MaxQuant Software for Ion Mobility Enhanced Shotgun Proteomics.

Ion mobility can add a dimension to LC-MS based shotgun proteomics which has the potential to boost proteome coverage, quantification accuracy and dynamic range. Required for this is suitable software that extracts the information contained in the four-dimensional (4D) data space spanned by m/z, retention time, ion mobility and signal intensity. Here we describe the ion mobility enhanced MaxQuant software, which utilizes the added data dimension. It offers an end to end computational workflow for the identification and quantification of peptides and proteins in LC-IMS-MS/MS shotgun proteomics data. We apply it to trapped ion mobility spectrometry (TIMS) coupled to a quadrupole time-of-flight (QTOF) analyzer. A highly parallelizable 4D feature detection algorithm extracts peaks which are assembled to isotope patterns. Masses are recalibrated with a non-linear m/z, retention time, ion mobility and signal intensity dependent model, based on peptides from the sample. A new matching between runs (MBR) algorithm that utilizes collisional cross section (CCS) values of MS1 features in the matching process significantly gains specificity from the extra dimension. Prerequisite for using CCS values in MBR is a relative alignment of the ion mobility values between the runs. The missing value problem in protein quantification over many samples is greatly reduced by CCS aware MBR.MS1 level label-free quantification is also implemented which proves to be highly precise and accurate on a benchmark dataset with known ground truth. MaxQuant for LC-IMS-MS/MS is part of the basic MaxQuant release and can be downloaded from http://maxquant.org.

Online Parallel Accumulation-Serial Fragmentation (PASEF) with a Novel Trapped Ion Mobility Mass Spectrometer.

In bottom-up proteomics, peptides are separated by liquid chromatography with elution peak widths in the range of seconds, whereas mass spectra are acquired in about 100 microseconds with time-of-flight (TOF) instruments. This allows adding ion mobility as a third dimension of separation. Among several formats, trapped ion mobility spectrometry (TIMS) is attractive because of its small size, low voltage requirements and high efficiency of ion utilization. We have recently demonstrated a scan mode termed parallel accumulation - serial fragmentation (PASEF), which multiplies the sequencing speed without any loss in sensitivity (Meier et al., PMID: 26538118). Here we introduce the timsTOF Pro instrument, which optimally implements online PASEF. It features an orthogonal ion path into the ion mobility device, limiting the amount of debris entering the instrument and making it very robust in daily operation. We investigate different precursor selection schemes for shotgun proteomics to optimally allocate in excess of 100 fragmentation events per second. More than 600,000 fragmentation spectra in standard 120 min LC runs are achievable, which can be used for near exhaustive precursor selection in complex mixtures or accumulating the signal of weak precursors. In 120 min single runs of HeLa digest, MaxQuant identified more than 6,000 proteins without matching to a library and with high quantitative reproducibility (R > 0.97). Online PASEF achieves a remarkable sensitivity with more than 2,500 proteins identified in 30 min runs of only 10 ng HeLa digest. We also show that highly reproducible collisional cross sections can be acquired on a large scale (R > 0.99). PASEF on the timsTOF Pro is a valuable addition to the technological toolbox in proteomics, with a number of unique operating modes that are only beginning to be explored.

Pharmacoproteomic characterisation of human colon and rectal cancer.

Most molecular cancer therapies act on protein targets but data on the proteome status of patients and cellular models for proteome-guided pre-clinical drug sensitivity studies are only beginning to emerge. Here, we profiled the proteomes of 65 colorectal cancer (CRC) cell lines to a depth of > 10,000 proteins using mass spectrometry. Integration with proteomes of 90 CRC patients and matched transcriptomics data defined integrated CRC subtypes, highlighting cell lines representative of each tumour subtype. Modelling the responses of 52 CRC cell lines to 577 drugs as a function of proteome profiles enabled predicting drug sensitivity for cell lines and patients. Among many novel associations, MERTK was identified as a predictive marker for resistance towards MEK1/2 inhibitors and immunohistochemistry of 1,074 CRC tumours confirmed MERTK as a prognostic survival marker. We provide the proteomic and pharmacological data as a resource to the community to, for example, facilitate the design of innovative prospective clinical trials.

Comprehensive and reproducible phosphopeptide enrichment using iron immobilized metal ion affinity chromatography (Fe-IMAC) columns.

Advances in phosphopeptide enrichment methods enable the identification of thousands of phosphopeptides from complex samples. Current offline enrichment approaches using TiO(2), Ti, and Fe immobilized metal ion affinity chromatography (IMAC) material in batch or microtip format are widely used, but they suffer from irreproducibility and compromised selectivity. To address these shortcomings, we revisited the merits of performing phosphopeptide enrichment in an HPLC column format. We found that Fe-IMAC columns enabled the selective, comprehensive, and reproducible enrichment of phosphopeptides out of complex lysates. Column enrichment did not suffer from bead-to-sample ratio issues and scaled linearly from 100 µg to 5 mg of digest. Direct measurements on an Orbitrap Velos mass spectrometer identified >7500 unique phosphopeptides with 90% selectivity and good quantitative reproducibility (median cv of 15%). The number of unique phosphopeptides could be increased to more than 14,000 when the IMAC eluate was subjected to a subsequent hydrophilic strong anion exchange separation. Fe-IMAC columns outperformed Ti-IMAC and TiO(2) in batch or tip mode in terms of phosphopeptide identification and intensity. Permutation enrichments of flow-throughs showed that all materials largely bound the same phosphopeptide species, independent of physicochemical characteristics. However, binding capacity and elution efficiency did profoundly differ among the enrichment materials and formats. As a result, the often quoted orthogonality of the materials has to be called into question. Our results strongly suggest that insufficient capacity, inefficient elution, and the stochastic nature of data-dependent acquisition in mass spectrometry are the causes of the experimentally observed complementarity. The Fe-IMAC enrichment workflow using an HPLC format developed here enables rapid and comprehensive phosphoproteome analysis that can be applied to a wide range of biological systems.

Phosphoproteome Profiling Reveals Molecular Mechanisms of Growth-Factor-Mediated Kinase Inhibitor Resistance in EGFR-Overexpressing Cancer Cells.

Although substantial progress has been made regarding the use of molecularly targeted cancer therapies, resistance almost invariably develops and presents a major clinical challenge. The tumor microenvironment can rescue cancer cells from kinase inhibitors by growth-factor-mediated induction of pro-survival pathways. Here we show that epidermal growth factor receptor (EGFR) inhibition by Gefitinib is counteracted by growth factors, notably FGF2, and we assessed the global molecular consequences of this resistance at the proteome and phosphoproteome level in A431 cells. Tandem mass tag peptide labeling and quantitative mass spectrometry allowed the identification and quantification of 22 000 phosphopeptides and 8800 proteins in biological triplicates without missing values. The data show that FGF2 protects the cells from the antiproliferative effect of Gefitinib and largely prevents reprogramming of the proteome and phosphoproteome. Simultaneous EGFR/FGFR or EGFR/GSG2 (Haspin) inhibition overcomes this resistance, and the phosphoproteomic experiments further prioritized the RAS/MEK/ERK as well as the PI3K/mTOR axis for combination treatment. Consequently, the MEK inhibitor Trametinib prevented FGF2-mediated survival of EGFR inhibitor-resistant cells when used in combination with Gefitinib. Surprisingly, the PI3K/mTOR inhibitor Omipalisib reversed resistance mediated by all four growth factors tested, making it an interesting candidate for mitigating the effects of the tumor microenvironment.

Chemical Proteomics Uncovers EPHA2 as a Mechanism of Acquired Resistance to Small Molecule EGFR Kinase Inhibition.

Tyrosine kinase inhibitors (TKIs) have become an important therapeutic option for treating several forms of cancer. Gefitinib, an inhibitor of the epidermal growth factor receptor (EGFR), is in clinical use for treating non-small cell lung cancer (NSCLC) harboring activating EGFR mutations. However, despite high initial response rates, many patients develop resistance to gefitinib. The molecular mechanisms of TKI resistance often remain unclear. Here, we describe a chemical proteomic approach comprising kinase affinity purification (kinobeads) and quantitative mass spectrometry for the identification of kinase inhibitor resistance mechanisms in cancer cells. We identified the previously described amplification of MET and found EPHA2 to be more than 10-fold overexpressed (p < 0.001) in gefitinib-resistant HCC827 cells suggesting a potential role in developing resistance. siRNA-mediated EPHA2 knock-down or treating cells with the multikinase inhibitor dasatinib restored sensitivity to gefitinib. Of all dasatinib targets, EPHA2 exhibited the most drastic effect (p < 0.001). In addition, EPHA2 knockdown or ephrin-A1 treatment of resistant cells decreased FAK phosphorylation and cell migration. These findings confirm EPHA2 as an actionable drug target, provide a rational basis for drug combination approaches, and indicate that chemical proteomics is broadly applicable for the discovery of kinase inhibitor resistance.

Optimized Enrichment of Phosphoproteomes by Fe-IMAC Column Chromatography.

Phosphorylation is among the most important post-translational modifications of proteins and has numerous regulatory functions across all domains of life. However, phosphorylation is often substoichiometric, requiring selective and sensitive methods to enrich phosphorylated peptides from complex cellular digests. Various methods have been devised for this purpose and we have recently described a Fe-IMAC HPLC column chromatography setup which is capable of comprehensive, reproducible, and selective enrichment of phosphopeptides out of complex peptide mixtures. In contrast to other formats such as StageTips or batch incubations using TiO2 or Ti-IMAC beads, Fe-IMAC HPLC columns do not suffer from issues regarding incomplete phosphopeptide binding or elution and enrichment efficiency scales linearly with the amount of starting material. Here, we provide a step-by-step protocol for the entire phosphopeptide enrichment procedure including sample preparation (lysis, digestion, desalting), Fe-IMAC column chromatography (column setup, operation, charging), measurement by LC-MS/MS (nHPLC gradient, MS parameters) and data analysis (MaxQuant). To increase throughput, we have optimized several key steps such as the gradient time of the Fe-IMAC separation (15 min per enrichment), the number of consecutive enrichments possible between two chargings (>20) and the column recharging itself (<1 h). We show that the application of this protocol enables the selective (>90 %) identification of more than 10,000 unique phosphopeptides from 1 mg of HeLa digest within 2 h of measurement time (Q Exactive Plus).

Establishment of a 3D cell culture model of primary bovine mammary epithelial cells extracted from fresh milk.

For the investigation of molecular processes underlying diseases of the bovine mammary gland, primary bovine mammary epithelial cells (pbMEC) are used. They are known to contribute to the innate immune system of the bovine mammary gland. The functionality of pbMEC depends on the maintenance of in vivo characteristics. So far, the optimization of pbMEC culture conditions was intended in a variety of experiments. For this purpose, most of the studies used stable cell lines or primary cells obtained from udder biopsies of slaughtered animals. By contrast, within our study, pbMEC of healthy and first lactating Brown Swiss cows were non-invasively isolated from fresh milk. The non-invasively isolated pbMEC were cultivated on the extracellular matrix-like scaffold Matrigel®. Further, they were challenged with different compositions of proliferation media, containing lactogenic hormones and/or the essential amino acid L-lysine. Changes in expression levels of genes coding for milk proteins and for components of the janus kinase/signal transducers and activators of transcription (JAK-STAT) and mTOR pathways were analyzed by RT-qPCR. The secreted proteins were analyzed by LC-MS/MS measurements. We showed for the first time the establishment of a physiologically functional 3D cell culture model of pbMEC isolated from fresh milk. This represents a primary cell culture model system, based on non-invasive cell collection, that can be used to unravel physiological processes in an unbiased manner.

The target landscape of clinical kinase drugs.

Kinase inhibitors are important cancer therapeutics. Polypharmacology is commonly observed, requiring thorough target deconvolution to understand drug mechanism of action. Using chemical proteomics, we analyzed the target spectrum of 243 clinically evaluated kinase drugs. The data revealed previously unknown targets for established drugs, offered a perspective on the "druggable" kinome, highlighted (non)kinase off-targets, and suggested potential therapeutic applications. Integration of phosphoproteomic data refined drug-affected pathways, identified response markers, and strengthened rationale for combination treatments. We exemplify translational value by discovering SIK2 (salt-inducible kinase 2) inhibitors that modulate cytokine production in primary cells, by identifying drugs against the lung cancer survival marker MELK (maternal embryonic leucine zipper kinase), and by repurposing cabozantinib to treat FLT3-ITD-positive acute myeloid leukemia. This resource, available via the ProteomicsDB database, should facilitate basic, clinical, and drug discovery research and aid clinical decision-making.