Identification of two novel human acyl-CoA wax alcohol acyltransferases: members of the diacylglycerol acyltransferase 2 (DGAT2) gene superfamily

A spatial portrait of the human sebaceous gland transcriptional program

Sebaceous glands (SG) and their oily secretion (sebum) are indispensable for maintaining skin structure and function, and their deregulation causes skin disorders including but not limited to acne. Recent studies also indicate that sebum may have important immunomodulatory activities and may influence whole-body energy metabolism. However, the progressive transcriptional changes of sebocytes that lead to sebum production have never been characterized in detail. Here, we exploited the high cellular resolution provided by sebaceous hyperplasia and integrated spatial transcriptomics, pseudo time analysis, RNA velocity, and functional enrichment to map the landscape of sebaceous differentiation. Our results were validated by comparison with published SG transcriptome data and further corroborated by assessing the protein expression pattern of a subset of the transcripts in the public repository Human Protein Atlas. Departing from four sebocyte differentiation stages generated by unsupervised clustering, we demonstrate consecutive modulation of cellular functions associable with specific gene sets, from cell proliferation and oxidative phosphorylation via lipid synthesis to cell death. Both validation methods confirmed the biological significance of our results. Our report is complemented by a freely available and browsable online tool. Our data provide the first high-resolution spatial portrait of the SG transcriptional landscape and deliver starting points for experimentally assessing novel candidate molecules for regulating SG homeostasis in health and disease.

Drug Targeting of Acyltransferases in the Triacylglyceride and 1-O-AcylCeramide Biosynthetic Pathways

Acyltransferase enzymes (EC 2.3.) are a large group of enzymes that transfer acyl groups to a variety of substrates. This review focuses on fatty acyltransferases involved in the biosynthetic pathways of glycerolipids and sphingolipids and how these enzymes have been pharmacologically targeted in their biologic context. Glycerolipids and sphingolipids, commonly treated independently in their regulation and biologic functions, are put together to emphasize the parallelism in their metabolism and bioactive roles. Furthermore, a newly considered signaling molecule, 1-O-acylceramide, resulting from the acylation of ceramide by DGAT2 enzyme, is discussed. Finally, the implications of DGAT2 as a putative ceramide acyltransferase (CAT) enzyme, with a putative dual role in TAG and 1-O-acylceramide generation, are explored. SIGNIFICANCE STATEMENT: This manuscript reviews the current status of drug development in lipid acyltransferases. These are current targets in metabolic syndrome and other diseases, including cancer. A novel function for a member in this group of lipids has been recently reported in cancer cells. The responsible enzyme and biological implications of this added member are discussed.

Skin single-cell transcriptomics reveals a core of sebaceous gland-relevant genes shared by mice and humans

BACKGROUND

Single-cell RNA sequencing (scRNA-seq) has been widely applied to dissect cellular heterogeneity in normal and diseased skin. Sebaceous glands, essential skin components with established functions in maintaining skin integrity and emerging roles in systemic energy metabolism, have been largely neglected in scRNA-seq studies.

METHODS

Departing from mouse and human skin scRNA-seq datasets, we identified gene sets expressed especially in sebaceous glands with the open-source R-package oposSOM.

RESULTS

The identified gene sets included sebaceous gland-typical genes as Scd3, Mgst1, Cidea, Awat2 and KRT7. Surprisingly, however, there was not a single overlap among the 100 highest, exclusively in sebaceous glands expressed transcripts in mouse and human samples. Notably, both species share a common core of only 25 transcripts, including mitochondrial and peroxisomal genes involved in fatty acid, amino acid, and glucose processing, thus highlighting the intense metabolic rate of this gland.

CONCLUSIONS

This study highlights intrinsic differences in sebaceous lipid synthesis between mice and humans, and indicates an important role for peroxisomal processes in this context. Our data also provides attractive starting points for experimentally addressing novel candidates regulating sebaceous gland homeostasis.

Applications of synthetic yeast consortia for the production of native and non-native chemicals

The application of microbial consortia is a new approach in synthetic biology. Synthetic yeast consortia, simple or complex synthetic mixed cultures, have been used for the production of various metabolites. Cooperation between the members of a consortium and cross-feeding can be applied to create stable microbial communication. These consortia can: consume a variety of substrates, perform more complex functions, produce metabolites in high titer, rate, and yield (TRY), and show higher stability during industrial fermentations. Due to the new research context of synthetic consortia, few yeasts were used to build these consortia, including Saccharomyces cerevisiae, Pichia pastoris, and Yarrowia lipolytica. Here, application of the yeasts for design of synthetic microbial consortia and their advantages and bottlenecks for effective and robust production of valuable metabolites from bioresource, including: cellulose, xylose, glycerol and so on, have been reviewed. Key trends and challenges are also discussed for the future development of synthetic yeast consortia.

Diacylglycerol acyltransferase 2 promotes the adipogenesis of intramuscular preadipocytes in goat

Diacylglycerol acyltransferase 2 (DGAT2) is the key enzyme that catalyzes the last step of triglyceride synthesis. However, its role in intramuscular fat (IMF) deposition in goat remains unclear. The purpose of this study was to explore the role of DGAT2 in regulating goat IMF deposition. In the present study, the expression of DGAT2 was highest in goat triceps brachii, and highest on the first day after oleic acid induction in goat intramuscular preadipocytes. The overexpression of DGAT2 promoted the accumulation of lipid droplets and triglyceride synthesis, accompanied by the expression upregulation of DGAT1, TIP47, ACC and ACOX1 significantly, and expression downregulation of AGPAT6, LPIN1, LPL, HSL, ATGL and ADRP significantly. In contrast, the silencing of DGAT2 decreased the accumulation of lipid droplets, inhibited the expression of DGAT1, GPAM, ADRP, AGPAT6, LPL, HSL, ATGL, ACC, FASN, ACOX1 significantly, and enhanced that of TIP47 significantly. Overall, these data underscore DGAT2 may play a potentially important role in lipid droplets formation and triglyceride accumulation, so as to maintain intramuscular fat deposition, beyond triglyceride synthesis in goat.

Antiadipogenic Effect of Citrus Flavonoids: Evidence from RNA Sequencing Analysis and Activation of AMPK in 3T3-L1 Adipocytes

Citrus fruits are rich in dietary flavonoids and have many health benefits, but their antiadipogenic mechanism of action and their impact on lipid metabolism remain unclear. In this study, we investigated the effect of citrus flavonoids, namely, hesperidin (HES), narirutin (NAR), nobiletin (NOB), sinensetin (SIN), and tangeretin (TAN), on preventing fat cell development by gene expression in 3T3-L1 adipocytes. Among the citrus flavonoids tested, HES and NAR significantly reduced fat storage and triglyceride levels and increased glucose uptake in 3T3-L1 adipocytes. Additionally, HES and NAR treatment increased the phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) while reducing the protein expression of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCR). Furthermore, in silico docking revealed that flavonoids activate AMPK. RNA sequencing analysis demonstrated that citrus flavonoids normalized the expression of 40 genes, which were either upregulated by more than 2-fold or downregulated by less than 0.6-fold including Acadv1, Acly, Akr1d1, Awat1, Cyp27a1, Decr1, Dhrs4, Elovl3, Fasn, G6pc, Gba, Hmgcs1, Mogat2, Lrp5, Sptlc3, and Snca to levels comparable to the control group. Altogether, HES and NAR among five citrus flavonoids showed antiadipogenic effects by regulating the expression of specific lipid metabolism genes partially restored to control levels in 3T3-L1 cells.

Sdr16c5 and Sdr16c6 control a dormant pathway at a bifurcation point between meibogenesis and sebogenesis

Genes Sdr16c5 and Sdr16c6 encode proteins that belong to a superfamily of short-chain dehydrogenases/reductases (SDR16C5 and SDR16C6). Simultaneous inactivation of these genes in double-KO (DKO) mice was previously shown to result in a marked enlargement of the mouse Meibomian glands (MGs) and sebaceous glands, respectively. However, the exact roles of SDRs in physiology and biochemistry of MGs and sebaceous glands have not been established yet. Therefore, we characterized, for the first time, meibum and sebum of Sdr16c5/Sdr16c6-null (DKO) mice using high-resolution MS and LC. In this study, we demonstrated that the mutation upregulated the overall production of MG secretions (also known as meibogenesis) and noticeably altered their lipidomic profile, but had a more subtle effect on sebogenesis. The major changes in meibum of DKO mice included abnormal accumulation of shorter chain, sebaceous-type cholesteryl esters and wax esters (WEs), and a marked increase in the biosynthesis of monounsaturated and diunsaturated Meibomian-type WEs. Importantly, the MGs of DKO mice maintained their ability to produce typical extremely long chain Meibomian-type lipids at seemingly normal levels. These observations indicated preferential activation of a previously dormant biosynthetic pathway that produce shorter chain, and more unsaturated, sebaceous-type WEs in the MGs of DKO mice, without altering the elongation patterns of their extremely long chain Meibomian-type counterparts. We conclude that the Sdr16c5/Sdr16c6 pair may control a point of bifurcation in one of the meibogenesis subpathways at which biosynthesis of lipids can be redirected toward either abnormal sebaceous-type lipidome or normal Meibomian-type lipidome in WT mice.

Decay of Skin-Specific Gene Modules in Pangolins

The mammalian skin exhibits a rich spectrum of evolutionary adaptations. The pilosebaceous unit, composed of the hair shaft, follicle, and the sebaceous gland, is the most striking synapomorphy. The evolutionary diversification of mammals across different ecological niches was paralleled by the appearance of an ample variety of skin modifications. Pangolins, order Pholidota, exhibit keratin-derived scales, one of the most iconic skin appendages. This formidable armor is intended to serve as a deterrent against predators. Surprisingly, while pangolins have hair on their abdomens, the occurrence of sebaceous and sweat glands is contentious. Here, we explore various molecular modules of skin physiology in four pangolin genomes, including that of sebum production. We show that genes driving wax monoester formation, Awat1/2, show patterns of inactivation in the stem pangolin branch, while the triacylglycerol synthesis gene Dgat2l6 seems independently eroded in the African and Asian clades. In contrast, Elovl3 implicated in the formation of specific neutral lipids required for skin barrier function is intact and expressed in the pangolin skin. An extended comparative analysis shows that genes involved in skin pathogen defense and structural integrity of keratinocyte layers also show inactivating mutations: associated with both ancestral and independent pseudogenization events. Finally, we deduce that the suggested absence of sweat glands is not paralleled by the inactivation of the ATP-binding cassette transporter Abcc11, as previously described in Cetacea. Our findings reveal the sophisticated and complex history of gene retention and loss as key mechanisms in the evolution of the highly modified mammalian skin phenotypes.

Cutaneous lesions in psoriatic arthritis are enriched in chemokine transcriptomic pathways

OBJECTIVES

Skin from people with psoriasis has been extensively studied and is assumed to be identical to skin from those with psoriatic arthritis (PsA). Chemokines and the CC chemokine scavenger receptor ACKR2 are upregulated in uninvolved psoriasis. ACKR2 has been proposed as a regulator of cutaneous inflammation in psoriasis. The aim of this study was to compare the transcriptome of PsA skin to healthy control (HC) skin and evaluate ACKR2 expression in PsA skin.

METHODS

Full-thickness skin biopsies from HC, lesional and uninvolved skin from participants with PsA were sequenced on NovaSeq 6000. Findings were validated using qPCR and RNAscope.

RESULTS

Nine HC and nine paired PsA skin samples were sequenced. PsA uninvolved skin was transcriptionally similar to HC skin, and lesional PsA skin was enriched in epidermal and inflammatory genes. Lesional PsA skin was enriched in chemokine-mediated signalling pathways, but uninvolved skin was not. ACKR2 was upregulated in lesional PsA skin but had unchanged expression in uninvolved compared with HC skin. The expression of ACKR2 was confirmed by qPCR, and RNAscope demonstrated strong expression of ACKR2 in the suprabasal layer of the epidermis in PsA lesions.

CONCLUSION

Chemokines and their receptors are upregulated in lesional PsA skin but relatively unchanged in uninvolved PsA skin. In contrast to previous psoriasis studies, ACKR2 was not upregulated in uninvolved PsA skin. Further understanding of the chemokine system in PsA may help to explain why inflammation spreads from the skin to the joints in some people with psoriasis.

Association of Early Clinical Response to Laser Rejuvenation of Photoaged Skin with Increased Lipid Metabolism and Restoration of Skin Barrier Function

Laser resurfacing treatments for photoaged skin have improved dramatically over the past decades, but few studies have examined the molecular mechanisms underlying differences in clinical response. Seventeen white female participants with moderate-to-severe photoaging received nonablative fractional laser treatment on the face and forearm once monthly for 6 months. Biopsies for microarray analysis were performed at baseline and 7 days after facial treatment and at baseline and 1, 7, 14, and 29 days after forearm treatment in each participant, resulting in 119 total samples. Participants were stratified into fast (n = 11) and slow (n = 6) responders on the basis of the presence of clinical improvement after the first treatment. Microarray analysis revealed the upregulation of genes associated with matrix metalloproteinases, collagen and extracellular components, TGF-β signaling, double-stranded RNA signaling, and retinoic acid synthesis after treatment that did not differ significantly between fast and slow responders. Cluster and enrichment analyses suggested significantly greater activation of lipid metabolism and keratinocyte differentiation in fast responders, who showed greater upregulation of acyltransferases, fatty acid elongases, fatty acid 2-hydroxylase, fatty acid desaturases, and specific keratins that may contribute to epidermal barrier function. These results create, to our knowledge, a previously unreported atlas of molecular changes that correlate with improvements in photoaging after laser therapy.

A yeast-based tool for screening mammalian diacylglycerol acyltransferase inhibitors

Dysregulation of lipid metabolism is associated with obesity and metabolic diseases but there is also increasing evidence of a relationship between lipid body excess and cancer. Lipid body synthesis requires diacylglycerol acyltransferases (DGATs) which catalyze the last step of triacylglycerol synthesis from diacylglycerol and acyl-coenzyme A. The DGATs and in particular DGAT2, are therefore considered potential therapeutic targets for the control of these pathologies. Here, the murine and the human DGAT2 were overexpressed in the oleaginous yeast Yarrowia lipolytica deleted for all DGAT activities, to evaluate the functionality of the enzymes in this heterologous host and DGAT activity inhibitors. This work provides evidence that mammalian DGATs expressed in Y. lipolytica are a useful tool for screening chemical libraries to identify potential inhibitors or activators of these enzymes of therapeutic interest.

TREM2 macrophages induced by human lipids drive inflammation in acne lesions

Acne affects 1 in 10 people globally, often resulting in disfigurement. The disease involves excess production of lipids, particularly squalene, increased growth of Cutibacterium acnes, and a host inflammatory response with foamy macrophages. By combining single-cell and spatial RNA sequencing as well as ultrahigh-resolution Seq-Scope analyses of early acne lesions on back skin, we identified TREM2 macrophages expressing lipid metabolism and proinflammatory gene programs in proximity to hair follicle epithelium expressing squalene epoxidase. We established that the addition of squalene induced differentiation of TREM2 macrophages in vitro, which were unable to kill C. acnes. The addition of squalene to macrophages inhibited induction of oxidative enzymes and scavenged oxygen free radicals, providing an explanation for the efficacy of topical benzoyl peroxide in the clinical treatment of acne. The present work has elucidated the mechanisms by which TREM2 macrophages and unsaturated lipids, similar to their involvement in atherosclerosis, may contribute to the pathogenesis of acne.

Effects of golden tomato extract on skin appearance-outlook into gene expression in cultured dermal fibroblasts and on trans-epidermal water loss and skin barrier in human subjects

SCOPE

Two experiments were performed to test the effects of rich tomato extract (Golden Tomato Extract, GTE) on human skin. In one experiment, the effects of this extract on gene expression in cultured human dermal fibroblasts were examined. In a second experiment, human subjects consumed the extract and trans-epidermal water loss (TEWL), and aspects of skin appearance were monitored.

METHODS AND RESULTS

Primary human dermal fibroblasts in culture were treated with the extract. After six hours, RNA was extracted, and gene expression was examined using Affymetrix Human Clariom D array processing. For the clinical study, 65 human subjects consumed a capsule once a day for 16 weeks, and various skin parameters were assessed at predetermined time intervals. Among the genes upregulated by GTE are genes that augment innate immunity, enhance DNA repair, and the ability to detoxify xenobiotics. GTE significantly reduced TEWL in subjects who had high TEWL at baseline, but it had no effect on TEWL in subjects who had lower TEWL at baseline.

CONCLUSIONS

Golden tomato extract may provide benefits to the skin by enhancing innate immunity and other defense mechanisms in the dermis and by providing antioxidants to the skin surface to optimize TEWL and the appearance of the skin.

Acyl-CoA:wax alcohol acyltransferase 2 modulates the cone visual cycle in mouse retina

The daylight and color vision of diurnal vertebrates depends on cone photoreceptors. The capability of cones to operate and respond to changes in light brightness even under high illumination is attributed to their fast rate of recovery to the ground photosensitive state. This process requires the rapid replenishing of photoisomerized visual chromophore (11-cis-retinal) to regenerate cone visual pigments. Recently, several gene candidates have been proposed to contribute to the cone-specific retinoid metabolism, including acyl-CoA wax alcohol acyltransferase 2 (AWAT2, aka MFAT). Here, we evaluated the role of AWAT2 in the regeneration of visual chromophore by the phenotypic characterization of Awat2^-/- mice. The global absence of AWAT2 enzymatic activity did not affect gross retinal morphology or the rate of visual chromophore regeneration by the canonical RPE65-dependent visual cycle. Analysis of Awat2 expression indicated the presence of the enzyme throughout the murine retina, including the retinal pigment epithelium (RPE) and Müller cells. Electrophysiological recordings revealed reduced maximal rod and cone dark-adapted responses in AWAT2-deficient mice compared to control mice. While rod dark adaptation was not affected by the lack of AWAT2, M-cone dark adaptation both in isolated retina and in vivo was significantly suppressed. Altogether, these results indicate that while AWAT2 is not required for the normal operation of the canonical visual cycle, it is a functional component of the cone-specific visual chromophore regenerative pathway.

The production of wax esters in transgenic plants: towards a sustainable source of bio-lubricants

Wax esters are high-value compounds used as feedstocks for the production of lubricants, pharmaceuticals, and cosmetics. Currently, they are produced mostly from fossil reserves using chemical synthesis, but this cannot meet increasing demand and has a negative environmental impact. Natural wax esters are also obtained from Simmondsia chinensis (jojoba) but comparably in very low amounts and expensively. Therefore, metabolic engineering of plants, especially of the seed storage lipid metabolism of oil crops, represents an attractive strategy for renewable, sustainable, and environmentally friendly production of wax esters tailored to industrial applications. Utilization of wax ester-synthesizing enzymes with defined specificities and modulation of the acyl-CoA pools by various genetic engineering approaches can lead to obtaining wax esters with desired compositions and properties. However, obtaining high amounts of wax esters is still challenging due to their negative impact on seed germination and yield. In this review, we describe recent progress in establishing non-food-plant platforms for wax ester production and discuss their advantages and limitations as well as future prospects.

The Studies in Constructing Yeast Cell Factories for the Production of Fatty Acid Alkyl Esters

Fatty acid alkyl esters have broad applications in biofuels, lubricant formulas, paints, coatings, and cosmetics. Traditionally, these esters are mostly produced through unsustainable and energy-intensive processes. In contrast, microbial production of esters from renewable and sustainable feedstocks may provide a promising alternative and has attracted widespread attention in recent years. At present, yeasts are used as ideal hosts for producing such esters, due to their availability for high-density fermentation, resistance to phage infection, and tolerance against toxic inhibitors. Here, we summarize recent development on the biosynthesis of alkyl esters, including fatty acid ethyl esters (FAEEs), fatty acid short-branched chain alkyl esters (FASBEs), and wax esters (WEs) by various yeast cell factories. We focus mainly on the enzyme engineering strategies of critical wax ester synthases, and the pathway engineering strategies employed for the biosynthesis of various ester products. The bottlenecks that limit productivity and their potential solutions are also discussed in this review.

Expression of Acyl-CoA wax-alcohol acyltransferase 2 (AWAT2) by human and rabbit meibomian glands and meibocytes

PURPOSE

Previously, we showed that Acyl-CoA wax-alcohol acyltransferase 2 (AWAT2), an essential enzyme required for meibum wax ester synthesis, was not expressed by immortalized human meibomian gland epithelial cells (hMGEC) in culture. To begin to understand the mechanisms controlling AWAT2 expression, we have analyzed its expression in human and rabbit meibomian glands and cultured meibocytes.

METHODS

Rabbit meibocyte progenitor cells (rMPC) were first grown in Cnt-BM.1 basal medium (Cellntec) supplemented with rhEGF, FGF10, and ROCK inhibitor (Y-27632 dihydrochloride), and then passed at 70-80% confluency with Accutase. Differentiation of rMPC to meibocytes (rMC) was induced by removal of Y-27632 and addition of 1 mM calcium with and without PPARγ agonists. RNA from the tissue, primary, passaged rMPC and differentiated rMC were obtained for AWAT2 qPCR analysis. Proteins and cells were evaluated for western blotting and neutral lipid synthesis, respectively. For comparison, human meibomian glands were separated for RNA and protein analysis. hMGEC was cultured to collect RNA and protein.

RESULTS

Rabbit rMPCs were successfully grown, passaged, and differentiated, showing a significant increase in lipid droplet accumulation. AWAT2 RNA was highly expressed in tissue but showed a -16.9 log2 fold decrease in primary and passaged rMPCs and was not induced by differentiation to rMC. By comparison, human meibomian glands showed high expression of AWAT2, and hMGEC expressed non-detectable levels of AWAT2 transcripts or protein.

CONCLUSIONS

AWAT2 expression is lost in cultured rMPC and rMC suggesting that cells in culture do not undergo complete meibocyte differentiation and require yet to be identified culture conditions.

Branched and linear fatty acid esters of hydroxy fatty acids (FAHFA) relevant to human health

Fatty acid esters of hydroxy fatty acids (FAHFAs) represent a complex lipid class that contains both signaling mediators and structural components of lipid biofilms in humans. The majority of endogenous FAHFAs share a common chemical architecture, characterized by an estolide bond that links the hydroxy fatty acid (HFA) backbone and the fatty acid (FA). Two structurally and functionally distinct FAHFA superfamilies are recognized based on the position of the estolide bond: omega-FAHFAs and in-chain branched FAHFAs. The existing variety of possible HFAs and FAs combined with the position of the estolide bond generates a vast quantity of unique structures identified in FAHFA families. In this review, we discuss the anti-diabetic and anti-inflammatory effects of branched FAHFAs and the role of omega-FAHFA-derived lipids as surfactants in the tear film lipid layer and dry eye disease. To emphasize potential pharmacological targets, we recapitulate the biosynthesis of the HFA backbone within the superfamilies together with the degradation pathways and the FAHFA regioisomer distribution in human and mouse adipose tissue. We propose a theoretical involvement of cytochrome P450 enzymes in the generation and degradation of saturated HFA backbones and present an overview of small-molecule inhibitors used in FAHFA research. The FAHFA lipid class is huge and largely unexplored. Besides the unknown biological effects of individual FAHFAs, also the enigmatic enzymatic machinery behind their synthesis could provide new therapeutic approaches for inflammatory metabolic or eye diseases. Therefore, understanding the mechanisms of (FA)HFA synthesis at the molecular level should be the next step in FAHFA research.

Evolutionary and biochemical characterization of a Chromochloris zofingiensis MBOAT with wax synthase and diacylglycerol acyltransferase activity

Wax synthase (WS) catalyzes the last step in wax ester biosynthesis in green plants. Two unrelated sub-families of WS, including the bifunctional acyltransferase and plant-like WS have been reported, but the latter is largely uncharacterized in microalgae. Here, we functionally characterized a putative plant-like WS (CzWS1) from the emerging model green microalga Chromochloris zofingiensis. Our results showed that plant-like WS evolved under different selection constraints in plants and microalgae, with positive selection likely contributing to functional divergence. Unlike jojoba with high amounts of wax ester in seeds and a highly active WS enzyme, C. zofingiensis has no detectable wax ester but a high abundance of WS transcripts. Co-expression analysis showed that C. zofingiensis WS has different expression correlation with lipid biosynthetic genes from jojoba, and may have a divergent function. In vitro characterization indicated that CzWS1 had diacylglycerol acyltransferase activity along with WS activity, and overexpression of CzWS1 in yeast and Chlamydomonas reinhardtii affected triacylglycerol accumulation. Moreover, biochemical and bioinformatic analyses revealed the relevance of the C-terminal region of CzWS1 in enzyme function. Taken together, our results indicated a functional divergence of plant-like WS in plants and microalgae, and the importance of its C-terminal region in specialization of enzyme function.

Diverse meibum lipids produced by Awat1 and Awat2 are important for stabilizing tear film and protecting the ocular surface

A lipid layer consisting of meibum lipids exists in the tear film and functions in preventing dry eye disease. Although the meibum lipids include diverse lipid classes, the synthesis pathway and role of each class remain largely unknown. Here, we created single and double knockout (KO and DKO, respectively) mice for the two acyl-CoA wax alcohol acyltransferases (Awat1 and Awat2) and investigated their dry eye phenotypes and meibum lipid composition. Awat2 KO and DKO mice exhibited severe dry eye with meibomian gland dysfunction, whereas Awat1 KO mice had mild dry eye. In these mice, specific meibum lipid classes were reduced: (O-acyl)-ω-hydroxy fatty acids and type 1ω wax diesters in Awat1 KO mice, wax monoesters and types 1ω and 2ω wax diesters in Awat2 KO mice, and most of these in DKO mice. Our findings reveal that Awat1 and Awat2 show characteristic substrate specificity and together produce diverse meibum lipids.

The molecular features of normal and atopic dermatitis skin in infants, children, adolescents, and adults

BACKGROUND

Although atopic dermatitis (AD) often presents in infancy and persists into adulthood, comparative characterization of AD skin among different pediatric age groups is lacking.

OBJECTIVE

We sought to define skin biopsy profiles of lesional and nonlesional AD across different age groups (0-5-year-old infants with disease duration <6 months, 6-11-year-old children, 12-17-year-old adolescents, ≥18-year-old adults) versus age-appropriate controls.

METHODS

We performed gene expression analyses by RNA-sequencing and real-time PCR (RT-PCR) and protein expression analysis using immunohistochemistry.

RESULTS

T_H2/T_H22 skewing, including IL-13, CCL17/thymus and activation-regulated chemokine, IL-22, and S100As, characterized the common AD signature, with a global pathway-level enrichment across all ages. Nevertheless, specific cytokines varied widely. For example, IL-33, IL-1RL1/IL-33R, and IL-9, often associated with early atopic sensitization, showed greatest upregulations in infants. T_H17 inflammation presented a 2-peak curve, with highest increases in infants (including IL-17A and IL-17F), followed by adults. T_H1 polarization was uniquely detected in adults, even when compared with adolescents, with significant upregulation in adults of IFN-γ and CXCL9/CXCL10/CXCL11. Although all AD age groups had barrier abnormalities, only adults had significant decreases in filaggrin expression. Despite the short duration of the disease, infant AD presented robust downregulations of multiple barrier-related genes in both lesional and nonlesional skin. Clinical severity scores significantly correlated with T_H2/T_H22-related markers in all pediatric age groups.

CONCLUSIONS

The shared signature of AD across ages is T_H2/T_H22-skewed, yet differential expression of specific T_H2/T_H22-related genes, other T_H pathways, and barrier-related genes portray heterogenetic, age-specific molecular fingerprints.

Production of branched-chain very-long-chain fatty acids by fatty acid elongases and their tissue distribution in mammals

Although most mammalian fatty acids (FAs) are straight-chain, there also exist branched-chain FAs such as iso- and anteiso-FAs, especially in the meibomian glands. Meibum lipids, which are secreted from the meibomian glands and are important for dry eye prevention, contain abundant branched-chain lipids, such as cholesteryl esters and wax esters with chain-lengths of ≥C21 (very-long-chain; VLC). However, the exact tissue distribution of branched-chain lipids or the enzymes involved in the production of branched-chain VLCFAs has remained poorly understood. Here, we revealed that FA elongases ELOVL1, ELOVL3, and ELOVL7, of the seven mammalian ELOVL isozymes, elongated saturated branched-chain acyl-CoAs. ELOVL3 was highly active toward iso-C17:0 and anteiso-C17:0 acyl-CoAs and elongated them up to iso-C23:0 and anteiso-C25:0 acyl-CoAs, respectively. ELOVL1 elongated both iso- and anteiso-C23:0 acyl-CoAs to C25:0 acyl-CoAs. By establishing a liquid chromatography-tandem mass spectrometry method capable of separating branched- and straight-chain lipids, we showed that essentially all of the cholesteryl esters and 88% of the wax esters in the mouse meibomian glands are branched. In Elovl1 mutant mice, the levels of ≥C24:0 branched-chain cholesteryl esters and ≥C25:0 branched-chain wax esters were decreased, indicating that ELOVL1 indeed elongates branched-chain VLC acyl-CoAs in vivo. In addition, substantial amounts of ceramides containing branched-chain FAs were present in the skin, meibomian glands, and liver. Our findings provide new insights into the molecular mechanisms that create FA and lipid diversity.

Deficiency in Acyl-CoA:Wax Alcohol Acyltransferase 2 causes evaporative dry eye disease by abolishing biosynthesis of wax esters

Lipids secreted by the meibomian glands (MGs) of the eyelids are essential to the protection of the eye's surface. An altered meibum composition represents the primary cause of evaporative dry eye disease (DED). Despite the critical importance of the meibum, its biosynthetic pathways and the roles of individual lipid components remain understudied. Here, we report that the genetic deletion of Acyl-CoA:wax alcohol acyltransferase 2 (AWAT2) causes the obstruction of MGs and symptoms of evaporative DED in mice. The lipid composition of the meibum isolated from Awat2^-/- mice revealed the absence of wax esters, which was accompanied by a compensatory overproduction of cholesteryl esters. The resulting increased viscosity of meibum led to the dilation of the meibomian ducts, and the progressive degeneration of the MGs. Overall, we provide evidence for the main physiological role of AWAT2 and establish Awat2^-/- mice as a model for DED syndrome that can be used in studies on tear film-oriented therapies.

Lipid polarity gradient formed by ω-hydroxy lipids in tear film prevents dry eye disease

Meibum lipids form a lipid layer on the outermost side of the tear film and function to prevent water evaporation and reduce surface tension. (O-Acyl)-ω-hydroxy fatty acids (OAHFAs), a subclass of these lipids, are thought to be involved in connecting the lipid and aqueous layers in tears, although their actual function and synthesis pathway have to date remained unclear. Here, we reveal that the fatty acid ω-hydroxylase Cyp4f39 is involved in OAHFA production. Cyp4f39-deficient mice exhibited damaged corneal epithelium and shortening of tear film break-up time, both indicative of dry eye disease. In addition, tears accumulated on the lower eyelid side, indicating increased tear surface tension. In Cyp4f39-deficient mice, the production of wax diesters (type 1ω and 2ω) and cholesteryl OAHFAs was also impaired. These OAHFA derivatives show intermediate polarity among meibum lipids, suggesting that OAHFAs and their derivatives contribute to lipid polarity gradient formation for tear film stabilization.

Network analysis uncovers putative genes affecting resistance to tick infestation in Braford cattle skin

Background

Genetic resistance in cattle is considered a suitable way to control tick burden and its consequent losses for livestock production. Exploring tick-resistant (R) and tick-susceptible (S) hosts, we investigated the genetic mechanisms underlying the variation of Braford resistance to tick infestation. Skin biopsies from four-times-artificially infested R (n = 20) and S (n = 19) hosts, obtained before the first and 24 h after the fourth tick infestation were submitted to RNA-Sequencing. Differential gene expression, functional enrichment, and network analysis were performed to identify genetic pathways and transcription factors (TFs) affecting host resistance.

Results

Intergroup comparisons of hosts before (Rpre vs. Spre) and after (Rpost vs. Spost) tick infestation found 51 differentially expressed genes (DEGs), of which almost all presented high variation (TopDEGs), and 38 were redundant genes. Gene expression was consistently different between R and S hosts, suggesting the existence of specific anti-tick mechanisms. In the intragroup comparisons, Rpost vs. Rpre and Spost vs. Spre, we found more than two thousand DEGs in response to tick infestation in both resistance groups. Redundant and non-redundant TopDEGs with potential anti-tick functions suggested a role in the development of different levels of resistance within the same breed. Leukocyte chemotaxis was over-represented in both hosts, whereas skin degradation and remodeling were only found in TopDEGs from R hosts. Also, these genes indicated the participation of cytokines, such as IL6 and IL22, and the activation of Wingless (WNT)-signaling pathway. A central gene of this pathway, WNT7A, was consistently modulated when hosts were compared. Moreover, the findings based on a genome-wide association study (GWAS) corroborate the prediction of the WNT-signaling pathway as a candidate mechanism of resistance. The regulation of immune response was the most relevant pathway predicted for S hosts. Members of Ap1 and NF-kB families were the most relevant TFs predicted for R and S, respectively.

Conclusion

This work provides indications of genetic mechanisms presented by Braford cattle with different levels of resistance in response to tick infestation, contributing to the search of candidate genes for tick resistance in bovine.

Role of DGAT enzymes in triacylglycerol metabolism

The esterification of a fatty acyl moiety to diacylglycerol to form triacylglycerol (TAG) is catalysed by two diacylglycerol O-acyltransferases (DGATs) encoded by genes belonging to two distinct gene families. The enzymes are referred to as DGAT1 and DGAT2 in order of their identification. Both proteins are transmembrane proteins localized in the endoplasmic reticulum. Their membrane topologies are however significantly different. This difference is hypothesized to give the two isozymes different abilities to interact with other proteins and organelles and access to different pools of fatty acids, thereby creating a distinction between the enzymes in terms of their role and contribution to lipid metabolism. DGAT1 is proposed to have dual topology contributing to TAG synthesis on both sides of the ER membrane and esterifying only the pre-formed fatty acids. There is evidence to suggest that DGAT2 translocates to the lipid droplet (LD), associates with other proteins, and synthesizes cytosolic and luminal apolipoprotein B associated LD-TAG from both endogenous and exogenous fatty acids. The aim of this review is to differentiate between the two DGAT enzymes by comparing the genes that encode them, their proposed topologies, the proteins they interact with, and their roles in lipid metabolism.

Protein engineering: Regulatory perspectives of stearoyl CoA desaturase

Stearoyl Co A desaturase (SCD) is a rate-limiting lipogenic enzyme that plays an integral role in catalyzing the synthesis of monounsaturated fatty acids, chiefly oleate and palmitoleate. Both contribute a major part of the biological membrane. Numerous SCD isoforms exist in mouse and humans, i.e., SCD-1 to SCD-4 and SCD-1 and SCD-5, respectively. From the biological viewpoint, hyperexpression of SCD1 cause many metabolic disorders including obesity, insulin resistance, hypertension, and hypertriglyceridemia, etc. Herein, an effort has been made to highlight the value of protein engineering in controlling the SCD-1 expression with the involvement of different inhibitors as therapeutic agents. The first part of the review describes Stearoyl CoA desaturase index and different SCD isoforms. Various regulatory aspects of SCD are reviewed in four subsections, i.e., (1) hormonal regulation, (2) regulation by dietary carbohydrates, (3) regulation by green tea, and (4) regulation via polyunsaturated fatty acids (PUFAs). Moreover, the regulation of Stearoyl CoA desaturase expression in the metabolism of fats and carbohydrates is discussed. The third part mainly focuses on natural and synthetic inhibitors. Towards the end, information is also given on potential future considerations of SCD-1 inhibitors as metabolic syndrome therapeutics, yet additional work is required.

Meibomian glands, meibum, and meibogenesis

Meibum is a lipid-rich secretion that is produced by fully differentiated meibocytes in the holocrine Meibomian glands (MG) of humans and most mammals. The secretion is a part of a defense mechanism that protects the ocular surface from hazardous environmental factors, and from desiccation. Meibomian lipids that have been identified in meibum are very diverse and unique in nature. The lipid composition of meibum is different from virtually any other lipid pool found in the human body. In fact, meibum is quite different from sebum, which is the closest secretion that is produced by anatomically, physiologically, and biochemically related sebaceous glands. However, meibum of mice have been shown to closely resemble that of humans, implying similar biosynthetic mechanisms in MG of both species. By analyzing available genomic, immunohistochemical, and lipidomic data, we have envisioned a unifying network of enzymatic reactions that are responsible for biosynthesis of meibum, which we call meibogenesis. Our current theory is based on an assumption that most of the biosynthetic reactions of meibogenesis are catalyzed by known enzymes. However, the main features that make meibum unique - the ratio of identified classes of lipids, the extreme length of its components, extensive ω-hydroxylation of fatty acids and alcohols, iso- and anteiso-branching of meibomian lipids (e.g. waxes), and the presence of rather unique complex lipids with several ester bonds - make it possible that either the activity of known enzymes is altered in MG, or some unknown enzymes contribute to the processes of meibogenesis, or both. Studies are in progress to elucidate meibogenesis on molecular level.

Synthesis of neutral ether lipid monoalkyl-diacylglycerol by lipid acyltransferases

In mammals, ether lipids exert a wide spectrum of signaling and structural functions, such as stimulation of immune responses, anti-tumor activities, and enhancement of sperm functions. Abnormal accumulation of monoalkyl-diacylglycerol (MADAG) was found in Wolman's disease, a human genetic disorder defined by a deficiency in lysosomal acid lipase. In the current study, we found that among the nine recombinant human lipid acyltransferases examined, acyl-CoA:diacylglycerol acyltransferase (DGAT)1, DGAT2, acyl-CoA:monoacylglycerol acyltransferase (MGAT)2, MGAT3, acyl-CoA:wax-alcohol acyltransferase 2/MFAT, and DGAT candidate 3 were able to use 1-monoalkylglycerol (1-MAkG) as an acyl acceptor for the synthesis of monoalkyl-monoacylglycerol (MAMAG). These enzymes demonstrated different enzymatic turnover rates and relative efficiencies for the first and second acylation steps leading to the synthesis of MAMAG and MADAG, respectively. They also exhibited different degrees of substrate preference when presented with 1-monooleoylglycerol versus 1-MAkG. In CHO-K1 cells, treatment with DGAT1 selective inhibitor, XP-620, completely blocked the synthesis of MADAG, indicating that DGAT1 is the predominant enzyme responsible for the intracellular synthesis of MADAG in this model system. The levels of MADAG in the adrenal gland of DGAT1 KO mice were reduced as compared with those of the WT mice, suggesting that DGAT1 is a major enzyme for the synthesis of MADAG in this tissue. Our findings indicate that several of these lipid acyltransferases may be able to synthesize neutral ether lipids in mammals.

Allosteric modulation of the substrate specificity of acyl-CoA wax alcohol acyltransferase 2

The esterification of alcohols with fatty acids is a universal mechanism to form inert storage forms of sterols, di- and triacylglycerols, and retinoids. In ocular tissues, formation of retinyl esters is an essential step in the enzymatic regeneration of the visual chromophore (11-cis-retinal). Acyl-CoA wax alcohol acyltransferase 2 (AWAT2), also known as multifunctional O-acyltransferase (MFAT), is an integral membrane enzyme with a broad substrate specificity that has been shown to preferentially esterify 11-cis-retinol and thus contribute to formation of a readily available pool of cis retinoids in the eye. However, the mechanism by which this promiscuous enzyme can gain substrate specificity is unknown. Here, we provide evidence for an allosteric modulation of the enzymatic activity by 11-cis retinoids. This regulation is independent from cellular retinaldehyde-binding protein (CRALBP), the major cis-retinoid binding protein. This positive-feedback regulation leads to decreased esterification rates for 9-cis, 13-cis, or all-trans retinols and thus enables preferential synthesis of 11-cis-retinyl esters. Finally, electron microscopy analyses of the purified enzyme indicate that this allosteric effect does not result from formation of functional oligomers. Altogether, these data provide the experimental basis for understanding regulation of AWAT2 substrate specificity.

Decreases in 15-lipoxygenase metabolites in Olmsted syndrome model rats

BACKGROUND

Olmsted syndrome (OS) is a congenital dermatosis characterized by palmoplantar keratoderma and periorificial keratotic plaque. TRPV3 (transient receptor potential vanilloid subtype 3) encodes a thermosensitive Ca²⁺ channel and is the causative gene of OS. However, the molecular mechanism that causes the pathological development of OS is unclear.

OBJECTIVE

We aimed to investigate the molecular mechanisms underlying OS pathology from the perspective of lipid metabolism.

METHODS

Comprehensive lipidomics and microarray analyses were conducted on tissue samples from a non-lesional skin area of OS model rats (Ht rats) and from wild type (WT) rats as the control.

RESULTS

Infiltration of leukocytes such as eosinophils and neutrophils and an increase in the fibrotic region were detected in the unaffected skin area of Ht rats compared with the WT rats. Among about 600 lipid species examined, the levels of 15-lipoxygenase (LOX) metabolites, the precursors of anti-inflammatory and pro-resolving lipid mediators, and dihydroceramides decreased by ≥16-fold in Ht rats compared with WT rats. Consistent with the decreases in the 15-LOX metabolites, expression levels of the genes that encode the 15-LOXs, Alox15 and Alox15b, were largely reduced. Conversely, increased expression levels were detected of Il36b, Ccl20, Cxcl1, and Cxcl2, which encode cytokines/chemokines, and S100a8 and S100a9, which encode the Ca²⁺ binding proteins that are implicated in epidermal proliferation.

CONCLUSION

The pro-inflammatory state in the unaffected skin of Ht rats caused by decreases in 15-LOX metabolites and increases in cytokines/chemokines may contribute to the pathogenesis of OS.

Primitive Extracellular Lipid Components on the Surface of the Charophytic Alga Klebsormidium flaccidum and Their Possible Biosynthetic Pathways as Deduced from the Genome Sequence

Klebsormidium flaccidum is a charophytic alga living in terrestrial and semiaquatic environments. K. flaccidum grows in various habitats, such as low-temperature areas and under desiccated conditions, because of its ability to tolerate harsh environments. Wax and cuticle polymers that contribute to the cuticle layer of plants are important for the survival of land plants, as they protect against those harsh environmental conditions and were probably critical for the transition from aquatic microorganism to land plants. Bryophytes, non-vascular land plants, have similar, but simpler, extracellular waxes and polyester backbones than those of vascular plants. The presence of waxes in terrestrial algae, especially in charophytes, which are the closest algae to land plants, could provide clues in elucidating the mechanism of land colonization by plants. Here, we compared genes involved in the lipid biosynthetic pathways of Arabidopsis thaliana to the K. flaccidum and the Chlamydomonas reinhardtii genomes, and identified wax-related genes in both algae. A simple and easy extraction method was developed for the recovery of the surface lipids from K. flaccidum and C. reinhardtii. Although these algae have wax components, their surface lipids were largely different from those of land plants. We also investigated aliphatic substances in the cell wall fraction of K. flaccidum and C. reinhardtii. Many of the fatty acids were determined to be lipophilic monomers in K. flaccidum, and a Fourier transform infrared spectroscopic analysis revealed that their possible binding mode was distinct from that of A. thaliana. Thus, we propose that K. flaccidum has a cuticle-like hydrophobic layer composed of lipids and glycoproteins, with a different composition from the cutin polymer typically found in land plant cuticles.

Sebaceous Gland Atrophy in Psoriasis: An Explanation for Psoriatic Alopecia?

In a transcriptome study of lesional psoriatic skin (PP) versus normal skin, we found a coexpressed gene module (N5) enriched 11.5-fold for lipid biosynthetic genes. We also observed fewer visible hairs in PP skin, compared with uninvolved nonlesional psoriatic skin or normal skin (P < 0.0001). To ask whether these findings might be due to abnormalities of the pilosebaceous unit, we carried out three-dimensional morphometric analysis of paired PP and nonlesional psoriatic skin biopsies. Sebaceous glands were markedly atrophic in PP versus nonlesional psoriatic skin (91% average reduction in volume, P = 0.031). Module N5 genes were strongly downregulated in PP versus normal skin (fold change < 0.25, 44.4-fold) and strongly upregulated in sebaceous hyperplasia (fold change > 4, 54.1-fold). The intersection of PP-downregulated and sebaceous hyperplasia-upregulated gene lists generated a gene expression signature consisting solely of module N5 genes, whose expression in PP versus normal skin was inversely correlated with the signature of IL17-stimulated keratinocytes. Despite loss of visible hairs, morphometry identified elongated follicles in PP versus nonlesional psoriatic skin (average 1.7 vs. 1.2 μm, P = 0.020). These results document sebaceous gland atrophy in nonscalp psoriasis, identify a cytokine-regulated set of sebaceous gland signature genes, and suggest that loss of visible hair in PP skin may result from abnormal sebaceous gland function.

Type I Diacylglycerol Acyltransferase (MtDGAT1) from Macadamia tetraphylla: Cloning, Characterization, and Impact of Its Heterologous Expression on Triacylglycerol Composition in Yeast

Acyltransferase enzymes have been reported as useful biotechnological tools in order to increase oil yield and modify fatty acid composition. Macadamia species are able to accumulate unusually high levels of palmitoleic acid that besides oleic acid amounts to over 80% of monounsaturated fatty acids in the seed oil. In this work, a gene encoding a type 1 acyl-CoA:diacylglycerol acyltransferase (DGAT1) was cloned from M. tetraphylla. DGAT activity of the protein encoded by MtDGAT1 was confirmed by heterologous expression in a yeast mutant. Fatty acid composition of triacylglycerols synthesized by MtDGAT1 was compared to that of DGAT1 enzymes from Arabidopsis and Echium, with the results suggesting a substrate preference for monounsaturated over polyunsaturated fatty acids. Characteristics of MtDGAT1 may contribute to biochemical mechanisms determining the particular fatty acid composition of Macadamia oil and also indicate the possibility of using this enzyme in biotechnological approaches where a reduction of polyunsaturated fatty acids in the oil is desired.

The pig X and Y Chromosomes: structure, sequence, and evolution

We have generated an improved assembly and gene annotation of the pig X Chromosome, and a first draft assembly of the pig Y Chromosome, by sequencing BAC and fosmid clones from Duroc animals and incorporating information from optical mapping and fiber-FISH. The X Chromosome carries 1033 annotated genes, 690 of which are protein coding. Gene order closely matches that found in primates (including humans) and carnivores (including cats and dogs), which is inferred to be ancestral. Nevertheless, several protein-coding genes present on the human X Chromosome were absent from the pig, and 38 pig-specific X-chromosomal genes were annotated, 22 of which were olfactory receptors. The pig Y-specific Chromosome sequence generated here comprises 30 megabases (Mb). A 15-Mb subset of this sequence was assembled, revealing two clusters of male-specific low copy number genes, separated by an ampliconic region including the HSFY gene family, which together make up most of the short arm. Both clusters contain palindromes with high sequence identity, presumably maintained by gene conversion. Many of the ancestral X-related genes previously reported in at least one mammalian Y Chromosome are represented either as active genes or partial sequences. This sequencing project has allowed us to identify genes--both single copy and amplified--on the pig Y Chromosome, to compare the pig X and Y Chromosomes for homologous sequences, and thereby to reveal mechanisms underlying pig X and Y Chromosome evolution.

MOGAT2: A New Therapeutic Target for Metabolic Syndrome

Metabolic syndrome is an ever-increasing health problem among the world’s population. It is a group of intertwined maladies that includes obesity, hypertriglyceridemia, hypertension, nonalcoholic fatty liver disease (NAFLD), and diabetes mellitus type II (T2D). There is a direct correlation between high triacylglycerol (triglyceride; TAG) level and severity of metabolic syndrome. Thus, controlling the synthesis of TAG will have a great impact on overall systemic lipid metabolism and thus metabolic syndrome progression. The Acyl-CoA: monoacylglycerolacyltransferase (MGAT) family has three members (MGAT1, -2, and -3) that catalyze the first step in TAG production, conversion of monoacylglycerol (MAG) to diacylglycerol (DAG). TAG is then directly synthesized from DAG by a Acyl-CoA: diacylglycerolacyltransferase (DGAT). The conversion of MAG → DAG → TAG is the major pathway for the production of TAG in the small intestine, and produces TAG to a lesser extent in the liver. Transgenic and pharmacological studies in mice have demonstrated the beneficial effects of MGAT inhibition as a therapy for treating several metabolic diseases, including obesity, insulin resistance, T2D, and NAFLD. In this review, the significance of several properties of MGAT physiology, including tissue expression pattern and its relationship to overall TAG metabolism, enzymatic biochemical properties and their effects on drug discovery, and finally what is the current knowledge about MGAT small molecule inhibitors and their efficacy will be discussed. Overall, this review highlights the therapeutic potential of inhibiting MGAT for lowering TAG synthesis and whether this avenue of drug discovery warrants further clinical investigation.

The bifunctional protein TtFARAT from Tetrahymena thermophila catalyzes the formation of both precursors required to initiate ether lipid biosynthesis

The biosynthesis of ether lipids and wax esters requires as precursors fatty alcohols, which are synthesized by fatty acyl reductases (FARs). The presence of ether glycerolipids as well as branched wax esters has been reported in several free-living ciliate protozoa. In the genome of Tetrahymena thermophila, the only ORF sharing similarities with FARs is fused to an acyltransferase-like domain, whereas, in most other organisms, FARs are monofunctional proteins of similar size and domain structure. Here, we used heterologous expression in plant and yeast to functionally characterize the activities catalyzed by this protozoan protein. Transient expression in tobacco epidermis of a truncated form fused to the green fluorescence protein followed by confocal microscopy analysis suggested peroxisomal localization. In vivo approaches conducted in yeast indicated that the N-terminal FAR-like domain produced both 16:0 and 18:0 fatty alcohols, whereas the C-terminal acyltransferase-like domain was able to rescue the lethal phenotype of the yeast double mutant gat1Δ gat2Δ. Using in vitro approaches, we further demonstrated that this domain is a dihydroxyacetone phosphate acyltransferase that uses preferentially 16:0-coenzyme A as an acyl donor. Finally, coexpression in yeast with the alkyl-dihydroxyacetone phosphate synthase from T. thermophila resulted the detection of various glycerolipids with an ether bond, indicating reconstitution of the ether lipid biosynthetic pathway. Together, these results demonstrate that this FAR-like protein is peroxisomal and bifunctional, providing both substrates required by alkyl-dihydroxyacetone phosphate synthase to initiate ether lipid biosynthesis.

Functional characterization of two structurally novel diacylglycerol acyltransferase2 isozymes responsible for the enhanced production of stearate-rich storage lipid in Candida tropicalis SY005

Diacylglycerol acyltransferase (DGAT) activity is an essential enzymatic step in the formation of neutral lipid i.e., triacylglycerol in all living cells capable of accumulating storage lipid. Previously, we characterized an oleaginous yeast Candida tropicalis SY005 that yields storage lipid up to 58% under a specific nitrogen-stress condition, when the DGAT-specific transcript is drastically up-regulated. Here we report the identification, differential expression and function of two DGAT2 gene homologues- CtDGAT2a and CtDGAT2b of this C. tropicalis. Two protein isoforms are unique with respect to the presence of five additional stretches of amino acids, besides possessing three highly conserved motifs known in other reported DGAT2 enzymes. Moreover, the CtDGAT2a and CtDGAT2b are characteristically different in amino acid sequences and predicted protein structures. The CtDGAT2b isozyme was found to be catalytically 12.5% more efficient than CtDGAT2a for triacylglycerol production in a heterologous yeast system i.e., Saccharomyces cerevisiae quadruple mutant strain H1246 that is inherently defective in neutral lipid biosynthesis. The CtDGAT2b activity rescued the growth of transformed S. cerevisiae mutant cells, which are usually non-viable in the medium containing free fatty acids by incorporating them into triacylglycerol, and displayed preferential specificity towards saturated acyl species as substrate. Furthermore, we document that the efficiency of triacylglycerol production by CtDGAT2b is differentially affected by deletion, insertion or replacement of amino acids in five regions exclusively present in two CtDGAT2 isozymes. Taken together, our study characterizes two structurally novel DGAT2 isozymes, which are accountable for the enhanced production of storage lipid enriched with saturated fatty acids inherently in C. tropicalis SY005 strain as well as in transformed S. cerevisiae neutral lipid-deficient mutant cells. These two genes certainly will be useful for further investigation on the novel structure-function relationship of DGAT repertoire, and also in metabolic engineering for the enhanced production of lipid feedstock in other organisms.

Fatty aldehyde and fatty alcohol metabolism: review and importance for epidermal structure and function

Normal fatty aldehyde and alcohol metabolism is essential for epidermal differentiation and function. Long-chain aldehydes are produced by catabolism of several lipids including fatty alcohols, sphingolipids, ether glycerolipids, isoprenoid alcohols and certain aliphatic lipids that undergo α- or ω-oxidation. The fatty aldehyde generated by these pathways is chiefly metabolized to fatty acid by fatty aldehyde dehydrogenase (FALDH, alternately known as ALDH3A2), which also functions to oxidize fatty alcohols as a component of the fatty alcohol:NAD oxidoreductase (FAO) enzyme complex. Genetic deficiency of FALDH/FAO in patients with Sjögren-Larsson syndrome (SLS) results in accumulation of fatty aldehydes, fatty alcohols and related lipids (ether glycerolipids, wax esters) in cultured keratinocytes. These biochemical changes are associated with abnormalities in formation of lamellar bodies in the stratum granulosum and impaired delivery of their precursor membranes to the stratum corneum (SC). The defective extracellular SC membranes are responsible for a leaky epidermal water barrier and ichthyosis. Although lamellar bodies appear to be the pathogenic target for abnormal fatty aldehyde/alcohol metabolism in SLS, the precise biochemical mechanisms are yet to be elucidated. Nevertheless, studies in SLS highlight the critical importance of FALDH and normal fatty aldehyde/alcohol metabolism for epidermal function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

A thraustochytrid diacylglycerol acyltransferase 2 with broad substrate specificity strongly increases oleic acid content in engineered Arabidopsis thaliana seeds

Diacylglycerol acyltransferase (DGAT) catalyses the last step in acyl-CoA-dependent triacylglycerol (TAG) biosynthesis and is an important determinant of cellular oil content and quality. In this study, a gene, designated TaDGAT2, encoding a type 2 DGAT (DGAT2)-related enzyme was identified from the oleaginous marine protist Thraustochytrium aureum. The deduced TaDGAT2 sequence contains a ~460 amino acid domain most closely related to DGAT2s from Dictyostelium sp. (45-50% identity). Recombinant TaDGAT2 restored TAG biosynthesis to the Saccharomyces cerevisiae H1246 TAG-deficient mutant, and microsomes from the complemented mutant displayed DGAT activity with C16 and C18 saturated and unsaturated fatty acyl-CoA and diacylglycerol substrates. To examine its biotechnological potential, TaDGAT2 was expressed under control of a strong seed-specific promoter in wild-type Arabidopsis thaliana and the high linoleic acid fad3fae1 mutant. In both backgrounds, little change was detected in seed oil content, but a striking increase in oleic acid content of seeds was observed. This increase was greatest in fad3fae1 seeds, where relative amounts of oleic acid increased nearly 2-fold to >50% of total fatty acids. In addition, >2-fold increase in oleic acid levels was detected in the triacylglycerol sn-2 position and in the major seed phospholipid phosphatidylcholine. These results suggest that increased seed oleic acid content mediated by TaDGAT2 is influenced in part by the fatty acid composition of host cells and occurs not by enhancing oleic acid content at the TAG sn-3 position directly but by increasing total oleic acid levels in seeds, presumably by limiting flux through phosphatidylcholine-based desaturation reactions.

Acyltransferases in bacteria

Long-chain-length hydrophobic acyl residues play a vital role in a multitude of essential biological structures and processes. They build the inner hydrophobic layers of biological membranes, are converted to intracellular storage compounds, and are used to modify protein properties or function as membrane anchors, to name only a few functions. Acyl thioesters are transferred by acyltransferases or transacylases to a variety of different substrates or are polymerized to lipophilic storage compounds. Lipases represent another important enzyme class dealing with fatty acyl chains; however, they cannot be regarded as acyltransferases in the strict sense. This review provides a detailed survey of the wide spectrum of bacterial acyltransferases and compares different enzyme families in regard to their catalytic mechanisms. On the basis of their studied or assumed mechanisms, most of the acyl-transferring enzymes can be divided into two groups. The majority of enzymes discussed in this review employ a conserved acyltransferase motif with an invariant histidine residue, followed by an acidic amino acid residue, and their catalytic mechanism is characterized by a noncovalent transition state. In contrast to that, lipases rely on completely different mechanism which employs a catalytic triad and functions via the formation of covalent intermediates. This is, for example, similar to the mechanism which has been suggested for polyester synthases. Consequently, although the presented enzyme types neither share homology nor have a common three-dimensional structure, and although they deal with greatly varying molecule structures, this variety is not reflected in their mechanisms, all of which rely on a catalytically active histidine residue.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Generation of a genome scale lentiviral vector library for EF1α promoter-driven expression of human ORFs and identification of human genes affecting viral titer

The bottleneck in elucidating gene function through high-throughput gain-of-function genome screening is the limited availability of comprehensive libraries for gene overexpression. Lentiviral vectors are the most versatile and widely used vehicles for gene expression in mammalian cells. Lentiviral supernatant libraries for genome screening are commonly generated in the HEK293T cell line, yet very little is known about the effect of introduced sequences on the produced viral titer, which we have shown to be gene dependent. We have generated an arrayed lentiviral vector library for the expression of 17,030 human proteins by using the GATEWAY® cloning system to transfer ORFs from the Mammalian Gene Collection into an EF1alpha promoter-dependent lentiviral expression vector. This promoter was chosen instead of the more potent and widely used CMV promoter, because it is less prone to silencing and provides more stable long term expression. The arrayed lentiviral clones were used to generate viral supernatant by packaging in the HEK293T cell line. The efficiency of transfection and virus production was estimated by measuring the fluorescence of IRES driven GFP, co-expressed with the ORFs. More than 90% of cloned ORFs produced sufficient virus for downstream screening applications. We identified genes which consistently produced very high or very low viral titer. Supernatants from select clones that were either high or low virus producers were tested on a range of cell lines. Some of the low virus producers, including two previously uncharacterized proteins were cytotoxic to HEK293T cells. The library we have constructed presents a powerful resource for high-throughput gain-of-function screening of the human genome and drug-target discovery. Identification of human genes that affect lentivirus production may lead to improved technology for gene expression using lentiviral vectors.

A look at diacylglycerol acyltransferases (DGATs) in algae

Triacylglycerols (TAGs) from algae are considered to be a potentially viable source of biodiesel and thereby renewable energy, but at the moment very little is known about the biosynthetic pathway in these organisms. Here we compare what is currently known in eukaryotic algal species, in particular the characteristics of algal diacylglycerol acyltransferase (DGAT), the last enzyme of de novo TAG biosynthesis. Several studies in plants and mammals have shown that there are two DGAT isoforms, DGAT1 and DGAT2, which catalyse the same reaction but have no clear sequence similarities. Instead, they have differences in functionality and spatial and temporal expression patterns. Bioinformatic searches of sequenced algal genomes reveal that most algae have multiple copies of putative DGAT2s, whereas other eukaryotes have single genes. Investigating whether these putative isoforms are indeed functional and whether they confer significantly different phenotypes to algal cells will be vital for future efforts to genetically modify algae for biofuel production.

ACAT2 and ABCG5/G8 are both required for efficient cholesterol absorption in mice: evidence from thoracic lymph duct cannulation

The metabolic fate of newly absorbed cholesterol and phytosterol is orchestrated through adenosine triphosphate-binding cassette transporter G5 and G8 heterodimer (G5G8), and acyl CoA:cholesterol acyltransferase 2 (ACAT2). We hypothesized that intestinal G5G8 limits sterol absorption by reducing substrate availability for ACAT2 esterification and have attempted to define the roles of these two factors using gene deletion studies in mice. Male ACAT2(-/-), G5G8(-/-), ACAT2(-/-)G5G8(-/-) (DKO), and wild-type (WT) control mice were fed a diet with 20% of energy as palm oil and 0.2% (w/w) cholesterol. Sterol absorption efficiency was directly measured by monitoring the appearance of [(3)H]sitosterol and [(14)C]cholesterol tracers in lymph after thoracic lymph duct cannulation. The average percentage (± SEM) absorption of [(14)C]cholesterol after 8 h of lymph collection was 40.55 ± 0.76%, 19.41 ± 1.52%, 32.13 ± 1.60%, and 21.27 ± 1.35% for WT, ACAT2(-/-), G5G8(-/-), and DKO mice, respectively. [(3)H]sitosterol absorption was <2% in WT and ACAT2(-/-) mice, whereas it was up to 6.8% in G5G8(-/-) and DKO mice. G5G8(-/-) mice also produced chylomicrons with ∼70% less cholesterol ester mass than WT mice. In contrast to expectations, the data demonstrated that the absence of G5G8 led to decreased intestinal cholesterol esterification and reduced cholesterol transport efficiency. Intestinal G5G8 appeared to limit the absorption of phytosterols; ACAT2 more efficiently esterified cholesterol than phytosterols. The data indicate that handling of sterols by the intestine involves both G5G8 and ACAT2 but that an additional factor (possibly Niemann-Pick C1-like 1) may be key in determining absorption efficiency.