These membranes exhibit a micron-scale segregation of lipids into ordered and disordered phases, excluding NPCs from the ordered phase. Balanced lipid saturation is required for NPC integrity, pore membrane curvature and nucleocytoplasmic transport. Oxygen deprivation amplifies the impact of saturated lipids, causing NE rigidification and rupture. Conversely, lipid droplets buffer saturated lipids to preserve NE architecture. Our study uncovers a fundamental link between lipid acyl chain structure and the integrity of the cell nucleus with implications for nuclear membrane malfunction in ischaemic tissues.

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.

Assignment of molecular origins of NOE signal at -3.5 ppm in the brain

PURPOSE

Nuclear Overhauser enhancemen mediated saturation transfer effect, termed NOE (-3.5 ppm), is a major source of CEST MRI contrasts at 3.5 ppm in the brain. Previous phantom experiments have demonstrated that both proteins and lipids, two major components in tissues, have substantial contributions to NOE (-3.5 ppm) signals. Their relative contributions in tissues are informative for the interpretation of NOE (-3.5 ppm) contrasts that could provide potential imaging biomarkers for relevant diseases, which remain incompletely understood.

METHODS

Experiments on homogenates and supernatants of brain tissues collected from healthy rats, that could isolate proteins from lipids, were performed to evaluate the relative contribution of lipids to NOE (-3.5 ppm) signals. On the other hand, experiments on ghost membranes with varied pH, and reconstituted phospholipids with different chemical compositions were conducted to study the dependence of NOE (-3.5 ppm) on physiological conditions. Besides, CEST imaging on rat brains bearing 9 L tumors and healthy rat brains was performed to analyze the causes of the NOE (-3.5 ppm) contrast variations between tumors and normal tissues, and between gray matter and white matter.

RESULTS

Our experiments reveal that lipids have dominant contributions to the NOE (-3.5 ppm) signals. Further analysis suggests that decreased NOE (-3.5 ppm) signals in tumors and higher NOE (-3.5 ppm) signals in white matter than in gray matter are mainly explained by changes in membrane lipids, rather than proteins.

CONCLUSION

NOE (-3.5 ppm) could be exploited as a highly sensitive MRI contrast for imaging membrane lipids in the brain.

Supplementation with protected kapok seed oil and choline chloride to improve the performance and lipid status of thin-tailed sheep

Background and Aim

Healthy meat production is an important aspect of increasing sheep productivity. This study aimed to examine the influence of protected kapok seed oil (KSO) in combination with choline chloride (CC) on the feed utilization, lipid status, and performance of thin-tailed sheep.

Materials and Methods

Thirty male thin-tailed sheep (approximately 6 months old, with an average body weight of 12.59 ± 1.48 kg) were divided into six treatment groups (five heads/treatment). Factor 1 consisted of two treatments: K1 (KSO supplementation at 10% supplementation and 75% protection level) and K0 (without KSO supplementation). Factor 2 consisted of three levels of CC: (C0: 0%; C1: 1.5% and C2: 3% feed dry matter (DM) basis supplementation levels). The variables measured were the DM consumption, DM digestibility, organic matter digestibility, nitrogen retention, daily body weight gain (DBWG), and blood and meat lipid status. The data were analyzed using analysis of variance in a completely randomized design in a factorial pattern of 2 × 3 × 5.

Results

Choline chloride supplementation (up to 3%) increased DM consumption in the K0C2 group. The CC and protected KSO (K1C2) supplementation combination resulted in the highest DM consumption level (p < 0.05). The protected KSO supplementation increased DBWG (the DBWG in the K1C0 group was higher than that in the K0C0 group, and the highest DBWG was found in the K1C2 group) (p < 0.05). Protected KSO and CC supplementation decreased cholesterol levels and increased the relative proportion of linoleic acid in meat (p < 0.05).

Conclusion

Combined supplementation with protected KSO and CC improved the feed utilization and performance of male thin-tailed sheep. There were increases in DBWG, decreases in intramuscular fat and cholesterol levels, and increases in meat linoleic acid levels.

SIR telomere silencing depends on nuclear envelope lipids and modulates sensitivity to a lysolipid

The nuclear envelope (NE) is important in maintaining genome organization. The role of lipids in communication between the NE and telomere regulation was investigated, including how changes in lipid composition impact gene expression and overall nuclear architecture. Yeast was treated with the non-metabolizable lysophosphatidylcholine analog edelfosine, known to accumulate at the perinuclear ER. Edelfosine induced NE deformation and disrupted telomere clustering but not anchoring. Additionally, the association of Sir4 at telomeres decreased. RNA-seq analysis showed altered expression of Sir-dependent genes located at sub-telomeric (0-10 kb) regions, consistent with Sir4 dispersion. Transcriptomic analysis revealed that two lipid metabolic circuits were activated in response to edelfosine, one mediated by the membrane sensing transcription factors, Spt23/Mga2, and the other by a transcriptional repressor, Opi1. Activation of these transcriptional programs resulted in higher levels of unsaturated fatty acids and the formation of nuclear lipid droplets. Interestingly, cells lacking Sir proteins displayed resistance to unsaturated-fatty acids and edelfosine, and this phenotype was connected to Rap1.

Building Asymmetric Lipid Bilayers for Molecular Dynamics Simulations: What Methods Exist and How to Choose One?

The compositional asymmetry of biological membranes has attracted significant attention over the last decade. Harboring more differences from symmetric membranes than previously appreciated, asymmetric bilayers have proven quite challenging to study with familiar concepts and techniques, leaving many unanswered questions about the reach of the asymmetry effects. One particular area of active research is the computational investigation of composition- and number-asymmetric lipid bilayers with molecular dynamics (MD) simulations. Offering a high level of detail into the organization and properties of the simulated systems, MD has emerged as an indispensable tool in the study of membrane asymmetry. However, the realization that results depend heavily on the protocol used for constructing the asymmetric bilayer models has sparked an ongoing debate about how to choose the most appropriate approach. Here we discuss the underlying source of the discrepant results and review the existing methods for creating asymmetric bilayers for MD simulations. Considering the available data, we argue that each method is well suited for specific applications and hence there is no single best approach. Instead, the choice of a construction protocol-and consequently, its perceived accuracy-must be based primarily on the scientific question that the simulations are designed to address.

Overproduction of palmitoleic acid from corn stover hydrolysate by engineered Saccharomyces cerevisiae

Palmitoleic acid (POA) has been widely applied to nutrition and pharmaceutical industry. However, high cost of scale-up fermentation restricts the extensive application of POA. Hence, we investigated the availability of corn stover hydrolysate (CSH) as carbon source in POA production by engineered S. cerevisiae. Although the yeast growth was inhibited to some extent by CSH, the POA production with CSH was slightly higher than that with pure glucose. The C/N ratio of 120 and addition of 1 g/L lysine raised the POA titer up to 2.19 g/L and 2.05 g/L, respectively. Two-stage cultivation could increase the POA titer by upregulating the gene expression of key enzymes in fatty acid synthesis pathway. A high POA content of 57.5% (v/v) and a highest POA titer of 6.56 g/L were achieved under the optimized conditions. These findings provide a feasible approach for sustainable production of POA or its derivatives from CSH.

Diet effects on longevity, heat tolerance, lipid peroxidation and mitochondrial membrane potential in Daphnia

The dietary supply of polyunsaturated fatty acids (PUFA) crucially affects animals' performance at different temperatures. However, the underlying physiological mechanisms are still insufficiently understood. Here, we analyzed lifespan and heat tolerance of four genotypes of Daphnia magna reared on either the green alga Scenedesmus obliquus that lacks long-chain (> C18) PUFA, or the heterokont alga Nannochloropsis limnetica that contains C20 PUFA, both either at saturating and near-starvation levels. A significant genotype-by-diet interaction in lifespan was observed at saturating diets. The C20 PUFA-rich diet eliminated differences in lifespan among genotypes on the PUFA-deficient diet. Corrected for body length, acute heat tolerance was higher at low than at high food concentration, at least in the older of the two age groups analyzed. Genotypes differed significantly in heat tolerance, but there were no genotype-by-diet interactions. As predicted, the C20 PUFA-rich diet resulted in higher lipid peroxidation (LPO) and a lower mitochondrial membrane potential (ΔΨ_m). LPO levels averaged across clones and rearing conditions were inversely related to acute heat tolerance. Yet, heat tolerance was higher on the PUFA-rich diet than on the PUFA-deficient diet, particularly in older Daphnia, indicating that the C20 PUFA-rich diet allowed Daphnia to compensate for higher LPO. In contrast, Daphnia with intermediate levels of ΔΨ_m showed the lowest heat tolerance. Neither LPO nor ΔΨ_m explained the diet effects on lifespan. We hypothesize that antioxidants present in the PUFA-rich diet may have enabled higher heat tolerance of Daphnia despite higher LPO, which may also explain the lifespan expansion of otherwise short-lived genotypes.

Engineering transmembrane signal transduction in synthetic membranes using two-component systems

Cells use signal transduction across their membranes to sense and respond to a wide array of chemical and physical signals. Creating synthetic systems which can harness cellular signaling modalities promises to provide a powerful platform for biosensing and therapeutic applications. As a first step toward this goal, we investigated how bacterial two-component systems (TCSs) can be leveraged to enable transmembrane-signaling with synthetic membranes. Specifically, we demonstrate that a bacterial two-component nitrate-sensing system (NarX-NarL) can be reproduced outside of a cell using synthetic membranes and cell-free protein expression systems. We find that performance and sensitivity of the TCS can be tuned by altering the biophysical properties of the membrane in which the histidine kinase (NarX) is integrated. Through protein engineering efforts, we modify the sensing domain of NarX to generate sensors capable of detecting an array of ligands. Finally, we demonstrate that these systems can sense ligands in relevant sample environments. By leveraging membrane and protein design, this work helps reveal how transmembrane sensing can be recapitulated outside of the cell, adding to the arsenal of deployable cell-free systems primed for real world biosensing.

Multimodal high-resolution nano-DESI MSI and immunofluorescence imaging reveal molecular signatures of skeletal muscle fiber types

The skeletal muscle is a highly heterogeneous tissue comprised of different fiber types with varying contractile and metabolic properties. The complexity in the analysis of skeletal muscle fibers associated with their small size (30-50 μm) and mosaic-like distribution across the tissue tnecessitates the use of high-resolution imaging to differentiate between fiber types. Herein, we use a multimodal approach to characterize the chemical composition of skeletal fibers in a limb muscle, the gastrocnemius. Specifically, we combine high-resolution nanospray desorption electrospray ionization (nano-DESI) mass spectrometry imaging (MSI) with immunofluorescence (IF)-based fiber type identification. Computational image registration and segmentation approaches are used to integrate the information obtained with both techniques. Our results indicate that the transition between oxidative and glycolytic fibers is associated with shallow chemical gradients (<2.5 fold change in signals). Interestingly, we did not find any fiber type-specific molecule. We hypothesize that these findings might be linked to muscle plasticity thereby facilitating a switch in the metabolic properties of fibers in response to different conditions such as exercise and diet, among others. Despite the shallow chemical gradients, cardiolipins (CLs), acylcarnitines (CAR), monoglycerides (MGs), fatty acids, highly polyunsaturated phospholipids, and oxidized phospholipids, were identified as molecular signatures of oxidative metabolism. In contrast, histidine-related compounds were found as molecular signatures of glycolytic fibers. Additionally, the presence of highly polyunsaturated acyl chains in phospholipids was found in oxidative fibers whereas more saturated acyl chains in phospholipids were found in glycolytic fibers which suggests an effect of the membrane fluidity on the metabolic properties of skeletal myofibers.

Age-related differences in response to plasma exchange in male rat liver tissues: insights from histopathological and machine-learning assisted spectrochemical analyses

This study aimed to examine the biological effects of blood plasma exchange in liver tissues of aged and young rats using machine learning methods and spectrochemical and histopathological approaches. Linear Discriminant Analysis (LDA) and Support Vector Machine (SVM) were the machine learning algorithms employed. Young plasma was given to old male rats (24 months), while old plasma was given to young male rats (5 weeks) for thirty days. LDA (95.83-100%) and SVM (87.5-91.67%) detected significant qualitative changes in liver biomolecules. In old rats, young plasma infusion increased the length of fatty acids, triglyceride, lipid carbonyl, and glycogen levels. Nucleic acid concentration, phosphorylation, and carbonylation rates of proteins were also increased, whereas a decrease in protein concentration was measured. Aged plasma decreased protein carbonylation, triglyceride, and lipid carbonyl levels. Young plasma infusion improved hepatic fibrosis and cellular degeneration and reduced hepatic microvesicular steatosis in aged rats. Otherwise, old plasma infusion in young rats caused disrupted cellular organization, steatosis, and increased fibrosis. Young plasma administration increased liver glycogen accumulation and serum albumin levels. Aged plasma infusion raised serum ALT levels while diminished ALP concentrations in young rats, suggesting possible liver dysfunction. Young plasma increased serum albumin levels in old rats. The study concluded that young plasma infusion might be associated with declined liver damage and fibrosis in aged rats, while aged plasma infusion negatively impacted liver health in young rats. These results imply that young blood plasma holds potential as a rejuvenation therapy for liver health and function.

Molecular simulations and NMR reveal how lipid fluctuations affect membrane mechanics

Lipid bilayers form the main matrix of functional cell membranes, and their dynamics underlie a host of physical and biological processes. Here we show that elastic membrane properties and collective molecular dynamics (MD) are related by the mean-square amplitudes (order parameters) and relaxation rates (correlation times) of lipid acyl chain motions. We performed all-atom MD simulations of liquid-crystalline bilayers that allow direct comparison with carbon-hydrogen (CH) bond relaxations measured with NMR spectroscopy. Previous computational and theoretical approaches have assumed isotropic relaxation, which yields inaccurate description of lipid chain dynamics and incorrect data interpretation. Instead, the new framework includes a fixed bilayer normal (director axis) and restricted anisotropic motion of the CH bonds in accord with their segmental order parameters, enabling robust validation of lipid force fields. Simulated spectral densities of thermally excited CH bond fluctuations exhibited well-defined spin-lattice (Zeeman) relaxations analogous to those in NMR measurements. Their frequency signature could be fit to a simple power-law function, indicative of nematic-like collective dynamics. Moreover, calculated relaxation rates scaled as the squared order parameters yielding an apparent κC modulus for bilayer bending. Our results show a strong correlation with κC values obtained from solid-state NMR studies of bilayers without and with cholesterol as validated by neutron spin-echo measurements of membrane elasticity. The simulations uncover a critical role of interleaflet coupling in membrane mechanics and thus provide important insights into molecular sites of emerging elastic properties within lipid bilayers.

Surface viscosities of lipid bilayers determined from equilibrium molecular dynamics simulations

Lipid membrane viscosity is critical to biological function. Bacterial cells grown in different environments alter their lipid composition in order to maintain a specific viscosity, and membrane viscosity has been linked to the rate of cellular respiration. To understand the factors that determine the viscosity of a membrane, we ran equilibrium all-atom simulations of single component lipid bilayers and calculated their viscosities. The viscosity was calculated via a Green-Kubo relation, with the stress-tensor autocorrelation function modeled by a stretched exponential function. By simulating a series of lipids at different temperatures, we establish the dependence of viscosity on several aspects of lipid chemistry, including hydrocarbon chain length, unsaturation, and backbone structure. Sphingomyelin is found to have a remarkably high viscosity, roughly 20 times that of DPPC. Furthermore, we find that inclusion of the entire range of the dispersion interaction increases viscosity by up to 140%. The simulated viscosities are similar to experimental values obtained from the rotational dynamics of small chromophores and from the diffusion of integral membrane proteins but significantly lower than recent measurements based on the deformation of giant vesicles.

Remodeling of yeast vacuole membrane lipidomes from the log (one phase) to stationary stage (two phases)

Upon nutrient limitation, budding yeast of Saccharomyces cerevisiae shift from fast growth (the log stage) to quiescence (the stationary stage). This shift is accompanied by liquid-liquid phase separation in the membrane of the vacuole, an endosomal organelle. Recent work indicates that the resulting micrometer-scale domains in vacuole membranes enable yeast to survive periods of stress. An outstanding question is which molecular changes might cause this membrane phase separation. Here, we conduct lipidomics of vacuole membranes in both the log and stationary stages. Isolation of pure vacuole membranes is challenging in the stationary stage, when lipid droplets are in close contact with vacuoles. Immuno-isolation has previously been shown to successfully purify log-stage vacuole membranes with high organelle specificity, but it was not previously possible to immuno-isolate stationary-stage vacuole membranes. Here, we develop Mam3 as a bait protein for vacuole immuno-isolation, and demonstrate low contamination by non-vacuolar membranes. We find that stationary-stage vacuole membranes contain surprisingly high fractions of phosphatidylcholine lipids (∼40%), roughly twice as much as log-stage membranes. Moreover, in the stationary stage, these lipids have higher melting temperatures, due to longer and more saturated acyl chains. Another surprise is that no significant change in sterol content is observed. These lipidomic changes, which are largely reflected on the whole-cell level, fit within the predominant view that phase separation in membranes requires at least three types of molecules to be present: lipids with high melting temperatures, lipids with low melting temperatures, and sterols.

How Do Cyclopropane Fatty Acids Protect the Cell Membrane of Escherichia coli in Cold Shock?

The cyclopropanation of unsaturated lipid acyl chains of some bacterial cell membranes is an important survival strategy to protect the same against drastic cooling. To elucidate the role of cyclopropane ring-containing lipids, we have simulated the lipid membrane of Escherichia coli (E. coli) and two modified membranes by replacing the cyclopropane rings with either single or double bonds at widely different temperatures. It has been observed that the cyclopropane rings provide more rigid kinks in the lipid acyl chain compared to the double bonds and therefore further reduce the packing density of the membrane and subsequently enhance the membrane fluidity at low temperatures. They also inhibit the close packing of other lipids and deleterious phase separation by strongly interacting with them. Therefore, this study has explained why E. coli bacterial strain, susceptible to freezing environments, relies on the cyclopropanation of an unsaturated chain.