Tissue Washing Improves Native Ambient Mass Spectrometry Detection of Membrane Proteins Directly from Tissue.

Native ambient mass spectrometry enables the in situ analysis of proteins and their complexes directly from tissue, providing both structural and spatial information. Until recently, the approach was applied exclusively to the analysis of soluble proteins; however, there is a drive for new techniques that enable analysis of membrane proteins. Here we demonstrate native ambient mass spectrometry of membrane proteins, including ß-barrel and α-helical (single and multipass) integral membrane proteins and membrane-associated proteins incorporating lipid anchors, by integration of a simple washing protocol to remove soluble proteins. Mass spectrometry imaging revealed that washing did not disrupt the spatial distributions of the membrane and membrane-associated proteins. Some delocalization of the remaining soluble proteins was observed.

High-Throughput Deconvolution of Native Protein Mass Spectrometry Imaging Data Sets for Mass Domain Analysis.

Protein mass spectrometry imaging (MSI) with electrospray-based ambient ionization techniques, such as nanospray desorption electrospray ionization (nano-DESI), generates data sets in which each pixel corresponds to a mass spectrum populated by peaks corresponding to multiply charged protein ions. Importantly, the signal associated with each protein is split among multiple charge states. These peaks can be transformed into the mass domain by spectral deconvolution. When proteins are imaged under native/non-denaturing conditions to retain non-covalent interactions, deconvolution is particularly valuable in helping interpret the data. To improve the acquisition speed, signal-to-noise ratio, and sensitivity, native MSI is usually performed using mass resolving powers that do not provide isotopic resolution, and conventional algorithms for deconvolution of lower-resolution data are not suitable for these large data sets. UniDec was originally developed to enable rapid deconvolution of complex protein mass spectra. Here, we developed an updated feature set harnessing the high-throughput module, MetaUniDec, to deconvolve each pixel of native MSI data sets and transform m/z-domain image files to the mass domain. New tools enable the reading, processing, and output of open format .imzML files for downstream analysis. Transformation of data into the mass domain also provides greater accessibility, with mass information readily interpretable by users of established protein biology tools such as sodium dodecyl sulfate polyacrylamide gel electrophoresis.

Native Ambient Mass Spectrometry Imaging of Ligand-Bound and Metal-Bound Proteins in Rat Brain.

Label-free spatial mapping of the noncovalent interactions of proteins in their tissue environment has the potential to revolutionize life sciences research by providing opportunities for the interrogation of disease progression, drug interactions, and structural and molecular biology more broadly. Here, we demonstrate mass spectrometry imaging of endogenous intact noncovalent protein-ligand complexes in rat brain. The spatial distributions of a range of ligand-bound and metal-bound proteins were mapped in thin tissue sections by use of nanospray-desorption electrospray ionization. Proteins were identified directly from the tissue by top-down mass spectrometry. Three GDP-binding proteins (ADP ribosylation factor ARF3, ARF1, and GTPase Ran) were detected, identified, and imaged in their ligand-bound form. The nature of the ligand was confirmed by multiple rounds of tandem mass spectrometry. In addition, the metal-binding proteins parvalbumin-α and carbonic anhydrase 2 were detected, identified, and imaged in their native form, i.e., parvalbumin-α + 2Ca2+ and carbonic anhydrase + Zn2+. GTPase Ran was detected with both GDP and Mg2+ bound. Several natively monomeric proteins displaying distinct spatial distributions were also identified by top-down mass spectrometry. Protein mass spectrometry imaging was achieved at a spatial resolution of 200 µm.

Native Ambient Mass Spectrometry Enables Analysis of Intact Endogenous Protein Assemblies up to 145 kDa Directly from Tissue.

Untargeted label-free interrogation of proteins in their functional form directly from their physiological environment promises to transform life sciences research by providing unprecedented insight into their transient interactions with other biomolecules and xenobiotics. Native ambient mass spectrometry (NAMS) shows great potential for the structural analysis of endogenous protein assemblies directly from tissues; however, to date, this has been limited to assemblies of low molecular weight (<20 kDa) or very high abundance (hemoglobin tetramer in blood vessels, RidA homotrimer in kidney cortex tissues). The present work constitutes a step change for NAMS of protein assemblies: we demonstrate the detection and identification of a range of intact endogenous protein assemblies with various stoichiometries (dimer, trimer, and tetramer) from a range of tissue types (brain, kidney, liver) by the use of multiple NAMS techniques. Crucially, we demonstrate a greater than twofold increase in accessible molecular weight (up to 145 kDa). In addition, spatial distributions of protein assemblies up to 94 kDa were mapped in brain and kidney by nanospray desorption electrospray ionization (nano-DESI) mass spectrometry imaging.

Discovery, validation, and diagnostic ability of multiple protein-based biomarkers in saliva and gingival crevicular fluid to distinguish between health and periodontal diseases.

AIM: To discover and validate differential protein biomarker expression in saliva and gingival crevicular fluid (GCF) to discriminate objectively between periodontal health and plaque-induced periodontal disease states. MATERIALS AND METHODS: One-hundred and ninety participants were recruited from two centres (Birmingham and Newcastle upon Tyne, UK) comprising healthy, gingivitis, periodontitis, and edentulous donors. Samples from the Birmingham cohort were analysed by quantitative mass spectrometry proteomics for biomarker discovery. Shortlisted candidate proteins were then verified by enzyme-linked immunosorbent assay in both cohorts. Leave-one-out cross validation logistic regression analysis was used to identify the best performing biomarker panels. RESULTS: Ninety-five proteins were identified in both GCF and saliva samples, and 15 candidate proteins were selected based upon differences discovered between the donor groups. The best performing panels to distinguish between: health or gingivitis and periodontitis contained matrix metalloproteinase-9 (MMP9), S100A8, alpha-1-acid glycoprotein (A1AGP), pyruvate kinase, and age (area under the curve [AUC] 0.970); health and gingivitis contained MMP9, S100A8, A1AGP, and pyruvate kinase, but not age (AUC 0.768); and mild to moderate and advanced periodontitis contained MMP9, S100A8, A1AGP, pyruvate kinase, and age (AUC 0.789). CONCLUSIONS: Biomarker panels containing four proteins with and without age as a further parameter can distinguish between periodontal health and disease states.

Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue.

Membrane proteins constitute around two-thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over-expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin-0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre-treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top-down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue.

Mass Spectrometry Detection and Imaging of a Non-Covalent Protein-Drug Complex in Tissue from Orally Dosed Rats.

Here, we demonstrate detection by mass spectrometry of an intact protein-drug complex directly from liver tissue from rats that had been orally dosed with the drug. The protein-drug complex comprised fatty acid binding protein 1, FABP1, non-covalently bound to the small molecule therapeutic bezafibrate. Moreover, we demonstrate spatial mapping of the [FABP1+bezafibrate] complex across a thin section of liver by targeted mass spectrometry imaging. This work is the first demonstration of in situ mass spectrometry analysis of a non-covalent protein-drug complex formed in vivo and has implications for early stage drug discovery by providing a route to target-drug characterization directly from the physiological environment.

Native Mass Spectrometry Imaging of Proteins and Protein Complexes by Nano-DESI.

Previously, we have demonstrated native mass spectrometry imaging (native MSI) in which the spatial distribution of proteins maintained in their native-like, folded conformations was determined using liquid extraction surface analysis (LESA). While providing an excellent testbed for proof of principle, the spatial resolution of LESA is currently limited for imaging primarily by the physical size of the sampling pipette tip. Here, we report the adoption of nanospray-desorption electrospray ionization (nano-DESI) for native MSI, delivering substantial improvements in resolution versus native LESA MSI. In addition, native nano-DESI may be used for location-targeted top-down proteomics analysis directly from tissue. Proteins, including a homodimeric complex not previously detected by native MSI, were identified through a combination of collisional activation, high-resolution MS and proton transfer charge reduction.

Self-Incompatibility Triggers Irreversible Oxidative Modification of Proteins in Incompatible Pollen.

Self-incompatibility (SI) is used by many angiosperms to prevent self-fertilization and inbreeding. In common poppy (Papaver rhoeas), interaction of cognate pollen and pistil S-determinants triggers programmed cell death (PCD) of incompatible pollen. We previously identified that reactive oxygen species (ROS) signal to SI-PCD. ROS-induced oxidative posttranslational modifications (oxPTMs) can regulate protein structure and function. Here, we have identified and mapped oxPTMs triggered by SI in incompatible pollen. Notably, SI-induced pollen had numerous irreversible oxidative modifications, while untreated pollen had virtually none. Our data provide a valuable analysis of the protein targets of ROS in the context of SI-induction and comprise a benchmark because currently there are few reports of irreversible oxPTMs in plants. Strikingly, cytoskeletal proteins and enzymes involved in energy metabolism are a prominent target of ROS. Oxidative modifications to a phosphomimic form of a pyrophosphatase result in a reduction of its activity. Therefore, our results demonstrate irreversible oxidation of pollen proteins during SI and provide evidence that this modification can affect protein function. We suggest that this reduction in cellular activity could lead to PCD.

High-Field Asymmetric Waveform Ion Mobility Spectrometry and Native Mass Spectrometry: Analysis of Intact Protein Assemblies and Protein Complexes.

High-field asymmetric waveform ion mobility spectrometry (FAIMS) enables the separation of ions on the basis of their differential mobility in an asymmetric oscillating electric field. We, and others, have previously demonstrated the benefits of FAIMS for the analysis of peptides and denatured proteins. To date, FAIMS has not been integrated with native mass spectrometry of folded proteins and protein complexes, largely due to concerns over the heating effects associated with the high electric fields employed. Here, we demonstrate the newly introduced cylindrical FAIMS Pro device coupled with an Orbitrap Eclipse enables analysis of intact protein assemblies up to 147 kDa. No evidence for dissociation was detected suggesting that any field heating is insufficient to disrupt the noncovalent interactions governing these assemblies. Moreover, the FAIMS device was integrated into native liquid extraction surface analysis (LESA) MS of protein assemblies directly from thin tissue sections. Intact tetrameric hemoglobin (64 kDa) and trimeric reactive intermediate deiminase A (RidA, 43 kDa) were detected. Improvements in signal-to-noise of between 1.5× and 12× were observed for these protein assemblies on integration of FAIMS.

LESA Cyclic Ion Mobility Mass Spectrometry of Intact Proteins from Thin Tissue Sections.

Liquid extraction surface analysis (LESA) is an ambient surface sampling technique that allows the analysis of intact proteins directly from tissue samples via mass spectrometry. Integration of ion mobility separation to LESA mass spectrometry workflows has shown significant improvements in the signal-to-noise ratios of the resulting protein mass spectra and hence the number of proteins detected. Here, we report the use of a quadrupole-cyclic ion mobility-time-of-flight mass spectrometer (Q-cIM-ToF) for the analysis of proteins from mouse brain and rat kidney tissues sampled via LESA. Among other features, the instrument allows multiple pass cyclic ion mobility separation, with concomitant increase in resolving power. Single-pass experiments enabled the detection of 30 proteins from mouse brain tissue, rising to 44 when quadrupole isolation was employed. In the absence of ion mobility separation, 21 proteins were detected in rat kidney tissue including the abundant α- and ß-globin chains from hemoglobin. Single-pass cyclic ion mobility mass spectrometry enabled the detection of 60 additional proteins. Multipass experiments of a narrow m/z range (m/z 870-920) resulted in the detection of 24 proteins (one pass), 37 proteins (two passes) and 54 proteins (three passes), thus demonstrating the benefits of improved mobility resolving power.

Electroporation and Mass Spectrometry: A New Paradigm for In Situ Analysis of Intact Proteins Direct from Living Yeast Colonies.

Yeasts constitute an oft-neglected class of pathogens among which the resistance to first-line treatments, attributed in part to mutations in efflux pumps, is rapidly emerging. Their thick, chitin-reinforced cell walls render cell lysis difficult, complicating their analysis and identification by methods routinely used for bacteria, including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Liquid extraction surface analysis mass spectrometry (LESA-MS) has previously been applied to the analysis of intact proteins from Gram-positive and Gram-negative bacterial colonies sampled directly on solid nutrient media. To date, a similar analysis of yeast colonies has not proved possible. Here we demonstrate the rapid release of intact yeast proteins for LESA-MS by electroporation using a home-built high-voltage device designed to lyse cells grown in colonies on agar media. Detection and identification of previously inaccessible proteins from baker's yeast Saccharomyces cerevisiae, as well as two clinically relevant yeast species (Candida glabrata and Cryptococcus neoformans), is shown. The electroporation approach also has the potential to be translated to other mass spectrometric analysis techniques, including MALDI and various ambient ionization methods.

Comprehensive LESA Mass Spectrometry Imaging of Intact Proteins by Integration of Cylindrical FAIMS.

The benefits of high field asymmetric waveform ion mobility spectrometry (FAIMS) for mass spectrometry imaging of intact proteins in thin tissue sections have been demonstrated previously. In those works, a planar FAIMS device coupled with a Thermo Elite mass spectrometer was employed. Here, we have evaluated a newly introduced cylindrical FAIMS device (the FAIMS Pro) coupled with a Thermo Fusion Lumos mass spectrometer for liquid extraction surface analysis mass spectrometry imaging of intact proteins in thin tissue sections from rat testes, kidney, and brain. The method makes use of multiple FAIMS compensation values at each location (pixel) of the imaging array. A total of 975 nonredundant protein species were detected in the testes imaging dataset, 981 in the kidney dataset, and 249 in the brain dataset. These numbers represent a 7-fold (brain) and over 10-fold (testes, kidney) improvement on the numbers of proteins previously detected in LESA FAIMS imaging, and a 10-fold to over 20-fold improvement on the numbers detected without FAIMS on this higher performance mass spectrometer, approaching the same order of magnitude as those obtained in top-down proteomics of cell lines. Nevertheless, high throughput identification within the LESA FAIMS imaging workflow remains a challenge.

In situ mass spectrometry analysis of intact proteins and protein complexes from biological substrates.

Advances in sample preparation, ion sources and mass spectrometer technology have enabled the detection and characterisation of intact proteins. The challenges associated include an appropriately soft ionisation event, efficient transmission and detection of the often delicate macromolecules. Ambient ion sources, in particular, offer a wealth of strategies for analysis of proteins from solution environments, and directly from biological substrates. The last two decades have seen rapid development in this area. Innovations include liquid extraction surface analysis, desorption electrospray ionisation and nanospray desorption electrospray ionisation. Similarly, developments in native mass spectrometry allow protein-protein and protein-ligand complexes to be ionised and analysed. Identification and characterisation of these large ions involves a suite of hyphenated mass spectrometry techniques, often including the coupling of ion mobility spectrometry and fragmentation techniques. The latter include collision, electron and photon-induced methods, each with their own characteristics and benefits for intact protein identification. In this review, recent developments for in situ protein analysis are explored, with a focus on ion sources and tandem mass spectrometry techniques used for identification.

Probing the Fundamentals of Native Liquid Extraction Surface Analysis Mass Spectrometry of Proteins: Can Proteins Refold during Extraction?

Native ambient mass spectrometry has the potential for simultaneous analysis of native protein structure and spatial distribution within thin tissue sections. Notwithstanding sensitivity, this information can, in principle, be obtained for any protein present with no requirement for a priori knowledge of protein identity. To date, native ambient mass spectrometry has primarily made use of the liquid extraction surface analysis (LESA) sampling technique. Here, we address a fundamental question: Are the protein structures observed following native liquid extraction surface analysis representative of the protein structures within the substrate, or does the extraction process facilitate refolding (or unfolding)? Specifically, our aim was to determine whether protein-ligand complexes observed following LESA are indicative of complexes present in the substrate, or an artifact of the sampling process. The systems investigated were myoglobin and its noncovalently bound heme cofactor, and the Zn-binding protein carbonic anhydrase and its binding with ethoxzolamide. Charge state distributions, drift time profiles, and collision cross sections were determined by liquid extraction surface analysis ion mobility mass spectrometry of native and denatured proteins and compared with those obtained by direct infusion electrospray. The results show that it was not possible to refold denatured proteins with concomitant ligand binding (neither heme, zinc, nor ethoxzolamide) simply by use of native-like LESA solvents. That is, protein-ligand complexes were only observed by LESA MS when present in the substrate.

Investigation of the 12-Month Stability of Dried Blood and Urine Spots Applying Untargeted UHPLC-MS Metabolomic Assays.

The use of dried blood spot (DBS) and dried urine spot (DUS) samples represents an attractive opportunity for researchers in biomedical metabolomics to collect whole blood and urine samples in the absence of a processing laboratory and so to allow collection in remote areas or in longitudinal studies away from the clinic. The 12-month stability of the thousands of metabolites present in these biofluids and the applicability of DBS and DUS samples for untargeted metabolomics applications has not previously been investigated in detail and compared to blood and urine samples. Here, the 12-month stability of DBS and DUS at different storage temperatures (-20, +4, and +21 °C) have been compared to plasma and urine biofluids stored at the same storage temperatures and time. Samples were analyzed applying complementary HILIC and C18 reversed-phase UHPLC-MS untargeted metabolomic assays. Results show that metabolites demonstrate increased stability in DBS and DUS compared to whole blood and urine at all storage temperatures and times. DBS and DUS stored at +21 °C are stable for up to 4 weeks but are not stable over a 1 year period. DBS and DUS showed good stability when stored at -20 °C for 1 year. We recommend that DBS and DUS samples are collected and transported within 28 days at room temperature and are stored for longer periods of time at -20 or -80 °C. The metabolomes of DUS samples and urine were very similar but the metabolome of DBS included additional metabolites not detected in plasma and therefore proposed to be released from cells in whole blood.

Application of High-Field Asymmetric Waveform Ion Mobility Separation to LESA Mass Spectrometry of Bacteria.

We have previously demonstrated the analysis of intact proteins directly from bacterial colonies (including Gram-negative and Gram-positive clinical isolates) grown on agar media by liquid extraction surface analysis mass spectrometry (LESA MS). Several challenges were identified in that work, including (1) interference of background signal derived from the nutrient media ( Escherichia coli), (2) a high density of protein peaks leading to the isolation of multiple protein precursor ions in a single window and consequent acquisition of composite tandem mass spectra ( Pseudomonas aeruginosa), and (3) the overabundance of secreted peptides suppressing peaks corresponding to proteins ( Staphylococcus aureus). Here, we present the coupling of high-field asymmetric waveform ion mobility spectrometry (FAIMS) separation into the LESA MS protocol, with the aim of resolving the aforementioned challenges and thus improving the capabilities of LESA MS for bacterial characterization. The results show that inclusion of FAIMS expands the range of detected proteins through separation of background peaks from protein signal, as well as through resolution of overlapping protein peaks which could not previously be isolated by LESA MS alone.

Direct Mass Spectrometry Analysis of Protein Complexes and Intact Proteins up to >70 kDa from Tissue.

Native liquid extraction surface analysis (LESA) mass spectrometry allows direct analysis of folded proteins and protein complexes from biological substrates, such as dried blood spots and thin tissue sections, by use of native-like extraction/ionization solvents. Previously, we have demonstrated native LESA mass spectrometry of folded proteins up to 16 kDa as well as the 64 kDa hemoglobin tetramer, from mouse tissues. With denaturing LESA solvents, the highest mass protein detected in tissue to date is ∼37 kDa. Here, we demonstrate native LESA mass spectrometry by use of a Q Exactive UHMR Hybrid Quadrupole-Orbitrap (QE-UHMR) mass spectrometer, pushing the upper mass limit of proteins detected in tissue to >70 kDa. Moreover, a protein trimer of 42 kDa was detected and its stoichiometry confirmed by higher energy collision dissociation (HCD). The benefits of inclusion of detergents in the LESA sampling solvent are also demonstrated.

Quantitative Imaging of Proteins in Tissue by Stable Isotope Labeled Mimetic Liquid Extraction Surface Analysis Mass Spectrometry.

Absolute quantification of proteins in tissue is important for numerous fields of study. Liquid chromatography-mass spectrometry (LC-MS) methods are the norm but typically involve lengthy sample preparation including tissue homogenization, which results in the loss of information relating to spatial distribution. Here, we propose liquid extraction surface analysis (LESA) mass spectrometry (MS) of stable isotope labeled mimetic tissue models for the spatially resolved quantification of intact ubiquitin in rat and mouse brain tissue. Measured ubiquitin concentrations are in agreement with values found in the literature. Images of rat and mouse brain tissue demonstrate spatial variation in the concentration of ubiquitin and demonstrate the utility of spatially resolved quantitative measurement of proteins in tissue. Although we have focused on ubiquitin, the method has the potential for broader application to the absolute quantitation of any endogenous protein or protein-based drug in tissue.

High Field Asymmetric Waveform Ion Mobility Spectrometry in Nontargeted Bottom-up Proteomics of Dried Blood Spots.

Despite the great potential of dried blood spots (DBS) as a source of endogenous proteins for biomarker discovery, the literature relating to nontargeted bottom-up proteomics of DBS is sparse, primarily due to the inherent complexity and very high dynamic range associated with these samples. Here, we present proof-of-concept results in which we have coupled high field asymmetric waveform ion mobility spectrometry (FAIMS) with liquid chromatography-tandem mass spectrometry (LC-MS/MS) for nontargeted bottom-up proteomics of DBS with the aim of addressing these challenges. We, and others, have previously demonstrated the benefits of FAIMS more generally in proteomics including improved signal-to-noise and extended proteome coverage, and the aim of the current work was to extend those benefits specifically to DBS. The DBS samples were either extracted by the more traditional manual "punch and elute" approach or by an automated liquid surface extraction (LESA) approach prior to trypsin digestion. The resulting samples were analyzed by LC-MS/MS and LC-FAIMS-MS/MS analysis. The results show that the total number of proteins identified increased by ∼50% for the punch and elute samples and ∼45% for the LESA samples in the LC-FAIMS-MS/MS analysis. For both the punch and elute samples and the LESA samples, ∼30% of the total proteins identified were observed in both the LC-MS/MS and the LC-FAIMS-MS/MS data sets, illustrating the complementarity of the approaches. Overall, this work demonstrates the benefits of inclusion of FAIMS for nontargeted proteomics of DBS.