Selective desorption/ionization of sulfatides by MALDI-MS facilitated using 9-aminoacridine as matrix

Hydroxylated and non-hydroxylated sulfatide are distinctly distributed in the human cerebral cortex

Sulfatide (ST) is a sphingolipid with an important role in the central nervous system as a major component of the myelin sheath. ST contains a structurally variable ceramide moiety, with a fatty acid substituent of varying carbon-chain length and double-bond number. Hydroxylation at the α-2 carbon position of the fatty acid is found in half the population of ST molecules. Recent genetic studies of fatty acid 2-hydroxylase (FA2H) indicate that these hydroxylated sphingolipids influence myelin sheath stability. However, their distribution is unknown. Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) enables the analysis of distinct distributions of individual ST molecular species in tissue section. We examined human cerebral cortex tissue sections with MALDI-IMS, identifying and characterizing the distributions of 14 ST species. The distribution analysis reveals that the composition ratios of non-hydroxylated/hydroxylated STs are clearly reversed at the border between white and gray matter; the hydroxylated group is the dominant ST species in the gray matter. These results suggest that hydroxylated STs are highly expressed in oligodendrocytes in gray matter and might form stable myelin sheaths. As a clinical application, we analyzed a brain with Alzheimer's disease (AD) as a representative neurodegenerative disease. Although previous studies of AD pathology have reported that the amount of total ST is decreased in the cerebral cortex, as far as the compositional distributions of STs are concerned, AD brains were similar to those in control brains. In conclusion, we suggest that MALDI-IMS is a useful tool for analysis of the distributions of various STs and this application might provide novel insight in the clinical study of demyelinating diseases.

Analysis of mammalian sphingolipids by liquid chromatography tandem mass spectrometry (LC-MS/MS) and tissue imaging mass spectrometry (TIMS)

Sphingolipids are a highly diverse category of molecules that serve not only as components of biological structures but also as regulators of numerous cell functions. Because so many of the structural features of sphingolipids give rise to their biological activity, there is a need for comprehensive or "sphingolipidomic" methods for identification and quantitation of as many individual subspecies as possible. This review defines sphingolipids as a class, briefly discusses classical methods for their analysis, and focuses primarily on liquid chromatography tandem mass spectrometry (LC-MS/MS) and tissue imaging mass spectrometry (TIMS). Recently, a set of evolving and expanding methods have been developed and rigorously validated for the extraction, identification, separation, and quantitation of sphingolipids by LC-MS/MS. Quantitation of these biomolecules is made possible via the use of an internal standard cocktail. The compounds that can be readily analyzed are free long-chain (sphingoid) bases, sphingoid base 1-phosphates, and more complex species such as ceramides, ceramide 1-phosphates, sphingomyelins, mono- and di-hexosylceramides, sulfatides, and novel compounds such as the 1-deoxy- and 1-(deoxymethyl)-sphingoid bases and their N-acyl-derivatives. These methods can be altered slightly to separate and quantitate isomeric species such as glucosyl/galactosylceramide. Because these techniques require the extraction of sphingolipids from their native environment, any information regarding their localization in histological slices is lost. Therefore, this review also describes methods for TIMS. This technique has been shown to be a powerful tool to determine the localization of individual molecular species of sphingolipids directly from tissue slices.

The modified-bead stretched sample method: development and application to MALDI-MS imaging of protein localization in the spinal cord

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) has been used to create spatial distribution maps from lipids, peptides, and proteins in a variety of biological tissues. MALDI-MSI often involves trade-offs between the extent of analyte extraction and desired spatial resolution, compromises that can adversely affect detectability. For example, increasing the extraction time can lead to unwanted analyte spatial redistribution. With the stretched sample method (SSM), the extraction period can be extended, resulting in reduced analyte redistribution while suppressing detection of cationic salt adducts. The SSM involves thaw-mounting a thin tissue section onto a substrate of small glass beads embedded in Parafilm M and then stretching the membrane to fragment the tissue into thousands of bead-sized pieces. Here, we applied the SSM method to MALDI-MSI using rat spinal cord as a model. We used surface-modified beads coated with trypsin or chymotrypsin in order to facilitate controlled digestion and detection of proteins. The enzymatic reactions were maintained by repeatedly condensing water on the stretched sample surface. As a result, new peptides formed by tryptic or chymotryptic protein digestion were detected and identified using a combination of MALDI-MSI and offline liquid chromatography tandem mass spectrometric analysis. Localization of these peptides indicated the distribution of their proteins of origin, including myelin basic protein, actin beta, and tubulin alpha chain. Additionally, we used uncoated beads to create distribution maps of many endogenous lipids and small peptides. The extension of the SSM using modified beads resulted in the creation of mosaic bead surfaces where adjacent beads were coated with different enzymes or other reactive chemicals, permitting investigation of the distributions of a wider range of analytes in biological samples within a single experiment.

Quantitative analysis of phospholipids using nanostructured laser desorption ionization targets

Since its introduction as an ionization technique in mass spectrometry, matrix-assisted laser desorption ionization (MALDI) has been applied to a wide range of applications. Quantitative small molecule analysis by MALDI, however, is limited due to the presence of intense signals from the matrix coupled with non-homogeneous surfaces. The surface used in nano-structured laser desorption ionization (NALDI) eliminates the need for a matrix and the resulting interferences, and allows for quantitative analysis of small molecules. This study was designed to analyze and quantitate phospholipid components of liposomes. Here we have developed an assay to quantitate the DPPC and DC(8,9)PC in liposomes by NALDI following various treatments. To test our method we chose to analyze a liposome system composed of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DC(8,9)PC (1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine), as DC(8,9)PC is known to undergo cross-linking upon treatment with UV (254 nm) and this reaction converts the monomer into a polymer. First, calibration curves for pure lipids (DPPC and DC(8,9)PC) were created using DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) as an internal standard. The calibration curve for both DPPC and DC(8,9)PC showed an R(2) of 0.992, obtained using the intensity ratio of analyte and internal standard. Next, DPPC:DC(8,9)PC liposomes were treated with UV radiation (254 nm). Following this treatment, lipids were extracted from the liposomes and analyzed. The analysis of the lipids before and after UV exposure confirmed a decrease in the signal of DC(8,9)PC of about 90%. In contrast, there was no reduction in DPPC signal.

Sphingomyelin is more sensitively detectable as a negative ion than phosphatidylcholine: a matrix-assisted laser desorption/ionization time-of-flight mass spectrometric study using 9-aminoacridine (9-AA) as matrix

Phospholipids (PLs) are increasingly analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) and imaging MS. Different classes of PLs are preferentially detectable either as positive or negative ions depending on the charges of their headgroups. Sphingomyelin (SM) and phosphatidylcholine (PC) occur in virtually all biological samples and both are assumed to be detectable with the same sensitivity (in the positive ion mode) because their headgroups are identical. We will show here that the detectabilities of PC and SM depend on the matrix used. In the presence of 2,5-dihydroxybenzoic acid (DHB) SM is more sensitively detectable in positive ion mode than PC while the use of 9-aminoacridine (9-AA) as matrix inverts the detectabilities. Our explanation is that the preferred generation of negative ions from SM if 9-AA is used as matrix results in a reduced yield of positive ions. It will also be shown that this is not only valid if a simplified model system is investigated, but also if, for instance, extracts from human erythrocytes are investigated. It will also be outlined that this finding is particularly important in the context of imaging studies where no previous separation of the lipids of interest can be performed.

Imaging of complex sulfatides SM3 and SB1a in mouse kidney using MALDI-TOF/TOF mass spectrometry

Sulfatides, a class of acidic glycosphingolipids, are highly expressed in mammalian myelin and in kidney, where they are thought to stabilize neuronal structures and signaling and to influence osmotic stability of renal cells, respectively. Recently, 9-aminoacridine (9-AA) has been introduced as a negative ion matrix that displays high selectivity for low complexity galactosylceramid-I(3)-sulfate sulfatides and that is suitable for quantitative analysis by matrix-assisted desorption/ionization (MALDI) mass spectrometry (MS). Analyzing acidic fractions of lipid extracts and cryosections from kidneys of wild type and arylsulfatase A-deficient (ASA -/-) mice, we demonstrate that 9-AA also enables sensitive on-target analysis as well as imaging of complex lactosylceramide-II(3)-sulfate and gangliotetraosylceramide-II(3), IV(3) bis-sulfate sulfatides by MALDI-TOF/TOF MS. Utilizing the MALDI imaging MS technique, we show differential localization in mouse kidney of (1) sulfatides with identical ceramide anchors, but different glycan-sulfate head groups but also of (2) sulfatides with identical head groups but with different acyl- or sphingoid base moieties. A comparison of MALDI images of renal sulfatides from control and sulfatide storing arylsulfatase A-deficient (ASA -/-) mice revealed relative expression differences, very likely reflecting differences in sulfatide turnover of the various renal cell types. These results establish MALDI imaging MS with 9-AA matrix as a label-free method for spatially resolved ex vivo investigation of the relative turnover of sulfatides in animal models of human glycosphingolipid storage disease.

Highly sensitive localization analysis of gangliosides and sulfatides including structural isomers in mouse cerebellum sections by combination of laser microdissection and hydrophilic interaction liquid chromatography/electrospray ionization mass spectrometry with theoretically expanded multiple reaction monitoring

Liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) is suitable for analysis of glycosphingolipids such as fragile gangliosides avoiding the use of the sialic acid elimination. However, it was not possible to distinguish the structural isomers such as GD1a and GD1b with reversed-phase LC/ESI-MS by hydrophobic interaction. Here we report an effective method for targeted analysis of theoretically expanded ganglioside molecular species including structural isomers by hydrophilic interaction liquid chromatography (HILIC)/ESI-MS with multiple reaction monitoring (MRM). As a result of MRM analysis of glycosphingolipid mixtures from porcine brain, each of the lipid classes was detected within 25 min in the following order: sulfatides > GM3 > GM2 > GM1 > GD3 > GD1a > GD2 > GD1b > GT1a > GT1b > GQ1b. For the advanced application, localization analysis of postnatal day 15 (P15) mouse cerebellum layered structures was carried out by combination of MRM and laser microdissection (LMD). As a result, GM3, GD1a, GT1b and GQ1b were abundantly detected in the molecular and granular layers, whereas GM1 was widely presented in each layered structure. These gangliosides were mainly composed of d18:1-18:0 and d18:1-20:0, but GM3 was d18:1-16:0 and d18:1-20:0. Meanwhile, sulfatide molecular species were mostly localized in the myelinated fibers and scarcely found in the molecular layer. These results suggested that our method is suitable to detect a variety of ganglioside classes and sulfatides with high sensitivity at the molecular species level and effective for localization analysis of these glycosphingolipids from mouse brain sections.

An update of MALDI-TOF mass spectrometry in lipid research

Although matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS)--often but not exclusively coupled with a time-of-flight (TOF) mass analyzer--is primarily established in the protein field, there is increasing evidence that MALDI MS is also very useful in lipid research: MALDI MS is fast, sensitive, tolerates sample impurities to a relatively high extent and provides very simple mass spectra without major fragmentation of the analyte. Additionally, MALDI MS devices originally purchased for "proteomics" can be used also for lipids without the need of major system alterations. After a short introduction into the method and the related ion-forming process, the MALDI mass spectrometric characteristics of the individual lipid (ranging from completely apolar hydrocarbons to complex glycolipids with the focus on glycerophospholipids) classes will be discussed and the progress achieved in the last years emphasized. Special attention will be paid to quantitative aspects of MALDI MS because this is normally considered to be the "weak" point of the method, particularly if complex lipid mixtures are to be analyzed. Although the detailed role of the matrix is not yet completely clear, it will be also explicitly shown that the careful choice of the matrix is crucial in order to be able to detect all compounds of interest. Two rather recent developments will be highlighted: "Imaging" MS is nowadays widely established and significant interest is paid in this context to the analysis of lipids because lipids ionize particularly well and are, thus, more sensitively detectable in tissue slices than other biomolecules such as proteins. It will also be shown that MALDI MS can be very easily combined with thin-layer chromatography (TLC) allowing the spatially-resolved screening of the entire TLC plate and the detection of lipids with a higher sensitivity than common staining protocols.

MALDI-TOF/MS analysis of archaebacterial lipids in lyophilized membranes dry-mixed with 9-aminoacridine

A method of direct lipid analysis by MALDI mass spectrometry in intact membranes, without prior extraction/separation steps, is described. The purple membrane isolated from the extremely halophilic archaeon Halobacterium salinarum was selected as model membrane. Lyophilized purple membrane were grinded with 9-aminoacridine (9-AA) as dry matrix, and the powder mixture was crushed in a mechanical die press to form a thin pellet. Small pieces of the pellet were then attached to the MALDI target and directly analyzed. In parallel, individual archaebacterial phospholipids and glycolipids, together with the total lipid extract of the purple membrane, were analyzed by MALDI-TOF/MS using 9-AA as the matrix in solution. Results show that 9-AA represents a suitable matrix for the conventional MALDI-TOF/MS analysis of lipid extracts from archaeal microorganisms, as well as for fast and reliable direct dry lipid analysis of lyophilized archaebacterial membranes. This method might be of general application, offering the advantage of quickly gaining information about lipid components without disrupting or altering the membrane matrix.

Dynamic simulation of cardiolipin remodeling: greasing the wheels for an interpretative approach to lipidomics

Cardiolipin is a class of mitochondrial specific phospholipid, which is intricately involved in mitochondrial functionality. Differences in cardiolipin species exist in a variety of tissues and diseases. It has been demonstrated that the cardiolipin profile is a key modulator of the functions of many mitochondrial proteins. However, the chemical mechanism(s) leading to normal and/or pathological distribution of cardiolipin species remain elusive. Herein, we describe a novel approach for investigating the molecular mechanism of cardiolipin remodeling through a dynamic simulation. This approach applied data from shotgun lipidomic analyses of the heart, liver, brain, and lung mitochondrial lipidomes to model cardiolipin remodeling, including relative content, regiospecificity, and isomeric composition of cardiolipin species. Generated cardiolipin profiles were nearly identical to those determined by shotgun lipidomics. Importantly, the simulated isomeric compositions of cardiolipin species were further substantiated through product ion analysis. Finally, unique enzymatic activities involved in cardiolipin remodeling were assessed from the parameters used in the dynamic simulation of cardiolipin profiles. Collectively, we described, verified, and demonstrated a novel approach by integrating both lipidomic analysis and dynamic simulation to study cardiolipin biology. We believe this study provides a foundation to investigate cardiolipin metabolism and bioenergetic homeostasis in normal and disease states.