Homeoviscous Adaptation and the Regulation of Membrane Lipids

Lipid Sorting and Organelle Identity

The sorting and trafficking of lipids between organelles gives rise to a dichotomy of bulk membrane properties between organelles of the secretory and endolysosome networks, giving rise to two "membrane territories" based on differences in lipid-packing density, net membrane charge, and bilayer leaflet asymmetries. The cellular organelle membrane dichotomy emerges from ER-to-PM anterograde membrane trafficking and the synthesis of sphingolipids and cholesterol flux at the trans-Golgi network, which constitutes the interface between the two membrane territories. Organelle homeostasis is maintained by vesicle-mediated retrieval of bulk membrane from the distal organelles of each territory to the endoplasmic reticulum or plasma membrane and by soluble lipid transfer proteins that traffic particular lipids. The concept of cellular membrane territories emphasizes the contrasting features of organelle membranes of the secretory and endolysosome networks and the essential roles of lipid-sorting pathways that maintain organelle function.

Comprehensive analysis of the composition of the major phospholipids during the asexual life cycle of the filamentous fungus Aspergillus nidulans

Filamentous fungi undergo significant cellular morphological changes during their life cycle. It has recently been reported that deletions of genes that are involved in phospholipid synthesis led to abnormal hyphal morphology and differentiation in filamentous fungi. Although these results suggest the importance of phospholipid balance in their life cycle, comprehensive analyses of cellular phospholipids are limited. Here, we performed lipidomic analysis of A. nidulans during morphological changes in a liquid medium and of colonies on a solid medium. We observed that the phospholipid composition and transcription of the genes involved in phospholipid synthesis changed dynamically during the life cycle. Specifically, the levels of phosphatidylethanolamine, and highly unsaturated phospholipids increased during the establishment of polarity. Furthermore, we demonstrated that the phospholipid composition in the hyphae at colony margins is similar to that during conidial germination. Furthermore, we demonstrated that common and characteristic phospholipid changes occurred during germination in A. nidulans and A. oryzae, and that species-specific changes also occurred. These results suggest that the exquisite regulation of phospholipid composition is crucial for the growth and differentiation of filamentous fungi.

What's past is prologue: FRAP keeps delivering 50 years later

Fluorescence recovery after photobleaching (FRAP) has emerged as one of the most widely utilized techniques to quantify binding and diffusion kinetics of biomolecules in biophysics. Since its inception in the mid-1970s, FRAP has been used to address an enormous array of questions including the characteristic features of lipid rafts, how cells regulate the viscosity of their cytoplasm, and the dynamics of biomolecules inside condensates formed by liquid-liquid phase separation. In this perspective, I briefly summarize the history of the field and discuss why FRAP has proven to be so incredibly versatile and popular. Next, I provide an overview of the extensive body of knowledge that has emerged on best practices for quantitative FRAP data analysis, followed by some recent examples of biological lessons learned using this powerful approach. Finally, I touch on new directions and opportunities for biophysicists to contribute to the continued development of this still-relevant research tool.

Gentamicin resistance to Escherichia coli related to fatty acid metabolism based on transcriptome analysis

Antibiotic overuse and misuse have promoted the emergence and spread of antibiotic-resistant bacteria. Increasing bacterial resistance to antibiotics is a major healthcare problem, necessitating elucidation of antibiotic resistance mechanisms. In this study, we explored the mechanism of gentamicin resistance by comparing the transcriptomes of antibiotic-sensitive and -resistant Escherichia coli. A total of 410 differentially expressed genes were identified, of which 233 (56.83%) were up-regulated and 177 (43.17%) were down-regulated in the resistant strain compared with the sensitive strain. Gene Ontology (GO) analysis classifies differential gene expression into three main categories: biological processes, cellular components, and molecular functions. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that the up-regulated genes were enriched in eight metabolic pathways, including fatty acid metabolism, which suggests that fatty acid metabolism may be involved in the development of gentamicin resistance in E. coli. This was demonstrated by measuring the acetyl-CoA carboxylase activity, plays a fundamental role in fatty acid metabolism, was increased in gentamicin-resistant E. coli. Treatment of fatty acid synthesis inhibitor, triclosan, promoted gentamicin-mediated killing efficacy to antibiotic-resistant bacteria. We also found that exogenous addition of oleic acid, which involved in fatty acid metabolism, reduced E. coli sensitivity to gentamicin. Overall, our results provide insight into the molecular mechanism of gentamicin resistance development in E. coli.

PAQR proteins and the evolution of a superpower: Eating all kinds of fats: Animals rely on evolutionarily conserved membrane homeostasis proteins to compensate for dietary variation

Recently published work showed that members of the PAQR protein family are activated by cell membrane rigidity and contribute to our ability to eat a wide variety of diets. Cell membranes are primarily composed of phospholipids containing dietarily obtained fatty acids, which poses a challenge to membrane properties because diets can vary greatly in their fatty acid composition and could impart opposite properties to the cellular membranes. In particular, saturated fatty acids (SFAs) can pack tightly and form rigid membranes (like butter at room temperature) while unsaturated fatty acids (UFAs) form more fluid membranes (like vegetable oils). Proteins of the PAQR protein family, characterized by the presence of seven transmembrane domains and a cytosolic N-terminus, contribute to membrane homeostasis in bacteria, yeasts, and animals. These proteins respond to membrane rigidity by stimulating fatty acid desaturation and incorporation of UFAs into phospholipids and explain the ability of animals to thrive on diets with widely varied fat composition. Also see the video abstract here: https://youtu.be/6ckcvaDdbQg.

The plasma membrane of focal adhesions has a high content of cholesterol and phosphatidylcholine with saturated acyl chains

Cellular functions, such as differentiation and migration, are regulated by the extracellular microenvironment, including the extracellular matrix (ECM). Cells adhere to ECM through focal adhesions (FAs) and sense the surrounding microenvironments. Although FA proteins have been actively investigated, little is known about the lipids in the plasma membrane at FAs. In this study, we examine the lipid composition at FAs with imaging and biochemical approaches. Using the cholesterol-specific probe D4 with total internal reflection fluorescence microscopy and super-resolution microscopy, we show an enrichment of cholesterol at FAs simultaneously with FA assembly. Furthermore, we establish a method to isolate the lipid from FA-rich fractions, and biochemical quantification of the lipids reveals that there is a higher content of cholesterol and phosphatidylcholine with saturated fatty acid chains in the lipids of the FA-rich fraction than in either the plasma membrane fraction or the whole-cell membrane. These results demonstrate that plasma membrane at FAs has a locally distinct lipid composition compared to the bulk plasma membrane.

Efficacy of acidified water-in-oil emulsions against desiccated Salmonella as a function of acid carbon chain-length and membrane viscosity

Sanitizing low-moisture food (LMF) processing equipment is challenging due to the increased heat resistance of Salmonella spp. in low-water activity (a_w) environments. Food-grade oils mixed with acetic acid have been shown effective against desiccated Salmonella. In this study, different hydrocarbon chain-length (C_n) organic acids were tested against desiccated Salmonella by using 1% v/v water-in-oil (W/O) emulsion as the delivery system for 200 mM acid. Fluorescence lifetime imaging microscopy (FLIM) was utilized with a BODIPY-based molecular rotor to evaluate membrane viscosity under environmental conditions such as desiccation and temperature elevation. Drying hydrated Salmonella cells to 75% equilibrium relative humidity (ERH) increased the membrane viscosity from 1,199 to 1,309 mPa·s (cP) at 22°C. Heating to 45°C decreased the membrane viscosity of hydrated cells from 1,199 to 1,082 mPa·s, and decreased that of the desiccated cells from 1,309 to 1,245 mPa·s. At both 22°C and 45°C, desiccated Salmonella was highly susceptible (>6.5 microbial log reduction (MLR) per stainless-steel coupon) to a 30-min treatment with the W/O emulsions formulated with short carbon chain acids (C_1-3). By comparison, the emulsion formulations with longer carbon chain acids (C_4-12) showed little to no MLR at 22°C, but achieved >6.5 MLR at 45°C. Based upon the decreased Salmonella membrane viscosity and the increased antimicrobial efficacy of C_4-12 W/O emulsions with increasing temperature, we propose that heating can make the membrane more fluid which may allow the longer carbon chain acids (C_4-12) to permeate or disrupt membrane structures.

Impacts of seasonal temperatures, ocean warming and marine heatwaves on the nutritional quality of eastern school prawns (Metapenaeus macleayi)

Ocean warming and marine heatwaves significantly alter environmental conditions in marine and estuarine environments. Despite their potential global importance for nutrient security and human health, it is not well understood how thermal impacts could alter the nutritional quality of harvested marine resources. We tested whether short-term experimental exposure to seasonal temperatures, projected ocean-warming temperatures, and marine heatwaves affected the nutritional quality of the eastern school prawn (Metapenaeus macleayi). In addition, we tested whether nutritional quality was affected by the duration of exposure to warm temperatures. We show the nutritional quality of M. macleayi is likely to be resilient to short- (28 d), but not longer-term (56 d) exposure to warming temperatures. The proximate, fatty acid and metabolite compositions of M. macleayi were unchanged after 28 d exposure to simulated ocean warming and marine heatwaves. The ocean-warming scenario did, however, show potential for elevated sulphur, iron and silver levels after 28 d. Decreasing saturation of fatty acids in M. macleayi after 28 d exposure to cooler temperatures indicates homeoviscous adaptation to seasonal changes. We found that 11 % of measured response variables were significantly different between 28 and 56 d when exposed to the same treatment, indicating the duration of exposure time and time of sampling are critical when measuring this species' nutritional response. Further, we found that future acute warming events could reduce harvestable biomass, despite survivors retaining their nutritional quality. Developing a combined knowledge of the variability in seafood nutrient content with shifts in the availability of harvested seafood is crucial for understanding seafood-derived nutrient security in a changing climate.

Membrane lipid changes in mouse blastocysts induced by ovarian stimulation, IVF and oocyte vitrification

RESEARCH QUESTION

Is the membrane lipid profile of mice blastocysts affected by ovarian stimulation, IVF and oocyte vitrification? Could supplementation of vitrification media with L-carnitine and fatty acids prevent membrane phospholipid changes in blastocysts from vitrified oocytes?

DESIGN

Experimental study comparing the lipid profile of murine blastocysts produced from natural mating, superovulated cycles or after IVF submitted or not to vitrification. For in-vitro experiments, 562 oocytes from superovulated females were randomly divided into four groups: fresh oocytes fertilized in vitro and vitrified groups: Irvine Scientific (IRV); Tvitri-4 (T4) or T4 supplemented with L-carnitine and fatty acids (T4-LC/FA). Fresh or vitrified-warmed oocytes were inseminated and cultured for 96 h or 120 h. The lipid profile of nine of the best quality blastocysts from each experimental group was assessed by multiple reaction monitoring profiling method. Significantly different lipids or transitions between groups were found using univariate statistics (P < 0.05; fold change = 1.5) and multivariate statistical methods.

RESULTS

A total of 125 lipids in blastocysts were profiled. Statistical analysis revealed several classes of phospholipids affected in the blastocysts by ovarian stimulation, IVF, oocyte vitrification, or all. L-carnitine and fatty acid supplements prevented, to a certain extent, changes in phospholipid and sphingolipid contents in the blastocysts.

CONCLUSION

Ovarian stimulation alone, or in association with IVF, promoted changes in phospholipid profile and abundance of blastocysts. A short exposure time to the lipid-based solutions during oocyte vitrification was sufficient to induce changes in the lipid profile that were sustained until the blastocyst stage.

Coral Lipidome: Molecular Species of Phospholipids, Glycolipids, Betaine Lipids, and Sphingophosphonolipids

Coral reefs are the most biodiversity-rich ecosystems in the world's oceans. Coral establishes complex interactions with various microorganisms that constitute an important part of the coral holobiont. The best-known coral endosymbionts are Symbiodiniaceae dinoflagellates. Each member of the coral microbiome contributes to its total lipidome, which integrates many molecular species. The present study summarizes available information on the molecular species of the plasma membrane lipids of the coral host and its dinoflagellates (phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), ceramideaminoethylphosphonate, and diacylglyceryl-3-O-carboxyhydroxymethylcholine), and the thylakoid membrane lipids of dinoflagellates (phosphatidylglycerol (PG) and glycolipids). Alkyl chains of PC and PE molecular species differ between tropical and cold-water coral species, and features of their acyl chains depend on the coral's taxonomic position. PS and PI structural features are associated with the presence of an exoskeleton in the corals. The dinoflagellate thermosensitivity affects the profiles of PG and glycolipid molecular species, which can be modified by the coral host. Coral microbiome members, such as bacteria and fungi, can also be the source of the alkyl and acyl chains of coral membrane lipids. The lipidomics approach, providing broader and more detailed information about coral lipid composition, opens up new opportunities in the study of biochemistry and ecology of corals.

Targeted Lipidomics Reveals a Novel Role for Glucosylceramides in Glucose Response

The addition of excess glucose to the diet drives a coordinated response of lipid metabolism pathways to tune the membrane composition to the altered diet. Here, we have employed targeted lipidomic approaches to quantify the specific changes in the phospholipid and sphingolipid populations that occur in elevated glucose conditions. The lipids within wildtype Caenorhabditis elegans are strikingly stable with no significant changes identified in our global mass spectrometry-based analysis. Previous work has identified ELO-5, an elongase that is critical for the synthesis of monomethyl-branched chain fatty acids (mmBCFAs), as essential for surviving elevated glucose conditions. Therefore, we performed targeted lipidomics on elo-5 RNAi-fed animals and identified several significant changes in these animals in lipid species that contain mmBCFAs as well as in species that do not contain mmBCFAs. Of particular note, we identified a specific glucosylceramide (GlcCer 17:1;O2/22:0;O) that is also significantly upregulated with glucose in wildtype animals. Furthermore, compromising the production of the glucosylceramide pool with elo-3 or cgt-3 RNAi leads to premature death in glucose-fed animals. Taken together, our lipid analysis has expanded the mechanistic understanding of metabolic rewiring with glucose feeding and has identified a new role for the GlcCer 17:1;O2/22:0;O.

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.

Molecular Mechanisms of Lipid-Based Metabolic Adaptation Strategies in Response to Cold

Temperature changes and periods of detrimental cold occur frequently for many organisms in their natural habitats. Homeothermic animals have evolved metabolic adaptation strategies to increase mitochondrial-based energy expenditure and heat production, largely relying on fat as a fuel source. Alternatively, certain species are able to repress their metabolism during cold periods and enter a state of decreased physiological activity known as torpor. By contrast, poikilotherms, which are unable to maintain their internal temperature, predominantly increase membrane fluidity to diminish cold-related damage from low-temperature stress. However, alterations of molecular pathways and the regulation of lipid-metabolic reprogramming during cold exposure are poorly understood. Here, we review organismal responses that adjust fat metabolism during detrimental cold stress. Cold-related changes in membranes are detected by membrane-bound sensors, which signal to downstream transcriptional effectors, including nuclear hormone receptors of the PPAR (peroxisome proliferator-activated receptor) subfamily. PPARs control lipid metabolic processes, such as fatty acid desaturation, lipid catabolism and mitochondrial-based thermogenesis. Elucidating the underlying molecular mechanisms of cold adaptation may improve beneficial therapeutic cold treatments and could have important implications for medical applications of hypothermia in humans. This includes treatment strategies for hemorrhagic shock, stroke, obesity and cancer.

Can marine heatwaves affect the fatty acid composition and energy budget of the tropical fish Zebrasoma scopas?

Marine heatwaves (MHWs) are extreme weather events featuring abnormally high seawater temperature, and expected to increase in frequency, duration and severity over this century. The impacts of these phenomena on physiological performance of coral reef species require understanding. This study aimed to evaluate the effects of a simulated MHW (category IV; ΔT = +2 °C, 11 days) (after exposure and 10-day recovery period) on fatty acid (FA) composition (as a biochemical indicator) and energy budget (i.e., growth, G, excretion (faecal, F and nitrogenous losses, U), respiration, R and food consumption, C) of a juvenile tropical surgeonfish species (Zebrasoma scopas). Significant and different changes were found under MHW scenario for some of the most abundant FA and respective groups (i.e., an increase in the contents of 14:0, 18:1n-9, ΣMonounsaturated (ΣMUFA) and 18:2n-6; and a decrease in the levels of 16:0, ΣSaturated (ΣSFA), 18:1n-7, 22:5n-3 and ΣPolyunsaturated (ΣPUFA)). The contents of 16:0 and ΣSFA were also significantly lower after MHW exposure compared to control (CTRL). Additionally, lower feed efficiency (FE), relative growth rate (RGR) and specific growth rate in terms of wet weight (SGRw), as well as higher energy loss for respiration were observed under MHW exposure conditions in comparison with CTRL and MHW recovery period. The energy proportion channelled for faeces dominated the mode of energy allocation, followed by growth in both treatments (after exposure). After MHW recovery, this trend was reversed, and a higher percentage was spent for growth and a lower fraction for faeces than in the MHW exposure period. Overall, FA composition, growth rates and energy loss for respiration of Z. Scopas were the physiological parameters most influenced (mainly in a negative way) by an 11-day MHW event. The observed effects in this tropical species can be exacerbated with increasing intensity and frequency of these extreme events.

Tuberculostearic Acid Controls Mycobacterial Membrane Compartmentalization

The intracellular membrane domain (IMD) is a laterally discrete region of the mycobacterial plasma membrane, enriched in the subpolar region of the rod-shaped cell. Here, we report genome-wide transposon sequencing to discover the controllers of membrane compartmentalization in Mycobacterium smegmatis. The putative gene cfa showed the most significant effect on recovery from membrane compartment disruption by dibucaine. Enzymatic analysis of Cfa and lipidomic analysis of a cfa deletion mutant (Δcfa) demonstrated that Cfa is an essential methyltransferase for the synthesis of major membrane phospholipids containing a C19:0 monomethyl-branched stearic acid, also known as tuberculostearic acid (TBSA). TBSA has been intensively studied due to its abundant and genus-specific production in mycobacteria, but its biosynthetic enzymes had remained elusive. Cfa catalyzed the S-adenosyl-l-methionine-dependent methyltransferase reaction using oleic acid-containing lipid as a substrate, and Δcfa accumulated C18:1 oleic acid, suggesting that Cfa commits oleic acid to TBSA biosynthesis, likely contributing directly to lateral membrane partitioning. Consistent with this model, Δcfa displayed delayed restoration of subpolar IMD and delayed outgrowth after bacteriostatic dibucaine treatment. These results reveal the physiological significance of TBSA in controlling lateral membrane partitioning in mycobacteria. IMPORTANCE As its common name implies, tuberculostearic acid is an abundant and genus-specific branched-chain fatty acid in mycobacterial membranes. This fatty acid, 10-methyl octadecanoic acid, has been an intense focus of research, particularly as a diagnostic marker for tuberculosis. It was discovered in 1934, and yet the enzymes that mediate the biosynthesis of this fatty acid and the functions of this unusual fatty acid in cells have remained elusive. Through a genome-wide transposon sequencing screen, enzyme assay, and global lipidomic analysis, we show that Cfa is the long-sought enzyme that is specifically involved in the first step of generating tuberculostearic acid. By characterizing a cfa deletion mutant, we further demonstrate that tuberculostearic acid actively regulates lateral membrane heterogeneity in mycobacteria. These findings indicate the role of branched fatty acids in controlling the functions of the plasma membrane, a critical barrier for the pathogen to survive in its human host.

Pseudomonas aeruginosa responds to altered membrane phospholipid composition by adjusting the production of two-component systems, proteases and iron uptake proteins

Membrane protein and phospholipid (PL) composition changes in response to environmental cues and during infections. To achieve these, bacteria use adaptation mechanisms involving covalent modification and remodelling of the acyl chain length of PLs. However, little is known about bacterial pathways regulated by PLs. Here, we investigated proteomic changes in the biofilm of P. aeruginosa phospholipase mutant (∆plaF) with altered membrane PL composition. The results revealed profound alterations in the abundance of many biofilm-related two-component systems (TCSs), including accumulation of PprAB, a key regulator of the transition to biofilm. Furthermore, a unique phosphorylation pattern of transcriptional regulators, transporters and metabolic enzymes, as well as differential production of several proteases, in ∆plaF, indicate that PlaF-mediated virulence adaptation involves complex transcriptional and posttranscriptional response. Moreover, proteomics and biochemical assays revealed the depletion of pyoverdine-mediated iron uptake pathway proteins in ∆plaF, while proteins from alternative iron-uptake systems were accumulated. These suggest that PlaF may function as a switch between different iron-acquisition pathways. The observation that PL-acyl chain modifying and PL synthesis enzymes were overproduced in ∆plaF reveals the interconnection of degradation, synthesis and modification of PLs for proper membrane homeostasis. Although the precise mechanism by which PlaF simultaneously affects multiple pathways remains to be elucidated, we suggest that alteration of PL composition in ∆plaF plays a role for the global adaptive response in P. aeruginosa mediated by TCSs and proteases. Our study revealed the global regulation of virulence and biofilm by PlaF and suggests that targeting this enzyme may have therapeutic potential.

Ectotherm mitochondrial economy and responses to global warming

Temperature is a key abiotic factor affecting ecology, biogeography, and evolution of species. Alterations of energy metabolism play an important role in adaptations and plastic responses to temperature shifts on different time scales. Mitochondrial metabolism affects cellular bioenergetics and redox balance making these organelles an important determinant of organismal performances such as growth, locomotion, or development. Here I analyze the impacts of environmental temperature on the mitochondrial functions (including oxidative phosphorylation, proton leak, production of reactive oxygen species(ROS), and ATP synthesis) of ectotherms and discuss the mechanisms underlying negative shifts in the mitochondrial energy economy caused by supraoptimal temperatures. Owing to the differences in the thermal sensitivity of different mitochondrial processes, elevated temperatures (beyond the species- and population-specific optimal range) cause reallocation of the electron flux and the protonmotive force (Δp) in a way that decreases ATP synthesis efficiency, elevates the relative cost of the mitochondrial maintenance, causes excessive production of ROS and raises energy cost for antioxidant defense. These shifts in the mitochondrial energy economy might have negative consequences for the organismal fitness traits such as the thermal tolerance or growth. Correlation between the thermal sensitivity indices of the mitochondria and the whole organism indicate that these traits experience similar selective pressures but further investigations are needed to establish whether there is a cause-effect relationship between the mitochondrial failure and loss of organismal performance during temperature change.

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.

Depleting Cationic Lipids Involved in Antimicrobial Resistance Drives Adaptive Lipid Remodeling in Enterococcus faecalis

The bacterial cell membrane is an interface for cell envelope synthesis, protein secretion, virulence factor assembly, and a target for host cationic antimicrobial peptides (CAMPs). To resist CAMP killing, several Gram-positive pathogens encode the multiple peptide resistance factor (MprF) enzyme that covalently attaches cationic amino acids to anionic phospholipids in the cell membrane. While E. faecalis encodes two mprF paralogs, MprF2 plays a dominant role in conferring resistance to killing by the CAMP human β-defensin 2 (hBD-2) in E. faecalis strain OG1RF. The goal of the current study is to understand the broader lipidomic and functional roles of E. faecalis mprF. We analyzed the lipid profiles of parental wild-type and mprF mutant strains and show that while ΔmprF2 and ΔmprF1 ΔmprF2 mutants completely lacked cationic lysyl-phosphatidylglycerol (L-PG), the ΔmprF1 mutant synthesized ~70% of L-PG compared to the parent. Unexpectedly, we also observed a significant reduction of PG in ΔmprF2 and ΔmprF1 ΔmprF2. In the mprF mutants, particularly ΔmprF1 ΔmprF2, the decrease in L-PG and phosphatidylglycerol (PG) is compensated by an increase in a phosphorus-containing lipid, glycerophospho-diglucosyl-diacylglycerol (GPDGDAG), and D-ala-GPDGDAG. These changes were accompanied by a downregulation of de novo fatty acid biosynthesis and an accumulation of long-chain acyl-acyl carrier proteins (long-chain acyl-ACPs), suggesting that the suppression of fatty acid biosynthesis was mediated by the transcriptional repressor FabT. Growth in chemically defined media lacking fatty acids revealed severe growth defects in the ΔmprF1 ΔmprF2 mutant strain, but not the single mutants, which was partially rescued through supplementation with palmitic and stearic acids. Changes in lipid homeostasis correlated with lower membrane fluidity, impaired protein secretion, and increased biofilm formation in both ΔmprF2 and ΔmprF1 ΔmprF2, compared to the wild type and ΔmprF1. Collectively, our findings reveal a previously unappreciated role for mprF in global lipid regulation and cellular physiology, which could facilitate the development of novel therapeutics targeting MprF. IMPORTANCE The cell membrane plays a pivotal role in protecting bacteria against external threats, such as antibiotics. Cationic phospholipids such as lysyl-phosphatidyglycerol (L-PG) resist the action of cationic antimicrobial peptides through electrostatic repulsion. Here we demonstrate that L-PG depletion has several unexpected consequences in Enterococcus faecalis, including a reduction of phosphatidylglycerol (PG), enrichment of a phosphorus-containing lipid, reduced fatty acid synthesis accompanied by an accumulation of long-chain acyl-acyl carrier proteins (long chain acyl-ACPs), lower membrane fluidity, and impaired secretion. These changes are not deleterious to the organism as long as exogenous fatty acids are available for uptake from the culture medium. Our findings suggest an adaptive mechanism involving compensatory changes across the entire lipidome upon removal of a single phospholipid modification. Such adaptations must be considered when devising antimicrobial strategies that target membrane lipids.

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.

Membrane lipid remodeling modulates γ-secretase processivity

Imbalances in the amounts of amyloid-β peptides (Aβ) generated by the membrane proteases β- and γ-secretase are considered as a trigger of Alzheimer´s disease (AD). Cell-free studies of γ-secretase have shown that increasing membrane thickness modulates Aβ generation, but it has remained unclear if these effects are translatable to cells. Here we show that the very long chain fatty acid erucic acid (EA) triggers acyl chain remodeling in AD cell models, resulting in substantial lipidome alterations which included increased esterification of EA in membrane lipids. Membrane remodeling enhanced γ-secretase processivity, resulting in the increased production of the potentially beneficial Aβ37 and/or Aβ38 species in multiple cell lines. Unexpectedly, we found that the membrane remodeling stimulated total Aβ secretion by cells expressing WT γ-secretase, but lowered it for cells expressing an aggressive familial AD mutant γ-secretase. We conclude that EA-mediated modulation of membrane composition is accompanied by complex lipid homeostatic changes that can impact amyloidogenic processing in different ways and elicit distinct γ-secretase responses, providing critical implications for lipid-based AD treatment strategies.

Regulation of membrane protein structure and function by their lipid nano-environment

Transmembrane proteins comprise ~30% of the mammalian proteome, mediating metabolism, signalling, transport and many other functions required for cellular life. The microenvironment of integral membrane proteins (IMPs) is intrinsically different from that of cytoplasmic proteins, with IMPs solvated by a compositionally and biophysically complex lipid matrix. These solvating lipids affect protein structure and function in a variety of ways, from stereospecific, high-affinity protein-lipid interactions to modulation by bulk membrane properties. Specific examples of functional modulation of IMPs by their solvating membranes have been reported for various transporters, channels and signal receptors; however, generalizable mechanistic principles governing IMP regulation by lipid environments are neither widely appreciated nor completely understood. Here, we review recent insights into the inter-relationships between complex lipidomes of mammalian membranes, the membrane physicochemical properties resulting from such lipid collectives, and the regulation of IMPs by either or both. The recent proliferation of high-resolution methods to study such lipid-protein interactions has led to generalizable insights, which we synthesize into a general framework termed the 'functional paralipidome' to understand the mutual regulation between membrane proteins and their surrounding lipid microenvironments.

Microbial membrane lipid adaptations to high hydrostatic pressure in the marine environment

The deep-sea is characterized by extreme conditions, such as high hydrostatic pressure (HHP) and near-freezing temperature. Piezophiles, microorganisms adapted to high pressure, have developed key strategies to maintain the integrity of their lipid membrane at these conditions. The abundance of specific membrane lipids, such as those containing unsaturated and branched-chain fatty acids, rises with increasing HHP. Nevertheless, this strategy is not universal among piezophiles, highlighting the need to further understand the effects of HHP on microbial lipid membranes. Challenges in the study of lipid membrane adaptations by piezophiles also involve methodological developments, cross-adaptation studies, and insight into slow-growing piezophiles. Moreover, the effects of HHP on piezophiles are often difficult to disentangle from effects caused by low temperature that are often characteristic of the deep sea. Here, we review the knowledge of membrane lipid adaptation strategies of piezophiles, and put it into the perspective of marine systems, highlighting the future challenges of research studying the effects of HHP on the microbial lipid composition.

The effect of rhamnolipids on fungal membrane models as described by their interactions with phospholipids and sterols: An in silico study

Introduction: Rhamnolipids (RLs) are secondary metabolites naturally produced by bacteria of the genera Pseudomonas and Burkholderia with biosurfactant properties. A specific interest raised from their potential as biocontrol agents for crop culture protection in regard to direct antifungal and elicitor activities. As for other amphiphilic compounds, a direct interaction with membrane lipids has been suggested as the key feature for the perception and subsequent activity of RLs. Methods: Molecular Dynamics (MD) simulations are used in this work to provide an atomistic description of their interactions with different membranous lipids and focusing on their antifungal properties. Results and discussion: Our results suggest the insertion of RLs into the modelled bilayers just below the plane drawn by lipid phosphate groups, a placement that is effective in promoting significant membrane fluidification of the hydrophobic core. This localization is promoted by the formation of ionic bonds between the carboxylate group of RLs and the amino group of the phosphatidylethanolamine (PE) or phosphatidylserine (PS) headgroups. Moreover, RL acyl chains adhere to the ergosterol structure, forming a significantly higher number of van der Waals contact with respect to what is observed for phospholipid acyl chains. All these interactions might be essential for the membranotropic-driven biological actions of RLs.

GPCR signaling regulates severe stress-induced organismic death in Caenorhabditis elegans

How an organism dies is a fundamental yet poorly understood question in biology. An organism can die of many causes, including stress-induced phenoptosis, also defined as organismic death that is regulated by its genome-encoded programs. The mechanism of stress-induced phenoptosis is still largely unknown. Here, we show that transient but severe freezing-thaw stress (FTS) in Caenorhabditis elegans induces rapid and robust phenoptosis that is regulated by G-protein coupled receptor (GPCR) signaling. RNAi screens identify the GPCR-encoding fshr-1 in mediating transcriptional responses to FTS. FSHR-1 increases ligand interaction upon FTS and activates a cyclic AMP-PKA cascade leading to a genetic program to promote organismic death under severe stress. FSHR-1/GPCR signaling up-regulates the bZIP-type transcription factor ZIP-10, linking FTS to expression of genes involved in lipid remodeling, proteostasis, and aging. A mathematical model suggests how genes may promote organismic death under severe stress conditions, potentially benefiting growth of the clonal population with individuals less stressed and more reproductively privileged. Our studies reveal the roles of FSHR-1/GPCR-mediated signaling in stress-induced gene expression and phenoptosis in C. elegans, providing empirical new insights into mechanisms of stress-induced phenoptosis with evolutionary implications.

The Plant Metabolic Changes and the Physiological and Signaling Functions in the Responses to Abiotic Stress

Global climate change has altered, and will further alter, rainfall patterns and temperatures likely causing more frequent drought and heat waves, which will consequently exacerbate abiotic stresses of plants and significantly decrease the yield and quality of crops. On the one hand, the global demand for food is ever-increasing owing to the rapid increase of the human population. On the other hand, metabolic responses are one of the most important mechanisms by which plants adapt to and survive to abiotic stresses. Here we therefore summarize recent progresses including the plant primary and secondary metabolic responses to abiotic stresses and their function in plant resistance acting as antioxidants, osmoregulatory, and signaling factors, which enrich our knowledge concerning commonalities of plant metabolic responses to abiotic stresses, including their involvement in signaling processes. Finally, we discuss potential methods of metabolic fortification of crops in order to improve their abiotic stress tolerance.

Bridge-Like Lipid Transfer Proteins (BLTPs) in C. elegans: From Genetics to Structures and Functions

In eukaryotic cells, lipid transfer can occur at membrane contact sites (MCS) to facilitate the exchange of various lipids between two adjacent cellular organelle membranes. Lipid transfer proteins (LTPs), including shuttle LTP or bridge-like LTP (BLTP), transport lipids at MCS and are critical for diverse cellular processes, including lipid metabolism, membrane trafficking, and cell signaling. BLTPs (BLTP1-5, including the ATG2 and VPS13 family proteins) contain lipid-accommodating hydrophobic repeating β-groove (RBG) domains that allow the bulk transfer of lipids through MCS. Compared with vesicular lipid transfer and shuttle LTP, BLTPs have been only recently identified. Their functions and regulatory mechanisms are currently being unraveled in various model organisms and by diverse approaches. In this review, we summarize the genetics, structural features, and biological functions of BLTP in the genetically tractable model organism C. elegans. We discuss our recent studies and findings on C. elegans LPD-3, a prototypical megaprotein ortholog of BLTP1, with identified lipid transfer functions that are evolutionarily conserved in multicellular organisms and in human cells. We also highlight areas for future research of BLTP using C. elegans and complementary model systems and approaches. Given the emerging links of BLTP to several human diseases, including Parkinson's disease and Alkuraya-Kučinskas syndrome, discovering evolutionarily conserved roles of BLTPs and their mechanisms of regulation and action should contribute to new advances in basic cell biology and potential therapeutic development for related human disorders.

Plant Plasma Membrane Proton Pump: One Protein with Multiple Functions

In plants, the plasma membrane proton pump (PM H⁺-ATPase) regulates numerous transport-dependent processes such as growth, development, basic physiology, and adaptation to environmental conditions. This review explores the multifunctionality of this enzyme in plant cells. The abundance of several PM H⁺-ATPase isogenes and their pivotal role in energizing transport in plants have been connected to the phenomena of pleiotropy. The multifunctionality of PM H⁺-ATPase is a focal point of numerous studies unraveling the molecular mechanisms of plant adaptation to adverse environmental conditions. Furthermore, PM H⁺-ATPase is a key element in plant defense mechanisms against pathogen attack; however, it also functions as a target for pathogens that enable plant tissue invasion. Here, we provide an extensive review of the PM H⁺-ATPase as a multitasking protein in plants. We focus on the results of recent studies concerning PM H⁺-ATPase and its role in plant growth, physiology, and pathogenesis.

Fatty acid composition and metabolic partitioning of α-linolenic acid are contingent on life stage in human CD3+ T lymphocytes

Introduction

Immune function changes across the life course; the fetal immune system is characterised by tolerance while that of seniors is less able to respond effectively to antigens and is more pro-inflammatory than in younger adults. Lipids are involved centrally in immune function but there is limited information about how T cell lipid metabolism changes during the life course.

Methods and Results

We investigated whether life stage alters fatty acid composition, lipid droplet content and α-linolenic acid (18:3ω-3) metabolism in human fetal CD3⁺ T lymphocytes and in CD3⁺ T lymphocytes from adults (median 41 years) and seniors (median 70 years). Quiescent fetal T cells had higher saturated (SFA), monounsaturated fatty acid (MUFA), and ω-6 polyunsaturated fatty acid (PUFA) contents than adults or seniors. Activation-induced changes in fatty acid composition differed between life stages. The principal metabolic fates of [¹³C]18:3ω-3 were constitutive hydroxyoctadecatrienoic acid synthesis and β-oxidation and carbon recycling into SFA and MUFA. These processes declined progressively across the life course. Longer chain ω-3 PUFA synthesis was a relatively minor metabolic fate of 18:3ω-3 at all life stages. Fetal and adult T lymphocytes had similar lipid droplet contents, which were lower than in T cells from seniors. Variation in the lipid droplet content of adult T cells accounted for 62% of the variation in mitogen-induced CD69 expression, but there was no significant relationship in fetal cells or lymphocytes from seniors.

Discussion

Together these findings show that fatty acid metabolism in human T lymphocytes changes across the life course in a manner that may facilitate the adaptation of immune function to different life stages.

Stress-Induced Phenoptosis: Mechanistic Insights and Evolutionary Implications

Evolution by natural selection results in biological traits that enable organismic adaptation and survival under various stressful environments. External stresses can be sometimes too severe to overcome, leading to organismic death either because of failure in adapting to such stress, or alternatively, through a regulated form of organismic death (phenoptosis). While regulated cell deaths, including apoptosis, have been extensively studied, little is known about the molecular and cellular mechanisms underlying phenoptosis and its evolutionary significance for multicellular organisms. In this article, we review documented phenomena and mechanistic evidence emerging from studies of stress-induced phenoptosis in the multicellular organism C. elegans and stress-induced deaths at cellular levels in organisms ranging from bacteria to mammals, focusing on abiotic and pathogen stresses. Genes and signaling pathways involved in phenoptosis appear to promote organismic death during severe stress and aging, while conferring fitness and immune defense during mild stress and early life, consistent with their antagonistic pleiotropy actions. As cell apoptosis during development can shape tissues and organs, stress-induced phenoptosis may also contribute to possible benefits at the population level, through mechanisms including kin selection, abortive infection, and soma-to-germline resource allocation. Current models can generate experimentally testable predictions and conceptual frameworks with implications for understanding both stress-induced phenoptosis and natural aging.

Effects of temperature and combinational exposures on lipid metabolism in aquatic invertebrates

Studies of changes in fatty acids in response to environmental temperature changes have been conducted in many species, particularly mammals. However, few studies have considered aquatic invertebrates, even though they are particularly vulnerable to changes in environmental temperature. In this review, we summarize the process by which animals synthesize common fatty acids and point out differences between the fatty acid profiles of vertebrates and those of aquatic invertebrates. Unlike vertebrates, some aquatic invertebrates can directly synthesize polyunsaturated fatty acids (PUFAs), which can be used to respond to temperature changes. Various studies have shown that aquatic invertebrates increase the degree of saturation in their fatty acids through an increase in saturated fatty acid production or a decrease in PUFAs as the temperature increases. In addition, we summarize recent studies that have examined the complex effects of temperature and combinational stressors to determine whether the degree of saturation in aquatic invertebrates is influenced by other factors. The combined effects of carbon dioxide partial pressure, food quality, starvation, salinity, and chemical exposures have been confirmed, and fatty acid profile changes in response to high temperature were greater than those from combinational stressors.

Adaptations to high pressure of Nautilia sp. strain PV-1, a piezophilic Campylobacterium (aka Epsilonproteobacterium) isolated from a deep-sea hydrothermal vent

Physiological and gene expression studies of deep-sea bacteria under pressure conditions similar to those experienced in their natural habitat are critical for understanding growth kinetics and metabolic adaptations to in situ conditions. The Campylobacterium (aka Epsilonproteobacterium) Nautilia sp. strain PV-1 was isolated from hydrothermal fluids released from an active deep-sea hydrothermal vent at 9° N on the East Pacific Rise. Strain PV-1 is a piezophilic, moderately thermophilic, chemolithoautotrophic anaerobe that conserves energy by coupling the oxidation of hydrogen to the reduction of nitrate or elemental sulfur. Using a high-pressure-high temperature continuous culture system, we established that strain PV-1 has the shortest generation time of all known piezophilic bacteria and we investigated its protein expression pattern in response to different hydrostatic pressure regimes. Proteogenomic analyses of strain PV-1 grown at 20 and 5 MPa showed that pressure adaptation is not restricted to stress response or homeoviscous adaptation but extends to enzymes involved in central metabolic pathways. Protein synthesis, motility, transport, and energy metabolism are all affected by pressure, although to different extents. In strain PV-1, low-pressure conditions induce the synthesis of phage-related proteins and an overexpression of enzymes involved in carbon fixation.

Lipid saturation induces degradation of squalene epoxidase for sterol homeostasis and cell survival

A fluid membrane containing a mix of unsaturated and saturated lipids is essential for life. However, it is unclear how lipid saturation might affect lipid homeostasis, membrane-associated proteins, and membrane organelles. Here, we generate temperature-sensitive mutants of the sole fatty acid desaturase gene OLE1 in the budding yeast Saccharomyces cerevisiae Using these mutants, we show that lipid saturation triggers the endoplasmic reticulum-associated degradation (ERAD) of squalene epoxidase Erg1, a rate-limiting enzyme in sterol biosynthesis, via the E3 ligase Doa10-Ubc7 complex. We identify the P469L mutation that abolishes the lipid saturation-induced ERAD of Erg1. Overexpressed WT or stable Erg1 mutants all mislocalize into foci in the ole1 mutant, whereas the stable Erg1 causes aberrant ER and severely compromises the growth of ole1, which are recapitulated by doa10 deletion. The toxicity of the stable Erg1 and doa10 deletion is due to the accumulation of lanosterol and misfolded proteins in ole1 Our study identifies Erg1 as a novel lipid saturation-regulated ERAD target, manifesting a close link between lipid homeostasis and proteostasis that maintains sterol homeostasis under the lipid saturation condition for cell survival.

Salinity and hydraulic retention time induce membrane phospholipid acyl chain remodeling in Halanaerobium congolense WG10 and mixed cultures from hydraulically fractured shale wells

Bacteria remodel their plasma membrane lipidome to maintain key biophysical attributes in response to ecological disturbances. For Halanaerobium and other anaerobic halotolerant taxa that persist in hydraulically fractured deep subsurface shale reservoirs, salinity, and hydraulic retention time (HRT) are important perturbants of cell membrane structure, yet their effects remain poorly understood. Membrane-linked activities underlie in situ microbial growth kinetics and physiologies which drive biogeochemical reactions in engineered subsurface systems. Hence, we used gas chromatography-mass spectrometry (GC-MS) to investigate the effects of salinity and HRT on the phospholipid fatty acid composition of H. congolense WG10 and mixed enrichment cultures from hydraulically fractured shale wells. We also coupled acyl chain remodeling to membrane mechanics by measuring bilayer elasticity using atomic force microscopy (AFM). For these experiments, cultures were grown in a chemostat vessel operated in continuous flow mode under strict anoxia and constant stirring. Our findings show that salinity and HRT induce significant changes in membrane fatty acid chemistry of H. congolense WG10 in distinct and complementary ways. Notably, under nonoptimal salt concentrations (7% and 20% NaCl), H. congolense WG10 elevates the portion of polyunsaturated fatty acids (PUFAs) in its membrane, and this results in an apparent increase in fluidity (homeoviscous adaptation principle) and thickness. Double bond index (DBI) and mean chain length (MCL) were used as proxies for membrane fluidity and thickness, respectively. These results provide new insight into our understanding of how environmental and engineered factors might disrupt the physical and biogeochemical equilibria of fractured shale by inducing physiologically relevant changes in the membrane fatty acid chemistry of persistent microbial taxa. GRAPHICAL ABSTRACTSalinity significantly alters membrane bilayer fluidity and thickness in Halanaerobium congolense WG10.

A conserved megaprotein-based molecular bridge critical for lipid trafficking and cold resilience

Cells adapt to cold by increasing levels of unsaturated phospholipids and membrane fluidity through conserved homeostatic mechanisms. Here we report an exceptionally large and evolutionarily conserved protein LPD-3 in C. elegans that mediates lipid trafficking to confer cold resilience. We identify lpd-3 mutants in a mutagenesis screen for genetic suppressors of the lipid desaturase FAT-7. LPD-3 bridges the endoplasmic reticulum (ER) and plasma membranes (PM), forming a structurally predicted hydrophobic tunnel for lipid trafficking. lpd-3 mutants exhibit abnormal phospholipid distribution, diminished FAT-7 abundance, organismic vulnerability to cold, and are rescued by Lecithin comprising unsaturated phospholipids. Deficient lpd-3 homologues in Zebrafish and mammalian cells cause defects similar to those observed in C. elegans. As mutations in BLTP1, the human orthologue of lpd-3, cause Alkuraya-Kucinskas syndrome, LPD-3 family proteins may serve as evolutionarily conserved highway bridges critical for ER-associated non-vesicular lipid trafficking and resilience to cold stress in eukaryotic cells.

High Throughput LC-MS Platform for Large Scale Screening of Bioactive Polar Lipids in Human Plasma and Serum

Lipids play a key role in many biological processes, and their accurate measurement is critical to unraveling the biology of diseases and human health. A high throughput HILIC-based (LC-MS) method for the semiquantitative screening of over 2000 lipids, based on over 4000 MRM transitions, was devised to produce an accessible and robust lipidomic screen for phospholipids in human plasma/serum. This methodology integrates many of the advantages of global lipid analysis with those of targeted approaches. Having used the method as an initial "wide class" screen, it can then be easily adapted for a more targeted analysis and quantification of key, dysregulated lipids. Robustness was assessed using 1550 continuous injections of plasma extracts onto a single column and via the evaluation of columns from 5 different batches of stationary phase. Initial screens in positive (239 lipids, 431 MRM transitions) and negative electrospray ionization (ESI) mode (232 lipids, 446 MRM transitions) were assessed for reproducibility, sensitivity, and dynamic range using analysis times of 8 min. The total number of lipids monitored using these screening methods was 433 with an overlap of 38 lipids in both modes. A polarity switching method for accurate quantification, using the same LC conditions, was assessed for intra- and interday reproducibility, accuracy, dynamic range, stability, carryover, dilution integrity, and matrix interferences and found to be acceptable. This polarity switching method was then applied to lipids important in the stratification of human prostate cancer samples.

Division of the role and physiological impact of multiple lysophosphatidic acid acyltransferase paralogs

Background

Lysophosphatidic acid acyltransferase (LPAAT) is a phospholipid biosynthesis enzyme that introduces a particular set of fatty acids at the sn-2 position of phospholipids. Many bacteria have multiple LPAAT paralogs, and these enzymes are considered to have different fatty acid selectivities and to produce diverse phospholipids with distinct fatty acid compositions. This feature is advantageous for controlling the physicochemical properties of lipid membranes to maintain membrane integrity in response to the environment. However, it remains unclear how LPAAT paralogs are functionally differentiated and biologically significant.

Results

To better understand the division of roles of the LPAAT paralogs, we analyzed the functions of two LPAAT paralogs, PlsC4 and PlsC5, from the psychrotrophic bacterium Shewanella livingstonensis Ac10. As for their enzymatic function, lipid analysis of plsC4- and plsC5-inactivated mutants revealed that PlsC4 prefers iso-tridecanoic acid (C₁₂-chain length, methyl-branched), whereas PlsC5 prefers palmitoleic acid (C₁₆-chain length, monounsaturated). Regarding the physiological role, we found that plsC4, not plsC5, contributes to tolerance to cold stress. Using bioinformatics analysis, we demonstrated that orthologs of PlsC4/PlsC5 and their close relatives, constituting a new clade of LPAATs, are present in many γ-proteobacteria. We also found that LPAATs of this clade are phylogenetically distant from principal LPAATs, such as PlsC1 of S. livingstonensis Ac10, which are universally conserved among bacteria, suggesting the presence of functionally differentiated LPAATs in these bacteria.

Conclusions

PlsC4 and PlsC5, which are LPAAT paralogs of S. livingstonensis Ac10, play different roles in phospholipid production and bacterial physiology. An enzyme belonging to PlsC4/PlsC5 subfamilies and their close relatives are present, in addition to principal LPAATs, in many γ-proteobacteria, suggesting that the division of roles is more common than previously thought. Thus, both principal LPAATs and PlsC4/PlsC5-related enzymes should be considered to decipher the metabolism and physiology of bacterial cell membranes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02641-8.

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.

In yeast, cardiolipin unsaturation level plays a key role in mitochondrial function and inner membrane integrity

Cardiolipin is the signature phospholipid of the mitochondrial inner membrane. It participates in shaping the inner membrane as well as in modulating the activity of many membrane-bound proteins. The acyl chain composition of cardiolipin is finely tuned post-biosynthesis depending on the surrounding phospholipids to produce mature or unsaturated cardiolipin. However, experimental evidence showing that immature and mature cardiolipin are functionally equivalents for mitochondria poses doubts on the relevance of cardiolipin remodeling. In this work, we studied the role of cardiolipin acyl chain composition in mitochondrial bioenergetics, including a detailed bioenergetic profile of yeast mitochondria. Cardiolipin acyl chains were modified by genetic and nutritional manipulation. We found that both the bioenergetic efficiency and osmotic stability of mitochondria are dependent on the unsaturation level of cardiolipin acyl chains. It is proposed that cardiolipin remodeling and, consequently, mature cardiolipins play an important role in mitochondrial inner membrane integrity and functionality.

Homeoviscous Adaptation of the Lipid Membrane of a Soil Bacterium Surviving under Diurnal Temperature Variation: A Molecular Simulation Perspective

A recent experiment has reported the lipidome remodeling of a soil-based plant-associated bacterium Methylobacterium extorquens due to diurnal temperature variations. The key adaptation strategy is the headgroup-specific remodeling of the acyl chain. To understand the idiosyncratic adaptation at the molecular level, we simulate the model membrane of the same bacterium using the reported lipidome compositions at four different experimental temperatures. We investigate the temperature-dependent packing density and fluidity of the membrane, the constancy of which is key to the homeoviscous adaptation. The results show that complex lipidome remodeling approximately preserves membrane properties under heat and cold stress. The headgroup-specific remodeling of the acyl chain serves to fine-tune the packing density and fluidity of the membrane at different temperatures. While lipids with strongly interacting headgroups are more abundant at higher temperatures, the lipidome is more dominated by lipids with weaker interacting headgroups at lower temperatures. This adaptation alleviates lipid membrane disruption caused by heat and cold stress. This study provides a molecular picture of the homeoviscous adaptation of the realistic lipid membrane of a soil-based bacterium.

Effect of Salinity on Leaf Functional Traits and Chloroplast Lipids Composition in Two C3 and C4 Chenopodiaceae Halophytes

Salt stress is one of the most common abiotic kinds of stress. Understanding the key mechanisms of salt tolerance in plants involves the study of halophytes. The effect of salinity was studied in two halophytic annuals of Chenopodiaceae Salicornia perennans Willd. and Climacoptera crassa (Bied.) Botsch. These species are plants with C₃ and C₄-metabolism, respectively. We performed a comprehensive analysis of the photosynthetic apparatus of these halophyte species at different levels of integration. The C₃ species S. perennans showed larger variation in leaf functional traits-both at the level of cell morphology and membrane system (chloroplast envelope and thylakoid). S. perennans also had larger photosynthetic cells, by 10-15 times, and more effective mechanisms of osmoregulation and protecting cells against the toxic effect of Na⁺. Salinity caused changes in photosynthetic tissues of C. crassa such as an increase of the mesophyll cell surface, the expansion of the interface area between mesophyll and bundle sheath cells, and an increase of the volume of the latter. These functional changes compensated for scarce CO₂ supply when salinity increased. Overall, we concluded that these C₃ and C₄ Chenopodiaceae species demonstrated different responses to salinity, both at the cellular and subcellular levels.

Simulated Docking Predicts Putative Channels for the Transport of Long-Chain Fatty Acids in Vibrio cholerae

Fatty acids (FA) play an important role in biological functions, such as membrane homeostasis, metabolism, and as signaling molecules. FadL is the only known protein that uptakes long-chain fatty acids in Gram-negative bacteria, and this uptake has traditionally been thought to be limited to fatty acids up to 18 carbon atoms in length. Recently however, it was found Vibrio cholerae has the ability to uptake fatty acids greater than 18 carbon atoms and this uptake corresponds to bacterial survivability. Using E. coli’s FadL as a template, V. cholerae FadL homologs vc1042, vc1043, and vca0862 have been computationally folded, simulated on an atomistic level using Molecular Dynamics, and docked in silico to analyze the FadL transport channels. For the vc1042 and vc1043 homologs, these transport channels have more structural accommodations for the many rigid unsaturated bonds of long-chain polyunsaturated fatty acids, while the vca0862 homolog was found to lack transport channels within the signature beta barrel of FadL proteins.

Solutions: how adaptive changes in cellular fluids enable marine life to cope with abiotic stressors

The seas confront organisms with a suite of abiotic stressors that pose challenges for physiological activity. Variations in temperature, hydrostatic pressure, and salinity have potential to disrupt structures, and functions of all molecular systems on which life depends. During evolution, sequences of nucleic acids and proteins are adaptively modified to "fit" these macromolecules for function under the particular abiotic conditions of the habitat. Complementing these macromolecular adaptations are alterations in compositions of solutions that bathe macromolecules and affect stabilities of their higher order structures. A primary result of these "micromolecular" adaptations is preservation of optimal balances between conformational rigidity and flexibility of macromolecules. Micromolecular adaptations involve several families of organic osmolytes, with varying effects on macromolecular stability. A given type of osmolyte generally has similar effects on DNA, RNA, proteins and membranes; thus, adaptive regulation of cellular osmolyte pools has a global effect on macromolecules. These effects are mediated largely through influences of osmolytes and macromolecules on water structure and activity. Acclimatory micromolecular responses are often critical in enabling organisms to cope with environmental changes during their lifetimes, for example, during vertical migration in the water column. A species' breadth of environmental tolerance may depend on how effectively it can vary the osmolyte composition of its cellular fluids in the face of stress. Micromolecular adaptations remain an under-appreciated aspect of evolution and acclimatization. Further study can lead to a better understanding of determinants of environmental tolerance ranges and to biotechnological advances in designing improved stabilizers for biological materials.

α-linolenic acid interconversion is sufficient as a source of longer chain ω-3 polyunsaturated fatty acids in humans: An opinion

α-linolenic acid (αLNA) conversion into the functionally important ω-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), has been regarded as inadequate for meeting nutritional requirements for these PUFA. This view is based on findings of small αLNA supplementation trials and stable isotope tracer studies that have been interpreted as indicating human capacity for EPA and, in particular, DHA synthesis is limited. The purpose of this review is to re-evaluate this interpretation. Markedly differing study designs, inconsistent findings and lack of trial replication preclude robust consensus regarding the nutritional adequacy of αLNA as a source of EPC and DHA. The conclusion that αLNA conversion in humans is constrained is inaccurate because it presupposes the existence of an unspecified, higher level of metabolic activity. Since capacity for EPA and DHA synthesis is the product of evolution it may be argued that the levels of EPA and DHA it maintains are nutritionally appropriate. Dietary and supra-dietary EPA plus DHA intakes confer health benefits. Paradoxically, such health benefits are also found amongst vegetarians who do not consume EPA and DHA, and for whom αLNA conversion is the primary source of ω-3 PUFA. Since there are no reported adverse effects on health or cognitive development of diets that exclude EPA and DHA, their synthesis from αLNA appears to be nutritionally adequate. This is consistent with the dietary essentiality of αLNA and has implications for developing sustainable nutritional recommendations for ω-3 PUFA.

Adaptation of Bacteria to Antineoplastic Agents Involves Persister Cells and Increases Resistance to Antibiotics

The increasing number of life-threatening infections observed in cancer patients has been ascribed to chemotherapy-induced neutropenia and to invasive medical procedures such as surgery and the application of catheters. In this study, it was questioned if the infections could also be favored by an increased resistance of bacteria due to the adaptation to antineoplastic agents used in chemotherapy. After exposure to several antineoplastic agents, it was observed that cells of Staphylococcus aureus, Mycobacterium vaccae, Pseudomonas aeruginosa, and Escherichia coli changed the fatty acid profile of their cellular membranes, produced exopolymeric substances, and formed aggregates that adhered to surfaces. Additionally, when exposed to high concentrations of these compounds, a persister sub-population could be identified. After adaptation to antineoplastic agents, the minimum inhibitory concentration (MIC) of several antibiotics increased considerably in the tested strains.

Coral Holobionts Possess Distinct Lipid Profiles That May Be Shaped by Symbiodiniaceae Taxonomy

Symbiotic relationships are very important for corals. Abiotic stressors cause the acclimatization of cell membranes in symbionts, which possess different membrane acclimatization strategies. Membrane stability is determined by a unique lipid composition and, thus, the profile of thylakoid lipids can depend on coral symbiont species. We have analyzed and compared thylakoid lipidomes (mono- and digalactosyldiacylglycerols (MGDG and DGDG), sulfoquinovosyldiacylglycerols (SQDG), and phosphatidylglycerols (PG)) of crude extracts from symbiotic reef-building coral Acropora sp., the hydrocoral Millepora platyphylla, and the octocoral Sinularia flexibilis. S. flexibilis crude extracts were characterized by a very high SQDG/PG ratio, a DGDG/MGDG ratio < 1, a lower degree of galactolipid unsaturation, a higher content of SQDG with polyunsaturated fatty acids, and a thinner thylakoid membrane which may be explained by the presence of thermosensitive dinoflagellates Cladocopium C3. In contrast, crude extracts of M. platyphylla and Acropora sp. exhibited the lipidome features of thermotolerant Symbiodiniaceae. M. platyphylla and Acropora sp. colonies contained Cladocopium C3u and Cladocopium C71/C71a symbionts, respectively, and their lipidome profiles showed features that indicate thermotolerance. We suggest that an association with symbionts that exhibit the thermotolerant thylakoid lipidome features, combined with a high Symbiodiniaceae diversity, may facilitate further acclimatization/adaptation of M. platyphylla and Acropora sp. holobionts in the South China Sea.

Dietary PUFA Preferably Modify Ethanolamine-Containing Glycerophospholipids of the Human Plasma Lipidome

The content of polyunsaturated fatty acids (PUFA) in complex lipids essentially influences their physicochemical properties and has been linked to health and disease. To investigate the incorporation of dietary PUFA in the human plasma lipidome, we quantified glycerophospholipids (GPL), sphingolipids, and sterols using electrospray ionization coupled to tandem mass spectrometry of plasma samples obtained from a dietary intervention study. Healthy individuals received foods supplemented with different vegetable oils rich in PUFA. These included sunflower, linseed, echium, and microalgae oil as sources of linoleic acid (LA; FA 18:2 n-6), alpha-linolenic acid (ALA; FA 18:3 n-3), stearidonic acid (SDA; FA 18:4 n-3), and docosahexaenoic acid (DHA; FA 22:6 n-3). While LA and ALA did not influence the species profiles of GPL, sphingolipid, and cholesteryl ester drastically, SDA and DHA were integrated primarily in ethanolamine-containing GPL. This significantly altered phosphatidylethanolamine and plasmalogen species composition, especially those with 38-40 carbons and 6 double bonds. We speculate that diets enriched with highly unsaturated FA more efficiently alter plasma GPL acyl chain composition than those containing primarily di- and tri-unsaturated FA, most likely because of their more pronounced deviation of FA composition from typical western diets.