Direct profiling of tissue lipids by MALDI-TOFMS

Cell and Tissue Imaging by TOF-SIMS and MALDI-TOF: An Overview for Biological and Pharmaceutical Analysis

The potential of mass spectrometry imaging (MSI) has been demonstrated in cell and tissue research since 1970. MSI can reveal the spatial distribution of a wide range of atomic and molecular ions detected from biological sample surfaces, it is a powerful and valuable technique used to monitor and detect diverse chemical and biological compounds, such as drugs, lipids, proteins, and DNA. MSI techniques, notably matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) and time of flight secondary ion mass spectrometry (TOF-SIMS), witnessed a dramatic upsurge in studying and investigating biological samples especially, cells and tissue sections. This advancement is attributed to the submicron lateral resolution, the high sensitivity, the good precision, and the accurate chemical specificity, which make these techniques suitable for decoding and understanding complex mechanisms of certain diseases, as well as monitoring the spatial distribution of specific elements, and compounds. While the application of both techniques for the analysis of cells and tissues is thoroughly discussed, a briefing of MALDI-TOF and TOF-SIMS basis and the adequate sampling before analysis are briefly covered. The importance of MALDI-TOF and TOF-SIMS as diagnostic tools and robust analytical techniques in the medicinal, pharmaceutical, and toxicology fields is highlighted through representative published studies.

Tissue fixation effects on human retinal lipid analysis by MALDI imaging and LC-MS/MS technologies

Imaging mass spectrometry (IMS) allows the location and abundance of lipids to be mapped across tissue sections of human retina. For reproducible and accurate information, sample preparation methods need to be optimized. Paraformaldehyde fixation of a delicate multilayer structure like human retina facilitates the preservation of tissue morphology by forming methylene bridge crosslinks between formaldehyde and amine/thiols in biomolecules; however, retina sections analyzed by IMS are typically fresh-frozen. To determine if clinically significant inferences could be reliably based on fixed tissue, we evaluated the effect of fixation on analyte detection, spatial localization, and introduction of artifactual signals. Hence, we assessed the molecular identity of lipids generated by matrix-assisted laser desorption ionization (MALDI-IMS) and liquid chromatography coupled tandem mass spectrometry (LC-MS/MS) for fixed and fresh-frozen retina tissues in positive and negative ion modes. Based on MALDI-IMS analysis, more lipid signals were observed in fixed compared with fresh-frozen retina. More potassium adducts were observed in fresh-frozen tissues than fixed as the fixation process caused displacement of potassium adducts to protonated and sodiated species in ion positive ion mode. LC-MS/MS analysis revealed an overall decrease in lipid signals due to fixation that reduced glycerophospholipids and glycerolipids and conserved most sphingolipids and cholesteryl esters. The high quality and reproducible information from untargeted lipidomics analysis of fixed retina informs on all major lipid classes, similar to fresh-frozen retina, and serves as a steppingstone towards understanding of lipid alterations in retinal diseases.

Innovation in drug toxicology: Application of mass spectrometry imaging technology

Mass spectrometry imaging (MSI) is a powerful molecular imaging technology that can obtain qualitative, quantitative, and location information by simultaneously detecting and mapping endogenous or exogenous molecules in biological tissue slices without specific chemical labeling or complex sample pretreatment. This article reviews the progress made in MSI and its application in drug toxicology research, including the tissue distribution of toxic drugs and their metabolites, the target organs (liver, kidney, lung, eye, and central nervous system) of toxic drugs, the discovery of toxicity-associated biomarkers, and explanations of the mechanisms of drug toxicity when MSI is combined with the cutting-edge omics methodologies. The unique advantages and broad prospects of this technology have been fully demonstrated to further promote its wider use in the field of pharmaceutical toxicology.

Mass spectrometry imaging of mice brain lipid profile changes over time under high fat diet

Overweight and obesity have been shown to significantly affect brain structures and size. Obesity has been associated with cerebral atrophy, alteration of brain functions, including cognitive impairement, and psychiatric diseases such as depression. Given the importance of lipids in the structure of the brain, here, by using 47 mice fed a high fat diet (HFD) with 60% calories from fat (40% saturated fatty acids) and 20% calories from carbohydrates and age-matched control animals on a normal chow diet, we examined the effects of HFD and diet-induced obesity on the brain lipidome. Using a targeted liquid chromatography mass spectrometry analysis and a non-targeted mass spectrometry MALDI imaging approach, we show that the relative concentration of most lipids, in particular brain phospholipids, is modified by diet-induced obesity (+ 40%of body weight). Use of a non-targeted MALDI-MS imaging approach further allowed define cerebral regions of interest (ROI) involved in eating behavior and changes in their lipid profile. Principal component analysis (PCA) of the obese/chow lipidome revealed persistence of some of the changes in the brain lipidome of obese animals even after their switch to chow feeding and associated weight loss. Altogether, these data reveal that HFD feeding rapidly modifies the murine brain lipidome. Some of these HFD-induced changes persist even after weight loss, implying that some brain sequelae caused by diet-induced obesity are irreversible.

Dual-polarity SALDI FT-ICR MS imaging and Kendrick mass defect data filtering for lipid analysis

Lipids are biomolecules of crucial importance involved in critical biological functions. Yet, lipid content determination using mass spectrometry is still challenging due to their rich structural diversity. Preferential ionisation of the different lipid species in the positive or negative polarity is common, especially when using soft ionisation mass spectrometry techniques. Here, we demonstrate the potency of a dual-polarity approach using surface-assisted laser desorption/ionisation coupled to Fourier transform-ion cyclotron resonance (SALDI FT-ICR) mass spectrometry imaging (MSI) combined with Kendrick mass defect data filtering to (i) identify the lipids detected in both polarities from the same tissue section and (ii) show the complementarity of the dual-polarity data, both regarding the lipid coverage and the spatial distributions of the various lipids. For this purpose, we imaged the same mouse brain section in the positive and negative ionisation modes, on alternate pixels, in a SALDI FT-ICR MS imaging approach using gold nanoparticles (AuNPs) as dual-polarity nanosubstrates. Our study demonstrates, for the first time, the feasibility of (i) a dual-polarity SALDI-MSI approach on the same tissue section, (ii) using AuNPs as nanosubstrates combined with a FT-ICR mass analyser and (iii) the Kendrick mass defect data filtering applied to SALDI-MSI data. In particular, we show the complementarity in the lipids detected both in a given ionisation mode and in the two different ionisation modes. Graphical abstract.

Mass spectrometry imaging of lipids: untargeted consensus spectra reveal spatial distributions in Niemann-Pick disease type C1

Mass spectrometry imaging (MSI) is a tool to rapidly map the spatial location of analytes without the need for tagging or a reporter system. Niemann-Pick disease type C1 (NPC1) is a neurodegenerative, lysosomal storage disorder characterized by accumulation of unesterified cholesterol and sphingolipids in the endo-lysosomal system. Here, we use MSI to visualize lipids including cholesterol in cerebellar brain tissue from the NPC1 symptomatic mouse model and unaffected controls. To complement the imaging studies, a data-processing pipeline was developed to generate consensus mass spectra, thereby using both technical and biological image replicates to assess differences. The consensus spectra are used to determine true differences in lipid relative abundance; lipid distributions can be determined in an unbiased fashion without prior knowledge of location. We show the cerebellar distribution of gangliosides GM1, GM2, and GM3, including variants of lipid chain length. We also performed MALDI-MSI of cholesterol. Further analysis of lobules IV/V and X of the cerebellum gangliosides indicates regional differences. The specificity achieved highlights the power of MSI, and this new workflow demonstrates a universal approach for addressing reproducibility in imaging experiments applied to NPC1.

AP-MALDI Mass Spectrometry Imaging of Gangliosides Using 2,6-Dihydroxyacetophenone

Matrix-assisted laser/desorption ionization (MALDI) mass spectrometry imaging (MSI) is widely used as a unique tool to record the distribution of a large range of biomolecules in tissues. 2,6-Dihydroxyacetophenone (DHA) matrix has been shown to provide efficient ionization of lipids, especially gangliosides. The major drawback for DHA as it applies to MS imaging is that it sublimes under vacuum (low pressure) at the extended time necessary to complete both high spatial and mass resolution MSI studies of whole organs. To overcome the problem of sublimation, we used an atmospheric pressure (AP)-MALDI source to obtain high spatial resolution images of lipids in the brain using a high mass resolution mass spectrometer. Additionally, the advantages of atmospheric pressure and DHA for imaging gangliosides are highlighted. The imaging of [M-H]^- and [M-H₂O-H]^- mass peaks for GD₁ gangliosides showed different distribution, most likely reflecting the different spatial distribution of GD_1a and GD_1b species in the brain. Graphical Abstract ᅟ.

Arachidonic acid containing phosphatidylcholine increases due to microglial activation in ipsilateral spinal dorsal horn following spared sciatic nerve injury

Peripheral nerve injury induces substantial molecular changes in the somatosensory system that leads to maladaptive plasticity and cause neuropathic pain. Understanding the molecular pathways responsible for the development of neuropathic pain is essential to the development of novel rationally designed therapeutics. Although lipids make up to half of the dry weight of the spinal cord, their relation with the development of neuropathic pain is poorly understood. We aimed to elucidate the regulation of spinal lipids in response to neuropathic peripheral nerve injury in mice by utilizing matrix-assisted laser desorption/ionization imaging mass spectrometry, which allows visualization of lipid distribution within the cord. We found that arachidonic acid (AA) containing [PC(diacyl-16:0/20:4)+K]⁺ was increased temporarily at superficial ipsilateral dorsal horn seven days after spared nerve injury (SNI). The spatiotemporal changes in lipid concentration resembled microglia activation as defined by ionized calcium binding adaptor molecule 1 (Iba1) immunohistochemistry. Suppression of microglial function through minocycline administration resulted in attenuation of hypersensitivity and reduces [PC(diacyl-16:0/20:4)+K]⁺ elevation in the spinal dorsal horn. These data suggested that AA containing [PC(diacyl-16:0/20:4)+K]⁺ is related to hypersensitivity evoked by SNI and implicate microglial cell activation in this lipid production.

Multistage Mass Spectrometry of Phospholipids using Collision-Induced Dissociation (CID) and Metastable Atom-Activated Dissociation (MAD)

We herein demonstrate an approach to gas phase ion manipulation that provides MS3-level CID spectra of phospholipid radical cations that are almost independent of the original charging adduct ions. In the MS2 He-MAD spectra of the protonated, sodiated and potassiated adducts of POPC, the different adducts induce different primary fragmentation pathways and provide significantly different spectra, as is commonly observed by other activation methods. In separate experiments, the even-electron adduct ions ([M+H]+, [M+Na]+, [M+K]+) of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) were first converted to radical cations [POPC]+• by using helium metastable atom-activated dissociation (He-MAD) to eject the charging adduct ions, then exposed to low-energy collision induced dissociation (CID) to induce extensive fragmentation along the acyl chains. Such charge-remote fragmentation is generally inaccessible through low-energy CID of the even-electron precursor ions. The combination of He-MAD and CID provides radical-induced spectra that show very major similarities and only minor differences, and therefore overcomes major differences in chemistry that are otherwise observed by the original adducting species. Collisional activation of even-electron [POPC+H]+ required higher CID amplitudes than odd-electron [POPC]+• to effect fragmentation-as expected-and the latter provided fragments within the acyl chains that were influenced by the double bond position.

Increased arachidonic acid-containing phosphatidylcholine is associated with reactive microglia and astrocytes in the spinal cord after peripheral nerve injury

Peripheral nerve injury (PNI) triggers cellular and molecular changes in the spinal cord. However, little is known about how the polyunsaturated fatty acid-containing phosphatidylcholines (PUFA-PCs) are regulated in the spinal cord after PNI and the association of PUFA-PCs with the non-neuronal cells within in the central nervous system (CNS). In this study, we found that arachidonic acid-containing phosphatidylcholine (AA-PC), [PC(16:0/20:4)+K](+), was significantly increased in the ipsilateral ventral and dorsal horns of the spinal cord after sciatic nerve transection, and the increased expression of [PC(16:0/20:4)+K](+) spatiotemporally resembled the increase of reactive microglia and the astrocytes. From the lipidomics point of view, we conclude that [PC(16:0/20:4)+K](+) could be the main phospholipid in the spinal cord influenced by PNI, and the regulation of specific phospholipid molecule in the CNS after PNI is associated with the reactive microglia and astrocytes.

Use of advantageous, volatile matrices enabled by next-generation high-speed matrix-assisted laser desorption/ionization time-of-flight imaging employing a scanning laser beam

RATIONALE

In mass spectrometry imaging (MSI) it is often desirable to analyse the same sample in both polarities to extract the most information. However, many matrices that produce high-quality spectra in matrix-assisted laser desorption/ionization (MALDI) are volatile, greatly limiting their use in long imaging experiments. We demonstrate that using a new high speed MALDI-MSI instrument, volatile matrices, including those that produce intense lipid signals in both positive and negative ion mode, can now be effectively used in MSI.

METHODS

A prototype Bruker rapifleX MALDI Tissuetyper™ time-of-flight (TOF) instrument was used for high-speed imaging. This allows acquisition rates up to 50 pixels/s made possible by use of a 10 kHz laser and two rotating mirrors that allow the laser beam to be moved over, and synchronised with, the rapidly moving sample. MSI experiments were performed on mouse brain sections using non-vacuum stable dithranol and 2,6-dihydroxyacetophenone (DHA) matrices with pixel sizes ranging from 10 × 10 µm(2) to 50 × 50 µm(2).

RESULTS

Both DHA and dithranol produced rich, complementary lipid spectra in both positive and negative ion modes. Due to the rapid acquisition speed of the instrument, both matrices could be effectively used for MSI despite their volatility. For example, an entire mouse brain could be imaged consecutively in both positive and negative ion mode with 50 × 50 µm(2) pixels in ~35 min. We demonstrate that these speeds make possible both faster and higher resolution imaging of biological tissues on practical timescales.

CONCLUSIONS

These high acquisition speeds now make possible whole new classes of matrices that are unstable under high vacuum for MALDI-MSI studies. This provides researchers with far greater range and flexibility in choosing the best matrix for the given sample and analytes that they wish to detect. In addition, such instruments allow MSI to be performed at higher resolution across larger areas on practical time scales.

Metabolomic Analysis of Oxidative and Glycolytic Skeletal Muscles by Matrix-Assisted Laser Desorption/IonizationMass Spectrometric Imaging (MALDI MSI)

Skeletal muscles are composed of heterogeneous muscle fibers that have different physiological, morphological, biochemical, and histological characteristics. In this work, skeletal muscles extensor digitorum longus, soleus, and whole gastrocnemius were analyzed by matrix-assisted laser desorption/ionization mass spectrometry to characterize small molecule metabolites of oxidative and glycolytic muscle fiber types as well as to visualize biomarker localization. Multivariate data analysis such as principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were performed to extract significant features. Different metabolic fingerprints were observed from oxidative and glycolytic fibers. Higher abundances of biomolecules such as antioxidant anserine as well as acylcarnitines were observed in the glycolytic fibers, whereas taurine and some nucleotides were found to be localized in the oxidative fibers.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Applications of mass spectrometry for cellular lipid analysis

Mass spectrometric analysis of cellular lipids is an enabling technology for lipidomics, which is a rapidly-developing research field. In this review, we briefly discuss the principles, advantages, and possible limitations of electrospray ionization (ESI) and matrix assisted laser desorption/ionization (MALDI) mass spectrometry-based methodologies for the analysis of lipid species. The applications of these methodologies to lipidomic research are also summarized.

Matrix coating assisted by an electric field (MCAEF) for enhanced tissue imaging by MALDI-MS

A novel technique, termed matrix coating assisted by an electric field (MCAEF), for enhancing tissue imaging by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was developed in this study. In this technique a static and uniform electric field is applied to sliced tissue sections during matrix spray-coating, resulting in the enrichment of positively or negatively chargeable analytes in the MALDI matrix layer. Experimental results show that MCAEF not only increased the sensitivity of lipid and protein detection across the board in the subsequent MALDI-MS analyses, but also resulted in successful imaging of a larger number of analytes. MALDI imaging enhancement with MCAEF was observed for various tissues (rat liver, rat brain, and porcine adrenal gland) and with different MALDI matrices (e.g., quercetin, 2-mercaptobenzothiazole, dithranol, 9-aminoacridine, and sinapinic acid) and the sensitivity increases were independent of the solvent compositions and pH values of the matrix solutions. Taking rat brain as an example, MCAEF led to the on-tissue detection and imaging of 648 identified lipids by combining positive and negative ion detection by MALDI-Fourier transform ion cyclotron resonance MS and with quercetin as the matrix, as compared to only 344 lipids without MCAEF. For protein imaging, up to 232 protein signals were successfully detected in rat brain tissue sections by MALDI-time-of-flight MS within a mass range of 3500 to 37 000 Da, as compared to 119 without MCAEF. MCAEF also enabled the detection of higher molecular-weight proteins. These results demonstrate the advantages of MCAEF for overall performance improvements in MALDI imaging and we believe that this technique has the potential to become a standard practice for MALDI tissue imaging.

MALDI-Ion Mobility Mass Spectrometry of Lipids in Negative Ion Mode

Profiling and imaging MALDI mass spectrometry (MS) allows detection and localization of biomolecules in tissue, of which lipids are a major component. However, due to the in situ nature of this technique, complexity of tissue and need for a chemical matrix, the recorded signal is complex and can be difficult to assign. Ion mobility adds a dimension that provides coarse shape information, separating isobaric lipids, peptides, and oligonucleotides along distinct familial trend lines before mass analysis. Previous work using MALDI-ion mobility mass spectrometry to analyze and image lipids has been conducted mainly in positive ion mode, although several lipid classes ionize preferentially in negative ion mode. This work highlights recent data acquired in negative ion mode to detect glycerophosphoethanolamines (PEs), glycerophosphoserines (PSs), glycerophosphoglycerols (PGs), glycerolphosphoinositols (PIs), glycerophosphates (PAs), sulfatides (STs), and gangliosides from standard tissue extracts and directly from mouse brain tissue. In particular, this study focused on changes in ion mobility based upon lipid head groups, composition of radyl chain (# of carbons and double bonds), diacyl versus plasmalogen species, and hydroxylation of species. Finally, a MALDI-ion mobility imaging run was conducted in negative ion mode, resulting in the successful ion mapping of several lipid species.

Intraoperative tissue identification using rapid evaporative ionization mass spectrometry

Rapid evaporative ionization mass spectrometry (REIMS) is an emerging technique that allows near-real-time characterization of human tissue in vivo by analysis of the aerosol ("smoke") released during electrosurgical dissection. The coupling of REIMS technology with electrosurgery for tissue diagnostics is known as the intelligent knife (iKnife). This study aimed to validate the technique by applying it to the analysis of fresh human tissue samples ex vivo and to demonstrate the translation to real-time use in vivo in a surgical environment. A variety of tissue samples from 302 patients were analyzed in the laboratory, resulting in 1624 cancerous and 1309 noncancerous database entries. The technology was then transferred to the operating theater, where the device was coupled to existing electrosurgical equipment to collect data during a total of 81 resections. Mass spectrometric data were analyzed using multivariate statistical methods, including principal components analysis (PCA) and linear discriminant analysis (LDA), and a spectral identification algorithm using a similar approach was implemented. The REIMS approach differentiated accurately between distinct histological and histopathological tissue types, with malignant tissues yielding chemical characteristics specific to their histopathological subtypes. Tissue identification via intraoperative REIMS matched the postoperative histological diagnosis in 100% (all 81) of the cases studied. The mass spectra reflected lipidomic profiles that varied between distinct histological tumor types and also between primary and metastatic tumors. Thus, in addition to real-time diagnostic information, the spectra provided additional information on divergent tumor biochemistry that may have mechanistic importance in cancer.

Imaging of lipids in rat heart by MALDI-MS with silver nanoparticles

Lipids are a major component of heart tissue and perform several important functions such as energy storage, signaling, and as building blocks of biological membranes. The heart lipidome is quite diverse consisting of glycerophospholipids such as phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylglycerols (PGs), cardiolipins (CLs), and glycerolipids, mainly triacylglycerols (TAGs). In this study, mass spectrometry imaging (MSI) enabled by matrix implantation of ionized silver nanoparticles (AgNP) was used to map several classes of lipids in heart tissue. The use of AgNP matrix implantation was motivated by our previous work showing that implantation doses of only 10(14)/cm(2) of 2 nm gold nanoparticulates into the first 10 nm of the near surface of the tissue enabled detection of most brain lipids (including neutral lipid species such as cerebrosides) more efficiently than traditional organic MALDI matrices. Herein, a similar implantation of 500 eV AgNP(-) across the entire heart tissue section results in a quick, reproducible, solvent-free, uniform matrix concentration of 6 nm AgNP residing near the tissue surface. MALDI-MSI analysis of either positive or negative ions produce high-quality images of several heart lipid species. In negative ion mode, 24 lipid species [16 PEs, 4 PIs, 1 PG, 1 CL, 2 sphingomyelins (SMs)] were imaged. Positive ion images were also obtained from 29 lipid species (10 PCs, 5 PEs, 5 SMs, 9 TAGs) with the TAG species being heavily concentrated in vascular regions of the heart.

Formal lithium fixation improves direct analysis of lipids in tissue by mass spectrometry

Mass spectrometry imaging is a powerful method for imaging and in situ characterization of lipids in thin tissue sections. Structural elucidation of lipids is often achieved via collision induced dissociation, and lithium-lipid adducts have been widely reported as providing the most structurally informative fragment ions. We present a method for the incorporation of lithium salts into tissue imaging experiments via fixation of samples in formal lithium solutions. The method is suitable for preparation of single tissue sections, or as an immersion fixation method for whole tissue blocks or organs prior to sectioning. We compare lithium adduct detection and MALDI-MSI of murine brain from analysis of tissues prepared in different ways. Tissues prepared in formal solutions containing lithium or sodium salts before coating in matrix via air-spray deposition are compared with fresh samples coated in lithium-doped matrix preparations by either dry-coating or air-spray deposition. Sample preparation via fixation in formal lithium is shown to yield the highest quality images of lithium adducts, resulting in acquisition of more informative product ion spectra in MALDI MS/MS profiling and imaging experiments. Finally, the compatibility of formal lithium solutions with standard histological staining protocols (hemotoxylin and eosin, Van Giessen and Oil Red O) is demonstrated in a study of human liver tissue.

Imaging mass spectrometry distinguished the cancer and stromal regions of oral squamous cell carcinoma by visualizing phosphatidylcholine (16:0/16:1) and phosphatidylcholine (18:1/20:4)

Most oral cancers are oral squamous cell carcinoma (OSCC). The anatomical features of OSCC have been histochemically evaluated with hematoxylin and eosin. However, the border between the cancer and stromal regions is unclear and large portions of the cancer and stromal regions are resected in surgery. To reduce the resected area and maintain oral function, a new method of diagnosis is needed. In this study, we tried to clearly distinguish the border on the basis of biomolecule distributions visualized by imaging mass spectrometry (IMS). In the IMS dataset, eleven signals were significantly different in intensity (p < 0.01) between the cancer and stromal regions. Two signals at m/z 770.5 and m/z 846.6 were distributed in each region, and a clear border was revealed. Tandem mass spectrometric (MS/MS) analysis identified these signals as phosphatidylcholine (PC) (16:0/16:1) at m/z 770.5 in the cancer region and PC (18:1/20:4) at m/z 846.6 in the stromal region. Moreover, the distribution of PC species containing arachidonic acid in the stromal region suggests that lymphocytes accumulated in response to the inflammation caused by cancer invasion. In conclusion, the cancer and stromal regions of OSCCs were clearly distinguished by use of these PC species and IMS analysis, and this molecular identification can provide important information to elucidate the mechanism of cancer invasion.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

MALDI profiles of proteins and lipids for the rapid characterisation of upper GI-tract cancers

AIM

To identify a reliable MALDI 'cancer fingerprint' to aid in the rapid detection and characterisation of malignant upper GI-tract disease from endoscopic biopsies.

METHODS

A total of 183 tissue biopsies were collected from 126 patients with or without oesophago-gastric malignancy and proteins and lipids separated by methanol/chloroform extraction. Peak intensities in the lipid and protein MALDI spectra from five types of samples (normal oesophageal mucosa from controls, normal oesophageal mucosa from patients with oesophageal adenocarcinoma, nondysplastic Barrett's oesophagus, oesophageal adenocarcinoma, normal gastric mucosa and gastric adenocarcinoma) were compared using non-parametric statistical tests and ROC analyses.

RESULTS

Normal oesophageal and gastric tissue generated distinct MALDI spectra characterised by higher levels of calgranulins in oesophageal tissue. MALDI spectra of polypeptides and lipids discriminated between oesophageal adenocarcinoma and Barrett's and normal oesophagus, and between gastric cancer and normal stomach. Many down-regulations were unique to each cancer type whilst some up-regulations, most notably increased HNPs 1-3, were common.

CONCLUSIONS

MALDI spectra of small tissue biopsies generated with this straightforward method can be used to rapidly detect numerous cancer-associated biochemical changes. These can be used to identify upper GI-tract cancers regardless of tumour location.

Orthogonal organic and aqueous-based washes of tissue sections to enhance protein sensitivity by MALDI imaging mass spectrometry

Imaging mass spectrometry (IMS) is an emergent and innovative approach for measuring the composition, abundance and regioselectivity of molecules within an investigated area of fixed dimension. Although providing unprecedented molecular information compared with conventional MS techniques, enhancement of protein signature by IMS is still necessary and challenging. This paper demonstrates the combination of conventional organic washes with an optimized aqueous-based buffer for tissue section preparation before matrix-assisted laser desorption/ionization (MALDI) IMS of proteins. Based on a 500 mM ammonium formate in water-acetonitrile (9:1; v/v, 0.1% trifluororacetic acid, 0.1% Triton) solution, this buffer wash has shown to significantly enhance protein signature by profiling and IMS (~fourfold) when used after organic washes (70% EtOH followed by 90% EtOH), improving the quality and number of ion images obtained from mouse kidney and a 14-day mouse fetus whole-body tissue sections, while maintaining a similar reproducibility with conventional tissue rinsing. Even if some protein losses were observed, the data mining has demonstrated that it was primarily low abundant signals and that the number of new peaks found is greater with the described procedure. The proposed buffer has thus demonstrated to be of high efficiency for tissue section preparation providing novel and complementary information for direct on-tissue MALDI analysis compared with solely conventional organic rinsing.

Bio-recognition and functional lipidomics by glycosphingolipid transfer technology

Through glycosphingolipid biochemical research, we developed two types of transcription technologies. One is a biochemical transfer of glycosphingolipids to peptides. The other is a physicochemical transfer of glycosphingolipids in silica gel to the surface of a plastic membrane. Using the first technology, we could prepare peptides which mimic the shapes of glycosphingolipid molecules by biopanning with a phage-displayed peptide library and anti-glycosphingolipid antibodies as templates. The peptides thus obtained showed biological properties and functions similar to those of the original glycosphingolipids, such as lectin binding, glycosidase modulation, inhibition of tumor metastasis and immune response against the original antigen glycosphingolipid, and we named them glyco-replica peptides. The results showed that the newly prepared peptides could be used effectively as a bio-recognition system and suggest that the glyco-replica peptides can be widely applied to therapeutic fields. Using the second technology, we could establish a functional lipidomics with a thin-layer chromatography-blot/matrix-assisted laser desorption ionization-time of flight mass spectrometry (TLC-Blot/MALDI-TOF MS) system. By transferring glycosphingolipids on a plastic membrane surface from a TLC plate, innovative biochemical approaches such as simple purification of individual glycosphingolipids, binding studies, and enzyme reactions could be developed. The combinations of these biochemical approaches and MALDI-TOF MS on the plastic membrane could provide new strategies for glycosphingolipid science and the field of lipidomics. In this review, typical applications of these two transfer technologies are introduced.(Communicated by Kunihiko SUZUKI, M.J.A.).

Analysis of neuropeptides by MALDI imaging mass spectrometry

Matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) is one of the most effective tools for localizing small molecules and compounds directly in thin tissue sections. MALDI IMS should be used when the distribution of molecular species is not known and to localize changes due to a disease process or a treatment. In recent years it has become increasingly clear that many pathological processes are not readily correlated to dramatic changes in protein levels. MALDI IMS can aid the localization of areas where the cellular concentration of proteins may be high enough to play an important biological role, but when the precise location is unknown. Here, we present a MALDI IMS protocol and data analysis of molecular imaging of multiple rat brain sections.

Cellular membrane phospholipids act as a depository for quaternary amine containing drugs thus competing with the acetylcholine/nicotinic receptor

We previously demonstrated that ammonium- or guanidinium-phosphate interactions are key to forming noncovalent complexes (NCXs) through salt bridge formation with G-protein coupled receptors (GPCR), which are immersed in the cell membrane's lipids. The present work highlights MALDI ion mobility coupled to orthogonal time-of-flight mass spectrometry (MALDI IM oTOF MS) as a method to determine qualitative and relative quantitative affinity of drugs to form NCXs with targeted GPCRs' epitopes in a model system using, bis-quaternary amine based drugs, α- and β- subunit epitopes of the nicotinic acetylcholine receptor' (nAChR) and phospholipids. Bis-quaternary amines proved to have a strong affinity for all nAChR epitopes and negatively charged phospholipids, even in the presence of the physiological neurotransmitter acetylcholine. Ion mobility baseline separated isobaric phosphatidyl ethanolamine and a matrix cluster, providing an accurate estimate for phospholipid counts. Overall this technique is a powerful method for screening drugs' interactions with targeted lipids and protein respectively containing quaternary amines and guanidinium moieties.

Spatiotemporal alteration of phospholipids and prostaglandins in a rat model of spinal cord injury

We determined quantitative and qualitative alterations in lipids during the occurrence and progression of spinal cord injury (SCI) in rats to identify potential clinical indicators of SCI pathology. Imaging mass spectrometry (IMS) was used to visualize twelve molecular species of phosphatidylcholine (PC) on thin slices of spinal cord with SCI. In addition, twelve species of phospholipids and five species of prostaglandins (PGs) were quantified by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) of lipid extracts from control/injured spinal cords. Unique distribution patterns were observed for phospholipids with different fatty acid compositions, and distinct dynamic changes were seen in both their amounts and their distributions in tissue as tissue damage resulting from SCI progressed. In particular, PCs containing docosahexaenoic acid localized to the large nucleus in the anterior horn region at one day post-SCI and rapidly decreased thereafter. In contrast, PCs containing arachidonic acid (AA-PCs) were normally found in the posterior horn region and were intensely and temporarily elevated one week after SCI. Lysophosphatidylcholines (LPCs) also increased at the same SCI stage and in regions with elevated AA-PCs, indicating the release of AA and the production of PGs. Moreover, LC-ESI-MS/MS analysis of lipid extracts from the spinal cord tissue at the impact site demonstrated a peak in PGE2 that reflected the elevation/reduction pattern of AA-PCs and LPC. Although further investigation is required, we suggest that invasive immune cells that penetrated from the impaired blood-brain barrier at 1-2 weeks post-SCI may have produced LPCs, released AA from AA-PCs, and produced PGs in SCI tissue at sites enriched in AA-PCs/LPC.

Enhanced sensitivity for high spatial resolution lipid analysis by negative ion mode matrix assisted laser desorption ionization imaging mass spectrometry

We have achieved enhanced lipid imaging to a ~10 μm spatial resolution using negative ion mode matrix assisted laser desorption ionization (MALDI) imaging mass spectrometry, sublimation of 2,5-dihydroxybenzoic acid as the MALDI matrix, and a sample preparation protocol that uses aqueous washes. We report on the effect of treating tissue sections by washing with volatile buffers at different pHs prior to negative ion mode lipid imaging. The results show that washing with ammonium formate, pH 6.4, or ammonium acetate, pH 6.7, significantly increases signal intensity and number of analytes recorded from adult mouse brain tissue sections. Major lipid species measured were glycerophosphoinositols, glycerophosphates, glycerolphosphoglycerols, glycerophosphoethanolamines, glycerophospho-serines, sulfatides, and gangliosides. Ion images from adult mouse brain sections that compare washed and unwashed sections are presented and show up to 5-fold increases in ion intensity for washed tissue. The sample preparation protocol has been found to be applicable across numerous organ types and significantly expands the number of lipid species detectable by imaging mass spectrometry at high spatial resolution.

Multi-dimensional mass spectrometry-based shotgun lipidomics and novel strategies for lipidomic analyses

Since our last comprehensive review on multi-dimensional mass spectrometry-based shotgun lipidomics (Mass Spectrom. Rev. 24 (2005), 367), many new developments in the field of lipidomics have occurred. These developments include new strategies and refinements for shotgun lipidomic approaches that use direct infusion, including novel fragmentation strategies, identification of multiple new informative dimensions for mass spectrometric interrogation, and the development of new bioinformatic approaches for enhanced identification and quantitation of the individual molecular constituents that comprise each cell's lipidome. Concurrently, advances in liquid chromatography-based platforms and novel strategies for quantitative matrix-assisted laser desorption/ionization mass spectrometry for lipidomic analyses have been developed. Through the synergistic use of this repertoire of new mass spectrometric approaches, the power and scope of lipidomics has been greatly expanded to accelerate progress toward the comprehensive understanding of the pleiotropic roles of lipids in biological systems.

Imaging of phospholipids in formalin fixed rat brain sections by matrix assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a valuable tool for the analysis of molecules directly from tissue. Imaging of phospholipids is gaining widespread interest, particularly as these lipids have been implicated in a variety of pathologic processes. Formalin fixation (FF) is the standard protocol used in histology laboratories worldwide to preserve tissue for analysis, in order to aid in the diagnosis and prognosis of diseases. This study assesses MALDI imaging of phospholipids directly in formalin fixed tissue, with a view to future analysis of archival tissue. This investigation proves the viability of MALDI-MSI for studying the distribution of lipids directly in formalin fixed tissue, without any pretreatment protocols. High quality molecular images for several phosphatidylcholine (PC) and sphingomyelin (SM) species are presented. Images correspond well with previously published data for the analysis of lipids directly from freshly prepared tissue. Different ionization pathways are observed when analyzing fixed tissue compared with fresh, and this change was found to be associated with formalin buffers employed in fixation protocols. The ability to analyze lipids directly from formalin fixed tissue opens up new doors in the investigation of disease profiles. Pathologic specimens taken for histologic investigation can be analyzed by MALDI-MS to provide greater information on the involvement of lipids in diseased tissue.

Protein and lipid MALDI profiles classify breast cancers according to the intrinsic subtype

Background

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has been demonstrated to be useful for molecular profiling of common solid tumors. Using recently developed MALDI matrices for lipid profiling, we evaluated whether direct tissue MALDI MS analysis on proteins and lipids may classify human breast cancer samples according to the intrinsic subtype.

Methods

Thirty-four pairs of frozen, resected breast cancer and adjacent normal tissue samples were analyzed using histology-directed, MALDI MS analysis. Sinapinic acid and 2,5-dihydroxybenzoic acid/α-cyano-4-hydroxycinnamic acid were manually deposited on areas of each tissue section enriched in epithelial cells to identify lipid profiles, and mass spectra were acquired using a MALDI-time of flight instrument.

Results

Protein and lipid profiles distinguish cancer from adjacent normal tissue samples with the median prediction accuracy of 94.1%. Luminal, HER2+, and triple-negative tumors demonstrated different protein and lipid profiles, as evidenced by permutation P values less than 0.01 for 0.632+ bootstrap cross-validated misclassification rates with all classifiers tested. Discriminatory proteins and lipids were useful for classifying tumors according to the intrinsic subtype with median prediction accuracies of 80.0-81.3% in random test sets.

Conclusions

Protein and lipid profiles accurately distinguish tumor from adjacent normal tissue and classify breast cancers according to the intrinsic subtype.

Multiplexed molecular descriptors of pressure ulcers defined by imaging mass spectrometry

The pathogenesis of impaired healing within pressure ulcers remains poorly characterized and rarely examined. We describe the results of a pilot study that applies matrix-assisted laser desorption/ionization imaging mass spectrometry technology for direct tissue analysis to evaluate proteomic signatures ranging from 2 to 20 kDa and phospholipids from 300-1,200 Da in focal regions within the wound microenvironment. Distinguishing molecular differences were apparent between upper vs. lower regions of ulcers and further contrasted against adjacent dermis and epidermal margins using protein profiles, ion density maps, principal component analysis and significant analysis of microarrays. Several proteins previously uncharacterized in pressure ulcers, the α-defensins (human neutrophil peptide [HNP]-1, -2, -3), are potential markers indicating whether the wound status is improving or being prolonged in a deleterious, chronic state. Thymosin β4 appears to be a favorable protein marker showing higher relative levels in adjacent dermis and maturing areas of the wound bed. Lipidomic examination revealed the presence of major lipid classes: glycerophosphocholines, glycerophosphoglycerols, glycerophosphoinositols, and triacylglycerols. Our pilot data examined from either a global perspective using proteomic or lipidomic signatures or as individual distributions reveal that imaging mass spectrometry technology can be effectively used for discovery and spatial mapping of molecular disturbances within the microenvironment of chronic wounds.

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.

Mapping lipid alterations in traumatically injured rat spinal cord by desorption electrospray ionization imaging mass spectrometry

Desorption electrospray ionization (DESI) mass spectrometry (MS) is used in an imaging mode to interrogate the lipid profiles of 15 μm thin tissue cross sections of injured rat spinal cord and normal healthy tissue. Increased relative intensities of fatty acids, diacylglycerols, and lysolipids (between +120% and +240%) as well as a small decrease in intensities of lipids (-30%) were visualized in the lesion epicenter and adjacent areas after spinal cord injury. This indicates the hydrolysis of lipids during the demyelination process due to activation of phospholipase A(2) enzyme. In addition, signals corresponding to oxidative degradation products, such as prostaglandin and hydroxyeicosatetraenoic acid, exhibited increased signal intensity by a factor of 2 in the negative ion mode in lesions relative to the normal healthy tissue. Analysis of malondialdehyde, a product of lipid peroxidation and marker of oxidative stress, was accomplished in the ambient environment using reactive DESI mass spectrometry imaging. This was achieved by electrospraying reagent solution containing dinitrophenylhydrazine as high-velocity charged droplets onto the tissue section. The hydrazine reacts selectively and rapidly with the carbonyl groups of malondialdehyde, and signal intensity of twice the intensity was detected in the lesions compared to healthy spinal cord. With a small amount of tissue sample, DESI-MS imaging provides information on the composition and distribution of specific compounds (limited by the occurrence of isomeric lipids with very similar fragmentation patterns) in lesions after spinal cord injury in comparison with normal healthy tissue allowing identification of the extent of the lesion and its repair.

Matrix-assisted laser desorption/ionization imaging mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a powerful tool that enables the simultaneous detection and identification of biomolecules in analytes. MALDI-imaging mass spectrometry (MALDI-IMS) is a two-dimensional MALDI-mass spectrometric technique used to visualize the spatial distribution of biomolecules without extraction, purification, separation, or labeling of biological samples. MALDI-IMS has revealed the characteristic distribution of several biomolecules, including proteins, peptides, amino acids, lipids, carbohydrates, and nucleotides, in various tissues. The versatility of MALDI-IMS has opened a new frontier in several fields such as medicine, agriculture, biology, pharmacology, and pathology. MALDI-IMS has a great potential for discovery of unknown biomarkers. In this review, we describe the methodology and applications of MALDI-IMS for biological samples.

Gangliosides' analysis by MALDI-ion mobility MS

The combination of ion mobility with matrix-assisted laser desorption/ionization allows for the rapid separation and analysis of biomolecules in complex mixtures (such as tissue sections and cellular extracts), as isobaric lipid, peptide, and oligonucleotide molecular ions are pre-separated in the mobility cell before mass analysis. In this study, MALDI-IM MS is used to analyze gangliosides, a class of complex glycosphingolipids that has different degrees of sialylation. Both GD1a and GD1b, structural isomers, were studied to see the effects on gas-phase structure depending upon the localization of the sialic acids. A total ganglioside extract from mouse brain was also analyzed to measure the effectiveness of ion mobility to separate out the different ganglioside species in a complex mixture.

Molecular imaging by mass spectrometry--looking beyond classical histology

Imaging mass spectrometry (IMS) using matrix-assisted laser desorption ionization (MALDI) is a new and effective tool for molecular studies of complex biological samples such as tissue sections. As histological features remain intact throughout the analysis of a section, distribution maps of multiple analytes can be correlated with histological and clinical features. Spatial molecular arrangements can be assessed without the need for target-specific reagents, allowing the discovery of diagnostic and prognostic markers of different cancer types and enabling the determination of effective therapies.

An introduction to sphingolipid metabolism and analysis by new technologies

Sphingolipids (SP) are a complex class of molecules found in essentially all eukaryotes and some prokaryotes and viruses where they influence membrane structure, intracellular signaling, and interactions with the extracellular environment. Because of the combinatorial nature of their biosynthesis, there are thousands of SP subspecies varying in the lipid backbones and complex phospho- and glycoheadgroups. Therefore, comprehensive or “sphingolipidomic” analyses (structure-specific, quantitative analyses of all SP, or at least all members of a critical subset) are needed to know which and how much of these subspecies are present in a system as a step toward understanding their functions. Mass spectrometry and related novel techniques are able to quantify a small fraction, but nonetheless a substantial number, of SP and are beginning to provide information about their localization. This review summarizes the basic metabolism of SP and state-of-art mass spectrometric techniques that are producing insights into SP structure, metabolism, functions, and some of the dysfunctions of relevance to neuromedicine.

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.