The series of conformational states adopted by rotorless F1-ATPase during its hydrolysis cycle.

F1Fo ATP synthase interchanges phosphate transfer energy and proton motive force via a rotary catalytic mechanism and isolated F1-ATPase subcomplexes can also hydrolyze ATP to generate rotation of their central γ rotor subunit. As ATP is hydrolyzed, the F1-ATPase cycles through a series of conformational states that mediates unidirectional rotation of the rotor. However, even in the absence of a rotor, the α and ß subunits are still able to pass through a series of conformations, akin to those that generate rotation. Here, we use cryoelectron microscopy to establish the structures of these rotorless states. These structures indicate that cooperativity in this system is likely mediated by contacts between the ß subunit lever domains, irrespective of the presence of the γ rotor subunit. These findings provide insight into how long-range information may be transferred in large biological systems.

Structural basis of promiscuous substrate transport by Organic Cation Transporter 1.

Organic Cation Transporter 1 (OCT1) plays a crucial role in hepatic metabolism by mediating the uptake of a range of metabolites and drugs. Genetic variations can alter the efficacy and safety of compounds transported by OCT1, such as those used for cardiovascular, oncological, and psychological indications. Despite its importance in drug pharmacokinetics, the substrate selectivity and underlying structural mechanisms of OCT1 remain poorly understood. Here, we present cryo-EM structures of full-length human OCT1 in the inward-open conformation, both ligand-free and drug-bound, indicating the basis for its broad substrate recognition. Comparison of our structures with those of outward-open OCTs provides molecular insight into the alternating access mechanism of OCTs. We observe that hydrophobic gates stabilize the inward-facing conformation, whereas charge neutralization in the binding pocket facilitates the release of cationic substrates. These findings provide a framework for understanding the structural basis of the promiscuity of drug binding and substrate translocation in OCT1.

Broadly neutralizing SARS-CoV-2 antibodies through epitope-based selection from convalescent patients.

Emerging variants of concern (VOCs) are threatening to limit the effectiveness of SARS-CoV-2 monoclonal antibodies and vaccines currently used in clinical practice; broadly neutralizing antibodies and strategies for their identification are therefore urgently required. Here we demonstrate that broadly neutralizing antibodies can be isolated from peripheral blood mononuclear cells of convalescent patients using SARS-CoV-2 receptor binding domains carrying epitope-specific mutations. This is exemplified by two human antibodies, GAR05, binding to epitope class 1, and GAR12, binding to a new epitope class 6 (located between class 3 and 5). Both antibodies broadly neutralize VOCs, exceeding the potency of the clinical monoclonal sotrovimab (S309) by orders of magnitude. They also provide prophylactic and therapeutic in vivo protection of female hACE2 mice against viral challenge. Our results indicate that exposure to SARS-CoV-2 induces antibodies that maintain broad neutralization against emerging VOCs using two unique strategies: either by targeting the divergent class 1 epitope in a manner resistant to VOCs (ACE2 mimicry, as illustrated by GAR05 and mAbs P2C-1F11/S2K14); or alternatively, by targeting rare and highly conserved epitopes, such as the new class 6 epitope identified here (as illustrated by GAR12). Our results provide guidance for next generation monoclonal antibody development and vaccine design.

Changes within the central stalk of E. coli F1Fo ATP synthase observed after addition of ATP.

F1Fo ATP synthase functions as a biological generator and makes a major contribution to cellular energy production. Proton flow generates rotation in the Fo motor that is transferred to the F1 motor to catalyze ATP production, with flexible F1/Fo coupling required for efficient catalysis. F1Fo ATP synthase can also operate in reverse, hydrolyzing ATP and pumping protons, and in bacteria this function can be regulated by an inhibitory ε subunit. Here we present cryo-EM data showing E. coli F1Fo ATP synthase in different rotational and inhibited sub-states, observed following incubation with 10 mM MgATP. Our structures demonstrate how structural transitions within the inhibitory ε subunit induce torsional movement in the central stalk, thereby enabling its rotation within the Fο motor. This highlights the importance of the central rotor for flexible coupling of the F1 and Fo motors and provides further insight into the regulatory mechanism mediated by subunit ε.

Mechanism of transcription modulation by the transcription-repair coupling factor.

Elongation by RNA polymerase is dynamically modulated by accessory factors. The transcription-repair coupling factor (TRCF) recognizes paused/stalled RNAPs and either rescues transcription or initiates transcription termination. Precisely how TRCFs choose to execute either outcome remains unclear. With Escherichia coli as a model, we used single-molecule assays to study dynamic modulation of elongation by Mfd, the bacterial TRCF. We found that nucleotide-bound Mfd converts the elongation complex (EC) into a catalytically poised state, presenting the EC with an opportunity to restart transcription. After long-lived residence in this catalytically poised state, ATP hydrolysis by Mfd remodels the EC through an irreversible process leading to loss of the RNA transcript. Further, biophysical studies revealed that the motor domain of Mfd binds and partially melts DNA containing a template strand overhang. The results explain pathway choice determining the fate of the EC and provide a molecular mechanism for transcription modulation by TRCF.

The long and the short of Huntington's disease: how the sphingolipid profile is shifted in the caudate of advanced clinical cases.

Huntington's disease is a devastating neurodegenerative disorder that onsets in late adulthood as progressive and terminal cognitive, psychiatric and motor deficits. The disease is genetic, triggered by a CAG repeat (polyQ) expansion mutation in the Huntingtin gene and resultant huntingtin protein. Although the mutant huntingtin protein is ubiquitously expressed, the striatum degenerates early and consistently in the disease. The polyQ mutation at the N-terminus of the huntingtin protein alters its natural interactions with neural phospholipids in vitro, suggesting that the specific lipid composition of brain regions could influence their vulnerability to interference by mutant huntingtin; however, this has not yet been demonstrated in vivo. Sphingolipids are critical cell signalling molecules, second messengers and membrane components. Despite evidence of sphingolipid disturbance in Huntington's mouse and cell models, there is limited knowledge of how these lipids are affected in human brain tissue. Using post-mortem brain tissue from five brain regions implicated in Huntington's disease (control n = 13, Huntington's n = 13), this study aimed to identify where and how sphingolipid species are affected in the brain of clinically advanced Huntington's cases. Sphingolipids were extracted from the tissue and analysed using targeted mass spectrometry analysis; proteins were analysed by western blot. The caudate, putamen and cerebellum had distinct sphingolipid changes in Huntington's brain whilst the white and grey frontal cortex were spared. The caudate of Huntington's patients had a shifted sphingolipid profile, favouring long (C13-C21) over very-long-chain (C22-C26) ceramides, sphingomyelins and lactosylceramides. Ceramide synthase 1, which synthesizes the long-chain sphingolipids, had a reduced expression in Huntington's caudate, correlating positively with a younger age at death and a longer CAG repeat length of the Huntington's patients. The expression of ceramide synthase 2, which synthesizes very-long-chain sphingolipids, was not different in Huntington's brain. However, there was evidence of possible post-translational modifications in the Huntington's patients only. Post-translational modifications to ceramide synthase 2 may be driving the distinctive sphingolipid profile shifts of the caudate in advanced Huntington's disease. This shift in the sphingolipid profile is also found in the most severely affected brain regions of several other neurodegenerative conditions and may be an important feature of region-specific cell dysfunction in neurodegenerative disease.

Analysis of sex-specific lipid metabolism of Plasmodium falciparum points to the importance of sphingomyelin for gametocytogenesis.

Male and female Plasmodium falciparum gametocytes are the parasite lifecycle stage responsible for transmission of malaria from the human host to the mosquito vector. Not only are gametocytes able to survive in radically different host environments, but they are also precursors for male and female gametes that reproduce sexually soon after ingestion by the mosquito. Here, we investigate the sex-specific lipid metabolism of gametocytes within their host red blood cell. Comparison of the male and female lipidome identifies cholesteryl esters and dihydrosphingomyelin enrichment in female gametocytes. Chemical inhibition of each of these lipid types in mature gametocytes suggests dihydrosphingomyelin synthesis but not cholesteryl ester synthesis is important for gametocyte viability. Genetic disruption of each of the two sphingomyelin synthase genes points towards sphingomyelin synthesis contributing to gametocytogenesis. This study shows that gametocytes are distinct from asexual stages, and that the lipid composition is also vastly different between male and female gametocytes, reflecting the different cellular roles these stages play. Taken together, our results highlight the sex-specific nature of gametocyte lipid metabolism, which has the potential to be targeted to block malaria transmission. This article has an associated First Person interview with the first author of the paper.

Small angle X-ray scattering analysis of ligand-bound forms of tetrameric apolipoprotein-D.

Human apolipoprotein-D (apoD) is a glycosylated lipocalin that plays a protective role in Alzheimer's disease due to its antioxidant function. Native apoD from human body fluids forms oligomers, predominantly a stable tetramer. As a lipocalin, apoD binds and transports small hydrophobic molecules such as progesterone, palmitic acid and sphingomyelin. Oligomerisation is a common trait in the lipocalin family and is affected by ligand binding in other lipocalins. The crystal structure of monomeric apoD shows no major changes upon progesterone binding. Here, we used small-angle X-ray scattering (SAXS) to investigate the influence of ligand binding and oxidation on apoD oligomerisation and conformation. As a solution-based technique, SAXS is well suited to detect changes in oligomeric state and conformation in response to ligand binding. Our results show no change in oligomeric state of apoD and no major conformational changes or subunit rearrangements in response to binding of ligands or protein oxidation. This highlights the highly stable structure of the native apoD tetramer under various physiologically relevant experimental conditions.

Molecular basis for RNA polymerase-dependent transcription complex recycling by the helicase-like motor protein HelD.

In bacteria, transcription complexes stalled on DNA represent a major source of roadblocks for the DNA replication machinery that must be removed in order to prevent damaging collisions. Gram-positive bacteria contain a transcription factor HelD that is able to remove and recycle stalled complexes, but it was not known how it performed this function. Here, using single particle cryo-electron microscopy, we have determined the structures of Bacillus subtilis RNA polymerase (RNAP) elongation and HelD complexes, enabling analysis of the conformational changes that occur in RNAP driven by HelD interaction. HelD has a 2-armed structure which penetrates deep into the primary and secondary channels of RNA polymerase. One arm removes nucleic acids from the active site, and the other induces a large conformational change in the primary channel leading to removal and recycling of the stalled polymerase, representing a novel mechanism for recycling transcription complexes in bacteria.

Cholesteryl ester levels are elevated in the caudate and putamen of Huntington's disease patients.

Huntington's disease (HD) is an autosomal dominant neurodegenerative illness caused by a mutation in the huntingtin gene (HTT) and subsequent protein (mhtt), to which the brain shows a region-specific vulnerability. Disturbances in neural cholesterol metabolism are established in HD human, murine and cell studies; however, cholesteryl esters (CE), which store and transport cholesterol in the brain, have not been investigated in human studies. This study aimed to identify region-specific alterations in the concentrations of CE in HD. The Victorian Brain Bank provided post-mortem tissue from 13 HD subjects and 13 age and sex-matched controls. Lipids were extracted from the caudate, putamen and cerebellum, and CE were quantified using targeted mass spectrometry. ACAT 1 protein expression was measured by western blot. CE concentrations were elevated in HD caudate and putamen compared to controls, with the elevation more pronounced in the caudate. No differences in the expression of ACAT1 were identified in the striatum. No remarkable differences in CE were detected in HD cerebellum. The striatal region-specific differences in CE profiles indicate functional subareas of lipid disturbance in HD. The increased CE concentration may have been induced as a compensatory mechanism to reduce cholesterol accumulation.

Distinct adaptations of a gametocyte ABC transporter to murine and human Plasmodium parasites and its incompatibility in cross-species complementation.

Parasites of the genus Plasmodium infect a wide range of mammalian hosts including humans, primates, bats and arboreal rodents. A hallmark of Plasmodium spp. is the very narrow host range, indicative of matching parasite-host coevolution. Accordingly, their respective genomes harbour many unique genes and gene families that typically encode proteins involved in host cell recognition and remodelling. Whether and to what extent conserved proteins that are shared across Plasmodium spp. also exert distinct species-specific roles remains largely untested. Here, we present detailed functional profiling of the female gametocyte-specific ATP-binding cassette transporter gABCG2 in the murine parasite Plasmodium berghei and compare our findings with data from the orthologous gene in the human parasite Plasmodium falciparum. We show that P. berghei gABCG2 is female-specific and continues to be expressed in zygotes and ookinetes. In contrast to a distinct localization to Iipid-rich gametocyte-specific spots as observed in P. falciparum, the murine malaria parasite homolog is found at the parasite plasma membrane. Plasmodium berghei lacking gABCG2 displays fast asexual blood-stage replication and increased proportions of female gametocytes, consistent with the corresponding P. falciparum knock-out phenotype. Strikingly, cross-species replacement of gABCG2 in either the murine or the human parasite did not restore normal growth rates. The lack of successful complementation despite high conservation across Plasmodium spp. is an indicator of distinct adaptations and tight parasite-host coevolution. Hence, incompatibility of conserved genes in closely related Plasmodium spp. might be more common than previously anticipated.

Honey bee caste lipidomics in relation to life-history stage and the long life of the queen.

Honey bees have evolved a system in which fertilised eggs transit through the same developmental stages but can become either workers or queens. This difference is determined by their diet through development. Whereas workers live for weeks (normally 2-6 weeks), queens can live for years. Unfertilised eggs also develop through the same stages but result in a short-lived male caste (drones). Workers and drones are fed pollen throughout their late larval and adult life stages, while queens are fed exclusively on royal jelly and do not eat pollen. Pollen has a high content of polyunsaturated fatty acids (PUFA) while royal jelly has a negligible amount of PUFA. To investigate the role of dietary PUFA lipids and their oxidation in the longevity difference of honey bees, membrane fatty acid composition of the three castes was characterised at six different life-history stages (larva, pupa, emergent and different adult stages) through mass spectrometry. All castes were found to share a similar membrane phospholipid composition during early larval development. However, at pupation, drones and workers increased their level of PUFA, whilst queens increased their level of monounsaturated fatty acids. After emergence, worker bees further increased their level of PUFA by 5-fold across most phospholipid classes. In contrast, the membrane phospholipids of adult queens remained highly monounsaturated throughout their adult life. We postulate that this diet-induced increase in membrane PUFA results in more oxidative damage and is potentially responsible for the much shorter lifespan of worker bees compared with long-lived queens.

Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84.

Fatty acid receptors have been recognized as important players in glycaemic control. This study is the first to describe a role for the medium-chain fatty acid (MCFA) receptor G-protein-coupled receptor (Gpr) 84 in skeletal muscle mitochondrial function and insulin secretion. We are able to show that Gpr84 is highly expressed in skeletal muscle and adipose tissue. Mice with global deletion of Gpr84 [Gpr84 knockout (KO)] exhibit a mild impairment in glucose tolerance when fed a MCFA-enriched diet. Studies in mice and pancreatic islets suggest that glucose intolerance is accompanied by a defect in insulin secretion. MCFA-fed KO mice also exhibit a significant impairment in the intrinsic respiratory capacity of their skeletal muscle mitochondria, but at the same time also exhibit a substantial increase in mitochondrial content. Changes in canonical pathways of mitochondrial biogenesis and turnover are unable to explain these mitochondrial differences. Our results show that Gpr84 plays a crucial role in regulating mitochondrial function and quality control.-Montgomery, M. K., Osborne, B., Brandon, A. E., O'Reilly, L., Fiveash, C. E., Brown, S. H. J., Wilkins, B. P., Samsudeen, A., Yu, J., Devanapalli, B., Hertzog, A., Tolun, A. A., Kavanagh, T., Cooper, A. A., Mitchell, T. W., Biden, T. J., Smith, N. J., Cooney, G. J., Turner, N. Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84.

HDX-MS reveals orthosteric and allosteric changes in apolipoprotein-D structural dynamics upon binding of progesterone.

Apolipoprotein-D is a glycosylated tetrameric lipocalin that binds and transports small hydrophobic molecules such as progesterone and arachidonic acid. Like other lipocalins, apolipoprotein-D adopts an eight-stranded ß-barrel fold stabilized by two intramolecular disulphide bonds, with an adjacent α-helix. Crystallography studies of recombinant apolipoprotein-D demonstrated no major conformational changes upon progesterone binding. Amide hydrogen-deuterium exchange mass spectrometry (HDX-MS) reports structural changes of proteins in solution by monitoring exchange of amide hydrogens in the protein backbone with deuterium. HDX-MS detects changes in conformation and structural dynamics in response to protein function such as ligand binding that may go undetected in X-ray crystallography, making HDX-MS an invaluable orthogonal technique. Here, we report an HDX-MS protocol for apolipoprotein-D that solved challenges of high protein rigidity and low pepsin cleavage using rigorous quenching conditions and longer deuteration times, yielding 85% sequence coverage and 50% deuterium exchange. The relative fractional deuterium exchange of ligand-free apolipoprotein-D revealed apolipoprotein-D to be a highly structured protein. Progesterone binding was detected by significant reduction in deuterium exchange in eight peptides. Stabilization of apolipoprotein-D dynamics can be interpreted as a combined orthosteric effect in the ligand binding pocket and allosteric effect at the N-terminus and C-terminus. Together, our experiments provide insight into apolipoprotein-D structural dynamics and map the effects of progesterone binding that are relayed to distal parts of the protein. The observed stabilization of apolipoprotein-D dynamics upon progesterone binding demonstrates a common behaviour in the lipocalin family and may have implications for interactions of apolipoprotein-D with receptors or lipoprotein particles. Statement: We reveal for the first time how apolipoprotein-D, which is protective in Alzheimer's disease, becomes more ordered when bound to a molecule of steroid hormone. These results significantly extend the understanding of apolipoprotein-D structure from X-ray crystallography studies by incorporating information on how protein motion changes over time. To achieve these results an improved protocol was developed, suitable for proteins similar to apolipoprotein-D, to elucidate how proteins change flexibility when binding to small molecules.

A High-Throughput Method for the Analysis of Erythrocyte Fatty Acids and the Omega-3 Index.

Omega-3 long-chain polyunsaturated fatty acids (n-3 LCPUFA) have several health benefits. In particular, low n-3 LCPUFA status is associated with cardiovascular disease (CVD) and led to the development of the omega-3 index that is the proportion of eicosapentaenoic acid and docosahexaenoic acid in the erythrocyte membranes, as a marker of CVD risk. Most methods used to measure the omega-3 index are laborious and time consuming. Therefore, the aim of this study was to develop a high-throughput method for the extraction and measurement of erythrocyte fatty acids and the omega-3 index. For sample extraction and quantification, two methods were used; a single-step extraction, degradation, and derivatization method by Lepage and Roy, followed by gas chromatography flame ionization detection (GC-FID), which is commonly used and a high-throughput method using an automated methyl tert-butyl ether extraction followed by electrospray ionization mass spectrometry. Both methods were first applied to the analysis of known concentrations of synthetic phospholipid (PL) mixtures to determine recovery and precision prior to their application in the analysis of human erythrocytes. The range of recoveries over five synthetic PL mixtures were 86.4-108.9% and the coefficient of variation was <10% (within-run) and ≤15.2% (between-run). Both methods showed high correlation (R = 0.993) for the omega-3 index and there was no systematic bias in the detection of omega-3 index using either method. The new high-throughput method described here offers considerable advantages in terms of simplicity and throughput compared to the GC-FID method and provides additional information on molecular PL concentrations.

Increasing Acyl CoA thioesterase activity alters phospholipid profile without effect on insulin action in skeletal muscle of rats.

Increased lipid metabolism in muscle is associated with insulin resistance and therefore, many strategies have been employed to alter fatty acid metabolism and study the impact on insulin action. Metabolism of fatty acid requires activation to fatty acyl CoA by Acyl CoA synthases (ACSL) and fatty acyl CoA can be hydrolysed by Acyl CoA thioesterases (Acot). Thioesterase activity is low in muscle, so we overexpressed Acot7 in muscle of chow and high-fat diet (HFD) rats and investigated effects on insulin action. Acot7 overexpression modified specific phosphatidylcholine and phosphatidylethanolamine species in tibialis muscle of chow rats to levels similar to those observed in control HFD muscle. The changes in phospholipid species did not alter glucose uptake in tibialis muscle under hyperinsulinaemic/euglycaemic clamped conditions. Acot7 overexpression in white extensor digitorum longus (EDL) muscle increased complete fatty acid oxidation ex-vivo but was not associated with any changes in glucose uptake in-vivo, however overexpression of Acot7 in red EDL reduced insulin-stimulated glucose uptake in-vivo which correlated with increased incomplete fatty acid oxidation ex-vivo. In summary, although overexpression of Acot7 in muscle altered some aspects of lipid profile and metabolism in muscle, this had no major effect on insulin-stimulated glucose uptake.

Identification of a novel tetrameric structure for human apolipoprotein-D.

Apolipoprotein-D is a 25 kDa glycosylated member of the lipocalin family that folds into an eight-stranded ß-barrel with a single adjacent α-helix. Apolipoprotein-D specifically binds a range of small hydrophobic ligands such as progesterone and arachidonic acid and has an antioxidant function that is in part due to the reduction of peroxidised lipids by methionine-93. Therefore, apolipoprotein-D plays multiple roles throughout the body and is protective in Alzheimer's disease, where apolipoprotein-D overexpression reduces the amyloid-ß burden in Alzheimer's disease mouse models. Oligomerisation is a common feature of lipocalins that can influence ligand binding. The native structure of apolipoprotein-D, however, has not been conclusively defined. Apolipoprotein-D is generally described as a monomeric protein, although it dimerises when reducing peroxidised lipids. Here, we investigated the native structure of apolipoprotein-D derived from plasma, breast cyst fluid (BCF) and cerebrospinal fluid. In plasma and cerebrospinal fluid, apolipoprotein-D was present in high-molecular weight complexes, potentially in association with lipoproteins. In contrast, apolipoprotein-D in BCF formed distinct oligomeric species. We assessed apolipoprotein-D oligomerisation using native apolipoprotein-D purified from BCF and a suite of complementary methods, including multi-angle laser light scattering, analytical ultracentrifugation and small-angle X-ray scattering. Our analyses showed that apolipoprotein-D predominantly forms a ∼95 to ∼100 kDa tetramer. Small-angle X-ray scattering analysis confirmed these findings and provided a structural model for apolipoprotein-D tetramer. These data indicate apolipoprotein-D rarely exists as a free monomer under physiological conditions and provide insights into novel native structures of apolipoprotein-D and into oligomerisation behaviour in the lipocalin family.

Mass spectrometry-directed structure elucidation and total synthesis of ultra-long chain (O-acyl)-ω-hydroxy fatty acids.

The (O-acyl)-ω-hydroxy FAs (OAHFAs) comprise an unusual lipid subclass present in the skin, vernix caseosa, and meibomian gland secretions. Although they are structurally related to the general class of FA esters of hydroxy FAs (FAHFAs), the ultra-long chain (30-34 carbons) and the putative ω-substitution of the backbone hydroxy FA suggest that OAHFAs have unique biochemistry. Complete structural elucidation of OAHFAs has been challenging because of their low abundance within complex lipid matrices. Furthermore, because these compounds occur as a mixture of closely related isomers, insufficient spectroscopic data have been obtained to guide structure confirmation by total synthesis. Here, we describe the full molecular structure of ultra-long chain OAHFAs extracted from human meibum by exploiting the gas-phase purification of lipids through multi-stage MS and novel multidimensional ion activation methods. The analysis elucidated sites of unsaturation, the stereochemical configuration of carbon-carbon double bonds, and ester linkage regiochemistry. Such isomer-resolved MS guided the first total synthesis of an ultra-long chain OAHFA, which, in turn, confirmed the structure of the most abundant OAHFA found in human meibum, OAHFA 50:2. The availability of a synthetic OAHFA opens new territory for future investigations into the unique biophysical and biochemical properties of these lipids.

Association of muscle lipidomic profile with high-fat diet-induced insulin resistance across five mouse strains.

Different mouse strains exhibit variation in their inherent propensities to develop metabolic disease. We recently showed that C57BL6, 129X1, DBA/2 and FVB/N mice are all susceptible to high-fat diet-induced glucose intolerance, while BALB/c mice are relatively protected, despite changes in many factors linked with insulin resistance. One parameter strongly linked with insulin resistance is ectopic lipid accumulation, especially metabolically active ceramides and diacylglycerols (DAG). This study examined diet-induced changes in the skeletal muscle lipidome across these five mouse strains. High-fat feeding increased total muscle triacylglycerol (TAG) content, with elevations in similar triacylglycerol species observed for all strains. There were also generally consistent changes across strains in the abundance of different phospholipid (PL) classes and the fatty acid profile of phospholipid molecular species, with the exception being a strain-specific difference in phospholipid species containing two polyunsaturated fatty acyl chains in BALB/c mice (i.e. a diet-induced decrease in the other four strains, but no change in BALB/c mice). In contrast to TAG and PL, the high-fat diet had a minor influence on DAG and ceramide species across all strains. These results suggest that widespread alterations in muscle lipids are unlikely a major contributors to the favourable metabolic profile of BALB/c mice and rather there is a relatively conserved high-fat diet response in muscle of most mouse strains.

Sterol Analysis by Quantitative Mass Spectrometry.

Analysis of sterols by mass spectrometry is a fundamental technique allowing for both qualitative and quantitative characterization of sterol molecular lipid species. Lipids are isolated from matrix or matrices by homogenization and solvent extraction, and converted into species amenable for ionization either by derivatization or adduct formation. Chromatogaphy (either gas or liquid phase) can assist with the resolution of sterols. Tandem mass spectrometry allows the precise identification of sterol lipid species, while comparison to internal standards added during extraction enables accurate quantification.