Identification of complex mixtures of sphingolipids in the stratum corneum by reversed-phase high-performance liquid chromatography and atmospheric pressure photospray ionization mass spectrometry

Lipids in the American Alligator stratum corneum provide insights into the evolution of vertebrate skin

In terrestrial vertebrates, the outermost layer of the skin, the stratum corneum (SC), provides a durable and flexible interface with the environment and is comprised of corneocytes embedded in lipids. However, the morphology and lipid composition of the SC varies throughout evolutionary history. Because crocodilians and birds phylogenetically bracket the Archosaurian clade, lipid composition in crocodilian SC may be compared with that of birds and other vertebrates to make inferences about broader phylogenetic patterns within Archosaurs while highlighting adaptations in vertebrate skin. We identified and quantified lipid classes in the SC of the American Alligator (Alligator mississippiensis) from three skin regions varying in mobility. Our results find similarities in lipid composition between alligator and avian SC, including a high percentage of cerebrosides, a polar lipid previously found only in the SC of birds and bats. Furthermore, polar lipids were more abundant in the most mobile region of the SC. Because polar lipids bind with water to increase skin hydration and therefore its pliability under physical stress, we hypothesize that selection for lipids in Archosaurian SC was driven by the unique distribution of proteins in the SC of this clade, and cerebrosides may have served as pre-adaptations for flight.

Extraction Optimization, UHPLC-Triple-TOF-MS/MS Analysis and Antioxidant Activity of Ceramides from Sea Red Rice Bran

As a new type of salt-tolerant rice, sea red rice contains more minerals, proteins, and lipid compounds, and, in particular, its by-product rice bran may be used to replace other commercial rice brans as the main source of ceramides (Cers). However, the extraction rate of Cers is generally low, and it is crucial to seek an efficient extraction method. This study optimized the ultrasonic-assisted extraction of Cers from sea red rice bran using response surface methodology (RSM) and obtained a Cers yield of 12.54% under optimal conditions involving an extraction temperature of 46 °C, an extraction time of 46 min, and a material–to-liquid ratio of 5 g/mL. The Cers content in sea red rice bran was preliminarily analyzed using thin-layer chromatography, and the Cers content was determined via UHPLC-Triple-TOF-MS/MS after purification and separation using silica column chromatography. Forty-six different types of Cers were identified in sea red rice bran, of which Cer 18:0/24:0 (2OH), Cer 18:0/26:0, Cer 18:0/26:0 (2OH), and Cer 18:0/24:0 accounted for 23.66%, 17.54%, 14.91%, and 11.96%. Most of the Cers structures were mainly composed of sphingadienine. A biological activity assay indicated that Cers extracted from sea red rice bran had significant antioxidant and anti-aging properties. These findings indicate that the extracted Cers show great potential for applications in the cosmetic and pharmaceutical industries.

Lipid composition of the stratum corneum in different regions of the body of Kuhl's pipistrelle from the Negev Desert, Israel

The most superficial epidermal layer in endotherms is the stratum corneum (SC), which is composed of dead corneocytes embedded in a lipid matrix with free fatty acids, cholesterol, ceramides, and cerebrosides; the lipid composition of the SC determines its permeability to water vapor. Lipids that are more polar, have longer hydrocarbon chains, and are less bulky are often packed in more ordered phase states to slow cutaneous evaporative water loss (CEWL); these lipids also resist transitions to more disordered phases at high ambient temperatures (T_a). In bats, wing and tail membranes (wing patagia and tail uropatagium, respectively) allow powered flight, but increase surface area, and hence CEWL, with implications for survival in arid environments. We captured Pipistrellus kuhlii from an arid habitat and measured the lipid composition of the SC of the plagiopatagium in the wing, the uropatagium, and the non-membranous region (NMR) of the body using thin layer chromatography and reversed phase high performance liquid chromatography coupled with atmospheric pressure photoionization mass spectrometry. The patagia contained more cholesterol and shorter-chained ceramides, and fewer cerebrosides than the NMR, indicating that the lipid phase transition temperature in the patagia is lower than in the NMR. Thus, at moderate T_a the lipids in the SC in all body regions will remain in an ordered phase state, allowing water conservation; but as T_a increases, the lipids in the SC of the patagia will more easily transition into a disordered phase, resulting in increased CEWL from the patagia facilitating efficient heat dissipation in hot environments.

Non-alcoholic fatty liver disease: Insights from sphingolipidomics

Non-alcoholic fatty liver disease (NAFLD) is a major clinical concern and its treatment consumes abundant resources. While accumulation of lipids in hepatocytes initiates the disease, this in itself is not necessarily harmful; rather, initiation of inflammation and subsequent fibrosis and cirrhosis are critical steps in NAFLD pathology. Mechanisms linking lipid overload to downstream disease progression are not fully understood; however, bioactive lipid metabolism may underlie instigation of proinflammatory signaling. With the advent of high-throughput, sensitive, and quantitative mass spectrometry-based methods for assessing lipid profiles in NAFLD, several trends have emerged, including that increases in specific sphingolipids correlate with the transition from the relatively benign condition of simple fatty liver to the much more concerning inflamed state. Continued studies that implement sphingolipid profiling will enable the extrapolations of candidate enzymes and pathways involved in NAFLD, either in biopsies or plasma from human samples, and also in animal models, from which data are much more abundant. While most data thus far are derived from targeted lipidomics approaches, unbiased, semi-quantitative approaches hold additional promise for furthering our understanding of sphingolipids as markers of and players in NAFLD.

The cutaneous lipid composition of bat wing and tail membranes: a case of convergent evolution with birds

The water vapour permeability barrier of mammals and birds resides in the stratum corneum (SC), the outermost layer of the epidermis. The molar ratio and molecular arrangement of lipid classes in the SC determine the integrity of this barrier. Increased chain length and polarity of ceramides, the most abundant lipid class in mammalian SC, contribute to tighter packing and thus to reduced cutaneous evaporative water loss (CEWL). However, tighter lipid packing also causes low SC hydration, making it brittle, whereas high hydration softens the skin at the cost of increasing CEWL. Cerebrosides are not present in the mammalian SC; their pathological accumulation occurs in Gaucher's disease, which leads to a dramatic increase in CEWL. However, cerebrosides occur normally in the SC of birds. We tested the hypothesis that cerebrosides are also present in the SC of bats, because they are probably necessary to confer pliability to the skin, a quality needed for flight. We examined the SC lipid composition of four sympatric bat species and found that, as in birds, their SC has substantial cerebroside contents, not associated with a pathological state, indicating convergent evolution between bats and birds.

Interfacial properties of avian stratum corneum monolayers investigated by Brewster angle microscopy and vibrational sum frequency generation

The outermost layer of skin, the stratum corneum (SC), contains a complex mixture of lipids, which controls the rate of cutaneous water loss (CWL) in reptiles, mammals, and birds. However, the molecular structure of SC lipids and how molecular configurations influence CWL is poorly understood. Here, the organization and structure of SC lipids extracted from birds were investigated by means of Langmuir films. Properties of lipids from the SC of arid and semi-arid adapted larks, known to have a low CWL, were compared with lipids extracted from the SC of mesic lark species with higher CWL to gain insight into how structure impacts CWL. Film properties were probed with surface pressure-area isotherms, Brewster angle microscopy (BAM), and vibrational sum frequency generation (VSFG). Results indicate organization and ordering of SC lipids in the arid-adapted hoopoe lark was vastly different from all other species, forming a miscible, rigid monolayer, whereas monolayers from semi-arid and mesic species were immiscible and disordered. Probing of interfacial water structure reveals that film morphology determines organization of water molecules near the monolayer; monolayers with a porous morphology had an increased population of water molecules that are weakly hydrogen-bonded. In general, CWL appears related to the miscibility and ordering of lipid components within the SC, as well as the ability of these lipids to interact with water molecules. From a broader perspective, CWL in larks appears linked to both the SC lipid composition and the aridity of the species' environment.

Organization of lipids in avian stratum corneum: Changes with temperature and hydration

In response to increases in ambient temperature (Ta), many animals increase total evaporative water loss (TEWL) through their skin and respiratory passages to maintain a constant body temperature, a response that compromises water balance. In birds, cutaneous water loss (CWL) accounts for approximately 65% of TEWL at thermoneutral temperatures. Although the proportion of TEWL accounted for by CWL decreases to only 25% at high Ta, the magnitude of CWL still increases, suggesting changes in the barrier function of the skin. The stratum corneum (SC) is composed of flat, dead cells called corneocytes embedded in a matrix of lipids, many of which arrange in layers called lamellae. The classes of lipids that comprise these lamellae, and their attendant physical properties, determine the rate of CWL. We measured CWL at 25, 30, 35, and 40 °C in House Sparrows (Passer domesticus) caught in the winter and summer, and in sparrows acclimated to warm and cold lab environments. We then used Fourier transform infrared spectroscopy to measure lipid-lipid and lipid-water interactions in the SC under different conditions of temperature and hydration, and correlated these results with lipid classes in the SC. As CWL increased at higher temperatures, the amount of gauche defects in lipid alkyl chains increased, indicating that lipid disorder is partially responsible for higher CWL at high temperatures. However, variation in CWL between groups could not be explained by the amount of gauche defects, and this remaining variation may be attributed to greater amounts of cerebrosides in birds with low CWL, as the sugar moieties of cerebrosides lie outside lipid lamellae and form strong hydrogen bonds with water molecules.

Lipid composition and molecular interactions change with depth in the avian stratum corneum to regulate cutaneous water loss

The outermost 10-20 µm of the epidermis, the stratum corneum (SC), consists of flat, dead cells embedded in a matrix of intercellular lipids. These lipids regulate cutaneous water loss (CWL), which accounts for over half of total water loss in birds. However, the mechanisms by which lipids are able to regulate CWL and how these mechanisms change with depth in the SC are poorly understood. We used attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) to measure lipid-lipid and lipid-water interactions as a function of depth in the SC of house sparrows (Passer domesticus Linnaeus) in the winter and summer. We then compared these molecular interactions at each depth with lipid composition at the same depth. We found that in both groups, water content increased with depth in the SC, and likely contributed to greater numbers of gauche defects in lipids in deeper levels of the SC. In winter-caught birds, which had lower rates of CWL than summer-caught birds, water exhibited stronger hydrogen bonding in deeper layers of the SC, and these strong hydrogen bonds were associated with greater amounts of polar lipids such as ceramides and cerebrosides. Based on these data, we propose a model by which polar lipids in deep levels of the SC form strong hydrogen bonds with water molecules to increase the viscosity of water and slow the permeation of water through the SC.

Analytical methods in sphingolipidomics: Quantitative and profiling approaches in food analysis

In recent years, sphingolipidomics has emerged as an interesting omic science that encompasses the study of the full sphingolipidome characterization, content, structure and activity in cells, tissues or organisms. Like other omics, it has the potential to impact biomarker discovery, drug development and systems biology knowledge. Concretely, dietary food sphingolipids have gained considerable importance due to their extensively reported bioactivity. Because of the complexity of this lipid family and their diversity among foods, powerful analytical methodologies are needed for their study. The analytical tools developed in the past have been improved with the enormous advances made in recent years in mass spectrometry (MS) and chromatography, which allow the convenient and sensitive identification and quantitation of sphingolipid classes and form the basis of current sphingolipidomics methodologies. In addition, novel hyphenated nuclear magnetic resonance (NMR) strategies, new ionization strategies, and MS imaging are outlined as promising technologies to shape the future of sphingolipid analyses. This review traces the analytical methods of sphingolipidomics in food analysis concerning sample extraction, chromatographic separation, the identification and quantification of sphingolipids by MS and their structural elucidation by NMR.

Skin lipids from Saudi Arabian birds

Skin lipids play an important role in the regulation of cutaneous water loss (CWL). Earlier studies have shown that Saudi desert birds exhibit a tendency of reduced CWL than birds from temperate environment due to adaptive changes in composition of their skin lipids. In this study, we used thin-layer chromatography (TLC) for separation and detection of non-polar and polar lipids from the skin of six bird species including sooty gull, brown booby, house sparrow, Arabian waxbill, sand partridge, and laughing dove. The lipids were separated and detected on Silica gel G coated TLC plates and quantified by using densitometric image analysis. Rf values of the non-polar lipids were as follows: cholesterol (0.29), free fatty acids (0.58), triacylglycerol (0.69), fatty acids methyl esters (0.84) and cholesterol ester (0.97). Rf values for the polar lipids were: cerebroside (0.42), ceramide (0.55) and cholesterol (0.73). The results showed the abundance of fatty acids methyl esters (47.75-60.46%) followed by triacylglycerol (12.69-24.14%). The remaining lipid compositions were as follows: cholesterol (4.09-13.18%), ceramide (2.18-13.27%), and cerebroside (2.53-12.81%). In conclusion, our findings showed that TLC is a simple and sensitive method for the separation and quantification of skin lipids. We also reported a new protocol for lipid extraction using the zirconia beads for efficient disruption of skin tissues. This study will help us better understand the role of skin lipids in adaptive physiology towards adverse climatic conditions.

Imaging mass spectrometry visualizes ceramides and the pathogenesis of dorfman-chanarin syndrome due to ceramide metabolic abnormality in the skin

Imaging mass spectrometry (IMS) is a useful cutting edge technology used to investigate the distribution of biomolecules such as drugs and metabolites, as well as to identify molecular species in tissues and cells without labeling. To protect against excess water loss that is essential for survival in a terrestrial environment, mammalian skin possesses a competent permeability barrier in the stratum corneum (SC), the outermost layer of the epidermis. The key lipids constituting this barrier in the SC are the ceramides (Cers) comprising of a heterogeneous molecular species. Alterations in Cer composition have been reported in several skin diseases that display abnormalities in the epidermal permeability barrier function. Not only the amounts of different Cers, but also their localizations are critical for the barrier function. We have employed our new imaging system, capable of high-lateral-resolution IMS with an atmospheric-pressure ionization source, to directly visualize the distribution of Cers. Moreover, we show an ichthyotic disease pathogenesis due to abnormal Cer metabolism in Dorfman–Chanarin syndrome, a neutral lipid storage disorder with ichthyosis in human skin, demonstrating that IMS is a novel diagnostic approach for assessing lipid abnormalities in clinical setting, as well as for investigating physiological roles of lipids in cells/tissues.

Atmospheric pressure photoionization as a powerful tool for large-scale lipidomic studies

Lipidomic studies often use liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) for separation, identification, and quantification. However, due to the wide structural diversity of lipids, the most apolar part of the lipidome is often detected with low sensitivity in ESI. Atmospheric pressure (APPI) can be an alternative ionization source since normal-phase solvents are known to enhance photoionization of these classes. In this paper, we intend to show the efficiency of APPI to identify different lipid classes, with a special interest on sphingolipids. In-source APPI fragmentation appears to be an added value for the structural analysis of lipids. It provides a detailed characterization of both the polar head and the non polar moiety of most lipid classes, and it makes possible the detection of all lipids in both polarities, which is not always possible with ESI.

Lipid composition of the stratum corneum and cutaneous water loss in birds along an aridity gradient

Intercellular and covalently bound lipids within the stratum corneum (SC), the outermost layer of the epidermis, are the primary barrier to cutaneous water loss (CWL) in birds. We compared CWL and intercellular SC lipid composition in 20 species of birds from desert and mesic environments. Furthermore, we compared covalently bound lipids with CWL and intercellular lipids in the lark family (Alaudidae). We found that CWL increases in birds from more mesic environments, and this increase was related to changes in intercellular SC lipid composition. The most consistent pattern that emerged was a decrease in the relative amount of cerebrosides as CWL increased, a pattern that is counterintuitive based on studies of mammals with Gaucher disease. Although covalently bound lipids in larks did not correlate with CWL, we found that covalently bound cerebrosides correlated positively with intercellular cerebrosides and intercellular cholesterol ester, and intercellular cerebrosides correlated positively with covalently bound free fatty acids. Our results led us to propose a new model for the organization of lipids in the avian SC, in which the sugar moieties of cerebrosides lie outside of intercellular lipid layers, where they may interdigitate with adjacent intercellular cerebrosides or with covalently bound cerebrosides.

Comprehensive quantification of ceramide species in human stratum corneum

One of the key challenges in lipidomics is to quantify lipidomes of interest, as it is practically impossible to collect all authentic materials covering the targeted lipidomes. For diverse ceramides (CER) in human stratum corneum (SC) that play important physicochemical roles in the skin, we developed a novel method for quantification of the overall CER species by improving our previously reported profiling technique using normal-phase liquid chromatography-electrospray ionization-mass spectrometry (NPLC-ESI-MS). The use of simultaneous selected ion monitoring measurement of as many as 182 kinds of molecular-related ions enables the highly sensitive detection of the overall CER species, as they can be analyzed in only one SC-stripped tape as small as 5 mm x 10 mm. To comprehensively quantify CERs, including those not available as authentic species, we designed a procedure to estimate their levels using relative responses of representative authentic species covering the species targeted, considering the systematic error based on intra-/inter-day analyses. The CER levels obtained by this method were comparable to those determined by conventional thin-layer chromatography (TLC), which guarantees the validity of this method. This method opens lipidomics approaches for CERs in the SC.

Cutaneous water loss and sphingolipids covalently bound to corneocytes in the stratum corneum of house sparrows Passer domesticus

The barrier to water loss from the skin of birds and mammals is localized in the stratum corneum (SC), the outer layer of the epidermis. The SC consists of corneocytes, each surrounded by a protein envelope, and a lipid compartment, formed by an extracellular matrix of lipids and by lipids covalently bound to the protein envelope. In mammals, covalently bound lipids in the SC consist of ω-hydroxyceramides attached to the outer surface of corneocytes. Evidence suggests that covalently bound lipids in the SC might be crucial for the establishment of a competent permeability barrier. In this study we assessed the composition of covalently bound lipids of the avian SC and their relationship to cutaneous water loss (CWL) in two populations of house sparrows, one living in the deserts of Saudi Arabia and the other in mesic Ohio. Previously, we showed that CWL of adult desert sparrows was 25%lower than that of mesic birds. In the present study we characterize covalently bound lipids of the SC using thin layer chromatography and high performance liquid chromatography coupled with atmospheric pressure Photospray® ionization mass spectrometry. Our study is the first to demonstrate the existence of sphingolipids covalently bound to corneocytes in the SC of birds. Although ω-hydroxyceramides occurred in the lipid envelope surrounding corneocytes, the major constituent of the covalently bound lipid envelope in house sparrows was ω-hydroxycerebrosides,ceramides with a hexose molecule attached. Sparrows from Saudi Arabia had more covalently bound cerebrosides, fewer covalently bound ceramides and a lower ceramide to cerebroside ratio than sparrows living in Ohio; these differences were associated with CWL.

Cutaneous water loss and sphingolipids in the stratum corneum of house sparrows, Passer domesticus L., from desert and mesic environments as determined by reversed phase high-performance liquid chromatography coupled with atmospheric pressure photospray ionization mass spectrometry

Because cutaneous water loss (CWL) represents half of total water loss in birds, selection to reduce CWL may be strong in desert birds. We previously found that CWL of house sparrows from a desert population was about 25% lower than that of individuals from a mesic environment. The stratum corneum (SC), the outer layer of the epidermis, serves as the primary barrier to water vapor diffusion through the skin. The avian SC is formed by layers of corneocytes embedded in a lipid matrix consisting of cholesterol, free fatty acids and two classes of sphingolipids, ceramides and cerebrosides. The SC of birds also serves a thermoregulatory function; high rates of CWL keep body temperatures under lethal limits in episodes of heat stress. In this study, we used high-performance liquid chromatography coupled with atmospheric pressure photoionization-mass spectrometry (HPLC/APPI-MS) to identify and quantify over 200 sphingolipids in the SC of house sparrows from desert and mesic populations. Principal components analysis (PCA) led to the hypotheses that sphingolipids in the SC of desert sparrows have longer carbon chains in the fatty acid moiety and are more polar than those found in mesic sparrows. We also tested the association between principal components and CWL in both populations. Our study suggested that a reduction in CWL found in desert sparrows was, in part, the result of modifications in chain length and polarity of the sphingolipids, changes that apparently determine the interactions of the lipid molecules within the SC.

Loss of functional ELOVL4 depletes very long-chain fatty acids (> or =C28) and the unique omega-O-acylceramides in skin leading to neonatal death

Mutations in elongation of very long-chain fatty acid-4 (ELOVL4) are associated with autosomal dominant Stargardt-like macular degeneration (STGD3), with a five base-pair (5 bp) deletion mutation resulting in the loss of 51 carboxy-terminal amino acids and truncation of the protein. In addition to the retina, Elovl4 is expressed in a limited number of mammalian tissues, including skin, with unknown function(s). We generated a knock-in mouse model with the 5-bp deletion in the Elovl4 gene. As anticipated, mice carrying this mutation in the heterozygous state (Elovl4(+/del)) exhibit progressive photoreceptor degeneration. Unexpectedly, homozygous mice (Elovl4(del/del)) display scaly, wrinkled skin, have severely compromised epidermal permeability barrier function, and die within a few hours after birth. Histopathological evaluation of the Elovl4(del/del) pups revealed no apparent abnormality(ies) in vital internal organs. However, skin histology showed an abnormally-compacted outer epidermis [stratum corneum (SC)], while electron microscopy revealed deficient epidermal lamellar body contents, and lack of normal SC lamellar membranes that are essential for permeability barrier function. Lipid analyses of epidermis from Elovl4(del/del) mice revealed a global decrease in very long-chain fatty acids (VLFAs) (i.e., carbon chain > or =C28) in both the ceramide/glucosylceramide and the free fatty-acid fractions. Strikingly, Elovl4(del/del) skin was devoid of the epidermal-unique omega-O-acylceramides, that are key hydrophobic components of the extracellular lamellar membranes in mammalian SC. These findings demonstrate that ELOVL4 is required for generating VLFA critical for epidermal barrier function, and that the lack of epidermal omega-O-acylceramides is incompatible with survival in a desiccating environment.

Localization of Fatty Acids with Selective Chain Length by Imaging Time-of-Flight Secondary Ion Mass Spectrometry

Localization of fatty acids in biological tissues was made by using TOF-SIMS (time-of-flight secondary ion mass spectrometry). Two cell-types with a specific fatty acid distribution are shown. In rat cerebellum, different distribution patterns of stearic acid (C18:0), palmitic acid (C16:0), and oleic acid (C18:1) were found. Stearic acid signals were observed accumulated in Purkinje cells with high intensities inside the cell, but not in the nucleus region. The signals colocalized with high intensity signals of the phosphocholine head group, indicating origin from phosphatidylcholine or sphingomyelin. In mouse intestine, high palmitic acid signals were found in the secretory crypt cells together with high levels of phosphorylinositol colocalized in the crypt region. Palmitic acid was also seen in the intestinal lumen that contains high amounts of mucine, which is known to be produced in the crypt cells. Linoleic acid signals (C18:2) were low in the crypt region and high in the villus region. Oleic acid signals were seen in the villi and stearic acid signals were ubiquitous with no specific localization in the intestine. We conclude that the results obtained by using imaging TOF-SIMS are consistent with known brain and intestine biochemistry and that the localization of fatty acids is specific in differentiated cells.